Co-Valorization of Bauxite Residue and Soybean Processing Waste via Supercritical Water Gasification

A multifunctional red mud bed, conventional acid gas removal, and reforming-coupled OXZEO olefin synthesis

Abstract

Douzha (okara) and bauxite residue are individually awkward wastes that become tractable when co-processed. Douzha is 80–85 wt% water, a liability for any dry thermochemical route but an asset for supercritical water gasification, where water is the reaction medium rather than a drying burden. Bauxite residue is alkaline and iron-rich, impounded at over 120 Mt per year globally with under 3% utilization; its Fe₂O₃ content makes it a credible redox mediator and its residual sodium alkalinity a tar cracker, yet that same alkalinity is what makes it hazardous and unsaleable. This study proposes a flowsheet in which the supercritical water gasifier performs both duties at once: it gasifies the biomass over a multifunctional red mud bed and, in the same pass, dealkalizes the residue, transferring sodium into a separable N-K-P-S brine.

Three design conflicts are identified and resolved by decision. The salt separator is committed as a purposeful product unit rather than a protective device, because deliberate alkali removal is what converts a disposal liability into a saleable sorbent and cementitious feed. The absence of CO in hydrothermal product gas, which forecloses direct OXZEO coupling, is resolved by an intermediate bi-reforming stage; the alternative of suppressing methanation at higher gasifier severity is evaluated and rejected. Calcium-based desulfurization is removed entirely, on the grounds that CaS hydrolyses back to Ca(OH)₂ and H₂S in hot pressurized water, and is replaced by a conventional Rectisol acid gas wash reaching 0.1 ppm total sulfur including COS, with CO₂ separation in the same unit.

Analysis of the solids budget shows the binding constraint on the design is not moisture but pumpability: douzha arrives at approximately 17.2 wt% solids, already within the 18–22 wt% window, so red mud dosing and straw loading compete directly for the remaining headroom. Product slate spans light olefins, an N-K-P-S fertilizer brine, elemental sulfur, and a tiered set of bauxite-derived materials from dealkalized residue through iron recovery to scandium and gallium. Feedstock geography is found to be unfavourable: no Chinese province holds red mud and douzha at scale together, and the Guangxi–Guangdong corridor is identified as the shortest credible pairing.

Mass and energy balances are not yet closed. All numerical process data in this draft is flagged as placeholder, indicative or literature accordingly.

Document: Process design study — working draft
Date: 27 July 2026
Status: Full report outline, with Sections 1, 2 and 3 drafted in full
Feedstock case: Douzha (okara) + soybean straw co-slurry
Compliance frame: ISO 14067 · EU CBAM · ISCC PLUS · RED III · China ETS / CCER
Affiliation: Developed during a process engineering internship with the Green Fuels & Chemicals division of The Hong Kong and China Gas Company (Towngas), and its green-methanol venture VENEX, across Foshan, Guangdong and Jungar Banner, Inner Mongolia.

A personal study. The analysis, the design decisions and any errors are the author's own, and it is not a Towngas or VENEX publication.

How to read the numbers

Placeholder. Placeholder — pending balance closure. The mass and energy balances are not yet written; these figures are structural placeholders, not results.

Indicative. Indicative — a real figure drawn from general engineering knowledge, not yet traceable to a primary source. Carried over from the report's own provenance convention; it must not survive into a final version without a citation.

Literature. Literature — an anchor value from a cited source, resolving to the references.

Act 1

Why this is being built

Why this is being built. A process concept in 2026 is judged not only on yield and cost but on whether its carbon claim survives third-party audit and border adjustment — and that verdict reaches back up the flowsheet into engineering decisions taken long before any gas is made.

Panel 1
The problem statement
Industrial decarbonisation has moved from voluntary target-setting to enforceable trade and disclosure instruments. A process concept in 2026 is evaluated not only on yield and cost but on whether its carbon claim survives third-party audit and border adjustment.
That changes engineering decisions upstream — it determines which allocation method may be used, which determines whether a waste feedstock enters the boundary burden-free.
Panel 2
ISO 14067
The product carbon footprint standard. Its decisive feature here is the allocation hierarchy: avoid allocation by system subdivision, then allocate by physical relationship, then by economic value — documenting the choice. The cut-off (zero-burden) method lets waste material enter a system burden-free.
For the same physical product, cut-off, 50/50 and substitution methods can produce footprints differing by more than 20%. ISO 14067 requires the allocation choice and rationale to be transparently documented; a footprint that does not disclose the method is non-compliant.
Whether red mud enters this system burden-free as a waste, or carries allocated Bayer-process burden as an aluminium co-product, is the largest single lever on the result.
Panel 3
CBAM
The EU Carbon Border Adjustment Mechanism entered its definitive period on 1 January 2026, phasing in through 2034 alongside withdrawal of free EU ETS allowances. Annex I scope remains aluminium, cement, electricity, fertilisers, hydrogen, and iron and steel.
Chemicals and polymers are not in scope for 2026; the Commission has proposed extending coverage from 1 January 2028 to roughly 180 downstream products with high steel or aluminium content, and plans a 2027 report evaluating extension to indirect emissions and further sectors including chemicals.
The counterintuitive implication: the olefins face no CBAM liability today, but recovered iron units entering a steel chain and brine-derived fertilizer sit inside existing Annex I categories. The exposure is on the by-products, not the main product.
Panel 4
ISCC PLUS, RED III, and China's carbon market
ISCC PLUS is the operative certification for bio-based chemicals and polymers outside RED-regulated fuel applications; it permits certified feedstock to be tracked through shared assets by mass balance rather than physical segregation. Its Mass Balance Guidance Document 1.0 went through public consultation with a final version expected mid-2026 — the credit-transfer rules are being rewritten on the same timeline as this design.
RED III thresholds apply where product gas is sold as fuel rather than feedstock. Domestically, China's national ETS is expanding to steel, cement and aluminium with absolute caps by 2027, and CCER has relaunched.
Note honestly that ISCC feedstock integrity at the self-declaration stage is a known weak point — and Act 2 shows exactly why that matters here.

Act 2

Why these two wastes

Two wastes, picked for each other rather than separately. Each one is awkward on its own in a way the other happens to fix, and the pairing is what makes a single reactor do two jobs at once. This section sets out why douzha and bauxite residue, what each contributes, and what the combination can actually be turned into.

Douzha

Too wet to burn, too perishable to store

Douzha leaves a soymilk or tofu line at 80–85 wt% water. Every dry thermochemical route has to evaporate that water first, and doing so costs more energy than the resulting gas carries — which is why douzha is normally fed to livestock at nominal value or paid to dispose of. It also spoils quickly, so it cannot be stockpiled or hauled far.

What it brings

In supercritical water the water stops being a cost and becomes the reaction medium. Douzha arrives at almost exactly the concentration the gasifier wants. Its fine hydrated fibre also forms a pumpable paste rather than a settling suspension, which is what makes it possible to feed solids into a 25 MPa reactor at all — douzha is the carrier that lets milled straw go in with it.

Bauxite residue

Alkaline, hazardous, and impounded rather than used

Bayer-process residue is generated at over 120 Mt per year globally and is overwhelmingly stored behind dams rather than sold. Its leachate runs at pH 12–13, driven by residual sodium, and that alkalinity is both why it is classified as hazardous and why almost nobody will take it. Utilization sits in the low single-digit percent.

What it brings

It arrives with roughly 30–60 wt% Fe₂O₃, which is a credible low-cost redox mediator, and with the alkali that cracks tars and pushes water-gas shift toward hydrogen. It is, in effect, a free catalyst that a refinery currently pays to store.

Why the pairing works

The gasifier does two jobs in one pass
The same conditions that gasify the biomass also strip sodium out of the red mud. Inorganic salt solubility collapses above the critical point, so the alkali that would otherwise foul the reactor leaves as a separable brine. One vessel is simultaneously a biomass converter and a bauxite residue treatment unit — and the treated residue comes out saleable, because dealkalization is exactly what was blocking its reuse.
Each feed supplies what the other lacks
Douzha is nitrogen-rich at C/N ≈ 5.8 and carries almost no lignin, so it gasifies readily but is thin on carbon density. Straw is the mirror image at C/N ≈ 60–80. Red mud brings iron and alkali but no carbon at all. Blended, they hit a combined C/N of 15–25, which puts the recovered brine straight into a fertilizer-compatible band rather than making it an effluent problem.
The catalyst is a waste, so the catalyst is free
Red mud gives hydrogen yields comparable to commercial alkali catalysts, and a Ni–Cu bimetallic on a red mud support has reached 21.88 mmol/g H₂. Ordinarily a gasification catalyst is a purchased consumable; here it is a material with a negative gate price. That inverts the usual economics — the plant is paid to accept its own catalyst.
The alkali problem and the sulfur problem partly cancel
Sulfur from douzha's cysteine and methionine partitions into the same alkaline brine as the sodium, so some of it leaves before the gas ever reaches the acid gas train. That is physics rather than a designed mechanism, and the downstream wash is deliberately sized without relying on it — but it is a real credit against the duty.

What can be made

What the pairing can be turned into. The commercial case does not rest on olefins alone, and framing it that way understates it — four saleable streams and one avoided cost leave the battery limits.

Stream	Origin	Basis of value
C₂–C₄ light olefins	From the reformed syngas over an oxide–zeolite catalyst	The bio-based premium over naphtha-cracker product, claimed by ISCC PLUS mass balance
N-K-P-S brine	The sodium, potassium and phosphate stripped out in the salt separator	Compound fertilizer — but the real driver is the avoided nitrogen-removal cost on the effluent
Dealkalized residue	The treated red mud leaving the solids loop	SO₂ sorbent, supplementary cementitious material or geopolymer precursor — and in every case it displaces landfill
Iron, scandium, titanium	Hydrometallurgy on the iron-depleted slag	Iron units by volume; scandium at ~84 ppm is the value driver, though market depth caps it
Elemental sulfur	Liquid-redox recovery on the acid gas	Modest revenue, but it replaces a spent-sorbent disposal stream with a commodity

None of this is free. The pairing creates a genuine conflict — the alkali that makes red mud useful is the same alkali that plugs supercritical reactors — and the design resolves it by treating salt removal as a product step rather than a protective one. That decision, and the two others it sits alongside, are set out with the flowsheet.

Act 3

Where the two wastes actually are

Not a synergy story. Two feedstocks, two completely different geographies — red mud is a handful of huge inland point sources, industrial douzha a handful of coastal beverage plants, fragmented tofu douzha follows people. Toggle the overlays; the point is the mismatch.

Every coordinate and capacity is flagged unverified — none confirmed against a primary source.

Overlays

Alumina refineries. Shandong, Shanxi, Henan, Guangxi and Guizhou hold roughly 95% of Chinese capacity. Large, few, permanently sited point sources — the easy half of the problem.

Douzha is not a crushing by-product: ~90% of imported GM soybean is crushed for oil and meal, producing none. It arises only from soymilk and tofu manufacture on non-GM domestic beans. A few large beverage plants — clean point sources with single-supplier traceability.

Larger tonnage, harder supply chain. Thousands of small workshops sited close to consumption, so this follows population rather than capacity. This is where the ISCC risk lives — fragmented, self-declaring sources are exactly where feedstock integrity fails at audit.

China refines alumina largely from imported bauxite, which is why capacity clusters behind the northern terminals — Yantai, Longkou, Rizhao, Qingdao — and behind Fangchenggang. The same terminals land the imported soybean crushed at Rizhao, Nantong and Dongguan. The ports are where the bauxite arrives, and where the wrong soybean arrives.

Off by default. Soybean straw follows cultivation — Heilongjiang, Jilin, Liaoning. Heilongjiang is also the non-GM food-grade origin: where the beans start, not where the douzha appears.

Reading the marks

Filled, sized by capacity. Only Fangchenggang (~2.4 Mt/y) has one — hence the single large mark.
Hollow — capacity unverified, i.e. every other site.
Dotted — neighbours and Taiwan, context only.

Haul-distance calculator

Great-circle distance to the nearest source in each active overlay.

The analytical payload

No province holds red mud and douzha at scale together. Red mud sits inland and north — Shandong, Shanxi, Henan — plus Guangxi and Guizhou. Industrial douzha sits in the Pearl River Delta, Shanghai and Wuhan. Straw is a thousand kilometres further north again.

Two pairings are worth examining. Guangxi to Guangdong is roughly 400–700 km and pairs real alumina capacity with three industrial soymilk plants — the shortest credible link. Henan is the dark horse: alumina refining and very high population density, so fragmented tofu douzha is generated on top of the red mud — no haul, but a hard-to-certify supply. Shandong is a trap: the largest alumina capacity in China, but its soybean industry crushes imported beans for meal, which yields no douzha.

Which stream moves is a live design decision. Red mud is dense, cheap and moves badly. Douzha is 80% water, moves worse, and spoils. This constrains siting more than any thermodynamic consideration in the flowsheet.

Act 4

What goes in

Douzha, soybean straw and red mud are characterized here on a common basis, and the blend ratio is set by three independent constraints rather than optimized against any one of them. Every heteroatom entering the system is given a named destination and a responsible unit — in a process whose products are sold against ISCC and ISO 14067 claims, "somewhere" has to be documented rather than assumed.

Douzha

A feed defined by its water

Douzha — 豆渣 in Chinese, okara in Japanese, biji in Korean — is the insoluble residue remaining after soybeans are ground, extracted and screened in soymilk and tofu manufacture. Roughly 1.1–1.2 kg arises per kg of soybeans processed into curd. Most is landfilled: the 70–85% moisture content makes it spoil within days and gives it negative value at the gate, which is precisely the favourable feedstock economics this project depends on.

Supply is ample in aggregate and constrained locally. A 100 t/d wet-feed plant consumes 36,500 t/y, so national arisings could in principle support of order seventy such plants. The binding constraint is collection radius against spoilage rather than tonnage — and this is the quantitative form of the industrial-versus-fragmented distinction the siting analysis draws. A single large soy beverage plant plausibly yields 150–330 t/d and can therefore supply a plant of this scale on its own, whereas the same tonnage aggregated from tofu workshops requires hundreds of collection points inside a spoilage-limited radius.

The defining physical property is water content. An independent characterization reports 46.3% dietary fibre, 17.8% protein, 5.9% lipid and 3.9% ash on a dry basis, which brackets the variability introduced by cultivar and extraction severity.

Yield per kg soybean (curd): 1.1–1.2 kgind.
China arisings: ~2.8 Mt/yind.
Japan / Korea arisings: 0.8 / 0.31 Mt/yind.
Large beverage plant output: 150–330 t/dind.
Moisture (wet basis): 82.8 wt%^[Li-2012]
Protein (wet basis): 6.34 wt%^[Li-2012]
Lipid (wet basis): 0.86 wt%^[Li-2012]
Ash (wet basis): 0.65 wt%^[Li-2012]
Protein (dry basis): ≈27 wt%^[Li-2012]
Crude fat (dry basis): ≈10 wt%^[Li-2012]
Soluble fibre (dry basis): ≈14 wt%^[Li-2012]
Insoluble fibre (dry basis): ≈42 wt%^[Li-2012]
Ash (dry basis): ≈3 wt%^[Li-2012]
C/N ratio: ≈5.8^[Li-2012]
Organic sulfur (dry basis): low tenths of a percentind.

The moisture is an asset, not a penalty
At 80–85 wt% water, douzha arrives at roughly the concentration a supercritical water gasifier wants. Any dry gasification route would spend more energy evaporating this water than the resulting gas contains — the standard argument for hydrothermal processing of wet feeds, and here it is decisive rather than marginal.
Lignin is essentially absent
Douzha's fibre is cellulosic and pectic rather than lignified. Lignin is the fraction most resistant to hydrothermal depolymerization and the dominant char precursor, so its absence predicts high carbon conversion and low char at comparatively mild severity.
The national arisings figure needs a basis before it is used
The ~2.8 Mt/y figure for China is reported without a stated basis and is most likely wet mass. It must be confirmed as wet or dry before it enters any capacity calculation, because the two differ by roughly fivefold.
Nitrogen and sulfur are structural
At roughly 27 wt% protein the C/N ratio is approximately 5.8, and the sulfur-bearing amino acids cysteine and methionine put organic sulfur into the feed. Both heteroatoms report to the aqueous and gas phases and both must be given a designed destination. A measured ultimate analysis of the specific douzha source is required.

Soybean straw

The dry-solids and carbon-density partner

Soybean straw is the field residue — stems, pods and petioles — left after grain harvest. It is lignocellulosic and carries a potassium- and silica-bearing ash typical of agricultural residues. Its C/N ratio is the mirror image of douzha's, which is the reason the two are blended rather than run separately.

Soybean straw is a demonstrated supercritical water gasification substrate in its own right. Catalytic supercritical water gasification over Ni-based catalysts with varied supports and promoters has been reported, with ZrO₂ the most effective support for H₂ yield and selectivity among activated carbon, carbon nanotubes, ZrO₂, Al₂O₃, SiO₂ and Al₂O₃–SiO₂ at 10 wt% Ni loading. That work is the closest published analogue to the biomass half of this system.

Moisture (as received): 10–15 wt%ind.
C/N ratio: ≈60–80ind.
Ash character: K- and Si-bearingind.
Best Ni support (H₂ yield): ZrO₂ at 10 wt% Ni^{[Okolie-2021]}

Red mud

Composition, alkalinity and three latent functions

Bauxite residue from the Bayer process is generated at over 120 Mt per year globally and is overwhelmingly impounded rather than used. Its composition varies with the source bauxite but sits in well-established ranges.

Leachate pH is 12.1–13.0, driven by the sodium content. This alkalinity is the primary reason the material is classified as hazardous and the primary barrier to its reuse — and it is what the gasifier is designed to remove.

Global generation: >120 Mt/y^{[Wang-Liu-2012]}
Leachate pH: 12.1–13.0^{[Wang-Liu-2012]}
Scandium content: ~84 ppm (16–230 ppm range)^[Zhang-2017]

The load-bearing claim

The dealkalization synergy — the strongest single argument

Supercritical water treatment of red mud has been shown to give enhanced dealkalization performance, and — critically — the dealkalized residue exhibits improved SO₂ capture capacity. That single result closes the loop three ways. It removes the sodium that would otherwise foul the reactor. It detoxifies the residue, converting a hazardous impoundment liability into a saleable material. And it produces, as the treated solid, a sulfur sorbent — meaning the process generates part of its own desulfurization capacity and an external sorbent product besides.^[Wang-2023]

This is the load-bearing claim of the whole concept and must be verified experimentally first. If supercritical dealkalization does not proceed at the residence times required for biomass gasification, the economics revert to those of a conventional wet-biomass gasifier with a cheap iron catalyst — a much weaker proposition.

Red mud composition and assigned function

Component	Typical range (wt%)	Representative Chinese Bayer residue	Function in this process
Fe₂O₃	30–60	36.87	Oxygen carrier / redox mediator; water-gas shift and tar cracking activity
Al₂O₃	10–20	22.11	Structural support; contributes to sintering resistance
SiO₂	3–50	18.72	Inert diluent; forms iron silicates that deactivate the redox couple
Na₂O	2–10	11.70	Alkali reservoir — tar cracking and WGS promotion; also the source of salt plugging and the target of dealkalization
TiO₂	trace–10	3.14	Inert here; a recovery target in the residue valorization block
CaO	2–8	2.49	Native alkalinity; contributes to but does not substitute for a dedicated sulfur sorbent
Sc	16–230 ppm	~84 ppm	Recovery target — sets a secondary revenue line

Ranges from Wang & Liu (2012); representative Chinese Bayer residue analysis and scandium content from Zhang et al. (2016, 2017).

Blend design constraints

The blend ratio is not a free variable. Three independent constraints bound it, and the design point is the intersection rather than an optimization of any single one.

Constraint	Bounding variable	Design window	Binding?
Slurry pumpability	Total solids; particle size distribution	18–22 wt% total solids; straw milled to <0.5 mmind.	Yes — upper bound
C/N ratio	Douzha : straw mass ratio (dry)	Combined C/N 15–25	Soft
Alkali loading	Red mud dose + straw K	Set by salt separator duty	Yes — couples all three feeds
Sulfur loading	Douzha protein fraction	Sets the sulfur train, not the blend	No

Constraint 1 — pumpability sets the upper bound on solids. Feeding solids into a 25 MPa reactor is the recurring practical failure of supercritical water gasification at scale. Douzha is the rheological advantage: its fine, hydrated fibre particles form a pumpable paste rather than a settling suspension, which is exactly the behaviour that corn starch paste was added to sewage sludge to create. Douzha can therefore act as the carrier medium for milled straw that would otherwise settle and bridge.

Constraint 2 — C/N balance. Unlike anaerobic digestion, supercritical water gasification is not inhibited by ammonia in the way a methanogenic consortium is, so C/N is not a hard operating constraint. It matters for two downstream reasons: the ammonia concentration in the aqueous effluent determines whether nitrogen recovery is worthwhile, and the nitrogen loading determines the corrosivity and composition of the salt-separator brine. Blending to a combined C/N of 15–25 puts the brine into a directly fertilizer-compatible band.

Constraint 3 — alkali loading. Red mud brings its own sodium; the straw brings potassium. Together these set the total alkali the salt separator must remove per unit of feed, and that duty scales the separator rather than the reactor. Alkali loading is the constraint that couples the biomass blend to the red mud dosing, and it is why the three feed streams have to be specified together rather than sequentially.

Contaminant inventory and assigned fate

Every heteroatom entering the system must leave it somewhere. The table assigns a destination to each element and names the unit responsible.

Element	Source	Form after gasification	Assigned fate and responsible unit
N	Douzha protein (dominant); straw (minor)	NH₃, some N₂; ammonia is the stable intermediate under hydrothermal conditions	Aqueous phase → salt separator brine → fertilizer-grade N-K-P-S product, or ammonium sulfate via the sulfur train
S	Douzha cysteine / methionine	Aqueous sulfide / sulfate under alkaline reducing conditions; balance as H₂S with traces of COS and thiophenes	Calcium-free three-stage train: bulk capture into the B3 brine as the S of N-K-P-S → dealkalized red mud polishing → ZnO guard to OXZEO specification
Na	Red mud (11.7 wt% as Na₂O)	Dissolved Na⁺, precipitating as salts above the critical point	Salt separator — designed removal, not a loss. Simultaneously dealkalizes the residue
K	Soybean straw ash	Dissolved K⁺, co-precipitating with Na	Salt separator brine — raises the fertilizer value of the brine
Cldata gap	Both biomass streams; process water	HCl / chloride	Corrosion driver, and competitor for red mud alkalinity in the polishing bed. Removing calcium eliminates the HCl-versus-Ca capacity problem but not the corrosion duty. Must be quantified before materials selection
P	Douzha phytate	Phosphate	Brine — completes the N-P-K fertilizer specification
Fe	Red mud	Reduced oxide phases (Fe₃O₄ / FeO)	Retained in the solid; re-oxidized in the regeneration loop or recovered carbothermically
Sc, Ti	Red mud	Unchanged oxides	Concentrated in the spent solid → leaching recovery as a secondary revenue line

Chloride deserves specific emphasis

It is the element most often left out of this kind of table, and its role changed when calcium was removed. In a calcium-based train chloride is a direct capacity competitor: HCl reacts not only with fresh CaO and Ca(OH)₂ but also with spent sorbent containing calcium carbonate, oxide and hydroxide, and after calcining and slaking the spent material recovers essentially the reactivity of pure Ca(OH)₂ toward HCl — so chloride consumes calcium inventory across the whole sorbent life cycle, not only the fresh charge. Eliminating calcium removes that failure mode, which is one of the arguments for the decision. Chloride does not become harmless, though: it remains a corrosion driver in a supercritical, alkaline, sulfide-bearing environment where internals are already specified in nickel-base alloys, and it remains a competitor for alkalinity in the polishing bed by the same chemistry that made it compete for lime.^{[Wang-Bjerle-1996]}

The solids budget

The binding constraint is pumpability, not moisture

The blend constraints above were expressed as independent bounds. Closing the arithmetic shows they are not independent, and that the pumpability ceiling is far more restrictive than it first appears. This is the single most consequential finding in Section 1, and it propagates into every downstream block.

Douzha arrives at approximately 17.2 wt% solids on an 82.8% moisture basis. That is already inside the 18–22 wt% design window with no dewatering whatsoever, which is a genuine argument for the feedstock and the reason no thickening step appears in B1. But it also means the solids budget is almost entirely consumed before any other component is added.

Take 100 kg of wet douzha as received: 17.2 kg of solids, of which roughly 0.5 kg is ash and 16.7 kg organic. Now dose red mud. At only 5 kg of residue the slurry is at 22.2 kg solids in 105 kg total — 21.1 wt%, already at the ceiling, with no straw in the blend at all. Red mud is inorganic dead weight in this budget: it consumes pumpability headroom without contributing carbon or hydrogen.

Component	Contributes solids	Contributes carbon	Effect on the budget
Douzha (as received, 17.2 wt% solids)	Yes — 17.2 wt%	Yes	Consumes ~78–96% of the available window on its own. Requires no dewatering.
Red mud	Yes	No	Dead weight. 5 kg per 100 kg douzha reaches the ceiling unaided.
Soybean straw	Yes	Yes	Competes directly with red mud for the remaining headroom.
Dilution water	Reduces wt%	No	Buys headroom, but must then be heated to 600 °C for no return.

Table 1.4 — Red mud dosing and straw loading are in direct competition. The constraint chain is: douzha solids + straw + red mud ≤ ~22 wt%.

Move the sliders — the ceiling binds fast

Douzha arrives at 17.2 wt% solids as received, so it consumes most of the 18–22 wt% window before anything else is added. Red mud buys no carbon; it only spends headroom.

Douzha (as received)100 kg

Soybean straw (dry)0 kg

Red mud (dry)5 kg

Dilution water0 kg

Total solids

21.1wt%

Inside the 18–22 wt% design window.

Solids: 22.2 kg
Total mass: 105 kg
Carbon-bearing share: 77%

Every route out of this is costly, and the four should be presented as a genuine trade rather than picking one silently. Diluting with water buys headroom but imposes a sensible-heat penalty on inert mass all the way to 600 °C. Cutting the red mud dose weakens both the catalytic function and the dealkalization duty — and dealkalization is the concept. Cutting straw sacrifices carbon density and the C/N benefit. Dewatering the douzha upstream is the obvious answer and the worst one: it is a hydrated gel that dewaters poorly, and mechanical dewatering is energy-intensive and incomplete.

This finding also corrects a framing error earlier in the work. Douzha was described as the slurry carrier that allows milled straw into the reactor. That remains true rheologically, but the headroom available for straw is much smaller than that framing implies, and in a red-mud-rich case it may approach zero. Section 5 must treat the douzha : straw : red mud : water split as a constrained optimization with the 22 wt% ceiling as an active constraint, not as four independently chosen dosing rates.

Slurry formulation

Whether dispersants and stabilizers are required

Coal–water slurry practice reaches for dispersants — naphthalene sulfonate condensates, lignosulfonates, polycarboxylates — to solve two problems this system does not have: wetting a hydrophobic solid, and suppressing viscosity at 60–70 wt% loading. Douzha is hydrophilic, arrives fully hydrated, and the target loading is around 20 wt%. On the biomass fraction, no wetting agent is required.

The feed additionally supplies its own surfactant. Residual soy protein and phospholipids are amphiphilic — soy lecithin is a commercial emulsifier derived from the same source material — so the organic fraction is substantially self-dispersing. This is a real and underused argument for the feedstock.

The suspension problem that does exist is the red mud, not the biomass. Haematite has a density near 5.2 g/cm³ and the residue is finely divided, so it will settle out of a matrix that comfortably suspends its own fibre. Any additive requirement therefore arises from mineral suspension stability, not from organic wetting — a distinction that determines which additive class is even relevant.

The additive trap: both standard choices inject the contaminant this flowsheet exists to remove

Sodium carboxymethyl cellulose is the documented stabilizer for supercritical water gasification slurry feeding, and it works — one overview reports it reaching above 40% carbon gasification efficiency at 540 °C and 25 MPa, and it is noted as more consistent and cheaper than tragacanth, gum arabic, guar or xanthan. But it is the sodium salt, and block B3 exists to strip sodium from this system. Dosing sodium into the feed to fix rheology, then paying separation duty to remove it again, is self-defeating. Lignosulfonate, the other standard dispersant, injects organically bound sulfur directly into the acid gas removal duty a Rectisol unit has just been committed to handling.

If a stabilizer proves necessary, xanthan gum is the clean selection: no sodium, no sulfur, strongly shear-thinning, effective at low dose. The literature notes xanthan delivers the same function at higher cost and lower batch consistency — an acceptable premium here, because the cheap option carries a process penalty rather than merely a price.

The first question, though, is whether any additive is needed. Slurries at 30–50 wt% loading are reported as shear-thinning while 10–20 wt% loadings shear-thicken, so a ~20 wt% blend sits near an unfavourable rheological transition and its behaviour cannot be assumed. A recirculating feed loop with in-line static mixing and minimal residence between mixer and pump suction may remove the requirement entirely. No additive is always preferable to a clean additive.

Co-feeding other wet organic wastes

Supercritical water gasification is a credible route for a wide range of wet organic wastes, so whether to widen the feed slate beyond douzha and soybean straw deserves an explicit answer rather than silence. The chemistry is largely permissive. The commercial answer is much narrower, and for a reason specific to this flowsheet.

The governing constraint is residue purity, not gas yield. The value case rests on selling dealkalized red mud as a supplementary cementitious material or sulfur sorbent, and on recovering iron, scandium and gallium from it. That is a materials-purity business. Any co-feed whose inorganic contaminants partition into the solid phase degrades the product the project depends on. Co-feeding therefore trades gas yield against residue saleability — and residue saleability is where the margin sits.

Co-feed	Case for	Why it is risky here	Verdict
Soybean straw	Same supply chain, no contaminant burden, complementary C/N	Competes only for the solids budget.	design basis
Pig manure	Proven supercritical water gasification feed; abundant; genuinely co-located with straw in the Northeast; adds N/K/P to the brine product; supercritical water destroys veterinary antibiotics and antibiotic resistance genes, which is a regulatory asset rather than merely neutral	Moderate Cu and Zn from feed additives will report to the solid. Best reported gas figures are at ~6 wt% feed concentration, far below this design point.	evaluate first
Food waste	High moisture and lipid content, energy-dense, abundant and urban	Chinese food waste carries high chloride from cooking salt, feeding directly into the corrosion and materials-selection problem already flagged.	caution
Sewage sludge	The closest chemistry match in the literature, with an established red mud co-gasification precedent and a phosphorus recovery credit	Heavy metals — Cd, Pb, Cr, Cu, Zn — partition into the solid and foreclose the cementitious and sorbent product routes. Rejected on residue purity, not on chemistry.	rejected
Human faeces / septage	Technically near-identical to sewage sludge	Certification and public-acceptance exposure. Compromises the ISCC claim and any fertilizer product derived from the brine.	rejected

Table 1.5 — Ranked by residue-purity risk rather than by gasification performance, because the residue is the product that carries the economics.

The five red-mud-producing provinces

Red mud arises from the Bayer process in alumina refining. Aluminium smelting consumes alumina electrolytically and generates no bauxite residue whatsoever — its wastes are spent pot lining, carbon anode residues and fluoride-bearing dust — so smelter locations are irrelevant to this feedstock and are excluded from the analysis entirely. The distinction matters because the two industries follow opposite siting logic: smelting chases cheap electricity, which is why provinces such as Xinjiang and Yunnan hold large aluminium capacity, whereas refining follows bauxite and caustic logistics.

Chinese alumina refining is highly concentrated. Shandong, Shanxi, Henan, Guangxi and Guizhou together account for approximately 95% of national capacity, with Shandong, Shanxi and Guangxi alone exceeding 70%. Red mud is generated at every refinery in these five provinces, and they therefore constitute the complete candidate set for siting this plant.

Province	Principal refining centres	Assessment against douzha supply
Guangxistrongest	Baise, Pingguo, Fangchenggang (~2.4 Mt/y)	Strongest candidate. Roughly 400–700 km from Pearl River Delta industrial soy beverage plants, pairing real refining capacity with traceable point-source douzha.
Henandark horse	Zhengzhou, Sanmenxia, Jiaozuo	Refining capacity plus very high population density, so fragmented tofu douzha is generated on top of the residue. No haul at all, but a self-declared supply chain that is the weakest control point in the project.
Shandongtrap	Binzhou / Zouping (Weiqiao), Chiping (Xinfa), Zibo	Largest provincial capacity and the largest red mud arising, but its soybean industry crushes imported beans for meal, which yields no douzha. Maximum residue, no local feedstock partner — an apparent match that is not one.
Shanxiweak	Jiaokou / Lüliang (Xinfa), Xiaoyi, Hejin	Major capacity, deep inland, no significant soy food processing within a spoilage-limited radius.
Guizhouweak	Guiyang, Zunyi (Chinalco)	Moderate capacity, no meaningful douzha supply nearby.

Table 1.6 — The five red-mud-producing provinces, ranked by suitability rather than by capacity. Centres and capacities are approximate and flagged as unverified pending a primary source.

The geography is unfavourable and the work states that plainly rather than presenting a synergy story. Douzha arises where soy foods are manufactured, soybean straw where soybeans are cultivated, and neither coincides with alumina refining. No province in the candidate set holds red mud and douzha at scale together. Which stream moves is therefore a live design decision, and it is constrained more tightly than any thermodynamic variable in the flowsheet: red mud is dense, cheap and travels badly; douzha is 80% water and spoils within days; straw is bulky and low in density. Section 9 must carry the Guangxi and Henan cases as distinct siting scenarios rather than assuming a single site.

Act 5

The plant

The plant, read top to bottom. The diagram on the left holds while the description scrolls; scroll position is process position. Every unit-operation symbol is drawn to Aspen / ISA-5.1 convention and chosen automatically from each block's type. Three blocks — B2, B3 and B6 — carry the load-bearing, unproven claims and are badged accordingly.

000est.Placeholder — pending balance closure000ind.Indicative — needs a citation000^[src]Literature — cited source

Feed preparation and slurry make-up

P0.1–25 MPa^[SWG-feed]

Straw is milled and blended into douzha to 18–22 wt% total solids; red mud is dosed; the slurry is pressurised by positive-displacement pump. Feed–effluent exchange recovers reactor outlet heat.

Douzha is the rheological enabler: its fine hydrated fibre forms a pumpable paste rather than a settling suspension, letting it carry milled straw that would otherwise bridge and settle.

Inlet streams

Tag	Stream	Phase	Components	Quantity
1	Douzha	Slurry	80–85 wt% water, protein-derived organics	—
2	Milled soybean straw	Solid	Lignocellulose	—
3	Red mud (fresh make-up)	Solid	Fe₂O₃, Al₂O₃, SiO₂, Na alkalinity	—
R1	Regenerated red mud recycle	Solid	Re-oxidised bed from B8, re-slurried and re-pressurised at make-up	—

Outlet streams

Tag	Stream	Phase	Components	Quantity
4	Pressurised feed slurry	Slurry	18–22 wt% total solids + red mud	—

Specific pumping energy: —est.

Context values

Discharge pressure: 25MPaest.

Note
No conversion metric here
This is a preparation block: report discharge pressure and specific pumping energy, not a conversion.

Supercritical water gasifier with red mud bed

Needs validation

T600–650 °Cest.P25 MPaest.

Biomass gasifies to CH₄, CO₂ and H₂. Red mud plays three roles simultaneously, and the page is honest that they are not equally well supported.

Concurrently the reactor dealkalizes the red mud — the same pass that gasifies the biomass begins the residue treatment that B3 completes.

Redox mediator
Requires qualification
Red mud is an established chemical-looping oxygen carrier (Fe₂O₃ typically >40 wt%), but that literature is atmospheric-pressure, high-temperature and dry. In supercritical water the oxidant is already present in vast excess, so lattice oxygen donation mediates the redox chemistry rather than supplying an oxygen deficit. Flagged as requiring qualification.
Gasification catalyst
Best supported
The best-supported role. Red mud addition gives H₂ yields comparable to commercial alkali catalysts; a Ni–Cu bimetallic on a red mud support reached 21.88 mmol/g H₂, 6.7× unpromoted Ni. Direct precedent exists in co-gasification of spirit-based distillers' grains with sewage sludge over red mud.
Alkali reservoir and tar cracker
Supported
Real but self-limiting, because the alkali dissolves then precipitates. The tar-cracking benefit is genuine but decays as sodium leaves the solid.

Inlet streams

Tag	Stream	Phase	Components	Quantity
4	Pressurised feed slurry	Slurry	Biomass + red mud, 25 MPa	—

Outlet streams

Tag	Stream	Phase	Components	Quantity
5	Supercritical effluent	Supercritical	CH₄ + CO₂ + H₂ (CO ≈ 0), reduced Fe phases, precipitating Na/K salts → B3	—

Carbon gasification efficiency: —est.
Planning-basis band: 50% (40–60 band)ind.
H₂ anchor (Ni–Cu on red mud): 21.88mmol/g^[NiCu-RM]

Context values

CLE reference — gas yield: 1.02Nm³/kg^[CLE-dry]
CLE reference — LHV: 12.06MJ/Nm³^[CLE-dry]
CLE reference — cold gas efficiency: 91.49%^[CLE-dry]
CLE reference — carbon conversion: 82.65%^[CLE-dry]

Needs validation
Needs validation
The three red-mud roles are not equally supported, and the redox role rests on dry, atmospheric literature applied to a wet, high-pressure medium.
Warning
Calibration warning — do not carry these efficiencies into this design
The 91.49% cold gas efficiency and 82.65% carbon conversion below are from dry, atmospheric-pressure chemical looping. They establish only that red mud is a credible oxygen carrier in that context; they are not achievable targets here. Two effects push the realistic figure much lower: supercritical gasification efficiency falls as feed concentration rises, and the headline results in that literature are reported at low loading — pig manure reaching 87.59% did so at 6 wt% feed with K₂CO₃ promotion, against roughly 20 wt% here. The one figure obtained under conditions resembling a real slurry feed is above 40% carbon gasification efficiency at 540 °C and 25 MPa.
Decision
A three-way conflict, and the planning basis chosen
Pumpability wants high solids, the energy balance wants high solids, and gasification efficiency wants low solids. The 18–22 wt% window was selected on the first two. A carbon gasification efficiency versus feed concentration curve for this specific blend is required, because the economically optimal point is set by that curve and not by the pumpability ceiling alone. Until it exists, carry 40–60% carbon gasification efficiency as the planning basis, flagged as an estimate.

Salt separator — a purposeful product unit

Needs validation

Supercritical; cooled-wall or cyclonic

Inorganic salt solubility collapses above the critical point, and salt deposition is the dominant plugging and corrosion failure mode of continuous supercritical water systems.

Design decision: this block is a primary product unit, not a protective device. Sodium removal is deliberate and metered, because supercritical water treatment of red mud gives enhanced dealkalization and detoxifies the residue for sale. The salt load is intentionally large, and red mud dosing is co-determined by this separator's duty rather than by catalytic requirement alone.

Two duties: dealkalization of the residue, and recovery of Na/K/P as a fertilizer-precursor brine. Sulfur partitions partly into the same brine as sulfide and sulfate — this is why the fertilizer product is N-K-P-S — but that is an incidental credit against the downstream acid gas removal duty, not a designed removal step, and B5 is sized without relying on it.

Inlet streams

Tag	Stream	Phase	Components	Quantity
5	Supercritical effluent	Supercritical	Gas + reduced Fe phases + dissolved/precipitating salts	—

Outlet streams

Tag	Stream	Phase	Components	Quantity
6	Product gas	Supercritical	CH₄ + CO₂ + H₂ → B4	—
7	N-K-P-S brine	Liquid	Na/K/P salts, partial sulfide/sulfate	—
8	Dealkalized solid	Solid	Detoxified red mud → B4	—

Sodium removal fraction: —est.
Brine concentration: —est.

Decision
Decision: product unit, not protection
B3 is sized as a primary product unit. Red mud dosing is co-determined by its dealkalization/brine duty, not by catalytic requirement alone.
Needs validation
Needs validation — the load-bearing claim
B3 remains the load-bearing claim of the concept and is unvalidated at biomass-gasification residence times.

Depressurization, phase separation, aqueous polishing

P3–25 MPaest.

Gas/liquid/solid split across a let-down valve and flash drum. The ammonia-bearing aqueous phase goes to nitrogen recovery; spent red mud goes to B8.

Inlet streams

Tag	Stream	Phase	Components	Quantity
6	Product gas	Supercritical	CH₄ + CO₂ + H₂, ~25 MPa	—
8	Dealkalized solid	Solid	Detoxified red mud	—

Outlet streams

Tag	Stream	Phase	Components	Quantity
9	Raw syngas	Gas	CH₄ + CO₂ + H₂ + H₂S/COS → B5	3MPaest.
10	Ammonia-bearing aqueous	Liquid	NH₃, dissolved organics → N recovery	—
11	Spent red mud	Solid	Reduced Fe phases → B8	—

Separation efficiency (per phase): —est.

Acid gas removal (Rectisol) and ZnO guard

T-60 to -30 °C^[Rectisol]P3 MPaest.

Two findings drive this block. First, the negative finding, because it is counterintuitive: in-bed calcium capture does not work in supercritical water. The hydrolysis CaS + 2H₂O ⇌ Ca(OH)₂ + H₂S is well established and is used deliberately as a CaS stabilization route; a supercritical water gasifier is close to an ideal reactor for running it. Lime dosed into B2 would capture sulfur and release it again. Moving calcium downstream into dry warm gas would function, but imports a consumable, creates a spent-sorbent disposal stream, and competes with HCl for capacity across the whole sorbent life cycle.

Second, the decision: calcium has been removed from the flowsheet entirely and replaced with a commercially proven acid gas wash. This is deliberately the least novel block in the flowsheet — the one place the design buys a vendor guarantee instead of inventing something.

Terminology note: "wet desulfurization" normally denotes limestone–gypsum wet flue gas desulfurization, which is an oxidising post-combustion SO₂ technology. The stream leaving B4 is a reducing syngas carrying H₂S and COS, so the correct standard analogues are the wet acid gas removal processes used in gasification and coal-to-chemicals practice — physical solvent or amine absorption — not WFGD.

S0 — aqueous credit
Supported
Sulfide/sulfate leaving in the B3 brine. Physics, not a mechanism. Reduces the B5 duty; B5 is sized without it.
S1 — primary wash: Rectisol chilled-methanol
Best supported
Purifies syngas to 0.1 ppm total sulfur including COS — exactly the OXZEO specification, in one guaranteed unit. COS matters specifically here: protein-derived sulfur in a CO₂-rich gas forms carbonyl sulfide, which amines handle poorly and Rectisol handles well. The decisive integration argument: this flowsheet needs CO₂ control independently of sulfur, since OXZEO co-produces CO₂ and B6 needs a metered CO₂ feed. Rectisol does acid gas removal and CO₂ separation in the same unit, collapsing two problems into one. At a 0.1 ppm H₂S target, reported CO₂ efficiency ranks Rectisol > Selexol > MDEA > sulfolane-MDEA. It is the incumbent technology in Chinese coal-to-chemicals.
S1-alt — selective MDEA
Supported
Near-ambient. A tertiary amine, fast with H₂S and slow with CO₂, reaching under 20 ppmv H₂S. Far cheaper, no refrigeration or methanol inventory — but misses the OXZEO spec alone, handles COS poorly, and gives no CO₂ control. Presented as a live technoeconomic alternative, not a rejected option.
S2 — ZnO guard
Best supported
Non-regenerable, immediately upstream of B6/B7. Cheap insurance with Rectisol; mandatory with MDEA.
S3 — sulfur recovery by liquid redox (LO-CAT type), not Claus
Supported
Claus wants acid gas at 10–13 vol% H₂S or above; this sulfur load will very likely fall short. Liquid redox suits small duties and yields saleable elemental sulfur rather than a disposal problem.

Inlet streams

Tag	Stream	Phase	Components	Quantity
9	Raw syngas	Gas	CH₄ + CO₂ + H₂ + H₂S/COS	—

Outlet streams

Tag	Stream	Phase	Components	Quantity
12	Clean syngas	Gas	CH₄ + H₂ + metered CO₂, ≤0.1 ppm S → B6	0.1ppm S^[Rectisol]
13	Separated CO₂	Gas	Rectisol CO₂ → B6 feed (recycle / vent split)	—
14	Elemental sulfur	Solid	From S3 liquid-redox recovery	—

Total sulfur at outlet: 0.1ppm^[Rectisol]
CO₂ split (recycle vs vent): —est.

Decision
Decision: calcium removed, Rectisol in
In-bed calcium capture fails in supercritical water (CaS hydrolysis). Calcium is removed from the flowsheet and replaced with a commercially proven acid gas wash — the least novel block by design.
Warning
Terminology: not WFGD
This is a reducing syngas with H₂S/COS. Do not call it 'wet desulfurization' — WFGD is oxidising post-combustion SO₂ control. The analogues are physical-solvent / amine acid gas removal.
Note
The cost of the decision
Rectisol is capital-intensive, cryogenic, and brings methanol inventory into a plant that otherwise has none — likely the largest CAPEX item after the hydrothermal island.

Bi-reformer

Needs validation

T800–900 °Cest.P1–3 MPaest.

Supercritical water gasification produces essentially no CO — water-gas shift equilibrium in a medium that is overwhelmingly water sits hard on the product side. OXZEO consumes CO. The two blocks are chemically incompatible as directly coupled units, and this block exists to resolve that.

Dry reforming alone (CH₄ + CO₂ → 2CO + 2H₂) gives H₂/CO ≈ 1, below the ratio the cited OXZEO systems run at; bi-reforming combines dry and steam reforming in one stage and delivers the target ratio without a separate adjustment step. Steam is free from the hydrothermal island, and steam co-feed is the principal coking mitigation, coking being the dominant Ni deactivation mode.

Inlet streams

Tag	Stream	Phase	Components	Quantity
12	Clean syngas	Gas	CH₄ + H₂	—
13	Separated CO₂	Gas	Dry-reforming oxidant, metered from B5	—
16	CO₂-rich recycle	Gas	OXZEO co-product CO₂, returned from B7	—
17	Steam	Gas	Free from hydrothermal island; coking mitigation	—

Outlet streams

Tag	Stream	Phase	Components	Quantity
15	Reformed syngas	Gas	CO + H₂ at OXZEO target ratio → B7	—

Reforming duty: —est.
CH₄ conversion: —est.
Outlet H₂/CO: —est.

Needs validation
Needs validation
The reformer resolves the CO deficit, but its duty, ratio and coking behaviour on this specific gas are unquantified.
Warning
The energy objection, stated plainly
Methanation in B2 is exothermic; reforming here is strongly endothermic and 200–300 °C hotter. The flowsheet spends energy making methane and more energy unmaking it. Heat integration limits how much of that round trip shows up as fuel; it does not remove it.
Decision
Decision: bi-reforming is the design basis
The alternative — operating B2 hotter and shorter with a low-methanation bed so the downstream duty collapses to reverse water-gas shift — is evaluated and rejected: higher-severity hydrothermal service worsens every materials problem; salt precipitation becomes more aggressive, attacking the B3 block the concept depends on; and the case rests on a kinetic assumption with no supporting result in the red mud literature. The diagram shows one committed architecture — no dashed alternative path.

OXZEO olefin synthesis

T400 °C^[OXZEO-1]P2.5–4 MPa^[OXZEO-2]

Oxide–zeolite bifunctional conversion of syngas to C₂–C₄ olefins, exceeding the Anderson–Schulz–Flory selectivity limit. Unconverted syngas is recycled.

Be honest about CO₂: the route co-produces substantial CO₂ via the CO-mediated pathway, and a published Comment in ACS Catalysis (2023) disputes 'low CO₂ emission' claims for direct syngas-to-olefins. In a standalone plant that CO₂ is a liability; here it recycles to B6 as dry-reforming oxidant. That integration is a genuine argument for this configuration — the recycle loop is shown prominently in the diagram.

Inlet streams

Tag	Stream	Phase	Components	Quantity
15	Reformed syngas	Gas	CO + H₂ at target ratio	—
18	Unconverted syngas recycle	Gas	Unconverted CO + H₂	—

Outlet streams

Tag	Stream	Phase	Components	Quantity
19	C₂–C₄ olefins	Gas	Light olefins → product slate	—
16	CO₂-rich recycle	Gas	OXZEO co-product CO₂ → B6	—
18	Unconverted syngas recycle	Gas	CO + H₂ → recycle	—

Per-pass CO conversion: —est.
Light-olefin selectivity: 80%^[OXZEO-1]
C₂/C₃ ratio: —est.

Context values

ZnCrOₓ–SAPO-34 CO conversion: 17%^[OXZEO-1]
ZnCr₂O₄@ZnOₓ CO conversion: 64%^[OXZEO-2]
ZnCr₂O₄@ZnOₓ light-olefin selectivity: 75%^[OXZEO-2]

Note
CO₂ honesty
The CO-mediated pathway co-produces substantial CO₂ (disputed 'low-CO₂' claims, ACS Catalysis 2023). Here it is not vented — it recycles to B6, which is the integration argument for this configuration.

Red mud regeneration and residue valorization

Air oxidation, then hydrometallurgy

Reduced iron phases are re-oxidised with heat recovery; a bleed stream leaves as product. The regenerated solid returns to B1 as stream R1 rather than to the reactor: it leaves B8 hot, oxidised and near atmospheric pressure, and has to be re-slurried and re-pressurised, so the recycle merges into slurry make-up and competes for the solids budget on exactly the same terms as fresh residue. The budget constraint is therefore on total red mud in circulation, not on make-up rate.

Note the synergy: B2 has already partially reduced Fe₂O₃ → Fe₃O₄/FeO using biomass-derived reductant, so the residue arrives at any ironmaking step pre-reduced at no marginal cost.

Inlet streams

Tag	Stream	Phase	Components	Quantity
11	Spent red mud	Solid	Reduced Fe phases (Fe₃O₄/FeO)	—
20	Air	Gas	Oxidant for re-oxidation	—

Outlet streams

Tag	Stream	Phase	Components	Quantity
R1	Regenerated red mud recycle	Solid	Re-oxidised red mud → re-slurried at B1	—
22	Residue bleed	Solid	Dealkalized residue → product slate (Tiers 1–3)	—

Oxidation heat recovery: —est.
Iron re-oxidation extent: —est.
Bleed fraction: —est.
Cycles to attrition failure: —est.

Note
Pre-reduction synergy
B2 has already partially reduced Fe₂O₃ using biomass-derived reductant, so residue destined for ironmaking arrives pre-reduced at no marginal cost.
Decision
The bleed fraction is a headline result, not a nuisance parameter
The recycle competes with the product. B3 strips sodium to make the residue saleable — but that same alkali supplied the tar-cracking and shift promotion that made red mud useful in B2. Recycled solid is therefore progressively less active on each pass, which inverts the usual chemical-looping logic: here the deactivation is the product being manufactured. A fast bleed keeps the circulating inventory alkaline and active but may release residue before it meets a saleable dealkalization specification; a slow bleed delivers fully dealkalized, saleable residue while leaving the design leaning entirely on iron. The bleed fraction sets the exchange rate between catalytic performance and residue product quality.
Warning
Does the air regeneration accomplish anything?
In supercritical water at 600–650 °C with water in vast excess, steam re-oxidises reduced iron directly by 3Fe + 4H₂O → Fe₃O₄ + 4H₂, so the reactor may hold the iron at a magnetite-like steady state without external help. If so, B8's air-oxidation step is not restoring oxygen capacity at all — it is performing heat recovery and conditioning the solid for metals recovery, which are real duties but different ones, and it should be sized against those. Open: this must be settled before B8 has a design basis.

Act 6

Why it works

Why the flowsheet is shaped the way it is. Almost everything distinctive here — why the biomass dissolves and reacts, why the product gas contains no CO, why the salts crash out, and why the red mud behaves as it does — follows from a single property change in the solvent. This section supplies that chemistry, and ends by listing three findings that contradict the flowsheet as drawn.

The solvent: one property change

Water has a critical point at 373.95 °C and 22.064 MPa, with a critical density of 322 kg/m³. Crossing it does not merely remove the phase boundary; it dismantles the hydrogen-bond network that gives liquid water its solvent character. Three consequences follow, and they are not independent — all three are expressions of the same structural collapse.

Property	25 °C, 0.1 MPa	400 °C, 25 MPa	600 °C, 25 MPa	Consequence
Density (kg/m³)	997	~167	~71	Gas-like transport, liquid-like collision frequency
Static dielectric constant εind.	78.4	~2	~1.2–1.5	Water becomes a non-polar solvent — behaves like hexane, not like water
Ionic product K_wind.	10⁻¹⁴	≈10⁻¹⁹ to 10⁻²¹	≈10⁻²³	Acid/base catalysis switched off; free-radical chemistry takes over
Organic miscibility	Poor	Complete	Complete	Single-phase reaction — no interphase mass transfer resistance
Inorganic salt solubility	High	Very low	Negligible	Salts precipitate — the B3 duty and the plugging hazard

Table 3.1 — Water properties across the operating range. Densities from steam tables at 25 MPa; dielectric and ionic-product values from the hydrothermal literature, with the 600 °C entries flagged as extrapolated and requiring a cited source before final issue.

One property change, two opposite consequences

The dielectric constant falling from 78 to roughly 2 is a single physical change with two opposite consequences, and the entire flowsheet architecture is a response to both at once. Because ε collapses, water can no longer stabilise ions by solvation. Non-polar organics therefore become fully miscible — which is why biomass depolymerises and reacts in a single homogeneous phase with no interphase resistance, and why the moisture that would cripple a dry gasifier becomes an asset. But the same loss of solvation power means dissolved inorganic salts have nothing holding them in solution, so they crash out — which is why sodium separation is available as a designed product operation and why plugging is the dominant failure mode.

The gasifier and the salt separator are therefore not two independent units that happen to sit in series. They are two faces of one solvent property, exploited in the same vessel train. That is the strongest physical argument for the concept.

The ionic-product collapse sets the reaction regime. Where K_w exceeds roughly 10⁻¹⁴, hydronium and hydroxide concentrations are high enough for acid- and base-catalysed ionic chemistry to dominate; where K_w falls far below that, homolytic free-radical pathways take over. K_w passes through a maximum near 250–300 °C, so a feed heated from ambient to 600 °C traverses an ionic regime on the way up and finishes in a radical regime. This is not a nuisance — it is the reason a single reactor can perform both hydrolytic depolymerisation and reforming, and the feed preheat profile is therefore a design variable rather than a utility calculation.

Reaction network in the gasifier

Stage 1 — ionic hydrolysis during preheat, 200–370 °C

In the near-critical region K_w is at or above its ambient value and water is simultaneously a strong acid and a strong base. Glycosidic bonds in douzha's cellulose and pectin, and in straw hemicellulose, hydrolyse to oligosaccharides and monosaccharides. Peptide bonds hydrolyse to free amino acids. Triglycerides hydrolyse to free fatty acids and glycerol. Because douzha carries essentially no lignin, the most hydrolysis-resistant and char-prone biopolymer is absent, and depolymerisation is close to complete before the critical point is reached — the central kinetic advantage of this feedstock.

Sugars entering this window are not stable in it. Retro-aldol cleavage, dehydration and isomerisation produce glyceraldehyde, pyruvaldehyde, glycolaldehyde, furfural and 5-hydroxymethylfurfural, together with organic acids — chiefly formic, acetic and lactic. Formic acid matters out of proportion to its concentration because it decomposes by two competing routes, dehydrogenation to CO₂ and H₂ or dehydration to CO and H₂O, and the branching ratio between them is one of the levers on early gas composition.

Stage 2 — free-radical gasification above the critical point

Beyond roughly 400 °C the ionic route is suppressed and homolytic bond scission dominates. Fragments undergo steam reforming, water-gas shift and methanation on their way to a gas that is close to thermodynamic equilibrium.

Why the product gas contains essentially no CO

This is the single most consequential result in the section, and it is a thermodynamic certainty rather than a catalyst artefact. Water-gas shift is mildly exothermic, so its equilibrium constant falls with temperature — but the equilibrium position depends on the water-to-carbon ratio as strongly as on temperature, and in supercritical water gasification that ratio is enormous. At 20 wt% solids the molar ratio of water to feed carbon is of order 15–25 to one, against roughly 3 to one in a conventional steam reformer. Le Chatelier applied to CO + H₂O ⇌ CO₂ + H₂ with water in vast excess drives the reaction essentially to completion, leaving CO at trace level.

The consequence is structural, not incidental

No adjustment of catalyst, temperature or residence time within the hydrothermal window recovers CO, because the water excess that makes the process work is the same thing that destroys CO. Any CO-consuming downstream chemistry — Fischer–Tropsch, classical methanol synthesis, OXZEO — is therefore foreclosed on a directly coupled basis. This is the origin of the second design conflict and the justification for block B6.

Nitrogen and sulfur speciation from a protein-rich feed

Douzha at roughly 27 wt% protein makes heteroatom chemistry a primary rather than a trace concern. Amino acids released by peptide hydrolysis follow two competing routes: deamination, which liberates ammonia and leaves a carboxylic acid, and decarboxylation, which liberates CO₂ and leaves an amine. Deamination dominates at hydrothermal severity, so nitrogen reports overwhelmingly as NH₃ rather than as N₂ or NOₓ. Because ammonia is highly soluble in the dense aqueous phase and is a base, it partitions strongly into the water and leaves with the B3 brine — which is why nitrogen recovery is an aqueous-phase operation in this flowsheet and not a gas-cleaning one.

Sulfur arrives almost entirely as cysteine and methionine. Thermolysis of cysteine gives H₂S directly; methionine gives methanethiol and dimethyl sulfide, which hydrolyse and reform toward H₂S under hydrothermal conditions. Two secondary species matter disproportionately downstream. Carbonyl sulfide forms readily from H₂S in a CO₂-rich environment by H₂S + CO₂ ⇌ COS + H₂O, and thiophenic sulfur can form by condensation of sulfide with carbonyl-bearing fragments. COS is the reason the design selects a physical solvent wash over an amine — amines remove COS poorly, and COS passes through to poison the OXZEO catalyst.

Char and coke: the specific risk douzha introduces

The absence of lignin removes the classical char precursor, but protein and reducing sugars together introduce a different one. Maillard condensation between amino groups and carbonyl carbons produces melanoidins — nitrogen-containing, highly conjugated, thermally robust polymers — and the near-critical preheat zone is close to ideal for forming them. Because they resist both hydrolysis and steam gasification, melanoidin formation converts feed carbon into a residue that reports to the solid phase and contaminates the red mud product.

An under-recognised risk, with a specific countermeasure

This argues for minimising residence time in the 200–300 °C window where Maillard kinetics peak, by using rapid direct mixing of the cold slurry with recycled supercritical effluent rather than slow indirect preheat. That is a heat-exchanger network decision made on chemical grounds. No quantitative melanoidin yield data exists for douzha under hydrothermal gasification conditions; it must be measured before the char balance can be closed.

Reaction	Stoichiometry	ΔH°₂₉₈	Role here
Steam reforming of fragments	CₙHₘOₖ + H₂O → CO + H₂	endothermic	Primary gas-forming step
Water-gas shift	CO + H₂O ⇌ CO₂ + H₂	−41	Drives CO to near zero
CO methanation	CO + 3H₂ ⇌ CH₄ + H₂O	−206	Exothermic; buffered by the water inventory
CO₂ methanation	CO₂ + 4H₂ ⇌ CH₄ + 2H₂O	−165	Consumes the H₂ that shift produced
Steam–carbon (char gasification)	C + H₂O → CO + H₂	+131	Consumes char; favoured at high severity
Boudouard (reverse)	2CO ⇌ C + CO₂	−172	Coke route; suppressed by low CO here
Methane cracking	CH₄ ⇌ C + 2H₂	+75	Negligible at 600–650 °C; matters in B6

Table 3.2 — Principal gas-phase reactions in the gasifier, ΔH°₂₉₈ in kJ/mol, endothermic positive. Standard values used for direction and relative magnitude only; Section 5 must use temperature-corrected values at operating conditions.

Iron redox and the alkali cycle

Red mud redox thermodynamics, and what buffers the iron

Iron oxide reduces in a defined sequence, Fe₂O₃ → Fe₃O₄ → FeO → Fe, and each step is governed not by absolute temperature but by the reducing potential of the gas, expressed as the H₂/H₂O ratio. Two constraints bear on this system. First, wüstite (FeO) is thermodynamically unstable below approximately 570 °C, disproportionating to metallic iron and magnetite, so at the lower end of the proposed 600–650 °C window FeO is only marginally accessible. Second, and decisively, the steam–iron equilibria run in reverse when water is in excess: 3Fe + 4H₂O ⇌ Fe₃O₄ + 4H₂ and 3FeO + H₂O ⇌ Fe₃O₄ + H₂ both proceed to the right when H₂/H₂O is small.

In supercritical water gasification H₂/H₂O is very small by construction — water is 80 wt% of the feed and hydrogen is a dilute product. The iron is therefore buffered at magnetite, not progressively reduced.

This resolves the open question on B8

If no lattice oxygen is lost across a pass, B8's air oxidation is not restoring oxygen transport capacity, because none was lost. Its real duties are heat recovery and conditioning the solid for metals recovery, and it should be sized against those. It also means the Fe³⁺/Fe²⁺ couple functions as a redox relay in the shift and reforming steps — a mediating role — rather than as a stoichiometric oxidant. The argument is qualitative and must be confirmed by a Gibbs-minimisation calculation over the Fe–O–H₂O system at 25 MPa before it is relied on.

The alkali contribution: a formate-mediated shift cycle

The alkali in red mud is not a generic promoter. Sodium carbonate and hydroxide participate in a stoichiometric formate cycle that constitutes a genuine homogeneous water-gas shift pathway, distinct from the surface redox mechanism operating on the iron:

Na₂CO₃ + H₂O → NaHCO₃ + NaOH
NaOH + CO → HCOONa
HCOONa + H₂O → NaHCO₃ + H₂
2NaHCO₃ → Na₂CO₃ + H₂O + CO₂

The cycle closes on sodium carbonate, so alkali is formally a catalyst — but only while it remains in contact with the reacting fluid. Because salts precipitate under these conditions, the alkali is continuously being removed from the reaction zone into the B3 brine. The formate pathway therefore has a finite life set by sodium residence in the solid, which is the mechanistic statement of the first design conflict: the same chemistry that promotes shift is the chemistry that dealkalizes the residue, and the two cannot be optimised independently.

Salt nucleation and deposition

Salt nucleation, growth and deposition

Salt behaviour in supercritical water divides into two classes, and the distinction determines separator design. Type 1 salts, of which sodium chloride is the archetype, retain appreciable solubility and form a dense brine that separates as a fluid phase. Type 2 salts, of which sodium sulfate and sodium carbonate are the relevant examples here, have very low solubility and precipitate directly as solids. Sodium sulfate is markedly less soluble than sodium chloride and correspondingly more prone to deposition.

This flowsheet is dominated by Type 2 behaviour, which is the harder case. The sodium arriving from red mud is present as carbonate, hydroxide and aluminosilicate-bound alkali; the sulfur partitioning into the aqueous phase appears as sulfide and sulfate; the potassium from straw ash accompanies it. The separator is therefore handling a mixed Na–K sulfate/carbonate system that precipitates as a solid rather than draining as a brine, and it must be designed to remove solids continuously.

The precipitation sequence is well characterised. Homogeneous nucleation initiates from a highly supersaturated solution, followed by mass-transfer-limited particle growth; molecular dynamics work resolves the early stages into ion pairs, then small ionic clusters, then large clusters, with nucleation complete within tens of picoseconds. Sodium sulfate particles are finer and more strongly aggregated than sodium chloride crystals, with primary particles of roughly 1–3 μm agglomerating into clusters up to about 20 μm.

Why a cooled wall is mechanism, not vendor preference

Hot surfaces present additional adsorption sites and therefore promote heterogeneous nucleation preferentially on high-temperature walls, where the resulting particles adhere. Deposition is thus not a random fouling process but a directed one, driven toward the hottest surface available. A cooled-wall separator inverts that gradient deliberately: by making the wall the coldest surface in the vessel, nucleation is driven into the bulk and onto a cold collection surface from which solids can be removed, rather than onto the process boundary.

Bi-reforming stoichiometry

Reaction	Stoichiometry	ΔH°₂₉₈	H₂/CO produced
Steam methane reforming	CH₄ + H₂O ⇌ CO + 3H₂	+206	H₂/CO = 3
Dry (CO₂) reforming	CH₄ + CO₂ ⇌ 2CO + 2H₂	+247	H₂/CO = 1
Bi-reforming (Olah stoichiometry)	3CH₄ + 2H₂O + CO₂ ⇌ 4CO + 8H₂	+659	H₂/CO = 2 exactly
Reverse water-gas shift	CO₂ + H₂ ⇌ CO + H₂O	+41	—
Methane cracking (coke)	CH₄ ⇌ C + 2H₂	+75	—
Boudouard (coke)	2CO ⇌ C + CO₂	−172	—
Carbon gasification (coke removal)	C + H₂O → CO + H₂	+131	—

Table 3.3 — Reformer reaction set, ΔH°₂₉₈ in kJ/mol. Bi-reforming is the combination 2×SMR + 1×DRM, and its H₂/CO ratio of exactly 2 is the reason it is preferred here.

Why H₂/CO = 2 is the correct target, and why dry reforming alone would waste half the carbon

Olefin synthesis from syngas must reject the oxygen that arrives bound to carbon monoxide. There are only two ways to do it, and they differ enormously in carbon efficiency:

Hydrogen-rich rejection as water

CO + 2H₂ → (–CH₂–) + H₂O

Requires H₂/CO = 2 exactly

Carbon efficiency 100% — every carbon atom fed reaches the product

Carbon-rich rejection as CO₂

2CO + H₂ → (–CH₂–) + CO₂

Operates at H₂/CO = 0.5

Carbon efficiency 50% — half the carbon fed leaves as carbon dioxide

Dry reforming alone delivers H₂/CO = 1, which sits between these limits and forces a substantial fraction of carbon out as CO₂ regardless of catalyst. Bi-reforming delivers H₂/CO = 2, which is precisely the stoichiometric requirement for water rejection with no CO₂ co-production. The match is exact and it is not a coincidence of convenience: it is the reason bi-reforming rather than dry reforming is the correct unit operation here, and it is a stronger argument than ratio-adjustment convenience. It also reframes the CO₂ selectivity problem in the OXZEO block as primarily a feed-ratio question rather than a catalyst question.

Carbon deposition and the thermodynamic carbon boundary

Coking is the dominant deactivation mechanism for nickel reforming catalysts and it proceeds by two routes with opposite temperature dependence. Methane cracking is endothermic and therefore worsens as temperature rises; the Boudouard reaction is exothermic and worsens as temperature falls. Between them they leave a window rather than a monotonic trend, and the practical consequence is that a reformer cannot be made coke-free by temperature alone.

The controlling variable is instead the oxidant-to-carbon ratio. Carbon deposition is suppressed thermodynamically when sufficient H₂O and CO₂ are present to gasify deposited carbon as fast as it forms. This is the mechanistic reason steam co-feed is the standard coking mitigation, and it is why bi-reforming is more robust than dry reforming in practice as well as more favourable stoichiometrically: the steam component both sets the H₂/CO ratio and suppresses carbon.

A genuine and unearned advantage

Steam is available at essentially zero marginal cost, because the hydrothermal island is already a water-dominated system. A conventional dry reforming installation must justify every mole of steam against its energy cost. Here the steam is a by-product of the process configuration, so the coke-suppressing oxidant-to-carbon ratio can be set generously without an economic penalty. Section 5 should quantify how far this offsets the reformer endotherm.

OXZEO: how the ASF limit is beaten

Why Fischer–Tropsch cannot reach these selectivities and OXZEO can

Classical Fischer–Tropsch synthesis builds hydrocarbons by successive CH₂ insertion on a single metal surface. Because each chain-growth step has the same probability, the product distribution follows Anderson–Schulz–Flory statistics and the achievable C₂–C₄ fraction is capped near 58% irrespective of catalyst optimisation. The oxide–zeolite approach evades that ceiling by physically separating the two functions: CO activation occurs on the oxide, and carbon–carbon coupling occurs inside the zeolite, where pore geometry rather than surface statistics governs chain length.

CO activation at oxygen vacancies on the oxide

The active site on the oxide component is the surface oxygen vacancy. Spinel ZnCr₂O₄ is reduced by CO or H₂ under reaction conditions to generate vacancies, and both CO and H adsorption strengthen markedly as vacancy concentration rises; two-coordinate oxygen vacancy sites have been identified as the active centres for CO activation and subsequent hydrogenation, generating a sequence of CHO, CH₂O, CH₂ and CH₂CO surface intermediates. Recent work confines a ZnOₓ overlayer on the ZnCr₂O₄ spinel specifically to control this vacancy population.

A direct and unwelcome implication for the sulfur specification

If the active site is a reduced-oxide oxygen vacancy, then sulfide chemisorption competes for exactly the coordination position the mechanism depends on — and zinc has a high affinity for sulfur. The same affinity that makes ZnO an effective guard bed sorbent makes the ZnCrOₓ component vulnerable. Deactivation should therefore be expected to be effectively irreversible rather than merely inhibitory, which is the mechanistic justification for the sub-ppm outlet specification and for choosing a wash that removes COS as well as H₂S. The sub-0.1 ppm figure remains an inference from this argument plus the Rectisol capability, not a cited deactivation study; a poisoning threshold measurement is required.

Ketene as the coupling intermediate, and the role of proximity

The species that crosses from oxide to zeolite has been identified as ketene, CH₂CO. Mechanistic work combining neural-network potentials with microkinetic simulation over ZnCrOₓ|SAPO-34 attributes ketene formation to two competing routes with strongly unequal weight: approximately 86% arises from methanol carbonylation at neighbouring zeolite acid sites, and approximately 14% from direct CHO*–CO* coupling on the oxide.

Bed architecture is a primary design variable, not a preparation detail

The dominant pathway requires the two functions to be close enough for a methanol-derived intermediate to reach a zeolite acid site before it is over-hydrogenated. Granule stacking versus intimate powder mixing versus a core–shell arrangement therefore has to be specified. If the oxide and zeolite are too far apart the system reverts toward methanol synthesis; if they are too intimate, the zeolite acid sites can over-hydrogenate intermediates to paraffins and the olefin-to-paraffin ratio collapses.

Shape selectivity in the CHA cage, and where the CO₂ comes from

SAPO-34 has the chabazite topology: large cages of roughly 0.94 nm connected through eight-membered-ring windows of approximately 0.38 nm. Coupling occurs inside the cages, but only species small enough to pass the windows can leave, which excludes branched and heavier products and delivers the light olefin cut directly. Selectivity is therefore a geometric property of the framework rather than a kinetic accident, which is why it is robust across the operating window while conversion is not.

The CO₂ co-production that the product slate flags as contested follows from the oxygen-rejection stoichiometry. Carbon monoxide brings one oxygen atom per carbon, and that oxygen must leave either as water, consuming hydrogen, or as carbon dioxide, consuming a second CO. The observed CO₂ selectivity is therefore primarily a statement about the H₂/CO ratio at the reactor inlet, and only secondarily about the catalyst. Feeding at H₂/CO ≈ 2 pushes rejection toward water and suppresses CO₂; feeding CO-rich forces the carbon-consuming route. This is the quantitative link between the reformer specification and the disputed carbon efficiency claim, and it is the strongest available response to the published criticism of low-CO₂ claims for direct syngas-to-olefins.

Deactivation modes

Three mechanisms belong in the risk register. Coke accumulation inside the CHA cages is the familiar methanol-to-olefins failure mode and is addressed by the same remedy — periodic oxidative regeneration, which requires a swing reactor arrangement. Zinc migration and volatilisation degrades the oxide component and sets an upper temperature limit independent of selectivity considerations. Sulfur poisoning at the oxygen vacancy is effectively terminal, which is why the guard bed is retained even when the primary wash is specified to 0.1 ppm.

Where the chemistry contradicts the flowsheet

Three findings in this section are not consistent with the block architecture as drawn, and the work is more useful for listing them plainly than for reconciling them prematurely.

The B8 air regeneration has no oxygen-capacity justification
The iron is buffered at magnetite by the steam–iron equilibrium in excess water, so no lattice oxygen is lost per pass. B8 should be re-scoped around heat recovery and metals conditioning, and the R1 recycle sized accordingly.
The Maillard route is an unquantified carbon sink specific to this feedstock
Melanoidin formation in the preheat zone is a char pathway that the lignin-free framing had implicitly dismissed. It argues for rapid direct-contact preheat, which is a change to the B1 heat exchanger network.
The catalyst bed architecture in B7 is unspecified but decisive
The dominant ketene route depends on oxide–zeolite proximity, so B7 cannot be treated as a generic fixed bed. The arrangement, and the olefin-to-paraffin consequence of getting it wrong, must be specified.

Act 7

What must not leave

A process that turns two waste streams into products still has to answer for what it releases. China regulates discharge by industry category and by receiving environment, and the applicable limit is often set by a special-limit regime rather than the headline national figure. This section names the instruments that would govern this plant and how the flowsheet is positioned against each — not the permit numbers, which cannot be set until the category and the site are fixed.

Standard designations below are named because they are the governing instruments. Specific numeric limits are deliberately not reproduced: the binding value depends on the permitted industry category, the receiving water body, and whether the site sits inside a key control area subject to tightened special limits. All of it requires verification against the current standard text and the local ecology and environment bureau before use.

Water discharge

GB 8978-1996
verify
Integrated Wastewater Discharge Standard
The default national instrument where no industry-specific standard applies. Sets tiered limits by discharge grade and by the receiving water body's protection class, and distinguishes first-class pollutants — which must be controlled at the workshop outlet rather than the site boundary — from second-class pollutants measured at the plant outlet.
GB 31571-2015
verify
Emission standard of pollutants for petroleum chemistry industry
The most likely governing standard if the plant is permitted as petrochemical rather than as a waste treatment facility. Carries a special-limit regime for key control areas that is materially tighter than the base limits, and is the standard against which an olefins train would normally be assessed.
GB 18918-2002
verify
Discharge standard of pollutants for municipal wastewater treatment plant
Applies if the aqueous effluent is discharged to, and co-treated by, a municipal works rather than released directly. Relevant because the ammonia-bearing stream from the phase separator is the obvious candidate for indirect discharge.

Air emissions

GB 16297-1996
verify
Integrated emission standard of air pollutants
The default instrument for stack and fugitive emissions where no sector standard governs. Sets both concentration limits and rate limits tied to stack height, and separately regulates unorganized (fugitive) emissions at the site boundary.
GB 37822-2019
verify
Standard for fugitive emission of volatile organic compounds
Governs unorganized VOC release — equipment leaks, storage tanks, loading, and open liquid surfaces. Directly relevant because the acid gas train brings a methanol inventory into a plant that would otherwise have none, and methanol handling is exactly the activity this standard targets.
GB 31570-2015
verify
Emission standard of pollutants for petroleum refining industry
Relevant to the fired reformer and to sulfur recovery tail gas. Together with GB 31571 it also carries the special-limit regime applied in key control areas.

Stream-by-stream disposition

Stream	Origin	Contaminants	Treatment route	Residual risk
Aqueous effluent	Phase separation after let-down (B4)	Ammonia, dissolved organics, residual COD, trace phenolics	Nitrogen recovery first, which is the point: ammonia is stripped and sold as fertilizer value rather than destroyed. The polished remainder carries a much lower nitrogen load into whatever biological or advanced-oxidation step follows.	COD and residual organics after nitrogen recovery are unquantified until the balance closes. Whether a biological step alone suffices, or advanced oxidation is required, is unresolved.
Salt brine	Salt separator (B3)	Na, K, phosphate, sulfide and sulfate	Sold as a fertilizer precursor rather than discharged. This is the single largest reason the flowsheet does not have a conventional brine disposal problem — the salt is a designed product, not an effluent.	Only works if the brine meets fertilizer product specification. If heavy metals from the residue partition into it, the stream reverts from product to hazardous waste and the economics change materially. Requires analysis.
Acid gas	Acid gas removal (B5)	H₂S, COS, CO₂	Liquid-redox recovery to elemental sulfur, chosen over Claus precisely because the sulfur load is expected to fall below the 10–13 vol% H₂S that Claus needs. Yields a saleable commodity rather than a spent-sorbent disposal stream.	Tail gas from the redox unit still requires treatment, and its sulfur slip sets whether a further polishing step is needed.
CO₂	Separated in the acid gas wash; co-produced by olefin synthesis	CO₂ with trace sulfur	Recycled to the reformer as dry-reforming oxidant rather than vented. This is the integration argument for the whole configuration: the CO₂ that makes the olefin route look bad in a standalone plant has a use here.	The recycle cannot absorb all of it. The vented fraction is the number that decides the carbon claim, and it is not yet quantified.
Spent solids	Red mud bleed from the regeneration loop (B8)	Iron oxides, alumina, silica, titania, trace heavy metals	Sold as SO₂ sorbent, cementitious material or metal recovery feed. Dealkalization is what makes this legal and commercial rather than a landfill line item.	Reclassification from hazardous waste to product is a regulatory determination, not a technical one, and it is the gating question for the entire residue value chain.
Fugitive VOC	Methanol inventory in the chilled solvent wash	Methanol vapour	Closed-loop solvent handling with leak detection and repair, which is the control regime GB 37822 is written around.	This is a liability the flowsheet imports by choosing a physical solvent wash. The cheaper amine alternative would avoid the methanol inventory entirely — one more input to that open capital decision.

The pattern is deliberate: almost every stream that would ordinarily be an effluent has been given a product destination instead. That is what makes the concept coherent — but it also means the environmental case and the commercial case stand or fall together. If the brine cannot be sold, it becomes a disposal cost; if the residue cannot be reclassified, it goes back to the impoundment. The discharge standards are therefore not a compliance afterthought but a constraint on whether the product slate exists at all.

Act 8

Product slate

After the last block the sticky diagram releases and the layout returns to full width. The slate spans three families — carbon-derived, heteroatom, and bauxite-derived — plus avoided disposal, which in the Chinese context is likely the largest single contributor and the thing that decides whether the feedstocks are wastes at all.

Carbon-derived

C₂–C₄ olefins

Polyolefins, ethylene oxide/glycol, propylene oxide, acrylonitrile, butadiene precursor.

The main product. Bio-attributed under ISCC PLUS mass balance; the premium sits here.

Downstream: polyolefins, ethylene oxide and glycol, propylene oxide, acrylonitrile, and butadiene precursors.

CO₂ from OXZEO

Large, contested, and here it has a home: recycle to B6.

The OXZEO route co-produces substantial CO₂. In a standalone plant that is a liability; here it recycles to B6 as dry-reforming oxidant, which is a genuine argument for this configuration.

C₅+ and paraffin tail

Internal fuel, best fired into the reformer endotherm.

Not sold. Best fired into the B6 reformer endotherm, where the plant already needs heat.

Bio-SNG

Only under the alternative fork — the base case the olefin route must beat.

Bio-substitute natural gas is the product of the rejected alternative architecture (methanation-favouring B2, no reforming). It is the base case the olefin route must beat, not a co-product of the committed design.

Heteroatom

N-K-P-S brine → compound fertilizer

Value driver is avoided nitrogen-removal cost on the effluent, not fertilizer price.

The recovered brine carries Na/K/P plus partitioned sulfur. Its value driver is the avoided nitrogen-removal cost on the effluent, not the fertilizer sale price.

Ammonia → ammonium sulfate or urea

Ammonium sulfate, or urea if you also want to consume CO₂.

The ammonia-bearing aqueous phase from B4 feeds nitrogen recovery. Convert to ammonium sulfate, or to urea if consuming additional CO₂ is desirable.

Elemental sulfur

From liquid-redox recovery on the B5 acid gas. A commodity, not a disposal stream.

From the S3 liquid-redox unit. Modest revenue, but the point is that it is a saleable commodity rather than a spent-sorbent disposal stream — one of the arguments for removing calcium.

Bauxite-derived — three tiers, inverse mass and value

Give this the most interactive depth. Volume falls and value rises as you descend the tiers.

T1Tier 1 — sell the dealkalized solid as-is

Highest volume, lowest margin, and the tier this process uniquely enables.

Dealkalization is what makes the residue saleable at all, so this is the tier the concept uniquely unlocks.

SO₂/H₂S sorbent — FGD, sinter plant, or fed back to the S3 bed.

Supplementary cementitious material — 10–20% clinker replacement in blended cements, 25–30% demonstrated; co-calcination with kaolinite gives 30% replacement at 88% of reference 28-day strength; 3–5% into clinker raw meal.

Geopolymer precursor — Na-silicate-activated systems reach comparable or better compressive strength than conventional binders, and red mud supplies its own sodium.

Heavyweight aggregate and radiation shielding; soil amendment; mine remediation.

The ceiling is alkalinity, not demand

Over 120 Mt/y produced globally, utilization below 3%. The uses exist; the alkalinity is what blocks them — which is exactly what this process removes.

T2Tier 2 — iron

Reduction roasting or smelting reduction; residual slag is building-material feed.

Reduction roasting + magnetic separation: 97.69% metallization, 81.40% recovery.

Smelting reduction (1500–1600 °C, lime/dolomite flux): pig iron Fe >90%, 90–95% recovery; recent work reports 98.14–98.36% Fe recovery meeting the steelmaking pig iron standard.

Residual slag is Al₂O₃–SiO₂–CaO–TiO₂ — building-material feed, so the route creates no new waste.

CBAM sits on the iron, not the olefins

Iron units entering a steel chain are Annex I goods. The olefins are not. The by-product carries the border-adjustment exposure.

T3Tier 3 — critical metals from the iron-depleted slag

Smelt first: it concentrates these roughly twofold. Acid leaching is complementary to iron recovery.

Smelt first — it concentrates these roughly twofold. Acid leaching dissolves Sc/REE/V/Ga while leaving iron in the solid, so the two routes are complementary.


Sc	16–230 ppm typical (~84 ppm Chinese sample); 100–800 ppm reported	Sc₂O₃ $3,000–5,000/kg	Al-Sc aerospace alloys, ScSZ fuel-cell electrolytes
REE	China 400–1,200 ppm; ~2× enriched over bauxite	varies	magnets, catalysts
Ga	50–100 ppm	$150–300/kg	GaAs/GaN, LEDs; strategically sensitive
V	—	V₂O₅ $8–15/kg	steel microalloying, redox flow batteries
Ti	3–10 wt%	low	grade too poor for pigment without upgrading

Scandium: the binding constraint is market depth, not resource

100 kt/y of residue at 84 ppm contains ~8.4 t Sc ≈ 13 t Sc₂O₃; at 60% recovery and $3,000/kg that is ~$23M/y. But global Sc₂O₃ demand is only of order tens of tonnes per year — a single plant this size could saturate the market and collapse the price. An option contingent on Al-Sc alloy demand growth, not a bankable revenue line. Verify the global demand figure against a USGS commodity summary.

Avoided disposal

Gate fees on douzha and red mud

Not revenue in the accounting sense, but likely the largest single contributor in the Chinese context.

It also determines whether the feedstocks are wastes — which determines zero-burden entry under ISO 14067, the largest single lever on the carbon result.

Act 9

State of the work

Decisions taken. The three design conflicts the concept turns on — the CO deficit, CaS hydrolysis, and red mud's three assigned roles — are settled. Each is presented in Act 3 as problem → decision → what it cost. They are not open.

Conflict 1 — the salt separator
The salt separator (B3) is a purposeful product unit, not a protective device. Sodium removal is deliberate and metered, and red mud dosing is co-determined by its dealkalization/brine duty.
Conflict 2 — the CO deficit
Bi-reforming (B6) is the design basis. Upstream methanation suppression — running B2 hotter and shorter — is evaluated and rejected. The flowsheet shows one committed architecture.
Conflict 3 — sulfur capture
Calcium is removed from the flowsheet in favour of a conventional acid gas wash (Rectisol, B5). In-bed calcium capture fails in supercritical water via CaS hydrolysis.

Honest state of the work. These are the items that remain genuinely open — not the settled conflicts above.

01
Rectisol versus MDEA is undecided
Rectisol meets the OXZEO specification outright and handles CO₂ in the same unit; MDEA is far cheaper but needs the guard bed and gives no CO₂ control. This is the largest open capital question. The answer also determines whether CO₂ separation is free or a separate unit, which changes the Section 5 energy balance — so the capital comparison is needed before that balance is drafted.
02
Measured ultimate analyses are missing
For the specific douzha and straw sources, including sulfur and chloride. Three values in Section 1 are currently indicative. Sulfur now sets the acid gas removal duty and the liquid-redox unit size; chloride sets materials selection and the red mud bed sizing.
03
Supercritical dealkalization is unvalidated
At biomass-gasification residence times. This is the load-bearing claim of the whole concept and should be verified experimentally first. If it does not hold, the concept reverts to a conventional wet-biomass gasifier with a cheap iron catalyst — a much weaker proposition, and the report should say so.
04
The ISO 14067 waste classification pathway is unconfirmed
For bauxite residue in the relevant jurisdiction. It determines whether zero-burden entry is available and therefore which of two very different value propositions the work is built to defend — a low-carbon-olefin framing, or an avoided-disposal and residue-treatment framing. That choice changes whether the plant is optimized for olefin yield or for red mud throughput.
05
Feedstock traceability must be designed in, not retrofitted
ISCC certification integrity at the feedstock self-declaration stage is a known weak point, and douzha sourced from many small tofu and soymilk producers is precisely the fragmented, self-declared supply profile where that weakness bites. A traceability protocol belongs in the procurement model from the start rather than at audit.
06
The EU Green Claims Directive status is contested
Its status is inconsistent across secondary sources and must be verified against a primary Commission source before any statement about mandatory verification of environmental claims is made.
07
Feedstock geography is unresolved
No site holds red mud and douzha together at scale, and the Guangxi–Guangdong versus Henan trade-off is undecided.

Disposition of external review comments

An external evaluation of the wider bio-SNG and methanol programme raised a set of technical and certification risks. Several are decisive for dry gasification with downstream methanation and are structurally absent from this flowsheet; several apply in transformed form; and the certification critique applies with full force. Recording which is which prevents the work from either ignoring the review or absorbing objections its own architecture already answers.

Objection	Applies here?	Disposition
High moisture destroys the energy balance, because vaporising feed water consumes chemical energy	inverted	This is the objection supercritical water gasification exists to answer. Water is the reaction medium, never vaporised across a phase boundary, and there is no latent heat penalty. Douzha's 80–85% moisture is the reason the feedstock was selected. The correct residual concern is not moisture but the solids ceiling and the sensible heat of pressurised water.
K- and Cl-rich Chinese straws form molten K-silicates, defluidising beds and fouling surfaces	transformed	There is no fluidised bed and no sand inventory to agglomerate, so the classical failure mode is absent. But the underlying chemistry reappears as salt precipitation in B3 — potassium from straw joins sodium from red mud in the separator duty. The problem is not eliminated; it is relocated into the block already designated to handle it, and it is one more reason B3 is the critical unit.
Cooling raw syngas from ~850 °C for cleaning then reheating for methanation is a severe thermodynamic penalty	overstated	Largely recoverable, and previously overstated in this work. Sensible-heat excursions are what feed–effluent exchanger trains are for. Two residues are not recoverable by exchange and should be reported separately: the Rectisol refrigeration duty is compressor shaft work, not heat, and no exchanger returns it; and the methane round trip is reaction enthalpy, which heat integration cannot touch at all.
Methanation exotherm creates runaway risk requiring TREMP-style staging and intercooling	no	There is no methanation reactor. Methane forms in situ in the aqueous phase, where the enormous thermal mass and heat capacity of supercritical water buffer the exotherm — an underappreciated safety advantage of the hydrothermal route. The equivalent exotherm management problem sits in B7, where OXZEO synthesis is exothermic, and in B8 air regeneration.
ISCC mass balance permits physical mixing with bookkeeping-only segregation, enabling multiple claiming	yes	Applies in full. Mass balance must be treated as a control weakness to be mitigated contractually, not as a compliance box.
No laboratory test distinguishes UCO from virgin oil post-processing; a China–EU mislabelling crisis has followed	yes	The direct analogue is acute. Douzha and soybean straw are as analytically indistinguishable from virgin soy processing streams as used cooking oil is from virgin palm oil, and the fraud incentive runs the same way. No downstream assay can defend this claim; integrity therefore rests entirely on chain-of-custody documentation.
Point-of-origin verification rests on self-declaration; small sources escape mandatory physical audit	yes	This is the sharpest applicable objection and it is decisive for feedstock strategy. It is the certification argument for sourcing industrial douzha from a small number of large soy beverage plants rather than aggregating from tofu workshops, and it should be elevated from a note to a stated procurement principle.
Certification bodies are paid by the producers they audit	unmitigable	A structural feature of the scheme that no project-level action removes. It should be acknowledged as residual risk borne by the offtaker rather than presented as managed.

Two objections are structurally voided by the hydrothermal architecture, one is relocated, one is conceded as worse than in the case criticised, and the entire certification critique applies.

Separating recoverable heat from irrecoverable work

The thermal excursions in this flowsheet are large but mostly recoverable, and an earlier draft overstated them by treating temperature swing as though it were energy loss. It is not. The gasifier effluent, the reformer feed and the reformer product all present high-grade sensible heat against streams that need heating, so a feed–effluent exchanger network recovers the majority of it. Pinch analysis should target that recovery explicitly rather than accepting the swing as a fixed debt.

Two items survive heat integration and should be reported on their own lines. The Rectisol refrigeration duty is compressor shaft work at −30 to −60 °C; no exchanger returns shaft work, and its cost is set by coefficient of performance rather than by approach temperature. The methane round trip is reaction enthalpy — exothermic methanation followed by endothermic reforming — and heat integration cannot recover it at all, because the loss is chemical rather than thermal.

The practical consequence is a narrower and more useful framing of the Rectisol-versus-MDEA question. It is primarily a capital and shaft-work comparison, not a heat-recovery one: MDEA removes the refrigeration work entirely but forgoes CO₂ separation and requires the ZnO guard. Both cases should be carried on a shaft-work basis as well as a capital one.

Report structure

3/10 drafted

The report is ten sections. Three are written; seven are specified but not yet drafted. This table is the honest state of the work — the argument each section has to carry, and whether it exists yet.

§	Section	Argument it must carry	Status
1	Feedstock characterization and blend design	Douzha, soybean straw and red mud characterized on a common basis; blend ratio set by three independent constraints — slurry pumpability, C/N, and alkali loading. Contaminant inventory (S, N, Cl, K, Na, P) with a designated fate for each element.	Drafted
2	Process concept and block architecture	Eight-block flowsheet. Red mud's three roles defined and their conflicts stated. The three design decisions: purposeful salt separator, bi-reforming stage, calcium-free sulfur train. Product slate and by-product dispositions.	Drafted
3	Thermodynamic and kinetic basis	Dielectric collapse as the unifying physics. Reaction network from ionic hydrolysis through free-radical gasification. Why CO ≈ 0. Nitrogen and sulfur speciation. The melanoidin char route. Iron buffered at magnetite by the steam–iron equilibrium. Salt nucleation and the cooled-wall rationale. Bi-reforming stoichiometry and the H₂/CO = 2 carbon-efficiency argument. OXZEO vacancy activation, ketene coupling and CHA shape selectivity.	Drafted
4	Red mud redox behaviour, catalysis and deactivation	Fe₂O₃ → Fe₃O₄ → FeO/Fe reduction sequence and which couple is accessible at 600–650 °C in supercritical water. Oxygen transport capacity versus the oxygen already available from water. Tar cracking by AAEM species. Sintering, Na loss, iron–silica interaction, and cycles to failure. Air regeneration and the heat it liberates.	Outlined
5	Mass and energy balance	Basis 100 t/d wet blend. Closed elemental balance on C/H/O/N/S/Na/K/Ca/Fe. Pumping and pressurization duty, feed–effluent heat recovery, reformer endotherm, compression to OXZEO pressure. Net energy ratio and the explicit cost of the make-methane-then-unmake-methane loop.	Outlined
6	Heteroatom management: S, N, Cl, alkali	The calcium-free sulfur train: aqueous partitioning in B3, dealkalized red mud polishing, ZnO guard sized for upstream underperformance. The CaS hydrolysis argument recorded as the reason calcium was removed. Nitrogen to NH₃ and its recovery route. Chloride as corrosion driver and competitor for red mud alkalinity. Sodium and potassium into the brine product.	Outlined
7	Reforming and OXZEO integration	Bi-reforming duty and catalyst selection; coking mitigation by steam co-feed. H₂/CO adjustment to the OXZEO optimum. Per-pass conversion, olefin selectivity, recycle architecture, and the CO₂ that OXZEO itself co-produces. Product separation and the propylene/ethylene ratio as a market variable.	Outlined
8	Regulatory and certification architecture	ISO 14067 boundary and allocation choices — the decisive one being whether red mud enters burden-free. CBAM scope reality check for 2026 versus the 2028 downstream extension. ISCC PLUS mass balance for bio-based olefins. RED III thresholds where gas is sold as fuel. China ETS and CCER interaction.	Outlined
9	Technoeconomics and market positioning	CAPEX for high-pressure hydrothermal service; avoided-cost framing for red mud and douzha disposal as the primary value drivers; olefin, brine-fertilizer, sorbent and scandium revenue lines; sensitivity to the reforming energy penalty.	Outlined
10	Risk register and experimental programme	Ranked kill-risks with the bench experiment that retires each. Salt plugging, red mud attrition, sulfur breakthrough, reformer coking and OXZEO deactivation as the top five. Staged validation from batch autoclave to continuous bench.	Outlined

Provenance convention

Values traceable to a specific source carry a citation resolving to the reference list. Values drawn from general engineering knowledge and not yet verified against a primary source are marked indicative, and must not survive into a final version without a citation. Open questions that change the design rather than merely the numbers are listed separately.

References

Sources consulted for the working draft. Full texts should in all cases be retrieved through institutional access before final citation.

✓ verified against a primary source · ° unverified attribution

✓ [Marrone-2004] Marrone, P. A., Hodes, M., Smith, K. A. & Tester, J. W. (2004). Salt precipitation and scale control in supercritical water oxidation — Part B: commercial/full-scale applications. Journal of Supercritical Fluids, 29, 289–312. doi:10.1016/S0896-8446(03)00092-5
✓ [Hodes-2004] Hodes, M., Marrone, P. A., Hong, G. T., Smith, K. A. & Tester, J. W. (2004). Salt precipitation and scale control in supercritical water oxidation — Part A: fundamentals and research. Journal of Supercritical Fluids.
✓ [Wang-Bjerle-1996] Wang, W., Ye, Z. & Bjerle, I. (1996). The kinetics of the reaction of hydrogen chloride with fresh and spent Ca-based desulfurization sorbents. Fuel, 75(2), 207–212. doi:10.1016/0016-2361(95)00242-1
✓ [Wang-Liu-2012] Wang, P. & Liu, D.-Y. (2012). Physical and chemical properties of sintering red mud and Bayer red mud and the implications for beneficial utilization. Materials, 5(10), 1800–1810. doi:10.3390/ma5101800
✓ [Okolie-2021] Okolie, J. A. & Dalai, A. K. et al. (2021). Catalytic supercritical water gasification of soybean straw: effects of catalyst supports and promoters. Industrial & Engineering Chemistry Research, 60(16), 5770–5782. doi:10.1021/acs.iecr.0c06177
° [Cheah-2010] Cheah, S., Carpenter, D. L. & Magrini-Bair, K. A. (2010). In-bed and downstream hot gas desulphurization during solid fuel gasification: a review. Fuel Processing Technology.
° [Li-2012] Li, B., Qiao, M. & Lu, F. (2012). Composition, nutrition, and utilization of douzha (soybean residue). Food Reviews International. Published under the Japanese term "okara" — search on that spelling to retrieve it.
° [Liu-2020] Liu, W. et al. (2020). Bauxite waste with low Fe₂O₃ and high Na concentration as a promising oxygen carrier in chemical looping combustion. International Journal of Energy Research.
° [Marrone-2019] Marrone, P. A. (2019). Supercritical water gasification: practical design strategies and operational challenges for lab-scale, continuous flow reactors.
° [Olah-2015] Olah, G. A., Goeppert, A., Czaun, M., Mathew, T., May, R. B. & Prakash, G. K. S. (2015). Catalytic bi-reforming of methane: from greenhouse gases to syngas.
° [Chein-2020] Chein, R.-Y. & Wang, C.-C. (2020). Renewable methanol synthesis through single-step bi-reforming of biogas. Industrial & Engineering Chemistry Research. doi:10.1021/acs.iecr.0c00755
° [Schubert-2010] Schubert, M., Regler, J. W. & Vogel, F. (2010). A novel concept reactor design for preventing salt deposition in supercritical water. Chemical Engineering Research and Design.
° [Wang-2023] Wang, X. et al. (2023). Red mud with enhanced dealkalization performance by supercritical water technology for efficient SO₂ capture. Journal of Environmental Management.
° [Zhang-2016] Zhang, N. et al. (2016). Recovery of scandium from bauxite residue — red mud: a review. Rare Metals.
° [Zhang-2017] Zhang, N. et al. (2017). Electron probe microanalysis for revealing occurrence mode of scandium in Bayer red mud. Rare Metals.
° [Zhang-2024] Zhang, Y. et al. (2024). Synergistic catalytic mechanism of red mud in the co-gasification of spirit-based distillers' grains and sewage sludge.
° [CLE-dry] Boosting agroforestry waste valorization: red mud oxygen carriers with tailored oxygen release for enhanced chemical looping gasification (2025). Processes, 13, 1716. doi:10.3390/pr13061716 — source of the dry chemical-looping reference set (1.02 Nm³/kg, 12.06 MJ/Nm³, 91.49% cold gas efficiency, 82.65% carbon conversion).
° [BCB-2023] Optimizing the gasification performance of biomass chemical looping gasification: enhancing syngas quality and tar reduction through red mud oxygen carrier (2023). Biomass Conversion and Biorefinery.
° [NiCu-RM] Red mud supported Ni-Cu bimetallic material for hydrothermal production of hydrogen from biomass (2024). Applied Catalysis A — 21.88 mmol/g H₂, 6.7× unpromoted Ni.
° [OXZEO-2] ZnOₓ overlayer confined on ZnCr₂O₄ spinel for direct syngas conversion to light olefins (2025). Nature Communications — 64% CO conversion at 75% light-olefin selectivity, 4.0 MPa, 400 °C, 68% H₂ / 27% CO feed.
° [OXZEO-1] Direct conversion of syngas to light olefins over a ZnCrOₓ + H-SSZ-13 bifunctional catalyst. ACS Omega — 80% light-olefin selectivity at 17% CO conversion, 400 °C, 2.5 MPa.
° [Salt-2024] Investigating salt precipitation in continuous supercritical water gasification of biomass (2024). Processes, 12, 935.
° [ISW-OC-2024] Industrial solid waste as oxygen carrier in chemical looping gasification technology: a review (2024).
° [CaS-hyd] US Patent 4,321,242 — Low sulfur content hot reducing gas production using calcium oxide desulfurization with water recycle; and US 4,686,090 — Desulfurizing of reducing gas stream using a recycle calcium oxide system. The specific patent carrying the CaS hydrolysis teaching must be confirmed before final issue.
° [Rectisol] Air Liquide / Lurgi. Rectisol process description — syngas purification to 0.1 ppm total sulfur including H₂S and COS; see also NETL Gasifipedia, Rectisol.
° [AGR-rank] Reconfiguration of acid gas removal process matching the integration of coal chemical industry with green hydrogen (2024). Separation and Purification Technology — source for the CO₂-efficiency ranking Rectisol > Selexol > MDEA > sulfolane-MDEA at a 0.1 ppm H₂S target.
° [MDEA] Bullin, J. A. et al. Selective absorption using amines. Bryan Research & Engineering — basis for MDEA selective H₂S removal behaviour and the sub-20 ppmv outlet figure. See also Moioli, S. et al., Assessment of MDEA absorption process for sequential H₂S removal, Politecnico di Milano.
° [SWG-feed] Supercritical reactor feed precedents (Marrone, 2019, and references therein): 30 wt% glucose, 18 wt% corn cob, 24 wt% coal–water slurry; twin piston pumps delivering 15% solids biomass slurry to 27 MPa; dewatered sewage sludge at 7.69 wt% solids requiring corn-starch paste and a cement pump.
✓ [Ketene-2025] Unraveling the mechanisms of ketene generation and transformation in syngas-to-olefin conversion over ZnCrOₓ|SAPO-34 catalysts (2025). Chemical Science — source for the 86.1% / 13.9% ketene route split and the CHO*–CO* coupling sequence.
° [Vacancy] Resolving the intricate mechanism and selectivity of syngas conversion on reduced ZnCr₂Oₓ: a quantitative study from DFT and microkinetic simulations. ACS Catalysis — source for two-coordinate oxygen vacancy sites as the CO activation centre.
✓ [Kw] Bandura, A. V. & Lvov, S. N. (2006). The ionization constant of water over wide ranges of temperature and density. J. Phys. Chem. Ref. Data — IAPWS-endorsed correlation for K_w.
° [Dielectric] Dielectric constant of supercritical water in a large pressure–temperature range (2020). Journal of Chemical Physics, 153, 101103.
° [SCWG-2015] Supercritical water gasification of biomass: a literature and technology overview (2015). Energies, 8, 859 — source for the ionic versus free-radical regime criterion at K_w ≈ 10⁻¹⁴ and for Na-CMC slurry feeding.
° [Salt-Type1] Continuous salt precipitation and separation from supercritical water. Part 1: Type 1 salts. Journal of Supercritical Fluids.
° [Salt-solubility] Solubility of Na₂SO₄, Na₂CO₃ and their mixture in supercritical water. Journal of Supercritical Fluids.
° [Hot-wall] Effects of superheated surface on the deposition behaviour of Na₂SO₄ in supercritical water (2023). Processes, 11, 1779 — source for preferential nucleation on hot surfaces and particle size ranges.
° [MD-nucleation] Molecular dynamics study on the nucleation characteristics of mixed Na₂SO₄/K₂SO₄ solution in supercritical water — source for the ion-pair / cluster nucleation sequence and timescale.
° [SFA-Claus] SFA Pacific, Inc. Process screening analysis of alternative gas treating and sulfur removal for gasification. NETL, December 2002 — basis for the Claus inlet-concentration constraint and the liquid-redox selection for small sulfur duties.
✓ [ISO 14067] ISO 14067:2018 — Greenhouse gases: Carbon footprint of products. Allocation hierarchy and disclosure requirements.
✓ [CBAM] Regulation (EU) 2023/956 establishing a Carbon Border Adjustment Mechanism; definitive period from 1 January 2026, Annex I scope. 2025 Commission proposal on downstream extension.
✓ [ISCC PLUS] ISCC PLUS certification scheme; Mass Balance Guidance Document 1.0 (public consultation, final version expected June 2026).
✓ [CN-ETS] China national ETS expansion to steel, cement and aluminium with absolute caps by 2027; CCER relaunch.
° [Sc-grade] Scandium in red mud: 16–230 ppm typical (~84 ppm Chinese sample); Sc₂O₃ price $3,000–5,000/kg. Global Sc₂O₃ demand of order tens of t/y — verify against a USGS commodity summary.
° [Fe-rec] Red mud iron recovery: reduction roasting + magnetic separation 97.69% metallization, 81.40% recovery; smelting reduction pig iron Fe >90%, 90–95% recovery, recent work 98.14–98.36% Fe recovery.
° [RM-SCM] Red mud as supplementary cementitious material: 10–20% clinker replacement (25–30% demonstrated); co-calcination with kaolinite 30% replacement at 88% of reference 28-day strength.
° [RM-OC] Red mud as a chemical-looping oxygen carrier: Fe₂O₃ content above roughly 40 wt% gives adequate oxygen transport capacity together with useful heat-carrying properties (Liu et al., 2020; industrial solid waste oxygen carrier review, 2024).

Co-Valorization of Bauxite Residue and Soybean Processing Waste via Supercritical Water Gasification

Why this is being built

The problem statement

ISO 14067

CBAM

ISCC PLUS, RED III, and China's carbon market

Why these two wastes

Douzha

Bauxite residue

Where the two wastes actually are

What goes in

Douzha

Soybean straw

Red mud

The dealkalization synergy — the strongest single argument

The binding constraint is pumpability, not moisture

Whether dispersants and stabilizers are required

The plant

Feed preparation and slurry make-up

Supercritical water gasifier with red mud bed

Salt separator — a purposeful product unit

Depressurization, phase separation, aqueous polishing

Acid gas removal (Rectisol) and ZnO guard

Bi-reformer

OXZEO olefin synthesis

Red mud regeneration and residue valorization

Why it works

Stage 1 — ionic hydrolysis during preheat, 200–370 °C

Stage 2 — free-radical gasification above the critical point

Why the product gas contains essentially no CO

Nitrogen and sulfur speciation from a protein-rich feed

Char and coke: the specific risk douzha introduces

Red mud redox thermodynamics, and what buffers the iron

Salt nucleation, growth and deposition

Carbon deposition and the thermodynamic carbon boundary

Why Fischer–Tropsch cannot reach these selectivities and OXZEO can

CO activation at oxygen vacancies on the oxide

Ketene as the coupling intermediate, and the role of proximity

Shape selectivity in the CHA cage, and where the CO₂ comes from

Deactivation modes

What must not leave

Product slate

Carbon-derived

C₂–C₄ olefins

CO₂ from OXZEO

C₅+ and paraffin tail

Bio-SNG

Heteroatom

N-K-P-S brine → compound fertilizer

Ammonia → ammonium sulfate or urea

Elemental sulfur

Bauxite-derived — three tiers, inverse mass and value

T1Tier 1 — sell the dealkalized solid as-is

T2Tier 2 — iron

T3Tier 3 — critical metals from the iron-depleted slag

Avoided disposal

Gate fees on douzha and red mud

State of the work

References

Writing...

Co-Valorization of Bauxite Residue and Soybean Processing Waste via Supercritical Water Gasification

Why this is being built

The problem statement

ISO 14067

CBAM

ISCC PLUS, RED III, and China's carbon market

Why these two wastes

Douzha

Bauxite residue

Where the two wastes actually are

What goes in

Douzha

Soybean straw

Red mud

The dealkalization synergy — the strongest single argument

The binding constraint is pumpability, not moisture

Whether dispersants and stabilizers are required

The plant

Feed preparation and slurry make-up

Supercritical water gasifier with red mud bed