Yes — that is a chemically coherent observation, and it exposes a possible alternative process concept that the HESC material does not appear to explore.
The HESC/CSIRO brown-coal gasification case treats Victorian brown coal as the feedstock, with oxygen-blown gasification followed by water-gas shift and hydrogen separation. The report expressly describes Option 4 as brown coal gasification using an oxygen-blown entrained-flow gasifier followed by a shift reactor, with CCS added to the coal-based routes. It then reports, for the preferred gasification option, 6.3 Mt/y raw coal, 3.7 Mt/y CO₂ generated, and 2.4 GL/y make-up process water to produce about 770 t/day H₂.
Your proposed modification is different. It treats the water in wet brown coal not merely as a nuisance to be evaporated or managed, but as an oxygen-and-hydrogen reagent. The simplified logic is:
Brown-coal/water-only limiting case
C+2H2O→CO2+2H2
That gives only 2 hydrogen molecules per CO₂ molecule.
But for a hydrocarbon-like polymer, approximated as polyethylene:
CH2+2H2O→CO2+3H2
That gives 3 hydrogen molecules per CO₂ molecule.
For a more hydrogen-rich plastic such as polypropylene, approximated per carbon as CH2 to CH2.0 or slightly higher depending on formulation, the same broad principle applies: the polymer contributes hydrogen as well as carbon. For many waste polymers and textiles, the oxygen content is far below the two oxygen atoms per carbon required to oxidise all carbon to CO₂. That oxygen can be supplied by water already present in wet brown coal, or by added steam.
The general relationship is useful. For a feedstock approximated as:
CHaOb
complete conversion with water to CO₂ and hydrogen can be written as:
CHaOb+(2−b)H2O→CO2+(2+a/2−b)H2
So the hydrogen yield per carbon/CO₂ molecule is:
2+a/2−b
That makes the point clear:
| Feed approximation | a | b | H₂ per CO₂ |
|---|
| Pure carbon | 0 | 0 | 2.0 |
| Cellulose-like biomass, CH1.67O0.83 | 1.67 | 0.83 | about 2.0 |
| Polyethylene-like plastic, CH2 | 2 | 0 | 3.0 |
| Methane, CH4 | 4 | 0 | 4.0 |
That explains why adding low-oxygen, hydrogen-rich polymer waste could improve the gross hydrogen per CO₂ ratio. It is not magic; it is simply using polymer hydrogen plus water hydrogen, while using water oxygen to oxidise carbon.
There is also a useful process implication. Wet brown coal’s moisture is usually treated as a severe efficiency penalty because energy is required to evaporate water. But in a co-gasification/reforming concept, some of that water could be a reactant. The 210-page report notes that the costs of recovering water from dryer condensate were not considered, despite the make-up water being less than the water evaporated during drying. That tends to support your instinct that the treatment of water in the brown-coal case may be too narrow.
The limitations are important, though.
First, adding plastics does not eliminate CO₂. It changes the ratio. Every carbon atom in the polymer still becomes CO₂ if the process is driven to full hydrogen production with shift conversion and carbon capture.
Second, waste polymer streams are messy. PVC introduces chlorine and HCl; PET contains oxygen; nylon contains nitrogen; textiles contain dyes, fillers, flame retardants and metals. Those contaminants can poison catalysts, corrode equipment, complicate syngas cleaning, and make the economics less simple than the stoichiometry.
Third, gasification does not automatically proceed by the clean global reactions above. Real gasifiers produce mixtures of CO, CO₂, H₂, CH₄, tars, char, soot and contaminants. The report itself relies on process simulation and gas separation/purification rather than a simple stoichiometric reaction model.
Fourth, the policy claim would change. A brown-coal-plus-waste-plastic system might be pitched less as “clean hydrogen from brown coal” and more as waste polymer destruction plus hydrogen recovery plus concentrated CO₂ capture. That may be more honest and technically defensible, but it would also make the carbon accounting more contested: fossil plastic carbon would still be fossil CO₂ unless permanently stored.
So the refined conclusion is:
Co-gasifying wet brown coal with low-oxygen waste polymers could, in principle, produce more hydrogen per molecule of CO₂ than gasifying brown coal alone, because the polymer contributes chemically bound hydrogen and the coal moisture can supply oxygen through steam reforming/shift chemistry. This would not make the process CO₂-free, but it could improve the hydrogen-to-carbon ratio and turn wet coal moisture from a drying burden into a process reagent.
That is a materially better idea than the simple HESC framing, but it would need a separate mass balance using actual brown-coal ultimate analysis, moisture content, polymer mix, oxygen-blown versus steam gasification assumptions, and contaminant-handling costs.