Biofuels & bioenergy
Hydrogen fuel from biomass
The gasification, shift and purification chain that turns biomass carbon into a hydrogen carrier, the thermodynamic ceiling on yield, and the narrow conditions under which it beats water electrolysis.
In every previous page of this cluster, the carbon of the biomass is the point — the fuel, the monomer, the product. Hydrogen production inverts that. Biomass gasified with steam, or biomethane reformed, is a scheme in which carbon serves as the reducing agent that strips hydrogen from water, and ends as CO2 to be vented, used or stored. Hydrogen itself carries about 120 MJ per kilogram, the highest gravimetric density of any chemical fuel, but it is a carrier, not a source: every route to it is an energy transaction.
The chain: gasify, shift, purify
Steam gasification runs C + H2O into CO + H2, absorbing about 131 kJ per mole — heat usually supplied by burning part of the feedstock, which is the first, unavoidable tax on yield. The water-gas shift then trades CO + H2O into CO2 + H2, releasing about 41 kJ per mole and converting every carbon monoxide into another hydrogen. The stoichiometric ceiling for cellulose plus steam is twelve moles of hydrogen per mole of cellulose — roughly fifteen percent of the cellulose mass, with all of it ending as CO2. Real plants sit well below: an operating Indian project converts twenty-five tonnes of waste into one tonne of hydrogen per day, about a quarter of that ceiling, the difference spent on process heat, lost to tars and char, or left unrecovered. Pressure-swing adsorption finishes the chain, lifting purity to the 99.97% that fuel cells demand — and proton-exchange fuel cells add a final constraint, tolerating carbon monoxide only at the level of parts per million, because CO poisons their platinum anodes.
The electrolysis benchmark
Hydrogen from biomass competes not with other biofuels but with splitting water directly. Electrolysis needs about 50–55 kWh of electricity per kilogram of hydrogen against a theoretical floor of 33 — it is clean at the stack and only as clean as its power. Biomass-to-hydrogen buys its reducing equivalents from a solid fuel instead of a wire, so it wins precisely where three conditions meet: waste or residue carbon that is already collected and cheap, electricity that is expensive or scarce, and a use for the concentrated biogenic CO2 stream the process delivers at pressure — including geological storage, which makes this the one hydrogen route that can go carbon-negative. Where any of those fails, electrolysis on cheap power is simpler and scales without hauling straw.
The biological fringe
Dark fermentation — microbes excreting hydrogen from sugars — exists and obeys a hard limit known for decades: at most four moles of hydrogen per mole of glucose, with real cultures managing two to three, the rest locked in acetate and butyrate. It is a research subject, not a route.
So the limiting quantities are a stoichiometric ceiling, a heat bill paid in feedstock, and a purity specification inherited from platinum. The syngas platform is shared all the way through cleanup; only the destination differs — and the destination chosen on the drop-in page or on the gas fermentation page competes for the very hydrogen this page wants free.