Biogas to power, heat and turquoise hydrogen

I have written elsewhere about sizing the digester honestly. Assume that discipline held: the anaerobic reactors now deliver a steady stream of biogas at 60–65 per cent methane. The remaining engineering question is the valuable one — what form of energy to turn it into. For most sites the answer is a combined heat and power engine; for a growing few, the more interesting answer runs through hydrogen.

The CHP baseline

A gas engine converts biogas at 36–40 per cent to electricity, and a further 40–45 per cent is recoverable as heat — roughly half from jacket cooling water at 85–95 °C and half from exhaust at 400–500 °C. That hot water is not a by-product; it is the digester's own heating supply, closing the loop that makes the whole plant self-sustaining, with surplus for sludge pre-heating or buildings. In the Gulf, where sewage arrives warm, the heat balance runs in surplus most of the year — which is precisely when absorption chilling on the jacket circuit becomes worth a study.

• Clean the gas first: hydrogen sulphide below ~200 ppm for engine warranties (biological scrubbing or activated carbon), and siloxanes removed unless piston deposits are an acceptable hobby.
• Size the engine at or slightly below steady gas make, with holder buffering — an engine cycling on a starved gas supply earns neither availability nor warranty.
• Count the displaced tariff, not the export tariff: behind-the-meter electricity and displaced heating fuel are where the business case lives.

The hydrogen route: turquoise, specifically

Methane pyrolysis splits CH₄ at 700–1,200 °C — thermally, catalytically or in plasma — into hydrogen and solid carbon: no CO₂ at the point of conversion, roughly 13 kilograms of carbon black per kilogram-mole of hydrogen, and an energy demand about a fifth of electrolysis per kilogram of H₂ because the oxygen bond is never broken. Fed with biomethane from upgraded digester gas, the accounting turns carbon-negative on paper: biogenic carbon enters as gas and leaves as a stable solid. That combination — modest energy input, solid sequesterable carbon, renewable feedstock — is why the route deserves the attention it is getting.

Route	Feed	Energy input	Carbon fate
Steam reforming (grey)	Natural gas	Moderate	CO₂ to atmosphere
Electrolysis (green)	Water + power	~50–55 kWh/kg H₂	None
Pyrolysis (turquoise)	Methane / biomethane	~10–13 kWh/kg H₂	Solid carbon black

The honesty clause: pyrolysis at works scale is early. Commercial units exist — plasma and molten-metal reactors are operating at demonstration and first-commercial scale — but the carbon-black offtake market, reactor lifetimes and biomethane upgrading costs all carry real uncertainty. A works considering it today should engineer the biogas system to keep the option open: upgrading-ready gas quality, plot space, and a CHP configuration that can cede fuel to a pyrolysis train later without rebuilding the energy centre.

A staged strategy

1. Build the CHP baseline now — it is bankable, warranted and closes the digester heat loop.
2. Upgrade a slipstream to biomethane when a vehicle-fuel or grid-injection offtaker signs, not before.
3. Pilot pyrolysis when the carbon product has a named buyer — the hydrogen sells itself; the carbon black decides the economics.
4. Keep the accounting biogenic and auditable end to end — the premium for carbon-negative hydrogen exists only while the paperwork survives scrutiny.