The Bug That Beats The Furnace

Anaerobic digestion is a biological process. Bacteria break down organic matter in the absence of oxygen, at 30 to 55 degrees Celsius, in four stages: hydrolysis, acidogenesis, acetogenesis and methanogenesis. The output is biogas at 50 to 70 per cent methane and 30 to 50 per cent carbon dioxide, plus a nutrient-rich digestate. Gasification is thermochemical. It runs at 700 to 1,200 degrees Celsius with controlled oxygen or steam, through drying, pyrolysis, oxidation and reduction. The output is syngas at 15 to 30 per cent hydrogen, 20 to 40 per cent carbon monoxide, 5 to 15 per cent carbon dioxide and 2 to 10 per cent methane, plus ash or slag and tar. One process is run by living organisms in water. The other is run by heat without it. That difference decides everything downstream of the gate.

The choice is made before the plant is drawn. It is a moisture reading. Digestion suits feedstock above 70 per cent moisture. Food waste sits at 70 to 85 per cent. Pig and cattle manure sits at 85 to 92. Sewage sludge sits at 92 to 97. Maize silage sits at 65 to 75. Gasification suits feedstock below 30 per cent moisture. Wood chips and pellets sit at 10 to 20 per cent. Straw and husks sit at 8 to 15. Refuse-derived fuel from municipal waste sits at 15 to 25. Plastics sit below 1. Above 50 per cent moisture, gasification without drying loses to the energy cost of drying, which runs 30 to 50 per cent of the feedstock energy. The crossover is one number on a moisture meter, and it is not a matter of taste.

Biogas carries 20 to 25 megajoules per normal cubic metre at roughly 55 per cent methane. It can go straight into a combined heat and power unit, or be upgraded to biomethane at 95 to 99 per cent methane for grid injection. Syngas carries 10 to 15 megajoules per normal cubic metre. It burns worse, and it does more. Syngas is the only biomass route to hydrogen, methanol and Fischer-Tropsch fuels. That is a platform, not a fuel. The wet process makes one product well. The hot process makes a thinner gas that becomes many products. Those are two businesses with two sets of buyers.

Per tonne of food waste, digestion returns 100 to 150 cubic metres of biogas. Manure returns 15 to 40. Sewage sludge returns 20 to 40. Maize silage returns 180 to 220. Per kilogram of dry feedstock, gasification returns 2.0 to 2.5 normal cubic metres of syngas from wood chips, 1.5 to 2.0 from straw and husks, 1.0 to 2.0 from refuse-derived fuel and 2.5 to 3.5 from plastics. The clocks differ more than the yields. Digestion takes 20 to 40 days at mesophilic temperature and 12 to 20 days at thermophilic. Gasification takes seconds to minutes. One is a farm with a holding time. The other is a steel reactor with a shift schedule.

The crossover is one number on a moisture meter, and it is not a matter of taste.

Thermal efficiency from feedstock energy to product energy runs 50 to 65 per cent for digestion as methane and 65 to 80 per cent for gasification as syngas. Electrical efficiency in a combined heat and power unit reverses the order: 35 to 42 per cent for digestion, 25 to 35 per cent for syngas in a gas engine. The system energy ratio runs 2.5 to 5.0 for digestion and 3.0 to 6.0 for gasification. The parasitic load runs 10 to 20 per cent for heating and mixing, against 15 to 30 per cent for air and oxygen compression and gas cleaning. The hot process wins the gas efficiency column and loses the electricity column. Anyone quoting one column without the other is selling something.

Below 500 kilowatts electrical, digestion costs EUR 4,000 to 7,000 per kilowatt. Gasification costs EUR 6,000 to 12,000. Between 500 kilowatts and 2 megawatts, digestion runs EUR 3,000 to 5,000 per kilowatt and gasification EUR 4,000 to 8,000. Above 2 megawatts, digestion runs EUR 2,500 to 4,000 and gasification EUR 3,000 to 6,000. Per tonne of feedstock a year, digestion costs EUR 100 to 300 and gasification EUR 200 to 500. Per normal cubic metre of gas, EUR 0.10 to 0.30 against EUR 0.15 to 0.40. Operating cost per tonne of input runs EUR 15 to 80 for digestion and EUR 30 to 120 for gasification. Digestion carries 30 to 50 per cent less capital per kilowatt. The money is not hiding. Digester tanks are 40 to 50 per cent of the digestion bill. In gasification the reactor is 30 to 40 per cent and the gas cleaning train is 25 to 35 per cent.

Digestion sells electricity at EUR 50 to 120 per megawatt hour, biomethane at EUR 60 to 90, and digestate as fertiliser replacement at EUR 5 to 15 per tonne. Gasification sells electricity at EUR 50 to 100, syngas as chemical feedstock at EUR 15 to 30 per megawatt hour, and biochar at EUR 50 to 500 per tonne depending on grade. Readiness is part of the price. Digestion sits at technology readiness level 9. Gasification sits at 7 to 9, depending on the scale and on how much gas cleaning the site needs. The ash from gasification usually goes to landfill. The digestate goes back to the field. One route closes a nutrient loop. The other closes a materials loop.

Corre and colleagues modelled both routes in Energy Conversion and Management in 2025. Digestion with biomethane upgrading reached minus 790 kilograms of carbon dioxide equivalent per functional unit under high fossil methane substitution. Hydrothermal gasification with a co-solid oxide electrolyser cell reached minus 1,100. Under zero fossil methane substitution, digestion can swing to plus 770. The environmental case is a displacement case. It depends on what is being replaced, not on which machine a reader prefers. I restructured a industrial group, and Entrade built 200 wood-gasification machines across the UK and Europe. The first units never set the price. The tenth build did. The hybrid version already exists and it is simple: the solid fraction of digestate at 25 to 40 per cent moisture goes to the gasifier, and the gasifier heat comes back to the digester. I place the probability that a developer picks the cheaper technology over the familiar one at close to zero. Read the moisture meter before you read the business plan. One number decides which machine gets built.