Biogas calculator: feedstock mix → methane, kW electric, heat & digestate

How this biogas calculator works

Anaerobic digestion turns organic matter into a mixture of methane and carbon dioxide inside a heated, sealed tank. How much gas you get depends almost entirely on what you feed: watery slurry contains little organic matter per tonne, while silage and food waste are concentrated energy. This calculator lets you build a feedstock mix of eight common materials and then follows the energy through the plant:

1. Biogas and methane from each feedstock, using mid-range literature yields per tonne of fresh matter and typical methane shares (FNR/KTBL);
2. Primary energy in the methane after a small slip loss;
3. either a CHP gas engine (electricity, heat, parasitic electricity and digester heating) or upgrading to biomethane for grid injection;
4. Digester sizing from the hydraulic retention time and the resulting organic loading rate;
5. Digestate output and the plant’s economics, including energy-crop purchases and gate fees earned on food waste.

The formulas

Worked example: dairy slurry plus maize silage

The default plant digests 30 t/day of cattle slurry (from roughly 500–600 dairy cows) and 12 t/day of maize silage:

• slurry gives 750 m³ of biogas a day and maize 2,400 m³, so the silage is 29% of the mass but 76% of the gas;
• total 3,150 m³/day at about 53% methane, i.e. 1,660 m³ of methane worth 6.0 GWh a year (683 kW continuously);
• a CHP engine at 40% electrical / 43% thermal efficiency running 8,000 h/yr is sized at about 300 kWe, generating 2.4 GWh of electricity (2.2 GWh exported after 8% own use) and 2.6 GWh of heat;
• with a 60-day retention time the digester needs about 2,500 m³, giving an organic loading rate of 2.45 kg VS/m³·d, comfortably in the stable range;
• the plant returns about 13,800 t of digestate a year to the fields.

Selling electricity at €0.15/kWh and 40% of the surplus heat at €0.04/kWh earns about €360,000 a year, but buying 4,380 t of maize at €40/t costs €175,000. At €4,500/kWe the plant pays back in about 13 years with an IRR under 3%. That is why farm biogas depends on tariffs, heat customers or cheap waste feedstocks.

How to read the results

• CHP capacity / biomethane output is the headline size of the plant.
• Methane content below 50% suggests a feedstock mix low in fats and proteins, or process problems.
• Organic loading rate: above about 4–4.5 kg VS/m³·d the risk of acidification and foaming rises; below 1, the digester is larger than needed.
• Energy flow chart shows how much of the methane energy becomes saleable electricity and heat; unused heat is the biggest lever on economics.
• Feedstock table compares each material’s share of mass versus share of gas.

Typical values

Tips to improve output and economics

• Use the heat. Selling or using the surplus heat (drying, greenhouses, district heating) can add 20–30% to revenue.
• Co-digest wastes. Food waste or crop residues raise gas yield and may earn a gate fee instead of costing money.
• Avoid dilution. Rainwater and wash water in slurry stores reduce yield per tonne and fill digester volume.
• Cover digestate storage. A gas-tight cover recovers residual gas (often 5–10% extra) and cuts methane emissions.
• Stable feeding in small, frequent portions keeps the microbes healthy and the methane share high.

Limitations

Yields are literature mid-points; real values vary ±20–30% with silage quality, storage losses, slurry dilution and retention time, so laboratory biomethane-potential (BMP) tests are recommended before investing. The model assumes a continuously stirred, mesophilic digester, constant year-round feeding and fresh matter density of about 1 t/m³. Seasonal heat demand of the digester, hydrogen-sulphide treatment, digestate processing and permits are not modelled. Subsidy schemes and limits on energy crops differ by country.

Method & assumptions

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