In short. A biogas generator is sized to the gas the digester can reliably produce, not to the load the site would like to carry. Raw biogas is roughly 50 to 70 percent methane with carbon dioxide, moisture and hydrogen sulphide, so it must be dried and desulphurised to the engine maker’s limits. Expect lower output than on natural gas, shorter oil intervals, and gas treatment as important as the genset.
What biogas is and why engines care
Biogas is the product of anaerobic digestion of organic material. In Telangana and Andhra Pradesh the practical sources are distillery spent wash, dairy and cattle waste, poultry litter, food-processing effluent, sugar industry press mud and the sludge digesters of sewage treatment plants. Depending on the feedstock and digester operation, the gas contains 50 to 70 percent methane, 30 to 45 percent carbon dioxide, hydrogen sulphide anywhere from a few hundred to several thousand parts per million, water vapour at saturation, traces of ammonia and, in sewage gas, siloxanes.
Each constituent affects the engine. Carbon dioxide dilutes the fuel, lowering the heating value to roughly 18 to 25 MJ/Nm³ against about 35 MJ/Nm³ for pipeline gas, which reduces the power a given engine can produce and slows combustion. Hydrogen sulphide forms sulphurous and sulphuric acid in the combustion chamber and crankcase, attacking bearings, valve guides and the exhaust system and acidifying the lubricating oil. Water condenses in pipework and fuel trains, carrying acid with it. Siloxanes burn to silica, which deposits as a hard glassy layer on pistons, valves and spark plugs and in the exhaust catalyst if one is fitted. A generator designed for treated natural gas should never be fed untreated biogas on the assumption that methane is methane.
Gas treatment is part of the power plant
| Contaminant | Effect on the engine | Typical treatment | What to verify |
|---|---|---|---|
| Hydrogen sulphide | Acidic oil, bearing and valve corrosion, exhaust corrosion | Iron oxide media, biological scrubber, activated carbon, air dosing into the digester headspace | Outlet H2S against the manufacturer’s limit in mg/Nm³ or ppm, media change interval |
| Moisture | Condensate in pipework, acid carry-over, regulator icing or fouling | Condensate traps, gas cooling with reheating, refrigerated dryer | Dew point margin below the lowest pipe temperature |
| Siloxanes | Silica deposits on pistons, valves, plugs and catalyst | Activated carbon or specialised adsorbent beds | Total silicon at the engine inlet, bed breakthrough monitoring |
| Particulates and foam | Fouled filters, blocked admission valves | Cyclone and coalescing filters, foam trap at the digester | Differential pressure across filters |
| Carbon dioxide | Low heating value, derated output | Accepted by the engine rating; removed only when upgrading to biomethane | Methane content trend from the analyser |
Digester gas is produced at very low pressure, often only 10 to 30 millibar in a floating dome or membrane gas holder, while most engine gas trains need a higher regulated inlet pressure. A gas blower or booster between the holder and the engine is therefore usual, and the gas holder volume acts as the buffer that lets the engine run steadily while production fluctuates through the day.
Size the generator to the gas, not the load
The usable generating capacity is set by the clean gas available continuously, not by the facility’s electrical demand. The estimate follows from the gas yield, its methane content and the engine’s efficiency.
P (kW) = V̇ × CH4 × 35.8 × η / 3.6
V̇ is the biogas flow in Nm³/h, CH4 is the methane fraction as a decimal, 35.8 MJ/Nm³ is the approximate lower heating value of pure methane, η is the electrical efficiency of the genset at that load as a decimal, and dividing by 3.6 converts MJ/h to kW. Worked example (illustrative): a distillery digester yields a steady 100 Nm³/h at 60 percent methane. The fuel energy is 100 × 0.6 × 35.8 = 2,148 MJ/h. At 36 percent electrical efficiency the output is 2,148 × 0.36 / 3.6 = about 215 kW. A 250 kW genset would run at a healthy load factor; a 500 kW set would spend its life at under half load, with poorer efficiency and more deposits. Design on the minimum dependable gas yield across the year, including feedstock lean periods and the monsoon, and treat any surplus gas as a flare or boiler duty rather than a reason to buy a larger engine.
Electrical integration
Most biogas plants in this region run in parallel with the DISCOM supply so that the engine can hold a steady load while the grid takes the balance, and so the plant’s own agitators, pumps and aerators are not subject to the engine’s transient limits. That requires synchronising, reverse power and anti-islanding protection, and the utility’s approval, all covered under integrated power systems. Where the plant must also operate in island mode during grid failure, the load steps from large pumps and blowers must be checked against the gas engine’s load acceptance, and a diesel set is often kept for black start and for the periods when gas production is low. Heat from the jacket water is routinely used to keep the digester at its working temperature, which makes nearly every biogas genset a small cogeneration plant, and the engine room cooling must be designed as described in heat rejection.
Operating and maintenance realities
Where the gas is upgraded to near pipeline quality the engine options widen, which is the subject of biomethane power. Sites dealing with municipal sewage or landfill gas have additional contaminant and variability issues covered in landfill and sewage gas power.
Common mistakes
- Buying the genset to match the plant’s connected load. The engine runs at a fraction of its rating, fouls, and never delivers the projected savings.
- Skipping desulphurisation because the gas analysis was done once on a good day. Oil acidifies within weeks and bearings and valve guides fail early.
- Feeding a natural-gas engine with raw biogas. Output falls, the engine misfires at the lean setting, and the warranty is void.
- Sizing the gas holder too small. The engine cycles on and off as production swings, with every start adding wear and every stop venting gas.
What to send us
Send the digester design data or operating log showing daily gas production over at least a few months, any gas analysis with methane and hydrogen sulphide figures, the electrical load list of the plant, and the DISCOM connection details. Include photographs of the gas holder, existing flare or boiler, and any diesel set on site with its nameplate, plus a note on whether digester heating is required. Use the application engineering enquiry and we will size the generating capacity to the gas and set out the treatment the engine will need.
Questions engineers ask
How much electricity can one cubic metre of biogas produce?
At 60 percent methane one normal cubic metre holds about 21 MJ, or roughly 6 kWh of fuel energy. A gas engine converting 35 percent of that gives about 2 kWh of electricity per cubic metre, with a further 2 to 2.5 kWh available as recoverable heat. Lower methane content or part-load operation reduces the figure.
What hydrogen sulphide level can a gas engine tolerate?
Manufacturers publish limits that vary widely, from a few hundred to over a thousand milligrams per cubic metre, often tied to shorter oil change intervals at higher levels. Treating the gas to well below the limit is cheaper over the engine’s life than running at the limit, because oil, bearings and exhaust components all last longer.
Can a biogas generator run the whole plant off-grid?
It can carry the plant if the gas yield supports the load and the engine can handle the starting currents of the largest pumps and blowers, which often means staged starting or soft starters. Most operators prefer grid-parallel operation with a diesel set for black start, so that gas production and electrical demand do not have to match minute by minute.
Why is a biogas engine derated compared with natural gas?
The carbon dioxide in biogas lowers the energy content of each cylinder charge and slows flame speed, so the same engine develops less power and may need retarded timing to avoid knock on a variable fuel. Purpose-built biogas engines compensate with higher compression ratios and dedicated calibration, but the rating is still fixed by the engine manufacturer for a stated gas quality.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.