Biogas CHP & Engine Sizing Calculator

What Is Biogas CHP and How Does It Work?

Biogas Combined Heat and Power (CHP), also called cogeneration, is the simultaneous generation of electricity and usable thermal energy from a single fuel source — biogas. Where a conventional power plant discards 55–65% of its fuel energy as waste heat, a CHP system captures this heat from the engine's cooling water (jacket water) and exhaust gas, achieving total fuel utilisation of 75–90%.

In a biogas CHP system, raw biogas from an anaerobic treatment unit is conditioned (H₂S removed to below 200 ppm, moisture dried) and fed to a reciprocating gas engine or micro-turbine. The engine drives a synchronous generator to produce electricity at the plant's operating voltage (typically 415V or 11 kV). The engine cooling circuit is connected to a hot water heat exchanger, and the exhaust is passed through a waste heat boiler or heat exchanger. Together, these systems recover 40–50% of the fuel energy as hot water or low-pressure steam.

The electricity produced offsets grid power purchases, and the recovered heat offsets fuel costs for process heating, boiler feed water pre-heating, or space conditioning. For an industrial facility with a large heat demand — a brewery, dairy, or food processing plant — CHP is almost always more economical than electricity generation alone. A 200 kW biogas CHP system recovering 240 kW of thermal energy delivers the equivalent of 440 kW of useful energy from biogas that might otherwise be flared or only partially utilised.

Sizing a Biogas Engine: From m³/day to kW Output

The starting point for biogas CHP sizing is the lower heating value (LHV) of methane — 35.8 MJ per cubic metre of pure CH₄ at standard conditions (0°C, 1 atm). Converting biogas flow to kW electrical output requires three steps: (1) isolate the methane fraction from the total biogas flow using the methane content percentage; (2) calculate the power equivalent by dividing the energy per hour by 3.6 MJ/kWh; and (3) apply the engine electrical efficiency to get net electrical output. This gives the continuous power rating required.

Standard commercial gas engine-generators are available in discrete sizes: 25, 50, 100, 160, 200, 250, 320, 400, 500, 630, 800, and 1,000 kW. The correct practice is to select the next available standard size above the calculated duty — this ensures the engine does not run at overload during peak biogas production periods and provides headroom for gas quality variation. A 15–20% upsize margin is generally recommended.

Engine availability — the fraction of time the engine operates after planned maintenance shutdowns — typically ranges from 85–95% for well-maintained units. This reduces the effective daily energy output from what the nameplate kW rating would suggest. A 200 kW engine at 90% availability produces 200 × 24 × 0.90 = 4,320 kWh per day, not 4,800 kWh. Always apply availability in financial projections.

Gas conditioning is non-negotiable for engine longevity. H₂S above 200 ppm causes rapid sulphuric acid formation in engine oil, corroding cylinder walls and bearings. Iron sponge, activated carbon, or biological desulphurisation can reduce H₂S to acceptable levels. Moisture removal to a dewpoint below 5°C prevents water accumulation in the gas train and engine. If siloxanes are present (common in landfill gas and some food waste digestates), specialised activated carbon or silica gel filters are required.

Biogas-to-Energy in Indian Industry: UASB and Anaerobic Digesters

India has thousands of industrial UASB (Upflow Anaerobic Sludge Blanket) reactors in operation across breweries, distilleries, paper mills, dairy plants, and food processing facilities. These reactors are mandatory for high-COD industrial effluent in many states, and virtually all of them produce significant quantities of biogas — much of which is currently flared or underutilised.

A 500 KLD brewery ETP with an inlet COD of 3,000 mg/L and 70% COD removal in the UASB can generate approximately 350–400 m³/day of methane, translating to 480–540 m³/day of biogas at 65–70% CH₄ content. At 38% electrical efficiency, this produces a continuous output of 110–125 kW — enough to meet 30–50% of the brewery's total electrical demand. With heat recovery, the combined annual energy value can exceed ₹80–100 lakhs for a single medium-scale plant.

Use the UASB Reactor Sizing Calculator to size your anaerobic reactor and estimate biogas production from your wastewater flow and COD data. Once you have the biogas m³/day figure, enter it here to size the CHP system and calculate the energy and financial return.

For distillery effluent (spent wash), biogas production can be exceptionally high — inlet COD of 80,000–100,000 mg/L in raw spent wash, treated in a high-rate anaerobic system, can generate over 2,000 m³/day of biogas from a 1,000 KLD plant. Many distilleries in India operate 1–5 MW biogas power plants on this feedstock, meeting 100% of their electrical demand with surplus available for export. The economics in such cases are transformative — full CAPEX recovery in 18–36 months and virtually zero net energy cost thereafter.

Carbon Credits and Green Certifications from Biogas CHP in India

Biogas CHP projects in India qualify for carbon credits through multiple mechanisms. The primary pathway is grid electricity displacement — every kWh generated from biogas avoids approximately 0.82 kg CO₂ equivalent that would otherwise have been produced by the Indian electricity grid (CEA 2023 grid emission factor). A 200 kW biogas CHP system running 8,000 hours per year avoids approximately 1,310 tCO₂e per year, which at current voluntary carbon market prices of USD 10–20/tCO₂e generates USD 13,000–26,000 (approximately ₹11–22 lakhs) per year in carbon revenue.

Under India's Carbon Credit Trading Scheme (CCTS), launched under the Energy Conservation (Amendment) Act 2022 and implemented by BEE, biogas-based electricity generation is an eligible activity for Indian Carbon Credits (ICCs). The ICCs are expected to trade on Indian bourses (BSE/NSE) from 2025–26. Simultaneously, projects that capture and utilise methane rather than allowing it to vent to the atmosphere earn additional credits for methane avoidance (methane has a global warming potential of 28 CO₂e over 100 years).

International certifications available for biogas CHP include the Verified Carbon Standard (VCS/Verra) using methodology VM0019 (biogas capture and power generation), Gold Standard for the Global Goals (GS4GG), and Clean Development Mechanism (CDM) methodologies AMS-I.C and AM0006 for smaller projects. Obtaining these certifications adds credibility for export markets and ESG reporting.

For industrial facilities considering upgrading from CHP to purified biomethane, our Compressed Biogas (CBG) solutions cover full biogas upgrading to CNG quality — an alternative pathway that eliminates engine maintenance while producing a marketable fuel commodity. The choice between CHP and CBG depends on your heat demand, electricity tariff, and access to CBG off-take markets.