CHP (CHP)

Combined Heat and Power (CHP), also called cogeneration, is the simultaneous production of electricity and useful thermal energy from a single fuel input. In a conventional grid-supplied system electricity is generated remotely at 35-42% efficiency, with the waste heat dumped at the power station, and process heat is generated separately on site by a boiler at 80-85% efficiency. Combining the two losses gives an overall fuel-to-useful-energy efficiency of about 45-50%. A CHP plant, by capturing the engine or turbine's exhaust and jacket heat for use on site, reaches 60-80% overall efficiency.

For CBG plants the most common configuration is a gas-engine genset (Jenbacher, Caterpillar, MWM, Cummins) running on raw or partly cleaned biogas, producing electricity for plant auxiliaries and the grid while its hot exhaust (around 450°C) and engine-cooling water (around 90°C) heat the digester. Engine electrical efficiency is typically 38-42%, recoverable thermal efficiency another 40-45%, and total CHP efficiency 78-85% on lower heating value of the fuel.

CHP economics in India hinge on three levers: the engine's load factor (above 7,500 hours per year is bankable), the spread between purchased grid power tariffs and self-generation cost, and whether the surplus power can be exported under MNRE's biomass power feed-in tariff or net-metering rules. The capital cost of a CHP module is roughly ₹4-6 crore per MW electrical, with a typical payback of 4-6 years when the heat is genuinely used on site.

The main trade-off is that CHP is only valuable when the thermal load matches the electrical load profile. A CBG plant that has fully upgraded its biogas to vehicle-grade gas no longer has fuel left for an engine, so CHP and full upgrading are mutually exclusive on the same gas stream. Most projects either pick CHP for captive use (raw biogas, no SATAT) or full upgrading for SATAT sale (no CHP, small auxiliary boiler for heat). Hybrid designs split the gas stream but pay capex for both lines.