Facility

Biogas is the gas produced when microbes break down organic matter without oxygen, a process called anaerobic digestion. It is mainly methane and carbon dioxide, with traces of hydrogen sulphide and water vapour. Raw biogas typically contains 50-70% methane.

Raw biogas is the untreated gas straight from the digester - roughly 50-70% methane and 30-45% carbon dioxide, plus trace hydrogen sulphide and water vapour. It must be cleaned and upgraded before it can be sold as CBG.

Methane (CH4) is the energy-carrying, combustible part of biogas, and the same molecule that makes up most of natural gas. The more methane a gas contains, the higher its fuel value.

It is the natural, oxygen-free breakdown of organic matter by microbes, producing biogas and a nutrient-rich residue called digestate. It happens inside a sealed tank called a digester.

The carbon-to-nitrogen ratio compares how much carbon a feedstock carries relative to nitrogen, and it controls how well the microbes work. The generally accepted optimum for anaerobic digestion is about 20:1 to 30:1. Too much carbon and the culture runs slow; too much nitrogen and it poisons itself with ammonia.

Raw biogas at 55-65% methane carries about 20-23 MJ per cubic metre, because only the methane fraction holds energy. Pure methane is about 35.8 MJ per cubic metre, and one cubic metre of biogas is roughly equivalent to 0.5-0.6 litres of diesel.

Carbon dioxide, nitrogen and water vapour in biogas are inert and carry no combustion energy, so calorific value scales almost linearly with methane content. Upgrading from about 60% to over 90% methane concentrates the energy per cubic metre, which is what makes CBG saleable as a vehicle fuel rather than a boiler gas.

An inoculum is a starter culture of active anaerobic microbes, usually digestate or slurry from a running digester, or cattle dung. Without seeding, a new digester can take months to establish stable methanogenesis, so the inoculum is what shortens the start-up lag.

Foaming is a stable layer of gas-filled bubbles on the digester surface, usually triggered by overloading, or by feed rich in surfactants, proteins or fats. It can block gas offtake, carry slurry into the gas lines and force an unplanned shutdown.

Secure a reliable feedstock supply first, then pick a site with land, water and a way to move gas out; choose digester and upgrading technology; get an offtake letter of intent from an OMC or a GAIL synchronisation agreement; clear the pollution board, PESO, factory and land approvals; then build and commission. Feedstock security is the make-or-break step, and it comes before the site, not after it.

Confirm feedstock availability with a long supply horizon, identify a site - typically 5-10 acres for a 5-10 TPD plant - and engage an oil marketing company with an Expression of Interest to secure a Letter of Intent. The LOI from IOCL, BPCL, HPCL, GAIL or IGL is the gateway to bankability.

Pollution control boards generally categorise CBG plants as White - the least polluting class - which simplifies Consent to Establish and Consent to Operate. Typical CBG scales also avoid full Environmental Clearance under the EIA notification. PESO remains the safety-critical approval regardless.

Efficiency depends far more on feedstock and operation than on technology, and many Indian plants run below design capacity mainly because of feedstock supply. Real output is a fraction of the lab BMP potential, so a plant quoted at its BMP is quoted at a number it will never see.

Feed the digester to schedule, check temperature, pH and gas quality, log flows and pressures, and inspect the scrubbers, compressors and safety systems. Consistent daily logging is what lets a problem be caught as a trend rather than as a shutdown.

A 5 TPD-class plant making about 12,000 cubic metres a day of raw biogas at 60% methane holds roughly 7,200 cubic metres of methane, which after upgrading and compression yields about 4,800-5,000 kg a day of saleable CBG. Volume falls through upgrading because CO2 and H2S are removed; energy density rises.