Science and Technology › Energy technology

Hydrogen and bioenergy

Fuel cells, ethanol blending and its feedstock, biogas, and aviation fuel made from biomass. Prelims has asked about the raw materials allowed under the National Policy on Biofuels and how green hydrogen is produced.

Foundation note: The colours of hydrogen

UPSC has asked

Prelims 2025: battery electric and fuel cell vehicles as alternative powertrains
Prelims 2023: uses of green hydrogen as fuel and in fuel cells
Prelims 2020: raw materials allowed under the National Policy on Biofuels

Nothing here changed from 1 to 30 September 2026.Show all

E20: Petrol blended with 20 per cent anhydrous ethanol by volume. The number is the ethanol share, so E20 (20 per cent) is a stronger blend than E10 (10 per cent).

Anaerobic digestion: The breakdown of organic matter by bacteria in the absence of oxygen, which gives off biogas.

Press mud: The solid residue left behind when sugarcane juice is filtered in a sugar mill.

second generation ethanol: Ethanol made from non-food biomass such as crop stubble or bagasse, by breaking down cellulose, so it does not compete with the food plate.

carburettor: An older fuel mixing device that cannot sense the extra oxygen ethanol brings and adjust for it, so the engine runs lean, with too little fuel for the air it draws in.

Green hydrogen: Hydrogen made by splitting water with electricity from renewable sources, so the making of it releases no carbon dioxide, unlike hydrogen reformed from natural gas.

Flex fuel vehicle: A vehicle that runs on any ethanol petrol blend from E20 to E100.

Compressed biogas: Biogas purified to about 96 per cent methane and compressed for use as fuel.