Identify the correct option from the following about the proportion of different gases in Biogas?

Correct — A, Methane > Carbon dioxide > Nitrogen. Anaerobic digestion of cattle dung and other organic waste yields a gas that is chiefly methane, typically 50 to 70 per cent by volume, with carbon dioxide next at roughly 30 to 40 per cent. Nitrogen appears only in traces, generally under a few per cent, along with small amounts of hydrogen, hydrogen sulphide and water vapour. Methane is the combustible fraction and is what makes biogas useful as a fuel; carbon dioxide is inert ballast that lowers the calorific value. The order in this option matches the analysis exactly.

• (b)Methane > Nitrogen > Carbon dioxide — This puts nitrogen above carbon dioxide. Nitrogen is present only in traces in biogas, since the digester is sealed against air, while carbon dioxide is produced in bulk by the same micro-organisms that make the methane.
• (c)Methane > Oxygen > Carbon monoxide — Oxygen cannot be a major component of biogas at all — the process is anaerobic, and any oxygen present would stop the methane-forming bacteria. Carbon monoxide is also not a normal constituent.
• (d)Nitrogen > Methane > Carbon dioxide — Nitrogen is never the largest fraction. If it were, the gas would have been diluted with air, and it would not burn.

Biogas is made by anaerobic digestion, in which micro-organisms break down organic matter in a sealed tank without oxygen. The process runs through four stages: hydrolysis of complex molecules, acidogenesis to short-chain acids, acetogenesis to acetate, hydrogen and carbon dioxide, and finally methanogenesis, where archaea produce methane. The residue left in the digester is a nutrient-rich slurry used as manure, so a biogas plant delivers fuel and fertiliser at once.

The item can be answered without recalling any percentage, because two of the four options can be struck out on principle: the process is anaerobic, so oxygen cannot be a major component, and the digester is sealed, so atmospheric nitrogen cannot dominate. That leaves methane against carbon dioxide, and methane must lead or the gas would not burn. India's programme has run under various names since the 1980s, most recently as the New National Biogas and Organic Manure Programme, and biogas upgraded to remove the carbon dioxide is sold as compressed biogas under the SATAT initiative launched in 2018.

• Biogas is roughly 50 to 70 per cent methane and 30 to 40 per cent carbon dioxide by volume.
• Nitrogen, hydrogen and hydrogen sulphide occur only in small amounts.
• Methane is the only combustible major component; carbon dioxide lowers the calorific value.
• Digestion is anaerobic and ends with methanogenesis carried out by archaea.
• The spent slurry from a digester is a nitrogen-rich manure, so the plant yields fuel and fertiliser together.

• Placing nitrogen above carbon dioxide by analogy with the composition of air.
• Admitting oxygen into a gas produced by an anaerobic process.
• Assuming biogas is pure methane, which would overstate its calorific value.

An ordering item. Two options can be eliminated from the anaerobic nature of the process alone, which leaves a single comparison to make.

• practice — not a real PYQ

  The gas that makes biogas combustible is

  • (a)carbon dioxide
  • (b)methane
  • (c)nitrogen
  • (d)hydrogen sulphide

  Answer(b) methane — it is the majority component and the only major combustible one.

• practice — not a real PYQ

  The slurry left behind in a biogas digester is chiefly used as

  • (a)a fuel
  • (b)a manure
  • (c)a building material
  • (d)an insecticide

  Answer(b) a manure — it is rich in nitrogen and phosphorus.