NDA (II) 2019 General Ability Test · Question 91 of 148

Correct — C, Methane. Biogas is produced by the anaerobic (oxygen-free) decomposition of organic matter - cattle dung, crop residue and sewage - by methanogenic bacteria. Its principal combustible constituent is methane (CH4), which makes up roughly 50-75% of the mixture and gives biogas its fuel value. Carbon dioxide is the second-largest component, but methane is the major constituent that actually burns.

• (a)Carbon dioxide — CO2 is the second-biggest component of biogas (about 25-50%), but it is non-combustible - a diluent, not the fuel. The question asks for the MAJOR constituent, which is methane.
• (b)Nitrous oxide — Nitrous oxide (N2O) is not a significant component of biogas. Only traces of nitrogen may be present; the gas that dominates the mixture is methane.
• (d)Oxygen — Biogas is made by ANAEROBIC digestion - in the absence of oxygen. Free O2 is essentially absent; if oxygen were present the methanogenic bacteria could not work.

Biogas (gobar gas) is a renewable fuel made by the anaerobic digestion of biomass such as cattle dung, sewage and crop waste by methanogenic bacteria. The mixture is dominated by methane, with carbon dioxide as the next-largest component and only traces of other gases.

The key word is 'major'. Both methane and carbon dioxide are present in large amounts, but only methane burns and it is the larger share, so it is the major constituent. Oxygen is ruled out immediately because biogas forms in the absence of oxygen.

• Biogas is about 50-75% methane (CH4) and 25-50% carbon dioxide, with traces of H2S, H2 and N2.
• It is produced by anaerobic digestion of biomass (dung, sewage, crop waste) by methanogenic bacteria.
• Methane is also the main constituent of natural gas / CNG and of marsh gas.
• Biogas is a clean, renewable fuel; the leftover slurry is used as a nitrogen-rich manure.

• Picking carbon dioxide because it is 'present' - it is the second-largest, but methane is the major (combustible) constituent.
• Confusing biogas with producer gas or water gas, which contain carbon monoxide and hydrogen.

Fuel-gas questions ask for the 'main / major constituent' - link each fuel to its dominant gas (biogas and CNG -> methane, LPG -> butane/propane, water gas -> CO + H2).

• practice — not a real PYQ

  Biogas is produced from organic waste chiefly by the process of

  • (a)combustion
  • (b)anaerobic digestion
  • (c)photosynthesis
  • (d)electrolysis

  Answer(b) anaerobic digestion - methanogenic bacteria act in the absence of oxygen.

• practice — not a real PYQ

  Which gas, the major constituent of biogas, is also the chief component of CNG?

  • (a)Ethane
  • (b)Propane
  • (c)Methane
  • (d)Hydrogen

  Answer(c) Methane - the dominant combustible gas in both biogas and CNG.