CDS (I) 2022 General Knowledge · Question 24 of 120

Correct — B, It is a non-renewable source of energy. Biogas is renewable, and the reason is in its raw material. It is made from cattle dung, crop residue and other biodegradable waste, all of which are produced continuously for as long as the animals are fed and the fields are cropped — nothing is being drawn down from a finite store the way coal or petroleum is. So the statement calling it non-renewable is the one that fails, and it is what the question asks for. The other three descriptions are accurate: methane is the main constituent, the gas is generated by microorganisms decomposing the dung in the absence of oxygen, and it burns cleanly without smoke or ash.

• (a)It is mainly composed of methane gas. — A correct statement. Biogas runs at roughly 50 to 80 per cent methane by volume, with 15 to 50 per cent carbon dioxide and small amounts of nitrogen, hydrogen and hydrogen sulphide. Methane is both the largest component and the one that carries the fuel value.
• (c)It is formed by the decomposition of cow dung in the absence of oxygen. — Also correct, and it names the process precisely. Anaerobic digestion is the breakdown of biodegradable material by microorganisms where there is no oxygen, and methanogens are the organisms that produce the methane at the end of it.
• (d)It burns without smoke and leaves no residue. — Correct, and it is the practical reason biogas is promoted as a domestic fuel. Firewood, dung cakes and coal all leave ash and fill a kitchen with smoke; biogas does neither, which matters for indoor air quality.

Renewable sources are replenished on a human timescale — sunlight, wind, flowing water, and biomass such as dung and crop residue. Non-renewable sources exist as a fixed stock laid down over geological time, which is coal, petroleum, natural gas and nuclear fuel. Biogas belongs firmly in the first group. It is produced by anaerobic digestion in a sealed plant, in which microorganisms break the dung down through a series of stages, methanogens producing methane at the last of them. The gas is drawn off for cooking or lighting, and the slurry left behind is a good manure, rich in the nitrogen and phosphorus the dung began with.

Sorting the four statements by whether they describe the fuel or classify it makes the item quick. Three are descriptions and easy to check against what a village biogas plant does; the fourth is a classification, and classification statements are where these questions usually hide the error. A distinction worth keeping is that biogas and natural gas are both mostly methane but sit on opposite sides of the renewable divide, because natural gas comes from a geological reservoir while biogas is made this week from this week's dung. The double benefit of the plant — a smokeless fuel and a manure that keeps the nutrients that burning dung cakes would destroy — is why the technology has been pushed in rural India for decades.

• Biogas is a renewable energy resource, made from dung and other biodegradable waste that is replenished continuously.
• It is roughly 50 to 80 per cent methane by volume, with 15 to 50 per cent carbon dioxide and traces of other gases.
• It is produced by anaerobic digestion — microorganisms breaking down biodegradable material where there is no oxygen.
• It burns without smoke and leaves no ash, unlike firewood, dung cakes or coal.
• The slurry remaining after digestion is used as manure, so the nutrients in the dung are kept rather than burnt away.

• Grouping biogas with natural gas because both are mostly methane, and inheriting the non-renewable label with it.
• Assuming any fuel that burns must be a fossil fuel.
• Forgetting that the process needs the absence of oxygen, and describing it as ordinary decay in the open.

As a statements item on the properties of biogas, as a question on its major constituent, or as a sorting exercise separating renewable from non-renewable sources.

• practice — not a real PYQ

  The gas that makes up the largest part of biogas by volume is

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

  Answer(b) methane — it accounts for roughly half to four-fifths of the mixture, and it is what gives the gas its fuel value.

• practice — not a real PYQ

  The slurry left behind in a biogas plant after the gas has been drawn off is best used as

  • (a)a building material
  • (b)manure for the fields
  • (c)fuel for a stove
  • (d)waste to be buried

  Answer(b) manure for the fields — the digestion removes the carbon as methane and carbon dioxide but leaves the nitrogen and phosphorus behind.