The main constituent of biogas used as a fuel in rural areas is which gas?

Introduction / Context:
Biogas plants are widely used in rural areas to convert animal dung and organic waste into combustible gas for cooking and lighting. This renewable energy source is a mixture of gases, but one main component is responsible for most of its fuel value. This question checks whether you know which gas predominates in biogas and gives it its heating power.

Given Data / Assumptions:

• The gas mixture is biogas produced by anaerobic digestion of organic matter.
• Options include butane, methane, hydrogen sulphide, carbon dioxide, and propane.
• We assume standard biogas composition as taught at school level.
• The focus is on the main or major component, not minor impurities.

Concept / Approach:
Biogas is produced when bacteria break down organic material in the absence of oxygen. The resulting gas mixture typically contains about 50 to 70 percent methane, 30 to 40 percent carbon dioxide, and small amounts of hydrogen sulphide and other gases. Methane is a highly combustible gas and is also the main component of natural gas. Other hydrocarbons like butane and propane are important in liquefied petroleum gas but are not the principal components of biogas. Hydrogen sulphide is present only as a minor, corrosive impurity.

Step-by-Step Solution:
Step 1: Recall that biogas plants use cow dung, kitchen waste, and other organic material in a sealed tank. Step 2: Anaerobic bacteria decompose this waste and release gas, which collects at the top of the digester. Step 3: Measurements of biogas composition show that methane makes up the bulk of the gas, typically more than half by volume. Step 4: Carbon dioxide is the next major component, but it is non combustible and does not contribute to the heating value. Step 5: Hydrogen sulphide is present in small amounts and causes smell and corrosion, while butane and propane belong to liquefied petroleum gas, not typical biogas. Step 6: Therefore, methane is correctly identified as the main constituent of biogas.

Verification / Alternative check:
Textbooks on environmental science and renewable energy consistently describe biogas composition in terms of methane and carbon dioxide percentages. Practical guides for biogas plants also emphasise methane content as an indicator of gas quality; higher methane content means better heating performance. Natural gas, compressed biogas, and similar fuels are mainly valued because of methane. This consistent description across sources confirms that methane is the main constituent of biogas.

Why Other Options Are Wrong:
Butane: A component of liquefied petroleum gas used in cylinders, not the principal component of biogas.
Hydrogen sulphide: Present only in small quantities in biogas and considered undesirable because it is corrosive and toxic.
Carbon dioxide: Present in significant amount but non combustible; it does not provide energy and cannot be called the main fuel constituent.
Propane: Another liquefied petroleum gas component; again not the main component of typical biogas from rural digesters.

Common Pitfalls:
Students sometimes confuse biogas with liquefied petroleum gas and are tempted to choose butane or propane. Another mistake is to select carbon dioxide simply because it is named first in some composition lists, without noticing that it is non combustible. Always associate methane with natural gas and biogas and remember that high methane percentages are what make such gas mixtures good fuels.

Final Answer:
The main constituent of biogas is methane.