Biogas
Explanation
Biogas is a renewable energy source produced through the biological breakdown of organic matter in the absence of oxygen. This process, known as anaerobic digestion, utilizes methanogenic bacteria to decompose biomass such as animal waste, agricultural residues, and sewage.
Option Analysis
- Coal is incorrect: Coal is a fossil fuel formed from the remains of ancient vegetation that have been subjected to intense heat and pressure over millions of years. It is not produced through the short-term anaerobic degradation of cattle dung.
- Biogas is correct: Biogas is directly produced from the anaerobic degradation of cattle dung (often referred to as "Gobar gas" in India). The process occurs in a biogas plant where bacteria break down the complex organic compounds in the dung to release a mixture of gases, primarily methane ($CH_4$) and carbon dioxide ($CO_2$).
- LPG is incorrect: Liquefied Petroleum Gas (LPG) is a mixture of hydrocarbon gases, specifically propane and butane. It is a byproduct of crude oil refining or natural gas processing, not biological degradation.
- CNG is incorrect: Compressed Natural Gas (CNG) is methane stored at high pressure. While chemically similar to the methane found in biogas, CNG is typically derived from fossil fuel deposits. The immediate product of cattle dung fermentation is classified as biogas.
Key Takeaway
Biogas is a biofuel generated by the anaerobic digestion of organic waste. Its primary constituent is methane ($50-75\%$), making it an excellent fuel for heating and electricity generation.