The major combustible component of gobar (cow- dung) gas is

1. Classification of Energy Resources (basic)

To understand energy in our daily lives, we first need to look at how we classify the resources that power our world. Broadly, energy resources are divided into two categories based on their availability and how long we have been using them: Conventional and Non-conventional sources.

Conventional sources, also known as non-renewable resources, are those that have been in common use for a long time but are finite. These include fossil fuels like coal, petroleum, and natural gas, as well as nuclear energy. Although nuclear energy is often discussed as a modern alternative, it is considered exhaustible because it relies on minerals like Uranium or Thorium, which are limited in supply INDIA PEOPLE AND ECONOMY, Mineral and Energy Resources, p.61. These sources generally have a higher environmental footprint due to carbon emissions or waste management challenges.

Non-conventional sources are those that are renewable and sustainable. These include solar, wind, tidal, geothermal, and biomass energy Geography of India, Energy Resources, p.27. These resources are more equitably distributed across the planet and are environment-friendly. A key example in rural India is biogas (gobar gas), which is produced through the anaerobic decomposition of organic matter like cattle dung. Chemically, biogas is a mixture, primarily composed of Methane (CH₄)—which makes up about 50% to 75% and serves as the main fuel—and Carbon Dioxide (CO₂) Science class X, Carbon and its Compounds, p.60.

Feature	Conventional (Non-renewable)	Non-conventional (Renewable)
Nature	Exhaustible; takes millions of years to form.	Inexhaustible; replenishable in a short time.
Examples	Coal, Petroleum, Natural Gas, Nuclear.	Solar, Wind, Biomass (Biogas), Geothermal.
Environment	Major source of pollution and CO₂.	Eco-friendly and sustainable.

Sources: INDIA PEOPLE AND ECONOMY, Mineral and Energy Resources, p.61; Geography of India, Energy Resources, p.27; Science class X, Carbon and its Compounds, p.60

2. Saturated Hydrocarbons: The Alkane Series (basic)

To understand carbon chemistry, we must first look at why carbon is so unique. Carbon is a "friendly" element that forms millions of compounds—more than all other elements combined! This is due to its tetravalency (having four valence electrons) and its ability to form stable bonds with itself and other elements Science, Carbon and its Compounds, p.62. The simplest of these compounds are hydrocarbons, which consist solely of carbon and hydrogen. When these atoms are linked by only single bonds, they are called saturated hydrocarbons or alkanes.

The alkane series follows a specific mathematical pattern for its chemical formula: CₙH₂ₙ₊₂. This means for every 'n' number of carbon atoms, there are twice as many hydrogen atoms plus two more to "saturate" the carbon's four bonds. For example, if you have 1 carbon atom, you have (2×1)+2 = 4 hydrogens, giving us Methane (CH₄). If you have 2 carbons, you get Ethane (C₂H₆) Science, Carbon and its Compounds, p.64. These chains can grow very long, forming the basis of many fuels we use daily.

No. of Carbon Atoms	Name	Formula
1	Methane	CH₄
2	Ethane	C₂H₆
3	Propane	C₃H₈
4	Butane	C₄H₁₀

In a practical sense, the simplest alkane, Methane, is an incredibly important fuel. It is the primary component of biogas (commonly known as Gobar gas) and Compressed Natural Gas (CNG) Science, Carbon and its Compounds, p.60. Biogas is produced through the anaerobic decomposition of organic matter, like cattle dung, and contains roughly 50% to 75% methane. While it also contains non-combustible Carbon Dioxide (CO₂), it is the methane that provides the energy for cooking and lighting in many rural areas.

Sources: Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.60; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.62; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.64

3. Biomass as a Rural Energy Source (basic)

To understand biomass, we must first look at it through the lens of biology and chemistry. Biomass is simply the organic matter derived from living organisms—primarily plants and animals. From a chemistry perspective, it is a reservoir of solar energy that plants have converted into chemical energy through photosynthesis. In rural India, this energy is harnessed from agricultural residues, cattle dung, and wood to provide a sustainable alternative to fossil fuels Environment and Ecology (Majid Hussain), BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.33.

The most sophisticated application of biomass in rural areas is the production of Biogas (popularly known as Gobar Gas). This occurs through a process called anaerobic decomposition—the breakdown of organic matter by bacteria in the absence of oxygen. The result is a clean-burning fuel that significantly improves rural hygiene and reduces the physical burden on women who otherwise collect fuelwood Environment and Ecology (Majid Hussain), Environmental Degradation and Management, p.53. Unlike burning raw wood, biogas provides high thermal efficiency and leaves behind a nutrient-rich slurry that acts as an excellent organic fertilizer.

Let’s break down the chemical composition of Biogas to understand why it is such an effective fuel:

Component	Percentage (%)	Role in Fuel
Methane (CH₄)	50% – 75%	The primary combustible component; provides the heat.
Carbon Dioxide (CO₂)	30% – 50%	Non-combustible; actually reduces the flame temperature.
Hydrogen (H₂) & Nitrogen (N₂)	Traces (<10%)	Minor constituents that do not significantly impact heating.

It is important to note that while burning biomass releases CO₂, it is considered carbon neutral in the short term. This is because the CO₂ released is roughly the same amount the plants absorbed during their growth, unlike fossil fuels which release "ancient" carbon stored for millions of years Environment (Shankar IAS), Renewable Energy, p.292. Beyond cooking, bio-energy can also be converted into electricity, helping rural areas achieve self-reliance and reducing environmental pollution INDIA PEOPLE AND ECONOMY (NCERT), Mineral and Energy Resources, p.64.

Sources: Environment and Ecology (Majid Hussain), BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.33; Environment and Ecology (Majid Hussain), Environmental Degradation and Management, p.53; Environment (Shankar IAS), Renewable Energy, p.292; INDIA PEOPLE AND ECONOMY (NCERT), Mineral and Energy Resources, p.64

4. Comparison of Gaseous Fuels: LPG vs. CNG (intermediate)

In our journey through everyday chemistry, understanding the gaseous fuels that power our kitchens and vehicles is essential. The two most common fuels we encounter are Liquefied Petroleum Gas (LPG) and Compressed Natural Gas (CNG). While both are hydrocarbons, their chemical composition and physical behavior differ significantly due to the length of their carbon chains.

LPG is primarily a mixture of Propane (C₃H₈) and Butane (C₄H₁₀). These molecules have three and four carbon atoms respectively Science Class X, Chapter 4, p.64. Because these gases can be easily liquefied under moderate pressure at room temperature, they are stored in liquid form in cylinders, making them highly portable for domestic use. In contrast, CNG is essentially Methane (CH₄), the simplest hydrocarbon with just one carbon atom. To store enough energy for a vehicle, methane must be compressed to very high pressures (200-250 bar), as it does not liquefy easily at room temperature.

A fascinating cousin to CNG is Biogas (or Gobar Gas). Like CNG, its primary combustible component is Methane, typically making up 50% to 75% of the volume Science Class X, Chapter 4, p.60. It is produced through the anaerobic decomposition of organic matter like cattle dung and farm waste Contemporary India II, Chapter 5, p.117. While it is an excellent renewable energy source, it also contains a significant amount of Carbon Dioxide (CO₂), which is non-combustible and slightly reduces its thermal efficiency compared to pure natural gas.

Feature	LPG (Liquefied Petroleum Gas)	CNG (Compressed Natural Gas)
Primary Component	Butane and Propane	Methane
Relative Density	Heavier than air (settles on floor)	Lighter than air (dissipates upwards)
Main Use	Cooking and Heating	Transport fuel (Buses, Cars)

Sources: Science Class X, Chapter 4: Carbon and its Compounds, p.64; Science Class X, Chapter 4: Carbon and its Compounds, p.60; Contemporary India II, Chapter 5: Minerals and Energy Resources, p.117

5. National Policy on Biofuels and SATAT (exam-level)

To understand the chemistry of modern energy, we must look at the National Policy on Biofuels. Biofuels are liquid or gaseous fuels produced from biomass (organic matter). The policy categorizes these into 'Basic Biofuels' (First Generation or 1G, like bio-ethanol and bio-diesel) and 'Advanced Biofuels' (Second Generation or 2G, Third Generation or 3G), providing financial incentives like Viability Gap Funding for the latter Indian Economy, Nitin Singhania, Infrastructure, p.453. From a chemical perspective, the policy focuses on converting complex carbohydrates and fats into simpler combustible molecules like Ethanol (C₂H₅OH) and Methane (CH₄).

One of the most significant shifts in India's policy is the expansion of permitted feedstocks for ethanol production. While traditionally only sugarcane juice was used, the government now allows damaged food grains (unfit for human consumption) such as broken rice, wheat, and corn, along with starch-rich materials like Cassava and Rotten Potatoes Indian Economy, Nitin Singhania, Infrastructure, p.465. In June 2023, the government further accelerated our climate goals by advancing the target of 20% Ethanol Blending in Petrol (E20) to the year 2025-26, moving it up from the original 2030 deadline Environment, Shankar IAS Academy, India and Climate Change, p.316.

Parallel to liquid fuels is the SATAT initiative (Sustainable Alternative Towards Affordable Transportation). This focuses on Compressed Bio-Gas (CBG). Chemically, biogas produced from anaerobic digestion of waste is purified to remove CO₂ and H₂S, leaving a high concentration of Methane (over 90%), which is then compressed. This allows agricultural residue and cattle dung to be used directly in the transport sector, reducing our dependence on imported Natural Gas and promoting a circular economy.

Biofuel Generation	Source Material (Feedstock)	Key Characteristic
1G (Basic)	Sugarcane, corn, edible oils, damaged grains.	Directly uses sugar/starch/oil from crops.
2G (Advanced)	Rice straw, wheat stalk, corn cobs (non-edible lignocellulose).	Uses agricultural waste; does not compete with food.
3G	Algae and microbes.	High yield; can be grown in wastewater/saline water.

Sources: Indian Economy, Nitin Singhania, Infrastructure, p.453, 465; Environment, Shankar IAS Academy, India and Climate Change, p.316

6. The Chemistry of Biogas (Gobar Gas) Production (intermediate)

At its heart, **Biogas** (often called **Gobar Gas** in rural India when cattle dung is used) is a product of **anaerobic decomposition**. This is a biological process where microorganisms break down organic matter—such as farm waste, animal dung, and shrubs—in the absolute absence of oxygen NCERT Contemporary India II, Chapter 5, p.117. Unlike aerobic composting which requires air, anaerobic digestion occurs in a closed 'digester' or plant, where complex organic compounds (carbohydrates, proteins, and lipids) are converted into a mixture of gases Environment Shankar IAS Academy, Chapter 1, p.6. The chemical profile of biogas is dominated by two main gases. The primary combustible component is **Methane (CH₄)**, which typically makes up **50% to 75%** of the total volume. Methane’s high concentration is what gives biogas its high **thermal efficiency**, making it far superior to burning raw dung cakes, kerosene, or charcoal NCERT Contemporary India II, Chapter 5, p.117. The second most abundant gas is **Carbon Dioxide (CO₂)**, making up about 30% to 50%. It is important to note that CO₂ is non-combustible; its presence actually acts as a diluent, slightly lowering the flame temperature compared to pure methane.

Component	Typical Percentage	Chemical Role
Methane (CH₄)	50% – 75%	The primary fuel; highly combustible.
Carbon Dioxide (CO₂)	30% – 50%	Non-combustible byproduct; lowers flame intensity.
Hydrogen (H₂)	< 1%	Trace combustible gas.
Nitrogen (N₂) / H₂S	Traces	Inert gases or impurities.

Beyond providing energy, the production of Gobar Gas offers a critical agricultural benefit. As the microbes digest the waste, they lower the **Carbon-to-Nitrogen (C:N) ratio** of the residue Environment Shankar IAS Academy, Chapter 30, p.364. This results in a final byproduct known as 'slurry' or compost, which is a high-quality organic manure that returns essential nutrients to the soil. Thus, the chemistry of biogas is a perfect example of a **short-term carbon cycle**, where carbon from plants is consumed by animals, converted to gas, and eventually returned to the atmosphere and soil Environment Shankar IAS Academy, Chapter 2, p.19.

Sources: NCERT Contemporary India II, Chapter 5: Minerals and Energy Resources, p.117; Environment, Shankar IAS Academy (10th Ed), Chapter 30: Agriculture, p.364; Environment, Shankar IAS Academy (10th Ed), Chapter 1: Ecology, p.6; Environment, Shankar IAS Academy (10th Ed), Chapter 2: Functions of an Ecosystem, p.19

7. Solving the Original PYQ (exam-level)

Now that you have mastered the basics of carbon compounds and renewable energy sources, this question serves as a perfect application of those building blocks. In your earlier lessons, you learned about anaerobic decomposition—the process where bacteria break down organic matter in the absence of oxygen. When this occurs with cattle dung in a biogas plant, it produces gobar gas. The key to solving this is identifying which specific molecule in that gaseous mixture actually provides the energy when ignited. As a UPSC aspirant, you must distinguish between the entire composition of the gas and its combustible (fuel) part.

To arrive at the correct answer, remember that while gobar gas is a mixture, methane (CH4) is its most dominant and efficient fuel component, typically comprising 50% to 75% of the total volume. This is why (A) methane is the correct answer. The reasoning is simple: for a gas to be a viable fuel source for rural cooking and lighting, it needs a high-energy hydrocarbon. Methane fits this role perfectly because of its clean-burning properties and high thermal efficiency, as noted in Science, Class X (NCERT).

UPSC often includes "distractor" options to test the depth of your knowledge. For instance, carbon dioxide is indeed a major part of biogas (30-50%), but it is non-combustible and actually acts to lower the flame temperature. Hydrogen is combustible but is only present in trace amounts (less than 10%), making it a minor contributor. Finally, propane is a common trap; while it is a major component of LPG (Liquefied Petroleum Gas), it is not a standard product of the biological fermentation of animal waste. Distinguishing between naturally fermented biogas and refined petroleum gases is a crucial skill for the Preliminary exam, as highlighted in Contemporary India II: Textbook in Geography for Class X (NCERT).