Question

Which hydrocarbon is mainly present in gobar gas?
A) Methane
B) Ethane
C) Propane
D) Butane

Answer 1

Hint: Biogas is a type of biofuel and is produced naturally from organic waste decomposition. This breakdown takes place under anaerobic conditions releasing a mixture of gases. It is highly flammable also and gives blue flame. So guess what the main component is.

Complete step by step answer:
When organic matter is broken down under anaerobic conditions (in the absence of oxygen) a blend of gases is produced, which is known as gobar gas. This organic matter we are talking about includes animal excreta, sewage, food waste, agricultural waste, manure, municipal waste, etc.
Hence, gobar gas is a renewable source of energy.

Now, you might be thinking how this organic matter is converted into gas. The organic matter undergoes anaerobic digestion or anaerobic fermentation by anaerobic organisms, methanogens and sulphate-reducing bacteria under controlled conditions to produce gobar gas.
Gobar gas is mainly composed of methane ($C{H_4}$), carbon dioxide ($C{O_2}$), small amounts of hydrogen sulphide (${H_2}S$) and trace amounts of nitrogen (${N_2}$), hydrogen (${H_2}$) and carbon monoxide ($CO$).

This gas is highly flammable due to the presence of methane. Its heat of combustion is (-890.4 $kJmo{l^{ - 1}}$). Its combustion reaction is:

$C{H_4} + 2{O_2} \to C{O_2} + 2{H_2}O$ , ∆H = -890.4kJ

The methane content of gobar gas is 50-75%, carbon dioxide content is 25-50%, etc.
Due to such high content of methane gobar gas is highly flammable and produces blue flame on burning. Thus, it can be used as a source of energy.

So, the correct option is: A) Methane.

Additional information:
In the way natural gas is compressed to form CNG, biogas or gobar gas can also be compressed after the removal of carbon dioxide. After this it can be used to power vehicles. Countries like the United Kingdom have adopted this method and their biogas is expected to be replacing approximately 17% of the vehicle fuel.

Note: Although all 4 options are hydrocarbons, all of them do not have high combustion energy. 25 times more heat can be captured by one pound of methane than by one pound of carbon dioxide. But this is not the case with other hydrocarbons.