Question

The calorific value of biogas is
A. 10 – 20 mj/${m^3}$
B. 23 – 28 mj/${m^3}$
C. 35 – 40 mj/${m^3}$
D. 5 – 10 mj/${m^3}$

Answer 1

Biogas is a clean fuel produced through anaerobic digestion, which includes a variety of organic waste such as animal, agricultural, domestic, and industrial. Anaerobic digestion consists of three steps: first is the breakdown of complex organic materials into simple organic substances and next is the conversion of decomposed matter into organic acids. And the last step is the conversion of acids into methane gases.

Complete answer: Biogas is a mixture of gases, which consist of methane, carbon dioxide, nitrogen, oxygen, and hydrogen sulphide. The percentage of methane is more important when compared to all gas mixtures. If methane content is higher, the calorific value will be higher too. The other gas constituents do not contribute to the energy content. The composition of biogas changes along with some parameters. Temperature plays a significant role in the composition of the gaseous mixture.
The calorific value of biogas lies in a range of 23 – 28 mj/${m^3}$.

Component	Chemical symbol	concentration
Methane	$C{H_4}$	50 – 70, % - vol
Carbon dioxide	$C{O_2}$	25 – 45, % - vol
Water vapour	${H_2}O$	2 – 7, % - vol
Oxygen	${O_2}$	< 2, % - vol
Nitrogen	${N_2}$	< 2, % - vol
Ammonia	$N{H_3}$	< 1, % - vol
Hydrogen	${H_2}$	< 1, % - vol
Hydrogen sulphide	${H_2}S$	20 – 20.000 ppm
Ppm: parts per million , % - vol: volumetric percentage

Hence, the correct answer is Option B.

Note: Apart from temperature, there are many such factors like carbon: nitrogen ratio, retention time, solid concentration, and types of feedstock which play important roles in the rate of biogas production. The methane content of biogas is very high, 60%. The calorific value will also be high enough which helps in power generation and energy to many applications.