What is biogas? How can biogas be obtained? Why is the use of biogas obtained from cow dung advised in preference to burning cow dung cakes?

(N/A) Biogas is a mixture of gases,primarily methane,$CO_2$,hydrogen,and hydrogen sulphide,produced by the anaerobic decomposition of organic matter.
Biogas is obtained by the anaerobic decomposition of cow dung (or other organic waste) in a biogas plant,which is a dome-like structure built with bricks.
The use of biogas is preferred over burning cow dung cakes because:
$(i)$ Biogas burns without smoke and does not cause air pollution,whereas burning cow dung cakes releases harmful smoke and pollutants.
$(ii)$ Biogas leaves no residue (ash) after combustion,whereas burning cow dung cakes leaves behind a significant amount of ash as residue.
$(iii)$ Biogas has a higher calorific value and is a more efficient fuel compared to cow dung cakes.