Answer: Biogas (methane) and ethanol (bioethanol) are two fuels obtained by microbial processes.
Biogas is produced when anaerobic bacteria decompose cattle dung, sewage and other organic waste in a biogas plant. Ethanol is produced by the fermentation of sugar or starch by yeast.
It is necessary to increase their use because the reserves of coal and petroleum are limited and will be exhausted, whereas these fuels are produced from waste and crops which are available again and again (renewable). They burn more cleanly, produce less pollution and help in the disposal of waste, so they reduce global warming.