Bio-CNG, also known as compressed biogas (CBG), is an upgraded version of the humble biogas, the dung-based version that serves as cooking fuel in many villages in India.
The first stage of the CBG process is pre-treatment. The waste is passed through a trommel screen to remove hard materials like coconut shells and pieces of wood.
The screened waste is shredded in a hammer mill and made into a slurry with water. This slurry is kept in the pre-digester tank in aerobic conditions for one to two days to attract microbes in a process called hydrolysis.
It is then transferred to an anaerobic digester where it is retained for 20 to 25 days. It is at this stage—methanogenesis—that biogas is generated.
This gas contains 65% methane, while the rest is carbon dioxide, hydrogen sulfide and water vapor. The gas is stored in a balloon and then taken to a gas upgradation area.
It is passed through a wet and dry scrubber to remove hydrogen sulfide and carbon dioxide, respectively. Methane, purified up to 95%, is obtained here which is then compressed at high pressure in cylinders and sent off to filling stations.
This highly purified methane is similar in chemical properties to CNG derived from petroleum sources and can thus be used in vehicles.
“Extracting CNG from fossil fuel sources involves sensitive equipment and polluting and costly processes like thermal fracking while CBG makes waste its feedstock,” said Subhashish Parida, an independent solid waste management expert.
Apart from biodegradable waste, agricultural residue, cow dung, chicken litter and press mud from sugar factories are also used as feedstock in CBG plants.
This story was produced with support from Internews' Earth Journalism Network. It's the second of a three-part series (parts one and three are also available). It was first published in Down To Earth on November 28, 2022, and has been lightly edited for length and clarity.
Banner image: CBG gives value to waste that is otherwise burned to reduce volumes / Credit: Ravleen Kaur.
Ravleen Kaur
