Can Bio-CNG click: A primer on this coming of age tech that can deal with air pollution at 3 levels

Apart from biodegradable waste, agricultural residue, cow dung and chicken litter and press mud from sugar factories are also used as feedstock in CBG plants

By Ravleen Kaur
Published: Tuesday 29 November 2022
CBG adds value to the waste, which is otherwise burnt to reduce volumes. Photo: Ravleen Kaur
CBG adds value to the waste, which is otherwise burnt to reduce volumes. Photo: Ravleen Kaur CBG adds value to the waste, which is otherwise burnt to reduce volumes. Photo: Ravleen Kaur

Bio-CNG, also known as Compressed Biogas (CBG), is an upgraded version of the humble biogas, the dung-based version of which 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-two days to attract microbes — the process is called hydrolysis.

It is then transferred to an anaerobic digester where it is retained for 20-25 days. It is at this stage — methanogenesis — that biogas is generated.

This gas contains 65 per cent methane, while the rest is carbon dioxide, hydrogen sulphide and water vapour. 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 sulphide and carbon dioxide, respectively. Methane, purified up to 95 per cent, 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 it’s feedstock,” said Subhashish Parida, an independent solid waste management expert.

Apart from biodegradable waste, agricultural residue, cow dung and chicken litter and press mud from sugar factories are also used as feedstock in CBG plants.

Benefits and limitations

India is the third largest importer of crude oil. In 2020-2021, India imported 54 per cent of its natural gas requirement, according to Petroleum Planning and Analysis Cell of the Ministry of Petroleum and Natural Gas.

“With global crude oil prices going up and shortage of CNG in international market, indigenous production of CBG only makes sense,” said Atma Ram Shukla, president of the Indian Biogas Association and former Advisor at the Ministry of New and Renewable Energy.

CBG is a decentralised energy form as it is produced closest to the point of consumption and unlike solar and wind energy, can be produced at all hours of the day, he added. 

CBG adds value to the waste, which is otherwise burnt to reduce volumes, said Anumita Roy chowdhury, air pollution expert and executive director at the Centre for Science and Environment, a non-profit.

“Fugitive methane emission from openly dumped waste also leads to landfill fires as it happened in Delhi earlier this year,” she added.

CBG is better than incineration-based waste to energy plants that release toxic emissions, she said.

The global warming potential of methane is 28 times more than that of carbon dioxide, according to the sixth assessment of the Intergovernmental Panel on Climate change. Landfill sites are a source of 20 per cent of the methane emission.

A 2019 study by the French Institute for Petroleum (IFPEN) compared the carbon footprint of the life cycle of CNG and biomethane-based vehicles to that of diesel, petrol and electric vehicles.

It concluded that biomethane is the best transportation option to preserve air quality, suggesting that biogas can play an important role in achieving climate neutrality in transportation.

The energy returned on energy invested (EROI) score for large biogas plants is 1.24 to 11.05, according to a study.

EROI is a ratio that measures the energy used to produce another energy source against the actual energy generated from that new source.

“This indicates that biogas systems have obvious energy-saving benefits,” said Prakash Singh, a research scholar at the Indian Institute of Technology, Guwahati.

A 2021 study claimed that if harnessed correctly, municipal solid waste (MSW) and wastewater energy can replace 4053.47 tonnes of India’s diesel consumption per day, the highest consumed transportation fuel in India.

The study titled ‘Biomethane plants based on municipal solid waste and wastewater and its impact on vehicle sector in India — An Environmental-economic-resource assessment’ was conducted by Singh and Ajay Kalamdhad.

“In fact, if CBG potential from all biomass is taken into account, 50 per cent of the current diesel usage in transport can be replaced,” claims Singh.

Emission comparison (amount of pollutant emitted per kilometre travelled) 


153.64 gram CO2 equivalent


133.16 gram CO2 equivalent


105 gram CO2 equivalent

Source: Prakash Singh, IIT Guwahati

Therefore, emission cuts of 28.16 grams of CO2 equalent can be achieved by travelling 1 km with CBG instead of diesel. India generates almost 62 million tonnes of MSW, roughly half of which is organic, according to Indian Biogas Association.

“CBG provides a very important service of managing our waste and producing organic manure, which can bring back the bio-content in our soil that has been overly laced with chemical fertilisers over the years,” said Shukla.

However, there are a few downsides too. Even though CBG and CNG are cheaper than petrol and diesel, the maintenance cost of gaseous fuel-based vehicles is higher than liquid fuel-based ones.

Theoretically, the calorific value of Bio-CNG is more than other transportation fuels. In practice, users have claimed that it is lower than CNG as it contains moisture.

“Normally, I would change the filter after the bus had run 18,000 km, but with Bio-CNG, I have to change it at 12-13,000 km,” said a city bus operator in Indore on conditions of anonymity.

The bus operator added that the moisture with the gas is possibly choking up the filter.

“Once the filter is choked, the bus is not able to pick up acceleration and I have to move it out of service which is also a loss to us. If I have realised the pickup issue in two months, six months down the line there might be an issue with the nozzle and engine too and that will mean a higher cost on maintenance,” he added.

Therefore, we have requested IMC for a meeting of all stakeholders and a comparison study on the performance of vehicles running on bio-CNG, he said.

A few concerns are also there about biogas plants becoming climate bombs, methane being its main component.

Because of its high global warming potential, any emission or leak of methane from the digester or pipelines can contribute to climate change significantly.

Previous studies have shown emissions in the range of 0.5-14 per cent of total produced biogas from large plants and up to 60 per cent from small-scale plants.

“This can be easily avoided with closed inlet and outlet tanks, airtight valves, regular checked tubing and body of digester for cracks and equipped flare unit when supply increase demand,” said Singh.

This is the second of a three-part series 

This story was produced with the support of Internews’ Earth Journalism Network

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