COMPRESSED BIOGAS (CBG): Its Potential as a Source of Green Energy in India
Compressed Biogas (CBG) is an enriched form of biogas containing more than 90% methane (v/v), carbon dioxide up to 4% (v/v) and other traces of gases such as hydrogen sulphide, moisture, oxygen, and nitrogen. It is produced through a series of processes like compression of raw biogas, removal of impurities (CO2 , H2 S), and storage of purified gas in a highpressure vessel at around 200–250 bars for the vehicular application. It is very similar to the compressed natural gas (CNG) in terms of its fuel properties, economics, engine performance, and emissions.
THE PROCESS OF CBG PRODUCTION IN THE COUNTRY
Biogas is produced through anaerobic digestion of biodegradable materials with methane (55%–65%) and carbon dioxide (25%–35%) as major constituents along with some traces of other gases like hydrogen, hydrogen sulphide, ammonia, oxygen, and water vapour. Except for methane and hydrogen, all are accounted as impurities, which reduce the calorific value of the fuel and corrodes the engine parts.
The process of removal of the impurities from the raw biogas is known as biogas cleaning and the adjustment of carbon dioxide to enhance the calorific value to an optimal level is called biogas purification which increases the methane concentration up to 80%–99% in the gas mixture. There are various technologies, such as water scrubbing, chemical absorption, pressure swing adsorption (PSA), membrane separation, cryogenic separation, and biological filtration method, used for CBG production worldwide. Though in India, water scrubbing and PSA systems are most prominently used for the same.
WATER SCRUBBING TECHNOLOGY
The working principle of water scrubbing is based on the solubility of different gaseous components present in raw biogas. In this process, water is used as an absorbent, because the solubility of CO2 in water is much higher than that of CH4 , and is almost 25–26 times higher at 25 °C and atmospheric pressure. Simultaneously, H2 S can also be removed since it is more soluble in water than that of CO2 . This technology consists of a vertical column, where water and pressurized gas, almost at around 9–10 bars is allowed to flow counter-currently. The water is supplied at the top of the column while the pressurized raw biogas is allowed to pass from the bottom. The column is filled with the packing materials to provide more surface contact area and retention time. Consequently, CO2 and H2 S get absorbed in the water and CH4 with some traces of other gases along with moisture left the purification column at the top. On the other hand, the impurities which are soluble in water leave the column at the bottom and sent to the regeneration tank, where water is depressurized, and CO2 released. The regenerated water is recirculated back to the scrubbing column, and CO2 is collected whereas the purified biogas is stored in a cylindrical gas storage vessel. The purified biogas is compressed in a high-pressure compressor at around 200–240 bars to produce CBG for vehicular application and subsequently stored in a high-pressure cylindrical vessel as same as CNG. The purification efficiency of this technology ranges from 88.9%–96% with less than 2% CH4 loss.
PRESSURE SWING ADSORPTION (PSA) TECHNOLOGY
The separation of impurities of the biogas is based on the adsorption of the different molecules in a solid surface as per the molecular size. This method is particularly used to remove N2, O2, CO2, and water vapour present in raw biogas. The purification process is completed in four sequential steps, viz., adsorption, depressurization, desorption, and pressurization. The dry biogas (after moisture removal) enters at the bottom of the adsorption column, where the impurities are adsorbed, and CH4 leaves at the top. When the adsorbent material gets fully saturated the pressure is released to desorb the CO2, and the raw biogas is sent to another vessel, in which the regeneration is already done. The CO2 is sent to the off-stream channel after desorption. Pressurization is done by equalizing the pressure with the depressurizing vessel. Activated carbon, activated charcoal, natural and synthetic zeolites or alumina, synthetic resins, silica gel, carbon molecular sieve, etc., are commonly used as an adsorbent material. This technique employs biogas purification up to 96%–98% methane concentration with 2%–4% CH4 loss.
POTENTIAL OF RAW MATERIALS FOR CBG
Biogas can be produced using any biodegradable biomass. India has a huge potential of these biodegradable materials for the production of CBG.
AN OVERVIEW OF CBG PLANTS IN INDIA
In India, researches on water scrubbing and PSA-based purification systems are common. Presently, there are 12 commercial CBG plants installed in the country with a cumulative CBG production capacity of 18,461.7 tonnes per year, which is only 0.06% of the total potential.
ENERGY EXPENDITURE AND ECONOMICS OF THE DIFFERENT CBG TECHNOLOGIES
A comparative study of the different purification technologies respective to the cost of biogas purification (`m-3), the maintenance cost of the plant (` year-1), energy expenditure for purification (kWh Nm-3 enriched gas), and purification efficiency (%). Amongst the various biogas purification technologies, water scrubbing technology is found to be most suitable concerning cost competitiveness, power consumption, and ease of operation.
Presently, 32 million tonnes of CBG potential is estimated in the country. Though, of the total estimated potential, only 0.06% CBG is being produced currently on an annual basis. The research on biogas purification and its utilization as a vehicular fuel and power production are getting more concern from the government bodies. Huge scope of setting up of CBG plants is available in the country.
Looking forward to this, on October 1, 2018, Shri Dharmendra Pradhan, Union Minister of Petroleum and Natural Gas, Government of India, announced 5,000 Compressed Biogas (CBG) plants would be set up by 2025 with an investment of `1700 billion under the scheme of “Sustainable Alternative Towards Affordable Transportation (SATAT)”. The scheme will open up a new window of green energy corridor in the transport sector and employment as well.
Mr Himanshu Kumar, Prof. V K Vijay, and Dr Ram Chandra, Centre for Rural Development and Technology, IIT Delhi; and Prof. P M V Subbarao, Department of Mechanical Engineering, IIT Delhi, India