The production of bio-CNG mainly takes place in three stages, out of which the first stage is extraction, the second stage is purification and the third stage is compression. Bio CNG is formed after passing through these three stages one after the other.
Stage 1 - Extraction
Bio-gas is produced from waste and bio-mass sources (such as agricultural residues, cattle dung, sugarcane press mud, municipal solid waste and sewerage treatment plant waste and Napier grass) through the process of anaerobic decomposition. Bio-CNG is the pure form of bio-gas. Bio-CNG has similar calorific value and other properties to compressed natural gas (CNG), Thus it can replace CNG in automotive, industrial and commercial areas. Ministry of Road Transport and Highways, Government of India had permitted usage of bio-compressed natural gas (Bio- CNG) for motor vehicles as an alternate composition of the compressed natural gas (CNG).
| IS 16087 : 2016 Standard | ||
|---|---|---|
| S No. | Characteristic | Requirement |
| 1. | Methane (CH4), minimum % 90.0% | 90.0% |
| 2. | Carbon Dioxide (CO2), maximum % | 4% |
| 3. | Carbon Dioxide (CO2) + Nitrogen (N2) + Oxygen (O2), maximum % | 10% |
| 4. | Oxygen (O2), maximum % | 0.5% |
| 5. | Total sulphur (including H2S) mg/m3, maximum % | 20 mg/m3 |
| 6. | Moisture mg/m3, maximum % | 5 mg/m3 |
Stage 2 - Purification
Production of biogas could be a continuous process. The utilization of biogas as an efficient energy source depends strongly on its methane concentration. Therefore, biogas purification is essential in order to have more energy per unit volume of compressed biogas and to get rid of the corrosive effect of H2S. Biogas purification process increases the methane concentration and decreases the carbon dioxide concentration in biogas, which in turn would result in higher calorific value. The purified biogas is compressed as CBG with methane content of more than 90%, and the CBG shall be complied to IS 16087:2016 specifications of BIS.
Some of the technologies which are prevalent in removal of carbon dioxide are Pressure Swing Adsorption (PSA), Vacuum Swing Adsorption (VSA), Water scrubbing, Membrane Separation and Chemical scrubbing - Monoethylammine (MEA) system.
Stage 3- Compression
The purified biogas with more than 90% of methane can be compressed at 250 bar, and transported in gas cylinders (cascades) for the end use. The compressed biogas will be filled and transported through cascades which is a group of high pressure cylindrical vessel. For delivery of CBG, 3000 litres metal cascades or cascades of higher capacity shall be used. Steel cylinder cascades and composite cascades may be used for transportation of CBG. For steel cylinder cascades, IS 7285 shall be applicable. If type-3 or type-4 composite cascades are used, respective BIS specifications like IS 15935 or relevant specifications shall be applicable.
