CBG process overview
Compressed Biogas (CBG) has a calorific value and properties similar to CNG, so it can be used as a green, renewable automotive and industrial fuel. A CBG plant is a biological plant first and a gas-processing plant second. If the microbes in the digester are not given consistent, well-prepared feed, no purifier or compressor downstream can fix the output.
- Feedstock
- Pretreatment
- Hydrolysis
- Anaerobic digestion
- Raw biogas
- Purification
- Upgradation
- Quality control
- Compression
- CBG
Step-by-step CBG plant process
1. Feedstock receiving & storage
Bales of paddy straw, harvested Napier grass, cattle dung, press mud or organic waste are received, weighed and stored. For seasonal residues like paddy straw, the plant needs a year-round storage and aggregation plan.
2. Pretreatment & size reduction
Lignocellulosic biomass such as straw and grass has long, tough fibres that resist microbial attack and can form floating layers in digesters. A typical pretreatment line for agri-residue CBG is:
- Bale handling
- Shredding
- Hammer mill
- Twin-screw shearing
- Slurry preparation
Mechanical shearing opens fibres and increases accessible surface area. Read more about biomass pretreatment.
3. Hydrolysis & feeding
Prepared biomass is mixed with water or recirculated digestate in a pretreatment/hydrolysis tank and fed to the digester at a controlled organic loading rate.
4. Anaerobic digestion
In CSTR digesters, microorganisms break down biodegradable organic matter in the absence of oxygen to produce raw biogas, mainly methane (CH₄) and carbon dioxide (CO₂). Agitators, heating, retention time and loading rate are designed around the feedstock.
5. Gas storage
Raw biogas is buffered in gas holders. Biovardhan integrates AMOCO membrane gas holders, as Authorized Channel Partner for India.
6. Purification & upgrading
Raw biogas goes through several cleaning stages before it can be sold as fuel:
- Moisture knock-out & cooling
- H₂S removal
- Activated-carbon polishing
- CO₂ separation
- Gas drying
- Quality analysis
- H₂S removal: hydrogen sulphide is toxic and corrodes compressors and cylinders. Depending on H₂S load, it is removed by biological desulphurisation, iron-based adsorption media, caustic scrubbing or a combination, with activated carbon as a final polishing step.
- Biological caustic scrubbers wash H₂S out with a mildly alkaline liquid. Bacteria then oxidise the captured sulphide to elemental sulphur and regenerate the caustic, which keeps chemical cost low for high-H₂S gas.
- Activated-carbon polishing works best when the gas is first cooled to drop out water, then slightly re-heated to a controlled relative humidity (about 60–70%) with a small, controlled oxygen dose.
- CO₂ separation: this raises methane purity to fuel grade, by water scrubbing, PSA or membranes (compared below).
- Gas drying: desiccant dryers bring the moisture dew point very low, so no water condenses in high-pressure cylinders.
- Quality control: online CH₄/CO₂/H₂S analysers plus periodic gas-chromatograph testing confirm the gas meets the BIS IS 16087:2025 specification for biomethane / CBG required by oil marketing companies.
7. Compression, odorisation & cascade filling
Dry, upgraded gas is boosted by reciprocating compressors, typically to around 250 bar, and filled into cascades of cylinders for dispensing or transport to fuel stations, or supplied to the CGD network where applicable. An odorant (mercaptan) is added for leak detection, and the filling station is licensed by PESO. More on compression & cascades.
8. Digestate management
Digestate is dewatered. The solid fraction becomes Fermented Organic Manure (FOM) and the liquid fraction can be processed into LFOM. See digestate value-addition solutions.
The biology inside the digester
Anaerobic digestion is a chain of four microbial steps. All of them must stay in balance for stable gas production:
| Stage | What happens | What can go wrong |
|---|---|---|
| 1. Hydrolysis | Enzymes break complex carbohydrates, proteins and fats into soluble sugars, amino acids and fatty acids | Slow for fibrous biomass, which is why pretreatment matters for straw and grass |
| 2. Acidogenesis | Acid-forming bacteria convert these into volatile fatty acids (VFAs), alcohols, H₂ and CO₂ | Overfeeding produces acids faster than they are consumed |
| 3. Acetogenesis | VFAs are converted mainly into acetic acid, hydrogen and CO₂ | Sensitive to hydrogen build-up |
| 4. Methanogenesis | Methanogenic archaea convert acetate and H₂/CO₂ into methane | The slowest, most sensitive microbes; pH or temperature shocks and ammonia inhibit them |
Key control parameters: mesophilic temperature (about 35–40 °C) held steady, pH close to neutral, a healthy VFA-to-alkalinity ratio, controlled organic loading rate, enough hydraulic retention time for the feedstock, and continuous mixing to prevent settling and scum. Biovardhan designs heating, agitation and monitoring around these parameters, and trains operators to read them daily.
Biogas upgrading technologies compared
| Technology | How it works | Typical considerations |
|---|---|---|
| Water scrubbing | CO₂ dissolves in pressurised water more readily than methane | Simple and robust; needs water and power for pumping and compression |
| PSA (pressure swing adsorption) | Adsorbent material captures CO₂ under pressure | Compact; needs good pre-cleaning of H₂S and moisture |
| Membrane separation | Selective membranes let CO₂ permeate faster than methane | Modular and scalable; staged designs control methane slip |
The right choice depends on capacity, power tariff, output specification and O&M capability. We evaluate the options in the DPR.
CBG feedstocks: what works and what to watch
| Feedstock | Strengths | Engineering watch-outs |
|---|---|---|
| Paddy straw / parali | Abundant in Punjab, Haryana, UP, MP and Rajasthan; reduces stubble burning | Seasonal; needs baling, storage, shredding and shearing; silica and lignin content |
| Napier grass | High-yield energy crop, multiple cuts a year, dependable supply | Needs land and cultivation planning; fibre opening matters |
| Cattle dung | Stable, buffered digestion; good inoculum for co-digestion | Lower gas yield per tonne; collection logistics from dairies and gaushalas |
| Press mud | Good biogas potential; concentrated at sugar mills | Seasonal with crushing season; wax and grit content |
| Food / mandi / MSW organic | High biodegradability; tipping-fee potential | Needs segregation; contaminants; risk of acidification |
| Mustard straw, wheat straw, sugarcane trash | Regional residues that diversify supply | Same fibre challenges as paddy straw; feedstock-wise trials advised |
| Distillery spent wash & agro-industrial effluents | Liquid, pumpable and available year-round at distilleries and food plants | High-strength and variable; needs effluent-specific digester design and pH control |
| Poultry litter | Nutrient-rich; useful as a co-substrate | High nitrogen can cause ammonia inhibition; grit and feathers need removal |
How much of each feedstock do you need for one kilogram of CBG? See our feedstock-to-CBG yield guide & calculator.
Pretreatment route by feedstock
Each feedstock enters the digester by a different route. Choosing the wrong one is a common cause of under-performing plants.
| Feedstock group | Typical pretreatment route |
|---|---|
| Crop residues & energy grass (paddy/wheat/mustard straw, Napier grass, sugarcane trash, maize stalk) | Bale opening / shredding → hammer milling → twin-screw shearing → mixing with water or recirculated digestate → hydrolysis tank (optionally heated, with about 1–2 days' residence) → digester |
| Manures & sludges (cattle dung, gaushala waste, press mud, poultry litter) | Receiving pit → screening of stones, sand and fibres → mixing / homogenisation → slurry feeding → digester |
| Municipal & market organic waste | Receiving → bag opening → sorting and removal of plastics, metals and inerts (magnetic, size and density separation) → size reduction → pulping → digester |
| Liquid effluents (spent wash, food-processing effluent) | Equalisation → pH and temperature conditioning → controlled pumping → effluent-specific digester |
Why CBG plants need standardisation
Today many CBG plants are designed according to the individual methodology of each EPC company. Plants processing similar feedstocks can end up with very different configurations and results. Biovardhan is working towards feedstock-specific, scientifically validated standard operating frameworks, with its in-house panel of scientists and technical experts:
- Data collection
- Process optimisation
- Standard guidelines
- Pilot validation
- Commercial deployment
This covers feedstock characterisation, particle-size optimisation, retention time and organic loading, gas-yield validation and digestate value addition. We welcome collaboration with plant owners, consultants and research institutions.
Frequently asked questions
Feedstock is received and pretreated (shredded, sheared, mixed into slurry), then digested anaerobically in digesters such as CSTRs where microbes produce raw biogas (mainly methane and CO₂). The raw biogas is stored, cleaned of H₂S and moisture, upgraded by removing CO₂ to raise methane content, and compressed into CBG. Digestate is dewatered and processed into FOM or LFOM.
Biogas is the raw gas from anaerobic digestion, typically a mix of methane and carbon dioxide with traces of H₂S and moisture. When it is purified to a high methane content and compressed, it is called Compressed Biogas (CBG). Bio-CNG is the same product, a renewable substitute for fossil CNG.
A Continuous Stirred Tank Reactor (CSTR) is a digester where feedstock is fed continuously and the contents are kept mixed by agitators, giving uniform temperature and contact between microbes and organic matter. It is widely used for CBG plants handling slurry-type feed.
Water scrubbing, pressure swing adsorption (PSA) and membrane separation are the common CO₂ removal routes. The best choice depends on plant size, power cost, methane-slip limits and output specification. We select and integrate the upgrading route for each project.
Yes. Co-digestion, for example cattle dung with paddy straw or Napier grass, is common and often improves stability. The pretreatment line and digester loading must be designed for the full feedstock mix.