Process Technology
A CBG facility is a chain, and its output is set by its weakest link. This is how the chain works, what each stage is actually doing, and where the engineering decisions matter most.
Incoming biomass is weighed, screened for contaminants and characterised for total solids, volatile solids and, where relevant, carbon-to-nitrogen ratio. Fibrous material is shredded to reduce particle size and increase the surface area available to the microbial population. Slurry is made up to a target solids concentration so the digester receives a consistent feed rather than a variable one.
Target 8–12% total solids for wet digestion
The prepared feed enters a digester — a CSTR or a plug flow reactor, chosen against the feedstock — where, in the absence of oxygen, four microbial stages run in sequence: hydrolysis, acidogenesis, acetogenesis and methanogenesis. Temperature, mixing and organic loading rate are held stable because methanogens are the slowest and most sensitive population in the chain, and everything downstream depends on them.
Mesophilic 35–40°C · 25–40 day hydraulic retention
Raw biogas carries hydrogen sulphide, which is corrosive to compressors, engines and pipework and poisons downstream adsorbents. It is removed biologically through controlled micro-aeration or biotrickling, and polished with iron-oxide media or activated carbon. This step protects every asset that follows it and is not somewhere to economise.
Reduce H₂S to single-digit ppm before upgrading
Gas leaving the digester is saturated with water vapour. Moisture is knocked out by cooling and condensation, then reduced further by chillers or adsorption drying. Dry gas prevents acid formation, protects adsorbent beds and membranes, and keeps water out of the high-pressure system.
Chilling and adsorption drying to a low dew point
Carbon dioxide is separated from methane to lift the gas from roughly 55–65% methane to above 90%, the point at which it becomes a vehicle-grade and grid-injectable fuel. Three routes are in common use — water scrubbing, pressure swing adsorption and membrane separation — and the right one depends on scale, utilities, and the tolerance for methane slip.
Upgrade to >90% CH₄, per IS 16087 for CBG
Upgraded gas is compressed in stages and filled into a cascade for cylinder loading, or metered into a city gas distribution network where a connection exists. Digestate leaves the other end of the plant as fermented organic manure — solid and liquid — for return to the catchment that supplied the feedstock.
Cascade filling at approximately 200–250 bar
Stage 02 in detail
Wet feedstock and dry feedstock do not belong in the same tank. The choice between CSTR and plug flow is made against what is actually arriving at the gate, not house preference.
A continuously stirred tank reactor holds a homogeneous, mechanically or gas-mixed slurry at wet solids concentration, giving the microbial population uniform conditions throughout the tank.
Best suited to
Feed moves in sequence along an elongated, largely unmixed vessel — a dry fermentation process suited to high-solids material that a CSTR cannot pump or stir efficiently.
Best suited to
Stage 05 in detail
There is no universally correct upgrading technology. There is only the one that suits your scale, your utilities and how much methane you can afford to lose out of the tail gas.
CO₂ is far more soluble in water than methane under pressure. Gas is contacted with water counter-current in a packed column; CO₂ dissolves and is stripped off in a flash and regeneration stage.
Strengths
Trade-offs
CO₂ is adsorbed onto a carbon molecular sieve at elevated pressure and released when the pressure is dropped. Multiple columns cycle in sequence so the output stream is continuous.
Strengths
Trade-offs
Hollow-fibre polymeric membranes let CO₂ permeate faster than methane. Staging the modules and recycling the permeate keeps methane losses down while reaching the required purity.
Strengths
Trade-offs
Operating parameters quoted across this page are indicative industry ranges. Project figures are confirmed against feedstock characterisation and detailed engineering.
Core Equipment
Continuously stirred tank or plug flow reactor with mechanical or gas mixing, insulation and heating loop. Sized against organic loading rate and selected against feedstock, not volume alone.
Biological and media-based H₂S removal with polishing, specified to the actual sulphur load of the feedstock rather than a nominal figure.
Buffer storage to decouple production from upgrading, with an enclosed flare for safe disposal during upset or maintenance.
Water scrubber, PSA train or membrane cascade, selected against scale, utilities and permissible methane slip.
Multi-stage compression with intercooling, high-pressure storage cascade and dispensing or grid metering.
Separation into solid and liquid fractions, drying and packing for fermented organic manure sale.
Special case
Municipal solid waste is the most politically attractive feedstock and the most operationally difficult. The organic fraction digests well; everything mixed in with it does not.
Segregation quality at source is the single variable that decides whether an MSW plant works. Where segregation is unreliable, front-end separation has to be engineered heavily — and the digestate is harder to sell as manure, which removes a revenue line the wet-feedstock case depends on.
We will build MSW-based plants, and we will tell you before we start exactly which municipal commitments the business case is resting on. It is a solvable problem, but not one to be optimistic about in a feasibility study.
Technical enquiry
Send a feedstock description and an approximate tonnage. We will come back with an indicative technology route and the questions that need answering before it can be firmed up.