Biogas Upgrading for CBG: How to Choose the Technology and Meet the Specification

At a glance. Choose a biogas upgrading technology on methane recovery, energy use, pre-treatment needs and how dry and pure the product must be, then design in the analyser and automatic shut-off that prove each batch is on specification. Amine scrubbing has the lowest methane slip but needs heat; water scrubbing, PSA and membranes run on electricity, and three-stage membrane trains reach 99–99.5% methane recovery with a dry product. In India, CBG must meet IS 16087, which sets at least 90% methane and tight limits on CO₂, oxygen, sulphur and moisture.

What does raw biogas contain, and what must CBG contain?

Raw biogas from anaerobic digestion is mostly methane and CO₂, saturated with water, with hydrogen sulphide that varies widely with feedstock, and traces of ammonia and, from landfill and sewage sludge, siloxanes. Upgrading removes CO₂; pre-treatment removes the rest.

For CBG sold through India's SATAT programme, the CBG white paper sets the product to IS 16087:2016 and delivery compressed to 250 bar in cascades:

IS 16087 characteristic	Requirement
Methane (CH₄)	90.0% minimum
Carbon dioxide (CO₂)	4% maximum
CO₂ + N₂ + O₂	10% maximum
Oxygen (O₂)	0.5% maximum
Total sulphur, including H₂S	20 mg/m³ maximum
Moisture	5 mg/m³ maximum

PNGRB's CBG integration paper notes that gas injected into city gas or pipeline networks must also meet the applicable PNGRB regulations, with the more stringent limit applying, and that BIS is revising IS 16087. Check the current edition before freezing a design.

Two limits catch plants out. The moisture limit requires very dry gas, and the combined CO₂ + N₂ + O₂ limit leaves little room for air. Most upgrading processes remove little nitrogen and only part of the oxygen, so air dosed into the digester for biological desulphurisation, or drawn in through leaks, can push an otherwise good plant off specification.

How do the main upgrading technologies compare?

Technology	Methane slip or recovery	Electricity, kWh per Nm³ raw gas	Heat	Operating pressure	H₂S before upgrading
Water scrubbing	About 1% slip	0.20–0.30	None	6–10 bar(a)	Tolerated; makers allow 300–2,500 ppm
Amine scrubbing	0.1% slip guaranteed, 0.06% measured	0.12–0.14	About 0.55 kWh/Nm³ for regeneration	1–2 bar(a) absorber	Removed upstream; design maximum about 300 ppm
Pressure swing adsorption	Median 1.8% loss without off-gas treatment	0.15–0.30	None	6–10 bar(a)	Removed upstream, usually activated carbon
Membranes, two-stage with recycle	98–99% recovery	0.20–0.30	None	6–20 bar(a)	Removed upstream, usually activated carbon
Membranes, three-stage	99–99.5% recovery	0.20–0.30	None	6–20 bar(a)	Removed upstream, usually activated carbon

The figures are from the Swedish Gas Technology Centre's review of commercial upgrading technologies (Bauer and co-workers, SGC report 270, 2013), with the water scrubber slip from Tobias Persson's technical review for IEA Bioenergy Task 37. The same authors summarise the field in Biofuels, Bioproducts and Biorefining. Organic physical scrubbing and cryogenic upgrading also exist but are less common at CBG scale.

Which technology suits which plant?

Amine scrubbing

Amine units give the lowest methane slip and a high-purity product at low pressure, but they need a steady heat supply to regenerate the solvent. They suit sites with surplus steam or hot water, such as distilleries and sugar mills. Oxygen reacts irreversibly with the amine, so landfill gas needs oxygen removal first, and the product gas must be dried.

Water scrubbing

Water scrubbers are simple and tolerate H₂S, but they need make-up water and treatment of the exhaust air, and the product leaves saturated with water. For a 5 mg/m³ moisture limit, a dryer is part of the package.

Pressure swing adsorption

PSA produces a dry gas with no liquids on site but needs H₂S and water removed beforehand. Its off-gas carries the lost methane, which often needs oxidation or recovery.

Membranes

Membrane units are dry, compact and modular, and run without solvents or water. Methane recovery depends on staging: a single stage loses noticeably more methane, while two-stage designs with recycle and three-stage designs that polish both retentate and permeate recover 98–99.5%. The SGC report notes that a membrane plant's product rarely needs a separate dryer. Pre-treatment must remove H₂S, water, compressor oil aerosols and, where present, siloxanes to protect the fibres.

What about the CO₂-rich off-gas?

Every technology produces a CO₂-rich stream carrying the slipped methane. Depending on local rules and economics, it goes to vent, a thermal oxidiser, or CO₂ recovery. Recovered biogenic CO₂ needs compression, drying and quality checks before it can be sold.

How do you prove every batch meets specification?

Laboratory spot checks cannot catch a drifting membrane stage or an air leak between samples. An online analyser on the product gas, measuring methane, CO₂, oxygen, H₂S and moisture, linked to an automatic shut-off valve that diverts off-specification gas before compression, protects both the customer and the plant. SCADA logging of the same data gives an auditable quality record and tracks methane recovery, uptime and energy use.

The sample system matters as much as the analyser. Moisture at a few mg/m³ and H₂S at ppm levels are easily distorted by wet, long or reactive sample lines, so the sample conditioning system should be short, clean and built from suitable materials, with calibration gas connections built in.

Step-by-step selection checklist

1. Measure the raw gas over weeks, not days: methane, CO₂, H₂S, oxygen, nitrogen, ammonia, siloxanes and flow profile.

2. Fix the product specification (IS 16087, PNGRB or the off-taker's) and the delivery pressure, such as 250 bar cascades or pipeline injection.

3. Establish whether surplus heat is available, which favours amine scrubbing, or whether electricity is the only energy input.

4. Set the methane recovery target and decide where the off-gas goes.

5. Design pre-treatment for H₂S, moisture, particles, compressor oil and siloxanes.

6. Choose a desulphurisation method that will not breach the nitrogen and oxygen limits.

7. Size for turndown, because digester output varies with feedstock and season.

8. Specify the online analyser, sample conditioning, automatic shut-off and SCADA.

9. Plan compression, drying where the technology needs it, and cascade filling or grid connection.

10. Factory-test the gas train and controls before dispatch to cut site work.

Frequently asked questions

What is the minimum methane content for CBG in India?

IS 16087:2016 requires at least 90.0% methane by volume. It also limits CO₂ to 4%, CO₂ + N₂ + O₂ to 10%, oxygen to 0.5%, total sulphur to 20 mg/m³ and moisture to 5 mg/m³. BIS is revising the standard, so confirm the current edition.

Which upgrading technology has the lowest methane loss?

Amine scrubbing, with guaranteed slip of about 0.1% and measured values as low as 0.06% in the SGC review. Three-stage membrane systems come next, with methane recovery of about 99–99.5%, and need no heat.

Do membrane upgrading systems need H₂S removal?

Yes. Membranes do not separate H₂S sufficiently and the fibres need protecting, so H₂S is removed upstream, usually with activated carbon or another desulphurisation step, along with water, oil aerosols and siloxanes.

Why does my CBG fail on oxygen or nitrogen?

The usual causes are air dosed into the digester for biological desulphurisation and air drawn in through leaks on the low-pressure side. Upgrading removes CO₂ efficiently but little nitrogen, so the combined CO₂ + N₂ + O₂ limit can be exceeded. Reduce air dosing, fix leaks or change the desulphurisation method.

Does upgraded biogas need drying?

It depends on the technology. Water and amine scrubbers deliver a water-saturated product that needs drying before compression, while PSA and membrane units produce dry gas. Either way, the 5 mg/m³ moisture limit should be verified with an online analyser.

What pressure is CBG compressed to?

Under the SATAT framework, CBG is compressed to 250 bar and supplied to retail outlets in cascades. Gas injected into a city gas or pipeline network is delivered at the pressure that network requires.

How Tech Inc. builds biogas upgrading and CBG trains

Our biogas upgrading and CBG gas train is a three-stage hollow-fibre membrane system in 250, 500 and 1,000 Nm³/h sizes. It runs from the raw gas blower and condensate knock-out, through pre-treatment for H₂S, moisture and particles, a membrane feed compressor with aftercooler and the membrane stages, to an online analyser, flow metering and CBG compression to 250 bar. PLC and SCADA with automatic shut-off of off-specification gas are built in. For plants already running, the CBG plant compliance and performance kit retrofits the analyser, sample system, SCADA and shut-off valve. Our wider gas conditioning skids include gas dehydration skids, and our PLC, RTU and SCADA packages handle plant-wide control.

Each train is engineered to the client's raw gas analysis and product specification, then designed, built and factory-tested in our own facilities. Tech Inc. is part of an engineering group founded in 1990, with offices in Canada, India, Estonia and Saudi Arabia, and our equipment is supplied to national oil companies and refineries. For how plant data feeds new-energy analytics, see AI in new energy.

Send your raw gas analysis, flow and product requirements to mail@techincresearch.com or call +1 (289) 622-1032 for a process proposal.