In short. Biomethane is biogas cleaned and upgraded to remove most of its carbon dioxide, hydrogen sulphide and moisture, leaving a gas of around 90 percent methane or more that behaves much like pipeline natural gas. Upgrading lets the fuel be compressed, injected into a gas grid or burned in a standard gas engine, but it costs energy and capital. For on-site electricity alone, treated raw biogas is usually the better route.
From biogas to biomethane
Raw biogas leaves a digester as a wet mixture of roughly 55 to 65 percent methane and 35 to 45 percent carbon dioxide with hydrogen sulphide and trace contaminants. Cleaning removes the hydrogen sulphide, water and particulates. Upgrading then separates the carbon dioxide, raising the methane content and with it the heating value to around 32 to 36 MJ/Nm³, close to pipeline gas. The product is called biomethane, renewable natural gas or, when compressed for transport and vehicle use in India, compressed biogas or CBG.
The Indian specification for biogas and biomethane intended for piped and automotive use is IS 16087, which sets limits on methane content, hydrogen sulphide, moisture, oxygen and other constituents. A project supplying gas to a city gas network or to a CBG offtaker will be held to that standard or to the offtaker’s own. A project burning the gas in its own engine is held to the engine manufacturer’s fuel specification, which may be less or more demanding on particular constituents. The two are not the same document, and a gas that meets one should still be checked against the other.
Upgrading technologies compared
| Technology | How it separates CO2 | Typical methane purity | Points to note |
|---|---|---|---|
| Pressurised water scrubbing | CO2 and H2S dissolve in water at pressure and are released in a stripper | 95 to 98 percent | Robust and common; needs water circulation and compression power; product gas is saturated and must be dried |
| Pressure swing adsorption | Adsorbent beds capture CO2 at pressure and release it on depressurisation | 95 to 98 percent | Needs H2S and moisture removed first; several beds cycling; moderate methane slip |
| Membrane separation | CO2 permeates polymer membranes faster than methane | 95 to 99 percent in multi-stage units | Compact, few moving parts, suited to small plants; purity and recovery trade against stages |
| Chemical (amine) scrubbing | CO2 reacts with an amine solution and is driven off by heat | Above 98 percent | Highest purity and low methane slip; needs heat for regeneration, which a CHP engine can supply |
Whichever method is used, the upgrading plant has a parasitic electrical demand, typically a few tenths of a kWh per normal cubic metre of raw gas, and loses a small percentage of the methane as slip. Both must be charged against the project’s energy balance.
Where the gas should go
The decision to upgrade is really a decision about the fuel’s best use. The question we put to a client is whether electricity, heat, pipeline gas or vehicle fuel is worth most at their site, and what infrastructure each pathway needs.
- Local generation on raw treated biogas in a purpose-built engine, described under biogas generators, avoids the upgrading plant entirely and uses the engine’s jacket heat for the digester. This is the simplest route when the plant needs the electricity itself.
- Local generation on biomethane allows a standard natural gas generator to be used, gives more stable combustion and full engine rating, and keeps the option of selling surplus gas. It only makes sense when upgrading is being built for another reason.
- Injection into a city gas distribution network or supply to a CBG offtaker under a long-term agreement converts the gas into revenue without any generating plant, and is the model the national CBG programme encourages.
- Compression and storage for vehicle fleets or for a remote generator moves the fuel to where it is needed, at the cost of compression energy and cylinder logistics.
None of these pathways is automatically available. Grid injection depends on a network being within reach and willing to accept the gas; CBG offtake depends on a contract; local generation depends on a load and a DISCOM interface.
Running a generator on biomethane
Once upgraded, biomethane can usually be used in engines approved for natural gas, but three checks remain. The first is methane number and heating value: biomethane is often purer in methane than pipeline gas, which actually raises the methane number and knock resistance, yet it may be slightly lower in heating value than a pipeline gas containing ethane and propane, so fuel flow and gas train sizing should use the real figures. The second is trace contaminants: residual hydrogen sulphide, oxygen from air dosing and siloxanes that slipped through must be below the engine manufacturer’s limits, with analyser records to prove it. The third is pressure and temperature: upgraded gas is often delivered at elevated pressure and must be regulated down to the engine’s inlet range with the regulator sized for the engine’s full flow.
The energy arithmetic is the same as for any gas engine. Worked example (illustrative): a plant produces 400 Nm³/h of raw biogas at 60 percent methane and upgrades it with 2 percent methane slip to 235 Nm³/h of 98 percent biomethane. The fuel energy in the product gas is 235 × 0.98 × 35.8 = about 8,240 MJ/h. If 60 Nm³/h of that biomethane feeds a natural gas genset at 38 percent electrical efficiency, the output is 60 × 0.98 × 35.8 × 0.38 / 3.6 = about 222 kW, and the remaining 175 Nm³/h is available for compression and sale. Upgrading power, drying and compression are deducted from the plant’s net electricity.
Fuel security and fallback
A biomethane-fed generator depends on the digester, the upgrading plant and the gas storage all working. Feedstock shortages, a scrubber fault or an analyser trip can stop gas supply with little warning. Sites that rely on the generator for essential loads should hold either a pipeline gas connection, an approved alternative gas such as LPG where the engine permits it, or a diesel set as the backstop, and should size gas storage to ride through at least the normal maintenance interruptions of the upgrading plant. The resilience logic is the same as for any single-source fuel supply described in the gas power systems overview.
Common mistakes
- Building an upgrading plant to feed a generator that could have run on treated raw biogas. The project carries capital and parasitic load that return nothing.
- Assuming any natural gas engine will accept the product gas without checking trace contaminants. Oxygen or residual siloxanes exceed the engine limit and warranty cover is lost.
- Taking the upgrading vendor’s methane recovery at face value. Real slip and off-spec periods reduce saleable gas and the business case with it.
- Promising a fixed emissions reduction without a defined boundary. Regulators and auditors ask for the method, and a bare percentage does not survive the question.
What to send us
Send the raw biogas production record and analysis, the intended gas pathway with any offtake or grid injection discussions, the upgrading vendor’s product gas specification if one exists, and the electrical and thermal loads on site. Include the DISCOM connection details, photographs of the digester and gas holder area, and the nameplate of any existing generator. Submit them through the application engineering enquiry and we will advise whether local generation, upgrading or a combination fits your gas and your loads.
Questions engineers ask
What is the difference between biogas and biomethane?
Biogas is the raw digester output of roughly 55 to 65 percent methane with carbon dioxide, moisture and hydrogen sulphide. Biomethane is biogas that has been cleaned and upgraded to around 90 percent methane or more by removing most of the carbon dioxide and contaminants, giving it a heating value and combustion behaviour close to pipeline natural gas.
Can a natural gas generator run on biomethane?
Usually yes, provided the biomethane meets the engine manufacturer’s fuel specification for methane content, heating value, hydrogen sulphide, oxygen, siloxanes and moisture, and is delivered at the required pressure. The engine rating and settings should be confirmed against an analysis of the actual product gas rather than a nominal specification.
How much energy does upgrading consume?
Water scrubbing, pressure swing adsorption and membrane plants typically draw a few tenths of a kilowatt-hour of electricity per normal cubic metre of raw biogas, mostly for compression, and amine plants need heat for regeneration instead. Over a year this parasitic demand is significant and must be subtracted from any electricity the project claims to produce.
Is biomethane power carbon neutral?
The engine still emits carbon dioxide when it burns biomethane. The emissions benefit arises because the carbon came recently from biomass rather than fossil reserves, and because methane that would otherwise escape is captured. Whether the result is described as low carbon or neutral depends on the accounting boundary, which should always be stated.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.