Biomethane: Biogas Upgrading and Grid Injection

Biomethane is biogas that has been upgraded to natural gas quality. Carbon dioxide, hydrogen sulphide and water are removed until the methane content meets grid requirements. The gas can then be injected into the natural gas grid, liquefied or used as a vehicle fuel. For a single farm plant this route is usually too expensive, because upgrading and grid connection only become economic from several hundred cubic metres of raw biogas per hour. For farmers, biomethane becomes interesting mainly in joint projects with raw gas collection pipelines.

From raw biogas to biomethane

Depending on feedstock, raw biogas from agricultural plants contains about 50 to 65 per cent methane. The rest is mostly carbon dioxide, plus water vapour, hydrogen sulphide and traces of other gases. Basic cleaning is enough for power generation in a combined heat and power (CHP) unit. For grid injection, however, the gas must meet the technical rules on gas quality, which in Germany are set mainly in the worksheets of the German gas and water association (DVGW).

Upgrading usually involves four steps: desulphurisation, drying, carbon dioxide removal and conditioning. Conditioning adjusts the calorific value to that of the grid gas, in H-gas grids often by adding small amounts of liquefied petroleum gas. The gas is then compressed, metered, odorised and injected. In many cases, compression, metering and injection are handled by the grid operator.

Upgrading itself also consumes energy. Depending on the technology, it needs electricity for compressors and pumps or heat to regenerate scrubbing solutions. A biomethane plant therefore has its own energy demand, which must appear in the business case just like the investment. The digesters still need heating, which without a CHP unit requires another heat source, such as a biogas-fired boiler.

Upgrading technologies compared

The best choice depends mainly on plant size, available heat and electricity prices. Where plenty of waste heat is available, amine scrubbing can play to its strengths. For small and medium plants, membrane systems have spread widely in recent years because of their modular design. For every technology, methane loss (methane slip) is a key criterion, as methane is a potent greenhouse gas. Permits and support schemes set limits here, and you should ask for guaranteed values in every quote.

Grid injection and grid connection

Injecting biomethane requires a connection to a gas grid with sufficient capacity. In rural distribution grids, summer gas demand is low, so injection is not always possible at all times. Solutions include reverse flow into higher pressure levels or gas storage at the plant.

Connection costs have so far been governed by the Gas Network Access Ordinance (Gasnetzzugangsverordnung, GasNZV): the grid operator bears 75 per cent and the connecting party 25 per cent, capped at 250,000 euros for pipelines up to one kilometre. These rules are in flux as German gas network law is being reorganised. Before making an investment decision, check the current position on the biogas page of the Federal Network Agency and with the relevant grid operator.

Minimum size: why a single farm plant is usually too small

Upgrading, the injection station, compressors and the grid connection carry high fixed costs that only spread out over large gas volumes. In practice, biomethane projects usually become economic only from several hundred standard cubic metres of raw biogas per hour. A small manure plant with a rated output of around 75 kilowatts, by contrast, produces only about 300,000 cubic metres of biogas a year, or roughly 30 to 40 cubic metres per hour. Even a larger farm plant often cannot reach the usual thresholds on its own. On top of that, running costs for electricity, heat, maintenance and metering weigh far more heavily per cubic metre of biomethane when gas volumes are small.

For most livestock farms, generating power in a CHP unit on site is therefore the obvious route; the technology is explained in the guide to CHP units for biogas. Biomethane is worth a closer look where several farms cooperate, or where an existing larger plant is looking for prospects after its payments under the Renewable Energy Sources Act (EEG) end. See the guide to biogas after the EEG.

Farm cooperatives and collection pipelines

One model for smaller farms is joint upgrading. Several farms produce raw biogas in their own digesters and pipe it through a raw gas collection line to a central upgrading plant. Alternatively, they deliver slurry and manure to a shared plant. This creates gas volumes large enough to support upgrading while keeping slurry transport short.

Such projects are organisationally demanding. Points to settle include:

1. Legal structure: often a limited partnership with a limited company as general partner (GmbH & Co. KG) or a cooperative in which the farms hold shares.
2. Pipeline construction: routes, rights of way and the cost per kilometre of raw gas pipeline determine the maximum distance between farms.
3. Gas quality and metering: each farm supplies different volumes and qualities, so billing needs suitable metering.
4. Term and exit: contracts should cover farm closure, succession or the failure of a partner.
5. Permits: raw gas pipelines, upgrading and injection raise their own permitting questions.

An early feasibility study by an independent engineering firm is especially important here. The guide to planning a farm biogas plant outlines the process.

Marketing: grid, bio-CNG and bio-LNG

Injected biomethane is delivered to customers on a balance basis via the gas grid. Typical markets are biomethane CHP units at sites with high heat demand, the heating market and transport. In transport, biomethane can count towards the greenhouse gas reduction quota, which can bring additional revenue depending on quota prices.

Without a grid connection, two routes remain. Bio-CNG is compressed biomethane that can be sold at filling stations for natural gas vehicles or methane-powered farm machinery, or transported in cylinder bundles. Bio-LNG is liquefied biomethane, used mainly in heavy goods transport. Liquefaction is technically demanding and only pays off at larger volumes. For a single farm, both routes remain niches that usually only work with additional customers or within a cooperative. Because revenues depend on quota prices, support conditions and gas prices that can change politically and in the market, any plan should calculate several scenarios. The guide to biogas profitability explains the basics. Suppliers of upgrading and injection technology are listed under biomethane and gas upgrading.