Biomethane: Overview, Production and Feedstocks

Biomethane: Overview, Production and Feedstocks

Biomethane is a renewable, high-purity form of methane gas produced from the anaerobic digestion or thermal gasification of organic matter. It is chemically identical to fossil natural gas (CH₄) and can be used as a direct replacement in gas grids, heating systems, or as a vehicle fuel. Unlike fossil gas, however, biomethane is derived from biological sources, making it a low-carbon or potentially carbon-neutral alternative.

Production of Biomethane

Biomethane is primarily produced through two main processes:

1. Anaerobic Digestion (AD)
   This is the most common method in the UK. Anaerobic digestion involves breaking down organic material in the absence of oxygen using naturally occurring microorganisms. The process produces biogas, a mixture typically consisting of 50–65% methane and 35–50% carbon dioxide, with trace impurities (such as hydrogen sulphide, ammonia, and water vapour).

   To produce biomethane, the raw biogas must be upgraded to remove CO₂ and contaminants. This results in a gas that is typically over 97% methane and suitable for injection into the natural gas grid or for use as a compressed natural gas (CNG) vehicle fuel.

2. Thermal Gasification
   This less common method involves heating dry biomass (such as wood chips or agricultural residues) at high temperatures in a low-oxygen environment to produce syngas – a mix of hydrogen, carbon monoxide, and some methane. Through a subsequent methanation process, this syngas is converted into biomethane. Although more complex and less commercially established than AD, gasification opens up additional feedstock options and may become more prevalent in the future.

Common Feedstocks

Biomethane can be derived from a wide range of organic materials, many of which are waste products. Typical feedstocks include:

• Agricultural Waste
  Livestock manure, slurry, and crop residues such as straw or maize silage are frequently used. These feedstocks are abundant in rural areas and can be locally sourced.

• Food Waste
  Both pre-consumer (from food processing and retail) and post-consumer (from households and catering) food waste can be digested. This not only produces energy but diverts biodegradable waste from landfill, reducing methane emissions from uncontrolled decomposition.

• Sewage Sludge
  Wastewater treatment facilities often use anaerobic digestion to process sewage sludge. The resulting biogas can be upgraded to biomethane or used to generate electricity and heat.

• Energy Crops
  Purpose-grown crops like maize or ryegrass are used in some cases, although their use is increasingly controversial due to land use and sustainability concerns. In the UK, the trend is shifting toward waste-based feedstocks where possible.

• Industrial Organic Waste
  Waste from breweries, dairies, food manufacturers, and other industries can serve as feedstock, offering both a disposal solution and energy recovery.

Sustainability and Benefits

Biomethane offers several environmental and strategic benefits:

• It significantly reduces greenhouse gas emissions compared to fossil gas, especially when made from waste.

• It supports the circular economy by recovering value from organic waste.

• It enhances energy security by reducing reliance on imported fossil fuels.

• When used as a transport fuel (especially in HGVs), biomethane offers immediate carbon and air quality benefits compared to diesel.

Biomethane is recognised by the UK government as a key component in the transition to net zero, particularly in hard-to-decarbonise sectors like heavy transport and off-grid heating.