How is biomethane produced?

Biomethane is an increasingly important source of fuel in the UK. It differs from fossil fuels that are recovered from underground sources in that it is produced from organic matter within a short period of time. The entire process for producing biomethane can take months or years, whereas fossil fuels such as coal and oil take millennia to form, as well as very specific high-pressure, low-oxygen conditions.

In today’s biogas production sites, production of the gas is carefully controlled and monitored over time, and certain techniques are often applied to accelerate the entire process so that the biogas production process is more cost and time efficient. The process starts when organic waste is placed in a sealed environment and the decomposition process is allowed to begin.

Biogas is produced during the anaerobic stage, so before this can happen all the oxygen in the waste must be used by aerobic microorganisms. Depending on the specifics of the site and the composition of the waste, this process can take a year or more. Once the initial aerobic decomposition stage is complete, anaerobic decomposition by methanogens can begin.

These methane producing bacteria devour waste and release methane and carbon dioxide as byproducts. In an average landfill, this stage can last for a decade or more, with peak production of methane taking place after this length of time. Some larger sites can remain productive for 50 years, though most will fall slightly short of this mark.

There are several factors that affect the rate at which a landfill will produce biogas and how long it will remain productive for. Firstly, the composition of the waste will have an impact on production. Tough, cellulosic waste such as garden clippings and other woody waste will decay more slowly than animal waste, for example.

Ambient temperatures also affect the rate of decomposition and therefore the rate of biogas production. Though the exothermic action of decomposition will eventually cause the site to heat up, a higher ambient temperature will kickstart this process by encouraging a more rapid rate of decomposition. Cold temperatures have the opposite effect, in that waste decomposes more slowly in cooler temperatures - though this is eventually negated by exothermic reactions within the decomposing waste.

Humidity also plays a role in decomposition rates at landfill sites. Moisture facilitates rapid decomposition, so sites that are in humid locations or contain a lot of moisture within the waste will begin producing biogas faster than sites that are in drier conditions. The fastest initial decomposition rates, therefore, take place in sites that are warm and humid, and contain waste that is softer and more easily digestible for methanogenic bacteria.

What is feedstock?

In this context, the answer to the question ‘what feedstock is used for biomethane production?’ is pretty much any biodegradable organic matter. While this is often woody material such as trimmings from the manufacturing sector or commercial garden waste, it also includes domestic and commercial food waste and agricultural waste. It is important to note that bacteria find it difficult to break down lignin - the tough component of plant cells that give woody plants their strength - so including high levels of this kind of waste can dramatically slow the rate of decomposition. 

Disadvantages of biogas

As with any form of renewable energy, biogas has its limitations. It's not a viable option in every area, and it requires an abundant supply of crop materials and organic, cellulosic waste. It isn't practical in urban settings, as it's unsuitable for larger-scale production in urban areas as the landfill sites required would take up a lot of space. The best place for biogas production is rural settings, where there is an abundant supply of waste and the site can be kept away from residential areas.

While these may seem like reasons not to rely on biogas as a fuel source, there are ways the issues presented can be overcome. The use of digesters allows the process to be better contained and can be a more efficient use of space and resources. Local sources of domestic food waste are the ideal feedstock for small-scale biogas production, meaning that it would be possible to produce clean, renewable, reliable fuel on a community scale.