Landfill Biogas. What a Load of Rubbish!

In 2013 the UK disposed of just under 39 million tonnes of waste into landfills (1). Although current waste regulations, such as the Landfill Directive (1999/31/EC), strictly control what type and volume of waste is disposed, landfills can have a negative environmental impact, especially if they start to produce greenhouse gases as the natural decomposition of organic waste occurs.

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Any organic matter which is deposited in landfills will begin to decompose as a result of anaerobic microbial action. A by-product of this process is the production of gas) (2). These two gasses contribute to the problem of global warming by increasing the strength of the atmospheric greenhouse effect. Methane (CH4) also has a potential explosive risk if sufficient quantities of gas build up over time and are unintentionally ignited. As a result, landfills which produce LFG have either passive vents to let gas escape or collection systems with flame stacks installed to burn off LFG.

LFG is collected by the use of tailored gas collection systems. A common gas collection system is composed of a series of gas collection wells placed throughout a landfill. As LFG is produced, collection wells offer a pathway of the least resistance (preferred pathway) for the gas to migrate. The spacing and number of the wells within a LFG collection system is dictated by site specific landfill characteristics, such as the area and volume of fill material, waste depth and density (3). To ensure continued operation and protection against system failure, LFG collection systems include extra collection wells in case one fails.

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There are two main types of collection systems, either a passive or an active collection system. A passive gas collection systems utilises variations in LFG pressure and concentration to vent and release landfill gas, into the atmosphere or a control system, through a series of collection and extraction wells (3). A passive collection system may also include horizontal wells located below the ground level to serve as conduits for gas movement within the landfill.

Landfill biogas 01Active gas collection systems also include vertical and horizontal gas collection wells similar to passive systems. However, the wells in an active collection system use valves to regulate gas flow through the system, allowing the operator to sample gas composition, pressure and manipulate LFG generation (2). Active LFG collection systems can also include gas boosters or pumps to increase/decrease gas flow to control LFG before it reaches the flare.

However, instead of wasting this negatively perceived by-product, LFG can be drawn off and converted into useable energy. Once collected, landfill gas can be processed and converted into Biogas, which then can be burned directly in a gas boiler to produce heat or burned in a combined heat and power (CHP) unit to produce heat and electricity (3). These specialist CHP units can be easily installed at landfill sites and the electrical energy produced sold directly back to the National Grid. The proximity of landfill sites to populated areas enables efficient energy transfer with low levels of transmission loss, thus easing grid capacity problems.

There are many companies producing electricity using LFG in the UK. Biffa operates over 100MW of generation capacity at 34 operational sites throughout the UK that utilise landfill gas to generate electricity (4), whilst Infinis, the sixth largest UK supplier of renewable energy, produces an aggregate generating capacity of 301MW, at 121 LFG electricity power sites across the UK (5).

Alternatively, the biogas used in LFG electrical generation systems can be cleaned to remove carbon dioxide (CO2) and other substances, to produce biomethane. Once upgraded to impart certain burn characteristics and odourised with a suitable stenching agent, biomethane can be injected into the national gas grid and used in the same way as natural gas (6). Biomethane can also be used as a fuel for vehicles, especially for vehicles which already run on fossil fuel produced methane. Changing to renewable biomethane can lead to a reduction in vehicular emissions and increased sustainability for transport and logistics operations.

Biogas and biomethane can also be used as an alternative fuel to power economic growth and as a tool to improve energy security. As a result, LFG has important part to play in creating renewable energy sources for the future, whilst also contributing to a reduction in greenhouse gas emissions.

References

  1. Department of Environment Food & Rural Affairs, DEFRA. (2015)UK Statistics on Waste. Available at: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/487916/UK_Statistics_on_Waste_statistical_notice_15_12_2015_update_f2.pdf (Accessed: 02 July 2016).
  2. Environment Agency (2009)Control of landfill gas containing low concentrations of methane – Science Report – SC030305/SR2. Edited by Environment Agency: Pg 10. Available at: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/291031/scho1008bouu-e-e.pdf
  3. Environment Agency (2004)Guidance on the management of landfill gas. Available at: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/321606/LFTGN03.pdf (Accessed: 02 July 2016).
  4. Biffa (2016)Gas to Energy. Available at: https://www.biffa.co.uk/about-us/operational-infrastructure/gas-to-energy/ (Accessed: 06 July 2016).
  5. Infinis (2016)Landfill gas (LFG), Our operations. Available at: http://www.infinis.com/our-business/LFG/our-operations/ (Accessed: 29 June 2016).
  6. IGEM News (2014)Injecting biomethane into the UK’s gas grid: The challenge of siloxanes. Available at: http://igem.org.uk/media/332466/injecting%20biomethane%20into%20the%20uk’s%20gas%20grid%20-%20the%20challenge%20of%20siloxanes.pdf (Accessed: 02 July 2016).

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