Taming of the beast: exploiting landfill gas

In the UK the exploitation of landfill gas as an energy source has been extremely successful. With a well designed and managed extraction system proving profitable in a matter of weeks, nearly all landfill gas extracted in the UK is now used to generate electricity for export to the national grid. More countries can learn from the UK’s experience. by Åsa Kolmert Strickland Since the beginning of the industrial revolution, the UK has exploited its natural resources by quarrying and open cast mining. Once depleted of resources, the pot holed countryside left behind was often converted to landfill for waste management. While landfilling has continued to this day, one thing that has changed is the awareness of the landfill gas: as a risk to the environment, to human health and in recent years to the atmosphere - but also as energy carrier and hence resource. Financial Incentives In the UK, nearly all landfill gas that is extracted is used for electricity generation, usually for export to the national grid. Through a combination of Renewables Obligation Certificates and Climate Change Levy Exemption for Renewables Certificates, a typical 1 MW landfill gas engine can earn £1000 to £1500 per day, on top of the price for the electricity. Installation of the infrastructure for a typical gas extraction system for a 1 ha landfill cell is £25,000 to £40,000, meaning the investment payback time is counted in weeks. Landfill gas well head Additionally, earlier this year, the UK Government announced its support for biomethane injection into the grid as part of the Renewable Heat Incentive (RHI). This sets out financial incentives for the generation of renewable heat by households, communities and industry. The RHI will be a world first. Designing a Gas Extraction System A landfill gas extraction system consists of vertical extraction wells (perforated pipes that penetrate the waste mass), connection pipes, and the gas plant. ‘Blowers’ which exercise a negative pressure on the gas field, and hence ‘sucking’ out the landfill gas, and condensate removal facilities are also important elements in gas extraction systems. Installation of a gas extraction system should ideally take place in a phased manner and match the methane generating capacity of the waste at all times. When the oxygen within the waste mass is depleted, the waste degradation enters the anaerobic phase. Conditions become favorable for methanogenic bacteria and methane production starts. Gas Well enclosure on closed landfill Landfills are live and changing beasts, and gas extraction system must be designed to adapt to the changing environment. Not only is the correct design important for the longevity of the system, but its careful installation and management will increase the yield. Before designing a landfill gas extraction system, it is important to estimate the gas generation rates. There are several software models available for this purpose. The one most often used in the UK is GasSim which is approved by the UK Environment Agency. The design of a landfill gas extraction system should consider factors such as landfill geometry, waste tipping operations, phasing of engineering (such as capping) and predicted yield. The sizing of pipes should be based on flow calculations, and the spacing of wells on an estimation of the zones of influence. The spacing of landfill gas wells will depend on factors such as waste depth and a gas well diameter. Commonly 40 metre spacing is used for ‘standard’ gas extraction wells, and eight metre for shallower pin wells. Condensate management is crucial to the efficiency of a landfill gas extraction system. Knowledge of leachate levels and waste moisture content in the waste mass is also helpful. To avoid problems with condensate blockages, pipes should be laid to adequate falls. The fall required varies depending on where the pipe is laid and the direction of the gas flow, from a minimum of 1 in 25 to a minimum of 1 in a 100. The pipe dimension should also be considered when the fall is calculated. To ensure long-term performance, the system must be installed to a good standard and in accordance with a construction plan. Drilling contractors with experience of drilling in waste should be used. Pipe sections should be joined using welded (butt or fusion) joints and the integrity of the welds should be checked by pressure testing. When the installation is completed, a survey of the installed system should be undertaken for future record. Landfills change as the waste degrades and settles. Waste settlement and movement can cause condensate blockages, bent well heads and leakage. Regular audits are therefore important to ensure that gas extraction is optimized at all times. Key to success The extraction and management of landfill gas is necessary to control the environmental impact and the greenhouse gas emissions in any landfill that accepts biodegradable waste. In many countries this is a requirement driven by legislation. The methane rich landfill gas is also an energy resource which has significant economic value. The UK market has gained useful experience over the years and landfill gas is being efficiently extracted at many landfills around the country. The key to success, reduced environmental impact and optimized revenue for the operator – is a well designed and managed gas extraction system. Asa Kolmert Strickland, associate, Caulmert Ltde-mail: AsaStrickland@caulmert.com More Waste Management World ArticlesWaste Management World Issue Archives