WET News

JANUARY 2015 WET NEWS 19 Electricity generation from biogas is a relatively inefficient process in comparison to direct com- bustion. GENeco took the process a stage further. A solution: Removing biogas impurities G ENeco designed and built a Gas to Grid plant that allows biomethane, produced during food waste and sewage treatment, to be injected directly into the national gas network. The gas is being used to supply 8,300 homes a year – and fuel a bus, known as the Bio-Bus. Every year GENeco treats 75Mm 3 of domestic sewage, 600,000m3 of commer- cial liquid waste and 35,000 tonnes of food waste. Organic matter from these wastes is processed via anaerobic diges- tion (AD), producing biogas comprising approximately 60% methane. The tech- nology of AD is well established and GENeco previously used biogas to gener- ate electricity using combined heat and power (CHP) engines. GENeco recognised electricity genera- tion from biogas is a relatively inefficient process in comparison to direct combus- tion, and the new Gas to Grid plant was developed to remove impurities from the biogas. The end product from the Gas to Grid plant is a gas that's 98% methane, an equivalent composition to natural gas. The first injection of biomethane into the national distribution network took place in November 2014. Digestate In anoxic environments, organic matter in waste breaks down to produce meth- ane. This natural process can be har- vested and optimised via AD to produce biogas – a combustible mixture of gases comprising approximately 60% methane. Every day, 56,000m 3 of biogas is pro- duced at Bristol sewage treatment works from the AD of domestic and commercial wastes. Not only does this practice recover the residual energy within waste, it also diverts huge volumes of waste from landfill. The by-product, a solid digestate, is spread to land as a fertiliser. Upgrading the biogas to biomethane through the Gas to Grid plant produces a versatile biofuel that can be used as a direct substitute for natural gas. Injection of the fuel into the national grid generates revenue for the company. The new ability to upgrade biogas into biomethane means GENeco can recover much more residual energy from waste streams. Maximising power production is to the benefit of both the public, commer- cial customers and the environment. Furthermore, GENeco's Gas to Grid plant is the first in the UK to extract biogas from food waste (unfit for human consumption), in addition to sewage sludge. The former feedstock is key to the operation's continued success, as food waste offers double the gas yield in com- parison to sewage sludge; digested mate- rial from food waste recycling is currently fuelling a third of the gas output from the plant. GENeco is already looking to expand on the success of the initial project and extend Gas to Grid production to its other sewage treatment sites across the South West region. The Gas to Grid plant actively diverts huge volumes of material from landfill and the le¡over liquid from the site's digesters is dewatered to produce a solid cake that is used as a nutrient-rich organic fertiliser, thus closing the process loop. n Utility Week™ Live incorporating IWEX will be the UK's first (and only) event dedicated to the business, operational and technical needs of the UK's water, electricity and gas utilities, from upstream, transmission and distribution through to domestic and non-domestic retail. 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