Case study - Biogen Greenfinch Westwood

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Courtesy of Geotech

Non-stop clean power generation uses smart machines and state-of-the-art technology to help keep running.

Keep it simple - and to keep it profitable, keep CHP engines running. That's just part of the formula for success at the BiogenGreenfinch-designed and run Westwood anaerobic digestion (AD) plant in Northamptonshire. Built and funded by parent company Bedfordia Group plc this £10 million-plus plant is set to take 45,000 tons a year of food waste, convert it to biogas and a nutrient rich biofertiliser and sell 1.7 megawatts of electricity to the national grid - enough to power 3000 homes. It went live in June 2009 and has run non-stop, 24/7 since then.

Beyond keeping the engines running, Westwood's CHPs have an impressive 99 percent uptime at 41 percent biogas to electricity and useable-heat efficiency. These high rates will make substantially positive profit impact over the several years of investment payback. Watching the biogas as it comes from the gasholder and travels towards the CHPs and water heating boilers is a Geotech static gas analyser, the Automated Extraction Monitoring System (AEMS). This AEMS equipment, like the whole site is powered by the electricity the CHPs produce, and it plays a key role in keeping the CHP engines running.

Westwood's biogas averages 60 percent methane (CH4), 40 percent carbon dioxide (CO2), about one percent oxygen (O2) with hydrogen sulphide (H2S) at trace levels below 100ppm. The static gas analysis AEMS unit is set to take a biogas reading every five minutes, 288 times a day. It feeds the data via Ethernet to Westwood's central monitoring and control systems. 'We watch the AEMS data gas trends and use CO2 levels as one of several indicators of digester performance and health. If the CO2 goes up then we examine the feed content and may adjust AD alkalinity. If H2S increases we 'air strip' and add ambient air, via Venturi, to the digester headspace, said David Woolgar, Director of Engineering. He also said that minimising the H2S in the biogas while keeping the O2 below critical levels helps keep the CHP engines running. They have a safety threshold engine-stop cut-off H2S level far higher than those ever reached.

In addition to Westwood's technical staff checking digester and engine performance, the CHP and the AEMS system are set to keep watch on the biogas and on themselves and have down-the-line remote access by Geotech to the AEMS panels and by the CHP manufactures to their engines. The two systems communicate with the central Westwood systems. So close is the system integration that when a Jenbacher CHP engine is to restart, for example after an oil change, it interrogates the AEMS data for the current percent CH4 content and adjusts its throttle setting for a start sequence at that level ensuring a safe, damage free and faultless engine restart.

The static gas analysis Geotech AEMS equipment is in use on many UK sites and others around the world. Applications include farm and food waste AD/biogas production, sewage sludge AD treatment, biogas harvesting and PPC compliance. With landfill gas and biogas energy-from-waste projects AEMS assists landfills and farmers in South America, Africa and Asia. With AEMS they earn valuable carbon credits under the Clean Development Mechanism, through a combination of robust and reliable performance coupled with easily auditable, secure and verifiable data collection and storage systems. Based on Geotech's continuously developing gas analysis technology, tested and proven in robust portable gas analysers for more than 20 years, AEMS operation is fully automated. It can analyse gas every six seconds. It self-purges and self-calibrates logs data and transfers it via Internet modem, Ethernet or both.

For servicing the AEMS gas analysis unit is simply swapped. According to David Woolgar, servicing the AEMS unit is easy with a service-exchange analyser inserted in moments and the Westwood unit sent for service and calibration within Geotech's seven-day turnaround. Westwood technicians also use a Geotech GA2000 portable gas analyser to check biogas from each of the digesters at tapping points and to double check the automated readings of the AEMS static gas analyser.

Westwood processes commercial food waste from names like Whitbread, Waitrose and Sainsbury's. On working with Sainsbury's John Ibbett, Chairman BiogenGreenfinch, said, 'Anaerobic Digestion is playing an ever-increasing role in recycling food waste in Britain; it is of significant environmental benefit and will help contribute toward a reduction in CO2 emissions. The Westwood site will not only dispose constructively of Sainsbury's own waste but will also be available to get rid of that produced by its suppliers, producing green electricity for thousands of people and in turn, green fertiliser for use on growing crops.'

Since it started running the Westwood plant now also receives, sorts and de-packages supermarket food for recycling and from food processors, dairy bi-products, cream, and vegetable oil production sludge. Fresh interest is coming from other sources countrywide looking for ethical, green, productive and non-landfill routes to disposing of food waste. It all goes to make biogas and bio-fertiliser and from the biogas, heat and electricity with AEMS checking the biogas 24/7.

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