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Microturbines Microturbines are based on the principles of large gas turbines. They include an improvement that contributes to an improved electrical efficiency - a heat exchanger attached to the exhaust. The recovered thermal energy pre-heats the inlet air, which is mixed with biogas. The air/gas mixture is burned and allowed to expand through a turbine section to perform work. A gearbox and other moving parts are replaced by inverter-based electronics allowing the turbine to be operated at high speeds. The shaft can rotate as fast as 96,000 rpm. The exhaust gas is directed to the heat exchanger and then can be piped to a second heat recovery unit for further heat recovery. Biogas must be compressed to approximately 585 kPa (85 psi) before use in a microturbine (Wiltsee and Emerson, 2004). The compressor may be included as a component of the microturbine system or may be run independently. Hydrogen sulfide does not need to be removed from the biogas. There are models of microturbines that are able to handle H2S levels as high as 7%, much higher than any gas produced by an agricultural anaerobic digester (Capstone, 2005). Other models are equipped with a fuel conditioner ‘on-board’ (Ingersoll-Rand, 2003). Water vapour can contain compounds that may harm the fuel injectors, fuel control valves or the compressor. Thus, the vapour must be removed from the biogas. This represents the biggest concern. It is important to note that microturbines may be easily switched to run on natural gas in periods of low biogas production. Compared to internal combustion engines, microturbines have a much lighter maintenance schedule. A continuously running microturbine requires as little as 24 hours of maintenance for every 8000 hours of operation (Wiltsee and Emerson, 2004). Due to the smaller number of moving parts, a microturbine produces relatively low levels of noise and vibration. Therefore, a heavy foundation is not required (Brandon, 2002). Microturbines are capable of achieving electrical efficiencies of 25 to 30 %. Increasing ambient air temperature and increasing altitude have a detrimental effect on the efficiency of microturbines, by decreasing the density of the air (NRC, 2002). Also, a large portion of the electrical efficiency comes from the inlet air/exhaust heat exchanger. This means the microturbine will need a warm-up period in order to maximize its potential efficiencies. The exhaust gas from the first heat exchange unit can be 230 to 315°C, depending on the model, ambient environment, and altitude (Wiltsee and Emerson, 2004). These secondary heat exchange units have been reported to reach thermal efficiencies of 45% (Willingham and Pipattanasomporn, 2003). It is important to note that all thermal energy available for recovery is in the form of the microturbine exhaust. This exhaust gas stream is concentrated and is hot enough to be used directly (e.g. in greenhouses or drying applications) (Anon, 2005b). Emission levels of NOx from a microturbine can be less than 9 ppm (NRC, 2002). 14PDF Image | Electricity and Heat Production Using Biogas from the Anaerobic Digestion of Livestock Manure - Literature Review
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