Alternative and Renewable Energy - Advanced Materials ...

Spiral 2: 3/28/2011independent power production, for instance, when there is no intention to sell the electricity to utilitycompanies. The heat can be used as electricity demand drops which limits product waste and capacityvariation. 248Steam turbines are used as the prime mover for the cogeneration process and are grouped as eithercondensing or noncondensing. Condensing turbines condense the system exhaust steam to a productliquid upon its exit from the turbine. With this system type it is necessary to extract any process steamrequired prior to its passing through the turbine. The extraction point is equipped with an extractionvalve and should be at the location where the steam has the desired pressure. A noncondensing turbinecreates process steam by acting as a pressure reducer. In order to properly select a turbine type,several system specifications must be known, including the boiler pressure, process steam requirements,electricity requirements, and the availability of cooling water. If the most important product is processsteam, a noncondensing turbine should be used and designed to produce the proper steam pressure andflow. 248The generation of electricity is a direct function of steam flow. However, with a noncondensing turbinean alternate power source is often needed to meet the variations in electrical load. A condensingturbine is required if the turbine-generator set is expected to meet all of the variations in both processsteam and electrical load. 248For condensing turbine systems, power is generated by steam flowing through the turbine; therefore,any process steam that is extracted prior to entrance to the turbine will decrease the powerproduction. This type of turbine readily handles product demand changes by increasing steam orelectricity production depending on what is needed. The disadvantages of condensing turbines includesystem size limitations, high costs, and overall cycle efficiency. For instance, they are not typicallyavailable in sizes below five MW. The cost is also higher due to the additional condensing component,piping, and water circulation system. Finally, due to the presence of the condensing unit, the overallefficiency is reduced because all of the usable heat within the exhaust stream is lost to the coolingmedium in the condenser. 248The annual maintenance costs for most turbines are based on a percentage of the capital investment,typically 2 to 2.5%, or as a multiplier of the power production in cents per kWh. When decidingwhether or not to utilize cogeneration systems it is necessary to consider the alternative of purchasingpower and separately generating steam. Some factors to consider include purchased electricity cost, fuelcost, and the overall investment. 24810.4 Biomass Conversion Processes for Power and Heat GenerationOnce the location, feedstock type, and feedstock supplier are determined, the next step is to determinethe basic plant set-up. Direct combustion is currently the preferred method for the conversion ofcellulosic and solid waste feedstocks. Therefore, the residues from forest product mills and woodsources (e.g., including urban wastes) are favored. The ash content of agricultural feedstocks often limitstheir use with direct combustion. Other desired feedstocks include MSW and herbaceous and woodenergy crops. Figure 132 highlights the available biomass conversion methods, their inputs, and theproducts. 235214