OCR Text |
Show Selective Catalytic Reduction (SCR) Selective Catalytic Reduction is a back-end clean up system which has been used on large units since the mid-1980's, particularly in California. In this process, the exhaust gas stream is sprayed with ammonia, and sent through a catalyst bed in the heat recovery steam generator. In a temperature range of 300-400oC, the ammonia in the presence of the catalyst reacts with the NOx in the exhaust to form nitrogen and water vapour. The SCR system needs the heat recovery steam generator to reduce the 500-600 °c exhaust to the required reaction temperature range. It is therefore only practical at this time for gas turbines in combined cycle applications where the load is fairly constant. SCR has a NOx removal efficiency of about 80 percent, and is typically used after water/steam injection systems, to reduce emissions from the 50 ppmv level to 10 ppmv. Adequate controls and a reliable continuous Emissions Monitoring system are needed to keep the ammonia inj ection rate at the proper setting, and to ensure the appropriate reaction temperature. Otherwise, emissions of unreacted ammonia (itself a pollutant) may increase over the normal 10 ppmv range. Operational and safety issues are associated with the associated ammonia handling facilities. Various types of catalyst beds are available, depending upon the required NOx removal efficiency and the temperature range. Titanium oxide, vanadium pentoxide or platinum are mounted on a substrate in a catalyst bed sized for an optimum flow velocity. Recently, more costly zeolite materials have been used for applications requiring a wider, higher temperature range (250-500 °C) and could potentially be used on simple cycle applications. SCR systems often are inappropriate for use in liquid-fuelled applications, where fuel sulphur leads to plugging of the downstream HRSG section and catalyst bed with sulphates of ammonia. The life of the SCR catalyst has been an issue, with lifespan on clean fuel of three years having been improved to 5 years with recent experience. Disposal of spent catalyst structures may be a problem if they are classified as hazardous waste. For large sizes, the capital cost for SCR systems is about $50 per kw installed. A system used to control NOx from 150 ppmv to 40 ppmv on a large gas turbine may achieve a cost of $2000 per tonne NO removal. A system using steam injection for control down to 50 pp~v, and SCR down to 10 ppmv, may also be used, but the marginal cost of the SCR portion of the removal would be much higher. Dry Low-NOx Combustors A more direct method of control is to modify the combustion process itself, by rearranging the airflow and fuel mixture inside the combustor to minimize the occurrence of high local peak flame temperatures. The need for this alternative technology arose out of some operational difficulties with water injection and SCR, especially for simple cycle plants in remote areas. Development |