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Show Appendix B NOx EMISSION CONTROL METHODS water and Steam Injection A common method of NO control on existing units is the addition of water or steam int~ the combustion zone to lower the flame temperature. The water must be very clean, thus requiring a high quality distillation system to remove impurities which can damage the downstream engine components. The water/fuel mass ratio can range up to about 1.2, with a ratio of 1.0 achieving about 70-80 percent NOx reduction. Above a 1.1 ratio the combustion efficiency may drop substantially, with a rapid rise in CO formation. Water injection may also contribute to pulsation and erosion in the combustion system, and must be carefully monitored in frequent inspections. Commonly cited emission limits of 42 ppmv on gasfired units and 75 ppmv on distillate oil were developed largely based on what was available with this control technology. Water can be more effective than steam in reducing NOx in some applications because of the heat absorbed by vaporization of the droplets. However, vaporization also requires more fuel to be burned, so that heat rate degrades by about 3 percent at a 1:1 ratio. output power due to increased mass flow may increase by 10 percent. Water injection is often used when steam is not available, in simple cycle applications such as peaking duty or more recently, pipeline compression (two projects in B.C.). It is estimated to cost about $2000-6000 per tonne of NOx removal for a water injection system on a new large unit, with the higher value applicable to isolated areas where the water acquisition/treatment costs are significant. Costs incurred for small engines will be about 50 percent higher. These values may be twice this amount for retrofit scenarios due to the number of modifications required for the existing unit and control system. The significant costs are associated with water transportation, treatment and disposal, modified combustor/ turbine and control system components, increased engine maintenance, and fuel penalty. steam injection is usually a preferred option on natural gas-fired combined cycles, where steam is readily available from the exhaust heat recovery system. For a given NOx reduction, steam/fuel mass ratio is about 50 percent higher than for water injection, but steam also has a less serious effect on component deterioration. It is also more efficient because the energy to vaporize the liquid is already derived from the turbine exhaust, instead of from the fuel with water injection. steam injection into the combustor will increase mass flow and generate a 20 percent power output gain, and a subsequent improvement in heat rate of up to 10 percent. Some units have the capability to add large amounts of steam at downstream locations to increase the mass flow through the power turbine, resulting in a 50 percent power augmentation for peaking applications. As with water injection the cost-effectiveness of steam injection for NOx control varies widely depending upon the application, but is usually in the same range. |