OCR Text |
Show 11 3.6 EMISSION LEVELS FOR OTHER CONTAMINANTS (CO, S02) The Guideline also recognizes the potential negative effects of other pollutants which may be emitted from turbines. These contaminants may result from implementing certain NOx control strategies, or from the type of fuel being burned. Carbon Monoxide Efforts to increase combustion efficiency, to burn out at high temperature as much of the carbon compounds as possible, have led to the subsequent formation of large quantities of thermal nitrogen oxides. When the process is changed to reduce NOx ' there is usually a significant increase in the amount of CO and unburned hydrocarbons. For turbines, research has shown that both NOx and CO can be minimized in a narrow temperature range near 1500oC, substantially lower than normal combustion temperatures. The developers of dry-low NOx combustors are faced with the challenge of staging and mixing the air and fuel to take advantage of this narrow temperature range, while at the same time ensuring flame stability during transient conditions. Most gas turbines presently produce less than 5 ppmv of CO as a result of obtaining good combustion efficiency. The CO emission target was debated at length, with some working group members questioning its need. A target of 50 ppmv was eventually agreed to, in an effort to ensure good combustion when NO~ control methods such as steam injection or DLN combustors are empioyed. Although some manufacturers claimed that these levels could be met over the normal operating range (50-100% load), confidence in this goal was not high and the 50 ppmv limit was chosen at a single power rating. Sulphur Dioxide Emission targets for sulphur dioxide (S02) were included to address S02 emissions associated with liquid fuels and any sulphurcontaining fuels such as syngas (coal-derived or biomass). The emission targets were chosen to reflect levels for large utility boilers contained in the recently updated N305 Guideline For Thermal Power Generation Plants, converted to energy output basis assuming an average 35% thermal efficiency. other contaminants Another cQllateral pollutant which could arise out of NO control techniques such as Selective Catalytic Reduction i~ ammonia, injected upstream of a catalyst bed. Off-design conditions and catalyst degradation will result in emissions of some unreacted ammonia. It was decided however that since SCR was not a technology that this Guideline would require, the inclusion of ammonia limits was not necessary. |