| OCR Text |
Show 12 In addition, nitrous oxide (N20) is a known greenhouse gas which is formed by combustion at lower temperatures of about 10000C in some catalytic combustion processes. It was not included as it is not presently judged to be relevant to gas turbines. The need for limits on both ammonia and nitrous oxide will be monitored for any future revision to the Guideline. 3.7 SIZE RANGES FOR EMISSION TARGETS The working group decided that emission targets should be based on what should be technically achievable with dry low-NOx (DLN) combustors on various types of gas turbines. It was first decided that units less than 3 MW would have less stringent emission targets due to the inherent difficulties in modifying their very small combustors, and the fact that they compete with reciprocating engines which produce much more NO. The total contribution of these small engines would only rep~esent about 2 percent of the industry's NO production. In this context the limits placed on these engine~ are more lenient. The emissions limits for large gas turbines were set at levels which were expected to be achieved by steam injection or DLN combustors over the next few years, without the need for back-end cleanup. Uncontrolled emissions of about 150-250 ppmv would have to be reduced to 25-35 ppmv, a sUbstantial reduction. Two-thirds of the installed and proposed power in Canada consists of these large gas turbines. Because of the wide range of sizes (1 MW to 150 MW), and the variety of types of combustion systems being used, it was felt by several members of the working group that an intermediate size rating should be established. This recognizes economies of scale for the largest units to have the greatest levels of NO} reduction, while medium-sized units with smaller combustors would nave to meet intermediate emission levels. The choice of this size cutoff was debated in the 10-20 MW range, and was finally chosen as 20 MW largely because there are very few commonly used machines near this rating. Therefore units from 3 MW to 20 MW would be reducing NOx from 100-200 ppmv levels down to 40-50 ppmv. Due to the need for these medium-sizes in remote applications such as gas pipelines, and because some major manufacturers are concentrating their DLN efforts on large units, they have been granted an interim target during 1994-96 which is about 50 percent higher than the final limits. Note however that one company selling these units will have some operational dry low-NOx turbines in 1994. |
