OCR Text |
Show maintain NOx emission, as measured by a continuous emissions monitor (CEM), at a target value of 0.28 IbflvlMBtu. No instrumentation is provided to monitor ammonia slip in the boiler. The SNCR is put into service at about 30% load (88,500 lb/h steam). At this load, only one level of injectors is in service. The system requires manual rotation of the injectors as load increases. At still higher loads (above 70%), a second level of injectors is manually inserted into the furnace. As load decreases, the second level of injectors is manually withdrawn from the furnace. As load further decreases, the injectors on the first level are manually rotated to their original position. Initial operation of the SNCR system began during plant startup in the Spring of 1992. By September of 1992, fabric-filter differential pressures gradually increased from a normal range of 4.0 to 6.5 inches water to over 12 inches water. This increase restricted boiler air flow to the point where rated boiler load was no longer attainable. Analysis of the bags revealed dense agglomeration of hygroscopic salts, which adhered stubbornly to the filter surface. In addition to the buildup of material on the filter bags, the first boiler inspection in March, 1993, revealed hard deposits plugging the tubular air heaters and plating the economizers. Operational experience at similar Cogentrix plants provided some insight into the problem at the Richmond plant. A plant similar to Richmond, but half the size and without an SNCR system, had been on line for about six years and never experienced any of the fabric-filter problems plaguing the Richmond plant. Also tubular air heaters and economizers experienced erosion, but not pluggage. Plant personnel suspected that high levels of ammonia slip from the SNCR system might be occurring. With pennission from the state of Virginia, the SNCR system was removed from two of the eight boilers. With the SNCR system operating, fabric-filter differential pressure would begin to increase within two weeks, and within four weeks the pressure would approach nine to ten inches water at rated load. After six weeks, the boiler would have to be taken out of service and the filter bags washed. Boilers operating without SNCR, displayed no change in the differential pressure of the fabric filter. During that period, the boilers were operated at about 89% of rated capacity. When urea injection commenced again, differential pressure had begun to increase within one week. Through late summer and early fall of 1993, more problems surfaced with the boiler tubular air heaters and economizers. Exit gas temperatures increased 20 to 30 degrees F, and pressure differential across the air heaters also increased, suggesting a worsening of the deposition in these areas. In October 1994, IGT as the technology developer together with Detroit Stoker Company, the manufacturing partner, and Cogentrix of Richmond, the host site, were awarded a contract by the Gas Research Institute to retrofit a boiler with the patented 3 |