SOx-NOx-Rox Box Flue Gas Cleanup Demonstration: A DOE Assessment

Abstract

The SNRB{trademark} test program demonstrated the feasibility of controlling multiple emissions from a coal-fired boiler in a single processing unit. The degree of emissions removals for SO{sub 2}, NO{sub x}, and particulates all exceeded the project goals. A high degree of removal for HAPs was also achieved. The SNRB system offers low space requirements, control of multiple pollutants, and operating flexibility. The pneumatic SO{sub 2} sorbent and ammonia injection systems are expected to have high reliability because of their mechanical simplicity. Despite these advantages, the SNRB process may not be an economic choice for applications involving SO{sub 2} removals above about 85%. For lower levels of SO{sub 2} removal, the projected economics for SNRB appear to be more favorable than those of existing processes which involve separate units for the same degree of control for SO{sub 2}, NO{sub x} , and particulates. Specific findings are summarized as follows: (1) SO{sub 2} removal of 85-90% was achieved at a calcium utilization of 40-45%, representing a significant improvement in performance over other dry lime injection processes. (2) When firing 3-4% sulfur coal, compliance with the 1990 CAAA Phase I SO{sub 2} emissions limit of 2.5 lb/10{sup 6} Btu was achieved with a Ca/S molar ratio of less than 1.0. For the Phase II SO{sub 2} emissions limit of 1.2 lb/10{sup 6} Btu, compliance was achieved with a Ca/S molar ratio as low as 1.5. Phase II compliance is the more relevant emissions limit. (3) When using NaHCO{sub 3} as the sorbent, the Phase II SO{sub 2} emissions limit was achieved at a Na{sub 2}/S molar ratio of less than 2.0 (NSR < 1.0). (4) Compliance with the Phase I NO{sub x} emissions limit of 0.45 lb/10{sup 6} Btu for Group 1 boilers was achieved at an NH{sub 3}/NO{sub x} ratio of 0.85, with an ammonia slip of 5 ppm or less. (5) Particulate collection efficiency averaged 99.9%, corresponding to an average emissions rate of 0.018 lb/10{sup 6} Btu. This is significantly lower than the NSPS value of 0.03 lb/10{sup 6} Btu. The high-temperature baghouse design incorporating an SCR catalyst for NO{sub x} reduction was demonstrated successfully. The technology is ready for commercial application. The key feature of the technology is control of SO{sub 2}, NO{sub x}, and particulates in a single process unit. However, this limits its commercial market to applications requiring control of all three components. Also, although the testing demonstrated greater than 90% SO{sub 2} capture, this was achieved at high sorbent/sulfur ratios. For applications requiring a high percentage of sulfur removal, a modern conventional FGD unit with LNBs for NO{sub x} control may be the preferred option.