Executive Summary: Phosphorous-Resistant SCR Catalysts for Air Staged Combustion of PRB Coal

摘要

Rapid SCR catalyst deactivation has been reported in air staged combustion boilers burning Powder River Basin (PRB) coal. It has previous been reported (1) that reducing conditions in staged combustion systems result in the release of gas-phase phosphorous species, which has been shown to result in accelerated SCR catalyst deactivation (2). This form of deactivation is analogous to that exhibited by arsenic-laden bituminous coals, where As interacts with vanadium reaction centers. While the gas-phase phosphorous levels in unstaged PRB units is around roughly 10 ppb, in staged units the levels range from 20 to 50 ppb or higher. Characterization of deactivated SCR catalysts by Raman spectroscopy revealed the presence of vanadium-phosphorus bonds, indicating a chemical deactivation mechanism attributed to the interaction of gas phase phosphorus with vanadium in the SCR catalyst. Here we report on a group of catalysts that have shown excellent resistance to deactivation by phosphorous in laboratory tests (3). These tests examined the effect of 100 ppm phosphoric acid vapor on SCR deNOx activity in fully-simulated flue gas streams. The oxidation rate of sulfur dioxide was measured using 500 ppm SO_2 in the simulated effluent. The catalysts tested were based on the conventional titania-supported vanadia-tungsta system doped with niobium, cobalt and/or molybdenum. No significant decrease in deNOx activity or increase in SO_2 oxidation was experienced under even very high phosphorous exposure. Commercialization of these simple modified formulations will quickly allow utilities to mitigate the detrimental effects of PRB-borne phosphorous on SCR catalysts.