OCR Text |
Show Introduction The combustion of fuels such as coal or heavy oils, i.e. fuels that contain chemically bound nitrogen and sulfur, produces a flue gas in which typically 98-99Y. of the sulfur oxides SO. with 1-2X present as S03 and the nitrogen oxides are roughly 95Y. NO and 5Y. NOe • There are, however, examples in which S~ is as high as lOY. of the SOx (1) and NO. as high as SOX of NOx (2). On cooling the S03 in the flue gas forms a sulfuric acid mist which can cause corrosion problems or, if NHa is present, can form N~HSO~ with resultant fouling problems. The former can be an especially severe problem in the operation of wet scrubbers. When SO. is removed from flue gas in a wet scrubber the gas temperature is necessarily below the acid dew point. On entering the scrubber the flue gas forms a acid mist. The mist particles, having an extremely low diffusion coefficient are not rem~ved efficiently by the scrubber and tend to pass through it, causing corrosion problems downstream of the scrubber. While this represents a significant problem for SOx control technology, NH~HSO~ formation is a problem for postcombustion NOx emission control. Several such processes are available, Thermal DeNOx, Urea Injection and Selective Catalytic Reduction, and all involve reducing NO in a manner that leaves traces of NH3 in the flue gas. This can lead to N~SO~ formation downstream of the NO reduction zone with resultant fouling problems. Because of the problems which S03 can cause, there has been a great deal of research aimed at elucidation t~e factors which determine the SOe/SOa ratio in post combustion gases (e.g. reference 3) but surprisingly there has been little or no research aimed at finding chemistry that would allow reducing the S~ to SOe in a practical manner. Similarly there has been many studies concerned with the factors which determine the NOe/NO ratio in combustion effluents (e.g. reference 4), but there has been little work relating to chemistry which could be used to convert NO to NOe in post combustion gases. Yano and Ito (5,6) have studied the reactions . which can occur in the exhaust of methanol fueled automobile engines and report that NO considerably accelerates the oxidation of residual methanol. They also note that this catalysis is accompanied by some limited NO to NOe conversion. To explain this they suggest that HOe free radicals formed by the oxidation of methanol react with NO via NO+HOe=NOe+OH. U.S. patent 4,350,669 (7) makes note of the fact that the scrubbers now in use to remove SOe from flue gas cannot remove NO but can remove NOR and the examples of this patent show that the low temperature oxidation of methanol (i.e. at temperatures of 200 to 700o C) can cause substantially complete NO to NOe conversion. The reaction times used, however, were in the range of 1.5 to S seconds, which is much longer than would be available in a practical combustion system. In this paper we report the discovery of a new homogeneous gas phase reaction in which methanol converts SOa to SOe. In the course of this reaction NO is converted to NO.. This new reaction is highly selective in that ppm concentrations of 503 and NO are converted by quantities of methanol only slightly greater than stiochiometry in the presence of massive amounts of 0.. The reaction is also rapid, capable of achieving better than 80Y. reduction in only SS milliseconds. The existence of this new reaction was predicted by computer modeling (8) and subsequent experiments were in reasonably good accord with the model's predictions. Experimental The bulk of our experiments were done using the flow system shown in Figure 1. This flow systems consisted of three SUbsystems: a system for blending fixed gases and vaporized liquids, a reactor, and an analytical train. |