OCR Text |
Show .. -, INTRODUCfION The current technology of choice - world-wide - for post-combustion NOx control is selective catalytic reduction (SCR) with ammonia. The application of SCR to coal-frred units has proven to be somewhat more difficult than its application to natural gas and oil-fired units due to S02 poisoning of the catalyst, catalytic oxidation of S02 to S03' and erosion and fouling of the catalyst by fly ash.[l] These problems could be diminished if S02 and particulate removal systems were placed upstream of the SCR reactor. Babcock & Wilcox (B&W) has developed and patented such a process known as the SOxNOx- ROx Box (SNRB) process. Briefly, this process consists ofa high-temperature baghouse employing woven, high-temperature bags, ammonia injection along with an SCR catalyst incorporated into each bag filter assembly, and the injection of either a calcium- or sodium-based sorbent upstream of the baghouse (Figure 1). Because the SNRB process achieves simultaneous SOx' NOx' and particulate removal in a single unit operation, SNRB offers reduced capital and operating costs, lower space requirements, and more operating simplicity than the conventional combination of pollution control technologies. B&W is currently being funded by the U.S. Department of Energy (DOE) and the Ohio Coal Development Office (OCDO) under the DOE Innovative Clean Coal Technology Demonstration program to demonstrate the feasibility of the SNRB technology in a 5-MWe field demonstration at a coal-frred utility boiler. The project involves a team approach with B&W, Norton Company (catalyst supplier), Minnesota Mining and Manufacturing - 3M (bag manufacturer), Ohio Edison (host utility), and the Electric Power Research Institute (EPRl). TECHNOLOGY DESCRIPTION A schematic representation of one possible arrangement of the SNRB process is depicted in Figure 2. In this version, hydrated lime (Ca(OH)2) is injected upstream of the economizer, at a flue gas temperature in the range of 9000 - l1000F. The Ca(OH)2 reacts with S02 to form Cas03 and Cas04 particulates. These reaction products are collected, along with the fly ash, in the hot baghouse. The baghouse utilizes high-temperature bags in order to operate in a temperature range of 7000 - 8500F. Anhydrous ammonia vapor is injected upstream of the baghouse to reduce NOll in the presence of a SCR catalyst The SCR catalyst is incorporated into each bag filter assembly, downstream of both S02 and particulate removal. Potential problems observed in past SCR operations such as S02 poisoning of the catalyst and erosion and fouling of the catalyst by fly ash are significantly diminished. In some applications, especially in the western U.S., a sodium- based sorbent such as sodium bicarbonate (NaHC03) may be the preferred sorbent. In this application, the NaHC03 would be injected downstream of the economizer, in a temperature range of 5000 - 8000F. Similar to the calcium-based SNRB system, the NaHC03 reacts with S02 to form sodium sulfate (N~S04) particulates which are collected in the hot baghouse. NO x removal would still be accomplished with Nl\ injection and a NOll reduction catalyst. |