OCR Text |
Show Modeling of the radical generation processes in a system which assumes uniform temperature and complete mixing can reveal temperature dependent tendencies which aid in experimental modeling. Current modeling results indicate that the generation of a reliable radical concentration profile will be obtained at relatively low plasma temperatures. Low plasma temperatures, at or near 2000 K, in combination with secondary gas injections should provide steady radical supply for a wide range of mixing time scales. Experiments have shown that maximum NOx reduction and maximum ammonia effectiveness occur at high ammonia flow rates and low plasma power. Relatively high NO concentrations will provide efficient removal for the small radical supply. The overall maximum which appears at maximum ammonia flow rate, however, is unexpected. As radical supply is increased well beyond the corresponding NO available, increased waste is expected. This effect may be caused by the complex interaction which occurs between hot and cool gases in the argon stream and by the amount of swirl imparted to these gases by the injected ammonia stream. Substitution of nitrogen as the carrier gas, replacing argon, was made with limited success. Torch operation with nitrogen gas, which is not as conductive as argon, required an increased power consumption. Operational power for nitrogen gas, at approximately 7 kW, was too high to promote the effective reduction of NO. A redesign of the torch nozzle for improVed arc stability must be completed to allow low power operation with nitrogen. Effective reduction is possible with NH3 in N2 carrier gas at low power. Nitrogen plasma is less expensive and provides additional radicals which may enhance NOx reduction in fuel rich NO-laden streams. CONCLUSIONS The removal of NOx by molecular radical reduction is an effective new technology for ultra-high emission control. Amidogen, a radical of ammonia, is the primary reducing agent which reacts with NO to fonn nitrogen gas and water. Radicals of methane and hydrogen have been used to improve less than optimal conditions. A stream of argon, or other carrier gas, is passed through an electrical arc in the PETC plasma torch. Ammonia and the secondary gases are introduced into this carrier gas stream and radicals are formed. The mixture is injected directly into post-combustion gases doped with NO. The injection of externally induced molecular radicals is designed to increase reduction effectiveness and decrease ammonia breakthrough. External generation, which requires less than 1 % of the energy available to the boiler, produces an effective supply of NO reducing radicals. Repeated NOx reduction of 85%, with individual data recorded as high as 94%, have been achieved with very little associated ammonia slip. Improvements in plasma energy efficiency and radical production capability, made as plasma generation technology advances, will increase the process viability. CONTRACf ACKNOWLEOOEMENT This paper is based on work performed under contract number DE-AC05-760ROOO33 between the U.S. Department of Energy and Oak Ridge Associated Universities and by grants from Allegheny Power Systems. 6 |