Reduction of nitrogen oxides from combustion source emissions utilizing chemical radical species

摘要

Injection of plasma-induced chemical radicals of ammonia, methane, hydrogen, steam and nitrogen into post-combustion gases for the purpose of NO(sub x) reduction has been investigated both numerically and experimentally. A kinetic mechanism has been developed for the fundamental reactions of C/H/N/O interaction in post-combustion gases. Numerical modeling has defined an optimal radical formation temperature of 2500 K, at which radical concentrations are formed and survive long enough to allow interaction with the flue gas NO. Comprehensive modeling has also explored the effectiveness and selectivity of ammonia derived radicals, particularly amidogen, in NO(sub x) reduction. Plasma induced radical generation eliminates the low temperature boundary found in current ammonia injection processes. Laboratory tests utilizing a DC arc discharge plasma generator have supported these results. Reduction of NO(sub x) from the product stream of a laboratory scale 1.4 MM BTU/hr combustor has been achieved in excess of 90%. Minimal torch power was used to produce this reduction with ammonia, ammonia/methane, and ammonia/hydrogen radicals at temperatures substantially below current ammonia injection mechanisms. Plasma induced radical generation does not appear to be limited by emission of unreacted ammonia, which is a primary limitation of Thermal-DeNO(sub x) and a substantial problem in inefficient catalytic reduction. Concurrent small scale testing employing an inductively heated plasma system have produced similar results. Numerical and experimental analyses of NO(sub x) reduction by nitrogen atom injection were also performed. Kinetic modeling predicted that nitrogen atom injection would be unsuccessful at any realistic nitrogen plasma temperature in the presence of carbon dioxide and oxygen. Carbon dioxide is a particular problem at lower temperatures. Experimental verification was provided during laboratory tests using nitrogen plasma and typical post-combustion gases.