FORMATION AND DECOMPOSITION OF N2O IN FLUIDIZED-BED BOILERS

Emissions of N2O from various coal fired appliances have been measured. The results show that conventional appliances generally emit low levels of N2O [<30 mg m(-3) (STP)]. Fluidized beds usually emit considerably higher levels, due to the lower combustion temperatures prevailing in such plant. In order to study the mechanisms underlying N2O formation and destruction, emissions from coal combustion in a circulating fluidized bed boiler (CFBC) with a thermal output >200 MW were measured under various operating conditions. The results show that the effects of limestone feedrate and of the secondary to primary air ratio on N2O emissions were not significant. The highest temperature in the system appears to be the main factor affecting N2O emission. In a load changing cycle N2O emission increased from about 30 mg m(-3) (STP) at full load to about 360 mg m(-3) (STP) at part load conditions. Additionally to these studies, thermal decomposition and reduction of N2O in the homogeneous gas phase, and on solid samples taken from the CFBC, have been studied in a laboratory fixed bed reactor at temperatures similar to those in the CFBC. It turned out that thermal decomposition took place in the homogeneous gas phase, which could be catalysed by solid materials, e.g. by bed ash or cyclone ash in the CFBC. In the presence of char, N2O was reduced in addition to thermal decomposition. Addition of CO increased N2O decomposition in the gas phase, but only in the absence of oxygen.