Formation of NOx and N2O in the fluidized-bed combustion of high- and low-rank coals

Abstract Fluidized-bed combustion (FBC) of coal is a technology which affords the opportunity for significant reduction of sulfur emissions from burning coal due to the incorporation of lime as a sorbent material. However, during the FBC of coal, nitrogen is released as oxides of nitrogen, both NO x (nitric oxide and nitrogen dioxide combined) and N 2 O (nitrous oxide). As well as being a greenhouse gas, N 2 O, like NO x , also interferes with the ozone chemistry in the upper atmosphere. Typically, in FBC systems, as the NO x level is lowered by utilizing lower-temperature regimes, the N 2 O level is correspondingly increased. This paper will describe a pilot plant investigation of the emission behavior of oxides of nitrogen from the combustion of seven coals, ranging from bituminous to lignite, including high-sulfur coals. The coals were burned in a 1-MWth circulating FBC over the temperature range of 730° to 930°C, excess air of 0% to 100%, superficial velocity of 3.6 to 7.0 m/s, and coal particle size of 125 to 500 microns. Continuous in situ monitoring of O 2 , CO, CO 2 , NO x , and N 2 O was conducted. A chemiluminescence technique was used for NO x , whereas N 2 O was detected by a nondispersive infrared analyzer. Nitrous oxide emissions are influenced most by the nature of the coal, with bituminous coals giving the highest emissions and subbituminous coals the lowest. Temperature is the next most influential parameter on emission levels–the higher the temperature, the lower the emission. Typical N 2 O emissions range from 10 to 180 ppm. Nitric oxide emissions, on the other hand, show the reverse trend with respect to temperature. The level of NO x typically ranges from 50 to 250 ppm. Coal type and temperature are also both influential factors in NO x generation. Other factors influencing the formation of NO x and N 2 O will also be discussed, along with possible mitigating factors for controlling the emissions. One interesting feature is the role of sulfur in the N 2 O-NO x chemistry, with respect to sorbent interaction. The interrelation between N 2 O and NO x formation presents some difficulties and challenges in selecting procedures to minimize the level of nitrogen oxides.