Evolution and reaction of coal fuel nitrogen during rapid oxidative pyrolysis and combustion

Abstract

An arc-jet fired entrained-flow reactor has been used to study the evolution and reaction of fuel nitrogen during the rapid oxidative pyrolysis and combustion of two narrowly sizegraded pulverized coals: Montana Rosebud subbituminous and Pittsburgh #8 HVA bituminous. Tar released by the Pittsburgh coal undergoes rapid secondary decomposition in the presence of ≈900 ppm background O 2 , partitioning into gaseous products and soot. The characteristic time for tar decomposition ranges from 16 msec at 1325 K to only 5 msec at 1625 K. The soot yeild reaches an asymptote at 1400 K, representing 30% of the total volatile yieldsoot nitrogen represents 30% of the total volatile nitrogen yield at 1200 K, and 20% at 1630 K. The remainder of volatile nitrogen is initially released to the gas phase as HCN. Pyrolysis of the Rosebud coal yeelds gaseous products, with fuel nitrogen released primarily as HCN. During combustion of the Rosebud coal under single particle conditions at oxygen concentrations as low as 0.5%, virtually all of the evolved fuel nitrogen is converted to NO. In contrast, 45% of evolved fuel nitrogen is converted to N 2 during combustion of the Pittsburgh coal under similar conditions. Combustion studies at 5% O 2 reveal significantly higher NO levels for 35 μm vs 68 μm Pittsburgh coal. Rapid and early removal of NO added to the reactor is observed to qualitatively follow the overall tar/soot yield. Heterogeneous reduction of NO on soot is a possible explanation for the effect of both coal type and particle size on N 2 formation.