Evolution and reaction of fuel nitrogen during the early stages of pulverized coal pyrolysis and combustion.

摘要

The pyrolysis and combustion of two U.S. coals has been studied in an argon arc-jet fired entrained-flow reactor. Narrowly size-graded Montana Rosebud subbituminous and Pittsburgh #8 HVA bituminous coals were used, with feed rates chosen to produce single particle conditions. Rapid mixing of coal particles into the hot gas stream resulted in heating rates similar to those expected in typical pulverized coal flames ({dollar}sim{dollar}10{dollar}sp5{dollar} K/sec).; Pyrolysis times for 55 {dollar}mu{dollar}m coal ranged from 50 msec at 1300K to 11 msec at 1750K. Pyrolysis of the Rosebud coal yielded primarily gaseous products, with low levels of soot and tar. The Pittsburgh coal released a substantial quantity of tar during pyrolysis. This tar experienced rapid secondary decomposition, with 60-80% converted to soot in times ranging from 12 to 27 msec under pyrolysis conditions. The addition of 900 ppm O{dollar}sb2{dollar} was found to increase the rate of secondary decomposition.; The Rosebud coal released fuel nitrogen primarily as HCN, with only low levels of soot/tar nitrogen detected. For the Pittsburgh coal, much of the volatile nitrogen was initially released as tar, which rapidly partitioned into HCN and soot. Soot nitrogen accounted for 40% of the volatile nitrogen at 1260K, decreasing to 22% at 1630K. Char nitrogen represented more than half of the original fuel nitrogen for both coals.; During the combustion of Rosebud coal, all released fuel nitrogen was converted to NO. Only 55% conversion was observed using the Pittsburgh coal for O{dollar}sb2{dollar} {dollar}<{dollar} 1.5%, with the remainder attributed to N{dollar}sb2{dollar}. At O{dollar}sb2{dollar} {dollar}>{dollar} 1.5%, significantly higher conversion to NO was observed for 35 {dollar}mu{dollar}m vs 68 {dollar}mu{dollar}m Pittsburgh coal. A complementary study of the removal of NO seeded into the flow found greater removal for 68 vs 35 {dollar}mu{dollar}m Pittsburgh coal for O{dollar}sb2{dollar} {dollar}>{dollar} 1%. Notable removal of seeded NO was observed under pyrolysis and low oxygen conditions for the Rosebud and Pittsburgh coals. Although less active than the coals, a pre-charred Utah bituminous coal removed NO via heterogeneous char reduction. The removal rate represented 60% of the diffusion-limited rate at 1260K, increasing to near 100% at 1640K.