FUEL NITROGEN CONVERSION DURING FUEL RICH COMBUSTION OF PULVERIZED COAL AND CHAR

摘要

The conversion of coal and char nitrogen has been investigated during fuel rich combustion. The experiments were done with the objective of clarifying the roles of NO, HCN, and NH₃, and char nitrogen in the post-combustion gases in the first, fuel rich stage of a staged combustor. The experimental apparatus includes a downflow combustor of 15 cm internal diameter and 180 cm length constructed of fibrous alumina insulation surrounding a central tube composed of vacuum- formed alumina cylinders. The combustion gases and solids were sampled in situ with a water-cooled and -quenched probe. Neither the combustor nor the sample probe were found to be reactive towards NO. Temperatures of the gases and walls were measured with Type K thermocouples and the particle temperatures were determined with a seven wavelength infrared pyrometer. Gas compositions were measured chromatographically using a 5A molecular seive for permanent gases (H₂, O₂, N₂, CO, and CH₄) and Poropak T for polar gases (CO₂ and HCN). A chemiluminescent analyzer measured NO. NH₃ and HCN were measured in the quench water with ion electrodes. The C, H, N, ash compositions of the char were measured with an elemental analyzer. Experiments of the fuel rich conversion of char nitrogen show that at all stoichiometries (SR = 0.8, 0.4) the concentrations of HCN and NH₃ in the post-flame gases are small compared to the concentration of NO. Char nitrogen conversion was stoichiometric or greater. NO destruction was found to be controlled by a heterogeneous mechanism involving the char carbon surface. The mechanism is deactivated by oxygen, an effect demonstrated by others. The fuel rich conversion of coal nitrogen was investigated with a Utah bituminous coal. At moderate fuel rich conditions (SR = 0.8), the residual char nitrogen conversion is 90 percent or greater and NH₃ and HCN concentrations were less than 20 ppmv. NO peaked at 1200 ppmv (1850 K) and declined to 600 (1580 K) ppmv over 1.8 seconds. Coal nitrogen conversion is dominated by NO formation at this stoichiometry. At extreme fuel rich conditions (SR = 0.4), coal nitrogen conversion is 85 percent. The gas is dominated by HCN, NO, and NH₃. HCN decayed from 600 ppm to 300 ppmv, NO from 350 to 50, and NH₃ increased from 200 to 375 ppmv, indicating that interconversion reactions in the gas phase are dominating. The kinetics which govern the volatile nitrogen reactions can be described by global homogeneous kinetics as follows: UNFORMATTED TABLE/EQUATION FOLLOWS: r₁ = d/dt[HCN] = -5.5x10¹⁷ exp(-83.3 K/RT)[HCN][H₂O]/[H₂]¹/² mole/cm³s r₂ = d/dt[NO] = -2.2x10¹⁶ exp(-54.4 K/RT)[NO][NH₃]/[H₂]¹/² d/dt[NH₃] = d/dt[NO] - d/dt[HCN] UNFORMATTED TABLE/EQUATION ENDS These yield rates for free radical reactions very similar to those determined in gas flame experiments, lending credence to their validity. A one-dimensional combustor model has been formulated which accounts for the heterogeneous combustion and gasification of the coal and char. This model includes the devolatilization of the coal and homogeneous oxidation of carbon monoxide and devolatilized species. The water-gas shift reaction is assumed to be equilibrated. The model also includes the mass, momentum and energy balances of the particles but obviates the solution of the combustor heat balance by using the measured gas temperature in the solution. The model accurately predicts the gas and elemental conversions and particle temperatures observed in the experiments, and supports the homogeneous and heterogeneous kinetics of post-combustion fuel nitrogen conversion.