Soot loading, temperature and size of single coal particle envelope flames in conventional- and oxy-combustion conditions (O2/N2 and O2/CO2)

Abstract

A fundamental laboratory study on the volatile-phase combustion of pulverized coal was conducted at the particle level. A primary goal has been to simultaneously assess soot volume fraction, fv, and soot temperature, T, in the diffusion flame (soot mantle) forming around a single burning bituminous coal particle, upon ignition of its volatile matter. This assessment was conducted with emission-based pyrometric methods. A secondary goal of this study has been to compare these radiative parameters, fv and T, in conventional air- and in simulated dry oxy-fuel combustion. Both fv, and T measurements were spatially averaged in the flame but temporally resolved throughout the combustion history of single particles. In addition, the size of the volatile envelope flames was assessed both pyrometrically and cinematographically. Combustion of three different bituminous coals took place with various oxygen partial pressures in nitrogen and in carbon dioxide background gases. Single particles, 75–90μm, were injected and burned in a transparent drop-tube furnace (DTF) at laminar-flow atmospheric-pressure and a wall temperature of 1400K. The free-falling bituminous coal particles heated up and devolatilized, their volatile matter ignited and formed bright envelope flames, often with distinctive soot contrails in the wakes of the flames. The radiative parameters fv, T and flame size, of these luminous envelope flames were assessed using different emission-based models for the analysis of the three-color pyrometric intensities. The particle envelope flames of all three coals were found to contain comparable to each other soot volume fractions, in the range of 20–90ppm. At identical furnace gas temperatures and identical O2 mole fractions, when the background N2 gas was replaced with CO2, the particle envelope flames of the bituminous coals were characterized by lower soot volume fractions, lower temperatures and bigger sizes. As the O2 mole fraction increased in either N2 or CO2 background gases, soot volume fractions increased to a maximum and then decreased, temperatures increased monotonically and flame sizes decreased. In CO2-based combustion, an oxygen mole fraction in the neighborhood of 35% was necessary to elevate the measured flame temperature to match that of conventional air-based combustion; however, the soot volume fraction was lower than that in air-based combustion regardless of the oxygen mole fraction.