Temperature, velocity and species profile measurements for reburning in a pulverized, entrained flow, coal combustor

Abstract

Nitrogen oxide emissions from pulverized coal combustion have been and will continue to be a regulated pollutant for electric utility boilers burning pulverized coal. Full scale combustion models can help in the design of new boilers and boiler retrofits which meet emissions standards, but these models require validation before they can be used with confidence. The objective of this work was to obtain detailed combustion measurements of pulverized coal flames which implement two NO reduction strategies, namely reburning and advanced reburning, to provide data for model validation. The data were also compared to an existing comprehensive pulverized coal combustion model with a reduced mechanism for NO reduction under reburning and advanced reburning conditions. The data were obtained in a 0.2 MW, cylindrical, down-fired, variable swirl, pulverized coal reactor. The reactor had a diameter of 0.76 m and a length of 2.4 m with access ports along the axial length. A Wyodak, sub-bituminous coal was used in all of the measurements. The burner had a centrally located primary fuel and air tube surrounded by heated and variably swirled secondary air. Species of NO, NO x , CO, CO 2 and O 2 were measured continuously. Aqueous sampling was used to measure HCN and NH 3 at specific reactor locations. Samples were drawn from the reactor using water quenched suction probes. Velocity measurements were obtained using two component laser doppler anemometry in back-scatter mode. Temperature measurements were obtained using a shielded suction pyrometer. A series of six or more radial measurements at six or more axial locations within the reactor provided a map of species, temperature, and velocity measurements. In total, seven reactor maps were obtained. Three maps were obtained at baseline conditions of 0, 0.5 and 1.5 swirl and 10% excess air. Two maps were obtained under reburning conditions of 0.78 stoichiometric ratio and 1.5 swirl and 0.9 stoichiometric ratio and 0.5 swirl. And finally, two maps were obtained under advanced reburning conditions both at the same operating condition of 1.05 stoichiometric ratio in the reburning zone followed by ammonia injection. Numerous effluent measurements were obtained to study the affect of natural gas injection location, stoichiometric ratio, and injection velocity on effluent NO. For advanced reburning, effluent measurements were obtained for a similar matrix of operating conditions with the additional variable of ammonia nitrogen to nitrogen in NO or nitrogen stoichiometric ratio (NSR). Baseline maps and effluent measurements were useful in establishing the flow patterns of the reactor and determining the effect of swirl on NO. At zero swirl the flame appeared to be lifted and therefore created higher NO. At 0.5 swirl the flame had transitioned to an attached flame with a central recirculation zone and produced lower NO. Detailed velocity measurements of the quarl were obtained which provided boundary conditions for modeling and helped explain why the flame was not attached at zero swirl. ...