Oxy-coal combustion: stability of coaxial pulverized coal flames in O2/CO2 environments

Oxy-coal combustion, in which air is replaced by an O2/ CO2 mixture, is one of the few technologies that may allow CO2 capture and sequestration technologies to be applied to existing coal-fired boilers. One issue of interest is to understand and predict the effects of near-burner-zone environment, now consisting of O2 and CO2 instead of O2 and N2, on flame stability. This dissertation is directed towards understanding how the composition of the coal transport medium (primary CO2 and O2) affects the ignition standoff distance in 40kW coaxial turbulent diffusion, oxy-coal flames, supported in a specially designed combustion test rig, rated at 100kW. The focus is on mechanisms of interaction between turbulent mixing in coaxial jets and coal particle ignition, rather than on each physical or chemical process taken individually. Experimental data focus on flame stability for a Utah Bituminous coal, although fragmentary results for a Powder River Basin (PRB) coal are also presented. Design considerations for the experimental rig and coal feeding system used are described in detail, and, because the emphasis was on flame stability under steady flow conditions, great care was exercised to demonstrate steady coal feeding behavior. A methodology to define and quantify the ignition behavior of laboratorycombustor-scale turbulent pulverized coal jets is developed and is described in detail. Flame stability is quantified by standoff distance, which is the distance between the burner tip and ignition point, and is defined by 6000 flame images obtained at the camera iv (EPIX SV5C10 CMOS) settings of 8.3 ms exposure time and 30 frames per second, which can correlate with the observations of human eyes and provide a physical description of a coaxial pulverized coal turbulent flame. Results are presented in the form of probability density function profiles of the measured standoff distance, as obtained from replicate runs, each consisting of 6000 photo images. Results show that flame stability is affected by primary PO2, secondary preheat temperature, secondary PO2, and transport diluent composition. Under certain conditions, multiple stand-off distances were observed for the same inputs. A targeted test, comparing different primary transporting media (O2/CO2 mixture and O2/N2 mixture), provided additional insight into flame stabilization under oxy-coal combustion. Taken together, the results led to the following inferred mechanisms: 1) At constant jet mixing aerodynamics, the composition of the primary jet fluid is very important in determining coal-jet stability in coaxial turbulent diffusion jet flames. Increasing primary PO2 always helps stabilize the flame, and in general, primary PO2 must exceed 10 vol% to allow this to occur. Not only is the concentration of O2 in CO2 important, but also the diluents containing the oxygen (CO2 versus N2). When CO2 is replaced by N2 in the primary jet, the primary jet need not contain O2 to allow stable flames. 2) The composition and the temperature of the secondary jet fluid are also very important. Flame stabilization is very sensitive to changes in secondary flow temperatures, from 489 K to 544 K, with the higher temperature leading to more stable flames. At higher secondary flow O2 concentrations (≥ 53 vol%), coaxial oxy-coal turbulent diffusion flames can be stabilized with zero oxygen