Numerical Simulation And Optimization Of Oxy-coal Combustion Test Rig

Carbon dioxide capture and storage technology has been consideredto be an effective method for carbon dioxide emission reduction in the nearfuture. Retrofitting of the existing conventional pulverized coalcombustion to oxy-coal combustion operation is suggested as one of thepromising technologies for carbon dioxide capture and storage in powerstation boilers. Air atmosphere is replaced by an O₂/CO₂mixture inoxy-coal combustion technology. Physical and chemical properties of CO₂and N2varies which cause the differences in pulverized coal ignition andflame stability, combustion reaction kinetics, furnace heat transfer andpollutant formation between oxy-coal combustion and conventional aircombustion. Oxy-coal combustion technology can be used not only innewly built power plant units but also in existing pulverized coal firedboilers when retrofit.Two different scales of pulverized coal combustion test systems werestudied in this paper, a40kW coaxial jets oxy-coal combustion test rig anda3MW air combustion tangential coal fired boiler. GAMBIT software wasused for setting up the three-dimensional structure and generating meshbased on the structure parameters and operation parameters of the twodifferent units and the numerical simulation was carried out with FLUENTsoftware.Numerical simulation on the effect of different oxygen concentrationsin the primary stream on the standoff distance was carried out for the40kW coaxial jets oxy-coal combustion test rig under given oxygenconcentration and excess oxidizer ratio conditions. The results show thatthe standoff distance is small and the flame is attached for primary stream oxygen concentration of20.9%. However, the standoff distance increasesand the flame is detached when primary stream oxygen concentrationdiminish to14.6%,10.0%,5.5%and0under the overall oxygenconcentration of40%and the excess oxidizer ratio of1.15. Highersecondary stream preheat temperature is helpful to form more stableflames under low primary stream oxygen concentration. The numericalresults agree well with the experimental ones in the University of Utah.Numerical simulation of both air combustion and oxy-coalcombustion was carried out for the3MW tangential pulverized coal firedboiler in order to match with the same air combustion furnace temperaturedistribution and heat flux distribution. Different cases were designed todetermine appropriate overall oxygen concentration for oxy-coalcombustion operation. Case of reduced primary stream oxygenconcentration was simulated considering pulverized coal safetransportation. Overall oxygen concentration as30%is appropriate tomatch the furnace temperature distribution and wall heat flux. Whenprimary stream oxygen concentration was reduced to21%, furnacetemperature level was very low. Numerical results show that lowtemperature distribution at low primary stream oxygen concentration couldbe improved by increasing the secondary stream preheat temperature.