OCR Text |
Show The summary of predicted different combustion species, unburned carbon, and temperature at the exit of the C E RF combustor is given in Table 4. It may be seen that the predicted unburned carbon in the present co-firing configurations increased with the amount of switch grass present in the coal/biomass blends because of the following reasons: (a) The long residence time (2.8 seconds) for the coal particles, which was sufficient for the coal particles to undergo complete combustion. The small residence time (1.6 seconds) for large switch grass particles which was enough for them to devolatilize but not sufficient to undergo any significant char oxidation. (b) The switch grass particles were considered as mono-sized ellipsoids with the minor axis as 1.0mm and an aspect ratio of 3. It should also be mentioned here that the increased burnout rate (or reduced unburned carbon) of 85% coal and 15% S W G (in column 2 of Table 4) compared to any other co-firing configuration is due to smaller thermal input into the system (please see the different feed rates in table 3) which will be corrected in future simulations at constant excess air. These simulation results will be confirmed in future C E R F cofiring tests in order to help validate the model. Table 4: Summary of Predicted Exit Temperature, Species and the Unburned Carbon for different biomass/coal cofiring configurations Exit Conditions Temperature (K) 02 (Dry %) C02(Dry%) CO (Dry ppmdv) H20 (%) Unburned Carbon on Ash free basis (Mass %) Coal: 100% SWG: 0% 2007 4.16 14.4 349 5.8 1.3 Coal: 8 5% SWG: 15% 1953 6.2 12.63 1 5.2 0.4* Coal: 6 6% SWG: 33% 1954 5.06 14 2 8.05 5.08 Coal: 3 3% SWG: 66% 1947 5.44 13.77 0 10.97 7.55 Coal:0% SWG: 100% 1952 6.2 14.7 0 13.08 9.15 lower total thermal input (refer to coal/biomass feed rate in Table 2) into the system NOx Reduction in CERF Combustor Exploratory simulations were performed to see the impact of reburning switch grass with coal on NO< emissions in the C E R F combustor. The coal and air feed rates used for the pulverized coal combustion (case-a in Figure 4) are 34 lb/hr (15 kg/hr) and 440 lb/hr (200 Kg/hr), respectively. The air feed rate for case-b through the transport and secondary ports was 140 kg/hr. The tertiary air was injected about 30 c m downstream of the biomass injection port at a rate of 55 kg/hr. It is predicted that the injection of 1 5 % of biomass (by weight) in the main heat release zone with an this fuel/air stoichiometry reduced the N O x by 4 6 % when compared with the pulverized coal combustion in the same combustor. In the present simulations, the mass transport of NO species includes the effects of convection, diffusion. production and consumption of N O and related species in the system. The effect of residence time in N O v mechanisms, a Lagrangian reference frame concept, is included through the convection terms in the governing equations written in Eulerian reference frame (refer to Fluent Users manual (1999) for details). The kinetic mechanisms of N O x formation and destruction are based on mean turbulent reaction rates which are calculated using the probability density function (PDF) approach. Figure 4 shows the temperature contours on a radial plane in the combustor for cases when (a) only pulverized coal is used, and (b) biomass is injected 90 c m downstream of the pulverized coal burner. The temperature inside the combustor in the latter (in general) is lower than the temperature in the former. In the latter case, a fuel rich zone is created by reducing the secondary air, and maintaining the global stoichiometry bv |