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Show 3.2 EXPLORATORY STUDIES ON CERF COMBUSTOR The Combustion and Environmental Engineering Facility (CERF) at the Federal Energy Technology Center is equipped with a pilot scale 500,000 BTU/hr combustor which was designed to achieve similarity with full-scale utility boilers, in terms of replicating typical specification ranges for burner relative mass flow, radiant furnace temperature distributions, gas residence time, and convective section gas velocity profiles among others. It is equipped with the state-of-the-art personal-computer-based data acquisition and process control system. The main components of the C E R F include, a vertically down-fired refractory lined combustion chamber with a horizontal convective section, movable block (variable) swirl burners, natural gas supply system, solid fuel preparation, storage and transport system, and the pulse-jet bag house for particulate control. The facility is designed to evaluate the following fuel characteristics, (i) transport, handling and storage, (ii) combustibility, including flame stability and carbon conversion efficiency, (ii) ash deposition rates, heat transfer properties (e.g., emissivity and thermal conductivity), and deposit removal characteristics (e.g., soot blowing requirements), and flue gas emissions such as S 0 2 , N O x , C 0 2 , C O , total hydrocarbons and particulates among others. A more detailed discussion of the C E R F is described elsewhere (Freeman et al., 1997, 1998). The CREF combustor is 52 cm in diameter and approximately 375 cm in length, which is represented by 80,000 hexahedral/tetrahedral grid cells in the present simulation as shown in Figure2. The coal, biomass and air feed rates used in this simulation are given in Table 3. The coal studied in these simulations was utility ground Pittsburgh#8 with mean diameter of 60 u.m (range 20-150 |im), and composition of 75.77 wt. % C, 5.44 wt. % H, wt. 7.96 % 0 , 1.45 wt. % N, 2.65 wt. % S, and 7.61 wt. % ash. The biomass used in this study was switch grass ( S W G ) with the composition of wt. 47.58 % C, 5.55 wt. % H, 40.03 wt. % 0 , 0.76 wt. % N, 0.11 wt. % S, and 5.9 wt. % ash. The ellipsoidal switch grass particles were assumed to be of 1.0 m m in diameter, with the length to diameter ratio of approximately 3. The heat content for the dry coal and biomass was 13,643 and 7.980 BTU/lb, respectively. Figure 2 C E R F Combustor The predicted temperature distribution on a central radial plane for various coal/biomass flow rates (listed in Table 3) inside the C E R F combustor is shown in Figure 3. It may be seen from Figure 3 that the larger biomass particles exhibit delayed combustion past the primary pulverized coal flame, resulting in still-burning particles/sparklers entering the convective section and the bottom ash hopper. These findings are qualitatively in agreement with the reported observations of cofiring biomass in the C E R F combustor by Freeman et al. (1999). The probable cause of the delayed combustion for S W G in the C E R F combustor is that the bigger size biomass particles are almost falling freely under the influence of gravity without experiencing significant resistance from the fluid. This results in a considerably reduced residence time for biomass particles to undergo any significant char oxidation. On the other hand, coal particles are smaller which are suspended most of the time in the bulk gas and hence they have a large enough residence time (2.8 s for coal compared to 1.6 s for biomass) to undergo almost complete char oxidation. The predicted temperatures may be slightly higher (of the order of 5-10% ) in general because the current simulation does not include the dissociation of species in eddy dissipation model. The computational time to converge these simulations was of the order of 3-4 days on a cluster of 3 dual processors, pentium-pro PCs running under the linux operating system. |
