Behavior of CFB Combustor

Experiments of burning have been conducted for different kinds of coal, including bituminous coal (No. 1), sub-bituminous coal (No. 2), caking coal (No. 3) and low-grade coal (No. 4). Proximate and ultimate coal analyses are listed in Table V. The operating behavior of this combustor will be briefly summarized below. 

Experimental results (Li et a/., 1991) indicate that the axial temperature distribution is highly dependent on the secondary-to-total air ratio. When this ratio is less than about 0.3, the axial temperature profile is essentially uniform except for the region approaching the exit of the combustor, as shown in Fig. 17. When this ratio exceeds about 0.4, however, the temperature in the middle of the combustor becomes lower than those at the two ends. On the other hand, experimental results also indicate that increasing the secondary air ratio is favorable for supressing NO emission. If this ratio is greater than 0.5, NO emission can be controlled to less than 65 ppm. Therefore, it is necessary to locate the secondary air inlet properly and choose the secondary air ratio in order to optimize between efficient combustion and low NO emission. 

Distributions of bed temperature along reduced bed height for various second air ratios (after Li et al., 1991). 

The residual carbon contents at different axial locations of the combustor were measured in the pilot plant tests (Li et al., 1991), as shown in Fig. 18. These data show that axial variations in carbon content with temperature (from 810 °C-923 °C) are as a whole rather slight, but mean carbon content increases with decreasing excess air ratio. Besides, for excess air ratios greater than 1.2, the carbon content at the top of the combustor is somewhat less than that at the bottom, while for excess air ratio less than 1.2, the opposite tendency is evident. In conclusion, for this improved combustor, an excess air ratio of 1.2 is considered enough for carbon burn-out, leading to reduced flue gas and increased heat efficiency as compared to bubbling fluidized bed combustion. That is probably attributable to bubbleless gas-solid contacting for increased mass transfer between gas and solids in the fast fluidized bed, as explained by combustion kinetics. 

In this pilot plant test, natural limestone containing 34% CaO, crushed to below 2 mm, was used as the S02 sorbent. Below a bed temperature of about 850°C, the S02 retention increases with Ca/S molar ratio. When this ratio approximates to 2, the S02 content in the flue gas is reduced to about 100 ppm, corresponding to an S02 retention efficiency of about 83%, as can be seen in Fig. 20. 

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