Acknowledgment
The authors wish to express their deep appreciation to Dr. C. Y. Wen, West Virginia University, for his helpful advice. The review of the manuscript by Dr. L. A. Davis, Nagoya University, is also deeply acknowledged. Nomenclature
A t = cross sectional area of bed, cm2 Co2 = oxygen concentration, mol/cm3 D = molecular diffusivity of oxygen in air, cm2/s d , = particle diameter, mm or cm E = NO, emission index, mol of NO,/g of feed coal f c = fractional conversion of char g = acceleration of gravity, cm/s2 K* = elutriation rate constant, g/(cm2.s) h , = combustion rate constant, cm/s hd = mass transfer coefficient, cm/s M c = molecular weight of carbon, g/mol r a = present particle radius, cm r , = present radius of unburned core, cm ro = initial coal radius, cm R = gas constant, cal/(mol-K) T = temperature, K u g = superficial air velocity, cm/s u,f = superficial velocity a t incipient fluidization, cm/s = weight of bed material, g X, = initial ash content X C = initial carbon content 0 = time, s
wb
0 = mean residence time, s X = air ratio = fraction of ash remaining on the particle
apparent density of ash, g/cm3 p c = apparent density of char, g/cm3 pg = density of gas, g/cm3 p , = density of particle, g/cm3 pa =
Literature Cited (1) Horio, M., Nenryo Kyohai-Shi, 56,871 (1977). ( 2 ) Chiba, T., Horio, M., Furusawa, T., Mori, S., Kagaku Kogaku, 42,652 (1978). ( 3 ) Jonke, A. A,, Vogel, G. J., Carls, E. L., Ramaswami, D., Anastasia, L., Jaary, R., Haas, M., AICHE Syrnp Ser., N o 126, 68, 241 (1972). (4) Furusawa, T., Honda, T., Takano, J., Kunii, D., PQC.Chern. Eng Congr., IProc.1, 2nd, 1236 (1977). ( 5 ) Fuksawa, T.., Honda, T., Takano, J., Kunii, D., “Fluidization”, Cambridge University Press, New York, 1978, p 316. (6) Horio, M.,Mori, S., Muchi, I., Proc. Int. C o n i Fluid. Bed Cornbust., 5th, 2,605 (1978). (7) Furusawa, T., Kunii, D., Oguma, A,, Yamada, N., Kagaku Kogaku Ronbunshu, 4,562 (1978). (8)Toba, Y., Ogisu, Y., Kawamura, Y., Nenryo Kyohai-Shi, 56,657 (1978). (9) Kuni;, D., Levenspiel, O., “Fluidization Engineering”, Wiley, New York, 1969. (10) Field, M. A., Gill, D. W., Morgan, B. B., Hawksley, P. G. W., “Combustion of Pulverized Coal”, BCURA, Leatherhead, 1967. (11) Zenz, F. A,, Weil, N. A., AIChE J., 4,472 (1958).
Receiiled for reuieu: J u l y 23, 1979. Accepted April 8, 1980.
An Experimental Study for Low-NO, Fluidized-Bed Coal Combustor Development. 2. Performance of Two-Stage Fluidized-Bed Combustion Toshimasa Hirama”, Minoru Tomita, and Tomio Adachi Government Industrial Development Laboratory, Hokkaido, Sapporo, 061-01, Japan
Masayuki Horio Department of iron and Steel Engineering, Nagoya University, Nagoya, 464, Japan ~~~
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A new concept of a low-polluting fluidized-bed combustor (FBC) with high combustion efficiency, the function-allotted dual FBC (FAD-FBC), was tested with an experimental two-stage FBC under atmospheric conditions. The two-stage FBC showed minimum NO, emission around a primary air ratio of 1.0, and the emission level was approximately 1/2 of that of the single-stage FBC. The CO emission level was lowered to less than 500 ppm even when the residual oxygen concentration in flue gas was 2%. The combustion efficiency reached 98% when the bed temperatures of both stages were 800 “C and the primary and total air ratios were 1.0 and 1.15, respectively. I t was found that the two-stage FBC system makes it possible to reduce NO, emission and to increase combustion efficiency to the levels of pressurized FBC. Future research needs for the development of FAD-FBC are also discussed. In recent years remarkable progress has been made in the mechanistic understanding of NO, emission from fluidizedbed combustion (1-6). Although many factors are still not clear, it is a t least an accepted fact that the emission is the 960
Environmental Science 8.Technology
overall result of the competition between NO, formation and decomposition. Because of the complicated network of NO, formation and decomposition processes, the emission level depends highly on the local atmosphere and the details of gas-solid contacting schemes. Therefore, we can state that the reduction of NO, emission from fluidized-bed combustion is a highly design-oriented subject. In part 1 (13)it has been confirmed that substoichiometric conditions are effective in lowering NO, emission. However, in order to assure a high efficiency of combustion it is necessary to inject secondary air so that the total air ratio is greater than unity. The final target of our work is to establish a twostage combustion system by applying a fluidized bed to each stage of the so-called “staged combustion”. The system is named FAD-FBC (function-allotted dual fluidized-bed combustor) because of the clearly separated functions of each stage, as described in part 1 (13). The objective of the present paper is to test the concept of FAD-FBC using a laboratory-scale, two-stage fluidized bed combustor. The NO, and CO emission characteristics, combustion efficiency, and ash particle elutriation are studied and discussed in comparison with the results in part 1 (13).Opti0013-936X/80/0914-0960$01.OO/O @ 1980 American
Chemical Society
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mum conditions for minimizing NO, emission while maintaining high combustion efficiency are researched experimentally by varying the air ratio to each stage and the temperatures of first and second beds. Comparisons of the performance of the two-bed combustor with the previously reported performance of bench/pilot scale plants are made, and future research needs are discussed.
Water
Carryover L
Experimental A schematic diagram of the experimental apparatus is shown in Figure 1.The inner diameter and the height of the first stage were 10.8 and 70 cm, respectively. In the second stage those values were 13.3 and 100 cm, respectively. Gas distributor, overflow tube, and screw feeder of the first stage were the same as used in part 1. For the gas distributor of the second stage, a stainless steel plate with 14 caps was used. The caps were made of stainless steel (0.6 cm i.d.) with 4 perforations of 0.3 cm diameter drilled in a lateral direction. Secondary air was introduced just above the gas distributor of the second stage through a cross-aranged stainless steel tube (1.5 cm i.d.) with 32 perforations of 0.2 cm diameter. The first-stage bed was composed of coal-derived ash. Inert silica sand with a size range of 0.35-1.0 mm was used as the bed material of the second stage. The bed temperatures of first and second stages were controlled manually by varying the water flow rate through each cooling tube immersed in the beds. Gas was sampled at two sites: the cyclone exit and just below the second-stage gas distributor. The procedures for gas analysis were explained in part 1. Experimental conditions were as follows: coal, Taiheiyo coal (same as in part 1);coal feed rate, 2.1 kg/h (230 kg/(m2.h), based on a cross section of the first stage); bed temperature of the first stage, t b l , = 800-950 "C; bed temperature of the second stage, t b 2 , = 800-900 "c;primary air ratio (the ratio of primary air to stoichiometric air), Al, = 0.65-1.15; and total air ratio, A, = 1.05-1.25. A few runs were also carried out with no particles in the second stage.
Experimental Results and Discussion NO, and CO Emissions. The effects of the bed temperature and air ratio on NO, emission were investigated for the minimization of NO, emission. Figure 2 shows the experimental results obtained by varying the air flow rate into each stage at the total air ratio, A, of 1.15. The NO, emission index is defined as the moles of NO, generated per gram of feed coal. Figure 2a shows the emission index for the gas sampled at the cyclone exit, E , namely, total NO, emission from the twostage FBC. Figure 2c shows the index for the gas sampled a t the inlet of the second stage, E l , namely, the NO, emission from the first stage, and Figure 2b shows the difference between E and E l , namely, the NO, emission from the second stage. The index E increases remarkably with a decrease in the primary air ratio, AI, under 0.9 and increases slightly with an increase in A1 over 1.0. E shows a minimum a t a A1 value of -1.0, and the minimum value of E decreases with a decrease in bed temperature. The precise picture of NO, emission from the two-stage FBC can be seen from the emission index of each stage, E1 and Ez. As shown in Figure 2c, the values of El are less than those from the single-stage runs as reported in part 1.The index E1 a t A 1 below 0.9 decreases with a rise in bed temperature of the first stage, t b l , but the index a t A1 above 0.9 increases with a rise in t b l . This tendency differs from the results reported in part 1 ( 1 3 )and is identical with the results of Jonke et al. ( 7 ) and Furusawa et al. (8).The value of E2 decreases extremely with an increase in A1 and with a decrease in the secondary air ratio, Az. I t becomes negative a t A1 > -0.9.
+Overflow
( u n i t : mm )
Figure 1. Experimental apparatus 101
1
(a) total emission
See(b) for legends
-
-
2
x
2nd bed material
,
silica sand I
P
l.900
90014
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0 1 06
oA 0
.--O1
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1.0 primary air ratio, Ai ( - )
Ob %m&yair
0.2 ratio, A? (-1
1.2 I
0
Figure 2. Effects of bed temperatures of first and second stages and primary air ratio on NO, emissions, when total air ratio is at a constant value of 1.15: (a) total emission; (b) emission in second stage; and (c) emission in first stage Volume 14, Number 8, August 1980
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Figure 3 shows the relation between the total NO, emission and total air ratio, A, a t XI = 1.0. The emission level from the two-stage FBC a t 800 "C is -l/> of that of the single-stage FBC. T h e NO, concentration in the flue gas a t X < 1.2 is lowered below 150 ppm, which is a corrected value to the residual oxygen concentration in the flue gas of 6%. Even when the bed temperature is 900 "C, the value of E from the two-stage FBC is considerably low as compared to the value from the singlestage FBC a t 800 "C. In dual-bed combustion several factors play important roles for effective NO, emission reduction. The supplemental reaction in the second stage is obvious, but the second stage, in addition, serves as a good atmosphere for NO, reduction in the first stage. Char holdup in the first stage is undoubtedly higher in the dual-bed system than that in the single-stage combustion due to the baffling of entrained particles by the second-stage distributor. The axial temperature profile is also changed by the thermal insulating effect of the second bed, as shown in Figure 4.As can be seen from this figure, in our single-stage runs the reactions initiated in the bed were probably quenched due to the low freeboard temperature. This is the most likely explanation for the less temperaturesensitive behavior of NO, emission from single-stage combustion (part 1, Figure 4) than that from the first stage of two-stage runs (this paper, Figure 2c). A reasonable explanation for the negative E2 a t XI > 0.9 is that the nitrogen compounds contained in the volatile matter of coal are almost completely converted to NO, in the first stage, and NO, decomposition by char in the second stage exceeds NO, generation from the burning of residual char. At X1 < 0.9 NO, generation from the gas-phase nitrogen compounds exceeds the NO, reduction by char, and, therefore, E2 becomes positive. This is supported by the fact that the observed E2 a t XI < 0.8 is much greater than the calculated value based on an assumption t h a t only the nitrogen in the fine char transported from the first stage to the second stage is converted to NO,. The fluidized particles in the second stage prolong the residence time of fine char and assure a thermal level for NO, decomposition. Figure 5 shows the relationship between CO emission level and residual oxygen level in flue gas. The broken line shows the results of single-stage runs. In the single-stage runs, CO concentration in the flue gas is -0.2% a t the residual oxygen concentration of 3%.On the other hand, in the two-stage FBC CO concentration is
