CO emission was as low as 500 ppm even for a residual oxygen concentration in flue gas of 2%. The values of combustion efficiency increase with an increase in primary air ratio and with a rise in bed temperature. For primary and total air ratios of 1.0 and 1.15, respectively, the value of combustion efficiency was 98% a t a bed temperature of 800 “C, which is roughly 4% higher than that of the single-stage FHC. The size of elutriated ash is smaller than that from a single-bed combustion. The elutriation rate of ash from the two-stage FBC is mainly controlled by the ash transportation rate from the first stage to the second stage. The presence of an additional bed and the two-stage combustion scheme are found to be highly effective in reducing NO, and CO emissions and in increasing combustion efficiency. I t has been confirmed by comparison of the present results with the data in the literature that the proposed FAD-FBC concept could be a new mode of atmospheric fluidized-bed combustion, which guarantees low pollution and high combustion efficiency equivalent to those of pressurized combustion.
tb2
= bed temperature of second stage,
“c
= superficial gas velocity in first stage, cm/s u02 = superficial gas velocity in second stage, cm/s 17 = overall combustion efficiency (heat available) u01
A = total air ratio (the ratio of total air to stoichiometric air) A1 = primary air ratio (the ratio of primary air to stoichiometric air) A 2 = secondary air ratio (the ratio of secondary air to stoichiometric air) Literature Cited (1) Lange, H B., J r , AIChE S l m p Ser N o 126, 68,17 (1972) (2) Edwards, H. W., AIChE S>mp Ser N o 126, 68,91 (1972)
( 3 ) De Soete. G G., Proc Int S>mp Combust, Combust I n s t , Ijth, iiq’isi -in48 ___ I
(4) Furusawa, T., Kunii, D.. Oguma, A., Yamada, N., Kagahu Kogahu
Ronbunshu, 4,562 (1978). (5) Horio, M., Mori. S.. Muchi. I.. Proc. Int. Conf. Fluid. Bed Combust., j t h , 2, 605 (1978). (6) Gibbs, B. M., Hedley, A. B., “Fluidization”, Cambridge University Press, New York, 1978, p 235. ( 7 ) Jonke, A. A., Vogel, G. J.,Carls, E. L., Ramaswami, D., Anastasia, L., Jarry, R., Haas. M., AIChE S y m p . Ser. No. 126, 68, 241 (1972). (8)Furusawa. T., Honda, T., Takano, J., Kunii, D., Pac. Chem. Eng. Congr , [Proc ] , 2 n d , 1236 (1977). (9) Toba. Y., Ogisu, Y., Kawamura, Y., N e n r J o K ~ o h a i - S h i56,657 , (1977). (10) -Gibbs, B. M., Pereira, F. J., Be&, J. M., Inst. Fuel S>,mp.,Ser. 1 , D-6 (1975). (11) Roberts, A. R., Stantan, J. E., Wilkins. D. M., Beacham, B., Hoy, H. R., Inst. Fuel Sq’mp., Ser. I , D-4 (1975). (12) Vogel, G. J., Swift, W. M., Montagna, J. C., Lenc, J. F., Jonke, A . A,, Inst. Fuel Sq’mp. Ser. I , D-3 (1975). (13) Hirama, T.,Tomita, M., Adachi, T.,Yamaguchi, H., Horio, M., Enciron. Sci. Technol., preceding paper in this issue. (14) Nutkis. M. S.. in ref 6. D 252. (15) Harrison, D., Aspinall, P.N.,Elder, J., Trans Inst Chem Eng , 52,213 (1974).
Acknoic l e d g m e n t
The authors wish to express their deep appreciation to Dr. C. Y. Wen, West Virginia University, for his helpful advice. The review of manuscript by Dr. L. A. Davis, Nagoya University, is also deeply acknowledged. N o me nc 1at u re
d, = particle diameter, mm E = NO, emission index for total emission, mol of NO,/g of coal E1 = NO, emission index for emission from first stage, mol of NO,/g of coal E2 = NO, emission index for emission from second stage, mol of NO,/g of coal t b l = bed temperature of first stage, “c
ReceLced for reuieu Jul> 23, 1979 Accepted April 8 , 1980
Some Etching Studies of the Microstructure and Composition of Large Aluminosilicate Particles in Fly Ash from Coal-Burning Power Plants L. D. Hulett” and A. J. Weinberger Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn. 37830
An etching method, using 1%HF, for removing glass phases from the aluminosilicate matrix of fly ash, leaving mullite and quartz phases, has been developed. It has been applied to a study of the structure of coarse aluminosilicate fly ash particles. Etched particles contain only crystalline residues, mullite in the acicular and chunky forms and quartz. Quantitative analyses with the scanning electron microscope (X-ray fluorescence induced by electron beam) indicate that the acicular mullite has a composition approximating 3(A1203).2(SiO2). Fe and Ti were isomorphically substituted for A1 and Si in the mullite. Na, Mg, K, and Ca were removed with the glass phases. There is reason to believe that trace elements in fly ash exist to a large extent as solid solutions and their chemical species are defined by the phases that contain them. This work describes a method for separating glass and crystal phases so that this thesis can be explored.
0013-936X/80/0914-0965$01 .OO/O
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Fly ash produced by the burning of coal in electric power plants consists mostly of aluminum-silicon-oxygen compounds, mixed with smaller amounts of sodium, magnesium, potassium, calcium, and titanium. This is commonly called the aluminosilicate matrix. Depending on coal sources, iron may also be a major component, occurring both as a separate, matrix of magnetic spinels (variations of Fe304) and other oxides, and as a dissolved form in the aluminosilicate. In addition to the elements of high concentration, which constitute the matrices, there are numerous trace elements which are considered detrimental to the environment. Considerable effort is being expended to determine valence states and compound forms, “chemical species”, of trace elements because these determine both their availabilities to the environment and their interactions with plant and animal life once they are released. The authors are speculating that trace elements exist to a
1980 American Chemical Society
Volume 14, Number 8, August 1980
965
large extent as solid solutions in the glass, mullite-quartz, and magnetic spinel phases. \'e have reported a certain degree of evidence for this ( I ) . Therefore, if physical and chemical methods can be developed for isolating the different phases so that they can be separately analyzed, we will have made progress in the trace element speciation problem. We have found an etching procedure that will preferentially dissolve glass phases in the aluminosilicate matrix and leave crystalline mullite and quartz as residues. Trace element analyses of separated phases will be published a t a later date. In the present paper we are reporting what we have learned about the morphologies, compositions, and distributions of mullite, quartz, and glass phases in the aluminosilicate matrix.
Experimental Fly ashes from four TVA plants, selected on the basis of firing conditions and coal types, were studied. Two plants, Bull Run and Kingston, use East Tennessee coal, which is low in calcium and iron, and they both use tangential firing of pulverized coal. The Johnsonville plant uses Western Kentucky coal and tangential firing. The Paradise plant uses cyclone firing of crushed coal from Western Kentucky sources. The samples from Bull Run, Kingston, and Johnsonville were collected from the mechanical cyclone, upstream from the electrostatic precipitators. The Paradise sample was taken from the inlet to the electrostatic precipitator. T o facilitate size fractioning, that is, eliminate sintering as much as possible, each specimen was washed with water. For three of the plants the washed particles were sieved into three fractions: (1) greater than 200 pm; (2) 100-200 pm; (3)
