C O M B U S T I O N AND FLAME 56:245-249 (1984)
Emi~ion of Submicron Carbon from Pulverized Coal Combustion System M. SADAKATA, Y. KUROSAWA, and T. SAKAI Department of Chemical Engineering, Gunma University, Kiryu, Gunma, Japan
Submicron unburned carbon (S.U.C.) from pulverized coal combustion is significant in pollution control problems since the S.U.C. tends to condense harmful trace elements and easily passes through an electrostatic precipitator. There have been some studies relating to S.U.C. from pulverized coal combustion. Mclean et al. [1 ] and Seeker et al.  observed the formation of soot from combustion of coal particles in a CH 4air premixed flame. Flagan and Taylor  measured the size distribution of submicron fly ash particles and suggested that 75 wt.% of submicron particles was carbon. Ubhayakar et al.  also suggested soot formation during devolatilization of pulverized coal in hot combustion gases. However, there are no reports on the emission of S.U.C. from a practical pulverized coal combustion system. In this study, emission of S.U.C. was mainly investigated by using an experimental furnace which was designed to produce combustion conditions similar to a practical boiler. Figure 1 shows the experimental furnace. The outer diameter of the furnace is 80 cm, the inner diameter 30 cm, and the length 3 m. Table 1 shows the combustion condition of this experiment. The proximate and elemental analysis of coals used in this experiment is shown in Table 2. The particle size distribution of the coal was 90% through 200 mesh. The approximate size distribution for B coal is shown in Table 3 as an example.
Copyright© 1984by The CombustionInstitute Published by ElsevierSciencePublishingCo., Inc. 52 VanderbiltAvenue,New York, NY 10017
The fly ash was sampled at the position just after the final heat exchanger. The sample was then passed through an Andersen cascade impactor where fly ash was classified into 8 stages. The carbon content of the fly ash in each stage was analyzed by an elemental analyzer that analyzed the carbon, hydrogen, and nitrogen content of the ash. The structure of the submicron fly ash and carbon was observed by an electron microscope that could enlarge 300,000 times. Figure 2 shows the relations between carbon content of the sampled fly ash and particle diameter. In this figure, the unburned carbon content C is defined as follows: weight of carbon in the sampled ash C(%) =
weight of the sampled ash including carbon
Note that the unburned carbon content increases slightly as the ash diameter increases for C and D coals. This is because the total combustion time for char particles lengthens as the surface-to-volume ratio decreases with an increase in particle diameter. However, it is notable here that for all four coals the carbon content again sharply increased in the submicron range as the particle diameter decreased. This result suggests that the formation mechanism of unburned carbon in the submicron range is different from that of the unburned char. Since the
M. SADAKATA ET AL
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Fig. 1. Experimental model furnace and flue system. TABLE 1
Experimental Conditions of Combustion Coal feed rate [kg/h] Air ratio [ - ] Furnace heat release intensity [MW/m3] Residence time in the furnace [s] Secondary air preheating [K] Peak temperature in the furnace [K] Furnace outlet temperature [K]
5 1.2 0.28 2.8 573 1673 1423
Characteristics of the Coals Examined Proximate analysis wt.% Coal A B C D E
3.2 5.1 5.8 2.6 4.6
20.3 14.5 8.3 11.7 10.1
41.3 45.3 29.5 31.1 28.6
38.4 40.1 62.2 57.2 61.3
0.93 0.89 2.09 1.83 2.14
a Fixed carbon (wt.%)/volatile matter (wt.%). b Water-free base.
Ultimate analysis wt.% (water-free) C
62.8 64.9 77.3 72.7 73.5
5.16 5.56 3.98 3.85 3.78
O 10.6 14.1 9.00 10.4 11.0
N 0.92 0.93 0.79 0.75 0.72
0.30 0.03 0.70 0.56 0.77
EMISSION OF SUBMICRON CARBON
z~ A Coal • B Coal
o C Coal 8o
0 IJ E 0
[ ~m ]
Fig. 2. Relation between carbon content and particle diameter for ash sampled by Andersen sampler.
size o f soot carbon formed in the gaseous phase is usually smaller than 0.1 grn, it is inferred that this carbon is soot carbon that is generated during the combustion of the volatile matter. The H/C mass ratio of the present submicron carbon was less than 0.02. According to a previous study , the H/C mass ratio of soot carbon is 0.005-0.042,
A Size Distribution of B Coal Size (~m)
>149 102-149 74-102 56-74 56>
2 4 7 22 65
which is very low compared with char carbon. Therefore, the level of the H/C ratio of the present sample supports the above inference. Figure 3a shows the transmission electron micrograph of unburned carbon that was collected at the backup filter of the Andersen cascade impactor where the smallest particles less than 0.43 ~m were collected. Note that the chainlike structure of 20-30 nm beads characteristic of sootlike carbon could be observed clearly. Figure 3b shows the electron micrograph of a spherical ash particle that was collected a 150 mm downstream from the burner in the furnace. Some fine chain structure can be observed on the surface of the ash particle. From these results, the unburned carbon that was observed at high fractions in the submicron range of fly ash particles is concluded to be a sootlike carbon which is formed mainly during the combustion of volatile matter.
M. S A D A K A T A ET AL
(b) Fig. 3. Transmission electron micrograph of (a) S.U.C. collected at the backup filter of the Andersen sampler (coal name; A; magnification, X 100,000), (b) S.U.C. collected 150 mm downstream from burner in the furnace (coal name, E; magnification, × 15,000).
EMISSION OF SUBMICRON CARBON
This work was supported in part by a Grant-inAid for Fundamental Scientific Research, Ministry o.f Education, Science and Culture, Japan, Projects 5 703500 7 and Tanigawa Foundation.
REFERENCES 1. McLean, W. J., Hardesty, D. R., and Pohl, J. H.,
249 3. Flagan, R. C., and Taylor, D. D., Eighteenth Symposium (International) on Combustion, The Combustion Institute, 1980, p. 1227. 4. Ubhayakar, S. K., Stickler, D. B., Von Rosenberg, C. W., Jr., and Gannon, R. E., Sixteenth Symposium [International) on Combustion, The Combustion Institute, 1976, p. 427. 5. Sakai, T., and Sugiyama, Y., Kogyo Kagaku Zasshi 68:736 (1965).
Eighteenth Symposium (International) on Combustion, The Combustion Institute, 1980, p. 1239. 2. Seeker, W. R., Samue!sen, G. S., Heap, M. P., and Trolinger, J. D., Eighteenth Symposium (International) on Combustion, The Combustion Institute, 1980, p. 1213.
Received 14 June 1983; revised 12 December 1983