Energy & Fuels 2007, 21, 2805-2808
2805
Investigation on the Release Characteristics of Light Hydrocarbon during Pulverized Coal Pyrolysis Junjie Fan, Zhongxiao Zhang,* Jing Jin, and Jianmin Zhang Institute of Combustion and Gasification, College of Power Engineering, UniVersity of Shanghai for Science and Technology, Shanghai 200093, China ReceiVed January 27, 2007. ReVised Manuscript ReceiVed June 6, 2007
In this paper, different coals were respectively heated up in a tube furnace by program temperature control under the reburning condition. The pyrolytic components of light hydrocarbon were analyzed using the gaschromatographic technique, which shows that, under the same pyrolytic condition, the pyrolytic content of light hydrocarbon obviously decreases with the increase of the coal carbonized degree and the peak temperature value of light hydrocarbon released decreases with the decrease of the coal particle diameter. The main component of pyrolytic gas is CH4, and other contents are less, such as C2(C2H4, C2H6), C3(C3H6, C3H8), and C4H10. When coals are pyrolyzed, there is a critical particle size (dc) for each coal types. If dh > dc, the total amount of light hydrocarbon increases, with the particle size decreasing. Conversely, if dh e dc, it tends to be saturated with the particle size decreasing.
1. Introduction The increase in the human population and rapid expansion of industry has brought great serious pollution to our environment.1 And the main source of the air pollution is that we burn coal directly, which will produce a mass of NOx.2 Coal reburning is a highly effective technology for NOx reduction emission. A great deal of research and practical utility indicated that gaseous fuel reburning could cut down on NOx exhaust to 50% or above. Along with profound investigation on reburning technology, recent research shows that pulverized coal also can be used as a reburning fuel and may even be better than natural gas.3,4 For one thing, using pulverized coal as a reburning fuel, CHi + NO f N2 + ... (i ) 1, 2, and 3) was the main reaction after the coal had been pyrolyzed in the reburning area. Accordingly, NOx could be effectively carried out.5 It is considered that coal reburning has potential to reduce the NOx from pulverized fuel (PF) power stations in our country. The alternative use of natural gas is less attractive because of the lower availability and less developed distribution system. Coal is our abundant resource and is widely distributed. Thus far, many studies have been done on NOx release control and modeling studies, but not much work has been done on the NOx reduction mechanism during pulverized coal reburning. Coal reburning combustion mainly includes three zones: the primary combustion zone, reburning zone, and burnout zone. In the reburning zone, a mass of light hydrocarbon will be pyrolyzed under the deoxidized atmosphere, which has a great * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Kim, Y. S.; Hwang, G. C.; Bae, S. Y. Korean J. Chem. Eng. 1999, 16 (2), 161-165. (2) Yan, L.; Boon, C. L.; Norihiko, M.; Masateru, N.; Masayoshi, S. Energy Fuels 2002, 16 (1), 155-160. (3) Burch, T. E.; Tillman, F. R.; Chen, W. Y. Energy Fuels 1991, 5 (2), 231-237. (4) Chen, W. Y.; Ma, L. AIChE J. 1996, 42 (7), 1968-1976. (5) Zhong, B. J.; Fu, W. B. J. Combust. Sci. Technol. 2000, 6 (1), 7784.
effect on NOx reduction during the processes of coal reburning. Therefore, it is important to understand the characteristic of light hydrocarbon during pulverized coal pyrolysis.6 Badzioch and Hawksley showed that under the condition of a lower oxygen concentration, the pyrolytic behavior of coal is the same as it is under the atmosphere condition of pure N2.7 Therefore, in this paper, light hydrocarbon releasing behaviors of pulverized coal were studied using gas chromatography under a pure N2 atmosphere. Effects of atmosphere, temperature, coal particle sizes, and coal types on coal pyrolysis behaviors were obtained. It not only has an important guidance function on technical design but also contributes to the understanding of the NOx reduction mechanism during pulverized coal reburning. 2. Experimental Section 2.1. Experimental System. Figure 1 is the scheme of the experimental system of the gas chromatogram. In the coal pyrolytic experiment, experimental coal weights of about 1 g were used, N2 was used as the carrier gas, the airflow was 200 mL/min (to ensure the same flux, D07-7B/ZM mass flowmeter and D08-4/ZM mass flow control were used to accurately measure and control air flux), the heating velocity was 20 °C/min, and the final pyrolytic temperature was 900 °C. Pyrolytic gases were respectively collected in the temperature range of ∼200-300, ∼300-400, ..., and ∼800900 °C. Light hydrocarbon compounds were analyzed using a GC7890II-Al2O3 chromatogram column and hydrogen flame ionization detector (column temperature was 60 °C; gas inlet room temperature was 100 °C; and detecting room temperature was 150 °C). 2.2. Coal Types. Three types of test coals were used: Longkou lignite, Shenfu bituminous coal, and Jincheng anthracite. The proximate and ultimate analyses of coals are shown in Table 1. (6) George, D.; Monthida, R.; Bruce, D. J. Energy Fuels 2006, 20 (5), 1997-2007. (7) Badzioch, S.; Hawksley, P. G. W. Ind. Eng. Chem. 1970, 9 (4), 521530.
10.1021/ef0700477 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/24/2007
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Figure 1. Scheme of the experimental system. 1, N2 cylinder; 2, pressure gauge; 3, mass flowmeter; 4, mass flow control; 5, tube furnace; 6, ceramic boat; 7, temperature controller; 8, gas container; 9, gas chromatogram; 10, computer. Table 1. Ultimate and Proximate Analyses of Coalsa proximate analysis (%)
ultimate analysis (%)
coal type
Mad
Aad
Vad
FCad
Cad
Had
Oad
Sad
Nad
Longkou lignite Shenfu bituminous coal Jincheng anthracite
8.40 1.76 2.81
22.10 30.11 19.44
39.70 23.06 7.00
29.80 45.07 70.75
49.90 53.68 71.53
3.40 3.31 1.94
14.22 9.66 2.49
0.60 0.51 0.93
1.38 0.97 0.85
a ad, air-dried basis; M , moisture content; A , ash content; V , volatile content; FC , fixed carbon; C , carbon content; H , hydrogen content; O , ad ad ad ad ad ad ad oxygen content; Sad, sulfur content; Nad, nitrogen content.
Figure 3. Effect of coal types on the release characteristics of light hydrocarbon during coal pyrolysis (particle size of 15 µm).
Figure 2. Distribution of the release characteristics of light hydrocarbon during coal pyrolysis (Longkou lignite, particle size of 15 µm; the y axis is the volume percentage of light hydrocarbon).
3. Results and Discussion 3.1. Distribution of the Release Characteristics of Light Hydrocarbon during Coal Pyrolysis. The distribution of the release characteristics of light hydrocarbon during coal pyrolysis is shown in Figure 2, and the average particle size of Longkou lignite is 15 µm. The y axis is the volume percent of light hydrocarbon within the gas mixture. It shows that the main component of pyrolytic gas is CH4, other contents such as C2(C2H4, C2H6), C3(C3H6, C3H8), and C4H10 are less, and the contents of light hydrocarbon show a decreasing trend with the increase of the C content in the coal. It also shows that Longkou lignite begins to be pyrolyzed when the temperature reaches 350 °C. When the temperature is up to ∼500-600 °C, pyrolytic velocity promptly increases and a mass of light hydrocarbon was released. As the temperature further heats up, the increment of light hydrocarbon contents do not continue to enlarge, but on the contrary, it tends to drop off. This is mainly because the greater part of aromatic hydrocarbon alkyl side chains and aliphatic chains in coal are already pyrolyzed to be light hydrocarbon and release at lower temperatures. Therefore, the upward trend of light hydrocarbon release is seen to increase up to a maximum value and then stabilize.
3.2. Effect of the Coal Type on the Release Characteristics of Light Hydrocarbon during Coal Pyrolysis. The effect of coal type on the release characteristics of light hydrocarbon is shown in Figure 3. Figure 3 indicates that, if coal types are different, there are large variations in the total amount of volatilized material and the temperature of the onset of coal pyrolysis. Longkou lignite shows the highest extent of pyrolysis and lowest initiation temperature, followed by Shenfu bituminous coal and then Jincheng anthracite. Contents of H, O, and N are higher in Longkou lignite and Shenfu bituminous coal, because their degrees of carbonization are lower. As the degree of carbonization increases, the bond strengths will also increase, which makes the structures more stable. This is accompanied by a higher activation energy for pyrolysis and a higher temperature for the start of light hydrocarbon release. Figure 3 indicates that Longkou lignite and Shenfu bituminous coal begin to be pyrolyzed when the temperature reaches 350 °C but it is 450 °C for Jincheng anthracite. Longkou lignite has the lowest extent of coalification. Its moisture8 and ash contents are higher, and the pore volume is greater, which make it easier for pyrolytic products to be released from the particles. The H/C, O/C, and N/C ratios and aliphatic compound content are greater than in the higher rank coals. There is a higher aliphatic component within the developing aromatic structure of the young coal. In other words, the aromatic structure9,10 contains alkyl side chains and a large density of bridge bonds, together with a greater number of (8) Iino, M. Energy Fuels 2002, 16, 1-2.
Light Hydrocarbon during PulVerized Coal Pyrolysis
heteroatom bonds. The alkyl chains, heteroatom bonds, and bridge bonds are weaker than the aromatic bonds,11 and this means that the average activation energy is lower for highvolatile coal12 and it is much easier for the low-rank coal to be pyrolyzed. Jincheng anthracite has the highest extent of coalification, which has resulted in a decrease in the H, O, and N contents and the development of a more aromatic structure.13 The acting forces of polarizable π electron (π-π) are continuously reinforced,14-17 and the size of the aromatic ring structures and the extent of condensation of the structure increases with the coal rank. Therefore, the aromatic structure of the anthracite is harder to be pyrolyzed. From this part of the study, it is obvious that Longkou lignite produces the highest amount of light hydrocarbons, followed by Shenfu bituminous coal and Jincheng anthracite. Therefore, lignite coal is the most ideal reburning fuel. 3.3. Effect of the Coal Particle Size on the Release Characteristics of Light Hydrocarbon during Coal Pyrolysis. The effect of the coal particle size on combustion has long been a research subject. Recently, the project of pulverized coal particle combustion provided a new way to understand the particle size effect.18 Longkou lignite, Shenfu bituminous coal, and Jincheng anthracite are respectively used as test coals in this experiment, and effect of the coal particle size on the release characteristics of light hydrocarbon during coal pyrolysis are shown in Figure 4. Figure 4 shows that the release velocity of light hydrocarbon increases with the diminishing of the coal particle size, and when the average particle size is about 30 µm, the total amount of light hydrocarbon released tends toward its maximum value. This shows that there exists an optimal particle size. If dh > dc, the total amount of light hydrocarbon increases, with the particle size decreasing. Conversely, if dh e dc, it is at the maximum value, and then further decreases in the particle size do not affect the result. Lots of new micropores were produced on the surface of forming char during coal pyrolysis, because of the release of volatile matters. Moreover, these micropores might be further enlarged by the reaction of the char. These changes increase the char surface area and enhance the capillary voidage and channel structure.19 Furthermore, with the diminishing of the coal particle size, light hydrocarbon might be through a more intensive method, “spray regime”, to be released from the granule interior to the exterior, which would enhance the roughness, voidage, and size of the coal coke surface. This is advantageous for light hydrocarbon to be released by way of free diffusion.20 (9) Eskay, T. P.; Britt, P. F.; Buchanan, A. C., III Energy Fuels 1997, 11 (6), 1278-1287. (10) Chen, R. H.; Kafafi, S. A.; Stein, S. E. J. Am. Chem. Soc. 1989, 111 (4), 1418-1423. (11) Suuberg, E. M.; Peters, W. A.; Howard, J. B. Ind. Eng. Chem. Process Des. DeV. 1978, 17 (1), 37-46. (12) Burnham, A. K.; Oh, M. S.; Crawford, R. W.; Samoun, A. M. Energy Fuels 1989, 3 (1), 42-55. (13) Fu, J. M.; Liu, D. H.; Sheng, G. Y. Textbook of Geochemistry of Coal-Formed Hydrocarbon; Science Press: Beijing, China, 1990. (14) Mao, J. X.; Mao, J. Q.; Zhao, S. M. Textbook of Clean Combustion of Coal; Science Press: Beijing, China, 1998. (15) Gorbaty, M. L. Fuel 1994, 73 (12), 1819-1828. (16) Qin, K. Z.; Guo, S. H.; Li, S. Y. Chin. Sci. Bull. 1998, 43 (18), 1912-1918. (17) Kashimura, N.; Takanohashi, T.; Saito, I. Energy Fuels 2006, 20 (4), 1605-1608. (18) Xiumin, J.; Chuguang, Z.; Jianrong, Q.; Jubin, L.; Dechang, L. Energy Fuels 2001, 15 (5), 1100-1102. (19) Xie, K. C. Textbook of Coal Structure and Its ReactiVity; Science Press: Beijing, China, 2002. (20) Fan, J. J.; Jin, J.; Zhong, H. Q. Boiler Technol. 2005, 36, 38-41.
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Figure 4. Effect of the coal particle size on the release characteristics of light hydrocarbon during coal pyrolysis.
Accordingly, the main reason of how to result this critical saturated phenomenon might include two aspects. First, for the heat-transfer angle, with the diminishing of the coal particle size, the coal-specific surface area, heat-transfer rate, and masstransfer velocity increase remarkably and the release distance of pyrolytic products becomes shortened, which were produced in the center of the coal granule. In addition, the escape work decreases, which is needed for light hydrocarbon to overcome the release. Therefore, the total amount of light hydrocarbon markedly increased. Second, for the secondary reaction21 to react, the secondary reaction of light hydrocarbon needs to occur in coal that mainly includes pyrolysis, cohesion, and polymerization.22 When dh is bigger than dc, coal particle sizes are relatively bigger and migration distances are longer; therefore, the needed escape work is bigger for light hydrocarbon. The main reactions for light hydrocarbon inside the coal are cohesion and polymerization. With a decreasing coal particle size, the surface area, heat-transfer rate, and mass-transfer velocity increase and the release distance (and time) for products of pyrolysis to be released from the particles becomes smaller. This reduces the extent of the secondary reactions of light hydrocarbons within the char structure; thus, the amount of light hydrocarbon increases. However, when dh is less than dc, the release amount of light hydrocarbon tends to be saturated; the (21) Serio, M. A.; Hamblen, D. G.; Markham, J. R.; Solomon, P. R. Energy Fuels 1987, 1 (2), 138-152. (22) Cen, K. F.; Yao, Q.; Luo, Z. Y. Textbook of AdVanced Combustion Dynamics; Zhejiang University Press: Hangzhou, China, 2002.
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main factors that effect the light hydrocarbon release is the secondary reactions. As the coal particle size is further diminished, the residence time of the pyrolytic decomposition product ceaselessly reduces and most light hydrocarbon (C2, C3, and C4) released without having the secondary reactions. Therefore, the amplitude of the total amount is unobvious and tends to be saturated. However, on the other hand, with the diminishing of the coal particle size, the demand for the milling apparatus increases and the cost for coal powder preparation also increases. Thus, in this experimental condition, having considered the aspects of combined economical efficiency, the optimum critical saturation particle size is 30 µm. 4. Conclusion In this paper, different coals were respectively subjected to be heated up in the tube furnace by program temperature control under the reburning condition with different coal particle sizes. The components of light hydrocarbon were analyzed using the
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gas-chromatographic technique, and some important conclusions were obtained as follows: (1) The main component of the pyrolytic gas is CH4, and other contents, such as C2(C2H4, C2H6), C3(C3H6, C3H8), and C4H10, are less. (2) The amount of light hydrocarbon formed during coal pyrolysis obviously decreases with the extent of coalification. (3) When coals are pyrolyzed, there is a critical particle size (dc) for each coal types. If dh > dc, the total amount of light hydrocarbon increases, with the particle size decreasing. However, once dh e dc, the release of light hydrocarbons has reached the limiting value and no further increases occur with further decreases in size. Acknowledgment. The research assistance received from the Fund for Doctor Startup of USST (X619), China, and the National Basic Research Program (2004CB217706-03), China, for this study is gratefully acknowledged. Furthermore, the authors thank the reviewers of this paper for many helpful and constructive comments. EF0700477
