Energy & Fuels 1996,9,866-869
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Study on Behaviors of Coal Nitrogen Converted to Tar Yong Chen, Hitoki Matsuda, Shigekatsu Mori,* and Masanobu Hasatani Department of Chemical Engineering, Nagoya University, Furo, Chikusa, Nagoya, Japan 464
Yu-Shen Xie Institute of Chemical Metallurgy, Academia Sinica, Bezjing, China Received March 8, 1 9 9 6
To develop a new method for reducing the NO, emissions in coal combustion processes, the possibility of removing the nitrogen with coal tar was confirmed by the experimental study of the formation behaviors of the tar and coal-N. Six different coals were pyrolyzed by using a free-fall pyrolyzer in an atmosphere of Ar gas a t various heating temperatures. It is explained that (1) both the tar yield and the conversions of coal-N to tar-N increase with increasing pyrolyzing temperature until about 873 K and then decreases as the temperature continuously increases; (2) the contents of nitrogen compounds such as carbazole, naphthoquinoline, and isoquinoline in the tar are larger a t 973 K than that at 1173 K. On the basis of the experimental results presented here, the NO, reduction effects in the combustion of the coal preremoved tar-N were estimated in both cases of high-temperature coal combustion as pulverized coal-fire combustion and low-temperature coal combustion as fluidized bed combustion. It is found that the conversion of coal-N to NO, in the case of high-temperature coal combustion can be reduced by about 20%, and it can be reduced by about 40% in the case of low-temperature coal combustion.
Introduction It is well-known that the nitrogen oxides are produced in the coal combustion processes mainly by the oxidation of the volatile-N and the char-N. The volatile-N is the major source of total NO, emissions in coal combustion, since the conversion of volatile-N t o NO, is larger than that of char-N to N0,.1,2 Sinozaki et al.3indicated from their experiments of coal combustion using circulating fluidized bed (CFB) that the combustion of lower fuel ratio (fixed carbodvolatile matter) coal shows higher formation of NO,. Recently, various NO, control techniques of coal combustion processes have been developed such as load reduction: low excess air comb~stion,~ twostage combustion,6-12flue gas recir~ulation,'~ fluidized
* Corresponding author.
Abstract published in Advance ACS Abstracts, July 15, 1995. (1)Pershing, D. W.; Wendt, J. 0. L. Symp. Int. Combust., fProc.116 1977, 389. (2) Pohl, J. H.; Sarofim, A. F. Symp. Int. Combust., fProc.116 1977, 491. (3)Sinozaki, S. Proc. 4th SCEJ (Soc. Chem. Eng., Jpn.) Symp. Circulating Fluidized Beds 1991, 43. (4) Ctvrtnicek, T. E.; Rusek, S. J. U.S. EPA Report No. EPA-BOO/ 7-77-006,January 1977. (5) Blakeslee, C. E.; Burbach, H. E. Presented a t the Air Pollution Control Association 65th Annual Meeting, Miami Beach, FL, June 1822, 1972. (6) Vatsly, J . U.S.EPA Report No. IERL-RTP-1083, October 1980, P 1. (7) Ando, J . U.S.EPA Report NO.EPA-600/7-79-205, August 1979. (8)Ishihara, Y.; Fukuzawa, H. The US.-Japan Exchange of NOx Control Technical Information Conference, Tokyo, May 25-27, 1981. (9) Azuhata, S.; Narato, K.; Kobayashi, H.; Arashi, N.; Morita, S.; Masai, T. Symp. Int. Combust., [ProcJ 21 1986. (10) Miyamae, S.; Kiga, T.; Ikebe, H.; Suzuki, K. 1986 Fall Meeting, WSSICI, Paper No. 86-6, Tucson (October 1986). (11)Crawford, A. R.; Manny, M. W.; Gregory, M. W.; Bartok, W. Presented at the EPA Symposium on Stationary Source Combustion, Atlanta, GA, September 1975. (12) Selker, A. P. Presented at the EPA Symposium on Stationary Source Combustion, Atlanta, GA, September 1975.
bed combustion,14low-NO, burners,15-17selective catalytic reduction,18 and so on. To reduce the NO, emissions in coal combustion processes, a new preremoved technology of nitrogen in the coal with tar is studied here. The experiments with six different coals were carried out by using a free-fall pyrolyzer in an atmosphere of Ar gas at various heating temperatures. The effects of pyrolysis temperature on the yields of pyrolysis products and the conversions of coal nitrogen to pyrolysis products are discussed. To support interpretation of the behaviors of nitrogen content in tar, the nitrogen compounds in tar are detected. Furthermore, based on the experimental results presented here, the NO, reduction effect in the combustion of the coal preremoved tar-N are estimated in both cases of high-temperature coal combustion as pulverized coal-fire combustion and low-temperature coal combustion as fluidized bed combustion.
Experiments Apparatus and Procedure. Figure 1 shows a schematic diagram of the experimental apparatus which consists of a free-fall pyrolyzer and a tar capture condenser. The reactor is a quartz tube with length 640 mm, inside diameter 44 mm. The reactor temperature is monitored with thermocouples which are set in different positions of 5,100, and 300 mm high from the reactor bottom, and the temperature is controlled by adjusting the electric power supplier. The pyrolysis is carried (13) Lim, K. J . et al. U.S.EPA Report No. EPA-600/7-80-075a,April 1980.
(14) Beer, J . M.; Sarofim, A. F.; Lee, Y. Y. 6th Int. Conf. Fluidized Bed Combust. 1980,3,942. (15) Heap, M. P.; Lowes, T. M.; Walmsley, R.; Bartelos, H. U S . EPA Report No. EPA-65012-73-021, 1973; p 141. (16) Armento, W. J.; Sage, W. L. AIChE Symp. Ser. 1975, 71, No. 148, 63. (17) Garg, A. Chem. Eng. Prog. 1994, January, 46. (18) Cho, S. M. Chem. Eng. Prog. 1994, January, 39.
0887-0624/95/2509-0866$09.00/0 0 1995 American Chemical Society
Energy & Fuels, Vol. 9, No. 5, 1995 867
Behavior of Coal Nitrogen Converted to Tar Pt-Pt Rh
cooled water
1
2
'
a"
' 473 573 613 773 873 973 107311731273 Temperature [K] (a)
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Figure 1. Schematic diagram of experimental apparatus. out in an atmosphere of Ar gas at the temperature range between 573 and 1273 K. When the reactor temperature rises to a prescribed temperature, coal particles are supplied from the hopper into the reactor at the feed rate of 0.5 g/min; the estimated heating rate of the coal particles is about 1000 Ws. Released volatile matter is carried to the condenser with Ar gas. The coal tar is obtained from the condensed volatile matter. The conversions of the coal nitrogen (coal-N) to the char nitrogen (char-N)and the tar nitrogen (tar-N) are determined by analyzing the nitrogen content in the char remaining in the reactor and the tar with CHN-corder (MT2-YANACO).The conversion of coal-N to the gas nitrogen (gas-N)is determined by the difference between the coal-N and the volatile-N (gas-N tar-N). The compositions of nitrogen compounds in the tar are detected by using liquid chromatography (HITACHI L-6200). Samples. In this study, six different coals of Sangei, Kaipin, Kaixi, Loy Yang, Newlands, and Kangra are pyrolyzed. The proximate analysis of these coals is determined by weighing the residue products from coal pyrolysis and coal burning in an electric furnace. The ultimate analysis is observed by CHN-corder (MT2, Yanaco). These data obtained here are shown in Table 1. It is found that the nitrogen content of the six coals range between 0.7 and 2.0 w t %, and it is independent of the fuel ratio (=fured carbodvolatile) of the coal. The sample coals are pulverized to a particle size 37-63 pm and they are air dried at 378 K for 24 h before the pyrolysis.
E
473 571 613 773 873 973 10731173127: Temperature [K] (b)
Figure 2. Effect of temperature on the distribution of pyrolysis products.
-
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Figure 3. Relation between the tar yield and fuel ratio.
It is also found from Figure 2 that the tar yield of Loy Yang coal is larger than that of Kaipin coal. In Figure 3, the maximum tar yield of six different types of coals observed here and the data presented by Tylerlg are shown by the relationship between the fuel ratio of coals. It is found from this figure that the tar yields decrease exponentially as the fuel ratio increases. Conversion of Coal-N. To show the effect of pyrolysis temperature on the conversions of coal nitrogen to pyrolysis products, the conversions of nitrogen of six coals to char-N, tar-N, and gas-N at various temperatures are shown in Figure 4. Similar to the effect of temperature on the tar yield, the conversion of coal-N to tar-N attains maximum value at about 873 K for any coals, and it decreases as the temperature rises above 973 K. It is also found in Figure 5 that the conversion of coal-N to tar-N increases exponentially with decreasing the fuel ratio.
Results and Discussion Products Distribution. The effect of pyrolysis temperature on the yields of pyrolysis products is observed for each of the six coals, and the effects of all coals show similar tendency. Typical yield data of products are shown in Figure 2, a and b, for Loy Yang and Kaipin coals, respectively. Tar yields increase with increasing temperature, and they decrease after reaching a maximum value at about 873 K. The gas yields sharply increase and the tar yield also sharply decrease above 973 K. It is considered for this reason that the gas is produced by secondary cracking reactions of the tar formed at low temperature.
Table 1. Properties of Various Coals coal Kaixi (Kx)
Sangei (SG) Kaipin (KP) Loy Yang (LY) Newlands (NL) Kangra (KG)
location China China China Australia Australia South Africa
proximate analysis (dry basis, w t %) FC VM ash fuel ratio 63.0 68.2 55.1 46.3 59.3 68.8
25.4 22.6 32.0 51.3 26.5 18.3
11.6 9.2 13.0 2.4 14.2 12.9
2.5 3.0 1.7 0.9 2.2 3.8
C 80.1 81.0 76.8 66.1 73.0 82.3
ultimate analysis (daf w t %) H N S+O(bydiffl
*
4.1 4.0 4.6 4.3 4.1 3.9
1.2 1.1 1.0 0.7 1.2 2.0
14.6 13.9 17.6 28.9 21.7 11.8
Chen et al.
868 Energy & Fuels, Vol. 9, No. 5, 1995 g
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SAMPLE I mg/lOml PACKING MATERIAL GH-CII COLUMNSIZE 4mmx150mm COLUMN TEMP.: 313K
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Figure 6. Chromatogram of nitrogen compounds in tar. 100 x90
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Figure 4. Effect of temperature on the conversion of coal-N. Y
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Figure 5. Relation between the conversion of coaLN to tarN.
Pohl et aL2have proposed from their pyrolysis experiments of Montana and Pittsburgh coals that most of the coal nitrogen evolution occurred at temperature above 1223 K. It can be also found in Figure 4 that the conversion of coal-N to gas-N increases quickly when the temperature rises above 1173 K. It is considered that the nitrogen compounds in coal consist of lowtemperature nitrogen and high-temperature nitrogen derivatives and they are released at temperatures below and above 1173 K, respectively, and the volatile-N in low-temperature (below 1173 K) coal combustion processes is less than that in high-temperature (above 1173 K) coal combustion processes. On the other hand, since the char works as a reducing agent to decompose NO to N2, the conversion of char-N to NO, is lower than that of volatile-N to NO,. Therefore, the conversion of coal-N to NO, in low-temperature combustion processes is lower than that in high-temperature combustion processes. Behavior of Nitrogen Compounds in Tar. To support interpretation of the behavior of tar-N, the nitrogen compounds in tar are detected and the effects of temperature on it are observed. Although tar-N is a complex mixture including different nitrogen compounds like quinoline, isoquinoline, 2-methylquinoline,
0.5
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Figure 7. Conversion of nitrogen to nitric oxides presented by Pohl et a1.2
carbazole, naphthoquinoline and so on,2o carbazole, quinoline, isoquinoline, and naphthoquinoline are detected by using Liquid Chromatography (LC). Figure 6 shows the observed chromatograms of nitrogen compounds in coal tar produced at 973 and 1173 K. The change of the content of compounds with temperature can be observed by chbge of absorbency of LC spectrum though to determine quantitatively the content of Ncompounds is difficult. The contents of nitrogen compounds like carbazole, naphthoquinoline, and isoquinoline are larger at 973 K than that at 1173 K, which is due to secondary cracking reactions of the nitrogen compounds. Estimation of Conversion of Coal-N to NO,. It was shown above that the conversion of coal-N to tar-N attains maximum value when the tar yield reaches a maximum at 873 K and that every coal employed here shows similar tendency. Pershing et al.l have indicated for the NO, emissions from coal combustor that the volatile-NO, formed from volatile-N is the major contributor to total fuel-NO,. Moreover, the conversion of coal nitrogen to NO, presented by Pohl et a1.2 is shown in Figure 7. It is shown from this figure that when the fuel equivalence ratio is 0.5 to 2.0, the conversion of char-N to NO, changes from 17 to 4.0%, volatile-N to NO, is 83 to lo%, and total coal-N to NO, is 100 to 14%. It can be concluded from this evidence that if the tar-N can be removed from the coal before the combustion, the conversion of volatile-N to NO, is diminished and then the conversion of coal-N to NO, can be controlled. For both the cases of the high-temperature coal combustion at 1700 K like the pulverized coal-fired combustion and of the low-temperature coal combustion at 1173 K like the fluidized bed combustion, this reduction effect is estimated by using the coal preremoved tar-N. Here, Loy Yang coal is selected as a typical example. At first, the conversion of coal-N to volatile-N at 1700 K is about 100%which was estimated
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Energy & Fuels, Vol. 9, No. 5, 1995 869
Behavior of Coal Nitrogen Converted to Tar Loy Yang 1
to tar
.
to volatile
50 40
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900 1100 1300 1500 1700 Temperature [K]
Figure 8. Estimation of the ratio of tar-N to volatile-N.
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6 0.4 9
0.3
3 0.2 F, b 0.1
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Figure 9. Estimation of the conversion of coal-N t o NO,.
by extrapolating the result on Figure 8 giving the temperature of 1700 K the ratio of tar-N to volatile-N is 20%. The conversion of coal-N to volatile-N a t 1173 K is 50%, and the ratio of tar-N to volatile-N is about 42.6%; they are obtained from Figure 8. Then, the conversions of nitrogen of 100 g of Loy Yang coal to NO, are estimated by using these data and the data of the conversion of coal-N t o NO, obtained from Figure 7.The results estimated here are summarized in Figure 9. The filled symbols show the case of untreated tar-N and the open symbols show the case of removed coal-N. The ~~
(19)Tyler, R. J. Fuel 1980,59, 218. (20) Xu,W. C.; Tomita, A. Jpn. Inst. Fuel 1988,67 (11, 14.
ranges enclosed by the closed and open five-point and four-point stars show the reduction effects of NO, in both cases of high-temperature coal combustion and lowtemperature coal combustion, respectively. It is found that the conversion of coal-N to NO, in the case of hightemperature coal combustion can be reduced by about 20%, and it can be reduced by about 40% in the case of low-temperature coal combustion with normal fuel equivalence ratio which is about 1.0.
Conclusions The pyrolysis of six different of coals were carried out by using free-fall pyrolyzer in an atmosphere of Ar gas at various heating temperature. The following results are explained. 1. Both the tar yield and the conversions of coal-N to tar-N increase with increasing pyrolysis temperature until about 873 K and then decrease as the temperature continuously increases. 2. The contents of nitrogen compounds such as carbazole, naphthoquinoline, and isoquinoline content in the tar are larger at 973 K than that a t 1173 K. Based on the experimental results presented here, the NO, reduction effect in the combustion of the coal preremoved tar-N was estimated in both cases of hightemperature coal combustion as pulverized coal-fire combustion and low-temperature coal combustion as fluidized bed combustion. It is found that the conversion of coal-N t o NO, in the case of high-temperature coal combustion can be reduced by about 20%, and it can be reduced by about 40% in the case of lowtemperature coal combustion. Acknowledgment. This paper is a part of work carried out by the International Research Team “Interfluid” supported by the New Energy Development Organization (NEDO), Japan, at the universities of Nagoya, Chubu, and Hamburg-Harburg. EF950042M
