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Study on Main Factors Influencing Acetylene Formation During Coal Pyrolysis in Arc Plasma
0957–5820/06/$30.00+0.00 # 2006 Institution of Chemical Engineers Trans IChemE, Part B, May 2006 Process Safety and Environmental Protection, 84(B3): 222– 226
www.icheme.org/psep doi: 10.1205/psep.05011
STUDY ON MAIN FACTORS INFLUENCING ACETYLENE FORMATION DURING COAL PYROLYSIS IN ARC PLASMA W. R. BAO , L. P. CHANG and Y. K. LU Key Laboratory of Coal Science and Technology (Taiyuan University of Technology), Ministry of Education and Shanxi Province, Taiyuan, P. R. China
A
cetylene from coal pyrolysis in arc plasma jet is simple and environmentally friendly with promising applications. A set of rank-ordered coal samples were selected in this study, in order to study the effects of volatile matter yield and the content of carbon, hydrogen and oxygen in raw coal on the yield of acetylene from arc plasma pyrolysis. The influence of feeding rate on the conversion of coal and acetylene yield was also investigated. The results showed that acetylene and carbon monoxide were the main gaseous products during coal pyrolysis under plasma conditions. A higher acetylene yield of 17 – 22% could be obtained from coal with volatile matter yield of 25– 40%. A high C/H mole ratio and low O/C mole ratio were favorable to the formation of acetylene. The percent conversion of coal, the yield of acetylene and the specific energy consumption (SEC) decreased as the feeding rate increased, but the trends in their changes were not identical. The relative volume fractions (RVF) in gaseous products of acetylene and carbon monoxide increased with increasing coal feeding rate until 5 g s21, while the RVFs of light hydrocarbons such as CH4, C2H4 and C3H6 gradually increased. The change in the selectivity for acetylene was not obvious when the feeding rate was less than 4 g s21. However, the portion of acetylene in the total gaseous products decreased rapidly when the coal feeding rate was further increased. Keywords: coal; plasma; pyrolysis; acetylene.
INTRODUCTION
1972; Qiu et al., 2004; Tian et al., 2001; Xie et al., 2002). These results showed that the yield of acetylene was affected by many factors such as coal properties and experimental conditions, and the yield and selectivity of acetylene formed in plasma jet were not high. Owing to the complexities of the structure and composition of coal, it is difficult to clearly understand the reaction mechanisms from the limited results of coal pyrolysis under plasma conditions. In order to reveal the influence of main factors on acetylene formation from coal pyrolysis under arc plasma, ten coals with different carbon contents and volatile matter yields were selected and used in the present experiments.
With its high energy density, high concentration of reactive active species and large unit processing capacity, the direct current arc plasma can cause many new reactions that would not proceed under ordinary conditions. It is these characteristics that lead to the essential difference in the production distribution from plasma pyrolysis from those through conventional pyrolysis of coal (Bond et al., 1963). Therefore, it is regarded as an attractive technique for the direct production of acetylene from coal or other carbonaceous materials. Acetylene is one of the most important and basic chemical raw materials with extensive application in chemical industry. Compared with the conventional processes for the production of acetylene from coal and calcium oxide, plasma pyrolysis not only simplifies the operation, but also avoids severe environmental pollution. Some studies of coal pyrolysis under plasma conditions have been carried out previously (Bao et al., 2004; Chakravartty et al., 1976; Dai et al., 1999; He et al., 2004; Lu et al., 2001; Nichoson and Littlewood,
METHODS AND MATERIALS Ten rank-ordered coal samples were selected from 10 major coal fields in China, i.e., Yangcheng (YC), Dongshan (DS), Jinyang (JY), Datong (DT), Xinzhao (XZ), Pingshuo (PS), Baode (BD), Yima (YM), Huolinhe (HLH) and Xianfeng (XF) mines. The coals were ground and sieved to the particle size of less than 0.074 mm. The results of their proximate and ultimate analyses are shown in Table 1. The experimental apparatus (Figure 1) mainly includes a power unit, an arc plasma generator, a bell and hopper, a reactor, a sampling system and a quenching unit.
Correspondence to: Dr Li-ping Chang or Dr Wei-ren Bao, Key Laboratory of Coal Science and Technology, Taiyuan University of Technology, No. 79 Yingze West Street, Taiyuan 030024, P. R. China. E-mail: [email protected]; [email protected]
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ACETYLENE FORMATION DURING COAL PYROLYSIS IN ARC PLASMA
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Table 1. Proximate and ultimate analyses of coals used in the present experiments. Ultimate analysis (wdaf%)
Proximate analysis (wt%) Coal type
Mada
Aad
Vdafb
H/C
O/C
C
H
Oc
SþN
YC DS JY DT XZ PS BD YM HLH XF
3.0 1.6 1.6 2.5 1.7 3.1 4.4 9.0 20.6 19.2
9.5 14.2 12.6 7.6 15.0 14.7 5.3 18.6 23.9 8.5
6.9 15.2 25.7 29.9 33.7 34.1 38.9 42.2 49.5 51.6
0.32 0.51 0.72 0.74 0.76 0.81 0.85 1.07 0.93 0.98
0.02 0.02 0.04 0.16 0.05 0.12 0.15 0.21 0.28 0.32
94.39 90.36 88.75 75.52 85.02 79.60 75.82 70.62 67.00 64.70
2.52 3.81 5.30 4.63 5.42 5.37 5.36 6.28 5.19 5.27
1.9 2.1 4.5 16.4 5.8 12.9 15.2 19.7 25.2 27.2
1.15 3.73 1.44 3.45 3.72 2.15 3.62 3.40 2.61 2.85
a
ad: air dry; bdaf: dry and ash-free; cby difference.
The tubular reactor made of stainless steel with a graphite lining has an inner diameter of 30 mm and a length of 400 mm. When the arc discharge was ignited and the arc column was generated, a plasma jet was formed after the working gases crossed the arc column. The coal was then injected into the reactor to blend with the plasma jet by carries gas. The power of the plasma generator was 55 kW. Coal sample was directly injected into the reactor with argon as the carrier gas through a single tube with the diameter of 5 mm located 20 mm downstream from the plane of the anode. The feeding rate was adjusted from 0.2 to 6.0 g s21 by the feeder according to scheduled experimental conditions. The flow rate of the working gas was 2.0 Nm3 h21 and 1.0 Nm3 h21 for Ar and H2, respectively. The flow rate of Ar carrier gas was 1.0 Nm3 h21. The gaseous products were collected by a sampling system and their compositions were analysed by a special gas chromatography with two channels and three detectors (two flame ionisation detectors and one thermal conductive detector). Argon was used as the tracer to estimate the amount of the components in the gaseous products, thus their corresponding yields could be calculated.
Figure 1. Schematic diagram of the experimental set-up for coal pyrolysis in arc plasma.
RESULTS AND DISCUSSION The Effects of Coal Properties on Acetylene Yield So far, there still exist some arguments regarding the mechanism of the formation of acetylene from coal pyrolysis under plasma conditions. Most researchers believed that the pyrolysis of coal involved two steps (Bao et al., 2004; Bittner and Wanzl, 1990; Lu et al., 2001; Xie et al., 2002). The volatiles were firstly released from coal when the temperature was over 1273 K with a heating rate higher than 106 K s21; the volatiles were further pyrolysed to form acetylene and other small fragments in the presence of atomic hydrogen or other high energy species, and the reaction time was less than 0.4 ms. It can be concluded that the release of volatiles from coal can directly affect the acetylene yield. A higher volatile matter yield was favorable to the formation of acetylene. The effects of volatile matter yield on the yield of acetylene are shown in Figure 2. The acetylene yield increased with increasing volatile matter yield, and reached about 22% at a volatile matter yield of 33.7%. However, the acetylene yield decreased when the volatile matter yield further increased. A low acetylene yield of 6.8, 6.0 and 4.3% was obtained from YM, HLH and XF coals with a high volatile matter yield of 42.20, 49.52 and 51.60%
Figure 2. The effect of volatile matter and carbon content on the yield of acetylene.
Trans IChemE, Part B, Process Safety and Environmental Protection, 2006, 84(B3): 222–226
224
BAO et al. radicals and a high carbon and hydrogen content would be favorable for the formation of acetylene. However, the fact that the yield of acetylene from the plasma pyrolysis of coals with higher volatile matter yield and hydrogen content was very low indicated that there existed competitive reactions between the formation of C2H2 and other gases, especially for the coal with high oxygen content. Therefore, it was deduced that CO and other light hydrocarbons must be also the main product except for C2H2, especially CO. This deduction will be sequentially discussed later. The Effect of Feeding Rate on the Conversion of Coal, Acetylene Yield and Selectivity
Figure 3. The effect of oxygen/carbon mole ratio on acetylene yield.
respectively. From Table 1, it could be seen that the carbon content showed a reversed trend with the volatile matter yield. But it was not also true that the acetylene yield increased monotonously with carbon content decreasing. There was a maximum value of acetylene yield at a carbon content of around 85%. These results show that there exist other influencing factors for the formation of acetylene from coal pyrolysis under plasma conditions except the volatile and carbon content. Figure 3 showed the effects of O/C atomic ratios of the substrate coals on their acetylene yields. Figure 4 showed the effects of H/C atomic ratios of the substrate coals on their acetylene yields. From Figures 3 and 4, it could be seen that a higher H/C ratio and lower O/C ratio are favorable to the formation of acetylene under the current experimental conditions. These results showed that a relatively high acetylene yield from the plasma pyrolysis of coal should have a proper content of carbon, hydrogen, oxygen content and volatile matter yield. Although the volatile matter yield played a key role in the extent of coal conversion and the yield of gaseous components, active carbon atom and hydrogen atom were the main
Figure 4. The effect of hydrogen/carbon mole ratio on acetylene yield.
The effects of feeding rate on the conversion of coal and acetylene yield at supplied energy were shown in Figure 5(a) and (b). The coal selected in the present experiments was DT coal with a medium volatile matter yield and H/C and O/C atomic ratios. It can be seen that both conversion of coal and acetylene yield gradually decrease
Figure 5. The conversion of coal and acetylene yield as a function of coal feeding rate.
Trans IChemE, Part B, Process Safety and Environmental Protection, 2006, 84(B3): 222–226
ACETYLENE FORMATION DURING COAL PYROLYSIS IN ARC PLASMA when the feeding rate is increased. When the coal feeding rate was higher than 2.0 g s21, the conversion of coal almost reached an asymptotic value, while the yield of acetylene decreased monotonously with increasing feeding rate. The yield of acetylene formed during coal plasma pyrolysis was obviously lower than the conversion of coal because not only acetylene but also carbon oxides and other light hydrocarbons were produced under the plasma conditions. The thermodynamics analysis showed that the products from coal plasma pyrolysis are governed by the temperature in the reactor. Methane is predominant among the products at 900 K and its yield decreases with increasing temperature. When temperature is above 1500 K, acetylene becomes the main product because the reaction becomes endothermic and the free energy of acetylene formation is less than that of the small molecular hydrocarbons such as methane and ethane. Once temperature reaches 3500 K, the reaction of acetylene formation overwhelms other reactions. Under the operation conditions of given energy supply, the feeding rate might affect the temperature of plasma reactor system and thus changed the conversion of coal and the yield of acetylene. The higher the coal feeding rate was used, the lower the temperature in the reactor system was. Change in the relative volume fraction (RVF) of gaseous products under plasma pyrolysis conditions were described in Figure 6. It is obvious that the acetylene and carbon monoxide were the main gaseous products and the volume concentration of all gaseous products increased with increasing feeding rate. The acetylene and carbon monoxide were predominant at low feeding rates. Their concentration increased quickly with feeding rate increasing and then slowly reached the asymptotic values. The concentration of other light hydrocarbons such as methane and ethylene increased slowly at low feeding rates and increased quickly at high feeding rates. This phenomenon is consistent with the thermodynamics analysis. The proportion of acetylene in gaseous hydrocarbon products is defined selectivity of acetylene. The relation between selectivity of acetylene and coal feeding rate was shown in Figure 7. It can be seen that the formation
Figure 6. The effect of coal feeding rate on the relative volume fraction (RVF) of gaseous components over the total product gases.
225
Figure 7. The selectivity of acetylene as a function of coal feeding rate.
of acetylene is not favorable at either very high or very low feeding rate. Good acetylene selectivities were observed at the feeding rates of 0.5 –4 g s21, showing a maximum selectivity of about 95%.
Figure 8. The relationship between coal feeding rate, acetylene yield and its specific energy consumption.
Trans IChemE, Part B, Process Safety and Environmental Protection, 2006, 84(B3): 222–226
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BAO et al. The Relationship Between the Yield and SEC of Acetylene
In order to achieve the smallest specific energy consumption (SEC) of acetylene and the maximum yield of acetylene, it was necessary to select a proper range of operating parameters. The relationship between SEC and the coal feeding rate or yield of acetylene are given in Figure 8(a) and (b). The SEC of acetylene decreased with increasing coal feeding rate, but it increased with increasing acetylene yield. When the feeding rate was slow, the pyrolysis of coal was nearly complete for definite heat enthalpy of the system, so the yield of acetylene was high. In the meantime, the RVF of acetylene in the reactive system was relatively low, so the SEC of acetylene was high. When coal feeding rate was high, the energy per unit mass of coal was reduced and the volatile matter could not be completely pyrolysed to form the acetylene, so the acetylene yield decreased and SEC of acetylene also increased. Combining Figures 5 and 6, it is found that the acetylene RVF increased with increasing coal feeding rate, but the acetylene yield decreased. It was therefore not advisable to pursue blindly a high yield of acetylene. The results in this experiment showed that the SEC and selectively of acetylene was optimal at an acetylene yield of 20– 30%. CONCLUSIONS The acetylene and carbon monoxide were the main gaseous products during coal pyrolysis under plasma conditions. A higher acetylene yield of 17– 22% could be obtained from the coal with a volatile matter yield of 25– 40%. The carbon content, H/C and O/C atomic ratios affect acetylene formation. There was an optimum range of carbon content of 75 – 90%. A higher H/C atomic ratio and a lower O/C atomic ratio favoured the formation of acetylene. The yield of acetylene was reduced by the formation of carbon monoxide or other hydrocarbons. The feeding rate had clear influence on acetylene formation. With increasing feeding rate, the RVF of acetylene among gaseous products increased. But the conversion of coal, acetylene yield and selectivity decreased when coal feeding rate was increased, while the yields of other light hydrocarbons such as CH4, C2H4 and C3H6 gradually
increased. The trend of the change in the specific energy consumption (SEC) of acetylene with coal feeding rate was different from that of the conversion of coal and acetylene yield. The changes in SEC were not obvious when the yield was less than 25%, and then increased rapidly with increasing acetylene yield. SEC and the selective of acetylene reached the optimality at coal feeding rate of 2 – 4 g s21. REFERENCES Bao, W.R., Lu, Y.K., Liu, S.Y. and Xie, K.C., 2004, Effects of properties on acetylene formation and coking in H2/Ar plasma pyrolysis, Journal of Fuel Chemistry and Technology, 32: 552 –555 (in Chinese). Bittner, D. and Wanzl, W., 1990, The significant of coal properties for acetylene formation in a hydrogen plasma, Fuel Processing Technology, 24(3): 311–316. Bond, R.L., Galbraith, I.F. and Lander, W.R., 1963, Production of acetylene from coal, using a plasma jet, Nature, 200: 1313–1314. Chakravartty, S.C., Dutta, D. and Lahiri, A., 1976, Reaction of coals under plasma conditions: direct production of acetylene from coal, Fuel, 55: 43–46. Dai, B., Fan, Y.S., Yang, J.Y. and Xiao, J.D., 1999, Effect of radicals recombination on C2H2 yield in process of coal pyrolysis by hydrogen plasma, Chem Eng Sci, 54(7): 957–959. He, X.J., Ma, T.C., Qiu, J.S., Sun, T.J., Zhao, Z.B., Zhou, Y. and Zhang, J.L., 2004, Mechanism of coal gasification in a steam medium under arc plasma conditions, Plasma Sources Science and Technology, 13: 446 –453. Lu, Y.K., Pang, X.Y., Bao, W.R. and Xie, K.C., 2001, Influence of granularity on coal pyrolysis in Ar/-H2 plasma, Journal of Fuel Chemistry and Technology, 29(1): 65–69 (in Chinese). Nichoson, R. and Littlewood, K., 1972, Plasma pyrolysis of coal, Nature, 236(6): 397–400. Qiu, J.S., He, X.J., Sun, T.J., Zhao, Z.B., Zhou, Y., Guo, S.H., Zhang, J.L. and Ma, T.C., 2004, Coal gasification in steam and air medium under plasma conditions: a preliminary study, Fuel Processing Technology, 85: 969 –982. Tian, Y.J., Xie, K.C. and Zhu, S.Y., 2001, Simulation of coal pyrolysis in plasma jet by CPD Model, Energy & Fuels, 15: 1354–1358. Xie, K.C., Lu, Y.K., Tian, Y.J. and Wang, D.Z., 2002, Study of coal conversion in an arc plasma jet, Energy Sources, 24: 1093–1098.
ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial supports of National Basic Research Program of China (2005CB221202) and Shanxi Province Program for Tackling Key Problems of Science and Technology (051161). The manuscript was received 30 September 2005 and accepted for publication after revision 30 December 2005.
Trans IChemE, Part B, Process Safety and Environmental Protection, 2006, 84(B3): 222–226
www.icheme.org/psep doi: 10.1205/psep.05011
STUDY ON MAIN FACTORS INFLUENCING ACETYLENE FORMATION DURING COAL PYROLYSIS IN ARC PLASMA W. R. BAO , L. P. CHANG and Y. K. LU Key Laboratory of Coal Science and Technology (Taiyuan University of Technology), Ministry of Education and Shanxi Province, Taiyuan, P. R. China
A
cetylene from coal pyrolysis in arc plasma jet is simple and environmentally friendly with promising applications. A set of rank-ordered coal samples were selected in this study, in order to study the effects of volatile matter yield and the content of carbon, hydrogen and oxygen in raw coal on the yield of acetylene from arc plasma pyrolysis. The influence of feeding rate on the conversion of coal and acetylene yield was also investigated. The results showed that acetylene and carbon monoxide were the main gaseous products during coal pyrolysis under plasma conditions. A higher acetylene yield of 17 – 22% could be obtained from coal with volatile matter yield of 25– 40%. A high C/H mole ratio and low O/C mole ratio were favorable to the formation of acetylene. The percent conversion of coal, the yield of acetylene and the specific energy consumption (SEC) decreased as the feeding rate increased, but the trends in their changes were not identical. The relative volume fractions (RVF) in gaseous products of acetylene and carbon monoxide increased with increasing coal feeding rate until 5 g s21, while the RVFs of light hydrocarbons such as CH4, C2H4 and C3H6 gradually increased. The change in the selectivity for acetylene was not obvious when the feeding rate was less than 4 g s21. However, the portion of acetylene in the total gaseous products decreased rapidly when the coal feeding rate was further increased. Keywords: coal; plasma; pyrolysis; acetylene.
INTRODUCTION
1972; Qiu et al., 2004; Tian et al., 2001; Xie et al., 2002). These results showed that the yield of acetylene was affected by many factors such as coal properties and experimental conditions, and the yield and selectivity of acetylene formed in plasma jet were not high. Owing to the complexities of the structure and composition of coal, it is difficult to clearly understand the reaction mechanisms from the limited results of coal pyrolysis under plasma conditions. In order to reveal the influence of main factors on acetylene formation from coal pyrolysis under arc plasma, ten coals with different carbon contents and volatile matter yields were selected and used in the present experiments.
With its high energy density, high concentration of reactive active species and large unit processing capacity, the direct current arc plasma can cause many new reactions that would not proceed under ordinary conditions. It is these characteristics that lead to the essential difference in the production distribution from plasma pyrolysis from those through conventional pyrolysis of coal (Bond et al., 1963). Therefore, it is regarded as an attractive technique for the direct production of acetylene from coal or other carbonaceous materials. Acetylene is one of the most important and basic chemical raw materials with extensive application in chemical industry. Compared with the conventional processes for the production of acetylene from coal and calcium oxide, plasma pyrolysis not only simplifies the operation, but also avoids severe environmental pollution. Some studies of coal pyrolysis under plasma conditions have been carried out previously (Bao et al., 2004; Chakravartty et al., 1976; Dai et al., 1999; He et al., 2004; Lu et al., 2001; Nichoson and Littlewood,
METHODS AND MATERIALS Ten rank-ordered coal samples were selected from 10 major coal fields in China, i.e., Yangcheng (YC), Dongshan (DS), Jinyang (JY), Datong (DT), Xinzhao (XZ), Pingshuo (PS), Baode (BD), Yima (YM), Huolinhe (HLH) and Xianfeng (XF) mines. The coals were ground and sieved to the particle size of less than 0.074 mm. The results of their proximate and ultimate analyses are shown in Table 1. The experimental apparatus (Figure 1) mainly includes a power unit, an arc plasma generator, a bell and hopper, a reactor, a sampling system and a quenching unit.
Correspondence to: Dr Li-ping Chang or Dr Wei-ren Bao, Key Laboratory of Coal Science and Technology, Taiyuan University of Technology, No. 79 Yingze West Street, Taiyuan 030024, P. R. China. E-mail: [email protected]; [email protected]
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ACETYLENE FORMATION DURING COAL PYROLYSIS IN ARC PLASMA
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Table 1. Proximate and ultimate analyses of coals used in the present experiments. Ultimate analysis (wdaf%)
Proximate analysis (wt%) Coal type
Mada
Aad
Vdafb
H/C
O/C
C
H
Oc
SþN
YC DS JY DT XZ PS BD YM HLH XF
3.0 1.6 1.6 2.5 1.7 3.1 4.4 9.0 20.6 19.2
9.5 14.2 12.6 7.6 15.0 14.7 5.3 18.6 23.9 8.5
6.9 15.2 25.7 29.9 33.7 34.1 38.9 42.2 49.5 51.6
0.32 0.51 0.72 0.74 0.76 0.81 0.85 1.07 0.93 0.98
0.02 0.02 0.04 0.16 0.05 0.12 0.15 0.21 0.28 0.32
94.39 90.36 88.75 75.52 85.02 79.60 75.82 70.62 67.00 64.70
2.52 3.81 5.30 4.63 5.42 5.37 5.36 6.28 5.19 5.27
1.9 2.1 4.5 16.4 5.8 12.9 15.2 19.7 25.2 27.2
1.15 3.73 1.44 3.45 3.72 2.15 3.62 3.40 2.61 2.85
a
ad: air dry; bdaf: dry and ash-free; cby difference.
The tubular reactor made of stainless steel with a graphite lining has an inner diameter of 30 mm and a length of 400 mm. When the arc discharge was ignited and the arc column was generated, a plasma jet was formed after the working gases crossed the arc column. The coal was then injected into the reactor to blend with the plasma jet by carries gas. The power of the plasma generator was 55 kW. Coal sample was directly injected into the reactor with argon as the carrier gas through a single tube with the diameter of 5 mm located 20 mm downstream from the plane of the anode. The feeding rate was adjusted from 0.2 to 6.0 g s21 by the feeder according to scheduled experimental conditions. The flow rate of the working gas was 2.0 Nm3 h21 and 1.0 Nm3 h21 for Ar and H2, respectively. The flow rate of Ar carrier gas was 1.0 Nm3 h21. The gaseous products were collected by a sampling system and their compositions were analysed by a special gas chromatography with two channels and three detectors (two flame ionisation detectors and one thermal conductive detector). Argon was used as the tracer to estimate the amount of the components in the gaseous products, thus their corresponding yields could be calculated.
Figure 1. Schematic diagram of the experimental set-up for coal pyrolysis in arc plasma.
RESULTS AND DISCUSSION The Effects of Coal Properties on Acetylene Yield So far, there still exist some arguments regarding the mechanism of the formation of acetylene from coal pyrolysis under plasma conditions. Most researchers believed that the pyrolysis of coal involved two steps (Bao et al., 2004; Bittner and Wanzl, 1990; Lu et al., 2001; Xie et al., 2002). The volatiles were firstly released from coal when the temperature was over 1273 K with a heating rate higher than 106 K s21; the volatiles were further pyrolysed to form acetylene and other small fragments in the presence of atomic hydrogen or other high energy species, and the reaction time was less than 0.4 ms. It can be concluded that the release of volatiles from coal can directly affect the acetylene yield. A higher volatile matter yield was favorable to the formation of acetylene. The effects of volatile matter yield on the yield of acetylene are shown in Figure 2. The acetylene yield increased with increasing volatile matter yield, and reached about 22% at a volatile matter yield of 33.7%. However, the acetylene yield decreased when the volatile matter yield further increased. A low acetylene yield of 6.8, 6.0 and 4.3% was obtained from YM, HLH and XF coals with a high volatile matter yield of 42.20, 49.52 and 51.60%
Figure 2. The effect of volatile matter and carbon content on the yield of acetylene.
Trans IChemE, Part B, Process Safety and Environmental Protection, 2006, 84(B3): 222–226
224
BAO et al. radicals and a high carbon and hydrogen content would be favorable for the formation of acetylene. However, the fact that the yield of acetylene from the plasma pyrolysis of coals with higher volatile matter yield and hydrogen content was very low indicated that there existed competitive reactions between the formation of C2H2 and other gases, especially for the coal with high oxygen content. Therefore, it was deduced that CO and other light hydrocarbons must be also the main product except for C2H2, especially CO. This deduction will be sequentially discussed later. The Effect of Feeding Rate on the Conversion of Coal, Acetylene Yield and Selectivity
Figure 3. The effect of oxygen/carbon mole ratio on acetylene yield.
respectively. From Table 1, it could be seen that the carbon content showed a reversed trend with the volatile matter yield. But it was not also true that the acetylene yield increased monotonously with carbon content decreasing. There was a maximum value of acetylene yield at a carbon content of around 85%. These results show that there exist other influencing factors for the formation of acetylene from coal pyrolysis under plasma conditions except the volatile and carbon content. Figure 3 showed the effects of O/C atomic ratios of the substrate coals on their acetylene yields. Figure 4 showed the effects of H/C atomic ratios of the substrate coals on their acetylene yields. From Figures 3 and 4, it could be seen that a higher H/C ratio and lower O/C ratio are favorable to the formation of acetylene under the current experimental conditions. These results showed that a relatively high acetylene yield from the plasma pyrolysis of coal should have a proper content of carbon, hydrogen, oxygen content and volatile matter yield. Although the volatile matter yield played a key role in the extent of coal conversion and the yield of gaseous components, active carbon atom and hydrogen atom were the main
Figure 4. The effect of hydrogen/carbon mole ratio on acetylene yield.
The effects of feeding rate on the conversion of coal and acetylene yield at supplied energy were shown in Figure 5(a) and (b). The coal selected in the present experiments was DT coal with a medium volatile matter yield and H/C and O/C atomic ratios. It can be seen that both conversion of coal and acetylene yield gradually decrease
Figure 5. The conversion of coal and acetylene yield as a function of coal feeding rate.
Trans IChemE, Part B, Process Safety and Environmental Protection, 2006, 84(B3): 222–226
ACETYLENE FORMATION DURING COAL PYROLYSIS IN ARC PLASMA when the feeding rate is increased. When the coal feeding rate was higher than 2.0 g s21, the conversion of coal almost reached an asymptotic value, while the yield of acetylene decreased monotonously with increasing feeding rate. The yield of acetylene formed during coal plasma pyrolysis was obviously lower than the conversion of coal because not only acetylene but also carbon oxides and other light hydrocarbons were produced under the plasma conditions. The thermodynamics analysis showed that the products from coal plasma pyrolysis are governed by the temperature in the reactor. Methane is predominant among the products at 900 K and its yield decreases with increasing temperature. When temperature is above 1500 K, acetylene becomes the main product because the reaction becomes endothermic and the free energy of acetylene formation is less than that of the small molecular hydrocarbons such as methane and ethane. Once temperature reaches 3500 K, the reaction of acetylene formation overwhelms other reactions. Under the operation conditions of given energy supply, the feeding rate might affect the temperature of plasma reactor system and thus changed the conversion of coal and the yield of acetylene. The higher the coal feeding rate was used, the lower the temperature in the reactor system was. Change in the relative volume fraction (RVF) of gaseous products under plasma pyrolysis conditions were described in Figure 6. It is obvious that the acetylene and carbon monoxide were the main gaseous products and the volume concentration of all gaseous products increased with increasing feeding rate. The acetylene and carbon monoxide were predominant at low feeding rates. Their concentration increased quickly with feeding rate increasing and then slowly reached the asymptotic values. The concentration of other light hydrocarbons such as methane and ethylene increased slowly at low feeding rates and increased quickly at high feeding rates. This phenomenon is consistent with the thermodynamics analysis. The proportion of acetylene in gaseous hydrocarbon products is defined selectivity of acetylene. The relation between selectivity of acetylene and coal feeding rate was shown in Figure 7. It can be seen that the formation
Figure 6. The effect of coal feeding rate on the relative volume fraction (RVF) of gaseous components over the total product gases.
225
Figure 7. The selectivity of acetylene as a function of coal feeding rate.
of acetylene is not favorable at either very high or very low feeding rate. Good acetylene selectivities were observed at the feeding rates of 0.5 –4 g s21, showing a maximum selectivity of about 95%.
Figure 8. The relationship between coal feeding rate, acetylene yield and its specific energy consumption.
Trans IChemE, Part B, Process Safety and Environmental Protection, 2006, 84(B3): 222–226
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BAO et al. The Relationship Between the Yield and SEC of Acetylene
In order to achieve the smallest specific energy consumption (SEC) of acetylene and the maximum yield of acetylene, it was necessary to select a proper range of operating parameters. The relationship between SEC and the coal feeding rate or yield of acetylene are given in Figure 8(a) and (b). The SEC of acetylene decreased with increasing coal feeding rate, but it increased with increasing acetylene yield. When the feeding rate was slow, the pyrolysis of coal was nearly complete for definite heat enthalpy of the system, so the yield of acetylene was high. In the meantime, the RVF of acetylene in the reactive system was relatively low, so the SEC of acetylene was high. When coal feeding rate was high, the energy per unit mass of coal was reduced and the volatile matter could not be completely pyrolysed to form the acetylene, so the acetylene yield decreased and SEC of acetylene also increased. Combining Figures 5 and 6, it is found that the acetylene RVF increased with increasing coal feeding rate, but the acetylene yield decreased. It was therefore not advisable to pursue blindly a high yield of acetylene. The results in this experiment showed that the SEC and selectively of acetylene was optimal at an acetylene yield of 20– 30%. CONCLUSIONS The acetylene and carbon monoxide were the main gaseous products during coal pyrolysis under plasma conditions. A higher acetylene yield of 17– 22% could be obtained from the coal with a volatile matter yield of 25– 40%. The carbon content, H/C and O/C atomic ratios affect acetylene formation. There was an optimum range of carbon content of 75 – 90%. A higher H/C atomic ratio and a lower O/C atomic ratio favoured the formation of acetylene. The yield of acetylene was reduced by the formation of carbon monoxide or other hydrocarbons. The feeding rate had clear influence on acetylene formation. With increasing feeding rate, the RVF of acetylene among gaseous products increased. But the conversion of coal, acetylene yield and selectivity decreased when coal feeding rate was increased, while the yields of other light hydrocarbons such as CH4, C2H4 and C3H6 gradually
increased. The trend of the change in the specific energy consumption (SEC) of acetylene with coal feeding rate was different from that of the conversion of coal and acetylene yield. The changes in SEC were not obvious when the yield was less than 25%, and then increased rapidly with increasing acetylene yield. SEC and the selective of acetylene reached the optimality at coal feeding rate of 2 – 4 g s21. REFERENCES Bao, W.R., Lu, Y.K., Liu, S.Y. and Xie, K.C., 2004, Effects of properties on acetylene formation and coking in H2/Ar plasma pyrolysis, Journal of Fuel Chemistry and Technology, 32: 552 –555 (in Chinese). Bittner, D. and Wanzl, W., 1990, The significant of coal properties for acetylene formation in a hydrogen plasma, Fuel Processing Technology, 24(3): 311–316. Bond, R.L., Galbraith, I.F. and Lander, W.R., 1963, Production of acetylene from coal, using a plasma jet, Nature, 200: 1313–1314. Chakravartty, S.C., Dutta, D. and Lahiri, A., 1976, Reaction of coals under plasma conditions: direct production of acetylene from coal, Fuel, 55: 43–46. Dai, B., Fan, Y.S., Yang, J.Y. and Xiao, J.D., 1999, Effect of radicals recombination on C2H2 yield in process of coal pyrolysis by hydrogen plasma, Chem Eng Sci, 54(7): 957–959. He, X.J., Ma, T.C., Qiu, J.S., Sun, T.J., Zhao, Z.B., Zhou, Y. and Zhang, J.L., 2004, Mechanism of coal gasification in a steam medium under arc plasma conditions, Plasma Sources Science and Technology, 13: 446 –453. Lu, Y.K., Pang, X.Y., Bao, W.R. and Xie, K.C., 2001, Influence of granularity on coal pyrolysis in Ar/-H2 plasma, Journal of Fuel Chemistry and Technology, 29(1): 65–69 (in Chinese). Nichoson, R. and Littlewood, K., 1972, Plasma pyrolysis of coal, Nature, 236(6): 397–400. Qiu, J.S., He, X.J., Sun, T.J., Zhao, Z.B., Zhou, Y., Guo, S.H., Zhang, J.L. and Ma, T.C., 2004, Coal gasification in steam and air medium under plasma conditions: a preliminary study, Fuel Processing Technology, 85: 969 –982. Tian, Y.J., Xie, K.C. and Zhu, S.Y., 2001, Simulation of coal pyrolysis in plasma jet by CPD Model, Energy & Fuels, 15: 1354–1358. Xie, K.C., Lu, Y.K., Tian, Y.J. and Wang, D.Z., 2002, Study of coal conversion in an arc plasma jet, Energy Sources, 24: 1093–1098.
ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial supports of National Basic Research Program of China (2005CB221202) and Shanxi Province Program for Tackling Key Problems of Science and Technology (051161). The manuscript was received 30 September 2005 and accepted for publication after revision 30 December 2005.
Trans IChemE, Part B, Process Safety and Environmental Protection, 2006, 84(B3): 222–226