ARTICLE pubs.acs.org/EF
Integrated Process of Coal Pyrolysis with CO2 Reforming of Methane by Dielectric Barrier Discharge Plasma Xinfu He, Lijun Jin, Ding Wang, Yunpeng Zhao, Shengwei Zhu, and Haoquan Hu* State Key Laboratory of Fine Chemicals, Institute of Coal Chemical Engineering, School of Chemical Engineering, Dalian University of Technology, No. 2 Linggong Road, Dalian 116024, PR China ABSTRACT: A novel method to increase the tar yield through an integrated process of coal pyrolysis with CO2 reforming of methane by dielectric barrier discharge plasma was put forward. The pyrolysis of Shenmu (SM) subbituminous coal was carried out under different atmospheres, including N2, H2, and different plasmas of H2, CH4, CH4/H2, CO2/H2, and CH4/CO2/H2 (1/1/2, MG) to check the effect of the integrated process on improving the tar yield. Two kinds of coal samples, SM and Huolinhe (HLH) lignite were used in the integrated process to investigate the effects of gas flow rate, pyrolysis time, H2 composition in the mixed gas (CH4/CO2/H2), and discharge power on product yields. The results showed that the effect of different atmospheres on increasing the tar yield generally has the following order, especially at the temperature range 400500 °C: MG plasma > CH4 plasma ≈ CH4/H2 plasma ≈ CO2/H2 plasma > H2 plasma > H2 > N2. The tar yield of SM coal pyrolysis under MG plasma at the discharge power of 40 W for 7 min is about 2.0 and 1.8 times as that under N2 and H2 at 400 °C, respectively.
’ INTRODUCTION Pyrolysis is the first step in the thermal degradation processes of coal conversion such as combustion, gasification, carbonization, and liquefaction, and it is also an important technique for coal conversion.1,2 The liquid product, tar, from low temperature pyrolysis of coal could be one of the most important sources of fuel oil and chemicals. In general, the tar yield in the coal pyrolysis process is low. To improve the tar yield, many methods, including changing the pyrolysis atmosphere,310 pretreatment of coal,11 catalytic pyrolysis,8,11 and catalytic hydropyrolysis,8,12 were explored. Our previous studies indicated that the tar yield can be remarkably increased by integrating the coal pyrolysis with partial oxidation13 or carbon dioxide reforming14,15 of methane (POM or CRM). However, POM is a strong exothermic reaction, and excessive oxidation or combustion of methane and sintering of the catalyst can easily happen because of the use of pure oxygen, which affects the stability and safety of the process. In contrast, the CRM process shows a promising way of using two main greenhouse gases to produce liquid fuels. Methane is one of the most stable organic gases; its activation has drawn extensive attention for decades. Transition metals such as Ni, Co, and Fe and noble metals loaded on MgO, Al2O3, SiO2, ZrO2, etc. have been used as the catalysts for CO2 reforming of methane.16 Because CRM is a strong endothermic reaction, it is usually operated at above 800 °C even if catalysts are used, which is higher than the optimal coal pyrolysis temperature range for high tar yield (500600 °C). It is necessary to develop a novel method for activating methane so that compatibility between the temperature for methane activation and that for high tar yield during coal pyrolysis can be achieved. The plasma conversion of methane has widely attracted attention since Capezzuto reported the decomposition of methane using radio frequency (RF) electrical discharges.17 RF discharges, corona discharge, arc discharge, and dielectric barrier discharge (DBD) are usually called nonequilibrium plasma, in which the r 2011 American Chemical Society
electron temperature significantly exceeds that of heavy particles (near ambient temperature). Ionization and chemical processes in such nonequilibrium plasmas are directly determined by electron temperature and, therefore, are not so sensitive to thermal processes and the temperature of the gas.18 DBD has numerous industrial applications such as ozone production, polymer treatment, pollution control, etc.19,20 because it can be operated at atmospheric pressure under different gases and the discharge can be initiated in large scale. Many studies have been carried out using DBD plasma for the direct conversion of methane,2123 or oxidative coupling of methane, or carbon dioxide/steam reforming of methane.2429 In this work, DBD was chosen as the discharge mode to find out whether the integrated process of coal pyrolysis with CO2 reforming of methane by DBD plasma could improve the tar yield.
’ EXPERIMENTAL SECTION Coal Sample. Two Chinese coal samples, Huolinhe (HLH) lignite and Shenmu (SM) subbituminous coal, were crushed and sieved to 4060 mesh for pyrolysis. The proximate and ultimate analyses of the coal samples are shown in Table 1. Apparatus and Procedures. Figure 1 shows the schematic diagram of the experimental setup and the reactor. The DBD reactor mainly consists of a quartz tube (Φ 12 2 mm), a stainless steel tube (Φ 3 0.5 mm) as the high voltage (HV) electrode, and a piece of stainless steel mesh coated on the outer wall of the quartz tube as the ground electrode; so, the discharge gap was 2.5 mm. The length of the discharge zone was about 30 mm. The coal sample (approximately 1.50 g) was put downstream of the discharge zone so that the gases activated by the discharge can react with it. A thermocouple was attached to the outer Received: June 4, 2011 Revised: August 1, 2011 Published: August 02, 2011 4036
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Table 1. Proximate and Ultimate Analyses of Coal Samples proximate analysis (wt %)
a
Table 2. Duplicated Experiment Results
ultimate analysis (wt %, daf) coal sample
Oa
experiments
char yield
tar yield
water yield
(%, daf)
(%, daf)
(%, daf)
coal sample
Mad
Ad
Vdaf
1
68.29
18.92
5.98
HLH
2.12
9.61
44.08
71.67 5.63 1.60 0.57 20.53
2
67.91
18.17
5.98
SM
5.06
5.36
35.31
80.98 4.51 1.05 0.39 13.07
3
68.82
18.77
5.98
average
68.34
18.62
0.67
2.13
1
75.13
18.92
3.28
2 3
74.47 74.84
18.03 18.33
3.28 3.28
average
74.81
18.43
0.44
2.46
C
H
N
S
HLH
By difference.
RSDa SM
RSD a
Figure 1. Schematic diagram of the experimental setup and the reactor. wall of the quartz tube at the center of the coal bed to monitor the pyrolysis temperature. The DBD plasma generator (CTP-2000K, Nanjing Suman Electronics Corp.) can provide a sine wave voltage with the maximum voltage 40 kV and an adjustable frequency between 10 and 40 kHz. The HV supplied to the DBD reactor was monitored and measured by a Tektronix 2022B digital storage oscilloscope sampling at 200 MHz. The HV signal was measured through two capacities (47 pF, 47 nF) where the voltage was reduced by 1000 times, and the discharge current was indirectly determined by measuring the voltage across a resistor (50 Ω) that was connected to the ground electrode. The power consumption in the DBD reactor was calculated using the voltage and current traces recorded and stored by the oscilloscope. A curve of point power was obtained by multiplying the corresponding current and voltage signals at the same phase angle and was averaged to give the discharge power (Pdis).30 The feed gas contains one or several kinds of the following gases: CH4, CO2, N2, and H2. The gas flow rate was controlled by a mass flow controller. All experiments were carried out at atmospheric pressure. During the experiment, the feed gas was first introduced to the reactor for several minutes; then, the gas discharge was initiated; and the reactor was loaded into the center of the preheated furnace (400650 °C). The heating time from the ambient temperature to the desired pyrolysis temperature was about 3 min, and the reactor was held at the temperature for a desired time. The liquid products, tar plus water, were collected in a cold trap at 10 °C, and then, the water in the liquid products was separated according to ASTM D95-05e1 (2005) using toluene as solvent. In this way, the tar and water yields can be calculated separately. The product yields are defined as follows: Ychar ðwt %, daf Þ ¼
Ytar ðwt %, daf Þ ¼
mchar mAad 100 mð1 Aad Mad Þ
mtar 100 mð1 Aad Mad Þ
Ywater ðwt %, daf Þ ¼
mwater 100 mð1 Aad Mad Þ
5.98 0
3.28 0
Relative standard deviation.
where Ychar, Ytar, and Ywater are the yields of char, tar, and water, respectively, in dry ash-free base and m, mchar, mtar, and mwater are the weights of the coal sample, char, tar, and water, respectively. Aad and Mad are the ash and moisture contents of the coal sample in air-dry base, respectively. Duplicated experiment results in Table 2 indicated that the experiment has a good repeatability, and most of the data in this paper are the average values of several repeated experiments.
’ RESULTS AND DISCUSSION Effect of Temperature under Different Atmospheres. The effect of pyrolysis temperature on char, tar, and water yields of SM coal under atmospheres of N2, H2, H2 plasma, CH4 plasma, CH4/H2 plasma, CO2/H2 plasma, and CH4/CO2/H2 (1:1:2, denoted as MG) plasma is shown in Figure 2. It can be seen from Figure 2a that the char yield under N2 is the highest at the investigated temperature range, while those under H2 and H2 plasma are almost equal and are the lowest when the temperature is above 550 °C. However, the char yield under H2 plasma is lower than that under H2, which is approximately the same as that under N2 below 500 °C. Other plasma atmospheres showed positive effects on coal conversion and displayed lower char yields than that under H2 plasma at the low temperature range (400500 °C). Figure 2b shows tar yields under different atmospheres. Higher tar yields can be achieved in the integrated process of coal pyrolysis with gas activation by DBD plasma than those in the single process of coal pyrolysis under N2 or H2. The tar yields under the studied atmospheres generally have the following order: MG plasma > CH4 plasma ≈ CH4/H2 plasma ≈ CO2/H2 plasma > H2 plasma > H2 > N2. A low pyrolysis temperature is more favorable for relatively increasing tar yields in the integrated process. From Figure 2c, the water yields under different atmospheres show a different trend from that of the tar yields and generally present in the following order: CO2/H2 plasma > MG plasma > H2 plasma > CH4 plasma > CH4/H2 plasma > H2 ≈ N2. The water yield under H2 plasma is obviously higher than that under H2, but it becomes closer as the temperature increases. It is noticeable that the water yield under CH4/H2 plasma is lower than that under CH4 plasma. During coal pyrolysis, the weak bridge bonds in the coal structure first ruptured and the depolymerization reaction occurred, which results in the formation of large amounts of free radicals.31 These radicals can be classified into three types: small 4037
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Figure 2. Effect of temperature on Ychar (a), Ytar (b), and Ywater (c) under different atmospheres (CH4/H2 = 1/2; CO2/H2 = 1/2; total gas flow rate, 240 mL/min; holding time, 7 min; Pdis, 40 W).
groups such as H•, •OH, •CH3, etc.; alkyl side-chain groups such as •CnH2n+1, •OCnH2n+1, substituted benzene, etc.; and large radical groups containing the framework of coal structure. The combination of the relatively small radicals such as H•, •OH, • CHx, •CnH2n+1, •OCnH2n+1, substituted benzene, and many other radicals leads to the formation of volatiles including tar and gas products. However, the combination of large radicals themselves or of large radicals and small radicals will form the solid product, char. Many processes were investigated to improve tar yields during coal pyrolysis, as mentioned in the Introduction.315 These processes have an essential characteristic in common: providing additional small hydrogen-rich radicals to stabilize the radicals decomposed from coal and preventing them from condensation and polymerization in the process of coal pyrolysis. The main chemically active species in nonequilibrium hydrogen plasma, H, H+, H2+, and H3+, have higher reactivity than molecular hydrogen,32 which can explain the lower char yield and higher tar yield under H2 plasma than that under H2 at low temperature range. However, as the temperature increases, the H2 molecule can be activated thermodynamically; this may be the reason why there is little difference in char and tar yields under H2 plasma compared with those under H2 at high temperature range. The higher water yield under H2 plasma may be ascribed to the high-energy H species excited by the discharge, which have a higher ability to react with oxygenous groups in coal.
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Under the CH4 plasma, tar and water yields are higher than those under H2 plasma below 500 °C. It is thought that CH4 was first dissociated to •CH3, •CH2, •CH, •C, and H• species in CH4 plasma33 and these activated species combined with free radicals ruptured from coal formed volatiles. •CHx species are larger than H species; thus, CH4 plasma has a better weight gain effect than H2 plasma, resulting in a higher tar yield. In CH4 plasma, the activated H species are fewer than those in H2 plasma; so, the water yield under CH4 plasma is lower than that under H2 plasma, especially at temperatures below 500 °C. However, the DBD under CH4 is very liable to convert to a stream discharge at high temperature (>500 °C), by which CH4 is mainly dissociated to •C and H• and will cause serious carbon deposits and further affect the discharge. Yao et al.34 found that the resistance of methane dramatically dropped when it was heated to 300 °C and caused the change of the power supply and the load. More tar may be formed under CH4 plasma if this problem could be avoided. Some studies35,36 indicate that the addition of H2 is beneficial to the methane coupling under nonequilibrium plasma, which can increase the CH4 conversion and the C2 hydrocarbons yield and reduce the carbon deposit. This is ascribed to the dilution effect, good thermal conductivity, and high heat capacity of the added H2. To eliminate carbon deposits and maintain a steady DBD, H2 was added to CH4 with a CH4/H2 ratio of 0.5. The results showed that char, tar, and water yields changed little after the addition of H2 to CH4 at low temperature range but the tar yield increased and the water yield decreased compared with that under CH4 plasma at high temperature range. This indicated that abundant H species activated by discharge effectively prevented further dissociation of the CHx radicals. The results suggested that the addition of H2 to CH4 not only has the effect of eliminating carbon deposits and stabilizing the discharge but also has the effect of increasing tar and decreasing water yields. The integrated process of coal pyrolysis with CO2 reforming of methane by plasma was also investigated. CO2 can inhibit carbon deposition, increase CH4 conversion, and form higher hydrocarbons, liquid products, and oxygenates during the plasma conversion of methane.37,38 The main problem with CH4/CO2 is the instability of the discharge and serious carbon deposition when the temperature exceeds 300 °C. So, H2 was added into the feed gas of CH4/CO2 with the CH4/CO2/H2 (MG) ratio of 1/1/2, and the total flow rate was kept at 240 mL/min. Coal pyrolysis under CO2/H2 (molar ratio 0.5) plasma was also studied to give a comparison among CH4/H2, CO2/H2, and MG plasma. It can be seen from Figure 2 that the tar yield under CO2/H2 plasma is about the same as that under CH4/H2 plasma at 400 °C; however, when temperature is higher than 400 °C, tar yield under CO2/H2 plasma is lower than that under CH4/H2 plasma, and the difference between them increases as the temperature increases. When the temperature rises to 600 °C, the tar yield under CO2/H2 plasma is about the same as that under N2. The water yield under CO2/H2 plasma is the highest in all the experiments, and it is about several times more than those under N2 and H2, especially at low temperature range. Even so, the water yield under CO2/H2 plasma is less than 6%. The results suggested that the CO2/H2 plasma has a positive effect on increasing the tar yield as well as the water yield and the low pyrolysis temperature is preferred to the integrated process for high tar yield. The high tar and water yields under CO2/H2 plasma may be explained as follows. On the one hand, CO2 and H2 can be 4038
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Figure 3. Increment analysis of Ytar and Ywater under different atmospheres.
excited or dissociated by plasma discharge and the excited species have sufficient high energy to react with each other or radicals cracked from coal to form volatiles. Compounds generated under CO2/H2 plasma not only contain CO and H2O but also contain CH3OH, HCHO, HCOOH, etc.3941 Abundant oxygenous radicals and CHx radicals form combined with the radicals ruptured from coal leading to the increase of the tar and water yields. On the other hand, the reverse water gas shift reaction and methanation reaction can easily occur at the experimental conditions thermodynamically and produce a large amount of water: CO2 þ H2 f CO þ H2 O
ΔH °298 ¼ 41:2 kJ=mol
CO2 þ 3H2 f CH3 OH þ H2 O ΔH °298 ¼ 49:5 kJ=mol CO2 þ 4H2 f CH4 þ 2H2 O ° ¼ 164:9 kJ=mol ΔH298
It can be found that coal conversion under MG plasma has little difference compared with that under CH4, CH4/H2, and CO2/H2 plasmas at low temperature range, while it is almost the same as that under H2 and H2 plasma at high temperature range (Figure 2a). The tar yield under MG plasma is the highest compared with those under other atmospheres at the same pyrolysis temperatures, and it is about 100 and 77% more than that under N2 and H2 at 400 °C, respectively. The increment of the tar yield under MG plasma is more evident at low temperature range, which can be seen from Figure 2b. The tar yield under MG plasma is about 2.0 and 1.1 times as that under N2 at 400 and 600 °C, respectively. The water yield under MG plasma is lower than that under CO2/H2 plasma but higher than those under any other atmospheres. As the temperature increases, the increment of the water yield becomes more obvious and approaches that under CO2/H2 plasma at 600 °C. The results
Figure 4. Effect of flow rate on Ychar, Ytar, and Ywater under MG plasma atmosphere (500 °C; 7 min; 40 W; CH4/CO2/H2 = 1/1/2).
indicate that the addition of CO2 and H2 to CH4 has the effect of improving coal conversion and product distributions. To illustrate the differences among the integrated processes under CH4/H2, CO2/H2, and MG plasma, increment analyses of the tar and water yields are shown in Figure 3. Δ1, Δ2, and Δ3 represent the yield differences between those under CH4/H2, CO2/H2, and MG plasma atmospheres and that under N2 atmosphere, respectively. Δ4 is the difference between the total yield increment under CH4/H2 and CO2/H2 plasma and that under MG plasma. (Δ1 = YCH4/H2P YN2, Δ2 = YCO2/H2P YN2, Δ3 = YMGP YN2, Δ4 = Δ1 + Δ2 Δ3.) The tar yield increment is generally higher than zero and has the order Δ3 > Δ1 > Δ2; this means that a plasma atmosphere is beneficial to increase tar yields and MG plasma is the most effective atmosphere on improving tar yield. The tar yield increment is obvious under 400 °C; however, as the pyrolysis temperature increases, the tar yield increment decreases. Δ4 becomes less than zero when the temperature is higher than 500 °C, indicating that the tar yield increment under MG plasma is higher than the total tar yield increment under CH4/H2 and CO2/H2 plasmas, which suggests the increment of tar yield under MG plasma is not a simple summation of that under CH4/H2 and CO2/H2 plasmas. From Figure 3b, it can be seen that the order of water yield increment is Δ2 > Δ3 > Δ1. The water yield increment under the CH4/H2 plasma decreases with the increasing temperature, while that under CO2/H2 and MG plasmas has the reverse trend. The water yield increment under MG plasma is less than the total water yield increment under CH4/H2 and CO2/H2 plasmas. Overall, in contrast with CH4/H2 and CO2/H2 plasmas, MG plasma can increase the tar yield to a maximum extent and decrease water yield compared with CO2/H2 plasma. Effect of Gas Flow Rate. Figure 4 shows the effect of total gas flow rate on product yields. For both HLH and SM coal, the char yield decreases while the water yield increases with the increase of the flow rate. The tar yields of the two coals show different trends: the tar yield of HLH coal decreases, while that of SM coal 4039
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Figure 5. Effect of pyrolysis time on Ychar (a), Ytar (b), and Ywater (c) under MG plasma atmosphere (500 °C; 240 mL/min; 40 W).
increases, as the flow rate increases. The residence time of gas is directly related with the gas flow rate. According to the analyses of the coal pyrolysis process, a higher gas flow rate is more conducive to the spill of the volatiles and can prevent them from cracking and starting a secondary reaction during volatilization. However, for the gas activation process, because the energy input is a constant when discharge power is fixed, the energy density will decrease with the increase of gas flow rate, and then, it will affect the activation level of the gas. However, too small of a gas flow rate will lead to serious carbon deposits because of the existence of CH4. The different effects of the gas flow rate on the tar yields of the two coals may relate to their properties. HLH lignite has lower aromaticity and higher ash and oxygen content than SM subbituminous coal, as can be seen from Table 2. With the increasing gas flow rate, the tar yield of HLH coal has a slight decrease, but the water yield obviously increases, which indicates that the hydrogen rich radicals in the gas combined with oxygenous groups in coal and the reaction inclined to generate water and gas. As SM coal has weak cohesiveness, the increase of the gas flow rate is favorable for generating tar. Effect of Pyrolysis Time. The effect of pyrolysis time (holding time at desired temperature) on the char, tar, and water yields under the MG plasma atmosphere was investigated, and the results are shown in Figure 5. It can be seen that, with the increase
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Figure 6. Effect of H2 content in the mixed gas (CH4/CO2/H2) on Ychar (a), Ytar (b), and Ywater (c) (500 °C; 7 min; 40 W; 240 mL/min; CH4/CO2 = 1).
of pyrolysis time, the char yield decreases gradually while water yield keeps on increasing. The tar yield increases about 3% when the pyrolysis time increases from 2 to 12 min, but then it increases slightly as the pyrolysis time further increases. It is thought that increasing the holding time leads to the formation of more free radicals from coal and improves tar yields in the integrated process of coal pyrolysis with CO2 reforming of methane.15 The obvious increase of the tar yield within 12 min is attributed to the combination of free radicals in the feed gas with those cracked from coal. As the pyrolysis time further increases, free radicals decomposed from coal gradually decrease, and those generated by discharge also decrease because the carbon deposit becomes more and more serious at the discharge zone, along with the increase of pyrolysis time, leading to the slight increase of the tar yield after 12 min. However, the condensation reaction between large groups from coal can last for a long time and result in the obvious increase of the water yield. Therefore, the appropriate pyrolysis time for the integrated process is important for obtaining more tar. Effect of H2 Content in the Mixed Gas. As described above, discharge under CH4/CO2 is instable and liable to turn to stream discharge when temperature exceeds 300 °C, resulting in serious carbon deposit and further affecting the discharge. The addition of H2 to the feed gas can restrain the formation of the carbon deposit and stabilize the discharge. The effect of H2 content in the mixed gas on the char, tar, and water yields was studied, and 4040
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completely dissociated to H and C, which will cause serious carbon deposit and further push the discharge to turn to stream discharge. In short, the tar yield can increase by increasing discharge power on the condition that the discharge is uniform and stable.
’ CONCLUSIONS Coal pyrolysis coupling with gas activation through DBD plasma has positive effects on the coal conversion and the tar yield compared with coal pyrolysis under N2 and H2, and different atmospheres have different effects on product yields. The tar yield of SM coal under MG plasma is the highest among all the atmospheres studied at the same temperature, and it is about 2.0 and 1.8 times as that under N2 and H2 at 400 °C, respectively. The increase of gas flow rate can increase the coal conversion and the water yield, but the tar yield of HLH lignite decreases while that of SM subbituminous coal increases. Increasing pyrolysis time can increase both the tar and water yields; 50% H2 in mixed gas (CH4/CO2/H2) and 40 W of discharge power are appropriate to stabilize the gas discharge and increase the tar yield. ’ AUTHOR INFORMATION Corresponding Author
*Telephone/Fax: +86-411-84986157. E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was performed with support of the National Natural Science Foundation of China (20576019 and 20776028), and the National Basic Research Program of China, the Ministry of Science and Technology, China (2011CB201301). Figure 7. Effect of discharge power on Ychar (a), Ytar (b), and Ywater (c) under MG plasma atmosphere (500 °C; 240 mL/min; 7 min).
the results are shown in Figure 6. The char yields of both SM and HLH coals decrease as the H2 content increases, the tar yields generally decrease with the increase of H2 content and become obvious when the H2 content exceeds 50%, and the water yields increase with the increase of H2 content. From Figure 6, it can be seen clearly that coal pyrolysis under CH4/CO2/H2 plasma is better than H2 plasma for producing tar. The most important role of H2 is to stabilize the discharge, and the addition of 50% H2 is appropriate and effective. Effect of Discharge Power. The effect of discharge power on product yields under the MG plasma is shown in Figure 7. With increasing discharge power, conversion and water yield of both coal types increase in different degrees. However, the tar yield of HLH coal has a maximum at the discharge power 40 W, while that of SM coal first increases and then becomes unchanged. Discharge power is one of the most important factors that affect the activation level of the feed gas. The energy and the quantity of the electrons in the discharge zone are directly determined by the input energy, which is dependent on the discharge power. As the discharge power increases, the number of high-energy electrons and the collision frequencies between the electrons and gas molecules increase, leading to formation of more activated species.4244 More radicals in the gas phase combining with those decomposed from coal will certainly increase the yield of volatiles. However, if the discharge power is too high, CH4 will be
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