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Co-liquefaction behavior of a sub-bituminous coal and sawdust
Energy 36 (2011) 6645e6650
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Co-liquefaction behavior of a sub-bituminous coal and sawdust Hengfu Shui*, Chuanjun Shan, Zhengyi Cai, Zhicai Wang, Zhiping Lei, Shibiao Ren, Chunxiu Pan, Haiping Li School of Chemistry & Chemical Engineering, Anhui Key laboratory of Coal Clean Conversion & Utilization, Anhui University of Technology, Ma’anshan 243002, Anhui Province, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 May 2011 Received in revised form 24 August 2011 Accepted 28 August 2011 Available online 5 October 2011
The co-thermolysis and co-liquefaction properties of Shenhua coal and sawdust were investigated in this study. The synergistic effect between Shenhua coal and sawdust in co-liquefaction was probed. TG/DTG analysis suggests that the sawdust, which has lower pyrolysis temperature, can promote the thermolysis of Shenhua coal, resulting in more volatile matter to be released from coal molecular structure during the co-thermolysis process. This will result in the larger weight losses of their mixture compared to the corresponding weighted mean values of individual pyrolysis. The individual liquefaction of Shenhua coal and sawdust shows that sawdust has higher liquefaction activity compared to Shenhua coal. It gives much higher liquefaction conversion and oil yield than Shenhua coal at the same liquefaction conditions. Co-liquefactions of Shenhua coal and sawdust at different conditions were carried out. The results suggest that there exists an obviously synergistic effect during the co-liquefaction, and this synergistic effect is the function of liquefaction conditions. At high liquefaction temperatures and long reaction times, the synergistic effect decreases because of the increase of liquefaction activity of coal and lack of hydrogen donating ability of the system at the conditions, resulting in the increase of the rate of retrogressive condensed reactions. The largest enhancements in conversion of 16.8% and oil yield of 11.4% comparing with corresponding calculated weighted mean values of the individual liquefaction of Shenhua coal and sawdust were obtained at 400 and 380 C, respectively in the co-liquefaction with 1/1 blending ratio of coal/sawdust. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Co-thermolysis Co-liquefaction Biomass Coal
1. Introduction Due to the increased demands and limited reserves of petroleum, the concern of energy security has intensified the interest in direct coal liquefaction (DCL) to produce alternative transportation fuels, especially for those countries which are short of oil resource but have abundant coal reserves, such as China, which is building the first and largest DCL plant by Chinese Shenhua Corporation in the world [1,2]. However, because DCL is usually conducted under severe reaction conditions with higher hydrogen consumption, this makes the cost of oil from DCL to be difficult to compete with that from crude oil. On the contrary, biomass such as sawdust is a cheap and renewable organic energy source, the liquefaction of biomass to convert into alternative transportation fuels has been paid more and more attentions [3e6]. Co-liquefaction of coal with biomass has gained increasing research interest due to the growing concerns over greenhouse gas emissions. Many studies demonstrated that there exists a positive synergetic effect in the co* Corresponding author. Tel.: þ86 5552311552; fax: þ86 5552311822. E-mail address: [email protected] (H. Shui). 0360-5442/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2011.08.046
liquefaction of coal with biomass. Co-liquefaction of coal with biomass can improve the yields and quality (such as H/C ratio) of the liquid products produced from coal under milder conditions of temperature and pressure [7e14], therefore greatly decreases the cost of oil from DCL. Coughlin et al. [7] believed that thermal depolymerization of lignin at relatively low temperatures leads to the formation of resonance stabilized phenoxy radicals, which then attack the coal causing scission of aliphatic carbonecarbon bonds in the coal. Matsumura et al. [8] found that co-liquefaction of lowrank coals and cellulose in the supercritical water medium resulted in enhanced coal conversion and increased yields/quality of the liquefaction products. Altieri et al. [9] studied co-liquefaction of a bituminous coal (Illinois #6) and lignin at 400 C in tetralin. They found that the filterable solids from co-liquefaction showed increased solubility in benzene. A greater portion of the benzenesoluble material was found to be pentane-soluble oil. Karaca et al [10e12] investigated co-liquefaction of a Turkish lignite with a cellulosic waste material in tetralin at different conditions. The highest increase in total conversion was obtained by co-processing the lignite with sawdust at 325 C. Lalvani et al. [13] also found synergistic effects in co-liquefaction of lignin and coal at 325 C and
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2 h. Guo et al. [14] investigated the co-liquefaction of lignite and sawdust under syngas and found that there were positive synergetic effects in the co-liquefaction process. Shenhua coal is a Chinese sub-bituminous coal and is the first chosen coal sample used in Shenhua DCL plant. In our previous work, the liquefaction behaviors of Shenhua coal have been widely researched [15e18]. In this study the co-liquefaction behaviors of Shenhua coal and biomass (sawdust) were investigated and some positive synergistic effects over coal conversion and oil yield were found.
Co-liquefaction products
Gas
Solid and liquid THF
THF soluble fractions (THFS)
2. Experimental section
THF insoluble fractions (THFI)
Hexane
2.1. Samples and reagents Shenhua coal (a Chinese sub-bituminous coal) used in this study was provided by Shenhua Group. Sawdust collected from a furniture factory was used as biomass feedstock, which was produced from China fir. The coal and sawdust as received were ground to pass through 200 and 60 meshes sieve respectively, stored under nitrogen atmosphere and dried under vacuum at 80 C overnight before use. The elemental and proximate analyses of Shenhua coal and sawdust used are provided in Table 1. All solvents used are commercial pure chemical reagent (Purity higher than 99.5%) without further purification. The purity of hydrogen used in this study is 99.9%.
Hexane soluble fractions (HS)
Hexane insoluble fractions (HI) Toluene
Toluene soluble fractions (AS)
Toluene insoluble fractions (PA)
Fig. 1. Fractionation procedure of liquefied product.
2.3. TG measurements 2.2. Liquefaction and product fractionation The liquefaction experiments were carried out in a 30 ml tubing reactor shaken vertically. 1.0 g of the dried coal or sawdust or the mixture of coal and sawdust loaded with 5% FeS catalyst was charged into the reactor together with 2 ml of tetralin. Before the liquefaction experiment, the reactor was sealed and flushed 3 times with hydrogen, followed by pressuring the system to the initial pressure of 5.0 MPa with hydrogen. The reactor, agitated vertically at 120 times per minute, was submerged into a eutectic salt bath, which had been heated to the desired temperature, and maintained at that temperature for desired reaction time. Then the reactor was quenched to ambient temperature in a water bath, and the overhead pressure in the reactor was released slowly. The liquefaction mixture was separated by Soxhlet solvent extraction with tetrahydrofuran (THF), n-hexane and toluene in turn. The n-hexane insoluble but toluene soluble fraction was defined as asphaltene (AS), and the toluene insoluble but THF soluble fraction was defined as preasphaltene (PA). The fractionation procedure is shown in Fig. 1. The conversion of feedstock was defined as the THF soluble fraction þ gas, which is calculated from the THF insoluble residue. Gas yield was calculated from the material weight difference before and after liquefaction with gas released. Oil yield was calculated as:
3. Results and discussion 3.1. Pyrolysis of coal, sawdust and their mixture Pyrolysis is the first stage of liquefaction of coal and biomass. Fig. 2 shows the TG/DTG curves of Shenhua coal and sawdust. The initial (Ti) and final (Tf) temperatures of pyrolysis can be determined from TG curves using tangent line method according to literature [19]. From Fig. 2 it can be observed that Ti of Shenhua coal is about 362 C, and the temperature of the biggest weight loss rate is 450 C. The Ti of sawdust is low, only about 260 C, and the 0.002 100 80
Weight (%)
The repeatability of the fractionation analyses is 1%.
0.000 -0.002 Coal DTG
60
-0.006
40
Table 1 Elemental and proximate analyses of coal and sawdust. Sample
Shenhua coal Sawdust a
By difference.
Proximate analysis (db),wt%
-0.004
Coal
Sawdust TG 20
Elemental analysis (daf), wt%
Sawdust DTG
M
A
V
FC
C
H
N
S
Oa
8.6 5.5
10.9 2.9
39.3 78.2
49.8 18.9
73.17 47.57
4.59 5.58
1.08 1.32
0.45 0.16
20.71 45.37
0
0
100
200
300
-0.008 -0.010
400
500
600
700
800
Temperature Fig. 2. TG/DTG curves of Shenhua coal and sawdust.
900
Weight loss rate (%/min)
Oil% ¼ Conversion e PA e AS e Gas
Thermo gravimetric (TG) analysis was carried out on a SHIMADZU TG60 analyzer. About 10 mg of sample was placed in an alumina pan and heated from 25 C to 800 C at a rate of 10 C/min under 50 ml/min nitrogen gas flow. The reproducibility of the TG experiment for the characteristic temperatures is 1 C.
H. Shui et al. / Energy 36 (2011) 6645e6650
temperature of the biggest weight loss rate is 360 C, which are much lower than the corresponding temperatures of Shenhua coal. The main pyrolysis temperature range (Ti w Tf) of Shenhua coal (362e750 C) is much higher than that of sawdust (260e420 C), and the releasing rate of volatile matter of Shenhua coal is much lower than that of sawdust as indicated by Fig. 2. The biggest weight loss rate of sawdust is much higher than that of Shenhua coal. The results suggest that sawdust is very easy to pyrolyze compared to Shenhua coal. Fig. 3 shows the TG/DTG curves of the mixture of Shenhua coal and sawdust with 1:1 by weight (Exp) and the corresponding calculated mean results (Cal) from the individual TG/DTG curve shown in Fig. 2. It can be observed from Fig. 3 that the experimental weight losses (Exp) for the mixture co-pyrolysis are larger than the corresponding calculated mean results (Cal), and the maximum difference appears at about 390 C. This suggests that much more volatile matters are released from coal molecular structure in the co-pyrolysis of Shenhua coal with sawdust compared to that from coal pyrolysis alone. Park et al. [20] also found that the yield and conversion of co-pyrolysis of sawdust and coal blend based on the volatile matters are higher than those of the sum of sawdust and coal individually. DTG also shows that the biggest Exp weight loss rate is larger than that of Cal value, and the Exp temperature of biggest weight loss rate shifts to lower temperature about 10 C comparing with that of Cal value. The results suggest that there exists a synergistic effect during the co-pyrolysis of Shenhua coal and sawdust. Sawdust can promote the pyrolysis of Shenhua coal and makes more volatile matters to be released from coal molecular structure, resulting in more weight loss of coal in the co-pyrolysis with sawdust compared to that of coal pyrolysis alone. Kim et al. [21] believed that thermolysis of the lignin results in the formation of phenoxy and other reactive radicals at the temperatures, which are too low for significant thermolysis of the coal matrix taking place, and such radicals are effective and active intermediates to depolymerize coal by cleaving methylene bridges. The larger of the biggest weight loss rate and the shifting of the temperature of the biggest weight loss rate to lower temperature for the Exp values comparing with those of the Cal values may support this mechanism. 3.2. The individual liquefaction of Shenhua coal and sawdust It is necessary to investigate the individual liquefaction of Shenhua coal and sawdust. Table 2 shows the liquefaction properties of Shenhua coal and sawdust at different temperatures. It can 0.002 100
Weight (%)
60
TG
-0.002
40 -0.004
DTG
20
TG DTG
0 0
100
200
300
400
500
600
700
800
Weight loss rate (%/min)
0.000
80
-0.006 900
Temperature ( ) Fig. 3. Experimental and calculated TG/DTG curves of Shenhua coal and sawdust mixture.
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Table 2 Liquefaction properties of Shenhua coal and sawdust at different temperatures. Temperature C
Gas wt% (daf)
Oil wt% (daf)
AS wt% (daf)
PA wt% (daf)
Conversion wt%(daf)
380
3.8 20.9 7.6 25.5 7.0 23.6
17.6 47.3 21.9 44.1 32.7 55.2
7.7 6.6 15.4 16.1 15.7 4.6
16.9 13.0 8.4 8.6 20.6 7.0
46.0 87.8 53.3 94.3 76.0 90.4
400 420
Coal Sawdust Coal Sawdust Coal Sawdust
Liquefaction time: 60 min.
be observed from Table 2 that with the increase of temperature, the liquefaction conversion of Shenhua coal increased obviously, and oil yield increased simultaneously. Coal liquefaction kinetics [15] suggests that Shenhua coal liquefaction is a complex multiphase catalytic reaction process, which contains series, parallel and retrogressive reactions. The series reactions of coal / PA / AS / oil and gas are main reactions of Shenhua coal liquefaction. PA formed from coal macromolecules thermolyzed converts into AS, oil and gas by hydrocracking reactions, resulting in an obvious decrease of PA from 16.9% of 380 C to 8.4% of 400 C. Further increasing liquefaction temperature to 420 C, PA increased from 8.4% of 400 C to 20.6% of 420 C. This may be because the retrogressive reactions take place markedly at 420 C [15]. Comparing with the liquefaction of Shenhua coal, the liquefaction conversions of sawdust were much higher, and the differences between their conversions at the same conditions decreased with the increase of temperature. For example, the conversion differences decreased from 41.8% of 380 C to 14.4% of 420 C. TG/DTG above suggests that the main pyrolysis temperature range of Shenhua coal is much higher than that of sawdust. Therefore the liquefaction activity of sawdust at low temperatures should be higher than that of Shenhua coal, and the differences of their liquefaction activities decrease with the increase of temperature. In the meantime, the gas and oil yields of sawdust liquefaction are much higher than those of corresponding of coal liquefaction respectively. The results suggest that sawdust has higher liquefaction activity compared to Shenhua coal. This is in agreement with the higher pyrolysis activity of sawdust indicated by TG/DTG above, because pyrolysis is the first step of liquefaction. Table 1 shows that H/C of sawdust is much higher than that of Shenhua coal. This may suggest that sawdust has higher hydrogen donation ability compared to Shenghua coal, and this will result in higher yields of oil and gas for sawdust liquefaction. Table 3 shows the liquefaction properties of Shenhua coal and sawdust at 400 C for different liquefaction times. It can be observed that for Shenhua coal liquefaction, increasing liquefaction time from 30 to 60 min, the liquefaction conversion increased slowly, but PA decreased obviously. Therefore the liquefaction appeared to be PA converting into oil and gas at this period. Further prolonging liquefaction time to 90 min, the conversion increased obviously from 53.3% of 60 min to 70.2% of 90 min with 16.9% Table 3 Liquefaction properties of Shenhua coal and sawdust at 400 C and different reaction times. Time min
Gas wt% (daf)
Oil wt% (daf)
AS wt% (daf)
PA wt% (daf)
Conversion wt%(daf)
30
4.5 20.6 7.6 25.5 7.1 24.9
18.9 48.3 21.9 44.1 31.9 54.1
14.7 3.9 15.4 16.1 13.8 11.2
12.9 13.8 8.4 8.6 17.4 8.8
51.0 86.6 53.3 94.3 70.2 99.0
60 90
Coal Sawdust Coal Sawdust Coal Sawdust
H. Shui et al. / Energy 36 (2011) 6645e6650
increments. In the same time, the yield of oil also increased from 21.9% of 60 min to 31.9% of 90 min with 10.0% increments. The results suggest that with the increase of liquefaction time, PA and AS formed will further hydrocrack to form light constituents as oil and gas. Therefore the increase of liquefaction conversion is mainly reflected by the increase of oil. In the case of sawdust liquefaction, the increase of conversion was negligible after 60 min, but oil gave an obvious increase due to the high liquefaction activity of sawdust. 3.3. Co-liquefaction of Shenhua coal and sawdust 3.3.1. Effect of blending ratio of coal/sawdust TG/DTG and individual liquefaction of Shenhua coal and sawdust results above show that there exists synergistic effect during the co-pyrolysis of Shenhua coal and sawdust, and sawdust has higher liquefaction activity compared to Shenhua coal. Therefore we further study the co-liquefaction properties of Shenhua coal and sawdust. The co-liquefactions were conducted at 400 C and 60 min. The comparisons of the experimental conversion and product distribution (Exp) among different mass blending ratios of coal/sawdust during co-liquefaction are shown in Table 4. Assuming that there is no interaction between Shenhua coal and sawdust during co-liquefaction, thus the liquefied product yield of co-liquefaction should be equal to the weighted mean value of the individual liquefaction of Shenhua coal and sawdust. In order to probe the synergistic effect during the co-liquefaction of Shenhua coal and sawdust, the weighted mean values of the co-liquefaction conversion and liquefied product yield were calculated based on the individual liquefaction result of Shenhua coal and sawdust shown in Tables 2 and 3, and are also provided in Table 4. Hereafter the weighted mean values are defined as calculated values expressed by Cal. It can be observed from Table 4 that the Exp conversions are higher than the Cal values at all blending ratios of coal/sawdust, and the (Exp-Cal) differences increase with the increase of the amount of sawdust blending. For example, the conversion differences increased from 14.2% of 5/1 blending ratio to 17.9% of 1/1 blending ratio of coal/sawdust. The result suggests that there really exist synergistic effects during the co-liquefaction of Shenhua coal and sawdust. It can also be observed from Table 4 that with the increase of the amount of sawdust blending, light products oil and gas increased obviously, but the heavy products PA and AS hardly changed. This is because sawdust is more easily to be liquefied and it gives more light products compared to Shenhua coal. Fig. 4 shows the effect of blending ratio of coal/sawdust on the (Exp-Cal) differences of product yield in the co-liquefaction of Shenhua coal and sawdust. It can be observed from Fig. 4 that the synergistic effect of the co-liquefaction of Shenhua coal and sawdust at different blending ratios of coal/sawdust is mainly reflected by the increases of PA and oil. With the decrease of blending ratio of coal/sawdust, the (Exp-Cal) differences have an increased tendency for oil and gas but a decreased tendency for AS. This means that in the co-liquefaction process of sawdust and coal, sawdust is beneficial for the promoting of the formation of oil,
14
Gas
Oil
AS
PA
12 10 8 6
Exp-Cal /%
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4 2 0 -2 -4
5:1
2:1
1:1
-6 -8 -10
Coal/sawdust Fig. 4. (Exp-Cal) differences of liquefied product yields from the co-liquefaction of Shenhua coal and sawdust at different coal/sawdust blending ratios.
because it can act as hydrogen-donors to prevent the recombination reactions [20]. Altieri [9] researched the co-liquefaction of lignin and bituminous coal at 400 C and found that far more of benzene-soluble material and pentene soluble oil were formed in the co-liquefaction compared to those of the same amounts of coal and lignin reacted individually. The results suggest that addition of sawdust can promote the liquefaction of Shenhua coal. This may be because the free radicals or intermediates formed from pyrolysis of sawdust at low temperatures can interact with coal, therefore to promote the thermolysis and hydrogenation liquefaction of coal, resulting in the enhancements of conversion and oil yield in the coliquefaction. It should be noted that the characterizations of liquefied products from co-liquefaction of coal and sawdust are quite different from that from the liquefaction of coal alone, especially in the contents of heteroatoms such as nitrogen and oxygen. Therefore it will affect the further hydro-treatment of the liquefied products from co-liquefaction of coal and sawdust. The characterizations of liquefied products from co-liquefaction of coal and sawdust over different catalysts will be reported elsewhere. 3.3.2. Effect of liquefaction temperature Temperature affects the co-liquefaction behaviors of Shenhua coal and sawdust. It has been demonstrated that co-liquefaction of Shenhua coal and sawdust at 1/1 blending ratio of coal/sawdust gives the largest synergistic effect with promoted conversion and oil yield. The effect of temperature on the co-liquefaction properties of Shenhua coal and sawdust at 1/1 blending ratio of coal/sawdust with 60 min was further studied, and the results are shown in Table 5. Table 5 shows that increasing temperature from 380 to 400 C, the conversion increased obviously from 77.2% to 91.7%, and PA and gas also increased from 10.7% to 20.2% and 8.0%e18.1%, respectively. At the same time, oil and AS had decreased tendency
Table 4 Effect of blending ratio of coal/sawdust on the co-liquefaction properties. Coal/sawdust (mass)
Gas wt%(daf)
Oil wt%(daf)
AS wt%(daf)
PA wt%(daf)
Conversion wt%(daf)
1/1
18.1 16.5 12.6 13.6 9.0 10.6
39.6 33.0 33.9 29.3 30.7 25.6
13.8 15.8 14.9 15.6 15.9 15.5
20.2 8.5 21.3 8.5 18.7 8.4
91.7 73.8 82.7 67.0 74.3 60.1
2/1 5/1
Exp Cal Exp Cal Exp Cal
Liquefactions were carried out at 400 C and 60 min.
H. Shui et al. / Energy 36 (2011) 6645e6650
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Table 5 Effect of temperature on the co-liquefaction properties. Temperature C 380
Exp Cal Exp Cal Exp Cal
400 420
Gas wt%(daf)
Oil wt%(daf)
AS wt%(daf)
PA wt%(daf)
Conversion wt%(daf)
8.0 12.4 18.1 16.5 16.7 15.3
43.7 32.4 39.6 33.0 47.4 43.9
14.8 7.2 13.8 15.8 14.0 10.2
10.7 14.9 20.2 8.5 13.7 13.8
77.2 66.9 91.7 73.8 91.8 83.2
Liquefaction time: 60 min; coal/sawdust: 1/1 in mass.
because of the large amounts of gas formed. Further increasing the temperature to 420 C, the changes of conversion, AS and gas were negligible, but PA decreased and oil increased obviously. The coliquefaction appears to be the converting of PA into oil at this temperature stage. It can also be observed from Table 5 that at all temperatures the Exp values for conversion and oil are higher than those of Cal values. This suggests that there are synergistic effects in the coliquefaction at different temperatures. In order to further probe the synergistic effects of various liquefied products in the coliquefaction at different temperatures, Fig. 5 shows the (Exp-Cal) differences of product yields from the co-liquefaction of Shenhua coal and sawdust at different liquefaction temperatures. It can be observed from Fig. 5 that the synergistic effects of various products in the co-liquefaction are a strong function of temperature. The differences gave the minimum values for gas and PA and the maximum values for oil and AS at 380 C, suggesting the favorable synergistic effect taking place at this temperature. Although the largest enhancement in Exp conversion of 17.9% compared to Cal value was obtained at 400 C. Increasing temperature to 420 C, the (Exp-Cal) differences of liquefied product yields obviously decreased, and the synergistic effects became weak. This is because the liquefaction activity of Shenhua coal increases at higher temperatures (more than 400 C) therefore weakening the synergistic effects caused by sawdust addition. However, co-liquefaction at lower temperatures (such as 380 C), the free radicals or intermediates formed from pyrolysis of sawdust at these temperatures may promote the thermolysis and hydrogenation liquefaction of coal, resulting in the synergistic effects occurring as enhancements of Exp conversion and oil yield compared to those of Cal values. 3.3.3. Effect of liquefaction time Runs were performed over different times for the coliquefaction of Shenhua coal and sawdust (1/1 by weight) at
400 C to investigate the effect of liquefaction time, and the results are shown in Table 6. With the increase of time from 30 to 60 min, the conversion increased obviously from 82.2 to 91.7%, and oil and AS hardly changed. Further prolonging time to 90 min, the conversion decreased to 83.5%, and oil kept little change, suggesting that longer liquefaction time is not beneficial for the coliquefaction process. In the same time, gas, AS and PA also decreased in certain extent. The results suggest that with the increase of liquefaction time, the retrogressive reactions take place markedly in the co-liquefaction. Fig. 6 shows the synergistic effects of various products in the coliquefaction at different times expressed by the (Exp-Cal) differences of liquefied product yields. It can be observed from Fig. 6 that the largest synergistic effects are obtained at 60 min, where, the (Exp-Cal) differences of oil and PA are much larger than those at 30 or 90 min respectively. The following reaction mechanism is proposed to explain the synergistic effects at different liquefaction times [13]: Sawdust (S) / free radicals or intermediates (I1) / products (1) (P1)
Table 6 Effect of time on the Co-liquefaction properties. Time min
Gas wt% (daf)
Oil wt% (daf)
AS wt% (daf)
PA wt% (daf)
Conversion wt%(daf)
30
13.0 12.5 18.1 16.5 15.1 15.0
38.0 33.6 39.6 33.0 40.3 43.0
13.0 9.3 13.8 15.8 9.5 12.5
18.2 13.4 20.2 8.5 18.6 13.1
82.2 68.8 91.7 73.8 83.5 84.6
60 90
Exp Cal Exp Cal Exp Cal
Liquefaction temperature: 400 C; coal/sawdust: 1/1 in mass.
14
15
Gas
Oil
AS
PA
Gas
Oil
AS
PA
12 10
10
8
Exp-Cal / %
Exp-Cal / %
6 5
0
-5
4 2 0 -2 -4
380
400
420
-6
30
60
90
-8 -10
-10
Temperature / Fig. 5. (Exp-Cal) differences of liquefied product yields from the co-liquefaction of Shenhua coal and sawdust at different liquefaction temperatures.
Time / min Fig. 6. (Exp-Cal) differences of liquefied product yields from the co-liquefaction of Shenhua coal and sawdust at different liquefaction times.
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Coal (C) / free radicals or intermediates (I2) / products (P2) (2) C þ I1 / products (P3)
(3)
The reactions (1) and (2) are hypothesized to occur when sawdust and coal are alone deploymerized. In the co-liquefaction, the intermediates from reaction (1) further depolymerize coal via reaction (3). The enhancements in conversion and liquefied products obtained are due to reaction (3). The concentration of I1 should be dependent on time, and should increase with time as sawdust is depolymerized. At a certain time, I1 should reach a maximum value after which it should decline as it is consumed in reactions (1) and (3). Thus the enhancements in coal conversion and liquefied products should also increase with an increase in I1 (i.e. with time) reaching a maximum value and then decline. 4. Conclusions TG/DTG suggests that there exists synergistic effect in the copyrolysis of Shenhua coal and sawdust. Sawdust can promote the thermolysis of Shenhua coal and makes more volatile matters to be released from coal molecular structure during the co-thermolysis process, resulting in the larger weight losses of their mixture compared to the calculated values of their individual pyrolysis. Sawdust has higher liquefaction activity than Shenhua coal. At the same liquefaction conditions, sawdust gives much higher liquefaction conversion and oil yield compared to Shenhua coal. There are obviously positive synergistic effects for conversion and oil in the co-liquefaction of Shenhua coal and sawdust, and the synergistic effects are the function of liquefaction conditions. The largest enhancements in conversion and oil yield compared to corresponding calculated weighted mean values of the individual liquefaction of Shenhua coal and sawdust were obtained at 400 and 380 C, respectively, with 1/1 blending ration of coal/sawdust and 60 min reaction times. The synergistic effects of co-liquefaction of Shenhua coal and sawdust may be attributed to the promotions of the thermolysis and hydrogenation liquefaction of coal by the free radicals or intermediates formed from pyrolysis of sawdust at low temperatures. Acknowledgments This work was supported by the Natural Scientific Foundation of China (20876001, 21076001, 20936007), National Basic Research Program of China (973 Program, 2011CB201302) and the State Key
Laboratory of Coal Conversion (Grant No. 11-12e904). Authors are also appreciative for the financial support from the Provincial Innovative Group for Processing & Clean Utilization of Coal Resource. References [1] Nolan P, Shipman A, Rui H. Coal liquefaction, Shenhua Group, and China’s energy security. Eur Manage J 2004;22(2):150e64. [2] Shui H, Cai Z, Xu C. Recent advances in direct coal liquefaction. Energies 2010; 3:155e70. [3] Qian Y, Zuo C, Jian T, He J. Structural analysis of bio-oils from sub-and supercritical water liquefaction of woody biomass. Energy 2007;32: 196e202. [4] Zhong C, Wei X. A comparative experimental study on the liquefaction of wood. Energy 2004;29:1731e41. [5] Sawayama S, Minowa T, Yokoyama SY. Possibility of renewable energy production and CO2 mitigation by thermochemical liquefaction of microalgae. Biomass and Bioenergy 1999;17(1):33e9. [6] Dote Y, Sawayama S, Inoue S, Minowa T, Yokoyama SY. Recovery of liquid fuel from hydrocarbon rich microalgae by thermochemical liquefaction. Fuel 1994; 73(12):1855e7. [7] Coughlin RW, Davoudzadeh F. Coliquefaction of lignin and bituminous coal. Fuel 1986;65:95e106. [8] Matsumura Y, Nonaka H, Yokura H, Tsutsumi A, Yoshida K. Co-liquefaction of coal and cellulose in supercritical water. Fuel 1999;78:1049e56. [9] Altieri P, Coughlin RW. Characterization of products formed during coliquefaction of lignin and bituminous coal at 400 C. Energy Fuels 1987;1: 253e6. [10] Karaca F, Bolat E. Coprocessing of a Turkish lignite with a cellulosic waste material 1. Fuel Process Technol 2000;64:47e55. [11] Karaca F, Bolat E. Coprocessing of a Turkish lignite with a cellulosic waste material 2. Fuel Process Technol 2002;75:109e16. [12] Karaca F, Bolat E, Diner S. Coprocessing of a Turkish lignite with a cellulosic waste material 3. Fuel Process Technol 2002;75:117e27. [13] Lalvani SB, Muchmore CB, Koropchak J, Abash B, Chivate P, Chavezt C. Ligninaugmented coal depolymerization under mild reaction conditions. Energy Fuels 1991;5:347e52. [14] Guo ZX, Bai ZQ, Bai J, Wang ZQ, Li W. Co-liquefaction of lignite and sawdust under syngas. Fuel Process Technol 2011;92:119e25. [15] Shui HF, Chen ZX, Wang ZC, Zhang DX. Kinetics of Shenhua coal liquefaction catalyzed by SO2¼ 4 /ZrO2 solid acid. Fuel 2010;89:67e72. [16] Wang ZC, Shui HF, Zhu YN, Gao JS. Catalysis of SO2 4 /ZrO2 solid acid for the liquefaction of coal. Fuel 2009;88:885e9. [17] Shui HF, Wang ZC. Effect of pre-swelling of coal on its solvent extraction and liquefaction properties. Fuel 2008;87:2908e13. [18] Shui HF, Liu JL, Wang ZC, Cao MX, Wei XY. Effect of pre-swelling of coal at mild temperatures on its hydro-liquefaction properties. Fuel Process Technol 2009;90:1047e51. [19] Yan WP, Chen YY. Experimental study on co-pyrolysis characteristics of lignite mixed with biomass mixture. J Power Eng 2006;26(6):865e93. [20] Park DK, Kim SD, Lee SH, Lee JG. Co-pyrolysis characteristics of sawdust and coal blend in a TGA and a fixed bed reactor. Bioresour Technol 2010;101(15): 6151e6. [21] Kim JW, Lalvani SB, Muchmore CB. Coliquefaction of coal and black liquor to environmentally acceptable liquid fuels. Energy Sources 1999;21(9):839e47.
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Co-liquefaction behavior of a sub-bituminous coal and sawdust Hengfu Shui*, Chuanjun Shan, Zhengyi Cai, Zhicai Wang, Zhiping Lei, Shibiao Ren, Chunxiu Pan, Haiping Li School of Chemistry & Chemical Engineering, Anhui Key laboratory of Coal Clean Conversion & Utilization, Anhui University of Technology, Ma’anshan 243002, Anhui Province, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 May 2011 Received in revised form 24 August 2011 Accepted 28 August 2011 Available online 5 October 2011
The co-thermolysis and co-liquefaction properties of Shenhua coal and sawdust were investigated in this study. The synergistic effect between Shenhua coal and sawdust in co-liquefaction was probed. TG/DTG analysis suggests that the sawdust, which has lower pyrolysis temperature, can promote the thermolysis of Shenhua coal, resulting in more volatile matter to be released from coal molecular structure during the co-thermolysis process. This will result in the larger weight losses of their mixture compared to the corresponding weighted mean values of individual pyrolysis. The individual liquefaction of Shenhua coal and sawdust shows that sawdust has higher liquefaction activity compared to Shenhua coal. It gives much higher liquefaction conversion and oil yield than Shenhua coal at the same liquefaction conditions. Co-liquefactions of Shenhua coal and sawdust at different conditions were carried out. The results suggest that there exists an obviously synergistic effect during the co-liquefaction, and this synergistic effect is the function of liquefaction conditions. At high liquefaction temperatures and long reaction times, the synergistic effect decreases because of the increase of liquefaction activity of coal and lack of hydrogen donating ability of the system at the conditions, resulting in the increase of the rate of retrogressive condensed reactions. The largest enhancements in conversion of 16.8% and oil yield of 11.4% comparing with corresponding calculated weighted mean values of the individual liquefaction of Shenhua coal and sawdust were obtained at 400 and 380 C, respectively in the co-liquefaction with 1/1 blending ratio of coal/sawdust. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Co-thermolysis Co-liquefaction Biomass Coal
1. Introduction Due to the increased demands and limited reserves of petroleum, the concern of energy security has intensified the interest in direct coal liquefaction (DCL) to produce alternative transportation fuels, especially for those countries which are short of oil resource but have abundant coal reserves, such as China, which is building the first and largest DCL plant by Chinese Shenhua Corporation in the world [1,2]. However, because DCL is usually conducted under severe reaction conditions with higher hydrogen consumption, this makes the cost of oil from DCL to be difficult to compete with that from crude oil. On the contrary, biomass such as sawdust is a cheap and renewable organic energy source, the liquefaction of biomass to convert into alternative transportation fuels has been paid more and more attentions [3e6]. Co-liquefaction of coal with biomass has gained increasing research interest due to the growing concerns over greenhouse gas emissions. Many studies demonstrated that there exists a positive synergetic effect in the co* Corresponding author. Tel.: þ86 5552311552; fax: þ86 5552311822. E-mail address: [email protected] (H. Shui). 0360-5442/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2011.08.046
liquefaction of coal with biomass. Co-liquefaction of coal with biomass can improve the yields and quality (such as H/C ratio) of the liquid products produced from coal under milder conditions of temperature and pressure [7e14], therefore greatly decreases the cost of oil from DCL. Coughlin et al. [7] believed that thermal depolymerization of lignin at relatively low temperatures leads to the formation of resonance stabilized phenoxy radicals, which then attack the coal causing scission of aliphatic carbonecarbon bonds in the coal. Matsumura et al. [8] found that co-liquefaction of lowrank coals and cellulose in the supercritical water medium resulted in enhanced coal conversion and increased yields/quality of the liquefaction products. Altieri et al. [9] studied co-liquefaction of a bituminous coal (Illinois #6) and lignin at 400 C in tetralin. They found that the filterable solids from co-liquefaction showed increased solubility in benzene. A greater portion of the benzenesoluble material was found to be pentane-soluble oil. Karaca et al [10e12] investigated co-liquefaction of a Turkish lignite with a cellulosic waste material in tetralin at different conditions. The highest increase in total conversion was obtained by co-processing the lignite with sawdust at 325 C. Lalvani et al. [13] also found synergistic effects in co-liquefaction of lignin and coal at 325 C and
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2 h. Guo et al. [14] investigated the co-liquefaction of lignite and sawdust under syngas and found that there were positive synergetic effects in the co-liquefaction process. Shenhua coal is a Chinese sub-bituminous coal and is the first chosen coal sample used in Shenhua DCL plant. In our previous work, the liquefaction behaviors of Shenhua coal have been widely researched [15e18]. In this study the co-liquefaction behaviors of Shenhua coal and biomass (sawdust) were investigated and some positive synergistic effects over coal conversion and oil yield were found.
Co-liquefaction products
Gas
Solid and liquid THF
THF soluble fractions (THFS)
2. Experimental section
THF insoluble fractions (THFI)
Hexane
2.1. Samples and reagents Shenhua coal (a Chinese sub-bituminous coal) used in this study was provided by Shenhua Group. Sawdust collected from a furniture factory was used as biomass feedstock, which was produced from China fir. The coal and sawdust as received were ground to pass through 200 and 60 meshes sieve respectively, stored under nitrogen atmosphere and dried under vacuum at 80 C overnight before use. The elemental and proximate analyses of Shenhua coal and sawdust used are provided in Table 1. All solvents used are commercial pure chemical reagent (Purity higher than 99.5%) without further purification. The purity of hydrogen used in this study is 99.9%.
Hexane soluble fractions (HS)
Hexane insoluble fractions (HI) Toluene
Toluene soluble fractions (AS)
Toluene insoluble fractions (PA)
Fig. 1. Fractionation procedure of liquefied product.
2.3. TG measurements 2.2. Liquefaction and product fractionation The liquefaction experiments were carried out in a 30 ml tubing reactor shaken vertically. 1.0 g of the dried coal or sawdust or the mixture of coal and sawdust loaded with 5% FeS catalyst was charged into the reactor together with 2 ml of tetralin. Before the liquefaction experiment, the reactor was sealed and flushed 3 times with hydrogen, followed by pressuring the system to the initial pressure of 5.0 MPa with hydrogen. The reactor, agitated vertically at 120 times per minute, was submerged into a eutectic salt bath, which had been heated to the desired temperature, and maintained at that temperature for desired reaction time. Then the reactor was quenched to ambient temperature in a water bath, and the overhead pressure in the reactor was released slowly. The liquefaction mixture was separated by Soxhlet solvent extraction with tetrahydrofuran (THF), n-hexane and toluene in turn. The n-hexane insoluble but toluene soluble fraction was defined as asphaltene (AS), and the toluene insoluble but THF soluble fraction was defined as preasphaltene (PA). The fractionation procedure is shown in Fig. 1. The conversion of feedstock was defined as the THF soluble fraction þ gas, which is calculated from the THF insoluble residue. Gas yield was calculated from the material weight difference before and after liquefaction with gas released. Oil yield was calculated as:
3. Results and discussion 3.1. Pyrolysis of coal, sawdust and their mixture Pyrolysis is the first stage of liquefaction of coal and biomass. Fig. 2 shows the TG/DTG curves of Shenhua coal and sawdust. The initial (Ti) and final (Tf) temperatures of pyrolysis can be determined from TG curves using tangent line method according to literature [19]. From Fig. 2 it can be observed that Ti of Shenhua coal is about 362 C, and the temperature of the biggest weight loss rate is 450 C. The Ti of sawdust is low, only about 260 C, and the 0.002 100 80
Weight (%)
The repeatability of the fractionation analyses is 1%.
0.000 -0.002 Coal DTG
60
-0.006
40
Table 1 Elemental and proximate analyses of coal and sawdust. Sample
Shenhua coal Sawdust a
By difference.
Proximate analysis (db),wt%
-0.004
Coal
Sawdust TG 20
Elemental analysis (daf), wt%
Sawdust DTG
M
A
V
FC
C
H
N
S
Oa
8.6 5.5
10.9 2.9
39.3 78.2
49.8 18.9
73.17 47.57
4.59 5.58
1.08 1.32
0.45 0.16
20.71 45.37
0
0
100
200
300
-0.008 -0.010
400
500
600
700
800
Temperature Fig. 2. TG/DTG curves of Shenhua coal and sawdust.
900
Weight loss rate (%/min)
Oil% ¼ Conversion e PA e AS e Gas
Thermo gravimetric (TG) analysis was carried out on a SHIMADZU TG60 analyzer. About 10 mg of sample was placed in an alumina pan and heated from 25 C to 800 C at a rate of 10 C/min under 50 ml/min nitrogen gas flow. The reproducibility of the TG experiment for the characteristic temperatures is 1 C.
H. Shui et al. / Energy 36 (2011) 6645e6650
temperature of the biggest weight loss rate is 360 C, which are much lower than the corresponding temperatures of Shenhua coal. The main pyrolysis temperature range (Ti w Tf) of Shenhua coal (362e750 C) is much higher than that of sawdust (260e420 C), and the releasing rate of volatile matter of Shenhua coal is much lower than that of sawdust as indicated by Fig. 2. The biggest weight loss rate of sawdust is much higher than that of Shenhua coal. The results suggest that sawdust is very easy to pyrolyze compared to Shenhua coal. Fig. 3 shows the TG/DTG curves of the mixture of Shenhua coal and sawdust with 1:1 by weight (Exp) and the corresponding calculated mean results (Cal) from the individual TG/DTG curve shown in Fig. 2. It can be observed from Fig. 3 that the experimental weight losses (Exp) for the mixture co-pyrolysis are larger than the corresponding calculated mean results (Cal), and the maximum difference appears at about 390 C. This suggests that much more volatile matters are released from coal molecular structure in the co-pyrolysis of Shenhua coal with sawdust compared to that from coal pyrolysis alone. Park et al. [20] also found that the yield and conversion of co-pyrolysis of sawdust and coal blend based on the volatile matters are higher than those of the sum of sawdust and coal individually. DTG also shows that the biggest Exp weight loss rate is larger than that of Cal value, and the Exp temperature of biggest weight loss rate shifts to lower temperature about 10 C comparing with that of Cal value. The results suggest that there exists a synergistic effect during the co-pyrolysis of Shenhua coal and sawdust. Sawdust can promote the pyrolysis of Shenhua coal and makes more volatile matters to be released from coal molecular structure, resulting in more weight loss of coal in the co-pyrolysis with sawdust compared to that of coal pyrolysis alone. Kim et al. [21] believed that thermolysis of the lignin results in the formation of phenoxy and other reactive radicals at the temperatures, which are too low for significant thermolysis of the coal matrix taking place, and such radicals are effective and active intermediates to depolymerize coal by cleaving methylene bridges. The larger of the biggest weight loss rate and the shifting of the temperature of the biggest weight loss rate to lower temperature for the Exp values comparing with those of the Cal values may support this mechanism. 3.2. The individual liquefaction of Shenhua coal and sawdust It is necessary to investigate the individual liquefaction of Shenhua coal and sawdust. Table 2 shows the liquefaction properties of Shenhua coal and sawdust at different temperatures. It can 0.002 100
Weight (%)
60
TG
-0.002
40 -0.004
DTG
20
TG DTG
0 0
100
200
300
400
500
600
700
800
Weight loss rate (%/min)
0.000
80
-0.006 900
Temperature ( ) Fig. 3. Experimental and calculated TG/DTG curves of Shenhua coal and sawdust mixture.
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Table 2 Liquefaction properties of Shenhua coal and sawdust at different temperatures. Temperature C
Gas wt% (daf)
Oil wt% (daf)
AS wt% (daf)
PA wt% (daf)
Conversion wt%(daf)
380
3.8 20.9 7.6 25.5 7.0 23.6
17.6 47.3 21.9 44.1 32.7 55.2
7.7 6.6 15.4 16.1 15.7 4.6
16.9 13.0 8.4 8.6 20.6 7.0
46.0 87.8 53.3 94.3 76.0 90.4
400 420
Coal Sawdust Coal Sawdust Coal Sawdust
Liquefaction time: 60 min.
be observed from Table 2 that with the increase of temperature, the liquefaction conversion of Shenhua coal increased obviously, and oil yield increased simultaneously. Coal liquefaction kinetics [15] suggests that Shenhua coal liquefaction is a complex multiphase catalytic reaction process, which contains series, parallel and retrogressive reactions. The series reactions of coal / PA / AS / oil and gas are main reactions of Shenhua coal liquefaction. PA formed from coal macromolecules thermolyzed converts into AS, oil and gas by hydrocracking reactions, resulting in an obvious decrease of PA from 16.9% of 380 C to 8.4% of 400 C. Further increasing liquefaction temperature to 420 C, PA increased from 8.4% of 400 C to 20.6% of 420 C. This may be because the retrogressive reactions take place markedly at 420 C [15]. Comparing with the liquefaction of Shenhua coal, the liquefaction conversions of sawdust were much higher, and the differences between their conversions at the same conditions decreased with the increase of temperature. For example, the conversion differences decreased from 41.8% of 380 C to 14.4% of 420 C. TG/DTG above suggests that the main pyrolysis temperature range of Shenhua coal is much higher than that of sawdust. Therefore the liquefaction activity of sawdust at low temperatures should be higher than that of Shenhua coal, and the differences of their liquefaction activities decrease with the increase of temperature. In the meantime, the gas and oil yields of sawdust liquefaction are much higher than those of corresponding of coal liquefaction respectively. The results suggest that sawdust has higher liquefaction activity compared to Shenhua coal. This is in agreement with the higher pyrolysis activity of sawdust indicated by TG/DTG above, because pyrolysis is the first step of liquefaction. Table 1 shows that H/C of sawdust is much higher than that of Shenhua coal. This may suggest that sawdust has higher hydrogen donation ability compared to Shenghua coal, and this will result in higher yields of oil and gas for sawdust liquefaction. Table 3 shows the liquefaction properties of Shenhua coal and sawdust at 400 C for different liquefaction times. It can be observed that for Shenhua coal liquefaction, increasing liquefaction time from 30 to 60 min, the liquefaction conversion increased slowly, but PA decreased obviously. Therefore the liquefaction appeared to be PA converting into oil and gas at this period. Further prolonging liquefaction time to 90 min, the conversion increased obviously from 53.3% of 60 min to 70.2% of 90 min with 16.9% Table 3 Liquefaction properties of Shenhua coal and sawdust at 400 C and different reaction times. Time min
Gas wt% (daf)
Oil wt% (daf)
AS wt% (daf)
PA wt% (daf)
Conversion wt%(daf)
30
4.5 20.6 7.6 25.5 7.1 24.9
18.9 48.3 21.9 44.1 31.9 54.1
14.7 3.9 15.4 16.1 13.8 11.2
12.9 13.8 8.4 8.6 17.4 8.8
51.0 86.6 53.3 94.3 70.2 99.0
60 90
Coal Sawdust Coal Sawdust Coal Sawdust
H. Shui et al. / Energy 36 (2011) 6645e6650
increments. In the same time, the yield of oil also increased from 21.9% of 60 min to 31.9% of 90 min with 10.0% increments. The results suggest that with the increase of liquefaction time, PA and AS formed will further hydrocrack to form light constituents as oil and gas. Therefore the increase of liquefaction conversion is mainly reflected by the increase of oil. In the case of sawdust liquefaction, the increase of conversion was negligible after 60 min, but oil gave an obvious increase due to the high liquefaction activity of sawdust. 3.3. Co-liquefaction of Shenhua coal and sawdust 3.3.1. Effect of blending ratio of coal/sawdust TG/DTG and individual liquefaction of Shenhua coal and sawdust results above show that there exists synergistic effect during the co-pyrolysis of Shenhua coal and sawdust, and sawdust has higher liquefaction activity compared to Shenhua coal. Therefore we further study the co-liquefaction properties of Shenhua coal and sawdust. The co-liquefactions were conducted at 400 C and 60 min. The comparisons of the experimental conversion and product distribution (Exp) among different mass blending ratios of coal/sawdust during co-liquefaction are shown in Table 4. Assuming that there is no interaction between Shenhua coal and sawdust during co-liquefaction, thus the liquefied product yield of co-liquefaction should be equal to the weighted mean value of the individual liquefaction of Shenhua coal and sawdust. In order to probe the synergistic effect during the co-liquefaction of Shenhua coal and sawdust, the weighted mean values of the co-liquefaction conversion and liquefied product yield were calculated based on the individual liquefaction result of Shenhua coal and sawdust shown in Tables 2 and 3, and are also provided in Table 4. Hereafter the weighted mean values are defined as calculated values expressed by Cal. It can be observed from Table 4 that the Exp conversions are higher than the Cal values at all blending ratios of coal/sawdust, and the (Exp-Cal) differences increase with the increase of the amount of sawdust blending. For example, the conversion differences increased from 14.2% of 5/1 blending ratio to 17.9% of 1/1 blending ratio of coal/sawdust. The result suggests that there really exist synergistic effects during the co-liquefaction of Shenhua coal and sawdust. It can also be observed from Table 4 that with the increase of the amount of sawdust blending, light products oil and gas increased obviously, but the heavy products PA and AS hardly changed. This is because sawdust is more easily to be liquefied and it gives more light products compared to Shenhua coal. Fig. 4 shows the effect of blending ratio of coal/sawdust on the (Exp-Cal) differences of product yield in the co-liquefaction of Shenhua coal and sawdust. It can be observed from Fig. 4 that the synergistic effect of the co-liquefaction of Shenhua coal and sawdust at different blending ratios of coal/sawdust is mainly reflected by the increases of PA and oil. With the decrease of blending ratio of coal/sawdust, the (Exp-Cal) differences have an increased tendency for oil and gas but a decreased tendency for AS. This means that in the co-liquefaction process of sawdust and coal, sawdust is beneficial for the promoting of the formation of oil,
14
Gas
Oil
AS
PA
12 10 8 6
Exp-Cal /%
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4 2 0 -2 -4
5:1
2:1
1:1
-6 -8 -10
Coal/sawdust Fig. 4. (Exp-Cal) differences of liquefied product yields from the co-liquefaction of Shenhua coal and sawdust at different coal/sawdust blending ratios.
because it can act as hydrogen-donors to prevent the recombination reactions [20]. Altieri [9] researched the co-liquefaction of lignin and bituminous coal at 400 C and found that far more of benzene-soluble material and pentene soluble oil were formed in the co-liquefaction compared to those of the same amounts of coal and lignin reacted individually. The results suggest that addition of sawdust can promote the liquefaction of Shenhua coal. This may be because the free radicals or intermediates formed from pyrolysis of sawdust at low temperatures can interact with coal, therefore to promote the thermolysis and hydrogenation liquefaction of coal, resulting in the enhancements of conversion and oil yield in the coliquefaction. It should be noted that the characterizations of liquefied products from co-liquefaction of coal and sawdust are quite different from that from the liquefaction of coal alone, especially in the contents of heteroatoms such as nitrogen and oxygen. Therefore it will affect the further hydro-treatment of the liquefied products from co-liquefaction of coal and sawdust. The characterizations of liquefied products from co-liquefaction of coal and sawdust over different catalysts will be reported elsewhere. 3.3.2. Effect of liquefaction temperature Temperature affects the co-liquefaction behaviors of Shenhua coal and sawdust. It has been demonstrated that co-liquefaction of Shenhua coal and sawdust at 1/1 blending ratio of coal/sawdust gives the largest synergistic effect with promoted conversion and oil yield. The effect of temperature on the co-liquefaction properties of Shenhua coal and sawdust at 1/1 blending ratio of coal/sawdust with 60 min was further studied, and the results are shown in Table 5. Table 5 shows that increasing temperature from 380 to 400 C, the conversion increased obviously from 77.2% to 91.7%, and PA and gas also increased from 10.7% to 20.2% and 8.0%e18.1%, respectively. At the same time, oil and AS had decreased tendency
Table 4 Effect of blending ratio of coal/sawdust on the co-liquefaction properties. Coal/sawdust (mass)
Gas wt%(daf)
Oil wt%(daf)
AS wt%(daf)
PA wt%(daf)
Conversion wt%(daf)
1/1
18.1 16.5 12.6 13.6 9.0 10.6
39.6 33.0 33.9 29.3 30.7 25.6
13.8 15.8 14.9 15.6 15.9 15.5
20.2 8.5 21.3 8.5 18.7 8.4
91.7 73.8 82.7 67.0 74.3 60.1
2/1 5/1
Exp Cal Exp Cal Exp Cal
Liquefactions were carried out at 400 C and 60 min.
H. Shui et al. / Energy 36 (2011) 6645e6650
6649
Table 5 Effect of temperature on the co-liquefaction properties. Temperature C 380
Exp Cal Exp Cal Exp Cal
400 420
Gas wt%(daf)
Oil wt%(daf)
AS wt%(daf)
PA wt%(daf)
Conversion wt%(daf)
8.0 12.4 18.1 16.5 16.7 15.3
43.7 32.4 39.6 33.0 47.4 43.9
14.8 7.2 13.8 15.8 14.0 10.2
10.7 14.9 20.2 8.5 13.7 13.8
77.2 66.9 91.7 73.8 91.8 83.2
Liquefaction time: 60 min; coal/sawdust: 1/1 in mass.
because of the large amounts of gas formed. Further increasing the temperature to 420 C, the changes of conversion, AS and gas were negligible, but PA decreased and oil increased obviously. The coliquefaction appears to be the converting of PA into oil at this temperature stage. It can also be observed from Table 5 that at all temperatures the Exp values for conversion and oil are higher than those of Cal values. This suggests that there are synergistic effects in the coliquefaction at different temperatures. In order to further probe the synergistic effects of various liquefied products in the coliquefaction at different temperatures, Fig. 5 shows the (Exp-Cal) differences of product yields from the co-liquefaction of Shenhua coal and sawdust at different liquefaction temperatures. It can be observed from Fig. 5 that the synergistic effects of various products in the co-liquefaction are a strong function of temperature. The differences gave the minimum values for gas and PA and the maximum values for oil and AS at 380 C, suggesting the favorable synergistic effect taking place at this temperature. Although the largest enhancement in Exp conversion of 17.9% compared to Cal value was obtained at 400 C. Increasing temperature to 420 C, the (Exp-Cal) differences of liquefied product yields obviously decreased, and the synergistic effects became weak. This is because the liquefaction activity of Shenhua coal increases at higher temperatures (more than 400 C) therefore weakening the synergistic effects caused by sawdust addition. However, co-liquefaction at lower temperatures (such as 380 C), the free radicals or intermediates formed from pyrolysis of sawdust at these temperatures may promote the thermolysis and hydrogenation liquefaction of coal, resulting in the synergistic effects occurring as enhancements of Exp conversion and oil yield compared to those of Cal values. 3.3.3. Effect of liquefaction time Runs were performed over different times for the coliquefaction of Shenhua coal and sawdust (1/1 by weight) at
400 C to investigate the effect of liquefaction time, and the results are shown in Table 6. With the increase of time from 30 to 60 min, the conversion increased obviously from 82.2 to 91.7%, and oil and AS hardly changed. Further prolonging time to 90 min, the conversion decreased to 83.5%, and oil kept little change, suggesting that longer liquefaction time is not beneficial for the coliquefaction process. In the same time, gas, AS and PA also decreased in certain extent. The results suggest that with the increase of liquefaction time, the retrogressive reactions take place markedly in the co-liquefaction. Fig. 6 shows the synergistic effects of various products in the coliquefaction at different times expressed by the (Exp-Cal) differences of liquefied product yields. It can be observed from Fig. 6 that the largest synergistic effects are obtained at 60 min, where, the (Exp-Cal) differences of oil and PA are much larger than those at 30 or 90 min respectively. The following reaction mechanism is proposed to explain the synergistic effects at different liquefaction times [13]: Sawdust (S) / free radicals or intermediates (I1) / products (1) (P1)
Table 6 Effect of time on the Co-liquefaction properties. Time min
Gas wt% (daf)
Oil wt% (daf)
AS wt% (daf)
PA wt% (daf)
Conversion wt%(daf)
30
13.0 12.5 18.1 16.5 15.1 15.0
38.0 33.6 39.6 33.0 40.3 43.0
13.0 9.3 13.8 15.8 9.5 12.5
18.2 13.4 20.2 8.5 18.6 13.1
82.2 68.8 91.7 73.8 83.5 84.6
60 90
Exp Cal Exp Cal Exp Cal
Liquefaction temperature: 400 C; coal/sawdust: 1/1 in mass.
14
15
Gas
Oil
AS
PA
Gas
Oil
AS
PA
12 10
10
8
Exp-Cal / %
Exp-Cal / %
6 5
0
-5
4 2 0 -2 -4
380
400
420
-6
30
60
90
-8 -10
-10
Temperature / Fig. 5. (Exp-Cal) differences of liquefied product yields from the co-liquefaction of Shenhua coal and sawdust at different liquefaction temperatures.
Time / min Fig. 6. (Exp-Cal) differences of liquefied product yields from the co-liquefaction of Shenhua coal and sawdust at different liquefaction times.
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Coal (C) / free radicals or intermediates (I2) / products (P2) (2) C þ I1 / products (P3)
(3)
The reactions (1) and (2) are hypothesized to occur when sawdust and coal are alone deploymerized. In the co-liquefaction, the intermediates from reaction (1) further depolymerize coal via reaction (3). The enhancements in conversion and liquefied products obtained are due to reaction (3). The concentration of I1 should be dependent on time, and should increase with time as sawdust is depolymerized. At a certain time, I1 should reach a maximum value after which it should decline as it is consumed in reactions (1) and (3). Thus the enhancements in coal conversion and liquefied products should also increase with an increase in I1 (i.e. with time) reaching a maximum value and then decline. 4. Conclusions TG/DTG suggests that there exists synergistic effect in the copyrolysis of Shenhua coal and sawdust. Sawdust can promote the thermolysis of Shenhua coal and makes more volatile matters to be released from coal molecular structure during the co-thermolysis process, resulting in the larger weight losses of their mixture compared to the calculated values of their individual pyrolysis. Sawdust has higher liquefaction activity than Shenhua coal. At the same liquefaction conditions, sawdust gives much higher liquefaction conversion and oil yield compared to Shenhua coal. There are obviously positive synergistic effects for conversion and oil in the co-liquefaction of Shenhua coal and sawdust, and the synergistic effects are the function of liquefaction conditions. The largest enhancements in conversion and oil yield compared to corresponding calculated weighted mean values of the individual liquefaction of Shenhua coal and sawdust were obtained at 400 and 380 C, respectively, with 1/1 blending ration of coal/sawdust and 60 min reaction times. The synergistic effects of co-liquefaction of Shenhua coal and sawdust may be attributed to the promotions of the thermolysis and hydrogenation liquefaction of coal by the free radicals or intermediates formed from pyrolysis of sawdust at low temperatures. Acknowledgments This work was supported by the Natural Scientific Foundation of China (20876001, 21076001, 20936007), National Basic Research Program of China (973 Program, 2011CB201302) and the State Key
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