- Email: [email protected]
Characterization of Zhundong lignite and biomass co-pyrolysis in a thermogravimetric analyzer and a fixed bed reactor
Energy 141 (2017) 2154e2163
Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Characterization of Zhundong lignite and biomass co-pyrolysis in a thermogravimetric analyzer and a fixed bed reactor Feiqiang Guo*, Xiaolei Li, Yan Wang, Yuan Liu, Tiantao Li, Chenglong Guo School of Electrical and Power Engineering, China University of Mining and Technology, 221116 Xuzhou, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 July 2017 Received in revised form 13 November 2017 Accepted 25 November 2017
The co-pyrolysis characteristics of Zhundong lignite and pine sawdust were investigated in a thermogravimetric analyzer and a fixed bed reactor. This study found that the obtained activation energies were generally lower than the calculated values. Particularly in the conversion range of 0.2e0.6, most of the relative deviation values was lower than 10% for the blends, indicating positive synergistic effect between Zhundong lignite and pine sawdust in volatiles release during non-isothermal pyrolysis. From the isothermal pyroylysis in the fixed bed reactor, the experimental values of gas yield were greater than the calculated, while both experimental tar and char yields became lower. Pronounced synergy effect occurred at ZD and PS mass ratio of 1:1 and 2:1 for tar and gas product, indicating that enough hydrogendonors could be provided to promote degradation reactions. The experimental yields of four main gas components, CO, H2, CO2 and CH4, were all higher than that of the calculated values. SEM results indicated both Zhundong lignite and pine sawdust residue chars became more porous, and metals salt in Zhundong lignite volatilized and condensed on the surface of Zhundong lignite and pine sawdust, which may perform as catalyst during co-pyrolysis. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Co-pyrolysis Zhundo ng lignite Pine sawdust Synergistic effect
1. Introduction Coal is always the most abundant and widely used fossil resource around the world. The newly discovered Zhundong coal in northwest China is a great energy resource with exploitable reserves of 164 Gt [1], which is predicted to meet the coal demand of China for the next one hundred years. However, direct combustion of Zhundong coal in traditional boilers is restricted for its high content of alkali metal, which lowers the ash fusion temperature of Zhundong coal and results in unacceptably slagging and fouling [2,3]. Additionally, another problem is that excessive utilization of coal leads to serious environmental impact in China, and therefore efficient clean coal conversion technologies and alternative energies must be developed. Recently, much attention was paid to pyrolysis/gasification of Zhundong coal for producing clean gas products at temperatures well below the ash fusion temperatures to circumvent ash melting. Biomass, a renewable and abundant energy resource with CO2free property, became one of the most attractive alternative of coal
* Corresponding author. E-mail address: [email protected] (F. Guo). https://doi.org/10.1016/j.energy.2017.11.141 0360-5442/© 2017 Elsevier Ltd. All rights reserved.
nowadays. Co-pyrolysis/gasification of coal and biomass has been considered as a bridge between energy productions based on fossil fuels and renewable fuels, and has been paid much attention in many countries [4,5]. There are many advantages in co-pyrolysis/ gasification of coal and biomass. The addition of biomass reduces coal usage and thus mitigate the environmental impacts [6]. Then, the production of gas products from coal is impeded by a low hydrogen to carbon molar ratio (H/C), and biomass with relative high hydrogen molar ratio could act as hydrogen donors during copyrolysis [7]. Besides, biomass has high content of volatiles and high thermochemical reactivity, which facilitates the co-pyrolysis as well [8]. Thus, co-pyrolysis/gasification of Zhundong coal with biomass may be an effective solution to their industrial utilization. Co-pyrolysis is the initial step in all the co-thermochemical conversion, including gasification and combustion, and comprehensive studies of the co-pyrolysis of biomass and coal is necessary when analyzing co-gasification. Previous investigations have been focused on the possible synergistic effects as well as the product distributions for better conversion of coal [9,10]. It generally believed that the synergistic effect occurred during co-pyrolysis due to the interaction between coal and rapidly evolved products form biomass at high temperatures [11]. Firstly, the rapidly evolved gases, including H2O, CO, CO2, H2, CH4, may influence the pyrolysis
F. Guo et al. / Energy 141 (2017) 2154e2163
of coal through reactions between gases and coal, resulting in variations in conversion, product distributions and reaction kinetics [7,12]. Particularly, the increased quantity of hydrogen produced from biomass pyrolysis may react with evolving species from coal, and thus prevents the undesirable recombination reaction and promotes an increase in coal conversion [13]. Secondly, the pyrolysis of coal may also be influenced by interactions between coal and char from biomass rapid pyrolysis as well. It has been reported that the physical characteristics of residual chars, such as specific surface area and pore size, were improved by adding biomass during coal pyrolysis [14]. An increase in the fractal dimensions of residual chars was also found with addition of biomass, which may improve their reactivity as well [15]. Besides, the presence of alkali and alkaline earth metals in residual char may have a catalytic effect on the reactivity and volatile product distribution during copyrolysis [16]. Co-pyrolysis of Zhundong coal and biomass mixtures has also been investigated by several researchers. Wan et al. [17] studied the co-pyorlysis of Zhundong brown coal and straw by using a singleparticle reactor system, and the results suggested an absence of synergistic effect. However, in our previous study [18], interaction was noted between Zhundong lignite and pine sawdust for generating the gas components using a micro fluidized bed reactor under isothermal conditions, particular for H2, CO and CH4, and the interaction differed for different gas components and different blending ratios. It has also been reported [3] that alkali metal in Zhundong coal released during pyrolysis and gasification process which might have catalytic effect and increase the gasification reaction rate. Also, the global level of production of biomass is enormous, particular the huge and readily collectable forestry biomass such as pine sawdust, can ensure that adequate supplies are available for co-utilization with Zhundong coal. Consequently, it is meaningful to obtain comprehensive knowledge about reaction kinetics, product distribution and surface morphology of residual chars from Zhundong coal and pine sawdust blends so as to forecast their reactivity. In the present work, non-isothermal and isothermal copyrolytic characteristics of Zhundong ligite and pine sawdust were explored applying a TGA and a drop tube fixed-bed reactor respectively in order to obtain comprehensive information about the co-pyrolysis conversion process. The synergistic effect was investigated on the basis of kinetic parameters and product distribution, and much attention was also paid to study the surface morphology of the residual chars from rapid pyrolysis. The obtained pyrolytic behavior of Zhundong coal and pine sawdust may be used to design and operate co-thermal conversion system of biomass and coal. 2. Material and methods 2.1. Sample preparation Zhundong lignite (ZD) used in this investigation was obtained from east Junggar Basin, Xinjiang province. Pine sawdust (PS) was chosen to represent easily available forestry biomass due to their abundance globally. Table 1 gives the proximate and ultimate analyses of ZD and PS. It is apparent that the H:C mole ratio for PS is relatively high in comparison with that of ZD, suggesting that pine sawdust in the blend may act as a hydrogen donor to the copyrolysis process, which may lead to the interaction between them. Meanwhile, ZD contained higher fixed carbon (50.73 wt%) than PS (17.69 wt%), while PS had much higher proportion of volatiles (81.05 wt%) than ZD (35.15 wt%). It is also evident that ZD has high ash content of 14.12%, while PS is a typical low ash biomass (1.26%). Therefore, the chemical components Zhongdong lignite ash
2155
Table 1 Ultimate and proximate analysis of samples. Sample Ultimate analysis (wt%, dry basis) Carbon Hydrogen Oxygen (diff) Nitrogen Sulfur Proximate analysis (wt%, dry basis) Volatiles Fixed carbon Ash
ZD
PS
49.26 3.71 30.34 1.02 1.55
51.05 5.78 39.62 0.75 1.54
35.15 50.73 14.12
81.05 17.69 1.26
were characterized by the X-Ray Fluorescence (XRF-1800, SHIMADZU, Japan), and the relative content of these elements were listed in Table 2, showing that the ash is predominantly aluminosilicate species with sulfur, iron and calcium. It has been reported [19] that calcium and magnesium are associated with high char reactivity although this catalytic activity depends on the chemical and physical forms they take. Besides, iron, calcium and potassium may have catalytic performance on biomass and coal pyrolysis as well [20], and thus in turn may contribute to the interaction between biomass. The samples were milled and sieved to obtain uniform particle size of 150e250 mm for both lignite and biomass, dried at 105 C for around 24 h and then kept in a desiccator before use. Lignite was blended with biomass in mass ratios of 1:3, 1:1 and 3:1.
2.2. Experimental apparatus and methods Non-isothermal pyrolysis characteristics of Zhundong lignite, pine sawdust and their blends were determined in TGA (Labsys Evo STA, France) under pure N2 (99.999%) flow rate of 60 mL/min, which can accurately record weight loss of 0.02 mg. Sample taken was 10 mg for each test and dynamic trials were performed at heating rate of 10, 20, 30 and 40 C/min from ambient to 900 C [21,22]. Each experiment was repeated at least three times to assure the repeatability of the results. To investigate isothermal co-pyrolysis characteristics of Zhundong lignite and pine sawdust, a laboratory-scale fixed bed reactor was designed and built, as shown in Fig. 1. The fixed-bed quartz reactor employed is 30 mm in diameter and consists of a porous plate to support the sample. Before the start of each experiment, a sample of 5 g was pre-loaded into sample feeder and the reactor was purged with N2 (99.999%) at 150 mL/min flow rate to ensure an inert atmosphere. The reactor was preheated to the final set point (600, 700 or 800 C) using an automatic PID temperature controller. Then, the reaction started as soon as the sample rapidly inserted into the tubular reactor. The sample pyrolyzed for around 20 min under isothermal conditions for each experimental run. When the test was finished, N2 was passed through the reactor continuously to avoid oxidation, until it had completely cooled to room temperature. The tar compounds were trapped in four gas bottles connected in series and filled with isopropyl alcohol (100 mL). The gas bottles were placed in an ice-water bath. Following the isopropyl alcoholwashing bottles, one bottle of water and two bottles of silica gel are
Table 2 XRF analysis of Zhundong lignite. Element
SiO2
Al2O3
SO3
Fe2O3
CaO
MgO
K2O
Na2O
TiO2
Content(wt.%)
49.62
20.50
15.40
5.18
4.28
2.01
1.83
0.51
0.48
2156
F. Guo et al. / Energy 141 (2017) 2154e2163
Sample feeder Thermocouple
Mass flowmeter
Temperature controller
N2 Gas sample bag
Electric furnace Gas meter
Micro pump
Ice water Tar trapping
Silica gel Fig. 1. Schematic diagram of the test unit for investigating fixed bed pyrolysis.
employed to avoid escaping of isopropyl alcohol vapor. As tar condensed on the pipelines and was difficult to collect, these measurements cannot be considered as precise. After reaction, the total solution of the four bottles were mixed together and dried by a rotary evaporator at 90 C. Then, the amount of residue tar was weighed and calculated. The syngas was collected by a sample bag and was analyzed by a gas chromatograph (SC-8000-010) with TCD detectors. Solid product in the reactor was collected and weighted after each experiment. The morphology of the obtained char samples was characterized by a Scanning Electron Microscope (SEM) (QuantaTM 250, American) to compare the surface characteristics and structure. Fourier Transforms Infrared Spectroscopy (FT-IR) (VERTEX 80v, German) analysis was carried out in the wavelength range of 4000e400 cm1 to obtain the surface functional groups of the char samples. At least three repeat runs were conducted under the same conditions to ensure the repeatability of the experiment.
3. Results and discussion 3.1. Structure analysis of Zhundong lignite and pine sawdust The major absorption bands of Zhundong lignite and pine sawdust obtained by FTIR spectra analysis were illustrated in Fig. 2, representing their typical structure. The characteristic bands locating in 3600e3000 cm1 indicate the existence of hydrogen contented bonds. The appearance of adsorption bands at 3420 cm1 associates with the hydroxyl (O-H), showing that pine sawdust has a much higher intensity in hydroxyl than that from Zhundong lignite. The blends in the range 3000e2800 cm1 originates from the vibration of aliphatic carbon (C-H), and pine sawdust also shows a higher intensity due to the high hydrogen content. However, there are two week peaks for Zhundong lignite corresponding to C-H stretching vibrations for asymmetric and symmetric modes. The bands at 1610 and 1032 cm1 for both two fuels indicates
the existence of aromatic structure C]C and C]O. More absorption bands were found in pine sawdust at 1510, 1368, 1316 and 1272 cm1 compared to Zhundong lignite, which represents the existence of different carboxylic groups and ether bonds. Consequently, although the two fuels show some similarities in structure, pine sawdust is composed of more aliphatic groups and hydroxyl while Zhundong lignite contains more aromatic groups. Particularly, the groups related to hydrogen and oxygen differed significantly, also indicating the difference of these two fuels in elements and chemical structure. The difference in structure leads to different chemical reactivity, which would affect the product distribution in pyrolysis.
3.2. Pyrolytic characteristics in TGA Both Zhundong lignite and pine sawdust belong to heterogeneous polymer compound. The main decomposed products of them are similar, mainly including combustible gas, tar, and char. The decomposition behaviors obtained by thermogravimetric analysis can provide significant information for the utilization of solid fuels in the pyrolysis process [23]. Fig. 3 shows the TG and DTG curves of Zhundong lignite, pine sawdust and their blend at a certain heating rate of 20 C/min. As a whole, the pyrolysis processes of the two pure fuels can be characterized by a three-stage thermal degradation, corresponding to only one peak in the DTG curves. ZD is obviously different from PS in chemical composition (Table 1). The volatile content in sawdust was much higher than Zhundong lignite, which could elevate the thermal reactivity effectively, leading to lower pyrolysis temperature zones and higher DTG values. It should be noted that co-pyrolysis of the blend samples comprise the behavior of individual component. The blend samples showed two obvious mass loss stages in TG curves, representing the decomposition behaviors of Zhundong lignite and pine sawdust respectively. Correspondingly, the DTG curves of the blend samples showed two evident peaks.
F. Guo et al. / Energy 141 (2017) 2154e2163
2157
Transimittance (%)
Pine Sawdust
C-H C=C C=O
Zhundong lignite
O-H
4000
3500
3000
2500
2000
1500
1000
500
-1
Wave number (cm ) Fig. 2. FTIR spectra of the pine sawdust and Zhundong lignite.
(a) 100
ZD ZD:PS=3:1 ZD:PS=1:3 ZD:PS=1:1 PS
60 40
ZD ZD:PS=3:1 ZD:PS=1:3 ZD:PS=1:1 PS
-10
-15
20 0
0
-5 DTG (%/min)
TG (%)
80
(b)
0
200
400
600
-20
800
0
200
o
400
600
800
o
Temperature ( C)
Temperature ( C)
Fig. 3. (a)TG and (b) DTG curves for Zhundong lignite, pine sawdust and their blends at heating rate of 20 C/min.
Kinetic parameters (activation energy and pre-exponential factor) of the co-pyrolysis mechanism were derived from the obtained TGA data. In this study, the Distributed Activation Energy Model (DAEM), which has been successfully applied to solid fuel pyrolysis such as biomass and coal [24,25], was introduced to deduce the kinetic parameters. The distribution function routinely assumes that solid fuel pyrolysis is composed of a series of reactions which are simultaneous occurrence of independent, irreversible and first order decomposition reactions [26]. The DAEM could be expressed as Eq. (1).
a¼
w ¼1 w0
Z∞ 0
0 exp@ A0
Zt
1
E dt Af ðEÞdE exp RT
(1)
0
where a is the extent of conversion at any time; w and w0 represent the weight loss and the total weight loss of the samples, respectively. f(E) is the distribution function of activation energy, which satisfies Eq. (2). A0 and E are the pre-exponential and activation energy.
Z∞ f ðEÞ dE ¼ 1
(2)
0
As mentioned in Section 2.2, the experiment in this study was conducted non-isothermally at four heating rates of 10, 20, 30 and 40 C/min. According to Miura and Maki [27], E and A0 in Eq. (1) could be calculated by Eq. (3).
b A R E þ 0:6075 ln 2 ¼ ln 0 E RT T
(3)
where b and T are the heating rate and temperature respectively. Table 3 presents the obtained activation energies for Zhundong lignite, pine sawdust and their blends as a function of conversion from 0.1 to 0.9 using the DAEM model. The overall fit of the data was good with correlation coefficients (R2) greater than 0.95 for all samples, indicating a strong applicability of the DAEM to describe the pyrolysis kinetics of Zhundong lignite, pine sawdust and their blends. The activation energies of the pure materials range from
2158
F. Guo et al. / Energy 141 (2017) 2154e2163
Table 3 Activation energies for Zhundong lignite, pine sawdust and their blend pyrolysis obtained by the Distributed Activation Energy Model.
a
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
ZD:PS ¼ 3:1
ZD
ZD: PS ¼ 1:1
ZD: PS ¼ 1:3
PS
Ea (kJ/mol)
R2
Ea (kJ/mol)
R2
Ea (kJ/mol)
R2
Ea (kJ/mol)
R2
Ea (kJ/mol)
R2
84.7 170.0 227.4 235.0 254.8 251.0 274.2 303.6 346.9
0.97 0.98 0.98 0.95 0.95 0.99 0.99 0.99 0.95
87.9 149.7 178.8 176.4 193.1 230.8 248.9 268.7 326.4
0.99 0.99 0.97 0.97 0.98 0.95 0.99 0.99 0.97
87.4 136.2 149.5 164.1 182.5 179.4 215.0 236.3 280.2
0.95 0.97 0.99 0.95 0.95 0.98 0.95 0.99 0.97
85.5 116.6 132.4 133.9 176.4 177.1 233.8 249.5 284.9
0.97 0.99 0.98 0.98 0.98 0.98 0.95 0.98 0.97
73.4 118.2 129.6 127.5 158.7 156.7 172.8 218.5 245.1
0.98 0.97 0.99 0.93 0.98 0.99 0.97 0.99 0.95
87.7 to 346.9 kJ/mol for Zhundong lignite and 73.4e245.1 kJ/mol for pine sawdust. As would be expected given coal’s higher activation energy than biomass, as the percent of Zhundong lignite in a mixture increases, so does the activation energy at each conversion level. The different activation energies between coal and biomass is mainly caused by their molecular structure. Biomass, which mostly comprises cellulose, hemicellulose, and lignin, is linked together with relatively weak ether bonds (R-O-R) with bond energies of 380e420 kJ/mol [28]. In comparison, the backbone of coal structure is made of polycyclic aromatic hydrocarbons and linked together by aromatic ring bonds (C]C), which are much stronger with a bond energy of 1000 kJ/mol [29]. Overall, the values of activation energies increase gradually with the increasing conversion (a) for the five samples, while the changing trend varies at different stages. In agreement with the literature [30,31], the changing trend of the activation energies showed three phases: decomposition in the first phase finished at a ¼ 0.2, corresponding to dehydration and depolymerization of macromolecules. The second phase, between a ¼ 0.3 and a ¼ 0.7, showed a relatively stable activation energy value, corresponding to the mian pyrolysis of the samples. The third phase corresponded to the end of the pyrolysis, a>0.7, when carbonization and polycondensation reactions mainly occurred and the disordered carbon structure became orderly gradually, leading to rapid decrease in the reaction activity and increase in activation energy values [32]. The obtained values are in good accord with activation energies of pyrolysis determined via the DAEM for similar biomass and coal samples across the literature. Activation energies obtained by Li et al. [33] ranged from 146 to 233 kJ/mol for the pyrolysis of Jerusalem artichoke using heating rates of 5, 10, 20 and 30 C/min de Jong et al. [34] used the DAEM to describe the pyrolysis wood pellet, finding activation energies on the order of magnitude of 136e299 kJ/mol. Cai et al. [31] found a range of 180e272 kJ/mol for eight lignocellulosic biomass samples via the DAEM. The DAEM was applied by Goldfarb et al. [30] to analyze the pyrolysis kinetics Pennsylvania coal and three different biomass samples, yielding activation energies ranging from 304 to 522 kJ/mol for coal, 164e304 kJ/mol for the biomasses, and 218e530 kJ/mol for the coalebiomass blends. Based on the obtained activation energies, the interaction of Zhundong lignite and pine sawdust can be described by predicting the activation energy of the blended samples. As suggested by Goldfarb et al. [30], if the blended samples experience no synergistic effect in terms of reaction kinetics, it can be expected that the activation energy of a blend would be the additive summation of each individual fuel’s contribution. In this study, the calculated activation energies (EC) of the blended samples were obtained by Eq. (4). In order to further show the interactions between biomass and coal, the relative deviation (d, %) between the calculated values and experimental values could be calculated by Eq. (5)
EC ¼ EP nP þ EZ ð1 nP Þ
(4)
d ¼ ðEa Þmeasured ðEa Þcalculated ðEa Þmeasured 100%
(5)
where EC is the calculated activation energy of a blend, kJ/mol. EP and EZ represent the experimental activation energy of the pure pine sawdust and Zhundong lignite, kJ/mol. nP denotes to the pine sawdust ratio of the blended samples. The comparison of calculated activation energies from experimental values of the blends to that calculated from weighted average of pure materials were shown in Fig. 4. For the blends of Zhundong lignite and pine sawdust, the values of calculated activation energy were generally higher than that of the experimental values, indicating that the activation energy is generally over predicted by Eq. (4). Particularly in the conversion range of 0.2e0.6, most of the relative deviation values was lower than 10% for the blends, indicating that the synergistic effect between Zhundong lignite and pine sawdust may exist, and this effect promotes the thermal decomposition of the blends. With the increase of the PS mass ratio from 25% to 75%, the variation trends of relative deviation were different to a certain extent. For the blend of ZD:PS ¼ 3:1, obvious deviation was observed in the conversion range of 0.2e0.4, indicating that lower biomass blending ratio can promote the volatiles release at the beginning of the main pyrolysis phrase. In comparison, when the mass ratio of PS increased to 50% and 75%, significant deviation was observed almost across the main pyrolysis phrase (0.3 a0.6). Particularly, the blend of ZD:PS ¼ 1:1 showed significant and stable deviation during the pyrolysis process, suggesting that the synergistic effects were more obvious during ZD and PS non-isothermal co-pyrolysis for easier volatiles release. Previous researchers reported similar synergy effect during copyrolysis of the coal blended with various biomass materials [21,22]. Potential explanations focused on the deceleration of repolymerization and cross linking reaction by hydrogen radicals and acceleration of demethoxylation reactions via inorganic matter in biomass [35,36]. 3.3. Characteristics of co-pyrolysis in a fixed bed reactor 3.3.1. Overall product yield Gas, char and tar (or bio-oil) are the primary products during the co-pyrolysis process. In this work, the yields of tar and gas was obtained according to the sampling process, and therefore the measuring units were different from the char yield. For the blends, the calculation values of the product yields (YCal) were obtained by Eq. (6) for the comparison with the experimentally derived values.
YCal ¼ YP nP þ YZ ð1 nP Þ
(6)
F. Guo et al. / Energy 141 (2017) 2154e2163
250 200 150 100 50
20
ZD:PS=3:1 ZD:PS=1:1 ZD:PS=1:3
10 Relative deviation (%)
300
Ea (kJ/mol)
(b)
ZD ZD:PS=1:3-Exp ZD:PS=1:3-Cal ZD:PS=1:1-Exp ZD:PS=1:1-Cal ZD:PS=3:1-Exp ZD:PS=3:1-Cal SD
(a) 350
2159
0
-10
-20
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0.1
0.2
0.3
Conversion (-)
0.4
0.6
0.5
0.7
0.8
0.9
Conversion (-)
Fig. 4. (a) Comparison of calculated and experimental Ea and (b) the corresponding relative deviation.
(a)
70
o
Exp-600 C o Cal-600 C o Exp-700 C o Cal-700 C o Exp-800 C o Cal-800 C
Char yield (wt. %)
60 50 40 30
will follow the behaviors of their parent materials in an additive manner. It was observed that the experimental product yields obtained from the co-pyrolysis of ZD/PS blends were somewhat different from predicted values. The char yields were a little lower than the predicted values, indicating that the co-pyrolysis of ZD with PS may have positive effect on volatiles releasing and lead to less char yield. This can be attributed to the fact that the H/C molar ratios of PS was significantly higher than that of ZD (Table 1), leading to much more hydrogen related chemical gropes, such as OH and C-H (Fig. 2). In consequence, larger amount of H and OH radicals generated from PS pyrolysis can act as hydrogen donor species, promoting the cracking of the aromatic compounds in the ZD [14]. Furthermore, the suppression of secondary reactions may happen as well, such as condensation, repolymerization and crosslinking reaction, leading to less secondary char formation [38,39]. Additionally, the interactions between ZD and PS also promote an additional decomposition of tar to enhance gas yield during the co-pyrolysis conditions. As a result, the experimental tar yield is lower than the calculated values from the additive model, which
(b)
o
Exp-600 C o Cal-600 C o Exp-700 C o Cal-700 C o Exp-800 C o Cal-800 C
150
Tar yield (mg/g)
where YP and YZ denote to the individual yield of pine sawdust and Zhundong lignite respectively. The experimental and calculated char yields from the pyrolysis of ZD, PS and their blends from 600 to 800 C are shown in Fig. 5 (a). Since biomass is mainly consist of volatiles, the char yield from PS pyrolysis was much lower in comparison with ZD at the same pyrolysis temperature. With increasing temperature, char yields of both PS and ZD decreased due to the primary thermal decomposition and secondary reactions of char with volatiles [37]. As a consequence, the volatilizes including the condensable tar and non-condensable gases increased gradually with the increasing temperature, as can be seen in Figs. 5 (b) and 6(a). In addition, the yield of char decreased with the increasing of PS blending ratio, while yield of tar and product gas showed the opposite trend, representing that the overall changing trend of product yields with the blending ratio of PS follows the pyrolysis characteristics of ZD and PS. Generally, if no interaction happens during the thermal decomposition of coal and biomass, the product yield of the blends
100
50
20 10
ZD
3:1
1:1
1:3
SD
0
ZD
3:1
ZD:PS Fig. 5. Experimental and calculated (a) char and (b) tar yields under different conditions.
1:1
ZD:SD
1:3
SD
2160
(a)
F. Guo et al. / Energy 141 (2017) 2154e2163
(b) 50
300
40
200 o
Exp-600 C o Exp-700 C o Exp-800 C
150 100
o
Cal-600 C o Cal-700 C o Cal-800 C
Gas yield (ml/g)
Gas yield ( ml/g )
250
50 0
Cal-H2
Exp-CO Exp-CH4
Cal-CO Cal-CH4
Exp-CO2
Cal-CO2
30
20
10 ZD
3:1
1:1
1:3
ZD
SD
3:1
100
Exp-CO Exp-CH4
Cal-H2 T=700oC Cal-CO Cal-CH4
Exp-CO2
Cal-CO2
Exp-H2
(d) 160 140 Gas yield (ml/g)
Gas yield (ml/g)
120
80 60
120
20
1:3
SD
ZD:PS
Exp-CO Exp-CH4
Cal-H2 T=800oC Cal-CO Cal-CH4
Exp-CO2
Cal-CO2
Exp-H 2
1:3
SD
60
20 1:1
SD
80
40
3:1
1:3
100
40
ZD
1:1
ZD:PS
ZD:SD
(c)
o
T=600 C
Exp-H2
0
ZD
3:1
1:1
ZD:PS
Fig. 6. Experimental and calculated yields of (a) total gas and gas components at (b) 600 C, (c) 700 C and (d) 800 C.
also leads to an increase in gas yield. As can be seen in Fig. 6(a), the calculated gas yield is obviously higher than the experimental values. The difference of char, tar and gas yield between the experimental and calculated values was most remarkable at 600 C, especially at ZD and PS ratio of 2:1 and 1:1. It has been reported by Zhang et al. [40] and Park et al. [38] that enough free radical and hydrogen-donors are generated at 600 C for synergy in copyrolysis of biomass and lignite. Also, higher biomass blending ratio conditions can provide sufficient hydrogen donors for hydrogenation during coal pyrolysis, leading to stronger interaction in co-pyrolysis. In this study, more pronounced synergy effect occurred at ZD and PS mass ratio of 1:1 and 2:1 for tar and gas product, indicating that enough hydrogen-donors could be provided to promote degradation reactions. However, the synergetic interaction was reduced at a lower coal and biomass blending ratio of 1:3 due to excess volatiles production, which was also observed in previous studies [41,42]. The changing trend obtained here are in good accord with the results determined via the fixed bed reactor for similar samples across the literature. Krerkkaiwan et al. [42] found the
experimental product yields obtained from the pyrolysis of the coal/biomass blends were different from predicted values, deviating the most at biomass and coal ratio of 1:1 with higher gas yield and lower char yield. Zhang et al. [40] investigated the co-pyrolysis of legume straw and Dayan lignite, finding that under the higher blending ratio conditions, the yields of chars are lower than the calculated values and consequently the yields of liquid and tar are higher. Park et al. [38] studied the co-pyrolysis characteristics of sawdust and coal in a fixed bed at isothermal condition, finding that the synergy to produce more volatiles was appeared at 500e700 C, and the maximum synergy exhibits with a sawdust blending ratio of 0.6 at 600 C. Meng et al. [15] investigated rapid co-pyrolysis of platanus wood and lignite in a drop tube fixed-bed reactor and found that the experimental values of gas volume yields were greater than the predicted, and the maximum gas volume yield exhibited with 50% biomass blending ratio at 1000 C. Fig. 6 (b)-(d) shows the experimental and calculated values of the four main gas components as a function of ZD/PS blending ratios at different temperature. It is apparent that the effect of ZD/PS blending ratios on the yields of the main gas components was
F. Guo et al. / Energy 141 (2017) 2154e2163
similar in the temperature range of 600e800 C. Pine sawdust is shown to produce about several times as much CO as Zhundong lignite, particularly at lower temperature. An increased CH4 yield was also observed with the increase of sawdust blending ratio, while CO2 and H2 decreased slightly. These results are largely linear with respect to the blend ratio of pine sawdust, which follow closely the results of [42]. For the four main gas components, the observed experimental yields were higher than that of the calculated values, indicating that the co-pyrolysis of Zhundong lignite and pine sawdust is in favor of their formation. Particularly at the ZD/PS blending ratio of 3:1 and 1:1, the deviation was significant, which also could be due to the sufficient hydrogen donors for the co-pyrolysis. Additionally, The OH radicals, formed during co-pyrolysis, can attack the aromatic rings in coal and also react with aliphatic species and combine with carbon atoms to form CO and H2 [14]. Furthermore, the observed synergetic effect could also be attributed to the roles of alkali and alkaline earth metallic species (AAEMs) present in the Zhundong lignite and Pine sawdust. From Table 1, it can be seen that ZD has high ash content, leading to more AAEM species, such as Fe, Ca and especially K. These AAEM species have been reported to act as catalysts for the decomposition of secondary char and char gasification, forming more small molecule gases [43,44]. It has also been verified that the potassium volatilizes and then condenses over the char surface during the co-pyrolysis process, and then promotes the gas production through both secondary decomposition and gasification of the char with the steam produced during pyrolysis [45,46].
2161
3.3.2. Characterization of char from co-pyrolysis The char after pyrolysis was characterized by SEM analysis to describe Zhundong lignite and pine char formation, as presented in Fig. 7. From Fig. 7(a) and (b), it was apparent that the Zhundong lignite char appeared to have a relatively smooth surface of dense hydrocarbon molecules with little pores, whereas the pine sawdust char showed a more porous structure with larger pores. The significant difference in char structure is generally attributed to the difference in volatiles and fixed carbon content between Zhundong lignite and pine sawdust (Table 1). For the chars derived from ZD/ PS ¼ 1:1, the morphology of obtained Zhundong lignite char clearly changed (Fig. 7 c), showing that a loose packed and more porous structure was formed as a result of the co-pyrolysis process. These changes are likely relevant to the increased reactivity of the blending samples, leading to more char conversion into tar and gas. More water was generated from biomass pyrolysis, which will promote its reactions with carbon in Zhundong lignite, also resulting in more porosity of Zhundong lignite char [47]. Additionally, the obtained pine sawdust char (Fig. 7 d) from the pyrolysis of ZD/PS ¼ 1:1 showed long fibrous structure with more micro pores, also indicating that the co-pyrolysis promote the volatiles releasing from biomass as well. Similar SEM results were also observed by Krerkkaiwan et al. [42], and they believed that copyrolysis increased reactivity of the chars derived from the coal and biomass blends. Zhu et al. [48] studied the co-pyrolysis of wheat straw and Shangwan coal, also finding that straw char particles had a porous and loose structure while the coal char particles kept an irregular shapeand relatively compact structure.
(a)
(b)
(c)
(d)
Fig. 7. The SEM images of pyrolysis chars obtainted at 700 C: (a) pure ZD char; (b) pure PS char; (c) ZD char at blending ratio of 1:1; (d) PS char at blending ratio of 1:1.
2162
F. Guo et al. / Energy 141 (2017) 2154e2163
ZD
C-O C-C
Transimittance (%)
ZD:PS=3:1
4000
ZD:PS=1:1
O-H
C-H ZD:PS=1:3 PS
3500
3000
2500
2000
1500
1000
-1
Wave number (cm ) Fig. 8. FT-IR spectra of co-pyrolysis char obtainted at T ¼ 700 C.
Interestingly, it was found that both Zhundong lignite and sawdust char surfaces showed obvious crystal particles after copyrolysis, which may due to the releasing of metal from the ash. The pine sawdust used in this work has a very small ash content of 1.26%, while the Zhundong lignite has a high ash content of 14.12% (Table 1). Also, the Zhundong lignite ash contains some metals with low melting point, such as potassium and sodium. During the copyrolysis, the metals salt in the coal ash volatilized under high temperature, and some of them condensed on the surface of lignite and pine sawdust, leading to catalytic pyrolysis for more volatiles formation. Thus, it can be speculated that the interaction existed during the co-pyrolysis of Zhundong lignite and pine sawdust, which made the char obtain some catalytic properties on their pyrolysis. More information on char evolution was obtained from the determination of the surface functional groups using FT-IR analysis on both pure and blended (ZD:PS ¼ 1:1) chars, as shown in Fig. 8. In comparison with the spectra of Zhundong lignite and pine sawdust in Fig. 2, most of the functional groups were decomposed after pyrolysis to form light gases such as CO and CO2. The five residue chars remained similar groups, including C-H, C-O and O-H groups. The O-H stretching vibration band at around 3428 cm1 indicates compounds such as phenols, alcohols or carboxylic acids. The O-H groups were decomposed when pine sawdust was pyrolyzed, while the decrease of these groups in Zhundong lignite was much weaker, also indicating the relative stability of lignite. It was observed that the C-H groups at the band around 2900 cm1 were almost disappeared for pine sawdust, representing its high reactivity during pyrolysis. However, two weak peaks still remained for Zhundong lignite at 3000-2800 cm1, and higher sawdust content in blends also decreased the concentration of C-H. The oxygen contained groups such as C]O and COOH at 1700e1200 cm1 were almost disappeared for both Zhundong lignite and pine sawdust, and the remaining groups also became much weaker. When the blending
ratio of PS was not higher than 50%, the content of aromatic groups (1100 cm1) become higher in the residue char. This indicated that more groups related to hydrogen and oxygen content released for the blending materials to form more gas components, and the residual char was mainly influenced by the decomposition characteristics of Zhundong lignite. However, when the blending ratio of pine sawdust was over 50%, the content of aromatic groups dropped down sharply, which followed the structure of pine sawdust, leading to more decomposition of aromatic groups. The results provided more direct evidence that the co-pyrolysis had a great influence on aromatic groups, which determined the final product formation. 4. Conclusion Co-pyrolysis of Zhundong coal with pine sawdust was performed in a thermogravimetric analyzer and a fixed bed reactor working under non-isothermal and isothermal conditions respectively to investigate the synergistic effect between them. From copyrolysis of Zhundong coal and pine sawdust blend in TGA, the synergistic effect was pronounced which lowered the activation energies for most of the blend samples, particularly in the conversion range of 0.2e0.6, most of the relative deviation values was lower than 10% for the blends. From the fixed bed reactor experiments, gas products increase while tar and char yield decreased in comparison with the calculated values from the additive model, representing the synergistic effect on the production distribution during co-pyrolysis. The difference of char, tar and gas yield between the experimental and calculated values was most remarkable at 600 C, especially at ZD and PS ratio of 2:1 and 1:1. For the four main gas components, CO, H2, CO2 and CH4, the observed experimental yields were higher than that of the calculated values. Particularly at the ZD/PS blending ratio of 3:1 and 1:1, the deviation was significant, indicating that the co-pyrolysis of Zhundong lignite
F. Guo et al. / Energy 141 (2017) 2154e2163
and pine sawdust is in favor of their formation. It was also observed that the metals salt in the coal ash volatilized under high temperature, which may condensed on the surface of lignite and pine sawdust, leading to catalytic pyrolysis and promoting the synergistic effect. Acknowledgement This work was financially supported by the Fundamental Research Funds for the Central Universities(2015XKMS055). References [1] Zhou J, Zhuang X, Alastuey A, Querol X, Li JH. Geochemistry and mineralogy of coal in the recently explored Zhundong large coal field in the Junggar basin, Xinjiang province, China. Int J Coal Geol 2010;82:51e67. [2] Jin H, Chen Y, Ge Z, Liu S, Ren C, Guo L. Hydrogen production by Zhundong coal gasification in supercritical water. Int J Hydrogen Energy 2015;40: 16096e103. [3] Song G, Song W, Qi X, Lu Q. Transformation characteristics of sodium of Zhundong coal combustion/gasification in circulating fluidized bed. Energy Fuels 2016;30:3473e8. [4] Li K, Zhang R, Bi J. Experimental study on syngas production by co-gasification of coal and biomass in a fluidized bed. Int J Hydrogen Energy 2010;35: 2722e6. [5] Wu Z, Wang S, Zhao J, Chen L, Meng H. Product distribution during Copyrolysis of bituminous coal and lignocellulosic biomass major components in a drop-tube furnace. Energy Fuels 2015;29:4168e80. [6] Wang J, Yan Q, Zhao J, Wang Z, Huang J, Gao S, et al. Fast co-pyrolysis of coal and biomass in a fluidized-bed reactor. J Therm Anal Calorim 2014;118: 1663e73. [7] Weiland NT, Means NC, Morreale BD. Product distributions from isothermal co-pyrolysis of coal and biomass. Fuel 2012;94:563e70. [8] Kan T, Strezov V, Evans TJ. Lignocellulosic biomass pyrolysis: a review of product properties and effects of pyrolysis parameters. Renew Sust Energ Rev 2016;57:1126e40. [9] Wu Z, Wang S, Zhao J, Chen L, Meng H. Thermochemical behavior and char morphology analysis of blended bituminous coal and lignocellulosic biomass model compound co-pyrolysis: effects of cellulose and carboxymethylcellulose sodium. Fuel 2016;171:65e73. [10] Oladejo JM, Adegbite S, Pang CH, Liu H, Parvez AM, Wu T. A novel index for the study of synergistic effects during the co-processing of coal and biomass. Appl Energy 2017;188:215e25. [11] Haykiri-Acma H, Yaman S. Interaction between biomass and different rank coals during co-pyrolysis. Ren Energy 2010;35:288e92. [12] Sonobe T, Worasuwannarak N, Pipatmanomai S. Synergies in co-pyrolysis of Thai lignite and corncob. Fuel Process Technol 2008;89:1371e8. €stro €m K, Chen G, Yu Q, Brage C, Rose n C. Promoted reactivity of char in co[13] Sjo gasification of biomass and coal: synergies in the thermochemical process. Fuel 1999;78:1189e94. [14] Yuan S, Dai ZH, Zhou ZJ, Chen XL, Yu GS, Wang FC. Rapid co-pyrolysis of rice straw and a bituminous coal in a high-frequency furnace and gasification of the residual char. Bioresour Technol 2012;109:188e97. [15] Meng H, Wang S, Chen L, Wu Z, Zhao J. Study on product distributions and char morphology during rapid co-pyrolysis of platanus wood and lignite in a drop tube fixed-bed reactor. Bioresour Technol 2016;209:273e81. [16] Liu Q, Hu H, Zhou Q, Zhu S, Chen G. Effect of inorganic matter on reactivity and kinetics of coal pyrolysis. Fuel 2004;83:713e8. [17] Wan K, Wang Z, He Y, Xia J, Zhou Z, Zhou J, et al. Experimental and modeling study of pyrolysis of coal, biomass and blended coalebiomass particles. Fuel 2015;139:356e64. [18] Guo FQ, Liu Y, Wang Y, Li X, Li T, Guo C. Characterization and kinetics for copyrolysis of Zhundong lignite and pine sawdust in a micro fluidized bed. Energy Fuels 2017;31:8235e44. [19] Ellis N, Masnadi MS, Roberts DG, Kochanek MA, Ilyushechkin AY. Mineral matter interactions during co-pyrolysis of coal and biomass and their impact on intrinsic char co-gasification reactivity. Chem Eng J 2015;279: 402e8. [20] Besson M, Gallezot P, Pinel C. Conversion of biomass into chemicals over metal catalysts. Chem Rev 2014;114:1827e70.
2163
[21] Chen C, Ma X, He Y. Co-pyrolysis characteristics of microalgae Chlorella vulgaris and coal through TGA. Bioresour Technol 2012;117:264e73. [22] Wu Z, Wang S, Zhao J, Chen L, Meng H. Synergistic effect on thermal behavior during co-pyrolysis of lignocellulosic biomass model components blend with bituminous coal. Bioresour Technol 2014;169:220e8. [23] Wu Z, Wang S, Zhao J, Chen L, Meng H. Thermal behavior and char structure evolution of bituminous coal blends with edible fungi residue during co-pyrolysis. Energy Fuels 2014;28:1792e801. [24] Wang S, Dai G, Ru B, Zhao Y, Wang X, Xiao G. Influence of torrefaction on the characteristics and pyrolysis behavior of cellulose. Energy 2017;120:864e71. [25] Mani T, Murugan P, Mahinpey N. Determination of distributed activation energy model kinetic parameters using annealing optimization method for nonisothermal pyrolysis of lignin. Ind Eng Chem Res 2009;48:1464e7. [26] Ulloa C, Gordon A, García X. Distribution of activation energy model applied to the rapid pyrolysis of coal blends. Fuel 2004;71:465e83. [27] Miura K, Maki T. A simple method for estimating f(E) and k0(E) in the distributed activation energy model. Energy Fuels 1998;12:864e9. [28] Blazej A, Kosik M, Phytomass A. Raw material for chemistry and biotechnology; ellis horwood series in polymer science and technology. Amsterdam: Ellis Horwood Ltd.; 1993. p. 173e221. [29] Sadhukhan AK, Gupta P, Goyal T, Saha RK. Modelling of pyrolysis of coalbiomass blends using thermogravimetric analysis. Bioresour Technol 2008;99:8022e6. [30] Goldfarb JL, Ceylan S. Second-generation sustainability: application of the distributed activation energy model to the pyrolysis of locally sourced biomassecoal blends for use in co-firing scenarios. Fuel 2015;160:297e308. [31] Cai J, Wu W, Liu R, Huber GW. A distributed activation energy model for the pyrolysis of lignocellulosic biomass. Green Chem 2013;15:1331e40. [32] Porada S. The reactions of formation of selected gas products during coal pyrolysis. Fuel 2004;83:1191e6. [33] Li L, Wang G, Wang S, Qin S. Thermogravimetric and kinetic analysis of energy crop Jerusalem artichoke using the distributed activation energy model. J Therm Anal Calorim 2013;114:1183e9. [34] de Jong W, Pirone A, Wojtowicz M. Pyrolysis of Miscanthus Giganteus and wood pellets: TG-FTIR analysis and reaction kinetics. Fuel 2003;82:1139e47. [35] Soncini RM, Means NC, Weiland NT. Co-pyrolysis of low rank coals and biomass: product distributions. Fuel 2013;112:74e82. [36] Yangali P, Celaya AM, Goldfarb JL. Co-pyrolysis reaction rates and activation energies of West Virginia coal and cherry pit blends. J Anal Appl Pyrolysis 2014;108:203e11. [37] Zhang H, Xiao R, Huang H, Xiao G. Comparison of non-catalytic and catalytic fast pyrolysis of corncob in a fluidized bed reactor. Bioresour Technol 2009;100:1428e34. [38] Park DK, Kim SD, Lee SH, Lee JG. Co-pyrolysis characteristics of sawdust and coal blend in TGA and a fixed bed reactor. Bioresour Technol 2010;101: 6151e6. [39] Ulloa CA, Gordon AL, Garcia XA. Thermogravimetric study of interactions in the pyrolysis of blends of coal with radiata pine sawdust. Fuel Process Technol 2009;90:583e90. [40] Zhang L, Xu S, Zhao W, Liu S. Co-pyrolysis of biomass and coal in a free fall reactor. Fuel 2007;86:353e9. [41] Howaniec N, Smolinski A, Stanczyk K, Pichlak M. Steam co-gasification of coal and biomass derived chars with synergy effect as an innovative way of hydrogen-rich gas production. Int J Hydrogen Energy 2011;36:14455e63. [42] Krerkkaiwan S, Fushimi C, Tsutsumi A, Kuchonthara Prapan. Synergetic effect during co-pyrolysis/gasification of biomass and sub-bituminous coal. Fuel Process Technol 2013;115:11e8. [43] Liu Y, Wang Y, Guo FQ, Li X, Li T, Guo C, et al. Characterization of the gas releasing behaviors of catalytic pyrolysis of rice husk using potassium over a micro-fluidized bed reactor. Energy Convers Manage 2017;136:395e403. [44] Guo FQ, Liu Y, Wang Y, Li X, Li T, Guo C. Pyrolysis kinetics and behavior of potassium-impregnated pine wood in TGA and a fixed-bed reactor. Energy Convers Manage 2016;130:184e91. [45] Wannapeera J, Fungtammasan B, Worasuwannarak N. Effects of temperature and holding time during torrefaction on the pyrolysis behaviors of woody biomass. J Anal Appl Pyrol 2011;92:99e105. [46] Hayashi JI, Iwatsuki M, Morishita K, Tsutsumi A, Li CZ, Chiba T. Roles of inherent metallic species in secondary reactions of tar and char during rapid pyrolysis of brown coals in a drop-tube reactor. Fuel 2002;81:1977e87. [47] Yang X, Yuan CY, Xu J, Zhang WJ. Co-pyrolysis of Chinese lignite and biomass in a vacuum reactor. Bioresour Technol 2014;173:1e5. [48] Zhu W, Song W, Lin W. Catalytic gasification of char from co-pyrolysis of coal and biomass. Fuel Process Technol 2008;89:890e6.
Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Characterization of Zhundong lignite and biomass co-pyrolysis in a thermogravimetric analyzer and a fixed bed reactor Feiqiang Guo*, Xiaolei Li, Yan Wang, Yuan Liu, Tiantao Li, Chenglong Guo School of Electrical and Power Engineering, China University of Mining and Technology, 221116 Xuzhou, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 July 2017 Received in revised form 13 November 2017 Accepted 25 November 2017
The co-pyrolysis characteristics of Zhundong lignite and pine sawdust were investigated in a thermogravimetric analyzer and a fixed bed reactor. This study found that the obtained activation energies were generally lower than the calculated values. Particularly in the conversion range of 0.2e0.6, most of the relative deviation values was lower than 10% for the blends, indicating positive synergistic effect between Zhundong lignite and pine sawdust in volatiles release during non-isothermal pyrolysis. From the isothermal pyroylysis in the fixed bed reactor, the experimental values of gas yield were greater than the calculated, while both experimental tar and char yields became lower. Pronounced synergy effect occurred at ZD and PS mass ratio of 1:1 and 2:1 for tar and gas product, indicating that enough hydrogendonors could be provided to promote degradation reactions. The experimental yields of four main gas components, CO, H2, CO2 and CH4, were all higher than that of the calculated values. SEM results indicated both Zhundong lignite and pine sawdust residue chars became more porous, and metals salt in Zhundong lignite volatilized and condensed on the surface of Zhundong lignite and pine sawdust, which may perform as catalyst during co-pyrolysis. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Co-pyrolysis Zhundo ng lignite Pine sawdust Synergistic effect
1. Introduction Coal is always the most abundant and widely used fossil resource around the world. The newly discovered Zhundong coal in northwest China is a great energy resource with exploitable reserves of 164 Gt [1], which is predicted to meet the coal demand of China for the next one hundred years. However, direct combustion of Zhundong coal in traditional boilers is restricted for its high content of alkali metal, which lowers the ash fusion temperature of Zhundong coal and results in unacceptably slagging and fouling [2,3]. Additionally, another problem is that excessive utilization of coal leads to serious environmental impact in China, and therefore efficient clean coal conversion technologies and alternative energies must be developed. Recently, much attention was paid to pyrolysis/gasification of Zhundong coal for producing clean gas products at temperatures well below the ash fusion temperatures to circumvent ash melting. Biomass, a renewable and abundant energy resource with CO2free property, became one of the most attractive alternative of coal
* Corresponding author. E-mail address: [email protected] (F. Guo). https://doi.org/10.1016/j.energy.2017.11.141 0360-5442/© 2017 Elsevier Ltd. All rights reserved.
nowadays. Co-pyrolysis/gasification of coal and biomass has been considered as a bridge between energy productions based on fossil fuels and renewable fuels, and has been paid much attention in many countries [4,5]. There are many advantages in co-pyrolysis/ gasification of coal and biomass. The addition of biomass reduces coal usage and thus mitigate the environmental impacts [6]. Then, the production of gas products from coal is impeded by a low hydrogen to carbon molar ratio (H/C), and biomass with relative high hydrogen molar ratio could act as hydrogen donors during copyrolysis [7]. Besides, biomass has high content of volatiles and high thermochemical reactivity, which facilitates the co-pyrolysis as well [8]. Thus, co-pyrolysis/gasification of Zhundong coal with biomass may be an effective solution to their industrial utilization. Co-pyrolysis is the initial step in all the co-thermochemical conversion, including gasification and combustion, and comprehensive studies of the co-pyrolysis of biomass and coal is necessary when analyzing co-gasification. Previous investigations have been focused on the possible synergistic effects as well as the product distributions for better conversion of coal [9,10]. It generally believed that the synergistic effect occurred during co-pyrolysis due to the interaction between coal and rapidly evolved products form biomass at high temperatures [11]. Firstly, the rapidly evolved gases, including H2O, CO, CO2, H2, CH4, may influence the pyrolysis
F. Guo et al. / Energy 141 (2017) 2154e2163
of coal through reactions between gases and coal, resulting in variations in conversion, product distributions and reaction kinetics [7,12]. Particularly, the increased quantity of hydrogen produced from biomass pyrolysis may react with evolving species from coal, and thus prevents the undesirable recombination reaction and promotes an increase in coal conversion [13]. Secondly, the pyrolysis of coal may also be influenced by interactions between coal and char from biomass rapid pyrolysis as well. It has been reported that the physical characteristics of residual chars, such as specific surface area and pore size, were improved by adding biomass during coal pyrolysis [14]. An increase in the fractal dimensions of residual chars was also found with addition of biomass, which may improve their reactivity as well [15]. Besides, the presence of alkali and alkaline earth metals in residual char may have a catalytic effect on the reactivity and volatile product distribution during copyrolysis [16]. Co-pyrolysis of Zhundong coal and biomass mixtures has also been investigated by several researchers. Wan et al. [17] studied the co-pyorlysis of Zhundong brown coal and straw by using a singleparticle reactor system, and the results suggested an absence of synergistic effect. However, in our previous study [18], interaction was noted between Zhundong lignite and pine sawdust for generating the gas components using a micro fluidized bed reactor under isothermal conditions, particular for H2, CO and CH4, and the interaction differed for different gas components and different blending ratios. It has also been reported [3] that alkali metal in Zhundong coal released during pyrolysis and gasification process which might have catalytic effect and increase the gasification reaction rate. Also, the global level of production of biomass is enormous, particular the huge and readily collectable forestry biomass such as pine sawdust, can ensure that adequate supplies are available for co-utilization with Zhundong coal. Consequently, it is meaningful to obtain comprehensive knowledge about reaction kinetics, product distribution and surface morphology of residual chars from Zhundong coal and pine sawdust blends so as to forecast their reactivity. In the present work, non-isothermal and isothermal copyrolytic characteristics of Zhundong ligite and pine sawdust were explored applying a TGA and a drop tube fixed-bed reactor respectively in order to obtain comprehensive information about the co-pyrolysis conversion process. The synergistic effect was investigated on the basis of kinetic parameters and product distribution, and much attention was also paid to study the surface morphology of the residual chars from rapid pyrolysis. The obtained pyrolytic behavior of Zhundong coal and pine sawdust may be used to design and operate co-thermal conversion system of biomass and coal. 2. Material and methods 2.1. Sample preparation Zhundong lignite (ZD) used in this investigation was obtained from east Junggar Basin, Xinjiang province. Pine sawdust (PS) was chosen to represent easily available forestry biomass due to their abundance globally. Table 1 gives the proximate and ultimate analyses of ZD and PS. It is apparent that the H:C mole ratio for PS is relatively high in comparison with that of ZD, suggesting that pine sawdust in the blend may act as a hydrogen donor to the copyrolysis process, which may lead to the interaction between them. Meanwhile, ZD contained higher fixed carbon (50.73 wt%) than PS (17.69 wt%), while PS had much higher proportion of volatiles (81.05 wt%) than ZD (35.15 wt%). It is also evident that ZD has high ash content of 14.12%, while PS is a typical low ash biomass (1.26%). Therefore, the chemical components Zhongdong lignite ash
2155
Table 1 Ultimate and proximate analysis of samples. Sample Ultimate analysis (wt%, dry basis) Carbon Hydrogen Oxygen (diff) Nitrogen Sulfur Proximate analysis (wt%, dry basis) Volatiles Fixed carbon Ash
ZD
PS
49.26 3.71 30.34 1.02 1.55
51.05 5.78 39.62 0.75 1.54
35.15 50.73 14.12
81.05 17.69 1.26
were characterized by the X-Ray Fluorescence (XRF-1800, SHIMADZU, Japan), and the relative content of these elements were listed in Table 2, showing that the ash is predominantly aluminosilicate species with sulfur, iron and calcium. It has been reported [19] that calcium and magnesium are associated with high char reactivity although this catalytic activity depends on the chemical and physical forms they take. Besides, iron, calcium and potassium may have catalytic performance on biomass and coal pyrolysis as well [20], and thus in turn may contribute to the interaction between biomass. The samples were milled and sieved to obtain uniform particle size of 150e250 mm for both lignite and biomass, dried at 105 C for around 24 h and then kept in a desiccator before use. Lignite was blended with biomass in mass ratios of 1:3, 1:1 and 3:1.
2.2. Experimental apparatus and methods Non-isothermal pyrolysis characteristics of Zhundong lignite, pine sawdust and their blends were determined in TGA (Labsys Evo STA, France) under pure N2 (99.999%) flow rate of 60 mL/min, which can accurately record weight loss of 0.02 mg. Sample taken was 10 mg for each test and dynamic trials were performed at heating rate of 10, 20, 30 and 40 C/min from ambient to 900 C [21,22]. Each experiment was repeated at least three times to assure the repeatability of the results. To investigate isothermal co-pyrolysis characteristics of Zhundong lignite and pine sawdust, a laboratory-scale fixed bed reactor was designed and built, as shown in Fig. 1. The fixed-bed quartz reactor employed is 30 mm in diameter and consists of a porous plate to support the sample. Before the start of each experiment, a sample of 5 g was pre-loaded into sample feeder and the reactor was purged with N2 (99.999%) at 150 mL/min flow rate to ensure an inert atmosphere. The reactor was preheated to the final set point (600, 700 or 800 C) using an automatic PID temperature controller. Then, the reaction started as soon as the sample rapidly inserted into the tubular reactor. The sample pyrolyzed for around 20 min under isothermal conditions for each experimental run. When the test was finished, N2 was passed through the reactor continuously to avoid oxidation, until it had completely cooled to room temperature. The tar compounds were trapped in four gas bottles connected in series and filled with isopropyl alcohol (100 mL). The gas bottles were placed in an ice-water bath. Following the isopropyl alcoholwashing bottles, one bottle of water and two bottles of silica gel are
Table 2 XRF analysis of Zhundong lignite. Element
SiO2
Al2O3
SO3
Fe2O3
CaO
MgO
K2O
Na2O
TiO2
Content(wt.%)
49.62
20.50
15.40
5.18
4.28
2.01
1.83
0.51
0.48
2156
F. Guo et al. / Energy 141 (2017) 2154e2163
Sample feeder Thermocouple
Mass flowmeter
Temperature controller
N2 Gas sample bag
Electric furnace Gas meter
Micro pump
Ice water Tar trapping
Silica gel Fig. 1. Schematic diagram of the test unit for investigating fixed bed pyrolysis.
employed to avoid escaping of isopropyl alcohol vapor. As tar condensed on the pipelines and was difficult to collect, these measurements cannot be considered as precise. After reaction, the total solution of the four bottles were mixed together and dried by a rotary evaporator at 90 C. Then, the amount of residue tar was weighed and calculated. The syngas was collected by a sample bag and was analyzed by a gas chromatograph (SC-8000-010) with TCD detectors. Solid product in the reactor was collected and weighted after each experiment. The morphology of the obtained char samples was characterized by a Scanning Electron Microscope (SEM) (QuantaTM 250, American) to compare the surface characteristics and structure. Fourier Transforms Infrared Spectroscopy (FT-IR) (VERTEX 80v, German) analysis was carried out in the wavelength range of 4000e400 cm1 to obtain the surface functional groups of the char samples. At least three repeat runs were conducted under the same conditions to ensure the repeatability of the experiment.
3. Results and discussion 3.1. Structure analysis of Zhundong lignite and pine sawdust The major absorption bands of Zhundong lignite and pine sawdust obtained by FTIR spectra analysis were illustrated in Fig. 2, representing their typical structure. The characteristic bands locating in 3600e3000 cm1 indicate the existence of hydrogen contented bonds. The appearance of adsorption bands at 3420 cm1 associates with the hydroxyl (O-H), showing that pine sawdust has a much higher intensity in hydroxyl than that from Zhundong lignite. The blends in the range 3000e2800 cm1 originates from the vibration of aliphatic carbon (C-H), and pine sawdust also shows a higher intensity due to the high hydrogen content. However, there are two week peaks for Zhundong lignite corresponding to C-H stretching vibrations for asymmetric and symmetric modes. The bands at 1610 and 1032 cm1 for both two fuels indicates
the existence of aromatic structure C]C and C]O. More absorption bands were found in pine sawdust at 1510, 1368, 1316 and 1272 cm1 compared to Zhundong lignite, which represents the existence of different carboxylic groups and ether bonds. Consequently, although the two fuels show some similarities in structure, pine sawdust is composed of more aliphatic groups and hydroxyl while Zhundong lignite contains more aromatic groups. Particularly, the groups related to hydrogen and oxygen differed significantly, also indicating the difference of these two fuels in elements and chemical structure. The difference in structure leads to different chemical reactivity, which would affect the product distribution in pyrolysis.
3.2. Pyrolytic characteristics in TGA Both Zhundong lignite and pine sawdust belong to heterogeneous polymer compound. The main decomposed products of them are similar, mainly including combustible gas, tar, and char. The decomposition behaviors obtained by thermogravimetric analysis can provide significant information for the utilization of solid fuels in the pyrolysis process [23]. Fig. 3 shows the TG and DTG curves of Zhundong lignite, pine sawdust and their blend at a certain heating rate of 20 C/min. As a whole, the pyrolysis processes of the two pure fuels can be characterized by a three-stage thermal degradation, corresponding to only one peak in the DTG curves. ZD is obviously different from PS in chemical composition (Table 1). The volatile content in sawdust was much higher than Zhundong lignite, which could elevate the thermal reactivity effectively, leading to lower pyrolysis temperature zones and higher DTG values. It should be noted that co-pyrolysis of the blend samples comprise the behavior of individual component. The blend samples showed two obvious mass loss stages in TG curves, representing the decomposition behaviors of Zhundong lignite and pine sawdust respectively. Correspondingly, the DTG curves of the blend samples showed two evident peaks.
F. Guo et al. / Energy 141 (2017) 2154e2163
2157
Transimittance (%)
Pine Sawdust
C-H C=C C=O
Zhundong lignite
O-H
4000
3500
3000
2500
2000
1500
1000
500
-1
Wave number (cm ) Fig. 2. FTIR spectra of the pine sawdust and Zhundong lignite.
(a) 100
ZD ZD:PS=3:1 ZD:PS=1:3 ZD:PS=1:1 PS
60 40
ZD ZD:PS=3:1 ZD:PS=1:3 ZD:PS=1:1 PS
-10
-15
20 0
0
-5 DTG (%/min)
TG (%)
80
(b)
0
200
400
600
-20
800
0
200
o
400
600
800
o
Temperature ( C)
Temperature ( C)
Fig. 3. (a)TG and (b) DTG curves for Zhundong lignite, pine sawdust and their blends at heating rate of 20 C/min.
Kinetic parameters (activation energy and pre-exponential factor) of the co-pyrolysis mechanism were derived from the obtained TGA data. In this study, the Distributed Activation Energy Model (DAEM), which has been successfully applied to solid fuel pyrolysis such as biomass and coal [24,25], was introduced to deduce the kinetic parameters. The distribution function routinely assumes that solid fuel pyrolysis is composed of a series of reactions which are simultaneous occurrence of independent, irreversible and first order decomposition reactions [26]. The DAEM could be expressed as Eq. (1).
a¼
w ¼1 w0
Z∞ 0
0 exp@ A0
Zt
1
E dt Af ðEÞdE exp RT
(1)
0
where a is the extent of conversion at any time; w and w0 represent the weight loss and the total weight loss of the samples, respectively. f(E) is the distribution function of activation energy, which satisfies Eq. (2). A0 and E are the pre-exponential and activation energy.
Z∞ f ðEÞ dE ¼ 1
(2)
0
As mentioned in Section 2.2, the experiment in this study was conducted non-isothermally at four heating rates of 10, 20, 30 and 40 C/min. According to Miura and Maki [27], E and A0 in Eq. (1) could be calculated by Eq. (3).
b A R E þ 0:6075 ln 2 ¼ ln 0 E RT T
(3)
where b and T are the heating rate and temperature respectively. Table 3 presents the obtained activation energies for Zhundong lignite, pine sawdust and their blends as a function of conversion from 0.1 to 0.9 using the DAEM model. The overall fit of the data was good with correlation coefficients (R2) greater than 0.95 for all samples, indicating a strong applicability of the DAEM to describe the pyrolysis kinetics of Zhundong lignite, pine sawdust and their blends. The activation energies of the pure materials range from
2158
F. Guo et al. / Energy 141 (2017) 2154e2163
Table 3 Activation energies for Zhundong lignite, pine sawdust and their blend pyrolysis obtained by the Distributed Activation Energy Model.
a
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
ZD:PS ¼ 3:1
ZD
ZD: PS ¼ 1:1
ZD: PS ¼ 1:3
PS
Ea (kJ/mol)
R2
Ea (kJ/mol)
R2
Ea (kJ/mol)
R2
Ea (kJ/mol)
R2
Ea (kJ/mol)
R2
84.7 170.0 227.4 235.0 254.8 251.0 274.2 303.6 346.9
0.97 0.98 0.98 0.95 0.95 0.99 0.99 0.99 0.95
87.9 149.7 178.8 176.4 193.1 230.8 248.9 268.7 326.4
0.99 0.99 0.97 0.97 0.98 0.95 0.99 0.99 0.97
87.4 136.2 149.5 164.1 182.5 179.4 215.0 236.3 280.2
0.95 0.97 0.99 0.95 0.95 0.98 0.95 0.99 0.97
85.5 116.6 132.4 133.9 176.4 177.1 233.8 249.5 284.9
0.97 0.99 0.98 0.98 0.98 0.98 0.95 0.98 0.97
73.4 118.2 129.6 127.5 158.7 156.7 172.8 218.5 245.1
0.98 0.97 0.99 0.93 0.98 0.99 0.97 0.99 0.95
87.7 to 346.9 kJ/mol for Zhundong lignite and 73.4e245.1 kJ/mol for pine sawdust. As would be expected given coal’s higher activation energy than biomass, as the percent of Zhundong lignite in a mixture increases, so does the activation energy at each conversion level. The different activation energies between coal and biomass is mainly caused by their molecular structure. Biomass, which mostly comprises cellulose, hemicellulose, and lignin, is linked together with relatively weak ether bonds (R-O-R) with bond energies of 380e420 kJ/mol [28]. In comparison, the backbone of coal structure is made of polycyclic aromatic hydrocarbons and linked together by aromatic ring bonds (C]C), which are much stronger with a bond energy of 1000 kJ/mol [29]. Overall, the values of activation energies increase gradually with the increasing conversion (a) for the five samples, while the changing trend varies at different stages. In agreement with the literature [30,31], the changing trend of the activation energies showed three phases: decomposition in the first phase finished at a ¼ 0.2, corresponding to dehydration and depolymerization of macromolecules. The second phase, between a ¼ 0.3 and a ¼ 0.7, showed a relatively stable activation energy value, corresponding to the mian pyrolysis of the samples. The third phase corresponded to the end of the pyrolysis, a>0.7, when carbonization and polycondensation reactions mainly occurred and the disordered carbon structure became orderly gradually, leading to rapid decrease in the reaction activity and increase in activation energy values [32]. The obtained values are in good accord with activation energies of pyrolysis determined via the DAEM for similar biomass and coal samples across the literature. Activation energies obtained by Li et al. [33] ranged from 146 to 233 kJ/mol for the pyrolysis of Jerusalem artichoke using heating rates of 5, 10, 20 and 30 C/min de Jong et al. [34] used the DAEM to describe the pyrolysis wood pellet, finding activation energies on the order of magnitude of 136e299 kJ/mol. Cai et al. [31] found a range of 180e272 kJ/mol for eight lignocellulosic biomass samples via the DAEM. The DAEM was applied by Goldfarb et al. [30] to analyze the pyrolysis kinetics Pennsylvania coal and three different biomass samples, yielding activation energies ranging from 304 to 522 kJ/mol for coal, 164e304 kJ/mol for the biomasses, and 218e530 kJ/mol for the coalebiomass blends. Based on the obtained activation energies, the interaction of Zhundong lignite and pine sawdust can be described by predicting the activation energy of the blended samples. As suggested by Goldfarb et al. [30], if the blended samples experience no synergistic effect in terms of reaction kinetics, it can be expected that the activation energy of a blend would be the additive summation of each individual fuel’s contribution. In this study, the calculated activation energies (EC) of the blended samples were obtained by Eq. (4). In order to further show the interactions between biomass and coal, the relative deviation (d, %) between the calculated values and experimental values could be calculated by Eq. (5)
EC ¼ EP nP þ EZ ð1 nP Þ
(4)
d ¼ ðEa Þmeasured ðEa Þcalculated ðEa Þmeasured 100%
(5)
where EC is the calculated activation energy of a blend, kJ/mol. EP and EZ represent the experimental activation energy of the pure pine sawdust and Zhundong lignite, kJ/mol. nP denotes to the pine sawdust ratio of the blended samples. The comparison of calculated activation energies from experimental values of the blends to that calculated from weighted average of pure materials were shown in Fig. 4. For the blends of Zhundong lignite and pine sawdust, the values of calculated activation energy were generally higher than that of the experimental values, indicating that the activation energy is generally over predicted by Eq. (4). Particularly in the conversion range of 0.2e0.6, most of the relative deviation values was lower than 10% for the blends, indicating that the synergistic effect between Zhundong lignite and pine sawdust may exist, and this effect promotes the thermal decomposition of the blends. With the increase of the PS mass ratio from 25% to 75%, the variation trends of relative deviation were different to a certain extent. For the blend of ZD:PS ¼ 3:1, obvious deviation was observed in the conversion range of 0.2e0.4, indicating that lower biomass blending ratio can promote the volatiles release at the beginning of the main pyrolysis phrase. In comparison, when the mass ratio of PS increased to 50% and 75%, significant deviation was observed almost across the main pyrolysis phrase (0.3 a0.6). Particularly, the blend of ZD:PS ¼ 1:1 showed significant and stable deviation during the pyrolysis process, suggesting that the synergistic effects were more obvious during ZD and PS non-isothermal co-pyrolysis for easier volatiles release. Previous researchers reported similar synergy effect during copyrolysis of the coal blended with various biomass materials [21,22]. Potential explanations focused on the deceleration of repolymerization and cross linking reaction by hydrogen radicals and acceleration of demethoxylation reactions via inorganic matter in biomass [35,36]. 3.3. Characteristics of co-pyrolysis in a fixed bed reactor 3.3.1. Overall product yield Gas, char and tar (or bio-oil) are the primary products during the co-pyrolysis process. In this work, the yields of tar and gas was obtained according to the sampling process, and therefore the measuring units were different from the char yield. For the blends, the calculation values of the product yields (YCal) were obtained by Eq. (6) for the comparison with the experimentally derived values.
YCal ¼ YP nP þ YZ ð1 nP Þ
(6)
F. Guo et al. / Energy 141 (2017) 2154e2163
250 200 150 100 50
20
ZD:PS=3:1 ZD:PS=1:1 ZD:PS=1:3
10 Relative deviation (%)
300
Ea (kJ/mol)
(b)
ZD ZD:PS=1:3-Exp ZD:PS=1:3-Cal ZD:PS=1:1-Exp ZD:PS=1:1-Cal ZD:PS=3:1-Exp ZD:PS=3:1-Cal SD
(a) 350
2159
0
-10
-20
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0.1
0.2
0.3
Conversion (-)
0.4
0.6
0.5
0.7
0.8
0.9
Conversion (-)
Fig. 4. (a) Comparison of calculated and experimental Ea and (b) the corresponding relative deviation.
(a)
70
o
Exp-600 C o Cal-600 C o Exp-700 C o Cal-700 C o Exp-800 C o Cal-800 C
Char yield (wt. %)
60 50 40 30
will follow the behaviors of their parent materials in an additive manner. It was observed that the experimental product yields obtained from the co-pyrolysis of ZD/PS blends were somewhat different from predicted values. The char yields were a little lower than the predicted values, indicating that the co-pyrolysis of ZD with PS may have positive effect on volatiles releasing and lead to less char yield. This can be attributed to the fact that the H/C molar ratios of PS was significantly higher than that of ZD (Table 1), leading to much more hydrogen related chemical gropes, such as OH and C-H (Fig. 2). In consequence, larger amount of H and OH radicals generated from PS pyrolysis can act as hydrogen donor species, promoting the cracking of the aromatic compounds in the ZD [14]. Furthermore, the suppression of secondary reactions may happen as well, such as condensation, repolymerization and crosslinking reaction, leading to less secondary char formation [38,39]. Additionally, the interactions between ZD and PS also promote an additional decomposition of tar to enhance gas yield during the co-pyrolysis conditions. As a result, the experimental tar yield is lower than the calculated values from the additive model, which
(b)
o
Exp-600 C o Cal-600 C o Exp-700 C o Cal-700 C o Exp-800 C o Cal-800 C
150
Tar yield (mg/g)
where YP and YZ denote to the individual yield of pine sawdust and Zhundong lignite respectively. The experimental and calculated char yields from the pyrolysis of ZD, PS and their blends from 600 to 800 C are shown in Fig. 5 (a). Since biomass is mainly consist of volatiles, the char yield from PS pyrolysis was much lower in comparison with ZD at the same pyrolysis temperature. With increasing temperature, char yields of both PS and ZD decreased due to the primary thermal decomposition and secondary reactions of char with volatiles [37]. As a consequence, the volatilizes including the condensable tar and non-condensable gases increased gradually with the increasing temperature, as can be seen in Figs. 5 (b) and 6(a). In addition, the yield of char decreased with the increasing of PS blending ratio, while yield of tar and product gas showed the opposite trend, representing that the overall changing trend of product yields with the blending ratio of PS follows the pyrolysis characteristics of ZD and PS. Generally, if no interaction happens during the thermal decomposition of coal and biomass, the product yield of the blends
100
50
20 10
ZD
3:1
1:1
1:3
SD
0
ZD
3:1
ZD:PS Fig. 5. Experimental and calculated (a) char and (b) tar yields under different conditions.
1:1
ZD:SD
1:3
SD
2160
(a)
F. Guo et al. / Energy 141 (2017) 2154e2163
(b) 50
300
40
200 o
Exp-600 C o Exp-700 C o Exp-800 C
150 100
o
Cal-600 C o Cal-700 C o Cal-800 C
Gas yield (ml/g)
Gas yield ( ml/g )
250
50 0
Cal-H2
Exp-CO Exp-CH4
Cal-CO Cal-CH4
Exp-CO2
Cal-CO2
30
20
10 ZD
3:1
1:1
1:3
ZD
SD
3:1
100
Exp-CO Exp-CH4
Cal-H2 T=700oC Cal-CO Cal-CH4
Exp-CO2
Cal-CO2
Exp-H2
(d) 160 140 Gas yield (ml/g)
Gas yield (ml/g)
120
80 60
120
20
1:3
SD
ZD:PS
Exp-CO Exp-CH4
Cal-H2 T=800oC Cal-CO Cal-CH4
Exp-CO2
Cal-CO2
Exp-H 2
1:3
SD
60
20 1:1
SD
80
40
3:1
1:3
100
40
ZD
1:1
ZD:PS
ZD:SD
(c)
o
T=600 C
Exp-H2
0
ZD
3:1
1:1
ZD:PS
Fig. 6. Experimental and calculated yields of (a) total gas and gas components at (b) 600 C, (c) 700 C and (d) 800 C.
also leads to an increase in gas yield. As can be seen in Fig. 6(a), the calculated gas yield is obviously higher than the experimental values. The difference of char, tar and gas yield between the experimental and calculated values was most remarkable at 600 C, especially at ZD and PS ratio of 2:1 and 1:1. It has been reported by Zhang et al. [40] and Park et al. [38] that enough free radical and hydrogen-donors are generated at 600 C for synergy in copyrolysis of biomass and lignite. Also, higher biomass blending ratio conditions can provide sufficient hydrogen donors for hydrogenation during coal pyrolysis, leading to stronger interaction in co-pyrolysis. In this study, more pronounced synergy effect occurred at ZD and PS mass ratio of 1:1 and 2:1 for tar and gas product, indicating that enough hydrogen-donors could be provided to promote degradation reactions. However, the synergetic interaction was reduced at a lower coal and biomass blending ratio of 1:3 due to excess volatiles production, which was also observed in previous studies [41,42]. The changing trend obtained here are in good accord with the results determined via the fixed bed reactor for similar samples across the literature. Krerkkaiwan et al. [42] found the
experimental product yields obtained from the pyrolysis of the coal/biomass blends were different from predicted values, deviating the most at biomass and coal ratio of 1:1 with higher gas yield and lower char yield. Zhang et al. [40] investigated the co-pyrolysis of legume straw and Dayan lignite, finding that under the higher blending ratio conditions, the yields of chars are lower than the calculated values and consequently the yields of liquid and tar are higher. Park et al. [38] studied the co-pyrolysis characteristics of sawdust and coal in a fixed bed at isothermal condition, finding that the synergy to produce more volatiles was appeared at 500e700 C, and the maximum synergy exhibits with a sawdust blending ratio of 0.6 at 600 C. Meng et al. [15] investigated rapid co-pyrolysis of platanus wood and lignite in a drop tube fixed-bed reactor and found that the experimental values of gas volume yields were greater than the predicted, and the maximum gas volume yield exhibited with 50% biomass blending ratio at 1000 C. Fig. 6 (b)-(d) shows the experimental and calculated values of the four main gas components as a function of ZD/PS blending ratios at different temperature. It is apparent that the effect of ZD/PS blending ratios on the yields of the main gas components was
F. Guo et al. / Energy 141 (2017) 2154e2163
similar in the temperature range of 600e800 C. Pine sawdust is shown to produce about several times as much CO as Zhundong lignite, particularly at lower temperature. An increased CH4 yield was also observed with the increase of sawdust blending ratio, while CO2 and H2 decreased slightly. These results are largely linear with respect to the blend ratio of pine sawdust, which follow closely the results of [42]. For the four main gas components, the observed experimental yields were higher than that of the calculated values, indicating that the co-pyrolysis of Zhundong lignite and pine sawdust is in favor of their formation. Particularly at the ZD/PS blending ratio of 3:1 and 1:1, the deviation was significant, which also could be due to the sufficient hydrogen donors for the co-pyrolysis. Additionally, The OH radicals, formed during co-pyrolysis, can attack the aromatic rings in coal and also react with aliphatic species and combine with carbon atoms to form CO and H2 [14]. Furthermore, the observed synergetic effect could also be attributed to the roles of alkali and alkaline earth metallic species (AAEMs) present in the Zhundong lignite and Pine sawdust. From Table 1, it can be seen that ZD has high ash content, leading to more AAEM species, such as Fe, Ca and especially K. These AAEM species have been reported to act as catalysts for the decomposition of secondary char and char gasification, forming more small molecule gases [43,44]. It has also been verified that the potassium volatilizes and then condenses over the char surface during the co-pyrolysis process, and then promotes the gas production through both secondary decomposition and gasification of the char with the steam produced during pyrolysis [45,46].
2161
3.3.2. Characterization of char from co-pyrolysis The char after pyrolysis was characterized by SEM analysis to describe Zhundong lignite and pine char formation, as presented in Fig. 7. From Fig. 7(a) and (b), it was apparent that the Zhundong lignite char appeared to have a relatively smooth surface of dense hydrocarbon molecules with little pores, whereas the pine sawdust char showed a more porous structure with larger pores. The significant difference in char structure is generally attributed to the difference in volatiles and fixed carbon content between Zhundong lignite and pine sawdust (Table 1). For the chars derived from ZD/ PS ¼ 1:1, the morphology of obtained Zhundong lignite char clearly changed (Fig. 7 c), showing that a loose packed and more porous structure was formed as a result of the co-pyrolysis process. These changes are likely relevant to the increased reactivity of the blending samples, leading to more char conversion into tar and gas. More water was generated from biomass pyrolysis, which will promote its reactions with carbon in Zhundong lignite, also resulting in more porosity of Zhundong lignite char [47]. Additionally, the obtained pine sawdust char (Fig. 7 d) from the pyrolysis of ZD/PS ¼ 1:1 showed long fibrous structure with more micro pores, also indicating that the co-pyrolysis promote the volatiles releasing from biomass as well. Similar SEM results were also observed by Krerkkaiwan et al. [42], and they believed that copyrolysis increased reactivity of the chars derived from the coal and biomass blends. Zhu et al. [48] studied the co-pyrolysis of wheat straw and Shangwan coal, also finding that straw char particles had a porous and loose structure while the coal char particles kept an irregular shapeand relatively compact structure.
(a)
(b)
(c)
(d)
Fig. 7. The SEM images of pyrolysis chars obtainted at 700 C: (a) pure ZD char; (b) pure PS char; (c) ZD char at blending ratio of 1:1; (d) PS char at blending ratio of 1:1.
2162
F. Guo et al. / Energy 141 (2017) 2154e2163
ZD
C-O C-C
Transimittance (%)
ZD:PS=3:1
4000
ZD:PS=1:1
O-H
C-H ZD:PS=1:3 PS
3500
3000
2500
2000
1500
1000
-1
Wave number (cm ) Fig. 8. FT-IR spectra of co-pyrolysis char obtainted at T ¼ 700 C.
Interestingly, it was found that both Zhundong lignite and sawdust char surfaces showed obvious crystal particles after copyrolysis, which may due to the releasing of metal from the ash. The pine sawdust used in this work has a very small ash content of 1.26%, while the Zhundong lignite has a high ash content of 14.12% (Table 1). Also, the Zhundong lignite ash contains some metals with low melting point, such as potassium and sodium. During the copyrolysis, the metals salt in the coal ash volatilized under high temperature, and some of them condensed on the surface of lignite and pine sawdust, leading to catalytic pyrolysis for more volatiles formation. Thus, it can be speculated that the interaction existed during the co-pyrolysis of Zhundong lignite and pine sawdust, which made the char obtain some catalytic properties on their pyrolysis. More information on char evolution was obtained from the determination of the surface functional groups using FT-IR analysis on both pure and blended (ZD:PS ¼ 1:1) chars, as shown in Fig. 8. In comparison with the spectra of Zhundong lignite and pine sawdust in Fig. 2, most of the functional groups were decomposed after pyrolysis to form light gases such as CO and CO2. The five residue chars remained similar groups, including C-H, C-O and O-H groups. The O-H stretching vibration band at around 3428 cm1 indicates compounds such as phenols, alcohols or carboxylic acids. The O-H groups were decomposed when pine sawdust was pyrolyzed, while the decrease of these groups in Zhundong lignite was much weaker, also indicating the relative stability of lignite. It was observed that the C-H groups at the band around 2900 cm1 were almost disappeared for pine sawdust, representing its high reactivity during pyrolysis. However, two weak peaks still remained for Zhundong lignite at 3000-2800 cm1, and higher sawdust content in blends also decreased the concentration of C-H. The oxygen contained groups such as C]O and COOH at 1700e1200 cm1 were almost disappeared for both Zhundong lignite and pine sawdust, and the remaining groups also became much weaker. When the blending
ratio of PS was not higher than 50%, the content of aromatic groups (1100 cm1) become higher in the residue char. This indicated that more groups related to hydrogen and oxygen content released for the blending materials to form more gas components, and the residual char was mainly influenced by the decomposition characteristics of Zhundong lignite. However, when the blending ratio of pine sawdust was over 50%, the content of aromatic groups dropped down sharply, which followed the structure of pine sawdust, leading to more decomposition of aromatic groups. The results provided more direct evidence that the co-pyrolysis had a great influence on aromatic groups, which determined the final product formation. 4. Conclusion Co-pyrolysis of Zhundong coal with pine sawdust was performed in a thermogravimetric analyzer and a fixed bed reactor working under non-isothermal and isothermal conditions respectively to investigate the synergistic effect between them. From copyrolysis of Zhundong coal and pine sawdust blend in TGA, the synergistic effect was pronounced which lowered the activation energies for most of the blend samples, particularly in the conversion range of 0.2e0.6, most of the relative deviation values was lower than 10% for the blends. From the fixed bed reactor experiments, gas products increase while tar and char yield decreased in comparison with the calculated values from the additive model, representing the synergistic effect on the production distribution during co-pyrolysis. The difference of char, tar and gas yield between the experimental and calculated values was most remarkable at 600 C, especially at ZD and PS ratio of 2:1 and 1:1. For the four main gas components, CO, H2, CO2 and CH4, the observed experimental yields were higher than that of the calculated values. Particularly at the ZD/PS blending ratio of 3:1 and 1:1, the deviation was significant, indicating that the co-pyrolysis of Zhundong lignite
F. Guo et al. / Energy 141 (2017) 2154e2163
and pine sawdust is in favor of their formation. It was also observed that the metals salt in the coal ash volatilized under high temperature, which may condensed on the surface of lignite and pine sawdust, leading to catalytic pyrolysis and promoting the synergistic effect. Acknowledgement This work was financially supported by the Fundamental Research Funds for the Central Universities(2015XKMS055). References [1] Zhou J, Zhuang X, Alastuey A, Querol X, Li JH. Geochemistry and mineralogy of coal in the recently explored Zhundong large coal field in the Junggar basin, Xinjiang province, China. Int J Coal Geol 2010;82:51e67. [2] Jin H, Chen Y, Ge Z, Liu S, Ren C, Guo L. Hydrogen production by Zhundong coal gasification in supercritical water. Int J Hydrogen Energy 2015;40: 16096e103. [3] Song G, Song W, Qi X, Lu Q. Transformation characteristics of sodium of Zhundong coal combustion/gasification in circulating fluidized bed. Energy Fuels 2016;30:3473e8. [4] Li K, Zhang R, Bi J. Experimental study on syngas production by co-gasification of coal and biomass in a fluidized bed. Int J Hydrogen Energy 2010;35: 2722e6. [5] Wu Z, Wang S, Zhao J, Chen L, Meng H. Product distribution during Copyrolysis of bituminous coal and lignocellulosic biomass major components in a drop-tube furnace. Energy Fuels 2015;29:4168e80. [6] Wang J, Yan Q, Zhao J, Wang Z, Huang J, Gao S, et al. Fast co-pyrolysis of coal and biomass in a fluidized-bed reactor. J Therm Anal Calorim 2014;118: 1663e73. [7] Weiland NT, Means NC, Morreale BD. Product distributions from isothermal co-pyrolysis of coal and biomass. Fuel 2012;94:563e70. [8] Kan T, Strezov V, Evans TJ. Lignocellulosic biomass pyrolysis: a review of product properties and effects of pyrolysis parameters. Renew Sust Energ Rev 2016;57:1126e40. [9] Wu Z, Wang S, Zhao J, Chen L, Meng H. Thermochemical behavior and char morphology analysis of blended bituminous coal and lignocellulosic biomass model compound co-pyrolysis: effects of cellulose and carboxymethylcellulose sodium. Fuel 2016;171:65e73. [10] Oladejo JM, Adegbite S, Pang CH, Liu H, Parvez AM, Wu T. A novel index for the study of synergistic effects during the co-processing of coal and biomass. Appl Energy 2017;188:215e25. [11] Haykiri-Acma H, Yaman S. Interaction between biomass and different rank coals during co-pyrolysis. Ren Energy 2010;35:288e92. [12] Sonobe T, Worasuwannarak N, Pipatmanomai S. Synergies in co-pyrolysis of Thai lignite and corncob. Fuel Process Technol 2008;89:1371e8. €stro €m K, Chen G, Yu Q, Brage C, Rose n C. Promoted reactivity of char in co[13] Sjo gasification of biomass and coal: synergies in the thermochemical process. Fuel 1999;78:1189e94. [14] Yuan S, Dai ZH, Zhou ZJ, Chen XL, Yu GS, Wang FC. Rapid co-pyrolysis of rice straw and a bituminous coal in a high-frequency furnace and gasification of the residual char. Bioresour Technol 2012;109:188e97. [15] Meng H, Wang S, Chen L, Wu Z, Zhao J. Study on product distributions and char morphology during rapid co-pyrolysis of platanus wood and lignite in a drop tube fixed-bed reactor. Bioresour Technol 2016;209:273e81. [16] Liu Q, Hu H, Zhou Q, Zhu S, Chen G. Effect of inorganic matter on reactivity and kinetics of coal pyrolysis. Fuel 2004;83:713e8. [17] Wan K, Wang Z, He Y, Xia J, Zhou Z, Zhou J, et al. Experimental and modeling study of pyrolysis of coal, biomass and blended coalebiomass particles. Fuel 2015;139:356e64. [18] Guo FQ, Liu Y, Wang Y, Li X, Li T, Guo C. Characterization and kinetics for copyrolysis of Zhundong lignite and pine sawdust in a micro fluidized bed. Energy Fuels 2017;31:8235e44. [19] Ellis N, Masnadi MS, Roberts DG, Kochanek MA, Ilyushechkin AY. Mineral matter interactions during co-pyrolysis of coal and biomass and their impact on intrinsic char co-gasification reactivity. Chem Eng J 2015;279: 402e8. [20] Besson M, Gallezot P, Pinel C. Conversion of biomass into chemicals over metal catalysts. Chem Rev 2014;114:1827e70.
2163
[21] Chen C, Ma X, He Y. Co-pyrolysis characteristics of microalgae Chlorella vulgaris and coal through TGA. Bioresour Technol 2012;117:264e73. [22] Wu Z, Wang S, Zhao J, Chen L, Meng H. Synergistic effect on thermal behavior during co-pyrolysis of lignocellulosic biomass model components blend with bituminous coal. Bioresour Technol 2014;169:220e8. [23] Wu Z, Wang S, Zhao J, Chen L, Meng H. Thermal behavior and char structure evolution of bituminous coal blends with edible fungi residue during co-pyrolysis. Energy Fuels 2014;28:1792e801. [24] Wang S, Dai G, Ru B, Zhao Y, Wang X, Xiao G. Influence of torrefaction on the characteristics and pyrolysis behavior of cellulose. Energy 2017;120:864e71. [25] Mani T, Murugan P, Mahinpey N. Determination of distributed activation energy model kinetic parameters using annealing optimization method for nonisothermal pyrolysis of lignin. Ind Eng Chem Res 2009;48:1464e7. [26] Ulloa C, Gordon A, García X. Distribution of activation energy model applied to the rapid pyrolysis of coal blends. Fuel 2004;71:465e83. [27] Miura K, Maki T. A simple method for estimating f(E) and k0(E) in the distributed activation energy model. Energy Fuels 1998;12:864e9. [28] Blazej A, Kosik M, Phytomass A. Raw material for chemistry and biotechnology; ellis horwood series in polymer science and technology. Amsterdam: Ellis Horwood Ltd.; 1993. p. 173e221. [29] Sadhukhan AK, Gupta P, Goyal T, Saha RK. Modelling of pyrolysis of coalbiomass blends using thermogravimetric analysis. Bioresour Technol 2008;99:8022e6. [30] Goldfarb JL, Ceylan S. Second-generation sustainability: application of the distributed activation energy model to the pyrolysis of locally sourced biomassecoal blends for use in co-firing scenarios. Fuel 2015;160:297e308. [31] Cai J, Wu W, Liu R, Huber GW. A distributed activation energy model for the pyrolysis of lignocellulosic biomass. Green Chem 2013;15:1331e40. [32] Porada S. The reactions of formation of selected gas products during coal pyrolysis. Fuel 2004;83:1191e6. [33] Li L, Wang G, Wang S, Qin S. Thermogravimetric and kinetic analysis of energy crop Jerusalem artichoke using the distributed activation energy model. J Therm Anal Calorim 2013;114:1183e9. [34] de Jong W, Pirone A, Wojtowicz M. Pyrolysis of Miscanthus Giganteus and wood pellets: TG-FTIR analysis and reaction kinetics. Fuel 2003;82:1139e47. [35] Soncini RM, Means NC, Weiland NT. Co-pyrolysis of low rank coals and biomass: product distributions. Fuel 2013;112:74e82. [36] Yangali P, Celaya AM, Goldfarb JL. Co-pyrolysis reaction rates and activation energies of West Virginia coal and cherry pit blends. J Anal Appl Pyrolysis 2014;108:203e11. [37] Zhang H, Xiao R, Huang H, Xiao G. Comparison of non-catalytic and catalytic fast pyrolysis of corncob in a fluidized bed reactor. Bioresour Technol 2009;100:1428e34. [38] Park DK, Kim SD, Lee SH, Lee JG. Co-pyrolysis characteristics of sawdust and coal blend in TGA and a fixed bed reactor. Bioresour Technol 2010;101: 6151e6. [39] Ulloa CA, Gordon AL, Garcia XA. Thermogravimetric study of interactions in the pyrolysis of blends of coal with radiata pine sawdust. Fuel Process Technol 2009;90:583e90. [40] Zhang L, Xu S, Zhao W, Liu S. Co-pyrolysis of biomass and coal in a free fall reactor. Fuel 2007;86:353e9. [41] Howaniec N, Smolinski A, Stanczyk K, Pichlak M. Steam co-gasification of coal and biomass derived chars with synergy effect as an innovative way of hydrogen-rich gas production. Int J Hydrogen Energy 2011;36:14455e63. [42] Krerkkaiwan S, Fushimi C, Tsutsumi A, Kuchonthara Prapan. Synergetic effect during co-pyrolysis/gasification of biomass and sub-bituminous coal. Fuel Process Technol 2013;115:11e8. [43] Liu Y, Wang Y, Guo FQ, Li X, Li T, Guo C, et al. Characterization of the gas releasing behaviors of catalytic pyrolysis of rice husk using potassium over a micro-fluidized bed reactor. Energy Convers Manage 2017;136:395e403. [44] Guo FQ, Liu Y, Wang Y, Li X, Li T, Guo C. Pyrolysis kinetics and behavior of potassium-impregnated pine wood in TGA and a fixed-bed reactor. Energy Convers Manage 2016;130:184e91. [45] Wannapeera J, Fungtammasan B, Worasuwannarak N. Effects of temperature and holding time during torrefaction on the pyrolysis behaviors of woody biomass. J Anal Appl Pyrol 2011;92:99e105. [46] Hayashi JI, Iwatsuki M, Morishita K, Tsutsumi A, Li CZ, Chiba T. Roles of inherent metallic species in secondary reactions of tar and char during rapid pyrolysis of brown coals in a drop-tube reactor. Fuel 2002;81:1977e87. [47] Yang X, Yuan CY, Xu J, Zhang WJ. Co-pyrolysis of Chinese lignite and biomass in a vacuum reactor. Bioresour Technol 2014;173:1e5. [48] Zhu W, Song W, Lin W. Catalytic gasification of char from co-pyrolysis of coal and biomass. Fuel Process Technol 2008;89:890e6.