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Experimental study on co-pyrolysis and gasification of biomass with deoiled asphalt
Energy 134 (2017) 301e310
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Experimental study on co-pyrolysis and gasification of biomass with deoiled asphalt Qian Zhang a, Qingfeng Li b, Linxian Zhang b, Zhongliang Yu b, Xuliang Jing c, Zhiqing Wang b, Yitian Fang b, Wei Huang a, * a b c
Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan, 030024, China State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, China State Key Laboratory of Coal-based Low Carbon Energy, ENN Group Co., Ltd, Langfang, 065000, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 December 2016 Received in revised form 27 May 2017 Accepted 27 May 2017 Available online 29 May 2017
The behavior of co-pyrolysis and gasification of biomass and deoiled asphalt (DOA) was investigated. The co-pyrolysis of three biomasses and DOA reflected no obvious synergetic effect on the char yield, but the char's graphite degree reduced greatly. For the DOA was melted and stuck to the biomass surface during pyrolysis, the co-pyrolysis char showed an obvious agglomeration. The gasification rate of the copyrolysis chars was greatly increased by the addition of biomass, and the gasification curve were much similar to that of a homogenous char, indicating the blends were quite uniform and the alkali and alkaline earth metals in biomass could catalyze DOA gasification greatly. Sunflower stalk which has the highest potassium content and mineral content promoted the gasification rate best. Kinetic analysis showed that the average E values of the co-pyrolysis chars increased compared with the pure biomass char. The co-gasification of DOA and biomass is a good choice for disposing DOA. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Deoiled asphalt Biomass Co-pyrolysis/gasification Catalytic effect
1. Introduction Biomass is an organic fuel that is in abundance, clean and renewable, and the development and utilization of biomass energy are increasingly valued worldwide [1]. Gasification is an advanced conversion mode for which the solid fuel could be converted to into useful syngas and then be converted into many other valuable products using a host of well-known reactions, e.g., steam-methane reforming to produce methane, the Fischer-Tropsch reaction for liquid fuels [2]. Biomass is favorable for gasification as it always presents high gasification reactivity for the low graphitization degree of the amorphous carbon structure and high inherent alkali content originating from the nutritional requirements during growth process [3]. However, a series of problems such as the low calorific value and high CO2 content of produced gas limited the development of gasification technology for biomass [4]. Deoiled asphalt (DOA) is a byproduct produced from solvent deasphalting process. The solvent deasphalting process has received a great deal of attention recently for it can remove
* Corresponding author. E-mail address: [email protected] (W. Huang). http://dx.doi.org/10.1016/j.energy.2017.05.157 0360-5442/© 2017 Elsevier Ltd. All rights reserved.
asphaltenes and metal compounds effectively from heavy oil and provide a better quality of deasphalted oil for further upgrading. With the increasing upgrading of heavy oil and the development of the process, the amount of DOA would increase markedly [5]. Gasification seems to be an ideal route for DOA utilization [6,7]. Different from biomass, DOA has a high calorific value, while the gasification reactivity of the char is very low, thus the energy consumption and the cost of gasification are very high due to the high gasification temperature needed. Based on this, the cogasification of biomass and DOA is proposed and it is thought to be a good way for solving the problems exist in individual gasification. Recently, co-utilization of biomass and coal has been conducted, and the issues presented in dealing with biomass/coal alone can be significantly reduced [3,8e11]. What's more, synergetic effects that biomass performed in coal gasification have been reported, and the co-gasification has been proposed as an energy production bridge between fossil fuels and renewable fuels [7]. Other researchers also studied the co-gasification of biomass with tire, petroleum coke and so on, and they found that biomasses have a good catalytic effect and could be used as a good additive to these resources utilization [12e17]. However, few researches focused on the cogasification of DOA with biomass. In our earlier study, the co-
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50:50 has been designated as 50CS50D, the blend of SS/DOA with a ratio of 50:50 has been designated as 50SS50D and the blend composed of SD/DOA with a ratio of 50:50 has been designated as 50SD50D.
gasification of DOA char and biomass char was investigated and the results showed that corncob with rich potassium exhibited a good catalytic effect on DOA char's gasification [6]. While for the coprocess of DOA and biomass in a large scale, it's easier to blend the raw DOA and biomass, and more types of biomass need to be studied to test the feasibility of this technology. Thermogravimetric analyzer (TGA) is a useful tool for studying gasification behaviors for the high precision and well-controlled experimental conditions. Many researchers have studied isothermal gasification process of biomass char, coal char under CO2 or steam conditions [3,7,13,15]. In isothermal mode, gasification is performed at a given temperature, reflecting the overall reaction characteristics of char in gasifier, and several temperatures must be chosen for a precise understanding of the kinetic characteristics. The non-isothermal gasification is usually conducted by a programmed reaction temperature variation, which reveals the gasification behavior of char under different temperatures and clarifies the relationship between heating rate and reaction characteristics [18]. Compared with the isothermal method, nonisothermal method is simple and easy because of avoiding the change of chemical and physical properties of the tested sample, and providing useful information via fewer experiments [19e23]. Miura and Silveston [23] confirmed the validity of the temperature programmed reaction (TPR) technique for analysis of non-catalytic gas-solid reaction. Other researchers also determined the kinetic parameters of the gasification of coal/biomass or blend chars in a series of non-isothermal experiments [20,21,24]. Thus in this paper, non-isothermal gasification experiments were employed. The aim of this work is to use a simple set of analysis to give a novel appraisal to the co-gasification process of DOA with several kinds of biomasses. The co-pyrolysis characteristics of DOA and biomass were first studied by a fixed bed reactor. Then the gasification reactivity of the char was evaluated using non-isothermal thermogravimetric analysis and the kinetic parameters were compared.
2.2. Char preparation Chars were prepared by a fixed-bed reactor as described earlier [25]. The crucible (35 mm i.d.10 mm deep) loaded with the samples (~0.5 g) is placed in the upper part of the closed reactor with a N2 flow rate of 450 mL/min to ensure the system air-free. Then the reactor is heated to 900 C with a N2 flow rate of 150 mL/min. The crucible is quickly pushed to the constant temperature zone of the reactor and stayed 30 min to ensure the completion of the devolatilization stage. After that, the crucible is lifted to the top of the reactor to make it rapidly cooled. Finally, the samples are taken out and measured, and then ground to less than 154 mm for use. All of the pyrolysis experiments were replicated three times for repeatability. The inorganic elements of the chars were determined by an inductively coupled plasma-atomic emission spectrometry (ICP-AES, iCAP 6300, Thermo Fisher Scientific) and listed in Table 2. Though there is a big difference for the minerals contained in different type of biomass chars, the major inorganic elementals are K, Ca, Si, Mg, Al, Na, and Fe. The DOA ash content (0.75%) is very low, and the mineral composition is mainly the Ni, V and Fe, and no K is detected [6]. 2.3. Gasification experiment CO2 gasification experiments were studied by a Setaram SETSYS TGA. About 5 mg of samples were put in an alumina crucible (8 mm i.d.6 mm deep), and then: (a) purge the TGA system with CO2 (99.8%) for 30 min; (b) ramp the furnace temperature to the final temperature with a heating rate of 10, 20, 40 C/min under CO2 (100 mL/min); (c) terminate the run when the mass of the sample does not change and decrease the furnace to room temperature. The data were obtained from step (b) subtract a blank run. All the gasification experiments were performed twice to verify the reproducibility, and the error of the experiments was within ±1%.
2. Experimental 2.1. Samples Deoiled asphalt (DOA, fine particles) was collected from the spraying granulation processes [5]. Corn stalk (CS), sunflower stalk (SS) and saw dust (SD) were chosen as the biomass samples. The CS and SS are major agro-wastes in North China, and the SD is a byproduct of the wood manufacturing industries. The proximate and ultimate analyses, along with high heating value (HHV) are given in Table 1. The biomasses contain more moisture and oxygen but less sulfur, and the heating value are much lower than that of DOA. The biomass was blended with DOA to prepare different binary blends with varying proportions of biomass/DOA, the mass ratios have been selected as 10:90, 30:70, 50:50, 70:30, and 90:10, respectively. The blend from co-pyrolysis of CS/DOA with a ratio of
2.4. Data analysis If there is no interaction between the two materials, then the char yield from the co-pyrolysis of the DOA and biomass could be calculated by Equation (1).
Ycalc ¼ xbiomass Ybiomass þ xDOA YDOA
(1)
where xbiomass and xDOA represent the ratio of biomass and DOA in the blends, respectively. The Ybiomass represents the biomass char yield and the YDOA represent the DOA char yield. In order to find out whether the components of the blends interacted during gasification process, the calculated and
Table 1 Proximate and Ultimate analyses of samples. Raw Sample
CS SS SD DOA
Proximate anal. (%, air dried basis)
Ultimate anal. (%, dry ash-free basis)
M
A
V
FCa
C
H
Oa
N
St
14.71 11.09 12.81 0.33
1.50 10.40 2.15 0.75
68.52 58.82 66.18 69.08
15.27 19.69 18.86 29.84
49.69 47.60 48.00 84.34
6.02 5.94 6.10 8.26
44.05 44.29 45.65 0.82
0.05 0.28 0.14 1.55
0.19 1.89 0.11 5.03
Note: a By difference; St the total sulfur content.
HHV (MJ/kg) 19.53 17.05 19.46 40.35
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Table 2 The amount of some inorganic elementals in the chars (wt.%, dry basis).
CS 50CS50D SS 50SS50D SD 50SD50D
Si
Al
Fe
Ca
Mg
K
Na
Ti
P
S
Cl
0.77 0.31 0.21 0.08 0.16 0.01
0.07 0.03 0.13 0.02 0.06 0.02
0.12 0.10 0.12 0.07 0.16 0.04
1.15 0.58 6.45 2.72 3.75 1.06
0.45 0.13 2.90 1.00 0.10 0.13
6.17 2.42 12.18 5.11 1.14 0.44
0.07 0.03 1.33 0.56 0.07 0.03
0.00 0.01 0.00 0.01 0.01 0.01
0.09 0.05 0.35 0.13 0.06 0.03
0.11 2.37 0.33 2.27 0.05 1.55
1.61 0.42 2.42 0.90 0.06 0.03
experimental dm/dt curves of blends are compared. The calculated curves of the blends are obtained by Equation (2).
ðdm=dtÞcalc ¼ a1 ðdm=dtÞbiomass þ a2 ðdm=dtÞDOA
(2)
where a1, a2 represent the ratio of biomass char or DOA char in the co-pyrolysis chars, which are calculated based on the pure char yield and the blending ratio, a1þ a2 ¼ 1; (dm/dt)biomass, (dm/dt)DOA are the reaction rate of pure biomass or DOA char at time t, respectively.
3. Results and discussion 3.1. Characterization of the co-pyrolysis of biomass and DOA 3.1.1. The char yields of the co-pyrolysis samples The char yield of the co-pyrolysis of biomass with DOA at different blending ratios are shown in Fig. 1, together with the calculated results from each pure chars on the basis of an absence of synergistic effect (the dotted line). For the pyrolysis of the pure samples, it shows a big difference on char yields. The DOA char yield (37.96%) is the highest compared with the biomasses, for the CS, SS, SD char yield are 17.06%, 29.43%, 20.22%, respectively. The four resource all show a relative high volatile content. From Fig. 1, the char yield of the blend fuels are in good agreement with the calculated results on the whole, though it shows some differences at some blending ratios. This indicates that the co-pyrolysis of DOA and biomass does not show significant synergetic effect on char yields. Zhu et al. studied the co-pyrolysis of coal and biomass in a spout-entrained reactor and found no synergy in the co-pyrolysis of coal and wheat straw [26]. While according to Wang et al., synergetic effect existed, for the tar yield increased and gas yield decreased during co-pyrolysis of corncob and lignite, and the interactions between corncob volatiles and lignite played a dominant role [27]. From the study of Krerkkaiwan et al., the co-pyrolysis of
coal/biomass resulted in higher gas yield and lower tar and char yields, and the synergetic effect could be explained by the transferring of active OH and H radicals from biomass to coal as well as the catalytic role of potassium (K) from biomass [28]. Since the composition of the volatiles were not detected, it's hard to say if there is any synergetic effect during the co-pyrolysis process, but no obvious effect is observed on the co-pyrolysis char yields of biomass and DOA clearly. 3.1.2. The morphology of the pure char and the blend chars Fig. 2 shows the snapshots of the raw samples and the chars. For different kind of resource, the char has a different appearance. The raw DOA is fine particles while its char is clearly bulked. It become glossy and have a tight surface. DOA is originated from petroleum, and melting is a prerequisite for the initiation of the pyrolysis [29]. The DOA is melted first, then the char is formed in massive compounds. While for biomasses, the particles have a different color, and after pyrolysis, the chars still keep as loose particles and show clearly volumetric reduction.
Char yield %
40
30
CS SS SD
20
10 0
20
40
60
80
100
Blending ratio of Biomass / % Fig. 1. The char yield of the co-pyrolysis of DOA and biomass with different ratios.
Fig. 2. The snapshots of the raw sample and chars.
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The snapshots of the chars from co-pyrolysis of biomass/DOA at different blending ratios are shown in Fig. 3. All the blended chars are agglomerated, especially when the blending ratio of DOA is higher. Only for the chars of biomass/DOA at a ratio of 90:10, they are still keep as pulverized chars and only little agglomeration appeared. From the micro point of view, the SEM images of the pure char and the chars of biomass/DOA blends of 50:50 are shown in Fig. 4. The biomass char still keep some long cellulosic particles after pyrolysis (Fig. 4(a), (b), (c)). The DOA char have a smooth and nonporous particles (Fig. 4(d)). The char from co-pyrolysis of biomass and DOA is mechanically grinded and the image shows that the char particles are more like the biomass char. The DOA melted and stuck to the biomass surface, the surface of the blends become smoother, and it's not easy to tell the difference between the biomass char and the blends. When we grinded them, the biomass char is more rigidity than the DOA char and the co-pyrolysis chars, for biomass is of highly fibrous structure with links [4]. The surface area for the CS, SS, SD chars are 4, 6, 17 m2/g, respectively. While for the DOA char and the co-pyrolysis chars, the surface area are all less than 1 m2/g, which mean that the melting of DOA blocked some of the channels on biomass chars. Fig. 5 shows the qualitative analysis of the composition and elemental mapping (S, K, Na, Ca and Mg) of the 50SS50D by SEMEDS. The location where the elements scanned is chosen randomly. The distribution of sulfur could be regarded as an existing signal of DOA. Then it could be concluded that the DOA melt and combined with biomass evenly, and the alkali and alkali earth metals (AAEM) distributed evenly on the surface of the copyrolysis chars. 3.1.3. The XRD pattern of the pure char and the blend chars Fig. 6 shows the X-ray diffraction patterns of pure chars and the co-pyrolysis chars. The DOA char has the highest graphite-like structure (the 002 peak is the sharpest), that is an important reason why it has a low gasification reactivity [6]. No significant peak (002 peak and 100 peak) appeared reflects that all the biomass
char are of the irregular carbon structure. For the blends, the 002 peak decreases greatly compared with the raw DOA char, this might be caused by the AAEM contained in biomass can be eroded into the surface and interior of the co-pyrolysis chars, thereby reduces the graphite degree and increases the active sites. 3.2. Gasification characteristics of the chars 3.2.1. Gasification characteristics of the pure chars The gasification rate of the pure chars of CS, SS, SD, DOA as a function of temperature are shown in Fig. 7. The initial and final reaction temperature are considered to be the temperature at which the rate of mass loss was 0.05%/ C [20]. Table 3 shows the initial, peak and final temperatures corresponding to the experimental reactivity plots. From a qualitative point of view, the chars have a different gasification characteristics, and all the curves presented a single peak, which corresponds to the maximum rate of mass loss. Compared the reaction rate curves of the chars, SS char has the highest gasification reactivity, the initial reaction temperature at 669 C is the lowest, so does the peak temperature (775 C) and final temperature (882 C). If we use the peak temperature to compare the gasification reactivity of the chars (the higher peak temperature means the lower gasification reactivity, according to Jing et al. [30]), then the four chars reactivity is in the order of SS >SD >CS >DOA. The DOA char have the lowest gasification reactivity, and when the DOA gasified initially (the onset temperature of 868 C), the three biomass char have already gasified completely. The lowest reactivity of the DOA char could be attributed to the low surface area, high graphite-like structure (Fig. 6) and low ash content [6]. While for the biomass char, the minerals contained in these biomasses are all rich in AAEM, as shown in Table 2, which is of good catalytic effect for gasification. Especially for SS, which has the highest potassium content (potassium is a very effective catalyst). The K content of CS is higher than SD, while the Al content is also higher than SD. As reported by Ding et al. [5], the existence of Al or Si would inhabit the gasification reaction. Moreover, SD has a larger content of Ca, which is also a good
Fig. 3. The snapshots of the chars from co-pyrolysis of biomass and DOA at different blending ratios.
Q. Zhang et al. / Energy 134 (2017) 301e310
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Fig. 4. The SEM images of the chars: (a) CS; (b) SS; (c) SD; (d) DOA; (e) 50CS50D; (f) 50SS50D; (g) 50SD50D.
catalyst. For these two reasons, the gasification reactivity of SD is a bit higher than CS. Compared with DOA, the irregular carbon structure of the biomass char is another reason for the high gasification reactivity [16,24]. 3.2.2. Gasification characteristics of the co-pyrolysis chars Fig. 8 shows the experimental and calculated gasification rate curves of different biomasses, blends and DOA. The gasification rate curves of the co-pyrolysis chars are located between the biomass and DOA gasification rate curves. If the blended chars does not interact with each other, just as the biomass and DOA gasified individually, the reaction rate curves of the blends would be the same as the calculated results (the dash line). However, the reaction rate of co-pyrolysis char shows a big difference with the calculated
result. At the initial reaction, the rate is a bit lower than the calculated, for the addition of DOA melt and block some of the pores of the biomass (as the images shows in Figs. 3 and 4), thus decreases the surface area, and the active sites of the biomass chars decreased. Another reason is the alkali metal in biomass could transfer to DOA surface after co-pyrolysis, the alkali content in biomass decreased and caused the biomass gasification rate shows a bit lower than that of the pure biomass char. From the calculated curves, the reaction rate would increases with the biomass reaction, and when the biomass char burns out, the reaction rate will decrease for the DOA char's gasification rate is very low, and two peak rate would appeared during the blends gasification. While in fact with the gasification proceeds, the blends gasification rate becomes higher than the calculated, and only a single peak
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Q. Zhang et al. / Energy 134 (2017) 301e310
Fig. 5. SEM-EDS elemental mapping of 50SS50D char. Each map depicts the relative abundance of each element, with brighter colors indicating greater abundance. The first, SEM image for area analysed; K-KA, potassium mapping; Na-KA, sodium mapping; S-KA, sulfur mapping; Mg- KA, magnesium mapping; Ca-KA, calcium mapping. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
100
Mass loss ( %)
80 DOA CS SS SD
60
40
20
0
1.5
DOA CS SS SD
o
Mass loss rate ( %/ C )
2.0
Fig. 6. X-ray diffraction patterns of pure chars and the co-pyrolysis chars.
appeared. This reflects that the addition of biomass greatly changes the blend chars gasification characteristics. Nemanova et al. found that co-gasification of petroleum coke and biomass did not occur simultaneously, but sequentially, and the synergetic effect observed could be attributed to the catalytic effect of the biomass ash that is rich in AAEM [16]. Rizkiana et al. found that biomass ash with higher content of AAEM and lower content of silica have higher catalytic activity on coal gasification [31]. In this paper, from Table 2, with the addition of biomass, the co-pyrolysis chars are rich in K, Ca and Mg, thus the gasification rate increases a lot than that of DOA. Moreover, the blends rate curves are more like one kind of
1.0
0.5
0.0 400
600
800 o Temperature ( C )
1000
1200
Fig. 7. The mass loss and mass loss rate profiles of pure chars gasification characteristics.
homogenous char, and before the biomass is completely gasified, the DOA gasification has already begun. This also indicates that the
Q. Zhang et al. / Energy 134 (2017) 301e310 Table 3 The initial, peak and final temperature of the chars at different heating rates. Samples
DOA CS 50CS50D SS 50SS50D SD 50SD50D
10 C min1
20 C min1
40 C min1
Tinitial
Tpeak
Tfinal
Tinitial
Tpeak
Tfinal
Tinitial
Tpeak
Tfinal
876 741 802 669 726 732 808
1080 872 936 775 865 865 964
1267 905 988 882 895 887 1006
897 770 830 697 743 769 830
1234 899 967 801 888 908 995
1313 939 1019 856 927 939 1042
913 802 849 722 757 785 858
1287 932 992 838 919 930 1041
1341 985 1070 920 968 969 1111
blends are quite uniform and the AAEM in biomass could transferred into DOA evenly. For a better understanding of the catalytic effect of the biomasses, the blends gasification rate are compared in Fig. 8(d). For 50SS50D, the peak temperature 865 C is the lowest, for 50CS50D the peak temperature is 936 C, 50SD50D the peak temperature is 964 C. The potassium content in the co-pyrolysis chars are also in the order of 50SS50D >50CS50D >50SD50D (Table 2), reflects that the potassium content is a good parameter to illustrate the catalytic effect of these biomasses. The catalytic effect of the biomasses on DOA gasification is in the order of SS >CS >SD. According to the study of co-gasification of biomass with coal, the Al and Si contained in coal would react with K in biomass and formed KAlSiO4 which is of no catalytic effect and would inhibit the
307
gasification rate [3,7,13]. While for DOA, the main ash component is Ni, V, Fe, almost has no Al and Si. Hence, no inhibition effect appeared during co-gasification of DOA and biomass.
3.2.3. Kinetics analysis According to ICTAC Kinetics Committee recommendations [24,32], the determination of the kinetic parameters from a single TPR run may lead to unreliable rate parameters. At least three TPR runs at different heating rates are required to estimate reliable parameters and accurate activation energies. Therefore, the gasification characteristics of the samples under the heating rate of 10, 20 or 40 C/min were studied (as shown in Fig. 9). The gasification rate curves with different heating rates are similar to each other, and with the increase of the heating rate, the curves moves to a higher temperature zone. Gasification characteristic parameters are shown in Table 3. The Tinitial, Tpeak and Tfinal of different chars all increase correspondingly with the heating rate increase. With the increase of heating rate, the temperature increases faster and individual reaction does not have enough time to reach completion or equilibrium, so they overlap with the adjacent higher temperature reaction [18,20,24]. Kinetic parameters are basic but necessary data for good design and operation of the gasification utility. Many kind of kinetics models has been successfully used in gasification, such as the homogenous model, shrinking core model and the random pore model [20,23]. However, for the co-gasification experiments in this
Fig. 8. Experimental and calculated gasification rate curves for different biomass, blends and DOA: (a) CS, 50CS50D, DOA; (b) SS, 50SS50D, DOA; (c) SD, 50SD50D, DOA; (d) comparison of the blended chars.
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Fig. 9. Gasification rate patterns of the pure chars and co-pyrolysis chars under different heating rates.
paper, the reaction is more complicated and none of the above models could satisfactorily fitted to the data of the samples. For the above models paid more attention on structural changes, and did not consider the catalytic effect of indigenous alkali minerals. Therefore, the iso-conversional method, which allow the activation energy (E) to be estimated as a function of a without pre-fixing the reaction model [33] were chosen in this paper. The Vyazovkin kinetic model was used to calculate E due to its relatively high dependability, which can be expressed by Equation (3).
b RA E ln 2 ¼ ln Ef ðaÞ RTa T
(3)
where b is the heating rate during gasification, a is the conversion ratio of the raw sample and a ¼ (m0 - mt)/(m0-mf), t is time and m0 and mf are the initial mass and final mass of the sample, respectively. mt is the mass of the sample at time t, A is the preexponential factor, R is the universal gas constant, Ta is the temperature for a conversion ratio a and f(a) is the integration function of the reaction model. The values of ln(b/T2) versus 1/T for constant a at several b values can be calculated using a linear regression. E can then be solved based on various a values. The activation energy (E) and correlation coefficient (R2) results of the individual components and mixtures based on the iso-conversional method are illustrated in Table 4 and the distribution of the E is shown in Fig. 10.
The R2 values are within a narrow range, suggesting that the values of E satisfy accuracy requirements. Accordingly, the E values vary with respect to temperature and extent of reaction, and higher apparent E values are generally due to the reaction of the less reactive components [34]. For biomasses, the average E values of the CS, SS, SD are 224, 175 and 191 kJ mol1, respectively. This is in correspondence with the gasification reactivity discussed above, the higher the gasification reactivity, the lower the E values. While the DOA has the lowest reactivity, the average E values are lowest, so it's hard to compare its reactivity by the E values as they come from different nature. The distribution of the E of the co-pyrolysis chars are more like that of the pure biomass char (Fig. 10), and the E values is higher than that of the biomass char. The overall trend of E for CS, SS and 50CS50D also indicates that the more reactive constituents gasified at earlier stages of conversion followed in the later stages (a > 0.3) by the less reactive components corresponding to higher E values. While the overall trend of E for SD, 50SS50D and 50SD50D decreases through the reaction reflects that the constituents gasified become more reactive with conversion. DOA is different as the E values decreases at first and then increase with the conversion larger than 0.6. It is hard to explain for the unreliability of the theoretical or mechanistic interpretation of the kinetic trends obtained from these complex materials [34].
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309
Table 4 Kinetic parameters of the char samples during CO2 gasification determined at three heating rates (10, 20 and 40 K min1) for iso-conversional reaction models.
a
DOA
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Average
CS
50CS50D
SS
SD
50SD50D
R2
E
R2
E
R2
E
R2
E
R2
E
R2
E
R2
211 200 182 168 161 160 164 190
0.924 0.979 0.996 0.999 1.000 1.000 1.000 0.995
208 235 242 240 236 228 220 211 198 224
0.996 0.998 0.999 1.000 1.000 1.000 1.000 1.000 1.000
241 255 263 268 268 261 248 233 221 251
0.942 0.954 0.959 0.969 0.982 0.991 0.995 0.998 1.000
117 175 188 190 188 185 184 187
0.877 0.989 1.000 0.998 0.996 0.994 0.992 1.000
288 264 255 251 247 243 240 236 228 250
0.996 1.000 0.999 0.999 0.998 0.998 0.997 0.997 0.998
207 197 189 189 190 191 190 187 179 191
0.988 0.991 0.997 0.999 0.999 0.996 0.991 0.985 0.982
303 261 232 222 218 212 205 196 187 226
0.997 0.999 0.987 0.983 0.981 0.975 0.971 0.967 0.966
179
175
activity of the biomass caused by ash interactions for which has happened during co-gasification of coal and biomass would not occur. The catalytic ability of the three types of biomasses is in the order of SS>CS>SD, which is consistent with the potassium content. The gasification kinetic analysis shows that for the copyrolysis chars, the average E values are greater than the pure biomass char. The co-gasification of DOA and biomass is a good choice for disposing DOA.
280 240
E ( kJ/mol)
50SS50D
E
200 160
CS SS SD
120
0.2
0.4
0.6 Conversion X
DOA 50CS50D 50SS50D 50SD50D 0.8
Acknowledgements The research is financially supported by the project of an international cooperation between China and Japan, “Technology development and process integration for high-efficiency utilization of low-rank coals based on mild depolymerization and exergy recuperation” (2013DFG60060) and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA07050100).
Fig. 10. The distribution of the active energy (E) vs carbon conversion X.
References 4. Conclusions and outlook In this paper, co-processing technology of biomass and DOA has been evaluated, and the corresponding conclusions are as follows. DOA is of high heating values, so the co-processing of DOA and biomass could avoid the problems derived from low calorific value products in biomass gasification. The co-pyrolysis of DOA and biomass did not show any significant synergistic effect on the char yield, but the co-pyrolysis char's graphite degree reduced greatly. What's more, the chars from co-pyrolysis of biomass/DOA showed an obvious agglomeration, for the DOA was melted during thermal conversion and stuck to the biomass surface. For one thing it's good for the elaborate of the catalytic gasification effect because the biomass and DOA could be contacted tightly, for another it should be noted that a block phenomenon occurs during the co-pyrolysis section. Thus more attention should be paid when choosing the pilot scale gasification technology to avoid the feeding problems and the reactive gas flowing problems caused by that, and these problems could be solved by varying the blending ratios or the gasification ways. Gasification experiments of the co-pyrolysis chars showed that all the three biomasses selected played a good catalytic role on improving the DOA gasification reactivity and make the reactivity of the blended char more like that of the biomass char. Different from other researcher's studies, before the biomass was completely gasified, the gasification of the DOA contained in the co-pyrolysis chars has already begun. All of this reflects that the blends are quite uniform and the AAEM in biomass could catalytic DOA gasification greatly. Further, since the ash content of the DOA is very low and it almost contains no Si and Al, the decrease of the catalytic
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Experimental study on co-pyrolysis and gasification of biomass with deoiled asphalt Qian Zhang a, Qingfeng Li b, Linxian Zhang b, Zhongliang Yu b, Xuliang Jing c, Zhiqing Wang b, Yitian Fang b, Wei Huang a, * a b c
Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan, 030024, China State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, China State Key Laboratory of Coal-based Low Carbon Energy, ENN Group Co., Ltd, Langfang, 065000, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 December 2016 Received in revised form 27 May 2017 Accepted 27 May 2017 Available online 29 May 2017
The behavior of co-pyrolysis and gasification of biomass and deoiled asphalt (DOA) was investigated. The co-pyrolysis of three biomasses and DOA reflected no obvious synergetic effect on the char yield, but the char's graphite degree reduced greatly. For the DOA was melted and stuck to the biomass surface during pyrolysis, the co-pyrolysis char showed an obvious agglomeration. The gasification rate of the copyrolysis chars was greatly increased by the addition of biomass, and the gasification curve were much similar to that of a homogenous char, indicating the blends were quite uniform and the alkali and alkaline earth metals in biomass could catalyze DOA gasification greatly. Sunflower stalk which has the highest potassium content and mineral content promoted the gasification rate best. Kinetic analysis showed that the average E values of the co-pyrolysis chars increased compared with the pure biomass char. The co-gasification of DOA and biomass is a good choice for disposing DOA. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Deoiled asphalt Biomass Co-pyrolysis/gasification Catalytic effect
1. Introduction Biomass is an organic fuel that is in abundance, clean and renewable, and the development and utilization of biomass energy are increasingly valued worldwide [1]. Gasification is an advanced conversion mode for which the solid fuel could be converted to into useful syngas and then be converted into many other valuable products using a host of well-known reactions, e.g., steam-methane reforming to produce methane, the Fischer-Tropsch reaction for liquid fuels [2]. Biomass is favorable for gasification as it always presents high gasification reactivity for the low graphitization degree of the amorphous carbon structure and high inherent alkali content originating from the nutritional requirements during growth process [3]. However, a series of problems such as the low calorific value and high CO2 content of produced gas limited the development of gasification technology for biomass [4]. Deoiled asphalt (DOA) is a byproduct produced from solvent deasphalting process. The solvent deasphalting process has received a great deal of attention recently for it can remove
* Corresponding author. E-mail address: [email protected] (W. Huang). http://dx.doi.org/10.1016/j.energy.2017.05.157 0360-5442/© 2017 Elsevier Ltd. All rights reserved.
asphaltenes and metal compounds effectively from heavy oil and provide a better quality of deasphalted oil for further upgrading. With the increasing upgrading of heavy oil and the development of the process, the amount of DOA would increase markedly [5]. Gasification seems to be an ideal route for DOA utilization [6,7]. Different from biomass, DOA has a high calorific value, while the gasification reactivity of the char is very low, thus the energy consumption and the cost of gasification are very high due to the high gasification temperature needed. Based on this, the cogasification of biomass and DOA is proposed and it is thought to be a good way for solving the problems exist in individual gasification. Recently, co-utilization of biomass and coal has been conducted, and the issues presented in dealing with biomass/coal alone can be significantly reduced [3,8e11]. What's more, synergetic effects that biomass performed in coal gasification have been reported, and the co-gasification has been proposed as an energy production bridge between fossil fuels and renewable fuels [7]. Other researchers also studied the co-gasification of biomass with tire, petroleum coke and so on, and they found that biomasses have a good catalytic effect and could be used as a good additive to these resources utilization [12e17]. However, few researches focused on the cogasification of DOA with biomass. In our earlier study, the co-
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50:50 has been designated as 50CS50D, the blend of SS/DOA with a ratio of 50:50 has been designated as 50SS50D and the blend composed of SD/DOA with a ratio of 50:50 has been designated as 50SD50D.
gasification of DOA char and biomass char was investigated and the results showed that corncob with rich potassium exhibited a good catalytic effect on DOA char's gasification [6]. While for the coprocess of DOA and biomass in a large scale, it's easier to blend the raw DOA and biomass, and more types of biomass need to be studied to test the feasibility of this technology. Thermogravimetric analyzer (TGA) is a useful tool for studying gasification behaviors for the high precision and well-controlled experimental conditions. Many researchers have studied isothermal gasification process of biomass char, coal char under CO2 or steam conditions [3,7,13,15]. In isothermal mode, gasification is performed at a given temperature, reflecting the overall reaction characteristics of char in gasifier, and several temperatures must be chosen for a precise understanding of the kinetic characteristics. The non-isothermal gasification is usually conducted by a programmed reaction temperature variation, which reveals the gasification behavior of char under different temperatures and clarifies the relationship between heating rate and reaction characteristics [18]. Compared with the isothermal method, nonisothermal method is simple and easy because of avoiding the change of chemical and physical properties of the tested sample, and providing useful information via fewer experiments [19e23]. Miura and Silveston [23] confirmed the validity of the temperature programmed reaction (TPR) technique for analysis of non-catalytic gas-solid reaction. Other researchers also determined the kinetic parameters of the gasification of coal/biomass or blend chars in a series of non-isothermal experiments [20,21,24]. Thus in this paper, non-isothermal gasification experiments were employed. The aim of this work is to use a simple set of analysis to give a novel appraisal to the co-gasification process of DOA with several kinds of biomasses. The co-pyrolysis characteristics of DOA and biomass were first studied by a fixed bed reactor. Then the gasification reactivity of the char was evaluated using non-isothermal thermogravimetric analysis and the kinetic parameters were compared.
2.2. Char preparation Chars were prepared by a fixed-bed reactor as described earlier [25]. The crucible (35 mm i.d.10 mm deep) loaded with the samples (~0.5 g) is placed in the upper part of the closed reactor with a N2 flow rate of 450 mL/min to ensure the system air-free. Then the reactor is heated to 900 C with a N2 flow rate of 150 mL/min. The crucible is quickly pushed to the constant temperature zone of the reactor and stayed 30 min to ensure the completion of the devolatilization stage. After that, the crucible is lifted to the top of the reactor to make it rapidly cooled. Finally, the samples are taken out and measured, and then ground to less than 154 mm for use. All of the pyrolysis experiments were replicated three times for repeatability. The inorganic elements of the chars were determined by an inductively coupled plasma-atomic emission spectrometry (ICP-AES, iCAP 6300, Thermo Fisher Scientific) and listed in Table 2. Though there is a big difference for the minerals contained in different type of biomass chars, the major inorganic elementals are K, Ca, Si, Mg, Al, Na, and Fe. The DOA ash content (0.75%) is very low, and the mineral composition is mainly the Ni, V and Fe, and no K is detected [6]. 2.3. Gasification experiment CO2 gasification experiments were studied by a Setaram SETSYS TGA. About 5 mg of samples were put in an alumina crucible (8 mm i.d.6 mm deep), and then: (a) purge the TGA system with CO2 (99.8%) for 30 min; (b) ramp the furnace temperature to the final temperature with a heating rate of 10, 20, 40 C/min under CO2 (100 mL/min); (c) terminate the run when the mass of the sample does not change and decrease the furnace to room temperature. The data were obtained from step (b) subtract a blank run. All the gasification experiments were performed twice to verify the reproducibility, and the error of the experiments was within ±1%.
2. Experimental 2.1. Samples Deoiled asphalt (DOA, fine particles) was collected from the spraying granulation processes [5]. Corn stalk (CS), sunflower stalk (SS) and saw dust (SD) were chosen as the biomass samples. The CS and SS are major agro-wastes in North China, and the SD is a byproduct of the wood manufacturing industries. The proximate and ultimate analyses, along with high heating value (HHV) are given in Table 1. The biomasses contain more moisture and oxygen but less sulfur, and the heating value are much lower than that of DOA. The biomass was blended with DOA to prepare different binary blends with varying proportions of biomass/DOA, the mass ratios have been selected as 10:90, 30:70, 50:50, 70:30, and 90:10, respectively. The blend from co-pyrolysis of CS/DOA with a ratio of
2.4. Data analysis If there is no interaction between the two materials, then the char yield from the co-pyrolysis of the DOA and biomass could be calculated by Equation (1).
Ycalc ¼ xbiomass Ybiomass þ xDOA YDOA
(1)
where xbiomass and xDOA represent the ratio of biomass and DOA in the blends, respectively. The Ybiomass represents the biomass char yield and the YDOA represent the DOA char yield. In order to find out whether the components of the blends interacted during gasification process, the calculated and
Table 1 Proximate and Ultimate analyses of samples. Raw Sample
CS SS SD DOA
Proximate anal. (%, air dried basis)
Ultimate anal. (%, dry ash-free basis)
M
A
V
FCa
C
H
Oa
N
St
14.71 11.09 12.81 0.33
1.50 10.40 2.15 0.75
68.52 58.82 66.18 69.08
15.27 19.69 18.86 29.84
49.69 47.60 48.00 84.34
6.02 5.94 6.10 8.26
44.05 44.29 45.65 0.82
0.05 0.28 0.14 1.55
0.19 1.89 0.11 5.03
Note: a By difference; St the total sulfur content.
HHV (MJ/kg) 19.53 17.05 19.46 40.35
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303
Table 2 The amount of some inorganic elementals in the chars (wt.%, dry basis).
CS 50CS50D SS 50SS50D SD 50SD50D
Si
Al
Fe
Ca
Mg
K
Na
Ti
P
S
Cl
0.77 0.31 0.21 0.08 0.16 0.01
0.07 0.03 0.13 0.02 0.06 0.02
0.12 0.10 0.12 0.07 0.16 0.04
1.15 0.58 6.45 2.72 3.75 1.06
0.45 0.13 2.90 1.00 0.10 0.13
6.17 2.42 12.18 5.11 1.14 0.44
0.07 0.03 1.33 0.56 0.07 0.03
0.00 0.01 0.00 0.01 0.01 0.01
0.09 0.05 0.35 0.13 0.06 0.03
0.11 2.37 0.33 2.27 0.05 1.55
1.61 0.42 2.42 0.90 0.06 0.03
experimental dm/dt curves of blends are compared. The calculated curves of the blends are obtained by Equation (2).
ðdm=dtÞcalc ¼ a1 ðdm=dtÞbiomass þ a2 ðdm=dtÞDOA
(2)
where a1, a2 represent the ratio of biomass char or DOA char in the co-pyrolysis chars, which are calculated based on the pure char yield and the blending ratio, a1þ a2 ¼ 1; (dm/dt)biomass, (dm/dt)DOA are the reaction rate of pure biomass or DOA char at time t, respectively.
3. Results and discussion 3.1. Characterization of the co-pyrolysis of biomass and DOA 3.1.1. The char yields of the co-pyrolysis samples The char yield of the co-pyrolysis of biomass with DOA at different blending ratios are shown in Fig. 1, together with the calculated results from each pure chars on the basis of an absence of synergistic effect (the dotted line). For the pyrolysis of the pure samples, it shows a big difference on char yields. The DOA char yield (37.96%) is the highest compared with the biomasses, for the CS, SS, SD char yield are 17.06%, 29.43%, 20.22%, respectively. The four resource all show a relative high volatile content. From Fig. 1, the char yield of the blend fuels are in good agreement with the calculated results on the whole, though it shows some differences at some blending ratios. This indicates that the co-pyrolysis of DOA and biomass does not show significant synergetic effect on char yields. Zhu et al. studied the co-pyrolysis of coal and biomass in a spout-entrained reactor and found no synergy in the co-pyrolysis of coal and wheat straw [26]. While according to Wang et al., synergetic effect existed, for the tar yield increased and gas yield decreased during co-pyrolysis of corncob and lignite, and the interactions between corncob volatiles and lignite played a dominant role [27]. From the study of Krerkkaiwan et al., the co-pyrolysis of
coal/biomass resulted in higher gas yield and lower tar and char yields, and the synergetic effect could be explained by the transferring of active OH and H radicals from biomass to coal as well as the catalytic role of potassium (K) from biomass [28]. Since the composition of the volatiles were not detected, it's hard to say if there is any synergetic effect during the co-pyrolysis process, but no obvious effect is observed on the co-pyrolysis char yields of biomass and DOA clearly. 3.1.2. The morphology of the pure char and the blend chars Fig. 2 shows the snapshots of the raw samples and the chars. For different kind of resource, the char has a different appearance. The raw DOA is fine particles while its char is clearly bulked. It become glossy and have a tight surface. DOA is originated from petroleum, and melting is a prerequisite for the initiation of the pyrolysis [29]. The DOA is melted first, then the char is formed in massive compounds. While for biomasses, the particles have a different color, and after pyrolysis, the chars still keep as loose particles and show clearly volumetric reduction.
Char yield %
40
30
CS SS SD
20
10 0
20
40
60
80
100
Blending ratio of Biomass / % Fig. 1. The char yield of the co-pyrolysis of DOA and biomass with different ratios.
Fig. 2. The snapshots of the raw sample and chars.
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The snapshots of the chars from co-pyrolysis of biomass/DOA at different blending ratios are shown in Fig. 3. All the blended chars are agglomerated, especially when the blending ratio of DOA is higher. Only for the chars of biomass/DOA at a ratio of 90:10, they are still keep as pulverized chars and only little agglomeration appeared. From the micro point of view, the SEM images of the pure char and the chars of biomass/DOA blends of 50:50 are shown in Fig. 4. The biomass char still keep some long cellulosic particles after pyrolysis (Fig. 4(a), (b), (c)). The DOA char have a smooth and nonporous particles (Fig. 4(d)). The char from co-pyrolysis of biomass and DOA is mechanically grinded and the image shows that the char particles are more like the biomass char. The DOA melted and stuck to the biomass surface, the surface of the blends become smoother, and it's not easy to tell the difference between the biomass char and the blends. When we grinded them, the biomass char is more rigidity than the DOA char and the co-pyrolysis chars, for biomass is of highly fibrous structure with links [4]. The surface area for the CS, SS, SD chars are 4, 6, 17 m2/g, respectively. While for the DOA char and the co-pyrolysis chars, the surface area are all less than 1 m2/g, which mean that the melting of DOA blocked some of the channels on biomass chars. Fig. 5 shows the qualitative analysis of the composition and elemental mapping (S, K, Na, Ca and Mg) of the 50SS50D by SEMEDS. The location where the elements scanned is chosen randomly. The distribution of sulfur could be regarded as an existing signal of DOA. Then it could be concluded that the DOA melt and combined with biomass evenly, and the alkali and alkali earth metals (AAEM) distributed evenly on the surface of the copyrolysis chars. 3.1.3. The XRD pattern of the pure char and the blend chars Fig. 6 shows the X-ray diffraction patterns of pure chars and the co-pyrolysis chars. The DOA char has the highest graphite-like structure (the 002 peak is the sharpest), that is an important reason why it has a low gasification reactivity [6]. No significant peak (002 peak and 100 peak) appeared reflects that all the biomass
char are of the irregular carbon structure. For the blends, the 002 peak decreases greatly compared with the raw DOA char, this might be caused by the AAEM contained in biomass can be eroded into the surface and interior of the co-pyrolysis chars, thereby reduces the graphite degree and increases the active sites. 3.2. Gasification characteristics of the chars 3.2.1. Gasification characteristics of the pure chars The gasification rate of the pure chars of CS, SS, SD, DOA as a function of temperature are shown in Fig. 7. The initial and final reaction temperature are considered to be the temperature at which the rate of mass loss was 0.05%/ C [20]. Table 3 shows the initial, peak and final temperatures corresponding to the experimental reactivity plots. From a qualitative point of view, the chars have a different gasification characteristics, and all the curves presented a single peak, which corresponds to the maximum rate of mass loss. Compared the reaction rate curves of the chars, SS char has the highest gasification reactivity, the initial reaction temperature at 669 C is the lowest, so does the peak temperature (775 C) and final temperature (882 C). If we use the peak temperature to compare the gasification reactivity of the chars (the higher peak temperature means the lower gasification reactivity, according to Jing et al. [30]), then the four chars reactivity is in the order of SS >SD >CS >DOA. The DOA char have the lowest gasification reactivity, and when the DOA gasified initially (the onset temperature of 868 C), the three biomass char have already gasified completely. The lowest reactivity of the DOA char could be attributed to the low surface area, high graphite-like structure (Fig. 6) and low ash content [6]. While for the biomass char, the minerals contained in these biomasses are all rich in AAEM, as shown in Table 2, which is of good catalytic effect for gasification. Especially for SS, which has the highest potassium content (potassium is a very effective catalyst). The K content of CS is higher than SD, while the Al content is also higher than SD. As reported by Ding et al. [5], the existence of Al or Si would inhabit the gasification reaction. Moreover, SD has a larger content of Ca, which is also a good
Fig. 3. The snapshots of the chars from co-pyrolysis of biomass and DOA at different blending ratios.
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305
Fig. 4. The SEM images of the chars: (a) CS; (b) SS; (c) SD; (d) DOA; (e) 50CS50D; (f) 50SS50D; (g) 50SD50D.
catalyst. For these two reasons, the gasification reactivity of SD is a bit higher than CS. Compared with DOA, the irregular carbon structure of the biomass char is another reason for the high gasification reactivity [16,24]. 3.2.2. Gasification characteristics of the co-pyrolysis chars Fig. 8 shows the experimental and calculated gasification rate curves of different biomasses, blends and DOA. The gasification rate curves of the co-pyrolysis chars are located between the biomass and DOA gasification rate curves. If the blended chars does not interact with each other, just as the biomass and DOA gasified individually, the reaction rate curves of the blends would be the same as the calculated results (the dash line). However, the reaction rate of co-pyrolysis char shows a big difference with the calculated
result. At the initial reaction, the rate is a bit lower than the calculated, for the addition of DOA melt and block some of the pores of the biomass (as the images shows in Figs. 3 and 4), thus decreases the surface area, and the active sites of the biomass chars decreased. Another reason is the alkali metal in biomass could transfer to DOA surface after co-pyrolysis, the alkali content in biomass decreased and caused the biomass gasification rate shows a bit lower than that of the pure biomass char. From the calculated curves, the reaction rate would increases with the biomass reaction, and when the biomass char burns out, the reaction rate will decrease for the DOA char's gasification rate is very low, and two peak rate would appeared during the blends gasification. While in fact with the gasification proceeds, the blends gasification rate becomes higher than the calculated, and only a single peak
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Fig. 5. SEM-EDS elemental mapping of 50SS50D char. Each map depicts the relative abundance of each element, with brighter colors indicating greater abundance. The first, SEM image for area analysed; K-KA, potassium mapping; Na-KA, sodium mapping; S-KA, sulfur mapping; Mg- KA, magnesium mapping; Ca-KA, calcium mapping. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
100
Mass loss ( %)
80 DOA CS SS SD
60
40
20
0
1.5
DOA CS SS SD
o
Mass loss rate ( %/ C )
2.0
Fig. 6. X-ray diffraction patterns of pure chars and the co-pyrolysis chars.
appeared. This reflects that the addition of biomass greatly changes the blend chars gasification characteristics. Nemanova et al. found that co-gasification of petroleum coke and biomass did not occur simultaneously, but sequentially, and the synergetic effect observed could be attributed to the catalytic effect of the biomass ash that is rich in AAEM [16]. Rizkiana et al. found that biomass ash with higher content of AAEM and lower content of silica have higher catalytic activity on coal gasification [31]. In this paper, from Table 2, with the addition of biomass, the co-pyrolysis chars are rich in K, Ca and Mg, thus the gasification rate increases a lot than that of DOA. Moreover, the blends rate curves are more like one kind of
1.0
0.5
0.0 400
600
800 o Temperature ( C )
1000
1200
Fig. 7. The mass loss and mass loss rate profiles of pure chars gasification characteristics.
homogenous char, and before the biomass is completely gasified, the DOA gasification has already begun. This also indicates that the
Q. Zhang et al. / Energy 134 (2017) 301e310 Table 3 The initial, peak and final temperature of the chars at different heating rates. Samples
DOA CS 50CS50D SS 50SS50D SD 50SD50D
10 C min1
20 C min1
40 C min1
Tinitial
Tpeak
Tfinal
Tinitial
Tpeak
Tfinal
Tinitial
Tpeak
Tfinal
876 741 802 669 726 732 808
1080 872 936 775 865 865 964
1267 905 988 882 895 887 1006
897 770 830 697 743 769 830
1234 899 967 801 888 908 995
1313 939 1019 856 927 939 1042
913 802 849 722 757 785 858
1287 932 992 838 919 930 1041
1341 985 1070 920 968 969 1111
blends are quite uniform and the AAEM in biomass could transferred into DOA evenly. For a better understanding of the catalytic effect of the biomasses, the blends gasification rate are compared in Fig. 8(d). For 50SS50D, the peak temperature 865 C is the lowest, for 50CS50D the peak temperature is 936 C, 50SD50D the peak temperature is 964 C. The potassium content in the co-pyrolysis chars are also in the order of 50SS50D >50CS50D >50SD50D (Table 2), reflects that the potassium content is a good parameter to illustrate the catalytic effect of these biomasses. The catalytic effect of the biomasses on DOA gasification is in the order of SS >CS >SD. According to the study of co-gasification of biomass with coal, the Al and Si contained in coal would react with K in biomass and formed KAlSiO4 which is of no catalytic effect and would inhibit the
307
gasification rate [3,7,13]. While for DOA, the main ash component is Ni, V, Fe, almost has no Al and Si. Hence, no inhibition effect appeared during co-gasification of DOA and biomass.
3.2.3. Kinetics analysis According to ICTAC Kinetics Committee recommendations [24,32], the determination of the kinetic parameters from a single TPR run may lead to unreliable rate parameters. At least three TPR runs at different heating rates are required to estimate reliable parameters and accurate activation energies. Therefore, the gasification characteristics of the samples under the heating rate of 10, 20 or 40 C/min were studied (as shown in Fig. 9). The gasification rate curves with different heating rates are similar to each other, and with the increase of the heating rate, the curves moves to a higher temperature zone. Gasification characteristic parameters are shown in Table 3. The Tinitial, Tpeak and Tfinal of different chars all increase correspondingly with the heating rate increase. With the increase of heating rate, the temperature increases faster and individual reaction does not have enough time to reach completion or equilibrium, so they overlap with the adjacent higher temperature reaction [18,20,24]. Kinetic parameters are basic but necessary data for good design and operation of the gasification utility. Many kind of kinetics models has been successfully used in gasification, such as the homogenous model, shrinking core model and the random pore model [20,23]. However, for the co-gasification experiments in this
Fig. 8. Experimental and calculated gasification rate curves for different biomass, blends and DOA: (a) CS, 50CS50D, DOA; (b) SS, 50SS50D, DOA; (c) SD, 50SD50D, DOA; (d) comparison of the blended chars.
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Fig. 9. Gasification rate patterns of the pure chars and co-pyrolysis chars under different heating rates.
paper, the reaction is more complicated and none of the above models could satisfactorily fitted to the data of the samples. For the above models paid more attention on structural changes, and did not consider the catalytic effect of indigenous alkali minerals. Therefore, the iso-conversional method, which allow the activation energy (E) to be estimated as a function of a without pre-fixing the reaction model [33] were chosen in this paper. The Vyazovkin kinetic model was used to calculate E due to its relatively high dependability, which can be expressed by Equation (3).
b RA E ln 2 ¼ ln Ef ðaÞ RTa T
(3)
where b is the heating rate during gasification, a is the conversion ratio of the raw sample and a ¼ (m0 - mt)/(m0-mf), t is time and m0 and mf are the initial mass and final mass of the sample, respectively. mt is the mass of the sample at time t, A is the preexponential factor, R is the universal gas constant, Ta is the temperature for a conversion ratio a and f(a) is the integration function of the reaction model. The values of ln(b/T2) versus 1/T for constant a at several b values can be calculated using a linear regression. E can then be solved based on various a values. The activation energy (E) and correlation coefficient (R2) results of the individual components and mixtures based on the iso-conversional method are illustrated in Table 4 and the distribution of the E is shown in Fig. 10.
The R2 values are within a narrow range, suggesting that the values of E satisfy accuracy requirements. Accordingly, the E values vary with respect to temperature and extent of reaction, and higher apparent E values are generally due to the reaction of the less reactive components [34]. For biomasses, the average E values of the CS, SS, SD are 224, 175 and 191 kJ mol1, respectively. This is in correspondence with the gasification reactivity discussed above, the higher the gasification reactivity, the lower the E values. While the DOA has the lowest reactivity, the average E values are lowest, so it's hard to compare its reactivity by the E values as they come from different nature. The distribution of the E of the co-pyrolysis chars are more like that of the pure biomass char (Fig. 10), and the E values is higher than that of the biomass char. The overall trend of E for CS, SS and 50CS50D also indicates that the more reactive constituents gasified at earlier stages of conversion followed in the later stages (a > 0.3) by the less reactive components corresponding to higher E values. While the overall trend of E for SD, 50SS50D and 50SD50D decreases through the reaction reflects that the constituents gasified become more reactive with conversion. DOA is different as the E values decreases at first and then increase with the conversion larger than 0.6. It is hard to explain for the unreliability of the theoretical or mechanistic interpretation of the kinetic trends obtained from these complex materials [34].
Q. Zhang et al. / Energy 134 (2017) 301e310
309
Table 4 Kinetic parameters of the char samples during CO2 gasification determined at three heating rates (10, 20 and 40 K min1) for iso-conversional reaction models.
a
DOA
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Average
CS
50CS50D
SS
SD
50SD50D
R2
E
R2
E
R2
E
R2
E
R2
E
R2
E
R2
211 200 182 168 161 160 164 190
0.924 0.979 0.996 0.999 1.000 1.000 1.000 0.995
208 235 242 240 236 228 220 211 198 224
0.996 0.998 0.999 1.000 1.000 1.000 1.000 1.000 1.000
241 255 263 268 268 261 248 233 221 251
0.942 0.954 0.959 0.969 0.982 0.991 0.995 0.998 1.000
117 175 188 190 188 185 184 187
0.877 0.989 1.000 0.998 0.996 0.994 0.992 1.000
288 264 255 251 247 243 240 236 228 250
0.996 1.000 0.999 0.999 0.998 0.998 0.997 0.997 0.998
207 197 189 189 190 191 190 187 179 191
0.988 0.991 0.997 0.999 0.999 0.996 0.991 0.985 0.982
303 261 232 222 218 212 205 196 187 226
0.997 0.999 0.987 0.983 0.981 0.975 0.971 0.967 0.966
179
175
activity of the biomass caused by ash interactions for which has happened during co-gasification of coal and biomass would not occur. The catalytic ability of the three types of biomasses is in the order of SS>CS>SD, which is consistent with the potassium content. The gasification kinetic analysis shows that for the copyrolysis chars, the average E values are greater than the pure biomass char. The co-gasification of DOA and biomass is a good choice for disposing DOA.
280 240
E ( kJ/mol)
50SS50D
E
200 160
CS SS SD
120
0.2
0.4
0.6 Conversion X
DOA 50CS50D 50SS50D 50SD50D 0.8
Acknowledgements The research is financially supported by the project of an international cooperation between China and Japan, “Technology development and process integration for high-efficiency utilization of low-rank coals based on mild depolymerization and exergy recuperation” (2013DFG60060) and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA07050100).
Fig. 10. The distribution of the active energy (E) vs carbon conversion X.
References 4. Conclusions and outlook In this paper, co-processing technology of biomass and DOA has been evaluated, and the corresponding conclusions are as follows. DOA is of high heating values, so the co-processing of DOA and biomass could avoid the problems derived from low calorific value products in biomass gasification. The co-pyrolysis of DOA and biomass did not show any significant synergistic effect on the char yield, but the co-pyrolysis char's graphite degree reduced greatly. What's more, the chars from co-pyrolysis of biomass/DOA showed an obvious agglomeration, for the DOA was melted during thermal conversion and stuck to the biomass surface. For one thing it's good for the elaborate of the catalytic gasification effect because the biomass and DOA could be contacted tightly, for another it should be noted that a block phenomenon occurs during the co-pyrolysis section. Thus more attention should be paid when choosing the pilot scale gasification technology to avoid the feeding problems and the reactive gas flowing problems caused by that, and these problems could be solved by varying the blending ratios or the gasification ways. Gasification experiments of the co-pyrolysis chars showed that all the three biomasses selected played a good catalytic role on improving the DOA gasification reactivity and make the reactivity of the blended char more like that of the biomass char. Different from other researcher's studies, before the biomass was completely gasified, the gasification of the DOA contained in the co-pyrolysis chars has already begun. All of this reflects that the blends are quite uniform and the AAEM in biomass could catalytic DOA gasification greatly. Further, since the ash content of the DOA is very low and it almost contains no Si and Al, the decrease of the catalytic
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