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Studies of fast co-pyrolysis of oil shale and wood in a bubbling fluidized bed
Energy Conversion and Management 205 (2020) 112356
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Studies of fast co-pyrolysis of oil shale and wood in a bubbling fluidized bed Bin Chen Xiao Yeb a b c
a,b
a,⁎
c,a
a
a
b
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, Xiangxin Han , Jianhui Tong , Mao Mu , Xiumin Jiang , Sha Wang , Jun Shen ,
Institute of Thermal Energy Engineering, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, People's Republic of China School of Mechanical and Automotive Engineering, Shanghai University of Engineering Science, Shanghai 201620, People's Republic of China School of Materials Science and Engineering, Jingdezhen Ceramic Institute, Jingdezhen 333403, People's Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Oil shale Biomass Fast co-pyrolysis, synergy
Fast co-pyrolysis characteristics of oil shale-wood blends were researched by a bubbling fluidized bed reactor in this paper. An on-line GASMET Fourier transform infrared (FTIR) spectrometer and a gas chromatography-mass spectrometer (GC–MS) were employed for analyzing gas and liquid products. The effect of different blending ratios of oil shale(S)/wood(W) (S:W = 1:0,3:1,1:1,1:3,0:1 in this paper) on the co-pyrolysis products was discussed. The effect of temperature on the characteristics of the co-pyrolysis of S:W = 3:1 was also investigated in this paper. According to the results, the interaction of oil shale and biomass influenced oxygen distribution in volatiles, promoting the generation of CO2 generation and inhibiting the conversion of oxygen-containing compounds like alcohols and acids in pyrolytic oil. The effects of minerals in oil shale and the free radicals generated from wood were concluded according to the experimental results. In addition, as the temperature increased from 430 °C–600 °C, the yield of oil reached maximum at 520 °C with stronger secondary cracking of kerogen and biomass macromolecules.
1. Introduction The everlasting strong demands for energies and inherent limited reserves of conventional fossil fuels have prompted the research about the utilization of alternative energy sources all over the world [1,2]. Oil shale can be converted to shale oil in pyrolysis, which is usually considered to be one of the most important potential substitute resources for petroleum [3,4]. As an appealing source of alternative energies, oil shale has attracted more and more attentions on its effective conversion and future possible applications [5–8]. During the pyrolysis process, the major organic matter in oil shale-kerogen, could be first converted to bitumen at the temperature of 350 °C, which could be cracked and evaporated in higher temperature of 400 °C–550 °C, generating gases, shale oil and coke. Shale oil mainly contained hydrocarbons, benzene derivatives, oxygenated compounds and PAH (polycyclic aromatic hydrocarbon) [9]. The performance parameters such as the pyrolysis temperature [10], pyrolysis pressure [11], reaction time and particle size [12] have been investigated in previous studies. However, the quality of shale oil needs to be further improved because of some defects like high viscosity, high molecular weight, poor stability, and so forth [5,6]. Catalysts provided an alternative lowerenergy pathway, which could improve the quality and properties of the
⁎
product [13]. It is reported that the internal minerals of oil shale, including carbonates, metallic oxides such as CaO, MgO and SiO2, exhibited different catalytic capability on shale oil generation [14]. The external catalysis such as zeolite [15], bentonite clay [16] and oil shale ash [17] could result in shorter chain, more single benzene rings or less nitrogen and sulphur containing aromatic compounds in shale oil. Compared with solid catalysts, H2 better promotes the generation of shale oil [18]. The ratio of H/C and oil yield could be obviously promoted through the donation of hydroge. But H2 is expensive and explosive, which requires a more complicated retorting system, pushing up the pyrolysis costs. So, an accessible and safety H donor would be required in the process of oil shale pyrolysis. Nowadays, biomass wastes also attract attentions from many researchers because of its large quantities, easy accessibility and great potential to play an important role in energy area [19,20]. The existing utilization methods were mainly retorting, direct combustion and forming pressed building materials [20]. In biomass thermal conversion, bio-oil, gases and coke are usually generated in 350 °C–400 °C, which is similar with the temperature range of bitumen generation in oil shale. The free radicals generated from the biomass could promote the decomposition of kerogen molecular by bombarding the weak bonds in it [21]. On the other hand, the bio-gas with higher heating
Corresponding author. E-mail address: [email protected] (X. Han).
https://doi.org/10.1016/j.enconman.2019.112356 Received 6 September 2019; Received in revised form 28 November 2019; Accepted 29 November 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.
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temperature of shell were −20 °C and 0 °C. Cyclone separator was twined by a heating band with temperature of 250 °C to mitigate heat loss. For retorting system, the height of dense-phase zone, sparse-phase zone and extension zone was 180 mm, 80 mm and 70 mm, respectively. High purity nitrogen was used in this experiment as inert gas. Quartz sand with particle size of < 0.5 mm was chosen as bed material. The fluctuation of feeding rate with the increase of rotation rate was shown in Fig. 2. The standard feeding mass was 100 g and the rotational speed of screw feeder was 24.3 r/min. The preset temperature of dense-phase zone, sparse-phase zone and bottom zone was set as 400 °C, 520 °C and 500 °C. The fluidization number nf was 3.0.
value escaped from the pyrolysis could be mixed with oil shale pyrolytic gas as a kind of heat source, improving the energy efficiency of retorting system. Individual pyrolysis of oil shale and biomass has been employed and investigated by many researchers all over the world [1–5,19,20,22,23]. Co-utilization of different fuels also has been investigated by many researchers, such as co-pyrolysis of coal and biomass [24]. Compared with coal, oil shale contains fewer aromatic hydrocarbons and more aliphatic hydrocarbons, such as long straight chains. But the research of the co-pyrolysis characteristics of oil shale-biomass blends is rare. In this work, fast co-pyrolysis characteristics of oil shale and wood were unveiled by a bubbling fluidized bed reactor. Moreover, a gas chromatography-mass spectrometry (GC–MS) and a gas analyzer combining a Gasmet Fourier transform infrared spectrometer (FTIR) with a MRU GmbH Vario Plus analyzer were used in the co-pyrolysis experiment to analyze the obtained oil and gases. The results could decipher the interaction characteristic of oil shale and biomass, promoting the utilization efficiency of oil shale and biomass wastes resources.
nf =
While nf is the fluidization number, U0 is the operating velocity of bubbling fluidized bed, Umf is the minimum fluidization velocity. 2.3. Product analysis The gases generated in bubbling fluidized bed were directly introduced into an online Gasmet DX-4000 FTIR spectrometer. Nitrogen spectrum was first recorded and set as a background. After that, the injected gases were analyzed by the spectrometer and the obtained spectra were handled, displayed and saved on a computer which was connected to the spectrometer by a RS232 cable. Except H2, most gases could be determined by this device. A MRU GmbH Vario Plus analyzer was connected to the spectrometer for detecting H2. The liquid products contained oil and aqueous phase, which must be separated by distillation extraction before the following detection. The obtained oil was analyzed on a model Agilent 6890 N GC/5973 N MS instrument, with injection port and detector temperature, flow rate, heating rate of 280 °C, 3.0 mL/min, 10 °C/min in gas chromatograph parameter setting, and electron energy, scanning range, temperature of 70 eV, 15–500 u, 230 °C in mass spectrometry parameter setting, respectively. The split ratio ns was chosen as 100:1.
2. Experimental section 2.1. Oil shale and biomass samples Oil shale used in this experiment was taken from Dachengzi mine in Huadian, China. Its kerogen is classified as lacustrine—Type I. According to the XRD (X-ray diffraction) results which have been given by previous studies [25], quartz and calcite are dominant minerals species and there are also minor amounts of muscovite, kaolinite, zeolite and pyrite. Also, biomass used in this work was wood pellet obtained from wood furniture factory in North Jiangsu, China. All samples were first air-dried, crushed, and sieved to a grain size of 3 mm. According to the national standards of China, proximate analysis, ultimate analysis and heating values of both samples were performed and shown in Table 1. The atomic H/C ratio was 1.58 in oil shale and 1.62 in wood. The blending ratios of oil shale with wood (S:W) were chosen as 1:0,3:1,1:1,1:3, and 0:1. The sample of S:W = 3:1 was heated in different temperatures as 430 °C, 460 °C, 490 °C, 520 °C, 550 °C and 600 °C to investigate the effect of temperature on the co-pyrolysis characteristic of the blends.
ns =
The fast pyrolysis system employed in the research is a bubbling fluidized bed, as shown in Fig. 1, which consisted of a temperature and gas control system, a retorting system, a condensation system and a gas analyzer system. The condensation system is composed of a tube-andshell heat exchanger and a refrigeration installation. The inlet and outlet temperature of tube were 500 °C and 0 °C and inlet and outlet
3. Results and discussion 3.1. Analysis of products distribution In this experiment, the operating temperature, total feed mass and rotational speed of screw feeder were set as 520 °C, 100 g and 24.3 r/ min respectively. The products of the co-pyrolysis system can be classified as gas, oil, water and semi-coke. The yield distributions of these over different blending ratios are shown in Fig. 3(a). It can be seen that pure oil shale (S:W = 1:0) could generate more semicoke and oil, less gas and water, according with the pyrolysis characteristic of oil shale and wood [8,19]. For oil shale, the yields of oil, gas and coke of are 19.72%, 12.46% and 66.78%, respectively. The corresponding yields of wood are 8.18%, 55.60% and 27.28%. In order to depict the synergistic effect between oil shale and wood, we adopt the synergistic index ξ [26] which in this paper is calculated as:
Table 1 Ultimate analysis and proximate analysis of oil shaleand wood on air dried basis. Oil shale
Wood
C H Od N St
30.43 4.00 8.78 0.63 0.94
44.78 6.05 35.85 0.43 0.29
Proximate analysis (mass%) Moisture 1.52 Ash 53.70 Volatile matter 40.31 Fixed carbon 4.47 HHV (MJ/kg) 12.59 H/C molar ratio 1.58 d–By difference; t–Total sulfur; HHV–high heat value
Vin Vout
While ns is split ratio, Vin is the vapour phase which were injected in chromatographic column, Vout is the residual vapour phase. The peaks in mass spectra were identified through comparison with National Institute of Standards and Technology—NIST 02 mass spectral data library.
2.2. Bubbling fluidized bed system
Ultimate analysis (mass%)
U0 Umf
8.52 4.08 71.70 15.70 14.73 1.62
Xadd = X1 × R1 + X2 × (1 − R1)
ξ=
2
X0 − Xadd Xadd
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Fig. 1. The sketch of bubbling fluidized bed pyrolysis system. I. Temperature and gas control system, II. Retorting system, III. Condensation system, IV. Gas analyzer system. 1. High purity nitrogen, 2. Mass flowmeters, 3. Temperature controller, 4. Thermocouple, 5. Feeding machine, 6. Fluidized bed, 7. Cyclone separator, 8. Preheater, 9. Semi-coke collecting pipe, 10. Tube-and-shell heat exchanger, 11. Refrigeration installation, 12. Buffer unit, 13. Mass flowmeters, 14. Gasmet, 15. Flue gas analyzer (MRU), 16. Exhaust pipe, 17. Observation port.
adding about 25% of wood during the pyrolysis of oil shale could inhibit the generation of semicoke, and promote the yield of oil. 3.2. Analysis of gas The gas products were scaled to volume per 100 g samples under standard conditions (one standard atmosphere, 0 °C), as shown in Table 2. There was more gas generated from the biomass during the pyrolysis. CO and CO2 are the main gases, and the yields are 7.24 L and 6.49 L respectively. For oil shale, the main pyrolysis gases are CO2, CH4 and C3H8, which occupy for volume yields of 1.47 L, 0.61 L and 2.39 L respectively. In addition, the heat value of each kind of gas was calculated, and the results are shown in Fig. 4. The pyrolysis gas of the wood (S:W = 0:1) has the highest heat value of 3237 J/g while the heat value of oil shale gas (S:W = 1:0) is just 838 J/g. Moreover, it’s noteworthy that the relation of heat value and blending ratios is approximately linear, indicating the subtle synergistic effect on the whole heat value of the pyrolysis gas. To make a further investigation, the yields of CO2, CO, H2O, CH4, C2H4, C2H6, C6H6 and C2H2 over time were drawn in Fig. 5. Consistent with previous results, CO and CO2 were the main pyrolysis gases of wood. In oil shale pyrolysis, CO2 was generated much more than CO. With the growth of oil shale content in the blending samples, C2H6, C2H2, C2H4 decreased and C6H6 increased, reflecting more aliphatic hydrocarbons in biomass and more aromatic compounds in oil shale. The yields of gas over different blending ratios were displayed in Fig. 6. The relationships of most gases with blending ratios didn’t follow a linear law, indicating the influence of synergistic effect on the copyrolysis. The change of synergistic index ξ over blending ratios was shown in Fig. 7. At the temperature of 520 °C, the main components in wood like cellulose, hemicelluloses and lignin were decomposed, generating CO2, CO and H2O through the process of decarbonylation, decarboxylation and dehydration [27]. For oil shale, oxygen-containing bonds like C-O-C were usually cracked and converted to C = O, which contacted other oxygen-containing free radicals generating CO2 or further cracked to generate CO [14]. But higher content of CO2 revealed that more C = O tended to form CO2 instead of CO in oil shale
Fig. 2. Change of feeding rate in different rotation rate.
Where X1, X2 are the products yield by wood and shale separately, and R1 is the blending ratio of wood, Xadd can be regarded as the theoretical mixture products without any synergistic effects, X0 is the observed experimental values. So ξ reflects the relative derivation of observation from Xadd. The change of ξ’s over different blending ratios are shown in Fig. 3(b). It can be seen that the synergistic effect promoted the generation of water and gas, and repressed the generation of coke. This might be because some free radicals escaping from the biomass were absorbed by the active sites on kerogen macromolecule, causing more volatiles and less semicoke [27]. Moreover, the absorbed free radicals also inhibited the recombining of themselves and repressed the generation of semicoke [28]. Synergistic effect inhibited the generation of oil slightly at the blending ratio of S:W = 1:3 and 1:1, while promoted it in S:W = 3:1. The synergistic index for gas reached maximum at the blending ratio of S:W = 1:3. For the generation of coke, the synergistic index reached minimum at the blending ratio of S:W = 3:1. This means 3
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(a) Products distribution with different S/W blends
(b) Synergistic index ȟ with different S/W blends
Fig. 3. Products distribution and synergistic index ξ with different S/W blends.
form some kinds of nitrogen-containing organic compounds like benzonitrile and indole [31]. Some active sites might be generated through the interactions between kerogen molecular and the free radicals generated from wood. These active sites would change the conversion path of NO2, causing the inhibiting effect during co-pyrolysis. The generation of unsaturated hydrocarbon gas such as C2H4 and C6H6 was promoted by co-pyrolysis. According to experimental [27] and theoretical [21] results, there were much ethylene radicals generated during the pyrolysis of oil shale. These free radicals would absorb -H free radicals or combine with other short hydrocarbons through cycloaddition and Diels-Alder reaction, forming C2H4 or benzene. The free radicals generated from wood offered more reactants, causing more C2H4 and C6H6. In addition, because of the highest content, the growth of CO2 was the major factor which evokes the increasing of gas product. But CO2 is a non-combustible gas with on heat value, which resulted in a linear relationship between the heat value and blending ratios in Fig. 4.
Table 2 Gas composition (mol%) of different S/W blends in the co-pyrolysis. CO2
CO
HCl
CxHy
H2O
H2
NO2
SO2 0.073 0.041 0.07 0.041 0.005
S:W S:W S:W S:W S:W
= = = = =
0:1 1:3 1:1 3:1 1:0
6.914 6.051 4.837 3.878 1.566
7.713 5.945 3.889 1.886 0.33
0.423 0.188 0.029 0.02 0.002
5.284 4.251 3.771 3.345 2.972
5.231 2.195 1.694 0.607 0.266
0.671 0.629 0.352 0.447 0.671
0.373 0.234 0.149 0.213 0.405
S:W S:W S:W S:W S:W
= = = = =
0:1 1:3 1:1 3:1 1:0
CH4 4.943 3.388 1.992 1.374 0.65
C2H2 0.008 0.004 0.007 0.002 0.001
C2H4 0.118 0.101 0.118 0.097 0.024
C6H6 0.021 0.181 0.192 0.256 0.309
C2H6 0.096 0.075 0.107 0.128 0.181
C3H8 0.586 0.756 1.8 2.259 2.546
HCN 0.01 0.003 0.004 0.005 0.001
3.3. Analysis of liquid Next, we analyze the liquid products during the co-pyrolysis reaction under different blending ratios. The relative contents are obtained from GC–MS. The bio-oil (generated by S:W = 0:1) contained more alkanes, oxygen and heteroatomic hydrocarbons, while correspondingly, the shale oil (generated by S:W = 1:0) contained more unsaturated hydrocarbons Fig. 8. This can be explained by the chemical components and the decomposition procedure of wood and oil shale. Acids, esters and alcohols were the major oxygen-containing organic products of wood. These products, especially acids, were usually derived from the decomposition of saccharides in the biomass [32] and further converted to esters or aldehydes [33]. The main oxygen hydrocarbons in oil shale products were ketones and esters, which were converted from oxygen bridge bonds like C-O-C. Moreover, the fragmentation of carbohydrate in wood and the secondary reactions of other oxygen-containing organic compounds also generate ketones [34]. Alkanes and alkenes were the major products generated from oil shale through the decomposition of kerogen [14,35,36]. The non-linear relationship between distribution of oil products and blending ratios suggests the existence of interaction within the wood and oil shale mixture. By comparing the differences between experimental values and theoretical values without synergistic effects in Fig. 9, we observed significant inhibition of oxygen hydrocarbons. By contrast, the distributions of heteroatomic and unsaturated hydrocarbons show different tendencies and varie oppositely. Under S:W = 1:3 and S:W = 1:1, heteroatomic hydrocarbon are inhibited and
Fig. 4. Heat value of pyrolysis gas in different S-W blends.
pyrolysis. The change of synergistic index ξ in Fig. 7 reflected that the interaction of oil shale and biomass increased the yield of CO2 during the co-pyrolysis. But the effect of synergy on CO generation was relatively insignificant. This might be because the oil shale promoted the conversion of oxygenates in wood [29], and more oxygen-containing free radicals contact the active site of C = O from kerogen intermediates, forming more CO2. Moreover, some –OH, –CH3 and -H free radicals were absorbed by the kerogen intermediates instead of forming H2O, resulting in the decrease of H2O, CH4, H2 and HCl. NO2 was generated from the reaction of nitrogen functional groups such as pyrrole/pyridine with oxygen functional groups in kerogen macromolecule [30]. The protein in wood also could be decomposed to 4
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(b) S : W = 0 : 1-(2)
(a) S : W = 0 : 1-(1)
(d) S : W = 1 : 3-(2)
(c) S : W = 1 : 3-(1)
(e) S : W = 1 : 1-(1)
(f) S : W = 1 : 1-(2)
(g) S : W = 3 : 1-(1)
(h) S : W = 3 : 1-(2)
(i) S : W = 1 : 0-(1)
(j) S : W = 1 : 0-(2)
Fig. 5. Fluctuation of gases yields with heating time.
5
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(a)
(b) Fig. 6. Effect of blending ratios on gas generation.
contained more C10-C19 and > = C20. The gaps between experimental and theoretical values reflected the obvious synergistic effect on the oil production—promoting the generation of C10-C19 and inhibiting the generation of C2-C9. Moreover, the curves of > = C20 followed a linear law, showing little synergistic effect on the generation of heavy components. Fig. 11 revealed that the synergistic effect also increased the oil’s H/C ratios, which was an important beneficial indicators to oil quality. In fast co-pyrolysis, the disproportionation, decarboxylation and decarbonylation reactions would happen together, generating a large number of free radicals [39]. These free radicals promoted the rupture of kerogen molecular by bombarded the bridge bonds between two long-carbon chains. –H or –CH3 radicals would combine with the
unsaturated hydrocarbons are promoted, while under S:W = 3:1, the former is promoted. The absorption and catalysis of alkali element like K+, Ca2+ in oil shale may promote more likely conversion of C = O to CO2, conducing the decrease of oxygen hydrocarbons products. CaO and MgO in oil shale also inhibited the conversion of acids through ketonization reaction. Moreover, high content of oil shale ash would decrease the yield of alcohols [27]. The mineral substance like calcite, dolomite and the alkali metals in oil shale could also influence the generation of benzene rings—inhibiting the formation of indol and promoting the generation of phenol and its derivatives [37,38]. Distribution of different length of carbon chains was shown in Fig. 10. Bio-oil (S:W = 0:1) contained more C2-C9, and shale oil
(a)
(b)
(c) Fig. 7. Effect of blending ratios on synergistic index ξ. 6
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Fig. 11. Atomic ratio of H/C with different blending ratios.
Fig. 8. Distribution of oil products with different blending ratios.
Fig. 9. Difference of experimental and theoretical value with different blending ratios. Fig. 12. Distribution of co-pyrolysis products (S:W = 3:1) with increasing temperature.
to 600 °C, the yield of semicoke decreased from 67.48% to 53.01% because of the stronger cracking and volatilization. The gas production increased from 16.28% to 22.86% for the stronger decomposition of biomass and oil shale macromolecules and secondary cracking in higher temperature. The secondary cracking also inhibited the generation of liquid products in higher temperature. So the yield of liquid products increased from 12.55% to 17.12% at the temperature range of 430 °C–520 °C, and then decreased to 15.93% when temperature reached 600 °C. The pyrolysis of biomass was an endothermic process, and higher temperature was more likely to promote the rupture of atomic bonds, causing the decomposition of organic compounds [40]. But as temperature further increased, the secondary reactions of organics would become stronger with more available energy, promoting the formation of gases like CO2 and CO and inhibiting the generation of organic products [34]. For the pyrolysis of oil shale, it started with the rupture of bridge bonds and then reformed the structure of kerogen (usually in 350 °C), forming the pyrolysis intermediates named bitumen [21]. When temperature continues to rise, the generated radical fragments coupled together or recombined to form volatiles or char [41]. The main purpose of the co-pyrolysis was to obtain the organic liquid products, but the oil yield began to decrease when temperature exceeded 520 °C. So, according to this results and previous researches [8,42], 520 °C could be considered as a reasonable temperature for the co-pyrolysis of oil shale and biomass. More details about the distribution of gas and liquid products with increasing temperature would be discussed later.
Fig. 10. Distribution of different carbon chains.
active points on the fragments of kerogen intermediates, generating more hydrocarbons instead of forming semicoke. Meanwhile, the free radicals repressed the generation of C2-C9 and promoted the generation of C10-C19. 3.4. Effect of temperature on the co-pyrolysis 3.4.1. Analysis of products distribution The yields of co-pyrolysis product (S:W = 3:1) over different temperature were shown in Fig. 12. As the temperature varied from 430 °C 7
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(a)
(b) Fig. 13. Effect of heating temperature on gas products (S:W = 3:1).
atomic ratios were shown in Fig. 15(b), from which it can be discovered that variation tendency of O/C and H/C were similar—showing a sharply decrease when temperature exceeded 500 °C. This might be attributed to dehydration of the organic compounds in biomass and oil shale [44]. The curves tended to smooth up when the temperature exceeded 550 °C, showing that higher temperature could not further enhance the dehydration. Considering the pyrolysis efficiency, oil yield and quality, 520 °C could be chosen as the normal temperature for the co-pyrolysis of oil shale and wood in industrial production.
3.4.2. Analysis of gas distribution The yields of gas such as H2O, CO, CO2, CH4, C2H6, C2H4, C2H2 and C6H6 over increasing temperature were drawn and shown in Fig. 13. CO2 increased from 5.82% to 9.1% at the temperature range of 430 °C–460 °C and kept steady at 460 °C–520 °C, and finally increased again from 9.53% to 14.32% at 520 °C–600 °C. This is because when temperature exceeded 520 °C, decarboxylation of wood and oil shale became stronger, making more oxygen functional groups depart from the macromolecule and form CO2 [40]. As the temperature increased, the generation of CO and some organic gases like CH4, C2H4 and C2H6 was more steadily than CO2. Other kinds of gases like C2H2 and C6H6 were increased more sharply when temperature exceeds 520 °C. Taylor [42] and Raley [43] thought that the molar ratio of C2H4/ C2H6 could reflect the degree of secondary cracking—higher C2H4/ C2H6 means stronger secondary cracking and lower yield of pyrolytic oil. The ratios of C2H4/C2H6 with increasing temperature in Fig. 14 show a stronger secondary cracking and higher production of unsaturated hydrocarbons after 520 °C.
4. Conclusions Fast co-pyrolysis of oil shale with wood was performed using a bubbling fluidized bed in this paper. The synergistic effect on the distributions of gas and liquid products was discussed and unveiled. The free radicals generated from wood played a critical role in oil shale pyrolysis. More volatiles and less semicoke were generated because of the combination of the free radicals and kerogen intermediates. CO2 generation was promoted through the combination of more oxygencontaining free radicals and C = O from kerogen. The absorption of some kinds of free radicals like –OH, –CH3 and -H resulted in the decrease of H2O, CH4, H2. The reaction pathways of oil shale pyrolysis were influenced by these free radicals, causing less production of NO2, more unsaturated hydrocarbon gas like C2H4, more long-carbon chains of C10-C19, less small molecular hydrocarbons of C2-C9, higher H/C ratios of oil, and so forth. Meanwhile, oil shale could also stabilize the free radicals from biomass, inhibiting their secondary reaction and recombination. The alkali element in oil shale promoted more conversion of C = O to CO2, causing a transfer of oxygen from liquid to gas products. Ethylene free radicals from oil shale promoted the Diels-Alder reaction and led to more aromatic hydrocarbons by cycloaddition. The temperature was also an important factor on the co-pyrolysis of oil shale and wood. Dehydration, decarboxylation and decarbonylation of organic compounds were influenced by it. The reasonable temperature was set as 520 °C, which could ensure the pyrolysis efficiency and oil production rate and quality simultaneously. The co-pyrolysis of oil shale and biomass had potential application prospect in the utilization of energies, which was beneficial to adjusting the thermal conversion process and promoting the energy utilization rate.
3.4.3. Analysis of liquid distribution Fig. 15(a) uncovered the effect of temperature on the distribution of different length of carbon chains. C10-C19 carbon chains were the major ingredient in co-pyrolysis oil—accounting for > 50% of all. The content increased from 54.13% to 58.87% as the temperature increased from 430 °C–600 °C. C2-C9 carbon chains increased from 11.23% to 25.78% as the temperature increased from 430 °C to 600 °C, while the carbon chains of > = C20 decreased from 35.18% to 15.74%. All kinds of curves are nearly horizontal after 520 °C. The curves of O/C and H/C
CRediT authorship contribution statement Bin Chen: Conceptualization, Methodology, Software, Funding acquisition, Writing - original draft, Writing - review & editing. Xiangxin Han: Conceptualization, Supervision, Software, Funding acquisition. Jianhui Tong: Software. Mao Mu: Validation, Formal analysis, Investigation. Xiumin Jiang: Funding acquisition. Sha Wang:
Fig. 14. Effect of heating temperature to C2H4/C2H6 (S:W = 3:1). 8
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Fig. 15. Effect of heating temperature on the characteristics of liquid products (S:W = 3:1).
Visualization. Jun Shen: Project administration. Xiao Ye: Software.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by Shanghai Sailing Plan (19YF1418000); the National Natural Science Foundation of China (51876122); the National Natural Science Foundation of China (51766004); the National Natural Science Foundation of China (51704194) References [1] Wang Qingqiang, Ma Yue, Li Shuyuan, Hou Jili, Shi Jian. Exergetic life cycle assessment of fushun-type shale oil production process. Energy Convers Manage 2018;164:508–17. [2] Jiang Haifeng, Deng Sunhua, Chen Jie, Zhang Mingyue, Li Shu, Shao Yifei, et al. Effect of hydrothermal pretreatment on product distribution and characteristics of oil produced by the pyrolysis of huadian oil shale. Energy Convers Manage 2017;143:505–12. [3] Yang Yu, Xiaofeng Lu, Wang Quanhai. Investigation on the cocombustion of low calorific oil shale and its semi-coke by using thermogravimetric analysis. Energy Convers Manage 2017;136:99–107. [4] Bai Fengtian, Sun Youhong, Liu Yumin, Li Qiang, Guo Mingyi. Thermal and kinetic characteristics of pyrolysis and combustion of three oil shales. Energy Convers Manage 2015;97:374–81. [5] Kılıc Murat, Pütün Ayse Eren, Uzun Başak Burcu, Pütün Ersan. Converting of oil shale and biomass into liquid hydrocarbons via pyrolysis. Energy Conversion Manage 2014;78:461–7. [6] Nazzal Jamal M. The influence of grain size on the products yield and shale oil composition from the pyrolysis of sultani oil shale. Energy Convers Manage 2008;49(11):3278–86. [7] Manara P, Bezergianni S, Pfisterer U. Study on phase behavior and properties of binary blends of bio-oil/fossil-based refinery intermediates: A step toward bio-oil refinery integration. Energy Convers Manage 2018;165:304–15. [8] Han XiangXin, Jiang XiuMin, Cui ZhiGang. Studies of the effect of retorting factors on the yield of shale oil for a new comprehensive utilization technology of oil shale. Appl Energy 2009;86(11):2381–5. [9] Huang Yiru, Han Xiangxin, Jiang Xiumin. Characterization of dachengzi oil shale fast pyrolysis by curie-point pyrolysis-gc-ms. Oil Shale 2015;32(2):134. [10] Na Jeong Geol, Im Cheol Hyun, Chung Soo Hyun, Lee Ki Bong. Effect of oil shale retorting temperature on shale oil yield and properties. Fuel 2012;95(1):131–5. [11] Sha Wang, Jiang Xiumin, Han Xiangxin, Tong Jianhui. Investigation of chinese oil shale resources comprehensive utilization performance. Energy 2012;42(1):224–32. [12] Solomon Peter R, Carangelo Robert M, Horn Eli. The effects of pyrolysis conditions on israeli oil shale properties. Fuel 1986;65(5):650–62. [13] Lee Hyung Won, Kim Young-Min, Jae Jungho, Jeon Jong-Ki, Jung Sang-Chul, Kim Sang Chai, Park Young-Kwon. Production of aromatic hydrocarbons via catalytic co-pyrolysis of torrefied cellulose and polypropylene. Energy Convers Manage 2016;129:81–8. [14] Chen Bin, Han Xiangxin, Jiang Xiumin. In situ FTIR analysis of the evolution of functional groups of oil shale during pyrolysis. Energy Fuels 2016;30(7):5611–6. [15] Williams Paul T, Chishti Hafeez M. Two stage pyrolysis of oil shale using a zeolite
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Studies of fast co-pyrolysis of oil shale and wood in a bubbling fluidized bed Bin Chen Xiao Yeb a b c
a,b
a,⁎
c,a
a
a
b
T
b
, Xiangxin Han , Jianhui Tong , Mao Mu , Xiumin Jiang , Sha Wang , Jun Shen ,
Institute of Thermal Energy Engineering, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, People's Republic of China School of Mechanical and Automotive Engineering, Shanghai University of Engineering Science, Shanghai 201620, People's Republic of China School of Materials Science and Engineering, Jingdezhen Ceramic Institute, Jingdezhen 333403, People's Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Oil shale Biomass Fast co-pyrolysis, synergy
Fast co-pyrolysis characteristics of oil shale-wood blends were researched by a bubbling fluidized bed reactor in this paper. An on-line GASMET Fourier transform infrared (FTIR) spectrometer and a gas chromatography-mass spectrometer (GC–MS) were employed for analyzing gas and liquid products. The effect of different blending ratios of oil shale(S)/wood(W) (S:W = 1:0,3:1,1:1,1:3,0:1 in this paper) on the co-pyrolysis products was discussed. The effect of temperature on the characteristics of the co-pyrolysis of S:W = 3:1 was also investigated in this paper. According to the results, the interaction of oil shale and biomass influenced oxygen distribution in volatiles, promoting the generation of CO2 generation and inhibiting the conversion of oxygen-containing compounds like alcohols and acids in pyrolytic oil. The effects of minerals in oil shale and the free radicals generated from wood were concluded according to the experimental results. In addition, as the temperature increased from 430 °C–600 °C, the yield of oil reached maximum at 520 °C with stronger secondary cracking of kerogen and biomass macromolecules.
1. Introduction The everlasting strong demands for energies and inherent limited reserves of conventional fossil fuels have prompted the research about the utilization of alternative energy sources all over the world [1,2]. Oil shale can be converted to shale oil in pyrolysis, which is usually considered to be one of the most important potential substitute resources for petroleum [3,4]. As an appealing source of alternative energies, oil shale has attracted more and more attentions on its effective conversion and future possible applications [5–8]. During the pyrolysis process, the major organic matter in oil shale-kerogen, could be first converted to bitumen at the temperature of 350 °C, which could be cracked and evaporated in higher temperature of 400 °C–550 °C, generating gases, shale oil and coke. Shale oil mainly contained hydrocarbons, benzene derivatives, oxygenated compounds and PAH (polycyclic aromatic hydrocarbon) [9]. The performance parameters such as the pyrolysis temperature [10], pyrolysis pressure [11], reaction time and particle size [12] have been investigated in previous studies. However, the quality of shale oil needs to be further improved because of some defects like high viscosity, high molecular weight, poor stability, and so forth [5,6]. Catalysts provided an alternative lowerenergy pathway, which could improve the quality and properties of the
⁎
product [13]. It is reported that the internal minerals of oil shale, including carbonates, metallic oxides such as CaO, MgO and SiO2, exhibited different catalytic capability on shale oil generation [14]. The external catalysis such as zeolite [15], bentonite clay [16] and oil shale ash [17] could result in shorter chain, more single benzene rings or less nitrogen and sulphur containing aromatic compounds in shale oil. Compared with solid catalysts, H2 better promotes the generation of shale oil [18]. The ratio of H/C and oil yield could be obviously promoted through the donation of hydroge. But H2 is expensive and explosive, which requires a more complicated retorting system, pushing up the pyrolysis costs. So, an accessible and safety H donor would be required in the process of oil shale pyrolysis. Nowadays, biomass wastes also attract attentions from many researchers because of its large quantities, easy accessibility and great potential to play an important role in energy area [19,20]. The existing utilization methods were mainly retorting, direct combustion and forming pressed building materials [20]. In biomass thermal conversion, bio-oil, gases and coke are usually generated in 350 °C–400 °C, which is similar with the temperature range of bitumen generation in oil shale. The free radicals generated from the biomass could promote the decomposition of kerogen molecular by bombarding the weak bonds in it [21]. On the other hand, the bio-gas with higher heating
Corresponding author. E-mail address: [email protected] (X. Han).
https://doi.org/10.1016/j.enconman.2019.112356 Received 6 September 2019; Received in revised form 28 November 2019; Accepted 29 November 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.
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temperature of shell were −20 °C and 0 °C. Cyclone separator was twined by a heating band with temperature of 250 °C to mitigate heat loss. For retorting system, the height of dense-phase zone, sparse-phase zone and extension zone was 180 mm, 80 mm and 70 mm, respectively. High purity nitrogen was used in this experiment as inert gas. Quartz sand with particle size of < 0.5 mm was chosen as bed material. The fluctuation of feeding rate with the increase of rotation rate was shown in Fig. 2. The standard feeding mass was 100 g and the rotational speed of screw feeder was 24.3 r/min. The preset temperature of dense-phase zone, sparse-phase zone and bottom zone was set as 400 °C, 520 °C and 500 °C. The fluidization number nf was 3.0.
value escaped from the pyrolysis could be mixed with oil shale pyrolytic gas as a kind of heat source, improving the energy efficiency of retorting system. Individual pyrolysis of oil shale and biomass has been employed and investigated by many researchers all over the world [1–5,19,20,22,23]. Co-utilization of different fuels also has been investigated by many researchers, such as co-pyrolysis of coal and biomass [24]. Compared with coal, oil shale contains fewer aromatic hydrocarbons and more aliphatic hydrocarbons, such as long straight chains. But the research of the co-pyrolysis characteristics of oil shale-biomass blends is rare. In this work, fast co-pyrolysis characteristics of oil shale and wood were unveiled by a bubbling fluidized bed reactor. Moreover, a gas chromatography-mass spectrometry (GC–MS) and a gas analyzer combining a Gasmet Fourier transform infrared spectrometer (FTIR) with a MRU GmbH Vario Plus analyzer were used in the co-pyrolysis experiment to analyze the obtained oil and gases. The results could decipher the interaction characteristic of oil shale and biomass, promoting the utilization efficiency of oil shale and biomass wastes resources.
nf =
While nf is the fluidization number, U0 is the operating velocity of bubbling fluidized bed, Umf is the minimum fluidization velocity. 2.3. Product analysis The gases generated in bubbling fluidized bed were directly introduced into an online Gasmet DX-4000 FTIR spectrometer. Nitrogen spectrum was first recorded and set as a background. After that, the injected gases were analyzed by the spectrometer and the obtained spectra were handled, displayed and saved on a computer which was connected to the spectrometer by a RS232 cable. Except H2, most gases could be determined by this device. A MRU GmbH Vario Plus analyzer was connected to the spectrometer for detecting H2. The liquid products contained oil and aqueous phase, which must be separated by distillation extraction before the following detection. The obtained oil was analyzed on a model Agilent 6890 N GC/5973 N MS instrument, with injection port and detector temperature, flow rate, heating rate of 280 °C, 3.0 mL/min, 10 °C/min in gas chromatograph parameter setting, and electron energy, scanning range, temperature of 70 eV, 15–500 u, 230 °C in mass spectrometry parameter setting, respectively. The split ratio ns was chosen as 100:1.
2. Experimental section 2.1. Oil shale and biomass samples Oil shale used in this experiment was taken from Dachengzi mine in Huadian, China. Its kerogen is classified as lacustrine—Type I. According to the XRD (X-ray diffraction) results which have been given by previous studies [25], quartz and calcite are dominant minerals species and there are also minor amounts of muscovite, kaolinite, zeolite and pyrite. Also, biomass used in this work was wood pellet obtained from wood furniture factory in North Jiangsu, China. All samples were first air-dried, crushed, and sieved to a grain size of 3 mm. According to the national standards of China, proximate analysis, ultimate analysis and heating values of both samples were performed and shown in Table 1. The atomic H/C ratio was 1.58 in oil shale and 1.62 in wood. The blending ratios of oil shale with wood (S:W) were chosen as 1:0,3:1,1:1,1:3, and 0:1. The sample of S:W = 3:1 was heated in different temperatures as 430 °C, 460 °C, 490 °C, 520 °C, 550 °C and 600 °C to investigate the effect of temperature on the co-pyrolysis characteristic of the blends.
ns =
The fast pyrolysis system employed in the research is a bubbling fluidized bed, as shown in Fig. 1, which consisted of a temperature and gas control system, a retorting system, a condensation system and a gas analyzer system. The condensation system is composed of a tube-andshell heat exchanger and a refrigeration installation. The inlet and outlet temperature of tube were 500 °C and 0 °C and inlet and outlet
3. Results and discussion 3.1. Analysis of products distribution In this experiment, the operating temperature, total feed mass and rotational speed of screw feeder were set as 520 °C, 100 g and 24.3 r/ min respectively. The products of the co-pyrolysis system can be classified as gas, oil, water and semi-coke. The yield distributions of these over different blending ratios are shown in Fig. 3(a). It can be seen that pure oil shale (S:W = 1:0) could generate more semicoke and oil, less gas and water, according with the pyrolysis characteristic of oil shale and wood [8,19]. For oil shale, the yields of oil, gas and coke of are 19.72%, 12.46% and 66.78%, respectively. The corresponding yields of wood are 8.18%, 55.60% and 27.28%. In order to depict the synergistic effect between oil shale and wood, we adopt the synergistic index ξ [26] which in this paper is calculated as:
Table 1 Ultimate analysis and proximate analysis of oil shaleand wood on air dried basis. Oil shale
Wood
C H Od N St
30.43 4.00 8.78 0.63 0.94
44.78 6.05 35.85 0.43 0.29
Proximate analysis (mass%) Moisture 1.52 Ash 53.70 Volatile matter 40.31 Fixed carbon 4.47 HHV (MJ/kg) 12.59 H/C molar ratio 1.58 d–By difference; t–Total sulfur; HHV–high heat value
Vin Vout
While ns is split ratio, Vin is the vapour phase which were injected in chromatographic column, Vout is the residual vapour phase. The peaks in mass spectra were identified through comparison with National Institute of Standards and Technology—NIST 02 mass spectral data library.
2.2. Bubbling fluidized bed system
Ultimate analysis (mass%)
U0 Umf
8.52 4.08 71.70 15.70 14.73 1.62
Xadd = X1 × R1 + X2 × (1 − R1)
ξ=
2
X0 − Xadd Xadd
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Fig. 1. The sketch of bubbling fluidized bed pyrolysis system. I. Temperature and gas control system, II. Retorting system, III. Condensation system, IV. Gas analyzer system. 1. High purity nitrogen, 2. Mass flowmeters, 3. Temperature controller, 4. Thermocouple, 5. Feeding machine, 6. Fluidized bed, 7. Cyclone separator, 8. Preheater, 9. Semi-coke collecting pipe, 10. Tube-and-shell heat exchanger, 11. Refrigeration installation, 12. Buffer unit, 13. Mass flowmeters, 14. Gasmet, 15. Flue gas analyzer (MRU), 16. Exhaust pipe, 17. Observation port.
adding about 25% of wood during the pyrolysis of oil shale could inhibit the generation of semicoke, and promote the yield of oil. 3.2. Analysis of gas The gas products were scaled to volume per 100 g samples under standard conditions (one standard atmosphere, 0 °C), as shown in Table 2. There was more gas generated from the biomass during the pyrolysis. CO and CO2 are the main gases, and the yields are 7.24 L and 6.49 L respectively. For oil shale, the main pyrolysis gases are CO2, CH4 and C3H8, which occupy for volume yields of 1.47 L, 0.61 L and 2.39 L respectively. In addition, the heat value of each kind of gas was calculated, and the results are shown in Fig. 4. The pyrolysis gas of the wood (S:W = 0:1) has the highest heat value of 3237 J/g while the heat value of oil shale gas (S:W = 1:0) is just 838 J/g. Moreover, it’s noteworthy that the relation of heat value and blending ratios is approximately linear, indicating the subtle synergistic effect on the whole heat value of the pyrolysis gas. To make a further investigation, the yields of CO2, CO, H2O, CH4, C2H4, C2H6, C6H6 and C2H2 over time were drawn in Fig. 5. Consistent with previous results, CO and CO2 were the main pyrolysis gases of wood. In oil shale pyrolysis, CO2 was generated much more than CO. With the growth of oil shale content in the blending samples, C2H6, C2H2, C2H4 decreased and C6H6 increased, reflecting more aliphatic hydrocarbons in biomass and more aromatic compounds in oil shale. The yields of gas over different blending ratios were displayed in Fig. 6. The relationships of most gases with blending ratios didn’t follow a linear law, indicating the influence of synergistic effect on the copyrolysis. The change of synergistic index ξ over blending ratios was shown in Fig. 7. At the temperature of 520 °C, the main components in wood like cellulose, hemicelluloses and lignin were decomposed, generating CO2, CO and H2O through the process of decarbonylation, decarboxylation and dehydration [27]. For oil shale, oxygen-containing bonds like C-O-C were usually cracked and converted to C = O, which contacted other oxygen-containing free radicals generating CO2 or further cracked to generate CO [14]. But higher content of CO2 revealed that more C = O tended to form CO2 instead of CO in oil shale
Fig. 2. Change of feeding rate in different rotation rate.
Where X1, X2 are the products yield by wood and shale separately, and R1 is the blending ratio of wood, Xadd can be regarded as the theoretical mixture products without any synergistic effects, X0 is the observed experimental values. So ξ reflects the relative derivation of observation from Xadd. The change of ξ’s over different blending ratios are shown in Fig. 3(b). It can be seen that the synergistic effect promoted the generation of water and gas, and repressed the generation of coke. This might be because some free radicals escaping from the biomass were absorbed by the active sites on kerogen macromolecule, causing more volatiles and less semicoke [27]. Moreover, the absorbed free radicals also inhibited the recombining of themselves and repressed the generation of semicoke [28]. Synergistic effect inhibited the generation of oil slightly at the blending ratio of S:W = 1:3 and 1:1, while promoted it in S:W = 3:1. The synergistic index for gas reached maximum at the blending ratio of S:W = 1:3. For the generation of coke, the synergistic index reached minimum at the blending ratio of S:W = 3:1. This means 3
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(a) Products distribution with different S/W blends
(b) Synergistic index ȟ with different S/W blends
Fig. 3. Products distribution and synergistic index ξ with different S/W blends.
form some kinds of nitrogen-containing organic compounds like benzonitrile and indole [31]. Some active sites might be generated through the interactions between kerogen molecular and the free radicals generated from wood. These active sites would change the conversion path of NO2, causing the inhibiting effect during co-pyrolysis. The generation of unsaturated hydrocarbon gas such as C2H4 and C6H6 was promoted by co-pyrolysis. According to experimental [27] and theoretical [21] results, there were much ethylene radicals generated during the pyrolysis of oil shale. These free radicals would absorb -H free radicals or combine with other short hydrocarbons through cycloaddition and Diels-Alder reaction, forming C2H4 or benzene. The free radicals generated from wood offered more reactants, causing more C2H4 and C6H6. In addition, because of the highest content, the growth of CO2 was the major factor which evokes the increasing of gas product. But CO2 is a non-combustible gas with on heat value, which resulted in a linear relationship between the heat value and blending ratios in Fig. 4.
Table 2 Gas composition (mol%) of different S/W blends in the co-pyrolysis. CO2
CO
HCl
CxHy
H2O
H2
NO2
SO2 0.073 0.041 0.07 0.041 0.005
S:W S:W S:W S:W S:W
= = = = =
0:1 1:3 1:1 3:1 1:0
6.914 6.051 4.837 3.878 1.566
7.713 5.945 3.889 1.886 0.33
0.423 0.188 0.029 0.02 0.002
5.284 4.251 3.771 3.345 2.972
5.231 2.195 1.694 0.607 0.266
0.671 0.629 0.352 0.447 0.671
0.373 0.234 0.149 0.213 0.405
S:W S:W S:W S:W S:W
= = = = =
0:1 1:3 1:1 3:1 1:0
CH4 4.943 3.388 1.992 1.374 0.65
C2H2 0.008 0.004 0.007 0.002 0.001
C2H4 0.118 0.101 0.118 0.097 0.024
C6H6 0.021 0.181 0.192 0.256 0.309
C2H6 0.096 0.075 0.107 0.128 0.181
C3H8 0.586 0.756 1.8 2.259 2.546
HCN 0.01 0.003 0.004 0.005 0.001
3.3. Analysis of liquid Next, we analyze the liquid products during the co-pyrolysis reaction under different blending ratios. The relative contents are obtained from GC–MS. The bio-oil (generated by S:W = 0:1) contained more alkanes, oxygen and heteroatomic hydrocarbons, while correspondingly, the shale oil (generated by S:W = 1:0) contained more unsaturated hydrocarbons Fig. 8. This can be explained by the chemical components and the decomposition procedure of wood and oil shale. Acids, esters and alcohols were the major oxygen-containing organic products of wood. These products, especially acids, were usually derived from the decomposition of saccharides in the biomass [32] and further converted to esters or aldehydes [33]. The main oxygen hydrocarbons in oil shale products were ketones and esters, which were converted from oxygen bridge bonds like C-O-C. Moreover, the fragmentation of carbohydrate in wood and the secondary reactions of other oxygen-containing organic compounds also generate ketones [34]. Alkanes and alkenes were the major products generated from oil shale through the decomposition of kerogen [14,35,36]. The non-linear relationship between distribution of oil products and blending ratios suggests the existence of interaction within the wood and oil shale mixture. By comparing the differences between experimental values and theoretical values without synergistic effects in Fig. 9, we observed significant inhibition of oxygen hydrocarbons. By contrast, the distributions of heteroatomic and unsaturated hydrocarbons show different tendencies and varie oppositely. Under S:W = 1:3 and S:W = 1:1, heteroatomic hydrocarbon are inhibited and
Fig. 4. Heat value of pyrolysis gas in different S-W blends.
pyrolysis. The change of synergistic index ξ in Fig. 7 reflected that the interaction of oil shale and biomass increased the yield of CO2 during the co-pyrolysis. But the effect of synergy on CO generation was relatively insignificant. This might be because the oil shale promoted the conversion of oxygenates in wood [29], and more oxygen-containing free radicals contact the active site of C = O from kerogen intermediates, forming more CO2. Moreover, some –OH, –CH3 and -H free radicals were absorbed by the kerogen intermediates instead of forming H2O, resulting in the decrease of H2O, CH4, H2 and HCl. NO2 was generated from the reaction of nitrogen functional groups such as pyrrole/pyridine with oxygen functional groups in kerogen macromolecule [30]. The protein in wood also could be decomposed to 4
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(b) S : W = 0 : 1-(2)
(a) S : W = 0 : 1-(1)
(d) S : W = 1 : 3-(2)
(c) S : W = 1 : 3-(1)
(e) S : W = 1 : 1-(1)
(f) S : W = 1 : 1-(2)
(g) S : W = 3 : 1-(1)
(h) S : W = 3 : 1-(2)
(i) S : W = 1 : 0-(1)
(j) S : W = 1 : 0-(2)
Fig. 5. Fluctuation of gases yields with heating time.
5
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(a)
(b) Fig. 6. Effect of blending ratios on gas generation.
contained more C10-C19 and > = C20. The gaps between experimental and theoretical values reflected the obvious synergistic effect on the oil production—promoting the generation of C10-C19 and inhibiting the generation of C2-C9. Moreover, the curves of > = C20 followed a linear law, showing little synergistic effect on the generation of heavy components. Fig. 11 revealed that the synergistic effect also increased the oil’s H/C ratios, which was an important beneficial indicators to oil quality. In fast co-pyrolysis, the disproportionation, decarboxylation and decarbonylation reactions would happen together, generating a large number of free radicals [39]. These free radicals promoted the rupture of kerogen molecular by bombarded the bridge bonds between two long-carbon chains. –H or –CH3 radicals would combine with the
unsaturated hydrocarbons are promoted, while under S:W = 3:1, the former is promoted. The absorption and catalysis of alkali element like K+, Ca2+ in oil shale may promote more likely conversion of C = O to CO2, conducing the decrease of oxygen hydrocarbons products. CaO and MgO in oil shale also inhibited the conversion of acids through ketonization reaction. Moreover, high content of oil shale ash would decrease the yield of alcohols [27]. The mineral substance like calcite, dolomite and the alkali metals in oil shale could also influence the generation of benzene rings—inhibiting the formation of indol and promoting the generation of phenol and its derivatives [37,38]. Distribution of different length of carbon chains was shown in Fig. 10. Bio-oil (S:W = 0:1) contained more C2-C9, and shale oil
(a)
(b)
(c) Fig. 7. Effect of blending ratios on synergistic index ξ. 6
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Fig. 11. Atomic ratio of H/C with different blending ratios.
Fig. 8. Distribution of oil products with different blending ratios.
Fig. 9. Difference of experimental and theoretical value with different blending ratios. Fig. 12. Distribution of co-pyrolysis products (S:W = 3:1) with increasing temperature.
to 600 °C, the yield of semicoke decreased from 67.48% to 53.01% because of the stronger cracking and volatilization. The gas production increased from 16.28% to 22.86% for the stronger decomposition of biomass and oil shale macromolecules and secondary cracking in higher temperature. The secondary cracking also inhibited the generation of liquid products in higher temperature. So the yield of liquid products increased from 12.55% to 17.12% at the temperature range of 430 °C–520 °C, and then decreased to 15.93% when temperature reached 600 °C. The pyrolysis of biomass was an endothermic process, and higher temperature was more likely to promote the rupture of atomic bonds, causing the decomposition of organic compounds [40]. But as temperature further increased, the secondary reactions of organics would become stronger with more available energy, promoting the formation of gases like CO2 and CO and inhibiting the generation of organic products [34]. For the pyrolysis of oil shale, it started with the rupture of bridge bonds and then reformed the structure of kerogen (usually in 350 °C), forming the pyrolysis intermediates named bitumen [21]. When temperature continues to rise, the generated radical fragments coupled together or recombined to form volatiles or char [41]. The main purpose of the co-pyrolysis was to obtain the organic liquid products, but the oil yield began to decrease when temperature exceeded 520 °C. So, according to this results and previous researches [8,42], 520 °C could be considered as a reasonable temperature for the co-pyrolysis of oil shale and biomass. More details about the distribution of gas and liquid products with increasing temperature would be discussed later.
Fig. 10. Distribution of different carbon chains.
active points on the fragments of kerogen intermediates, generating more hydrocarbons instead of forming semicoke. Meanwhile, the free radicals repressed the generation of C2-C9 and promoted the generation of C10-C19. 3.4. Effect of temperature on the co-pyrolysis 3.4.1. Analysis of products distribution The yields of co-pyrolysis product (S:W = 3:1) over different temperature were shown in Fig. 12. As the temperature varied from 430 °C 7
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(a)
(b) Fig. 13. Effect of heating temperature on gas products (S:W = 3:1).
atomic ratios were shown in Fig. 15(b), from which it can be discovered that variation tendency of O/C and H/C were similar—showing a sharply decrease when temperature exceeded 500 °C. This might be attributed to dehydration of the organic compounds in biomass and oil shale [44]. The curves tended to smooth up when the temperature exceeded 550 °C, showing that higher temperature could not further enhance the dehydration. Considering the pyrolysis efficiency, oil yield and quality, 520 °C could be chosen as the normal temperature for the co-pyrolysis of oil shale and wood in industrial production.
3.4.2. Analysis of gas distribution The yields of gas such as H2O, CO, CO2, CH4, C2H6, C2H4, C2H2 and C6H6 over increasing temperature were drawn and shown in Fig. 13. CO2 increased from 5.82% to 9.1% at the temperature range of 430 °C–460 °C and kept steady at 460 °C–520 °C, and finally increased again from 9.53% to 14.32% at 520 °C–600 °C. This is because when temperature exceeded 520 °C, decarboxylation of wood and oil shale became stronger, making more oxygen functional groups depart from the macromolecule and form CO2 [40]. As the temperature increased, the generation of CO and some organic gases like CH4, C2H4 and C2H6 was more steadily than CO2. Other kinds of gases like C2H2 and C6H6 were increased more sharply when temperature exceeds 520 °C. Taylor [42] and Raley [43] thought that the molar ratio of C2H4/ C2H6 could reflect the degree of secondary cracking—higher C2H4/ C2H6 means stronger secondary cracking and lower yield of pyrolytic oil. The ratios of C2H4/C2H6 with increasing temperature in Fig. 14 show a stronger secondary cracking and higher production of unsaturated hydrocarbons after 520 °C.
4. Conclusions Fast co-pyrolysis of oil shale with wood was performed using a bubbling fluidized bed in this paper. The synergistic effect on the distributions of gas and liquid products was discussed and unveiled. The free radicals generated from wood played a critical role in oil shale pyrolysis. More volatiles and less semicoke were generated because of the combination of the free radicals and kerogen intermediates. CO2 generation was promoted through the combination of more oxygencontaining free radicals and C = O from kerogen. The absorption of some kinds of free radicals like –OH, –CH3 and -H resulted in the decrease of H2O, CH4, H2. The reaction pathways of oil shale pyrolysis were influenced by these free radicals, causing less production of NO2, more unsaturated hydrocarbon gas like C2H4, more long-carbon chains of C10-C19, less small molecular hydrocarbons of C2-C9, higher H/C ratios of oil, and so forth. Meanwhile, oil shale could also stabilize the free radicals from biomass, inhibiting their secondary reaction and recombination. The alkali element in oil shale promoted more conversion of C = O to CO2, causing a transfer of oxygen from liquid to gas products. Ethylene free radicals from oil shale promoted the Diels-Alder reaction and led to more aromatic hydrocarbons by cycloaddition. The temperature was also an important factor on the co-pyrolysis of oil shale and wood. Dehydration, decarboxylation and decarbonylation of organic compounds were influenced by it. The reasonable temperature was set as 520 °C, which could ensure the pyrolysis efficiency and oil production rate and quality simultaneously. The co-pyrolysis of oil shale and biomass had potential application prospect in the utilization of energies, which was beneficial to adjusting the thermal conversion process and promoting the energy utilization rate.
3.4.3. Analysis of liquid distribution Fig. 15(a) uncovered the effect of temperature on the distribution of different length of carbon chains. C10-C19 carbon chains were the major ingredient in co-pyrolysis oil—accounting for > 50% of all. The content increased from 54.13% to 58.87% as the temperature increased from 430 °C–600 °C. C2-C9 carbon chains increased from 11.23% to 25.78% as the temperature increased from 430 °C to 600 °C, while the carbon chains of > = C20 decreased from 35.18% to 15.74%. All kinds of curves are nearly horizontal after 520 °C. The curves of O/C and H/C
CRediT authorship contribution statement Bin Chen: Conceptualization, Methodology, Software, Funding acquisition, Writing - original draft, Writing - review & editing. Xiangxin Han: Conceptualization, Supervision, Software, Funding acquisition. Jianhui Tong: Software. Mao Mu: Validation, Formal analysis, Investigation. Xiumin Jiang: Funding acquisition. Sha Wang:
Fig. 14. Effect of heating temperature to C2H4/C2H6 (S:W = 3:1). 8
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Fig. 15. Effect of heating temperature on the characteristics of liquid products (S:W = 3:1).
Visualization. Jun Shen: Project administration. Xiao Ye: Software.
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