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Syngas production by chemical looping gasification of rice husk using Fe-based oxygen carrier
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Syngas production by chemical looping gasification of rice husk using Fe-based oxygen carrier Xiangneng Huang a, Jiawei Wu a, Mingfeng Wang a, Xiaoqian Ma b, Enchen Jiang a, Zhifeng Hu a, * a b
College of Materials and Energy, South China Agricultural University, Guangzhou 510642, China School of Electric Power, South China University of Technology, Guangzhou 510640, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 October 2019 Received in revised form 17 November 2019 Accepted 18 November 2019 Available online xxx
The chemical looping gasification (CLG) of rice husk was conducted in a fixed bed reactor to analyze the effects of the ratio of oxygen carrier to rice husk (O/C), temperature, residence time and preparation methods of Fe-based oxygen carriers. The yield of gas, H2/CO, lower heating value of syngas (LHV), conversion efficiency and performance parameters were analyzed to obtain CLG reaction characterization and optimal reaction conditions. Results showed that when O/C increased from 0.5 to 3.0, the gas production, H2/CO, CO2 yield and carbon conversion efficiency gradually increased, while the yield of H2, CO and CH4 and LHV gradually decreased. At the same time, a highest gasification efficiency was obtained when O/C was 1.5. As increasing temperature, the gas production, CO yield, carbon conversion efficiency and gasification efficiency gradually increased, while the yield of H2, CH4 and CO2, H2/CO and LHV gradually decreased. Sintering and agglomeration was obvious when the temperature was higher than 850 C. When the reaction time increased from 10 min to 60 min, the gas production, CO yield, carbon conversion efficiency and gasification efficiency gradually increased, but the yield of H2, H2/CO and LHV decreased, among which 30 min was the best reaction residence time. In addition, coprecipitation was the best preparation method among several preparation methods of oxygen carrier. Finally, O/ C of 1.5, 800 C, 30 min and coprecipitation preparation method of oxygen carrier were the optimal parameters to obtain a gasification efficiency of 26.88%, H2 content of 35.64%, syngas content of 56.40%, H2/CO ratio of 1.72 and LHV of 12.25 MJ/Nm3. © 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Chemical looping gasification Rice husk Biomass Oxygen carrier Syngas
1. Introduction Rice husk, an important by-product of rice processing [1], is a kind of low cost and abundant biomass resource. The rice husk production is about 140 million tons per year in the world [2] and about 40 million tons per year in China [3]. Due to the lack of rice husk utilization technology, a large amount of rice husk produced annually in China cannot be reasonably developed and utilized. Nowadays, more and more researchers believe that energy utilization of rice husk could reduce pollutants and increase economic benefits [4]. Chemical looping gasification (CLG) is a novel gasification technology. The needed oxygen during the gasification is provided by lattice oxygen of oxygen carrier. Through controlling the ratio of
* Corresponding author. E-mail address: [email protected] (Z. Hu).
lattice oxygen to fuel, solid fuel is gasified and converted into syngas with H2 and CO as the main components [5]. CLG has several advantages as following [6]: (1) The recycling of oxygen carrier provides oxygen for fuel gasification without additional oxygen; (2) The heat released by the oxygen carrier in the air reactor can provide heat for fuel gasification; (3) Oxygen carrier plays a catalytic role in gasification reaction; (4) CLG can reduce the emission of toxic gases such as nitrogen oxides and sulfur compounds. Oxygen carrier is the key factor and important condition of CLG [7]. It is necessary to select suitable oxygen carrier. Fe-based oxygen carrier is widely used in CLG process [8e11] because of good reactivity, stable cyclicity and low cost [12e14]. Huang et al. [15] confirmed that the Fe-based oxygen carrier used in the study could produce syngas from experimental biomass, which has a good application prospect. Qi et al. [16] analyzed the gas production in a fixed bed system and indicated that Fe2O3 increased the concentration of CO and CO2 in the CLG process. In the process of chemical
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cycle, the thermal efficiency is improved by 5%e6% after utilizing iron oxide as oxygen carrier [17]. Zhu et al. [18] found that the efficient conversion of CO2 and CH4 to syngas can be achieved on the basis of reduced ferric oxygen carrier. CH4 conversion efficiency and syngas yield (syngas output/CH4 input) in the dry reforming stage reached 98.32% and 3.84, respectively, when the ratio of CH4 and CO2 is 1.00 and the iron reduction degree is 33%. Reaction conditions are also important factors affecting CLG [19]. The maximum gas production (1.22 Nm3/kg) was obtained by CLG of rice straw under the optimal ratio of steam and biomass (S/ B) of 2.8 and the reaction temperature of 900 C [20]. Hu et al. [21] conducted CLG of wheat straw with Fe-based oxygen carrier to produce syngas, which indicated that the H2/CO ratio, gas production volume, H2 þ CO concentration and carbon conversion efficiency reached the maximum under the optimal load of 60% Fe2O3, S/B ratio of 0.8 and O/C ratio of 1.0. The optimal carbon conversion efficiency and syngas production was obtained during CLG of biomass with S/B ratio of 0.8, temperature of 800 C and oxygen carrier of natural iron ore [22]. Chemical looping gasification and reforming of gaseous fuel was studied and developed extensively because of high reactivity and conversion efficiency between oxygen carrier and fuel gas [23]. At present, China has abundant solid fuels such as biomass and coal, lacking gaseous fuels such as natural gas and syngas [24]. The research on CLG of solid fuels has positive significance for energy efficient utilization and environment-friendly society. In this paper, CLG of rice husk was conducted in a fixed bed reactor to analyze the effects of the ratio of oxygen carrier to rice husk (O/C), temperature, residence time and preparation methods of Fe-based oxygen carrier. In addition, gas yield, H2/CO, LHV, conversion efficiency, SEM, XRD and TG/DTG were analyzed to obtain CLG reaction performance and the optimal reaction parameters.
and Fe2O3 with a molar ratio of Al and Fe ¼ 2:1. The third oxygen carrier is prepared by impregnation method. The chemical reagents are Al2O3 and Fe(NO3)39H2O with a molar ratio of Al and Fe ¼ 2:1. The fourth oxygen carrier is prepared by coprecipitation method. The chemical reagents are Al(NO3)39H2O and Fe(NO3)39H2O with a molar ratio of Al and Fe ¼ 2:1. After drying, calcining, pulverizing and sieving, the particle diameter of different oxygen carrier is less than 200 mm. 2.3. Experimental procedure Fig. 1 shows the reaction device of CLG. The experimental device is assembled as shown in Fig. 1. Rice husk was mixed with oxygen carrier at a mechanical mixer for 20 min in order to ensure the uniformity of samples. And then they were placed in the center of quartz tube. Before experiment, N2 was ventilated into the quartz tube for 20 min at 100 ml/min in order to maintain the inert atmosphere. When the reactor is heated to the desired temperature, the quartz tube with oxygen carrier and rice husk is loaded into the reactor for 30 min. During the experiment, N2 was ventilated into the quartz tube at 100 ml/min. After condensation, the flue gas was collected by gas collecting bag and then was measured by Gas Chromatograph.
2.4. Methods The evaluation indexes of CLG mainly include the following equations: (1) The lower heating value (QLHV,kJ/Nm3) of the gas product is calculated as follows:
QLHV ¼ 126 VCO þ 188 VCO2 þ 359 VCH4 þ 594 VC2 H4 2. Materials and methods
(1)
2.1. Biomass raw material
where, VCO , VH2 , VCH4 and VC2 H4 are respectively the volume fraction of CO, H2, CH4 and C2H4 in gas products, %.
Rice husk is provided by Qilin north farm of South China Agricultural University. The proximate analysis, elemental analysis and LHV of rice husk are shown in Table 1. After pulverizing and sieving, the particle diameter of rice husk is less than 200 mm.
(2) Gas production (Gv , Nm3/kg) is defined as the volume (standard conditions) of gas products produced by biomass (per unit mass) after gasification. The calculation is as follows:
2.2. Oxygen carrier The analytical Fe2O3, SiO2 and Al2O3 power were calcined at 900 C for 4 h. After pulverizing and sieving, the particle diameters of Fe2O3, SiO2 and Al2O3 were less than 200 mm. There are four types of preparation method for oxygen carrier in this paper. The first oxygen carrier is prepared by mechanical blending method. The chemical reagents are SiO2 and Fe2O3 with a molar ratio of Si and Fe ¼ 2:1. The second oxygen carrier is prepared by mechanical blending method. The chemical reagents are Al2O3
Proximate analysisa (wt, %)
Elemental analysisa (wt, %)
LHV (kJ/kg)
Moisture Ash Volatile Fixed carbon
C H Ob N S
13839.54
a b
On wet basis. Calculated by difference.
40.78 5.575 52.41 0.82 0.415
Vg mb
(2)
where, Vg is the volume (standard conditions) of gas product obtained by gasification, Nm3. Vg is the biomass mass in the gasification process, kg. (3) Carbon conversion efficiency (hc , %) is defined as proportion of the carbon from gaseous products to the carbon from biomass during CLG process. The calculation is as follows:
hc ¼
Table 1 The proximate analysis, elemental analysis and LHV of rice husk.
8.43 12.33 65.53 13.71
Gv ¼
12 ðVCO þ VCO2 þ VCH4 þ VC2 H4 Þ Gv n ðT1 =TÞ C%
(3)
where, n is the molar volume (standard conditions) of gas, L/mol. T1 is the temperature when the gas concentration is measured, K. C% is the carbon content in biomass. (4) Gasification efficiency (h, %) is defined as the proportion of LHV of gaseous products to LHV of biomass during CLG process. The calculation is as follows:
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Fig. 1. The schematic diagram of biomass CLG reaction.
h¼
LHV Gv Qb
(4)
where, Qb is LHV of biomass, kJ/kg. 3. Results and analysis 3.1. The effect of oxygen carrier to material ratio on CLG This section focused on the influence of O/C on CLG performance under different O/C (0.5, 1.0, 1.5, 2.0, 2.5 and 3.0). Meanwhile, the reaction temperature was 800 C and the total amount of oxygen carrier and rice husk was 5 g for each experiment. The effect of O/C on gas content is shown in Fig. 2(a). The percentage of gaseous products increased from 29.15% to 44.43% as O/C increasing from 0.5 to 3. This was because the oxygen carrier provided lattice oxygen for rice husk to promote gasification. In addition, the oxygen carrier also provided partial catalysis to promote the thermal cracking of rice husk and tar [25]. Moreover, Fe element in oxygen carrier was beneficial to reduce the content of organic macromolecular compounds in tar [26]. The effect of O/C on gas concentration and LHV is shown in Fig. 2(b). When O/C increased from 0.5 to 3.0, the yield of H2, CO and CH4 in syngas decreased by 15.19%, 42.43% and 31.56%, respectively, while the CO2 yield increased by 48.84%. This phenomenon could be explained as following: Firstly, the increase of oxygen carrier would promote the reactions of H2 þ 3 Fe2O3 / 3 H2O þ 2 Fe3O4 and CH4 þ 4 Fe2O3 / 2 H2O þ CO2 þ 8 FeO, leading to the consumption of more H2 and CH4. Therefore, the yield of H2 and CH4 decreases gradually with the increase of oxygen carrier. Secondly, the increase of oxygen carrier will promote the reactions of CO þ 3 Fe2O3 / CO2 þ 2 Fe3O4, CO þ Fe2O3 / CO þ 2 FeO, CH4 þ 4 Fe2O3 / 2 H2O þ CO2 þ 8 FeO and C þ 2 Fe2O3 / CO2 þ 4 FeO, leading to the consumption of more CO. In other words, higher O/C provided more oxygen, converting C, CO, and CH4 to CO2. With the increase of oxygen carrier, the CO yield gradually decreased and CO2 increased. The yield of C2H4 decreased by 45.32% as O/C increasing from 0.5 to 3.0. Due to the increase of oxygen carrier, the reduction reaction of oxygen carrier was promoted, resulting in the decrease of C2H4. They were consistent with the results of literature [25]. There is a positive correlation between LHV and the yield of combustible gas (Eq. (1)). In addition, the yield of H2, CO, CH4 and
C2H4 decreased while CO2 yield increased since increasing oxygen carrier. Therefore, LHV decreased gradually. Table 2 shows the effect of O/C on H2/CO. As shown in Table 2, with the increase of O/C, H2/CO firstly increased from 1.28 to 1.58, which indicated that a series of reactions in favor of H2 generation occurred in the hydrogen production zone: 3 Fe þ 3y FeO þ y H2O / (x þ y) Fe3O4 þ y H2, CO þ H2O / CO2 þ H2, C þ 2 H2O / CO2 þ 2 H2. However, when O/C was greater than 2.0, H2/CO declined and became stably, which may be because excessive oxygen carrier promoted the reaction of CO þ 3 Fe2O3 / CO2 þ 2 Fe3O4. The effect of O/C on efficiency and gas yield is shown in Fig. 2(c). When O/C increased from 0.5 to 3, gas yield increased by 30% (from 0.270 Nm3/kg to 0.351 Nm3/kg). This is because higher O/C provided more lattice oxygen for rice husk in the reaction process. Moreover, excess oxygen carrier also provided partial catalyst effect, which promoted the cracking of rice husk and tar, resulting in the increase of gas yield. With the increase of O/C, the carbon conversion efficiency increased continuously, which was mainly due to the CLG of biomass and chemical looping combustion reaction between the addition of oxygen carrier and the syngas generated from gasification: C þ 3 Fe2O3 / CO þ 2 Fe3O4, C þ 6 Fe2O3 / CO2 þ 4 Fe3O4, C þ H2O / CO þ H2, CO þ 3 Fe2O3 / CO2 þ 2 Fe3O4, CO þ Fe2O3 / CO2 þ 2 FeO, CH4 þ 4 Fe2O3 / 2 H2O þ CO2 þ 8 FeO [27]. In other words, more Fe-based oxygen carrier not only provided more lattice oxygen for rice husk and promoted the oxidation of pyrolysis products, but also had catalytic cracking effect on tar and carbon, promoting the increase of CO2, H2, CO, CH4, C2H4 and gas yield. This is also consistent with the results of literatures [24,28,29]. As increasing O/C, the gasification efficiency increased to 28.72% (O/C ¼ 1.5) firstly and then decreased evidently. This is mainly because when O/C increased from 0.5 to 1.5, the combustible gas concentration just reduced by 8.27%, on the contrary, the gas content increased obviously by 35.12%. These dual effects increased the gasification efficiency. However, when O/C increased to 3.0, although the gas content continued to increase by 16.67%, the combustible gas concentration reduced significantly by 21.98% because excessive lattice oxygen was provided for the combustion. Moreover, LHV increased and gas production decreased with the increase of O/C. The gasification efficiency was affected by the dual effects of LHV and gas production [30]. Based on the above comprehensive study of gas concentration, gas yield, LHV, carbon conversion efficiency and gasification efficiency, the results indicated that 1.5 is the optimal O/C under the
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X. Huang et al. / xxx (xxxx) xxx Table 2 The effect of O/C on H2/CO.
Fig. 2. The effect of O/C on gas content, gas concentration, LHV, efficiency and gas yield.
experimental condition. 3.2. The effect of temperature on CLG Based on Section 3.1, keeping O/C ¼ 1.5, the experiments were
O/C
H2/CO
0.5 1 1.5 2 2.5 3
1.28 1.36 1.56 1.58 1.55 1.55
conducted under different temperature of 700 C, 750 C, 800 C, 850 C, 900 C and 950 C. The effect of temperature on gas content is shown in Fig. 3(a). The percentage of gaseous products increased from 24.72% to 53.82% as temperature increasing from 700 C to 950 C. In addition, when the temperature increased from 700 C to 750 C, from 750 C to 800 C, from 800 C to 850 C, from 850 C to 900 C and from 900 C to 950 C, the percentage of gaseous products increased by 3.82%, 10.37%, 6.49%, 3.32% and 5.11%, respectively. This was because high temperature would promote biomass thermal reactions, which was conducive to improving the reaction rate of pyrolysis and gasification [31,32], resulting in the increase of gaseous products. However, when the temperature was too high, sintering and agglomeration would occur on the oxygen carrier [33,34], weakening the activity of oxygen carrier. Therefore, the improvement on the percentage of gaseous products became weaken at higher temperatures. The effect of temperature on gas concentration and LHV is shown in Fig. 3(b). When the temperature increased from 700 C to 950 C, the yield of H2, CO2 and CH4 in syngas decreased by 28.17%, 13.64% and 37.36%, respectively, however the CO yield increased by 137.51%. As shown in Table 3, the ratio of H2/CO decreased with the temperature increasing. This trend could be explained as following: Firstly, the reduction reaction between carbon and oxygen carrier (C þ 3 Fe2O3 / CO þ 2 Fe3O4) and carbonization reactions (C þ H2O / CO þ H2 and C þ CO2 / 2 CO) were both endothermic reactions, leading to more CO and H2 production at higher temperature. Secondly, the reduction reactions between CO and oxygen carrier (CO þ 3 Fe2O3 / CO2 þ 2 Fe3O4 and CO þ Fe2O3 / CO2 þ 2 FeO) were exothermic reactions. High temperature would cause the reaction to proceed in the reverse direction, producing more CO and consuming more CO2. Thirdly, the reduction reaction between CH4 and oxygen carrier (CH4 þ 3 Fe2O3 / 2 H2 þ CO þ 2 Fe3O4), the reforming reaction of CH4 (CH4 þ H2O / CO þ 3 H2) and the reduction reaction between H2 and oxygen carrier (H2 þ Fe2O3 / H2O þ 2 FeO) were endothermic reactions. High temperature will promote the reaction to proceed in the positive direction, consuming more CH4, H2 and producing more CO. Therefore, as increasing the temperature, the yield of H2, CO2 and CH4 gradually decreased, while the CO yield increased. As shown in Fig. 3(b), because of the decrease of H2, CH4 and C2H4, LHV decreased with the increase of temperature, especially higher than 850 C. It indicated that high temperature (>850 C) was not conducive to the formation of high LHV gas. The effect of temperature on carbon conversion efficiency and gasification efficiency is shown in Fig. 3(c). Temperature has a significant impact on carbon conversion efficiency and gasification efficiency. When the temperature increased from 700 C to 950 C, carbon conversion efficiency and gasification efficiency increased by 130.41% and 90.84% (from 18.15% to 41.82% and from 19.55% to 37.31%), respectively. High temperature promoted the biomass pyrolysis, gasification and reduction reaction rate leading to the increase of gaseous products and the C-containing gas
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5 Table 3 The effect of temperature on H2/CO. Temperature ( C)
H2/CO
700 750 800 850 900 950
2.50 2.08 1.56 1.08 0.91 0.76
after CLG at 700 C, which was the main conversion (Fe2O3/ Fe3O4) of Fe-based oxygen carrier in chemical looping reaction [37]. When the temperature increased to 800 C, some Fe3O4 gradually reduced to FeO because high temperature promoted the reactions between oxygen carrier with biomass and pyrolysis products. When the temperature was 850 C, a few Fe were detected while Fe3O4 and FeO were the main component. In addition, as shown in Fig. 5, a dense layer was formed by many particles at 850 C, causing the sintering and agglomerate of oxygen carrier. However, clear and large particles were observed at 800 C. The results of XRD and SEM indicated that Fe would be reduced deep from FeO and it was liable to cause the sintering and agglomerate. When the temperature increased to 900 C and 950 C, more Fe were formed in oxygen
Fig. 3. The effect of temperature on gas content, gas concentration, LHV, carbon conversion efficiency and gasification efficiency.
concentration. Therefore, carbon conversion efficiency and gasification efficiency of CLG were improved with increasing the temperature. These results were consistent with the relevant literatures [35,36]. The XRD characterization of oxygen carrier under different temperature is shown in Fig. 4. All Fe2O3 were reduced to Fe3O4
Fig. 4. The XRD characterization of oxygen carrier under different temperature. (A: Fe3O4, B: FeO, C: Fe, D: Fe2SiO4).
Please cite this article as: XiangnengJiaweiMingfengHuangWuWang, Syngas production by chemical looping gasification of rice husk using Febased oxygen carrier, , https://doi.org/10.1016/j.joei.2019.11.009
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Fig. 5. The SEM images of solid residues under different temperature.
carrier. Moreover, few Fe2SiO4 were detected, which was also an important reason for sintering and agglomerate because of the low melting point [38]. The results exhibited the characteristics of stepwise reduction (Fe2O3 / Fe3O4 / FeO / Fe) in CLG process, which was consistent with the literature [39]. Furthermore, high temperature (850 C, 900 C and 950 C) should be avoided because sintering would affect the re-oxidation and recycling capacity of oxygen carrier. Based on the above comprehensive study of gas concentration, gas yield, LHV, XRD, SEM, carbon conversion efficiency and gasification efficiency, the results indicated that 800 C is the optimal reaction temperature under the experimental condition.
3.3. The effect of residence time on CLG Based on the above sections, the experiments were conducted under different residence time of 10 min, 20 min, 30 min, 40 min, 50 min and 60 min to obtain the best residence time when O/C was 1.5 and temperature was 800 C. The effect of residence time on gas content is shown in Fig. 6(a). It can be seen that the percentage of gaseous products increased with the increase of residence time. When the residence time increased from 10 min to 30 min, the percentage of gaseous products increased by 21.53%, which was a significant increase. This is mainly because there were sufficient reaction materials during this period, leading to a larger contact probability between oxygen carrier and rice husk. In other words, biomass pyrolysis and gasification were easier to react. When the residence time increased from 30 min to 60 min, the percentage of gaseous products slightly increased by 7.64%. It could be explained that the quantity of oxidized oxygen carrier and rice husk decreased after 30 min, resulting in the weakness of gas production capacity [36]. Therefore, the first 30 min is the main stage of CLG on rice husk. The effect of residence time on gas concentration is shown in Fig. 6(b). When the residence time increased from 10 min to 60 min, the CO yield increased gradually and increased by 26.98%, while the H2 yield decreased gradually and decreased by 11.74%. This may be because there were more char would be converted into gaseous products under longer residence time leading to more CO production. However, hydrogen in biomass was generally released during the release process of volatiles, which was the first stage of gasification. Moreover, more CO and other gaseous products were generated in the later stage, which would dilute the H2 concentration. Therefore, the H2 yield decreased as increasing the residence time. In addition, as shown in Table 4, when the residence
time increased from 10 min to 60 min, H2/CO decreased continuously and decreased by 30.64%. Unlike CO, the CO2 yield increased firstly and then decreased because the gasification of char (C þ CO2 / 2 CO) played a major role under longer residence time. According to the above analysis, under the residence time of 10 min, the percentage of gaseous products was the lowest and most of the reactions had not been completed. Therefore, 10 min was not suitable for CLG and it would not be discussed in this section. The effect of residence time on efficiency and LHV is shown in Fig. 6(c). When the residence time increased from 20 min to 60 min, the carbon conversion efficiency increased from 24.31% to 31.93%, while the gasification efficiency and LHV decreased from 30.19% to 18.90% and 14.28 MJ/Nm3 to 7.25 MJ/Nm3, respectively. There were more char would be converted into gaseous products under longer residence time, resulting in the increase of carbon conversion efficiency. However, a stable gasification equivalent ratio would be formed between the oxygen carrier and rice husk under a longer residence time so that the gasification reaction gradually tended to be stable [35]. Moreover, there were more carrier gas in the gaseous production under a longer residence time leading to the decrease of gasification efficiency and LHV. As shown in Fig. 6(c), when the residence time increased from 20 min to 30 min (increased by 10 min), the carbon conversion efficiency increases by 12.88%, while LHV and gasification efficiency decreases by 16.06% and 4.88%, respectively. However, when the residence time increased from 30 min to 40 min (increased by the same 10 min), the carbon conversion efficiency just increased by 2.02%, while LHV and gasification efficiency decreased evidently by 20.87% and 20.43%, respectively. The results revealed that the gasification performance was not reasonable when the residence time was longer than 30 min. In addition, as shown in Fig. 6(c) and Table 4, H2/CO, LHV and gasification efficiency were fairly low when the residence time were 50 min and 60 min. Based on the above comprehensive study of gas concentration, gas yield, LHV, carbon conversion efficiency and gasification efficiency, the results indicated that 30 min is the optimal residence time under the experimental condition.
3.4. The effect of oxygen carriers prepared by different methods on CLG Based on the above sections, the experiments were conducted under different preparation methods of mechanical blending method, impregnation method, coprecipitation method and SiO2 þ Fe2O3 method to obtain the optimal method when O/C,
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Table 4 The effect of residence time on H2/CO.
Fig. 6. The effect of residence time on gas content, gas concentration, efficiency and LHV.
temperature and residence time was 1.5, 800 C and 30 min, respectively. The effect of oxygen carriers on gas content is shown in Fig. 7(a). The gas content of SiO2 þ Fe2O3 method was the highest (32.81%), followed by coprecipitation method (29.95%) and impregnation method (26.84%). Mechanical blending method was the lowest.
Residence time (min)
H2/CO
10 20 30 40 50 60
1.73 1.66 1.56 1.48 1.34 1.20
The effect of oxygen carriers on gas concentration is shown in Fig. 7(b). The H2 yield of coprecipitation method was the highest (35.64%), followed by impregnation method (35.32%). Moreover, the CO yield of mechanical blending method was the highest (26.79%), followed by coprecipitation method (20.76%). Futhermore, the CH4 and C2H4 yield of coprecipitation method were the lowest (7.58% and 1.53%), followed by SiO2 þ Fe2O3 method (9.13% and 1.64%). In addition, the CO2 yield of impregnation method was the lowest (33.75%), followed by coprecipitation method (34.49%). As shown in Table 5, H2/CO of impregnation method was the highest (1.79), followed by coprecipitation method (1.72), which were beneficial to the synthesis of chemical products. Meanwhile, the syngas content (CO þ H2) of coprecipitation method was the highest (56.40%), followed by impregnation method (55.05%). The results indicated that coprecipitation was the best preparation method to produce syngas of high H2/CO and CO þ H2. The effect of oxygen carriers on efficiency and LHV is shown in Fig. 7(c). Carbon conversion efficiency of SiO2 þ Fe2O3 method was the highest (25.39%), followed by coprecipitation method (23.06%). Further, gasification efficiency of coprecipitation method was the highest (26.88%), followed by impregnation method (25.52%). In addition, LHV of impregnation method was the highest (13.56 MJ/ Nm3), followed by coprecipitation method (12.94 MJ/Nm3). After comprehensive analysis of carbon conversion efficiency, gasification efficiency and LHV, coprecipitation was the best preparation method to obtain a high CLG performance. The results can be explained by the analysis of Fig. 8. As shown in Fig. 8, before the reaction, Fe2O3 and Al2O3 were detected in the oxygen carriers of mechanical blending method, impregnation method and coprecipitation method. After CLG, Fe2O3 of SiO2 þ Fe2O3 method were converted to Fe3O4 and FeO. There were many Fe3O4 remaining on the oxygen carrier, which could be deeply converted to FeO. Therefore, the CO2 yield of SiO2 þ Fe2O3 method was the highest (as shown in Fig. 7(b)). Meanwhile, many Fe3O4 remaining on the oxygen carrier revealed that this method did not exert thoroughly the redox ability of oxygen carrier. After CLG, all Fe2O3 of mechanical blending method and impregnation method were converted to FeO, which revealed that the redox ability of oxygen carrier could be fully developed. However, with a low melting point, FeO was easier to sinter with residues at high temperature and affect the subsequent reduction reaction. After CLG, FeAl2O4 and few Al2O3 were detected on the oxygen carrier of coprecipitation method. This is because Fe2O3 and Al2O3 were evenly distributed and combined through coprecipitation method. Under this condition, FeO, converted from Fe2O3 after CLG, was easy to react with Al2O3 and formed FeAl2O4 crystal at high temperature. Through the coprecipitation method, all Fe3þ of oxygen carrier were converted to Fe2þ, which could exert thoroughly the redox ability. In addition, the reduced oxygen carrier formed FeAl2O4 crystal with a better thermal stability and crystal structure [40], which was conducive to subsequent reduction reactions. Therefore, as shown in Fig. 7(b) and (c), coprecipitation was the best preparation method to obtain a high CLG performance. Fig. 9 shows the TG-DTG characterization under different
Please cite this article as: XiangnengJiaweiMingfengHuangWuWang, Syngas production by chemical looping gasification of rice husk using Febased oxygen carrier, , https://doi.org/10.1016/j.joei.2019.11.009
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X. Huang et al. / xxx (xxxx) xxx Table 5 The effect of method on H2/CO and CO þ H2. O/C
H2/CO
CO þ H2 (%)
Mechanical blending Impregnation coprecipitation SiO2 þ Fe2O3
0.94 1.79 1.72 1.49
51.91 55.05 56.40 49.07
Fig. 8. XRD characterization of different oxygen carriers before and after reaction (A: Fe2O3, B: Al2O3, C: Fe3O4, D: FeO, E: FeAl2O4).
Fig. 7. The effect of oxygen carriers on gas content, gas concentration, efficiency and LHV.
oxygen carriers. The whole trends of TG-DTG characterization were similar under different oxygen carriers, which had three main stages. The first stage was from 30 C to 200 C. TG curves decreased slightly in this stage. This weight loss was because the evaporation of residual water and internal water in rice husk at about 90 C [41]. Therefore, the first stage of CLG was the drying
stage and the differences among these four oxygen carriers were not obvious. The second stage was from 200 C to 400 C, which was the pyrolysis stage of CLG. This stage was the main stage of weight loss because rice husk was rich in cellulose and lignin. Moreover, the first step of degradation was the initial decomposition of hemicellulose and cellulose (200e270 C), while the second step of degradation was the decomposition of lignin and cellulose (270e370 C) [42]. In addition, Fe2O3 promoted the pyrolysis of rice
Please cite this article as: XiangnengJiaweiMingfengHuangWuWang, Syngas production by chemical looping gasification of rice husk using Febased oxygen carrier, , https://doi.org/10.1016/j.joei.2019.11.009
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CLG performance. When O/C increased from 0.5 to 3.0, the gas production, H2/CO, the CO2 yield and carbon conversion efficiency gradually increased, while the yield of H2, CO, CH4 and LHV gradually decreased. Further, the gasification efficiency was the highest when O/C was 1.5. Moreover, high temperature was beneficial to improve gas production, carbon conversion efficiency and gasification efficiency. However, higher temperature was easier to cause sintering, which reduced the activity of oxygen carriers. This study showed that 800 C was the best temperature for gas production, LHV, high conversion efficiency, good H2/CO and strong performance of oxygen carrier. In addition, longer reaction time was also beneficial to improve gas production, carbon conversion efficiency and gasification efficiency, but 30 min was the best reaction residence time. Among SiO2 þ Fe2O3 method, mechanical blending method, impregnation method and coprecipitation method, oxygen carrier of coprecipitation method could convert and form FeAl2O4 crystal after CLG. Meanwhile, coprecipitation method was the optimal preparation method to obtain the highest CLG performance. Acknowledgements The authors are grateful to National Natural Science Foundation of China (51806068, 51706074), Guangdong Natural Science Foundation (2017A030310370), Guangzhou Science and Technology Project (201804010205) and Guangdong Key Laboratory of Clean Energy Technology (2017B030314127) for the financial support. References
Fig. 9. The TG- DTG characterization under different oxygen carriers (N2 atmosphere, heating rate ¼ 30 C/min).
husk [16]. Therefore, TG curves decreased sharply in this range. However, the differences among these four oxygen carriers were not obvious. The third stage was from 400 C to 900 C, which was the gasification stage of CLG. This stage occur the gasification of char and the redox reaction between char and oxygen carrier. The weight loss and DTG of coprecipitation method was the highest, which indicated that oxygen carrier of coprecipitation method was more beneficial to the gasification in CLG. The results of TG-DTG characterization had also proved that coprecipitation was the best preparation method to obtain a high CLG performance. Based on the above comprehensive study of gas concentration, gas yield, LHV, XRD, carbon conversion efficiency, gasification efficiency and TG-DTG characterization, the results indicated that coprecipitation method is the optimal preparation method under the experimental condition. 4. Conclusions This paper was expected to obtain the optimal reaction characterization and reaction conditions for CLG on rice husk. The results indicated that O/C, temperature, residence time and preparation method of oxygen carrier could significantly affect the
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Please cite this article as: XiangnengJiaweiMingfengHuangWuWang, Syngas production by chemical looping gasification of rice husk using Febased oxygen carrier, , https://doi.org/10.1016/j.joei.2019.11.009
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Syngas production by chemical looping gasification of rice husk using Fe-based oxygen carrier Xiangneng Huang a, Jiawei Wu a, Mingfeng Wang a, Xiaoqian Ma b, Enchen Jiang a, Zhifeng Hu a, * a b
College of Materials and Energy, South China Agricultural University, Guangzhou 510642, China School of Electric Power, South China University of Technology, Guangzhou 510640, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 October 2019 Received in revised form 17 November 2019 Accepted 18 November 2019 Available online xxx
The chemical looping gasification (CLG) of rice husk was conducted in a fixed bed reactor to analyze the effects of the ratio of oxygen carrier to rice husk (O/C), temperature, residence time and preparation methods of Fe-based oxygen carriers. The yield of gas, H2/CO, lower heating value of syngas (LHV), conversion efficiency and performance parameters were analyzed to obtain CLG reaction characterization and optimal reaction conditions. Results showed that when O/C increased from 0.5 to 3.0, the gas production, H2/CO, CO2 yield and carbon conversion efficiency gradually increased, while the yield of H2, CO and CH4 and LHV gradually decreased. At the same time, a highest gasification efficiency was obtained when O/C was 1.5. As increasing temperature, the gas production, CO yield, carbon conversion efficiency and gasification efficiency gradually increased, while the yield of H2, CH4 and CO2, H2/CO and LHV gradually decreased. Sintering and agglomeration was obvious when the temperature was higher than 850 C. When the reaction time increased from 10 min to 60 min, the gas production, CO yield, carbon conversion efficiency and gasification efficiency gradually increased, but the yield of H2, H2/CO and LHV decreased, among which 30 min was the best reaction residence time. In addition, coprecipitation was the best preparation method among several preparation methods of oxygen carrier. Finally, O/ C of 1.5, 800 C, 30 min and coprecipitation preparation method of oxygen carrier were the optimal parameters to obtain a gasification efficiency of 26.88%, H2 content of 35.64%, syngas content of 56.40%, H2/CO ratio of 1.72 and LHV of 12.25 MJ/Nm3. © 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Chemical looping gasification Rice husk Biomass Oxygen carrier Syngas
1. Introduction Rice husk, an important by-product of rice processing [1], is a kind of low cost and abundant biomass resource. The rice husk production is about 140 million tons per year in the world [2] and about 40 million tons per year in China [3]. Due to the lack of rice husk utilization technology, a large amount of rice husk produced annually in China cannot be reasonably developed and utilized. Nowadays, more and more researchers believe that energy utilization of rice husk could reduce pollutants and increase economic benefits [4]. Chemical looping gasification (CLG) is a novel gasification technology. The needed oxygen during the gasification is provided by lattice oxygen of oxygen carrier. Through controlling the ratio of
* Corresponding author. E-mail address: [email protected] (Z. Hu).
lattice oxygen to fuel, solid fuel is gasified and converted into syngas with H2 and CO as the main components [5]. CLG has several advantages as following [6]: (1) The recycling of oxygen carrier provides oxygen for fuel gasification without additional oxygen; (2) The heat released by the oxygen carrier in the air reactor can provide heat for fuel gasification; (3) Oxygen carrier plays a catalytic role in gasification reaction; (4) CLG can reduce the emission of toxic gases such as nitrogen oxides and sulfur compounds. Oxygen carrier is the key factor and important condition of CLG [7]. It is necessary to select suitable oxygen carrier. Fe-based oxygen carrier is widely used in CLG process [8e11] because of good reactivity, stable cyclicity and low cost [12e14]. Huang et al. [15] confirmed that the Fe-based oxygen carrier used in the study could produce syngas from experimental biomass, which has a good application prospect. Qi et al. [16] analyzed the gas production in a fixed bed system and indicated that Fe2O3 increased the concentration of CO and CO2 in the CLG process. In the process of chemical
https://doi.org/10.1016/j.joei.2019.11.009 1743-9671/© 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Please cite this article as: XiangnengJiaweiMingfengHuangWuWang, Syngas production by chemical looping gasification of rice husk using Febased oxygen carrier, , https://doi.org/10.1016/j.joei.2019.11.009
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cycle, the thermal efficiency is improved by 5%e6% after utilizing iron oxide as oxygen carrier [17]. Zhu et al. [18] found that the efficient conversion of CO2 and CH4 to syngas can be achieved on the basis of reduced ferric oxygen carrier. CH4 conversion efficiency and syngas yield (syngas output/CH4 input) in the dry reforming stage reached 98.32% and 3.84, respectively, when the ratio of CH4 and CO2 is 1.00 and the iron reduction degree is 33%. Reaction conditions are also important factors affecting CLG [19]. The maximum gas production (1.22 Nm3/kg) was obtained by CLG of rice straw under the optimal ratio of steam and biomass (S/ B) of 2.8 and the reaction temperature of 900 C [20]. Hu et al. [21] conducted CLG of wheat straw with Fe-based oxygen carrier to produce syngas, which indicated that the H2/CO ratio, gas production volume, H2 þ CO concentration and carbon conversion efficiency reached the maximum under the optimal load of 60% Fe2O3, S/B ratio of 0.8 and O/C ratio of 1.0. The optimal carbon conversion efficiency and syngas production was obtained during CLG of biomass with S/B ratio of 0.8, temperature of 800 C and oxygen carrier of natural iron ore [22]. Chemical looping gasification and reforming of gaseous fuel was studied and developed extensively because of high reactivity and conversion efficiency between oxygen carrier and fuel gas [23]. At present, China has abundant solid fuels such as biomass and coal, lacking gaseous fuels such as natural gas and syngas [24]. The research on CLG of solid fuels has positive significance for energy efficient utilization and environment-friendly society. In this paper, CLG of rice husk was conducted in a fixed bed reactor to analyze the effects of the ratio of oxygen carrier to rice husk (O/C), temperature, residence time and preparation methods of Fe-based oxygen carrier. In addition, gas yield, H2/CO, LHV, conversion efficiency, SEM, XRD and TG/DTG were analyzed to obtain CLG reaction performance and the optimal reaction parameters.
and Fe2O3 with a molar ratio of Al and Fe ¼ 2:1. The third oxygen carrier is prepared by impregnation method. The chemical reagents are Al2O3 and Fe(NO3)39H2O with a molar ratio of Al and Fe ¼ 2:1. The fourth oxygen carrier is prepared by coprecipitation method. The chemical reagents are Al(NO3)39H2O and Fe(NO3)39H2O with a molar ratio of Al and Fe ¼ 2:1. After drying, calcining, pulverizing and sieving, the particle diameter of different oxygen carrier is less than 200 mm. 2.3. Experimental procedure Fig. 1 shows the reaction device of CLG. The experimental device is assembled as shown in Fig. 1. Rice husk was mixed with oxygen carrier at a mechanical mixer for 20 min in order to ensure the uniformity of samples. And then they were placed in the center of quartz tube. Before experiment, N2 was ventilated into the quartz tube for 20 min at 100 ml/min in order to maintain the inert atmosphere. When the reactor is heated to the desired temperature, the quartz tube with oxygen carrier and rice husk is loaded into the reactor for 30 min. During the experiment, N2 was ventilated into the quartz tube at 100 ml/min. After condensation, the flue gas was collected by gas collecting bag and then was measured by Gas Chromatograph.
2.4. Methods The evaluation indexes of CLG mainly include the following equations: (1) The lower heating value (QLHV,kJ/Nm3) of the gas product is calculated as follows:
QLHV ¼ 126 VCO þ 188 VCO2 þ 359 VCH4 þ 594 VC2 H4 2. Materials and methods
(1)
2.1. Biomass raw material
where, VCO , VH2 , VCH4 and VC2 H4 are respectively the volume fraction of CO, H2, CH4 and C2H4 in gas products, %.
Rice husk is provided by Qilin north farm of South China Agricultural University. The proximate analysis, elemental analysis and LHV of rice husk are shown in Table 1. After pulverizing and sieving, the particle diameter of rice husk is less than 200 mm.
(2) Gas production (Gv , Nm3/kg) is defined as the volume (standard conditions) of gas products produced by biomass (per unit mass) after gasification. The calculation is as follows:
2.2. Oxygen carrier The analytical Fe2O3, SiO2 and Al2O3 power were calcined at 900 C for 4 h. After pulverizing and sieving, the particle diameters of Fe2O3, SiO2 and Al2O3 were less than 200 mm. There are four types of preparation method for oxygen carrier in this paper. The first oxygen carrier is prepared by mechanical blending method. The chemical reagents are SiO2 and Fe2O3 with a molar ratio of Si and Fe ¼ 2:1. The second oxygen carrier is prepared by mechanical blending method. The chemical reagents are Al2O3
Proximate analysisa (wt, %)
Elemental analysisa (wt, %)
LHV (kJ/kg)
Moisture Ash Volatile Fixed carbon
C H Ob N S
13839.54
a b
On wet basis. Calculated by difference.
40.78 5.575 52.41 0.82 0.415
Vg mb
(2)
where, Vg is the volume (standard conditions) of gas product obtained by gasification, Nm3. Vg is the biomass mass in the gasification process, kg. (3) Carbon conversion efficiency (hc , %) is defined as proportion of the carbon from gaseous products to the carbon from biomass during CLG process. The calculation is as follows:
hc ¼
Table 1 The proximate analysis, elemental analysis and LHV of rice husk.
8.43 12.33 65.53 13.71
Gv ¼
12 ðVCO þ VCO2 þ VCH4 þ VC2 H4 Þ Gv n ðT1 =TÞ C%
(3)
where, n is the molar volume (standard conditions) of gas, L/mol. T1 is the temperature when the gas concentration is measured, K. C% is the carbon content in biomass. (4) Gasification efficiency (h, %) is defined as the proportion of LHV of gaseous products to LHV of biomass during CLG process. The calculation is as follows:
Please cite this article as: XiangnengJiaweiMingfengHuangWuWang, Syngas production by chemical looping gasification of rice husk using Febased oxygen carrier, , https://doi.org/10.1016/j.joei.2019.11.009
X. Huang et al. / xxx (xxxx) xxx
3
Fig. 1. The schematic diagram of biomass CLG reaction.
h¼
LHV Gv Qb
(4)
where, Qb is LHV of biomass, kJ/kg. 3. Results and analysis 3.1. The effect of oxygen carrier to material ratio on CLG This section focused on the influence of O/C on CLG performance under different O/C (0.5, 1.0, 1.5, 2.0, 2.5 and 3.0). Meanwhile, the reaction temperature was 800 C and the total amount of oxygen carrier and rice husk was 5 g for each experiment. The effect of O/C on gas content is shown in Fig. 2(a). The percentage of gaseous products increased from 29.15% to 44.43% as O/C increasing from 0.5 to 3. This was because the oxygen carrier provided lattice oxygen for rice husk to promote gasification. In addition, the oxygen carrier also provided partial catalysis to promote the thermal cracking of rice husk and tar [25]. Moreover, Fe element in oxygen carrier was beneficial to reduce the content of organic macromolecular compounds in tar [26]. The effect of O/C on gas concentration and LHV is shown in Fig. 2(b). When O/C increased from 0.5 to 3.0, the yield of H2, CO and CH4 in syngas decreased by 15.19%, 42.43% and 31.56%, respectively, while the CO2 yield increased by 48.84%. This phenomenon could be explained as following: Firstly, the increase of oxygen carrier would promote the reactions of H2 þ 3 Fe2O3 / 3 H2O þ 2 Fe3O4 and CH4 þ 4 Fe2O3 / 2 H2O þ CO2 þ 8 FeO, leading to the consumption of more H2 and CH4. Therefore, the yield of H2 and CH4 decreases gradually with the increase of oxygen carrier. Secondly, the increase of oxygen carrier will promote the reactions of CO þ 3 Fe2O3 / CO2 þ 2 Fe3O4, CO þ Fe2O3 / CO þ 2 FeO, CH4 þ 4 Fe2O3 / 2 H2O þ CO2 þ 8 FeO and C þ 2 Fe2O3 / CO2 þ 4 FeO, leading to the consumption of more CO. In other words, higher O/C provided more oxygen, converting C, CO, and CH4 to CO2. With the increase of oxygen carrier, the CO yield gradually decreased and CO2 increased. The yield of C2H4 decreased by 45.32% as O/C increasing from 0.5 to 3.0. Due to the increase of oxygen carrier, the reduction reaction of oxygen carrier was promoted, resulting in the decrease of C2H4. They were consistent with the results of literature [25]. There is a positive correlation between LHV and the yield of combustible gas (Eq. (1)). In addition, the yield of H2, CO, CH4 and
C2H4 decreased while CO2 yield increased since increasing oxygen carrier. Therefore, LHV decreased gradually. Table 2 shows the effect of O/C on H2/CO. As shown in Table 2, with the increase of O/C, H2/CO firstly increased from 1.28 to 1.58, which indicated that a series of reactions in favor of H2 generation occurred in the hydrogen production zone: 3 Fe þ 3y FeO þ y H2O / (x þ y) Fe3O4 þ y H2, CO þ H2O / CO2 þ H2, C þ 2 H2O / CO2 þ 2 H2. However, when O/C was greater than 2.0, H2/CO declined and became stably, which may be because excessive oxygen carrier promoted the reaction of CO þ 3 Fe2O3 / CO2 þ 2 Fe3O4. The effect of O/C on efficiency and gas yield is shown in Fig. 2(c). When O/C increased from 0.5 to 3, gas yield increased by 30% (from 0.270 Nm3/kg to 0.351 Nm3/kg). This is because higher O/C provided more lattice oxygen for rice husk in the reaction process. Moreover, excess oxygen carrier also provided partial catalyst effect, which promoted the cracking of rice husk and tar, resulting in the increase of gas yield. With the increase of O/C, the carbon conversion efficiency increased continuously, which was mainly due to the CLG of biomass and chemical looping combustion reaction between the addition of oxygen carrier and the syngas generated from gasification: C þ 3 Fe2O3 / CO þ 2 Fe3O4, C þ 6 Fe2O3 / CO2 þ 4 Fe3O4, C þ H2O / CO þ H2, CO þ 3 Fe2O3 / CO2 þ 2 Fe3O4, CO þ Fe2O3 / CO2 þ 2 FeO, CH4 þ 4 Fe2O3 / 2 H2O þ CO2 þ 8 FeO [27]. In other words, more Fe-based oxygen carrier not only provided more lattice oxygen for rice husk and promoted the oxidation of pyrolysis products, but also had catalytic cracking effect on tar and carbon, promoting the increase of CO2, H2, CO, CH4, C2H4 and gas yield. This is also consistent with the results of literatures [24,28,29]. As increasing O/C, the gasification efficiency increased to 28.72% (O/C ¼ 1.5) firstly and then decreased evidently. This is mainly because when O/C increased from 0.5 to 1.5, the combustible gas concentration just reduced by 8.27%, on the contrary, the gas content increased obviously by 35.12%. These dual effects increased the gasification efficiency. However, when O/C increased to 3.0, although the gas content continued to increase by 16.67%, the combustible gas concentration reduced significantly by 21.98% because excessive lattice oxygen was provided for the combustion. Moreover, LHV increased and gas production decreased with the increase of O/C. The gasification efficiency was affected by the dual effects of LHV and gas production [30]. Based on the above comprehensive study of gas concentration, gas yield, LHV, carbon conversion efficiency and gasification efficiency, the results indicated that 1.5 is the optimal O/C under the
Please cite this article as: XiangnengJiaweiMingfengHuangWuWang, Syngas production by chemical looping gasification of rice husk using Febased oxygen carrier, , https://doi.org/10.1016/j.joei.2019.11.009
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X. Huang et al. / xxx (xxxx) xxx Table 2 The effect of O/C on H2/CO.
Fig. 2. The effect of O/C on gas content, gas concentration, LHV, efficiency and gas yield.
experimental condition. 3.2. The effect of temperature on CLG Based on Section 3.1, keeping O/C ¼ 1.5, the experiments were
O/C
H2/CO
0.5 1 1.5 2 2.5 3
1.28 1.36 1.56 1.58 1.55 1.55
conducted under different temperature of 700 C, 750 C, 800 C, 850 C, 900 C and 950 C. The effect of temperature on gas content is shown in Fig. 3(a). The percentage of gaseous products increased from 24.72% to 53.82% as temperature increasing from 700 C to 950 C. In addition, when the temperature increased from 700 C to 750 C, from 750 C to 800 C, from 800 C to 850 C, from 850 C to 900 C and from 900 C to 950 C, the percentage of gaseous products increased by 3.82%, 10.37%, 6.49%, 3.32% and 5.11%, respectively. This was because high temperature would promote biomass thermal reactions, which was conducive to improving the reaction rate of pyrolysis and gasification [31,32], resulting in the increase of gaseous products. However, when the temperature was too high, sintering and agglomeration would occur on the oxygen carrier [33,34], weakening the activity of oxygen carrier. Therefore, the improvement on the percentage of gaseous products became weaken at higher temperatures. The effect of temperature on gas concentration and LHV is shown in Fig. 3(b). When the temperature increased from 700 C to 950 C, the yield of H2, CO2 and CH4 in syngas decreased by 28.17%, 13.64% and 37.36%, respectively, however the CO yield increased by 137.51%. As shown in Table 3, the ratio of H2/CO decreased with the temperature increasing. This trend could be explained as following: Firstly, the reduction reaction between carbon and oxygen carrier (C þ 3 Fe2O3 / CO þ 2 Fe3O4) and carbonization reactions (C þ H2O / CO þ H2 and C þ CO2 / 2 CO) were both endothermic reactions, leading to more CO and H2 production at higher temperature. Secondly, the reduction reactions between CO and oxygen carrier (CO þ 3 Fe2O3 / CO2 þ 2 Fe3O4 and CO þ Fe2O3 / CO2 þ 2 FeO) were exothermic reactions. High temperature would cause the reaction to proceed in the reverse direction, producing more CO and consuming more CO2. Thirdly, the reduction reaction between CH4 and oxygen carrier (CH4 þ 3 Fe2O3 / 2 H2 þ CO þ 2 Fe3O4), the reforming reaction of CH4 (CH4 þ H2O / CO þ 3 H2) and the reduction reaction between H2 and oxygen carrier (H2 þ Fe2O3 / H2O þ 2 FeO) were endothermic reactions. High temperature will promote the reaction to proceed in the positive direction, consuming more CH4, H2 and producing more CO. Therefore, as increasing the temperature, the yield of H2, CO2 and CH4 gradually decreased, while the CO yield increased. As shown in Fig. 3(b), because of the decrease of H2, CH4 and C2H4, LHV decreased with the increase of temperature, especially higher than 850 C. It indicated that high temperature (>850 C) was not conducive to the formation of high LHV gas. The effect of temperature on carbon conversion efficiency and gasification efficiency is shown in Fig. 3(c). Temperature has a significant impact on carbon conversion efficiency and gasification efficiency. When the temperature increased from 700 C to 950 C, carbon conversion efficiency and gasification efficiency increased by 130.41% and 90.84% (from 18.15% to 41.82% and from 19.55% to 37.31%), respectively. High temperature promoted the biomass pyrolysis, gasification and reduction reaction rate leading to the increase of gaseous products and the C-containing gas
Please cite this article as: XiangnengJiaweiMingfengHuangWuWang, Syngas production by chemical looping gasification of rice husk using Febased oxygen carrier, , https://doi.org/10.1016/j.joei.2019.11.009
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5 Table 3 The effect of temperature on H2/CO. Temperature ( C)
H2/CO
700 750 800 850 900 950
2.50 2.08 1.56 1.08 0.91 0.76
after CLG at 700 C, which was the main conversion (Fe2O3/ Fe3O4) of Fe-based oxygen carrier in chemical looping reaction [37]. When the temperature increased to 800 C, some Fe3O4 gradually reduced to FeO because high temperature promoted the reactions between oxygen carrier with biomass and pyrolysis products. When the temperature was 850 C, a few Fe were detected while Fe3O4 and FeO were the main component. In addition, as shown in Fig. 5, a dense layer was formed by many particles at 850 C, causing the sintering and agglomerate of oxygen carrier. However, clear and large particles were observed at 800 C. The results of XRD and SEM indicated that Fe would be reduced deep from FeO and it was liable to cause the sintering and agglomerate. When the temperature increased to 900 C and 950 C, more Fe were formed in oxygen
Fig. 3. The effect of temperature on gas content, gas concentration, LHV, carbon conversion efficiency and gasification efficiency.
concentration. Therefore, carbon conversion efficiency and gasification efficiency of CLG were improved with increasing the temperature. These results were consistent with the relevant literatures [35,36]. The XRD characterization of oxygen carrier under different temperature is shown in Fig. 4. All Fe2O3 were reduced to Fe3O4
Fig. 4. The XRD characterization of oxygen carrier under different temperature. (A: Fe3O4, B: FeO, C: Fe, D: Fe2SiO4).
Please cite this article as: XiangnengJiaweiMingfengHuangWuWang, Syngas production by chemical looping gasification of rice husk using Febased oxygen carrier, , https://doi.org/10.1016/j.joei.2019.11.009
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Fig. 5. The SEM images of solid residues under different temperature.
carrier. Moreover, few Fe2SiO4 were detected, which was also an important reason for sintering and agglomerate because of the low melting point [38]. The results exhibited the characteristics of stepwise reduction (Fe2O3 / Fe3O4 / FeO / Fe) in CLG process, which was consistent with the literature [39]. Furthermore, high temperature (850 C, 900 C and 950 C) should be avoided because sintering would affect the re-oxidation and recycling capacity of oxygen carrier. Based on the above comprehensive study of gas concentration, gas yield, LHV, XRD, SEM, carbon conversion efficiency and gasification efficiency, the results indicated that 800 C is the optimal reaction temperature under the experimental condition.
3.3. The effect of residence time on CLG Based on the above sections, the experiments were conducted under different residence time of 10 min, 20 min, 30 min, 40 min, 50 min and 60 min to obtain the best residence time when O/C was 1.5 and temperature was 800 C. The effect of residence time on gas content is shown in Fig. 6(a). It can be seen that the percentage of gaseous products increased with the increase of residence time. When the residence time increased from 10 min to 30 min, the percentage of gaseous products increased by 21.53%, which was a significant increase. This is mainly because there were sufficient reaction materials during this period, leading to a larger contact probability between oxygen carrier and rice husk. In other words, biomass pyrolysis and gasification were easier to react. When the residence time increased from 30 min to 60 min, the percentage of gaseous products slightly increased by 7.64%. It could be explained that the quantity of oxidized oxygen carrier and rice husk decreased after 30 min, resulting in the weakness of gas production capacity [36]. Therefore, the first 30 min is the main stage of CLG on rice husk. The effect of residence time on gas concentration is shown in Fig. 6(b). When the residence time increased from 10 min to 60 min, the CO yield increased gradually and increased by 26.98%, while the H2 yield decreased gradually and decreased by 11.74%. This may be because there were more char would be converted into gaseous products under longer residence time leading to more CO production. However, hydrogen in biomass was generally released during the release process of volatiles, which was the first stage of gasification. Moreover, more CO and other gaseous products were generated in the later stage, which would dilute the H2 concentration. Therefore, the H2 yield decreased as increasing the residence time. In addition, as shown in Table 4, when the residence
time increased from 10 min to 60 min, H2/CO decreased continuously and decreased by 30.64%. Unlike CO, the CO2 yield increased firstly and then decreased because the gasification of char (C þ CO2 / 2 CO) played a major role under longer residence time. According to the above analysis, under the residence time of 10 min, the percentage of gaseous products was the lowest and most of the reactions had not been completed. Therefore, 10 min was not suitable for CLG and it would not be discussed in this section. The effect of residence time on efficiency and LHV is shown in Fig. 6(c). When the residence time increased from 20 min to 60 min, the carbon conversion efficiency increased from 24.31% to 31.93%, while the gasification efficiency and LHV decreased from 30.19% to 18.90% and 14.28 MJ/Nm3 to 7.25 MJ/Nm3, respectively. There were more char would be converted into gaseous products under longer residence time, resulting in the increase of carbon conversion efficiency. However, a stable gasification equivalent ratio would be formed between the oxygen carrier and rice husk under a longer residence time so that the gasification reaction gradually tended to be stable [35]. Moreover, there were more carrier gas in the gaseous production under a longer residence time leading to the decrease of gasification efficiency and LHV. As shown in Fig. 6(c), when the residence time increased from 20 min to 30 min (increased by 10 min), the carbon conversion efficiency increases by 12.88%, while LHV and gasification efficiency decreases by 16.06% and 4.88%, respectively. However, when the residence time increased from 30 min to 40 min (increased by the same 10 min), the carbon conversion efficiency just increased by 2.02%, while LHV and gasification efficiency decreased evidently by 20.87% and 20.43%, respectively. The results revealed that the gasification performance was not reasonable when the residence time was longer than 30 min. In addition, as shown in Fig. 6(c) and Table 4, H2/CO, LHV and gasification efficiency were fairly low when the residence time were 50 min and 60 min. Based on the above comprehensive study of gas concentration, gas yield, LHV, carbon conversion efficiency and gasification efficiency, the results indicated that 30 min is the optimal residence time under the experimental condition.
3.4. The effect of oxygen carriers prepared by different methods on CLG Based on the above sections, the experiments were conducted under different preparation methods of mechanical blending method, impregnation method, coprecipitation method and SiO2 þ Fe2O3 method to obtain the optimal method when O/C,
Please cite this article as: XiangnengJiaweiMingfengHuangWuWang, Syngas production by chemical looping gasification of rice husk using Febased oxygen carrier, , https://doi.org/10.1016/j.joei.2019.11.009
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Table 4 The effect of residence time on H2/CO.
Fig. 6. The effect of residence time on gas content, gas concentration, efficiency and LHV.
temperature and residence time was 1.5, 800 C and 30 min, respectively. The effect of oxygen carriers on gas content is shown in Fig. 7(a). The gas content of SiO2 þ Fe2O3 method was the highest (32.81%), followed by coprecipitation method (29.95%) and impregnation method (26.84%). Mechanical blending method was the lowest.
Residence time (min)
H2/CO
10 20 30 40 50 60
1.73 1.66 1.56 1.48 1.34 1.20
The effect of oxygen carriers on gas concentration is shown in Fig. 7(b). The H2 yield of coprecipitation method was the highest (35.64%), followed by impregnation method (35.32%). Moreover, the CO yield of mechanical blending method was the highest (26.79%), followed by coprecipitation method (20.76%). Futhermore, the CH4 and C2H4 yield of coprecipitation method were the lowest (7.58% and 1.53%), followed by SiO2 þ Fe2O3 method (9.13% and 1.64%). In addition, the CO2 yield of impregnation method was the lowest (33.75%), followed by coprecipitation method (34.49%). As shown in Table 5, H2/CO of impregnation method was the highest (1.79), followed by coprecipitation method (1.72), which were beneficial to the synthesis of chemical products. Meanwhile, the syngas content (CO þ H2) of coprecipitation method was the highest (56.40%), followed by impregnation method (55.05%). The results indicated that coprecipitation was the best preparation method to produce syngas of high H2/CO and CO þ H2. The effect of oxygen carriers on efficiency and LHV is shown in Fig. 7(c). Carbon conversion efficiency of SiO2 þ Fe2O3 method was the highest (25.39%), followed by coprecipitation method (23.06%). Further, gasification efficiency of coprecipitation method was the highest (26.88%), followed by impregnation method (25.52%). In addition, LHV of impregnation method was the highest (13.56 MJ/ Nm3), followed by coprecipitation method (12.94 MJ/Nm3). After comprehensive analysis of carbon conversion efficiency, gasification efficiency and LHV, coprecipitation was the best preparation method to obtain a high CLG performance. The results can be explained by the analysis of Fig. 8. As shown in Fig. 8, before the reaction, Fe2O3 and Al2O3 were detected in the oxygen carriers of mechanical blending method, impregnation method and coprecipitation method. After CLG, Fe2O3 of SiO2 þ Fe2O3 method were converted to Fe3O4 and FeO. There were many Fe3O4 remaining on the oxygen carrier, which could be deeply converted to FeO. Therefore, the CO2 yield of SiO2 þ Fe2O3 method was the highest (as shown in Fig. 7(b)). Meanwhile, many Fe3O4 remaining on the oxygen carrier revealed that this method did not exert thoroughly the redox ability of oxygen carrier. After CLG, all Fe2O3 of mechanical blending method and impregnation method were converted to FeO, which revealed that the redox ability of oxygen carrier could be fully developed. However, with a low melting point, FeO was easier to sinter with residues at high temperature and affect the subsequent reduction reaction. After CLG, FeAl2O4 and few Al2O3 were detected on the oxygen carrier of coprecipitation method. This is because Fe2O3 and Al2O3 were evenly distributed and combined through coprecipitation method. Under this condition, FeO, converted from Fe2O3 after CLG, was easy to react with Al2O3 and formed FeAl2O4 crystal at high temperature. Through the coprecipitation method, all Fe3þ of oxygen carrier were converted to Fe2þ, which could exert thoroughly the redox ability. In addition, the reduced oxygen carrier formed FeAl2O4 crystal with a better thermal stability and crystal structure [40], which was conducive to subsequent reduction reactions. Therefore, as shown in Fig. 7(b) and (c), coprecipitation was the best preparation method to obtain a high CLG performance. Fig. 9 shows the TG-DTG characterization under different
Please cite this article as: XiangnengJiaweiMingfengHuangWuWang, Syngas production by chemical looping gasification of rice husk using Febased oxygen carrier, , https://doi.org/10.1016/j.joei.2019.11.009
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X. Huang et al. / xxx (xxxx) xxx Table 5 The effect of method on H2/CO and CO þ H2. O/C
H2/CO
CO þ H2 (%)
Mechanical blending Impregnation coprecipitation SiO2 þ Fe2O3
0.94 1.79 1.72 1.49
51.91 55.05 56.40 49.07
Fig. 8. XRD characterization of different oxygen carriers before and after reaction (A: Fe2O3, B: Al2O3, C: Fe3O4, D: FeO, E: FeAl2O4).
Fig. 7. The effect of oxygen carriers on gas content, gas concentration, efficiency and LHV.
oxygen carriers. The whole trends of TG-DTG characterization were similar under different oxygen carriers, which had three main stages. The first stage was from 30 C to 200 C. TG curves decreased slightly in this stage. This weight loss was because the evaporation of residual water and internal water in rice husk at about 90 C [41]. Therefore, the first stage of CLG was the drying
stage and the differences among these four oxygen carriers were not obvious. The second stage was from 200 C to 400 C, which was the pyrolysis stage of CLG. This stage was the main stage of weight loss because rice husk was rich in cellulose and lignin. Moreover, the first step of degradation was the initial decomposition of hemicellulose and cellulose (200e270 C), while the second step of degradation was the decomposition of lignin and cellulose (270e370 C) [42]. In addition, Fe2O3 promoted the pyrolysis of rice
Please cite this article as: XiangnengJiaweiMingfengHuangWuWang, Syngas production by chemical looping gasification of rice husk using Febased oxygen carrier, , https://doi.org/10.1016/j.joei.2019.11.009
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CLG performance. When O/C increased from 0.5 to 3.0, the gas production, H2/CO, the CO2 yield and carbon conversion efficiency gradually increased, while the yield of H2, CO, CH4 and LHV gradually decreased. Further, the gasification efficiency was the highest when O/C was 1.5. Moreover, high temperature was beneficial to improve gas production, carbon conversion efficiency and gasification efficiency. However, higher temperature was easier to cause sintering, which reduced the activity of oxygen carriers. This study showed that 800 C was the best temperature for gas production, LHV, high conversion efficiency, good H2/CO and strong performance of oxygen carrier. In addition, longer reaction time was also beneficial to improve gas production, carbon conversion efficiency and gasification efficiency, but 30 min was the best reaction residence time. Among SiO2 þ Fe2O3 method, mechanical blending method, impregnation method and coprecipitation method, oxygen carrier of coprecipitation method could convert and form FeAl2O4 crystal after CLG. Meanwhile, coprecipitation method was the optimal preparation method to obtain the highest CLG performance. Acknowledgements The authors are grateful to National Natural Science Foundation of China (51806068, 51706074), Guangdong Natural Science Foundation (2017A030310370), Guangzhou Science and Technology Project (201804010205) and Guangdong Key Laboratory of Clean Energy Technology (2017B030314127) for the financial support. References
Fig. 9. The TG- DTG characterization under different oxygen carriers (N2 atmosphere, heating rate ¼ 30 C/min).
husk [16]. Therefore, TG curves decreased sharply in this range. However, the differences among these four oxygen carriers were not obvious. The third stage was from 400 C to 900 C, which was the gasification stage of CLG. This stage occur the gasification of char and the redox reaction between char and oxygen carrier. The weight loss and DTG of coprecipitation method was the highest, which indicated that oxygen carrier of coprecipitation method was more beneficial to the gasification in CLG. The results of TG-DTG characterization had also proved that coprecipitation was the best preparation method to obtain a high CLG performance. Based on the above comprehensive study of gas concentration, gas yield, LHV, XRD, carbon conversion efficiency, gasification efficiency and TG-DTG characterization, the results indicated that coprecipitation method is the optimal preparation method under the experimental condition. 4. Conclusions This paper was expected to obtain the optimal reaction characterization and reaction conditions for CLG on rice husk. The results indicated that O/C, temperature, residence time and preparation method of oxygen carrier could significantly affect the
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Please cite this article as: XiangnengJiaweiMingfengHuangWuWang, Syngas production by chemical looping gasification of rice husk using Febased oxygen carrier, , https://doi.org/10.1016/j.joei.2019.11.009