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Experimental study of cyclone pyrolysis – Suspended combustion air gasification of biomass
Accepted Manuscript Short Communication Experimental Study of Cyclone Pyrolysis - Suspended Combustion Air Gasification of Biomass Yijun Zhao, Dongdong Feng, Zhibo Zhang, Shaozeng Sun, Xinwei Zhou, Jiyi Luan, Jiangquan Wu PII: DOI: Reference:
S0960-8524(17)31161-6 http://dx.doi.org/10.1016/j.biortech.2017.07.065 BITE 18485
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
6 June 2017 11 July 2017 12 July 2017
Please cite this article as: Zhao, Y., Feng, D., Zhang, Z., Sun, S., Zhou, X., Luan, J., Wu, J., Experimental Study of Cyclone Pyrolysis - Suspended Combustion Air Gasification of Biomass, Bioresource Technology (2017), doi: http://dx.doi.org/10.1016/j.biortech.2017.07.065
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Experimental Study of Cyclone Pyrolysis - Suspended Combustion Air Gasification of Biomass Yijun Zhaoa, Dongdong Fenga,*, Zhibo Zhanga, Shaozeng Suna, Xinwei Zhoua, Jiyi Luanb, Jiangquan Wua a
School of Energy Science and Engineering, Harbin Institute of Technology, Harbin, 150001, China b
School of Mechanical Engineering, Jiamusi University, Jiamusi, 154003, China
Corresponding Author: Dongdong Feng* Email: [email protected]; Tel: +86-18746057656;
Fax: +86-451-8641-2528;
Harbin Institute of Technology, 92, West Dazhi Street, Harbin 150001, P.R. China.
Abstract: Based on the original biomass cyclone gasifier, the cyclone pyrolysis-suspension combustion gasification technology was constituted with a bottom wind ring to build the biochar suspension combustion zone. This technology decouples the biomass pyrolysis, gasification (reduction reaction) and combustion (oxidation reaction) within the same device. With the feed amount and total air fixed, the effect of air rate arrangement on temperature distribution of the gasifier, syngas components and gasification parameters was studied. With the secondary air rate (0.20) and bottom air rate (0.50), the gasification efficiency was best, with gas heating value of 5.15 MJ/Nm3, carbon conversion rate of 71.50%, gasification efficiency of 50.80% and syngas yield of 1.29 Nm3/kg. The device with biochar for the tar catalytic cracking was installed at the gasifier outlet, effectively reducing the tar content in syngas, with a minimum value of 1.02 g/Nm3. Keywords: Biomass; Air gasification; Cyclone pyrolysis; Suspended Combustion; Tar reforming.
1. Introduction Thermochemical conversion is one of the main technical approaches to the utilization of biomass energy (Dayton, 2002), including the pyrolysis of biomass macromolecules, gasification of
pyrolysis biochar, steam conversion of gaseous products, secondary cracking of liquid products, oxidation reaction of gas-solid combustible products with oxygen during gasification. In general, the thermal chemical transformation of biomass takes place at the same time and space, and the various chemical reactions that occur at each stage would be completely coupled so that the reactions are naturally end-to-end. The intermediate and final products are completely mixed under the promotion or inhibition effect of the chemical reactions and it is so difficult to be regulated for a single stage of reaction to optimize the overall conversion process. Xu et al. (Xu et al., 2009; Zhang et al., 2010; Zhang et al., 2013) proposed that decoupling ideas could be applied to the field of thermochemical conversion, separating the interrelated chemical reactions through isolation or grading patterns, thereby promoting the improvement of technical effects. Decoupling reaction refers to the pyrolysis of biomass, the reduction of biochar with H2O/CO2 and the combustion reaction of gasification biochar taking place in different regions. Based on this idea, many scholars have studied the new thermochemical conversion technology of biomass. Xiao et al.(Xiao et al., 2017) describes pine sawdust steam gasification in a novel decoupled dual loop gasification system. Zhang et al. (Zhang & Pang, 2017) investigates the correlation between the devolatilization stage and the gasification stage in a 100kw dual fluidized bed gasifier. Kraussler et al. (Kraussler et al., 2016) reports dual fluidized-bed biomass steam gasification technology is a suitable route for polygeneration process which can produce H2. Wang et al. (Wang et al., 2010) demonstrates the dual-bed gasification technology on a pilot plant for producing middle caloric fuel gas using granular coal. Based on the idea of reaction decoupling, combined with the our previous study of biomass cyclone gasification (Sun et al., 2009; Zhao et al., 2012), a composite gasification technology of cyclone pyrolysis-suspension combustion was presented in this paper. The gasification system is mainly consisted of two parts, namely the upper cyclone pyrolysis zone and the lower suspension combustion zone (biochar reduction and combustion), as shown in Fig. 1. According to the traditional cyclone gasification, the carrying air flow (primary air) with the biomass fuel would rotate
downward from the upper part of gasifier and form a cyclone flow field in the pyrolysis zone. Due to the effect of cyclone flow field, the biomass pyrolysis gas-phase product would be separated gradually from the solid-phase biochar, with a high concentration of biochar area formed in the bottom of gasifier. It makes a solid foundation for building the enhanced partition between the pyrolysis zone and the combustion zone. The main air flow (bottom air) was added to the bottom of gasifier by the bottom wind ring and the main suspending combustion zone was formed. The compound gasification technology of cyclone pyrolysis-suspension combustion strengthens the partition in the gasifier. On the one hand, after the biomass material enters the upper part of gasifier, the pyrolysis and gasification (reduction) reactions occur. According to the characteristics of cyclone flow field, the generated syngas is inverted upward from the lower part of gasifier, and is precipitated from the central exhausting pipe, as shown in Fig. 2. The main air is fed from the lower part of gasifier, with a certain distance from the end of central exhausting pipe. Oxygen (Air) is mainly consumed by the combustion of gasified biochar, thereby reducing the oxygen consumption of the gasification syngas in the upper part of gasifier. On the other hand, the upper part of suspension combustion zone is mainly for reduction reaction of the downstream biochar and the upstream carbon dioxide, while the lower part is the main combustion zone of the biochar with the main air. The increasing of CO2 concentration in the main combustion zone enhances the reduction reaction between pyrolysis biochar and carbon dioxide. In addition, in the middle of the cyclone pyrolysis - suspension combustion zones, the addition of steam can strengthen the reduction rate by the biochar-H2O interaction to improve the quality of syngas. According to the needs of the energy flow, the addition of additional oxygen and/or air could satisfy the heat need for the endothermic reaction in the upper part of gasifier. Thus, the cyclone pyrolysis-suspension combustion gasification technique actually uses a graded approach to decouple pyrolysis, gasification (reduction reaction), combustion (oxidation reaction) within the same device. It achieves the thermochemical cascade conversion of biomass.
2. Materials and Methods 2.1. Material preparation Rice husk, from the northeast of China, was used for the experimental study of cyclone pyrolysis - suspended combustion air gasification of biomass. The proximate and ultimate analyses of rice husk are listed in Table 1.
2.2. Cyclone Pyrolysis - Suspended Combustion Air Gasification The cyclone pyrolysis-suspended combustion gasifier is a self-heating system, in which the heat required for the gasification reaction is provided by the heat of its own reaction, without the need for external heat source, as shown in Fig. 2. The gasifier consists of six parts: the main body of gasifier, the air supplying system, the feeding system, the ignition system, the temperature measurement system and the tar cracking device. The furnace is cast from the mullite and the center exhausting pipe is made of 2520 stainless steel. The upper pyrolysis zone of gasifier is with an outer diameter of 340mm, the center exhaust pipe is 140mm in diameter and the total height of furnace is 3500mm. The air required for the test was provided by the air compressor. To ensure the stability of the air flow, the compressed air was pressurized by the gas tank before entering the gasifier, and divided into the primary air (carrying material particles tangential feeding), the secondary air (adjusting the gasifier temperature distribution) and the bottom air (providing enough air for the combustion of gasification biochar). Mixed with the primary air, the rice husk particles were feed tangentially into the gasifier by a screw feeder. With the draft fan at the end of the flue, the micro-negative pressure was achieved by adjusting the opening of ventilator inlet valve during the operation of the gasifier. It is the most important process for the stable operation whether the gasifier could be started simply and quickly or not. Two materials were used successively to preheat the cyclone gasification system. The lower part of gasifier is equipped with a movable grate. The wood solidification particles (diameter of 5mm, length of 20mm) are stacked on the grate. The high-temperature syngas formed
during the combustion of solidification particle would be used to preheat gasifier from bottom to top, and after combustion the ash may fall down from the voids of the grate into the hopper. When the inlet of gasifier reached the ignition temperature of biomass material, the rice husk was feed from the upper inlet and the furnace would be further preheated from top to bottom, and then adjust the air flows for the biomass gasification. Before the experiment, the biochar catalyst was added in the tar cracking device. When the gasifier was stable, the flap was rotated in 90°, so that the biochar could fall into the horizontal flue with the screen device, which could react with the syngas without changing the gas flow and gasifier pressure. There are 10 temperature measurement points arranged along the axial direction in the gasifier, numbered from T1 to T10, respectively, as shown in Fig. 2. The first point (T1) is at the same level as the upper inlet of the gasifier, and the distance between each measuring point is 150 mm. The temperature measurement point at the outlet of gasifier is T11, which is used to monitor the flue gas temperature of the gasifier and also for the tar cracking over biochar. Based on the previous investigation of biomass cyclone gasification (Sun et al., 2009; Zhao et al., 2012), the amount of biomass feeding quantity and total air flow were fixed. The effect of air rate arrangement on the gasification efficiency of rice husk during the cyclone pyrolysis - suspension combustion was studied emphatically. The operating parameters are shown in Table 2.
2.3. Sampling and analysis After the syngas was separated by the cyclone separator, the tar was trapped in four gas bottles connected in series and filled with a mixture of HPLC-grade chloroform and methanol (4:1, v/v), as shown in Fig. 2. The tar content in the syngas was determined by evaporating the solvents and water at 35 °C for 4 h (Min et al., 2011). Then, the syngas was sampled by a gas sampling bag and analyzed by a Gas Chromatograph-Mass Spectrometer (GC-MS) instrument (6890N/59751, Agilent, USA).
3. Results and discussion 3.1. Temperature distribution in gasifier The effect of bottom air rate on the temperature distribution in gasifier can be seen in Fig. 3. When the bottom air rate was 0.40-0.50, the whole temperature distribution of the gasifier showed a parabolic one with high intermediate temperature and low temperature at both ends. The biomass particles were pyrolyzed firstly in the cyclone pyrolysis zone of the gasifier. The reactivity of pyrolysis gas-phase product (volatile) with oxygen is significantly greater than that of pyrolysis biochar. In order to minimize the burning of volatile, the primary air rate (0.3) was used just to meet the ability for carrying material and the organization of cyclone flow field. The heat required for the biomass pyrolysis was mainly from the heat radiation downstream of the pyrolysis zone, so the temperature from T1 to T5 showed a gradual increase trend. It can be seen that the T1-T5 temperatures maintained at 650~900 oC in Fig. 3, meeting the needs of rice husk pyrolysis temperature (Kook et al., 2016; Makwana et al., 2015) and avoiding the slagging effectively. The secondary air vent is at the same level as the fifth temperature measurement point (T5), and a strong oxidation reaction occurred between the fifth temperature point (T5) and the seventh temperature measurement point (T7) according to the highest temperatures. When the bottom air rate was 0.40~0.50, the maximum temperature of the gasifier changed by about 20 °C. And then the temperature gradually decreased, suggesting that the suspended combustion zone did not go on a strong oxidation reaction. The possible reason is that rice husk gasification has a low reactivity of biochar and a lower burning rate in the lower oxygen atmosphere. When the bottom air rate is 0.55, the temperature distribution between T1 and T7 did not change a lot, while the temperature between T7 and T10 increased obviously. It is shown that the combustion rate of rice husk gasification in the suspended combustion zone is obviously improved under this oxygen concentration. When the bottom air rate is 0.60, the regional temperature distribution between T1 and T7 decreased obviously, which were not conducive
to the biomass pyrolysis. However, the temperature of the region between T7 and T10 showed a further increase. It can be seen that the bottom air rate of 0.55 is favorable for the combustion of rice husk in the suspended combustion zone.
3.2. Syngas components and gasification parameters The variation of the volume fraction of gas components and the calorific value of gas under different bottom air rate can be seen in Table 3. The small molecular hydrocarbons such as CH4, C2H2, C2H4 and C2H6 are mainly derived from the pyrolysis of rice husk and the secondary cracking of macromolecule hydrocarbons under high temperature. With the increase of bottom air rate, the volume fraction of CH4, C2 H2, C2H4 and C2H6 in syngas increased. The less the bottom air rate, namely the more secondary air rate, the higher the temperature of pyrolysis zone and the more the thermal cracking reaction of macromolecule hydrocarbons, which is beneficial to improve the production of pyrolysis syngas. The H2 in the biomass gasification syngas is mainly derived from the primary pyrolysis of rice husk and the secondary thermal cracking of macromolecule hydrocarbons, and the reduction reaction between the pyrolytic biochar and steam (H2O) released by the drying of biomass particles. At the temperature ≥ 800 oC, the biochar-H2O interaction can be carried out effectively (Keown et al., 2008). In this experiment, the temperatures between T5 and T7 is 800~900 oC, which is favorable for the biochar-H2O interaction. With the secondary air rate (0.20-0.30), the temperatures of the middle gasifier were higher, and the corresponding H2 content was more. When the secondary air rate is 0.10, the temperature between T1 and T7 obviously decreased, and the corresponding H2 content also decreased. As the experimental gasification agent was the air without the addition of steam, when the bottom air rate increased and the temperature of suspended combustion zone rose, the H2 content did not change. In the further studies, the steam would be added to this area to increase the H2 content. The CO was mainly from the devolatilization reaction of biomass, the reduction reaction of biochar, and the incomplete combustion of pyrolysis biochar. With the bottom air rate from 0.40 to
0.50, the temperatures of the gasification furnace between T1 and T7 were similar and the content of CO in the gas-phase product was basically the same. This region belongs to the anoxic combustion, and the combustion reaction of the gaseous product was dominant, and the share of CO from the incomplete combustion of biochar is limited. The secondary air rate in the middle of gasifier reduced from 0.30 to 0.20, which reduces the combustion fraction of CO generated from the volatiles leading to the CO content increases from 19.51% to 21.78%. When the bottom air rate increased to 0.55~0.60, the CO content in the syngas reduced and the CO2 content increased, indicating that the suspended combustion zone reacted strongly with air, which was consistent with the temperature field analysis. The heating value of the gas was mainly determined by the contents of CmHn, CO and H2, and the contents of these gases were directly affected by the temperature distribution of gasifier. Gasification temperatures (850~900 oC) for a reasonable biomass gasification temperature were both conducive to pyrolysis of the biochar gasification reaction, but also to effectively prevent fly ash slagging. In this experiment, when the secondary air rate was 0.20 and the bottom air rate was 0.50, the heating value of syngas from the rice husk gasification was 5.15 MJ/Nm3. The carbon conversion, gasification efficiency and syngas yield in the cyclone pyrolysis suspension combustion is an important indicator of the biomass air gasification. With the increase of the bottom air rate, the carbon conversion rate, gasification efficiency and syngas yield during the rice husk air gasification increased firstly and then decreased, as shown in Fig. 4. With the bottom air rate (0.50), they would reach the maximum, as 71.50%, 50.80%, 1.29Nm3/kg, respectively.
3.3. Tar content in syngas With the bottom air rate from 0.40 to 0.60, the T11 measured at the outlet of the gasifier was 707 o
C, 739 oC, 748 oC, 735 oC and 705 oC. With the decrease of the bottom air rate in the suspended
combustion zone, the tar content in the syngas decreased first and then increased, as shown in Fig. 5. The main reason is that with the bottom air rate is less than 0.50, the overall temperature of the
gasifier and the amount of tar should be basically the same. The more the secondary air was added from the middle of gasifier, the greater the likelihood that the tar molecules produced from the rice husk pyrolysis were oxidized by oxygen before being excluded from the central cylinder. The tar content in the syngas was the lowest when the bottom air rate was 0.50, and the tar content is 3.24 g/Nm³ without the biochar catalyst, as shown in Fig. 5. According to the temperature distribution of gasifier in Fig. 3, when the bottom air rate was 0.50, the reaction rate of biochar combustion in the suspended combustion zone was slower. Part of the air was too late to react with biochar, whereas promote the oxidation reaction of the tar molecules in the central exhausting pipe thus reduced the tar content, which was in consistent with the temperature distribution at the outlet of gasifier. When the bottom air rate was increased to 0.55-0.60, the temperature in the cyclone pyrolysis zone reduced and the tar yield of the biomass pyrolysis increased. At the same time, the combustion rate of rice husk gasification biochar was accelerated, and the consumption of oxygen in the suspended combustion zone was relatively complete. All of these lead to an increase in tar content. When the biochar catalyst was added for tar reforming at the outlet of the gasifier, the tar content was significantly decreased. With the bottom air rate (0.50), the tar content was the lowest at 1.02 g/Nm3 and the rate of tar removal was 68.58%.
4. Conclusions Based on the original cyclone gasifier, the bottom wind ring was equipped to form the biochar suspension combustion zone. With the feed amount and total air fixed, the effect of air rate arrangement on temperature distribution in gasifier, syngas components and gasification parameters was studied. With secondary air rate (0.20) and bottom air rate (0.50), the gasification efficiency was best, with gas heating value of 5.15 MJ/Nm3, carbon conversion rate of 71.50%, gasification efficiency of 50.80% and syngas yield of 1.29 Nm3/kg. Addition of the tar cracking device with biochar catalyst at gasifier outlet effectively reduced tar content in syngas, with a minimum value of 1.02 g/Nm3.
Acknowledgments The Collaborative Innovation Center of Clean Coal Power Plant with Poly-generation, National key R&D program of China (2016YFE0102500), the National Natural Science Foundation innovation research group Heat Transfer and Flow Control (51421063) and the CSC scholar (201606120136) are gratefully acknowledged.
References 1. Dayton, D. 2002. A review of the literature on catalytic biomass tar destruction. US DOE NREL Report Golden, CO, 510-32815. 2. Keown, D.M., Hayashi, J.-I., Li, C.-Z. 2008. Drastic changes in biomass char structure and reactivity upon contact with steam. Fuel, 87, 1127-1132. 3. Kook, J.W., Choi, H.M., Kim, B.H., Ra, H.W., Yoon, S.J., Mun, T.Y., Kim, J.H., Kim, Y.K., Lee, J.G., Seo, M.W. 2016. Gasification and tar removal characteristics of rice husk in a bubbling fluidized bed reactor. Fuel, 181, 942-950. 4. Kraussler, M., Binder, M., Schindler, P., Hofbauer, H. 2016. Hydrogen production within a polygeneration concept based on dual fluidized bed biomass steam gasification. Biomass and Bioenergy (In press). 5. Makwana, J., Joshi, A.K., Athawale, G., Singh, D., Mohanty, P. 2015. Air gasification of rice husk in bubbling fluidized bed reactor with bed heating by conventional charcoal. Bioresource technology, 178, 45-52. 6. Min, Z., Asadullah, M., Yimsiri, P., Zhang, S., Wu, H., Li, C.-Z. 2011. Catalytic reforming of tar during gasification. Part I. Steam reforming of biomass tar using ilmenite as a catalyst. Fuel, 90 , 1847-1854. 7. Sun, S., Zhao, Y., Tian, H., Ling, F., Su, F. 2009. Experimental study on cyclone air gasification of wood powder. Bioresource technology, 100, 4047-4049.
8. Wang, Y., Dong, W., Dong, L., Yue, J., Gao, S., Suda, T., Xu, G. 2010. Production of middle caloric fuel gas from coal by dual-bed gasification technology. Energy & Fuels, 24, 2985-2990. 9. Xiao, Y., Xu, S., Song, Y., Shan, Y., Wang, C., Wang, G. 2017. Biomass steam gasification for hydrogen-rich gas production in a decoupled dual loop gasification system. Fuel Processing Technology, 165, 54-61. 10. Xu, G., Murakami, T., Suda, T., Matsuzaw, Y., Tani, H. 2009. Two-stage dual fluidized bed gasification: its conception and application to biomass. Fuel Processing Technology, 90, 137-144. 11. Zhang, J., Wang, Y., Dong, L., Gao, S., Xu, G. 2010. Decoupling gasification: approach principle and technology justification. Energy & Fuels, 24, 6223-6232. 12. Zhang, J., Wu, R., Zhang, G., Yu, J., Yao, C., Wang, Y., Gao, S., Xu, G. 2013. Technical review on thermochemical conversion based on decoupling for solid carbonaceous fuels. Energy & Fuels, 27, 1951-1966. 13. Zhang, Z., Pang, S. 2017. Experimental investigation of biomass devolatilization in steam gasification in a dual fluidised bed gasifier. Fuel, 188, 628-635. 14. Zhao, Y., Sun, S., Che, H., Guo, Y., Gao, C. 2012. Characteristics of cyclone gasification of rice husk. international journal of hydrogen energy, 37, 16962-16966.
Tables Table 1. Proximate and ultimate analyses of rice husk. Proximate analysis (wt%)
Sample
Mad.
Aad.
Vad.
Ultimate analysis (wt%)
FCad.
Cad.
Rice husk 6.86 17.00 60.92 15.22
Had.
Oad.(diff)
37.35 4.40
34.05
Nad.
St,ad.
0.20 0.14
Note: ad.=air dry basis, diff.=by difference.
Table 2. Operating parameters of the experiment. Conditions
ER
1
Feeding quantity Total air flow Primary Secondary Bottom (kg/h)
(m3/h)
air rate
air rate
air rate
0.29
36.59
34.29
0.30
0.30
0.40
2
0.29
36.59
34.29
0.30
0.25
0.45
3
0.29
36.59
34.29
0.30
0.20
0.50
4
0.29
36.59
34.29
0.30
0.15
0.55
5
0.29
36.59
34.29
0.30
0.10
0.60
Table 3. Effect of bottom air rate on the syngas components. CO2
CH4 C2H4 C2H2 C2H6
CO
H2
N2
Calorie value
(%)
(%)
(%)
(%)
(%)
(%)
(%)
(%)
(MJ/Nm3)
0.40
12.38 3.19
1.31
0.12
0.17
19.51 3.91 59.41
5.11
0.45
12.42 3.07
1.21
0.11
0.17
20.07 3.89 59.07
5.07
0.50
12.85 2.91
1.16
0.10
0.12
21.78 3.89 57.21
5.15
0.55
13.14 2.73
1.11
0.09
0.09
19.81 2.60 60.43
4.62
0.60
13.24 2.61
1.03
0.08
0.07
19.12 2.49 61.36
4.41
Bottom air rate
Figures
Fig. 1. Schematic diagram of cyclone pyrolysis - suspended combustion gasification system.
Fig. 2. Schematic diagram of the experimental gasifier.
o
Temperature ( C )
900
800 Bottom air rate: 0.40 0.45 0.50 0.55 0.60
700
600 1
2
3 4 5 6 7 8 Temperature measuring point (Tx)
9
10
Fig. 3. Effect of bottom air rate on the temperature distribution.
1.35
70
1.30
65
1.25
60
Synthetic gas yield
1.20
55
1.15
50
1.10
45
Gasification efficiency
1.05
40
3
Carbon conversion rate / gasification efficiency (%)
Carbon conversion rate
Synthetic gas yield(Nm /kg)
1.40
75
1.00 0.40
0.45
0.50 Bottom air rate
0.55
0.60
Fig. 4. Effect of bottom air rate on carbon conversion, gasification efficiency and syngas yield.
3
Tar content in syngas (g/Nm )
6 5
Without biochar catalyst Tar refroming over biochar
4 3 2 1 0 0.40
0.45
0.50
0.55
0.60
Bottom air rate
Fig. 5. Tar content in syngas from the biomass gasification.
Highlights
Bottom wind ring was equipped for the biochar suspension combustion zone. The tar cracking device with biochar was installed at the gasifier outlet. Two materials were used successively to preheat the cyclone gasification system. With bottom air rate as 0.50, the gasification efficiency was the best.
S0960-8524(17)31161-6 http://dx.doi.org/10.1016/j.biortech.2017.07.065 BITE 18485
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
6 June 2017 11 July 2017 12 July 2017
Please cite this article as: Zhao, Y., Feng, D., Zhang, Z., Sun, S., Zhou, X., Luan, J., Wu, J., Experimental Study of Cyclone Pyrolysis - Suspended Combustion Air Gasification of Biomass, Bioresource Technology (2017), doi: http://dx.doi.org/10.1016/j.biortech.2017.07.065
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Experimental Study of Cyclone Pyrolysis - Suspended Combustion Air Gasification of Biomass Yijun Zhaoa, Dongdong Fenga,*, Zhibo Zhanga, Shaozeng Suna, Xinwei Zhoua, Jiyi Luanb, Jiangquan Wua a
School of Energy Science and Engineering, Harbin Institute of Technology, Harbin, 150001, China b
School of Mechanical Engineering, Jiamusi University, Jiamusi, 154003, China
Corresponding Author: Dongdong Feng* Email: [email protected]; Tel: +86-18746057656;
Fax: +86-451-8641-2528;
Harbin Institute of Technology, 92, West Dazhi Street, Harbin 150001, P.R. China.
Abstract: Based on the original biomass cyclone gasifier, the cyclone pyrolysis-suspension combustion gasification technology was constituted with a bottom wind ring to build the biochar suspension combustion zone. This technology decouples the biomass pyrolysis, gasification (reduction reaction) and combustion (oxidation reaction) within the same device. With the feed amount and total air fixed, the effect of air rate arrangement on temperature distribution of the gasifier, syngas components and gasification parameters was studied. With the secondary air rate (0.20) and bottom air rate (0.50), the gasification efficiency was best, with gas heating value of 5.15 MJ/Nm3, carbon conversion rate of 71.50%, gasification efficiency of 50.80% and syngas yield of 1.29 Nm3/kg. The device with biochar for the tar catalytic cracking was installed at the gasifier outlet, effectively reducing the tar content in syngas, with a minimum value of 1.02 g/Nm3. Keywords: Biomass; Air gasification; Cyclone pyrolysis; Suspended Combustion; Tar reforming.
1. Introduction Thermochemical conversion is one of the main technical approaches to the utilization of biomass energy (Dayton, 2002), including the pyrolysis of biomass macromolecules, gasification of
pyrolysis biochar, steam conversion of gaseous products, secondary cracking of liquid products, oxidation reaction of gas-solid combustible products with oxygen during gasification. In general, the thermal chemical transformation of biomass takes place at the same time and space, and the various chemical reactions that occur at each stage would be completely coupled so that the reactions are naturally end-to-end. The intermediate and final products are completely mixed under the promotion or inhibition effect of the chemical reactions and it is so difficult to be regulated for a single stage of reaction to optimize the overall conversion process. Xu et al. (Xu et al., 2009; Zhang et al., 2010; Zhang et al., 2013) proposed that decoupling ideas could be applied to the field of thermochemical conversion, separating the interrelated chemical reactions through isolation or grading patterns, thereby promoting the improvement of technical effects. Decoupling reaction refers to the pyrolysis of biomass, the reduction of biochar with H2O/CO2 and the combustion reaction of gasification biochar taking place in different regions. Based on this idea, many scholars have studied the new thermochemical conversion technology of biomass. Xiao et al.(Xiao et al., 2017) describes pine sawdust steam gasification in a novel decoupled dual loop gasification system. Zhang et al. (Zhang & Pang, 2017) investigates the correlation between the devolatilization stage and the gasification stage in a 100kw dual fluidized bed gasifier. Kraussler et al. (Kraussler et al., 2016) reports dual fluidized-bed biomass steam gasification technology is a suitable route for polygeneration process which can produce H2. Wang et al. (Wang et al., 2010) demonstrates the dual-bed gasification technology on a pilot plant for producing middle caloric fuel gas using granular coal. Based on the idea of reaction decoupling, combined with the our previous study of biomass cyclone gasification (Sun et al., 2009; Zhao et al., 2012), a composite gasification technology of cyclone pyrolysis-suspension combustion was presented in this paper. The gasification system is mainly consisted of two parts, namely the upper cyclone pyrolysis zone and the lower suspension combustion zone (biochar reduction and combustion), as shown in Fig. 1. According to the traditional cyclone gasification, the carrying air flow (primary air) with the biomass fuel would rotate
downward from the upper part of gasifier and form a cyclone flow field in the pyrolysis zone. Due to the effect of cyclone flow field, the biomass pyrolysis gas-phase product would be separated gradually from the solid-phase biochar, with a high concentration of biochar area formed in the bottom of gasifier. It makes a solid foundation for building the enhanced partition between the pyrolysis zone and the combustion zone. The main air flow (bottom air) was added to the bottom of gasifier by the bottom wind ring and the main suspending combustion zone was formed. The compound gasification technology of cyclone pyrolysis-suspension combustion strengthens the partition in the gasifier. On the one hand, after the biomass material enters the upper part of gasifier, the pyrolysis and gasification (reduction) reactions occur. According to the characteristics of cyclone flow field, the generated syngas is inverted upward from the lower part of gasifier, and is precipitated from the central exhausting pipe, as shown in Fig. 2. The main air is fed from the lower part of gasifier, with a certain distance from the end of central exhausting pipe. Oxygen (Air) is mainly consumed by the combustion of gasified biochar, thereby reducing the oxygen consumption of the gasification syngas in the upper part of gasifier. On the other hand, the upper part of suspension combustion zone is mainly for reduction reaction of the downstream biochar and the upstream carbon dioxide, while the lower part is the main combustion zone of the biochar with the main air. The increasing of CO2 concentration in the main combustion zone enhances the reduction reaction between pyrolysis biochar and carbon dioxide. In addition, in the middle of the cyclone pyrolysis - suspension combustion zones, the addition of steam can strengthen the reduction rate by the biochar-H2O interaction to improve the quality of syngas. According to the needs of the energy flow, the addition of additional oxygen and/or air could satisfy the heat need for the endothermic reaction in the upper part of gasifier. Thus, the cyclone pyrolysis-suspension combustion gasification technique actually uses a graded approach to decouple pyrolysis, gasification (reduction reaction), combustion (oxidation reaction) within the same device. It achieves the thermochemical cascade conversion of biomass.
2. Materials and Methods 2.1. Material preparation Rice husk, from the northeast of China, was used for the experimental study of cyclone pyrolysis - suspended combustion air gasification of biomass. The proximate and ultimate analyses of rice husk are listed in Table 1.
2.2. Cyclone Pyrolysis - Suspended Combustion Air Gasification The cyclone pyrolysis-suspended combustion gasifier is a self-heating system, in which the heat required for the gasification reaction is provided by the heat of its own reaction, without the need for external heat source, as shown in Fig. 2. The gasifier consists of six parts: the main body of gasifier, the air supplying system, the feeding system, the ignition system, the temperature measurement system and the tar cracking device. The furnace is cast from the mullite and the center exhausting pipe is made of 2520 stainless steel. The upper pyrolysis zone of gasifier is with an outer diameter of 340mm, the center exhaust pipe is 140mm in diameter and the total height of furnace is 3500mm. The air required for the test was provided by the air compressor. To ensure the stability of the air flow, the compressed air was pressurized by the gas tank before entering the gasifier, and divided into the primary air (carrying material particles tangential feeding), the secondary air (adjusting the gasifier temperature distribution) and the bottom air (providing enough air for the combustion of gasification biochar). Mixed with the primary air, the rice husk particles were feed tangentially into the gasifier by a screw feeder. With the draft fan at the end of the flue, the micro-negative pressure was achieved by adjusting the opening of ventilator inlet valve during the operation of the gasifier. It is the most important process for the stable operation whether the gasifier could be started simply and quickly or not. Two materials were used successively to preheat the cyclone gasification system. The lower part of gasifier is equipped with a movable grate. The wood solidification particles (diameter of 5mm, length of 20mm) are stacked on the grate. The high-temperature syngas formed
during the combustion of solidification particle would be used to preheat gasifier from bottom to top, and after combustion the ash may fall down from the voids of the grate into the hopper. When the inlet of gasifier reached the ignition temperature of biomass material, the rice husk was feed from the upper inlet and the furnace would be further preheated from top to bottom, and then adjust the air flows for the biomass gasification. Before the experiment, the biochar catalyst was added in the tar cracking device. When the gasifier was stable, the flap was rotated in 90°, so that the biochar could fall into the horizontal flue with the screen device, which could react with the syngas without changing the gas flow and gasifier pressure. There are 10 temperature measurement points arranged along the axial direction in the gasifier, numbered from T1 to T10, respectively, as shown in Fig. 2. The first point (T1) is at the same level as the upper inlet of the gasifier, and the distance between each measuring point is 150 mm. The temperature measurement point at the outlet of gasifier is T11, which is used to monitor the flue gas temperature of the gasifier and also for the tar cracking over biochar. Based on the previous investigation of biomass cyclone gasification (Sun et al., 2009; Zhao et al., 2012), the amount of biomass feeding quantity and total air flow were fixed. The effect of air rate arrangement on the gasification efficiency of rice husk during the cyclone pyrolysis - suspension combustion was studied emphatically. The operating parameters are shown in Table 2.
2.3. Sampling and analysis After the syngas was separated by the cyclone separator, the tar was trapped in four gas bottles connected in series and filled with a mixture of HPLC-grade chloroform and methanol (4:1, v/v), as shown in Fig. 2. The tar content in the syngas was determined by evaporating the solvents and water at 35 °C for 4 h (Min et al., 2011). Then, the syngas was sampled by a gas sampling bag and analyzed by a Gas Chromatograph-Mass Spectrometer (GC-MS) instrument (6890N/59751, Agilent, USA).
3. Results and discussion 3.1. Temperature distribution in gasifier The effect of bottom air rate on the temperature distribution in gasifier can be seen in Fig. 3. When the bottom air rate was 0.40-0.50, the whole temperature distribution of the gasifier showed a parabolic one with high intermediate temperature and low temperature at both ends. The biomass particles were pyrolyzed firstly in the cyclone pyrolysis zone of the gasifier. The reactivity of pyrolysis gas-phase product (volatile) with oxygen is significantly greater than that of pyrolysis biochar. In order to minimize the burning of volatile, the primary air rate (0.3) was used just to meet the ability for carrying material and the organization of cyclone flow field. The heat required for the biomass pyrolysis was mainly from the heat radiation downstream of the pyrolysis zone, so the temperature from T1 to T5 showed a gradual increase trend. It can be seen that the T1-T5 temperatures maintained at 650~900 oC in Fig. 3, meeting the needs of rice husk pyrolysis temperature (Kook et al., 2016; Makwana et al., 2015) and avoiding the slagging effectively. The secondary air vent is at the same level as the fifth temperature measurement point (T5), and a strong oxidation reaction occurred between the fifth temperature point (T5) and the seventh temperature measurement point (T7) according to the highest temperatures. When the bottom air rate was 0.40~0.50, the maximum temperature of the gasifier changed by about 20 °C. And then the temperature gradually decreased, suggesting that the suspended combustion zone did not go on a strong oxidation reaction. The possible reason is that rice husk gasification has a low reactivity of biochar and a lower burning rate in the lower oxygen atmosphere. When the bottom air rate is 0.55, the temperature distribution between T1 and T7 did not change a lot, while the temperature between T7 and T10 increased obviously. It is shown that the combustion rate of rice husk gasification in the suspended combustion zone is obviously improved under this oxygen concentration. When the bottom air rate is 0.60, the regional temperature distribution between T1 and T7 decreased obviously, which were not conducive
to the biomass pyrolysis. However, the temperature of the region between T7 and T10 showed a further increase. It can be seen that the bottom air rate of 0.55 is favorable for the combustion of rice husk in the suspended combustion zone.
3.2. Syngas components and gasification parameters The variation of the volume fraction of gas components and the calorific value of gas under different bottom air rate can be seen in Table 3. The small molecular hydrocarbons such as CH4, C2H2, C2H4 and C2H6 are mainly derived from the pyrolysis of rice husk and the secondary cracking of macromolecule hydrocarbons under high temperature. With the increase of bottom air rate, the volume fraction of CH4, C2 H2, C2H4 and C2H6 in syngas increased. The less the bottom air rate, namely the more secondary air rate, the higher the temperature of pyrolysis zone and the more the thermal cracking reaction of macromolecule hydrocarbons, which is beneficial to improve the production of pyrolysis syngas. The H2 in the biomass gasification syngas is mainly derived from the primary pyrolysis of rice husk and the secondary thermal cracking of macromolecule hydrocarbons, and the reduction reaction between the pyrolytic biochar and steam (H2O) released by the drying of biomass particles. At the temperature ≥ 800 oC, the biochar-H2O interaction can be carried out effectively (Keown et al., 2008). In this experiment, the temperatures between T5 and T7 is 800~900 oC, which is favorable for the biochar-H2O interaction. With the secondary air rate (0.20-0.30), the temperatures of the middle gasifier were higher, and the corresponding H2 content was more. When the secondary air rate is 0.10, the temperature between T1 and T7 obviously decreased, and the corresponding H2 content also decreased. As the experimental gasification agent was the air without the addition of steam, when the bottom air rate increased and the temperature of suspended combustion zone rose, the H2 content did not change. In the further studies, the steam would be added to this area to increase the H2 content. The CO was mainly from the devolatilization reaction of biomass, the reduction reaction of biochar, and the incomplete combustion of pyrolysis biochar. With the bottom air rate from 0.40 to
0.50, the temperatures of the gasification furnace between T1 and T7 were similar and the content of CO in the gas-phase product was basically the same. This region belongs to the anoxic combustion, and the combustion reaction of the gaseous product was dominant, and the share of CO from the incomplete combustion of biochar is limited. The secondary air rate in the middle of gasifier reduced from 0.30 to 0.20, which reduces the combustion fraction of CO generated from the volatiles leading to the CO content increases from 19.51% to 21.78%. When the bottom air rate increased to 0.55~0.60, the CO content in the syngas reduced and the CO2 content increased, indicating that the suspended combustion zone reacted strongly with air, which was consistent with the temperature field analysis. The heating value of the gas was mainly determined by the contents of CmHn, CO and H2, and the contents of these gases were directly affected by the temperature distribution of gasifier. Gasification temperatures (850~900 oC) for a reasonable biomass gasification temperature were both conducive to pyrolysis of the biochar gasification reaction, but also to effectively prevent fly ash slagging. In this experiment, when the secondary air rate was 0.20 and the bottom air rate was 0.50, the heating value of syngas from the rice husk gasification was 5.15 MJ/Nm3. The carbon conversion, gasification efficiency and syngas yield in the cyclone pyrolysis suspension combustion is an important indicator of the biomass air gasification. With the increase of the bottom air rate, the carbon conversion rate, gasification efficiency and syngas yield during the rice husk air gasification increased firstly and then decreased, as shown in Fig. 4. With the bottom air rate (0.50), they would reach the maximum, as 71.50%, 50.80%, 1.29Nm3/kg, respectively.
3.3. Tar content in syngas With the bottom air rate from 0.40 to 0.60, the T11 measured at the outlet of the gasifier was 707 o
C, 739 oC, 748 oC, 735 oC and 705 oC. With the decrease of the bottom air rate in the suspended
combustion zone, the tar content in the syngas decreased first and then increased, as shown in Fig. 5. The main reason is that with the bottom air rate is less than 0.50, the overall temperature of the
gasifier and the amount of tar should be basically the same. The more the secondary air was added from the middle of gasifier, the greater the likelihood that the tar molecules produced from the rice husk pyrolysis were oxidized by oxygen before being excluded from the central cylinder. The tar content in the syngas was the lowest when the bottom air rate was 0.50, and the tar content is 3.24 g/Nm³ without the biochar catalyst, as shown in Fig. 5. According to the temperature distribution of gasifier in Fig. 3, when the bottom air rate was 0.50, the reaction rate of biochar combustion in the suspended combustion zone was slower. Part of the air was too late to react with biochar, whereas promote the oxidation reaction of the tar molecules in the central exhausting pipe thus reduced the tar content, which was in consistent with the temperature distribution at the outlet of gasifier. When the bottom air rate was increased to 0.55-0.60, the temperature in the cyclone pyrolysis zone reduced and the tar yield of the biomass pyrolysis increased. At the same time, the combustion rate of rice husk gasification biochar was accelerated, and the consumption of oxygen in the suspended combustion zone was relatively complete. All of these lead to an increase in tar content. When the biochar catalyst was added for tar reforming at the outlet of the gasifier, the tar content was significantly decreased. With the bottom air rate (0.50), the tar content was the lowest at 1.02 g/Nm3 and the rate of tar removal was 68.58%.
4. Conclusions Based on the original cyclone gasifier, the bottom wind ring was equipped to form the biochar suspension combustion zone. With the feed amount and total air fixed, the effect of air rate arrangement on temperature distribution in gasifier, syngas components and gasification parameters was studied. With secondary air rate (0.20) and bottom air rate (0.50), the gasification efficiency was best, with gas heating value of 5.15 MJ/Nm3, carbon conversion rate of 71.50%, gasification efficiency of 50.80% and syngas yield of 1.29 Nm3/kg. Addition of the tar cracking device with biochar catalyst at gasifier outlet effectively reduced tar content in syngas, with a minimum value of 1.02 g/Nm3.
Acknowledgments The Collaborative Innovation Center of Clean Coal Power Plant with Poly-generation, National key R&D program of China (2016YFE0102500), the National Natural Science Foundation innovation research group Heat Transfer and Flow Control (51421063) and the CSC scholar (201606120136) are gratefully acknowledged.
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Tables Table 1. Proximate and ultimate analyses of rice husk. Proximate analysis (wt%)
Sample
Mad.
Aad.
Vad.
Ultimate analysis (wt%)
FCad.
Cad.
Rice husk 6.86 17.00 60.92 15.22
Had.
Oad.(diff)
37.35 4.40
34.05
Nad.
St,ad.
0.20 0.14
Note: ad.=air dry basis, diff.=by difference.
Table 2. Operating parameters of the experiment. Conditions
ER
1
Feeding quantity Total air flow Primary Secondary Bottom (kg/h)
(m3/h)
air rate
air rate
air rate
0.29
36.59
34.29
0.30
0.30
0.40
2
0.29
36.59
34.29
0.30
0.25
0.45
3
0.29
36.59
34.29
0.30
0.20
0.50
4
0.29
36.59
34.29
0.30
0.15
0.55
5
0.29
36.59
34.29
0.30
0.10
0.60
Table 3. Effect of bottom air rate on the syngas components. CO2
CH4 C2H4 C2H2 C2H6
CO
H2
N2
Calorie value
(%)
(%)
(%)
(%)
(%)
(%)
(%)
(%)
(MJ/Nm3)
0.40
12.38 3.19
1.31
0.12
0.17
19.51 3.91 59.41
5.11
0.45
12.42 3.07
1.21
0.11
0.17
20.07 3.89 59.07
5.07
0.50
12.85 2.91
1.16
0.10
0.12
21.78 3.89 57.21
5.15
0.55
13.14 2.73
1.11
0.09
0.09
19.81 2.60 60.43
4.62
0.60
13.24 2.61
1.03
0.08
0.07
19.12 2.49 61.36
4.41
Bottom air rate
Figures
Fig. 1. Schematic diagram of cyclone pyrolysis - suspended combustion gasification system.
Fig. 2. Schematic diagram of the experimental gasifier.
o
Temperature ( C )
900
800 Bottom air rate: 0.40 0.45 0.50 0.55 0.60
700
600 1
2
3 4 5 6 7 8 Temperature measuring point (Tx)
9
10
Fig. 3. Effect of bottom air rate on the temperature distribution.
1.35
70
1.30
65
1.25
60
Synthetic gas yield
1.20
55
1.15
50
1.10
45
Gasification efficiency
1.05
40
3
Carbon conversion rate / gasification efficiency (%)
Carbon conversion rate
Synthetic gas yield(Nm /kg)
1.40
75
1.00 0.40
0.45
0.50 Bottom air rate
0.55
0.60
Fig. 4. Effect of bottom air rate on carbon conversion, gasification efficiency and syngas yield.
3
Tar content in syngas (g/Nm )
6 5
Without biochar catalyst Tar refroming over biochar
4 3 2 1 0 0.40
0.45
0.50
0.55
0.60
Bottom air rate
Fig. 5. Tar content in syngas from the biomass gasification.
Highlights
Bottom wind ring was equipped for the biochar suspension combustion zone. The tar cracking device with biochar was installed at the gasifier outlet. Two materials were used successively to preheat the cyclone gasification system. With bottom air rate as 0.50, the gasification efficiency was the best.