Effect of the Moisture Content in Coal on the Pyrolysis Behavior in

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Effects of Moisture content in Coal on Pyrolysis Behavior in indirectly Heated Fixed-Bed Reactor with Internals Erfeng Hu, Xi Zeng, Dachao Ma, Fang Wang, Xiaojian Yi, Yuan Li, and Xiaoheng Fu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02780 • Publication Date (Web): 23 Jan 2017 Downloaded from http://pubs.acs.org on January 24, 2017

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Effects of Moisture content in Coal on Pyrolysis Behavior in indirectly Heated Fixed-Bed Reactor with Internals Erfeng Hu1, 2, Xi Zeng2*, Dachao Ma3, Fang Wang2, Xiaojian Yi4, Yuan Li5, Xiaoheng Fu1 1. School of Chemical & Environmental Engineering, China University of Mining & Technology (Beijing), Beijing, 100083, China 2. State Key Laboratory of Multi-Phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China 3. School of Environment, Guangxi University, Nanning 530004, China 4. Department of Overall Technology of NO.1 Research Institute, China North Industries Group Corporation, Beijing 100072, China 5. Department of Chemical Engineering, University of New Brunswick, Fredericton E3B 5A3, Canada *Authors to whom correspondence should be addressed. Phone/Fax: +86-10-8254-4905. E-mail: xzeng@ ipe.ac.cn (Xi Zeng).

Abstract: The fixed-bed reactor with internals has been proposed to enhance the pyrolysis performance for coal. In this study, the pyrolysis behavior of different coal moisture contents and the reaction mechanism, were investigated in an indirectly heated fixed-bed reactor with internals. The results showed that, at a furnace temperature of 900 °C, the increased coal moisture content went from 0.41 wt.% to 11.68 wt.% and significantly modified the temperature fields, thereby prolonging the pyrolysis time to reach 500 °C, and then enhancing the condensation and trapping of the coal at the bed center. Therefore, the tar yield and light tar content were raised from 9.21 wt.% and 63.7 wt.% to 10.74 wt.% and 64.5 wt.% respectively. However, when the coal moisture content exceeded 16.77 wt.%, the tar yield and light tar content decreased to 8.55 wt.% and 62.0

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wt.% respectively. In addition, the HHV (Higher heating value) of char with internals was dramatically higher than without internals, and the char HHV in reactor with internals rose primarily, then decreased with the increase in the coal moisture; meanwhile, its fixed carbon content of char showed an increase and then followed by a decline were observed. In contrast, the pyrolysis products varied slightly in the reactor without internals. Keywords：Coal pyrolysis;

Fixed-bed reactor;

Coal moisture content; Reaction mechanism

1. Introduction

The increased demand for energy and the constant depletion of oil resources have encouraged some countries to explore other alternative resources. As a high-efficiency clean technology for converting coal volatiles into oil, pyrolysis, has attracted much attention from researchers with various research backgrounds [1-4]. Up to now, many coal pyrolysis technologies have been developed over the past century, mainly including the direct-heating pyrolysis technology, such as Toscoal technology

[5]

technology [6-7], LFC technology

[12]

[8-9]

, COED technology

[10-11]

, and Encoal technology

, DG and

indirect-heating pyrolysis technology, such as W-D technology, Koppers technology [13] and MRF technology [14-15]. The heat carrier in former can be solid heat carrier, such as char or hot ash, and the gaseous heat carrier, such as pyrolysis gas or flue gas. These technologies have obvious advantages in high tar yield due to fast heating rate. However, the unstable operation and poor tar quality always limits the application in industry. On the other hand, the indirect heating


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technologies may have the disadvantages of low heating rate and tar yield, the tar produced is farily good with more light components and less dust. So the problems faced by this kind of pyrolysis technologies is how to enhance the heat transfer in reactor, and thus improve the heating rate. Based on this concept, a newly configured fixed-bed reactor mounted with internals has been proposed and developed by the institute of Process & Engineering, Chinese Academy of Science. By the special internals, the heat transfer in pyrolyzer can be enhanced, and the flow of pyrolysis gas produced is also regulated effectively to reduce the secondary reaction [16]. The results showed that the yields and qualities of tar and gas are increased significantly. This reactor was also applied to oil shale pyrolysis by Lin et al.

[17-18]

and Lai et al.

[19]

, and the results also showed that the

quality and yield can be improved. Notwithstanding, a limited work has been performed to investigate the effects of coal moisture content on the pyrolysis behavior in a fixed-bed reactor with internals. Yip et al.

[20]

reported that high coal moisture content adversely affects the energy efficiency

and the coal utilization processes. Dryden and Sparham

[21]

and Jones et al.

[22]

, investigated the

effect of water during the pyrolysis process using steam and found that the tar yield increased with the presence of steam. Hayashi et al.

[23]

reported that the absorbed water in rapid pyrolysis not

only takes part in thermochemical reactions that convert the water into hydroxyls of liquids but also suppresses the conversion of carbon into liquids. Kerbs et al. [24] suggested that in a two-stage carbonization reactor, the various amounts of additional water to the coal significantly changed the heat transfer and consequently both the primary tar yield and the composition of the primary and secondary tars. Tyler

[25]

researched the effects of brown coal moisture on pyrolysis behavior in a

small fluidized-bed reactor and indicated moisture associated with the coal having little effect on pyrolysis yields. Based on literature review, the effect of low-rank coal moisture content on


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pyrolysis behavior in existing coal utilization processes, should be an important consideration in various technologies being developed, especially those deploying reactors in which the coal experiences a long residence time

[16]

. However, considering the new configuration for the

proposed indirectly heating fixed-bed reactor with internals and thus the difference in reaction behavior with that in the existing pyrolyzer, it become necessary and essential to study the pyrolysis characteristics of coal with different moisture contents in this process. The objective of the present work is to investigate the effects of moisture content on coal pyrolysis behavior in the indirectly heated fixed-bed reactor with internals, which was also compared with the results from that without internals. By analyzing the tar components and char characteristics, the working mechanism of coal moisture content in the pyrolysis process was also discussed.

2 Experimental Section 2.1 Materials The sub-bituminous coal sample was from Yilan coal mining, located in Heilongjiang province of China, whose main characterization data were listed in Table 1. From it, one can see that the coal had a relatively high ash content and tar yield (11.8 wt.%)determined by the Cray-King assay. Prior to use, the coal sample was crushed below 5 mm, and then stored in a sealed bag. 2.2 Apparatus and methods A schematic plot for the experimental system and two types of fixed-bed reactors (Reactor A and B) are shown in the Fig.1. Both reactors were made of 304-type stainless steel with the same inner diameter of 100 mm and effective volume of 1500 mL for coal samples. Reactor A was a conventional fixed bed without internals, whereas reactor B was fitted with a central gas collection


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pipe and four metallic heating plates that were 35 mm wide and 120 mm high. The metallic plates were perpendicular to the reactor wall and at 90° to one another. Figure 2 shows the main flow patterns of gaseous pyrolysis products in reactors A and B. It is evident that the internals in reactor B can change the flow direction of pyrolysis product from pyrolysis sites to central gas collection pipe. On the contrary, in reactor A, the coal near the reactor wall became high-temperature char first, so that the pyrolysis products escaped from the char bed because of its higher gas permeability. The high-temperature char also contains some metal oxides

[26]

which catalytically

crack the pyrolysis products. Therefore, the internals in reactor B made the pyrolysis products escape from the central low-temperature coal bed, thus decreasing the secondary reaction of pyrolysis products. The pyrolysis facility and method are basically similar to that previously reported [16]. The coal samples with different moisture contents were prepared the night before and then stored in the sealed bags. Before the test, the reactor loaded with coal was connected with gas collection system. Then, the reactor was quickly put into the furnace (1) when the furnace was heated to 900 °C. The gaseous pyrolysis products were rapidly cooled down by the condenser（4） and subsequently formed liquid products in the bottle (5). The other non-condensable gaseous products were further absorbed in the three acetone absorption bottles (6-8). The scrubbed gas was metered by a wet gas meter (11) and the sampled gas was analyzed every 5 min by GC after removing moisture and sulfur species in the bottles (13-14) respectively. All the tests were terminated to ensure that the highest tar yield could be attained with the shortest reaction time when the coal bed center reached 500 °C [16].


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After every test, the warm acetone was used to wash the condenser pipe and then mixed with cool acetone solution in the bottles (6-8). Then the mixed solution was evaporated to remove acetone and further obtain the recovery tar. The tar in the bottle (5) was separated by oil-water delamination. The tar moisture was determined by the water-toluene distillation method, and the pyrolysis water referred to the tar moisture and the water separated from the bottle (5). The gas volume was recorded on the wet gas meter to calculate gas yield. Finally, the cooled char was collected to calculate the char yield. 2.3 Analysis and characterization The dehydrated tar was analyzed in a GC- MS spectrometer (Shimadza QP 2010 Ultra) to get the chemical components. The GC column temperature was first heated to 50 °C in 5 min and further to 280 °C at 6 °C/min, and then held for 10 min at 280 °C. The MS scanning range was from 20 to 900 m/z and the solvent delay time was 1.7 min. The relative content of components was calculated with the peak area percentage, i.e. the peak area proportion to total peak area. The tar was further analyzed in a simulated distillation GC (Agilent 7890) to determine the fraction distribution according to the boiling points. The noncondensable gas was analyzed using a micro GC (Agilent 3000A) to obtain the concentrations of H2, CH4, CO, CO2, C2H4, C2H6, C3H6, and C3H8. The calorific value of char was measured using an oxygen bomb calorimeter (Shanghai Jichang XRY-1B). The pyrolysis product yields (wt.%) were defined against the dry weight of coal (M), and calculated according to the following equations. Tar yield = Mtar/M

Eqn…..

(1)


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Gas yield = V*ρ/M

Eqn…..

ρ=∑ρi*Vi

Eqn…..

(2) (3)

Water yield= Mwater/M

Eqn…..

(4)

Char yield = Mchar/M

Eqn…..

(5)

where Mtar is the mass of coal tar obtained from the bottle (5) and the recovery tar. The V is the pyrolysis gas volume recorded in a wet gas meter (11), ρ is the average density based on the average composition of gas. ρi and Vi refer to the gas density and volume of individual gas species i in the gas at 273 K and 1.0 atm. Mwater is the mass of pyrolysis water separated from the bottle (5) and the tar moisture. Mchar is the mass of char collected from the reactor. The relative errors of the experiments are less than 3%.

3. Results and Discussion 3.1 Heating characteristics for coal Fig.3 compares the heating curves of coal in reactor center for both reactors under different moisture content conditions. The increased coal moisture content prolonged the pyrolysis time for the central coal to reach 500 °C in both reactors, and the pyrolysis time for the coal in reactor center to reach 500 °C in reactor B (with internals) was shorter than that of reactor A under the same conditions. When the coal moisture content was 0.38 wt.%, the coal pyrolysis times for reactor A and B were 56 min and 29 min respectively; and the coal pyrolysis times for reactor A and B rose to 67 min and 37 min respectively as the coal moisture content increased to 11.68 wt.%. The explanation is that the increased coal moisture contents, under the same reactor and constant heating temperature condition, inevitably resulted in experiencing a longer pyrolysis period and


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carrying more heat away from the reactor, which finally modified the temperature fields and therefore prolonged the pyrolysis time to obtain more heat for water evaporation. This results also showed that the coal in reactor B experienced a faster heating than that of reactor A owing to the presence of the internals. The heating plate, one part of the internals, can carry much heat from the heating furnace into the central low-temperature coal bed and facilitate the process of heat transfer. In addition, the central gas collection pipe, the other part of the internals, changed the flow direction of pyrolysis products and therefore increased the sensible heat transfer carried by the gas stream into the central low-temperature coal bed. Moreover, the increase of the moisture resulted in a longer stagnation period for coal drying at 100 °C in both reactors. For example, in reactor B, the coal dehydration time increased from 6 min to 13 min as the coal moisture content rose from 0.41 wt.% to 11.68 wt.%, whereas the dehydration time in reactor A increased from 15 min to 22 min, also suggesting the fast heating rate in the former. 3.2 Pyrolysis product characterization and working mechanism investigation Fig.4 summarizes the product distribution of coal with different moisture contents in both reactors. In reactor B (with internals), with the increase of coal moisture content, the yields of pyrolysis water and gas increased while the char yield decreased, and the tar yield increased first, then decreased. However, the pyrolysis products varied slightly in reactor A (without internals) when varying the coal moisture content. The increase of coal moisture from 0.41 wt.% to 11.68 wt.% raised the tar yield from 9.21 wt.% to 10.74 wt.% for reactor B; however, the tar yield decreased to 8.55 wt.% when the coal moisture content further increased to 16.77 wt.%. Increasing the coal moisture content (from 0.41 wt.% to 16.77 wt.%) elevated the pyrolysis water and gas yields from 8.20 wt.% and 10.02 wt.% to 11.78 wt.% and 15.18 wt.% respectively,


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accompanying a progressive decline in char yield (from 72.57 wt.% to 64.49 wt.%). Compared with reactor A, reactor B can provide a higher tar yield and lower pyrolysis gas yield under the same moisture conditions, indicating that internals facilitated the heat transfer and changed the flow direction of pyrolysis products, and therefore suppressed the secondary reaction. Similar results were also reported by Zhang et.al [16]. The increased coal moisture content modified the temperature fields in both reactors. However, owing to the presence of internals in reactor B, many volatiles can be trapped on the central low-temperature coal bed. For example, in reactor B, the increased coal moisture content from 0.41 wt.% to 11.68 wt.% raised the dehydration stagnation time at 100 °C from 6 min to 13 min, and when the former coal bed center reached 280 °C, the latter coal bed center still remained at 100 °C. Therefore, the increase of coal moisture content enhanced the condensation process and trapped more volatiles on the central low-temperature coal bed. As the central low-temperature coal samples were heated up, the condensed tar was re-evaporated and escaped from the reactor and in turn increased the tar yield. However, high coal moisture content resulted in decreasing the average pyrolysis temperature and further weakening the effect of condensation process, finally decreasing the tar yield. In contrast, the pyrolysis product flowed to the high-temperature char bed and therefore the heavy components in tar cannot be trapped in reactor A. Consequently, the increased coal moisture content caused a negligible change on pyrolysis products for reactor A. Moreover, in reactor B, the central gas collection pipe, one part of the internals adopted, changed the flow direction of gaseous products and made the gas escape from the central low-temperature coal bed. Therefore, the pyrolysis gas in reactor B can be thought as the reactive


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gas and was beneficial to increase the tar yield. Similar results have been demonstrated by Braekman et al.

[27]

, who studied the effect of using coke-oven gas on tar yield, and this results

showed that tar yield was increased and higher than in nitrogen or methane. In another study, Minkova et al. [28] found that the water not only played a vehicular role for volatiles by penetrating the porous structure of coal, but also accelerated the desorption of low molecular-weight compounds trapped within the macro-molecular network. Generally, at high temperature, increasing water produced plenty of H radicals, which enhanced the reforming reaction of heavy components in tar and inhibited the repolymerization reaction, thus elevating the tar yield

[29]

.

Based on the above discussion, the results fully indicate that appropriate increase in coal moisture content in reactor B had the advantage for raising the tar yield. Fig.5 compares the light tar fraction (boiling point below 360 °C) varying with the coal moisture content for reactors A and B. With the increase of coal moisture content, the light tar fraction decreased for reactor A, whereas the light tar fraction for reactor B increased first, then decreased. The increased coal moisture content from 0.41 wt.% to 11.68 wt.% raised the light tar fraction from 63.7 wt.% to 64.5 wt.% for reactor B, whereas further increasing the coal moisture content to 16.77 wt.% dropped the light tar fraction to 62.0 wt.%. In contrast, the light tar fraction for reactor A decreased from 63.3 wt.% to 61.7 wt.% with the increase of coal moisture content (from 0.41 wt.% to 11.68 wt.%). Based on the GC-MS spectrum data, the tar can be grouped into the ≥C20 components, C15-C20 components, C9-C14 components and ≤C8 components. Fig.6 shows the varying concentration of such tar species in terms of peak area percentage for reactors A and B with different coal moisture contents. With the increase of coal moisture content, the


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concentration of C9-C14 components decreased while the ≥C20 components concentration increased for reactor A. For reactor B, the concentration of C9-C14 components increased first, then decreased, while the ≥C20 components concentration decreased first, then increased. And other components for both reactors varied marginally. The increased coal moisture content for reactor B from 0.41 wt.% 11.68 wt.% raised the concentration of C9-C14 components from 61.63 wt.% to 69.44 wt.% while decreasing the ≥C20 components concentration from 15.00 wt.% to 6.87 wt.%. As the coal moisture content further increased to 16.77 wt.%, the concentration of C9-C14 components decreased to 57.52 wt.% while the ≥C20 components concentration increased to 20.82 wt.%. The results also showed that the concentration of C9-C14 components was higher while the ≥C20 components concentration was lower for reactor B than for reactor A under the same conditions. The GC-MS results agreed well with the simulated distillation results (shown in Figure 5), and both of those results were caused by flow pattern differences in reactors A and B. In reactor A (without internals), due to higher gas permeability of the char, the gaseous pyrolysis products flow into the char located in the outer layer that not only has the high temperature but also contains some metal oxides [26], which catalytically crack tar. Therefore, the increased coal moisture content decreased the heating rate of coal bed and prolonged the residence time of volatile precursors within the particles, and further accelerated intraparticle volatile cracking and raised the heavy tar fraction and ≥C20 components. In contrast, in reactor B, the increased coal moisture content modified the temperature fields and prolonged the residence time of central low-temperature coal bed. Due to the utilization of internals, the distribution of flow fields was changed, which caused the gaseous pyrolysis product flow from the pyrolysis sites into the central


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low-temperature coal bed. Therefore, the condensation process was enhanced and further more volatiles were trapped on the central low-temperature coal bed with the increase in coal moisture content. As the low-temperature coal was heated up gradually, the condensed components were re-evaporated and cracked into low molecular-weight compounds, and in turn increased the light tar fraction and C9-C14 components. However, excessively high moisture content of coal resulted in decreasing the average pyrolysis temperature and further weakening the effect mentioned above, finally decreasing the light tar fraction and C9-C14 components. Moreover, the increased coal moisture content prolonged the residence time of produced H radials during the reforming reactions and produced much plenty of H radicals, which not only enhanced the tar reforming reaction but also offered an important stabilizer for tar intermediate to prevent the repolymerization reaction, thus producing more light tar fraction and C9-C14 components [30-32]. Significantly, the produced pyrolysis gas used as the reactive gas for reactor B, contained H2 and CO, promoted the formation of di-aromatic compounds, which was beneficial to produce high-quality tar with many light tar fraction and C9-C14 components [33-34]. In another study, Minkova et al.

[28]

also found that the water partially depolymerized the coal network to release

more low molecular-weight components. Consequently, the results suggest that appropriate increase in coal moisture content in reactor B was beneficial to increase the light tar fraction and C9-C14 components. Fig.7 and Fig.8 show the average composition of pyrolysis gas and its calorific value respectively. The increased coal moisture content did not vary the average composition and calorific value of pyrolysis gas greatly. Compared with reactor A (without internals), reactor B can


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provide a lower H2 content and higher CH4 content, indicating that internals can suppress the secondary reaction of pyrolysis products. The HHV of pyrolysis gas for the case with internals (reactor B) was overall higher than for the case without internals (reactor A), and it agreed well with the lower H2 content in the average composition of pyrolysis gas for reactor B (shown in Fig.7).

3.3 Pyrolysis char characterization Fig.9 shows the calorific value variation of char under different coal moisture contents for reactors A and B. With the increase in coal moisture content, the HHV of char for reactor B increased first, then decreased, whereas the HHV of char varied slightly for reactor A. The char HHV was higher for reactor B than for reactor A under the same conditions. With the increase of coal moisture content from 0.41 wt.% to 11.68 wt.%, the char HHV increased from 19462.92 kJ/kg to 20405.85 kJ/kg for reactor B, whereas further increasing the coal moisture content to 16.77 wt.% dropped the light tar fraction to 19592.58 kJ/kg. Similar observation was also found by Krebs, who proposed a model of temperature fields within the coal mass to explain the effects of coal moisture content on carbon deposition formation [24]. It is postulated that the increased coal moisture content enhanced the condensation process and trapped more volatiles on the central low-temperature coal bed. As the low-temperature coal was heated up gradually, the condensed tar was re-evaporated and cracked, which resulted in depositing substantial carbon on the char surface and in turn increased the char HHV. Fig.10 shows the fixed carbon variation of char for reactors A and B under different coal moisture contents. The increased coal moisture content for reactor B raised the fixed carbon


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content first, then decreased, whereas the fixed carbon did not vary greatly for reactor A. The increase of coal moisture content from 0.41 wt.% to 11.68 wt.% elevated the fixed carbon content from 53.11 wt.% to 56.51 wt.% for reactor B, whereas further increasing the coal moisture content to 16.77 wt.% dropped the fixed carbon content to 54.22 wt.%. The results suggest that the increased coal moisture content enhanced the condensation process and deposited more carbon on the char surface, and it coincided with the char HHV results shown in Figure 8. However, excessive increase of the coal moisture content decreased average pyrolysis temperature and weakened the carbon deposition effect, finally dropping the fixed carbon content. Compared with reactor B, the reactor A provided a lower fixed carbon content. This suggests that the coal in reactor A experienced a longer pyrolysis time, which not only released more volatiles, but also accelerated the gasification process and further consumed more carbon. 4 Conclusions The effects of moisture content in coal on pyrolysis behavior in an indirectly heated fixed-bed reactor with (B) and without (A) internals were studied. The results showed that, with the increase of moisture content, both of the dehydration residence time and pyrolysis time of coal in the two reactors were prolonged. In the reactor B, for the range of 0.41 wt.% - 11.68 wt.%, the increase of coal moisture content elevated the tar yield, light tar fraction and components of C9-C14. However, further increase of the coal moisture content decreased the tar yield, light tar fraction and components of C9-C14 in reactor B. While in the reactor A, under the same conditions, the pyrolysis product varied marginally, and the H2 content was higher while the CH4 content was


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lower than that of reactor B under the same conditions. Mechanism study clarified that the increase of coal moisture content in reactor B modified the temperature fields, which enhanced the condensation process and trapped more volatiles on the central low-temperature coal bed of reactor B, thus increasing the tar yield and quality. Furthermore, pyrolysis gas in reactor B can be thought as the reactive gas and was also beneficial to increase the tar yield and quality due to internals. At high temperature, the increased coal moisture content also produced plenty of H radicals, which enhanced the reforming reaction of heavy components in tar and inhibited the repolymerization reaction, finally elevating the tar yield and quality. The water in reactor B not only played a vehicular role for volatiles by penetrating the porous structure of coal, but also accelerated the desorption of low molecular-weight compounds trapped within the macro-molecular network. However, high coal moisture content unfortunately resulted in decreasing the average pyrolysis temperature and further weakening the effect mentioned above, finally decreasing the tar yield. The results also showed that, with the increase of coal moisture content, the char HHV and fixed carbon content varied slightly in reactor A, whereas the char HHV and fixed carbon content in reactor B showed an increase, then dropped but it was higher than that of reactor A. Those suggests that internals enhanced the condensation process and trapped more volatiles on the central low-temperature coal bed, finally resulting in depositing substantial carbon on char surface.

ACKNOWLEDGEMENT The authors gratefully acknowledge the financial support provide by the National High-tech Research and Development Projects (2015AA050505-02), the Major Program of National Natural


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Science Foundation of China (U1302273), and State Scholarship Fund of China (N0. 201506430030). The deepest gratitude also goes to Professor Guangwen Xu (Institute of Process Engineering, CAS, Beijing, China), Professor Ying Zheng (University of Edinburgh, UK), and Emily Christine Montgomery (University of New Brunswick, Canada), for their recommendations.

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Energy & Fuels

Chinese]. [16] Zhang, Chun, Rongcheng Wu, and Guangwen Xu. Coal pyrolysis for high-quality tar in a fixed-bed pyrolyzer enhanced with internals [J]. Energy & Fuels, 28.1 (2013): 236-244. [17] Lin, Lanxin, et al. Pyrolysis in indirectly heated fixed bed with internals: The first application to oil shale [J]. Fuel Processing Technology. 138 (2015): 147-155. [18] Lin, Lanxin, et al. Oil shale pyrolysis in indirectly heated fixed bed with metallic plates of heating enhancement [J]. Fuel. 163 (2016): 48-55. [19] Lai, Dengguo, et al. Pyrolysis of oil shale by solid heat carrier in an innovative moving bed with internals [J]. Fuel. 159 (2015): 943-951. [20] Yip, Kongvui, Hongwei Wu, and Dong-ke Zhang. Effect of inherent moisture in collie coal during pyrolysis due to in-situ steam gasification [J]. Energy & Fuels. 21.5 (2007): 2883-2891. [21] Dryden, I. G. C., and G. A. Sparham. Carbonization of coals under gas pressure [J]. BCURA Monthly Bull. 27.1 (1963): 1-11. [22] Jones, J. F., M. R. Schmid, and R. T. Eddinger. Fluidized-bed pyrolysis of coal [J]. Chem. Eng. Prog. 60.6 (1964). [23] Hayashi, Jun-ichiro, et al. Effect of sorbed water on conversion of coal by rapid pyrolysis [J]. Energy & fuels. 13.3 (1999): 611-616. [24] Krebs, Véronique, et al. Effects of coal moisture content on carbon deposition in coke ovens [J]. Fuel. 75.8 (1996): 979-986. [25] Tyler, Ralph J. "Flash pyrolysis of coals. 1. Devolatilization of a Victorian brown coal in a small fluidized-bed reactor [J]. Fuel. 58.9 (1979): 680-686. [26] Franklin, Howard D., William A. Peters, and Jack B. Howard. Mineral matter effects on the rapid pyrolysis and hydropyrolysis of a bituminous coal. 1. Effects on yields of char, tar and light gaseous volatiles [J]. Fuel. 61.2 (1982): 155-160. [27] Braekman-Danheux, Colette, et al. Coal hydromethanolysis with coke-oven gas: 2. Influence of the coke-oven gas components on pyrolysis yields [J]. Fuel. 74.1 (1995): 17-19. [28] Minkova, Venecia, et al. Effect of water vapour on the pyrolysis of solid fuels: 1. Effect of water vapour during the pyrolysis of solid fuels on the yield and composition of the liquid products [J]. Fuel. 70.6 (1991): 713-719. [29] Hayashi, Jun-Ichiro, et al. Roles of inherent metallic species in secondary reactions of tar and char during rapid pyrolysis of brown coals in a drop-tube reactor [J]. Fuel. 81.15 (2002): 1977-1987. [30] Wang, Fang, et al. Jetting pre-oxidation fluidized bed gasification process for caking coal: Fundamentals and pilot test [J]. Applied Energy. 160 (2015): 80-87. [31] Qin, Yu-Hong, Jie Feng, and Wen-Ying Li. Formation of tar and its characterization during air–steam gasification of sawdust in a fluidized bed reactor [J]. Fuel.89.7 (2010): 1344-1347. [32] Minkova, V., et al. Effect of water vapour and biomass nature on the yield and quality of the pyrolysis products from biomass [J]. Fuel Processing Technology. 70.1 (2001): 53-61. [33] Wang, Pengfei, et al. Analysis of coal tar derived from pyrolysis at different atmospheres [J]. Fuel.104 (2013): 14-21. [34] Van der Hoeven, T. A., H. C. De Lange, and A. A. Van Steenhoven. Analysis of hydrogen-influence on tar removal by partial oxidation [J]. Fuel.85.7 (2006): 1101-1110.


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Table1 Characterization of coal used in experiments Proximate analysis (wt.%) Mad

Ad

Vdaf

FCdaf

4.61 34.97 51.83 48.17 * Determined

Ultimate analysis (daf, wt.%) C

H

N

S

O*

70.96 6.23 1.57 0.63 20.61

G-K (d, wt.%)† Tar 11.8

by element mass balance; †Tar yield from Gray-King assay test.


Water Gas Char 7.0

6.8

74.4

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Energy & Fuels

Fig.1. Schematic plots for experimental system and two reactors A and B. 1.Furnace; 2. Reactor; 3. Pressure gauge; 4. Condenser; 5. Collection bottle; 6,7,8. Acetone absorption bottle; 9. Filter; 10. Buffer bottle; 11. Vacuum pump; 12. Wet-type flow meter; 13. Sodium bicarbonate wash bottle; 14. Silica gel wash bottle; 15. Thermal couple; 16,17,18,19. Valve; 20. Sampling port; 21. Emission port. A. Conventional fixed bed reactor; B. Fixed bed reactor with internals.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig.2. Flow patterns of gaseous pyrolysis products in reactors A (without internals) and B (with internals).


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1000 Furnace

Temperature (° C)

1 800 600

0.41% Mad

3.51% Mad

7.98% Mad

11.68% Mad

Reactor B 13 min

400

6 min

200 0

0

5

10

15

20

25

30

35

40

1000 Furnace

2

Temperature (° C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

800 600

0.41% Mad

3.51% Mad

7.98% Mad

11.68% Mad

Reactor A 22 min

400

15min

200 0

0

5 10 15 20 25 30 35 40 45 50 55 60 65 70

Time (min) Fig. 3. Heating curves of coal in the reactor center in reactors A (2) and B (1).


Energy & Fuels

16 tar

water

gas

char

12

70

10

68

8 6

66

Reactor B

8

66

6

2

5

0 18

0

0 10

12

14

16

12

68

5 8

14

10

64

6

16

char

70

4

4

gas

72

64

2

water

14

72

0

tar

74

Yield (wt.%)

74

Yield (wt.%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Reactor A

4 2

0

2

4

6

8

10

0 12

Inherent moisture (wt.%)


Fig. 4. Effects of different coal moisture content on yields of pyrolysis products in reactors A and B.


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Energy & Fuels

Light tar yield (wt.%) Light tar yield (wt.%)

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65 64

Reactor B

63 62 61

64

Reactor A

63 62 61 60

0

2

4

6

8

10 12 14 16 18

Inherent moisture (wt.%) Fig. 5. Effects of different coal moisture content on light tar yields in reactors A and B.


70

70

60

60

50

C9- C14 ≥ C21

40

Reactor B

≤ C8

50

C15- C20

40 30

30

20 20 10

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70

Area percentage (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Area percentage (%)

Energy & Fuels

70

60

C9-C14

≤ C8

≥ C21

C15- C20

60 50

50 40

Reactor A

40

30

30

20

20

10

10 0 0

2

4

6

8

10

12

14

16

18

0

0 0

2

4

6

8

10

12



Fig. 6. Peak area percentage of major components in tar varying with inherent moisture in reactors A and B.


40

20

50

20

35

18

45

18

40

16

35

14

30

12

Gas composition (vol.%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Gas composition (vol.%)

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16

30

14

25

12

20

H2 CO2

CH4 C2+C3

10

CO

8

15

6

10

Reactor B

4

5

2

0

0

0

2

4

6

8

10

12

14

16

18

25 15 10

CH4 C2+C3

CO

8 6 4

Reactor A

5 0


10 H2 CO2

20

0

2

4

2 6

8

10

12

0 14


Fig.7. Effects of different coal moisture content on gas composition for coal in reactors A and B


-1

Gas HHV (MJ•kg )

24

Reactor B 22 20 18 1 0

-1

Gas HHV (MJ•kg

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

)

Energy & Fuels

Reactor A 22 20 18 1 0

0

2

4

6

8 10 12 14 16 18


Fig. 8. Effects of different coal moisture contents on HHV of pyrolysis gas in reactors A and B.


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-1

Char HHV (MJ•kg

21 20

Reactor B

19 18 17 16 1

) -1

Char HHV (MJ•kg

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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)

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20

Reactor A

19 18 17 16 1 0

0

2

4

6

8 10 12 14 16 18

Inherent moisture (wt.%) Fig. 9. Effects of different coal moisture content on HHV of char in reactors A and B.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fixed carbon (wt.%) Fixed carbon (wt.%)

Energy & Fuels

60

Reactor B

55 50 45 10 5 60

Reactor A 55 50 45 10 5 0

0

2

4

6

8 10 12 14 16 18

Inherent moisture (wt.%) Fig. 10. Effects of different coal moisture content on fixed carbon of char in reactors A and B.


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Effect of the Moisture Content in Coal on the Pyrolysis Behavior in

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