- Email: [email protected]
Coal pyrolysis and its mechanism in indirectly heated fixed-bed with metallic heating plate enhancement
Fuel 185 (2016) 656–662
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Coal pyrolysis and its mechanism in indirectly heated fixed-bed with metallic heating plate enhancement Erfeng Hu a,b,c, Chuanqiang Zhu d, Kyle Rogers c, Xue Han c, Jie Wang a, Jing Zhao a, Xiaoheng Fu a,⇑ a
School of Chemical and Environmental Engineering, China University of Mining & Technology (Beijing), Beijing 100083, China State Key Laboratory of Multi-Phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China c Department of Chemical Engineering, University of New Brunswick, Fredericton E3B 5A3, Canada d China Everbright International Limited, Shenzhen 518000, China b
h i g h l i g h t s The tar yield and light fraction increased then decreased when increasing the number of plates. Working mechanism of metallic plates was proposed and verified by an in-house design. An excessive number of plates resulted in aggravating the cracking of primary products.
a r t i c l e
i n f o
Article history: Received 8 May 2016 Received in revised form 29 June 2016 Accepted 28 July 2016 Available online 11 August 2016 Keywords: Coal pyrolysis Metallic heating plates Secondary reaction Reaction mechanism
a b s t r a c t A metallic plate has been devised to enhance the heat transfer; this in turn increases the yield and quality of coal tar (Zhang et al., 2013). This work is devoted to investigating the pyrolysis of Yilan subbituminous coal and developing a working mechanism for an indirectly heated fixed bed reactor with different number of metallic plates. Increasing the number of metallic heating plates enhanced the heat transfer from the high-temperature reactor wall to the low-temperature central coal layer and thereby shortened the reaction time. Meanwhile, the yields of tar and light fraction were increased then decreased. The results of the pressure drop experiment demonstrated that the metallic plates weakened dense stacks of coal particles and raised the particle interstices, and therefore lowered the gas diffusion resistance. It verified that many gaseous pyrolysis products escaped from the central low-temperature coal layer, indicating the suppressed secondary reactions to the primary products. When the number of plates was 8, the correspondingly added high-temperature surface was 113%; it achieved the optimal matching between the secondary reactions of pyrolysis products and the fields of the temperature as well as the gas flow inside the reactor, and it also had the highest tar yield (6.60 wt.% dry basis) and the maximum content of light tar (below the boiling points of 360 °C) of 76.4 wt.% and C4–C9 components of 43.52 wt.%. The BET results showed that the char had the biggest surface area and volume for this case with 8 plates. Consequently, adding suitable metallic plates into the indirectly heated fixed bed reactor obviously enhances the heat transfer and also achieves higher yield and quality of tar. Ó 2016 Published by Elsevier Ltd.
1. Introduction Due to large quantities of low-rank coal resources but limited reserves of oil and gas, advanced coal utilization technologies have attracted more and more attention from researchers. Pyrolysis is the critical core technology for cascade utilization of low-rank coal; it is becoming a promising research focus for energy systems, especially in China.
⇑ Corresponding author. E-mail address: [email protected] (X. Fu). http://dx.doi.org/10.1016/j.fuel.2016.07.115 0016-2361/Ó 2016 Published by Elsevier Ltd.
A large number of fundamental and technical studies of coal pyrolysis have been done for decades. Many fundamental aspects, including the coal rank [2], coal particle diameter [3,4], coal mineral composition and content [5], pyrolysis atmosphere [6], as well as the heating rate [7,8], coal composition [9,10], and operating conditions [11–14], were extensively investigated. In the technical aspects, mostly the existing coal pyrolysis technologies based on solid heat transfer (char or hot ash), such as Toscoal [15]and DG [16,17], have been examined. There are also ones using gaseous heat carrier, such as COED [18,19], Encoal [20] and LFC [21,22]. There are drawbacks, however, that include complex process,
657
E. Hu et al. / Fuel 185 (2016) 656–662
unstable operation and especially poor oil quality in those technologies, which limit their expansion. Because of no heating medium, relatively simple process and good oil quality, indirect heating technology may be a good way to upgrade low rank coal if its low heat transfer efficiency can be solved. Elevating the heating temperature, accelerating the heating speed or reducing the material particle size accelerates the heat transfer. Khan [23] reported that in an indirectly heated fixedbed reactor, the tar yield increased when the heating temperature rose from 500 °C to 649 °C, whereas the tar yield decreased with increased heating temperature above 650 °C [1]. Peters and Bertling [24] found the tar yield increased when the heating rate was raised. Hu et al. [25] found fine particles facilitated heat transfer but aggravated secondary reactions, resulting from the low gas permeability. Zhang et al. [1] found the metallic plate accelerated the heat transfer and raised the tar yield in fixed-bed reactors. Lin et al. [26] determined that appropriately increasing the number of metallic plates reinforced the heating transfer of oil shale and also increased yield and quality of shale oil in a newly configured reactor with internals. Nonetheless, little work has been done to evaluate how the number of metallic heating plates in coal pyrolysis would affect the pyrolysis products. What is more, the investigation of the working mechanism is absolutely necessary for the coal pyrolysis process. This study aims to investigate the coal pyrolysis behaviors in an indirectly heated fixed-bed reactor with different number of metallic plates, and the components of tar and microcharacteristics of char are analyzed. In addition, the working mechanism of plates is also studied and verified by an in-house design. 2. Experimental 2.1. Materials The sub-bituminous coal sample from Yilan, Heilongjiang, China, was crushed into sizes below 5 mm for tests and stored in a sealed bag for later use. The main property data for the coal sample are listed in Table 1. The coal had high ash content, and its volatile content was about 27 wt.%. The Gray-King gave a tar yield of 7.88 wt.% against dry coal mass. 2.2. Apparatus and procedure The left inset of Fig. 1 exhibits the fixed-bed reactor and metallic plates used in this paper. They were both made of 304-type stainless steel. The right inset of Fig. 1 presents a schematic diagram of the test system. It consisted mainly of an electric furnace (1), a reactor (2), a pressure gauge (3), a cooling and adsorption system (4–8), a filter (9), a vacuum pump (11), a sodium bicarbonate washing bottle for removing sulphureted hydrogen (13), and a silica gel bottle (14). Cotton wool was used to insulate both reactor and furnace. The reactor was 45 mm in inner diameter and its total heating surface area which was provided by reactor heating wall was 254 cm2 for coal samples. The metallic plates were perpendicular to the reactor wall and mutually at the same angle. The plate was 15 mm in width and 120 mm in height. As shown in Table 2,
corresponding to the number of 0, 4, 8 and 12 of the added metallic plates, the added high-temperature heating surface reached 0, 57%, 113% and 170% of the reactor heating wall, and those were denoted as a, b, c and d respectively. The results of the previous study have demonstrated that Yilan coal had the highest yield of tar and gas when the central temperature reached 500 °C [1]. Consequently, each test was ended when the same central position reached 500 °C. As illustrated in Fig. 1, coal was fed into the reactor and then the reactor was connected with gas collection and purification at the beginning of the experiment; at the end of experiments, cooled char was weighed to calculate the yield. The product in the collection bottle (5) was also weighed, and then the water in the bottle (5) was poured into a clean beaker. The pyrolysis products attached to the condenser (4) were washed by acetone, and then mixed with acetone solution from the bottles (6, 7, 8). The acetone in the solution from the bottles (6, 7, 8) was removed by a vacuum rotary evaporator. The recovery tar was mixed with the preceding tar in the collection bottle (5). The moisture content in tar mixture was further determined and water-free tar yield was obtained by using toluene as azeotrope according to the standard method. The moisture contained in tar mixture and the preceding water poured into a clean beaker were calculated as the pyrolysis water. The gas volume was recorded on a wet gas meter to calculate pyrolysis gas yield. The experiments were repeated and the relative errors were less than 3%. Experimental procedures were substantially similar to those of Zhang [1]. This study also measured the pressure drop across a char or a coal bed in a N2 flow of different fluxes. A quartz tube with a sintered orifice was used as gas distributor and was 40 mm in diameter and 400 mm in height. A total of 130 mm in height of coal or char was loaded into the tube to measure the pressure drop across the material bed at different N2 fluxes (see Fig. 2). Then the metallic plates were loaded in the tube with coal or char and measurement of the pressure drop was conducted by varying the number of plates. 2.3. Products analysis The micro GC (Agilent 3000 A) detected the composition of sampled non-condensable gases. Tar in experiments was removed in water and collected into a vial. Then, the dehydrated tar entered the micro GC (Agilent 7890), which can determine fraction distribution for diverse boiling points. The light tar consisted of those components with boiling points below 360 °C. The composition of dehydrated tar was also measured by using a GC–MS spectrometer (Shimadzu QP 2010 Ultra). The injector and detector reached a temperature of 280 °C, and the column of GC was heated to 50 °C in 5 min and further to 280 °C at 6 °C/min. Finally the column was kept at 280 °C for 10 min. The scanning range was from 20 to 900 m/z and the delay time of solvent was 1.7 min. The relative content of components was evaluated with the peak area percentage, i.e., the peak area proportion to total peak area. The calorific value of char was measured with a Shanghai Jichang XRY-1B oxygen bomb calorimeter. The char surface morphology was examined by an atomic force microscope (AFM) equipped with a scanning electron microscope (SEM JSM-6700 F,
Table 1 Proximate and ultimate analyses for the tested Yilan coal. Proximate analysis (ar, wt.%)
a b
Ultimate analysis (daf, wt.%) a
Mt
A
V
FC
C
H
N
S
O
4.61
42.08
27.24
26.07
69.15
7.28
2.00
0.80
20.77
Determined by element mass balance. Tar yield from Gray-King assay test.
G-K (d, wt.%)b
HHV(kJkg
Tar
Coal
7.88
16,214
1
)
658
E. Hu et al. / Fuel 185 (2016) 656–662
Fig. 1. Schematic diagram of the experimental apparatus.
Table 2 Comparison of added heating surface area in the reactor mounted with different heating plates. Added plate number
Added heating surface area (%)
a b c d
0 4 8 12
0 57 113 170
Furnace 800 Plate number Adding heating surface area (%)
Temperature (°C)
No.
1000
a 0 0
b 4 57
c 8 113
d 12 170
600
d
c b
a
400
200
0 0
5
10
15
20
25
30
Time (min) Fig. 3. Heating curve of central coal in the reactor mounted with different added heating surface areas or plates.
1. Quartz reactor 2. Sample particles 3. Pressure gauge 4. Mass flow meter 5. Valve 6. Gas cylinder. Fig. 2. Schematic plot for the pressure drop experiment.
Japan). The specific surface areas of the samples were obtained from N2 adsorption measurement using a Micromeritics system (BET ASAP2020M, America). 3. Results and discussion 3.1. Heating characteristics for coal The heating curves of central coal for the reactor mounted with 0, 4, 8 and 12 heating plates are shown in Fig. 3. The correspondingly added heating surface areas were 0, 57%, 113% and 170% of the original heating surface area provided by the reactor wall (line
a, b, c and d). Increasing the number of metallic plates shortened the reaction time evidently. At a heating furnace temperature of 900 °C, the time for the central coal bed to reach 500 °C in the reactor without plates was longest at nearly 23.5 min (line a). When the number of plates in the reactor further increased to 4, 8 and 12, the time to reach 500 °C was shortened to 22.3 min, 21.4 min and 17.8 min respectively (line b, c and d). It is postulated that using heating plates resulted in the discrepancy of heat transfer. Heat radiation and conduction were the main means of heat transfer for the reactor without metallic plates. When metallic plates were installed into the reactor, it accelerated the process of heat conduction from the reactor wall to the central low-temperature coal bed because the coefficient of plates was much higher than that of coal and char. Finally, even the lowtemperature coal in the reactor center directly contacted the metallic high-temperature plates and was further heated quickly. 3.2. Pyrolysis product distribution and working mechanism Fig. 4 summarizes the pyrolysis product distribution of coal the reactor in the reactor with different added heating surface areas.
E. Hu et al. / Fuel 185 (2016) 656–662
80
char
tar
gas
water
12
79
10
77 8
76 75
6 74 73 5 0
Yield (wt.%)
Yield (wt.%)
78
4
0
20
40
60
80
100
120
140
160
0
Added heating surface area (%) Fig. 4. Effect of different added heating surface areas on yields of pyrolysis products.
With the increase of the added heating surface area, the pyrolysis gas yield increased but the char yield decreased; the pyrolysis water did not vary much, whereas the tar yield increased then decreased. The tar yield rose from 4.60 wt.% to 6.60 wt.% with the increase of the added heating surface area from 0 to 113% (8 plates). However, the tar yield decreased to 6.23 wt.% when the added heating surface area further increased to 170% (12 plates). The pyrolysis gas yield rose from 10.47 wt.% to 12.03 wt.% but the char yield lowered from 77.61% wt.% to 74.19 wt.% corresponding to the increase of the added heating surface area from 0 to 170%. Evaluating the gas permeability through a fixed-bed quartz tube filled with either coal and metallic plates or char was further conducted. This was performed by measuring the pressure drop across the particle bed according to the approach described in Section 2.2. The pressure drops of the coal with different number of plates and the char are compared in Fig. 5. As expected, the pressure drops increased with raising the number of plates, and it demonstrated that the gas permeability increased. When adequate number of plates was installed into the quartz tube, the gas permeability of coal would be superior to that of char. The pressure drop for coal and char in the quartz tube without any plates were 110 Pa and 10 Pa respectively, and it showed that the char had a better gas permeability than coal. Therefore, the gas produced by coal pyrolysis process in the fixed-bed reactor tended to flow to the hightemperature char bed. When the number of plates was increased
659
to 1, 2, 3 and 4 and the high-temperature heating surface areas were increased by 30%, 60%, 90% and 120%, the pressure drop decreased to 60 Pa, 40 Pa, 20 Pa and 5 Pa, respectively. The result suggests that the metallic plates prevented dense stacking pattern and increased the particle interstices, and then decreased the gas diffusion resistance. Moreover, the boundary effect between metallic plate and coal may be another cause for the result. In the indirectly heated fixed-bed reactor, the coal near the reactor heating wall was pyrolyzed first and then became some high-temperature char. Because of the lower gas permeability of the char, the pyrolysis gas produced by the inner coal sample tended to escape from the first formed char. The char not only has cavities structure, but it also contains some inherent metallic minerals. The cavities prolong the residence time when the volatile products of pyrolysis pass through the char layer, and then facilitate the bond with active site of the char. In addition, the metal species on char are mainly alkaline and alkaline earth metals which have a catalysis effect on tar cracking. Those mentioned above cause the further catalytical pyrolysis of the volatile products [27]. Consequently, the tar yield in the reactor without any added heating surface area was lowest at 4.60 wt.%. As the added heating surface area increased, the stacking pattern became loose and the particle interstices rose; it accelerated the flow of gaseous pyrolysis products from the high-temperature reaction wall to the low-temperature central coal layer and appreciably suppressed the secondary reactions. Thus, the tar yields in the reactor with added heating surface area of 57% and 113% were elevated to 6.04 wt.% and 6.60 wt.% respectively. Besides, increasing the number of metallic plates can enhance the heat transfer and heating rate. High heating rates shortened the residence time of volatile precursors within the particles and suppressed intraparticle volatiles cracking and deposition in the char [28–30]. As a result, more heat was transferred to the inside of the coal particles, and thus pyrolysis reaction might easily occur for the conditions with appropriate number of metallic plates [31]. Increasing the number of metallic plates not only reduced breakdown of alkyl substituents to pyrolysis gas, but also limited the retrogressive reaction of the aromatic ring systems leading to the formation of char [29]. Hence, it was postulated that increasing the number of metallic plates inside the reactor inhibited the macromolecular aggregation and increased the tar yield [31]. However, when the added heating surface area further increased to 170%, the tar yield decreased to 6.23 wt.%. This can be explained by the excessive number of heating plates resulted in raising the average pyrolysis temperature, thus exacerbating the cracking of pyrolysis volatile products. In conclusion, adding metallic plates into the reactor obviously shortened the reaction time and reinforced the heat transfer from the high-temperature reactor wall to the low-temperature central coal bed. Finally, the volatile products escaped from the coal surface quickly and the degree of pyrolysis was enhanced, and therefore the char yield deceased but the gas yield increased. 3.3. Pyrolysis gas and tar characterization
Fig. 5. Comparison of the pressure drop of char and coal with different heating plates.
Pyrolysis gas average composition and its calorific value are shown in Figs. 6 and 7. With the increase of added heating surface area, the composition and calorific value of gas did not greatly vary. The contents of C2 + C3 and CO were maintained at an almost constant value, i.e., 8 vol.% and 13.5 vol.% respectively. Whereas increasing the added heating surface area elevated the content of CO2. This was because the formation of CO2 was from decarboxylation reaction and the decomposition of minerals. The increase of added heating surface area raised the average pyrolysis temperature of coal and accelerated the decomposition of minerals. When the added heating surface area increased from 0 to 57%, the content of H2 rose from 35.83 vol.% to 38.27 vol.% but the content of
660
E. Hu et al. / Fuel 185 (2016) 656–662
40
30 15 25 14 20 15
H2
CH4
CO2
CO
C2+C3
13
10
12
5
11
Gas composition (vol.%)
16
Light tar fraction (wt.%)
a- 0 plate c- 8 plates
35
Gas composition (vol.%)
80
17
20
40
60
80
100 120 140 160 180
c
75
b
d
70
a 65
60 5 0
0
b- 4 plates d- 12 plates
0
20
40
60
80
100
120
140
160
180
Added heating surface area (%)
Added heating surface area (%) Fig. 8. Variation of yield of light tar with different added heating surface areas. Fig. 6. Effect of different added heating surface areas on pyrolysis gas composition.
23000
a- 0 plate c- 8 plates c
a 22000
b- 4 plates d- 12 plates d
HHV(kJ⋅Nm
-3
)
b
21000
20000
0
0
20
40
60
80
100 120 140 160 180
then many volatile products escaped from the central lowtemperature coal bed. Thus, it suppressed the secondary reactions of pyrolysis products and in turn increased the light tar fraction yield. Whereas an excessive number of heating plates exacerbated the cracking of tar and consequently decreased the light tar fraction yield. Fig. 9 compares the GC–MS spectrum of tar from pyrolysis in the reactor with different added heating surface areas. No obvious differences can be identified because of a large quantity of peaks. Based on the GC–MS data the tar can be further grouped into 6C8 component, C9–C14 component, C15–C20 component and PC20 component. Fig. 10 compares the concentration of such tar species in terms of peak area percentage. With the increase of heating surface area, the concentrations of C15–C20 component and PC20 component fluctuated slightly; the concentration of 6C8 component marginally decreased, while the concentration of
Added heating surface area (%) Fig. 7. Variation of calorific value of pyrolysis gas with different added heating surface areas.
CH4 decreased from 30.26 vol.% to 25.86 vol.%. The explanation is that much heating surface area raised the average pyrolysis temperature and thereby increased the pyrolysis degree. As the added heating surface area further increased to 113%, it achieved the optimal matching between secondary reaction of pyrolysis products and the gas permeability of the coal bed, and therefore the content of H2 decreased to the lowest value (35.00 vol.%) but the content of CH4 increased to 29.92 vol.%. However, the H2 content slightly increased to 36.44 vol.% with raising the added heating surface area to 170%, while the CH4 decreased to 28.22 vol.%. This suggests that the excessive number of heating plates exacerbated the tar cracking and thereby produced more small-molecule gaseous products. HHV of gas for the indirectly heated fixed-bed reactor ranged between 21553.69 kJ/Nm3 and 22202.09 kJ/Nm3, and the HHV of gas for the case with 113% added heating surface area (8 heating plates) was highest at 22202.09 kJ/Nm3 because of the lowest H2 content and relatively high CH4 content. Fig. 8 shows the light tar fraction (boiling point below 360 °C) varying with the added extra heating surface area. With the increase of the added heating surface area, the light tar fraction increased then decreased. The light tar fraction increased from 66.0 wt.% to 76.4 wt.% as the added heating surface area rose from 0 to 113%; when the added heating surface area further increased to 170%, the light tar fraction decreased to 70.3 wt.%. This may indicate that increasing the added heating surface area raised the particle interstices and lowered the gas diffusion resistance, and
Fig. 9. GC–MS spectrum in coal tar varying with added heating surface area.
661
E. Hu et al. / Fuel 185 (2016) 656–662
16600
a- 0 plate c- 8 plates
a
b- 4 plates d- 12 plates
-1
)
16400
HHV (kJ⋅kg
C9–C14 component increased then lowered. The concentration of C9–C14 was lowest at 41% with added heating surface area of 0, and the concentration increased to the highest value of 43.52% when the added heating surface area rose to 113%. As the added heating surface further elevated to 170%, the concentration decreased to 42.59%. It coincided with the simulated distillation results shown in Fig. 8, indicating that increasing the number of plates suppressed the secondary reactions of pyrolysis products and elevated the yield of light tar fraction. An excessive number of plates, however, weakened the effect.
16200
d
b c
16000
15800
3.4. Pyrolysis char characterization
46
35
45
30
44
25 20
43
15
42
C9-C14 C15-C20
41
≥C21 ≤C8
10
Area percentage (%)
Area percentage (%)
Fig. 11 compares the calorific value of char under different added heating surface areas. Increasing the heating surface area prolonged the average pyrolysis temperature of coal and released many volatile products, and thus decreased the HHV of char. This suggests that the HHV of char for the case without any plates was dramatically higher than for the case with some plates. When the added heating surface area increased to 113% (8 plates), it reached the optimal matching between heat transfer and field of pyrolysis temperature, and the most volatile products were released; therefore, the HHV of char under this case was lowest (16003.4 kJ/Nm3). Table 3 compares the morphological characteristics of coal and char produced under different conditions. With the increase of added heating surface area, the BET area and total volume increased then decreased, whereas the average pore size showed a reverse trend. When the added heating surface area increased from 0 to 113%, the BET surface area and total volume of char increased from 6.324 m2g 1 and 1.983 ⁄ 10 2 cm3g 1 to 14.487 m2g 1 and 2.748 ⁄ 10 2 cm3g 1 respectively, but the average size decreased from 12.51 nm to 7.588 nm. This suggests that the increase of added heating surface area raised the average pyrolysis temperature andrate, and released many volatile products as well as generating a large quantity of fine pores [32,33]. When the added heating surface area further increased to 170%, the BET surface area and total volume of char steeply decreased, and were 3.546 m2g 1 and 1.716 ⁄ 10 2 cm3g 1 respectively, but the average pore size substantially increased to 19.35 nm. This indicates that the fine pores diminished while a few large pores formed. The results suggest that an excessive number of plates would elevate the temperature of pyrolysis, and then the higher temperature blocked the releasing and cracking of volatiles, which finally covered the surface and closed the fine pores.
0
0
20
40
60
80
100 120 140 160 180
Added heating surface area (%) Fig. 11. Calorific value variations of char varying with added heating surface area. Table 3 Morphology structure characteristics of coal and different chars.
Raw coal 0 Plate 4 Plate 8 Plate 12 Plate
Added heating surface area (%)
BET area (m2g 1)
Total volume (cm3g 1)
0 57 113 170
11.051 6.342 7.229 14.487 3.546
1.833 ⁄ 10 1.983 ⁄ 10 2.094 ⁄ 10 2.748 ⁄ 10 1.716 ⁄ 10
2 2 2 2 2
Average pore size (nm) 6.635 1.251 ⁄ 101 1.148 ⁄ 101 7.588 1.935 ⁄ 101
4. Conclusions The differences in coal pyrolysis behaviors in an indirectly heated fixed bed reactor with different numbers of metallic plates were studied and the working mechanism of metallic plates was investigated in a quartz tube. The added heating surface area was varied from 0 (0 plates) to 170% (12 plates) by installing different numbers of metallic plates. With the increase of plate numbers or added heating surface area, the tar yield, light tar fraction and component of C9–C14 increased then decreased. With 8 plates and the added heating surface area of 113%, the optimal matching was achieved between secondary reaction of pyrolysis products and permeability of gas flow. Therefore, it had the highest tar yield, most light tar fraction and greatest components of C9–C14 (i.e., about 6.6 wt.%, 76.4 wt.% and 43.52% respectively). What is more, the char in this case also had the biggest BET surface area and total volume but the smallest average pore size. Analyzing the working mechanism of metallic plates in a quartz tube showed that increasing the heating surface area prolonged the average pyrolysis temperature of coal and raised the particle interstices, and thus loosened the stacking pattern of coal and lowered the gas diffusion resistance. Finally many gaseous pyrolysis products escaped from the central low-temperature coal layer, essentially indicating the suppressed secondary reactions to the primary products. Besides, metallic plates enhanced the heat transfer and heating rate. As a result, more heat was transferred to the inside of the coal particles. However, an excessive number of plates resulted in raising the average pyrolysis temperature, thus aggravating the cracking of primary products and lowering the tar yield and quality.
5 40 5 0
Acknowledgement 0
20
40
60
80
100 120
140 160
0 180
Added heating surface area (%) Fig. 10. Peak area percentage of major components in coal tar varying with added heating surface area.
The authors gratefully acknowledge the financial support provided by the ‘‘Strategic Priority Research Program” of CAS on clean and high efficiency utilization of low-rank coal (XDA07050400), and State Scholarship Fund of China (No. 201506430030). The
662
E. Hu et al. / Fuel 185 (2016) 656–662
deepest gratitude also goes to Professor Guangwen Xu (Institute of Process Engineering, CAS, Beijing, China) and Professor Ying Zheng (University of New Brunswick, Canada), for their recommendations. References [1] Zhang C, Wu R, Xu G. Coal pyrolysis for high-quality tar in a fixed-bed pyrolyzer enhanced with internals. Energy Fuels 2013;28(1):236–44. [2] Furimsky E, Vancea L, Belanger R. Effect of coal rank on structure of tars from low-temperature pyrolysis of Canadian coals. Ind Eng Chem Prod Res Dev 1984;23(1):134–40. [3] Zhu W, Song W, Lin W. Effect of the coal particle size on pyrolysis and char reactivity for two types of coal and demineralized coal. Energy Fuels 2008;22 (4):2482–7. [4] Cui L, Lin W, Yao J. Influences of temperature and coal particle size on the flash pyrolysis of coal in a fast-entrained bed. Chem Res Chin Univ 2006;22 (1):103–10. [5] Franklin HD, Peters WA, Howard JB. Mineral matter effects on the rapid pyrolysis and hydropyrolysis of a bituminous coal. 1. Effects on yields of char, tar and light gaseous volatiles. Fuel 1982;61(2):155–60. [6] Wang P, Jin L, Liu J, et al. Analysis of coal tar derived from pyrolysis at different atmospheres. Fuel 2013;104:14–21. [7] Cui Zhen LTZ, Chao W. Study on the effects of coal diameters and heating rate on the coal pyrolysis. Coal Convers 2005;1:003. [8] Na Chang. Study on the effects of heating rate and temperature to coal pyrolysis. Coal Convers 2012;35(3):1–5. [9] Li CZ, Bartle KD, Kandiyoti R. Vacuum pyrolysis of maceral concentrates in a wire-mesh reactor. Fuel 1993;72(11):1459–68. [10] Sun QL, Li W, Chen HK, et al. The variation of structural characteristics of macerals during pyrolysis. Fuel 2003;82(6):669–76. [11] Roy C, De Caumia B, Kalkreuth W. Vacuum pyrolysis of Prince Mine coal, Nova Scotia, Canada. Fuel 1985;64(12):1662–6. [12] Hirajima T, Chan EW, Whiteway SG. Vacuum and atmospheric pressure TGA on an eastern Canadian coal. Fuel 1986;65(6):844–8. [13] Yang H, Chen H, Ju F, et al. Influence of pressure on coal pyrolysis and char gasification. Energy Fuels 2007;21(6):3165–70. [14] Hu E, Zhang C, Wu R, Fu X, Xu G. Pyrolysis of coal with different moisture contents in fixed-bed reactor with internals. J Chem Ind Eng 2015;71 (7):638–45. [15] Atwood MT, Schulman BL. The TOSCOAL process-pyrolysis of western coals and lignites for char and oil production. Prepr Pap Am Chem Soc Div Fuel Chem 1977;22:233–52.
[16] Li Q, Li R, Ma Z, Chen J. New progress of the U.S. LFC technology of low rank coal upgrading with cogeneration of coal liquids. China Min Mag 2010;12 (2):82–7. [17] Zhang J, Wu R, Zhang G, Yu J, Yao C, Wang Y, et al. Technical review on thermochemical conversion based on decoupling for solid carbonaceous fuels. Energy Fuels 2013;27(4):1951–66. [18] Eddinger R, Jones J, Blanc F. Development of the COED process. Chem Eng Prog 1968;64(10):33–8. [19] Strom A, Eddinger R. COED plant for coal conversion. Chem Eng Prog 1971;67 (3):75–80. [20] Shamsi A, Shadle LJ, Seshadri KS. Study of low temperature oxidation of buckskin subbituminous coal and derived chars produced in ENCOAL process. Fuel Process Technol 2004;86(3):275–92. [21] Li Q, Li R, Ma Z, Chen J. New progress of the U.S. LFC technology of low rank coal upgrading with cogeneration of coal liquids. China Min Mag 2010;12:82–7. [22] Zhang J, Wu R, Zhang G, Yu J, Yao C, Wang Y, et al. Technical review on thermochemical conversion based on decoupling for solid carbonaceous fuels. Energy Fuels 2013;27(4):1951–66. [23] Khan MR. Production of high quality liquid fuels from coal by mild pyrolysis of coal-lime mixtures. Fuel Sci Technol Int 1987;5(2):185–231. [24] Peters W, Bertling H. Kinetics of rapid degasification of coals. Fuel 1965;44 (5):317–31. [25] Hu G, Fan H, Liu Y. Experimental studies on pyrolysis of Datong coal with solid heat carrier in a fixed bed. Fuel Process Technol 2001;69(3):221–8. [26] Lin L, Lai D, Guo E, et al. Oil shale pyrolysis in indirectly heated fixed bed with metallic plates of heating enhancement. Fuel 2016;163:48–55. [27] Zeng X, Wang Y, Yu J, et al. Gas upgrading in a downdraft fixed-bed reactor downstream of a fluidized-bed coal pyrolyzer. Energy Fuels 2011;25 (11):5242–9. [28] Yip Kongvui, Wu Hongwei, Zhang Dong-ke. Effect of inherent moisture in collie coal during pyrolysis due to in-situ steam gasification. Energy Fuels 2007;21(5):2883–91. [29] Kershaw John R et al. Fluorescence spectroscopic analysis of tars from the pyrolysis of a Victorian brown coal in a wire-mesh reactor. Energy Fuels 2000;14(2):476–82. [30] Li Chun-Zhu, Bartle Keith D, Kandiyoti Rafael. Characterization of tars from variable heating rate pyrolysis of maceral concentrates. Fuel 1993;72(1):3–11. [31] Gai Ronghua et al. Effect of inherent and additional pyrite on the pyrolysis behavior of oil shale. J Anal Appl Pyrol 2014;105:342–7. [32] Singla PK, Miura S, Hudgins RR, et al. Pore development during carbonization of coals. Fuel 1983;62(6):645–8. [33] Yang Haiping et al. Influence of pressure on coal pyrolysis and char gasification. Energy Fuels 2007;21(6):3165–70.
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Coal pyrolysis and its mechanism in indirectly heated fixed-bed with metallic heating plate enhancement Erfeng Hu a,b,c, Chuanqiang Zhu d, Kyle Rogers c, Xue Han c, Jie Wang a, Jing Zhao a, Xiaoheng Fu a,⇑ a
School of Chemical and Environmental Engineering, China University of Mining & Technology (Beijing), Beijing 100083, China State Key Laboratory of Multi-Phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China c Department of Chemical Engineering, University of New Brunswick, Fredericton E3B 5A3, Canada d China Everbright International Limited, Shenzhen 518000, China b
h i g h l i g h t s The tar yield and light fraction increased then decreased when increasing the number of plates. Working mechanism of metallic plates was proposed and verified by an in-house design. An excessive number of plates resulted in aggravating the cracking of primary products.
a r t i c l e
i n f o
Article history: Received 8 May 2016 Received in revised form 29 June 2016 Accepted 28 July 2016 Available online 11 August 2016 Keywords: Coal pyrolysis Metallic heating plates Secondary reaction Reaction mechanism
a b s t r a c t A metallic plate has been devised to enhance the heat transfer; this in turn increases the yield and quality of coal tar (Zhang et al., 2013). This work is devoted to investigating the pyrolysis of Yilan subbituminous coal and developing a working mechanism for an indirectly heated fixed bed reactor with different number of metallic plates. Increasing the number of metallic heating plates enhanced the heat transfer from the high-temperature reactor wall to the low-temperature central coal layer and thereby shortened the reaction time. Meanwhile, the yields of tar and light fraction were increased then decreased. The results of the pressure drop experiment demonstrated that the metallic plates weakened dense stacks of coal particles and raised the particle interstices, and therefore lowered the gas diffusion resistance. It verified that many gaseous pyrolysis products escaped from the central low-temperature coal layer, indicating the suppressed secondary reactions to the primary products. When the number of plates was 8, the correspondingly added high-temperature surface was 113%; it achieved the optimal matching between the secondary reactions of pyrolysis products and the fields of the temperature as well as the gas flow inside the reactor, and it also had the highest tar yield (6.60 wt.% dry basis) and the maximum content of light tar (below the boiling points of 360 °C) of 76.4 wt.% and C4–C9 components of 43.52 wt.%. The BET results showed that the char had the biggest surface area and volume for this case with 8 plates. Consequently, adding suitable metallic plates into the indirectly heated fixed bed reactor obviously enhances the heat transfer and also achieves higher yield and quality of tar. Ó 2016 Published by Elsevier Ltd.
1. Introduction Due to large quantities of low-rank coal resources but limited reserves of oil and gas, advanced coal utilization technologies have attracted more and more attention from researchers. Pyrolysis is the critical core technology for cascade utilization of low-rank coal; it is becoming a promising research focus for energy systems, especially in China.
⇑ Corresponding author. E-mail address: [email protected] (X. Fu). http://dx.doi.org/10.1016/j.fuel.2016.07.115 0016-2361/Ó 2016 Published by Elsevier Ltd.
A large number of fundamental and technical studies of coal pyrolysis have been done for decades. Many fundamental aspects, including the coal rank [2], coal particle diameter [3,4], coal mineral composition and content [5], pyrolysis atmosphere [6], as well as the heating rate [7,8], coal composition [9,10], and operating conditions [11–14], were extensively investigated. In the technical aspects, mostly the existing coal pyrolysis technologies based on solid heat transfer (char or hot ash), such as Toscoal [15]and DG [16,17], have been examined. There are also ones using gaseous heat carrier, such as COED [18,19], Encoal [20] and LFC [21,22]. There are drawbacks, however, that include complex process,
657
E. Hu et al. / Fuel 185 (2016) 656–662
unstable operation and especially poor oil quality in those technologies, which limit their expansion. Because of no heating medium, relatively simple process and good oil quality, indirect heating technology may be a good way to upgrade low rank coal if its low heat transfer efficiency can be solved. Elevating the heating temperature, accelerating the heating speed or reducing the material particle size accelerates the heat transfer. Khan [23] reported that in an indirectly heated fixedbed reactor, the tar yield increased when the heating temperature rose from 500 °C to 649 °C, whereas the tar yield decreased with increased heating temperature above 650 °C [1]. Peters and Bertling [24] found the tar yield increased when the heating rate was raised. Hu et al. [25] found fine particles facilitated heat transfer but aggravated secondary reactions, resulting from the low gas permeability. Zhang et al. [1] found the metallic plate accelerated the heat transfer and raised the tar yield in fixed-bed reactors. Lin et al. [26] determined that appropriately increasing the number of metallic plates reinforced the heating transfer of oil shale and also increased yield and quality of shale oil in a newly configured reactor with internals. Nonetheless, little work has been done to evaluate how the number of metallic heating plates in coal pyrolysis would affect the pyrolysis products. What is more, the investigation of the working mechanism is absolutely necessary for the coal pyrolysis process. This study aims to investigate the coal pyrolysis behaviors in an indirectly heated fixed-bed reactor with different number of metallic plates, and the components of tar and microcharacteristics of char are analyzed. In addition, the working mechanism of plates is also studied and verified by an in-house design. 2. Experimental 2.1. Materials The sub-bituminous coal sample from Yilan, Heilongjiang, China, was crushed into sizes below 5 mm for tests and stored in a sealed bag for later use. The main property data for the coal sample are listed in Table 1. The coal had high ash content, and its volatile content was about 27 wt.%. The Gray-King gave a tar yield of 7.88 wt.% against dry coal mass. 2.2. Apparatus and procedure The left inset of Fig. 1 exhibits the fixed-bed reactor and metallic plates used in this paper. They were both made of 304-type stainless steel. The right inset of Fig. 1 presents a schematic diagram of the test system. It consisted mainly of an electric furnace (1), a reactor (2), a pressure gauge (3), a cooling and adsorption system (4–8), a filter (9), a vacuum pump (11), a sodium bicarbonate washing bottle for removing sulphureted hydrogen (13), and a silica gel bottle (14). Cotton wool was used to insulate both reactor and furnace. The reactor was 45 mm in inner diameter and its total heating surface area which was provided by reactor heating wall was 254 cm2 for coal samples. The metallic plates were perpendicular to the reactor wall and mutually at the same angle. The plate was 15 mm in width and 120 mm in height. As shown in Table 2,
corresponding to the number of 0, 4, 8 and 12 of the added metallic plates, the added high-temperature heating surface reached 0, 57%, 113% and 170% of the reactor heating wall, and those were denoted as a, b, c and d respectively. The results of the previous study have demonstrated that Yilan coal had the highest yield of tar and gas when the central temperature reached 500 °C [1]. Consequently, each test was ended when the same central position reached 500 °C. As illustrated in Fig. 1, coal was fed into the reactor and then the reactor was connected with gas collection and purification at the beginning of the experiment; at the end of experiments, cooled char was weighed to calculate the yield. The product in the collection bottle (5) was also weighed, and then the water in the bottle (5) was poured into a clean beaker. The pyrolysis products attached to the condenser (4) were washed by acetone, and then mixed with acetone solution from the bottles (6, 7, 8). The acetone in the solution from the bottles (6, 7, 8) was removed by a vacuum rotary evaporator. The recovery tar was mixed with the preceding tar in the collection bottle (5). The moisture content in tar mixture was further determined and water-free tar yield was obtained by using toluene as azeotrope according to the standard method. The moisture contained in tar mixture and the preceding water poured into a clean beaker were calculated as the pyrolysis water. The gas volume was recorded on a wet gas meter to calculate pyrolysis gas yield. The experiments were repeated and the relative errors were less than 3%. Experimental procedures were substantially similar to those of Zhang [1]. This study also measured the pressure drop across a char or a coal bed in a N2 flow of different fluxes. A quartz tube with a sintered orifice was used as gas distributor and was 40 mm in diameter and 400 mm in height. A total of 130 mm in height of coal or char was loaded into the tube to measure the pressure drop across the material bed at different N2 fluxes (see Fig. 2). Then the metallic plates were loaded in the tube with coal or char and measurement of the pressure drop was conducted by varying the number of plates. 2.3. Products analysis The micro GC (Agilent 3000 A) detected the composition of sampled non-condensable gases. Tar in experiments was removed in water and collected into a vial. Then, the dehydrated tar entered the micro GC (Agilent 7890), which can determine fraction distribution for diverse boiling points. The light tar consisted of those components with boiling points below 360 °C. The composition of dehydrated tar was also measured by using a GC–MS spectrometer (Shimadzu QP 2010 Ultra). The injector and detector reached a temperature of 280 °C, and the column of GC was heated to 50 °C in 5 min and further to 280 °C at 6 °C/min. Finally the column was kept at 280 °C for 10 min. The scanning range was from 20 to 900 m/z and the delay time of solvent was 1.7 min. The relative content of components was evaluated with the peak area percentage, i.e., the peak area proportion to total peak area. The calorific value of char was measured with a Shanghai Jichang XRY-1B oxygen bomb calorimeter. The char surface morphology was examined by an atomic force microscope (AFM) equipped with a scanning electron microscope (SEM JSM-6700 F,
Table 1 Proximate and ultimate analyses for the tested Yilan coal. Proximate analysis (ar, wt.%)
a b
Ultimate analysis (daf, wt.%) a
Mt
A
V
FC
C
H
N
S
O
4.61
42.08
27.24
26.07
69.15
7.28
2.00
0.80
20.77
Determined by element mass balance. Tar yield from Gray-King assay test.
G-K (d, wt.%)b
HHV(kJkg
Tar
Coal
7.88
16,214
1
)
658
E. Hu et al. / Fuel 185 (2016) 656–662
Fig. 1. Schematic diagram of the experimental apparatus.
Table 2 Comparison of added heating surface area in the reactor mounted with different heating plates. Added plate number
Added heating surface area (%)
a b c d
0 4 8 12
0 57 113 170
Furnace 800 Plate number Adding heating surface area (%)
Temperature (°C)
No.
1000
a 0 0
b 4 57
c 8 113
d 12 170
600
d
c b
a
400
200
0 0
5
10
15
20
25
30
Time (min) Fig. 3. Heating curve of central coal in the reactor mounted with different added heating surface areas or plates.
1. Quartz reactor 2. Sample particles 3. Pressure gauge 4. Mass flow meter 5. Valve 6. Gas cylinder. Fig. 2. Schematic plot for the pressure drop experiment.
Japan). The specific surface areas of the samples were obtained from N2 adsorption measurement using a Micromeritics system (BET ASAP2020M, America). 3. Results and discussion 3.1. Heating characteristics for coal The heating curves of central coal for the reactor mounted with 0, 4, 8 and 12 heating plates are shown in Fig. 3. The correspondingly added heating surface areas were 0, 57%, 113% and 170% of the original heating surface area provided by the reactor wall (line
a, b, c and d). Increasing the number of metallic plates shortened the reaction time evidently. At a heating furnace temperature of 900 °C, the time for the central coal bed to reach 500 °C in the reactor without plates was longest at nearly 23.5 min (line a). When the number of plates in the reactor further increased to 4, 8 and 12, the time to reach 500 °C was shortened to 22.3 min, 21.4 min and 17.8 min respectively (line b, c and d). It is postulated that using heating plates resulted in the discrepancy of heat transfer. Heat radiation and conduction were the main means of heat transfer for the reactor without metallic plates. When metallic plates were installed into the reactor, it accelerated the process of heat conduction from the reactor wall to the central low-temperature coal bed because the coefficient of plates was much higher than that of coal and char. Finally, even the lowtemperature coal in the reactor center directly contacted the metallic high-temperature plates and was further heated quickly. 3.2. Pyrolysis product distribution and working mechanism Fig. 4 summarizes the pyrolysis product distribution of coal the reactor in the reactor with different added heating surface areas.
E. Hu et al. / Fuel 185 (2016) 656–662
80
char
tar
gas
water
12
79
10
77 8
76 75
6 74 73 5 0
Yield (wt.%)
Yield (wt.%)
78
4
0
20
40
60
80
100
120
140
160
0
Added heating surface area (%) Fig. 4. Effect of different added heating surface areas on yields of pyrolysis products.
With the increase of the added heating surface area, the pyrolysis gas yield increased but the char yield decreased; the pyrolysis water did not vary much, whereas the tar yield increased then decreased. The tar yield rose from 4.60 wt.% to 6.60 wt.% with the increase of the added heating surface area from 0 to 113% (8 plates). However, the tar yield decreased to 6.23 wt.% when the added heating surface area further increased to 170% (12 plates). The pyrolysis gas yield rose from 10.47 wt.% to 12.03 wt.% but the char yield lowered from 77.61% wt.% to 74.19 wt.% corresponding to the increase of the added heating surface area from 0 to 170%. Evaluating the gas permeability through a fixed-bed quartz tube filled with either coal and metallic plates or char was further conducted. This was performed by measuring the pressure drop across the particle bed according to the approach described in Section 2.2. The pressure drops of the coal with different number of plates and the char are compared in Fig. 5. As expected, the pressure drops increased with raising the number of plates, and it demonstrated that the gas permeability increased. When adequate number of plates was installed into the quartz tube, the gas permeability of coal would be superior to that of char. The pressure drop for coal and char in the quartz tube without any plates were 110 Pa and 10 Pa respectively, and it showed that the char had a better gas permeability than coal. Therefore, the gas produced by coal pyrolysis process in the fixed-bed reactor tended to flow to the hightemperature char bed. When the number of plates was increased
659
to 1, 2, 3 and 4 and the high-temperature heating surface areas were increased by 30%, 60%, 90% and 120%, the pressure drop decreased to 60 Pa, 40 Pa, 20 Pa and 5 Pa, respectively. The result suggests that the metallic plates prevented dense stacking pattern and increased the particle interstices, and then decreased the gas diffusion resistance. Moreover, the boundary effect between metallic plate and coal may be another cause for the result. In the indirectly heated fixed-bed reactor, the coal near the reactor heating wall was pyrolyzed first and then became some high-temperature char. Because of the lower gas permeability of the char, the pyrolysis gas produced by the inner coal sample tended to escape from the first formed char. The char not only has cavities structure, but it also contains some inherent metallic minerals. The cavities prolong the residence time when the volatile products of pyrolysis pass through the char layer, and then facilitate the bond with active site of the char. In addition, the metal species on char are mainly alkaline and alkaline earth metals which have a catalysis effect on tar cracking. Those mentioned above cause the further catalytical pyrolysis of the volatile products [27]. Consequently, the tar yield in the reactor without any added heating surface area was lowest at 4.60 wt.%. As the added heating surface area increased, the stacking pattern became loose and the particle interstices rose; it accelerated the flow of gaseous pyrolysis products from the high-temperature reaction wall to the low-temperature central coal layer and appreciably suppressed the secondary reactions. Thus, the tar yields in the reactor with added heating surface area of 57% and 113% were elevated to 6.04 wt.% and 6.60 wt.% respectively. Besides, increasing the number of metallic plates can enhance the heat transfer and heating rate. High heating rates shortened the residence time of volatile precursors within the particles and suppressed intraparticle volatiles cracking and deposition in the char [28–30]. As a result, more heat was transferred to the inside of the coal particles, and thus pyrolysis reaction might easily occur for the conditions with appropriate number of metallic plates [31]. Increasing the number of metallic plates not only reduced breakdown of alkyl substituents to pyrolysis gas, but also limited the retrogressive reaction of the aromatic ring systems leading to the formation of char [29]. Hence, it was postulated that increasing the number of metallic plates inside the reactor inhibited the macromolecular aggregation and increased the tar yield [31]. However, when the added heating surface area further increased to 170%, the tar yield decreased to 6.23 wt.%. This can be explained by the excessive number of heating plates resulted in raising the average pyrolysis temperature, thus exacerbating the cracking of pyrolysis volatile products. In conclusion, adding metallic plates into the reactor obviously shortened the reaction time and reinforced the heat transfer from the high-temperature reactor wall to the low-temperature central coal bed. Finally, the volatile products escaped from the coal surface quickly and the degree of pyrolysis was enhanced, and therefore the char yield deceased but the gas yield increased. 3.3. Pyrolysis gas and tar characterization
Fig. 5. Comparison of the pressure drop of char and coal with different heating plates.
Pyrolysis gas average composition and its calorific value are shown in Figs. 6 and 7. With the increase of added heating surface area, the composition and calorific value of gas did not greatly vary. The contents of C2 + C3 and CO were maintained at an almost constant value, i.e., 8 vol.% and 13.5 vol.% respectively. Whereas increasing the added heating surface area elevated the content of CO2. This was because the formation of CO2 was from decarboxylation reaction and the decomposition of minerals. The increase of added heating surface area raised the average pyrolysis temperature of coal and accelerated the decomposition of minerals. When the added heating surface area increased from 0 to 57%, the content of H2 rose from 35.83 vol.% to 38.27 vol.% but the content of
660
E. Hu et al. / Fuel 185 (2016) 656–662
40
30 15 25 14 20 15
H2
CH4
CO2
CO
C2+C3
13
10
12
5
11
Gas composition (vol.%)
16
Light tar fraction (wt.%)
a- 0 plate c- 8 plates
35
Gas composition (vol.%)
80
17
20
40
60
80
100 120 140 160 180
c
75
b
d
70
a 65
60 5 0
0
b- 4 plates d- 12 plates
0
20
40
60
80
100
120
140
160
180
Added heating surface area (%)
Added heating surface area (%) Fig. 8. Variation of yield of light tar with different added heating surface areas. Fig. 6. Effect of different added heating surface areas on pyrolysis gas composition.
23000
a- 0 plate c- 8 plates c
a 22000
b- 4 plates d- 12 plates d
HHV(kJ⋅Nm
-3
)
b
21000
20000
0
0
20
40
60
80
100 120 140 160 180
then many volatile products escaped from the central lowtemperature coal bed. Thus, it suppressed the secondary reactions of pyrolysis products and in turn increased the light tar fraction yield. Whereas an excessive number of heating plates exacerbated the cracking of tar and consequently decreased the light tar fraction yield. Fig. 9 compares the GC–MS spectrum of tar from pyrolysis in the reactor with different added heating surface areas. No obvious differences can be identified because of a large quantity of peaks. Based on the GC–MS data the tar can be further grouped into 6C8 component, C9–C14 component, C15–C20 component and PC20 component. Fig. 10 compares the concentration of such tar species in terms of peak area percentage. With the increase of heating surface area, the concentrations of C15–C20 component and PC20 component fluctuated slightly; the concentration of 6C8 component marginally decreased, while the concentration of
Added heating surface area (%) Fig. 7. Variation of calorific value of pyrolysis gas with different added heating surface areas.
CH4 decreased from 30.26 vol.% to 25.86 vol.%. The explanation is that much heating surface area raised the average pyrolysis temperature and thereby increased the pyrolysis degree. As the added heating surface area further increased to 113%, it achieved the optimal matching between secondary reaction of pyrolysis products and the gas permeability of the coal bed, and therefore the content of H2 decreased to the lowest value (35.00 vol.%) but the content of CH4 increased to 29.92 vol.%. However, the H2 content slightly increased to 36.44 vol.% with raising the added heating surface area to 170%, while the CH4 decreased to 28.22 vol.%. This suggests that the excessive number of heating plates exacerbated the tar cracking and thereby produced more small-molecule gaseous products. HHV of gas for the indirectly heated fixed-bed reactor ranged between 21553.69 kJ/Nm3 and 22202.09 kJ/Nm3, and the HHV of gas for the case with 113% added heating surface area (8 heating plates) was highest at 22202.09 kJ/Nm3 because of the lowest H2 content and relatively high CH4 content. Fig. 8 shows the light tar fraction (boiling point below 360 °C) varying with the added extra heating surface area. With the increase of the added heating surface area, the light tar fraction increased then decreased. The light tar fraction increased from 66.0 wt.% to 76.4 wt.% as the added heating surface area rose from 0 to 113%; when the added heating surface area further increased to 170%, the light tar fraction decreased to 70.3 wt.%. This may indicate that increasing the added heating surface area raised the particle interstices and lowered the gas diffusion resistance, and
Fig. 9. GC–MS spectrum in coal tar varying with added heating surface area.
661
E. Hu et al. / Fuel 185 (2016) 656–662
16600
a- 0 plate c- 8 plates
a
b- 4 plates d- 12 plates
-1
)
16400
HHV (kJ⋅kg
C9–C14 component increased then lowered. The concentration of C9–C14 was lowest at 41% with added heating surface area of 0, and the concentration increased to the highest value of 43.52% when the added heating surface area rose to 113%. As the added heating surface further elevated to 170%, the concentration decreased to 42.59%. It coincided with the simulated distillation results shown in Fig. 8, indicating that increasing the number of plates suppressed the secondary reactions of pyrolysis products and elevated the yield of light tar fraction. An excessive number of plates, however, weakened the effect.
16200
d
b c
16000
15800
3.4. Pyrolysis char characterization
46
35
45
30
44
25 20
43
15
42
C9-C14 C15-C20
41
≥C21 ≤C8
10
Area percentage (%)
Area percentage (%)
Fig. 11 compares the calorific value of char under different added heating surface areas. Increasing the heating surface area prolonged the average pyrolysis temperature of coal and released many volatile products, and thus decreased the HHV of char. This suggests that the HHV of char for the case without any plates was dramatically higher than for the case with some plates. When the added heating surface area increased to 113% (8 plates), it reached the optimal matching between heat transfer and field of pyrolysis temperature, and the most volatile products were released; therefore, the HHV of char under this case was lowest (16003.4 kJ/Nm3). Table 3 compares the morphological characteristics of coal and char produced under different conditions. With the increase of added heating surface area, the BET area and total volume increased then decreased, whereas the average pore size showed a reverse trend. When the added heating surface area increased from 0 to 113%, the BET surface area and total volume of char increased from 6.324 m2g 1 and 1.983 ⁄ 10 2 cm3g 1 to 14.487 m2g 1 and 2.748 ⁄ 10 2 cm3g 1 respectively, but the average size decreased from 12.51 nm to 7.588 nm. This suggests that the increase of added heating surface area raised the average pyrolysis temperature andrate, and released many volatile products as well as generating a large quantity of fine pores [32,33]. When the added heating surface area further increased to 170%, the BET surface area and total volume of char steeply decreased, and were 3.546 m2g 1 and 1.716 ⁄ 10 2 cm3g 1 respectively, but the average pore size substantially increased to 19.35 nm. This indicates that the fine pores diminished while a few large pores formed. The results suggest that an excessive number of plates would elevate the temperature of pyrolysis, and then the higher temperature blocked the releasing and cracking of volatiles, which finally covered the surface and closed the fine pores.
0
0
20
40
60
80
100 120 140 160 180
Added heating surface area (%) Fig. 11. Calorific value variations of char varying with added heating surface area. Table 3 Morphology structure characteristics of coal and different chars.
Raw coal 0 Plate 4 Plate 8 Plate 12 Plate
Added heating surface area (%)
BET area (m2g 1)
Total volume (cm3g 1)
0 57 113 170
11.051 6.342 7.229 14.487 3.546
1.833 ⁄ 10 1.983 ⁄ 10 2.094 ⁄ 10 2.748 ⁄ 10 1.716 ⁄ 10
2 2 2 2 2
Average pore size (nm) 6.635 1.251 ⁄ 101 1.148 ⁄ 101 7.588 1.935 ⁄ 101
4. Conclusions The differences in coal pyrolysis behaviors in an indirectly heated fixed bed reactor with different numbers of metallic plates were studied and the working mechanism of metallic plates was investigated in a quartz tube. The added heating surface area was varied from 0 (0 plates) to 170% (12 plates) by installing different numbers of metallic plates. With the increase of plate numbers or added heating surface area, the tar yield, light tar fraction and component of C9–C14 increased then decreased. With 8 plates and the added heating surface area of 113%, the optimal matching was achieved between secondary reaction of pyrolysis products and permeability of gas flow. Therefore, it had the highest tar yield, most light tar fraction and greatest components of C9–C14 (i.e., about 6.6 wt.%, 76.4 wt.% and 43.52% respectively). What is more, the char in this case also had the biggest BET surface area and total volume but the smallest average pore size. Analyzing the working mechanism of metallic plates in a quartz tube showed that increasing the heating surface area prolonged the average pyrolysis temperature of coal and raised the particle interstices, and thus loosened the stacking pattern of coal and lowered the gas diffusion resistance. Finally many gaseous pyrolysis products escaped from the central low-temperature coal layer, essentially indicating the suppressed secondary reactions to the primary products. Besides, metallic plates enhanced the heat transfer and heating rate. As a result, more heat was transferred to the inside of the coal particles. However, an excessive number of plates resulted in raising the average pyrolysis temperature, thus aggravating the cracking of primary products and lowering the tar yield and quality.
5 40 5 0
Acknowledgement 0
20
40
60
80
100 120
140 160
0 180
Added heating surface area (%) Fig. 10. Peak area percentage of major components in coal tar varying with added heating surface area.
The authors gratefully acknowledge the financial support provided by the ‘‘Strategic Priority Research Program” of CAS on clean and high efficiency utilization of low-rank coal (XDA07050400), and State Scholarship Fund of China (No. 201506430030). The
662
E. Hu et al. / Fuel 185 (2016) 656–662
deepest gratitude also goes to Professor Guangwen Xu (Institute of Process Engineering, CAS, Beijing, China) and Professor Ying Zheng (University of New Brunswick, Canada), for their recommendations. References [1] Zhang C, Wu R, Xu G. Coal pyrolysis for high-quality tar in a fixed-bed pyrolyzer enhanced with internals. Energy Fuels 2013;28(1):236–44. [2] Furimsky E, Vancea L, Belanger R. Effect of coal rank on structure of tars from low-temperature pyrolysis of Canadian coals. Ind Eng Chem Prod Res Dev 1984;23(1):134–40. [3] Zhu W, Song W, Lin W. Effect of the coal particle size on pyrolysis and char reactivity for two types of coal and demineralized coal. Energy Fuels 2008;22 (4):2482–7. [4] Cui L, Lin W, Yao J. Influences of temperature and coal particle size on the flash pyrolysis of coal in a fast-entrained bed. Chem Res Chin Univ 2006;22 (1):103–10. [5] Franklin HD, Peters WA, Howard JB. Mineral matter effects on the rapid pyrolysis and hydropyrolysis of a bituminous coal. 1. Effects on yields of char, tar and light gaseous volatiles. Fuel 1982;61(2):155–60. [6] Wang P, Jin L, Liu J, et al. Analysis of coal tar derived from pyrolysis at different atmospheres. Fuel 2013;104:14–21. [7] Cui Zhen LTZ, Chao W. Study on the effects of coal diameters and heating rate on the coal pyrolysis. Coal Convers 2005;1:003. [8] Na Chang. Study on the effects of heating rate and temperature to coal pyrolysis. Coal Convers 2012;35(3):1–5. [9] Li CZ, Bartle KD, Kandiyoti R. Vacuum pyrolysis of maceral concentrates in a wire-mesh reactor. Fuel 1993;72(11):1459–68. [10] Sun QL, Li W, Chen HK, et al. The variation of structural characteristics of macerals during pyrolysis. Fuel 2003;82(6):669–76. [11] Roy C, De Caumia B, Kalkreuth W. Vacuum pyrolysis of Prince Mine coal, Nova Scotia, Canada. Fuel 1985;64(12):1662–6. [12] Hirajima T, Chan EW, Whiteway SG. Vacuum and atmospheric pressure TGA on an eastern Canadian coal. Fuel 1986;65(6):844–8. [13] Yang H, Chen H, Ju F, et al. Influence of pressure on coal pyrolysis and char gasification. Energy Fuels 2007;21(6):3165–70. [14] Hu E, Zhang C, Wu R, Fu X, Xu G. Pyrolysis of coal with different moisture contents in fixed-bed reactor with internals. J Chem Ind Eng 2015;71 (7):638–45. [15] Atwood MT, Schulman BL. The TOSCOAL process-pyrolysis of western coals and lignites for char and oil production. Prepr Pap Am Chem Soc Div Fuel Chem 1977;22:233–52.
[16] Li Q, Li R, Ma Z, Chen J. New progress of the U.S. LFC technology of low rank coal upgrading with cogeneration of coal liquids. China Min Mag 2010;12 (2):82–7. [17] Zhang J, Wu R, Zhang G, Yu J, Yao C, Wang Y, et al. Technical review on thermochemical conversion based on decoupling for solid carbonaceous fuels. Energy Fuels 2013;27(4):1951–66. [18] Eddinger R, Jones J, Blanc F. Development of the COED process. Chem Eng Prog 1968;64(10):33–8. [19] Strom A, Eddinger R. COED plant for coal conversion. Chem Eng Prog 1971;67 (3):75–80. [20] Shamsi A, Shadle LJ, Seshadri KS. Study of low temperature oxidation of buckskin subbituminous coal and derived chars produced in ENCOAL process. Fuel Process Technol 2004;86(3):275–92. [21] Li Q, Li R, Ma Z, Chen J. New progress of the U.S. LFC technology of low rank coal upgrading with cogeneration of coal liquids. China Min Mag 2010;12:82–7. [22] Zhang J, Wu R, Zhang G, Yu J, Yao C, Wang Y, et al. Technical review on thermochemical conversion based on decoupling for solid carbonaceous fuels. Energy Fuels 2013;27(4):1951–66. [23] Khan MR. Production of high quality liquid fuels from coal by mild pyrolysis of coal-lime mixtures. Fuel Sci Technol Int 1987;5(2):185–231. [24] Peters W, Bertling H. Kinetics of rapid degasification of coals. Fuel 1965;44 (5):317–31. [25] Hu G, Fan H, Liu Y. Experimental studies on pyrolysis of Datong coal with solid heat carrier in a fixed bed. Fuel Process Technol 2001;69(3):221–8. [26] Lin L, Lai D, Guo E, et al. Oil shale pyrolysis in indirectly heated fixed bed with metallic plates of heating enhancement. Fuel 2016;163:48–55. [27] Zeng X, Wang Y, Yu J, et al. Gas upgrading in a downdraft fixed-bed reactor downstream of a fluidized-bed coal pyrolyzer. Energy Fuels 2011;25 (11):5242–9. [28] Yip Kongvui, Wu Hongwei, Zhang Dong-ke. Effect of inherent moisture in collie coal during pyrolysis due to in-situ steam gasification. Energy Fuels 2007;21(5):2883–91. [29] Kershaw John R et al. Fluorescence spectroscopic analysis of tars from the pyrolysis of a Victorian brown coal in a wire-mesh reactor. Energy Fuels 2000;14(2):476–82. [30] Li Chun-Zhu, Bartle Keith D, Kandiyoti Rafael. Characterization of tars from variable heating rate pyrolysis of maceral concentrates. Fuel 1993;72(1):3–11. [31] Gai Ronghua et al. Effect of inherent and additional pyrite on the pyrolysis behavior of oil shale. J Anal Appl Pyrol 2014;105:342–7. [32] Singla PK, Miura S, Hudgins RR, et al. Pore development during carbonization of coals. Fuel 1983;62(6):645–8. [33] Yang Haiping et al. Influence of pressure on coal pyrolysis and char gasification. Energy Fuels 2007;21(6):3165–70.