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Two-dimensional combustion modelling and experimental research on oil shale semicoke
Fuel 256 (2019) 115891
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Full Length Article
Two-dimensional combustion modelling and experimental research on oil shale semicoke
T
Yiqun Huanga, Man Zhanga, Boyu Denga, Hao Konga, Yanjun Zhangb, Junfu Lyua, Hairui Yanga, , ⁎ Yan Jinc, ⁎
a
Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, State Key Laboratory of Power Systems, Department of Energy and Power Engineering, Tsinghua University, Beijing 100084, China b State Key Laboratory of Efficient and Clean Coal-fired Utility Boilers (Harbin Boiler Company Limited), Harbin 150046, China c Taiyun Science and Technology University, Taiyuan 030024, China
ARTICLE INFO
ABSTRACT
Keywords: Oil shale Semicoke Combustion Diffusivity Anisotropy
Due to high ash content and anisotropy of bedding structure, the effects of ash thickness and bedding structure anisotropy on combustion of oil shale semicoke were crucial. A two-dimensional mathematical model of oil shale semicoke particle combustion was developed and validated by combustion experiments of cylindrical Huadian oil shale semicoke particles on the self-designed Macro thermal gravimetric analyzer (TGA), with a weighting capacity over 100 g. There was obvious anisotropy during the combustion of cylindrical oil shale semicoke particles due to the difference of the effective diffusivity of oxygen between the directions parallel and perpendicular to the bedding planes. Combustion was faster along the direction parallel to the bedding planes than that perpendicular to the bedding planes. Theoretically, thicker oil shale semicoke particles with a ratio of diameter to thickness much less than one burnt faster, since they had a higher overall effective diffusivity of oxygen within the particles. A low oxygen concentration was harmful for the oil shale semicoke burnout due to a quite low conversion rate at the initial combustion stage.
1. Introduction Oil shale, as one of the most promising energy resources [1], has gained much more attentions as reserves of conventional fossil fuels decrease gradually. Crude oil could be produced by oil shale retort with plenty of semicoke as byproduct. Semicoke is a potentially harmful solid waste, often containing some toxic organic compounds and heavy metals, disposal of which can result in severe environmental contamination [1]. Combustion was considered as a good way to utilize oil shale semicoke, not only avoiding environmental contamination caused by landfill, but also recovering the heat in carbonaceous residues [2]. The description of oil shale semicoke combustion, especially the prediction of burnout rate and burnout time, was crucial to the industrial application of oil shale semicoke combustion. The ash content of oil shale semicoke can be higher than 70% in most cases, leading to a tight porous structure in ash particle of oil shale semicoke after combustion [3]. Thus, the diffusion resistance within ash particles is high, and usually dominates the combustion process, leading to a slow combustion rate [4]. Oil shale semicoke showed a similar combustion character like the shrinking-core model, and for
⁎
large oil shale semicoke particles, such as a diameter greater than 1.5 mm, an unreacted core will remain in the particle after a long-time combustion, proving the difficulty in burn out [5]. Combustion kinetics and mass transfer of oxygen in the ash layer of oil shale semicoke simultaneously played important roles in oil shale semicoke combustion. Lots of researches have been conducted to obtain reaction intrinsic kinetics of oil shale semicoke through combustion of fine oil shale semicoke particles on thermal gravimetric analyzer (TGA) [6–8]. It was found that partial pressure of oxygen, which represented oxygen concentration in gas mixture, had great influence on combustion kinetics of oil shale semicoke, and some kinetic models considering particle pressure of oxygen were proposed [9–12]. However, there were great difference between the kinetic parameters calculated by different researchers, including activation energy and influence factor of particle pressure of oxygen, due to the effects of shale original difference and other operation conditions [11]. There are numerous small pores in the oil shale semicoke particles, with different structures and diameters from 1 nm to 100 nm [13–15]. Both molecular diffusion and Knudsen diffusion occurred in the mass transfer of oxygen in the ash layer. Thus, the diffusivity of oxygen in the
Corresponding authors. E-mail addresses: [email protected] (H. Yang), [email protected] (Y. Jin).
https://doi.org/10.1016/j.fuel.2019.115891 Received 27 February 2019; Received in revised form 22 July 2019; Accepted 25 July 2019 Available online 01 August 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
Fuel 256 (2019) 115891
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oil shale semicoke ash layer should be variable with the pore structure and temperature. For instance, Yan et al. [16] found that the effective diffusivity of oxygen in the ash layer of oil shale semicoke would decrease with the ash layer thickness, which might be the cause of the existence of unreacted core in oil shale semicoke particles after longtime combustion. Thus, although oil shale semicoke showed a similar combustion character like the shrinking-core model, the one-dimensional shrinking-core model couldn’t yield a good agreement with the experiments if a constant diffusivity was used [17]. Besides, the effective diffusivities of oxygen in the ash layer were related to the diffusion directions, due to the bedding structure of oil shale [18]. However, so far, there were few models developed to consider the anisotropy of mass transfer of oxygen in the ash layer [19]. In this paper, a two-dimensional mathematical model of oil shale semicoke particle combustion was developed, which considered the effects of ash thickness and bedding structure anisotropy on the effective diffusivities. Cylindrical oil shale semicoke particles with different geometric sizes were combusted in a Macro-TGA to validate the twodimensional combustion model. Also, the burnout rate and burnout time of oil shale semicoke with different geometric sizes under different temperatures were analyzed. These results might help for the design of industrial application of oil shale semicoke combustion.
Fig. 1. Physical model of a cylindrical oil shale semicoke particle.
dependent reaction order. The energy balance for oil shale semicoke particle was written as the following:
[
YO2 = t
,
T = t
·(
T) +
dX ( dt
0)
H
(4) (5)
z
·(De YO2 ) +
dX ( dt MC
0)
(6)
where MC was the molar mass of carbon. The heat and mass flux at the center of the particle, r = 0, was defined as zero. The heat and mass flux at the particle surface could be calculated as the following:
q = hT (T
2.1. Effective diffusivity model
T ) + e (T 4 - T 4 )
m = hm (YO2,
An effective diffusivity model developed in our previous work was adopted to describe mass transfer of oxygen in the ash layer, which considered the effects of diffusion temperature and ash layer thickness. The effective diffusivity of oxygen at any ash layer location δ along the direction parallel to the bedding planes, De(δ) could be calculated from:
YO2 )
(7) (8)
where hT was the convective heat coefficient, e was the surface emissivity, σ was Stefan-Boltzmann constant, T∞ was the reactor temperature, hm was the convective mass coefficient, YO2,∞ was the oxygen concentration at the atmosphere. Heat and mass transfer coefficients were calculated based on Ranz-Marshall correlations. The simulation platform COMSOL Multiphysics was used to numerically solve the governing system. In all calculations completed for this work, grid size was set to be 1.5 × 10−4 m and time step was set to be 0.01 s. This combination yielded accurate results, while making the computation time acceptable.
(1)
where δ was the thickness of ash layer. And the effective diffusivity of oxygen at any ash layer location δ along the direction perpendicular to the bedding planes is 1/3 of that along the direction parallel to the bedding planes: (2)
2.3. Physical and chemical properties
This effective diffusivity model showed that the effective diffusivity would increase with diffusion temperature and decrease with ash layer thickness.
Some physical and chemical property values were required for the evaluation of the model, such as thermal conductivity and reaction heat of combustion. The physical and chemical property values used in the model were listed in Table 1, which are mostly based on Huadian oil shale in literatures.
2.2. Mathematical formulation The two-dimensional mathematical model was established on the basis of the cylindrical oil shale semicoke particle shown in Fig. 1. The mass balance for oil shale semicoke particle was written as the following:
dX = k (1 - X ) nYOm2 dt
r
0 )] Cp
where Cp was the heat capacity of solid residue, λ was the thermal conductivity of oil shale semicoke particle, △H was the reaction heat of oil shale semicoke combustion, r was the radial position, z was the axial position. The mass balance for oxygen in the oil shale semicoke particles was written as the following:
In this section, a two-dimensional mathematical model incorporating combustion reactions, oxygen mass transfer in the ash layer and external mass transfer was presented. In consideration of the bedding structure of oil shale and the model simplicity, oil shale semicoke particle was assumed to be cylindrical and isotropic along the circumferential direction. Besides, any change of oil shale semicoke particle shape and size was ignored. The heating up inside the particle during the combustion took place only by the conduction avenue, ignoring the convection, radiation and mass transfer inside the pores.
De ( ) = 5.3 × 10 7 (T /293)1.42exp( - 998.5 )
+ X(
=
2. Mathematical model
De ( ) = 1.6 × 10 6 (T /293)1.42exp( - 998.5 )
0
3. Experiment 3.1. Apparatus In order to investigate the characteristics of Huaidan oil shale semicoke and validate the two-dimensional mathematical model, thermogravimetric experiments of cylindrical Huadian oil shale semicoke particles with different geometric sizes were conducted. For the sample mass greater than several grams, a Macro-TGA with a weighting
(3)
where X was the conversion rate, k was the reaction rate constant, YO2 was the oxygen concentration, n was the residual carbon conversion rate-dependent reaction order, and m was the oxygen concentration2
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3.2. Samples
Table 1 Physical and chemical properties. Variable ρ0 Cp λ △H MC e σ
Particle density before combustion (m3/kg) Heat capacity of solid residue (J/(kg K)) Thermal conductivity of oil shale semicoke particle (W/(m K)) Reaction heat per kg fixed carbon of oil shale semicoke combustion (J/kg) Molar mass of carbon (g/mol) Surface emissivity Stefan-Boltzmann constant
Value
Reference
1200 2000 0.13
Measured [20] [20]
9 × 106
[21]
12 0.9 5.67 × 10-8
Constant Esimated Constant
Huadian oil shale was pyrolyzed under N2 atmosphere at 1073 K for 2 h to yield oil shale semicoke. The proximate properties of the Huadian oil shale semicoke were listed in Table 2. There were still a little volatiles remained in oil shale semicoke, which were kerogen or bitumen not released after pyrolysis. Oil shale semicoke particles were polished as cylinder, with the diameter d from 5 mm to 10 mm and the thickness z from 1.67 mm to 10 mm. A typical sample was shown in Fig. 3. After each combustion experiment, residue sample was grinded to fine powder with a diameter less than 120 μm. The fine powder was then combusted again to a constant weight, and the mass loss rate was used to calculate the real conversion rate in the combustion experiments of cylindrical particles. Besides, there were tree thin quartz pillars with a diameter of 1 mm and a height of 10 mm at the tray center. And the cylindrical particles were supported by these pillars on the round surface of cylindrical particles to ensure a good mass transfer environment.
capacity over 100 g was designed and manufactured, as shown in Fig. 2. The core component was an electrical heating furnace installed on a sliding rail, whose movement and heating could be controlled by the computer. A quartz tube was placed in the furnace to serve as the gas inlet and reaction chamber. The sample tray was placed on the analytical balance with an accuracy of 0.1 mg. The analytical balance was under the furnace and the quartz tube and connected to the computer to record the mass loss history. A thermocouple was installed near the sample tray to measure the temperature and provide temperature control signals. In each experiment, the electrical heating furnace was moved to the highest position and heated to the given temperature (873–1073 K). The gas mixture of 21% O2 and 79% Ar was opened with a volume flow rate of 2 L/min and the sample was placed on the tray. When the given temperature was obtained, the furnace was moved downward quickly to the lowest position, forming a good seal. Meantime, the analytical balance started to record the mass data. Blank experiment was conducted before each experiment to exclude the disturbance of buoyancy and gas flow.
4. Results and discussion 4.1. Experimental results and model validation Cylindrical Huadian oil shale semicoke particles with different geometry sizes were combusted at a constant combustion temperature of 1073 K. Several typical experimental results were shown in Fig. 4. As expected, smaller particles burnt faster. The conversion rate history curve of larger particle was flatter, showing greater diffusion resistance in ash layer during the combustion. Cylindrical Huadian oil shale semicoke particles with a diameter of 7.5 mm and a thickness of 2.5 mm were combusted at certain combustion temperatures of 873, 973 and 1073 K. The experimental results were shown in Fig. 5. Higher combustion temperature would lead to
Evacuation
850oC Ar O2 Gas mixing chamber Sliding rail
Sample tray
Ar O2
Analytical balance Computer Cooling water tank Fig. 2. Schematic of Macro-TGA. 3
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1
Table 2 The proximate analysis of the Huadian oil shale semicoke. Aar/%
Var/%
FCar/%
0.74
80.63
8.45
10.18
d=7.5 mm z=2.5 mm
0.8
Conversion X
Mar/%
T=1073 K
0.6
T=973 K 0.4
T=873 K 0.2
0
Experimental results Modelling results 0
100
200
300
400
500
600
Time (s) Fig. 5. Conversion rate histories of Huadian oil shale semicoke particles with the same geometry sizes at different combustion temperatures.
Fig. 3. Typical sample in the experiments.
1100 1000
Conversion X
d=7.5 mm z=2.5 mm
T=1073 K
0.8
Temperature (K)
1
d=7.5 mm z=7.5 mm
0.6
900 800 700 600
0.4
d=10 mm z=10 mm
500
0.2
0
T=1073 K d=7.5 mm z=2.5 mm
100
200
300
400
500
Modelling results 0
100
200
300
400
500
Time (s)
Experimental results Modelling results 0
Experimental results
Fig. 6. Particle surface heating histories of Huadian oil shale semicoke particles.
600
Time (s)
Fig. 7 showed the comparison of cross-sectional image between experimental samples (oblique section view) and modelling prediction (vertical section view) after long-time combustion (2200 s) for cylindrical particle with a diameter of 10 mm and a thickness of 10 mm at 1073 K. Even though the sample weight nearly kept constant at the end of combustion, there was still a black unreacted core in the particle. The model also predicted this unreacted core (blue zone in Fig. 7.B), caused by a huge diffusion resistance in the particle center. Besides, Fig. 8 (black solid line and red dash line) shows a good agreement on conversion rate between experimental results and modelling results. In order to investigate the effects of ash layer thickness on the oxygen diffusion, the modelling results with an effective diffusivity model ignoring the effects of ash layer thickness (De = 1.6 × 10 6 (T /293)1.42 ) was compared with that with the effective diffusivity model considering the effects of ash layer thickness (De = 1.6 × 10 6 (T /293)1.42exp( - 998.5 ) ), and the experimental results for the cylindrical particle with a diameter of 10 mm and a thickness of 10 mm at 1073 K in Fig. 8. At the initial stage of combustion (< 100 s), whether considering the negative effects of ash layer thickness on the effective diffusivity or not would hardly affect the modelling results. However, when the reaction enters into the control stage of ash layer diffusion resistance (after 200 s), the modelling prediction ignoring the negative effects of ash layer thickness on the effective diffusivity obviously overestimated the conversion rate, and the deviation compared with the experimental results became greater with the reaction continues. By contrast, when the negative effects of ash layer thickness on the effective diffusivity
Fig. 4. Conversion rate histories of Huadian oil shale semicoke particles with different geometry sizes at a constant combustion temperature of 1073 K.
faster combustion and a higher conversion rate after 600 s. While, different from the negative effects of geometry size, lower combustion temperature not only leaded to slower combustion but also a lower final conversion rate. As shown in Fig. 5, after a fast combustion at the first 100 s, the conversion rate increased little at 873 K and 973 K, showing that chemical reaction resistance played a greater role in the combustion process and the combustion temperature was not high enough to oxidize more fixed carbon in the oil shale semicoke particle. Though there were a little deviation in some cases, the modelling results agreed well with experimental results in general, showing the model could yield an acceptable prediction of the conversion rate history of Huadian oil shale semicoke combustion. In the combustion of cylindrical particle with a diameter of 7.5 mm and a thickness of 2.5 mm, another K-type thermocouple was installed at the center of the upper round surface and the surface temperature history was recorded. The experimental results and modelling results were shown in Fig. 6. The center temperature of the upper round surface increased quickly at the first tens of seconds and then increased at a lower rate. After about 200 s, the center temperature of the upper round surface reached the atmosphere temperature 1073 K. The modelling results also agreed well with the experimental results, showing a reasonable prediction in heat transfer. 4
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Y. Huang, et al.
A. Experimental sample with oblique section view
B. Modelling prediction with vertical section view
Fig. 7. Comparison of cross-sectional image between experimental samples and modelling prediction after long-time combustion (2200 s) for cylindrical particle with a diameter of 10 mm and a thickness of 10 mm at 1073 K.
Due to the difference of the effective diffusivity between the directions parallel and perpendicular to the bedding planes, the oxygen concentration at the plane z = 0 was higher than that at the plane r = 0, as shown in Fig. 10 A. Thus, combustion was faster along the direction parallel to the bedding planes than perpendicular to the bedding planes, as shown in Fig. 10 B. As a consequence, the reaction frontier advanced more quickly along the direction parallel to the bedding planes than that perpendicular to the bedding planes, as shown in Fig. 9.
1 T=1073 K d=10 mm z=10 mm
Conversion rate X
0.8 0.6 0.4
Experimental Results Modelling with effects of ash layer Modelling without effects of ash layer
0.2 0
0
500
1000 Time (s)
1500
4.3. Effects of geometric size As discussed in Section 4.2, there was obvious anisotropy during the combustion of cylindrical oil shale semicoke particles. Thus, geometric sizes might have important effects on the combustion rate and burnout time. Combustion times reaching X = 0.2 of cylindrical particles with a same volume of 110.4 mm3 but different geometric structures at a combustion temperature of 1073 K were compared to investigate effects of geometry sizes on the combustion rate. The geometric sizes in the modelling cases were listed in Table 3. Four different surface areas of the cylindrical modelling samples were set to investigate the effects of surface area. Two ratio of diameter to thickness was included for each surface area to investigate the effects of anisotropy. The cylindrical sample with a higher ratio of diameter to thickness and the same surface area was called as thin and the opposite was called as thick. As shown in Fig. 11, with the surface area increasing, the combustion time reaching X = 0.2 decreased obviously, due to more surface area for heating by the atmosphere and oxygen diffusing. Since the cylinder surface area was least when the diameter equaled to the thickness at the same volume, a higher difference between the diameter and the thickness would lead to faster combustion. Besides, with the same surface area, the conversion time of the thicker particle was always less than that of the thinner particle. When the particle was thicker which meant that the diameter was less than the thickness, the decrease of the effective diffusivity along the direction parallel to the bedding planes due to the ash layer thickness increasing was relatively slighter than that perpendicular to the bedding planes. In addition, the effective diffusivity along the direction parallel to the bedding planes was about three times than that perpendicular to the bedding planes. Thus, the overall effective diffusivity of oxygen within a thicker cylindrical particle was higher than that within a thinner cylindrical particle. As a consequence, a thicker cylindrical particle burnt faster than a thinner particle with the same volume. However, oil shale semicoke particles in industry application were usually thin with a high ratio of diameter to thickness from 2.5 to 6[19]. It was difficult to increase combustion rate by choosing thick oil
2000
Fig. 8. Effects of the effective diffusivity model on the modelling results.
were considered, the modelling results showed an obvious slower increase of conversion rate after 200 s, and agreed better with the experimental results. Thus, it was necessary for an accurate prediction to consider the negative effects of ash layer thickness on the effective diffusivity. Overall, the two-dimensional mathematical model could predict well the combustion of cylindrical Huadian oil shale semicoke with different geometry sizes at different combustion temperatures, which was validated by good agreements with experimental results including conversion rate, surface heating history and cross-sectional image. 4.2. Intraparticle combustion process The validated model could be used to analyze the intraparticle combustion process. Fig. 9 showed the details about particle heat transfer, conversion rate and oxygen diffusion within the cylindrical particle with a diameter of 10 mm and a thickness of 10 mm at 1073 K, in the view of vertical section. As shown in Fig. 9.A, the particle was heated quickly and reached a uniform temperature distribution quickly. Fig. 9.B and .C showed similar distribution at the same time, meaning that it was oxygen diffusion with the particle that controlled the combustion process. Combustion reaction rate was much faster than oxygen diffusion rate, thus oxygen was consumed immediately once they diffused into unreacted zone within the particle. Fig. 10 showed oxygen concentration and conversion rate histories on the plane z = 0 (the center plane parallel to the bedding planes) and the plane r = 0 (the center plane perpendicular to the bedding planes).
5
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Y. Huang, et al.
A. Temperature field (K)
B. Conversion rate field
C. Oxygen concentration field (mol/m3) Fig. 9. Intraparticle details during the combustion.
shale semicoke particles with a ratio of diameter to thickness much less than one. So, thinner oil shale semicoke particles with a ratio of diameter to thickness much greater than one were more suitable for burnout.
concentration within the particles increased and then combustion reaction also accelerated because the combustion reaction rate was related to the oxygen concentration. It was worth mentioning that at a combustion temperature of 873 K, when the oxygen concentration decreased to 5%, the conversion time was significantly longer than that under the oxygen concentration of 10% to 21%, due to a much lower conversion rate at the initial stage. A low oxygen concentration was harmful for the oil shale semicoke burnout.
4.4. Effects of oxygen concentration Since oxygen diffusion within the ash layer usually dominated combustion of oil shale semicoke, oxygen concentration in the atmosphere might also play important roles, which determined the highest oxygen concentration within the ash layer and affected oxygen diffusion also. Fig. 12 showed the conversion histories of oil shale semicoke particles with a diameter of 7.5 mm and a thickness of 2.5 mm at 873 K under different oxygen concentrations from 5% to 21%. With the oxygen concentration in the atmosphere increasing, the oxygen
5. Conclusion In this paper, two-dimensional mathematical model of oil shale semicoke particle combustion was developed, which considered the effects of ash thickness and bedding structure anisotropy on the effective diffusivities. 6
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Y. Huang, et al.
1
0.3
B
0.8
Conversion rate X
O2 concentration (mol/m3)
A 0.2
0.1
z=0 r=0
0.6 0.4 0.2
z=0 r=0 0
0
100
200
300
400
500
0
600
Time (s)
0
100
200
300 Time (s)
400
500
600
Fig. 10. Comparison of oxygen concentration (A) and conversion rate (B) histories between planes along different directions (T = 1073 K, d = 10 mm, z = 10 mm). 0.30
Thickness (mm)
Diameter/Thickness
Surface area (mm2)
3.44 7.50 3.16 8.00 2.80 8.70 2.29 10.00
11.88 2.50 14.08 2.20 17.94 1.86 26.82 1.41
0.29 3.00 0.22 3.64 0.16 4.68 0.09 7.11
147
0.25
Conversion X
Diamter (mm)
2.50E-03
155 170
2.00E-03
0.20
1.50E-03
0.15
O2-5% O2-10%
0.10
O2-15%
0.05
201
0.00
5.00E-04
O2-21% 0
200
400
600
800
0.00E+00 1000
Time (s)
40 thick
Conversion time (s)
1.00E-03
Conversion rate dX/dt (s-1)
Table 3 The geometric sizes of the modelling samples with a same volume of 110.4 mm3.
Fig. 12. Conversion histories of oil shale semicoke particles with a diameter of 7.5 mm and a thickness of 2.5 mm at 873 K under different oxygen concentrations from 5% to 21%.
thin
30
Acknowledgement
20
Financial support of this work by the National Natural Science Foundation of China (U1810126) are gratefully acknowledged.
10
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Surface area (mm2) Fig. 11. Conversion times reaching X = 0.2 of cylindrical particles with a same volume but different surface areas and ratios of diameter to thickness (T = 1073 K).
The two-dimensional mathematical model was validated with experimental results including conversion rate, surface heating history and cross-sectional image for combustion of cylindrical Huadian oil shale semicoke with different geometry sizes at different combustion temperatures. There was obvious anisotropy during the combustion of cylindrical oil shale semicoke particles. Combustion was faster along the direction parallel to the bedding planes than direction perpendicular to the bedding planes. Theoretically, thicker oil shale semicoke particles with a ratio of diameter to thickness much less than one burnt faster, since they had a higher overall effective diffusivity of oxygen within the particles. Oxygen concentration affected significantly on the combustion rate at the initial combustion stage and a lower oxygen concentration would lead to a longer burnout time.
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Full Length Article
Two-dimensional combustion modelling and experimental research on oil shale semicoke
T
Yiqun Huanga, Man Zhanga, Boyu Denga, Hao Konga, Yanjun Zhangb, Junfu Lyua, Hairui Yanga, , ⁎ Yan Jinc, ⁎
a
Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, State Key Laboratory of Power Systems, Department of Energy and Power Engineering, Tsinghua University, Beijing 100084, China b State Key Laboratory of Efficient and Clean Coal-fired Utility Boilers (Harbin Boiler Company Limited), Harbin 150046, China c Taiyun Science and Technology University, Taiyuan 030024, China
ARTICLE INFO
ABSTRACT
Keywords: Oil shale Semicoke Combustion Diffusivity Anisotropy
Due to high ash content and anisotropy of bedding structure, the effects of ash thickness and bedding structure anisotropy on combustion of oil shale semicoke were crucial. A two-dimensional mathematical model of oil shale semicoke particle combustion was developed and validated by combustion experiments of cylindrical Huadian oil shale semicoke particles on the self-designed Macro thermal gravimetric analyzer (TGA), with a weighting capacity over 100 g. There was obvious anisotropy during the combustion of cylindrical oil shale semicoke particles due to the difference of the effective diffusivity of oxygen between the directions parallel and perpendicular to the bedding planes. Combustion was faster along the direction parallel to the bedding planes than that perpendicular to the bedding planes. Theoretically, thicker oil shale semicoke particles with a ratio of diameter to thickness much less than one burnt faster, since they had a higher overall effective diffusivity of oxygen within the particles. A low oxygen concentration was harmful for the oil shale semicoke burnout due to a quite low conversion rate at the initial combustion stage.
1. Introduction Oil shale, as one of the most promising energy resources [1], has gained much more attentions as reserves of conventional fossil fuels decrease gradually. Crude oil could be produced by oil shale retort with plenty of semicoke as byproduct. Semicoke is a potentially harmful solid waste, often containing some toxic organic compounds and heavy metals, disposal of which can result in severe environmental contamination [1]. Combustion was considered as a good way to utilize oil shale semicoke, not only avoiding environmental contamination caused by landfill, but also recovering the heat in carbonaceous residues [2]. The description of oil shale semicoke combustion, especially the prediction of burnout rate and burnout time, was crucial to the industrial application of oil shale semicoke combustion. The ash content of oil shale semicoke can be higher than 70% in most cases, leading to a tight porous structure in ash particle of oil shale semicoke after combustion [3]. Thus, the diffusion resistance within ash particles is high, and usually dominates the combustion process, leading to a slow combustion rate [4]. Oil shale semicoke showed a similar combustion character like the shrinking-core model, and for
⁎
large oil shale semicoke particles, such as a diameter greater than 1.5 mm, an unreacted core will remain in the particle after a long-time combustion, proving the difficulty in burn out [5]. Combustion kinetics and mass transfer of oxygen in the ash layer of oil shale semicoke simultaneously played important roles in oil shale semicoke combustion. Lots of researches have been conducted to obtain reaction intrinsic kinetics of oil shale semicoke through combustion of fine oil shale semicoke particles on thermal gravimetric analyzer (TGA) [6–8]. It was found that partial pressure of oxygen, which represented oxygen concentration in gas mixture, had great influence on combustion kinetics of oil shale semicoke, and some kinetic models considering particle pressure of oxygen were proposed [9–12]. However, there were great difference between the kinetic parameters calculated by different researchers, including activation energy and influence factor of particle pressure of oxygen, due to the effects of shale original difference and other operation conditions [11]. There are numerous small pores in the oil shale semicoke particles, with different structures and diameters from 1 nm to 100 nm [13–15]. Both molecular diffusion and Knudsen diffusion occurred in the mass transfer of oxygen in the ash layer. Thus, the diffusivity of oxygen in the
Corresponding authors. E-mail addresses: [email protected] (H. Yang), [email protected] (Y. Jin).
https://doi.org/10.1016/j.fuel.2019.115891 Received 27 February 2019; Received in revised form 22 July 2019; Accepted 25 July 2019 Available online 01 August 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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oil shale semicoke ash layer should be variable with the pore structure and temperature. For instance, Yan et al. [16] found that the effective diffusivity of oxygen in the ash layer of oil shale semicoke would decrease with the ash layer thickness, which might be the cause of the existence of unreacted core in oil shale semicoke particles after longtime combustion. Thus, although oil shale semicoke showed a similar combustion character like the shrinking-core model, the one-dimensional shrinking-core model couldn’t yield a good agreement with the experiments if a constant diffusivity was used [17]. Besides, the effective diffusivities of oxygen in the ash layer were related to the diffusion directions, due to the bedding structure of oil shale [18]. However, so far, there were few models developed to consider the anisotropy of mass transfer of oxygen in the ash layer [19]. In this paper, a two-dimensional mathematical model of oil shale semicoke particle combustion was developed, which considered the effects of ash thickness and bedding structure anisotropy on the effective diffusivities. Cylindrical oil shale semicoke particles with different geometric sizes were combusted in a Macro-TGA to validate the twodimensional combustion model. Also, the burnout rate and burnout time of oil shale semicoke with different geometric sizes under different temperatures were analyzed. These results might help for the design of industrial application of oil shale semicoke combustion.
Fig. 1. Physical model of a cylindrical oil shale semicoke particle.
dependent reaction order. The energy balance for oil shale semicoke particle was written as the following:
[
YO2 = t
,
T = t
·(
T) +
dX ( dt
0)
H
(4) (5)
z
·(De YO2 ) +
dX ( dt MC
0)
(6)
where MC was the molar mass of carbon. The heat and mass flux at the center of the particle, r = 0, was defined as zero. The heat and mass flux at the particle surface could be calculated as the following:
q = hT (T
2.1. Effective diffusivity model
T ) + e (T 4 - T 4 )
m = hm (YO2,
An effective diffusivity model developed in our previous work was adopted to describe mass transfer of oxygen in the ash layer, which considered the effects of diffusion temperature and ash layer thickness. The effective diffusivity of oxygen at any ash layer location δ along the direction parallel to the bedding planes, De(δ) could be calculated from:
YO2 )
(7) (8)
where hT was the convective heat coefficient, e was the surface emissivity, σ was Stefan-Boltzmann constant, T∞ was the reactor temperature, hm was the convective mass coefficient, YO2,∞ was the oxygen concentration at the atmosphere. Heat and mass transfer coefficients were calculated based on Ranz-Marshall correlations. The simulation platform COMSOL Multiphysics was used to numerically solve the governing system. In all calculations completed for this work, grid size was set to be 1.5 × 10−4 m and time step was set to be 0.01 s. This combination yielded accurate results, while making the computation time acceptable.
(1)
where δ was the thickness of ash layer. And the effective diffusivity of oxygen at any ash layer location δ along the direction perpendicular to the bedding planes is 1/3 of that along the direction parallel to the bedding planes: (2)
2.3. Physical and chemical properties
This effective diffusivity model showed that the effective diffusivity would increase with diffusion temperature and decrease with ash layer thickness.
Some physical and chemical property values were required for the evaluation of the model, such as thermal conductivity and reaction heat of combustion. The physical and chemical property values used in the model were listed in Table 1, which are mostly based on Huadian oil shale in literatures.
2.2. Mathematical formulation The two-dimensional mathematical model was established on the basis of the cylindrical oil shale semicoke particle shown in Fig. 1. The mass balance for oil shale semicoke particle was written as the following:
dX = k (1 - X ) nYOm2 dt
r
0 )] Cp
where Cp was the heat capacity of solid residue, λ was the thermal conductivity of oil shale semicoke particle, △H was the reaction heat of oil shale semicoke combustion, r was the radial position, z was the axial position. The mass balance for oxygen in the oil shale semicoke particles was written as the following:
In this section, a two-dimensional mathematical model incorporating combustion reactions, oxygen mass transfer in the ash layer and external mass transfer was presented. In consideration of the bedding structure of oil shale and the model simplicity, oil shale semicoke particle was assumed to be cylindrical and isotropic along the circumferential direction. Besides, any change of oil shale semicoke particle shape and size was ignored. The heating up inside the particle during the combustion took place only by the conduction avenue, ignoring the convection, radiation and mass transfer inside the pores.
De ( ) = 5.3 × 10 7 (T /293)1.42exp( - 998.5 )
+ X(
=
2. Mathematical model
De ( ) = 1.6 × 10 6 (T /293)1.42exp( - 998.5 )
0
3. Experiment 3.1. Apparatus In order to investigate the characteristics of Huaidan oil shale semicoke and validate the two-dimensional mathematical model, thermogravimetric experiments of cylindrical Huadian oil shale semicoke particles with different geometric sizes were conducted. For the sample mass greater than several grams, a Macro-TGA with a weighting
(3)
where X was the conversion rate, k was the reaction rate constant, YO2 was the oxygen concentration, n was the residual carbon conversion rate-dependent reaction order, and m was the oxygen concentration2
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3.2. Samples
Table 1 Physical and chemical properties. Variable ρ0 Cp λ △H MC e σ
Particle density before combustion (m3/kg) Heat capacity of solid residue (J/(kg K)) Thermal conductivity of oil shale semicoke particle (W/(m K)) Reaction heat per kg fixed carbon of oil shale semicoke combustion (J/kg) Molar mass of carbon (g/mol) Surface emissivity Stefan-Boltzmann constant
Value
Reference
1200 2000 0.13
Measured [20] [20]
9 × 106
[21]
12 0.9 5.67 × 10-8
Constant Esimated Constant
Huadian oil shale was pyrolyzed under N2 atmosphere at 1073 K for 2 h to yield oil shale semicoke. The proximate properties of the Huadian oil shale semicoke were listed in Table 2. There were still a little volatiles remained in oil shale semicoke, which were kerogen or bitumen not released after pyrolysis. Oil shale semicoke particles were polished as cylinder, with the diameter d from 5 mm to 10 mm and the thickness z from 1.67 mm to 10 mm. A typical sample was shown in Fig. 3. After each combustion experiment, residue sample was grinded to fine powder with a diameter less than 120 μm. The fine powder was then combusted again to a constant weight, and the mass loss rate was used to calculate the real conversion rate in the combustion experiments of cylindrical particles. Besides, there were tree thin quartz pillars with a diameter of 1 mm and a height of 10 mm at the tray center. And the cylindrical particles were supported by these pillars on the round surface of cylindrical particles to ensure a good mass transfer environment.
capacity over 100 g was designed and manufactured, as shown in Fig. 2. The core component was an electrical heating furnace installed on a sliding rail, whose movement and heating could be controlled by the computer. A quartz tube was placed in the furnace to serve as the gas inlet and reaction chamber. The sample tray was placed on the analytical balance with an accuracy of 0.1 mg. The analytical balance was under the furnace and the quartz tube and connected to the computer to record the mass loss history. A thermocouple was installed near the sample tray to measure the temperature and provide temperature control signals. In each experiment, the electrical heating furnace was moved to the highest position and heated to the given temperature (873–1073 K). The gas mixture of 21% O2 and 79% Ar was opened with a volume flow rate of 2 L/min and the sample was placed on the tray. When the given temperature was obtained, the furnace was moved downward quickly to the lowest position, forming a good seal. Meantime, the analytical balance started to record the mass data. Blank experiment was conducted before each experiment to exclude the disturbance of buoyancy and gas flow.
4. Results and discussion 4.1. Experimental results and model validation Cylindrical Huadian oil shale semicoke particles with different geometry sizes were combusted at a constant combustion temperature of 1073 K. Several typical experimental results were shown in Fig. 4. As expected, smaller particles burnt faster. The conversion rate history curve of larger particle was flatter, showing greater diffusion resistance in ash layer during the combustion. Cylindrical Huadian oil shale semicoke particles with a diameter of 7.5 mm and a thickness of 2.5 mm were combusted at certain combustion temperatures of 873, 973 and 1073 K. The experimental results were shown in Fig. 5. Higher combustion temperature would lead to
Evacuation
850oC Ar O2 Gas mixing chamber Sliding rail
Sample tray
Ar O2
Analytical balance Computer Cooling water tank Fig. 2. Schematic of Macro-TGA. 3
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1
Table 2 The proximate analysis of the Huadian oil shale semicoke. Aar/%
Var/%
FCar/%
0.74
80.63
8.45
10.18
d=7.5 mm z=2.5 mm
0.8
Conversion X
Mar/%
T=1073 K
0.6
T=973 K 0.4
T=873 K 0.2
0
Experimental results Modelling results 0
100
200
300
400
500
600
Time (s) Fig. 5. Conversion rate histories of Huadian oil shale semicoke particles with the same geometry sizes at different combustion temperatures.
Fig. 3. Typical sample in the experiments.
1100 1000
Conversion X
d=7.5 mm z=2.5 mm
T=1073 K
0.8
Temperature (K)
1
d=7.5 mm z=7.5 mm
0.6
900 800 700 600
0.4
d=10 mm z=10 mm
500
0.2
0
T=1073 K d=7.5 mm z=2.5 mm
100
200
300
400
500
Modelling results 0
100
200
300
400
500
Time (s)
Experimental results Modelling results 0
Experimental results
Fig. 6. Particle surface heating histories of Huadian oil shale semicoke particles.
600
Time (s)
Fig. 7 showed the comparison of cross-sectional image between experimental samples (oblique section view) and modelling prediction (vertical section view) after long-time combustion (2200 s) for cylindrical particle with a diameter of 10 mm and a thickness of 10 mm at 1073 K. Even though the sample weight nearly kept constant at the end of combustion, there was still a black unreacted core in the particle. The model also predicted this unreacted core (blue zone in Fig. 7.B), caused by a huge diffusion resistance in the particle center. Besides, Fig. 8 (black solid line and red dash line) shows a good agreement on conversion rate between experimental results and modelling results. In order to investigate the effects of ash layer thickness on the oxygen diffusion, the modelling results with an effective diffusivity model ignoring the effects of ash layer thickness (De = 1.6 × 10 6 (T /293)1.42 ) was compared with that with the effective diffusivity model considering the effects of ash layer thickness (De = 1.6 × 10 6 (T /293)1.42exp( - 998.5 ) ), and the experimental results for the cylindrical particle with a diameter of 10 mm and a thickness of 10 mm at 1073 K in Fig. 8. At the initial stage of combustion (< 100 s), whether considering the negative effects of ash layer thickness on the effective diffusivity or not would hardly affect the modelling results. However, when the reaction enters into the control stage of ash layer diffusion resistance (after 200 s), the modelling prediction ignoring the negative effects of ash layer thickness on the effective diffusivity obviously overestimated the conversion rate, and the deviation compared with the experimental results became greater with the reaction continues. By contrast, when the negative effects of ash layer thickness on the effective diffusivity
Fig. 4. Conversion rate histories of Huadian oil shale semicoke particles with different geometry sizes at a constant combustion temperature of 1073 K.
faster combustion and a higher conversion rate after 600 s. While, different from the negative effects of geometry size, lower combustion temperature not only leaded to slower combustion but also a lower final conversion rate. As shown in Fig. 5, after a fast combustion at the first 100 s, the conversion rate increased little at 873 K and 973 K, showing that chemical reaction resistance played a greater role in the combustion process and the combustion temperature was not high enough to oxidize more fixed carbon in the oil shale semicoke particle. Though there were a little deviation in some cases, the modelling results agreed well with experimental results in general, showing the model could yield an acceptable prediction of the conversion rate history of Huadian oil shale semicoke combustion. In the combustion of cylindrical particle with a diameter of 7.5 mm and a thickness of 2.5 mm, another K-type thermocouple was installed at the center of the upper round surface and the surface temperature history was recorded. The experimental results and modelling results were shown in Fig. 6. The center temperature of the upper round surface increased quickly at the first tens of seconds and then increased at a lower rate. After about 200 s, the center temperature of the upper round surface reached the atmosphere temperature 1073 K. The modelling results also agreed well with the experimental results, showing a reasonable prediction in heat transfer. 4
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A. Experimental sample with oblique section view
B. Modelling prediction with vertical section view
Fig. 7. Comparison of cross-sectional image between experimental samples and modelling prediction after long-time combustion (2200 s) for cylindrical particle with a diameter of 10 mm and a thickness of 10 mm at 1073 K.
Due to the difference of the effective diffusivity between the directions parallel and perpendicular to the bedding planes, the oxygen concentration at the plane z = 0 was higher than that at the plane r = 0, as shown in Fig. 10 A. Thus, combustion was faster along the direction parallel to the bedding planes than perpendicular to the bedding planes, as shown in Fig. 10 B. As a consequence, the reaction frontier advanced more quickly along the direction parallel to the bedding planes than that perpendicular to the bedding planes, as shown in Fig. 9.
1 T=1073 K d=10 mm z=10 mm
Conversion rate X
0.8 0.6 0.4
Experimental Results Modelling with effects of ash layer Modelling without effects of ash layer
0.2 0
0
500
1000 Time (s)
1500
4.3. Effects of geometric size As discussed in Section 4.2, there was obvious anisotropy during the combustion of cylindrical oil shale semicoke particles. Thus, geometric sizes might have important effects on the combustion rate and burnout time. Combustion times reaching X = 0.2 of cylindrical particles with a same volume of 110.4 mm3 but different geometric structures at a combustion temperature of 1073 K were compared to investigate effects of geometry sizes on the combustion rate. The geometric sizes in the modelling cases were listed in Table 3. Four different surface areas of the cylindrical modelling samples were set to investigate the effects of surface area. Two ratio of diameter to thickness was included for each surface area to investigate the effects of anisotropy. The cylindrical sample with a higher ratio of diameter to thickness and the same surface area was called as thin and the opposite was called as thick. As shown in Fig. 11, with the surface area increasing, the combustion time reaching X = 0.2 decreased obviously, due to more surface area for heating by the atmosphere and oxygen diffusing. Since the cylinder surface area was least when the diameter equaled to the thickness at the same volume, a higher difference between the diameter and the thickness would lead to faster combustion. Besides, with the same surface area, the conversion time of the thicker particle was always less than that of the thinner particle. When the particle was thicker which meant that the diameter was less than the thickness, the decrease of the effective diffusivity along the direction parallel to the bedding planes due to the ash layer thickness increasing was relatively slighter than that perpendicular to the bedding planes. In addition, the effective diffusivity along the direction parallel to the bedding planes was about three times than that perpendicular to the bedding planes. Thus, the overall effective diffusivity of oxygen within a thicker cylindrical particle was higher than that within a thinner cylindrical particle. As a consequence, a thicker cylindrical particle burnt faster than a thinner particle with the same volume. However, oil shale semicoke particles in industry application were usually thin with a high ratio of diameter to thickness from 2.5 to 6[19]. It was difficult to increase combustion rate by choosing thick oil
2000
Fig. 8. Effects of the effective diffusivity model on the modelling results.
were considered, the modelling results showed an obvious slower increase of conversion rate after 200 s, and agreed better with the experimental results. Thus, it was necessary for an accurate prediction to consider the negative effects of ash layer thickness on the effective diffusivity. Overall, the two-dimensional mathematical model could predict well the combustion of cylindrical Huadian oil shale semicoke with different geometry sizes at different combustion temperatures, which was validated by good agreements with experimental results including conversion rate, surface heating history and cross-sectional image. 4.2. Intraparticle combustion process The validated model could be used to analyze the intraparticle combustion process. Fig. 9 showed the details about particle heat transfer, conversion rate and oxygen diffusion within the cylindrical particle with a diameter of 10 mm and a thickness of 10 mm at 1073 K, in the view of vertical section. As shown in Fig. 9.A, the particle was heated quickly and reached a uniform temperature distribution quickly. Fig. 9.B and .C showed similar distribution at the same time, meaning that it was oxygen diffusion with the particle that controlled the combustion process. Combustion reaction rate was much faster than oxygen diffusion rate, thus oxygen was consumed immediately once they diffused into unreacted zone within the particle. Fig. 10 showed oxygen concentration and conversion rate histories on the plane z = 0 (the center plane parallel to the bedding planes) and the plane r = 0 (the center plane perpendicular to the bedding planes).
5
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Y. Huang, et al.
A. Temperature field (K)
B. Conversion rate field
C. Oxygen concentration field (mol/m3) Fig. 9. Intraparticle details during the combustion.
shale semicoke particles with a ratio of diameter to thickness much less than one. So, thinner oil shale semicoke particles with a ratio of diameter to thickness much greater than one were more suitable for burnout.
concentration within the particles increased and then combustion reaction also accelerated because the combustion reaction rate was related to the oxygen concentration. It was worth mentioning that at a combustion temperature of 873 K, when the oxygen concentration decreased to 5%, the conversion time was significantly longer than that under the oxygen concentration of 10% to 21%, due to a much lower conversion rate at the initial stage. A low oxygen concentration was harmful for the oil shale semicoke burnout.
4.4. Effects of oxygen concentration Since oxygen diffusion within the ash layer usually dominated combustion of oil shale semicoke, oxygen concentration in the atmosphere might also play important roles, which determined the highest oxygen concentration within the ash layer and affected oxygen diffusion also. Fig. 12 showed the conversion histories of oil shale semicoke particles with a diameter of 7.5 mm and a thickness of 2.5 mm at 873 K under different oxygen concentrations from 5% to 21%. With the oxygen concentration in the atmosphere increasing, the oxygen
5. Conclusion In this paper, two-dimensional mathematical model of oil shale semicoke particle combustion was developed, which considered the effects of ash thickness and bedding structure anisotropy on the effective diffusivities. 6
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1
0.3
B
0.8
Conversion rate X
O2 concentration (mol/m3)
A 0.2
0.1
z=0 r=0
0.6 0.4 0.2
z=0 r=0 0
0
100
200
300
400
500
0
600
Time (s)
0
100
200
300 Time (s)
400
500
600
Fig. 10. Comparison of oxygen concentration (A) and conversion rate (B) histories between planes along different directions (T = 1073 K, d = 10 mm, z = 10 mm). 0.30
Thickness (mm)
Diameter/Thickness
Surface area (mm2)
3.44 7.50 3.16 8.00 2.80 8.70 2.29 10.00
11.88 2.50 14.08 2.20 17.94 1.86 26.82 1.41
0.29 3.00 0.22 3.64 0.16 4.68 0.09 7.11
147
0.25
Conversion X
Diamter (mm)
2.50E-03
155 170
2.00E-03
0.20
1.50E-03
0.15
O2-5% O2-10%
0.10
O2-15%
0.05
201
0.00
5.00E-04
O2-21% 0
200
400
600
800
0.00E+00 1000
Time (s)
40 thick
Conversion time (s)
1.00E-03
Conversion rate dX/dt (s-1)
Table 3 The geometric sizes of the modelling samples with a same volume of 110.4 mm3.
Fig. 12. Conversion histories of oil shale semicoke particles with a diameter of 7.5 mm and a thickness of 2.5 mm at 873 K under different oxygen concentrations from 5% to 21%.
thin
30
Acknowledgement
20
Financial support of this work by the National Natural Science Foundation of China (U1810126) are gratefully acknowledged.
10
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The two-dimensional mathematical model was validated with experimental results including conversion rate, surface heating history and cross-sectional image for combustion of cylindrical Huadian oil shale semicoke with different geometry sizes at different combustion temperatures. There was obvious anisotropy during the combustion of cylindrical oil shale semicoke particles. Combustion was faster along the direction parallel to the bedding planes than direction perpendicular to the bedding planes. Theoretically, thicker oil shale semicoke particles with a ratio of diameter to thickness much less than one burnt faster, since they had a higher overall effective diffusivity of oxygen within the particles. Oxygen concentration affected significantly on the combustion rate at the initial combustion stage and a lower oxygen concentration would lead to a longer burnout time.
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