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Pyrolysis of pulverized coal to acetylene in magnetically rotating hydrogen plasma reactor
Fuel Processing Technology 167 (2017) 721–729
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Research article
Pyrolysis of pulverized coal to acetylene in magnetically rotating hydrogen plasma reactor Jie Ma, Baogen Su, Guangdong Wen, Qiwei Yang, Qilong Ren ⁎, Yiwen Yang, Huabin Xing Key Laboratory of Biomass Chemical Engineering of Ministry of Education, College of Chemical and Biological Engineering, Zhejiang University, Zheda Road 38, Hangzhou 310027, China
a r t i c l e
i n f o
Article history: Received 29 September 2016 Received in revised form 17 June 2017 Accepted 20 June 2017 Available online 24 August 2017 Keywords: Acetylene Coal Pyrolysis Rotating plasma
a b s t r a c t This paper presents a clean and one-step way to produce acetylene by plasma pyrolysis of coal. A self-designed MW-scale rotating plasma reactor is used to investigate the pyrolysis of pulverized coal, characterized by the upstream injection of coal particles into the rotating plasma. The rotating plasma arc has a stirring effect and the upstream injection facilitates particle transportation, allowing for a good mixing of coal with plasma and high heattransfer efficiency. The effects of magnetic coil current, coal flow rate and synergetic parameters on the pyrolysis were studied, and the optimum operating conditions of pyrolysis process were screened by taking consideration of acetylene mole fraction, yield, specific energy requirement and coal conversion rate. The minimum specific energy requirement of 9.50 kWh/kg-C2H2 was achieved with a magnetic coil current of 875 A. © 2017 Published by Elsevier B.V.
1. Introduction Acetylene is one important raw material in chemical industries, which is used for the production of polyvinyl chloride (PVC), 1, 4butanediol, vinyl acetate, etc. [1] The partial oxidation (POX) method and calcium carbide (CaC2) method are currently the two main approaches to produce acetylene [2,3]. The calcium carbide process is energy-intensive with considerable CO2 emissions [4]. Because of the abundant resources of coal, calcium carbide method is still the dominant method to produce acetylene in China. Plasma conversion of carbonaceous materials such as coal, biomass and hydrocarbons, has been the research focuses recently [5–11].The target products vary from low hydrocarbons (C2H2, C2H4) to syngas or hydrogen [12,13], which depends on the process, reacting medium and feed compositions. Depending on the differences in the temperatures of electron (Te) and ions (Ti), plasmas could be categorized as thermal (Te − Ti) and non-thermal plasmas (Te ≫ Ti) [14]. The nonthermal plasma is widely applied to low temperature process such as environmental pollutants treatment [15,16]. However, the thermal plasma with characteristics of high enthalpy, high chemical reactivity and adjustable medium [17], provides an effective way for the highly endothermic process such as the production of acetylene [18]. Chen et al. investigated the pyrolysis of coal to acetylene with a V-shaped plasma torch up to 2.12 MW, and the acetylene volume fraction of 9.4% was achieved [19]. Yan et al. conducted a comprehensive research in the plasma pyrolysis of coal in a lab-scale plasma reactor, and results
⁎ Corresponding author. E-mail address: [email protected] (Q. Ren).
http://dx.doi.org/10.1016/j.fuproc.2017.06.022 0378-3820/© 2017 Published by Elsevier B.V.
indicated that the acetylene yield was sensitive to the coal rank and compositions [20]. Generally, thermal plasma conversion of chemicals suffers from poor mixing efficiency in heterogeneous reaction system, which is attributed to the asymmetric and inhomogeneous nature of the thermal plasma. To overcome the disadvantage, the mixing efficiency could be improved by adjusting the injection angle of coal particle, and the particle heating efficiency depends strongly on the inlet design [21]. Furthermore, external magnetic field is also one promising way to disperse the plasma arc [22]. The rotating plasma is driven by the Lorentz force caused by the external magnetic field, which helps to improve the plasma uniformity. Furthermore, magnetically rotating plasma has a stirring effect and will enhance the mixing efficiency between the reactants and the hot plasma jet. The mechanism of plasma pyrolysis of coal (Fig. 1) includes the evaporation of the moisture, devolatilization of the volatile components and char reaction [23]. The primary pyrolysis products can be classified as light gas, tar and char in the coal chemical percolation devolatilization (CPD) model [24–26]. The primary volatiles will undergo secondary reactions, producing secondary pyrolysis products. The reaction of volatile is the crucial step for the formation of acetylene. Generally, acetylene yield is determined by both the content and constituent of the volatile components released from coal particles [18,27,28]. Traditional hydropyrolysis of coal is carried out under high pressure [28–30], with methane as the primary product. The plasma hydropyrolysis of coal is carried out at atmospheric pressure, high temperature and relatively short residence time (varying from several milliseconds in a lab-scale reactor [20] to around 18.84 milliseconds in a pilot-scale reactor [19]), with acetylene as the primary product. Owing to the unique feature of descending trend of the Gibbs energy with increasing temperature,
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Fig. 1. Schematic view of the plasma pyrolysis of coal to acetylene.
when the temperature in the reaction system is above 1600 K, acetylene becomes the main light hydrocarbon component among the pyrolysis products [31]. Various parameters affecting the plasma pyrolysis of coal such as plasma gas type, coal properties, specific enthalpy input and particle size [20,27,28,32], are mostly investigated in lab-scale reactors. Due to the high experimental cost, few pilot-scale plasma reactors are reported. In summary, the yield and the specific energy requirement (SER) of acetylene can be concluded as three crucial factors: (1) sufficient energy input, which is accounted for the effective heat transfer between the plasma and coal particles, and achieving high heating rate and high volatile yield; (2) compatible coal loading ratio, which is associated with coal conversion efficiency and acetylene yield; (3) coal type, which determines the amount and the constituents of the volatile matters released from coal particles. Most available experiments have been carried out with the coal particles injected downstream of the plasma jet. However, it is difficult for the particle to be transported into the hot plasma core zone due to the thermophoretic force. Generally, coal particles could be injected at the downstream (at the plasma jet downstream) or upstream (pre-mixed with carrying gas) of the plasma jet, and the pre-mixed method showed a better conversion efficiency [33]. This paper focuses on the plasma pyrolysis of pulverized coal with one pilot-scale (~1 MW) magnetically rotating plasma reactor. The main advantage of the rotating plasma is that the coal particles are injected at the upstream of the plasma, leading to good mixing and heat transfer efficiency. Effect of magnetic coil current and coal flow rate under different power input on the pyrolysis performance were investigated. In particular, the comprehensive effects of the coal/H2 ratio and specific energy input were investigated to optimize the pyrolysis process. 2. Experimental section 2.1. Coal properties Coal properties are listed in Table 1, and coal particle size distribution is analyzed by the Winner 318A laser particle instrument (Jinan Winner Particle Technology Co., Ltd.). The particle size distribution is shown in Fig. 2, with the average size of 30 μm. 2.2. Experimental reactor and procedures The plasma arc reactor (Fig. 3) includes the electrodes, external magnetic coils and feed inlet system. The rated power is ~ 1 MW with the IGBT (Insulated Gate Bipolar Transistor) based power supply. The cathode is graphite with diameter of 2.0 cm, and the annular anode is
copper with chamber diameter of 10.0 cm and length of 40.0 cm. To reduce the erosion caused by the hot plasma jet, the anode chamber is cooled by circling water, the reactor is wrapped with magnetic coils to generate axial magnetic field. The plasma arc reactor was operated under atmospheric pressure, driven by a direct current (DC) power source. Coal particles were premixed with the carrying gas and introduced into the thermal plasma, underwent ultra-fast pyrolysis within the range of several milliseconds. In order to prevent the decomposition of acetylene, the hot pyrolysis gas was quenched immediately below the reactor outlet with water spraying. The products gas was analyzed by two gas chromatographs (Shimadzu GC-2010 Plus and Kexiao GC-1690). The gaseous hydrocarbons were determined by flame ionization detector (FID) with a capillary column (HP-AL/S) and CO was determined by thermal conductivity detector (TCD) with a packed column (PLOT 5A).The gas mole fraction is presented in moisture free basis. Product gas yield is calculated as weight based as below: Yi ¼
mi 100% mcoal
SER ¼
SE ¼
ð1Þ
P mC 2 H2
ð2Þ
P Q H2
ð3Þ
Y sum ð%Þ ¼ Y C2 H2 þYCO þYCH4 þYC2 H4
ð4Þ
The subscript of i represents a certain gas species, m is the mass flow rate in kg/h, mcoal is coal feed rate in air-dried basis, P is the plasma input Table 1 Proximate and ultimate analysis of coal.a Proximate analysis
Ar.%
Fixed carbon Volatile matter Moisture Ash Ultimate analysis C H O N S
46.8 38.06 4.40 10.74 Daf. % 76.88 5.53 15.36 1.11 1.12
a
Ar. As received, Daf. Dry ash free.
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Fig. 2. Coal particle size distribution.
power in kW, SER is the specific energy requirement (SER) of acetylene in kWh/kg-C2H2, SE is the specific energy (SE) input of hydrogen in kWh/Nm3-H2, QH2 is the flow rate of hydrogen in Nm3, Ysum represents the total yield of carbon-containing species to evaluate the coal conversion rate.
Fig. 4. Effect of magnetic coil current on the composition of product gas and quench temperature. (Filled symbol-mole fraction, hollow symbol-quench temperature).
flow rate is 590 Nm3/h, and the power input is ~790 kW. The average residence time could be estimated by the average gas velocity according to the energy input [19], and the calculated value is around 8.5 ms.
3. Results and discussion 3.1. Effect of magnetic coil current Experiments were conducted to study the effect of magnetic coil current on pyrolysis (Figs. 4–6), with the coal flow rate is 346 kg/h, H2
Fig. 5. Effect of magnetic coil current on the yield of product gas.
Fig. 3. Schematic view of the plasma reactor.
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Fig. 6. Effect of magnetic coil current on the acetylene yield and SER.
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Fig. 7. Thermodynamic analysis of the plasma pyrolysis of coal.
The magnetic flux density is expressed as below: B ¼ ð4:5 IB −3:41Þ=20000
ð5Þ
where B is the magnetic flux density in Tesla, IB is the magnetic coil current in A. Fig. 4 shows that both the mole fraction of CO and C2H2 increase slowly with the increase of the magnetic coil current. The CH4 mole fraction increases at the beginning (IB b 750 A), and then shows a decrease trend when the magnetic coil current is larger than 750 A. Attributed to the ultra-fast reaction rate in gas phase, the reaction is in quasi-equilibrium state [34]. The composition of the product gas is sensitive to the system temperature, when the temperature is going up, the mole fraction of CH4 will decrease (e.g. point a and b in Fig. 4). The CH4 mole fraction or the ratio of C2H2/CH4 could be used as a reference index to assess the system temperature before quench [35]. It should be noted that the
coal pyrolysis temperature was much higher than the pyrolysis gas quench temperature. Taking the IB = 750 A for example (Fig. 4), the calculated average pyrolysis temperature by the computational fluid dynamics (CFD) method [5,36] was 2450 K, and the quench temperature was estimated to be 1790 K from the ratio of C2H2/CH4 [37]. The conversion rate of coal (expressed as Ysum) increases with the increase in magnetic coil current (Fig. 5), and the conversion rate is larger than 50%. Increasing the magnetic coil current results in larger Lorentz force and higher rotating speed of the plasma arc, the plasma will be dispersed and expanded toward the radial position, which will enhance the homogeneity of the plasma jet [38] that is favorable for the conversion of coal. Also the swirl flow [36] caused by the rotating plasma could prolong the residence time of the coal particles, and has a positive effect for the release of the volatile matters. Acetylene yield increases with the increase of magnetic coil current (Fig. 6), whereas the variation of acetylene SER has a decreasing trend. The minimum SER is 9.50 kWh/kg-C2H2. The thermodynamic analysis was based on the minimum Gibbs energy method [35], using the Chemkin software with a homogeneous model. The quench temperature was assessed by the effective system C/H ratio and the gas compositions according the thermodynamic method [37]. The CH4 mole fraction will decrease with the increase in temperature, whereas the C2H2 mole fraction keeps steady above 1800 K. The optimum range of equilibrium temperature is 1800–3000 K (Fig. 7), which is consistent with the results in literatures [35,39]. The rotational speed of arc can be represented by the following equation [40]. −8=9
v ¼ 78I 4=9 B0:6 ρ0
φ−1=3
ð6Þ
where v is the arc velocity, I is the arc current, B is the magnetic induction, ρ0 is gas density, φ can be represented by the axial gas velocity va as follows: φ¼
1 þ va 1 þ va
Fig. 8. Effect of coal feed rate on the composition of product gas and quench temperature. (Filled symbol-mole fraction, hollow symbol-quench temperature).
ð7Þ
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725
Fig. 9. Effect of coal feed rate on the yield of product gas.
Taking the IB = 750 A for example, the calculated arc velocity was relatively high with a value of 443.8 m/s, which was around 1413 r/s. It is clearly shown that the rotating frequency will increase with the increase in the magnetic coil current, and is beneficial for the coal conversion and acetylene production. However, excessive magnetic flux density will result in higher rotating speed for the plasma and larger centrifugal force for the particle, which will increase the deposition rate of coal residue on the reactor wall.
the magnetic coil current IB = 750 A. Since hydrogen is the plasma medium, the voltage and power input will increase with the increase in the hydrogen flow rate. Operating conditions in Figs. 8–11 is differentiated by a, b, c as below: (a). H2 269 Nm3/h (SE-2.31 kWh/Nm3-H2); (b). H2 401 Nm3/h (SE-1.60 kWh/Nm3-H2); (c). H2 561 Nm3/h (SE-1.19 kWh/Nm3-H2).
3.2. Effect of coal flow rate Experimental investigation of the impact of coal flow rate on the pyrolysis was carried out under three different hydrogen flow rate, with
Higher coal feed rate will increase the amount of volatile matters released from the coal particles, resulting in the increase in the mole fraction of the product gas (Fig. 8). It should be noted that the oxygen content in the coal is 15.36 Daf. %, the theoretical maximum CO yield
Fig. 10. Effect of coal feed rate on the acetylene yield.
Fig. 11. Effect of coal feed rate on the SER.
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from coal is 22.82 wt%. And the CO yield in Fig. 8 is much larger than this value, especially at lower coal flow rate. The extra CO originates from the reaction of quenching water with both the coal and the product gas. As shown in Fig. 9, the coal conversion and the yield of carbon monoxide decrease with the increase of coal feed rate. On the contrary, the methane and ethylene yield increase meanwhile (except for some points of CH4 yield in Fig. 9(b)). Firstly, when increasing coal feed rate, the bulk gas phase temperature (quench temperature in Fig. 8) will decrease due to the heat exchange between coal and plasma jet, which is negative for the conversion of coal. Secondly, the increasing amplitude of product gas yield is relatively smaller than that of coal feed rate. As shown in Fig. 9, the coal conversion rate is a N b N c at the same coal flow rate, the highest value of coal conversion rate is ~80% in Fig. 9(a). As shown in Fig. 10, the acetylene yield of condition-a pyrolysis maintains stable at the beginning, then decreases slightly when the coal feed rate is larger than 275 kg/h. And for the condition b and c, the acetylene yield decreases almost linearly with the increase in coal feed rate. Plasma pyrolysis of coal is an energy-intensive process, specific energy input of the plasma will affect both the devolatilization of coal and the pyrolysis reactions in the bulk phase. The specific energy input in condition-a is sufficient for the conversion of coal, and changing the coal feed rate has little effect on the acetylene yield. However, higher specific energy input will result in higher system temperature and higher acetylene degradation rate, thus the acetylene yield in condition-a is lower than that of condition-b and condition-c under smaller coal feed rate (b275 kg/h). Although it is not able to quantitatively determine the conversion rate of the coal char, the reaction rate of the solid char is about two magnitude lower than that of the gaseous reactions. Furthermore, the residence time of the coal particle is relatively short in the scale of several milliseconds. The major part of coal conversion is the high temperature volatile fraction, which is much higher than the proximate analysis value of volatile matter. Volatile yield is sensitive to the particle heating rate, final temperature and subsequent holding time. The Q-factor [41] is the ratio of the actual volatile yield compared to the proximate analysis value. High temperature and high heating rate result in higher volatile yield (higher Q-factor) [24,42], which is consistent with the results in entrained flow gasification [43]. Lower specific energy input, which means lower reaction temperature
and lower particle heating rate, has a negative effect on the conversion of coal. Since the total amount of acetylene will increase with the coal feed rate, the SER of acetylene will decrease correspondingly. As discussed above, at lower coal flow rate, higher specific energy input will lead to a higher SER value (Fig. 11). Meanwhile, higher specific energy input will lead to a higher coal conversion rate, but it does not mean a higher acetylene yield and a cost effective SER. The principle for the energy input is to achieve efficiently conversion of coal, optimum environment for the production of acetylene with relatively lower degradation rate of acetylene. 3.3. Effect of synergetic parameters Plasma pyrolysis of coal is affected by various operating parameters, such as power input, coal flow rate and hydrogen flow rate. However, single parameter is not adequate to assess the pyrolysis process. Because the plasma pyrolysis is a complex process consisting of a series of reactions in homogeneous or heterogeneous phase, such as the gas-solid heat transfer, the devolatilization of the coal, and the turbulence chemistry interaction in the bulk phase. The SE of hydrogen and coal/H2 feed ratio are selected as the macroscopic parameters to evaluate the process. The 3d spatial distributions of the pyrolysis results derived from over 60 experimental data points are shown in Figs. 12–16. As shown in Fig. 12, the mole fraction of acetylene will increase with the increase in coal/H2 ratio. The mole fraction of acetylene is above 5.4% with the SE N 1.0 kWh/Nm3-H2 and coal/H2 N 0.7 kg/Nm3. Higher coal loading means more volatile matters released to the gas phase. However, acetylene yield (Fig. 13) shows a decrease trend with increasing the coal/H2 ratio. The highest acetylene yield locates in the zone of coal/H2 b 0.7 kg/Nm3 and the SE N 1.0 kWh/Nm3-H2. The variation of acetylene yield with specific energy input shows different trend under different coal/H2 rate. For example, acetylene yield shows a growing trend at the coal/H2 ratio of 0.6 kg/Nm3, but a decreasing trend at the coal/H2 ratio of 1.4 kg/Nm3. As is expected, the coal conversion will increase with the increase in energy input [20]. However, the acetylene yield does not show a monotonic increasing trend with the increase in SE. The gas temperature will
Fig. 12. Acetylene mole fraction with variation of operating conditions.
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Fig. 13. Acetylene yield with variation of operating conditions.
increase with increase of the SE, which will enhance the heat transfer between the gas and coal particle, the release of the volatile matter and the pyrolysis of the volatile matters. On the other hand, acetylene degradation rate will increase under higher temperature. As shown in Fig. 14, the higher the coal/H2 ratio, the lower the acetylene SER. However, it is not appropriate for the pyrolysis when the coal/H2 ratio is larger than 1.0 kg/Nm3, as the acetylene yield and coal conversion rate are in the low level. The plasma pyrolysis process should be evaluated by four indexes as below: the mole fraction of acetylene, the acetylene yield, the acetylene SER and the coal conversion rate. Comparing with the Fig. 13 and Fig. 15, there is an overlapping zone with both higher acetylene yield and coal conversion rate, with the SER in a relatively lower range. The optimum
operating conditions are screened with abundant experimental data, and the SE range is (1.40–1.90 kWh/Nm3-H2)with coal/H2 ratio (0.40– 0.70 kg/Nm3),where the quench temperature is 1810–1900 K (Fig. 16), and the values of acetylene mole fraction (5%–6.5%), yield (N22%), coal conversion (~75%) and SER (11–15 kWh/kg-C2H2). The plasma pyrolysis of gaseous hydrocarbon is an ultra-fast reaction with the time scale b 1.0 ms [27]. And the coal devolatilization process completes in 2–10 ms, which indicates that the devolatilization is the rate controlling step in the pyrolysis of coal [20,32]. Since the yield of volatile matter is sensitive to the particle heating rate, and the C2H2 yield is significantly affected by the gas phase temperature, thus compatible coal loading under specific energy input is the key parameter in the pyrolysis.
Fig. 14. Acetylene SER with variation of operating conditions.
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Fig. 15. Coal conversion rate with variation of operating conditions.
4. Conclusion Pulverized coal was pyrolyzed with one self-designed MW-scale magnetically rotating hydrogen plasma. The coal particle underwent high heating rate with a higher volatile yield within several milliseconds. Magnetic coil current has a positive effect on the conversion of coal. Increasing the magnetic coil current will enhance the mixing efficiency between plasma jet and coal particle, which is favorable for the production of acetylene. The minimum SER is 9.50 kWh/kg-C2H2 with the magnetic coil current of 875 A. Acetylene mole fraction shows a growing trend with increasing the coal feed rate, whereas the acetylene yield and coal conversion rate show a negative trend. Also increasing the specific energy input is favorable for the conversion of coal. Since
the acetylene yield is affected by both the production and degradation rates, it does not mean higher acetylene yield will be obtained with higher specific energy input. The combination of SE and coal/H2 loading ratio are chosen as the macroscopic parameters to assess the pyrolysis process. The optimum operating conditions are located at the range of SE (1.40– 1.90 kWh/Nm3-H2) and coal/H2 ratio (0.40–0.70 kg/Nm3),with the values of acetylene mole fraction (5%–6.5%), coal conversion rate (~ 75%) and SER (11–15 kWh/kg-C2H2). The results will not only promote the research and application in the clean coal conversion to acetylene process, but also has an important guidance for biomass studies and solid waste utilization based on knowledge derived from this research.
Fig. 16. Quench temperature with variation of operating conditions.
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Research article
Pyrolysis of pulverized coal to acetylene in magnetically rotating hydrogen plasma reactor Jie Ma, Baogen Su, Guangdong Wen, Qiwei Yang, Qilong Ren ⁎, Yiwen Yang, Huabin Xing Key Laboratory of Biomass Chemical Engineering of Ministry of Education, College of Chemical and Biological Engineering, Zhejiang University, Zheda Road 38, Hangzhou 310027, China
a r t i c l e
i n f o
Article history: Received 29 September 2016 Received in revised form 17 June 2017 Accepted 20 June 2017 Available online 24 August 2017 Keywords: Acetylene Coal Pyrolysis Rotating plasma
a b s t r a c t This paper presents a clean and one-step way to produce acetylene by plasma pyrolysis of coal. A self-designed MW-scale rotating plasma reactor is used to investigate the pyrolysis of pulverized coal, characterized by the upstream injection of coal particles into the rotating plasma. The rotating plasma arc has a stirring effect and the upstream injection facilitates particle transportation, allowing for a good mixing of coal with plasma and high heattransfer efficiency. The effects of magnetic coil current, coal flow rate and synergetic parameters on the pyrolysis were studied, and the optimum operating conditions of pyrolysis process were screened by taking consideration of acetylene mole fraction, yield, specific energy requirement and coal conversion rate. The minimum specific energy requirement of 9.50 kWh/kg-C2H2 was achieved with a magnetic coil current of 875 A. © 2017 Published by Elsevier B.V.
1. Introduction Acetylene is one important raw material in chemical industries, which is used for the production of polyvinyl chloride (PVC), 1, 4butanediol, vinyl acetate, etc. [1] The partial oxidation (POX) method and calcium carbide (CaC2) method are currently the two main approaches to produce acetylene [2,3]. The calcium carbide process is energy-intensive with considerable CO2 emissions [4]. Because of the abundant resources of coal, calcium carbide method is still the dominant method to produce acetylene in China. Plasma conversion of carbonaceous materials such as coal, biomass and hydrocarbons, has been the research focuses recently [5–11].The target products vary from low hydrocarbons (C2H2, C2H4) to syngas or hydrogen [12,13], which depends on the process, reacting medium and feed compositions. Depending on the differences in the temperatures of electron (Te) and ions (Ti), plasmas could be categorized as thermal (Te − Ti) and non-thermal plasmas (Te ≫ Ti) [14]. The nonthermal plasma is widely applied to low temperature process such as environmental pollutants treatment [15,16]. However, the thermal plasma with characteristics of high enthalpy, high chemical reactivity and adjustable medium [17], provides an effective way for the highly endothermic process such as the production of acetylene [18]. Chen et al. investigated the pyrolysis of coal to acetylene with a V-shaped plasma torch up to 2.12 MW, and the acetylene volume fraction of 9.4% was achieved [19]. Yan et al. conducted a comprehensive research in the plasma pyrolysis of coal in a lab-scale plasma reactor, and results
⁎ Corresponding author. E-mail address: [email protected] (Q. Ren).
http://dx.doi.org/10.1016/j.fuproc.2017.06.022 0378-3820/© 2017 Published by Elsevier B.V.
indicated that the acetylene yield was sensitive to the coal rank and compositions [20]. Generally, thermal plasma conversion of chemicals suffers from poor mixing efficiency in heterogeneous reaction system, which is attributed to the asymmetric and inhomogeneous nature of the thermal plasma. To overcome the disadvantage, the mixing efficiency could be improved by adjusting the injection angle of coal particle, and the particle heating efficiency depends strongly on the inlet design [21]. Furthermore, external magnetic field is also one promising way to disperse the plasma arc [22]. The rotating plasma is driven by the Lorentz force caused by the external magnetic field, which helps to improve the plasma uniformity. Furthermore, magnetically rotating plasma has a stirring effect and will enhance the mixing efficiency between the reactants and the hot plasma jet. The mechanism of plasma pyrolysis of coal (Fig. 1) includes the evaporation of the moisture, devolatilization of the volatile components and char reaction [23]. The primary pyrolysis products can be classified as light gas, tar and char in the coal chemical percolation devolatilization (CPD) model [24–26]. The primary volatiles will undergo secondary reactions, producing secondary pyrolysis products. The reaction of volatile is the crucial step for the formation of acetylene. Generally, acetylene yield is determined by both the content and constituent of the volatile components released from coal particles [18,27,28]. Traditional hydropyrolysis of coal is carried out under high pressure [28–30], with methane as the primary product. The plasma hydropyrolysis of coal is carried out at atmospheric pressure, high temperature and relatively short residence time (varying from several milliseconds in a lab-scale reactor [20] to around 18.84 milliseconds in a pilot-scale reactor [19]), with acetylene as the primary product. Owing to the unique feature of descending trend of the Gibbs energy with increasing temperature,
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Fig. 1. Schematic view of the plasma pyrolysis of coal to acetylene.
when the temperature in the reaction system is above 1600 K, acetylene becomes the main light hydrocarbon component among the pyrolysis products [31]. Various parameters affecting the plasma pyrolysis of coal such as plasma gas type, coal properties, specific enthalpy input and particle size [20,27,28,32], are mostly investigated in lab-scale reactors. Due to the high experimental cost, few pilot-scale plasma reactors are reported. In summary, the yield and the specific energy requirement (SER) of acetylene can be concluded as three crucial factors: (1) sufficient energy input, which is accounted for the effective heat transfer between the plasma and coal particles, and achieving high heating rate and high volatile yield; (2) compatible coal loading ratio, which is associated with coal conversion efficiency and acetylene yield; (3) coal type, which determines the amount and the constituents of the volatile matters released from coal particles. Most available experiments have been carried out with the coal particles injected downstream of the plasma jet. However, it is difficult for the particle to be transported into the hot plasma core zone due to the thermophoretic force. Generally, coal particles could be injected at the downstream (at the plasma jet downstream) or upstream (pre-mixed with carrying gas) of the plasma jet, and the pre-mixed method showed a better conversion efficiency [33]. This paper focuses on the plasma pyrolysis of pulverized coal with one pilot-scale (~1 MW) magnetically rotating plasma reactor. The main advantage of the rotating plasma is that the coal particles are injected at the upstream of the plasma, leading to good mixing and heat transfer efficiency. Effect of magnetic coil current and coal flow rate under different power input on the pyrolysis performance were investigated. In particular, the comprehensive effects of the coal/H2 ratio and specific energy input were investigated to optimize the pyrolysis process. 2. Experimental section 2.1. Coal properties Coal properties are listed in Table 1, and coal particle size distribution is analyzed by the Winner 318A laser particle instrument (Jinan Winner Particle Technology Co., Ltd.). The particle size distribution is shown in Fig. 2, with the average size of 30 μm. 2.2. Experimental reactor and procedures The plasma arc reactor (Fig. 3) includes the electrodes, external magnetic coils and feed inlet system. The rated power is ~ 1 MW with the IGBT (Insulated Gate Bipolar Transistor) based power supply. The cathode is graphite with diameter of 2.0 cm, and the annular anode is
copper with chamber diameter of 10.0 cm and length of 40.0 cm. To reduce the erosion caused by the hot plasma jet, the anode chamber is cooled by circling water, the reactor is wrapped with magnetic coils to generate axial magnetic field. The plasma arc reactor was operated under atmospheric pressure, driven by a direct current (DC) power source. Coal particles were premixed with the carrying gas and introduced into the thermal plasma, underwent ultra-fast pyrolysis within the range of several milliseconds. In order to prevent the decomposition of acetylene, the hot pyrolysis gas was quenched immediately below the reactor outlet with water spraying. The products gas was analyzed by two gas chromatographs (Shimadzu GC-2010 Plus and Kexiao GC-1690). The gaseous hydrocarbons were determined by flame ionization detector (FID) with a capillary column (HP-AL/S) and CO was determined by thermal conductivity detector (TCD) with a packed column (PLOT 5A).The gas mole fraction is presented in moisture free basis. Product gas yield is calculated as weight based as below: Yi ¼
mi 100% mcoal
SER ¼
SE ¼
ð1Þ
P mC 2 H2
ð2Þ
P Q H2
ð3Þ
Y sum ð%Þ ¼ Y C2 H2 þYCO þYCH4 þYC2 H4
ð4Þ
The subscript of i represents a certain gas species, m is the mass flow rate in kg/h, mcoal is coal feed rate in air-dried basis, P is the plasma input Table 1 Proximate and ultimate analysis of coal.a Proximate analysis
Ar.%
Fixed carbon Volatile matter Moisture Ash Ultimate analysis C H O N S
46.8 38.06 4.40 10.74 Daf. % 76.88 5.53 15.36 1.11 1.12
a
Ar. As received, Daf. Dry ash free.
J. Ma et al. / Fuel Processing Technology 167 (2017) 721–729
Fig. 2. Coal particle size distribution.
power in kW, SER is the specific energy requirement (SER) of acetylene in kWh/kg-C2H2, SE is the specific energy (SE) input of hydrogen in kWh/Nm3-H2, QH2 is the flow rate of hydrogen in Nm3, Ysum represents the total yield of carbon-containing species to evaluate the coal conversion rate.
Fig. 4. Effect of magnetic coil current on the composition of product gas and quench temperature. (Filled symbol-mole fraction, hollow symbol-quench temperature).
flow rate is 590 Nm3/h, and the power input is ~790 kW. The average residence time could be estimated by the average gas velocity according to the energy input [19], and the calculated value is around 8.5 ms.
3. Results and discussion 3.1. Effect of magnetic coil current Experiments were conducted to study the effect of magnetic coil current on pyrolysis (Figs. 4–6), with the coal flow rate is 346 kg/h, H2
Fig. 5. Effect of magnetic coil current on the yield of product gas.
Fig. 3. Schematic view of the plasma reactor.
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Fig. 6. Effect of magnetic coil current on the acetylene yield and SER.
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Fig. 7. Thermodynamic analysis of the plasma pyrolysis of coal.
The magnetic flux density is expressed as below: B ¼ ð4:5 IB −3:41Þ=20000
ð5Þ
where B is the magnetic flux density in Tesla, IB is the magnetic coil current in A. Fig. 4 shows that both the mole fraction of CO and C2H2 increase slowly with the increase of the magnetic coil current. The CH4 mole fraction increases at the beginning (IB b 750 A), and then shows a decrease trend when the magnetic coil current is larger than 750 A. Attributed to the ultra-fast reaction rate in gas phase, the reaction is in quasi-equilibrium state [34]. The composition of the product gas is sensitive to the system temperature, when the temperature is going up, the mole fraction of CH4 will decrease (e.g. point a and b in Fig. 4). The CH4 mole fraction or the ratio of C2H2/CH4 could be used as a reference index to assess the system temperature before quench [35]. It should be noted that the
coal pyrolysis temperature was much higher than the pyrolysis gas quench temperature. Taking the IB = 750 A for example (Fig. 4), the calculated average pyrolysis temperature by the computational fluid dynamics (CFD) method [5,36] was 2450 K, and the quench temperature was estimated to be 1790 K from the ratio of C2H2/CH4 [37]. The conversion rate of coal (expressed as Ysum) increases with the increase in magnetic coil current (Fig. 5), and the conversion rate is larger than 50%. Increasing the magnetic coil current results in larger Lorentz force and higher rotating speed of the plasma arc, the plasma will be dispersed and expanded toward the radial position, which will enhance the homogeneity of the plasma jet [38] that is favorable for the conversion of coal. Also the swirl flow [36] caused by the rotating plasma could prolong the residence time of the coal particles, and has a positive effect for the release of the volatile matters. Acetylene yield increases with the increase of magnetic coil current (Fig. 6), whereas the variation of acetylene SER has a decreasing trend. The minimum SER is 9.50 kWh/kg-C2H2. The thermodynamic analysis was based on the minimum Gibbs energy method [35], using the Chemkin software with a homogeneous model. The quench temperature was assessed by the effective system C/H ratio and the gas compositions according the thermodynamic method [37]. The CH4 mole fraction will decrease with the increase in temperature, whereas the C2H2 mole fraction keeps steady above 1800 K. The optimum range of equilibrium temperature is 1800–3000 K (Fig. 7), which is consistent with the results in literatures [35,39]. The rotational speed of arc can be represented by the following equation [40]. −8=9
v ¼ 78I 4=9 B0:6 ρ0
φ−1=3
ð6Þ
where v is the arc velocity, I is the arc current, B is the magnetic induction, ρ0 is gas density, φ can be represented by the axial gas velocity va as follows: φ¼
1 þ va 1 þ va
Fig. 8. Effect of coal feed rate on the composition of product gas and quench temperature. (Filled symbol-mole fraction, hollow symbol-quench temperature).
ð7Þ
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725
Fig. 9. Effect of coal feed rate on the yield of product gas.
Taking the IB = 750 A for example, the calculated arc velocity was relatively high with a value of 443.8 m/s, which was around 1413 r/s. It is clearly shown that the rotating frequency will increase with the increase in the magnetic coil current, and is beneficial for the coal conversion and acetylene production. However, excessive magnetic flux density will result in higher rotating speed for the plasma and larger centrifugal force for the particle, which will increase the deposition rate of coal residue on the reactor wall.
the magnetic coil current IB = 750 A. Since hydrogen is the plasma medium, the voltage and power input will increase with the increase in the hydrogen flow rate. Operating conditions in Figs. 8–11 is differentiated by a, b, c as below: (a). H2 269 Nm3/h (SE-2.31 kWh/Nm3-H2); (b). H2 401 Nm3/h (SE-1.60 kWh/Nm3-H2); (c). H2 561 Nm3/h (SE-1.19 kWh/Nm3-H2).
3.2. Effect of coal flow rate Experimental investigation of the impact of coal flow rate on the pyrolysis was carried out under three different hydrogen flow rate, with
Higher coal feed rate will increase the amount of volatile matters released from the coal particles, resulting in the increase in the mole fraction of the product gas (Fig. 8). It should be noted that the oxygen content in the coal is 15.36 Daf. %, the theoretical maximum CO yield
Fig. 10. Effect of coal feed rate on the acetylene yield.
Fig. 11. Effect of coal feed rate on the SER.
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from coal is 22.82 wt%. And the CO yield in Fig. 8 is much larger than this value, especially at lower coal flow rate. The extra CO originates from the reaction of quenching water with both the coal and the product gas. As shown in Fig. 9, the coal conversion and the yield of carbon monoxide decrease with the increase of coal feed rate. On the contrary, the methane and ethylene yield increase meanwhile (except for some points of CH4 yield in Fig. 9(b)). Firstly, when increasing coal feed rate, the bulk gas phase temperature (quench temperature in Fig. 8) will decrease due to the heat exchange between coal and plasma jet, which is negative for the conversion of coal. Secondly, the increasing amplitude of product gas yield is relatively smaller than that of coal feed rate. As shown in Fig. 9, the coal conversion rate is a N b N c at the same coal flow rate, the highest value of coal conversion rate is ~80% in Fig. 9(a). As shown in Fig. 10, the acetylene yield of condition-a pyrolysis maintains stable at the beginning, then decreases slightly when the coal feed rate is larger than 275 kg/h. And for the condition b and c, the acetylene yield decreases almost linearly with the increase in coal feed rate. Plasma pyrolysis of coal is an energy-intensive process, specific energy input of the plasma will affect both the devolatilization of coal and the pyrolysis reactions in the bulk phase. The specific energy input in condition-a is sufficient for the conversion of coal, and changing the coal feed rate has little effect on the acetylene yield. However, higher specific energy input will result in higher system temperature and higher acetylene degradation rate, thus the acetylene yield in condition-a is lower than that of condition-b and condition-c under smaller coal feed rate (b275 kg/h). Although it is not able to quantitatively determine the conversion rate of the coal char, the reaction rate of the solid char is about two magnitude lower than that of the gaseous reactions. Furthermore, the residence time of the coal particle is relatively short in the scale of several milliseconds. The major part of coal conversion is the high temperature volatile fraction, which is much higher than the proximate analysis value of volatile matter. Volatile yield is sensitive to the particle heating rate, final temperature and subsequent holding time. The Q-factor [41] is the ratio of the actual volatile yield compared to the proximate analysis value. High temperature and high heating rate result in higher volatile yield (higher Q-factor) [24,42], which is consistent with the results in entrained flow gasification [43]. Lower specific energy input, which means lower reaction temperature
and lower particle heating rate, has a negative effect on the conversion of coal. Since the total amount of acetylene will increase with the coal feed rate, the SER of acetylene will decrease correspondingly. As discussed above, at lower coal flow rate, higher specific energy input will lead to a higher SER value (Fig. 11). Meanwhile, higher specific energy input will lead to a higher coal conversion rate, but it does not mean a higher acetylene yield and a cost effective SER. The principle for the energy input is to achieve efficiently conversion of coal, optimum environment for the production of acetylene with relatively lower degradation rate of acetylene. 3.3. Effect of synergetic parameters Plasma pyrolysis of coal is affected by various operating parameters, such as power input, coal flow rate and hydrogen flow rate. However, single parameter is not adequate to assess the pyrolysis process. Because the plasma pyrolysis is a complex process consisting of a series of reactions in homogeneous or heterogeneous phase, such as the gas-solid heat transfer, the devolatilization of the coal, and the turbulence chemistry interaction in the bulk phase. The SE of hydrogen and coal/H2 feed ratio are selected as the macroscopic parameters to evaluate the process. The 3d spatial distributions of the pyrolysis results derived from over 60 experimental data points are shown in Figs. 12–16. As shown in Fig. 12, the mole fraction of acetylene will increase with the increase in coal/H2 ratio. The mole fraction of acetylene is above 5.4% with the SE N 1.0 kWh/Nm3-H2 and coal/H2 N 0.7 kg/Nm3. Higher coal loading means more volatile matters released to the gas phase. However, acetylene yield (Fig. 13) shows a decrease trend with increasing the coal/H2 ratio. The highest acetylene yield locates in the zone of coal/H2 b 0.7 kg/Nm3 and the SE N 1.0 kWh/Nm3-H2. The variation of acetylene yield with specific energy input shows different trend under different coal/H2 rate. For example, acetylene yield shows a growing trend at the coal/H2 ratio of 0.6 kg/Nm3, but a decreasing trend at the coal/H2 ratio of 1.4 kg/Nm3. As is expected, the coal conversion will increase with the increase in energy input [20]. However, the acetylene yield does not show a monotonic increasing trend with the increase in SE. The gas temperature will
Fig. 12. Acetylene mole fraction with variation of operating conditions.
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Fig. 13. Acetylene yield with variation of operating conditions.
increase with increase of the SE, which will enhance the heat transfer between the gas and coal particle, the release of the volatile matter and the pyrolysis of the volatile matters. On the other hand, acetylene degradation rate will increase under higher temperature. As shown in Fig. 14, the higher the coal/H2 ratio, the lower the acetylene SER. However, it is not appropriate for the pyrolysis when the coal/H2 ratio is larger than 1.0 kg/Nm3, as the acetylene yield and coal conversion rate are in the low level. The plasma pyrolysis process should be evaluated by four indexes as below: the mole fraction of acetylene, the acetylene yield, the acetylene SER and the coal conversion rate. Comparing with the Fig. 13 and Fig. 15, there is an overlapping zone with both higher acetylene yield and coal conversion rate, with the SER in a relatively lower range. The optimum
operating conditions are screened with abundant experimental data, and the SE range is (1.40–1.90 kWh/Nm3-H2)with coal/H2 ratio (0.40– 0.70 kg/Nm3),where the quench temperature is 1810–1900 K (Fig. 16), and the values of acetylene mole fraction (5%–6.5%), yield (N22%), coal conversion (~75%) and SER (11–15 kWh/kg-C2H2). The plasma pyrolysis of gaseous hydrocarbon is an ultra-fast reaction with the time scale b 1.0 ms [27]. And the coal devolatilization process completes in 2–10 ms, which indicates that the devolatilization is the rate controlling step in the pyrolysis of coal [20,32]. Since the yield of volatile matter is sensitive to the particle heating rate, and the C2H2 yield is significantly affected by the gas phase temperature, thus compatible coal loading under specific energy input is the key parameter in the pyrolysis.
Fig. 14. Acetylene SER with variation of operating conditions.
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Fig. 15. Coal conversion rate with variation of operating conditions.
4. Conclusion Pulverized coal was pyrolyzed with one self-designed MW-scale magnetically rotating hydrogen plasma. The coal particle underwent high heating rate with a higher volatile yield within several milliseconds. Magnetic coil current has a positive effect on the conversion of coal. Increasing the magnetic coil current will enhance the mixing efficiency between plasma jet and coal particle, which is favorable for the production of acetylene. The minimum SER is 9.50 kWh/kg-C2H2 with the magnetic coil current of 875 A. Acetylene mole fraction shows a growing trend with increasing the coal feed rate, whereas the acetylene yield and coal conversion rate show a negative trend. Also increasing the specific energy input is favorable for the conversion of coal. Since
the acetylene yield is affected by both the production and degradation rates, it does not mean higher acetylene yield will be obtained with higher specific energy input. The combination of SE and coal/H2 loading ratio are chosen as the macroscopic parameters to assess the pyrolysis process. The optimum operating conditions are located at the range of SE (1.40– 1.90 kWh/Nm3-H2) and coal/H2 ratio (0.40–0.70 kg/Nm3),with the values of acetylene mole fraction (5%–6.5%), coal conversion rate (~ 75%) and SER (11–15 kWh/kg-C2H2). The results will not only promote the research and application in the clean coal conversion to acetylene process, but also has an important guidance for biomass studies and solid waste utilization based on knowledge derived from this research.
Fig. 16. Quench temperature with variation of operating conditions.
J. Ma et al. / Fuel Processing Technology 167 (2017) 721–729
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