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Gasification of preheated coal: Experiment and thermodynamic equilibrium calculation
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Journal of the Energy Institute xxx (2018) 1e9
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Gasification of preheated coal: Experiment and thermodynamic equilibrium calculation Q8
Chen Liang a, b, Qinggang Lyu a, b, *, Yongjie Na a, b, Xiaofang Wang a a b
Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing, 100190, China University of Chinese Academy of Sciences, Beijing, 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 April 2018 Received in revised form 22 July 2018 Accepted 24 July 2018 Available online xxx
A new process integrating a circulating fluidized bed (CFB) reactor and an entrained bed reactor was proposed for gasification of preheated coal. The CFB reactor as a preheater was successfully used in clean coal combustion. In this study, gasification of preheated coal was tested in a bench-scale test rig, which consisted of a CFB preheater and a down flow bed (DFB) gasifier. The effects of operating parameters of the preheater and gasifier were revealed via thermodynamic equilibrium calculations. A stable preheating process was obtained in the CFB preheater at the O2/C molar ratio of 0.31 and higher gasification reactivity was gained in preheated char owing to the improvement in intrinsic reactivity, specific surface area and total pore volume. Effective gasification of preheated char was achieved in the DFB gasifier at 1100 C and the total O2/C molar ratio of 0.67, meanwhile the CO þ H2 yield and carbon conversion increased. Thermodynamic equilibrium calculations revealed when the gasification reaction rates varied little above 1100 C and the same carbon conversion was achieve in gasifier, lowering the temperature would lead to an increase in cold gas efficiency and a decrease in O2 demand. © 2018 Published by Elsevier Ltd on behalf of Energy Institute.
Keywords: Gasification Coal Circulating fluidized bed preheater Down flow bed gasifier
1. Introduction Coal will still supply 26% of global energy mix and over 55% of China's energy demand in 2022, and is expected to contribute to energy production until the end of 2100 [1,2]. Gasification, as an efficient and clean technology to covert coal into hydrogen, electricity and chemical products, has attracted more attention [3,4]. The coal gasification technology can be mainly categorized by the reactor into three types: fixed bed, fluidized bed and entrained bed [5]. Firstly, a fixed bed reactor with a given volume has a high conversion rate, but is limited by a low operating temperature below 1000 C and tar problems in the product gas [6,7]. Secondly, a fluidized bed reactor is characterized by uniform temperature distributions, efficient heat exchange, good gas-solid mixing and strong back-mixing of solids [8,9]. However, its application is limited by the low carbon conversion because the fluidized bed gasifier should be operated below 1000 C to avoid slagging and defluidization [5,10]. Thirdly, an entrained bed reactor is characterized by an extremely high operating temperature beyond 1350 C, so the ash is removed as molten slag [11]. The high operating temperature contributes to increasing the carbon conversion and cold gas efficiency [12e14], but causes the corrosion of burners and refractories [5]. It is well known that temperature is an essential parameter in gasification operation. Reaction rates of gasification reactions and carbon conversions usually increase with the increasing temperature. However increases in reaction rates obviously slow and an overlapping exists in the curves of coal/char conversion against time when the temperature is over 1100 C [4,15]. Meanwhile, oxygen demands increase faster and serious corrosion of refractories are caused when the temperature is beyond 1300 C and more sensible heat can't be transferred into chemical energy in product gas. Therefore, operating at 1100e1200 C maybe is more economic in gasification. According to these characteristics, the gasification of preheated coal was proposed as a new concept of gasification process to realize this operating temperature, which combines a circulating fluidized bed (CFB) preheater [16] and an entrained bed gasifier. Using CFB as preheaters was proposed by the Institute of Engineering Thermophysics, Chinese Academy of Sciences (IET, CAS) and have been applied for clean and efficient combustion of pulverized coal since 2008 [17e19]. Non-ignitable coal can be burnt out steadily with less
* Corresponding author. Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing, 100190, China. E-mail address: [email protected] (Q. Lyu). https://doi.org/10.1016/j.joei.2018.07.006 1743-9671/© 2018 Published by Elsevier Ltd on behalf of Energy Institute.
Please cite this article in press as: C. Liang, et al., Gasification of preheated coal: Experiment and thermodynamic equilibrium calculation, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.07.006
Q1 Q2
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NOX emission [20e22]. The CFB preheater can be applied to coal gasification because morphological properties of coal can be improved in a CFB preheater to enhance oxidation reactions in the gasifier [23]. Meanwhile the gasifier can be operated at 1100e1200 C to increase reaction rates compared with fluidized bed gasification, and to reduce oxygen demands and avoid serious corrosion of refractories compared with entrained bed gasification [5]. Moreover, coal handling processes for sub-bituminous and lignite with poor grindability [24] can be simplified by using a CFB preheater fed by fuel with wide-range particle size distribution. In this study, a bench-scale test rig was built for gasification of preheated coal and tested through a series of experiments. The preheating and gasification processes were characterized experimentally and the preliminary results are introduced. The effects of carbon conversion, CH4 yield and temperature were evaluated through thermodynamic equilibrium calculations. 2. Experimental 2.1. Test rig The test rig for gasification of preheated coal consisting of a CFB preheater, a down flow bed (DFB) gasifier and an auxiliary system was shown schematically in Fig. 1. The CFB preheater consisted of a riser (internal diameter ¼ 900 mm, height ¼ 1500 mm), a cyclone and a loop seal. The outlet of the cyclone was connected to the inlet of the DFB gasifier, which was 260 mm in diameter and 3000 mm in height. The primary air was supplied through an air distributor to the bottom of the riser to maintain the preheater operating at about 900 C. Pulverized coal was fed to the riser by a screw at 240 mm above the air distributor and was preheated in the CFB preheater. Gas products and solid products from the preheater were called preheater gas and preheated char, respectively. The preheater gas carried the preheated char to a nozzle at the top of the DFB gasifier. The secondary air was supplied though the nozzle to provide oxygen for the gasification of preheated char. The CFB preheater and the DFB gasifier were installed with four and five thermocouples, respectively. Gas and solid products were collected at seven sampling points: one at the preheater outlet to sample preheater gas and preheated char, one at the cooler outlet to sample the final product gas and fly ash, and five at 100, 400, 900, 1400 and 2400 mm below the nozzle, respectively. The test rig was coated with 150 mm-thick aluminum silicate fabric to reduce heat loss. 2.2. Coal and sample characteristics Shenmu bituminous coal from Shaanxi province of China was used in the experiments. The characteristics of coal are shown in Table 1. The ash composition is listed in Table 2. The Shenmu coal has low ash content, high volatiles and high calorific value, which is fit for gasification. The particle sizes of coal and silica sand are 0e0.5 mm (50% cut size d50 ¼ 230.8 mm) and 0.1e0.5 mm, respectively. 2.3. Experimental In the starting procedure, silica sand (3 kg) was added into the preheater riser as bed materials and then fluidized by the air supply. The electrical heater was turned on to heat the CFB to 550 C, and coal was fed continuously. When the CFB temperature rose to 800 C, the heater was turned off, and the coal and primary air were adjusted to switch the running state of preheater from combustion to gasification. The procedure of increasing the preheater temperature was similar to CFB gasification technology operations [25,26]. During the starting of the preheater, the gasifier was heated by the product of preheater. When the starting of the preheater was finished, the secondary air was injected into the gasifier to combust the preheated char and preheater gas and to increase the gasifier temperature. When the gasifier
Fig. 1. Schematic diagram of the test rig. 1. Air compressor; 2. Electrical heater; 3. Screw feeder; 4. Riser; 5. Cyclone; 6. Loop seal; 7. CFB sampling port; 8. Nozzle; 9. Down flow bed; 10. Sampling point (100 mm); 11. Sampling point (400 mm); 12. Sampling point (900 mm); 13. Sampling point (1400 mm); 14. Sampling point (2400 mm); 15. Water cooler; 16. Bag filter; 17. Final sampling point; 18. Water tank.
Please cite this article in press as: C. Liang, et al., Gasification of preheated coal: Experiment and thermodynamic equilibrium calculation, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.07.006
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Table 1 The characteristics of Shenmu bituminous coal a. Proximate analysis (wt %)
Ultimate analysis (wt %)
Qnet,
Mar
Aar
Var
FCar
Cdaf
Hdaf
Odaf
Ndaf
Sdaf
6.90
4.80
32.79
55.51
81.38
4.70
12.51
1.00
0.41
a
ar
(MJ/kg)
27.68
ar, as received basis; M, moisture; A, ash; V, volatile matter; FC, fixed carbon; daf, dry ash free basis.
temperature reached 1000 C, the secondary air was adjusted to keep gasifier under gasification condition. Then the operating parameters were adjusted to running conditions. The coal feeding rate was 5.5 kg/h in experiments of this work. The preheater gas and product gas were deashed by a glass fiber filter and dried before online tests. Main gas components including H2, CO, CH4, CO2 and N2 were detected on an Agilent GC 3000 gas chromatograph. The gas yield, calorific value (Qgas,net), carbon conversion (XC) and cold gas efficiency (h) were calculated as follows:
ygas;i ¼ ðxi 0:79qair Þ
xN2 Mcoal
ði ¼ CO; H2 ; CH4 ; CO2 Þ
Nm3 =kg
(1)
Qgas;net ¼ xH2 10:79 þ xCO 12:64 þ xCH4 35:90 MJ=Nm3
(2)
XC ¼ ygas;CO þ ygas;CO2 þ ygas;CH4 12=ðCad 22:4Þ 100%
(3)
h ¼ Qgas;net ygas
. Qar;net 100%
(4)
where ygas,i and ygas are the yields (Nm3/kg) of component i and gas product, respectively; xi is the volume percentage (%) of i; qair is the volume flow rate (Nm3/h) of air as gasifying agent; Mcoal is the coal feeding rate (kg/h); Qar,net is the net calorific value of coal (MJ/kg). 3. Results and discussion 3.1. Characteristics of preheating process in the CFB preheater Pulverized coal with particle sizes of 0e0.5 mm was used to characterize the CFB preheater at the O2/C molar ratio of 0.31 and the fluidization velocity of 1.50 m/s. A stable operation at 900 C for more than 3 h was obtained in the preheater. The preheating process was like an incomplete CFB gasification process [25] at low O2/C molar ratios. Once fed into the preheater riser, the pulverized coal was rapidly heated by the bed material, which consisted of circulating char and silica sand, and was pyrolyzed into char and gas. The char was mechanically cracked and incompletely gasified in the riser. The preheater gas was generated during the pyrolysis and incomplete gasification. The small-size char from the riser, which was not captured by the cyclone, was carried by the preheater gas and left the preheater from the outlet as preheated char. The large-size char from the riser was captured by the cyclone and sent back to the riser by the loop seal as bed materials. The cyclone separation efficiency was designed to achieve a mass balance in the preheater so as to ensure the stable operation of the preheater. The particle size distributions of pulverized coal and preheated char were tested by a Malvern Mastersizer 2000 laser analyzer and shown in Fig. 2. The particle sizes of pulverized coal declined and the 50% cut size d50 decreased from 0.231 to 0.161 mm. Due to the particle fragmentation caused by cracking and reactions in the riser and the separation of cyclone, coal was pulverized in the preheating process, which contributed to the gasification in DFB and reducing pulverizing cost. Pore surface area and pore volume distributions of the pulverized coal and preheated char are shown in Fig. 3, which were analyzed by an automatic specific surface area/pore size distribution measurement (ASAP 2020). N2 adsorption isotherms at 196 C were measured for the relative pressure (P/P0) range from 0.01 to 0.99. Specific surface areas were obtained using the BrunauereEmmetteTeller (BET) model, and the pore size distributions were obtained using the DollimoreeHeal (DH) model. The surface areas and volumes of width-varying pores were enlarged in the preheater, especially at the width of 20e40 nm. As a result, the specific surface areas increased from 13.92 to 263.35 m2/g and the total pore volumes increased from 0.23 ✕ 107 to 2.01 ✕ 107 m3/g, while the average pore width decreased from 67.17 to 30.54 nm, indicating a large number of mesoporous were formed in the preheated char. Proximate analyses showed 82.31% of volatiles in pulverized coal were released in the preheater. The sharp increase in specific surface area and total pore volume was attributed to the intense coal devolatilization at a fast heating rate in the preheater. As pores and their surfaces are main places for heterogeneous gasification reactions (e.g. R6 and R7 in Table 3), the enlargement of the specific surface area and total pore volume accelerated reactions of the preheated char in the gasifier. The gasification reactivity of intrinsic char from the preheated char and pulverized coal was evaluated via thermogravimetric analyses (TGA) at atmospheric pressure. Samples were heated in N2 atmosphere to 600 C and kept for 1 h to remove volatiles. Then the atmosphere was switched to 10 mol% CO2 in N2 [27,28]. After that, a conversion of 5% was excluded to eliminate effects of gas change and possible remaining volatiles. Thereby, the intrinsic reactivity of char was tested. The reaction rate r was calculated from the weight loss data as follows: Table 2 Ash composition of Shenmu bituminous coal (wt %). SiO2
Al2O3
Fe2O3
CaO
MgO
TiO2
SO3
P2O5
K2O
Na2O
29.32
13.37
10.96
31.59
1.08
0.59
11.00
0.21
0.43
1.45
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Fig. 2. Size distributions of the pulverized coal and preheated char.
Fig. 3. Pore surface area and pore volume distributions of the pulverized coal and preheated char.
Table 3 Main reactions involved in coal gasification [26,27] a. No.
Reaction
R1 R2 R3 R4 R5 R6 R7 R8 R9
Char oxidation reaction (I) Char oxidation reaction (II) Gas oxidation reaction (I) Gas oxidation reaction (II) Gas oxidation reaction (III) Boudouard reaction Steam char gasification Water gas shift reaction (WGS) Methanation reaction
a
Q7
DH1350 (kJ/mol) C þ O2/CO2 C þ 0.5O2/CO CO þ 0.5O2/CO2 H2 þ 0.5O2/H2O CH4 þ 2O2/CO2 þ 2H2O C þ CO2#2CO C þ H2O#CO þ H2 CO þ H2O#CO2 þ H2 CO þ 3H2#CH4 þ H2O
395.37 114.21 281.16 249.69 802.63 þ166.94 þ135.48 31.46 227.61
DH1350, the standard enthalpy of reaction at the temperature of 1350 K (1076.85 C).
r ¼ dm=ðdt,mÞ
(5)
where m is the instantaneous mass of the sample. The reaction rate of intrinsic char from the preheated char and pulverized coal were matchable before 950 C (Fig. 4). As the temperature rose, the Boudouard reaction was enhanced and both reaction rates were obviously accelerated. However, the reaction rate of preheated char was higher and increased faster than that of the pulverized coal at higher temperatures. It indicates the intrinsic reactivity of the preheated char was improved in comparison with pulverized coal, especially at above 950 C. Therefore, the gasification reactivity of preheated char was enhanced by the improvement of intrinsic reactivity, specific surface area and total pore volume during the preheating. It was also confirmed by research that decreases in the activation energy were contributed for char by the heat treatment compared with coal [29,30]. Reaction rates of preheated char and the carbon conversion in the gasifier will be promoted by the enhancement. 3.2. Characteristics of gasification process in the DFB gasifier The O2/C molar ratio in the CFB preheater was 0.31 as mentioned above, and that in the DFB gasifier was 0.36 in the experiment. After pulverized coal with particle sizes of 0e0.5 mm was fed into the preheater, the resulting preheated char and preheater gas at 900 C were injected to the top of the gasifier through the annular channel of the nozzle. The secondary air was injected through the central channel of Please cite this article in press as: C. Liang, et al., Gasification of preheated coal: Experiment and thermodynamic equilibrium calculation, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.07.006
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Fig. 4. Observed reaction rate of the preheated char and pulverized coal with increasing temperatures in CO2 (balance gas N2).
the nozzle with a velocity of 18 m/s to provide oxygen for the gasification of preheated char. Temperatures in the gasifier were stabilized owing to no igniting problem of preheated char. A temperature profile in the gasifier is presented in Fig. 5. The injection of the secondary air accelerated oxidation and induced a temperature rise by about 195 C from the top position to 100 mm below the nozzle. The oxidation was sustained until 500 mm below the nozzle, leading to the temperature rising to 1100 C. The endothermic gasification reactions and heat loss led to a temperature drop along the flowing direction from 500 mm below the nozzle to the bottom. As present in Fig. 5, the specific surface areas and total pore volumes of solids sampled at different positions along the DFB changed in the same trend of sharply decreasing from the top to 100 mm below the nozzle. The reason was that pores at the outer layer of char particles, which were mainly formed in the preheating process, were rapidly consumed in the fast oxidation. The fast oxidation was verified by the temperature profile. Then the oxidation slowed down from 100 to 500 mm below the nozzle due to the oxygen consumption. The further devolatilization of preheated char was induced simultaneously. The temperature rise was also decelerated in this section, which proved the weakening of oxidation. Therefore, pores were generated and the specific surface areas and total pore volumes increased. The specific surface areas and total pore volumes decreased continuously after 500 mm below the nozzle, but at gradually slower rates. Pores were consumed by the Boudouard reaction R6 and the consumption slowed down from 500 mm below the nozzle to the bottom because the reaction was decelerated at lower temperatures. Gas yields, carbon conversions and cold gas efficiencies of the CFB preheater and the DFB gasifier are shown in Fig. 6. The back-mixing of char and gas and the uniform temperature distribution in the preheater strongly enhanced the destruction of the pyrolysis-induced tar [31] and no tar was detected in the preheater gas. Because the homogeneous reactions between the preheater gas and oxygen, such as R3, R4 and R5, were much faster than the heterogeneous reactions between preheated char and oxygen [29,32], such as R1 and R2, CO, H2 and CH4 in the preheater gas were consumed first to form numerous CO2. However, reactions of preheated char was verified by the change of total pore volume of solids sampled at different positions in the gasifier. Through the gasifier the CO yield increased from 0.33 to 0.44 Nm3/kg and the CO þ H2 yield increased from 0.59 to 0.65 Nm3/kg. It indicates that Boudouard reaction was enhanced and the preheated char was effectively gasified at 1100 C in the gasifier. The CO2 yield also increased from 0.26 to 0.61 Nm3/kg because of the fast oxidation of the preheater gas and preheated char. As a result of the increase in CO yield and CO2 yield, carbon conversion was strongly improved. The increase in CO þ H2 yield and the decrease in CH4 yield made the cold gas efficiencies little different. The changes of Gibbs free energy, DG, in oxidation reactions among the preheated char, preheater gas and secondary air at different temperatures [26,27] are present in Fig. 7. The DG of the reactions from 700 to 1400 C are negative, indicating these reactions are possible. The DG of reaction R2 is the lowest above 800 C and decreases with the temperature rise, indicating R2 owns the strongest driving force and
Fig. 5. Temperature profile, BET specific surface area and total pore volume of sampled solids at different positions along the DFB gasifier.
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Fig. 6. Gas yield, carbon conversion and cold gas efficiency of the CFB preheater and the DFB gasifier.
the highest possibility of occurrence in thermodynamic analysis. If the preheated char is separated from the preheater gas, the preheated char will be concentrated and R2 will be promoted. The separation will be tested in our future study. 4. Thermodynamic equilibrium calculations 4.1. Effects of carbon conversions and CH4 yields in the preheater and the gasifier Thermodynamic equilibrium in the preheated coal gasification was calculated with the aid of ChemCAD. A Gibbs reactor was used to simulate the gasification output based on the Gibbs free energy minimization method [33e35]. Due to that the preheating included a pyrolysis and an incomplete gasification process, the preheater was modeled by a yield reactor and a Gibbs reactor. Coal was first decomposed into conventional components in the yield reactor and the resulted components were gasified with the first air in the Gibbs reactor to obtain the product of preheater. Then the product of the preheater was gasified with the second air in the gasifier, which was model by a Gibbs reactor as well. Temperatures and carbon conversions in the calculations were specified according to experimental results [36,37] or inputted as variables. Main gas products in air gasification were CO, CO2, H2, CH4 and N2. When thermodynamic equilibrium of gas composition was reached beyond 800 C, CH4 was consumed almost completely due to the reverse shift of R9 [35]. But in experiments CH4 was mainly generated from pyrolysis [38,39] and did not reach equilibrium below 1000 C. Therefore, the CH4 yield of the preheater was input in calculations. The carbon conversion and CH4 yield are important indicators of operations, which represent the intermediate and final statuses of carbon and CH4. Their effects on gas yield were investigated by keeping the heat losses of the preheater and the gasifier both at 5% of Qar,net. The preheater and gasifier temperatures were 900 C and 1100 C, respectively. Yields of product gas at different carbon conversions and CH4 yields in the preheater along with different final carbon conversions and CH4 yields in the gasifier are shown in Fig. 8. At a certain final carbon conversion and CH4 yield, the gas yields were unchanged with the varying carbon conversion or CH4 yield in the preheater. However, the CO yield and H2 yield both increased with the increasing final carbon conversion and decreased with the increasing final CH4 yield. Therefore, when the temperature and heat loss were constant, gas yields were decided by the final carbon conversion and final CH4 yield regardless of the intermediate conversion of carbon and CH4 in the preheater. 4.2. Effects of preheating temperature and gasifier temperature Effects of the preheating temperature were investigated by keeping the heat losses of the preheater and gasifier both at 5% of Qar,net. The gasifier temperature was set at 1100 C. Carbon conversions in the preheater and gasifier were 40% and 90%, respectively. CH4 yield in the preheater was 0.06 Nm3/kg and CH4 reached thermodynamic equilibrium in the gasifier. As displayed in Fig. 9, the CO yield, H2 yield and cold gas efficiency were unchanged with the preheating temperature from 800 to 1000 C. Though the products of the preheater were decided by thermodynamic equilibrium at different preheating temperatures, products of the preheater would eventually reached thermodynamic equilibrium in the gasifier at 1100 C. So the final products and their sensible heat were same at different preheating temperatures. Therefore, if the thermodynamic equilibrium was reached in the gasifier at a certain temperature and final carbon conversion, the final gas yield and cold gas efficiency would not be affected by the variation of carbon conversion, CH4 yield or temperature in the preheater. The effects of final carbon conversions and gasifier temperatures on cold gas efficiency were investigated by keeping the heat losses of the preheater and gasifier both at 5% of Qar,net (Fig. 10). As mentioned above, reaction rates show small difference when the temperature is above 1100 C, so the gasifier temperature was chose to range from 1100 to 1300 C in calculations. At a fixed gasifier temperature, a higher carbon conversion required a higher O2/C molar ratio. But the cold gas efficiency still increased with the increasing final carbon conversion and O2/C molar ratio. It's for the reason that the CO and H2 yield increased with the increasing final carbon conversion (Fig. 8). However, at a certain final carbon conversion, a decrease in the cold gas efficiency was induced by the temperature rise (Fig. 10). When the gasifier temperature increased, a higher O2/C molar ratio was required, leading to a higher extent of oxidation. More chemical energy was transferred to sensible heat instead of chemical energy of product gas, resulting in a reduction in CO yield, H2 yield and cold gas efficiency. Therefore, when the gasification reaction rates varied little above 1100 C and the same final carbon conversion was achieved in the gasifier, Please cite this article in press as: C. Liang, et al., Gasification of preheated coal: Experiment and thermodynamic equilibrium calculation, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.07.006
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Fig. 7. Changes of Gibbs free energy for oxidation reactions at different temperatures.
Fig. 8. Gas yield of product gas at different carbon conversions (preheater CH4 yield ¼ 0.06 Nm3/kg, final CH4 yield ¼ 0 Nm3/kg) and CH4 yields (preheater carbon conversion ¼ 40%, final carbon conversion ¼ 99.99%) in the preheater and gasifier.
Fig. 9. Gas yields of product gas and cold gas efficiency at different preheating temperatures (gasifier temperature ¼ 1100 C, preheater CH4 yield ¼ 0.06 Nm3/kg).
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Fig. 10. Cold gas efficiency at different final carbon conversions and at different temperatures in the gasifier (Final CH4 yield ¼ 0 Nm3/kg). 16 17 lowering the temperature would lead to an increase in cold gas efficiency and a decrease in O2 demand. Considering that reaction rates in 18 the gasifier was improved by the enhancement of gasification reactivity from the preheater, the gasification of preheated coal made it 19 possible to further increase the cold gas efficiency. 20 21 22 4. Conclusions 23 24 Gasification of preheated coal combining a CFB preheater and an entrained bed gasifier was put forward. A bench-scale test rig on 25 Shenmu bituminous coal was tested. The preheating process in the preheater and the gasification in the DFB gasifier were investigated. 26 Thermodynamic equilibrium calculations were conducted to reveal the effects of operating parameters of the preheater and the gasifier. 27 28 (1) The stable operation of the CFB preheater was obtained. The gasification reactivity of preheated char was increased due to the 29 improvement of intrinsic reactivity, specific surface area and total pore volume. 30 (2) Effective gasification of preheated char was achieved in the gasifier at about 1100 C meanwhile the CO þ H2 yield and carbon 31 conversion increased. 32 (3) Thermodynamic equilibrium calculations revealed when the same final carbon conversion was achieved in the gasifier, lowering the 33 temperature would lead to an increase in cold gas efficiency and a decrease in O2 demand. 34 35 Acknowledgements 36 37 Q3 Q4 This work was financially supported by the Innovation Guide Fund of Institute of Engineering Thermophysics, CAS. 38 39 References 40 41 [1] IEA, World Energy Outlook 2017, IEA, 2017. 42 [2] IEA, Coal 2017, IEA, 2017. [3] C.Z. Li, Importance of volatile-char interactions during the pyrolysis and gasification of low-rank fuels - a review, Fuel 112 (2013) 609e623. 43 44 Q5 [4] M.F. Irfan, M.R. Usman, K. 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Technol. 91 (2010) 915e920. 50 [10] N. Mahinpey, A. Gomez, Review of gasification fundamentals and new findings: reactors, feedstock, and kinetic studies, Chem. Eng. Sci. 148 (2016) 14e31. 51 [11] S. Wu, S. Huang, Y. Wu, J. Gao, Characteristics and catalytic actions of inorganic constituents from entrained-flow coal gasification slag, J. Energy Inst. 88 (2015) 93e103. 52 [12] F. Weiland, H. Hedman, M. Marklund, H. Wiinikka, O. Ohrman, Pressurized oxygen blown entrained-flow gasification of wood powder, Energy Fuels 27 (2013) 932e941. [13] A. Tremel, J. Stemann, M. Herrmann, B. Erlach, H. Spliethoff, Entrained flow gasification of biocoal from hydrothermal carbonization, Fuel 102 (2012) 396e403. 53 [14] S. Xu, Y. Ren, B. Wang, Y. Xu, L. Chen, Development of a novel 2-stage entrained flow coal dry powder gasifier, Appl. Energy 113 (2014) 318e323. 54 [15] T. Liu, Y. Fang, Y. Wang, An experimental investigation into the gasification reactivity of chars prepared at high temperatures, Fuel 87 (4e5) (2008) 460e466. 55 [16] J. Wang, J.G. Zhu, Q.G. Lu, Experimental study on combustion characteristics and NOX emissions of pulverized anthracite preheated by circulating fluidized bed, J. Therm. Sci. 20 (2011) 355e361. 56 [17] Y. Yao, J.G. Zhu, Q.G. Lu, Z.X. Zhou, Experimental study on preheated combustion of pulverized semi-coke, J. Therm. Sci. 24 (2015) 370e377. 57 [18] J.G. Zhu, Z.Q. Ouyang, Q.G. Lu, Numerical Simulation on pulverized coal combustion and NOX emissions in high temperature air from circulating fluidized bed, J. Therm. 58 Sci. 22 (2013) 261e268. [19] J.G. Zhu, Q.G. Lu, T.Y. Niu, G.L. Song, Y.J. Na, NO emission on pulverized coal combustion in high temperature air from circulating fluidized bed - an experimental study, 59 Fuel Process. Technol. 90 (2009) 664e670. 60 [20] J. Zhu, Z. Ouyang, Q. Lu, An experimental study on NOX emissions in combustion of pulverized coal preheated in a circulating fluidized bed, Energy Fuels 27 (2013) 61 7724e7729. [21] Y. Yao, J. Zhu, Q. Lu, Experimental study on nitrogen transformation in combustion of pulverized semi-coke preheated in a circulating fluidized bed, Energy Fuels 29 62 (2015) 3985e3991. 63 [22] Z. Ouyang, J. Zhu, Q. Lu, Y. Yao, J. Liu, The effect of limestone on SO2 and NOX emissions of pulverized coal combustion preheated by circulating fluidized bed, Fuel 120 64 (2014) 116e121. 65 [23] Z. Ouyang, J. Zhu, Q. Lu, Experimental study on preheating and combustion characteristics of pulverized anthracite coal, Fuel 113 (2013) 122e127. Please cite this article in press as: C. Liang, et al., Gasification of preheated coal: Experiment and thermodynamic equilibrium calculation, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.07.006
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Contents lists available at ScienceDirect
Journal of the Energy Institute journal homepage: http://www.journals.elsevier.com/journal-of-the-energyinstitute
Gasification of preheated coal: Experiment and thermodynamic equilibrium calculation Q8
Chen Liang a, b, Qinggang Lyu a, b, *, Yongjie Na a, b, Xiaofang Wang a a b
Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing, 100190, China University of Chinese Academy of Sciences, Beijing, 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 April 2018 Received in revised form 22 July 2018 Accepted 24 July 2018 Available online xxx
A new process integrating a circulating fluidized bed (CFB) reactor and an entrained bed reactor was proposed for gasification of preheated coal. The CFB reactor as a preheater was successfully used in clean coal combustion. In this study, gasification of preheated coal was tested in a bench-scale test rig, which consisted of a CFB preheater and a down flow bed (DFB) gasifier. The effects of operating parameters of the preheater and gasifier were revealed via thermodynamic equilibrium calculations. A stable preheating process was obtained in the CFB preheater at the O2/C molar ratio of 0.31 and higher gasification reactivity was gained in preheated char owing to the improvement in intrinsic reactivity, specific surface area and total pore volume. Effective gasification of preheated char was achieved in the DFB gasifier at 1100 C and the total O2/C molar ratio of 0.67, meanwhile the CO þ H2 yield and carbon conversion increased. Thermodynamic equilibrium calculations revealed when the gasification reaction rates varied little above 1100 C and the same carbon conversion was achieve in gasifier, lowering the temperature would lead to an increase in cold gas efficiency and a decrease in O2 demand. © 2018 Published by Elsevier Ltd on behalf of Energy Institute.
Keywords: Gasification Coal Circulating fluidized bed preheater Down flow bed gasifier
1. Introduction Coal will still supply 26% of global energy mix and over 55% of China's energy demand in 2022, and is expected to contribute to energy production until the end of 2100 [1,2]. Gasification, as an efficient and clean technology to covert coal into hydrogen, electricity and chemical products, has attracted more attention [3,4]. The coal gasification technology can be mainly categorized by the reactor into three types: fixed bed, fluidized bed and entrained bed [5]. Firstly, a fixed bed reactor with a given volume has a high conversion rate, but is limited by a low operating temperature below 1000 C and tar problems in the product gas [6,7]. Secondly, a fluidized bed reactor is characterized by uniform temperature distributions, efficient heat exchange, good gas-solid mixing and strong back-mixing of solids [8,9]. However, its application is limited by the low carbon conversion because the fluidized bed gasifier should be operated below 1000 C to avoid slagging and defluidization [5,10]. Thirdly, an entrained bed reactor is characterized by an extremely high operating temperature beyond 1350 C, so the ash is removed as molten slag [11]. The high operating temperature contributes to increasing the carbon conversion and cold gas efficiency [12e14], but causes the corrosion of burners and refractories [5]. It is well known that temperature is an essential parameter in gasification operation. Reaction rates of gasification reactions and carbon conversions usually increase with the increasing temperature. However increases in reaction rates obviously slow and an overlapping exists in the curves of coal/char conversion against time when the temperature is over 1100 C [4,15]. Meanwhile, oxygen demands increase faster and serious corrosion of refractories are caused when the temperature is beyond 1300 C and more sensible heat can't be transferred into chemical energy in product gas. Therefore, operating at 1100e1200 C maybe is more economic in gasification. According to these characteristics, the gasification of preheated coal was proposed as a new concept of gasification process to realize this operating temperature, which combines a circulating fluidized bed (CFB) preheater [16] and an entrained bed gasifier. Using CFB as preheaters was proposed by the Institute of Engineering Thermophysics, Chinese Academy of Sciences (IET, CAS) and have been applied for clean and efficient combustion of pulverized coal since 2008 [17e19]. Non-ignitable coal can be burnt out steadily with less
* Corresponding author. Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing, 100190, China. E-mail address: [email protected] (Q. Lyu). https://doi.org/10.1016/j.joei.2018.07.006 1743-9671/© 2018 Published by Elsevier Ltd on behalf of Energy Institute.
Please cite this article in press as: C. Liang, et al., Gasification of preheated coal: Experiment and thermodynamic equilibrium calculation, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.07.006
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NOX emission [20e22]. The CFB preheater can be applied to coal gasification because morphological properties of coal can be improved in a CFB preheater to enhance oxidation reactions in the gasifier [23]. Meanwhile the gasifier can be operated at 1100e1200 C to increase reaction rates compared with fluidized bed gasification, and to reduce oxygen demands and avoid serious corrosion of refractories compared with entrained bed gasification [5]. Moreover, coal handling processes for sub-bituminous and lignite with poor grindability [24] can be simplified by using a CFB preheater fed by fuel with wide-range particle size distribution. In this study, a bench-scale test rig was built for gasification of preheated coal and tested through a series of experiments. The preheating and gasification processes were characterized experimentally and the preliminary results are introduced. The effects of carbon conversion, CH4 yield and temperature were evaluated through thermodynamic equilibrium calculations. 2. Experimental 2.1. Test rig The test rig for gasification of preheated coal consisting of a CFB preheater, a down flow bed (DFB) gasifier and an auxiliary system was shown schematically in Fig. 1. The CFB preheater consisted of a riser (internal diameter ¼ 900 mm, height ¼ 1500 mm), a cyclone and a loop seal. The outlet of the cyclone was connected to the inlet of the DFB gasifier, which was 260 mm in diameter and 3000 mm in height. The primary air was supplied through an air distributor to the bottom of the riser to maintain the preheater operating at about 900 C. Pulverized coal was fed to the riser by a screw at 240 mm above the air distributor and was preheated in the CFB preheater. Gas products and solid products from the preheater were called preheater gas and preheated char, respectively. The preheater gas carried the preheated char to a nozzle at the top of the DFB gasifier. The secondary air was supplied though the nozzle to provide oxygen for the gasification of preheated char. The CFB preheater and the DFB gasifier were installed with four and five thermocouples, respectively. Gas and solid products were collected at seven sampling points: one at the preheater outlet to sample preheater gas and preheated char, one at the cooler outlet to sample the final product gas and fly ash, and five at 100, 400, 900, 1400 and 2400 mm below the nozzle, respectively. The test rig was coated with 150 mm-thick aluminum silicate fabric to reduce heat loss. 2.2. Coal and sample characteristics Shenmu bituminous coal from Shaanxi province of China was used in the experiments. The characteristics of coal are shown in Table 1. The ash composition is listed in Table 2. The Shenmu coal has low ash content, high volatiles and high calorific value, which is fit for gasification. The particle sizes of coal and silica sand are 0e0.5 mm (50% cut size d50 ¼ 230.8 mm) and 0.1e0.5 mm, respectively. 2.3. Experimental In the starting procedure, silica sand (3 kg) was added into the preheater riser as bed materials and then fluidized by the air supply. The electrical heater was turned on to heat the CFB to 550 C, and coal was fed continuously. When the CFB temperature rose to 800 C, the heater was turned off, and the coal and primary air were adjusted to switch the running state of preheater from combustion to gasification. The procedure of increasing the preheater temperature was similar to CFB gasification technology operations [25,26]. During the starting of the preheater, the gasifier was heated by the product of preheater. When the starting of the preheater was finished, the secondary air was injected into the gasifier to combust the preheated char and preheater gas and to increase the gasifier temperature. When the gasifier
Fig. 1. Schematic diagram of the test rig. 1. Air compressor; 2. Electrical heater; 3. Screw feeder; 4. Riser; 5. Cyclone; 6. Loop seal; 7. CFB sampling port; 8. Nozzle; 9. Down flow bed; 10. Sampling point (100 mm); 11. Sampling point (400 mm); 12. Sampling point (900 mm); 13. Sampling point (1400 mm); 14. Sampling point (2400 mm); 15. Water cooler; 16. Bag filter; 17. Final sampling point; 18. Water tank.
Please cite this article in press as: C. Liang, et al., Gasification of preheated coal: Experiment and thermodynamic equilibrium calculation, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.07.006
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Table 1 The characteristics of Shenmu bituminous coal a. Proximate analysis (wt %)
Ultimate analysis (wt %)
Qnet,
Mar
Aar
Var
FCar
Cdaf
Hdaf
Odaf
Ndaf
Sdaf
6.90
4.80
32.79
55.51
81.38
4.70
12.51
1.00
0.41
a
ar
(MJ/kg)
27.68
ar, as received basis; M, moisture; A, ash; V, volatile matter; FC, fixed carbon; daf, dry ash free basis.
temperature reached 1000 C, the secondary air was adjusted to keep gasifier under gasification condition. Then the operating parameters were adjusted to running conditions. The coal feeding rate was 5.5 kg/h in experiments of this work. The preheater gas and product gas were deashed by a glass fiber filter and dried before online tests. Main gas components including H2, CO, CH4, CO2 and N2 were detected on an Agilent GC 3000 gas chromatograph. The gas yield, calorific value (Qgas,net), carbon conversion (XC) and cold gas efficiency (h) were calculated as follows:
ygas;i ¼ ðxi 0:79qair Þ
xN2 Mcoal
ði ¼ CO; H2 ; CH4 ; CO2 Þ
Nm3 =kg
(1)
Qgas;net ¼ xH2 10:79 þ xCO 12:64 þ xCH4 35:90 MJ=Nm3
(2)
XC ¼ ygas;CO þ ygas;CO2 þ ygas;CH4 12=ðCad 22:4Þ 100%
(3)
h ¼ Qgas;net ygas
. Qar;net 100%
(4)
where ygas,i and ygas are the yields (Nm3/kg) of component i and gas product, respectively; xi is the volume percentage (%) of i; qair is the volume flow rate (Nm3/h) of air as gasifying agent; Mcoal is the coal feeding rate (kg/h); Qar,net is the net calorific value of coal (MJ/kg). 3. Results and discussion 3.1. Characteristics of preheating process in the CFB preheater Pulverized coal with particle sizes of 0e0.5 mm was used to characterize the CFB preheater at the O2/C molar ratio of 0.31 and the fluidization velocity of 1.50 m/s. A stable operation at 900 C for more than 3 h was obtained in the preheater. The preheating process was like an incomplete CFB gasification process [25] at low O2/C molar ratios. Once fed into the preheater riser, the pulverized coal was rapidly heated by the bed material, which consisted of circulating char and silica sand, and was pyrolyzed into char and gas. The char was mechanically cracked and incompletely gasified in the riser. The preheater gas was generated during the pyrolysis and incomplete gasification. The small-size char from the riser, which was not captured by the cyclone, was carried by the preheater gas and left the preheater from the outlet as preheated char. The large-size char from the riser was captured by the cyclone and sent back to the riser by the loop seal as bed materials. The cyclone separation efficiency was designed to achieve a mass balance in the preheater so as to ensure the stable operation of the preheater. The particle size distributions of pulverized coal and preheated char were tested by a Malvern Mastersizer 2000 laser analyzer and shown in Fig. 2. The particle sizes of pulverized coal declined and the 50% cut size d50 decreased from 0.231 to 0.161 mm. Due to the particle fragmentation caused by cracking and reactions in the riser and the separation of cyclone, coal was pulverized in the preheating process, which contributed to the gasification in DFB and reducing pulverizing cost. Pore surface area and pore volume distributions of the pulverized coal and preheated char are shown in Fig. 3, which were analyzed by an automatic specific surface area/pore size distribution measurement (ASAP 2020). N2 adsorption isotherms at 196 C were measured for the relative pressure (P/P0) range from 0.01 to 0.99. Specific surface areas were obtained using the BrunauereEmmetteTeller (BET) model, and the pore size distributions were obtained using the DollimoreeHeal (DH) model. The surface areas and volumes of width-varying pores were enlarged in the preheater, especially at the width of 20e40 nm. As a result, the specific surface areas increased from 13.92 to 263.35 m2/g and the total pore volumes increased from 0.23 ✕ 107 to 2.01 ✕ 107 m3/g, while the average pore width decreased from 67.17 to 30.54 nm, indicating a large number of mesoporous were formed in the preheated char. Proximate analyses showed 82.31% of volatiles in pulverized coal were released in the preheater. The sharp increase in specific surface area and total pore volume was attributed to the intense coal devolatilization at a fast heating rate in the preheater. As pores and their surfaces are main places for heterogeneous gasification reactions (e.g. R6 and R7 in Table 3), the enlargement of the specific surface area and total pore volume accelerated reactions of the preheated char in the gasifier. The gasification reactivity of intrinsic char from the preheated char and pulverized coal was evaluated via thermogravimetric analyses (TGA) at atmospheric pressure. Samples were heated in N2 atmosphere to 600 C and kept for 1 h to remove volatiles. Then the atmosphere was switched to 10 mol% CO2 in N2 [27,28]. After that, a conversion of 5% was excluded to eliminate effects of gas change and possible remaining volatiles. Thereby, the intrinsic reactivity of char was tested. The reaction rate r was calculated from the weight loss data as follows: Table 2 Ash composition of Shenmu bituminous coal (wt %). SiO2
Al2O3
Fe2O3
CaO
MgO
TiO2
SO3
P2O5
K2O
Na2O
29.32
13.37
10.96
31.59
1.08
0.59
11.00
0.21
0.43
1.45
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Fig. 2. Size distributions of the pulverized coal and preheated char.
Fig. 3. Pore surface area and pore volume distributions of the pulverized coal and preheated char.
Table 3 Main reactions involved in coal gasification [26,27] a. No.
Reaction
R1 R2 R3 R4 R5 R6 R7 R8 R9
Char oxidation reaction (I) Char oxidation reaction (II) Gas oxidation reaction (I) Gas oxidation reaction (II) Gas oxidation reaction (III) Boudouard reaction Steam char gasification Water gas shift reaction (WGS) Methanation reaction
a
Q7
DH1350 (kJ/mol) C þ O2/CO2 C þ 0.5O2/CO CO þ 0.5O2/CO2 H2 þ 0.5O2/H2O CH4 þ 2O2/CO2 þ 2H2O C þ CO2#2CO C þ H2O#CO þ H2 CO þ H2O#CO2 þ H2 CO þ 3H2#CH4 þ H2O
395.37 114.21 281.16 249.69 802.63 þ166.94 þ135.48 31.46 227.61
DH1350, the standard enthalpy of reaction at the temperature of 1350 K (1076.85 C).
r ¼ dm=ðdt,mÞ
(5)
where m is the instantaneous mass of the sample. The reaction rate of intrinsic char from the preheated char and pulverized coal were matchable before 950 C (Fig. 4). As the temperature rose, the Boudouard reaction was enhanced and both reaction rates were obviously accelerated. However, the reaction rate of preheated char was higher and increased faster than that of the pulverized coal at higher temperatures. It indicates the intrinsic reactivity of the preheated char was improved in comparison with pulverized coal, especially at above 950 C. Therefore, the gasification reactivity of preheated char was enhanced by the improvement of intrinsic reactivity, specific surface area and total pore volume during the preheating. It was also confirmed by research that decreases in the activation energy were contributed for char by the heat treatment compared with coal [29,30]. Reaction rates of preheated char and the carbon conversion in the gasifier will be promoted by the enhancement. 3.2. Characteristics of gasification process in the DFB gasifier The O2/C molar ratio in the CFB preheater was 0.31 as mentioned above, and that in the DFB gasifier was 0.36 in the experiment. After pulverized coal with particle sizes of 0e0.5 mm was fed into the preheater, the resulting preheated char and preheater gas at 900 C were injected to the top of the gasifier through the annular channel of the nozzle. The secondary air was injected through the central channel of Please cite this article in press as: C. Liang, et al., Gasification of preheated coal: Experiment and thermodynamic equilibrium calculation, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.07.006
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Fig. 4. Observed reaction rate of the preheated char and pulverized coal with increasing temperatures in CO2 (balance gas N2).
the nozzle with a velocity of 18 m/s to provide oxygen for the gasification of preheated char. Temperatures in the gasifier were stabilized owing to no igniting problem of preheated char. A temperature profile in the gasifier is presented in Fig. 5. The injection of the secondary air accelerated oxidation and induced a temperature rise by about 195 C from the top position to 100 mm below the nozzle. The oxidation was sustained until 500 mm below the nozzle, leading to the temperature rising to 1100 C. The endothermic gasification reactions and heat loss led to a temperature drop along the flowing direction from 500 mm below the nozzle to the bottom. As present in Fig. 5, the specific surface areas and total pore volumes of solids sampled at different positions along the DFB changed in the same trend of sharply decreasing from the top to 100 mm below the nozzle. The reason was that pores at the outer layer of char particles, which were mainly formed in the preheating process, were rapidly consumed in the fast oxidation. The fast oxidation was verified by the temperature profile. Then the oxidation slowed down from 100 to 500 mm below the nozzle due to the oxygen consumption. The further devolatilization of preheated char was induced simultaneously. The temperature rise was also decelerated in this section, which proved the weakening of oxidation. Therefore, pores were generated and the specific surface areas and total pore volumes increased. The specific surface areas and total pore volumes decreased continuously after 500 mm below the nozzle, but at gradually slower rates. Pores were consumed by the Boudouard reaction R6 and the consumption slowed down from 500 mm below the nozzle to the bottom because the reaction was decelerated at lower temperatures. Gas yields, carbon conversions and cold gas efficiencies of the CFB preheater and the DFB gasifier are shown in Fig. 6. The back-mixing of char and gas and the uniform temperature distribution in the preheater strongly enhanced the destruction of the pyrolysis-induced tar [31] and no tar was detected in the preheater gas. Because the homogeneous reactions between the preheater gas and oxygen, such as R3, R4 and R5, were much faster than the heterogeneous reactions between preheated char and oxygen [29,32], such as R1 and R2, CO, H2 and CH4 in the preheater gas were consumed first to form numerous CO2. However, reactions of preheated char was verified by the change of total pore volume of solids sampled at different positions in the gasifier. Through the gasifier the CO yield increased from 0.33 to 0.44 Nm3/kg and the CO þ H2 yield increased from 0.59 to 0.65 Nm3/kg. It indicates that Boudouard reaction was enhanced and the preheated char was effectively gasified at 1100 C in the gasifier. The CO2 yield also increased from 0.26 to 0.61 Nm3/kg because of the fast oxidation of the preheater gas and preheated char. As a result of the increase in CO yield and CO2 yield, carbon conversion was strongly improved. The increase in CO þ H2 yield and the decrease in CH4 yield made the cold gas efficiencies little different. The changes of Gibbs free energy, DG, in oxidation reactions among the preheated char, preheater gas and secondary air at different temperatures [26,27] are present in Fig. 7. The DG of the reactions from 700 to 1400 C are negative, indicating these reactions are possible. The DG of reaction R2 is the lowest above 800 C and decreases with the temperature rise, indicating R2 owns the strongest driving force and
Fig. 5. Temperature profile, BET specific surface area and total pore volume of sampled solids at different positions along the DFB gasifier.
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Fig. 6. Gas yield, carbon conversion and cold gas efficiency of the CFB preheater and the DFB gasifier.
the highest possibility of occurrence in thermodynamic analysis. If the preheated char is separated from the preheater gas, the preheated char will be concentrated and R2 will be promoted. The separation will be tested in our future study. 4. Thermodynamic equilibrium calculations 4.1. Effects of carbon conversions and CH4 yields in the preheater and the gasifier Thermodynamic equilibrium in the preheated coal gasification was calculated with the aid of ChemCAD. A Gibbs reactor was used to simulate the gasification output based on the Gibbs free energy minimization method [33e35]. Due to that the preheating included a pyrolysis and an incomplete gasification process, the preheater was modeled by a yield reactor and a Gibbs reactor. Coal was first decomposed into conventional components in the yield reactor and the resulted components were gasified with the first air in the Gibbs reactor to obtain the product of preheater. Then the product of the preheater was gasified with the second air in the gasifier, which was model by a Gibbs reactor as well. Temperatures and carbon conversions in the calculations were specified according to experimental results [36,37] or inputted as variables. Main gas products in air gasification were CO, CO2, H2, CH4 and N2. When thermodynamic equilibrium of gas composition was reached beyond 800 C, CH4 was consumed almost completely due to the reverse shift of R9 [35]. But in experiments CH4 was mainly generated from pyrolysis [38,39] and did not reach equilibrium below 1000 C. Therefore, the CH4 yield of the preheater was input in calculations. The carbon conversion and CH4 yield are important indicators of operations, which represent the intermediate and final statuses of carbon and CH4. Their effects on gas yield were investigated by keeping the heat losses of the preheater and the gasifier both at 5% of Qar,net. The preheater and gasifier temperatures were 900 C and 1100 C, respectively. Yields of product gas at different carbon conversions and CH4 yields in the preheater along with different final carbon conversions and CH4 yields in the gasifier are shown in Fig. 8. At a certain final carbon conversion and CH4 yield, the gas yields were unchanged with the varying carbon conversion or CH4 yield in the preheater. However, the CO yield and H2 yield both increased with the increasing final carbon conversion and decreased with the increasing final CH4 yield. Therefore, when the temperature and heat loss were constant, gas yields were decided by the final carbon conversion and final CH4 yield regardless of the intermediate conversion of carbon and CH4 in the preheater. 4.2. Effects of preheating temperature and gasifier temperature Effects of the preheating temperature were investigated by keeping the heat losses of the preheater and gasifier both at 5% of Qar,net. The gasifier temperature was set at 1100 C. Carbon conversions in the preheater and gasifier were 40% and 90%, respectively. CH4 yield in the preheater was 0.06 Nm3/kg and CH4 reached thermodynamic equilibrium in the gasifier. As displayed in Fig. 9, the CO yield, H2 yield and cold gas efficiency were unchanged with the preheating temperature from 800 to 1000 C. Though the products of the preheater were decided by thermodynamic equilibrium at different preheating temperatures, products of the preheater would eventually reached thermodynamic equilibrium in the gasifier at 1100 C. So the final products and their sensible heat were same at different preheating temperatures. Therefore, if the thermodynamic equilibrium was reached in the gasifier at a certain temperature and final carbon conversion, the final gas yield and cold gas efficiency would not be affected by the variation of carbon conversion, CH4 yield or temperature in the preheater. The effects of final carbon conversions and gasifier temperatures on cold gas efficiency were investigated by keeping the heat losses of the preheater and gasifier both at 5% of Qar,net (Fig. 10). As mentioned above, reaction rates show small difference when the temperature is above 1100 C, so the gasifier temperature was chose to range from 1100 to 1300 C in calculations. At a fixed gasifier temperature, a higher carbon conversion required a higher O2/C molar ratio. But the cold gas efficiency still increased with the increasing final carbon conversion and O2/C molar ratio. It's for the reason that the CO and H2 yield increased with the increasing final carbon conversion (Fig. 8). However, at a certain final carbon conversion, a decrease in the cold gas efficiency was induced by the temperature rise (Fig. 10). When the gasifier temperature increased, a higher O2/C molar ratio was required, leading to a higher extent of oxidation. More chemical energy was transferred to sensible heat instead of chemical energy of product gas, resulting in a reduction in CO yield, H2 yield and cold gas efficiency. Therefore, when the gasification reaction rates varied little above 1100 C and the same final carbon conversion was achieved in the gasifier, Please cite this article in press as: C. Liang, et al., Gasification of preheated coal: Experiment and thermodynamic equilibrium calculation, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.07.006
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Fig. 7. Changes of Gibbs free energy for oxidation reactions at different temperatures.
Fig. 8. Gas yield of product gas at different carbon conversions (preheater CH4 yield ¼ 0.06 Nm3/kg, final CH4 yield ¼ 0 Nm3/kg) and CH4 yields (preheater carbon conversion ¼ 40%, final carbon conversion ¼ 99.99%) in the preheater and gasifier.
Fig. 9. Gas yields of product gas and cold gas efficiency at different preheating temperatures (gasifier temperature ¼ 1100 C, preheater CH4 yield ¼ 0.06 Nm3/kg).
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Fig. 10. Cold gas efficiency at different final carbon conversions and at different temperatures in the gasifier (Final CH4 yield ¼ 0 Nm3/kg). 16 17 lowering the temperature would lead to an increase in cold gas efficiency and a decrease in O2 demand. Considering that reaction rates in 18 the gasifier was improved by the enhancement of gasification reactivity from the preheater, the gasification of preheated coal made it 19 possible to further increase the cold gas efficiency. 20 21 22 4. Conclusions 23 24 Gasification of preheated coal combining a CFB preheater and an entrained bed gasifier was put forward. A bench-scale test rig on 25 Shenmu bituminous coal was tested. The preheating process in the preheater and the gasification in the DFB gasifier were investigated. 26 Thermodynamic equilibrium calculations were conducted to reveal the effects of operating parameters of the preheater and the gasifier. 27 28 (1) The stable operation of the CFB preheater was obtained. The gasification reactivity of preheated char was increased due to the 29 improvement of intrinsic reactivity, specific surface area and total pore volume. 30 (2) Effective gasification of preheated char was achieved in the gasifier at about 1100 C meanwhile the CO þ H2 yield and carbon 31 conversion increased. 32 (3) Thermodynamic equilibrium calculations revealed when the same final carbon conversion was achieved in the gasifier, lowering the 33 temperature would lead to an increase in cold gas efficiency and a decrease in O2 demand. 34 35 Acknowledgements 36 37 Q3 Q4 This work was financially supported by the Innovation Guide Fund of Institute of Engineering Thermophysics, CAS. 38 39 References 40 41 [1] IEA, World Energy Outlook 2017, IEA, 2017. 42 [2] IEA, Coal 2017, IEA, 2017. [3] C.Z. Li, Importance of volatile-char interactions during the pyrolysis and gasification of low-rank fuels - a review, Fuel 112 (2013) 609e623. 43 44 Q5 [4] M.F. Irfan, M.R. Usman, K. 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