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Experimental Study of Coal Pyrolysis under the Simulated HighTemperature and High-Stress Conditions of Underground Coal Gasification Tian-Hong Duan, Zuo-Tang Wang,* Zhen-Jia Liu, Ya-Zhou Chen, and Zhen-Bin Fu Key Laboratory of Deep Coal Resource Mining, School of Mines, Ministry of Education of China, China University of Mining and Technology, Xuzhou 221116, China ABSTRACT: Coal pyrolysis under high-temperature and high-stress conditions was studied experimentally in this work. Which experimental table should be chosen was studied, and parameter settings such as the final pyrolysis temperature, heating rate, and stresses were investigated. Pyrolysis experiments of Xinglongzhuang gas coal and Huating long-flame coal were conducted using an improved Paterson gas-medium high-pressure and high-temperature testing (Paterson HPT) system at temperatures of up to 1000 °C and under different stresses conditions. The results were compared to those of coal pyrolysis experiments without any stress and using a GR.TF80/11 tube furnace. The average heating rates used in these experiments were as low as 0.44 K/min. The produced gas and semicoke (coke) yields both increased with increasing stress, while the tar yield decreased. However, the variations in the yields of the pyrolysis products induced by the same increase in stress gradually decreased as the stress increased and nearly reached a peak at approximately 750 °C. The concentrations of CH4 and CO2 in the pyrolysis gas gradually increased with increasing stress, while the H2 and CO concentrations gradually decreased. Regarding the variations in H2, CO, CH4, and CO2, the Huating coal displayed significantly more variations than the Xinglongzhuang coal for the same increase in stress, indicating its higher sensitivity. The variation curve for each gas component under each stress state was similar to the corresponding curve in the stress-free state, indicating that temperature is a dominant factor in coal pyrolysis and that the effects of stress-induced secondary reactions are relatively small.

1. INTRODUCTION The development and application of underground coal gasification (UCG) technologies have emerged as a main direction of energy acquisition research.1 UCG integrates well construction, mining, and gasification into a unified process rather than separating the individual processes of well construction and coal mining, transportation, washing, processing, and gasification.2−5 In comparison to conventional coal mining methods, UCG increases miner safety, does not require the treatment of coal refuse and tailings, and reduces the amounts of dust, noise, and wastewater pollution. In addition, methane emissions are reduced.6−8 The resources that are buried deep underground are inaccessible for traditional mining methods.9,10 However, these kinds of resources can be utilized by UCG. Therefore, UCG is an efficient method for directly converting deep coal resources into clean energy11 and reducing carbon emissions.12−14 However, the technology has several limitations. It can have significant environmental consequences including aquifer contamination and ground subsidence. While a framework that can eliminate or reduce these environmental risks can be constructed from current knowledge, it is important to proactively address this constraint on siting and operation of any future UCG projects.15 The spontaneous combustion of coal seams and UCG are both complex physical−chemical reactions of coal seams under stress in situ, including coal seam pyrolysis. Coal seam pyrolysis reactions significantly affect gasification parameters, such as the gas yield from UCG, the calorific value and composition of the resulting syngas, and the consumption of the gasification agent. When a coal seam spontaneously combusts, the escape of the © XXXX American Chemical Society

resulting gases significantly influences the occurrence and development of spontaneous combustion fires in the coal seam. Basic experimental research on coal seam pyrolysis under hightemperature and high-stress conditions (heating under triaxial stress) has significant engineering value for improving the calorific value and yield of syngas, calculations of the gasification parameters of UCG, and the control of spontaneous combustion fires in coal seams. In addition, this research can assist with investigations of the occurrences of spontaneous combustion fires in coal seams and the mechanisms by which these fires can spread.16 Nearly all previous coal pyrolysis theories and experiments have been based on stress-free conditions. However, in situ coal seam pyrolysis occurs under in situ stress; coal seams have relatively high levels of continuity and compactness, and the porosity, permeability, and thermal conductivity of the coal all undergo significant changes. Current studies of surface pressurized gasification show that the production of gas and solid products increases, while the production of liquid products decreases as the pressure or residence time increases.17,18 The results of experiments performed by Roberts, Harris, and Wall indicate that the effects of the pyrolysis pressure and heating rate on char gasification rates are more likely to be due to effects of structure and surface area (depending on the reaction conditions), resulting in consequential effects on the diffusion of reactants to the char Received: September 1, 2016 Revised: January 19, 2017

A

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Energy & Fuels Table 1. Quality Analyses of the Two Types of Coala proximate analysis/wt %

a

ultimate analysis/wt %

coal

Mad

Aad

Vad

FCad

Cad

Had

Oad

Nad

St,ad

Xinglongzhuang coal Huating coal

2.13 9.10

11.62 7.80

30.66 35.00

55.59 48.10

73.65 72.00

4.67 4.20

8.55 14.70

1.24 0.80

0.27 0.50

Air-dried basis is abbreviated as ad in this table.

Figure 1. Coal pyrolysis apparatus in a stress-free state.

ditions. Moreover, these studies did not compare coal pyrolysis under stress to coal pyrolysis without stress. In this study, coal pyrolysis tests were conducted under high-temperature and high-stress conditions, with a maximum final pyrolysis temperature of 1000 °C. A relevant experimental method was developed, and the rules of coal seam pyrolysis during UCG were identified. None of these results have been reported in the existing literature.

surface, rather than on the intrinsic reactivity of the coal chars. 19 Experiments performed by Bauman and Deo demonstrated that the energy requirement of in situ processing methods for producing oil from oil shale can be reduced significantly by following a period of pyrolysis with in situ coke combustion.20 However, the effects of in situ stress on UCG pyrolysis, particularly compared with the effects of inertial gas pressure on surface pressurized pyrolysis, have rarely been studied. In addition, without special treatment, coal seam pyrolysis that occurs with UCG has a very slow heating rate, which was recorded as 0.02−0.2 K/min by Elliot.21 However, in typical conventional surface pyrolysis, the pyrolysis heating rate varies between several °C/s and 106 °C/s. In surface pyrolysis, the tar yield gradually decreases with the heating rate, while the gas yield gradually increases.22,23 The effects of this unique UCG heating rate on pyrolysis have yet to be theoretically or experimentally investigated. It is impractical to directly study coal seam pyrolysis in situ. Laboratory simulation is an effective and feasible solution to this problem. Zhao, Wan, Qu, and Feng et al. conducted coal pyrolysis tests under high-temperature and high-stress conditions and showed that high stress results in higher gas yields than under stress-free conditions.16,22,23 However, the tests relied on an XPS-20MN, which is a servo-controlled hightemperature and high-pressure triaxial rock testing system with a maximum final pyrolysis temperature of only 600 °C.24 However, at higher temperatures, the pyrolysis of coal plays an increasingly important role in the overall coal conversion process.25 The secondary reactions of tar, an initial pyrolysis product, begin to intensify at approximately 600 °C, and the secondary reactions of hydrocarbon molecules occur at higher temperatures. Final temperatures of in situ coal seam pyrolysis can reach 1000 °C. Moreover, recent research has primarily focused on the mechanical characteristics of coal, such as its elastic modulus, Poisson’s ratio, permeability, and thermal cracking under high-temperature and high-stress conditions. Pyrolysis gas composition and tar and semicoke (coke) yields have not been studied. The employed heating rates did not conform to the characteristics of UCG. Therefore, the data obtained in these tests differed from the coal seam pyrolysis data obtained under high-temperature and high-stress con-

2. COMPARISON OF COAL PYROLYSIS EXPERIMENTS WITH OR WITHOUT STRESS To account for the effects of stress on coal pyrolysis, stress-free coal pyrolysis experiments were conducted using the same methods and parameters as the coal pyrolysis experiments conducted under high-temperature and high-stress conditions; the methods and parameters maintained included the coal samples, the heating rate, and the final pyrolysis temperature. The only differences between the two types of experiments were that stress-free coal pyrolysis experiments did not involve exerting stress on the samples and that the experiments used different types of pyrolysis equipment. To eliminate the effect of the surface oxide layer, cylindrical samples of ϕ35 mm × 70 mm were extracted using an emery drill bit from the center of a large lump of coal and immediately covered with wax. However, before the experiments, we polished each coal sample using sandpaper. Therefore, the wax was removed before the experiments and did not affect the pyrolysis process. Each sample was drilled a hole with a diameter of 5 mm in the center, then polished and dried before conducting experiments. Each sample weighed approximately 90 g. These experiments cannot be performed using a thermogravimetric analyzer. Additionally, syngas produced by the tests is too light compared with a gas sac or a gas bag, which leads to very large measurement errors. We did not test the mass balances of the tests during these experiments. To guarantee the accuracy of the experimental data, the two types of experiments were each performed three times, and the average of the three results was used. The coefficients of variation of product yields in our experiments were controlled to be less than 12%. Obviously erroneous data were discarded, and the experiment was repeated. B

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Figure 2. Improved Paterson gas-medium high-pressure and high-temperature testing system.

2.1. Stress-Free Coal Pyrolysis Experiments. 2.1.1. Types of Coal Used in the Experiments. Low rank coal exhibits higher gasification reaction activity in comparison with high rank coal in UCG. Therefore, in UCG, the gasification of low rank coal is favorable. Therefore, low rank coal should be used in these experiments. Since the experiments performed in this study require a large amount of time, only the long-flame coal of the Huating coal group from the Huating coal mine and the gas coal of the Yanzhou coal group from the Xinglongzhuang coal mine of China were used in this study. Lignite was not used in these experiments. The results of the proximate analysis and the ultimate analysis of the coal samples are shown in Table 1. 2.1.2. Experimental Method. 2.1.2.1. Experimental Equipment and Methods. The equipment used in the stress-free coal pyrolysis experiment are shown in Figure 1 and includes a GR.TF80/11 tube furnace. The produced gas was purified and dried using two iced saline condensers, a cotton lint filter, and a silica gel filter. The tar in the produced gas was passed through two iced saline condensers and condensed inside because of the sharp drop in temperature. The purified produced gas was collected in a gas sac or an aluminum foil gas bag. After the produced gas was cooled to room temperature, its volume was measured using a wet flowmeter and subsequently converted to a standard state. The main components in the syngas produced in UCG are H2, CO, CO2, and CH4. N2 may also be included in syngas if air is used as a gasification agent. Additionally, syngas

also includes low concentrations of H2S (normally 100 °C), which influences the production of coal pyrolysis gas. Considering the above factors, a rheometer with a solid confining pressure medium is not suitable for this experiment, whereas a liquid medium high-temperature and high-pressure rock triaxial experimental system can only reach limited temperatures (typically less than 300 °C).28 Because accurate measurements of the stress−strain properties of most materials require that gas be the confining pressure loading medium, Professor Mervyn Paterson of Australia National University designed the first-generation Paterson high-pressure and high-temperature (HPT) rheometer in 1963.28 After nearly half a century of development, a number of modifications to the prototype apparatus were made to increase its maximum operation temperature and achieve a better temperature distribution. In the early 1980s, this apparatus was routinely used to characterize the plastic flow of minerals and their aggregates (the ability to independently control the pore pressure and the capability for torsion experiments were added later).29,30 The equipment used in this study is shown in Figure 2. The cooling, purification, drying, collection and analysis devices used for pyrolysis and the collection and analysis devices used for tar are the same as those used in the surface stress-free coal pyrolysis experiments described in section 2.1. When conducting coal pyrolysis

Figure 3. A diagram of the experimental sample setup.

shown in Figure 3, two sets of 3 mm Al2O3 ceramic gaskets (7), Al2O3 ceramic pistons (3), and ZrO2 ceramic pistons (2) were installed symmetrically on both ends of the sample before the experiment to reduce the amount of heat lost through conduction and guarantee a uniform temperature distribution along the sample’s long axis. The Al2O3 gaskets, the Al2O3 pistons, the ZrO2 pistons, and the coal samples were all perforated to ensure that the volatiles can flow through these D

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Energy & Fuels parts to reach the following devices. Finally, the package was wrapped in a 0.25 mm aluminised iron cylinder (to prevent corrosion by the H2S in the produced gas) to isolate the sample from the confining pressure medium (argon). After the sample had been installed and sealed, the high-temperature furnace was closed and buckled up. Then, an R type (Pt 13% Rh−Pt) thermocouple was placed 3 mm above it to measure and control the temperature in the experiment. Next, the iced brine condenser, filters, flowmeter, chromatograph, and gas bag were connected and installed. Before each experiment, the system was purged with argon, and airtightness was verified. Once the system was verified to be airtight, the contact point between the axial driver and the piston was adjusted. Then, an ice water mixture was put into the iced brine condensers. Next, a load was applied according to the predefined heating rate and stress state. The weight loss of the coal sample was determined after tests were finished at each final pyrolysis temperature. During these tests, argon (99.9999%) was used as the carrier gas. The flow rate of argon was set to 300 mL/min, and the average retention time of pyrolysis gas was 27.7 s, which is greater than in stress-free coal pyrolysis experiments. Since the ice in the iced brine condensers melted easily, ice was added from time to time. 2.2.2.1. Final Pyrolysis Temperature Determination. Compared to gasification processes that use air as a gasification agent, the maximum final temperature during coal seam pyrolysis during UCG can be achieved using the pure oxygen-steam method. However, an oxygen concentration that is too high reduces the economy of UCG. In the results reported by Liu et al., who conducted experiments using Huating coal, desirable gasification parameters were obtained when the ratio of steam to oxygen was between 1.5 and 2.31 However, the smaller the steam to oxygen ratio, the higher the underground gasifier temperature and coal seam pyrolysis temperature. On the basis of a numerical simulation of the pure oxygen-steam gasification method using Huating coal, Duan et al. discovered that the final pyrolysis temperature of the coal seam reached a maximum value of 977 °C when the ratio of steam to oxygen was minimal (i.e., 1.5).32 After the temperature reached 900 °C, the main pyrolysis products only changed slightly. Therefore, the highest final pyrolysis temperature in the experiment was set to 1000 °C. The secondary reactions of the initial product of coal pyrolysis intensified after reaching 600 °C. In a surface pressurized coal pyrolysis experiment, Sun et al. discovered that the pressure only had an effect above a critical temperature (560 °C for Hanqiao bituminous coal and 680 °C for Yanquan anthracite).33 Therefore, the minimum final pyrolysis temperature was set to 600 °C in this experiment. Under normal conditions, the production of coal pyrolysis gas was scarce at temperatures above 850 °C. Beyond 900 °C, the main pyrolysis products only exhibited minor changes; therefore, it is not necessary to use 950 °C as the final pyrolysis temperature. From 600 to 1000 °C, the final pyrolysis temperature is set every 50 °C (except at 950 °C). After each experiment reaches the final temperature, that temperature is maintained for 30 min. 2.2.2.2. Heating Rate Determination. Figure 4 shows the structure of a typical UCG gasifier. In UCG, the underground coal seams that undergo gasification are initially ignited. The gasification agent is then blasted into the inlet hole (1) to burn and gasify the coal seams. Finally, the gas is discharged through the outlet hole (8). On the basis of differences in the major

Figure 4. Diagram of the three UCG zones.

chemical reactions, temperature, and gas composition, the gasification area can be roughly divided into three zones: the oxidation zone (4); the reduction zone (6); and the pyrolysis and drying zone (5).34 When oxygen has gradually been depleted in the oxidation zone, the oxidation zone (4) ends, and the gasification area advances into the reduction zone (6). When the oxygen concentration reaches 0, the coal is ready for pyrolysis, as shown in Figure 4. Subsequently, the heated coal around the gasification channel toward the outlet and in the larger part of the coal seam (the area with 45° diagonal lines) experiences different degrees of pyrolysis. This part of the coal seam is commonly called the pyrolysis and drying zone (5). In the pyrolysis and drying zone (5), the coal near the outlet has a relatively low temperature and the coal near the oxidation zone has a relatively high temperature. Therefore, the temperature in the junction between the oxidation zone and the reduction zone is the maximum coal pyrolysis temperature in the pyrolysis and drying zone, i.e., the final pyrolysis temperature. As the gasification working face moves, the oxidation, reduction, and pyrolysis and drying zones gradually shift toward the gasifier outlet; thus, the coal in the pyrolysis and drying zone gradually increases in temperature. Therefore, before gasification, the coal experiences a pyrolysis process involving a gradual increase from a low temperature to the final pyrolysis temperature. The heating rate of the coal seam pyrolysis occurring during UCG can be calculated using eqs 1 and 2.

HR = TG × V

(1)

where HR represents the underground gasification pyrolysis heating rate in K/min, TG represents the temperature gradient between the two points in the pyrolysis and drying zone in °C/ cm, and V represents the gasification working face moving speed in cm/min. In addition, TG = TD/L

(2)

where TD represents the temperature difference between two points in the pyrolysis and drying zone in °C, L represents the distance between two points in the pyrolysis and drying zone in E

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Energy & Fuels cm, and V is related to the size and shape of the combustion section, the flow rate of the gasification agent, the stress state and gasification reactivity of the coal seam, and the type of gasification agent.35 The average moving speeds of the underground gasification system when using the air-steam methods employed in the Mazhuang coal mine (Xuzhou) and the high gas mine in Zhongliangshan (Chonging) were 2.1 cm/ h and 1.3 cm/h, respectively. The speeds of the gasification working faces of two alternative gasifiers in pilot UCG projects that used the air-steam method in Liuzhuang Colliery (Tangshan) were 0.85 cm/h and 2.03 cm/h.36 When the airsteam method was used, the gasification working face moved at a relatively slow speed; therefore, this study took more time. When calculating the heating rate in this study, V was set to 3.76 cm/h, which was used in the UCG industrial trial in Huating that employed the pure oxygen-steam continuous method. The TG data were based on data from a study conducted by Yang.37 These data showed that the temperature gradient was greater closer to the flame and vice versa. Therefore, the heating rate of coal during UCG is not stable and gradually increases with temperature, in contrast with the constant heating rate used in surface pyrolysis experiments. When the pyrolysis temperature is less than 300 °C, coal seam pyrolysis is not sufficiently active to begin the decomposition process. To reduce the time required by these experiments, the heating rate from 18 to 300 °C was accelerated 10-fold to 1.18 K/min, and the heating rates in the other temperature ranges were accelerated 2-fold. The calculated heating rates are shown in Table 2. The average heating rates of these experiments are as low as 0.44 K/min.

stress. After these experiments were completed, we assumed that the in situ coal seam burial depth had changed and conducted additional triaxial high-temperature and high-stress experiments for the same coals to investigate the influences of stress on coal pyrolysis. The coal seam burial depth that is favorable for UCG is normally greater than 100 m. To examine variations in experimental data with burial depth, the burial depths of both types of coal were set at three evenly spaced distances. Thus, Huating coal had burial depths of 136, 200, and 264 m, while Xinglongzhuang coal had burial depths of 400, 500, and 600 m. The stresses on the two types of coal throughout the experiment are presented in Table 3 and were tested using combinations of eight final temperatures and three stresses.

3. RESULTS This manuscript mainly studied the effects of the dependence of coal seam pyrolysis on underground stress in UCG. Therefore, the effects of the dependence on pressure are discussed in detail. There are many papers to introduce regarding the coal pyrolysis in dependence on temperature.38,39 Therefore, we did not focus on this issue in this manuscript. 3.1. Effects of Stress on Coal Pyrolysis Yields. The pyrolysis gas yields per tonne of coal (Nm3/t) of the two types of coal under each stress state are shown in Figure 5. The diagram shows that the produced gas yield curves under different stresses were similar to those obtained in the stressfree experiment. The gas yield curves of the two types of coal with or without stress clearly exhibit segmented characteristics. The lines gradually climb from 600 °C, increase very rapidly between 700 and 800 °C, somewhat decrease between 800 and 850 °C, and flatten above 850 °C. This pattern occurs because the primary temperature range of CO production is 700−800 °C and that of H2 is 700−850 °C. Figure 5 also shows that the pyrolysis gas yields of the two types of coal demonstrate a consistent pattern; i.e., the gas yields gradually increased at each temperature with increasing stress. Moreover, the pyrolysis gas yield increments did not form a linear relationship with the stress increment. The distances between the curves show that the increment of yield of pyrolysis gas that resulted from the same incremental increase in stress gradually diminished as the stress increased. Overall, the pyrolysis gas yield of Huating coal was slightly greater than that of Xinglongzhuang coal. Furthermore, in the case of Xinglongzhuang coal, the increment in the produced gas yield induced by the same incremental stress increase reached a peak at 800 °C; the incremental yields of produced gas under the stress states 1, 2, and 3 were 26.5, 36.00, and 41.55 N m3/t, respectively. The peak for Huating coal was reached at 750 °C, and its incremental produced gas yields under the stress states 4, 5, and 6 were 26.63, 33.96, and 40.96 N m3/t, respectively. The tar yields (wt %) versus temperature under each stress state are shown in Figure 6. The graphs show the similarities between the tar yield curves under different stresses and the tar yield curve under a stress-free state. At 600 °C and higher, the tar yield began to rapidly decrease. After 750 °C, the rate of decline decreased further, and after exceeding 900 °C, the condensing device collected virtually no tar. Figure 6 also shows that the tar yields of the two types of coal followed a consistent pattern: with increasing stress, the tar yield gradually decreased at each temperature. Moreover, the tar yield did not decrease linearly with increasing stress. The distances between the curves show that the unit increase in stress gradually diminished the

Table 2. Experimental Heating Rates temperature range (°C)

TD (°C)

TG (K/cm)

calculated heating rate (K/min)

heating rate (K/min)

heating time (min)

18−300 300−400 400−600 600−750 750−850 850−900 900−1000

282 100 200 150 100 50 100

1.88 1.93 2.23 2.39 2.73 6.23 12.53

0.118 0.121 0.140 0.150 0.171 0.391 0.785

1.18 0.24 0.28 0.30 0.34 0.78 1.57

239.36 413.41 714.29 500.00 291.90 64.00 63.69

2.2.2.3. Stress Setting. In an in situ coal seam, Xinglongzhuang coal bears a vertical stress of 12.50 MPa and a horizontal stress of 13.78 MPa (stress state 2, Table 3), while Huating coal bears a vertical stress of 3.40 MPa and a horizontal stress of 5.52 MPa (stress state 4, Table 3). Triaxial high-temperature and high-stress experiments were initially performed for the two types of coal under actual underground Table 3. Stress Settings in the Experiment burial depth (m)

stress state no.

axial stress (MPa)

confining stress (MPa)

Xinglongzhuang coal

400

stress state 1

10.00

11.50

Huating coal

500 600 136 200 264

stress stress stress stress stress

12.50 15.00 3.40 5.00 6.60

13.78 16.05 5.52 6.95 8.41

coal types

state state state state state

2 3 4 5 6

F

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Figure 5. (a, b) Effects of stress on the pyrolysis gas yield per tonne of coal.

Figure 6. (a, b) Effects of stress on tar yield.

Figure 7. (a, b) Effects of stress on semicoke (coke) yield.

G

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Figure 8. (a, b) Effects of stress on the H2 content of pyrolysis gas.

Figure 9. (a, b) Effect of stress on the CO content of pyrolysis gas.

tar yield reduction. The stress increase decreased the tar yield. Xinglongzhuang coal reached its maximum tar yield at 750 °C under stress states 1 and 2 and at 700 °C under stress state 3, with values of 2.19, 3.63, and 4.36 wt %, respectively. Huating coal reached its maximum decrement at 750 °C under stress states 4 and 5 and at 700 °C under stress state 6, with corresponding values of 1.51, 3.01, and 4.42 wt %, respectively. However, after the final pyrolysis temperature exceeded 600 °C, the absolute tar yield gradually decreased as the final pyrolysis temperature increased. When the variation in the tar yield induced by an increase in the stress was measured, the relative variations in the tar yield should be considered. As shown in eq 3, the rate of decrease of the tar yield at a temperature T is defined as the ratio of the decrease of the tar yield in stress state i to the tar yield in a stress-free state when the final pyrolysis temperature T is multiplied by 100%.

RR i = (Tar − Tar)/Tar × 100% i

(3)

where RRi represents the relative variation in tar yield in stress state i (%), Tari represents the tar yield in stress state i (wt %), Tar represents the tar yield in a stress-free state (wt %). For both types of coal, RRi peaked at 750 °C. At 750 °C, RR1 = 31.20%, RR 2 = 51.76%, and RR 3 = 61.68% for Xinglongzhuang coal and RR4 = 21.98%, RR5 = 43.81%, and RR6 = 59.53% for Huating coal. Figure 7 shows the variations of the semicoke (coke) yield with temperature for each stress state. The diagram shows that the semicoke (coke) yield at each temperature gradually increased as the stress increased for both types of coal. In addition, the semicoke (coke) yield did not increase linearly in response to increases in stress. The distances between the curves show that the increase in the semicoke (coke) yield due H

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Figure 10. (a, b) Effects of stress on the CO2 content of pyrolysis gas.

Figure 11. (a, b) Effect of stress on the CH4 content of pyrolysis gas.

pyrolysis gas yield always increased, the semicoke (coke) yield slightly increased, and the tar yield decreased, regardless of the type of coal, when the pressure increased.40 3.2. Effects of Stress on the Major Components of Gas from Coal Pyrolysis. Figure 8 shows a comparison of the H2 content of pyrolysis gas versus temperature for various stresses and for a stress-free state. This diagram clearly shows that the curves representing high-temperature and high-stress conditions are similar to the curve for the stress-free state; i.e., the H2 content increased with temperature for both Xinglongzhuang coal and Huating coal, and greater stress resulted in lower H2 contents for both types of coal. However, the decrease in the H2 content differed with temperature. At temperatures below 650 °C, the H2 content was not sensitive to variations in the stress, while at temperatures of 650 °C and higher, the decreases in the H 2 content became more apparent. Furthermore, the variations in the H2 contents of the two

to a unit increase in the stress gradually decreased as the stress increased. During the experiments, carbon deposits were found in the quartz tube furnace and in the Paterson HPT system when the final pyrolysis temperature exceeded 800 °C. Increments in the semicoke (coke) yield that were induced by the increases in stress peaked at 750 °C for both types of coal. Compared to the stress-free state, the semicoke (coke) yield of the Xinglongzhuang coal in stress states 1, 2, and 3 increased by 0.63, 1.00, and 1.26%, respectively, compared to the stress-free state, and those for the Huating coal in stress states 4, 5, and 6 increased by 0.59, 0.95, and 1.19%, respectively. The experimental results for the yields of the above pyrolysis products under stress were similar to those of a surface pressurized coal pyrolysis experiment under a nitrogen atmosphere. Liu and Feng extensively studied the pressurized pyrolysis of different types of coal and discovered that the I

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Energy & Fuels

the tar precursor had undergone thermal decomposition during pyrolysis at the elevated pressure.41 This idea was supported by the finding that the rate of variations in the pyrolysis product yield versus the yield in a stress-free state nearly always peaked at 750 °C during our experiments and can be explained as follows: some tar decomposed into gas, which leads to an increase in the gas yield, and other parts of the tar are converted to semicoke (coke), which leads to an increase in the semicoke (coke) yield. Furthermore, stress-induced tar decomposition reaches its highest speed at approximately 750 °C. Therefore, variations in the yields of the three types of products are maximized at nearly the same temperature. Furthermore, stress closes the fracture inside the coal and reduces the possibility of pyrolysis gas escape during the pyrolysis process. Therefore, the time it takes for the initial pyrolysis products to escape is prolonged, and the macromolecular hydrocarbons are split into small molecular hydrocarbons as they escape, leading to further increases in the gas yield. After conducting pressurized pyrolysis experiments on Shenfu coal, Wang et al. concluded that pressurization prolonged the residence time of macromolecular volatile matter inside the coal and that high temperatures (>700 °C) boosted the decomposition of macromolecular volatile matter.42 The slight increase in semicoke (coke) yields with increasing stress was also observed during surface pressurized pyrolysis and can be explained via the following mechanisms. The increase of stress intensifies the polycondensation reaction that occurs during the pyrolysis process. In addition to the concurrent carbon deposition reaction, this leads to an increase in the semicoke (coke) yield with an increase in stress. Therefore, the mechanisms behind the variations in coal pyrolysis products under high-temperature and high-stress conditions are similar to those of surface pressurized pyrolysis. The curves representing variations in the H2, CO, CH4, and CO2 contents with stress were similar to those obtained from pressurized pyrolysis experiments by Shuai, Peng, and Zou.43 Tao et al. also observed a similar phenomenon. The influences of pressure on the yields of gaseous hydrocarbon products derived from Fushun and Xianfeng coal pyrolysis in an anhydrous and confined system were investigated by Tao et al. The hydrogen gas yields from Fushun and Xianfeng lignite decreased by 76.5 and 75.9%, respectively, when the pressure increased from 50 to 250 MPa.44 This phenomenon can be explained as follows. First, coal produces macromolecular gaseous products during pyrolysis. The further decomposition of these gases at high temperatures produces methane. Next, the escape of volatile matter from the coal is prolonged by stress, which results in the following secondary reactions that occur under self-generated pore pressure:

types of coal only became relatively identifiable under relatively high-stress conditions (stress states 3 and 6). Figure 9 shows a comparison of the curves representing the CO contents of pyrolysis gas versus temperature for various stresses and for a stress-free state. The diagram shows that the curves representing high-temperature and high-stress conditions are similar to the curves representing a stress-free state; greater stress resulted in lower CO concentrations in the pyrolysis gas for both types of coal. However, the decrease in the CO content was different at each temperature. At temperatures below 700 °C, the CO content was not very sensitive to stress variations. However, at 700 °C and higher, the increase in the CO content became conspicuous. Furthermore, under relatively low-stress conditions (in stress states 1, 2, and 4), the variations in the CO contents of the two types of coal were undetectable. However, with further increases in the stress, the variations became relatively significant. In comparison to Xinglongzhuang coal, Huating coal displayed a more significant decrease in CO content for a given increment in stress, indicating its higher sensitivity. Figure 10 shows a comparison of the variations of the CO2 contents of pyrolysis gas with temperature for various stresses and for a stress-free state. The diagram shows that the curve representing high-temperature and high-stress conditions is similar to the curve representing a stress-free state. A higher level of stress resulted in pyrolysis gas with a higher CO2 content for both types of coal; however, there was no clear relationship between the decreases in the CO2 content and the temperature. For Xinglongzhuang coal and Huating coal at temperatures below 750 and 650 °C, respectively, the CO2 content was not sensitive to variations in stress. Moreover, the CO2 content varied significantly only when the two types of coal were under greater stresses (stress states 3 and 6). In comparison with Xinglongzhuang coal, the Huating coal displayed a more significant incremental change in CO2 content for a given incremental change in stress, indicating its higher sensitivity. Figure 11 shows a comparison of the variations of the CH4 contents of the pyrolysis gas with temperature under various states of stress and in the stress-free state. The diagram shows that the curve representing high-temperature and high-stress conditions was essentially the same as the curve representing a stress-free state; a higher level of stress resulted in pyrolysis gas with a higher CH4 content for both types of coal. For the Xinglongzhuang and Huating coal, the CH4 content was not sensitive to variations in the stress at temperatures below 750 and 800 °C, respectively.

4. DISCUSSION Coal pyrolysis proceeds under a slow heating rate and under stress in the UCG process, which leads to the slow escape of initial pyrolysis products during the process and subsequent secondary reactions involving the initial pyrolysis products. The most conspicuous secondary reaction is that of tar at high temperatures. Under stress, the boiling points of some components of tar increase, which extends the amount of time it takes for these components to escape from the coal grain, intensifying the secondary reactions. This process is similar to the secondary reaction of tar in surface pressurized pyrolysis. An experiment conducted by Chen et al. showed a similar phenomenon: the pyrolysis of low rank coal resulted in the production of more pyrolysis gas, particularly CO2, at 50 bar than at atmospheric pressure. These authors suggested that

CO + 3H 2 ⇄ CH4 + H 2O

(4)

C + 2H 2 ⇄ CH4

(5)

2CO + 2H 2 ⇄ CH4 + CO2

(6)

Reactions 4−6 all produce methane; therefore, the CH4 content increases significantly, and its decomposition temperature is relatively high. Consequently, the CH4 content always increased as the stress increased in this experiment. These secondary reactions consume CO and H2 and produce CO2; thus, the CO and H2 contents decrease as the stress increases, and the CO2 content increases with increasing stress. As demonstrated in Figures 8−11, stresses have demonstrable effects on the changes in the gas content only after the J

DOI: 10.1021/acs.energyfuels.6b02168 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

(PAPD), the Fundamental Research Funds for the Central Universities (China University of Mining and Technology) under Grant No. 2014YC03. Professor Bingxiang Huang’s contributions in revising this paper and funding support were quite acknowledged.

temperature exceeds a certain value. This phenomenon also appears in the surface pressurized pyrolysis experiments. In a surface pressurized coal pyrolysis experiment, Sun and Xiong et al. discovered that the critical temperatures for Hanqiao bituminous coal and Yanquan anthracite are 560 and 680 °C, respectively.33 This shows that the secondary reactions of volatile matter are inert below a certain temperature, and only above a certain temperature do their activities begin to show. The curves representing the gas content as a function of temperature are essentially consistent with the curves for a stress-free state, which shows that the effects of temperature on the yields of the pyrolysis products and each component are more significant than the effects of stress. This finding indicates that temperature remains the dominant factor in coal pyrolysis and that the effects of stress-induced secondary reactions are relatively small.

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5. CONCLUSIONS To study the rules of in situ coal pyrolysis under hightemperature and high-stress conditions, this study explored related experimental methods and fulfilled related model experiments. Which experimental table should be chosen was studied, and the parameter settings, such as the final pyrolysis temperature, heating rate, and stresses, were investigated. Pyrolysis experiments of Xinglongzhuang gasification coal and Huating long-flame coal were conducted using an improved Paterson HPT system under temperatures of up to 1000 °C and under different stress conditions. The results were compared to the results obtained in stress-free coal pyrolysis experiments using a GR.TF80/11 tube furnace. The average heating rates of these experiments were as low as 0.44 K/min. The produced gas and semicoke (coke) yields both increased with increasing stress, while the tar yield decreased. However, the variations in the pyrolysis product yields induced by a unit increment of the stress gradually decreased as the stress increased and nearly reached their peak values at approximately 750 °C. The concentrations of CH4 and CO2 in the pyrolysis gas gradually increased with increasing stress, while the H2 and CO concentrations gradually decreased with increasing stress. The variations of H2, CO, CH4, and CO2 were significantly greater for the Huating coal than for the Xinglongzhuang coal under the same increases in stress, indicating that the Huating coal is more sensitive. The variation curves for each gas component under each stress state were similar to the corresponding curve obtained from the stress-free state, which indicates that temperature remains the dominant factor in coal pyrolysis and that the effects of stress-induced secondary reactions are relatively small.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel.:+86 13512560307; fax: +86 516 83590567; e-mail: [email protected]. ORCID

Zuo-Tang Wang: 0000-0001-6505-7604 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the collaborative funding support from the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions K

DOI: 10.1021/acs.energyfuels.6b02168 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.6b02168 Energy Fuels XXXX, XXX, XXX−XXX