A Case Study of Ulanqab Lignite for Underground Coal Gasification

Oct 29, 2014 - Ulanqab lignite from Inner Mongolia, northern China, with the focus being on the variation of coal permeability under dehydrating and h...
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Variation of Coal Permeability under Dehydrating and Heating: A Case Study of Ulanqab Lignite for Underground Coal Gasification Shuqin Liu,*,† Shangjun Zhang,† Feng Chen,‡ Caihong Wang,† and Mingyue Liu† †

School of Chemical and Environmental Engineering, China University of Mining and Technology (Beijing), Beijing 100083, People’s Republic of China ‡ State Key Laboratory of Coal-Based Low Carbon Energy, ENN Group Company, Limited, Langfang, Hebei 065001, People’s Republic of China ABSTRACT: Underground coal gasification (UCG) is a promising option for the recovery of low-rank and inaccessible coal resources, during which coal permeability plays an important role in various aspects, including the method of channel construction, fire face extension, and gas yield. In this paper, experimental studies were conducted examining the permeability of Ulanqab lignite from Inner Mongolia, northern China, with the focus being on the variation of coal permeability under dehydrating and heating below 400 °C. In addition, the specific surface area and the pore volume of the heated samples were analyzed. The results indicate that the permeability of the lignite in the direction parallel to the bedding is 3 times greater than that observed perpendicular to the bedding, while the permeability values of three high-rank coals considered in this study exhibit the opposite trend. With a decrease in the water content of the lignite sample, the permeability exhibits an increasing “S-curved” trend. The permeability of the coal sample in the as-received state (ARS) is determined to be nearly 100 times higher than that in the water-saturated state (WSS). When the ARS sample is heated to over 300 °C, the permeability sharply increases and attains 2000 mD at 400 °C, while the lowest value remains between 200 and 300 °C. The temperature affects the permeability of lignite by changing the specific surface area and the pore volume. Draining, preheating, and high-pressure air fracturing are proposed as efficient methods for improving the permeability of the lignite regarding the UCG process.

1. INTRODUCTION

Combustion linkage based on vertical boreholes or directional boreholes is primarily used in the present UCG trials. The gasifying coal has certain natural permeability, and it can be regarded as a natural layer that is separated by pores and fractures. Combustion linkage aims to strengthen the separating degree of the coal by heat and pressure and finally build a channel in the coal seam between two boreholes. The sketch of the reverse combustion linkage is shown in Figure 1. After the coal at the bottom of one borehole is ignited, the gasifying agent, including oxygen or air, is injected through another borehole. Next, flames form in the coal seam and move toward the injection borehole. With the consumption of the coal by combustion, the channel between two boreholes begins to

Underground coal gasification (UCG) involves the conversion of coal in the seam into combustible fuel gas. UCG is potentially a clean method of coal conversion and has been adopted to recover the coal resources that are not minable or are uneconomically mined.1−5 In recent years, many theoretical studies regarding the UCG process have been conducted. Meanwhile, field tests of the UCG have been widely performed in China, Australia, and South Africa at commercial scale, which has received more attention worldwide.6 The unit reactor of the UCG process consists of the injection and production boreholes drilled from the surface into the coal seam. The important step in the construction of the underground reactor is establishing a channel between the injection and production boreholes, which is called “channel linkage”. The objective of creating the gasification channel is to ensure continuous air injection into the coal seam and the discharge of combustible gas from the reactor, which provides the high-temperature condition for the gasification process in the coal seam. Dependent upon the deposition conditions of the coal seam and the physical and chemical properties of the coal,7 different methods could be used for linking the boreholes, such as combustion linkage, electrolinkage, hydrocracking, directional drilling, explosive linkage, and chemical liquid cracking. During the UCG process, the cost of linkage including drilling always accounts for 20−30% of the total cost. Therefore, it is necessary to develop technically and economically reasonable linking methods according to the coal seam properties. © XXXX American Chemical Society

Figure 1. Sketch of the reverse combustion linkage in the UCG process. Received: July 28, 2014 Revised: October 28, 2014

A

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However, the other properties of lignite, such as high permeability and abundant pore structure, make it more suitable for UCG. The reported studies on the permeability of coal are relatively extensive, with most of them targeting coal bed methane mining and CO2 sequestration,12−24 while a few papers involve UCG.25,26 In this paper, taking the lignite from Ulanqab, Inner Mongolia, as the test sample, the permeability and pore structure of the lignite were studied under heating conditions up to 400 °C. In comparison to three typical Chinese coals (anthracite from Xiyang, bituminous coal from Fushun, and sub-bituminous coal from Datong), the effects of the metamorphic grade, the bending direction, and the water content were also investigated. The results produced are expected to provide the theoretical basis for channel linkage and process control during the UCG process.

form, making it easier for the diffusion of the gasifying agent to the coal surface to improve the intensity of the combustion and gasification. Studies on the reverse combustion linkage have been performed,8,9 and the application potential of the method has been discovered. The permeability of coal, i.e., the ability of fluid to pass through the porous medium of the coal body, is the physical and chemical basis for accomplishing the technological process of linkage. When the fluid in the pore is single liquid phase, the permeability measured is the absolute permeability, which is an intrinsic property of the coal and depends upon the pore structure of the coal as well as the stress applied. When more than one phase of fluids is flowing though the coal pore structure, relative permeability for each phase is required to be characterized. During the UCG process, when the injected high-pressure gas passes through the coal seam, the existence of water in the coal pore will compete with the gas, thereby reducing the ability of gas to move. Therefore, the permeability in this paper for the wet samples refers to the gas effective permeability, which changes with the water saturation of the coal. During the UCG process, diffusion of oxygen to the surface of coal and heterogeneous reaction between the solid coal and oxygen are the most crucial steps, which largely depend upon the ability of gas to pass through the coal seam, i.e., the permeability of coal. The permeability of coal is influenced by many natural factors, such as coal maceral, fracture, coal porosity, and stress by surrounding rocks.10 Among these factors, the development of fractures and pores inside the coal plays a leading role, especially under dehydrating and heating. The study on the permeability of coal not only plays an important role in the construction of the gasifier but also has a guiding meaning for the operation of the UCG process. It has been discovered from the tests in the former Soviet Union that the permeability of the Moscow suburb lignite varies dramatically with the water content. The permeability sharply increases by several hundreds or thousands of times as the water content in the coal decreases from the maximum to 20%. When the water content declines from 20 to 8%, the permeability increases approximately 9 times. As the value reduces from 8 to 2%, the growth of the permeability is more than that in the second stage and less than that in the first stage. In another research study on the permeability of Lisichansk bituminous coal, the permeability growth during the drying stage was found to be more obvious than that of the Moscow suburb lignite.11 The results of studies on the fractures and permeability of the coal seam could provide the basis for the decision of an efficient linking scheme as well as for further improvement of the UCG process practically. For example, boreholes can be arranged according to the trend of the major fracture, the coal seam can be preheated to a proper temperature to make it more permeable, and the injection pressure of a gasifying agent during the linking process can be rationally determined by the regularity that the permeability of the coal seam declines with the increment of the burial depth. Currently, there are five UCG pilot projects around the world, located in China, Australia, Uzbekistan, and South Africa, most of which are focused on the lignite. In China, there are more than 100 billion tons of lignite resources in Inner Mongolia, which are not suitable for mining because of the technical difficulty and the high cost of mining as a result of the lignite properties of high moisture, high ash, and soft rock roof.

2. GEOLOGICAL SETTING OF THE COALFIELD The strata in the study area are Lower Jining Group in Mesoarchean (Ar2J1), Paleogene Oligocene Huerjing Formation (E3h), Neogene Miocene Hannuoba Formation (N1h), Neogene Pliocene Baogedawula Formation (N2b), and Quaternary Holocene. The coal-bearing strata are a set of sedimentary sequences of terrestrial clastic rocks formed in lake and swamp facies and overlain by the Quaternary strata. In addition, the Jining Group is the basement of the coal-bearing strata, and the major lithology is granite at depths of 202.95− 565.25 m. The main coal seam in this area occurs in the Lower Huerjing Formation (E3h1). It is 7.05 m thick on average and minable in most coalfields with 0−12 partings of an accumulative thickness of 1.79 m. The dip angle of the coal seam is less than 5°. The roof of the coal seam is siltstone and dark gray carbonaceous mudstone with clastic organic debris, while the bottom of the coal seam is a thin layer of mudstone close to the lowest basement of granite and gneiss. The coal is identified as lignite, with an ignition point of 268 °C and heat value ranging from 13.37 to 16.72 MJ/kg.27 3. EXPERIMENTAL SECTION 3.1. Coal Sample Preparation. Test coal samples were prepared from the coal core samples obtained during the drilling process in the UCG field site. To prevent weathering of lignite, original coal cores were wrapped carefully with polyethylene film, stored in poly(vinyl chloride) (PVC)-sealed containers, and then transported to the laboratory. After being fixed by the gripper, a coring machine and diamond cutter were used to cut them into cylindrical shapes with a diameter of 50 mm and a height of 101 mm in directions both parallel and perpendicular to the bedding. Next, the end faces were ground and polished with a stone mill and made to be parallel to each other until the standard size (Φ 50 × 100 mm) is reached, as shown in Figure 2. Reasonable accuracy in the parallelism and smoothness of the specimen were in accordance with the International Society of Rock Mechanics (ISRM) standards. The prepared specimens were wrapped

Figure 2. Lignite specimen for the permeability measurement. B

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Table 1. Proximate and Ultimate Analyses of Ulanqab Lignite (on an Air-Dried Basis) proximate analysis (wt %)

ultimate analysis (wt %)

M

A

V

FC

C

H

O

N

S

15.14

23.96

27.22

33.68

44.66

3.03

10.46

0.59

2.16

in several layers of plastic and then stored in a sealed bag until used in the permeability test. Lignite, which is abundant in pores and natural fractures, is brittle and low in strength. As a result, the samples were quite easy to crush during the process of drilling, cutting, and grinding. In addition, the sample integrity was also influenced by the mechanical vibration. Therefore, the success rate of sample preparation was lower than 30%. For the study on the effect of water content on permeability, samples in the water-saturated state (WSS) and samples with different water contents were prepared. The original specimens were immersed in water for 48 h to be saturated with water. Next, to avoid coal cracking in the existence of air, the WSS samples were placed in the vacuum drying chamber (40 °C) in parallel for different drying periods. Finally, the samples with different water contents were obtained and directly used for the experiments. The water content of each sample could be calculated by the mass difference before and after drying. The proximate and ultimate analyses of the lignite are listed in Table 1, which indicates that the test lignite is high in the volatile, ash, and moisture contents and medium in the sulfur content. 3.2. Permeability Measurement Apparatus. Experiments were performed using a high-temperature triaxial permeability testing system developed and designed by Shanxi Yilong Mining Equipment Technology Development Co., Ltd. (China), and the schematic diagram is shown in Figure 3. The main apparatus is 0.8 ton in weight

and the water content is calculated by mass balance. However, for the test lignite, because of its higher permeability, the continuous gas− water experiments are difficult to be conducted. For the UCG process, what we are concerned about are the initial injection pressure and the flow rate of gas, which partly depend upon the coal permeability at a certain water content, and therefore, the effective permeability of coal was determined using different wet samples. In the permeability test at room temperature, the prepared coal sample with a known water content was initially placed into the triaxial compression chamber and then the axial and lateral pressures were loaded to 7.0 and 4.2 MPa, respectively, according to the coal seam deposit condition (280 m in deep with a coefficient of lateral pressure of 0.6). Next, high-pressure nitrogen was gradually introduced from the bottom of the chamber until the selected pore pressure is reached. The discharge water method was used to measure the flow rate of discharged gas, and the injection and outlet pressure were recorded simultaneously. For the study of the effect of the temperature, coal samples were heated at a rate of 10 °C/min to the set temperature and held for 4 h. Gaseous products were released from coal in this stage, which could be detected by a gas chromatograph. When the flow rate of the outlet gas was steady, the permeability measurement was performed at different pore pressures. When the previous test was finished, the specimen was then heated to the next temperature and the procedure was repeated, as mentioned above. Because of the influence of sulfur corrosion on the sealing circle and the consideration for the low-temperature heat treatment related to UCG, the maximum testing temperature in this study was 400 °C and temperature interval was 50 °C. According to the gasification pressure adopted in the field test, the pore pressures selected were 1.0, 2.0, and 3.0 MPa, respectively. A total of 20 min was required for stabilizing the specimen between two pore pressures to reflect the effect of pore pressure accurately. It is assumed that the gas permeation through the sample is an isothermal process and that the ideal gas law is applicable. In this condition, the permeability obeys Darcy’s law

K= Figure 3. High-temperature triaxial permeability testing system: (1) pressure gauge, (2) cylinder, (3) support frame, (4) steel tube, (5) value, (6) heating wire, (7) muff, (8) specimen, (9) thermocouple, (10) sealing packing, (11) temperature and pressure controller, (12) oil pump, and (13) measuring equipment of gas flow.

2QP0Lμ (P12 − P2 2)A

(1)

where K, μ, L, Q, and A are the permeability, dynamic viscosity of nitrogen, length of the sample, gas flow rate, and cross-section area of the sample, respectively. P0, P1, and P2 are the atmospheric pressure, injection pressure, and outlet pressure, respectively. Because of the coal properties of heterogeneity and brittleness, the deviation for coal permeability measurement is relatively higher and the conventional testing method and standards for the oil−gas relative permeability determination are not suitable for the coal samples and must be improved.28−30 In each testing condition, parallel experiments were performed. If the data deviation between the two tests or the two samples was less than 10% and the same trend was acquired, then the average was used as the final value. Otherwise, additional sample tests would be performed until the deviation was lower than 10%. 3.4. Specific Surface Area and Pore Volume Measurements. Permeability measurement at a pore pressure of 3.0 MPa at different temperatures was repeated to prepare the heated coal samples for nitrogen adsorption experiments, which were performed on a BUILDER SSA-4300 physical adsorption analyzer (Beijing, China). Small pieces of post-experiment samples were selected to meet the size of the sample tube. High-purity nitrogen was used as the adsorbed gas. The specific surface area and pore volume were obtained from the nitrogen adsorption isotherm line according to the Brunauer− Emmett−Teller formula and the Barrett−Joyner−Halenda method, respectively.

and 1410 × 1410 × 2100 mm in size. The apparatus consists of a support framework, heating system, triaxial compression chamber, pressure controller, temperature controller, and measuring system. The lateral and axial stresses up to 31.5 MPa are applied by the hydraulic pump (10MCY14-1B) and 63 MPa by the hand pump. Salt is used as the pressure medium. In the pressure controller part, the load pressure is stabilized by two hydraulic accumulators: one for the axial load and the other for the lateral load. A hand pump is used for fine increases in pressure, and an atmospheric valve is used for pressure relief. The coal sample is heated by a peripheral electric furnace with two temperature-controlling systems. The maximum temperature can attain 650 °C with an accuracy of ±5 °C. The circulating water system is equipped for the cooling of the oil cylinder. The loading pressure, axial displacement of the sample, and temperature are recorded automatically. 3.3. Measurement Procedure. Generally, for the gas−water twophase penetration process in the coal and rock, changes of relative permeability against water saturation are always performed by the steady-state method or non-steady-state method at room temperature C

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4. RESULTS AND DISCUSSION 4.1. Effect of the Metamorphic Grade on the Permeability. The permeability and porosity of the test lignite and the three coals11 used for comparison are listed in Table 2. The permeability data were acquired at room

Table 3. Permeability of Lignite and the Compared Coals in Different Bedding Directionsa coal type WSS

Table 2. Permeability and Porosity of Ulanqab Lignite and Three Compared Coals coal type porosity (vol %) permeability (mD)

Ulanqab lignite

Datong sub-bituminous coal

Fushun bituminous coal

ARS

33

25

6

704.3

24.3

10.0

2.8

Datong sub-bituminous coal

Fushun bituminous coal

Xiyang anthracite

6.5 0.5 704.3 209.2

0.2 3.1 24.3 99.6

4.5 81.0 10.0 110.2

0.1 1.5 2.8 5.1

a ∥, parallel to the bedding; ⊥, perpendicular to the bedding; WSS, water-saturated state; and ARS, as-received state.

Xiyang anthracite

42

∥ ⊥ ∥ ⊥

Ulanqab lignite

bedding. On the contrary, for Datong sub-bituminous coal, Fushun bituminous coal, and Xiyang anthracite, the permeability perpendicular to the bedding is higher than that parallel to the bedding. For the WSS samples, the changes in permeability follow the same trend, while the difference between the two directions becomes greater compared to those for the ARS samples. The permeability of the WSS lignite in the direction parallel to the bedding is more than 10 times that perpendicular to the bedding direction. On the basis of the superior permeability of the lignite parallel to the bedding, it can be inferred that a high-pressure air-fracturing method and a high-pressure combustion linkage can be used to construct the gasification channel in the coal seam of lignite. During the process, between two vertical boreholes, high-pressure airflow passes through the seam parallel to the bedding and penetrates into the surrounding coal seam to burn; as a result, less airflow is lost to the surrounding strata, which, in turn, shortens the linkage time of the adjacent boreholes. However, the other three coals do not have this advantage, and high-pressure fracturing becomes uncertain. 4.3. Effect of the Water Content in Coal on Permeability. In addition to the metamorphic grade and the bedding direction, the water content of coal is also a major factor of influence on the permeability. As mentioned above, lignite generally contains abundant water,33 especially in Inner Mongolia, China. For the UCG process in the coal seam of lignite, because of the existence of massive water, the injected air or oxygen must overcome the water resistance when they penetrate into the coal fractures and pores and expand to the lateral sides of the gasification tunnel. Therefore, the initial injection pressure and volume partly depend upon the coal permeability at a certain water content state, which, in turn, will provide an important base for pretreatment of the coal seam before UCG. To simulate the original state of the lignite coal seam, the ARS coal samples were dipped in water to fill the fractures, capillary pores, and outer surfaces of the coal sample with water. It can be seen from Table 3, in comparison to the ARS samples, the permeability of the WSS samples remarkably decreases by several to hundreds of times. The lignite permeability of the ARS samples is more than 100 times that of the ARS samples parallel to the bedding and over 400 times that perpendicular to the bedding. With an increase of the metamorphic grade, the influence of the water content on the permeability decreases. For the compared anthracite, the permeability is reduced by 28 times in the direction parallel to the bedding when the state changes from ARS to WSS and by 3.4 times in the direction perpendicular to the bedding. To further investigate the influence of water in coal, the water in WSS samples was removed via vacuum drying, and then the changes of the effective permeability of nitrogen with residual

temperature and at a pore pressure of 3.0 MPa in the direction parallel to the beddings. It is clear that the lower the metamorphic grade of coal, the higher the permeability. The permeability of lignite is 704.3 mD, which is 251.5, 70.4, and 29.0 times that of anthracite, bituminous coal, and subbituminous coal, respectively. The changes in porosity exhibit the same trend. The porosity of lignite reaches 42%, approximately 7 times that of anthracite. The relationship between permeability and porosity clearly indicates that the ability of gas to penetrate through the coal depends upon the porosity, which is mainly related to the coal rank, pore volume, and pore interconnectivity. During coal metamorphosis, weak chemical bonds break and side chains become shorter. Meanwhile, the condensation degree of the aromatic rings in coal tends to be higher. Coal metamorphosis has a complex effect on the variation of the pore structure, but in general, the total pore volume decreases as the metamorphic grade increases. However, when the carbon content in coal exceeds 88%, the total pore volume begins to increase again. For the four coals, the same trend is found for changes in permeability and porosity. In addition to coal rank, permeability may vary with coal maceral. Many exogenous cracks were reported to have been discovered in coal with a higher percentage of vitrinite. The vitrinite exists mostly in the shapes of bands and lenses and is easily cracked into cubic blocks before exogenous cracks form.31 4.2. Effect of the Coal Bedding on Permeability. Coal fractures are formed under all types of natural geological stress. These fractures can be divided into endogenic and exogenous fractures based on their origins, and the former can be classified into face and butt cleats for fractures parallel and perpendicular to the bedding direction, respectively. During UCG and coal bed methane mining, coal fractures are the main penetration channels for water and gas transportation, which is the main internal factor affecting the permeability of coal seam. Studies have been conducted on the rock mechanics characteristics perpendicular and parallel to the bedding,32 while few studies have been focused on the variation of coal permeability with the direction of bedding. The permeability of the test lignite in different bedding directions is presented in Table 3. The values for the other three coals in the literature are also listed for comparison.31 To simulate the original state of underground lignite, water-saturated samples were prepared and the permeability values of the samples were measured for comparison to those in the as-received state (ARS). For the ARS samples, the permeability of the lignite parallel to the bedding is 3 times that of the lignite perpendicular to the D

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and the van Genuchten−Mualem (VGM) model are the two commonly used relative permeability models, which are obtained through integration of the Brooks−Corey model with the Burdine method and from the integration of the van Genuchten model with the Mualem method, respectively. The Brooks−Corey model has always been used for the consolidated porous media, while the van Genutchen model has been used for the unconsolidated porous media.34 There are a few improved relative permeability models for coal reservoirs that have been developed, but they are more suitable for coals with lower permeability. Thus, in this study, experimental data were fitted with the BCB model and the VGM model. Because the bound water effect at the end of the coal permeability measurement is serious, the absolute permeability was obtained using the trend extrapolation method. Taking the residual moisture content as water saturation, the effective water saturation (Sew) was calculated.35 The fitting results and the correlation coefficients (R2) are presented in Table 4. It is obvious that experimental data are well-fitted with the VGM model based on the values of R2. R2 values of fitting with the VGM model are 0.954 and 0.926 in the direction parallel to the bedding and perpendicular to the bedding, respectively. However, for BCB models, the values of R2 are both lower than 0.9. It is inferred from the results that draining, dewatering, and even air preheating are effective methods to increase the permeability and are beneficial to the preparation and processing of UCG in the lignite seam, such as ignition and channel connection. Water removal from high moisture coal in UCG can not only lead to an obvious increase of permeability but also reduce the water in the reaction area, which, in turn, will reduce the energy consumption for water evaporation during gasification. 4.4. Effect of the Temperature and Pore Pressure on Permeability. The molecular structure of coal appears to be a three-dimensional network of condensed aromatic and hydroaromatic units connected by weak bonds. The structural variations in coal during the thermal treatment process related to UCG, such as drying and pyrolysis, may lead to changes in its permeability.36 Therefore, the effect of the temperature (up to 400 °C) on the permeability of Ulanqab lignite was studied, and the results are shown in Figures 5 and 6. It can be observed that the variation of permeability with the temperature has the same profile parallel and perpendicular to bedding. There are two peak values of permeability, which occur at 100−150 and 400 °C, the maximum temperature. In the direction parallel to bedding, after reaching the first peak value, 2241.0 mD, at 100 °C and a pore pressure of 3.0 MPa, the permeability begins to decrease sharply to the minimum value, at which it remains from 200 to 300 °C. When the temperature rises above 300 °C, the permeability is found to increase linearly and rapidly to the maximum, 2496.0 mD, at a pore pressure of 3.0 MPa. In the direction perpendicular to the bedding, the first peak value of 570.2 mD occurs at 150 °C.

water were considered. The relationships between the water content and permeability for the lignite parallel and perpendicular to the bedding are shown in Figure 4. It is

Figure 4. Effect of the residual moisture on the permeability of the test lignite.

clear that, as the residual water decreases, the permeability exhibits an increasing trend of an “S curve”. As the water content declines from 34.9 to 8.5%, the permeability of coal parallel to the bedding increases rapidly from 812.5 mD to a maximum value of 2211.2 mD. For the sample perpendicular to the bedding, a significant increase in permeability occurs, from 282.4 to 588.5 mD, as the water content changes from 20.7 to 12.6%. In addition, the permeability of coal parallel to the bedding is higher than that perpendicular to the bedding at the same moisture content. Water inside the coal is divided into free phase and bound phase. The former can be further classified into external water and inherent water. External water refers to both the water adsorbed to the outer surface of coal and in the larger capillary pores (with a diameter larger than 100 nm), while inherent water exists in smaller pores. During the vacuum-drying process, the water loss from the outer surface of coal and the large capillary pores gradually occurs because the water absorption by larger pores significantly contributes to the water content of coal, while the inherent water and combined water are not released in this condition. On the basis of the above data, it is clear that external water has a significant effect on coal permeability, especially for lignite. With the removal of water upon drying, additional fractures and larger pores are exposed. These exposed fractures and pores reduce the masstransfer resistance and favor the passing of gas through the fissures, thereby improving permeability. The changes of permeability are closely related to larger pores in this stage. To describe the effect of the water content on coal permeability quantitatively, modeling of the relative permeability with water saturation was performed. The relative permeability, the ratio of effective permeability to absolute permeability, is often expressed as a function of the wetting phase saturation. The Brooks−Corey−Burdine (BCB) model

Table 4. Fitting Results of Experimental Data with the VGM and BCB Models formula η

VGM

krg = (1 − Sew) (1 − Sew

BCB

krg = (1 − Sew)2(1 − Sew1 + (2/λ))

1/m 2m

)

results (∥)

R2 (∥)

results (⊥)

R2 (⊥)

η = −0.302 m = 1.126 λ = 4.213

0.954

η = −1.791 m = 1.503 λ = −4.894

0.926

E

0.853

0.801

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coal inner surface. Water evaporation leads to the formation of micropores and the increment of the specific surface area and pore volume. During this stage, the adsorptive gases (such as N2, CH4, and CO2) inside the organic matrix of coal begin to flow. These gases expand under heating, which, in turn, enlarge the pore size and increase the pore volume of large pores. Meanwhile, the adsorptive gases in the closed pores break out under the action of thermal stress, resulting in the formation of massive micropores. Therefore, the first peak value of coal permeability is observed. However, the frame structure of coal is hardly affected during this stage. From 200 to 300 °C, lignite pyrolysis commences, during which the frame structure of coal begins to break with the formation of tar and emission of gases, including CH4, CO, and CO2. Part of the combined water in minerals begins to release. However, in this stage, the coal framework begins to soften and the fractures between frames are closed under the action of tristress, which represents surrounding stress in the coal seam deposition condition. The liquid products cannot be discharged at the temperature range and, instead, block the micropores and fractures in coal, hinder gas flow through coal, and result in the lowest permeability. When the temperature exceeds 300 °C, the decomposition and cracking of coal are accelerated and the emission of gas increases. The framework of coal tends to be broken up with the formation of a large number of fractures. In addition, during lignite pyrolysis, little colloid is formed; therefore, the phenomena of melting, swelling, and caking hardly occur. Therefore, a sharp rise of permeability occurs above 300 °C. This enhanced permeability favors diffusion and mass transfer of gasification (air or oxygen) toward the coal body and enhances the gasification reactions in the infiltration channels. Under natural conditions, coal permeability is closely related to cleat or large pores. On the basis of the above analysis, both larger pores and micropores are found to change upon heating. To further understand the effect of thermal treatment on lignite, the specific surface area and pore volume of postexperiment samples were measured and analyzed with the nitrogen adsorption method. As seen from Figure 7, the microscopic structure of the heated samples is affected significantly by the temperature. In

Figure 5. Variation of the permeability with the temperature in the direction parallel to the bedding.

Figure 6. Variation of the permeability with the temperature in the direction perpendicular to the bedding.

Above 300 °C, the permeability rises rapidly and attains a value of 1971.0 mD at 400 °C. With the change of the temperature, the effect of the pore pressure on permeability ranging from 1.0 to 3.0 MPa was also taken into consideration. As the pore pressure increases, water in coal is displaced gradually by the injected gas. The effective stress has also decreased, which may lead to an increase in permeability.30 For the samples perpendicular to the bedding, an improvement in pore pressure leads to an increase in permeability, especially over 300 °C. The maximum permeability is 1971.0 mD when the pore pressure reaches 3.0 MPa, which is nearly 1.6 and 2.0 times that at 2.0 and 1.0 MPa, respectively. For the samples parallel to the bedding, a slight increment of permeability with increasing pore pressure is observed below 300 °C. However, from 350 to 400 °C, the influence of the pore pressure becomes weak. In addition, there exists a temperature point where permeability is hardly affected by the pore pressure. This temperature, defined as the “threshold temperature”, is 300 and 350 °C for the samples perpendicular and parallel to the beddings, respectively. It is clear that permeability of coal is not monotonically increasing with the temperature because of the comprehensive effect of the chemical changes of coal upon heating and the action of stress. As the coal samples are heated from room temperature to 100 °C, the external water and the water in large capillary pores (>100 nm) and on the outer surface of coal is lost gradually. Above 100 °C, the inherent water evaporates gradually, including water in smaller capillary pores and on the

Figure 7. Variation of the permeability, specific surface area, and pore volume with temperature.

the temperature range of 100−400 °C, the changes in specific surface area and pore volume exhibit the same profile with the variation in permeability. A remarkable decrease in specific surface area and pore volume are observed from 100 to 200 °C. From 200 to 300 °C, the specific surface area and pore volume F

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bedding, respectively. When the ARS lignite sample is heated from room temperature to 400 °C, the permeability was observed to increase by up to 90 times, while retaining the lowest value between 200 and 300 °C. The variation of permeability with temperature results from the changes of the specific surface area and pore volume during the process of dehydrating and pyrolysis. The variation of permeability occurs with the increase in pore pressure over a wide temperature range. However, at the threshold temperature of approximately 300 °C, the permeability was observed to be hardly affected by the pore pressure. From these conclusions, it is suggested that draining, preheating, and high-pressure air fracturing are efficient methods for improving the permeability of the lignite coal seam, which favors both the channel linkage and gasification capacity in the UCG process.

are of the lowest value and change slightly. However, above 300 °C, the specific surface area and pore volume reach maximum values of 9.16 m2/g and 0.43 cm3/g, respectively, at 400 °C, i.e., the specific surface area and pore volume are increased by 2.1 and 29.7 times compared to those at 300 °C, respectively. On the basis of these results, it can be concluded that the permeability of the lignite is related to the changes in the specific surface area and the coal pore structure under thermal treatment. In addition to the cleat, the formation of micropores may also contribute to the variation of permeability. To further understand their relationship, correlation between permeability and pore volume, specific surface area, and pore diameter were analyzed by SPSS 17.0, and the results are listed in Table 5. The correlation coefficient and significance level are

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Table 5. Correlation Analysis between Permeability and the Pore Volume, Specific Area, and Pore Diameter pore parameter correlation coefficient significance level

pore volume specific surface area 0.905 0.002

0.786 0.021

pore diameter

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0.500 0.207

Notes

two factors that evaluate the correlation degree between two variables. When the correlation coefficient is close to 1, it suggests that the two variables are well-correlated. With regard to the significance level, the results obtained above will be considered statistically significant if it is close to 0. From Table 5, it is obvious that the permeability is well-correlated to the pore volume statistically, with a correlation coefficient of 0.905 and a significance level of 0.002. Unlike sedimentary rock (such as sandstone) or magmatic rock (such as granite), coal is one of the organic rocks with pore-fracture dual-porosity structure. Obvious differences exist in the variation of permeability between the coal and those inorganic rocks under the interaction of heat and pressure. The coal structure tends to deform upon heating because of the developed microstructure. When pressure is exerted on the heated coal, the pores and fractures tend to close. In addition, the permeability may be changed because of alterations in the coal structure, with the occurrence of pyrolysis above 300 °C. Therefore, under the combined action of heat and pressure, changes of coal are of great significance for the UCG process. Gas emission at confining pressures upon heating will be discussed in other papers.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support provided by the National Basic Research Program of China (973 Program) (2014CB238905), the National High-Technology Research and Development Program of China (863 Program) (2011AA050105), and the Fundamental Research Funds for the Central Universities (2009QH13).

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5. CONCLUSION The permeability of Ulanqab lignite parallel to the bedding was determined to be 704.3 mD, which is 251.5, 70.4, and 29.0 times that of the permeability of the compared anthracite, bituminous coal, and sub-bituminous coal, respectively. For both WSS and ARS samples, the permeability parallel to the bedding is higher than that perpendicular to the bedding. However, the opposite tendency was observed for the compared coals. Significant differences in permeability were observed in different bedding directions for the WSS samples. With the decrease of moisture, the permeability of the lignite was found to exhibit an increasing trend, following a “S curve”. The permeability of the ARS sample is nearly 100 times higher than that of the WSS samples parallel to the bedding and approximately 400 times higher than that of the samples perpendicular to the bedding. At a minimum moisture content of 8.5%, the permeability of lignite was found to peak at 2211.2 and 642.4 mD for samples parallel and perpendicular to the G

dx.doi.org/10.1021/ef501696e | Energy Fuels XXXX, XXX, XXX−XXX

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dx.doi.org/10.1021/ef501696e | Energy Fuels XXXX, XXX, XXX−XXX