Evaluation of Coal Combustion Zone and Gas Energy Recovery for

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Evaluation of Coal Combustion Zone and Gas Energy Recovery for Underground Coal Gasification (UCG) Process Fa-qiang Su,†,‡ Akihiro Hamanaka,‡ Ken-ichi Itakura,*,§ Gota Deguchi,∥ Kohki Sato,§ and Jun-ichi Kodama⊥ †

School of Energy Science and Engineering, Henan Polytechnic University, 2001 Century Avenue, Jiaozuo, Henan 454-003, China Center of Environmental Science and Disaster Mitigation for Advanced Research, and §Graduate School of Engineering, Muroran Institute of Technology, 27-1 Mizumoto, Muroran 050-8585, Japan ∥ Underground Resources Innovation Network, NPO, Higashi-ku, Sapporo 007-0847, Japan ⊥ Graduate School of Engineering, Hokkaido University, Kita 13, Nishi 8, Kita-ku, Sapporo, 060-8628, Japan ‡

ABSTRACT: During underground coal gasification (UCG) operations, evaluation of coal gasification cavity evolution and precise control of the underground reactor are important for efficient gasification. It is also essential to estimate the energy recovery of a UCG system and the whole gasification process to ensure an effective combustion and gasification rate. An experimental simulation of UCG using an artificial coal seam comprising a compacted broken coal block was conducted using ex situ UCG models. The main goal of the experiments was the establishment of evaluation methods for the gasification zone and energy recovery during UCG. To investigate the distribution and extent of fracture activity, and to evaluate the propagation of the combustion area in the UCG reactor, we used acoustic emissions (AE) monitoring. This was combined with traditional measurements of temperature variation and product gas content. This paper presents the results of AE analysis of the fracturing activities and damage mechanisms of the coal seam with respect to the UCG operations. From the results of AE source location, we found that the position and area of the crack concentration area, i.e., the gasification zone, can be inferred with comparative accuracy. This is important for in situ practical application of underground coal gasification. In addition, use of the distribution characteristics of AE information over time can also provide advanced warning, and help in timely adjustment of the operational parameters. The results of gas energy recovery were estimated with a proposed stoichiometric method based on measured product gas composition. Quantitative evaluation results include the gas quantities, coal consumption, and heating value yield of the produced synthesis gas. The coal consumption of the obtained energy recovery results also meets the estimated results when calculating the gasification volume with AE source locations (in an error range of about 10%). Therefore, the applied AE monitoring and gas energy recovery approaches may be considered attractive options for evaluating the coal gasification process and developing a safe and efficient UCG system.

1. INTRODUCTION Underground coal gasification (UCG) is a technique that allows the utilization of coal reserves, particularly those at great depths or in complicated geological conditions where mining is not economical. It creates a combustion reactor in an underground coal seam that converts coal in situ into a combustible gaseous product comprising hydrogen, methane, and carbon monoxide through the same chemical reactions that occur in surface gasifiers.1 The synthesized gas is useful either as a fuel or as a chemical feedstock. Many health, safety, and environmental hazards posed by traditional coal mining methods do not apply to UCG. A typical closed UCG system includes a coal seam into which two boreholes are drilled: one for injecting air or oxygen for the in situ burning of coal and the other to extract the gaseous products. Following the proposal of UCG in the 1910s,2 more than 100 years of experimentation and practice in UCG1,3−16 have made it one of the most popular methods for coal utilization, especially because of its advantages of being safe, being costeffective,1,17 and having low environmental impact. The development of CO2 capture and storage technology18−20 has aroused much research interest in UCG. The directional © XXXX American Chemical Society

drilling technologies adopted in oil industries also provide significant technical support for connecting the injection well and production well deep underground.21,22 Hydrogen production from underground coal gasification is also regarded as an effective and a low investment approach, since hydrogenrich gas could be attained through distinct UCG operations.23−28 Recently, many noteworthy experimentally simulated UCG systems have been developed and have achieved acceptable outputs.25,29−32 Research on UCG has also been conducted using mathematical modeling to simulate gasification reaction processes and the product gas.33−36 However, there is relatively little experimental and theoretical research on fracturing activities occurring in coal gasification processes and there is a lack of a widely applicable method for evaluating the energy recovery of UCG. Even recent UCG technology presents certain problems that are attributable to the fracturing activity occurring in underground coal seams and rock masses. Gasification processes associated with coal and rock fractures Received: August 2, 2016 Revised: December 1, 2016 Published: December 2, 2016 A

DOI: 10.1021/acs.energyfuels.6b01922 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. (a) UCG experimental system (a) with a Linking-hole model; (b) UCG experimental system with a Linking-hole model and a Coaxial-hole model.

have advanced with UCG development. The gasification cavity induces many cracks, and the development of cracks inside coal and rock seams results from stress changes around the cavity. These fracture activities can cause well collapse, water pollution, subsidence, and gas leakage to the surface; therefore, UCG development close to residential zones may be dangerous. In other words, monitoring and controlling fracturing activity around a gasification cavity are necessary to maintain safe UCG. In contrast to traditional coal mining and surface coal gasification, the UCG process occurs underground; consequently, we focus on evaluating the combustion zone and visualizing the phenomena and processes of fracturing occurring during coal gasification. In addition, we provide a theoretical calculation for estimating the gasification efficiency. Different from the laboratory-scale model experiments of previous work,37 larger-scale UCG systems were designed and established to simulate UCG conditions with a distinct linkinghole type (Linking-hole model and Coaxial-hole model) in the artificial coal seam. The constructed crack models provide information on crack initiation and the clustered region inside the reactor. Coal consumption, gas quantities, heating value, and gasification rate were also obtained using a proposed stoichiometric approach38,39 based on the results of product gas composition measurements. These monitoring and estimation methodologies are expected to give guidance for evaluating the underground combustion zone and gasification efficiency.

Table 1. Proximate and Ultimate Analysis of Coal Samples No.

Parameter

Bibai coal

Proximate analysis/[wt %] 1 Fixed moisture 2 Ash content 3 Volatile matter 4 Fixed carbon 5 Total sulfur Ultimate analysis /[wt %] 6 Carbon 7 Hydrogen 8 Nitrogen 9 Oxygen 10 Heating capacity/[MJ/kg]

3.20 17.42 37.97 41.41 2.05 81.85 6.08 1.98 9.99 25.43

were made of two 12.5 mm thick fire resistant boards. Steel tubes of 34 or 45 mm diameter were used for the injection and production wells and were placed as shown in the figure. In order to establish a link between the three wells in Linking-hole model, a horizontal gasification channel (cardboard tube) of 45 mm diameter was placed at the bottom of the coal seam. For comparison, System (b) was constructed to investigate the effect of linking types of gasification channel with the Linking-hole and Coaxial-hole models. A stainless steel wire mesh was preinstalled to connect the injection well of hole 1 and the production hole. This was used instead of the cardboard tube used in System (a) to increase the support strength and prevent channel collapse in the case of overpressure. In addition, gas leakage was observed from several positions across the concrete block structure during the gasification process in System (a); therefore, we employed a steel box type coal seam holder in this system. Table 2 presents a description of the four conducted model tests in these two systems. As shown in Figure 1(b), for Test 2 and Test 3, the coal seam was ignited at the bottom of Injection-hole 1 and Injection-hole 2, respectively. In Test 4, the production hole was replaced with a manufactured coaxial-pipe for gas injection and production. The auxiliary hole of System (a) was employed for coal seam reignition during Test 1. During the experiments, the temperature changes, AE activities, and production gas contents were measured successively at distinct gasification agent feeding rates. The vertical cross sections of these two simulated coal seams and the surrounding rock are shown in Figure 2. The typical dimensions and structure of the gasifier applied in the simulations of underground gasification are portrayed. The internal artificial coal seams used in these two trials were simulated based on the compacted pulverized coal and had sizes of 3.77 m (length) × 0.90 m (width) × 0.55 m (height) and 2.75 m (length) × 0.60 m (width) × 0.55 m (height). A box-type coal holder made of steel plates was placed in the concrete block chamber, with its top face exposed, for preparing the coal seam

2. MATERIALS AND METHODS 2.1. Description of the simulated UCG gasifiers. After several small-scale UCG model trials,37,38 two large dimension systems were constructed using artificial coal seams (Figure 1). As shown for these two units, 1,500−2,000 kg of the small size coal block was pressed and tamped into the concrete block (firebrick) structure (Figure 1(a); Linking-hole model), or supported in a steel box (Figure 1(b); Linking-hole and Coaxial-hole models). To better simulate the practical conditions of the target coal seam, the coal samples used in this study were collected by excavating the coal seam in situ from an open-cast coal mine (Sunago Coal Mine of Japan); however, the coal seam broke in the excavation process because the target coal belongs to the soft coal seam. The characteristics of the coal are listed in Table 1, where results of proximate and ultimate analysis, i.e., elemental analysis; contents of fixed moisture, carbon, and ash; and the heating capacity are given. The coal used here had a high ash content (17.42%) and a considerable amount of volatile matter (37.97%). The experimental unit of System (a) was designed with a rectangular shape and possessed the following external dimensions: 4.3 m (length) × 1.3 m (width) × 1.0 m (height). Walls of the reactor B


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Energy & Fuels Table 2. Operating Conditions of the Four Tests Test No.

Test 1

System No.

Artificial coal seam of System (a)

Model types Ignition location

Linking-hole model Bottom of Injection hole & auxiliary hole 220 (122 h of operation)

Operation time/h

Test 2

Test 3

Artificial coal seam of System (b) Linking-hole model Bottom of Injection hole 1

Artificial coal seam of System (b) Linking-hole model Bottom of Injection hole 2

58

61

Test 4 Artificial coal seam of System (b) Coaxial-hole model Bottom of Production hole & Coaxial hole 49

Figure 2. Vertical cross sections of simulated coal seam: (a) System (a); (b) System (b). and surrounding layers. The steel holder had thermal insulation (plaster board) at all of its internal faces. 2.2. Gas supply and experimental procedure. In the ignition phase, the lighted charcoal was placed at the bottom of the coal seam via the injection hole. Pure oxygen was then supplied and the char sample and surrounding coal were heated from ambient temperature to about 200 °C at an oxygen feeding rate of 5−10 L/min. The temperature was continuously increased in order to dry and ignite the coal. Then, the pure oxygen supply was maintained for about 10 min to ensure successful ignition. After ignition, the gasification agent (an air/oxygen mixture) was supplied with an air compressor and oxygen separator. High temperature steam was produced by a pressure cooker. Figure 3 shows the feeding gas flow rate with respect to the operation time in these tests. The hot product gas was passed through the drain tank, in which steam and other condensable gases were filtered out. The dry and clean synthesized gas was then pumped to a gas chromatograph (Inficon, Micro GC 3000A) for automatic analysis every 1 h. All the analysis results were transferred to a control computer for real-time monitoring. To follow the propagation of the combustion flame and variation in the temperature of the coal seam during the UCG process, the experimental units were equipped with sets of 22 or 20 thermocouples, deployed as presented in Figure 4. There were 15 thermocouples (T1−T8 and T11−T17) uniformly located in a plane at different distances from the linking-hole. The temperatures at shallow locations were measured by six thermocouples (T21−T26) installed on both sides of the gasification channel. The thermocouples TG1 and TG2 were used for monitoring the temperatures of the product gas stream. In the second system, thermocouples T11−T13 and T21−T23 were inserted at either end of the steel box, as shown in Figure 4(b). The thermocouples denoted as T51 and T52 were set on either side of Injection-hole 2, each at the same depth. The temperature profiles

Figure 3. Feed gas flow during the gasification experiments: (a) System (a); (b) System (b). recorded by the thermocouples were crucial for controlling process development and cavity growth. C


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Figure 4. Deployment of thermocouples in the experimental reactor: (a) System (a); (b) System (b). 2.3. Monitoring the fracturing activity with acoustic emission. Acoustic emissions (AE) are commonly defined as transient elastic waves within many materials such as ceramics, rocks, concrete, and metals. They are caused by the energy released when a material undergoes stress. Therefore, an event source is the phenomenon that releases elastic energy into the material, which then propagates as an elastic wave. As many detected AE activities can be attributed to thermal stress during the UCG process, it is also useful to estimate the progress of the gasification process and special events such as collapse of the coal seam in the cavity and extensive propagation of the gasification zone. Figure 5 shows an AE signal and the commonly used parameters of AE techniques. When the AE sensor triggers a signal over a certain level (i.e., the threshold), an AE event is captured. The maximum amplitude of the event is the greatest measured voltage in a waveform. The counts of the AE event, i.e., the ring down count refer to the number of emitted pulses if the amplitude of the signal is greater than the threshold. The time period between the rising edge of the first count and the falling edge of last count is the duration of the AE event. Furthermore, the time period between the peak of the AE event and the falling edge of the last count is termed the dead time. The AE

Figure 5. AE parameters. events show the number of microcracking noises (signals); in fact, the AE counts reflect the magnitude of such events. Source location has been widely studied from many years. This technique makes it clear when and where microfailure phenomena occur. Furthermore, the scale of observed activity can be accurately determined, meaning that the effect of wave attenuation with distance D


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travel-time-difference method37,40 is used for source location by using six or seven sensors. For the purpose of detecting the damage extent and fracturing activity occurring during the UCG process, AE sensors were installed into the coal seam, i.e., one sensor for measuring the AE count rates and events, and six or seven sensors (in each test) for the AE waveforms signals recorded in the oscilloscope (GR-7000; Keyence Co., Osaka, Japan). In terms of calculation, estimates included picking the times of P wave arrivals, rejecting the noise signals of waveform data and selecting the data where waveforms from six/seven AE sensors were available were used to locate the AE sources. AE sources were located three-dimensionally under the assumption that P-wave velocity of coal in the gasification zone is constant in all directions and that the signal path from source to sensor is straight. The locations of the AE sensors within these two experimental units are presented in Figure 7. The sensors AE1−AE7 were used in the early period of Test 1 and AE8−AE12 were replaced according to the propagation of the gasification area in the later period. In System (b), three sensors (square red symbols marked in the figure) were employed in Test 2, Test 3, and Test 4 to obtain the AE count rate.

can be considered. The purpose of source location is to determine the true coordinates of the source and the origin time for AE events based on the sensor coordinates and AE arrival times and velocities. Figure 6

Figure 6. Typical AE sensor array and signals.

3. RESULTS AND DISCUSSION The four tests of simulated underground coal gasification in the artificial coal seams were performed to compare the process results of temperature change, high temperature zone evolution, and AE activity occurring during the gasification process, and to explore the estimation methods of energy recovery during UCG. 3.1. Temperature and acoustic emission. Owing to the double reignition of the coal seam during Test 1 of the Linkinghole model experiment conducted in System (a), distinct

shows a typical sensor array and AE signals. An analytical technique, generally known as the travel-time-difference method, is commonly used for source location analysis as shown eq 1.

(Xi − Xs)2 + (Yi − Ys)2 + (Zi − Zs)2 = Vi (ti − Ts)

(1)

where Xi, Yi, and Zi (i = 1, 2, 3, ..., n) are the sensor ordinates, ti and Vi are the AE arrival times and velocities, respectively; XS, YS, and ZS are the true coordinates of the source, and TS is the true origin time. The least-squares method can be adopted when the more than four sensors are used for three-dimensional location. In this study, the least-squares

Figure 7. Arrangement of AE sensors in the experimental reactor: (a) System (a); (b) System (b). E


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Figure 8. Temperature profiles during the gasification process of Test 1.

Figure 9. AE activities during the gasification process of Test 1.

Figure 10. Temperature change rate and AE count rate with respect to the operation time of Test 1.

F


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Figure 11. Temperature profiles (a), as well as temperature change rate and AE count rate (b) during the gasification process of Test 2.

rate of the temperature with respect to the operation time was also investigated. Figure 10 clearly depicts the relation between the temperature fluctuations and fracturing activities. Again, the figure clearly shows the effect of temperature change on the AE activities in the coal. In particular, we found that the effect during the period of drastic temperature increase or decrease is much higher than the effect generated by gradual changes in temperature (during the period of slow and smooth rise in temperature). Figures 11, 12, and 13, respectively, present the temperature fluctuations and AE activities during the three experiments, Test 2, Test 3, and Test 4, conducted in System (b). Based on the thermocouple locations (see Figure 4) and temperature profiles of Test 2, the gasification process could be divided into two phases. In the first phase, during the 0−10 h period of operation, the temperatures of T21, T22, and T33 increased steadily and reached their peak values. It can be inferred that the gasification area moved from the bottom portion to the upper portion along Injection-hole 1. From Figure 11(b), we can see that the AE count rate increased rapidly according to the increasing temperature in the reactor. In the second phase, the temperatures of T42 and T43 continued to slowly increase after a temporary decrease. This indicates that the combustion front moved toward the direction of the production well. With more than 45 h of operation, the temperature profiles and produced gas compositions suggest that the gasification process was close to termination. Consequently, we ended the supply of

periodic increases and decreases of temperature inside the gasifier were observed, as shown in Figure 8. Figure 9 presents the variation in monitored AE parameters, i.e., count rates and events recorded during the experiment, wherein the AE events show the number of cracks occurring inside the coal seam and the count rate depicts the magnitude of event damage. The results show that the number of AE events and the AE count rate increased during the whole operation. These AEs were generated along with the crack initiation and extension around the coal combustion area under the influence of local temperature change. After the first ignition, the combustion area was expanded along the axis of a linking hole because the temperature of T2 increased to 600 °C. The other expansion direction was toward the upper portions of T21 and T24. However, from the temperature change, the combustion and gasification area was limited to the region around the bottom of the injection hole and near the linking hole. Therefore, the second ignition was conducted at the bottom of the auxiliary hole. After the second ignition, the temperatures of T4, T5, T6, T14, and T15 showed a drastic increase. The results show that the combustion and gasification zone occurred around the intermediate hole. After the final ignition at the production hole, the T6 and T15 temperatures rose rapidly. However, the temperatures around the hole were, relatively, not high; T8, which is closest to the production hole, showed little change. These results show that the combustion and gasification zone is smaller than the first and second ignition points. The change G


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coaxial pipeline melted under the high temperature and blocked the drill hole. The coal seam was reignited after clearing the molten metal residue and installing a spare pipe. After about 13 h, the temperature of T13 rapidly increased and reached about 1200 °C. It can be inferred that the gasification area formed in the upper part of the coal seam. Consequently, the temperatures of T12, T11, and T31 increased following the combustion, and the gasification zone moved from the upper seam to the downside and expanded from the Coaxial-hole. During the periods 144 h−149 h in Test 1 and 25 h−32 h in Test 2, high temperature saturated steam was blown into the reactor to investigate its effect on gasification. It can be seen that the temperature inside the reactor declined considerably (200−400 °C within about 5 h of operation) and the AE was active during these two phases, as presented in Figure 10 and Figure 11(b). We also monitored the fire extinguishing process with AE. A drastic decrease in the temperature of the reaction zone from above 1000 °C to between 100 and 200 °C could result in the end of the gasification process. The extinguishing effect of carbon dioxide and nitrogen is investigated in the following sections. Around the fire extinguishing phase, the AEs increased considerably and the temperature in the gasification zone decreased. The experimentally obtained results demonstrate that many AE events were generated during coal combustion. The AE activity was closely related to the change of local temperature inside the coal. 3.2. Evolution of gasification zone. AE is the stress wave produced by the sudden internal initiation of the cracking of

reaction gas and used carbon dioxide to extinguish the combustion. In a further Linking-hole model experiment, Test 3, we attempted to ignite the coal around the connection location of Injection-hole 1 and the bottom linking-hole by injecting pure oxygen into Injection-hole 2; however, the coal seam became ignited at the bottom of Injection-hole 2. This may be caused by the excessive feed rate of reactant gas (pure oxygen)41 or the pressure difference occurring at these two locations of the gasification channel. As the temperature profiles show for the initial period (see Figure 12), the temperatures of T43 and T42 underwent a brief period of rapid increase and reached approximately 400−500 °C. Then, after about 10 h of operation, a significant increase (more than 1300 °C) occurred at the temperatures of T46, T36, T51, and T52 around Injection-hole 2. After about 10 h of the experiment, the temperatures recorded by thermocouples T35 and T45 (at each side of the combustion zone) gradually increased to approximately 600−800 °C. Corresponding to these periods of fluctuating temperature, the AEs were also active. From the temperature profiles of Coaxial-model Test 4 (Figure 13(a)), we can clearly see that the recorded temperatures of the gasifier were lower during the initial stage. The temperature of thermocouples T11 and T31 (located around the ignition area at the bottom of the Coaxial-hole) only showed a short period (about 3 h) of increase after the start of the experiment. In this case, we suspended the experiment and found that the inner pipe of the H


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Figure 14. Located AE sources with temperature contour profiles during the first stage of Test 1.

coal; it is caused by local temperature changes in the gasification reactor. Evolution of the gasification zone could be caused by crack initiation, crack growth, and crack cluster, which form the necessary space and extend unceasingly to allow for further gasification. In the present work, the AE source was located by using the arrival times of several event signals, which were received by an array of acceleration sensors positioned in a well-proportioned manner at the surface of the UCG artificial coal seam. A 3D model of AE sources was used to demonstrate the detection,

location, and development of the gasification zone based on the movement of the high temperature area (as given by the temperature measuring system). The location results from AE sources in Test 1 of System (a) and the other three tests of System (b) are presented in Figures 14, 16, and 17, respectively. The color spheres in these models depict the located sources, wherein, the extent of damage, i.e., the relative energy emitted from cracking, can be differentiated by the sphere sizes. I


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Figure 15. AE sources located at the top face of the coal seam and the burning of plasterboard during Test 1.

Figure 16. Located AE sources with temperature contour profiles during the second stage of Test 1.

Figure 17. Located AE sources with temperature profiles during the gasification process of Test 2, Test 3, and Test 4 in System (b).

The AE sources of the early stage of Test 1 (Figure 14) are represented by red spheres. It can be seen that the damage sources were concentrated in the vicinity of the injection hole (ignition location) and expand around the linking-hole. The gray spheres in this model gave the clustered AE during the last period of the experiment. According to the plane view

distribution of located AE sources, it appears that most AEs clustered at the surface of the coal seam. Indeed, gas leakage was observed during this time period. After the experiment, we excavated the covered concrete and found the burned out plaster board, as shown in Figure 15. The white contour line in Figure 15 (plane view distribution of AE sources located on the J


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Figure 18. Variations of gas composition and heating value with respect to operation time of Test 1.

upper face of the coal seam) highlights the approximate region of the damaged face. However, from an alternative perspective, we found that the potential damage/danger area around the gasifier could be predicted based on where crack initiation occurred and extended; therefore, if certain protection measures, e.g., adjustment of the operational parameters, were adopted, excessive damage in nontarget areas could be prevented. Figure 16 presents the AE sources located in the later stage after igniting the coal seam at the bottom of the auxiliary hole. The AEs clustered around the auxiliary hole and formed a relatively large combustion zone in the direction of the production hole. We also investigated high temperature distribution using the measurement results of thermocouples located at three levels of the coal seam: 0.2, 0.3, and 0.4 m (Figure 4(a)). The obtained temperature contour profiles are presented in the upper portions of Figures 14 and 16. It can be seen that for the two stages in Test 1, location accuracy is good for both the high temperature distribution and the AE sources. Figure 17 presents the AE sources located in Test 2, Test 3, and Coaxial model Test 4, wherein, AE clustering regions appeared in the vicinity of Injection holes 1 and 2 and expanded around the Coaxial hole, respectively. As can be seen from the sources clustered in Test 4, the gasification zone was limited to the upper part of the coal seam; this can also be inferred from the temperature variations of Test 4 (see Figure 13). The upper portion of Figure 17 shows the variation in temperature on a vertical plane at a seam depth of 0.4 m in System (b). The observed temperature profile coincides with the results of the AE source locations of these three gasification zones in Test 2, Test 3, and Test 4. The results of located AE sources provide more information relating to the gasification degree and the volume calculation of the gasification zone. The space distribution characteristics of

the AE sources, which are produced by local thermal stress during UCG operation, allow for improved evaluation of the formation and evolution of the gasification zone and can even provide an advanced warning if extensive damage occurs. For the in situ UCG, a real-time monitoring and control system is necessary for safe and efficient gasification. From our study, the AE technology is verified as a reliable method for monitoring and evaluating the gasification process in laboratory-scale UCG when combined with the traditional measurements. The earliest application of AE analysis probably occurred in seismology.42 Elastic waves produced by an earthquake were analyzed to characterize fault movements in terms of energy release, location, and depth. The AE technique has also been used to detect the impending failure of mine shafts.43,44 For the real UCG, a type of transducer with the low frequency microseismicity, i.e., a geophone, which is functionally similar to the AE accelerometer employed in our work, could be used for monitoring the fracturing activity and gasifier’s structure in underground. The geophone is a ground motion transducer that can convert the ground damage and movement in the voltage signal. As with the AE sensor, parameters such as event, count rate, and relative energy can be obtained from the geophone by processing the recorded voltage signals. Many different types of geophone are now being manufactured nowadays and are used for the measurement of machine vibration, earthquakes, oil exploration, and conventional mining. In our follow-up study of UCG, the newtype three-component geophones will be employed. A moving coil type geophone is being built, comprising one component vertical and two components horizontal (allowing for output voltage acquisition in X, Y, and Z directions). It has the potential to offer high-performance fracture exploration and K


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Figure 19. Variations in gas composition with respect to operation time of Test 2, Test 3, and Test 4 in System (b).

reaction was that of H2O (g) and CO2 being reduced to H2 and CO (endothermic reduction reaction) at high temperatures25,28 when they interfaced with the incandescent coal. During the steam stage, a gradual drop in temperatures (300−400 °C) of the reactor was observed. However, the effect of steam injection was too small to enhance this reduction reaction and the CO remain was the dominant combustible gas content at this time. For System (b), an electronic dehumidifier was used to reduce the effects of moisture during gas chromatography analysis. Figure 19 presents the variation in the percentage composition of combustible components with respect to the operation time during Tests 2, 3, and 4. During the course of Linking-hole model Tests 1 and 2, a relatively high quality gas content of CO was observed in the stable gasification process. After igniting the coal, the temperatures in these two tests maintained rapid growth until they reached approximately 1200−1300 °C, the maximum CO content obtained was rather high above 50%. After injecting the steam in Test 2 after about 25 h of the experiment, although the temperature profiles inside reactor decreased nearly 400 °C, there was no obvious change in the gas concentrations. The amount of the steam supplied in this test was also not sufficient for sustaining the remarkable reduction reaction. The variation in H2 content in these two tests at different times also shows a similar concentration (about 10%). The concentration of gas produced during Test 4 is low compared to the results of the two tests with the Linkinghole model. The same trends in the temperature changes and variation of the major combustible gas content (CO) could be observed over these three tests. 3.3.2. Gas energy. For UCG operation, estimations of cavity volume and level of coal consumption are indispensable for controlling and managing gas production. Coal consumption creates underground cavities and subsequent continuous

provide higher precision three-dimensional data during real UCG. 3.3. Energy recovery. 3.3.1. Gas composition. The combustible components and variation in the heating value of the product gas during the gasification process of Test 1 from the first stage to the second stage are shown in Figure 18. For about 200 h, reactant gas (oxygen) was supplied to the reaction zone at a flow rate of 20−30 L/min to sustain the gasification process. During the initial gasification period, the content of noncombustible gas (nitrogen and carbon dioxide) was greater than 60%, which engendered a low heating value (average 1.36 MJ/m3, 0−40 h). Overall, the heating value rose with an increase in the oxygen flow rate. Although the noncombustible gas contents remained at a high level during the 70−90 h period, the heating value was high at about 4.77 MJ/m3. The gas was mainly composed of combustible components such as CO, H2, and CH4. After about 45 h of the experiment, the temperature began to decline gradually. Although the supply rate of the reactant gas was increased to 55 L/min, the decreasing trend in temperature did not change. As a result, the heating value of the produced gas also showed a significantly decreasing trend after about 95 h of operation. Therefore, the coal seam was reignited using the auxiliary hole at about 122 h. After the reignition phase, the maximum temperatures of approximately 1400 °C were recorded by thermocouples set in reactor, which resulted in a significant increase in the heating value. The combustible gas components were comprised mainly of CO (22%). At about 144 h, steam was injected into the gasifier to investigate its influence on the gasification effect. Subsequently, the combustible gas content, especially of CO and H2, showed a considerable increase. The gas heating value also increased continuously. This increase may have resulted from the coal reduction reaction. At this time, the major L


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Energy & Fuels growth during the gasification process. However, no direct in situ measurement method exists. As a conventional UCG control method, the inlet gas content and flow rate are adjusted using information obtained from analysis of the product gas. The amount of coal consumption was also estimated using analysis of the product gas. Therefore, it is difficult to estimate and visualize the cavity growth and its configurations, the fracturing conditions around the combustion reactor, and the gasification zone in the coal seam. For this study, coal consumption was estimated by calculating the cavity volumes formed in the gasifiers (using AE techniques), and with the method of estimating the coal gasification reaction process based on stoichiometry (using product gas composition).38,39 The chemical reaction that takes place during UCG is given by the material balance equation shown in eq 2. The dry and clean syngas produced from the UCG process used in this formula contains H2, CO, CO2, and CH4 (N2, O2 free), with concentrations expressed, respectively, as p, q, r, and s, which were obtained from the gas analysis results.

parameters in this work. Moreover, a definition of the rate of energy recovery (Rg) is also proposed to better evaluate the energy exchange efficiency in this experiment, wherein, Vd and Qu are the dry gas production rate and unit heating value, respectively, and Qc is the heating capacity of the applied coal.

CHmOn + αO2 + βH2O → γH2 + δCO + εCO2 + ηCH4

Table 5. Comparison of Rate of Energy Recovery in the Four Tests

R g = (Vd × Q u)/(Q c) × 100%

The results show that the total quantities of synthesized gas produced in System (a) and System (b) of the simulated UCG experiments are 235.354 m3 (Test 1), 87.970 m3 (Test 2), 77.461 m3 (Test 3), and 31.655 m3 (Coaxial model of Test 4). As presented in Table 4, the rate of coal consumption in the Coaxial-hole model Test 4 is about 0.679 kg/h, which equals about 60−70% of the rate obtained in the Linking-hole model tests. During the entirety of Test 1, the coal consumed in System (a) is estimated at about 168.855 kg, with an actual operating time of about 122 h. The rates of energy recovery (Table 5) in these two systems are 32.49% for System (a) and an average of 53.11% for the gasification process in System (b).

(2)

In this reaction, α and β, respectively, denotes the balance coefficients of O2 and H2O, and m and n are given by the ultimate analysis of coal samples. In addition, γ, δ, ε, and η, respectively, represents the gas outputs of H2, CO, CO2, and CH4. Average gas composition and heating value of the product gas during the gasification process of these four tests are presented in Table 3. The average heating values obtained in

Gas production rate[m3/kg] Unit heating value [MJ/m3] Heating capacity [MJ/kg] Rate of energy recovery [%]

Avg gas composition Test Test Test Test

1 2 3 4

CO2/%

CO/%

CH4/%

H2/%

Avg Heating Value (MJ/m3)

68.2 44.3 40.7 45.9

18.4 37.7 37.0 39.2

4.5 4.9 8.1 2.1

8.0 11.9 12.6 12.1

5.65 8.78 10.23 7.57

the Test 2 and Test 3 Linking-hole models of System (b) are 8.78 MJ/m3 and 10.23 MJ/m3, respectively. With a Coaxialhole, Test 3 yields a relatively low heating value under the similar operational conditions. The product gas compositions are similar to those of the small-scale UCG model experiments using coal block that were conducted in our previous work37 and they gained the higher heating values. The obtained heating value in the present work is also of the same order as that of the results reported in the literature.25,45 Table 4 summarizes the detailed results of gas energy recovery and the relevant Table 4. Estimated Gas Energy Recovery Results in the Four Tests

Rate of coal consumption/ [kg/h] Amount of coal consumption/ [kg] Quantity of gas product/[m3] Heating value of gas product/ [MJ/h]

Test 1

Test 2

Test 3

Test 4

1.384

1.279

1.178

0.679

168.855

60.090

52.433

21.391

235.354 11.433

87.970 17.048

77.461 18.582

31.655 8.047

Test 1

Test 2

Test 3

Test 4

1.394 5.927 25.43 32.49

1.452 9.109 25.43 52.01

1.471 10.675 25.43 61.75

1.447 8.008 25.43 45.57

To investigate the cavity volume of the gasification zone, the approach of Delaunay Triangulation46 was applied on the basis of the located AE source results. The Delaunay Triangulation is used simply to satisfy the geometry of the triangular elements by the finite element method and widely used as an automatic element division method. The 3D tetrahedron mesh models could be built using the point cloud data of AE sources and displayed by OpenGL (open graphics library), as shown in Figure 20. The analysis results of the cavity volumes are given in Table 6. The coal consumed during the experiments can also be obtained from the measured apparent density of coal (0.9 g/ cm3). A comparison of coal consumption in the four tests of the estimated results according to the AE source locations and gas energy recovery of stoichiometry is presented in Table 6. The percentage errors indicated in the table show that the percentage error between the two assessment methods is only about 9−13% in the case of Test 2, Test 3, and Test 4 in System (b), whereas for Test 1, it is less than 20%. This is not only due to the distinct construction of each system and the operating parameters, but is also the result of gas leakage, as this directly affects the estimated results of gas energy recovery. The characteristic observation specific to Test 1 was the significant gas leakage that appeared during several periods of the gasification process. As a whole, by taking this effect into account, the applied estimation methods are also able to predict the coal consumed during UCG. The equilibrium constant has often been used to understand the composition of gasification products. However, this is only available to the stable state, and it is not applicable to the analysis of the constantly fluctuating, transient composition of gases during operation. Estimated coal consumption based on stoichiometry using gas composition supports the experimental observations. However, an error exists among the calculated results and might result from moisture evaporation within the

Table 3. Average Gas Composition and Heating Values of Product Gas in the Four Tests Test No.

(3)

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Energy & Fuels

Figure 20. Stereotactic volumetric model constructed by the Delaunay Triangulation method.

gasification zone close to the coaxial-hole under the present experimental conditions. 4. The estimation methods of AE monitoring and gas energy recovery described here may be feasible options. Estimated coal consumption based on the stoichiometric method using product gas composition supports the results obtained from the located AE sources, in an acceptable error range (about 10%). AE monitoring techniques combined with the proposed stoichiometric approach applied in the present work provide a potential approach for estimating the underground cavity volume and coal consumption during the UCG process and for visualizing cavity configuration and fracturing extent. Such approaches can be used to control an underground gasifier.

Table 6. Comparison of Coal Consumption in the Four Tests Cavity volume by AE/ [cm3] Coal consumption by AE/ [kg] Coal consumption by gas energy/[kg] Error percentage [%]

Test 1

Test 2

Test 3

Test4

221462.9

75662.3

51170.7

26050.6

199.3

68.1

46.1

23.5

168.9

60.1

52.4

21.4

18.0

13.3

−12.0

9.8

coal (inherent moisture), the tar filtered by the drain tank, and volatile matter. These estimated results provide a fair evaluation of coal consumption and energy recovery.

■

4. CONCLUSIONS The feasibility of evaluation methods using acoustic emission (AE) techniques and a stoichiometric approach for the gasification process was studied in ex situ artificial coal seam UCG models at operating conditions similar to UCG. The following conclusions can be obtained from the experimental study: 1. The change in thermal stress is responsible for AE initiation and extension from the gasification zone. AE activity is closely related to the local temperature change inside the UCG reactor. 2. Results on the evolution of the gasification zone based on the AE clustered region and temperature profiles were provided. It was observed that expansion and movement of the combustion zone could be evaluated because the same tendency was observed for changes in the temperature distribution within the coal. 3. We also investigated the effects of various design and operating parameters on the temperature change, evolution of the gasification zone, and product gas composition. From the gas composition and temperature profiles, it can be concluded that the heating value increased with the increase of CO, H2, and CH4 because of the injection of saturated steam (occurrence of an exothermic reaction) as the temperature dropped inside the gasifier. Considerable amounts of combustible gas components were produced in the Linking-hole model tests, whereas the Coaxial-hole model of Test 4 yielded a relatively low heating value and formed a limited

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel.: +81-143-465424; Fax: +81-143-46-5499. ORCID

Fa-qiang Su: 0000-0001-9377-0271 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the Japanese Society on UCG, Mikasa City, Center of Environmental Science and Disaster Mitigation for Advanced Research of Muroran Institute of Technology and a Grant-in-Aid for Scientific Research (b), 21360441, from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The authors gratefully acknowledge their support.

■

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