Experimental Study of Shaftless Underground ... - ACS Publications

2390

Energy & Fuels 2007, 21, 2390-2397

Experimental Study of Shaftless Underground Gasification in Thin High-Angle Coal Seams Lanhe Yang* College of Resources and Geosciences, China UniVersity of Mining and Technology, Xuzhou, Jiangsu ProVince 221008, China

Shuqin Liu, Li Yu, and Jie Liang School of Chemistry and EnVironmental Engineering, China UniVersity of Mining and Technology, Beijing 100083, China ReceiVed May 5, 2007

The purpose of this article is to study the feasibility of shaftless underground gasification in thin high-angle coal seams and the reliability of a new two-stage technique for pushing-through gasification galleries with firepower permeation. This paper first presents the theory, design, and technical details of a field experiment that tests the new process, which consists of blind-hole electric ignition of an underground gasifier, a forward and backward firepower seepage method, and gasification of a small shaftlike gasifier. The results of the experiment are then analyzed. It is concluded that the bigger the blast intensity, the faster the moving speed of the fire source and the higher the average temperature of the gallery; however, if the blast intensity exceeded a critical value, the moving speed of the fire source (pushing-through speed) and the average temperature exhibited the drop tendency. Under the experimental conditions, the average leakage rate for the blasting was 81.89%, with an average volume of discharge of 93.82 m3/h and an average gallery diameter of 0.40 m. Along with the progression of the firepower pushing-through process, the air blast leakage rate dramatically declined. In addition, the heating value of coal gas rose with the increase of gas flow and approached stabilization. The experiment showed that the heating value of underground water gas produced by two-phase gasification could reach 12.00 MJ/m3, in which the content of H2 exceeded 48% and those of CO and CH4 were 13% and 7%, respectively.

1. Introduction Underground coal gasification is a process by which underground coal is converted into combustible gas in a way that has many advantages over conventional surface gasification. The advantages, which range from sound safety and less pollution to higher efficiency and lower costs, are making the process more and more attractive throughout the world. A growing number of theoretical and experimental studies on underground coal gasification techniques1-26 demonstrate the progress being made in the field. There are two main types of underground * Corresponding author. Fax: +86 516 83590998. Tel.: +86 516 83885762. E-mail: [email protected]. (1) Zhao, S. G.; Zhang, S. W. The Technology Keys for Commercialization of Underground Coal Gasification. Coal Process. Comprehen. Util. 2003, 17 (1), 36-38. (2) Jiang, D. P. Rational Knowledge and Practical Operation of the Underground Gasification of Coal. Coal Mine Des. 2001, 21 (4), 9-11. (3) Chu, M.; Liang, J.; Yu, L. Feasibility of Producing Coal-Synthetic Fuels by Underground Coal Gasification Gas. Coal ConVers. 2000, 23 (4), 18-21. (4) Yu, L.; Liu, S. Q. Thoughts on Commercialization of the LLTSUCG New Technique. Sci. Technol. ReV. 2003, 2, 51-53. (5) Sun, B. Z.; Xu, Z. L.; Zhang, B. Trend of Coal Underground Gasification. Coal 2002, 11 (3), 1-3. (6) Wang, B. Y. Analysis of Underground Coal Gasification Power Generation Demonstration Project on Lineng Group. Clean Coal Technol. 2004, 10 (1), 26-28. (7) Liu, B. Y.; Qiu, P. Study on the Underground Coal Gasification Technology. Clean Coal Technol. 2003, 9 (2), 23-29. (8) Liang, J.; Chang, J.; Liu, S. Q.; Yang, Z. Gray Prediction of Underground Coal Gasification Process. J. Chin. UniV. Min. Technol. 2003, 32 (6), 608-611.

coal gasification: shaft gasification and shaftless gasification. The former requires laborers to work underground to engineer the gasifier, while the latter process avoids this labor by, instead, (9) B. M. Increasing Efficiency of Underground Coal Gasification. Collect. Translat. Works Min. Technol. 1988, 9 (2), 17-19. (10) Franke, F. H. Survey on Experiment Laboratory Work on UCG. Proceedings of the Twelfth Annual Underground Coal Gasification Symposium, Washington, D.C. 1986; pp 131-133. (11) Beyer, L. G., Guntermann, K.; Gudenau, H. W.; Wenzel, W.; Franke, F. H. Large Scale Apparatus for Simulating UCG. Proceedings of the Twelfth Annual Underground Coal Gasification Symposium, Washington, D.C. 1986; pp 142-150. (12) Yeary, D.; Riggs, J. B. Experimental Study of Lateral Cavity Growth Mechanisms. Proceedings of the Twelfth Annual Underground Coal Gasification Symposium, Washington, D.C. 1986; pp 158-163. (13) Skocypec, R. D.; Cook, D. W.; Engler, B. P. Char Consumption in the Underground Gasification of Eastern Bituminous Coal. Proceedings of the Twelfth Annual Underground Coal Gasification Symposium, Washington, D.C. 1986; pp 164-178. (14) Brych, J. Rock Mechanics for Underground Coal Gasification in Belgium. Proceedings of the Twelfth Annual Underground Coal Gasification Symposium, Washington, D.C. 1986; pp 188-195. (15) Guntermann, K.; Gudenau, H. W.; Franke, F. H. An integrated UCGsimulation model of laboratory work and mathematical modeling. Proceedings of the 12th Annual Underground Coal Gasification Symposium, Washington, D.C. 1986; pp 207-216. (16) Dufaux, A. Modeling of the UCG process at Thulin on the basis of thermodynamic equilibrium and isotopic measurements. Fuel 1990, 69 (5), 624-632. (17) Thorsness, C. B.; Britten, J. A. Analysis of material and Energy Balances for the Rocky Mountain-1 UCG Field Test; Report Nos. 44-49, U.S. DOE, Ws7405sEng-48, Lawrence Livermore National Laboratory: Livermore, CA, 1984. (18) Debelle, B.; Malmendier, M. Modeling of flow at Thulin underground coal gasification experiments. Fuel 1992, 71 (2), 95-104.

10.1021/ef700231p CCC: $37.00 © 2007 American Chemical Society Published on Web 06/23/2007

Gasification in Thin High-Angle Coal Seams

adopting pushing-through techniques to form the underground gasifier. In China, shaftless underground coal gasification was tried, both in the 1960s and the 1980s;27-29 however, due to the limitations of the pushing-through techniques used at those times, the results were largely ineffective. The former Soviet Union also conducted experimental research on underground coal gasification but mainly employed electric pushing-through techniques, hydraulic fracturing, or oxygen-enriched pushingthrough techniques to build gasification galleries, which resulted in enormous energy consumption, as well as high cost and low profits.29 These techniques also made it notoriously difficult to control the course of the pushing-through process as it proceeds from one end of the gasifier to the other. Considering the advantages and disadvantages of the methods mentioned above, this paper develops a kind of pushing-through technique with forward and backward firepower seepage and successfully tests this method in the thin high-angle coal seams of Mazhuang Mine, Xuzhou Mining Industry Group, Jiangsu Province, China. 2. Design of Underground Coal Gasification 2.1. Coal Seams to be Gasified. Together, the Shihezi Formation and the Shanxi Formation of the Permian Period constitute the major coal-bearing formations of Mazhuang Mine. These formations include six coal seams, five of which (nos. 1, 2, 3, 5, and 6) are major commercial coal seams. Because the no. 1 and 2 seams are already used up and the no. 5 and 6 seams are too deep to efficiently gasify within the scope of this experiment, the no. 3 coal seam was selected as the object seam for the underground gasification experiment. The third seam is a steep coal layer, with a dip angle of 65-73°, a thickness of 1.20 m, a strike of N15 °E, and a slope of N75 °W. The coal type of this seam is gas-fat coal, whose proximate analysis is shown in Table 1. The roof of the coal layer is composed of sandstone, mudstone, and siltstone; the floor is composed of carbonaceous (19) Edwani, S. H. The Expression, Simulation and Appraisal of the Thermo-physical Model on the Underground Coal Gasification. Collect. Translat. Works Min. Technol. 1985, 6 (3), 84-93. (20) Guntermann, K. The comprehensive experimental model and mathematical model on the underground coal gasification. Collect. Translat. Works Min. Technol. 1988, 9 (3), 6-10. (21) Thomas, F. E. Research and Development on Underground Gasification of Texas Lignite. In Underground Coal Gasification: The State of the Art; William, B. K., Robert, D. G., Ed.; American Institute of Chemical Engineers: New York, 1983; Vol. 79, pp 66-77. (22) Martin, J. M.; Layne, A. W.; Siriwardane, H. J. Thermo-mechanical Modeling of Ground Movements Associated with Large Underground Coal Gasification Cavities in Thin Coal Seams. Proceedings of the 10th Annual Underground Coal Gasification Symposium, Fallen Leaf Lake, CA, 1984; pp 295-307. (23) Yang, L. H.; Liang, J.; Liu, Z. H. The Detection and Computer Identification of Moving Speed of Working Face of UCG. Proceedings of the 29th International Symposium on Computer Applications in the Mining Industries, Beijing, China; A.A. Balkema Publishers: Netherlands, 2001; pp 447-480. (24) Bloomstran, M. A. The Underground Gasification Experiment in Rocky Mountain-1. Collect. Translat. Works Min. Technol. 1988, 9 (1), 11-14. (25) Alexander, P.; Coots, T.; David, B. Comparative Study on the Measuring Methods Adopted in the Gasification of Horizontal and Steep Coal Layers. Collect. Translat. Works Min. Technol. 1985, 5 (2), 107115. (26) Chandelle, V. The Pushing Through Project of the Gasification Channel in Tulin Coal Field. Collect. Translat. Works Min. Technol. 1992, 13 (2), 5-7. (27) Yu, L.; Liang, J.; Yu, X. D. Progress in the Coal Underground Pneumatolysis Technology. Sci. Technol. ReV. 1999, 4, 33-35. (28) Yang, L. H.; Yu, L.; Liang, J. The Coal Gasification in the Discarded Mines and Comprehensive Utilization of Its Product Gas. Min. World 1995, 16 (1), 20-24. (29) Yang, L. H.; Song, Q. Y.; Li, Y. J. Underground Coal Gasification Project; Song, D. Y., Ed.; China University of Mining & Technology Press: Xuzhou, 2001; pp 169-177.

Energy & Fuels, Vol. 21, No. 4, 2007 2391 Table 1. Coal Proximate Analysis (%) analytical base water ash volatile content content matter 4.90

13.08

26.77

C

H

O

N

S

heating value (MJ/m3)

66.56 3.92 10.09 0.84 0.59 26.10-28.81

mudstone. The horizon of the third seam is -52 to -150 m, whereas the present production horizon of Mazhuang Mine has reached -160 to -270 m; therefore, the burial of the third coal seam is shallow, with a comparatively small thickness, which makes it difficult to be excavated by conventional mining methods. As a result, the third seam had been discarded underground. It is estimated that approximately 15 600 tons of coal reserves, similarly, cannot be mined using conventional methods. This discarded coal, however, provides a very good resource for the present experiment. 2.2. Position of the Gasifier. The underground gasifier was deployed (1) above the northern end of the material roadway of the former no. 301 coalface which is at an elevation of -40 m, (2) in the residual outcropping coal pillar of the third seam at an elevation of -20 to -40 m, and (3) 120-230 m away from the mine goaf. The bottom of the gasifier was positioned within reach of caving and fissure zones that helped to form percolation galleries during gasification. At the same time, the upper boundary of the gasifier was positioned close to the lower boundary of the weathering zone of the coal outcrop, which allowed for a lowering of the cohesiveness of the coal, facilitated the development of the fissures in the seam, and, therefore, accelerated the gasification reaction. 2.3. Structure of the Gasifier. According to the characteristics of the geological structure, the storage conditions of the coal seams, and the economic and technological indicators of the experiment, it was determined that a U-shaped underground gasifier was optimal. The gasifier consists of a gasification gallery, inlet, outlet, and monitoring (temperature-measuring) holes (Figure 1). The net diameter of the inlet and outlet holes is 159 mm, and that of the monitoring hole is 50 mm; at an average depth of 82.26 m, the target layer of the holes is the floor of the seam. The surface distance between the inlet hole and outlet hole is 40 m, the pushingthrough length is 30 m, and the designed section area of firepower permeation is 2.5-3.0 m.2 The underground gasifier and the existing production system of the mine are in a state of separation. In order to increase the reliability of the seal, an additional sealed wall was set up beside the sealed wall for the former crossdrift of the mining area at the -40, -80, and -160 m levels. A demolition pressure test under water for the isolation wall, a static water pressure test for the gasifier, and a test for the leakage of the gasifier indicated that the shielding tightness between the underground gasifier and the former goaf was sound and that the sealing conditions were reliable.

Figure 1. Structure of the gasifier.

2392 Energy & Fuels, Vol. 21, No. 4, 2007

Yang et al. Table 2. Gas Compositions and Heating Value for the Forward and Backward Combustion

Figure 2. Blind-hole electric ignition system.

3. Pushing-Through Process 3.1. Blind-Hole Ignition. For the shaftless gasifier of the Mazhuang colliery, the ignition of the coal seam had to withstand two disadvantages: the increase of water content at the bottom of the borehole while drilling and the absence of a gasification gallery to discharge smoke. Water vaporization absorbs heat and increases thermosteresis. At the same time, the flue-gas reduces the oxygen concentration on the surface of the carbon particles and lowers the heat produced. To overcome these disadvantages and ignite the coal seam quickly, forced ignition is usually adopted in shaftless gasifiers: air is blasted through a borehole containing incandescent coal. By this method, comparatively high blasting pressure is needed to press the smoke into the seams. If the heat produced by the incandescent coal is not enough to offset the heat radiation from water evaporation, the ignition tends to fail. Initially, the field test failed because of excessive steam and insufficient air blasting pressure. Later, when blind-hole electric ignition was adopted, the field test was successful. The ignition system is shown in Figure 2. The igniter is made of chromel-filament and wrapped up in kindling materials (e.g., wood soaked in gas, cotton yarn, etc.). The blind-hole ignition system gives the following advantages: (1) it quickly produces naked light, avoiding the explosion from the slow temperature rise process during which oil is evaporated into oil gas, when resistor ignition is adopted; (2) it guarantees no short circuiting by utilizing the chromel-filament in series in a circuit with an electrical current of about 14 A, two homopolar wires in the holes, and two power-driven appliances used for protection in the ground; (3) the igniter provides constant external heat for the ignition area over a long time, which maintains comparatively high-temperature conditions for the ignition area; (4) the air-supply sebific duct transports the fresh air to the ignition area directly and simultaneously discharges flue-gas. When igniting, air was supplied in a pulse of 0-0.3 MPa at a time interval of 15 min, which was conducive to, both, the expansion of the fire area and a timely discharge of the flue gas and had no impact on the oxygen concentration of the fire area. Successful ignition was indicated when gas with a heating value of over 1.30 MJ/m3 30-32 passed through the nos. 1, 2, and 3, S, and N boreholes. (30) Wu, R. Y. Coal Gasification; Huang, W., Ed.; China University of Mining & Technology Press: Xuzhou, 1988; pp 68-73. (31) Wen, W. Y. The Introduction to the Combustion and Gasification of Solid Fuel; China Industry Press: Beijing, 1965; pp 145-149.

time (h)

mode of wind blasting

H2

1.5 1.5 2.0 2.0

backward forward backward forward

2.91 9.38 4.29 6.67

gas compositions (%) CO CH4 CO2 O2 1.63 4.17 0.00 3.56

1.72 5.19 2.56 2.91

19.24 15.28 13.80 18.97

0.04 0.08 4.37 1.10

N2


74.73 65.89 74.98 66.80

1.29 4.00 1.65 2.61

3.2. Forward and Backward Combustion Pushing-Through. The pushing-through techniques usually adopted in shaftless gasification include firepower pushing-through, electric power pushing-through, hydrofracture, and directional drilling. Firepower pushing-through, i.e., seepage pushing-through by means of air firepower, is often used for its simplicity. There are two different types of firepower pushing-through processes: forward combustion percolation and backward combustion percolation. During forward combustion percolation, the forefront of the flame of the burning coal expands in the same direction with that of the air current. The consumption rate of coal determines the moving speed of the forefront of the flame.33,34 With the consumption of the coal, the combustion area moves down to the place in the gallery where the coal density is comparatively high. During backward combustion percolation, however, the forefront of the flame expands in the opposite direction to that of the air current and the velocity of the forefront of the flame depends on the transmission rate from the upstream heat to the air current. The upstream coal burns when being heated to its burning point, which obtains oxygen from the residual coal (semicoke) and results in incomplete combustion of the coal seams.29,35,36 Therefore, forward combustion consumes all the combustible substances, but backward combustion passes through the combustible substances and only consumes part of them. The important difference between the two processes is that while backward combustion tends to lead to a narrow gallery with a fixed diameter, forward combustion expands with a relatively wider forefront. Thus, forward combustion can enlarge the fire source, while backward combustion can form a relatively regular gallery. As a result, the forward and backward interchanging combustion penetration can form a regular gallery with a comparatively wider diameter. In the field test, this interchange pushing-through method was used. Fifty hours after the ignition of the gasifier, pushing-through of the gasification channel began: air was mainly supplied through the N hole but was supplemented by the nos. 3, 2, and 1 holes, and it gradually drew the fire source from the S hole to the N hole. The interchange between the forward and backward air supply depended on the gas compositions; i.e., when the total volume fraction of CO, CH4, and CO2 was more than 18%,29 the backward air supply was employed; when the total volume fraction of CO, CH4, and CO2 was less than 18%,29 the forward (32) Yang, L. H. Study on the Method of Seepage Combustion in Underground Coal Gasification; Song, D. Y., Ed.; China University of Mining & Technology Press: Xuzhou, 2001; pp 73-79. (33) Liu, S. Q.; Liang, J.; Yu, L. Reaction Character and Influence Factor of CO2 in Underground Coal Gasification. J. Chin. UniV. Min. Technol. 2000, 29 (6), 606-609. (34) Yang, L. H.; Liang, J. TEM (Pulse Transient Electrimagnetic Method) Method of Combustion Space Area in Underground Coal Gasification. J. Nanjing UniV. Sci. Technol. 2001, 25 (2), 200-204. (35) Yang, Z.; Liang, J.; Li, X. Z. Study of Burning Control System of Underground Gasification. Coal ConVers. 2002, 25 (4), 32-35. (36) Liu, S. Q. Study on the Regularity of CO2 Production in the Process of Underground Coal Gasification and on the Disposal Method of CO2. Ph.D. Thesis, China University of Mining & Technology, Beijing, 2000.


Energy & Fuels, Vol. 21, No. 4, 2007 2393

Table 3. Simplified Table for the Major Pushing-Through Process of the Gasification Gallery backward combustion air-blast holes discharge holes

phases 1st phase

N and nos. 3, 2, and 1 auxiliary blasting 2nd phase N and nos. 3 and 2 auxiliary blasting 3rd phase N and no. 3 auxiliary blasting 4th phase N

S

forward combustion air-blast holes discharge holes S

N and no. 1 auxiliary discharging S and no. 1 auxiliary S and no. 1 auxiliary N and nos. 2 and 3 discharging blasting auxiliary discharging S and nos. 1 and 2 S and nos. 1 and 2 N and no. 3 auxiliary discharging auxiliary blasting auxiliary discharging S and nos. 1, 2, and 3 S and nos. 1, 2, and 3 N auxiliary discharging auxiliary blasting

Table 4. Gas Compositions and Heating Value during the Course of the Pushing-Through Process Gas compositions (%) H2

CO

CH4

CO2

O2

N2


2-16

1-12

0-7

3-24

0-5

60-79

1.26-5.44

air supply was employed so as to increase the temperature of either the fire source or the fire area and to expand the range of the fire area. The experiment showed that adopting such an interchanging method could control the effective diameter of the gasification galleries within a specified range. The gas compositions and heating value of the gas produced by the forward and backward combustion are shown in Table 2. From the table, it was concluded that the gas quality of forward combustion is better than that of backward combustion. It is clear that when backward firepower seepage is switched forward, the temperature conditions can be greatly improved. The pushing-through process, gas compositions, and heating value are shown in Tables 3 and 4, respectively. 3.3. Gasification of the Small Shaftlike Gasifier and Continuous Pushing-Through to the N Hole. With the completion of the pushing-through process between the no. 1 hole and the S hole, a small shaftlike gasifier was formed. This made the next stage in the gasification experiment possible: gasification of the small shaftlike gasifier. Pushing-through between the no. 1 hole and the N hole continued at the same time as the gasification of the small shaftlike gasifier was conducted. When air was continuously pumped through the no. 1 hole, both the heating value and the gas flow from the S hole steadily rose. After 40 h of pumping, the heating value of the gas had raised from the previous 0.17 MJ/m3 to more than 3.35 MJ/m3, and the flow of the gas from the previous 60 m3/h increased to 120 m3/h. From this point on, the heating value of the gas was relatively stable, but the gas flow continued to rise until it reached the maximum 300 m3/h. Figure 3 shows the changes of the heating value of the gas over the 72-h course of the experiment. Figure 4 shows the rules of the heating value as a function of the gas flow. For shaftless underground gasification, the monitoring of the pushing-through speed (or moving speed of the fire source) and the temperature of the gasifier is crucial. During the test, a

Figure 3. Change of the gas heating value.

volume of blast (m3/h)

leakage rate (%)

pushed- through holes

318-542

96.24-91.86

no. 1-S

389-584

91.89-85.36

nos. 2-1

540-575

88.33-56.67

nos. 3-2

540

59.67-38.53

N-no. 3

radioactive radon detecting technique37 was employed to measure the moving speed of the fire source and the change of temperature, as the gasification gallery was pushed-through, the results of which are shown in Figure 5. As the firepower penetration proceeded, the effective wind volume blasted into the gasifier continued to rise, the moving velocity of the flame working face gradually increased, and the fire area approached close to the N hole. In the latter stage of the experiment, the gallery between the N hole and the S hole was linked. The change and distribution of the temperature field of the gallery is shown in Figure 6. The flowing quantity and loss rate for the underground gas during pushing-through are shown in Figure 7. With the aid of a seismic longitudinal wave transmission survey technique,34 the curve for the expansion of the diameter of the gallery was obtained, as shown in Figure 8. 3.4. Two-Phase Gasification. On the basis of the research of a model test conducted in the laboratory, the following twophase gasification technique38 was tested in the field. During the first phase, air was blasted through the inlet hole (the N hole) at a blast rate of 270-310 m3/h, producing air gas and discharging it from the S hole. After an ideal temperature field was formed in the gasifier, the second phase began. During the second phase, steam was blasted through the N hole, gradually increasing the blast volume at rates of 50, 75, 125, 200 m3/h, respectively, with corresponding intervals of 10, 20, and 40 min.

Figure 4. Relation between the gas flow and heating value.

Figure 5. Relation between the moving velocity of the fire source and temperature to the blast volume.


Yang et al.

Figure 8. Change curve for the channel diameter.

Figure 6. Change and distribution of the temperature field during pushing-through of the gasification channel.

high enough for the decomposition reaction to continue; whenever the temperature dropped to 700 °C, it was necessary to cease steam-blasting immediately and resume air-blasting through the N hole. This combination of air- and steam-blasting was repeated until the end of the experiment when all the coal was used up. The compositions and heating value of air gas in the first phase and those of water gas in the second phase are shown in Figures 9 and 10, respectively. 4. Analysis and Discussions

Figure 7. Air inflow, gas displacement, and leakage rate vs time diagram.

When the steam met the incandescent coal layers, a decomposition reaction occurred:

C + H2O(g) f CO + H2 131.5 MJ/kmol

(1)

Because reaction 1 was endothermic, the temperature of the coal seams in the gasifier gradually declined due to the continuous blasting of steam. The overall reaction speed of reaction 1 increased with the rise in temperature. When the temperature rose above 1000 °C, reaction 1 was characterized by high-speed diffusion;31,39 when the temperature was below 700 °C, on the other hand, the reaction speed was very low.31,39 From these observations, it was concluded that the temperature of the coal seams should be as high as possible during the course of air blasting in an attempt to cause the decomposition reaction during steam blasting to proceed quickly. For the same reasons, the temperature change of the coal seams during steam blasting was monitored closely to ensure that the temperature remained (37) Yang, L. H.; Yu, L.; Liang, J.; Yu, X. D.; Chen, Q. H. Radon Measurement TechnologysNew Method to Predicate and Measure High Temperature Zone of Underground Coal Gasification. Coal Sci. Technol. 2001, 29 (12), 12-15. (38) Liu, S. Q. UCG Model Test of Huating Coal with Oxygen-steam as Gasification Agent. J. Southeast UniV. 2003, 33 (3), 355-358. (39) Kou, G.; Chen, H. Z.; Wang, Z. Q. Coal Gasification Project; Gao, W. J., Ed.; Machinery Industry Press: Beijing, 1992; pp 103-107.

4.1. Gas Flow and Heating Value. Table 4 shows that there are obvious fluctuations of the various compositions and heating value of the gas during forward and backward firepower pushing-through. This is because the time for the forward and backward air supply is determined by the gas quality and there is a time lag when the method is adopted in the gasifier. When the forward air supply is reversed, there is a considerable distance between the N hole and the fire source, as well as great resistance for the air movement. So, the gas discharged from the S hole contains more retorting compositions within a short time and has a higher heating value. When the air reaches the fire source, however, the gas discharged from the S hole contains mainly air gas and has a declining heating value. Over the course of the experiment, it was observed that backward air-blasting cannot guarantee the formation of a high temperature. As a result, air gas quality is degraded. The quality of the gas can be gradually improved only when forward air-blasting is restored. Figure 3 shows that there are some fluctuations of the gas heating value. Likewise, the flow of the gas also fluctuates as it increases. This is because the continual expansion of the gasification gallery causes the irregular caving of the ash-dreg and coal seams during the gasification of high-angle coal seams,40 leads to changes in the resistance of the gasification gallery, influences the wind blasting quantity and temperature in the gasifier, and makes the gas compositions and heating value fluctuate. Figure 4 shows that the increase in the gas flow quantity is accompanied by a rise in the heating value. The reason for this is that the body of coal is above the gasification gallery, and the test is conducted in high-angle coal seams. Due to the effect of high temperature as the gasification proceeds, the body of coal expands, cracks, and continuously falls on to the bottom of the gasification gallery with the help of gravity, causing the gasification gallery gradually to move upward and the air-blasting quantity to increase. This, in turn, causes a large (40) Liu, S. Q.; Liang, J.; Yu, L. Study on Auxiliary Gasification Technology of CO2 Control in the Process of Underground Coal Gasification. Coal ConVers. 1999, 22 (4), 50-54.



Figure 9. Change curves for the air gas composition and heating value.

Figure 10. Change curves for the water gas composition and heating value.

number of gasification agents to come into contact with the fresh coal seams, provides a large specific surface needed for the effective combustion-gasification,32 intensifies the oxidizationreduction reactions on gas-solid surfaces, increases the combustible compositions, and, last, increases the heating value of the gas. From the fact that the heating value of the gas approaches stabilization with the increase of the air or gas flow in the shaftless percolation gallery, it was concluded that with an increase of gas flow there will be a corresponding increase in the heating value of the gas; however, when the gasification gallery is pushed-through by firepower percolation, the heating value of the gas approaches a stable value. 4.2. Moving Speed of Fire Source. The experiment shows that the effect of firepower penetration pushing-through depends on the moving speed of the fire source. Figure 5 shows that, during the process of fire seepage, when the volume of the blast is increased to 250 m3/h, the moving speed of the fire source almost rises linearly. When the volume of blasting air is between 300 and 500 m3/h, the moving speed of the fire source remains virtually stable. When the volume of blasting air continues to increase, the moving speed of the fire source takes on the drop tendency. The highest temperature in the pushing-through process is closely related to the volume of blasting air. The experiment shows that, when the volume of blast is between 50 and 75 m3/h, the average temperature is 800 °C. Within a specified range, with the rise in the flowing quantity of the air blasted,

the average temperature gradually increases to 1160 °C. Apparently, the rise in temperature expedites the moving speed of the fire source. However, when the volume of the blast reaches 550 m3/h, the temperature stops rising. This is because the continued increase in the heat carried away with the discharged gas hinders the further rise in the temperature (Figure 5). This research shows that the temperature in the pushingthrough process depends on the heat separated out in the combustion from the wall plane and is also determined by the heat consumed, including the heat used to preheat the gas phase passing through the gallery. At the very beginning of the process, the heat released surpasses that consumed, so the temperature in the gallery rises; as the volume of blasting air further increases, there is a balance between the heat separated out and that consumed, so the temperature in the gallery remains stable. As the volume of the blast continues to increase, the growth rate for the heat released is lower than that for the heat consumed, so the temperature declines. The experiment further shows that the moving speed of the fire source depends on the heating speed of the combustion section in front of the oxidation area, whereas the heating speed depends on the heat loaded on the surface of the nonburned section along the gallery. Obviously, the higher the temperature of the gallery, the easier the heat exchange by heat conduction and radiation between the nonburned and the oxidation areas, the smaller the heat loss rate from the surface of the gallery to the air-current, as well as the higher the speed of the fire source. 4.3. Temperature Field. According to the rationale of underground coal gasification, the temperature in the oxidation zone is normally between 950 and 1300 °C, and the temperature in the reduction zone and the dry and distillation zone usually falls between 700 and 950 °C and 300 and 700 °C, respectively. From Figure 6, it was concluded that, in the initial phase of pushing-through the gallery, the heating value of the gas was low; this is indicated by the fact that the scale of the temperature field is small; the small scale, in turn, indicates the presence of an oxidation zone and the near absence of any reduction and the dry and distillation zones. As gasification proceeded, the flame working face gradually moved forward, the length of the pushing-through gallery increased by degrees, the permeability of the coal seams rose continuously, and the combustion effect of the fire source intensified. Thus, the reduction zone and dry and distillation zone formed little by little.


Compared to the initial phase of pushing-through the gallery, the “three zones” characteristics were relatively obvious at the time when the nos. 1 and 2 holes became linked. When the pushing-through between the no. 2 hole and the no. 3 hole was completed, the combustion intensity in the fire area continued to increase, with the three zones distribution scope further expanding, such combustible compositions as H2, CO, and CH4 contents increasing, and the gas heating value rising. When the flame working face moved to the vicinity of the N hole, the pushing-through process in the gasification gallery was complete. At that time, the three zones were maximally lengthened, the conditions for producing gas were remarkably improved, and the heating value of the gas reached 5.41 MJ/m3. The temperature and distribution scope of the dry and distillation zone of the gasification gallery are obviously better when formed by means of firepower penetration than when formed by the techniques employed in underground shaft-gasification; the gas of the former enjoys a higher retort gases content. Figure 6 shows that, as the experiment proceeded, the temperature fluctuated more and more obviously; this is mainly because of the forward and backward interchange permeating combustion, the auxiliary air-blasting for each monitoring hole, and the gradual rise in the effective air blasted into the coal seams, and it is what led to the uneven combustion of the fire source in different sections. 4.4. Leakage Rate. The air-blasting leakage rate is a relatively key technical indicator for firepower-permeation pushingthrough of shaftless gasification galleries. Figure 7 shows that, during the course of the pushing-through, the leakage rate is over 88.00% at most times, with an average of 81.89%. As the firepower permeation proceeds, the leakage rate tends to decline; in the final stage of the experiment above, the leakage rate dropped dramatically; at the end of pushing-through, the leakage rate dropped to 38.53%. Corresponding to this change, in the initial and middle stages of the pushing-through, the amount of the discharged gas increased slowly, while, in the final stage of the pushing-through, as the leakage rate dropped sharply, the gas production rose remarkably, at an average rate of 93.82 m3/h. However, from Figure 7, it can be seen that, while the flow of the blasted air is kept stable, there seems to be no obvious relationship between the leakage rate and the discharging flow and volume of the blast. Judging from this, the proceeding state of the firepower pushing-through had an important effect on the leakage rate and gas production. In a strict sense, the air-blasting leakage rate depends on the airtightness of the coal-rock layers in the gasification panel. For shaftless underground gasification, the changes in the effective permeability limit the moving state of the flame working face and the leakage rate of the gasifier. The research proved that the natural permeability of the seam is 40-60 times that of the surrounding rocks.29,41 As a function of the gradual increase of the pushing-through length, the temperature in a yet non-pushed-through section rose steadily, the primary cracks, microcrevices, and micropores in the coal seams further expanded, and the permeability of the seams increased sharply. Meanwhile, as expected, the temperature had very little effect on the permeability of the surrounding rocks, so wind losses due to the rock permeability were very minimal and the volume of gas discharged and the speed of the pushingthrough process, under the same air-blast volume, increased. The above changes are especially obvious and acute in the final (41) Liu, S. Q. Environmental Benefits of Underground Coal Gasification. J. EnViron. Sci. 2002, 14 (2), 284-288.

Yang et al.

stage of firepower penetration, when the distance between the fire source and the N hole was gradually decreasing. 4.5. Gallery Diameter. As shown in Figure 8, the diameters for the gallery took on a fluctuating change, with an average of 0.40 m and a maximum of 0.57 m. In the vicinity of the N hole, nos. 3, 2, and 1 holes, and S hole, the diameters for the gallery are comparatively large, 0.45, 0.48, 0.43, 0.52, and 0.57 m, respectively. The changes in the cross section of the gallery mainly resulted from the following four causes: (1) Shortly after ignition, the pushing-through process was mainly forward combustion, with a high fire source temperature, big fire area, and fast gallery expansion; therefore, the diameter was the biggest near the ignition hole (the S hole). (2) During the course of drilling, the surrounding coal body was loosened in advance, forming new loose cracks and exposing part of the new coal seams. Moreover, given the continuous auxiliary air-blasting from the nos. 3, 2, and 1 holes during the pushing-through process, the relatively sufficient air quantity, and good combustion conditions, the expanding diameter near each drilling hole was bigger than that of other areas. (3) As backward combustion proceeded, the porosity of the coal seams increased and the permeability improved. Nevertheless, because of the strong heterogeneous anisotropism of the coal seams media, the reverse nonuniform combustion effect intensified in the backstreaming direction, which is one of the main reasons why the expanding cross section of the gallery was uneven. (4) When the forward combustion was reversed during the pushing-through process, the resistance to the air current rose sharply, which caused not only the problem of the lag for the gasification agent to reach the flame working face but also the low air-blast volume, resulted in the dwindling of the combustion intensity for the fire source and decrease in the temperature of the coal seams, and thus reduced the expanding rate of the channel. Over time, the temperature of the coal seams increased little by little. The high-temperature steam and gas produced in the pushing-through process simultaneously diffused along the surface of the surrounding media and exchanged heat, making the peripheral coal seams dry with an increasing permeability. Thus, the combustion intensity in the fire area gradually increased and improved the expanding conditions for the diameter of the gallery accordingly. It is the very intermittent rise and fall that ensures the size of the cross section of the gallery within a specified range. It may be inferred that the interchangeable forward and backward firepower-permeating combustion provides guarantees for shaftless underground coal gasification. 4.6. Gas Compositions and Heating Value. Figure 9 shows that, in the air gas, O2 content was generally less than 1%, and except O2, there were erratic fluctuations for the other compositions, in which, N2 and CO2 contents varied most erraticly, falling between 36.90% and 62.98% and 1.08% and 26.50%, respectively. The range of fluctuation of the combustible compositions was relatively low; the average H2, CO, and CH4 contents were 13.73%, 8.61%, and 5.10%, respectively. From Figure 9, it can also be seen that, corresponding to the changes in the compositions, the air gas heating value also took on a relatively large amplitude of variation, varying around the axis of 5.30 MJ/m3, with a rising trend and a maximum of 6.88 MJ/ m3. As stated above, the occurrences of the coal layers have an important influence on the gas compositions and heating value. This experiment has shown that the gasifier scale has a profound effect on the stability of the gas quality. Given the characteristics of the shaftless underground gasification process, the unevenness and irregularity of the size and shape of the cross section of the gasification gallery formed from firepower permeation resulted


in a large amount of resistance on the air current and a high range of fluctuation of the gas flow quantity, which affected the stability of the gas-solid oxidation and reduction intensity as well as the erratic fluctuation of the gas compositions and heating value. The length of the gallery is far greater during normal gasification, when it is between the N and S holes, than during gasification of the small pit gasifier, when it is between the no. 1 and S holes. Apparently, the longer the gasification gallery, the more obvious the above effect; thus, the more erratic the fluctuation of the gas compositions and heating value will be. Because the average temperature in the longer N to S gasifier was high and the oxidation, reduction, and distillation effects were strong, the average gas heating value was higher than that produced in the small pit gasifier. Figure 10 shows that there was no O2 in the water gas; N2 content ranged from 0% to 8%. At the initial phase of supplying steam, the N2 content dropped sharply, while, with the continuous blasting of steam, the drop extent for the N2 content declined. This is mainly because, N2, produced in the first phase of production and left over inside the interstices, was not completely replaced by H2O(g) in the second phase. Compared with that of the air gas, the content of combustible compositions improved a lot: H2 content remarkably rose to 50% and then approached stabilization, exhibiting a rising trend with a maximum content of 58.15%; most of the CH4 content was over 7%, with a maximum of 10.27%; most of the CO content was above 13%, with a maximum of 20.50%; the gas heating value rose to over 12.00 MJ/m3 at a sharply increasing rate of speed and, then, fluctuated around the axis of 12.00 MJ/m3, with a maximum of 13.67 MJ/m3. The reasons are attributed mainly to the fact that the steam injected in the second phase came into contact with the incandescent coal seams. The decomposition reaction between the steam and carbon occurred in the oxidation zone and reduction zone with a high temperature, producing a large quantity of H2 and CO. Because the gasification gallery was long and the cross section large, the range of the distillation action along the direction of the whole gallery expanded. The exit gas escaped due to the influence of N2 on its heating value. Therefore, H2 and CO concentrations in the gas-phase improved greatly. Meanwhile, under the catalytic effect of some metals or some metallic oxidants in the ash dreg, a certain hydrogenation of carbon and methanization occurred, resulting in the increase of the CH4 content and further improving the gas heating value. 5. Conclusions (1) This research shows that it is feasible to conduct shaftless underground gasification in thin high-angle coal seams and that


the adoption of the forward and backward firepower-penetration combustion pushing-through techniques described in this paper will ensure a highly manageable pushing-through process. Under the experimental conditions, when the sum total of volume percentages for CO, CO2, and CH4 in the gas was more than 18%, backward combustion was employed; otherwise, forward combustion was employed. (2) According to the results of the experiment, within a specified range, with the increase in the volume of the blast, the moving speed of the fire sources will increase, with the average temperature in the gallery gradually rising. However, when the volume of the blast exceeds 500 m3/h, the moving speed of the fire source (pushing-through speed) will tend to decline; as the volume of the blast further increases to 550 m3/ h, the process temperature will no longer rise. (3) The experiment data indicate that under the experimental conditions, the average leakage rate for the air-blasting is 81.89% with the average diameter for the gallery being 0.4 m. As firepower permeation proceeds, the leakage rate decreases in a stepwise way. In the final stage of the experiment, both the degree of decline of the leakage rate and the discharge volume of the gas increase drastically, an increase which puts the mean value of the discharge volume at 93.82 m3/h; the diameter for the gallery fluctuates. In the vicinity of each borehole, the diameter for the gallery is large. (4) The heating value for the air gas produced in the gasifier rises with the increase in the gas flowing quantity and approaches stabilization. Under the experimental conditions, the two-phase gasification produces underground water gas with a medium-high heating value, in which H2 is over 48% and CO and CH4 are over 13% and 7%, respectively, with an average heating value of 12.00 MJ/m3. Acknowledgment. This work was supported by the National Natural Science Foundations of China (Ratification Nos.: 59906014, 50276066, 20207014, and 50674084). The technical contributions of professor Guo Chuwen and Dr. Yang Guoyong are gratefully acknowledged by the authors. The authors also gratefully thank professor Zhang Weilian and Wang Jialian for helpful discussion. Support received for this research from the Engineering Research Centre (ERC) for Clean Coal Technology of the China University of Mining and Technology (CUMT) is gratefully acknowledged. We would also like to thank Brandon Conlon for editing the English version of the paper. EF700231P

Experimental Study of Shaftless Underground ... - ACS Publications

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