The morphological expansion of the gasification cavity is essentially the burning and spalling of the coal surface. Different dimensions reflect different combustion and spalling rates in different parts of an irregular cavity. The temperature field is the most intuitive element to use to study the cavity. During the test, the overburden pressure and lateral pressure remain constant, and the mechanical constraints are fixed. Coal seams are sufficiently dense to prevent the reaction gases from communicating with the outside atmosphere. The influence of external temperature is negligible because the coal seam is covered with thermal insulation material.
3.1. Morphological Evolution of Gasification Cavity
Gasification cavity shape is monitored in real time by collecting temperature data from sensors. The results are analyzed by the visual processing of temperature data at 20 min intervals. Temperature values and their corresponding coordinates are matched and plotted. By generating a two-dimensional temperature field, the cross-sectional structure of the cavity is reconstructed.
An approximate relationship between the temperature and the physical boundary is established by comparing the shape of the foam blocks with the temperature field diagrams at the appropriate time. Therefore, the internal cavity shape diagram is obtained by calculating the temperature field (see
Figure 7).
In accordance with the shape diagram of the cross-section, the high-temperature zone above the ignition point expands rapidly. An arc-shaped gasification cavity is formed. The high-temperature zone begins to expand downstream of the gas flow when the arc reaches a certain height. This condition continues for a certain period of time, and a water drop shape is formed, which gradually shrinks from the ignition point to the outlet. In the late stage of cavity formation, the air flow joints appear in the upper part of the coal seam layer so that the high-temperature zone expands into the layer. Because the bottom of the coal seam is mudstone, the similar material is not combustible and the gasification cavity can rarely expand downward.
By observing the cross-sectional shape at different stages, the evolution law of the cavity shape during the whole process is analyzed (see
Figure 8). The evolution law is discussed in three aspects, including expansion around the ignition point, gas flow channel enlargement and the upward evolution to the roof.
The internal space is limited in the early stage of the gasification process. A large amount of gasification agent enters the space in the form of gas. The coal material is ignited by the high temperature. The cavity expands slowly due to the narrow flow channel and limited gas discharge. The horizontal area of the cavity increases gradually. For a long time, the cavity body near the ignition point expands hemispherically. This hemispherical part dominates most of the volume expansion of the cavity and is the site of the gasification reaction.
In the next stage, the high-temperature gas migrates along the flow channel to the gas outlet point. The channel size expands to form an obvious dominant channel. At this point, the asymmetric distribution around the ignition point appears, and the space near the outlet point increases. As the reaction continues, the channel becomes conical, and the entire cavity begins to resemble a water droplet. The conical slanted plane connects the top of the cavity and the gas outlet point under the action of the high-temperature gas flow.
The expansion rate slows down due to the distance from the ignition point after the top of the cavity reaches a certain height. High-temperature gas penetrates the coal seam along the horizontal bedding due to the layer separation of the upper coal seam material. The temperature of the upper part increases sharply in the horizontal direction in the case of height limitation. Moreover, the stagnation time of the high-temperature atmosphere near the top of the cavity is longer, and the separation becomes considerable. Therefore, due to the change in the thermal-physical properties of the upper coal seam layers in the later stage of cavity development, large-scale caving damage may occur.
3.2. Distribution of Temperature Field
There is a dynamic equilibrium between the temperature field distribution in the gasification cavity and the surrounding rock. The temperature field is significantly affected when the test system is disturbed or the gasification agent injection conditions are changed. The internal gas flow field is also an important factor affecting the temperature field. Therefore, the adjustment of the injection condition is made gradually after the system is stable. The disturbance of a sudden change in the injection condition must be avoided.
During the cavity generation process, a stable gasification agent flow is maintained to stabilize the temperature. The temperature value from each sensor is loaded into the processing terminal. The monitoring of Path A is used to analyze the temperature changes in the vertical direction inside the cavity. Both Path B and Path C are used to study the temperature changes in the horizontal direction but at different heights (see in
Figure 4). In this paper, the temperature changes along vertical Path A (points TP7, TP26, TP40 and TP47), horizontal Path B (points TP1, TP2, TP3, TP4, TP5, TP6, TP7, TP8 and TP9) and horizontal Path C (points TP22, TP23, TP24, TP25, TP26 and TP27) at different times are analyzed.
The temperature gradually decreases in a vertical direction from the bottom to the top of the cavity, as shown in
Figure 9. Between the inside of the cavity and the surrounding material, there is a remarkable temperature difference. The highest temperature occurs at the bottom of the cavity near the ignition point. The top of the cavity expands successively through points TP7, TP26 and TP40 above the ignition point. The temperature increases rapidly after the sensor of the corresponding point is exposed into the cavity, and the maximum temperature monitored in the cavity is approximately 1150 °C.
As shown in
Figure 10, except for the ignition point, the temperature along Path B in the cavity changes relatively little in the horizontal direction. The temperature of the points along the flow direction increases sequentially because the gas flow pushes the heat toward the outlet point. The horizontal temperature gradient is small.
Path B is near the open gas flow channel; Path C is above Path B. Compared to those along Path B, points along Path C have lower temperatures. The temperature at the higher position is influenced by the irregular boundary of the cavity, and there is also an obvious difference in temperature between the inside of the cavity and the surrounding material (see
Figure 11).
Points TP5 and TP40 (see
Figure 5,
Figure 9 and
Figure 10) are selected for comparison. Both points are located at a distance of 10 cm from the ignition point. TP5 is located downstream of the gas stream and TP40 is located directly above the ignition point. The temperature of the two points increases simultaneously at 420 min of test time. The temperature of TP5 reaches the peak of about 250 °C after the test lasts about 500 min, and the cavity extends to the TP5 position. Since TP5 is close to the gas passage, the high-temperature gas is not retained in the cavity for a long time. The gas generated by the gasification and combustion is quickly discharged through the gas channel with an excessive amount of the gasification agent. As a result, the temperature of TP5 drops to a lower value of about 200 °C. On the other hand, TP40 is far from the outlet point, the residence time of the high-temperature gas is longer, and the temperature continues to rise to more than 400 °C until the end of the test. After the high-temperature region reaches the coal seam roof, the TP47 position set in the roof material also heats up simultaneously. In the late stage of cavity formation, the temperature of the points around the gas injection site is relatively higher, whereas the temperature of the areas toward the outlet point is severe.
In addition, the temperature field at the top of the cavity expands in a wide range after the test duration reaches 600 min (see
Figure 7d). However, the temperature in the cavity decreases significantly. It can be concluded that a large amount of heat is diffusing into the separated layers of the coal seam and the exposed roofing.
3.3. Genetic Analysis
The expansion of the cavity is mainly due to the combination of coal combustion and spalling, according to the analysis of the evolution of the cavity and the distribution of the temperature field.
The first aspect of the combustion refers to the combustion and gasification reaction that takes place on the coal wall: the consumption of the coal body leads to the enlargement of the cavity. This occurs especially in the early stages, when the cavity size is small and the inner surfaces are close to the ignition point. The second aspect of spalling refers to the high temperature acting on the coal wall, which weakens the physical and mechanical properties of coal and rock materials. Coal fragments are separated from the massive coal block under the conditions of self-weight and the overburden pressure. The spalling effect is the main factor of the cavity development, and the cavity volume increases continuously. As the roof is increasingly exposed, the top coal spalling phenomenon becomes more pronounced. The combustion and gasification reactions continue as the small fragments fall into the cavity. The reactions take place at the bottom, close to the ignition point.
In the hemispherical cavity state, the high-temperature gas slowly flows to the outlet point, causing the additional erosion of the coal wall downstream. The upper arc of the cavity extends gradually to the outlet, and the radiant is gently tapered to a sloping surface. As more gasification agent is injected, the horizontal consumption becomes more significant, and the cavity axial length becomes longer.
The volume in the cavity increases rapidly when the dominant gas channel between the injection and the outlet points is wide enough. The gasification agent reacts sufficiently and flows out through the conical channel at a rapid rate. A part of the heat is taken away by the gas flow and the maximum temperature is reduced. Both lateral and upward expansions are observed while forming the conical sloped surface of the cavity. The vertical and horizontal temperature gradients inside the cavity are determined by gas flow conditions and affect the final cavity shape. As shown in
Figure 12, the final cavity is the combined result of the temperature field, flow field and stress field in the coal seam.
The analysis of the temperature distribution shows large temperature gradients in the vertical direction, as shown in
Figure 9. After the initial ignition, the temperature at the bottom of the chamber is much higher than the temperature in the top of the cavity and in the upper layer, and the expansion is slow. In the horizontal direction, the amount of high-temperature gas increases slowly along the direction of gas flow, and the expansion rate also increases accordingly so that the position of the strongest expansion moves toward the outlet point.
The final water drop shape is analyzed based on the foam reconstructed cavity model. Different injection flow rates are tested using pure oxygen as the gasification agent. As shown in
Table 1, the cavity size is significantly affected. Increasing the injection flow rate helps to expand the cavity space under the same conditions.
Maintaining a constant gasification agent injection results in a smaller change rate of the height and a smaller slope of the cavity shape as the cavity expands; the inner surface area becomes larger, and the heat required for expansion becomes greater. When a certain volume is reached, the constant injection can no longer support the continuous expansion of the cavity. The shape of the cavity is basically stable and does not change any more.
Using a fixed injection flow rate of 2.4 L/min, gasification agents with different oxygen proportions are tested. The cavity shape is obtained, as shown in
Figure 13 and
Table 2. In the cross-section of the axial view, the cavity is centered on the prefabricated gas flow channel, which means that the empty space in the coal seam material expands in the radial direction. The initial sectional shape of the cavity is roughly semicircular. In the later stage, due to the severe spalling of coal material, the vertical height
H1 in the gas ignition point cross-section is greater than the transverse width
W1.
The cavity space around the ignition point is significantly larger than the cavity space around the gas outlet point. Higher oxygen content results in a greater size in all dimensions. The gasification agent has the highest concentration at the gas injection point so the reaction is intense and the heat release is the largest. Therefore, in the process of cavity development control, reaction and expansion states around the ignition point should be adjusted to cover more coal seam area.
The development of the second cavity is more complicated due to the existence of a full-size cavity in the downstream direction when the gas injection point is retracted to the second ignition point. During the test, the gasification agent flow rate and the oxygen content are adjusted to control the temperature fields in the cavity and surrounding materials. This aims to maximize the high-temperature zone, increase gasification range and improve the exploitation rate of the coal seams.
The gasification agent is maintained in contact with the fresh coal wall by continuously moving the position of the gas injection point during the second cavity generation test. As the violent reaction continues and the temperature rises, the cavity expands to form a semi-elliptical cavity, as shown in
Figure 14. The cavity height does not reach the roof, but more coal seam material is involved in the reaction.