Characteristics of Pyrolysis and Low Oxygen Combustion of Long Flame Coal and Reburning of Residues

To further understand the problems of coal combustion and coalfield fire reignition, this paper researched the reaction characteristics of coal pyrolysis and low oxygen combustion and the reburning oxidation characteristics of residual structure by thermal analysis methods. The results show that temperature promotes both pyrolysis and low oxygen combustion reactions, but low oxygen combustion reaction is more sensitive to temperature changes. As the constant temperature rises, the mass reduction rate of low oxygen combustion of coal samples reaches 80% on average, which is 4 times that of pyrolysis, and the variations of thermogravimetric parameters are also significantly higher than those of pyrolysis. However, the higher the pyrolysis degree of the residues, the stronger their oxidizability, which greatly enhances the intensity and concentration of the secondary combustion, and the mass of residues is reduced by 90% on average. Conversely, because the combustible components are continuously consumed during low oxygen combustion, the reburning characteristics of residues become less obvious. For instance, the weight loss rate slows down, the burning becomes dispersed, and the burning intensity is weakened. In addition, the heat release is reduced from 8662 to 444.5 J/g, and the change trend is just opposite to that of pyrolysis. The above results show that as the constant temperature rises, the pyrolysis reaction greatly shortens the reburning process, while the low oxygen combustion reaction largely inhibits the reburning.


Introduction
In the world energy consumption structure, coal resources occupy the dominant position. The total reserve of low-rank bituminous coal resources accounts for about 51.23% in China, but spontaneous combustion and fire zone reignition are easy to occur in the mining process of low-rank coal. Currently, the coal combustion can be identified by infrared temperature measurement and index gas analysis [1], but the cause of reburning in coal fire area has not been found. In essence, the reburning characteristics of coal are still unclear. In particular, the reburning characteristics of long flame coal should be focused on. Long flame coal belongs to bituminous coal with high volatile matter and low rank. Moreover, it has developed pores, strong gas adsorption and strong spontaneous combustion tendency [2][3][4][5]. In addition, the combustion of long flame coal mainly occurs in hidden areas such as goaf, and the oxidation process is slow under low oxygen concentration. It is difficult to discern the fire source in the early stage and control the fire area to further expand. The incomplete treatment of the fire area easily induces the secondary combustion of coal samples. After coal samples underwent pyrolysis or low oxygen combustion reaction, their internal structure and composition were changed, and a series of combustible substances were generated. This affected the tendency of re-combustion and changed the process of secondary combustion. Thus, this not only causes a large amount of resource wastes, but also induces extremely destructive gas explosion disasters [6][7][8][9][10]. Considering the resource utilization [11,12] and disaster prevention, it is necessary to study the residual structure characteristics and reignition characteristics of pyrolysis and low oxygen combustion of long flame coal at different temperatures.
At present, the research on coal's spontaneous combustion mechanism and its reaction characteristics is relatively mature [13][14][15][16], and the research of the reignition characteristics of coalfield fire areas mainly focuses on the basic macro-experiments of reignition conditions and indicators [17][18][19][20][21][22]. However, the ignition sources of reignition in fire zone mainly come from pyrolysis or low oxygen combustion reaction, but related researches mainly discuss the internal structure change law of coal pyrolysis or low oxygen combustion process affected by different gas environments [23][24][25]. The explorations on the characteristics of pyrolysis and low oxygen combustion residues and reburning oxidation characteristics are lacking. In addition, in many previous studies, it was found that macroscopic analysis of thermal analysis technology [26][27][28][29][30] has become the main research method of coal pyrolysis and combustion. Thermal analysis methods can repeatedly reflect the whole process of coal pyrolysis and combustion, accurately record experimental data, and quantitatively characterize experimental results. Wang [31] studied non-isothermal and isothermal combustion tests of four coal chars in a thermogravimetric analysis (TGA) device, and the results showed that the combustion characteristics of bituminous coal char was better than that of anthracite char, and both increase of heating rate and increase of combustion temperature could obviously improve combustion characteristics of coal char. It illustrated that the lower the coal rank, the more prominent the combustion characteristics, so the research on the characteristics of long flame coal was very representative. Zhu [32] analyzed the effects of sample quality and heating rate on the coal combustion process through thermogravimetric experiments. This provided an important reference for choosing an appropriate heating rate for the thermogravimetric experiment. Wei [33] used thermal analysis technology, and found that coal particle size refinement enhanced the ignition, burnout, and maximum combustibility of coals. Liu [34] analyzed the effect of methane on the coal-oxygen reaction and found that methane as an inert gas delayed the combustion reaction. Arenillas [35] combined thermogravimetric analysis and mass spectrometry to analyze the pyrolysis characteristics of six kinds of perhydrate coal with H/C ratio between 0.83 and 1.07. Chen [36,37] tested the ignition point kinetic parameters of 12 different coals and analyzed their combustion performance. At the same time, the influence of coal rank and pulverized coal particle size on the ignition mechanism of pulverized coal was studied based on the ignition point temperature. Liu [38,39] tested the TG and DTG curves of bituminous coal and anthracite coal under the experimental conditions of 3.3-21% oxygen concentration and analyzed the influence of different oxygen concentrations on the combustion performance of coal samples based on the variation characteristics of ignition temperature, burnout temperature, and kinetic parameters. It was worth noting that the article clarified that 5% oxygen concentration was the highest threshold for low oxygen combustion. Ma [40] compared the consumption rates of oxidation and pyrolysis by using TGA data under a lean oxygen gradient of 0-21% and established an oxidation-pyrolysis competition mechanism for low-rank coal. Based on the above research, the thermal analysis method can well-characterize the coal pyrolysis and combustion process. Besides, the experimental conditions which were designed provide a reference for related combustion research, and the defined related characteristic parameters can also reflect the characteristics of the combustion reaction.
To sum up, the existing research focuses more on the combustion characteristics and reignition phenomenon of coal with different coal ranks, oxygen concentrations, and temperatures [41]. The lack of research on the characteristics of coal pyrolysis and low oxygen combustion residues, and the ambiguity of influence of pyrolysis and combustion reactions on the reburning of residues, make it difficult to restrict the further expansion of the coal field fire area. Therefore, this paper took typical low-rank bituminous coal, long flame coal, as the research object to carry out pyrolysis and low oxygen combustion experiments in different constant temperatures. The macroscopic quality characteristics of products of coal pyrolysis and low oxygen combustion were analyzed by thermal analysis. Furthermore, to reveal the reignition process and ignition tendency characteristics of lowrank bituminous coal in fire zone, the reignition oxidation experiments of coal pyrolysis and low oxygen combustion residues were also carried out by thermal analysis. The main purpose is to deepen the understanding of the reburning characteristics of pyrolysis and low oxygen combustion residues at different constant temperatures, and to guide the prevention and control of long flame coal reburning and the safe and efficient mining.

Coal Samples
The bituminous coal is a main energy source because of its large proportion and wide distribution in China. However, bituminous coal leads far too easily to spontaneous combustion of coals and reignition of coal fire. Long flame coal, which belongs to bituminous coal with the lowest metamorphism, has a high tendency of spontaneous combustion. Therefore, this paper took the long flame coal from the LiuHuangGou coalfield (LHG) in Xinjiang province as experimental coal samples. Before the experiment, the impurities on the surface of the coal samples were polished and peeled off, and then the inner core of the coal samples were ground into particles with particle size of 0.038-0.074 mm. To eliminate the influence of moisture, coal samples were placed in a DZF-700 high-temperature vacuum-drying oven. The results of proximate and ultimate analysis of coal samples are shown in Table 1.

Thermal Analysis Experiment
Thermal analysis technology can reflect the characteristics of material weight and heat flow at different temperatures in the pyrolysis and combustion process of long flame coal. They mainly include thermogravimetric analysis (TG), differential thermal analysis (DTG), and differential scanning calorimetry (DSC). In this paper, the TA-Q600 synchronous thermal analyzer, equipped with a AST10 series mass flow controller with extremely high precision and a MF-4 dynamic stable gas distribution device, was selected as the experimental instrument.
The thermal analysis experiment mainly included two parts: constant temperature experiment of pyrolysis and low oxygen combustion of coal samples and temperature programmed reburning experiment of residues. The total flow rate of reaction gas was always kept at 100 mL/min. The proportion of mixed gas was adjusted by the mass flow controller under different experimental conditions. Coal samples' pyrolysis and low oxygen combustion reaction were carried out respectively under pure nitrogen and mixed gas environment (5% O 2 concentration and 95% N 2 concentration). Each group of 10 mg coal samples was heated respectively to 350, 400, 450, and 500 • C at the heating rate of 10 • C /min. Long flame coal will undergo pyrolysis reaction in the temperature range of 350-500 • C, and thermal condensation will occur if the temperature exceeds this range. In order to determine the changes in thermogravimetric characteristics at each stage of pyrolysis and low oxygen combustion, all reactions were kept at constant temperatures until the mass was nearly constant. Then, the residues of pyrolysis and combustion at each constant temperature condition were cooled to 30 • C for reburning oxidation experiments. In a dry air environment, each group of residues was heated to 800 • C at the heating rate of 5 • C/min, and change characteristics of the mass and heat release of the residues were analyzed during the reburning oxidation process. Under the same oxidation experimental conditions, a group of blank control experiments of raw coal was carried out. The experiment program is shown in Table 2.

Characteristics of Coal Pyrolysis and Low Oxygen Combustion
Restricted areas such as goafs are usually in an oxygen-depleted state in coal mines. In this section, to analyze the thermogravimetric characteristics of coal samples in different reaction processes, the experiments of pyrolysis and low oxygen combustion were carried out at different constant temperatures. It is beneficial to further study the oxidation characteristics of residues.
It is found from Figure 1 that the overall mass of the coal sample gradually decreases under the same constant temperature conditions, as the pyrolysis and low oxygen combustion reactions proceed. The coal pyrolysis process has two rapid weight loss phases, which have experienced dry dehydration and rapid pyrolysis phases, respectively. Subsequently, the mass of the coal sample tends to be stable, and the internal structural reaction reaches an equilibrium state. Under the same constant temperature conditions, the first mass decline time of low oxygen combustion is later than pyrolysis', and the change range of the thermogravimetric curve of low oxygen combustion is smaller. Although oxygen consumption is greater than oxygen absorption during combustion, the effect of weight gain by oxygen absorption obviously delays the mass decline process of coal samples. With the steady heating of constant temperature, the combustion process is gradually accelerated, volatile matter is rapidly precipitated, and the secondary decline is obviously increased. In addition, the change trend of pyrolysis mass of coal samples is similar under different constant temperature conditions. As the temperatures rises, the pyrolysis reaction energy increases, and the macromolecular structure is gradually activated for reaction. This leads to an increase in the ratio of volatilization analysis. When the constant temperature is increased from 350 to 500 • C, the remaining mass of pyrolysis drops from 86.59% to 70.92%. Moreover, the increase of constant temperatures activates the low oxygen combustion reaction to a greater extent, so the mass of coal samples decreases greatly. Although the existence of oxygen is not the decisive factor for the change of thermogravimetric characteristics, oxygen increases the change range of the mass in low oxygen combustion reaction under the same high-temperature conditions. Additionally, the residual mass is far less than pyrolysis'. When the constant temperature reaches 450 • C, the consumption of combustible substances increases, and the mass of the coal samples finally decreases by 80% in low oxygen combustion reaction, because the interior of coal samples is changed greatly, and the reactants increase. In order to better characterize the experimental process and quantitatively describe the pyrolysis and low oxygen combustion characteristics, relevant characteristic parameters were used for characterization [42,43]. The relevant parameters include: initial separation residue of volatile components weight, Wv%, indicating the mass fraction of combustible substances in coal samples. Since the pyrolysis and low oxygen combustion reactions are less affected by temperature in the early stage, the experiment result shows that Wv is 87.93% and 87.68%, respectively. Residual value of oxidation end coal sample is Wo%, that is, the residual mass of the raw coal at the end of the temperature program oxidation. Because it is only related to coal type, the value of Wo is always 7.859. In addition, constant temperature starting point of coal sample residual value, Wc%, and residual weight value at the end of pyrolysis or low oxygen combustion, Wp%, are also significant characteristic parameters, and their values are as shown in Figure 2. It can be seen that the low oxygen combustion is more obviously affected under the same temperature condition. Besides, the coal samples start to enter the combustion stage earlier than the pyrolysis reaction with the promotion of oxygen. After the coal samples are burned, a large number of macromolecules participate in the reaction, and the reaction intensity is obviously higher than that of pyrolysis. When the constant temperature reaches 450 • C, the residual mass of coal samples in the process of low oxygen combustion is much lower than pyrolysis', and the internal components are almost non-combustible. Based on the analysis of the above results, to more concretely describe the reaction details of pyrolysis and low oxygen combustion, it is necessary to further calculate the variation values of characteristic parameters. Residual weight difference of pyrolysis or low oxygen combustion is ∆Wvp%, and ∆W vp = W v − W p , which indicates the change in residual weight of the coal sample that has not entered the constant temperature period, that is, the loss of volatile matter. Residual weight difference at constant temperature is ∆Wcp%, and ∆W cp = W c − W p , which represents the mass loss of coal samples during constant temperature pyrolysis or low oxygen combustion. The results of ∆Wvp and ∆Wcp are shown in Figure 3.
As can be seen from Figure 3, the pyrolysis residual weight difference, ∆Wvp, is lower than that of low oxygen combustion reaction at the same temperature. Under the combined action of temperature and oxygen, the low oxygen combustion reaction promotes great changes in the internal structure of coal samples, that is, intermolecular chemical bonds break and recombine constantly, and the rate of volatilization analysis rises continuously. As the temperature increases, the residual weight difference curves of pyrolysis and low oxygen combustion both show an upward trend as a whole. In the early stage of thermal decomposition, the increasing temperature promotes the pyrolysis, and the generated amount of small molecular structure increases rapidly. In the late stage of thermal decomposition, coal colloids generate semi-coke, so the growth rate of pyrolysis residual weight difference gradually becomes slow. As the temperature rises, the change of the residual weight difference increases obviously in low oxygen combustion, far exceeding the pyrolysis'. In addition, after entering the constant temperature stage, the residual weight difference, ∆Wcp, of the low oxygen combustion is also much larger than the pyrolysis under the same constant temperature condition, which indicates that the combustion intensity is still higher than the pyrolysis. With the increase of constant temperature, values of ∆Wcp of the two reactions both appear in a similar trend of first increasing and then decreasing. The ∆Wcp of pyrolysis has a peak value at 450 • C, indicating that the number of molecular structures reaches the maximum at that temperature, and vast decomposable macromolecular structures are massively consumed. However, the combustion reaction is more concentrated, and the constant temperature residual weight difference, ∆Wcp, is relatively advanced, which changes at 400 • C at the earliest. Constant temperature at 450 • C is the turning point of coal sample combustion characteristics. The internal structure of coal sample is rapidly transformed, and many small molecular structures are involved in combustion, which consumes a good deal of combustible components. After 500 • C, the residual structure of the coal sample is in burnout state, and almost contains no combustible volatile components.  It not only is an important characterization parameter of coal residue characteristics, but also a key index of reburning. Due to the different effects of oxygen and temperature, there are some differences in the change trend of the ∆Wpo between pyrolysis and low oxygen combustion residues. Under the same temperature condition, because long flame coal has long branch chains, which is easy to generate tar after thermal decomposition and then remain in the coal body, the ∆Wpo value of pyrolysis is larger. Due to the oxidation of oxygen, the internal structure of small molecules is rapidly transformed and continuously consumed in low oxygen combustion. In addition, temperature can promote both pyrolysis and low oxygen combustion. With the rising constant temperature, the macromolecular structure participating in the reaction increases, and the precipitation of volatile matter increases, which lead to the decrease of coal sample mass. Both pyrolysis and combustion reactions show obvious turning points at around 450 • C. In the later stage of pyrolysis reaction, the coal colloid began to decompose rapidly and solidified into coke at the same time, and the degree of volatilization analysis decreased slightly, so the degree of residual weight reduction of coal samples was relatively reduced. However, in the combustion reaction, oxygen greatly promotes the oxidation process, the combustion reaction intensity increases continuously and volatiles and fixed carbon and other substances burn rapidly, consuming plentiful combustible components. After the constant temperature reaches 450 • C, the combustible components of low oxygen combustion residues are only about 10%, while that of pyrolysis is still higher than 70% in the same period. Therefore, low oxygen combustion makes great changes in the internal structure of coal samples, and the rapid loss of reactants also causes the combustion reaction to slow down or even extinguish. It can be inferred that the extremely few combustible components are difficult to maintain reburning.

Thermogravimetric Characteristics of Residues
It can be seen that the temperature directly determines the number and types of reaction components in coal samples from the previous experimental results. The composition and structure of the products of pyrolysis and low oxygen combustion reactions are different at various constant temperatures. Therefore, the different residues show different oxidation characteristics after contacting and reacting with oxygen. Different residues were heated and oxidized for thermogravimetric analysis by the synchronous thermal analyzer. Figures 5 and 6 show the TG-DTG results of pyrolysis and low oxygen combustion residues at different constant temperatures. As shown in Figure 5, the reburning thermogravimetric curves of pyrolysis and low oxygen combustion residues show obvious differences under the same constant temperature condition. With the increase of oxidation temperature, both reactions have experienced three stages: steady development, rapid decline, and second stabilization, but the mass decline rate of pyrolysis residues is obviously greater than that of low oxygen combustion. When it starts to enter the oxidation stage, the mass of pyrolysis residues shows an upward trend. Although the activity of the group has not been activated, there are obvious phenomenon about dry degassing and weight gain by oxygen absorption. The low oxygen combustion residues do not appear to have increasing weight, because when the residues undergo re-oxidation reaction, the effect of oxygen consumption exceeds that of oxygen absorption. At a constant temperature of 350-400 • C, the low oxygen combustion reaction is close to the pyrolysis reaction, so the thermogravimetric curves of the residues almost coincide with that of the pyrolysis residues during re-oxidation. When the constant temperature reaches 450 • C, the first low oxygen combustion reaction consumes a large amount of macromolecular structure, resulting in fewer residual combustible components. In order to better clarify this phenomenon, as can be seen from Figure 6, the peak value of the DTG curve of the low oxygen combustion residues gradually decreases and the peak width gradually increases. This explains that the secondary combustion is becoming more and more dispersed, the combustion is gradually slowing down, the combustion intensity is greatly reduced, and the stage of rapid decline of residue mass is no longer obvious. The mass change of pyrolysis residues at constant temperature at 500 • C is only about 20%. This is because combustible components account for a relatively small proportion of residues, and the mass change of residues at low oxygen combustion is mainly the decrease of combustible components. The pyrolysis residues experienced different constant temperature conditions in the early stage. As the pyrolysis temperature increased, the reaction energy continually increased, and the macromolecular structures in coal sample were activated, resulting in the increasing of volatilization analysis. However, the combustion concentration intensity of pyrolysis residues at each constant temperature changes little, the peak value and peak width of DTG curve fluctuate within a certain range, and the residual weight value is finally between 3% and 10%. In order to further explore the influence of coal pyrolysis and low oxygen combustion on the oxidation process, the evolution law of oxidation characteristics was quantitatively characterized in the reaction process of residual structure. In this section, oxidation characteristic parameters are used to quantitatively analyze the thermogravimetric characteristics of reignition oxidation of residues generated by pyrolysis and combustion at different temperatures. Figure 7 reflects the change trend of the ignition point and burnout temperature of pyrolysis and low oxygen combustion residues. The ignition temperature, T i , and the burnout temperature, T h , respectively represent the initial temperature at which the coal sample residue enters the violent oxidation and the end temperature of the oxidation reaction. The results show that compared with raw coal, pyrolysis and low oxygen combustion residues enter the intense oxidation stage earlier at lower oxidation temperatures. Due to the long chain of long flame coal, the pyrolysis reaction breaks the weak chemical bonds, and the residual amount of small molecular structure is increasing. This is why the ignition point temperature of the coal sample is greatly advanced. Especially, when the pyrolysis temperature reaches 450 • C, the ignition temperature even drops to 265.56 • C during oxidation. Furthermore, the pyrolysis reaction makes the residues no longer undergo drying and dehydration, and sufficient combustible substances ensure that the oxidation reaction is fully carried out, which makes the burnout temperature of the pyrolysis residues slightly increase. However, the low oxygen combustion reaction greatly delays the re-oxidation process of the residues. Compared with raw coal and pyrolysis residues, the burnout temperature of the low oxygen combustion residues is increased by at least 140 • C. As the constant temperature is higher, the combustible components of the residues are less, and the reburning phenomenon becomes less obvious.  Figure 8 reflects the change trend of the maximum speed of weightlessness, d Wmax = (dw/dT) max %/ • C, and maximum weightlessness temperature, T Wmax • C, during the oxidation reaction process. These characteristic parameters further characterize the combustion intensity of different residues. Due to the large difference between pyrolysis and low oxygen combustion reactions, the residues exhibit different characteristics when they lose weight during oxidation. The maximum speed of weightlessness of the pyrolysis residue is higher than that of the raw coal, and the speed has been increasing with the rising of the constant temperature, which indicates that the pyrolysis reaction facilitates the oxidation reaction of the residues. On the contrary, the maximum speed of weightlessness of the low oxygen combustion residues is continuously decreasing, and the intensity of the reoxidation reaction is far less than that of the low oxygen combustion. When the pyrolysis residues of different constant temperature are reburned, the maximum weightlessness temperature is stable at about 450 • C, while the T Wmax of the low oxygen combustion residues have been increasing, and the maximum value even reaches 619 • C. In addition, it is worth noting that when coal samples are combusted at a constant temperature of 350 • C, the combustion reaction is close to pyrolysis due to the weak impact of the low temperature, which has a similar promotion effect on the oxidation reaction of residues. Combustion half peak width, ∆T 1/2 • C, characterizes the concentration degree of coal sample combustion. The smaller the value, the more concentrated the coal sample combustion is. As shown in Figure 9, with the increasing constant temperature, the combustion half peak width of the pyrolysis residues gradually becomes smaller, from 39.63 to 34.78. This indicates that the higher the degree of coal pyrolysis, the more concentrated the oxidation combustion. The combustion half peak width, ∆T 1/2 , of the low oxygen combustion residues shows the trend of first increasing and then decreasing. Low oxygen combustion mainly carries out physical reactions such as dehydration and drying at lower temperature. Therefore, when it is combusted again, the residue of low oxygen combustion at 350 • C is more concentrated, and the combustion half peak width is lower than that of raw coal and pyrolysis residue. With the continuous increase of temperature, the reaction accelerates rapidly under the combined action of temperature and oxygen, and a large number of small molecular structures are consumed, which leads to the dispersion of the reaction when the residue burns again. Although the combustion half-peak width of the residues decreases at 500 • C, the combustible components of the residues account for a relatively small proportion, and the combustion reaction only exists for a short time and even extinguishes quickly.

Heat Release Law of Residual Reignition Oxidation
Due to the complex and changeable structure of coal, the internal structure of the residue is not the same, and the law of heat released by the breakage of the chemical bonds between molecules is also different. In this section, the law of heat release of the residual structure at different temperatures is analyzed during the oxidative combustion process.
As shown in the DSC curve of oxidation of residues in Figure 10, the heat release of residues from pyrolysis and low-oxygen combustion is significantly different under different constant temperature conditions. At the constant temperature of 350 • C, the change trend of the DSC curve of the low oxygen combustion residue is similar to that of pyrolysis. Besides, the exothermic peak is higher, and the peak width is smaller at that temperature, and this indicates that the combustion intensity is the highest and the combustion is most concentrated. The pyrolysis reaction has no obvious effect on the initial exothermic temperature of coal residues, and the exothermic temperature is about 400 • C. As the constant temperature rises, the overall change trend of the DSC curve of the pyrolysis residues is similar, but the peak value increases, and the peak width continues to shrink. The DSC curve change trend of the low oxygen combustion residues shows obvious differences. The turning zone of low oxygen combustion characteristics is 400-450 • C, and then the peak value of the exothermic curve begins to become smooth and shows a linear downward trend. This means that the combustible components of residues are almost consumed, and the combustion reaction is weak.
As can be seen from Figure 11, the heat release of residues of pyrolysis and low oxygen combustion shows opposite trends. Under the action of temperatures, the pyrolysis reaction causes the weak chemical bonds to break in the coal structure, which increases the aromaticity of the skeleton and accumulates more combustible components. Hence, this lays a material foundation for the reburning of pyrolysis residues. Through the oxidation heating experiment, the heat release of the pyrolysis residue shows a gradually increasing trend. This is because the pyrolysis residue structure is further polymerized by high-temperature oxidation, which increases the degree of coal aromatization. The heat release increases from 7000 to 8554 J/g, a total increase of 1.22 times. On the contrary, when the low oxygen combustion residues are reburned, the total heat release shows the trend of first stabilizing and then decreasing. When coal samples were burned in a low oxygen combustion temperature range of 350-400 • C, the reaction was relatively gentle, and there were more combustible components remaining. The heat release of residues is close to pyrolysis at the same period during reburning. However, as the low oxygen combustion temperature increases, the heat release drops sharply. When the low oxygen combustion residues are reburned at 500 • C, the heat release is near to 0. This shows that the combustible volatiles of the coal sample are basically burned out during the low oxygen combustion stage, and there is almost no secondary combustion condition. The higher the pyrolysis temperature, the higher the heat generated when the residue undergoes secondary oxidation. To a certain extent, it indicates that the higher the degree of pyrolysis of the coal samples, the easier it is for the residual structure of pyrolysis to reignite, and the combustion intensity will be more intense. On the contrary, as the constant temperature increases, low oxygen combustion becomes more complete, and the residual combustibles gradually decrease, resulting in a more dispersed secondary combustion.

Conclusions
In this paper, the long flame coal samples from the LiuHuangGou coalfield in Xinjiang province were selected to separately study the characteristics of pyrolysis and low oxygen combustion at constant temperature condition and the reburning oxidation characteristics of residues at temperature rising condition. Relying on thermal analysis technology, the mass characteristics of coal samples at 350-500 • C were analyzed, and the change of characteristics of combustion parameters and heat release laws of residues at 30-800 • C were revealed. The results show that the increase of constant temperatures can promote both pyrolysis and low oxygen combustion, while the pyrolysis reaction accelerates the combustion process of residues and the low oxygen combustion hinders reburning. This has important guiding significance for the prevention of spontaneous combustion of lowrank bituminous coal and the reignition of residues and the safe unsealing of fire areas.
In the pyrolysis and low oxygen combustion stages of coal samples, the low oxygen combustion reaction is more intense than the pyrolysis, and the thermogravimetric characteristics of the coal samples change more obviously under the same constant temperature conditions. In addition, as the constant temperature rises, the intensity of the pyrolysis reaction is relatively small, and the variation range of related characteristic parameters is far less than that of low oxygen combustion. During the pyrolysis process, constant temperature starting point of coal sample residual value, Wc, or residual value of pyrolysis end coal sample, Wp, and mass of combustible components, ∆Wpo, gradually decrease, and residual weight difference of pyrolysis, ∆Wvp, and residual weight difference at constant temperature, ∆Wcp, gradually increase. These parameters all have obvious turning points at around 450 • C. Nevertheless, the change of characteristic parameters of low oxygen combustion reaction is similar to pyrolysis, only at low temperature (<400 • C). With the continuous increasing of constant temperature, the low oxygen combustion reaction strengthens, and the mass of coal samples decreases greatly. The change rate of combustion characteristics, such as residual weight difference at constant temperature, ∆Wcp, and mass of combustible components, ∆Wpo, is accelerated, and both have obvious turning points in the range of 400~450 • C. This illustrates that the low oxygen combustion reaction is sufficient, and the combustion intensity reaches the maximum. Above 450 • C, the residual structure of the coal samples is in a burning state, and the residual combustible volatile component content is small.
In the reburning oxidation stage of the residues, the pyrolysis reaction enhances the oxidation performance and the tendency to reignite the residues. Pyrolysis generates a large number of flammable molecular structures to lay the material foundation for the reburning of the residues, and the aromatization of the coal structure improves the oxidation performance of the residues. However, the low oxygen combustion reaction greatly inhibits the reburning of the residues. The low oxygen combustion reaction consumes a mass of combustible components in the coal samples, reducing the possibility of reburning of the residues. As the oxidation temperature increases, the mass of pyrolysis residues is greatly reduced, which is much larger than that of low oxygen combustion residues. With the increasing of the constant temperature, the ignition temperature, T i , burnout temperature, T h , and combustion half-peak width, ∆T 1/2 , gradually decrease, and maximum speed of weightlessness, d Wmax , the combustion peak, and the heat release increase. It illustrates that the higher the degree of pyrolysis of the residues, the more concentrated combustion and the greater the intensity, and the higher degree of reburning oxidation. On the contrary, the more sufficient the low oxygen combustion is, the more combustible chemical components are consumed, so the less combustible structures remain in the reoxidation process and the more asymmetric the distribution is. Along with the increasing of the constant temperature, the characteristic parameters of residual reburning become smaller. This shows that the secondary combustion is more and more dispersed, and the intensity is gradually reduced. Additionally, after the constant temperature exceeds 450 • C, the reburning conditions of the residues are harsher, the combustion reaction is weaker, and it is hard to reburn.