Authors: Wanting Zhao, Ziwen Dong, Xian Wu, Song Kong, Yue Yang, Zhenya Zhang, Feifei Yin, Chuanwen Sun
Categories: Article
Source: ACS Omega
Oxidation Risk of Coal with Variable Soaking Time under the Influence of Oxygen Concentration
Primary and secondary low-temperature oxidation experiments
were
conducted on lignite immersed in water for short (50 days) and long
(200 days) periods under three different oxygen concentrations (8,
18, and 21%), which provided a theoretical basis for the identification
and risk judgment of a spontaneous combustion zone in goaf. The results
indicate that in both the short-term water-immersed coal (STWIC) and
long-term water-immersed coal (LTWIC), the apparent activation energy
(Ea) for the three stages of secondary oxidation is lower than that
for primary oxidation under 18% oxygen concentration, suggesting a
greater risk of spontaneous combustion. The STWIC and LTWIC that have
experienced a primary oxidation in the goaf are more prone to secondary
oxidation spontaneous combustion when the oxygen concentration is
8 and 18%, while the risk of secondary oxidation spontaneous combustion
is lower than that of primary oxidation at 21%. During the experimentation,
a temperature inflection point was observed, which decreased with
an increase in the oxygen concentration conditions. Beyond this temperature
inflection point, the oxygen consumption rate and heat release intensity
during the primary oxidation of STWIC surpassed those during secondary
oxidation. Moreover, the generation rates of CO and CO2 during the secondary low-temperature oxidation process of LTWIC
were lower than those of STWIC.
The majority of mine fires in China are attributed to internal-caused fire, and the occurrence of spontaneous combustion poses a substantial risk to production safety as well as detrimental effects on mineral resources and the ecological environment. Moreover, the goaf area is highly susceptible to coal spontaneous combustion, contributing to more than 60% of the overall incidents of spontaneous combustion fires in coal mines.^1^ The infiltration of groundwater will result in the accumulation of a significant amount of water within the goaf formed after mining the overlying coal seam. Consequently, when coal seams are mined in close proximity, it becomes necessary to drain the overlying goaf, leading to substantial air leakage.^2−5^ Furthermore, the utilization of fully mechanized mining technology during the residual coal mining process in thick coal seams may lead to substantial air leakage.^6,7^ As the volume of air changes, the concentration of oxygen carried in the airflow changes as well. During coal mining and storage, low-temperature oxidation (LTO) of coal often occurs in an oxygen-deficient environment, leading to self-heating and subsequent self-ignition.^8^ Therefore, it is crucial to investigate the LTO process of water-immersed coal under lean levels of oxygen.
Spontaneous combustion of coal is a multifaceted physical and chemical phenomenon, influenced not only by its own exothermic oxidation but also by external factors such as groundwater, high-temperature geothermal energy, and variations in air volume.^9,10^ Qiao et al.^11^ investigated the impact of immersion on the spontaneous combustion of metamorphic coals, with a primary focus on elucidating the activation energy for oxidation and the associated thermal effects. The oxidation characteristics and stage division of coal were investigated by Nie et al. through a programmed heating experiment conducted at different oxygen concentrations.^12^ Song et al.^13^ investigated how water immersion affected the pore structure of coal with varying degrees of metamorphism. During the coal LTO process, Zhou et al.^14^ conducted an investigation on the effects of varying oxygen concentrations on group changes. According to Song et al.,^15^ the soaking process influences the coal pore structure and LTO characteristics, leading to a greater risk of spontaneous combustion in coal due to water immersion. Xu et al.^16^ conducted an analysis on the oxidation and spontaneous combustion properties of long flame coals following their exposure to water immersion and subsequent air drying. Zhao et al.^17^ examined the impact of oxygen concentration and heating rate on the spontaneous combustion characteristics of coal. Liu et al.^18^ investigated the thermal properties of low-temperature oxidation and the evolution of key functional groups in bituminous coal under varying oxygen concentrations, specifically focusing on a lean-oxygen environment. During the LTO process, Lu et al.^19^ conducted the oxidation kinetics characteristics of long-term immersed coal.
Coal may be affected by many factors or measures such as cooling and hypoxia during the oxidation process so that the temperature is cooled to normal temperature before reaching the ignition point, and the oxidation reaction with oxygen occurs again when the accumulation thickness, oxygen concentration, and air leakage intensity are appropriate, that is, secondary oxidation. The schematic diagram of secondary oxidation spontaneous combustion of water-immersed coal is shown in Figure 1. The secondary LTO process of coal, as indicated above, may potentially induce spontaneous combustion in coal. By preoxidizing coal with different degrees of metamorphism under a 100 mL/min airflow, Liang et al.^20^ prepared aerobic heated coal. Bu et al.^21^ examined how the duration of immersion impacts the combustion properties of high-temperature preoxidized coal. Through programmed heating experiments, Niu et al.^22^ explored the impact of preoxidation temperatures on coal’s secondary oxidation properties. The results showed that coal’s ability to self-heat was inhibited by oxidation temperatures below 110 °C. Lu et al.^23^ studied the kinetics of coal oxidation during a reoxidation process at low temperatures. By conducting temperature-programmed heating experiments, Liu et al.^24^ analyzed the parameters related to spontaneous combustion characteristics of bituminous coal when subjected to varying preoxidation temperatures and air volumes. An investigation was conducted by Niu et al.^25^ on the thermal characteristics and group transformation mechanism of secondary oxidation of oxidized coal in deep mines. Xu et al.^26^ examined how different preoxidation temperatures impacted the secondary oxidation of water immersion coal samples. Liu and colleagues^27^ conducted a study on the impact of water immersion and preoxidation on the secondary oxidation properties of coal. Their findings revealed that these two factors can be effectively combined to expedite the LTO process of coal.
Figure 1 Schematic diagram of secondary oxidation spontaneous combustion of water-immersed coal.
In summary, the spontaneous combustion process of coal is complicated by external environmental factors. When mining residual coal in goaf, it is crucial to consider the single-to-multiple-oxidation spontaneous combustion of coal under conditions such as water immersion and varying oxygen concentrations. However, there is a lack of research on the secondary oxidation process of soaked coal under different oxygen concentration conditions. Therefore, this study investigates the low-temperature primary and secondary oxidation processes of coal with varying oxygen concentrations and soaking times, providing detailed analysis of the kinetic characteristics of oxidation and limit parameters for coal spontaneous combustion. The findings from this research offer theoretical insights for preventing and controlling spontaneous combustion in gobs under complex conditions while also contributing to identifying areas at risk for spontaneous combustion.
This study used fresh raw coal samples from the Zuoyun Donggucheng Coal Mine of the Coal Import and Export Group Company in Shanxi, China. The coal sample exhibits moisture, ash, volatile matter, and fixed carbon contents of 12.18, 14.07, 34.32, and 39.45% correspondingly. In order to prevent the effects of mechanical forces generated by the crusher crushing the coal samples, hydraulic equipment was used to let the lignite samples reduce to a size of less than 10 mm at a pressure of 25 MPa. Put 200 g of evenly stirred crushed coal into a 1 L wide mouthed bottle, fill it with pure water, seal, and store it, and shake it three times a day. According to the difference of immersion time, coal samples are divided into two groups that were immersed in water for durations of 50 and 200 days correspondingly. The wet coal is then filtered and air-dried at room temperature for 48 h. Experimental brief flowchart is shown in Figure 2. Coal samples were appropriately marked before conducting experiments on LTO. S was designated as the short-term immersion time of 50 days and L as the long-term immersion time of 200 days. Subscripts 1 and 2 represent the number of oxidations, where 1 signifies the initial oxidation and 2 signifies the second oxidation. The varying air intake oxygen concentrations are indicated at the end. To avoid the interference of external moisture, the experimental coal sample was placed in a vacuum drying oven and dried at a constant temperature of 40 °C for 8 h before the oxidation experiment.
Figure 2 Experimental brief flowchart.
A dynamic gas distribution system was used to prepare atmospheres with different oxygen concentrations in the airflow at 8, 18, and 21% in order to design a control experiment that is as close to engineering reality as possible. Since spontaneous combustion of goaf is often caused by the LTO process,^28^ the reoxidation process of lignite is explored range from 40–170 °C.
Preoxidize water-immersed coal samples using a programmed heating device at a rate of 0.5 °C/min under an airflow of 150 mL/min containing different oxygen components. The coal sample undergoes heating at ambient temperature until it reaches a maximum of 170 °C, at which point the heating process is terminated. Then, an equal airflow of nitrogen gas is introduced to lower the temperature of the coal sample and expel the gas produced during the primary LTO procedure. On the primary oxidized coal sample is subjected to a secondary oxidation process and ensures that the heating rate, air intake flow rate, and steps in the experiment are consistent. Gas detection and analysis are performed at intervals of 10 °C during the primary and secondary oxidation processes of water-immersed coal samples.
The program heating system consists primarily of a modified electric hot blast constant temperature drying oven, FD-HQ02 dynamic gas distribution system (Suzhou Friends Experimental Equipment Company), and multichannel temperature tester. By using a gas chromatograph (GC-6900), the chromatographic analysis system analyzes the outlet gas. To monitor the coal temperature, the temperature sensor is connected to half of the axial position of the cylindrical coal sample tank.
During
the LTO process of coal, products such as CO, CO2, hydrocarbon
gas, and water vapor are produced. The oxidative spontaneous combustion
status of coal can be evaluated by monitoring the variations in oxygen
consumption rate and gaseous byproduct emission during the oxidation
process.^29^
In the LTO process of coal, oxygen is continuously consumed unidirectionally; therefore, the oxygen consumption rate is a good indicator of coal oxidation intensity. For analysis and calculation of oxygen consumption rate, oxygen concentration change is only one of the basic parameters.^29^ The calculation method for oxygen consumption rate is as follows^30,31^
where VO2~~(T) is the rate of oxygen consumption by the
coal sample, (mol·cm^–3^·s^–1^); Q represents the air supply volume, 150 mL/min; v represents the volume of coal sample, (cm^3^); CO2~~^in^ and CO2~~^out^ represent the oxygen concentration
at the air intake and outlet of the coal sample, respectively (mol/mL).
The changes in the concentration of the oxygen volume and oxygen consumption rate with coal temperature are depicted in Figure 3. During the primary LTO process of short-term water-immersed coal (STWIC), when the coal temperature is below 110 °C, the oxygen consumption rate is highest under an air intake oxygen concentration of 18%, followed by 8%, and lowest at 21%. However, when the coal temperature exceeds 110 °C, there is a positive correlation between the air intake oxygen concentration and the oxygen consumption rate. As coal temperature escalates, the oxygen consumption rate of STWIC during secondary LTO exhibits a maximum under an inlet oxygen concentration of approximately 18%, followed by 8%, and the lowest at 21%. This indicates that the variation pattern of oxygen consumption rate with oxygen concentration during the secondary LTO of STWIC differs from that during the primary LTO process. Notably, the coal-oxygen reaction rate peaks under an inlet oxygen concentration of 18%, rendering the strongest likelihood of spontaneous combustion due to secondary oxidation. Under the condition of 8% oxygen concentration at the air intake, for STWIC, when the coal temperature is below 140 °C, the oxygen consumption rate of secondary oxidation is relatively close to that of primary oxidation. However, when the coal temperature exceeds 140 °C, the oxygen consumption rate of secondary oxidation is significantly lower than that of primary oxidation. A similar trend is observed in the comparison of oxygen consumption rates between secondary and primary oxidation for STWIC under oxygen concentrations of 18 and 21% at the air intake, albeit with temperature turning points corresponding to 130 and 90 °C, respectively. This indicates the existence of a temperature turning point that increases as the oxygen concentration decreases. Above this turning point, the oxygen consumption rate of primary oxidation in STWIC exceeds that of secondary oxidation.
Figure 3 Variation curve of the oxygen volume concentration and oxygen consumption rate of each sample with coal temperature.
The oxygen consumption rates of the primary and secondary oxidation of LTWIC before 90 °C increase with the increase of air intake oxygen concentration conditions, while the oxygen consumption rates after 90 °C are greatly affected by oxidation times and are not positively correlated with the change of imported oxygen concentration conditions. Under conditions of 8, 18, and 21% oxygen concentration at the air intake, the secondary oxidation oxygen consumption rate of LTWIC begins to exceed that of primary oxidation when the coal temperature surpasses 80, 90, and 140 °C, respectively. Moreover, the temperature range within which the secondary oxidation rate surpasses the primary oxidation rate diminishes as the air intake oxygen concentration increases. The oxygen consumption rate during the secondary LTO process of LTWIC is consistently higher than that of STWIC solely under a 21% air intake oxygen concentration condition, indicating a faster coal-oxygen reaction rate and thus a heightened risk of spontaneous combustion under such conditions.
The apparent activation energy (Ea) refers to the minimum energy
required for initiating and sustaining a chemical reaction between
coal and oxygen at a certain temperature. In a specific oxidation
stage, the higher the Ea, the more energy is required for the oxidation
reaction, and the possibility of sustained development decreases relatively.
Using ln[ln(CO2~~^in^/CO2~~^out^)] as the dependent
variable and 1/T as the independent variable, by
calculating its slope through eq 2, the Ea for oxygen consumption can be determined^32^
where A represents the pre-exponential factor, s^–1^; Ea represents the apparent activation energy, J/mol; R represents the molar gas constant, 8.314 J/(mol·k); and T is the thermodynamic temperature, K.
Based on the changes in oxygen
concentration and oxygen consumption rate depicted in Figure 3, the reaction process between
coal and oxygen can be categorized into three distinct stages. (I)
The slow oxygen consumption stage occurs in the range from 40–90
°C, during which the oxygen concentration gradually decreases
as the coal temperature rises, and the oxygen consumption rate increases
relatively slowly. (II) The accelerated oxygen consumption stage spans
the temperature range of 90–140 °C, representing a transitional
period from the slow to rapid oxygen consumption stage. (III) The
rapid oxygen consumption stage occurs between 140 and 170 °C,
characterized by a significant drop in oxygen concentration and a
drastic increase in the oxygen consumption rate. In different oxidation
stages, a linear fit was performed with ln[ln(CO2~~^in^/CO2~~^out^)] as the y-axis and 1/T as the x-axis, as shown in Figure 4. According to the slope of the fitting curve,
the Ea for different oxidation stages is calculated, as shown in Table 1.
Figure 4 Relationship between ln[ln(C
O2~~^in^/CO2~~^out^)] and 1/T in different reaction stages.
The Ea of the first stage, second stage, and third
stage for the
S2-8% coal sample decreased by 56.39, 40.30, and 41.68%
compared to the S1-8% coal sample, respectively. The observed
reduction in Ea for the first, second, and third stages of S2-18% coal sample was 76.05, 19.95, and 26.84%, respectively, when
compared to S1-18% coal sample. This indicates that the
minimum energy required for the secondary LTO process of STWIC under
8% and 18% air intake oxygen concentrations is lower compared with
the primary oxidation process. The Ea required for the coal-oxygen
reaction of S2-21% coal sample after primary oxidation
decreased by 16.6315 kJ/mol in the first stage, indicating that the
coal-oxygen reaction can be facilitated more easily. The second stage
results in a relatively small loss of energy due to water evaporation
for coal sample S2-21%, and the Ea decreases by 26.9761
kJ/mol compared with coal sample S1-21%. In the third stage,
there was an observed increase of 8.0408 kJ/mol in the Ea for the
S2-21% coal sample compared to that of the S1-21% coal sample. This may be attributed to the significant consumption
of active functional groups during the primary oxidation process of
the S2-21% coal sample, leading to a higher energy barrier
that needs to be overcome in the rapid oxygen consumption stage.
The Ea of the first, second, and third stages of the L2-18% coal sample decreased by 11.33, 90.46, and 51.48% compared to
the L1-18% coal sample. The observed reduction in E for
the first, second, and third stages of the L2-21% coal
sample was 9.84, 38.08, and 44.16%, respectively, when compared to
the L1-21% coal sample. This indicates that the minimum
energy required for the secondary oxidation process of LTWIC under
18% and 21% air intake oxygen concentrations is lower, compared to
the primary oxidation process. In the first stage, the Ea required
for the coal-oxygen reaction of L2-8% coal sample after
primary oxidation decreased by 3.6235 kJ/mol, indicating that the
coal-oxygen reaction is more easily facilitated. In the second stage,
the Ea of the L2-8% coal sample increased by 2.5648 kJ/mol
compared to the L1-8% coal sample, which may be attributed
to a greater energy requirement for enhanced reaction intensity between
coal and oxygen. In the third stage, the Ea of the L2-8%
coal sample was reduced by 36.8087 kJ/mol compared to the L1-8% coal sample, suggesting that oxidative spontaneous combustion
of it is more likely to occur.
The observed reduction in Ea
for the first, second, and third stages
of the L1-21% coal sample was 88.33, 77.06, and 5.74% lower
than that of the S1-21% coal sample, respectively. The
Ea of the first stage, second stage, and third stage for the L2-21% coal sample were reduced by 45.28, 68.27, and 54.79%
compared to the S2-21% coal sample, respectively. As a
result, both the primary oxidation and secondary oxidation processes
of LTWIC require less energy than those of STWIC under 21% oxygen
concentration at the air intake. In the first stage, the Ea of the
primary and secondary oxidation reactions of LTWIC under the condition
of 18% air intake oxygen concentration increased by 71.97 and 92.43%,
respectively, compared to those of STWIC. Similarly, under 8% air
intake oxygen concentration, the Ea for the primary and secondary
oxidation reactions of LTWIC increased by 50.06 and 76.01%, respectively,
in comparison to STWIC. This suggests that the energy required for
both primary and secondary coal oxidation during the slow oxygen consumption
stage is lower in STWIC compared to LTWIC, under conditions of air
intake conditions with 8 and 18% oxygen concentration. In the second
stage, the Ea of the primary and secondary oxidation reactions of
LTWIC under 18% air intake oxygen concentration decreased by 17.81
and 90.20%, respectively, compared to STWIC. However, under 8% air
intake oxygen concentration, the Ea for the accelerated oxygen consumption
stages of the primary and secondary oxidation reactions of LTWIC increased
by 48.36 and 71.69%, respectively, in comparison to STWIC. This suggests
that, at 18% air intake oxygen concentration, both the primary and
secondary oxidation reactions of LTWIC require less energy for coal-oxygen
reactions during the accelerated oxygen consumption stage than STWIC,
whereas the opposite is true under 8% air intake oxygen concentration.
In the third stage, the Ea of the primary and secondary oxidation
reactions of LTWIC under 18% air intake oxygen concentration were
reduced by 58.72 and 72.62%, respectively, compared to STWIC. Under
8% air intake oxygen concentration, the Ea for the primary and secondary
oxidation reactions during the rapid oxygen consumption phase of LTWIC
decreased by 43.64 and 38.33%, respectively, compared to STWIC. This
indicates that, under both 8 and 18% air intake oxygen concentrations,
LTWIC requires less energy for both primary and secondary oxidation
reactions during the rapid oxygen consumption phase compared to STWIC,
with a greater reduction in activation energy observed under 18% air
intake oxygen concentration.
Low-rank lignite is oxidized during the oxidation process where
the alkyl chain is first oxidized to a carbonyl group (-C=O),
the carbonyl group further produces CO and carboxyl groups, and then
these carboxyl groups break to produce CO2 or continue
to oxidize to organic oxygenated compounds.^33^ Carbon monoxide is one of the most sensitive gas indicators of the
coal oxidation status.^30^ A higher rate
of CO2 generation indicates a relatively higher oxidation
intensity of lignite. The calculation method for the rate of CO and
CO2 generation is as follows.^24,27,30^
where VCO(T) represents the gas rates of production of CO, (mol·cm^–3^·s^–1^); CCO^out^ is the gas
outlet’s CO volumetric concentration, (mol/mL); Q represents the air supply volume, 150 mL/min; vn is the volume of the reactor, (cm^3^). VCO2~~(T) represents the gas
rates of production of CO2, (mol·cm^–3^·s^–1^); CCO^out^ and CCO2~~^out^ is the gas outlet’s CO2 volumetric concentration,
(mol/mL); Q represents the air supply volume, 150
mL/min; vn is the volume of the reactor (cm^3^).
The changes of the CO and CO2 generation rate
with the coal temperature are shown in Figure 5. With the increase in temperature, the concentration
and rate of CO generation exhibit a relatively slow growth prior to
90 °C, followed by an accelerated increase range from 90–140
°C, and a notable sharp increase trend beyond 140 °C. According
to the stage-wise changes shown in Figure 4, the coal sample might have a critical temperature
around 90 °C and a dry cracking temperature around 140 °C.
Figure 5 Variation curve of CO and CO
2production rates of each sample with the coal temperature.
It is generally believed that a higher concentration
of oxygen
in the environment is more favorable for the generation of CO2 in coal samples.^34−36^ During the primary oxidation
process, the rate of CO2 generation in LTWIC increases
as the oxygen concentration at the air intake decreases. Similarly,
STWIC exhibits a similar pattern from 40 to 130 °C during a primary
oxidation process, indicating that water immersion alters the CO2 generation pattern in coal-oxygen reactions. After the coal
temperature of STWIC exceeds 130 °C, the CO2 generation
rate from primary oxidation is maximized when the air intake oxygen
concentration is set at 18%, followed by the condition of 8% air intake
oxygen concentration, and the lowest is observed under the condition
of 21% air intake oxygen concentration. The CO2 production
during the primary oxidation process of both LTWIC and STWIC is less
favorable at an oxygen concentration of 21% in the air intake compared
to when the oxygen concentration at the air intake is 8% or 18%. During
the secondary oxidation process, the maximum rate of CO2 generation by LTWIC occurs at the air intake oxygen concentration
of 18% range from 40 to 170 °C, followed by 8%, and the lowest
rate is observed at 21%. Within the temperature ranges of 40 to 80
°C and 160 to 170 °C, the rate of CO2 generation
through secondary oxidation of STWIC is greater at an air intake oxygen
concentration of 8% than that at 21%, whereas the opposite trend is
observed in the range from 80 to 160 °C. When the oxygen concentration
at the air intake is 18%, the rate of secondary oxidation of STWIC
to generate CO2 is consistently higher than when the oxygen
concentration at the air intake is 8% and 21% during LTOP. This implies
that the production of CO2 during the secondary oxidation
process of LTWIC and STWIC is more favorable when the oxygen concentration
condition at the air intake is 18%, as opposed to 8% or 21%.
When STWIC is exposed to an air intake oxygen concentration of
21%, the production rates of CO and CO2 during the secondary
LTO process exceed those during the primary oxidation process. Under
an air intake oxygen concentration of 18%, the rate of CO generation
exceeds that of primary oxidation during the secondary LTO of STWIC,
whereas the rate of CO2 generation surpasses that of primary
oxidation only prior to 140 °C, with a reversal observed thereafter.
At an air intake oxygen concentration of 8%, the rate of CO generation
during the secondary low-temperature oxidation process of STWIC exceeds
that of primary oxidation before 140 °C, but it is significantly
lower than that of primary oxidation after 140 °C. Meanwhile,
the rate of CO2 generation during secondary oxidation consistently
surpasses that of primary oxidation. This indicates that the production
rates of CO and CO2 are influenced not only by the coal
temperature but also by the number of oxidation times and oxygen concentration
conditions.
When the oxygen concentration of LTWIC at the air
intake is 21%,
the rate of secondary low-temperature oxidation to CO and CO2 is lower than that of primary low-temperature oxidation. Under an
air intake oxygen concentration of 18%, the rate of CO generation
exceeds that of primary oxidation during the secondary LTO of LTWIC,
whereas for the CO2 generation rate, it surpasses primary
oxidation only prior to 140 °C and the opposite is true after
140 °C. At an air intake oxygen concentration of 8%, the rate
of CO generation during the secondary LTO process of LTWIC exceeds
that of primary oxidation only before 150 °C, reversing thereafter,
whereas the rate of CO2 generation via secondary oxidation
is consistently lower than that of primary oxidation. This shows that
the production rates of CO and CO2 in the secondary LTO
process are primarily influenced by coal temperature when the oxygen
concentration at the air intake is 18%, while the immersion time condition
has a relatively minor impact.
Under an oxygen concentration
of 21% at the air intake, the rates
of CO and CO2 generation during the secondary LTO process
of LTWIC are higher than those of STWIC before 90 °C, yet significantly
lower after 90 °C. When the oxygen concentration at the air intake
is 18%, the rate of CO and CO2 generation during the secondary
LTO process of LTWIC is higher than that of STWIC before 150 °C,
but the opposite is true after 150 °C. When the oxygen concentration
at the air intake is 8%, the rate of CO and CO2 generation
during the secondary LTO process of LTWIC exceeds that of STWIC before
reaching 160 °C; however, this trend reverses beyond 160 °C.
This indicates the existence of a temperature turning point that increases
with the decrease in oxygen concentration, after which the rate of
secondary LTO generating CO and CO2 in LTWIC is significantly
lower than that in STWIC.
Heat release capacity of coal is an important indicator for assessing its tendency toward spontaneous combustion. The actual heat release intensity of the coal and oxygen composite reaction falls between the minimum and maximum.^37^ In considering the propensity for coal spontaneous combustion under the most unfavorable conditions, it is more advantageous for the prevention and control of coal mine fires. This article elucidates the oxidative heat release capacity of coal based on its maximum heat release intensity (Q). The calculation method for Q is as follows.^32,37^
where Q is the maximum estimated
intensity of the heat release, J/(cm^3^·s); ΔH^CO^ is the average exothermic heat produced by
a single mole of CO, 311.9 kJ/mol; ΔH^CO2^ is the average amount of heat generated by a single
mole of CO2, 446.7 kJ/mol.
The oxidation of carbonyl groups to carboxyl groups is the main exothermic source during low-temperature oxidation, while the formation of CO is an endothermic reaction.^38^ Variation of Q for STWIC and LTWIC with the temperature under different experimental conditions is illustrated in Figure 6. The variation trend of the Q of coal with temperature is almost consistent with the variation trend of oxygen consumption rate with temperature. Under conditions of oxygen concentrations of 8, 18, and 21% at the air intake, the Q of the primary oxidation of STWIC is greater than that of secondary oxidation after the coal temperature exceeds 140, 130, and 90 °C, respectively. In comparison to the primary oxidation process, STWIC undergoes less impact from heat absorption due to water evaporation during the secondary oxidation process, thereby exhibiting a greater Q prior to the aforementioned temperature turning point. The Q of secondary oxidation of STWIC after the temperature inflection point is lower than that of primary oxidation, which may be related to the consumption of a large amount of active groups during the primary oxidation process. During the LTO process of LTWIC, when the oxygen concentration at the air intake is 8%, the Q of secondary oxidation is higher than that of primary oxidation only range from 80 to 160 °C. Similarly, when the air intake’s oxygen concentration reaches 18%, the Q of secondary oxidation surpasses that of primary oxidation in the temperature range between approximately 90 and 120 °C. Furthermore, at an oxygen concentration of 21% at the air intake, the Q of secondary oxidation surpasses that of primary oxidation approximately within 140 to 150 °C. This indicates that, with increasing air intake oxygen concentration, the initial temperature point where the Q of secondary oxidation exceeds that of primary oxidation gradually shifts toward higher temperatures for LTWIC. Moreover, the temperature range where secondary oxidation surpasses primary oxidation decreases as the air intake oxygen concentration increases.
Figure 6 Difference of Q in primary oxidation and secondary oxidation of each sample under different oxygen concentration conditions.
The
various analyses of the results from the programmed temperature increase
experiment provide strong guidance for assessing the risk of the spontaneous
combustion of coal. However, as the results are based on specific
experimental conditions and the influencing factors of SC of residual
coal in goaf areas of coal mines are numerous and highly variable,
the applicability of the experimental results is greatly limited.
To address this, limited spontaneous combustion parameters can be
used, which utilizes comprehensive analysis based on the experimental
results to expand the scope of application of the research and enhance
the guidance value for the prevention and control of complex and variable
dangers in production sites. The parameters for the spontaneous combustion
limit of coal include minimum floating coal thickness (hmin), lower limit oxygen concentration (Cmin^O2^), and upper limit air leakage intensity (qmax).^9,27^ When the thickness of residual
coal in goaf reaches the minimum spontaneous combustion thickness,
and the oxygen concentration is above the lower limit value while
the air leakage intensity is below the upper limit value, there is
a possibility for coal to undergo heat storage spontaneous combustion.^39,40^
where hmin represents
minimum floating coal thickness, cm; ρg represents
the air density, 1.29 kg/m^3^; Cg represents the specific heat capacity of air, 0.001J/(kg·K); q represents the air leakage intensity, 0.025 cm/s; T represents the temperature of residual coal in goaf, °C; Ty represents the temperature
of rock mass in goaf, 20 °C; λe is the effective
thermal conductivity of float coal, J/(cm·s·K); Qt represents the heat release
intensity of coal at a temperature of T, J/(cm^3^·s); Cmin^O2^ represents lower limit oxygen
concentration, %; Ce^O2^ represents the oxygen concentration
in the air, 21%; h is the coal thickness, 90 cm; qmax is upper limit air leakage intensity, cm/s.
Taking the actual coal thickness of 90 cm, leakage air velocity
of 0.025 cm/s, and surrounding rock temperature of 20 °C as an
example, combined with experimental results on heat release rate under
different conditions, the variation curves of limit parameters shown
in Figures 7, 8, and 9 were calculated and
plotted. The lower the hmin, the lower
the Cmin^O2^, the higher the qmax, and the range and risk of spontaneous combustion in the goaf is
greater. The spontaneous combustion of coal is a continuous oxidation
process, so the danger of coal spontaneous combustion is discussed
by measuring the extreme values of three parameters of spontaneous
combustion limit in the whole LTO range (40–170 °C).
Figure 7 Difference of h
minin the primary oxidation and secondary oxidation of each sample under different oxygen concentration conditions.
Figure 8 Difference of C
min^O2^ in primary and secondary oxidations of each sample under different oxygen concentration conditions.
Figure 9 Difference of q
maxin the primary oxidation and secondary oxidation of each sample under different oxygen concentration conditions.
During the process of primary and secondary oxidation
of STWIC,
both the maximum value of hmin and the
maximum value of Cmin^O2^ decrease initially
and then increase with the increase in the oxygen concentration at
the air intake. However, the minimum value of qmax increases initially and then decreases with the increase
in the oxygen concentration at the air intake. Therefore, regardless
of primary or secondary oxidation, the risk of spontaneous combustion
of STWIC is highest under the condition of 18% air intake oxygen concentration,
followed by 8% air intake oxygen concentration, and finally 21% air
intake oxygen concentration. In comparison to the primary oxidation
process, the secondary oxidation process of STWIC under 8 and 18%
air intake oxygen concentrations reduces the maximum value of hmin and the maximum value of Cmin^O2^, while increasing the minimum value of qmax. Additionally, under conditions of 21% air intake oxygen concentration,
this trend is reversed. This suggests that under a 21% air intake
oxygen concentration, the likelihood of spontaneous combustion due
to secondary oxidation of STWIC after primary oxidation is reduced.
However, at air intake oxygen concentrations of 8% and 18%, the risk
of SC from secondary oxidation of STWIC is higher than that from primary
oxidation.
During the process of the primary and secondary oxidation
of LTWIC,
both the maximum value of hmin and the
maximum value of Cmin^O2^ decrease with an increase in the
oxygen concentration at the air intake. However, the minimum value
of qmax increases with an increase in
the oxygen concentration at the air intake. This indicates that regardless
of primary or secondary oxidation processes, the risk of spontaneous
combustion in LTWIC increases with the rise in oxygen concentration
at the air intake. The comparative patterns of the limiting spontaneous
combustion parameters between the secondary oxidation and primary
oxidation of LTWIC under the same air intake oxygen concentration
conditions are consistent with those of STWIC. Compared to STWIC,
under air intake oxygen concentrations of 8 and 18%, the maximum value
of the MFCT and the maximum value of the Cmin^O2^ are
both higher for LTWIC, regardless of the primary or secondary oxidation
process, while the minimum value of the qmax is lower. However, this pattern reverses under the condition of
a 21% air intake oxygen concentration. This indicates that regardless
of the primary or secondary oxidation process, the risk of spontaneous
combustion of LTWIC under 21% air intake oxygen concentration is greater
than that of STWIC. However, under 8 and 18% air intake oxygen concentration
conditions, the risk of spontaneous combustion of LTWIC, whether during
the primary or secondary oxidation process, is lower than that of
STWIC.
W.Z.: Data curation, writing—original draft, and formal analysis. Z.D.: Conceptualization, methodology, and funding acquisition. X.W.: Methodology and validation. S.K.: Software and project administration. Y.Y.: Conceptualization and methodology. Z.Z.: Resources and funding acquisition. F.Y.: Supervision and methodology. C.S.: Software and writing—review and editing.
This work was supported by the National Natural Science Foundation of China (No. 51804107); the Open Research Fund Program of Liaoning Key Laboratory of mining environment and disaster mechanics (No. MEDM2023-B-4).
The authors declare no competing financial interest.