Oxygen
functional groups play a key role in the process of coal
spontaneous combustion, and aldehyde groups are one of the main oxygen
functional groups, but their reaction pathways are still unclear.
Based on the quantum chemical calculation method, this study used
the density functional theory (DFT) of Gaussian software to explore
the oxidation and self-reaction pathways of aldehyde groups in the
process of coal spontaneous combustion. The Ph–CH2–CHO was selected as the characterization of a coal molecule
containing the aldehyde group, and the results showed that the C–H
bonds of the aldehyde groups formed by s–sp2 hybridization
are the active sites. During the oxidation reaction process, the hydrogen
atoms in aldehyde groups can be captured by oxygen to generate the
−·CO free radicals. The enthalpy change and activation
energy of the reaction are 136.87 and 149.53 kJ/mol, respectively,
indicating that the reaction can occur in the middle and later stage
of coal spontaneous combustion (70–200 °C), which can
greatly enhance the self-heating of the reaction system. During the
self-reaction process, aldehyde groups can react with the −·CH2 free radicals and the ·OH free radicals, and both reactions
can generate the −·CO free radicals, but the thermal
effects are not obvious. The activation energies of the two reactions
are 63.76 and 22.23 kJ/mol, respectively, which indicates that the
former can occur in the middle stage of coal spontaneous combustion
(30–70 °C) and the latter can occur in the initial stage
of coal spontaneous combustion (room temperature). One part of the
generated −·CO free radicals will directly undergo
decarbonylation to generate CO, and the enthalpy change and activation
energy are 9.62 and 37.69 kJ/mol, respectively. This reaction can
be regarded as the main source of CO in the initial stage of coal
spontaneous combustion (room temperature). Another part of the generated
−·CO free radicals can adsorb free O atoms to
generate the −COO· free radicals and undergo a decarboxylation
reaction to generate CO2. The total enthalpy change and
activation energy of these reactions are 6.12 and 73.11 kJ/mol, respectively,
which can occur in the middle stage of coal spontaneous combustion
(30–70 °C). The results can be helpful to the study of
coal spontaneous combustion mechanism.
The “three stages” division of coal spontaneous combustion is fuzzy and lacks adequate risk and warning levels corresponding to its divisions; additionally, the targeted prevention measures for each stage have not been described. To address the shortcomings of the “three stages” division, the “five stages” division was proposed to more clearly analyze the stage changes of the spontaneous combustion of coal. The “five stages” method divides the process of the spontaneous combustion of coal into five stages, including: the latent stage, heat accumulating stage, evaporation stage, active stage, and hypoxic stage. The critical point of each stage was determined using adiabatic oxidation experiments and programmed heat experiments. As the critical point of the latent stage, the temperature of zero activation energy is approximately 55–70°C. In the heat accumulating stage, the critical point is the temperature (approximately 90°C) where the external moisture of coal evaporates violently while the internal moisture of coal has not yet fully evaporated. During the evaporation stage, the temperature (approximately 105°C) where the internal moisture has evaporated completely represents the end of this stage and the start of the active stage (105–170°C). When the oxygen concentration drops to 5%, the spontaneous combustion of coal enters the hypoxic stage. Thus, an oxygen concentration of 5% represents the critical point of the start of the hypoxic stage (above 170°C). After the analysis of each stage, risk and warning levels were determined. Considering the major prevention measures of the spontaneous combustion of coal, a staged warning and disposal table was created.
The coal spontaneous
combustion phenomenon seriously affects the
safety production of coal mines. Aiming at the problem of complex
coal molecular structure and incomplete reaction sequences at present,
the mechanisms and thermodynamic parameters of coal spontaneous combustion
chain reactions were explored by combining experimental detections
and molecular simulations. First, the active groups on the surface
of coal were obtained by Fourier transform infrared spectroscopy (FTIR),
mainly including methyl (−CH3), methylene (−CH2), methyne (−CH), phenolic hydroxyl (−ArOH),
alcohol hydroxyl (−ROH), carboxyl (−COOH), aldehyde
(−CHO), and ether (−O−), and the coal molecular
models containing functional groups and radicals were established.
According to the charge density, electrostatic potential, and frontier
orbital theories, the active sites and active bonds were obtained,
and a series of reactions were given. The thermodynamic and structural
parameters of each reaction were explored. In the chain initiation
reaction stage, O2 chemisorption and the self-reaction
of radicals play a leading role. In this stage, heat gradually accumulates
and various radicals begin to generate, where the intramolecular hydrogen
transfer reaction of a peroxide radical (−C–O–O·)
can produce the key hydroxyl radical (−O·). In the chain
propagation reaction stage, O2 and −O· continuously
consume active sites to accelerate the reaction sequences and increase
the temperature of coal, and index gases such as CO and CO2 generate, causing the chain cycle reactions to gradually form. The
chain termination reaction stage is the formation of stable compounds
such as ethers, esters, and quinones, which can inhibit the development
of chain reactions. The results can further explain the reaction mechanism
of coal spontaneous combustion and provide references for the development
and utilization of chemical inhibitors.
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