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.
Understanding
the adsorption and diffusion of CO2 and
N2 in lignite at high temperature is of great significance
for fire prevention and control. Considering the influence of temperature
on coal structure, molecular structure models of lignite at 298.15,
323.15, 423.15, and 523.15 K were constructed by molecular mechanics
and dynamics, and grand canonical Monte Carlo molecular simulation
was conducted for single-component and two-component systems under
different temperatures, pressures, and gas ratios. The adsorption
capacity was positively correlated with the pressure and molar ratio
of CO2 but negatively correlated with the temperature.
The adsorption amount of CO2 (1.060 mmol/g) was generally
larger than that of N2 (0.069 mmol/g), showing a greater
selectivity. However, CO2 was more sensitive to temperature,
and the adsorption amount decreased faster with the increase in temperature.
At high temperature, the adsorption amount of CO2 and N2 is basically equal, both of which are at a low level. The
CO2 isosteric heat of adsorption (7.46–8.84 kcal/mol)
varies significantly with temperature. The interaction energy is consistent
with the change trend of adsorption quantity, and van der Waals energy
plays a dominant role in adsorption. Injecting CO2 and
N2 at the high temperature stage has a poor extinguishing
effect, which can only dilute oxygen content and exchange heat, and
the advantage of CO2 will be lost. It may be more effective
using liquid nitrogen which has the properties of low temperature
and high specific heat capacity. The results are of great significance
to improve the efficiency of fire prevention and suppression in underground
coal mines.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.