Study on the Formation Mechanism of Acetaldehyde during the Low-Temperature Oxidation of Coal

Abstract

The threshold dilution ratio of acetaldehyde is much larger than those of other odor compounds generated during the spontaneous combustion process and so it is the most important odorant. Studying the mechanism by which acetaldehyde is generated can provide the necessary theoretical support for acetaldehyde-based odor analysis. In the present work, the release of acetaldehyde was monitored while heating lignite, long-flame coal, and coking coal specimens under either air or nitrogen. The data show that acetaldehyde was primarily produced by the oxidation of active sites in the coal rather than by the pyrolysis of oxygen-containing functional groups. Based on quantum chemistry and coal−oxygen reaction theory, the transition state approach was used to further study the formation of acetaldehyde during the low-temperature oxidation of coal. Using density functional theory, three different coal molecule structures were modeled and optimized structures for acetaldehyde formation and the energies, bond lengths, and virtual frequencies of each reaction stagnation point were obtained at the B3LYP-D3/6-311G** and M062X-D3/Def2-TZVP levels. The results indicate that the low-temperature oxidation of coal to generate acetaldehyde involves the capture of H atoms from aliphatic side chains to generate peroxy radicals. These radicals then attack unsaturated C atoms through complex inversions to generate peroxides. In the third step of this process, the O−O single bonds in the peroxides break in response to thermal energy to form carbonyl groups. Finally, specific C−C or C−O bonds on the aliphatic side chains are thermally cleaved to generate acetaldehyde.

Influence of Hydroxyl Groups on the Oxidative Reaction Characteristics of Active Groups in Lignite at Room Temperature

“…Currently, coal remains an important source of energy and industrial raw material in many countries, serving as a vital foundational energy resource. − However, during the mining process, spontaneous combustion accidents often occur in the process of coal mining, and its spontaneous combustion properties have become one of the important issues in the field of coal mine safety. , The mechanism of coal spontaneous combustion involves complex physical and chemical reactions . The relationship between coal structure, properties, and reactivity is particularly important for understanding and controlling the complex multistage processes of coal combustion. − The chemical and physicochemical changes resulting from coal oxidation primarily depend on different oxidation conditions. − During the process of the coal–oxygen interaction, active groups are specific functional groups or atomic groups with the highest reactivity and are most prone to chemical reactions.…”

“… 5 , 6 The mechanism of coal spontaneous combustion involves complex physical and chemical reactions. 7 The relationship between coal structure, properties, and reactivity is particularly important for understanding and controlling the complex multistage processes of coal combustion. 8 − 10 The chemical and physicochemical changes resulting from coal oxidation primarily depend on different oxidation conditions.…”