Low-temperature Oxidation Of Coal-surface-groups Based On Model Compounds

Spontaneous combustion of coal (SCC) is a serious hazard in the the coalindustry. The study on characteristics and mechanism of spontaneous combustion ofcoal has a critical significance for the preventing and reducing losses on this disaster.Low-temperature oxidation is an important stage for coal spontaneous combustion,which controls the occurrence and development of spontaneous combustion. As aresult, the research targeting at low-temperature oxidation of coal plays a key role inprevention of self-heating hazards. It’s well known that coal is a complexorganic-fossil-fuel which constituted by similar but different molecular structures.Generally, the coal is considered as the porous polymer with large internal surfacearea. Given this property, direct investigation into the internal structural changes ofcoal during the low-temperature oxidation period is often difficult to succeed.According to theories on Coal Chemistry, in the combining process of oxygenand coal, O-containing functional groups, such as side-chains and bridging bonds aresusceptible to be oxidized, while the chemical properties of aromatic rings arerelatively stable. As a consequence, the model compounds which contained a benzenering and a representative group of coal were used in this paper for studyingspontaneous combustion properties of coal. In order to simplify the complexity ofbiological macromolecules into small molecules of simple structure and study theiroxidation performance, the model compounds: phenetole, phenylacetaldehyde,phenylpropanol, benzoic acid, diphenylmethane and diphenylsulfide are adopted asthe substitute material for simulating the oxidation properties of active groups in coal.The programmed-heating experiments of six kinds of model compounds werecarried out. The results show that the phenylacetaldehyde and diphenyl methane havehigher oxidative activity than others. In the light of data on GC, FTIR and GC/MS,the outcome shows that there is a critical temperature in low-temperature oxidation ofmodel compounds. The oxidative activity of model compounds will sharply increaseonce the temperature over a certain value. Besides, there is a time delay between themaximum O2consumption and the generation of oxidation products, which alsoindicate that, like self-ignite of coal, the oxidation of model compounds is acomplicated reaction with multiple steps. The oxidation products of modelcompounds contain CO, CO2, benzene, phenol and other intermediate products. Thereaction pathways of model compounds were inferred on the basis of the compositionof oxidation products. Oxidation kinetics such as activation energy and pre-exponential factor wereanalyzed, according to the O2consumption rate of model compounds. In addition,their thermal characteristics were analyzed by the TG/DTA. Similar with the coal, theweight of model compounds will slightly increase at initial period. Then the TG curveappears a significant decrease with the temperature rising continuously, and finallykeep stable. It is worth noting that more than one peak were shown in the DTA curveduring the low-temperature process, which suggested that the oxidation of modelcompounds has complicated thermal-variation in different temperature stage.On the basis of oxidation properties of active groups in coal, five differentantioxidants (BHT, propyl gallate, phytic, sodium benzoate and ascorbic acid) wereemployed for inhibiting the low-temperature oxidation of model compounds, and alsoapplied to the coal samples. The outcomes indicate that the BHT and phytic have welleffect on the inhibition of coal oxidation. Further, the inhibition mechanism ofantioxidants was analyzed.