Formation Mechanism Of BTEXN During One Low Rank Coal Pyrolysis

As basic organic chemical materials, BTEXN (benzene, toluene,ethylbenzene, xylene, and naphthalene) are widely used in all kinds of chemicalengineering technology fields, such as pesticide, plastic, medicine. In addition tomaking from the petrochemical industry, these compounds can also be obtainedthrough coal pyrolysis. Coal is mainly composed of two parts: organic matterand minerals, and there are also some low molecular weight compoundsscattered inside the macromolecular network structure. Generally,macromolecular network structure would undergo thermal decomposition duringcoal pyrolysis, meanwhile the minerals and low molecular weight compoundshave important impacts on coal tar yields and composition. Therefore, furtherinvestigation of the influence of minerals and low molecular weight compoundson BTEXN distribution and formation during coal pyrolysis is of vitalimportance.In this work, one low rank coal from Kazakhstan (named as KZ) waspreprocessed by HCl–HF to obtain its demineralized sample. Then, in order toget separate low molecular weight compounds and the macromolecular networkstructure, the raw and demineralized coals were extracted in pyridine byGerhardt Soxtherm macro414Soxhlet respectively. Pyrolysis-gaschromatograph/mass spectrometer (Py-GC/MS) was used to analyze theformation and distribution characteristics of BTEXN during raw coal,demineralized coal, and residual coal pyrolysis online. By comparing theformation and distribution principles of the BTEXN before and afterdemineralization and extraction, the inherent relationship between minerals and the BTEXN formation was revealed, and the effect of low molecular weightcompounds on the BTEXN formation was also got. Moreover, the catalyticmechanism of minerals and the influence mechanism of low molecular weightcompounds on the BTEXN formation were discussed. The main conclusions areas follows:(1) The total yield of BTEXN from KZ pyrolysis is1.1wt.%. With thepyrolysis temperature increasing from500oC to1000oC, the yield of BTEXNfirst increases, reaches a maximum value at700oC, then decreases. Themaximum value accounts for57%of the total yield. The most abundant BTEXNspecies in coal pyrolysis are benzene and toluene.(2) The inherent minerals have an obvious catalytic effect on the BTEXNformation during KZ pyrolysis. Ca, Fe-based minerals in KZ have a goodcatalytic activity on the decomposition of condensed aromatics and phenols tolight aromatic hydrocarbons during pyrolysis, and the final product compositionand distribution derived from coal pyrolysis are changed.(3) The main components of the low molecular weight compounds fromKZ are straight-chain aliphatic hydrocarbons, aromatic hydrocarbons, andoxygen-containing compounds. The amount of BTEXN from low molecularweight compounds pyrolysis accounts for16%of the demineralized parent coalpyrolysis, which indicates that the BTEXN formation during KZ pyrolysis ismainly from the thermal decomposition of the macromolecular networkstructure. The low molecular weight compounds are helpful to the BTEXNgeneration. As hydrogen-rich components, low molecular weight compounds,especially the aliphatics pyrolysis could provide a plenty of activated hydrogen,which could be coupled with the free radical fragments to promote the BTEXNformation.(4) The existing form of low molecular weight compounds in coal, the wayand position connecting with macromolecular skeleton structure, havesignificant impacts on its hydrogen donating ability. The aliphatics can be cracked to generate activated hydrogen under the catalysis of minerals, then theability that low molecular weight compounds provide activated hydrogen isenhanced. Therefore, the presence of minerals and low molecular compoundscould simultaneously promote the BTEXN generation.