PAhs Formation And Its Catalytic Cracking Property During Coal Pyrolysis

Polycyclic Aromatic Hydrocarbons (PAHs) from the coal conversion have attracted increasing attention due to carcinogenicity, teratogenicity, and mutagenicity which affect human’s health and environment when they were released into the environment. Pyrolysis as the starting step plays a key role in coal thermal conversions, where include combustion, gasification and liquefaction, and especially for coking which is a pyrolysis process entirely. Research on the formation, emission and catalytic cracking property of PAHs during coal pyrolysis has significant effect on pollutants control and clean coal utilization at present and in the futureGerhardt Soxtherm macro414Soxhlet was used to fully extract the PAHs from15kinds of Chinese raw coals different in rank by the CH2C12, GC-MS was used to analyze the free PAHs extracted from raw coals and PY (Pyro-probe CDS5250)-GC-MS was used to online pyrolysis analysis of the raw coals, their extraction residues and extracts respectively. The influence of alkaline metal oxides which are inexpensive and easily available on the PAHs catalytic cracking was also investigated. Main contents as follow:(1) The Gerhardt Soxtherm macro414was used to extract the PAHs from raw coals by dichloromethane, a complete method for the extraction and analysis of PAHs from raw coals was established. The method has the characters of high extraction efficiency, time saving, high solvent recovery, high sensitivity and environmental protection.(2) The relationship between the distribution of free PAHs in raw coals and coals properties were studied, and the PAHs Toxicity Equivalent (TEQ) was also mentioned. The results showed that the properties of raw coals have significant effect on the distribution of free PAHs; The free PAHs in the raw coals were mainly dominated by4,5-ring PAHs, and bituminous coal has the highest concentration of TEQ, which may pollute the environment more easily;(3) PAHs emitted from15kinds of Chinese coals different in rank during pyrolysis were studied. The results suggest that the lower-ring (2,3-rings) PAHs prevailed over the higher-ring PAHs during the coals pyrolysis; the amounts and species of PAHs under the same conditons of coal pyrolysis depend on coal properties; Temperature has significant effect on the distribution of PAHs from coal pyrolysis and the emission amount of PAHs showed a single peak at800℃with increasing temperature. The emission amount of PAHs decrease with increasing heating rate at the same pyrolysis temperature.(4) The source of the PAHs from coal pyrolysis is discussed by comparing the difference among the amounts and species of the PAHs from the pyrolysis of raw coals, their extraction residues and extracts, the free PAHs from raw coals. The main conclusions are as follows:the16PAHs mainly came from complex chemical reactions of coal pyrolysis rather than from the free PAHs in the raw coals; the higher-ring (5,6-ring) PAHs were prone to decompose at certain temperature; NaP was formed by the reaction of coal pyrolysis, and the increase of the total PAHs was mainly the contribution of the2,3-ring PAHs; The breakdown of the macromolecular network and the free radicals condensation reaction were the main causes which gave rise to the forming of PAHs in the coal pyrolysis.(5) The precursors of PAHs was studied by comparing the omposition of PS coal extract and its extract pyrolysis. The results indicated that the extract of PS raw coal is mainly composed of aromatic compounds which accounted for70.87wt%of the total extract. Aromatic compounds mainly contains16PAHs and their derivatives of isomerization and alkylation, part of the derivatives which generate low-ring PAHs during the pyrolysis are the precursor of a small ring PAHs.(6) The effect of calcium oxide(CaO) and iron oxide(Fe2O3) on the16PAHs emitted from the coal pyrolysis were analyzed under different operation conditions including temperature, quality and partical size of catalyst. The results showed that the leading effect factor of CaO and Fe2O3on catalytic action of PAHs decomposition was temperature; at600℃, Fe2O3had the best catalytic cracking effect to16PAHs; while under the CaO catalytic cracking effect, the best temperature is700℃.