Influence of different state alkali and alkaline earth metal on chemical structure of Zhundong coal char pyrolyzed at elevated pressures

The investigation on the evolution of coal char structure is the prerequisite in evaluating the catalytic action of alkali and alkaline earth metal (AAEM) during pressurized pyrolysis. In the study, Zhundong raw coal samples were first treated using the chemical fractionation method, and then pyrolyzed at 0.1 MPa-3 MPa in a pressurized thermogravimetric analyzer (PTGA). In order to understand the effect of AAEM on the chemical structure of the samples at elevated pyrolysis pressures, the Fourier transform infrared spectroscopy (FT-IR), the Raman spectroscopy and a series of original structure parameters were employed. It is indicated that over 1 MPa, water-soluble AAEM inhibited the decomposition of phenolic hydroxyl group, while ion exchangeable AAEM inhibited the decomposition of carboxyl functional groups and catalyzed the cracking of aliphatic long chains. The influence of pressure on conversion of functional groups was various at different pyrolysis temperatures. Under 1 MPa, the pressure had distinct influences on the carbon skeleton structure of coal char and mainly promoted the graphitization process of samples during pyrolysis. Besides, the ratios of phenolic hydroxyl, ether and aliphatic aromatic in char were not sensitive with elevated pressure.