Detailed Investigation on Sodium (Na) Species Release and Transformation Mechanism during Pyrolysis and Char Gasification of High-Na Zhundong Coal

A typical type of Zhundong coal with high sodium (Na) concentration was pyrolyzed, and the pyrolyzed char was gasified at temperatures of 850–950 °C in a laboratory-scale electrically heated tube-quartz fluidized-bed reactor, to study the releasing characteristics and transformation mechanism of Na species in various occurrence modes including the H2O-soluble, NH4Ac-soluble, HCl-soluble, and insoluble Na species. The various Na species in collected residues were quantified using a method of sequential chemical extraction combined with an inductive coupled plasma equipped atomic emission spectrometer. The crystalline phases in residues were identified through the X-ray diffraction analysis. The releasing characteristics and transformation mechanism of various Na species were in detail discussed through the qualitative and quantitative analyses of the experimental results. It was shown that the H2O-soluble and NH4Ac-soluble Na species mainly contributed to release of total Na species during pyrolysis, while the H2O-soluble and insoluble Na species did that during char gasification, and a larger quantity of Na species was released at the stage of char gasification. Also, different occurrence of Na species displayed different releasing and transformation characteristics, showing a strong relationship with processes of pyrolysis and gasification. A chart describing the releasing characteristics and transformation mechanism of various Na species was given. It was speculated that the H2O-soluble Na species in the form of Na2SO4 and NaCl were more likely to be released into gas in the form of ultrafine particulates or at molecular/atomic scales through the particle pores along with the gas flow, with the NaCl partially being transformed to the NH4Ac-soluble Na species during pyrolysis and char gasification. The NH4Ac-soluble Na species was more likely to be transformed to the insoluble Na species after a series of reactions between radicals from volatiles and Na species during pyrolysis, and was released accompanying with the consumption of the char substrate during gasification. Besides, various Na species showed enrichment in residue particles during pyrolysis and char gasification. This work can provide a comprehensive understanding of various Na species release and transformation mechanism for the Zhundong coal gasification, which is essentially beneficial to optimize the gasification process in industry for controlling the ash-related problems.