Occurrence and transformation of sodium and calcium species in mild liquefaction solid product of Hami coal during pyrolysis

摘要

To better understand the occurrence and chemical transformation of sodium and calcium species in mild liquefaction solid product (MLS) of Hami coal (HM) during pyrolysis, sequential chemical extraction combined with SEM-EDS and XRD analysis of low temperature ash were used in this work. Results show that the addition of polymer dispersant in sequential chemical extraction help to accurately determine the content of different occurrence modes of sodium and calcium species because of the hydrophobicity and high cohesiveness of samples. Compared with HM char, the smooth and compacted structure of MLS char makes it easier for calcium, sodium and sulfur to enrich on the surface of char during MLS pyrolysis. The release of sodium is much higher than that of calcium, and the release of both sodium and calcium in MLS during pyrolysis is lower than that in HM. Besides pyrolysis temperature, the transformation of calcium and sodium species is also greatly influenced by other minerals. Due to the enrichment of minerals and the addition of Fe-S catalyst in the liquefaction process, the chemical transformation behaviors of minerals during MLS pyrolysis are more complex than that of HM coal. For organic bound calcium (AS-Ca), besides partial volatilization into gas phase, it may convert into hydrochloric acid soluble calcium (HS-Ca). The content of HS-Ca decreases over 600 degrees C due to the decomposition of CaCO3, while the formation of CaS leads to a significantly increase of HS-Ca during MLS pyrolysis.