Effect of Ca2 + species with different modes of occurrence on direct liquefaction of a calcium-rich lignite

Abstract

Low-rank coals concurrently contain organically bound alkali and alkaline earth metals (AAEM species) and discrete mineral matters. In this work, the influences of exchangeable calcium cations and discrete calcium salts on direct liquefaction of a Ca2+-rich lignite were investigated. The raw coal was partially demineralized by ammonium acetate solution and certain amounts of Ca2+ were loaded into the demineralized coal (DMC) by ion-exchanged or physical mixing method separately. The amount of exchangeable Ca2+ and -COOH groups were finely correlated by FTIR spectra. Temperature programmed liquefaction of various samples was performed to obtain the product distributions. Experimental results of liquefaction demonstrate that exchangeable Ca2+ present in coal are adverse to direct liquefaction process and Ca2+ loaded by physical mixing method have less detrimental effects on the oil yield compared with the exchangeable ones. The reason could be attributed to that exchangeable Ca2+ accelerate generation of CO2 and free radical fragments through coal pyrolysis, which enhance the unbalance between generation rate of free radicals and that of their combination with hydrogen donors, and eventually results in the formation of tetrahydrofuran-insoluble portion (THFISP). Therefore, oil generation during liquefaction of the lignite is mainly inhibited by exchangeable Ca2+ and the influences of discrete calcium salts are negligible.