The promotion effect of pyrolysis conditions on alkali metal pretreatment during pyrolysis of sub-bituminous Zhundong coal: Carbon engulfment control and subsequent sodium transformation

Abstract

Alkali metal pretreatment in pyrolysis is proposed to address alkali-induced issues in high-alkali coal utilization, on the basis of staged fuel combustion. However, the newfound carbon engulfment remains an intractable problem. This work investigated the effect of pyrolysis conditions on enhancing alkali metal pretreatment during Zhundong coal pyrolysis over 500–1000 °C in a laboratory fixed-bed apparatus, which focused on heating rate, volatile-char interaction, and CH4 atmosphere. Multiple analytical methods were integrated to relate the facilitation effect of pyrolysis conditions on sodium pretreatment with the suppression of carbon engulfment. The results indicated carbon engulfment primarily blocked mesopores with a size <5 nm to retain large quantities of water-soluble sodium species. It resulted from coal swelling induced by the fusibility of semi-fusinites. Fortunately, volatile-char interaction and CH4 atmosphere eliminated carbon engulfment through the intra-particle reaction between gasification agents/active radicals and char matrix. They fully eliminated carbon engulfment at 800 °C, which mainly reopened mesopores with a diameter of 4–5 nm. They also promoted the formation of sodium silicates/aluminosilicates. A slow heating rate weakened carbon engulfment by limiting the fusibility of semi-fusinites, but to a lower extent. Volatile-char interaction and CH4 atmosphere were proposed as universal enhancements for alkali metal pretreatment.