Releasing behavior of mercury in coal during mild thermal treatment

Abstract

Mild coal thermal upgrading is one of the strategies for the emission control of mercury prior to coal utilization. Two Chinese coals have been selected to investigate the removal and emission behaviors of mercury in the process of coal mild thermal upgrading. Ontario Hydro Method (OHM) has been used to identify the emission behavior of different forms of mercury under rapid and temperature-programmable thermal upgrading treatments. The releasing behaviors of mercury associated with different matters in coal during thermal upgrading are identified by two-step acid washing pretreatment. The results indicate that Hg-silicate in coal is found to release between 300-450<span style="font-family: Symbol;mso-ascii-font-family:" times="" new="" roman";mso-hansi-font-family:"times="" roman";="" mso-char-type:symbol;mso-symbol-font-family:symbol"="" lang="EN-GB">°C. Hg-sulfide is found to release between 400-600<span style="font-family:Symbol;mso-ascii-font-family:" times="" new="" roman";mso-hansi-font-family:="" "times="" roman";mso-char-type:symbol;mso-symbol-font-family:symbol"="" lang="EN-GB">°C by mild thermal upgrading, however, oxidizing atmosphere has positive effect on lowering the releasing temperature for Hg-sulfide. Hg⁰ is the major form of mercury emitted under N₂ atmosphere by coal thermal upgrading. Temperatures increasing from 200-500<span style="font-family:Symbol;mso-ascii-font-family:" times="" new="" roman";mso-hansi-font-family:="" "times="" roman";mso-char-type:symbol;mso-symbol-font-family:symbol"="" lang="EN-GB">°C under weak oxidizing atmosphere not only have significant effect on increasing the total mercury releasing ratio but also improving the proportion of Hg²⁺ in the flue gas, especially for coal with high Hg-sulfide compounds.

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