Behavior of boron release and change in the occurrence mode of boron during fixed-bed pyrolysis of coals

Behavior of boron (B) release and change in the occurrence mode of B during fixed-bed pyrolysis of seven coals with 60–82 mass%-daf C have been studied mainly to understand B chemistry in coal utilization. The sequential leaching experiments of the coals with H2O, CH3COOH (AcOH) and HCl solutions show that the proportion of H2O-soluble B species is higher with lower rank coal with a smaller carbon content (C%), whereas the B insoluble in these solvents exhibits the reverse C% dependence. The density separation of some coals with a sink-float technique also reveals that the concentration of B element tends to increase with decreasing density of coal particles. When each coal used is pyrolyzed at 10 °C/min up to 1200 °C, the extent of B release depends significantly on the kind of coal and ranges from approximately 5–70%, it at 900 °C being lower at a higher content of SiO2 or Al2O3 in coal. About 60–100% of the B present in the chars after pyrolysis at 900 °C is insoluble in the above-mentioned three solvents in many cases, and the rest exists in the forms of H2O- and/or AcOH-/HCl-soluble B. In the fixed-bed gasification of the 900 °C-chars with CO2 under the conditions of 900 °C and 0.1 MPa, char conversion increases with increasing reaction time, but no significant B release is observed in almost all cases. It is thus probable that the B in char is retained and enriched in the residual char even at the latter stage of gasification.