Insights in mechanisms of carbonaceous microparticles formation from black liquor hydrothermal conversion

Abstract

The study addresses the formation of secondary hydrochar and the morphology of secondary microparticles from black liquor. The black liquor solution was hydrothermally converted at 350 °C and 16.5 MPa in batch reactors running from 15 min to 24 h at different heating and cooling rates. Spherical carbon microparticles were obtained at a short reaction time (< 90 min) and then coalesced to form aggregates. At a very long reaction time (24 h), spherical carbon microparticles were obtained once again. The carbonaceous solid was enriched in carbon during the first 4 h of reaction. In parallel, the concentrations of total organic carbon and total phenols in the solution decreased drastically. The formation of solids was mainly due to chemical reactions occurring during the first 4 h. Subsequent changes in the solids were mainly due to physical reorganization since the chemical compositions of solutions were almost stable.