Effects of Secondary Reactions on the Destruction of Cellulose-Derived Volatiles during Biomass/Coal Co-gasification

The purpose of this study was to reveal the structural evolution of cellulose-derived tars in the biomass/coal co-gasification environment. A two-stage reactor was employed, where the pyrolytic vapors of cellulose were produced in the top stage and the secondary reactions of these vapors took place in the bottom stage under a range of conditions. The roles of anthracite char and steam were examined by implementing three operational modes in the bottom stage: thermal cracking (TC), catalytic cracking (CC), and catalytic reforming (CR), over a temperature range from 600 to 900 °C. Anthracite char was effective in enhancing tar reduction at lower temperatures (≤700 °C), above which its effect diminished. Tar yields under the CC and CR modes were comparable in the studied temperature range, suggesting the minimum effects of steam on the tar amount. However, gel permeation chromatography coupled with a diode array detector (GPC–DAD) and gas chromatography–mass spectrometry (GC–MS) characterization of tars showed that both anthracite char and steam had significant influence on the composition of tar, in terms of the molecular weight distribution and aromatic cluster sizes. Specifically, at a high temperature (900 °C), the presence of anthracite char facilitated the reduction of aromatic compounds, especially those with larger aromatic ring systems (≥3 fused benzene rings). In addition, the used anthracite chars showed lowered specific surface areas, which was postulated to be the main reason for their slightly reduced gasification reactivity.