Effect of KCl doping on the slow pyrolysis of lignin

This study examines the effect of KCl on the slow pyrolysis of lignin. Commercial alkali lignin was chosen as reference compound and it was demineralized through a washing procedure using two different leaching media, distilled water and a HF water solution at 3 wt.%. The ICP/MS analyses of the demineralized samples showed that the HF washing procedure was more effective than the water washing in reducing the K ions contents of the raw lignin. The HF demineralized sample was doped with an amount of KCl corresponding to 0.6 wt.% of K ion using the wet impregnation procedure. Subsequently, thermogravimetric (TG) analysis and slow pyrolysis tests of raw, demineralized and doped lignin samples were performed at a heating rate of 5 °C/min up to 900 °C and 700 °C, respectively. Gas species release was monitored along the pyrolysis temperature. The reduction of K ions slightly affected the pyrolytic behaviour of the raw sample by delaying both the onset of the lignin decomposition reactions and the maximum weight loss rate by about 10 °C. Moreover, the reduction of the alkali metals content enhanced the weight loss in the range of 330 and 450°C, thus affecting only negligibly the solid residue yields at 700 °C. Finally, it was observed that the KCl addition promotes decarboxylation reactions and dehydrogenation of the char matrix and depresses the carbon monoxide production.