Formation Mechanisms of Three-Phase Products and Soot During the Pyrolysis of Biomass at High Temperatures

To study the pyrolysis/gasification characteristics of biomass under high temperature conditions, in particular the formation mechanism of soot during the process, rapid pyrolysis of wheat straw and poplars sawdust was conducted in a lab-scale drop-tube furnace (DTF) at 900-1 300℃. Three-phase products including gas, liquid and solid were collected and characterized to study the influences of temperature and biomass origin. Special attention was paid to the soot formation during the pyrolysis. The results showed that the yields of soot from wheat straw and poplars sawdust were 0.28%-2.40% and 0.34%-6.30% (dry biomass) respectively, with an increasing trend with the temperature, while the yield of char was 2.8%-7.3% and 0.29%-2.9% respectively, with a decreasing trend. More soot was formed due to the high level of lignin and cellulose components in sawdust, and more char was produced due to the high ash and extracts contents in wheat straw. Raising temperature favored the production of uncondensed gas: for wheat straw the production rate of uncondensed gas ranged from 47%-69% and for sawdust the data were between 59%-77%. It was found that temperature has a significant effect on tar destruction. The tar dominated by aromatic compounds is decomposed completely at 1 200℃. When the pyrolysis temperature is relatively low (900-1 100℃), the soot is formed through the combined mechanism of light hydrocarbon decomposition and heavy tar polycondensation. However when temperature is higher than 1 100℃, most of the soot is formed by the decomposition of light hydrocarbon.