A Detailed Experimental Study on the Thermal Decomposition Behaviour of Wood Pellets Under Inert and Oxidative Conditions in a Fixed Bed Reactor

Abstract

In this work, spruce wood pellets are pyrolysed in an electrically heated fixed bed reactor. Experimental campaigns have been conducted to investigate the influence of final pyrolysis temperature (600-800°C), heating rate (5-20 K/min) and purge gas composition (none, 100% N₂ and 90/10% N₂/O₂). The instrumentation of the reactor includes transient temperature measurements in the reactor (3 locations in the vertical direction) and inside the pellets bed (3 locations in the radial direction) throughout the thermal decomposition process. Gas measurements are carried out for permanent gases (using a GC), condensables are condensed and collected and the remaining solids are also collected. Hence, the mass balance can be established. The detailed experimental results make them useful for validation of thermal decomposition modelling approaches. The experimental results show evidence of endothermal cellulose decomposition reactions as well as the exothermal char formation process. The occurrence of these two processes overlap at high heating rates and when oxygen is used in the purge gas. The two processes can be separated visually by decreasing the heating rate to 5 K/min. The separation shown in the temperature curves is confirmed by the gas analysis. The yields of CO and CH₄ show a visible shoulder in the higher temperature region. The endothermic plateau visible on the temperature readings can be reduced by increasing the heating rate. Oxygen present in the purge gas will further reduce the visibility of the plateau. The amount of CO₂ formed during experiments shows dependency on the oxygen in the purge gas but appears independent of the applied heating rate. The comprehensive experimental results provide both useful knowledge and a validation basis for further modelling work.