Mass loss rates for wood chips at isothermal pyrolysis conditions: A comparison with low heating rate powder data

Abstract

Spruce chips of three different thicknesses were pyrolyzed isothermally in a vertical furnace macro-TGA at 574 to 676K, which is the temperature range relevant for char production. The measured mass loss data was analyzed in terms of mass loss rate, thermal lag and char yield as a function of chip size and pyrolysis temperature. The char yield decreased with increasing temperature and there was no significant difference in char yield as a function of sample thickness, ranging from 1mm to 7mm. Thermal lag was present for all chip sizes above 600K. At 574K the data suggests that chips below 1mm in thickness are decomposing at rates governed by reaction kinetics. An isoconversional kinetic model based on low heating rate data of spruce powder was adopted to analyze the data. The model predicted lower mass loss rates than those measured for the chips, suggesting that the pyrolysis process of wood proceeds through a network of parallel reactions. Despite this, the model could predict the final char yield of the wood chips with an accuracy above 80%. The predictive capability of the isoconversional reaction rate expression is promising since the procedure to derive such a rate expression is straight-forward, compared to the conventional model-fitting methods. The data and modeling approach presented in this work is important to the field of biomass pyrolysis as it covers the temperature range and chip sizes relevant for pyrolysis in multi-staged gasification plants which has been given little attention.