Combustion (pyrolysis with oxygen) and pyrolysis without oxygen are two potential methods to convert wood into biofuels or biochemicals. To evaluate which is preponderant to convert wood into biofuels or biochemicals and provide guidance for optimization of product yield, the pyrolysis characteristics and kinetics of typical hardwood (black walnut) are comparatively investigated in nitrogen and air employing thermogravimetric analysis (TGA) and differential scanning calorimeter (DSC). Two model-free methods including Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS) method are applied to obtain the kinetic parameters, and a model-fitting method called Coats-Redfern (CR) method is employed to estimate the reaction mechanism. The black walnut pyrolysis in nitrogen may be divided into two stages with the threshold of conversion rate α = 0.4, but that in air may be separated into three stages with the thresholds of α = 0.25 and 0.7. The reaction mechanism for pyrolysis in nitrogen may be assumed random nucleation and its subsequent growth, but that in air may be assumed random nucleation and its subsequent growth followed by chemical reaction. The average activation energy and natural logarithm of pre-exponential factor for the whole pyrolysis process in nitrogen and air are 211.59 and 187.73 kJ/mol and 32.33 and 28.36 min-1, respectively.
Keywords: Biofuels; Kinetics; Mechanism; Pyrolysis; Thermogravimetry; Wood biomass.