Modeling and prediction of devolatilization and elemental composition of wood during mild pyrolysis in a pilot-scale reactor

Abstract

Mild pyrolysis, operated at 200–300 °C in an inert atmosphere, is a promising technology to produce sustainable materials (i.e., heat treated woods for construction and building) and solid fuels (i.e., torrefied woods or biochars for combustion and gasification). To aid in process and reactor design, the aim of this work is to conduct thermal degradation kinetics of wood. A two-step kinetics model is developed to predict the elemental composition (C, H, and O) and devolatilization dynamics of wood materials during heat treatment in a pilot-scale reactor by kinetic analysis. A hardwood poplar (Populus nigra) and a softwood fir (Abies pectinata) sever as feedstock, and the experiments are carried out at 200–230 °C with a heating rate of 0.2 °C min−1 in a low-pressure environment (200 hPa). The predictions in the weight losses of the woods are in a good agreement with the experimental data. The evolutions of solids, volatiles, elements (C, H, and O), and the heating values of treated woods are further analyzed. The predictions suggest that the intermediate solid is the main product, and almost all the woods are converted when the treatment temperature is as high as 230 °C. The devolatilization process, which is responsible for the mass loss of wood, can be clearly identified, and the volatile liberation amounts from poplar and fir at 230 °C are 17.05 and 12.44 wt%, respectively. The predicted HHVs of treated woods from the empirical formula are between 19.62 and 20.55 MJ kg−1, and the enhancement factors at the end of treatment are between 1.01 and 1.07 which is close to torrefied wood after light torrefaction. During the treatment, the extents of decarbonization, dehydrogenation, and deoxygenation in fir are all smaller than those in poplar, resulting from the lower intensity of devolatilization in the former.