Computational Study of the Yield of Solid Wood Pyrolysis Products under High Pressure

Abstract

The kinetic model of wood pyrolysis under pressure is discussed in the present paper taking into account the diffusion of the resulting gas-phase products (i.e. heavy hydrocarbons) and their decomposition reactions. This model is based on a simplified mechanism of wood pyrolysis, including two parallel chemical reactions, viz. the primary decomposition reaction of wood biomass with the formation of solid and gaseous components and the thermal decomposition reaction in the biomass pores of hydrocarbons formed in the primary process. The model takes into account the diffusion processes of the primary pyrolysis products from the resulting pores and thermal decomposition in the pores of these products. Based on the developed model, a computer program for calculating the main parameters of the pyrolysis process under pressure was created and the mass yield of solid pyrolysis products under various conditions was calculated. The calculation took into account the main parameters that affect the yield of solid wood biomass products, viz. temperature and pyrolysis pressure, particle sizes, porosity, etc. The calculations demonstrated that the increase of the pressure at which the pyrolysis of wood biomass is carried out causes an increase of the formation of the amount of solid products, which corresponds to the available experimental data. It was established that at a pressure of 1 atm when a sample size is of 0.025 m, the maximum yield of solid products is observed at the temperature of 600 °C. As the pressure increases the maximum yield increases, while the temperature at which the maximum is reached decreases. So, at a pressure of 10 atm when a particle size is of 0.025 m, the maximum yield of solid products is observed at the temperature of about 500 °C, and it is higher than that at 1 atm by 1.18 times. It was also determined that the temperature of the maximum yield of charcoal decreases with increasing sizes of pyrolyzable samples. Thus, when a sample size is of 0.5 m, this temperature is about 400 °C at 10 atm.

Keywords

• wood
• thermal decomposition
• overpressure
• secondary reactions
• reaction rate constants
• mass yield