Effects of Pressure and Heating Rate on Products Release Rates and Yields during Biomass Pyrolysis in Thermally Thick Regime

Abstract

Numerical investigation of the synergetic effects of reactor pressure and heating rate on biomass pyrolysis in thermally thick regime has been carried out. Wood cylinders  were modeled as two-dimensional porous solids. Transport equations, solid mass conservation equations, intra-particle pressure generation equation and energy conservation equation were coupled and simultaneously solved to simulate the pyrolysis process and the accompanying physical phenomena. First order Euler Implicit Method (EIM) was used to solve the solid mass conservation equations. The transport, energy conservation and intra-particle pressure generation equations were discretized by Finite Volume Method (FVM). The generated set of linear equations was solved by Tri-Diagonal Matrix Algorithm (TDMA). Intra-particle fluid flow velocity was estimated by Darcy’s law. Findings revealed that increase in reactor pressure does not significantly affect biomass primary decomposition reactions for all heating rates considered (10, 20, 30, 40 and 50 K/s). In the vacuum region (0.0001 and 0.01 atm), increase in pressure had no significant effect on the release rates and yields of product species at all heating rates. In the pressurized region, increase in pressure (from 10 to 100 atm) inhibited intra-particle secondary reactions. Pressure increase from vacuum to atmospheric and from atmospheric to pressurized condition caused some changes in product distribution. The degrees of intra-particle secondary reactions at atmospheric and pressurized conditions were much higher than at vacuum conditions. Primary tar release rates and yields at atmospheric and pressurized conditions were much lower than at vacuum conditions while gas and secondary tar release rates and yields were higher at atmospheric and pressurized conditions. Intra-particle secondary reactions were more sensitive to change in heating rate at vacuum conditions than at atmospheric and pressurized conditions.

Keywords: Biomass, pyrolysis, pressure, heating rate, intra-particle secondary reactions, thermally thick regime