Numerical Investigation of the Combined Impact of Reactor Pressure and Heating Rate on Evolution and Yields of Biomass Pyrolysis Products in Thermally Thin Regime

Abstract

Combined effects of reactor pressure (0.0001, 0.01, 1, 10, 100 atm) and heating rate (10, 20, 30, 40 and 50 K/s) on biomass pyrolysis characteristics at a final reactor temperature of 973 K in thermally thin regime have been numerically investigated. Wood pellets (  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 pyrolysis. Solid mass conservation equations were solved by first order Euler Implicit Method (EIM). Finite Volume Method (FVM) was used to discretize the transport, energy conservation and pressure generation equations, and the resulting linear simultaneous equations were solved by Tri-Diagonal Matrix Algorithm (TDMA). Intra-particle fluid flow velocity was estimated by Darcy’s law. Results showed that pressure does not have any significant effect on biomass primary disintegration reactions at all heating rates. Increase in heating rate accelerated the rate of biomass primary conversion. In the vacuum region, increase in pressure did not have any significant effect on tar, gas and secondary tar release rates and yields at all heating rates. Increase in pressure from vacuum to atmospheric, and from atmospheric to pressurized condition diminished tar release rate and yield but favoured gas and secondary tar release rates and yields at all heating rates. Findings further showed that volatiles intra-particle secondary reactions products generation rates at atmospheric and pressurized conditions were ten times and over higher than those at vacuum conditions. It was concluded that at and above atmospheric pressure conditions, pressure can significantly influence the rate of generation, yield and percentage composition of product species.

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