Numerical Investigation of the Effect of Reactor Severity on Biomass Pyrolysis Characteristics in Thermally Thick Regime

Abstract

Effect of reactor peak temperature on biomass pyrolysis in thermally thick regime with a constant heating rate of 30 K/s, reactor pressure of 1 atm and reactor peak temperature ranging from 500 to 1000 ^oC in a fixed-bed reactor has been numerically investigated. 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. 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. Results showed that increase in reactor temperature from 500 to 600 ^oC and from 600 ^oC to 700 ^oC favoured primary tar intra-particle secondary reactions, causing corresponding increase in the release rate and yield of gas and secondary tar. Increase in reactor temperature beyond 700 ^oC had no significant influence on primary tar intra-particle secondary reactions and on the release rates and yields of volatiles and primary tar. Char yield, however, decreased continually with increase in reactor temperature. The highest gas yield (50.10%) was obtained at 700 ^oC, primary tar yield (33.91%) at 500 ^oC and char yield (14.79%) at 500 ^oC.

Keywords: Biomass, pyrolysis, intra-particle secondary reactions, thermally thin regime