Numerical Investigation of the Effects of Reactor Pressure on Biomass Devolatilization in Thermally Thick Regime

Abstract

Effects of reactor pressure on biomass devolatilization in thermally thick regime were numerically investigated in this study. Wood pellet  was modeled as a two-dimensional porous solid and pyrolysis was simulated at a heating rate of 30 K/s and final reactor  temperature of 973 K for five different reactor pressures [vacuum; 0.0001 and 0.01 atm, atmospheric;1 atm and  pressurized; 10 and 100 atm, regions]. Transport equations, kinetic models, intra-particle pressure generation equation and energy conservation equation were coupled and simultaneously solved to simulate the pyrolysis process. Solid mass conservation equations were solved by first order Euler Implicit Method. Darcy’s law was used to estimate intra-particle flow velocity. Finite Volume Method was used to discretize the transport, energy conservation and pressure generation equations. Results showed that even in thermally thick regime, increase in reactor pressure does not affect the rate of primary tar generation. Findings also revealed that the rates of generation of secondary products at atmospheric and pressurized regions are not significantly different. Further increase in reactor pressure in the pressurized region resulted in a slight reduction in the peak of secondary products generation rate. Results further showed that increase in reactor pressure reduced intra-particle temperature gradient especially in the pressurized region, thereby causing process time elongation. As would be expected, tar release rate decreased with increase in reactor pressure while gas release rate increased in both atmospheric and pressurized regions.

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