Biomass fast pyrolysis in a shaftless screw reactor: A 1-D numerical model

The thermochemical conversion of biomass can be effective for flexible and programmable production of electric and thermal power. Only a few models have been developed so far in the literature to describe the behavior of a screw reactor system designed for biomass fast pyrolysis. The temperature profile plays a crucial role in particular for fast pyrolysis purposes. Hence, a complete heat transfer model is required to that aim. This paper is focused on numerical modeling of a shaftless screw pyrolyzer with special focus on the kinetic framework, as well as the description of heat and mass transfer phenomena. A steady-state model with constant wall temperature has been developed to generate temperature profile and conversion patterns along the reactor. Residence time distribution input has been considered to take into account non-perfect mass conveying characteristics. The model, including all the different heat flux mechanisms such as conduction, convection and radiation, is based on a four parallel Distributed Activation Energy Model. The structure includes the three major biomass pseudo-component occurring in the biomass thermal degradation, and namely cellulose hemicellulose and lignin, along with the moisture evaporation process. Numerical results have been compared with experimental data of spruce wood pellet fast pyrolysis obtained in a lab-scale screw reactor. Numerical temperature profiles for both gas and solid phase, are in good agreement with experimental data. The results obtained allow for demonstrating that the selected framework gives realistic conversion rates for all the fast pyrolysis products namely bio-oil, char, and syngas. The maximum bio-oil production from ground spruce wood has been observed at 500 °C, with yield in the range of 64%. Moreover, the results show a strong dependence on wall temperature, gas-solid heating rate, and screw geometry.