Biomass fast pyrolysis in screw reactors: Prediction of spent coffee grounds bio-oil production through a monodimensional model

Abstract

In the context of renewable sources exploitation, the thermochemical conversion of biomass may give a significant contribution to the flexible and programmable production of electric and thermal power. From this perspective, the biomass fast pyrolysis conversion process is more than a promising technology but only few models have been developed so far to describe the behavior of a screw reactor system. This paper is thus focused on numerical modeling of a shaftless screw fast pyrolyzer with special attention on the residence time distribution and the definition of the kinetic framework, as well as the heat and mass transfer phenomena representation. A steady-state model with constant wall temperature has been developed to generate temperature profile and conversion patterns along the reactor. Residence Time Distribution evaluation has been developed as well to take into account non-ideal mass conveying characteristics of the proposed reactor design. The reaction framework, considering the conductive, convective and radiative heat transfer mechanisms, is based on a 4 parallel Distributed Activation Energy Model. The structure includes the three major biomass pseudo-component occurring in the biomass thermal degradation, namely cellulose hemicellulose and lignin, together with the moisture evaporation process. The numerical results are compared with results collected experimentally from the fast pyrolysis of spent coffee grounds in a lab-scale screw reactor. Numerical temperature profiles for both the gas and solid phase are in good agreement with the experimental ones. The peak bio-oil production has been observed in the range of 500 °C. The results also show a strong dependence of results on wall temperature and gas-solid heating rate.