Simplified model of torrefaction-grinding process integrated with a power plant

A simple method is presented to formulate the total energy consumption of a biomass torrefaction process operating as a standalone unit or integrated with a downstream power plant. New expressions are derived to estimate the volatiles heating value, mass yield at autothermal operation, and mass yield at minimum energy consumption. Experimental data reported in the literature for grinding torrefied beech in a knife-mill are correlated to estimate the specific grinding energy. The presented model accurately predicts the critical moisture content and the auxiliary heat input of a 150 MW_th torrefaction plant producing torrefied woodcutting. Illustrative numerical results are presented for an integrated application using willow and beech as two woody feedstocks. The autothermal solid yield is found to be higher than that at optimum energy consumption. The autothermal solid yield declines with moisture content and it is approximately the same for both feedstocks. For every percentage point increase in the power plant efficiency, the optimum solid yield increases 0.67% and 0.15% for willow and beech, respectively, whereas the energy consumption decreases by 7.1 kJ/kg (willow) and 5.75 kJ/kg (beech). The results suggest that the design of torrefaction plants is highly feedstock-dependent. The method presented can be applied to any type of feedstock and both upstream and downstream applications. It is a simple and effective tool that can be used to have a quick estimate of optimum operating point and energy consumption at early design stages.