Integrated torrefaction vs. external torrefaction – A thermodynamic analysis for the case of a thermochemical biorefinery

Integrated and external torrefaction is analyzed and compared via thermodynamic modeling. In this paper, integrated torrefaction is defined as torrefaction integrated with entrained flow gasification. External torrefaction is defined as the decentralized production of torrefied wood pellets and centralized conversion of the pellets by entrained flow gasification. First, the syngas production of the two methods was compared. Second, the two methods were compared by considering complete biorefineries with either integrated torrefaction or external torrefaction. The first part of the analysis showed that the biomass to syngas efficiency can be increased from 63% to 86% (LHV-dry) when switching from external torrefaction to integrated torrefaction. The second part of the analysis showed that the total energy efficiency (biomass to methanol + net electricity) could be increased from 53% to 63% when switching from external torrefaction to integrated torrefaction. The costs of this increase in energy efficiency are as follows: 1) more difficult transport, storage and handling of the biomass feedstock (wood chips vs. torrefied wood pellets); 2) reduced plant size; 3) no net electricity production; and 4) a more complex plant design.