The influence of torrefaction on the combustion behavior of selected agricultural and waste-Derived solid biomass fuels

The utilization of agricultural and waste-derived solid biomass fuels for generation of sustainable energy is widely hampered by their “difficult” combustion behavior. Torrefaction can be a well suitable procedure to improve the fuel properties of these fuels. In this study the combustion behavior of five widely available untreated and torrefied biomass fuels - forest residues (FR), sugar cane leaves (SCL), green waste (GW), winter wheat straw (ST) and sunflower husks (SFH) - were investigated by the application of a multi-step laboratory approach (chemical analyses, fuel indices and fixed-bed lab-scale reactor combustion tests). The investigations revealed that SFH (untreated and torrefied) can be recommended as a fuel directly applicable in grate furnaces (also without flue gas recirculation) because of the low slagging tendency, but high aerosol emission needs to be considered. Also FR, SCL and GW (untreated and torrefied) can be applied under consideration of the slagging tendency, whereas ST is not recommendable concerning its severe slagging tendency. However, for all fuels investigated the applicability in grate fired plants needs special considerations, since they do not behave like natural stem wood. Therefore, an appropriate validation by means of experimental investigations with a lab-scale combustion facility is highly recommended before using such fuels. The combustion tests reveal the measures which must be taken to ensure an undisturbed and continuous operation on grate furnaces. By execution of such combustion tests the advantages of torrefied material (e.g., higher calorific value, better storage feasibility, etc.) can be fully exploited.