Co-firing based on biomass torrefaction in a pulverized coal boiler with aim of 100% fuel switching

Torrefied biomass has several benefits, such as higher energy density, good grindability, higher flowability and uniformity. The process of torrefaction moves the chemical and physical properties of raw biomass close to that of bituminous coal, which allows co-utilization with high substitution ratios of biomass in the existing coal-fired boilers without major modifications. In this study, a torrefaction based co-firing system was proposed and studied. Devolatilization and char oxidize kinetics of the torrefied biomass have been investigated experimentally. CFD modeling of co-firing with varying substitutions of torrefied biomass in a pulverized coal boiler have been carried out. To figure out the boiler performance when co-firing torrefied biomass, five different cases were involved and simulated, coal only, 25% biomass, 50% biomass, 75% biomass, and 100% biomass on thermal basis, respectively. The results showed torrefaction is able to provide a technical option for high substitution ratios of biomass in the co-firing system. The case-study pulverized coal boiler could be fired 100% torrefied biomass without obvious decreasing of the boiler efficiency and fluctuation of boiler load. More positively, the net CO2 and the NOx emissions significantly reduced with increasing of biomass substitutions in the co-firing system.