Characterization of torrefied fuels for direct co-firing in large pulverized fuel boilers

The torrefaction process involves heating biomass in the absence of oxygen to a temperature of 200°C to 300°C thereby reducing the moisture and volatile content of biomass. This leaves behind a dry, relatively more hydrophobic, brittle and carbonized solid, similar to coals. Torrefied biomass fuels have energy densities approaching those of coals. For torrefied biomass, the transport and logistics costs are reduced and significant investments on separate storage facilities, milling and feeding equipment required for non-thermally treated biomass materials can be minimized. Torrefied biomass fuels can be potentially co-fired with coal at higher percentages in conventional coal boilers with minimal modifications. The work investigates the milling and co-firing properties of torrefied biomass from lab scale tests to demonstration tests in a utility boiler. The milling tests include development of a lab-scale method for characterizing grindability of torrefied biomass and its validation in industrial scale mills (hammer mils, bowl mill and a ball and ring grinder).
De-volatilization and combustion tests were performed for torrefied biomass in order to understand the differences there are compared to non-torrefied biomass and coals, especially as it relates to combustion reactivity, NOX formation and reduction. Co-firing tests established whether there are secondary synergetic effects observed for fuel mixtures of coal and torrefied biomass. The tests were performed in an entrained flow reactor (20kW), a pilot scale 500kW combustion test rig and a 420MW Utility boiler.