Enhancing biomass + coal Co-firing scenarios via biomass torrefaction and carbonization: Case study of avocado pit biomass and Illinois No. 6 coal

Co-firing of biomass with coal is a short-term solution to increase renewables in energy generation portfolios. Fuel blending uses existing infrastructure for coal combustion to reduce economic costs and net CO2 and SOx emissions. However, the lower heating value and higher reactivity (at lower temperatures) for raw biomass than coal could lead to fuel segregation, resulting in burn-out at lower temperatures, loss of steam generation efficiency, and fouling. To probe whether torrefaction/carbonization may solve issues related to fuel segregation, this study analyzed the possibility of co-firing a series of avocado biomass samples carbonized at 200, 300, 400, 500 and 600 °C, with Illinois No. 6 coal. Overall, the H/C ratio and average activation energy of oxidation of the biomass decreased as pyrolysis temperature increased, while surface area and higher heating value increased. Low temperature pyrolysis (300 °C) produced a biochar with similar characteristics to the coal, virtually eliminating fuel segregation as noted through derivative thermogravimetric curves with singular peak reactivities. As carbonization temperature increases, the energy input required to carbonize the biomass increases, and there may be issues with reverse fuel segregation, where the biomass begins to resemble a much higher rank coal than often available in the United States.