Second-generation sustainability: Application of the distributed activation energy model to the pyrolysis of locally sourced biomass–coal blends for use in co-firing scenarios

Abstract

While first generation biofuels paved the way for a vision of a renewable energy future, their competition for arable land limited widespread applicability. Second generation fuels, made from a variety of carbonaceous wastes, are considerably more “sustainable” in a land competition sense, but require a higher degree of processing to extract energy. Here we extend the idea of second-generation sustainability by investigating blends of coal and biomasses found within 20miles of coal-fired power stations in the Northeast United States for use in co-firing scenarios that would limit long-range transport of biomass. A commercial high volatile bituminous Pennsylvanian coal was blended at 90, 80, and 50wt% with one of three biomasses: feed corn stover from a local farm, brewer’s spent grains from Redhook Brewery, or cocoa shells from the Lindt chocolate factory. The Distributed Activation Energy Model was applied to analyze the pyrolysis kinetics of the solid fuels and blends, yielding activation energies as a function of mass fraction conversion ranging from 304 to 522kJ/mol for coal, 164 to 304kJ/mol for the biomasses, and 218 to 530kJ/mol for the coal–biomass blends. Overall, the peak reaction rates and temperatures for the primarily biomass decomposition stages were linearly correlated with the percent biomass in the blend. Such an additive scheme did not represent the blends’ kinetics, instead over-predicting the activation energies. Synergy was noted between the fuels, in that the biomass does appear to be promoting the devolatilization of the coal at lower temperatures.