Impact of Co-firing Coal and Biomass on Mixed Char Reactivity under Gasification Conditions

This work focuses on the gasification behaviors of the chars formed during devolatilization when co-firing coal/biomass blends. In this experimental and modeling investigation, Wyodak coal, a sub-bituminous coal from the Powder River Basin, was the coal employed and corn stover was the biomass. Torrefied corn stover was also used in selected tests. Chars were produced by injecting pulverized fuel particles into a laboratory-scale entrained flow reactor, in which a reducing environment was established at ∼1550 K (1277 °C) and 1 atm. The injected fuel was either pure coal, pure biomass, or a premixed blend of the two. Char particles were extracted from the reactor just subsequent to devolatilization and then analyzed for specific surface areas and reactivity to CO₂, H₂O, and O₂. The results indicated that the specific surface areas of the mixed chars were lower than the specific surface areas of the chars produced from the pure fuels. The mixed char reactivities were also outside the bounds of the reactivities of the chars produced from the pure fuels, being lower in CO₂-containing environments and higher in environments containing H₂O. The cause of this impact of the fraction of biomass in the coal/biomass blend on mixed char specific surface area and reactivity is a consequence of devolatilizing biomass particles altering the environment in which devolatilizing coal particles are exposed. Mixed char specific surface area and reactivity models were developed that accurately characterize the observations. The models employ a parameter N, which represents the number of coal particles that are affected by a single biomass particle during devolatilization for the fuel blend that yields the lowest mixed char specific surface area. A value of 3 was determined for the Wyodak coal/corn stover blend, and a value of 0.2 was determined for the Wyodak coal/torrefied corn stover blend, meaning that it takes five torrefied corn stover particles to affect a single coal particle, reflecting the reduction in the volatile matter of the torrefied material.