Since the 1990s, mountain pine beetles have infested mature pine trees in the forests of western North America. Fast pyrolysis is an encouraging method to convert the beetle killed pine trees into bio-oils. In this study, an unsteady-state mathematical model is developed to simulate fast pyrolysis under concentrated solar radiation. Conservation equations of total mass, species, and energy, coupled with the chemical kinetics model, have been developed and solved to simulate fast pyrolysis of cylindrical biomass pellets in a quartz reactor exposed to various radiant heating fluxes. This study demonstrates the importance of the secondary reactions on fast pyrolysis products.