The overall goal of this study was to scale-up a patented downdraft gasifier system for development of a mobile power plant. A 100 kg/h up-scaled downdraft gasifier system was designed, built and evaluated using switchgrass and eastern redcedar as feedstocks. The effect of equivalence ratio (0.15–0.30) were studied for each of biomass feedstock. Performance parameters, including composition, syngas yield, heating value, tar content, and gasification efficiencies were measured and analyzed. Results showed that biomass type and equivalence ratio have a significant influence on the syngas compositions and calorific value. Maximum gasification efficiencies for switchgrass and redcedar were 64 and 80% at optimal equivalent ratios of 0.22 and 0.24, respectively. Based on syngas quality, the performance of the scaled-up gasifier system was superior to the laboratory-scale reactor.