Recent environmental regulatory sharp curbs on fossil fuel power plants have obliged industries to incorporate alternative sources of fuels for energy production. Cost and recovery of synthetic catalysts are major challenges in carbonaceous materials catalytic gasification. Biomass rich in alkali metals can be added as fuel and also to provide inexpensive natural catalysts to boost fossil fuel gasification. Biomass/fossil fuel co-gasification could provide bridging energy production based on renewable and fossil fuels. In this work, CO2 co-gasification of switchgrass and sawdust with coal and fluid coke was conducted in a thermogravimetric analyzer. Gasification kinetics were inhibited or enhanced, depending on the potassium concentration in the mixture. For low K/Al and K/Si molar ratios, the coal ash sequestered the biomass potassium needed for KAlSiO4 formation, and thus, no catalytic effect was observed until the biomass-to-coal mass ratio reached 3:1, where the switchgrass ash supplied enough potassium to more than satisfy the minerals in the coal ash. For high K/Al and K/Si molar ratios, unreacted residual potassium acted as catalyst, enhancing coal gasification. Fluid coke contained much lower Al and Si than for the coal. Hence, the gasification kinetics of fluid coke were significantly augmented by blending the coke with switchgrass due to the abundance of potassium in the biomass. (C) 2014 Elsevier Ltd. All rights reserved.