Recent environmental sharp curbs on fossil fuel energy systems such as coal power plants due to their greehhouse gas emissions have compelled industries to include renewable fuels. Biomass/coal co-gasification could provide a transition from energy production based on fossil fuels to renewables. A low-ash coal and switchgrass rich in potassium were selected on the basis of previous thermogravimetric studies to steam co-gasify 50:50 wt% coal:switchgrass mixtures in a pilot scale bubbling fluidized bed reactor with silica sand as the bed material at similar to 800 and 860 degrees C and 1 atm. With the switchgrass added to coal, the hydrogen and cold gas efficiencies, gas yield and HHV of the product gas were enhanced remarkably relative to single-fuel gasification. The product gas tar yield also decreased considerably due to decomposition of tar catalyzed by switchgrass alkali and alkaline earth metals. Switchgrass ash therefore can act as inexpensive natural catalysts for steam gasification and assist in operating at lower temperatures without being penalized by an increase in product tar yield. An equilibrium model over-predicted hydrogen and under-predicted methane concentrations. However, an empirically kinetically-modified model was able to predict the product gas compositions accurately. (C) 2015 Elsevier Ltd. All rights reserved.