This thesis is an exploration into the technical and economic issues related to a syngas fermenting biorefinery producing 50 metric tons per day of hydrogen gas and 12 metric tons per day of PHA, a biodegradable plastic. In addition, an optimization study on the bioreactor is performed by varying reactor diameter and stirring speed.;The analysis assumed switchgrass costing {dollar}55/Mg as feedstock, and the bacterium Rhodospirillum rubrum to ferment the syngas. With a hydrogen market value of {dollar}1.90/kg assumed, 95% pure PHA was determined producible for approximately {dollar}2.05/kg. Grassroots capital for the biorefinery was estimated to be {dollar}55 million, with annual operating costs at {dollar}8.3 million. For a constant volumetric mass transfer coefficient of 0.05 s-1 the optimum reactor size was a 14.8 m diameter being stirred at 0.6 rev/s. Producing PHA by this method was found to be less expensive than processes using sugar fermentation.