Techno-economic analysis of utilization of stillage from a cellulosic biorefinery

Cellulosic biorefinery stillage contains waste water with dissolved unutilized fermentable sugars and, mainly, lignin of the biomass. Currently, the best use of this lignin is through direct combustion to supply industrial process energy; however, fast pyrolysis has recently attracted researcher's interest as it has the potential to convert lignin into bio-oil and bio-char. It is essential that these alternative routes be thoroughly analyzed for their techno-economic feasibilities and bottlenecks, which was the main objective of this study. Stillage from a cellulosic biorefinery of a113.5 million liters per year (30 million gallons per year) butanol production facility was considered in this study. Experimental and modeling data for both fast pyrolysis and direct combustion systems were gathered from recent publications and used for analysis. Modeling software, SuperPro Designer, was used to develop process models and economic analyses of both systems. The estimated stillage processing cost ($/l butanol produced) of a direct combustion system and fast pyrolysis system were found to be 0.15 and 0.17, respectively, including byproducts credits. Plant size, anaerobic digester retention time, the moisture content of solid stillage, turbine and boiler efficiencies, and cost of waste treatment chemicals were the most sensitive input parameters. At the present state of these technologies, direct combustion is a more economically feasible option than fast pyrolysis; however, both could be of interest to commercial cellulosic biorefineries, interested in lignin utilization alternatives, due to the marginal difference in stillage utilization costs.