Sustainability assessment of cellulosic biorefinery stillage utilization methods using emergy analysis

Energy can be recovered from stillage from cellulosic biorefineries in different ways, including direct combustion and fast pyrolysis. These different energy conversion routes require different level of inputs from natural resources, non-renewable resources, and economic services. Due to the high energetic and economic costs of stillage recovery methods, it is essential to perform a sustainability analysis of these different options before commercial deployment. Thus, the main objective of this study was to assess the relative sustainability and environmental impact of fast pyrolysis and direct combustion systems for the beneficial use of waste stillage using emergy analysis. The estimated emergy sustainability indices of direct combustion and fast pyrolysis were 0.09 and 0.07, respectively, where the renewable fraction of stillage was the most influential input parameter. Additionally, the net product transformity for direct combustion and fast pyrolysis were 7.06E+05 and 2.61E+05 seJ/J, respectively. Overall, a 23% higher emergy sustainability index for the direct combustion compared to the fast pyrolysis and a 63% lower overall product transformity for the fast pyrolysis compared to the direct combustion suggests that both systems, at the current state of the technology, offer differing advantages for stillage utilization depending upon the desired end products and uses.