(368b) Optimization and Analysis of Carbon Capture in Ethanol Biorefineries

Bioenergy with carbon capture and sequestration (BECCS) is a promising method for global warming mitigation that combines fuel production, to replace current fossil fuels, with permanent carbon dioxide removal (CDR). While typical industrial power plants that use natural gas as fuel have only a single point of carbon dioxide emissions, an ethanol biorefinery has three point sources of carbon dioxide emissions: from fermentation (~99 wt%), in biogas (~73 wt% CO₂), and in flue gas from solid residue combustion (~20 wt%). The relative flows of these streams are dependent on both the feedstock and pretreatment method used. However, previous studies on carbon capture in biorefineries have limited their analyses to biorefineries with a fixed feedstock and pretreatment technology, and predetermine from which of the three point sources of carbon dioxide.

Previous studies also often assume a fixed capacity of the biorefinery, typically of 2,000 tons of feedstock per day. However, increasing capacity has two important effects: it decreases fixed costs due to favorable sublinear scaling of equipment costs, but also increases the feedstock cost as larger transportation distances are required. Different methods of transportation such as by truck or rail, and the total distance of transport also affect the overall emissions balance and energy consumption of the entire biorefinery and supply chain system.

To address the limitations of previous studies, we use a mixed-integer nonlinear programming (MINLP) model of an ethanol biorefinery and supply chain system. By solving thousands of optimization problems with varying parameter values, we analyze the impact of feedstock selection, pretreatment method, biorefinery capacity, and carbon sequestration credit on the economic, energetic, and environmental performance of an ethanol biorefinery with carbon capture.

We find that biorefineries using feedstocks and pretreatment methods that lead to higher emissions from fermentation and anaerobic digestion tend to have lower average costs of capturing carbon dioxide. Biorefineries with high biomass to ethanol yields, such as those using dilute acid pretreatment, can capture high percentages of the carbon in the feedstock if energy is purchased, but less excess energy available from residue combustion limits carbon capture at an energetically self-sufficient biorefinery. Biorefineries processing feedstocks with high lignin content or that have low energy requirements create the most excess energy, which enables higher capture rates. We also find the inclusion of carbon capture technologies can significantly increase the cost-optimal biorefinery capacity.