An MILP model to identify optimal strategies to convert soybean straw into value-added products

Soybean is a highly valuable global commodity due to its versatility and numerous derivative products. During harvest, all non-seed materials become �straw�. Currently, this waste is primarily used for low-value purposes such as animal feed, landfilling, and incineration. To address this, the present work proposes a conceptual biorefinery aimed at converting soybean straw into higher-value products. The study began with data collection to identify potential conversion routes. Based on this information, a superstructure was developed, comprising seven conversion routes: four thermochemical routes (pyrolysis, combustion, hydrothermal gasification, and liquefaction), two biological routes (fermentation and anaerobic fermentation), and one chemical route (alkaline extraction). Each process was evaluated based on product yields, conversion times, and associated capital and operating costs. Using this data, an MILP (Mixed-Integer Linear Programming) optimization model was built in Pyomo using CPLEX as the solver. The variables in this problem were the amounts of biomass processed by each conversion process. The objective function was to maximize the Net Present Value (NPV). The optimization considered a maximum raw material supply of 232,500 metric tons per year. As a result, pyrolysis was identified as the most profitable route, yielding US$447 million, producing biochar, bio-oil and gas as products. It was observed that only if the price of the optimum product was 46% lower would the optimum conversion process change. In conclusion, soybean straw offers significant potential for value-added applications in a biorefinery context, with pyrolysis emerging as the most profitable route.