Steam pretreatment: a robust method for processing mixtures of hybrid poplar and wheat straw. Sugar yields, economics and environmental implications

To ensure that future lignocellulosic biomass biorefineries are sustainable and economically feasible, it is necessary to reduce the cost of raw materials and to improve the process yields. A low cost feedstock will be diverse, changing as a function of seasonality and price and will most likely be available from multiple sources to the biorefinery. Unfortunately, little attention has been shown to determining the influence of processing mixed biomass during steam pretreatment (SP) and how mixed biomass will influence the economic performance of a biomass to ethanol facility. In this study, hybrid poplar (HP) and wheat straw (WS) in three different proportions were impregnated with 3% SO₂ and steam pretreated at 195˚C for 5 minutes. Surprisingly, after fractionation, on average 14% more sugar was recovered using mixed feedstock than either single biomass, showing the robustness of SP for processing different mixtures of HP and WS. After SP and saccharification, mixed biomass showed on average 20% higher sugar yields than single HP and WS.

Since feedstock cost and sugar yields are key factors determining the economic performance of a biorefinery, the NREL techno-economic model was modified to calculate the unit ethanol cost obtained by mixing HP and WS. Mixed biomass ethanol cost ranged from $2.89 to 3.05 USD/gal, and was on average 17% less expensive than using single HP and WS. Additionally, life cycle assessment was used to determine the environmental impacts of using mixed HP and WS during the bioconversion process.