Microwave-assisted alkali pretreatment, densification and enzymatic hydrolysis of canola straw and oat hull

Abstract

Production of bioethanol from lignocellulosic biomass is very challenging due to the complex nature of the feedstocks. Pretreatment is a necessary step for efficient and effective conversion of lignocellulosic biomass to biofuel. This study investigated the effects of microwave-assisted alkali pretreatment, microwave pretreatment (distilled water) and untreated canola straw and oat hull on the pellet quality, and the conversion of microwave pretreated and alkali treated substrates into sugar for cellulosic bioethanol production. Microwave pretreatments were carried out by immersing the biomass in dilute alkali solutions (NaOH and KOH) at various concentrations of 0, 0.75 and 1.5% (w/v) for microwave-assisted times of 6, 12, and 18 min, and at a fixed microwave power of 713 W. Alkali treatments were carried out by soaking and the same alkali used for the microwave pretreatment. The biomass and alkali concentrations remained unchanged. Chemical composition analysis of canola straw and oat hull showed that the application of microwave-assisted alkali pretreatment on the biomass disrupted and broke down the lignocellulosic structure of canola straw and oat hull compared to microwave pretreatment and alkali treatment. Images acquired using a scanning electron microscope (SEM) revealed the structural changes caused by microwave-assisted alkali pretreatment on biomass samples. Pellet characteristics such as density, tensile strength and dimensional stability were used to describe the pellets produced from single pelleting technique. The results showed that the interaction effects of alkali concentration and microwave heating time significantly affected the physical characteristics of canola straw and oat hull pellets. Upon enzymatic saccharification, microwave-assisted alkali pretreatment and alkali treatment of canola straw and oat hull on enzymatic saccharification were investigated. Microwave-assisted alkali pretreatment showed the highest glucose yield in treatment combinations of 1.5% NaOH/18 min for canola straw and 0.75% NaOH/18 min for oat hull sample ground in a1.6 mm hammer mill screen size. Overall, microwave/NaOH pretreatment resulted in better physical quality pellets from both canola straw and oat hull samples than microwave/KOH pretreatment. Also, microwave pretreatment and alkali treatment using NaOH solution at different concentrations resulted in higher glucose yields compared to KOH in both feedstocks.