Development of the Steam Pretreatment Process for Production of Ethanol from Lignocellulosic Biomass

Sammanfattning: At a time when natural petroleum resources are on the verge of depletion, and concern about climate change is increasing, the potential of lignocellulosic material for the production of bioethanol is well recognized. Ethanol can be produced from biomass through enzymatic hydrolysis and fermentation. However, to achieve high yields and high ethanol concentration, pretreatment of the cellulosic biomass is necessary. Pretreatment improves the enzymatic digestibility of biomass and has a key role in the ethanol production cost. In this work, steam pretreatment of acid-impregnated lignocellulosic materials, such as spruce and sugarcane bagasse, was studied with the aim of increasing the sugar yield while reducing the cost of pretreatment in terms of energy demand and capital cost. Several ways of increasing the overall sugar yield in dilute-acid steam pretreatment were investigated. Steam pretreatment of sugarcane bagasse stored with lactic acid for one month resulted in increased overall sugar yield without the addition of SO2. This also allows the stillage from the biomass-to-ethanol process to be used in the production of biogas through anaerobic digestion. Pretreatment of spruce with lactic acid was not successful, with or without the addition of SO2. However, the addition of a metal salt (FeSO4) in dilute-acid steam pretreatment of spruce (using SO2 or H2SO3) increased the overall sugar yield by improving the acid hydrolysis during pretreatment. Two-step steam pretreatment process results in increased recovery of hemicellulose sugars due to milder conditions in the first step, and improves digestibility of cellulose using a higher severity in the second step. When material was pressed to a high DM content, washing could be omitted, and pressing had no negative effects on either the second pretreatment step or the enzymatic hydrolysis. With the lower DM achieved by filtration, washing was necessary to avoid degradation of too much of the sugars in the second step. Combing the two-step process into a one-step, dilute-acid steam pretreatment of spruce in which the temperature was increased linearly or step-wise between the two-levels, eliminated the need for intermediate washing and separation steps, and resulted in high sugar and ethanol yields. Similar ethanol yields were achieved when pretreatment was run in one step with a linear or step-wise temperature profile, but at a shorter time compared to the one-step constant temperature pretreatment. This results in a lower capital cost of the process. The results showed that larger wood chips (up to 6 mm) could be used in pretreatment instead of smaller ones (down to 1 mm) without influencing the overall sugar yield, which results in a lower energy demand for size reduction/chipping. Moreover, an impregnation time of 5 min resulted in the same overall sugar yield, following enzymatic hydrolysis, as impregnation for 20 min.