Process development and environmental assessment within softwood based biorefineries

Abstract: We urgently need to move from a fossil-based to a bio-based society. From a Swedish perspective, a promising way forward are biorefineries with an integrated production of materials, chemicals, and energy from the softwood species Scots pine (Pinus sylvestris) and Norway spruce (Picea abies). A diverse set of process routes will be necessary to produce all intermediates and commercial products. In this thesis, the focus is the production of phenolic compounds through extraction, and production of fermentable sugars through steam explosion pretreatment with subsequent enzymatic hydrolysis. Although renewable, the annual biomass production is limited, and efficient biorefinery processes are essential. By understanding the structural or chemical differences within the feedstock, processes can be optimized further. In this thesis, it is shown how more juvenile tissue, or sapwood, of Scots pine required lower severity in steam explosion pretreatment than older tissue, heartwood, or knots for the production of fermentable sugars. Pretreating different wood tissues of Scots pine at different severity can thus increase the overall yield of fermentable sugars. Bark is chemically significantly different from wood and the production of fermentable sugars from bark is not feasible using the same processes as for wood. In this thesis, it is shown how removing water-soluble extractives prior to steam explosion pretreatment, and enzymatic hydrolysis can increase the yield of glucose by 30 % and 11 % in the enzymatic hydrolysis of bark from Norway spruce and from Scots pine, respectively. To develop new process technologies with favorable environmental profiles, it is essential to quantitatively assess the potential environmental impact of different process choices through life cycle assessment (LCA), early during process development. In this thesis, the potential environmental impact of the production of cationized tannins from high-pressure hot water extract of Norway spruce bark was evaluated. The potential environmental impacts when using the three different extraction technologies: hot water extraction, ultrasound extraction, and supercritical extraction, for the production of phenolic compounds, were also assessed and compared. For the production of 1 kg cationized tannins the overall contribution to climate change was estimated to be between 1.2-4.7 kg CO2 eq. The potential impact on climate change for the three extraction technologies were estimated to 0.48-0.68, 5.9-11, and 5.8-6.3 kg CO2-eq. per kg of produced phenolic compounds for hot water extraction, ultrasound assisted extraction, and supercritical fluid extraction, respectively. The need for chemical solvents and reagents were identified as primary hotspots, i.e. critical aspects to be prioritized for action, in all studied systems and for most of the studied impact categories. Simple, water-based systems, therefore, have substantial advantages. In addition, it will be necessary with sustainably produced solvents and reagents to reach a sustainable biobased economy.