Consequential Life Cycle Assessment of hemicellulose extraction from kraft mill for pentane and higher hydrocarbons production

The sustainable utilization of forest resources stands as a critical topic in biorefineries, addressing contemporary global environmental challenges such as climate change, land use, water scarcity, and the depletion of fossil resources. To enhance the sustainability of biomass-based plants, capable of competing with the use of fossil resources, this thesis explores the potential of elevating side-streams to high-value end-products. The primary focus lies in augmenting the sustainability of a forest biomass-based biorefinery through the production of chemicals derived from hemicellulose. This involves a comprehensive investigation into the environmental impacts through a consequential Life Cycle Assessment. In the product system considered, hemicellulose is extracted from birch wood via pre-hydrolysis during a Kraft pulping process, followed by its conversion into Furfural utilizing Beta-Zeolites as catalysts. Subsequently, Furfural undergoes direct hydro-processing to obtain pentane, higher hydrocarbons and gaseous by-products comprising <C5 hydrocarbons, facilitated by a combination of Pd/C and ZSM-5 Zeolites. Post-extraction, wood chips undergo Kraft pulping to obtain Dissolving Grade Pulp, utilized in textile production. The incorporation of a pre-hydrolysis step proves to be an effective way of de-bottlenecking the recovery boiler of the pulp mill which represent the bottleneck of the whole process, enhancing pulp production while concurrently generating bio-based chemicals. Environmental performances and hot spots in this alternative process are investigated through a Consequential Life Cycle Assessment. Two scenarios are examined, A first scenario (S1-Burn) addressing pre-hydrolysis coupled with the kraft pulping process to yield unbleached pulp without the integration of the hydrocarbons value chain, in this case the pre-hydrolysis liquor is burnt co-generation facility to recover heat and power. The second scenario (S2-HC) investigates the potential environmental impacts of upgrading hemicellulose to hydrocarbons and burning the hemicellulose residues in the same co-generation plant, producing heat and power with high efficiency. A Contribution Analysis was performed, which provides insights into the contribution of each analysis group in the product system to various impact categories. Performance in both scenarios is further explored through five different sensitivity analysis, investigating the performances when the pre-hydrolysis liquor and the hemicellulose residues are burnt in the recovery boiler of the plant, the benefits of locating the integrated kraft mill in an European context, the drawbacks of decreasing the yield of hydrocarbons by 10%, the impacts of decoupling the hydrocarbons value chain process from the pulp mill, and the sensitivity to a change in the impact assessment method. Five distinct impact categories were investigated to assess the efficacy of the different scenarios: Climate change [Kg CO2 eq.], Land use [Pt], Water use [m3 depriv.], Resource use – fossil [MJ], and Resource use – mineral and metals [Kg Sb eq.]. The findings reveal that S2-HC, under an average European context, delivers the best performances in the climate change and fossil resources use impact categories, with a benefit of (-) 0.721 KgCO2 eq and (-) 6.85 MJ respectively. The best performances in Land use and mineral resources use are delivered by S1-Burn in a Swedish + Finland context, with an impact of (+) 84.8 Pt and (+) 4.06E-06 Kg Sb eq. S1-Burn under an average European context delivers the best outcomes in the water use with a benefit of (-) 2.54E-02 m3 depriv. The results highlight that the production of pentane and higher hydrocarbons from hemicellulose coupled with the recovery of heat and power from a co-generation plant through the incineration of hemicellulose residuals, can significantly enhances the environmental performance of a kraft mill within an averag European scenario.