Energy Efficiency Measures in an average Scandinavian Kraft Pulp Mill with Hemicelluloses Pre-Extraction - Hot Water and Dilute Acid Extraction

Rapid increase in energy demand results in conventional sources of energy e.g. fossil fuels, becoming limited. This issue has been shown to cause an impact on both the energy cost and the environment. Likewise, industry’s irrational energy use can decrease revenues and increase the environmental impacts. The economic and environmental issues are crucial for energy intensive industries like pulp and paper industries in North America and Europe. Moreover, lower costs for pulp and paper production accompanied with the fast growth rate of mainly Asian and South American forests calls for modifications in the older mills. Converting a conventional Kraft pulp mill into an Integrated Forest Biorefinery (IFBR) has been mentioned in the literature as a potential to compensate the older mills production and energy costs. The IFBR provides co-production of a wider range of products such as ethanol, carbon fibers, polymers and diesel fuel with pulp (Fernando, et al. 2006, Van Heiningen 2006, Huang, et al. 2010, Berntsson, et al. 2008). In this thesis, a biorefinery concept focusing on hemicelluloses pre-extraction for co-production of pulp and ethanol was studied. In the pre-extraction process, hemicelluloses are extracted from hardwood chips prior to cooking and with further purifications are converted into ethanol. During the extraction, acetic acid is formed as byproduct. Several hemicelluloses pre-extraction processes have been studied in literature; the focus of this thesis is, however, on hot water and dilute acid extraction methods. Opportunities to implement the pre-extraction process in an average Scandinavian Kraft Pulp Mill have been analyzed. The studied mill is a model mill developed in the FRAM program: Bleached Market Kraft Pulp mill – Type mill. The objective of this Thesis is to investigate opportunities for increasing energy efficiency in the mill, and, evaluate the integration potential of the pre-extraction process into the mill. The results of pinch analysis for the mill shows a pinch temperature of 107°C and a minimum hot utility demand of 164.3 MW while the current hot utility demand of the mill is 179 MW. This results in 14.7 MW potential for steam savings. All pinch violations were identified, they mainly consisted in heating streams below the pinch with live LP steam. A total of 14 MW of pinch violations were solved through 3 retrofits, i.e. 95% of the total potential for hot utility savings. The energy efficiency measures were then evaluated for the extraction process and ethanol production. The IFBR showed the lowest heating demand among other possibilities. Major conclusion from this thesis is that an IFBR based on hemicellulose extraction and co-production of pulp and ethanol has lower income than the conventional pulp mill manily due to a 20% overall pulp yield loss. Increasing the wood input to the process or improving the amount of pulp production, may raise the profitability of the IFBR