Energy Efficiency Measures in an average Swedish Kraft Pulp Mill with hemicelluloses Pre-Extraction - Near-Neutral and Alkaline Extraction

In recent years, Northwestern pulp mills have been experiencing a decrease in competitiveness on the global market. Countries with different climates have access to larger and cheaper wood feedstock and thus produce cheaper products. Furthermore, recent global concerns such as limited fossil fuels supply and greenhouse gases emissions have put even more pressure on European forest industries. This situation creates the urgency and the opportunity for North-American and European pulp mills to increase their revenues and their energy efficiency. This aim can be reached by upgrading a traditional pulp mill into an Integrated Forest Biorefinery (IFBR), producing higher value-added products in addition to pulp and paper (van Heiningen, 2006). In this Thesis, a biorefinery with hemicelluloses pre-extraction aiming at producing simultaneously pulp and ethanol was studied. In this process, hemicelluloses are extracted from wood chips prior to cooking and further processed into ethanol. Acetic acid is formed as byproduct. Although several hemicelluloses pre-extraction processes are reported in literature, the focus was put on Near-Neutral and Alkaline extraction to minimize the impact on pulp. The biorefinery was based on a model mill developed in the FRAM program: Bleached Market Kraft Pulp mill – Type mill. One constraint was that pulp yield and quality should remain unchanged, therefore hardwood was chosen as raw material. The objectives of this Thesis were to investigate opportunities to increase energy efficiency in the pulp mill itself in a first time, then energy integration options between the pulp mill and the hemicelluloses processes. Concerning the Kraft pulp mill, Pinch Analysis showed a pinch temperature of 107°C and a minimum hot utility demand of 159 MW. Compared to the mill’s current consumption, potential heat savings amount to 28 MW, i.e. 15% of the current hot utility demand. All pinch violations were identified, they mainly consisted in heating streams below the pinch with live LP steam. A total of 20.5 MW of pinch violations were solved through 4 retrofits, i.e. 73% of the total potential for hot utility savings. Energy integration of hemicelluloses pre-extraction for ethanol production was then investigated. For this part, only three retrofits were considered in the mill, with hot utility savings of 16.2 MW. Energy integration of the Near-Neutral process allows saving 10.8 MW and yields an IFBR with 190.7 MW as hot utility demand. A cash flow analysis was finally performed to compare three different scenarios for the new biorefinery. Conclusions that can be drawn from this Thesis are that hot utility savings potential exists in the mill studied, which is encouraging since it represents an average Scandinavian hardwood Kraft pulp mill. According to the results, 15% of the mill’s current hot utility demand can be saved. Cash flow analysis of the pulp+ethanol plant showed that an IFBR with unchanged electricity geenration, but increased steam production through an increased load on the bark boiler was the configuration that yielded the largest income from other products than pulp. This income was found to be 2.6 times greater than in the original FRAM mill