Chemical and Biochemical Biorefineries in Kraft Pulp Mills – Process Integration and Economics for Three Concepts
Some of the advantages of integrating biorefinery concepts with kraft pulp mills are that the utility system can be shared and that mass and energy streams can be exported from the pulp mill to the biorefinery or vice versa. These measures may result in lower investments and operating costs for the biorefinery compared with stand-alone operations. However, the implementation of biorefinery concepts can interact and occasionally interfere with pulp production.
In this thesis, assessment results for the integration of three chemical and biochemical biorefinery concepts are presented. The studied concepts include (1) “near-neutral” hemicellulose extraction and upgrading of the hemicellulose-containing stream to ethanol, (2) conversion of a kraft pulp mill to a dissolving pulp mill, and (3) high-solids ethanol production next to a kraft pulp mill. The results of this work show that efficient heat integration within and between the pulp mill and the biorefinery can result in a substantial decrease in utility demand and thereby energy costs. However, changes in composition/flowrates of some material streams at the pulp mill that arise from implementing the biorefinery concept appear to play a crucial role in the technical and economic feasibility of these concepts.
For the first biorefinery concept, efficient heat integration can make the processes self-sufficient in terms of steam; however, exporting chemicals from the pulp mill to the biorefinery can destabilize the sodium and sulfur balance in the pulp mill. In the second biorefinery concept, the modified cooking conditions at the pulp mill result in a lower pulp yield. Although the wood input could, in principle, be increased to compensate for the lower yield, the limited capacity of the equipment may result in a pulp production reduction by up to 40% in the worst-case scenario. For the third biorefinery concept, the potential advantages of operating at high-gravity conditions in terms of energy use are almost negligible in practice because very efficient heat integration is possible for both high-solids loading and a more conventional solids loading. Therefore, these advantages cannot compensate for the likely lower ethanol yield in the high-gravity process.
The results of this work demonstrate the importance of extending the focus from only the biorefinery reactor to include the entire biorefinery and pulp mill (including the energy system and the available equipment capacity) when evaluating the feasibility of biorefineries. The results are also a reminder of the importance of conducting system studies during the initial development of new processes and in parallel with experimental work.