Process integration of near-neutral hemicellulose extraction in a Scandinavian kraft pulp mill - Consequences for the steam and Na/S balances

While in a conventional kraft pulp mill, most of the hemicellulose and lignin fraction of the wood is burned in the recovery boiler to produce steam, in a biorefinery it can partially be used to produce added-value products. In this paper, the most important consequences of integrating a bioethanol production plant with a model pulp mill are presented in terms of steam and Na/S balances. The model mill represents an average Scandinavian hardwood kraft pulp mill, and the bioethanol plant is based on the "near-neutral" hemicellulose pre-extraction method. Regarding the steam balance, a comprehensive heat integration study is performed. Implementing hemicellulose extraction increases the net steam demand by 48 MW. However, process integration at the mill and the bioethanol plant individually leads to significant steam savings, and a corresponding net increase of steam by only 3 MW. Additional steam savings can be achieved if the total integration of the two processes (between the pulp mill and the bioethanol plant) is considered (3 MW), resulting in a biorefinery with no increase of steam demand. As regards the Na/S balance, it is shown that green liquor export from the mill to the bioethanol plant results in severe disruptions in the sodium and sulphur balance of the mill. Different attempts to solve this problem are discussed, but are very costly and/or negatively affect the water and steam balance of the mill.