Effects of hemicellulose extraction on the kraft pulp mill operation and energy use: Review and case study with lignin removal

The implementation of wood extraction prior to pulping (pre-hydrolysis), with subsequent recovery of hemicellulose, is expected to affect the operation of a conventional kraft pulp mill. The magnitude of impacts will depend especially on the extraction conditions. In this specific work, the consequences of integrating the auto-hydrolysis process are studied using a detailed mill balance. A softwood pulp mill in Finland was used as a reference. With 14.1% of wood extracted, the wood demand increased by 15.5% and the steam generation in the recovery boiler by 13.5%. The removal of approximately 17% of lignin from black liquor would put the flue gas side of the boiler back to the original required capacity. In turn, this would enable the simultaneous recovery of lignin and hemicellulose. To make this process economically feasible, the extra revenue from the sales of lignin and hemicellulose products would need to compensate for the additional operating costs in the pulp mill.

► We did a brief review on hemicellulose extraction, prior to cooking, in kraft pulp mills.
► We performed mill balances before and after integrating the auto-hydrolysis process.
► We examine changes in the operation of a Finnish softwood mill.
► The lignin removal can be used to decrease the increased heat load caused by auto-hydrolysis.
► We noticed that the attractiveness of this technology depend on technical and economic factors.