Biorefinery in a kraft pulp mill: From bioethanol to cellulose nanocrystals

Wood, composed of cellulose, lignin and hemicelluloses, is the main raw material used to produce cellulosic fibres. Mills producing cellulosic fibres, also called kraft mills, look very much like a “biorefinery ” plant: isolated cellulose is almost pure, and then exploited today mainly as paper; hemicelluloses and lignin are degraded and solubilised during the process as a very complex mixture, called “black liquor”, which is burnt in a recovery boiler. The heat thus produced covers largely the energy needs of the kraft mill, which may then become a net producer of electricity. The aim of this paper is to present two examples of research pathways aiming at enlarging the products portfolio a pulp mill could offer. The first example deals with the definition of hemicelluloses extraction from softwood, prior to its transformation into cellulosic paper pulp by the kraft process. The hemicelluloses fraction (in the case of softwood species, mainly composed of C6 sugars) should be extracted in a relatively pure form, whereas the cellulose fraction would continue to be obtained as fibres for papermaking. Thus, lignin becomes the main component of the “black liquor ” and might continue to be used as fuel for the energy needs of the mill. The extracted hemicelluloses are hydrolysed into hexoses and then fermented into bio-ethanol. This type of second generation bio-ethanol should eventually substitute the first bio-ethanol generation produced today from food raw materials. In the study, softwood chips were submitted to various hydrolysis treatments. Temperature, time and pH wer