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(May 2011)
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II - Chemical Processes

II - 5 - Biorefinery in a cellulosic fibres production mill

Wood, composed of cellulose, lignin and hemicelluloses, is the main raw material used to produce cellulosic fibers. Mills producing cellulosic fibres, also called kraft mills, look very much like a “biorefinery” plant: cellulose is isolated almost pure, and then valorised principally as paper; hemicelluloses and lignin are degraded and solubilized during the process as a very complex mixture, called “black liquor”, which is burnt in a recovery boiler. The heat thus produced covers very largely the energy needs of the kraft mill which may then be a net producer of electricity. The aim of this work is to define an extraction process of the hemicelluloses from softwood, prior to its transformation into cellulosic paper pulp by the kraft process. The hemicelluloses fraction (mainly composed of C6 sugars in the case of softwood species) should be extracted in a relatively pure form, whereas the cellulose fraction would continue to be obtained as fibres for papermaking. The lignin becomes then 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 [Figure 1]. This type of second generation bio-ethanol should eventually substitute first generation bio-ethanol produced today from food raw materials. This work is done in the frame of Institut Carnot Énergies du Futur.

Tomorrow’s kraft biorefinery mill

Figure 1 - Tomorrow’s kraft biorefinery mill

The work has associated one post-doctoral fellow and one Ph.D student, in collaboration with Scion (new zelander forestry institute) and co-sponsored by EGIDE for the exchange of researchers. Hydrolysis trials were conducted under different conditions of pH, time and temperature. In particular acid hydrolysis (addition of sulphuric acid) was compared to autohydrolysis, in which aqueous solution is submitted to high temperature (160°C) leading to the formation of acetic acid and natural acidification. Acid hydrolysis yields much higher quantities of monomers than autohydrolysis because of pH difference. Thus if the focus is to produce ethanol directly on the hydrolysate without any additional hydrolysis stage, the hydrolysis should be conducted under stronger conditions for a high yield of monomers [Figures 2 & 3]: by varying the hydrolysis time and the acid amount, about 160 kg monomers/ton of wood can be obtained. The hydrolysates were fermented into ethanol [Figure 4].

Effect of the type of hydrolysis on the amount of monomers dissolved   Formation of monomers and oligomers during acid hydrolysis 
   (160°C with 0.7, 1.4 and 3.5% H2SO4)
Figure 2 - Effect of the type of hydrolysis on the amount of monomers dissolved   Figure 3 - Formation of monomers and oligomers during acid hydrolysis
(160°C with 0.7, 1.4 and 3.5% H2SO4)
Fermentation of hydrolysates into ethanol
Figure 4 - Fermentation of hydrolysates into ethanol

Production of about 89 liters ethanol/ton of wood could be obtained for the highest acid charge. The lowest charges led to a maximum of about 77 liters/ton. Fermentation yields close to theoretical value were obtained, indicating that the presence of fermentation inhibitors, if any, was not significant.The next step will concern the study of the effect of acid hydrolysis on the subsequent cooking and bleaching processes to produce cellulosic fibres.

Cellulosic pulps for paper production are obtained by wood delignification. Full bleaching requires the complete removal of coloured residual lignin and chromophores. Only a small amount of bleached pulp is used after purification for pure chemical cellulose production. This last application offers a great added value and has today a growing interest due to the opportunities offered by the synthesis of bio-polymers and derivatives originating from biomass. However bleached paper pulps in chemical applications are limited by hemicelluloses contamination, lack of reactivity linked to fibrous structure and too high degree of polymerisation. Preliminary bleaching step necessary to chemical applications is difficult. Oxygenated reagents such as oxygen and hydrogen peroxide are generally not sufficient and full bleaching requires the use of chlorinated reactants responsible for the production of toxic organochlorinated compounds. This project aims at developing a novel catalytic environmentally-friendly bleaching process using hydrogen peroxide. This new process would be implemented in parallel to the current bleached kraft pulp production line. Two academic laboratories from Grenoble University, the LGP2 and the DCM, drive this project with industrial partners.

A new project has started in October 2009, complementary to project (1) described above. Contrary to softwoods hardwoods hemicelluloses mostly contain pentoses from which ethanol fermentation is uneasy. However, pentoses have broad chemical applications potential like in green surfactants, polymers, additives, etc. This project presents similarities as (1) in the sense that pentoses extraction should impact as low as possible the production of paper cellulose fibers. Further chemical developments using pentoses will be made through collaboration with industrial partners.

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