LGP2 - Study of the possibilities to produce hemicellulosic ethanol in a biorefinery
Published on July 8, 2014
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June 16, 2014
Jérémy Boucher defended his University of Grenoble Ph.D. thesis entitled "Study of the possibilities to produce hemicellulosic ethanol in a biorefinery".
This thesis was prepared at the LGP2 Laboratory, under the supervision of Christine Chirat, Associate Professor, and of Dominique Lachenal, Professor, at Grenoble INP-Pagora.
This thesis deals with the production of bioethanol of second generation. Bioethanol is a sustainable fuel which can substitute gasoline. Nowadays, it is exclusively produced from food resources, which can have harmful fallout on food prices. That is why the development of a second generation ethanol, from lignocellulosic material, is mandatory. Among all the possible ways to produce such ethanol, this study is focused on the production of ethanol from hemicelluloses in a kraft pulp mill using softwood as raw material.
Softwood hemicelluloses are mainly constituted of galactoglucomannanes (GGM). These polymers are made of sugars which can be fermented into ethanol when they are into their monomer form. During the kraft process, cellulose is produced by removing the lignin from wood. The lignin is then burnt to produce energy. The GGM are also degraded and solubilised during the kraft process but their combustion is not very profitable. The aim of this study is to extract these hemicelluloses prior to the kraft process, hydrolyze them into monomers and then ferment them into ethanol.
The efficiency of different extraction treatments was evaluated. The two processes performed in acidic medium, autohydrolysis (hot water extraction) and acid hydrolysis (in the presence of sulfuric acid) are the most interesting. The acid hydrolysis enables the extraction and the hydrolysis into monomers of most of the GGM. The sugars can therefore be fermented directly. The autohydrolysis requires a secondary hydrolysis to achieve the depolymerization of the extracted GGM. This secondary hydrolysis is performed with an acid as catalyst. This two-step treatment shows two advantages: the cellulose is less damaged and the degradation of sugars extracted is reduced. This degradation has to be avoided because the degradation products cannot be fermented and they are known to inhibit the microorganisms used for the fermentation, which is one of the main issues for the production of second generation ethanol.
The direct fermentations of the hydrolysates, performed with a wild strain of Saccharomyces cerevisiae, allow to reach good yields after 24 hours. However, it might be necessary to increase the concentrations in sugars in the hydrolysates prior to the fermentation in order to raise the profitability of the process. Three processes were compared: evaporation, decrease of the L/W ratio and recycling of the hydrolysate. The evaporation is very efficient and enables to remove acetic acid and furfural, which are inhibitors of the fermentation. Recycling the hydrolysate increases the concentrations in sugars, but also their degradation. Reducing the L/W ratio is also efficient to increase the concentration.
The fermentation of a hydrolysate produced after an autohydrolysis and a secondary hydrolysis and concentrated to reach 100 g/L of hexoses was not successful because of the high concentration in inhibitors. Several strains of S. cerevisiae were acclimatized to this kind of hydrolysate in order to achieve the fermentation.
A complete process to produce ethanol and cellulose from wood chips was developed by taking into consideration industrial constraints. This study is a first step before its implementation in a kraft pulp mill.