Résumé
The use of renewable energy sources is becoming increasingly necessary, to address the impacts of earth global warming. The conversion of biomass into energy can be achieved in a number of ways, for example in producing fuel thanks to various technologies: biologically mediated reactions, gasification, pyrolysis or combustion, each having specific advantages and disadvantages. In our study, we focused on the biological transformation, the anaerobic digestion that could process biomass and produces a gas rich in methane. This transformation results also in the production of digestates rich in mineral elements. The anaerobic digestion is a well-known process particularly considering the treatment of wastewater sludge. Processing lignocellulosic biomass is a relatively new development of this technology. In order to optimize the biological transformations, physic and / or chemical pretreatments need to be implemented. Such pretreatments could be mechanical (milling), physical (thermal), chemical (acid or alkali), or a combination of them. Palmovski (2000), Muller (2003), Mshandete (2006), showed the significant positive influence of the particle size diminution on biogas production. Nevertheless, these conclusions are disputed since Pommier (2010) noted no difference neither on the biogas production quantity, nor on the biogas production rate. Theses various conclusions may be due to the specific surface variation which is, depending on the milling type, reduced by roughly one order of magnitude. Our purpose is to determine if a significant variation of the mean diameter of particles (more than one order of magnitude) could increase the maximum biogas production value or /and the kinetics. For that, an innovative ultra-fine milling process was applied to raw biomass, reducing the particles to some micrometers. Wheat straw was milled with a Retsch type cutting mill. The initial fraction had a mean diameter of 804 µm and was compared to smaller fraction of respectively 612 and 45 µm of mean diameter. The fractions were characterized biochemically and physically (TS / VS, Fourier Transform Infra Red spectroscopy, water activity...) before implementing biological methane potential (BMP) experiments. No significant differences could be observed on the maximum methane production (302.3 + 3.9 mLCH4/g VS to 327.7 + 15.8 mLCH4/g VS) whereas the biogas production kinetics constants differ (increase of the constants from 0.035 to 0.049 h-1 respectively for the fractions going from 804 µm to 45 µm). The BMP tests are usually implemented to compare maximum methane production values. In order to compare the kinetics, further experiments need to be performed, consisting in successive addition of substrate in batches reactors. Moreover, specific surface area measurements will provide a better comparison criterion than the mean diameter to test the effect of milling on biogas production.