Lignocellulose fractionation

To compete with the traditional fossil-based refineries, biorefineries have to exploit optimally raw materials firom plants and create multiple value chains. Therefore, the concept of a whole-plant biorefinery appears as a more convenient model. Despite the diversity of oil crops, such as soy, rapeseed, sunflower, and palm, the whole-plant biorefinery concept can be applied similarly to all of them. Differences can emerge due to the nature of the plant and the way to recover its seeds containing the vegetable oil. Palm trees, for instance, remain in the soil, and their fruits are harvested, where plants such as rapeseed or sunflower are cut every year. In both cases, the first step of the biorefinery process is to separate the oil-rich seeds firom the lignocellulosic fraction of the plant. 

The first stage of the biorefinery is often the separation of the main components of the plant. Most of the time, this stage has been performed on the field by the harvesting unit. In the case of rapeseed, the seeds are separated from the rest of the plant (stem, leaves), whereas in the case of other plants such as sunflower, the seeds have to go through an additional stage to separate the husk from the seed kernel that is then used for oil recovery. The husk is then treated as a lignocellulosic fraction. 

The solid output of the mash separation unit has a particle size >10 mm and joins the oversized fraction of the first 80 mm screening step for composting. They are mixed with the solid digestate and with the oversized lignocellulosic fraction originating from a parallel green waste composting process. 

Zhu, Z., Sathitsuksanoh, N., Vinzant, T., Schell, D.J., McMillan, J.D., Zhang, Y.-H.P., 2009. Comparative study of com stover pretreated by dilute acid and cellulose solvent-based lignocellulose fractionation enzymatic hydrolysis, supramolecular structure, and substrate accessibility. Biotechnology and Bioengineering 103, 715—724. 

Both in the USA and the EU, the introduction of renewable fuels standards is likely to increase considerably the consumption of bioethanol. Lignocelluloses from agricultural and forest industry residues and/or the carbohydrate fraction of municipal solid waste (MSW) will be the future source of biomass, but starch-rich sources such as corn grain (the major raw material for ethanol in USA) and sugar cane (in Brazil) are currently used. Although land devoted to fuel could reduce land available for food production, this is at present not a serious problem, but could become progressively more important with increasing use of bioethanol. For this reason, it is important to utilize other crops that could be cultivated in unused land (an important social factor to preserve rural populations) and, especially, start to use cellulose-based feedstocks and waste materials as raw material. 

A hydrolysis step is involved in the pulp industry in order to concentrate the cellulose from wood. This uses large-scale processes whereby a liquid fraction, the lignocellulose, is formed as a by-product in the process, and contains high levels of phenolic components and their derivatives. These compounds also constitute an environmental problem due to their possible introduction into rivers, lakes, and/or seas. Chlorophenols from the cellulose bleaching process have traditionally attracted most of the interest in the analysis of industrial waste because of their high toxicity. 

Levulinic acid and formic acid are end products of the acidic and thermal decomposition of lignocellulosic material, their multistep formation from the hexoses contained therein proceeding through hydroxymethylfurfural (HMF) as the key intermediate, while the hemicellulosic part, mostly xylans, produces furfural.A commercially viable fractionation technology for the specific... 

One of the favored organisms for study of cellulolysis by Trichoderma is T. reesei. Consequently, many mutant strains which hyperproduce cellulase have been obtained by treatment with ultraviolet light, gamma irradiation, the linear accelerator, diethyl sulphate and N-methyl-N -nitro-N-nitroso-guanidine (7). Whereas much of the study of T. reesei has been with cellulose as substrate, it is relevant to consider the other fractions of natural lignocelluloses hemicellulose and holocellulose (the combined cellulose and hemicellulose fraction). 

As far as the ethylene glycol lignin is concerned, it has been shown to be a native-like lignin which can be produced and recovered by direct solvolytic treatment of the initial lignocellulosic substrate. It would also be possible to remove the hemicelluloses via an aqueous/steam treatment prior to solvolytic separation of the lignin and cellulose. Such an option would facilitate the recovery of the three main constitutive fractions of lignocellulosics in significant yields. Work in this direction is now underway. 

A major problem in the commercialization of this potential is the inherent resistance of lignocellulosic materials toward conversion to fermentable sugars (4). To improve the efficiency of enzymatic hydrolysis, a pretreatment step is necessary to make the cellulose fraction accessible to cellulase enzymes. Delignification, removal of hemicellulose, and decreasing the crystallinity of cellulose produce more accessible surface area for cellulase enzymes to react with cellulose (5). 

The development of pretreatment technologies that are tuned to the characteristics of the biomass is still needed. Ideally, lignocellulose should be fractionated into multiple streams that contain valuable compounds in concentrations that make purification, utilization, and/or recovery economically feasible. Predictive pretreatment models should enable the design of this step to match both the biomass feedstock and the fermentation technology. 

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