Patent Application
20190249362
The present invention relates to the field of the degradation of agricultural residues, and more particularly of the degradation of lignocellulosic agricultural residues such as straw.
The invention also relates to a composition making it possible to degrade these lignocellulosic residues, a treatment process employing such a composition, and also the use thereof as biological treatment of said residues.
At the present time, agricultural plant residues are being increasingly upgraded in processes of anaerobic digestion, leading to the production of a biogas comprising methane.
Among these crop residues, lignocellulosic residues are among the slowest to be degraded during anaerobic digestion. This is because this lignocellulosic biomass comprises on the one hand cellulose fibers and hemicellulose fibers, forming holocellulose, and on the other hand a matrix based on lignin, a heteropolymer which constitutes a protective barrier for the cellulose and hemicellulose fibers. Thus, the anaerobic biodegradation of straw (which is lignocellulosic residue) in digesters (to produce methane) may be slow and exceed 40 days. For some cereal straws, such as rapeseed straw, anaerobic biodegradation is even slower.
Moreover, this rapeseed straw is very rarely used as bedding or feed for animals and is commonly left in the fields after the cereals have been harvested. This straw therefore constitutes an abundantly available biomass which would be useful to upgrade.
There is therefore a need to pretreat this lignocellulosic biomass, in particular before introducing it into anaerobic digesters.
A first aim of the invention is therefore to propose a process for treating these lignocellulosic residues in order to facilitate or accelerate the digestion thereof in anaerobic digesters such as methanizers.
Another aim of the invention is to propose an agent and a process making it possible to destructure the lignin-based matrix of these lignocellulosic residues, with a view to increasing the accessibility of the cellulosic fibers, consequently enabling degradation of the cellulosic fibers released in this way.
Different types of processes for destructuring lignocellulosic biomass are known to date:
The inventors focused on the latter category of processes, due to the following advantages: (i) they require minimal energy investment, (ii) are inexpensive and (iii) have specificity of action.
With a view to developing such a biological treatment, the inventors, during their research of isolating microorganism strains that may have lignolytic capacity, surprisingly discovered a composition that makes it possible to significantly increase the accessibility of the cellulose of the lignocellulosic residues to cellulases, and therefore to effectively degrade such residues. It has also proved that such a composition was in particular effective with regard to rapeseed straw residues.
The present invention therefore relates to a composition for the treatment of agricultural plant residues, especially lignocellulosic residues, of straw type, characterized in that it comprises a liquid mixture of an aqueous solution containing NPK mineral salts, able to form a nutritive medium for endogenous microorganisms of said agricultural residues to be treated, and of a source of lignin originating from a process of alkaline delignification of lignocellulosic biomasses.
Advantageously, the source of lignin is lignin originating from the Kraft process of paper pulp industries, referred to as Kraft lignin.
The examples presented below show synergy between the NPK nutritive salts and the Kraft lignin to increase the accessibility of the cellulose of lignocellulosic residues to cellulases.
The concentration of lignin in said liquid mixture is preferably between 0.1 g/l and 3 g/l, preferably between 1 g/l and 2.5 g/l, even more preferably between 2 g/l and 2.5 g/l. The lignin, preferably the Kraft lignin, is advantageously added into the solution of NPK mineral salts.
Regarding the aqueous solution containing the NPK mineral salts, it preferably comprises from 10 to 300 mmol/l of potassium K, from 20 to 200 mmol/l of phosphorus P and from 5 to 100 mmol/l of nitrogen N.
The aqueous solution containing the NPK mineral salts comprises the molar ratios N:P of advantageously between 1:10 and 1:1 and P:K of between 1:5and 10:1.
Preferably, in the aqueous solution containing the NPK mineral salts, nitrogen N is present in the form of ammonium ions and phosphorus P in the form of phosphate ions.
The present invention also relates to the process for treating agricultural plant residues, especially lignocellulosic residues, of straw type, characterized in that it comprises a step of treatment by bringing said lignocellulosic residue into contact, in aerobic medium, with a composition as described above, said treatment leading to an increase in the accessibility of the cellulose of said lignocellulosic residue to cellulases.
The tests carried out show that this accessibility of the cellulose may be multiplied by between 5 and 8 times, in just three days of aerobic treatment.
The lignocellulosic agricultural plant residues may be chosen from cereal straw such as wheat, rapeseed, corn, barley, oat or rye straw, preferably rapeseed straw.
This straw may be used in crude form, uncut, or cut just to reduce the bulk thereof or to adapt the length thereof to the dimensions of the reactor. It does not therefore require any particular preparation.
The process according to the invention is advantageously carried out in a reactor, in which said lignocellulosic residues are placed, continuously impregnated by said aqueous mixture, having a redox potential of greater than 100 mV, preferably of between 150 mV and 500 mV.
Surprisingly, this aerobic treatment may be carried out solely in the presence of endogenous microorganisms of said agricultural residues to be treated, that is to say without the introduction of microorganisms into the reactor other than those initially present within said residues.
According to an advantageous feature of the process, the ratio of the chemical oxygen demand (COD) of the lignocellulosic residues to the chemical oxygen demand (COD) of the starting aqueous mixture is between 4 and 50, preferably between 5 and 20, more preferably still between 6 and 10.
The composition according to the invention, and also the treatment process described above, may especially be used:
The process according to the invention thus makes it possible to contribute to upgrading large amounts of lignocellulosic biomass, such as cereal straw, which were hitherto not utilized.
The invention will be understood on reading the following description of exemplary embodiments, with reference to the appended drawings, in which:
and
An example of composition according to the present invention is given by the mixture comprising the following constituents:
The different constituents are introduced as follows: the Kraft lignin is suspended in the aqueous solution prepared after dissolution of all the nutritive salts mentioned above.
An example reactor used to carry out the process according to the present invention is presented in
The lignocellulosic residues 11 to be treated are placed on a screen 10 arranged a certain distance from the bottom 9 of the reactor. Since the treatment in said reactor 1 takes place in aerobic medium, an injection of air is provided in the bottom 9 of the reactor by means of the air inlet 7. The discharged air and the emitted gases leave the reactor via the outlet 8.
The reactor is temperature-controlled to a temperature of between 22 and 38° C., more preferentially of around 30° C. The pH of the solution is neutral (between 6.5 and 7.5). The redox potential of the solution is likewise monitored, and must be at least 100 mV. The solid lignocellulosic residue 11 placed on the screen 10 is permanently impregnated with the aqueous solution, which is continuously sprayed over said residue, but is separated from the stagnant solution at the bottom of the reactor before recycling thereof.
The amount of solution is of course adapted to the amount of lignocellulosic residues: in order to be able to permanently moisten, the ratio of solids of the mass of residue relative to the amount of liquid must not be greater than 2.
The flow rate of the aqueous solution is adjusted to approximately 100 ml per minute. The air is also permanently injected at a flow rate of 25 ml per minute.
The above conditions of flow rate of liquid and of air injected were applied to a reactor with a volume of 3.3 l into which 100 g of rapeseed straw residues were introduced with 400 ml of solution recirculated at a rate of 100 ml per minute.
Different tests were carried out with various aqueous nutritive solutions of M9 or NaCl in the presence or absence of Kraft lignin and/or in the presence or absence of lignolytic bacteria:
Lignin=Kraft lignin at 2.5 g/l in the nutritive solution or the solution of NaCl
Bacteria=108/ml of solution
Aerobic treatments were carried out for 11 days with measurements respectively on days 3, 5, 7 and 11 of the degree of cellulose accessibility (CAC). This measurement of the accessibility of the residues to cellulase was carried out according to the method of Hong and Zhang (Langmuir, 23, 12535-12540, 2007), on samples immediately after they were taken (not frozen).
The results obtained are presented in
On the contrary, the bacteria additions made only provide a slight improvement in this parameter of accessibility of the cellulose over the same periods of aerobic treatment. It should also be noted that the composition according to the invention enables a most significant improvement in the cellulose accessibility after just three days of aerobic treatment.
In order to supplement these results, analyses of the different fractions of the lignocellulosic biomass obtained after 11 days of treatment with these different solutions are presented in
The partitioning was performed using the Van Soest process (Van Soest et al. J Dairy Sci, 74, 3583-3597, 1991). This process, by partitioning the residues chemically using neutral or acidic detergents, makes it possible to determine the respective cellulosic, hemicellulosic or lignin fractions and also the water-soluble fraction.
In light of these results, it is noted that the lignin fraction remains constant whereas the cellulosic and hemicellulosic fractions decrease. The increase in the cellulose accessibility by means of the composition according to the invention is therefore associated with a destructuring of this lignocellulosic biomass, which leads to a release of the cellulose fibers.
This release of the cellulose fibers thus makes it possible to envisage subsequent treatments such as anaerobic treatments, for example digestions with a view to producing methane, simple sugars or else bioethanol.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1660188 | Oct 2016 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FR2017/052841 | 10/16/2017 | WO | 00 |
