METHOD FOR CLEANING A REACTOR FOR PROCESSING A LIGNOCELLULOSIC BIOMASS

Abstract

The present invention relates to a processing method at acidic or neutral pH in a reactor (4) for processing lignocellulosic biomass (P), said process including a continuous cleaning phase of the reactor which comprises introducing a basic aqueous solution (EB) into said reactor containing the biomass being processed.

Claims

1. A process for the treatment at acidic or neutral pH in a treatment reactor (4) of a lignocellulosic biomass (P), said process comprising a phase of continuous cleaning of the reactor which comprises the introduction of a basic aqueous solution (EB) into said reactor containing the biomass being treated.

2. The process as claimed in claim 1, characterized in that the basic aqueous solution (EB) is heated before introduction into the treatment reactor (4), in particular to a temperature of at least 40° C., in particular of at least 80° C.

3. The process as claimed in claim 1, characterized in that the introduction of the basic solution into the treatment reactor (4) is carried out in said reactor, the internal volume of which is at a temperature of at least 120° C., in particular of at least 140° C.

4. The process as claimed in claim 1, characterized in that the phase of cleaning the treatment reactor (4) with the basic solution (EB) has a duration of between 15 minutes and 8 hours, in particular between 1 and 3 hours.

5. The process as claimed in claim 1, characterized in that, during the phase of cleaning the treatment reactor (4), the residence time in said reactor of the biomass (P) impregnated with basic aqueous solution (EB) is between 5 and 15 minutes.

6. The process as claimed in claim 1, characterized in that, during the phase of cleaning the treatment reactor (4), the flow rate of the basic aqueous solution (EB) at the inlet of said reactor is adjusted so that the solids content SC of the biomass (P) decreases during its passage through the reactor, from a value of 30% to 60% SC, in particular 50% SC, to a value of 15% to 25% SC, in particular 20% SC.

7. The process as claimed in claim 1, characterized in that the concentration of base of the basic aqueous solution (EB) is adjusted so as to increase the pH of the biomass (P) entering the reactor (4) from an acidic pH range of between 0.5 and 3, preferably in the vicinity of 3, to a basic pH range of between 8 and 14, preferably in the vicinity of 13.

8. The process as claimed in claim 1, characterized in that the phase of continuous cleaning of the treatment reactor (4) is carried out according to a given frequency and/or when a threshold value of a physicochemical or rheological characteristic of the reaction medium in said reactor is exceeded.

9. The process as claimed in claim 1, characterized in that a change is introduced to one of the physical, chemical or rheological characteristics or the nature of the biomass (P) feeding the treatment reactor (4) during at least a part of its cleaning phase.

10. The process as claimed in claim 1, characterized in that the treatment is a cooking with steam explosion, and in that the heat of the steam (V) is thermally exhausted, at least in part, at the outlet of a separation device (5) positioned at the outlet of the treatment reactor (4), by recovery via a heat exchanger, in order to heat the or one of the aqueous solutions (E, EA, EB) used in said process, the portion of steam (V) which is condensed, in particular via a condenser, preferably being recycled as make-up water.

11. The process as claimed in claim 1, characterized in that the biomass (P) is impregnated with an acidic aqueous solution (EA) in an impregnation reactor (3) before its introduction into the treatment reactor which is a pretreatment reactor (4).

12. The process as claimed in claim 1, characterized in that, during at least a part of the phase of continuous cleaning of the treatment reactor (4), the acid content of the acidic aqueous solution (EA) brought into contact with the biomass during its prior impregnation in the impregnation reactor (3) is reduced or eliminated.

13. The process as claimed in claim 11, characterized in that, during at least a part of the phase of continuous cleaning of the treatment reactor (4), the acidic aqueous solution (EA) brought into contact with the biomass (P), during its prior impregnation in the impregnation reactor (3), is replaced by a basic aqueous solution (EB), in particular the same as that which is injected during said phase into the treatment reactor (4), or by an aqueous solution (E) of neutral pH.

14. The process as claimed in claim 1, characterized in that several, in particular two, impregnation reactors (3,3′) are used in parallel to impregnate the biomass (P) with an aqueous solution before its introduction into the pretreatment reactor, a first impregnation reactor (3) being fed with acidic aqueous solution (EA) or with aqueous solution (E) of neutral pH, and a second impregnation reactor (3′) being fed with basic aqueous solution (EB), the two reactors (3,3′) operating alternately, the second reactor (3′) being operational during at least a part of the phase of cleaning the pretreatment reactor (4).

15. The process as claimed in claim 1, characterized in that a separation is carried out between biomass (P) and aqueous phase in liquid or vapor form at the outlet of the pretreatment reactor (4) by a separation device (5), and in that the cleaning phase comprises, after the introduction of the basic aqueous solution (EB) into the pretreatment reactor (4), at least one rinsing of the separation device by an aqueous solution (E), in particular between 1 and 10 successive rinsings.

16. A plant for implementation of the process as claimed in claim 1, characterized in that it comprises, from upstream to downstream, a reactor for impregnation (3) of lignocellulosic biomass (P) in fluid connection with a vessel (1) for preparation of an acidic aqueous solution (EA) or a neutral aqueous solution (E), a treatment reactor which is a reactor for pretreatment (4) of the impregnated biomass, in fluid connection with a vessel for preparation (6) of a basic aqueous solution (EB), and a device for separation (5) of the pretreated biomass which is positioned downstream of the pretreatment reactor (4) and which is associated with means for rinsing by an aqueous solution (E).

Description

LIST OF FIGURES

[0053] FIG. 1 represents the block diagram of the cleaning process according to the invention of a reactor for the pretreatment of biomass inserted into a process for the pretreatment of biomass.

[0054] FIG. 2 represents a first alternative form of the process according to FIG. 1.

[0055] FIG. 3 represents a second alternative form of the process according to FIG. 1.

[0056] FIG. 4 represents a third alternative form of the process according to FIG. 1.

[0057] FIG. 5 represents a fourth alternative form of the process according to FIG. 1.

[0058] FIG. 6 represents a fifth alternative form of the process according to FIG. 1.

[0059] FIG. 7 represents a sixth alternative form of the process according to FIG. 1.

[0060] FIG. 8 represents a seventh alternative form of the process according to FIG. 1.

[0061] FIG. 9 represents an eighth alternative form of the process according to FIG. 1.

[0062] FIG. 10 represents a ninth alternative form of the process according to FIG. 1.

[0063] FIG. 11 represents a tenth alternative form of the process according to FIG. 1.

DESCRIPTION OF THE EMBODIMENTS

[0064] The figures are very diagrammatic; the same references correspond to the same components from one figure to another. The reactors are represented in the spatial position which they substantially occupy in the operational position.

[0065] The process for cleaning a reactor for the treatment of biomass is illustrated here in the context of a process for the treatment of biomass intended to produce alcohols, in particular biofuel of the bioethanol type, according to the stages of the process according to FIG. 1 described quickly below.

[0066] It is a pretreatment, in the sense known in the field of the conversion of lignocellulosic biomass. An embodiment of this process is described in more detail, for example, in the patent WO 2018/015227, to which reference will be made if necessary.

[0067] It should be noted that the cleaning process according to the invention can be applied in the same way to any reactor for the pretreatment of biomass and more generally to any reactor which is intended to treat a lignocellulosic biomass, the treatment having the object of modifying one of its chemical, physical or rheological characteristics, and generally being operated at high temperature.

[0068] The process for the treatment of biomass taken here as an example and represented in FIG. 1 comprises a first stage of impregnation of the biomass in a vertical reactor 3, followed by a stage of pretreatment of the biomass, once impregnated, in a horizontal steam explosion reactor 4.

[0069] These stages of the process are carried out continuously and are described in detail below using the description of the items of equipment used to implement it: [0070] A vessel for preparation 1 of an impregnation liquor containing a chemical catalyst is provided, which liquor is formed from water E and from catalyst A which will feed it; the catalyst in this case is a strong acid of concentrated sulfuric acid type in an aqueous phase, this vessel making it possible to feed the impregnation reactor 3 with a mixture of water E and of chemical catalyst A, [0071] A conical screw 2 (also called plug screw or sealing screw) for feeding with fresh biomass (in this instance wheat straw P) into the impregnation reactor 3, [0072] A line for feeding the reactor with impregnation liquor connecting the vessel for preparation of liquor 1 and the impregnation reactor 3, [0073] An impregnation reactor 3 equipped with two upward transportation screws (not represented) making it possible for the biomass to pass from the impregnation zone in the lower part of the reactor to the draining zone in the upper part of the reactor, and to bring the impregnated and drained biomass to the reactor outlet located at the top of the reactor.

[0074] This impregnated and drained biomass is subsequently sent to the pretreatment by a feedline emerging in a second conical screw 2′, [0075] This second conical screw 2′ feeds a pretreatment reactor 4 with impregnated biomass, [0076] The pretreatment reactor 4 treats the impregnated biomass by steam explosion, [0077] A water circuit for washing the conical screws 2,2′ of the impregnation reactor 3 and of the pretreatment reactor 4, represented symbolically in FIG. 1 by water inlets E at said screws, is provided, [0078] A means for separation of the steam 5 is fed by the reactor 4 with biomass which has undergone cooking by steam explosion, for example of cyclone type, with at the high outlet steam V and at the low outlet the pretreated/exploded biomass, also called must (or marc) M.

[0079] This must M exhibits at this stage a sufficient accessibility of the cellulose to enzymes to be treated by enzymatic hydrolysis for the production of 2G sugars. The conditions of the enzymatic hydrolysis and of the consecutive or simultaneous fermentation which follow this separation (not represented in FIG. 1) are suitable for the desired products and are known to a person skilled in the art.

[0080] The use of the pretreatment technique described above results in the deposition of different types of biomass (wheat straw here, but also miscanthus, poplar, and the like), which accumulate at/adhere to the surface of at least one of the transportation screws internal to the pretreatment reactor 4. These deposits undergo a cooking over times longer than the normal residence time of the biomass in the reactor, and are transformed into a residue, which can be called “coke” here. This “coke” can create various operating problems, such as blockages of the outlet orifice of the reactor 4 or an increase in the frictional actions of the transportation screw(s) in question on the wall of the chamber in which they are housed, and this may result in a reduction in the performance characteristics of the pretreatment unit in its entirety as represented in FIG. 1.

[0081] The definition of the composition of the “coke” has proven to be problematic because it concerns a residue, the appearance and the composition of which change over time: at the start of a production cycle, the material which is deposited is biomass; it thus has essentially the same characteristics as the biomass which continues its journey through the reactor 4 and toward the downstream stages. The deposit which is formed by adhesion to the internal wall of the reactor 4 will remain for a much longer time under the cooking conditions (temperature in particular) than desired. The effect of the temperature affects the composition and the morphology of the residue, which will change toward an increasingly “cooked” residue. The more the residue is “cooked”, the more compact it is and the more it adheres to the walls of the reactor.

[0082] These “coke” deposits are cumulative: the longer the continuous operating time of the tool, the greater the amount of coke deposited, and the more the “layers” of coke close to the wall will change toward a very hard solid. These deposits thus bring about a fouling phenomenon, by increasing the thickness of the walls and by reducing the useful volume of the reactor. Depending on the configuration of the cooking reactor, and in particular the type of internal in place, there may be observed interference with the rotation of certain elements, such as the screw, or one at least of the screws for transportation of the biomass in the reactor being cooked. This interference is observed in particular by an increase in the power of the motor rotating the screw.

[0083] Throughout the production, it can also happen that a part of this residue, which is more or less hardened, detaches from the wall of the reactor, under the effect, for example, of the rotation of the screw or of one at least of the transportation screws internal to the reactor or of the passage of the biomass through the reactor: thus, particles with a much greater density than the bed of biomass being cooked can be caused to detach and to be entrained toward the outlet orifice of the reactor, which can generate blockages or operating problems downstream. Despite these detachments, it is found that the deposits continue to increase over time during a given production cycle.

[0084] After stopping, cooling and opening the cooking reactor 4, it could be found that the coke exists in two forms: a hard form in direct contact with the internal walls of the reactor and a more friable form which covers the hard coke. The difference between these two cokes is found in their elemental compositions, as shown in table 1 below.

TABLE-US-00001 Friable coke Hard coke Carbon content (%) 44.57 65.07 Hydrogen content (%) 5.85 4.67 Oxygen content (%) 34.63 24.58

[0085] It is observed that the percentage of carbon contained in hard coke is higher than that in friable coke, while an opposite trend is noted for the oxygen content, and similar values are noted for the hydrogen content. It emerges from this that friable coke is, as it were, the precursor of dense coke.

[0086] The invention consists in continuing the operation of the two impregnation 3 and pretreatment 4 reactors, while carrying out the chemical cleaning of the reactor 4 in order to extract this coke C and/or to slow down its formation.

[0087] This cleaning does not require the opening of the reactor and the mechanical cleaning of the interior of the reactor, as was the case previously. This cleaning according to the invention, described in detail below, is thus faster, more economical and safer, since it makes it possible to limit the operating risks associated with the assembling and dismantling of the unit and, above all, since it makes it possible not to stop the production.

[0088] An example of implementation of the process according to the invention and its alternative forms are explained with the help of the combined figures. It requires the following additional items of equipment, with respect to those already described, in the light of FIG. 1: [0089] A vessel for preparation 6 of a cleaning liquor EB containing a base. This vessel 6 makes it possible to feed the pretreatment reactor 4 with basic solution at a certain concentration. It is fed with water E and with base B (for example a base B in the form of a concentrated aqueous KOH solution), the contribution of which is adjusted in order to obtain a liquor in the desired amount and at the desired concentration of base/pH. [0090] A line for feeding the reactor 4 with cleaning liquor connecting the vessel for preparation of cleaning liquor 6 and the pretreatment reactor 4 to be cleaned, if appropriate preheating it by ad hoc items of equipment (heating resistors surrounding the pipes, for example), with items of equipment suitable for injecting the cleaning liquor into the reactor 4 under pressure, [0091] An inlet for rinsing water ER for the cyclone 5.

[0092] The course of the implementational example of the cleaning process according to the invention comprises two consecutive sequences: [0093] Sequence 1: Injection of the preheated basic liquor EB into the reactor 4 while the reactor is being fed with the acidic biomass.

[0094] The injection conditions are as follows: [0095] the basic solution EB is an aqueous KOH solution, with a KOH concentration of 1% to 50% by weight of KOH, preferably from 5% to 12% by weight of KOH, with respect to the water [0096] the flow rate of the solution EB into the reactor is between 100 and 500 kg/h, in particular approximately 300 kg/h [0097] the degree of filling by the biomass impregnated with the basic solution EB of the reactor 4 is from 20% to 90%, in particular approximately 30% [0098] the temperature at which the solution EB is injected into the reactor 4 is between 80° C. and 200° C., in particular approximately 130° C. [0099] the temperature of the reactor 4 is between 150° C. and 220° C., in particular approximately 200° C. [0100] the duration of this sequence is between 15 minutes and 8 hours; it is in particular 2 hours [0101] the residence time of the solution EB in the reactor 4 is between 5 and 15 minutes, and in particular approximately 10 minutes. [0102] Sequence 2: Cleaning the cyclone 5 by rinsing with the water ER to complete the cleaning.

[0103] The term “water flush” can be used, insofar as the rinsing consists, in this implementational example, in spraying water under pressure into the cyclone, which water is subsequently rapidly discharged.

[0104] The operating conditions for this sequence are as follows: [0105] number of rinsing operations: from 1 to 10, for example equal to 2 [0106] temperature of the rinsing water: 20° C. to 80° C., for example 20° C. (i.e. either a temperature at ambient or close to ambient, or a higher temperature requiring preheating of the rinsing water ER)

[0107] In the cleaning phase, a must is obtained at the outlet of the cyclone 5 which is no longer the conventional acidic must M but a basic must M1.

[0108] The frequency of the cleaning procedure can vary widely depending on the type and size of the pretreatment reactor 4, on the type of biomass being treated, and the like. For example, the cleaning can be triggered when the torque of one of the transportation screws internal to the reactor increases by more than 15%, with respect to the torque observed at the start of production. It can also be triggered after a given period, which can range from 2 hours to 4 months of production.

[0109] Different alternative forms can be introduced to the example of cleaning process described above, while remaining within the scope of the invention, some of which are described in detail below (some at least of these alternative forms can be alternative or cumulative):

[0110] A—During cleaning, the concentration of acid A of the vessel for preparation 1 of the impregnation liquor can be reduced to a zero concentration optionally, that is, ultimately, an impregnation which is carried out only with water.

[0111] B—The aqueous washing liquor E for the screw 2′ (which is neutral) bringing the impregnated biomass into the reactor 4 can be recycled into the vessel for preparation 6 of the basic solution EB during the cleaning, which makes it possible to reduce the additional water consumption due to the cleaning, as represented in FIG. 2.

[0112] C—The biomass can be impregnated with a basic liquor during the cleaning sequence, either with the same basic liquor EB as that prepared in the vessel 6 or a different basic liquor, in particular in terms of concentration of base B. This alternative form makes it possible to reduce the amount of pure basic solution to be introduced into the pretreatment reactor 4, since there will no longer be, or will be less, acid to be neutralized in order to reach the targeted basic pH. However, a certain amount of basic liquor will be removed in the pressate (which corresponds to the water extracted from the screw 2′).

[0113] Thus, it may still be necessary to top up with basic liquor EB directly in the reactor 4 (via the vessel 6).

[0114] FIG. 3 illustrates this alternative form: the vessel 6 for preparation of basic liquor EB still has two inlets, one for the concentrated base B (concentrated KOH), the other for the water, but here it also has two outlets: one outlet to the pretreatment reactor, as above, and one outlet to the impregnation reactor 3. With this configuration, it is possible to feed the impregnation reactor 3 either with the acid solution EA from the vessel 1 in production mode or with the basic solution EB from the vessel 6 in production+cleaning mode.

[0115] The vessel 6 can thus simultaneously feed the two reactors 3 and 4, or at least for a common period during the cleaning of the reactor 4. It is also possible to anticipate and begin to feed one of the reactors with basic solution EB before the other, in particular the impregnation reactor 3 before the start of cleaning by the solution EB of the pretreatment reactor 4.

[0116] D—It is also possible to combine the two preceding alternative forms, as represented in FIG. 4, with, at the same time, the recycling of the (basic) pressate at the outlet of the screw 2′ in the vessel 6 for preparation of basic liquor EB, and the feeding by this same vessel 6 of the two reactors 3 and 4 during at least a part of the cleaning. The impregnation of the biomass, during the cleaning, is carried out with basic liquor by changing the impregnation reactor during the cleaning.

[0117] E—It is also possible to use two impregnation reactors 3,3′ operating alternately, as represented in FIG. 5. As in the alternative form C, the biomass is impregnated not with an acidic liquor EA but with a basic liquor EB during at least a part of the cleaning of the reactor, indeed even also a little before, in the following way: in production mode, the biomass is brought into the pretreatment reactor 3 fed with acidic liquor by the vessel 2, and in production+cleaning mode (during all or part of the cleaning), the biomass is rerouted to the impregnator 3′, which itself is fed with basic liquor EB from the vessel 6. A second dedicated impregnation reactor 3′ is thus used for the cleaning. This embodiment exhibits the advantage, in comparison with the alternative form C, of reducing the transition times between acidic impregnations and basic impregnations.

[0118] F—The alternative forms E and B can be combined, that is to say the two impregnation reactors 3,3′ can be used and the water extracted from the screw 2′ can be recycled in the vessel 6 for preparation of basic liquor EB.

[0119] G—The must (also called marc) can be recycled, in particular in the context of the alternative form E having two impregnation reactors: the basic pretreated biomass M1 which exits from the separation device 5 during the cleaning of the pretreatment reactor 4. This is because, during this period, it is basic. It is then possible to wash this must M1 at the outlet of the separation device 5 with water: it becomes a washed basic must M1′, as represented in FIG. 6; and to extract a basic aqueous phase E1 therefrom which is recycled in the vessel 6 for preparation of the basic liquor. The impregnation, during the cleaning, is carried out with a basic liquor by changing the impregnation reactor and the vessel for preparation of liquor.

[0120] H—Another alternative form consists in using two separation devices 5,5′ (cyclone) operating alternately, as represented in FIG. 7: a cyclone 5′ is added which is dedicated to the treatment of the basic marc M1. In production mode, the cyclone 5 is operational; it treats an acidic marc M; in production+cleaning mode, the output of the reactor 4 is switched to the second cyclone 5′, which will thus separate only basic marc M1. The advantage of this alternative form is to reduce the transition time between the two modes. FIG. 7 combines this alternative form with the alternative form G: the basic marc, once separated in the cyclone 5′, is also washed in order to recycle the basic aqueous washing liquor E1 to the vessel for preparation 6 of basic liquor.

[0121] J—This alternative form emerges from the preceding alternative form E having two impregnation reactors 3,3′, with the following difference: In production mode, use is made of the conventional impregnation reactor 3 fed with acidic solution EA by the vessel 1. In production+cleaning mode, the system is switched here to the second impregnation reactor 3′ which is fed only with water, as represented in FIG. 8: during the cleaning, the biomass is thus impregnated only with an aqueous solution at neutral pH (and not a basic solution EB).

[0122] K—The invention also applies to processes for the pretreatment of biomass without prior preimpregnation with a liquor (reference is then made to self-hydrolysis): in this case, the biomass P, after having optionally undergone a treatment of mechanical (grinding, and the like), thermal (drying) or humidification type, is introduced directly into the pretreatment reactor 4, as represented in FIG. 9.

[0123] L—This alternative form, illustrated in FIG. 10, combines the recycling of the pressate E1 of the alternative form E with that of the aqueous washing liquor for the screw 2′ of the alternative form B to the vessel 6 for preparation of the basic liquor EB. Both the consumption of water and of base required for the cleaning according to the invention are thus more substantially reduced.

[0124] M—This alternative form, represented in FIG. 11, recommends a thermal integration of the process, by condensation of the steam V at the outlet of the cyclone 5. This steam V is used to heat the basic liquor EB circulating in pipes between the vessel 6 and the pretreatment reactor 4 via a heat exchanger (not represented). It is also used to reduce the amount of water used in the vessel 6 by recovering the condensate resulting from the cooling of the steam exiting from the cyclone, via a condenser (not represented).

[0125] N—According to another alternative form, it is possible to choose to inject biomass impregnated with acidic liquor EA into the pretreatment reactor 4 from the impregnation reactor 3 in production mode, and to directly inject the nonimpregnated biomass P into the pretreatment reactor 4 in production+cleaning mode, by then stopping the feeding of biomass impregnated with acidic liquor.

[0126] O—According to yet another alternative form, which can be combined with all the others, it is possible to choose to inject, into the pretreatment reactor, a given impregnated biomass in production mode and to inject another biomass, impregnated or not with a liquor, in production+cleaning mode. For example, in production, a straw-type biomass is chosen and, in production+cleaning mode, a more abrasive poplar-based biomass is chosen: a temporary increase is thus brought about in the cleaning time, the abrasive nature of the biomass, in order to help to more easily detach solid coke residues from the walls.

EXAMPLES

Example 1 not in Accordance with the Invention

[0127] It uses the configuration presented in FIG. 1, without the addition of the vessel for preparation of a basic aqueous liquor (KOH) specific to the invention.

[0128] It concerns a mechanical cleaning of the pretreatment reactor with stopping of the production and opening of the reactor, according to an earlier solution.

[0129] It was carried out with wheat straw as biomass. The characteristics and composition of the feedstock are as follows: [0130] Solids content: 91.07% [0131] Biomass flow rate: 65 kg SC/h

[0132] The operating conditions for producing pretreated biomass are as follows: [0133] Impregnation for the production in the impregnation reactor 3:

[0134] Acidic H.sub.2SO.sub.4 solution flow rate: 1.5 kg/h (pH of approximately 1) [0135] Steam explosion of the impregnated biomass in the pretreatment reactor 4:

[0136] Residence time: 5 min

[0137] Production duration: 72 hours [0138] Mechanical cleaning according to prior art:

[0139] Temperature drop time: 48 hours

[0140] Opening and dismantling time: 8 hours

[0141] Cleaning time: 8 hours

[0142] Time for reassembling the reactor: 8 hours

[0143] The weight of coke C produced is 16 kg occupying a volume of 0.012 m.sup.3 in the reactor 4, i.e. a reduction in the reactor volume of 8.7% and a coke production throughput of 222 g/h.

Example 2 in Accordance with the Invention

[0144] It uses the configuration presented in FIG. 1, with the addition of the vessel for preparation of a basic aqueous liquor (KOH) specific to the invention.

[0145] The characteristics and composition of the wheat straw feedstock are identical to those of the wheat straw used in example 1.

[0146] The operating conditions are described in detail below:

[0147] Mode 1=production: [0148] Impregnation for the production in the impregnation reactor 3:

[0149] Acidic solution flow rate: 1.5 kg/h (H.sub.2SO.sub.4) [0150] Steam explosion of the impregnated biomass in the pretreatment reactor 4:

[0151] Residence time: 5 min

[0152] Production duration: 20 hours

[0153] The pressate E1 resulting from the screw 2′ is completely recycled to the vessel 1 for preparation of the acidic aqueous solution.

[0154] After 20 hours of production, the cleaning sequence is carried out under the following conditions:

[0155] Mode 2=production+cleaning: [0156] Impregnation in the impregnation reactor 3:

[0157] Acidic solution flow rate (acidic solution EA): 1.5 kg/h (H.sub.2SO.sub.4) [0158] Steam explosion of the impregnated biomass in the pretreatment reactor 4:

[0159] Residence time: 10 min

[0160] Temperature in the reactor: 200° C.

[0161] KOH flow rate: sufficient to lower the SC down to the saturation value of the biomass Duration of the cleaning: 2 hours

[0162] Concentration of KOH in the liquor: sufficient to change the biomass from a pH of 3 to a pH of 13

[0163] Number of cycles: 3 cycles of mode 1 (production) and of mode 2 (production+cleaning) [0164] Cleaning of the cyclone 5 (separation device)

[0165] Number of water flushes: 2, after each cleaning operation

[0166] After 3 cycles of mode 1+mode 2, for a total production duration of 66 h (60 hours of production and 6 hours of cleaning), the cleaning proved to be effective.

[0167] This is because the weight of coke C recovered at the end of the procedure (thus after these 3 cycles) does not exceed 3 kg, i.e. a coke production throughput of 39 g/h.

[0168] Thus, the cleaning procedure made it possible to reduce the production throughput from 222 g/h in 72 hours to only 39 g/h after a production of 60 h separated from 3 cleaning operations each of 2 hours.

Example 3 in Accordance with the Invention

[0169] It is identical to example 2 except that the production does not last 20 hours but 80 hours.

[0170] After 3 production and cleaning cycles for a total operating time of 246 hours (240 hours of production mode and 6 hours of production+cleaning mode), the cleaning proved to be effective.

[0171] This is because the weight of coke C recovered at the end of these three cycles does not exceed 3 kg, i.e. a coke production throughput of 41 g/h.

[0172] Thus, the cleaning procedure made it possible to reduce the production throughput from 222 g/h in 72 h to only 41 g/h after a production of 240 hours separated from 3 cleaning operations each of 2 hours.

Example 4 in Accordance with the Invention

[0173] The feedstock is still wheat straw, the characteristics and composition of which are as follows: [0174] Solids content: 88.30% by weight [0175] Biomass flow rate: 65 kg SC/h

[0176] The operating conditions are described in detail below:

[0177] Mode 1=production: [0178] Impregnation for the production in the impregnation reactor 3:

[0179] Acidic solution flow rate: 1.5 kg/h [0180] Steam explosion of the impregnated biomass in the pretreatment reactor 4:

[0181] Residence time: 5 min

[0182] Production duration: 80 hours

[0183] After 80 hours of production, the cleaning sequence is set in motion under the following conditions:

[0184] Mode 2=production+cleaning: [0185] Impregnation in the impregnation reactor 3:

[0186] Acidic solution flow rate: 1.5 kg/h [0187] Steam explosion of the impregnated biomass in the pretreatment reactor 4:

[0188] Residence time: 10 min

[0189] Temperature of the reactor: 200° C.

[0190] KOH flow rate: sufficient to lower the SC down to the saturation value of the biomass

[0191] Duration of the cleaning: 2 h

[0192] Concentration of the KOH in the liquor: sufficient to change the biomass from a pH of 3 to a pH of 13

[0193] Number of cycles: 8 cycles of mode 1 (production) and of mode 2 (production+cleaning) [0194] Cleaning of the cyclone 5

[0195] Number of water flushes: 2

[0196] After these 8 cycles, for a total operating time of 656 hours (640 hours of production and 16 hours of production+cleaning), the cleaning proved to be effective.

[0197] This is because, in total, 14.96 kg of coke C were recovered, i.e. a coke production throughput of 22 g/h.

Example 5 in Accordance with the Invention

[0198] It was carried out with SRC poplar wood, the characteristics and composition of which are as follows, with the configuration of FIG. 1 with addition of the vessel 6 for preparation of basic liquor, as for example 2: [0199] Solids content: 55.50% by weight [0200] Biomass flow rate: 80 kg SC/h

[0201] Mode 1=production: [0202] Impregnation for the production in the impregnation reactor 3:

[0203] Acidic solution (2.5% by weight) flow rate: 2.7 kg/h [0204] Steam explosion of the impregnated biomass in the pretreatment reactor 4:

[0205] Residence time: 7.5 min

[0206] Production duration: 60 hours

[0207] After 60 hours of production, the cleaning sequence is set in motion under the following conditions:

[0208] Mode 2=production+cleaning: [0209] Impregnation in the impregnation reactor 3:

[0210] Acidic solution (2.5% by weight) flow rate: 2.7 kg/h [0211] Steam explosion of the impregnated biomass in the pretreatment reactor 4:

[0212] Residence time: 7.5 min

[0213] Temperature of the reactor: 200° C.

[0214] KOH flow rate: sufficient to lower the SC down to the saturation value of the biomass Duration of the cleaning: 2 h

[0215] Concentration of the KOH in the liquor: sufficient to change the biomass from a pH of 3 to a pH of 13

[0216] Number of cycles: 3 cycles of mode 1 and of mode 2 [0217] Cleaning of the cyclone 5 [0218] Number of water flushes: 2

[0219] After these 3 cycles, for a total operating time of 186 hours (180 hours of production and 6 hours of cleaning), the cleaning proved to be effective.

[0220] This is because, in total, 8 kg of coke were recovered, i.e. a coke production throughput of 121 g/h.

[0221] In conclusion, the cleaning according to the invention avoids having to stop the production, with all the disadvantages which are connected with this (loss of time, loss of yield, more burdensome servicing by operators), or, at the very least, makes it possible to very significantly space out the complete cleaning operations with stopping of production.