Method for treating lignocellulosic biomass by impregnation and steam explosion
11236403 · 2022-02-01
Assignee
- Institut National De La Recherche Agronomique (Paris, FR)
- Ifp Energies Nouvelles (Rueil-Malmaison, FR)
- AGRO INDUSTRIES RECHERCHE ET DEVELOPPEMENT (Pomade, FR)
Inventors
Cpc classification
C08H8/00
CHEMISTRY; METALLURGY
International classification
Abstract
The invention concerns a continuous method for treating a lignocellulosic biomass before enzymatic hydrolysis, said method comprising a transfer of crushed then pressed biomass to a step of impregnation by an acid liquor, the impregnated biomass dripping over the area containing the impregnation liquor, then the resulting wet biomass is pressed and sent to a steam explosion step. The impregnation liquor is prepared in a dedicated area from the used liquor from the dripping operation and/or that from the pressing operation, prior to the explosion step. The invention also concerns the facility for implementing the method.
Claims
1. A continuous process for treating a lignocellulosic biomass for the production of sugary juices, carried out before an enzymatic hydrolysis, the process comprising: 1) in a first transfer zone, transferring a milled biomass to a first impregnation zone of an impregnation step, said first transfer zone and first impregnation zone being separated by a plug of biomass preventing liquid upflow from said first impregnation zone to the first transfer zone, 2) carrying out the impregnation step under atmospheric pressure in 2 superposed impregnation zones operating at a temperature of 10-95° C., the residence time of the biomass in said step being 1 min-60 min, and said first impregnation zone receives the pressed biomass resulting from the first transfer zone and it contains an impregnation liquor having a pH of between 0.1 and 7, and transferring said biomass to a second impregnation zone, located above said first impregnation zone, in which the impregnated biomass is drained, so as to produce a resulting wet biomass having a total solids content of between 15 wt % and 40 wt %, and a separate liquor, 3) in a second transfer zone, transferring the wet biomass to a steam explosion step, the steam explosion step having each of a cooking zone which comprises a reactor, an expansion zone and a separating zone, said second transfer zone being positioned in a chamber separated, on the one hand, from the impregnation zones of said impregnation step and, on the other hand, from the zones of the steam explosion step, the second impregnation zone and the reactor of the steam explosion step being separated in a leaktight manner by a plug of biomass, and in said second transfer zone, pressing the wet biomass so as to increase the total solids content of the biomass to 40% by weight or more, and produce a resulting liquor, 4) a steam explosion step comprising cooking the biomass in the cooking zone for 1-30 min by injection of steam with a specific steam consumption of between 0.05 and 10 tonnes/tonne of biomass total solids entering said zone, said cooking zone being at a temperature of 150-250° C. and a pressure of 0.5-4 MPa, then, in the expansion zone, expanding the biomass resulting from the cooking zone, then, in the separation zone, separating the steam from the biomass, 5) a step of producing an acid liquor using a preparation device suitable for, receiving the resulting liquor from the second transfer zone and/or from the first impregnation zone, and receiving inputs of acid and/or water that are determined so as to maintain the pH at the inlet of the first impregnation zone at a value between 0.1 and 7, and introducing said acid liquor into the first impregnation zone.
2. The process as claimed in claim 1, wherein said impregnation step is carried out in a single step.
3. The process as claimed in claim 1, wherein: the impregnation step is carried out in 2 impregnation zones positioned vertically one above the other and along the same axis, separated so as to allow the liquor from the second zone to pass to the first impregnation zone and so as to retain the biomass in the second impregnation zone, the level of the liquid of the first impregnation zone ensuring the separation between said 2 zones, and said biomass is transferred by means of a screw from said first impregnation zone into said second impregnation zone.
4. The process as claimed in claim 1, wherein the liquor level is kept constant by input of acid liquor.
5. The process as claimed in claim 1, wherein the acid liquor is only a solution of sulfuric acid.
6. The process as claimed in claim 1, wherein the acid liquor has a pH of between 0.1 and 2.
7. The process as claimed in claim 1, wherein said step of preparing the acid liquor receives liquor extracted from the second transfer zone and optionally liquor from the first impregnation zone.
8. The process as claimed in claim 1, wherein said step of preparing the acid liquor receives liquor from the first impregnation zone and optionally liquor extracted from the second transfer zone.
9. The process as claimed in claim 1, wherein, in said second transfer zone for transferring the wet biomass, the total solids content of the biomass after pressing is between 40% and 70% by weight.
10. The process as claimed in claim 1, wherein the cooking zone is a horizontal tubular reactor, and is provided with one or more screws that transport the biomass from the second transfer zone upstream to the expansion zone downstream.
11. The process as claimed in claim 1, wherein the milled biomass has a size of at most 300 mm.
12. The process as claimed in claim 1, wherein at least one portion of the biomass resulting from the steam explosion step is subjected to an enzymatic hydrolysis and sugary juices are obtained.
13. The process as claimed in claim 12, in which at least one portion of the sugary juices is subjected to an alcoholic fermentation.
14. The process as claimed in claim 1, wherein, in said second transfer zone for transferring the wet biomass, the total solids content of the biomass after pressing is greater than 40%.
15. A unit for the continuous treatment of a lignocellulosic biomass before enzymatic hydrolysis, comprising: 1) a first transfer zone for transferring a milled biomass, with pressing, to a first impregnation zone of an impregnation reactor, said first transfer and first impregnation zones being separated by a plug of biomass preventing liquid upflow from said first impregnation zone to the first transfer zone, 2) an impregnation reactor comprising 2 superposed impregnation zones, the second impregnation zone being located above the first impregnation zone said first impregnation zone containing an acid liquor and being provided with an inlet opening for the pressed biomass resulting from the first transfer zone, the impregnation reactor being provided with a screw that transfers said biomass from said inlet opening in the first impregnation zone to an outlet opening of the second impregnation zone, the second impregnation zone located above the liquor of the first impregnation zone and being provided with screen(s) that allow the liquid to flow from the second impregnation zone into the first impregnation zone and that retain a wet biomass wetted by the liquor in the second zone, 3) a second transfer zone for transferring the wet biomass, with pressing, to a steam explosion zone, said second transfer zone being positioned in a chamber separated, on the one hand, from the impregnation zones and, on the other hand, from the steam explosion zone, the second impregnation zone and the steam explosion zone being separated by a plug of pressed biomass so as to increase the total solids content of the biomass to 40% by weight or more, said plug preventing liquid upflow from said first zone to the second transfer zone, and said second transfer zone being provided with a line for drawing off the spent liquor separated from the wet biomass during the pressing, 4) the steam explosion zone comprising a screw for transferring the biomass through the following successive zones: a biomass cooking zone provided with a line for conveying the pressed biomass resulting from the second transfer zone and a steam injection line, an expansion zone for expanding the biomass resulting from the cooking zone, a separation zone for separating the steam from the biomass, 5) an acid liquor preparation zone provided with a line conveying the spent liquor drawn off from the second transfer zone to the steam explosion zone and/or a line conveying the spent liquor drawn off from the first impregnation zone, with a line conveying the acid and/or a line conveying the water, with a line for recycling said prepared acid liquor to the first impregnation zone, with a stirring system and optionally with a heating means.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1)
(2)
(3)
DESCRIPTION OF THE FIGURES WHICH WILL BE FOLLOWED USING THE FOLLOWING REFERENCES
(4)
(5)
(6)
(7)
(8) The milled biomass is introduced via the line 1 into the process and transferred (first transfer zone 2) into the impregnator 3 and more specifically into the first impregnation zone 3a of the impregnator. This zone contains liquid conveyed by the line 4 and which originates from the liquor preparation device (step) 6. This device 6 receives, via the line 5, the liquor originating from the second impregnation zone 3b of the impregnator and which has been separated from the biomass, and is provided with lines 7 and 8 for the addition of water and acid respectively. The wet biomass obtained after separation of the liquid is transferred from the second transfer zone 9 to the next step of the process, which is usually a step of pretreatment by steam explosion, carried out for example in the zone 10.
(9) Single Recirculation Loop with One or More Other Downstream Liquid Streams
(10) Other liquid streams separated after the impregnation step (or after the impregnator), and more specifically after the second impregnation zone of this step, may also be recycled as impregnation after passing through the liquor preparation step (device). Advantageously, the characteristics of the stream are measured by sensors (of flow rate, pH, etc.) before introducing the stream into the preparation step. The control of the device enables the addition of water and/or of acid in the correct proportions for preparing the liquor with appropriate characteristics. Preferably, said stream is the pressate (liquid resulting from the second transfer zone for transferring wet biomass to the steam explosion step). Indeed, this stream is a spent liquor that still contains acid which is therefore recoverable.
(11) The operation of a recycling of liquid downstream of the impregnation is the following (with reference to
(12) In
(13) However, in this arrangement from
(14) This arrangement is preferably used when there is little difference between the fresh liquor entering the first zone and the spent liquor separated from the biomass in the pressate (or more generally in this other stream).
(15) Recirculation Loop Referred to as a Double Recycling Loop
(16) It combines the preceding 2 recycling loops in order to benefit from the combined advantages thereof.
(17)
(18) Thus, in
(19) Similarly, the downstream liquor 11 (which is for example the pressate resulting from the second transfer zone) is sent to the preparation zone. Thus, the liquor preparation zone receives two (or more) spent liquors and makes it possible to mix them.
(20) The sensors cited above send information necessary for determining the appropriate amounts of water and acid to be added in order to obtain a recycled liquor having the pH and flow rate that are desired for the first zone of the impregnator.
(21) According to the invention, the liquor preparation step may therefore be carried out in various ways: with withdrawal of liquor from the impregnator and recycling of liquor into the first impregnation zone of the impregnator; with withdrawal of one or more liquids present downstream of the impregnation (such as the pressate) and recycling into the first impregnation zone of the impregnator; this arrangement is advantageous, especially in the case where there is little difference in characteristics between the downstream liquor and the fresh liquor, with double recycling, i.e. with withdrawal of liquor from the impregnator and recycling of liquor into the first impregnation zone of the impregnator and also withdrawal of one or more liquids present downstream of the impregnation and recycling into the first impregnation zone of the impregnator. This arrangement is widely preferred.
(22) Zone for Transferring with Pressing to the Steam Explosion Step
(23) The wet biomass resulting from the second impregnation zone of the impregnation step is transferred continuously to the steam explosion step via a second transfer zone.
(24) The second transfer zone is separate from the impregnation zones and is not positioned in the chamber of one or other of the impregnation zones. For example, in the case of an impregnator comprising 2 zones (impregnation and draining zones), the transfer zone is outside of the impregnator.
(25) This second transfer zone is also separate from the steam explosion zones.
(26) In the same way as in the first transfer zone conveying the biomass to the impregnation step, the second impregnation zone and the reactor where the steam explosion step takes place are separated by a plug of biomass. Said plug prevents liquid upflow from the steam explosion zones first zone to the transfer zone or even further upstream.
(27) This leaktightness is ensured in the second transfer zone between said second impregnation zone of the impregnation step and the first zone of the steam explosion step.
(28) One means customarily used for this purpose is a screw referred to as plug screw known to a person skilled in the art. The conical part of the screw is here connected to the first steam explosion zone. The non-conical part is connected to said second impregnation zone.
(29) The formation of a plug of biomass ensures the leaktightness to the pressure of the steam explosion reactor, thus preventing dangerous escapes of steam.
(30) In said second transfer zone, the wet biomass is pressed so as to increase the TS content of the biomass. The total solids content of the biomass achieved is generally at least 40% by weight, preferably it is greater than 40% by weight and even more preferably it is from 40% by weight to 70% by weight.
(31) The second transfer zone is also provided with a line for drawing off the spent liquor (referred to as pressate) separated from the wet biomass during the pressing.
(32) The spent liquor is advantageously recycled into the impregnation step, as was described above.
(33) It should be noted that the drained biomass does not undergo a mechanical treatment before hydrolysis that aims to reduce the sizes.
(34) The Steam Explosion Step Carried Out in a Steam Explosion Zone
(35) It generally employs: a cooking zone, in which the biomass is bought into contact for 1-30 min with steam with a specific steam consumption of 0.05-10 tonnes/tonne of biomass total solids, said zone being at a temperature of 150-250° C. and a pressure of 0.5-4 MPa, then an expansion zone for expanding the biomass resulting from the cooking zone, then a separation zone for separating the steam from the biomass.
(36) The steam recovered is advantageously recycled after compression to the steam explosion step, or optionally is recycled to the utilities of the site.
(37) Preferably, this step is carried out in a tubular reactor that is horizontal (i.e. which may be very slightly inclined for the flow of the liquid).
(38) The biomass cooking zone is provided with a line for conveying the pressed biomass resulting from the second transfer zone and a steam injection line. The cooking is carried out at high temperature and under pressure. This pressurization is carried out by injection of steam so as to achieve a pressure of 0.5-4 MPa. The cooking temperature is generally 150-250° C. Preferably, the conditions are regulated so that the cooking time is limited to 1-30 min.
(39) This step employs a reactor which is provided with a screw for transferring the biomass through the successive zones. The screw ensures the transport of the biomass continuously, the speed of the screw being regulated in order to fulfil the residence time conditions.
(40) At the end of the screw (at the end of the reactor), the biomass is entrained very rapidly by the steam to an expansion zone in a line referred to as a blowline which has a reduced diameter relative to the cooking zone.
(41) The expansion zone comprises a line in which the biomass circulates and passes through a cross section restriction member then, after having cleared the restriction, undergoes an abrupt expansion.
(42) The blowline has a cross section restriction member which may be an orifice or a valve with an adjustable opening (diaphragm valve for example) that allows a small flow area. At this restriction, the biomass arrives with a very high transport speed, and undergoes a rapid and large pressure variation, then an abrupt expansion after having cleared the restriction, which destructures the cooked biomass. This is why it is referred to as steam explosion.
(43) This restriction sets the steam flow rate and therefore the specific steam consumption (steam flow rate/dry biomass flow rate). Generally, this specific steam consumption is 0.05-10 tonnes/tonne of total solids.
(44) Once the expansion zone is passed, the biomass is entrained by the steam through the remainder of the blowline which has a larger diameter than the restriction (or which returns to its diameter upstream of the restriction) and which conveys the biomass to a zone for separating the steam, for example via a cyclone.
(45) The exploded biomass resulting from the separation zone now has a sufficient accessibility of the cellulose to enzymes in order to be treated by enzymatic hydrolysis for the production of 2G sugars.
(46) The conditions of the enzymatic hydrolysis and of the consecutive or spontaneous fermentation are suitable for the desired products and are known to a person skilled in the art.
(47) This process according to the invention finds a particular advantageous application in a process for preparing sugars from lignocellulosic biomass and in the process for producing ethanol from said sugary juices.
(48) Such processes are known. A process for preparing sugars from lignocellulosic biomass comprises a pretreatment, which is advantageously a steam explosion, followed by an enzymatic hydrolysis. The process for producing ethanol from sugars further comprises an alcoholic fermentation of said sugars.
(49) In a process according to the invention, at least one portion of the biomass resulting from the steam explosion step is subjected to an enzymatic hydrolysis and sugary juices are obtained. Preferably, said biomass is introduced without an intermediate mechanical step into the enzymatic hydrolysis reactor.
(50) In a process according to the invention, at least one portion of the biomass resulting from the steam explosion step is subjected to an enzymatic hydrolysis, sugary juices are obtained and at least one portion of the sugary juices is subjected to an alcoholic fermentation.
EXAMPLES
(51) In the examples described below, the acronym “TS” denotes the total solids content which is measured according to the standard ASTM E1756-08(2015) “Standard Test Method for Determination of Total Solids in Biomass”.
Example 1 (Comparative)
(52) In this example 1, the biomass treated is straw previously milled on a 50 mm screen. The milled straw has a TS of 90.2% and a glucose content estimated at 39.9 g per 100 g of TS and a xylose content estimated at 26.6 g per 100 g of TS.
(53) The straw is treated according to a process not in accordance with the invention.
(54) In a first step, the milled straw is bought into contact in tanks over 4 hours with an acid liquor preheated to 70° C. The contacting operation (impregnation of the liquor) is carried out in batch mode in the following manner: a tank with a capacity of 1 m.sup.3 is filled with 62.1 kg of milled straw (i.e. 56 kg TS), then the screen is positioned on the milled straw and weighted down with 4 weights of 2 kg each. 685 kg of acid liquor containing 4.72 kg of H.sub.2SO.sub.4 acid and the remainder of water, are then introduced into the tank. After 4 hours of contact, the tank is emptied by gravity drainage. The mean amount of liquor extracted is 469 kg. The drained biomass is then transferred into a buffer hopper, then poured onto a conveyor belt and finally introduced into a conical screw conveyor for transfer to the cooking tool continuously, at a mean flow rate of 203.2 kg/h. In order to maintain the feed rate, a tank is prepared every 82 minutes. During the passage in the conical screw conveyor, a plug of biomass is formed, ensuring the leaktightness with the cooking reactor maintained at 190° C. Liquid is extracted from the biomass during the compression thereof in the conical screw conveyor, at a rate of 133.6 kg/h on average. The cooking reactor is maintained at 190° C. by addition of steam, at a mean flow rate of 384.7 kg/h. On leaving the reactor, the biomass is rapidly depressurized and collected in a tank at atmospheric pressure. During the transfer between the cooking reactor and the collection tank, the pretreated substrate is separated from the gas phase in a cyclone. The collected flow rate of biomass thus pretreated is 85.5 kg/h. Its TS is measured at 40.7%. The vapors separated at the top of the cyclone are condensed and a condensate is collected with a mean flow rate of 368.8 kg/h.
(55) The pretreated biomass has a potential xylose content of 20.2 g per 100 g TS, of which 17.5 g per 100 g TS are in the form of xylose monomer and soluble oligomers. Thus, the xylose balance shows a conversion of 91.3% of the xylose initially present. The process of example 1, not in accordance with the invention, requires the following specific consumptions (relative to 1 kg TS treated) water: 12.14 kg of water/kg TS treated sulfuric acid: 84.3 g/kg TS treated steam: 9.4 kg/kg TS treated
(56) Furthermore, the process of example 1 has considerable equipment requirements: a minimum volume for the contacting of 2.2 m.sup.3. The volume to be pumped in order to fill the contacting tanks is around 0.5 m.sup.3/h.
Example 2 (in Accordance with the Invention)
(57) In example 2, the biomass treated is straw identical to that of example 1. The milled biomass is introduced at a mean flow rate of 45.4 kg/h into the impregnation tool via a first transfer zone in which it is compressed. The compressed biomass is introduced into an impregnation tool having a working height of 2 meters, equipped with two parallel screws enabling a vertical conveying of the biomass at an equivalent linear velocity of 106 m/h. The total working volume of the impregnation tool is 78 liters, with a volume of the first impregnation zone (referred to as submerged zone) of 45 liters. At 5 cm below the liquid level, 412 kg/h of acid liquor are added, this acid liquor originates from a zone for preparing said liquor. The temperature of the acid liquor injected is 80° C. Thus, in the first impregnation zone, the biomass is bought into contact with an acid liquor. The mean temperature in the first impregnation zone is 73.5° C.
(58) At the bottom of the impregnation tool, liquid is drawn off (spent liquor), at a flow rate of 232.1 kg/h. In accordance with the invention, this liquid is sent back to the liquor preparation zone The impregnated biomass leaves by the top of the impregnation reactor with a flow rate of 225.3 kg/h.
(59) Into the zone for preparing the acid liquor, three streams are thus conveyed: the spent liquor drawn off from the impregnation tool (at a rate of 232.1 kg/h), the sulfuric acid (expressed as 100% by weight equivalent) at a flow rate of 2.52 kg/h, and the water at a flow rate of 177.4 kg/h. These incoming streams are mixed in a stirred tank having a unit volume of 560 liters, the prepared liquor is drawn off (at a rate of 412 kg/h), sent to a heat exchanger in order to heat it to the temperature of 80° C., then injected into the impregnation tool as described above.
(60) The straw impregnated with acid liquor is then conveyed on a belt then transferred into the cooking tool via a transfer zone in a conical screw. During this transfer, a liquid (spent liquor) flows at a flow rate of 127.8 kg/h.
(61) In the cooking reactor, steam is injected at a flow rate of 347 kg/h. This injection of steam makes it possible to maintain the temperature of the reactor at 190° C. On leaving the reactor, the biomass is rapidly depressurized and collected in a tank at atmospheric pressure. During the transfer between the cooking reactor and the collection tank, the pretreated substrate is separated from the gas phase in a cyclone. The collected flow rate of substrate thus pretreated is 92.8 kg/h. Its TS is measured at 38.5%. The vapors separated at the top of the cyclone are condensed so as to produce a condensate with a condensate flow rate of 351.8 kg/h.
(62) The pretreated substrate has a potential xylose content of 19.8 g per 100 g TS, of which 17.3 g per 100 g TS are in the form of xylose monomer and soluble oligomers. Thus, the xylose balance shows a conversion of 91.7% of the xylose initially present.
(63) The process of example 2, in accordance with the invention, requires the following specific consumptions (relative to 1 kg TS treated) water: 4.34 kg of water/kg TS treated sulfuric acid: 61.5 g/kg TS treated steam: 8.5 kg/kg TS treated
(64) Thus, the process according to the invention of example 2 makes it possible to achieve the same degrees of conversion of the xylose in the pre-treatment step as example 1 while ensuring a significant reduction of the consumptions of water and acid, respectively a 64% reduction in the consumption of water and a 27% reduction in the consumption of acid.
(65) Furthermore, the process of example 2 has reduced equipment requirements: a minimum volume for the impregnation of the acid liquor of less than 1 m.sup.3 including the liquor preparation tank and the impregnation tool. The volume to be pumped for pumping and drawing off the liquors around the impregnation is around 0.6 m.sup.3/h.
Example 3 (in Accordance with the Invention)
(66) Example 3 uses poplar wood as biomass in the form of chips having a mean size of 50 mm long and 10 mm thick and the initial TS content of which is 55.7%. Before its treatment, the biomass is at ambient temperature, i.e. around 20° C.
(67) The biomass is introduced into the same impregnation tool as example 2, via a first transfer zone at a mean flow rate of 140.2 kg/h. The compressed biomass is introduced into an impregnation tool having a working height of 2 meters, equipped with two parallel screws enabling a vertical conveying of the biomass at an equivalent linear velocity of 106 m/h. At the bottom of the impregnation tool, 163.9 kg/h of acid liquor are added, this acid liquor originates from a preparation zone. The temperature of the acid liquor injected is 80° C. Thus, in the first impregnation zone, the biomass is bought into contact with an acid liquor. The mean temperature in the first impregnation zone is 55.9° C. On leaving the impregnation tool, the impregnated biomass is extracted at a flow rate of 304 kg/h. Its TS is measured at 27.1% TS according to the standard ASTM E1756.
(68) The impregnated biomass is then conveyed on a belt then transferred into the cooking tool via a transfer zone in a conical screw. During this transfer, a liquid (spent acid liquor) flows at a flow rate of 161.7 kg/h. In accordance with the invention, this liquid referred to as pressate, is collected and pumped to the acid liquor preparation zone. In the cooking reactor, steam is injected at a flow rate of 416.9 kg/h. This injection of steam makes it possible to maintain the temperature and the pressure of the reactor, at 200° C. and 1.49 MPa respectively. On leaving the cooking reactor, the biomass is rapidly depressurized and collected in a tank at atmospheric pressure. During the transfer between the cooking reactor and the collection tank, the pretreated substrate is separated from the gas phase in a cyclone. The collected flow rate of substrate thus pretreated is 145.9 kg/h. Its TS is measured at 49.9%. The vapors separated at the top of the cyclone are condensed so as to produce a condensate with a flow rate of around 413.3 kg/h.
(69) The pressate extracted from the second transfer zone for transferring to the cooking reactor is sent to the liquor preparation zone. This zone consists of a mixing tank and a heat exchanger. The mixing tank is fed by a line for make-up of water, a line for make-up of concentrated H.sub.2SO.sub.4 acid solution and a line conveying the recycled pressate, and has a line for drawing off prepared liquor. The heat exchanger is placed on the prepared liquor line, going from the mixing tank to the impregnator and therefore makes it possible to reheat the liquid leaving the tank and to control the temperature of the liquor introduced into the impregnation tool independently of the temperature of the mixing tank and of its inputs. In the configuration used, the make-ups of water and of H.sub.2SO.sub.4 (100%) are respectively 1.2 kg/h and 1 kg/h. The outlet temperature of the exchanger is 80° C. for the acid liquor. Thus, the implementation of the process according to the invention makes it possible to treat the wood with minimized consumptions of acid and of water. Specific consumption of acid is 12.8 g/kg of incoming TS and the specific consumption of water is 15.4 g/kg TS.
Example 4 (in Accordance with the Invention)
(70) Example 4 treats the same poplar wood as example 3. The process thus treats a nominal flow rate of 140.2 kg/h of poplar wood in the form of chips having a mean size of 50 mm long and 10 mm thick, the initial TS content of which is 55.7% and which was stored before treatment at a temperature of 20° C. The biomass is introduced into the same impregnation tool as example 2 via a first transfer zone. The compressed biomass is introduced into an impregnation tool having a working height of 2 meters, equipped with two parallel screws enabling a vertical conveying of the biomass at an equivalent linear velocity of 106 m/h. At the bottom of the impregnation tool, 570 kg/h of acid liquor are added, this acid liquor originating from a liquor preparation zone. The temperature of the acid liquor injected is 80° C. Thus, in the impregnation tool, the transferred biomass is bought into contact with an acid liquor. At 15 cm below the liquid level, spent liquor is drawn off, at a flow rate of 407 kg/h. This spent liquor is sent to the liquor preparation zone in accordance with the invention.
(71) The mean temperature in the first impregnation zone is 70.1° C. On leaving the impregnation tool, the impregnated biomass is extracted at a flow rate of 303.1 kg/h. Its TS is measured at 27.2% according to the standard ASTM E1756.
(72) The impregnated biomass is then conveyed on a belt then transferred into the cooking tool via a transfer zone in a conical screw. During this transfer, a liquid (spent liquor) flows at a flow rate of 161.7 kg/h. This liquid, referred to as pressate, is collected and pumped to the acid liquor preparation zone. In the cooking reactor, steam is injected at a flow rate of 414.2 kg/h. This injection of steam makes it possible to maintain the temperature and the pressure of the reactor, at 200° C. and 1.49 MPa respectively. On leaving the reactor, the biomass is rapidly depressurized and collected in a tank at atmospheric pressure. During the transfer between the cooking reactor and the collection tank, the pretreated biomass is separated from the gas phase in a cyclone. The collected flow rate of pretreated biomass is 142.5 kg/h. Its TS is measured at 51.1%. The vapours separated at the top of the cyclone are condensed providing a condensate having a flow rate of 413.1 kg/h.
(73) In accordance with the invention, the pressate extracted from the second transfer zone for transferring to the cooking reactor is sent to the liquor preparation zone. This zone consists of a mixing tank and a heat exchanger. The mixing tank is fed by a line for make-up of water, a line for make-up of concentrated H.sub.2SO.sub.4 acid solution, a line conveying the recycled pressate, and a line conveying the liquor drawn off from the first impregnation zone. The tank also has a line for drawing off prepared liquor. The contents of the mixing tank is maintained at 1100 kg. The heat exchanger is placed on the prepared liquor line, going from the mixing tank to the impregnator and therefore makes it possible to reheat the liquid leaving the tank and to control the temperature of the liquor introduced into the impregnation tool independently of the temperature of the mixing tank and of its inputs. In the configuration used, the make-ups of water and of H.sub.2SO.sub.4 are respectively 1.2 kg/h and 1 kg/h. The outlet temperature of the exchanger is 80° C. for the acid liquor.
(74) Thus, the setting up of a drawing-off of the spent liquor in the first impregnation zone and a drawing-off of the pressate with the recycling thereof into the liquor preparation tank makes it possible to increase the temperature in the first impregnation zone by 14.2° C. while maintaining the temperature heating the liquor prepared at 80° C., i.e. below the bubble point of said liquor.