DESCRIPTION

BACKGROUND

The transfer of a ligno-cellulosic feedstock from a low pressure environment to a high pressure environment is a typical problem of the pulp and paper industry. Many solutions have been developed for moving hardwood and softwood chips from a chip bin at atmospheric pressure to a pressurized vessel, where the wood chips are treated to release the cellulosic fibers. However, the treatment often involves the use of steam or vapor at the pressure of many atmospheres, thereby there is the need to prevent the loss of steam or vapor from the pressurized vessel.

Some methods known in the art for solving the problem involve the use of a piston for compressing the feedstock in a conduct isolated from the pressurized vessel by means of valves system.

As an example, SE469536 discloses a method and an apparatus for feeding a feedstock such as wood chips and the like into a collecting container under pressure, the feedstock being transported in a pulsing flow from a charging bin through an input valve to a cylindrical input chamber. The feedstock is then discharged axially from this chamber to the collecting container through an output valve by means of a feeding piston. This method has a main drawback of operating in a discontinuous way. Moreover, there is the need of operating the valve and the piston in a synchronized way thereby subjecting the apparatus to fault risk. The valve is a gate valve and it is operated frequently during normal operation of the apparatus, thereby limiting the lifetime of the gate valve.

WO03013714A1 discloses a method and apparatus for the transfer of feedstock products between zones of different pressure by means of a piston screw. The invention is especially suitable for the transfer of- low density biomass such as straw. Inventor recognizes that straw, while being a large biomass resource, has not yet been intensively exploited, because its properties makes it very difficult to transport into, through and out of pressurized equipment. According to the inventor, the main obstacles for transferring a straw feedstock are the low density of the straw and the fact that straw is a non-flowing product and it has very strong bridging properties. The method is based on a sluice system which comprises at least one sluice chamber and two pressure locks of which at least one at any time secures a pressure tight barrier between the two pressure zones. Product portions are force loaded from the first zone into a sluice chamber by means of a piston screw, the axis of which is practically in line with the axis of the sluice chamber, and the product portions are force unloaded from the sluice chamber and into the second pressure zone by means of said piston screw or a piston or by means of gas, steam or liquid supplied at a pressure higher than that of the second pressure zone. The operation procedure results are complicated, subjecting the apparatus to fault risk of the two pressure locks, which are operated frequently during normal operation of the apparatus. The patent discloses thereby a method for reducing or eliminating the risk of bridging by forcing the product through the critical zones of the apparatus. A further problem solved by the method is compressing low density products, such as straw, to a higher density in order to obtain a suitable capacity within reasonable dimensions. Another solution disclosed in the prior art for transferring a feedstock involves the formation of a slurry of the feedstock, by mixing the feedstock with a liquid such as water or a liquor obtained from the downstream process. The slurry is then pumped into the pressurized vessel by means of slurry pumps. As an example, US6325890 discloses a system and a method for feeding comminuted cellulosic fibrous material such as wood chips to the top of a treatment vessel such as a continuous digester. The system and method provide enhanced simplicity, operability, and maintainability by eliminating the high pressure transfer device conventionally used in the prior art. Instead of a high pressure transfer device the steamed and slurried chips are pressurized using one or more slurry pumps.

Plug screws are devices which are able to compact the feedstock to form a feedstock plug, which preserves a pressure difference at its ends. By using a plug screw device, there is no longer the need to use a valve to seal the pressurized vessel during continuous operation, because the feedstock plug is able to prevent pressure losses. A valve is often used for sealing the pressurized vessel in case the feedstock plug fails in preserving the pressure difference. US2009173005A1 discloses a transport system for the introduction of biomass into a gasifier comprising a plug screw that forces the biomass into the gasifier, wherein the plug screw is formed such that the biomass is compressed in order on the one hand for it to be conveyed against a pressure in the gasifier and, on the other hand, to leave the gas and bed material in the gasifier, having a gate valve adjacent to the plug screw and which closes when the plug screw stops so that heat, vapor and gas cannot escape. The gate valve should prevent backflow during the operational phases in which the feedstock plug being fed does not itself establish the isolation. A main drawback of the gate valve is the need to be operated by an external control as a response to a pressure drop detected by the control by means of a sensor. For being effective, in the case of failure of the feedstock plug in preserving the pressure drop, the gate valve must be operated quickly to seal the gasifier and to avoid the complete flashing of the gasifier. If some feedstock is located in the gap of the gate valve, the gate valve may be subjected to serious damage.

WO2008008296A2 discloses a gasifier feed system including a buffer assembly that receives material, such as comminuted cellulosic material to be fed to a gasifier. The feed system includes a structure that pre- processes the material for delivery to the gasifier. From a buffer bin, the cellulosic material is fed to a plug screw feeder. Once in the screw feeder, a plug of the cellulosic material is formed and is moved toward a discharge end of the screw feeder. A back stop or blowback valve (blowback damper) is disposed adjacent the discharge end of the plug screw feeder. The back stop serves to close off the plug screw feeder from downstream equipment if the flow of material or density of the plug inside the plug screw feeder is lost. That is, the back stop comprises a preloaded valve or cone that closes the feeder in the event of a pressure drop in the feeder. The back stop serves to prevent the escape of any gas from the gasifier into the atmosphere or other equipment upstream of the gasifier. The back stop or blowback valve is activated directly from the pressure drop, thereby avoiding the need of an active control system and sensor.

While the patent application WO2008008296A2 discloses the use of a conic back stop or blowback valve adjacent to outlet of the plug screw feeder to solve the problem of feeding a generic comminuted cellulosic material to a high pressure environment, inventors have surprisingly found that the disclosed configuration fails to work and is jammed, or obstructed, in the case of low density feedstocks such as straw. Moreover, the patent application does not give any teaching on a working method to operate the back stop or blowback valve in the case of a low density feedstock having an enhanced aptitude to bridging.

SUMMARY

It is disclosed a system for feeding a ligno-cellulosic feedstock (50) to a pressurized vessel (200) having a vapor or gas phase of at least one fluid maintaining a vessel pressure greater than or equal to 1 bar. The pressurized vessel comprises a feedstock inlet (230), a feedstock outlet (220), a top and a bottom relative to the force of gravity, wherein the feedstock outlet is closer to the bottom of the pressurized vessel than the feedstock inlet.

The feeding system further comprises a plug forming system ( 100) comprising a plug forming device (105) located in a plug forming chamber (130), the chamber having a first end (HOB) and a second end (1 10A), said first end having a first end operating pressure which is lower than the vessel pressure and said second end being connected to the feedstock inlet and having a second end operating pressure which is greater than the first operating pressure. Said plug forming device is capable of receiving the ligno-cellulosic feedstock at the first end pressure; advancing the ligno- cellulosic feedstock through the plug forming chamber; and forming a plug from the ligno-cellulosic feedstock which prevents the passage of the vapor or gas phase from the pressurized vessel.

The feeding system further comprises a sealing head located inside the pressurized vessel, which is movable at a seal opening speed between a sealing position located at the feedstock inlet and a rest position, wherein the sealing head in the sealing position isolates the pressurized vessel from the plug forming chamber. The seal opening speed is fast enough so that the sealing head disengages from the feedstock before or at the moment the sealing head reaches the rest position. The sealing head in the rest position is located at a distance far enough away from the feedstock inlet (sealing head) so that the feedstock entering the pressurized vessel does not contact the sealing head.

It is also disclosed that the distance between the sealing position and the rest position may be greater than a value selected from the group consisting of 50cm, 70cm, 80cm, 90cm, 100cm, 120cm, and 150cm. It is further disclosed that the sealing head may have a flat surface (300B) and the flat surface of the sealing head in the sealing position seals the pressurized vessel (200) from the plug forming chamber (130).

It is also disclosed that the sealing head may have a right conic or trunked conic shape having an inclined surface, and the inclined surface of the sealing head in the sealing position seals the pressurized vessel from the plug forming chamber.

It is further disclosed that the sealing head may be linearly moved by a sealing rod (310) along a rod axis (320) which is perpendicular to the feedstock inlet (230). It is also disclosed that the vessel pressure may be greater than a value selected from the group consisting of 2 bar, 4bar, 5 bar, 7 bar, and 10 bar.

It is further disclosed that the plug forming device may comprise a plug screw (105) which rotates around a rotation axis ( 120) to form the feedstock plug (55) at the second end (1 10A), and that the rotation axis (120) may be parallel to the rod axis (320).

It is also disclosed that the pressurized vessel (200) may further comprise means for linearly moving the sealing piston and means for inserting fluids in liquid or vapor phase in the pressurized vessel.

It is further disclosed that the pressurized vessel may further comprise means for detecting the feedstock plug at the feedstock inlet (230) and means for removing the feedstock plug (55) at the feedstock inlet (230). It is also disclosed that the vessel pressure may be obtained by injecting steam in the pressurized vessel.

It is further disclosed that the feedstock outlet of the pressurized vessel may be connected to a pressurized reactor. It is also disclosed that the system may further comprise means for loading the ligno-cellulosic feedstock into the plug forming chamber (130).

It is further disclosed that preferably the ligno-cellulosic feedstock feeding the system may have a bulk density and the bulk density is less than a value selected from the group consisting of 300kg/ m ³ , 250 kg/m ³ , 200 kg/m ³ , 150 kg/m ³ , 100 kg/m ³ , 75 kg/m ³ , and 50 kg/m ³ .

It is also disclosed that preferably the ligno-cellulosic feedstock may be composed of chips of commutated ligno-cellulosic biomass, the chips having a mean aspect ratio, and the mean aspect ratio of the chips is greater than a value selected from the group consisting of 3: 1 , 5: 1 , 10: 1 , 15: 1 , 20: 1, 30: 1 , and 40: 1.

It is further disclosed that preferably the ligno-cellulosic feedstock is a straw, and that the straw may be selected from the group consisting of wheat straw, rice straw, barley straw, and bagasse.

It is also disclosed a method for continuously feeding the ligno-cellulosic feedstock (50) to the pressurized vessel (200), said method comprising: forming the feedstock plug (55) at the feedstock inlet (230) of the pressurized vessel, wherein the feedstock plug dynamically prevents a loss of the operating pressure; inserting the feedstock plug in the pressurized vessel through the feedstock inlet; in the pressurized vessel, removing by gravity the feedstock plug from the feedstock inlet. During the continuous feeding of the ligno-cellulosic feedstock, any contact between the sealing head and the feedstock plug is avoided so as to prevent a feedstock bridge of the feedstock plug in the pressurized reactor while the feedstock plug enters the pressurized vessel. It is also disclosed a method for starting the feeding of the ligno-cellulosic feedstock (50) to the pressurized vessel (200), said method comprising: placing the sealing head in the sealing position (350B); inserting pressurized steam in the pressurized vessel to reach the vessel pressure, wherein the vessel pressure on the sealing head generating a sealing force on the sealing head which substantially seals the pressurized vessel from the plug forming device; applying a recall force to the sealing head having a recall force component which is opposite to the sealing force; inserting the ligno-cellulosic feedstock in the plug forming device (100) to form the feedstock plug (55), wherein the feedstock plug exerts a plug force on the sealing head which is opposite to the sealing force; when the plug force substantially equals the sealing force, moving the sealing head in the rest position at a velocity (VB) which is higher than the velocity of the feedstock plug entering the pressurized vessel.

The recall force component is less than the sealing force, and it is applied when the sealing force generated by the pressure in the pressurized vessel is greater than the recall force. BRIEF DESCRIPTION OF DRAWINGS

Figure 1 is a schematic representation of equipment for transferring a ligno-cellulosic feedstock disclosed in the prior art.

Figure 2 is a schematic representation of the operating principle of the equipment for transferring a ligno-cellulosic feedstock disclosed in the prior art.

Figure 3 is a schematic representation of the operating principle of the equipment for transferring a ligno-cellulosic feedstock disclosed in the prior art.

Figure 4 is a schematic representation of a preferred embodiment of the disclosed equipment for transferring a ligno-cellulosic feedstock .

Figure 5 is a schematic representation of the operating principle of the disclosed equipment for transferring a ligno-cellulosic feedstock .

Figure 6 is a schematic representation of the operating principle of the disclosed equipment for transferring a ligno-cellulosic feedstock . Figure 7 is a schematic representation of the operating principle of the disclosed equipment for transferring a ligno-cellulosic feedstock .

DETAILED DESCRIPTION

The present invention is related to equipment and method for the efficient transfer of a ligno-cellulosic feedstock (50) from a low pressure environment to a high pressure environment.

This specification discloses equipment and methods for transferring a ligno-cellulosic feedstock (50). The equipment and method prevents the formation of feedstock bridging inside a pressurized vessel. Even if the disclosed system and methods work properly with any ligno-cellulosic feedstock, the advantages offered by the present invention are evident in the case of a low density feedstock, such as straw, when the systems and methods disclosed in the prior art fail to work.

The disclosed invention arises from the failure of the prior art equipment disclosed in patent application WO2008008296A2 to work with varying feedstocks. The invented equipment was the result of many months of failures and experiments using industrial scale equipment and rates.

. The equipment used in the experiments, according to the teaching of WO2008008296A2, is schematically represented in Figure 1 and comprised a plug forming system ( 100) having two ends, one of which was connected and communicating with a vessel, which could be pressurized by steam insertion (210). The plug forming system comprised a plug screw (105) which compressed the feedstock (50) to form a feedstock plug (55) at the end of the plug forming system connected to the inlet of the vessel, while the feedstock entered the plug forming system at the other end. When the plug was correctly formed, it was able to substantially seal or isolate the vessel from the plug forming system, preventing pressure drop and steam loss from the vessel when the vessel was pressurized with steam. In the vessel, a piston (300) having a conic shape head (300A) was linearly moved along its main axis (320) from a rest position (355A) to a sealing position (350A) in contact with the inlet of the vessel. The piston axis (320) and the plug screw axis (120) were on the same straight line. The rest position is the position of the piston in operating condition of the equipment, when the feedstock plug isolated the pressurized vessel (200). The distance between the tip of the conic piston head in the rest position and the feedstock inlet (230) could be varied up to approximately 120cm. The feedstock inlet had a circular shape with a diameter of approximately 60cm. This configuration is commonly used in the pulp and paper industry and apparently works well with wood chips or low aspect ratio particles.

Continuous feeding operations

Inventors performed a set of continuous experiments on different types of ligno-cellulosic feedstocks, by varying the distance between the tip of the conic piston head in the rest position and the feedstock inlet from a position adjacent to the inlet of the vessel, that is according to the teaching of WO2008008296A2, and up to approximately 120cm. Inventors have found the following results: - Wood feedstock (softwood and hardwood): the feedstock was composed of chips having a high density approximately in the range of 400-800Kg/m ³ and linear dimensions ranging from a few centimeters to more than 10cm. As very well known in the art and in the common practice, wood chips are compact and hard. The plug screw (105) compressed the wood chips to form a plug entering the vessel which was a dense, rigid feedstock block. The feedstock block was pushed by the plug screw against the adjacent conic piston head (300A) and it was crushed by friction into small pieces which fell by gravity to the bottom of the vessel. Inventors noted that the presence of the conic piston adjacent to the feedstock inlet (230) was useful for breaking the feedstock block in small fragments. In the absence of the conic piston head adjacent to the feedstock inlet, the feedstock fell down in big blocks under the action of gravity force. Therefore, the feedstock of this invention may be considered the ligno-cellulosic biomass which is not wood chips, wherein at least 75% by dry weight are not wood chips. In particular, the comminuted lingo- cellulosic biomass.

Straw: the feedstock was composed by chips having a very small bulk density approximately in the range of about 50-200Kg/m ³ , having a mean length of a few tens of centimeters and a transversal section with linear dimensions of a few millimeters. Depending also on humidity, the chips were flexible, thereby being able to accumulate a certain amount of stresses without breaking. Water or other liquids may be added to the straw feedstock to improve the plug sealing capability. Inventors noted that straw chips, even if compressed by the plug screw ( 105) in conditions to form a plug able to isolate the pressurized vessel (200), formed an elastic network which behaved in a completely different way with respect to the rigid wood chips plug while entering the vessel. Namely, when pushed by the plug screw against the conic piston head (300A) disposed adjacent to the feedstock inlet, the straw block did not break in pieces, but expanded around the piston head, as represented in Figure 3. It is inventors' opinion that, unlike the compressed wood block, the compressed straw block is able to release the mechanical stress induced by the plug screw without breaking due to its elastic properties. Moreover, once in contact with the conic piston head adjacent to the feedstock inlet (230), the straw block forms bridges or a bridge on the inclined surface of the piston head which prevent the break-up of the block. As a consequence, the pressurized vessel was fully jammed upwards relative to gravity by the straw feedstock and the feeding was stopped. Inventors found that for feeding the system with a straw feedstock it was necessary to remove the conic piston head from an adjacent position. Experiments performed with two different piston heads, namely the original conic piston head and a flat piston head (300B), placed at a distance sufficient to avoid the contact with the feedstock block, succeeded in obtaining continuous straw feedstock. These experiments evidenced that the presence of the adjacent piston head was the reason of the failure of straw feeding. The minimum rest position to avoid the contact varies depending among other parameters on the feedstock inlet size, the humidity of the feedstock, and the compression of the feedstock plug (55). A person skilled in the art can easily determine the minimum distance allowed between the sealing head and the feedstock inlet according to the experimental setup used. Inventors believe that in the original feeding configuration of the adjacent piston head, the straw block may discharge the gravity force on both the ends of the macroscopic bridge that is formed between the feedstock inlet and the piston head, thereby preventing the break-up and the fall by gravity of the feedstock plug. Inventors remark that this is a bridging effect which is completely different from the straw bridging reported in the prior art, which occurs in feeding the plug forming device or in compressing the feedstock in the plug forming device. Start-up operations:

Inventors performed experiments for defining a successful method for starting the equipment with different ligno-cellulosic feedstocks.

The feedstock inlet (230) was sealed by the conic piston head (300A) to prevent the pressure drop of the pressurized vessel (200) in the absence of the feedstock plug (55), as represented in Figure 2, according to WO2008008296A2, which teaches that the back stop valve serves to prevent the escape of any gas from the gasifier, which is a pressurized vessel.

In a first start-up procedure, after the feedstock plug (55) was formed, the piston head was pushed in the continuous feeding position by the feedstock plug pressure, as soon as the feedstock plug pressure exceeded the steam pressure in the pressurized vessel (200). Inventors have found that:

Wood feedstock: the first start-up procedure was successfully operated. When the conic piston head (300A) was stopped in the continuous operating position, which is the rest position (355A) of the piston head adjacent to the feedstock inlet, the wood plug was correctly broken up into small pieces and continuous feeding operation was reached.

- Straw feedstock: the first start-up procedure failed to work. The straw block formed a bridge with the surface of the piston head preventing the break-up of the feedstock block and the vessel was jammed up by the feedstock. Moreover, once the feedstock bridging was formed in the startup operation, it was maintained even moving the piston head in a rest position in which it did not occur in continuous operation.

In a second start-up procedure performed after many attempts and opening and closing of the vessel, when the feedstock plug (55) was formed, the piston head was removed at a speed which was greater that the speed of the feedstock block entering the pressurized vessel (200), thereby avoiding enough contact between the feedstock block entering the vessel and the surface of the piston head to prevent bridging. It is preferable that any contact is avoided. The second start-up procedure was successfully operated both for wood and straw feedstock, and continuous feeding operation was reached in both cases.

Equipment and method

Many conversion processes of ligno-cellulosic feedstock (50) to useful conversion products require to treat the feedstock at a pressure greater than the pressure at which the feedstock is stored, which is typically atmospheric pressure. Conversion processes may comprise a treatment, or pre-treatment, of the ligno-cellulosic feedstock in the presence of steam or other fluids in vapor phase. In some processes, the pressure is generated by gas or vapor produced in the conversion of the ligno-cellulosic feedstock or added from an external source. The pressure is determined also by the process temperature, thereby in many cases it is necessary to handle a conversion pressure ranging from a few bars to many tens or hundreds of bars. Conversion processes are therefore conducted in one, or more than one, pressurized vessel (200). Moreover, conversion processes are often conducted in a continuous way, wherein the ligno-cellulosic feedstock enters the pressurized vessel in a continuous or semi continuous way without substantially affecting the pressure in the vessel and preventing the loss of pressure from the pressurized vessel. According to one aspect of the invention, it is disclosed equipment for continuously transferring a ligno-cellulosic feedstock (50) into a pressurized vessel (200), preventing the pressure loss from the vessel, with such equipment including the pressurized vessel.

According to another aspect of the invention, it is disclosed equipment for avoiding bridging effects of the ligno-cellulosic feedstock (50) at the inlet (230) of the pressurized vessel, which may cause the stop of the conversion system. As disclosed in the prior art, the bridging effect is particularly important in the case of straw as the feedstock. According to another aspect of the invention, it is disclosed equipment for improving feeding systems of the prior art, wherein a conic back stop or blowback valve adjacent to the inlet (230) of the feedstock is used.

With reference to Figure 4, the disclosed equipment comprises a vessel which can support a vessel pressure greater than or equal to 1 bar. Depending on the used conversion process of the ligno-cellulosic feedstock (50), the vessel pressure can be greater than 2 bar, preferably greater than 4 bar, even more preferably greater than 5 bar, even yet more preferably greater than 7 bar, and most preferably greater than 10 bar. The vessel may be realized in stainless steel or any other material or materials which can support a pressure greater than the process pressure, and which is compatible with the process temperature and other process constrains, such as being resistant to corrosion of chemical agents involved in the process. Preferably, the vessel has a cylindrical shape. The vessel is pressurized by one or more fluids, preferably a compressible fluid(s), which may be in gas or vapor phase. Nitrogen and Hydrogen are examples of gas which may be used to pressurize the vessel. Preferably, fluids are introduced directly in vapor phase into the vessel. More preferably, the vessel is pressurized by steam, which may be injected continuously or semi-continuously into the vessel through a steam inlet (210). The pressurized vessel (200) may further comprise means for removing the one or more fluids from the pressurized vessel. Fluids removal may occur in normal operation, or may be necessary for reducing an overpressure condition which could be created in the vessel. The vessel comprises a feedstock inlet (230) for introducing the feedstock into the vessel and a feedstock outlet (220) for removing the feedstock from the vessel. The feedstock inlet and outlet are essentially portholes in the vessel, preferably having a circular shape of diameter greater than 50cm. As the feedstock is removed from the feedstock inlet by gravity force during normal operation of the feeding system, the feedstock outlet is placed closer to the bottom of the vessel than the feedstock inlet. Preferably, the feedstock outlet is placed at the bottom or close to the bottom of the vessel. If necessary, the vessel may comprise means for conveying the feedstock to the feedstock outlet. The vessel, also called a pressurized vessel, is connected to a plug forming system (100), which is used to process the feedstock (50) before entering the pressurized vessel (200). The plug forming system comprises a plug forming chamber (130) containing a plug forming device. The plug forming chamber is realized in a material, or materials, able to support a pressure greater than the pressure in the pressurized vessel. Stainless steel is a preferred material. The plug forming chamber is characterized by two ends, the second one (1 10A) of which is connected to the feedstock inlet (230). In operating conditions, when the feedstock enters the pressurized vessel, preferably in a continuous or semi continuous way, the first end (HOB) has ah operating pressure which is lower than the pressure in the pressurized vessel and the second end has a pressure which is greater than the first operating pressure. Preferably, the pressure at the first end is atmospheric pressure. Thereby, in operating condition a difference of pressure is present between the two ends of the plug forming chamber. The difference of pressure is realized by means of the plug forming device, preferably without the need of sealing means others than the feedstock itself.

The plug forming system (100) may be connected to a feedstock storing system, such as a bin, directly or through a conveying system (400). Thereby, the equipment may comprise means for inserting the feedstock (50) in the plug forming chamber ( 130), preferably close or at the first end (HOB), and at the same pressure which is the first end operating pressure.

The plug forming device in the plug forming chamber (130) is a movable device able to receive the feedstock (50) at the first pressure, transporting or conveying the feedstock through the plug forming chamber, and forming a feedstock plug (55) at the second end (1 10A). The feedstock plug is a portion of the feedstock able to substantially prevent the passage of the pressurized fluid or fluids in gas or vapor phase from the pressurized vessel (200). Stated in another way, the feedstock plug is a sealing means which dynamically maintains the difference of pressure between the pressurized vessel and the plug forming chamber. By the expression "dynamically maintaining a difference of pressure" it is meant that the feedstock is continuously forming the feedstock plug at the feedstock inlet (230) to maintain the difference of pressure, and that the portion of feedstock forming the feedstock plug will change over time. The feedstock plug is realized by subjecting the feedstock to a mechanical treatment comprising a compression step, or compression forces or forces, to form a compressed feedstock having a density higher than the feedstock entering the plug forming chamber. While the compression occurs mainly at the second end of the plug forming chamber, a certain degree of compression may be applied to the feedstock also while conveying the feedstock to the second end of the plug forming chamber. The mechanical treatment may further comprise the application of shear stress which further improves the sealing properties of the plug. Water or other process liquids may be added to the feedstock before or during the plug forming step, as a wet feedstock has improved sealing properties. Thereby, the plug forming system (100) may comprise means for injecting process liquids into the plug forming chamber, as well as means for draining and removing excess of liquids from the plug forming chamber.

In a preferred embodiment, the plug forming device comprises a plug screw ( 105). The plug screw is a specific kind of screw widely diffused in the pulp and paper industry which, by rotating around its own axis (120), transports a feedstock (50) from a first end of the screw to the second end of the screw, while compacting the feedstock at the second end (1 10A) to form a feedstock plug (55). Any kind of plug screw may be used as plug forming device. The advantage of the plug screw is that it may operate in a continuous way, being the feedstock continuously inserted in and removed from the plug forming chamber (130). One embodiment is to have the size of the first end of the plug forming system smaller than the second end, so that the feedstock must compress to pass through the second end.

In another embodiment, the plug forming device comprises a linear piston (300) linearly moving along its axis (320) from the first end ( HOB), where feedstock (50) enter the plug forming chamber (130), to the second end. In this case plug formation occurs mainly through compression of the feedstock. A plug forming system (100) comprising a linear piston is inherently discontinuous. The key feature of the disclosed system for feeding the pressurized vessel (200) is a sealing head inside the pressurized vessel which is kept in a specific position during normal operation of the feeding system. In operating condition of inserting the feedstock (50) in the pressurized vessel in a continuous way, the sealing head is maintained in a rest position (355B) so that the feedstock entering the vessel does not contact the sealing head. The rest position is the position of the plug operating in continuous conditions of the feeding system. It is understood that 355A is the rest position of the prior art, while 355B is the rest position of an embodiment of this equipment. The rest position may vary depending on the type of vessel and the plug forming system design/ configuration, the feedstock type, and operating parameters. A person skilled in the art knows how to determine the rest position according to the present disclosure, on the basis of a few simple trials. The rest position is a position in which the sealing head is not adjacent to the feedstock inlet (230). Preferably the distance between the sealing head and the feedstock inlet is greater than 60cm, more preferably greater than 70cm, even more preferably greater than 80cm, even yet more preferably greater than 90cm, even yet more preferably greater than 100cm, most preferably greater 120 cm, being greater than 150cm the even most preferred value.

The sealing head can be moved from the rest position (355B) to a sealing position (350B), in which the sealing head is in contact with the feedstock inlet (230). In the sealing position, as represented in Figure 5, the sealing head isolates the vessel from the plug forming chamber (130). The sealing head may be moved by an actuator which is operated by an external control. The sealing head may be moved along a generic trajectory, provided that the contact between the feedstock (50) and the sealing head is avoided when the sealing head is in the rest position. The actuator moves the sealing head from the sealing position to the rest position at a seal opening speed sufficient to disengage the sealing head from the feedstock entering the vessel. This means that the seal opening speed is typically higher than the speed of the feedstock entering the vessel, at least in the portion of the vessel wherein the contact between the feedstock and sealing head may occur. The sealing head is used to maintain the pressure or start up the equipment before the feedstock plug is formed or if it loses its sealing integrity. In a preferred embodiment, the sealing head is the head of a head rod, or a piston (300), and it is linearly moved from the rest position (355B) to the sealing position (350B) and vice versa along a trajectory which is normal to the feedstock inlet (230). In this case the plug forming device comprises a plug screw (105), the piston axis (320) and the plug screw axis (120) are preferably parallel, more preferably they lie substantially on the same straight line.

When the feedstock plug (55) loses its integrity i.e. fails to prevent the passage of the pressurized gas or vapor from the pressurized vessel (200) to the plug forming chamber ( 130), the sealing head is driven to the sealing position (350B). In one embodiment, the driving force is the pressure drop in the pressurized vessel, specifically by the pressure drop in the portion of the pressurized vessel close to the feedstock inlet (230). Thereby, the sealing head can be a passive control system which does not require to be operated by an actuator for preventing the complete flashing of the pressurized vessel. Moreover, a sensor for detecting the plug fail, such as a pressure sensor, may not needed. Thereby, the piston head may be operated either in an active way, that is by means of the actuator, or in a passive way under the effect of the pressure drop. The sealing head may have different shapes, provided that it may substantially isolate by contact the pressurized vessel (200) from the plug forming chamber (130). Preferably, the sealing head comprises a flat surface (300B), and the flat surface contacts the feedstock inlet (230) in the sealing position (350B). A sealing head with a flat surface has the advantage of being integrated in a compact vessel. In another embodiment, the sealing head has a conic or trunked conic shape, and the contact with the feedstock inlet occurs through the inclined surface of the cone or trunked cone. In another embodiment, the sealing head is in a wedge shape, where the thicker part of the wedge is at the top and the thinner part is the at the bottom so as to force any feedstock towards the downward direction (relative to gravity).

The feedstock outlet (220) may then be connected to another pressurized vessel or reactor, directly or by means of a connection stage, and the feedstock (50) may be conveyed by a conveying screw (400). The second pressurized vessel may be pressurized at a pressure different from the pressurized vessel pressure, thereby the feeding of the feedstock to the second reactor may require an additional feeding system.

The disclosed feeding system may be operated in a continuous way, wherein the pressure in the pressurized vessel is greater than the pressure at the first end (HOB) of the plug feeding chamber (130). The feedstock (50) is inserted into the plug forming chamber under process conditions to form continuously a feedstock plug (55) which substantially prevents the gas or vapor to escape from the pressurized vessel (200). Process conditions comprise for instance a combination of humidity of the feedstock, insertion rate of the feedstock in the plug forming chamber, and rotation speed of the plug screw, which can be easily determined by a person skilled in the art by means of a simple set of experimental tests. In particular, the feedstock plug is sufficiently compact to prevent the flashing of the pressurized vessel, which is a rapid and potentially dangerous loss of pressure from the vessel. The pressure in the pressurized vessel is maintained by inserting the fluid in vapor or gas phase, preferably steam, in a continuous or semi-continuous way. The feedstock plug may be inserted continuously in the pressurized vessel. The insertion may occur also at a variable speed, even if preferably the speed is maintained constant. The insertion speed will vary depending on the operating condition of the plug system device. In the pressurized vessel, the feedstock is removed then by gravity force, in the sense that the compacted feedstock is broken under the action of its own weight. While operating in a continuous way of preventing the passage of the gas or vapor from the pressurized vessel to the plug forming chamber, the sealing head is kept in a position to avoid any contact with the feedstock entering the pressurized vessel, as schematically represented in Figure 7.

For starting the feeding of the ligno-cellulosic feedstock (50) to the pressurized vessel (200), the sealing head is moved to the sealing position (350B) by means of the actuator to isolate the vessel as indicated in Figure 5. If necessary, the actuator may exert the force needed to seal the vessel. Once the vessel is isolated, the vapor or gas is inserted in the vessel. While increasing the pressure in the vessel, the vapor or gas exerts a sealing force on the sealing head which pushes the sealing head against the feedstock inlet (230). A recall force (B of (VB)) acting in the opposite direction of the sealing force is then applied to the sealing head by means of the actuator, the intensity of the force being less than the sealing force of the vapor or gas. The ligno-cellulosic feedstock is inserted into the plug forming device to form the feedstock plug (55), the insertion being started after or before the vessel has been isolated by the sealing head. The feedstock plug exerts a plug force under the action of the plug forming device, said force having an opposite direction to the sealing force. As the intensity of the plug force substantially equals the sealing force, the recall force B moves the sealing head in the rest position (355B) at a speed (VB) sufficient to disengage the sealing head from the feedstock plug. The startup operating method is schematically represented in Figure 6.

In another embodiment, the recall force B (of (VB)) is started while the feedstock plug (55) is entering the pressurized vessel (200), the sealing head starting moving under the action of the feedstock plug, provided that the recall force is sufficient to disengage the sealing head from the feedstock plug.

The disclosed equipment may feed any kind of ligno-cellulosic feedstock (50) to the pressurized vessel (200). The advantages are evident in the case of a ligno-cellulosic feedstock comminuted in chips, wherein the chips are characterized by a low bulk density, as determined according to the standard ASAE 269.5, which defines universal methods and procedures for measuring unit density, bulk density, durability, and moisture content of fibrous and non-fibrous material for bulk handling in the feed and non- feed industries. The bulk density may be less than 300kg/m ³ , preferably less than 250 kg/m ³ , more preferably less than 200 kg/m ³ , even more preferably less than 150 kg/m ³ , even yet more preferably less than 100 kg/m ³ , most preferably less than 75 kg/m ³ , being less than 50 kg/m ³ the even most preferred value. The bulk density is measured at a moisture content of 10%. The ligno-cellulosic feedstock is stored at a moisture content of about 10% and it is preferably poured with water before insertion in the plug forming chamber (130) to reach a moisture content in the range of 20% to 40%, more preferably 25% to 35%. Optionally, the wetting may occur at least partially also in the plug forming chamber. The increase in moisture content is important for improving the capability of the feedstock to form the desired plug.

Even if the disclosed equipment may feed comminuted ligno-cellulosic feedstock (50) composed by chips of any shape, the advantages are evident in the case of elongated chips, being favorable of the formation of bridging effects. The comminuted ligno-cellulosic feedstock may be characterized by the mean aspect ratio of the chips, wherein the aspect ratio of a chip is defined as the ratio of its longest size and the mean size in the section transversal to the longest size. The average is done on a sampling of the feedstock having a statistical relevance. As an example, in the case of wheat straw, the chip may be as long as some tens of centimeter and the mean transversal size is typically a few millimeters. The mean aspect ratio may be more than 3: 1 , preferably more than 5: 1, more preferably more than 10: 1 , even more preferably more than 15: 1 , even yet more preferably more than 20: 1 , most preferably more than 30: 1 , being more than 40: 1 the even most preferred value.

Preferably, the ligno-cellulosic feedstock is a straw, more preferably the straw is selected from the group consisting of wheat straw, rice straw, barley straw, and bagasse. Ligno-cellulosic feedstock

In general, a ligno-cellulosic feedstock (50), or ligno-cellulosic biomass, can be described as follows:

Apart from starch, the three major constituents in plant biomass are cellulose, hemicellulose and lignin, which are commonly referred to by the generic term lignocellulose. Polysaccharide-containing biomasses as a generic term includes both starch and ligno-cellulosic biomasses. Therefore, some types of feedstocks can be plant biomass, polysaccharide containing biomass, and ligno-cellulosic biomass which may or may not contain starch. Polysaccharide-containing biomasses according to the present invention include any material containing polymeric sugars e.g. in the form of starch as well as refined starch, cellulose and hemicellulose. Relevant types of biomasses for deriving the claimed invention may include ligno-cellulosic biomasses derived from agricultural crops selected from the group consisting of starch containing grains, refined starch; corn stover, bagasse, straw e.g. from rice, wheat, rye, oat, barley, rape, sorghum; softwood e.g. Pinus sylvestris, Pinus radiate; hardwood e.g. Salix spp. Eucalyptus spp.; tubers e.g. beet, potato; cereals from e.g. rice, wheat, rye, oat, barley, rape, sorghum and corn; waste paper, fiber fractions from biogas processing, manure, residues from oil palm processing, municipal solid waste or the like. In one embodiment, the ligno-cellulosic biomass feedstock (50) used in the process is from the family usually called grasses. The proper name is the family known as Poaceae or Gramineae in the Class Liliopsida (the monocots) of the flowering plants. Plants of this family are usually called grasses, or, to distinguish them from other graminoids, true grasses. Bamboo is also included. There are about 600 genera and some 9,000- 10,000 or more species of grasses (Kew Index of World Grass Species).

Poaceae includes the staple food grains and cereal crops grown around the world, lawn and forage grasses, and bamboo. Poaceae generally have hollow stems called culms, which are plugged (solid) at intervals called nodes, the points along the culm at which leaves arise. Grass leaves are usually alternate, distichous (in one plane) or rarely spiral, and parallel- veined. Each leaf is differentiated into a lower sheath which hugs the stem for a distance and a blade with margins The leaf blades of many grasses are hardened with silica phytoliths, which helps discourage grazing animals. In some grasses (such as sword grass) this makes the edges of the grass blades sharp enough to cut human skin. A membranous appendage or fringe of hairs, called the ligule, lies at the junction between sheath and blade, preventing water or insects from penetrating into the sheath. Grass blades grow at the base of the blade and not from elongated stem tips. This low growth point evolved in response to grazing animals and allows grasses to be grazed or mown regularly without severe damage to the plant. Flowers of Poaceae are characteristically arranged in spikelets, each spikelet having one or more florets (the spikelets are further grouped into panicles or spikes). A spikelet consists of two (or sometimes fewer) bracts at the base, called glumes, followed by one or more florets. A floret consists of the flower surrounded by two bracts called the lemma (the external one) and the palea (the internal). The flowers are usually hermaphroditic (maize, monoecious, is an exception) and pollination is almost always anemophilous. The perianth is reduced to two scales, called lodicules, that expand and contract to spread the lemma and palea; these are generally interpreted to be modified sepals.

The fruit of Poaceae is a caiyopsis in which the seed coat is fused to the fruit wall and thus, not separable from it (as in a maize kernel).

There are three general classifications of growth habit present in grasses; bunch-type (also called caespitose), stoloniferous and rhizomatous. The success of the grasses lies in part in their morphology and growth processes, and in part in their physiological diversity. Most of the grasses divide into two physiological groups, using the C3 and C4 photo synthetic pathways for carbon fixation. The C4 grasses have a photosynthetic pathway linked to specialized Kranz leaf anatomy that particularly adapts them to hot climates and an atmosphere low in carbon dioxide.

C3 grasses are referred to as "cool season grasses" while C4 plants are considered "warm season grasses". Grasses may be either annual or perennial. Examples of annual cool season are wheat, rye, annual bluegrass (annual meadowgrass, Poa annua and oat). Examples of perennial cool season are orchard grass (cocksfoot, Dactylis glomerata), fescue (Festuca spp), Kentucky Bluegrass and perennial ryegrass (Lolium perenne). Examples of annual warm season are corn, sudangrass and pearl millet. Examples of Perennial Warm Season are big bluestem, indian grass, bermuda grass and switch grass. One classification of the grass family recognizes twelve subfamilies: These are 1) anomochlooideae, a small lineage of broad-leaved grasses that includes two genera (Anomochloa, Streptochaeta); 2) Pharoideae, a small lineage of grasses that includes three genera, including Pharus and Leptaspis; 3) Puelioideae a small lineage that includes the African genus Puelia; 4) Pooideae which includes wheat, barley, oats, brome-grass (Bronnus) and reed-grasses (Calamagrostis); 5) Bambusoideae which includes bamboo; 6) Ehrhartoideae, which includes rice, and wild rice; 7) Arundinoideae, which includes the giant reed and common reed; 8) Centothecoideae, a small subfamily of 1 1 genera that is sometimes included in Panicoideae; 9) Chloridoideae including the lovegrasses (Eragrostis, ca. 350 species, including teff), dropseeds (Sporobolus, some 160 species), finger millet (Eleusine coracana (L.) Gaertn.), and the muhly grasses (Muhlenbergia, ca. 175 species); 10) Panicoideae including panic grass, maize, sorghum, sugar cane, most millets, fonio and bluestem grasses; 1 1) Micrairoideae and 12) Danthoniodieae including pampas grass; with Poa which is a genus of about 500 species of grasses, native to the temperate regions of both hemispheres. Agricultural grasses grown for their edible seeds are called cereals. Three common cereals are rice, wheat and maize (corn). Of all crops, 70% are grasses.

Sugarcane is the major source of sugar production. Grasses are used for construction. Scaffolding made from bamboo is able to withstand typhoon force winds that would break steel scaffolding. Larger bamboos and Arundo donax have stout culms that can be used in a manner similar to timber, and grass roots stabilize the sod of sod houses. Arundo is used to make reeds for woodwind instruments, and bamboo is used for innumerable implements. Another ligno-cellulosic biomass feedstock may be woody plants or woods. A woody plant is a plant that uses wood as its structural tissue. These are typically perennial plants whose stems and larger roots are reinforced with wood produced adjacent to the vascular tissues. The main stem, larger branches, and roots of these plants are usually covered by a layer of thickened bark. Woody plants are usually either trees, shrubs, or lianas. Wood is a structural cellular adaptation that allows woody plants to grow from above ground stems year after year, thus making some woody plants the largest and tallest plants. These plants need a vascular system to move water and nutrients from the roots to the leaves (xylem) and to move sugars from the leaves to the rest of the plant (phloem). There are two kinds of xylem: primary that is formed during primary growth from procambium and secondary xylem that is formed during secondary growth from vascular cambium.

What is usually called "wood" is the secondary xylem of such plants.

The two main groups in which secondary xylem can be found are:

1) conifers (Coniferae): there are some six hundred species of conifers. All species have secondary xylem, which is relatively uniform in structure throughout this group. Many conifers become tall trees: the secondary xylem of such trees is marketed as softwood.

2) angiosperms (Angiospermae): there are some quarter of a million to four hundred thousand species of angiosperms. Within this group secondary xylem has not been found in the monocots (e.g. Poaceae). Many non- monocot angiosperms become trees, and the secondary xylem of these is marketed as hardwood.

The term softwood is used to describe wood from trees that belong to gymnosperms. The gymnosperms are plants with naked seeds not enclosed in an ovary. These seed "fruits" are considered more primitive than hardwoods. Softwood trees are usually evergreen, bear cones, and have needles or scale like leaves. They include conifer species e.g. pine, spruces, firs, and cedars. Wood hardness varies among the conifer species.

The term hardwood is used to describe wood from trees that belong to the angiosperm family. Angiosperms are plants with ovules enclosed for protection in an ovary. When fertilized, these ovules develop into seeds. The hardwood trees are usually broad-leaved; in temperate and boreal latitudes they are mostly deciduous, but in tropics and subtropics mostly evergreen. These leaves can be either simple (single blades) or they can be compound with leaflets attached to a leaf stem. Although variable in shape all hardwood leaves have a distinct network of fine veins. The hardwood plants include e.g. Aspen, Birch, Cherry, Maple, Oak and Teak. Therefore, in one embodiment, a suitable naturally occurring ligno- cellulosic biomass may be selected from the group consisting of the grasses and/orwoods or mixtures thereof. In one embodiment, naturally occurring ligno-cellulosic biomass can be selected from the group consisting of the plants belonging to the conifers, angiosperms, Poaceae and families. Another preferred naturally occurring ligno-cellulosic biomass may be that biomass having at least 10% by weight of it dry matter as cellulose, or more preferably at least 5% by weight of its dry matter as cellulose.