Abstract:
A method for torrefaction of lignocellulosic biomass using a torrefaction reactor vessel having stacked trays, the method including: continuously feeding the biomass to an upper inlet of the torrefaction reactor vessel such that the biomass material is deposited on an upper tray of a plurality of trays stacked vertically within the reactor; as the biomass moves across an upper surface of each of the trays, heating and drying the biomass material with a gas injected into the vessel, wherein the gas is substantially non-oxidizing of the biomass, is under a pressure of at least 20 bar gauge and at a temperature of at least 200° C.; cascading the biomass down through the trays by passing the biomass through an opening in each of the trays to deposit the biomass on a lower tray; discharging torrefied biomass from a lower outlet of the torrefaction reactor vessel, and circulating gas extracted from a lower elevation of the reactor vessel to an upper region of the reactor vessel.

Description:
CROSS RELATED APPLICATION 
       [0001]    The application claims priority to U.S. Ser. No. 61/374,412 filed on Aug. 17, 2010 which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention generally relates to systems and methods for torrefaction of lignocellulosic material, such as wood and other biomass, and more particularly relates to a pressurized reactor vessel for the torrefaction of such material. 
         [0003]    Torrefaction can be used to convert biomass, e.g., wood, to an efficient fuel having increased energy density relative to the input biomass. For example, wood generally contains hemicellulose, cellulose and lignin. Torrefaction removes moisture and low weight organic volatile components from wood. Torrefaction may also depolymerize the long polysaccharide chains of the hemicellulose portion of biomass and produce a hydrophobic solid combustible fuel product with an increased energy density (on a mass basis) and improved grindability. Torrefaction changes the chemical structure of the biomass to a form suitable for burning in coal fired facilities. Torrefied wood or biomass has characteristics similar to low rank coals and can be compacted to high grade fuel pellets. 
         [0004]    Torrefaction refers to the thermal treatment of biomass, usually in an oxygen deprived atmosphere (which is referred to herein as an “inert atmosphere”), at relatively low temperatures of 200 degrees Celsius (° C.) to 400° C. A torrefaction process is described in related U.S. Provisional Patent Application Ser. No. 61/235,114, the entirety of which is incorporated by reference. 
         [0005]    Unpressurized reactor vessels with multiple trays have been used for torrefaction, as is described in U.S. Patent Application Publication 2010/0083530 (the &#39;530 Application). The &#39;530 Application states that torrefaction should be performed in a reactor vessel operating at atmospheric pressure. By stating that it is advantageous to operate the vessel at atmospheric pressure, the &#39;530 Application teaches that vessels should not be operated at above-atmospheric pressures. See &#39;530 Application, para. 0061. 
         [0006]    Pressurized reactor vessels with multiple trays have been used in pulp mills to delignify pulp by oxidation. Examples of a pulping reactor vessel with multiple trays are disclosed in U.S. Pat. Nos. 3,742,735 (&#39;735 Patent) and 3,660,225 (&#39;225 Patent). Multiple tray vessels allow pulp to cascade through the vertical arrangement trays in the reactor. The trays allow the pulp to cascade in discrete batches down through the vessel. An oxygen rich environment in the pulping reactor promotes delignification and bleaching of the pulp. The &#39;735 Patent and &#39;225 Patent do not suggest using a pulping reactor vessel for torrefaction of wood or other biomass material. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0007]    A difficulty with unpressurized reaction vessels is the low mass of gas at atmospheric pressure. The ability of a gas to transfer heat to a biomass is proportional to the mass of the gas. The greater its mass, the faster a gas can heat the biomass. A large reaction vessel is needed to heat biomass with a gas at atmospheric pressure because a large volume of gas is necessary to heat the biomass. 
         [0008]    The mass of a gas at atmospheric pressure is substantially less than the mass of gas at a substantial pressure, such as above 20 bar gauge (290 psig). The volume of gas under substantial pressure needed to heat biomass to a certain temperature is much less than the volume of atmospheric gas needed to heat the biomass. A small pressurized vessel may be used to heat biomass, as compared to a similar but unpressurized vessel. 
         [0009]    Pressurized reaction vessels require seals and other devices to keep the gas and materials in the vessel under pressure. Similarly, pressure transfer devices are required at the input to or in the feed systems for a pressurized vessel to pressurize the material being fed to the vessel. Further, pressurized reaction vessels require pressurized gases and conduits for the pressurized gases. 
         [0010]    A novel reaction vessel has been conceived for torrefaction of biomass having vertically stacked trays for drying and heating biomass using an inert hot gas under substantial pressure. The vessel may be substantially smaller than a reaction vessel for torrefaction performed at atmospheric pressure. The inert pressurized gas may be circulated through the vessel and through pressurized conduits that reheat the gas. 
         [0011]    A method for torrefaction of lignocellulosic biomass using a torrefaction reactor vessel ( 10 ,  70 ,  100 ) having stacked trays ( 42 ,  74 ,  102 ,  104 ), the method including: continuously feeding the biomass to an upper inlet ( 14 ) of the torrefaction reactor vessel such that the biomass material is deposited on an upper tray of a plurality of trays stacked vertically within the reactor; as the biomass moves across an upper surface of each of the trays, heating and drying the biomass material with a gas ( 18 ) injected into the vessel, wherein the gas is substantially non-oxidizing of the biomass, is under a pressure of at least 20 bar gauge and at a temperature of at least 200° C.; cascading the biomass down through the trays ( 42 ,  74 ,  102 ,  104 ) by passing the biomass through an opening ( 46 ) in each of the trays to deposit the biomass on a lower tray; discharging torrefied biomass from a lower outlet ( 16 ,  81 ,  116 ) of the torrefaction reactor vessel, and circulating gas ( 30 ,  31 ,  24 ,  64 ,  76 ,  77 ,  78 ,  79 ) extracted from a lower elevation of the reactor vessel to an upper region ( 15 ) of the reactor vessel. 
         [0012]    The gas may be superheated steam, nitrogen or carbon dioxide. The biomass may be pressurized before being fed to the vessel with a pressure transfer device. The upper trays may be a mesh, screen or have perforations and the heating and drying of the biomass includes passing the gas through the biomass and the trays. The trays below the upper trays may be solid such that the gas does not pass through the tray. The gas may be adjacent a lower tray of the trays. 
         [0013]    The gas may be injected into the vessel at two elevations wherein the gas is hotter when injected at a lower elevation of the two elevations than the gas injected at an upper elevation of the two elevations. At an elevation of the vessel below from which the gas is extracted, the biomass may continue to cascade down through the trays. The gas may be injected in the biomass to purge oxygen from the biomass, wherein the injection occurs before the biomass enters the vessel. 
         [0014]    A torrefaction pressurized reactor vessel assembly ( 10 ,  70 ,  100 ) has been conceived comprising: a stack of trays ( 42 ,  74 ,  102 ,  104 ) housed within the vessel; a source of a pressurized, reduced oxygen gas ( 18 ) coupled to the vessel to allow the gas to flow into at least an upper region of the vessel, wherein the gas is at a pressure of at least 20 bar gauge and at a temperature of at least 200° C.; an upper inlet ( 14 ) to the vessel through which biomass enters the pressurized reactor vessel, wherein a chute ( 54 ) aligned with the upper inlet directs the biomass from the inlet to an upper tray of the stack of trays; a scraper device ( 52 ) associated with an upper surface on each of the trays, wherein at least one of the scraper device and tray rotates within the vessel; a lower outlet ( 16 ,  81 ,  116 ) in the vessel through which torrefied biomass is discharged from the torrefaction reactor vessel, and a gas circulation system ( 30 ,  31 ,  24 ,  64 ,  76 ,  77 ,  78 ,  79 ) through which gas extracted from a lower elevation in the vessel flow to an upper elevation ( 15 ) of the reactor vessel. 
         [0015]    In the torrefaction pressurized reactor vessel assembly, the source of a pressurized, reduced oxygen gas ( 18 ) may be a source of at least one of superheated steam, carbon dioxide and nitrogen. A pressure transfer device ( 22 ) may pressurize and feed the biomass to the reactor vessel. At least one of the upper trays of the stack of trays ( 42 ,  74 ,  102 ) may be mesh, screen or have perforations. The trays ( 104 ) below the upper trays may be solid such that the gas does not pass through the tray. The lower elevation ( 72 ) of the vessel at where the gas is extracted for circulation may be adjacent a lower tray ( 104 ). The gas circulation system further comprises a heat exchanger. A vertical rotating shaft ( 44 ) may extend through a center axis of the reactor vessel and at least one of the scraper device and trays are fixed to the shaft. Further, a lower region of the vessel may be devoid of trays and receiving the biomass material. In addition, a biomass supply bin ( 12 ) may receive the pressurized, reduced oxygen gas to the biomass supply bin from a conduit ( 90 ) leading from the source of the gas. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a schematic diagram of a pressurized treatment vessel receiving biomass from a feed system. 
           [0017]      FIG. 2  is a schematic diagram of a second embodiment of a pressurized treatment vessel receiving biomass from a feed system. 
           [0018]      FIG. 3  is a schematic diagram of a third embodiment of a pressurized treatment vessel. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]      FIG. 1  is schematic diagram of a pressurized treatment vessel  10  for receiving, from a storage bin  12 , biomass material, such as wood chips, wood pulp and other comminuted cellulosic material. The biomass enters the pressurized treatment vessel through an upper inlet  14 . The upper inlet may be a high pressure transfer device that allows biomass at atmospheric pressure to be transferred into the high pressure vessel. Alternatively, the biomass may be pressurized by a high pressure feeder (HPF) or a series of chip pumps  22  as the pressurized biomass flows through a conduit  23  to the upper inlet which may be an open valve. 
         [0020]    Within the vessel, the biomass is subjected to a torrefaction reaction and is discharged as torrefied biomass  14  from a lower discharge outlet  16  of the vessel. Before the torrefaction reaction occurs in the vessel, the biomass may be dried and heated in an inert environment to a temperature of 200° C. to 400° C. The biomass may be dried and heated in a separate dryer  11  or in an upper drying zone  15  of the vessel  10 . Within the pressurized treatment vessel  10 , the biomass may be heated in an upper drying zone  13  of the vessel. The biomass may be indirectly heated by a heat exchanger  9  having surfaces in contact with the biomass. The heat exchanger may be in the dryer or vessel. Alternatively, the biomass may be directly heated with an oxygen deprived gas  18 , e.g., super-heated steam, injected into the vessel or dryer. 
         [0021]    The vessel may be operated at a relatively high above-atmospheric pressure, such as above 20 bar gauge (290 psig). Pressurizing the vessel  10  allows for higher gas temperatures in the vessel and increases the amount of gas per unit volume available to react with the biomass. 
         [0022]    The volume of hot inert gas needed for the vessel is dramatically reduced in a pressurized reaction vessel  10  as compared to a vessel operating at atmospheric pressure. Pressurizing the treatment vessel reduces the volume of hot gas needed to heat the biomass by a factor of two (2) to thirty-five (35) as compared to a vessel at atmospheric pressure. The reduction factor for the vessel depends on the pressure in the vessel. 
         [0023]    Because of the reduced volume of hot gas needed in the pressurized reactor, the volume and hence the size and cost of the vessel  10  may be significantly reduced as compared to a vessel operating at atmospheric pressure. A pressurized vessel in which a hot gas is injected provides effective and economical heat transfer from the gas to the biomass in the vessel. 
         [0024]    The vessel  10  may be pressurized by injecting an inert gas  18 , e.g., oxygen starved gas, at a pressure of up to 35 bar gauge (barg), such as in a range of 20 barg to 35 barg. The oxygen starved gas (also referred to as the inert gas) may be substantially nitrogen, a carbon oxide or steam. The pressurized vessel  10  operates in an inert gas environment in which a heated pressured gas  18  circulates through the vessel to directly heat the biomass and promote a torrefaction reaction with the biomass. 
         [0025]    The upper inlet  14  to the pressurized vessel may be coupled to a continuous feed, pressure isolation device  20 , such as a conventional rotary valve or plug screw feeder, to feed the biomass into the pressurized vessel from a source of biomass at atmospheric pressure. The vessel  10  operates in a gas phase in which the dried biomass remains dry in the vessel. 
         [0026]    The biomass may be fed to the inlet  14  to the vessel at a temperature of 80° C. to 120° C., for example, or higher if a dryer  11  heats the biomass before entering the vessel. The biomass is heated in the vessel by a pressurized, hot and oxygen starved gas  18 . The gas entering the vessel may be at a temperature in a range of 200° C. to 600° C. and may particularly be in a range of 250° C. to 400° C. or a range of 300° C. to 380° C. The hot gas  18  may be injected to the vessel through a gas input manifold  24  including nozzles arranged at an upper level of the vessel  10 . 
         [0027]    The hot inert gas  18  may be injected into the pulp in the feed system  26  such as in the inlet downstream of the pressure isolation device or downstream of a high pressure transfer device  28 . If there is a high pressure transfer device  28 , the pressure isolation device may be unnecessary at the inlet to the vessel  10 . 
         [0028]    The hot gas  18  flows with the biomass in the vessel and directly heats the biomass to a temperature that promotes a torrefaction reaction in the pulp. The hot gas and any gas generated in the reactor are exhausted from the reactor at a bottom gas vent manifold  30 . The gas may exhaust from the vessel at a temperature of about 280° C. A portion  32  of the exhausted gas is removed from the vessel for use outside of the torrefaction system. Another portion of the exhausted gas is indirectly heated in a heat exchanger  34  (or other heat transfer device) and returned to the gas input manifold  24  at the top of the vessel  10 . The heat exchanger  34  may add heat energy to heat the exhausted gas from about 280° C. to 300° C. to 380° C., for example. Reheating and recalculating the exhausted gas reduces the amount for additional pressurized heated gas  38  required to be supplied to the gas input manifold of the vessel. 
         [0029]    The biomass enters the pressure treatment vessel  10  through the upper inlet  14 , which may be a single inlet orifice or an array of inlet orifices in the top or upper portion of the vessel. The biomass may have been previously dried before entering the vessel or the biomass may be dried in an optional drying zone  15  in an upper region of the vessel. Below the drying zone, the vessel includes a torrefaction zone  40   
         [0030]    The vessel  10 , including the drying zone  15  (if any) and the torrefaction zone  40 , includes a stack of generally trays  42  each of which are mounted on a center vertical shaft  44  extending through the vessel. The trays may be circular discs having a generally planar upper support surface. The trays  42  may each include an opening  46 , such as a pie-shaped open section of a circular disc. The open section may be one or more openings in each tray. The open section  46  allows biomass on the upper surface of the tray to fall through to an underlying tray. 
         [0031]    The open sections  46  (also referred to as “openings”) of each tray preferably are not vertically aligned with the openings  46  in the trays immediately above and below the tray. If the openings were vertically aligned, the biomass may fall from one open section and immediately through the open section in the underlying tray without resting on the support surface of the underlying tray. 
         [0032]    The open sections  46  may be vertically staggered such that each opening is over a trailing region  47  of the upper section of the tray immediately below the opening. The trailing region  47  of a tray is adjacent and behind the open section  46  in the direction of rotation  56  of the tray. By aligning an open section  46  above a trailing region  47  on a lower tray, the biomass falls through the open section and onto the trailing region. As the tray turns, the biomass slides across the entire upper surface of the tray in an arc-shaped path from the trailing region to the open section. Maintaining the biomass on the upper surface of each tray maximizes the retention period of the biomass on tray and, thus, allows the biomass to be heated and dried 
         [0033]    The trays  42  may rotate with the shaft  44 . Alternatively, the trays may be stationary and mounted to the sidewall of the vessel and a scraper device  52  may rotate with the shaft and across the upper surface of each tray. The shaft  44  is rotatably driven by a gear and motor assembly  50  which may be at the lower base of the vessel  10 . The rotational speed of the shaft may be adjusted to control the flow rate of the biomass through the vessel. The rotational speed of the shaft and tray governs the retention period of the biomass on each tray. A fast rotational speed will cause the biomass to flow faster through the vessel as compared to a slow rotational speed. 
         [0034]    The trays  42  may be perforated, wire frames with an open network of support beams or otherwise open to allow hot gases to pass through the trays and biomass on the trays. Allowing hot gas to pass through the biomass and trays promotes the exposure of the surfaces of the biomass particles to the hot gases. 
         [0035]    The biomass is heated in the vessel by exposure to the hot gases. The biomass may reach a temperature sufficient to promote torrefaction within thirty (30) seconds to twenty (20) minutes after entering the vessel. 
         [0036]    The flow rate of inert gas needed to increase the temperature of the biomass moving on the trays is greater than the flow rate of inert gas needed to maintain the biomass as the temperature desired for torrefaction. To provide an high flow rate of the inert gas through an upper portion of the torrefaction zone  40 , perforated trays may be use to enhance the exposure of hot gas to the biomass and allow the gas to pass through the trays. The solid trays  42  may be used below the elevation of the vessel at which the biomass reaches the desired torrefaction temperature. The use of solid trays in the middle and lower regions of the torrefaction zone  40  assists in confining the hot gases in the upper elevations of the vessel. By confining the high flow rate of hot inert gasses to the upper elevations of the torrefaction zone and the drying zone  15 , the volume of gas needed in the vessel may be optimized to that necessary to heat the biomass up to the desired temperature for torrefaction. 
         [0037]    The trays may be optionally heated, such as with electric heating coils  48  to provide indirect heat to the biomass. The heating coils  48  are arranged on the upper surface of the trays and electrically connected through the shaft to a source of electric power. 
         [0038]    The biomass may be retained in the treatment vessel  10  for a period of five (5) to one-hundred (100) minutes. The retention time starts as the biomass enters the upper inlet  14  and ends as the biomass is discharged through the outlet  16  at the bottom of the vessel. Biomass continually flows through the vessel. As biomass enters the upper inlet, biomass already in the vessel is on each tray and biomass at the bottom of the vessel is being discharged through the outlet  16 . 
         [0039]    Immediately below the inlet  14  and in the vessel  10  may be a chute  54  that receives the biomass from the inlet and directs the biomass to the trailing section  47  portion of the upper tray. The chute ensures that biomass entering the vessel is retained on the upper tray for nearly a full rotational period of the tray. 
         [0040]    A scraper device  52 , such as arms extending radially outward from the shaft, may extend over the upper surface of each tray and be fixed to the outer wall of the vessel. The scraper device may not rotate with the shaft and trays. If the scraper device does not rotate, it may be affixed to the shaft  44  by a collar  60  that is rotatably mounted on the shaft and rests on the upper surface of each tray. If the scraper device rotates, it may be affixed to the shaft and the stationary trays may be fixed to the wall of the vessel rather than the shaft. 
         [0041]    The scraper device  52  forces biomass to slide across the upper surface of the tray as the tray rotates. The biomass slides across the tray until the biomass reaches the opening  46  in the tray and falls to the next lower tray in the vessel. 
         [0042]    A conventional bottom scraper device (not shown) may be positioned in a bottom portion of the vessel  10 . The bottom scraper device directs biomass that has cascaded through each of the trays into the discharge outlet  16 . The bottom scraper device may be fixed to and rotate with the shaft to move the biomass in the bottom of the vessel into the outlet  16 . 
         [0043]    The discharge outlet  16  may be in or near the bottom of the vessel. The shape of the discharge outlet may be conical, hemispherical, elliptical or a chute formed of geometric panels (such as disclosed in U.S. Pat. No. 5,000,083). 
         [0044]    The flow of heated gas into, through and from the pressure reaction vessel  10  may be configured to promote the flow of hot, pressurized gases through the trays in the upper elevations of the vessel  10  where the biomass is being heated to the desired temperature for torrefaction. As shown in  FIG. 1 , the hot inert gas may be injected into the upper section of the vessel  10  through an input manifold  24  that has one or more gas injection nozzles  64  arranged at the same elevation on the vessel or at various elevations such as the elevations of the upper trays used to increase the temperature of the biomass. The introduced hot inert gas may be supplied just to the top of the vessel as shown in  FIG. 1 , or also to multiple elevations of the vessel as shown in  FIG. 2 . 
         [0045]    If gas flows to multiple elevations, the inert gas flowing to each elevation may be a gas source at a temperature, pressure or composition that is different from the gas sources supplying the gas nozzles at other elevations of the vessel. For example, the hot inert gas introduced to the uppermost elevation of the vessel may be at a temperature slightly, e.g., 10° C. to 40° C., hotter than the temperature, e.g., 100° C., of the biomass being fed to the vessel. The hot inert gases introduced at succeeding lower elevations of the vessel may be increasingly hotter so as to be slightly above the temperature of the biomass in the vessel that is proximate to the injected hot gas. By injecting the inert gas at a temperature slightly above the biomass being heated by the gas, the efficiency of heating can be increased as compared to injecting gas at a single temperature which may be substantially hotter than the incoming biomass to the vessel. 
         [0046]    Hot gases in the pressure vessel may be extracted, e.g., purged, from various elevations in the vessel. The hot gases include the inert gases injected into the vessel and gases, e.g., steam, generated by the heated biomass in the vessel. These gases may be extracted through the bottom gas vent manifold  30 . Rather than or in addition to extracting the hot gases from a bottom gas vent manifold  30 , the gases may be extracted from one or more outlets  31  between elevations at which there are trays  42 . The trays  42  proximate to the gas outlets  31  may be the trays on which the biomass reaches the desired temperature for the torrefaction reaction, e.g., 250° C. to 300° C. Extracting gases through outlets  31  at elevations in the vessel at which the biomass reaches the desired temperatures, allows the hot gas to be directed to and circulated through the upper portion of the vessel  10  where the temperature of the biomass is raised to the desired temperature. 
         [0047]    The portions of the vessel below the gas outlets  31  are pressurized and maintain the biomass at the desired torrefaction temperature. Hot inert gas from the upper regions of the vessel will flow down to the lower portions of the vessel to maintain the biomass at the desired temperature in the lower portions of the vessel. In addition, a relatively small amount of hot inert gas may be injected into the those lower portions of the vessel through one or more inlet nozzles  77  arranged on the sidewall of the vessel, as is shown in  FIG. 2 . 
         [0048]    As shown in  FIG. 2 , a pressurized treatment vessel  70  may have an upper portion  72  having perforated trays  74 , gas inlets  76 ,  77  at upper and mid-elevations of the vessel to receive hot, inert gas and gas outlets  78  at lower elevations of the vessel to purge gases from biomass in the vessel. The upper portion  72  corresponds to the volume in the vessel in which the temperature of the biomass is raised to the temperature desired for the torrefaction reaction. The uppermost portion of the vessel may include a drying section, similar to the drying section  15  in the vessel  10  shown in  FIG. 1 . 
         [0049]    In  FIG. 2 , the upper vessel portion  72  receives inert, hot gas at gas inlets  76  and  77  that may be arranged at different elevations of the upper vessel portion and at various positions on the top of the vessel or around the circumference of the vessel. The inert, hot gas for the gas inlets is provided by recovering inert gas from exhaust vents  78  at lower elevations of the vessel  70  and from an external source  18  of inert gas. 
         [0050]    Circulation conduits  79 , e.g., pipes, external to the vessel transport inert gas transport from the lower elevations of the vessel to the upper elevation of the vessels and allow inert gas to be added to the circulation from the gas source  18 . Heat energy may be added to the inert gas in the circulation conduits by heat exchangers  34  and  85 . The heat exchanger  34  increases the temperature of the extracted inert gas extracted from the lower elevations of the vessel so that the gas may be reintroduced into the upper elevations of the vessel at higher temperatures to dry and promote torrefaction of the biomass. 
         [0051]    The heat exchanger  85  may be used to increase the temperature of the inert gas fed via nozzles  77  to the lower trays  74  in the upper portion  72  of the vessel as compared to the upper trays in the upper portion. The lower trays in the upper portion may receive the hottest inert gas to heat the biomass on the lower trays to the desired torrefaction temperature. The upper trays  74  in the upper portion  72  of the vessel receive a slightly cooler inert gas from gas nozzles  76  because the biomass has not yet reached the desired torrefaction temperature and is cooler than the biomass on the lower trays. Heat energy is conserved by adding inert gas at temperatures slightly, e.g., 10° C. to 20° C., above the biomass temperature adjacent the gas inlets  76 ,  77 . 
         [0052]    The lower portion  80  of the vessel is maintained at a pressure and temperature sufficient to promote the torrefaction reaction of the biomass, but need not increase the temperature of the biomass in the lower portion. The lower portion  80  may be a generally open chamber without trays. The flow rate or volume of hot gas needed to maintain the pressure and temperature in the lower vessel portion  80  may be only that sufficient to allow the biomass to flow downward in the portion  80  and stay at a desired temperature. 
         [0053]    in addition, hot inert gas  85  may be added to the lower portion  80 , such as at the discharge port  81 . The hot inert gas  85  may be circulated gas directed from the conduits  79 . A heat exchanger  34  adds heat energy to the circulated gas by indirectly transferring steam heat from a steam source  87  that flows through the heat exchanger and to a recovery device  89 . 
         [0054]    The lower portion  80  of the vessel  70 , below the gas outlet(s)  78 , may have trays  42  with solid surfaces which isolate the lower portion from the upper portion of the vessel. Alternatively, the lower portion  80  of the vessel may have no trays and enclose a relatively open volume in which the hot biomass is retained while the torrefaction reaction continues to occur and be completed before the biomass is discharged from the vessel. 
         [0055]    The lower portion  80  of the vessel may be shaped to facilitate the flow of biomass down through the vessel. The geometry, e.g., cross-sectional geometry, of the lower portion  80  of the vessel  70  may be a substantially circular cross-section open top  82  and a substantially rectangular cross-section open bottom discharge  84  for the biomass. The lower portion  80  may have opposite side non-vertical gradually tapering planar side walls  86  that make an angle with respect to vertical of about 20 degrees to 30 degrees. These angles may be set depending upon the particular biomass material handled by the chip bin  11 , such as the particular species of wood chips commonly fed to the bin. Between the opposite planar side walls  86 , are opposite curved side walls  86  connected the planar side walls. The planar side walls may each be generally triangular in plan view. These planar sidewalls may be arranged vertically in diamond shape as shown in  FIG. 2 . 
         [0056]    The discharge port  81  for the vessel  70  may be coupled to a screw conveyor  83  that delivers the torrefied biomass  14  from the vessel  70  to a vessel or other process. A screw conveyor  83  may meter the flow of torrefied biomass  14  from the vessel to a collection vessel or other process. 
         [0057]    The shaft  44  may have a lower end within the vessel  70 . The lower end is driven by a gear box and motor  96  also within the vessel. Brackets, e.g. radial ribs, within the vessel support the lower end of the shaft and the gear box and motor within the vessel. By mounting the end of the shaft and gearbox and motor in the vessel, pressurized shaft seals become unnecessary between the shaft and openings in the vessel. 
         [0058]    A portion of the hot pressurized inert gas extracted from the vessel  70  may flow through conduit  90  to the chip feed system  14 . The hot gas flows through the chip feed system to purge air from the chip feed system. For example, the hot gas may flow into a lower gas manifold  92  into one or more locations in the chip bin  12 . The gas from the manifold enters the chip bin and forces the air in the bin out through an upper air vent  94  and to a conventional non-condensable gas handling system. The hot gas entering the chip bin adds heat to the biomass and thereby reduces the heat energy needed to be added to the biomass in the vessel  70 . 
         [0059]      FIG. 3  is a schematic diagram of a pressurized treatment vessel  100  for torrefaction of biomass. The vessel  100  is similar to the vessels  10  and  70  shown in  FIGS. 1 and 2 , except that the trays  102 ,  104  are stationary. The scraper bars  106  are fixed to the shaft  108  that is rotated by a gear box and motor  96 . The scraper bars may be solid rods, frames with supporting ribs or other radially extending rigid or semi-rigid arm. As the scraper bars  106  rotate with the shaft, the bars sweep the biomass over the surface of the stationary trays. As the biomass slides over the trays, the biomass is dried and heated by hot inert gases circulating through the vessel and fed to one or more upper gas inlets  112  from an inert gas source  110 . 
         [0060]    There may be one or more scraper bars arranged in a radial array on each tray  102 ,  104 . The scraper bars may extend radially outward or extend at an angle of zero to 90 degrees with respect to a radial line. The bars may be straight, curved, concave, convex or other shape that facilitates the movement of biomass over the trays. 
         [0061]    At least the upper trays  102  may be perforated, mesh, screens or other open frames to promote the flow of hot inert gases to flow through the biomass and the vessel. The lower trays  104  may be solid to slow the flow of hot inert gases from the upper to the lower regions of the vessel. Alternatively, the lower trays  104  may be open frames as are the upper trays  102 . 
         [0062]    The trays may be supported by the inner surface of the wall  112  of the pressure vessel. The inner surface of the wall  112  may include hangers, ridges or other support surfaces  114  on which rest the outer rim of the trays. The trays may be removed, replaced and repositioned in the vessel by opening the vessel and sliding the trays in and out of the vessel. The open section  46  of each tray  102 ,  104 , allows the tray to slide past the shaft  108  from removal and installation. 
         [0063]    The lowermost tray may have a center chute  116  to direct biomass to the lower portion  80  of the vessel. The lowermost tray may be an inverted cone with the center discharge chute  116  to allow biomass to flow directly to a center discharge outlet  116  of the vessel. 
         [0064]    The biomass flowing through the chute  116  drops into an optional lower portion  80  of the vessel. The biomass may form a pile in the lower portion which temporarily retains the biomass in the lower portion. While in the pile, the biomass continues to undergo the torrefaction reaction. The torrefied biomass is discharged from an outlet  116 . 
         [0065]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.