Abstract:
A system for pre-conditioning of biomass for subsequent torrefaction of the biomass comprises a burner producing combustion gases. A feed screw unit has an inlet for receiving the biomass, an outlet for outletting the biomass, and a feed screw for displacing the biomass from the inlet to the outlet. A sleeve surrounds and is in heat exchange relation with at least part of the feed screw unit. A pneumatic circuit receives combustion gases from the burner, the pneumatic circuit connected to an inlet of the sleeve for directing combustion gases therein to heat the biomass by indirect contact via the heat exchange relation, the pneumatic circuit having a pipe section extending from the outlet of the feed screw unit to a torrefaction reactor with combustion gases flowing from the outlet of the sleeve to the torrefacton reactor to convey the biomass and the combustion gases to the torrefaction reactor.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application is a continuation of U.S. Non-Provisional Application Ser. No. 14/403,795 filed on Nov. 25, 2014 which is a National Phase Entry of PCT/CA2013/050402 filed May 27, 2013 which claims priority on U.S. Provisional Application Ser. No. 61/651,862, filed on May 25, 2013, the entire contents of which is incorporated herewith by reference. 
     
    
     FIELD OF THE APPLICATION 
       [0002]    The present application relates to the transformation of biomass into products of increased energy density (e.g., combustion products) and increased carbon content, and more particularly to a method and apparatus therefor. 
       BACKGROUND OF THE ART 
       [0003]    In the torrefaction of biomass, products of increased energy density and increased carbon content are produced by the thermal treatment of the biomass. Torrefaction may decompose reactive content from the biomass (e.g., hemicellulose content), remove organic volatile compounds and/or moisture from the biomass. Hence, the products resulting from torrefaction have an increased energy density and carbon content that is well suited for various applications, such as efficient combustion. However, the thermo-transformation of biomass into fuel may be problematic, for instance due to the flammable nature of the end product. 
       SUMMARY OF THE APPLICATION 
       [0004]    It is therefore an aim of the present disclosure to provide a method and apparatus that addresses issues associated with the prior art. 
         [0005]    Therefore, in accordance with the present application, there is provided a system for pre-conditioning of biomass for subsequent torrefaction of the biomass comprising: at least one burner producing combustion gases; a feed screw unit having an inlet configured for receiving the biomass, an outlet configured for outletting the biomass, and a feed screw configured for displacing the biomass from the inlet to the outlet; a sleeve surrounding and in heat exchange relation with at least part of the feed screw unit; a pneumatic circuit receiving combustion gases from the at least one burner, the pneumatic circuit connected to an inlet of the sleeve for directing combustion gases therein to heat the biomass in the feed screw unit by indirect contact via the heat exchange relation, the pneumatic circuit having a pipe section extending from the outlet of the feed screw unit to a torrefaction reactor with combustion gases flowing from the outlet of the sleeve to the torrefacton reactor to convey the biomass and the combustion gases to the torrefaction reactor. 
         [0006]    Further in accordance with another embodiment of the present disclosure, there is provided a method for the torrefaction of biomass comprising: receiving biomass having a given moisture content; heating the biomass in a generally inert environment by indirect contact; subsequently torrefying the biomass by exposing the biomass to a flow of combustion gases in the generally inert environment; and outletting the biomass with a reduced moisture content. 
         [0007]    Further in accordance with the present disclosure, heating the biomass by indirect contact comprises circulating the biomass in a conduit surrounded by a heated sleeve. 
         [0008]    Still further in accordance with the present disclosure, wherein heating the biomass comprises directing the combustion gases in the heated sleeve. 
         [0009]    Still further in accordance with the present disclosure, circulating the biomass in the conduit comprises conveying the biomass with a feed screw. 
         [0010]    Still further in accordance with the present disclosure, heating the biomass in a generally inert environment comprises inletting the biomass in the conduit by operating a rotary valve. 
         [0011]    Still further in accordance with the present disclosure, heating the biomass comprises heating the biomass to a temperature ranging from to 250 C to 400 C. 
         [0012]    Still further in accordance with the present disclosure, exposing the biomass to a flow of combustion gases comprises circulating the biomass in a cyclonic flow. 
         [0013]    Still further in accordance with the present disclosure, circulating the biomass in a cyclonic flow comprises exposing the biomass to an annular vortex of the combustion gases in the cyclonic flow to increase a resident time of the biomass in the cyclonic flow. 
         [0014]    Still further in accordance with the present disclosure, torrefying the biomass comprises exposing the biomass to a temperature ranging from 300 C to 500 C by exposing the biomass to the combustion gases. 
         [0015]    Still further in accordance with the present disclosure, the biomass is cooled after the outletting by conveying the biomass in a feedscrew unit. 
         [0016]    Still further in accordance with the present disclosure, outletting the biomass comprises operating a rotary valve to control an amount of outlet biomass. 
         [0017]    Still further in accordance with the present disclosure, the biomass is dried prior to heating the biomass by indirect contact. 
         [0018]    Still further in accordance with the present disclosure, drying the biomass comprises at least one of mixing and recirculating biomass within a chamber of a reactor while exposing the biomass to hot air. 
         [0019]    Still further in accordance with the present disclosure, drying the biomass comprises drying the biomass to a moisture content ranging from 20% to 40%. 
         [0020]    Still further in accordance with the present disclosure, drying the biomass comprises heating air by heat exchange with combustion gases used for at least one of heating the biomass by indirect contact and torrefying the biomass. 
         [0021]    Still further in accordance with the present disclosure, a temperature of the combustion gases is controlled used for of heating the biomass by indirect contact and torrefying the biomass, by operating a heat exchanger with refrigerant in a pneumatic circuit in which the combustion gases circulate. 
         [0022]    Still further in accordance with the present disclosure, a condensate from the heat exchanger is collected to remove moisture from the combustion gases. 
         [0023]    Still further in accordance with the present disclosure, operating the heat exchanger comprises operating the heat exchanger adjacent to an outlet of the torrefying of the biomass. 
         [0024]    Still further in accordance with the present disclosure, torrefaction gases are collected from the step of exposing the biomass to a flow of combustion gases, whereby exposing the biomass to a flow of combustion gases comprises exposing the biomass to a flow of combustion gases and of torrection gases 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  is a schematic diagram of a thermo-transformation system in accordance with the present disclosure; 
           [0026]      FIG. 2  is a schematic diagram of a pre-drying stage of the thermo-transformation system of  FIG. 1 ; and 
           [0027]      FIG. 3  is a flowchart of a method for the thermo-transformation of biomass products. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0028]    Referring to  FIG. 1 , there is illustrated a thermo-transformation system  10 . The thermo-transformation system  10  is used to perform the thermo-transformation (i.e., torrefaction) of biomass. The biomass feedstock may be in any appropriate format, such as sawdust, pellets, flakes, chips, etc. The biomass may have been screened and passed through a sieve to be within a given range of granulometry. The moisture content of the biomass may be between 20% and 60%, with a range of optimal operation being between 25-40%. For instance, the biomass may originate from wood, agricultural residues, recycled wood, compost, etc. The biomass feedstock may be predried if necessary to reach an appropriate moisture content for being subjected to torrefaction in the thermo-transformation system  10 . A predrying stage is shown as an example hereinafter. 
         [0029]    The thermo-transformation system  10  comprises a conditioning stage  12  and a torrefaction stage  13 . The stages are interrelated by a pneumatic circuit  14 . A condensing unit may be used with the pneumatic circuit  14  to remove condensate from synthesis gases in the pneumatic circuit  14 , and regulate a temperature thereof. As shown in  FIG. 2 , a pre-drying stage  16  may be located upstream of the conditioning stage  12  to pre-dry biomass feedstock prior to feeding same to the conditioning stage  12 . The conditioning stage  12  is the biomass inlet of the system  10  and increases the temperature of the biomass feedstock. The torrefaction stage  13  is the biomass outlet of the system  10  and thermally transforms the biomass. The pneumatic circuit  14  displaces the biomass between the conditioning stage  12  and the torrefaction stage  13  (e.g., at a range of 2000-4000 ft/min). Moreover, the pneumatic circuit  14  provides heat in the form of combustion gases to drive the torrefaction stage  13 . 
         [0030]    The conditioning stage  12  comprises an inlet  20 . In the illustrated embodiment, the inlet  20  is a funnel that may be used with a hopper, a conveyor, bulk bags, or any other appropriate apparatus or format in which the biomass will be fed to the system  10 . However, in the illustrated embodiment, the biomass is in the form of sawdust. A rotary valve  21  is at a bottom of the inlet  20  and interfaces the inlet  20  to a feed screw unit  22 . Therefore, the rotary valve  21  controls the feed rate of the biomass feedstock to the feed screw unit  22 . 
         [0031]    The feed screw unit  22  may comprise any appropriate feed screw, namely an endless screw within a cylindrical conduit, in addition to an actuator. The actuation of the endless screw (i.e., rotation) will result in the movement of the biomass along the cylinder to an outlet  23  at an opposed end of the feed screw unit  22 . The feed screw unit  22  may comprise a double or twin feed screw to increase the throughput of biomass in the conditioning stage  12  (e.g., 10-25 Hz). 
         [0032]    The feed screw unit  22  may further have a sleeve  24  to heat the mass moving in the cylindrical conduit. An inlet of the sleeve  24  is at an upstream end of the feed screw unit  22 , whereby the flow of combustion gases (a.k.a., flue gas) in the sleeve  24  is in the same direction as that of the biomass. As shown in  FIG. 1 , a screw-like path may be defined by the insertion of baffle walls within the sleeve  24  (for instance forming a spiral path), increasing the time of residency of the hot air in the sleeve  24 . A water injection unit may also be provided at the inlet  20  or outlet  23 , within the rotary valve  21  (e.g., an annular nozzle). The water injection unit may be used to lower the temperature of the biomass, for instance if the temperature within the outlet screw unit  22  is above a predetermined threshold (a suitable range of temperatures being from 250 C to 400 C). Other methods are also considered to lower the temperature in the feed screw unit  22 , such as having a part of the combustion gases bypass the sleeve  24 , via bypass  25 . 
         [0033]    Accordingly, by the presence of the sealed rotary valve  21  and by the use of the feed screw unit  22  with heated sleeve  24 , the biomass circulating in the feed screw unit  22  will be exposed to high temperatures in an inert environment (i.e., low oxygen). The sealed rotary valve  21  may limit the infiltration of oxygen into the system  10 . For instance, the biomass is exposed to a temperature higher than a temperature of condensation of tar. 
         [0034]    Therefore, at the outlet  23 , the temperature of the biomass has raised. The outlet  23  may be positioned on an underside of the cylinder of the feed screw unit  22 . A continuous feed of conditioned biomass is as a result dropped out of the feed screw unit  22 , and will be transported to the torrefaction stage  13  by the pneumatic conveyor  14 , as described in further detail hereinafter. 
         [0035]    According to an embodiment, the torrefaction stage comprises one or more cyclonic bed reactors  30  (a.k.a., torrefaction reactor  30 ), with of the cyclonic bed reactors being illustrated in  FIG. 1 . The torrefaction stage  13  may comprise one or more of the torrefaction reactors  30 , or any other torrefaction apparatus. In the torrefaction stage  13 , the biomass is exposed to combustion gases, during at least a minimum time of residency. The combustion gases are at any appropriate temperature to have a torrefying effect on the biomass. As an example, the combustion gases are at a temperature ranging between 300 and 500° C., although temperatures outside this range may be appropriate as well in certain circumstances. As a result of the exposure to the combustion gases, the biomass is torrefied: the level of moisture is substantially reduced, and volatile organic compounds are removed. The volatile organic compounds form the torrefaction gases composed of condensable and non-condensable gases. The cyclonic bed reactors  30  may be similar in configuration to the filtration apparatus described in US patent application publication no. 2011/0239861, incorporated herewith by reference. 
         [0036]    More specifically, the torrefaction reactor  30  may be broadly described as having a casing defining an inner cavity with an upper cylindrical portion, and a lower hopper portion connected to the upper cylindrical portion. The inlet is in the upper cylindrical portion for feeding a flow of gas and the biomass into the inner cavity. The inlet is positioned with respect to the casing to cause movement of the biomass in a downward spiral path in the casing. A solids outlet is at a bottom of the lower hopper portion for outletting the biomass from the casing. A gas outlet is in the upper cylindrical portion to exhaust gases from the casing. There is an annular arrangement of ports (i.e., a pair of sustentation rings, although one or more are possible) in a wall of the lower hopper portion or the cylindrical portion of the casing to inject gas into the inner cavity (i.e., torrefaction gases, combustion gases). The ports are oriented so as to guide these other gas into following a path at least partially vertical when entering the inner cavity to disrupt the movement of the solids in the downward spiral path. Hence, the gases injected through the ports of the sustentation ring may increase the residency time of the biomass in the reactor  30 . For instance, the ports have a vertical component in their orientation, to guide the gases upwardly, and in the spiral path. 
         [0037]    US patent application publication no. 2011/0239861 describes a filtration configuration at an upper end of the support wall of the filtration apparatus. The cyclonic bed reactor  30  may have a different filtration configuration, or even limited or no filtration. 
         [0038]    A feed screw unit  31 A may be located at the bottom of the reactors  30  to collect the dry thermo-transformed biomass exiting from the torrefaction reactors  30 , and to cool off the biomass. A second screw unit  31 B may be used to cool off the biomass. A water injection unit may also be provided adjacent to an upstream end of the second screw unit  31 B. The water injection unit may be used to lower the temperature of the biomass, for instance if the temperature within the second screw unit  31 B is above a predetermined threshold. In an embodiment, a cooling fluid (e.g., water, air) is in heat exchange with the outer surface of the unit  31 , for the direct or indirect contact cooling of the biomass. A rotary valve  32  may be positioned at the outlet of the feed screw unit  31 , thereby minimizing gas leaks at the outlet of the torrefaction reactors  30 , and controlling the torrefied biomass output rate. The system  10  may be provided with multiple feed screw units  31 , for instance with one for each of the torrefaction reactors  30 . 
         [0039]    The use of rotary valves  21  and  32  at the inlet and the outlet of the system  10  reduces and/or prevents oxygen infiltration in the system  10 , thereby helping in preserving an inert environment to avoid combustion of the biomass during torrefaction. The feed screw units  31 A and  31 B, or like mechanism, are used to reduce the temperature of the biomass, to reduce the risk of combustion of the biomass when exposed to oxygen at the outlet of the system  10 . 
         [0040]    As an alternative to the torrefaction reactors  30 , any other configuration of reactor may be used in the torrefaction stage  13  to expose the biomass to combustion gases. For instance, a rotary drum reactor may be operated, or air conveyors may be used provided they have sufficient length to respect the residency time of the biomass, and thus allow sufficient exposure of the biomass to combustion gases. However, the torrefaction reactors  30  are well-suited for being used with a continuous feed of biomass from the conditioning stage  12 . 
         [0041]    The pneumatic circuit  14  comprises an air conveyor  40  extending from the outlet of the feed screw unit  22  to the inlet of the cyclonic bed reactors  30 . Accordingly, the biomass flows to the reactors  30  as entrained by a flow of combustion gases (a.k.a. flue gases), and torrefaction gases emanating from the torrefaction of the biomass. More, specifically, the pneumatic circuit  14  has return pipes  41  collecting gases exhausted by the torrefaction reactors  30 , whereby the synthesis gases (i.e., syngas) circulating in the pneumatic circuit  14  are a mixture of combustion gases and torrefaction gases. The gases collected at the exhaust are generally hot, with some humidity and generally without airborne dust, as the reactors  30  typically perform some form of filtration. A fan  42  in the return pipes  41  ensures that the flow of gases is of sufficient magnitude in the circuit  14  to cause the movement of the biomass. The fan  42  may be one of numerous fans in the circuit  14 . A reactor branch  43  diverges from the return pipes  41  and feeds some gases to the reactors and more specifically to the sustentation ring of the reactors  30 . The return pipes  41  converge to a single return pipe also labeled  41 , which return pipe connects to the inlet of the air conveyor  40 . 
         [0042]    A burner branch  44  diverges from the return pipe  41  and is connected to a burner  45 . Hence, the burner branch  44  feeds combustion gases and/or torrefaction gases to the burner  45 . According to an embodiment, the burner  45  is a combustion burner. Gases exhausted by the torrefaction reactors  30  (i.e, combustion gases and/or torrefaction gases or synthesis gases (syngas)) may be fed into the combustion chamber of the burner  45  through an annular vortex to raise the resident time and combustion efficiency. A fresh air intake  46  is also associated to the burner  45 , for instance to adjust the amount of oxygen fed to the burner  45  for efficient combustion. An external fuel (natural gas, fuel oil, propane, etc) may be used to start the process and to maintain a pilot flame into the combustion chamber of the burner  45 . A feed pipe  47  relates the burner  45  to the sleeve  24  of the conditioning stage  12 . The sleeve  24  may have an exhaust pipe  48  to exhaust some of the combustion gases from the pneumatic circuit  14 . 
         [0043]    Hence, at the exit of the conditioning stage  12 , the biomass is exposed to synthesis gases. The biomass feedstock exiting the conditioning stage  12  has a reduced moisture content, whereby the thermal transformation reaction of the biomass feedstock is initiated when it reaches the air conveyor  40  of the conveyor pneumatic circuit  14 , and continues in the torrefaction reactors  30  or like apparatus. There may be some flash evaporation of the moisture in the biomass when it reaches the air conveyor  40  of the conveyor pneumatic circuit  14 . 
         [0044]    Torrefaction gases emanating from the biomass may be directed to the burner  45 , to be part of the combustion. Hence, the torrefaction gases are used to produce heat for both stages  12  and  13 . The use of rotary valves  21  and  32  reduce the amount of oxygen entering the system  10 . 
         [0045]    Still referring to  FIG. 1 , a condensing unit  15  has water-cooled combustion gas condenser  50  that may be provided as branching off from the pneumatic circuit  14 . In  FIG. 1 , the condenser  50  receives synthesis gases from the return pipe  41 , but may be located elsewhere in the pneumatic circuit  14 . The condenser  50  is used to condensate humidity in the syngas resulting from the torrefaction process, and may hence be located in proximity of the torrefaction reactors  30 . Moreover, the condenser  50  may regulate the temperature of the syngas/combustion gas by its heat capacity. 
         [0046]    The combustion gases enter the condenser  50 . In an embodiment, the condenser  50  is configured with respect to a feed pipe  51  such that the synthesis gases enter tangentially via an upper portion of the condenser  50 . The outlet  52  is equipped with a coil  53  of refrigerant, such as a glycol cooled coil. In an embodiment, outside surfaces of the coil  53  have a double wall jacket with cooling glycol. However, any suitable type or configuration of coil or heat exchanger is considered for the condenser  50 . To prevent clogging, the condenser  50  may be equipped with a self-cleaning blow back system with appropriate injection nozzles. The heat recuperated by the coil  53  may be used for heating purposes. In  FIG. 1 , one or more heating units  54  of the type having a coil and fan is shown, although other arrangements are considered as well. A return pipe  55  may then direct the combustion gases to the pneumatic circuit  14 . An appropriate draining circuit may then be used to collect the condensate. 
         [0047]    Referring to  FIG. 2 , the pre-drying stage  16  is shown in greater detail, and may optionally be used to pre-dry biomass feedstock to a suitable moisture content (e.g., 25% to 40%). The pre-drying stage may be comprises of any type of dryers, e.g. rotary dryers, belt dryers or flash dryers. In the illustrated embodiment, the pre-drying stage  16  essentially comprises a dryer  60  defining a chamber in which the biomass feedstock is exposed to hot air, with mixing features operated by motor  60 A (e.g., a screw, etc). The dryer  60  has a dryer inlet  61 , a recirculating outlet  62  (with rotary valve  62 A or equivalent) and a dryer outlet  63  (with rotary valve  63 A or equivalent), with both outlets  62  and  63  being in a bottom of the dryer  60 . The stage  16  further comprises a pneumatic circuit  64  in association with the dryer  60  to provide hot air, a flow of biomass, and a conveying flow for recirculation of biomass. The dryer  60  and the circuit  64  form a generally hermetic unit, so as to limit air infiltration causing heat loss. 
         [0048]    In an embodiment, the pneumatic circuit  64  has an air inlet  64 A, followed by a heat exchanger  65  to heat the air from the inlet  64 A. The circuit  64  has appropriate piping to direct the heated air from the heat exchanger  65  to the reactor inlet  61 . By way of the piping, a biomass source converges with the circuit  64 . The biomass source may comprise a hopper, a funnel and a rotary valve  66 A or equivalent, to control the amount of biomass entering the circuit  64 . The piping of the pneumatic circuit  64  is also fluidly connected to the recirculating outlet  62 , with the rotary valve  62 A controlling the amount of biomass recirculating via the circuit  64 . The piping of the pneumatic circuit  64  then reaches the inlet  61 , to discharge a mix of fresh biomass and recirculated biomass, in the flow of hot air. The recirculation of the biomass is performed to expose all biomass to hot air and thus promote uniform temperature condition of the biomass. A portion of the biomass may exit the reactor  60  via the reactor outlet  63 , with the rotary valve  63 A controlling the amount of biomass exiting the stage  16 . A conveyor  67  may then feed the pre-dried biomass to the conditioning stage  12 . 
         [0049]    In the illustrated embodiment of  FIG. 2 , the heat exchanger  65  may receive combustion gases from the pneumatic circuit  14  ( FIG. 1 ), for instance combustion gases that are to be exhausted, to recuperate heat therefrom. It is also possible to use any appropriate source of heat, for instance independent of the pneumatic circuit  14 , to heat the air in the stage  16 . For instance, electric coils may be used. 
         [0050]    A filtration unit  68  with blowback may be provided to remove dust and airborne particles from the reactor  60 . A water injection unit  69  may be used to extinguish a fire. Referring to  FIG. 3 , there is illustrated at  70  a method for thermo-transformation (i.e., torrefaction) of biomass. 
         [0051]    According to step  71 , the biomass may be pre-dried to reach a suitable temperature or reduce its humidity content, if necessary. 
         [0052]    According to step  72 , the biomass is received in a sawdust format. 
         [0053]    According to step  73 , the biomass is exposed to a high temperature by indirect contact (e.g., temperature above the temperature of condensation of tar), whereby the biomass feedstock is heated and its moisture content may be reduced. In an embodiment, the biomass that is exposed to these conditions is a continuous feed of biomass. 
         [0054]    According to step  74 , the biomass is directly exposed to a high-temperature combustion gas flow (a.k.a., flue gases) and residual torrefaction gases, with low static pressure, subsequent to step  52 . As a result, the biomass feedstock undergoes thermo-transformation, by which the chemical structures of the biomass may be broken (i.e., lignin, cellulose, hemi-cellulose). Volatile organic compounds may be vaporized after the two stages, thereby improving the condition of the biomass for combustion. Moisture may further evaporate from the biomass. For example, flash evaporation of moisture in the biomass may occur. 
         [0055]    According to step  75 , the biomass is outlet with a reduced moisture content. The biomass may be subjected to a cooling stage. It is pointed out that the biomass may be exposed to an inert environment (i.e., negligible level of oxygen) in steps  74  and/or  75 . 
         [0056]    The resulting torrefied biomass may be in any appropriate format. For instance, the torrefied biomass is in a sawdust state, although it could be in flakes, granules, pellets or the like. The torrefied biomass may be used in any appropriate application. For example, the torrefied biomass may be used as a fuel in combustion. Applications include non-exclusively co-firing in large coal power plants, heavy fuel oil substitution, partial substitute for coke in carbon anodes, blast furnaces, iron ore pellets, activated carbon for gas purification, gold purification, metal extraction and many other applications, soil amendment and soil remediation (mining site rehabilitation), among numerous possibilities. 
         [0057]    While the methods and systems described herein have been described and shown with reference to particular steps performed in a particular order, it will be understood that these steps may be combined, subdivided or reordered to form an equivalent method without departing from the teachings of the present invention. Accordingly, the order and grouping of the steps is not a limitation of the present invention. 
         [0058]    Modifications and improvements to the above-described embodiments of the present invention may become apparent to those skilled in the art. The foregoing description is intended to be exemplary rather than limiting. The scope of the present invention is therefore intended to be limited solely by the scope of the appended claims.