Abstract:
Biomass is gasified to generate syngas. The syngas is subjected to thermal cracking. Heat from syngas exiting a thermal cracking stage is transferred to syngas entering the thermal cracking stage. Biomass gasification apparatus may include a thermal pathway connected to transfer heat from an outlet of a thermal cracking process to an inlet of the thermal cracking process. Energy efficiency is enhanced. Syngas may be used as fuel for engines or fuel cells, burned to yield heat, or processed into a fuel.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Patent Application No. 61/075,685 filed on 25 Jun. 2008 entitled GENERATING CLEAN SYNGAS FROM BIOMASS and U.S. Patent Application No. 61/098,643 filed on 19 Sep. 2008 and entitled GENERATING CLEAN SYNGAS FROM BIOMASS. For purposes of the United States, this application claims the benefit of Application Nos. 61/075,685 and 61/098,643 under 35 U.S.C. §119. 
    
    
     TECHNICAL FIELD 
     The invention relates to generating syngas from biomass. Embodiments of the invention provide methods and apparatus for generating syngas from biomass. The methods and apparatus can provide for cracking of certain fractions within the syngas. Syngas so produced may be used in a wide range of applications. 
     BACKGROUND 
     Combustible gases can be generated by thermo-chemical conversion of biomass. Biomass may be any suitable carbon-containing fuel. Non-limiting examples of biomass include: wood (in any suitable form including sawdust, shavings, pellets, chips, other wood residue and the like), municipal waste, sewage, coal, bitumen, fossil fuels, food waste, plant matter or the like. Combustible gases may be liberated from biomass by heating the biomass in an oxygen-reduced atmosphere. The heating may be done by partially oxidizing the biomass or by way of a separate heat source. 
     The heating causes the biomass to release combustible gases (sometimes called “syngas”, “synthesis gas”, “producer gas”, or “product gas”). 
     Combustible gases produced from biomass may be used for various applications. For example, the gases may be burned to generate heat, processed to make synthetic fuels (the synthetic fuels may comprise gaseous, liquid or solid fuels), used to run an engine, used as a fuel for a fuel cell, used as a fuel to run a turbine, or the like. 
     Gases liberated from biomass may include fractions, such as tars and heavier hydrocarbons, that can condense in ducts and other equipment. This can cause significant operational and maintenance problems. 
     There is a need for practical and energy-efficient methods and apparatus for producing clean syngas from biomass. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings illustrate non-limiting example embodiments of the invention. 
         FIG. 1  is a flow chart illustrating a method according to an example embodiment of the invention. 
         FIG. 2  is a block diagram illustrating an apparatus according to an example schematic embodiment of the invention. 
         FIG. 3  is a block diagram illustrating an apparatus according to another example embodiment of the invention. 
         FIG. 4  is a block diagram illustrating an apparatus according to another example embodiment of the invention. 
         FIG. 4A  is a process diagram illustrating schematically an apparatus according to a further embodiment of the invention. 
         FIG. 5  is a schematic cross-sectional illustration of an example thermal cracking unit and associated systems. 
         FIG. 6  is a block diagram illustrating an apparatus according to another example embodiment of the invention. 
         FIG. 7  is a schematic cross-sectional illustration of an example thermal cracking unit and associated systems. 
     
    
    
     DESCRIPTION 
     Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense. 
       FIG. 1  illustrates a method  10  for gasifying biomass according to an example embodiment of the invention. At block  12  method  10  introduces biomass  11  into a chamber in which the biomass can be heated. The biomass may comprise any suitable type of biomass. In an example embodiment, the biomass comprises wood and block  12  comprises placing the wood into a gasification chamber. In some embodiments the wood is supplied in the form of small pieces. In some embodiments the small pieces are pieces up to about 1 or 2 inches (about 25 to 50 mm) in size. In some embodiments, the small pieces are inhomogeneous pieces ranging from ⅛ inch to about 3 or 4 inches (about ½ mm to about 10 cm) in size. In some embodiments, the introduction of biomass in block  12  is performed substantially continuously. In other embodiments the introduction of biomass is performed intermittently (e.g. periodically, on demand, or the like). 
     In some embodiments block  12  comprises introducing the biomass upwardly through an opening in a floor of a gasification chamber. In such embodiments the biomass may form a heap on the floor of the gasification chamber. 
     In block  14  the biomass is gasified by raising the biomass to an elevated temperature under reduced oxygen conditions to produce raw syngas. In some embodiments the biomass is heated by performing partial oxidation of the biomass. In such embodiments, air or another oxygen-containing gas may be introduced into the gasification chamber in an amount sufficient to permit partial oxidation of the biomass. In other embodiments the biomass is heated by applying heat from an external source to pyrolize the biomass. 
     In some embodiments a temperature of the biomass is maintained at a temperature below a temperature at which ash from the biomass would melt to yield slag. The melting point of the ash from some types of biomass is in the range of approximately 2100° F. to about 2200° F. For example, in some embodiments the temperature of the biomass is prevented from exceeding about 1800° F. in block  14 . Temperature of the biomass may be controlled by controlling a concentration of oxygen in air or other gas being introduced into the biomass (e.g. blast air). 
     In block  16  the raw syngas is drawn off. In some embodiments the raw syngas is drawn off through a duct connected to receive syngas from a gasification chamber in which block  14  is performed. In some embodiments the temperature of the raw syngas is lower than 900° F. (about 480° C.) at the point where it exits the gasification chamber. In some embodiments the temperature of the raw syngas is in the range of 300° F. (about 150° C.) up to about 1000° F. (about 540° C.) at the point where it exits the gasification chamber. 
     In block  18  the temperature of the raw syngas is boosted. In some embodiments, block  18  comprises heating the syngas with heat extracted from the syngas at a downstream location. In some embodiments block  18  comprises boosting a temperature of the raw syngas by at least 800° F. (about 430° C.). In some embodiments block  18  comprises boosting a temperature of the raw syngas by 1000° F. (about 540° C.) or more. 
     In some embodiments, the heated raw syngas has a temperature of at least 1600° F. (about 870° C.). In some embodiments, a temperature of the heated raw syngas is at least about 1200° F. (about 650° C.). In some embodiments a temperature of the heated raw syngas is at least about 1300° F. (about 700° C.). 
     In block  20  the heated raw syngas is passed to a cracking stage. 
     In block  22  the raw syngas is treated in the cracking stage. Conditions in the cracking stage promote the breakdown of heavy or tar fractions within the raw syngas. Tars and/or other heavier hydrocarbons present in the syngas may be broken down in the cracking stage. The cracking stage may facilitate breakdown of such fractions by providing oxidants that promote breakdown of the condensing fractions at the temperature at which the cracking stage is operated. The cracking stage may also or alternatively facilitate breakdown of such fractions by providing energy from a plasma torch to promote breakdown of the condensing fractions at the temperature at which the cracking stage is operated. The cracking stage may additionally include one or more of:
         one or more catalysts; and,   injection of steam or other chemicals that combine with carbon to avoid or reduce soot formation.
 
In some embodiments, a temperature of the syngas in the thermal cracking stage is at least 1600° F. (about 870° C.). Cracking stages in some example embodiments operate at temperatures in the range of about 1600° F. (about 870° C.) to about 2100° F. (about 1150° C.).
       

     In some embodiments, steam is injected into the syngas in or upstream from the cracking stage. Introduction of steam can reduce soot formation. Steam can react with tar, which could otherwise form soot particles, to form carbon monoxide. 
     Where block  22  involves mixing the syngas with an oxidant, the oxidant may be preheated. In some embodiments, preheating the oxidant comprises heating the oxidant with heat extracted from the syngas at downstream location. The oxidant may, for example, comprise oxygen, ambient air, or mixtures thereof with one another and/or with other gases such as steam or the like. 
     As a result of the incoming raw syngas being preheated, suitable temperatures for thermal cracking may be maintained in cracking block  22  with a reduced requirement for heat to be generated in block  22  or a reduced requirement for energy to be otherwise supplied to the syngas in block  22  or both. Where heat is generated in block  22  by exothermic oxidation of syngas, desired temperatures may be reached with a reduced fraction of stoichiometric air (oxidant) introduced into the cracking stage. In some embodiments, the incremental equivalence ratio in the cracking stage is less than 0.15. In some embodiments the incremental equivalence ratio in thermal cracking block  22  is in the range of 0.10-0.15. 
     In some embodiments, thermal cracking comprises using a plasma torch to provide some or all of the energy required to thermally crack the incoming raw syngas. Any suitable type of plasma torch may be used. For example, a plasma arc may be used to heat a small amount of inert gas, which may then be blown into the cracking chamber. Alternatively, a reducing gas or an oxidizing gas may be introduced by way of the plasma torch. Where heat is generated in block  22  by operation of a plasma torch, the desired temperature may be reached with a reduced amount of energy applied to the plasma torch as a result of the incoming raw syngas being preheated. In some embodiments, complete cracking of the syngas may be achieved in the absence of oxygen and/or water. However, the presence of oxygen and/or water may prevent or reduce the formation of soot as a product of the cracking process. In some embodiments, heat may be generated in block  22  by a combination of the operation of a plasma torch and the exothermic oxidation of syngas in the presence of a sub-stoichiometric amount of oxidant. In some such embodiments, the incremental equivalence ratio in the cracking stage may be less than 0.15, and may be less than 0.10. 
     In some embodiments, a temperature rise experienced by the syngas upon entering the thermal cracking stage is about 1000° F. (about 550° C.) or less. This temperature rise can be smaller in some cases, for example 500° F. (about 260° C.) or less or 400° F. (about 200° C.) or less in some embodiments. In an example embodiment, heated syngas enters the cracking stage at a temperature of about 1200° F. (about 650° C.) and reaches a temperature of about 2100° F. (about 1150° C.) in the cracking stage. In this example embodiment, the temperature rise experienced by the syngas upon entering the thermal cracking stage is about 900° F. (about 480° C.). In another example embodiment, heated syngas enters the cracking stage at a temperature of about 1300° F. (about 700° C.) and reaches a temperature of about 2000° F. (about 1100° C.) in the cracking stage so that the temperature rise in the cracking stage is about 700° F. (about 370° C.). 
     The cracking stage may comprise a stage in which syngas passes into and through a volume within a thermal cracking chamber. The thermal cracking chamber may comprise a refractory-lined vessel. In an example embodiment, the thermal cracking chamber comprises a cylindrical chamber having axial inlet and outlet ports and oxidant ports located in an area near to the inlet port. In some embodiments the oxidant ports enter the thermal cracking chamber radially. In some embodiments there are two or more sets of oxidant ports, each set may comprise a plurality of oxidant ports spaced circumferentially around the thermal cracking chamber. 
     In some embodiments, the thermal cracking chamber may include a plasma torch. In some embodiments that include a plasma torch the oxidant ports are omitted. Some embodiments that include a plasma torch include a pathway for adding steam into or upstream from the thermal cracking chamber. Oxidant may be introduced into the cracking chamber through oxidant ports, if present, and/or through the plasma torch. 
     In some embodiments the thermal cracking stage is performed in a thermal cracking chamber that is in a separate structure from the gasification chamber. 
     In block  24  the syngas passes out of the cracking stage. 
     In block  26  some heat is removed from the syngas. Block  26  may comprise passing the syngas through a heat exchanger, for example. In some embodiments heat extracted at block  26  is used to boost the temperature of raw syngas in block  18 . In some embodiments heat extracted at block  26  is used to boost the temperature of oxidant introduced in block  22 . 
     In block  28  the syngas is optionally further cooled and/or filtered. 
     In block  29  the syngas is provided as a fuel to one or more of:
         a burner (block  29 A)   an engine (block  29 B)   a fuel cell (block  29 C)   a turbine (block  29 D)   a process for making synthetic fuel (block  29 E)   etc.       

       FIG. 2  shows schematically apparatus  30  according to an example embodiment of the invention. Apparatus  30  comprises a biomass supply  32  and a gasification chamber  34 . A conveyor  35  carries biomass from biomass supply  32  to gasification chamber  34 . Syngas is generated within gasification chamber  34  by heating biomass, either indirectly or by partial oxidation of the biomass. 
     A thermal cracking unit  40  receives raw syngas from gasifier chamber  34 . Thermal cracking unit  40  comprises a chamber within which the syngas is maintained at a temperature sufficient to break down tars, heavier hydrocarbons and the like. A controlled amount of an oxidant (for example, air or another gas containing oxygen) is introduced into thermal cracking unit  40 . Water, which may be in the form of steam, may be introduced into or upstream from thermal cracking unit  40 . Injection of water may be unnecessary in cases where sufficient water is already present (e.g. in cases where biomass from supply  32  has a sufficiently-high moisture content).  FIG. 2  shows an example source of steam  33  connected to inject steam into the syngas upstream from cracking unit  40 , where the syngas is thermally cracked. 
     A heat path  41  carries heat from syngas exiting thermal cracking unit  40  to raw syngas that has not yet been processed in thermal cracking unit  40 . 
     In the illustrated embodiment, an oxidant supply system  42  is connected to supply air or another oxidant into thermal cracking unit  40 . A second heat path  43  carries heat from syngas exiting thermal cracking unit  40  to oxidant being supplied to thermal cracking unit  40  by oxidant supply system  42 . 
     Syngas from thermal cracking unit  40  is delivered to one or more of:
         A burner  45 A.   An engine  45 B. Engine  45 B may be an internal combustion engine or a turbine for example. In some embodiments engine  45 B drives a generator  46  to generate electrical power.   A fuel cell  45 C.   A chemical process  45 D. Chemical process  45 D may take syngas as a raw material and process the syngas into a synthetic fuel.
 
A storage tank  44  is optionally provided.
       

       FIG. 6  shows schematically apparatus  31  according to a further example embodiment of the invention. Apparatus  31  is similar to apparatus  30 , except that a plasma torch  47  is provided to supply heat to syngas within a thermal cracking unit  39  to maintain syngas in thermal cracking unit  39  at a temperature sufficient to break down tars, heavier hydrocarbons and the like. Apparatus  31  includes many of the same components as apparatus  30 , which have like reference numerals in  FIG. 6 . Additionally, apparatus  31  is illustrated without an oxidant supply system. However, in some embodiments, apparatus  31  may include an oxidant supply system similar to oxidant supply system  42  for thermal cracking unit  39 . As with apparatus  30 , water, optionally in the form of steam, may be introduced into or upstream of thermal cracking unit  39 . 
       FIG. 3  shows a gasification apparatus  50  according to an example embodiment of the invention. Apparatus  50  comprises a biomass supply  52  and a gasification chamber  54 . A conveyor  55  carries biomass from biomass supply  52  to gasification chamber  54 . 
     In the illustrated embodiment, the biomass is heated (at least in part) by partial oxidation of the biomass in a controlled atmosphere within gasification chamber  54 . An oxidant supply system  56  supplies oxidant (which may, for example, comprise air, a mixture of air and/or oxygen with steam and/or flue gas, or the like) to gasification chamber  54 . A controller  57  regulates operation of conveyor  55  and oxidant supply system  56  to maintain proper conditions for gasification of the biomass. In alternative embodiments, other means, such as an indirect heater, may be provided for heating biomass within gasifier chamber  54 . 
     A duct  58  carries raw syngas from gasification chamber  54  to the cold side (i.e., second side) of a first heat exchanger  59 . The temperature of the raw syngas is increased in first heat exchanger  59 . The heated raw syngas then passes through a duct  61  to a thermal cracking unit  60 . Thermal cracking unit  60  may be constructed and operated in substantially the same manner as thermal cracking unit  40  of  FIG. 2 , or as thermal cracking unit  39  of  FIG. 6 , for example. 
     First heat exchanger  59  may comprise, for example, a gas-gas heat exchanger (such as a shell-and-tube or plate heat exchanger) or a heat exchanger in which an intermediate circulating heat exchange medium carries heat to the incoming syngas. Apparatus  50  of  FIG. 3  includes an optional second heat exchanger  69 . 
     Syngas exits thermal cracking unit  60  into duct  62  which eventually connects to the hot side (i.e., first side) of first heat exchanger  59 . In the illustrated embodiment, duct  62  has a first part  62 A that carries syngas from thermal cracking unit  60  to a hot side of second heat exchanger  69  and a second part  62 B that carries the hot syngas from second heat exchanger  69  to the hot side of first heat exchanger  59 . In first heat exchanger  59  heat from the hot syngas that has exited thermal cracking unit  60  is transferred to raw syngas that is being moved from duct  58  into thermal cracking unit  60  through first heat exchanger  59 . 
     Syngas exiting the hot side of first heat exchanger  59  is delivered to an engine  66  (or other end use or storage) by way of a filter  64  and a cooler  65 . When the syngas is used as fuel for an engine it is usually desirable that the syngas be at a temperature of about 110° F. (about 45° C.) or less at the point where it is taken into the engine. 
     In the illustrated embodiment, air or another oxidant is delivered to thermal cracking unit  60 . The air is conveyed through the cold side of a second heat exchanger  69  before it enters thermal cracking unit  60 . Hot syngas that has exited thermal cracking unit  60  passes through the hot side of second heat exchanger  69 . 
     In the illustrated embodiment, the hot syngas that has exited thermal cracking unit  60  passes first through the hot side of second heat exchanger  69  and then through the hot side of first heat exchanger  59 . This order is not mandatory. In other embodiments, the syngas passes first through first heat exchanger  59  and then through second heat exchanger  69 . In still other embodiments the syngas is divided into streams that pass through first heat exchanger  59  and second heat exchanger  69  in parallel. 
     In apparatus  50  first heat exchanger  59  provides a heat path that carries heat from syngas exiting thermal cracking unit  60  to raw syngas that has not yet been processed in thermal cracking unit  60 . 
     In apparatus  50 , gasification chamber  54  may take any of a variety of forms. In some embodiments, gasification chamber  54  is an updraft gasification chamber and raw syngas is drawn off at a location that is above the biomass from which the syngas is being generated. In some embodiments gasification chamber  54  comprises a bottom-fed gasification chamber. Non-limiting examples of bottom-fed gasification chambers of types that may be used in apparatus  50  are described in the following patents and patent applications:
         U.S. Pat. No. 6,120,567   US 2004/0107638   PCT/US2007/011965   CA 1380910   CA 2486318.
 
Other types of gasification chamber may be provided for the generation of raw syngas. A wide range of gasification chambers useful for gasifying biomass is described in the technical literature in the field of biomass gasification.
       

       FIG. 4  shows apparatus  70  according to an alternative embodiment. The apparatus of  FIGS. 3 and 4  have a number of common components. These components have the same reference numbers in  FIG. 4  as in  FIG. 3 . Apparatus  70  optionally includes a plasma torch  76  to heat syngas in thermal cracking unit  60 . Apparatus  70  comprises a first heat exchanger  72  and a second heat exchanger  74 . The cold side of first heat exchanger  72  is in the path taken by raw syngas flowing from gasifier chamber  54  to thermal cracking unit  60 . The hot side of second heat exchanger  74  is in the path taken by hot syngas that has exited thermal cracking unit  60 . Air passing through the cold side of second heat exchanger  74  receives heat from the hot syngas on the hot side of second heat exchanger  74 . The air is heated to a temperature in excess of 1600° F. (about 870° C.) for example. Some of the heated air is provided as an oxidant to thermal cracking unit  60  by way of path  75 . Some of the heated air may optionally be provided to plasma torch  76 . The rest of the heated air passes through the hot side of first heat exchanger  72 . In doing so, heat is transferred from the hot air to the raw syngas passing through the cold side of first heat exchanger  72 . The hot air (now at a reduced temperature) may be exhausted, applied to drying or preheating biomass, applied for other heating functions, or the like. 
     Blowers, adjustable valves and the like may be provided to maintain suitable flow of air, syngas and other fluids as required. These components are understood by those of skill in the art and are not illustrated here to avoid obscuring the invention. 
       FIG. 4A  is a process diagram illustrating apparatus for producing clean syngas that is similar to that depicted in  FIG. 4 .  FIG. 4A  shows a cooler  64 A that comprises a condenser to remove water vapor and a filter  65 A. In the illustrated embodiment, a plasma torch  76 A is provided in the thermal cracker to provide energy to heat the syngas at the cracking stage. 
       FIG. 5  shows schematically a thermal cracking unit  80  comprising a vessel  81  lined with a layer  82  of refractory material. A thermal cracking unit like thermal cracking unit  80  may be used in any of the embodiments described above (although this is not mandatory—other styles of thermal cracking unit may be used). 
     Syngas can enter vessel  81  through an inlet  84 A and, after processing in thermal cracking unit  80  can exit at outlet port  84 B. A catalyst structure  85  is optionally provided in vessel  81 . Oxidant is introduced into vessel  81  by way of two rings of radially-oriented ports  86 . Air is supplied to ports  86  by a blower  87  that feeds manifolds  88  by way of a control valve  89 .  FIG. 5  shows an optional preheater  91  (which may comprise a heat exchanger) that heats air before the air is introduced into thermal cracking unit  80 . Preheater  91 , if present, may be provided at any suitable location in the path taken by air being delivered into thermal cracking unit  80 . 
     In the embodiment illustrated in  FIG. 5 , a controller  90  controls the degree of opening of valve  89  in response to signals from at least one temperature sensor  92  that measures a temperature in thermal cracking unit  80  and mass flow sensors  94 A and  94 B that monitor the mass flow of air into vessel  81 . Controller  90  may comprise a suitable programmable or hard-wired process controller, a programmed computer control system, or the like. Controller  90  controls the influx of air into vessel  81  to maintain a desired temperature within vessel  81 . 
       FIG. 5  shows an optional steam inlet line  95  connected to supply steam into vessel  81  by way of a manifold  88 . The influx of steam is controlled by a valve  96  operated in response to signals from controller  90 . In alternative embodiments that include steam injection, steam is injected at other locations into and/or upstream from thermal cracking unit  80 . 
       FIG. 7  shows schematically a thermal cracking unit  100 . Thermal cracking unit  100  includes many of the same components as thermal cracking unit  80 , and these components are referred to by the same reference numerals as in  FIG. 5 . A thermal cracking unit  100  may be used in any of the embodiments described above. 
     In the illustrated thermal cracking unit  100 , syngas enters vessel  81  through inlet  84 A and exits through outlet port  84 B after it has been processed in thermal cracking unit  100 . A plasma torch  99  and an optional catalyst structure  85  are provided in vessel  81 . Plasma torch  99  is operated to heat syngas within thermal cracking unit  100 , and thereby maintain the syngas at a temperature sufficient to break down tars, heavier hydrocarbons and the like. A controller  97 , which may be any suitable programmable or hard-wired process controller, a programmed computer control system, or the like, may be used to regulate the operation of plasma torch  99  in response to temperature sensor  92 . In some embodiments, thermal cracking unit  100  may further include ports for introducing oxidant into vessel  81 . In such case, a suitable controller may likewise be used to regulate both the influx of air into vessel  81  (as described with reference to thermal cracking unit  80 ) and the operation of plasma torch  99 , to maintain a desired temperature within the vessel  81 . 
     The embodiment illustrated in  FIG. 7  also includes optional steam inlet  95  connected to supply steam into vessel  81  through manifold  88  and port  86 . Valve  96  controls the influx of steam into vessel  81  in response to signals from controller  97 . Alternatively, steam may be injected at other locations into and/or upstream from thermal cracking unit  100 . 
     Where a component (e.g. a chamber, duct, assembly, device, etc.) is referred to above, unless otherwise indicated, reference to that component (including a reference to a “means”) should be interpreted as including as equivalents of that component any component which performs the function of the described component (i.e., that is functionally equivalent), including components which are not structurally equivalent to the disclosed structure which performs the function in the illustrated exemplary embodiments of the invention. 
     As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. For example, features from any of the embodiments described herein may be combined with features of other embodiments described herein to provide further embodiments.