Abstract:
A system and method capable of efficient production of synthesis gas from biomass materials in a manner which can be scaled to relatively large throughput capacities. the system is operable to compact a loose biomass material and simultaneously introduce the compacted biomass material into entrances of internal passages of multiple parallel reactors, heat the compacted biomass material within the reactors to a temperature at which organic molecules within the compacted biomass material break down to form ash and gases comprising carbon monoxide and hydrogen gases, inhibit combustion of the compacted biomass material when heated within the internal passages of the reactors, conduct the carbon monoxide and hydrogen gases through the reactors in a direction opposite the movement of the compacted biomass through the reactors, and remove the ash from the reactor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/212,624, filed Apr. 14, 2009, and is related to U.S. Provisional Application No. 61/129,352, filed Jun. 20, 2008. The contents of these prior patent applications are incorporated herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     This invention was made with government support under USDA Grant number 68-3A75-7-607. Under 37 CFR §401.14, the Government may have certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to the conversion of organic lignocellulosic material (biomass) into a useful gas-phase fuel, and more particularly to a system and method capable of scalable and continuous conversion of biomass materials into useful gas-phase fuels. 
     Biomass gasification is a well-known process for producing synthesis gas (syngas), which as also known in the art is a gas mixture containing varying amounts of carbon monoxide (CO) and hydrogen gas (H 2 ). Though having a lower energy density than natural gas, syngas is suitable for use as a fuel source. 
     Within a biomass gasifier, a carbonaceous material typically undergoes pyrolysis, during which the carbonaceous material is heated to release volatiles and produce char. Combustion then occurs during which the volatiles and char react with oxygen to form carbon dioxide (CO 2 ) according to the reaction
 
C+O 2 →CO 2  
 
     The next process is the gasification process, during which the char reacts with carbon dioxide and steam (H 2 O) to produce carbon monoxide and hydrogen gas via the reaction
 
C+H 2 O→H 2 +CO
 
     Consequently, the biomass gasification process employs oxygen or air to combust some of the biomass and produce carbon monoxide and energy, the latter of which is utilized to convert the remaining biomass to hydrogen and additional carbon monoxide. 
     Various types of gasifier designs are known. The most common type of gasifier used in biomass gasification is believed to be an up-draft design (counter-current) design, in which air, oxygen and/or steam flows upward through a permeable bed of biomass and counter-currently to the flow of ash and other byproducts of the reaction. Typical up-draft gasifiers have significant technical shortcomings. First, the introduction of air into the hot gasification chamber partly combusts the biomass, yielding a lower overall heating value compared to pure gasification. Second, if air is used as the gasification agent, nitrogen in the air is a diluent that reduces the energy content per unit volume of the output gas, making the output gas inconvenient for use in gas turbines, for storage, and for subsequent chemical processing. Third, tars and phenolic hydrocarbons produced in an up-draft gasifier require removal to reduce emissions, avoid fouling of a gas turbine, and avoid catalyst poisoning when used to create liquid fuels. The removal equipment adds to system complexity and size, with the result that for economic reasons the gasifier is usually limited to large installations. Because biomass is a low-energy content fuel and is dispersed geographically, a large-scale gasifier requires transport and storage of the biomass, which negatively affects the economic payback for the system. 
     If biomass gasification is performed at a sufficiently high temperature, all organic material can be broken down into simple molecules, such as carbon monoxide and hydrogen, and combustion byproducts can be avoided if the organic material is broken down predominantly through anaerobic pyrolysis. The latter is significant because it reduces toxic effluents and simplifies downstream syngas clean-up requirements. As noted above, it is also advantageous to minimize the introduction of air gases, since air is predominantly nitrogen. Achieving high temperatures efficiently without the introduction of air for partial combustion requires a system design which maximizes heat transfer, minimizes heat loss, and provides for continuous operation. These advantages have been realized for small scale systems (for example, up to 10 tons of biomass per day), as reported in U.S. patent application Ser. No. 12/357,788, whose contents are incorporated herein by reference. However, it would be desirable if such advantages could also be realized with a system and process that are scalable to systems that are larger by at least two orders of magnitude, for example, about 1000 to 10,000 tons/day. 
     BRIEF DESCRIPTION OF INVENTION 
     The present invention provides a system and method capable of efficient production of synthesis gas from biomass materials in a manner which can be scaled to relatively large throughput capacities. 
     According to a first aspect of the invention, the system includes multiple parallel reactors each defining an internal passage, and means for compacting a loose biomass material and simultaneously introducing the compacted biomass material into entrances of the internal passages of the reactors. The system further includes means for heating the compacted biomass material within the reactors to a temperature at which organic molecules within the compacted biomass material break down to form ash and gases comprising carbon monoxide and hydrogen gases, means for inhibiting combustion of the compacted biomass material when heated by the heating means within the internal passages of the reactors, gas passages within the internal passages of the reactors for conducting the carbon monoxide and hydrogen gases through the reactors in a direction opposite the movement of the compacted biomass through the reactors, and means for removing the ash from the reactor. 
     According to a second aspect of the invention, the method includes compacting a loose biomass material and simultaneously introducing the compacted biomass material into entrances of internal passages of multiple parallel reactors, heating the compacted biomass material within the reactors to a temperature at which organic molecules within the compacted biomass material break down to form ash and gases comprising carbon monoxide and hydrogen gases, inhibiting combustion of the compacted biomass material when heated by the heating means within the internal passages of the reactors, conducting the carbon monoxide and hydrogen gases through the reactors in a direction opposite the movement of the compacted biomass through the reactors, and removing the ash from the reactor. 
     Other aspects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  represents a perspective view of an array of reactor tubes according to an embodiment of this invention. 
         FIG. 2  represents a cross-sectional view of an array of reactor tubes similar to that shown in  FIG. 1 , but showing the tubes coupled to a feed hopper and to syngas and ash removal passages according to an embodiment of this invention. 
         FIG. 3  is a schematic of a biomass reactor incorporating a neutral atmospheric pressure capability in accordance with another aspect of this invention. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
       FIGS. 1 and 2  represent basic concepts relating to a scalable biomass gasifier system in accordance with preferred aspects of the invention.  FIG. 1  represents four parallel reactor tubes  12  of a scalable biomass gasifier system  10  represented in  FIG. 2 . The reactor tubes  12  serve as the containment vessel in which the gasification process occurs, producing syngas as a desired product and dry ash as a byproduct. In  FIGS. 1 and 2  further represent the tubes  12  configured in an array (linear, planar, or convex surface), into which biomass particles (not shown) are delivered from a bulk hopper  14  via any suitable type of feeder device  16 , represented in  FIG. 2  as an auger though other methods of delivery are also within the scope of the invention, such as through the use of a ram or by gravity feed. The reactor tubes  12  are packed together so that incoming biomass does not enter the interstices  18  between adjacent tubes  12 . For example, a deflector  20  is represented in  FIG. 1  as being located within the interstice  18  at the throats  22  of the tubes  12  to serve as a guide for incoming biomass and prevent biomass from entering the interstice  18 . 
     As evident from  FIG. 2 , the array of reactor tubes  12  are arranged so that their throats  22  lie on a common two-dimensional (2-D) surface (either Euclidian or Riemann). The arrangement of the reactor tubes  12  may be on a rectilinear grid, or may be in a close-packed configuration, or other geometric arrangement as may be necessary for coupling with the biomass hopper  14 . Though shown as round, the throats  22  of the tubes  12  may have a rectilinear cross-section. The throats  22  are further represented as having a flared shape, which is advantageous to minimize backflow of syngas through the tubes by promoting compaction of the biomass as it enters the tubes  12 , as well as to allow for the creation of the interstices  18  between the individual reactor tubes  12  to accommodate heating elements  24 . The heating elements  24  can be of a variety of types, including but not limited to resistance heaters, radiant heaters including heat lamps, plasma heaters and electromagnetic heaters. The heating elements  24  are preferably arranged within the interstices  18  and optionally surrounding the outermost tubes  12  of the array so that the tubes  12 , and particularly the heated zones  28  located near the midsections of the tubes  12 , capture substantially all of the heat energy generated by the elements  24  and the overall temperature of the entire array is as uniform as possible across surfaces parallel to the common 2-D surface of the array. Though the array of tubes  12  are represented in  FIGS. 1 and 2  as horizontally oriented, the tubes  12  can be oriented at various angles with respect to each other and with respect to gravity (vertical). 
       FIG. 2  further shows the reactor tubes  12  fitted with gas lines  26  that gather syngas near the end of each tube  12  farthest from the throats  22  where the biomass entered the tubes  12 . Each gas line  26  is shown as being routed back through the heated zones  28  of the tubes  12  so that any heavy hydrocarbon molecules in the syngas can be cracked into smaller molecules (such as carbon monoxide and hydrogen) by the additional heating achieved with the heating elements  24 . The resulting syngas is then drawn from these gas lines  26 , such as with blowers (not shown), and collected in an outlet passage  30 , after which the syngas may be passed through a particulate filter before being delivered to a holding tank, downstream process, prime mover, fuel cell, or any other suitable destination. Steam may be injected into the tubes  12  to assist in the conversion of char to syngas via the known water-gas shift reaction. The continuous supply of biomass to the tubes  12  pushes the dry ash byproduct of the reaction through the tube  12  and into a manifold  32 , which can employ gravity and/or other conveying means (such as the auger represented in  FIG. 2 ) to remove the ash from the system  10 . The tubes  12  may be optionally sealed to prevent backflow of syngases toward the tube throats  22 , as well as to allow for maintenance. Ports within the reactor tubes  12  that serve as entrances to the gas lines  26  are preferably oriented and located outside the heated zones  18  to reduce the likelihood that biomass material will enter the gas lines  26 . 
     The diameters of the reactor tubes  12  are preferably selected such that the heating elements  24  are as nearly as possible able to uniformly heat the biomass material across the cross-section of the tubes  12  and within the time period required for the biomass material to travel through the tubes  12  and become pyrolized. Thus, the lengths and diameters of the reactor tubes  12  are interdependent based on this common concept. 
     The biomass conveyed from the hopper  14  into the open throats  22  of the reactor tube  12  is preferably size-reduced, as is typically the case for corn stover, wood chips, gin trash, dry distillers grain solids, and mess hall organic waste. The particle size of the biomass material is preferably limited to about one-sixth of the diameters of the reactor tubes  12 . The hopper  14  is shown in  FIG. 2  as being in intimate contact with the flared ends at the throat  22  of each tube  12  within the tube array, such that the common 2-D surface defined by the array is either a boundary of the hopper  14  or protrudes into the hopper  14 . It is advantageous that the amount of biomass material in the hopper  14  be maintained at a sufficient level to ensure that there is always biomass available to every reactor tube  12  in the array, such that backflow of syngas is minimized. The biomass within the hopper  14  may be stirred to maintain the material at a uniform height within the hopper  14 , especially if the common 2-D surface of the tube array is substantially normal to the earth&#39;s surface. Alternatively, it is foreseeable that the reactor tubes  12  could be vertically oriented and the common 2-D surface of the tube array arranged and configured to form the bottom of the hopper  14 , allowing the biomass material to be conveyed downward by gravity and/or other conveying means to the reactor tube throats  22 . 
     As previously noted, a preferred aspect of the invention is to maintain the tubes  12  with what is essentially a plug of biomass material, preferably located within the throat  22  of each reactor tube  12  to inhibit backflow of syngas through the tubes  12 . Plugging the tubes  12  with the biomass also serves to better contain the heat within the reactor tubes  12  to promote the gasification reaction and reduce the risk of a fire in the hopper  14 . The degree to which the tube throats  22  are tapered, the degree to which the feeder devices  16  are capable of packing the biomass material into the throats  22 , and the distance of the feeder devices  16  from the openings of the throats  22  will all affect the axial length and density of the biomass plug within the tubes  12 . It can be appreciated that there may be more than one combination of these three factors which provide the desired or optimal performance in a given configuration. To address the contingency that an individual tube  12  becomes starved of biomass material, the tubes  12  may be equipped with means (not shown) for closing individual tubes  12 . Such closing means may include, but is not limited to, driving the corresponding feeder device  16  into the throat  22  of the starved tube  12  to promote a better seal, provide a knife valve at or near the throat  22  to seal a starved tube  12 , and/or closing a valve (not shown) through which syngas is drawn from the starved tube  12 . Each of these closing means, individually or in combination, may be employed to minimize the risk of fire, minimize back-diffusion of the desired syngas product, and minimize heat loss to promote process efficiency and reduced hazard risks. 
     Further features of the system  10  and of the tubes  12  of the system  10  are discussed below, some of which are similar to or derived from certain process and design parameters reported in U.S. patent application Ser. No. 12/357,788. 
     The temperature of pyrolysis employed by this invention can vary, but preferably ranges from about 850 to about 1150° C. Within the reactor tubes  12 , there is preferably a temperature profile which most effectively converts the solid biomass into syngas. This profile may entail the creation of a two- or three-zone heater arrangement where, for example, biomass encounters a first heating zone after it enters the tube  12  where the biomass is heated to nearly its volatilization temperature (typically around 350° C.), then enters a second heating zone where its temperature is quickly ramped up to the full pyrolysis temperature to effect a rapid cracking of molecules before they can form heavy or toxic compounds. In an optional third heating zone, the temperature is lower than the second zone so that mineral ash remaining after pyrolysis will not form low-melting point glasses that may not flow readily through the reactor tubes  12 . 
     If the primary axes of the reactor tubes  12  are horizontal, it may be advantageous for the axes to tilt downward toward the ends of the tubes  12  opposite their throats  22 . The purpose of this slope is to encourage any gasses, rolling debris, or packed ash to be conveyed to the ash manifold  32  coupled to the ends of the reaction tubes  12 . If the primary axes of the reactor tubes  12  are essentially vertical, it may be advantageous to provide the tubes  12  with one or more spikes (not shown) that project into the interiors of the tubes  12  so that biomass material falling into the tubes  12  impinges the spikes to break up any large biomass chunks as well as impede the flow of biomass material through the tubes  12  and thereby increase the residence time of the biomass material within the hottest (second) zones of the tubes  12 . In addition, a grate (not shown) can be located at or near the base of each spike to assure that little or no biomass material falls entirely through the reactor tube  12  without becoming gasified. 
     To minimize energy input and equipment complexity, the system  10  can be configured to have a neutral atmospheric pressure reactor configuration, such as represented in  FIG. 3 . Such a configuration can be accomplished by utilizing highly sensitive differential pressure sensors  36  at the reactor inlet and/or the ash removal section of the system to monitor and adjust the volumetric rate of gaseous discharge via a blower or vacuum pump used to draw the syngas from the gas lines  26  into the passage  30 . In this manner, the aforementioned plug of biomass material forms a seal at the inlet section  38  of the array of tubes  12 , while also an ash removal system  40  located downstream of the manifold  32  promotes the formation of an ash plug to effectuate a seal within the manifold  32 . This can be done by having a section of pipe where the transport mechanism (piston, auger, etc.) through the manifold  32  is absent and the ash is forced through this portion thus compacting the ash slightly. An alternative is to taper the manifold  32  to force or compact the ash similarly to an extrusion process. The integrity of the seal at a given pressure is a function of leakage rate due to the porosity/composition of the biomass or ash plug. The porosity of the plug can be adjusted by the degree of compaction of the material being transported. This capability is particularly desirable from the stand point of eliminating the need for a lock hopper system to mitigate the ingress of air into the reactor tubes  12  or unwanted leakage of syngas from the ash removal section  40  by ensuring that the system  10  operates within inlet and outlet pressures within certain limits. As should be understood, particularly in view of the foregoing discussion, the ingress of air into the reactor tubes  12  would have an unwanted diluent effect on the syngas produced, thus reducing its heating value and leading to an overall net energy efficiency decrease, while leakage of syngas from the process would introduce potentially significant safety issues and have a net overall decrease in net energy efficiency, especially if the leakage is such as to reduce the energy production capabilities of the system. 
     A closed-loop control system (not shown), with suitable parameters (such as a PID controller or another known to those skilled in these art) may be used to introduce a controlled amount of water or water vapor (including steam) based on properties of the syngas. These properties may include, but are not limited to, the moisture content of the syngas, the amount of liquid condensed out in a condenser, the conductivity of the gas, or other means known to those skilled in the art. There are also means by which the output gas properties, such as pressure or temperature, can be used in a chemical and/or mechanical system to regulate the amount of water introduced. Introduction of the water may be accomplished in many ways, including but not limited to injection, osmosis, control valve, diffusion, or wicking/capillary action. 
     Waste heat generated from the heating elements  24  and lost from the tubes  12  may be harvested and used for a variety of purposes. The gas effluent may also be run through a heat exchanger, heat pipe, or other means of heat transfer to provide heat which can be used to advantage in the overall method. The waste heat can be conveyed in many ways, including but not limited to a working fluid, a heat pipe (single-phase or two-phase), a conductive media such as metal or diamond, by radiation, or by convection of a suitable working fluid. The harvested waste heat may be used, as nonlimiting examples, to dry incoming biomass, heat of the reactor tubes  12 , and heat devices used to remove liquid and/or solid residues from the system. Waste heat may also be harvested in more useful forms, such as for the purpose of running a Stirling engine for mechanical work, operating a thermoelectric cooler (Peltier effect) for electrical power, or used outside the system  10  for essentially any desired purpose. 
     Waste heat, including exhaust gasses from a prime mover or SOFC, can be particularly useful for drying a biomass material that has a high moisture content. Injection of hot, dry air into the hopper  14  could be used for this purpose to obtain several benefits, including driving-off excess moisture in the biomass material and separating or fluffing the biomass material to avoid bridging or rat-holing. 
     While the invention has been described in terms of a specific embodiments, it is apparent that other forms could be adopted by one skilled in the art. For example, the physical configuration of the system  10  and its components could differ from that shown, and materials and processes other than those noted could be used. Therefore, the scope of the invention is to be limited only by the following claims.