Abstract:
A downdraft gasifier is disclosed. The gasifier includes a biomass section that accepts and stirs raw biomass materials, a pyrolysis and tar cracking section having an inner cylinder for receiving biomass and an outer surrounding cylinder for gases from the biomass, and a char gasification section for receiving biomass and gases from the pyrolysis and tar cracking section. The char gasification section provides a grating and scraper for passing gases and ash and retaining biomass for char gasification on the grate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 11/773,617, entitled “DOWNDRAFT GASIFIER WITH INTERNAL CYCLONIC COMBUSTION CHAMBER”, filed Jul. 5, 2007. 
     This application claims the priority of U.S. Provisional Patent Application No. 61/076,180, entitled “GASIFICATION OF SWITCHGRASS USING A DOWNDRAFT REACTOR,” filed Jun. 27, 2008, the contents of which are hereby incorporated by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with U.S. Government support under USDA/CSREES Grant No. 00-52104-9662, USDA/CSREES Grant No. 2001-34447-10302, USDA/CSREES Grant No. 2002-34447-11908, USDA/CSREES Grant No. 2003-34447-13162, USDA/CSREES Grant No. 2004-34447-14487, USDA/CSREES Grant No. 2005-34447-15711, USDA/CSREES Grant No. 2006-34447-16939, and USDA/CSREES Grant No. 2008-34447-19201 awarded by the Department of Agriculture and under DOT/OST Grant No. DTOS59-07-G-0053 awarded by the Department of Transportation. The Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     This disclosure relates to gasification of biomass materials in general and, more specifically, to gasification by downdraft gasifiers. 
     BACKGROUND OF THE INVENTION 
     Biomass may be converted into useful gas products such as carbon monoxide (CO), carbon dioxide (CO 2 ), hydrogen (H 2 ), and others. There are multiple processes by which the raw biomass materials may be gasified. These include pyrolysis, tar cracking, and char gasification. Heating the biomass material under the proper circumstances such that the desired gases are released without being oxidized or otherwise consumed is one commonality among certain of the various gasification methods. 
     In order to obtain useful quantities of gases from raw biomass material, the gasification process must be implemented in such as way as to operate in a steady state. The desirable gases, or production gases, should more or less be output at a steady rate. Improper handling and processing of the biomass can result in a suboptimal amount of the raw biomass being gasified. Unacceptably high levels of undesirables can also be produced and taint the output gases if the production process is not controlled. 
     SUMMARY OF THE INVENTION 
     The invention disclosed and claimed herein, in one aspect thereof, comprises a downdraft gasifier. The gasifier includes a biomass section that accepts and stirs raw biomass materials, a pyrolysis and tar cracking section having an inner cylinder for receiving biomass and an outer surrounding cylinder for gases from the biomass, and a char gasification section for receiving biomass and gases from the pyrolysis and tar cracking section. The char gasification section provides a grating and scraper for passing gases and ash and retaining biomass for char gasification on the grate. 
     In some embodiments, the biomass section is arranged superior to the pyrolysis and tar cracking section, and the pyrolysis and tar cracking section is arranged superior to the char gasification section. In some embodiments, the inner cylinder defines a plurality of perforations on at least a portion thereof. A biomass feeding unit may selectively provide biomass through an airlock to the biomass section. 
     A cyclone separator may remove particulate from the gas leaving the char gasification section. An ash chamber may be provided below the char gasification section that catches ash and solid matter falling through the grate. An ash conveyor may remove ash from the ash chamber to a remote ash chamber. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating one embodiment of a gasification system according to aspects of the present disclosure. 
         FIG. 2  is a schematic diagram illustrating one embodiment of a gasification combustion chamber for use with the gasification system of  FIG. 1 . 
         FIG. 3  illustrates an exemplary temperature profile of a downdraft gasifier constructed according to  FIG. 1 . 
         FIG. 4  illustrates the pressure drop and volumetric concentrations of various output gases from a gasifier constructed according to  FIG. 1 . 
         FIG. 5  is a flow diagram illustrating an embodiment of a gasification process according to the present disclosure. 
         FIG. 6  is a schematic diagram illustrating another embodiment of a gasification system according to aspects of the present disclosure. 
         FIG. 7 . illustrates a temperature profile of the gasification system of  FIG. 6 . 
         FIG. 8 . illustrates the variation of gas composition with time for the gasifier of  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to  FIG. 1 , a schematic diagram illustrating one embodiment of a gasification system according to aspects of the present disclosure is shown. The gasifier system  100  comprises three primary components: a biomass feeding unit  102 ; a combustion chamber  104 ; and a separator  106 . These primary components may further comprise a number of subcomponents, which will be described in detail below. The system  100  is operable to accept biomass as an input product and provide useful gases as an output product. The producer gas may be a mixture of carbon monoxide (CO), carbon dioxide (CO 2 ), hydrogen (H 2 ), and possibly other gases. In one embodiment, the gasification system  100  operates to convert biomass material into the desired gases by means of pyrolysis and tar cracking. This result may be achieved by creating high temperatures within the combustion chamber  104 . This causes the biomass material to break down into a number of materials, including ash and gases. 
     The biomass feeding unit  102  accepts the biomass intake product for processing by the system  100 . Biomass materials suitable for use with the system  100  may include, but are not limited to, woodchips, sewage or sludge, and refuse from the processing of plant matter. The gasification system may also operate using input biomass from plants grown with the specific purpose of being fed into the gasification system  100 . 
     The biomass feeding unit  102  comprises a hopper  108  and an agitator  110  with an agitator drive unit  112 . The dimensions and specific shape of the hopper  108  may vary in accordance with the needs of the end user. In the present embodiment, the hopper  108  has a tapered cylindrical shape. The agitator  110  may be a bladed or impellor type agitator or another type of agitator suitable for the biomass used with the gasification system  100 . It is also understood that stirrers, conveyors, or other implements could be used to ensure ready delivery of biomass material into the gasifier  100 . In the present embodiment, where the agitator  110  is a rotational agitator, the agitator drive unit  112  may be selected according to the duty cycle and torque requirements necessary to agitate the chosen biomass material. Some embodiments will provide a variable speed agitator. The agitator may be selectively operable such that it operates only when needed to insure proper feeding of the biomass. 
     In the present embodiment, a screw drive  114  serves to move biomass from the hopper  108  to an airlock  118 . In the present embodiment, a screw drive  114  is powered by a screw drive powering unit  116 . The screw drive powering unit  116  may be pneumatic, electrical, or powered by another source. The screw drive may be selectively operable and/or of variable speed so that feeding of the biomass may be properly controlled. In other embodiments, the screw drive  114  may be replaced with other conveyance means, such as conveyor belt, a slip stick movement device, or another suitable conveyance. 
     The air lock  118  serves to control the intake of biomass from the hopper  108  to the rest of the gasification system  100 . The air lock  118  also serves to prevent back flow of the gases from combustion chamber  104 . The airlock  118  may be electrically or mechanically powered. The airlock  118  may be remotely controllable, such as with an electronic relay. 
     Beyond the airlock  118  is another screw drive  120 . The screw drive  120  is powered by another screw drive power unit  122 . These may be similar to the screw drive  114  and screw drive powering unit  116 . As before, in embodiments other than the one shown in  FIG. 1 , the screw drive  120 , as well as the screw drive powering unit  122 , could be replaced with other conveyance means. In some embodiments, the airlock  118 , agitator  110 , and the screw drives  114 ,  120  will operate in concert to ensure proper delivery of biomass to the combustion chamber  104 . 
     When the biomass material leaves the biomass feeding unit  102 , it is fed into the combustion chamber  104 . The combustion chamber  104  provides a number of additional steps in the gasification process, which will be described in more detail below. A biomass section  124  may be provided near the top of the combustion chamber  104 . In one embodiment, the biomass section  124  serves to guide or direct the entering biomass material into the remainder of the combustion chamber  104 . 
     A stirrer  128  may be provided starting at the biomass section  124 . The stirrer may proceed further into the depths of combustion chamber  104 . The stirrer  126  may be made from a suitably heat resistant material able to withstand high temperatures necessary in the combustion chamber  104 . Blades or other agitating means may be provided on the stirrer  126 . The stirrer  126  is powered by a stirrer drive unit  128 . The stirrer drive unit may once again be electrical, pneumatic, mechanical or powered by another source. The biomass section  124  may be cylindrical, conical, or may have another shape. In one embodiment, the shape of the biomass section  124  serves to feed biomass material at the appropriate speed and volume down into a tar cracking section  130 . 
     The tar cracking section  130  may be generally cylindrical in shape and may provide an inner chamber  135 , defined by an inner cylindrical wall  132 . The inner wall  132  and an outer wall  134  may define an annular outer chamber  133 . It can be seen that the inner wall  132  may also feature perforations  134  that aid in the heating of the biomass material. As solid biomass in the inner chamber  135  ispyrolysed, the gases may escape the inner chamber  135  through the perforations  134  in the inner wall  132  into the annular chamber  133 . 
     It can be seen that, in the embodiment shown, the stirrer  126  proceeds at least part of the way through the inner chamber  135 . In this way, stirring or agitation is provided starting at the biomass section and proceeding through at least a portion of the tar cracking section  130 . This reduces and/or eliminates hot spots that would prevent efficient pyrolysis and tar cracking within the combustion chamber  104 . 
     In the present embodiment, the combustion chamber  104  is heated in part by the combustion of propane. The propane heating may only be necessary to initiate the gasification process. In the present embodiment, propane enters through the fuel inlet  136  into the combustion chamber  104  where it may be ignited to produce heat. Although propane is used in the present example, it is understood that other fuel sources may be utilized, including but not limited to, natural gas, refined fuels, and other petroleum products. 
     It may be important to carefully control oxygen content within the combustion chamber  104 . An air inlet  138  is provided for oxygenating the environment of the combustion chamber  104 . An additional function of the air inlet  138  may be to provide heated air for furthering the gasification processes of the system  100 . Some embodiments will provide a heater  140  for preheating the air entering the combustion chamber  104 . The heater  140  may be gas or electrical powered or, in some embodiments, may be based off of the waste heat generated by another outside process. In some embodiments, the heater  140  will preheat the air to up to 300° C. or greater. A compressor  142  may also be provided for delivering the air into the combustion chamber  104  at the appropriate pressure. Pressurizing the ambient air will also heat the air to a certain degree, which may be useful in the gasification process. The compressor  142  can be electrical, pneumatic, or powered by another source. In the present embodiment, the heater  140  follows the compressor  142  resulting in higher efficiencies resultant from the heater  140  operating on compressed, and therefore hotter, air. 
     Various components of the system  100 , may be insulated for increased efficiency or productivity. For example, the air inlet  138  may be insulted. Similarly, all or a portion of the combustion chamber  104  may be insulated. In one embodiment, a ceramic wool blanket insulation (not shown) of about 25 mm thickness will be utilized. In other embodiments, different materials that are suitably heat resilient may be utilized. Additionally, the thickness of any insulation used may be varied based upon a number of factors including the desired reaction temperature, the ambient air temperature, efficiency concerns, and others. 
     Below the tar cracking section  130  is a char gasification section  144 . In the present embodiment, the char gasification section  144  is separated from the tar cracking section by an annulus  146 . This component may be optional depending upon the nature of the biomass material being utilized. In the present embodiment, the annulus  146  serves to guide the partially gasified biomass into the char gasification section  144 . 
     The biomass material in the char gasification section  144  falls down onto a grating  148 . The grating  148  serves as a separation step to separate the solid material from the gases created in the combustion chamber  104 . It can be seen that the raw gases and ash are allowed to escape via a conduit  152  and travel to the separator  106 . The remaining solid biomass material will remain trapped by the grating  148  where additional char gasification will occur. As the biomass further gasifies, the ash and gases produced will pass through the grating and out the conduit  152 . 
     It can be seen in  FIG. 1  that the biomass section  124 , the tar cracking section  130 , and the char gasification section  144  may be arranged in a generally vertical fashion. The present embodiment provides the tar cracking section  130  in between the biomass section  124  and the char gasification section  130 . In this configuration, gravity may serve to feed the biomass through the combustion chamber resulting in down draft type gasification process. The combustion and gasification in the combustion chamber  104  may serve to create swirls, vortices, and other cyclonic gas flows. These may be controlled and/or aided by the stirrer  126  and perforations  133  in the inner chamber wall  132  of the tar cracking section  130 . This serves to prevent cold spots in the combustion chamber  104 , particularly as the size of the process is scaled up. 
     The configuration of the combustion chamber  104  also helps to ensure substantially complete transformation of the biomass material into gases and ash. The gases will include producer gas and possibly waste gas. The ashes will contain substantially no organic material. Nevertheless, as a practical consideration, means may be provided for clearing any solid material captured on the grating  148  that is not consumed by char gasification. In one embodiment, this may be an access portal  150  located near the grating  148  on the char gasification section  144  of the combustion chamber  104 . The access portal  150  may also allow for servicing, inspection, and/or replacement of the grating  148  and other components on the interior of the combustion chamber  104 . 
     The separation section  106  provides a separator  154  for separating the production gas from the ash in the raw gas stream coming from the conduit  152 . In one embodiment, the separator  154  is a cyclonic separator, but other separators may be utilized. The separator may be mechanical and may be electrically, pneumatically, or otherwise powered. The separated production gas is removed by the outlet  156 . The present embodiment illustrates a burner  158  that consumes the production gas coming from the outlet  156 . Thus, heat and other power may be provided for another process. However, it is understood that the production gas may be stored, utilized in a different manner, or further refined downstream of the gasification system  100 . A storage chamber  160  is provided for catching and/or holding the ash from the separator  154 . The ash may be useful in other processes and can therefore be retained until needed. In the present embodiment, an access portal  162  is provided for periodically removing the ash from the storage chamber  160 . It is understood, however, that other means may be utilized, such as conveyor belts or screw drives. 
     Referring now to  FIG. 2 , a schematic diagram illustrating one embodiment of a gasification combustion chamber for use with the gasification system of  FIG. 1  is shown. It should also be noted that this combustion chamber may also be utilized with the gasification system of  FIG. 6  discussed below. It can be seen that the combustion chambers  200  and  104  are similar. Once again, a three-section embodiment is shown. The sections or chambers include the biomass section  124 , the tar cracking section  130 , and the char gasification section  144 . A stirrer  126  is provided, driven by a stirrer drive unit  128 . The fuel inlet  136  is shown, along with the air inlet  138 . A grating  148  is provided near the bottom end of the char gasification section  144 . Gases and ash escape through the gas conduit  152 . It will be appreciated that the combustion chamber  200  may be utilized in the gasification system  100  of  FIG. 1 , directly replacing the combustion chamber  104  illustrated in  FIG. 1 . 
     As has been described, in one embodiment biomass is provided to the combustion chamber  200  through a biomass feeding unit. Biomass enters the combustion chamber  200  through an inlet  202 . In  FIG. 2 , a biomass column  204  is illustrated to show one possible route for the biomass material through the combustion chamber  200 . It can be seen that the stirrer  126  may serve to stir the biomass  204 . As before, propane gas is introduced through the inlet  136 . In the present embodiment, the propane is supplied near the top of the tar cracking section  130 , and is used only for initial firing at start up of the process. 
     The tar cracking section  130  is once again formed by inner cylindrical walls  132  and an outer cylindrical wall  134 . An inner chamber  135  is bounded by the inner wall  132  and an annular chamber  136  is formed between the inner wall  132  and outer wall  134 . In the present embodiment, the entirety of the inner chamber  132  is provided with perforations  134 . Various degrees of perforation of the inner chamber  132  may be utilized depending upon the raw biomass material being utilized. Some embodiments may provide for an adjustment of the degree of perforation using a sleeve or other means, for example. In the present embodiment, tar loaded pyrolysis gases are allowed to escape from the biomass  204  column through the perforations  134  where they are mixed with preheated air from the air inlet  138 . The pressurized gas entering the tar cracking section  130  provides high temperature turbulence and swirling combustion flows, allowing tar cracking to occur. 
     The high temperature combustion products being produced in the tar cracking section  130  feed through the annulus  146  into the char gasification section  144 . In the present embodiment, the char gasification section  144  provides for additional biomass decomposition by char gasification reactions. In some embodiments, temperatures of up to 1200° C. are attained in the char gasification section  144 . 
     It can thus be appreciated that biomass entering the combustion chamber  200  will undergo a continuous process whereby the gasification process begins as early as the biomass section  124 . As the biomass is consumed, it is allowed to fall with the aid of the stirrer  126  into the tar cracking section where a majority of the pyrolysis of the process may occur. As the partially consumed biomass exits the tar cracking section  130 , it is allowed to fall downward into the gasification chamber  144  where it may land on the grating  148 . In some embodiments, the reaction of remaining biomass in the column  204  continues on the grating  148 . Gases and heat escaping downward through the combustion chamber  104  and out through the conduit  152  provide energy for the char gasification process on the grating  148 . Thus, a substantially complete reduction process will occur such that gases and essentially inorganic material, or ash, are allowed to flow freely through the conduit  152 . 
     Table 1 shows the characteristics of pine wood pellets that may be used as a feedstock (biomass) for operation of the gasification system of the present disclosure. Table 2 illustrates a summary of a number of gasification tests conducted utilizing a system constructed in accordance with  FIG. 1 . The table includes the temperatures reached by various locations within the system  100 , as well as the gases produced in percentage by volume thereof. It can be seen that, in some of the tests, tar content and particulates were measured. Efficiency and mass balance percentages are also shown. The mass balance percentages may not add up to exactly 100 due to measurement limitations and rounding errors in equipment. 
     Referring now to  FIG. 3 , an illustration of an exemplary temperature profile of a downdraft gasifier constructed according to aspects of the present disclosure is shown. The measurements of  FIG. 3  were taken with a gasifier built according to the present disclosure. Referring also to  FIG. 4 , the pressure over time of various output gases from the gasifier is shown. With reference to  FIGS. 3 and 4 , it can be seen that within 60 min from system start time, the gasifier system operation was stabilized.  FIG. 4  reveals that, throughout the test period of three hours, concentration levels of all gases were stable. The present embodiment produces gases with a heating value in the range of 1277 to 1423 kcal/m 3 . Volumetric CO, H 2 , and CO 2  concentrations are in the range of 21-23%, 11-13%, and 13-13.5% percent, respectively. Tar cracking zone temperatures were maintained close to 1000° C. Hot gas efficiency ranged from 63 to 81 percent. Average producer gas flame temperatures were approximately 780° C. Tar and particulate contents in the raw producer gas were in the range of 5 to 12 g/m 3  and 0.4 to 0.45 g/m 3 , respectively. It can be seen that the results corresponding to the performance of a gasifier constructed according to the present disclosure are comparable to the performance of a conventional throat type downdraft gasifier. This relationship is illustrated for reference in Table 3. 
     Referring now to  FIG. 5 , a flow diagram illustrating one method of a gasification process according to the present disclosure is shown.  FIG. 5  illustrates a simplified version of one gasification method that may be accomplished by the systems of the present disclosure. At step  502 , biomass is input to the system. At step  504 , the biomass will be stirred and heated. Stirring could be done in a biomass chamber, for example. Heating could be accomplished by a propane flame and/or heated air, or by other means. Pyrolysis begins at step  506 . However, it is understood that stirring and heating may continue even as pyrolysis occurs. 
     At step  508  tar cracking occurs. As before, it is understood that pyrolysis may still be occurring when tar cracking has begun. Stirring and heating of the biomass as shown at step  504  may also still be occurring. With reference back now to  FIG. 1 , it can be seen in the combustion chamber  104  of the system  100  that stirring and heating at  504 , pyrolysis at step  506 , and tar cracking at step  508  may be simultaneously and/or continuously occurring. 
     Char gasification begins at step  510 . Although char gasification is illustrated as the last of the actual gasification steps, referring again to  FIG. 1 , it will be clear that the char gasification at step  510  can occur simultaneously with stirring and heating at step  504 , pyrolysis at step  506 , and/or tar cracking at step  508 . 
     Following the reduction of substantially all of the biomass through pyrolysis, tar cracking, and/or char gasification, the raw gases will be separated from the ash contained therein at step  512 . Following removal of the ash at step  512 , the gas may be output at step  514 . As previously described, the output gas may have a number of uses, such as immediate consumption, storage, and/or further refining. 
     Referring now to  FIG. 6 , a schematic diagram illustrating another embodiment of a gasification system according to aspects of the present disclosure is shown. It can be seen that the system of  FIG. 6  is similar in some regards to the system of  FIG. 1  described above. Differences between the embodiments will be discussed herein. The gasifier system  600  comprises a biomass feeding unit  601 , a multi-stage combustion chamber  614 ′; and a separator  634 . Combustion chamber  614  has an inner lining of high temperature refractory. The biomass feeding unit  601  comprises a hopper  602  and a stirrer  604 . The hopper  602  of the present embodiment is cylindrical in shape. 
     A screw drive  604  serves to move biomass from the hopper  602  to an airlock  606 . As with previous embodiments, the air lock  606  serves to control the intake of biomass from the hopper  601  to the rest of the gasification system  600  and serves to prevent unwanted gases (e.g., air) from entering the combustion chamber  614 . Another screw drive  608  delivers biomass to the combustion chamber  614 . As with previous embodiments, the screw drives could be replaced with other conveyance means and may be air powered, electrically powered, or power by other mechanical means. 
     In the present embodiment, the gasification reactor or combustion chamber  614  comprises a biomass section  610  near the top, a pyrolysis and tar cracking (PTC) zone  622  near the middle, and a char gasification chamber  624  near the base. Similar to previous embodiments discussed with regard to  FIGS. 1 and 2 , the PTC zone  622  comprises a twin cylinder unit extended downward to the top of the gasification chamber  624  with an annular space between the cylinders. The inner cylinder is perforated and holds the biomass column. Tar-loaded pyrolysis gases enter into the annular space. Air (possibly compressed and/or heated as in  FIG. 1 ) enters an inlet  620  and is tangentially mixed with the pyrolysis gases. High temperatures in the PTC zone  622  also facilitate biomass pyrolysis. Propane gas, or other fuel, may be supplied at a gas inlet  618  near the top of the PTC zone  622  for initial firing. 
     A suitably heat resistant stirrer  612  may be provided starting at the biomass section  605  and proceed into the PTC zone  622 . It can be seen that, in the embodiment shown, the stirrer  126  proceeds at least part of the way through the inner chamber  135 . In this way, stirring or agitation is provided starting at the biomass section  610  and proceeding through at least a portion of the PTC zone  622 . This reduces and/or eliminates hot spots that would prevent efficient pyrolysis and tar cracking within the combustion chamber  104 . Various components of the system  600  may also be insulated for increased efficiency or productivity. For example, in the present embodiment, the gasification reactor  614 , piping, and a cyclone separator  634  are insulated with a 25-mm thick ceramic wool blanket. 
     The char gasification section  624  may be separated from the PCT zone  622  by an annulus  623 . In the present embodiment, the annulus  623  serves to guide the partially gasified biomass into the char gasification section  624 . In the present embodiment, the biomass material in the char gasification section  624  may be stirred by a stirrer  626 . This may help break up any large chunks of biomass material remaining as the biomass falls down onto a grating  628 . The grating  628  serves as a separation step to separate the solid material from the gases created in the combustion chamber  104 . The grating  628  may be a wire mesh and may also be provided with a rotating scraper  630 . The rotating scraper may provide a circular opening in the center (not shown). Remaining biomass material may be further reduced to gases and ash on the grating  628 . 
     Raw gases and ash will pass through the grating  628 . Ashes will tend to fall into the ash chamber  632  while gases may be drawn into the cyclonic separator  634 . Here, particulates remaining in the gas stream may be removed. Separated production gas may be consumed by a burner  158 . Thus, heat and other power may be provided for other processes. However, it is understood that the production gas may be stored, utilized in a different manner, or further refined downstream of the gasification system  600 . 
     A tar and particulate measurement system  636  may be provided for monitoring the gases leaving the cyclonic separator  634 . Further testing of the producer gas can be conducted using a device such as a gas chromatograph. In order to properly monitor and control the system  600 , various other sensors may be placed at needed locations. Without limitation, these may include temperature and pressure probes, mass flow meters, thermocouples, and rotational sensors. 
     Ash that is collected in the ash chamber  632  may be removed by screw conveyor  640  to a remote ash storage chamber  642 . Here the ash may be stored until discarded or removed for use in another process. 
     The embodiment of  FIG. 6  should increase CO and H 2  concentrations and reducing CO 2  relative to other gasification methods. For testing of the device shown in  FIG. 6 , switchgrass, at approximately 11.6% dry basis moisture content, was chopped using a Haybuster H-1000 tub grinder (DuraTech Industries International, Inc. Jamestown, N.D.) using a screen with a 25-mm hole size. For bulk density determination, the material was poured into a 473-ml container from 100 mm above the container. The bulk density was determined by dividing the weight of the material by the container volume. Biomass proximate and ultimate analyses were performed by Hazen Research Inc, Golden, Colo. 
     Test preparation started with loading 5 kg of wood charcoal onto the grate  630 . The gasification reactor  614  was then completely filled with chopped switchgrass. The hopper  602  was also kept full with the biomass. The gasifier  600  was preheated using propane for about five minutes. When the temperature in PTC zone  622  reached approximately 600° C., preheating was discontinued. The desired air flow was then set. Within thirty minutes, the reactor temperature profile stabilized. 
     During each test, biomass fuel level in the gasification reactor  614  was maintained by intermittently operating the biomass feeding system  601 . Reactor temperature profile, temperature of the producer gas at the exit of the cyclone reactor and that of the flame, pressure drops across the gasification reactor and the whole system, air flow rate, and amount of biomass loaded before and during the tests were closely monitored. The maximum test duration was six hours. Producer gas sampling began once the system was stabilized as indicated by the reactor temperature profile. For gas analysis, samples were taken every 10-15 minutes. At the end of each experiment, solid residues remaining in the reactor and in the particulate chamber and the biomass remaining in the hopper were quantified to estimate the fuel consumption rate and to determine the overall mass balance. Gas flow rate was determined by a nitrogen balance. The gas calorific values were determined using the volumetric gas composition values from gas chromatograph and the theoretical heating values of all the combustible components. Gasifier efficiencies, equivalence ratios and mass balances were calculated as follows:
 
 CGE=[PCE /( DBE+ASE )]*100   Eqn. 1
 
 HGE=[(PCE+PSE )/( DBE+ASE )]*100   Eqn. 2
 
 ER =AIR/( DBIR*STADB )   Eqn. 3
         Where,       

     CGE=Cold gas efficiency, % 
     HGE=Hot gas efficiency, % 
     ER=Equivalence ratio 
     PCE=Chemical energy in dry producer gas, kcal/h 
     PSE=Sensible energy in dry producer gas, kcal/h 
     DBE=Dry biomass energy, kcal/h 
     ASE=Hot air sensible energy, kcal/h 
     AIR=Air input, Nm 3 /h 
     DBIR=Dry biomass input, kg/h 
     STADB=Stochiometric air requirement for dry biomass, Nm 3 /kg of dry biomass
 
Mass balance, %=(Total mass out/Total mass in)*100   Eqn. 4
 
     Table 4 shows the characteristics of switchgrass used in the study. Chopped switchgrass is a low bulk density biomass with ash content and elemental composition comparable to most of the crop residues. Low bulk density poses major challenge to ensure proper material flow in the reactor and the hopper. Agitators have been used to facilitate the material flows in the biomass hopper and the gasification reactor. 
     The major operating parameters and results of the gasification tests are presented in Table 5.  FIGS. 7 and 8  show typical cases of temperature and gas composition profiles. Within one hour from system start-up, the gasifier operation was stabilized. The tar cracking temperatures were between 1003 and 1110° C. Gas components of greatest interest (volume basis) were CO: 19.2-24.4%, H 2 : 9.7-12.0%, CO 2 : 7.9-13.7% and CH 4 : 2.5-4.5%. Dry product gas yield ranged from 1.7 to 1.8 Nm 3 /kg dry biomass. Specific gasification rates varied from 507 to 736 m 3 /h of dry gas per square meter combustion zone area. Hot gas and cold gas efficiencies were: 63-89% and 52-78%, respectively. Average producer gas flame temperatures were around 8000C. The lower heating value of the gas ranged from 1160 to 1673 kcal/Nm 3 . 
     Among the four levels of specific air input rates (kg of air/h-sq. m of combustion zone area) tested to date, 542 kg/h-sq. m of combustion zone area resulted in the highest system performance: average values for hot gas and cold gas efficiencies of 89% and 72% respectively; lower heating value of gas: 1566 kcal/Nm 3 ; and CO, H 2  and CO 2  concentrations: 23%, 12% and 9%, respectively. The corresponding average specific gasification rate was 663 cu. m dry gas/h-sq. m of combustion zone area. As the specific air input rate increased to 647 kg/h-sq. m of combustion zone area, CO 2  concentration increased 14% while the CO and H 2  concentrations decreased (19 and 10% respectively). The average lower heating value of gas also decreased up to 1160 kcal/Nm 3 . The corresponding specific gasification rate was 736 cu. m dry gas/h-sq. m of combustion zone area. Specific air input rate of 542 kg of air/h-sq. m of combustion zone area provided optimal reaction environment in the gasifier for CO 2  and water vapor reactions with carbon, and as a result produced gas with higher levels of CO and H 2  concentrations. At this level of specific air input, the gas tar and particulate content at the gasifier exit were: 18 and 2.5 g/Nm 3 , respectively. For wood pellets based gas these values were 5-12 g/Nm 3  and 0.4-0.45 g/Nm 3 , respectively [4]. 
     Lower bulk density and higher volatiles in the chopped switchgrass as compared to wood pellets, is one reason for higher levels of tars. Another major reason for higher levels of tars in the gas is the shifting of high temperature zone downward below the PTC section  622  because of the low density nature of the chopped biomass. In general, the system performance was consistently good regarding CO and H 2  concentrations and gasification efficiencies as shown in Table 5. The differences in the mass balance closure figures is attributed to measurement errors in collection and quantification of the incoming and outgoing streams of the gasifier system. 
     Among the four levels of specific air input rates, a level of 542 kg/h-sq. m of combustion zone area resulted into highest performance: average values for hot gas and cold gas efficiencies of 82% and 72% respectively; lower heating value of gas: 1566 kcal/Nm 3 ; and CO, H 2  and CO 2  concentrations: 23%, 12% and 9%, respectively. The corresponding average specific gasification rate was 663 cu. m dry gas/h-sq. m of combustion zone area. 
     As the specific air input rate increased to 647 kg/h-sq. m of combustion zone area, CO 2  concentration increased 14% while the CO and H 2  concentrations decreased 19 and 10% respectively. CO and H 2 % increased up to 24% &amp; 12% (by volume), respectively while CO 2 % decreased from earlier concentration of 18% to 8%. 
     
       
         
               
             
               
               
               
             
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Wood pellet characteristics 
               
               
                   
               
             
             
               
                 Proximate, 
               
               
                 (weight %, dry basis) 
               
             
          
           
               
                   
                 Moisture content 
                 7.5 ± 0.1 
               
               
                   
                 Volatile matter 
                 82.2 ± 0.6  
               
               
                   
                 Fixed carbon 
                 17.6 
               
               
                   
                 Ash 
                  0.2 ± 0.03 
               
               
                   
                 Higher heating value, kcal/kg a   
                 5075 
               
             
          
           
               
                 Ultimate a   
               
               
                 (weight %, dry basis) 
               
             
          
           
               
                   
                 Carbon C 
                 52.13 ± 1.7  
               
               
                   
                 Hydrogen H 
                 6.36 ± 0.3  
               
               
                   
                 Oxygen O 
                 41.23 
               
               
                   
                 Nitrogen N 
                 0.07 ± 0.03 
               
               
                   
                 Sulphur S 
                 0.01 
               
               
                   
                 Diameter (mm) 
                 6.0 
               
               
                   
                 Length (mm) 
                 10-35 
               
               
                   
                 Bulk density (kg/m 3 ) 
                 660 
               
               
                   
                   
               
               
                   
                   a BIOBIB. 1992. A database for biofuels. Available at: www.vt.tuwien.ac.at/Biobib/biobib.html. Accessed 8 May 2006. 
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Summary of typical gasification operation 
               
             
          
           
               
                   
                 Test 1 
                 Test 2 
                 Test 3 
                 Test 4 
               
               
                   
                   
               
             
          
           
               
                 Equivalence ratio 
                 0.18 
                 0.21 
                 0.23 
                 0.17 
               
               
                 Fuel feed rate, kg/h 
                 17.0 
                 14.8 
                 13.0 
                 18.1 
               
               
                 Input air temperature, C. 
                 216 ± 4  
                 205 ± 3  
                 216 ± 17 
                 219 ± 4  
               
               
                 Tar cracking zone (TCZ) 
                 854 ± 43 
                 896 ± 38 
                 866 ± 48 
                 800 ± 48 
               
               
                 temperature, (Ave.), C. 
               
               
                 TCZ temp. (Max.), C. 
                 966 
                 1001.7 
                 1002 
                 975 
               
               
                 Char gasification (CG) 
                 706 ± 38 
                 770 ± 22 
                  556 ± 208 
                 708 ± 50 
               
               
                 chamber top, Ave., C. 
               
               
                 CG chamber top, (Max), C. 
                 793 
                 819 
                 786 
                 844 
               
               
                 CG chamber mid, (Ave.) C. 
                 742 ± 27 
                 790 ± 26 
                  607 ± 181 
                 731 ± 25 
               
               
                 CG chamber mid, (Max.) C. 
                 789 
                 827.7 
                 768 
                 769 
               
               
                 Gas temperature after cyclone 
                 352 ± 4  
                 383 
                 350 ± 7  
                 356 ± 26 
               
               
                 separator, C. 
               
               
                 Flame temp. (Ave.), C. 
                 770 ± 25 
                 780 ± 31 
                 777 ± 30 
                 777 ± 24 
               
               
                 Flame temp. (Max.), C. 
                 813 
                 843.4 
                 829 
                 829 
               
               
                 Pressure drop across 
                 11.0 ± 0.6 
                 12.0 ± 0.4 
                 10.4 ± 0.4 
                 10.4 ± 0.3 
               
               
                 gasifier, Inch of water 
               
             
          
           
               
                 Gas composition, % vol. 
               
             
          
           
               
                 CO 
                 22.7 ± 0.9 
                   21 ± 0.9 
                 21.2 ± 2.1 
                 21.6 ± 1.3 
               
               
                 H 2   
                 10.9 ± 1.6 
                 11.9 ± 2.3 
                 11.6 ± 1.7 
                 12.4 ± 2.2 
               
               
                 CH 4   
                  3.4 ± 0.7 
                   3 ± 0.7 
                  3.1 ± 0.8 
                  3.6 ± 1.1 
               
               
                 CO 2   
                 13.4 ± 0.9 
                 13.3 ± 1.1 
                 13.4 ± 0.6 
                 13.1 ± 1.0 
               
               
                 N 2   
                 48.8 ± 1.7 
                 50.3 ± 1.8 
                   50 ± 2.1 
                 48.3 ± 3.5 
               
               
                 C 2 H 2   
                 ND* 
                  0.1 ± 0.2 
                 ND* 
                  0.2 ± 0.4 
               
               
                 C 2 H 4   
                  0.5 ± 0.1 
                  0.4 ± 0.2 
                  0.5 ± 0.1 
                  0.7 ± 0.3 
               
               
                 C 2 H 6   
                  0.2 ± 0.3 
                  0.1 ± 0.1 
                  0.1 ± 0.3 
                  0.1 ± 0.1 
               
               
                 LHV gas (kcal/Nm 3 ) †   
                 1369 
                 1277 
                 1293 
                 1423 
               
               
                 Dry gas yield (Nm 3 /kg) 
                 1.69 
                 1.88 
                 2.16 
                 1.60 
               
               
                 Tar content, g/Nm 3   
                 Not 
                 7.5 
                 5 
                 12 
               
               
                   
                 measured 
               
               
                 Particulates, g/Nm 3   
                 Not 
                 0.45 
                 0.4 
                 0.4 
               
               
                   
                 measured 
               
               
                 Hot gas efficiency, % 
                 63.2 
                 71.6 
                 80.7 
                 60.5 
               
               
                 Cold gas efficiency, % 
                 56.3 
                 63 
                 71.9 
                 54 
               
               
                 Mass balance, % 
                 98 
                 101 
                 105 
                 94 
               
               
                   
               
               
                 *Not detected; 
               
               
                   † Nm 3  refers to a cubic meter of gas at a standard temperature of 0° C. and pressure of 1 atm 
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Gasifier performance comparison with other published 
               
               
                 data on conventional downdraft gasification systems 
               
             
          
           
               
                   
                 Air-to-fuel 
                   
                   
                   
               
               
                   
                 ratio, 
                 Tar cracking 
                 % Volume 
               
             
          
           
               
                 Feedstock 
                 Nm 3 /kg 
                 Temp, ° C. 
                 CO 
                 H 2   
                 Tar, g/Nm 3   
               
               
                   
               
             
          
           
               
                 Hazelnut 
                 1.46 
                 1050 
                 21 
                 13.1 
                 3.0 
               
               
                 shells 
               
               
                 Sewage 
                 2.3 
                 1077 
                 10.6 
                 10.9 
                 6.26 
               
               
                 sludge 
               
               
                 Wood chips 
                 Equivalence 
                 1000 
                 24 
                 14 
                 No data 
               
               
                   
                 ratio of 0.38 
               
               
                 Pine wood 
                 Equivalence 
                 1000 
                 21 
                 12 
                 5.0 
               
               
                 pellets (this 
                 ratio of 0.23 
               
               
                 study) 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
             
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Switchgrass characteristics 
               
             
          
           
               
                   
                 % db 
               
               
                   
                   
               
             
          
           
               
                   
                 Moisture content 
                 11.6 
               
               
                   
                 Carbon 
                 49.67 
               
               
                   
                 Hydrogen 
                 5.27 
               
               
                   
                 Oxygen 
                 40.31 
               
               
                   
                 Nitrogen 
                 0.57 
               
               
                   
                 Sulphur 
                 0.07 
               
               
                   
                 Ash 
                 4.11 
               
               
                   
                 Lower heating value, kcal/kg 
                 4118 
               
               
                   
                 Bulk density, kg/m 3   
                 138 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                 Summary of results for gasification tests 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Specific air input rate, 
                 437 
                 542 
                 542 
                 647 
               
               
                 kg of air/h-sq. m 
               
               
                 Specific gasification rate, 
                 507 
                 639 
                 688 
                 736 
               
               
                 cu. m dry gas/h-sq. m 
               
               
                 Equivalence ratio 
                 0.22 
                 0.22 
                 0.20 
                 0.23 
               
               
                 Fuel feed rate, kg/h 
                 12.8 
                 16.7 
                 18.4 
                 19.7 
               
               
                 Air temperature, C. 
                 16.0 ± 0.8 
                 10.1 ± 1.1 
                 25.0 ± 3.0 
                 20.0 ± 2.0 
               
               
                 Tar cracking temp. (CG1), C. 
                 1078 ± 101 
                 1050 ± 76  
                 1003 ± 135 
                 1110 ± 107 
               
               
                 CG1 temperature (max.), C. 
                 1261 
                 1169 
                 1318 
                 1336 
               
               
                 Temperature below grate, C. 
                 355 ± 91 
                  374 ± 120 
                 360 ± 57 
                 441 ± 69 
               
               
                 Flame temperature, C. 
                 717 ± 49 
                 768 ± 34 
                 764 ± 50 
                 813 ± 30 
               
               
                 Flame temperature (max.), C. 
                 791 
                 815 
                 920 
                 871 
               
               
                 Pressure drop across 
                  4 ± 3 
                  9 ± 5 
                 15 ± 6 
                 16 ± 8 
               
               
                 gasifier, cm of water 
               
               
                 CO, % vol. 
                 23.5 ± 3.2 
                 24.4 ± 1.3 
                 22.1 ± 4.3 
                 19.2 ± 1.6 
               
               
                 H 2 , % vol. 
                 10.9 ± 1.9 
                 11.4 ± 0.7 
                 12.0 ± 1.4 
                  9.8 ± 1.2 
               
               
                 CH 4 , % vol. 
                  3.9 ± 0.6 
                  3.3 ± 0.5 
                  4.5 ± 0.2 
                  2.5 ± 0.7 
               
               
                 CO 2 , % vol. 
                  7.9 ± 1.9 
                  7.8 ± 1.1 
                 11.2 ± 0.8 
                 13.7 ± 0.4 
               
               
                 N 2 , % vol. 
                 52.9 ± 1.2 
                 52.1 ± 2.1 
                 48.5 ± 3.5 
                   54 ± 2.4 
               
               
                 LHV gas, kcal/Nm 3   
                 1437 
                 1458 
                 1673 
                 1160 
               
               
                 Dry gas yield, kg/kg of dry 
                 1.8 
                 1.8 
                 1.7 
                 1.7 
               
               
                 biomass 
               
               
                 Hot gas efficiency, % 
                 79 
                 75 
                 89 
                 63 
               
               
                 Cold gas efficiency, % 
                 69 
                 65 
                 78 
                 52 
               
               
                 Mass balance, % 
                 89 
                 87 
                 93 
                 89 
               
               
                   
               
             
          
         
       
     
     Thus, the present invention is well adapted to carry out the objectives and attain the ends and advantages mentioned above as well as those inherent therein. While presently preferred embodiments have been described for purposes of this disclosure, numerous changes and modifications will be apparent to those of ordinary skill in the art. Such changes and modifications are encompassed within the spirit of this invention as defined by the claims.