Abstract:
A biomass gasifier includes a wall structure defining a gasification chamber having a biomass inlet at an upper end thereof and a char outlet at a lower end. A generally U-shaped char tube has a base and first and second substantially vertical limbs. The first limb of the char tube is positioned to receive char from the char outlet of the gasification chamber and the second limb terminates upwardly at a level above the char outlet of the gasification chamber. A blower has a suction side connected to the second limb of the char tube for inducing a flow of gas through the gasification chamber and the char tube and also has a pressure side for connection to a consuming device.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims benefit of the date of filing of Provisional Application No. 60/807,030, the entire disclosure of which is hereby incorporated herein by reference for all purposes. 
     
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to a biomass gasifier. 
         [0003]    Combustible gas can be obtained from agricultural crop waste and other biomass by a process known as gasification.  FIG. 1  illustrates a conventional downdraft biomass gasifier, based on the disclosure in U.S. Pat. No. 6,647,903. The biomass is typically cellulose based and contains, in addition, volatile organic compounds (VOCs), tars and resins. Ground or chopped biomass is placed in a hopper  2  at the top of a gasification chamber  4  having a restricted outlet  8  at the bottom of the chamber and is ignited. A blower  10  draws air into the gasification chamber to support combustion of the biomass. The blower is controlled so that the rate of supply of oxygen is not sufficient for complete combustion of the biomass. As the biomass is heated, water and volatile compounds are released from the biomass, which forms a smoldering char composed primarily of carbon. The volatile compounds are oxidized, supplying heat to the biomass. Carbon in the char reacts with oxygen in reactions that produce carbon dioxide and carbon monoxide: 
         [0000]      C+O 2 →CO 2 +heat 
         [0000]      2C+O 2 →2CO+heat  (1) 
         [0004]    Subsequently, carbon dioxide reacts with carbon in the char to produce carbon monoxide in a highly endothermic carbon reaction: 
         [0000]      Heat+C+CO 2 →2CO  (2) 
         [0005]    Gasification thus produces a gas that contains CO and can be used as fuel. The fuel gas is discharged at the pressure side of the blower  10 . Incombustible ash is discharged downward from the gasification chamber through a char tube  12  that terminates in an ash pit. The ash that accumulates in the ash pit partially blocks the char tube and prevents excessive air entering the gasification chamber. As ash is removed from the vicinity of the lower end of the char tube, more char and ash can enter the char tube at its upper end, permitting fuel to enter the gasification chamber from the hopper. 
         [0006]    The conventional downdraft gasifier shown in  FIG. 1  is subject to disadvantage due to bridging of the fuel and char in the gasification chamber, preventing proper feeding of biomass to the zone at which the carbon reaction occurs. Further, in operation of the conventional downdraft gasifier, flow of gas through the char tube may create channels such that the gas passes through the char tube too rapidly to permit the carbon reaction (2) to occur. In addition, the composition of gas delivered to the carbon reaction zone may be inconsistent, because the temperature distribution in the gasification chamber is not consistent. 
       SUMMARY OF THE INVENTION 
       [0007]    According to the present invention there is provided a biomass gasifier comprising a wall structure defining a gasification chamber having a biomass inlet at an upper end thereof and a char outlet at a lower end, a generally U-shaped char tube having a base and first and second substantially vertical limbs, the first limb of the char tube being positioned to receive char from the char outlet of the gasification chamber and the second limb terminating upwardly at a level above the char outlet of the gasification chamber, and a blower having a suction side connected to the second limb of the char tube for inducing a flow of gas through the gasification chamber and the char tube and also having a pressure side for connection to a consuming device. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: 
           [0009]      FIG. 1  is a vertical sectional view of a conventional gasifier, 
           [0010]      FIG. 2  is a vertical sectional view of a gasifier embodying the present invention, and 
           [0011]      FIG. 3  is a sectional view taken on the line  3 - 3  in  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    The gasifier shown in  FIGS. 2 and 3  has walls that surround a gasification chamber  20  and an adjacent gas exit chimney  24 . At its upper end, the gasification chamber  20  is provided with a hopper  26  for feeding ground or chopped biomass to the gasification chamber and at its lower end, the gasification chamber is provided with a downwardly tapering funnel  28  that is lined with refractory material and leads to a generally U-shaped char tube  30  located in an ash receptacle  32  provided with a drain valve  34 . 
         [0013]    Air inlet tubes project into the gasification chamber in three zones, namely an upper zone  36  about the center of the chamber, an intermediate zone  38  somewhat below the center of the chamber, and a lower zone  40  just above the entrance to the char tube. The air inlet tubes open into the gasification chamber for introducing air into the gasification chamber and are provided with valves  46  for controlling flow of air into the tubes. 
         [0014]    Vibrators  50  are provided for agitating the contents of the gasification chamber. A vibrator may comprise a motor that is attached to a wall of the gasification chamber and an out of balance weight that is rotated by the motor for vibrating the wall of the chamber. Alternatively, a vibrator may comprise a poker that projects into the gasification chamber and is driven to move in reciprocating or other periodic fashion by a motor located outside the chamber. Temperature probes (not shown) may be provided in the gasification chamber to enable an operator to monitor changes in temperature at the different levels in the gasification chamber. 
         [0015]    The char tube  30  has an inclined base  52 , one vertical limb  54  extending upward from the base into the lower end of the funnel  28  and an opposite vertical limb  56  extending upward from the base into the chimney  24  and terminating at above the lower end of the funnel. The inclined base of the char tube extends downward from the lower end of the limb  54  to a location beyond the lower end of the limb  56  and at which the char tube is provided with a controllable outlet valve  58 . The base  52  of the char tube is formed with small weep holes for draining water from the char tube into the ash receptacle  32 . 
         [0016]    The gas exit chimney is connected to the suction side of a blower  62  via a suction tank  66  provided with a drain valve  68 . The pressure side of the blower  62  is connected to a pressure tank  70 , having a gas outlet connected through a valve  72  to one or more consuming devices (not shown) and also having a drain outlet  74 . The blower  62  induces a flow of gas from the gasification chamber through the char tube and the gas exit chamber and delivers the gas to the pressure tank  70 . 
         [0017]    Operation of the gasifier is started by loading ground or chopped cellulose-based biomass, such as agricultural waste, into the gasification chamber by way of the hopper to a suitable level, which may be above the lower end of the hopper, inserting a heat source into the gasification chamber, and energizing the vibrators and the blower. The heat source may be, for example, a propane torch that is inserted into the gasification chamber by way of one of the air inlet tubes. The heat source ignites the biomass. The blower induces a flow of air into the gasification chamber through the air inlet tubes, onward through the char tube and the gas exit chamber. The air flow induced by the blower provides oxygen to support combustion of the biomass. 
         [0018]    The temperature distribution in the gasification chamber depends on the distribution of the supply of air to the gasification chamber. If the temperature at a given location in the gasification chamber is sufficient for the biomass to smolder, supply of air to that location will accelerate the exothermic oxidation reaction, thus increasing the temperature, whereas if the supply of air is discontinued the reaction is starved of oxygen and the smolder may be extinguished. By providing several air inlet tubes at different levels in the gasification chamber, it is possible to control the temperature distribution in the gasification chamber. 
         [0019]    After first igniting the biomass, the air supply valves  46  are controlled to supply air principally through the bottom air inlet tube  76  to promote the combustion reaction in the lower zone  40 , just above the entrance to the char tube  30 . Radiant heat from the reacting char heats the biomass that is above the lower zone  40 . Sufficient air is supplied to the upper zone  36  that the temperature in the upper zone attains a sufficient level that water and VOCs present in the biomass are vaporized. Some of the VOCs may be combusted in oxygen present in the upper zone. The temperature established in the intermediate zone  38  is sufficient to complete evaporation of volatile compounds and tars and may also complete the combustion of volatile compounds and partially combust the tars and resins, leaving a hot carbon char. 
         [0020]    The heat generated in the intermediate and lower zones is sufficient to sustain the reactions in those zones and to supply heat to the biomass in the upper zone for evaporating water and low temperature volatile compounds and partial consumption of those compounds. 
         [0021]    In the lower zone  40 , sufficient air is supplied that the temperature reaches a level of 1370° C. (2500° F.) or higher. In this zone, any remaining tars and resins are incinerated such that the major components of the gas entering the char tube are CO, CO 2  and water vapor, both from the biomass and from the incoming air. Some of the carbon in the char is oxidized, leaving a residue of incombustible ash containing some unreacted carbon. 
         [0022]    In the char tube, the carbon dioxide reacts with previously unreacted carbon to generate carbon monoxide in accordance with reaction (2). Some water vapor also reacts with the carbon in the char tube: 
         [0000]      Heat+C+H 2 O→CO+H 2   (3) 
         [0000]    Both these reactions are highly endothermic and accordingly the temperature of the gas falls from over 1300° C. to about 57° C. (135° F.). Water vapor that does not react in accordance with reaction (3) condenses at the bottom of the char tube. Some of the condensed water drains into the ash pit through the weep holes in the char tube and water that does not drain into the ash pit drains down the char tube to the drain valve  58 , which is periodically opened to allow the water to drain from the char tube. 
         [0023]    The char that is created by the reactions described above is finely divided and behaves substantially in the manner of a liquid. The char fills the base and the taller limb of the char tube to a height above the inlet to the shorter limb  54 . The gas flow leaves the gasification chamber through the char tube by permeating through the bed of finely divided char (and ash that may also be present). Because of the nature of the char and ash, gas channels are not created in the base of the char tube. 
         [0024]    The taller limb  56  of the char tube diverges upward, allowing expansion of the gas into the gas exit chimney. The flow of gas is sufficiently vigorous that the most of the ash is entrained in the gas flow leaving the char tube. Some ash settles from the gas flow into the ash pit. The ash that falls into the ash pit forms a slurry with water that drains from the char tube. The ash slurry can be removed from the ash pit through the drain valve  34 . Most of the ash that is entrained in the gas flow is carried into the suction tank  66 , in which it settles. Residual water vapor condenses in the suction tank. Ash slurry can be removed from the suction tank by the drain valve  68 . 
         [0025]    The blower  62  draws a steady flow of air (containing insufficient oxygen for complete combustion of the biomass) into the gasification chamber with the contents of the hopper and gasification chamber serving as a throttle so that air enters preferentially through the air inlet tubes. Due to the reactions that occur in the gasification chamber, the composition of the gas progressively changes as oxygen is removed and reaction products are added to the gas flow. 
         [0026]    Establishing three distinct zones of operation in the gasification chamber is important to ensuring that the gas delivered to the char tube has a consistent composition. Thus, by progressively heating the biomass and incinerating tars and resins, the only reaction that is needed at the third zone is oxidation of the carbon in the char so that the gas entering the char tube contains only carbon monoxide, carbon dioxide and water vapor. Remaining water vapor and entrained ash precipitate from the gas in the suction tank  66 , and accordingly the gas delivered to the pressure tank  70  is clean and dry. The drain valve  74  can be used to remove liquid or solid material from the pressure tank should the need arise. 
         [0027]    The vibrators keep the biomass and char in an agitated state, preventing formation of bridges and gas channels, and 
         [0028]    It will be appreciated that the invention is not restricted to the particular embodiment that has been described, and that variations may be made therein without departing from the scope of the invention as defined in the appended claims, as interpreted in accordance with principles of prevailing law, including the doctrine of equivalents or any other principle that enlarges the enforceable scope of the claims beyond the literal scope. Unless the context indicates otherwise, a reference in a claim to the number of instances of an element, be it a reference to one instance or more than one instance, requires at least the stated number of instances of the element but is not intended to exclude from the scope of the claim a structure or method having more instances of that element than stated.