Abstract:
A gasifier for the manufacture of producer gas has a chamber ( 10 ) for the combustion of material, the chamber ( 10 ) being provided with a plurality of tuyeres ( 30 ) for the introduction of combustion air to the chamber. The tuyeres ( 30 ) are disposed at an acute angle to the wall of the chamber ( 10 ) so that air is directed both around and upwardly of the chamber. Each tuyeres ( 30 ) is configured to produce a jet stream wherein the air swirls along the length of the stream.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is the U.S. National Stage Application of International Application PCT/GB99/01671, filed May 27, 1999, which international application was published on Dec. 2, 1999 as International Publication WO99/61561 in the English language. The International Application claims priority of Great Britain Application 9811415.0, filed May 28, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to gasifiers and in particular to improvements in or relating to gasifiers whereby improved combustion may take place therewithin. The invention further relates to a method of operating a gasifier. 
     The production of producer gas (a mixture of about 30% carbon monoxide and 70% nitrogen, though other gases may also be present) is usually performed with a so-called gasifier, in which pyrolysis is performed. The gasifier comprises a chamber provided with air inlets in such a way that the burning of fuel within the chamber take place under controlled conditions. Originally, gasifiers used coal or coke as a fuel source, but interest in gasifiers has recently increased since they may be used for the disposal of various kinds of waste organic solid matter whilst yielding producer gas, which in turn may drive a combined heat and power unit, to yield both heat and electricity. Thus, the solid fuel may typically comprise wood or wood derivatives, straw, poultry litter, dried sewage sludge and refuse-derived combustible material, to mention but a few. 
     In a gasifier chamber, the solid fuel is reduced to a bed of carbon at a temperature of above 1000° C., a stream of air being passed through the bed with the combustion conditions set so that the oxygen in the air combines with the carbon to form carbon monoxide. Other gases such as methane and hydrogen may also be produced, depending upon the chemical composition of the fuel employed. 
     A product of the combustion in a gasifier is ash, but this is of relatively low density and of small volume compared to the solid fuel supplied to the gasifier. It is consequently relatively easy to dispose of, especially since it is wholly sterile. By contrast, the producer gas may be used for a variety of purposes, though since it is toxic in view of the carbon monoxide content, it must nevertheless be treated with care. For example, the collected producer gas may immediately be used in an internal combustion engine for the generation of electricity, without being stored for long periods or otherwise handled. 
     Though the principle upon which a gasifier operates is well known and understood, it is important that the combustion conditions are closely controlled in order that the production of carbon monoxide is optimised and that the carbon dioxide content of the producer gas is minimised. It has now been established that by controlling the actual introduction of air into the oxidation zone of a gasifier, it is possible to improve the gasifier performance, so increasing the overall efficiency of plant using this equipment. 
     Gasifier designs have been proposed in NL-A8900939 and CH-A-237348. In both of these prior designs, arrangements are made to introduce combustion air into a gasifier in such a way that the air is not directed strictly radially with respect to the axis of the combustion chamber, in an attempt to enhance combustion within the chamber. The present invention stems from attempts further to improve the production of producer gas. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, there is provided a gasifier for the production of producer gas from combustible material, comprising a chamber into which said material is introduced and a plurality of tuyères disposed at or adjacent the lower end of the chamber for the introduction of air into the combustion chamber, each such tuyère defining an axis along which air is projected into the chamber, at least some of the tuyères being configured to impart a rotational motion about said axis along which the air is projected into the chamber from each such tuyère, whereby the combustion air is projected in the form of a jet stream which swirls about the length of the stream. 
     Preferably, at least some of the tuyères are disposed with their respective axes at an acute angle (when projected on to a horizontal plane containing the respective tuyère) to a tangent to the wall of the chamber at the location of that tuyère, the axis of projection of each said tuyère being in the same sense with respect to the axis of the chamber, whereby the projected air also tends to swirl around the chamber. 
     It will be appreciated that the gasifier of this invention is able to achieve better reduction of the fuel, by improving the interaction between the introduced air and the hot carbonised bed at the bottom of the gasifier chamber. This interaction may be achieved either solely by causing the introduced air to rotate along the axis of projection into the gasifier, or by additionally causing the introduced air to swirl around the lower region of the chamber. By adopting both of these measures of this invention, the interaction of the air with the bed at the bottom of the gasifier is much enhanced, so giving better control of the combustion of the fuel. In turn, this leads to more complete combustion in the oxidation zone, giving a greater oxygen deficiency in the reduction zone and raising the proportion of carbon monoxide in the resultant producer gas. 
     Most preferably, all of the tuyères lie at substantially the same acute angle to the chamber wall where the respective tuyère projects therethrough. Thus, by having the angle of projection of each said tuyère in the same sense with respect to the axis of the chamber, the introduced air tends to swirl around the chamber interacting with all portions of the relatively hot carbon bed. This effect is enhanced by providing the tuyères in a uniform distribution equi-spaced around the chamber. 
     Advantageously, each tuyère is directed both at a non radial angle to the axis of the chamber and also upwardly of the chamber. In this way, penetration of the entire hot carbon bed by the introduced air may be assured. 
     The chamber may be of general circular cross-sectional shape, at least in the region of the tuyères, with the axis of the chamber extending generally vertically. That chamber may have a lower wall of a generally conical shape and which supports a bed of the combustible material, said tuyères being mounted in that lower conical wall. At the other end of the chamber, there may be provided an inlet orifice for combustible material, the upper portion of the chamber serving as a hopper for the material loaded thereinto. The inlet orifice advantageously is fitted with a slide valve, to permit charging of the hopper whilst the gasifier is in operation. 
     Each tuyère is preferably in the form of a nozzle projecting through the chamber wall, the bore of the nozzle being configured to cause the air flow therethrough to rotate about its length. This may be achieved by providing an insert within each said nozzle, the insert comprising a plate the width of which is substantially the same as the nozzle internal diameter and the plate being twisted along its length. 
     This invention extends to a method of operating a gasifier for the production of producer gas from combustible material, which gasifier comprises a chamber into which said material is introduced and a plurality of tuyères disposed at or adjacent the lower end of the chamber, in which method air is introduced into the combustion chamber through the tuyères and each tuyère is configured to impart a rotational motion about the axis of introduction of the air so that the combustion air is projected in the form of a jet stream which swirls about the length of the introduced stream. In addition, the invention provides for the air being introduced through tuyères at least some of which have their respective axes at an acute angle (when projected onto a horizontal plane containing the respective tuyère) to a tangent to a wall of the chamber at the location of that tuyère, whereby the air will tend to swirl around the chamber, simultaneously with the rotation of the air about its own axis. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       By way of example only, one specific embodiment of gasifier constructed and arranged in accordance with the present invention will now be described in detail, reference being made to the accompanying drawings in which: 
         FIG. 1  is a diagrammatic vertical section through the embodiment of the gasifier; 
         FIG. 2  is a plan view on the lower wall of the gasifier chamber shown in  FIG. 1 , with parts removed for clarity; 
         FIG. 3  is a detailed view on an enlarged scale through said lower wall; 
         FIG. 4  is an end-view on a tuyère of the gasifier of  FIGS. 1  to  3 ; and 
         FIG. 5  illustrates an insert plate of the tuyère nozzle of FIG.  4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring initially to  FIG. 1 , there is shown diagrammatically an embodiment of gasifier arranged for the production of producer gas from a solid combustible material serving as a fuel, such as wood chippings, logs, coal or similar materials, poultry litter, dried sewage sludge or a refuse derived fuel. The gasifier comprises a combustion chamber  10  having a generally conical lower wall  11  provided with a flange  12  around its upper periphery. A hopper  13  has a corresponding flange  14  at its lower periphery and which is secured by bolts (not shown) to flange  12  of the lower wall  11 . The upper end of the hopper  13  is closed by a slide valve assembly  15 , which permits recharging of the hopper with more solid fuel whilst operation of the gasifier continues. An actuator  15 A is mounted to one side of the hopper, to effect opening and dosing of the slide valve assembly. 
     In an alternative arrangement (not shown) the hopper has a simple lid which may be secured in position and a fuel feed arrangement may be provided to supply fuel into the upper part of the hopper. 
     The lower wall  11  is carried on a base structure  16  which defines a plenum chamber  17  provided with an air inlet  18  and six lighting ports  19 , each normally closed by a respective cap  20 , the ports  19  being equi-spaced around the plenum chamber  17 . The central region of the lower wall  11  communicates through opening  22  with a tube  23  within which the producer gas is formed during operation of the gasifier, the tube  23  leading to a lower chamber  24 . A producer gas outlet pipe  25  passes through an outer wall  26  of the lower chamber  24  and there is provided a port  27  to that chamber, normally closed by a blanking plate  28  but through which access to the chamber may be gained for example for ash removal and servicing. 
     An automated ash removal system is fitted below the tube  23 . This comprises an eccentric grate assembly  40  mounted on a shaft  41  rotatably supported below the base wall of the lower chamber  24 . The shaft  41  also carries a scraper bar  42  having chains which serve to plough collected ash into a discharge chute  43  below lower chamber  24 . A motor  44  is drivingly connected by chain  45  to the shaft  41  to effect rotation of both the grate assembly  40  and the scraper bar  42 . A discharge auger  46  takes discharged ash from the chute  43 , through a water seal provided at the bottom of that chute. 
     Air enters the plenum  17  through inlet  18  and passes into the combustion chamber  10  through a plurality of tuyères  30 , provided in the conical lower wall  11  of the chamber. As best seen in  FIG. 2 , six such tuyères  30  are provided, equi-spaced around the opening  22  in alignment with the lighting ports  19 . The tuyères all lie at substantially the same angle to the vertical axis  31  of the combustion chamber  10  and also all lie at substantially the same angle to a horizontal radius of the chamber intersecting the respective tuyère. Thus, air entering the combustion chamber will tend to swirl around the chamber in a counter-clockwise direction and at the same time to rise upwardly within the chamber. 
     Each tuyère  30  is fitted with an insert  33  so as to impart a spin on the jet of air issuing from the tuyère into the combustion chamber  10 . The insert is in the form of a plate having a width substantially equal to the diameter of the tuyère, as shown in  FIG. 4 , and is twisted through 90° along its length. In this way, the air passing through the tuyère will be caused to swirl about the axis of the tuyère. 
     In operation, solid fuel pieces are loaded into the hopper  13  through the slide valve assembly  15  and then the air flow is commenced by reducing the pressure at the producer gas outlet  25 . This draws air through inlet  18  into the plenum  17 , the air then swirling around that plenum  17  and assisting cooling of the lower plate  11 , when operation of the gasifier has been established. From the plenum  17 , the air is drawn through the tuyères  30  into the bed on the lower wall  11  and down through opening  22 , tube  23  and into the lower chamber  24  by the reduced pressure at the producer gas outlet  25 . The gasifier is lit through at least one of the ports  19 , for example with a gas torch, and the combustion within the chamber  10  is established. When fully operational, the upper region  35  of the gasifier will be at around 500° C. and will serve as a distillation zone for high molecular weight hydrocarbons. Below that, there is a carbonisation zone  36  operating at around 600° C., where the solid fuel is converted to charcoal, by burning off other matter. On and immediately above the lower wall  11 , there is established an oxidation zone  37 , operating at around 1200° C., where the carbon is burned in air to form CO 2 . The hot carbon then falls through opening  22  into tube  23  and on to grate  40 , there being a deficiency of oxygen in the tube  23  to continue the combustion of the carbon and so a reduction process takes place, reducing the CO 2  to CO. The final producer gas mixture leaves the lower chamber  24  through pipe  25 . 
     During operation of the gasifier, the motor  47  is operated intermittently slowly to drive the grate assembly  40 . The eccentricity of this assembly grinds any large pieces which then fall through the lower chamber  24  and are ploughed into the discharge chute  43 . The motor may be operated continuously, depending upon the ash content of the fuel source. 
     A typical producer gas composition obtained from using wood as a fuel source may be as follows: 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 GAS 
                   % by weight 
               
               
                   
                   
               
             
             
               
                   
                 Nitrogen 
                     45-54 
               
               
                   
                 carbon monoxide 
                 18-25 
               
               
                   
                 hydrogen 
                 13-15 
               
               
                   
                 water vapour 
                 10-15 
               
               
                   
                 carbon dioxide 
                  5-10 
               
               
                   
                 methane 
                 3-5 
               
               
                   
                   
               
             
          
         
       
     
     By adopting the measures as described above concerning the disposition of the tuyères and also the internal configuration thereof, and so improving the introduction of air into the oxidation zone through the tuyères, it is found the carbon dioxide content may significantly be reduced, with a consequent improvement in the carbon monoxide content of the producer gas.