Patent Publication Number: US-2016238238-A1

Title: Combustion Chamber for Burning Solid Fuels

Description:
TECHNICAL FIELD 
     This invention relates to a combustion chamber for burning solid fuels having a volatiles content and, in particular, to a combustion chamber for burning solid fuels having a high volatiles content, and also to a fire chamber incorporating such a combustion chamber. 
     By solid fuels having a high volatiles content in this context is meant solid fuels such as wood pellets, pellets made from switchgrass, miscanthus, maize stalks, straw or the like, and nut shells such as almond shells, all collectively referred to as solid biomass fuels, and also fossil fuels such as peat or bituminous coal. In contrast, an example of a low volatiles solid fuel would be anthracite coal. 
     BACKGROUND ART 
     In recent years, worldwide concern has arisen relating to climatic changes ascribed to the increase in atmospheric carbon dioxide released by fossil fuels. The price of such fuels has been increasing, and projections as to the remaining world supplies of such fossil fuels have led to increased interest in the development of devices utilising alternative fuels. Furthermore, use of renewable fuels in such devices could slow down the increase in carbon dioxide levels in the atmosphere. 
     Devices, which burn wood are known. However, cord wood burns inefficiently in conventional combustion chambers and is inconvenient to use in comparison with oil or gas burning devices. 
     The use of biomass fuels is also increasing. However, again, biomass fuel tends to burn inefficiently in conventional devices and this is due to its high volatiles content. Much of the heat is released into the burning gases (the flames) and is lost up the chimney or flue. 
     Another reason that conventional boilers and stoves are unsuitable for burning pelleted biomass fuels such as wood pellets, wood chips, etc., is that such pellets tend to smoulder at reduced output particularly when they have a relatively high moisture content. Consequently devices have been developed for burning these fuels more efficiently. 
     Thus, for example, a typical wood pellet stove includes a hopper, an auger, a firebox or grate, a combustion fan and a heat exchanger which, respectively, store, feed, burn the fuel and transfer the heat to the space to be heated. The auger operates in a timed manner to control the delivery of the pellet fuel from the hopper into the firebox. The rate of delivery of the fuel to the firebox is matched to the rate of consumption of the fuel for a particular output. The combustion fan provides a measured amount of air to the firebox. An example of such a stove is the Pellet stove Mod. 1000 manufactured by Caminetti Montegrappa s.r.l of via A. da Bassano, July 9, 36020 Pove Del Grappa (VI), Italy. 
     However, a problem with such stoves and boilers is that the transfer of heat from the burning volatiles is relatively low, particularly at low output, and acceptably high efficiencies can only be achieved by passing the flue gases through extensive heat exchangers. 
     It is an object of the present invention to overcome the disadvantages of the devices hereinbefore described. 
     DISCLOSURE OF INVENTION 
     Accordingly, the invention provides a combustion chamber for burning solid fuels having a high volatiles content, the combustion chamber comprising an enclosed hollow body in which the fuel is to be burnt, the body having a fuel inlet, a primary air inlet, a secondary air inlet and a volatiles outlet mounted therein, the volatiles outlet having a plurality of apertures, such that, in use, the burning volatiles exit the apertures in a turbulent flow resulting in efficient combustion of the volatiles. 
     An advantage of the combustion chamber according to the invention is that the volatiles have to exit the chamber via the apertures in the volatiles outlet and this causes an increase in the velocity of the burning volatiles as they exit. It also causes the turbulent flow of volatiles. The result is that the volatiles burn more efficiently and at a higher temperature than is achieved in a conventional device burning the same fuel. A consequence of this greater efficiency is that the levels of harmful products, such as the nitrous oxides, in the flue gases are reduced relative to known devices. 
     A combustion chamber according to the invention can be used in devices such as boilers, air heaters and stoves, in hotplates, in devices for providing a source of heat for an industrial process, in an incinerator or the like. 
     Preferably, the apertures are of differing sizes. 
     Having apertures of differing sizes in the volatiles outlet results in more efficient mixing of the volatiles in the turbulent flow leading to more efficient combustion. 
     The apertures can be arranged in a particular pattern so as to optimise the performance of the combustion chamber in a particular application. 
     Further, preferably, the secondary air inlet is adjacent the volatiles outlet. 
     The positioning of the secondary air inlet can be important for a particular application of the combustion chamber and such positioning will affect the characteristics of the turbulent flow exiting from the volatiles outlet. 
     In one embodiment of the invention the volatiles outlet has means for temporarily restricting the flow of volatiles therethrough. 
     An advantage of the restricting means is that efficient combustion can be maintained at varying outputs. Thus, at low output the volatiles outlet can be restricted so as to maintain a turbulent flow of volatiles therethrough. 
     Preferably, upstanding formations on the outer surface of the body adjacent the apertures are shaped so as to direct the emerging volatiles along the outer surface. 
     By diverting the burning volatiles along the outer surface of the body the body is maintained at a higher temperature which leads to more efficient combustion of the fuel. This is a requirement at low output particularly when the fuel has a relatively high moisture content. 
     In circumstances where the problem of high moisture content in the fuel is acute, the primary air can also be preheated by the burning volatiles by passing the primary air supply duct through or against the burning volatiles. 
     In a further embodiment of the invention, two or more volatiles outlets are mounted in the hollow body. 
     The provision of a number of volatiles outlets results in more even distribution of the heat from the burning fuel. 
     This arrangement also lends itself to maximising the efficiency of the combustion chamber at all available outputs, while restricting the emission of oxides of nitrogen by reducing the peak temperature of combustion. 
     In another aspect of the invention there is provided a fire chamber for a device for burning solid fuels having a high volatiles content, the fire chamber comprising a housing, a combustion chamber within the housing, the combustion chamber having an enclosed hollow body in which the fuel is to be burnt, the body having a fuel inlet, a primary air inlet, a secondary air inlet and a volatiles outlet mounted therein, the volatiles outlet having a plurality of apertures, such that, in use, the burning volatiles exit the apertures in a turbulent flow resulting in efficient combustion of the volatiles. 
     By employing the combustion chamber with the volatiles outlet, which causes a turbulent flow in the emerging volatiles, the fire chamber is heated more efficiently than a conventional fire chamber consuming a similar amount of fuel. 
     Preferably, an inner heat transferring surface of the housing is shaped about the combustion chamber so as to define a channel therebetween, such that, in use, the burning volatiles exiting the apertures are forced to circulate around the combustion chamber within the channel. 
     An advantage of this arrangement is that the heat released by the volatiles is transferred both to the heat transferring surface and back into the combustion chamber. This provides a means for increasing the temperature of the zone into which the fresh fuel is introduced, particularly at low output, resulting in an increase in the overall temperature of combustion. 
     In one embodiment of the fire chamber according to the invention, a set of upstanding curved formations is mounted on the inner heat transferring surface around the combustion chamber within the channel. 
     The shape and positioning of the set of upstanding curved formations within the channel further directs the circulation of the volatiles around the combustion chamber and also enhances the turbulent flow of the volatiles. 
     In a further embodiment of the fire chamber according to the invention, the apertures are of differing sizes. 
     In a further embodiment of the fire chamber according to the invention, the secondary air inlet is adjacent the volatiles outlet. 
     In a further embodiment of the fire chamber according to the invention the volatiles outlet has means for temporarily restricting the flow of volatiles therethrough. 
     Preferably, upstanding formations on the outer surface of the body adjacent the apertures are shaped so as to direct the emerging volatiles along the outer surface. 
     In a further embodiment of the fire chamber according to the invention, two or more volatiles outlets are mounted in the hollow body. 
     This arrangement results in the efficient circulation of the burning volatiles around the combustion chamber. 
     At certain outputs, the tail of the flame emerging from each volatiles outlet will run into the flame emerging from the next volatiles outlet. This arrangement can provide a means for achieving complete combustion at the tail of each flame. It can also cause a reduction in the peak temperature of combustion, thereby preventing the formation and emission of nitrous oxides. 
     Preferably, the volatiles outlets are arranged equidistantly around the surface of the hollow body. 
     The equidistant arrangement of the volatiles outlets optimises the flame merging effect described above. 
     Alternatively, the volatiles outlets are positioned about the surface of the hollow body so as to facilitate the optimisation of the turbulent flow of volatiles thereabout for each output setting. 
     Thus, for a particular output setting the volatiles flow through a selected number of the volatiles outlets could be restricted so as to optimise the flame merging effect while minimising the emission of nitrous oxides. 
     In a further embodiment of the fire chamber in accordance with the invention, means for moving the combustion chamber towards and away from the inner heat transferring surface is provided. 
     An advantage of this arrangement is that when a low output is required the combustion chamber can be moved closer to the inner heat transferring surface. This has the effect of restricting the flow of volatiles through the volatiles outlet. Conversely, when a higher output is required the combustion chamber can be moved further away from the inner heat transferring surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be further illustrated by the following description of embodiments thereof, given by way of example only with reference to the accompanying drawings in which: 
         FIG. 1  is a vertical section through a boiler containing a combustion chamber and a fire chamber in accordance with the invention; 
         FIG. 2  is a horizontal section on line II-II of  FIG. 1 ; 
         FIG. 3  is a vertical section through a second embodiment of a combustion chamber in accordance with the invention; 
         FIG. 4  is a vertical section through a third embodiment of a combustion chamber in accordance with the invention; and 
         FIG. 5  is a horizontal section on line V-V of  FIG. 4 . 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     Referring to  FIG. 1  there is illustrated, generally at  10 , a combustion chamber in accordance with the invention, the combustion chamber  10  having an enclosed hollow body  11 , which is generally circular in cross-section. The hollow body  11  has a cylindrical wall section  12 , and a top section  13  at end  14  of the cylindrical wall section  12 . The cylindrical wall section  12  narrows at end  15  to form a frusto-conical section  16 , which terminates in a neck section  17 , within which the ash collects in use to be removed through an exit pipe  18 , which is regulated by a valve  19 . 
     A primary air inlet  20  is connected to the neck section  17  at point  21  and is regulated by a valve  22  housed therein. 
     A fuel inlet  23  is mounted in the top section  13  at point  24  and in this embodiment the fuel inlet  23  also acts as a secondary air inlet. 
     Four volatiles outlets  25  (one visible) are mounted in the cylindrical wall section  12 , adjacent the top section  13 . Each volatiles outlet  25  has a plurality of apertures  26  of differing sizes arranged in a plate  27 . The plate  27  is made of tungsten to withstand the heat generated in use. 
     A grate  28  is mounted within the hollow body  11  and supports the wood pellets  29  to be burnt. As the wood pellets  29  burn, they break up, fall through the grate  28  and are held on a mesh  30  while they burn for a further period, until they finally fall through the mesh  30  as ash (not shown), to be collected in the neck section  17 . 
     In the embodiment illustrated the combustion chamber  10  forms part of a fire chamber, shown generally at  31 , in accordance with the invention. The fire chamber  31  has a water jacket  32  having a heat transfer surface  33  which encircles the combustion chamber  10 . Around the area of cylindrical section  12 , in which the volatiles outlets  25  are mounted, the heat transfer surface  33  is formed so as to create a channel  34  between the heat transfer surface  33  and the cylindrical section  12 . 
     The water jacket  32  has a water inlet  35  and a water outlet  36 . 
     In use the wood pellets  29  are introduced into the combustion chamber  10  through the fuel inlet  23  at a rate appropriate for the required heat output of the device. Primary air at the appropriate pressure is introduced into the combustion chamber  10  via the primary air inlet  20  and is blown up through the mesh  30 , the grate  28  and the pellets  29 . Thus, the primary combustion takes place above the grate  28  in the area of the top section  13 . Again, depending on the output required, secondary air is introduced into the combustion chamber  10  through the fuel inlet  23  and mixes with the volatiles above the pellets  29 . The burning volatiles then exit the combustion chamber  10  through the volatiles outlets  25  in a turbulent flow and circulate around the cylindrical section  12  raising the temperature of both the heat transfer surface  33  and the combustion chamber  10  itself. 
     Thus, the burning of the volatiles is concentrated in the area above the pellets  29  and in the channel  34 . The burning volatiles will remain in this area due to thermal buoyancy until they start to cool. As the volatiles cool they drop down in the combustion chamber  10  and the exhaust gases are vented through a flue  37 , which is regulated by a paddle valve  38 . 
     Referring to  FIG. 2  the arrangement of the volatiles outlets  25  around the cylindrical section  12  can be seen more clearly. The arrows  39  indicates the path of the turbulent flow of the volatiles through the channel  34  around the combustion chamber  10 . The volatiles outlets  25  are offset in the cylindrical section  12  such that the turbulent flow of volatiles, as it exits the volatiles outlets  25  is already directed around the combustion chamber  10 , as desired. 
     Referring to  FIG. 3  there is illustrated, generally at  40 , a second embodiment of a combustion chamber in accordance with the invention. The combustion chamber  40  is designed to burn wood pellets and to provide a blown flame and is suitable for use as a replacement for an oil burner in an oil-fired heating boiler. 
     The combustion chamber  40  has an enclosed hollow body  41 , which has a generally circular cross-section and a domed top section  42 . A frusto-conical section  43  extends laterally from the top section  42  and terminates in a volatiles outlet  44  having a plurality of apertures  45 . A fuel inlet  46  is located at point  47  on the top section  42  and a primary air inlet  48  is located at point  49  on the body section  41 . 
     A secondary air inlet  50  is mounted in the frusto-conical section  43  and is positioned such that a secondary air nozzle  51  is located, within the hollow body  41 , adjacent the volatiles outlet  44 . 
     A grate  52  is mounted within the hollow body  41  and supports the wood pellets  53  to be burnt. A drop-on umbrella shaped plate  54  is mounted centrally on the grate  52  at position  55  below the fuel inlet  46 . In use, the drop-on plate  54  prevents the burning wood pellets  53  from being crushed by fresh pellets  53  dropping from the fuel inlet  46  and also helps to disperse the pellets  53  over the grate  52 . 
     In use, the burning of the pellets  53  on the grate  52  results in burning volatiles above the grate  52 . These volatiles mixed with the primary and secondary air are forced through the apertures  45  of the volatiles outlet  44  and exit as a rapidly burning turbulent flow, which can be directed onto a heat transferring surface within the fire chamber of the heating boiler. 
     Referring to  FIG. 4  there illustrated generally at  60 , a third embodiment of a combustion chamber in accordance with the invention, the combustion chamber  60  having an enclosed hollow body  61 , which is generally circular in cross-section. The hollow body  61  has a cylindrical wall section  62  and a top section  63  at end  64  of the cylindrical section  62 . 
     A volatiles outlet  65  is mounted in the top section  63  and has a plurality of apertures  66  therein. A pipe  67  passes through a central opening  68  in the top section  63 . The pipe  67  serves as a fuel inlet  69  and a secondary air inlet  70 . 
     A grate  71  is mounted within the hollow body  61  and supports the wood pellets  72  to be burnt. 
     A primary air inlet  73  is mounted at the bottom end  74  of the hollow body  61 . 
     In the embodiment illustrated the combustion chamber  60  forms part of a fire chamber, shown generally at  75 , in accordance with the invention. The fire chamber  75  has a water jacket  76  having an inner heat transferring surface  77  which encircles the combustion chamber  60 . The inner heat transferring surface  77  has an upper section  78 , which has an inverted frusto-conical shape and a lower cylindrical section  79 . 
     A set of upstanding curved formations  80  is mounted on an internal surface  81  of a top section  82  of the inner heat transferring surface  77 . The set of upstanding curved formations  80  is arranged equidistantly around the combustion chamber  60  and this can be more clearly seen with reference to  FIG. 5 . 
     A means (not shown) for moving the combustion chamber along its vertical axis is provided. Thus, when a low output is required the combustion chamber  60  can be moved closer to the top section  82  of the inner heat transferring surface  77 . This has the effect of restricting the flow of volatiles through the volatiles outlet  65 . Conversely, when a higher output is required the combustion chamber  60  can be moved further away from the top section  82  of the inner heat transferring surface  77 . 
     In use the wood pellets  71  are introduced into the combustion chamber  60  through the fuel inlet  69  at a rate appropriate for the required heat output of the device. Primary air at the appropriate pressure is introduced into the combustion chamber  60  via the primary air inlet  73  and is blown up through the grate  71  and the pellets  72 . Thus, the primary combustion takes place above the grate  71 . Again, depending on the output required, secondary air is introduced into the combustion chamber  60  through the secondary air inlet  70  and mixes with the volatiles above the pellets  72 . The burning volatiles then exit the combustion chamber  60  through the volatiles outlet  65  in a turbulent flow and circulate around the fire chamber  75  raising the temperature of both the inner heat transferring surface  77  and the combustion chamber  60  itself. 
     The shape and positioning of the set of upstanding curved formations  80  within the fire chamber  75  causes the volatiles to circulate around the combustion chamber  60  and also enhances the turbulent flow of the volatiles. As the volatiles cool they drop down to the end  74  of the combustion chamber  60 , where they pass through an exit pipe (not shown).