Abstract:
A method of converting animal litter biomass into useful energy includes the steps of both combusting and pyrolising the biomass, of mixing the resultant combustible gas from the pyrolysis step with the gases from the combustion step, of igniting the mixture of gases ad and feeding the hot mixture of gases to a thermal recuperator; and apparatus therefor.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    Benefit of U.S. Provisional Application for Patent Ser. No. 60/795,572, filed on Apr. 28, 2006, is hereby claimed. 
     
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to the conversion of animal litter biomass into useful energy. 
         [0003]    More specifically, the invention has particular, but not exclusive, reference to such conversion of poultry litter that essentially consists of a mixture of wood shavings, straw and fecal matter. The expression ‘poultry’ as used herein means domestic foul of various kinds, inter alia chickens, turkeys, ducks and geese. The invention may advantageously be used for the conversion of biomass derived from the keeping of other farm animals, such as pigs and bovines. 
       BACKGROUND OF THE INVENTION 
       [0004]    Great emphasis is now placed by both government and the general public upon the need to reduce the disposal of waste that might otherwise provide useful feedstock for varied processes in industry and generally. Recycling of metals, plastics and paper is now mandatory in many countries to ensure that the optimum utilization of waste materials may be achieved by reuse in appropriate processes. Equally, attention has been focused upon so called renewable energy resources to provide ‘green power’ and among such resources biomass constitutes a major element since agricultural waste is a continuous and renewable source of fuel in the right process. Many and varied techniques have been proposed and indeed adopted on a commercial basis for the destruction of poultry litter biomass to produce energy by heat generation and/or gasification for use on site or for distribution to appropriate facilities for use. 
         [0005]    Generally, techniques of this kind involve relatively complex plant, for example fluidized bed combustors using bubbling bed technology, or gasifiers, and necessarily incur high capital and running costs. 
         [0006]    Accordingly, there is a need for a simpler and therefore cheaper approach to the conversion of animal litter biomass into useful energy. 
       SUMMARY OF THE INVENTION 
       [0007]    It is therefore a general object of the present invention to provide a method of and apparatus for the conversion of animal litter biomass into useful energy. 
         [0008]    A further object of the present invention is to provide such a method and apparatus that employ relatively simple process steps and equipment of comparatively simple construction with concomitant cost benefits. 
         [0009]    According to a first aspect of the present invention there is provided a method of converting animal litter biomass into useful energy comprising the steps of feeding said biomass in comminuted form into a primary combustion zone comprising an upper region and a lower region, partially combusting the biomass at the lower region thereof at an elevated temperature in the presence of a combustion-sustaining medium thereby to provide heat into the upper region thereof for pyrolysis to generate combustible gases, of subjecting the lower partially combusted incandescent biomass to air flow to generate combustion gases, of mixing the combustion gases and the combustible gases, of igniting the resultant mixture of gases thereof to produce a hot gaseous product, and of passing the hot gaseous product to a thermal recuperator for expending and exploiting the heat energy thus produced. 
         [0010]    The solid residue arising from combustion and pyrolisation is discharged and may be employed further, for example for collection for use in horticultural or agricultural applications. 
         [0011]    The mixing of the gaseous products before their egress from the process seeks to secure the suppression of pollutants by heat destruction and enhanced rate of combustion. 
         [0012]    According to a second aspect of the present invention there is provided an apparatus for the conversion of animal litter biomass into useful energy, the apparatus comprising a primary combustion zone incorporating a chamber of frusto-conical form, a feed entry to the primary combustion zone, a feed conduit contiguous with said zone and connected to the feed entry at a relatively upper part of the zone, a combustible gas outlet for the primary pyrolysis combustion zone, a rotatable grate situated at the base of the primary combustion zone, a means for rotating the grate, a combustion-sustaining fluid distributor located at said base, a secondary normal combustion zone on the grate circumscribing the base of the primary pyrolysis combustion zone, a combustion exhaust gas outlet from the secondary combustion zone, a gas mixing zone downstream of the combustible gas outlet and the combustion exhaust gas outlet wherein in use the respective combustion gases and the combustible gas mix, an ignition station for the resultant mixture of gases, the gas mixing zone comprising at least a partial tube bank for reheating and suppression of pollutant emissions, and a final gas discharge flue for the resultant gases. 
         [0013]    Conveniently, the gas mixing zone includes a heat exchange zone for expending and exploiting the heat energy of the hot mixture thus produced. 
         [0014]    The apparatus may be of generally cylindrical form. 
         [0015]    Conveniently, there is also provided a means of comminuting the feedstock which may include a hopper with a rotating armature provided with blades suitable for effecting the break up of the animal litter biomass introduced thereto. A rotatable disc may be disposed beneath the hopper outlet for feeding the comminuted feedstock into the feed conduit. 
         [0016]    The combustion-sustaining medium distributor may be of foraminous conical form enclosing a plenum chamber therebeneath to which a suitable conduit for the medium, e.g. air, is connected. 
         [0017]    The gas mixing zone may be provided with a full tube bank of coiled tubes or may in the alternative have a demi-bank of coiled tubes extending substantially half way around the zone. 
         [0018]    Other objects and advantages of the present invention will become apparent from a careful reading of the detailed description provided herein, with appropriate reference to the accompanying drawings. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    Further aspects and advantages of the present invention will become better understood with reference to the description in association with the following Figures, in which similar references used in different Figures denote similar components, wherein: 
           [0020]      FIG. 1  is a diagrammatic vertical cross section of an apparatus for the conversion of animal litter biomass in accordance with the present invention; 
           [0021]      FIG. 2  is a horizontal section taken through one embodiment of the apparatus shown in  FIG. 1 ; 
           [0022]      FIG. 3  is a horizontal section taken through an alternative embodiment of the apparatus shown in  FIG. 1 ; and 
           [0023]      FIG. 4  is a fragmented view on the line  4 - 4  in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]    With reference to the annexed drawings the preferred embodiment of the present invention will be herein described for indicative purpose and by no means as of limitation. 
         [0025]    Referring first to  FIG. 1  there is shown diagrammatically an apparatus  1  for the conversion of animal litter biomass into useful energy, the apparatus including a generally hollow cylindrical body  2  enclosing a primary combustion zone  4  of frusto-conical form vertically oriented centrally within the body  2 , the zone  4  having an upper region  5   a  and a lower region  5   b . The zone  4  has connected at its top part a feed tube  6  which registers with an opening  7  in the top  8  of the body  2 . 
         [0026]    A feed hopper  10  is disposed vertically above the opening  7  and has a rotatable drive spindle  9  carrying a plurality of comminuting paddles  11 ; the hopper  10  has an outlet  13  feeding onto a rotating feed table  14  provided with a discharge station  15  selectively aligned with the opening  7  and the outlet  13 . In use feedstock, namely the biomass, is fed into the hopper  10  wherein it is comminuted prior to delivery to the combustion zone  4 . 
         [0027]    Alternatively, as it would be obvious to one skilled in the art, the feedstock, typically fed from a conveyor (not shown), can be directed into the hopper  10  free of any spindle  9  and paddles  11 , or even directly fed into the feed tube  6  through opening  7 , without deviating from the scope of the present invention. 
         [0028]    At the base of the combustion zone  4  there is provided a circular rotatable grate  20  the diameter of which is greater than the base dimension of the zone  4  which is spaced therefrom to provide a path in use for the centrifugal conveyance of incandescent biomass. The biomass located in the upper region of the combustion zone  4  is heated and dried by the heat generated by the incandescent biomass to form a pyrolysis zone. At the center of the grate  20  is a foraminous conical plenum chamber  22  for the introduction of air into the combustion zone  4 , an air inlet pipe  24  being connected thereto and extending therefrom the exterior of the body  2 . A drive shaft  26  for the grate  20  extends through the pipe  24  for rotating the grate. A solid waste collection area  30  is defined beneath the rotatable grate  20  and a discharge outlet  32  is provided for that area and leads to a removal station  34 . 
         [0029]    A secondary, normal combustion zone  40  is defined externally of the primary zone  4  and is provided with perforated air distributor tubes or sparge pipes  42  to provide air for the combustion. 
         [0030]    Both the zones  4  and  40  are contained within a central cylindrical core  50 , which is provided with suitable outlets  52  and  54  respectively for said zones, a gas discharge conduit  53  connecting the zone  4  to the outlet  52 . 
         [0031]    A gas mixing chamber  60  is defined between the core  50  and the wall of the body  2  at the exit from the zones  4  and  40  and in this chamber the gases arising from pyrolysis and the combustion zones are mixed. Thence the mixture of gases flows through into a heat exchange zone  59  including a tube bank  61  formed in the annular space between the core  50  and the wall of the body  2 . In  FIG. 2  the post-combustion zone tube bank  61  extends substantially throughout the whole of the annular space, the tubes  61 ′ being held in tube plates  63  and being disposed in spaced relation one to the other with baffles  62 ′ accommodated within the interstices between the tubes. The successive baffles  62 ′ are alternately raised and lowered along the tubes  61 ′ to force the hot gaseous product to vertically flow through the tubes  61 ′, as illustrated by the alternating orientations of arrows around the baffles  62 ′ in  FIG. 2 . In  FIG. 3  the annular space is only partially, e.g. half, filled with a demi-tube bank  61  with tubes  61 ″, the remainder of the space being provided with offset baffles  62 ″. Baffles  62 ′ and  62 ″, essentially vertically oriented, are provided as shown to enhance in practice the turbulence of gases and to lengthen residence to time thereby to optimize the various steps of mixing and suppression of noxious emissions and heat transfer efficiency. 
         [0032]    In use comminuted poultry litter biomass is fed from the hopper  10  into the feed tube  6  of the combustion zone  4  and descends onto the grate  20  and forms a bed of biomass typically extending up into the tube as shown in order to prevent heat losses there through. The temperature of the zone  4  is initially raised to a level appropriate for the ignition of the biomass and air is provided through the plenum  22 . The biomass is burned only at the bottom (lower region  5   b ) of the zone (smothered combustion) and that combustion energy allows the pyrolysis phenomenon in the biomass column thereabove (upper region  5   a ), and the hot gases of pyrolysis exit therefrom through the conduit  53  and the outlet  52  into the mixing chamber  60 . 
         [0033]    An incandescent layer or cake of biomass gradually radiates on the rotating grate  20  to pass from the pyrolysis zone  4  into the normal combustion zone  40  supplied with air through sparge pipes  42 , the gas being generated passing through the outlet  54  into the mixing chamber  60  after providing more energy to the biomass in zone  4  for pyrolysis. 
         [0034]    In the embodiment of  FIG. 2  the gas from zone  4  and the combustion gases emanating from zone  40  are mixed in the chamber  60  and pass through the tubes  61 ′ of the substantially full tube bank  61 . The gases after their passage through the tubes  61 ′ enter into a further, ignition chamber  68 , separated from the mixing chamber  60  by separation wall  67 , provided with a pilot flame igniter  69  and any remaining unburnt gas is combusted and all the gases from the chamber  68  pass over the tubes  61 ′, thereby effectively reheating the mixture of gases from pyrolysis zone  4  and those from combustion zone  40  passing through the tubes, thus improving efficiency and contributing to the destruction of any harmful elements that might give rise to atmospheric pollution. All gaseous products of combustion exhaust through a common outlet  70  of the body  2  whence they pass to a thermal recuperator (not shown) for appropriate usage. 
         [0035]    In the embodiment of  FIG. 3  the tube bank  61  is not so long but the gases from zones  4  and  40  are again mixed in chamber  60  and pass through the tubes  61 ″. The mixture of gases then flow through a zone in which turbulence is promoted by the baffles  62 ″ and eventually enter an ignition chamber  68  wherein any remaining unburnt gas is ignited by the igniter  69 . The resulting gas exhausts from the chamber  68  into a space  71  ( FIG. 4 ) defined over the offset baffles  62 ″ before flowing around the tubes  61 ″ of the tube bank  61  to provide reheat for the gases passing therethrough for the same purpose as described above in relation to  FIG. 2 , the final mixture of gases then exhausting through outlet  70  for passage to a thermal recuperator. 
         [0036]    The baffles are provided to improve turbulence and simultaneously increase residence time of the gases to enhance heat transfer and the consumption of potentially noxious elements in the gases. 
         [0037]    The present invention thus provides a relatively simple and yet effective means of disposing of a waste biomass product in a useful ‘green’ energy way, thus reducing the time and cost of conventional disposal. 
         [0038]    Although the present invention has been described with a certain degree of particularity, it is to be understood that the disclosure has been made by way of example only and that the present invention is not limited to the features of the embodiments described and illustrated herein, but includes all variations and modifications within the scope and spirit of the invention as hereinafter claimed.