Method and apparatus for the improved combustion of biomass fuels

Abstract

A cylindrical furnace having a vertical axis controls combustion. Solid fuel, particulates, and gases inside the furnace rotate around the axis, inducing radial stratification using centrifugal forces. Fuel and particulates drag on the wall of the cylinder, slipping in and out of suspension, thereby increasing particle residence times. The solid particles comprise combustible fuel particles, and non-combustible ash and contaminants. Control of the temperature of non-combustible particles and the wall surface prevents these non-combustible particles from adhering to, and building up on, the furnace wall. It is also advantageous to control the gas temperature leaving the furnace to minimize temperature-driven corrosion of downstream heat-exchange surfaces. Method and apparatuses are described to control the gas, non-combustible particle, and wall temperatures. The furnace can be integrated into a stand-alone boiler or as a combustor in which a portion of the pyrolysis gas from the combusting fuel is burned in a separate vessel.

1 . A furnace burning biomass fuel defined by a cylindrical enclosure with a vertical axis, a top of said enclosure disposed to conduct hot gases to heat-absorbing surfaces or to a separate means for further combustion, in which solid fuel particles and combustion air are injected into said furnace and said fuel burns releasing gaseous products of combustion, in which noncombustible particles are present, in which said gases are induced to rotate around the vertical axis and said rotation entrains at least some of said fuel and noncombustible particles to rotate with said gases, and an average residence time of said particles in said cylindrical enclosure is greater than an average residence time of said gases in said cylindrical enclosure, and in which an air-to-fuel ratio is controlled at least at one combustion air injection elevation to control a temperature above the elevation of said combustion air injection. 
     
     
         2 . The furnace of  claim 1  in which at least one melting temperature has been determined for at least one composition of said noncombustible particles and said melting temperature is used to determine a desired range of said controllable temperature. 
     
     
         3 . The furnace of  claim 2  in which said cylindrical enclosure is internally lined with refractory and said controllable temperature is the temperature of said refractory measured at least at one location within said cylindrical enclosure. 
     
     
         4 . The furnace of  claim 2  in which said controllable temperature is a furnace exit gas temperature measured at least at one location at an exit of said cylindrical enclosure. 
     
     
         5 . The furnace of  claim 2  in which said controllable temperature is a temperature of said noncombustible particles extracted from said cylindrical furnace. 
     
     
         6 . The furnace of  claim 2  in which said controllable temperature is a temperature of said gas measured within a cylindrical height of said furnace. 
     
     
         7 . The furnace of  claim 1  in which said air-to-fuel ratio is controlled by regulating a quantity of said combustion air injected at least at one of said air injection elevations. 
     
     
         8 . The furnace of  claim 7  in which an injection velocity of said combustion air is controlled independently of a flow of said combustion air. 
     
     
         9 . The furnace of  claim 1  in which said air-to-fuel ratio is controlled by diluting said combustion air with oxygen-depleted gas taken from a furnace flue gas. 
     
     
         10 . The furnace of  claim 9  in which an injection velocity of said diluted combustion air and said furnace flue gas is controlled independently of a flow of said diluted combustion air and said furnace flue gas. 
     
     
         11 . A furnace burning biomass fuel defined by a cylindrical enclosure with a vertical axis, a bottom of said enclosure being conically shaped and truncated at a floor, a top of said enclosure disposed to conduct hot gases to heat-absorbing surfaces or to a separate means for further combustion, in which solid fuel particles and combustion air are injected into said furnace and said fuel burns releasing gaseous products of combustion, in which noncombustible particles are present, in which said gases are induced to rotate around the vertical axis and said rotation entrains at least some of said fuel and noncombustible particles to rotate with said gases, and tan average residence time of said particles in said cylindrical enclosure is greater than an average residence time of said gases in said cylindrical enclosure, and in which an air-to-fuel ratio is controlled at least at one combustion air injection location within said conically shaped bottom to control a temperature within said conically shaped bottom. 
     
     
         12 . The furnace of  claim 11  in which at least one melting temperature has been determined for at least one composition of said noncombustible particles and said melting temperature is used to determine a desired range of said controllable temperature. 
     
     
         13 . The furnace of  claim 12  in which said conically shaped bottom is internally lined with refractory and said controllable temperature is the temperature of said refractory measured at least at one location within said conically shaped bottom. 
     
     
         14 . The furnace of  claim 12  in which said controllable temperature is a gas temperature measured at least at one location above a top of said conically shaped bottom but below a lowest level of fuel injection. 
     
     
         15 . The furnace of  claim 12  in which said controllable temperature is a temperature of said noncombustible particles extracted from said conically shaped bottom. 
     
     
         16 . The furnace of  claim 12  in which said controllable temperature is a temperature of said gas measured within said conically shaped bottom. 
     
     
         17 . The furnace of  claim 11  in which said air-to-fuel ratio is controlled by regulating a quantity of said combustion air injected at least at one of said combustion air injection locations. 
     
     
         18 . The furnace of  claim 17  in which the injection velocity of said combustion air is controlled independently of the flow of said combustion air. 
     
     
         19 . The furnace of  claim 11  in which said air-to-fuel ratio is controlled by diluting said combustion air with oxygen-depleted gas taken from a furnace flue gas. 
     
     
         20 . The furnace of  claim 19  in which an injection velocity of said diluted combustion air and said furnace flue gas is controlled independently of a flow of said diluted combustion air and said furnace flue gas. 
     
     
         21 . The furnace of  claim 1  in which said separate means for further combustion is an existing boiler. 
     
     
         22 . The furnace of  claim 1  in which said furnace is integrated into a stand-alone boiler. 
     
     
         23 . The furnace of  claim 1  in which said heat absorbing surfaces are contained in a separate heat recovery steam generator. 
     
     
         24 . A furnace burning biomass fuel defined by a cylindrical enclosure with a vertical axis, a top of said enclosure disposed to conduct hot gases to heat-absorbing surfaces or to a separate means for further combustion, in which solid fuel particles and combustion air are injected into said furnace and said fuel burns releasing gaseous products of combustion, in which noncombustible particles are present, in which said gases are induced to rotate around the vertical axis and said rotation entrains at least some of said fuel and noncombustible particles to rotate with said gases, and an average residence time of said particles in said cylindrical enclosure is greater than an average residence time of said gases in said cylindrical enclosure, and in which a momentum of said injected combustion air and fuel is controlled, either together or separately, to control a rotation of said rotating gases. 
     
     
         25 . The furnace of  claim 24  in which furnace flue gas is also injected, either separately or as a dilutant to said combustion air or as transport media for said fuel, and in which a momentum of said injected furnace flue gas is controlled, either separately or together with said combustion air and fuel, to control a rotation of said rotating gases. 
     
     
         26 . A furnace burning biomass fuel defined by a cylindrical enclosure with a vertical axis, a top of said enclosure disposed to conduct hot gases to heat-absorbing surfaces or to a separate means for further combustion, in which solid fuel particles and combustion air are injected into said furnace and said fuel burns releasing gaseous products of combustion, in which noncombustible particles are present, in which said gases are induced to rotate about the axis of said cylindrical enclosure and said rotation entrains at least some of said fuel and noncombustible particles to rotate with said gases, and an average residence time of said particles in said cylindrical enclosure is greater than an average residence time of said gases in said cylindrical enclosure, and in which a rotational velocity of said rotating gases is measured at least at one location within said cylindrical enclosure, and in which said rotational velocity measurement is used as an input to a control loop to regulate said rotational velocity. 
     
     
         27 . A furnace burning biomass fuel defined by a cylindrical enclosure with a vertical axis, a bottom of said enclosure conically shaped and truncated at a floor, a top of said enclosure disposed to conduct hot gases to heat-absorbing surfaces or to a separate means for further combustion, in which solid fuel particles and combustion air are injected into said furnace and said fuel burns releasing gaseous products of combustion, in which a portion of said injected fuel at least partially fills said bottom, in which noncombustible particles are present, in which said gases are induced to rotate around the vertical axis and said rotation entrains at least some of said fuel and noncombustible particles to rotate with said gases, and an average residence time of said particles in said cylindrical enclosure is greater than an average residence time of said gases in said cylindrical enclosure, in which said fuel residing in said bottom is induced to rotate about the vertical axis, and in which an injection of combustion air, furnace flue gas, or steam, is regulated to control a rotational velocity of said rotating fuel. 
     
     
         28 . The furnace of  claim 27  in which the rotational velocity of said rotating fuel is measured at least at one location within said conically shaped bottom, and in which said velocity measurement is used as an input to a control loop to regulate said rotational velocity. 
     
     
         29 . (canceled) 
     
     
         30 . A method of operating a furnace, comprising:
 injecting solid fuel particles and combustion air into a cylindrical furnace;   burning said fuel and releasing gaseous products of combustion and noncombustible particles,   inducing said gases to rotate around a vertical axis of said cylindrical furnace, wherein said rotation entrains at least some of said fuel and noncombustible particles to rotate with said gases, and wherein an average residence time of said particles in said cylindrical enclosure is greater than an average residence time of said gases in said cylindrical furnace; and   controlling an air-to-fuel ratio at least at one combustion air injection elevation to control a temperature above the elevation of said combustion air injection.   
     
     
         31 . The method of  claim 30  further comprising determining at least one melting temperature for at least one composition of said noncombustible particles and using said melting temperature to determine a desired range of said controllable temperature. 
     
     
         32 . The method of  claim 31  in which said cylindrical furnace is internally lined with refractory and controlling the controllable temperature is a temperature of said refractory measured at least at one location within said cylindrical furnace. 
     
     
         33 . The method of  claim 31  in which said controllable temperature is a furnace exit gas temperature measured at least at one location at an exit of said cylindrical furnace. 
     
     
         34 . The method of  claim 31  in which said controllable temperature is a temperature of said noncombustible particles extracted from said cylindrical furnace. 
     
     
         35 . The method of  claim 31  in which said controllable temperature is a temperature of said gas measured within a height of said cylindrical furnace. 
     
     
         36 . The method of  claim 30  in which said air-to-fuel ratio is controlled by regulating a quantity of said combustion air injected at least at one of said combustion air injection elevations. 
     
     
         37 . The method of  claim 30  in which said air-to-fuel ratio is controlled by diluting said combustion air with oxygen-depleted gas taken from a furnace flue gas. 
     
     
         38 . The method of  claim 37  in which an injection velocity of said combustion air is controlled independently of a flow of said combustion air. 
     
     
         39 . The method of  claim 38  in which the injection velocity of said diluted combustion air and said furnace flue gas is controlled independently of the flow of said diluted combustion air and said furnace flue gas.