Abstract:
A portable air curtain incinerator for burning biomass, such as cleared logs and vegetation, is equipped with heat recovery panels along at least one of the sidewalls of the incinerator&#39;s firebox, and a heat transfer medium is cycled through the heat recovery panels and expanded to its gaseous phase by heat released during incineration of the biomass. The gaseous medium is directed to drive the turbine of a generator to generate electricity. The heat transfer medium is condensed, preferably using a local source of cooling water, and the cycle is repeated. The air curtain incinerator may include its own generator, or multiple incinerators may be coupled to a single shared generator.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates generally to incineration of vegetative waste (biomass) and generation of electrical power therefrom. 
       BACKGROUND OF THE INVENTION 
       [0002]    Vegetative waste, in particular wood waste, has long been a difficult problem for community landfills and lumbering operations. Grinding the wood waste reduces its volume, but is expensive and extremely harmful to the environment, and it fails to reduce the amount of wood waste. Grinding ten tons of logs yields ten tons of wood chips. In the context of the massive tree kill currently befalling forests in the western United States due to insect infestation and climate change, the approach of grinding, chipping and hauling the wood waste actually spreads the problem. 
         [0003]    Fireboxes and fire pits have been used to burn vegetative waste at clearing sites. In order to reduce ash and smoke released during waste incineration (particulate release), a flow of high velocity air has been used to provide an “air curtain” over a fire pit or firebox in which the waste is burned. U.S. Pat. Nos. 4,756,258 and 5,415,113 describe portable apparatus for air curtain incineration. The former patent teaches a fan and manifold assembly that can be towed to and positioned at the edge of a fire pit, whereas the latter patent teaches a firebox, fan, and manifold assembly mounted on a support frame for transport to a desired clearing site for incineration of waste without the need to dig a fire pit. These portable solutions offer clean burning, and they minimize the need to transport the waste. 
         [0004]    U.S. Pat. No. 6,536,360 by the present applicant discloses an air-curtain firebox incinerator designed to capture waste heat for useful applications. The heat is recovered from the side walls of the firebox, which radiate between 400 and 600 degrees Fahrenheit, using heat transfer panels to heat circulating water. The heated water is typically pumped to a radiator located in the building or greenhouse where air is heated to either warm a facility or provide process heat. 
         [0005]    U.S. Pat. No. 7,063,027 also by the present applicant provides a self-contained, transportable air curtain incinerator for combustion of low calorific value waste. The incinerator comprises a transportable frame supporting a firebox, a fuel supply tank, a fuel-burning electric power generator in communication with the fuel supply tank, at least one fuel-burning burner unit in communication with the fuel supply tank for directing a flame into a combustion chamber defined by the firebox, and an air curtain blower powered by the generator for providing a sheet of high velocity air flow generally across an open top of the firebox. Thus, the generator runs on energy from burning fuel in the fuel tank, not on energy from combustion of waste in the firebox. The generated power is used locally in the incinerator apparatus to power the air curtain blower. 
         [0006]    Even with the advances mentioned above, biomass incineration facilities currently suffer from three main drawbacks: 1) the waste has to go through a grinder and then a chipper to a achieve particular size acceptable to the incinerator and only about 80% of that waste is acceptable for the incinerator; 2) the incinerators use natural gas to burn the waste; and 3) at today&#39;s rates, if the waste has to be transported more than fifty miles to an incinerator the costs are prohibitive. What is needed is a biomass incineration facility that can be setup at a temporary location and operated until the waste transportation costs are too high, and then the whole facility can be easily moved to a new location. The incinerators should not require any fuels to augment burning, and they should accept 100% of the waste materials without any need to process the waste before it is placed into an incinerator. In addition, the incineration facility should allow for capture of energy produced by incinerating the biomass and conversion of that energy to electric power as an economic and environmental benefit. 
       SUMMARY OF THE INVENTION 
       [0007]    In accordance with a first embodiment of the present invention, a portable air curtain incinerator is equipped with heat recovery panels along at least one of the sidewalls of the incinerator&#39;s firebox, and a heat transfer medium is cycled through the heat recovery panels. The heat transfer medium may be expanded to its gaseous phase by heat released during incineration of wood waste or other biomass, and the gaseous medium is directed to drive the turbine of a generator to generate electricity. The electricity may then be conditioned for internal use and/or sale to a utility company. The heat transfer medium is condensed, preferably using a local source of cooling water, and the cycle is repeated. 
         [0008]    Alternatively, the heat transfer medium in the heat recovery panels may be kept under pressure so that the heat transfer medium remains in its liquid phase. In that case, the pressurized heat transfer medium is directed to a heat transfer unit containing a refrigerant. The heat from the pressurized heat transfer medium causes the refrigerant to expand, which drives the turbine of a generator to generate electricity. 
         [0009]    A second embodiment of the invention comprises a group of portable air curtain incinerators each equipped with heat recovery panels, and a single shared power generator and cooling station connected to the incinerators to receive gaseous or liquid heat transfer medium from each of the incinerators, wherein energy from multiple incinerators may be combined to drive the turbine of the single generator, and a common cooling system may be shared by all the incinerators. Here again, electric power may be conditioned and used for private purposes or sold for public use. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING VIEWS 
         [0010]    The invention will be explained further with reference to drawing figures in which: 
           [0011]      FIG. 1  is a perspective view of a portable air-curtain incinerator having an on-board power generator in accordance with a first embodiment of the present invention; 
           [0012]      FIG. 2  is a side view of the incinerator shown in  FIG. 1 ; 
           [0013]      FIG. 3  is a schematic view of the incinerator shown in  FIGS. 1 and 2 ; and 
           [0014]      FIG. 4  is a schematic view of a second embodiment of the present invention, wherein multiple air curtain incinerators are in communication with a single shared power generator. 
           [0015]      FIG. 5  is a schematic end view of an incinerator having a heat recovery roof. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]      FIGS. 1-3  show a portable air-curtain incinerator  10  capable of cleanly converting biomass into electrical power in accordance with a first embodiment of the present invention. Incinerator  10  generally comprises a firebox  12 , an air curtain manifold  14  arranged to direct a curtain of high-velocity airflow over an open top of firebox  12 , and an equipment deck  16  adjacent the firebox. Equipment deck  16  supports a fuel tank  18 , an engine  20  running on fuel stored in fuel tank  18  or powered by electricity from a local power grid, and a fan  22  driven by engine  20  to generate airflow through air curtain manifold  14 . An electrical generator (not shown) may also be included for starting the engine  20  in the case where the portable air-curtain incinerator  10  is being used in a remote connection, with no access to a local power grid. Incinerator  10  may be constructed generally as described in commonly-owned U.S. Pat. No. 5,415,113, the entire disclosure of which is incorporated herein by reference. However, modifications for recovering waste heat and generating electrical power may be implemented as described below in accordance with the present invention. 
         [0017]    For recovering waste heat, the sidewalls of firebox  12  are equipped with heat recovery panels  24  having tubing  26  for conducting a flowing heat transfer medium such as an environmentally benign refrigerant or a water solution. For sake of simplicity, the present description refers to a refrigerant, however it will be understood that other heat transfer media may be used. Panels  24  are insulated on the inner exposed side with refractory material. Heat recovery panels  24  may be formed as disclosed in commonly-owned U.S. Pat. No. 6,536,360, the entire disclosure of which is incorporated herein by reference. Alternatively, retrofittable heat recovery panels may be mated to existing thermo-ceramic firebox side panels. Each heat recovery panel includes an inlet port  28  and an outlet port  30 . Ports  28  and  30  are fitted with suitable coupling hardware for connecting hose or tubing lines thereto. As may be understood, the outlet port  30  of a given panel  24  may be connected by hose or tubing lines  32  to the inlet port  28  of a next panel, and so on, to provide a continuous flow path for heat transfer medium to traverse substantially the entire length of a sidewall of firebox  12 . Retrofittable heat recovery panels may be formed using stainless steel to inhibit corrosion. 
         [0018]    As shown in  FIG. 5 , heat may also be recovered from above the exhaust plume  51  of the firebox  12  by constructing a partial heat recovery roof  52 . Due to the forces of the air curtain  53 , the exhaust plume  51  (or compression of exhaust gases) rises up from the wall opposite the air curtain manifold  14 . The exhaust plume  51  covers the entire length of the firebox  12 , and approximately 20% of the width of the firebox  12 . The temperature of the exhaust plume  51  reaches over 1800 degrees Fahrenheit (982.2 degrees Celsius). The partial heat recovery roof  52  may use similar heat recovery panels as the sidewalls of the firebox  12  to recover heat from the incineration of waste. 
         [0019]      FIG. 3  shows that tubing  26  and connecting lines  32  are part of a closed refrigerant loop generally indicated by numeral  34 . The refrigerant starts at a pump  36  as a liquid and is cycled through heat recovery panels  24  along a sidewall of firebox  12 . Heat from incineration of biomass within firebox  12  is transferred to the refrigerant, causing the refrigerant to change from liquid phase to gaseous phase and rapidly expand. The rapidly expanding gas is conveyed to a high speed generator  38  on equipment deck  16 , causing the generator&#39;s turbine to spin at a very high rate to generate high frequency alternating current (AC Power). The gaseous refrigerant exits generator  38  and travels through a condensing portion  40  of loop  34 , which serves to condense the refrigerant to its liquid phase. 
         [0020]    Alternatively, the heat transfer medium may be kept under pressure so that it remains in a liquid phase after being heated by the incineration of biomass. The pressurized heat transfer medium is directed to a heat transfer unit containing a refrigerant. The heat from the pressurized heat transfer medium causes the refrigerant to change from liquid phase to gaseous phase and rapidly expand. The rapidly expanding gas is conveyed to a high speed generator  38  on equipment deck  16 , causing the turbine to generate electric power. In this case, the heat transfer medium may be water, kept under pressure, which reaches a temperature about 275 degrees Fahrenheit (135 degrees Celsius); however, other liquid mediums may be used. 
         [0021]    Condensing portion  40  may be embodied in a variety of ways depending upon the location and use of incinerator  10 . The simplest and lowest cost system is to run the refrigerant line through a cool water (about 78 degrees Fahrenheit; 25.6 degrees Celsius) bath  42  where cooling water is drawn from a local pond, stream, lake or well. In this system no cooling water is consumed, lost or contaminated in the process but the water returned to the source will see about a 10 degrees Fahrenheit (5.6 degrees Celsius) increase in temperature. A variant of this would be to use a portable water tank or truck to circulate the water for cooling. The size of the tanker would depend on the size of firebox  12  and the amount of power being generated. Another option is an evaporative cooler which uses a small amount of water run over the heat exchange coils to cool the refrigerant. A further option is an air blast cooler which uses air fans to blow cooling air across a radiator and cool the refrigerant. This option would not use water, but would consume more of the electricity produced by incinerator  10 . Waste hot air from the air blast cooler may be used to warm a building or greenhouse. 
         [0022]    Generator  38  may include a single stage turbo expander, rated, for example, at 28,000 RPM, and a high speed two-pole rare earth magnet alternator providing, for example, a 100 kWe minimum output. By way of further example, the electrical output may be 380-480 V line-to-line rms 3 phase 4 wire 50/60 Hz 100 kWe minimum. 
         [0023]    Generator  38  outputs into a power conditioning module  44  located on equipment deck  16 . Power conditioning module  44  controls, distributes and conditions the power coming from generator  38 . Power conditioning module  44  may be a PE modulated solid state module programmable to user requirements. First the power is distributed within the incinerator system itself to charge the batteries and to run all the pumps, valves, fans and electronics of the system. This consumes approximately 10% of the available power (except for an air blast cooling system, which would consume an additional 10%). The other 90% is then conditioned for output to the local power grid. Power can be provided at almost any voltage and frequency required, but the most common is 480V three-phase AC power. Power output is dependent in part on the capacity of the incinerator firebox  12 . Using an existing firebox configuration such as the Model S220 FireBox available from Air Burners LLC, incinerator  10  will consume between three and six tons of wood waste per hour and is expected to yield a minimum output of about 100 kWe. If a larger firebox configuration is used, such as the Model S327 FireBox from Air Burners LLC, incinerator  10  may generate between 175 and 300 kWe. Incinerator  10  is fully self-contained and easily transportable, making its use possible at multiple sites or communities. On-site connections include the electrical grid and possibly a source of cooling water. 
         [0024]      FIG. 4  shows an alternative embodiment  100  of the present invention, wherein multiple air curtain incinerators  110  are in communication with a single shared power generator  138 . Generator  138  may be part of a power generation and cooling station  150  located near air curtain incinerators  110 . Station  150  is shown as further including a condensing system in the form of a cooling water bath  142  (other condensing systems may be used as discussed above), and a power conditioning module  144 . Gas phase refrigerant is carried by conduit  132  from incinerators  110  to station  150  to rotate the turbine of generator  138  to generate electrical power. Power conditioning module  144  converts the generated power for distribution along line  152  to the local power grid for general use and along lines  154  to incinerators  110  for powering components of each incinerator  110  that run on electrical power. Refrigerant is cooled and returned to its liquid phase as it is conveyed through cooling water bath  142 . Conduits  133 , equipped with suitable pumping hardware (not shown), carry the condensed refrigerant back to the incinerators  110  to repeat the cycle. As will be appreciated, the embodiment of  FIG. 4  requires only one generator for a group of fireboxes, and a large portion of the generated electricity may be sold to a utility company at a profit. It may also be possible to realize additional income from generating and selling carbon credits on the open market (e.g. the Carbon Credit Exchange or “CCX”). 
         [0025]    Advantageously, in both embodiments described above, the refrigerant is contained in a closed system and is not expelled or replenished. 
         [0026]    The present invention provides a portable system for generating power from large scale biomass incineration. The present invention reduces wood waste by 98%; ten tons of logs in yields about two-hundred pounds of ash out (a clean natural ash which is a highly desirable recycled product for agriculture, growers, nurseries and is also a good landfill cover). The invention also captures energy from the wood waste and converts it to electricity, providing an additional income from the sale of that electricity. The present invention is useful in almost every landfill, transfer station or forestry operation. Air curtain incineration is a well-tested and proven technology that allows for natural burning of clean wood waste while protecting our environment from the smoke typically associated with open burning. Of course, the wood waste has enormous energy potential that may now be realized by the present invention, and significant amounts of electricity may be made available in remote locations.