Patent Publication Number: US-2007095259-A1

Title: Method for oxygen enriched low NOx, low CO2 and low CO combustion of pulverized solid fuel suspended in a preheated secondary fluid hydrocarbon fuel

Description:
FIELD OF THE INVENTION  
      This invention relates specifically to the method and combination of devices for the combustion of a pulverized solid hydrocarbon fuel that results in CO2 and CO emissions reduction, combustion efficiency and energy transfer efficiency improvement, with increased flame stability and turn-down capability in combustion systems operating with such solid hydrocarbon fuels. Specifically, this invention is directed to a method and combination of devices for an oxygen enriched combustion process of pulverized coal (PC), including sub-bituminous coals (SBC), bituminous coals (BC), petroleum coke/coal blends (PCCB) and biomass or biomass/coal blends (BCB), that results in general combustion efficiency improvements, in reductions of NO.sub.x, CO.sub.2 and CO emissions, and the increase in transfer efficiency of combustion products and energy into heat, thrust, rotational or other mechanical force.  
     BACKGROUND OF THE INVENTION  
      The combustion of solid hydrocarbon fuels such as coal, wood waste and biomass results in the generation of harmful emissions of greenhouse gases (CO.sub.2), nitrogen oxides (NO.sub.x) and carbon monoxides (CO) which are formed as a result of the oxidation of the fuel and combustion air mixture during the process of combustion. Greenhouse gases and Nitrogen oxides released into the atmosphere as a result of such combustion method significantly contribute to acid rain, and accelerate the photochemical reactions responsible for smog and the increase in harmful ground level ozone concentrations.  
      All existing prior art and established industry practice however makes use of the established combustion method and devices for combustion of pulverized coal including the combustion of pulverized coal in a manner which reduces CO emissions. The most advanced of all such methods teach the suspension of the pulverized solid fuel in an ambient temperature or preheated fluid of combustion air, which has significant major disadvantages.  
      Generally, when a solid fuel like coal dust is suspended in air, such air is diverted from the combustion air stream and may represent up to one third or more of the volume of combustion air required for efficient combustion of the fuel. It is obvious that, whenever such amount of combustion air is preheated, the oxygen ratio in such preheated portion of combustion air is drastically effected, and may be reduced from an ambient 21% to as low as 5% or less. Therefore, such practice will reduce the overall oxygen ratio in such combustion process from 21% down to 10% or less, reducing efficiency.  
      U.S. Pat. No. 3,788,796, Classification 431, to Brett C. Krippene et al. teaches an improved pulverized fuel burning method and apparatus having means for enhancing the fuel-air mixture passing therethrough and including three separately controlled passageways delivering the air necessary for combustion of the fuel.  
      U.S. Pat. No. 4,761,132 Classification 431, to Khinkis teaches a process and apparatus for oxygen-rich combustion wherein a first portion of about 5 to about 40 percent of the total fuel to be cracked and combusted is introduced to a cracking chamber where it is combusted and cracked at a temperature below about 2200.degree. C. to produce a cracked products mixture. Oxygen-rich gas of greater than about 30 volume percent oxygen is introduced to the cracking chamber in about 5 to about 50 percent of the stoichiometric requirement for complete combustion of the first portion of fuel introduced to that chamber. Cracked products mixture, a second remaining portion of fresh fuel and oxidizer having sufficient oxygen for substantially complete combustion of the combustible portion of the cracked products mixture and the fresh fuel is introduced to a combustion chamber wherein the combustible portion of the cracked products mixture and the fresh fuel is combusted. The process and apparatus provide a controllable, highly luminous, high temperature and high kinetic energy flame in the combustion chamber resulting in enhanced heat transfer rate to the furnace load, increased furnace specific production rate, increased furnace thermal efficiency, and reduced nitrogen oxides pollutant emissions.  
      U.S. Pat. No. 5,829,369, Classification 431, to Sivy et al. teaches a pulverized coal burner having a pulverized fuel transport nozzle surrounded by a transition zone which shields a central oxygen-lean fuel devolatilization zone from the swirling secondary combustion air. The transition zone acts as a buffer between the primary and the secondary combustion air streams to improve the control of the air-burner mixing and the flame stability by providing a limited recirculation region between primary and secondary combustion air streams. The limited recirculation regions transport evolved NO.sub.x back towards the oxygen-lean fuel devolatilization zone for reduction to molecular nitrogen. The burner may be configured to fire a combination of fossil fuels, for example, pulverized coal delivered through the primary zone with a small amount of natural gas being injected through the transition zone, wherein the natural gas constitutes between about 5-15% of the burner thermal input.  
      U.S. Pat. No. 5,799,594, Classification 431, to Dernjatin et al. teaches a method and apparatus for combustion of pulverized fuel in a tangentially fired boiler in which an air-deficient mixture of fuel and primary air is introduced through a fuel feeding pipe tangentially into the furnace of the boiler, forming a reducing flame, and at least one stream of combustion air is injected into the furnace. Although significant strides have been made in the area of pulverized coal combustion to reduce NO.sub.x emissions generated by the combustion of pulverized coal, NO.sub.x emissions from coal fired facilities continues to be problematic. In addition, it is frequently the case that methods and apparatuses for reducing NO.sub.x emissions from heating apparatuses in which the fuel is pulverized coal undesirably impact other elements of the combustion process such as flame stability, turn down capability, CO.sub.2 emissions, and combustion efficiency.  
      U.S. Pat. No. 6,244,200, Classification 110, to Rabovitser et al. teaches a combustion method and apparatus for low-NO.sub.x combustion of a pulverized solid fuel in which combustion products including combustion air from a partial oxidation combustor are mixed with a pulverized solid fuel, thereby preheating the pulverized solid fuel and resulting in devolatilization of at least a portion of the pulverized solid fuel. The preheated pulverized solid fuel and the devolatilization products are then burned in a burner firing directly into a combustion chamber.  
      Although significant strides have been made in the area of pulverized coal combustion to reduce nitrogen oxides NO.sub.x emissions generated by the combustion process, both greenhouse gas CO.sub.2 and NO.sub.x emissions from coal fired combustion facilities continues to be problematic. In addition, it is frequently the case that methods and devices for reducing NO.sub.x emissions from combustion systems in which the fuel is a pulverized coal, still undesirably impact other elements of the combustion process such as flame stability, turn down capability, CO.sub.2 emissions, and general energy combustion and transfer efficiency.  
      It would also be more desirable to provide a combustion method which is designed to reduce CO.sub.2 submissions before ignition and combustion rather than through a post combustion amine scrubbing process, which is connected to a substantial heat or energy penalty.  
     SUMMARY OF THE INVENTION  
      It is one object of this invention to provide a method and devices for pulverized solid hydrocarbon or biomass fuel combustion, especially pulverized coal, which results in a reduction of CO, CO.sub.2 and NO.sub.x emissions compared to conventional pulverized coal combustion methods and devices.  
      It is another object of this invention to provide a method and devices for pulverized coal combustion which will significantly reduce said harmful emissions by improving the ratio of oxygen in the fuel/combustion air mixture during the combustion process, compared to conventional pulverized coal or coal combination combustion methods and devices.  
      It is yet another object of this invention to provide a method and devices for pulverized coal combustion which increases said oxygen ratio by preheating the specified fluidized hydrocarbon fuel mix volume while precooling the specified combustion air volume, thereby providing increased combustion performance and combustion kinetics such as improved flame stability and greater turn down and energy conversion or energy transfer capabilities compared to known methods and devices.  
      It is a further object of this invention to provide a method and devices for pulverized coal combustion which increases the energy conversion and energy transfer efficiency during the combustion process by improving the flow characteristics of all fluids operating in the combustion system.  
      These and other objects of this invention are addressed by this method for low-CO, low NO.sub.x and low CO.sub.2 combustion of a pulverized solid fuel in which the pulverized fuel is suspended and fluidized in a secondary fluid hydrocarbon fuel for delivery and ignition in the burner or combustion area of a combustion system. Prior to ignition, the fluidized mixture of pulverized solid fuel and fluid hydrocarbon fuel is substantially preheated to between ambient and the fuel mixture&#39;s auto-ignition temperature, while the combustion air is substantially precooled to a temperature between ambient and minus 50 degrees F.  
      In accordance with a particularly preferred embodiment of this invention, the pulverized and fluidized solid fuel is coal. It should further be noted that the step of specifically mixing the pulverized solid fuel into a secondary fluid or liquid fuel is required in order to provide the most ideal fuel mixture condition for the purpose of fuel preheating prior to ignition.  
      It is one object of this invention to provide a combination device for treatment of a pulverized solid fuel utilizing the method of this invention, comprising a fluid hydrocarbon fuel supply conduit for the transport of fluidized pulverized solid fuel to the combustion chamber of the combustion system. Said fuel supply conduit is equipped with a fuel turbulator assembly to alter the flow characteristics of said fluid pulverized solid fuel from laminar to turbulent, and is further equipped with a heat exchanger assembly to increase the operating temperature of said fluid pulverized solid fuel prior to ignition to an optimal operating temperature, ranging from ambient to the auto-ignition temperature of the fuel mixture.  
      It is another object of this invention to provide a further combination device for treatment of combustion air utilizing the method of this invention, comprising a combustion air supply conduit for the transport of combustion air to the burner or combustion chamber of the combustion system. Said air supply conduit is equipped with an air turbulator assembly to alter the flow characteristics of said combustion air from laminar to turbulent, and is further equipped with a heat exchanger assembly to reduce the temperature of said combustion air to an optimal operating temperature ranging between ambient and minus 50 degrees Fahrenheit.  
      It is yet another object of this invention to provide a further combination device for treatment of the energy transfer medium of the system, utilizing the method of this invention and comprising an energy transfer operating medium conduit for the transport of converted energy through the energy transfer conduit of the combustion system.  
      Said energy transfer operating medium conduit is equipped with a turbulator assembly to alter the flow characteristics of any operating medium, should said medium be a fluid, or should such medium be combined with other combustion oxidation products, which may sometimes be desirable, converting the ambient flow characteristics of said fluid operating medium from its ambient laminar to a more appropriate turbulent condition, and said conduit is further equipped with the means to efficiently transfer converted energy to the operating medium or its related mechanical operating means.  
      It is yet one more object of this invention to provide a further combination device operating as a turbulator for the treatment of the flue exhaust gas stream before exiting the combustion system, providing conditions for utilizing any waste energy contained in the exhaust gases by altering the ambient flow characteristics of said exhaust gases from their ambient laminar to a turbulent condition, so that any waste energy remaining in the exhaust gas stream may be more efficiently transferred for heating the fuel mixture or for cooling the combustion air of the mechanism.  
      It is the paramount object of this invention that no portion of the specified combustion air volume, which may most desirably be at the stoiciometric ratio, be ever preheated. Contrary to the established combustion practice of preheating up to 60% of combustion air volume, which reduces the oxygen ratio in the air volume to between 1% and 15%, the disclosed precooling of combustion air to a maximum of minus 50 degrees F. will instead reverse such negative condition by increasing air density and in turn increasing the oxygen ratio in the volume of air from an ambient 21 to 35 volume percent or more, providing a significant oxygen enrichment of 20 volume percent above ambient.  
      The combustion industry is now beginning to be more aware of “Advances in Industrial Energy-Efficiency Technologies” and it is becoming more apparent that “Energy is wasted when ambient air is used in the combustion of fossil fuels”. This of course includes preheated combustion air with a much-reduced oxygen content per volume of air.  
      It is therefore the main object of this invention to provide the means to provide a more efficient combustion method which reduces harmful emissions, especially CO, NO.sub x and CO. sub 2, by reducing the amount of pulverized solid hydrocarbon fuel required to generate and transfer energy from such fuel to an operating medium.  
      It is the further paramount object of this invention that any pulverized fuel be fluidized and suspended only in a secondary fluid hydrocarbon fuel instead of in any portion of the required or specified combustion air volume, as is the established combustion practice. Thereby, when preheating a combination fuel mixture only, the combustion air oxygen ratio is not detrimentally effected.  
      As detailed herein, a large variety and combination of embodiments of the method and apparatus of this invention are possible. However, embodiments which are not specifically described herein but which fall within the scope of the invention defined by the claims hereof are deemed to be contemplated and within the scope of this invention.  
      In addition, while the detailed description disclosed herein is directed to coal as the preferred pulverized solid fuel, it will be understood that the method and devices of this invention may be applied to other solid fuels, such as biomass, wood waste and other pulverized solid hydrocarbon fuels, which may be each or all combined in a secondary fluid fuel like natural gas, propane gas or fuel oil, and, thus, applications to such and all other pulverized solid fuels are deemed to be within the scope of this invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      These and other objects and features of this invention will be better understood from the following detailed description taken in conjunction with the disclosed drawings  
       FIG. 1  is a schematic diagram showing the basic low emission pulverized solid fuel mixture combustion process of this invention as well as the related elements required to operate the various devices of this invention for carrying out the method and process;  
       FIG. 2  is another schematic diagram showing the basic low emission pulverized solid fuel mixture combustion process of this invention detailing the preferred and alternative means of the fuel preheating and combustion air precooling process and devices required to operate the method of this invention;  
       FIG. 3  is a sectional view through a fuel preheating or combustion air precooling component, incorporating one of the means to alter a fluid flow from laminar to turbulent.  
       FIG. 4  is a further sectional view through a fuel preheating or combustion air precooling component, incorporating an alternative means to alter a fluid flow from laminar to turbulent. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
      As previously stated, the pulverized solid fuel combustion method and devices of this invention are applicable to a variety of furnaces, boilers and other combustion mechanisms. By the term “pulverized solid fuel,” we mean a solid fuel having particle sizes whereby the particles are capable of being suspended in a carrier fluid, which must never be air, but must always be a secondary fluid hydrocarbon fuel, by which the particles are conveyed to the combustion chamber. In order to conform to established and accepted combustion process operating standards for pulverized coal, particle sizes should typically be such that about 90% of the particles can pass through a 200 mesh screen, that is less than about 75 microns. In the case of biomass, for a carrier fluid velocity corresponding to the carrier fluid velocity for pulverized coal, the particle sizes are such that about 90% of the particles can pass through a 100 mesh screen, that is less than about 150 microns.  
      Referring to  FIG. 1 , there is shown a schematic view of the method for low emissions combustion of pulverized solid hydrocarbon fuel in accordance with one embodiment of this invention comprises crushing solid fuel  1  in a solid fuel pulverizer  2 , and introducing such pulverized hydrocarbon fuel into a carrier fluid consisting of a secondary fluid hydrocarbon fuel  3  into a fluid fuel mixer  4 . In accordance with one preferred embodiment of this invention, the fluid carrier fuel may be natural gas or fuel oil. The pulverized solid fuel and secondary fluid fuel mixture is transported from fuel mixer  4  to fuel turbulizer  5 , a device in which the flow characteristics of the fuel is changed from laminar to turbulent. The fuel turbulizer also provides the means of a heat exchanger, preheating the tubulated pulverized fluid fuel mixture to a temperature range between ambient and the fuel auto-ignition temperature, before the fuel enters the combustion chamber or burner  8  for ignition. The turbulated fuel provides a significantly more superior condition for heat transfer. Combustion air  6  is directed through air turbulizer  7 , a device in which the flow of combustion air is changed from laminar to turbulent. The air turbulizer also provides the means of a heat exchanger, precooling the turbulated combustion air to a temperature range between ambient and minus 50 degrees Fahrenheit, before the air enters the combustion chamber or burner  8  for ignition. The turbulated combustion air provides a significantly more superior condition for energy transfer.  
      Referring to  FIG. 2 , there is shown a further schematic view of the method for combustion of pulverized solid hydrocarbon fuel, wherein, after oxidation of the air and fuel mixture in combustion chamber or burner  8 , the combustion products  9  and the converted energy are transferred to one of a variety of operating systems, before entering the exhaust gas turbulator  10 . Any waste energy remaining in the exhaust gases may then be available for further conversion, whereby fuel turbulator  5  may have the means for energy transfer from said exhaust gas turbulator  10  in order to preheat fuel by way of direct heat transfer from the exhaust products. Air turbulator  7  may have means for energy transfer from exhaust gas turbulator  10  in order to precool combustion air by way of thermochemical compression. Fuel turbulator  5  and air turbulator  7  may also acquire the required energy for heating and cooling by making use of independent operating means  12  and  13  as already known in prior art.  
      Referring to  FIG. 3 , there is shown a sectional view of the surface configuration  14  of a typical turbulator assembly shown as  5 ,  7  or  10 , wherein the surface of the turbulator assembly is equipped with protrusions in the form of convoluted corrugations  15 , placed in a direction opposing the general fluid flow direction such as to alter any adjacent laminar fluid flow from laminar to turbulent.  
      Referring to  FIG. 4 , there is shown another sectional view of the surface configuration  14  of a typical turbulator assembly indicated as  5 ,  7  or  10 , wherein the surface of the turbulator assembly is now equipped with alternative protrusions in the form of convoluted dimples  16 , placed in a direction to most effectively oppose the general fluid flow direction such as to alter any adjacent laminar fluid flow from laminar to turbulent.