Abstract:
A thermally efficiency regenerative air preheater  250  extracts more thermal energy from the flue gas exiting a solid fuel fired furnace  26  by employing an alkaline injection system  276 . This mitigates acid fouling by selectively injecting different sized alkaline particles  275  into the air preheater  250 . Small particles provide nucleation sites for condensation and neutralization of acid vapors. Large particles are injected to contact and selectively adhere to the heat exchange elements  542  and neutralize liquid acid that condenses there. When the deposit accumulation exceeds a threshold, the apparatus generates and utilizes a higher relative percentage of large particles. Similarly, a larger relative percentage of small particles are used in other cases. Mitigation of the fouling conditions permits the redesign of the air preheater  250  to achieve the transfer of more heat from the flue resulting in a lower flue gas outlet temperature without excessive fouling.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. Application Ser. No. 61/245,822 “Exhaust Process and Heat Recovery System” by James W. Birmingham and Kevin J. O&#39;Boyle filed Sep. 25, 2009 and incorporates the material of the priority application to the extent that it does not contradict the present application. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates generally to an exhaust processing and heat recovery (EPHR) system and method for use with fossil fuel fired furnaces. More particularly, the present invention relates to an EPHR system in which alkaline particles are introduced into a flue gas stream to allow additional heat extraction and reduce fouling of air preheater equipment. 
         [0004]    2. Discussion of Related Prior Art 
         [0005]    Many power generation systems are powered by steam that is generated via furnaces fired by fossil fuels, such as, for example, coal or oil. A typical power generation system is generally depicted in the diagram shown in  FIG. 1A . 
         [0006]      FIG. 1A  shows a power generation system  10  that includes a steam generation system  25  and an exhaust processing and heat recovery system (EPHRS)  15  and an exhaust stack  90 . The steam generation system  25  includes a furnace  26 . The EPHRS  15  may include a regenerative air preheater  50 , a particulate removal system  70  and a scrubber system  80 . A forced draft (FD) fan  60  is provided to introduce air into the cold side of the air preheater  50  via inlet  51 . The particulate removal system  70  may include, for example, an electrostatic precipitator (ESP), and/or a fabric filter system (Bag House), or the like. Scrubber system  80  may include, for example, a wet or dry flue gas desulphurization (WFGD/DFGD) systems. 
         [0007]    The regenerative air preheater  50  helps increase the thermal efficiency of furnace  26 , thereby reducing its operating costs and emissions of greenhouse gases. An air preheater  50  is a device designed to heat air before it is introduced to another process such as, for example, the combustion chamber of a furnace  26 . There are different types of regenerative air preheaters, including those that include moving or rotating heat exchange elements, such as, for example, the Ljungstrom® air preheater. Other regenerative air preheaters utilize fixed heat exchange elements and/or internally rotating hoods or ductwork that is fixed to rigid air and/or gas ducts. 
         [0008]      FIG. 1B  and  FIG. 1C  are diagrams generally depicting a conventional rotary regenerative preheater  50 . The typical air preheater  50  has a rotor  512  rotatably mounted in a housing  524 . The rotor  512  is formed of diaphragms or partitions  516  extending radially from a rotor post  518  to the outer periphery of the rotor  512 . 
         [0009]    The partitions  516  define compartments  520  there between. These partitions  516  contain heat exchange element basket assemblies  522 . Each basket assembly  522  includes one or more specially formed sheets of heat transfer surfaces that are also referred to as heat exchange elements  542 . The surface area of the heat exchange elements  542  is significant, typically on the order of several thousand square feet. 
         [0010]    In a typical rotary regenerative air preheater  50 , the flue gas stream, FG 1  and the combustion air stream, A 1 , enter the rotor  512  from opposite ends/sides of the air preheater  50  and pass in opposite directions over heat exchange elements  542  that are housed within the basket assemblies  522 . Consequently, the cold air inlet  51  and the cooled flue gas outlet  54  are at one end of the air preheater  50  (generally referred to as the cold end  544 ) and the hot flue gas inlet  53  and the heated air outlet  52  are at the opposite end of the air preheater  50  (generally referred to as the hot end  546 ). Sector plates  536  extend across the housing  524  adjacent the upper and lower faces of the rotor  512 . The sector plates  536  divide the air preheater  50  into an air sector  538  and a flue gas sector  540 . 
         [0011]    The arrows shown in  FIG. 1B  and  FIG. 1C  indicate the direction of the flue gas stream FG 1 /FG 2  and the air stream A 1 /A 2  through the rotor  512 . The flue gas stream FG 1  entering through the flue gas inlet  53  transfers heat to the heat exchange elements  542  in the basket assemblies  522  mounted in the compartments  520  positioned in the flue gas sector  540 . The heated basket assemblies  522  are then rotated to the air sector  538  of the air preheater  50 . The stored heat of the basket assembly  522  is then transferred to the air stream A 1  entering through the air inlet  51 . The cold flue gas FG 2  stream exits the preheater  50  through the flue gas outlet  54  and the heated air stream A 2  exits the preheater  50  through the air outlet  52 . 
         [0012]    Referring back to  FIG. 1A , air preheater  50  heats the air introduced via FD fan  60 . Flue gas (FG 1 ) emitted from the combustion chamber of the furnace  26  is received by the air preheater via inlet  53 . Heat is recovered from the flue gas (FG 1 ) and is transferred to input air (A 1 ). Heated air (A 2 ) is fed into the combustion chamber of the furnace  26  to increase the thermal efficiency of the furnace  26 . 
         [0013]    During the combustion process in furnace  26 , sulfur in the fuel used to fire the furnace  26  is oxidized to sulfur dioxide (SO 2 ). After the combustion process, some amount of SO 2  is further oxidized to sulfur trioxide (SO 3 ), with typical amounts on the order of 1% to 2% going to SO 3 . The SO 2  and SO 3  will be passed from the combustion chamber of the furnace  26  and into the exhaust flue as part of the flue gas FG 1  that is then emitted from the steam generating system  25  and received by the inlet  53  of air preheater  50 . The presence of iron oxide, vanadium and other metals at the proper temperature range allows this oxidation to take place. Selective catalytic reduction (SCR) is also widely known to oxidize a portion of the SO 2  in the flue gas FG 1  to SO 3 . 
         [0014]    As heat is being recovered/extracted by the air preheater from the flue gas FG 1 , the temperature of the flue gas FG 1  is reduced. It is desirable to remove the maximum amount of heat from the flue gas and transfer it to the heated air going to the furnace or the fuel pulverizer mills to optimize the thermal efficiency of the power plant. Additional heat extraction allows for the design/use of particulate collection equipment, gaseous cleanup equipment, ducting and stacks downstream of the flue gas outlet that are rated for lower temperature ranges and reduced volumetric flow rates. The lower temperature rating and lower flow rate mean that tremendous cost savings can be realized by not having to provide equipment capable of withstanding higher temperatures and higher flow rates. However, the lower flue gas temperature range may result in excessive condensation of sulfur trioxide (SO 3 ) or sulfuric acid vapor (H 2 SO 4 ) that may be present in the flue gas. As a result, sulfuric acid may accumulate on surfaces of the heat exchange elements  522  of the air preheater  50 . Fly ash in the flue gas stream can be collected by the condensed acid that is present on the heat transfer surfaces. This acid causes fly ash to stick more tightly to surfaces. This “fouling” process impedes the air and flue gas flow thru the air preheater, resulting in increased pressure drop through the air preheater plus lower heat transfer effectiveness. 
         [0015]    After a period of time, accumulations of acid and flyash on surfaces of the air preheater  50  grow so large that they must be removed in order to maintain the thermal performance and an acceptable pressure drop the air preheater. This is typically accomplished by periodically (for example, 3 times daily) “sootblowing” the heat transfer surface with compressed air or steam to remove the deposits that have accumulated on the heat transfer surface while the air preheater is operating. In addition, if required, washing the air preheater with water may be conducted during an outage of the steam generation system  25  when the furnace  26  is shut down and maintenance operations are performed. 
         [0016]    A potential benefit to reducing the flue gas outlet temperature is that the particulate removal system  70  and scrubbing equipment  80  may be designed for a lower operating temperature. The lower temperature flue gas also has a lower volumetric flow rate. The reduction in flue gas temperature, volume and acidity reduce operating and capital costs that are associated with equipment designed for the higher volumetric flow rates, higher operating temperatures, or higher SO 3 /H 2 SO 4  concentrations in the flue gas. These conditions would exist if the acid were not condensed and/or neutralized to prevent excessive fouling of the heat transfer surfaces. Once the flue gas exhaust has passed through particulate removal and scrubbing operations, it is then ready for introduction to the exhaust stack  90  for elevation and dispersion over a wide geographic area. 
         [0017]    Extraction of heat from flue gases is beneficial and is used for performing various operations in a typical plant. However, in existing coal and/or oil fired steam generation systems, it is costly to remove additional heat from the exhaust gas stream. Excessive reduction of the flue gas temperature without consideration for the additional condensation of H 2 SO 4  vapors in the flue gas, will result in excessive fouling of the heat transfer surfaces in the air preheater. Thus, a need exists in the industry to address the aforementioned deficiencies and inadequacies. 
       SUMMARY OF THE INVENTION 
       [0018]    The invention may be embodied as a method of extracting heat from a flue gas stream FG 1  having acidic material and flue gas particulates using an air preheater  250  having a flue gas inlet  253 , flue gas outlet  254  and a plurality of heat exchange surfaces  542 , comprising the steps of: 
         [0019]    receiving a flue gas stream FG 1  into the flue gas inlet  253  of the air preheater  250 ; 
         [0020]    calculating a mass flow rate of acid material passing in the flue gases FG 1 ; 
         [0021]    calculating a mass flow rate of alkaline particles  275  to be injected into the flue gas stream FG 1  to neutralize the acidic material; 
         [0022]    injecting alkaline particles  275  with a distribution of particles sizes at the calculated mass rate into the flue gas stream upstream of the air preheater  250 ; 
         [0023]    calculating a degree of accumulation of particulates; 
         [0024]    based upon the degree of accumulation of particulates, adjusting at least one of a size distribution of the alkaline particles  275  being injected into the flue gases, and the mass flow rate at which the alkaline particles  275  are injected into the flue gases; 
         [0025]    thereby reducing accumulation of flue gas particulates on the heat exchange elements  542 , plus reducing fouling within the air preheater, and thereby increasing the thermal efficiency of the air preheater  250 . 
         [0026]    The degree of fouling may be calculated by measuring a pressure drop across the air preheater  250  from the flue gas inlet  253  to the flue gas outlet  254  and comparing the measured pressure drop to at least one predetermined threshold. 
         [0027]    When using a rotary air preheater having a rotor that is rotated by an motor powered by electric current I of varying voltage V, the degree of fouling may be calculated by measuring the voltage V and electric current I, and comparing the measured current at the measured voltage to a predetermined current for the same voltage to determine a current difference. The current I difference is compared to prestored conversion information to determine a degree of fouling. 
         [0028]    The present invention may also be embodied as a method of reducing fouling of an air preheater  250  used in recovering heat from a furnace  26  that creates flue gases with acidic materials and flue gas particulates, comprising the steps of: 
         [0029]    providing an air preheater  250  coupled to said furnace  26  to receive said flue gases FG 1  at a flue gas inlet  253 , pass them over a plurality of heat exchange plates  542  and exhaust said flue gases out of a flue gas outlet  543 ; 
         [0030]    sensing or calculating a mass flow rate of acidic material in said flue gases; 
         [0031]    calculating a mass flow rate of alkaline particles required to adequately neutralize the acidic materials in the flue gases; 
         [0032]    injecting the alkaline particles  275  at the calculated mass flow rate into flue gases entering the air preheater  250 ; 
         [0033]    sensing a pressure drop from the flue gas inlet  253  to the flue gas outlet  254  of the air preheater  250 ; 
         [0034]    increasing the mass rate of alkaline particles  275  injected into the flue gases when the sensed pressure drop is greater than a predetermined threshold, and 
         [0035]    decreasing the mass rate of alkaline particles  275  injected into the flue gases when the sensed pressure drop is lower than a predetermined threshold; and 
         [0036]    repeating the steps above during operation of the furnace  26  to reduce fouling of the air preheater  250  allowing it to more efficiently extract heat. Additional heat, beyond the levels that are achieved with current air preheater design technologies, can be extracted from the flue gas as a result of reducing the gas outlet temperature of the heat exchanger without excessive fouling or corrosion activities within the air preheater that would exist if the SO 3 /H 2 SO 4  were not condensed and neutralized by the alkaline material injected into the flue gas stream upstream of the air preheater. 
         [0037]    The present invention may also be embodied as an exhaust processing and heat recovery (EPHR) system  215  for more efficiently recovering heat from a furnace  26  producing heated flue gases FG 1  having acid vapors and entrained flue gas particulates comprising: 
         [0038]    an air preheater  250  coupled to said furnace  26 , the air preheater  250  having: 
         [0039]    an flue gas inlet  253  adapted to receive said flue gases FG 1 , 
         [0040]    a plurality of heat exchange plates  522  for extracting heat from the flue gases; and 
         [0041]    a flue gas outlet  254  for exhausting the flue gas stream FG 2  after it has passed over the heat exchange plates  522 ; 
         [0042]    flue gas sensors  310  to monitor physical and chemical conditions within the flue gases; 
         [0043]    pressure drop sensors  301 ,  303  adapted to measure the drop in pressure from the air preheater inlet  253  to the air preheater outlet  254 ; 
         [0044]    an alkaline injection system  276  responsive to control signals from another device, for introducing alkaline particles  275  into a flue gas stream FG 1  upstream of an air preheater  250  when actuated; and 
         [0045]    a PLC controller  305  adapted to calculate a mass flow rate of alkaline particles  275  based upon the sensed flue gas conditions; and adapted to control the alkaline injection system  276  to inject the calculated mass flow rate of alkaline particles  275  to neutralize the acidic materials in the flue gases. 
         [0046]    The present invention may also be embodied as an efficient, low cost furnace system having: 
         [0047]    a. a fossil fuel furnace that produces heated flue gases; 
         [0048]    b. an air preheater coupled to the furnace, adapted to receive the heated flue gases, neutralize acids in the heated flue gases, extract heated combustion air for the furnace, extract additional heated air to be used elsewhere in the system, reduce flue gas temperature below a flue gas acid dew point, reduce the volume of flue gases exiting the preheater; and 
         [0049]    c. flue gas processing equipment coupled to, and downsteam of the air preheater that are more compact and less costly than those used on systems that do not have air preheaters that neutralize flue gas acids. 
         [0050]    Other systems, methods, features, and advantages of the present invention will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0051]    The present invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings in which: 
           [0052]      FIG. 1A  is a diagram depicting a typical steam generation system and associated exhaust processing equipment. 
           [0053]      FIG. 1B  is a diagram depicting a perspective view, partially broken away, of a conventional rotary regenerative air preheater. 
           [0054]      FIG. 1C  is a schematic diagram depicting a further perspective view of the conventional rotary regenerative air preheater of  FIG. 1B . 
           [0055]      FIG. 2A  is a diagram generally depicting one embodiment of an exhaust processing and heat recovery system in accordance with the invention. 
           [0056]      FIG. 2B  is a diagram generally depicting a further embodiment of an exhaust processing and heat recovery system in accordance with the invention. 
           [0057]      FIG. 3  is a schematic diagram depicting an embodiment of an air preheater having an auxiliary air inlet. 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0058]    The purpose of this invention is to provide a means to extract more heat from the flue gas as it passes through the gas side of the regenerative air heater without the heat transfer surfaces of the equipment downstream of the regenerative air preheater becoming excessively fouled or corroded. 
         [0059]    The present invention is directed to control the amount of acid that is condensed and accumulated on heat transfer elements of an air preheater and to thereby improve the effectiveness of the air preheater in extracting heat from a flue gas stream FG 1  from the combustion chamber of, for example, a furnace. A further aspect of the invention is directed to controlling the “wetness” of the deposit on the heat transfer surfaces so that the deposit can be maintained in a condition that allows it (the deposit) to be easily removed while the air preheater is in operation. A further aspect of the proposed invention is directed to an air preheater that is configured to allow for the distribution of additional heat extracted from the flue gas stream FG 1  due to the increased efficiency of the air preheater in extracting heat from the flue gas stream. 
         [0060]    Reduction of the SO 3  concentration entering the air heater, plus an additional means to extract heat from the flue gas as it passes through the air preheater will have several benefits: (1) the volumetric flue gas flow leaving the air heater will be lower, (2) the preheat temperatures of the air side flows (generally called primary and secondary air) can be increased, and (3) Additional energy in the form of preheated air can be made available for use elsewhere in the plant. Potential uses of this additional energy are: preheating boiler feedwater, drying pulverized coal, conveying the pulverized coal to the burners, supplying energy to post-combustion CO 2  capture systems, reheating stack gas to reduce visible water vapor plume or for other uses where heat is needed within a power plant. 
         [0061]      FIG. 2A  and  FIG. 2B  are diagrams generally depicting embodiments of an exhaust processing and heat recovery system  215  in accordance with the proposed invention.  FIG. 2A  is a diagram depicting one embodiment of an EPRS  215  that is includes an alkaline injection system  276  to interactively introduce a sorbent of alkaline particles  275  into the flue gas stream FG 1  prior to FG 1  being received by the air preheater  250  via inlet  253 . Alkaline injection system  276  has the ability to selectively introduce various size distributions of alkaline particles  275  in the sorbent. 
         [0062]    In this embodiment, the EPRS  215  includes a regenerative air preheater  250 , a particulate removal system  70  and a scrubber system  80 . An FD fan  60  is provided to introduce an air stream A 1  into the cold side of the air preheater  250  via inlet  251 . The particulate removal system  70  may include, for example, an electrostatic precipitator (ESP), and/or a fabric filter system (bag house), or the like. Scrubber system  80  may include, for example, a wet or dry flue gas desulphurization (WFGD/DFGD) system. 
         [0063]    During operation of the EPRS  215 , sulfur trioxide (SO 3 ) and water vapor (H2O) in the flue gas FG 1  can combine to form an acid vapor in the operating temperature range of the flue gas upstream of the air preheater  250 . Once the flue gas containing this acid vapor reaches the air preheater  250  it will come in contact with, condense and accumulate on, various surfaces in the air preheater  250 , including heat transfer elements ( 542  of  FIG. 1B ) when it is cooled below its acid dew point temperature. This accumulation of condensed acid will “foul” the air preheater operation by collecting and retaining flyash particles on the surface of the heat transfer surface, thus impeding the flow of flue gas FG 1  through the air preheater  250 . This results in an excessive pressure drop through the air preheater and overall drop in effective transfer of heat from the flue gas stream FG 1  to the input air stream A 1 . 
         [0064]    The acid vapor and condensed acid may be referred to collectively as ‘acidic material’. 
         [0065]    One embodiment of the present invention employs flue gas sensors  310  that monitor physical and chemical parameters of the flue gas. Depending upon their use they may be located at the inlet or outlet, or other location within the air preheater  250 . 
         [0066]    A programmable logic controller (“PLC controller”)  305  reads the sensor information and determines a proper mass flow rate to neutralize the acidic material in the flue gases. This mass flow rate may also be determined by calculation from air and fuel firing conditions that are transmitted from the furnace by various methods of data communication in use in fossil fuel fired furnaces. It may also control an alkaline injection system  276  causing it to inject the calculated mass flow rate of correctly sized alkaline material into the flue gases upstream of the flue gas inlet  253 . 
         [0067]    Alkaline particles  275 , such as powdered limestone or other alkaline materials are introduced as a sorbent into the flue gas stream FG 1  upstream of the air preheater  51  (i.e. before the flue gas stream FG 1  reaches the air preheater  50 ). These particles serve as condensation sites within the flue gas stream FG 1  for the acid vapors, and then function to neutralize the condensed acid. Both the condensation and neutralization of the acid occurs inside the air preheater when the flue gas is cooled to a temperature that will initiate condensation of the acid vapor. Introducing an adequate mass quantity, for example, 1% to 25% mass ratio of alkaline particles to flyash concentration into the flue gas stream FG 1  as it passes through the air preheater  250  causes most of the acid to neutralize. However, introducing alkaline material into the flue gas stream strictly on a stoichiometry basis does not result in the most effective control of fouling caused by the build-up of acid within the air preheater  250 . In order to more effectively control the creation and build up of acid within the air preheater, it is proposed that the alkaline particles that are introduced into the flue gas stream FG 1  have a varying range of sizes (diameters). 
         [0068]    By measuring the temperature gradient of the flue gas as it passes thru the heat transfer surfaces within the air preheater, and controlling the mass quantity, and size distribution of the alkaline particles that are introduced into the flue gas stream FG 1 , it is possible to control the extent to which acid condenses and remains on the heat transfer surface and in the flue gas as the flue gas passes through the air preheater  250 . 
         [0069]    The size of fly ash particles, produced from the typical combustion of coal, varies from below 0.01 microns to over 100 microns. The smaller diameter particles of fly ash or other particulate material in the flue gas stream FG 1 , generally less than 5 microns in diameter, tend to provide a good nucleus for condensation and potential neutralization of H2SO 4  vapor that may exist in the flue gas stream FG 1 . 
         [0070]    If the condensation results in a deposit on the heat transfer surface that cannot be removed by cleaning methods employed while the air preheater is in operation, the deposit will accumulate to the point where the normal operation of the air preheater cannot be maintained. However, when the condensation process is combined with the neutralization process that can occur when an adequate mass quantity of alkaline materials of the proper particle size distribution are injected into the flue gas stream, successful operation of the air heater can be maintained. The neutralization process will result in the reduction in the amount of acid that remains on the heat transfer surface and embedded in the particulate deposits within the air preheater. 
         [0071]    An important factor in the effectiveness of the control of fouling within the air preheater is the location where the flue gas particulates and alkaline particles in the flue gas contact the various heat transfer surfaces of the air preheater exchange elements ( 542  of  FIG. 1B ), as well as the size of these particles. Smaller particles have a greater tendency to follow the flue gas flow and a lesser tendency to strike the surface of heat exchange elements. Large particles, generally greater than 15 microns, have more momentum and a greater tendency to impact the surface of the heat exchange elements. Large particles also have a greater tendency to fall off (without accumulating thereon) the surfaces of the heat exchange elements if there is little or no acid present on the surface of the particle or on the surface of the heat exchange elements. The large particles can also act to “scrub”, or erode, small particles from the air preheater surfaces, such as the heat transfer elements if the small particles are not strongly bonded to the surface. 
         [0072]    Injection of alkaline particles downstream of the air preheater is typically done to control SO 3  plume emissions and to enhance mercury removal by the bag house or precipitator. However, this does not impact the fouling of the air preheater. 
         [0073]    In the present invention, the alkaline particles are injected into the ductwork upstream of the gas inlet to the air preheater. They must be distributed via the injection system to insure that there is an adequate supply of the alkaline material is evenly dispersed throughout the cross-section of the ductwork to insure the condensation and neutralization processes can occur once the flue gas stream enters the air preheater and is cooled to its dew point temperature or comes in contact with the heat transfer surfaces within the air heater that are below the acid dew point temperature. 
         [0074]    When flue gases containing sulfur trioxide and water vapor are at a temperature that is below the acid dew point, sulfuric acid condensates to a liquid. Condensation will occur on surfaces within the air preheater having temperatures that are below the local dew point temperature, and upon further cooling, it may also occur within the gas stream itself. 
         [0075]    When the gas stream reaches a supersaturated state, sulfuric acid may condense by self-nucleation in the absence of entrained particulates. This generally occurs when the flue gas temperature is below the local acid dew point. If the gas stream contains entrained particles, these particles act as nucleation sites, and condensation occurs at temperatures closer to the local dew point. 
         [0076]    In general, and when present, the small particles are the first to produce condensate when it appears within the gas stream. This is due to the fact that small particles have higher surface area to volume ratios, and this allows them to more closely follow flue gas temperature during cooling. Large particles have lower ratios that cause them to retain more heat, and upon cooling, they remain warmer than the surrounding flue gas. Therefore, in order to preferentially condense and chemically neutralize acid on an injected alkaline particle—as opposed to condensing on native flyash with little neutralizing capacity due to its composition, the size of the particle should be small compared to the majority of the native fly ash particles. 
         [0077]    As previously stated, acid condensation begins on heat transfer surfaces with temperatures at or below the acid dew point. In order to adequately consume this acid to a level that results in a deposit on the heat transfer surfaces that can be removed by sootblowing or water washing, the alkaline particles must be deposited on the acid-wetted heat transfer surfaces at a suitable rate that adequately neutralizes the acid in the flyash. Thus, at this location the role of the alkaline particle has little in common with that of an optimum nucleation site, and its size requirements are different. 
         [0078]    The physical momentum of the gas-entrained particles is the means by which the majority of the particles reach the surfaces of the heat transfer elements within the air preheater. Assuming that all particles have the same density, and travel through the air preheater with a velocity equal to that of the surrounding flue gas, small particles have a lesser momentum due to their lower mass. Therefore, given equal quantities entrained in flue gas, small particles will have a lesser deposition rate on the heat transfer surfaces. If greater deposition rates are required to consume acid condensed on the heat transfer surface, a large alkaline particle size may be preferable compared to increasing the quantity of small alkaline particles in the gas stream. 
         [0079]    Optimum injection rates for alkaline particles may be achieved when the size distribution of the particles accounts for the two different purposes presented above. This size distribution is likely to be bimodal including ranges of both small and large particle sizes. 
         [0080]    It is possible to further locate where within the air preheater acid will condense. 
         [0081]    It is also possible to calculate and alter the alkaline particle distribution to ‘target’ locations with the air preheater to deposit the alkaline particles. 
         [0082]    As flue gas passes through the air preheater, it cools. This causes a temperature gradient to be created. Knowing the inlet temperature and the outlet temperature, one can estimate the gradient across the air preheater. 
         [0083]    As flue gas passes through the air preheater, it loses flow velocity. Again, this velocity gradient may be estimated knowing the inlet velocity and the outlet velocity. 
         [0084]    The alkaline particles are subject to the force of the flowing flue gases. The flue gas force exerted on a particle depends upon the flue gas velocity, the particle&#39;s wind resistance and the weight of the particle. 
         [0085]    The particles also have momentum due to their motion. The momentum of the particle is based upon the particle&#39;s velocity and mass. 
         [0086]    When the flue gas force is not great enough to change the momentum of the particle directing it away from a surface, the particle impacts the surface. If the surface has condensed acid, the particle is very likely to stick to the surface. If the particle is an alkaline particle, it neutralizes some of the condensed acid. 
         [0087]    Smaller particles have high surface area/mass ratio, and therefore a large wind resistance per unit mass. Larger particles have a smaller surface area to mass ratio, and have less wind resistance per unit mass and are less affected by the flue gas force. 
         [0088]    For the same velocity, particles with greater mass have a larger momentum. 
         [0089]    Assuming the same density for all particles, larger particles have larger mass. 
         [0090]    As particles travel through the air preheater, they lose velocity. If the flue gas forces become weak enough (due to the lower velocity) so that they cannot alter the momentum of the particle away from a surface, the particles impact surfaces within the air preheater. 
         [0091]    The distance that the particles travel through the air preheater before impacting a surface is dependent upon the particle size. Very small particles may be carried with the flue gas out of the preheater without impacting a surface at all. Therefore, the particle size is indicative of the location that a particle will be deposited and particle size distribution indicates how many particles will be deposited at various locations within the air preheater. If the particle size distribution is continuous in a proper size range, then the particles will blanket a contiguous region within the air preheater. Therefore, if one determines the location where the acids will condense, the particle size distribution may be chosen to deposit the majority of particles in the locations where acid is expected to condense. 
         [0092]    The mass quantity of alkaline material, as well as the particle size distribution of the alkaline material, are factors in controlling the degree of fouling within the air preheater. The overall quantity of alkaline material introduced into the flue gas stream FG 1  must be adequate, however the particle size distribution must also be provided so that the alkaline particles actually contact the heat transfer surface locations within the air preheater at points where the acid condensation/accumulation tends to occur. As the acid in the flue gas stream FG 1  is neutralized and consumed, the accumulations become less sticky and can be more easily removed with soot blowing and/or water washing technologies. Without condensed acid present in the flue gas stream FG 1 , or on the heat transfer surface, particles, such as fly ash, do not form a deposit with strong adhesion properties on the surface of the heat exchange elements and thus, will not accumulate on the heat exchange elements to the thickness that will impede the flow of flue gas FG 1  thru the air preheater. The less that the flow of flue gas FG 1  thru the air preheater is impeded, the more heat the air preheater can extract from the flue gas stream FG 1 . 
         [0093]    In one embodiment of the proposed invention, alkaline particles are introduced into the flue gas stream FG 1  have a bi-modal particle size distribution. These alkaline particles include “small” particles and “large” particles. The small particles are preferably sized to be within a range of 1 micron-15 microns in diameter, while the large particles are sized to be within a range of 15 microns to 150 microns. In general, all particles introduced into the flue gas stream FG 1  will be within a size range of 1 microns to 250 microns in diameter. The mass quantity of alkaline material required to be injected into FG 1  is a function of the SO 3 /H 2 SO 4  concentration in FG 1 , the flue gas flow rate, the mass quantity of flyash in FG 1 , and the chemical composition of the flyash in FG 1 . In general, the higher the concentration of SO 3 /H 2 SO 4  in FG 1 , the higher the mass quantity of alkaline material that must be injected. Flyash with a higher alkaline content will generally require less injection of alkaline material into FG 1  because the native alkalinity of the fly ash will aid the neutralization and consumption of H 2 SO 4  in the flue gas stream. The alkaline particles are preferably introduced into the flue gas stream FG 1  before the flue gas stream FG 1  reaches the air preheater. Flue gas sensors  310  may include a flue gas flow rate sensor, a particulate concentration sensor, and/or a sampling sensor, for measuring the alkalinity of the flue gas particulates. 
         [0094]    These particles may be introduced into the flue gas stream FG 1  via, for example, as a dry material or as a liquid slurry that is injected via a distribution system, such as, for example, spray nozzles or injection devices (injectors) for introducing the particles into the flue gas stream FG 1 . The distribution system may be installed in the gas inlet ductwork leading to the air preheater. The distribution system is preferably configured to result in a uniform and adequate distribution of alkaline material across the flue gas stream FG 1  as it enters the air preheater. Alkaline distribution system  276  may employ compressed air to be utilized as a transport medium for the dry injection, or water supplied via a pump(s) could be used as the transport medium for the wet injection. Dry injection is the preferred method of introducing the alkaline particles into FG 1 , but a wet system designed to provide adequate dwell time in FG 1  for the evaporation of the water and drying of the alkaline particles is also a suitable method. 
         [0095]    The mass quantity per unit time of alkaline sorbent injected can be controlled by monitoring several operating parameters associated with the air preheater and plant operation. This information can be collected from the overall plant control system, or obtained by the installation of specific data collection instrumentation. This input is provided to a PLC controller  305  controlling an alkaline injection system  276 . The quantity of sorbent to be injected will be a function of the mass flow rate and temperature of the flue gas entering the air heater, plus the concentration of the SO 3  and water vapor in the flue gas entering the air heater. The content of SO 3  in the flue gas entering the air preheater could be calculated from the sulfur content of the fuel, air/fuel ratio in the furnace, plus the temperature of the flue gas leaving the furnace and catalyst system installed upstream of the air preheater. The content of SO 3  in the flue gas can be calculated from the combustion efficiency characteristics of the fuel firing system. Most of these parameters may be read from an industrial system controller (not shown) that is used to operate the furnace  26 , directly measured in the flue gas stream by flue gas sensors  310 , or measured by means of wet chemistry or other suitable instrumentation that is commercially available. As a general rule, the lower the temperature of the flue gas leaving the air preheater, the lower the temperature of the heat transfer surfaces within the air preheater. Therefore, the amount of acid condensed and accumulated on the heat transfer surfaces will increase as the gas outlet temperature is decreased. As a result, lower gas outlet temperature or lower heat transfer surface temperature operation will require a higher rate of sorbent mass flow injection to prevent excessive fouling of the air preheater with a deposit that is too “wet” to be removed. 
         [0096]    An added benefit of the large alkaline particles may be their natural tendency to aid in the “scrubbing” of deposits present on the heat transfer surfaces. Once again, the particle size that produces the scrubbing affect will have little in common with the size of an optimum nucleation site, and may not have the same size as a particle destined to consume acid condensed on the heat transfer surface. 
         [0097]    The above parameters are measured and fed as inputs to the PLC controller  305 . The PLC controller  305  can be used to control the particle size distribution and/or the amount of alkaline sorbent injected into the air preheater over the entire operating range. For example, as the mass flow of flue gas entering the air preheater  250  is reduced, the PLC controller  305  will recalculate the quantity of sorbent required as a result of this change while also factoring in the current status of the other parameters being measured to complete the calculation of the required quantity of sorbent mass flow and its associated particle size distribution, and send a signal to the alkaline injection system to adjust the quantity of sorbent injected or the distribution of the particle sizes. If the sulfur content of the fuel is reduced (or increased), this input would be fed to the PLC controller  305 , and in combination of knowing the current status of the other parameters noted above, the quantity and sizing of sorbent to be injected would be adjusted. 
         [0098]    The flue gas sensors  310  may include a flow rate sensor to determine the rate the flue gas is flowing through the preheater  250 , a particulate concentration sensor for measuring flue gas particulates, temperature sensors, and optionally sampling sensors to determine chemical properties of the flue gas particulates. PLC controller  305  reads information from these sensors to interactively calculate the proper mass flow rate of the alkaline particles  275  to be injected by alkaline injection system  276 . 
         [0099]    It would be desirable to change the particle size distribution of the sorbent being injected in order to optimize the location of the sorbent deposition on the heat transfer surface. The objective is to predict the location of the mass distribution of condensed acid on the heat transfer surface, and size the sorbent particles so their momentum would enhance the distribution of the sorbent material on the heat transfer surface in direct relation to the distribution location of the condensed acid. In this manner, the ratio of sorbent material of the proper sizing can be deposited on the heat transfer surface in the optimum location to react with the amount of condensed acid at a given location. 
         [0100]    In addition to the above control logic, a pressure drop across the air preheater  250  would be continuously measured by sensors  301 ,  303  and compared to the calculated threshold (as defined in an algorithm installed in the PLC controller  305 ) as a function of the flue gas and air side flow rates and temperatures. 
         [0101]    The predicted pressure drop vs. time relationship that would be desired to exist between sootblowing cycles of the heat transfer surface would also be an input to the PLC controller  305 . If the actual pressure drop increased at a faster rate, it would be indicative of a buildup of flyash deposit and sulfuric acid on the heat transfer surface due to an inadequate mass quantity of sorbent injection, incorrect particle size distribution of the sorbent material, or improper operation of the alkaline injection system  276 . 
         [0102]    The PLC controller  305  would increase the sorbent injection rate in an attempt to return the pressure drop across the air heater vs. time relationship to the proper level. In addition, the sizing of the sorbent material would be altered by evaluating the various operating parameters used to control the system, and sending the proper signal to the pulverizing system to alter the sizing of the sorbent material as determined by the algorithm in the PLC controller  305 . Note that the sorbent particle sizing process would not be applicable if the sorbent was injected via a slurry or solution. 
         [0103]    Conversely, if the rate of pressure drop increase was below the predicted level based on actual operating conditions as calculated in PLC controller  305 , the sorbent injection rate would be decreased to reduce operating costs. 
         [0104]    During the sootblowing cycle, the flyash that has accumulated on the heat transfer surface since the last sootblowing cycle should be removed, and the resulting pressure drop across the air preheater would be reduced. However, if the deposit is too “wet” due to the presence of non-neutralized sulfuric acid, it will not be removed during the sootblowing cycle. Therefore, for a given flue gas flow rate and temperature, if the air preheater pressure drop vs. time relationship is greater than the standard profile that would be entered into the PLC controller  305 , it would indicate that not enough sorbent is available in the flue gas, and/or the particle size distribution of the sorbent material is incorrect for the current operating conditions. A signal would be sent from the PLC controller  305  to the alkaline injection system  276  to increase the sorbent injection rate and/or alter the sorbent particle size distribution. 
         [0105]    If the proper mass rate of alkaline particles  275  is being provided according to PLC controller  305 , and the pressure drop exceed the calculated threshold, a larger relative ratio of large to small particles is provided as a sorbent  275 . More of the large particles will come in contact with the heat transfer surfaces and neutralize and consume the acids holding particulates to the surfaces. If the sensed pressure drop is below the threshold, a smaller relative ratio of large to small alkaline particles is provided, allowing for more small particles to act as nucleation sites in the flue gases. 
         [0106]    PLC controller  305  may optionally control a pulverizer  277  to direct the pulverizer to grind of alkaline particles  275  of a desired size or a distribution of sizes. 
         [0107]    Other operating parameters that could be integrated into the PLC controller  305  to determine the sorbent injection rate are the voltage and amperage of the electric motor that is used to drive the rotor ( 512  of  FIG. 1B ) of the air preheater  250 . As the mass of particulate deposits increase on the heat transfer surface of the air preheater, the overall weight of the rotor will increase. For a given voltage to the motor, this will cause the amperage draw by the motor to increase due to the additional friction in the rotor support bearing system as a result of the increased weight of the rotor on the bearing assembly. Therefore, the rotor drive motor voltage and amperage would be continuously measured and fed to the PLC controller  305  and included in the overall calculation to determine the mass injection rate and particle size distribution of the sorbent. The PLC control logic would include the target amperage to be maintained, and the range of acceptable amperage swing that could result from the normal accumulation of flyash on the heat transfer surface that would occur during the sootblowing cycles for the heat transfer surface. The PLC controller  305  would include the calculation methods to accommodate voltage swings that might occur, and therefore, adjust the target amperage level to be maintained as a function of the actual voltage levels if necessary. 
         [0108]    As noted above, the introduction of alkaline particles into the flue gas stream FG 1  greatly increases the effectiveness of the air preheater in capturing more heat from the flue gas stream FG 1  and reduces the fouling of the heat transfer surface. This permits the gas outlet temperature of the flue gas leaving the air heater to be reduced. Practical design and cost limitations tend to determine the temperature at which the preheated air will leave the air preheater. However, the maximum gas outlet temperature reduction can be achieved while maintaining the desired air temperature leaving the air preheater by increasing the mass flow of air passing through the air preheater. In view of this, some provisions may be made to distribute excess heat in the form of additional heated air side mass flow to operations other than furnace operations. 
         [0109]    In a further embodiment of the proposed invention (See  FIG. 2B ), an air preheater  250  is provided that is configured to distribute heat extracted from the flue gas FG 1  to the furnace  26  via air stream A 2  and to other purposes via auxiliary air stream(s) A 3  and/or B 2 . Possible uses for these auxiliary air streams may include, for example, coal mill drying and grinding operations and/or preheating boiler feed water, site heating or cooling processes, preheating of the air entering the air preheater by direct recirculation of a portion of the heated air leaving the air heater to the inlet side of the air preheater so that it is mixed with the ambient air prior to increase the temperature of the air flow entering the air heater, indirect heating of the ambient air via the use of a heat exchanger wherein a portion of the hot air leaving the air heater is used to preheat the incoming ambient air prior to entry into the regenerative air preheater. There are additional uses such as off site uses district heating for industrial processes requiring a source of heated air, and thermal energy provided to CO 2  capture systems, including but not limited to, chilled ammonia or amine injection processes. 
         [0110]    With reference to  FIG. 2B , the EPRS  215  includes a regenerative air preheater  250 , a particulate removal system  70  and a scrubber system  80 . An FD fan  60  is provided to introduce an air stream A 1  into the cold side of the air preheater  250  via inlet  251 . As described above, the particulate removal system  70  may include an ESP and/or a fabric filter system, or the like. Scrubber system  80  may include a WFGD/DFGD system. 
         [0111]    In this embodiment, an additional FD fan  260  is provided to introduce an auxiliary air stream B 1  into the cold side of the air preheater  250  via inlet  256 . 
         [0112]      FIG. 3B  is a diagram generally depicting further details an air preheater  250  configured to provide an alternate stream of heated air to certain predefined operations other than to the furnace combustion chamber. 
         [0113]    With reference to  FIG. 3 , air preheater  250  is configured to include an inlet  251  for receiving an air stream A 1  and an auxiliary air inlet  256  for receiving an auxiliary air stream B 1 . An outlet  252  for outputting a heated air stream A 2  to a furnace ( 26  of  FIG. 2B ). An auxiliary outlet  255  is also provided for outputting a second stream of heated air B 2  to one or more predetermined operations or pieces of equipment such as a mill ( 270  of  FIG. 2B ). By having two separate outlets  252  and  255 , heated air streams A 2  and B 2  may be separately controlled and heat extracted from the flue gas stream FG 1  that is greater than is needed for proper operation of the furnace ( 26  of  FIG. 2B ). Heated air streams A 3 , B 2  may be easily routed for use in other uses associated with the steam plant operations, or other plant related operations. Further, by providing two air inlets A 1  and B 1 , it is possible to selectively or variably control air input to the air preheater. The principles and concepts disclosed and claimed herein are applicable to all air preheater devices/systems, including but not limited to bi-sector, tri-sector and quad-sector air preheater devices and systems. 
         [0114]    It should be emphasized that the above-described embodiments of the present invention, particularly, any “preferred” embodiments, are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the invention. Many variations and modifications may be made to the above-described embodiment(s) of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of this disclosure and the present invention and protected by the following claims.