Abstract:
A method for increasing the efficiency of a steam generator system including a boiler and a regenerative air preheater. The method including determining a reduced rate of acid accumulation in the preheater which may be achieved by injecting an SO 3  neutralizing additive material into flue gas generated by the boiler. A new maximum allowable clean condition pressure drop is calculated based on the reduced rate of acid accumulation. Modified heat exchange element baskets are created having an increased heat transfer efficiency, compared to conventional heat exchange element basket assemblies, and a maximum allowable clean condition pressure drop substantially equal to the calculated new maximum allowable clean condition pressure drop. The conventional heat exchange element basket assemblies are replaced with modified heat exchange element basket assemblies. When the boiler is operating, the additive material is added to the flue gas.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to a steam generating system having a coal or oil fired boiler and a regenerative air preheater. More particularly, the present invention relates to a steam generating system having a boiler and a rotary regenerative air preheater. 
     During the combustion process in the boiler, the sulfur in the fuel is oxidized to SO 2 . After the combustion process, some amount of SO 2  is further oxidized to SO 3 , with typical amounts on the order of 1 to 2% going to SO 3 . The presence of iron oxide, vanadium and other metals at the proper temperature range produces this oxidation. Selective catalytic reduction (SCR) is also widely known to oxidize a portion of the SO 2  in the flue gas to SO 3 . The catalyst formulation (primarily the amount of vanadium in catalyst) impacts the amount of oxidation, with rates ranging from 0.5% to over 1.5%. Most typical is around 1%. Therefore plants firing a high sulfur coal with a new SCR can see a large increase in the SO 3  emissions, which produce a visible plume, local acidic ground level problems and other environmental issues. 
     Regenerative air preheaters condense or trap a portion of the SO 3  in the flue gas. The SO 3  is condensed as sulfuric acid at temperatures typically below 300° F. Cold end acidic fouling of regenerative air preheaters creates a gradual increase in pressure drop. Sootblowing is generally utilized to reduce the rate of pressure drop build-up, but after some period of operation the air preheater must be cleaned by water washing. This is most typically accomplished by having an outage and shutting down the boiler. The maximum amount of pressure drop increase which is acceptable depends on the limitations of the existing fans, either the forced draft (air side), or induced draft (gas side) fans. The maximum acceptable pressure drop across the air preheater imposes limits on the design of the air preheater, principally limiting the number and type of heat exchange elements, thereby limiting the thermal efficiency of the air preheater. 
     SUMMARY OF THE INVENTION 
     Briefly stated, the invention in a preferred form is a method for increasing the efficiency of a steam generator system including a boiler producing a flow of flue gas containing SO 3 . An air preheater includes an air inlet and a flue gas outlet defining a cold end and a flue gas inlet and an air outlet defining a hot end. The flow of flue gas is received by the flue gas inlet, carried through heat exchange element basket assemblies, and discharged from the flue gas outlet, such that the flow of flue gas creates a pressure drop across the air preheater. A portion of the SO 3  carried in the flue gas forms an acid which accumulates in the cold end of the air preheater, with the rate of acid accumulation depending on the amount of SO 3  carried in the flue gas. The accumulating acid causes the pressure drop across the air preheater to increase from a maximum allowable clean condition pressure drop to a maximum allowable dirty condition pressure drop over the operating cycle of the steam generator system. The method comprises the steps of determining a reduced rate of acid accumulation which may be achieved by injecting an SO 3  neutralizing or SO 3  reactant additive material into the flue gas. A new maximum allowable clean condition pressure drop is calculated based on the reduced rate of acid accumulation. Modified heat exchange element baskets are created. The modified baskets have an increased heat transfer efficiency, compared to the conventional heat exchange element basket assembly, and a maximum allowable clean condition pressure drop substantially equal to the calculated new maximum allowable clean condition pressure drop. The conventional heat exchange element basket assemblies are replaced with modified heat exchange element basket assemblies. When the boiler is operating, the additive material is added to the flue gas. 
     Creating a modified heat exchange element basket includes identifying how the conventional heat exchange element basket assemblies may be modified to increase the heat transfer surface area and heat transfer. The cost of effecting each identified modification is determined. Finally, it is determined which of the identified modifications will most cost effectively produce the new maximum allowable clean condition pressure drop to provide the increased efficiency desired. 
     The steam generator system also generally includes fans for pushing and pulling the flue gas through the boiler. The maximum output of the limiting fan determines the maximum allowable dirty condition pressure drop (ΔP max ). The new maximum allowable clean condition pressure drop may be determined by calculating the increase in the pressure drop over the operating cycle attributable to the reduced rate of acid accumulation and subtracting the increase in the pressure drop over the operating cycle from the maximum allowable dirty condition pressure drop. Alternatively, the new maximum allowable clean condition pressure drop may be determined by calculating the increase percent decrease in acid accumulation over the operating cycle attributable to the reduced rate of acid accumulation (% ΔP) and determining the maximum allowable dirty condition pressure drop with the formula ΔP max /(1+% ΔP). 
     It is an object of the invention to provide a cost effective steam generating system in which a large percentage of SO 3  emitted by the boiler is removed in the installed regenerative air preheater. 
     It is also an object of the invention to provide a steam generating system in which fouling and corrosion problems associated with SO 3  removal are minimized. 
    
    
     Other objects and advantages of the invention will become apparent from the drawings and specification. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
     FIG. 1 is a perspective view, partially broken away, of a rotary regenerative air preheater; 
     FIG. 2 is a schematic diagram of a system in accordance with the invention; 
     FIG. 3 is a flow diagram of a method for increasing the efficiency of the air preheater of FIG. 1; and 
     FIG. 4 is a perspective view of portions of three heat exchange elements of a heat exchange element basket assembly of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The majority of steam generating systems utilize regenerative air preheaters to increase the boiler efficiency, with the largest portion being a rotary regenerative air preheater. This type of air preheater features rotating heat exchange elements. A different type of regenerative air preheater utilizes fixed heat exchange elements and internally rotating hoods or ductwork fixed to the rigid air and gas ducts. The subject invention relates to boiler systems equipped with either type of regenerative air preheater. To facilitate discussion, the inventive arrangement will be discussed in combination with a rotary regenerative air preheater. 
     With reference to FIG. 1 of the drawings, a conventional rotary regenerative preheater is generally designated by the numerical identifier  10 . The air preheater  10  has a rotor  12  rotatably mounted in a housing  14 . The rotor  12  is formed of diaphragms or partitions  16  extending radially from a rotor post  18  to the outer periphery of the rotor  12 . The partitions  16  define compartments  20  therebetween for containing heat exchange element basket assemblies  22 . 
     In a typical rotary regenerative heat exchanger  10 , the hot flue gas stream  28  and the combustion air stream  34  enter the rotor  12  from opposite ends and pass in opposite directions over the heat exchange elements  42  housed within the heat exchange element basket assemblies  22 . Consequently, the cold air inlet  30  and the cooled flue gas outlet  26  are at one end of the heat exchanger, referred to as the cold end  44 , and the hot flue gas inlet  24  and the heated air outlet  32  are at the opposite end of the air preheater  10 , referred to as the hot end  46 . Sector plates  36  extend across the housing  14  adjacent the upper and lower faces of the rotor  12 . The sector plates  36  divide the air preheater  10  into an air sector  38  and a flue gas sector  40 . The arrows of FIG. 1 indicate the direction of the flue gas stream  28  and the air stream  34  through the rotor  12 . The hot flue gas stream  28  entering through the flue gas inlet duct  24  transfers heat to the heat exchange elements  42  in the heat exchange element basket assemblies  22  mounted in the compartments  20  positioned in the flue gas sector  40 . The heated heat exchange element basket assembles  22  are then rotated to the air sector  38  of the air preheater  10 . The stored heat of the heat exchange element basket assemblies  22  is then transferred to the air stream  34  entering through the air inlet duct  30 . The cold flue gas stream exits the preheater  10  through the flue gas outlet duct  26  and the heated air stream exits the preheater  10  through the air outlet duct  32 . 
     Regenerative air preheaters  10  condense or trap a portion of the SO 3  carried in the flue gas. Acidic fouling of the cold end  44  of the air preheater  10  creates a gradual increase in pressure drop across the air preheater  10 . Sootblowing is generally utilized to reduce the rate of pressure drop build-up, but after some period of operation the air preheater  10  must cleaned by water washing. This is most typically accomplished during an annual outage when the boiler  48  is shut down. 
     The amount of pressure drop increase which is acceptable depends on the most limiting of either the forced draft (air side) fan(s)  49 , or induced draft (gas side) fan(s)  50 . The design of the heat exchange element basket assemblies  22  must account for the increase in pressure drop over the twelve month period between outages. That is, the number, size, and/or type of heat exchange elements  42  carried in the basket assemblies  22  is in part set by the value of the pressure drop across the air preheater  10  in the clean condition. For example, if a maximum pressure drop of 8 inches is allowed by the limiting fan  49  or  50  and the acidic fouling will cause the pressure drop to double over the twelve month period, the maximum allowable pressure drop of the air preheater  10  in the clean condition is 4 inches. A heat exchange element basket assembly  22  for such an air preheater  10  will include fewer heat exchange elements  42  and/or heat exchange elements  42  which are less efficient in transferring heat than a heat exchange element basket assembly  22  which may sustain a greater pressure drop in the clean condition. 
     In a system for increasing efficiency of steam generator system having a regenerative air preheater  10 , an additive material  52  is injected into the hot flue gas stream  28  to remove or significantly reduce the amount of SO 3  prior to the cold end  44 . The SO 3  reaction may occur prior to the hot end  46 , or during the temperature reduction within the heat exchange elements  42  (but prior to the heat exchange elements  42  reaching the acidic condensation temperature), or some combination of the two. Such additive materials  52  include solutions containing a bisulfite, or a sulfite. Alternatively, the additive material  52  may be an alkaline sorbent such as magnesium oxide or calcium oxide. 
     Reducing the amount of SO 3  reduces the rate of cold end acidic fouling, thereby reducing the rate of increase in the pressure drop and consequently reducing the pressure drop across the air preheater  10  at the end of the twelve month period (or any desired design time period) of the operating cycle. The limiting fan  49  or  50  will therefore have additional capacity which can be used to allow a revision in the heat exchange elements  42  that increases the efficiency of such elements  42  while increasing the pressure drop attributable to the heat exchange elements  42 . Addition of the additive material  52  produces a significant reduction in the rate of pressure drop increase, for example by at least by 25%. 
     The efficiency of the air preheater  10  is increased, thereby increasing the efficiency of the entire steam generator system, by replacing some or all of the existing heat exchange elements  42  with new, more efficient, heat exchange elements  42 ′. As explained above, the new heat exchange elements  42 ′ generate a greater pressure drop in the air/gas flow. Accordingly, the total increase in the pressure drop attributable to the new heat exchange elements  42 ′ is set to be equal to or less than the reduction in pressure drop attributable to the reduction in acidic fouling of the cold end  44 . In this manner, the total pressure drop across the air preheater  10  at the end of the design period between steam generator system outages will be the same as the total pressure drop for a conventional steam generator system having equivalent pressure drop limitations. 
     For example, if the additive material  52  injected into the hot flue gas steam  28  produces a twenty-five percent (25%) reduction in acidic fouling of the cold end  44  of an air preheater  10  having a 4 inch pressure drop in the clean condition, the increase in pressure drop over the operating cycle will be 3 inches (25% less than the 4 inch increase discussed above), providing a total pressure drop across the air preheater  10  at the end of the operating cycle of 7 inches. Accordingly, more efficient heat exchange elements  42 ′ may be substituted for the conventional heat exchange elements  42 . The allowable clean condition pressure drop of the “improved” air preheater  10  may be determined by the following formula: 
     
       
         Δ P   max /(1+% Δ P  increase) 
       
     
     Where ΔP max  is the maximum allowable pressure drop at the end of the operating cycle and % ΔP increase is the percentage increase in pressure drop over the operating cycle after addition of the additive material  52 . For the example above, the allowable clean condition pressure drop would therefore be 
     
       
         8 inches/(1+0.75)=4.57 inches 
       
     
     With an initial, clean condition pressure drop of 4.57 inches, a pressure drop increase of seventy-five percent (75%) over twelve months produces 8 inches of pressure drop, leaving no excess fan capacity. 
     The efficiency of a heat exchange element basket assembly  22  may be increased in a number of ways. The area of the surface available for transferring heat may be increased by increasing the depth or flow length  54  of the heat exchange elements  42 ′ (FIG. 4) within a basket assembly  22  by using a special basket design that provides a greater total depth  54  for the heat exchange elements  42 ′ by reducing the space occupied by supports and/or handling bars. The spacing  56  between the heat exchange elements  42 ′ may be reduced and/or the thickness  58  of the sheet material forming the heat exchange elements  42 ′ may be reduced to allow the basket assembly  22  to contain a greater number of heat exchange elements  42 ′. Heat exchange elements  42 ′ may be used which have a larger length factor. Although costly, the rotor  12  may be modified to provide for a greater depth  54  for the heat exchange elements  42 ′. The design of the rotor  12  may also be modified to reduce the number of layers of heat exchange element basket assemblies  22 , thereby reducing the number of support bars and also reducing rotor volume attributable to clearance gaps. 
     The efficiency may also be increased by increasing the heat transfer coefficient of the heat exchange element basket assemblies  22 . The heat transfer coefficient of a basket assembly  22  may be increased by lowering the porosity, for example by increasing the number of heat exchange elements  42 ′. Increasing the number of heat exchange elements  42 ′ in a basket assembly  22  not only increases the total surface for heat exchange, it decreases the total flow area  60  resulting in a higher flow velocity and a higher heat transfer coefficient. The heat exchange elements  42 ′ may have a rougher heat transfer surface to produce turbulence in the flow. Heat exchange element features such as indentations  62  on notches, a greater undulation height  64 , or a steeper undulation angle  66  may be used to roughen the surface. Alternatively, the heat exchange elements  42 ′ may include flow interrupters or boundary layer trips (e.g. punched tabs or expanded metal) to produce turbulence in the flow. 
     It should be appreciated that reducing the thickness  58  of the sheet material from which the heat exchange elements  42 ′ are manufactured will increase the porosity of the basket assembly  22  in the absence other changes to the basket assembly design. That is, the thinner heat exchange elements  42 ′ create a larger flow area  60 , producing a lower flow velocity. 
     In summary, the efficiency of a regenerative air preheater  10  may be increased by first determining  68  the reduction in the rate of cold end fouling which may be achieved by injecting an additive material  52  into the hot flue gas stream  28  that reduces the amount of SO 3  which may be retained in the cold end  44  of the air preheater  10  (FIG.  3 ). For a given reduction in the rate of fouling, a new allowable clean condition pressure drop is calculated  70 . The various ways of increasing the heat transfer surface area and the heat transfer coefficient for the particular preheater design are evaluated to determine  72  which modifications to the heat exchange element basket assembly design will most cost effectively produce the calculated clean condition pressure drop and thereby increase the heat transfer efficiency. Heat exchange element basket assemblies  22 ′ incorporating the selected modifications are installed  74  in the air preheater  10 . During operation of the steam generating system, additive material  52  is injected  76  into the flue gas stream  28  proximate to the flue gas inlet duct  24 . The additive material  52  reacts  78  with the SO 3  present in the flue gas stream  28  such that the amount of acid produced and deposited in the cold end  44  is substantially equal to the amount calculated in step  68 . 
     While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.