Abstract:
A boiler system includes combustion air conduit for directing the flow of combustion air through a first air heater (e.g., A-side heater) to a boiler and flue gas conduit for directing the flow of flue gas generated by the boiler to a second air heater (e.g., B-side heater). The combustion air conduit is configured to direct the majority of the combustion air to the first air heater, and the flue gas conduit is configured to direct the majority of the flue gas from the boiler to the second air heater.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a system and method for improving the operation of a boiler. 
   2. Description of the Background 
     FIG. 1  shows a conventional boiler system that includes two sets of fans, an “A” set and a “B” set. The A-side set of fans includes forced draft fan  101   a , which includes a silencer  105   a  for reducing noise. The forced draft fan  101   a  forces combustion air through combustion air conduit  109 . Combustion air conduit  109  includes duct sections  111   a ,  111   b ,  111   c ,  111   d ,  111   e , and  111   f , which are shown as solid lines with the arrows showing the direction in which combustion air from the forced draft fan  101   a  is forced. The combustion air conduit  109  also includes shutoff damper  113   a , which can be used to reduce or shut off completely the flow of combustion air between duct sections  111   a  and  111   b.    
   Air preheater  115   a  is located between duct sections  111   b  and  111   c . Air preheater  115   a  heats combustion air flowing from fan  101   a  before the air reaches duct section  111   c.    
   Duct sections  111   d  and  111   e  branch from duct section  111   c . Duct section  111   d  directs combustion air into the A-side air heater  119   a . Air heater  119   a  is divided into two parts, as shown conceptually, by the dashed line in FIG.  1 . Air heater  119   a  may be any suitable device for heating combustion air, such as a CE LJUNGSTROM regenerative air preheater, for example. An exemplary air heater  119   a  may include a large disk of stacked steel baskets that rotate from the hot side of the air heater  119   a  toward the cold side of the air heater  119   a . The hot side of the air heater  119   a  (conceptually shown on the left of the dashed line bisecting air heater  119   a  in  FIG. 1 ) receives hot flue gas. The cold side of air heater  119   a  (shown on the right of the dashed line bisecting air heater  119   a  in  FIG. 1 ) receives combustion air via the duct section  111   d  of the combustion air conduit  109 . Air heater  119   b  a heats the combustion air  119   a  it receives from duct section  111   d . The combustion air heated by air heater  119   a  is forced out of air heater  119   a  and is directed by duct section  111   f  if to the boiler  121 , which bums a mixture of combustion air received from duct section  111   f  and fuel. The burning of fuel and combustion air generates hot flue gas, which is directed away from boiler  121  by flue gas conduit  129 , shown as a dashed line. Flue gas conduit  129  includes duct section  123   a  which directs flue gas from boiler  121  to the hot side of the A-side air heater  119   a . Flue gas conduit  129  also includes duct section  123   b  for directing flue gas away from the air heater  119   a , duct sections  123   c  and  123   j , which branch from duct section  123   b , shutoff damper  127   a  for controlling the volume of flue gas flowing between duct sections  123   c  and  123   d , duct section  123   d  for directing the flue gas to an A-side induced flow fan  131   a ; duct section  123   e  for directing flue gas away from induced draft fan  131   a , shutoff damper  133   a ; for controlling the volume of flue gas flowing between duct sections  123   c  and  123   f  and duct sections  123   f  and  123   g  for directing flue gas toward the stack  135 . 
   The A-side induced draft fan  131   a  pulls flue gas away from the boiler  121  in the direction shown by the arrows along flue gas conduit  129 . Induced draft fan  131   a  may be implemented by any suitable fan for that purpose. Shutoff dampers  127   a  and  133   a  perform the same or similar function as shutoff damper  113   a . Shutoff dampers  115   a ,  127   a , and  133   a  may be implemented by any suitable device for shutting off, opening, and varying the flow of air and/or gases through conduit. 
   The B-side of the boiler system in  FIG. 1  includes B-side forced draft fan  101   b , the silencer  105   b  of the forced draft fan  101   b , air preheater  115   b , B-side air heater  119   b , B-side induced draft fan  131   b , and shutoff dampers  113   b ,  127   b , and  133   b . These B-side components may be implemented in the same manner as the corresponding A-side components and perform the same or similar function for the B-side as the A-side components perform for the A-side. 
   Combustion air conduit  109  includes duct section  111   g  for directing the flow of combustion air from the fan  101   b  to the shutoff damper  113   b , duct section  111   h  for directing combustion air from the shutoff damper  113   b  to the air preheater  115   b , duct section  111   i  for directing air away from the air preheater  115   b , duct sections  111   k  and  111   j  which branch from duct section  111   i  to duct section  111   e  and to air heater  119   b  respectively, and duct section  111   l  which directs combustion air from the air heater  119   b  to the boiler  121 . Together, the duct sections  111   e  and  111   k  form combustion air crossover conduit, which allows combustion air to flow between the A-side and the B-side of the boiler system. 
   Flue gas conduit  129  includes duct section  123   h  for directing flue gas from the boiler  121  to the hot side of the air heater  119   b  (conceptually shown as the portion of the air heater  119   b  on the right side of the dotted line that bisects the air heater  119   b ), duct section  123   i  for directing flue gas from the air heater  119   b , duct section  123   k  which branches from duct section  123   i  and directs flue gas toward duct section  123   j , duct section  123   l  which branches from duct section  123   i  and directs flue gas toward the shutoff damper  127   b , duct section  123   m  for directing flue gas from the shutoff damper  127   b  to the induced air fan  131   b , duct section  123   n  for directing flue gas from the fan  131   b  to the shutoff damper  133   b , and duct section  123   o  for directing flue gas to the duct section  123   g  and eventually on to the stack  135 . The shutoff dampers  113   b ,  127   b , and  133   b  may also be considered part of the flue gas conduit  129  and regulate flow in the same manner as the corresponding counterpart shutoff dampers on the A-side (i.e., shutoff dampers  113   a ,  127   a , and  133   a ). The duct sections  123   j  and  123   k  form flue gas crossover conduit for permitting the flow of flue gas between the A-side and the B-side of the boiler system in FIG.  1 . 
   During normal operation, the forced draft fans  101   a  and  101   b  push combustion air through the cold sides of the air heaters  119   a  and  119   b  and toward the boiler  121 . The induced draft fans  131   a  and  131   b  pull flue gas away from the boiler through the hot sides of the air heaters  119   a  and  119   b , and toward the stack  135 . A problem with the operation of this and other boiler systems is that acid, such as sulphuric acid, condenses in the flue gas. In this respect, the acid dew point is important because when the flue gas and components in contact with the flue gas have a temperature below the acid dew point, acid condenses out of the flue gas. Condensed acid corrodes the components of the boiler system that it contacts and also increases the opacity of the flue gas. If the opacity is regulated (by state or federal agencies, for example), then it is normally desirable to decrease the opacity of the flue gas. 
   As the boiler system starts up or shuts down, the flue gas temperature in the air heaters  119   a  and  119   b  as well as the flue gas output ductwork temperatures (i.e., the portions of the flue gas conduit  129  downstream of the air heaters  119   a  and  119   b  and the induced draft fans  131   a  and  131   b ) are below the sulphuric acid dew point. As noted, this allows sulphuric acid to condense and collect on the colder surfaces. During startup and initial unit loading, this condensed sulphuric acid is re-volatized when flue gas temperature rises to approximately 240°. At this temperature, the re-volatized sulphuric acid is a combination of fine droplets and gas. The fine droplets cause elevated opacity in the stack  135 . As the temperature continues to rise above the acid dew point (approximately 270° F.) all of the sulphuric acid becomes gaseous and no longer contributes to opacity. 
   Several solutions for reducing sulphuric acid mist have been proposed. One such proposal is to use a chemical additive that reduces the formation of sulphuric acid. These additives, such as magnesium-oxide, are injected into the flue gas during startup and shutdown of the boiler system. This approach has several problems, not the least of which is that the use of additives may pose environmental risks and/or be prohibited by law. 
   Another approach is to increase the temperature of the combustion air and/or the flue gas during startup and shutdown. These techniques attempt to reduce the effect of the ambient temperature on the time it takes to warm the flue gas above the acid dew point. An example of one such system is described in U.S. Pat. No. 4,932,464, which is incorporated herein by reference in its entirety. Other techniques for increasing flue gas temperature involve air heater bypass. Air heater bypass may be implemented, for example, by causing combustion air to flow around, rather than through, an air heater. For example, extra ductwork could be added to the boiler system of  FIG. 1  to cause combustion air to flow directly from duct section  111   d  to  111   f  and from duct section  111   j  to duct section  111   l , without passing through either of the air heaters  119   a  and  119   b . Shutoff dampers may be used along this extra duct work to vary the amount of bypass. Air heater bypass causes the air heaters to increase in temperature as a result of receiving less of the combustion air, which is relatively cooler than the hot flue gasses received on the hot sides of the air heaters. This solution, however, is costly because it requires extra ductwork to be able to control the bypass of air flow around the air heaters. Additionally, it may not be feasible, or even possible, to retrofit existing boiler systems with the ductwork required to enable air heater bypass. 
   SUMMARY OF THE INVENTION 
   The present invention provides a boiler system and method for operating a boiler system. The system on which the method is based, includes combustion air conduit for directing the flow of combustion air through a first air heater (e.g., A-side air heater) to a boiler and flue gas conduit for directing the flow of flue gas generated by the boiler to a second air heater (e.g., B-side air heater). The combustion air conduit is configured to direct the majority of the combustion air to the first air heater, and the flue gas conduit is configured to direct the majority of the flue gas from the boiler to the second air heater. 
   According to one embodiment, the combustion air conduit directs all of the combustion air to the first heater, and the flue gas conduit directs all of the flue gas through the second air heater. In other configurations, the flue gas conduit and the combustion air conduit may be configured to direct anywhere from 50 to 100% of their flow to either the first or second fan. 
   According to another embodiment, different combustion air conduit and flue gas conduit configurations are achieved through the use of shutoff dampers positioned along crossover conduits. With such embodiments, one such crossover conduit may form part of the combustion air conduit and a second crossover conduit may form part of the flue gas conduit. 
   Accordingly, the present invention advantageously emulates an air heater bypass. In some instances, it is not even necessary to retrofit an existing boiler system with new ductwork to achieve advantages of the present invention. The present invention advantageously increases flue gas temperatures, which in turn reduces condensation, including sulfuric acid mist in the flue gas and along the path of the flue gas. The reduction in condensation reduces corrosion and thereby prolongs the life of the boiler system, and in particular, the portions of the boiler system that are most susceptible to corrosion caused by acid condensation. The reduction in acid mist also causes a corresponding reduction in the opacity of the flue gas in the lower portions of the stack. Thus, sensors or other devices for measuring opacity will not be affected by the acid mist and therefore will not incorrectly detect the acid mist as particulate waste. 
   Additionally, the present invention virtually eliminates the effect of the ambient temperature on the time it takes to warm up the back end temperature of the boiler. Accordingly, there is minimal time difference between summer and winter heat-up times. The present invention is also used when the boiler is shut down to keep the air heater and ductwork temperatures above the acid dew point during the time that fuel is being combusted. This also minimizes the corrosion of components and sulfuric acid condensation which may cause increases in opacity during the next startup and initial loading period. In certain implementations of the present invention, when the boiler is shut down, combustion stops while the flue gas temperature is still above the acid dew point. 
   Lastly, the present invention also allows the flue gas to remain hot or above the acid dew point when the unit is at minimum generation. This reduces opacity and corrosion when the boiler system is at minimum load. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
       FIG. 1  is a schematic illustration of a conventional boiler system; 
       FIG. 2  is a schematic illustration of a boiler system configured in accordance with one embodiment of the present invention; 
       FIG. 3  is a flow chart illustrating boiler system startup according to an embodiment of the invention; and 
       FIG. 4  is a flow chart describing boiler system shutdown according to an embodiment of the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, and more particularly, to  FIG. 2  thereof, there is shown a conceptual diagram of an exemplary boiler system of the present invention. According to  FIG. 2 , the A-side of the boiler system includes forced draft fan  101   a , the A air heater  119   a , and the induced draft fan  131   a . The B-side of the boiler system of  FIG. 1  includes forced draft fan  101   b , air heater  119   b , and induced draft fan  131   b.    
   According to one embodiment, the air heaters  119   a  and  119   b  work by passing flue gas on the hot side (the side to the left of the dashed line bisecting air heater  119   a  and the side to the right of the dashed line bisecting air heater  119   b ) and bypassing combustion air on the cold side, which is opposite the hot side. The air heaters  119   a  and  119   b  are designed to absorb waste heat from the flue gas flowing through the flue gas conduit  129  and transfer this heat to the relatively cold combustion air flowing through the combustion air conduit  109 . The transfer of heat is achieved by continuously rotating heat transfer elements within the air heaters  119   a  and  119   b . In one embodiment, these heat transfer elements are specially formed metal plates, and thousands of these elements are spaced and compactly arranged within 24 sector-shaped compartments of a radially divided cylindrical shell, called a rotor, within each of the air heaters  119   a  and  119   b.    
   The housing surrounding the rotor is provided with duct connections at both ends, and is sealed by radial and circumferential sealing members, forming an air passage through one-half of the air heater (the cold side) and a gas passage through the other half of the air heater (the hot side). As the rotor slowly revolves the mass of elements alternately through the gas and air passages, heat is absorbed by the element surfaces passing through the hot stream of flue gas; then, as the element surfaces are carried through the stream of combustion air on the cold side, they release the stored heat and increase the temperature of the combustion air flowing through the combustion air conduit  109 . The cooler the ambient air, the more heat is extracted from the flue gas, and the longer It takes to warm up the air heaters  119   a  and  119   b  and the portions of the flue gas conduit  129  that lead from the air heaters  119   a  and  119   b  to the stack  135 . A regenerative type air heater is described in “Combustion Fossil Power Systems, A Reference Book on Fuel Burning and Steam Generation,” edited by Joseph G. Singer, 3 rd  ed., Combustion Engineering. Inc., Windsor, Conn., 1981, which is incorporated herein by reference in its entirety. As mentioned previously, the air heater  119   a  and  119   b  may be implemented by LJUNGSTROM regenerative air preheaters (e.g., Model #30-VI-57-1/2) or any other suitable device. 
   As shown in  FIG. 2 , an example of the inventive boiler system configuration may be implemented by closing crossover damper  117  along the combustion air crossover formed by duct sections  111   e  and  111   k  and by closing the crossover damper  125  positioned along the flue gas crossover formed by duct sections  123   j  and  123   k . Additionally, the A-side forced draft fan  101   a  is turned on while the B-side forced draft fan  101   b  is turned off. However, both the A-side and B-side air heaters  119   a  and  119   b  are used in this implementation. With the crossover damper  117  closed, the B-side air heater  119   b  receives no combustion air since the B-side forced draft fan  101   b  is off. The dampers  113   a ,  113   b ,  117 ,  125 ,  133   a , and  133   b  maybe implemented as electronically controlled multi-leaf louver type dampers or any other suitable devices for regulating air or gas flow. 
   On the other hand, the A-side induced draft fan  131   a  is turned off, the B-side induced draft fan  131   b  is turned on, and the crossover damper  125  is closed. This structure causes all of the flue gas to be extracted through the B-side air heater  119   b  by the B-side induced draft fan  131   b . Thus, the A-side air heater  119   a  receives all of the combustion air, while the B-side air heater  119   b  receives all of the flue gas. The configuration in  FIG. 2  effectively causes all of the combustion air to bypass the B-side air heater  119   b  and causes all of the flue gas to bypass the A-side air heater  119   a . In alternate embodiments of the invention, the various dampers shown in  FIG. 2  may be altered to cause different effective amounts of bypass, infinitely variable between 0 and 100%, of the combustion air and/or the flue gas. 
     FIG. 3  describes the method steps for implementing the present invention according to one embodiment. In step  301 , the A-side forced draft fan  101   a  is turned on, and in step  303 , the B-side induced draft fan  131   b  is turned on to extract flue gas from the boiler  121  to the air heater  119   b  and on to the stack  135 . Fans  101   b  and  131   a  remain off. In step  305 , the crossover damper  117  is closed to direct 100% of the flow of combustion air from the fan  101   a  to the air heater  119   a  and to shut off the flow of combustion air to the air heater  119   b . In step  307 , the crossover damper  125  is closed to direct 100% of the flow of flue gas from the boiler  121  to the air heater  119   b  and to prevent flue gas from flowing through air heater  119   a . Once steps  301 ,  303 ,  305  and  307  are performed, the B-side air heater  119   b  will increase in temperature rapidly because it receives none of the relatively cool combustion air flowing through the combustion air conduit  109 . As a result, the flue gas flowing through the flue gas conduit  129  and all components in contact with the flue gas increase in temperature relatively quickly relative to a conventional boiler system. Thus, the flue gas and the components in contact with the flue gas are raised above the acid dew point much more quickly than in a conventional boiler system, and as a result, acid mist, opacity, and corrosion is reduced. 
   Steps  301 ,  303 ,  305 , and  307  may be performed in any order; however, in one preferred embodiment, steps  305  and  307  are performed prior to steps  301  and  303 , and combustion begins in step  308  after steps  301 ,  303 ,  305 , and  307  are complete. 
   Once the gas exiting air heater  119   b  reaches 300° to 320° F., in step  309  the crossover damper  117  is throttled open to unrestrict the flow of air from fan  101   a  to air heater  119   b . The crossover damper  117  is throttled open to maintain the gas temperature of 300° to 320° F. until the crossover damper  117  is 100% open. It should be noted that different temperatures may be preferred in different applications. For example, if the distance from the air heater  119   b  to the stack  135  increases, then the temperature of gas exiting the air heater  119   b  may have to be increased during this step to account for the additional cooling time of the gas on its way to the stack  135 . 
   Once the crossover damper  117  is 100% open, in step  311  the crossover damper  125  is throttled open to unrestrict the flow of flue gas from boiler  121  to air heater  119   a . The crossover damper  125  is throttled open such that the air heater  119   b  output temperature of 300° to 320° F. is maintained while the air heater  119   a  is warmed. 
   Once the crossover damper  125  is all the way open, in step  313 , the forced draft fan  101   b  and the induced draft fan  131   a  are started and normal operation begins. 
     FIG. 4  shows how the boiler system of  FIG. 2  may be shut down according to one embodiment. Shut down is generally the reverse of start up. In step  401 . the flows of induced draft fan  131   a  and the forced draft fan  101   b  are lowered. Depending on the type of fan, this may be accomplished by adjusting the pitch of the fans, by closing control dampers of the fans, or adjusting the speeds of the fans, for example. Then, in step  403  the crossover damper  125  is throttled closed to shut off the flow of flue gas to the air heater  119   a . Once the crossover damper  125  is 100% closed, in step  405  the crossover damper  117  is throttled closed to shut off the flow of combustion air to the air heater  119   b.    
   As before, the crossover dampers  117  and  125  are throttled in steps  403  and  405  to maintain the temperature of the gas leaving the air heater (in this case, the air heater  119   b ) at 300° to 320° F. Once the crossover damper  117  is throttled 100% closed, combustion of fuel in the boiler  121  is ceased in step  407 . Then the remaining active fans, the A-side forced draft fan  101   a  and the B-side induced draft fan  131   b , are lowered. The result of shutting down the system in the manner depicted in  FIG. 4  is that the temperature of air heater  119   b  increases rapidly so that the flue gas remains above the dew point for as long as possible after the supply of fuel to the boiler is stopped in step  407 . 
   Accordingly, it can be appreciated that the present invention provides a novel system and method for increasing flue gas temperatures during startup and shutdown without requiring additional hardware necessary for conventional air heater bypass. Further, if the combustion air crossover conduit and the flue gas crossover conduit in an existing boiler system, such as that shown in  FIG. 1 , are already fitted with crossover dampers  117  and  125 , then no additional hardware is required. The only necessary changes are to alter the configuration of the crossover conduits using the crossover dampers to obtain the inventive configuration. If crossover dampers  117  and  125  are not present, then the flow of air and gas can be manipulated according to the invention by simply controlling the flow of air with the forced draft fans  101   a  and  101   b  and the induced draft fans  131   a  and  131   b , for example. Thus, there are numerous, if not infinite, number of ways the present invention could be implemented. 
   Testing of the inventive system yielded favorable results when compared to a conventional boiler system. Using the invention, on a hot startup it took only 5-8 minutes to heat up the back end temperatures above the acid dew point, compared to 4.5 hours with a conventional system. On a cold startup, it took only 1.5 to 2 hours to heat up the back end temperatures above the acid dew point, compared to 9 hours with a conventional system. Thus, the invention makes it possible for a relatively inexpensive modification to a conventional boiler system to greatly reduce acid mist, corrosion, and opacity. 
   Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. For example, it is not always necessary to use induced draft fans in a boiler system. On the other hand, it is also possible to add booster fans or use more than two induced draft fans with a single boiler system or unit. As another example, additional ductwork, including various dampers, may be added to achieve a higher level of variability or control of the flow in either the combustion air conduit  109  or the flue gas conduit  129 . It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.