Abstract:
A system for drying pulverized high moisture fuel for use in a selective catalytic reduction system equipped combustion system is provided. The combustion system includes a mill for pulverizing fuel, an air heater, two fuel gas streams at different temperatures, a booster air heater and a fuel duct for feeding dried pulverized fuel to a combustion furnace.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 13/923,633, filed Jun. 21, 2013, which claims priority to U.S. Provisional Application No. 61/699,484, filed Sep. 11, 2012, both of which are incorporated by reference herein in their entireties 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to a booster air heater for use with high moisture fuels, and more specifically to a booster primary air (PA) heater for use with a selective catalytic reduction (SCR) system equipped combustion system fueled by high moisture fuels such as sub-bituminous coal and lignite. 
     BACKGROUND OF THE INVENTION 
     In suspension firing solid fuel, such as coal and lignite, the fuel must be first pulverized before it can be introduced into the furnace in a stream of air commonly termed “primary air” (PA). Such pulverization is accomplished in a mill wherein the solid fuel is simultaneously pulverized and dried in the PA stream. Accordingly, the PA stream entering the mill must be heated to a temperature high enough to ensure sufficient drying of the fuel within the mill. Typically the PA stream is heated in an air preheater prior to entering the mill. As such, the PA stream is heated using heat in a heat exchange relationship from hot flue gas leaving the system boiler. However, in a system requiring a selective catalytic reduction (SCR) system located between the boiler gas exit and the air heater gas inlet, the maximum temperature of flue gas entering the air heater gas inlet is that allowed for efficient operation of the SCR system. Providing flue gas having a maximum temperature of that allowed for efficient operation of the SCR system is insufficient for pulverizing and drying high moisture fuels. 
     Therefore, it is the general object of the present invention to provide a method and apparatus capable of achieving a sufficiently high PA temperature and quantity to assure adequate drying of high moisture fuels in systems requiring a selective catalytic reduction (SCR) system located between the boiler gas exit and the air heater gas inlet. 
     It is a specific object of the present invention to provide a method and apparatus for adequately heating the PA flow for high moisture fuel drying and pulverizing while not affecting associated SCR system catalysts. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the above described deficiencies and disadvantages of the prior art in accomplishing the above-identified objectives through a method and apparatus for providing the requisite temperature and quantity of primary air (PA) to a mill for drying and pulverizing high moisture fuel for purposes of furnace combustion. 
     According to the present invention, PA is first heated from an ambient temperature to a higher temperature in an air heater. The air heater heats the incoming ambient temperature PA through heat exchange. The heat source for the air heater comes from combustion flue gas flowing from a boiler furnace. A selective catalytic reduction (SCR) system is located between the boiler furnace and the air heater. This overall system arrangement provides the gas temperature necessary to ensure the SCR system catalyst is chemically active. However, many SCR catalysts are heat sensitive or adversely affected by higher temperatures, thereby requiring limits to be placed on the maximum temperature of flue gas allowed to flow through the SCR system. By setting a limit on the maximum temperature of flue gas allowed to flow through the SCR system, the temperature of the flue gas entering the air heater from that SCR system is thereby also limited. However, according to embodiments of the present apparatus, a booster air heater is provided. The heat source for this booster air heater is not limited by the SCR system operating temperature, since the heat source of the booster air heater is from flue gas drawn from a system location upstream of the SCR system, such as from a system location upstream of an economizer. The booster air heater is thus capable of efficiently increasing the temperature of the PA prior to its flow into a mill for high moisture fuel pulverization and drying, and use of the fuel in a combustion system furnace. 
     The present system is a selective catalytic reduction system equipped combustion system fueled by high moisture fuel. The system comprises a mill for pulverizing high moisture fuel to obtain pulverized fuel, an air heater operable to heat primary air to an increased temperature, a booster air heater operable to heat primary air of an increased temperature to a higher temperature, and a fuel duct for passage of higher temperature primary air through the mill to dry the pulverized fuel and to carry the dry pulverized fuel from a mill outlet to a combustion furnace. The high moisture fuel referred to is one or more fuels selected from the group consisting of sub-bituminous coal and lignite. The air heater referred to uses flue gas from a selective catalytic reduction system as a heat source and is also operable to heat secondary air to an increased temperature for use as combustion air in said combustion furnace. The flue gas heat source under full system load has a temperature in the range of 700° F. to about 750° F. upon entering said selective catalytic reduction system. Under partial system load operation, the flue gas heat source has a temperature in the range of 550° F. to about 650° F. upon entering said catalytic reduction system. The heated primary air increased temperature is a temperature in the range of 400° F. to about 500° F. and its higher temperature is a temperature in the range of 700° F. to about 800° F. The temperature of the primary air at the mill outlet is in the range of 160° F. to about 220° F. 
     The present method is for drying pulverized high moisture fuel used to fuel a selective catalytic reduction system equipped combustion system. The method comprises pulverizing in a mill a high moisture fuel to produce a moist pulverized fuel, heating primary air in an air heater to an increased temperature, heating primary air of an increased temperature in a booster air heater to a higher temperature, drying the moist pulverized fuel using higher temperature primary air heater to produce a dry pulverized fuel, and feeding the dry pulverized fuel to a combustion furnace useful for power generation. The high moisture fuel is one or more fuels selected from the group consisting of sub-bituminous coal and lignite. The noted air heater uses flue gas from a selective catalytic reduction system as a heat source and is also operable to heat secondary air to an increased temperature for use as combustion air in said combustion furnace. Under full load, the flue gas has a temperature in the range of 700° F. to about 750° F. upon entering the selective catalytic reduction system. Under partial load, the flue gas has a temperature in the range of 550° F. to about 650° F. upon entering the catalytic reduction system. The increased temperature of the primary air is a temperature in the range of 400° F. to about 500° F. and the higher temperature of the primary air is a temperature in the range of 700° F. to about 800° F.18. The primary air temperature at said mill outlet is in the range of 160° F. to about 220° F. 
     Further objects and features of the present invention will be apparent from the description and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described in more detail with reference to the appended drawings. 
         FIG. 1  is a schematic side view illustrating a pulverized fuel-fired boiler operated in accordance with the prior art. 
         FIG. 2  is a schematic side view illustrating a pulverized fuel-fired boiler operated in accordance with one embodiment of the present invention. 
         FIG. 3  is a schematic side view illustrating a pulverized fuel-fired boiler operated in accordance with another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the prior art of  FIG. 1 , there is illustrated a pulverized fuel-fired steam generator combustion system  10  having a furnace  11  defining an interior chamber  12  therein wherein pulverized fuel (PF) is burned thereby generating hot flue gas (FG). FG generated in interior chamber  12  flows therefrom through a fluidly connected exit duct  14 . Exit duct  14  includes in fluid communication therewith an economizer  16  and a by-pass duct  18  that separates at fluid connection  20  to circumvent and by-pass economizer  16 . As such, all or a portion of FG flowing from interior chamber  12  flows through economizer  16  and none or a portion of FG flowing from interior chamber  12  may be allowed to flow through by-pass duct  18 . 
     In order to control the flow of FG through economizer  16  is a control valve  22  downstream with regard to the flow of FG from economizer  16  into economizer exit duct  24 . Likewise, to control the flow of FG through by-pass duct  18  is a control valve  26  upstream with regard to the flow of FG to fluid connection  28  of by-pass duct  18  with economizer exit duct  24 . Control valves  22  and  26  work in cooperation to control the flow and thereby the temperature of FG. For example, if cooler FG is desired, all FG may be made to flow through economizer  16  by fully opening control valve  22  and fully closing control valve  26 . Likewise, if warmer FG is desired, control valve  22  may be partially closed and control valve  26  is at least partially opened to allow a portion of FG to by-pass economizer  16 . The system  10  is designed such that no by-pass of FG through by-pass duct  18  is needed when system  10  is operated at full load. With system  10  operating at partial loads, such as at times of low system  10  demand, the temperature of FG flowing from economizer  16  may fall below that needed by the SCR catalyst (not shown) for proper performance. In such cases, valve  26  of by-pass duct  18  is opened to allow or increase FG flow through by-pass duct  18  and thereby increase the temperature of FG flowing into the SCR system  30  to ensure proper SCR catalyst performance. 
     Economizer  16  is used to cool FG to a lower temperature, typically a temperature lower than 700° F. to 750° F. so as to be within a temperature range suitable for proper SCR system  30  operation. FG flows from economizer  16  through fluidly connected economizer exit duct  24  to fluidly connected SCR system  30 . 
     SCR system  30  operates as a typical SCR system to remove nitrous oxides (NOx) and like pollutants from the FG. Fluidly connected to SCR system  30  is duct  32 . Duct  32  is fluidly connected to an air heater  34  used to heat both primary air (PA) and secondary air (SA). PA is ambient temperature air from the environment that is heated and used in a mill for drying and pulverizing fuel. SA is ambient temperature air from the environment that is heated and used as combustion air in furnace  11 . 
     As illustrated in  FIG. 1 , FG flows from SCR system  30  to air heater  34  via duct  32 . FG is used as a source of heat for air heater  34 . As such, FG of a temperature flows into air heater  34  via duct  32 , and at a lower temperature than that of flowing in, flows out of air heater  34  via fluidly connected duct  46 . 
     PA of an ambient temperature is pulled into combustion system  10  by a fluidly connected fan  36  within fluidly connected intake duct  38  of air heater  34 . SA of an ambient temperature is drawn into combustion system  10  by a separate fluidly connected fan  36   a  within a separate fluidly connected intake duct  38   a  of air heater  34 . Accordingly, ambient PA and ambient SA are heated in air heater  34  by heat exchange, wherein FG serves as the source of heat as described above. From air heater  34 , PA flows through fluidly connected duct  40  to mill  42  and SA flows through fluidly connected duct  48  to furnace  11 . 
     Mill  42  is where fuel is pulverized for combustion in furnace  11 . When high moisture fuel such as sub-bituminous coal and lignite is pulverized, moisture is released. For proper entrainment and flow of pulverized fuel though duct  44  fluidly connected between mill  42  and furnace  11 , moisture released upon high moisture fuel pulverization must be dried. The heated PA flowing through duct  40  to mill  42  is thus used for such purpose as described in more detail below. 
     Furnace  11  is fired by injecting pulverized fuel into the furnace  11  through burners  50 . The amount of fuel injected into the furnace  11  is controlled in response to combustion system  10  load demand, such as for example, the demand on a steam generator (not shown) to provide a total heat release necessary to yield a desired stream generation for a given steam generator design (not shown). 
     In pulverized fuel firing, as illustrated in  FIG. 1 , solid fuel such as coal and/or lignite is fed from a storage bin (not shown) at a controlled rate, depending on combustion system  10  demand therefor, through feeder  52  to the mill  42  where the fuel is pulverized to a fine powder-like particle size. In a typical pulverized fuel-fired furnace  11 , PA is supplied to the mill  42  for transporting the pulverized fuel from the mill  42  to the burners  50  to be injected into the furnace  11  and burned as a suspension therein. As mentioned previously, the PA supplied to the mill  42  is first preheated in the air heater  34  wherein the PA is passed in heat exchange relationship with the FG leaving the furnace through exit ducts  14 ,  18 ,  24  and  32 . As the PA sweeps through the mill  42 , the comminuted coal is entrained therein and dried by the heat content of the PA. However, because the PA heated by the FG flowing through air heater  34  has a maximum obtainable temperature limited by the temperature of the FG following from SCR system  30 , PA is often times ineffective in adequately drying the pulverized fuel as the required mill outlet temperature of 160° F. to 220° F. cannot be achieved. 
     To address the problem associated with inadequate drying of high moisture fuels in combustion systems  10  equipped with SCR systems  30  as described above and illustrated in  FIG. 1 , the present apparatus embodiment is disclosed herein and illustrated in  FIG. 2 . The present apparatus schematically illustrated in  FIG. 2  has features in common with those illustrated in  FIG. 1 . As such, features illustrated in  FIG. 2  common to those of  FIG. 1  are signified using the same numbers but with the number “2” preceding them. 
     Referring now to  FIG. 2 , there is illustrated a pulverized high moisture fuel-fired steam generator combustion system  210  having a furnace  211  defining an interior chamber  212  therein wherein pulverized fuel (PF) is burned thereby generating hot flue gases (FG). FG generated in interior chamber  212  flows therefrom through a fluidly connected exit duct  214 . Exit duct  214  includes in fluid communication therewith an economizer  216  and a by-pass duct  218  that separates from exit duct  214  at fluid connection  220  to circumvent and by-pass economizer  216 . As such, all or a portion of FG flowing from interior chamber  212  flows through economizer  216  and none or a portion of FG flowing from interior chamber  212  may flow through by-pass duct  218 . Exit duct  214  also includes in fluid communication therewith a booster air heater  254  and a by-pass duct  260  that separates from exit duct  214  at fluid connection  258  to circumvent and by-pass economizer  216 . FG flowing through booster air heater  254  through heat transfer is cooled prior to flowing outwardly through fluidly connected exit duct  262 , which rejoins economizer exit duct  224  downstream of fluid connection  228 . As such, all or a portion of FG flowing from interior chamber  212  flows through economizer  216  and none or a portion of FG flowing from interior chamber  212  may flow through by-pass duct  218  and/or by-pass duct  260 . At full load, combustion system  210  preferably operates with up to about 50 percent of FG flowing through by-pass duct  260  and almost no FG flowing through by-pass duct  218 . 
     In order to control the flow of FG through economizer  216  is a control valve  222  downstream with regard to the flow of FG from economizer  216  into economizer exit duct  224 . Likewise, to control the flow of FG through by-pass duct  218  is a control valve  226  upstream with regard to the flow of FG to fluid connection  228  of by-pass duct  218  with economizer exit duct  224 . To control the flow of FG through exit duct  262  is a control valve  268  upstream with regard to the flow of FG to fluid connection  264 . Control valves  222 ,  226  and  268  work in cooperation to control the flow of FG. For example, if cooler FG is desired, all FG may be made to flow through economizer  216  and booster air heater  254  by fully opening control valves  222  and  268 , while fully closing control valve  226 . Likewise, if warmer FG is desired, control valve  222  and/or  268  are partially closed and control valve  226  is at least partially opened to allow a portion of FG to by-pass economizer  216  and/or booster air heater  254 . If hot FG is desired, control valves  222  and  268  could be fully closed and control valve  226  fully opened to by-pass economizer  216  and booster air heater  254 . However, in the case of the current embodiment wherein an SCR system  230  is used for FG treatment, such hot FG must be avoided to protect SCR operation. SCR operation is dependent upon properly functioning catalysts, which are heat sensitive. Allowing hot FG to flow through an SCR system  230  can destroy expensive catalysts. 
     Economizer  216  and booster air heater  254  are used to cool FG to a lower temperature, preferably a temperature lower than 700° C. to 750° C. so as to be within a temperature range suitable for proper SCR system  230  operation. FG flows from economizer  216  and booster air heater  254 , through fluidly connected economizer exit duct  224  and fluidly connected exit duct  262 , respectively, to fluidly connected SCR system  230 . 
     SCR system  230  operates as a typical SCR to remove nitrous oxides (NOx) and like pollutants from the FG. Fluidly connected to SCR system  230  is duct  232 . Duct  232  is fluidly connected to a primary air (PA) air heater  234 . As such, FG flows from SCR system  230  to air heater  234  via duct  232 . Duct  232  is fluidly connected to an air heater  234  used to heat both primary air (PA) and secondary air (SA). PA is ambient temperature air from the environment that is heated and used in a mill for drying and pulverizing fuel. SA is ambient temperature air from the environment that is heated and used as combustion air in furnace  211 . 
     As illustrated in  FIG. 2 , FG flows from SCR system  230  to air heater  234  via duct  232 . FG is used as a source of heat for air heater  234 . As such, FG of a temperature flows into air heater  234  via duct  232 , and at a lower temperature than that of flowing in, flows out of air heater  234  via fluidly connected duct  246 . 
     PA of an ambient temperature is pulled into combustion system  210  by a fluidly connected fan  236  within fluidly connected intake duct  238  of air heater  234 . SA of an ambient temperature is drawn into combustion system  210  by a separate fluidly connected fan  236   a  within a separate fluidly connected intake duct  238   a  of air heater  234 . As an alternative, SA could also be drawn into combustion system  210  by fan  236  in intake duct  238 . Accordingly, ambient PA and ambient SA are heated in air heater  234  by heat exchange, wherein FG serves as the source of heat as described above. From air heater  234 , PA flows through fluidly connected duct  240  to enter fluidly connected booster air heater  254 . Although PA has been heated in air heater  234 , the PA is of a lower temperature than that of the FG flowing through booster air heater  254  from duct  260 . As such, through heat transfer from hot FG, PA is heated further by booster air heater  254  prior to flow through fluidly connected duct  256  to mill  242 . SA flows through fluidly connected duct  248  to furnace  211 . 
     Mill  242  is where fuel is pulverized for combustion in furnace  211 . When high moisture fuel, such as sub-bituminous coal and/or lignite is pulverized, moisture is released. For entrainment and flow of pulverized fuel though duct  244  fluidly connected between mill  242  and furnace  211 , moisture released upon high moisture fuel pulverization must be dried. The heated PA flowing through duct  240  to booster air heater  254  and then mill  242 , is effectively used for such purpose as described in more detail below. 
     The furnace  211  is fired by injecting pulverized fuel into the furnace thru burners  250 . In accordance with conventional practice, the amount of fuel injected into the furnace  211  is controlled in response to load demand on the steam generator to provide the total heat release necessary to yield a desired stream generation for the given steam generator design. 
     In pulverized fuel firing, as illustrated in  FIG. 2 , solid fuel, such as coal and/or lignite, is fed from a storage bin (not shown) at a controlled rate, depending on the demand therefor by combustion system  210 , through feeder  252  to the mill  242  where the fuel is pulverized to a fine powder-like particle size. In pulverized fuel-fired furnace  211 , PA is supplied to the mill  242  for transporting the pulverized fuel from the mill  242  to the burners  250  to be injected into the furnace  211  and burned as a suspension therein. As mentioned previously, the PA supplied to the mill  242  is first preheated in the air heater  234  wherein the PA is passed in heat exchange relationship with the FG leaving the furnace  211  through exit ducts  214 ,  218 ,  260 ,  262 ,  224  and  232 , and then heated a second time in booster air heater  254  wherein the PA is passed in heat exchange relationship with the FG leaving the furnace through exit ducts  260  and  262 . As the PA sweeps through the mill  242 , the comminuted pulverized fuel is entrained therein and dried by the heat content of the PA heated by heat transfer from the FG flowing through air heater  234  and booster air heater  254 . A mill  242  outlet temperature in the range of about 160° F. to about 220° F. is needed to ensure adequate fuel drying. Thus, the maximum obtainable temperature of the PA is no longer limited by the temperature of the FG flowing from SCR system  230 . For this reason, PA is economically heated to a higher temperature than otherwise possible and is thereby effective in drying pulverized high moisture fuel even with SCR system  230  treatment of FG. 
     Now referring to the apparatus of another embodiment illustrated in  FIG. 3 . The apparatus schematically illustrated in  FIG. 3  has features in common with those illustrated in  FIG. 1 . As such, features illustrated in  FIG. 3  common to those of  FIG. 1  are signified using the same numbers but with the number “3” preceding them. 
     Referring now to  FIG. 3 , there is illustrated a pulverized fuel-fired steam generator combustion system  310  having a furnace  311  defining an interior chamber  312  therein wherein pulverized fuel (PF) is burned thereby generating hot flue gas (FG). FG generated in interior chamber  312  flows therefrom through a fluidly connected exit duct  314 . Exit duct  314  includes in fluid communication therewith an economizer  316  and a by-pass duct  318  that separates from exit duct  314  at fluid connection  320  to circumvent and by-pass economizer  316 . As such, all or a portion of FG flowing from interior chamber  312  flows through economizer  316  and none or a portion of FG flowing from interior chamber  312  may flow through by-pass duct  318 . Exit duct  314  also includes in fluid communication therewith a booster air heater  354  and a by-pass duct  360  that separates from exit duct  314  at fluid connection  358  upstream of fluid connection  320  to circumvent and by-pass economizer  316 . Fluid connection  358  is arranged in exit duct  314  upstream with regard to the flow of FG of fluid connection  320  since a larger portion and a larger flow of FG is typically desired through by-pass duct  360  than that of by-pass duct  318 . FG flowing through by-pass duct  360  to booster air heater  354  is cooled through heat transfer prior to flowing outwardly from booster air heater  354  through fluidly connected exit duct  362 . Exit duct  362  is fluidly connected to a second SCR system  380 . As such, a portion of FG flowing from interior chamber  312  flows through economizer  316 , and none or a portion of FG flowing from interior chamber  312  may flow through by-pass duct  318 , and a significant portion of FG flowing from interior chamber  312  flows through by-pass duct  360 . At full load, combustion system  310  preferably operates with up to about 50 percent of FG flowing through by-pass duct  360  and almost no FG flowing through by-pass duct  318 . 
     In order to control the flow of FG through economizer  316  is a control valve  322  downstream with regard to the flow of FG from economizer  316  into economizer exit duct  324 . Likewise, to control the flow of FG through by-pass duct  318  is a control valve  326  upstream with regard to the flow of FG to fluid connection  328  of by-pass duct  318  with economizer exit duct  324 . To control the flow of FG from booster air heater  354  through exit duct  362  is a control valve  378  upstream with regard to the flow of FG to second SCR system  380 . 
     By-pass duct  360  includes a by-pass duct  372  fluidly connected thereto at fluid connection  370 . By-pass duct  372  by-passes booster air heater  354  for fluid connection to exit duct  362  at fluid connection  376 . Upstream of fluid connection  376  in by-pass duct  372  is a control valve  374 . Likewise, upstream of fluid connection  376  in exit duct  362  is control valve  378 . 
     Control valves  322 ,  326 ,  374  and  378  work in cooperation to control the flow of FG. For example, if cooler FG is desired, all FG may be made to flow through economizer  316  and booster air heater  354  by fully opening control valves  322  and  378 , while fully closing control valves  326  and  374 . Likewise, if warmer FG is desired, control valve  322  and/or  378  are partially closed and control valves  326  and  374  are partially opened to allow a portion of FG to by-pass economizer  316  and/or booster air heater  354 . If hot FG is desired, control valves  322  and  378  could be fully closed and control valves  326  and  374  fully opened to by-pass economizer  316  and booster air heater  354 . However, in the case of the current embodiment wherein SCR systems  330  and  380  are used for FG treatment, such hot FG must be avoided to protect SCR operation. SCR operation is dependent upon properly functioning catalysts, which are heat sensitive. Allowing hot FG to flow through an SCR systems  330  and  380  can destroy expensive catalysts. 
     Economizer  316  and booster air heater  354  are used to cool FG to a lower temperature, typically a temperature lower than 700° C. to 750° C. so as to be within a temperature range suitable for proper SCR system  330  and  380  operation. FG flows from economizer  316  and booster air heater  354 , through fluidly connected economizer exit duct  324  and fluidly connected exit duct  362 , respectively, to fluidly connected SCR systems  330  and  380 , respectively. 
     SCR systems  330  and  380  operate as typical SCRs to remove nitrous oxides (NOx) and like pollutants from the FG. Having two SCR systems  330  and  380  in parallel is of particular benefit to enable more efficient combustion system  310  operation. Fluidly connected to SCR system  330  is duct  332 . Duct  332  is fluidly connected to a primary air (PA) air heater  334 . As such, FG flows from SCR system  330  to air heater  334  via duct  332 . FG is used as a source of heat for air heater  334 . FG flows into air heater  334  via duct  332  and out of air heater  334  via fluidly connected duct  346 . 
     Similarly, fluidly connected to SCR system  380  is duct  382 . Within duct  382  prior to fluidly connecting to duct  332  at fluid connection  386  is control valve  384 . Control valve  384  is useful to control the volume of FG flowing through SCR system  380  and into duct  332 . 
     PA is ambient air from the environment pulled into the combustion system  310  by a fluidly connected fan  336  within fluidly connected intake duct  338  of air heater  334 . Ambient PA is heated by the FG prior to flow through fluidly connected duct  340 . Heated PA flowing through duct  340  enters fluidly connected booster air heater  354 . Although PA has been heated in air heater  334 , the PA is of a lower temperature than that of the FG flowing through booster air heater  354  from duct  360 . As such, through heat transfer from hot FG, PA is heated further by booster air heater  354  prior to flow through fluidly connected duct  356  to mill  342 . 
     SA of an ambient temperature is drawn into combustion system  310  by a separate fluidly connected fan  336   a  within a separate fluidly connected intake duct  338   a  of air heater  334 . As an alternative, SA could also be drawn into combustion system  310  by fan  336  in intake duct  338 . Accordingly, ambient PA and ambient SA are heated in air heater  334  by heat exchange, wherein FG serves as the source of heat as described above. From air heater  334 , SA flows through fluidly connected duct  348  to furnace  311 . 
     Mill  342  is where fuel is pulverized for combustion in furnace  311 . When high moisture fuel such as sub-bituminous coal and/or lignite is pulverized, moisture is released. For entrainment and flow of pulverized fuel though duct  344  fluidly connected between mill  342  and furnace  311 , moisture released upon high moisture fuel pulverization must be dried. The heated PA flowing through duct  340  to booster air heater  354  and then mill  342 , is effectively used for such purpose as described in more detail below. 
     Furnace  311  is fired by injecting pulverized fuel into the furnace through burners  350 . The amount of fuel injected into the furnace  311  is controlled in response to load demand on the steam generator to provide the total heat release necessary to yield a desired stream generation for the given steam generator design. 
     In pulverized fuel firing, as illustrated in  FIG. 3 , solid fuel such as coal and/or lignite is fed from a storage bin (not shown) at a controlled rate, depending upon the demand therefor by combustion system  310 , through feeder  352  to the mill  342  where the fuel is pulverized to a fine powder-like particle size. In fuel-fired furnace  311 , PA is supplied to the mill  342  for transporting the pulverized fuel from the mill  342  to the burners  350  to be injected into the furnace  311  and burned as a suspension therein. As mentioned previously, the PA supplied to the mill  342  is first preheated in the air heater  334  wherein the PA is passed in heat exchange relationship with the FG leaving the furnace through exit ducts  314 ,  318 ,  360 ,  362 ,  324 ,  372 ,  382  and  332 , and then heated a second time in booster air heater  354  wherein the PA is passed in heat exchange relationship with the FG leaving the furnace through exit ducts  360  and  362 . As the PA sweeps through the mill  342 , the comminuted fuel is entrained therein and dried by the heat content of the PA heated by heat transfer from the FG flowing through air heater  334  and booster air heater  354 . A mill  342  outlet temperature in the range of approximately 160° F. to about 220° F. is needed to ensure the fuel is adequately dried for use. Thus, the maximum obtainable temperature of the PA is no longer limited by the temperature of the FG following from SCR system  330 . For this reason, PA is economically heated to a higher temperature than otherwise possible and is thereby effective in drying pulverized high moisture fuel even with SCR systems  330  and  380  treatment of FG. 
     In a method of using the apparatus illustrated in  FIG. 2 , FG is cooled in economizer  216  to a temperature in the range of about 700° F. to about 750° F. and booster air heater  254  to a temperature in the range of about 700° F. to about 750° F. prior to contact and treatment in SCR system  230 , which operates at a temperature in the range of about 700° F. to about 750° F. at full load. With operation at part loads, SCR system  230  must be maintained at a temperature in the range of about 550° F. to about 650° F. to ensure adequate catalyst performance. After contact and treatment in SCR system  230 , FG is further cooled in air heater  234  to a temperature in the range of about 250° F. to about 300° F. prior to exit through duct  246 . PA is heated through heat exchange in air heater  234  to a temperature in the range of about 400° F. to about 500° F. and booster air heater  254  to a temperature in the range of about 700° F. to about 800° F. prior to passage through mill  242  to dry high moisture fuel pulverized therein, to obtain dried pulverized fuel. Mill  242  outlet temperature must be maintained at a temperature in the range of about 160° F. to about 220° F. 
     In a method of using the apparatus illustrated in  FIG. 3 , FG is cooled in economizer  316  to a temperature in the range of about 700° F. to about 750° F. and booster air heater  354  to a temperature in the range of about 700° F. to about 750° F. prior to contact and treatment in SCR systems  330  and  380 , which operate at a temperature in the range of about 700° F. to about 750° F. After contact and treatment in SCR systems  230  and  380 , FG is further cooled in air heater  334  to a temperature in the range of about 250° F. to about 300° F. prior to exit through duct  346 . PA is heated through heat exchange in air heater  334  to a temperature in the range of about 400° F. to about 500° F. and booster air heater  354  to a temperature in the range of about 700° F. to about 800° F. prior to passage through mill  342  to dry high moisture fuel pulverized therein, to obtain dried pulverized fuel. Mill  342  outlet temperature must be maintained at a temperature in the range of about 160° F. to about 220° F. 
     While the preferred embodiment has been shown and described in relation to a pulverized fuel-fired steam generator, the present invention may apply to any of a number of combustion systems wherein pulverized fuel is burned and various modifications may be made thereto by those skilled in the art without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the invention has been described by way of illustration and is to be limited only in accordance with the claims appended hereto