Abstract:
Methods of reducing emissions levels during steady-state conditions are disclosed for use with a dry scrubber desulfurization system. A dry calcium hydroxide powder is injected into the gas flowpath and watered in the spray dryer absorber. The resulting slurry is then deposited on the filter bags in the baghouse. This can be done at lower temperatures than the spray dryer absorber would otherwise be operable, enabling desulfurization to occur earlier in the combustion process, particularly during startup of a cold boiler at ambient temperature. The operation of the boiler can also be backed up, made up, trimmed, or augmented depending on various operating scenarios.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Application Ser. No. 61/540,795, filed on Sep. 29, 2011. The disclosure of this application is hereby fully incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    The present disclosure generally relates to the removal of particulates and other contaminants from flue gas produced during combustion using a dry scrubber flue gas desulfurization system during normal operation. In particular, this disclosure relates to new and useful methods and systems for capturing sulfur dioxide (SO 2 ), sulfur trioxide (SO 3 ), HCl, and other acid gases by injecting dry sorbent into a gas stream and passing the gas stream through a spray dryer absorber to disperse the sorbent in a baghouse during the use of a pollutant-forming fossil fuel in a combustion system. 
         [0003]    During combustion, the chemical energy in a fuel is converted to thermal heat, which can be used in various forms for different applications. The fuels used in the combustion process can include a wide range of solid, liquid, and gaseous substances, including coal, oil (diesel, No. 2, Bunker C or No. 6), natural gas, wood, tires, biomass, etc. 
         [0004]    Combustion transforms the fuel into a large number of chemical compounds. Water (H 2 O) and carbon dioxide (CO 2 ) are the primary products of complete combustion. However, other combustion reactions with chemical components in the fuel result in undesirable byproducts. Depending on the fuel used, such byproducts may include particulates (e.g. fly ash), acid gases such as sulfur oxides (SO x ) or nitric oxides (NO x ), metals such as mercury or arsenic, carbon monoxide (CO), and hydrocarbons (HC). The emissions levels of many of these byproducts are regulated by governmental entities, such as the U.S. Environmental Protection Agency (EPA). 
         [0005]    Several different technologies exist for removing such byproducts from the flue gas. In one method, known as spray drying chemical absorption or dry scrubbing, an aqueous alkaline solution or slurry, which has been finely atomized, is sprayed into the hot flue gas downstream of the combustion chamber in which the fuel was combusted. The alkaline reagent reacts with the pollutants, and particulates are formed. The water evaporates and cools the hot flue gas. The exiting cleaned flue gas typically has a moisture content of about 10% to about 15%. The flue gas then travels to a particulate collection device, generally a baghouse, where the particulates are removed from the flue gas, which is then sent to a stack. 
         [0006]    When a combustion system, such as a boiler having a furnace, is started up from cold conditions such as ambient temperatures, the furnace usually burns natural gas or diesel (No. 2) oil to “warm up” the boiler before switching over to coal. A furnace temperature of about 400° F. to about 500° F. is needed before coal can start to be burned. Due to various startup conditions and safety requirements, the furnace can be started and stopped several times before attaining steady-state operations. Complete startup can take anywhere from 8 hours to up to 2 days to complete, depending on the problems encountered. 
         [0007]    The dry scrubbing desulfurization process does not work well at low temperatures. In particular, the temperature of the flue gas typically needs to be at least 220° F. to use the spray dryer absorber, so that the water can be completely evaporated. During startup, the temperature of the flue gas that passes to the spray dryer absorber may be below this threshold temperature, yet SO x  and other pollutants are still being produced. In addition, the furnace generally reaches the coal operating temperature of 400° F. to 500° F. before the flue gas attains a temperature of 220° F. in the spray dryer absorber. This results in higher SO x  emissions during startup. In addition, the baghouse generally requires 30 to 60 minutes of operation after the spray dryer absorber has started to accumulate significant alkaline material and achieve significant SO 2  removal. 
         [0008]    Previously, emissions regulations did not cover “upset” periods such as startup, shutdown, and malfunction. However, it would be desirable to reduce such emissions due to increasing regulatory restrictions. Methods that can reduce such emissions during startup would be very helpful. 
       BRIEF DESCRIPTION 
       [0009]    Disclosed herein are various methods and systems for reducing SO x  emissions during steady-state operating conditions in a pollution control system that uses a dry scrubber for desulfurization. Briefly, a dry calcium hydroxide powder is injected into the flue gas while the combustion chamber is at normal operating conditions (i.e. high temperatures). The powder is injected upstream of the spray dryer absorber. The resulting calcium hydroxide powder is then collected in a downstream baghouse to form a filter cake that is useful in reducing SO x  emissions. This can be used to augment the desulfurization capacity of the dry scrubber or to trim emissions. 
         [0010]    Disclosed in embodiments is a method for reducing combustion emissions produced during normal operating conditions in a combustion system. The combustion system has a gas flowpath that extends from a combustion chamber through a spray dryer absorber to a baghouse downstream of the spray dryer absorber. Flue gas produced by the combustion chamber flows through the gas flowpath. A dry calcium hydroxide powder is mixed into a transport gas, typically air, and is pneumatically conveyed to an injection location downstream of the combustion chamber and upstream of the baghouse where the dry calcium hydroxide powder is blown into and mixed with the flue gas in the gas flowpath. Water is sprayed into the flue gas in the spray dryer absorber to humidify and reduce the temperature of the flue gas. The flue gas then passes through the baghouse, where the calcium hydroxide powder is deposited in the baghouse to form a filter cake that reduces combustion emissions. 
         [0011]    In particular embodiments, no liquid is added to the flue gas between the injection location and the spray dryer absorber. 
         [0012]    The water sprayed into the spray dry scrubber may come from a recycle system for recycling solids from the baghouse. The water may also be in the form of an alkaline slurry, rather than just water. 
         [0013]    Sometimes, the gas flowpath extends through an air preheater located between the combustion chamber and the spray dryer absorber. The injection location can be located between the air preheater and the spray dryer absorber. Alternatively, the injection location is upstream of the air preheater. A particulate collection device may also be located between the air preheater and the spray dryer absorber with the injection location downstream of the particulate collection device. 
         [0014]    The injection location can also be between the spray dryer absorber and the baghouse. 
         [0015]    The baghouse downstream of the spray dryer absorber may be a pulse jet fabric filter or reverse gas fabric filter. 
         [0016]    The amount of dry calcium hydroxide powder mixed into the flue gas varies over time depending on an emissions level in the flue gas (i.e. a trim scenario). 
         [0017]    The water sprayed into the flue gas in the spray dryer absorber may be in the form of water, especially when an alkaline slurry is not being sprayed in the spray dryer absorber (i.e. a malfunction or augment scenario). 
         [0018]    The flue gas entering the spray dryer absorber may have a temperature of about 220° F. or higher. The flue gas exiting the furnace may have a temperature of 400° F. or higher. 
         [0019]    Also disclosed are methods for operating a boiler that uses a spray dryer absorber to clean a flue gas. A dry calcium hydroxide powder is mixed into the flue gas at an injection location downstream of the boiler and upstream of the spray dryer absorber. Water is then sprayed into the flue gas in the spray dryer absorber to form a cleaned particulate-containing flue gas. The particulates in the particulate-containing flue gas are then deposited in the baghouse to form a filter cake that reduces combustion emissions. This can be used as a back-up to ensure desulfurization, or to trim the emissions level in a manner that allows for quick response to changing levels, during routine maintenance of the spray dryer absorber, or to supplement/replace the lime slurry typically used for flue gas desulfurization. 
         [0020]    The amount of dry calcium hydroxide powder injected into the flue gas can be determined by comparing an emissions level to a predetermined value. 
         [0021]    The water sprayed into the spray dryer absorber may be in the form of simply water (i.e. H 2 O), or in the form of an alkaline slurry (i.e. water plus an alkaline sorbent like calcium hydroxide). The water can also come from a recycle system for recycling solids from the baghouse, or through auxiliary nozzles when the atomizer is not operating. In some embodiments, the flue gas entering the spray dryer absorber has a temperature of about 220° F. or higher, i.e. during conditions in which the alkaline slurry can be sufficiently evaporated. The flue gas exiting the furnace may have a temperature of 400° F. or higher. 
         [0022]    These and other non-limiting characteristics are more particularly described below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The following is a brief description of the drawings, which are presented for the purposes of illustrating the exemplary embodiments disclosed herein and not for the purposes of limiting the same. 
           [0024]      FIG. 1  is a diagram illustrating a conventional boiler with a dry desulfurization system. 
           [0025]      FIG. 2  is a diagram illustrating a combustion system with a dry desulfurization system and a calcium hydroxide powder injection system as described in the present disclosure. 
           [0026]      FIG. 3  is an illustration of a filter bag in a pulse jet fabric filter. 
           [0027]      FIG. 4  is a cutaway view of a spray dryer absorber. 
           [0028]      FIG. 5  is an illustration of the major components of a dry sorbent injection system. 
           [0029]      FIG. 6  is an emissions vs. time graph showing actual emissions with calcium hydroxide injection and estimated emissions without calcium hydroxide injection. 
           [0030]      FIG. 7  is a general process diagram illustrating the methods of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    A more complete understanding of the components, processes, and apparatuses disclosed herein can be obtained by reference to the accompanying drawings. These figures are merely schematic representations based on convenience and the ease of demonstrating the present disclosure, and are, therefore, not intended to indicate relative size and dimensions of the devices or components thereof and/or to define or limit the scope of the exemplary embodiments. 
         [0032]    Although specific terms are used in the following description for the sake of clarity, these terms are intended to refer only to the particular structure of the embodiments selected for illustration in the drawings, and are not intended to define or limit the scope of the disclosure. In the drawings and the following description below, it is to be understood that like numeric designations refer to components of like function. 
         [0033]    The singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. 
         [0034]    As used in the specification and in the claims, the term “comprising” may include the embodiments “consisting of” and “consisting essentially of.” 
         [0035]    All ranges disclosed herein are inclusive of the recited endpoint and independently combinable (for example, the range of “from 250° F. to 400° F.” is inclusive of the endpoints, 250° F. and 400° F., and all the intermediate values). The endpoints of the ranges and any values disclosed herein are not limited to the precise range or value; they are sufficiently imprecise to include values approximating these ranges and/or values. 
         [0036]    As used herein, approximating language may be applied to modify any quantitative representation that may vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about” and “substantially,” may not be limited to the precise value specified, in some cases. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value. The modifier “about” should also be considered as disclosing the range defined by the absolute values of the two endpoints. For example, the expression “from about 2 to about 4” also discloses the range “from 2 to 4.” 
         [0037]    The term “hydrated lime” refers to calcium hydroxide, also known as Ca(OH) 2 . The term “hydrated” when used here does not mean that molecular water is present. 
         [0038]    The term “lime slurry” is used to refer to a mixture of calcium hydroxide with water. Other calcium sorbents include, for example, limestone or quicklime. The term “limestone” refers to calcium carbonate, also known as CaCO 3 . The term “quicklime” refers to calcium oxide, CaO. 
         [0039]    The present disclosure refers to components which are “upstream” and “downstream” of other components. These two terms are relative to another named component. A given component is “upstream” of a named component if a flowpath runs through the given component before running through the named component. Similarly, a given component is “downstream” of a named component if a flowpath runs through the given component after running through the named component. 
         [0040]    The present disclosure relates to various methods and systems for reducing SO x  emissions during steady-state operating conditions in a pollution control system that uses a dry scrubber for desulfurization. Very generally, a flue gas is generated by a combustion system containing a combustion chamber in which fuel is combusted. A dry calcium hydroxide powder can be injected into the flue gas while the combustion chamber is at normal operating conditions (i.e. high temperatures). The powder is injected upstream of the spray dryer absorber. The resulting calcium hydroxide powder is then collected in a downstream baghouse to form a filter cake that is useful in reducing SO x  emissions. 
         [0041]    Generally, it is considered that such methods can be used in any system in which combustion occurs. The combustion can be used for any purpose, for example to generate power, produce a certain product, or simply to incinerate a given fuel. Exemplary combustion systems in which the present methods may be applicable include power generation systems that use a boiler having a furnace as the combustion chamber; cement kilns; electric arc furnaces; glass furnaces; smelters (copper, gold, tin, etc.); pelletizer roasters; blast furnaces; coke oven batteries; chemical fired heaters; refinery ovens; and incinerators (medical waste, municipal solid waste, etc.). The term “combustion chamber” is used herein to refer to the specific structure within the system in which combustion occurs. 
         [0042]      FIG. 1  generally illustrates an exemplary power generation system with a boiler  100  and a downstream desulfurization system  110 . A fossil fuel  112 , such as coal from a pulverizer  111 , and air  114  are burned in the furnace  105 , resulting in the generation of a flue gas  120 . The flue gas  120  passes an economizer  116  used to preheat the water used in the boiler to produce steam and to cool the flue gas  120 . Other heat transfer surfaces upstream of the economizer  116  are not shown. The flue gas  120  then enters a selective catalytic reduction (SCR) system  130 , which may or may not be present, to remove nitrogen oxides (NO x ) from the flue gas  120 . Next, the flue gas  120  passes through an air preheater  140  to further cool the flue gas  120  and heat the air  114  entering the furnace  105 . After passing through the air preheater  140 , the flue gas  120  typically has a temperature of about 250 to about 400° F. (121 to 204° C.). Sometimes the flue gas  120  then passes through a particulate collection device  150  to collect fly ash and other large particles. The flue gas continues into a dry scrubber or spray dryer absorber  160 . Here, an atomized alkaline slurry  162  is sprayed into the flue gas to react with sulfur oxides (SO x ) and to further cool the flue gas  120  to a range of about 140 to about 210° F. (60 to 99° C.). The water in the slurry is evaporated, and the resulting cleaned and particle-laden flue gas  120  is conveyed to a particulate collection device  170 , such as a baghouse or an electrostatic precipitator, to remove the particles from the flue gas  120 . The cleaned flue gas  120  is then sent to a stack  180 . If desired, a recycle stream  172  from the particulate collection device  170  can be used to collect the alkaline particles from the baghouse and mix them with water  176  in a recycle tank  180  to make the alkaline slurry  162  which is used in the spray dryer absorber  160 . Alternatively, fresh slurry  164  can be used in the spray dryer absorber  160 . Particles can also be removed from the particulate collection device  170  for disposal, indicated here with reference numeral  174 . 
         [0043]    In the methods of the present disclosure, calcium hydroxide is deposited in the baghouse to provide and enhance high-efficiency removal of acids during normal operations (i.e. steady-state operating conditions). In this regard, the flue gas must travel through the filter cake formed on the filter in the baghouse, which provides intimate contact between the flue gas and the alkaline calcium hydroxide product and promotes the absorption of vapor-phase acid gases (such as SO x ) in the flue gas by the filter cake. Depending on the operating scenario, the dry calcium hydroxide powder can be used to augment the desulfurization capability of the desulfurization system, or can be used to trim the emissions level of the overall power generation system. More generally, the present methods can be used to remove particulates from the flue gas. 
         [0044]    The term “steady-state operating conditions” is used herein to refer to periods when the temperature of the flue gas passing through the spray dryer absorber is 220° F. (approx. 104° C.) or higher. 
         [0045]      FIG. 2  generally illustrates an exemplary system of the present disclosure having a combustion system  200 , a downstream desulfurization system  210 , and a dry calcium hydroxide powder injection system  290 . Similar to  FIG. 1 , air  214  and coal  212  from a pulverizer  211  are burned in the combustion chamber  205 , resulting in the generation of a flue gas  220 . Generally speaking, the flue gas is a carrier gas that travels along a gas flowpath. The flue gas passes an economizer  216  (other heat transfer surfaces upstream of the economizer are not shown) and a SCR system  230  which may or may not be present that removes NO x  from the flue gas. The flue gas passes through an air preheater  240  and continues into the spray dryer absorber  260 . If desired, an optional particulate collection device  250  can be located between the air preheater  240  and the spray dryer absorber  260  to collect fly ash and other large particles. In the spray dryer absorber  260 , an atomized alkaline slurry  262 , such as a lime slurry, is sprayed into the flue gas  220  to clean and cool the flue gas. The resulting cleaned and particle-laden flue gas  220  is conveyed to a baghouse  270  to remove the particles from the flue gas. The cleaned flue gas  220  is then sent to a stack  280 . If desired, a recycle stream  272  from the baghouse  270  can be used to collect the unreacted alkaline particles from the baghouse and mix them with water  276  in a recycle tank  280  to make the alkaline slurry  262  which is used in the spray dryer absorber. Alternatively, fresh slurry  264  can be used in the spray dryer absorber  260 . Particles from the baghouse can also be disposed of, shown here with reference numeral  274 . 
         [0046]    The combustion chamber  205  is upstream of the air preheater  240 , which is upstream of the spray dryer absorber  260 . A baghouse  270  is downstream of the spray dryer absorber  260 . Put another way, the spray dryer absorber  260  is located between the air preheater  240  and the baghouse  270 . The SCR system  230 , if present, is located between the furnace  205  and the air preheater  240 . 
         [0047]    The present methods contemplate that a gas flowpath  220  is present between the combustion system and the desulfurization system. Flue gas flows through or travels along the gas flowpath. A dry calcium hydroxide powder is injected into the flue gas at an injection location downstream of the combustion chamber  205  and upstream of the baghouse  270 . Water is sprayed into the carrier gas in the spray dryer absorber  260  to cool and humidify the flue gas. This water may be simply water (i.e. H 2 O) or water in the form of the alkaline slurry (containing water and alkaline sorbent). The calcium hydroxide powder is then deposited in the baghouse  270  to form a filter cake that is used to reduce the emissions. 
         [0048]    The dry calcium hydroxide powder injection system  290  includes a calcium hydroxide supply source  292 . It is contemplated that calcium hydroxide powder can be injected into the desulfurization system in three different locations A, B, C. These three injection locations are all downstream of the combustion chamber  205  and upstream of the baghouse  270 . In particular, the temperature of the flue gas/carrier gas should be less than 1000° F. to maintain the stability of the hydrated lime. 
         [0049]    The first injection location A is downstream of the air preheater  240  and upstream of the spray dryer absorber  260 . Put another way, injection location A is between the air preheater  240  and the spray dryer absorber  260 . The optional particulate collection device  250  should be upstream of the injection location A. 
         [0050]    The second injection location B is downstream of the combustion chamber  205  and upstream of the air preheater  240 . The second injection location B may also be described as being downstream of the SCR system  230 . 
         [0051]    The third injection location C is downstream of the spray dryer absorber  260 . Put another way, injection location C is between the spray dryer absorber  260  and the baghouse  270 . 
         [0052]    Dry calcium hydroxide powder may also be simultaneously injected at the various locations identified above. Referring back to  FIG. 2 , the water that is sprayed in the spray dryer absorber  260  can come from a separate water source, or in some embodiments can come from the recycle system  280 , or comes from alkaline slurry  262 . 
         [0053]    The optional particulate collection device  250  may in various embodiments be either an electrostatic precipitator (ESP) or a baghouse. Different types of baghouses are known in the art, for example a reverse gas fabric filter, a shake deflate fabric filter, and a pulse jet fabric filter. 
         [0054]    The baghouse  270  downstream of the spray dryer absorber  260  is desirably a pulse jet fabric filter (PJFF) or a reverse gas fabric filter. In this regard, a baghouse is preferable to an ESP at this location due to the desulfurization ability of the baghouse compared to an ESP. In other words, a baghouse can capture pollutants that are in the vapor phase, whereas an ESP only traps particles and does not significantly capture vapor-phase pollutants. Generally, all of the flue gas entering the baghouse  270  should pass through the filter cake so that acid gases such as SO 2 , SO 3 , and HCl can be removed. 
         [0055]      FIG. 3  is a schematic illustration of a pulse jet fabric filter. A baghouse generally contains multiple compartments, with each compartment containing up to several hundred long, vertically supported, small diameter fabric bags. In a pulse jet fabric filter (PJFF), the bags  320  hang from a tubesheet  330 . The flue gas containing particulates flows from outside the bag (indicated as solid arrows) to inside the bag (indicated as outlined arrows). The flue gas passes through the porous bag material, leaving the particulates behind to form a filter cake  340  on the exterior of the bag. A pulse of compressed air can be directed into the bag from the open top  322 , causing a shock wave to travel down the length of the bag and dislodge the filter cake. 
         [0056]    Calcium hydroxide is used because its salt is not soluble in water. In contrast, sodium sorbents are generally soluble and thus are less desirable. In addition, calcium hydroxide is safer than quicklime, which gives off heat when combined with water. 
         [0057]    Applicants have determined that the reactivity of powdered calcium hydroxide is comparable to the reactivity of calcium hydroxide in a lime slurry. This allows the dry desulfurization system to be operated acceptably in various conditions. In particular, the dry calcium hydroxide powder injection system allows for normal operations of the boiler when there is a failure in the alkaline slurry supply system. Calcium hydroxide powder can be added in larger quantities when compared to alkaline slurry, to make up for the loss of the alkaline slurry and maintain acceptable emissions levels. For example, if the atomizer clogs, the atomizer can be removed and a backup atomizer can be installed to continue spraying water into the flue gas. Alternatively, water can be introduced through auxiliary nozzles. The calcium hydroxide powder can be used to maintain desulfurization capability in the baghouse. 
         [0058]    Another operating scenario is to operate the spraying of the alkaline slurry in the spray dryer absorber so as to maintain emissions levels close to a predetermined value. As emissions near or exceed the predetermined value, the calcium hydroxide powder can be immediately added to trim the emissions level back down to an acceptable level. 
         [0059]    Yet another operating scenario may occur where the operating plant has a limited supply of alkaline slurry. Here, the calcium hydroxide powder can be used to augment the atomizer slurry to maintain acceptable emissions levels. 
         [0060]    Typically, it is more desirable to inject the calcium hydroxide powder upstream of the spray dryer absorber  260  (i.e. injection locations A or B) because the spray dryer absorber helps to properly disperse the powder throughout the baghouse  270 .  FIG. 4  is a cutaway view of a spray dryer absorber  400  typically used in desulfurization systems. The spray dryer absorber typically has a housing  410  with a frustoconical shape, with the apex of the cone at the bottom of the absorber. However, spray dryer absorbers may also have a flat bottom instead of the cone. The flue gas  420  coming from the air heater can be split into two streams  422 ,  424 , although this is not always the case and is not necessary for the present disclosure. One stream  422  is directed to an upper gas disperser  430  which has an annular shape. The other stream  424  is directed to a lower gas disperser  440 . The atomizer  450  extends through the center of the roof of the absorber housing, and sprays the lime slurry into the flue gas. The flue gas enters the spray dryer absorber  400  through the gas dispersers. The spray dryer absorber is designed to assure good mixing of the flue gas with the slurry, and is sized to provide sufficient residence time for drying the slurry to produce free-flowing solids without internal deposits. The mixing and turbulence imparted to the calcium hydroxide powder by the spray dryer absorber assures better dispersion of the calcium hydroxide throughout the filter bags in the baghouse. Water is added into the spray dryer absorber by the atomizer  450  to the dry calcium hydroxide powder to form a calcium hydroxide slurry. The water is needed in the baghouse for the filter cake to attain its full desulfurization ability since the reaction mechanism for SO 2  absorption requires the presence of molecular water. The evaporated calcium hydroxide slurry exits the spray dryer absorber through outlet  460  and proceeds to the baghouse. 
         [0061]      FIG. 5  is a schematic diagram of a typical dry sorbent injection system for hydrated lime. Hydrated lime can either be delivered  510  by truck or by rail (truck unloading is illustrated here). Ambient air  512  is drawn into the truck to pick up the hydrated lime and transfer the reagent to a storage silo  520 . The reagent flows from the storage silo  520  through a series of valves  522 , feeders  524 , and hoppers  526 ,  528  into a rotary airlock  530  where the reagent is mixed with the transport gas  540  to be pneumatically conveyed to the injection location into the gas flowpath (see  FIG. 2 ). The transport gas, typically air, is provided by transport air blowers  542  that pass the transport gas through air coolers  544  to reduce the air temperature to prevent premature calcination of the reagent. It should be noted that in the present system, no liquids are injected into the gas flowpath between the injection location and the spray dryer absorber. This is in contrast to prior systems where solutions and slurries have been injected into the flue gas upstream of a wet or dry scrubber; see for example U.S. Pat. No. 6,126,910 to Wilhelm. This is also in contrast to a system where a dry calcium sorbent has been injected and then humidified with water in ductwork; see for example U.S. Pat. No. 5,165,903 to Hunt. In these prior systems, the desired purpose is to remove selected pollutants from the flue gas before entering the desulfurization system. In contrast, the purpose of the present methods is to provide an alternate source of alkali reagent (hydrated lime), increase hydrated lime concentration in the spray dry absorber and to coat the baghouse with calcium hydroxide in order to provide desulfurization and enhanced desulfurization capability. Adding water or liquid before the spray dryer absorber may result in the undesirable condition of calcium hydroxide falling out of the gas and failing to travel to the baghouse. 
         [0062]    The methods of the present disclosure improve the capability of the desulfurization system to respond to and operate within acceptable acid gas emissions levels by providing a means to react in a timely manner to variations in emissions levels. One recurring theme in maintaining combustion system operations is the time needed to fix a given problem. Calcium hydroxide powder can be quickly added and a good response is obtained. The methods also provide a dry sorbent that does not require adding water to the process. 
         [0063]      FIG. 7  is a general process diagram illustrating the methods of the present disclosure. A combustion system  700  contains a combustion chamber  705  in which combustion occurs and results in the generation of a flue gas. The flue gas travels along a gas flowpath  720  through a spray dryer absorber  760  to a baghouse  770  downstream of the spray dryer absorber. Dry calcium hydroxide powder is mixed with the flue gas (in the gas flowpath  720 ) between the combustion chamber  705  and the baghouse  770 . For example, the calcium hydroxide powder can be added upstream of the spray dryer absorber (reference numeral  794 ) or downstream of the spray dryer absorber (reference numeral  796 ). Inside the spray dryer absorber  760 , water (reference numeral  762 ) is sprayed into the flue gas to humidify and cool the flue gas. The flue gas is passed to the baghouse  770 . The calcium hydroxide captures pollutants or particulates in the flue gas. The cleaned flue gas is sent to a stack  780  or similar device for release into the atmosphere. 
         [0064]    Designs for practicing the methods of this disclosure are within the ordinary skill of the art. The valves, piping, sensors, connections, and fittings needed to permit the practice of these methods are also generally commercially available. 
       EXAMPLE 
       [0065]    A 120 MWg (gross megawatts) power plant had the layout seen in  FIG. 2 . The use of calcium hydroxide powder was implemented during startup and as a replacement for lime slurry. The calcium hydroxide powder was injected at injection locations A and C. Actual stack SO 2  emissions are shown in  FIG. 6 . The y-axis is the amount of SO 2  emitted, in units of lb/MBtu (pounds per million BTUs). The x-axis is the time of day, i.e. from midnight (0:00) to 12:00 pm. The regulated stack SO 2  emission limit of 0.09 lb/MBtu is shown for reference. Two lines are shown: one for the actual emissions and one for the estimated emissions if calcium hydroxide powder had not been injected. It should be noted that startup was attempted three times on this figure: at about 12:30 am, about 2:45 am, and about 5:45 am. 
         [0066]    The present disclosure has been described with reference to exemplary embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the present disclosure be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.