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    Lesson 7Dry Scrubbing Systems

    Goal

    To familiarize you with the types, operating characteristics and collection efficiency associatedwith dry scrubbing systems.

    ObjectivesAt the end of this lesson you will be able to do the following:

    1. Name three industrial processes where dry scrubbers are primarily used

    2. Briefly describe how dry sorbent and spray dryer absorbers operate to collect gaseousemissions

    3. Name two types of atomizers used in spray dryers

    4. Name and describe at least three operating parameters that affect the performance of dryscrubbing systems

    5. Briefly describe operation and maintenance problems associated with spray dryer absorbers

    Introduction

    Dry scrubbing systems control acid gas emissions (SO 2, HCl, HF, etc.) and are used primarilyon utility and industrial boilers, municipal waste combustors, medical waste incinerators, andsome refinery processes. Of course, wet scrubbing systems can also function effectively asacid gas collectors. Regardless of whether scrubber acid gas control systems operate wet ordry, they have a mechanism for introducing alkaline material into the exhaust gas to reactwith the acid gases present. Dry scrubbing systems are discussed in this lesson, while wetflue gas desulfurization systems (wet acid gas control systems that remove SO 2) are discussedin Lesson 9.

    Up to this point, you have been learning about wet scrubber designs. In wet scrubbers, liquiddroplets provide the primary targets for collecting particles and gases. To facilitate this

    process, gas streams are saturated with moisture; therefore wet scrubbing systems release asteam plume when exiting the stack. Also, wet scrubbers require a system of pipes and

    pumps for collecting, treating, and recirculating the scrubbing liquid. In contrast, as theirname implies, dry scrubbers either operate completely dry or use much smaller amounts of

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    liquid than wet scrubbers. In some semi-dry designs, liquid is added to the alkaline material,creating a slurry. High scrubber temperatures evaporate the moisture before the gases andreaction products leave the scrubber. Therefore, dry scrubbing systems do not have a stacksteam plume or waste water handling/disposal requirement.

    There are a number of different dry scrubbing systems designs. However, all consist of twomain sections or devices: (1) a device to introduce the acid gas sorbent material into the gasstream, and (2) a particulate-matter control device to remove reaction products, excesssorbent material and any other particulate pollutants in the flue gas. Dry scrubbing systemscan be categorized as dry sorbent injectors (DSIs) or as spray dryers [also called semi-dryscrubbers or spray dryer absorbers (SDAs)]. Since dry scrubbing systems only remove gases,a separate device is always required to remove particles. The particulate control devices aregenerally fabric filters or electrostatic precipitators (ESPs).

    Dry sorbent injection involves the addition of a dry alkaline material (usually hydrated limeor soda ash) into the gas stream to react with any acid gases that are present. The sorbent can

    be injected directly into the flue gas duct ahead of the particulate control device or into anopen reaction chamber. The acid gases react with alkaline sorbents to form solid salts whichare removed in the particulate control device.

    In spray dryer absorbers, the flue gases are introduced into an absorbing tower (dryer) wherethe gases are contacted with a finely atomized alkaline slurry [usually a calcium-basedsorbent such as Ca(OH) 2 or CaO]. Acid gases are absorbed by the slurry mixture, and reactto form solid salts. The heat of the flue gas is used to evaporate all the water droplets leavinga non-saturated (i.e. dry) flue gas exiting the absorber tower. The effect of cooling andhumidifying the hot gas stream increases collection efficiency over simple dry injection.

    Gas Removal Mechanisms

    In dry scrubbing, acid gas is removed by the mechanisms of adsorption and absorption. Indry injection systems, where adsorption is the primary removal mechanism, pollutant gasmolecules adhere to the surface area of the alkaline particles. Thus, the reaction between theacid gas and the alkaline material takes place on the surface of these alkaline particles. Thealkaline materials are generally calcium hydroxide or sodium-based reagents that have theconsistency of a fine powder. These fine particles have large surface areas to aid in adsorbingthe acid gases.

    In spray dryer systems, absorption is the predominant collection mechanism. Lesson 2describes the general process of gaseous pollutants being absorbed by liquid droplets.Absorption can occur in conjunction with a chemical reaction if a reagent has been added tothe scrubbing liquid. Spray dryer absorbers utilize this principle. First, the acid gas dissolvesin the alkaline slurry droplets, then reacts with the alkaline material dissolved therein to formsolid salts. Because the acid gases react to form new compounds, additional acid gases can

    be absorbed by the liquid. Also, when the liquid droplets evaporate, the acid gases continueto react (by adsorption) with the solid alkaline materials remaining in the SDA.

    Adsorption and absorption are similar mass transfer processes in that the acid gases must first be brought into contact with the alkaline sorbent material, be provided ample reaction sitesand time, and finally, be removed from the gas stream. Intimate contact between the alkalinesorbent and acid gases is important for effective gas removal. With dry injection, solid

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    powder-like sorbent is dispersed in either the furnace area, exhaust duct, or in a reactionchamber. Dispersion is generally provided by injecting the sorbent through a venturi devicecountercurrent to the flow of the gas stream to create turbulence. In spray dryers, the alkalinesorbent slurry is dispersed as a mist of tiny liquid droplets in the reaction or drying vessel.Due to their fine spray mists, spray dryers provide much more contact area than dry injectorsfor gas absorption to occur. Also, spray dryer absorbers provide more effective mixing ofacid gases with the alkaline sorbent than dry sorbent injectors because it is easier to mix a gaswith a liquid than with a solid. Spray dryer absorbers have some disadvantages; the injection(atomization) equipment required by spray dryer absorbers is much more complicated andexpensive to operate.

    Residence or reaction time can be enhanced in these applications in a number of ways. In dryinjectors, the sorbent is often injected directly into the furnace or ductwork. To extend theresidence time, reaction or holding vessels can be added to the dry sorbent injection system.Spray dryers always have a reaction or drying chamber to assure a dry gas stream leaving thechamber. Also, in both systems, the particulate control device will provide an additional areafor the acid gases to further react with the sorbent.

    In addition, both the absorption and adsorption processes are temperature dependent: thecooler the flue gas, the more effectively the acid gases will react with the sorbents. Spraydryer absorbers cool the gas stream and therefore, can achieve higher removal efficienciesthan dry injection with no cooling.

    Stoichiometry

    An important parameter in the operation of a dry scrubbing system is the amount of alkalinematerial feed into the system. The amount of sorbent required is a function of the following:

    1. The type of sorbent used

    2. The inlet and outlet acid gas levels (the outlet level is determined by removalrequirements)

    3. The effectiveness of the dry scrubbing system design

    The amount of sorbent added is usually reported on a molar basis as the stoichiometric ratio of sorbent to acid gases.

    Although the sorbents are either calcium- or sodium-based solids, the exact chemical reactionthat occurs depends on the type of sorbent used and the injection point in the process.Presently the most widely used dry scrubbing system is the calcium-based hydrated lime[Ca(OH) 2]. A slurry of hydrated lime and water is injected into the spray dryer and reactswith the acid gases in a simplified manner as follows:

    Ca(OH) 2 + SO 2 → CaSO 3(s) + H 2O (7-1)

    Ca(OH) 2 + 2HCl → CaCl 2(s) + 2H 2O (7-2)

    As you can see from the above reactions, one mole of calcium hydroxide [Ca(OH) 2] willneutralize one mole of SO 2, whereas one mole of calcium hydroxide will neutralize twomoles of HCl.

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    To compute the pounds of calcium hydroxide required to neutralize a given weight of SO 2 orHCl, the molecular weight of each component must be utilized. For example, the molecularweights of SO 2, HCl, and Ca(OH) 2 are as follows:

    Compound Molecular Weight(lb/lb-mole)

    SO 2 64

    HCl 36

    Ca(OH) 2 74

    Therefore, one pound of calcium hydroxide can neutralize 0.86 pounds of SO 2 (64 divided by74) or 0.97 pounds of HCl (36 times 2 divided by 74).

    In computing the stoichiometric ratio of a system, all the acid compounds in the waste streamneed to be accounted for. Also, the above equations are for the stoichiometric quantities ofsorbent. The actual use of sorbent will be above these quantities because of normalinefficiencies in operation; contact of sorbent and acid gases is never ideal and distribution ofacid gases in the flue gas is often not uniform (especially in incineration systems). The actualstoichiometric ratios can range from as low as 1.5 to 4.0 dependent on system design andrequired removal efficiencies.

    Similar type reactions occur with sodium-based compounds. For semi-dry systems usingcaustic soda (NaOH) the following simplified reactions can be written:

    SO 2 + 1/2 O 2 + 2NaOH → Na 2SO 4 + H 2O (7-3)

    HCl + NaOH → NaCl + H 2O (7-4)

    Also, sorbents react with different acids at different rates. For example, sorbents react withchlorides at a faster rate than with SO 2. Therefore, in waste streams that have both SO 2 andHCl, the HCl is removed at a higher rate than the SO 2.

    To test your knowledge of the preceding section, answer the questions in Part 1 of the Review Exercise.

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    Dry Injection

    Dry sorbent injection (DSI) is a process used to control acid gases by injecting a powderedsorbent into the flue gas stream. The sorbent can be injected into the furnace, boiler area orthe ductwork/reaction chamber prior to the air pollution control device. The injection pointdepends on the type of sorbent and required reaction time. For example, some sorbents needto be injected at elevated temperatures to undergo a decomposition reaction before they caneffectively remove the acid gas. Figure 7-1 shows a schematic of a typical dry injectionsystem.

    Figure 7-1. Components of a dry injection system

    The dry sorbent injection system is a very simple system that consists of a dry sorbent storagetank, a weight feeder to meter the required amount of sorbent, a blower and transfer line, andan injection device such as a venturi. The dry sorbent material is blown through a pneumaticline to the injection area where transfer through the pneumatic line provides fluidization ofthe sorbent material. Injection into the duct is generally done countercurrent to the gas flow

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    to create added turbulence and promote mixing. An expansion/reaction chamber may beincluded to increase the residence time of the acid gases to react with sorbent.

    The simple dry injector process described above is capable of achieving moderate control ofacid gases for example 50% SO 2 and 90% HCl removal on municipal and medical wastecombustors. The acid gas removal efficiencies can be increased by cooling and/orhumidifying the flue gas stream. Exhaust gases from industrial boilers or refuse combustorscan range from 600 oF to 400 oF. The flue gases can be cooled (and the humidity increased) byusing a heat exchanger or a dry quench chamber upstream of the injection point. Cooling theflue gas temperature increases the rate of reaction between the sorbent and acid gases. But,the temperature must be maintained high enough (300-350 oF) to ensure that all the waterdroplets used to quench are evaporated.

    Recycling a portion of the collected particles and unreacted sorbent is another method used toincrease overall effectiveness of dry scrubbing systems. As stated previously, it is difficult tomix a dry solid and a gas stream; therefore, additional sorbent (above stoichiometric amount)must be injected. As a result, there is unreacted sorbent captured in the baghouse orelectrostatic precipitator. In some instances a portion of this waste stream is recycled back tothe injection point.

    In order to achieve high removal efficiencies using relatively inexpensive calcium sorbents,most dry injection systems have to operate at higher stoichiometric ratios than a spray dryerwould. For example, stoichiometric ratios of 2.0 to 4.0 are used on municipal wastecombustors to achieve moderate acid gas control. This increased sorbent usage limits theirapplication to smaller sources such as medical waste incinerators. Table 7-1 lists somefacilities that have installed dry injection acid gas control systems.

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    Table 7-1. Examples of dry injection systems on medical and municipalwaste incinerators

    Facility location Incinerator Control device Combustor

    Facility name City State manufacturer manufacturer size, lb/hr

    Baltimore Baltimore MD Consumat Procedaire 6000

    Trumbull Hospital Warren OH Joy Joy 765

    Erlanger N Chattanooga TN Basic BACT 1176

    Evanston Hospital Evanston IL Basic United McGill 1176

    Florida Hospital Orlando FL Basic Mikropul FF 1176

    Mediwaste West Babylon NY U.S. WasteSystems

    Interel 2000

    Northwest Hospital Seattle WA Consumat Consumat 1200

    HealthcareIncinerators Fargo ND Consumat Consumat 1200

    Incindere Spring Hill LA Consumat Consumat 1500

    BiomedicalServices

    Mathews NC Consumat Consumat 1500

    WMI Terrel TX Disc International ERA-Tech 1300

    Midway Stroud OK Basic United McGill 6588

    Sparrow Hospital Lansing MI Econotherm Airopulse 1200

    Thermtec Elyria OH Therm Tec Donalson 1000-1200

    Thermtec Cincinnati OH Therm Tec Donalson 1000-1200

    WMI Northwood OH Joy 2000 TES ERA-Tech 1525

    WMI W. Carrolton OH Joy 2000 TES ERA-Tech 1525

    WMI Germantown WI Joy 2000 TES ResearchCottrell

    1525

    WMI Apopka FL Joy 2500 TES United McGill 1910

    MorristownMemorial Hospital

    Morristown NJ ThermAll, Inc. ThermAll, Inc. 800

    Swedish HospitalMed. Ctr.

    Seattle WA Therm-tec Mikropul FF 800

    Hamot Erie PA BICO >1000

    Borgess Kalamazoo MI Cleaver Brooks Cleaver BrooksDI/ Mikropul FF

    650

    Note: All systems are a dry injector followed by a fabric filter.

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    Spray Dryer Systems

    In the spray drying process, an alkaline slurry [usually Ca(OH) 2] is injected into a spray dryerchamber through either a rotary atomizer or two-fluid nozzle injectors.

    The atomized slurry droplets contact the hot flue gas in the spray dryer chamber (See Figure7-2). The water in the alkaline (lime) slurry evaporates to cool the flue gas, and the limereacts with the acid gases in the flue gas to form calcium- or sodium-based salts. Thereaction or absorption chamber is designed to provide sufficient contact and residence time to

    produce a dry product leaving the chamber. The particulate exiting the chamber contains flyash, calcium salts and unreacted lime that must be sent to a particulate control device, usuallya fabric filter or electrostatic precipitator (ESP).

    Figure 7-2. Spray dryer absorber

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    Collected solid reaction products from the system are sometimes recycled to the feed of thespray dryer to reduce alkaline sorbent use. Figure 7-3 provides a diagram of a typical spraydrying system. The major components of a typical spray drying system are:

    • Alkaline (lime) storage and slaking system

    • Alkaline mixing and feed tanks

    • Atomizer (rotary or nozzle)

    • Spray dryer chamber

    • Particulate control device (e.g. baghouse)

    • Recycle system (optional)

    Figure 7-3. Components of a spray dryer absorber system

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    Operating and Design Parameters

    Key design and operating parameters that affect spray dryer design and/or performanceare:

    • Flue gas flow rate and composition

    • Temperature of flue gas exiting the boiler and spray dryer

    • Alkaline stoichiometric ratio

    • Alkaline (lime or sodium) properties

    • Required removal efficiency

    The most important parameter in sizing the spray dryer is to ensure an adequate gasresidence time at maximum gas flow rate to prevent wet solids at the spray dryer outlet.Gas residence times are generally in the range of 10 to 15 seconds for most commercial

    systems. Due to the large volume of gases that must be treated from utility boilers, theseinstallations often have multiple dryers per boiler.

    The spray dryer outlet temperature is controlled by the amount of water injected eitherwith the alkaline slurry or as makeup water. The key to achieving good SO 2 removal is tomaintain the temperature of the flue gas exiting the spray dryer as close above its dew

    point (adiabatic saturation) as possible without actually saturating the flue gas.Generally, 20 - 30 °F above the adiabatic saturation point is a good target range. This willenhance the reaction yet still prevent condensation. The amount of water that canevaporate in a spray dryer is dependent on the incoming flue gas temperature and to alesser extent on the moisture content.

    The alkaline feed rate is a function of the incoming acid gas levels and the requiredremoval efficiency. The stoichiometric ratio is defined as the molar ratio of alkaline (i.e.calcium) in the spray dryer feed to the amount of acid gases (SO 2 and HCl) present. Forexample, at a ratio of 1.0 the moles of calcium are equal to the moles of incoming HCland SO 2. However, due to inefficiencies in the mixing process, more than the theoreticalamount of alkaline material is required to assure compliance with applicable standards.Thus, stoichiometric feed rates of 1.5 to 2.5 have been used to achieve SO 2 removal levelin the 75 to 85% range and HCl removal efficiencies of 95% on municipal wastecombustors. For utility and industrial boilers, sulfur removal guarantees by spray dryervendors have ranged from 60 to 90%. Table 7-2 lists information on operating spraydryer systems at utility boilers.

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    Table 7-2. Summary of spray-dryer applications

    Size Unit Type 1

    Atomizersper Dryer/Number of

    Dryers RT2

    (s) T3

    ( F) T

    4

    ( F)

    Spray-Dryer

    Diameter(ft)

    Utilities (size in Megawatts)

    440(each)

    Antelope Valley 1, 2

    Basin Electric

    R 1/5 12 310 20 46

    110 Riverside 6, 7

    Northern States Power

    R 1/1 10 350 Var.5

    46

    60 Stanton 10

    United Power

    R 3/1 10 323 20 38

    450 Craig 3

    Colorado-Ute

    N 12/4 NR6 NR 25 NR

    280 Rawhide 1

    Platte River Power

    R 1/3 11 276 NR 46

    320 Holcomb 1

    Sunflower Coop.

    R 1/3 10.6 249 50 51

    44 Shiras 3

    City of Marquette

    R 1/1 10 265 25 36

    270 North Valmy

    Sierra Pacific Power

    Idaho Power

    R 3/3 NR 260-300

    30 NR

    570 Laramie River 3

    Basin Electric

    N 12/4 8 286 23 55

    370 Springerville 1, 2

    Tucson Electric

    R 1/3 12 256 20 46

    575 GRDA R 3/4 12 310 20 52

    Industries (size in acfm)

    75,000 Argonne National Lab

    Argonne, IL

    R 1/1 12 330-340

    ≈20 25

    90,500 Container Corp.Philadelphia, PA

    R 1/1 NR 350 NR NR

    46,500

    (3 units)

    Fairchild Air ForceBase

    Spokane, WA

    R 1/1 NR 375 ≈25 20

    Continued on next page

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    Table 7-2. (continued)Summary of spray-dryer applications.

    Size Unit Type 1

    Atomizersper Dryer/

    Number ofDryers RT2

    (s) T3

    ( F) T

    4

    ( F)

    Spray-Dryer

    Diameter (ft)

    Industries (size in acfm)

    167,000 General Motors

    Buick Division

    Flint, MI

    R 1/1 NR 300 NR 32

    48,600 Griffis Air Force Base

    Rome, NY

    R 1/1 NR 400 NR 22

    44,400 Malstrom Air ForceBase

    Great Falls, MI

    R 1/1 NR 325 ≈35 20

    40,000 Strathmore Paper

    Woronco, MA

    N 4/1 NR NR NR NR

    62,000

    96,000

    University ofMinnesota

    N

    N

    1/1

    1/1 12 375 20 24

    97,000 Rockwell International

    Columbus, OH

    R 1/1 12 450 30 30

    205,000 M. M. Carbon

    Long Beach, CA

    R 1/1 10 405 90-120

    36

    81,710 Ohio State UniversityColumbus, OH

    N NR/1 NR 400 NR 24

    1. R = rotary; N = nozzle.2. Residence time.3. Flue-gas temperature at entrance.4. Approach to saturation at exit.5. Varies.6. Not reported.Source: Huang et al. 1988.

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    Spray Drying Equipment

    In a spray drying system, there are a number of system components. Three of the majorcomponents are the atomizer, spray dryer chamber and particulate control system. Anoverview of these systems is provided in the following sections and was adapted fromSpray-Dryer Flue-Gas-Cleaning System Handbook (Huang et al. 1988).

    Atomizers

    Currently, two types of atomizers are used in spray dryers for acid gas removal:rotary disks or wheels and dual-fluid nozzles. In either case, the purpose of theatomizer is to break the sorbent slurry into a cloud of fine droplets to promoteintimate sorbent contact with the acid bases.

    In the rotary atomizer , the slurry is fed into the top of the rotating wheel or disk.Centrifugal force causes the slurry to form a thin film on the internal surface of thecavity. As the slurry emerges from the cavity through abrasion-resistant inserts in the

    side of the wheel, the liquid is atomized into discrete droplets that are propelledradially outward. These droplets, generally 25-150 µm in diameter, dry rapidly in thehot flue gas within the spray dryer. Figure 7-4 shows an example of a typicalatomizer wheel used in spray dryers (Huang 1988).

    Figure 7-4. Example of rotary atomizer used in spray-dryer FGDsystems

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    For FGD spray dryer applications, atomizer wheels range from 8 to 16 inches indiameter and have rotational speeds from 7,000 to 20,000 revolutions per minute(rpm). Due to the highly abrasive nature of the slurry (which can consist of eitherslaked lime [Ca(OH) 2] or slaked lime plus recycled fly ash/reacted product), thewheels are constructed of corrosion- and abrasion-resistant materials, including

    ceramic inserts in the vanes or nozzles.

    In dual-fluid pneumatic nozzle atomization , the slurry feed is injected into the body of a nozzle and is entrained into a high-velocity, high-pressure air stream asshown in Figures 7-5 and 7-6 (Maurin 1983). The high-velocity air impacts on theslurry-feed stream, resulting in the production of fine droplets. The air stream andslurry comprise the two fluids. The size of liquid droplets produced decreases as thecompressed air pressure and relative velocity of the liquid to air increases.

    Figure 7-5. Two-fluid nozzle atomizer (nozzle body)

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    Figure 7-6. Two-fluid nozzle atomizer (high pressure air stream)

    The mean droplet size for both atomizing systems has been shown to be the same,indicating that the systems perform similarly. Likewise, the capacity of a nozzlesystem for atomization of slurries is the same as that for a rotary atomizer.

    Nevertheless, rotary atomizers and pneumatic nozzles have somewhat differentadvantages and disadvantages (Huang 1988):

    1. Rotary atomizers, with their higher capacity per unit, will have a simpler pipingsystem. In a rotary-atomizer system, usually only one feed pipe per atomizer is

    used; whereas in a nozzle-type atomizer, there will be an individual feed pipe(and valve) to each nozzle. In very large installations, this results in a complex

    piping system.

    2. Pneumatic nozzle atomizers are much easier to maintain than rotary atomizerswhile the system is on-line because the individual feed lines have isolation andcontrol valves. With multiple nozzles, it is possible to isolate an individualnozzle, remove it for cleaning or replacement, and then return the cleaned or newnozzle to service without reducing the gas flow to the system or bypassing thegas flow to another spray dryer.

    3. The net-energy requirements of a rotary atomizer and a set of pneumatic nozzlesare approximately the same, but the method by which this energy is applied isdifferent. For a rotary atomizer, the atomization energy is supplied via a motorcoupled to the atomizing wheel with a gear and/or belt drive. For a pneumaticatomizer, the energy of atomization is produced primarily by the pressure of theatomizing air. Hence, the energy is supplied through an air compressor that mayalso supply air for instrumentation or other purposes.

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    4. A spare rotary atomizer is often required as a backup in case of failure. In a pneumatic nozzle system, the required spares consist of nozzles and an extra aircompressor. For a smaller single rotary-atomizer unit, the relative cost of a spareatomizer would be substantial.

    Spray-Dryer ChamberThe atomization method chosen will affect the design of the spray-dryer chamber,including the physical dimensions. For a rotary-atomizer type of spray dryer ,which projects the droplets radially outward and perpendicular to the gas flow, thelength-to-diameter ratio of the dryer (L/D) is typically 0.8:1. Figure 7-7(a) illustratestwo typical configurations of rotary atomizer spray dryers. The droplets deceleraterapidly due to the drag forces of the downward-moving flue gas and eventually attainthe speed and direction of the flue gas. To avoid wall deposition, the designed radialdistance between the atomizer and the dryer wall must be sufficient to allow foradequate drying of the largest droplets. This is accomplished by proper choice of theL/D, droplet size, and residence time.

    For a two-fluid pneumatic nozzle spray dryer [shown in Figure 7-7(b)], whichatomizes the droplets in the direction of the gas flow (downward), the L/D istypically 2:1. In this case, sidewall deposition is a minor problem.

    Typically, industrial boiler spray dryers have diameters of 25-30 ft, whereas utilityspray dryers have diameters of 40-50 ft. Currently, the maximum diameter of aninstalled spray dryer is about 60 ft. In general, if the gas-flow rate is large enoughthat a single unit greater than 40-50 ft in diameter would be specified, then theinstallation of multiple spray dryers should be considered. In utility systems wherethe gas flow can range from 1-2 million acfm, multiple spray dryers are common.Multiple spray dryers are installed for easy maintenance and high reliability.

    Flue gas may enter a spray dryer in one of three patterns relative to the slurrydirection: cocurrent, countercurrent, or mixed. In cocurrent spray dryers, all of thegas enters through a roof gas disperser in the top of the vessel, where its rotation iscontrolled by angled vanes that direct the gas around the atomizer [shown in Figure7-7(a)]. This type of gas distribution precisely controls the exit gas temperature sincethe gas and slurry travel in the same direction. This is the most common flow patternused in acid gas control systems.

    In countercurrent spray dryers, the gas enters from the bottom of the vessel and isdirected at the atomized liquid above. Although uncommon in utility or industrialflue-gas control systems, these spray dryers have the advantage of a much higherdrying capacity than the cocurrent system.

    Another type of spray dryer, the compound-gas disperser or mixed, is offered by onemanufacturer as an option in specific applications. This type of spray dryer issometimes used on very large units as an alternative to multiple rotary atomizers toobtain efficient contact between the hot gas and the liquid droplets.

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    Particulate-Matter Collection

    A spray-dryer system is not complete without a means of particulate-matter collection. Notonly is a well-designed particulate-matter control system needed to meet emissionsrequirements, but it also aids in acid-gas removal. Acid gases are removed when the flue gascomes in contact with lime-containing particles in the fabric filter or ESP. Fabric filters have

    been used on the majority of acid gas control systems, due to their ability to obtain slightlyhigher residual acid gas removal than ESPs.

    Regardless of the type of particulate control device, an important design feature is tominimize potential heat loss in the fly ash collection system. The fly ash contains unreactedalkaline sorbent along with calcium (or sodium) sulfates, and in the case of wasteincinerators, it also contains calcium chlorides. These materials are very hygroscopic and canresult in corrosion problems or ash plugging of equipment if condensation occurs. Addinginsulation, hopper heaters and reducing air in-leakage are essential to prevent operational

    problems with the ash handling system.

    Maintenance Problems

    Except for rotary atomizers in spray dryers, dry injectors and spray dryer absorbers arerelatively simple devices with few moving parts. ( Note : Maintenance associated with anatomizer is specific to the type and manufacturer of the atomizer and is not covered in thislesson). The primary maintenance problem associated with any dry scrubbing system is

    potential plugging in the solid or slurry transport systems. Manufacturers of the varioussystems provide suggested maintenance and inspection schedules for each component. Theseschedules should be followed and information recorded to aid in documenting the systemoperation.

    Dry scrubbing systems involve transporting a solid or slurry (which can be 10 to 40% solids)in small pipes; therefore, plugging problems could occur in a number of locations. The mostcommon locations of plugging problems are in "dead" areas of the solid or slurry piping,valves and the atomizer. Dead areas of piping are associated with tees going to spare pumpsor a cleanout port. In these areas, flow only occurs occasionally and provides an area forsolids to buildup and block transport lines. Eliminating the tees is not practical sinceredundancy is needed (and often mandated) in order to assure continual operation of thescrubbing system. Also, certain tees are installed specifically to allow quick access to pipinginternals in order that a specific length of pipe can be flushed with water to dislodge buildup.Flexible rubber hosing and quick-type connectors have been used to try and minimize line

    plugging. Flexible piping is not as susceptible to plugging as solid pipe, and with the use ofquick connectors, the flexible piping can be installed or removed quickly to flush out areas orto connect spare components.

    Plugging problems associated with valves and atomizers in slurry systems are minimized byusing screens in transport lines to remove solids. However, these screens must be

    periodically checked and cleaned or else they will cause plugging. Atomizing systems areoften designed so that they can be flushed with water during operation (this will temporaryreduce potential acid gas removal efficiency). Atomizers should also be designed so that theycan be replaced in a short timeframe.

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    Another area of maintenance with semi-dry scrubbing systems is the lime slaking system.Lime slaking is the process of mixing controlled amounts of water and lime in a mixingvessel (slaker). The lime and water react (an exothermic process) to form the lime slurrywhich is then screened of grit, stored in agitated mixing tanks and then metered to theatomizer.

    With the slaking system, plugging and the quality of slurry produced are the two biggestmaintenance concerns. Plugging of dead spots and pumps can occur as already discussed. Inaddition, the grit screening process is of concern because if the screen is damaged, then largequantities of grit can get into the entire lime-slurry transport system causing extensive

    plugging and/or abrasion wear problems. Slurry quality is dependent on the quality of limeand slaking water utilized in addition to the mechanical action of the slaker. Both the limeand water should be of high quality (limited contaminants or other chemicals present) to

    prevent adverse reactions that can result in scaling, plugging or reduced acid gas removalefficiencies. The mechanical action of the slaker will determine how efficiently the slakingreaction occurs. The slaker should be frequently inspected to ensure that it is operating asdesigned.

    Plugging of lime slurry transport components can also occur due to a lack of slurry movement(i.e. during standby periods) when solids could settle out or the calcium could have time toreact and form scale. During extended downtimes, lines and storage tanks should be drainedand flushed where practical. Also, manufacturers recommend periodic cleaning in acid ofscreens and other components that are prone to plugging problems.

    Summary

    Dry scrubbing systems are used to control acid gas emissions primarily from combustionsources such as utility and industrial boilers and municipal and medical waste incinerators.Dry scrubbing systems only remove acid gases and therefore must be followed by a

    particulate control device (ESP or fabric filter) prior to exhausting the gases to theatmosphere.

    Dry scrubbing systems can be categorized as dry sorbent injectors (DSI) or as semi-dryscrubbers (also referred to as spray dryer absorbers or spray dryers). Dry sorbent injectioninvolves the addition of a dry alkaline material (usually hydrated lime or soda ash) into thegas stream to react with any acid gases that are present. The sorbent can be injected directlyinto the flue gas duct ahead of the particulate control device or into an open reaction chamber.The acid gases are adsorbed onto and react with alkaline sorbents to form solid salts whichare removed in the particulate control device.

    In spray dryer absorbers (SDAs) the flue gases are introduced into an absorbing tower (dryer)where the gases are contacted with a finely atomized alkaline slurry: usually a calcium-basedsorbent such as Ca(OH) 2 or CaO. Acid gases are absorbed by the slurry droplets and react toform solid salts. The heat of the flue gas is used to evaporate all the water droplets, leaving anon-saturated (i.e. dry) flue gas exiting the absorber tower. The effect of cooling andhumidifying the hot gas stream increases collection efficiency over simple dry injection.

    The major components of a spray dryer absorber are the atomizer, spray dryer chamber andthe particulate control device. Two types of atomizers are currently utilized for acid gasremoval: rotary disks (wheel type) and dual-fluid nozzles. In either case, the purpose of the

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    atomizer is to break the sorbent slurry into a cloud of fine droplets. The spray chamber isdesigned based on the type of atomizer utilized. Spray chambers used with rotary atomizersare shorter but fatter than those used with two-fluid nozzle atomizers. Both ESPs and

    baghouses have been used with spray dryer absorbers. An important design feature of the particulate control device is to minimize potential heat loss in the fly ash collection system to prevent potential plugging problems.

    An important parameter in the operation of a dry scrubbing system is the amount of alkalinematerial feed into the system. The amount of sorbent required is a function of the type ofsorbent used, inlet and outlet (required removal) acid gas levels and the effectiveness of thedry scrubbing system design. The amount of sorbent added is generally reported as thestoichiometric ratio on a molar basis of sorbent to acid gases. A stoichiometric ratio of 1:1would be used under ideal conditions; in practice more than the theoretical amount must beutilized to assure compliance with required acid gas removal levels.

    Except for rotary atomizers in spray dryers, dry scrubbing systems are relatively simpledevices with few moving parts. The primary maintenance problem is potential plugging inthe solid or slurry transport lines. Plugging can occur whenever there are bends orrestrictions in piping.

    To test your knowledge of the preceding section, answer the questions in Part 2 of the Review Exercise.

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    Review ExercisePart 1

    1. Dry scrubbing systems are used to remove ____________________ ____________________ from flue gas streams.

    2. True or False? In dry scrubbing systems no water or slurry is ever used.

    3. In dry scrubbing, the following mechanisms are applicable:

    a. Absorption b. Adsorptionc. Impactiond. a and b, only

    4. In general, higher acid gas removal efficiencies are achievable as the operating temperatureof the dry scrubbing system:

    a. Increases b. Decreasesc. Does not changed. All of the above

    5. The ratio of the sorbent materials injected into the spray dryer relative to the acid gases present is referred to as the ____________________ ____________________.

    6. The alkaline sorbent used in spray drying systems is:

    a. Calcium based b. A form of lime or soda ashc. Sodium basedd. a and b, onlye. a, b, and c

    7. In a scrubbing system, HCl reacts ____________________ with the sorbent than SO 2 does.

    a. Faster b. Slowerc. At the same rated. None of the above

    Part 2

    8. True or False? Dry sorbent injection is a very simple process that involves injecting a solidinto the flue gas.

    9. Spray dryer gas residence times are generally in the range of:

    a. 1 to 2 seconds b. 10 to 15 secondsc. 1 to 2 minutesd. a or b

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    10. True or False? The spray dryer outlet temperature should be maintained as close above itsadiabatic saturation temperature as possible without risking condensation to obtain best acidgas removal rates.

    11. The amount of water that can evaporate in a spray dryer is dependent on the:

    a. Acid gas levels b. Sorbent typec. Incoming temperatured. All the above

    12. For a given system design, what is the alkaline sorbent feed rate a function of? ________________________________________ and ________________________________________

    13. True or False? Spray dryers can operate at stoichiometric ratios of less than 1.0 and achievevery high (90+) removal efficiencies.

    14. The two types of atomizer systems used on spray dryers are ____________________ ____________________ and ____________________ ____________________ ____________________.

    15. True or False? The droplet size produced and power consumption of a rotary atomizer anddual-fluid nozzle system are essentially the same.

    16. The spray chamber length for a dual fluid nozzle system should be ____________________than for a system with a rotary atomizer.

    a. Shorter b. Longerc. About the samed. Any of the above

    17. The particulate matter control device on spray drying systems removes particles and can aidin additional ____________________ ____________________ removal.

    18. Fly ash collection systems on spray dryers must be properly insulated and heated to preventcondensation which could cause:

    a. Plugging b. Corrosionc. Reentrainmentd. a and b, only

    19. The primary maintenance problem with dry scrubbing systems is:

    a. Plugging in the sorbent transport system b. Scalingc. Corrosiond. Erosion

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    20. Lime slurry is dependent on the mechanical action of the slaker and the quality of the:

    a. Water b. Limec. Soda ashd. a and b, only

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    Review Exercise AnswersPart 1

    1. Acid gasesDry scrubbing systems are used to remove acid gases from flue gas streams.

    2. FalseIn dry scrubbing systems, water or slurry is sometimes used. Semi-dry systems (also calledspray dryer absorbers) use an alkaline slurry.

    3. d. a and b, onlyIn dry scrubbing, the following mechanisms are applicable: absorption and adsorption.

    4. b. DecreasesIn general, higher acid gas removal efficiencies are achievable as the operating temperatureof the dry scrubbing system decreases.

    5. Stoichiometric ratio

    The ratio of the sorbent materials injected into the spray dryer relative to the acid gases present is referred to as the stoichiometric ratio.

    6. e. a, b, and cThe alkaline sorbent used in spray drying systems can be any of the following:

    • Calcium based

    • A form of lime or soda ash

    • Sodium based

    7. a. FasterIn a scrubbing system, HCl reacts faster with the sorbent than SO 2 does.

    Part 2

    8. TrueDry sorbent injection is a very simple process that involves injecting a solid into the fluegas.

    9. b. 10 to 15 secondsSpray dryer gas residence times are generally in the range of 10 to 15 seconds.

    10. TrueThe spray dryer outlet temperature should be maintained as close above its adiabaticsaturation temperature as possible without risking condensation to obtain best acid gas

    removal rates.11. c. Incoming temperature

    The amount of water that can evaporate in a spray dryer is dependent on the incomingtemperature.

    12. Incoming acid gas levelsRemoval rateFor a given system design, the alkaline sorbent feed rate is a function of the incoming acidgas levels and removal rate .

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    13. FalseSpray dryers cannot operate at stoichiometric ratios of less than 1.0 and achieve very high(90+) removal efficiencies. They must operate at ratios above 1.0 to be effective.

    14. Rotary atomizers (and) dual-fluid nozzles (or two-fluid nozzles)The two types of atomizer systems used on spray dryers are rotary atomizers and dual-fluid

    nozzles.15. True

    The droplet size produced and power consumption of a rotary atomizer and dual-fluid nozzlesystem are essentially the same.

    16. b. LongerThe spray chamber length for a dual-fluid nozzle system should be longer than for a systemwith a rotary atomizer because of the type of spray pattern required by dual-fluid nozzle.

    17. Acid gasThe particulate matter control device on spray drying systems removes particles and can aidin additional acid gas removal.

    18. d. a and b, onlyFly ash collection systems on spray dryers must be properly insulated and heated to preventcondensation which could cause plugging and corrosion.

    19. a. Plugging in the sorbent transport systemThe primary maintenance problem with dry scrubbing systems is plugging in the sorbenttransport system.

    20. d. a and b, onlyLime slurry is dependent on the mechanical action of the slaker and the quality of the waterand lime.

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    Bibliography

    Apple, C., and M. E. Kelly. 1982, April. Mechanisms of Dry SO 2 Control Processes . EPA-600/7-82-026, NTIS PB 82-196924. U.S. Environmental Protection Agency.

    Huang, H., J. W. Allen, C. D. Livengood, W. T. Davis, and P. S. Farber. 1988. Spray-Dryer Flue-Gas-Cleaning System Handbook . U.S. Department of Energy. Publication No. ANL/ESD-7.Energy Systems Division, Argonne National Laboratory.

    Maurin, P. G., et al. 1982, April. Two-fluid nozzle vs. rotary atomization for dry-scrubbing systems.Chemical Engineering Progress . (pp. 51-59).

    U.S. Environmental Protection Agency. 1982, September. Flue Gas Desulfurization - Spray Dryer Process . Sulfur Oxides Control Technology Series. EPA 625/8-82-009.

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