Section 4

NO

x

 Controls

EPA/452/B-02-001
i

Section 4.2

NO

x

 Post- Combustion

EPA/452/B-02-001
ii

Introduction

Nitrogen oxides (NOx) are gaseous pollutants that are primarily formed through combustion

process. While flue gas is within the combustion unit, about 95% of the NOx

 exists in the form of

nitric oxide (NO). The balance is nitrogen dioxide (NO2

), which is unstable at high temperatures.

Once the flue gas is emitted into the atmosphere, most of the NOx is ultimately converted to NO2.

NOx in the atmosphere reacts in the presence of sunlight to form ozone (O3), one of the criteria

pollutants for which health-based National Ambient Air Quality Standards have been established.

Since ozone formation requires sunlight and high temperatures, ozone formation is greatest in

summer months.

NOx is generated in one of three forms; fuel NOx, thermal NOx, and prompt NOx. Fuel

NOx

 is produced by oxidation of nitrogen in the fuel source. Combustion of fuels with high nitrogen

content such as coal and residual oils produces greater amounts of NOx

 than those with low

nitrogen content such as distillate oil and natural gas. Thermal NOx is formed by the fixation of

molecular nitrogen and oxygen at temperatures greater than 3600°F (2000°C). Prompt NOx

forms from the oxidation of hydrocarbon radicals near the combustion flame and produces an

insignificant amount of NOx.

Selective Noncatalytic Reduction (SNCR) and Selective Catalytic Reduction (SCR) are

post-combustion control technologies based on the chemical reduction of nitrogen oxides (NOx)

into molecular nitrogen (N2) and water vapor (H2O). The primary difference between the two

technologies is that SCR utilizes a catalyst to increase the NOx removal efficiency, which allows the

process to occur at lower temperatures. The technologies can be used separately or in combination

with other NOx

 combustion control technologies such as low NOx burners (LNB) and natural gas

reburn (NGR). SNCR and SCR can be designed to provide NOx reductions year-round or only

during summer months, when ozone concerns are greatest.

This section presents design specifications and a costing methodology for SNCR and

SCR applications for large industrial boilers (greater than 250 MMBtu/hr). Its primary purpose is

to provide study-level cost estimates. These estimates can be used to compare the approximate

costs of SNCR, SCR, and alternative NOx control technologies. The costing methodologies are

based on cost estimations for SNCR and SCR applications to utility boilers, which were developed

by the Office of Research and Development (ORD), U.S. Environmental Protection Agency (EPA),

Research Triangle Park, NC.

As a prelude to the cost methodology, the section describes the process chemistry,

performance parameters, and system components of SNCR and SCR. In addition, impacts to the

boiler performance and facility operations resulting from the installation of SNCR and SCR are

presented. The section also estimates important underlying design parameters including the

normalized stoichiometric ratio, catalyst volume, and reagent consumption. Lastly, it presents
iii

assumptions and equations for estimating capital costs, annual operation and maintenance costs,

and annualized costs. SNCR and SCR are discussed in separate chapters, however, the chapter

on SCR builds on the concepts discussed in the SNCR chapter.

Information on key aspects of the design of SNCR and SCR systems is considered

proprietary by vendors, including methods for estimating certain design parameters and costs.

This information is not available to develop cost methodologies for SNCR and SCR. In order to

obtain estimates of the proprietary design parameters and costs, it is necessary to develop

mathematical correlations from available data using regression and curve fitting techniques. These

expressions are derived in EPA reports prepared by The Cadmus Group, Bechtel Power, Inc. and

SAICin SNCR and SCR References [1, 2 and 3] from documented comprehensive SNCR and

SCR performance data and costs based on quotations provided by suppliers and facilities.
2-1

EPA/452/B-02-001

Chapter 2

Selective Catalytic Reduction

Daniel C. Mussatti

Innovative Strategies and Economics Group

Air Quality Strategies and Standards Division

Office of Air Quality Planning and Standards

U.S. Environmental Protection Agency

Research Triangle Park, NC 27711

Dr. Ravi Srivastava

Office of Research and Development

U.S. Environmental Protection Agency

Research Triangle Park, NC 27711

Paula M. Hemmer and

Randy Strait

E.H. Pechan & Associates, Inc.

Durham, NC 27707

October 2000
2-2

Contents

2.1 Introduction ...........................................................................................................................................2-3

2.2 Process Description ...............................................................................................................................2-4

2.2.1 Sampling Systems ....................................................................................................................2-5

2.2.2 SCR Performance Parameters ..................................................................................................2-9

2.2.3 SCR System Configurations ..................................................................................................2-18

2.2.4 High Dust SCR System Equipment .......................................................................................2-23

2.2.5 SCR System Auxiliary Equipment .........................................................................................2-26

2.2.6 Other Considerations .............................................................................................................2-28

2.3 Design Parameters ...............................................................................................................................2-30

2.4 Cost Analysis .......................................................................................................................................2-40

2.4.1 Total Capital Investment ........................................................................................................2-41

2.4.2 Total Annual Costs.................................................................................................................2-44

2.5 Example Problem ................................................................................................................................2-50

2.5.1 Design Parameter Example ....................................................................................................2-51

2.5.2 Cost Estimation Example .......................................................................................................2-54

References ..................................................................................................................................................2-59
2-3

1 Cost are in 1997 dollars

2.1 Introduction

Selective Catalytic Reduction (SCR) has been applied to stationary source, fossil fuelfired, combustion units for emission control since the early 1970s and is currently being used in

Japan, Europe, and the United States. It has been applied to large ($250 million British thermal

units per hour (MMbtu/hr)) utility and industrial boilers, process heaters, and combined cycle gas

turbines. There has been limited application of SCR to other combustion devices and processes

such as simple cycle gas turbines, stationary reciprocating internal combustion engines, nitric acid

plants, and steel mill annealing furnaces [4]. In the United States, SCR has mainly been applied to

coal- and natural gas-fired electrical utility boilers ranging in size from 250 to 8,000 MMbtu/hr (25

to 800 megawatts (MW)). SCR can be applied as a stand-alone NOx

 control or with other

technologies such as combustion controls. SCR systems have experienced relatively few operational or maintenance problems [1].

SCR is typically implemented on stationary source combustion units requiring a higher

level of NOx reduction than achievable by SNCR or combustion controls. Theoretically, SCR

systems can be designed for NOx removal efficiencies up to 100 percent (%). Commercial coal-

, oil- and natural gas-fired SCR systems are often designed to meet control targets of over 90%.

However, maintaining this efficiency is not always practical from a cost standpoint. In practice,

SCR systems operate at efficiencies in the range of 70% to 90%. [4]

SCR capital costs vary by the type of unit controlled and the fuel source. Capital costs for

retrofit of coal-fired electric utility boilers range from 501 to 702 dollars per kilowatt ($/kW) for

wall- and tangential-fired boilers and 501

 to 801 $/kW for cyclone and wet bottom boilers. New

utility boiler capital costs are generally less than 401 $/kW. Retrofit of oil- and gas- fired utility

boilers range from 251 to 301 $/kW since they have lower NOx and particulate emissions. Typical

operation and maintenance costs are less than 0.11 cents per kilowatt-hour. Capital costs for

installing SCR on new industrial boilers range from 40001 to 60001 $/MMBtu/hr for small oil and

gas-fired units to over 10,0001

 $/MMBtu/hr for large coal-fired units. Gas turbines have capital

costs of 301 to 1001 $/kW depending on the size of the turbine. The cost of retrofitting a gas-fired

reciprocating internal combustion engine is approximately 1251 $/horsepower, which is less than

retrofit of combustion control technology for this type of unit. [4]

Although the cost data used in this report are based on retrofit applications of SCR systems on existing utility coal-fired boilers, this report’s costing procedure accounts for the major

differences between a new and retrofit installation. Therefore, the costing procedure is suitable for

retrofit or new boiler applications of SCR on all types of large ($250 MMBtu/hr) coal-fired industrial boilers.The costing methodology incorporates certain approximations, consequently, it should

be used to develop study-level (± 30%) cost estimates of SCR applications.
2-4

2.2 Process Description

Like SNCR, the SCR process is based on the chemical reduction of the NOx molecule.

The primary difference between SNCR and SCR is that SCR employs a metal-based catalyst

with activated sites to increase the rate of the reduction reaction. A nitrogen based reducing agent

(reagent), such as ammonia or urea, is injected into the post combustion flue gas. The reagent

reacts selectively with the flue gas NOx

 within a specific temperature range and in the presence of

the catalyst and oxygen to reduce the NOx into molecular nitrogen (N2

) and water vapor (H2O).

The use of a catalyst results in two primary advantages of the SCR process over SNCR.

The main advantage is the higher NOx

 reduction efficiency. In addition, SCR reactions occur

within a lower and broader temperature range. However, the decrease in reaction temperature

and increase in efficiency is accompanied by a significant increase in capital and operating costs.

The cost increase is mainly due to the large volumes of catalyst required for the reduction reaction.

Figure 2.1 shows a simplified process flow schematic for SCR. Reagent is injected into

the flue gas downstream of the combustion unit and economizer through an injection grid mounted

in the ductwork. The reagent is generally diluted with compressed air or steam to aid in injection.

The reagent mixes with the flue gas and both components enter a reactor chamber containing the

catalyst. As the hot flue gas and reagent diffuse through the catalyst and contact activated catalyst

sites, NOx in the flue gas chemically reduces to nitrogen and water. The heat of the flue gas

provides energy for the reaction. The nitrogen, water vapor, and any other flue gas constituents

then flow out of the SCR reactor. More detail on the SCR process and equipment is provided in

the following sections.

There are several different locations downstream of the combustion unit where SCR systems can be installed. Flue gas temperature and constituents vary with the location of the SCR

reactor chamber. SCR reactors located upstream of the particulate control device and the air

heater have higher temperatures and higher levels of particulate matter. An SCR reactor located

downstream of the air heater, particulate control devices and flue gas desulfurization system is

essentially dust- and sulfur-free but its temperature is generally below the acceptable range. In this

case, reheating of the flue gas may be required which significantly increases the SCR operational

costs. Section 2.2.3 discusses the various SCR system configurations.
2-5

Economizer

SCR

Air Heater

Anhydrous

Ammonia

Tank

Electric Vaporizer

Dilution Air

Blower

Ammonia

Injection

Grid

Static Gas Mixer

Static Gas

Mixer

Economizer Bypass

ID FAN

Ammonia/Air

Line

AIR

ESP

SCR Bypass

Ammonia/Air

Mixer

Ammonia Vapor Line

GAS

LIQUID

Figure 2.1: SCR Process Flow Diagram [6, 22]

2.2.1 Reduction Chemistry, Reagents and Catalyst

The reducing agent employed by the majority of SCR systems is gasphase ammonia (NH3

) since it penetrates the catalyst pores more readily than aqueous urea.

The ammonia, either in anhydrous or aqueous form, is vaporized before injection by a

vaporizer. Within the appropriate temperature range, the gas-phase ammonia then decomposes into free radicals including NH3

 and NH2. After a series of reactions, the ammonia

radicals come
2-6

into contact with the NOx and reduce it to N2 and H2O. The global representation of these reactions is given below. Note that the NOx

 is represented as nitrogen oxide (NO) since it is the

predominant form of NOx within the boiler. The ammonia reaction equation is represented by:

2 2 1

2

NO + NH + O N H O 3 2 22 2 3 catalyst  →  + (2.1)

The equation indicates that 1 mole of NH3

 is required to remove one mole of NOx. The

catalyst lowers the required activation energy for the reduction reaction and increases the

reaction rate. In the catalytic reaction, activated sites on the catalyst rapidly adsorb ammonia and gas phase nitric oxide to form an activated complex. The catalytic reaction, represented by Equation 2.1, occurs resulting in nitrogen and water, which are then desorbed to

the flue gas. The site at which the reaction occurs is then reactivated via oxidation.

The high temperature of the flue gas converts the ammonia to free radicals and

provides the activation energy for the reaction. The reaction also requires excess oxygen,

typically 2 to 4 percent, to achieve completion. NOx

 reduction with ammonia is exothermic,

resulting in the release of heat. However, because the NOx concentration in the flue gas at

the inlet of the SCR is typically 0.02 to 0.01 percent by volume, the amount of heat released

is correspondingly small. Thermodynamic equilibrium is not a limiting factor in NOx reduction if the flue gas is within the required temperature range. [1]

Reagent

The SCR system can utilize either aqueous or anhydrous ammonia for the reduction

reaction. Anhydrous ammonia is nearly 100% pure ammonia. It is a gas at normal atmospheric temperature, therefore, it must be transported and stored under pressure. Anhydrous

ammonia often requires special permits for transportation and storage.

SCR applications using aqueous ammonia generally transport and store it at a concentration of 29.4% ammonia in water, though some recent applications utilize a 19% solution [1]. The use of aqueous ammonia reduces transport and storage problems. In addition,

certain locations may not require permits for concentrations below 28%. Aqueous ammonia

however, requires more storage capacity than anhydrous ammonia. While the 29.4% solution has substantial vapor pressure at normal air temperatures, to provide sufficient ammonia vapor to the SCR system generally requires a vaporizer.

The cost of the reagent impacts the annual costs of an SCR system. Depending on

the local permitting requirements and on owner preference, either anhydrous or aqueous

ammonia may be used in the SCR process. This presentation is valid for anhydrous or

aqueous ammonia, however, the design parameters and cost analysis assume aqueous ammonia as a reagent since it is more commonly employed. Table 2.1 gives the properties of

anhydrous ammonia and the properties of a 29.4% aqueous ammonia solution.
2-7

Table 2.1: Ammonia Reagent Properties

Property Anhydrous Ammonia [15,16] Aqueous Ammonia

Liquid or gas at normal air Liquid Liquid

 temperature

Concentration of reagent 99.5 % (by weight) 29.4 % (by weight of NH3)

normally supplied

Molecular weight of reagent 17.03 17.03 (as NH3)

Ratio of ammonia to 99.5% (by weight of NH3) 29.4 % (by weight of NH3)

Solution

Density of liquid at 60 °F 5.1 lb/gal 7.5 lb/gal

Vapor pressure @ 80 °F 153 psia 14.6 [15, p. 3]

Flammability limits in air 16-25 % NH3

 (by volume) 16-25 % NH3 (by volume)

Short-term exposure limit 35 ppm 35 ppm

Odor Pungent odor @ 5 ppm or Pungent odor @ 5 ppm or

more more

Acceptable materials for Steel tank, rated for at least Steel tank, rated for at least 25

storage 250 psig pressure (no copper psig pressure (no copper or

or copper-based alloys, etc.) copper-based alloys, etc.)

Catalyst

SCR catalysts are composed of active metals or ceramics with a highly porous structure. Within the pores of the catalyst are activated sites. These sites have an acid group on

the end of the compound structure where the reduction reaction occurs. As stated previously, after the reduction reaction occurs, the site reactivates via rehydration or oxidation.

Over time, however, the catalyst activity decreases, requiring replacement of the catalyst.

Catalyst designs and formulations are generally proprietary. Both the catalyst material and

configuration determine the properties of the catalyst.

Originally, SCR catalysts were precious metals such as platinum (Pt). In the late

1970s, Japanese researchers used base metals consisting of vanadium (V), titanium (Ti),

and tungsten (W), which significantly reduced catalyst cost. In the 1980’s, metal oxides

such as titanium oxide (TiO2), zirconium oxide (ZrO2), vandium pentoxide (V2O5), and
2-8

silicon oxide (SiO2) were employed to broaden the reaction temperature range. Zeolites, crystalline alumina silicates, were also introduced for high temperature (675°F to 1000°F) applications,

however, zeolites tended to be cost prohibitive.

Improvements to the catalyst formulations decrease unwanted side reactions such as

sulfur oxide conversions (SO2 to SO3) and increase the resistance to flue gas poisons. Newer

catalyst designs also increase catalyst activity, surface area per unit volume, and the temperature range for the reduction reaction. As a consequence, there is a corresponding decrease in the required catalyst volumes and increase in the catalyst operating life. For coalfired boiler applications, SCR catalyst vendors typically guarantee the catalyst for an operating life ranging between 10,000 hours to 30,000 hours [11]. Applications using oil and

natural gas have a longer operating life, over 32,000 hours [19]. In addition, operating experience indicates that actual catalyst deactivation rates are lower than the design specifications [1].

 Catalyst formulations include single component, multi-component, or active phase

with a support structure. Most catalyst formulations contain additional compounds or supports to give thermal and structural stability or to increase surface area [4]. Catalysts configurations are generally ceramic honeycomb and pleated metal plate (monolith) designs in

a fixed-bed reactor, which provide high surface area to volume ratio. Pellet catalyst in fluidized beds are also available. Pellets have greater surface area than honeycombs or pleated

plates but are more susceptible to plugging. This limits the use of pellets to clean burning

fuels such as natural gas.

Catalyst elements placed in a frame form a catalyst module. The modules stack

together in multiple layers to create a reactor bed of the total required catalyst volume. A

typical module is 3.3 ft to 6.6 ft in area (1 m x 2 m) and 3.3 ft (1 m) in height. A crane hoists

the large catalyst modules into the reactor from either the interior or exterior of the reactor,

depending on the reactor design.

Catalyst replacement is infrequent, generally less than one layer per year for fixed

bed designs. Most SCR manufacturers offer a disposal service. The catalyst is either reactivated for reuse or its components are recycled for other uses [4]. If the catalyst cannot be

recycled or reused, the facility operator must dispose of the spent catalyst in an approved

landfill. In the United States, most catalyst formulations are not considered hazardous waste

[4].

Catalysts greatly accelerate the NOx

 reduction reaction rate, but some catalysts have

more favorable properties for a given application. Performance requirements that drive the

choice of catalyst include reaction temperature range, flue gas flow rate, fuel source, catalyst activity and selectivity, and catalyst operating life. In addition, the design must consider

the cost of the catalyst, including disposal costs since catalyst costs can account for 20% or

more of the capital costs for a SCR system [1].
2-9

2.2.2 SCR Performance Parameters

The rate of the reduction reaction determines the amount of NOx removed from the

flue gas. The major design and operational factors that affect the NOx

 removal performance

of SCR are similar to those presented in Chapter 1 SNCR. The factors discussed previously for

SNCR include the following:

• Reaction temperature range;

• Residence time available in the optimum temperature range;

• Degree of mixing between the injected reagent and the combustion gases;

• Molar ratio of injected reagent to uncontrolled NOx

;

• Uncontrolled NOx concentration level; and

• Ammonia slip.

The majority of the discussion regarding SNCR design and operational factors is

valid for the SCR process excepting small variations due to the use of a catalyst and the

reaction chamber being separate from the combustion unit. Additional design and operational factors to consider, which are specific to the SCR process, include:

• Catalyst activity

• Catalyst selectivity

• Pressure drop across the catalyst

• Catalyst pitch

• Catalyst deactivation

• Catalyst management

The major differences between SNCR and SCR are discussed below.

Temperature

The NOx

 reduction reaction is effective only within a given temperature range. The

use of a catalyst in the SCR process lowers the temperature range required to maximize the

NOx reduction reaction. At temperatures below the specified range, the reaction kinetics

decrease and ammonia passes through the boiler (ammonia slip). At temperatures above the

specified range, nitrous oxide (N2O) forms and catalyst sintering and deactivation occurs.

In an SCR system, the optimum temperature depends on both the type of catalyst

utilized in the process and the flue gas composition. For the majority of commercial catalysts (metal oxides), the optimum temperatures for the SCR process range from 480 °F to

800 °F (250 °C to 427 °C) [11]. Figure 2.2 is a graph of the NOx

 removal efficiency as a

function of temperature for a typical metal oxide-type catalyst [11]. The figure shows the rate of
2-10

the NOx removal increases with temperature up to a maximum between 700°F to 750°F (370°C

to 400°C). As the temperature increases above 750°F, the reaction rate and resulting NOx removal efficiency begin to decrease.

Figure 2.2: NOx Removal versus Temperature [11]

As flue gas temperature approaches the optimum, the reaction rate increases and less

catalyst volume achieves the same NOx removal efficiency. Figure 2.3 shows the change in the

required catalyst volume versus temperature [10]. There is approximately a 40% decrease in the

required catalyst volume as flue gas temperature increases from 600 °F (320 °C) to the optimum

range, 700 to 750 °F (370 °C to 400 °C). This decrease in catalyst volume also results in a

significant decrease in capital cost for the SCR system.

The relationships between flue gas temperature, catalyst volume, and NOx removal

are complicated functions of the catalyst formulation and configuration. The physical and

chemical properties of each catalyst are optimized for a different operating conditions. For

a given catalyst formulation, the required catalyst volume and/or temperature range can

even change from one manufacturer of the catalyst to another. The selection of catalyst,

therefore, is critical to the operation and performance of the SCR system.

50

55

60

65

70

75

80

85

90

95

500 550 600 650 700 750 800 850 900

Flue Gas Temperature, °F

NOx Removal Efficiency, %
2-11

Figure 2.3: Change in Catalyst Volume vs. Temperature [10]

Since the optimum temperature window of the SCR process is lower than that of SNCR,

the reagent injection into a reactor chamber occurs downstream of the combustion unit, rather than

inside the combustion unit. As discussed previously, there are several options for the location of the

SCR reactor. The flue gas temperature at each of these locations is different. Most designs install

the reactor downstream of the economizer and prior to the air preheater, where the flue gas is at

the appropriate temperature for metal oxide-based catalysts. Reheating of the flue gas may be

required for reactors located downstream of the air pre-heater. Reheating significantly increases

the SCR operational costs.

Boiler operation at reduced loads decreases the gas flow rate. At reduced gas flow

rates, the economizer outlet gas temperature decreases because boiler heat transfer surfaces

absorb more heat from the flue gas. Typical SCR systems tolerate temperature fluctuations

of ± 200°F (± 93°C) [1]. At low boiler loads, however, the temperature can decrease below

the optimum range. For example, a coal-fired utility boiler has an economizer exit flue gas

temperature of 690°F (366°C) at 100% load, but only 570°F (300°C) at 50% load [1]. For

low-load operations, an economizer bypass can be used to raise the flue gas temperature.

An economizer bypass diverts part of the hot flue gas from within the economizer through

a bypass duct and mixes it with the relatively cooler flue gas exiting the economizer. An

economizer feedwater bypass also raises the flue gas temperature. The use of an economizer bypass results in less energy transfer to the feedwater for steam generation, consequently, there is a small reduction in boiler efficiency. Lower boiler efficiencies require

more fuel to be burned to meet the required boiler steam output.

Residence Time and Space Velocity

0

5

10

15

20

25

30

35

40

45

600 625 650 675 700 725 750

Flue Gas Temperature, °F

% Change in Catalyst Volume 
2-12

Residence time is the time the reactants are within the reactor. Higher residence times

generally result in higher NOx removal rates. Temperature also affects the required residence time.

The required residence time decreases as the temperature approaches the optimum temperature

for the reduction reaction. Residence time is often expressed as space velocity, the inverse of

residence time. The space velocity of a reactor is experimentally determined from the measured

flue gas flow rate divided by the superficial volume of the catalytic reactor. The NOx removal

efficiency increases with decreasing space velocity, i.e. increasing catalyst volume, for a given flue

gas flow rate.

The optimal residence time for an SCR system is a function of the number of active

catalyst sites available for the reduction reaction and the gas flow rates within those active

sites (interstitial flow rate). The “area velocity” is a parameter used by SCR vendors which

relates the number of sites and the interstitial flow rate to residence time. The area velocity

is defined as the space velocity divided by the catalyst pore surface area (specific surface

area). For coal-fired boilers, typical specific surface areas range from 90 to 3,800 square

feet per cubic feet (ft2

/ft3) (300 to 1,200 square meters per cubic meters (m2/m3)) [1]. Increasing the catalyst specific surface area increases the NOx

 removal for a given flue gas

flow rate. This can be accomplished by either increasing the catalyst volume, which increases the reactor size, or increasing the pore space of the catalyst, which generally increases the catalyst cost.

Degree of Mixing

The reagent must be dispersed and mixed throughout the flue gas to ensure sufficient contact between the reactants. Mixing is performed by an injection system which

injects pressurized gas-phase ammonia into the flue gas. The injection system controls the

spray angle, velocity and direction of the injected reagent. Some systems inject the ammonia with a carrier fluid such as steam or air to increase penetration into the flue gas. Injection

systems are application specific. Numeric modeling of the flue gas and reagent flow optimizes the design of the injection system. (See 2.2.6).

Mixing of the flue gas and ammonia occurs before entering the SCR reactor. If

mixing is not adequate, the NOx reduction is inefficient. SCR designs must incorporate

adequate duct length between the ammonia injection and the reactor inlet to allow for mixing. Mixing patterns can be improved by:

• Installation of static mixers upstream of the reactor;

• Increase the energy imparted to the injected fluids;

• Increase the number of injectors and/or injection zones; and

• Modify the nozzle design to improve the reagent distribution, spray angle,

and direction.
2-13

Actual Stoichiometric Ratio

The actual stoichiometric ratio (the moles of reagent injected per mole of uncontrolled

NOx

) defines the quantity of reagent needed to achieve the targeted NOx reduction. According to

Equation 2.1, the theoretical stoichiometric ratio for the reduction reaction with ammonia is equal

to 1. This assumption of a 1 to 1 linear relationship between the quantity of reagent and the NOx

removed is good up to about 85% NOx reduction [11]. After 85%, the removal efficiency begins

to level off and more than the theoretical amount of ammonia is required for additional NOx removal. This is due to the portion of NOx

 that is in the form of NO2 rather than NO and reaction

rate limitations. SCR systems typically employ a stoichiometric ratio 1.05 moles of ammonia per

mole of NOx

 [1]. Because capital and operating costs depend on the quantity of reagent consumed, the actual stoichiometric ratio is an important design parameter that is determined by the

SCR designer.

Uncontrolled NOx Concentration

The concentration of the reactants also affects the reaction rate of the NOx reduction

process. In general, higher uncontrolled NOx inlet concentrations result in higher NOx removal efficiencies [1] due to reaction kinetics. However, NOx

 levels higher than approximately 150 parts per million (ppm), generally do not result in increased performance. Low

NOx

 inlet levels result in decreased NOx removal efficiencies because the reaction rates are

slower, particularly in the last layer of catalyst [1]. In general, though, SCR performs better

than SNCR on sources with low uncontrolled NOx levels such as natural gas-fired boilers.

For a given NOx removal efficiency, higher NOx levels at the SCR inlet require more

catalyst volume. For example, to achieve 90% NOx removal, requires10% more catalyst at

inlet NOx level of 1.7 lb/MMBtu versus inlet levels of 0.8 lb/MMBtu [9]. SCR is generally

more cost effective for sources which emit less NOx, since the required catalyst volume is

minimal.

Ammonia Slip

Ammonia slip refers to the excess reagent passing through the reactor. Ammonia in

the flue gas causes a number of problems which were discussed in Chapter 1 SNCR, including health effects, visibility of the stack effluent, salability of the fly ash, and the formation

of ammonium sulfates. Limits on acceptable ammonia slip, imposed by either regulatory

limits or by design requirements, place constraints on SCR performance.

Ammonia slip does not remain constant as the SCR system operates but increases as

the catalyst activity decreases. Properly designed SCR systems, which operate close to the

theoretical stoichiometry and supply adequate catalyst volume, maintain low ammonia slip levels,

approximately 2 to 5 ppm. Reliable instrumentation for monitoring ammonia slip is currently being
2-14

developed but not commercially available [11]. One method to quantify ammonia slip is to determine the ammonia concentration in collected fly ash [13].

Catalyst Activity

Catalyst activity is a measure of how much the catalyst accelerates the NOx

 reduction reaction rate. A higher catalyst activity results in a faster reaction rate and more NOx

removal. Catalyst activity is a function of many variables including catalyst composition

and structure, diffusion rates, mass transfer rates, gas temperature, and gas composition

[14]. As the catalyst activity decreases, the NOx reduction reaction rate also decreases. This

results in lower NOx removal and higher ammonia slip levels.

The following equation describes the deactivation of the catalyst activity, K, with

time, t [8]:

K = Ko e

(t

/τ) (2.2)

where Ko is the original catalyst activity and τ is the catalyst operating life time constant. Figure 2.4

shows a typical catalyst deactivation curve based on Equation 2.2. As the catalyst activity decreases, the NOx

 removal efficiency is usually kept constant by injecting more ammonia, thereby

increasing the ammonia slip. When the ammonia slip reaches the maximum design or permitted

level, new catalyst must be installed. ⋅B

0

5

10

15

20

25

0 10,000 20,000 30,000 40,000 50,000 60,000

Operating Hours

Catalyst Activity, K

Figure 2.4: Typical Catalyst Deactivation per Equation 2.2 with KO = 24.12; τ = 55,000
2-15

Catalyst Reaction Selectivity

SCR favors the NOx

 reduction reaction over competing reactions given the reactants are

at the appropriate temperature and oxygen is present. However, competing reactions still occur

and the catalyst accelerates these reactions as well. Each catalyst has different chemical reaction

selectivity properties. In general, catalysts promote the formation of two undesirable compounds,

sulfur trioxide (SO3) and nitrous oxide (N2O). SO3 is formed by the oxidation of SO2 to SO3.

Sulfur oxides (SOx) are regulated under the 1990 Clean Air Act. SO3 reacts with ammonia in the

flue gas to form ammonia sulfates. Ammonium sulfur salts deposit on the catalyst and on downstream equipment such as the air preheaters. N2

O is both an ozone depletor and a greenhouse gas

but is currently not regulated.

Pressure Loss

The flue gas pressure decreases as the flue gas flows across the catalyst. The decrease

in pressure is a function of the length of the catalyst and the catalyst configuration. Deposition of

fly ash and other particulates on the catalyst over time increases this pressure drop across the

catalyst. The flue gas pressure can be increased by installing new draft fans or by upgrading

existing fans. To minimize the pressure loss across the catalyst, the SCR reactor ductwork can

be expanded and flow rectifiers and turning vanes can be installed. Pressure loss is of greater

concern in turbine applications which rely on air flow, rather than heat transfer, to generate

power.

Catalyst Pitch

Catalyst pitch is a term used in association with honeycomb and metal plate catalyst

and affects the flue gas velocity in interstitial spaces [1]. As shown in Figure 2.5 pitch,

represented as p, is the width of the catalyst cell plus the cell wall thickness, a. For a given flow

rate, wider pitch will result in lower interstitial gas velocities. Appropriate catalyst pitch is important

to assure that ash will not deposit and bridge over catalyst cells and pores. Plugging of the catalyst

reduces the effective surface area by decreasing the number of active sites available for the NOx

reduction reaction.

Figure 2.5: Pitch for a Honeycomb Catalyst Configuration
2-16

Catalyst Deactivation

Catalysts lose their activity over time for various reasons. The primary mechanisms for

catalyst deactivation and surface area loss are discussed below.

Poisoning - Certain fuel constituents which are released during combustion act as catalyst

poisons. Catalyst poisons include calcium oxide and magnesium oxide, potassium, sodium, arsenic, chlorine, fluorine, and lead. These constituents deactivate the catalyst by

diffusing into active pore sites and occupying them irreversibly. Catalyst poisoning represents the main cause of catalyst deactivation.

Thermal Sintering - High flue gas temperatures within the SCR reactor cause sintering, a

permanent loss of catalyst activity due to a change in the pore structure of the catalyst.

Thermal sintering can occur at temperatures as low as 450°F (232°C). The amount of

thermal sintering depends on the composition and structure of the catalyst. Newer catalyst

materials are less susceptible to thermal sintering which increases their operating life.

Blinding/Plugging/Fouling - Ammonia-sulfur salts, fly ash, and other particulate matter in

the flue gas cause blinding, plugging or fouling of the catalyst. The particulate matter deposits on the surface and in the active pore sites of the catalyst. This results in a decrease

of the number of sites available for NOx reduction and an increase in flue gas pressure loss

across the catalyst.

Erosion - Impingement of particulate matter and high interstitial gas velocities erode the

catalyst material. Catalysts with hardened leading edges or increased structural strength

are less susceptible to erosion. Increasing catalyst strength through hardening, however,

reduces the number of active pore sites.

Aging - Catalyst aging is a change in the physical and chemical properties of the catalyst

pores that occurs over time.

There are a number of measures which can be taken to decrease the rate of deactivation and

deterioration of the catalyst. These measures are discussed in the following section.
2-17

Catalyst Formulation - Each catalyst formulation has different physical and chemical properties. Catalyst formulations with the following properties will have decreased deactivation.

• Increased activity per unit volume

• Greater thermal resistance

• Chemical and physical resistance to poisons

• Wider thermal operating range

• Greater structural strength and hardened leading edges

• Lower interstitial velocities (i.e., wider catalyst pitch)

To obtain the optimum catalyst formulation and SCR design for an application, the catalyst

supplier and SCR vendor should be informed of the fuel constituents, such as sulfur, chlorine, fluorine, alkali, and trace metals. These fuel and ash constituents can be determined

by chemical analyses. The associated analytical data can then be used to modify the catalyst composition, to determine catalyst volume, and to design the SCR reactor components.

Soot Blowers - Deposits on the surface of the catalyst can be dislodged by soot blowers

which are generally installed between each catalyst layer and operated on a periodic basis,

such as once a week.

Turning Vanes and Rectifier Grids - Particulate matter can be removed from the flue gas by

gas-flow turning vanes and flow rectifier grids near the front of the catalyst layer. Particles

impact the surface of the vanes or grid and fall out of the flue gas stream. In addition to

removing particles, turning vanes and flow rectifier grids decrease the linear velocity of the

flue gas and align its vector with the flow path of the catalyst.

Catalyst Management Plan

Catalyst deactivation is an inherent part of the SCR process. As the catalyst activity

decreases with time, the NOx

 reduction reaction rate decreases and ammonia slip increases.

When the ammonia slip level reaches the design limit, the catalyst must be replaced or new

catalyst must be added. The catalyst life is the time the catalyst activity for a given catalyst

volume (layer volume) maintains ammonia slip below the design limit. Currently, vendor-guaranteed life for a catalyst layer in coal-fired applications is approximately three years [10]. The actual

catalyst layer lifetimes being experienced in such applications are in the 5 to 7 year range, depending on the condition of untreated flue gas [1]. Gas- and oil-fired applications experience even

longer catalyst layer lifetimes.
2-18

A catalyst management plan, as shown in Figure 2.6, schedules periodic replacement of

catalyst to maintain ammonia slip limits. Most catalyst management plans call for the SCR reactor

design to provide two or more layers filled with catalyst and one or more empty or spare catalyst

layers. When the initial catalyst layers deactivate to the point ammonia slip reaches the maximum

design value, the facility adds catalyst to the empty layer. Catalyst addition is managed so that the

total catalyst activity of all the layers (the two or three older catalyst layers plus new catalyst layers)

is sufficient to meet the ammonia slip requirement for a relatively long period of time. As the

catalyst continues to deactivate, ammonia slip begins to rise. When ammonia slip again reaches the

maximum design value, one of the older catalyst layers is removed and replaced with new catalyst.

Figure 2.6 shows a catalyst management plan for an application in which the maximum design

value of ammonia slip is 2 ppm [7].

Catalyst cost is a significant portion of the annual cost of operating an SCR system. For

SCR designs which utilize a catalyst management plan, only a fraction of the total catalyst inventory, rather than the entire volume, is replaced at any one time. This distributes the catalyst replacement costs more evenly over the lifetime of the system.

Figure 2.6: Typical Catalyst Management Plan [5]

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Year

NH3 Slip, ppm

Replace 

Layer 1

Add 

Layer 4

Replace 

Layer 2

Replace 

Layer 4

Replace 

Layer 3
2-19

2.2.3 SCR System Configurations

Electric utility and large industrial boiler applications implement several different SCR system configurations, including high-dust, low-dust, and tail-end arrangements. SCR configurations

for gas turbine applications are dependant on the type of engine cycle, such as combined-cycle or

simple cycle. The various configurations for boilers and gas-fired turbines are discussed below. In

addition, there are two different SCR reactor designs; full SCR and in-duct SCR which are also

discussed.

High-Dust SCR

Figure 2.7 shows a high-dust SCR system for coal-fired boiler applications. The SCR

reactor location is downstream of the economizer and upstream of the air heater and particulate

control devices. The flue gas temperature in this location is usually within the optimum temperature

window for NOx

 reduction reactions using metal oxide catalysts. In this configuration, however,

the flue gas contains particulates when it enters the SCR reactor.

Coal-fired boilers generally use a vertical SCR reactor, where the flue gas flows downward through the catalyst. The reactor generally contains multiple layers of catalyst. The volume of

catalyst required varies with each installation, as discussed previously. Sootblowers are installed

to remove particulates from the catalyst surfaces. For design which utilize a honeycomb catalyst,

the catalyst pitch is typically about 7 to 9 mm (compared with 3 or 4 mm for gas-fired boilers) to

allow easy passage of ash particles without deposition and for ease of cleaning with sootblowers.

To obtain uniform gas flow and remove particulates, high-dust SCR designs usually include turning

vanes and a flow-rectifying grid in the ductwork prior to the reactor.

AIR HEATER

BOILER

SECONDARY

AIR

FLUE GAS

SCR

REACTOR

AMMONIA

INJECTION

GRID

ECONOMIZER

BYPASS

Figure 2.7: High Dust SCR Arrangement, [4]
2-20

A hopper at the bottom of the SCR reactor collects ash and particulates separated from

the flue gas stream. The hopper outlet connects to the plant fly ash handling system for periodically

removal of the accumulated ash. Flue gas exits the reactor via an opening at the top of the hopper

and is directed to the air heater inlet. Some designs eliminate the need for hoppers by keeping flue

gas velocities high enough in these areas so fly ash remains entrained in the flue gas.

Natural gas-fired and distillate oil-fired boilers generate flue gas that is relatively

free of dust and SO2

 (for low-sulfur oil). Consequently, SCR systems for these boilers place

the reactor upstream of the air heater, the high-dust SCR configuration.

Low-Dust SCR

Coal-fired units with an electrostatic precipitator (ESP) located upstream of the air heater

(hot-side ESP), typically use a low-dust SCR configuration. Figure 2.8 shows a low dust configuration, which locates the SCR reactor downstream of the ESP. In this location, the flue gas is

relatively dust free. The ash removed by the ESP typically contains arsenic, alkali metals, and other

constituents that are detrimental to catalyst performance and life.

A low-dust SCR system increases catalyst life by reducing concentrations of particulates and catalyst poisons in the SCR reactor. In addition, low-dust SCR configurations

do not need ash hoppers. For designs employing honeycomb catalyst, the catalyst pitch can

be reduced to approximately 4 to 7 mm, resulting in lower catalyst volume. Longer catalyst

life, lower catalyst volume and the elimination of the ash hopper mean lower costs for lowdust SCR compared to high-dust configurations. The only disadvantage of low dust SCR is the

HOT

ELECTROSTATIC

PRECIPITATOR

AMMONIA

INJECTION

GRID

ECONOMIZER

BYPASS

BOILER

FLUE GAS

SECONDARY TO AIR BOILER

SCR

REACTOR

Figure 2.8: Low-Dust SCR Arrangement, [4]
2-21

temperature drop of the flue gas as it flows through the ESP. Flue gas temperatures generally do

not decrease to the point where reheating is required. However, an increase in the size of the

existing economizer bypass duct may be required to maintain the flue gas temperature within the

optimum range.

Tail-end SCR

Early installations of coal-fired boilers in Europe and Japan employ tail-end SCR

configurations. This configurations places the SCR reactor downstream of all air pollution

control equipment installed on a unit Figure 2.9 depicts a tail-end system for a plant with a

particulate control device and a wet flue gas desulfurization (FGD) system. The air pollution control equipment removes most flue gas constituents detrimental to SCR catalyst before it enters the SCR reactor. However, because the flue gas temperature at the tail-end is

below the range required for the ammonia/NOx reaction, the flue gas needs to be reheated.

Tail end SCR systems use oil- or natural gas-fired duct burners or steam coil gas heaters for

reheating. Some of the energy used to reheat the gas is recovered in a recuperating gas-togas heater.

A tail-end system typically costs the most of the three SCR systems because of the

additional equipment and operational costs required for flue gas reheating and heat recovery. Operating experience and the availability of improved catalysts for the high-dust SCR

system makes the tail-end SCR system the least attractive of the three options. New low

temperatures catalysts are currently being developed and may make tail end systems a more

cost effective option in the future [25].

ID FAN

FGD

SCRUBBER

VESSEL TO

STACK

RECUPERATING

GAS-TO-GAS

HEATER

SCR

REACTOR

AMMONIA

INJECTION

GRID

DUCT BURNER

FUEL

SECONDARY

AIR

AIR HEATER

PARTICULATE

CONTROL

DEVICE

BOILER

Figure 2.9: Tail-end SCR Arrangement, [4]
2-22

Gas Turbines

Natural gas-fired turbine applications frequently use SCR technology for post-combustion NOx

 control. There are two basic gas turbine configurations; combined cycle (cogeneration cycle) and simple cycle. The majority of SCR systems are installed as combined

cycle applications. A typical combined-cycle SCR design places the reactor chamber after

the superheater within a cavity of the heat recovery steam generator system (HRSG) as

shown in Figure 2.10. The flue gas temperature in this area is within the operating range for

base metal-type catalysts . Some combined-cycle gas turbine designs locate the SCR reactor

downstream of the HRSG and prior to the economizer at temperatures ranging from 350°F to

400°F. Simple cycle applications of SCR place the reactor chamber directly at the turbine exhaust

where the flue gas temperature is in the range of 850°F to 1000°F (450°C to 540°C). This requires

the use of a high temperature catalyst such as zeolite. [4]

SCR Reactor Designs

The reactor design affects capital and operating costs of the SCR system and the

catalyst management plan. There are two different types of SCR reactors; full SCR and induct SCR. Full SCR designs house the catalyst in a separate reactor chamber. The boiler

flue gas must be ducted from the economizer outlet, to the SCR reactor, then to the air

heater inlet. A separate reactor allows a large volume of catalyst to be installed in layers,

which increases NOx reduction and catalyst lifetime. It also increases the duct length avai

able for the mixing of reactants prior to entering the reactor chamber. A separate reactor,

Figure 2.10: SCR Arrangement for a Combined Cycle Gas Turbine, [4]

Steam SCR

Injectors

Drum

Clean

Gas

Water

SCR Evaporator Economizer

Catalyst

Superheater

Turbine

Exhaust
2-23

however, requires a large amount of space adjacent the boiler to install the reactor and ductwork.

The additional ductwork often necessitates upgrades to the draft fan system.

In-duct (in-line) SCR systems house the reactor within the plant’s existing ductwork

rather than in a separate reactor chamber. The ductwork is generally enlarged to provide

sufficient room for the catalyst. In-duct systems save on costs for the ductwork, reactor

chamber, and ID fan. In-duct designs limit catalyst volume and mixing length, therefore,

they are commonly used in conjunction with other NOx control technologies [19]. Catalyst

erosion is generally higher for in-duct systems. Installation and maintenance of in-duct systems typically require more boiler outages. Natural gas-fired boilers, which have low catalyst volumes, frequently employ in-duct systems. Coal-fired boilers may apply in-duct SCR

reactors where space limitations restrict the installation of a full reactor [19].

2.2.4 High Dust SCR System Equipment

The majority of SCR designs utilize Thermal DeNOx

®, which is a an ammoniabased NOx

 reduction system developed and patented by Exxon Research and Engineering

Company in 1975. The SCR system has five basic steps to accomplish. These steps are:

• Receiving and storage of the ammonia;

• Vaporizing the ammonia and mixing with air;

• Injecting the ammonia/air mixture at appropriate locations;

• Mixing the ammonia/air with flue gas; and

• Diffusing the reactants into the catalyst and reducing the NOx.

While the basic steps in an SCR system are similar for all configurations, the system

design and equipment specifications are somewhat different. A discussion of the SCR system design and equipment is given below for an aqueous ammonia-based, high dust configuration, full reactor SCR for a large ($2,500 MMBtu/hr) coal-burning industrial boiler. The

SCR process steps, related auxiliary equipment, and the potential impacts of SCR operation on

existing plant equipment are also discussed. A simplified system flow schematic is presented in

Figure 2.1 and Figure 2.7 and a list of equipment is presented in Table 2.2.

Ammonia Storage and Vaporization

Aqueous ammonia is typically available as a 19% to 29.4% solution in water. Generally, a 29.4 percent solution is used in this application. Anhydrous ammonia is nearly

100% pure ammonia and stored as a liquid under pressure. Table 2.1 presents the properties

of aqueous and anhydrous ammonia.
2-24

Table 2.2: Major Equipment List for an SCR Application

Item Description/Size

SCR reactors (1 to 2) Vertical flow type, 805,000 acfm capacity, 44 ft.× 44 ft.× 31 ft. high

(excluding outlet duct and hoppers), equipped with 9,604 ft3 of

ceramic honeycomb catalyst, insulated casing, sootblowers, hoppers,

and hoisting mechanism for catalyst replacement

Anhydrous ammonia tank Horizontal tank, 250 psig design pressure, storage tanks 15,000 gal,

(1 or more) 34-ton storage capacity

Air compressor (2) Centrifugal type, rated at 3,200 acfm and 30 hp motor

Vaporizers (2) Electrical type, rated at 80 kW

Mixing chamber Carbon steel vessel for mixing or air and ammonia

Ammonia injection grid Stainless steel construction, piping, valves and nozzles

Ammonia supply piping Piping for ammonia unloading and supply, carbon steel pipe: 1.0"

diameter, with valves and fittings

Sootblowing steam Steam supply piping for the reactor soot-piping blowers, 2" diameter

pipe with an on-off control valve and drain and vent valved connections

Air ductwork Ductwork between air blowers, mixing chamber, and ammonia

injection grid, carbon steel, 14 in. dia., with two isolation butterfly

dampers and expansion joints

Flue gas ductwork Ductwork modifications to install the SCR modifications reactors,

consisting of insulated duct, static mixers, turning vanes, and expansion joints

Economizer bypass Ductwork addition to increase flue gas temperature during low loads

consisting of insulated duct, flow control dampers, static mixers,

turning vanes, expansion joints, and an opening in the boiler casing

Ash handling Extension of the existing fly ash handling modifications system:

modifications consisting of twelve slide gate valves, twelve material

handling valves, one segregating valve, and ash conveyor piping

Induced draft fans Centrifugal type, 650,000 acfm at 34" wg and 4,000 hp motor

Controls and Stand-alone, microprocessor-based controls for the SCR system with

instrumentation feedback from the plant controls for the unit load, NOx emissions,

etc., including NOx analyzers, air and ammonia flow monitoring

devices, ammonia sensing and alarming devices at the tank area,

and other miscellaneous instrumentation

Electrical supply Electrical wiring, raceway, and conduit to connect the new equipment

and controls to the existing plant supply systems

Electrical equipment System service transformer OA/FA/-60 Hz, 1,000/1,250 kVA (65 °C)

Foundations Foundations for the equipment and ductwork/piping, as required

Structural steel Steel for access to and support of the SCR reactors and other

equipment, ductwork, and piping
2-25

Facilities receive anhydrous or aqueous ammonia via a tank-truck or rail car and pump it

into one or more storage tanks. Ammonia is typically stored as a liquid in horizontal cylindrical

tanks. The aqueous ammonia tank is an enclosed tank rated for only slightly elevated pressure,

while the anhydrous ammonia tank is a pressure vessel rated for at least 250 pounds per square

inch gauge (psig). The anhydrous ammonia tank can be filled only to about 85% of its total volume

to allow for a vapor space above the liquid level. The tanks are equipped with level and temperature indicators, a manway, vent, and access ladder, and other appurtenances. The applicability of

heat tracing, insulation, and seismic design criteria are determined based on site-specific conditions. The tank should be mounted on a concrete pad and surrounded by a spill containment

structure such as a dike.

 SCR applications on large boilers generally require one to five tanks with volumes

ranging from 10,000 to 20,000 gallons per tank to maintain sufficient volume for 1 to 3

weeks of SCR operations. The ammonia storage tank may be sized for 3 to 30 days of

storage.

 The high end of the range would be used in conservative design practice. Alternatively, if ammonia distributors are located nearby and considered reliable, the plant owner

might opt for a smaller tank, sized for fewer days of ammonia storage.

Aqueous ammonia is vaporized by pumping it to a vessel where it mixes with hot

air. The air from the dilution air fan is heated in an electric heater or other heat exchanger

(e.g., steam). In most aqueous ammonia applications, the ammonia-air mixture leaves the

vaporizer vessel at about 300 °F. The vaporization energy required for aqueous ammonia is

much greater than that required for anhydrous ammonia because the water in the aqueous

ammonia solution also must be vaporized.

If anhydrous ammonia is used, it is fed to the electrical vaporizer by gravity, and the

vaporized gas is returned to the storage tank vapor space. Vapor is drawn from the vapor

space and piped to the ammonia/air mixer. Alternatively, liquid anhydrous ammonia may be

pumped to a vaporizer and piped to the ammonia/air mixer.

Ammonia (aqueous or anhydrous), diluted with air at a ratio of about 20:1 (air:NH3

),

is transported to the ammonia injection grid. The high proportion of air helps ensure good

mixing of air and ammonia and keeps the mixture below the flammable limit.

Ammonia Injection

For either aqueous or anhydrous ammonia, the ammonia-air mixture is directed

through a flow-balancing skid to the ammonia injection grid (AIG) where it is injected

under pressure. The flow-balancing skid consists of flow meters and manual valves to adjust the flow to each part of the AIG.
2-26

The AIG consists of a network of pipes or lances connected in parallel, and perforated with several holes or nozzles. The lances are placed in a grid formation across the

width and height of the ductwork. The lances and holes are sized to distribute the ammonia

uniformly into the flue gas. The spray angle and velocity of the injection control the trajectory of the ammonia. Injectors are subject to high-temperatures and to flue gas impingement

which cause erosion, corrosion, and structural integrity degradation. Therefore, injectors

are generally constructed of stainless steel and designed to be replaceable. Multiple injection zones may be utilized to increase the distribution of ammonia.

The ammonia can be injected with a low energy or high energy system. A lowenergy system uses little or no pressurized air while a high-energy system uses large amounts

of compressed air or steam to inject and vigorously mix the solution with the flue gas. AIG

systems in large boilers typically use high-energy systems. High energy systems are more

expensive to build and operate since they require a larger compressor, a more robust injection system, and consume more electric power.

Uniform distribution and mixing with flue gas is critical to maintain desired low

levels of ammonia slip. Cold gas flow modeling and numerical flow modeling are generally performed for the AIG and SCR system to ensure uniform mixing and dispersion before the gases enter the SCR reactor. If duct length is inadequate to ensure thorough mixing,

or results from the model study indicate poor gas mixing characteristics, devices such as

turning vanes or static gas mixers may be added.

An essential part of an AIG system is the controller used to regulate ammonia injection. Boiler load, inlet NOx

, and inlet gas temperatures set the feed-forward signal to establish the base ammonia injection rate. A feedback signal measuring the SCR outlet NOx

concentration is used to trim the base ammonia injection rate.

Catalytic Reduction of NOx

The catalytic reduction of NOx

 in the SCR reactor occurs when the NOx and ammonia in the flue gas contact the catalyst layers. The catalyst itself is the key component of the

SCR system. The catalyst composition, type (honeycomb or plate), and physical properties

affect performance, reliability, catalyst quantity required, and cost. However, because the

SCR system supplier and catalyst supplier must guarantee catalyst life and performance,

most catalyst characteristics are selected by the SCR system supplier.

2.2.5 SCR System Auxiliary Equipment

SCR Inlet and Outlet Ductwork

 In retrofit installations, new ductwork is required to integrate the SCR system with

the existing equipment. In high-dust SCR systems, the reactor is located between the econo-
2-27

mizer outlet, the air heater inlet. In the low-dust SCR, the SCR reactor is located between

the outlet duct of the particulate control device and the the air heater inlet duct. In the tailend SCR, the ductwork tie ins are downstream of the FGD system and also require the

integration of the flue gas reheating equipment.

SCR Bypass Duct

Low load boiler operations can decrease the temperature at the SCR reactor inlet

below the SCR operating range. In addition, startup and shutdown of the boiler causes

drastic temperature fluctuations. For these operating conditions, an SCR bypass may be

required to route the flue gas around the reactor chamber. The bypass prevents catalyst

poisoning and fouling during periods when the SCR is not operating. This system must

include zero-leakage dampers to protect the catalyst from flue gas leakage and deactivation

during SCR shut down. A bypass system may also be considered for seasonal operation of

the SCR system.

Sootblower

In coal-fired boilers, sootblowers are usually installed in the SCR reactor to remove

particulates that may mask or block active catalyst surfaces and gas passages. Sootblowing

helps maintain acceptable flue gas pressure drop in the SCR reactor by keeping the catalyst

gas passages free of particulate. Sootblowers also keep the air heater gas passages open and

thereby reduce system pressure drop. This is especially true for SCR retrofits where the air

heater plate spacing is generally narrow, making it more susceptible to fouling or clogging

by ammonia-sulfur salts.

Retractable rake-type sootblowers which use steam or air for blowing are utilized in

SCR designs. The sootblowers are typically located above each catalyst layer. Sootblowing

is usually performed on one catalyst layer or part of one catalyst layer at a time. Sootblowing

of all the catalyst layers takes between 30 minutes to 2 hours, but it is usually done infrequently. In European SCR installations, sootblowing is done approximately once or twice a

week [18].

Economizer Bypass Duct

Although the SCR reaction occurs within a temperature window of 600°F to 750°F

(320°C to 400°C), the catalyst for a given application is designed for a somewhat narrower

range, the economizer outlet temperature at normal boiler operating load. Maintaining the

flue gas temperature within the required window is essential for optimizing the NOx reduction reaction. When the economizer outlet flue gas temperature decreases because the plant

is operating at reduced loads, the temperature can be raised using an economizer bypass.
2-28

The economizer bypass duct generally has a modulating damper to regulate the amount of

hot bypass gas flow to be mixed with the cooler economizer outlet flue gas. The lower the boiler

load, the more this damper opens, thus admitting more hot gas. The economizer outlet duct also

needs a modulating damper to provide enough backpressure to allow the required volume of gas

to flow through the bypass. The main design considerations for an economizer bypass involve

maintaining the optimum gas temperature and ensuring uniform mixing of the two gas streams prior

to entering the SCR reactor.

Upgraded or New Induced Draft (ID) Fan

The new ductwork and the SCR reactor’s catalyst layers decrease the flue gas pressure. In order to maintain the same flow rate through the duct work, additional energy is

required. The existing induced draft (ID) fan may be unable to provide the required increase

in static pressure. In such cases, an upgraded or new ID fan is installed. The existing fan

and motor foundation also may need modification. Replacement involves installation of a

new fan or booster fan. In all cases, SCR systems require additional electric power for the

ID fan equivalent to approximately 0.3 percent of the plant’s electric output. Refer to Equation (2.49) for a more precise estimate of the additional electric power needs.

2.2.6 Other Considerations

Formation of SOx

Sulfur trioxide (SO3) forms during the combustion of fuels which contain sulfur. It

reacts with ammonia in the flue gas downstream of the reactor (ammonia slip) to form

ammonium bisulfate and ammonium sulfate. The amount formed depends on the sulfur

content of the fuel and the amount of ammonia slip. Ammonium bisulfate condenses as the

flue gas stream temperatures lowers. It then deposits on the SCR catalyst and downstream

equipment such as the air heater, ducts, and fans. Ammonia slip limits are generally imposed as part of the SCR design requirements to avoid impacts on downstream equipment.

There are several methods for limiting the impact of ammonia-sulfur salt deposition. Soot blowers can be installed between catalyst layers to remove surface deposits by

blowing air or steam across the catalyst. Increased acid washing of the air preheater and

other equipment may be required to remove deposits. However, more frequent acid washing

generates additional wastewater which must be disposed or treated by the plant. The sulfur

content of the flue gas can be decreased by coal desulfurization processes or fuel switching.

Lastly, the flue gas temperature may be raised to prevent condensation.

Elevated SO3

 concentrations raise the acid dew point of the flue gas. This phenomenon potentially leads to more corrosion on the air heater’s cold-end surfaces if the flue gas

temperature is below the acid dew point. To protect against this possibility, the cold-end

baskets of the air heater can be replaced with enamel-coated baskets.
2-29

Ammonia sulfates also deposit on the fly ash. Ammonia content in the fly ash greater than

5 ppm can result in off-gassing which would impact the salabilty of the ash as a byproduct and the

storage and disposal of the ash by landfill. [10] (See Chapter 1 SNCR)

Formation of Arsenic Oxide

Arsenic oxides (As2O3), formed during combustion of fuel containing arsenic, cause

catalyst deactivation by occupying active pore sites. Coal burning boilers are particularly

susceptible to arsenic poisoning. Limestone (CaCO3

) can be injected into the flue gas to

generate the solid Ca3

(AsO4)2, which does not deposit on the catalyst and can be removed

from the flue gas with a precipitator.

Retrofit Versus New Design

Retrofit of SCR on an existing boiler has higher capital costs than SCR installed on

a new boiler system. The magnitude of the cost differential is a function of the difficulty of

the retrofit. A large part of the capital costs are not impacted by retrofit including ammonia

storage, vaporization, and injection equipment costs. The increase in cost is primarily due

to modifications to existing ductwork, the cost of structural steel and reactor construction,

auxiliary equipment costs, such as additional fans, and engineering costs. In addition, significant demolition and relocation of equipment may be required to provide space for the

reactor. These costs can account for over 30% of the capital costs associated with SCR [9].

Retrofit costs for cyclone or wet bottom wall-fired boilers are somewhat higher than retrofit

costs for dry bottom wall- or tangentially-fired boilers [4]. Differential retrofit cost for SCR

in Germany is approximately $200 per MMBtu/hr (20 $/kW) [4].

Combustion Unit Design and Configuration

Boiler size is one of the primary factors that determines the SCR system capital

costs. In addition, boiler configuration influences SCR costs. Boiler configurations that

split the flue gas flow for two or more air preheaters and/or particulate removal systems

require more than one SCR reactor. Additional reactors substantially increase capital costs.

Boiler operations that have varying operating load, frequent startup/shutdowns, or seasonal

operations require an SCR bypass. Additional ductwork, dampers, and control systems

increase the SCR system capital costs. The SCR system may require modifications to draft

fans and/or installation of additional fans. This increases both capital and operating costs of

the SCR system. In addition, boiler and duct modifications may be required for implosion

protection to accommodate increased draft requirements. [9]

Fuel Source

Industrial boilers use coal, distillate oil, residual oil, and natural gas. The fuel type

and grade affects the SCR design and, therefore, the capital costs of the SCR system. Fuels
2-30

with high heating value have higher gas flow rates, which in turn results in an increase of the

required SCR reactor size and catalyst volume. Coal-fueled applications are more costly than oiland natural gas-fired boilers due to their higher flue gas flow rates. [9]

The quantity of nitrogen, fly ash and pollutants in the flue gas stream varies according to the type and grade of the fuel. This affects the volume of catalyst required as well as

the catalyst design, composition and rate of deactivation. Coal flue gas contains a greater

amount of fly ash, sulfur dioxide, sulfur trioxide, arsenic and other trace pollutants than oil

and natural gas. Natural gas is the cleanest fuel and contains the least amount of nitrogen,

therefore, burning natural gas results in the least amount of NOx

 and pollutants in the flue

gas stream. The amount of nitrogen and pollutants in oil-based fuels sources varies with the

grade and type of oil, either refined or residual.

Modeling of the SCR System

Computational fluid dynamics (CFD) and chemical kinetic modeling are performed

as part of the design process for SCR (See Chapter 1 SNCR). In addition to CFD and chemical kinetic modeling, three dimensional, physical flow modeling, also referred to as cold

flow modeling, is generally required. Cold flow modeling ensures that the flow through the

SCR reactor provides adequate residence time, achieves uniform mixing of flue gas and

ammonia, minimizes linear velocities to prevent catalyst erosion, and minimizes pressure

drop across the catalyst layers. It involves constructing a model of the ammonia injection

system, mixing area and reactor chamber. In a high-dust configuration, this involves modeling from the economizer outlet to the inlet of the air heater. Typical model scales range

from a 1:10 ratio to a 1:12 ratio for large electric utility boilers.

2.3 Design Parameters

SCR system design is a proprietary technology. Extensive details of the theory and

correlations that can be used to estimate design parameters such as the required catalyst

volume are not published in the technical literature [4]. Furthermore, the design is highly

site-specific. In light of these complexities, SCR system design is generally undertaken by

providing all of the plant- and boiler-specific data to the SCR system supplier, who specifies the required catalyst volume and other design parameters based on prior experience and

computational fluid dynamics and chemical kinetic modeling. [1]

This section presents a step-by-step approach to estimate design parameters based

on a procedure developed in the draft EPA report Selective Catalytic Reduction for NOx

Control on Coal-fired Boilers [1]. This procedure assumes SCR system size and cost are

based on three main parameters: the boiler size or heat input, the required level of NOx

reduction, and the catalyst volume. The approach to SCR sizing described in this section is

based on the catalyst volumes for a base case and several sensitivity cases developed to
2-31

where Table 2.3 gives the HHV for various coals. A conversion of 10-6 MMBtu/Btu is

required for HHV values.

 Table 2.3: High Heating Values for Various Coals

T ype of Coal Energy Content (Btu/ lb)

Lignite 5000-7500

Subbituminous 8000-10,000

Bituminous 11,000 - 15,000

Anthracite 14,000

If the boiler produces electricity, then its heat input can be estimated using the boiler net

plant heat rate, in Btu per kilowatt-hr (Btu/kWh) NPHR:

support the cost estimating procedures in Reference [1]. Although this approach is based on SCR

data for utility boilers, it provides sufficient accuracy and detail to develop the capital and annual

cost estimates for SCR as applied to industrial boilers.

Boiler Heat Input

The primary cost estimation parameter in the methodology presented in Reference

[1] is the maximum potential heat released by the boiler or heat input rate, QB, expressed as

million British thermal units per hour (MMBtu/hr). It is obtained from the high heating

value, HHV, of the fuel in Btu per pound (Btu/lb) multiplied by the maximum fuel consumption rate in pounds per hour (lb/hr), m& fuel :

Q HV m B fuel = & (2.3)

Q B NPHR B MW = (2.4)

where BMW is the boiler megawatt (MW) rating at full load capacity. Note that if NPHR is

not known (e.g., a cogeneration unit), a value of 9,500 Btu/kWh can be used as a reasonable
2-32

estimate. Using this value, the heat input rate, QB, is:

(2.5)

System Capacity Factor

The total system capacity factor, CFtotal, is a measure of the average annual use of the

boiler in conjunction with the SCR system. CFtotal is given by:

CF

total 

 = CF

plant

 CF

SNCR (2.6)

For industrial boilers, the capacity factor of the boiler, CFplant, is the ratio of actual quantity

of fuel burned annually to the potential maximum quantity of fuel burned annually in pounds.

CFplant is given by:

B

CF

actual m

maximum m plant

fuel

fuel

= (2.7)

SCR can be operated year-round or only during the specified ozone season. The capacity facor

for the SCR system, CFSCR, is the ratio of the actual number of SCR operating days, t

SCR, to the

total number of days per year:

CF

t

SCR

SCR = 365 days (2.8)

Uncontrolled NOx and Stack NOx

Uncontrolled NOx, represented as NOxin, is the NOx concentration in the flue gas

after any reductions provided by combustion controls, but prior to the SCR system. The

uncontrolled NOx concentration, obtained from analyzing the boiler flue gas stream, is generally given in pounds per MMBtu (lb/MMBtu) of NO2

 [1].

The stack NOx, represented as NOxout is the required NOx emission limit at the stack

outlet. It is generally set by the plant or regulatory limits and also given in pounds per

MMBtu (lb/MMBtu) of NO2

 [1].

Q . B ( ) ( ) MMBtu

hr = 9 5 MMBtu

MWh B (MW) MW
2-33

NOx Removal Efficiency

The NOx

 removal efficiency, represented as ηNOx, is determined from the uncontrolled

NOx

 level of the boiler at maximum heat input rate, CFplant = 1.0, and the required stack emission

limit.

The equation for the NOx removal efficiency is given by:

η NOx

x

in xout

x

in

NO NO

NO =

−

(2.9)

The required NOx

 removal efficiency is one of the most influential parameters on the overall SCR

system cost [9].

Actual Stoichiometric Ratios

The Actual Stoichiometric Ratio (ASR) indicates the actual amount of reagent needed

to achieve the targeted NOx reduction. Typical ASR values are higher than theoretical values

due to the complexity of the reactions involving the catalyst and limited mixing. Higher

ASR values generally result in increased NOx reduction. The ASR is an important parameter in SCR system design because it establishes the reagent use of the SCR system. The

ASR is defined as:

B

ASR moles of equivalent NH injected

mole of uncontrolled NO

3

x

= (2.10)

For estimating purposes, the moles of NOx

 are equivalent to the moles of NO2. Note that the

moles of equivalent NH3 in Equation 2.10 the moles of NH3 that will be released from the

reagent. When using ammonia as the reagent, the equivalent moles of NH3 injected is equal

to the moles of ammonia injected.

In a design developed by a system supplier, the ASR would be adjusted to account

for temperature, residence time, degree of mixing, catalyst activity, and allowable ammonia

slip for a specific boiler. No equation for estimating ASR was available for SCR. The value

for ASR in a typical SCR system is approximately:

ASR = 1.05 (2.11)
2-34

This value incorporates design margins for ammonia slip and the small amount of NO2 in

the boiler flue gas which requires two moles of NH3

 per mole of NO2 instead of one mole of

NH3 per mole of NO as shown in Equation 2.1.

Flue Gas Flow Rate

The full-load flue gas flow rate, including the typical design margin of 5 to 15 percent, is used to size the SCR reactors and associated catalyst inventory. This flow rate

should be obtained from test data or a combustion calculation.

If flow rate data are not available, an approximation of the flue gas flow rate to each

of the SCR reactors can be calculated. The estimation of the volumetric flow rate in actual

cubic ft. per minute (acfm) should be based on the expected operating gas temperature at the

SCR inlet. The equation for the volumetric flow rate is:

( )

( ) q

qQ T

n fluegas

fuel B

SCR

= +

+ °

460

460 700 F (2.12)

where nSCR is the number of SCR reactors chambers and qfuel is the fuel volumetric flow rate

for the boiler in units of ft3/min - MMBtu/hr and Q

b

 is given in MMBtu/lb.

The fuel flow rate qfuel, is based on combustion calculations using typical fuels, typical

boiler parameters (e.g., 20 % excess air), and typical SCR flue gas pressure (-10 in. w.g)

[20]. Estimated values for qfuel are given in Table 2.4 for various types of coal.

Note that, in general, the number of reactors, nSCR, is site specific. One SCR reactor

per boiler unit is typically required in high-dust system designs. However, two SCR reactors

may be needed to treat flue gas from a boiler equipped with two air preheaters. The system

designs developed for the base and sensitivity cases of this report use one reactor. Studylevel costs of a two-reactor system are expected to be similar to the cost of a corresponding

one-reactor system because the catalyst, ammonia, economizer bypass, and ID fan costs are

essentially identical.

Table 2.4: Estimated Values of Volumetric

Flow Rate for Various Coals

Coal Type Estimate value of qfuel

 (ft3/min-MMBtu/hr)

Eastern Bituminous 484

Powder River Basin 516

Lignite 547
2-35

Space Velocity and Area Velocity

The space velocity is defined as the inverse of the residence time given by the equation:

 V

V

A area

space

specific

= (2.15)

Aspecific is given in units of length2

/length3 and must be provided by the catalyst manufacturer.

Theoretical NOx Removal

Equation 2.9 defines the NOx removal efficiency. However, in SCR NOx removal efficiency changes with catalyst activation. The following theoretical equation allows for estimation of

the NOx removal efficiency, ηNOx, based on the catalyst activity constant, Kcatalyst, at a specified

time, t [1]. The theoretical NOx removal efficiency is:

V

ResidenceTime space = 1 (2.13)

Space velocity is calculated from the experimentally measured flue gas volumetric flow

rate at the reactor inlet, represented as qflue gas, and the reactor volume, represented as Volreactor

,

given by the equation:

 V q

Vol space

flue gas

reactor

= (2.14)

SCR system designers and vendors use the concept of area velocity, Varea, to account

for the reaction being limited to active catalyst sites. The area velocity is calculated from the

specific surface area of the catalyst per catalyst volume, Aspecific, in the following equation:

η NOxSR (1- e

a ) (2.16)

where

a

K A

V

catalyst specific

space

=  −



 



 (2.16a)

Both Kcatalyst and Aspecific are generally provided by the catalyst manufacturer.
2-36

 Slip ASR ( ) NOx = −η (2.17)

Catalyst Volume

The theoretical catalyst volume required for the SCR system is based on the factors

discussed in Section 2.2 Process Description. Equation 2.17 can be rearranged to determine

the theoretical catalyst volume [8]. Substituting the definition of space velocity into Equation 2.17, the volume of the catalyst is given by:

 Vol

q ASR

K A catalyst

fluegas

NOx

catalyst specific

=

− ×−

















































×

ln 1

η

(2.18)

An empirical equation was developed in Reference [1] as a function of several sensitivity

variables. The sensitivity variables were determined from catalyst volume estimates obAccording to this equation, the NOx

 removal efficiency increases with increasing NH3/

NOx ratio and decreasing space velocity (i.e., increasing catalyst volume for a given gas flow rate).

In addition, the equation shows that as the activity of the catalyst decreases over time, the NOx

removal also decreases.

The theoretical ammonia slip in parts per million by volume can be calculated from

the value of ηNOx, using the equation [14]:

tained from catalyst suppliers for base and sensitivity cases. Adjustment factors for these

variables were then developed using regression techniques.

The empirical equation for catalyst volume is given below:

Vol Q Slip NO S

T

N catalyst B adj adj xadj adj

adj

SCR

= ×× × × × × 2 81 . η (2.19)

and the adjustment factors include:

• NOx efficiency adjustment factor:

η η ( ) adj = +× 0 2869 1058 . . (2.20)
2-37

• NOx adjustment factor for inlet NOx:

NO NO x ( ) adj x

in

=+ × 08524 0 3208 . . (2.21)

• Ammonia slip adjustment factor for ammonia slips between 2 and 5 ppm:

Slip Slip ( ) adj =− × 12835 0 0567 . . (2.22)

• Sulfur in coal adjustment factor:

S S ( ) adj =+ × 0 9636 0 0455 . . (2.23)

where S is the sulfur content of the fuel by weight fraction.

• The temperature adjustment factor for gas temperatures other than 700°F:

T . T. T ( ) ( ) adj = − ×+ × × − 1516 0 03937 2 74 10 5 2 . (2.24)

where T is the temperature of the flue gas at the reactor inlet in degrees Fahrenheit (°F).

SCR Reactor Dimensions

The cross sectional area of the SCR reactor is sized for the flow rate of the flue gas

in acfm and the superficial velocity. A typical value for the superficial velocity is 16 feet per

second (960 ft/min). Using this value for velocity the equation for the catalyst cross-sectional area is given by:

A q

ft

sec min

catalyst

fluegas = 



 



 × 



 

 16  60 sec (2.25)
2-38

The SCR reactor cross-sectional area is approximately 15 percent greater than the catalyst

cross-sectional area to account for the module geometry and hardware:

A A SCR catalyst = × 115. (2.26)

The actual dimensions of the SCR depend on the module arrangement in the catalyst layer. The

typical cross-sectional dimensions of a module are 3.3 feet wide by 6.6 feet long. Therefore, the

SCR plan dimensions are approximately multiples of these dimensions. Depending on the number

of modules in width and in length, the SCR reactor may be square or rectangular.

For the purposes of this report, the SCR reactor can be treated as a square. The

screening costs are valid for rectangular SCR reactors as long as the aspect ratio (length

divided by width) is not too large. Industry standard aspect ratios are between 1.0 and 1.5.

For a square reactor, the length, l, and width, w, are estimated by:

lw A = = ( ) SCR

1 2/ (2.27)

An initial value for the number of catalyst layers is estimated first. This estimate is then

checked by calculating the catalyst height for each layer. The initial estimate for the number

of catalyst layers can be determined from the total catalyst volume, the cross sectional area

of the catalyst, and estimating the height of the catalyst element. A nominal height for the

catalyst, h′

layer, is 3.1 feet. A first estimate for the number of catalyst layers, nlayer, is:

 n Vol

h A layer

catalyst

layer catalyst

= ′ × (2.28)

This value of nlayer is then rounded to the nearest integer. In addition, there must be at least

two catalyst layers.

The height of each catalyst layer is calculated using the estimated number of layers.

This must result in the height of a catalyst layer, hlayer, to be within the standard industry

range of 2.5 to 5.0 feet. The height of a catalyst layer is calculated from the following

equation:

h

Vol

n A layer 1 catalyst

layer catalyst

= ×

















+ (2.29)

where 1 foot is added to account for space required above and below the catalyst material

for module assembly.
2-39

The number of catalyst layers calculated above does not include any empty catalyst layers

for the future installation of catalyst. An empty catalyst layer is recommended for use with a catalyst management plan. The total number of catalyst layers includes all empty catalyst layers that will

be installed:

nnn total layer empty = + (2.30)

The height of the SCR reactor, including the initial and future catalyst layers, the flowrectifying layer, space for sootblowers and catalyst loading, but excluding the inlet and

outlet ductwork and hoppers is determined from the equation:

B

h n ch c SCR total layer = ++ ( ) 1 2 (2.31)

where the constants are based on common industry practice of c1

 = 7 and c2 = 9.

Estimating Reagent Consumption and Tank Size

The rate of reagent consumption or mass flow rate of the reagent, , generally expressed as pounds per hour (lb/hr), can be calculated using the inlet NOx in lb/MMBtu and

heat input rate in MMBtu/hr.

m& NO Q NSR M

M SR reagent

x

in B NOx reagent

NOx T

= η

(2.32)

where the M

reagent is the molecular weight of ammonia (17.03 gram per mole) and MNOx is

the molecular weight of NO2 (46.01 gram per mole). The molecular weight of NO2 is

used because the NOx emissions, NOxin, are given in lb/MMBtu of NO2.

For ammonia, the mass flow rate of the aqueous reagent solution, is given by:

& & m

m

C sol

reagent

sol

= (2.33)

where Csol is the percent concentration of the aqueous reagent solution by weight.
2-40

The solution volume flow rate, qsol, generally expressed as gallons per hour (gph), is:

qsol = m& sol

sol

sol ρ ν (2.34)

where ρsol is the density of the aqueous reagent solution in lbs per cubic foot, 56.0 lb/ft3 for a 29%

solution ammonia at 60°F. The specific volume of a 29% solution ammonia at 60°F is 7.481

gal.ft3

.

The total volume stored in the tank, or tanks, is based on the volume that the SCR

system requires for operating a specified number of days. The volume stored on site for the

number of operating days, t, is:

 Tank Volume q t sol = (2.35)

Note that the tank volume is typically based on full-load operation, so the capacity factor is

not included in Equation 2.36. A common on site storage requirement is for 14 days of SCR

operation.

2.4 Cost Analysis

The cost-estimating methodology presented here provides a tool to estimate studylevel costs for high-dust SCR systems. Actual selection of the most cost-effective option

should be based on a detailed engineering study and cost quotations from the system suppliers. The costs presented here are expressed in 1998 dollars.

The cost estimating equations presented in this section are based on equations developed by The Cadmus Group, Bechtel Power, Inc. and SAIC in the draft EPA report,

Selective Catalytic Reduction for NOx Control on Coal-fired Boilers, [1]. These equations

follow the costing methodology of Electric Power Research Institute (EPRI)[23]. In the

EPRI method, both the purchased equipment cost (PEC) and direct installation cost are

estimated together. This methodology is different from the EPAAir Pollution Control Cost

Manual methodology, which estimates equipment costs and installation costs separately.

Due to the limited availability of equipment cost data and installation cost data, the equations for SCR capital costs were not reformulated.

The capital and annual cost equations were developed for coal-fired wall and tangential utility and industrial boilers with heat input rates ranging from 250 MMBtu/hr to 6000

MMBtu/hr (25 MW to 600 MW). The SCR system design is a high-dust configuration with one

SCR reactor per combustion unit. It utilizes anhydrous ammonia as the reagent with an allowed
2-41

ammonia slip in the range of 2 to 5 ppm.. The catalyst is a ceramic honeycomb with an operating

life of 3 years at full load operations. The cost equations are sufficient for NOx reduction efficiencies up to 90% . A correction factor for a new installation versus a retrofit installation is included to

adjust the capital costs. [1]

The cost information presented in this report is based on using ceramic honeycomb

catalyst for the base case. In general, more catalyst volume is required for an SCR system

using plate catalyst, although the unit cost of plate catalyst is lower than honeycomb. Thus,

any difference in capital cost is expected to be within the accuracy of a study-level cost

estimate.

This report is based on the high-dust SCR system because it is the most common

design. A low-dust configuration would cost somewhat less because the required catalyst

volume is smaller and ash hoppers on the SCR reactor are not required. The cost methodology is valid for a low-dust SCR system because the cost reductions are expected to be

within the range of uncertainty for study-level costs. The costs for the tail-end arrangement,

however, cannot be estimated from this report because they are significantly higher than the

high-dust SCR systems due to flue gas reheating requirements.

2.4.1 Total Capital Investment

Total Capital Investment (TCI) includes direct and indirect costs associated with

purchasing and installing SCR equipment. Costs include the equipment cost (EC) for the

SCR system itself, the cost of auxiliary equipment, direct and indirect installation costs,

additional costs due to installation such as asbestos removal, costs for buildings and site

preparation, offsite facilities, land, and working capital. In general, SCR does not require

buildings, site preparation, offsite facilities, land, and working capital. A more detailed

discussion of capital costs can be found in Section 1, Chapter 2 of this Manual.

Direct Capital Costs

Direct capital costs (DCC) include purchased equipment costs (PEC) such as SCR

system equipment, instrumentation, sales tax and freight. This includes costs associated

with field measurements, numerical modeling and system design. It also includes direct

installation costs such as auxiliary equipment (e.g., ductwork, fans, compressor), foundations and supports, handling and erection, electrical, piping, insulation, painting, and asbestos removal. Table 2.2 presents a list of the equipment and installation requirements for an

aqueous ammonia-based, high-dust SCR system.

The direct capital cost equation is based on data for one representative boiler and several
2-42

sensitivity cases [2]. The direct capital cost equation includes the affect of specific design parameters on the SCR cost through the use of adjustment factors. Adjustment factors are based on

correlations of the sensitivity cases as functions of the catalyst volume, reactor height, and ammonia flow rate [2]. The design parameters and their affect on the capital cost equation are listed

below:

• NOx removal (catalyst volume);

• Inlet NOx (catalyst volume);

• Ammonia slip (catalyst volume);

• Sulfur content in coal (catalyst volume);

• Flue gas temperature (catalyst volume);

• Catalyst management plan (cost of the SCR reactor);

• New plant versus retrofit (cost of the ductwork, structural steel, foundations); and

• With and without an SCR bypass, e.g., for seasonal operation (cost of ductwork)

The equation for direct capital cost (DCC) including the adjustment factors in dollars is:

 DC Q f h f NH rate f new f bypass ( )( ) ( ) ( ) ( ) Q f Vol B MMBtu

hr

SCR

B

catalyst = + + ++





















 



 + 



 





$ , .3 380 3500

3

0 35

(2.36)

Note that in the capital cost equation only the catalyst volume factor is not multiplied by the

boiler heat input scaling factor (3500/QB)0.35. The catalyst cost in $/ft3 is simply based on the

volume of catalyst used and does not incorporate an economy of scale. The design parameter adjustment factors are given in the equations listed below.

Adjustment for the SCR reactor height:

( ) ( ) fh h SCR ft MMBtu

hr

SCR MMBtu

hr

= 













 − − $. $ . 612 187 9

(2.37)

Adjustment for the ammonia flow rate:

f NH rate ( ) lb hr

m

Q

reag

B MMBtu

hr

3

3 = 



 



 − 



 





$411

/

& $47.

(2.38)
2-43

for a retrofit

( ) ( ) f new MMBtu

hr

= $0

(2.39)

for a new boiler

( ) ( ) f new MMBtu

hr

= − $728

(2.40)

Adjustment for installing an SCR bypass.

no bypass installed

( ) ( ) f bypass MMBtu

hr

= $0

(2.41)

bypass is installed

( ) ( ) f new MMBtu

hr

= $127

(2.42)

The capital cost for the initial charge of catalyst:

f Vol Vol CC ( ) catalyst catalyst initial = (2.43)

where Volcatalyst is in ft3 and CCinitial is the cost of the initial catalyst, currently estimated at 240 $/ft3

for a ceramic honeycomb catalyst.

Indirect Capital Costs

Indirect installation costs are those associated with installing and erecting the control system equipment but do not contribute directly to the physical capital of the installation. This generally includes general facilities and engineering costs such as construction and contractor fees,

preproduction costs such as startup and testing, inventory capital and any process and project

contingency costs. As explained in the Manual Section 1 Introduction, average values of indirect

installation factors are applied to the direct capital cost estimate to obtain values for indirect installation costs. These costs are estimated as a percentage of the TCI. The equations for indirect

installation costs are presented in Table 2.5.
2-44

Table 2.5: Capital Cost Factors for an SCR Application

Description Data or Formula

Boiler Size (MMBtu/hr) QB

NOx Removal Efficiency ηNOx

Cost Year December 1998

T otal Direct Capital Costs ($) A (see eq. 2.37-2.44)

Indirect Installation Costs

 General Facilities ($) 0.05 × A

 Engineering and Home Office Fees ($) 0.10 × A

 Process Contingency ($) 0.05 × A

T otal Indirect Installation Costs ($) B = A × (0.05 + 0.10 + 0.05)

Project Contingency ($) C = (A + B)× 0.15

T otal Plant Cost ($) D = A + B+ C

Allowance for Funds During Construction ($) E = 0 (Assumed for SCR)

Royalty Allowance ($) F = 0 (Assumed for SCR)

Preproduction Cost ($) G = 0.02 × (D + E)

Inventory Capital* ($) H = V ol reagent (gal) × C ost reagent ($/gal)

Initial Catalyst and Chemicals ($) I = 0 (Assumed for SCR)

T otal Capital Investment (T CI) ($) TC I = D + E + F + G + H + I

* Cost for ammonia stored at site, i.e., the first fill of the reagent tanks

The initial capital cost includes the cost of the initial reagent volume, where RC is the

reagent cost in dollars per gallon, given by the equation:

Initial Capital Cost Vol RC reagent = (2.44)

2.4.1 Total Annual Costs

Total annual costs (TAC) consist of direct costs, indirect costs, and recovery

credits. Direct annual costs are those proportional to the quantity of waste gas processed

by the control system. Indirect( fixed) annual costs are independent of the operation of the

control system and would be incurred even if it were shut down. No byproduct recovery

credits are included because there are no salvageable byproducts generated from the SCR

[2]. Each of these costs is discussed in the sections below. A more detailed discussion of

annual costs can be found in Section 1, Chapter 2 of this Manual.
2-45

Design parameters are estimated using the maximum annual heat input rate of the boiler

to ensure adequate sizing of the SCR system. Annual costs are calculated using the average heat

input rate of the boiler and SCR system using CFtotal. This ensures that annual costs are based

on the actual operating conditions rather than the design case.

Direct Annual Costs

Direct annual costs (DAC) include variable and semivariable costs. Variable direct

annual costs account for purchase of reagent and electrical power. Semivariable direct

annual costs include operating and supervisory labor cost, maintenance cost, and catalyst

replacement cost. These costs are discussed individually below. Equations for these

variable cost items were derived in Reference [1].

Operating costs also result from small decreases in boiler efficiency due to operation of the economizer bypass. The economizer bypass operation depends on the flow rate

of gas bypassed at full and partial loads and the boiler’s capacity factor. Another operating cost is incurred for the steam or electric power used for compressed air, as required

for the relatively infrequent operation of sootblowers. These operating costs are generally

small and site-specific. Therefore, they are not discussed in this report.

DAC 

Annual

Maintenance

Cost

Annual 

Cost

Annual 

Electricity

Cost

Annual

Water

Cost

Annual

Catalyst

Cost

= 













 +















 +















 +















 +















 Reagent (2.45)

Operating and Supervisory Labor

The SCR reactor is a stationary device with no moving parts. Further, the SCR

system incorporates only a few pieces of rotating equipment (e.g., pumps, motors, etc.).

Therefore, the existing plant staff can operate the SCR from an existing control room. In

general, operation of an SCR system does not require any additional operating or supervisory labor.

Maintenance

The annual maintenance labor and material cost in dollars per year ($/yr), including nozzle tip replacement for the injectors, is assumed to be 1.5% of the Total Capital

Investment (TCI) in dollars. The equation is given by:

Annual Maintenance Cost = 0.015 TCI (2.46)
2-46

Reagent Consumption

The annual cost of ammonia purchase in $/yr is estimated using the ammonia volume

flow rate, the capacity factor, and the cost of reagent in dollars per gallon, Costreag

:

where

B

t CF

hr

yr ops plant = 8760 

(2.47a)

Utilities

The electrical power consumption in kilowatts is estimated for SCR equipment,

ammonia vaporization, and additional ID fan power [1]. It is based on linear regression of

electrical power consumption data correlated to the uncontrolled NOx concentration,

NOxin, ASR and the boiler heat input, QB.

Power NO P B x duct [ ] ( ) in = ++ 0105 05 . . Q n P NO total catalyst x η ∆ ∆ (2.48)

The term, (NOxin × ηNOx), accounts for the ammonia vaporization and dilution air blower.

The terms 0.5 × (∆Pduct + ntotal × ∆Pcatalyst ), account for the additional fan power required to

overcome the additional pressure drop of the SCR and ductwork. Typical values of ∆Pduct range

between 2 to 3 inches of water; ∆Pcatalyst usually ranges from 0.75 to 1 inch of water per

catalyst layer.

The annual cost of electricity is estimated from the equation:

Annual Electricity Cost Power Cost t elect op = (2.49)

where Costelect is the electricity cost in dollars per kilowatt-hour ($/kWh).

Catalyst Replacement

The catalyst life is a function of the catalyst activity and ammonia slip. As the

catalyst activity decreases with time, the ammonia slip increase until it reaches the design

limit and new catalyst must be added. Catalyst life is usually specified when purchasing

reagent reag op Annual Reagent Cost = q Cost t (2.47)
2-47

the catalyst. A catalyst layer is typically guaranteed for 16,000 to 24,000 operating hours. This

cost methodology assumes a guaranteed catalyst life of 24,000 hours or 3 years.

If the SCR does not have an empty catalyst layer, all of the catalyst layers must be

replaced at the end of 24,000 operating hours. This very conservative assumption has

been used in the SCR costs developed in the References [2] and [3]. If the SCR includes a

spare catalyst layer, then only one catalyst layer is replaced at the end of 24,000 hours.

Most SCR designs include a spare catalyst layer. The cost for catalyst replacement in all

the SCR reactors for a given boiler, nSCR, is given by:

Catalyst Replacement Cost n Vol

CC

R SCR catalyst

replace

layer

= (2.50)

where Volcatalystt is in ft3 CCreplace is the cost of catalyst in dollars per foot cubed ($/ft3) and

Rlayer is a factor for catalyst replacement. Rlayer = 1 for full replacement and Rlayer = nlayer for

replacing 1 layer per year.

Because the catalyst is replaced every few years, the annual catalyst cost for all

reactors is a function of the future worth of the catalyst given by:

Annual Catalyst Replacement Cost

Catalyst

Replacement

Cost

=





















 FWF (2.51)

where FWF is the future worth factor. Future worth is used because the annual catalyst

replacement cost is accrued starting in the first year of operation, while catalyst replacement purchases occur every few years. To account for the time value of money, the FWF

amortizes the catalyst cost over the years preceding the actual catalyst purchase [21].

Because the money is allocated in advance of the purchase, the sum of the annual catalyst

replacement costs is less than the purchase price of the catalyst. The future worth factor,

FWF is given by:

( ) FWF i

i Y = + −





 



 1

1 1 (2.52)

where i is the assumed interest rate and Y is the term in years.
2-48

The term, Y is given by the equation:

Y

h

h

catalyst

year

= (2.53)

where hcatalyst is the operating life of the catalyst in hours and h

year

 is the number of hours

per year the SCR is operated. The value of Y estimated from the equation is then rounded

to the nearest integer.

Indirect Annual Costs

In general, indirect annual costs (fixed costs) include the capital recovery cost,

property taxes, insurance, administrative charges, and overhead. Capital recovery cost is

based on the anticipated equipment lifetime and the annual interest rate employed. An

economic lifetime of 20 years is assumed for the SCR system. The remaining life of the

boiler may also be a determining factor for the system lifetime.

In many cases property taxes do not apply to capital improvements such as air

pollution control equipment, therefore, for this analysis, taxes are assumed to be zero

[19]. The cost of overhead for an SCR system is also considered to be zero. An SCR

system is not viewed as risk-increasing hardware (e.g., a high energy device such as a

boiler or a turbine). Consequently, insurance on an SCR system is on the order of a few

pennies per thousand dollars annually [19]. The administrative charges, covering sales,

research and development, accounting, and other home office expenses, incurred in

operation of an SCR system are relatively insignificant for the cost estimation procedure

presented here. Finally, there are two categories of overhead, payroll and plant. Payroll

overhead includes expenses related to labor employed in operation and maintenance of

hardware; whereas plant overhead accounts for items such as plant protection, control

laboratories, and parking areas. Because this procedure assumes that no additional labor

is needed in operation of an SCR system, payroll overhead is zero and plant overhead is

considered to be negligible.

Using these assumptions, indirect annual costs in $/yr, IDAC, can be expressed as:

 IDAC = CRF TCI (2.54)
2-49

where TCI is the total investment and CRF is the capital recovery factor and defined by:

where i is the interest rate, and n is the year the cost is incurred.

Total Annual Cost

The total annual cost (TAC) for owning and operating an SCR system is the sum of

direct and indirect annual costs as given in the following equation:

The tons of NOx removed annually are

NO Removed NO Q t x x

in = NO B op x η 

(2.57)

where NOx is in lb/MMBtu, Q

B

 is in MMBtu/hr and tops is in hours and a conversion of 2,000

lb/ton is required.

The cost in dollars per ton of NOx removed per year is:

where TAC is in $/yr and NOx

 removed is tons/yr.

B

( )

( ) CRF

i i n

= n

+

+ i −

1

1 1 (2.55)

Cost Effectiveness 

TAC

NOx

= Removed (2.58)

Total Annual Cost

Direct

Annual

Cost

Indirect

Annual

Cost

=





















+



















 (2.56)
2-50

 2.5 Example Problem

An example problem, which calculates both the design parameters and capital and

annual costs, is presented below. The design basis is a retrofit SCR system being applied

to a 1000 MMBtu/hr, wall-fired, industrial boiler firing subbituminous coal. The following assumptions are made to perform the calculations:

Fuel High Heating Value 10,000 Btu/lb

Maximum Fuel Consumption Rate 1.0 x 105 lb/hr

Average Annual Fuel Consumption 4.38 x 108

 lb

Number of SCR operating days 155 days

Plant Capacity Factor 50%

Uncontrolled NOx Concentration 0.86 lb/MMBtu

Required Controlled NOx

 Concentration 0.13 lb/MMBtu

Acceptable Ammonia Slip 2.0 ppm

Fuel Volume Flow Rate, Eastern Bituminous 484 ft3/min per MMBtu/hr

Fuel Heating Value 12,696 Btu/lb

Fuel Sulfur Content 1.0% by weight

Fuel Ash Content 7.7% by weight

ASR 1.05

Stored Ammonia Concentration 29%

Number of Days of Storage for Ammonia 14 days

Pressure Drop for SCR Ductwork 3 inches w.g.

Pressure Drop for each Catalyst Layer 1 inch w.g.

Temperature at SCR Inlet 650°F

In addition to these assumptions, the estimated economic factors for the cost equations are:

Cost year December, 1998

Equipment Life 20 years

Annual Interest Rate 7%

Catalyst Cost, Initial 240 $/ft3

Catalyst Cost, Replacement 290 $/ft3

Electrical Power Cost 0.05 $/kWh

29% Ammonia Solution Cost 0.101 $/lb

Operating Life of Catalyst 24,000 hours

Catalyst Layers 2 full, 1 empty
2-51

2.5.1 Design Parameter Example

Boiler Calculations

The boiler annual heat input rate, QB, is calculated from the High Heating Value for

subbituminous coal given in Table 1.3 and the maximum fuel consumption rate, m& fuel :

QB

Btu

lb

lb

hr

Btu

MMBtu

MMBtu

lb = = 10 000 100 000

10

1000 6

, , , 

The plant capacity factor is calculated from the maximum and annual average fuel consumption:

( ) CF

lb

plant lb

hr

h = × × = = 4 38 10

1 10 8760

05 50%

8

5

. . r

yr

The SCR sytem capacity factor is calculated from themonths of SCR operation, 5 months:

CF days

SNCR = == 155

365 0 42 42 days . %

The total capacity factor including both plant and SCR capacity factors is given by:

CFtotal =× = = 05 0 42 0 21 21% .. .

The flue gas flow rate using Equation (2.12) is:

( ) ( )

( ) q

F

F flue gas =

 ft

min-

+

+ = acfm

MMBtu

hr

MMBtu

hr

484

1000 460 650

460 700 1 463138

3

× ×°

° ×

,

,

The NOx removal efficiency, ηNOx, is calculated from the inlet NOx concentration and the required
2-52

controlled NOx concentration using Equation (2.16):

η NOx =

 lb

MMBtu

 - MMBtu

lb

MMBtu

 = =

086 013

086

085 85

. .

.

. %

SCR Reactor Calculations

The catalyst volume using Equation (2.20) and the equations for each adjustment factor

is:

Volcatalyst = 2.81 × 1,000 MBtu/hr

× (0.2869 + (1.058 × 0.85)) (ηNOx

)

× (0.8524 + (0.3208 × 0.86)) (NOxin)

× (1.2835 - (0.0567 × 2.0)) (Slip)

× (0.9636 + (0.0455 × 1.0)) (Sulfur)

× (15.16 - (0.03937 × 650) + (0.0000274 × 6502)) (Temperature)

= 5089 ft3

The catalyst and SCR cross-sectional areas using Equations (2.26) and (2.27) are:

Acatalyst = acfm

ft

s smin

 = ft 463138

16 60

482 2 ,





 



 



 





ASCR = ft = ft 115 482 554 2 2 . ×

The length and width of the reactor using Equation (2.28) is:

l = w = ( = ft 554 235 1 2 ) / .

The first estimate of the number of catalyst layers using Equation (2.29) is:

n . . layer = = 5089

31 482

34

×

Rounding this value gives, nlayer = 3.
2-53

Checking the actual catalyst height using Equation (2.29):

h . layer = ×+ ft

 ft = 5089

3 482

1 45

3

This value is between the design height limits of 2.5 and 5 feet.

The total number of catalyst layers is determined by Equation (2.30) with 1 empty catalyst layer:

n = total 314 + = 

The SCR height, excluding the outlet duct and hoppers using Equation (2.31) is:

h = ft SCR 4 7 4 5 9 55 + . + = × ( )

Reagent Calculations

The mass flow rate of the reagent is calculated using the molecular weight of the

reagent, 17.03 g/mole and NO2

, 46.01g/mole. For an ASR of 1.05, the reagent mass flow

rate is given by Equation (2.32):

&

. ,

.

m =

 lb

MMBtu MMBtuhr g

mole

g

mole

 = lb

hr reagent

086 1 000 105 17 03

46 01

334

× ×× . .

The mass flow rate of 29% aqueous ammonia solution is given by Equation (2.33):

& . m , sol = 

lb

hr

 = 

lb

hr

334

0 29

1152

The solution volume flow rate can then be calculated from Equation (2.34) where p is the

density of the29% aqueous ammonia solution, 56.0 lb/ft3

 at 60oF and the specific volume of the

solution is 7.481 gal/ft3.

qsol = 

lb

hr gal

ft

lb

ft

 = gph

1152 7 481

56 0

154

3

3

, .

.

×
2-54

The total volume stored in the tank, or tanks, is based on the volume that the SCR system

requires for 14 days of operation. The on site storage requirement is given by Equation (2.35):

Tank Volume = gph days hr

day

154 14 = gal 24× × 51 744 



 

 () , 

The on site storage requirement for ammonia is 51,744 gallons per 14 days of operation.

2.5.2 Cost Estimation Example

Once the SCR system is sized, the capital and annual costs for the SCR system can be

estimated. The DCC are estimated using the Equation (2.36):

DC NH = [ ] MMBtu

hr f hSCR + f + f new + f bypass

 MMBtu / hr

 MMBu / hr

 + Volcatalyst

1000 3 380 3

3500

1000

0 35

, , ( )( )( )( )

,

,

.

( )

× +

× 



 





where the funcions in units of dollars per MMBtu/hr are given by:

f ( ) (. ) . [ ] $ h = - = MMBtu / hr SCR 612 55 187 9

149

×

f(NH )= MMBtu

hr

- MMBtu

hr

 = MMBtu

hr

3

334

1000

3

90

lb

hr

lb hr

. ×





















$411

/

$47 $

f( ) , new = for retrofit 0

 f( ) , bypass = for no bypass 0

The function Volcatalyst is the initial cost of the catalyst given by Equation (2.43):

f Volcatalyst ( ) $ = ft , , ft = 3 5089

240

× 3 $1221360
2-55

Substituting the value obtained for the factors gives:

DC = [ ] MMBtu

hr

+ + ++

MMBtu

1 000 3 380 149 90 0 0 + =

3 500

1 000

221 360 832 000

0 35

, , $ ,

, $1, , $6, ,

.

× ×





 





Indirect capital costs are estimated from the direct capital costs using the following

equations in Table 2.5:

Table 2.5: Indirect Capital Cost Estimations

Indirect Installation Costs

General Facilities ($) $6,832,000 × 0.05 = $341,600

Engineering and Home Office Fees ($) $6,832,000 × 0.10 = $683,200

Process Contingency ($) $6,832,000 × 0.05 = $341,600

T otal Indirect Installation Costs ($) $341,600+ $683,200 + $341,600 = $1,366,400

Project Contingency ($) ($6,832,000 + 1,366,400) × 0.15 = $1,229,760

T otal Plant Cost ($) $6,832,000 + $1,366,400 + $1,229,760 = $9,428,160

Preproduction Cost ($) $9,428,160 × 0.02 = $188,562

Inventory Capital ($) $0.101/lb × 1,152 lb/hr × 24hr/day × 14 days = $39,094

T otal Capital Investment (T CI) ($) $9,428,160 + $188,562 + $39,094 = $9,655,816

Annual costs are based on the economic factors listed above. In addition, the SCR system is

assumed to operate for 5 months of the year with a boiler loading of 50%, resulting in a total

capacity factor of 27%. The annual variable costs are given by:

Maintenance Cost =

yr

 = 

yr

0 015

9 601261 . 144 019 , , $ ,

×
2-56

An estimate for power consumption is given by:

Power [ ] .= MMBtu / hr

MMBtu / hr

kW

 + + = kW 1000

9 5

(. . ) . ( ) 086 085 05 3 4 1 445 × ×

Electricity and reagent solution cost can then be estimated from Equation (2.49) and (2.47):

 Electricity Cost . $ ,

yr

= kW

 hr

yr kWh 445 = 8 760

0 27

0 05 52 626

× ××

, .$

Reagent Solution Cost =

lb

hr

hr

yr lb = yr

1152 8 760 0 27 0101

275196

× ×× , .. $$ ,

The catalyst is replaced every few years, the annual catalyst cost for all reactors is a function of

the future worth of the catalyst given by:

FWF =

 + - 

0 07 = 1

1 0 07 1

014 6 . ( .)× .

It is assumed that 1 layer of catalyst is replaced per year, therefore, Rlayer = 3. The operating life

of the catalyst is given as 24,000 hours and the estimated SCR operation hours per year is

3,720 hours which gives the value of Y in Equation (2.53) as:

Y = hours

hours = = 24000

3720

64 6 .

The cost of the catalyst replacement, for one layer replaced per year, is given by Equation

(2.50) and (2.51):

Annual Catalyst Replace Cost = ft = yr014 1 5089

290

3

871 3

3

.

$

$68,

×× ×

























ft
2-57

The total variable direct annual cost, the sum of the cost of the reagent, electricity, and catalyst

replacement, is given by Equation (2.45):

Total Variable Direct Cost =

yr

+

yr

+

yr = yr

$52, $275, $68, $396, 626 196 871 693

and the total direct annual cost is given by the sum of the maintenance and variable cost:

Total Direct Annual Cost =

yr

+

yr = yr

$144, $396, $541 837 693 530 ,

In estimating total indirect annual costs, it is assumed that the property tax factor,Ftax, and

the overhead factor, Fovhd, are both zero. The capital recovery factor, CRF, is defined by:

CRF = 0 07 1 0 07

1 0 07 1

0 0944

20

20

.( .)

( .) . +  + - =

and the indirect annual costs (IDAC) are calculated from Equation (2.55) and the TCI:

Indirect Annual Cost =

yr = yr

0 0944

655 816 . 412 $9, , $911,

×

The total annual cost is the sum of the direct annual and indirect annual costs given by Equation

(2.56)

Total Annual Cost = 41,530

yr

 + 

yr

 = 

yr

$ $ , $, , 5 911 412 1 452 942
2-58

The annual cost in terms of NOx removed can be calculated using the total annual cost and the

tons of NOx removed annually by Equation (2.58):

 NOx remove tons d =

 lb

MMBtu / hr

MMBtu

hr

hr

yr

lb

ton

 = 

086

085 1000 0 27 8 760

2 000

864

. ., . ,

,

×× ××

and Equation (2.59)

Cost of NOx Removal = yr

tons = tons

$, ,

$1,

1 452 492

864

682
2-59

References

[1] Selective Catalytic Reduction for NOx Control on Coal-fired Boilers, Draft Report, prepared for the U.S. Environmental Protection Agency by The Cadmus

Group, Inc., Bechtel Power Corporation, and Science Applications International

Corporation. May 1998.

[2] Investigation of Performance and Cost of NOx Controls as Applied to Group 2

Boilers. Revised Draft Report, prepared for the U.S. Environmental Protection

Agency by The Cadmus Group, Inc., Bechtel Power Corporation, and Science

Applications International Corporation. August 1996.

[3] Cost Estimates for Selected Applications of NOx

 Control Technologies on Stationary Combustion Boilers, Draft Report and Responses to Comments on the Draft

Report. prepared for the U.S. Environmental Protection Agency by The Cadmus

Group, Inc., and Bechtel Power Corporation. June 1997.

[4] Institute of Clean Air Companies (ICAC). Selective Catalytic Reduction (SCR)

Control of NOx

 Emissions, Prepared by the ICAC SCR Committee, November

1997.

[5] U.S. EPA. Analyzing Electric Power Generation Under the CAAA. EPA Office

of Air and Radiation, March 1998.

[6] Rummenhohl, V., H. Weiler, and W. Ellison. Experience Sheds Light on SCR

O&M Issues. Power Magazine, September 1992.

[7] Pritchard, S., C. DiFrancesco, and T.R. von Alten. SCR Catalyst Performance

Under Severe Operating Conditions. Presented at Power-Gen International ‘96

Conference and Exhibition, ASME, Orlando, FL. December 1996.

[8] Harmgart, S., M. Meierer, and J. Fahlke. In-Service Assessment of DeNOx

Catalysts. Presented at the International Joint Power Generation Conference and

Exposition ‘98, ASME, Baltimore, MD, 1998.

[9] Cochran, J.R., M.G. Gregory, and V. Rummenhohl. The Effect of Various Parameters on SCR System Cost. Presented at Power-Gen ’93, the Fourth International

Power Generation Exhibition & Conference, ASME, Dallas, Texas, November

1993.

[10] Electric Power research Institute. Technical Feasibility and Cost of Selective

Catalytic Reduction (SCR) NOx

 Control. EPRI GS-7266, Final Report, May

1991.
2-60

[11] Rosenberg, H.S. and J. H. Oxley. Selective Catalytic Reduction for NOx Control

at Coal-fired Power Plants. ICAC Forum ’93, Controlling Air Toxics and NOx

Emissions, Baltimore, MD, February 24-26, 1993.

[12] Singer, J.G. (ed.) Combustion Fossil Power Systems. Third edition, Combustion

Engineering, Inc., Windsor, CT, 1981.

[13] Sigling, R. and R. Johnson. Current Practices for Monitoring Ammonia Slip from

SCR Processes. Presented at the 1998 Conference on Selective Catalytic and

Non-catalytic Reduction for NOx

 Control, U.S. Department of Energy, Federal

Energy Technology Center, Pittsburgh, PA, May 21-22, 1998.

[14] Cho, S.M. and S. Z. Dubow. Design of a Selective Catalytic Reduction System

for NOx

 Abatement in a Coal-Fired Cogeneration Plant. Proceedings of the

American Power Conference, Chicago, IL, pp. 717-722, April 1992.

[15] ANSI K-61.1-1989. Safety Requirements for the Storage and Handling of Anhydrous Ammonia. Compressed Gas Association, Arlington, VA, March 1989.

[16] Tanner Industries, Inc. Storage & Handling of Anhydrous Ammonia. Philadelphia, PA, Revision 5/96.

[17] Tanner Industries, Inc. Storage & Handling of Ammonium Hydroxide. Philadelphia, PA, Revision 10/95.

[18] Broske, D., G. Offen, and E. Cichanowicz. An Assessment of European Experience with Selective Catalytic Reduction for NOx

 Control. Presented at the “NOx

Controls for Utility Boilers Workshop,” Baltimore, MD, August 1998, (“Proceedings: 1998 NOx

 Control Workshop,” EPRI report number TR-1113561998.).

[19] Staudt, J.E. Status Report on NOx

 Control Technologies and Cost Effectiveness

for Utility Boilers. Published by Northeast States for Coordinated Air Use Management (NESCAUM) and Mid-Atlantic Regional Air Management Association

(MARAMA), June 1998.

[20] S.C. Stultz and J. B. Kitto, eds. Steam, Its Generation and Use. 40th edition,

Babcock& Wilcox, Inc., Barberton, Ohio, 1992.

[21] Green, D.W. and R.H. Perry, eds. Perry’s Chemical Engineers’ Handbook. 6th

edition, McGraw Hill, Inc., 1984.

[22] Tonn, D.P. and T. A. Uysal. 2200 MW SCR Installation on New Coal-Fired
2-61

Project. Presented at the Institute of Clean Air Companies ICAC Forum ‘98,

Durham, North Carolina, March 18-20, 1998.

[23] Electric Power Research Institute. Technical Assessment Guide. Vol. 1, Revision 7, 1993.

[24] Babb, Bob, Ed Angelini and Scot Pritchard. Implemenation of SCR System at

TVA Paradise Unit 2, Presented at the Institute of Clean Air Companies ICAC

Forum 2000, Washington D.C., March 23-24, 2000.

[25] Stenger, H. G., et al, Low Temperature Selective Catalytic Reduction of NO,

Presented at the Institute of Clean Air Companies ICAC Forum 2000, Washington

D.C., March 23-24, 2000.
TECHNICAL REPORT DATA

(Please read Instructions on reverse before completing)

1. REPORT NO.

 452/B-02-001

2. 3. RECIPIENT'S ACCESSION NO.

4. TITLE AND SUBTITLE

The EPA Air Pollution Control Cost Manual

5. REPORT DATE

January, 2002 

6. PERFORMING ORGANIZATION CODE

7. AUTHOR(S)

 Daniel Charles Mussatti

 

8. PERFORMING ORGANIZATION REPORT NO.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

 U.S. Environmental Protection Agency

 Office of Air Quality Planning and Standards

 Air Quality Standards and Strategies Division

 Innovative Strategies and Economics Group

 Research Triangle Park, NC 27711

10. PROGRAM ELEMENT NO.

11. CONTRACT/GRANT NO.

 

12. SPONSORING AGENCY NAME AND ADDRESS

 Director

 Office of Air Quality Planning and Standards

 Office of Air and Radiation

 U.S. Environmental Protection Agency

 Research Triangle Park, NC 27711

13. TYPE OF REPORT AND PERIOD COVERED

Final

14. SPONSORING AGENCY CODE

EPA/200/04

15. SUPPLEMENTARY NOTES

Updates and revises EPA 453/b-96-001, OAQPS Control Cost Manual, fifth edition (in English only)

16. ABSTRACT

In Spanish, this document provides a detailed methodology for the proper sizing and costing of numerous air

pollution control devices for planning and permitting purposes. Includes costing for volatile organic

compounds (VOCs); particulate matter (PM); oxides of nitrogen (NOx); SO2, SO3, and other acid gasses;

and hazardous air pollutants (HAPs). 

17. KEY WORDS AND DOCUMENT ANALYSIS

a. DESCRIPTORS b. IDENTIFIERS/OPEN ENDED TERMS c. COSATI Field/Group

Economics

Cost

Engineering cost

Sizing

Estimation

Design

Air Pollution control

Incinerators

Absorbers

Adsorbers

Filters

Condensers

Electrostatic Precipitators

Scrubbers

18. DISTRIBUTION STATEMENT

 Release Unlimited

19. SECURITY CLASS (Report)

 Unclassified

21. NO. OF PAGES

1,400

20. SECURITY CLASS (Page)

 Unclassified

22. PRICE

EPA Form 2220-1 (Rev. 4-77) PA Form 2220-1 (Rev. 4-77) PREVIOUS EDITION IS OBSOLETE