Selective Catalytic Reduction (SCR) - Innovative … · Selective Catalytic Reduction (SCR) for the removal of NO x (NO and NO 2) This primer includes: • SCR and Catalyst Basics

Selective Catalytic Reduction (SCR)

for the removal of NOx (NO and NO2)

This primer includes:

• SCR and Catalyst Basics

• SCR Design Considerations

• Catalyst Management

Trade names and companies mentioned are for illustration and clarification purposes but not for endorsement.

10 Commerce Drive Pelham, AL 35124

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www.innovativecombustion.com

1

SCR and Catalyst Basics


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Catalysis: a modification and especially increase in the rate of a chemical reaction induced by material unchanged chemically at the end of the reaction.


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Basic chemical reactions:

4NO + 4NH3 + O2 4N2 + 6H2O

2NO2 + 4NH3 + O2 3N2 + 6H2O

NO + NO2 + 2NH3 2N2 + 3H2O

The key reductant is ammonia, the ammonia molecules need to be thoroughly mixed with NOx in the

flue gas, this is essential for high NOx removal efficiency for all SCR systems. Thus, the mixing

system and ammonia injection grid design is closely related to removal efficiency. In general cold flow

modeling and Computational Fluid Dynamics (CFD) modeling are conducted to ensure that the system

has the least amount of maldistribution and ash dropout for a range of conditions. Due to

maldistribution in NH3 and NOx, a minute amount of NH3 in single digit ppm will exit the catalyst

layers, this is usually referred to as ammonia slip or slip.

CHEMISTRY


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SCR catalyst Economizer gas outlet with NOX

Ammonia (NH3) mixed with NOX

Exit consists of N2 + H2O.

Minor unreacted ammonia

/slip

Ammonia (NH3)

Ammonia Injection Grid (AIG)

NH3 reacts

with NO on

an active site

Typical SCR Process and Schematic


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Most commercial SCR catalysts are referred to as titania-vanadia base catalyst (since the 1970’s). Basic

material is similar to ceramic; the main component is titanium oxide (TiO2) with minor components such as

tungsten. As for the SCR reaction, the active sites are vanadium oxides (V2O5, V2O3).

There are many physical forms: homogeneous (extruded honeycombs), and heterogeneous (plates, corrugated).

In addition there are different pitches and/or cell openings for different applications (cleaner low dust flue gas

vs. high dust solid fuel boilers.)

The catalysts are packaged at different depths in the direction of gas flow. Some applications require multiple

layers of catalyst to meet the removal efficiency and expected operating life. Catalysts are sold by volumes in

cubic meters (m3). However, the effective specific geometric surface area is used for catalyst design, expressed

in m2 per m3. The total surface area, Acat in m2 is derived from specific area (m2 per m3) times total catalyst

volume (m3). This Acat is also used to derive the Area Velocity, AV (m/hr), a key design parameter and for

activity calculation. AV is defined as Flue Gas Flow rate in standard condition divided by Acat. Note: Internal surface area (pore volume) and pore distribution varies for different manufacturers, the

standard measurement is the BET surface area, m2/gm of material. [Brunauer–Emmett–Teller

theory/model]. BET surface area is used for quality control as well as for determining the exposed

catalyst sample condition.

SCR Catalysts


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In addition to physical differences, there are ranges in activity which is reflected by the bulk concentration of

vanadium in the formulation. High V concentrations will have higher activity and NOX removal efficiency for a

given catalyst volume, however, due to the concomitant oxidation reaction, a small amount SO2 is converted to

SO3. In general the ammonia slip will react with SO3 and form a sticky salt, ammonia bisulfate (ABS) due to its

high melting point. This is especially troublesome for downstream heat exchange surfaces (lower bulk flue

gas/metal temperature) such as air heater baskets and finned tubes.

Note: ABS reaction – NH

3 + SO

3 + H

2O (NH

4)HSO

4 (ABS) ; usually under [NH

3] << [SO

3]

SCR Catalysts, contd.


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SCR Catalyst Types and Physical Configurations

HETEROGENEOUS HOMOGENEOUS


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Plate

(Hitachi)

Rolled

Coated

Honeycomb (extruded)

(Cormetech, Ceram)

Extruded

Coated

Corrugated

(Haldor Topsoe)

Composite

Hybrid

SCR Catalyst Types and Physical Configurations

HETEROGENEOUS HOMOGENEOUS


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Plate-type structure

Flexible plates

Rectangular opening

Wall thickness: 0.6 to 0.8 mm

Pitch: 5 to 7 mm

Plate Pitch - center line to center line from one plate or wall to the next.

Honeycomb structure

Rigid

Square openings


Pitch: 2 to 9.2 mm

Hybrid plate-type structure

Rigid

Corrugated openings


Pitch: 2 to 12 mm

pitch pitch

SCR Catalyst Pitch


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Dust Load

gr/dscf

Plate Pitch Honeycomb Pitch Corrugated Pitch

< 2 5.0 mm 6.7 mm (22 cell) 5.8 mm

2 to 6 5.5 mm 7.4 mm (20 cell) 7.2 mm

6 to 10 6.0 mm 8.2 mm (18 cell) 8.3 mm

10 to 12 6.2 mm 9.2 mm (16 cell) 9.3 mm

> 12 6.5 mm NA > 9.3 mm

12 to 16 6.5 mm NA 10.3 mm

16 to 20 6.5 mm NA 12 mm

SCR Catalyst Pitch and Dust Loading


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• Catalyst elements arranged and packed in steel frames. o Plate – 2 levels of 8 element boxes

o Honeycomb – 72 monoliths

o Corrugated – 2 to 3 levels of 8 element boxes.

• Standardized cross-section module o Possible to interchange corrugated and

plate element boxes in most modules.

• Possible to interchange catalyst types within reactor

• Module height varies with the catalyst monolith heights, different for different catalyst suppliers.

• Most applications have a top grid/mesh designed onto the modules.

SCR Catalyst Blocks and Modules

SCR Design Considerations


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SCR Design Parameters

Both new and retrofit applications have the same basic design criteria and considerations.

However, some retrofits may have more stringent and available height, width and depth as well

as access to the reactor.

Projects are structured in many fashions, e.g., SCR system suppliers, EPC contractor, A/E firm as

sole owner engineer, A/E firm as partner, catalyst supplier and manufacturer as process design

engineer and catalyst supplier, catalyst manfacturer as supplier only with pass through guarantees

by A/E firm.

Most SCR guarantees are for a specific NOX removal efficiency (ETA, η) and/or Stack NOX

emissions for a certain cumulative operating hours. In general this is called ‘life’, even though

the catalyst proper is still capable of reducing NOX but not at a high level at the design

conditions.

Note: η = [ (NOX)in

– (NOX)out

]/ (NOX)in


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• Performance Requirements (and Guarantees)

• NOx reduction (80 – 95%) (or Stack NOX limit, ppmvdc) and

• the associated operating life (8,000 to 24,000 hours).

• NH3 slip allowed, 2 to 5 ppm, typically lower values for high sulfur fuel.

• SO2 oxidation allowed, 0.1 to 1.0% per initial catalyst charge.

• Pressure drop limit, usually 1 to 1.5” wc per layer.

Note on mercury emission (MATS): SCR catalysts do oxidize elemental mercury to

the oxidized form, the latter is readily removed in the wet scrubber system.

Certain system suppliers do guarantee mercury oxidation.



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• Flue Gas Operating Conditions

• Flue gas temperature and flue gas flow rate, and operating range (minimum operating temperature for low loads, some employ economizer bypass.)

• NOx inlet concentration and flue gas composition

• Fuel characteristics (affects catalyst volume)

• Fly ash concentration and characteristics (issues with large particle ash/popcorn ash)

• Other potential poisons in fuel and from the combustion process (affects catalyst volume due to deactivation.)



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• SCR Reactor

• Initial catalyst charge (e.g., 2 + one spare layer, 3 + 1, etc.)

• Reactor size – layers, catalyst depth, modules per layer

• Optimize effective cross-section to mitigate erosion potential for high erosional ash, most applications are in the 15 to 17 actual feet per second range.

• Plant configuration – high or low dust, AIG only, AIG/Mixers

• Governing - flue gas ammonia to NOX distribution entering first layer, a reasonably low maldistributionnote (5%) is required for high removal efficiency.

• Sealing for intra- and inter-module contact surface as well as side-walls and grating and floor.

Note: The actual local ammonia to NOx mole ratio (stoichiometric ratio, SR) at

the catalyst inlet is a key design flue gas parameter for all SCR systems. It is

usually normalized to 1.0, thus, NSR, and is expressed in percent, in RMS or Std

Deviation divided by the mean value or Co-variance.



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• Governing - flue gas ammonia to NOX distribution entering first layer and its effect on slip and removal efficiency.


0

2

4

6

8

10

12

0

20

40

60

80

100

120

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

amm

on

ia s

lip

, pp

m

rem

ov

al

stoichiometry

Effect of maldistribution on removal and slip

higher maldistrbution will shift

removal to a lower value for the

same stoichiometry (and catalyst

volume.)

higher maldistrbution will shift

the ammonia slip to a higher value

for the same stoichiometry (and

Catalyst volume.)


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Catalyst Management


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SCR Catalyst Management

All applications are aimed at providing the design performance with the installed catalyst layer(s) for a

period beyond the guaranteed/expected operating life. Should there be a need to replace a layer, the

plan should yield the best option and fit in with the outage schedule. The users need to have a

comprehensive catalyst management plan (CCMP) in addition to a catalyst replacement plan from the

suppliers. Within the industry, the latter is also called ‘Catalyst Management Plan (CMP)’.

A CCMP has many essential components:

• Actively study SCR performance trending and evaluation of key SCR indicators.

• Conduct periodic full load SCR performance test for removal and ammonia slip under design

(guaranteed) SCR conditions.

• Perform SCR reactor outage inspection with documentation of the system: reactor flow devices,

AIG, and catalyst (appearance and deposition, seals/bypasses, modules and sidewalls) mapping.

• Follow the SCR shutdown procedures and SCR catalyst outage protection.

• Perform periodic sample extraction analysis (either built-in sample log or coring of sample):

1. Catalyst Activity Analysis (deactivation leads to low activity, K)

2. Physical and Chemical Analyses (root causes for deactivation)

• Review the supplier’s ‘CMP’ with data from the first three items.


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Example - supplier’s ‘Catalyst Management Plan’

0

2

4

6

8

10

12

14

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

0 8,760 17,520 26,280 35,040 43,800

NH

3-s

lip

, p

pm

Rel

ati

ve

Act

vit

y

Hours

EXAMPLE -- Expected Replacement strategy

'Catalyst Management Plan'

Relative Activity

NH3-slip

add spare 1 layer

add spare 2layer replace layer 1 replace layer 1


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• SCR performance trending and evaluation (e.g., CEMS data, operating NOx removal,

ammonia consumption, ammonia slip (if equipped), pressure differential.)

• Most systems should not have sudden changes in performance at the same load,

deactivation should be gradual.

• Abrupt changes should be immediately investigated (e.g, low removal efficiency,

ammonia consumption, ammonia slip, ammonia in ash, increase in pressure drop.)

• Full SCR Reactor performance test for removal and ammonia slip under design (guaranteed)

SCR conditions. The results should be compared to the design and guaranteed values, certain

corrections from test to design may apply by using supplier’s performance correction curves.

• SCR performance should exceed that of design (guaranteed) parameters during the

initial/acceptance performance test.

• SCR performance should meet the design (guaranteed) parameters during the ‘end of

life’ performance test.

• Non-performance should be discussed with the supplier.


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Catalyst Activity (K note) Analysis

• Full bench is recommended due to the larger sample and less wall effect.

• Micro-reactor is also used, however, extrapolation and certain correlations entered into

deriving the activity.

• There are only few independent test labs (e.g., SRI, FERCo, EON.)

• Most manufacturers and regenerators have their own test lab, both for quality control and

aftermarket analysis.

• In general the deactivation rate and actual remaining activity is expressed in K/Ko, where

K is the initial known activity from the manufacturer.

Note:

K is defined as -AV * ln (1 - η )

Relative Activity, K = e-a x t

, where: a is the deactivation rate, t in 10,000 hours

Only two ‘standards’ (Test Protocols) are widely accepted and followed, VGB ad EPRI.

(There are no SCR test standards/protocol such as ASME, ASTM,…)


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Catalyst Activity (K note) Analysis, contd.

Application – High Dust Time, hr Relative Activity,

Kt/Ko

PRB firing 16,000 0.65 – 0.70

Lignite firing 16,000 0.50 – 0.55

Bituminous with 10 – 20% bio-fuels

co-firing

24,000 0.70 – 0.75

Bio-fuels firing 10,000 0.30 – 0.60

Application - Low Dust 24,000 0.85 – 0.95

Note: Compare full size performance and test sample activity to this table and the supplier’s

plan/curve.


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Example - supplier’s ‘Catalyst Management Plan’, typical strategy is to provide Design NOX removal as

catalyst layers deactivate, note the change in operating ammonia slip through the management cycle.

0

2

4

6

8

10

12

14

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

0 8,760 17,520 26,280 35,040 43,800

NH

3-s

lip

, p

pm

Re

lati

ve

Ac

tvit

y

Hours

EXAMPLE -- Expected Replacement strategy

'Catalyst Management Plan'

Relative Activity

NH3-slip

add spare 1 layer

add spare 2layer replace layer 1 replace layer 1


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Example – activity, K/Ko trend and the value of sample test results.

0.50

0.60

0.70

0.80

0.90

1.00

1.10

- 5,000 10,000 15,000 20,000 25,000

time, hours

EXAMPLE - Expected Relative Activity (deactivation) and Actual Test Sample results

Relative Activity, low deactivation application

test sample RA

HIGH DEACTIVATION samples

High Deactivation Application

Poly. (Relative Activity, low deactivation

application)


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Catalyst Sample Physical Analyses

• Physical analysis usually includes a determination of internal surface area (BET), in

m2/gm.

• The results will support whether the loss in activation is due to a decrease in active

surface and/or other mechanisms impeding the reactants from reaching the active sites.

Catalyst Sample Chemical Analyses

• Bulk elemental analysis as well as acid soluble elemental analysis is useful to determine

foreign elements in the exposed catalyst.

• ICP is one technique to determine a panel of interested elements (for example, known

poisons - alkalis, phosphorus, arsenic (coal units), chromium.)

• Scanning EM is also used to scan the surface for poisons and blinding material (e.g.,

calcium sulfate), this diagnostic technique is also applied to cut sections so as to

understand the profile of different poisons and its deposition gradient inside the catalyst.


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Actions:

• Non-performance could be in the catalyst proper with low activity and/or the process

condition, one common problem is the localized maldistribution of ammonia and NOX. In

short, there is either not enough ammonia molecules to react with NOX molecules and/or too

many ammonia molecules present for too few NOX molecules.

• AIG tuning involves the testing and adjusting the injection grid ammonia flows to achieve

design maldistribution.

• When the total catalyst volume (and its reactor potential) is below the required performance,

actions such as root cause analysis, tuning, replacement and/or regeneration options should

be evaluated as soon as possible; third party consultants and catalyst suppliers should be

engaged to ensure the best course has been chosen.

• If the performance is marginal, certain short-term measures such as de-rating and inlet NOX

tuning, and AIG tuning should be considered in order to extend the run-time for the next

planned or unplanned catalyst replacement.


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SCR Catalyst Management – Action:

• AIG tuning considerations

• Need to collect basic SCR design information including the design maldistribution, removal and

flue gas conditions, AIG design and CFD/flow model results.

• Ensure that SCR outlet test provisions are available, best to have some type of installed grid, if not,

then test probes should be assembled so as to provide an adequate grid. (Most end-users do not

have inlet sample grid. Although turning off the ammonia feed is the best way to obtain the inlet

NOX profile and value, this is not done in practice since it will violate the permit in most

circumstances. Some form of inlet NOX is usually available, if not, a single inlet probe to monitor

the process condition is a good alternative.) Check: individual flow control devices are operable;

zone or point flow indications are operating and indicating (e.g., orifices, Magnehelic.)

• Test unit should be at design load, removal efficiency should be slightly below guaranteed, and SCR

operates at stable removal rate.

• Test should be conducted within a reasonable time frame so as to lower the temporal changes, lower

test duration will improve the accuracy of the spatial distribution and the final analysis for the

ammonia to NOX distribution. Then the test protocol will require multiple analyzers to sample

multiple individual points simultaneously.

• Collect and convert raw data so as to derive the maldistribution value. AIG tuning is an iterative

process involving adjusting ammonia flows and evaluating the results. (Examples below.)


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• Example 1. As-found

Distribution.

• Without actual inlet NOX

distribution, one can

assume uniform

distribution, as reflected

in the second section with

40 ppm for all points.

Example: with an outlet

NOX of 10 ppm and inlet

NOX of 40 ppm assuming

no slip (at less than max

removal), the inlet

ammonia equals to (40-

10) 30 ppm. Local

stoichiometry = 30/40 or

0.75.

example AIG tuning

NOX (ppm) at the Outlet test grid

Probe

1 2 3 4 5 6 7 8

Point 1 10 2 6 6 10 3 2 9

Point 2 7 2 6 5 8 4 1 6

Point 3 9 2 6 5 10 7 1 9

Point 4 7 1 5 7 9 5 2 9

'most PROBABLE' INLET NOX AVG.= 40 ppm

LOCAL "inlet NH3 at individual point, ASSUMING no ammonia slip condition

30 38 34 34 30 37 38 31

33 38 34 35 32 36 39 34

31 38 34 35 30 33 39 31

33 39 35 33 31 35 38 31

mathematically the local stoichiometry ratio (SR) WILL THEN BE

0.75 0.95 0.85 0.85 0.75 0.925 0.95 0.775

0.825 0.95 0.85 0.875 0.8 0.9 0.975 0.85

0.775 0.95 0.85 0.875 0.75 0.825 0.975 0.775

0.825 0.975 0.875 0.825 0.775 0.875 0.95 0.775

with an average Stoichiometry of 0.858594


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• Example 1, contd.

• Local SR is then normalized as

shown in the table. Example:

0.75/0.8586 = 0.874

• Mapping shows high ammonia

flows to Lances 2 and 7, these

should be decreased. Should

also increase flow to Lance 5.

• After changing flows, wait for 1

hour before testing for outlet

profile, again the process is

iterative and the same calculation

applies after each adjustment.

‘Final’ result is shown in

Example 2.

Point 4

Point 3

Point 2

Point 1

1 2 3 4 5 6 7 8

example - NSR distribution

0.850-0.900 0.900-0.950 0.950-1.000 1.000-1.050 1.050-1.100 1.100-1.150

mathematically the Normalized SR (NSR) distribution WILL THEN BE

Probe

1 2 3 4 5 6 7 8

Point 1 0.874 1.106 0.990 0.990 0.874 1.077 1.106 0.903

Point 2 0.961 1.106 0.990 1.019 0.932 1.048 1.136 0.990

Point 3 0.903 1.106 0.990 1.019 0.874 0.961 1.136 0.903

Point 4 0.961 1.136 1.019 0.961 0.903 1.019 1.106 0.903

with an average Stoichiometry of 1

sample standard deviation of 0.084

OR A MALDISTRIBUTION OF 8.4%


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• Example 2: as-left,

successful tuning test

results due to the

available lances for the

targeted zones.

example AIG tuning - 2

NOX (ppm) at the Outlet test grid

Probe

1 2 3 4 5 6 7 8

Point 1 7 4 6 6 6 4 5 7

Point 2 5 7 5 5 7 4 7 6

Point 3 5 6 6 5 7 7 7 7

Point 4 6 4 5 7 6 7 5 6

'most PROBABLE' INLET NOX AVG.= 40 ppm

LOCAL "inlet NH3 at individual point, ASSUMING no ammonia slip condition

33 36 34 34 34 36 35 33

35 33 35 35 33 36 33 34

35 34 34 35 33 33 33 33

34 36 35 33 34 33 35 34

mathematically the local stoichiometry ratio (SR) WILL THEN BE

0.825 0.9 0.85 0.85 0.85 0.9 0.875 0.825

0.875 0.825 0.875 0.875 0.825 0.9 0.825 0.85

0.875 0.85 0.85 0.875 0.825 0.825 0.825 0.825

0.85 0.9 0.875 0.825 0.85 0.825 0.875 0.85

with an average Stoichiometry of 0.853906


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• Example 2 after tuning, contd.

• This tuning iteration has lowered the

maldistribution from 8.4% to 3% as

calculated below. The 3% is a very low

value, usually 5% is used for a well

designed AIG and mixing system. The

‘final’ mapping results should be

documented.

• AIG valve positions should be recorded

and ‘locked’ in position.

• The end result of AIG tuning will be

higher removal for the same ammonia

slip value, also this will effectively

‘lengthen’ the ‘life’ of the catalyst since

it can now meet higher removal

efficiency with the same process

conditions.

mathematically the Normalized SR (NSR) distribution WILL THEN BE

Probe

1 2 3 4 5 6 7 8

Point 1 0.966 1.054 0.995 0.995 0.995 1.054 1.025 0.966

Point 2 1.025 0.966 1.025 1.025 0.966 1.054 0.966 0.995

Point 3 1.025 0.995 0.995 1.025 0.966 0.966 0.966 0.966

Point 4 0.995 1.054 1.025 0.966 0.995 0.966 1.025 0.995

with an average Stoichiometry of 1

sample standard deviation of 0.030

OR A MALDISTRIBUTION OF 3.0%

Point

4

Point

3

Point

2

Point

1

1 2 3 4 5 6 7 8

example - NSR distribution

0.850-0.900 0.900-0.950 0.950-1.000 1.000-1.050 1.050-1.100 1.100-1.150


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Documents

Selective Catalytic Reduction (SCR) - Innovative … · Selective Catalytic Reduction (SCR) for the removal of NO x (NO and NO 2) This primer includes: • SCR and Catalyst Basics