Selective Catalytic Reduction for NOX

How to Design Selective Catalytic Reduction (SCR)
for NOX



What is a Selective Catalytic Reduction System?

  • Nitrogen Oxides are reduced to Nitrogen by Ammonia in the presence of catalyst
  • Technology available since 1950s. First tried out in Japan in 1963. The first commercial installation began in 1978
  • Best available control technology (BACT) for reduction of NOx (can accomplish greater than 95% reduction)
  • Ammonia and NOx mixing methods are discussed in the previous post

What are the chemical reactions in SCR

  • The following are the major reactions taking place:

4NO + 4 NH3+ O= 4 N2+6H2O               (1) Dominant Reaction

6NO + 4NH3 =  5 N2+6H2O                          (2)

NO + NO2+ 2 NH=  2 N2+3H2O                (5)

2NO2+4 NH3+O= 3 N2+6H2O                  (3)

6NO2+ 8 NH= 7 N2+12H2O                       (4)

  • Flue gas passes through an ammonia injection grid
  • Ammonia vapor is dispersed and mixes with NOx
  • Flue gases pass through the catalyst bed
  • NOx is converted to Nitrogen and Water vapor


Effect of NH3 Slip on NOx Reduction Efficiency

  • To reduce the  NOx emissions and to meet regulatory limits, ammonia (NH3) is used to react with these NOx molecules at high temperatures to produce molecular nitrogen (N2) and water vapor
  • The NOx removal efficiency depends on NH3 slip

What is the catalyst Bed?

  • Base metals (V-Mo-W)
  • Honeycomb or plate corrugated
  • Blocks packaged into metal frames for handling

Schematic of Selective Catalytic Reduction System

Catalyst Operating Temperature

  • Ideal operating range: 500-800°F (260-427 °C)
  • Greater catalyst volume required outside the ideal operating range
  • Must not neglect alternative operating conditions: turndown or decoking

SCR Catalyst Chemicals

  • Vanadium-Titanium based catalyst
    • 350-850°F (177-454 °C) (most optimum 675-840°F (357-449 °C)
    • Vanadium-Titanium coating on ceramic honeycomb or metallic plate
    • Homogenous monolithic honeycomb
  • Cormetech
  • Haldor Topsoe

September 26, 2000 (8)

Structure of SCR Catalyst

  • There are different of SCR structures been employed in industries to ge uniform velocity and reaction with catalyst
  • Honeycomb, Plate type and corrugated structures reduces fluctuation in fluid flow


What is NOx Removal Efficiency

  • Typical inlet NOx  values  for in gas/oil-fired units are given below
    • Oil: 250-300 ppmvd
    • Gas: 150-200 ppmvd
  • NOx reduction efficiency should be more than 95%
  • The desired ammonia slip: NH3 slip < 5-10 ppmvd
  • Average value at Outlet NOx:  5-15 ppmvd

Correcting NOx Emissions to Standard O2 Concentration

  • In Europe NOx is frequently expressed as mg/Nm3.
  • To convert NOx reading from ppmvd to mg/Nm3,

                    NOx (mg/Nm3)= 2.05 * NOx (ppmvd)

Converting NOx from ppmvd to lb per MMBtu

  • In order to convert NOx from ppmvd to lb./MMBtu, the volume of dry products in SCF per MMBtu (DSCF) must be known
  • The equation is DSCF for gas fuels=8,740/MMBtu (DSCM 0.235/MJ )

Definition of NOx Removal Efficiency

  • The NOx removal efficiency is defined as the ratio of the concentration of  NOx removed to the amount NOx in flue gas
  •  Its depends on updo space velocity and the molar ratio of Ammonia to NOx
  • Lower space velocity higher conversion
  • High molar ratio higher conversion

Key Parameters for SCR System 

  • Type of catalyst
  • Area of catalyst surface exposed to flue gas
  • The residence time of the gas in the reactor
  • Amount of ammonia injected upstream
  • Degree of mixing of ammonia and NOx
  • Fuel sulfur content
  • Dust loading of flue gases

Space Velocity for Catalyst

  • Linear velocity
  • Space velocity-most popular measure
  • Typical space velocities- 20,000-40,000/hr (5.56-11.11/sec.)
  • Pressure Drop
    • Typically, 2-4 inches W.C. (50.8-102 mm WC)

Catalyst Surface Area

  • Performance depends upon:
    • Diffusion surface area of the catalyst
      • The larger the area, the better will be SCR performance
    • Characteristics of the catalyst
    • Cells per inch (higher cell density provides more surface area)

Oxygen Concentration

  • As indicated in the original reactions, the presence of oxygen is needed for denitrification
  • As oxygen concentration increases, catalyst performance improves till it reaches an asymptotic value
  • Effect significant only when O2 is less than 3%

Catalyst Life

  • Typical guaranteed life of 3-5 years
  • If the activity is depleted, another module could be added for increasing the activity
  • Catalyst disposal is normally taken care by the catalyst vendor

SCR Catalyst Deactivation

  •  Various mechanisms is possible for deactivation of SCR
    • Poisoning
    • Blockage
    • Physical destruction
  • When the catalyst surface or the pores of the catalyst are blocked, flue gas/NOx cannot contact the catalyst

Catalyst Poisons

  • Selective Poisoning
    • SO2 or SO3 gets absorbed on the active surfaces of the catalyst and renders it inactive
  • Non Selective Poisoning
    • Foreign substances on both catalyst and carrier
    • Dust, Soot, Oil, Mist, Phosphorus components
    • Arsenic compounds
  • Alkali Metals
    • Alkali metals chemically attach to the active
  • catalyst pore sites and cause blinding. Sodium and potassium are of prime concern

Catalyst Fouling

  • Catalyst bed pressure drop should be monitored for any fouling
  • Fouling can occur due to:
    • Burning dirty fuel
    • Sulfate deposition
    • Excessive fuel-rich operation
    • Surface dust or dirt in flue gases

Side Reactions

  • Ammonium Bisulfate and Ammonium Sulfate compounds
  • Ammonium bisulfate is a sticky compound
  • Ammonium sulfate is a dry solid
  • Ammonia also reacts with Hydrochloric acid to form Ammonium chloride

SCR System Components

  • Following major components:
    • Ammonia injection grid
    • SCR reactor
    • Ammonia control and dilution skid
  • Both aqueous and anhydrous ammonia are used as reactants

  • The main components of SCR are shown in the following figure

SCR catalyst fitting

Ammonia Flow Control Unit (AFCU)

    • Contains the equipment and instrumentation necessary for the control and injection of ammonia in the flue gas stream
    • Mainly Consists of
    • Vaporizer/Atomizer
    • Dilution Air Fan
    • Mixer

Hot AFCU - Integrated Flow Solutions

NH3 Reagent

  • Aqueous NH3
    • Contains 19-29 wt.% NH3 solution mixed with water
    • Safer to store compared to anhydrous NH3
    • No special handling or permitting
    • Typically 4-5 times expensive on basis of wt. of pure NH3
  • Anhydrous NH3
    • Contains 99.5 vol.% minimum NH3
    • Stored as compressed liquid gas (Typically 20-265 psig (1.41-18.6 kgf/cm2))
    • Classified as hazardous material & requires special handling
    • Less energy intensive as less/no water is to be vaporized

Dilution Medium

    • Fluid used to disperse reactant within the flue gas stream
    • Air or Flue Gas
    • Air used is at least 20 times the amount of NH3
    • Air blowers provide 55-60 in WC (1,397-1,524 mm H2O) pressure
    • Maximum pressure is provided to ensure good mixing
    • Two 100% capacity blowers are typically provided
    • Each blower is provided with an inlet filter/silencer and a differential pressure indicator to monitor pressure drop

Aqueous NH3– Gaseous Injection

  • Pumped from the storage tanks and mixed with a heated air stream in an NH3 vaporizer and mixer
  • Primary system components are
    • Aqueous NH3 storage tank
    • Carrier gas supply
    • NH3 gas supply
    • Filters or Strainers
    • Air heater or NH3– Air vaporizer

NH3 Vaporizer and Mixer

  • Should provide an even distribution of aqueous NH3 with dilution stream
  • Feed must be filtered and metered
  • The material in contact with NH3 must be 304L SS minimum
  • Elements and components should provide a minimum time between maintenance cleaning of 60 months


Anhydrous NH3– Gaseous Injection

AIG Design Criteria

  • Ammonia to NOx concentration at the catalyst faces is generally  maintained between ± 5-10% RMS for better NO removal efficiency
  • Mixing time of 50 to 1000 mill-seconds is taken in most the industrial applications
  • Flue gas temperature over the face of the catalyst bed is uniform, with ± 20°F (11°C)
  • Velocity distribution at SCR inlet needs to be  uniform for better mixing of ammonia and flue gas

Scope of CFD Modeling for SCR

  • The velocity and pressure drop across the ammonia and flue gas mixing units can be predicted based on CFD results
  • Species transport models in any CFD solver without reaction need to be selected.
  • After numerical simulation, we can show with contours of RMS of ammonia concentration of flue
  • CFD users must study the following subject for SCR modeling


CFD Results - Effect of AIG lances for NH3-NOx mixing

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