How to Design Selective Catalytic Reduction (SCR)
for NO_X

 

 

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+ O₂  = 4 N₂+6H₂O               (1) Dominant Reaction

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

NO + NO₂+ 2 NH₃  =  2 N₂+3H₂O                (5)

2NO₂+4 NH₃+O₂  = 3 N₂+6H₂O                  (3)

6NO₂+ 8 NH₃  = 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

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

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

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

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
  • Turbulent flow physics and its modeling
  • Basic of emissions and formation: we need to understand the level of NOx formed
  • Mass Transfer: species transport modeling
  • Understanding of Basic CFD Models: Numerical schemes
  • Ammonia and NOx mixing method at the inlet of SCR