Table of Contents
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+ 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)
- Diffusion surface area of the catalyst
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