Ammonia and NO  (NH3-NO) Mixing for SCR
Design of  DeNOx Mixer

Introduction

• Combustion due to fossil fuels results in various forms of pollutions like CO2, NOx, SOx , particulate matter which is discussed in the posts of basic of pollitions.
• Nitrogen oxide (NO_x) pollution has been significant. Many industries are working on various techniques to reduce  NOx emissions from fossil fuel-fired furnace
• Selective catalytic reduction  (SCR) denitration (de-NO_x) technology is a well-proven denitration method
• The SCR NO removal system is placed after the economizer of boiler

 

We will discuss the following topic in details

• Ammonia injection and Its location
• SCR Performance Goals are discussed
• Frequently asked questions for SCR Design
  1. Why do you mix for SCR
  2. How to find the mal-distribution of NH3 in mixing
  3. How Do You Mix NH3 with Flue Gas
  4. Where Do You Mix
  5. Ammonia-to-NOx Ratio
  6. NOx stratification
• Ammonia Injection
  1. Fine Grid Ammonia Injection
  2. Coarse Grid Ammonia Injection
• Vaporized Ammonia Injection vs Direct Injection
• Different Methods of Mixing
• Scope of CFD Modeling for Optimum Design of mixing

Location of Ammonia Injection Grid (AIG) and Mixing Systems

• The AIG system is placed after the economizer and before the air preheater in the boiler of the thermal power plant.

• The location of the ammonia injection grid (AIG) is shown below

SCR Performance Parameters

The following are typical performance goals of SCR systems in industries

• Uniform ammonia-to-NOx ratio
• Uniform velocity at AIG and catalyst
• Vertical flow entering catalyst
• The uniform temperature at the catalyst
• Capture LPA with screen/baffles
• Minimize pluggage potential
• Minimize pressure loss
• Minimize erosion potential ammonia Injection & Gas Mixing to Achieve Molar Ratio Distribution Goals

Why Do You Mix for SCR?

• Ammonia must be dispersed and mixed thoroughly with the flue gas to maximize contact between the reactants
• NOx removal rate is highly dependent on the level of mixing

How to find the Mal-distribution of NH3 in mixing

Distributions Descriptions for NH₃ and NOx

• The non-uniform distribution is commonly stated as X% of the Data falling within Y% of the Mean (Arithmetic Average):
  • 80% of points to be within 10% of the Mean
  • 80% of points to be within 20% of the Mean
  • 85% of points to be within 15% of the Mean
  • 100% of points to be within 25% of the Mean
• Often more than one is applied to a single distribution goal: i.e. velocity or NH3/NOx molar ratio at a given location
• System blending and reactor performance can be more easily correlated when the distributions are adequately defined by a single parameter

Normal Distribution Relationship of mixing

• Need to measure many points to capture ±3 σ

• The relative Effect of velocity, mole ratio, and temperature on the SCR Reactor Efficiency is about 70% 

How Do You Mix NH3 with Flue Gas?

• Control the flow streams at the injection location
  • Multi-point injection
  • Nozzle design
  • Diffusion + turbulence
• Churn up the flow after the injection
  • Induce high turbulence
  • Create shear forces
  • Generate swirls or vortices

 Where Do You Mix?

• Ammonia NOx mixing system is placed upstream of AIG
• In some cases, various maxing plates or baffles are placed after AIG to get uniform mixing at upstream of SCR

What is Ammonia-to-NOx Ratio

• Ammonia-to-NOx ratio at the catalyst inlet plane should be “uniform”
• Allows optimal NOx reduction with a minimum ammonia slip
• Typical goal is %RMS < 5% or deviation within +/-5% of mean
• Can be highly influenced by velocity patterns

Poor Mixing 

• When mixing of ammonia and NO is not uniform due to nonuniform velocities

Better Mixing

• Using baffles or mixing plates better mixing is possible

 

Scope of CFD Modelings to Design the AIG Mechanism

• CFD simulation helps to prefix the concentration of ammonia and NOx after mixing.
• The following figure shows the comparison of existing and proposed system

 

• The comparison between the existing and proposed system at the inlet of SCR is shown below.
• In the existing system there is a poor mixing of NH3 and NO due to less number of AIG lances and nozzles

 

What is NOx Stratification

• NOx is not necessarily uniform at the boiler exit and it is a function of
  • Boiler design
  • Burner air flow balance
  • Coal pipe balance
  • Mills out-of-service
• Solutions
  • Mix the NOx prior to the NH3 injection “Pre-mixer”
  • Mix the NOx and the NH3
  • Tune the NH3 to the NOx profile  Consistency overload range important

 

Ammonia Injection Techniques

• Two basic strategies are used for ammonia injection in SCRs
  • Dense grid of injection pipes
  • Coarse grid of injection pipes with mixers
• The following are examples of AIG grids and mixing plateau after it

Dense Grid Ammonia Injection for Uniform Mixing

• Many injection lances with multiple nozzles per lance
  • Depending on SCR size, could have 50-100 lances per reactor
  • Typically 6-10 nozzles per lance
  • Hundreds of discrete injection points
• Often no mixer or only a “local” mixer
• Lances grouped into zones for tuning
• Benefits of dense grid injection
  • More tunable for maximum NOx reduction
  • No negative influence on velocity or fly-ash distribution at catalyst

 

The disadvantage of Dense AIG Grid 

• Plug gage of nozzles
• Requires very good velocity profile at AIG location
• Tuning is not as predictable as sometimes envisioned
  • Velocity distribution issues
  • Unequal flow per nozzle
  • The low resolution of the reactor outlet sample grid
• Valve issues over time

 Coarse Grid Ammonia Injection

• Fewer injection lances compared to the dense grid by a factor of 5-10
  • Depending on SCR size, could have 5, 10, 20 lances per reactor
  • Some systems have just 1 injection point per lance
  • Others have multiple nozzles per lance (2 to 10)
• Lances located immediately upstream of a static mixer
• Often multiple stages of static mixers
• Benefits of coarse grid injection
  • Fewer nozzles and larger openings less prone to pluggage
  • Mixing and high turbulence reduce the sensitivity of gradients
    • Does not need as much tuning?

More consistent performance over the load range

Coarse Grid AIG Issues

• The following are examples of coarse AIG grid issues

 

Comparison of Vaporized Ammonia Injection and Direct Injection

• Vaporized Ammonia Injection
  • Utilizes vaporizer skid to get ammonia into the gaseous form prior to injection
  • Need to ensure ammonia is properly vaporized and mixed with dilution air ○
  • More common but higher capital cost
• Direct Injection
  • inject aqueous ammonia directly in liquid form without dilution air or vaporization
  • relies on heat from flue gas for vaporization
  • Need special spray nozzles to ensure proper vaporization and mixing
  • The major concern is about liquid ammonia impingement on walls, mixer

Types of Mixers

• Shear Mixers
• Swirl-Shear Mixers
• Vortex Mixers

 Configuration of static mixers

• The swirl-type mixer is more effective than the line-type mixer with respect to the enhancement of mixing performance
• The swirl-type mixer with a vane angle of 45º is the most suitable model

Swirl-Shear Mixers

 Vortex Mixers

• Different geometries are used to create vortex flows for better mixing of NH3 and NOx
• This vortex plate can result in high-pressure drop

 CFD Modeling for effective and optimum design

• CFD represents an efficient and effective tool to optimize the design of the Mixer/Injector configuration as well as of the flow conditioning internals like guiding vanes.
• CFD helps a lot to design and optimize a test model configuration.

Model mixing

• Model mixing is used to tune the Mixer/Injector configuration and the flow conditioning internals
• Model mixing allows determining the basis for scale-up and the guarantee values (typically NH3 homogeneity, pressure drop, etc.), as well as dust deposition behavior

Case study Sulzer Mixer-Injector for SCR

• The Sulzer SMV Mixer in combination with the patented Sulzer Ammonia Injector represents a proven, highly efficient, and reliable technology to distribute ammonia into flue gas in front of the SCR Reactor of coal or gas-fired boiler applications
• Typical Layout for Coal Fired Boiler

 

Static Mixer in the SCR process

1. SMV side-Mixer for NH3 evaporation
2. Patented Sulzer Ammonia Injector, no trimming, no plugging, no maintenance
3. SMV Flue Gas Premixer for NOX and Temperature Distribution
4. Main Mixer for Ammonia mixing
5. Even distribution of NOX, Ammonia, and Temperature at the catalyst face

• Inspection on an SMV Mixer with Sulzer Ammonia Injection

 

CFD Modeling of Pressure Drop across SCR

Pressure Drop Equation

• The catalyst is the core of SCR reactor.
• For, a honeycomb type SCR catalyst filter is adopted. If the catalyst filter is constructed physically from a numerical simulation without any simplifications, the grid of the model will reach a level that is beyond the calculation capabilities of most computing systems.
• Therefore, an approach of a porous media model is adopted to simulate the flow in the catalyst filter
• The mass and momentum transfer in the radial direction are elected when the axial velocity is dominant in the SCR filter (Jeong et al., 2005).
• In the simple model, the pressure change (drop) is calculated by a combination of Darcy’s Law and an additional inertial loss term along with the SCR filter in ANSYS FLUENT

where ∆p is the pressure drop (pa), µ is the laminar fluid viscosity, α is the permeability content of the medium,  C₂ is the pressure-jump coefficient, v is the velocity  (ms/s) normal to the porous face, and ∆m is the thickness of the porous medium.

• To obtain the two unknown values, α and C2 , a quadratic equation of pressure drop is derived from the calculation of the simple part cell analysis as shown in the following figure.
• Pressure drops  in the SCR filter is shown with respect to inlet gas velocity

 

Effect of Mixer Geometry on Turbulent flow Characteristic

• Turbulent flow occurs when instabilities in a flow are not sufficiently damped by viscous effect and the fluid velocity at each point in the flow shows random fluctuations with time and space
• Turbulence is described as fluctuations in a fluid flow. When working with chemicals as in an SCR reactor, typically a high level of flow fluctuations is better for good mixing of urea-water-solutions (UWS) with exhaust gases.
• It is common to define the relative turbulent intensity for the velocity as follows

Where,

• u′ is the root-mean-square (RMS) of the turbulent velocity fluctuations ( Uavg ) at a particular location over a specified period of time
• is the average of the velocity for the instantaneous value (U) at the same location over the time period.
• The standard deviation of the temporal variation of Reynolds averaged velocity (σ ) and its relative intensity ( U = σ /Uavg ) are adopted.

 

Relationship between pressure drop and uniform flow

• A uniform flow means less variation in velocity and Urea-water-solution variation in the upstream of catalyst filter is necessary to get maximum performance  (de-NOx mixer) and a minimum NH3 slip in the SCR system.
• The flow uniformity index is commonly used to describe the degree of flow distribution  for after-treatment applications
• In this article, a similar definition is adopted to evaluate the distribution degree of water concentration at a certain location along the pipe
• Uniformity index for the water concentration (UI_C)

Where C_i is the local concentration of water, C is the average concentration of water, and n is the number of cells. Higher UIc present  better fluid mixing

Summary

• Different configurations of AIG lances and mixing plates have developed to improve the mixing of Ammonia with flue gas containing NOx
• Uniform velocity at the inlet of SCR ensures better NOx removal efficiency
• The desired RMS in deviation of NH₃/NO is a 5% at the inlet of SCR for better mixing
• Due to adding a mixing mechanism due to baffles or plates, overall pressure drops increase compared the existing system

References

1. P.E. Kanthan Rajendran,  Ammonia Injection and Mixing Systems 101, 2018 NOx Combustion CCR Round, St Louis MO (2018)
2. Refer to the handbook of Sulzer, Mixing and  Reaction Technology, 2015