Table of Contents
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 (NOx) 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-NOx) 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
- Why do you mix for SCR
- How to find the mal-distribution of NH3 in mixing
- How Do You Mix NH3 with Flue Gas
- Where Do You Mix
- Ammonia-to-NOx Ratio
- NOx stratification
- Ammonia Injection
- Fine Grid Ammonia Injection
- 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
- SMV side-Mixer for NH3 evaporation
- Patented Sulzer Ammonia Injector, no trimming, no plugging, no maintenance
- SMV Flue Gas Premixer for NOX and Temperature Distribution
- Main Mixer for Ammonia mixing
- 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 (UIC)
Where Ci 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
- P.E. Kanthan Rajendran, Ammonia Injection and Mixing Systems 101, 2018 NOx Combustion CCR Round, St Louis MO (2018)
- Refer to the handbook of Sulzer, Mixing and Reaction Technology, 2015