How to model Pollutants from Industries like NOx, SOx and Soot Particles ?
Introduction to Pollutants
Basics of combustion and Pollutions
Understanding combustion processes is important to control pollution formation. Flame characteristics and maximum temperature of flue gases depend on fuel composition, equivalent ratio, air fuel mixing mechanisms and burners configuration. For nonpremixed combustion, pollution is a matter of concern due to incomplete combustion. SOx and Soots are observed for burning of heavy hydrocarbon fuels, liquid or solid fuel containing sulfur and ash.
Before going to Pollutants modeling, CFD users or Engineers need to understand the following subjects
 Basic of Combustion: Selection of fuels, Thermodynamics, Chemical Kinetics, Fluid mad Heat Transfer, Species Transport, Slow or fast chemistry compared to flow time, Ait fuel mixing (Premixed or nonpremixed flames)
 Formation and controlling of Pollutants: Identification of harmful pollutants like NO, SOx and Soots, Reasons for formation of pollutants,Design Burners
 Combustion Modeling: Turbulence models, mechanism of chemical kinetic, Modeling of turbulent chemistry interaction (TCI) models
 Turbulent Multiphase Combustion: Majority of pollutions (NOx, SOx and soots) due to combustion of liquid and solid fuels which contains high carbon to hydrogen (C/H) ration compared gaseous fuels. For, gaseous fuels, air fuel mixing is in single phase which is more fast compared that for liquid or solid of multiphase combustion.
Major Pollutants from Combustion
Basic of pollution and reason for formation of NOx, SOx and Soots are explained the previous post. These three pollutants cause serious detrimental effects to environments. Emission norms are becoming more strict in many countries.
 Nitrogen Oxide (NOx)
 Nitrogen oxide comprises majorly NO (nitric oxide). But Two other oxide of nitrogen such as Nitrogen Peroxide and N_{2}O (Nitrous oxide) can be formed
 Nitric oxide (NO) reacts with oxygen or excess air to form NO_{2}
 NOx is Highly regulated pollutant Which can results photochemical smog, This pollutant contributes ozone depletion and acid rain
 There are four different mechanisms to form NOx: Thermal, Prompt, Fuel and N_{2}O route
 Soot Particles
 Soot particles are generally formed due to incomplete combustion or pyrolysis in rich mixture of hydrocarbon fuel with strong temperature gradients in combustion chamber
 Aggregates of Spherical carbon particles are observed within flames. Soot observed in domestic fires or chimneys contains few aggregates but large amounts of particulate fragments of coke or char are found.
 Formation of soot particle is a complex phenomenon: nucleation, surface growth, coalescence, aggregation
 Sulphur Oxide (Sox)
 Sulphur oxides (SOx) is a colourless compounds of sulphur and oxygen molecules
 Sox compounds consist of SO_{2}, SO, SO_{3 }
 Example: The smoke containing sulphur oxides emitted due to combustion of marine fuel. This smoke will further react with oxygen to form NO2 and can lead to sulphuric acid as a major cause of acid rain
Steps for CFD Modelings of Pollutants in Solver
 As the reaction pathways are extremely complicated, pollutants are modeled by semiempirical mechanisms
 CFD results are sensitive to model inputs. Unless a detailed validation is performed, pollutant modeling can be used and parametric study or scaling analysis
 After complete convergence flow and temperature, NOmodel solves species transport equations as a postprocessing simulation. NO typically appears in low concentrations
 NO chemistry does not have significant influence on the predicted velocity, pressure, temperatures and concentrations of major combustion products
 Correct NO modeling depends on accurate combustion products
 Pollutants can be modeled based on selection of variables and turbulent chemistry interaction (TCI)
 Postprocessing
 Transport equation of pollutants (like NOx and Sox) are solved without solving flow velocity, turbulent variables and composition fields
 In some cases, radicals such as H, O, can be needed in pollutant formation and they are calculated based on equilibrium or partial equilibrium assumptions
 ANSYS FLUENT and Star CCM have NOx, SOx and Soot formation models
 Fully coupled Simulation
 Detailed chemistry contains the submechanism of pollutant formation
 Transport equations of pollutant species solved together with those of major species
 Computationally more expensive than postprocessing
 Decoupled detailed chemistry
 Pollutant Postprocessing using a Chemical Kinetic Finite Rate Mechanism
 NOx Module in CFD Solver
 Solve NOmodel transport equations as a postprocessing step
 NO typically appears in low concentrations
 Thus, NO chemistry will have a negligible influence on the predicted flow field, temperatures and major combustion product concentrations
 An accurate combustion solution is a necessary prerequisite to correctly predict NOx
Overview of NOx Formation Mechanism
Thermal NOx

 Oxidation of atmospheric nitrogen
 Formed at high temperatures by the Zeldovich mechanism
 User chooses [O] model as equilibrium or partial equilibrium (recommended)
 Thermal NO formation proposed by Zeldovich Mechanism is given as
Prompt NOx

 Produced in lower temperature, fuel rich regions
 Formed predominantly through the intermediate HCN
Fuel NOx


 Oxidation of volatile nitrogen to HCN and/or NH3
 Oxidation of char nitrogen to NO, HCN and/or NH3
 FLUENT solves an additional transport equation for HCN and/or NH3 mass fraction

NOx from intermediate N2O

 Favorable under elevated pressure and oxygen rich conditions
 Additional transport equation for N2O is solved if steady state approximation is not used
Method to reduce NOx in power plant
 Ammonia or Urea is injected over flue gas to reduce NOx.
 The ammonia reacts with NO. N2 and H20 are formed at the outlet
 Ammonia injection is placed after furnace
Transport Equations for NOx Modeling
Species transport for NOx and NH3
 For thermal and prompt NO predictions, only the NO mass fraction transport equation is solved
 For fuel NO mechanisms, transport equations for NO and HCN and/or NH3 are necessary
 In above equation species j can be NO, HCN, NH3
 The mean reaction rate terms need to be modelled
 Modelling Turbulence chemistry interactions: PDF approach for turbulence chemistry interaction
 Reaction expressions may be functions of species concentrations which are not calculated in the combustion simulation (O and OH radicals)
 Customized UserDefined Kinetics can be used for NOx Rate using UDF in ANSYS FLUENT
Modeling of TurbulentChemistry Interaction (TCI)
 The relationship of NOx formation rate with temperature and species concentrations is highly nonlinear
 Temperature and composition fluctuations are considered using the probability density functions (PDF) which considers their statistical fluctuations in the turbulent flow of flue gases after combustion
 The PDF approach is used for obtaining the mean turbulent reaction rates (ω) after the integration over suitable parametric range:
 where ω is the NOx formation rate and V1, V2, … are temperature and/or the various species concentrations present
 P is the probability density function
Calculation of Mean NO Production rate by the PDF Approach
Single Variable Approach
 For a single variable approach, the variable V1 can be mixture fraction or temperature
 Mixture fraction is recommended for nonpremixed (diffusion flame) and partially premixed model
 Temperature is recommended for species transport model
Two variable PDF
 For a two variable PDF, choices for V1 and V2 include
 Fuel Mass Fraction, temperature
 Temperature, O2 Mass Fraction
 Fuel Mass Fraction, O2 Mass Fraction
The PDF Approach for NOx Calculation
 PDF’s comprises mean and variance of species variables which are assumed to be twomoment beta functions
 During simulation, mean values are computed from the data of initial combustion
 For species transport model, the variance of variable V is calculated in either of two ways
 The transport equation of variance for any variable V is given as follows
 By approximating same production and dissipation rate in the above transport equation, the following algebraic expression is presented
NO Reburn Model
 NO can be reduced in fuel rich zones by reaction with hydrocarbons
Modeling of NO Reduction
 10% – 20% reburn fuel added above the main combustion zone
 After reburn zone, additional air added to complete combustion
 Significant reductions possible (> 50%) when flue gas consists of high initial NO
 Reburn rate is the sink term NO transport equation. It is a strong function of temperature and concentrations of NO, CH, CH_{2}, CH_{3}, O_{2 }and HCN. Reburning of NO occurs for when temperature of flue in the range of 1600K – 2100K
 The reburn rate of NO can be computed with Eddy dissipation model (EDM) considering partial equilibrium for hydrocarbon (CH) radicals
NO Reduction by Selective NonCatalytic Reduction (SNCR)
 NH_{3 }or Urea is the reductant injected to convert NO to N_{2}
 Simplified pathway for NH_{3 }– SNCR
 Reductant are oxidized to NO and selectivity of reduction reactions decreases with an increase in flue gas temperature
 If NH_{3 }is added as an intermediate species for fuel N. Natural SNCR can be observed if the SNCR model is not activated
 The temperature range for SNCR: 1073 K < T < 1373 K
https://www.afs.enea.it/project/neptunius/docs/fluent/html/ug/node643.htm
NO Reduction by SNCR with UREA injection
 Simplified seven steps reaction mechanism can be used for modelling of SNCR with Urea injection
 Additional species transport equations need to be solved for Urea, HNCO and NCO are solved
 Urea injection is defined as a multi component mixture droplet. Urea will be released in the gas phase based on the Arrhenius reaction with devolatilization law
 Two methods for the Urea decompositions are set to NH_{3} and HNCO
 Rate limiting reaction
 User specified decomposition
 For simplified modelling Urea injection (CO(NH_{2})2), droplets are considered as combusting particle or wet combustion
 For wet combustion, initially water will be released following phase change with evaporation or boiling law
Numerical Solution Strategy in CFD solver
 Steady state Solver
 Ensure the convergence of the combustion solution with high resolution of solid angles for radiative heat transfer
 Disable all other equations like momentum, turbulent variables, energy, multiphase and combustion
 Solve only for NOx equations
 Set pollutant models for with NO mechanism
 Refer ANSYS FLUENT User guide for NOx set up
 with unit value as under relaxation factor (URF = 1) and solve NOx equation
 Set model formation parameters in ANSYS FLUENT
 Unsteady Solver
 Solve the NOx equation at the end of the time step by using the pollutant post processing option
 Set pollutant with unit under relaxation factor, URF = 1
 Turbulencechemistry interaction
 Use the mixture fraction option for nonpremixed and partiallypremixed combustion models
 Use the temperature option for species transport model
Case study for NOx: CFD Results
 CFD results are presented with and without urea injection for boiler or furnace
 Without ammonia or urea injection, the concentration of NO is higher in top portion of boiler
 The follwoing CFD results show that, NO concentration is less after ammonia injection in boiler
SOx Modeling
 If fuel contains Sulfur then SOx is formed during or after combustion
Soot Modeling
 Aircraft engines burning hydrocarbonbased fuels emit Particulate Matter (PM) emissions
 These fine (PM2.5) particles have a great impact on human health. They affect lungs and are carcinogenic
 They are also known to affect the climate/ global warming
 Soot deposition on combustor liners can adversely affect durability, heat transfer and performance of combustor
Main Soot Models in Fluent
 Soot affects the radiation absorption. Enable SootRadiation option in the Soot panel
 Soot Models: 1) MossBrookers Model, 2) Method of Moments
MossBrookes model
 Solve transport equations for the soot mass fraction and (normalized) number density
 MossBrookes model was originally developed for methene CH_{4}
 For higher hydrocarbon MossBrookesHall extension is available
 Two equation model
 Soot mass fraction
 The Nuclei concentration ‒ Constructed and validated for methane flames
 Soot formation is modelled using the species transport equations solved considering concentration of normalized soot radical nuclei ( b_{nuc}) and mass fraction of soot particles (Y_{soot })
 In above equation, M presents the soot mass concentration and N is the soot particle number density
 These variables (M and N) are used to model soot formation such as inception, coagulation, growth and oxidation rates
 The equation for soot mass concentration (M)
 As per Moss–Brookes model, the inception rate is a linear function of the local concentration of the acetylene (C_{2}H_{2})
 The extended model of soot formation, the inception rate considers the rate of formation of two and threeringed aromatics from radicals of acetylene, benzene, and phenyl
Selection of MossBrookesHall Model in FLUENT
 Two equation model:
 Soot mass fraction
 The Nuclei concentration
 Nucleation:
 Precursors : C_{6}H_{6}, C_{2}H_{2} , C_{6}H_{5}
 Surface Growth
 SGS species: C_{2}H_{2}
 Oxidation: Lee model: OH radical, O_{2}
 MossBrookesHall Model: Setting of Soot model in FLUENT
Method of Moments for Soot Modeling
Governing Equation for Method of Soot Moments
 The soot size distribution due to incomplete combustion is defined based on the concentration moment of the particle number density function for a given number of moments and recast as
 In above equation, M_{n} presebts soot size distribution for n^{th} moment and N_{t} is the particle density of the size class “i”.
 Soot concentrations are calculated in the physical space by solving the following transport equation
 The source term (S_{n}) considers the altogether of nucleation, coagulation, surface growth, and oxidation source term.
 This term is related to soot statistics and can be determined as
 Nucleation: it is generally called as a coagulation process between two precursor species for soot
 Coagulation: the assumption is a coalescent coagulation. The particles after the collision will remain a sphere with an increased diameter. The source term related to coagulation for the n = 0_{th} moment
For higher moments of soot particles, n ≥ 2
Where, α_{i,j }is termed as the collision frequency
 Surface Growth and Oxidation: due to nucleation the soot formation is negligible as compared to the overall soot formation. The major route of the soot formation is driven by surface growth. Simultaneously, the soot particles can lose its mass due to oxidation with O_{2} or OH species. Mainly, the chemical kinetics controls the overall process of surface growth and oxidation and it is difficult to model higher order of complexities
Setting of Moment Model in ANSYS FLUENT

 Interpolative closure for soot diameter and surface area
 3 to 6 moment transport equations solved in ANSYS FLUENT
 Source terms consider realistic chemical and physical processes Available with species transport and flamelet models
 Coupled to the flow through radiation heat transfer
 Aggregation can be included
Case study for Soot CFD results
 The follwoing figures shows the concentration of carbon and soot particles for coal fired burners
 Higher concetration of soot are away from inlet due to delayed or improper mixing process
Summary
 Understanding combustion process, fuel composition its CFD modeling is important for pollution modeling
 Before setting the pollution simulation, first ensure numerical convergence of combustion simulation including radiative heat fluxes
 To save simulation cost, pollutants are solved as species transport in a commercial finite element method (FVM) solver based on single or two variables PDF functions
 Selection of NOx mechanism and correct CFD input date like equivalent ration, carbon index, are important
 SOX and Soot models can simulated for liquid or solid fuels using ANSYS FLUENT
References
 Rohit Saini and Ashoke De , Soot Predictions in Higher Order Hydrocarbon Flames: Assessment of SemiEmpirical Models and Method of Moments, Springer Publication (2017)
You Tube Video Lecture
 You tube video lectures for same topic of pollution modeling
 Yout tube Video for ANSYS FLUENT set up for NOx, SOx
Well written and clearly explained how to select NOx Soot models in ANSYS FLUENT