Combustion and Reactions Chapter 7: Multiphase Combustion and Pollutant Formation
This chapter develops the modeling framework for multiphase combustion systems involving liquid sprays, coal particles, evaporation, devolatilization, heterogeneous reactions, and pollutant formation. The central challenge is coupling particle dynamics, turbulence, chemistry, heat transfer, and radiation inside reacting flows. The chapter introduces Discrete Phase Modeling (DPM), spray combustion physics, coal combustion mechanisms, NOx formation pathways, and selective catalytic reduction (SCR) systems used for emission control.
Context and Motivation
Many industrial combustion systems involve:
Liquid droplets
Solid particles
Coal dust
Fuel sprays
Ash and soot
Pollutant formation
Examples include:
Diesel engines
Coal-fired power plants
Gas turbines
Industrial furnaces
SCR aftertreatment systems
Unlike single-phase gaseous combustion:
Fuel exists initially as discrete particles or droplets
Additional processes appear:
Injection
Atomization
Evaporation
Devolatilization
Char oxidation
Particle heating
Radiation coupling
The combustion process therefore becomes a strongly coupled multiphysics problem.
1. Discrete Phase Modeling (DPM)
Core Philosophy
The most important framework for dilute particle flows is:
Discrete Phase Modeling (DPM)
In DPM:
Gas phase solved in Eulerian framework
Particles tracked individually in Lagrangian framework
This creates:
Eulerian–Lagrangian coupling
Continuous vs Dispersed Phase
Two phases coexist:
Continuous Phase
Gas flow
Solved on mesh
Discrete Phase
Particles or droplets
Tracked as trajectories
Particles exchange:
Mass
Momentum
Energy
with the surrounding gas.
Why DPM Works
DPM assumes:
Particle volume fraction is relatively low
Particle–particle interaction negligible
Typically:
Volume loading below roughly 10%
Physically:
Particles “feel” the flow
But particles do not strongly interfere with each other
One-Way vs Two-Way Coupling
One-Way Coupling
Gas affects particles
Particles do not affect gas
Valid when:
Particle loading small
Two-Way Coupling
Particles also modify:
Flow momentum
Temperature
Species concentrations
Required when:
Heat release significant
Evaporation important
Mass loading large
This is essential for realistic combustion simulations.
2. Particle Dynamics and Life Cycle
Particle Motion
Particle trajectories are determined by:
Drag
Gravity
Pressure-gradient forces
Turbulent dispersion
Additional body forces
Particles continuously evolve as they travel.
Particle Life Cycle
A combusting particle typically undergoes:
Injection
Heating
Evaporation or devolatilization
Chemical reaction
Burnout or escape
The particle state changes continuously:
Diameter
Temperature
Velocity
Composition
Turbulent Dispersion
Particles entering turbulent eddies experience:
Random lateral dispersion
Stochastic trajectory fluctuations
This strongly affects:
Mixing
Residence time
Combustion distribution
Discrete Random Walk Model
One common approach:
Particles interact statistically with turbulent eddies
Physically:
Simulates random turbulent scattering
Important insight:
Too few stochastic samples produce noisy source terms and convergence problems
3. Spray Combustion
Spray Physics
Liquid fuel combustion begins with:
Injection through nozzle
Atomization into droplets
Droplets then:
Heat up
Evaporate
Mix with oxidizer
Ignite
Combustion therefore depends heavily on:
Spray quality
Droplet size distribution
Injection velocity
Turbulence
Atomization
The liquid jet breaks into droplets due to:
Aerodynamic instabilities
Surface tension effects
Turbulent stresses
Smaller droplets:
Evaporate faster
Mix faster
Burn more efficiently
Droplet Heating and Evaporation
The droplet receives heat from surrounding gases.
This energy:
Raises droplet temperature
Eventually causes phase change
Evaporation rate depends strongly on:
Temperature
Relative velocity
Surface area
Spray Combustion in Engines
The ECN Spray A diesel case illustrates the full process:
High-pressure liquid injection
Spray penetration
Evaporation
Autoignition
Turbulent combustion
Important physical insight:
→ Ignition delay depends strongly on:
Ambient temperature
Mixing rate
Detailed chemical kinetics
Chemistry Complexity
Detailed diesel chemistry may involve:
Hundreds of species
Hundreds of reactions
This creates:
Severe numerical stiffness
Acceleration methods such as:
ISAT
Dynamic mechanism reduction
become necessary
4. Coal Combustion
Coal Flame Structure
Coal combustion is fundamentally multiphase.
The fuel particle undergoes multiple stages:
Heating
Drying
Devolatilization
Char combustion
The volatile gases burn in the gas phase, while remaining solid carbon burns heterogeneously
Devolatilization
As coal heats:
Volatile compounds are released
These gases:
Mix with air
Burn similarly to gaseous flames
The remaining particle becomes:
Char
Char Combustion
Char burns at the particle surface through heterogeneous reactions.
The reaction rate depends on:
Oxygen diffusion
Surface kinetics
Temperature
At high temperatures:
Diffusion often limits the process
Swirl-Stabilized Coal Burners
Industrial coal burners often use:
Strong swirl
Swirl creates:
Central recirculation zones
These zones:
Trap hot gases
Improve ignition
Stabilize combustion
Coal Combustion Modeling
Important submodels include:
Devolatilization model
Char oxidation model
Radiation model
Turbulence–chemistry interaction
Several devolatilization approaches exist:
Constant rate
Single-rate kinetics
Competing-rate models
CPD models
Sequential Solution Strategy
Coal combustion simulations are numerically difficult.
A stable workflow usually proceeds gradually:
Cold flow
Reactive flow
Radiation
Particle radiation
This improves convergence robustness
5. Radiation in Combustion
Why Radiation Matters
Combustion temperatures are high enough that:
Radiative heat transfer becomes dominant
In furnaces:
Radiation may dominate over convection
Participating Media
Important radiating species:
CO2
H2O
Soot
Coal particles
Particles strongly modify:
Temperature distribution
Heat fluxes
Flame stability
Particle Radiation
Coal particles:
Emit
Absorb
Scatter radiation
Ignoring this effect can significantly distort:
Furnace temperatures
Heat transfer prediction
6. Pollutant Formation
Importance of Pollutants
Combustion generates:
NOx
CO
Unburned hydrocarbons
Soot
NOx is one of the most important pollutants because:
Strongly regulated
Harmful to health
Contributes to smog and acid rain
7. NOx Formation Mechanisms
Several pathways produce NOx.
Thermal NOx
Produced at:
High temperatures
Mechanism:
Nitrogen from air reacts with oxygen radicals
Strong temperature dependence:
Small temperature increase → huge NO increase
This is why:
Peak flame temperature is critical
Prompt NOx
Occurs in:
Fuel-rich regions
Forms rapidly through:
Hydrocarbon radical chemistry
Usually important near:
Flame fronts
Fuel NOx
Occurs when:
Fuel itself contains nitrogen
Very important in:
Coal combustion
Fuel nitrogen converts into:
HCN
NH3
NO
N2O Mechanism
Relevant mainly for:
Lean flames
High-pressure systems
Usually smaller contribution in conventional flames.
8. Pollutant Modeling Philosophy
Postprocessing Assumption
Pollutant concentrations are usually:
Much smaller than main species
Therefore:
Pollutants assumed not to significantly alter main flow field
This allows:
→ Pollutant calculations using frozen combustion fields
This greatly reduces computational cost.
Sensitivity to Temperature
NOx prediction is extremely temperature-sensitive.
Even small temperature errors may produce:
Large NO prediction errors
Therefore:
Radiation
Turbulence
Chemistry
Boundary conditions
must all be reasonably accurate.
Engineering Reality
Absolute NOx prediction is difficult.
However:
CFD is extremely valuable for:
Parametric studies
Design optimization
Trend analysis
9. Selective Catalytic Reduction (SCR)
Purpose
SCR systems reduce NOx emissions after combustion.
Widely used in:
Diesel engines
Power plants
Industrial exhaust systems
Operating Principle
A reducing agent (typically ammonia or urea) reacts with NOx over a catalyst surface:
NOx converted into:
Nitrogen
Water
SCR Process Stages
Typical sequence:
Urea injection
Atomization
Evaporation
Thermal decomposition
Mixing with exhaust
Catalytic reduction
Importance of Mixing
Uniform ammonia distribution before catalyst is critical.
Poor mixing causes:
Low NOx conversion
Ammonia slip
Catalyst inefficiency
Porous Catalyst Modeling
The catalyst is usually represented as:
Porous medium
This captures:
Pressure loss
Flow redistribution
Species conversion
without resolving tiny catalyst channels directly.
10. Numerical and CFD Aspects
Mesh Requirements
Critical regions:
Injection zone
Flame stabilization region
Shear layers
Near-wall heat transfer
Catalyst regions
Sprays often require:
Strong local refinement
Tracking Parameters
Particle tracking depends on:
Time step
Step length
Maximum tracking steps
Too large time steps:
Inaccurate trajectories
Poor evaporation prediction
Boundary Conditions
Particles reaching walls may:
Escape
Reflect
Trap
Form liquid film
Wall interaction models become important in:
Engine sprays
Fuel impingement
SCR systems
Steady vs Unsteady Tracking
Steady tracking:
Lower computational cost
Unsteady tracking:
Required for:
Spray breakup
Transient combustion
Pulsed injection
Double Precision
Combustion problems often require:
Double precision
Especially for:
High temperature gradients
Stiff chemistry
Complex geometries
Physical Interpretation and Engineering Intuition
Combustion Efficiency Depends on Mixing
Whether in sprays, coal flames, or SCR systems:
→ Mixing controls performance.
Particle Size Is Critical
Smaller particles:
Heat faster
Evaporate faster
Burn faster
Large particles:
Longer residence times
Incomplete combustion risk
Swirl Is Primarily a Stabilization Tool
Swirl generates:
Internal recirculation
This:
Increases residence time
Recirculates hot products
Stabilizes flames
Radiation Cannot Be Ignored
At furnace temperatures:
Radiation strongly modifies temperature fields
Ignoring it:
Produces unrealistic flame behavior
Pollutants Are Extremely Sensitive
NOx formation depends on:
Local temperature
Oxygen availability
Residence time
Very small modeling errors may strongly affect emissions prediction.
Applications
Diesel engine combustion
Coal-fired power plants
Industrial furnaces
Gas turbines
SCR aftertreatment systems
Spray combustors
Biomass combustion
Limitations and Assumptions
DPM
Assumes dilute dispersed phase
Limited particle–particle interaction
Spray Models
Sensitive to breakup assumptions
Initial droplet distribution uncertain
Coal Models
Coal properties highly variable
Devolatilization models approximate
NOx Models
Extremely temperature sensitive
Strong dependence on combustion accuracy
SCR Models
Catalyst chemistry simplified
Mixing assumptions important
Study Priorities
If short on time:
DPM philosophy (Eulerian–Lagrangian coupling)
One-way vs two-way coupling
Spray evaporation process
Coal devolatilization and char combustion
Swirl stabilization
Radiation importance
Thermal vs prompt vs fuel NOx
SCR operating principle
Key Takeaways
Multiphase combustion couples particle physics with reacting flow dynamics.
DPM tracks particles individually while solving gas flow in Eulerian form.
Spray combustion depends heavily on atomization and evaporation.
Coal combustion involves devolatilization followed by char oxidation.
Swirl stabilizes flames through recirculation zones.
Radiation becomes dominant in high-temperature furnaces.
NOx formation is extremely sensitive to local flame temperature.
SCR systems reduce NOx through catalytic reactions using ammonia or urea injection.
Accurate multiphase combustion CFD requires careful coupling of turbulence, chemistry, radiation, and particle dynamics.