Basics of Emissions and Its Control

 

Understanding Basics of Emissions and

 Its CFD Modeling 

(Modeling the Unwanted combustion to understand Environmental Pollution)

  

1. Introduction to Emissions

  • The term of Emissions is  used to describe the gases and particles which are put into the air or emitted by various sources.
  • An emission comprises one or more specific greenhouse gases or air pollutants originated from all source categories in a certain geographical area and within a specified time span
Is Chinese industry polluting India? Beijing's media outlet hints ...
Thermal Power Plant
 Hot gases and particles emitted from the power plant
  • Details of pollutants emitted from power plant and automobile  have been explained the following video:

 

2. Major sources of Pollutant Emissions 

  • Natural sources of emissions: volcano, wild fires, forest
  • Area /region based emission: construction works, farm fields burning
  • Mobile /transport vehicle: cars, trucks ,buses and aero-planes
  • Stationary Industries: Coal or oil Thermal power plant, Cement plant, chemical and processing industries, Oil refineries
Pie chart of total U.S. greenhouse gas emissions by economic sector in 2017. 27 percent is from electricity, 28 percent is from transportation, 22 percent is from industry, 12 percent is from commercial and residential, and 10 percent is from agriculture.

3. Quantification of Emissions

  • Emission index” (EI): The EIi for a certain chemical species is defined as the ratio of the mass of the pollutant species i to the mass of fuel burned
  • Assuming that CO2, CO, and unburned hydrocarbons are the major combustion products and all other species are negligible Emission index” (EI)
  •   EI for automobile or power generation applications: mass species emission(MSE, units g/(kW-h)) is used and its definition is

where  mf is the fuel mass flow rate (kg/h), W is the brake power generated (kW)

  •  For furnace applications: the level of pollution is often expressed as
  •      The natural gas industry uses the following expression
 

4. Classification of Pollution 

a) Outdoor Pollution:

  • There are many resources of pollution
  • Natural resources mountain volcano, atmospheric air flow with dusts, land sliding
  • Man made resources: transport vehicle, stationary power plant, chemical and  process industries

Chapter 2 indoor and outdoor air pollution

  • Industries are examples of outdoor pollution
Exxonmobil invests £140m after local tempers flare | Scotland ...
 
b) Indoor Pollution:
  • Airborne particle in home
  • Flue gases and dust generated in kitchen due to cooking and unwanted vegetables
  • Dust from broken walls and floors
  • Dust from the sandal, shoes  etc.
Indoor air quality | Catalyst Magazine
Don't Let Indoor Air Fool You Silently | KENT Blog
Fig. 4  Examples of indoor pollution
 

Major Types of Air Pollutants

 Air Pollutants According to the level of danger

 5. Key Parameters Controlling the Pollutants

 

5.1 Key Parameters controlling the Pollution

  •     Temperature and residence time, τres, are two important parameters influencing the formation of pollutants. Temperature affects the onset of certain chemical reactions and consequently, the formation of certain chemical species.
  •    Since combustion temperature is a strong function of mixture composition, i.e. equivalence ratio (ϕ), pollutant formation can be influenced by controlling reactant mixture composition
  •        In order to complete chemical reactions in a combustion device, sufficient time must be provided for the reactants to react, i.e. the reactants must remain in the combustor longer than the time they need to react.
  •    Residence time (τres): it is the amount of time that reactants reside inside the combustor
  •     Chemical time (τchem): it is the amount of time that the reactants need to react. Chemical time is inversely proportional to reaction rate which depends on temperature and mixture composition. Typical values of chemical time are of the order of milliseconds
 

 5.2 Effect of relative magnitudes of residence time and chemical time

  •        τres >= τchem : Combustion is completed, i.e., most CO is oxidized to form  CO2. There will be low CO emission at the exit of the combustor.
  •  Mole fraction of species for τres >= τchem
 
  •         τres  τchem : Combustion is incomplete. CO emissions will be high.
  •          Mole fraction of species for τres < τchem

 5.3 Effect of Temperature on Emissions

  • Temperature peaks near the stoichiometric equivalence ratio, ϕ = 1
  • Trends of emission versus ϕ show that lean combustion can achieve low emissions
  • Emissions reductions and potential difficulties in achieving lean combustion: flame stability becomes an issue when combustion temperature is low
  • Effect of flame temperature on emissions
  • Effect of flammability on on emissions 
 
 
  • Effect of flame temperature on emissions

 5.4 CO Emissions

  • CO emissions are minimized at slightly lean equivalence ratios;
  • The reaction rate constant of the “wet” CO oxidation route increases with temperature but not a strong function of temperature
Wet route: CO+OH => CO2+H
Dry route: CO+O2 => CO2 +O
  • Effect of equivalent ratio on CO emissions
 
  • Effect of temperature on reaction rate

6. NOx Emissions and its Reduction

 
  •  NOx is formed when the temperature of flue gases is high
China Energy Group

6.1 Methods of NOx Formation 

  • Thermal NOx: NO is formed by the reaction of molecular nitrogen from the combustion air with atomic oxygen at high temperature (> 1800K)
  • Prompt NOx: NO is formed by attach of hydrocarbon radical (CH) on N2
  • Fuel NOx: NO is formed  if the fuel contains organically bound nitrogen

6.2 Effect of Temperature on NOx

  • Predicted NO formation rate versus temperature showing less thermal NO formation when temperature is below 1,800 K.
  • The right branch is for rich mixtures (equivalence ratio >1) and the left is for lean combustion
  • Effect of equivalent ratio and temperature on NOx emissions
  •       Effect of  temperature on types NOx emissions
           
 
6.3 Technical Methods of NOx Reduction
 
  • Combustion Modification and Post Combustion Controls

 

 

a) Using Staged air for combustion in Boiler

 Air staging in boiler to control NOx

 
 
 

 

 Ammonia (NH3) Injection in Flue Gases

 
                        
 
                      
c) Staged Air/Fuel Burner
 
 
       A techno-economic analysis of the application of continuous staged ...
 
 
 
 
Refer :
 
d) Water Injection
 
 
 
e) Flue Gas Re-circulation
Schematic (left) and concept (right) of the present low-NO x ...
 
 

7. Measurement of Emissions 

7.1 Vehicular Emissions:

  • Vehicular emissions can be measured by exahusting the flue gases into the wind tunnel
  • Sample can checked using different analyzers for emissions
  •  Measurement of emissions from a vehicle 
(a) Wind tunnel testing
 
(b) Car emissions measurement
(c) Two wheeler emissions measurement
 
ATS Emission Monitor Exhaust Gas Analyzer, 206A, Rs 137500 /piece ...
  • Flue gas analyzers and measurement principles. 
 
 
 

4.2 Power Plant Emissions:

 
  • Power plant emissions can be measured by sampling flue gases generated by boiler at different locations of ducts.
  • The following parameters are commonly measured or conterminously monitored in thermal power plant to control the emissions
  •  Flue gas monitoring systems for boiler
Flue Gas & Emissions Analyzers | Nova Gas
 
  • The Continuous Emission Monitoring System (CEMS) is a facility to measure flow, dust, concentration of air pollutants (such as SO2, NOx, CO etc), and other parameters according to requirements. Required parameters depend on the type of stationary source.

 

 

8. CFD Modeling of Pollutants

  

For more detail refer: Rection Eng.International Pollution Modeling

 

1) Modeling Emission from the Terrain
Modelling NOX concentrations through CFD-RANS in an urban hot-spot ...
 

2) Using CFD for Cleaner Fireplace Design

Using CFD for Cleaner Fireplace Design > ENGINEERING.com

Refer:Using CFD for Cleaner Fireplace Design using FloEFD

3) CFD Modeling  of Smoke from the Chimney

 

🥇 ANSYS CFX - Smokestack - Multi Species - YouTube

References

 

Books

    •  Sara McAllister, Jyh-Yuan Chen, A. Carlos Fernandez-Pello, Fundamentals of Combustion Process, Springer Publication
    • F. El-mahallay, S. El-Din HabikFuna, Fundamentals and Technology of Combustion, Elsevier Publication
    • Stephen Turn, An Introduction to Combustion: Concepts and Applications, Tata Mac Graw hill Publication

     Research Articles

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