Understanding Basics of Emissions and
Modeling the Unwanted combustion to understand Environmental Pollution
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
- Hot gases and particles emitted from coal-fired thermal power plants. Solid Fuel contains a high carbon index and unburnt carbon particles are exhausted at higher altitudes
- The majority of air pollution is due to coal-fired thermal plants. Hence such power generation stations are away from the cities or public areas.
- Details of pollutants emitted from the power plants and automobiles have been explained the following video:
Major sources of Pollutant Emissions
- Natural sources of emissions: volcano, wildfires, 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
- The majority of air pollution is due to transportation and thermal power plant for electricity generation
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
Classification of 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
- Industries are examples of outdoor 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.
Fig. 4 Examples of indoor pollution
Parameters Controlling the Pollutants
- 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
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
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 emissions
- Effect of flame temperature on 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
NOx Emissions and its Reduction
- NOx is formed when the temperature of flue gases is high
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
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
Technical Methods of NOx Reduction
- Many industrial practices have been developed to reduce NO pollutions
Combustion Modification and Post Combustion Controls
Ammonia (NH3) Injection in Flue Gases
Staged Air/Fuel Burner
Flue Gas Re-circulation
Measurement of 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
Flue gas analyzers and measurement principles
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
- The Continuous Emission Monitoring System (CEMS) is a facility to measure flow, dust, the 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.
- Fore more details, click here: Horiba Continuous Emission Monitoring System
Measurement points for thermal power plant
CFD Modeling of Pollutants
For more detail refer: Rection Eng.International Pollution Modeling
1 Modeling Emission from the Terrain
Using CFD for Cleaner Fireplace Design
CFD Modeling of Smoke from the Chimney
- CFD modeling of Pollutants is given on this website. NOx, SO and Soot modeling is possible in a CFD simulation
- We need to understand turbulent multiphase combustion
- Sara McAllister, Jyh-Yuan Chen, A. Carlos Fernandez-Pello, Fundamentals of Combustion Process, Springer Publication
- F. El-mahallay, S. El-Din Habik, Funa, Fundamentals and Technology of Combustion, Elsevier Publication
- Stephen Turn, An Introduction to Combustion: Concepts and Applications, Tata Mac Graw hill Publication
- J. Valentine, M. Cremer, K.Davis,J. J. Letcatvits, S.Vierstra, CFD Modeling for Design of NOX Control System for Two Boiler, Reaction Engineering International
- Z. Man, F.U/Zhenbo, L. Yuzven, Li Jibao, CFD Study of NOx Emissions in a Model Commercial Aircraft Engine Aircraft, Chinese Journal of Aeronautics, 25 (2012) 854-863
- R. Zadghaffaria J .S. Moghaddas, Z. Rahimiaharas, Numerical Investigation of a Burner Configuration to Minimize Pollutant Emissions, APCBEE Procedia, 3 (2012) 177-181(2012) 854-863
7 thoughts on “Basics of Emissions and Its Control”
Very well compiled information on vehicular emission. Covering more on emission index will be still useful
Thanks for your feedback, for vehicular emission refer:https://cfdflowengineering.com/modern-combustion-technologies-in-automobiles/
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