3a. Stoichiometric, thermodynamic and kinetic considerations in pollutant formation.

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3a. Stoichiometric, thermodynamic and kinetic
considerations in pollutant formation.
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MOTOR VEHICLE EMISSIONS

• Exhaust (tailpipe) (CO, NOx, HC, PM)
• Products of the combustion process
• transformed by exhaust after-treatment (if
  present)
• Evaporative (VOC)
• Resting
• Diurnal heat build
• Hot soak
• Running
• Refuelling

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Refuelling Emissions
CO2 Estimated based on fuel consumption
Two Processes Combustion (Exhaust
System) Evaporation (Fuel Storage and Delivery
System)
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COMBUSTION DEVICES

• Convert chemical energy in fuel to thermal (heat)
  and/or mechanical energy (work)
• Stationary (heaters, boilers, kilns,
  incinerators)
• Mobile (motor vehicles, aircraft, trains, ships)
• Internal combustion engines - the working
  fluid is the fuel-air mixture
• External combustion engines - the working fluid
  is typically steam generated by heat transfer
• Premixed vs diffusion flames in combustion devices

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Premixed vs diffusion flames

• Premixed fuel and air are mixed before arriving
  at the flame
• Diffusion fuel and air meet at the flame,
  approaching it from opposite sides

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Figure 7.10 Heinsohn Kabel

• Diffusion flame

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Figure 7.11 Heinsohn Kabel

• Premixed flame

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POLLUTANT FORMATIONStoichiometry,
Thermodynamics, Kinetics

• Stoichiometry what are the quantitative
  relationships between reactants and products
• Thermodynamics what is the ultimate ratio of
  products to remaining reactants
• Kinetics how fast will the reactants react to
  give products

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Figure 7.3 (7.9) de Nevers

• Stochiometry of combustion for CxHy

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Table 10.2 (7.1) de Nevers

• Combustion data for HC fuels

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Figure 10.13 (7.4) de Nevers

• Adiabatic flame temperature for HC fuels

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Figure 10.16 (7.5) de Nevers

• Effect of air-fuel ratio and quality of mixing on
  composition of combustion gases

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POLLUTANT FORMATION, CO and HCStoichiometric
considerations

• Stoichiometric A/F ratio based on chemical
  formula
• predicts only CO2 and H2O formation
• If A/F less than stoichiometric we are sure to
  get some CO and HC (unburned fuel and partial
  products of combustion)
• We can get some CO and HC even when A/F is
  slightly higher than stoichiometric (poor mixing,
  frozen equilibrium)
• Fuel-Air mixture will not burn if
• Too little air (Lower flammability limit, LEL)
• Too much air (Upper flammability limit, UEL)
• LEL and UEL not easily predictable

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COMBUSTION IN IC ENGINES

• Air/Fuel ratio, mass of air per mass of fuel,
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• Equivalence ratio
• ? (A/F)stoichiometric / (A/F) actual
  ? 1/ ?
• ? ? 1 for gasoline engines most of the time,
• ? gt 1 (fuel rich) during high power demand and
  start
• ? lt 1 (fuel lean) for diesel most of the time,
• ignition - combustion - extinction
• sequence repeated 102 103 times a minute
  unsteady combustion

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Figure (13.2) de Nevers

• Emissions and fuel consumption vs lambda

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Thermodynamics What will happen?
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• We also have by definition

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Thermodynamics What will happen?
n.b. this is a simplified form for ideal gases
where p are in atm and P1 atm
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• The temperature dependency of the equilibrium
  constant is given by the vant Hoff equation
• where the superscript 0 refers to the standard
  condition of 1 atm
• If the heat of reaction is constant we can
  integrate to get

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Table 6-8 Wark, Warner Davis

• Equilibrium constants for the CO-CO2 oxidation
  process

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Figure E7-3 Heinsohn Kabel

• Equilibrium concentration of CO vs T at three
  different pressures

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POLLUTANT FORMATION, CO Thermodynamic Kinetic
considerations

• Consider that C in fuel first reacts to form CO,
  then reacts further to form CO2
• Thermodynamics of CO oxidation suggest virtually
  all C will go to CO2 at low temperature (say 500
  K) but measurable amounts of CO will remain at
  high temperature (say 2000 K)
• Thermodynamics tells us what will happen
  ultimately, not how fast we will get there
• The lower the temperature, the slower the rate at
  which the ultimate fate is approached
• Equilibrium established at high temperature may
  persist over long periods even though the
  temperature has dropped frozen equilibrium

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Kinetics How fast will it happen?

• A B -----gt C D
• reaction rate, r mols of A reacting per unit
  time per unit volume
• t time
• CA concentration of A, mols per unit volume
• k reaction rate constant
• a, b order of reaction with respect to A and B

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Arrhenius equation

• k0 frequency factor
• E activation energy, J/mol
• ko and E determined experimentally for each
  reaction
• R ideal gas constant, 1.987 J/mol.K
• T temperature, K
• k increases with temperature
• high E, high sensitivity to temperature

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FREE RADICAL CHEMISTRY

• Free radicals that play significant roles in
  atmospheric and flame chemistry
• OH? hydroxyl
• HO2? hydroperoxyl
• CH3? methyl
• O? single oxygen
• H? hydrogen
• The ? represents an unpaired electron, not to be
  confused with the extra or deficient electrons in
  ions such as OH- or H
• The ? notation is occasionally omitted when it is
  clear from the context that one is dealing with
  free radicals

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FREE RADICAL CHEMISTRY

• Free radical reactions
• Initiation reactions that initiate radicals
  through the action of sunlight or high
  temperature
• Propagation and Branching reactions that
  consume one radical but produce other radicals
• Termination reactions that annihilate radicals

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Reactions of radicals

• Initiation

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Reactions of radicals

• Propagation and branching

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Reactions of radicals

• termination

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POLLUTANT FORMATION, CO

• Formation
• Oxidation

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Reactions of fuel molecules

• Complex set of reactions that proceed through
  free radical mechanisms
• Description of complete kinetics very difficult
• Two-step overall reaction mechanism formulation

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POLLUTANT FORMATION, HC Kinetic considerations

• The H in the fuel goes to H2O faster than the C
  goes to CO2
• In steady combustion with seconds of residence
  time at the high temperature range, HC emissions
  not a problem
• IC engine combustion is very unsteady
• ignition - combustion - burnout
• repeated at millisecond time-frame
• Low temperatures near cylinder walls

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POLLUTANT FORMATION, thermal NO

• Source of both the nitrogen and oxygen is the air
  used for combustion
• A/F ratio still has effect on NO formation
  because it determines
• the maximum temperature reached
• the oxygen avaliable to react with nitrogen
• Reasonably well understood thermodynamic and
  kinetic effects in formation

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Table 12.1 (12.2) de Nevers, Table 15.3 Cooper
Alley)

• Equilibrium constants for the formation of NO and
  NO2

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Table 12.2 (12.3) de Nevers (Table 15.4 Cooper
Alley)

• Calculated equilibrium concentrations of NO and
  NO2

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ZELDOVICH MECHANISM for NO

• At the high temperatures in a flame the nitrogen,
  oxygen, and water molecules can dissociate
• There are also reactions that involve very
  reactive, short lived, intermediate species such
  as CH3, etc.
• The Zeldovich mechanism assumes that most of the
  NO is formed following the completion of these
  reactions involving fuel molecules

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ZELDOVICH MECHANISM for NO FORMATION
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SIMPLIFIED ZELDOVICH MECHANISM
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SIMPLIFIED ZELDOVICH MECHANISM
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NO concentration as a function of time from
simplified Zeldovich mechanism

Equations 12.17 (12.16) 12.18 (12.17) de Nevers
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Example 12.2 de Nevers

• 78 N2, 4 O2
• held at 2000 K for 1 s
• Calculated NO 610 ppm
• This is in the right range of observed values.
• For more accurate values we need to know
  time-temperature history

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Thermal, Fuel, Prompt NO

• If the fuel contains nitrogen, some will end up
  as NO, called fuel NO to distinguish its source
• The highly reactive species such as CH3 found in
  flames of carbonaceous fuels interact rapidly
  with nitrogen and oxygen to form prompt NO
• NO formed by the Zeldovich mechanism is highly
  sensitive to temperature and is called thermal NO
• Thermal NO dominates the scene at high
  temperatures

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Fuel NO

• If the fuel contains nitrogen, some will end up
  as NO, called fuel NO to distinguish its source
• N in fuel ---gt HCN ---gt NH, NH2
• NO/O2 ratio determines fate of fuel N
• Keep O2 low in in high temperature zones

O2
NO
NO H2O
N2 H2O
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Definitions - PM

• Soot
• Carbonaceous particles produced through gas-phase
  combustion process
• Coke or cenospheres
• Carbonaceous particles formed as a result of
  direct pyrolysis of liquid hydrocarbon fuels
• Particulate Matter (PM)
• Particles that can be collected on the probes of
  measuring instruments such as filters
• Originate from a variety of sources

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Soot Formation in Combustion

• Conversion of a hydrocarbon fuel with molecules
  containing a few carbon atoms into a carbonaceous
  agglomerate containing some millions of carbon
  atoms in a few milliseconds
• Transition from a gaseous to solid phase
• Smallest detectable solid particles are about 1.5
  nm in diameter (about 2000 amu) .

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Soot Formation in Combustion
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Soot Photomicrographs
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POLLUTANT FORMATIONQuantitative predictive
ability

• CO2 excellent, basic stoichiometry
• CO and NO Reasonably well understood formation
  mechanisms Quantitative predictions possible for
  well defined combustors IC engines quite
  challenging due to the nature of the combustor
• HC and PM Qualitative understanding of factors
  affecting formation, quantitative predictions for
  practical combustors not available.

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POLLUTANT FORMATIONEmpirical Estimation

• Emission factors quantify the emissions of a
  particular pollutant per unit activity
• g CO emitted per km driven
• g NO emitted per million kJ of energy generated
• g of CO2 produced per ton of pulp
• Based on measurements on similar applications
• The better the similarity between the measurement
  set-up and the estimation set-up the better the
  estimate
• The more number of measurements, the better the
  estimate
• U.S. EPA Document AP-42 provides comprehensive
  tabulation for many applications