CLL725 Air pollution control Engineering Dr. Divesh Bhatia Instructor • Dr. Divesh Bhatia • Block 1, Room 315 • dbhatia@chemical.iitd.ac.in • Tel: x-1456 Reference Books • R.M. Heck, R.J. Farrauto, S.T. Gulati, “Catalytic Air pollution control: Commercial Technology” Third Edition, John Wiley & Sons (2009). • J. B. Heywood, “Internal Combustion Engine Fundamentals” McGrawHill (2014). • J.H. Seinfeld, S.N. Pandis, “Atmospheric Chemistry and Physics: From Air Pollution to Climate Change” Wiley (2016) Marking scheme and attendance policy • Mid-Term – 30% • Quizzes (unannounced) – 10% • Project (Report and presentation) – 15% • Major – 40% • Attendance – 5 marks for attendance > 90%, 2.5 marks for attendance between 75% and 90%) • Bonus 15 marks for publishable project work (needs to be discussed in advance) • In case you are absent for the class, quiz will not be taken again • Best (π − 1) out of π quizzes. ‘π’ is typically 3 or 4 • If attendance is less than 50% before the exams (mid-term or major), you will be ineligible to appear for the respective exam. In case you appear for the exam, the marks will not be counted towards your final score • Minimum passing marks for D grade – 30% • Audit pass criteria – Score 40% marks AND 75% attendance • Reminor / remajor / I grade requests will not be accepted for attendance (before respective examination) below 75%. No attendance relaxations for first week. • Proxy attendance: 10 marks will be deducted • 9: 05 AM is the last time that you can enter the class How to reduce Air pollution? Fuel (Refinery operations)) Engine Aftertreatment device Reactors on wheels Atmosphere What can we do as chemical engineers? Fuel (Refinery operations)) Engine Composition of fuel (sulphur content, etc.)) Fuel properties (density, viscosity, volatility, etc.) CO (g/kWh) Decrease density from 855 to 828 kg/m3 +5.0% HC(g/kWh) +14.3% NOx (g/kWh) PM (g/kWh) −3.6% −1.6% (Stati stically not significant) Aftertreatment device Atmosphere What can we do as chemical engineers? Fuel ) Engine Kinetics and thermodynamics of pollutant formation NOx reduction techniques (Exhaust gas recirculation) Mixing of fuel and air Aftertreatment device Atmosphere What can we do as chemical engineers? Fuel Engine Aftertreatment device Catalyst development and kinetics Process control and sensor development Diffusion in catalyst Atmosphere What can we do as chemical engineers? Fuel Engine Aftertreatment device Atmosphere Atmospheric chemistry / Kinetics of reactions Source apportionment Dispersion of pollutants Overview Fuel / emission source Engine • Air pollution from mobile and stationary sources • Effect of combustion engine on engine-out emissions • Effect of fuel type on air quality • Kinetics, thermodynamics and mechanism of pollutant formation • Household pollutants and control of indoor air quality • Emissions from power plants • Fuel quality standards • Modeling of pollutant profiles from engine • Reduction of engine-out emissions • Regulations, methods for quantification • What is going on in industry? • What can you do as a chemical engineer? Focus on India. • Where are we heading? Aftertreatment device Atmosphere • Automotive catalysts (Technology implementation and active research) • Atmospheric chemistry / Kinetics of atmospheric reactions • Preparation of catalysts • Photochemical cycle • πππ₯ reduction technologies • Regulations, methods for quantification • Modeling of aftertreatment devices • Diesel particulate filters • Emission control in power plants • Meteorological aspects / dispersion Relevant organizations • Automobile • • • • • • • • • • • • Mercedes Benz Research and Development Cummins emission solutions Maruti – Suzuki Nissan General Electric Tata Mahindra & Mahindra UOP – Honeywell BASF catalysts Sud Chemie Ecocat Umicore The Indian reality Sources of air pollution • • • • • • • Classified as stationary sources and mobile sources Transportation sector Power plants Chemical industry Construction dust Indoor fuel burning (wood, dung, etc.) Natural sources (wildfire, volcanic activity, human breathing causing enhanced πΆπ2 levels indoors, etc.) Classification of air pollutants • Primary pollutants • Emitted directly from source • Carbon monoxide generation during burning of a fossil fuel (wood) • Other examples include smoke, dust • Secondary pollutants • Formed in atmosphere by chemical interactions between primary pollutants and normal atmospheric constituents • Sulphur trioxide formation by oxidation of Sulphur dioxide • ππ2 + ½ π2 Υ ππ3 • Chemical and photochemical reactions • Influenced by reactant concentration, moisture content, degree of photoactivation, meteorological conditions Examples of secondary pollutants • Photochemical smog formation – reaction between nitrogen oxides and volatile organic compounds in the presence of sunlight • Peroxyacetyl nitrate (PAN) formation • Enhanced by using ethanol-blended fuel because of aldehyde emissions • ~10 − 50 times more toxic than π3 • More stable than ozone and capable of long range transport • Stable at cold temperatures and decomposes to release NOx at warm temperatures • Serves as a carrier of πππ₯ into rural regions and causes ozone formation • Ozone in the lower atmosphere can have adverse effects on human health and plants • Acid rain (discussed later) https://www.sciencedirect.com/science/article/pii/B9780123822253004333 Anthropogenic vs natural sources • Anthropogenic sources concentrated in specific locations • 95 − 98% atmospheric CO in an urban area comes from human activities • Resulting CO levels are 50 − 100 times higher than characteristics global values of 0.1 − 0.5 ppm Pollutants of major concern • Carbon monoxide • Hydrocarbons • Nitrogen oxides • Sulphur oxides • Particulate matter • Carbon dioxide (not considered an air pollutant, but its possible influence on global climate patterns are of great concern) Carbon monoxide • Colourless and odourless gas • Formation • Fossil fuel combustion • Incomplete combustion produces CO • πΆ + ½ π2 Υ πΆπ • Sources • • • • • Internal combustion engines (vehicles) Boilers (power plants) Open burning of waste Natural and man made fires Volcanic activity • Harmful effects • CO is a known carcinogen • It can reduce oxygen carrying capacity of blood to vital organs such as brain • Never ever burn coal in a closed room. The odourless and colourless Carbon monoxide can kill you !! Hydrocarbons Potential for oxidant formation • Gaseous and volatile liquid hydrocarbons of particular interest • Amongst alkenes, ethylene and propylene are the prominent pollutants • Aromatics include benzene, toluene, m-xylene • Hydrocarbons undergo chemical reactions forming photochemical oxidants (such as π3 and PAN) • Methane has low photochemical activity compared to other hydrocarbons • Non-methane hydrocarbon concentration of interest Heywood page 598 Nitrogen oxides • NO and ππ2 collectively referred to as πππ₯ • Formation • NO formation is endothermic • Occurs at high temperature (~1600π πΆ) Nitrogen oxides • NO and ππ2 collectively referred to as πππ₯ • Formation • NO formation is endothermic • Occurs at high temperature (~1600π πΆ) Nitrogen oxides • Thermal πππ₯ • Formed by high temperature oxidation of atmospheric π2 in post-flame burned gases • Formation is dependent on temperature 1 1 • π2 + π2 β ππ is highly endothermic (βπ» = 90 ππ½/πππ) 2 2 • Zeldovich mechanism • π2 + π. Υ π. + ππ • π2 + π. Υ ππ + π. • Fuel πππ₯ • • • • Source of Nitrogen for NO formation is the fuel itself Oxidation of fuel-bound Nitrogen at relatively low temperatures Certain classes of oils and coal contain significant Nitrogen Some of the major shale oil deposits in the United States contain 2 to 4% nitrogen (crude oils usually contain 0.1 to 0.2% Nitrogen) • Nitrogen concentrated in the residual fractions • Likely to be burned in large combustion systems such as power plants or industrial boilers • Concentration low in transportation fuels Flagan page 179 − 180 Air replaced by a mixture of π2, Ar, and πΆπ2 Nitrogen oxides (Prompt NOx) • Formed by reaction of atmospheric nitrogen with radicals such as πΆ. πΆπ». πΆπ»2 . • • • • • π2 + πΆπ». Υ π»πΆπ + π. Not seen for CO and π»2 flames Large concentrations of HCN near the reaction zone in fuel rich flames Occurs at low temperature, fuel-rich conditions and short residence times Reactions proceed at a rate comparable to fuel oxidation (reactions have relatively low activation energy) • Formed at flame front within the flame reaction zone • Rate of oxidation of fuel usually sufficiently rapid that fuel radicals such as CH. are at low concentrations and CH.+π2 reaction is negligible • Significance • • • • Prompt NOx is when fuel is in excess Prompt NOx adds complication that nitrogen can be emitted in forms other than NO (such as HCN) Contribution estimated to be about 5 − 10% As NOx reduced to low levels, relative importance of prompt NOx expected to increase (limits the effectiveness of NOx controls) Flagan page 174 − 176 Sulphur oxides • Sources • Fuel (petroleum, coal) contains sulphur compounds which can oxidize to form ππ2 • Industrial processes (processing of sour gas – natural gas or any other gas containing significant amounts of π»2π) • Volcanic eruptions • Harmful effects • Adverse respiratory disorders • Acid rain formation Particulate matter • Exists as a liquid or solid in the atmosphere (except pure water) under normal conditions • Is of microscopic or submicroscopic size but larger than molecular dimensions • Sources • Direct emissions of particles • Emissions of certain gases that condense as particles • Emissions of gases that undergo chemical transformation to a species that condenses as a particle • Isoprene has largest global atmospheric emissions of all non-methane VOCs • Readily oxidized in the atmosphere by OH, ππ3 and π3 • Oxidation product methacrolein condenses on existing particles and can react even further • Includes acids (nitrates and sulphates), organic chemicals, metals, and soil or dust particles • Particulate matter from vehicles comprised of soluble organic fraction (SOF) and elemental carbon besides other elements Seinfeld page 22,29 Particulate matter • Coarse particles • Larger than 2.5 microns and less than 10 microns in diameter • Referred to as ππ10 • Fine particles • Less than 2.5 microns • Referred to as ππ2.5 • Once airborne, particles can change their size and composition by • Chemical reaction • Condensation of vapor species • Coagulating with other particles Seinfeld page 56 Particulate matter - effects • Classified as group 1 carcinogen • Small particles biggest threat since they can enter the lungs and some may even enter the bloodstream • Inhibit visibility at high levels Aerosols • Aerosols are suspensions of solid or liquid particles in a gas (usually air) • The particulate portion of an aerosol is referred to as Particulate Matter • Aerosols can affect climate and trace gas concentrations • Role in climate • Ability to reflect solar radiation back to space – Cooling effect • Role as cloud condensation nuclei • Cooling effect may be sufficiently large to mask the warming effect of greenhouse gas increases over industrialized regions of the Northern Hemisphere • Aerosols interact chemically with atmospheric gases • Volcanic aerosols in the stratosphere participate in the catalytic destruction of ozone by chlorine compounds Spatial and temporal scales • Lifetime of a species is the average time that a molecule resides in the atmosphere before removal • Chemical transformation to another species counts as removal • Lifetimes vary from less than a second to many years • Depends on the effectiveness of the removal processes • Hydroxyl radical (OH) has a spatial transport scale of only about 1 cm and lifetime of less than 0.01 s • Methane (πΆπ»4 ) can become close to uniformly mixed over the entire Earth (lifetime of about 10 years) • Microscale • Dispersion of a chimney plume • Flow regime in the wake of a large building • Mesoscale • Land-sea breezes, mountain-valley winds • Synoptic Scale • Motions of whole weather systems • Global Scale • Phenomena occurring on scales exceeding 5 × 103 km Spatial and temporal variation • Relatively long-lived species have a spatial uniformity • Strategically located sampling sites adequate to characterize spatial distribution and temporal trend • Urban areas can require tens of monitoring stations over an area of hundreds of square kilometers • Polar ice core samples reveal global average concentrations of previous eras • Gases such as πΆπ2 and πΆπ»4 that are long-lived in the atmosphere and therefore distributed rather uniformly over the globe • πΆπ2 and πΆπ»4 concentrations remained essentially unchanged from ~10,000 years ago until roughly 300 years ago • About 300 years ago methane levels began to climb, and about 100 years ago levels of both gases began to increase markedly Seinfeld page 3 http://www.dpccairdata.com/dpccairdata/display/index.php Problems of relevance • Local air pollution effects from gases • Anthropogenic aerosols (urban smog) • Ground level ozone • Antarctic ozone hole / depletion of stratospheric ozone • Ozone in the stratosphere protects living organisms from harmful solar ultraviolet radiation • Essentially complete disappearance of ozone in the Antarctic stratosphere during the austral spring • Acid rain • Global warming from greenhouse gases • • • • Global increase in concentrations of gases such as πΆπ2, πΆπ»4, π2π Greenhouse gases absorb infra-red radiation from earth’s surface Re-radiate a portion of the radiation back to the surface Act as thermal insulators • Cooling effect of aerosols Fuel, engine and emissions Spark ignition engine • Compression ratio = Volume at bottom dead center / Volume at top dead center • Typical compression ratio for a spark ignited gasoline engine is less than 15: 1 Working of an internal combustion engine Gas composition of gasoline engine outlet • CO levels are in the range of a few vol% • Unburned hydrocarbons range between a few hundred and a few thousand vppm • πππ₯ ranges between a few hundred and a few thousand vppm • Approximately 0.3 moles π»2 per mole of CO • Quantity of pollutants affected by air to fuel ratio in addition to other operating conditions • Lambda ratio π = π΄/πΉ πππ‘π’ππ / π΄/πΉ π π‘πππβπππππ‘πππ • For insufficient air, π < 1 (referred to as rich conditions) • For excess air, π > 1 (referred to as lean conditions) 1 π • Fuel air equivalence ratio π = = πΉ/π΄ πππ‘π’ππ / πΉ/π΄ π π‘πππβπππππ‘πππ Effect of air/fuel ratio • Under rich conditions, CO and HC emissions are high • Incomplete burning of fuel • NOx is maximum just lean of stoichiometric • Adiabatic flame temperature is maximum just rich of stoichiometric • Why can’t we operate very lean? • Actual operating region of combustion defined by flame stability • Combustion becomes unstable under excessive lean conditions • Engine misfiring will cause erratic engine operation and sharp increase in HC emissions • Gasoline engines operated close to stoichiometric?? Farrauto page 103 − 104 Fuel-air equivalence ratio Increasing O2 Kummer et al., 1980 (Progress in Energy and combustion science) Equivalence ratio (contd.) Effect of air/fuel ratio • Under rich conditions, CO and HC emissions are high • Incomplete burning of fuel • NOx is maximum just lean of stoichiometric • Adiabatic flame temperature is maximum just rich of stoichiometric • Why can’t we operate very lean? • Actual operating region of combustion defined by flame stability • Combustion becomes unstable under excessive lean conditions • Engine misfiring will cause erratic engine operation and sharp increase in HC emissions • Gasoline engines operated close to stoichiometric Fuel-air equivalence ratio Increasing O2 • Highest power levels obtained with slightly rich-of-stoichiometric mixtures and no recycled exhaust Heywood page 570 http://nptel.ac.in/courses/112104033/1 Farrauto page 103 − 104 Kummer et al., 1980 (Progress in Energy and combustion science) Modeling of pollutant profiles Internal Combustion Engine – a reactor from a chemical engineering perspective • • • π(ππ π) ππ‘ ππ = ππ‘ ππΈ = ππ‘ = πΉπππ − πΉπππ’π‘ + πππ£ π π π, π π-crank angle, π – revolution speed παΆ βπππ‘ − παΆ π + σ π»πππ πΉπππ − σ π»πππ’π‘ πΉπππ’π‘ Heat transfer to coolant Shaft work Kumar et al., 2011 (AIChE Journal) Reaction scheme ππ£ = πΈ − π π΄π π π • Volume of combustion chamber / control volume changes with time • Time-dependent temperature and pressure Kumar et al., 2011 (AIChE Journal) Modeling results Kumar et al., 2011 (AIChE Journal) Modeling results Intake Compression Power Exhaust • Increase in pressure during compression • Significant increase in pressure due to combustion Modeling results Kumar et al., 2011 (AIChE Journal) Combustion thermodynamics • Equilibrium composition of combustion gases strong function of temperature and equivalence ratio • CO and π»2 unreacted under equilibrium conditions at high temperatures and stoichiometric conditions Adiabatic combustion of kerosene Doubt Stoichiometric combustion of kerosene Equilibrium composition • Reduces adiabatic flame temperature • Dissociation reactions have large heats of reaction 1 • πΆπ2 β πΆπ + 2 π2 βπ» = 283 ππ½/πππ • π»2 π β π»2 + 2 π2 βπ» = 242 ππ½/πππ 1 • CO, NO and unburnt hydrocarbons emitted from combustion systems • Chemical equilibrium not maintained as combustion products cool Flagan page 98 − 100 Formation of pollutants • In general, concentrations of pollutants in Internal Combustion Engine (ICE) exhaust differ from values calculated assuming chemical equilibrium • Detailed chemical mechanisms of pollutant formation and kinetics important in determining emissions levels • Formation and consumption of CO, organic compounds, particulates intimately coupled with primary fuel combustion process • Knowledge of combustion chemistry required • Formation and consumption of πππ₯ and πππ₯ not part of fuel combustion process • Reactions which produce these species take place in an environment created by combustion reactions Heywood page 568 πππ₯ formation • NO forms throughout the high temperature burned gases behind the flame • Chemical reactions involving nitrogen and oxygen atoms and molecules • Increase in NO formation rate with increase in temperature • Reactions involving NO freeze with decrease in temperature during expansion stroke • NO concentrations in excess of equilibrium values Heywood page 568 Factors affecting NO emissions • Fuel air equivalence ratio • Burnt gas fraction of in-cylinder unburnt mixture • Depends on amount of diluent such as recycled exhaust gas and residual gas fraction • Spark timing • Effect explained by change in time history of temperature and oxygen concentration in the burned gases and early part of the expansion stroke Heywood page 581 Equivalence ratio • Maximum burnt gas temperature occurs around fuel/air equivalence ratio of 1.1 • Oxygen concentrations low at this equivalence ratio • With decrease in fuel / air equivalence ratio (mixture becomes lean), oxygen concentration increases • Peak NO emissions around fuel / air equivalence ratio of 0.9 • Substantial NO decomposition occurs from peak concentrations in rich mixtures • In lean mixtures, NO concentrations freeze early in expansion process – little NO decomposition occurs • Gas conditions in lean mixtures at time of peak pressure are especially significant Heywood page 581 − 582 Burned gas fraction • Burned gases – residual gas from previous cycle and any exhaust gas recycled to intake • Act as charge diluents • Temperature reached after combustion varies inversely with burned gas mass fraction • Decrease in temperature due to charge dilution which results in lower NO levels • Residual gas fraction influenced by valve timing (especially extent of valve overlap) Heywood page 582 − 583 Burned gas fraction • Increase of heat capacity of cylinder charge per unit mass of fuel • High specific heat of πΆπ2 and π»2π • Specific heat of exhaust gas higher than air due to substantial amounts of πΆπ2 and π»2π • Plotting vs. diluent heat capacity – collapse to single curve • Effect of charge dilution almost entirely due to heat capacity of diluting gases • Negative effect of charge dilution • Reduction in oxygen concentration, decrease in combustion rate • Beyond a limit, it causes misfired combustion resulting in low fuel efficiency and higher hydrocarbon emissions Heywood page 583 Spark timing • Significant effect on NO emissions • Advancing spark timing • Increases peak cylinder pressure • More fuel is burned before top dead center, peak pressure moves closer to top dead center • Retarding spark timing • Decreases peak cylinder pressure • High peak cylinder pressure results in high peak burned gas temperatures, resulting in high NO formation rates Heywood page 585 − 586 NO formation in SI engines • Vigorous mixing of fuel and air with residual gas during intake process • Fuel air ratio and concentration of diluent approximately uniform • NO formation studied by coupling NO formation kinetics with burned gas temperature distribution and pressure • Mixture which burns early compressed to higher temperatures as cylinder pressure continues to rise • Mixture which burns later compressed primarily as unburnt mixture and ends up after combustion at lower burnt gas temperature • Results in figure obtained by assuming that no mixing takes place between mixture elements burning at different times Heywood page 578 − 580 NO formation in SI engines 1 • ππ = β«Χ¬β¬0 ππ π ππ₯π • Earlier burning fractions of charge contribute much more to exhaust NO • In absence of vigorous bulk gas motion, highest NO concentrations occur near to the spark plug ππ ππ π Final frozen NO mass fraction in the element of charge which burned when mass fraction burned was xb Average NO mass fraction Heywood page 580 − 581 Hydrocarbon emissions from SI engines • Consequence of incomplete combustion of hydrocarbon fuel • HC emissions rise rapidly as mixture becomes rich • With very lean mixtures, HC emissions can rise rapidly due to incomplete combustion or misfire Crevice • Sources • • • • Crevice Flame quenching at combustion chamber walls Solubilization and desorption of fuel by engine oil Incomplete combustion due to bulk quenching of flame • Most sources result in unburnt HC close to combustion chamber walls (not in bulk of cylinder gases) • Distribution expected to be non-uniform Heywood page 570 Sources of hydrocarbon emissions • Crevice • Increase of cylinder pressure during combustion and compression • Forces gas in the cylinder into crevices (narrow volumes) Crevice • Volume between piston and cylinder wall • Most of this gas escapes primary combustion process • Entrance to crevices too narrow for flame to enter • Unburnt hydrocarbons proportional to total crevice volume • Decreased to very low values as all crevices were filled with solid material • Flame quenching at combustion chamber walls • Partially burnt fuel – air mixture left at wall Heywood page 570 Sources of hydrocarbon emissions (contd.) • Solubilization and desorption of fuel by engine oil • Oil layers on walls of bomb cause increase in residual unburnt hydrocarbons • Additional HC in experiments with oil films are primarily fuel molecules • Additional HC directly proportional to amount of oil placed on walls of reactor and solubility of specific fuel in oil • Results show solubilization of fuel in oil prior to ignition followed by desorption after combustion Heywood page 600 − 603 Carbon monoxide formation • Controlled primarily by fuel / air equivalence ratio • With rich fuel-air mixtures, insufficient oxygen to burn carbon • Spark ignited (SI) engines operate close to stoichiometric at partial load and fuel rich at full load • CO emissions are significant • CO levels observed in SI engine exhaust gases lower than maximum values measured within combustion chamber • Significantly higher than equilibrium values for exhaust conditions • Imply kinetically controlled processes • CO concentrations in the immediate postflame burned gases close to equilibrium • CO oxidation (πΆπ + ππ» β πΆπ2 + π») may not remain locally equilibrated as burned gases cool during expansion and exhaust strokes • Freezing of CO oxidation due to decrease in temperature during expansion stroke Heywood page 592 − 594 Particulate emissions in SI engines • Three classes: Lead, Organic particulates and sulphates • Lead • Use of leaded gasoline results in particulates with significant mass fraction of lead • Particulate emissions higher when engine is cold • Organic particulates • Soot emissions result from combustion of overly rich mixtures • Sulphates • Sulphur in fuel oxidized to ππ2 • ππ2 oxidized to ππ3 which combines with water at ambient temperature to form sulphuric acid aerosol • In properly adjusted SI engines, soot in the exhaust is not a significant problem Heywood page 626 Compression ignition engine • Only air (no fuel) is injected during intake stroke • Intake air heated by compression • Fuel is injected during power stroke • Compression ratios are higher as compared to spark ignition engines. Greater than about 22: 1 Farrauto page 239 Diesel engines • High fuel efficiency • High compression ratio • Operate very lean of stoichiometric (air to fuel ratio higher than stoichiometric) Converter inlet temperature during NEDC cycle (emission testing) • Lean nature results in cooler combustion with less gaseous CO and HC emissions • Design of combustion process and the fact that fuel is injected as a liquid, results in high particulate emission levels Farrauto page 239 Emissions from SI and CI engines SI engine CI engine • CO and HC emissions lesser in CI engine as compared to SI engines • PM formation in CI engine • HCCI engine – homogenous charge compression ignition • Combines advantages of both CI and SI engines Pollutant formation in CI engines • Compression ignition combustion process divided into 4 stages • Ignition delay • Time between start of injection and ignition • Premixed/rapid combustion • Mixing controlled combustion • Rate governed by fuel-air mixing • Late combustion phase • Rate governed by mixing of residual combustibles with π2 and kinetics of oxidation process Heywood page 621 Pollutant formation in CI engines • Three distinct regions in a fully developed diesel spray • Fuel rich core where fuel-air equivalence ratio is richer than rich flammability limits • Flammable region where fuel-air equivalence ratio lies between rich and lean flammability limits • Lean flame out region where fuel-air equivalence ratio is less than the lower flammability limits Pollutant formation in CI engines • NO formed in high temperature burned gas region • CO formed in fuel rich mixture in flammable region • Unburnt hydrocarbons • Formed in region where flame quenches on walls • Mixture too lean due to excessive dilution with air • Soot forms in fuel-rich spray core where fuel vapor heated by hot burned gases • Soot oxidized in the flame zone where it contacts with unburned oxygen • Number density of particles goes through a maximum as a result of particle coagulation and possibly oxidation Heywood page 633 Pollutant formation in CI engines • NO formed in high temperature burned gas region • CO formed in fuel rich mixture in flammable region • Unburnt hydrocarbons • Formed in region where flame quenches on walls • Mixture too lean due to excessive dilution with air • Soot forms in fuel-rich spray core where fuel vapor heated by hot burned gases • Soot oxidized in the flame zone where it contacts with unburned oxygen • Number density of particles goes through a maximum as a result of particle coagulation and possibly oxidation Heywood page 635 Solubilization and desorption in engine oil • π₯π = ππ π» • π₯π = π • ππ,π ππ,π = = ππ,π π π ππ» ππ,π π,π +ππ ≈ ππ π π π»π ππ,π ππ (1) (2) Fuel molecules Oil layer (3) • Equation (3) valid if diffusion is rapid enough • ππ ≈ • • • • πΏ2 π· βͺ πΏ π −1 Oil film thickness varies between 1 and 10 microns D - order of 10−10 π2/π at 300 πΎ and 10−9 π2/π at 400 K Characteristic diffusion time – 10−3 to 10−1 s For thinnest oil layers, equilibrium would be achieved • π₯π Mole fraction of fuel vapour in oil • ππ Partial pressure of fuel vapour in gas • π» Henry’s constant • ππ,π Number of moles of fuel in cylinder • ππ,π Number of moles of fuel dissolved in oil • ππ Number of moles of oil • π Volume of cylinder • πΏ Oil layer thickness • π· Diffusion coefficient of fuel vapour in oil • π Engine speed Heywood page 609 Sources of hydrocarbon emissions (contd.) • Poor combustion quality / Incomplete combustion • Flame extinction source of HC emissions under certain engine operating conditions • If pressure and temperature fall too rapidly, flame can be extinguished • Bulk quenching most likely to occur at • Light load where engine speed is low and residual gas fraction is high • High dilution with excessive EGR • Substantially retarded combustion • Effect of deposits • Deposit buildup on combustion chamber walls is known to increase HC emissions • Absorption and desorption of hydrocarbons by deposits Heywood page 610 − 612 Hydrocarbon oxidation • Measurements of in-cylinder HC concentrations show levels about 1.5 − 2 times the average exhaust level • Points towards oxidation of unburnt hydrocarbons • ππ»πΆ,ππ₯ = [π»πΆ] π[π»πΆ] − ππ‘ • In-cylinder gas temperature prior to blowdown generally exceeds 1250 K • Characteristic HC oxidation time calculated to be less than 1 ms • During blowdown, temperature falls rapidly to less than 1000 K • ππ»πΆ,ππ₯ is greater than about 50 ms • Mixing of unburnt HC with bulk cylinder gases limits the amount of oxidation rather than the reaction kinetics • Large fraction of HC leaving crevice regions expected to survive with little further oxidation Heywood page 614 − 616 HC oxidation in exhaust system • Oxidation of HC requires a temperature in excess of 600 oC with a residence time of 50 ms or longer • Potential for HC oxidation decreases because of a steady decrease in gas temperature • Exhaust system modified by installing insulated pipe • Incomplete HC oxidation result in increased CO levels • CO oxidation commences because of heat released by HC oxidation • Air addition to ensure enough π2 available for oxidation Heywood page 616 − 617
