question

What are some of the reasons for needing estimates of air pollutant emissions?

answer

1) Implementation plan or control strategy development.
2.) Emission cap and trade activities.
3.) Early Reduction Program desiogn.
4.) Emission trends analysis and projections.
5.) Permit limit determination
6.) Info for the public
7.) Emisssion statement/ fee ollection.

question

What do you do when you have emission factors?

answer

you relate quantity of pollutant emitted to the activity associated with the release of the pollutant ( Ex: tons of fuel going into a factory, and pounds of pollutant going out of the factory)

question

What is an emission factor?

answer

1.) A representative value that relates the quantity of a pollutant released with an activity associated with the release of that pollutant.
2.) Averages of available data of acceptable quality.
3.) Generally assumed to be representative of long term averages for all facilities in the soure category
4.) they do NOT represent indvidual facilities.

question

Review Question: Why are NOx emissions reported as equivalent mass of NO2?

answer

Use the AP-42 emission factor for SO2 to calculate the SO2 emission rate for a coal with 2.5 weight percent of sulfur ( S) in units of lb SO2/ton of coal.

question

Explain what it means when you have emission factors from mass balance.

answer

1.) Some emission factors can be estimated based on a mass balance.
2.) This can be done for conserved species such as sulfur, when we know that the specie in the duel is emitted in a particular form.

question

Using the mass balance emission factors give an example of emissions of SO2.

answer

Example: Coal
- Organic sulfur: chemically bound to organic matter, easily combusted and converted to SO2.
- Pyritic sulfur: bound with mineral matter. Some may be retained with bottom ash.
- Typically, over 95% of S in fuel is emitted as SO2.

question

What are some details about ash emissions?

answer

1.) the 1977 NSPS for electric geerating unit large steam boilers is 0.03 lb PM/10^6 BTU.
2.) Not all ash is emitted from the boiler.
3.) about 20% or more is " bottom ash"
4.) About 80% is " fly ash"
5.) A particulate matter control device ( E.g., ESP, FF), may achieve >99%

question

Estimating emissions based upon Mass Balances will...

answer

1.) Focus on combustion sources.
2.) Pollutant formation:
- Due to impurities in fuels ( e.g., sulfur, chlorine, arsenic,etc.)
-Due to incomplete combustion ( e.g., CO, unburned hydrocarbons, aldehydes, dioxin, etc.)
- Due to the combustion process ( e.g., NOx, CO)

question

What does the term Heating value mean, and Fuel composition:

answer

Heating value is heat release during combustion.
Fuel Composition is equivalent molecular formula.
- Gaseous fuels- vol-% of onstituent compounds
- Ultimate analysis: wt-% of maor elements and components ( C, H, N, O, S, moisture, ash)

question

Converting an Ultimate Analysis to an equivalent molecular formula

answer

1.) Use 100 lb ( or g) as a basis.
2.) List Molecular Weight ( MW) of each element.
3.) Calculate moles of each element per 100 lb (g) of fuel
4.) Calculate moles of each element per mole of carbon
Write the equivalent formula

question

What do you need to know about mass balance for combustion:

answer

1.) Stoichiometric combustion: Exactly enough air to completely combust the fuel.
2.) Air is omprised of 21 vol-% O2, 79 vol-% N2.
3.) Approximately 3.76 moles of N2 per mole of O2.

question

Empiracal emission factors

answer

1.) For many pollutants, a mass balance is not an adequate basis for estimating an emission factor.
2.) Formation of NO from the thermal fixation of N2 and 02 is endothermic and depends on a chemical mechanism involving free radicals.
3.) Formation of CO depends on factors such as air-to-fuel ratio and flame temperature.

question

How is a locomotive engine tested?

answer

One method is an engine dynamometer

question

How is a car tested?

answer

the typical method used for regulatory purposes is the chassis dynamometer

question

Where do you get emission factors from stationary sources, highway vehicles, nonroad vehicles, aircraft, locomotives, and marine sources?

answer

stationary sources: AP-42
Highway vehicles: mobile6 or the new MOVES model
Nonroad vehicles: construction, farm, and industrial equipment- NONROAD model ( later, MOVES)
Aircraft- international civil aviation organization database, landing and take off cycle.
Locomotives- Modal emission factors by notch position from reports and papers. freight or passenger duty cycls.
Marine sources- emissions based on modes of operation, activity data from port operations.

question

How good are emission factors?

answer

AP-42 emission factors have a quality rating
Emission factors for other cources are typically not rated.
AP-42 EPA emission test rating goes from A-D.

question

AP-42 Emission factor ratings are:

answer

1.) qualitative
2.) They do not convey any quantitative information regarding the precision or accuracy of the emission factor
3.) Emission factors should be used to estimate average emission rates for many similiar sources
they are not intended for use for an individual source

question

Emission inventory formula

answer

EI= Sum of ( EFi x AFi)

where EI= Emission inventory
EFi= Emission factor for source category, i.
AFi= Activity factor for source category i.

question

What are some challenges in emission inventories?

answer

1) they may not be representative of a particular region, operating condition, season.
2.) Might not be for the time period of interest.
3.) Might not be for the geographic region or resolution of interest
4.) May be interested in a projection of future emissions, based on eonomic and population growth.
5.) May not be for the chemical species of interest, may have to speciate

question

National inventories for selected pollutants include:

answer

Sulfur Dioxide, Nitrogen oxides, carbon monoxide, particulate matter, and volatile organic compounds.

question

Info on the equilibrium:

answer

1.) Net reaction rate is zero.
2.) Assumes sufficient time has elapsed for forward and reverse reactions to equilibrate
3.) Provides insight into driving forces for the formation of a given compound.
4.) May not be achieved in a real system.

question

Equilibrium and thermal NO formation

answer

Overall reaction ( Look at forumala)
Equilibrium constant, K, is a function of temperature.

question

Equilibrium constant

answer

For an ideal gas, can derive the equilibrium constant from:
First law of thermodynamics
energy balance ( energy conservation)
Second law of thermodynamics
constraint of direction of energy flow
driving forces

question

Equilibrium constant is

answer

1.)Not a constant
2.) Depends upon temperature
Exothermic reaction: Decreases with temperature
Endothermic reaction: inreaceses with temperature.

question

Equilibrium Calculations:

answer

1.) Approximate solution will tend to underestimate the products
2.) Does not account for depletion of reactants.
3.) Depletion of reactants reduces driving force for formation of the products.
4.) No equilibrium is sensitive to initial mole fractions.

question

NO equilibrium is:

answer

NO formation is endothermic
NO levels increase with temperature
At a given temperature, NO levels increase with increasing )2 levels.
NO control: Avoid high tempoeratures
avoid high O2 levels.

question

Equilibrium for CO

answer

Formation of CO from CO2 is endothermic
Occurs at very high temperatures
CO2 becomes unstable

question

CO Emissions

answer

With sufficient excess air, CO emissions due to incomplete combustion should be low
CO can be formed at high tempoeratures
as flue gas cools, CO will tend to oxidize CO2
if flue gas cools too rapidly, the oxidation cannot be completed.

question

CO emissions:

answer

furnaces tend to have low Co emissions because there is suffiecient residence time for CO to oxidize as flue gas is cooled
Internal Combustion engines tend to have high CO emissions because the exhaust gases are cooled too rapidly.

question

Equilibrium between NO and NO2

answer

formation of NO2 from NO is exothermic
favored as the temperature is lowered
at combustion temperatures, NO is the main component of NOx
At atmospheric temperatures, NO2 becomes the main component of NOx

question

Why might equilibrium not be achieved?

answer

Residence time < time required to reach equilibrium

question

What does chemical equilibrium do?

answer

Provides insigh into qualitative aspects of pollutant formation and prevention ex.
temperature, and concentration
need to also consider chemical kinetics.

question

Chemical Kinetics

answer

Gas molecules react only when they come close enough for direct energy exchance leading to bond breaking
close enough means a few angstroms

question

Chemical Kinetics and collisions:

answer

Collisions between molecules are short in duration compared to the mean time between collisions
Collisions must occur with sufficient energy to break bonds

question

Conditions for a chemically kinetic reaction:

answer

Collision
Sufficient energy to break bond
orientation of molecules

question

Is the total collision rate= reaction rate?

answer

Only if every collision results in a reaction
Actual speed of each molecule is different
Speed distribution described by the maxwell boltzman distribution function

question

What else affects the reaction rate of chemical kinetics?

answer

Strength of chemical bonds
orientation of molecules during collision

question

What has to be true about energy required in chemical kinetics?

answer

Energy required to create activated complex increases as the relative orientation of the molecules deviates from the most favorable orientation

question

Chemical kinetics an fraction of molecules:

answer

Fraction of molecules with sufficient energy of relative motion may change substantially and nonlinearly with temperature

question

CK and energy

answer

Total energy is constant
Kinetic vs potential
High potential energy when a bond is broke, but not yet re-formed ( actviated complex)

question

CK and elementary process

answer

For an elementary process, the reaction order is predicatable.
Elementary process: a simple event in which some kind of transformation ours.
Example: Two molecules colliding and reacting.

question

Elementary process ( order and molecularity)

answer

Order and molecularity are the same
Unimolecular= First order overall
Bimolecular= second order overall
termolecular= third order overall

question

Reaction mechanisms are:

answer

Sequence of elementary proesses
each step is unimolecular, bimolecular, termolecular

question

Reaction mechanisms and products

answer

The products can form no faster than the slowest step
rate determining step

question

Pseudo steady state assumption is:

answer

highly reactive species are consumed as rapidly as they are formed.

question

Examples of pseudo steady state assumption

answer

free radicals, activated complexes

question

Kinetics of NO formation

answer

Simplified Zeldovich kinetics of thermal NO Formation

question

Details of thermal NO Formation

answer

At high temperatures, free radical are present
formation of free radical is endothermic
Once formed they are high reactive

question

Thermal NO formation and equilibrium

answer

Equilibrium can be used to estimatte the amounts of these free radicals
in reality, radical may be formed due to a collision of a molecule with some other molecule M:

question

Assumptions of the simplified Zeldovich mechanism:

answer

Oxygen radicals attack nitrogen molecules
Nitrogen radicals attack oxygen molecules

question

More assumptions of the simplified Zeldovich mechanism

answer

O radicals are in equilibrium with O2
N radicals are in a pseudo steady state
Overall reaction rate is given by:

question

Insights from chemical kinetics

answer

Actual compound formed may be muh less than the equilibrium value
reaction rates can be very slow at low temperatures
at high temperatures, may more rapidly approach equilibrium.

question

Air pollution meterology and scale of problems

answer

Macroscale: 1000s of km
Mesosccale: 100s of km
Micro scale: ~ 1km

question

Geographic and temporal scales

answer

Acid deposition- seasonal, annual, 1000km
troposhperic ozone- daily, 50-100 km
CO due to highways- hourly, local

question

Temperature profiles in the atmosphere

answer

Atmosphere is comprised of:
troposphere: 0 to approx 15km
Stratosphere: to about 50 km
Mesosphere: to about 85 km
thermosphere:more than about 85 km

question

Troposphere

answer

Temperature decreases with altitde
Air is typically unstable
sunlight heats surface
Air near surface is heated
Buoyancy
Convection- vertical motion
contains about 90% of all air

question

Troposphere and planteary boundary layer

answer

Approx the lower 500 meters of the atmosphere
Winds influences by frictional drag of the earths surface.

question

Stratosphere

answer

Contains the ozone layer
Absorbtion of sunlight by ozone causes warming
temperature increases with altitude
air is stable
contains about 9.5% of all air.

question

Temperature Inversion

answer

When warm air overlies cooler air
stable with little vertical mizing

question

Mesosphere

answer

Temperature decreases as altitude increases
less than 1 percent of total air mass
small meteors burn
contains lowest layers of ionosphere

question

Thermosphere

answer

Air is heated by X rays and other radiation
Temperature increases with altitude
highly ionized

question

Temperature in the lower atmosphere

answer

Temperature profile-
Temperature vs altitude
determines atmospheric stability ie tendency of an air parcel to rise or fall.
Less stability, more motion, leads to better dispersion of air pollution
stable atmospheric conditions can result in build up of pollutants

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