CE2100 Air Pollution Study Guide

3/27/25 Masters and Ela (Chap. 7.1-7.4), AirTrends_Flyer23.pdf, Masters and Ela: Pages 404-405; 410-413; 431– 436

What is the difference between a primary and secondary pollutant?

Primary Pollutant

• substances that are emitted directly into the atmosphere

• eg nitrogen oxides and hydrocarbons are emitted when fuels are burned

• most enter the atmosphere due to combustion, evaporation, or grinding and abrasion

• combustion example: automobile exhaust emissions and power plant stack gases

• evaporation example: gasoline, paints, cleaning fluids

• grinding and abrasion example: dust is kicked up when land is plowed and asbestos fibers flake off of pipe insulation

• combustion accounts for most emissions

Secondary Pollutant

• substances that are created by various physical processes and chemical reactions that take place in the atmosphere

• eg ozone is created when nitrogen oxides and hydrocarbons react with each other in the atmosphere

(pg 369 of textbook)

Which criteria air pollutant(s) is/are secondary pollutants?

• ozone (O₃)

  • ground-level ozone: harmful to health

    • which is different from stratospheric ozone that protects health by shielding us from the sun’s UV radiation

(pg 369 of textbook)

What is a Corporate Average Fuel Economy (CAFE) standard?

• enacted in 1975

• required fuel efficiency for each manufacturer’s fleet of new automobiles to average at least 27.5 miles per gallon (mpg) within a 10-year period

• American Council for an Energy Efficient Economy estimates raising CAFE standards by 5 percent annually until 2012 and by 3 percent per year thereafter could save 67 billion barrels of oil over the next 40 years

  • 10 to 20 times greater than the potential oil supply from the Arctic National Wildlife Refuge

(pg 404 of textbook)

Carbon Monoxide (CO)

• one of the six criteria pollutants (pg 371 of textbook)

• Over two-thirds of the mass of all the pollutant emissions in the United States (pg 380 of textbook)

Primary Source

• produced when carbonaceous fuels are burned under less than ideal conditions

• incomplete combustion when any of the following four variables are not kept sufficiently high: oxygen supply, combustion temperature, gas residence time at high temperature, and combustion chamber turbulence

• over 80% emissions from transportation sector

• Highway vehicles (13 million tons, 30.8%)

• Industrial and other processes (12.9 million tons, 30.4%)

• Non-road mobile (11.6 million tons, 27.5%)

• Stationary fuel combustion (4.8 million tons, 11.3%)

Impacts

• Health

  • breathing elevated levels of it reduces the amount of oxygen reaching the body’s organs and tissues

  • for those with heart disease, this can result in chest pain or other symptoms that lead to hospital admissions and emergency department visits

• Environmental

  • can contribute to the formation of carbon dioxide CO₂ and ozone, which are greenhouse gases that warm the atmosphere

Lead (Pb)

• one of the six criteria pollutants (pg 371 of textbook)

Primary Source

• most emissions in the past from motor vehicles burning gasoline containing the antiknock additive, tetraethyllead Pb(C₂H₅)₄

• major remaining sources: industrial processes (particularly metal-processing plants) and leaded fuels for aviation and nonroad vehicles (pg 397 of textbook)

Impacts

• Health

  • depending on the level of exposure, it may harm the developing nervous system of children, resulting in lower IQs, learning deficits, and behavioral problems

  • long-term exposure to higher levels may contribute to cardiovascular effects, such as high blood pressure and heart disease in adults

• Environmental

  • elevated amounts accumulated in soils and fresh water bodies can result in decreased growth and reproductive rates in plants and animals

Nitrogen Dioxide (NO₂)

• one of the six criteria pollutants (pg 371 of textbook)

• of the nitrogen oxides, this gas poses the greatest health concern

Primary Source

• Stationary fuel combustion (2.2 million tons, 32.6%)

• Highway vehicles (1.7 million tons, 24.3%)

• Non-road mobile (1.6 million tons, 22.7%)

• Industrial and other processes (1.4 million tons, 20.3%)

Impacts

• Health

  • short-term exposure can aggravate respiratory diseases, particularly asthma, leading to respiratory symptoms, hospital admissions, and emergency department visits

  • long-term exposure may contribute to asthma development and potentially increase susceptibility to respiratory infections

• Environmental

  • can react with volatile organic compounds (VOCs) to form ozone and with ammonia and other compounds to form particle pollution, resulting in ecological impacts

  • deposition of this pollutant contributes to the acidification and nutrient enrichment (eutrophication, nitrogen saturation) of soils and surface waters

  • contributes to ozone formation

  • contributes to direct and indirect effects on vegetation, soils, and animals

Ground-Level Ozone (O₃)

• one of the six criteria pollutants (pg 371 of textbook)

• most abundant of the photochemical oxidants (pg 384 of textbook)

Primary Source

• When NO_x, VOCs, and sunlight come together, they produce this photochemical oxidant

Impacts

• Health

  • depending on the level of exposure, it can cause respiratory effects, like coughing and difficulty breathing

  • aggravates asthma and lung diseases, like emphysema, leading to medication use, hospital admissions, and emergency department visits

  • studies in locations with elevated concentrations report associations with deaths from respiratory causes

  • long-term exposure is linked to aggravation of asthma

  • likely one of the many causes of asthma development

• Environmental

  • damages vegetation by injuring leaves, reducing photosynthesis, impairing reproduction and growth, and decreasing crop yields

  • damage to plants may alter ecosystem structure, reduce biodiversity, and decrease plant uptake of CO₂

  • is a greenhouse gas that contributes to the warming of the atmosphere

Sulfur Dioxide (SO₂)

• one of the six criteria pollutants (pg 371 of textbook)

• among the sulfur oxides, it is the most commonly occurring in the atmosphere and the one most clearly associated with human health effects

Primary Source

• about 86% of the 15 million tons per year of emissions result of fossil fuel combustion in stationary sources, and most emitted by coal-fired power plants

• only about 5% from highway vehicles

• significant noncombustion sources associated with petroleum refining, copper smelting, and cement manufacture

• Stationary fuel combustion (1 million tons, 62.1%)

• Industrial and other processes (0.6 million tons, 36.1%)

• Non-road mobile (0 million tons, 1.1%)

• Highway vehicles (0 million tons, 0.7%)

Impacts

• Health

  • short-term exposures linked with respiratory effects including difficulty breathing and increased asthma symptoms

  • its effects are particularly problematic for people with asthma while breathing deeply, such as when exercising or playing

  • short-term exposures connected to increased emergency department visits and hospital admissions for respiratory illnesses, particularly for at-risk populations like children, older adults, and those with asthma

  • contributes to particle formation with associated health effects

• Environmental

  • form particle pollution resulting in associated ecological effects

  • deposition of this pollutant contributes to the acidification of soils and surface waters and mercury methylation in wetland areas

  • acidification can cause injury to vegetation and species loss in aquatic and terrestrial systems

Particulate Matter

• one of the six criteria pollutants (pg 371 of textbook)

• specifically particles with an aerodynamic diameter of less than or equal to 10 micrometers (PM₁₀) (since 1987) or particulates smaller than 2.5 microns (PM_2.5) (since 1997)

Primary Source

• Direct PM_2.5

  • Stationary fuel combustion (0.9 million tons, 51.3%)

  • Industrial and other processes (0.7 million tons, 39.4%)

  • Non-road mobile (0.1 million tons, 5.6 %)

  • Highway vehicles (0.1 million tons, 3.7%)

• Direct PM₁₀

  • Industrial and other processes (1.1 million tons, 47.9%)

  • Stationary fuel combustion (0.9 million tons, 39.7%)

  • Highway vehicles (0.2 million tons, 8.2%)

  • Non-road mobile (0.1 million tons, 4.3%)

Impacts

• Health

  • short-term exposure, particularly PM_2.5, can cause harmful effects on the cardiovascular and respiratory systems, including heart attacks, irregular heartbeat, aggravated asthma, and chronic obstructive pulmonary disease (COPD), decreased lung function, and increased respiratory symptoms, like irritation of the airways, coughing, or difficulty breathing

  • short-term exposures and related health effects can result in emergency department visits, hospitalizations, and premature death in people with heart or lung disease

  • long-term exposures can lead to the development of cardiovascular diseases, like atherosclerosis, respiratory diseases like asthma, nervous system effects (eg cognitive effects), lung cancer, and premature mortality

• Environmental

  • main cause of reduced visibility (haze) in parts of the US, including many national parks and wilderness areas

  • can be carried over long distances by wind and settle on soils or surface waters

  • effects of settling include making lakes and streams acidic, changing the nutrient balance in coastal waters and large river basins, depleting nutrients in soil, damaging sensitive forests and farm crops, and affecting the diversity of ecosystems

  • can stain and damage stone and other materials, including culturally important objects like statues and monuments

  • can contribute to climate effects, including radiative forcing

  • indirect effects on cloud brightness and changes in precipitation

What is methyl tertiary butyl ether (MTBE)?

• its use is motivated by the Clean Air Act Amendments of 1990 which required 2% oxygen (by weight) in fuels used in areas that have high levels of carbon monoxide CO pollution

• oxygenates used to reformulate gasoline, which helps reduce tailpipe emissions of carbon monoxide CO and hydrocarbons HC as they encourage more complete combustion

• it is the preferred oxygenate because of its higher octane number and lower volatility than ethanol

• however, it’s water soluble and does not biodegrade easily

• leaking underground fuel tanks and fuel spills allowed it to contaminate groundwaters and reservoirs, creating a potential drinking water hazard

• studies with rats and mice suggest drinking it may cause gastrointestinal irritation, liver and kidney damage, and nervous system effects

• some evidence that inhalation while pumping gasoline can cause nausea, dizziness, mental confusion, and headaches

• California banned its use as of 2003

(pg 413 of textbook)

How does MTBE relate to air pollution and water pollution?

• used to reformulate gasoline, which helps reduce tailpipe emissions of carbon monoxide CO and hydrocarbons HC as it encourages more complete combustion

• it’s water soluble and does not biodegrade easily

• leaking underground fuel tanks and fuel spills allowed it to contaminate groundwaters and reservoirs, creating a potential drinking water hazard

(pg 413 of textbook)

Which of the criteria pollutants has seen the greatest and least change in emissions?

Based on emission totals:

• Greatest: sulfur dioxide SO₂ (93% decrease since 1990)

• Least: direct particulate matter 10 microns PM₁₀ (27% decrease since 1990)

Relative to the national ambient air quality standards, for which criteria pollutant(s) are we doing the best?

Based on concentration averages:

• sulfur dioxide SO₂ 1-Hour (92% decrease since 1990)

• lead Pb 3-month average (87% decrease since 1990)

• carbon monoxide CO 8-hour (79% decrease since 1990)

Based on emission totals:

• sulfur dioxide SO₂ (93% decrease since 1990)

• nitrogen oxides NO_x (73% decrease since 1990)

• carbon monoxide CO (71% decrease since 1990)

Relative to the national ambient air quality standards, for which criteria pollutant(s) are we doing the worst?

Based on concentration averages:

• ozone O₃ 8-Hour (18% decrease since 1990)

• particulate matter 2.5 microns PM_2.5 24-Hour (29% decrease since 1990)

• particulate matter 10 microns PM₁₀ 24-Hour (29% decrease since 1990)

Based on emission totals:

• direct particulate matter 10 microns PM₁₀ (27% decrease since 1990)

• direct particulate matter 2.5 microns PM_2.5 (28% decrease since 1990)

How do trends in air pollution emissions compare to other socio-economic trends?

• Between 1970 and 2023, the combined emissions of the six common pollutants (PM_2.5 and PM₁₀, SO₂, NO_x, VOCs, CO and Pb) dropped by 78%

• progress occurred while the U.S. economy continued to grow, Americans drove more miles and population and energy use increased

• Progress occurred while U.S. economic indicators remain strong.

• Gross domestic product increasing the most (1st) as six common pollutants decrease (+321%)

• Vehicles miles traveled increasing (2nd highest) as six common pollutants decrease

• Population slightly increasing (3rd highest) as six common pollutants decrease

• Energy consumption increasing but also going down (4th highest) as six common pollutants decrease

• CO₂ emissions increasing but steadily going down (5th highest) as six common pollutants decrease

• Six common pollutants (aggregate emissions) decreasing more and more each year (-78%)

(https://gispub.epa.gov/air/trendsreport/2024/#home)

Name and describe various approaches to removing particulate matter from point sources

• appropriate device dependent on factors like, particle size, concentration, corrosivity, toxicity, volumetric flow rate, required collection efficiency, allowable pressure drops, and costs

• cyclone collectors

  • also known as centrifugal collectors, they are most commonly used for relatively large particles

  • particle-laden gas enters tangentially near the top of the cyclone

  • as gas spins in the cylindrical shell, centrifugal forces cause the particles to collide with the outer walls, and gravity causes them to slide down into a hopper at the bottom

  • spiraling gases then exit the collector from the top

  • efficiencies of cyclones can be above 90% for particles larger than 5 micrometers, but the efficiency drops off rapidly for small particle sizes that pose a greater concern for human health

  • not efficient enough to meet emission standards

  • however, they are relatively inexpensive and maintenance free, making them ideal as precleaners for more expensive and critical final control devices, like baghouses and electrostatic precipitators

• electrostatic precipitators

  • used by most utility power plants to collect really small particles

  • one configuration uses vertical wires that are placed between parallel collector plates

  • plates are grounded and wires are charged up to a very high (negative) voltage of about 100,000 V

  • intense electric field created near the wires causes a corona discharge, ionizing gas molecules in the air stream

  • negative ions and free electrons move toward the grounded plates, and along the way, some attach themselves to passing particulate matter

  • particles now carry a charge, causing them to move under the influence of the electric field to a grounded collecting surface

  • particles are removed from the collection electrode either by gravitational forces, by rapping, or by flushing the collecting plate with liquids

  • may have hundreds of parallel plates, with total collection areas measured in tens of thousands of square meters

  • particles in the gas stream acquire negative charge as they pass through the corona and are then attracted to the grounded collecting plates

  • can remove more than 98% of the particles passing through them, including particles of submicron size

  • some efficiencies greater than 99.9%

  • can handle large flue-gas flow rates with little pressure drop

  • relatively low operation and maintenance costs

  • versatile as it can operate on both solid and liquid particles

  • expensive and take up a lot of space

  • area requirements increase nonlinearly with collection efficiency (using Deutsch-Anderson equation)

• baghouses

  • major competition electrostatic precipitators have for efficient collection of small particles is fabric filtration

  • dust-bearing gases pass through fabric filter bags, which are suspended upside down in a large chamber called a baghouse

  • baghouse may contain thousands of bags, which are often distributed among several compartments

    • allows individual compartments to be cleaned while others remain in operation

  • part of the filtration is accomplished by the fabric itself

  • significant part of the filtration is caused by the dust that accumulates on the inside of the bags

  • efficiencies approach 100% removal of particles as small as 1 micrometer

  • substantial quantities of particles as small as 0.01 micrometers are also removed

  • some disadvantages include its large size, expensive cost, and it can be harmed by corrosive chemicals in the flue gases

  • they also cannot operate in moist environments

  • potential for fires or explosions if dust is combustible

(pg 432-436 of textbook)

How does a three-way catalytic converter work?

• to function correctly, it must operate within a very narrow band of air/fuel ratios near the stoichiometric point or else its ability to reduce all three pollutants at once is compromised

• uses precise electronic feedback control systems to monitor the composition of exhaust gas and feeds that information to a microprocessor-controlled carburetor or fuel-injection system (closed-loop control system)

  • malfunction in this control system turns on the dashboard engine light

• sensor monitors the exhaust that moves through the catalytic converter, sensor sends a sensor voltage to the electronic controller, which sends an electronic feedback signal to the actuator that controls the air/fuel metering system (where are air fuel first enter the system)

• very effective once they are warmed up, but when they are cold, and when there are spurts of sudden acceleration or deceleration, they can release excessive amounts of pollution that may offset their benefits

(pg 410-412 of textbook)

What is a three-way catalytic converter’s purpose?

• three-way means it handles all three pollutants: carbon monoxide CO, hydrocarbons HC, and nitrogen oxides NO_x

• the approach most favored by automobile manufacturers to achieve emission standards set by the Clean Air Act

• it is able to oxidize hydrocarbons and carbon monoxide to carbon dioxide, while reducing nitrogen oxides NO_x and nitrogen gas N₂ all in the same catalyst bed

• very effective in controlling emissions

• advantage of allowing the engine to operate at near stoichiometric conditions where engine performance and efficiency are greatest

• must operate within a very narrow band of air/fuel ratios near the stoichiometric point or else its ability to reduce all three pollutants at once is compromised

• uses precise electronic feedback control systems to monitor the composition of exhaust gas and feeds that information to a microprocessor-controlled carburetor or fuel-injection system (closed-loop control system)

  • malfunction in this control system turns on the dashboard engine light

(pg 410-412 of textbook)

How did catalytic converters lead to dramatic reductions in lead emissions?

• catalysts are quickly destroyed if leaded fuels are burned, resulting in no pollution control

• development of catalytic converters led to the virtual elimination of tetraethyllead as an octane booster in gasoline

• lead “poisons” the catalyst and renders it ineffective by coating its surface, which keeps the exhaust gases from making contact

(pg 412 of textbook)

How are acidic gases controlled?

controlling nitrogen oxides NO_x emissions

• nitrogen oxides formed partly by the oxidation of nitrogen in the fuel itself (fuel NO_x) and partly by the oxidation of nitrogen in the combustion air (thermal NO_x)

• coal-fired plants responsible for ¼ of total NO_x emissions in the US, and they emit roughly twice as much fuel NO_x as thermal NO_x

• modifying the combustion process reduces both sources of nitrogen oxides NO_x

• low excess air

  • amount of air made available for combustion is carefully controlled at the minimum amount required for complete combustion

  • can be retrofitted onto some boilers at a modest cost, yielding from 15 to 50% lower NO_x emissions

• low NO_x burner technology

  • new, second-generation technology that promises greater NO_x removal efficiencies and can be retrofitted onto more existing furnaces

  • employ a staged combustion process that delays mixing the fuel and air in the boiler

  • in the first stage of combustion, fuel starts burning in an air-starved environment, causing fuel-bound nitrogen to be released as nitrogen gas N₂ rather than NO_x

  • next stage introduces more air to allow complete combustion of the fuel to take place

  • potential NO_x reductions of 45 to 60% likely

• limestone injection multistage burner (LIMB) technology

  • combustion modification that incorporates a staged burner for NO_x control and limestone injection for sulfur dioxide SO₂ control

  • under development but promising

• selective catalytic reduction (SCR)

  • a form of precombustion controls that can augment or replace low-NO_x burners

  • widely used in Europe and Japan, recently applied to high-sulfur content US coal

  • ammonia NH₃ is injected into boiler flue gas, and the mix is passed through a catalyst bed where nitrogen oxides NO_x and ammonia NH₃ react to form nitrogen and water vapor

  • SCR for 90% NO_x removal could add $66/kW (1996 dollars) to capital cost of a 250-MW coal-fired power plant, resulting in a 30-year, levelized extra cost of about 0.3 cents/kWh

controlling sulfur dioxide SO₂ emissions

• flue gas desulfurization (FSD) technologies can be categorized as being either wet or dry depending on the phase where the main reactions occur

• FSD can also be categorized as either throwaway or regenerative, depending on whether or not sulfur from the flue gas is discarded or recovered in a usable form

• most of these scrubbers that operate in the US use wet, throwaway processes

• in most wet scrubbers, finely pulverized limestone CaCO₃ is mixed with water to create a slurry that is sprayed into the flue gases

• flue gas SO₂ is absorbed by the slurry, which produces calcium sulfite CaSO₃ or a calcium sulfate CaSO₄ precipitate

• precipitate is removed from the scrubber as a sludge

• about 90% of the sulfur dioxide SO₂ can be captured from the flue gas using limestone in wet scrubbers

• wet scrubbers sometimes use lime CaO instead of limestone in the slurry, which can achieve greater SO₂ removal efficiencies up to 95%

• however, lime is more expensive than limestone and is therefore not widely used

• dry scrubbers must use lime, making their increased cost a reason as to why they aren’t widely used

• wet scrubbers can capture very high fractions of flue gas SO₂, but they are expensive and cost $200 million for a large power plant

• if wet scrubbers are installed on older plants, with less remaining lifetime, their capital costs must be amortized over a shorter period of time, and annual revenue requirements increase accordingly

• scrubbers reduce the net energy delivered to transmission lines as the energy to run scrubber pumps, fans, and flue gas reheat systems require about 5% of the total power produced by the plant

• scrubbers are subject to corrosion, scaling, and plugging problems, reducing the power plant’s reliability

• scrubbers use large amounts of water and create large volumes of sludge with the consistency of toothpaste

  • sludge treatment involves oxidation of calcium sulfite to calcium sulfate, precipitating easier thickening and vacuum filtration

  • calcium sulfate, aka gypsum, can be reused in the construction industry

(pg 431-432 of textbook)