Energy and the Environment

1. Energy and the Environment Fall 2012 Instructor: Xiaodong Chu Email：chuxd@sdu.edu.cn Office Tel.: 81696127

2. Flashbacks of Last Lecture • Several manufacturers currently produce passenger cars (乘用车)and pickup trucks that are battery-powered • They have limited passenger- or freight-carrying capability compared to conventional vehicles and have much smaller travel range between recharges of the onboard (车载) energy supply • The traction motor (牵引电机) can regenerate a partial battery charge during periods of vehicle deceleration

3. Flashbacks of Last Lecture • Hybrid vehicles are those that combine conventional power sources (SI or CI engines) with electric motors to power the vehicle • The motor/generator can store energy in a battery bank when excess power is available, during deceleration or when the power need is less than what the combustion engine can deliver, and can deliver extra power to the wheels when it is temporarily needed for acceleration or hill climbing

4. Flashbacks of Last Lecture • Prototypes of electric drive vehicles whose electric power is supplied by fuel cells have been under development for several decades • The synthetic fuel transformation penalties diminish the fuel efficiency advantage of fuel cells compared to conventional internal combustion engines in vehicles fueled by conventional hydrocarbon fuels

5. Flashbacks of Last Lecture • Vehicle emissions to the atmosphere are of two kinds: exhaust emissions (废气排放/尾气排放) and evaporative emissions(蒸发排放) • The first are the combustion gases (燃烧气体)emitted while the engine is running • The second are emissions of fuel vapors (燃油蒸气)from the fuel supply system and the enginewhen the engine isnot operating • The federal government regulates both of these emissions by requiring the manufacturers of new vehicles sold in the United States to provide the technology needed to limit these emissions for the useful life (使用寿命) of the vehicle and to warrant the performance of these control systems

6. Environmental Effects of Fossil Fuel Use: Air Pollution • After leaving the smoke stack (烟囱) or exhaust pipe (排气管), the primary air pollutants disperse into the atmosphere by turbulent diffusion(湍流扩散) , advection by winds(风的平流), and transform into secondary pollutants by chemical reactions among themselves and with atmospheric species • The estimation of the concentration of pollutants in space and time is called air-quality modeling(空气质量模型)

7. Environmental Effects of Fossil Fuel Use: Air Pollution

8. Environmental Effects of Fossil Fuel Use: Air Pollution • A basic information necessary for air-quality modeling is the wind statistics for the modeling domain and the dispersion characteristics of the atmosphere • Multiyear wind statistics are necessary for predicting the advection by winds of pollutants from the sources(源) to the receptor(受体)

9. Environmental Effects of Fossil Fuel Use: Air Pollution • In the atmosphere, dispersion occurs mostly by turbulent or eddy diffusion(湍流或涡流扩散) • The cause for turbulent diffusion is either mechanical or thermal • Mechanical turbulence is due to wind shears(风切变) in the free atmosphere (自由大气) or friction experienced by winds blowing over the ground surface and obstacles, such as tree canopies(树冠), mountains, and buildings • In the lower troposphere, the temperature is usually higher near the ground and declines with altitude

10. Environmental Effects of Fossil Fuel Use: Air Pollution • When the temperature in the upper layers is much colder than that in the lower layers, upper air parcels (气块)fall downward due to their larger density, and lower air parcels move upward • The steeper the temperature gradient(温度梯度), the more the turbulent intensity(湍流强度), which is called an unstable condition • A temperature gradient that is equal to the dry adiabatic lapse rate(干绝热递减率)is called a neutral condition, and it leads to moderate turbulence • A temperature gradient that is less steep than the dry adiabatic lapse rate, or even a positive gradient, is called a stable condition, in which there is minimal or no turbulence at all

11. Environmental Effects of Fossil Fuel Use: Air Pollution • When an effluent (排放物) from a power plant stack leaves the top exit of the stack, it immediately begins to mix with the surrounding atmosphere • This mixing process begins to dilute(稀释) the concentration of the stack gas, the more so the more atmospheric air is entrained in the stack plume(烟羽) • The concentration of the plume gas, a mixture of the principal products of combustion and the air pollutants of interest, steadily declines with downwind distance from the stack exit • Measurements of the pollutant gas concentrations downwind of stacks show that, at any distance downwind, they are maximum at the plume centerline and decline with vertical or lateral distance(竖向或横向距离) from the plume centerline (the Gaussian plume model)

12. Environmental Effects of Fossil Fuel Use: Air Pollution

13. Environmental Effects of Fossil Fuel Use: Air Pollution • Photo-oxidants (光氧化剂) are a class of secondary air pollutants formed from some of the primary pollutants emitted by fossil fuel combustion • They irritate (刺激)and destroy (oxidize) the respiratory tract(呼吸道), eyes, skin, animal organs, vegetation tissues(植被), and materials and structures • The major representative of this class of chemicals is ozone, O3, but other compounds are included: ketones(酮), aldehydes(醛), alkoxy radicals (RO)(氧自由基), peroxy radicals (RO2)(过氧自由基), peroxyacetyl nitrate (PAN)(过氧硝酸), and peroxybenzoyl nitrate (PBN)(硝酸过氧化苯甲酰)

14. Environmental Effects of Fossil Fuel Use: Air Pollution • Tropospheric ozone(对流层臭氧) (the “bad” ozone) is to be distinguished from stratospheric ozone(平流层臭氧) (the “good” ozone) • Tropospheric ozone is mainly formed as a consequence of fossil fuel combustion, while stratospheric ozone is formed naturally by photochemical reactions under the influence of solar ultraviolet radiation(太阳紫外线辐射) • Some of the tropospheric ozone may be due to intrusion of stratospheric ozone into the troposphere, which constitutes a background level of ozone • Compared to concentrations of ozone in an urban polluted atmosphere, the background ozone is a small fraction of that concentration

15. Environmental Effects of Fossil Fuel Use: Air Pollution • The only precursor (前体) that can initiate ozone formation in the troposphere is nitrogen dioxide, NO2 • The sum of NO and NO2 is termed nitric oxides, NOx • Nitric oxides are formed primarily in the combustion of fossil fuels • The other precursors to sustain the formation are hydrocarbon molecules(烷烃分子) and other organic compounds, collectively known as volatile organic compounds (VOC)(挥发性有机化合物)

16. Environmental Effects of Fossil Fuel Use: Air Pollution • Researchers developed models that attempted to show how observed concentrations of ozone and other smog(烟雾) ingredients are reached as a function of precursor concentrations, meteorological conditions, and insolation • The model that has been used most frequently in the past is called the Empirical Kinetic Modeling Approach (EKMA)(经验动力学模拟方法) • The shortcomings of EKMA are that it models the ozone concentrations only within a particular air column(空气柱), and only for a single day • Regulatory agencies in many countries rely now on more sophisticated models that cover much larger areas than a single air column and also cover a longer time period

17. Environmental Effects of Fossil Fuel Use: Air Pollution • Acid deposition (酸沉降)is popularly termed acid rain • Acid deposition is a more appropriate term because acidic matter can be deposited on the ground not only as rain but also in other kinds of precipitation—for example, snow, hail(冰雹), and fog—and in dry form • The deposition by precipitation(降水)is called wet deposition; the direct impaction on land and water of acidic gaseous molecules (酸性气体分子) and acidic aerosols(酸性气溶胶) is called dry deposition • Acid deposition is a secondary pollutant, because it is a result of transformation of primary emitted pollutants

18. Environmental Effects of Fossil Fuel Use: Air Pollution • Power plant, industrial, commercial, residential, and mobile sources emit the precursors of acid deposition, namely, sulfur and nitrogen oxides (SOx and NOx) • The precursors are advected by winds and are dispersed by turbulent diffusion • During transport in the air, the precursors react with various oxidants present in the air and water molecules to form sulfuric and nitric acid (H2SO4 and HNO3) • The acids are deposited on land and water in the dry and wet form

19. Environmental Effects of Fossil Fuel Use: Air Pollution • Small particles (also called fine particles) less than 1–2 μm in diameter settle very slowly on the ground and can travel hundreds to thousands of kilometers from their emitting sources • A part of the fine particles are emitted directly from industrial, commercial–residential, and transportation sources, which are called primary particles(初级粒子) • The majority of fine particles is a product of gas-to-particle (气粒转化) transformation processes, including photochemical processes, which are called secondary particles(二次粒子)

20. Environmental Effects of Fossil Fuel Use: Air Pollution • The composition of the fine particles varies from region to region, depending on the precursor emissions • The small particles are efficient scatterers(散射体) of light • The scattering efficiency is dependent upon the wavelength • Maximum scattering efficiency for visible light (400–750 nm) occurs with particles less than 1 μm in diameter, the so-called submicron particles (亚微米粒子) • Light scattering prevents distant objects from being seen, which is called visibility impairment(能见度降低)

21. Environmental Effects of Fossil Fuel Use: Air Pollution • Visibility impairment is a significant and unpleasant side effect of fossil fuel use • Visibility improvement can be accomplished by reducing fine-particle and gaseous precursor emissions from fossil fuel combustion and other fossil fuel usage • In practice this means improved emission control devices for primary particles, SO2, NOx, and VOC

22. Environmental Effects of Fossil Fuel Use: Water Pollution • The consumption of fossil fuel entails a significant impact on water quality and water usage • The contamination of water starts at the mining and extraction stage, through transport and refining, all the way to leaching into the groundwater of ash(灰) and scrubber sludge(污泥) left behind after combustion of fossil fuels

23. Environmental Effects of Fossil Fuel Use: Water Pollution • The most serious water pollution problem associated with coal use is acid drainage(酸性排水) from mines, especially surface mines, coal piles, and coal washing • Precipitation falling on open coal seams (露天煤层) and on coal piles will leach out mineral matter, which contains acids, toxic elements, and often radioactive isotopes

24. Environmental Effects of Fossil Fuel Use: Water Pollution • Coal washing increases the heating value per unit mass of coal by removing the incombustible mineral matter • Coal washing removes pyritic sulfur(黄铁矿硫), which can amount up to 50% of the sulfur content of the raw coal • The most widely used technique for coal washing is mass separation(质量分离) • By flushing crushed coal in a stream of water, the mineral matter settles out, while the lighter coal particles float in the stream

25. Environmental Effects of Fossil Fuel Use: Water Pollution • Although much of the mineral matter of coal is removed at the mine mouth, coal delivered to a power plant or other facilities still contains adhering mineral matter, simply called ash • The ash content can amount to anywhere between 1% and 15% of the coal weight • Even oil contains ash, amounting to 0.01–0.5%by weight • After combustion of the coal particle(煤粉) or oil droplet(油滴), the mineral matter remains uncombusted, and either falls to the bottom of the boiler or is blown out with the flue gas as fly ash • In modern pulverized-coal-fired boilers(煤粉锅炉), about 90% of the mineral matter forms fly ash, and 10% bottom ash

26. Environmental Effects of Fossil Fuel Use: Water Pollution • Coal-burning power plants and industrial boilers are required to be equipped with flue gas desulfurization (烟气脱硫) devices • For high-sulfur-content coal (≥0.6% by weight), a wet limestone scrubber (湿石灰石洗涤) is necessary • A slurry of limestone (石灰石浆) is injected from sprinklers (喷头) at the top of the scrubber • Flue gas containing SO2 and other sulfur compounds flows countercurrent to the limestone spray • A sludge is collected in the bottom of the scrubber, consisting of wet calcium sulfate (gypsum)(硫酸钙) and calcium sulfite(亚硫酸钙) along with unreacted limestone

27. Environmental Effects of Fossil Fuel Use: Water Pollution

28. Environmental Effects of Fossil Fuel Use: Water Pollution • Power plants, must reject some of the fuel energy in the form of heat transfer to a cold reservoir • About one-third of the inherent heating value of the fuel is rejected via the steam condenser to the cold reservoir • Another third is rejected to the atmosphere via the stack gas, and only one-third is transformed into useful work • The cold reservoir is usually a water body • Some power plants and industrial boilers are located near surface waters, such as a river, lake, or ocean, which can be used for absorbing the heat from the condenser

29. Environmental Effects of Fossil Fuel Use: Water Pollution • In addition to sulfur and nitrogen oxides, there are other combustion products that escape from smoke stacks and eventually are deposited on land and water, which may cause deleterious health and environmental effects • Two cases are the atmospheric deposition of toxic metals and polycyclic aromatic hydrocarbons (PAH)(多环芳烃)

30. Environmental Effects of Fossil Fuel Use: Water Pollution • Fly ash particles may contain toxic metals, such as arsenic, cadmium, mercury, lead, selenium, vanadium, and zinc(砷、镉、汞、铅、硒、钒和锌) • These metals are found in small particles, less than 1 μm in diameter • Because of their small size, these particles are little affected by gravity and can be transported over large distances, hundreds to thousands of kilometers • Eventually, they are deposited in dry or wet form on land and water • From the land, toxic metals may leach into groundwater, or run off into streams, lakes, or ocean, entering the food chain

31. Environmental Effects of Fossil Fuel Use: Water Pollution • PAH are organic compounds consisting of two or more fused benzene rings (苯环) • Naphthalene(萘) has two rings, anthracene (蒽) three, pyrene (芘) and chrysene (屈) four, benzopyrenes(苯并芘) five, perylene (苝) six, and coronene(晕苯) seven • Some of the PAH are known or suspected carcinogens(可疑致癌物), notably benzopyrene • PAH are a product of incomplete combustion • Rivers, lakes, and coastal waters surrounded by urban-industrial areas are especially affected by PAH deposition from the myriad of combustion sources in those areas

32. Environmental Effects of Fossil Fuel Use: Water Pollution • PAH are not emitted as much from big centralized combustion sources as from small, dispersed sources • Large power plants, industrial boilers, and municipal incinerators are easily controlled for preventing emissions of products of incomplete combustion, of which PAHare a part • The control involves “good engineering practice,” which is combustion in excess air, thorough mixing of fuel and air, high flame temperature, and sufficient residence time in the combustion chamber

33. Environmental Effects of Fossil Fuel Use: Land Pollution • In regard to fossil fuel use, the heaviest toll on land impact is due to coal mining—in particular, surface mining, also called strip mining • Because the coal seam is rarely on the surface itself, this requires removing the “overburden” • The latter can amount to up to 100 m of soil, sand, silt(淤泥), clay (粘土), and shale(页岩) • Some coal seams appear in hill sides and river banks

34. Environmental Effects of Fossil Fuel Use: Land Pollution • Surface mining leaves behind enormous scars on the landscape, not to mention the disruption of the ecosystem that existed before mining started, or the loss of other possible uses of land instead of mining • Deep shaft mining (矿井开采) also places a burden on the land

35. Environmental Effects of Fossil Fuel Use: Land Pollution • The mined coal is brought to the surface, where it is crushed and washed • The removed mineral matter accumulates in enormous piles that despoil the landscape • Nowadays, strict regulations pertain in many countries as to the disposal of the coal mine slag and restoration of the landscape • The vast number of derricks(井架) associated with oil and gas exploration and exploitation is neither an aesthetically pleasing sight nor helpful to the ecology that existed on the land prior to oil and gas mining