Fossil Fuels Our Energy Resource of Convenience

Transcription

1 Lecture 13 March 5, 2019 Fossil Fuels Our Energy Resource of Convenience Natural Gas Coal Petroleum History of U.S. Energy Consumption by Source EJ = J State Rankings

2 Natural Gas Natural gas provides ~30% of all the energy used in the United States. It is especially important in homes, where it supplies nearly half of all the energy used for cooking, heating, and for fueling other types of home appliances. In 1821 in Fredonia, New York, William A. Hart drilled a 27 foot deep well in an effort to get a larger flow of gas from a surface seepage of natural gas. This was the first well intentionally drilled to obtain natural gas. In 1885, Robert Bunsen invented a burner that mixed air with natural gas. The "Bunsen burner" showed how gas could be used to provide heat for cooking and warming buildings. Natural gas can be found in a variety of different underground formations, including: shale formations, sandstone beds, and coal. Some of these formations make it more difficult and more expensive to produce natural gas Natural Gas Methane and small amounts of ethane, propane and butane Typically CH 4 takes up 60-80% Exact composition varies with the source Crude natural gas also contains small quantities of higher alkanes, CO 2, N 2, H 2 S, and noble gases Crude gas is treated to remove CO 2, sulfur compounds, water vapor, and hydrocarbons with MW greater than ethane Propane and butane are byproducts from crude natural gas. The liquid form of these products is liquefied petroleum gas, or LPG. He is another useful by-product. 2

3 Natural Gas On average, a typical coalburning power plant in 2013 was about 33% efficient in converting heat energy into electrical power. A gas-fired plant was about 42% efficient. And in natural gas combined-cycle power plants in which waste heat from a natural gas turbine is used to power a steam turbine generation may be as much as 60% efficient. Natural Gas Extraction - Hydraulic Fracturing.. as per EPA The Energy Policy Act of 2005 excluded hydraulic fracturing, except when diesel fuels are used, for oil and gas production from permitting under the UIC Program. This was because of concern about the risks to drinking water from diesel fuels. 3

4 'Fracking' yields fuel, fear in Northeast By Sarah Hoye and Steve Hargreaves, CNN Accessing the natural gas involves the controversial process of hydraulic fracturing, or "fracking." Fracturing requires drillers to pump large amounts of water mixed with sand and chemicals into the shale formation under high pressure to depths 8,000 feet or greater or even wells less than 1,000 feet, according to the U.S. Environmental Protection Agency. This process fractures the shale around the well, which allows the natural gas to flow freely, according to the Pennsylvania Department of Environmental Protection. Over the past few years, technological advances and increased profit margins have spurred increased use of hydraulic fracturing, according to the EPA. The U.S. Department of Energy estimates shale gas will make up more than 20 percent of the nation's total natural gas supply by Gas Land The Industrial Revolution played a major role in expanding the use of coal. During the first half of the 1800s, the Industrial Revolution spread to the United States. Steamships and steam-powered railroads were becoming the chief forms of transportation, and they used coal to fuel their boilers. America has more coal than any other fossil fuel resource. 1/4 of of all the known coal in the world is in the United States. The United States has more coal that can be mined than the rest of the world has oil that can be pumped from the ground. NYTimes: s/subjects/c/coal/index.html?8qa 4

5 Coal Is it really Cheap? Mountain Top Removal Virtual fly-over Coal Type of coal Carbon (%) Water (%) Fuel value (MJ/kg) Peat 5 90 Very low Lignite Low Bituminous Coal 65 3 High Anthracite 90 3 High Note: values may vary considerably with the source of coal With each step of transformation from peat to anthracite, chemical reactions occur. Volatile compounds are released, the water content of the material decreases; and the carbon content increases. At the same time, the materials becomes harder and brighter. 5

6 Composition of coal Composed primarily hydrocarbons and small amounts of O-, N-, and S-containing compounds. Compared with petroleum, coal contains a higher percentage of aromatic hydrocarbons. Why not Coal? Coal provides nearly 50% of the electrical generating fuel in the United States and similar percentages apply around the world. Coal is more abundant than oil, in fact, coal reserves are far more abundant than oil reserves. Coal burning is creating serious air pollution around the world, and with coal production rising with no end in sight, not just carbon dioxide but more immediate and deadly pollutants should be cleaned out of the burning process. This is why we need clean coal technology NYTimes: s/subjects/c/coal/index.html?8qa 6

7 ss/15coal.html ユ Carbon Capture and Sequestration (CCS) Involves capturing CO2 gases and transporting them to geological storage sites ユ Integrated Gasification Combined Cycle (IGCC) Involves CO 2 capture and lower emissions of SO 2 and NOx ユ Fluidized Bed Combustion (FBC) technology utilizes low grade, variable quality coal plus biomass and municipal waste to meet emissions requirements Clean coal is feasible but expensive Mounting Costs Slow the Push for Clean Coal 7

8 TAR SANDS Garth Lenz: The true cost of oil of_beauty_and_devastation?language=en How Keystone XL Got (So) Political WASHINGTON President Obama on Tuesday rejected an attempt by lawmakers to force his hand on the Keystone XL oil pipeline, using his veto pen to sweep aside one of the first major challenges to his authority by the new Republican Congress. With no fanfare and a 104-word letter to the Senate, Mr. Obama vetoed legislation to authorize construction of a 1,179-mile pipeline that would carry 800,000 barrels of heavy petroleum a day from the oil sands of Alberta to ports and refineries on the Gulf Coast. s/ /howkeystone-xl-got-so-political.html 8

9 Petrolium 9

10 Where does fuel energy reside? Fuel energy exists as chemical energy, residing in the bonds that hold the atoms together. When fossil fuels are burned in an adequate supply of O 2, they are oxidized to CO 2 and H 2 O. In this combustion process, chemical bonds are broken, new bonds are formed. The net result is energy is released as heat. The amount of energy released can be estimated by accounting bond energies Average Bond Dissociation Energies Source: Spiro & Stigliani Bond Enthalpy kj/mol H-H 432 O=O 494 O-H 460 C-H 410 C-O 360 C=O 799 Bond Enthalpy kj/mol C O 1071 C-C 347 C=C 611 C C 519 N=O 623 N O

11 From Chemistry in Context Book (Note: Some average bond energy values are slightly different) 4.4 Consider: 2 H 2 + O 2 2 H 2 O Bonds breaking 2 H-H + O=O Bonds forming 4 O-H =494 kj 482 kj/mole of O 2 consumed 2x432 kj =864 kj 4x460 kj =1840 kj 482/2moles of H 2 =241 kj/mole of H 2 Or kj/g H 2 =482 kj

12 CH 4(g) + 2 O 2(g) CO 2(g) + 2 H 2 O (l) Exothermic reaction Net energy change is kj/mol CH 4 The products are lower in energy than the reactants Exothermic reaction: E is a negative value. 4.3 Estimate energy release from fuel burning CH O 2 CO 2 + 2H 2 O Energy _ released product BE i BE reac tan t j 2(C=O)=2x799 2x2x(O-H)=4x460 4(C-H)=4x410 2(O=O)=2x kj/mol 12

13 Expressing the energy content of fuels CH O 2 CO 2 + 2H 2 O Reaction enthalpy Energy content per mol fuel Energy content per gram fuel Energy released per mol O 2 reacted 810 kj 810 kj/mol 810kJ / mol 50.6kJ / g 16g / mol 810kJ 405kJ / mol _ O2 _ reacted 2mol _ O2 _ reacted Energy released per mol CO 2 formed 810kJ 810kJ / mol _ CO2 _ formed 1mol _ CO2 _ formed # of mol CO 2 formed per 1000kJ 1000 kj energy released kJ / mol _ CO2 _ formed Estimate energy content of petroleum Approximate the chemical composition of petroleum as saturated hydrocarbons The approximate combustion reaction is expressed as -CH 2 -CH O 2 2CO H 2 O 2C-C 4C-H Result: 43.6 kj/gram petroleum; 1.6 moles of CO 2 released per 1000kJ produced. 13

14 How do different fuels Stack up? Energy Content (kj) Reaction per mole per mole per gram moles of Enthalpy O 2 fuel fuel CO 2 per 1000kJ Hydrogen H 2 + O 2 =2H 2 O Gas CH 4 + 2O 2 =CO 2 +2H 2 O Petroleum (-CH 2 -) +3O 2 = 2CO 2 + 2H 2 O Coal (-CH-) + 5O 2 = 4CO 2 + 2H 2 O Ethanol C 2 H 5 OH + 3O 2 = 2CO 2 + 3H 2 O Cellulose (-CHOH-) + O 2 = CO 2 + H 2 O One of the drawbacks to petroleum is that it must be refined before use

15 How do we use each barrel (42 gal) of petroleum? This 1.25 gal includes nonrenewable feedstocks for all plastics, pharmaceutical s, fabrics and other carbonbased products. Over 87% of each barrel is used for transportation and heating. 4.8 Combustion Under normal operating conditions, the firing of the spark plug initiates the burning of the fuel air mixture in the combustion chamber. Once the plug fires, a wall of flame starts outward from the plug in all directions at once. This flame front moves evenly and rapidly throughout the entire combustion chamber until the entire fuel/air mixture is burned. This even, rapid progress of the burning fuel/air mixture is highly dependent on the octane rating of the gasoline. 15

16 Octane Rating Some components of gasoline begin to burn before they are ignited by the spark plug -This premature ignition produces a knocking sound. The tendency of gasoline to cause knocking is rated according to an arbitrary scale known as the octane rating. Isooctane (2,2,4-trimethylpentane) is particularly resistant to premature ignition. It burns without causing knocking, therefore it is assigned an octane rating of 100. The straight-chain alkane n-heptane, in contrast, causes serious knocking and is given an octane rating of 0. The gasoline fraction from the distillation tower, known as straight-run gasoline, has an octane rating of 50-55, much too low for today s automobiles, which requires a rating between 87 and 90. Pump Octane Pump octane numbers are what you see on the yellow decal on the pumps at the gas stations. Pump octane number is the average of the Research Octane Number (RON) and the Motor Octane Number (MON) Pump Octane = (RON + MON)/2 MON=82, RON = 91 Pump octane =87 MON is a measure of the gasoline s ability to resist knock under severe operating conditions. RON is a measure of the gasoline s ability to resist knock under less severe operating conditions. 16

17 Octane numbers of selected hydrocarbons Name Formula Octane rating N-butane C 4 H N-pentane 2-methylbutane N-hexane 2-methylpentane 2,3-dimethylbutane N-heptane 2-methylhexane 2,3-dimethylpentane 2-methylheptane 2,3-dimethylhexane Isooctane C 5 H C 6 H C 7 H C 8 H Benzene C 6 H Chemistry affecting octane rating Combustion is a radical chain process. Pre-mature ignition is caused by radical generation from fuel components at the relatively lower temperature before ignition. The methylene groups (-CH 2 -) in straight chain hydrocarbons are more susceptible to attack by thermally activated oxygen molecules to produce hydrocarbon radicals. In contrast, branched-chain hydrocarbons are more resistant to radical formation because branching increases the fraction of the H atoms that are on methyl groups, CH 3. C-H in CH 3 has a bond energy of 423 kj/mol C-H in CH 2 - has a bond energy of 410 kj/mol 17

18 To increase the octane rating Cracking catalytic reforming addition of octane enhancers. Refining petroleum Cracking: higher-boiling fractions (kerosene or gas-oil) of petroleum are converted to gasoline. C 14 H o C C 7 H 16 + C 7 H 14 Kerosene size catalyst Gasoline size Alkylation: Lower-boiling fractions of petroleum are converted to gasoline CH 3 CH=CH 2 + C 3 H o C 8 CH 3 CH 2 CH 2 -C 3 H 7 catalyst 18

19 Octane enhancers Before 1975, the most widely used octane enhancer was tetraethyl lead (TEL). Adding as little as 0.1% of TEL per gallon of gasoline can increase the octane rating by 10 to 15 points Other octane enhancers Organometallic compounds: methyllcyclopentadienyl manganese tricarbonyl (MMT) Alcohols: methanol, ethanol Ethers: MTBE (methyl-t-butyl ether) ethyl-t-butyl ether (ETBE). Gasoline Additives Regulation to eliminate octane enhancing tetraethyl lead (TEL) created a need to find substitutes. H H H C H C H O H Ethanol (ethyl alcohol) To Reduce CO emission (The extra O helps combustion) during winter months H 3 C O CH 3 C CH 3 CH 3 To boost octane rating (MTBE has an octane rating of 116) MTBE methyl tertiary-butyl ether MTBE is an environmental hazard

20 N.H. Sues 22 Major Oil Companies Granite State: Gasoline Additive MTBE Pollutes Water CONCORD, N.H, Oct. 6, 2003 (AP) The state sued 22 major oil companies Monday because of the gasoline additive MTBE, which has been found to pollute water, Gov. Craig Benson said. The lawsuit claims that the oil companies have added increasing amounts of the additive to gasoline, even though they knew years ago it would contaminate water supplies. New Hampshire's groundwater and surface waters are under attack, Attorney General Peter Heed said at a news conference with Benson. A glass of water sits on the counter at Falls Diner in Hoosick Falls. The diner's water was tainted with MTBE from 1997 to 2000 Source: Timesunion.com /story.asp?storyid= New Hampshire is the first state to sue gasoline companies over MTBE, Benson said, although municipal utilities have sued. In California, Sacramento County and 10 water utilities filed suit last week against major gas companies over potential MTBE contamination. Utilities in the South Tahoe, California, area reached a $28 million settlement with oil companies last year over MTBE pollution. Convert coal to cleaner fuels Increase H/C ratio of the coal C + H 2 O CO + H kj x2 CO + H 2 O CO 2 + H kj CO + 3H 2 CH 4 + H 2 O kj 2C + 2H 2 O CH 4 + CO kj Steam-reforming (900 o C) 20

21 Is natural gas a better fuel than coal and petroleum? Advantages: Relatively inexpensive to process as a fuel Burns cleanly, leaves no residue Weight-for-weight, has a higher heat output Emits less CO 2 per unit of energy Does not produce oxides of sulfur 21