Fossil Fuels: Origin, Composition, and Environmental Impact

FOSSIL FUELS

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• Coal, oil and natural gas are fossil fuels.
• formed by the reduction of biological compounds that contain carbon, hydrogen, nitrogen, sulfur and oxygen.
• formed over millions of years, from the remains of dead organisms in anaerobic conditions (without oxygen).
• Coal was formed from dead plant material.
• Oil and natural gas were formed from dead marine organisms.
• Oil (crude oil / petroleum) is a complex mixture of straight-chain, branched, cyclic, and aromatic hydrocarbons.
• Natural gas is composed mainly of methane, with varying amounts of ethane, propane, and butane as well as hydrogen sulfide.
• Fossil fuels release large amounts of energy in exothermic combustion reactions.
• They also produce large amounts of carbon dioxide, CO2, which is a greenhouse gas. 2
• Greenhouse gases contribute to the enhanced greenhouse effect.

ADVANTAGES AND DISADVANTAGES OF FOSSIL FUELS

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COAL

Advantages of Coal

Coal is relatively inexpensive Coal has a high specific energy and high energy density Coal can be converted into liquid fuels (coal liquefaction) and gaseous fuels (coal gasification) Coal is distributed throughout the world

Disadvantages of Coal

Coal is finite (non-renewable) When burned, coal produces CO2 which is a greenhouse gas When burned, coal produces SO2 which causes acid deposition The mining of coal can cause environmental damage 4

CRUDE OIL / PETROLEUM PRODUCTS

Advantages of Petroleum Products

Petroleum products are relatively inexpensive Petroleum products generally have high specific energies and energy densities Ease of transport (gasoline) Can be used as feedstocks for petrochemicals

Disadvantages of Petroleum Products

Crude oil is finite (non-renewable) When burned, petroleum products produce CO2 which is a greenhouse gas Drilling for and transporting crude oil can cause environmental damage Uneven distribution worldwide 5

NATURAL GAS

Advantages of Natural Gas

Natural gas is a relatively clean fuel Natural gas has a higher specific energy than coal and oil Ease of transport in pressurized containers Does not contribute to acid deposition

Disadvantages of Natural Gas

Natural gas is finite (non-renewable) When burned, natural gas produces CO2 which is a greenhouse gas Lower energy density than coal and oil Uneven distribution in the world 6

EVIDENCE FOR GLOBAL WARMING

• the graphs below show the concentration of CO2 in the atmosphere since 1960 and the change temperature since 1860. in global
• increasing CO2 levels and the earth's average temperature show a clear correlation, but wide variations in the surface temperature of the earth have occurred frequently in the past.

Atmospheric CO2 at Mauna Loa Observatory

Scripps Institution of Oceanography NOAA Earth System Research Laboratory 380 PARTS PER MILLION 360 340 - NORA 320 January 2012 1960 1970 1980 1990 2000 2010

Global Temperatures

-0.6 1860 1880 1900 1920 1940 1960 1980 2000 YEAR 0.6 Temperature Anomaly (℃) 0.4 - Annual Average Five Year Average 0.2 0 -0.2

CO2 and the Greenhouse Effect

CO2 is a greenhouse gas, which means that it traps heat energy inside the Earth's atmosphere. This is known as the greenhouse effect (figure 4) energy from the Sun greenhouse gases in the atmosphere Figure 4 Greenhouse gases reduce the amount of heat radiated into space

• Nitrogen, N2, and oxygen, O2, make up over 99% of atmospheric air.
• Neither N2 nor O2 have the ability to absorb infrared radiation that enters the Earth's atmosphere from the Sun.
• carbon dioxide, CO2, constitutes approximately 0.04% of the atmosphere.
• Despite the small proportion of CO2, the increase in the concentration of this greenhouse gas is causing significant damage to our environment.
• A CO2 molecule can absorb infrared radiation, resulting in the vibration of bonds within the molecule after which the molecule will emit infrared radiation back into the atmosphere. 8

Global Temperature Increase

• Some of this radiation will be directed towards the Earth's surface, increasing the global temperature.
• Other greenhouse gases include methane, nitrous oxide, water vapour and fluorinated substances such as hydrofluorocarbons.
• As a result of the greenhouse effect, average temperatures worldwide are increasing, known as global warming.
• The main cause of global warming is the increase in the levels of greenhouse gases, in particular, carbon dioxide (figure 5) and, to a lesser extent, methane.

Annual Mean Atmospheric CO2 Levels

Figure 5 Annual mean atmospheric CO2 levels recorded at Mauna Loa Observatory 420 415 410 405 CO2/ppm 400 395 390 385 380 375 2004 2006 2008 2010 2012 2014 2016 2018 2020 2022 year 9

Greenhouse Gas Emissions by Energy Source

The combustion of different fuels releases different quantities of greenhouse gases (figure 6). greenhouse gas emissions/tonnes CO2 per GWh 1600 1400 average emissions intensity 1200 I range between studies 1000 - ... 800 -------. 4 600 400 4 200 0 - lignite coal oil natural solar PV biomass nuclear hydro- wind gas electric A Figure 6 Greenhouse gas emissions by energy source 10

Historic CO2 Levels

420 current 380 340 CO2/ ppm highest historic CO2 level 300 1950 2 N 260 220 180 0 400 350 300 250 200 150 100 50 thousands of years before today (0 = 1950) A Figure 7 Historic CO2 levels during the last three glacial cycles, as reconstructed from ice cores

• Figure 7 shows the historical patterns of atmospheric CO2 concentration during the last 3 glacial cycles constructed by the analysis of ice- core samples.
• Current CO2 levels are much higher than historical peaks, suggesting that human-made emissions are largely responsible for the additional quantities of carbon dioxide in the atmosphere. 11

Fuel Choice and Specific Energy

• Many different sources of fuel are used in everyday life.
• The choice of fuelsdepends on: - the economic development of nations - the natural resources available.
• Each fuel has a different specific energy: the amount of heat energy released per mass of the fuel.
• wood, a traditional means of generating energy for cooking and heating, has the lowest specific energy of all common fuels.

Specific Energy of Different Fuels

60 45 specific energy / Mj kg-1 30 15 0 wood coal ethanol biodiesel crude oil diesel gasoline natural gas A Figure 9 The specific energy of different fuels.Common fuels vary in composition.

Hydrocarbon Chain Length and Combustion

• The chain length of hydrocarbons present in these fuels also varies.
• The longer the hydrocarbon chain, the greater the tendency of the fuel to undergo incomplete combustion.
• Incomplete combustion results in the release of: - poisonous carbon monoxide &/or elemental carbon & produces a smaller amount of heat energy per unit mass of the fuel if compared to the complete combustion of the same hydrocarbon.
• Larger hydrocarbons have a reduced volatility due to stronger London (dispersion) forces (LDFs). This affects the way the hydrocarbon molecules interact with the oxygen molecules and the type of combustion that occurs.