Combustion of Fossil Fuels
From Thermal-FluidsPedia
Fossil fuel, whether in the form of coal, oil, or natural gas, reacts with oxygen in the air when burned, forming a number of products. In its simplest form, this chemical reaction can be written as:
- Fuel + Oxidizer ->Product
Th is reaction always produces heat. Chemical reactions that produce heat are called exothermic reactions. In addition to carbon and hydrogen, fossil fuels may contain oxygen and traces of other elements, such as sulfur and metals, which complicate the combustion. In the present analysis, we assume that all impurities have been removed and the reactions are such that all the fuels are burned to produce carbon dioxide and water vapor.
For example, methane, gasoline, and coal burn in oxygen as:
- Methane: CH4+ 2 O2 -> CO2 + 2 H2O
- Gasoline: C8H18 + 12.5 O2 -> 8 CO2 + 9 H2O
- Coal: C+O2 -> CO2
Such reactions, where the oxidizer is just enough to convert all fuels to carbon dioxide and water vapor, are called stoichiometric. When the reactant mixture has fuel or oxygen in excess of what is required for stoichiometric ratio, the mixture is fuel rich (oxygen lean) or oxygen rich (fuel lean), respectively. If air were used instead of oxygen, the reaction would remain the same except that the nitrogen in the air would appear unburned in the product. For example, the stoichiometric combustion of gasoline in air would be:
- C8H18+ 12.5 (O2 + 3.76 N2) -> 8 CO2 + 9 H2O +47.0 N2
(Air is a mixture of 1 part oxygen and 3.76 parts nitrogen).
On the mass basis:
114 kg gasoline + 1,716 kg air -> 352 kg carbon dioxide + 162 kg water vapor + 1,316 kg nitrogen
or
1 kg gasoline + 15.1 kg air -> 3.1 kg carbon dioxide + 1.42 kg water vapor + 11.54 kg nitrogen
Question: As a result of fossil fuel consumption, roughly 6 billion tons of carbon is produced each year. It is estimated from geological evidence that each year about 30 million tons of carbon goes into the formation of new fossil sediments. What is the signifi cance of this data in estimating the lifetime of remaining fossil fuels?
Answer: Th e rate of consumption of fossil fuels is 200 times faster than the rate they are replenished by nature. Th is means fossil fuels are gradually being depleted and must be considered as a nonrenewable source of energy.
Example: Calculate the stoichiometric air to fuel ratio necessary for burning the natural gas in air. Natural gas consists mainly of methane.
Solution: Th e stoichiometric reaction of methane is
CH4+ 2(O2+3.76 N2) CO2 + 2H2O +7.52 N2
16 kg methane+ 275 kg air -> 44 kg carbon dioxide + 36 kg water vapor + 211 kg nitrogen or 1 kg methane+ 17.2 kg air -> 2.75 kg carbon dioxide + 2.25 kg water vapor + 13.2 kg nitrogen
Note that although methane and gasoline have very different structures, they have a similar air/fuel ratio and produce roughly the same amount of carbon dioxide per mass of fuel burned.
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Incomplete Combustion
If chemical reactions are completed fully, there would be litt le pollution and hydrocarbons would be a relatively clean source of fuel. Unfortunately, these reactions do not reach completion and other gases such as nitric oxides (NO and NO2) and carbon monoxide (CO) are always present. Because reactions take longer to reach completion than the time available in the combustion chamber, some hydrocarbons might also remain unburned in the product. Therefore, products will be exhausted before reaching equilibrium. The degree to which the reaction completes depends upon such variables as the combustion temperature and pressure, residence time, and mixture ratios. Th e details of the processes are outside the scope of this book and will not be discussed further. Th ose interested should consult more advanced texts on the subject (1).
Heating Values
Table 1. Heating Values for
Common Fuels*Fuel Calorific Value
(kJ/kg)Coal 15,000-30,000 Kerosene 43,000 Gasoline 44,000 Methane 55,600 Propane 50,500 Coal Gas 34,000 Methanol 19,800 Garbage** 19,800 Wood 20,000 Hydrogen 143,000 * Calorifi c values are approximate and can
vary somewhat depending on fuel purity.
** Garbage from a typical
Heating value (also called calorific value or heat of combustion) refers to the amount of thermal energy that is released by burning a unit amount of fuel (a). For gaseous and liquid fuels such as methane and gasoline, the heat of combustion is usually given per unit volume (37,000 kJ/m3 for methane and 39,000 kJ/lit for gasoline), while for solids such as coal, it is expressed per unit mass (15,000 to 30,000 kJ/kg) (b). Roughly speaking, we can show the following relations between the energy releases of various forms of fossil fuels:
1 liter of petroleum ~ 1 kg of coal ~ 1 m3 of natural gas
1 gal of petroleum ~ 14 pounds of coal ~ 150 ft 3 of natural gas
It is interesting to note that, per unit mass, different fossil fuels have relatively similar calorific values (Table 1). Th is is logical due to the fact that all fossil fuels are hydrocarbons. Except for a slight variation in energy trapped between different bonds which form their molecules, these hydrocarbons are expected to release similar amounts of heat. For comparison, the heating values of hydrogen, firewood, and of garbage from a typical American household are also given. As we saw in Biomass Energy, as much as 75% of the average American garbage content is paper, foodstuff , and yard waste; like fossil fuels, these derive their energy from living matt er and thus can be burned in a manner similar to fossil fuels. Hydrogen is not a fossil fuel and does not contain carbon. Its high heating value is att ributed to the much higher energy content trapped in H-H bonds relative to H-C bonds common in fossil fuels.
References
(1) See for example, Seinfeld, J. H. Air Pollution: Physical and Chemical Fundamentals, McGraw-Hill, Inc., 1975.
(2) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005
Additional Comments
(a) Heating values are expressed either as higher (gross) or lower (net) heating values, depending on whether the products of combustion are cooled back to their initial reactant temperature, and whether water in the combustion product is condensed out. If water in the exhaust is in the form of vapor, the heating value is called net or lower heating value (LHV). If the water vapor is cooled so all the water is condensed out, then additional heat is available and the heating value is called gross or higher heating value (HHV). For most reactions the exhaust is relatively hot therefore litt le condensation takes place and LHV is a more realistic number to use. Th e HHV is most appropriate in certain devices such as natural gas furnaces used in residential units and air conditioners that use condensers.
(b) BTU equivalents of various energy sources are: 1 bbl crude oil = 5.8 million Btu; 1000 cu. ft gas = 1.03 million Btu; 1 kWh electricity = 3,400 Btu; 1 metric ton coal = ~30 million Btu.
Further Reading
Berkowitz, N., Fossil Hydrocarbons: Chemistry and Technology, Elsevier Academic Press, 1997.
Deff eyes, K. S., Hubbert’s Peak: Th e Impending World Oil Shortage, Princeton University Press, Princeton, N. J., 2001.
Campbell, C. J., Th e Coming Oil Crisis, Multi-Science Publishing Company, 2004.
Tariq Ali, Th e Clash of Fundamentalisms: Crusades, Jihads and Modernity, Verso, 2002.
Pelletiere, S., Iraq and the International Oil System: Why America Went to War in the Gulf, Praeger Publishing, 2001.
Oil and Gas Journal, Technology, news, statistics, special reports, and analysis (http://ogj.pennnet.com).
Journal of Petroleum Technology, The official journal of Society of Petroleum Engineers, Dallas.
The Petroleum Engineer, Petroleum Engineer Pub. Co.
Journal of Petroleum Science and Engineering, Elsevier, covers the fields of petroleum (and natural gas) exploration, production and flow.
External Links
National Energy Technology Laboratory: Th e Strategic Center for Coal (http://www.netl.doe.gov/coal).
National Petroleum Technology Office (http://www.npto.doe.gov).
US Geological Survey (http://www.usgs.gov).
Organization of Petroleum Exporting Countries (OPEC) (http://www.opec.org).
Society of Petroleum Engineers (http://sae.org).