Carbon Footprint Calculations: An Application of Chemical Principles

In the Classroom

Carbon Footprint Calculations: An Application of Chemical Principles Richard S. Treptow Department of Chemistry and Physics, Chicago State University, Chicago, Illinois 60628 [email protected]

Global warming is considered the most serious environmental problem confronting our world today. Most climate scientists attribute the rise in average global temperature since the beginning of the industrial age to the emission of greenhouse gases into the atmosphere by human activity. These gases trap heat emitted in the form of infrared radiation from the earth's surface. The subject of global warming is treated briefly in chemistry textbooks (1, 2) and in greater detail in monographs (3-6) and at Web sites (7-12). Articles on related topics have appeared in this Journal (13-18). The purpose of this article is to illustrate how basic principles taught in general chemistry can be used to calculate greenhouse gas emissions from a variety of sources. Our attention will focus on carbon dioxide, the major heat-trapping gas. Calculating its emissions is an important first step in developing strategies for preventing global warming. General chemistry courses emphasize the use of units, dimensional analysis, and scientific laws in performing calculations. We will use these basic skills as we seek to quantify carbon dioxide emissions. Stoichiometry, thermochemistry, and the ideal gas law will be called into action. We will often convert the units of our calculated quantities into the units commonly used by the U.S. news media, industry, or government. Our overall goal is to demonstrate the central role of chemistry in confronting one of humanity's greatest challenges. Carbon Footprint Defined Carbon footprint is a currently popular term used to express the quantity of greenhouse gas emitted by a specific source. The word “carbon” is used because the most important greenhouse gas is carbon dioxide, and “footprint” reminds us that the activity under consideration leaves an unintended mark on our planet. In its broadest definition, a carbon footprint is a measure of the total quantity of all greenhouse gases emitted by a specific entity or process over a period of one year. In actual practice, carbon dioxide is often the only gas considered and the time period need not be a year. We can speak of the carbon footprint of a person, a factory, or an entire country. Organizations and the events they sponsor also have footprints, as do automobiles, airplanes, and computers. A carbon footprint may take into account only the most direct emissions associated with the source of interest, such as the smoke rising from the chimney of a factory, or it may be more inclusive and consider less direct emissions, such 168

Journal of Chemical Education

_

_

Figure 1. Portland cement factory.

as the exhaust from trucks that deliver raw materials to the factory. CO2 Emission Factors Carbon footprint calculations require detailed information about the pollution source. A useful property to know is the CO2 emission factor of the source. This factor expresses the mass of carbon dioxide emitted for a given quantity of activity: CO2 emission factor ¼

mass of CO2 emitted quantity of activity

(1Þ

The “quantity of activity” can be expressed in various ways. For example, if the source is an automobile, the quantity of activity can be a gallon of gasoline burned. If the source is a power plant, it can be a kilowatt-hour of electricity generated. Emission factors are sometimes called emission coefficients in the environmental literature. Cement Production Many industrial processes emit large quantities of carbon dioxide from the chemical reactions they carry out. The U.S. cement industry, for example, emits about 4.6 1010 kg CO2 per year as reported by the U.S. Environmental Protection Agency (19). Portland cement, a common product manufactured worldwide (Figure 1), will be investigated. In the first step of Portland cement production, limestone, CaCO3, is heated in a kiln to about 1300 °C. The limestone decomposes into lime, CaO, in a reaction known as calcination or decarbonation: (2Þ CaCO3 ðsÞ f CaOðsÞ þ CO2 ðgÞ

_

Vol. 87 No. 2 February 2010 pubs.acs.org/jchemeduc r 2010 American Chemical Society and Division of Chemical Educaation, Inc. 10.1021/ed8000528 Published on Web 01/12/2010

In the Classroom

After the reaction is complete, additional inorganic components are added to the lime to produce the cement. The final product is approximately 62% CaO by mass. We can calculate the emission factor for cement production as ! 1 mol CO2 44:01 g CO2 1 mol CaO 56:08 g CaO 1 mol CaO 1 mol CO2 ! 62 g CaO g CO2 (3Þ ¼ 0:49 100 g cement g cement The result could be called a gravimetric emission factor. It expresses the quantity of carbon dioxide emitted in terms of the mass of product manufactured. Although the units used to express this factor would be popular among chemists, we may wish to convert them into units more common in industrial and governmental literature: ! ! 0:49 g CO2 1 kg 106 g kg CO2 (4Þ ¼ 490 1 tonne 1 g cement 103 g tonne cement In other words, 490 kg of CO2 are emitted for each metric tonne of cement produced. The emission factor just calculated accounts for only a fraction of the total carbon dioxide emitted by a cement factory. Additional factors would be needed to determine the factory's total carbon footprint. They would include, for example, the carbon dioxide emitted by the fuel that heats the kilns and generates the electricity required throughout the factory. When all additional sources are taken into account, the emission factor rises to 700-1000 kg CO2 per tonne cement.

Figure 2. Pumping gasoline into an automobile. Table 1. CO2 Emission Factors for Liquid Hydrocarbons and Fossil Fuels

Alkanes

C8 H18 ðlÞ þ 12:5O2 ðgÞ f 8CO2 ðgÞ þ 9H2 OðlÞ

(5Þ

We can calculate the emission factor for this reaction from its stoichiometry and the density of octane: ! 8 mol CO2 44:01 g CO2 1 mol C8 H18 1 mol C8 H18 1 mol CO2 114:23 g C8 H18 ! 0:6986 g C8 H18 103 cm3 1 cm3 C8 H18 1L ¼ 2:153 103

g CO2 L C8 H18

(6Þ

Note that the emission factor is based upon the volume of octane burned and could be called a volumetric emission factor. If expressed in the units common in the United States it becomes 18.0 pounds CO2 per gallon of octane.

r 2010 American Chemical Society and Division of Chemical Educaation, Inc.

_

Hexane (C6H14)

16.7

Octane (C8H18)

18.0

Decane (C10H22)

18.8

Dodecane (C12H26)

19.4

Tetradecane (C14H30)

19.8

Alkenes and Aromatics 1-Hexene (C6H12)

17.6

1,2-Hexadiene (C6H10)

19.2

Benzene (C6H6)

24.7

Fossil Fuels

Gasoline Combustion Gasoline is the most commonly encountered fossil fuel in the modern world (Figure 2). The emission factor for its combustion is worthy of our consideration. Gasoline is a mixture of many hydrocarbon compounds in variable concentrations. For simplicity let us consider octane, one of its many components. Octane combustion can be expressed as

CO2 Emission Factor/(lb/gal)

Compound or Fuel

Gasoline

19.6

Kerosene

21.5

Diesel Fuel

22.4

The emission factors for the combustion of liquid hydrocarbon compounds typical of those present in gasoline are shown in Table 1. The factors were calculated by the method just illustrated. They demonstrate that the emission factor of a hydrocarbon increases with the percentage of carbon it contains. This percentage increases with both molecular size and the extent of multiple bonding. The emission factors for three common liquid fossil fuels as reported by the U.S. Energy Information Administration (20) are also listed in Table 1. The values for the fuels are consistent with the generalizations we derived from the compounds. For example, it is reasonable that gasoline would have a factor of 19.6 lb per gal. Gasoline consists primarily of alkanes with 5-12 carbon atoms, but alkenes and aromatics are also present. The factors for kerosene and diesel fuel would be expected to be slightly greater than that of gasoline since they contain somewhat larger molecules. Chemistry students can be assigned to calculate the annual carbon footprint of their family automobile or pickup truck. They will need to determine the fuel efficiency of the vehicle and to estimate its miles driven per year. Its footprint can then be calculated using the appropriate emission factor from Table 1. Note that the value obtained is based on tailpipe carbon dioxide emissions only. A much more challenging assignment would be to include the carbon dioxide emitted during production and

pubs.acs.org/jchemeduc

_

Vol. 87 No. 2 February 2010

_


169

In the Classroom Table 2. CO2 Emission Factors for Gaseous Hydrocarbons and Fossil Fuels CO2 Emission Factor lb/ft3

Compound or Fuel

lb/(106 Btu)

Methane (CH4)

0.116

115

Ethane (C2H6)

0.232

131

Propane (C3H8)

0.348

138

Natural Gas

0.121

117

Flare Gas

0.134

121

distribution of the gasoline. Footprints of this more inclusive type are called “well-to-wheels”. Natural Gas Combustion Natural gas is the common fossil fuel pumped from underground deposits and piped by distribution companies to buildings, factories, and power plants where it is burned for its heat content. The gas is often cited as the cleanest fossil fuel; nevertheless, its combustion does emit carbon dioxide. Our discussion of natural gas will begin with methane, its principal component. Methane combustion occurs by the reaction CH4 ðgÞ þ 2O2 ðgÞ f CO2 ðgÞ þ 2H2 OðlÞ

(7Þ

As a first step in expressing the emission factor, the mass of carbon dioxide emitted per mole of methane is calculated: 1 mol CO2 44:01 g CO2 g CO2 (8Þ ¼ 44:01 1 mol CO4 1 mol CO2 mol CH2 Fuel gases are commonly measured in the United States in terms of their volume in cubic feet at a pressure of 1 atm and a temperature of 60 °F. We can use the ideal gas law and a unit conversion factor to calculate the volume of 1 mol CH4 in cubic feet under these conditions: V ¼ ¼

nRT P ð1 mol CH4 Þ½0:0821 L atm=ðmol KÞð289 KÞ 1 atm ! 1ft3 28:32 L

¼ 0:838 ft3 CH4

(9Þ

The emission factor for burning methane can then be calculated: ! 44:01 g CO2 1 mol CH4 1 lb 453:6 g 1 mol CH4 0:838 ft3 CH4 ¼ 0:116

lb CO2 ft3 CH4

(10Þ

Fuel gases are also commonly measured in terms of the quantity of heat they produce upon burning. We can use thermochemical principles to calculate the emission factor for methane on this basis. The heat produced by burning methane at constant pressure is given by ΔrxnH° for the combustion 170


_


_

Figure 3. Natural gas meter.

reaction. This enthalpy change can be calculated from tabulated ΔfH ° values for the reactants and products (21) by use of Hess's law. The result is ΔrxnH ° = -891 kJ. The heat produced is therefore 891 kJ per mol CO2 emitted. The emission factor can then be determined: 1 mol CO2 44:01 g CO2 g CO2 (11Þ ¼ 4:94 10 - 2 891 kJ 1 mol CO2 kJ If we choose to express the factor in units common in U.S. industry it becomes ! 4:94 10 - 2 g CO2 1 lb 1:055 kJ 453:6 g 1 Btu 1 kJ 10 lb CO2 (12Þ ¼ 115 1 million Btu 1 million Btu The Btu is the British thermal unit. The two emission factors calculated for methane are listed in Table 2. The first is a volumetric emission factor. The second, on the other hand, is a thermometric emission factor based upon the quantity of heat produced by burning the gas. Table 2 also lists the emission factors calculated for ethane and propane expressed in the same two ways. Note that the volumetric emission factor of ethane is twice that of methane. This can be understood since a cubic foot of ethane has the same number of molecules as a cubic foot of methane at the same temperature and pressure with the assumption that they are ideal gases. Because each molecule of ethane produces two molecules of carbon dioxide, its emission factor should be twice as large. In contrast, the thermometric emission factor of ethane is only slightly larger than that of methane. This can be explained by considering the energies of the bonds broken and formed in the combustion reactions. The factors for natural gas and flare gas as reported by the EIA (20) are also listed in Table 2. Flare gas is an unwanted byproduct released from oil wells, refineries, chemical plants, and landfills. It is usually burned or “flared” rather than being emitted directly into the atmosphere. The emission factors listed for natural gas and flare gas are consistent with the fact that they are both composed mostly of methane and lesser quantities of ethane and propane. Students can be assigned to calculate the quantity of carbon dioxide emitted from their homes by their burning of natural gas for one month. If a home has a gas meter (Figure 3), the volume of gas delivered in cubic feet can be easily read. The volumetric emission factor listed for natural gas in Table 2 can then be use


_


In the Classroom

for the calculation. Alternatively, if a gas company bill has recently been sent to the home, it will likely state the monthly gas consumption in Btu's or therms. The thermometric emission factor would then be applied. Other Greenhouse Gases After students have learned the basic principles, they may be encouraged to determine their own carbon footprints by use of the “carbon calculators” available at Web sites (22-24). These calculators estimate a person's carbon footprint on the basis of their vehicle use, air travel, household heating and electricity, food choices, and waste recycling practices. Most carbon calculators include only carbon dioxide emissions in their accounting, and they typically report their results in the unit tons CO2. More advanced calculators, however, include other greenhouse gases, such as methane, nitrous oxide, and halocarbons. Furthermore, the quantity of each gas emitted is expressed in terms of its total heat-trapping effect over its lifetime in the atmosphere. The quantity is then expressed in terms of the mass of carbon dioxide that would have an equivalent heat-trapping effect. A unit commonly used for this purpose is tons of carbon dioxide equivalent, which is abbreviated tons CO2-eq. With the quantity of each gas expressed in a single unit, the total greenhouse gas emissions can be calculated and reported as the person's carbon footprint. Conclusion The CO2 emission factors were calculated for several different sources of carbon dioxide in the environment. Each calculation began with a balanced chemical equation for an emission reaction. Various chemical principles were then applied to express the factors in different ways. For example, the mass of carbon dioxide emitted was based upon the mass of cement produced by a cement factory, the volume of gasoline consumed by an automobile, or the heat produced by burning natural gas. The emission factors provided a pathway for calculating the carbon footprint of each source. Hopefully, future articles in this Journal will continue to demonstrate the role of chemistry in building understanding of global warming. We must rise to the challenge of protecting our planet's climate. Literature Cited 1. Eubanks, L. P.; Middlecamp, C. H.; Heltzel, C. E.; Keller, S. W. Chemistry in Context: Applying Chemistry to Society, 6th ed.; McGraw-Hill Higher Education: Boston, 2009; pp 100-149.


_

2. Baird, C.; Cann, M. Environmental Chemistry, 3rd ed.; W. H. Freeman: New York, 2004; pp 166-251. 3. Houghton, J. Global Warming: The Complete Briefing, 3rd ed.; Cambridge University Press: Cambridge, 2004. 4. Turco, R. P. Earth Under Siege: From Air Pollution to Global Change, 2nd ed.; Oxford University Press: Oxford, 2002. 5. Wayne, R. P. Chemistry of Atmospheres, 3rd ed.; Oxford University Press: Oxford, 2000. 6. Atmospheric Chemistry and Global Change; Brasseur, G. P., Orlando, J. J., Tyndall, G. S., Eds.; Oxford University Press: Oxford, 1999. 7. Intergovernmental Panel on Climate Change. http://www.ipcc.ch (accessed Aug 2009). 8. Union of Concerned Scientists. Global Warming. http://www. ucsusa.org/global_warming/ (accessed Aug 2009). 9. U.S. Environmental Protection Agency. Climate Change. http:// www.epa.gov/climatechange (accessed Aug 2009). 10. NASA Earth Observatory. http://earthobservatory.nasa.gov/ (accessed Aug 2009). 11. The Encyclopedia of Earth. http://www.eoearth.org (accessed Aug 2009). 12. Global Warming. www.global-greenhouse-warming.com/ (accessed May, 2009). 13. Burley, J. D.; Johnston, H. S. J. Chem. Educ. 2008, 85, 224A– 224B. 14. Bozlee, B. J.; Janebo, M.; Jahn, G. J. Chem. Educ. 2008, 85, 213– 217. 15. Oliver-Hoyo, M. T.; Pinto, G. J. Chem. Educ. 2008, 85, 218–220. 16. Kauffman, J. M. J. Chem. Educ. 2004, 81, 1229–1230. 17. Weston, R. E., Jr. J. Chem. Educ. 2000, 77, 1574–1577. 18. Elrod, M. J. J. Chem. Educ. 1999, 76, 1702–1705. 19. U.S. EPA. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006. http://www.epa.gov/climatechange/emissions/downloads/ 08_Industrial.pdf (accessed Aug 2009). 20. U.S. EIA. Fuel and Emission Source Codes and Emission Coefficients. http://www.eia.doe.gov/oiaf/1605/coefficients.html (accessed Aug 2009). 21. CRC Handbook of Chemistry and Physics, 81st ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 2000-2001; pp 5-4-5-60. 22. U.S. EPA. Personal Emissions Calculator. http://www.epa.gov/ climatechange/emissions/ind_calculator.html (accessed Aug 2009). 23. World Resources Institute. SafeClimate. http://www.safeclimate. net/calculator (accessed Aug 2009). 24. The Nature Conservancy. http://www.nature.org/initiatives/ climatechange/calculator (accessed Aug 2009).


_


_


171

Carbon Footprint Calculations: An Application of Chemical Principles

Recommend Documents