Anthropogenic carbon dioxide emissions

July 20, 2012, 12:23 pm
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This article will discuss the contribution of carbon fuel in the discharge of carbon dioxide into the atmosphere.

Chemistry of carbon compounds, that is organic chemistry, provides a basis for biological life on our planet. It can be accounted for by the unique ability of carbon atoms to combine with each other and with the atoms of other elements, forming practically innumerable combinations. It suffices to say that in the 20th century alone chemists managed to synthesize more than one million organic compounds that had not existed before. Organic compounds include all kind of fuels (except anthracite) conventionally used by humans, such as wood, oil, oil-cracking products, coal, peat, etc. Fuel of organic nature releases heat in the course of the oxidation reaction called burning. Quite another nature is possessed by nuclear fuel, the employment of which to produce heat and electric power was only mastered in the second half of the 20th century. Nuclear fuel releases energy as a result of fission of the nuclei of uranium-235, a uranium isotope.

Combustion of a carbon-containing fuel is a process of oxidizing the oxidizable components of the fuel by the atmospheric oxygen (23.2% by mass). The products of burning a carbon-containing fuel are carbon dioxide (CO2), and water, when a hydrocarbon is burned. Oxidation of carbon and hydrogen is accompanied by generation of heat, qCO2 and qH2O. The heat released as a result of oxidizing (burning) 1 kilogram (kg) of fuel is called the specific heat of combustion (heating value).

It is for the release the heat that fuel is usually burned. It generally forms the reaction of oxidation of a hydrocarbon fuel, which can be presented as:

K·[CnHm + (n+m/4)O2 = n(CO2 + qCO2) + (m/2)(H2O + qH2O)], (1)

where n?1 is the number of carbon atoms and m?0 is the number of hydrogen atoms contained in a molecule of the fuel and entering into the oxidation reaction; and K is the coefficient taking only two values, 1 or 2. When (n+m/4) is an integer, then the coefficient K = 1. When (n+m/4) is a fractional number, then the coefficient K = 2.

Reaction (1) releases carbon dioxide into the atmosphere thus contributing to the greenhouse effect. In addition to carbon dioxide (CO2), discharged gases can contain oxides of any substances that are present in the fuel as admixtures, such as sulfur dioxide. These facts are mentioned to support once again the idea that when less fuel is burned, less harmful gases are discharged into the atmosphere. The nitrogen oxides result from the nitrogen of the air (75.5% by weight) and the high temperature at which these oxides are synthesized in the boiler or furnace.

As is clear from Eq. (1) and the Law of Conservation of Mass, weight of the gaseous discharge from the engines of cars, aircrafts and ships, and from power-producing plants, especially from thermal power plants (TPP), exceeds the weight of the fuel-burned carbon. To calculate the mass of carbon dioxide discharged into the atmosphere when burning 1 ton (t) of fuel, as well as the mass of oxygen consumed from the air to oxidize 1 t of hydrocarbon fuel, we can use Equations (2) and (3). The equations hold true within a sufficiently wide range of the fuel compositions burned, and represent regression models obtained by processing the data of stoichiometric calculation carried out by Equation (1). In calculating, it was assumed that going out of the chemical reaction of the fuel oxidation amounts to 100%. In the case of coal being burned we shall assume that it is anthracite, which is practically one hundred percent carbon. It should be noted that the dependent variable is a dimensionless value since it defines the number of tons of the reaction product or reagent formed by burning a ton of fuel.

CO2 = 0.0366·C%, (2)

where C% is the percent content of carbon in a ton of fuel. (C% varies from 0 to 100%).

The mass of oxygen consumed to oxidize one ton of hydrocarbon fuel amounts to:

O2 = 7.94 - 0.0528 ·C% - 0.00045/H%, (3)

where H% is the percent content of hydrogen in a ton of fuel. (H% varies from 0.1 to 99.9%).

Since the air contains 23.2% oxygen by mass, the mass of air used to oxidize 1 ton of hydrocarbon fuel amounts to:

Air = 34.2 - 0.227643·C% - 0.00194/H%, (4)

Let us examine a couple of illustrative examples. When burning one ton of anthracite coal, 3.66 tons of carbon dioxide (CO2) are formed. In the process, 2.66 tons of oxygen from the air are consumed to oxidize the carbon. When burning one ton of natural gas (assuming it is 100% methane), 2.74 tons of CO2 are formed. In the process, 3.99 tons of oxygen are consumed from the air to oxidize the fuel.

As follows from Equation (1), it is more expedient to burn hydrocarbon fuel than to burn coal, as one obtains more heat but less carbon dioxide in the process. Ideally, from both an economic and ecological point of view, it would be more profitable to burn hydrogen. Combustion of one ton of hydrogen releases 3 to 4 times more heat, and, most important, the product of the oxidation of hydrogen is just plain water.

Let us estimate the amount of carbon dioxide discharged into the environment by power-producing units, both mobile and stationary. We will assume that the discharge goes on non-stop, every hour, every day, month by month, year by year. Since the fuel used is quite non-uniform in composition, and, consequently, is non-uniform as to the amount of heat released though burning (see Equation (1)), when performing calculations in power engineering they use, as a standard, the heating value of the conventional fuel, which is 7,000 kilocalories per kilogram (kcal/kg), which is equal to 29.3 megajoules per kilogram (MJ/kg) or 12,605 British thermal units per pound (BTU/lb). Anthracite has heating value of 26 to 33 MJ/kg (11,180 to 14,190 BTU/lb), which is close to that of the conventional fuel.

Let us estimate how much carbon dioxide is discharged into the atmosphere when producing electric power. We will assume "standard" consumption of conventional fuel to produce 1 kilowatt-hour (kWh) of electric power amounts to 0.405 kg (0.000405 t), which results in discharging 0.0015 tons of carbon dioxide into the atmosphere while extracting 0.0011 ton of oxygen from the air. Let us remember that to run a 100-watt (W) lightbulb throughout a year (= 876 kWh) we will have to burn 0.355 tons of conventional fuel while discharging 1.3 tons of CO2 into the atmosphere, extracting 0.95 tons of oxygen from the air, and passing 4.1 tons of air by the burners.

What about the ecological impact of an ordinary car? The heating value of gasoline is 44 to 46 MJ/kg, which is 1.5 to 1.6 times higher than that of the conventional fuel. It means that having driven your car for 100 kilometers (km) and having used 5 liters (4.3 kg) of gasoline, you will have discharged 0.016 ton of CO2 into the air and will have consumed 0.012 tons of oxygen from the air.

Let us shift the scale and character of our discussion and examine a real-life application of these calculations. According to data published by the Ministry of Fuel Resources and Power Engineering of the Ukraine, in January 2007 consumption of electric power declined 6.4%, or by 16,890,000,000 kWh. This means that at least 6.84 million tons of conventional fuel was left intact, while discharge of CO2 into the atmosphere was 25.1 million tons less, and the air retained 18.2 million tons of oxygen.

Further Reading

Glossary

Citation

Prisyazhniuk, V. (2012). Anthropogenic carbon dioxide emissions. Retrieved from http://www.eoearth.org/view/article/150129

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