Text for Discussion:
Carbon Balance: Basis for the calculation of greenhouse effect gases emission.
Omar Campos Ferreira.
The calculation of greenhouse effect gases emission related with energy use is based on the mass of fuel consumed and on specific emission coefficients found in technical publications. The practical units for measuring the quantities of commercialized liquid and gaseous fuels are the respective volumes and densities and the fuels are characterized by properties that guarantee it adequacy to each type of use. A set of these properties constitute the technical specification formulated by agencies that normalize and inspect the market. So the National Petroleum Agency, that substituted the National Department of Fuels, publishes periodically the specifications for the fuels under its responsibility. In the case of gasoline, for example, the relevant properties are the calorific value, the density, the octane number, the sulfur content, the distillation curve, etc…These properties are generally dependent on the molecular structure of the fuel components that is in most cases one or more mixtures of hydrocarbons eventually improved by addictives in order to correct small specification deviations.
The specific emission of greenhouse effect gases depends on the carbon content of the fuel so that the emission calculation by the pool of fuels should take into account not only the carbon content of the hydrocarbon present but also the different mixtures that satisfy the specifications. Therefore it seems that the detailed description of the sources of greenhouse effect gases is a hard task, fortunately dispensable when it is desired to calculate just the quantity of carbon emitted. The present report deals with the problem of calculating the carbon content of petroleum product fuels since natural gas and mineral coal, that are also important emission sources, are simpler substances for which it is sufficient to consider the impurities, most of them non combustible.
Petroleum is divided, by refining, cracking and reforming into a large number of fuels for different use in the different sectors of the economy. In a certain measure, the crude oil composition determines the refining structure whereas the economy of the process gives priority to the lower cost products. Therefore most of the petroleum products are obtained by fraction distillation that separates the different components according to the vapor tension or, in other words, according to the vaporization temperature, by groups appropriate for the different uses.
So, along the distillation column, the temperature profile determines the composition profile of the mixture. Therefore, there are no alterations of the molecular structure but only an enrichment of the mixture in determined hydrocarbons according to the temperature in each point. In what concerns cracking, the molecular structure is modified: a heavy hydrocarbon is broken into lighter ones and there is frequent alteration of the chemical links, from simple links to double or triple ones. Finally, in reform molecules of other elements are introduced in the original structure, as in the case of hydrogenation.
In all the operations described above, the number of atoms in each element is conserved, what exempts the elementary balance from perturbations induced by them. In this way the carbon mass is maintained constant in transformation whatever may be the physical properties of the fuels, what is an important information for the calculation of fuel loss in transformation or addition of other substances, such as water, aiming at increasing the volume of the product.
The calculation of the total carbon content in petroleum can be made based on the difference between the higher and lower calorific values that can be determined using a flow calorimeter in which the combustion water is collected and weighted. According to the usual definition, the difference is due to the vaporization of the combustion water that is not recovered in the open processes (for example, internal combustion engines). Using the calorific values of the petroleum consumed in Brazil, supplied by the National Energy Balance (base year 2001), and neglecting the contributions from impurities for water formation (as H2S) , we calculate the hydrogen content as being 0.130, not considering impurities (sulfur between 0.0005 and 0.05), the method gives a carbon content of 0.870, a result that is in agreement with, including the mentioned incertitude, the Handbook of Chemical’s Engineer, McGraw Chemical Engineering Series, 1973, for oil with low sulfur content (0.867).
For the petroleum products it is possible to use the specifications in order to obtain the probable carbon content. For comparison purposes we have calculated this value for automotive gasoline (without alcohol) by the two ways as described below.
1 –Using the calorific values
HCV= 11.220 kcal/kg LCV= 10.550 kcal/kg.
HCV – LCV = 670 kcal/kg
Mass of water produced: m = 670 kcal/kg / 540 kcal/kg = 1,24 kg water/ kg gasoline.
Mass of hydrogen/kg gasoline = 1,24/9 = 0,140 kg hydrogen / kg gasoline
Hydrogen content = 0,149 = 14,9%.
2 –Using the gasoline specifications and determining the probable composition by groups of hydrocarbons:
The relevant properties are
- High Calorific Value11.220 kcal/kg.
- Density = 0,742.
- Limits for aromatics: 0 – 40%.
- Limits for olefins: 0 – 20%
- Initial boiling point: 30 a 40 °C.
- Final boiling point 190 a 215 °C.
For this calculation, we have listed paraffins (alkanes) hydrocarbons with boiling point within the specified curve and the more abundant cyclic hydrocarbons (aromatics) presented in the following table.
Of the olefins, only pentane has its properties listed in the consulted manuals , necessary for the calculation and since they are minority in specification we don’t include it, which furthermore has a HCV and density close to those of the paraffins hydrocarbons. The two groups of hydrocarbons are represented in the equations by the average values of the calorific value and of the density, a procedure that is justified by the fact that its distribution in gasoline is unknown.
It should be observed that the paraffins should be represented by octane and the aromatics by toluene. With the average values of the HCV properties we build the system of two equations that is sufficient for determining the participation of the two hydrocarbons groups:
11.450 x + 10.170 (1-x) = 11.220
0,691 x + 0,870 (1-x) = 0,742,
whose solution is : x = 0,75 (proportion of paraffins).
Finally we calculate the fraction of hydrogen in gasoline as:
F = 0,75 x 0,158 + 0,25 x 0,09 = 0,141.
The difference between the two results is about 6% that we consider adequate for calculating the emission of greenhouse effect gases taking into account the larger incertitude effect of the atmospheric carbon sinks.
 Chemicals´ Engineers Handbook – Perry, R. H e Chilton, C. H – McGRAW-HILL KOGAKUSHA, LTD./1973 e Handbook of Chemistry and Physics – Weast, R.C and Astle, M.J. – CRC Press/1981.
 Principle of Insufficient Reason.
Graphic Edition/Edição Gráfica:
Tuesday, 11 November 2008.