Text for Discussion:
EstimatE oF Carbon CONTENT In DRY Natural GAS UsING THE DifFerenCE BETWEEN THE HIGH AND LOW HEAT VALUES
Omar Campos Ferreira.
The Economy and Energy – NGO staff has been developing methods for determining the carbon content in fuels as part of studies for the General Coordination on Climate Change of the Ministry of Science and Technology regarding the atmospheric carbon inventory. The subject is a relevant one in what concerns the Brazilian attitude vis-à-vis the Kyoto Protocol because the establishment of mechanism for clean development was an initiative of the Brazilian Delegation to the Kyoto Conference.
The Brazilian energy matrix is one of the cleanest in the world regarding both the emission of chemical pollutants (CO, non-burned hydrocarbons, SOx, NOx, etc.) and the emission of greenhouse effect gases (CO2, HC, NMOCV´s), except for doubts relative to methane emission. This quality of the matrix is a consequence of the use of hydroelectricity and biomass fuels and it can have important economical effects on the Energy Sector when the Kyoto Protocol comes into force. Then Brazil may sell carbon credits to countries that for some reason cannot reduce their emission to the 1991 level. The position of the United States is contrary to the implementation of the Clean Development Mechanisms because they fear the stagnation of its economy since fossil fuels represents the largest share of energy conversion.
The main objective of the present report is the monitoring of the official information about the carbon content in the products of Transformation Centers (oil refineries, gasification plants, electric power plants, coke plants and distilleries) using the mass/energy balance, the legal specifications of fuels and the respective high and low heat values, checking the coherence of these data through the physical-chemical properties of the components of the fuel mixture.
In the nº 43 issue of the Economy & Energy periodical () we have proposed a quick method for determining the carbon content and we gave examples of its application on crude oil and automotive gasoline, obtaining 6% of uncertainty which is typical of the method. This approximation was assigned to uncertainties regarding the fuels specifications and field measurements, in general larger than the uncertainties regarding data from technical publications. However, there are other sources of uncertainties, systematic ones, related to different interpretations of the high and low heat values and even to the explicit - or not - change of the reference physical state.
A compilation of the usual definitions shows that the oldest Thermodynamic texts define the low heat value as the difference between the high heat value and the latent heat of water vapor condensation (L=539 kcal/kg) that is formed in combustion, what is equivalent to considering as reference state that of the combustion product mixture at 100°C and 1 atm when the water vapor is condensed. Other authors, considering that the fuel is initially at 25°C and 1 atm, subtract from the high heat value the condensation latent heat and the cooling heat of the combustion products at the original temperature of the fuel and, in this case, assigning to the combustion gases the average specific heat, between 100°C and 25°C, and assuming the stoichiometric composition of the mixture (fuel + air), it is necessary to take into account the hydrogen content of the original fuel in order to obtain the masses of water vapor and of other gases present as combustion products, what would make the calorific values method meaningless.
Therefore, the verification of the present proposal represents an evaluation of the influence of all these factors on the quick calculation result of the carbon content that, like other quick calculations, can be used as a first approximation, considering that the concentration of greenhouse effect gases is not simply due to the calculated emission of these gases, since the biosphere has reduction mechanisms that are not well known yet.
However, the application of the method to natural gas using data from the 2002 Brazilian Energy Balance did not produce results consistent with the other technical publications within the incertitude range evaluated for the exemplified cases, casting doubts on the validity of the method. In the present study we have tried to remove these doubts by applying the method to dry natural gas (or processed gas, according to several nomenclatures) verifying at each step the data consistency.
Data concerning the Candeias Natural Gas Processing Plant (UPGN) were taken from the site that is supported and supervised by PETROBRÁS, as declared on the main page.
1 – Conversion of volumetric composition data to mass composition.
2 – Calculation of the high and low heat values, the specific mass and the mixture density relative to that of air using data from the Chemical Engineers´ Handbook, ed. Mc Graw-Hill, 1973 for the physical-chemical properties of substances contained in natural gas.
3 – Calculation of carbon content in natural gas and comparison with the IPCC data.
Table 1 – Composition of processed NG of Candeias.
*Calculated by the molecular mass
Relative density in air: 0,791/1,293 = 0,612.
UPGN density = 0,61.
Relative difference 0,002/0,612 = 0,03 (0,3 %).
Table 2 – Heat values.
· Previous table
Calculated HHV = 12650 kcal/kg = 10010 kcal/m3
HHV UPGN = 12070 Kca/kg = 9549 kcal/m3
Relative difference = 0,048 (4,8%)
Calculated LHV = 11430 kcal/kg = 9041 kcal/m3
LHV UPGN = 10090 kcal/kg = 8621 kcal/m3
Relative difference = 0,049 (4,9%)
Table 3 – Carbon content calculated by the mixture composition
Table 4 – Carbon content calculated by the heat values of the UPGN.
Notes: L1 is the water vapor condensation heat at 100°C and 1 atm. L2 is the sum of L1 and the cooling heat of the combustion products at 25°C and 1 atm. All heat values are referred to the mass unit (kcal/kg) of the respective substance.
The largest relative difference between the carbon content calculation results using the fuel composition and using the high and low heat values is less than 0.07 (7%) which is not substantially different from the relative difference for petroleum and automotive gasoline (6%) and it is smaller than the relative uncertainty concerning the high heat value of the processed natural gas (8.6%).
In order to compare our calculations with the IPCC results we have taken the result that presents the largest difference relative to the carbon content calculated using the NG composition (0.789 in Table 4) and have calculated the carbon mass in tones corresponding to 1 TJ of liberated heat.
1 TJ = 1012 J = 0,239 x 1012 cal.
Mass of NG that liberates 1 TJ in complete combustion = 0.239 x 1012 cal /12 x 109 cal/t NG = 19,9 t NG = 19,9 x 0,789 = 15,7 t C /TJ.
The value published by IPCC is 15,5 t C / TJ.
Therefore, we believe that the quick method seems reliable within the accuracy used for calculating carbon balances. Furthermore it permits to take into account differences of fuels from different origins and, in the case of Brazil, to take into account the differences of fuel specifications along time.
Graphic Edition/Edição Gráfica:
Tuesday, 11 November 2008.