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Text for Discussion:
Evaluation of CO2 Emissions Using the Top-Down Extended Process Between 1970 and 2004
1 - Introduction
This report is part of the data survey for the Carbon Balance revision, object of the Partnership Contract 13.0020.00/2005 between the Economy and Energy – e&e –organization and the Ministry of Science and Technology – MCT.
It is specifically listed as Goal 1 – “Estimate of C and CO2 Emissions in Energy Use and Transformation from 1970 to 2004 by the Top Down Extended Process”.
The activities foreseen for this phase are: use of carbon balance in energy production, transformation and use in Brazil in order to evaluate emissions in the period and using the above mentioned approach.
2 - Perliminaries
The MCT has previously signed the No 01.0065.00-2003 Agreement with the e&e Organization aiming at calculating the carbon balance in the Brazilian Energy Matrix regarding the Brazilian Inventory of the greenhouse effect gases. One of its objectives was to detect eventual incoherence in the initial Inventory by comparing the Top-Down Extended (TDE) and Bottom-Up (using coefficients) processes.
In the first calculation, the Top-Down process was extended so that it would evaluate carbon emissions in the transformation and consumption of energy. The Bottom-Up methodology using coefficients was applied to the emissions inventory from 1990 to 1994. So, emission coefficients were deduced for the various gases responsible for the greenhouse effect gases by unit of consumed energy and by fuel listed in the Energy Balance.
The result of the previous agreement identified a series of incoherence and omissions that the present Agreement will try to resolve. The objective of the present study is to consolidate a Carbon Balance that can be an instrument to emissions evaluation, helping the establishment of policies for that area.
This Partnership Contract foresees as the first goal the elaboration of the Carbon Balance using the TDE methodology. From the previous study it was concluded that the application of this methodology permitted the calculations of emissions by a process equivalent to the Top-Down by the IPCC methodology using some lines of the Carbon Balance and retention and oxidizing coefficients. This methodology was adopted in Brazil for calculating its inventory and the corresponding results are presented in Reference 1.
Comparison of results has shown a good agreement of data regarding emitted carbon and its CO2 equivalent, around 1.3%. However, concerning analysis by fuel some divergences were detected which is tried to be clarified in the present study.
Furthermore, the present Partnership Contract foresees the improvement and use of computer programs to automate emissions calculations aiming at assisting in the calculation of the next inventories. It is expected that once the incoherence is eliminated in the use of coefficients or conflicting procedures in the two methodologies, some points that need clarification will be defined, including experimental measurements.
This technical note calculates the Top-Down emissions, revises some of the coefficients and presents the calculation methodology so that future studies can easily incorporate changes in the technical coefficients that are considered convenient.
3 - Methodology
The IPCC (Intergovernmental Panel on Climate Change) Top-Down methodology makes the accounting of the primary and secondary fuel that are introduced in the economic system in order to satisfy the needs of the human activities (even the non-commercial ones) and of how much carbon leaves the system. Once introduced in the national economy, in a determined year, the carbon content of a fossil fuel either is emitted to the atmosphere or is retained in some way, for example, through the increase of fuel stock, its transformation in non-energy products or its partial retention in non-oxidized form in the combustion residues.
The use of the Top-Down (TD) methodology recommended by IPCC in its 1996 revision permits to estimate the CO2 emissions as a function of data regarding the energy supply in the country and a few data concerning their form of use. The data used are from BEN (National Energy Balance) edited by the Ministry of Mines and Energy – MME.
The values supplied by BEN are originally given in natural units that correspond to those used in their origin (mass in tons and volumes in cubic meters). In some cases, where energy sources are grouped, the units used are ton equivalent petroleum (tep) and a special criterion must be established for calculating emissions.
The Top-Down (TD) methodology calculates the Apparent Consumption of a country by energy source from the equation:
Apparent Consumption = Production + Imports – Exports – International Bunkers + Stock Variation
In practice this concept coincides with data from the Total Internal Supply BEN/MME where:
Total Internal Supply = Production + Imports – Exports (in BEN Bunkers are included) – Non Used – Re-injection
The “Non Used” and “Re-injection” concepts refer specifically to the Natural Gas accounting that, as will be examined below, can be separately treated. However, in order to calculate using the TD process, production excludes re-injection of Natural Gas in the wells and the non-used energy (gas flow to the atmosphere or burned in flares during extraction are not accounted for by TD and are treated separately). This equivalence will be shown below when a practical case will be analyzed.
In a simplified way, the Top-Down methodology can be described as follows:
a) Calculation of fuels apparent consumption in their natural measurement units;
b) Conversion of the apparent consumption to a common energy unit - terajoules(TJ);
c) Transformation of the apparent consumption of each fuel to carbon content by multiplying by the specific emission factor of that fuel;
d) Calculation of the carbon amount of each fuel destined to non-energy utilization and subtraction of that amount from the carbon of the apparent consumption in order to calculate the real carbon content that can be emitted;
e) Correction of this value in order to consider the incomplete combustion, calculating the true carbon quantity that is actually oxidized in the combustion
f) Conversion of the oxidized carbon to CO2 emissions.
In the adopted methodology, the emissions are calculated by multiplying the values - expressed in energy relative to the final use of the energy sources and some transformation – by coefficients adequate to the fuels used in Brazil or default coefficients recommended IPCC.
4 - Use of the ben_eec Program in the Ca;culation of Carbon Content
4.1 calculation of Energy Data Using the ben_eec Program
The BEN / MME (49 sectors and 46 accounts) data base supplies data in natural units from 1970 until 2005. These data are annually published and constitute the National Energy Balance presently elaborated by the Energy Research Enterprise – EPE, for the Ministry of Mines and Energy - MME.
The ben_eec program whose operation manual was presented in the Technical Note 3, annex to the Report No1 of the Partnership Contract E&E/MCT, was previously developed by ECEN Consultoria Ltda for the Economy and Energy Organization e&e. This program was already used in several energy planning studies and it is available at http://ecen.com in the previous version. The present version improves the calculation structure and facilitates the data revision.
Using original data expressed in natural units (mass, volume or, in case of aggregations, in ton equivalent petroleum – tep) and coefficients for each energy source and year, the program converts data to energy units (Mcal) using energy/mass or energy/volume coefficients. Presently, the MME used as base the low heat value (LHV) but previously the values were given in high heat value (HHV). These data are used in the conversion of the usual data from the energy source balance that is the ton equivalent petroleum presently defined as:
1 tep = 10.000 Mcal with the energy content measured in LHV values
Previously, BEN/MME used the relationship:
1 tep (old) = 10.800 Mcal with the energy content measured in HHV values
Furthermore, the adopted equivalence for electricity took into account the quantity of fuel oil necessary for generating electricity. Presently, the conversion is made directly in energy. Thus the following equivalence:
1 kWh=860 kcal=0,086 tep (new)
Previously, the adopted equivalence was:
1 kWh was equivalent to 3132 kcal = 0,29 tep (old)
The previous literature (such as reference document concerning the initial inventory of greenhouse effect gases emissions) used old units such energy coefficients relative to HHL and values in tep (old). For this reason, the program supplies to the user results in this unit and in tep (new) and LHV. It should be noted that the LHV and HHV are not equivalent and may have other applications for the user.
The ben_eec program also converts energy to equivalent energy (that takes into account the average relative efficacy of the energy sources in each sector) and in carbon content that is used for calculating emissions and test the coherence based on the conservation of carbon atoms in the involved chemical reactions in the use of the energy source.
The MME supplies annually the values used in the conversion to tep. This means that the old tep and HHV coefficient values are available until 2002. As these values were subsequently converted to new tep, the new tep and the implicit LHV values are also available for all years. Based on these values of the LHV/HHV ratio for each fuel and extrapolating to the years subsequent to 2002 it is possible to supply data for all the years in the old and new units. For each j fuel and i year, there is a coefficient c(i.j) such that
[Energy in tep] (i,j) = [Quantity in Natural Units] (i,j) × c(i.j)
Furthermore, the r(i,j) = [Energy in new tep](i,j) / [Energy in old tep ](i,j) ratio permits to convert from new tep to old tep and, based on the energy equivalence previously mentioned, it is possible to supply the energy values in Mcal for both the LHV and HHV. This conversions are automatically carried out by the program. Besides that, the ben_eec software permits to construct tables chosen by the user and make several aggregations, including renewable and non-renewable energy that are important regarding emissions accounting.
In Table 4.1 it is illustrated how the program calculates the values in different energy equivalence in the different presentation formats. In this technical not are not shown examples regarding electricity, hydraulic and nuclear energies that are of no interest to the present study since in these cases there is neither carbon content nor emissions directly involved.
Table 4.1: Illustration of calculation carried out by the ben_eec program in energy and carbon conten
The example refers to a line of the spreadsheet for 1994 (Total Energy Supply). The table shows the original data (natural units indicated in the first line), the conversion factors indicated as new tep (10.000 Mcal) and old tep (10.800 Mcal). In the first column are shown the conversion formula as well as the factors regarding carbon content calculation that will be commented in what follows.
4.2 Calculation of Carbon Content in the ben_eec Program
The carbon content calculation is carried out using coefficients given in tons of contained carbon per contained energy (LHV) for the different energy sources (last line). These coefficients can in principle have different average values for each year. The usual unit of these coefficients is ton of contained carbon per Terajoule, tC/TJ.
In Brazil, considering the inadequate production profile of petroleum products consumption (mainly after the substitution of fuel oil and gasoline after the 1973 and 1979 oil prices crises) and the “dieselization” of the automobile fleet, fuels have rather had a varied utilization. The same happens to fuels of local use such as biomass products intensely used in Brazil. However, the usual practice is to adopt a fixed value (for each fuel) of the carbon mass/ energy coefficient for all years with a few exceptions that will be commented below. It should be noted that the energy / natural unit coefficients variation available for some years in BEN has already some of the fuel variations mentioned above.
Besides the energy in the different equivalences, the program calculates for each column (fuel) the carbon content using the original energy balance data as illustrated in Table 4.1 (year 1994) for the Total Energy Supply account that corresponds, as shown, to Final Consumption in the Top-Down approach. The carbon content value is shown in the last column.
The carbon content is calculated by multiplying the energy content of each fuel (converted to TJ) by the factor shown in the last line. The procedure is valid for all lines (accounts) in the same column.
The conversion factor for calculating the contained carbon mass is based on the conversion of the energy value in Tcal based on the LHV (D in the table) by the 4.1868 TJ/Tcal ratio corresponding to the equivalence of these units. Since the energy content and the contained carbon mass ratio is given by the z factor in Table 4.1, given in Terajoule per ton of carbon, one has:
1 TJ/tC = 4.1868 Tcal/tC = 0. 0041868 Tcal/mil tC = 0.0041868 Tcal/Gg of C
It should be mentioned that it was not possible to use in the present study the LHV adopted by BEN to define ton equivalent petroleum (tep) which was not available in the calculation of the initial inventory (COPPE/MCT reference report). For this reason, when one compares the data from the present study with those of the inventory it should be considered that even though the primary data source is the same, conversions to the LHV are slightly different from those of the HHV.
It is also shown in one of the lines of Table 3.1 the high heat value/low heat value ratio (HHV/LHV) resulting from the coefficients used by the MME. The values are (according to the organizers of the Balance) based on international values. The values oscillate around 1.06. It was expected that they would depend on the hydrogen content of the fuel since the difference between the two types of heat values (see Technical Note No 2) produces water in the combustion process due to the combination of contained hydrogen and the atmospheric oxygen.
5 - Carbon Balance
The Application of the described methodology to all lines of the Energy Balance permits the generation of contained carbon spreadsheets for all fuels in all accounts. Since the carbon share that is not released to the atmosphere by the energy use of fossil fuels is very small (around 1%), the Carbon Balance spreadsheets already give a good idea of the emissions by sector or by fuel. They also permit to detect possible errors in the carbon accounting that might influence emissions calculations.
The contained carbon obtained, as shown, is a function of the tep data in the Energy Balance (converted to TJ) and of the emission factor of each fuel (in tC/TJ). In the analysis made in the ambit of the previous agreement with the MCT it was pointed out the inconvenience of using generic coefficients (for liquid and solid biomass) suggested by the IPCC. This subject was object of the Technical Note No 2 that is also part of Report No 1 of the Partnership Contract that originated the present study and was published in the Nº 57 e&e periodical. Some of the emission factors for biomass calculated in the NT 2 were adopted in the present study. In what follows the values adopted in the present study are discussed and they are compared with those used in a previous e&e study, with those used by COPPE and with the consolidation carried out by the MCT team.
5.1 Energy Content per Contained Carbon in Fuels
The IPCC supplies emission factors for energy sources (f tC/TJ) for primary and secondary liquid fossils, for primary and secondary solid fossil, natural gas and solid, liquid and gaseous biomass.
Table 5.1 present carbon emission factors used in the present study and its comparison with other values suggested by IPCC, those used by COPPE (ref), those calculated by MCT and those calculated by e&e. When the emission factor was calculated by e&e this value was used (gray), when not calculated, it was adopted the most repeated values among the options (green) and in case of discrepant values among the different suggestion, the IPCC value was adopted (yellow).
In the same table it is presented the correspondence between the different energy sources as presented by the IPCC and the adopted in the present study.
z = Carbon Emission Factor
From the energy source data in (new) tep converted to TJ one obtains the carbon mass for each of the fuels that are part of the Energy Balance and one can construct the Carbon Balance for each year. The carbon balance in 2004 is presented in Table 5.2 as example.
(*) The values recommended by IPCC refer generically to liquid or solid biomass, those adopted here are those of NT1
6 - Calculation of Top-Down Emissions
6.1 - Carbon Retained in the Non-Energy Use
The carbon retained correspond to the non-energy use. The fuels that have non-energy use are: natural gas, naphtha, illumination kerosene, anhydrous alcohol, hydrated alcohol, refinery gas, asphalt, lubricants, other non-energy petroleum products and tar. In this type of use not always there is carbon retention and the IPCC methodology recommends the use of some coefficients (percent of mass) to take into account emissions that can occur due to natural evaporation (and subsequent conversion to CO2 in the atmosphere) or by burning and degradation of wastes. When they are not supplied, one can use coefficients based on available information. In the present case, the option was to repeat whenever possible the values from the COPPE/MCT report (ref) as follows (Table 6.1):
C = fraction of retained carbon.
Table 6.1 – Fractions of stored carbon
These values also deserve some comments. For example, hydrated and anhydrous alcohol used in non-energy uses include alcohol used for cleaning (domestic and others) whose end is obviously oxidation in the atmosphere, as already considered in the lubricants case. The same is true for kerosene. Analyzing the subject extrapolates the objectives of the present study. Similarly to these energy sources (used in non-energy ends), other retentions should be considered. For this purpose it was adopted for this coefficient the value 1 for all energy sources (shown in red in the table shown below) so that eventual cases of non-energy uses where part of the carbon is not emitted can be detected
6.2 - Non-Oxidized Carbon
In practice the difference between fuel apparent consumption and that retained in non-energy products is the carbon available to be emitted in the combustion. But not all carbon will be oxidized because combustion is not complete and a fraction of non-oxidized carbon will remain in the ashes. The fraction of oxidized carbon (that generates CO2 directly or through the degradation of other compounds in the atmosphere) varies according to the fuel. In the adopted methodology this correction is made through the multiplication by a factor called oxidizing factor suggested by the IPCC.
Table 6.2 illustrates the calculation process, presents the results for 1994 and shows the oxidizing factors values used. In the present study were adopted the oxidizing factors recommended by IPCC. These values were maintained for all years and for most of the energy sources except for two specific cases: natural gas and firewood.
a) Natural Gas
Natural gas contains liquids that must be separated from the consumed natural gas (as such or in the form of dry gas) whose oxidizing is more complete. For liquids of natural gas the 0.99 factor is used (1% is not oxidized) for gases the 0.995 factor is used (5% is not oxidized). The oxidizing factor to be used for the total natural gas will be a weighted average of the share of the two consumption types. At the bottom of Table 6.3 is shown the way to calculate the fraction of liquids and obtain the average oxidizing factor.
The fraction of liquids extracted from natural gas can be obtained by adding the transformation of natural gas in the “Natural Gas Plants” and “Other Transformations”. The emissions obtained by applying the average factor are, as expected, equal to the sum of the values obtained separately for liquids of natural gas and natural gas for direct use (in most cases, dry natural gas).
Table TTable 6.2: E missions by the Top-Down Process Using the Carbon Balance (Gg/year) shown in Annex 2
Emissions by the Top Dow Process Using the Carbon Balance (Gg/ano)
Table 6.3 - Equivalence of emissions calculation procedure using factors proportional to the use
flgn = Liquids fraction GN = -(C+E)/A = 0,234 flc = Fraction of firewood for charcoal= -D/A=0,441
H=flgn*0,99+(1-flgn)*0,995 =0,994 H=flc*0,89+(1-flc)*0,87 = 0,879
For firewood used in charcoal production and firewood for direct use two oxidizing factor are used. The fraction of the former is obtained from firewood transformed in the charcoal units divided by the total supply. The average oxidizing is analogous to the previous case and is also shown in Table 6.3.
The data obtained by using this methodology are compared with those of the COPPE/MCT reference report (Table 6.4). It can be noticed that the changes introduced did not alter substantially the results for the fossil sources (2% in the total) but alter significantly emissions associated with biomass, mainly solid biomass.
Table 6.4 Comparison COPPE/MCT with present Study
Comparison of COPPE/MCT with present Study (continuation Table 6.4)
Comparison of COPPE/MCT with present Study (conclusion Table 6.4)
CARBON BALANCE - 2004 Contained Carbon Gg/year