Economy & Energy
Year IX -No 51:
August-September 2005   
ISSN 1518-2932

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Natural Gas in Bolivia: Risks and Opportunities

Petroleum in Brazil

Carbon Balance: The Top-Down and Bottom-Up Emissions Accounting Methodologies

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Carbon Balance:
The Top-Down and Bottom-Up Emissions Accounting Methodologies

Carlos Feu Alvim, Frida Eidelman e Omar Campos Ferreira

Introduction

The Economy and Energy Organization has carried out together with the Ministry of Science and Technology a study on the carbon balance of energy use and transformation. The publication of its results has been made by the e&e periodical. So it has already been published:

• Carbon Balance in the Production, Transformation and Use of Energy in Brazil – Methodology and Results of the Top-Bottom Process from 1970 to 2002 (e&e No 48)
• Carbon Balance in the Energy Transformation Centers (e&e No 50).

In the present issue we present  the results corresponding to the adopted accounting process that comprises the Top-Down approach and the use of coefficients calculated in the Brazilian inventory from 1990 to 1994 for estimating emission from 1970 to 2002 by the Bottom-Up process. The results can then be compared in order to evaluate the deficiencies and possible incoherence in the use of the two methodologies.

For this purpose data and comparison of results two computer programs have been used that permit to list the energy and carbon balance data for the whole period in different configurations.

In order to obtain data by the Bottom-up process the benemis_e program was uswd. This program previously made and updated to include the year 2002 in the MCT’s agreement permits to quantify emissions by type of fuel in the different sectors. It permits as well a survey of emissions from 1970 to 2002 and includes results for CO2, CO, CH4, and NMVOCs[1] that contain carbon besides N2O and NOx[2] , that are of no interest for the carbon balance. These data are in the Final Report of the Carbon Balance Project presented to the MCT and will soon be published in e&e. For emission calculations the program uses coefficients obtained from the Brazilian energy Balance (BEN) data on consumption and transformation and from Bottom-Up calculations carried out for the inventory. These coefficients were obtained from the inventory results and supplied to e&e by Branca Americano of the General Coordination on Global Climate Change – MCT. Subsequently the results obtained from the program were checked and some discrepancies were found and accordingly corrected by the Coordination.

Using the benemis_e, it is possible to make tables and graphics for each type of emitted gas by “account” (consuming sector and some transformation centers) that present for the different fuels and “accounts” the energy and emissions by type of greenhouse effect gases.

The present version of the benemis_e program was updated and it presents the carbon contained in the fuels (before emissions) and the results of the Top-Down and Bottom-up approximations of emitted carbon and the absolute and relative difference between the two methods.

In the  Bottom-Up approximation one starts from the carbon contained in each of the gases of interest, obtained from the chemical composition itself and from the atomic masses. In the case of NMVOCs it was simply assumed the carbon mass/ total mass ratio (based on the carbon contained in cetane). The coefficients used were the following:

CO₂ (12/44=0,2727), CO (12/28=0,4286), CH₄ (12/16=0,75)  e

 NMVOC (0,8)

The sum of the carbon compounds masses obtained by using these coefficients can be compared to the mass obtained using the Top-Down process that is the net carbon mass (except for that retained in the non energy uses) multiplied by the oxidation factor. Both values should be equal.

Some doubts may remain about non oxides (CH₄ e NMVOC), namely if they are included in the oxidation factor used in the Top-Down methodology. The IPCC (International Panel on Climate Change- Revised 1996 IPCC Guidelines) indications are that the oxidation factor refers neither to the time of use or to the atmosphere. The residue one wants to exclude is that which remains in the ashes (non degradable) or the carbon fixed in some equipment or product.

Therefore, the corrected retention value should be compared to emissions value calculated in each sector from emissions obtained in the use and transformation equipment. In order to do that it is necessary to extend the Top-Down technique to the whole consumption matrix and in transformations where there exists calculated emissions in the respective units. 

Extension  of the Top-Down Approach to BEN’s  Sectorial Data.

In the  Top-Down approach  the aim is to detect emissions associated to the apparent consumption of primary fuels in a country.  In the case of secondary products the internal production is considered, since they come from the already considered primary source. As shown above, this corresponds to the total value shown in the “gross internal offer” line of BEN. An evaluation of emissions from losses can be made separately. The methodology name corresponds exactly to the procedure adopted by those who take as base an energy balance table that is consolidated in the form where these information are contained. In the case of the Brazilian balance this approach goes up to the eighth of the 46 lines that correspond to “accounts” of the energy balance.

The carbon balance carried out in the present article corresponds to:

·         Surveying the carbon content of the energy balance fuels;

 ·       Testing the balance coherence of the transformation centers where these emissions are not calculated; 

·      Extending the Top-Down approach down to the final of the table, including the transformation and consumption sectors;

·       Comparing the results obtained with the “reconstituted” values using coefficients (emission/obtained energy) extracted from evaluations of the inventory that used Bottom-Up  methodology.

Therefore extending the Top-Down process to the energy sources set is part of the carbon balance. One starts from the fact that when an oxidation factor is applied on a raw material in that approach it is implicit that it can represent the average oxidation values of the fuels in its direct use. That is, assuming that 1% of the carbon from petroleum does not undergo oxidation implies that the same happens to the average of petroleum products.  The  present extension assumes that in all petroleum products (gasoline, fuel oil, diesel oil, LPG, etc.) 1% of its carbon does not undergo oxidation, This was already implicit in the original method because when net exports were subtracted this hypothesis was adopted. Naturally this is not exactly true since there is no reason to suppose that – considering extreme cases – fuel oil undergoes oxidation in the same way as LPG whose oxidation is closer to that of natural gas (99.5% of emission and 0.5% of no oxidation). Actually, the deviations detected in the balance made were such that the same comparison with contained carbon was already useful regarding the desired diagnosis.

The oxidation factor proposed for raw material (primary energy) was applied here to all products (secondary energy) in the consumption and transformation steps when direct emissions are calculated (charcoal plants and electricity generation in public service and auto-producers plants).

This approximation permits to produce a set of the expected emissions data by consumption sector by energy source and compare these values with the expected emissions of sectorial evaluations in the Bottom-Up process. Since it is expected that the values based on the two methodologies are coherent, comparing them in a carbon balance is a good test for the estimations made. Analyzing consumption data can – as it happened in the transformations case – identify improvements that should be introduced in one or the other methodology.

Spreadsheets in the 49X46 format [3] relative to the expected emissions were here produced through the extension of the Top-Down concept using the final energy spreadsheet in new tep (10000 Mcal). They can be directly compared with the results of the Bottom-Up methodology.

The results were obtained using the benemis-e program adapted to produce carbon balance values in its “Emission in the Year” spreadsheet. A program that can produce complete graphics along time, as the one for emitted gases and energy, can be made from the existing structure.

Annex 5 presents for chosen years the values obtained from the two approaches with automatic indications (colored ones) for the balance results. In what follows we point out some results.

Reconstitution of the Bottom-Up Process through Coefficients.

The methodology and the results of the Bottom-Up process reconstitution from 1970 to 2002) are presented in Annex 4 of the MCT report and will be published in this periodical. Furthermore, the benemis-e program produces outputs that can generate tables and graphics for chosen periods and in different  organizations. It was made before the definite publication of the inventory results (November 2004) and had not been compared to them.

This comparison is necessary since the purpose is to compare the results obtained using the two methodologies. Concerning the Top-Down methodology the calculations were completely remade in the present article and they are coherent with the previous results. In the Bottom-Up reconstituted results it was used coefficients extracted from the “Initial National Communication to the United Nations Convention” held in Brasília in 2004.

The “account” values are compared in Table 1 and show good agreement in the consumption sectors. The differences detected in the Transformation and Energy Sector are mainly due to the interpretation given to BEN’s “other primary” item which were here considered as renewable sources except for those produced in transformations that occur in natural gas plants and refineries. The comparison suggests that it is convenient to revise the criterion and the necessary modification of the benemis-e program. It should be noticed that one of the results expected here is actually the correction of coefficients and the distribution of emissions by energy source in the program.

Table 1: Comparison of Calculated CO2 Emissions  by Sector 
(1000Gg/year) - without Biomass

 Table 3.1.5 of Brazilian Declaration

                The same comparison can be made concerning the emissions by energy source. Again problems arise about how to interpret the grouping. In fuels where it is possible to calculate emissions in the data organization as that of BEN, there is no problem as can be observed in the first ten lines of Table 2. In fact, it is natural to have a perfect agreement of the results, considering that one is following the same path used for calculating the coefficients. Only subsequent corrections of the energy or emissions data justify some discrepancies; further differences should be due to programming errors or to coefficients transcription.

 Table 2: Comparison of Calculated CO2 Emissions   (“Bottom-Up” Methodology) –
1000 Gg/year –  1990 and 1994

                 In energy source grouping (Table 3) one can observe differences regarding other secondary petroleum products (corresponding to petroleum coke, coke gas, refinery gas and other secondary petroleum products as listed in the Brazilian Declaration) and other regarding mineral coal (tar and gas plant coke in the mentioned Declaration) important percent differences are observed. In the “other fossil” item there is the problem of including this source as a renewable one. Concerning the inventory, part of them are considered as fossil sources but it does not identify them.[4] The difference is not quantitatively important but it should be investigated. difference

               Since the inventory does not record CO2 emissions from biomass, CH4 emissions obtained from the benemis-e program were compared to that of the inventory. The coherence is very good, as expected. The exception (similar to previous cases) is related to the “other” item where the grouping criteria could be different. One can conclude from the comparison that the values calculated by the program can be considered as representative of the Bottom-Up approach and of the Inventory results.

Table 3: Comparison of Calculated CH4 Emissions   (“Bottom-Up” Methodology) –
1000 Gg/year –  1990 and 1994

                 Conclusion and Next Results

 The methodology developed and the results obtained indicate that it is reliable in what concerns emissions estimations in the period (from 1990 to 1994) considered in the national inventory and that it is adequate for extrapolation the results to previous and subsequent years. In the next e&e issues it will be presented the modifications concerning the treatment of data suggested by the present analysis and the comparison of results. It will be presented as well the emissions evolution of the different greenhouse effect gases in the 1970-2004 period.

Graphic Edition/Edição Gráfica:
MAK
Editoração Eletrônica

Revised/Revisado:
Tuesday, 11 November 2008.