Economy & Energy
Year IX -No 57:
August - September
2006 
ISSN 1518-2932

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Carbon Content in Biomass Fuel

A Model for National Development

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seta.gif (5908 bytes)No 57 Em Português  

Texts for Discussion:

Carbon Content in Biomass Fuel

Omar Campos Ferreira.

Introduction

This report is part of the data survey for the Carbon Balance revision concerning the Partnership Contract 13.0020.00/2005 signed by the Economy and Energy Organization - e&e - and the Ministry of Science and Technology -MCT.

The Carbon Balance in the energy area was made using the Energy Balance BEN/MME data and emission coefficients used for calculating the emission inventory of the greenhouse effect gases.

Brazil is a member of the United Nations Framework Convention on Climate Change by which it is committed to survey periodically the emissions of greenhouse effect gases. The e&e Organization and the MCT have signed a Partnership Contract (13.0020.00/2005) in order to find eventual errors in this periodical emissions inventory included in the Declaration of the country presented to the Convention.

In Brazil biomass is a significant share of the Brazilian energy matrix. The CO2 emissions from biomass are not accounted for[1] as green effect gases since for its production this gas is extracted from the atmosphere. The accounting of the gases emitted by biomass are calculated since methane is included in the inventory. On the other hand understanding the mechanisms of carbon recycling in the atmosphere through biomass is important for understanding the global warming phenomenon.

The importance of biomass in the Brazilian energy matrix makes it necessary a more careful treatment of its emissions because the standard approach of the IPCC (International Panel on Climate Change) is directed to energy profiles where biomass is less important. Just to enhance the relevance of this issue, it should be remembered that almost one third of our primary energy comes from biomass and sugarcane products (even not considering sugarcane tops) already holds the second place regarding primary energy supply in the country.

The objective of the present report is to obtain the most adequate coefficients for carbon content per contained energy of the main biomass energy sources (measured by the low energy value) in tons of carbon per Tera-Joule (t C/ TJ). Due to the production diversity of these fuels, the information available in the technical literature do not have the coherence necessary to produce data for the National Communication to the Convention. We tried in each case to use information from a single source, based on laboratory experiments with sufficient specification of the samples and methods used, comparing the results with those of the IPCC and investigating the eventual discrepancies and including the performance of tests and measurements.

 

a) Firewood

Firewood is composed of cellulose, hemi-cellulose and lignin in varied proportions, according to the vegetal species, and minor substances such as resins, plant nutrients and other substances. Therefore it is natural to find a large variation of its physical and chemical characteristics in particular the carbon and hydrogen contents and the high and low heat values that are used for the evaluation of greenhouse effect gases emissions according to the methodology adopted by IPCC. A search regarding articles published at the Internet has produced heat values varying from 4,700 (eucalypt, acacia, gravilea) to 6,870 kcall/kg (mimosa), woods of industrial use, without the explicit value of humidity content; regarding common firewood for residential use there are no information that permit the evaluation of carbon content and the heat values. According to the 2004 National Energy Balance about 63% of firewood is transformed into charcoal and used as energy source in the food and beverage, paper and cellulose and ceramics industries; the most used species for these ends is eucalyptus, extensively studied by steel companies of Minas Gerais (Acesita, Belgo-Mineira and Mannesmann, among others) and by the Fundação Centro Tecnológico de Minas Gerais – CETEC. The technical publications of CETEC contain important information about the charcoal production process and it is practically the only public source of data about the physical-chemical properties of eucalyptus. Therefore, the emission coefficient of eucalyptus can be considered as representing the energy use of firewood.

Knowing the carbon, hydrogen and oxygen content and the high heat value of a fuel permits to calculate its low heat value which is the basis for the values recommended by IPCC regarding carbon content per energy and it is also presently used by BEN/MME to express energy in ton equivalent petroleum (1 tep = 10000 Mcal).

The method for measuring the heat value is based on the energy balance, in the complete combustion of a sample, generally with pure oxygen, at a constant volume, and in the heat transfer to the water’s calorimeter. The difference between the high heat value (HHV) and the low one (LHV) is due to the final state of the combustion gases mixture and the water vapor formed when hydrogenated substances are burned[2]. If the thermal equilibrium state of the combustion products and the water of the calorimeter occurs without the condensation of water vapor, the heat value measured is the low one; if the vapor is condensed and the mixture is cooled to the initial temperature (generally the ambient one, taken as 25°C), a larger heat quantity is transferred to the calorimeter and the result is the high heat value. The equation that links both heat values is:

PCS = PCI + m(c ΔT + L),                    (1)

where m is the combustion water mass, ΔT is the difference between the ambient temperature and the equilibrium temperature before condensation and L is the condensation latent heat of the water vapor.

The heat value of eucalyptus are taken from the following information sources:

-          “Fonte primária de energia – madeira combustível”, Martins, H; SPT 01, CETEC, 1980 – PCS = 4.700 kcal/kg;

-          “State of the Art on Charcoal Production in Brazil”, Almeida, M. R. et al, Florestal Acesita, 1982 – PCS = 4.200 kcal/kg (calculado pelo balanço de massa na carbonização);

-          “Produção de energia do fuste de Eucalyptus grandis”, Vale, A. T et al, UnB, 1997 – PCS = 4.640 kcal/kg;

-          “Principles of World Science”, Côté, N. A et al, Springer Verlag, 1962 – PCS = 4,500 kcal/kg. 

        The average value from these data is HHV = 4,510 ± 220 kcal/kg and the relative certitude is 5% (220/4510).

Hydrogen content of firewood is shown in the table below:

 Table 1 – Physical-chemical characteristics of dry firewood of Eucalyptus Grandis (SPT-008)

Element

Carbon

Oxygen

Hydrogen

Content % in mass

50

44

6

Each gram of hydrogen generates 9 g of water; so, the combustion of 60g contained in 1 kg of firewood generates 540g of water. The LHV is:

            LHV = HHV – water mass x (1 kcal/kg.°C * 75°C + 540 kcal/kg), in the present case:

            LHV = 4,510 – 0.540 x 615 = 4,178 kcal/kg.[3]   

The emission coefficient of firewood calculated using these data is:

      fc = 0.500kgc /(4,178 kcal/kg * 4,186 kJ/kcal) = (0.495 / 18.3 * 106) kgc / MJ          = 28.6 tC /TJ.

                The relative incertitude regarding fc is also 5%, since the other parameters in the equation above are physics constants.

This result differs in less than 5% from the coefficient recommended by IPCC, difference compensated by the incertitude concerning the high heat value.

 

b) Charcoal

Revision of carbon balance relative to charcoal has the purpose of proposing a coherent set of emission coefficients to be used in future works. Charcoal production generates as byproducts: insoluble tar, that can be economically recovered to be used as fuel, pyroligneous liquid, that contains water, soluble tar, acetic acid methanol and gases. The proportion of the substances resulting from pyrolisis varies according to the quality of the firewood and the pyrolisis temperature. Tar is not well characterized and together with the variation of its composition according to the carbonization temperature makes it difficult to evaluate its contribution to the global emission factor.

The characteristics of the substances are evaluated at 400°C, the typical pyrolisis temperature commercially used in Brazil. The proportion of the different substances can be described with good approximation by the reaction equation[4]:

 2C42 H66 O28         3C16 H10 O2 + 28H2O + 5CO2 + 3CO + C28H46O9.

The content of the elements of interest calculated by the equation above are satisfactorily close to those mentioned in SPT-008 of CETEC for dry firewood as shown below:

 

Table 2 – Elements composition of Eucalyptus grandis.

Element

Carbon

Oxigen

Hydrogen

Equation

0.495

0.440

0.064

SPT 008

0.500

0.433

0.061

It should be observed that the firewood heating phase up to the pyrolosis temperature is not included in the equation. Practically a fraction of the firewood loaded in the carbonization kiln, estimated to be 5% in mass, is burned with the open kiln in atmospheric conditions, emitting predominantly CO2  that can be calculated using the appropriate coefficient[5]. Using the equation above together with the heating values from the Technical Publications makes it possible to calculate emission coefficients of firewood, charcoal and the liquid and gaseous effluents.

 

 Table 3 – Data for calculating emission coefficients for firewood and charcoal.

 

Firewood

Charcoal

Water

CO2

CO

Tar+ pyroligneous liquid

Mass – g

2036

702

504

220

84

526

Carbon mass– g

1008

576

-

60

36

336

C content

0.496

0.821

-

0.273

0,429

0.639

PCS – kcal/kg

4510

6940

-

0

2350

6610 (tar)

PCI

4178

6750

-

0

2350

6390(tar)

Emis. Coef

tC/TJ

28.6

29.1

-

0

43.6

23.9

 

c) Sugarcane Products

The emission coefficient for alcohol calculated by the chemical formula (18.8 tC/TJ) differs slightly from the values calculated in the emissions balance (18.5 tC/TJ), suggesting that carbon accounting in the input and products is correct. However, the coefficient used in the inventory is quite lower (14.81 tC/TJ) since it took into account only the CO2 content emitted by vehicles.

The consolidated balance of distilleries (Table 16, pg. 36 of the Final Report) considers as input: sugarcane juice, molasses and other recoveries and as product anhydrous and hydrated alcohol. Bagasse, that represents the fibrous part of sugarcane is not accounted for in the distillery system. Molasses is a byproduct of the sugar industry which shows that there has been some difficulties in treating the sugar-alcohol industry as a system. As it is not our purpose to include sugar production in the present phase, we have considered as input for alcohol production sugarcane ( fermenting sugars – represented by sucrose – fibers and water), as product and co-product, alcohol (in anhydrous equivalent) and the remaining bagasse; as waste there are combustion gases, represented by CO2, fermenting gas (actually CO2) and glycerol, considered as carbon compounds waste such as vinhoto. The following table shows the carbon compounds flows in the distilleries, carbon content of the considered substances and the carbon balance.

 

Table 4 – Data for carbon balance in alcohol distilleries.

 

Component

t/t Sugarcane

C content

Mass C

LHV  TJ/t

f C

 

Input

Sucrose

0.1401

0.4214

- 0.059

 

 

Fibers

0.1401

0.4432

- 0.062

 

 

 

Products

Alcohol

0.060

0.522

+ 0.031

0.0278

18.8

Remaining

Bagasse

0.0222

0.443

+ 0.014

 0.0183*

24.2

 

Wastes

CO2 comb.

0.190

0.273

+ 0.052

 

 

CO2 fermen.

0.073

0.273

+ 0.020

 

 

Glycerol vinhoto

0.0053

0.3914

+ 0.002

 

 

                                                                Σ MC =- 0.001 ( 0.8 %)    

* dry base  

1 – “Balanço das emissões de gases do efeito estufa na produção e uso do etanol no Brasil” SMA/SP – 2004.

2 – “Análise Exergética da Produção de Etanol da Cana-de-Açúcar”, Esteves, O.A. – MSc Thesis– CCTN/UFMG-1995.

3 –“Tratamento de Efluentes na Indústria Sucroalcooleira”, CTC Copersucar-1995.

4 – Formulas: Sacarose C12H22O11 – Glycerol C3H8O3

Using the described treatment, carbon balance in alcohol distilleries presents a relative deviation of less than 1%.

 

Conclusion

Table 4 (Summary) Comparison between the values recommended by IPCC and those proposed in the present report in tC/TJ

 

Present report

IPCC

Reference report COPPE/MCT

Firewood

28.6

29.9

29.9

Charcoal

29.1

29.9

29.9

Tar+ pyroligneous

23.9

-

-

Ethyl alcohol

18.8

-

14.81

Remaining Bagasse

24.2

29.9

29.9


 

[1] Naturally this does not include the eventual destruction of the accumulated native forests biomass.

[2] The water usually contained in the firewood is not part of this calculations, being eliminated by the preliminary drying of the sample in a muffle or being excluded from the sample mass.

[3] The 2001 National Energy Balance indicates LHV= 3,100 kcal/kg for commercial firewood with 25% of humidity.

[4] SPT-008 mentions as the source of information the work of Klar, M Technologie de la distillation du bois”, Librairie Poly Technique, Paris, 1925.

[5] “Emissions of greenhouse effect gases in the production and use of charcoal”, Ferreira, O.C -E&E, nº 20, 2000.

Graphic Edition/Edição Gráfica:
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Editoração Eletrônic
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Revised/Revisado:
Tuesday, 11 November 2008
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