Economia & Energia
No 20 - Maio - Junho 2000
Greenhouse Effect Gases in the
It was shown in the previous section that charcoal is predominantly used in the production of pig iron and steel. The integrated plants presently show a trend to use mineral coal coke. It is known that the Belgo-Mineira's charcoal plant in Monlevade, Minas Gerais, is deactivating its charcoal blast furnace in favor of only one coke blast furnace. If this trend is confirmed, charcoal will be restricted to the market of independent pig iron producers, to the production of ferroalloy in some regions where still exist reserves of planted forests or exploitable native forests under the handling regime and to scrap complementation in electric arc furnaces.
The study on primary iron previously mentioned shows however that charcoal could support export efforts concerning pig iron to be used in electrical furnaces whose world demand will grow up to 63 million tons in 2010. The following data about the integrated biomass-seamless tubing system were obtained from reference (1).
Wood for charcoal production comes from a 58,000 hectares plantation with several eucalyptus species (E. Camaldulensis, Cloesiana, Urophylla and Pellita) selected because they adapt well to the soil and climate of Minas Gerais' northwest region. Moderrn forestry practices were followed aiming at preserving part of the native savannah and the fauna, producing good quality charcoal and at convenient costs. The following photo shows a Mannesmann Florestal S A' plantation where one can see islands of native forest connected by ecological corridors that facilitate the transit of large animals and preserve birds and insects that have the role of biological weed controllers. The productivity reached in old plantations is 9 tons/hectare.year of dry wood and 14 tons/hectare.year in the most recent ones that use improved seedlings. It is expected to reach 18 tons/hectare.year by using clones already available commercially (1).
In the present practice, eucalyptus is cut down in the 7 0, 14 0 and 21 0 years without necessity of replanting. Therefore, there is a permanent stock of standing wood while there is metallurgical production, corresponding to 6 years of plant growth. Once the cut down is carried out, the roots, smaller branches and leaves are left in place and constitute an additional carbon stock. Carbon inventory calculations are carried out based on plant development kinetics (1) and on the wood elementary analysis (2).
Wood Elementary Analysis (% of dry mass)
The graphic shows the carbon mass contained in the log at the time of cut down (between 72 and 84 months) is approximately equal to the mass contained in the remaining parts of the tree. The following figure shows schematically the mass balance in the process (1).
Carbon inventory (by ton of cut down log, dry base)
The table above shows that for each ton of carbon put into circulation by the productive process the plantation stores 6.8 tons of carbon in the branches and in the non processed parts.
EMISSION OF GREENHOUSE GASES EFFECT IN CHARCOAL PRODUCTION
The calculation of gas mass emitted is made from elementary analysis of non condensable gases, representing 25% of carbonized dry wood mass, and is given below (2)
The conversion parameters, already presented in the Partial Report, are the following:
In metric unit, 1 ton of pig iron requires 0.725 of charcoal produced from 3.6 tons of wood..
In the present practice, 5% of the wood mass put into the furnace is burned to heat the furnace load. The composition of the emitted smoke in this phase is not known. Considering the small mass burned it is supposed the complete conversion of carbon into CO2 equivalent.
With these data the calculated emission to produce charcoal is shown below:
EMISSION IN THE REDUCTION FROM IRON ORE TO PIG IRON
Referring to the emission of 1 t of wood put into the furnace and taking into account the loss of 10% charcoal (4) in handling and transportation, the mass of charcoal that gets into the blast furnace is 0.17 t. The charcoal specific consumption is 2.9 m3 / ton of pig iron (5) or 0.725 t of charcoal / ton of pig iron, therefore the mass of pig iron produced by ton of wood put into the furnace is 0.23 t. The typical carbon content in pig iron is 4.3 % of the mass.
With these data, the carbon balance in reduction is presented below:
The gas composition from a charcoal blast furnace and the gaseous emission by ton of wood put into the furnace are presented below.
Gaseous emission in charcoal reduction by ton of wood put into the furnace
TOTAL EMISSION IN CHARCOAL PRODUCTION AND IN REDUCTION
In the table below the relevant emissions in charcoal production and in iron ore reduction by ton of wood put into the furnace are consolidated.
It is useful to express emission by ton of iron pig produced and this is shown in the table below:
COMPARED EMISSION IN THE COMPLETE CYCLE OF STEEL PRODUCTION USING CHARCOAL AND MINERAL COAL
Steel production includes ore reduction (blast furnace) and de-carbonization of primary iron (oxygen basic furnace). The following diagram (R) referring to the production route of MANNESMANN S A presents a comparison of CO2 emissions in the cycle with coke and with charcoal. Data refer to plants using 80% of ore fines sinter and 20% of granulated ore in the blast furnace and 20% scrap in the oxygen basic furnace.
In order to compare the calculated results shown previously with those from the above work (1), the emissions are expressed in carbon content mass since it does not discriminate the carbon compounds emitted, and the comparison is limited to the carbonization and reduction steps.
The relative difference between the two results is about 10 % and it can be explained by the adoption of different indexes since the dispersion of values mentioned in the consulted works is higher than this difference.
The authors conclude that the compared analysis of the coke and charcoal routes endorses the establishment of an international credit or bonus for carbon abatement and oxygen regeneration. According to the above presented diagram, the coke route liberates 1.65 t of CO2 and fixes 1.536 t of O2 by ton of steel produced, while the charcoal route abates 16.336 t of O2 and regenerates 1.536 t of O2 by ton of steel produced in the complete cycle, from eucalyptus planting up to steel production. Furthermore, the coke route liberates 7 kg of sulfur dioxide (SO2) and this emission is practically absent in the charcoal route.
The question under examination would allow for more refined studies, including direct (activation of engines used in charcoal modern industry, for example) and indirect (energy used in extraction and refining or nutrients applied to planted forest help, for example) energy inputs in what concerns charcoal. This type of study was applied to production of alcohol from sugar cane and it showed that the exergetic efficiency of the industrial phase is of the same order of magnitude as that of the best industrial processes, while the agricultural phase efficiency is over 400%. (6), considering the fatality of solar radiation on earth and the hydrological cycle, namely exempting solar energy and rain of exergetic cost.
It should be noticed that electric furnace reduction with mixed load of pig iron and scrap would reduce emission proportionally to the scrap used. However, this advantage is real only if electricity is from renewable origin (hydroelectric or thermoelectric from biomass) since the best thermodynamic cycles have efficiency of about 50%, i. e., in order to produce 1kWh of electricity it is necessary to use at least 1,900 kcal and that the industrialized countries obtain from converting fossil fuels and the corresponding emission of greenhouse effect gases, as shown in a previous work (e&e).
The considerations above show the singular situation of Brazil in what concerns a movement to establish the bonus system for carbon abatement and the concurrent oxygen regeneration, avoiding the use of nuclear electricity with risks as serious or even worse than those associated with fossil fuels. An economic study using the equivalent energy concept or even better the exergy concept would permit to quantify the bonus value. This is a long study that surpasses the dimension of the present report.
The conditions for producing and using charcoal in metallurgy examined in this report indicate that the charcoal industry can reach full maturity as a function of the anticipated rise of petroleum price that would pull the prices of the other energy sources. International studies that have been consulted consider as possible the return of energy economy based on mineral coal for producing synthetic liquid fuels. (7).
In the same way as fuel alcohol, charcoal competes with a fossil fuel-reducer with cost necessarily lower and that in its turn competes with another fossil fuel, natural gas, whose use has been growing due to its multiple applications. Therefore, charcoal should be considered for its ecological and social advantages since the sector uses large low-qualified manpower, occupies land of marginal value not suitable for agriculture production, besides generating income in regions where employment alternatives are not particularly favorable to the worker. The potential for carbon abatement and oxygen regeneration together with the better quality of pig iron from charcoal as source of virgin metal for electric arc furnaces, qualifies this fuel as a motivating factor for international negotiations related to global climate.
In the mid-seventies the João Pinheiro Foundation, an organ connected with the Planning Secretariat of the Minas Gerais Government, defined a study and research program aiming at characterization of charcoal, optimization of the carbonization process and the improvement of furnaces used in the sector. The entity responsible for executing the program was Fundação Centro Tecnológico de Minas Gerais- CETEC - which worked together with the Instituto Estadual de Florestas. (IEF) - has developed the laboratory work (wood and charcoal analysis, friability assays , calorific power determination), economical studies about wood and charcoal production and formulated projects of Technical Standards proposed to the Brazilian Association of Technical Standards. Several technical meetings were promoted by CETEC with the participation of metallurgical enterprises (ACESITA, MANNESMANN, BELGO-MINEIRA, among others) and equipment manufacturers.
A program for personnel formation was established between the MG Federal University ( UFMG, Department of Metallurgical Engineering) and ACESITA. It had as result a special approach for charcoal in dissertation works (12 dissertations presented between 1981 and 1998, with larger concentration in the eighties, related with process mathematical modeling, energy diagnosis, thermal treatment, injection of coal fines, sinter production, coke and charcoal mixture, etc.). A collection of these works can be found in the CETEC Technical Publications series (n0 04 to 08) and it constitutes an important source on the subject.
ACESITA has operated in this period a battery of experimental carbonization furnaces, complementing the resources of CETEC and UFMG. It has carried also experiments with Otto and Diesel engines using coal gas (gazogene) with results considered as satisfactory at the time. It carried out research on the use of charcoal in irrigation pumps and in engine-generator group. Engine emission tests were not carried out because at that time the pertinent legislation was not established yet
With the effects of the petroleum shocks gone, research was gradually abandoned and the Charcoal Program followed the same path as the Alcohol Program. Presently, few integrated metallurgy enterprises still consider this as an alternative to coke, among them MANNESMANN. Charcoal consumption by independent producers of pig iron, shown in the graphic below, also present a decreasing trend.
1- CO2, O2, and SO2 OVERALL BALANCE FOR THE IRON AND STEEL PRODUCTION THROUGH THE USE OF BIOMASS OR COAL BASED ON INTEGRATED PROCESSES. Ronaldo Santos Sampaio 1 Maria Emilia Antunes Resende
2 - Produção e Utilização de Carvão Vegetal. Publicação Técnica n0 8 - CETEC -1982
3 - COMPETITIVIDADE E PESRPECTIVAS DA INDUSTRIA MINEIRA DE FERRO-GUSA. SINDIFER /FIEMG - 1997
4 - STATE OF THE ART REPORT ON CHARCOAL PRODUCTION IN BRAZIL. FLORESTAL ACESITA S. A - 1982
5 - ANUÁRIO ABRACAVE (several years)
6 - ANÁLISE EXERGÉTICA DA PRODUÇÃO DE ETANOL
DA CANA DE AÇUCAR
7 - ENERGY IN A FINITE WORLD. International Institute for Applied System Analysis - 1981
8 - BALANÇO ENERGÉTICO NACIONAL. Ministry of Mines and Energy - 1999
(*)This work is part of the study on emission of greenhouse effect gases in the period between 1990 and 1997 made for the Brazilian Ministry of Science and Technology and PNUD.