Economia & Energia
No 20 - Maio - Junho 2000
|No 20 Em Português|
EMISSION OF GREENHOUSE EFFECT GASES IN VEGETAL COAL PRODUCTION
The importance of biomass fuel for abating the carbon released to the atmosphere as CO2, CO and CH4 is acknowledged by nations that participate in international climate meetings. Proposals regarding ways to motivate the use of biomass in developing countries with compensations from the industrialized countries have been presented but there has been no consensus about the matter, in spite of the greenhouse effect worsening. The decrease in petroleum prices from the mid-eighties on, favoring the international transport of goods, has strongly contributed to increasing the emission of greenhouse effect gases in all sectors of productive activities, both by transportation vehicles and by industrial consumption of fossil fuels. Nevertheless, one cannot be sure that petroleum prices will be kept low in the near future, therefore it is predictable the return to the use of biomass as fuel source, particularly in Brazil.
Charcoal can be considered as an energy source vector of ample use, so much so that after the first petroleum price shock (1973), the Federal Government stimulated the substitution of fuel oil by coal in several sectors of the industrial production and charcoal had an expressive participation in this effort. However, metallurgy is the best market niche because it favors the production of pig iron practically without sulfur, phosphor or other undesirable elements. The present work will consider as priority its use in metallurgy.
Pyrolysis or distillations of dry wood or other vegetal biomass in controlled atmosphere and convenient temperature produces charcoal and volatile, partially condensable material.
From condensation results the pyroligneous liquid containing pyroligneous acid and insoluble tar. The pyroligneous liquid is composed of pyroligneous acid, an aqueous solution of acetic and formic acids, methanol and soluble tar and minor constituents. The non-condensable volatile matter consists of gaseous carbon compounds (CO2, CO, CnHm) and nitrogen. Analysis of charcoal and of the volatile matter shows that its composition depends strongly on the carbonization temperature of the vegetal species that supplies the wood and on the age of the tree. Therefore, charcoal produced from native species shows a certain fluctuation of the physic-chemical and mechanical properties, undesirable in the production of pig iron. The evolution of metallurgical technology lead naturally to the necessity of standardizing the wood through the planting of selected species aiming at improving the charcoal yield and its carbon content (fixed carbon), density and other mechanical properties required for its use in furnaces.
The carbonization process can be delineated in 4 phases:
The physical, chemical and mechanical properties of carbon (composition, reactivity to CO2, density, compression resistance, etc.) depend on the wood composition and structure, on the humidity, on the log dimensions, on the carbonization temperature, on the load heating rate in the furnace and other less relevant variables. The furnace operation mode to obtain good quality charcoal is still an empiric technique due to the many factors to be taken into consideration and the means of process monitoring compatible with the production structure. Actually, the necessity of producing charcoal at prices competitive with those of coke, in the present economical conjuncture, imposes certain rusticity on the furnaces and on the manipulation of raw material and products. The state of the art in the seventies, when the importance of biomass fuels became evident, was not much different of that existing in the forties, when the Belgo-Mineira Plant was installed in Monlevade. Considerable improvement efforts were made in the eighties and the companies that persisted in those efforts, motivated by the excellent quality of the charcoal for producing pig iron and ferroalloy, show today a marked progress both in forestry practices and the carbonization process.
The most simple furnace model is a masonry construction in the shape of a beehive with holes for air intake. Loading is made in batches, wood is cut in logs 1.0 and 2.0 meters long. The wood diameter is a function of the age of the tree and of planting interval and it is desirable to have small diameters in order to assure the low friability of the charcoal produced (information used in the present work mention diameters between 10 and 3 cm). The wood is pre-dried in open air until it reaches 25-30% humidity.
Closing progressively the holes for air intake makes the air control. Figure 1 shows the most simple, low-cost masonry furnace used mainly for charcoal production from native wood, where the holes for controlling air intake can be seen.
In masonry furnaces the progress of the carbonization process is evaluated by the smoke color that escapes from the holes. The complete carbonization process, from loading the batch until its removal lasts about 8 days.
Figures 2 and 3 show schematically larger furnaces and the one with 4 m diameter is used as slope furnace in uneven areas (closing is partly carried out through the steep side). The 5-meter-diameter furnace, with a better control of air intake and longer useful life, is used by the metallurgy industries with own production in batteries with 36 to 108 furnaces.
CARBONIZATION PRODUCTS AND ENERGY EFFICIENCY
In laboratory experiments, carbonization of Eucalyptus Grandis (the species adopted in most of plantations in the seventies and eighties) produces (% in mass, dry base) (1):
Pyroligneous acid is composed of acids (acetic and formic), soluble tar, a small proportion of methanol (about 1%) and water. The total proportion of tar (soluble + non-soluble) is 12%.
The main constituents of the non-condensable gases (in mass %) are (2):
Energy balance for the above production profile, referred to 100 g of humid wood is the following:
h = (25 x 6.800) / 336.000 = 0,51
It should mentioned that the calorific power obtained from BEN is the result of experiments carried out by Belgo-Mineira, Acesita and INT and is higher than that calculated based on the composition obtained in laboratory.
In the present state of the art, insoluble tar and pyroligneous acid are recovered in the proportion of 140 kg / t of charcoal or 4% of mass of carbonized wood (MCW). Pyroligneous acid is used for other industrial ends. Tar, that can substitute fuel oil, is destined for other industrial uses due to the low price of fuel oil. Calculating only the recovered tar, efficiency would be:
h = (25 x 6.800 + 3,2 x 6.000) / 336.000 = 0,56
It is verified that charcoal production can be an efficient way of substituting fuel oil in conjunctures of petroleum shortage as in the mentioned seventies crisis, by deploying the technology already dominated by the large producers. For comparison purposes we mention methanol production from natural gas, a capital-intensive process, whose energy efficiency is 65%.
The complete analysis of the production, distribution and use of charcoal, taking into consideration the economical (capital cost, distribution cost, etc.) and social (income distribution) aspects and using the more appropriate methodology of exergetic analysis instead of the energy analysis based simply on the Energy Conservation Principle - which includes only the energy aspect - was not carried out yet. Considering simply the energy efficiency leads in general to partial conclusions, in general not favorable to biomass industrialization processes.
The following figure shows schematically the integrated process, from the forest to the end product, used by Mannesmann S. A in the production of seamless tubes.
The wood used in charcoal production at the start of the metallurgical industry came exclusively from native forests. Noble wood such as jacarandá and angico were carbonized according to the technology of that time, with low charcoal yield. However, since the establishment of the Real Fábrica de Ferro in Ipanema - SP in 1818, Frederico Varnhagen, in a document to the Regent Prince, manifested his preoccupation about forest conservation (3) as industry's economical measure. Carbonization was carried out in rustic furnaces and it consisted on the ignition of stacks of wood that were covered with earth as soon as the combustion mass was sufficient for completing the carbonization. Since there was no air control, carbonization was irregular and charcoal got mixed with semi-burnt wood.
In the forties, eucalyptus planting practices started in Minas Gerais with the objective of supplying charcoal to the metallurgical plants in the state that already produced half a million tons of steel per year. Charcoal production technology from planted forests has developed pari passu with the steel production technology, encouraged by the Federal Government via income taxes in the sixties. The incentive method had several weaknesses and the most serious was that of not connecting the forest planting activity with the end use of the wood. Since any enterprise could benefit from the incentive, even if it did not consume the raw material produced, the country formed a considerable forest mass for industrial purposes, estimated in 4 million hectares of different Eucalyptus and Pinus species. A new push concerning forestall activity occurred in the seventies soon after the two petroleum price shocks when the industrial use of firewood and charcoal as substitutes for imported fuel oil and metallurgical coal was stimulated. The graphic that follows shows the increasing path of charcoal consumption in the eighties and its fall in the nineties. It is inferred that its use in metallurgy determined the evolution of this process.
The burst of demand growth started in the seventies, induced by the establishment of the Minas Gerais Government Charcoal Program , including the development of forestall techniques, production and charcoal characterization methods and introduction of innovations in the carbonization process.
It was then established the cooperation practice among metallurgical enterprises (Acesita, Belgo-Mineira and others), governmental agencies (State Forest Institute, CETEC), University (UFMG, UFV) and equipment manufacturers, that resulted in the introduction of several innovations from soil preparation to recovery of charcoal by-products. Evaluation of gains from this effort in Acesita can be represented by the following scheme:
The following photo shows a modern carbonization installation of Mannesmann S A, where "Missouri" type furnaces equipped with porticos for wood unloading can be seen. About 50% of these furnaces have tar recovery systems.
The metallurgical sector is the main charcoal consumer, as shown in the following graphic (total consumption, industrial and metallurgical). Therefore, the perspectives of the charcoal industry are in some way linked to the perspectives of the world steel market, since Brazil exports 40% of its gross steel production.
The following graphic made using data from Iron & Steel Statistic Bureau shows the evolution of the world steel production from 1960 on. It can be verified that production developed according to the logistic law and has taken more than 90 % of its own niche.
The saturation of the world steel market, suggested by the graphic, coincides on time with the peak of petroleum extraction (J.C. Campbell, 'The coming oil crisis", Multiscience Publishing Company, 1997), an auspicious fact for the environmentalists and a bad omen for the economists. The coincident facts could be forecasted with reasonable safety by acknowledging the respective inflection points of the curves, both logistic ones, that describe the integrated steel demand and the integrated petroleum reserves discoveries that can be exploited using the available technology and with current costs ("conventional" oil). This forecast reflects an economical conjuncture subject to physical restrictions (finite reserve of petroleum) and can be considered as safe due to the conjuncture. Reversion of expectancies depends on technological changes concerning energy conversion or the introduction of a new source since conservation, if isolated considered and in the necessary scale, would have negative effects on the world economy, or still changes on the steel production technology. Some of these transformations are already under way (fuel cell, electricity co-generation, combined thermodynamic cycles, information technology and others).
In the metallurgical sector, besides improvements in the coal coking process and iron ore reduction, it is under development steel recycling in electrical arc furnaces using scrap steel and an additional load of up to 40 % of virgin metal. Virgin metal sources will be pig iron and the pre-reduced material produced by direct reduction using natural gas or steam coal. It is estimated that CO2 emission in the electrical furnace recycling 40% of scrap corresponds to 25 % of the emission verified in the traditional production (blast furnace and oxygen basic furnace). Specialists on the metallurgical sector have identified opportunities for pig iron produced in charcoal furnaces whose properties are superior to those of the competitive primary metal sources. Analysis made from the energy balance indicates that primary metal could reach 63 million tons in 2010.
The possibility that charcoal will continue to be an important CO2 sink depends obviously on its competitiveness with fossil fuels produced since the economical criterion is a priority in most analyses. Therefore, the ecological and social advantages of charcoal production and use, as the sole CO2 absorber, among all fuels-reductions used in the metallurgical industry and as employer of less qualified labor, will be explored by the interested enterprises and governments.
Presently, there is a forestall deficit relative to wood consumption as shown in the following graphic, made from the ABRACAVE Annual Bulletin (1999) (not edited yet). The gap between consumption and planting started in the mid eighties and it can be related to the petroleum price decrease and consequent decrease of metallurgical coal export prices that was reduced from 50 to about 32 dollars per ton between 88 and 97. The linking between prices of these two fuels verified in the last years seems to be severed by the arrival of natural gas, including in the metallurgical sector through the direct reduction with CO externally generated in reduction furnaces.
The following graphic reflects the dynamics of charcoal shift relative to mineral coal coke, now delivered to the Minas Gerais plants at 95 dollars per tone. Studies carried out by the Minas Gerais Iron Industry Trade Union - SINDIFER - and the Industry Federation - FIEMG - in 1997 show that the competitive limit price for charcoal would be R$ 25/m3 and indicate a set of measures necessary for guaranteeing these conditions. Some are internal to the system (quality promotion, reduction of average transport distances, mechanization, reduction of specific consumption, development of correlated industries, use of scrap in blast furnace) and others require support from incentive agencies (financing schemes, with interests close to the international ones, incentive to forestall fomentation, etc.).
A study carried out by SINDIFER, the State Institute of Forest (IEF) and the Metallurgical Plants Association for Forestal Fomentation (ASIFLOR) concerning reforesting 525,000 hectares in Sete Lagoas, Divinópolis and Vale do Jequitinhonha areas of influence, in Minas Gerais, with investments of R$ 389 million, regards the following objectives:
The study considers some ways of plantation financing already tried with promising results. They are the following:
The mentioned study, taken as example, shows that reforesting activity is not grievous and may become attractive again from the economical point of view if the CO2 absorption bonus system will be implemented or, apart from that, coke price will follow the now ascending petroleum price.
1 - "State of the Art Report on Charcoal
Production in Brazil"
Fig. 1 - Furnaces used by small producers.
Fig. 2 - Slope furnace
Fig. 3 - Furnace used by integrated metallurgy plants.
1 - Opening for load ignition 2 -
Openings for air control
Fig. 4 - Furnace with external combustion chamber
Fig. 5 - Installation for tar recovery
1 - Furnace 2 - Washing tower
3 - Cyclone