Energy Vol. 12, No. lO/ll, pp. 1177-1181, 1987 03~5442/87 $3.00 +O.CHl

Printed in Great Britain Pergamon Journals Ltd

TECHNOLOGICAL APPROACHES TO ENERGY SAVING IN BLAST-FURNACE OPERATIONS IN THE IRON AND STEEL

INDUSTRY OF THE U.S.S.R.

N. I. PERLOV

The Central Research Institute of Ferrous Metallurgy, 2nd Baumanskaya 9/23, Moscow 107843, U.S.S.R.

Abstract-Opportunities for saving energy in blast-furnace operation are discussed. Among the most important are improving the processes of pellet making and sinter making, reducing sulfur inputs, restricting the variability in the characteristics of the iron ore raw material, prereducing the iron ore outside the furnace, and increasing the internal pressure in the blast furnace. Quantitative estimates of the effect on the coke rate of small changes in conventional parameters of blast-furnace operation are presented. The discussion concludes with a consideration of the use of supplementary hydrocarbons (natural gas, residual oil, reducing gases from steam conversion or fuel pyrolysis, pulverized coal) to displace coke, and it is explained that, except in the case of pulverized coal, heat must be added to compensate for the low enthalpy of exiting gases; blast-gas enriched in oxygen is helpful.

1. INTRODUCTION

At present the iron and steel industry consumes about 9% of the fuel and 10% of the electricity produced in the U.S.S.R. Accordingly, using energy efficiently is a major task, requiring, in particular: (1) improving the quality of blast-furnace raw materials; (2) installing less energy-intensive processes during modernization; (3) increasing production capacity at the principal metallurgical facilities; and (4) automating the control of processes and of product quality.

Increasing the energy efficiency of the most energy-consuming facilities is often achieved in existing plants by improving the use of secondary energy sources, for example, by minimizing the heat lost in hot waste gases, by minimizing the heat radiated through refractory linings of metallurgical furnaces, and by cooling the highly thermally stressed components. Naturally, this approach is based on feasibility studies that take capital costs into account. Often, waste heat is best used in adjacent production processes. Alternatively, waste heat can produce electricity, steam, hot water, or refrigerants, either to use within the plant or to sell to outside consumers.

TSNIIChermet, together with other research institutes of the iron and steel industry of the U.S.S.R., strives to develop and foster the installation of energy-conserving processes and to improve the quality of metal products. Here, we report on our studies of energy efficiency in blast-furnace operation. A companion article’ describes energy-saving opportunities in other aspects of steel making.

2. FUEL SAVING IN THE PREPARATION OF THE RAW MATERIALS FOR BLAST FURNACES

Future increases in the volume of production of beneficiated raw materials for blast furnaces in the U.S.S.R. are expected to come primarily from increases in the production of agglomerates, especially pellets. To make the production of agglomerates more energy efficient, the capacity of pelletizing machines is being increased. Efforts are also being made in sintering plants to increase the thickness of the sintered layer of the ore mixture; experiments show that increasing this thickness to 400mm decreases the fuel rate 6-8% while increasing output 8-10%.

Industrial tests suggest that several promising approaches to saving energy in the preparation of blast-furnace inputs, if implemented jointly, could reduce specific fuel consumption by 15%. Six of these approaches are listed below, with estimated fuel savings if pursued independently given in parentheses: (1) throttling the flow of cold air into the

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ignition hood through the vacuum chambers (8%); (2)introducing a new rolling process incorporating solid fuel into pellets (5%); (3)enhancing the recirculation of gas to improve the heat pattern for roasting the pellets (5-8%); (4) using layered gas firing, while simultaneously simplifying the roasting machine (556%); (5) in the pellet-roasting machines, improving the longitudinal air seal between the moving trays (pallets) and the machine housing (electricity savings, separate savings not quantified); and (6)adding anthracite (up to 15%), either as fines or ground with limestone or bentonite (savings of coke breeze, separate savings not quantified).

Looking ahead, combined heating by gaseous and solid fuels looks promising, especially when the air is preheated either at a ring cooler of the sinter or at special heat exchangers fueled by blast-furnace gas. A combination of measures to improve the preparation of the mixture, to blend the mixture materials, and to further prepare the sintering mixture can decrease the specific fuel consumption by another 5%, allowing total savings of up to 20% in the production of sinter or pellets to be achieved by relatively simple measures.

3. FUEL SAVINGS IN BLAST-FURNACE IRON MAKING

Improving iron-ore beneficiation

The iron ore used as a raw material for the blast-furnace process should have the highest possible iron content and the lowest possible content of gangue. The gangue should also have a favorable chemical composition. However, process difficulties can arise as the iron content of the burden increases, because desulfurization of the hot metal is inhibited as the slag yield decreases. The theoretical limit of beneficiation of iron ores is 72.4%. In the range from 53 to 60%, a 1% increase of iron content in the ore decreases the coke rate by about 1.5%, or 8 kg of coke per tonne of hot metal. This decrease can vary from 2.5% for a lean burden and raw fluxes to 1.2% for a rich burden and a fully fluxed sinter. Coke savings also increase with increases in the silicon content of the ore.

Removing raw jluxes from the blast-furnace burden

The consumption of raw fluxes in the blast-furnace burden depends on a set of factors: the amount of SiOz, CaO, and other materials in the gangue, the composition of the coke ash, the percent of sinter in the burden, the alkalinity of the slag and sinter, and the quality of the flux itself. Increasing the fluxing of the sinter and using it in the blast-furnace burden contributes to decreased raw flux consumption and thus to coke savings. From experimental data, per tonne of hot metal, removal of 100 kg of raw limestone from the burden reduces the use of coke by 20-50 kg (reduces the coke rate by 20-50 kg/t).

Reducing the ash content of coke

More extensive coal beneficiation can decrease the ash content in the coal blend. The optimum level of beneficiation can be found from a cost-benefit analysis that considers the costs of removing coke non-combustibles both in the blast furnace and outside of it. Japanese data suggest that decreasing the ash content of the coke by 1% will improve the coke rate by 5-10 kg/t in blast-furnace operation, reducing the skip coke rate by 5-8 kg/t (or l.O-1.6%).

Reducing the sulfur content of coke and iron-ore materials

Decreasing the sulfur content in the blast-furnace burden significantly reduces the consumption of energy. Sulfur in the coke typically contributes up to 90% of the sulfur entering the blast furnace. With increased use of fluxed sinter, gas-phase pick-up of sulfur by the hot metal and slag has increased, although at present no more than 4-5% of the sulfur is removed immediately with the blast-furnace gases. One way to reduce sulfur transfer into the hot metal is to increase slag alkalinity, but this increases the amount of slag and therefore requires additional coke consumption.

The relationship between the sulfur content of coke and the coke rate may be more

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subtle than was first anticipated. One study at TSNIIChermet showed that a 0.1% increase in sulfur content within the 0.67-1.17% range of concentrations causes a 7 kg/t increase in the coke rate; in the 1.17-1.67% range, lOkg/t; and in the 1.67-2.17% range, 17 kg/t. However, another study by DonNIIChermet and UCHIN at Blast Furnace No. 12 of the Dzerzhinsky iron and steel works and at Blast Furnace No. 3 of the Enakievo iron and steel works revealed a much weaker dependence: the sulfur content was in the 1.4-1.8% range, and an increase in sulfur content of 0.1% gave rise to an increase in coke rate of only 1.6-2.0 kg/t.

The effect of a change in sulfur content on the coke rate appears to be enhanced when the blast heating occurs at lower temperatures or with a higher consumption of flux.

Reducing the output fractions of cast iron and ferroalloys

The coke rate for production of pig iron is 1.3 times less than the coke rate for production of cast iron and 3-4 times less than the coke rate for production of blast-furnace ferroalloys. In the future the output of cast iron from blast furnaces will diminish as a result of competition from cast iron made in induction furnaces from steel and iron wastes and from pig iron. The production of ferrosilicon in blast furnaces has already stopped, and the production of blast-furnace ferromanganese is decreasing due to competition from electric-arc furnace products.

Using larger fractions of partly reduced (metallized) raw materials

The prereduction of iron ore outside the blast furnace is a promising way to decrease the coke rate. A 10% increase in the fraction of partly oxidized pellets (iron content, 67-69%, and degree of metallization, 38-41%) has been found to decrease the coke rate 15-20 kg/t.

Improving the blending, classification and mechanical strength of iron-ore raw material

Iron-ore agglomerates now constitute most of the iron-ore burden of blast furnaces. They are less easily blended than are raw iron ores. As a result, the blending of ores and concentrates at mining, beneficiation, and sinter plants is important.

Stabilization of the quality of the iron ore (in terms of chemical and physical composition, particle size, fines content, and material strength) is essential for efficient blast-furnace operation. Increasing the variability of the iron content from *0.5% to + 1% increases the coke rate by 1.9-2.3%; increasing it from +0.5% to 2.0% increases the coke rate by 3.9-4.0%; and decreasing it from f0.5% to f0.3% decreases the coke rate by 0.7-1.0%. Blast-furnace melting with low slag yield and minimum heat storage is accomplished by decreasing iron content variations in iron-ore materials to f 0.2%. This requires care in storing and blending crushed ore, concentrates, and sintered ore.

Improving the physicomechanical properties of the metal burden requires screening the fines, limiting the content of large pieces, and strengthening the agglomerated material. Experimental data suggest that a decrease in the content of fines (O-5 mm) in agglomerated iron-ore materials by 1% reduces the coke rate by 0.5-1.0%.

Increasing the internal pressure in the blast furnace

Increases in gas pressure can be used to force sharp increases of blast-furnace output coupled with modest decreases in coke consumption, or to achieve larger decreases of coke consumption but with smaller increases of output. This measure is subject to diminishing returns. For example, increasing the gauge pressure from 60-80 kPa (from 0.6 to 0.8 atm) raises output 5-9% while decreasing the coke rate by 0.5-2.5%, but further raising the pressure to 100 kPa (l.Oatm), raises output only an additional 3.9%, while the coke rate is lowered an additional 1.9%.

A gauge pressure of 200-220 kPa (2-2.2atm) has been attained in a number of present day blast furnaces. With a well-prepared burden, pressure increases are not limited by process factors but by designs of the furnace and hot-air stove and by the durability of double bells and hoppers.

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Increasing the blast temperature

The temperature of the blast has reached the range of llO&1200°C during the last decade. When the blast temperature is over lOOO”C, studies show that each increase of 10°C brings about, on average, a 0.23% decrease in coke rate (1.26 kg/t of hot metal).

Introducing external desulfurization

External desulfurization refers to processes and technologies used outside the furnace to decrease the sulfur content of the hot metal. With conventional processes, the sulfur content in the blast furnace is controlled by limestone additions (forming sulfurous slag) and by limiting the sulfur content of the coke. External desulfurization facilitates the production of low-sulfur steels, and allows low-sulfur fuels to be replaced by their high- sulfur equivalents, thus widening the range of supplementary fuels and kinds of coke which can be used in a blast furnace. External desulfurization reduces coke use by 2% for each decrease of sulfur content by 0.1%.

4. PARTIALLY REPLACING COKE WITH OTHER FUELS IN THE BLAST FURNACE

In many countries optimization of the blast-furnace temperature to minimize total costs has led to the combined injection of gaseous, liquid, and solid fuels. Injecting supplementary fuel will reduce the coke rate, lower the temperature of hearth gas, and decrease the output of the furnace, so these variables must be studied carefully. The initial coke rate, the blast temperature, and the concentration of oxygen in the blast all affect these relationships; enriching the oxygen in the blast, for example, widens the range of conditions where the use of supplementary fuels is advantageous.

Injecting supplementary fuel into the hearth both diminishes direct-reduction reactions in favor of indirect-reduction reactions in the charge column and also decreases carbon reactions generally in the tuyeres. Which of these two effects makes a larger contribution to the reduction of the coke rate depends on the ratio of carbon to hydrogen in the supplementary fuel: low C:H ratios (characterizing natural gas, residual oil, and pyrolysis or conversion gases) in supplementary fuels improve the coke rate by enhancing indirect- reduction reactions, and high C:H ratios (characterizing pulverized coal) improve the coke rate by reducing the carbon reactions at the tuyeres.

When natural gas is the supplementary fuel, coke carbon is replaced by methane carbon. There is 1.7 times more combustion product by volume, the recovery of oxygen from iron oxides is enhanced, and indirect reduction increases. For example, direct and indirect reduction of ore are roughly equally important in conventional processes with only coke as fuel and with air at atmospheric pressure, but injecting supplementary natural gas permits direct reduction to drop to 30-40%. However, additional heat must be introduced to compensate for the reduced enthalpy of the gas leaving the tuyeres.

Consider the following example: assume the heating value of 1 kg of coke is 0.99 kg of coal equivalent, or kgce (29.1 MJ/kg, where 1 kgce is 29.3 MJ) and the heating value of natural gas is 1.17 kgce/m3 (34.4 MJ/m3) at 1 atm and 25°C. Experiments show that each additional cubic meter of natural gas injected in the blast furnace will replace 0.86-0.90 kg of coke, and that 0.32-0.38 kgce (9.4-l 1.1 MJ) of additional fuel will be required for each kg of coke displaced. The net result is a savings of about 0.50 kg of coke, on a fuel- equivalent basis, per cubic meter of natural gas added. This displacement ratio can be improved, however, if the blast is enriched with oxygen, to the point where 0.80 kg of coke is displaced per cubic meter of natural gas added.

Residual fuel oil behaves much like natural gas when it is used as a supplementary fuel. Its hydrocarbons break down in the hearth to carbon monoxide and hydrogen, thus reducing the temperature of hearth gases and necessitating increased hearth preheating and oxygen enrichment. Strictly in terms of heat content, 1.38 kg fuel oil will displace 1 kg of coke if the heat content of the fuel oil is 1.37 kgce/kg (40.3 MJ/kg) and that of the coke is 0.99 kgce/kg (29.1 MJ/kg). However, to increase the enthalpy of the material leaving the tuyeres, 0.27-0.37 kgce (7.9-10.8 MJ) of additional heat will be required, per kg of coke

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saved, when residual oil is used and the blast furnace is operated at atmospheric pressure. When injecting finely pulverized coal, coke carbon is replaced by coal carbon. Compared

with natural gas, pulverized coal requires less temperature compensation, and pulverized coal combustion is not accompanied by a major increase in the volume of combustion products. The coefficient of coke substitution varies from 0.92 to 1.03 kg of pulverized coal per kg of coke saved, resulting in a decrease in the specific consumption of fuel, assuming that the heating value of the coal is 5500 kcal/kg (23.1 MJ/kg). However, the complexity of the plants that grind coal and problems in delivery of powdered coal into the blast- furnace hearth make the potential for using pulverized coal as a supplementary fuel in blast furnaces very problematic.

At present the possibility of injecting hot reducing gas into blast furnaces is gaining ever increasing attention. This gas is constituted almost completely of carbon monoxide and hydrogen, produced either by conversion of steam or carbon dioxide or a mixture of the two, or by pyrolysis of fuel. Preparation of the reducing gases outside the blast furnace, making it possible to avoid heat expenditures for gas decomposition in the combustion zone, renders these gases very suitable for reduction, smelting, and slag formation in a blast furnace. A considerable saving in coke can be achieved, while increasing the blast- furnace output. However, the reducing gases will also increase the specific fuel consumption, as the coefficient of coke substitution is only 0.35 kg coke/m3 of reducing gases, for gases of 4000 kcal/m3 (16.8 MJ/m3).

Other supplementary fuels are being considered. Coke-oven gas is not attractive, because of its high sulfur content and the capital and operating costs for compression. There are other additives, however, which improve the economic performance of the blast furnace and improve the coke rate, but increase the total specific fuel consumption.

REFERENCES

I. N. P. Lyakishev and N. I. Perlov, Energy 12 (lO/ll), 1169 (1987).