Reactivity of Peat Coke and the Effect of Ore and Flux Additives on It

K. K. Stramkovskaya, V. K. Borodin, V. M. Dinelt

(Presented by Professor, Doctor I. V. Gebler)

The gasification of milled and fine-grained peat, and the production of durable peat coke for metallurgical purposes, as well as the energy-technological utilization of peat, open up new prospects for the development of peat resources and also create the possibility for the development of the chemical industry in regions having large peat reserves.

Current trends in the development of blast-furnace production (low-shaft furnaces and the use of oxygen blast) raise the question of obtaining so-called fuel-smelting materials in order to create close contact between the ore and the reducing agent and to provide the necessary conditions for slag formation [1, 2]. Fuel-smelting materials are briquettes or moldings made from fine ores and fuel, or from fuel and fluxes. Such fuel, as shown by the work of I. V. Gebler and S. I. Smolyaninov [3], can be successfully obtained from peats of deposits in Tomsk Oblast and will make it possible to use poor dusty iron ores of the West Siberian basin for smelting metal without their preliminary sintering. Blast-furnace smelting in low-shaft furnaces on peat-smelting material with the use of oxygen blast will make it possible to obtain not only pig iron, but also a large quantity (up to 1000 m³/t of peat) of high-calorific (2500 kcal/m³ and higher) gas (the gas-blast-furnace process). In addition, in this process tar may be obtained, serving as a source of raw material for producing such valuable products as waxes, paraffin, phenols, neutral oils, and others.

Peat-smelting materials prepared from peat of the Tagan bog and ore of the Bakchar deposit of Tomsk Oblast, both without the addition of flux and with its addition, showed acceptable mechanical strength and thermal stability for use in the blast-furnace process [3].

In evaluating blast-furnace fuel and fuel as a raw material for gasification, great importance is attached to its reactivity.

The purpose of the present work was to investigate the effect of temperature, contact time, and also additions of ore and fluxes on the reactivity of peat coke.

Reactivity was determined with respect to carbon dioxide and water vapor in the temperature range from 500 to 900°. In view of the fact that the final pyrolysis temperature affects the reactivity...

…of the coke residue; the samples under investigation were first coked in a muffle furnace at the temperature at which the reactivity was subsequently determined. The contact time of the reagent with the coke was studied within the range from 1.5 to 7.7 sec, calculated for the empty tube and for the volume of reagent reduced to normal conditions.

Fig. 1. Schematic of the apparatus. 1—reaction tube; 2—grate; 3—thermocouple sheath; 4—valve for feeding nitrogen; 5—valve for feeding CO₂; 6—rheometer; 7—reactor head; 8—fused quartz; 9—electric furnace; 10—condenser; 11—trap; 12—inclined manometer; 13—millivoltmeter; 14—burettes.

In selecting a laboratory method for determining the reactivity of peat-coke samples, Chizhevsky’s method [4] was taken as the basis. A feature of the apparatus arrangement we adopted (Fig. 1) is that it made it possible to determine the reactivity of a sample both toward carbon dioxide and toward water vapor in a single reaction tube. In addition, water, rather than steam, was fed into the reaction tube, which considerably simplified this unit. Accurate dosing of the water was effected with the aid of the inclined manometer 12 and the burettes 14, in which the water was under constant nitrogen pressure.

The reactor consisted of a quartz tube 16 mm in diameter and 420 mm long, with a removable head 7 and a condenser 10 made of Pyrex glass. Through a fitting in the reactor head, the quartz thermocouple sheath 3 was easily introduced into the layer of the carbon charge. The thermocouple was inserted into the sheath after the reactor had been charged. Fuel (fraction of particle size 3–5 mm) was placed in the reaction tube between layers of fused quartz, the height of the layer being 10 mm.

This reactor design is convenient to operate and simple to manufacture, and also permits rapid reloading of the fuel. Heating of the reactor to the experimental temperature was carried out under a weak stream of dried nitrogen. At the experimental temperature, the nitrogen feed was stopped and carbon dioxide or water was introduced into the system. After five minutes had elapsed, gas was sampled for analysis. The chemical activity, characterized by the degree of decomposition of CO₂, was calculated from the equation

\[ R_{\mathrm{CO_2}} = \frac{\mathrm{CO}\cdot 100}{2\mathrm{CO}+\mathrm{CO_2}}, \]

where CO and CO₂ are the percentage contents of the components in the gas. In the experiments the carbon dioxide flow rate was maintained at 0.12; 0.24; 0.48; 0.6 l/min, which corresponded to nominal contact times of 7.7; 3.8; 1.92 and 1.5 sec.

To prepare peat briquettes, raw peat from the Tagan bog of Tomsk Oblast was used, having the following composition: \(A^c\)—8.87; \(V^r\)—71.0%; \(C^r\)—53.80%; \(H^r\)—6.32%; \(O^r\)—36.55%; \(N^r\)—3.00%; \(S^r\)—0.27%.

Fluxes, ore, or both together were added to the peat. Limestone was used as the flux. The ore had the following composition: Fe₂O₃—50.28%; Al₂O₃—4.01; SiO₂—26.16; FeO—1.14; CaO—1.7; MgO—0.21; MnO—0.3. The flux and ore were first ground to…

passing through a 0.25 mm sieve. For thorough mixing and comminution, the raw peat with the indicated additives was passed twice through a meat grinder. The briquettes were formed on a lever press under a pressure of 2 kg/cm², dried, and subjected to pyrolysis at the corresponding temperatures. To determine the reactivity, samples were selected from pure peat, from peat with additions of 2 and 3% flux, 3% ore, and a sample with the simultaneous presence of 2% ore and 1% flux by weight of raw peat. These samples had comparatively high mechanical strength.

The amount of mineral matter in the tested coke samples, characterized by their ash content, is given in Table 1.

Table 1

The results of determining the reactivity of peat-coke samples with respect to CO₂, presented in the graphs of Figs. 2 and 3, indicate their high activity. Peat coke begins to react with carbon dioxide at a temperature below 500°, and at a temperature of 900° practically complete reduction of carbon dioxide is achieved with a contact time of 3.8 sec. Meanwhile, according to the data of Klassen, Adams, and Haskins [5], for charcoal almost complete reduction of CO₂ to CO occurs only at a temperature of 1100° and a contact time of 5 seconds.

Fig. 2. Chemical activity with respect to CO₂ as a function of temperature for samples with different amounts of fluxes and ore; contact time 3.8 sec.
○—peat coke; △—peat coke with 3% fluxes; ×—peat coke with 3% ore.

Fig. 3. Chemical activity with respect to CO₂ as a function of contact time for samples with different amounts of fluxes and ore. ○—peat coke; ▲—peat coke with 3% fluxes; ×—peat coke with 3% ore.

The addition of flux in an amount of one percent of the mass of raw peat changed the reactivity only slightly, whereas the addition of three percent flux markedly lowered the reactivity. This is apparently associated with the decomposition of limestone, which proceeds most vigorously at a temperature of 750–900°. The carbon dioxide liberated in this process increases its amount in the gas obtained after the reaction and thus, as it were, lowers the reactivity. When ore alone was added, and also when ore was added together with a small amount of flux, the reduction reaction of CO₂ was accelerated. The reactivity of peat-coke samples with ore proved to be higher than that of peat coke without ore, all other conditions being the same. This is consistent with a number of works in which it is indicated that, among the oxides present in ash, the oxides of Fe, Mn, Cr, Ni, and others accelerate the reduction of CO₂ [6].

A high ash content in coke and the presence of readily fusible components in it have an unfavorable effect on reactivity. However, so long as the process develops in the kinetic region, the ash shell has no effect on its rate. The latter will be determined by the nature of the mineral substances and by their influence on the strength of the surface complex. In contrast to the combustion of ash-free fuel, the transition from the kinetic regime to the diffusion regime must occur at lower temperatures, the greater the content of mineral substances and the denser the ash cover [7]. In our experiments, the large increase in chemical activity with increasing temperature, regardless of the contact time of CO₂ with the coke (Fig. 3), indicates that the process of interaction of CO₂ with carbon is in the kinetic regime at temperatures up to 800°. With an increase in temperature from 800 to 850°, the increase in reactivity per one degree of temperature rise decreased, which indicates the onset of transition of the processes to the diffusion regime [8]. In addition, in the temperature interval 500–800° the dependence of \(\lg R_{\mathrm{CO_2}}\) on \(\frac{1}{T}\) (Fig. 4) for peat coke is quite strictly linear, which also indicates that, at these temperatures, the process proceeds in the kinetic region. When the temperature is raised to 850–900°, a deviation of this dependence from linearity is obtained, which indicates the transition of the process, under these conditions, to the diffusion regime. The strictly linear character of the dependence of the logarithm of reactivity on temperature made it possible to calculate the apparent activation energy of the interaction of CO₂ with carbon of peat coke, which in the temperature interval 500–800° was equal to 26000 cal/mole.

Fig. 4. Dependence of the logarithm of the reactivity of peat with respect to CO₂ on temperature.

V. N. Ershov and V. V. Pomerantsev [8], as a result of studying the reaction

\[ \mathrm{C} + \mathrm{CO_2} = 2\mathrm{CO} \]

on various samples of coals and cokes, obtained activation-energy values from 33000 to 51000 cal/mole.

The low activation energy of this reaction on peat coke indicates its high activity.

In experiments to determine reactivity with respect to water vapor for peat coke and coke with additives, the water consumption is established

constant, equal to 2 g/min, which corresponded to a steam-blast velocity of 0.25 m/sec. The comparatively high steam consumption made it possible to neglect changes in its partial pressure over the height of the fuel layer.

The change in chemical activity, or degree of decomposition of steam, as a function of temperature is given in Table 2.

Table 2

The data obtained indicate a catalytic action of the ore. The presence of ore accelerates the reaction of steam decomposition.

In a blast furnace, iron ores are reduced by two reactions: indirect reduction—by means of carbon monoxide—and direct reduction—by means of solid carbon. In the shaft and in the upper part of the bosh, indirect reduction takes place; in the lower part of the furnace, where the temperature is very high, direct reduction predominates. Reduction reactions with the aid of gases proceed at a higher rate, and indirect reduction is more effective than direct reduction. The more successfully and completely CO₂ is converted into CO, the faster the reactions of ore reduction by carbon monoxide proceed, and the more prepared the ores are when they enter those zones of the blast furnace in which reduction by solid carbon predominates. Thus, an increase in the reactivity of the fuel has a favorable effect on ore reduction in the upper parts of the blast furnace [9]. The high reactivity of peat-smelting materials is especially favorable in carrying out a gas-blast process, when, along with smelting metal, the aim is to obtain a high-calorific gas.

Conclusions

1. Peat coke has the highest reactivity toward CO₂ of all fuels, reaching, at a temperature of 900° and a contact time of 3.8 sec, practically 100%, whereas complete reduction of CO₂ on charcoal occurs only at a temperature of 1100° and a contact time of 5 seconds.

2. The addition of ore increases the reactivity of peat coke. The reaction of carbon dioxide reduction by carbon proceeded in the kinetic regime in the temperature range 500–860° even with large additions of ore.

3. The addition of small amounts of fluxes did not affect the reactivity of the fuel. With the addition of a large amount of flux, the reactivity of peat coke in the temperature range 500–900° decreased somewhat.

4. With the simultaneous presence of ore and fluxes, the peat-smelting material had a high reactivity, greater than without the indicated additives.

5. It was found that the apparent activation energy of the process of interaction of peat coke with carbon dioxide in the temperature interval 500–900° averages 26000 cal/mol.

1. The high reactivity of peat coke, both with additions of ore and fluxes and without additives, indicates the advisability of using peat in the gas–blast-furnace process.

References

1. L. M. Tsylev, B. A. Gess, A. M. Chernyshev, Yu. I. Borisov, and L. V. Borodina. New Principles of Coal Coking. Publishing House of the USSR Academy of Sciences, 89, 1955.

2. L. M. Sapozhnikov. New Principles of Coal Coking. Publishing House of the USSR Academy of Sciences, 5, 1955.

3. N. V. Gebler, S. I. Smolyaninov, V. E. Potapenko, and V. N. Kosolapov. “The Effect of Additions of Ore and Flux on the Properties of Peat as a Metallurgical Fuel,” article included in this volume.

4. E. M. Taits, N. G. Titov, and N. V. Shpigakov. Methods for Evaluating Coals as Raw Material for Industrial Use. Ugletekhizdat, 146, 1949.

5. D. D. Ginzburg. Gas-Generator Installations, Part 1, 26, 1936.

6. O. A. Esin and P. V. Geld. Physical Chemistry of Pyrometallurgical Processes, Part 1. Metallurgizdat, 1950.

7. F. Chukhanov. Proceedings of the USSR Academy of Sciences, Department of Technical Sciences, No. 4, 562, 1953.

8. V. N. Ershov and V. V. Pomerantsev. Journal of Applied Chemistry, 14, 1951.

9. N. P. Chizhevsky. Selected Works, Vol. 1. Publishing House of the USSR Academy of Sciences, 401, 1955.