Proceedings of the Tomsk Polytechnic Institute named after S. M. Kirov, Holder of the Order of the October Revolution and the Order of the Red Banner of Labor

Vol. 250　　　　　　　　　　　　　　　　　　　　　　　　　　　1975

On the Influence of Iron Oxides on Hydrogen Formation Processes during Peat Pyrolysis

S. I. Smolyaninov, V. I. Lozbin

(Presented by the Scientific-Methodological Seminar of the Faculty of Chemical Technology)

Considerable attention in the literature is given to the question of the influence of mineral additives on the process of thermal decomposition of the organic mass of solid fuels. By adding salts or alkalis, a definite directionality of the process of pyrolysis of peat and coals is achieved [1—3]. The question of fuel pyrolysis with additions of iron oxides occupies a special place in this case, since the industrial use of such mixtures (regardless of the method of preparation: pellets, molded forms, briquettes, etc.) provides for a large percentage of the ore component in the charge.

Thermal treatment of a charge based on peat and iron ore is characterized by the release of a considerable quantity of vapor-gas products in a comparatively low-temperature region. The presence of iron oxides determines the direction in which gas-formation reactions proceed: first, through oxidation of the peat and the components of the tar and gas formed by the oxygen of the oxides and, second, through a possible catalytic influence of the oxides on the mechanism of a complex of secondary reactions. All this ultimately determines the yield and composition of the products of thermal decomposition.

To study the influence of iron oxides on gas-evolution processes during peat pyrolysis, samples of peat-ore moldings were prepared from peat of the Tagan deposit and Bakchar iron ore. The characteristics of the initial materials are presented in Tables I and II.

Table 1

Technical and elemental analysis of peat

The studies were carried out on a combined thermal-analysis setup [4]. Analysis conditions: heating rate 5 deg/min, T — 1200, TG — 1000, DTG — 1/3, sample weight 1200—2500 mg, medium — nitrogen, gas analysis on an KhT-2M chromatograph, carrier gas—air.

It is generally accepted that one of the important characteristics of the process of thermal decomposition of fuels is the yield of volatiles over temperature intervals. Literature data on the yield of volatiles from solid fuels in the presence of iron oxides are rather contradictory; however, most authors note an increase in the yield of volatiles in the interval up to 600°C [5–6]. Processing the results of studies of the thermal decomposition of peat–ore moldings showed a decrease in the total yield of volatiles over temperature intervals up to 600°C in comparison with the yield of volatiles for the initial peat (Table III).

Table 2

Analysis of Bakchar ore

At the same time, a change is observed in the rate of total evolution of volatiles: the ratio between the first and second maxima of the DTG curve for peat and peat–ore moldings changes markedly; the deviation of the DTG curve increases sharply in the interval 300–400°C (Fig. 1) during pyrolysis of peat–ore moldings. The temperature curve shows noticeable heat absorption, from which one may conclude that the process in this interval is endothermic.

Table 3

Total yield of volatiles
over temperature intervals for peat and TPM

Chromatographic analysis of the gaseous products of pyrolysis of peat–ore moldings showed a sharp increase in CO₂ in comparison with the initial peat, as well as the appearance of hydrogen in the gas. The maximum rates of evolution of CO₂ and H₂ coincide in temperature with the maximum of carbon monoxide in the gas (Fig. 2). At the same time, in the presence of iron oxides a smaller amount of carbon monoxide is evolved than from the initial peat.

The decrease in CO in the gas, as well as the endothermic character of the process, indicate the occurrence of reduction reactions of iron oxides (according to Baikov [7]):

\[ 3\,\mathrm{Fe_2O_3} + \mathrm{CO} \to 2\,\mathrm{Fe_3O_4} + \mathrm{CO_2} - \Delta H, \]

\[ \frac{1}{4}\,\mathrm{Fe_3O_4} + \mathrm{CO} \rightleftharpoons \frac{3}{4}\,\mathrm{Fe} + \mathrm{CO_2} - \Delta H, \]

which partly accounts for the increase in carbon dioxide in the gas.

On the other hand, V. N. Erikh and V. K. Pazhitnov note that the conversion of carbon monoxide

\[ \mathrm{CO} + \mathrm{H_2O} \rightleftharpoons \mathrm{CO_2} + \mathrm{H_2} + \Delta H \]

proceeds in the interval 300–500°C at a sufficiently high rate on catalysts—iron oxides, while an excess of water vapor increases the degree of conversion of CO [8]. Under our conditions there are all the prerequisites for this reaction to proceed toward the formation of CO₂ and H₂:

Fig. 1. Dependence of the total rate of release of volatiles from peat on temperature

Fig. 2. Dependence of the rate of formation of the main components of the peat pyrolysis gas on temperature

a) the catalyst is iron oxides activated in the process of interaction with carbon monoxide;

b) the maximum rate of CO evolution, which should shift the process to the right;

c) an increase in the rate of evolution of pyrogenetic water (according to V. E. Raevsky [1]) at a temperature of 350°C, which should also shift the process toward the formation of hydrogen.

Further heating of the peat-ore pellets is characterized by a reduced yield of carbon monoxide in comparison with the original peat. The rate of CO₂ evolution over the entire interval is somewhat higher, which indicates the occurrence of reduction reactions. In the interval 400–550°C a second maximum in the rate of hydrogen evolution appears. V. Nauke [9] notes in this interval the highest rate of formation of ethane, heavy hydrocarbons, and tar during peat pyrolysis.

Comparing the obtained results with data from a number of authors who investigated the oxidative dehydrogenation of hydrocarbons [10–14], it may be assumed that the formation of hydrogen in this temperature interval is caused by oxidation of tar hydrocarbons by the oxygen of iron oxides and partly by conversion of heavy tar hydrocarbons on iron oxides.

A number of compounds apparently rather similar in their chemical properties take part in these processes, since the process proceeds within a rather narrow temperature interval, and the function of the rate of hydrogen evolution with temperature has one clearly expressed maximum. The consequence of these reactions should probably be a change in the composition of the pyrolysis tar toward an increase in oxygen-containing components, on the one hand, and an increase in low-molecular-weight compounds, on the other.

Conclusions

1. The studies carried out on the pyrolysis of peat mixed with ore from the Bakchar deposit showed a significant decrease in the total yield of volatiles from peat at low pyrolysis temperatures.

2. The increase in the rate of evolution of carbon dioxide and hydrogen and the decrease in the rate of formation of carbon monoxide in the interval 300–400°C make it possible to suppose that, alongside the reactions of reduction of iron oxide, conversion processes occur on the active centers of freshly reduced \(\mathrm{Fe}_3\mathrm{O}_4\).

3. Comparison of the results of chromatographic analysis with literature data leads to the supposition that, in the presence of iron oxides, reactions of catalytic dehydrogenation of hydrocarbons of the primary tars may occur, with formation of hydrogen in the pyrolysis gas.

LITERATURE

1. V. E. Rakovskii. General Chemical Technology of Peat. Moscow–Leningrad, 1949.

2. M. E. Ravich, N. P. Chizhevskii. Selected Works of the Academy of Sciences of the USSR, 1, 373, 1939.

3. Z. F. Pechkovskaya. Proceedings of IGI, Academy of Sciences of the USSR, 8, 239, 1958.

4. S. I. Smolyaninov, V. I. Lozbin, Ya. A. Belikhmaer. Izvestiya TPI, vol. 233, published by TSU.

5. G. P. Geid, L. M. Limarenko. Scientific Works of the Dneprovsk Institute, A3, 1959.

6. G. P. Geid, Yu. S. Pavlyuk. In: Metallurgy and Coke Chemistry. Kiev, 1966.

7. O. A. Esin, P. V. Geld. Physical Chemistry of Pyrometallurgical Processes, Part 1, Moscow, 1962.

8. V. N. Erikh, V. K. Pazhitnov. Chemistry of Petroleum and Artificial Liquid Fuel. Leningrad, 1955.

9. V. Nauke. New Studies on Peat Pyrolysis. Abstracts of Translations. GEI, 1961.

10. K. Ellis. Chemistry of Petroleum Hydrocarbons and Their Derivatives, Vol. II, 905, 908, 943, 944, 949, 1068. ONTI, Moscow, 1938.

11. N. N. Vorozhtsov. Fundamentals of the Synthesis of Intermediate Products and Dyes, 592, GKhI, Moscow, 1955.

12. L. Reggel, I. Wender and R. Raymond. Fuel, 3, 229, 1964.

13. B. S. Altshuler, V. L. Rabinovich. In: Thermal and Oxidative Pyrolysis of Fuels and HPM, 192, Moscow, 1966.

14. Z. A. Rumyantseva et al. KhTT, 6, 11, 1969.