Effect of Iron Oxide Additives on the Process of Volatile Release from Peat

EFFECT OF IRON OXIDE ADDITIVES ON THE PROCESS OF RELEASE OF VOLATILES FROM PEAT

S. I. Smol’yaninov, V. I. Lozbin, Ya. A. Belikhmaer

(Presented by the scientific-methodological seminar of the KhTF)

In assessing the prospective possibility of using peat in a chemical-metallurgical process, it appears advisable to study the effect of iron oxide additives on the process of thermal decomposition of peat.

Similar studies have been carried out repeatedly [1—5], with the majority of authors clearly showing a significant increase in the total yield of volatiles during coking of fuels with additions of iron oxides. At the same time, a decrease in tar yield and changes in its qualitative composition are noted. In most cases these factors are explained by the catalytic influence of the additives on the pyrolysis process of the organic mass of fuels, which, in our view, is insufficiently definite; therefore we have carried out a series of analyses in order to elucidate the mechanism of the influence of additives on the process of volatile release during semicoking of peat.

Peat samples with iron oxide additions in amounts of 5, 30, and 50% by dry peat mass were prepared by the TPM method [6]. The investigations were carried out on a derivatograph; the sample charge in all cases was 700 mg, the balance sensitivity was 500 mg, the final temperature was 600°C, the DTG galvanometer sensitivity was 1/3, and the heating rate was about 5 deg/min. The sample was heated in a nitrogen atmosphere. The pyrolysis products were continuously removed from the reaction space by means of a gasometer and analyzed for the content of CO₂, CO, H₂, and CH₄ at various moments of the experiment.

Results and Discussion

In the present work, as in those described above, a regular increase in the total yield of volatiles was observed with an increase in the amount of iron oxide in the analyzed sample (Table 1).

The influence of iron oxide additions begins to appear already at temperatures somewhat above 200°C (Fig. 1). The temperature of the maximum value of the rate of change in weight increases with increasing hematite content in the sample.

The temperature displacement of the maxima of the gas-evolution rate is caused, in all probability, by chemical interaction between iron oxide and the products of thermal decomposition of peat. In this temperature interval the most probable reaction is the reduction of hematite to magnetite by carbon monoxide:

\[ 3\mathrm{Fe}_2\mathrm{O}_3 + \mathrm{CO} \to 2\mathrm{Fe}_3\mathrm{O}_4 + \mathrm{CO}_2 - \Delta H . \]

Table 1

Effect of Fe\(_2\)O\(_3\) additions on the total yield of volatiles at 600°C

Fig. 1. Effect of iron oxide on the rate of weight change during thermal decomposition of peat.
1 — initial peat; 2 — peat + 20% iron oxide; 3 — peat + 30% iron oxide; 4 — peat + 50% iron oxide.

This reaction on the equilibrium diagram of iron oxides with the gas phase of CO and CO\(_2\) corresponds to curve 1 (Fig. 2), conventionally shown by a dashed line near the axis of 100% CO\(_2\) content; this indicates that the stability of the oxide Fe\(_2\)O\(_3\) is so low that the equilibrium gas phase contains insignificant amounts of CO [7]. The thermodynamic probability of this reaction occurring at low temperatures was determined by a number of authors. Thus, according to [8], \(\Delta Z\) and \(K_p\) for this reaction can be approximately determined as:

\[ \Delta Z = -52130 - 41.0\,T\,\lg(-12470 - 9.81\,T\ \text{cal.}), \]

\[ K_p = \frac{2726}{T} + 2.144; \]

for a temperature of 473°K (200°C):

\[ \Delta Z = 17110\ \text{cal.}, \]

\[ K_p = 7.904, \]

which indicates that the reaction of formation of $\mathrm{Fe_3O_4}$ from $\mathrm{Fe_2O_3}$ in the presence of small amounts of CO in the gas will proceed irreversibly.

Confirmation of the occurrence of this reaction is the presence of a considerable amount of CO in the products of peat pyrolysis (it begins to be released at $180^\circ\mathrm{C}$) and the increase in the rate of evolution of $\mathrm{CO_2}$ in these temperature intervals (Fig. 3).

It is also possible to show the thermodynamic probability of the irreversible course of the reaction of reduction of iron oxide by hydrogen according to the scheme:

Fig. 2. Equilibrium diagram of iron oxides with a gas phase of CO and $\mathrm{CO_2}$.

\[ 3\mathrm{Fe_2O_3}+\mathrm{H_2}\longrightarrow 2\mathrm{Fe_3O_4}+\mathrm{H_2O}-\Delta H, \]

which will also have a definite influence on the total yield of volatiles.

In the reactions considered above, an increase in the yield of volatiles is assumed to occur as a result of oxidation reactions of the gaseous products of pyrolysis—carbon monoxide and hydrogen—by the oxygen of hematite.

In the works of V. E. Rakovskii and co-workers [9], the possibility of reduction of iron oxides by the liquid products of peat pyrolysis was shown. The occurrence of these reactions with the formation of thermally unstable products can explain the decrease in tar yield noted by most authors. And, finally, it is difficult to deny the catalytic-

Fig. 3. Effect of iron oxide on the rate of evolution of $\mathrm{CO_2}$ during the thermal decomposition of peat.
1 — initial peat, 2 — peat + 30% iron oxide.

... the influence of oxides on the course of pyrolysis processes of the components of the vapor–gas phase, which may exert a definite effect on the qualitative composition and quantitative ratios of the individual components of the tar and gas.

In conclusion, it should be noted that additions of hematite have a substantial influence on the quantitative and qualitative composition of the products of peat pyrolysis. The experimental results obtained make it possible, with a certain degree of probability, to judge the occurrence of such processes as the oxidation of hydrogen and carbon monoxide by oxygen from iron oxide. The influence of hematite additions on tar pyrolysis and the processes taking place in it remains, for the time being, a matter of conjecture, and further investigations in this direction are necessary.

Conclusions

1. The thermodynamic probability of interaction of iron oxide with the gaseous products of peat pyrolysis—carbon monoxide and hydrogen—has been demonstrated.

2. Experimental results have confirmed the occurrence of reactions involving the oxidation of carbon monoxide and hydrogen by the oxygen of iron oxide, with the formation of an additional amount of volatile products.

LITERATURE

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

2. A. S. Bruk et al. On the catalytic action of various additives during the pyrolysis of complex organic compounds. VIII Mendeleev Congress on General and Applied Chemistry. Section of Chemistry and Chemical Technology of Fuels. Publishing House of the Academy of Sciences of the USSR, Moscow, 1959.

3. Yu. N. Skvortsov. The influence of iron-ore additions on the properties of coke from fat coal. Proceedings of the Moscow Institute named after D. I. Mendeleev, issue 28, 79–83, 1959.

4. G. P. Gei, L. M. Litvinenko. Coking of coals with additives and their influence on the yield of chemical coking products. Scientific Transactions of the Dnepropetrovsk Institute, 43, 1959.

5. L. A. Agroskin, A. K. Shelkov. Expansion of the coal base for coking. Metallurgizdat, Moscow, 37, 1962.

6. S. I. Smolyaninov, G. G. Krinitsyn, S. G. Maslov. Prospective evaluation of peat from a number of deposits in Tomsk Oblast as raw material for the metallurgical processing of Bakchar iron ores. Izvestiya TPI (in press).

7. S. I. Filippov. Theory of Metallurgical Processes. Metallurgizdat, Moscow, 1967.

8. O. A. Esin, P. V. Geld. Physical Chemistry of Pyrometallurgical Processes, Metallurgizdat, Moscow, part 1, 1962.

9. V. E. Rakovskii et al. Reduction of iron oxides by products of pyrogenetic decomposition of fuels. KhTT, No. 3, 102, 1967.