INFLUENCE OF CERTAIN FACTORS ON THE YIELD OF CHEMICAL PRODUCTS FROM PEAT PYROLYSIS

S. I. Smolyaninov, G. I. Kravtsova

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

To resolve the question of the chemical potential of a possible chemical-metallurgical process based on peat, it is necessary to know the yields of the chemical products of peat pyrolysis as a function of various factors that may be important in this process.

The most important of these factors, as an analysis of the literature on the pyrolysis of solid fuel shows, should be considered to be the nature of the fuel, the final temperature of peat heating, and the temperature conditions of secondary pyrolysis of the volatile products of thermal decomposition of the organic matter of peat.

The influence of the composition of the initial fuel on the yield of products of thermal decomposition obtained from this fuel is manifested under any temperature regimes of fuel processing [1]. With sufficient observance of the identity of the conditions of thermal decomposition of different peats, the influence of the composition of the initial fuel is so great that it makes it possible to establish quite definite regularities.

A very great influence on the process of thermal decomposition of fuel, as many investigators have shown [2–5], is exerted by iron oxides. The influence of iron and its oxides has been studied by many authors in connection with the production of iron-coke, and therefore the main attention was devoted to the quality of the coke and very little to the quality of the chemical products.

In the present work, the influence of the following factors on the yield of chemical products of peat pyrolysis was studied: the nature of the peat, the final heating temperature and pyrolysis temperature, additions of iron ore, the type of ore, and the method of preparing the peat-ore mixture*).

Two characteristic peat samples from regions located near the Bakchar iron-ore basin were selected as the starting material (Table 1).

From the indicated peats and Bakchar iron ore, fuel-smelting materials were prepared on the basis of machine-formed peat with different contents of the mineral part: 12, 19, 35, and 50% iron ore in a mixture with raw \((W^p—85\%)\) peat. Several samples were prepared using, as the ore component, magnetite iron-ore concentrate from the Abagur concentrating plant (Novokuznetsk).

*) T. A. Mironova and O. P. Mosina took part in the experimental work.

The studies were carried out in an apparatus assembled analogously to that used to determine the yield of chemical products from coal carbonization [6]. The final heating temperature of the samples was varied from 400 to 850°C. Experiments were also conducted with different temperatures of the pyrolysis chamber (400–800°C). At the beginning of the experiment, the main furnace (before the quartz tube with the fuel charge was placed in it) was heated to 250°C, and then the temperature was raised to the final heating temperature at a rate of 5 deg/min. At the final temperature the sample was held for 30 min. The furnace for pyrolysis of volatile products was maintained at the specified temperature throughout the entire experiment.

Table 1

Characteristics of the peat samples studied

In all, more than 180 experiments were carried out. Because of lack of space, only a small part of the experimental data is presented in this article. The yields of chemical products are given recalculated to the organic mass of the fuel. Clearly, such recalculations introduce a considerable error, especially for compositions with a high content of mineral components; the yields of water and carbon dioxide are assigned to the organic mass quite conditionally, since during significant pyrolysis of peat–ore mixtures reduction reactions of iron oxides occur, and “mineral” oxygen from these oxides also enters the indicated products.

Thus, the results presented do not claim to characterize fully the absolute yields of the chemical products of pyrolysis; however, on the basis of the experimental data obtained, general regularities in the change of product yields with variation of the pyrolysis conditions can be identified.

1. Influence of the final heating temperature.
   With an increase in the final heating temperature (Fig. 1a) of the peat, the rate and depth of thermal destruction of the organic matter of the peat increase, which, naturally, leads to a decrease in the yield of coke and pyrogenetic water; the gas yield increases; the yields of tar and gas gasoline pass through a maximum; the yield of carbon dioxide increases somewhat; the yields of unsaturated compounds, ammonia, and hydrogen sulfide change little.

2. Influence of the temperature of secondary pyrolysis.
   The data of Fig. 1b show that with an increase in the temperature of secondary pyrolysis, the yield of semicoke increases somewhat, evidently owing to cracking of volatile products; the yields of tar and unsaturated compounds decrease, while the gas yield correspondingly increases; the yields of ammonia and hydrogen sulfide increase somewhat owing to cracking of the corresponding organic compounds; the yield of gas gasoline passes through a maximum, which, as is known, is characteristic of consecutive reactions. The yields of carbon dioxide and water change practically little.

3. Influence of additions of iron ore.

Table 2

Chemical composition of the Abakur concentrate and Bakchar iron ore

It is seen from Table 3 that additions of iron ore affect the yields of chemical products of peat pyrolysis in different ways. Thus, the yield of tar with the addition of 12% iron ore decreases somewhat in comparison with pure peat, and then increases as the amount of iron ore is increased.

The yield of gas gasoline from peat of a low degree of decomposition changes little (not shown in the table); for peat of a high degree of decomposition, at a heating temperature of up to 600°C, it decreases as the additions of iron ore are increased (Table 3), and then increases somewhat with increasing temperature (not shown in the table).

Fig. 1. Influence of the temperature of heating peat and pyrolysis of volatile products on the yield of chemical products.
× — coke, ○ — tar, □ — gas and losses, ● — gas gasoline, ■ — pyrogenetic water, △ — carbon dioxide, ▲ — ammonia, ⊕ — hydrogen sulfide, [[unclear: symbol]] — unsaturated hydrocarbons; a — without pyrolysis, b — pyrolysis of volatile products; pure peat, degree of decomposition 15% (Table 1).

The yields of ammonia and hydrogen sulfide apparently decrease somewhat. The yields of carbon dioxide and water increase the more, the higher the content of iron ore.

4. Influence of the nature of the peat.

This factor is revealed quite distinctly for the two selected types of peat of low and high degree of decomposition. The yields of pyrolysis products of these peat samples without additions of iron ore are a characteristic reflection of the properties of the starting material (Tables 3 and

Fig. 1). In this respect it is interesting to note the difference in the behavior of the volatile products of thermal decomposition of these peats during secondary pyrolysis. The tar obtained from peat of a low degree of decomposition is less thermostable and, upon thermal decomposition, gives higher yields of gas and lower yields of gas gasoline.

1. Influence of the type of ore.

The influence of this factor can apparently manifest itself, with close contact between the organic and mineral parts, not only in the form of a catalytic effect but also as direct chemical reaction. Brown iron ores have a considerably stronger oxidizing potential (Bakchar ore) compared with magnetite (Abagur concentrate), which apparently may also be reflected in differences in the yields of the chemical products of pyrolysis.

Analysis of the data obtained (Table 3) shows that this influence can be quite appreciable. Thus, the yield of tar is higher for TPM obtained

Table 3

Yields of chemical products of peat pyrolysis

Notes: 1) Final heating temperature 600°C, temperature of pyrolysis of volatiles 400°C; 2) For the characteristics of the peat samples see Table 1, and for the characteristics of the iron-ore samples see Table 2.

on the basis of Bakchar ore, and this difference increases with an increase in the content of iron oxides in the mixtures. A mechanical mixture, as was to be expected, does not give such a clear picture. As for the content of unsaturated hydrocarbons, the dependences here are completely opposite. This testifies to the reality of the assumption of a profound chemical interaction between the organic and mineral parts in mixtures with raw peat at low temperatures.

And, finally, the yields of carbon dioxide and pyrogenetic water vividly reflect the easier reducibility of brown iron ores.

1. Effect of the method of preparation of peat–ore mixtures.

Analysis of the data obtained (Table 3) shows that the influence of this factor on the yield of chemical products from peat pyrolysis is significant and characteristic.

The yield of tar proves to be greatest from fuel-flux materials obtained by forming raw peat (TPM); a mechanical mixture gives, apparently, the lowest tar yield. An analogous dependence is observed in the gas yield. All that has been established above applies fully also to the yields of gas gasoline, and in this case an especially sharp difference is observed for TPM. The mechanical mixture and thermobriquettes differ from one another to a considerably smaller degree.

In the pyrolysis of TPM as a whole, a higher yield of volatile products and a lower yield of undetermined products are observed, which, quite probably, may indicate a possible intensification of reactions of the hydrogen-redistribution type. All that has been said above is confirmed when the influence of the factor under consideration on the yields of water and carbon dioxide is examined. Their maximum yields are obtained in the pyrolysis of TPM; moreover, this circumstance is the more noticeable the higher the content of iron ore and the higher its reducibility.

Conclusions

1. A study has been carried out of the influence, on the yields of chemical products from peat pyrolysis, of the final heating temperature and the pyrolysis temperature, additions of iron ore, the type of ore, and the method of preparing peat–ore concentrates over a broad range of conditions.

2. It has been found that additions of iron ore to raw peat lead to a change in the yield of chemical products during pyrolysis.

3. When brown iron ores (Bakchar ore) are added to peat, higher yields of tar and gas gasoline are obtained than when magnetite (Abagur concentrate) is added.

4. It has been shown that the method of preparing peat–ore mixtures has a substantial influence on the yield of chemical products from peat pyrolysis.

Literature

1. V. E. Rakovskii. Chemistry and Technology of Primary Tars and Peat. Publishing House of the Academy of Sciences of the BSSR, 1948.

2. N. P. Chizhevskii. Coking of coal charges with inorganic additives. Selected Works of the Academy of Sciences of the USSR, vol. 1, pp. 373–376, 1959.

3. A. K. Isasheshi. Study of the influence of catalysts on the yield of primary tar in the dry distillation of peat. Proceedings of the Grozny Polytechnic Institute, No. 5, pp. 81–84, 1957.

4. A. S. Bruk et al. On the catalytic action of various additives in the pyrolysis of complex organic compounds. Eighth 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, 1959.

5. V. E. Rakovskii. General Chemical Technology of Peat. Gosenergoizdat, 1949.

6. L. D. Gluzman, I. I. Edelman. Control of Coke-Chemical Production. Metallurgizdat, pp. 116–120, 1957.