COKE FROM PEAT OF THE TAGANSKOYE BOG

I. V. GEBLER and I. S. KARPENKO

General conditions for coking peat.

As is known, coke can be obtained from peat (and is obtained in practice) that is suitable for cupola furnaces in a mixture with foundry anthracite, and by itself, without additions, for blast furnaces of low capacity, both as charcoal coke and in furnaces for hard-coal coke. Experiments carried out under the direction of Prof. M. A. Pavlov in the blast furnaces of “Vyksa” and “Kosaya Gora” gave quite favorable results.

Not all varieties of peat are suitable for producing coke, but only those that have a definite botanical composition and a definite degree of decomposition. In addition, because peat lacks caking ability, it can be used for coking only in the form of a briquette, i.e., machine-formed peat, or hydropeat. Cut peat is not suitable for coking; milled peat must first be briquetted.

Depending on its nature, sphagnum peat from raised bogs with a degree of decomposition not below medium is considered the best for coking. Since the coke yield is about 30% of the weight of the peat (with a moisture content on the order of 15%), and since the mineral substances pass entirely into the coke, in order that the coke not be too high in ash, the ash content in the peat is usually strictly limited. The desirable ash content of peat is not above 3%, and, as an upper limit, 4–5% is permitted. The sulfur content should not be above 0.2% ¹).

The most favorable initial moisture content of peat going to coking lies within the range of 14–16%; in this connection the peat must be subjected to artificial drying.

As for the coking temperature, the most favorable for obtaining lump, strong coke is considered to be a temperature on the order of 700°.

With regard to the furnace system for coking peat, furnaces with internal heating are suitable, for example, Pinch-system furnaces, which have been mastered at the peat-coking plant at Redkino station, Oktyabrskaya Railway.

Coke is also burned from peat by artisanal methods: pit, pit-bonfire, bonfire methods, and in furnaces without recovery of volatile products, for example, in so-called Maryanka furnaces. All these methods are simple and cheap in terms of initial costs, but they are uneconomical because of the loss of volatile products.

The peat briquette going to coking must be well formed: dense and strong, without cracks, and must not crumble. The mass for forming such a brick must be well worked and uniformly mixed so that inclusions of large stems, twigs, and the like do not get into the brick.

¹) The phosphorus content must not exceed 0.04–0.05%.

Characteristics of the Peat of the Taganskoye Bog

The Taganskoye bog is located 13 km southeast of the city of Tomsk, on the left bank of the Tom River.

The botanical composition is, on average, hypnum–sedge, varying somewhat in different horizons of the deposit to sedge–hypnum and hypnum peat (at great depths). The degree of decomposition is, generally speaking, medium; above 1.5 m through the thickness of the peat deposit it is low.

For investigation, a sample of cut peat from the workings of the previous summer was delivered to the laboratory, taken from the 5th quarry canal (in an amount of 10–15 kg). The moisture content of this peat was from 30 to 35%. From this peat, briquettes were prepared by wet molding on a laboratory brick-molding machine. For this purpose the peat was mixed with water, which was added in an amount of about 50%, so that the total moisture content of the peat mass was about 80–85%. The briquettes molded in this way were dried in the laboratory; over the course of 20 days their moisture content decreased to 16–17%. During drying, the room temperature fluctuated within the limits of 14–18°C, and the relative humidity (determined by dry- and wet-bulb thermometers) was 67–70%. The dried briquettes (with the indicated moisture content of 16–17%) had dimensions, after considerable shrinkage, of approximately 60 · 33 · 20 mm.

Analysis of the briquettes gave the following figures:

Table 1

The ash melting temperature, as determined by the silicate laboratory, was 1470°.

As can be seen, the ash content in this peat considerably exceeds the limits set for peat intended for coking, which is connected with the character of the bog, which is not a raised bog. The sulfur content also exceeds the usual requirements. The predominantly hypnum–sedge composition of the peat, in turn, does not guarantee the production of peat of fully satisfactory quality; for this, as indicated above, sphagnum peat is required. However, the literature contains indications of the possible suitability, for coking purposes, of peat from transitional and lowland bogs; the corresponding work, however, has not gone beyond the stage of laboratory experiments. On the other hand, in the absence of peat of proper quality and given the need to have peat coke as a product obtained from local fuel to replace imported hard-coal coke, it nevertheless seemed desirable to test this peat with respect to obtaining coke from it and to establish the basic properties of the latter, so that subsequently, if necessary, the peat coke could be tested directly in a cupola furnace. For this purpose, experiments were carried out on obtaining coke from peat.

Coking of Tagansk Peat and Characteristics of the Coke

Peat briquettes with a moisture content of 16–17%, prepared in the manner described above, were subjected to coking. The process was carried out in a muffle—

... furnace in two ways, namely by slow and rapid coking. In the first case the peat was placed in a cold muffle, which was gradually heated to the final temperature, taken as 700°. In rapid coking the peat was placed in a muffle heated to 700°, and coking was completed at this same temperature when the evolution of volatile products ceased; the latter were not collected and, on leaving the opening in the door of the closed muffle, burned. The almost complete disappearance of the flame was taken as the sign that the process at the given temperature was finished. The coke was removed from the muffle after it had completely cooled.

In several parallel experiments the following average figures were obtained:

Table 2

As can be seen, the coke yields in both cases are very close. Taking the average coke yield on the dry mass of peat as, in round numbers, 41%, and the moisture content of the air-dry peat in production as 35%, the gross coke yield according to laboratory data will be 26%, or, rounded, 25% on air-dry peat.

In appearance, the coke, while retaining the original shape of the briquette, consists of dense slabs of black-gray color, possessing a certain ring. Coke from rapid coking is distinguished by a strongly developed fissuring, as a result of which it easily breaks up into small pieces. To characterize the technical strength, the coke was tested in a Broch and Nedelman laboratory crusher; for comparison, under the same conditions the peat briquette, hard-coal coke, and wood charcoal (birch) were tested for mechanical strength. The test was carried out as follows. The material under test, in the form of pieces of the 25–15 mm class, was treated in the crusher and then screened; the yield of coke larger than 15 mm (the residue on a sieve with openings 15 mm in diameter), in %, was taken as an indicator of mechanical strength.

The figures obtained were as follows:

Peat coke from rapid coking, as can be seen, has very low strength, breaking up under mechanical action into small pieces along cracks; these cracks are formed as a result of the vigorous evolution of a large quantity of volatile substances, which occurs when the peat briquette is introduced into the retort, where a high temperature (700°) has already been established.

With slow coking the strength of the coke exceeds that of wood charcoal, but is much less than the strength of hard-coal coke.

Charcoal, as is known, is successfully used for blast-furnace smelting; therefore it may be assumed that an analogous use of this peat coke (from slow coking), in view of its strength, is also possible.

The peat briquette possesses very great strength.

In production, the conditions of gradual coking are met by the operating regime of shaft furnaces with internal heating (for example, Pintsch furnaces), where the material being coked successively passes (from top to bottom) through the zones of the shaft with increasing temperature.

The composition of the coke (from slow coking) was found to be as follows:

Table 3

The apparent specific gravity of peat coke was found to be 0.744, and the true specific gravity 2.08; hence the porosity of the coke is 64.23%, i.e., considerably higher than the porosity of hard-coal metallurgical coke, for which it may be taken on average as about 50%. As a result, the peat coke showed a high rate of combustion. The corresponding determinations were made, for comparison, for peat, peat coke, hard-coal coke, and charcoal. For this purpose, equal portions of fuels ground to 3–2 mm were burned in open crucibles in a muffle, where a temperature of 700° was maintained. Every 10 minutes the crucibles were removed from the muffle, cooled (closed), and weighed. The figures thus obtained, showing the loss in weight, characterized the rate of combustion of the fuels. These figures are given in the following table:

Table 4

In its combustion character, peat coke is close to charcoal, but the total burning time for the former is shorter, which is connected with the smaller quantity of combustible mass owing to the higher ash content. Peat (in the form of a briquette) rapidly loses a large amount of volatiles and burns the fastest. Hard-coal coke has the lowest combustion rate.

On the basis of the experiments carried out, the following conclusions may be drawn:

1. Taganskoye peat, at a moisture content of about 80%, is readily formed under ordinary conditions on a belt machine, yielding, in an air-dry state, a dense and strong briquette possessing sufficient thermal stability, i.e., the ability to burn as a lump without disintegrating into fines; consequently, it should be a lump fuel conveniently burned in ordinary furnaces with a grate.

The same thermal stability, mechanical strength, and high ash-melting temperature (1470°) give grounds to suppose the possib—

the possibility of using peat briquette for smelting cast iron in cupola furnaces; however, difficulties may arise here because of the very large amount of volatiles, which gives a great deal of gas and, at the same time, causes a very rapid decrease in the solid residue of the fuel, which, apparently, should make it difficult to make proper use of the reaction volume of the cupola furnace.

In connection with this, it seems more probable that peat briquette would be used in the cupola furnace in a mixture with another fuel (foundry anthracite, blast-furnace coal, hard-coal coke), as is indicated in the literature.

For a final judgment on this question, experimental melts in a cupola furnace are necessary.

1. Tagansky peat formed as briquettes, despite its botanical composition not being entirely suitable for coking purposes (mainly sphagnum), under an appropriate coking regime yields lump coke with mechanical strength and combustibility close to the corresponding properties of wood (birch) charcoal.

2. Coke from Tagansky peat differs from standard peat coke by a considerably higher (approximately double) ash content (as well as sulfur content), which accordingly results in a lower calorific value of the working fuel; this will lead to a high consumption of this coke in the cupola furnace. The very possibility of its use in the cupola furnace and the production regime must be established experimentally.