STUDY OF THE EFFECT OF HEATING TEMPERATURE ON THE DISTRIBUTION OF SULFUR IN COAL PYROLYSIS PRODUCTS

N. M. Smolyaninova, N. E. Smolyutin, S. I. Smolyaninov

(Presented by Prof., Doctor of Technical Sciences I. V. Gebler)

Sulfur in solid fossil fuels is a harmful impurity in all methods of their thermal processing. Therefore, serious attention is given to the problem of obtaining pyrolysis products of solid fuels with the lowest sulfur content, especially in coke production. One method is desulfurization of the initial raw material; however, ordinary beneficiation methods are entirely ineffective when the removal of organic sulfur is involved, and also of pyritic sulfur if the latter is present in coals in the form of the finest inclusions, finely distributed in the organic substance. The content of these types of sulfur in coals may reach considerable values, which limits their use for processing.

Another line of approach may be the study of pyrolysis conditions that ensure the production of a given product with a minimum amount of sulfur.

The study of the distribution of sulfur among all products of coal pyrolysis is of great practical importance, since the sulfur content often determines the technology for the further processing of these products. For example, even a small content of sulfur compounds in the so-called “crude benzol” from coking makes it necessary to remove these compounds before its final processing into pure products, which is associated with additional capital investment and other costs.

It should be said that a considerable number of works by various authors [1, 2] have been devoted to the behavior of sulfur during coal pyrolysis. The investigations carried out have been reduced, in the main, to determining the ratio between volatile and nonvolatile sulfur at temperatures. The available data on the distribution of sulfur among all pyrolysis products usually relate to some single temperature. Most often such determinations were made for coking products.

We have carried out an investigation of the effect of temperature on the distribution of sulfur in all the main practically separable products of pyrolysis.

For the investigation, a run-of-mine high-sulfur coal from the May 1 Mine of the Kizel Basin was taken, with the following characteristics: analytical moisture—1.52 percent, ash content of the dry fuel—21.9 percent, yield of volatile matter on combustible mass—43.5 percent, total sulfur content in the dry fuel—9.78 percent, content of individual forms of sulfur in the dry fuel: sulfate—0.19 percent, pyritic—7.92 percent, organic—1.67 percent, degree of grinding—0–5 mm.

Pyrolysis experiments with this coal were carried out at temperatures of 500, 600, 700, and 800° without overheating and with overheating of the under-vault space.

The experiments were carried out in a “semi-industrial” crushed-coal laboratory unit. The unit consists of a furnace in the form of a steel rectangular chamber with a capacity of about 2 kg of coal and a capture system. Heating of the chamber is effected by placing it in a special fireclay block, assembled from 4 separate parts: two side walls, the hearth, and the roof; in grooves in these parts heating spirals made of nichrome are arranged.

A characteristic feature of this furnace design is the separate regulation of the heating of each of the chamber walls, the hearth, and the roof.

The capture system included hot and cold electrostatic precipitators and absorbing apparatus. The electrostatic precipitators serve for the separate recovery of heavy and light tar in order to facilitate its dewatering. The design of the electrostatic precipitators is shown in Fig. 1. They consist of glass tubes with receivers, placed in special glass jackets with double walls, which serve as coolers. The first electrostatic precipitator is cooled with hot glycerin at a temperature of 105°, which circulates in the cooler jacket by means of a circulation pump. The second electrostatic precipitator, used for separating and condensing vapors of light tar and water, is cooled with tap water. To improve heat exchange, distilled water is poured into the jacket. The electrostatic precipitators are supplied with high-voltage direct current (6,000–10,000 volts).

Fig. 1. Diagram of the electrostatic precipitators
1—thermoregulator; 2—circulation pump; 3—hot electrostatic precipitator;
4—cold electrostatic precipitator; 5—limiting resistance; 6—step-up transformer.

The absorbing apparatus consists of cotton filters, used to monitor the purification of the gas from tar; columns with calcium chloride for capturing water; Drechsel bottles with a solution of cadmium acetate, in which hydrogen sulfide is captured. Ammonia is captured with 10-percent sulfuric acid; the gas is purified from carbon dioxide with ascarite; and, finally, columns filled with activated carbon serve to absorb gas benzine.

In carrying out the experiments, the yields of solid residue, tar, pyrogenetic water, gas gasoline, and gas (with losses) were determined. These results are given in Table 1.

Table 1

The separated products were analyzed for sulfur content: in the solid residue—by the accelerated method [3], in the tar—by the double-combustion method [4], in the gas gasoline—by the lamp method, and in the pyrogenetic water—iodometrically. The results of the analyses are given in Table 2. As can be seen from the table, the sulfur content of the solid residue, in comparison with the sulfur content of the initial coal, is insignificant and decreases uniformly with increasing experimental temperature. The sulfur content in the tar, depending on the temperature, passes through a minimum occurring at temperatures of 600–700°, while the tar obtained when the charge is overheated contains somewhat more sulfur. The sulfur content in the gas gasoline increases continuously with increasing temperature, and a noticeable increase in the sulfur content of this product is observed when secondary pyrolysis is intensified. The sulfur content of the pyrogenetic water follows the same regularities.

Table 2

Data on the distribution of sulfur in the pyrolysis products, as percentages of the total sulfur of the coal, are given in Table 3.

Table 3

The proportion of sulfur going into the solid residue decreases continuously with increasing temperature. Correspondingly, the proportion of sulfur passing into the gas increases. It is interesting to note that our data are in full agreement with the conclusions of E. S. Krym and G. I. Beskina [5] on the independence of the hydrogen sulfide yield from the conditions of subsequent pyrolysis (the overwhelming amount of sulfur in the gas is accounted for by hydrogen sulfide). However, this cannot be said of the other volatile products. With deeper processes of subsequent pyrolysis (overheating of the arch), the proportion of sulfur passing into these products increases noticeably.

As regards the influence of the heating temperature proper, the proportion of sulfur passing into the tar is maximal at a temperature of 600–700° and begins to decrease with a further rise in temperature.

The proportion of sulfur passing into the water increases continuously with increasing experimental temperature. The same phenomenon is also observed for gas gasoline.

Conclusions:

1. The distribution of sulfur in the process of pyrolysis of Kizel coal and the sulfur content of the products obtained have been studied as a function of the experimental temperature and the conditions of secondary pyrolysis.

2. It has been shown that, with increasing temperature, the proportion of volatile sulfur increases and its distribution among the volatile products changes.

3. It has been established that the conditions of secondary pyrolysis do not affect the yield of hydrogen sulfide, but have a noticeable effect on the sulfur content and on changes in the proportion of sulfur in the tar, gas gasoline, and water.

4. An enlarged laboratory installation has been designed, distinguished by the construction of the furnace (separate heating of the chamber walls, hearth, and arch) and by a system of electrofilters for fractional condensation of the tar.

LITERATURE

1. Yu. R. v s k i i A. Z. Sulfur of Hard Coals. Ugletekhizdat, 1948.

2. T i s s e n D. Collection Chemistry of Solid Fuel, edited by N. M. Karavaev. Izd-vo inostr. lit., vol. 1, 70, 1951.

3. G r a c h e v a N. M. et al., Accelerated Methods of Coal Analysis. Ugletekhizdat, 1954.

4. V o l y n s k i i N. P. and Ch u d a k o v a I. K. Transactions of the Petroleum Institute, Publishing House of the Academy of Sciences of the USSR, 8, 88, 1956.

5. K r y m E. S. and B e s k i n a G. I. Transactions of UKhIN, Metallurgizdat, issue 3, 182, 1951.

Corrections and Noted Misprints