Centrifugal Three-Stage Needle Tar Separator for Water Gas Purification

I. V. Gebler, N. N. Norkin

Water gas is produced in gas generators with a horizontal grate and manual operation. The fuel is hard coal from the Aralichevskoye deposit: the coal is lean, partly semi-anthracitic, and does not cake. The average composition of the coal is: \(W^{p} — 9.0\%\); \(Ac — 16.0\%\); \(S_{\text{tot.}} — 0.8\%\); \(C^{r} — 88.1\%\); \(H^{r} — 4.0\%\); \(N^{r} — 1.7\%\); \(O^{r} — 5.3\%\); volatiles on the combustible mass — \(11\%\). The technological regime of gasification is as follows: the period of air (hot) blast is from 2 to 3 min, the period of steam (cold) blast is from 3 to 4 min, and is sometimes extended to 5–6 min. The period of steam blowing (gasification) averages 65% of the total cycle, and the ratio of the gasification time to the air-blast time is 1.76.

Before charging, the coal is screened through a 25 mm sieve; fines smaller than 25 mm are burned in steam-boiler furnaces, while material larger than 25 mm is charged into the gas generator. The average gasification intensity is \(150\ \text{kg}/\text{m}^{2}\cdot\text{h}\). The quantity of ash and slag (raked out) averages 18% of the coal charged. The water gas has a temperature of approximately \(900^\circ\text{C}\) at the generator outlet. The gases from the air blast are burned in the furnace of a steam boiler.

The moisture content in the water gas is about \(5.2\ \text{g}/\text{m}^{3}\), which corresponds to a water-gas saturation temperature of \(45^\circ\text{C}\); however, the gas temperature is usually about \(20^\circ\text{C}\), and therefore the moisture in the gas is present both as vapor and in the form of a finely dispersed mist.

The amount of hydrogen sulfide in the water gas is: before purification, from 0.53 to \(2.0\ \text{g}/\text{m}^{3}\), and after purification, from 0.01 to \(0.13\ \text{g}/\text{m}^{3}\).

The tar content in the water gas was determined by passing the water gas through two series-connected U-shaped glass tubes filled with cotton wool. Since, together with the tar, the cotton wool also absorbs the water vapor present in the gas, after a certain quantity of gas (from 100 to 1000 liters) had been passed through the absorption tubes, the latter were heated for 1.5–2 hours in an electric drying oven at a temperature of \(105^\circ\text{C}\) and then weighed, neglecting the removal at this temperature of part of the light fraction. Constancy of the weight of the tubes after heating served as an indication that moisture had been removed from the cotton wool.

The tar content in the gas depends on the point at which the gas sample is taken and on the gasification regime. Thus, in samples taken directly at the outlet from the generator, the tar content varies from 0.1 to \(0.56\ \text{g}/\text{m}^{3}\).

depending on the gasification time. The minimum tar content corresponds to the gasification period before a fresh portion of coal is charged into the gas generator, when almost pure coke is present in the generator; the maximum tar content corresponds to the gasification period immediately after a fresh portion of coal has been charged, when the most intensive release of volatiles from the coal occurs.

In samples taken after gas cleaning in a scrubber irrigated with water, a tar content from 0.05 to 0.19 g/m³ was found, depending on the gasification time. In samples taken from the gas pipeline of the plant’s chemical laboratory and in the institute laboratory, i.e., after the gas had passed through a fairly long and branched network of gas pipelines, the tar content in the water gas is more or less constant, does not depend on the gasification time, and is on average 0.04 g/m³. The tar is a very viscous dark-brown mass. The specific gravity of the tar, determined with a pycnometer at 20°C, is 1.24 g/cm³. The free carbon content in the tar is on average 11.8%; the moisture content in the tar is about 30%. Fractional distillation established the following group composition of the tar. The first fraction up to 180°C amounts to 36%, including: water 32.7%, light oil 3.3%; the second fraction, in the interval 230–300°C, heavy oil 2.6%; the third fraction, in the interval 300–320°C, the first anthracene fraction 3.7%; and the fourth fraction, in the interval 320–345°C—the second anthracene fraction—9.2%; and finally pitch in an amount of 48.5%. The significant yield of cyclic compounds (anthracene fractions) and the large pitch residue indicate that the tar is a secondary, high-temperature tar.

Despite the fact that the amount of tar in the water gas supplied to the burners is insignificant, on average 0.04 g/m³, cleaning the water gas of tar is absolutely necessary, since atmospheric burners have a small hole diameter (about 1 mm and less), become clogged with tar after a short time, and the phenomenon of flame “retreat” is observed, i.e., a sharp reduction in flame length and finally complete extinction of the gas burner. However, cleaning the gas of tar was not carried out, except for preliminary scrubber cleaning.

The authors developed a method for removing small amounts of tar from water gas by treating the gas in a centrifugal three-stage tar separator, which has been operating quite successfully at the plant for more than 6 years.

The main principles adopted in developing the design of the tar separator are the following: the tar in the water gas is in a mist-like state, i.e., it consists of bubbles filled with gas inside and covered on the outside with a tar shell. When the gas is cooled during scrubber cleaning, the gas pressure inside the bubble decreases and the surface tension of the tar shell increases, as a result of which the tar mist is very stable and is not destroyed during scrubber cleaning. The stability of the tar bubble can be disrupted by a static method—when the pressure changes sharply, the bubble may burst—and by a dynamic method—a sharp impact against an obstacle may tear the bubble apart. As such an obstacle leading to rupture of the bubble, the authors selected sharp needles. Therefore, in the three-stage tar separator three rotating disks and three stationary disks are provided with a large number of sharp needles.

The main component of the tar separator is a vertical shaft, on which three disks equipped with needles are rigidly mounted. The gas enters from below through the central pipe into the first stage and pro-

Centrifugal
three-stage tar
separator for gas

Explosion-proof electric motor
\(N = 1\ \text{kW};\)
\(n = 1500\ \text{rpm}\).

M 1:5

SPECIFICATION

passes between the moving and stationary disks. The torn-off resinous films of the bubbles are thrown by centrifugal force toward the walls of the tar separator and coalesce. The gas then passes through the second and third stages and, after purification, is discharged upward through a fitting located in the cover. The tar is discharged from below and from the first stage.

The overall dimensions of the tar separator are: height 1320 mm, diameter 310 mm, shaft speed 1500 rpm, motor power 1 kW. The capacity of the tar separator is 500 m³/hour of water gas; the amount of tar extracted from the gas is approximately 20 g/hour. The hydraulic resistance of the tar separator is 850 mm water column. Since the tar is fairly viscous and its mobility is low, over time it accumulates in the apparatus, as a result of which the hydraulic resistance of the apparatus increases with time. Therefore, after two to three weeks of operation, the tar separator is steamed out.