On the Determination of the Slag-Forming Capacity of Solid Fuels

K. K. Stramkovskaya, I. B. Berman, V. I. Tishkin

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

A very important factor, often decisive in assessing the suitability of a fuel for gasification, is the behavior of the fuel ash in the gas generator, since the intensity of gasification and the quality of the gas depend to a considerable extent on this. The characteristics of ash fusibility obtained by the previously adopted standard method are wholly unindicative of the behavior of ash under practical gasification conditions. Under the latter conditions, the mineral impurities are not mixed together, and changes proceed in accordance with their individual properties. In addition, the time factor and the nature of the gaseous medium are of substantial importance in slag formation [1, 2].

The method for determining the slag-forming capacity of fuels developed by the All-Union Scientific Research Institute of Liquid Fuel and Gas, VNIGI (VNIINT¹), is closer to the conditions of fuel gasification. This method makes it possible to trace the change in ash with respect to sintering and slag formation under various regimes and to reveal the individual properties and behavior of separate ash components at the initial stage of slag formation. The initial stage of slag formation is conventionally taken to be that period of time which is necessary for gasifying carbon out of pieces of fuel of specified sizes. The duration of this period depends on the intensity of gasification and on the properties of the fuel, i.e., on its reactivity and on the quantity and distribution of mineral inclusions [3]. However, the apparatus proposed by VNIGI is cumbersome, comparatively complicated to manufacture (it consists of 82 parts), and inconvenient in operation. As for the experimental procedure, it can be considerably simplified and accelerated.

The aim of the present work was to develop a more improved apparatus and an accelerated procedure for determining the slag-forming capacity of fuels.

The apparatus proposed for this purpose is clearly shown in Fig. 1. The procedure for determining slag-forming capacity proposed by VNIGI [3] has, in principle, been adopted by us as well, but certain changes have been introduced into it. To determine the slag-forming capabil—

Fig. 1. TPI apparatus for determining the slag-forming capacity of solid fuels.

1 — branch pipe for supplying the blast; 2 — cover; 3 — asbestos cord; 4 — body (steel 18KhN9T); 5 — grate (steel 18KhN9T); 6 — bottom; 7 — branch pipe for gas outlet; 8 — nut for fastening the thermocouple.

... a sample of the fuel being tested, with a particle size of 6–3 mm, is taken, brought to the air-dry state, and placed in the apparatus in a volume of 125 cm³.

The thermal preparation of the fuel was carried out in the same apparatus; for this purpose an electric furnace with an open nichrome coil of 0.5 mm cross-section and 8 mm length was slipped over it. The heating temperature of the furnace is regulated by a transformer. After the thermal preparation is completed, the electric furnace is removed from the apparatus and, instead of it, air measured by rheometers is supplied from above to the air blower. The temperature during gasification is recorded by a galvanometer with a platinum–platinum-rhodium thermocouple. The maximum temperature in the gasification chamber and the amount of fused particles in relation to the total weight of ash and slag were taken as the resultant indices and plotted graphically. The slag-forming capacity of the fuel was evaluated according to a conventional grid (Fig. 2), which we borrowed from the method of VNIGI.

Fig. 2. Curves of the slagging intensity of fuels.
1 — SSK coal from the Chernaya Gora mine (Kuzbass). 2 — TK coal from the Dimitrov mine (Kuzbass). 3 — Norilsk hard coal (Mine 18). 4 — Cheremkhovo long-flame coal (from Combine 16). ○ — Results of determinations in the VNIGI apparatus. ▲ — Results of determinations by the accelerated method in the TTI apparatus. — — — Boundary lines separating the slagging regions.

In the graph: vertical axis — Ash caking, $\theta$, %. Horizontal axis — Load on the grate, kg/m²·h. The regions are marked: I — region of ashes that slag intensively; II — region of ashes of medium slagging intensity; III — region of high-melting ashes.

Under the graph: Maximum temperature reached during gasification:

1. 1475; 1500; >1500
2. 1470; >1500; >1500
3. 1530; 1440; 1290
4. 1040; 1190

VNIGI. The graph grid is conventionally divided by boundary lines into regions: I — intensively slagging ashes; II — ashes of medium slagging intensity; and region III — high-melting ashes. Depending on the region in which the points of the ash-slagging curve of the fuel under test are located, we assign it to one or another group.

The test is carried out at gasification rates of 200, 400, 600, and 800 kg/m²·h. The air consumption for blasting is determined from the content of organic matter in the solid residue.

According to the VNIGI procedure, thermal preparation of the fuel in an electric furnace is carried out for 3 hours at a temperature of 800–850°, after which the prepared fuel is transferred into the gasification chamber, into which air is then supplied. In our apparatus the operations of thermal preparation and gasification are combined, which accelerates and simplifies the analysis. In addition, the smaller fuel charge—125 cm³ instead of 500—makes it possible to reduce substantially the time required for thermal preparation.

Thus, the observations we carried out showed that the duration—

Vertical axis: Degree of slagging, \(B\), %

Horizontal axis: Load on the grate, in kg/m²·h

Maximum temperature attained during gasification:

Fig. 3. Curves of the slagging intensity of coals from the Gorely and Moshchnyi seams.
I — region of intensely slagging ashes. II — region of ashes of medium slagging intensity. III — region of refractory ashes.

Table 1

heating time greater than 40 min, both for lean fuels and for fuels with a high yield of volatile matter, has no effect on the slag-forming capacity. Therefore the thermal preparation of the fuel was carried out by us as follows: 125 cm³ of fuel of the specified size was heated to 800° over 20 min and held at this temperature for another 20 min; then its gasification was carried out by the procedure described above. After the fuel had burned out, the apparatus was cooled. The slag was screened into the size classes \(>6\), \(6—3\), and \(1—0\); the yield of the \(>6\) class was determined, and the results were plotted graphically.

For the fuels whose characteristics are given in Table 1, we determined the slag-forming capacity in the VNIGI apparatus and in the apparatus described by us.

The results shown in the graph (Fig. 2) for determining the slag-forming capacity of fuels in the TPI apparatus show good agreement with the data for determining this index in the VNIGI apparatus; however, the apparatus proposed by us is distinguished by exceptional simplicity of manufacture, is convenient in operation, and makes it possible to determine the gasification indices of solid fuel many times more rapidly.

A check of the slag-forming-capacity readings for reproducibility showed that the absolute deviations in parallel experiments lay within the range from 0.39 to 0.75, while the relative errors varied in the interval from 0.79 to 2.5%.

In addition to slag-forming capacity, the proposed apparatus can be used to determine the thermal stability of fuel and its reactivity with respect to air.

Among Kuzbass coals, we had the opportunity to determine the gasification parameters of coals from the Gorely and Moshchnyi seams, mined by the Krasnobrodsky and Karaulny open-pit coal mines. The characteristics of these coals are given in Table 2.

Table 2

The mechanical strength and thermal stability of these coals are given in Table 3.

The results of determining the slag-forming capacity of these coal samples, shown in Fig. 3, indicate that all these coals lie

in the region of ashes of medium slagging intensity; however, the coals of the Gorely seam are the most difficult to slag. The coals of the Moshchny seam slag much more strongly.

The investigated coals of the Gorely seam, belonging to grade T, are low-ash (their ash content ranges from 4.00 to 11.7%), low-sulfur, and possess fairly high mechanical strength and thermal stability. At the same time, having shown a low slag-forming capacity, these coals, mined by open-pit operations, can quite well be used as a raw material for gasification.

Table 3

| Sample No. | Name of coal seam | Shattering index | \multicolumn{3}{c}{Thermal stability according to VNIIGI} |
|---:|---|---:|---:|---:|---:|
| | | | $K^{\mathrm{b}}_{p}$ | $K^{\mathrm{z}}_{p}$ | $K^{\mathrm{t}}_{p}$ |
| 1 | Gorely | 74 | 14.6 | 2.3 | 0.7 |
| 1 | Moshchny | 57 | — | — | — |

The coals of the Moshchny seam, belonging to grade SS, having a somewhat higher slag-forming capacity and low mechanical strength, are less valuable as a raw material for gasification.

Conclusions

1. An apparatus has been developed for determining the slag-forming capacity of solid fuels, and an accelerated method has been proposed for determining this parameter.

2. A characterization is given of the gasification properties of Kuzbass hard coals mined by the Krasnobrodsky and Karaulny open-pit coal mines.

LITERATURE

1. M. V. Kontorov. Gas Generators and Gas-Generator Stations in the Metallurgical Industry. Metallurgizdat, 1958.

2. E. M. Taits, N. G. Titov, N. V. Shishanov. Methods for Evaluating Coals as Raw Materials for Industrial Use. Ugletekhizdat, 1949.

3. A. L. Mikheev, R. V. Pukhlikova, T. E. Yaroslav. Chemistry and Technology of Artificial Liquid Fuel and Gas, issue III, 59, 1951.

1. GOST 9271—59. ↩