Gray-King Assay Characterisation of Nigerian Enugu and Polish Bellview Coals for Co-carbonisation

MATERIALS AND METHODS

Materials: Samples of Enugu and Poland’s Bellview coals were used for the Gray-King assay tests. The Bellview coal was imported from Poland for the Ajaokuta steel plant, while the Enugu coal was obtained from Enugu coal mines in Nigeria.

Methods: The tests were conducted in the Fuels and Energy Laboratory of the National Metallurgical Development Centre, Jos, Nigeria. Coal samples pulverized to pass 250 micron were used for both proximate analysis and Gray-King assay tests.

Proximate analysis (NMDC manual, 1994): Proximate analysis involves determination for moisture, ash, volatile matter and fixed carbon.

Moisture: The silica crucible for the test was preheated at 105°C for 1 h. One gram of coal sample was then placed in the crucible and heated at 105°C for 1 h. The loss in weight accounts for the moisture content.

Volatile matter: The standard crucible was preheated in the muffle furnace at 970°C for 7 min. The crucible was cooled in the desiccators and 1 g coal sample was placed in it with three drops of benzene. The crucible (with the lid on) was now placed in the muffle furnace at 970°C for 7 min. The loss in weight accounts for the volatile content of the coal.

Ash: The standard silica crucible was preheated at 825°C for 1 h. It was the cooled in the desiccators. One gram of coal sample was placed in the crucible (with the lid on) and it was heated at 825°C for 1 h. The incombustible residue gives the ash content of the coal.

Fixed carbon: The fixed carbon is obtained from the relation:

(1)

Proximate analysis results are presented in Table 1.

Gray-King assay test: Five gram of coal was placed in a silica retort, 300 mm long and 19 mm internal diameter. One end of the retort is round and closed and at the other end, a side arm is fused which serves as an exit for the tarry vapours and gases released during the experiment. With the retort held horizontally, the coal was spread over the length in an even layer. The retort was then introduced horizontally into the tube furnace maintained at 325°C. The furnace temperature was raised at the rate of 5°C per minute till a temperature of 600°C was attained. The furnace was maintained at this temperature for 15 min. The retort was then taken out, allowed to cool and the coke pencil produced was brought out. It was now compared with the standard profiles which are designated A, B, C, D, E, F, G, G1, G2, ..., G9 (Moitra et al., 1972). The results of Gray-King Assay tests are presented in Table 2.

The percentage coke and tar yield are calculated as follows:

(2)

(3)

Table 1:	Result of proximate analysis
Ad-as determined, db- dried basis, daf- dried ash free

Table 2:	Gray-King assay test parameters

The Gray-King coke type results has been reported not to be susceptible to weight of samples, but to such other factors as the state of oxidation and size of the coal samples (Moitra et al., 1972).

DISCUSSION

The moisture content of 8.15 and 0.57% determined for Enugu and Bellview coals respectively falls within the range required for coals to be carbonized for metallurgical cokemaking at Ajaokuta (Federal Ministry of Mines, Power and Steel (FMPS) circular, 1992). The moisture contents for the two coals are also lower than the 12.80% determined for the low rank Australian Loy Yang coal (Xiaofeng et al., 1998). However, Enugu coal moisture far exceeds the 1.5-2.2% for the Indian Jharia coal samples (Moitra et al.,1972; Prasad et al., 1992). On the basis of their moisture contents, the two coals can be considered suitable for inclusion in blends for cokemaking.

The ash content of 7.57 and 5.56% for Enugu and Bellview respectively are far lower than 20 to 25% for Indian Jharia coal samples and 17.52% for Indian prime coking coals (Prasad et al., 1992). The ash for the two coals also satisfy the 10% upper limit for coals to be carbonized at Ajaokuta (Federal Ministry of Mines, Power and steel circular, 1992). The ash content of Lafia coal, 15.9%, determined by Afonja (1996) also exceeds those of Enugu and Bellview coals. The ash contents of the two coals fall within the 7.2 to 9.6% range for main coals carbonized in Japan to produce coke (Katsuhiko et al., 1987). The tolerable ash contents of the imported Bellview and Nigerian Enugu coals strongly indicate that cokeable blends with acceptable ash contents may be produced from both.

The volatile matter of both Enugu and Bellview coals exceeds the limits of 30.3% for coals to be carbonized for cokemaking at Ajaokuta. However, the Bellview coal volatile matter is lower than 36-40% for Indian Jharia samples and 33-48% for coking coals indicated by Aderonpe (1996). The volatile content of Bellview is also lower than 39.4 and 41.8% for high volatile coals carbonized in Japan to produce coke that ensured the most efficient blast furnace operation (Katsuhiko et al., 1987). The successful carbonization of coals with specified volatiles exceeding the upper limit of 30.3% for Ajaokuta suggests that cokeable blends of Bellview and Enugu that satisfy international cokemaking practice may be produced despite their relatively high volatile contents.

The Gray-King coke type of G determined for Bellview coal is the same with Canadian Green Hill coal being carbonized in India. But the Indian Sudamih, Australian Utah and Polish Weglokoks coals with G1, G2 and G5 Gray-King Coke type numbers, respectively have better Gray-King coking power than Bellview coal (Parthasarathy, et al., 1987). The Gray-King coke type B determined for Enugu is lower than D for Indian Dugda II coal sample for cokemaking in India (Parthasarathy et al., 1987). Bellview coal has the same G value as the medium coking Kargali, Swang and prime coking Patherdith coals in use at Indian Sail plant for cokemaking (Parthasarathy et al., 1992). As expected, the coke yield for the medium coking Bellview exceeds that of non-coking Enugu, while the tar yield of the latter exceeds the former (Moitra et al., 1972). The high tar yield of Enugu coal may be an advantage in sourcing tar as a binder in the production of formed coke (Afonja, 1991). The Bellview coal with G as its Gray-King coke type may therefore be considered as cokeable while Enugu coal is obviously non-coking and may only blend with Bellview to an extent constrained by its poor plastic behaviour. The higher value of tar yield for Enugu coal in comparison with Bellview coal confirms it to be a lower rank coal with higher content of peripheral groups that produce tar (Moitra et al., 1972).

CONCLUSIONS

The proximate analysis, coke and tar yields obtained for Enugu and Bellview coals, showed that the latter is a medium coking bituminous coal suitable for cokemaking, while the former is a non-caking sub-bituminous coal with a higher tar content useable as a binder in formed coke production. The volatile content of Bellview coal can be reduced below the upper limit of 30.3% by blending with a low volatile prime coking bituminous coal. Bellview coal may also accommodate an amount of Enugu coal in blend to the extent constrained by the zero plastic property of the latter.

ACKNOWLEDGMENT

The authors hereby gratefully acknowledge the experimental contributions of Late Mr. Dauda Magaji of the National Metallurgical Development Centre (NMDC), Jos.