Coals are organic detrital sedimentary rocks that originate from a variety of plant material and different tissues deposited in more or less aquatic locations (Loison et al., 1989). Coal vary in rank depending on the degree of maturity attained during metamorphism. A coal is characterized by a number of chemical, physical, physico-chemical and petrographic properties.
In proximate analysis of coals, moisture, ash, volatile matter and fixed carbon contents are determined under strictly standardized conditions. It is the simplest means of characterizing coals for utilization. Water exists in coal either as free surface water, surface adsorbed, capillary condensed water and water bound to the coal molecule by chemisorptions or hydrogen bonding with the reactive groups. The free surface water evaporates out of coal when it is kept exposed to the atmosphere, while the other forms of water are retained in coal depending on the atmospheric humidity. They are released only at high or temperatures. The determination of moisture content is important not only to assess the commercial value of coal but also because of its relationship to other properties (Moitra et al., 1972). Ash in coals are due to mineral matter which are either co-deposited with the coal-forming vegetable matter or gets into coal by subsequent infiltration. On incineration of coal, the organic matter burns off leaving the product of oxidation of the mineral matter. The ash in coking blends should be as low as possible.
The volatile products obtained during the pyrolysis of coal under specified conditions are known as volatile matter. It consists mainly of hydrogen, methane, carbon monoxide, carbon dioxide, hydrocarbon, tar, water vapours, nitrogen, ammonia, hydrogen sulphide etc. It is mainly derived from the peripheral groups of the coal molecule up to a pyrolysis temperature of 600 to 700°C. Immature coals have higher weights of these peripheral groups and thus their volatiles are more than for mature coals in which these groups are fewer. The yield of volatiles is therefore an approximate measure of rank (Moitra et al., 1972).
When bituminous coals are heated, they develop plastic properties at about 350°C and as a result exhibit fluidity, swelling, agglutinating properties, expansion and contraction in volume and after carbonization produce coherent residues of varying strengths depending on the rank of the coals. Gray-King coke assay test is one of the tests developed to measure this physico-chemical characteristic of coals (Moitra et al., 1972).
The efforts to source coals for cokemaking at the Nigerian Ajaokuta integrated
Steel Plant, led to location of coal deposits with proven reserves of 54, 74
and 22 million tons at Enugu, Okaba and Lafia, respectively. Of these three
deposits, only Lafia coal is medium coking but with intolerable ash and sulphur
contents of 22.91 and 1.90%, respectively. The non-caking Enugu and Okaba coals
however contain acceptable levels of ash and sulphur (Anonymous, 1987). The
Bellview coal was imported from Poland to produce cokeable blends with Nigerian
coals. In this research, Bellview and Enugu coals were subjected to Gray-King
assay tests to study their blendability for cokemaking and tar yielding capacity.
Assay analysis was a key parameter determined in the study to utilize the Canadian
Hat Creek coal. The tar yield of the coal was determined as 6. 8% (http://www.em.gov.bc).
The current international market price of US$300 per ton for coal makes research
in the area of co-carbonization of coking coals with sub-bituminous coals more
MATERIALS AND METHODS
Materials: Samples of Enugu and Polands 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:
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:
||Weight of semi-coke residue in the test tube
||Weight of tar and water collected
||Weight of sample
|| Result of proximate analysis
|Ad-as determined, db- dried basis, daf- dried ash free
|| 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).
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).
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.
The authors hereby gratefully acknowledge the experimental contributions of Late Mr. Dauda Magaji of the National Metallurgical Development Centre (NMDC), Jos.