Coal can be classified as coking or non coking depending on its behavior when heated to a temperature of about 7000C to 10000C in absence of air. If coal softens, de-volatilizes and solidifies into porous hard solid coke, it is classified as coking or caking coal. If the residue left on heating in absence of air be weakly coherent or powdery mass then it termed as non-coking.
The process of heating of prepared coking coal charge or blend of coking coals in absence of air to a temperature of ~700˚C or ~1000˚C to pass through different stages such as softening, swelling and re-solidification into coherent cellular mass called coke is known as carbonization.
Coke is one of the important raw materials for the Blast Furnace. Coke plays very important role in Blast furnace like physical, chemical and thermal. It acts as fuel, generate coke oven gas which acts as reducing agent and supports the overlaying burden in the furnace. Therefore properties of coke (hot and cold strength) are varying important for efficient operation of blast furnace.
The properties of weathered coal have adverse effect on coke properties. Weathering of coal start as coal is mined and it will continue till it is charged in coke oven to produce coke. In mines coal exists in a water-saturated, oxygen-free environment, any disturbance of this environment such as a change in the temperature, moisture content or oxygen partial pressure, result in change of chemical properties, physical stability. This dynamic behavior of coal is termed ‘weathering ’ and includes the aerial oxidation of the organic and mineral matter (chemical weathering), the microbial oxidation of pyrite (biological weathering) and changes in the moisture content that result in particle size degradation (physical weathering). During transportation and stockpiling, coal is in contact with air for periods of time that may exceed 6 months. During this time, reaction with oxygen in the presence of water, sunlight and possibly elevated temperatures may take place. Different coal follows different trends during weathering. The weathered coal affect its beneficiation process as due to oxidation coal surface property get changed and coal surface become more hydrophilic.
Materials and Method:
In this study 6 types of coking coal used for this experiment. Collected coal sample first screened through (4.75 mm) screen and sample which passes through 4.75 mm screen and then stored in open tray and kept at open in lab atmosphere for a total period of 3 months. Evaluation of weathering is planned to be carried out at an interval of 15 days. It means after fixed interval a representative sample withdrawn for study the effect of weathering.
Methods to Monitor Coal Weathering:
Coal is heterogeneous in nature, and weathering of coal strongly depends on coal rank and seam. In the present study the following properties of coking coal have been evaluated with time to monbitor weathering behavior. The properties are:
ü Proximate analysis
ü Change in caking properties of coal (free swelling index)
ü pH measurement
ü Determine the oxygen content
ü Degree of oxidation by alkali extraction method.
Representative 60 mesh samples have been prepared for the proximate analysis and carried out as per ASTM method.
Free Swelling Index (FSI)
FSI test have been carried out with same sample (-60 mesh) which was prepared for proximate analysis. Test conducted as per ASTM D720.
In this test 2 gm (-60 mesh coal) and 20 ml distilled water has been heated to 150 ˚C for one hrs. Then it is cooled by tap water indirectly. The pH has been determined by pH meter.
Determination of oxygen content
This test carried out with 60 mesh sample as per ASTM D 3176.
Degree of oxidation by alkali extraction
This test can be carried out with same sample (-60 mesh) as per ASTM D 5263.
Experiment & Results:
Coking coal of size (-4.75mm) stored in Lab to monitor the coal weathering effects. Representative sample were withdrawn from tray at a fixed interval. Then theses were dried in oven at 38 ˚C till constant weight. These samples are then reduced by coning and quartering then grinded in ball mill. Grinded sample were screened through 60 mesh. The proximate analysis involves determination of moisture, VM, ASH and fixed carbon using above mentioned procedure also found small changes in ASH with time. In case of VM, it is found that VM increases with time. This is in agreement with may be due to fact that changes in VM depends on sensitivity of aliphatic and alicyclic to oxidation, and how these groups are combined, and it differs from coal to coal. Following tables shows Proximate, GCV, FSI, S, pH, oxygen and transmittance (degree of oxidation) value measured at different time for coking coal.
For all sample (A-F), FSI value decrease maximum 1 to 1.5 in a span of 4 months storage periods. Figure 1 shows changes in FSI with time for all coals:
pH as a technique to monitor coal weathering. We observe change in pH and put effort to correlate it with loss in calorific value. These changes in pH value may be due to release of sulfur and acid produced during oxidation. It was observe that GCV value reduces with decreasing the pH value of the following samples. Figure 4 & 5 show changes in pH and GCV with time.
It was observe that degree of oxidation increases with increasing the oxygen content of the following samples. Figure 4 & 5 show changes in oxygen content and transmittance (%T) with time.
Oxidation also reduces the calorific value of coals as indicated below:
1. During carbonization process of coal, FSI is the important coke making properties. FSI decreases during weathering resulting poor coke quality. Therefore, it is a good indicator to monitor coal weathering.
2. Volatile matter increases during weathering by marginal amount. They may also be useful to monitor moderate coal weathering because it is directly related to degree of oxidation.
3. pH value appear to be a good indicator for moderate coal weathering. It decreases appreciably during weather process leading to oxidation of coal.
4. Weather and resulting Oxidation reduces the calorific value of coal. In this present study there have been losses of calorific value as low as 110 Kcal/Kg for each 1%increase in oxygen content.