Coal is an inhomogeneous sedimentary rock that is formed as a result of the accumulation and precipitation of plant and tree residues in swamps and changing over millions of years under high temperature and pressure, by chemical and physical effects. It mainly contains carbon (C), with small amounts of sulfur (S), oxygen (O), hydrogen (H) and nitrogen (N). In addition to these, coal also contains inorganic substances in its content.
Coal is a type of fuel that can behave differently depending on its formation process and content. For this reason, coal must be analyzed under laboratory conditions before being used as a fuel. These tests must be carried out by competent operators in laboratories using methods and devices that comply with international standards in terms of the accuracy and reliability of the results.
Chemical analyzes applied to coal and coke enable the content of the fuel to be determined. Based on the determined features, the efficiency, economic value, usage areas and damages to the environment of the coal to be used are determined.
Chemical analyzes applied to coal are given below.
1. Calorific Value (ASTM D5865, ISO 1928)
It is the determination of the amount of energy released at the end of the combustion process of the coal sample. It can be analyzed using a calorimeter instrument.
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2. Moisture (ASTM D3173, ISO1172)
It is the amount of water in the coal that reduces the combustion efficiency. It is calculated by determining the mass loss by heating the sample in a controlled manner. It can be analyzed using a moisture oven or moisture analyzer.
3. Sulfur (ASTM D4239, ISO 351)
It is an elemental analysis method to evaluate the sulfur emissions after the combustion of coal and to determine the damage it may cause to the environment. The sulfur content affects the upper calorific value. It can be obtained as a result of the titration process performed with an elemental analyzer or methods in accordance with the standards.
4. Volatile Matter (ASTM D3175, ISO 562)
It is the analysis of volatile substances except water in the coal content. Samples with high volatile matter pose a risk of self-ignition. It is an important parameter for deciding the combustion condition. It is calculated by determining the mass loss by heating the sample in a controlled manner.
5. Ash (ASTM D3174)
Non-combustible inorganic substances contained in coal. It is calculated by determining the mass loss as a result of the controlled heating and burning of the sample using a muffle furnace.
6. Fixed Carbon (ASTM D5142, ISO 17246)
Fixed carbon is combustible solid waste contained in coal. It is calculated gravimetrically. It is found by subtracting the percentages of ash, volatile matter and moisture from the total mass of the sample.
7. Hydrogen (ASTM D5373)
Hydrogen is a combustible element in coal. It affects the lower calorific value of coal. It is calculated using an elemental analyzer.
8. Hardgrove Grindability Index (HGI)
HGI is the determination of the resistance of coal to cracking. A certain amount of coal samples, which are the same in size, are placed inside the grinding unit and crushed under pressure. As a result of the process, the HGI value is determined by using the smallest coal size.
9. Crumb Test
While the coal is brought to dimensions suitable for analysis, it may cause a certain amount to be lost as a result of crumbling in the processes it goes through. To determine this, the economic damage can be prevented by performing a crumbling test. Coals of the dimensions determined during the test are dropped from a certain height to the plate. The comminuted parts are separated, and the friability is determined for each size by proportioning it to the total mass.
10. Buoyancy – Immersion Test
It is used to determine the ash content and yield of clean coal. Additionally, it is used when determining whether coal is suitable for demolition.
The tests applied to the above-mentioned coal and coke samples play a very active role in ensuring that the manufacturer, seller, and user have quality products, preventing economic problems and reducing the damage to the environment by using the right combustion systems. Before the coal is used as a fuel, the specified chemical analyzes must be carried out under reliable laboratory conditions and the results obtained must be evaluated.