Fossil fuels (oil, natural gas, coal) are not just energy raw materials; they are also the raw materials in which the main inputs of many industries (such as paint, plastic, pharmacy, cosmetics, iron-steel, aluminum, etc.) are produced. Coal is a mineral and rock with a different chemical and physical structure. It is composed of mostly carbon, hydrogen and oxygen and contains a small amount of sulfur and nitrogen.
Other components are ash-forming inorganic compounds and mineral substances. Based on the current production levels of the proved coal reserves they are globally available and have long lifetimes compared to other fossil fuels. Price stability, ease of transportation, the convenience of storage facilities, safety and reliability in terms of ease of use, cheapness, and continuity of supply to the user make it an indispensable energy source.
This position of coal will continue to play an important role in terms of sustainable development and energy reliability in the future as in the past. Since it is an inexpensive and competitive fuel in electricity generation, approximately 41% of the world’s electricity production is provided by coal.
Other uses are concentrated in the heating, iron-steel and cement industries. Therefore, coal is the most common of the fuels used for electricity production. It is estimated that the high usage of coal for electricity generation will not change in the foreseeable future.
Coal is still seen as a reliable source of energy and especially developing countries as Turkey are planning to increase their consumption rates. To reduce the energy dependence the share of domestic resources must be increased and the exploration studies must be accelerated.
Production of clean fuel using advanced coal technologies was included among the 2023 targets of Turkey. To reduce the coal (especially lignite) scarcity, which exists today and will increase gradually in the coming years, the production level has to be increased.
Coals are very diverse in terms of carbonization process and bedding, moisture content, ash and volatile content, fixed carbon content, sulfur, and mineral substance contents, as well as geological, petrographic, physical, chemical and thermal properties.
Generally, in Turkey, coal is evaluated as lignite and hard coal reserves. Hard coals are produced by Turkey Hard Coal Enterprises (TTK) and lignites are operated by Electricity Generation Company (EÜAŞ), Turkish Coal Enterprises (TKI), and the private sector.
In our country, although natural gas and oil reserves are quite limited, there are 1.3 billion tons of hard coal with 506 million tons are visible and 14.2 billion tons of lignite reserves with approximately 13.9 billion tons of visible. As a result of the exploration and reserve development studies carried out in recent years, a large number of reserves were found. General Directorate of Mineral Research and Exploration (MTA) continues to find new sites and try to increase coal reserves.
The coals extracted from the mines contain various types and amounts of non-coal inorganic materials as well as coal grains of various sizes. Inorganic materials in the run of mine coal might be originated from the coal vein and/or the geological structure (quartz, clay minerals, carbonate minerals, sulfur minerals, etc.) or foreign materials that get mixed with the coal during the excavation of coal. For the production of coal with low ash and sulfur content that does not create air pollution, these impurities in the coal must be removed.
The lignite with high ash and sulfur content and low thermal value, which constitute the main energy source of our country, are first removed from their inorganic impurities in coal washing plants. To produce electrical energy they are ground to a fine size and burned in thermal power plant boilers.
Coal washing techniques can be simple, as well as more complex and advanced techniques in line with the demands of the consumption area.
In coarse size coal enrichment, the upper size is chosen as 150 or 100 mm according to the gap of the screen and is generally enriched with heavy media system and jigs. As fine coal enrichment methods, heavy media cyclones, fine coal jigs, shaking tables, water cyclones, spirals, cones, and flotation can be given.
The wastes generated during coal washing and preparation have different characteristics. While coarse grains are usually stored in bulk in the open piles, more watery and fine-grained mud-like wastes are filtered and then sent to waste dams.
In a typical coal preparation plant, a fine-sized coal waste (below 0.5 mm and approximately 20% of the total feed) contains a high amount of coal and discharged to tailing pond.
They cannot be used due to high processing costs. In parallel with the use of coal, the amount of waste increases and causes an important problem for the environment. The type, character and discharge limits of these wastes are very important for successful waste management.
Thanks to the developing coal cleaning technologies, the coal found in these wastes can be used efficiently with low ash and pyritic sulfur content. Coal in waste is an energy source and waste storage problems and cost elements can be reduced by re-processing.
Generally, physical enrichment devices such as 1)Coal spiral, 2)Shaking tables, 3)Water cyclone, 4)Centrifugal gravity separators, 5)Hydraulic separators can be used to recover fine coal from coal washing plant wastes.
Besides, other alternative methods such as flotation and oil agglomeration are preferred. Especially the flotation method is applied for cleaning material below 0.5 mm. The fact that the clean coal surface has hydrophobic (non-wetting) feature and it is a natural floatable material the separation of schist and pyrite become easy.
The clean coal obtained from the flotation circuit is sent directly to the vacuum filter. Wastes are transported either into a waste pool or thickener to recover water. Today, with the advances in flotation technology, it is possible to produce low ash (less than 4%) clean coals from coal waste below 0.045 mm.
The PhD study titled “Investigation of the Effects of Ultrasonic and Mechanical Vibration Mechanisms in the Dewatering of Small-Sized Coals” was completed in 2011 at the Istanbul Technical University, Mineral Processing Engineering Department by the corresponding author.
As a result of this study, it was understood that clay and schists deposited in coal waste (below 0.5 mm) were separated from the coal during the dewatering process by sieving and the fine coal was also recovered. The ash content of the feed material was 41.07% and the upper heat value was 2760 Kcal/kg.
As a result of the operations, the ash content has decreased by approximately 11.66% and the upper heat value has increased by approximately 1337 kcal/kg. Thus, a clean coal with an upper thermal value of about 4100 kcal/kg is obtained, while a final tailing with an upper thermal value of 1050 kcal/kg is discarded.
All these methods aim to obtain the very small size grains formed by the crumbling of coal. Generally, the inorganic material (mostly clay, silicate, carbonate, etc.) is found in this material and disposed of as final waste.
Coal obtained as a result of these recovery processes is used as fuel for energy production, and the material that comes out as waste can be used as raw material and additive for different industries. These final wastes of inorganic matter are in natural form and usually contain clay and schists.
For example, if the amount of clay in these wastes is high, it can be used in the ceramic and brick industry depending on their quality. The economic added value is provided by the production of coal for energy production and as well as the production of additives and raw materials for various sectors.
Herewith, the environmental effects of these wastes can be reduced to a minimum. Lack of oil and natural gas reserves from fossil sources will keep coal in the foreground in the future. Not only in terms of the quantity of the reserve compared to other fossil sources, but coal deposits are distributed to various regions of our country, cheap raw materials per kW/hour in electricity generation and safe in transportation.
They have also an extremely positive effect on economic, regional and cultural development. These factors make coal the most important fossil energy source of our country. An energy production-consumption balance must be established as the short and mediumterm based on our resources.
In other words, it is important to reduce the share of fuels such as imported coal, gas and oil, which are met from the outside and to implement processes that respect the environment and produce less waste.
Assoc. Prof. Dr. Fırat Burat
Istanbul Technical University
Faculty of Mines
Mineral Processing Engineering Department
Recycling, Separation and Enrichment Group
Dr. Mühendis Mustafa Özer
Istanbul Technical University
Faculty of Mines
Mineral Processing Engineering Department
Recycling, Separation and Enrichment Group
