Clean Coal Technologies
According to the research carried out by World Energy Council, the mineable coal reserve in the world is around 900 billion tonnes. In the total reserve amount, anthracite and bituminous coal, sub-bituminous coal, and lignite coal have a reserve of 400 billion, 290 billion, and 210 billion tonnes, respectively.
Considering the total coal production in 2018, the remaining coal reserve worldwide is expected to have a lifetime of 135 years. As a result of a substantial increase in coal exploration activities carried out in recent years, the total lignite resource in Turkey has been increased significantly. By the end of 2019, Turkey’s total coal resource is around 21 billion tonnes, including lignite, asphaltite, and hard coal (MTA, 2019) (TTK, 2018). Since the feasibility studies have been completed only for one-third of the coal resource, a minor part of the total resource could be classified as a reserve.
Turkey is at the low level in hard coal and mid-level in lignite coal in terms of reserve and production amounts worldwide. Although Turkey has 19 billion tonnes of lignite coal, which is quite high in terms of reserve amount, the coal quality and specification show a variation with different ranges in calorific value. At this point, 2% of the lignite reserve is with below 1000 kcal/kg, 65% of the reserve is between 1000 and 2000 kcal/kg, 24% of the reserve is between 2000 and 3000 kcal/kg, 6% of the reserve is between 3000 and 4000 kcal/kg, and 3% of the reserves is above 4000 kcal/kg. In brief, the lignite reserve generally has low calorific value, high ash content, and high humidity (moisture) content. In addition, 40% of the reserve has sulfur content higher than 2%. Therefore, the lignite reserve in Turkey is preferably used in thermal power plants due to its low quality (1).
In the future, lignite reserves can be used effectively considering environmental, financial, and sales parameters (sustainable and long-lasting) only if they can be converted into high value-added products by reducing lignite-induced carbon emission and environmental pollution via national and innovative technologies.
The technologies necessary to achieve these targets are called “Clean Coal Technologies”. Clean Coal Technologies can be defined as technologies that can minimize the environmental impacts of coal in preparation and usage stages and increase the efficiency and added value of the final product.
Using these technologies, it is possible to reduce coal-induced emissions and waste and increase the amount of energy and product per unit tonne of coal. Clean coal technologies can be classified into four main groups:
1. Technologies related to Preparation/Beneficiation/Concentration/Liquefaction of Coal:
These technologies are generally referred with the names of crushing/grinding/screening/sizing, wet or dry washing, coal-water mixtures, dewatering/drying, liquefaction, and briquetting.
Since these technologies improve coal content by reducing humidity, sulfur, and ash and increasing quality and calorific value, the final product becomes more environment-friendly, and transportation and treatment costs are decreased.
The most commonly used technology in coal's beneficiation or washing process is the gravitational separation of impurities such as clay and stone in the coal washing plants. The photo shows a coal washing plant located in the license area of ELI Soma.
2. Advanced Technologies related to Coal Gasification, Integrated Gasification Combined Cycle Technologies (IGCC), Underground Gasification:
Using coal gasification technologies, synthesis gas is obtained by performing partial combustion (oxidation) instead of full combustion. In this gasification process carried out in surface reactors, sources such as carboniferous coal, biomass/wastes, and natural gas are converted to artificial gas called synthesis gas, having the main components of carbon monoxide (CO), hydrogen (H2), and methane (CH4).
Differently from other combustion technologies, synthesis gas is obtained from the release of sulfur and hydrogen sulfide (H2S), and chemicals such as elemental sulfur or sulfuric acid are produced in this process. In other words, they cease to be pollutants and turn into products. Other pollutants such as particles, nitrous oxide (NOx), carbon oxysulfide (COS), and carbon dioxide (CO2) can also be retained or stored at 95-99%.
There is also a potential to generate electricity from the produced synthesis gas by applying various chemicals (methanol, dimethyl ether, hydrogen, olefins, ammonia, etc.), liquid fuels (including jet fuel), and gas engine or IGCC technology.
Therefore, it is observed that almost all of the products produced from petroleum can also be produced from coal. In the coal gasification technologies with an increased usage rate in the world in recent years, the production of various chemical and liquid fuels and the electricity conversion efficiency are growing. At the same time, the air pollutant emission values remain at very low levels. In addition, since they require a few amounts of water, produce a minor amount of solid waste, and have high value-added product potential, these advantages will further increase the significance and utilization rate of the gasification process in clean coal technologies in the future.
The primary types of gasifiers are as follows: a) Fixed/Moving bed gasifier (downdraft, updraft), b) Fluidized bed gasifier, c) Entrained bed gasifier, d) Melt-type gasifier, e) Plasma gasifier. TKI established a pilot gasification plant in the type of moving bed gasifier within the operation area of GLI Tuncbilek, with a capacity of 250 kg/h. This type of gasification plant is extensively used in the world. In this plant, some pilot-scale R&D studies on gasification of coal, gas purification, electricity generation with a gas engine, and methanol production are performed using the coal samples gathered from the sites of TKI and other fields. (3)
Another project conducted on coal gasification in TKI Soma is the project entitled “Liquid Fuel Production from Biamass and Coal Mixtures – TRIJEN” supported by TUBITAK-KAMAG-1007.
In this project, a national clean coal technology research for obtaining synthesis gas, conditioning of gas after purification, and achieving 6-7 barrels of diesel fuel per day with proper catalyzers is performed by feeding 250 kg/h of coal and biomass into a circulating fluidized bed-type gasification plant.
Moving and fluidized gasifiers are the most common types of gasifiers and have different production targets and capacities. TRIJEN plant established in Soma can be examined from figure (2).
Gasification application by injecting air or oxygen directly into the coal seams without extracting coal and conveying the resultant gas to the surface is called Underground Coal Gasification (UCG). It is possible to produce value-added products and generate electricity using this technology, similar to surface gasification technologies.
3. Technologies related to Improving Thermal Plant Efficiency and Emission Control:
These technologies are called High-Efficiency Low Emission (HELE) technologies and help to retain SO2 using flue gas desulfurization systems at 90-97%, NOx using various NOx retaining technologies at 90-95%, and particulate matter (PM) using Electrostatic Precipitator (ESP) technologies up to 99% efficiency.
4. Technologies to Capture, Use, and Store Carbon Dioxide (CCS/CCUS):
As a part of the fight against climate change, especially in achieving the goals of the Paris Agreement, these types of technologies have been studied extensively. They are critical in the future of coal mining concerning its capability to reduce CO2 emission to zero. Various CO2 capturing and storing systems via coal gasification are available. It can be stated that Coal Gasification Technologies are integrated into carbon dioxide capturing technologies.
Motivation and implementation of gasification technologies in Turkey may help to save billions of USD in import expenditures. The gasification technology used extensively in many countries since 1995 became one of the most developing energy technologies. The number of gasifiers in 600 gasification plants, operated commercially in 28 countries, is about 850.
Percentile shares of coal, petroleum coke, and biomass/waste gasified in the gasification plants, with a total installed power of 150,000 MWt in 2015, are 49%, 36%, and 15%, respectively. The statistics on the utilization of the synthesis gas produced in these plants can be examined in Figure 2. It is foreseen that the total installed power of the gasification plants will be 370,000 MWt by the end of 2020.
Ammonia, the primary raw material of chemical fertilizers, is the most produced chemical among the products obtained from synthesis gas, and it is followed by methanol. These two are the products that Turkey imports almost entirely and increase the current account deficit.
