Energy and Coal
COAL FORMATION
Coal is a sedimentary rock composed of plant-derived organic substances and inorganic compounds. Coal is formed due to the successive accumulation and sedimentation of plant and tree residues and their alteration by chemical and physical effects over a period of millions of years. Coal can also be defined as sedimentary rock composed of organic and inorganic compounds formed due to the compression and solidification of plant residues and inorganic minerals under high pressure and temperature.
Coal is formed with a process called coalification consisting of some physical (pressure, precipitation, etc.) and chemical (heat, deterioration, and transformation, etc.) events. The formation process of coal can be briefly summarized as follows: The first phase is the accumulation and precipitation stage of plant communities, the second phase is the stage where biochemical and geochemical deteriorations become available, and the third phase is the stage where physical, petrographic and chemical properties of coal are formed by thermochemical transformations (Figure 1).
Geological time is one of the critical parameters in the coalification process, and the carboniferous, permian, and crater-tertiary periods are the periods when the coalification process is active.
The degree of carbonization is called Rank, and the one with the lowest degree of carbonization, i.e. coalification, (rank) is peat coal. This coal type comprises incompletely solidified, light brown, porous organic sediments with high water content.
Coal types formed over a more extended period than peat coal are lignite, sub-bituminous coal, hard coal, and anthracite. Their hardness values also increase in this order, and peat coal is the youngest, and anthracite is the oldest coal formation. Quality and thermal values of coal increase with age as the rank increases. The formation of coal and the related processes are illustrated in Figure 2.
In brief, the proportion of inorganic substances in coal content is always desired to be low regardless of coal’s utilization area. Coal consists of nonhomogeneous microscopic organic (maceral) and inorganic (mineral) particles. Maceral, i.e. organic substances, consists of sub-groups such as vitrinite (huminite), liptinite (rezinite), inertinite (fusinite). It has more carbon (C) and less hydrogen (H), oxygen (O), sulfur (S), and nitrogen (N) elements by percentage. Mineral or inorganic substances include clay minerals, quartz, pyrite, calcite, and dolomite.
PHYSICAL AND CHEMICAL PROPERTIES OF COAL
PHYSICAL PROPERTIES OF COAL
Density: Inorganic substances and moisture content change the density values. Coal density ranges between 1.1 and 2.2 gr/cm3. The density of peat coal is generally assumed to be 1.0 gr/cm3. Porosity: Its value changes from 3% to 25% according to coal rank, i.e. degree of coalification. Gas absorption: Coal can absorb water vapor, ethyl alcohol, and benzene vapor at room temperature. Lignite coal is suitable for the absorption of gas and vapor in virtue of its chemical structure.
Run-off-mine lignite coal can absorb air and CO2, 1.5 times its volume. The reflectivity of Coal: This value is assigned according to the actual coalification degree of basin coal. Fluorescence Property of Coal: This property is in a reverse relation with reflectivity. Peat coal with low reflectivity has a high fluorescence value. The substances with high fluorescence values appear in lighter colors (light green, etc.), while those with low fluorescence values appear in darker colors (red, dark brown, etc.). It is generally utilized to distinguish liptinites from inorganic substances (clay, etc.) and detail their properties.
CHEMICAL PROPERTIES OF COAL
Coking: Coal formations with a high coalification, i.e. degree of coalification (hard coal), soften first under heat treatment. Afterward, they release their gas content by swelling and harden again. The highly porous and light substance obtained from these treatments is called "coke". Moisture (Humidity) content: Variety in coal moisture content can be explained regarding that the functional groups in hydrophilic characteristics decrease to negligible values with the advance in coalification level. Coal has different moisture measures such as body moisture, rough moisture, and molecular water.
Its value is calculated by taking the difference between the weights of coal before and after heating it to 105-1100C in special furnaces. Volatile substance content: When coal is heated in an oxygen-free environment, it is exposed to chemical changes, and volatile substances in coal content, including non-combustible gases such as carbon dioxide and water vapor, and tar vapor are released. The gaseous and liquid substances released upon the heating process are called volatile matters of coal, where the proportion of volatile matter in weight is called the volatile matter ratio. It is calculated by measuring the difference between the coal weights before and after heating it to 950±250C (the USA, ASTM standard). Ash Content: When a coal-burning process is completed, the mineral substances contained in coal content undergo some fundamental changes, and inorganic waste ash is formed as a result. The ash content in terms of % determines the coal quality, and the quality decreases as the ash content increases. Ash content classification for dry samples is as follows:
- < 5% very low,
- 5-10% low,
- 10-20% moderate,
- 20-30% high,
- 30-50% very high.
Fixed Carbon Content: This property cannot be determined directly. It is calculated indirectly by subtracting the sum of the percentage values of moisture, ash, and volatile matter from 100% (% water + % mineral substance + % volatile matter + % fixed carbon (C) = 100%). Coal Calorific Value: Calorific value of a fuel type is the amount of heat released from the complete combustion of a unit mass of fuel. Carbon and Hydrogen Content: The organic substance of coal contains carbon (C) 70-95% and hydrogen (H) 2-6% by weight. Sulfur Content: It is one of the parameters determining the coal quality. The sulfur types contained in coal are organic and inorganic (sulfite and sulfate) sulfurs. The organic sulfur proportion by weight is less than 3%, while the "S" ratio of the sulfates is generally less than 0.1%. In calculating the "S" amount, the coal is burned at 13,500C, and the S amount in the coal is converted to SO2. Then, SO2 is converted to sulfuric acid, and the total amount of S is calculated from the obtained sulfuric acid. Nitrogen content: Coal's nitrogen content is generally related to protein content and originates from plants rich in nitrogen content.
The nitrogen content in coal does not variate proportionally with the age of the coal. Nitrogen content is determined by the Kjeldahl method. Using sulfuric acid, nitrogen is converted to ammonium sulfate, and the amount of resultant ammonium sulfate is calculated. Oxygen Content: It is available in organic substances of coal, water, clay, and carbonate minerals. The amount of organic oxygen determines coal rank, i.e. the degree of coalification (carbonization). Lignite, hard coal, and anthracite contain 25%, 10%, and 3% oxygen on average, respectively (Ediger, 2014). Calculation of the oxygen content by Ediger (2014) is presented as follows:
O2 (%) = 100 – (C + H + N + S org.)%
COAL IN THE WORLD AND IN TURKEY
Global coal production has nearly doubled in the recent thirty-five years. The growth in coal production is mainly due to the electrical energy demand in Asian countries, especially China. In the last ten years, the total electrical energy production in the Asia-Pacific Region has been doubled, and coal has been used as the primary source in electricity generation intensively.
When the global distribution of coal resource types is investigated, it is observed that bituminous coal and anthracite have the highest share with 45%, and followed by sub-bituminous coal with 32% and lignite with 23% (Figure 4).
With a continuous upward trend for 14 years from 1998 to 2013, global coal production decreased by 5.3% (7,324 million tonnes) in 2016 compared to the previous year. However, the production again increased by 3.26% in 2017 by achieving a total coal production of 7,563 million tonnes (IEA, 2019). The increase rate of production from 2000 to 2014 is around 73%.
In the same observation period, steam coal and coking coal production increased by 85.4% and 77.7%, respectively. On the other hand, lignite production dropped by 4% (Figure 5) (IEA, 2019).
China alone accounted for 45% (3,397.2 million tonnes) of the overall coal production in 2017. India, which took over second place from the USA, achieved a share of 10% (725.5 million tonnes). The USA had a share of 9% (702.7 million tonnes) followed by Australia with a share of 7% (499.5 million tonnes).
The rest of the production share list is as follows: Indonesia (494.7 million tonnes), the Russian Federation (387.7 million tonnes), the Republic of South Africa (256.8 million tonnes), and Germany (175.1 million tonnes) (Figure 6). The total share of these eight countries in global coal production is close to 90%.
In terms of coal resource and production amounts, Turkey can be evaluated at a moderate level in lignite and a low level in hard coal (anthracite). The most significant hard coal resource of Turkey is located in and around Zonguldak. According to the up-to-date reports of MTA, the hard coal resource of Turkey is approximately 1.52 billion tonnes, and 736 million tonnes of the total resource can be classified as a measured resource (TTK, 2020).
Since the feasibility studies have been completed only for one-third of the coal resources, a small part can be considered reserve. An observable increase in exploration and reserve development studies has been achieved in recent years. The total coal resource in Turkey is approximately 20.84 billion tonnes (MTA, 2020; TKI 2020) (Figure 7)
Approximately 19.32 billion tonnes of the total coal resource is classified as lignite. Its sectoral distribution can be examined in Figure 8.
ENERGY
Energy is briefly defined as the triggering power or the capability to do work. Every production activity requires a certain amount of energy. Sources capable of producing energy with various methods and techniques are called energy sources, and they are classified in different ways:
A- Energy Sources According to Sustainability Aspect (permanence, depletion): It is a classification type of energy sources according to their renewability aspects.
1- Renewable (Alternative) Energy Sources: They are renewable energy sources such as solar, wind, hydraulic, geothermal, biomass, hydrogen, wave, and tidal energies.
2- Nonrenewable (Fossil, Conventional) Energy Sources: These are also called primary sources or conventional sources. They are characterized as nonrenewable and can be used one time or depleted. Sources such as coal, bituminous shale, fuel oil, natural gas, uranium, and thorium are included in this group.
B- Energy Sources According to Their Convertibility: It is the classification type regarding whether sources can be directly or indirectly used as an energy source.
1- Primary Energy Sources: Sources that can produce energy directly without changing their main characteristics when used are included in this group. Coal, nuclear, biomass, hydraulic, and wave energy are examples of primary energy sources.
2- Secondary Energy Sources: Sources that need conversion to another energy source for utilization are included in this category. Electricity, gasoline, diesel, fuel oil, coke, and LPG are examples of secondary energy sources.
C- Energy Sources According to Their Being Located at Underground/Surface: Energy sources can be classified whether they are located originally underground or surface.
1- Underground Energy Sources: Sources such as coal, fuel oil, natural gas, geothermal, bituminous shale, and nuclear (radioactive) sources are included in this group.
2- Surface Energy Sources: Sources such as solar, wind, and biomass are examples of surface energy sources.
D- Energy Sources by Physical State: It is the classification type regarding the physical state of a source at room condition.
1- Solid Energy Sources: Sources such as coal, wood, biomass waste, and uranium.
2- Liquid Energy Sources: Sources such as petroleum, LPG, diesel, and biodiesel.
3- Gaseous Energy Sources: Sources such as natural gas, methane gas, and biogas.
PRIMARY ENERGY SOURCES AND COAL
Primary energy supply increased 2.2 times globally for 44 years between 1973 and 2017 and reached 13,511 mtoe (million tons equivalent oil) in 2017. The total energy supply in 2017 increased by 2% compared to the previous year (BP, 2018). The energy supply increased by 34% between 2000 and 2017, where about 75% of this growth was stemmed from the Asian countries.
In these 17 years, energy supply had a jump by 169% in China and 87% in India, while it decreased by 7.7% in the European Union (EU) and 2% in the USA (Figure 8) (BP, 2018). The most crucial change in the energy supply is apparently based on that the primary energy supply decreased or remained stable in the developed countries, whereas it increased continuously in Asia-Pacific countries, especially China and India. In the period between 1973 and 2017, the percentile share of natural gas increased from 16% to 23%, the share of nuclear energy increased from 0.9% to 4%, and the share of renewable energy sources increased from 1.9% to 4%, while the percentile share of petroleum decreased from 46.2% to 34% (BP 2018, p.9). In the same period, the percentage of coal increased 3.5 points from 24.5% to 28% (Figure 9).
It is observed from the distribution of primary energy sources supplied globally in 2017 that petroleum has ranked first with a share of 34%, followed by coal with 28% and natural gas with 23%.
Other sources are hydraulic with 7%, renewable with 4%, and nuclear with 4%, respectively (Figure 10).
The most observable development between 2000 and 2017 is related to the weight of coal in the total energy supply. At this point, the share of petroleum decreased from 36.5% to 34%, the share of nuclear decreased from 6.7% to 4%, while the share of natural gas increased by 2.4 points from 20.6% to 23%, and the share of coal increased by 4.9 points from 23.1% to 28% (Figure 11) (BP, 2018).
The International Energy Agency estimates that the world's primary source supply will increase by 45.3% between 2017 and 2040, reaching the level of 19,637 mtoe, and there will not be significant variations in the source distribution.
On the other hand, according to the Current Policies Scenario, it is forecasted that the shares of petroleum and coal will be 27.5% and 27.1, respectively. These sources will be followed by natural gas with a share of 24%. In addition, in 2040, nuclear energy and other sources are expected to have a share of 5.3% and 16.1%, respectively (Figure 12) (IEA, 2018).
In Turkey, energy production has increased by 34%, and energy consumption has also increased by 39.7% in the recent decade. The percentage of the domestic output to meet the energy consumption increased from 26.4% to 27.6% between 2008 and 2018.
The share of domestic coal production to meet energy consumption was 10.38% in 2017 and 11.52% in 2018. Since domestic energy production can meet the energy consumption at a low rate, it has been inevitable to increase the rate of energy import.
In 2018, 27.6% of Turkey’s energy consumption was met by domestic energy sources, while 72.4% of the consumption, which is a significant portion, was provided by the import energy sources (Figure 14).
References: 1. Ünalan, G., 2010; Kömür Jeolojisi. 2. Ediger, V.Ş, 2014; TKİ ve Kömürün Tarihçesi ile Türkiye Kömür Stratejileri. 3. TKİ 2018 Kömür Sektör Raporu. 4. TKİ 2019 Faaliyet Raporu. 5. EİGM, 2019; Denge Tabloları; retrieved from https://www.eigm.gov.tr/tr-TR/Denge-Tablolari/Denge-Tablolari. 6. BP. (2019). Statistical Review of World Energy 2019, 68th edition. BP.
Retrieved from https://www.bp.com/en/global/corporate/energy-economics/statistical-review-of-world-energy.html 7. IEA. (2020, 07 7). Headline Energy Data. Retrieved from IEA: https://www.iea.org/subscribe-to-data-services/world-energy-balances-and-statistics. Prepared by: Zeki OLGUN – Head of the Department of Research, Planning and Coordination (APK) Metin AKTAN – Head of the Directorate for Strategy Development Neslihan UÇAR – Mining Engineer July 2020.
