The expected demand for hard coal intended for the households will progressively be decreasing. This is directly related to the introduced anti-smog resolutions, as well as the growing level of environmental awareness. However, it should be noted, that the use of the modern home heating boilers will result in an increase in the demand for medium coal sizes. The shortfall of this type of coal is already observed on the market. Therefore, its import is necessary. One of the solutions to increase the supply of the medium coal sizes is the production of coal briquettes. Moreover, their use will consequently lead to reduced emissions.
The paper presents a comparison of emissions from the combustion of coal briquettes and hard coal in home heating boilers. The briquettes were characterized by significantly lower emissions than hard coal (by 52% on average). The particulate matter emissions were lower by 70%. This may significantly contribute to improving air quality in Poland and in addition, limit the occurrence of smog. The possibility of further emission reduction by using low-emission fuels as briquette components was presented. The average relative emission reduction compared to hard coal for the analyzed fuels was estimated as follows: 62% for coal char, 57% for coke, 51% for charcoal/biocarbon, 49% for anthracite, 45% for torrefied biomass, and 33% for peat.
Furthermore, the issue of the mercury content in the analyzed fuels was discussed. The lowest mercury content was found in biomass fuels, in particular biomass after thermal treatment (torrefied biomass, biocarbon, and charcoal). Fuels produced from hard coal in the pyrolysis process (coal char and coke) were characterized by very low mercury content as well.
In the processes of coal mining, preparation and combustion, the rejects and by-products are generated. These are, among others, the rejects from the coal washing and dry deshaling processes as well as the coal combustion by-products (fly ash and slag). Current legal and industry regulations recommend determining the content of mercury in them. The regulations also define the acceptable content of mercury. The aim of the paper was to determine the mercury content in the rejects derived from the coal cleaning processes as well as in the combustion by-products in respect of their utilization. The mercury content in the representative samples of the rejects derived from the coal washing and dry deshaling processes as well as in the coal combustion by products derived from 8 coal-fired boilers was determined. The mercury content in the rejects from the coal washing process varied from 54 to 245 μg/kg, (the average of 98 μg/kg) and in the rejects from the dry deshaling process it varied from 76 to 310 μg/kg (the average of 148 μg/kg). The mercury content in the fly ash varied from 70 to 1420 μg/kg, (the average of 567 μg/kg) and in the slag it varied from 8 to 58 μg/kg (the average of 21 μg/kg). At the moment, in light of the regulations from the point of view of mercury content in the rejects from the coal preparation processes and in the coal combustion by-products, there are no significant barriers determining the way of their utilization. Nevertheless, in the future, regulations limiting the maximum content of mercury as well as the acceptable amount of leachable mercury may be introduced. Therefore, preparing for this situation by developing other alternative methods of using the rejects and by-products is recommended.
Węgiel kamienny wydobywany w kopalni (węgiel surowy) tworzy substancja organiczna i mineralna. Przed bezpośrednim wykorzystaniem urobek musi być poddany procesom wzbogacania. Stosowane procesy wzbogacania mają na celu usunięcie skały płonnej, pirytu, a także przerostów. Do wzbogacania węgla kamiennego stosowane mogą być zarówno metody wzbogacania na mokro, jak i na sucho. W pracy przedstawiono wybrane ekologiczne i ekonomiczne aspekty procesu suchego odkamieniania węgla kamiennego przy wykorzystaniu separatora powietrzno-wibracyjnego i separatora optyczno-rentgenowskiego. Zastosowanie nowoczesnych urządzeń do suchego odkamieniania, tj. separatory powietrzno-wibracyjne i separatory optyczno-rentgenowskie, umożliwią obniżenie emisji pierwiastków ekotoksycznych ze spalania węgla kamiennego. Wydzielenie pirytu pozwala na obniżenie zawartości siarki, a także innych pierwiastków ekotoksycznych, m.in. rtęci, arsenu, talu, czy ołowiu. Generalnie pod względem ekonomicznym technologia suchego odkamieniania cechuje się niższymi nakładami inwestycyjnymi i kosztami eksploatacyjnymi w porównaniu do metod wzbogacania na mokro. Instalacje suchego odkamieniania są dobrym rozwiązaniem dla inwestycji o krótkim okresie planowanej eksploatacji i/lub dla instalacji o małej wydajności, a także w przypadku ograniczonej dostępności do wody. Dla instalacji o dłuższym okresie eksploatacji i o wyższych wydajnościach, efektywność inwestycji jest wyższa dla metod wzbogacania na mokro. Istnieje również możliwość suchego odkamieniania węgla na dole w kopalni przy użyciu tzw. kruszarek Bradforda. Wydzielony produkt w postaci grubych kamieni może znaleźć zagospodarowanie na dole kopalni, np. do podsadzania wyrobisk.
Coal combustion processes are the main source of mercury emission to the environment in Poland. Mercury is emitted by both power and heating plants using hard and brown coals as well as in households. With an annual mercury emission in Poland at the level of 10 Mg, the households emit 0.6 Mg. In the paper, studies on the mercury release in the coal and biomass combustion process in household boilers were conducted. The mercury release factors were determined for that purpose. For the analyzed samples the mercury release factors ranged from 98.3 to 99.1% for hard coal and from 99.5% to 99.9% for biomass, respectively. Due to the high values of the determined factors, the amount of mercury released into the environment mainly depends on the mercury content in the combusted fuel. In light of the obtained results, the mercury content in the examined hard coals was 6 times higher than in the biomass (dry basis). Taking the calorific value of fuels into account, the difference in mercury content between coal and biomass decreased, but its content in coal was still 4 times higher. The mercury content determined in that way ranged from 0.7 to 1.7 μg/MJ for hard coal and from 0.1 to 0.5 μg/MJ for biomass, respectively. The main opportunity to decrease the mercury emissions from households is offered by the use of fuels with a mercury content that is as low as possible, as well as by a reduction of fuel consumption. The latter could be obtained by the use of modern boilers as well as by the thermo-modernization of buildings. It is also possible to partially reduce mercury emissions by using dust removal devices.
Nowadays, actions allowing for a reduction of anthropogenic mercury emission are taken worldwide. Great emphasis is placed on reducing mercury emission from the processes of energochemical coal conversion, mainly from the coal combustion processes. One of the methods which enable a reduction of anthropogenic mercury emission is the removal of mercury from coal before its conversion. It should be pointed out that mercury in hard coal may occur both in the organic and mineral matter. Therefore, a universal method should allow for the removal of mercury, combined in both ways, from coal. In the paper, a concept of the hybrid mercury removal process from hard coal was presented. The idea of the process is based on the combination of the coal cleaning process using wet or dry methods (first stage) and the thermal pretreatment process at a temperature in the range from 200 to 400 °C (second stage). In the first stage, a part of mercury occurring in the mineral matter is removed. In the second stage, a part of mercury occurring in the organic matter as well as in some inorganic constituents characterized by a relatively low temperature of mercury release is removed. Based on the results of the preliminary research, the effectiveness of the decrease in mercury content in coal in the hybrid process was estimated in the range from 36 to 75% with the average at the level of 58%. The effect of the decrease in mercury content in coal is much more significant when mercury content is referred to a low heating value of coal. So determined, the effectiveness was estimated in the range from 36 to 75% with the average at the level of 58%.
Work is being carried out on possibilities of limiting the content of mercury in hard coal products by gravity concentration of run-of-mine coal in the Branch of the Institute of Mechanized Construction and Rock Mining in Katowice and on the Faculty of Energy and Fuels of the AGH University of Science and Technology in Krakow. Under domestic industrial conditions, gravity concentration is carried out with heavy medium liquids and in jigs. Preliminary - pilot studies have shown the possibility of mercury removal also by using the dry deshaling method involving vibratory air separators. Mercury is mainly found in the pyrite and the rubble formed by the mineral carbon, but also in the organic carbon. Some of it is located in layers of coal roof fields, which in the course of their exploitation go to coal. The mercury removal efficiency during the gravity concentration process will depend on the decomposition of the listed components in the density fractions. The paper presents the results of investigations of total mercury and total sulphur content in the separated coal fractions from four mines. These contents were determined in fractions: –1.5 g/cm3 (conventionally clean coal – concentrate), 1.5–1.8 g/cm3 (conventionally middlings) and +1.8 g/cm3 (conventionally rock – waste). The results are summarized in Tables 3–5 and in Charts 1–4. Conversely, graphs 5-8 show the relationship between mercury content and total sulphur content in the tested coal samples. The study, which can be called a preliminary analysis of the susceptibility of the coals to gravity concentration, showed that the dry deshaling method on the vibratory air separators would allow significant amounts of mercury accumulated in the middlings and waste fractions to be removed.