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.
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%.
In recent years, more and more attention has been paid to the quality of produced coal size categories for energy purposes. This is important from the perspective of promoting clean coal technologies which aim at changing the perception of coal as a fuel friendly for the environment. This is specifically because hard coal resources in Poland allow the national energy security to be guaranteed on the basis of energy production based on hard coal. Fine coals upgraded at coal processing facilities in the separation process in fine coal jigs are mainly used in energy production from coal. In the article, an analysis of hard coal upgrading in a jig regarding the optimum recovery of a useful fraction in the concentrate (combustible and volatile matter) and non-useful fraction in tailings (ash and sulfur) was conducted. Based on the industrial testing of a fine coal jig, the granulometric and densimetric analysis of the taken samples of concentrate, middlings and tailings of coal was conducted in laboratory conditions. Yields of products were calculated in separated size-fractions of separation products, and ash content and total sulfur content were determined in them. Based on the results of granulometric, densimetric and chemical analyses of the obtained size-fractions, the balance of separation products and appropriate calculations, Fuerstenau upgrading curves which allowed the process to be evaluated and a comparison of the results of hard coal upgrading regarding the optimum recovery of the organic phase in the concentrate and mineral components in tailings to be drawn. The obtained results were evaluated on the basis of different criteria for changing the device’s hydrodynamic operational conditions. The ash content and total sulfur content were analyzed as non-useful substances.