At present and in the future the problem of native energy carriers supply of the country is one of the most urgent. It is connected not only with the oil and gas reserves in Ukraine but with the partial reduction of coal mining industry and closing down of old coal mines.

   With these conditions, searching for new alternative energy resources takes on major significance, in particular, involving the so-called salting coals (considerable supplies of which have been found in the West and North Donbas) in the fuel and energy balance.

   Salting or ‘alkaline’ coals are coals with abnormally high alkaline metals content, mainly sodium. The essence of the salting coals problem lies in several undesirable technological and ecological consequences of their treatment, connected with detrimental impurities occurrence in the coals. These detrimental impurities are fusible chemical compounds of sodium and chlorine (mainly halite).

The essence of salting coals problem

    he main difficulties connected with usage of alkaline coal fields lie in the following. Firstly, they are conditioned by the scorifying power of salting coals. Depositing on the work surfaces of steam generating units, fusible chemical compounds containing in salting coals make the conditions for forming a firm slag blanket, which cannot be removed with the help of the usual procedures of purification.

   The second negative phenomenon that happens during salting coals combustion is intense metal corrosion, which increases with a rise in temperature and sodium chloride content in the coal.

    Thirdly, the ecological problem of salting coals is of great importance. During their combustion hydrochloric acid, molecular chlorine and other chemical compounds, which have different levels of toxicity, escape into the atmosphere.

World’s experience of salting coals treatment

   There are two fundamentally different lines of investigation of effective and environmentally friendly methods of salting coals treatment, which are being carried out at present. The first one assumes development of new ways of salting coals direct use without sodium and chlorinated addition agents removal. The second line assumes preliminary desalting with the subsequent use of the conditioned product in furnaces.

   The first line of research includes salting coals combustion, gasification, liquefaction, semicoking and their complex treatment with humate escape.

   The second line of research, which provides for preliminary coal cleaning from salt (desalting), has been done in several scientific research centres. Most of the suggested solutions are based on different variants of salting coal wet washing.

Processes and methods of salting coals direct treatment

   1. Salting coals gasification technologies.

   Researchers of Dnepropetrovsk Chemical – Technological Institute have done the research into salting coals usefulness of Novomoskovsk coal field in order to get reformed fuel by gasification. The basic method used is the method of coal gasification in the boiling bed under pressure.

While using this method, significant corrosion of different grades was observed caused by aggressive effect of water steam enriched with sulfureted, alkaline components and chlorine compounds. This circumstance is an essential obstacle, which restrains the application of this technology to practice.

   2. Technology of salting coal combustion.

    It is aimed at reduction of chlorinated gases content in salting coals combustion materials down to the amount received with non-salting coals and the corresponding corrosion reduction of the furnace heating surfaces. The experiments have showed the effectiveness of the process.

   3. Technologies of salting coals complex treatment.

   The engineering solution of the suggested technology includes three stages of processing. The first one – extraction of humic acid salts. The second one – carbonization of coal without humic acid. The third stage is gasification of the coal without humic acid carbonate with carbon dioxide in order to get reducing gases. The conducted tests have proved the prospects of the technology.

Domestic research in the field of coals desalting.

   1. SIMR technology of coal desalting by wet washing.

    Scientists of State Institute of Mineral Resources have developed the technology of coal desalting by wet washing which presupposes usage of the production-run equipment for concentration plants in the desalting process.

   2. RPA “Hymec” ‘desalting-hydrotransport’ technology.

   The technology presupposes a combination of hydrotransport and desalting technologies. The disadvantages of the technology are, first of all, the detected effect of secondary salt concentration in the coal after 50 km of its hydrotransportation and, secondly, the intense breakage of the salting coal during hydrotransportation that causes the rise of class content – 0,074 mm up to 35-40%.

DonNTU ‘wet washing – oil agglomeration’ technology

    Researchers of Donetsk National Technical University have developed an improved technology of coal desalting, which is based on the above mentioned technologies but which eliminates their defects. The main idea of this way is the combination of ‘wet washing’ and ‘oil agglomeration’ processes. The key difference of the technology is fractional reagent delivery into the process.

   The process flowsheet includes two-stage crushing with preliminary screening, the first stage of reagent delivery to hammer crusher feeding, desalting in conditioners, the second stage of reagent delivery to coal-supply line and filtration in centrifuges or vacuum filters. The coal agglomerate after dehydration takes ejection.

   Due to the application of an organic binding reagent, the process flowsheet of coal dehydration becomes considerably simpler and the equipment decreases in the number.

Fingings

   The theoretical study has allowed formulating some basic principles and specific features of coal desalting process using various technologies. The elements of the theory of coal desalting in aquatic environment developed by the researches of DonNTU are as follow:

   1. The analysis of an elementary event of coal-water contact has showed that maximal hydration of salting coal is possible on the external surface and in macropores of the coal phase.

   2. Effective salt extraction is possible only when the salts are extracted out of coal macropores and fractures of the coal substance. Coal internal surface opening is necessary to involve dissolving of the salt components contained in meso- and micripores.

   3. The theoretical interpretation has been put on the discovered phenomenon of the secondary salt accumulation at the coal stage during combined ‘washing-hydrotransport’ process.

   4. The equation of desalting effectiveness evaluation through the parameters of coal dehydration, salt concentration in the source coal, solution and suspension fluidity has been proposed.

   5. The combined ‘coal washing desalting – oil agglomeration’ technology has been substantiated in theory.

Links

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