Sulfur in coal is a contaminant. Over the past few decades, growing concerns over the detrimental effects of acid rain and sulfur oxide emission on the environment due to coal combustion have led to the development of methods for reduction of sulfur in coal prior to combustion. However, for effective desulfurization of coal, knowledge of different forms of sulfur is required in addition to the total sulfur content. Accurate determination of forms of sulfur is imperative to evaluate the extent at which sulfur can be reduced in coal. The primary forms of sulfur in coal are pyritic sulfur (FeS2), sulfate sulfur (CaSO4. 2H2O, FeSO4. 7H2O) and organic sulfur (mercaptan, the others, disulfide and aromatic systems containing thiophene rings).Of the two predominant forms, pyrite and organic, the former is easier to remove by physical and chemical methods without altering the structure of coal. Organic sulfur, being a part of the macromolecular structure of coal, cannot be removed, analyzed or separated without the loss or destruction of the macromolecular network. As a result, pyritic sulfur concentration is used to assess the quantity of sulfur that can be removed from coal. The aim of this present work was to develop an easy and time-efficient method to determine the concentration of different forms of sulfur in coal.
Results and Discussion
According to ASTM D-2492 standard procedure, sulfate sulfur in coal is soluble in dilute HCl whereas pyritic and organic sulfur is not attacked. Moreover, dilute nitric acid dissolves sulfate and pyritic sulfur but affects organic sulfur slightly. The sulfate and pyritic sulfurs are determined directly while the organic sulfur is obtained by subtracting the total sulfur from the sum of the sulfate and pyritic sulfur. We have used a different time-efficient and less hazardous approach for determination of sulfate and pyritic sulfur as the ASTM or IS method is very tedious and time-consuming. The results were matched with that obtained using ASTM and are methods.
Total sulfur was determined using LECO sulfur analyzer.
To determine sulfate sulfur, the sample was boiled with saturated sodium carbonate solution for an hour. The solution was filtered and the filtrate was acidified with HCl. From the resulting solution, sulfate was precipitated as BaSO4 using BaCl2 and sulfur was calculated from the weight of BaSO4.
The precipitate obtained from the sulfate determination was boiled with dilute nitric acid for 30 minutes and the resulting solution was filtered. Boiling with nitric acid converts iron pyrite to ferric sulfate and sulphuric acid. As a result, sulfur was determined from the solution gravimetrically as barium sulfate by using barium chloride.
2FeS2+ 10HNO3 Fe2(SO4)3+ H2SO4+ 10NO + 4H2
Direct determination of organic sulfur is not possible without destruction of the coal structure as it is part of the coal network and no economically suitable process exists for the extraction and removal of all the components of organic sulfur. Therefore, organic sulfur was calculated by subtracting the value of the sum of sulfate and pyritic sulfur from total sulfur.
% Organic Sulphur = % Total Sulphur – (% Sulfate Sulphur + % Pyritic Sulphur)
|Sample||Sulfate sulfur||Pyritic sulfur||Organic sulfur|
|Lab method||ASTM||IS||Lab method||ASTM||IS||Lab method||ASTM||IS|
In this present work, we have developed an easy and time-efficient method for the determination of forms of sulfur in coal. The results are in good agreement with those obtained using ASTM and are methods. Since the organic sulfur was calculated from sulfate, pyritic and total sulfur content, accurate determination of the latter is very important. As such, we have repeated the experiments thrice and the reliability and repeatability of the method were quite good. Therefore, the method can be used successfully for the determination of forms of sulfur in coal in the near future.