Bazı Türk linyitlerinin kükürt içeriklerinin azaltılması

Abstract

Birincil enerji kaynağımız durumundaki linyitlerimiz yüksek oranda kükürt içermektedir. Isıl değerlerinin düşük olması nedeniyle çok miktarda yakılması, atmosferdeki kükürt dioksit emisyonlarının hızla artmasına neden olmaktadır. Atmosferdeki kükürt dioksit emisyonlarının belirli değerlere ulaş ması, canlı hayatı için tehlike oluşturmaktadır. Ekolojik dengeyi bozan etken lerden biri olan asit yağmurları da büyük ölçüde atmosferdeki kükürt dioksit emisyonlanndan kaynaklanmaktadır. Kaliteli kömürlerimizin rezervinin gitgide azalması ve bu kömürlerin kalan rezervlerine zor ulaşılması nedeniyle madenciliği güçleşmektedir. Geçmişte düşük kalitesi nedeniyle pek rağbet görmemiş kömürlerin özellikle büyük şehirlerde yaygın olarak yakılması, hava kalitesinin düşmesinin önde gelen nedenlerinden biridir. Atmosfere bırakılan kükürt dioksit miktannın azaltılması amacıyla geliş tirilmiş bazı önlemler mevcuttur. Bu önlemler arasında kükürt dioksit tutma özelliğine sahip sorbentlerin de kullanıldığı akışkan yataklı yakma sistemlerinin kullanılması ve baca gazından kükürt dioksit gideren süreçlerin uygulanması yer almaktadır. Ülkemizde küçük ölçekli sanayinin yaygın olması ve konutlann büyük bölümünde bireysel ısınmanın söz konusu olması nedeniyle sözü edilen bu yöntemlerin şu an için yaygın olarak uygulanması mümkün görülmemek tedir. Kömürün içerdiği kükürtlü bileşiklerin yanma öncesinde kömürden uzaklaştınlması, diğer yöntemlere göre ülkemiz koşullanna daha uygundur. Fiziksel, kimyasal veya biyolojik yollarla kömürdeki kükürtlü bileşiklerin uzak laştınlması mümkündür. Biyolojik ve fiziksel yolla kömürün organik yapısı ile bağ oluşturmamış ve kömür içinde çok ince boyutta dağılmamış kükürt uzaklaştırabilmektedir. Linyitlerimizin içerdiği toplam kükürt içerisinde organik kükürdün büyük paya sahip olması nedeniyle, yeterli miktarda kükürtsüzleştir- menin sadece kimyasal yolla gerçekleştirilebileceği görülmektedir. Pahalı kimyasalların kullanıldığı kimyasal yolla kükürt giderme yöntemlerinin uygulanabilme olasılığı yoktur. Bu çalışmada Gediz, Göynük ve Tunçbilek yörelerinden alınmış olan üç ayrı linyit numunesinin, esas reaktifleri oksijen ve su olan kimyasal bir süreç yardımıyla kükürt içeriklerinin giderilmesi araştırılmıştır. Ortama ilave edilen trona, boraks, uçucu kül, sodyum hidroksit, amonyum hidroksit ve kalsiyum hidroksit gibi kolayca temin edilebilen maddeler yardımıyla etkin bir şekilde kükürt giderme sağlanmıştır. Kömürden kükürt giderme işleminin kinetiği oluşturulmuş ve kükürt giderme işleminden geçirilmiş kömürün yanma özellikleri incelenmiştir.

Sulphur, in the form of its element or combined with other elements, is a nutrient for both plant and animal life. However, in recycling sulphur back to nature, ecological soundness requires that there must be no excess at any given point in the cycle. In general, the fate of sulphur dioxide emissions involves photo-oxidation in the atmosphere to form sulphur trioxide which, under humidifying conditions, becomes sulphuric acid or sulphates aerosol. Residual sulphur dioxide and sulphuric acid or sulphates are scavenged from the atmosphere by vegetation, eventually being discharged into the sea by rivers, along with sulphur accumulated from weathering rocks and sulphur applied as fertilizer. Some of the sulphate is directly deposited into the ocean by rain or dust. When an excess occurs, the atmosphere to land portion of the sulphur cycle is unsound. The sulphur in coal is classified into organic and inorganic sulphur. The organic sulphur is chemically bonded to the hydrocarbon matrix of the coal while the inorganic sulphur is distributed in the coal, mainly as loose of pyrite. The inorganic sulphur occurs as iron disulphides, FeS2, with a small amount occurring as sulphates., in the form of szomolnokite (FeS04H20), rozenite (FeS044H20), melanterite (FeS047H20), coquimbite (Fe2(S04)39H20), roemerite (FeS04Fe(S04)314H20), jarosite ((Na,K)Fe3(S04)2(OH)6) and halotrichite (FeAbCSO^^^O). The iron sulphate is soluble in water and its concentration increases with the length of exposure of the coal to air. The disulphides appear as small crystals that are very widely distributed in the coal substance. They exist in two crystalline forms, pyrite and marcasite. The pyrite has a cubic structure with a specific density of 5.0, and the marcasite is rhombic with a specific density of 4.87. While the marcasite crystals are intimately coated with coal substance, the pyrite is generally present in the coal as loose particles. The pyrite is more stable and slightly less reactive than marcasite. Organic sulphur in coal is categorized according to the type of functional group in which it appears. There are five functional groups: 1- Mercaptan or thiol, RSH, 2- Sulphide or thio-ether, RSR', 3- Disulphide, RSSR', 4- Aromatic systems, 5- Y-thiopyrone systems ??> Where R and R' designate alkyl or aryl groups. Thiol and disulphide are likely secondary products because they are thermally rather unstable. The incorporation of sulphur in coal is a four-step mechanism: 1- Contact between the organic coal substance and sulphates, 2- Ingress of ferruginous solution 3- Formation of iron sulphates, and 4- Transformation of iron sulphates to pyrite and organic sulphur. The combustion of coal for the generation of electric power and process heat makes necessary the control of pollutants which are potentially harmful to human being or the environment. Emissions of sulphur dioxide from coal combustion may be controlled by one or more of the following alternatives: 1- The use of low sulphur content coal; 2- The pretreatment of coal to remove sulphur; 3- The retention of sulphur during combustion; 4- The post-combustion treatment of flue gases; 5- The conversion of coal into a liquid or gaseous form. Reserves of coals with low sulphur content are limited. The costs of coal conversion processes preclude the use of coal derived liquid or gaseous fuels. Processes which retain sulphur during combustion are currently being developed. The sulphur retention property of some sorbent materials such as limestone and dolomite is being employed in fluidised bed combustion. The most widely adopted method of controlling emissions of sulphur dioxide is flue gas desulphurization (FGD). However, the reliability of flue gas scrubbers is often such that two or three systems in series are required to ensure sufficient control of sulphur dioxide. The cost of flue gas desulphurization systems xvixi represents a significant proportion of the equipment cost of a coal fired power station. This proportion increases as the size of the plant decreases making FGD prohibitively expensive for the small industrial coal fired boiler. The remaining option for controlling sulphur emissions is that of pretreating coal to remove sulphur before combustion. Cleaning of coal before combustion is performed by applying of one or more of the physical, chemical and biological methods. The biological sulphur removal from coal bases on the attack of a micro-organism to the sulphur content of coal. Two groups of micro-organisms are involved in the removal of sulphur from coal. One group of microbes functions at near room temperature while the other group is strictly thermophilic. Incapability for the organic sulphur removal, requirement of long time and water pollution are some of the problems with the biological sulphur removal. The well-known physical cleaning methods can be classified as float/sink separation, hydraulic and pneumatic methods, separation based on surface properties, selective oil agglomeration, solvent partitioning, magnetic cleaning and electrostatic separation. Since the density of the organic matter in coal is considerably lower than that of the pyrite, the two materials are frequently separated by adding crushed coal to a liquid medium of intermediate density. The organic coal particles float because they are more dense than the medium. Hydraulic methods which employ jigs, wet concentration tables hydrocyclones and other types of equipment are the most widely used for cleaning coal. Several coal cleaning methods including froth flotation, oil agglomeration, and solvent partitioning take advantage of the differences between the organic and inorganic components of coal. In the froth flotation method, a suspension of fine coal particles in water is aerated with air bubbles. The hydrophobic coal particles become attached to these bubbles and are moved to surface of the suspension where they are recovered in a froth. In the oil agglomeration method, a small amount of fuel oil is added to an agitated suspension of coal particles in water to coat the hydrophobic particles selectively. The oil-coated particles stick together and form relatively large floes or agglomerates which can be separated from the unagglomerated particles by screening the suspension. In the solvent partitioning method, a relatively large amount of an organic solvent is mixed with a suspension of coal in water. The organic particles are transferred to the solvent phase. The liquids can be separated by decantation. XiX Since the organic material in coal is diamagnetic, whereas pyrite is slightly paramagnetic, the separation of these components by magnetic methods is theoretically possible. However, because of the difference in magnetic susceptibility of coal and pyrite is very small, the separation is not satisfactory with conventional magnetic separators. Therefore, various treatments have been applied for increasing the magnetic susceptibility of pyrite particles. Electrostatic methods can be used to separate discrete particles of materials with different dielectric properties. Separation is achieved when charged particles are subjected to the action of an intense electrical field. Physical coal cleaning methods have been used to remove pyritic sulphur from coal in the coal industry. However, because coal is inevitably rejected with the higher density material of high sulphur content the yield of coal may be reduced by as much as 40 %. In addition, as much as 50 % of the sulphur in coal may be associated with the organic portion of coal and is therefore not removed. To remove finely disseminated pyritic sulphur, and organic sulphur, chemical desulphurization methods must be applied. Chemical desulphurization methods can be classified as oxidative treatments and reductive treatments. Many oxidizing agents are sufficiently strong to convert the pyritic sulphur in coal to forms that can be readily removed in a gaseous stream or aqueous solution. Some of these reagents are also capable of converting part of the organic sulphur to extractable forms. Hydrogen is the principal reducing agent which has been considered for desulphurization coal. High temperatures are employed which lead to the production of hydrogen sulphide. Generally, pressures near atmospheric are used when the main objective is to desulphurize rather than to liquefy or gasify. Since almost all of the reagents used are expensive and their recovery is impossible, chemical desulphurization methods have been applied only on bench scale. The applicability of chemical desulphurization methods to coal economically, depends on the use of cheap and abundant reagents. One of the chemical desulphurization methods is oxydesulphurization. This method is a chemical technique by which all rank of coal can be desulphurized in acidic or basic solutions containing dissolved oxygen under pressure. In the present study, lignite samples from Gediz, Göynük and Tunçbilek were extracted in the aqueous solutions prepared by using some cheap, commercial grade reagents and some natural materials. Removal of total sulphur and sulphur forms, and recovery of coal were observed. NaOH, NH4OH, Ca(OH)2, trona, borax and fly ash were used to increase the sulphur removal capacity of the oxydesulphurization process. XX The experiments in which trona solutions were used, the parameters were selected between 0.05-0.30 M equivalent alkalinity of Na2C03, 423- 473 K, 0-1 MPa partial pressure of oxygen, and 2.5-60.0 min. The ranges of the parameters of the experiments performed using ammonia solutions were selected as 0.125-10.000 M concentration of ammonia solution, 0.0-1.5 MPa partial pressure of oxygen, 403-473 K temperature and 10-60 min reaction time. The sulphur removal from the lignite samples using NaOH was investigated under these conditions: 423-498 K, 1.0-1.5 MPa partial pressure of oxygen, 0.0625-1.0000 M, 30 min. Fly ash extracted in water at 473 K was also used to increase the sulphur removal potential of the oxydesulphurization process. In these experiments the amount of the fly ash varied between 5-30 g. The effects of some parameters including the partial pressure of oxygen, temperature, and time on the removal of the total sulphur and sulphur forms, and solid product yield were investigated in the ranges of 0.0-1.5 MPa, 403-498 K, and 15-90 min, respectively. Borax and Ca(OH)2 were used together and separately. The effects depending on the amount of each was studied individually and cumulatively. The addition of any kind of the mentioned chemicals or natural materials improved the extent of the sulphur removal of the process. The optimum concentrations or amounts were determined as 0.1 M equivalent Na2C03 for trona solution, 0.125 M for ammonia solution, 0.25 M for NaOH solution, 5 g for fly ash, 5 g for borax, and the mixture consisting 1.250 g of borax and 0.375 g of Ca(OH)2. The desulphurization of Gediz lignite sample by means of water at 473 K, under 1 MPa partial pressure of oxygen for 30 min resulted in 41.8 % total sulphur removal, 76.6 % pyritic sulphur removal, and 14.7 % organic sulphur removal. Solid product yield was calculated as 92.5 %. The experiment carried out by using 0.1 M equivalent Na2C03 solution supplied the sulphur removals of 62.5, 82.3, and 47.1 % in the total, pyritic, and organic sulphur contents of Gediz lignite under the conditions of 473 K, 1 MPa partial pressure of oxygen, and 30 min. The Solid product was recovered in the ratio of 85.6 %. 59.1 % of total sulphur content, 71.4 % of pyritic sulphur content, and 48.1 % of organic sulphur content of Gediz lignite were removed and 88.8 % of solid product was recovered after desulphurization using ammonia solution with 0.125 M, at 423 K, under 1 MPa partial pressure of oxygen for 30 min. The solution prepared by the extraction of 5 g of fly ash in water under pressure at 473 K, was shown as an excellent solution which can be used in XXX oxydesulphurization process. Total, pyritic and organic sulphur removals were performed in considerable levels without important loss of solid product when the solution derived from fly ash was used at 473 K, under 1 MPa partial pressue of oxygen for 30 min. Solid product yield, removals of total-, pyritic- and organic sulphur contents were 91.1, 50.4, 91.5, and 21.5 %, respectively. Desulphurization of coal with NaOH solutions having a concentration of more than 0.25 M causes the decomposition of coal drastically. The oxydesulphurization carried out under the conditions of 423 K, 1 MPa partial pressure of oxygen, and 30 min using 0.25 M NaOH supplied a total sulphur removal of 56.4 %. Under these conditions solid product yield was determined as 80.9 %. 5 g of Gediz lignite and 5 g of borax were mixed and then this mixture was subjected to oxydesulphurization at 473 K, under 1 MPa partial pressure of oxygen for 30 min. Total sulphur removal and solid product yield had the values of 72.9 and 64.8 %, respectively. The oxydesulphurization of the mixture containing 5 g of Gediz lignite and 0.75 g of Ca(OH)2 was investigated at 473 K, under 1 MPa partial pressure of oxygen for 30 min. It was shown that, 56.4 % of the total sulphur content of coal was removed and 84.4 % of coal was recovered after this process. 5 g of Gediz lignite, 1.25 g of borax and 0.375 g Ca(OH)2were mixed together. The oxydesulphurization was applied to this mixture at 423 K, under 1 MPa partial pressure of oxygen for 30 min. By this process, 52.4 % of the total sulphur content of Gediz lignite was eliminated and 94.9 % of the solid product was recovered. It was shown that, the higher the partial pressure of oxygen, the higher desulphurization ratio will be. In order to determine the optimum values of time and temperature, solid product yield must be considered. Oxydesulphurization kinetics was investigated under the conditions of an oxygen partial pressure of 1 MPa, with a 0.10 M equivalent Na2C03 solution of trona at temperatures between 423-473 K. It is concluded that the organic sulphur removal follows the first-order kinetics with respect to the removable part of organic sulphur content and the kinetics of pyritic sulphur removal was controlled by the diffusion of oxygen through the hematite layer which is formed on the pyrite particles during the oxydesulphurization process. Thermogravimetric analysis was used and differential thermo- gravimetric analysis was derived to characterize the combustion behaviour of original and desulphurized lignite samples. The effects of desulphurization conditions on some combustion properties of the desulphurized coal samples were investigated.