Oksitleyici Kükürt Giderme Yöntemlerinin Bazı Türk Linyitlerinin Fiziksel Ve Kimyasal Özelliklerine Etkilerinin Araştırılması

Abstract

Kömürün yakılmasından kaynaklanan sorunlar ile kömürün içerdiği kükürt arasında yakın bir ilişki bulunmaktadır. Kömür yakıldığında, içerdiği kükürdün bir kısmı, kömürün inorganik yapısının bileşimine bağlı olarak, külü tarafından tutulmakta, geriye kalanı ise, kükürt oksit bileşikleri halinde yakma ortamını terk etmektedir. Türk linyitleri yüksek oranlarda kükürt içerdiğinden, kükürt içerikleri azaltılmadan yakılmaları, önemli miktarda çevre kirliliğine neden olmaktadır. Kömürün kükürt içeriğinin etkin şekilde azaltılması, kimyasal kükürt giderme süreçlerinin uygulanması sonucunda sağlanabilmektedir. Bu süreçler, kömürün içerdiği kükürtlü bileşiklerin bazı kimyasal maddeler ile tepkimeye sokulması veya karmaşık yapılı kükürtlü bileşiklerin kömürden kolayca uzaklaştırılabilen daha basit yapılı bileşiklere dönüştürülmesine dayanmaktadır. Ancak, kullanılan kimyasal maddeler sadece kükürtlü bileşikleri değil, aynı zamanda kömürün organik ve inorganik yapısını da etkilemektedir. Bunun sonucu olarak, kömürün pek çok özelliğinde önemli değişiklikler meydana gelmektedir. En yaygın olarak uygulanan kimyasal kükürt giderme süreçleri, kömürün oksitleyici bir ortamda, çeşitli kimyasal maddeler ile etkileşim içine sokularak, içerdiği kükürtlü bileşiklerin suda çözünebilir hale getirildiği süreçlerdir. Kömürün gözenekli yapıya sahip olması nedeniyle, oksijen hem kömürün gözeneklerinde adsorplanmakta, hem de kömür ile kimyasal bağlar oluşturmaktadır. Bu nedenle, kömürün oksijen içeriğinde artış meydana gelmektedir. Oksitlenme sonucunda, kömürün gözenek yapısını ve yüzey özelliklerini etkileyen çeşitli oksijen içeren fonksiyonel gruplar oluşmaktadır. Oksitlenmiş kömürün ısıl değerinde önemli ölçüde kayıplar ortaya çıktığı gibi, kömürün birçok fiziksel ve kimyasal özelliğinde de değişiklikler meydana gelmektedir. Bu değişiklikler, kömürün yanması, koklaştırılması, pirolizi, sıvılaştırılması, flotasyonu, kırılganlığı, su tutma kapasitesi, kendiliğinden tutuşması v.b. gibi birçok özelliğini etkilemektedir. Bu çalışmada, Ağaçlı-İstanbul, Bolluca-İstanbul, Tavşanlı-Kütahya, Malkara- Tekirdağ ve Yeniköy-İstanbul linyit numunelerine, 423 K'de, 1.5 MPa kısmi oksijen basıncı altında ve 500 rpm karıştırma hızında, 60 dakika süre ile arı su veya 0.2 M Na2C03 çözeltilerinin kullanıldığı oksitleyici kükürt giderme yöntemleri uygulanmıştır. Bu koşullara maruz bırakılan linyit numunelerinin geri kazanım oranları, numunelerin içerdiği fonksiyonel gruplarda meydana gelen değişimler, gözeneklilik, yoğunluk, ortalama gözenek boyut dağılımı, tutuşma sıcaklığı, maksimum yanma hızı, en hızlı yanma sıcaklığı, maksimum yanma hızına ulaşma süresi, yanmanın bitiş sıcaklığı gibi özelliklerine etkileri araştırılmıştır.

Combustion of high-sulfur coals is the most important reason of the atmospheric pollution. When coal is burnt, a small part of total sulfur content is fixed by ash depending on the composition of the mineral species in coal, and others are likely to form the sulfur oxide emissions. Turkish lignites usually contain high sulfur. Therefore, combustion of Turkish lignites brings about environmental pollution. Sulfur compounds found in coal can be classified into two categories, namely organic sulfur and inorganic sulfur. The sulfur organically bound to carbon is a part of the organic matrix of coal. Therefore it is called as "organic sulfur". Inorganic sulfur is subdivided into pyritic sulfur, and sulfate compounds. The most part of the inorganic sulfur is pyritic sulfur that mainly consisted of pyrite and marcasite. Inorganic sulfur is distributed through the coal structure in the form of either discrete particles or finely disseminated particles. Some physical, chemical or biological methods have been applied to coal in order to reduce sulfur content. Conventional physical and biological methods usually have the potential of inorganic sulfur removal from coal. The removal efficiency of these methods depends strongly on the particle size of coal to be used. The lower the particle size, the higher the removal efficiency will be. Physical methods base on separation of organic matrix and inorganic sulfur compounds from each other considering some differences in the physical properties of the two parts. Physical methods involve crushing or comminution of the coal followed by separation of the inorganic sulfur from the organic part. Although dry separation methods are sometimes applied, wet methods are common, particularly with small particle sizes. Since the density of the organic matter in coal is considerably lower than that of pyrite, the two parts are frequently separated by adding crushed coal to a liquid medium of intermediate density. The organic coal particles float because they are less dense, whereas pyrite particles sink because they are more dense than the medium. -iX- Hydraulic methods employing jigs, wet concentration tables, hydrocyclones and other types of equipment are the most widely used of all industrial methods for pyrite removal. These methods base on the difference in settling rates. Since pyrite particles have greater density, they sink faster than coal particles of the same size. Several physical desulfurization methods including froth flotation, oil agglomeration, and solvent partitioning take advantage of the difference in surface properties between the organic matrix and pyrite. Organic matrix is hydrophobic, whereas pyrite is hydrophilic. In the froth flotation method, a suspension of fine coal particles in water is aerated with numerous small air bubbles. The hydrophobic coal particles become attached to these bubbles and are buoyed to the surface of the suspension where they are recovered in the froth. Pyrite is left behind in the aqueous suspension. In the oil agglomeration method, a small amount of fuel oil is added to a suspension of coal particles in water to coat the hydrophobic particles selectively. The-oil coated particles stick together and form relatively large flocks or agglomerates which can be separated from the unagglomerated pyrite particles by screening. 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, while pyrite particles remain in the aqueous phase. The liquids can be separated by decantation. Since the organic matrix of coal is strongly diamagnetic and pyrite is paramagnetic, magnetic cleaning can be applied to coal in order to remove pyrite particles. Application of "High Gradient Magnetic Separation (HGMS) Technique" brings about important improvements in the magnetic susceptibility between organic matrix and pyrite. In biological methods, sulfur is removed from coal via microbiological means. For this purpose, various micro-organisms have been used. Biological methods are time consuming and they can be applied only on bench- scale. Pyrite particles can be removed from coal by means of physical or biological methods unless they are finely disseminated, whereas organic sulfur is almost inert to these methods. Break of the bond between carbon and sulfur is carried out when chemical methods applied. Chemical methods base on the reaction of sulfur compounds with some reagents, or conversion of complex sulfur compounds into simpler ones, which are easy to remove from coal. Well-known chemical desulfurization techniques are as follows: PETC - Oxidation, AMES - Wet Oxidation, LEDGEMONT - Oxidation ARCO - Oxidation TRW MEYERS - Oxidation JPL Clorinolysis - Oxidation, -X- KVB - Oxidation - Displacement, BATELLE Hydrothermal - Displacement, TRW Gravimelt - Displacement, General Electric Microwave Displacement, IGT - Hydrodesulphurisation, Chemical Comminution. Reagents of chemical desulfurization methods not only effect the sulfur compounds, but also organic and inorganic structure of coal can be affected. Thus, important changes take place in some properties of coal. The chemical desulfurization processes, which are extensively applied, are usually oxidative techniques. Since coal has a porous structure, oxygen can be adsorbed in the pores as well as it forms some chemical bonds with coal. For this reason, oxygen content of coal highly augments. As a result of oxidation, some functional groups such as carboxyl, hydroxyl, and carbonyl are formed, which have remarkable effects on pore structure and surface properties of coal. Important losses occur in the calorific value of oxidised coal. Furthermore, some changes take place in the physical and chemical properties of coal. These changes effect combustion, coking, pyrolysis, liquefaction, flotation, friability, water holding capacity, spontaneous combustion, etc. The mechanical strength of coal gradually decreases as a result of oxidation. Hence, oxidized coal samples become more fragile than their original state. Some functional groups such as -COOH and/or -OH are formed on the coal surface owing to oxidation. Since these groups are hydrophilic, they have negative effects on the floatability of coal. In other words, the extent of oxidation determines floatability of coal. Another aspect of the hydrophilic character of oxidised coal is the enhanced moisture-holding capacity. Water molecules interact -COOH and -OH groups and form some complex structures increasing moisture holding capacity. Oxidation also affects spontaneous combustion of coal. Ignition temperatures of low rank coals are low because of their high tendency toward oxidation. The heat resulting from oxidation of FeS2 initiates spontaneous combustion of coal. In case of further oxidation, non-aromatic components and hydroaromatic components of coal decompose. This fact leads to changes in the pyrolytic behaviour of coal. Pyrolysis products vary depending on coal properties and oxidative conditions. Oxidation plays an important role in the plasticity of coking coals. After oxidation, plasticity decreases or completely disappears. Thus, coking behaviour determining the quality of coke to be obtained is considerably affected regarding oxidation. Another effect of oxidation is on solvolytic extraction by which coal is converted into liquid products in the temperature range of 473-673 K. Both amount and -Xi- distribution of the liquid products are extremely sensitive to oxidation degree of the parent coal. In this study Ağaçlı-Istanbul, Bolluca-Istanbul, Tavşanlı-Kütahya, Malkara-Tekirdağ and Yeniköy-Istanbul lignite samples were treated with either distilled water or 0.2 M Na2CÜ3 solutions at 423 K under 1.5 MPa partial pressure of oxygen in a 1 litre PARR autoclave for 60 min. After this period, the autoclave was cooled and its contents were filtered. Treated lignite samples were washed with hot distilled water until soluble sulfates were eliminated and the pH of the washings became neutral. The coal was dried at 383 K for 24 h in a vacuum oven under nitrogen atmosphere. In the design of industrial coal-fired furnaces, it is of importance to have an assessment of the reactivity of the intended fuel. Alternatively, if it is proposed to change the fuel supply for an existing installation, it is advantageous to have a test which enables the burning characteristics of the candidate fuels to be compared with the original in terms of reactivity or burning rate. Thermogravimetric Analysis was applied to original and treated lignite samples to compare their burning characteristics. F.T-i.r. technique was applied to determine the differences between functional groups of the original and oxydesulphurized lignite samples. Mercury intrusion porosimetry was used to identify the differences between some porosimetric properties of the original and oxydesulphurised lignite samples. Changes in the functional groups of the lignite samples, porosity, density, mean particle size, ignition temperature, max. burning rate, temperature of max. burning rate, period for the max. burning rate, and end of burning were investigated and compared. It is concluded that some important changes took place in the physical and chemical characteristics of lignite samples related to oxidation. In case of alkaline conditions, these changes were extremely evident. As the humic acids are highly soluble under alkaline conditions, most of the organic structures of lignite samples could not be recovered after oxidative treatments. For example, 17.5 % of Ağaçlı lignite, 17.9 % of Yeniköy lignite, 19.1 % of Malkara lignite, 21.9 % of Bolluca lignite were recovered after treatment with 0.20 M Na2CC«3 solution under 1.5 MPa partial pressure of oxygen at 423 K for 60 min. Whereas, recovery of these lignite samples were higher than that in acidic conditions. Some of Na2CC>3 could not be eliminated from lignite samples. Consequently, mineral matter content of the treated lignites samples increased. The presence of the retaining Na2CC<3 caused some additional peaks at DTG burning profiles and F.T.-i.r. spectra. Since, some mineral species were eliminated from lignite under acidic conditions, decrease took place in the ash ratios of lignite samples after acidic treatment. It is concluded that Yeniköy lignite sample is the most thermally reactive lignite sample. But, Tavşanlı lignite sample is the least thermally reactive lignite sample. Oxydesulphurization under acidic conditions showed important effect on the ignition temperature of the lignite samples. -Xii- Important changes were determined in the functional groups of the lignite samples after oxidation. C=C and C-H groups were decreased, whereas C=0, C-0 and O-H groups were increased. Since some of the organic matrix of the lignite samples decomposed, mineral species of these lignite samples relatively increased. -Xiii-