Mineral maddenin Türk linyitlerinin yanma özelliklerine etkisi
Effect of the mineral matter on the combustion properties of Turkish lignites
- Tez No: 39528
- Danışmanlar: PROF.DR. SADRİYE KÜÇÜKBAYRAK
- Tez Türü: Doktora
- Konular: Kimya Mühendisliği, Chemical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1994
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 334
Özet
ÖZET Ülkemizde, düşük değerli, yani kül ve nem içeriği yüksek, ısıl değeri ise düşük olan linyitlerden, kaliteli linyitlere kadar çok çeşitli linyitler bulunmaktadır ve ne yazıktır ki düşük değerli linyitlerin toplam rezerv içindeki payı oldukça yüksektir. Uygun koşulların sağlanmadığı yakma sistemlerinin kullanılması, yenilenemeyen bir enerji kaynağı olan kömürden yararlanma oranını önemli ölçüde düşürmekte ve çevre kirliliğine neden olmaktadır. Kömürün yakılması konusunda çok sayıda araştırma yapılmış olmasına karşın, henüz tam olarak aydınlatılmamış olaylar vardır. Kömürün mineral içeriğinin yanma özelliklerine etkisi konusundaki bilgiler henüz çok sınırlıdır. Türk linyitlerinin mineral içeriklerinin yüksek olması, konunun ülkemiz açısından önemini artırmaktadır. Türk linyitleri mineral bileşenlerinin çeşitliliği açısından da çok zengindir. Bu çalışma, mineral maddenin Türk linyitlerinin yanma özelliklerine etkilerini araştırmaya yönelik olarak gerçekleştirilmiştir. Varılan sonuçların genelleştirilebilmesi amacıyla 25 değişik yöreden toplanmış, farklı fiziksel ve kimyasal özelliklere sahip linyit numuneleri kullanılmıştır. Orijinal linyit numuneleri ile mineral maddesi giderilmiş haldeki linyit numunelerine diferensiyel termal analiz ve termogravimetrik analiz uygulanmıştır. Elde edilen verilerden mineral bileşenlerin, linyit numunelerinin yanma davranımlanna etkileri araştırılmıştır. Mineral maddenin char' m yanmasına etkisini araştırmak amacıyla, linyit numuneleri önce azot atmosferinde karbonize edilmiş ve sonra aynı şartlar altında diferensiyel termal analiz ve termogravimetrik analiz uygulanmıştır. Mineral maddenin giderilmesi linyit ve yankok numunelerinin yanma eğrileri ile yanma profillerinin daha yüksek sıcaklıklara kaymasına neden olmaktadır. Mineral madde içeriği linyit numunelerinin ve yankoklann yanma özelliklerini önemli ölçüde etkilemektedir. Tutuşma sıcaklığı, ağırlık kayıp hızı, yanma ısısının çıkış hızı gibi önemli yanma özellikleri mineral bileşenlerinin derişimine bağlı olarak değişmektedir. Uçucu maddenin karbonizasyon esnasında çıkması, mineral bileşenlerinin yankoktaki derişimini artırdığından, orijinal ve mineral maddesi giderilmiş linyit numunelerinden üretilen yankoklann yanma özellikleri arasındaki farklar, bunlann üretildiği orijinal ve mineralsiz linyitlere oranla daha fazladır. xıv
Özet (Çeviri)
EFFECT OF THE MINERAL MATTER ON THE COMBUSTION PROPERTIES OF TURKISH LIGNITES SUMMARY Burning coal in order to generate heat can be traced back at least as far as the 12tn century; and some studies of combustion mechanisms - at that time mostly concerned with prevention of dust explosions in coal mines - began to be undertaken almost 150 years ago. Since then, however, and more especially since World War II, such more fundamental investigations have come to center on how coal burns in different combustion systems, and have therefore focused attention on phenomena associated with combustion of single particles and particle 'clouds' as well as on combustion in various types of fixed and fluidized fuel beds. The vastly better experimental and computational facilities now available to researches have also allowed extensive modeling and theoretical analyses of combustion processes. How and how fast (or completely) a coal burns depends upon the nature and composition of the coal, on the form in which it is burned, and on ambient combustion conditions. The combustion of coal, either in a bed or as pulverized coal in a suspension, is primarily a matter of combustion of carbon and volatile matter. It is agreed that the burning of coal involves three stages: 1. The release of the volatile matter resulting from the heating of the coal, 2. The burning of the volatile matter in the gas phase, and 3. The burning of the solid residue of carbonaceous matter (char). These processes occur sequentially to some extent; however, there is always some overlap between the stages. Char burnout is the slowest step, so there is practical interest in determining the factors that influence its rate. The reactions of char with oxygen have generally been described as governed by the following controlling processes: 1. Mass transfer (by diffusion) of oxygen to reaction sites, 2. Chemisorption of oxygen on the carbon surface, reaction of chemisorbed oxygen with carbon to form products, and desorption of products from the carbon surface,3. Mass transport of the gaseous products from the carbon surface. In the absence of mass transport limitations, the intrinsic char reactivity is controlled by (a) the char active surface area, and (b) catalysis by impurities. Thermal analysis plays an important role in determining the combustion characteristics of coal. In recent years the applications differential thermal analysis (DTA), derivative thermogravimetry (DTG) to the study of solid fuels has gained a wide acceptance amongst researchers in the field of energy conversion. DTA covers those techniques which record the temperature difference between a substance and a thermally inert material when the two substances are undergoing identical temperature changes“ within an environment which is heated and cooled in a controlled ratio. This method was developed following the perfection of thermocouples as precise temperature gauges. The thermal analysis technique of thermogravimetry (TG) is one of in which the change in sample mass (mass-loss or gain) is determined as a function of temperature and/or time. Three modes of thermogravimetry are commonly used: a. Isothermal thermogravimetry, in which the sample mass is recorded as a function of time at constant temperature, b. Quasi-isothermal thermogravimetry, in which the sample is heated to constant mass at each of a series of increasing temperatures, and c. Non isothermal thermogravimetry, in which the sample is heated in an environment whose temperature is changing in a predetermined manner, preferably at a linear rate. Non isothermal thermogravimetry has been widely used for evaluating burning properties of coals and chars. A plot of the rate of weight loss against temperature while burning a sample in air has been referred to as a ”burning profile“. Burning profiles obtained under a set of standard conditions provide detailed information from the onset of oxidation to complete burnout and are useful for predicting the relative ranking of fuels with regard to their combustion reactivities. Fuels with similar burning profiles have comparable burning characteristics in large coal-fired furnaces. The DTA determinations of coal in air are essentially exothermic and their configurations, although influenced by compositional variations, both organic, inorganic and rank, represent the large amounts of heat generated by burning. The information obtainable under these conditions is relatable to combustion characteristics, ignition temperatures, and relative resultant heats of combustion. The term ”combustion curves" has been suggested for the application of DTA to the monitoring of burning coal to distinguish it from the TG curves (burning profiles). XVICoal contains significant and variable amounts of largely incombustible mineral matter which could influence the combustion characteristics of it. (Mineral matter is generally considered to be the sum of all inorganic minerals (discrete phases) and elements that are present in coal). Thus, all elements in coal except organically combined C, H, O, N and S are classified by this definition as mineral matter. This adequately classifies most inorganic elements in coals, those that are structurally bound within various minerals, but some other elements are also combined in the organic matter. The mineral matter of coal primarily includes clays, shales, pyrite, quartz, calcite and lesser amounts of other materials, depending on the chemical and mineralogical composition. It occurs in many forms and sizes, which may be seen by the naked eye or occur in micron-sized particles that require an optical or electron microscope to observe. The purpose of this study was to examine the effect of the mineral matter on the combustion properties of 25 Turkish lignite samples from various parts of Türkiye. The lignite samples are from: All coal samples were prepared as for the ASTM standard proximate analysis, i.e. less than 0.25 mm. XVXlAccording to the ASTM standards the moisture content of the lignite samples varies between 2.0-48.0 %; the ash content between 6.2-40.6 %; the volatile matter content between 22.2-46.4 %; the fixed carbon between 11.8-53.4 %; and the heat content between 10.3-27.7 MI The mineral matter content of the lignite samples varies between 7.68-47.97 % on dry basis. The demineralization of the lignite samples was performed by treatment with hydrochloric and hydrofluoric acids. Differential thermal analysis and thermogravimetric analysis were used to characterize the combustion of the original and demineralized lignite samples. The same tests have been applied to chars prepared from original and demineralized lignite samples under nitrogen atmosphere. Differential Thermal Analysis was carried out using a Shimadzu DTC 40 analyzer. 20 mg lignite samples were spread uniformly on the bottom of crucible and the furnace heated at a constant 10 K/min. while being swept by air at a rate of 40 cc/min. until 1073 K and held for 10 minutes at constant temperature. Reference material was a-Alumina. Thermogravimetric analysis was carried out using a Shimadzu TG 4 1 thermal analyzer. 40 mg lignite samples were spread uniformly on the bottom of the crucible made of alumina. The flow rate of air was fixed at 40 cc/min. The chart speed was selected as 5 mm/min.. The temperature was raised with a heating rate of 40 K/min. to 1273 K and held for 30 minutes at this temperature. The DTG curves of the samples were derived from TG applications. The DTA (combustion curves) and DTG (burning profiles) curves of the original and demineralized lignite samples and of the chars prepared from original and demineralized lignite samples were compared and discussed. Experiments have shown that the mineral matter content of the lignite samples plays an important role in determining the combustion characteristics such as ignition temperature, burnout time, temperature at which the maximum rate of mass-loss occurred, combustion curve maximum peak temperature and heat generation rate. After demineralization the combustion curves and the burning profiles of the lignite and char samples are shifted to higher temperatures. Data indicated that the effects of the mineral matter content on combustion characteristics are more pronounced at higher temperatures. The catalytic effects of CaO, MgO, Na:0 and K20 were compared. CaO is the most effective one. xvniMixed catalysts are more effective than single catalysts. Data indicated that the catalytic effects of the mineral matter are for char combustion more pronounced than for lignite combustion. xix
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