Linyitlerin temal özelliklerinin araştırılması

Abstract

Kömür, hemen hemen tüm tüketim alanlarında ısı etkisine maruz bırakılmaktadır. Bu nedenle, kömürün termal özelliklerinin bilinmesi uygulanan proseslerin verimliliğini artırmak açısından oldukça önemlidir. Diferensiyel termal analiz, kömürün termal özelliklerinin belirlenmesi amacıyla en yaygın olarak uygulanan yöntemdir. Bu çalışmada, Türkiye'nin 24 değişik yöresinden toplanmış olan linyit kömürlerine diferensiyel termal analiz yöntemi uygulanmıştır. Farklı özelliklerdeki linyitlerin DTA eğrileri de önemli farklılıklar göstermiştir. Linyit numunelerinin ısı profillerinin incelendiği bu çalışmada, ısıtma hızı ile numune miktarının değiştirilme sinin DTA eğrilerinin karakterine ve şekline olan etkileri da araştırılmıştır.

Thermal analyses are those instrumental dynamic analysis methods that monitor the physical and chemical tranformations which take place in the structure of a substance being heated or cooled. On this principle, a large array of instrumental methods has been developed based on variations in mass, volume, and tempe rature between the sample under analysis and a thermally inert substance. Among previously mentioned methods, those that have given the most encouraging results in compositional analysis are Diffe rential Thermal Analysis (DTA),Thermogrevimetry (TG) and Derivative Thermogravimetry (DTG). 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 or cooled in a controlled ratio. This method was developed following the perfection of ther mocouples as precise temperature gauges. When the differences in heat capacity and heat conduction between a sample and a simultaneously heated inert substance are ignored, the sample and inert substance should be at the same tem perature during heating as long as there is no reaction in the sample. Such is the case with a small sample and an inert reference material that does not differ greatly from the sample chemically. At the free ends of two thermocouples, the welded ends of which are imbedded in the sample and the inert substance, respec tively, there is initially no potential. The graph of the differen ce in voltage as a function of time and/or temperature is parallel to the axis. As soon as an endothermic or exothermic reaction begins, the heating of the sample either remains below or rises beyond the furnace temperature. The sample remains colder or beco mes warmer than the inert reference material. The potential is measured at the free ends of the thermocouples differential connec tions, and the deviation from the zero line on the DTA curve is thus obtained. Differential thermal analysis, since it is a dynamic tempe rature technique, has a large number of factors which can affect the resulting experimental curves. The DTA curve is dependent on two general categories of variables: VI 1. Instrumental factors. a) Furnace atmosphere b) Furnace size and shape c) Sample- hol der material d) Sample-holder geometry e) Heating rate f) Speed and response of recording instrument g) Thermocouple location 2. Sample characteristics a) Particle size b) Thermal conductivity c) Heat capacity d) Packing density e) Swelling or shrinkage of sample f) Amount of sample g) Effect of diluent h) Degree of crystal! i nity With world energy consumption increasing, petroleum resour ces and reserves limited, and nuclear sources strongly questioned, the emphasis of fuel research has turned increasingly to coal. World production of coal is more than that of any other sing le mineral material and most of this is either burned directly as fuel or carbonized to produce secondary fuels by heating in the absence of air. Thermal treatment is thus of great importance in coal utilization, and the application of thermal analysis is of immediate interest to the coal industry. Thermal decomposition characteristics of coals control the behaviour, utilization and va lue of these fuels. The thermal analysis of solid fuels is complicated by the heterogeneous nature of the materials themselves. Apart from vari ations in rank, most coals are visibly heterogeneous, portions vn which are brigh (vitröin and clarain), or dull (durain), or powdery (fusain) being easily recognized. Differences are found in the ther mal behaviour of the various macera! s and in that of the macroscopic types which are combinations of macerals. Thermal behaviour may also be modified by the presence of mineral materials. Thermal analysis methods hava the advantage of direct appli cation to whole coal samples which (except for crushing) have not been pretreated in any way. Thus such problems as the loss, altera tion and imperfect concentration/removal of particular constituents are obviated and the resultant curves represent all of the original constituents in the sample. Pyrolysis is a fundamental process in the combustion, carbo nization and gasification of coal. The progress of coal pyrolysis begins with the evolution of moisture and occluded gases up to tem peratures of about 573 K; this is followed by active thermal decom position during which gaseous hydrocarbons, tarry materials, ammonia and water of decomposition are evolved. At 875-973 K-the approximate temperature limit of most so-called low- temperature carbonization processes-this stage is virtually complete. If the temperature is raised further the character of the decomposition changes and the evolved volatile products consist almost entirely of hydrogen. Carbonization of static beds of coal is usually carried out at low rates of heating, up to 10 K/min; such rates are common in DTA which is thus well suited to studies on coal pyrolysis. DTA curves for coals represent very complex thermal decompo sition, involving many successive or concurrent chemical reactions, the liberation of gases and the volatilization of moisture, tarry materials, etc. It would therefore be expected that the curves would be more complex than those for a material undergoing a simple physical change, such as the quartz inversion, and that they would be affected to a considerable extent by experimental conditions. The purpose of this study was to investigate the differenti al thermal analysis of Turkish lignites. The lignite samples used are from: 1) Çayırhan-Ankara, 2) Aşkale-Erzurum, 3) Bağyaka-Muğla, 4) Merkeşler-Bolu, 5) Çan-Çanakkale, vm 6) Dodurga-Çorum, 7) Akpı nar- Istanbul, 8) Çiftalan-îstanbul, 9) Gediz- Kütahya, 10) lig m- Konya, 11) Mengen-Bolu, 12) Mihalıçcık-Eskişehir, 13) Milas-Muğla, 14) Orhaneli -Bursa, 1 5 ) Şeyi tömer- Kütahya, 16) Soma-Manisa, 17) Tepe baş i -Konya, 18) Ttnaz-Muğla, 19) Karakaya-Tekirdağ, 20) Tünebil ek-Kütahya, 21) Yatağan-Muğla, 22) Merzifon- Amasya, 23) Sorgun-Yozgat, 24) Yeni köy- İstanbul. The volatile matter of the lignite samples varies between 24.95-55.02 %, the ash content between 7.61-43.19 % and the heat content between 14.81-29.24 MJ. The thermal properties of the -1 ignite samples were investiga ted in the temperature range cf 293-1223 K by using Differential - Thermal Analysis. DTA were carried out under nitrogen atmosphere (flow rate 30 cc/min). The parameters tested for their effects on DTA curves are heating rate and sample size. Runs were made at heating rates of 5, 10 and 50 K/min using sample sizes of 10, 20 and 40 mg. IX All coal samples ground to pass a 0.25 mm sieve, were lightly packed into a platin sample holder. Reference material was * -Alumina. The thermal characteristics of the lignite sample undergoing pyrolysis in an inert atmosphere were compared and discussed in this study.