Kömürün kısa analizinin TGA yöntemiyle saptanması

Abstract

Kömürün ısıl değerinin saptanması ile kısa analizi kömür teknolojisinde kömür numunesinin kalitesi ve tipi konusunda ön bilgileri veren en uygun yöntemler olarak kabul edilmektedir. Kömürün, nem, uçucu madde, kül ve sabit karbon içerikleri kısa analiz sonucu saptanan özelliklerini oluşturmaktadır. Bu özelliklerin saptanması amacıyla uygulanmakta olan çeşitli analiz yöntemlerinden en yaygın olarak kullanılanlar ASTM (American Society for Testing and Materials) ve ISO (International Standarts Organiza tion) standartlarıdır. Son on yılda birbirinden bağım sız çeşitli araştırma grupları kömürün kısa analizini gerçekleştirmede termogravimetri ' den yararlanma yolunu seçmişlerdir. Ülkemizin 25 değişik yöresinden toplanmış olan linyit numunelerine hem ASTM standardına uygun kısa analiz testleri ve hem de TG yöntemi uygulanmıştır. TG yönteminde tüm kısa analiz sonuçlarına aynı analiz numunesi kullanılarak ve büyük zaman tasarrufu sağlamak suretiyle ulaşılabilmekte ve kömürün sabit karbon içeriği standartlarda önerildiği gibi farktan değil, deneysel olarak saptanabilmektedir. Linyit numunelerinin TG eğrilerinden ve ASTM standartlarına uygun olarak yürütülmüş olan analizleri sonu cunda saptanan nem, uçucu madde, kül ve sabit karbon içerikleri deneysel hata sınırları içinde kalacak kadar az farklılıklar göstermektedir. Ayrıca, azot atmosferlerinde elde edilmiş olan TG eğrilerinden yararlanılarak linyit numunelerinin uçucu madde çıkış hız profilleri türetilmiş ve incelenmiştir.

The proximate analysis of coal uıas deueloped as a simple means for determining the distribution of products obtained during heating under standard conditions. it is the most used type of analysis for characterizing coals in connection uıith their utilization. The procedural details of the individual tests uthich go to make up the proximate analysis vary some uıhat among the various standard specifications in cammon use. Proximate analysis separates the compounds into four groups: 1)- Moisture, 2)- Uolatile matter, 3)- Fixed carbon, and 4)- Ash. The fixed carbon is a calculated figüre obtained by subtracting from 1DD the sum of the percentages of moisture, volatile matter, and ash. tılater is present in coal in more than öne form. Different ranks of coal contain varying quantities of uater present in part as a mechanical and in part as a physical mixture. The mechanically admixed uater on the surface and inside cracks and large capillaries has a normal vapor pressure, uhich is termed as "free uater". The physically held uater in the internal pare structure of the coal is termed as "bound uıater". This form of moisture is held in the smaller capillaries ör pores. vıı The volatile matter obtanied during pyrolysis of coal cansists mainly of the combustible gases, hydrogen, carbon monoxide, methane, and other hydrocarbons; tar vapors; and soma incombustible gases, sunh as carbon dioxide and uater uapor. it does not include the mois- ture in coal that can be removed by heating to tempera- tures slightly higher than the bailing point of uater, but does include the mater formed during thermal decom- position of the coal substance. The compositian af the volatile matter varies greatly for different ranks of coal, uıith the proportion of incombustible gases increasing as the rank decreases. The volatile matter is öne of the most important determinations in coal analysis, because volatile matter is used as a parameter in certain classification systems and for evaluating coals for combustion and carbonization. Ash is the residue derived from the mineral matter during complete incineration of the coal. it is different in chemical composition and usually is less than the mineral matter originally present in the coal. During incineration, various uıeight changes take place such as loss of uıater of constitution of the silicate minerals, loss of carbon dioxide from carbonate minerals, oxidation of iron pyrites to iron oxide, and fixation of oxides of sülfür by bases such as calcium and magnesium. Thermal analyses are those instrumental dynamic analysis methods that monitör the physical and chemical tranformations uıhich take place in the structure of a substance being heated ör cooled. Dn this principle,a large array of instrumental methods has been developed based on variations in mass, volume, and temperature betmeen the sample under analysis and a thermally inert substance. Among preuiously mentioned methods, those that haue given the most encouraging results in compositional analysis are Differential Thermal Analysis (DTA), Thermo- grevimetry (TE) and Derivative Thermogravimetry (DTG). DTA covers those techniques uhich record the tempe¬ rature difference betuıeen a substance and a thermally inert material uhen the tuo substances are undergoing identical temperature changes uithin an environment which is heated ör cooled in a controlled ratio. This method uıas dev/eloped folloufing the perfection of thermocoupleB as precise temperature gauges. vııı The thermal analysis technique of thermogravimetry (TG) is öne in uhich the change in sample mass(mass- loss ar gain) is determined as a function of temperature and/or time. ThreE modes of thermogravimetry are commonly used: a)- Isothermal thermogravimetry, in uhich the sample mass is recorded as a function of time at constant temperature, b)- Quasi-isothermal thermogravimetry, in uıhicn the sample is heated to constant mass at each of a series of increasing temperatures, and c)- Dynamic thermogravimetry, in uhich the sample isheated in an environment ujhase temperature is changing in a predetermined manner, preferably at a linear rate. The resulting mass-change versus temperature curve provides information concerning the thermal stability and composition of the initial sample, the thermal stability, and composition of any intermediate compounds that may be formed, and the composition of the residue, if any. To yield usuful information uıith this technique, the sample must evolve a volatile product, uhich can originate by various physical and chemical processes. The rate of decompositian reactipns occurring in coal under the action of heat may best be studied by means of TG. The uıeight curues obtained make it possible to deriue the rate of loss in uıeight as a function of temperature and time. Since ali the measurements of the proximate analysis involve yeight changes under specified conditions of temperature, time and atmosphere, it seemed that the technique of thermogravimetry uould be ideally suited to this type of analysis. The three measured parameters, i.e. moisture, volatile matter, and as.h are ali determined in dublicate on separate aliquots of the analysis sample, making the ıx The mean difference in volatile matter content Df the lignite samples between the TG and ASTM methods is 1.63 %; the minimum and maximum differences are 0.1% and 4.6 % respectively. The mean difference in fixed carbon content of the lignite samples between the TG and ASTM methods is 1.47 %', the minimum and maximum differences are D.D % and 4.8 %, respectively. Using TG curves of the lignite samples the DTG curves in the range of volatile matter release were derived. The devolatilization behaviour of the samples umre compared and discussed. Differential Thermal Analysis was carried out using a Shimadzu DTC 40 analyser. 20 mg lignite samples of -0.25 mm mere spread uniformly on the bottom of the cruble and the furnace heated at a constant 10 H/min while being swept by air at a rate of 40 cc/min until 1073 H and held for 10 minutes at constant temperature. The area under DTA curves was measured to estimate the calorific value of the lignite samples. There is a good relationship between the area under the DTA curves and the calorific value of the samples measured by using oxygen bomb calorimeter. Xll The thermal analysis technique of thermogravimetry (TG) is öne in uhich the change in sample mass(mass- loss ar gain) is determined as a function of temperature and/or time. ThreE modes of thermogravimetry are commonly used: a)- Isothermal thermogravimetry, in uhich the sample mass is recorded as a function of time at constant temperature, b)- Quasi-isothermal thermogravimetry, in uıhicn the sample is heated to constant mass at each of a series of increasing temperatures, and c)- Dynamic thermogravimetry, in uhich the sample isheated in an environment ujhase temperature is changing in a predetermined manner, preferably at a linear rate. The resulting mass-change versus temperature curve provides information concerning the thermal stability and composition of the initial sample, the thermal stability, and composition of any intermediate compounds that may be formed, and the composition of the residue, if any. To yield usuful information uıith this technique, the sample must evolve a volatile product, uhich can originate by various physical and chemical processes. The rate of decompositian reactipns occurring in coal under the action of heat may best be studied by means of TG. The uıeight curues obtained make it possible to deriue the rate of loss in uıeight as a function of temperature and time. Since ali the measurements of the proximate analysis involve yeight changes under specified conditions of temperature, time and atmosphere, it seemed that the technique of thermogravimetry uould be ideally suited to this type of analysis. The three measured parameters, i.e. moisture, volatile matter, and as.h are ali determined in dublicate on separate aliquots of the analysis sample, making the ıx The mean difference in volatile matter content Df the lignite samples between the TG and ASTM methods is 1.63 %; the minimum and maximum differences are 0.1% and 4.6 % respectively. The mean difference in fixed carbon content of the lignite samples between the TG and ASTM methods is 1.47 %', the minimum and maximum differences are D.D % and 4.8 %, respectively. Using TG curves of the lignite samples the DTG curves in the range of volatile matter release were derived. The devolatilization behaviour of the samples umre compared and discussed. Differential Thermal Analysis was carried out using a Shimadzu DTC 40 analyser. 20 mg lignite samples of -0.25 mm mere spread uniformly on the bottom of the cruble and the furnace heated at a constant 10 H/min while being swept by air at a rate of 40 cc/min until 1073 H and held for 10 minutes at constant temperature. The area under DTA curves was measured to estimate the calorific value of the lignite samples. There is a good relationship between the area under the DTA curves and the calorific value of the samples measured by using oxygen bomb calorimeter.