Linyit-su karışımlarının incelenmesi

Abstract

Toplam rezervi 8 milyar ton olan linyit kömürü, ülkemiz fosil enerji kaynaklan içinde birinci sırada yer almakta ve hem günümüzde hem de yakın gelecekte vazgeçemeyeceğimiz bir enerji kaynağı durumundadır. Türk linyitlerinin yumuşak karakterde olması nedeniyle, üretimleri, hazırlanmaları, taşınmaları ve depolanma ları sırasında çok miktarda toz kömür oluşumu söz konusudur. Yurdumuzdaki kömür işletmelerinin çoğunda değerlendirilemeyen toz kömür stoklan vardır. Enerji tüketiminin hızla arttığı günümüz koşullannda, yenilenemeyen bir enerji kaynağı olan linyitlerimizin en iyi şekilde değerlendirilmesi gerektiğinden, çeşitli proseslerde oluşan toz kömürün de değerlendirilmesi gerekmektedir. Kömür tozlannın değerlendirilmesi amacıyla geliştirilmiş ve dünyada, uygulanmakta olan üç yöntem vardır: 1) Toz kömür yakma sistemlerinde yakılması, 2) Çeşitli katkı maddeleri kullanarak biriketlenmesi ve 3) Kömür-sıvı kanşımlan halinde yakılması. Kömür-sıvı kanşımı yakıtlann kullanılması sonucunda petrole olan bağımlılığın azaltılması hedeflenmektedir. Bu tip yakıtlar, geniş bir kullanım alanına sahiptir. Bu çalışmanın temel amacı, Türk linyitleri kullanılarak linyit-su kanşımlannın hazırlanması ve yanma davranımlannın belirlenmesidir. Bu amaçla; öncelikle, uygun kanşımın hazırlanmasında en önemli değişkenlerden biri olan tanecik boyut dağılımının, kanşımın reolojik özelliklerini nasıl etkilediği araştınlmış ve kanşım için en uygun tane boyut dağılımı belirlenmiştir. Daha sonra, kömür-su kanşımlannın hazırlanmasında kullanılabilecek en uygun dispersantlar araştınlmış ve bazılannın linyit numuneleri tarafından adsorbsiyonu, farklı pH ve sıcaklık koşullannda incelenmiştir. Ayrıca, mineral maddenin giderilmesinin ve linyit numunelerine uygulanan oksidasyonun dispersant adsorbsiyonuna olan etkileri de araştınlmıştır. Sonuçta, bazı Türk linyitleri için, en uygun dispersant oranlan ve adsorbsiyonun en yüksek oranda gerçekleştiği koşullar saptanmış ve bu koşullarda gerçekleşebilecek değişimlerin, kanşımın reolojik özelliklerine etkileri belirlenmiştir. KSK'nın yanma davranımı, bu yakıtın gelecekte kullanımını belirleyen en önemli etkendir. Hazırlanmış olan linyit-su kanşımlannın yanma davrammlan, tek bir yakıt damlasının yanmasının izlendiği bir teknik kullanılarak belirlenmiştir. Damla boyutunun, kanşımdaki linyit derişiminin ve finn sıcaklığının damlanın yanma olayına olan etkileri sistematik olarak incelenmiştir. Yakıtın farklı su içeriğinin, farklı damla çapının ve çeşitli finn sıcaklıklannın, yakıt damlasının çeşitli yanma kademelerine etkileri incelenmiştir. Yakıtın yanma kademeleri, yakıtın özellikleri ile ilişkil endirilmiştir. Aynca, yakıt damlasının yanma özellikleri (yanma hızı vs. gibi), çeşitli değerlendirmeler sonucunda belirlenmiştir. Yapılan incelemeler sonucunda, linyit-su kanşımının yakılmasında karşılaşılan bazı sorunlann çözümüne yönelik öneriler oluşturulmuştur. XI Yozgat-Sorgun lignite sample was oxidized at 473 K for 48 hours under dry air atmosphere to examine the effect of oxidation on dispersant adsorption. Structural changes in the oxygene containing functional groups were determined by analyzing FTIR spectra of the original and oxidized lignite samples. Oxidation caused the formation of acidic functional groups on the lignite samples leading to an advers effect on dispersant adsorption. The effect of mineral matter content of the lignite sample on dispersant adsorption was also investigated and it was observed that decreasing the mineral matter content of the lignite decreases the adsorption of dispersant. To determine combustion characteristics of the slurries prepared using different Turkish lignites, suspended single droplet combustion technique has been used. The effect of droplet size, lignite fraction in the slurry and furnace temperature on slurry combustion were studied systematically. Various combustion stages including evaporation, ignition delay, flame lifetime, char and overall combustion periods were related with the characteristics of the slurry. The results were also compared with the literature. The burning rate coefficients were calculated for overall and gas-phase combustion stages. All the results observed on the combustion of LWS droplet were summarized as follows: During the overall combustion process disintegration or disruption of either the droplet or the char into two or more portions was not observed. There is a linear, increasing relationship between the combustion stages and diameters of the LWS droplets. There is a linear, reciprocal relationship between the ignition delay time and the volatile matter content of a LWS droplet. The more increase in the volatile matter content of the slurry, the lower the ignition delay time of the droplet. Thus, using lignites of higher volatile matter content in the preparation of the slurry can be recommended as a way of reducing the ignition delay time of the droplet, which is one of the main drawback of the fuel during combustion. Since the total moisture content is sum of the moisture content of the coal and water added in the preparation of the slurry and, it affects the ignition delay time adversely, in the stuides performed to find a suitable coal in the preparation of slurry, the inherent moisture content of the coal must be considered as an important parameter as the other characteristics of the coal, such as volatile matter content, calorific value etc. The change of duration of visible flame lifetime with the furnace temperature was found to be formulated. The higher the volatile matter content of the fuel, the longer the flame lifetime of the droplet. As far as the flame stability, which is one of the main subject trying to beachived in CWS technology, is considered, using coal of higher volatile matter content may solve the problem of the flame instability which is found in the combustion of CWS in practical systems. This is encouraging for the usage of XVI ÖZET Toplam rezervi 8 milyar ton olan linyit kömürü, ülkemiz fosil enerji kaynaklan içinde birinci sırada yer almakta ve hem günümüzde hem de yakın gelecekte vazgeçemeyeceğimiz bir enerji kaynağı durumundadır. Türk linyitlerinin yumuşak karakterde olması nedeniyle, üretimleri, hazırlanmaları, taşınmaları ve depolanma ları sırasında çok miktarda toz kömür oluşumu söz konusudur. Yurdumuzdaki kömür işletmelerinin çoğunda değerlendirilemeyen toz kömür stoklan vardır. Enerji tüketiminin hızla arttığı günümüz koşullannda, yenilenemeyen bir enerji kaynağı olan linyitlerimizin en iyi şekilde değerlendirilmesi gerektiğinden, çeşitli proseslerde oluşan toz kömürün de değerlendirilmesi gerekmektedir. Kömür tozlannın değerlendirilmesi amacıyla geliştirilmiş ve dünyada, uygulanmakta olan üç yöntem vardır: 1) Toz kömür yakma sistemlerinde yakılması, 2) Çeşitli katkı maddeleri kullanarak biriketlenmesi ve 3) Kömür-sıvı kanşımlan halinde yakılması. Kömür-sıvı kanşımı yakıtlann kullanılması sonucunda petrole olan bağımlılığın azaltılması hedeflenmektedir. Bu tip yakıtlar, geniş bir kullanım alanına sahiptir. Bu çalışmanın temel amacı, Türk linyitleri kullanılarak linyit-su kanşımlannın hazırlanması ve yanma davranımlannın belirlenmesidir. Bu amaçla; öncelikle, uygun kanşımın hazırlanmasında en önemli değişkenlerden biri olan tanecik boyut dağılımının, kanşımın reolojik özelliklerini nasıl etkilediği araştınlmış ve kanşım için en uygun tane boyut dağılımı belirlenmiştir. Daha sonra, kömür-su kanşımlannın hazırlanmasında kullanılabilecek en uygun dispersantlar araştınlmış ve bazılannın linyit numuneleri tarafından adsorbsiyonu, farklı pH ve sıcaklık koşullannda incelenmiştir. Ayrıca, mineral maddenin giderilmesinin ve linyit numunelerine uygulanan oksidasyonun dispersant adsorbsiyonuna olan etkileri de araştınlmıştır. Sonuçta, bazı Türk linyitleri için, en uygun dispersant oranlan ve adsorbsiyonun en yüksek oranda gerçekleştiği koşullar saptanmış ve bu koşullarda gerçekleşebilecek değişimlerin, kanşımın reolojik özelliklerine etkileri belirlenmiştir. KSK'nın yanma davranımı, bu yakıtın gelecekte kullanımını belirleyen en önemli etkendir. Hazırlanmış olan linyit-su kanşımlannın yanma davrammlan, tek bir yakıt damlasının yanmasının izlendiği bir teknik kullanılarak belirlenmiştir. Damla boyutunun, kanşımdaki linyit derişiminin ve finn sıcaklığının damlanın yanma olayına olan etkileri sistematik olarak incelenmiştir. Yakıtın farklı su içeriğinin, farklı damla çapının ve çeşitli finn sıcaklıklannın, yakıt damlasının çeşitli yanma kademelerine etkileri incelenmiştir. Yakıtın yanma kademeleri, yakıtın özellikleri ile ilişkil endirilmiştir. Aynca, yakıt damlasının yanma özellikleri (yanma hızı vs. gibi), çeşitli değerlendirmeler sonucunda belirlenmiştir. Yapılan incelemeler sonucunda, linyit-su kanşımının yakılmasında karşılaşılan bazı sorunlann çözümüne yönelik öneriler oluşturulmuştur.

CWSs, the typical feed for advanced combustion systems such as gas turbines, are more dilute (50-55%) because of the lower packing efficiency. Fines are beneficial in slurries to provide stability against settling and to increase the solid loading; however, they represent a drawback in slurry preparation because grinding energy and therefore cost is higher. The question for the combustion technology is how do they impact atomization and burning. CWS typically requires a particle size distribution with less than 74um (200 mesh); this is standard boiler feed size. A micronized coal grind with a median particle diameter less than 15um and 98% less than 44um (325 mesh) is sometimes produced by benefication processes in the preparation of fuel for advanced combustion systems. Preferred candidate coals for CWS fuels have high heating values, high combustible volatiles, high ash fusion temperatures, high Hardgrove Grindability Indicies, and low inherent moisture, ash, and sulfur contents and low slagging and fouling potentials. The manufacture of CWSs can be integrated with coal benefication processes such as froth flotation to produce a high heating value fuel that contains a low quantity of inorganic material and sulfur. Dewatering is not required before use. The physical and combustion characteristics of the CWS vary depending on the following factors: 1. The type of coal used in the preparation of the CWS, 2. Coal size, shape, and particle size distribution, 3. The quality of inorganic material in the coal after sizing and cleaning, 4. The solid content of the mixture, 5. The stability of the fuel, and 6. The chemical additives present to modify the viscosity and stability of the mixture. These factors influence the fuel transportation, storage, and flow properties, the atomization and flame characteristics, the energy density, and the emissions; they must be considered when developing a suitable fuel specification. Low oil prices affect overall system economics even when fuel costs marginally favor CWS. A significant number of conversions to CWS are not expected until the price differential between oil and coal widens considerably. Combustion behavior of CWS fuels is the most important factor effecting the future usage of this fuel. Combustion trials, even on the smallest practicable scale of about 0.5 MW, require significant quantities of fuel, perhaps 2 t of each formulation. Consequently, it is expensive and time consuming to investigate wide variations in fuel composition. These problems can be overcome by using a technique for monitoring the combustion of single droplets of fuel. Suspended single droplet technique is widely used. The main advantage of this technique is that, droplet lifetime history, ignition delay, flame structure, centre and surface temperature of the droplet and burning rate can be investigated and a good comparative result for different fuels under the same conditions can be obtained. An understanding of these combustion characteristics enables burner manufacturers to develop equipment to ensure successful commercial application of CWS fuels. With an improved understanding of the combustion fundamentals better control of the combustion system may become possible. Observing of droplet lifetime reveals details of the mechanism of the slurry combustion. These mechanisms in general also apply to large scale practical situations in boiler and furnaces if the rate of heating is of the same order of magnitude. Clearly there will be some differences between the two systems because of different conditions and influence of droplet- droplet interactions but the general features have been found to be comparable. Therefore the single droplet studies have been used to provide much useful information about the behavior of various types of CWS. The combustion mechanism of CWS is similar to that of pulverized coal with an additional stage of water evaporation. While evaporation occurs rapidly, it still delays coal ignition. The various stages of the combustion of CWS can be described as follows:. Injection of the CWS droplet,. Drying of the CWS droplet,. Agglomeration and swelling during the coal plasticity period,. Localized ignition followed by spread of ignition,. Volatile flame formation,. Rotation induced by volatile evolution,. Extinction of volatile flame and ignition of char,. Fragmentation both during devolatilization and char burnout, and. Ash shedding and completion of char burnout. Although the water content of CWS seems considerably to have an adverse effect on the combustion of CWS droplets, the heat required to vaporize the water in a CWS is 3-4% of the heat of combustion and as such should have no substantial impact on the overall combustion process. However, this water is evaporated at the outset of the process and may affect ignition. Its presence complicates ignition and flame stability. Coal type, particle size, coal/water ratio, combustion stoichiometric ratio, and secondary air swirl number can affect combustion characteristics, such as carbon burnout and nitrogen oxide formation. Both particle size and type of coal will influence both ignition and burning characteristics of the fuel. XIV CWSs, the typical feed for advanced combustion systems such as gas turbines, are more dilute (50-55%) because of the lower packing efficiency. Fines are beneficial in slurries to provide stability against settling and to increase the solid loading; however, they represent a drawback in slurry preparation because grinding energy and therefore cost is higher. The question for the combustion technology is how do they impact atomization and burning. CWS typically requires a particle size distribution with less than 74um (200 mesh); this is standard boiler feed size. A micronized coal grind with a median particle diameter less than 15um and 98% less than 44um (325 mesh) is sometimes produced by benefication processes in the preparation of fuel for advanced combustion systems. Preferred candidate coals for CWS fuels have high heating values, high combustible volatiles, high ash fusion temperatures, high Hardgrove Grindability Indicies, and low inherent moisture, ash, and sulfur contents and low slagging and fouling potentials. The manufacture of CWSs can be integrated with coal benefication processes such as froth flotation to produce a high heating value fuel that contains a low quantity of inorganic material and sulfur. Dewatering is not required before use. The physical and combustion characteristics of the CWS vary depending on the following factors: 1. The type of coal used in the preparation of the CWS, 2. Coal size, shape, and particle size distribution, 3. The quality of inorganic material in the coal after sizing and cleaning, 4. The solid content of the mixture, 5. The stability of the fuel, and 6. The chemical additives present to modify the viscosity and stability of the mixture. These factors influence the fuel transportation, storage, and flow properties, the atomization and flame characteristics, the energy density, and the emissions; they must be considered when developing a suitable fuel specification. Low oil prices affect overall system economics even when fuel costs marginally favor CWS. A significant number of conversions to CWS are not expected until the price differential between oil and coal widens considerably. Combustion behavior of CWS fuels is the most important factor effecting the future usage of this fuel. Combustion trials, even on the smallest practicable scale of about 0.5 MW, require significant quantities of fuel, perhaps 2 t of each formulation. Consequently, it is expensive and time consuming to investigate wide variations in fuel composition. These problems can be overcome by using a technique for monitoring the combustion of single droplets of fuel. Suspended single droplet technique is widely used. The main advantage of this technique is that, droplet lifetime history, ignition delay, flame structure, centre and surface temperature of the droplet and burning rate can be investigated and a good comparative result for different fuels under the same conditions can be obtained. An understanding of these combustion characteristics enables burner manufacturers to develop equipment to ensure successful commercial application of CWS fuels. With an improved understanding of the combustion fundamentals better control of the combustion system may become possible. Observing of droplet lifetime reveals details of the mechanism of the slurry combustion. These mechanisms in general also apply to large scale practical situations in boiler and furnaces if the rate of heating is of the same order of magnitude. Clearly there will be some differences between the two systems because of different conditions and influence of droplet- droplet interactions but the general features have been found to be comparable. Therefore the single droplet studies have been used to provide much useful information about the behavior of various types of CWS. The combustion mechanism of CWS is similar to that of pulverized coal with an additional stage of water evaporation. While evaporation occurs rapidly, it still delays coal ignition. The various stages of the combustion of CWS can be described as follows:. Injection of the CWS droplet,. Drying of the CWS droplet,. Agglomeration and swelling during the coal plasticity period,. Localized ignition followed by spread of ignition,. Volatile flame formation,. Rotation induced by volatile evolution,. Extinction of volatile flame and ignition of char,. Fragmentation both during devolatilization and char burnout, and. Ash shedding and completion of char burnout. Although the water content of CWS seems considerably to have an adverse effect on the combustion of CWS droplets, the heat required to vaporize the water in a CWS is 3-4% of the heat of combustion and as such should have no substantial impact on the overall combustion process. However, this water is evaporated at the outset of the process and may affect ignition. Its presence complicates ignition and flame stability. Coal type, particle size, coal/water ratio, combustion stoichiometric ratio, and secondary air swirl number can affect combustion characteristics, such as carbon burnout and nitrogen oxide formation. Both particle size and type of coal will influence both ignition and burning characteristics of the fuel.