Çeşitli katkı maddelerinin kömür küllerinin ergime sıcaklıklarına etkisi

Abstract

Kömür yakma sistemlerinde karşılaşılan sinterlesme ve aglomerasyon olayları, yanmanın sürekliliği açısından problem yaratmakta ve sistemin verimim olumsuz yönde etkilemektedir. Aglomerasyon, kül taneciklerinin birbirine yapışıp, katı bir kütle haline gelmesi olayıdır ve özellikle akışkan yataklı yakma sistemlerinde karşılaşılan önemli bir. problemdir. Temel olarak kömürün anorganik yapı özelliklerinden kaynaklanmaktadır. Bu nedenle JmL ergime sıcakhldarı ve aglomerasyon özellikleri arasında bir etkileşim olduğu sarınmaktadır. Bu çalışmada, 9 linyit ile bir bitümlü şistin küllerinin anorganik yapılan ile özgül sıcaklıkları olarak bilinen, başlangıç deformasyon (İT), yumuşama (ST), yan küreselleşme (HT) ve akma {FT) sıcaklıkları arasındaki ilişki incelenmiştir. Çalışmada, hem orjinal, hem de belirli oranlarda, Na2C03, K2C03, CaO ve MgO katılarak bileşimleri değiştirilmiş olan "katkılı küller" göz önüne alınmıştır. Küllerin anorganik yapılan, standart yöntemler kullanılarak belirlenmiştir. Özgül sıcaklıkları LECO AF 500 model bir cihaz kullanılarak, ASTM D 1857 ' ye göre oksitleyici atmosferde ölçülmüştür. Aglomerasyon sıcaklıkları ise 10 cm çapında bir akışkan yatakta ölçülmüştür. Orjinal küllerin İT değerleri 1377-1576 K, ST değerleri 1414-1801 K, HT değerleri 1460-1814 K, FT değerleri 1482- 1844 K arasında değişmektedir. Özgül sıcaklıkları en yüksek olan kömü Eynez linyiti, en düşük olan ise Göynük bitümlü şisti olmuştur. Orjinal küllerin anorganik yapılan ile özgül sıcaklıklan arasında sistematik herhangibir ilişki görülmemiştir. Genel olarak, Na20 içerikleri yüksek olan kömürlerin çoğunlukla daha düşük sıcaklıklarda, buna karşın SİO2 oranlan yüksek ve Fe203 oranlan düşük kömürler ise daha yüksek sıcaklıklarda ergimiştir. Ancak bu durum incelene bütün kömürler için, geçerli değildir. Aynı tür küllerin yapısal değişimlerini özgül sıcaklıklara etkisini görmek amacıyla, bazı küllere değişik oranlarda Na2C03, K2CO3, CaO ve MgO katılarak hazırlanan katkılı küllerle yapılan ölçümlerin sonuçlan, sıcaklıklan en fazla etkileyen katkının Na2C030İduğunu göstermiştir. Toplam Na20 içeriğine bağlı olarak, özgül sıcaklıklarda genelde bir düşme eğilimi gözlenmiştir. Na20 katkısı bazı kömürlerin ergime sıcaklıklarını 120 K kada^düşürmüştür. Kömürlerin aglomerasyon sıcaklıkları, deformasyon sıcaklıklarına (IT) allında kalmaktadır. Aglomerasyon sıcaklıkları İT değerlerinden 175-346 K, ergime sıcaklıklarından ise, yaklaşık olarak 346-551 K daha düşük olmuştur. Akışkan yataklarda yanan kömür taneciklerinin sıcaklığının ölçülen ortalama yatak sıcaklığından yaklaşık olarak, 100-150 K daha yüksek olduğu göz önüne alındığında, aglomerasyonun deformasyon sıcaklığında veya ona çok yalan sıcaklık seviyelerinde başladığı anlaşılmaktadır.

In coal processing, coal ash may cause serious problems such as clinker trouble in fluidized bed gasifiers, sintering in stoker combustors, agglomeration in fluidized bed combustors, fouling of heating surface in combustors, vaporization of harmful elements from the ash. Therefore, prediction of ash behavior is of vital importance, particularly for combustion systems In fluidized bed combustion systems, agglomeration (sintering) of ash (coal) and bed material particles can cause severe problems. As a result of agglomeration, deposition can form on heat transfer surfaces and bed walls. Accumulation of sticky ashes on surfaces can cause substantial decrease in heat transfer coefficient, disturb air distribution and consequently fluidization in the bed. In the most serious cases, sintering of particles can lead to heavy agglomerate mass which finally disturb fluidization and combustion completely. In these cases the system has to be dismantled in order to remove agglomerates and restore the normal operation. The measurement of so called characteristic ash temperatures, namely initial deformation temperature (IT), softening temperature (ST), hemispherical temperature (HT) and fusion temperature (FT) is considered to be one of the best methods to estimate the behavior of coal ash in coal combustors in practice. The agglomeration is a process which describes formation of agglomerates from particles under heating. The earlier works suggest that, in fluidized bed combustors, agglomeration can occur as a result of partial melting and stickiness of bed particles due to the operating of fluidized bed at temperatures higher than the melting point of coal. But later research indicate that agglomeration can occur at temperatures much lower than the ash fusion temperature. The temperature at which agglomeration starts is called first sintering temperature. The properties and behavior of ashes are remarkably different among coals and since coal ash is extremely complex mixture of minerals it is not possible to predict the fusion behavior of such mixture from its chemical composition with a reasonable accuracy. Many correlations have been developed to estimate the softening and fusion behavior of ashes from their chemical composition, in general, all of them suggest that the softening and fusion temperatutes increase as the amounts of asidic oxides (SİO2, AI2O3) in the ash increase. Therefore, it is essential to study the individual coal deposit in order to see how they exactly behave in practical combustion. The objectives of this work is to determine the characteristic ash temperatures of some major Turkish lignites, to investigate the relationship between their temperatures and inorganic compositions and relationship between ash fusion temperatures and inorganic compositions and relationship between ash fusion temperature and agglomeration temperature of some selected coals. The effect of addition of some metal oxides which are the major component of ashes of coals on the characteristic temperatures is also investigated. Ten coal samples studied were taken from the major lignite deposits in various districts of Turkey. One sample out of ten is oil shale. The samples are ; 1 ) Beypazarı - Ankara 2 ) Çan - Çanakkale 3 ) Elmalı ( Soma ) - Manisa 4 ) Eynez - Manisa 5 ) Deniş ( Soma ) - Manisa 6 ) Göynük - Bolu 7 ) Göynük Oil shale - Bolu 8 ) Kısrakdere - Manisa 9 ) Tunçbilek - Kütahya 10) Yatağan - Muğla The following analyses were carried out in order to characterize coal samples and ashes : Proximate analysis, elemental analysis, measurement of characteristic temperatures and chemical analysis of ashes. Coal samples were crushed and sieved to - 200 mesh and ashes were prepared using the TS 330 procedures. Ash compositions were determined according to ASTM procedure D 2795 - 86. Ash fusion measurements were made by using a LECO AF - 500 type ash fusion furnace with digital read - out, sensivity of 5K and a maximum temperature of 1844 K according to the ASTM procedure D 1857 - 68. Ash cones were prepared and put into the furnace and were heated to the fusion temperature with a heating rate of 8 K/ min. under oxidizing gas atmosphere. The results are presented as the an average values of repeated measurements. The results of chemical analysis of ashes and ash fusion measurements are presented in Table 1 and Table 2 respectively. As seen from Table 2, initial deformation temperatures and fusion temperatures of lignites change between 1377 - 1573 K and 1482 - 1844 K, respectively. The fusion temperature of Eynez lignite was out of the temperature range of ash fusion furnace and it could not be measured exactly. Comparison of Table 1 and Table 2 indicated that, in general, coals with higher Si02 and lower Fe203 contents such as Eynez, Elmalı, Kısrakdere, Tunçbilek and Yatağan lignites have higher initial deformation and fusion temperatures while coals with higher Na20 percentages have lower fusion temperatures. However, no regular relationship is seen between chemical composition and critical temperatures of ashes. For example, in spite of their high Na20 percentages, Eynez, Kısrakdere and Tunçbilek lignites have fusion temperatures higher than that those with lower Na20 percentages. In Figure 1, the initial deformation temperatures are given as a function of total percentage of (Na20 + K20 + CaO + MgO). These oxides are called basic oxides which are considered, chemically, to be the most active components and have the leading role in agglomeration (sintering) behavior of ash. It is clear from figure that no regular relationship between composition and deformation temperature can be descibed for original coal ashes. The initial deformation temperature differs widely even for ashes having nearly the same total basic oxide percentage. A set of experiments was carried out by using of Na2C03, K2CO3, CaO and MgO as additives to ashes in order to investigate the relationship between the X U.m&<*J..l I X-/.CA, I 1^-TV J X^VS^ IT : Initial deformation temperature, ST : Softening temperature, HT temperature, FT :Fusion temperature. Hemispherical composition and fusion charteristics for the individual ashes. In these experiments, CaO and Na20 contents of all ashes were increased by adding of CaO and Na2C03. K2C03 and MgO were added only to Beypazarı lignite ash. The characteristic temperatures of Beypazarı Elmah, Eynez, Tunçbilek and Yatağan lignites and Göynük oil shale decreased regularly with increasing CaO content while no considerable changes was observed for other lignites. A maximum decrease of 130 K was determined in fusion temperature with an increase of 9% in CaO for Eynez and Yatağan lignites. The Na20 content of ashes was increasd by using Na2CÛ3. Increasing of the amount of Na20 lowered the characteristic temperatures of Beypazarı, Çan, Eynez, Kısrakdere, Tunçbilek and Yatağan lignite ashes. Temperature profiles of Elmah XI lignite ash had a minimun around 8% Na20 below which temperatures decreased while above which increased slightly with increase in Na20 contest. Increasing of Na20 content by 10%, caused a 120 K and 180 K reduction in fusion temperatures of Beypazarı and Eynez lignite, respectively, which were the highest reductions determined. Other ashes were not affected significantly by the Na20. 10 15 20 25 30 %{CaO+ MgO+ Na20+ K20} 35 40 Figure 1. Change of fusion temperature of oginal coal ashes with (Na20 + K20 + CaO + MgO) % Another topic studied in this work is the relationship between the agglomeration and characteristic temperatures of lignites. For this purpose, a set of experiments was performed in which the agglomeration temperatures (Tag) of Beypazarı, Çan, Eynez, Kisrakdere, Göynük and Yatağan lignites were determined in a 10 cm id fluidized bed combustor. Results of agglomeration measurements are compiled in Table 3. The difference between agglomeration temperature and characteristic temperatures are also included in the Table. The differences described as AT, = IT - Tag, AT2 = ST - TAG, AT3= HT - TAG, AT4 = FT - TAG Agglomeration tempertures listed in Table 3 are lowest temperatures where first agglomerates formed. Data in the table indicate that agglomeration of lignite can occur at temperatures several hundred degrees below fusion temperature for a lignite in a fluidized bed coal combustor. Values of ATi, AT2, AT3 > AT4 show that the agglomeration behavior of lignites with different ash compositions vary widely and can not be explained on the basis of ash fusion temperature. XII Table 3. Agglomeration Temperature of Some Lignites.