Linyit yakan değişik dağıtıcı elekli akışkan yataklarda ayrışma ve aglomerasyon

Abstract

Bu çalışmanın amacı Çan ve Yatağan linyitlerinin akışkan yataklı yanma sürecindeki davranımları ve çeşitli parametrelerin bu davranımlara etkilerini incelemeye yönelik olmuştur. Çeşitli parametrelerin etkilerinin araştırılmasında yatak içi sıcaklık dağılımları ve yatak basınç düşüşü değerlerinden yararlanılmıştır. Çalışmalar da yatak malzemesi olarak kuartz (kum) ve kireçtaşı kullanılmıştır Deneyler paslanmaz çelik malzemeden yapılmış 0.188 m iç çapında silindirik akışkan yatakta delikli düz, delikli eğik ve nozul tipi olmak üzere üç farklı dağıtıcı elek kullanılarak gerçekleştirilmiştir Yanma süreci bilgisayar ve data veri toplama ünitesi yardımıyla kontrol edilmiştir. Değişik tanecik boyutundaki yatak malzemesi ve kömür ile farklı ayrışma rejimleri oluşturulmaya çalışılmıştır. Yatak malzemesi tanecik boyutunun 6.3x1 0"4- 1x1 0"3 m ve kömür tanecik boyutunun 1x10"3-2x10"3 m olarak seçildiği ikili sistemlerde linyit yüzüşen zengin ayrışma rejimi gözlenmiştir. Düşük hızlarda delikli eğik dağıtıcı elek kullanılan yataklarda linyit yüzüşen zengin ayrışma eğilimi artmaktadır. Linyit yüzüşen zengin sistemlerde yatak malzemesi olarak kireçtaşı kullanılması ayrışma eğilimini azaltmaktadır. Bunun yanı sıra akışkanlaştırma hava hızının artırılması daha tekdüze bir sıcaklık dağılımına sebep olmaktadır. Yatak malzemesi tanecik boyutunun SxlO^-e.SxIO"4 m ve kömür tanecik boyutunun 4x10"3-5x10"4 m olarak seçildiği sistemlerde yatak malzemesi olarak kireçtaşı kullanılması durumunda linyit batışan zengin ayrışma rejimi oluşmaktadır. Bu ikili sistemde kireçtaşı yoğunluğunun azalması sonucu tanecik boyutu farkı ön plana çıkarak ayrışma rejimini belirlemede etkin rol almaktadır. Linyit yüzüşen zengin sistemlerde belirgin bir ayrışma sürecinden sonra ag- lomerasyon görülmektedir. Aktif yatak yüzeyi ile dağıtıcı elek civarındaki kesitler arasındaki sıcaklık farkları ayrışma eğilimi şiddetini belirlemede ve aglomerasyon süre cinin gelişmesini izlemede bir gösterge olarak kullanılabilmektedir. Linyit batışan zengin sistemlerde aglomerasyon öncesinde belirgin bir sıcaklık farklılaşması görülmemektedir. Çalışmalar sonucunda karışma etkinliği en iyi olan dağıtıcı eleğin delikli düz dağıtıcı elek olduğu görülmüştür. Bu çalışmada kullanılan Çan linyiti Yatağan linyiti ne kıyasla daha düşük sıcaklıklarda aglomere olabilmektedir. Kömürün anorganik yapısında bulunan Na20 oranı aglomerasyonda önemli rol oynamaktadır.

The main purpose of this study is to investigate the fluidised bed combustion behaviour of lignites which constitute one of the main energy resources of Turkey. Eventhough there has been a lot of studies on fluidised bed combustion, there are still problems that need to be solved. Agglomeration is one of the leading problems. Agglomeration is defined as the sticking of lignite, ash and bed material due to various reasons which causes a solidified state in the fluidised bed. Agglomeration terminates the combustion in fluidised beds. During the experimental studies two important lignite reserves of the country namely Çan and Yatağan were combusted in a fluidised bed using three different distributor plates. For each of these different distributor plates the changes in the combustion process due to changes in various parameters were investigated. The effect of parameters on combustion was investigated using the bed temperature distribution and pressure drop measurements. Also investigation of the surface behaviour of the bed, bed inventory, and agglomerated bed were helpful for the understanding of deviations from normal operation. The Çan lignite samples were obtained from the coal mines of Turkish Coal Works (TKİ) Çanakkale Çan Region Administration. The coal was recovered from galery type mines. The other coal samples were obtained from the line feeding Yatağan power station from the sieve exit position. During the experiments quartzsand and limestone were used as bed materials. The lignite and bed materials were subjected to various characteristic analysis relevant to fluidised bed combustion using Turkish Standarts and ASTM if the former is not existing. The lignite samples were crushed and classified in two different particle size distributions of 1x10"3-2x10"3 and 4x10"3-5x10"3 m. The limestone and quartz sand on the other hand were classified as 3x1 O^-o.Sxl 0"4 and 6.3x1 0"4-1x10"3 m. The particle size distribution of lignite and bed material were selected such that in a continous fluidised bed combustion experiment if would be expected to have lignite flotsam and jetsam rich segregation systems by appropriate choice of lignite and bed material particle sizes. VIII - The experimental set up used during the study was mainly consisted of the following parts. 1- Fluidised Bed 2- Preheating System 3- Lignite Feeding System 4- Pressure Monitoring System 5- Cyclone and Draft Fan 6- Control Units. The fluidised bed was manufactured from a 0.1 88 m diameter and 1 m long cylindrical mild steel. At various heights holes were drilled for the purpose of temperature and pressure measurements and feeding and discharging purposes. The three types of distributor plates used in this study were. a) Perforated Plate b) Perforated Slanted Plate c) Nozzle Type Plate. On top of the perforated plate, hole, having 9x1 0-4 m diameter were located on square pitch arragement. The ratio of open area to the total cross sectional area was chosen to be 1.9 %. In the case of perforated slanted plate holes of 1x10"3 m were distributed using a equilateral triangular pitch. The radio of open area was taken to be 1.8 %. The plate was manufacture such that it made a 25° angle to the horizontal plane. On top of the nozzle type distributor plate 37 nozzles were fixed. In each of the nozzle there were 3 holes 120° from each other. The diameter of the holes were 2x1 0-3 m. The ratio of the open area was 1.3 %. All the components of the fluidised bed combustion system were computer controlled. The data acquisition and control was accomplished by use of a programme developed for this study. During the cold bed hydrodynamic studies, the pressure drop measurements were found to increase with increasing fluidisation velocity. The perforated plate was found to give the highest pressure drop under comparable conditions compared to the other two distributor plates. This was mainly due to the higher number of perforations which resulted in a higher amount of frictional loss. Experiments were repeated for various particle sizes for both bed materials. The experimentally measured values of the pressure drop were compared to the calculated values obtained from the equations proposed by Qureshi and Creasy in the literature. It has been observed that for the empty bed the distributor pressure drop values obtained from experimental measurements and from Qureshi - ix - and Creasy equation were in reasonable agreement for perforated and slanted perforated plate. Pressure drop measurements under loaded conditions and calculated values were in reasonable agreement for slanted and nozzle type plates. However, there was considerable differences for the plane perforated plate. The equation which calculates the pressure drop for loaded beds is calculated from the empty bed equation by using a factor of 0.70. Therefore, choice of an alternative design may easily deviate from the results of the equation. in literature the stability criterion for the fluidised beds were based on the "critical pressure ratio Rc ". The critical pressure is defined as Rc=APdVAPb in this equation APd* and APb represent the loaded distributor and bed pressure drops under a defined air velocity, respectively. The Rc values obtained for various distributor plates were plotted. The graphical representation showed that the performance of each of the three distributor plates were well above the critical value. Therefore it is understood that the inclination of the distributor plate to segregation would not be vigorous. Of course high Rc values provide stability at the expense of power consumption. Therefore in real operation it should be optimised. The combustibility characteristics of the two lignites used in the present work was investigated using differential thermogravimetric method by obtaining burning profiles. The results of this investigation showed that the combustibility of the two lignites did not differ considerably from each other. However the shoulder effect observed the maximum combustion temperature and completion of the combustion was at 450°C for Çan and 520°C for Yatağan. This difference was consistent with the nature of the anorganic matrix of the two lignites. It was possible to forsee from burning profiles that aglomeration would start at lower temperatures for Çan compared to Yatağan lignite. In the combustion runs the preheating system was used to raise the temperature of the bed above the ignition temperature of lignite. The lignite was burned together with LPG and later LPG was cut off. In order to see the lignite flotsam rich segregation the particle size of lignite and bed material were kept at 1x10"3-2x10"3 m and 6.3x1 O^-lxlO"3 m respectively. It is well known that the distributor plate type and fluidisation velocity effect the fluidisation quality, mixing, and segregation tendency. In order to investigate the effect of the three types of distributor plates on segregation, experiments were carried out under similar conditions. The experiments that were carried out using 1x10"3-2x10"3 m lignite and SxlO^-o-SxIO"4 - x- m bed material particle sizes and perforated distributor plate showed a relatively weak segregation tendency consequently the time taken for the establishment of the process took relatively longer time. The strongest segregation tendency was observed for the slanted perforated plate. For this type of distributor plate of the lower end of the bed accumulation of the coarser (jetsam) particles were observed which affected the circulation behaviour in the bed. The nozzle type distributor plate evidently was between the other two types of distributor plates as far as the segregation tendency is conserned. The segregation was followed from the differential temperatures measured at 0.1 and 0.01 m levels from the distributor plate. The time taken between the start of defluidisation and agglomeration is termed as segregation time. Comparing the start of segregation times showed that it was longest for the slanted, perforated plate and shortest for the perforated plate. Apart from distributor plate characteristics fluidisation velocity was found to be an important parameter affecting the mixing and segregation in the bed. For all of the distributor plates an increase in the fluidisation velocity improved mixing and detained segregation. For this reason at times it was necessary to run the experiments under relatively low fluidisation velocities to observe segregation. This was especially true for the lignite jetsam rich systems. The pressure drop measurements at hot operation conditions showed that under good mixing condition the pressure drop was fairly uniform. Increases in the pressure drop observed at low fluidisation velocities were attributed to the ash accumulation. Replacing quartz with limestone as bed material resulted in a decrease in the segreation tendency for all distributor plates. One of the reasons for the improvement in bed performances was attributed to thermal schock and mechanical impact on the limestone particles after which the difference between the densities of bed material and lignite become narrower. Density is cited as dominant factor in determining the hydrodynamics of segreation. For the experiments in which lignite and bed material particles sizes were kept as 4x10"3-5x10"3 m an 3x1 0"4-6.3x1 0"4 m the differential temperatures values decreased indicating a reduction in segregation tendency. In the binary system where quartz sand was used as the bed material there was a distinct lignite flotsam rich segregation regime. However the particle size of the quartz not changing with thermal and mechanical changes played a determining role on the segregation behaviour. This effect can be better understood if results of limestone are compared. - xi - The main factor with regard to bed material is the density gradient closing which magnifies the effect of the relative particle sizes of bed material and lignite. During the course an the experiment it has been observed that the initial segregation mode dictated by particle sizes and densities of the binary system may change to an another regime. This has been observed for Çan combustion in which case the starting lignite jetsam rich type segregation gradually changed to flotsam rich type system. This was attributed to the accumulation of ash in the bed together with the loss of bed material through overflow. In due course larger ash material accumulated at the level of distrubutor plate acted as the jetsam component. Investigation of the ashes of Çan and Yatağan lignite showed that the former had lower affinity for attrition. In all experiments it was realised that segregation was a prestage of agglomeration. Also there was an obvious relation between the intensity of segregation and agglomeration. However global generalisation is not appropiate since there was differences between the effect of distributor plates to segregation and agglomeration. The tendency to agglomerate increased after the start of defluidisation and development of segregation. This was true for both lignite flotsam and jetsam rich segregation systems. It has been realised that one other important parameter affecting agglomeration is the bed material/lignite ratio. As this ratio decreased the tendency to agglomerate increased. High fluidisation velocities decreased the agglomeration tendency. The effect of fluidisation velocity has got differences for different distributor plates. It has been pointed out that the fluidisation characteristics of perforated distributor plates was better than the others. As a matter of fact at velocities as low as 0.5 m/s combustion could be maintained without agglomerating the bed. This was not possible for the other two distributor plates. During the process of agglomeration sudden drop in pressure has been realised. Also the concentration of lignite at the lower and higher levels of the bed related to the segregation mode has been observed. Agglomeration was a much faster phenomenon for the lignite jetsam rich system. The distributor plate effect and the structure of the ash was important factors in the determination of agglomeration temperature.