PROMENE RASPODELE VELIČINE ČESTICA LIGNITA U CIRKULACIONOM FLUIDIZOVANOM SLOJU. Beogradu Mike Alasa 12-14, Beograd, Srbija

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1 PROMENE RASPODELE VELIČINE ČESTICA LIGNITA U CIRKULACIONOM FLUIDIZOVANOM SLOJU M. Paprika *, M. Komatina **, S. Nemoda *, M. Mladenović *, B. Repić * i G. Živković * * Laboratorija za termotehniku i energetiku Instituta za nuklearne nauke Vinča, Univerzitet u Beogradu Mike Alasa 12-14, Beograd, Srbija ** Mašinski fakultet Univerziteta u Beogradu, Kraljice Marije 18, Beograd, Srbija Apstrakt: Raspodela čestica goriva je veoma važan faktor u sagorevanju u cirkulacionom fluidizovanom sloju (CFS) jer direktno definiše elemente kotlovskih postrojenja sa CFS koji su u vezi sa aerodinamikom sloja, emisijom zagađivača, erozijom, naslagama i prljanjem kotlovskih površina. U radu su razmotreni su uzroci i posledice promene raspodele koksnog ostatka čvrstog goriva u cirkulacionom fluidizovanom sloju, kao i razlike u ponašanju pri fragmentaciji u mehurastom i cirkulacionom fluidizovanom sloju. Dati su rezultati eksperimenata i simulacije raspodele veličine čestica koksnog ostatka lignita Kolubara. Raspodela veličine i broja čestica koksnog ostatka uglja Kolubara u fluidizovanom sloju se značajno razlikuje od raspodele veličine i broja čestica uglja na ulazu u sloj. Dat je pregled poređenja rezultata eksperimenata i simulacije primarne fragmentacije uglja Kolubara za tri karakteristične temperature i različite granulacije. Takođe, dat je pregled koeficijenata Vejbulove raspodele za sitniju i krupniju sekciju fragmenata na osnovu koje se može izvršiti predviđanje raspodele koksnog ostatka za različite uslove sloja i prvobitne veličine čestica. Ključne reči: ugalj, fluidizovani sloj, devolatilizacija, matematički model, primarna fragmentacija SIZE DISTRIBUTION CHANGES OF LIGNITE IN CIRCULATING FLUIDIZED BED M. Paprika *, M. Komatina **, S. Nemoda *, M. Mladenović *, B. Repić * and G. Živković * * University of Belgrade, Institute of Nuclear Sciences Vinča, Laboratory for Thermal Engineering and Energy Mike Alasa 12-14, Belgrade, Serbia ** Belgrade University, Mechanical Faculty, Kraljice Marije 18, Belgrade, Serbia Abstract: Particle size distribution (PSD) is a very important parameter in circulating fluidized bed (CFB) combustion, because it directly defines systems in relation to bed dynamics, pollutant formation, corrosion, erosion, slagging, and fouling. In the paper, the causes and consequences of the change of particle size distribution are discussed, as well as the differences of fragmentation behavior in bubbling (BFB) and circulating (CFB) fluidized bed. The results of the change of PSD

2 of lignite Kolubara are given. The char particle size distribution differs significantly from the inlet coal particle size distribution. An overview of comparison of the experimental and model results for three characteristic temperatures and granulations are given. Also, the table of Weibull coefficients for the fine and large char sections is given. It can be used to predict char particle size distribution for various conditions of fluidized bed and original coal size. Key words: coal, fluidized bed, devolatilization, mathematical model, primary fragmentation 1. INTRODUCTION Characteristics of the Kolubara lignite basin indicate the future exploitation in complex coal-bearing series, with distinct stratification of coal seam. In that view, the quality fluctuations of the coal, excavated by current equipment, are inevitable. Also, there is a growing need for utilization coal reserves with the heating value from 3500 to 5300 kj / kg and off-reserve coal with heating value below 3500 kj / kg. At the same time, already adopted regulations for power plant emissions impose the necessity of reducing emissions below the values for the boilers burning pulverized fuels, without desulphurization and measures taken to reduce emissions of oxides of nitrogen [1]. Development of surface mining in Kolubara and Kostolac-Kovin basin should ensure coal supply to the power plant blocks included in the revitalization which will extended their lifespan of about 15 years. For the construction of new power plant blocks it is necessary increase capacity in coal mines, also. In the both mines, the next deposits have ever more difficult exploitation and they have to be selectively excavated. The coal obtained in that way has a lower calorific value. Combustion of this coal is far better suited to CFS furnaces and boilers, because the combustion of a poor quality coal in FB is favorable from the environmental aspect, which is going to have a growing importance in future [1]. For these reasons, the next generation of boilers utilizing Kolubara coal has to be generally more flexible in view of coal quality, and the circulating fluidized bed (CFS) technology is responding to these issues. An understanding of the behavior of coal particles during combustion process in a wide range of temperatures and particle sizes are necessary to optimize combustion and to develop reliable models of processes, as well as to solve practical, constructional issues. The devolatilization, as the first stage of the combustion, has an important role in the process. The primary fragmentation is an important concomitant of the devolatilization that affects the distribution of fuel along the furnace height, the formation of ash, and pollutant emissions. In that respect, the results of the primary fragmentation investigation of the coal Kolubara are given here. Investigation is conducted by experiments and a mathematical model, described in detail elsewhere [2]. 2. PARTICLE SIZE DISTRIBUTION Combustion and devolatilization of solid fuels in fluidized bed depend on the characteristics of the individual particles that participate in the process. The number of these particles is enormous, and it is impossible to create a model of the process, which takes into account a detailed description of the behavior of the individual solid fuels particles. The complexity of the numerical procedure would jeopardize its viability. However, the characteristics of individual particles must be taken into account and models that accurately describe the process cannot be based solely on the averaged values of the sample. Likewise, the interpretation of the experimental results and the conclusion of the process, which is based on the mean values of the particle size, cannot be accurate. Flow characteristics of solid particles in a fluidized bed (as well as in other suspensions of gas and solid particles) vary, depending on the geometrical and physical properties of the particles, for both particles of inert material and the solid fuel particles. Geometric particle characteristics: size, size distribution and shape, affect the flow through the thrust force, the rate of formation and

3 dissipation of bubbles. The solid fuel particles in the fluidized bed are usually polydisperse and irregular in shape. Physical characteristics affect the adsorption, elastic and plastic deformation, cracking, heat transfer properties and materials. These physical characteristics are not always independent of each other, for example specific gravity depends on the composition, the active area of the size, etc. Therefore, it is necessary to establish a specific scheme for describing the characteristics of particles in a population that will allow a sufficiently detailed insight into the characteristics of individual particles, and at the same time, be economical enough not to have to strictly define the size of each individual particle The process as the process of fragmentation of the primary fragmentation of the starting particles is primarily dependent on the size of particles that participate in the process, as well as by other characteristics such as composition, bulk density, brittleness, etc. In order to describe and give a value to the primary fragmentation, as well as to measure and compare the experimental results, the particle size distribution (PSD) is used. It is a mathematical way to describe the change in the number and size of the solid fuel. 3. PRIMARY FRAGMENTATION The majority of authors classify the fragmentation into primary and secondary. Primary fragmentation happens in the initial moments of the combustion process, due to the thermal shock suffered by the particles when they are introduced into the bed. Thermal stresses produced in the structure of the particles and the interior pressure rise due to the volatiles release, lead to their fragmentation in smaller sized particles. The secondary fragmentation takes place during the combustion process, and it is due to the structural fragility of the particles, provoked by the combustion process and by inter-particle collisions [3]. The results show that ignoring the phenomenon of fragmentation, diffusive and kinetic data can be overestimated by about 5 10% [4, 5]. The main parameters of the primary fragmentation are used for its characterization and they are defined as follows Primary fragmentation ratio (intensity, particle multiplication factor):. Here, is the number of the char particles and is the number of the original coal particles. Changing ratio of coal particle size (variation factor of fed particles):. Here, is the mass fraction of particles with size, is the total number of size classes, is the average diameter of coal particles with size after the fragmentation, and is the diameter of the original coal particles. Primary fragmentation index:. That is the most comprehensive parameter of the process, taking into account the changes in number and size of particles. Coal combustion processes in CFB boiler are very complex, undergoing the following interrelated sequences: heating and drying, devolatilization and volatile combustion, swelling and fragmentation, and char combustion, the latter process being the longest one. The unburned carbon content in the fly ash is believed to be the final results for characterizing the combustion efficiency. Here, an assumption was made that the most of the unburned carbon in the fly ash originates in the process of primary fragmentation. Understanding this process, and being able to predict the amount of fine char particles produced in it, can help to optimize the combustion process and make predictions of the combustor performance in a broad range of operating conditions. The higher carbon content in the fly ash directly decreases the combustion efficiency of CFB boilers. The carbon content in the fly ash depends on the coal rank strongly [6]. At the same time, the increase of the bed temperature and residence time can promote the chemical reaction rate and consequently increase the combustion efficiency. Recycling the ash from an electrostatic

4 precipitator (ESP) to burnout fine high reactive particles (originated in primary fragmentation) can improve the overall combustion efficiency, whilst, for the low-reactivity char particles (originated in the attrition), even if they are fed back to the furnace, their burnout will not increase much [6]. 4. EXPERIMENTS gasni analizator 80 mm gas za fluidizaciju FS distributor izolacija N 2 N 2 voda Figure 1 Experimental apparatus Experiments were carried out in a furnace with an electric heater (Figure 1). Upon reaching steady state with the desired conditions in the furnace (temperature 600 C, 800 C or 850 C), portions of the coal particles of a certain granulation were brought into the fluidized bed through the upper opening of the apparatus. After completion of the process of devolatilization the entire FB material is discharged from the furnace into a container. The content of the fluidized bed (sand and char) is cooled in the stream of nitrogen, which prevents further combustion. Also, the container with the bed material is placed in a vessel with water, to expedite cooling. After cooling, the bed material was sieved and the char particle distribution was measured. Each coal sample was photographed before and after devolatilization process (Figure 2), and the photographs were analyzed using the computer program for image analysis, giving statistical data on the number, area, perimeter, shape factor, average diameter, and estimated volume of the elements in the image. Table 1 Proximate and ultimate analysis of Kolubara coal, dry ash free Proximate analysis ash (%) volatile (%) char (%) C fix (%) combustible (%) heating value (kj/kg) Ultimate analysis C (%) H (%) N (%) S (%) O (%) In the experiments that investigate characteristics of the primary fragmentation of the coal Kolubara, the particle size and fluidized bed temperature were varied, because these factors have the greatest impact on the primary fragmentation [7]. The tested granulations were: 4-4,75mm, 6,3-

5 mass fraction (-) 8mm, 8-10mm, 10-15mm, for the FB temperatures 600 o C and 850 o C, and granulations 7-10mm, 10-13mm, 13-16mm for the FB temperature 800 o C. The three characteristic fluidized bed temperatures were selected: 600 o C, 800 o C and 850 o C. Fluidized bed facilities start at temperatures close to 600 C, so this temperature is chosen to examine the behavior of coal during devolatilization at the start-up. Additionally, this temperature is a critical from the aspect of the bed agglomeration [8]. The temperatures 800 o C and 850 o C were chosen because they are close to the operating temperatures of fluidized bed facility. The literature has noted that the degree of fluidization does not affect the processes occurring in the particle, especially not the primary fragmentation [9], so the selected fluidization ratio was N=2. The mass of the portion is adjusted in a way that a minimum concentration of oxygen in the experiment was between 1 and 3% and the average about 8%. 5. RESULTS AND DISCUSSION Observations of the primary fragmentation of Kolubara coal in the experiments testing concur with those recorded previously in the literature about the fragmentation of lignites. A coal Kolubara particle fragmentizes into a smaller number of larger pieces, and it often happens that the particles only crack and do not break. The section of smaller fragments is not that rich. Results of particle size distribution can be displayed in two ways, by showing: mass fraction of classes or the cumulative distribution of the mass fraction. Figure 2 shows the comparison of experimental results and the model for the distribution of mass fraction of particles, and Figure 3 cumulative distribution of mass fraction size from 4 to 4.75 mm, before and after devolatilization in a fluidized bed temperature of 850oC. 0,7 0,6 0,5 0,4 0,3 0,2 0,1 T fb =850 o C, D c = mm ulaz inlet eksperiment experiment model particle diameter (mm) Figure 2 Distribution of mass fraction of particles size of 4-4,75 mm before and after devolatilization in a fluidized bed of temperature of 850 o C, the results of experiments and the model

6 cumulative mass fraction (-) 1,2 1 0,8 0,6 T fb =850 o C, D c = mm ulaz inlet eksperiment experiment model 0,4 0, particle diameter (mm) Figure 3 Cumulative distribution of mass fraction of particles size of 4-4,75 mm before and after devolatilization in a fluidized bed of temperature of 850 o C, the results of experiments and the model Table 2 The values of the Pearson correlation coefficient of experimental and model results, for char PSD temperature size 600 o C 850 o C mm mm mm mm o C mm mm mm 0.77 Table 2 shows the values of the Pearson correlation coefficient of experimental and model results for specific experimental conditions. As a typical lignite coal, Kolubara does not fragmentize very extremely. The results of the model and experiment show the same tendency - a coal particle exfoliates at the beginning of devolatilization, producing a large number of fine fragments, whilst in the continuation of the process, the parent particles sometimes break down into a smaller number of larger pieces, and sometimes does not break at all. 6. CONCLUSION The main consequence of the coal breakage during the first stage of combustion, the so-called primary fragmentation, is the change of the particle size distribution of the char population in comparison to the original size of the coal particles. This change affects the fluidized bed aerodynamics, the distribution of fuel along the furnace height, ash particle size distribution, and losses due to the fly ash. Ignoring the phenomenon of fragmentation can lead to errors in modeling the processes in both BFB and CFB furnaces. An analysis of the process and excerpt of experimental results is offered in the paper. The losses due to unburned carbon so far were mainly attributed to the process of attrition, or neglected. However, depending on the initial particle size and FB temperature, as shown here, the section of the fine char particles is not negligible, and it may represent a contribution to elutriable carbon. In designing of a high-efficiency FB unit that burns coal Kolubara, an estimation of quantity of fine char particles has to be carried out. The particle size distribution of ash is one of the factors that affect the aerodynamics of the FB, especially in the case of CFB. The char particle size

7 distribution, as shown in the above manner, can enhance its assessment. Considering that, it can be expected that the majority of Kolubara char particles are going to be concentrated in the lower levels of furnace space in the dense zone, and that the volatiles and finer char particles are burning in the freeboard. ACKNOWLEDGMENTS The authors thank the Ministry of Education, Science and Technological Development of Serbia for enabling funding of the projects TR33042 Fluidized bed combustion facility improvements as a step forward in developing energy efficient and environmentally sound waste combustion technology in fluidized bed combustors and III Development and improvement of technologies for energy efficient and environmentally sound use of several types of agricultural and forest biomass and possible utilization for cogeneration. REFERENCES 1. Prof. dr Simeon Oka, d.i., rukovodilac Studije, et al., Istraživanje mogućnosti primene sagorevanja u cirkulacionom fluidizovanom sloju (cfs) u kotlovima u Elektroprivredi Srbije, S. Oka, Editor. 2006: Belgrade. 2. Paprika, M.J., et al., Prediction of Coal Primary Fragmentation and Char Particle Size Distribution in Fluidized Bed. Energy & Fuels, (9): p Scala, F., R. Chirone, and P. Salatino, Combustion and Attrition of Biomass Chars in a Fluidized Bed. Energy & Fuels, (1): p Pereira, C.C. and C. Pinho, Influence of particle fragmentation and non-sphericity on the determination of diffusive and kinetic fluidized bed biochar combustion data. Fuel, (0): p Nikku, M., et al., Three-dimensional modeling of fuel flow with a holistic circulating fluidized bed furnace model. Chemical Engineering Science, (0): p Xiao, X., et al., Research on Carbon Content in Fly Ash from Circulating Fluidized Bed Boilers. Energy & Fuels, (4): p Milijana Paprika, M.K., Franz Winter, Dragoljub Dakić, Factors Affecting Primary Fragmentation During Combustion of Serbian Coals in 18th International Conference on Fluidized Bed Combustion. 2005: Toronto, Ontario, Canada. p Al-Otoom, A.Y., et al., Experimental Options for Determining the Temperature for the Onset of Sintering of Coal Ash. Energy & Fuels, (1): p Sasongko, D. and J.F. Stubington, Significant factors affecting devolatilization of fragmenting, non-swelling coals in fluidized bed combustion. Chemical Engineering Science, (16): p