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Experimental and Numerical Investigation of the Primary Fragmentation of a Lignite During FB Devolatilization Milijana J. Paprika, Mirko S. Komatina, Dragoljub V. Daki#, Goran S. Živkovi#, and Milica R. Mladenovi# Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef5024803 • Publication Date (Web): 03 Apr 2015 Downloaded from http://pubs.acs.org on April 8, 2015
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Experimental and Numerical Investigation of the Primary Fragmentation of a Lignite during FB Devolatilization Milijana Paprika1*, Mirko Komatina2, Dragoljub Dakić3, Goran Živković1 and Milica Mladenović1 1
Vinča Institute of Nuclear Sciences, University of Belgrade, PO Box 522, 11001 Belgrade,
Serbia 2
Faculty of Mechanical Engineering, University of Belgrade, Kraljice Marije 16, 11000
Belgrade, Serbia, 3
Innovation Center of Mechanical Faculty, University of Belgrade, Kraljice Marije 16, 11000
Belgrade, Serbia Keywords: lignite coal, primary fragmentation, fluidized bed, devolatilization
Abstract The paper presents a comparison between experimental and model results of primary fragmentation of a lignite coal in fluidized bed. In the experiments, the char particle size distribution and the general indicators of primary fragmentation (intensity and index) were determined. The same parameters were calculated using a mathematical model of the process, fed by data of the fuel (the amount of volatiles and fixed carbon), fluidized bed temperature, and
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inlet particle size distribution. The size distribution and number of the char particles in fluidized bed significantly differ from the size distribution and number of inlet coal particles. Char population has a bimodal distribution - a separate distributions for the smaller and for the larger set of fragments. The experimental and model results show the same tendency - a coal particle partially breaks at the beginning of devolatilization, giving a large number of fine fragments, whilst in the continuation of the process the rest of the parent particle sometimes breaks down into a smaller number of larger pieces, and sometimes does not fragment at all. The review of the Weibull distribution coefficients enables prediction of the char particle size distribution for the characteristic fluidized bed conditions and inlet coal particle sizes.
INTRODUCTION The primary fragmentation is the breakage of solid fuel particles during the devolatilization. This process is caused and accompanied by complex chemical processes1, heat and mass transfer, and thermoelastic phenomena in porous material of the fuel2. The primary fragmentation in the fluidized bed (FB) reactors affects the fuel distribution along the furnace height3, ash particles granulation4, carbon content in the fly ash5, and sulfur capture6. The general indicators of the process, as introduced by Zhang et. al.7, are: primary fragmentation intensity (Nf), changing ratio of fuel size (Fd), and primary fragmentation index (Sf). The intensity of the primary fragmentation (Nf) is the measure of change of the number of particles during devolatilization. It is defined as the ratio of the total number of coal particles after the devolatilization process (Nout) to the number of particles initially introduced into the fluidized bed (Nin).
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N f = N out / N in
(1)
The changing ratio of fuel size (Fd) characterizes the changes in the particle sizes, taking into account possible coal swelling. It is defined as: n
Fd = ∑ X i d i / d c
(2)
i =1
Here, Xi is the mass fraction of char with the average diameter di; dc denotes the average diameter of coal particle introduced to the fluidized bed. If the coal is a non-swelling, Fd is less than 1, i.e. the average diameter of fragments is smaller than the average diameter of the original particles. In the case of a swelling coal, the parameter Fd can be equal to or greater than 1. The primary fragmentation index is a comprehensive parameter, taking into the account changes both in number and size of particles. It is defined as: S f = N f / Fd
(3)
This parameter increases with the increase of the number of particles after the primary fragmentation and with the decrease of their diameter. The following characteristics of the process of primary fragmentation can be established: 1. The most influential factors are: coal type, coal particle size, and FB temperature. The primary fragmentation intensity increases monotonically with the particle size7-9 and FB temperature8-10, whilst the effect of the bed pressure is nonmonotonous11. When considering the coal type in experimental and theoretical investigations, it turned out that no special characteristics of coal (amount of volatiles or carbon, mechanic characteristics, porosity, type of pores, etc.) can be singled out as the critical to the process12, 13. Several authors proposed a combined influence of a number of factors as key to the primary fragmentation process, for example: amount of volatiles and total porosity7, 8, combination of coal size and diameter of convective pores14 or
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combination of coal’s mechanical characteristics15,
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16
. The influences of the
fluidization regime (bubbling or circulating), can be neglected, because this breakage is taking place in the “splash“zone17, 18. However, the authors in the paper19 state that cyclic heating, characteristic for the circulated fluidized bed (CFB), cause a repeated change in temperature gradients inside the particle, which significantly increases the intensity of fragmentation. 2. There are two apparent patterns of primary fragmentation: exfoliation of the outer layer of the particle, which produces relatively small fragments, and breakage of the central part of the particle. This results in bimodal particle size distribution (PSD) of char, with independent coefficients: one distribution for smaller and one for larger fractions2, 11, 20. Earlier, the fraction of fine fragments were ignored, or attributed to secondary fragmentation21. This applies to the usual FB conditions (dc≥1mm, up to 900oC), as well as for the higher temperatures and smaller coal particles. 3. In the oxidizing atmosphere of the fluidized bed a partial overlapping of the coal devolatilization and char combustion processes occur, i.e. overlapping of two types of fragmentation - the primary and secondary. The fragments produced as a result of experiments in oxidizing atmosphere are smaller and more numerous than those obtained in an inert atmosphere7. The aim of this research was to determine general characteristics of the coal primary fragmentation and char particle size distribution in a certain range of characteristic temperatures and inlet granulations. It was conducted with the Serbian lignite Kolubara, a coal with the largest share in the coal reserves in Serbia. The investigation was conducted through experiments and a model of the primary fragmentation, developed by the authors, described in detail earlier2. Also, the paper includes a discussion on how the fragmentation behavior of the coal is going to affect combustion process in a FB furnace.
EXPERIMENT
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Experiments were carried out in a furnace with an electric heater (Figure 1). Upon reaching the steady state with the desired conditions in the furnace (temperature 600°C or 800°C), portions of the coal particles of a certain granulation were brought into the fluidized bed through the upper opening of the apparatus. After the end of the process of devolatilization the entire FB material was discharged from the furnace into a water cooled container. The content of the fluidized bed (sand and char) was cooled in the stream of nitrogen, which was aimed at preventing the further combustion of the char particles. After the cooling, the bed material was sieved and coal and inert material separated. Each coal sample was photographed before and after the devolatilization process (Figure 2). The batches of coal (and later char) were placed on the photographic paper and exposed to light. This method of photographing was chosen in order to avoid shadows of the particles in the photos. The photos were scanned; and further measurement has been done by the image analysis program ImageJ. The main measured quantity was the area of the particle projection. The diameter of the particle was determined as the diameter of the circle with same area (as the measured one); the volume of the particle was calculated as the volume of the sphere with that diameter. Further, the mass fraction of a class size was determined on the basis of the particle volume. The measurement uncertainty analysis took into account the calibration of the image analysis program, the position of the particle on the photo paper and the possibility of continued combustion of the char particles after the end of the devolatilization process. The estimated relative uncertainty of the volume measurement for the FB temperature of 600°C is 0.058, and for the FB temperature of 800°C is 0.202. Table 1 contains the proximate and ultimate analysis of the coal. In the experiments, the particle size and fluidized bed temperature were varied because these factors have the greatest impact on
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the primary fragmentation22. The tested granulations were: 4.76-7 mm, 7-10 mm, 10-13 mm, 1318 mm. The two characteristic fluidized bed temperatures were selected for the testing: 600oC and 800oC. 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 agglomeration23. The temperature 800oC was chosen because it is close to the operating temperatures of fluidized bed facility. The literature has noted that the fluidization ratio (ratio between the apparent and minimal fluidization rate) does not affect the processes occurring in the particle, especially not the primary fragmentation14, so the selected fluidization ratio was 2. The number of particles in an investigated category was varied from 80-100, for the two larger inlet coal granulations; and 120-194 for the smaller granulations. In order to have enough particles in a sample, the experiment with a particle granulation on a certain temperature was carried out in a number of repetitions (usually 4). The mass of the coal in a batch introduced to bed was adjusted in a way that a minimum concentration of oxygen in the experiment was between 1% and 3%, and the average of about 8%. MODEL Primary fragmentation models generally consist of a series of partial differential equations, thereby considering all the relevant processes in the fuel particles during devolatilization14-16, 24. They require a large, in praxis usually unattainable, set of input parameters. The model used in this paper is primarily designed for engineering purposes, but at the same time it takes into account all the relevant thermo-mechanical processes occurring in the particle during the devolatilization. The input of the model is: the amount of volatiles in the coal, the amount of fixed carbon in the coal, the initial coal particle size distribution, the fluidized bed temperature.
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The output of the model is: char particle size distribution and the main primary fragmentation parameters (the intensity, changing ratio of fuel size, and index)2. In order to enable a comparison between distributions, and to provide an opportunity for incorporating the model into a mass balance calculation of an FB facility, the model results are presented in the form of Weibull's distribution. The char particle size distribution has a bimodal distribution, i.e. it is divided into two sections (smaller and larger fragments)2. The cumulative distribution function is:
< = 1 − 1 − −⁄ +
(4)
+ 1 − −⁄ . where < is the cumulative mass of fragments smaller than diameter D, ∑ is the total mass of the fragments, = ∑ ⁄∑ is the share of large particles in the total mass, = 0 !"# ≤ , and = 1 !"# > , α, β shape, scale factor of Weibull distribution Df is the diameter separating the fine and large char section, f, l are subscripts that refer to section of smaller and larger fragments, respectively.
RESULTS AND DISCUSSION The main result of the investigation is the char particle size distribution (PSD), obtained after the devolatilization of a coal batch, for the characteristic FB temperatures and distributions of the tested lignite. For each inlet coal particle size, the fragments (char particles) were divided into 22 class sizes, and for each of them the number and mass of the fragments were determined. Figures
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3 and 4 present PSDs of the char particles, obtained after the devolatilization on the FB temperature of 600°C and 800°C, for four inlet coal granulations. The results on primary fragmentation indicators – intensity and index – are presented in Figures 5 (FB temperature 600°C) and 6 (FB temperature 600°C). For the lower tested FB temperature, the PSD of the fragments of the smallest inlet coal granulation is the least disperse, whilst the PSD of the largest granulation is the most disperse (Figure 3). The primary fragmentation intensity and index for the FB temperature 600°C (Figure 5) are increasing monotonously with the inlet coal diameter. It illustrates an observation, which was often recorded in the literature, that the larger particles fragmentize more intensively, and that the distributions of char particles are more disperse for larger initial particles. However, although it is expected that the increase in fluidized bed temperature has the same effect (more disperse distributions and increase of the intensity and index), the experimental results obtained after the devolatilization on FB temperature 800°C (Figures 4 and 6) in some points do not follow expected tendencies. The experimental results were compared to the model results, and the Pearson’s coefficient of agreement FB temperature 600°C were: 0.90, 0.89, 085, and 0.88 (for the tested inlet coal sizes: 4.76-7 mm, 7-10 mm, 10-13 mm, and 13-18 mm). The comparative results of the char particle size distribution of the experiments and the model for the inlet coal granulation 7-10 mm for the FB temperature of 600°C are presented in Figure 7. The Pearson’s correlation coefficient for the experimental and model results for the FB temperature 800°C was acceptable only for the least disperse inlet granulation of 4.76-7 mm, and it was 0.96. It indicates the possibility that during the experiments, at the higher FB temperatures, there is a loss of char particles. In the first stage of primary fragmentation - the particle exfoliation - a number of fine char particles are being produced. In the case of devolatilization of larger fractions of coal, there is a high probability that
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these fine char particles produced during the exfoliation usually start to combust while the original, “parent” particle still devolatilizes. In this way, devolatilization of the larger coal particles overlaps with the combustion of smaller char particles, which leads to loss of some char particles. The ash originated in this process leaves the furnace as the flying ash, or it is destroyed during the sieving and char particle measurement (the ash particles are much more fragile than char particles). The coefficients of Equation 4 for the FB temperature of 850oC are given in Table 2. The shape and scale factors of Weibull distribution (Table 2, α, β) uniformly decrease with the decreasing of the initial coal diameter.
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. Given the results of the experiments and model, some general observations of the primary fragmentation of Kolubara lignite can be made: Because it is a typical lignite coal, the primary fragmentation is not very intense. The results of the model and experiment show the same tendency - a coal particle breaks 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 do not break at all. Considering that, it can be expected that the majority of char particles are going to be concentrated in the lower levels of furnace interior, and that the volatiles and finer char particles are burning in the freeboard.
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The experimental and the model results show that the particle size distribution of char has a bimodal distribution, i.e. it is divided into two sections: the smaller and larger fragments. So far, the losses due to unburned carbon were mainly attributed to the process of attrition, or neglected. However, depending on the initial particle size and FB temperature, 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.
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Figure 1 Experimental apparatus
Figure 2 Photos of coal samples before/after devolatilization
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Figure 3 Particle size distributions for FB temperature 600°C, experimental results
Figure 4 Particle size distributions for FB temperature 800°C, experimental results
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Figure 5 Primary fragmentation intensity and index for FB temperature 600°C, experimental results
Figure 6 Primary fragmentation intensity and index for FB temperature 800°C, experimental results
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Figure 7 Comparison of the experimental and model results of particle size distribution for the FB temperature 600°C, inlet coal granulation 7-10mm
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Table 1 Proximate and ultimate analysis of Kolubara coal, dry ash free Proximate analysis volatile (%)
Fixed C (%)
Total sulfur (%)
Higher heating value (kJ/kg)
Lower heating value (kJ/kg)
66.79
33.21
1.01
28172.59
26275.48
Ultimate analysis C (%)
H (%)
56.85
N (%)
8.43
S (%)
1.42
O (%) 1.62
31.68
Table 2 Results of the simulation: the parameters of the Weibull distribution depending on the initial particle size of the Kolubara coal, for FB temeprature 850oC Dc
A
Df
αs
βs
αl
βl
[mm]
[-]
[mm]
[-]
[mm]
[-]
[mm]
15
0.934
7
4.80
5.28
5.70
16.39
10
0.953
5
4.75
3.62
4.98
14.96
8
0.974
4
4.70
2.98
4.23
13.37
6
0.997
3
5.25
2.02
3.39
12.42
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AUTHOR INFORMATION Corresponding Author * Milijana Paprika, PhD, Researcher Vinča Institute of Nuclear Sciences PO Box 522 11001 Belgrade, Serbia Phone: +381 11 3408 336 Fax: +381 11 24 53 670 e-mail:
[email protected] Funding Sources Funds of Ministry of Education, Science and Technological Development of Serbia have been used to support the research. ACKNOWLEDGMENT 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 42011 “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”.
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