Energy & Fuels 1998, 12, 689-696
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Experimental Study on Self-Desulfurization Characteristics of Biobriquette in Combustion Guoqing Lu, Heejoon Kim, Jianwei Yuan, Ichiro Naruse,* and Kazutomo Ohtake† Department of Ecological Engineering, Toyohashi University of Technology, Toyohashi 441-8580, Japan
Mitsushi Kamide Department of Resource and Energy, Hokkaido Industrial Institute, Sapporo 060, Japan Received October 8, 1997
A new kind of biobriquette, with scallop shell as desulfurizer, was developed in this study, and its self-desulfurization behavior was studied by combustion experiments. For comparison and further understanding on the self-desulfurization characteristics, the biobriquettes with Tsukumi limestone and calcium hydroxide as desulfurizers were also investigated experimentally. The influence of the furnace temperature, types of coal and desulfurizer, and structure of added desulfurizer on the behavior of desulfurization was elucidated by measuring the time concentration history of SO2 emission in combustion flue gas and calculating the desulfurization efficiency. The desulfurization efficiency was not sensitive to the temperature in the range 973-1173 K. However, the efficiency was strongly affected by coal type, and it changed from about 25 to 67% for the eight tested types of coals under the same experiment conditions. The desulfurization efficiency has been found to also be a function of the calcination temperature of desulfurizer. On the basis of experimental results, a shrinking-core reaction model was used to simulate the desulfurization process during the char combustion of biobriquette by a finite volume numerical method. The calculated results generally agreed with the experimental results. Finally, an improvement on the biobriquette structure, namely dual layered biobriquette, was proposed and tested in order to improve the desulfurization efficiency.
Introduction It has been recognized that coal has the highest potential as a future energy source among fossil fuels in the world.1 Low-grade coals with high fuel sulfur content (2 mass %, daf [dry ash-free]), high fuel nitrogen content (1.2 mass %, daf), high ash content (15 mass %, dry basis), and low heating value (16 700 kJ/kg) are being directly burned in domestic stoves and smallcapacity industrial boilers in many countries. This causes not only lung disease for people living near this area but also environmental pollution, especially acid rain. For example, in China, the largest country of coal consumption, about 76% of energy sources is supplied by coal,2 and more than 90% of the total SOx emission is from coal combustion, among that, electrical power plants are responsible for 30%, and the remainder is covered by domestic stoves and small-capacity industrial boilers.3,4 The coal with high fuel sulfur content burned directly in domestic stoves and small-capacity industrial †
Deceased. (1) Kawakami, Y. J. Jpn. Energy 1991, 72, 136. (2) Xu, X. C.; Zhang X. Y. Proceedings of the 2nd International Symposium on Coal Combustion; China Machine Press: Beijing, 1991; p 8. (3) Sadakata, M.; Kim, H. J.; Hashimoto, S.; Matui, K.; Xu, X. Report of New Program for Promotion of Basic Sciences Studies of Global Environmental Charge with Special Reference to Asia and Pacific Region to Ministry of Japan; 1995, 11, 67.
boilers causes harmful particulate pollution and a great deal of acid rain in the southwest region of China.5 To enhance effective utilization of low-grade coals and to control emissions of environmental pollutants, a lot of work has been done to find simple and economical methods for lower emission combustion of coals. For this aim, a new artificial solid fuel called biobriquette has been developed as one of the very promising composite fuels in the pollution-free coal combustion in Japan. The biobriquette is comprised of about 80 mass % of low-grade coal and about 20 mass % of biomass (like discarded wood chips, bark, agricultural waste and so on) and is briquetted under a high pressure. By comparing its combustion characteristics with those of coal briquette or lumpy coal, some merits have been revealed. That is, the biobriquette is excellent in ignitability and combustibility, it generates little smoke and particulate during the combustion process, and it has also better combustion controllability and higher combustion efficiency.4,5 If a desulfurizer is mixed into it, the SO2 emission from its combustion can be obviously reduced. This self-desulfurization method is simple, economical, and efficient and needs no remaking (4) Sugawara, K.; Abe, K.; Sugawara, T.; Sholes, M. A. J. Jpn. Energy 1995, 77, 205. (5) Zhang, C.; Xu, G.; Lian, X. Report of Tesinghua University, China 1995.
S0887-0624(97)00194-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/20/1998
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(or retrofitting) combustor and no attaching desulfurization equipment. Hence, the biobriquette is considered to be very suitable for domestic stoves and some small-capacity industrial boilers in some developing countries. In the previous research,6-12 the desulfurization characteristics of coal briquettes with desulfurizers were studied and discussed. It was pointed out that when limestone is used as the desulfurizer, the reasonable Ca/S molar ratio is 2 to 2.5, and if the ratio is more than 4, its increase will not elevate the desulfurization efficiency during the combustion of coal briquette. The sulfur dioxide emission can be reduced by nearly one-half with the addition of limestone to coal briquette. Ca(OH)2 was found to be a more effective desulfurizer than CaCO3. The optimal desulfurization temperature is in the range 1073-1273 K. When temperature is higher than 1273 K, the desulfurization efficiency is considerably decreased, presumably due to the decomposition of CaSO4. The coal briquette combustion under rapid heating conditions leads to relatively higher desulfurization efficiency than that under slow heating conditions. However, the desulfurization characteristics of biobriquette and their dependence on the furnace temperature, the types of coal and desulfurizer, and the structure of added desulfurizer have not yet been well studied in detail. Recently, for the effective reuse of aquatic product wastes such as sea shells, the waste scallop shell, containing CaCO3 as the main component similar to limestone, has been adopted as a new desulfurizer, and its desulfurization capability has been proved to be better than that of limestone.13 In this study, a new kind of biobriquette was developed that used the wasted scallop shell as the desulfurizer. The desulfurization characteristics of this kind of biobriquette were investigated experimentally and compared with those of biobriquettes with Tsukumi limestone and calcium hydroxide as desulfurizers. The influences of furnace temperature and types of coal and desulfurizer were also studied by measuring the time concentration history of SO2 emission in flue gases and calculating the desulfurization efficiency. To have a deep understanding of the self-desulfurization characteristics, a shrinking-core reaction model was developed in this study based on the experimental results. The model was used to simulate the self-desulfurization process during the char combustion of biobriquette in combination with a finite volume numerical method. Finally, an improvement on the biobriquette structure was proposed and tested to increase the desulfurization efficiency. (6) Maruyama, T., J. Jpn. Energy 1995, 74, 70. (7) Kim, H. J.; Hashimoto, S.; Ona, S.; Matsui, K.; Sadakada, M. J. Jpn. Energy 1997, 76, 205. (8) Lu, C. M.; Cheng, S. Q.; Li, S. Y.; Shao, Y. L.; Xu, B. S. Proceedings of the 3rd International Symposium on Coal Combustion; Science Press: Beijing, 1995; p 609. (9) Zhuang, Y. H.; Shen, D. X.; Xiao, P. L. Processing and Utilization of High-Sulfur Coals III; Elsevier: 1990; p 653. (10) Zhang, B.; Zhang, Q.; Liu, S. Proceedings of the 8 International Conference on Coal Science; Elsevier: 1995; p 1645. (11) Uemiya, S.; Itoh, K.; Kojima, T. Proceedings of the 8 International Conference on Coal Science; Elsevier: 1995; p 1835. (12) Ahmed, M. S.; Attia, Y. A. Processing and Utilization of HighSulfur Coals V Elsevier: 1993; p 379. (13) Lu, G. Q.; Kim, H. J.; Naruse, I.; Ohtake, K.; Kamide M. Kagaku Kogaku Ronbunshu 1997, 23, 954.
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Figure 1. Schematics diagram of experimental apparatus.
Experimental Apparatus and Procedures The experimental apparatus is schematically shown in Figure 1. It is composed of an electrically heated batch furnace, temperature controllers, a digital balance, and a flue gas analyzing system. The time history of biobriquette mass and the time concentration histories of SO2, NOx, CO, CO2, and O2 in the flue gas were continuously measured during the combustion process, respectively, by the digital balance and the flue gas analyzing system. The combustion air was preheated to a desired temperature by a packed bed of alumina balls located at the bottom of the furnace. The biobriquette sample was placed in a basket, which was linked up with the upper digital balance, and was positioned in the center of the furnace axis. At the beginning of combustion, the electrical furnace that had the desired temperature was moved upward to heat the fuel in order to avoid the mass fluctuation. The influence of buoyancy on the indication of measured mass was neglected. In the experiments the furnace temperature (Tf) was set at a given level which ranged from 873 to 1173 K. The combustion experiments were carried out in the atmosphere of the forced flow air with enough oxygen in order that the sulfur in biobriquette was oxidized as much as possible. The flow rate of combustion air (Qair) was fixed at 10 L/min for the various sample combustions. The properties of tested coal, biomass, and desulfurizers are shown in Table 1 and Table 2, respectively. The experimental conditions are summarized in Table 3. The eight tested coals have a wide range of fuel ratio [the ratio of fixed carbon (FC) content to volatile matter (VM) content] from 0.82 to 4.27, and their total sulfur contents range from 0.8 to 3.2 mass % (daf). The biomass used for biobriquettes in this study is the wasted bark chip whose sulfur content (daf) is almost zero. Tsukumi limestone and the scallop shell, whose particle diameter was in the range 297-420 µm, were used as desulfurizers. To make a comparison, calcium hydroxide was also used as a desulfurizer, which had a purity over 96.0% and particle
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Table 1. Properties of Tested Coals and Biomass proximate analysis [mass %, dry basis] sample coal
biomass
PA SB MI DT NL BR BJ SZ XM DS bark
ultimate analysis [mass %, daf]
sulfur [mass %, daf]
heating value [kJ/kg]
ash
VM
FC
fuel ratio
C
H
N
total
combustible
QH
QL
28.4 2.7 13.7 16.6 17.2 17.6 20.5 5.6 4.9 15.3 2.6
41.0 53.6 41.6 38.2 36.6 32.7 19.6 21.5 21.6 16.1 90.6
30.6 43.7 44.7 45.2 46.2 49.2 59.9 72.9 73.5 68.6 6.8
0.75 0.82 1.08 1.18 1.26 1.48 3.05 3.37 3.41 4.27 0.08
51.0 71.8 77.4 82.2 79.2 81.9 83.9 85.4 85.2 84.8 42.5
4.3 5.2 5.5 5.5 4.9 5.1 4.2 4.1 3.9 4.2 5.5
1.0 1.6 1.3 1.9 1.7 1.8 1.0 0.9 1.0 1.1 0.5
10.8 1.9 3.2 1.1 0.8 1.1 2.2 2.7 2.2 2.9 0.0
9.3 1.3 2.6 0.5 0.3 0.4 1.6 2.1 1.7 2.1 0.0
12200 25400 23700 23600 21300 25700 28300 33500 31300 27500 18200
11400 24200 23500 23300 21100 24700 27400 32700 30400 26700 17000
Table 2. Properties of Used Desulfurizers [mass %] moist.
SiO2
Al2O3
Fe2O3
MgO
CaO
SO3
lossa
0.1 0.4
0.16 0.03
0.042 0.024
0.027 0.012
0.52 0.22
55.44 53.07
0.0043 0.53
45.53 45.44
Tsukumi limestone Scallop shell
Ca(OH)2 a
insoluble (in HCl)
Cl
SO4
Na
K
Mg
Pb
Cr
Ca(OH)2
Mn
Fe
As
0.1
0.01
0.05
0.05
0.05
1
0.003
0.005
96.0
0.001
0.02
0.0000005
Mass loss after calcination. Table 3. Experimental Conditions furnace temperature [K] flow rate [m3/min] Qair QN2 biomass content [mass %] compression pressure [MPa] particle diameter [m] coal biomass desulfurizer diameter [m] Tsukumi limestone scallop shell calcium hydroxide Ca/S ratio
873, 973, 1073, 1173 1 × 10-3 1 × 10-3 20 245.2 less than 1 × 10-3 less than 1 × 10-3 297∼420 × 10-6 297∼420 × 10-6 less than 25 × 10-6 0, 1, 3, 5
diameter under 25 µm. The biobriquette sample was cylindrically shaped under a high pressure, with a dimension of 16 mm in both diameter and height. A biobriquette sample weights about 5 g. To quantitatively compare the desulfurization processes under different conditions, the definition of desulfurization efficiency is introduced here as
ηSOx ) 1 - SO2(Ca/S)n)/SO2(Ca/S)0)
(1)
where SO2(Ca/S)0) is the SO2 emission from the combustion of biobriquette without desulfurizer and SO2(Ca/S)n) the SO2 emission from the combustion of biobriquette with desulfurizer. Ca/S in the equation represents the molar ratio of calcium to sulfur. It is clear that ηSOx represents the extent of the decreasing of SO2 emission by desulfurizers based on the SO2 emission without desulfurizers.
Results and Discussion Combustible Sulfur and Noncombustible Sulfur. The sulfur in coal can be classified as combustible sulfur and noncombustible sulfur. This classification is very important for the discussion of biobriquette desulfurization. According to the Japanese Industrial Standard, in this study, the sulfur in coal is subdivided into the total sulfur (weight loss of sulfur when coal sample is burned in the atmosphere of pure oxygen at 1723 K) and the combustible sulfur (weight loss of sulfur when coal sample is burned in the atmosphere of pure oxygen
Figure 2. Combustible sulfur vs total sulfur.
at 1073 K). The noncombustible sulfur is the difference between the total sulfur and the combustible sulfur. Table 1 gives the contents of the total sulfur and the combustible sulfur of the eight tested coals. It can be seen from this table that the combustible sulfur content does not directly depend on the fuel ratio of coal but is quite linear with the total sulfur content as shown in Figure 2. The correlation between the total sulfur and the combustible sulfur can be expressed as follows
SComb. ) 0.89STotal - 0.35
(2)
where SComb. is the combustible sulfur content (mass %, daf) and STotal the total sulfur content (mass %, daf). Influence of Temperature on Desulfurization Efficiency. First of all, the influence of furnace temperature on the desulfurization efficiency was investigated in the case of Ca/S ) 3. As shown in Figure 3, the desulfurization efficiency increased about 14% as the furnace temperature increased from 873 to 1073 K and then saturated over 1073 K. Although the furnace temperature of 873 K seems to be not high enough to start the calcination of CaCO3 to CaO, the internal temperature of a burning biobriquette is much higher than the furnace temperature during biobriquette combustion. So, it is important to know the time-temperature profile inside the burning biobriquette. Figure 4
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Figure 3. Change of desulfurization efficiency with furnace temperature.
Figure 4. Time profiles of temperature at the surface, half a radius, and center of a burning biobriquette at 873 K of furnace temperature.
shows the time-temperature profiles at three points (center, half the radius, and surface) of a burning biobriquette at the furnace temperature of 873 K. When the biobriquette is heated suddenly by radiation and convection heat transfer in a hot air stream, the volatile is evolved, ignited, and burned near the biobriquette. The surface temperature rises very fast, and internal temperature is also rapidly raised by heat conduction. But the evolution of volatile is an endothermic process, and the heat of its combustion is carried off by the flowing air, so that the surface temperature does not rise so much over the ambient temperature. When the volatile is almost burned out, the oxygen in the air stream reaches the solid surface and the char combustion starts. As char combustion going on, the combustion heat accumulates inside the particle, which makes the center temperature rise and reach a level of 1200 K. It should be noted that the internal temperature of the burning particle takes over the calcination temperature of Tsukumi limestone soon after char combustion starts. During the whole char combustion process, the internal temperature almost always remains higher than the calcination temperature of limestone and around the optimal temperature of the CaO desulfurizing reaction. This fact can well explain why the desulfurization efficiency at the furnace temperature of 873 K has reached about 55%. It implies that the biobriquette has good self-desulfurization characteristics even at rather lower furnace temperatures. The time-temperature history corresponds to the unburnt fraction history as shown in Figure 5 where
Lu et al.
Figure 5. Unburnt fraction vs combustion time.
Figure 6. Time history of SO2 concentration in flue gas with various Ca/S values.
the unburnt fraction was defined on the ash-free basis. It can be found that the biobriquette combustion process obviously has two stages: the first stage with a rapid mass loss, and the second stage with a slower mass loss. In the first stage, the volatile evolves out from biobriquette and burns around its surface. In the second stage, the char combustion occurs slowly inside the biobriquette and is controlled by oxygen diffusion.14 It will be found later that the self-desulfurization of biobriquette has different characteristics in these two combustion stages. Influences of Ca/S Ratio and Coal Type on Desulfurization Behavior. The time-concentration history of SO2 emission in the flue gas of biobriquette combustion with Ca/S as a parameter was measured and is shown in Figure 6. In the figure the area under the SO2 concentration profile corresponds to the total SO2 emission. It is found that the time-concentration history of SO2 emission in the flue gas also has two stages, which do correspond to the two combustion stages, namely the volatile combustion stage and the char combustion stage. The experimental results show that the SO2 emission in the first stage does not change very much with the variation of Ca/S and that only about 7% of SO2 formed in the volatile combustion stage is captured at Ca/S ) 5 (see the smaller figure where the “total desulfurization efficiency” denotes the efficiency during the whole biobriquette combustion process, while the “desulfurization efficiency in char” for that during the char combustion stage). In contrast, the (14) Lu, G. Q.; Kim, H. J.; Naruse, I.; Ohtake, K.; Kamide M. Kagaku Kogaku Ronbunshu 1997, 23, 404.
Self-Desulfurization Characteristics of Biobriquette
Figure 7. Change of desulfurization efficiency with different coal types.
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Figure 9. Time history of SO2 concentration in flue gas with different desulfurizers.
Figure 10. Change of desulfurization efficiency with different desulfurizers. Figure 8. Relation between desulfurization efficiency and sulfur fraction in fixed carbon.
SO2 emission in the second stage (char combustion) does change with the variation of Ca/S. It is known that the desulfurization can occur after the calcination of CaCO3 in desulfurizer to CaO. Because of the short time and the low temperature in the volatile combustion stage, the calcination is not well completed, and thus only a small amount of SO2 can be captured. On the other hand, most of the SO2 formed during the char combustion can be effectively captured because of the longer time and higher temperature for the calcination. The desulfurization efficiency in this stage depends on the Ca/S ratio if desulfurizer is uniformly distributed in biobriquette. The influence of coal type on the desulfurization efficiency is illustrated by experimental results as shown in Figure 7. It is observed that the desulfurization efficiency is an approximately linear function of the fuel ratio. It implies that the biobriquette made of coals with less volatile matter may have better self-desulfurization results. It can be understood from the above discussion that the SO2 formed during volatile combustion cannot be effectively captured. However, there exists an exception for BJ coal which has lower fuel ratio but much higher desulfurization efficiency than DS coal. By examining the sulfur distribution in volatile and char, it is found that BJ coal has higher sulfur content in char than DS coal as shown in Figure 8. It suggests that the sulfur content in char becomes a rather important factor. In other words, the higher the sulfur content in char, the higher the desulfurization efficiency under the same combustion conditions.
Desulfurization Characteristics of Various Desulfurizers. As a kind of typical and abundant sea shell, the wasted scallop shell was used as a new desulfurizer for the biobriquette in this study. To compare the desulfurization characteristics among the scallop shell, Tsukumi limestone, and calcium hydroxide, the time concentration histories of SO2 emission in flue gas during combustion of biobriquettes with different desulfurizers at Ca/S ) 1 were measured, and the results are shown in Figure 9. Comparing with that of limestone, the desulfurization reaction of the scallop shell starts earlier, namely in the late volatile combustion stage, and it has better desulfurization behavior during the char combustion. The desulfurization efficiencies for the tested desulfurizers are summarized in Figure 10 as a function of Ca/S. The shell has a higher desulfurization efficiency than limestone at Ca/S < 3. Especially at Ca/S ) 1, the efficiency can reach about 3 times as high as that of limestone. At Ca/S > 3, however, the efficiencies of both shell and limestone become almost same. For calcium hydroxide, it keeps the highest efficiency among the tested desulfurizers and can capture SO2 even during the volatile combustion. One of the main reasons for this can be explained by the characteristics of calcination as shown in Figure 11. It can be seen that Tsukumi limestone is calcined rapidly to calcium oxide when the temperature is above 1023 K. As was shown in Figure 4 and discussed above, the internal temperature of the burning char exceeds this calcination temperature, which results in a desulfirization efficiency of about 55% even with the furnace temperature of 873 K. Calcium hydroxide has the lowest calcination temperature of only about 673
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the briquetting compression pressure, and its calculation formula has been derived previously.13 To simulate the desulfurization process in char combustion by this model, it can be assumed that (1) sulfur and desulfurizer are uniformly distributed in biobriquette, (2) only C + O2 f CO2 and S + O2 f SO2 reactions take place during the char combustion, and (3) the diffusion rate of flue gas toward the outside of the biobriquette is equal to that of oxygen toward the inside. The formed SO2 at the flame sheet is captured by desulfurizers during the char burning process and the diffusing process through the ash layer. The desulfurization reaction is considered as Figure 11. Calcination conversion of shell, limestone, and calcium hydroxide with temperature increase.
K, which makes the SO2 capture effective even in the volatile combustion stage. Conversely, both limestone and scallop shell seem to have difficulty in capturing the SO2 formed below the temperature of 973 K because their calcination temperatures are above this temperature. This result will become the limitation on the further increase of desulfurization efficiency and can also be considered the reason why scallop shell and limestone have almost the same desulfurization efficiencies at Ca/S > 3. Modeling and Calculation of Desulfurization for Biobriquettes. As discussed above, the biobriquette combustion process appears in two stages, namely the volatile combustion stage and the char combustion stage. In our previous study,13 a volume reaction model was developed to describe the volatile combustion and a shrinking-core reaction model to describe the char combustion. Since the desulfurization mainly happens in the char combustion stage, the shrinking-core reaction model is adopted here as a base for the prediction of self-desulfurization efficiency in the char combustion stage. According to the shrinking-core reaction model, the char combustion is proceeding from the surface toward the inside, controlled by the oxygen diffusion in both the ash layer inside the biobriquette and the gas boundary layer around the biobriquette. To simplify the simulation, the biobriquette is considered in a spherical shape. It has been shown in our previous study14 that a cylindrically shaped biobriquette, with the diameter and height being the same as 2R, has nearly the same combustion process as a spherically shaped biobriquette with the same diameter as 2R. As the flame sheet moves from the surface of the biobriquette toward an unburnt core with the radius of rc, the elapsed char combustion time tc can be calculated by the following equation:15
tc ) (R/AC0)[1/3(1/Kc - R/Dc)(1 - rc3/R3) + (R/2Dc)(1 - rc2/R2)] (3) In this equation, R is the radius of biobriquette, Kc the oxygen diffusion coefficient in the gas boundary layer, Dc the effective oxygen diffusion coefficient in the ash layer, A the volume of reactable char per mole oxygen, and C0 the oxygen concentration in atmosphere. Dc is strongly related to the porosity in ash layer which in turn depends on the amount of biomass addition and
CaO + SO2 + 1/2O2 f CaSO4
(4)
and the conversion rate of CaO is taken from the result of Kojima et al.16 as
dxCaO/dt ) kSO2 (D - xCaO)CSO2
(5)
kSO2 ) 403.7 exp(-6750/T)
(6)
Here, xCaO is the converted fraction of CaO to CaSO4, CSO2 the molar fraction of SO2, t the reaction time, and D the maximum conversion fraction of CaO to CaSO4 which takes values of 0.87 for scallop shell and 0.36 for limestone as determined in previous experiments.17 The captured SO2 during the reaction time ∆t can then be calculated by integrating eq 5 as
∆W ) FCaODkSO2 exp(-kSO2CSO2∆t) CSO2 [mol SO2 m-3] (7) where FCaO is the molar density of CaO in char. To calculate the desulfurization efficiency, a numerical method is used, in which the spherical biobriquette is divided into a great number (N) of small spherical shells with the same thickness (∆r ) R/N). One hundred fifty spherical shells were used in the calculation in order to make some of the above-mentioned assumptions be reasonable. The totally captured SO2 should be the sum of those captured during char burning at each shell and those captured during the diffusion process through each shell. According to eq 7, the captured SO2 during char burning at shell i is calculated as
∆Wi ) FCaODkSO2CSO2(0)(∆Vi)(∆ti) [mol SO2]
(8)
where ∆Vi is the volume of shell i, ∆ti the char burnout time of shell i, and CSO2(0) the molar concentration of SO2 on the flame sheet. The uncaptured SO2 during the char burning at shell i, if it exists, will diffuse toward the outside and will be captured by residual desulfurizer in the ash layer. The captured SO2 during (15) Levenspiel, O. Chemical Reaction Engineering; Wiley Int. Edition: New York, 1972; p 362. (16) Kojima, T.; Take, K.; Kunii, D.; Furusawa, T. J. Chem. Eng. Jpn. 1985, 18, 432. (17) Naruse, I.; Nishimura, K.; Ohtake, K. Kagaku Kogaku Ronbunshu 1995, 21, 904.
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Figure 13. A cross section of dual layered biobriquette. Figure 12. Desulfurization efficiencies of char combustion for different desulfurizers.
the diffusion process through shell j is calculated as
∆Wij ) FCaODkSO2 exp(-kSO2CSO2(ij)tj) CSO2(ij)(∆Vj)(∆tij) [mol SO2] (9) where ∆Vj is the volume of shell j, ∆tij the diffusion time through shell j, and CSO2(ij) the molar concentration of SO2 when the combustion products of small shell i diffuse through the small shell j. In the calculation, the SO2 concentrations, CSO2,i and CSO2,ij, are assumed to be constant during the desulfurization at each shell. By summing these two parts of captured SO2 through all shells, the totally captured SO2 can be calculated, and then the desulfurization efficiency can be obtained. Figure 12 shows both the calculated and the experimental measured desulfurization efficiencies during the char combustion for various desulfurizers. By examining the figure, it is seen that for the scallop, the calculated value is higher than its experimental result at Ca/S < 1.5, and for Tsukumi limestone, the calculated value is higher than its experimental result at Ca/S < 2.5, but the former is closer to its experimental result than the latter. When the Ca/S ratio is larger than 3, the calculated values for the two desulfurizers are well consistent with their experimental results. In the case of lower Ca/S ratio, the desulfurization reaction has to come more deeply into the inside of the desulfurizer, but in the simulation, the fact that the plugging of pores due to the CaSO4 formation on the surface of the desulfurizer prevents some desulfurization reaction from further occurring in the inside of desulfurizer was ignored. This might be the reason for the higher calculated desulfurization efficiency than the experimental results at lower Ca/S ratio. Moreover, both calculation and experimental results indicate that increasing the Ca/S ratio does not have an obvious benefit for elevating the desulfurization efficiency during the char combustion at Ca/S > 3. In addition, an important phenomenon is that the scallop almost has the same desulfurization efficiency as the calcium hydroxide during the char combustion stage. However, the mean diameter of the former is about 14 times larger than that of the latter. This suggests that the scallop has quite a great potential for desulfurization. In Figure 9, calcium hydroxide presents a larger desulfurization capability only because it can capture some SO2 during the volatile combustion while the scallop shell cannot. Method to Improve Desulfurization for a Biobriquette. As mentioned above, an earlier calcination
Figure 14. Time history of SO2 concentration in flue gas with different addition methods of desulfurizers.
Figure 15. Comparison of desulfurization efficiencies for different desulfurizers and addition methods.
reaction could result in a higher desulfurization efficiency. For prompting calcination, a kind of dual layered biobriquette was proposed and manufactured, which has a part inside without desulfurizer covered by a mixture of biomass and desulfurizer as shown in Figure 13. Figure 14 illustrates the desulfurization behaviors of both a normal biobriquette (in which the desulfurizer was uniformly scattered) and a dual layered biobriquette at Ca/S ) 1. This figure indicates that the mixture on the surface of biobriquette is easier to incinerate and prompts the calcination of CaCO3 to CaO, making it possible to capture a part of SO2 during the volatile combustion. Figure 15 also shows that the efficiency varies with the different structure of added desulfurizers and Ca/S values. The desulfurization efficiency of the dual layered biobriquette rises obviously at Ca/S < 3, and become about 3 times as high as that of a normal biobriquette at Ca/S ) 1. However, both dual layered biobriquette and normal biobriquette have almost the same desulfurization efficiency when Ca/S
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> 3. Therefore, the desulfurization efficiency of the dual layer biobriquette is limited when Ca/S > 3. This might suggest that some sulfur in volatile has been released before the calcination reaction of desulfurizer. In addition, the experimental results also show that the dual layered biobriquette does not improve the desulfurization efficiency when scallop shell is used as the desulfurizer. This might be because the calcination temperature of scallop shell is lower than that of limestone.
Acknowledgment. With deep regret we wish to acknowledge the untimely death of our coauthor, Professor Kazutomo Ohtake, who passed away in an airplane accident on September 26, 1997, in Indonesia. We mourn his passing away with deep grief and appreciate his guidance and efforts in this study. The partial funding by Grant-in-Aid for Scientific Research (A) and Industry Research Institute on Global Environment, Japan, is also acknowledged.
Conclusions
Nomenclature
The self-desulfurization characteristics of biobriquettes with different desulfurizers have been discussed in detail by both experiments and numerical calculations. The following conclusions can be derived. 1. The self-desulfurization efficiency for a biobriquette increases as the furnace temperature rises up till 1073 K. Above this temperature, however, the increase of desulfurization efficiency becomes unremarkable. The biobriquette shows good self-desulfurization characteristics at rather lower temperature. 2. The desulfurization efficiency is strongly affected by coal types and varies almost linearly with the fuel ratio. At the same time, it also depends on the sulfur content in char. 3. The desulfurization reaction mainly occurs during the char combustion. A shrinking-core reaction model has been used to predict the desulfurization efficiency during the char combustion. The calculated results are generally in agreement with the experimental results. 4. The scallop shell has better self-desulfurization characteristics than Tsukumi limestone, and even better than calcium hydroxide during the char combustion. The desulfurization efficiency can be improved by changing the biobriquette structure from a normal one to a dual layered one when limestone is used as the desulfurizer and the Ca/S value is less than 3.
A ) volume of char reactable per mole of oxygen, m3 mol-1 C0 ) molar concentration of oxygen, mol m-3 CSO2 ) molar fraction of SO2 D ) maximum conversion fraction of CaO to CaSO4 Dc ) effective diffusion coefficient of oxygen in ash layer, m2 s-1 Kc ) mass transfer coefficient of oxygen in gas boundary layer, m2 s-1 kSO2 ) reaction rate between SO2 and CaO, s-1 R ) radius of biobriquette, m rc ) radius of unburnt core in biobriquette, m SComb. ) combustible sulfur content STotal ) total sulfur content SO2(Ca/S)0) ) SO2 emission without addition of desulfurizer, kg SO2(Ca/S)n) ) SO2 emission with addition of desulfurizer, kg T ) temperature in biobriquette, K tc ) elapsed char combustion time, s xCaO ) converted fraction of CaO to CaSO4 ∆t ) reaction time, s ∆V ) volume of a spherical shell, m3 ∆W ) captured sulfur by desulfurizer, mol ηSOx ) desulfurization efficiency FCaO ) molar density of CaO in char, mol m-3 EF970194C
