Energy & Fuels 2003, 17, 1239-1243
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Development of White-Biocoalbriquettes with High Desulfurization Function Heejoon Kim,*,† Guoqing Lu,‡ Tianji Li,† and Masayoshi Sadakata‡ Department of Ecological Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi, 441-8580, Japan, and Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan Received December 24, 2002
To control the pollutant emission from coal combustion from both domestic stoves and middleand small-scale boilers in some developing countries, an artificial solid fuel called biocoalbriquette was researched and developed experimentally. However, it is necessary to further increase the desulfurization efficiency of the biocoalbriquette in order to effectively popularize it in these countries. For this reason, white-biocoalbriquette was proposed and researched in this study for a more effectively and economically artificial solid fuel. This white-biocoalbriquette is a composite solid fuel of both coal and biomass. A little limestone is mixed in its inside, and its surface is coated by a thin layer of desulfurizer. The white-biocoalbriquette looks white and will change people’s traditional concept of coal from black and dirty to new and clean. The experimental results show that the white-biocoalbriquette has successfully improved the desulfurization characteristics for this kind of artificial solid fuel from coal due to effectively controlling SO2 emission in both volatile combustion and char combustion. The temperature fluctuation in a combustion chamber hardly has an obvious influence on the desulfurization efficiency. Finally, the desulfurization efficiency of the white-biocoalbriquette has been achieved nearly 90%. At present, it is the highest desulfurization efficiency in a dry desulfurization process.
Introduction Currently, coal serves as a main energy supply in some countries in the world, and will continue to do so in the foreseeable future. Associated with sharp industrialization and rapid economic development in both China and other developing countries, coal is becoming an increasingly important energy resource, and air pollution and water contamination have become an urgent task to be solved. Acid rain is one of the most serious air pollutants, and it also directly results in water contamination. Direct combustion of low-grade coals with high sulfur content is an important cause of acid rain. Therefore, it is a pressing task to study and develop new technologies for the pollution-free utilization of coals in these countries. For this purpose, biocoalbriquette, an artificially produced solid fuel was developed. The biocoalbriquette is produced from a mixture of low-grade coal, biomass, binder, and some additives such as pulp black liquor and limestone or scallop shell. The combustion, desulfurization, and denitrification characteristics of the biocoalbriquette have been reported in detail in our previous studies.1-7 However, increasing desulfurization efficiency the same * Corresponding author. † Toyohashi University of Technology. ‡ The University of Tokyo. (1) Naruse, I.; Kim, H. J.; Lu, G. Q.; Yuan, J. W.; Ohtake, K. TwentySeventh Symposium (International) of the Combustion Institue 1998, 2973. (2) Lu, G. Q.; Kim, H. J.; Yuan, J. W.; Naruse, I.; Ohtake, K.; Kamide, M. Energy Fuels 1998, 12, 689. (3) Lu, G. Q.; Kim, H. J.; Naruse, I.; Ohtake, K.; Kamide, M. Kagaku Kogaku Ronbunshu (Japanese) 1997, 23, 954.
as a wet desulfurization process has been required in order to popularize the biocoalbriquette in China, because coals with high sulfur content (sulfur content: over 4 mass %) are being burned directly in domestic stoves in the southwest region of China. In practical popularization, as the desulfurization efficiency of the biocoalbriquette can reach about 75%, SO2 concentration in the flue gas from the above coal combustion in domestic stoves will be over 1000 ppm. For this reason, a white-biocoalbriquette was researched and developed in this study. It should have a more effective desulfurization function and should also be cheap. This whitebiocoalbriquette is also a composite solid fuel of both coal and biomass. A little limestone is mixed in its inside, and its surface is coated by a white thin layer of desulfurizer. The white-biocoalbriquette is white and will change people’s traditional concept of coal from black and dirty to new and clean. In this study, the desulfurization characteristics of the white-biocoalbriquette were experimentally investigated. The desulfurization efficiency was compared with that of the previous biocoalbriquette. The experimental results show that the white-biocoalbriquette can increase the (4) Lu, G. Q.; Toyama, T.; Kim, H. J.; Naruse, I.; Ohtake, K.; Kamide, M. Kagaku Kogaku Ronbunshu (Japanese) 1997, 23, 404. (5) Lu, G. Q.; Wang, Q. Y.; Sakamoto, K.; Kim, H. J.; Naruse, I.; Yuan, J. W.; Maruyama, T.; Kamide, M.; Sadakata, M. Energy Fuels 2000, 14, 1133. (6) Kim, H. J.; Naruse, I.; Lu, G. Q.; Ohtake, K.; Kamide, M. Kagaku Kogaku Ronbunshu (Japanese) 1998, 24, 779. (7) Kim, H. J.; Lu, G. Q.; Naruse, I.; Yuan, J. W.; Ohtake, K. J. Energy Resourc. Technol., ASME (American Society of Mechanical Engineers) 2001, 123, 27.
10.1021/ef020300x CCC: $25.00 © 2003 American Chemical Society Published on Web 08/21/2003
1240 Energy & Fuels, Vol. 17, No. 5, 2003
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Table 1. Properties of Tested Coals and Biomass proximate analysis [mass %, dry basis] sample coal
NT BJ SZ cornstalk pulp liquor
biomass binder
ultimate analysis [mass %, daf]
calorific value [kJ/kg]
ash
VM
FC
fuel ratio [ - ]
C
H
N
S
QH
QL
15.4 20.5 5.6 2.1 1.2
21.8 19.6 21.5 88.7 70.6
62.8 59.9 72.9 9.2 28.2
2.90 3.05 3.37 0.10 0.4
85.3 83.9 85.4 40.0 32
3.4 4.2 4.1 5.2 3.4
1.2 1.0 0.9 2.1 0.14
4.6 2.2 2.7 0.2 3.0
23300 28300 33500 16700 -
20800 27400 32700 15800 -
Table 2. Properties of Used Desulfurizers various components [mass %] desulfurizer
moist.
SiO2
Ai2O3
Fe2O3
MgO
CaO
SO3
lossa
Tsukumi limestone Scallop
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
desulfurizer
insoluble (in HCl)
Cl
SO4
Na
K
Mg
Pb
Cr
Ca(OH)2
Mn
Fe
As
Ca(OH)2
0.1
0.01
0.05
0.05
0.05
1
0.003
0.005
96.0
0.001
0.02
5 × 10-7
a
Mass loss after calcination.
desulfurization efficiency nearly 90%. Such desulfurization efficiency is almost the same as that of a wet desulfurization process. Experimental Section In this study, the experimental apparatus and methods were the same as that of the reference.1 That is, the electrically heated batch furnace was preheated to a predetermined temperature and then the furnace was moved upward to heat the sample and to start the combustion experiments. The white-biocoalbriquettes and ordinary biocoalbriquettes were positioned in the center of the furnace. The combustion air was forced into the furnace and was preheated through the packed bed, at an air rate of 1 × 10-2 m3/min, which provided enough ambient oxygen for the complete combustion of the sample. The flue gas composition and the sample mass during its combustion process were continuously measured and recorded by a gas-analyzing system, a digital balance, and a computer, respectively. Table 1 and Table 2 show the properties of three main tested coals, biomass, desulfurizers, and pulp liquor as a binder in this study. BJ and SZ coals are of Chinese origin (the properties of the other coals can be referred in the reference1). Especially, NT coal has quite high sulfur content, 4.6 mass % (daf) and came from the southwest region of China. Agricultural waste, cornstalk from Japan, was used as biomass. The coals and cornstalk were ground to the diameter range of under 1 mm, and were briquetted at a certain mass ratio of coal to biomass. Pulverized limestone and scallop were adopted as desulfurizers. For convenience in comparison, a chemistry reagent, Ca(OH)2 was also used as a desulfurizer. The particle size of both the limestone and the scallop shell were in the range of 297 to 429 µm, and that of Ca(OH)2 was under 25 µm. In this study, a very thin layer of Ca(OH)2 was used to coat the surface of biocoalbriquettes, that is, the white-biocoalbriquette was created. As reported in our previous study,1,2 the desulfurization efficiency is defined as follows:
ηSOx)[1 - SO2(Ca/S)n)/SO2(Ca/S)0)]
(1)
where SO2(Ca/S)0) is the SO2 emission from a sample combustion without desulfurizer and SO2(Ca/S)n) is that with desulfurizer Ca/S ) n. The Ca/S represents the molar ratio of calcium to sulfur in the sample. The flue gas composition and the sample mass during the combustion process were continuously measured and recorded by the gas-analyzing system and the balance, respectively.
Figure 1. Volatile matter-dependence of desulfurization efficiency.
Results and Discussion First, the desulfurization efficiencies of the biocoalbriquettes made of various coal types were investigated experimentally and are summarized in Figure 1. In the experiments, the mass ratio of coal to biomass (coal:BM) is 8:2 in each sample, and the desulfurizer, limestone, was mixed at Ca/S ) 3 in the samples. The chamber temperature (Tf) was 1073 K. From the experimental results, the desulfurization efficiency was found to decrease approximately linearly with increasing volatile matter in the coal. The biocoalbriquette made of BJ coal appears in the best desulfurization result in the experiments. It is very important to understand the desulfurization behavior in the biocoalbriquette combustion process. For example, Figure 2 shows the time concentration histories of SO2 emission in the flue gas of the burning biocoalbriquettes made of BJ coal with different types of desulfurizers and different addition methods as experimental parameters where the horizontal axis and vertical represent SO2 emission concentration in the flue gas and the SO2 emission time during sample combustion. In the figure, the mass ratio of BJ coal to biomass (coal:BM) is 8:2 in each sample, the limestone, scallop and Ca(OH)2 used as desulfurizers were mixed
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Figure 3. Comparison of desulfurization efficiency vs various desulfurizers.
Figure 2. Time concentration histories of SO2 emission in flue gas with different Ca desulfurizers and addition methods.
at Ca/S ) 3 in the samples, respectively, and a biocoalbriquette without any desulfurizers (Ca/S ) 0) was used as a comparison basis of desulfurization efficiency. The temperature (Tf) of chamber combustion in the furnace was 1073 K. Figure 2a shows that the SO2 emission profiles in the flue gas during the combustion of different samples obviously had two parts, namely, the volatile combustion and the char combustion. In the volatile combustion stage, the SO2 emission increased and decreased rapidly, but it kept lower values for a long time in the char combustion stage. The experimental results show that the limestone did not have desulfurization function and the scallop shell appeared in a little of the desulfurization function in the volatile combustion stage although Ca/S reached 3. Simultaneously, both of them effectively reduced the SO2 emission in the char combustion. However, the SO2 emission line with the desulfurization of the scallop shell
was a little lower than that of the limestone in the char combustion. These results demonstrated that the scallop shell had a better desulfurization capacity than the limestone. In Figure 2b, the Ca(OH)2 was added as a desulfurizer at the same value of Ca/S ) 3. It was observed that the Ca(OH)2 appeared to effectively reduce the SO2 emission from the volatile combustion and obviously improved the desulfurization. Through the above experimental results, it was concluded that the desulfurizers all have an excellent desulfurization function in the char combustion stage; however, only the Ca(OH)2 could partly capture the SO2 from the volatile combustion and further increase the desulfurization efficiency. Therefore, to develop a biocoalbriquette with high desulfurization efficiency, the research should focus on improving the desulfurization function in the volatile combustion stage. For this reason, the white-biocoalbriquette was created. Figure 2c simultaneously shows the time concentration histories of SO2 emission in the flue gas from both the white-biocoalbriquette and the ordinary biocoalbriquette under the same combustion conditions. The only difference between the white-biocoalbriquette and biocoalbriquette was that a smaller amount of limestone (Ca/S ) 1) was mixed in the white-biocoalbriquette, and the surface of the white-biocoalbriquette was coated thoroughly by a very thin layer of Ca(OH)2. The amount of Ca(OH)2 used was about 0.2 g (Ca/S is nearly equal to 1) and the total Ca/S ratio was 2, whereas the Ca/S in the biocoalbriquette was 3. The figure shows clearly that the desulfurization reaction happened in both the volatile combustion and the char combustion. Most of the SO2 emitted from the volatile combustion and of SO2 emitted from the char combustion were successfully desulfurized despite a lower Ca/S ratio than that of the ordinary biocoalbriquette. Figure 3 compares the desulfurization efficiency under the same conditions for the whitebiocoalbriquette with that for the biocoalbriquette. It was found that the desulfurization efficiency of whitebiocoalbriquettes reached about 92% when the total Ca/S was 2. This desulfurization efficiency increase exceeded above 15% that of ordinary biocoalbriquettes. The reason the white-biocoalbriquette has a high desulfurization efficiency can be explained as follows. In general, within 15 s after the white-biocoalbriquette comes into the combustion chamber at 1073 K, the surface temperature of the white-biocoalbriquette will surpass 673 K, at which temperature coal pyrolysis just
1242 Energy & Fuels, Vol. 17, No. 5, 2003
Figure 4. Comparison of desulfurization efficiency vs Ca/S, various desulfurizers, and addition methods.
begins.4 That is, volatile release in coal and Ca(OH)2 decomposition occur simultaneously. The CaO from Ca(OH)2 decomposition can react directly with H2S [CaO + H2S f CaS + H2] and SO2 [CaO + SO2 + 1/2O2 f CaSO4] in the volatile combustion stage.5 In addition, as the char combustion follows a shrinking-core reaction model, the SO2 not captured in the char combustion stage has also reacted with this thin layer when going through the thin layer. These are reasons why the white-biocoalbriquette has a more effective desulfurization function than the ordinary biocoalbriquette with the limestone mixed inside. The limestone, scallop shell, and Ca(OH)2 continued to be adopted as desulfurizers in manufacturing the biocoalbriquette and the white-biocoalbriquette. The effects on the experimental results by changing the inside Ca/S of both the white-biocoalbriquette and the biocoalbriquette are shown in Figure 4. In these experiments, the symbols (circle, square, and triangle) represent the experimental results of mixing the desulfurizers into the samples. The symbol star is that of mixing the limestone into the white-biocoalbriquettes and coating the Ca(OH)2 on its surface. The amount of Ca(OH)2 coated on the surface kept constant, about 0.2 g. This figure shows that the desulfurization efficiency of whitebiocoalbriquettes was not increased and it was saturated despite increasing the inside Ca/S ratio from 1 to 3 (total Ca/S changed from 2 to 4). This is quite different from the inside mixture of the desulfurizer. Coating desulfurizer on the surface promoted the desulfurizer’s calcinations, and also increased contact opportunity between the desulfurizer particle and flue gas. For applying this new desulfurization technology to low-grade coals with high sulfur content, in this study, the white-biocoalbriquettes were also manufactured with NT coal (sulfur content 4.6%, daf) from the southwest region of China, and the desulfurization experiments were performed. The desulfurization results of both the white-biocoalbriquette with NT coal and the white-biocoalbriquette with BJ coal (sulfur content 2.2%, daf) are shown in Figure 5. In these experiments, the inside Ca/S was changed from 1 to 3 and the amount of Ca(OH)2 coated on the surface was kept constant, about 0.2 g. The desulfurization efficiency of the white-biocoalbriquette with NT coal appeared in
Kim et al.
Figure 5. Change of desulfurization efficiency with inside Ca/S and coal type.
Figure 6. Change of desulfurization efficiency with furnace temperature.
the same change tendency as the white-biocoalbriquette with BJ coal. It varied from about 85% (Ca/S ) 1) to about 91% (Ca/S ) 3). The desulfurization efficiency of the white-biocoalbriquette with NT coal was a little lower than that of the white-biocoalbriquette with BJ coal at Ca/S less than 1.5. Moreover, another type of coal, SZ coal with a different fuel ratio, 3.37 (see Table 1) originating from China was tested for examining this new technology of coating a desulfurizer on the surface in the same conditions. The tested results further confirmed that in the range of 2-5% sulfur content (daf), the desulfurization efficiency was always over 85%. The combustion in domestic stoves often has a temperature fluctuation. With changing the furnace temperature from 873 to 1173 K, the desulfurization experiments were carried out to simulate practical combustion in domestic stoves. Here, NT coal with high sulfur content was used again. Figure 6 shows the temperature dependence of desulfurization efficiency. It was revealed from the experimental results that the efficiency kept high values and was almost not influenced by the furnace temperature. The results demonstrated that the white-biocoalbriquette has good desulfurization characteristics and is suitable for domestic stoves. Figure 7 shows the sulfur distribution, desulfurization behavior, and desulfurization efficiency of the white-biocoalbriquette. In the figure, the horizontal axis
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the horizontal axis and the two curves, it was found that although the white-biocoalbriquette contains more sulfur than the biocoalbriquette, the SO2 emission duringcombustion of the former is less than that of the latter. Almost no SO2 emission during the char combustion of the white-biocoalbriquette was observed. The black solid circle in the column of the figure gives the desulfurization efficiency of the white-biocoalbriquette. From Figure 7, it can be seen clearly that NT coal not only has high sulfur content but also, its volatile contains high sulfur content up to nearly 40%. The coating desulfurizer on the surface could control SO2 emission from both volatile combustion and char combustion. The desulfurization efficiency still amounted to nearly 90%. Conclusions Figure 7. Sulfur distribution and desulfurization efficiency
is the combustion time, and the left vertical axis presents the time concentration history of SO2 in the flue gas during the combustion of white-biocoalbriquettes with NT coal. The right vertical axis indicates the sulfur distribution and desulfurization efficiency. The column in the figure shows that the char in the white-biocoalbriquette contains about 60% of its total sulfur content, and the rest, about 40% sulfur exists in the volatile. To increase the breaking strength, a little wasted black liquor was mixed into the white-biocoalbriquette. The ordinary biocoalbriquette and whitebiocoalbriquette were briquetted, respectively. The former was made of 4 g of NT coal and 1 g of the pulverized cornstalk, and the latter was made of 4 g of NT coal, 1 g of the pulverized cornstalk, 1 g of the black liquor, and Tsukumi limestone (Ca/S ) 1, inside mixture) and about 0.2 g of Ca(OH)2 coated on the surface. The time concentration histories of SO2 emission in the flue gas during the combustion of both the biocoalbriquette and the white-biocoalbriquette are represented by a solid curve and dash curve, respectively, in the figure. By comparing the two curves and the two areas between
The white-biocoalbriquette was proposed and researched in this study for a more effective and economical artificial solid fuel. This white-biocoalbriquette is a composite solid fuel of both coal and biomass. A little limestone is mixed in its inside, and its surface is coated by a thin layer of desulfurizer. This white-biocoalbriquette looks white and will change people’s traditional concept of coal from black and dirty to new and clean, and it increases the desulfurization efficiency to almost the same level as a wet desulfurization process according to effectively controlling SO2 emission in both volatile combustion and char combustion. Moreover, the temperature fluctuation in a combustion chamber almost does not have an obvious influence on the desulfurization efficiency. Finally, the white-biocoalbriquette is considered to be one of the most effective technologies to improve the desulfurization efficiency for artificial solid fuels. Acknowledgment. The authors acknowledge the financial support of The Japan Society for the Promotion of Science for the present study. EF020300X
