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Volatilization characteristics of boron compounds during coal combustion
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Proceedings of the
Proceedings of the Combustion Institute 34 (2013) 2831–2838
Combustion Institute www.elsevier.com/locate/proci
Volatilization characteristics of boron compounds during coal combustion Naoki Noda a, Shigeo Ito a, Yoko Nunome b, Yasuaki Ueki b, Ryo Yoshiie b, Ichiro Naruse b,⇑ a
Central Research Institute of Electric Power Industry (CRIEPI), 2-6-1 Nagasaka, Yokosuka 240-0196, Japan b Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Available online 21 July 2012
Abstract Boron behavior during coal combustion was studied experimentally and theoretically. Seven types of coal with different boron concentrations were pyrolyzed or burned, using an electrically heated drop tube furnace. In order to understand the effect of coal type on the boron component of ash, chemical equilibrium calculations were made to determine which boron compounds contributed to forming the boron compounds found in the ash particles. Additionally, XAFS analyses were also carried out to evaluate the results obtained by the chemical equilibrium calculations. Some boron compounds in coal were volatilized, and others remained in the molten coal ash after combustion. Both the chemical equilibrium calculations of the boron compounds under the combustion condition and the boron analysis of the ash particles by the XAFS suggested that the boron compound contained in the ash was mainly B2O3. The boron compounds were concentrated in the molten slag during combustion. The enrichment of boron into the ash as particles was related to the amount of slag formed. The index, (Base/Acid ratio) (Ash content), correlates well with the volatilization fraction of boron even in the actual power plants. Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Coal combustion; Trace elements; Boron; Ash; XAFS
1. Introduction Recently, emissions of trace elements from coal combustion have been viewed as a global environmental issue. Boron compounds in coal are categorized as one of the volatile elements of combustion. Especially in the coal combustion process, most boron compounds are volatilized to the gaseous ⇑ Corresponding author. Address: Department of Mechanical Science and Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan. Fax: +81 52 789 5123. E-mail address: [email protected] (I. Naruse).
phase in the combustor. After that, some volatilized boron compounds are enriched into clinkers in the combustor and/or fly ash particulates in the flue gas [1]. The remaining gaseous boron compounds are almost completely absorbed by a wet flue gas desulfurizer (FGD) [2], since most boron compounds are water-soluble. Consequently, the boron compounds are separated into the clinkers in the combustor, the fly ash particulates collected by an electrostatic precipitator and an effluent from the wet-FGD. Although the boron compounds have little adverse effect on the human body, it is harmful to take them in over a long term. While, boron is an essential plant nutrient, nearly all plants show some symptoms of boron
1540-7489/$ - see front matter Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.proci.2012.07.018
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toxicity. Therefore, the World Health Organization (WHO) has provided a guideline for boron concentration in drinking water [3]. Since the boron concentration in surface water continues to increase due to discharge from industries, Japan has set an effluence standard for boron under the Water Pollution Control Law [4]. Because of this, in Japan not only electric power generation companies, but also industrial companies with coal combustion boilers have to import coals with lower boron concentration. The enrichment characteristics of boron in the fly ash particulates, the boron absorption efficiency in the wet-FGD effluent and the leaching characteristics of boron depend on coal types, combustion conditions and so forth. This means that it is necessary to understand, at the fundamental level, boron behavior during coal combustion. In earlier investigations, boron appeared to be completely converted to the gas phase in the furnace by combustion, this being assumed because of the high volatility of boron [5,6]. Previous research on boron relating to coal had focused only on its partitioning in a coal combustion process [7], leaching characteristics from the coal ash of high sulfur coal [8], removal of boron from effluents by use of ion-exchange resins [9], preremoval of boron from coal by washing the coal with a variety of acids [10] and so forth. Although Kashiwakura et al. [11] reported that all of the boron compounds were volatilized during coal combustion; our study [1] shows that some of the boron compounds are contained in the clinkers in the pulverized coal combustion boiler. Considering the present situation, precise boron behavior during coal combustion has not been studied sufficiently. The boron behavior during combustion should be considered as a two steps process; the volatilization process of boron compounds from coal during combustion, and the heterogeneous condensation of gaseous boron in the furnace and flue. In this study, seven types of coal with different boron concentrations were pyrolyzed burned, using an electrically heated drop tube furnace (DTF). The boron concentrations contained in the ash particles sampled were analyzed. In order to determine the effect of coal type on the boron evolution characteristics, chemical equilibrium calculations were made to investigate which boron compounds contributed to forming the boron compounds remaining in the ash particles. Additionally, XAFS analyses were also carried out to evaluate the results obtained by the chemical equilibrium calculations.
facility mainly consists of a screw feeder for the coal, electric furnaces and a sampling section. The pulverized coal of 40 lm median diameter is constantly fed at around 27.8 mg/s. In the combustion experiments, air was supplied as an oxidizer. The furnace temperature was kept at 1673 K, and the stoichiometric combustion air ratio was controlled at 1.24, which is nearly the same as that in practical pulverized coal (PC) combustion boilers. Under this experimental condition, the residence time of coal particles was about 4 s. In the pyrolysis experiments, on the other hand, nitrogen at 0.124 103 m3/s was supplied to the furnace. The furnace temperature was maintained at 1473 K. Under this condition, it took about 2.6 s of residence time. In all the experiments, the ash particles were sampled by a dust filter. The boron concentrations in both the ash particles sampled and the bottom ash accumulated in the furnace exit were analyzed by an inductively coupled plasma-atomic emission spectrometry (ICP-AES). The evolution fraction of boron from the coal during combustion is
2. Experimental setup and procedures Figure 1 shows a schematic diagram of an electrically heated drop tube furnace (DTF). This
Fig. 1. Schematic of an electrically heated drop tube furnace.
N. Noda et al. / Proceedings of the Combustion Institute 34 (2013) 2831–2838
calculated by the following equation, based on the boron content in the coal and ash particles. RBA
C B;ash F ash ¼1 C B;coal F coal
ð1Þ
C B;char F char C B;coal F coal
ð2Þ
RBC ¼ 1
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3. Results and discussions 3.1. Volatilization characteristics of boron in coal under the pyrolysis and combustion conditions Coals A, B and C, which had different coal properties, were selected for the pyrolysis and combustion experiments. Figure 2 shows the fraction of boron volatilization under the pyrolysis and combustion conditions. From this figure, the boron in Coal A and C is not volatilized under the pyrolysis condition. For Coal B, a little boron is volatilized. These results suggest that the boron in coal may not appear in the volatile matter. Under the combustion condition, on the other hand, more than half of the boron is volatized for the three coals tested. The volatilization fraction of boron depends on the coal type. Comparing the volatilization fractions under both conditions, the boron in coal seems to be contained mainly in the fixed carbon or the mineral matter.
Here, RBA, RBC, Fcoal, Fash, Fchar, CB,coal, CB,char and CB,ash denote volatilization fraction of boron under pyrolysis condition [–], volatilization fraction of boron under combustion condition [–], coal feed rate [kg/s], char emission rate [kg/s], ash emission rate [kg/s], boron concentration in the coal [mg/kg coal], boron concentration in the char [mg/kg ash] and boron concen tration in the ash [mg/kg ash], respectively. In order to analyze the respective concentrations of boron in the combustible and ash of raw coals, a low temperature plasma asher was also applied to make ash samples without heating treatment. The boron fraction in the ash of raw coal was defined as the boron concentration in the ash obtained by the plasma asher. Table 1 .shows the coal samples tested in this study. The boron concentration in the coals selected varies from 16 to 134 mg B/kg enabling us to determine the effect of coal type on the volatilization characteristics of boron during combustion. There are also a variety of ash constituents. In the experiment on volatilization characteristics of the boron in pyrolysis or combustion process, three kinds of coal (Coal A, B and C) were used because of the differences in ash composition. In the experiment for comparison with the ash of the utility boiler, five kinds of coal (Coal B, D, E, F and G) were used because of the similar property of ash composition with the utility boiler.
3.2. Presence part of boron in coal The results of Fig. 2 show that the boron in coal is contained in the fixed carbon or the mineral matter in the coal. Therefore, the boron present in the coal was analyzed, using the plasma asher at low temperature. Figure 3 shows the fraction of boron in the mineral matter for three types of coal. The residual boron is defined as the boron contained in the fixed carbon. It can be seen from the figure that Coal C has the highest boron fraction (in the mineral matter) of the three. The boron fraction in the mineral matter also depends on the coal type. The results in Fig. 3 also show that the boron content in the fixed carbon for Coal A is the highest of the three. From Fig. 2, however, the boron contained in Coal B tends to
Table 1 Properties of coal tested. Coal A
Coal B
Coal C
Coal D
Coal E
Coal F
Coal G
Proximate analysis Ash (wt.% dry) Fixed carbon (wt.% dry) Volatile matter (wt.% dry) Fuel ratio (–)
4.9 58.2 36.9 1.58
9.3 61.3 29.5 2.08
6.1 57.9 36.1 1.60
6.8 41.4 51.2 0.81
10.2 56.6 32.3 1.75
4.9 50.7 44.8 1.13
13.1 49.6 37.2 1.33
Ash compositions SiO2 (wt.% dry) Al2O3 (wt.% dry) Fe2O3 (wt.% dry) CaO (wt.% dry) MgO (wt.% dry) Na2O (wt.% dry) K2O (wt.% dry)
28.6 9.5 7.82 34.7 1.56 1.57 1.20
61.2 29.7 3.16 0.92 0.60 0.13 0.06
23.5 8.8 7.88 45.8 1.67 0.77 1.75
54.5 26.3 6.87 1.96 0.69 0.35 0.75
57.3 25.3 4.77 1.94 0.82 0.43 1.24
50.7 25.8 9.61 1.02 0.75 0.30 1.45
76.5 19.2 2.38 1.60 0.73 0.32 0.73
24
96
16
48
22
Trace element Boron (mg B/kg)
134
116
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Fig. 2. Fraction of boron volatilization under the pyrolysis and combustion conditions.
Fig. 3. Fraction of boron in the mineral matter for three types of coal.
be volatilized easily during combustion. In other words, the tendency of the volatilization characteristics shown by Fig. 2 is different from that shown by Fig. 3. This tendency disagreement is caused by the difference in the form of the boron compounds present in the three coals tested. Thus it is necessary to characterize or analyze the form of the boron compounds in the mineral matter of the respective raw coals. 3.3. Estimation of boron compounds in the mineral matters of raw coal by chemical equilibrium calculations As discussed above, the presence of boron compounds in the mineral matter of raw coal may affect the boron volatilization during pyrolysis
and combustion. Therefore, chemical equilibrium calculations were made to determine the presence and form of the boron compounds for the three coals, using the FactSage (ver. 5.5). One of the features of this software is the ability to calculate quantitative amounts of gas, liquid solid and molten slag chemical compounds. The input compositions were determined, based on coal properties shown in Table 1. In this calculation all components in Table 1 were used. The pressure was kept at ambient pressure. The temperature was varied from 600 to 1400 °C. The SLAG-C which is the only slag containing boron compounds was selected as the dataset relating to the molten slag. Generally, practical PC boilers apply a two-staged combustion method to reduce NOx concentration during combustion. Hereby, the stoichiometric combustion air ratios were given as 0.8 and 1.24 for the reducing and oxidizing atmospheres, respectively. Figure 4 shows the calculation results for Coal A, B and C under the reducing and oxidizing conditions. In this figure, the vertical axis is the fraction of each boron compound to the total boron given. The labels (gas), (solid) and (slag) indicate gas, solid and molten slag phases, respectively. In the temperature range greater than 1000 °C, which is the temperature in a furnace, metaboric acid (HBO2) and sodium and potassium boron compounds are produced as gaseous boron compounds for the three coal samples. Boron trioxide (B2O3) is also formed in the molten slag phase for the three. It seems that boron exists as B2O3 in the furnace. In the temperature range less than 1000 °C, on the other hand, Mg3B2O6 is formed as a solid boron compound for Coal A and C, while, (Al2O3)9(B2O3)2 is produced for Coal B. Comparing the results for differences in the reaction atmosphere, the boron fraction as the molten slag phase under the reducing condition is higher than that under the oxidizing condition. Comparing the boron fraction in the molten slag phase under the oxidizing condition with the volatilization fraction of boron obtained in Fig. 2 under the combustion condition, the order of the boron fraction in the molten slag phase calculated qualitatively agrees with the order of the volatilization fraction of boron obtained experimentally. This result suggests that the coal with low potential of formation of slag containing boron, which corresponds to Coal B, shows high emission of boron compounds during combustion. Generally, the particle temperature in the combustion zone is higher than 1000 °C, so that the boron in coal will be molten in the slag formed during combustion. As the boron in the molten slag is hard to evolve from the coal even at high temperature, the boron in Coal C did not volatilize even under the combustion condition shown in Fig. 2. Additionally, the boron fraction in the molten slag under the reducing condition is higher than that under the oxidizing condition
N. Noda et al. / Proceedings of the Combustion Institute 34 (2013) 2831–2838
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Fig. 4. Equilibrium calculation results of boron fractions for Coal A, B and C.
mentioned above. Therefore, the boron contained in Coal C is more difficult to evolve as a gaseous phase. Summarizing the effect of coal type on the boron volatilization, the presence
of boron in coal as well as the possibility of melting of the mineral matter in coal will contribute to boron compounds evolving as a gaseous phase.
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3.4. XAFS analysis of boron compounds in the ash particles In order to evaluate the validity of the equilibrium calculations obtained above, the actual boron compounds contained in the ash particles were analyzed by an X-ray absorption fine structure (XAFS). The qualitative analysis of boron compounds was conducted in the Kyushu Synchrotron Light Research Center (SAGA-LS) [12]. The bottom ash in the furnace (clinker ash) sampled at 1400 °C during combustion was analyzed, using Beam Line No. 12 in SAGA-LS. In assigning the boron compounds, H3BO3, B2O3, K2B4O7 and Na2B4O7 were also analyzed as they are the standard boron compounds. The coordination number of H3BO3 and B2O3 is three. In K2B4O7 and Na2B4O7, two and three as the coordination number co-exists, but two is dominant for Na2B4O7. Figure 5 shows the XAFS spectra of boron in the ash particles and standard boron compounds. From the figure, all of the samples have a first peak, whose energy corresponds to 194 eV. While, the second, third and forth peaks shift to the lower energy a little bit with the decrease of the coordination number with the respective boron compounds. The ash sample shows peaks at 194, 202, 212 and 220 eV. Comparing the peaks obtained for the standard boron
Fig. 5. XAFS spectra of boron in the ash particles and standard boron compounds.
compounds, the peaks with H3BO3 and B2O3 agree well with those with the ash sample. From the results of the chemical equilibrium calculations of those boron compounds, H3BO3 showed high volatility under the combustion condition. Therefore, B2O3 should be taken as the main boron compound in the ash particles. This analytical result by the XAFS is in good agreement with the results obtained by the equilibrium calculations shown in Fig. 4. 3.5. Effect of coal types on the boron volatilization during combustion Summarizing the results mentioned above, the volatilization characteristics of boron are affected by the melting behavior of mineral matter in the coal. Therefore, fractions of the molten slag in the mineral matter for all of the coals shown in Table 1 first were calculated using the chemical equilibrium theory. In this calculation, we assumed the virtual coal property for the investigation of the effect by the difference of coal ash composition. Therefore, the coal ash composition under the condition of same ash content and B content was changed. The condition of same ash content and B content with Coal A were used in this calculation. The stoichiometric combustion air ratio and temperature were set at 0.8 and 1000 ° C, which is high in the ratio of B2O3 in Fig. 4, respectively. Figure 6 shows the relationship between the amount of slag and the volatilization fraction of boron for each coal. It can be seen from the figure that the volatilization fraction of boron decreases with an increasing amount of slag. Consequently, the slag formation of the coal
Fig. 6. Relationship between the amount of slag calculated and the volatilization fraction of boron obtained experimentally for each coal.
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particles during combustion contributes to reducing the boron volatilization. 3.6. Correlation between the boron volatilization and the ash compositions in the commercialized pulverized coal combustion boilers In order to estimate the volatilization characteristics of boron in commercial PC combustion boilers, an index, (defined below) is easier and better for evaluating the boron volatilization than the amount of slag obtained by the chemical equilibrium calculations. Generally, the amount of molten slag relates to not only the ash content, but also the melting temperature of mineral matter in the coal. It is known that the melting temperature correlates with the Base/Acid ratio of ash (B/ A ratio) as defined by the following equation: B=A ratio ¼ ðFe2 O3 þ CaO þ MgO þ Na2 O þ K2 OÞ=ðSiO2 þ Al2 O3 þ TiOÞ
ð3Þ
For coals with higher B/A ratio, the melting point of ash is lower. This means that the mineral matter in the coal will be molten at low temperature, and form the molten slag easily. It is generally accepted that the amount of slag affects the volatilization fraction of boron from coal. We studied the relationship between the (B/A ratio) (Ash content), which shows the amount of slag, and the volatilization fraction of boron obtained by both the DTF and the commercialized PC combustion boilers. The result is shown in Fig. 7. In a utility boiler where coal whose B/ A in the ash is under 1.0 generally is used, we choose five kinds of coal (Coal B, D, E, F and G) for this experiment. The volatilization frac-
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tions of boron in practical power plants were obtained by analysis of boron concentrations in the clinker ash samples. From the figure, the index of (B/A ratio) (Ash content) correlates well with the volatilization fraction of boron even in the actual power plants. This index, proposed in this study, can estimate the volatilization fraction of boron from coals without depending on the coal types and scale of the combustion facility. 4. Conclusions The volatilization characteristics of boron during combustion were studied experimentally and theoretically. Seven types of coal with different boron concentrations were pyrolyzed or burned, using an electrically heated drop tube furnace (DTF). In order to understand the effect of coal type on the boron evolution characteristics, chemical equilibrium calculations were made to determine which boron compounds contributed to forming the boron compounds remaining in the ash particles. Additionally, XAFS analyses were also carried out to evaluate the results obtained by the chemical equilibrium calculations. The following results were obtained. (1) Some of boron compounds in coal are volatilized, and the other is remained in the molten coal ash during combustion. (2) Both the chemical equilibrium calculations of the boron compounds under the combustion condition and the boron analysis of the ash particles by the XAFS suggest that the boron compound contained in the ash is mainly B2O3. (3) The boron compounds are found in the molten slag during combustion. (4) The enrichment characteristics of boron in the ash particles relate to the slag amount formed, the amount of ash in the coal and the base/acid ratio of the ash. (5) The index of (B/A ratio) (Ash content) correlates well with the volatilization fraction of boron even in the actual power plants. References
Fig. 7. Relationship between (B/A ratio) (Ash content) and the volatilization fraction of boron in the drop tube furnace and the commercialized pulverized coal combustion boilers.
[1] N. Noda, S. Ito, Powder Technol. 180 (1–2) (2008) 227–231. [2] S. Ito, T. Yokoyama, Sci. Total Environ. 368 (1) (2006) 397–402. [3] World Health Organization (WHO), Guidelines for Drinking Water Quality, Second addendum to Third Edition, 2008. [4] The Ministry of Environment, 2007, available at http://www.env.go.jp/press/press.php?serial=8210. [5] R. Meiji, Fuel Process. Technol. 38 (1–3) (1994) 199– 217. [6] L.B. Clarke, L.L. Sloss, IEA Coal Research IEACR, 1992.
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[7] C. Cheng, P. Hack, P. Chu, et al., Energy Fuels 23 (10) (2009) 4805–4816. [8] S. Mukherjee, S.K. Srivastava, Energy Fuels 19 (3) (2005) 882–891. [9] A. Ohki, K. Yamada, T. Furuzono, T. Nakajima, H. Takanashi, Energy Fuels 25 (8) (2011) 3568– 3573.
[10] L. Zhang, H. Kawashima, T. Takanohashi, T. Nakazato, I. Saito, H. Tao, Energy Fuels 22 (2) (2008) 1183–1190. [11] S. Kashiwakura, T. Takahashi, Fuel 90 (4) (2011) 1408–1415. [12] Kyushu Synchrotron Light Research Center, available at http://www.saga-ls.jp/?page=206.
Proceedings of the
Proceedings of the Combustion Institute 34 (2013) 2831–2838
Combustion Institute www.elsevier.com/locate/proci
Volatilization characteristics of boron compounds during coal combustion Naoki Noda a, Shigeo Ito a, Yoko Nunome b, Yasuaki Ueki b, Ryo Yoshiie b, Ichiro Naruse b,⇑ a
Central Research Institute of Electric Power Industry (CRIEPI), 2-6-1 Nagasaka, Yokosuka 240-0196, Japan b Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Available online 21 July 2012
Abstract Boron behavior during coal combustion was studied experimentally and theoretically. Seven types of coal with different boron concentrations were pyrolyzed or burned, using an electrically heated drop tube furnace. In order to understand the effect of coal type on the boron component of ash, chemical equilibrium calculations were made to determine which boron compounds contributed to forming the boron compounds found in the ash particles. Additionally, XAFS analyses were also carried out to evaluate the results obtained by the chemical equilibrium calculations. Some boron compounds in coal were volatilized, and others remained in the molten coal ash after combustion. Both the chemical equilibrium calculations of the boron compounds under the combustion condition and the boron analysis of the ash particles by the XAFS suggested that the boron compound contained in the ash was mainly B2O3. The boron compounds were concentrated in the molten slag during combustion. The enrichment of boron into the ash as particles was related to the amount of slag formed. The index, (Base/Acid ratio) (Ash content), correlates well with the volatilization fraction of boron even in the actual power plants. Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Coal combustion; Trace elements; Boron; Ash; XAFS
1. Introduction Recently, emissions of trace elements from coal combustion have been viewed as a global environmental issue. Boron compounds in coal are categorized as one of the volatile elements of combustion. Especially in the coal combustion process, most boron compounds are volatilized to the gaseous ⇑ Corresponding author. Address: Department of Mechanical Science and Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan. Fax: +81 52 789 5123. E-mail address: [email protected] (I. Naruse).
phase in the combustor. After that, some volatilized boron compounds are enriched into clinkers in the combustor and/or fly ash particulates in the flue gas [1]. The remaining gaseous boron compounds are almost completely absorbed by a wet flue gas desulfurizer (FGD) [2], since most boron compounds are water-soluble. Consequently, the boron compounds are separated into the clinkers in the combustor, the fly ash particulates collected by an electrostatic precipitator and an effluent from the wet-FGD. Although the boron compounds have little adverse effect on the human body, it is harmful to take them in over a long term. While, boron is an essential plant nutrient, nearly all plants show some symptoms of boron
1540-7489/$ - see front matter Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.proci.2012.07.018
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toxicity. Therefore, the World Health Organization (WHO) has provided a guideline for boron concentration in drinking water [3]. Since the boron concentration in surface water continues to increase due to discharge from industries, Japan has set an effluence standard for boron under the Water Pollution Control Law [4]. Because of this, in Japan not only electric power generation companies, but also industrial companies with coal combustion boilers have to import coals with lower boron concentration. The enrichment characteristics of boron in the fly ash particulates, the boron absorption efficiency in the wet-FGD effluent and the leaching characteristics of boron depend on coal types, combustion conditions and so forth. This means that it is necessary to understand, at the fundamental level, boron behavior during coal combustion. In earlier investigations, boron appeared to be completely converted to the gas phase in the furnace by combustion, this being assumed because of the high volatility of boron [5,6]. Previous research on boron relating to coal had focused only on its partitioning in a coal combustion process [7], leaching characteristics from the coal ash of high sulfur coal [8], removal of boron from effluents by use of ion-exchange resins [9], preremoval of boron from coal by washing the coal with a variety of acids [10] and so forth. Although Kashiwakura et al. [11] reported that all of the boron compounds were volatilized during coal combustion; our study [1] shows that some of the boron compounds are contained in the clinkers in the pulverized coal combustion boiler. Considering the present situation, precise boron behavior during coal combustion has not been studied sufficiently. The boron behavior during combustion should be considered as a two steps process; the volatilization process of boron compounds from coal during combustion, and the heterogeneous condensation of gaseous boron in the furnace and flue. In this study, seven types of coal with different boron concentrations were pyrolyzed burned, using an electrically heated drop tube furnace (DTF). The boron concentrations contained in the ash particles sampled were analyzed. In order to determine the effect of coal type on the boron evolution characteristics, chemical equilibrium calculations were made to investigate which boron compounds contributed to forming the boron compounds remaining in the ash particles. Additionally, XAFS analyses were also carried out to evaluate the results obtained by the chemical equilibrium calculations.
facility mainly consists of a screw feeder for the coal, electric furnaces and a sampling section. The pulverized coal of 40 lm median diameter is constantly fed at around 27.8 mg/s. In the combustion experiments, air was supplied as an oxidizer. The furnace temperature was kept at 1673 K, and the stoichiometric combustion air ratio was controlled at 1.24, which is nearly the same as that in practical pulverized coal (PC) combustion boilers. Under this experimental condition, the residence time of coal particles was about 4 s. In the pyrolysis experiments, on the other hand, nitrogen at 0.124 103 m3/s was supplied to the furnace. The furnace temperature was maintained at 1473 K. Under this condition, it took about 2.6 s of residence time. In all the experiments, the ash particles were sampled by a dust filter. The boron concentrations in both the ash particles sampled and the bottom ash accumulated in the furnace exit were analyzed by an inductively coupled plasma-atomic emission spectrometry (ICP-AES). The evolution fraction of boron from the coal during combustion is
2. Experimental setup and procedures Figure 1 shows a schematic diagram of an electrically heated drop tube furnace (DTF). This
Fig. 1. Schematic of an electrically heated drop tube furnace.
N. Noda et al. / Proceedings of the Combustion Institute 34 (2013) 2831–2838
calculated by the following equation, based on the boron content in the coal and ash particles. RBA
C B;ash F ash ¼1 C B;coal F coal
ð1Þ
C B;char F char C B;coal F coal
ð2Þ
RBC ¼ 1
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3. Results and discussions 3.1. Volatilization characteristics of boron in coal under the pyrolysis and combustion conditions Coals A, B and C, which had different coal properties, were selected for the pyrolysis and combustion experiments. Figure 2 shows the fraction of boron volatilization under the pyrolysis and combustion conditions. From this figure, the boron in Coal A and C is not volatilized under the pyrolysis condition. For Coal B, a little boron is volatilized. These results suggest that the boron in coal may not appear in the volatile matter. Under the combustion condition, on the other hand, more than half of the boron is volatized for the three coals tested. The volatilization fraction of boron depends on the coal type. Comparing the volatilization fractions under both conditions, the boron in coal seems to be contained mainly in the fixed carbon or the mineral matter.
Here, RBA, RBC, Fcoal, Fash, Fchar, CB,coal, CB,char and CB,ash denote volatilization fraction of boron under pyrolysis condition [–], volatilization fraction of boron under combustion condition [–], coal feed rate [kg/s], char emission rate [kg/s], ash emission rate [kg/s], boron concentration in the coal [mg/kg coal], boron concentration in the char [mg/kg ash] and boron concen tration in the ash [mg/kg ash], respectively. In order to analyze the respective concentrations of boron in the combustible and ash of raw coals, a low temperature plasma asher was also applied to make ash samples without heating treatment. The boron fraction in the ash of raw coal was defined as the boron concentration in the ash obtained by the plasma asher. Table 1 .shows the coal samples tested in this study. The boron concentration in the coals selected varies from 16 to 134 mg B/kg enabling us to determine the effect of coal type on the volatilization characteristics of boron during combustion. There are also a variety of ash constituents. In the experiment on volatilization characteristics of the boron in pyrolysis or combustion process, three kinds of coal (Coal A, B and C) were used because of the differences in ash composition. In the experiment for comparison with the ash of the utility boiler, five kinds of coal (Coal B, D, E, F and G) were used because of the similar property of ash composition with the utility boiler.
3.2. Presence part of boron in coal The results of Fig. 2 show that the boron in coal is contained in the fixed carbon or the mineral matter in the coal. Therefore, the boron present in the coal was analyzed, using the plasma asher at low temperature. Figure 3 shows the fraction of boron in the mineral matter for three types of coal. The residual boron is defined as the boron contained in the fixed carbon. It can be seen from the figure that Coal C has the highest boron fraction (in the mineral matter) of the three. The boron fraction in the mineral matter also depends on the coal type. The results in Fig. 3 also show that the boron content in the fixed carbon for Coal A is the highest of the three. From Fig. 2, however, the boron contained in Coal B tends to
Table 1 Properties of coal tested. Coal A
Coal B
Coal C
Coal D
Coal E
Coal F
Coal G
Proximate analysis Ash (wt.% dry) Fixed carbon (wt.% dry) Volatile matter (wt.% dry) Fuel ratio (–)
4.9 58.2 36.9 1.58
9.3 61.3 29.5 2.08
6.1 57.9 36.1 1.60
6.8 41.4 51.2 0.81
10.2 56.6 32.3 1.75
4.9 50.7 44.8 1.13
13.1 49.6 37.2 1.33
Ash compositions SiO2 (wt.% dry) Al2O3 (wt.% dry) Fe2O3 (wt.% dry) CaO (wt.% dry) MgO (wt.% dry) Na2O (wt.% dry) K2O (wt.% dry)
28.6 9.5 7.82 34.7 1.56 1.57 1.20
61.2 29.7 3.16 0.92 0.60 0.13 0.06
23.5 8.8 7.88 45.8 1.67 0.77 1.75
54.5 26.3 6.87 1.96 0.69 0.35 0.75
57.3 25.3 4.77 1.94 0.82 0.43 1.24
50.7 25.8 9.61 1.02 0.75 0.30 1.45
76.5 19.2 2.38 1.60 0.73 0.32 0.73
24
96
16
48
22
Trace element Boron (mg B/kg)
134
116
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Fig. 2. Fraction of boron volatilization under the pyrolysis and combustion conditions.
Fig. 3. Fraction of boron in the mineral matter for three types of coal.
be volatilized easily during combustion. In other words, the tendency of the volatilization characteristics shown by Fig. 2 is different from that shown by Fig. 3. This tendency disagreement is caused by the difference in the form of the boron compounds present in the three coals tested. Thus it is necessary to characterize or analyze the form of the boron compounds in the mineral matter of the respective raw coals. 3.3. Estimation of boron compounds in the mineral matters of raw coal by chemical equilibrium calculations As discussed above, the presence of boron compounds in the mineral matter of raw coal may affect the boron volatilization during pyrolysis
and combustion. Therefore, chemical equilibrium calculations were made to determine the presence and form of the boron compounds for the three coals, using the FactSage (ver. 5.5). One of the features of this software is the ability to calculate quantitative amounts of gas, liquid solid and molten slag chemical compounds. The input compositions were determined, based on coal properties shown in Table 1. In this calculation all components in Table 1 were used. The pressure was kept at ambient pressure. The temperature was varied from 600 to 1400 °C. The SLAG-C which is the only slag containing boron compounds was selected as the dataset relating to the molten slag. Generally, practical PC boilers apply a two-staged combustion method to reduce NOx concentration during combustion. Hereby, the stoichiometric combustion air ratios were given as 0.8 and 1.24 for the reducing and oxidizing atmospheres, respectively. Figure 4 shows the calculation results for Coal A, B and C under the reducing and oxidizing conditions. In this figure, the vertical axis is the fraction of each boron compound to the total boron given. The labels (gas), (solid) and (slag) indicate gas, solid and molten slag phases, respectively. In the temperature range greater than 1000 °C, which is the temperature in a furnace, metaboric acid (HBO2) and sodium and potassium boron compounds are produced as gaseous boron compounds for the three coal samples. Boron trioxide (B2O3) is also formed in the molten slag phase for the three. It seems that boron exists as B2O3 in the furnace. In the temperature range less than 1000 °C, on the other hand, Mg3B2O6 is formed as a solid boron compound for Coal A and C, while, (Al2O3)9(B2O3)2 is produced for Coal B. Comparing the results for differences in the reaction atmosphere, the boron fraction as the molten slag phase under the reducing condition is higher than that under the oxidizing condition. Comparing the boron fraction in the molten slag phase under the oxidizing condition with the volatilization fraction of boron obtained in Fig. 2 under the combustion condition, the order of the boron fraction in the molten slag phase calculated qualitatively agrees with the order of the volatilization fraction of boron obtained experimentally. This result suggests that the coal with low potential of formation of slag containing boron, which corresponds to Coal B, shows high emission of boron compounds during combustion. Generally, the particle temperature in the combustion zone is higher than 1000 °C, so that the boron in coal will be molten in the slag formed during combustion. As the boron in the molten slag is hard to evolve from the coal even at high temperature, the boron in Coal C did not volatilize even under the combustion condition shown in Fig. 2. Additionally, the boron fraction in the molten slag under the reducing condition is higher than that under the oxidizing condition
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Fig. 4. Equilibrium calculation results of boron fractions for Coal A, B and C.
mentioned above. Therefore, the boron contained in Coal C is more difficult to evolve as a gaseous phase. Summarizing the effect of coal type on the boron volatilization, the presence
of boron in coal as well as the possibility of melting of the mineral matter in coal will contribute to boron compounds evolving as a gaseous phase.
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3.4. XAFS analysis of boron compounds in the ash particles In order to evaluate the validity of the equilibrium calculations obtained above, the actual boron compounds contained in the ash particles were analyzed by an X-ray absorption fine structure (XAFS). The qualitative analysis of boron compounds was conducted in the Kyushu Synchrotron Light Research Center (SAGA-LS) [12]. The bottom ash in the furnace (clinker ash) sampled at 1400 °C during combustion was analyzed, using Beam Line No. 12 in SAGA-LS. In assigning the boron compounds, H3BO3, B2O3, K2B4O7 and Na2B4O7 were also analyzed as they are the standard boron compounds. The coordination number of H3BO3 and B2O3 is three. In K2B4O7 and Na2B4O7, two and three as the coordination number co-exists, but two is dominant for Na2B4O7. Figure 5 shows the XAFS spectra of boron in the ash particles and standard boron compounds. From the figure, all of the samples have a first peak, whose energy corresponds to 194 eV. While, the second, third and forth peaks shift to the lower energy a little bit with the decrease of the coordination number with the respective boron compounds. The ash sample shows peaks at 194, 202, 212 and 220 eV. Comparing the peaks obtained for the standard boron
Fig. 5. XAFS spectra of boron in the ash particles and standard boron compounds.
compounds, the peaks with H3BO3 and B2O3 agree well with those with the ash sample. From the results of the chemical equilibrium calculations of those boron compounds, H3BO3 showed high volatility under the combustion condition. Therefore, B2O3 should be taken as the main boron compound in the ash particles. This analytical result by the XAFS is in good agreement with the results obtained by the equilibrium calculations shown in Fig. 4. 3.5. Effect of coal types on the boron volatilization during combustion Summarizing the results mentioned above, the volatilization characteristics of boron are affected by the melting behavior of mineral matter in the coal. Therefore, fractions of the molten slag in the mineral matter for all of the coals shown in Table 1 first were calculated using the chemical equilibrium theory. In this calculation, we assumed the virtual coal property for the investigation of the effect by the difference of coal ash composition. Therefore, the coal ash composition under the condition of same ash content and B content was changed. The condition of same ash content and B content with Coal A were used in this calculation. The stoichiometric combustion air ratio and temperature were set at 0.8 and 1000 ° C, which is high in the ratio of B2O3 in Fig. 4, respectively. Figure 6 shows the relationship between the amount of slag and the volatilization fraction of boron for each coal. It can be seen from the figure that the volatilization fraction of boron decreases with an increasing amount of slag. Consequently, the slag formation of the coal
Fig. 6. Relationship between the amount of slag calculated and the volatilization fraction of boron obtained experimentally for each coal.
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particles during combustion contributes to reducing the boron volatilization. 3.6. Correlation between the boron volatilization and the ash compositions in the commercialized pulverized coal combustion boilers In order to estimate the volatilization characteristics of boron in commercial PC combustion boilers, an index, (defined below) is easier and better for evaluating the boron volatilization than the amount of slag obtained by the chemical equilibrium calculations. Generally, the amount of molten slag relates to not only the ash content, but also the melting temperature of mineral matter in the coal. It is known that the melting temperature correlates with the Base/Acid ratio of ash (B/ A ratio) as defined by the following equation: B=A ratio ¼ ðFe2 O3 þ CaO þ MgO þ Na2 O þ K2 OÞ=ðSiO2 þ Al2 O3 þ TiOÞ
ð3Þ
For coals with higher B/A ratio, the melting point of ash is lower. This means that the mineral matter in the coal will be molten at low temperature, and form the molten slag easily. It is generally accepted that the amount of slag affects the volatilization fraction of boron from coal. We studied the relationship between the (B/A ratio) (Ash content), which shows the amount of slag, and the volatilization fraction of boron obtained by both the DTF and the commercialized PC combustion boilers. The result is shown in Fig. 7. In a utility boiler where coal whose B/ A in the ash is under 1.0 generally is used, we choose five kinds of coal (Coal B, D, E, F and G) for this experiment. The volatilization frac-
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tions of boron in practical power plants were obtained by analysis of boron concentrations in the clinker ash samples. From the figure, the index of (B/A ratio) (Ash content) correlates well with the volatilization fraction of boron even in the actual power plants. This index, proposed in this study, can estimate the volatilization fraction of boron from coals without depending on the coal types and scale of the combustion facility. 4. Conclusions The volatilization characteristics of boron during combustion were studied experimentally and theoretically. Seven types of coal with different boron concentrations were pyrolyzed or burned, using an electrically heated drop tube furnace (DTF). In order to understand the effect of coal type on the boron evolution characteristics, chemical equilibrium calculations were made to determine which boron compounds contributed to forming the boron compounds remaining in the ash particles. Additionally, XAFS analyses were also carried out to evaluate the results obtained by the chemical equilibrium calculations. The following results were obtained. (1) Some of boron compounds in coal are volatilized, and the other is remained in the molten coal ash during combustion. (2) Both the chemical equilibrium calculations of the boron compounds under the combustion condition and the boron analysis of the ash particles by the XAFS suggest that the boron compound contained in the ash is mainly B2O3. (3) The boron compounds are found in the molten slag during combustion. (4) The enrichment characteristics of boron in the ash particles relate to the slag amount formed, the amount of ash in the coal and the base/acid ratio of the ash. (5) The index of (B/A ratio) (Ash content) correlates well with the volatilization fraction of boron even in the actual power plants. References
Fig. 7. Relationship between (B/A ratio) (Ash content) and the volatilization fraction of boron in the drop tube furnace and the commercialized pulverized coal combustion boilers.
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