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Behavior of selenium in the flue gas of pulverized coal combustion system: Influence of kind of coal and combustion conditions
Fuel Processing Technology 167 (2017) 388–394
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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Research article
Behavior of selenium in the flue gas of pulverized coal combustion system: Influence of kind of coal and combustion conditions Takuya Furuzono a, Tsunenori Nakajima b,⁎, Hiroki Fujishima b, Hirokazu Takanashi b, Akira Ohki b a b
Coal & Environmental Research Laboratory, Idemitsu Kosan Co., Ltd., 3-1 Nakasode, Sodegaura, Chiba 299-0267, Japan Department of Chemistry, Biotechnology, and Chemical Engineering, Graduate School of Science and Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890-0065, Japan
a r t i c l e
i n f o
Article history: Received 30 March 2017 Received in revised form 5 July 2017 Accepted 15 July 2017
Keywords: Pulverized coal combustion Flue gas Selenium Distribution Kind of coal
a b s t r a c t Five kinds of coals (mainly low Se levels) were pulverized and combusted in a turbulent furnace combustion test rig, which had a burner on the top and a gas cooling pipe with two sampling ports at the side near the bottom. A sampling train according to ISO 17211:2015 was attached to the ports to assess the distribution of Se in the gas and solid phase (coal ash) in the flue gas. The fraction of Se in the solid phase (R Ses) in the flue gas is given by R Ses = Ses/(Ses + Seg), where Ses and Seg are the concentrations of Se in the solid and gas phases, respectively. For all coals, R Ses increased with a decrease in the flue gas temperature from 300 °C to 100 °C, and the difference in R Ses by the temperature was greatly dependent on the kinds of feed coals. The effects of the contents of minerals in feed coals and the combustion conditions upon R Ses were studied. The partitioning of Se from the gas to solid phase in the flue gas was elucidated in terms of the chemical and physical adsorption of Se on coal ash. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The discharge of Se compounds into the environment can have negative impacts on human health. Although Se is an essential element for humans and other animals at low levels of uptake, a high level of uptake can cause serious damage to living organisms [1,2]. In several countries, Se is listed in national pollutant chemical inventories, such as the United Kingdom's Pollution Inventory, Australia's National Pollution Inventory, and the Pollution Release and Transfer Register of Japan. In the future, regulations related to the control of Se emissions into air will be formed in many countries. Combustion of coal is a major anthropogenic emission source of volatile toxic trace elements such as Hg and Se. It has been reported that various hazardous trace elements are distributed in the by-products of coal combustion systems, such as coal fly ash, flue gas desulfurization wastewater, and exhaust gas, and these elements are discharged into the environment in both gas and solid states [3–5]. The behavior of Hg in coal combustion systems has been well studied. The chemical speciation of Hg in coal combustion flue gas as well as the distribution of Hg in the combustion systems have been reported [6–10]. Moreover, the partitioning of Hg from the gas to solid phase (coal ash) in the flue gas has been investigated [11–13]. Compared with Hg, studies on the behavior of Se in coal combustion systems are scarce. Senior et al. investigated the behavior of some trace
⁎ Corresponding author. E-mail address: [email protected] (T. Nakajima).
http://dx.doi.org/10.1016/j.fuproc.2017.07.019 0378-3820/© 2017 Elsevier B.V. All rights reserved.
elements, including Se, in a coal combustion system by using a cascade impactor to collect particulate ashes [14,15]. They suggested that Se is almost completely vaporized in the flame zone to form SeO2, while Se still exists in the gas phase in low temperature zones, such as at the entrance to electrostatic precipitators (ESPs), although the partitioning of Se from the gas to solid phase (coal ash) occurs considerably. However, compared with other elements, the mass balance of Se in the low temperature zones is not very high. A similar experiment was carried out by Seams et al. who reported that the concentration of Se in coal ash was dependent on its particle size whereas the relationship between the concentration of Se and that of Ca or Fe in size-segregated ash samples was not clear [16,17]. There have been some reports on the partitioning of Se from the gas to solid phase in the flue gas of coal combustion based on model and simulation studies. Lopez-Anton et al. investigated the retention of Se on the surface of coal ash using reference materials such as activated carbon and limestone, and suggested that the Se retention may be influenced by the presence of carbonaceous particles and Ca compounds in the ash [18,19]. Senior et al. reported that the partitioning of Se from the gas to solid phase may be ascribed to the reaction of gaseous Se with Ca and Fe compounds in feed coal in the flame zone, whereas the coagulation of gaseous Se leads to physical adsorption on coal ash in the low temperature zone [20]. In addition, Raeva et al. reported that the presence of Ca in feed coal promotes the Se retention in the solid phase due to common acid-base chemistry in coal combustion [21]. Noda et al. examined the behavior of some trace elements, including Se, in the flue gas of coal combustion [22]. They used US Environmental Protection Agency (EPA) method 29 [23], which allows for the
T. Furuzono et al. / Fuel Processing Technology 167 (2017) 388–394 Table 1 Properties of feed coals.
Table 2 Combustion conditions. Unit
Inherent moisture Ash Volatile matter Fixed carbon Ash composition SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O Se
389
wt% (air dried)
wt% of ash
mg/kg (dry basis)
Feed coal A
B
C
D
E
3.3 12.3 34.4 50.0 60.4 23.3 4.28 3.20 1.06 0.32 1.40 0.40
4.8 13.0 32.7 50.0 47.8 41.8 3.84 2.11 0.39 0.23 0.51 2.55
3.1 12.2 31.4 53.3 56.8 21.0 6.51 3.50 1.47 0.60 2.04 0.27
2.4 15.7 26.0 55.9 46.6 29.6 5.95 6.95 0.96 0.36 0.60 0.49
7.2 3.9 43.0 46.0 38.1 22.6 18.5 7.37 2.35 1.14 0.95 0.05
determination of distribution of the elements in gas and solid phase in the flue gas. However, it has been noted that EPA and related methods are not particularly effective in terms of Se speciation in coal combustion systems [24,25]. The main reason for this is the difficulty in sampling of Se in the gas phase, especially in the low temperature zone, because of the reduction of SeO2 to Se0 by SO2, which is usually present in large quantities in the flue gas. The Se0 species tends to adhere to the tubing and vessel walls in the gas sampling train. To prevent the problem, a new official method for the sampling and determination of Se in the flue gas was recently issued (ISO 17211:2015) [26]. This is an improved method for EPA method 29 in terms of the addition of a rinse process to attain the complete recovery of Se adhered to the tubing and vessel walls. Thus, the sampling of Se in the gas phase in the presence of SO2 can be effectively carried out. The Se contamination in desulfurization wastewaters has been one of the big problems in Japanese coal fired power plants. Therefore, the use of coals with low Se levels is currently recommended in Japan. In this study, the ISO method was applied to the combustion of various coals, including low Se level coals, in a turbulent furnace combustion test rig to
Primary air Secondary air Over-fire air Over-fire air port (distance from the furnace top)
Unit
Condition 1
Condition 2
Condition 3
Nm3/h
6 52 0 –
6 36 16 116
6 36 16 166
cm
assess the distribution of Se in the gas and solid phase (coal ash) in the flue gas. The influence of flue gas temperature, kind of coal, and combustion conditions on the behavior of Se was investigated in terms of the partitioning of Se from the gas to solid phase in the low temperature zone, which is modeled for NOx removal systems (ca. 300 °C) and ESPs (ca. 100 °C).
2. Experimental 2.1. Coal samples Five coals (termed A–E), which had been imported for use in Japanese coal-fired power plants, were tested. Analysis of these coals was conducted based on JIS M8812 [27] and the results are shown in Table 1. The concentration of Se in the coals varied from 2.55 to 0.05 mg/kg. The ash composition was obtained by X-ray fluorescence analysis (Zetium, PANalytical V. B.) after low temperature ashing of the coal. The determination of Se in the coal was conducted as follows. The digestion of coal sample was carried out using a microwave processor (ETHOS 1, Milestone Inc.) with the acidic mixture of HNO3/H2O2. Detailed conditions of the microwave processing were described in our previous study [28]. The concentration of Se in the resulting solution was analyzed by hydrate generation atomic absorption spectrometry (HG-AAS; HYD-20 hydride generator, Nippon Jarrell-Ash Co. Ltd. and solar S2 spectrometer, Thermo Scientific Inc.) [29].
Fig. 1. Schematic of the turbulent furnace combustion test rig with sampling ports I and II to which the Se sampling train (Fig. 2) was attached.
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Fig. 2. Schematic of the Se sampling train.
2.2. Turbulent furnace combustion test rig A schematic of the turbulent furnace combustion test rig used is shown in Fig. 1. The combustor had a 300 mm inner diameter and a 2800 mm length with a burner on the top and a gas cooling pipe with sampling ports at the side near the bottom. Coal samples were pulverized to fine particles (b 75 μm) and dried prior to combustion. The coal feed rate was approximately 6 kg/h. The combustion conditions are listed in Table 2, and condition 1 was usually used. When the influence of combustion conditions on the behavior of Se was examined, the flow rate of secondary air was varied while the position of over-fire air port was changed. To adjust the O2 concentration (to 4.2%) in the combustion air, the coal feed speed was controlled. The flue gas samples were collected from two sampling ports (I and II) of the cooling pipe. The temperature of flue gas at each sampling port was measured by a type K thermocouple probe and those for ports I and II were ca. 300 and 100 °C, respectively.
2.3. Sampling and determination of Se in the gas and solid phases
ash was measured by a similar method described in Section 2.1 for feed coals except for the use of the acidic mixture of HNO3/HF/H2O2 [30]. On the other hand, Se in the gas phase was adsorbed by a mixture of 5% HNO3 and 10% H2O2 in two subsequent impingers. The amount of Se found in the absorbent solution of the second impinger was always b10% of that in the first impinger, and the sum of those Se amounts was used. The ISO method involved a rinse procedure for the tubing and vessel walls within the train using an oxidant solution containing 10 g/L KMnO4 and 1 M H2SO4. The Se content in the gas phase was calculated from the sum of the amounts of Se found in the absorbent solutions and in the rinse solution. When coal was combusted in the turbulent furnace, the amount of bottom ash was about 1/20–1/30 of that of the fly ash which was captured by the bag filter after the sampling ports (Fig. 1). In some cases, it was confirmed that the Se concentration in the bottom ash was below the detection limit. It is proposed that the distribution of Se to the bottom ash can be negligible.
2.4. Measurement of particle size distribution and unburned carbon content in coal ash
According to ISO 17211:2015, a sampling train was set up as shown in Fig. 2, and the inlet of the train was connected to sampling ports I and II. The flue gas of coal combustion was passed through the sampling train for 60 min, and the flow rate was regulated by using an adjustable vacuum pump (ca. 30 L/min). In the train, Se in the solid phase (coal ash) was captured by a cylindrical filter paper (outside diameter 25 mm, length 90 mm, No. 88RH, Advantec). The Se concentration in the coal
The particle size distribution and median diameter of the coal ash samples obtained were measured by a laser diffraction-type particle size analyzer (LA-920; Horiba, Ltd.). The unburned carbon content was determined according to JIS M8812 [27]. The coal ash samples were heated to 815 °C in an electric furnace and the unburned carbon content was obtained from the weight loss during the heating procedure.
Fig. 3. Recovery of Se in the gas phase.
Fig. 4. Mass balance of Se in the flue gas.
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Fig. 5. Influence of the flue gas temperature and kind of feed coal on R Ses. Mass balance of Se in the flue gas for each case is shown in parentheses.
3. Results and discussion 3.1. Sampling of Se in the gas and solid phases in coal combustion flue gas Firstly, the availability of a rinse process in the sampling of Se in coal combustion flue gas was examined when the ISO method was applied to the turbulent furnace (combustion condition 1) with coals A and B, which had different Se levels (0.40 and 2.55 mg/kg, respectively). Sampling port I was used. The theoretical concentration of Se in the gas phase was obtained from the difference between the amount of Se found in the feed coal and that in the coal ash (solid phase) captured by the filter in the sampling train, considering the regulated flow rate of the flue gas. Based on the theoretical Se concentration, the recovery of Se was calculated for both the absorbent and rinse solutions. As shown in Fig. 3, the recovery of Se in the gas phase is acceptable for both coals when the recovery in the absorbent solution and that in the rinse solution were summed, and it is confirmed that the rinse process is essential for the complete recovery of Se. Secondly, the mass distribution of Se in the gas and solid phase was examined by using the ISO method. The amount of Se in the gas phase was obtained with the aid of the rinse process as described above. For both sampling ports I and II, the mass balance of Se in the flue gas is
Fig. 6. Plot of R Ses (port I) vs. the contents of Ca and Fe in feed coal.
Fig. 7. Plot of R Ses (port I) vs. the contents of Na and K in feed coal.
acceptable when the mass distribution in the gas phase and that in the solid phase are summed, as shown in Fig. 4. 3.2. Influence of flue gas temperature and kind of coal on the partitioning of Se from the gas to solid phase The influence of flue gas temperature and kind of coal on the partitioning of Se from the gas to solid phase was examined. When five kinds of coals (A–E) were combusted under combustion condition 1, the flue gas samples were collected at ports I and II. The fraction of Se in solid phase (R Ses) is given by the following equation. R Ses ¼ Ses = Ses þ Seg
where Ses and Seg are the concentrations of Se in the solid and gas phases, respectively, in the flue gas. Fig. 5 shows the influence of flue gas temperature and kind of coal on R Ses as well as the mass balance of Se in the flue gas. For port I, an incomplete mass balance closure was obtained when coals with low Se levels (D and E) were used. For all five coals, R Ses increased with a decrease in the flue gas temperature from 300 °C (port I) to 100 °C (port II). However, R Ses varied among the coal types, especially for the samples collected at port I. In a coal combustion flue gas, Se in the gas phase probably reacts with specific inorganic components in the feed coal in the high temperature zone over 300 °C. By contrast, it is anticipated that Se in the gas phase is physically adsorbed and condensed on the surface of coal ash (solid phase) at lower temperatures (around 100 °C). It is rational that chemical absorption favorably occurs in the relatively high temperature zone while physical absorption is promoted in the low temperature zone. To test this hypothesis, the influence of the content of specific inorganic elements (Ca, Fe, Na, and K) in the feed coal on R Ses at ca. 300 °C (port I) was investigated. The results are shown in Figs. 6 and 7. When the feed coal contained larger amounts of Ca and Fe, the R Ses value increased. By contrast, there was no notable correlation between the contents of Na and K in the feed coal and R Ses. It is proposed that Ca and Fe in the feed coal may react with Se in the gas phase to form stable nonvolatile Se species, such as CaSeO4 and solid solution in Fe-containing silicate glasses [20]. The effect of temperature upon the partitioning of Se from the gas to solid phase was further examined. For the coal ash samples captured by the filter in the sampling train at port II, the particle size distribution and median particle diameter were measured by a laser diffraction-type
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Fig. 8. Particle size distribution and median particle diameter of coal ash samples collected at port II.
particle size analyzer. The results are shown in Fig. 8. The difference between R Ses at ca. 300 °C (port I) and R Ses at ca. 100 °C (port II) was greatly dependent on the kind of the feed coal. Fig. 9 shows the plot of ΔR Ses = R Ses (port II) − R Ses (port I) vs. the median particle size of coal ash. As the particle size of coal ash became larger, ΔR Ses decreased, suggesting that more Se species can be physically adsorbed on the surface of smaller ash particles at a lower temperature (ca. 100 °C).
Consequently, the physical adsorption of Se on coal ash is predominant at the low temperature; adsorbed SeO2 may be reduced to Se0 by SO2 in the flue gas. 3.3. Influence of the presence of unburned carbon in coal ash on the partitioning of Se from the gas to solid phase It is known that unburned carbon particles contaminated in coal ash usually have larger specific surface areas than the ash particles themselves [31,32]. To probe the influence of the presence of unburned carbon in coal ash on the partitioning of Se from the gas to solid phase, three different combustion conditions were tested. Coals C and D were combusted under combustion conditions 1, 2, and 3 (Table 2), and the flue gas samples were collected at ports I and II. The contents of unburned carbon in the coal ash captured by the filter in the sampling train are given in Table 3. Combustion condition 1 was operated with a Table 3 Unburned carbon content in coal ash samples. Unburned carbon content (wt%)
Coal C
Coal D Fig. 9. Plot of ΔR Ses vs. the median particle diameter of coal ash.
Condition 1 Condition 2 Condition 3 Condition 1 Condition 2 Condition 3
Port I
Port II
7.8 13.4 14.9 13.4 15.3 20.5
7.6 13.7 14.8 13.2 15.2 20.4
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appears that the behavior of Se in the flue gas is not affected by the gas component.
4. Conclusions
Fig. 10. Influence of coal combustion conditions on R Ses (Coal C).
larger amount of secondary air supply (Table 2), and thus the feed coal was burned under measurable oxygen-rich condition in the main combustion zone. Therefore, when combustion condition 1 was used, the unburned carbon content was significantly smaller than those for conditions 2 and 3. Under any combustion condition, the unburned carbon contents for coal D were higher than those for coal C. In Figs. 10 and 11, the values of R Ses obtained under each combustion condition are shown for coals C and D, respectively. It was found that R Ses for condition 1 tended to be lower than those for either condition 2 or 3. In addition, especially for port I, the degree of R Ses for coal C was lower than that for coal D, which had a higher content of unburned carbon (Table 3). It is apparent that the partitioning of Se from the gas to solid phase is promoted by the presence of unburned carbon in coal ash, and the unburned carbon physically adsorbs the Se species. When coal is burned, the Se species present in coal is vaporized into the gas phase to form SeO2 [20], and as the temperature of flue gas is lowered, the partitioning of Se from the gas to solid phase occurs. It is proposed that Se in the gas phase reacts with Ca and Fe in feed coals in the high temperature zone over 300 °C while the physical adsorption of Se on the surface of coal ash takes place in the low temperature zone (ca. 100 °C). The presence of unburned carbon in coal ash may promote the physical absorption of Se. Since the O2 concentration was adjusted to 4.2% after the supply of over-fire air described previously, the main composition of flue gas was almost constant for all combustion conditions. In some cases, the coal ash was sampled near the over-fire air port, and the Se concentration in the ash was almost constant. Therefore, it
Fig. 11. Influence of coal combustion conditions on R Ses (Coal D).
The behavior of Se in a coal combustion system was examined in terms of the partitioning of Se from the gas to solid phase in the flue gas. The sampling of Se in the flue gas was performed according to ISO 17211:2015, and it was confirmed that 74–106% of the Se recovery was attained even when coals with low Se levels were used. The partitioning of Se to the solid phase in the flue gas increased with a decrease in the flue gas temperature. The partitioning of Se to the solid phase tended to be higher at 300 °C when the contents of Ca and Fe in feed coals were higher, while that was affected by the particle size of coal ash and the presence of unburned carbon at 100 °C. The partitioning of Se from the gas to solid phase in the flue gas was elucidated in terms of the chemical and physical adsorption of Se. It is anticipated that Se in the gas phase reacts with Ca and Fe in feed coals in the high temperature zone over 300 °C; the temperature is equivalent to that in NOx removal systems. On the other hand, the physical adsorption of Se on the surface of coal ash takes place in low temperature zone (ca. 100 °C); the temperature is equivalent to that in ESPs.
Acknowledgements This work was supported by New Energy and Industrial Technology Development Organization (NEDO), Japan for “Innovative Zero-emission Coal-fired Power Generation Project. Fundamental Technology Development for Zero-emission Coal-fired Power Generation. Research and Development for Reduction of Environmental Impact Relating to Trace Elements in Coal on Coal Utilization Process (P07021).” (Japanese FY2007-2013). References [1] C.G. Fraga, Relevance, essentiality and toxicity of trace elements in human health, Mol. Asp. Med. 26 (2005) 235–244. [2] L.H. Foster, S. Sumar, Selenium in health and disease: a review, Crit. Rev. Food Sci. Nutr. 37 (1997) 211–228. [3] L.B. Clarke, L.L. Sloss, Trace elements - emissions from coal combustion and gasification, IEA Coal Research IEACR/49, 1992. [4] Q. Tang, G. Liu, Z. Yan, R. Sun, Distribution and fate of environmentally sensitive elements (arsenic, mercury, stibium and selenium) in coal-fired power plants at Huainan, Anhui, China, Fuel 95 (2012) 334–339. [5] A. Clemens, J. Deely, D. Gong, T. Moore, J. Shearer, Partitioning behaviour of some toxic trace elements during coal combustion—the influence of events occurring during the deposition stage, Fuel 79 (2000) 1781–1784. [6] K.C. Galbreath, C.J. Zygarlicke, Mercury transformations in coal combustion flue gas, Fuel Process. Technol. 65 (2000) 289–310. [7] Y.E. Yudovich, M.P. Ketris, Mercury in coal: a review part 2. Coal use and environmental problems, Int. J. Coal Geol. 62 (2005) 135–165. [8] J.H. Pavlish, L.L. Hamre, Y. Zhuang, Mercury control technologies for coal combustion and gasification systems, Fuel 89 (2010) 838–847. [9] N. Omine, C.E. Romero, H. Kikkawa, S. Wu, S. Eswaran, Study of elemental mercury re-emission in a simulated wet scrubber, Fuel 91 (2012) 93–101. [10] S. Niksa, N. Fujiwara, Y. Fujita, K. Tomura, H. Moritomi, T. Tuji, S. Takasu, A mechanism for mercury oxidation in coal-derived exhausts, J. Air Waste Manage. Assoc. 52 (2002) 894–901. [11] P. Abad-Valle, M.A. Lopez-Anton, M. Diaz-Somoano, R. Juan, B. Rubio, J.R. Garcia, S.A. Khainakov, M.R. Martínez-Tarazona, Influence of iron species present in fly ashes on mercury retention and oxidation, Fuel 90 (2011) 2808–2811. [12] P. Abad-Valle, M.A. Lopez-Anton, M. Diaz-Somoano, M.R. Martinez-Tarazona, The role of unburned carbon concentrates from fly ashes in the oxidation and retention of mercury, Chem. Eng. J. 174 (2011) 86–92. [13] J.C. Hower, C.L. Senior, E.M. Suuberg, R.H. Hurt, J.L. Wilcox, E.S. Olson, Mercury capture by native fly ash carbons in coal-fired power plants, Prog. Energy Combust. Sci. 36 (2010). [14] C.L. Senior, L.E. Bool, S. Srinivasachar, B.R. Pease, K. Porle, Pilot scale study of trace element vaporization and condensation during combustion of a pulverized sub-bituminous coal, Fuel Process. Technol. 63 (2000) 149–165. [15] C.L. Senior, L.E. Bool, J.R. Morency, Laboratory study of trace element vaporization from combustion of pulverized coal, Fuel Process. Technol. 63 (2000) 109–124. [16] W.S. Seames, An initial study of the fine fragmentation fly ash particle mode generated during pulverized coal combustion, Fuel Process. Technol. 81 (2003) 109–125.
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Contents lists available at ScienceDirect
Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Research article
Behavior of selenium in the flue gas of pulverized coal combustion system: Influence of kind of coal and combustion conditions Takuya Furuzono a, Tsunenori Nakajima b,⁎, Hiroki Fujishima b, Hirokazu Takanashi b, Akira Ohki b a b
Coal & Environmental Research Laboratory, Idemitsu Kosan Co., Ltd., 3-1 Nakasode, Sodegaura, Chiba 299-0267, Japan Department of Chemistry, Biotechnology, and Chemical Engineering, Graduate School of Science and Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890-0065, Japan
a r t i c l e
i n f o
Article history: Received 30 March 2017 Received in revised form 5 July 2017 Accepted 15 July 2017
Keywords: Pulverized coal combustion Flue gas Selenium Distribution Kind of coal
a b s t r a c t Five kinds of coals (mainly low Se levels) were pulverized and combusted in a turbulent furnace combustion test rig, which had a burner on the top and a gas cooling pipe with two sampling ports at the side near the bottom. A sampling train according to ISO 17211:2015 was attached to the ports to assess the distribution of Se in the gas and solid phase (coal ash) in the flue gas. The fraction of Se in the solid phase (R Ses) in the flue gas is given by R Ses = Ses/(Ses + Seg), where Ses and Seg are the concentrations of Se in the solid and gas phases, respectively. For all coals, R Ses increased with a decrease in the flue gas temperature from 300 °C to 100 °C, and the difference in R Ses by the temperature was greatly dependent on the kinds of feed coals. The effects of the contents of minerals in feed coals and the combustion conditions upon R Ses were studied. The partitioning of Se from the gas to solid phase in the flue gas was elucidated in terms of the chemical and physical adsorption of Se on coal ash. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The discharge of Se compounds into the environment can have negative impacts on human health. Although Se is an essential element for humans and other animals at low levels of uptake, a high level of uptake can cause serious damage to living organisms [1,2]. In several countries, Se is listed in national pollutant chemical inventories, such as the United Kingdom's Pollution Inventory, Australia's National Pollution Inventory, and the Pollution Release and Transfer Register of Japan. In the future, regulations related to the control of Se emissions into air will be formed in many countries. Combustion of coal is a major anthropogenic emission source of volatile toxic trace elements such as Hg and Se. It has been reported that various hazardous trace elements are distributed in the by-products of coal combustion systems, such as coal fly ash, flue gas desulfurization wastewater, and exhaust gas, and these elements are discharged into the environment in both gas and solid states [3–5]. The behavior of Hg in coal combustion systems has been well studied. The chemical speciation of Hg in coal combustion flue gas as well as the distribution of Hg in the combustion systems have been reported [6–10]. Moreover, the partitioning of Hg from the gas to solid phase (coal ash) in the flue gas has been investigated [11–13]. Compared with Hg, studies on the behavior of Se in coal combustion systems are scarce. Senior et al. investigated the behavior of some trace
⁎ Corresponding author. E-mail address: [email protected] (T. Nakajima).
http://dx.doi.org/10.1016/j.fuproc.2017.07.019 0378-3820/© 2017 Elsevier B.V. All rights reserved.
elements, including Se, in a coal combustion system by using a cascade impactor to collect particulate ashes [14,15]. They suggested that Se is almost completely vaporized in the flame zone to form SeO2, while Se still exists in the gas phase in low temperature zones, such as at the entrance to electrostatic precipitators (ESPs), although the partitioning of Se from the gas to solid phase (coal ash) occurs considerably. However, compared with other elements, the mass balance of Se in the low temperature zones is not very high. A similar experiment was carried out by Seams et al. who reported that the concentration of Se in coal ash was dependent on its particle size whereas the relationship between the concentration of Se and that of Ca or Fe in size-segregated ash samples was not clear [16,17]. There have been some reports on the partitioning of Se from the gas to solid phase in the flue gas of coal combustion based on model and simulation studies. Lopez-Anton et al. investigated the retention of Se on the surface of coal ash using reference materials such as activated carbon and limestone, and suggested that the Se retention may be influenced by the presence of carbonaceous particles and Ca compounds in the ash [18,19]. Senior et al. reported that the partitioning of Se from the gas to solid phase may be ascribed to the reaction of gaseous Se with Ca and Fe compounds in feed coal in the flame zone, whereas the coagulation of gaseous Se leads to physical adsorption on coal ash in the low temperature zone [20]. In addition, Raeva et al. reported that the presence of Ca in feed coal promotes the Se retention in the solid phase due to common acid-base chemistry in coal combustion [21]. Noda et al. examined the behavior of some trace elements, including Se, in the flue gas of coal combustion [22]. They used US Environmental Protection Agency (EPA) method 29 [23], which allows for the
T. Furuzono et al. / Fuel Processing Technology 167 (2017) 388–394 Table 1 Properties of feed coals.
Table 2 Combustion conditions. Unit
Inherent moisture Ash Volatile matter Fixed carbon Ash composition SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O Se
389
wt% (air dried)
wt% of ash
mg/kg (dry basis)
Feed coal A
B
C
D
E
3.3 12.3 34.4 50.0 60.4 23.3 4.28 3.20 1.06 0.32 1.40 0.40
4.8 13.0 32.7 50.0 47.8 41.8 3.84 2.11 0.39 0.23 0.51 2.55
3.1 12.2 31.4 53.3 56.8 21.0 6.51 3.50 1.47 0.60 2.04 0.27
2.4 15.7 26.0 55.9 46.6 29.6 5.95 6.95 0.96 0.36 0.60 0.49
7.2 3.9 43.0 46.0 38.1 22.6 18.5 7.37 2.35 1.14 0.95 0.05
determination of distribution of the elements in gas and solid phase in the flue gas. However, it has been noted that EPA and related methods are not particularly effective in terms of Se speciation in coal combustion systems [24,25]. The main reason for this is the difficulty in sampling of Se in the gas phase, especially in the low temperature zone, because of the reduction of SeO2 to Se0 by SO2, which is usually present in large quantities in the flue gas. The Se0 species tends to adhere to the tubing and vessel walls in the gas sampling train. To prevent the problem, a new official method for the sampling and determination of Se in the flue gas was recently issued (ISO 17211:2015) [26]. This is an improved method for EPA method 29 in terms of the addition of a rinse process to attain the complete recovery of Se adhered to the tubing and vessel walls. Thus, the sampling of Se in the gas phase in the presence of SO2 can be effectively carried out. The Se contamination in desulfurization wastewaters has been one of the big problems in Japanese coal fired power plants. Therefore, the use of coals with low Se levels is currently recommended in Japan. In this study, the ISO method was applied to the combustion of various coals, including low Se level coals, in a turbulent furnace combustion test rig to
Primary air Secondary air Over-fire air Over-fire air port (distance from the furnace top)
Unit
Condition 1
Condition 2
Condition 3
Nm3/h
6 52 0 –
6 36 16 116
6 36 16 166
cm
assess the distribution of Se in the gas and solid phase (coal ash) in the flue gas. The influence of flue gas temperature, kind of coal, and combustion conditions on the behavior of Se was investigated in terms of the partitioning of Se from the gas to solid phase in the low temperature zone, which is modeled for NOx removal systems (ca. 300 °C) and ESPs (ca. 100 °C).
2. Experimental 2.1. Coal samples Five coals (termed A–E), which had been imported for use in Japanese coal-fired power plants, were tested. Analysis of these coals was conducted based on JIS M8812 [27] and the results are shown in Table 1. The concentration of Se in the coals varied from 2.55 to 0.05 mg/kg. The ash composition was obtained by X-ray fluorescence analysis (Zetium, PANalytical V. B.) after low temperature ashing of the coal. The determination of Se in the coal was conducted as follows. The digestion of coal sample was carried out using a microwave processor (ETHOS 1, Milestone Inc.) with the acidic mixture of HNO3/H2O2. Detailed conditions of the microwave processing were described in our previous study [28]. The concentration of Se in the resulting solution was analyzed by hydrate generation atomic absorption spectrometry (HG-AAS; HYD-20 hydride generator, Nippon Jarrell-Ash Co. Ltd. and solar S2 spectrometer, Thermo Scientific Inc.) [29].
Fig. 1. Schematic of the turbulent furnace combustion test rig with sampling ports I and II to which the Se sampling train (Fig. 2) was attached.
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Fig. 2. Schematic of the Se sampling train.
2.2. Turbulent furnace combustion test rig A schematic of the turbulent furnace combustion test rig used is shown in Fig. 1. The combustor had a 300 mm inner diameter and a 2800 mm length with a burner on the top and a gas cooling pipe with sampling ports at the side near the bottom. Coal samples were pulverized to fine particles (b 75 μm) and dried prior to combustion. The coal feed rate was approximately 6 kg/h. The combustion conditions are listed in Table 2, and condition 1 was usually used. When the influence of combustion conditions on the behavior of Se was examined, the flow rate of secondary air was varied while the position of over-fire air port was changed. To adjust the O2 concentration (to 4.2%) in the combustion air, the coal feed speed was controlled. The flue gas samples were collected from two sampling ports (I and II) of the cooling pipe. The temperature of flue gas at each sampling port was measured by a type K thermocouple probe and those for ports I and II were ca. 300 and 100 °C, respectively.
2.3. Sampling and determination of Se in the gas and solid phases
ash was measured by a similar method described in Section 2.1 for feed coals except for the use of the acidic mixture of HNO3/HF/H2O2 [30]. On the other hand, Se in the gas phase was adsorbed by a mixture of 5% HNO3 and 10% H2O2 in two subsequent impingers. The amount of Se found in the absorbent solution of the second impinger was always b10% of that in the first impinger, and the sum of those Se amounts was used. The ISO method involved a rinse procedure for the tubing and vessel walls within the train using an oxidant solution containing 10 g/L KMnO4 and 1 M H2SO4. The Se content in the gas phase was calculated from the sum of the amounts of Se found in the absorbent solutions and in the rinse solution. When coal was combusted in the turbulent furnace, the amount of bottom ash was about 1/20–1/30 of that of the fly ash which was captured by the bag filter after the sampling ports (Fig. 1). In some cases, it was confirmed that the Se concentration in the bottom ash was below the detection limit. It is proposed that the distribution of Se to the bottom ash can be negligible.
2.4. Measurement of particle size distribution and unburned carbon content in coal ash
According to ISO 17211:2015, a sampling train was set up as shown in Fig. 2, and the inlet of the train was connected to sampling ports I and II. The flue gas of coal combustion was passed through the sampling train for 60 min, and the flow rate was regulated by using an adjustable vacuum pump (ca. 30 L/min). In the train, Se in the solid phase (coal ash) was captured by a cylindrical filter paper (outside diameter 25 mm, length 90 mm, No. 88RH, Advantec). The Se concentration in the coal
The particle size distribution and median diameter of the coal ash samples obtained were measured by a laser diffraction-type particle size analyzer (LA-920; Horiba, Ltd.). The unburned carbon content was determined according to JIS M8812 [27]. The coal ash samples were heated to 815 °C in an electric furnace and the unburned carbon content was obtained from the weight loss during the heating procedure.
Fig. 3. Recovery of Se in the gas phase.
Fig. 4. Mass balance of Se in the flue gas.
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Fig. 5. Influence of the flue gas temperature and kind of feed coal on R Ses. Mass balance of Se in the flue gas for each case is shown in parentheses.
3. Results and discussion 3.1. Sampling of Se in the gas and solid phases in coal combustion flue gas Firstly, the availability of a rinse process in the sampling of Se in coal combustion flue gas was examined when the ISO method was applied to the turbulent furnace (combustion condition 1) with coals A and B, which had different Se levels (0.40 and 2.55 mg/kg, respectively). Sampling port I was used. The theoretical concentration of Se in the gas phase was obtained from the difference between the amount of Se found in the feed coal and that in the coal ash (solid phase) captured by the filter in the sampling train, considering the regulated flow rate of the flue gas. Based on the theoretical Se concentration, the recovery of Se was calculated for both the absorbent and rinse solutions. As shown in Fig. 3, the recovery of Se in the gas phase is acceptable for both coals when the recovery in the absorbent solution and that in the rinse solution were summed, and it is confirmed that the rinse process is essential for the complete recovery of Se. Secondly, the mass distribution of Se in the gas and solid phase was examined by using the ISO method. The amount of Se in the gas phase was obtained with the aid of the rinse process as described above. For both sampling ports I and II, the mass balance of Se in the flue gas is
Fig. 6. Plot of R Ses (port I) vs. the contents of Ca and Fe in feed coal.
Fig. 7. Plot of R Ses (port I) vs. the contents of Na and K in feed coal.
acceptable when the mass distribution in the gas phase and that in the solid phase are summed, as shown in Fig. 4. 3.2. Influence of flue gas temperature and kind of coal on the partitioning of Se from the gas to solid phase The influence of flue gas temperature and kind of coal on the partitioning of Se from the gas to solid phase was examined. When five kinds of coals (A–E) were combusted under combustion condition 1, the flue gas samples were collected at ports I and II. The fraction of Se in solid phase (R Ses) is given by the following equation. R Ses ¼ Ses = Ses þ Seg
where Ses and Seg are the concentrations of Se in the solid and gas phases, respectively, in the flue gas. Fig. 5 shows the influence of flue gas temperature and kind of coal on R Ses as well as the mass balance of Se in the flue gas. For port I, an incomplete mass balance closure was obtained when coals with low Se levels (D and E) were used. For all five coals, R Ses increased with a decrease in the flue gas temperature from 300 °C (port I) to 100 °C (port II). However, R Ses varied among the coal types, especially for the samples collected at port I. In a coal combustion flue gas, Se in the gas phase probably reacts with specific inorganic components in the feed coal in the high temperature zone over 300 °C. By contrast, it is anticipated that Se in the gas phase is physically adsorbed and condensed on the surface of coal ash (solid phase) at lower temperatures (around 100 °C). It is rational that chemical absorption favorably occurs in the relatively high temperature zone while physical absorption is promoted in the low temperature zone. To test this hypothesis, the influence of the content of specific inorganic elements (Ca, Fe, Na, and K) in the feed coal on R Ses at ca. 300 °C (port I) was investigated. The results are shown in Figs. 6 and 7. When the feed coal contained larger amounts of Ca and Fe, the R Ses value increased. By contrast, there was no notable correlation between the contents of Na and K in the feed coal and R Ses. It is proposed that Ca and Fe in the feed coal may react with Se in the gas phase to form stable nonvolatile Se species, such as CaSeO4 and solid solution in Fe-containing silicate glasses [20]. The effect of temperature upon the partitioning of Se from the gas to solid phase was further examined. For the coal ash samples captured by the filter in the sampling train at port II, the particle size distribution and median particle diameter were measured by a laser diffraction-type
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Fig. 8. Particle size distribution and median particle diameter of coal ash samples collected at port II.
particle size analyzer. The results are shown in Fig. 8. The difference between R Ses at ca. 300 °C (port I) and R Ses at ca. 100 °C (port II) was greatly dependent on the kind of the feed coal. Fig. 9 shows the plot of ΔR Ses = R Ses (port II) − R Ses (port I) vs. the median particle size of coal ash. As the particle size of coal ash became larger, ΔR Ses decreased, suggesting that more Se species can be physically adsorbed on the surface of smaller ash particles at a lower temperature (ca. 100 °C).
Consequently, the physical adsorption of Se on coal ash is predominant at the low temperature; adsorbed SeO2 may be reduced to Se0 by SO2 in the flue gas. 3.3. Influence of the presence of unburned carbon in coal ash on the partitioning of Se from the gas to solid phase It is known that unburned carbon particles contaminated in coal ash usually have larger specific surface areas than the ash particles themselves [31,32]. To probe the influence of the presence of unburned carbon in coal ash on the partitioning of Se from the gas to solid phase, three different combustion conditions were tested. Coals C and D were combusted under combustion conditions 1, 2, and 3 (Table 2), and the flue gas samples were collected at ports I and II. The contents of unburned carbon in the coal ash captured by the filter in the sampling train are given in Table 3. Combustion condition 1 was operated with a Table 3 Unburned carbon content in coal ash samples. Unburned carbon content (wt%)
Coal C
Coal D Fig. 9. Plot of ΔR Ses vs. the median particle diameter of coal ash.
Condition 1 Condition 2 Condition 3 Condition 1 Condition 2 Condition 3
Port I
Port II
7.8 13.4 14.9 13.4 15.3 20.5
7.6 13.7 14.8 13.2 15.2 20.4
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appears that the behavior of Se in the flue gas is not affected by the gas component.
4. Conclusions
Fig. 10. Influence of coal combustion conditions on R Ses (Coal C).
larger amount of secondary air supply (Table 2), and thus the feed coal was burned under measurable oxygen-rich condition in the main combustion zone. Therefore, when combustion condition 1 was used, the unburned carbon content was significantly smaller than those for conditions 2 and 3. Under any combustion condition, the unburned carbon contents for coal D were higher than those for coal C. In Figs. 10 and 11, the values of R Ses obtained under each combustion condition are shown for coals C and D, respectively. It was found that R Ses for condition 1 tended to be lower than those for either condition 2 or 3. In addition, especially for port I, the degree of R Ses for coal C was lower than that for coal D, which had a higher content of unburned carbon (Table 3). It is apparent that the partitioning of Se from the gas to solid phase is promoted by the presence of unburned carbon in coal ash, and the unburned carbon physically adsorbs the Se species. When coal is burned, the Se species present in coal is vaporized into the gas phase to form SeO2 [20], and as the temperature of flue gas is lowered, the partitioning of Se from the gas to solid phase occurs. It is proposed that Se in the gas phase reacts with Ca and Fe in feed coals in the high temperature zone over 300 °C while the physical adsorption of Se on the surface of coal ash takes place in the low temperature zone (ca. 100 °C). The presence of unburned carbon in coal ash may promote the physical absorption of Se. Since the O2 concentration was adjusted to 4.2% after the supply of over-fire air described previously, the main composition of flue gas was almost constant for all combustion conditions. In some cases, the coal ash was sampled near the over-fire air port, and the Se concentration in the ash was almost constant. Therefore, it
Fig. 11. Influence of coal combustion conditions on R Ses (Coal D).
The behavior of Se in a coal combustion system was examined in terms of the partitioning of Se from the gas to solid phase in the flue gas. The sampling of Se in the flue gas was performed according to ISO 17211:2015, and it was confirmed that 74–106% of the Se recovery was attained even when coals with low Se levels were used. The partitioning of Se to the solid phase in the flue gas increased with a decrease in the flue gas temperature. The partitioning of Se to the solid phase tended to be higher at 300 °C when the contents of Ca and Fe in feed coals were higher, while that was affected by the particle size of coal ash and the presence of unburned carbon at 100 °C. The partitioning of Se from the gas to solid phase in the flue gas was elucidated in terms of the chemical and physical adsorption of Se. It is anticipated that Se in the gas phase reacts with Ca and Fe in feed coals in the high temperature zone over 300 °C; the temperature is equivalent to that in NOx removal systems. On the other hand, the physical adsorption of Se on the surface of coal ash takes place in low temperature zone (ca. 100 °C); the temperature is equivalent to that in ESPs.
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