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Emission Behavior of Particulate Matter during Co-combustion of Coal and Biomass in a Drop Tube Furnace† Quanbin Wang, Hong Yao, Dunxi Yu,* Li Dai, and Minghou Xu* State Key Laboratory of Coal Combustion, Huazhong UniVersity of Science and Technology (HUST), Wuhan 430074, China ReceiVed August 20, 2006. ReVised Manuscript ReceiVed NoVember 17, 2006
Coal/biomass co-combustion experiments were carried out in a laboratory-scaled drop tube furnace to understand the emission characteristics of particulate matter with an aerodynamic diameter of less than 10 µm (PM10). The furnace was electrically heated, and the temperature was maintained at 1423 K. Two different atmospheres, N2/O2 ) 4:1 and 1:1, were conducted. The coal/biomass ratio was kept at 75:25% (wt %), and four types of coal/biomass blends were selected as fuels in these experiments. The fuel feeding rate was adjusted to about 0.3 g/min. The results show that the particle-size distribution of PM10 from the co-combustion of coal and biomass is bimodal, with one peak at about 4.3 µm and the other at about 0.1 µm. With the increase of the oxygen ratio, the total concentration of PM10 increases significantly and the percentage of PM1.0 (particle size below 1.0 µm) in PM10 decreases greatly, while that of PM1.0+ (particle size between ∼1.0-10 µm) increases. In addition, the majority of alkali, sulfur, and chlorine are enriched in PM1.0. With the increase of the oxygen concentration, the ratio of S/Cl increases and a higher sulfating extent is found in PM1.0. Meanwhile, the alkali content in PM1.0 decreases, while there is a significant increase in PM1.0+. In addition to the combination with chlorine or sulfur, part of alkalis in coarse particles is found to form other alkali salts.
Introduction Increasing concerns over climate change and global warming have prompted the development of utilization technologies of clean, renewable sources of energy. Co-combustion of coal and biomass fuels has been popular in recent years because of its potential benefits for both the environment and the economics of power generation. On one hand, biomass is considered CO2 neutral and can inherently reduce the CO2 net production when it is fired with the primary fuels. On the other hand, if the biomass processing costs can be well-controlled, the total fuel costs can also be greatly reduced.1 More fundamental information about co-combustion of coal and biomass can be found in two newly published papers.1,2 Coal is the dominant energy source in China and accounts for more than 60% of its total energy consumption. The wide use of coal has led to a serious air pollution problem that is receiving much concern. Every year, there is a large amount of biomass available in China, and it is estimated that the total biomass energy resources are about 437 million tones of coal equivalent (Mtec). Therefore, there is great potential for China to convert biomass resources into energy to ensure its energy-supply security and environmental sustainability. Because of its many advantages, co-combustion of coal and biomass is regarded as an effective renewable energy option and is now being developed in China. Despite the simplicity of the co-combustion concept, its application in pulverized fuel boilers is still associated with †
Presented at the 2006 Sino-Australia Symposium on Advanced Coal Utilization Technology, July 12-14, 2006, Wuhan, China. * To whom correspondence should be addressed. Telephone: +86-2787545526. Fax: +86-27-87545526. E-mail:
[email protected] (D.Y.);
[email protected] (M.X.). (1) Sami, M.; Annamalai, K.; Wooldridge, M. Prog. Energy Combust. Sci. 2001, 27, 171. (2) Baxter, L. Fuel 2005, 84, 1295.
many technical issues, including fuel ignition, flame stability, ash deposition, particulate matter (PM) emission, etc.3 Among these issues, PM has received much concern in recent years because of its adverse effects on both human health and the environment.4 PM with an aerodynamic diameter of 10 µm or less (PM10) can be easily inhaled and is especially harmful to human health. Fernandez et al.5 found that inhalation of fine particles from the co-combustion of coal and biomass caused much greater lung damage in mice than that of coal ash or biomass ash alone. Jime´nez et al.6,7 investigated the effect of co-combustion of coal and biomass on the properties of PM and particle formation processes and pointed out that the alkali metal (i.e., potassium) in biomass was responsible for the key element in the fine particles emitted in co-combustion of the coal and biomass blend. However, the emission behavior of PM during co-combustion of coal and biomass has not been wellclarified yet.8-10 The aim of this study was to investigate the properties of PM10 from the co-combustion of coal and biomass and the effect of oxygen content on the emission behavior of PM1.0 (PM less than 1.0 µm) and PM1.0+ (PM between 1.0 and 10 µm). The enrichment of alkalis, sulfur, and chlorine, which are the key elements to form fine particles, was also discussed in this study. The results could be very useful to the understanding of PM formation during co-combustion of coal and biomass. (3) Moghtaderi, B.; Meesri, C.; Wall, T. F. Fuel 2004, 83, 745. (4) Johansson, L. S. Ph.D. Thesis, Chalmers University of Technology, Gothenburg, Sweden, 2002. (5) Fernandez, A.; Wendt, J. O. L.; Witten, M. L. Fuel 2005, 84, 1320. (6) Jime´nez, S.; Ballester, J. Aerosol Sci. Technol. 2005, 39, 811. (7) Jime´nez, S.; Ballester, J. Combust. Flame 2005, 140, 346. (8) Spliethoff, H.; Unterberger, S.; Hein, K. R. G. Clean Air 2004, 5, 113. (9) Demirbas, A. Prog. Energy Combust. Sci. 2005, 31, 171. (10) Demirbas, A. Prog. Energy Combust. Sci. 2004, 30, 219.
10.1021/ef060410u CCC: $37.00 © 2007 American Chemical Society Published on Web 01/13/2007
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Wang et al. Table 1. Composition Analysis of Used Coals and Biomass
Figure 1. Schematic diagram of the DTF.
Experimental Section In this study, two typical Chinese coals, namely, LA and PB, and two types of biomass, sawdust (SD) and straw (SW), were selected. The raw coals and biomass were pulverized to 100-200 µm and less than 200 µm, respectively. All samples were dried prior to use. The properties of raw coals and biomass are shown in Table 1. In comparison to coal samples, biomass has lower contents of fixed carbon and ash but higher contents of volatile matter and moisture. Before the experiments, a cold flow calibration test was carried out to make sure that the feeding rate of the raw fuel was uniform. First, in a unit time, at the same volume flow rate, the mass of coal and biomass injected from the micrometer-pulverized fuel feeder was measured, respectively. Second, one coal sample was mixed with one biomass, with a mass ratio of coal/biomass being 3:1, and the coal/biomass blend was tested in the same way. If the mass of the blend was approximately equal to the sum of the 75% coal’s weight alone and the 25% biomass’s weight alone, the blend was assumed to be well-mixed.
a
ad ) air dry base.
During the experiment, the well-mixed fuels were injected from the top of the drop tube furnace (DTF). The feeding rate in each test was about 0.3 g/min, and the tests were carried out at the temperature of 1423 K with two different atmospheres, that is, N2/ O2 ) 4:1 and 1:1. The DTF used is shown in Figure 1. It has a height of 2000 mm and an inner diameter of 56 mm. The DTF is electrically heated, and the wall temperature can be adjusted. The residence time of the fuel particle is estimated to be about 2 s. For each run of the tests, the burnout of samples is above 98%. The produced solid particles pass through a probe that is nitrogenquenched and water-cooled to prevent further reactions inside. The solid residues are first collected by a cyclone with a cutoff diameter of 10 µm. The particles less than 10 µm are then separated by a Dekati low-pressure impactor (DLPI). The DLPI has 13 stages, whose 50% cutoff diameters are 0.0281, 0.0565, 0.0944, 0.154, 0.258, 0.377, 0.605, 0.936, 1.58, 2.36, 3.95, 6.60, and 9.8 µm, respectively.
Figure 2. Particle-size distributions of PM10 obtained at N2/O2 ) 4:1 and 1:1 for blends.
Co-combustion of Coal and Biomass in a DTF
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Table 2. Emission Concentration of PM1.0 and PM1.0+ N2/O2 ) 4:1
N2/O2 ) 1:1
fuel
PM1.0/PM10 (%)
PM1.0+/PM10 (%)
PM10 (mg/m3)
PM1.0/PM10 (%)
PM1.0+/PM10 (%)
PM10 (mg/m3)
LA plus SW PB plus SW LA plus SD PB plus SD
72.27 68.21 59.74 74.49
27.73 31.79 40.26 25.51
36.53 30.51 29.61 46.25
54.11 47.96 37.66 56.83
45.89 52.04 62.34 43.17
63.2 60.34 57.81 58.72
Figure 3. S/Cl ratios and the concentration of Cl and S (wt % × 10) in PM1.0 and PM1.0+ at different atmospheres.
The major elements in PM10 collected on each stage of the DLPI were analyzed by an Eagle III X-ray microfluorescence spectrometer (XRF, EDAX, Inc.). For elemental analysis, Teflon filters were used. To obtain particle-size distributions, aluminum filters were used. In addition, the maximum PM10 mass collected on each stage should be kept less than 1 mg to avoid particle rebounce.
Results and Discussion Emission Characteristics of PM10. Figure 2 gives the particle-size distributions of PM10 during combustion of four coal/biomass blends at 1423 K with two different atmospheres. It is clear that PM10 collected during combustion of different fuels has a similar bimodal size distribution, with two peaks at around 4.3 and 0.1 µm, respectively. As shown in Table 2, the amount of PM1.0 accounts for more than one-half of that of PM10 at the atmosphere of N2/O2 ) 4:1. It indicates that most of the PM10 was present as submicrometer particles under this atmosphere. However, with the increase of the oxygen concentration, the percentage of the PM1.0 in PM10 decreases greatly, while the percentage of PM1.0+ increases remarkably. It is reported that PM1.0 is formed by vaporization and subsequent condensation of volatile elements and PM1.0+ is formed by included mineral coalescence, char fragmentation, and excluded mineral fragmentation.11-12 The increase of the oxygen concentration can enhance mineral matter vaporization, especially during the combustion of biomass. However, the molten large coal ash particles are expected to capture very fine PM from the combustion of coal and biomass at higher oxygen concentrations (high temperatures), resulting in the decrease of PM1.0 in PM10. Moreover, it is also obvious that the total concentration of PM10 increases significantly with an increasing oxygen ratio. (11) Russell, N. V.; Mendez, L. B.; Wigley, F.; Williamson, J. Fuel 2002, 81, 657. (12) Yan, L.; Gupta, R. P.; Wall, T. F. Fuel 2001, 80, 1333.
It is suggested that, when the oxygen concentration increases, large particles tend to break up to form much more fine ones that are smaller than 10 µm. Notably, the concentration of PM10 from co-combustion of LA and SD was the lowest, which may be caused by the low content of ash in this mixed fuel. Sulfur, Chlorine, and Alkalis in PM10. Sulfur and Chlorine in PM10. Figure 3 shows the mass ratio of S/Cl and concentrations of elemental chlorine and sulfur in PM1.0 and PM1.0+ during co-combustion of the coal/biomass blend at different atmospheres. The S/Cl ratio has been used as an index of the Cl deposition tendency.13 It seems that the increase of the S/Cl ratio is mostly due to the increase of sulfur, because the chlorine concentration roughly remains constant. Moreover, most sulfur and chlorine are enriched in PM1.0, and the S/Cl ratio increases with the increase of the oxygen concentration. One of the possible reasons is that part of alkali chlorides, with the presence of sulfur or SO3, could be oxidized to form alkali sulfate and hydrogen chloride when the oxygen concentration is increased.14-17 Therefore, it can be shown that the higher oxygen content could cause a more intensive sulfating in PM1.0.14 Content of Alkali Metals in PM10. In comparison to coal, biomass contains more potassium and sodium, which are easily vaporized as chlorides to form fine particles.10 As can be seen in Figure 4, in the atmosphere of N2/O2 ) 4:1, PM1.0 from cocombustion of coal and biomass contains the majority of alkali metals, with an exceptional case of the blend of LA and SD, PM1.0 from which has a lower content of potassium. When the oxygen ratio is changed into N2/O2 ) 1:1, the content of alkalis in PM1.0+ increases greatly, while the alkali content in PM1.0 reduces, except for potassium from co-combustion of LA and SW or LA and SD. This might be attributed to the fact that alkalis as alkali chlorides can be trapped by aluminosilicates,18-20 as shown in the following equation:
Al2O3‚2SiO2 + 2MCl + H2O f M2O‚Al2O3‚2SiO2 + 2HCl (1) where M is Na or K, the aluminosilicates can react with alkali chlorides in the presence of H2O, leading to the incorporation of more alkalis into PM1.0+. As for the LA and SW blend or the LA and SD blend, the differences in the potassium concentration may be caused by the higher content of calcium in LA coal compared with PB coal. A high content of calcium has been found to enhance potassium release by competing for silicates and phosphates, resulting in the vaporization of more potassium, followed by condensation to form fine particles.21 Occurrence of Alkalis in PM10. Because the coal/biomass blend contains a relatively higher content of alkalis, the influence (13) Makkonen, P. Ph.D. Thesis, Lappeenranta University of Technology, Finland, 1999. (14) Aho, M.; Silvennoinen, J. Fuel 2004, 83, 1299. (15) Nielsen, H. P.; Frandsen, F. J.; Dam-Johansen, K. Energy Fuels 1999, 13, 1114. (16) Christensen, K. A.; Stenholm, M.; Livbjerg, H. J. Aerosol Sci. 1998, 29, 421. (17) Boonsongsup, L.; Iisa, K.; Frederick, W. J., Jr. Ind. Eng. Chem. Res. 1997, 36, 4212. (18) Nielsen, H. P.; Baxter, L. L.; Sclippab, G.; Morey, C.; Frandsen, F. J.; Dam-Johansen, K. Fuel 2000, 79, 131. (19) Aho, M. Fuel 2001, 80, 1943. (20) Coda, B.; Aho, M.; Berger, R.; Hein, K. R. G. Energy Fuels 2001, 15, 680.
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Figure 4. Concentration of K and Na (mg/m3) in PM1.0 and PM1.0+.
Figure 5. Mole ratios of (Na + K)/Cl and (Na + K)/(Cl + 2S) (mol/mol) in PM1.0 and PM1.0+ at different atmospheres.
of heavy metals, such as Pb and Cd, is not discussed here. This study just focuses on the interactions between alkalis and sulfur or chlorine. In Figure 5, the occurrence of the alkalis in PM10 as chlorides is studied by calculating (Na + K)/Cl molar ratio as a function of the particle size for different tests. The alkali/ chlorine molar ratio is 1 if all of the alkalis and chlorine are bound as alkali chlorides. While if the Alkali/chlorine molar ratio is more than 1, it indicates that much of the alkalis exist in other compounds rather than chlorides. Similarly, the occurrence of alkalis as chlorides and sulfates can also be implied by calculating the molar ratio (Na + K)/(Cl + 2S). This ratio is 1 if all of the alkalis are bound as sulfates and chlorides and if all of the Cl and S are bound in alkalis.22 As shown in Figure 5a, the (Na + K)/Cl molar ratio is more than 1. That is to say that Cl is almost totally combined with alkalis. However, Figure 5b shows that the molar ratio (Na + K)/(Cl + 2S) in PM1.0 is smaller than 1. This indicates that the alkalis are partially bound as chlorides and partially associated with S. With the increase of the oxygen concentration, the molar ratio (Na + K)/(Cl + 2S) mostly increases to more than 1 in PM1.0+. That means that in PM1.0+ a portion of alkalis is present as other alkali salts besides the combination with chlorine or sulfur, such as M2O‚Al2O3‚2SiO2 in eq 1. The increase of the oxygen concentration leads to a greater coalescence of the included minerals during char combustion, resulting in the (21) Knudsen, J. N. Ph.D. Thesis, Technical University of Denmark, Lyngby, Denmark, 2004. (22) Lind, T.; Kaupponen, E. I.; et al. The Sixth International Symposium and Exhibition of Gas Cleaning at High Temperatures; Osaka, Japan, 2005; pp 592-598.
increase of the aluminosilicate content in coarse particles and the formation of other alkali salts. Conclusion The emission behavior of PM10 derived from the cocombustion of coal and biomass in a laboratory-scaled DTF has been investigated. The characteristics of PM10 and several important elements, such as S, Cl, Na, and K, in the PM10 were discussed. The results show that PM10 has a bimodal size distribution with two peaks at around 4.3 and 0.1 µm, respectively. With the increase of the oxygen concentration, the total concentration of PM10 from the co-combustion increases significantly, which suggests that the large particles tend to break up to form much more PM10, and the percentage of PM1.0 decreases greatly, while that of PM1.0+ increases remarkably. The majority of alkalis, sulfur, and chlorine are enriched in PM1.0. With the increasing oxygen concentration, the S/Cl ratio increases and the sulfating extent has also been enhanced in PM1.0. Meanwhile, the alkali content in PM1.0 decreases, while a significant increase is found in PM1.0+. In addition, besides the combination with chlorine or sulfur, part of alkalis in PM1.0+ is also present as other alkali salts. Acknowledgment. This work was supported by the National Key Basic Research and Development Program of China (2002CB211602) and the National Natural Science Foundation of China (50325621). Partial support from the Programme of Introducing Talents of Discipline to Universities (“111” project, number B06019) and the Natural Science Foundation of Hubei Province (2006ABC002) is also acknowledged. EF060410U
