Energy Fuels 2010, 24, 1677–1681 Published on Web 03/04/2010
: DOI:10.1021/ef900941b
Number Size Distribution of Particles Emitted from Two Kinds of Typical Boilers in a Coal-Fired Power Plant in China Xiaoyu Liu,†,‡ Wei Wang,*,‡ Hongjie Liu,‡ Chunmei Geng,‡ Wenjie Zhang,‡ Hongqi Wang,† and Zhong Liu§ †
College of Water Sciences, Beijing Normal University, Beijing, China, ‡Chinese Research Academy of Environmental Sciences, Beijing, China, and §School of Energy & Power Engineering, North China Electric Power University, Beijing, China Received August 27, 2009. Revised Manuscript Received February 23, 2010
Ultrafine particles (Dp < 100 nm) are suspected to have considerably stronger impacts on human health in recent studies, and the coal-fired power plants are one of the major anthropogenic sources of the ultrafine particles. In order to characterize the ultrafine particles emitted from power plants, comprehensive field research on the number size distribution for boilers with and without low NOx burners (LNBs) was conducted online in a pulverized coal-fired power plant in China. With the use of an engine exhaust particle sizer (EEPS) and an aerodynamic particle sizer (APS), the lower limit of particles measured is extended to 5.6 nm. The particle number concentrations decrease with increasing size, and the nucleation mode particles are dominant in the number concentration. Additionally, the size distributions for boilers with or without LNBs are compared. Results show that the concentrations of ultrafine particles are notably higher for the boiler with LNBs than those for the boiler without LNBs. In addition, the shapes of particle size distributions (PSDs) for the two boilers are quite different in the PSDs measured with the EEPS but uniform in the PSDs measured by the APS. This suggests that LNBs favor the formation of fine particles, which should be considered in the emission control measures of coal-fired power plants.
health,6-9 their effects on visibility,10 the direct and indirect effects on radiative balance, and global climate change.11 Compared with the surface area and mass concentration, the number concentration of particles is believed to be a reasonable measure for description of fine particles, which may be a determinant for negative impacts on the environment and health in recent studies.12 Historically, the present particle emission standards for thermal power plants are based on mass concentration of total particulate matter, which gives no differentiation on particle sizes especially those fine and ultrafine particles. Thus, in order to evaluate the influence of coal-burning boilers on the environment quantitatively, it is necessary to characterize the number concentration of particles emitted from coal-burning boilers. This could also provide scientific basis for the establishment of proper pollution control strategies. However, very few data of coalfired power plants particulate emission associated with ultrafine particle number size distributions were available. The earlier field studies on the coal-fired power plants were mainly focused on the particulate mass concentration and size-limited by the measurement techniques, so that very fine particles ( 1 μm) increased while the fine particle number (17-550 nm) remained close to constant but a decrease in geometric mean diameter (GMD) was observed. However, the existing data, concerning the characteristics of PM in the flue gas of coal-fired power plants as a function of load, are comparatively limited. Low NOx burners (LNBs) are widely used in China’s power plants to control NOx emission, and the penetration of LNBs increased gradually over time under the strengthened requirements of China’s NOx emission standards. All existing and new boilers with capacity equal to or larger than 300 MW have been equipped with LNBs since 1995, and boilers smaller than 300 MW but equal to or larger than 100 MW have begun to install LNBs.22,23 However, few studies focus on the LNBs’ impacts on PM emission. This study was aimed at investigating the particles, especially ultrafine particles, emitted from a typical pulverized coal-fired power plant using precision particle sizer with high time resolution. The number concentration and PSDs, as well as the nucleation modes (Dp < 20 nm), were obtained. The emissions of PM at different loads in a boiler were also measured. Furthermore, the conditions for boilers with and without LNBs were first investigated for clarity about the effects of LNBs on PM emission.
and contained six pulverized-coal boilers with a capacity of 200 MW each (Shanghai East Boiler Factory). The combustion fuel was Datong bituminous coal, which is the main power coal in China. The raw coal was pulverized by a direct-feed fan mill system up to an average diameter of around 100 μm, and the coal powder was combusted with impellerless burners. Boiler 1 (B1) and Boiler 2 (B2) selected in this study were the same boilers built in 1980s, and the combustion system of B1 was reconstructed for reducing NOx emissions by installing LNBs in 2004. LNBs offered a stage of fuel-rich and a stage of fuel-lean combustion. The combustion processes, which the fuel-air ratio was off-stoichiometric or deviated from stoichiometry, reduced conversion of nitrogen into NOx. Each boiler was equipped with a twin chamber five-field electrostatic precipitator (ESP) as a particulate control device. 2.2. Instrumental. The 5.6-560 nm particles were measured by an engine exhaust particle sizer (EEPS 3090, TSI Inc.) with 32 channels. The sample flow rate of the EEPS was 10 L/min, and the time resolution was 0.1 s (set as 1 s in this study). In order to extend the PSDs data collected by the EEPS, an aerodynamic particle sizer (APS 3310A, TSI Inc.) with working range of 0.47-30 μm diameter was employed simultaneously. APS had 57 channels with 5 L/min of the sample flow rate and 1 min time resolution. The working diameter range of EEPS and APS were slightly overlapped at 470-560 nm. In order to decrease the particle concentrations and gas temperatures from the stack, a two-stage dilution system was applied to try to preserve the gas and particle conditions.17,24 Filtered and dried air was used as the dilution gas. In the firststage diluter, the dilution gas was heated to stack temperature to minimize the unrealistic condensation of the flue gas caused by temperature drop. The secondary dilution was carried out with unheated dilution gas to cool the sample to a final temperature. The dilution rate was related to both stack pressure and dilution air pressure, and a dilution factor of ∼1:85 was used in this work. Both EEPS and APS were connected to the dilution system. Extracting samples were taken by the isokinetic aerosol sampling probe of the dilution system and then directed to the particle sizers. All sampling lines were as short as possible to avoid losses of the large particles. The sample flow rates of the dilution system and particle sizers were calibrated before and after sampling. 2.3. Measurement Plan. Measurements were taken at the outlet of the ESP. A combustion analyzer (model KM9106, Kane Inc.) was employed to measure O2, CO2, CO, NOx, and SO2 in the flue gas and flue gas temperature at the ESP outlet. Details of the measured parameters were listed in Table 1. The CO, NOx, and SO2 concentration, as well as all particle number concentrations were converted to 7% O2 dry gas at normal temperature (0 °C) and pressure (101.3 kPa). The testing loads for B1 were 100%, 85%, and 70% and for B2, the load was kept as 100%. Although the sampling for the four measuring cases was not performed simultaneously; the operating conditions (such as dilution rates, temperature of heated dilution air, sampling flow) were set to perform consistently, in order to ensure comparability of measurement data. At 100% load for the two boilers, the total feeding rates of coal were the same, 95.1 t/h for each.
2. Experimental Section 2.1. Power Plant. The power plant was located in Shanxi province, one of the main coal-producing areas in China. The plant was typical of those operated from the 1980s until recently (17) Yi, H.; Hao, J.; Duan, L.; Li, X.; Guo, X. J. Air Waste Manage. Assoc. 2006, 56 (9), 1243–1251. (18) Yi, H.; Hao, J.; Duan, L. Fuel 2007, 87 (10-11), 2050–2057. (19) Wang, H.; Hao, Z.; Zhuang, Y.; Wang, W.; Liu, X. Energy Fuels 2008, 22, 1636–1640. (20) Yin, C.; Rosendahl, L. A.; Kær, S. K. Prog. Energy Combust. Sci. 2008, 34, 725–754. (21) Pagelsa, J.; Strand, M.; Rissler, J.; Szpila, A.; Gudmundsson, A.; Bohgard, M.; Lillieblad, L.; Sanati, M.; Swietlicki, E. J. Aerosol Sci. 2003, 34, 1043–1059. (22) Zhang, H.; Li, J.; Xu, X.; Zeng,R.;Yue, G. Chin. J. Power Eng. 2005, 25, 125-130. (23) Hao, J.; Tian, H.; Lu, Y. Environ. Sci. Technol. 2002, 36, 552–560.
(24) Moisio, M. Ph.D. Thesis, Tampere University of Technology, Tampere Finland, 1999; Publication 279.
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Aitken mode. The accumulation mode comes from coagulation of the Aitken mode and condensation of the hightemperature vaporous gases. Higher running load implies more coal consumption and higher combustion temperature. The higher temperature increases the vaporization of inorganic elements in the coal. Accordingly, more potential nucleant and condensable compounds will exist in the exhaust gas and a great deal of nuclei particles will be generated. In addition, higher temperature is beneficial to the occurrence of the oxidation process and consequently conduces to the decreasing of the particle diameter.28 With the comparison of the 5.6-560 nm size range between B1 and B2 with 100% load, the number concentrations of particles sized 60 nm for B1 account for only 27.6% of those for B2. In particular, averaged number concentrations of particles in the nucleation mode and the Aitken mode for B1 are 2.01 108 and 2.61 107 N cm-3, respectively, and nearly 20.2 and 12.3 times higher than those for B2 (Table 3). Considering similarly designed boilers, the same testing load, precipitator, and coal used; the differences of EEPS PSDs for B1 boiler and B2 boiler are mainly due to the LNBs. It indicates that the LNBs (on B1 boiler) in these measurements favor the formation of nuclei. Although the LNBs have a lower flame temperature to control NOx emission, they do increase the formation of ultrafine particles. The multistage combustion technology applied in LNBs offers a stage of fuel-rich combustion condition, which means the air-fuel rate is larger than 1. Fuel-rich combustion is efficient to reduce NOx emission, nevertheless the reducing condition results in more volatilization of ash contents in the coal and then condensed to form nucleation mode particles. In addition, soot, the major type of combustion-derived ultrafine PM, forms under fuel-rich conditions in which hydrocarbon fragments have a greater chance of colliding with other hydrocarbon fragments and growing, rather than being oxidized to CO, H2, CO2, and H2O.9 The effect on increasing nucleation mode particles cannot be compensated by reducing the temperature.29 The number concentrations of particles in the nucleation mode and Aitken mode for B1 are notably higher than those for B2, while it is the opposite for the accumulation mode. It is implied that the accumulation mode in this study may be formed more by the dominant mechanism of condensation of high-temperature gases rather than coagulation of Aitken mode particles. This may explain why in B2, with relatively high burning temperature, more particles are formed in the accumulation mode. 3.2. APS Particle Number Size Distribution. The 0.4730 μm PSDs emitted from B1 and B2 with 100% running load are shown in Figure 3. Both boilers produce an accumulation mode, with a peak at size range of 0.7-0.8 μm. The total number concentrations of the particles at 0.47-30 μm are 3-5 orders of magnitude lower than that of 5.6560 nm particles (Table 2). The curves are uniform in shape but vary slightly different in the peak of the size distribution, as the peak is 0.778 μm for B1 and 0.835 μm for B2. The peak and concentration median diameter for B1 are smaller than
Figure 1. EEPS number size distributions of particles for B1 and B2.
Figure 2. The 5th/95th box plot of the total number concentrations of emitted particles for B1 and B2.
3. Results and Discussion 3.1. EEPS Particle Number Size Distribution. Figure 1 presents representative 5.6-560 nm number PSDs for different loads of B1 (with LNBs) and the normal condition of B2 (without LNBs). As shown in the figure, the particle number concentrations decrease with increasing size. The curves of PSDs from B1 are similar, which show several small peaks at 10, 30, and 100 nm, respectively, while the PSDs from B2 show only one peak at 10 nm. The average values, standard deviation, and standard error of the total number concentration for B1 and B2 at different working conditions are shown in Figure 2. The total number concentrations for B1 boiler vary between 1.66 108 and 2.49 108 N cm-3, which is an order of magnitude higher than those for the B2 boiler (1.18 107 N cm-3). The mean number concentrations are given in Table 2, as well as the concentration median diameter and geometric standard deviation values. Compared with different loads, the concentration of the particles for B1 shows an increasing trend with the increasing running load. Additionally, for higher running loads, the concentration median diameter shifts to smaller size (see Table 2). This shows that the boiler tends to emit more fine particles under higher running loads. EEPS gains the PSDs at nucleation mode particles (Dp < 20 nm),25-27 Aitken mode (20 nm < Dp < 100 nm) and a part of accumulation mode (the size range of accumulation mode is 100 nm < Dp < 2 μm, but the EEPS can only be used to measure particles below 560 nm). The particles in the nucleation mode are formed when gaseous precursors (produced in the boiler by combustion) nucleate and grow into nuclei in the dilution tunnel and then coagulate and grow into particles of the (25) Kulmala, M.; Vehkamaki, H.; Petajda, T.; Dal, M. M.; Lauri, A.; Kerminen, V. M.; Birmili, W.; McMurry, P. H. J. Aerosol Sci. 2004, 35 (2), 143–176. (26) Hussein, T. Rep. Ser. Aerosol Sci. 2005, 74, 1–35. (27) Hussein, T.; Dal, M. M.; Petaja, T.; Koponen, I. K.; Paatero, P.; Aalto, P. P.; Hameri, K.; Kulmala, M. Boreal Environ. Res. 2005, 10 (5), 337–355.
(28) Wehner, B.; Bond, T. C.; Birmili, W. Environ. Sci. Technol. 1999, 33 (21), 3881–3886. (29) Flagan, R. C.; Seinfeld, J. H. Fundamentals of Air Pollution Engineering; Prentice Hall: Englewood Cliffs, NJ, 1998.
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Table 2. Concentration Median Diameter, Geometric Standard Deviation, and Mean Number Concentration Measured for Emitted Particles 0.47 μm < Dp < 30 μm
5.6 nm < Dp < 560 nm CMD (nm) B1 100% B1 85% B1 70% B2 100%
8.58 8.62 9.46 10.85
GSD
mean no. concn (N/cm3)
CMD (μm)
GSD
mean no. concn (N/cm3)
1.59 1.71 1.94 2.58
2.49 10 2.18 108 1.66 108 1.18 107
0.94 0.81 0.8 0.99
2.48 1.82 2.18 2.02
2.00 103 2.46 103 320 2.94 104
8
Table 3. Typical Number Concentration of Particles Measured for Each Mode (N cm-3)
EEPS APS
mode
B1 100%
B2 100%
nuclei mode Aitken mode accumulation mode accumulation mode coarse mode
2.01 10 2.61 107 1.02 104 1.67 103 330
9.45 106 1.96 106 7.04 104 2.68 104 2.60 103
8
Figure 3. APS number size distributions of particles for B1 and B2.
Figure 5. Typical APS PSDs (A) and EEPS PSDs (B) variation with time. Color represents concentration in cm-3.
consequence of the melting and coalescence of bulk mineral matter in the original fuel particles. Therefore, higher temperature will promote the concentration of particles in both accumulation and coarse modes. In view of the total size range in this study (5.6 nm∼30 μm), both B1 and B2 produce a nucleation mode and an accumulation mode, but only B1 produces an Aitken mode. The nucleation mode particles hardly survive in an “aged” atmosphere and might easily disappear by condensation. In this study, they are captured because these particles are newly formed in the dilution tunnel by condensation and coagulation of nucleating precursors in the high-temperature flue gas. In the B2 boiler, less particles of the nucleation mode are produced, resulting in insufficient nucleation mode particles to grow into the Aitken mode. Figure 4 presents the typical curves of cumulative mass concentration (calculated from number concentration, assuming all particles are spheres, with the density as 1 g/cm3) and number concentration in the measurements. It suggests that the mass concentration of ultrafine particles contributes less to the total mass (accounts for about 2% of the total
Figure 4. Typical accumulative percent of particles mass concentration (A) and number concentration (B).
those for B2, which further reveals the tendency of LNBs to form fine particles. In terms of the 0.47-30 μm particles, the number concentrations of particles in the accumulation mode (0.47-2 μm) and the coarse mode (2-30 μm) for B2 are nearly 15 and 6.9 times higher than those for B1, respectively (Table 3). The higher concentrations in the accumulation and coarse modes for B2 are probably attributable to the relatively higher burning temperature. As previous discussed, the accumulation mode particles in this study maybe mainly come from condensation of high-temperature gases. The coarse mode particles (Dp > 2 μm) are a 1680
Energy Fuels 2010, 24, 1677–1681
: DOI:10.1021/ef900941b
Liu et al.
by condensation and coagulation in the dilution tunnel. It is implied that in order to avoid the impact of re-entraining, online particle emission measurements for boilers with ESP should continue for at least two periods of last electric field rapping.
mass) (Figure 4A) but contributes most to the total number concentration (accounts for more than 99% of the total number) (Figure 4B). Historically, the emission standard of air pollutants for thermal power plants is only based on the total mass concentration of particulate matter. It neglects the huge number of ultrafine particles which possibly cause more health risk for their large number and huge surface area. Thus, study of the number concentration distribution of ultrafine particles is of great significance for evaluating the contribution of power plants emission on the ambient atmospheric particulate matter, which may provide a scientific basis for particle emission control. 3.3. PSDs Variation with Time. With the use of high timeresolution particle sizer spectrometers (1 s resolution for EEPS and 1 min resolution for APS in this study) for continuous online sampling, the variation of the particles emitted from boilers with time was detected. Although the boiler was at steady state in each sampling process, relatively regular vibration occurred for the APS PSDs with time but little for the EEPS PSDs, as shown in Figure 5. This may be caused by the rapping process of the ESP collection plate. When the rapping system is working, the dust cake is shaken off from the ESP surface. Some particles may be re-entrained in the upstream flue gas and then could be captured again in the next field. When the last electric field is rapped, some escaping particles will be re-entrained into the stack gas, resulting in the instantaneous increase of measured particle concentration.12,30 The concentrations of particles with sizes lower than 20 nm change little with time, as they are formed
4. Conclusions The number size distributions of particles sized 5.6 nm ∼30 μm for boilers with (B1) and without LNBs (B2) were measured online in a pulverized coal fired power plant in China. Both B1 and B2 produced a nucleation mode and an accumulation mode, but only B1 produced an Aitken mode. Particles at nucleation mode were monitored in this study, which was not easy in “aged” air. For B1, the number concentrations of
