Influence of Boiler Load on Generation Characteristics of PM2.5

Apr 6, 2016 - Fine particulate matter (PM2.5) was sampled at the furnace outlet of a 660 MW boiler using a two-stage sampling dilution system. The num...
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Influence of Boiler Load on Generation Characteristics of PM2.5 Generated by a 660 MW Pulverized Coal Boiler Qian Du,* Heming Dong, Lipeng Su, Zhifeng Zhao, Donghui Lv, and Min Wang School of Energy Science and Engineering, Harbin Institute of Technology, 92, Xidazhi Street, Harbin 150001, People’s Republic of China ABSTRACT: Fine particulate matter (PM2.5) was sampled at the furnace outlet of a 660 MW boiler using a two-stage sampling dilution system. The number and mass concentrations of PM2.5 were measured at different applied loads using an electrical lowpressure impactor to determine how boiler load affects PM2.5 production. Size-segregated elemental compositions of particles were measured by energy-dispersive X-ray spectroscopy. The results indicate that PM2.5 contains both ultrafine and central-mode particles. The number concentration depends on the ultrafine particles (PM0.38), and the mass concentration depends on the central-mode particles (PM0.38−2.5). The Na, S, and Ca concentrations decrease with increasing particle size, but Si, Al, and K concentrations in the central-mode particles are higher than in the ultrafine mode. The Fe and Mg concentrations are roughly independent of particle size. The trends of elemental behavior suggest that the central-mode particles are primarily formed from fine residual ash particles coated by heterogeneous condensation of vaporized minerals. WIth the boiler load increasing, the number and mass concentrations of PM2.5, PM0.38, and PM0.38−2.5 increased, which was mainly due to the higher furnace temperature. The average size of PM0.38 tended to increase with increasing load, but the PM0.38−2.5 average size decreased: this difference is attributed to their different mechanisms of formation.

1. INTRODUCTION The presence of fine particulate matter of aerodynamic diameters less than 2.5 μm, referred to as PM2.5, in the atmosphere has attracted widespread attention in China. According to news reports for 2014, no city in China met the World Health Organization safety limits (less than 10 μg/m3) for annual average levels of PM2.5. Because of its small volume, PM2.5 has a large surface area that can absorb many harmful substances (bacteria, organic matter, etc.) and can enter the alveolar tissue of lungs.1 The main sources of PM2.5 are automobile exhausts and boiler flue gases. The Chinese electricity energy structure is based on thermal power, specifically coal combustion, and PM2.5 produced by coal-fired power stations is usually rich in large amounts of toxic heavy metal elements.2 The most recent emission standard for air pollutants from thermal power plants in China (GB13223−2011) limits the PM emission concentrations to a maximum of 30 mg/m3 in general areas and 20 mg/m3 in key areas. This is the same as the emission limits for newly constructed boilers given in Directive 2001/80/EC of the European Parliament and that for new emission sources (0.14−0.15 lb/MWh, about 20 mg/m3) promulgated by the United States government in 2005. There is still a disparity between the PM2.5 emissions in China and other developed countries. In America, most power plants are equipped with an electric precipitator, and in addition to a conventional electrostatic precipitator, the application of new technologies such as a wet electrostatic precipitator are extensive;3 only part of power plants use a fabric filter, but because the deducting efficiency is higher and the adaptability of coal ash is wider, the use of a fabric filter becomes inflated.4 Conventional electrostatic precipitators are the most widely used in China. Many researchers found that there is a minimum value of sizesegregated deducting efficiency of electrostatic precipitators between 0.1 and 1 μm,5,6 so only using electrostatic precipitators © 2016 American Chemical Society

is difficult to achieve ideal reduction of PM2.5 emission. In order to implement more stringent emission standards, like America, China is ongoing the development and improvement of electrostatic precipitation technology and promoting fabric filter. Design and improvement of the dust collector depends on the characteristics of coal ash especially small size particles (such as PM2.5), so research into the characteristics of PM2.5 produced by pulverized coal boilers, which is the main furnace application in Chinese electricity production, is of great significance. Previous studies have shown the importance of small particles in dust emissions from coal-fired boilers. Initial research focused on submicron particles: Markowski7 and McElroy8 both found a peak in particle diameters in the vicinity of 0.1 μm in their respective measurements of the size distribution of particle from coal-fired boiler. Linak et al.9 studied the mass and compositions of particles generated by three coals and a residue fuel oil and found that the size distributions of the coals showed a central mode with aerodynamic diameters between 0.8 and 2.0 μm. Although the origins of the central-peak particles were less clear, the results of elemental analysis indicated that they may be produced by fine mineral inclusions in the coal that were released in the process of char fragmentation and burnout. In subsequent research on PM2.5 by Linak,10 various methods were used for particle sampling and size classification to confirm the presence of this central-mode component in coal-fired particles, given the importance of these particles on PM2.5 emissions. Other studies,11−13 that were focused on dust-removal device penetration or elements emission characteristics of PM, also indicated the key role of PM2.5 in the emission characteristics of flue-gas Received: October 17, 2015 Revised: April 3, 2016 Published: April 6, 2016 4300

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Energy & Fuels Table 1. Proximate and Ultimate Analyses of the Fuel Coal Proximate analysis (wt %)

a

ultimate analysis (wt %)

heating value(MJ/kg)

rank

Mara

Varb

Aarc

FCard

Car

Sar

Har

Nar

Oar

Qgr,ar

Qnet,ar

bitumite

3.38

25.5

30.1

41.02

51.5

1.65

3.06

0.7

9.58

20.13

19.43

Mar means moisture, as received. bVar means volatile, as received. cAar means ash, as received. dFCar means fixed carbon, as received.

Table 2. Ash Analysis of the Fuel Coal SiO2 (%)

Al2O3 (%)

Fe2O3 (%)

CaO (%)

MgO (%)

SO3 (%)

TiO2 (%)

K2O (%)

Na2O (%)

P2O5 (%)

MnO2 (%)

48.94

25.74

6.04

6.26

2.16

3.26

1.24

1.38

0.26

0.22

0.12

The elemental concentrations (S, Si, Al, Ca, Fe, Na, K, and Mg) of particles collected on the first nine channels of the ELPI were measured by energy-dispersive X-ray spectroscopy. The particle samples were collected on a polycarbonate substrate and were sprayed with a 25 nm Au coating. Each sample was measured three times, and the average was taken as the test result.

particles. There is, however, no investigation of the relationship between boiler load and emission characteristics of PM2.5 which are important to the design and operation of the dust collector. In this paper, the number and mass concentrations of PM2.5 generated by a 660 MW pulverized coal boiler were determined in on-site tests. The size-segregated aerosol content was analyzed and the characteristics of PM2.5 generated under different loads were analyzed and compared.

3. RESULTS AND DISCUSSION 3.1. Generation and Formation of PM2.5. The number and mass concentration distributions of PM2.5 were measured at the outlet of the furnace (before the dust-removal device) at stabilized boiler loads of 400 MW, 550 MW, 600 MW, and 630 MW, as shown in Figures 2 and 3, respectively;

2. EXPERIMENTAL SECTION 2.1. Equipment and Materials. A 660 MW supercritical aircooled generator was employed. The boiler (model HG-2210/ 25.4-YM16) is a once-through boiler for variable supercritical pressure operation and comprises a single furnace. Four low NOx direct-flow burners are installed near the middle of the furnace walls to provide a wall-tangential flame. The steam parameters of the single reheating unit are 25.4 MPa/57 °C/569 °C and the maximum continuous rating of the boiler is 2210 t/h. The coal powder system comprises six medium-speed pulverizers and a positive-pressure direct-firing unit. Proximate and ultimate analyses of the fuel coal are given in Table 1, the ash analysis is given in Table 2. To investigate the PM2.5 generation characteristics of this pulverized coal boiler, the measuring point was taken at the outlet of boiler, prior to the dust collector. 2.2. Experimental Methods. As shown in Figure 1, particles in the flue gas were sampled using an isokinetic sampling probe.

Figure 2. Number concentration distribution of PM2.5 under different loads.

Figure 1. Schematic of two-stage dilution system for PM2.5 measuring. Particles with diameters greater than 10 μm were separated by a preseparation cyclone and the flue gas was then passed into two dilutors and diluted by filtered air. The first-stage dilutor required the air to be heated to avoid condensation of condensable particulate matter, and the second-stage dilutor required the air to enter at ambient temperature to ensure that the dilution gas was in the operatingtemperature range (−5 to 45 °C) of the electrical low-pressure impactor (ELPI; Dekati, Finland). After dilution, the particulates were size-classified measured by the ELPI. The ELPI measuring range is from 30 nm to 10 μm. Particles in this size range are divided into 12 channels; only the first nine channels were used in this study. The ELPI collected one data point per second; measuring one set of data generally took about 3 min; five sets of data were collected under each load. The reported PM2.5 concentration under each load is the average of the five measurements for the load.

Figure 3. Mass concentration distribution of PM2.5 under different loads and the compositions of the major elements of particle collected on channel 2 and channel 9.

size-segregated elemental compositions of particles on each channel are presented in Figures 4 and 5; changes in the number and mass concentrations of PM2.5 with load are shown in Figure 6. As shown in Figure 2, the particle number concentration distribution of PM2.5 produced by the boiler comprised two 4301

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coated with condensate formed by heterogeneous condensation of vaporized minerals.6 These fine residual ash particles mainly include particles formed by melting and coalescence of inherent minerals in fine coal particles or small fragments produced by fragmentation of coal during combustion, fine external mineral particles, and small particles formed by fragmentation of large external mineral particles.18 The ratio of the number concentration of PM0.38/PM2.5 is greater than 97.5%, so the number concentration of PM2.5 mainly depends on PM0.38, implying that the number concentration of PM2.5 mainly depends on the ultrafine mode particles. Figure 3 shows the mass concentration distribution of PM2.5 under different loads. The mass concentration profile at a load of 400 MW has a single peak at 0.12 μm, but this moves to 0.2 μm under higher loads. The mass concentration of PM0.38 comprised a small proportion of the total mass concentration of PM2.5: the ratio of mass concentration of PM0.38/PM2.5 was less than 16% for each load. The mass concentration of PM2.5 therefore mainly depends on the central-mode particles, PM0.38−2.5. The compositions of the major elements of the particles collected on ELPI channels 2 (ultrafine mode) and 9 (central mode) are also shown in Figure 3. The compositions of the particles comprising these two modes are very different: the ultrafine mode particles contain more S and Ca and less Si and Al than the central-mode particles. This result is in agreement with that of Linak et al.6 and supports the proposal that PM0.38 and PM0.38−2.5 are generated by different mechanisms. As shown in Figures 4 and 5, the Na, S, and Ca concentrations decreased from 0.45%, 10.41%, and 18.08% to 0.22%, 1.32%, and 5.23%, respectively, as the particle size increased from 0.076 to 1.99 μm. In contrast, the Si, Al, and K concentrations in the PM0.38 fraction were lower than those in PM0.38−2.5. For PM0.38 (ultrafine mode particles), the Si and Al concentrations were approximately 35.16% and 19.59%, respectively, and essentially independent of particle size. For PM0.38−2.5 (centralmode particles), the Si and Al concentrations increased from 37.29% and 27.08% to 45.86% and 32.45%, respectively, with increasing particle size. This observation is same as that reported from the laboratory study of Yu et al.19 that used a drop-tube furnace. In addition, the Fe and Mg concentrations in particles reporting to the different channels were roughly the same. Enrichment of Na, S, and Ca in ultrafine particles has generally been shown in previous studies.20,21 Na and Ca favor vaporization during pulverized coal combustion to form ultrafine particles. In flue gas, SO2/SO3 may react with vaporized metal to cause S enrichment in ultrafine particles.22 The volatilities of Si and Al are much lower than those of Na, S, and Ca, so their concentrations in the central mode are higher than in the ultrafine mode. K usually occurs in bituminous coal in minerals that are difficult to vaporize,23 so K enrichment is found in central-mode particles. The volatilities of Fe and Mg may be higher than those of Si and Al, but are lower than Na, S, and Ca, so their concentrations in the ultrafine-mode particles are similar to those of central-mode particles. In this work, the Si and Al concentrations in the PM0.38 component are much higher than those reported by Yu et al.15 This discrepancy is likely related to the different combustion temperatures pertaining in a drop-tube furnace (1420−1620K) and pulverized coal boiler (about 1700−1800K) and higher temperatures are conducive to vaporization of this two minerals.17 The Si and Al concentrations in the PM0.38−2.5 particles show similar tendencies to those reported by Yu et al.15 As mentioned

Figure 4. Size-segregated aerosol content of S, Si, Al, Ca, and Fe in PM2.5.

Figure 5. Size-segregated aerosol content of Na, K, and Mg in PM2.5.

Figure 6. Total number and mass concentration of PM2.5 under different loads.

components: PM0.38, which represents particles smaller than 0.38 μm in aerodynamic diameter, and PM0.38−2.5, which represents particles between 0.38 and 2.5 μm. There is a clear peak at 0.12 μm in PM0.38, but PM0.38−2.5 has no peak, and the number concentration of each channel in this range shows a gentle downward trend. According to the trimodal distribution theory of the distribution of particles produced by coal combustion (ultrafine, central, and coarse modes),14,15 PM0.38 belongs to the ultrafine mode and is formed by inorganic minerals through a vaporization−condensation mechanism.16 In a high-temperature combustion environment, some coal minerals are gasified and, when the temperature drops, these gaseous minerals form a large number of nanoparticles by homogeneous nucleation. Nanoparticles in the boundary region of burning char particles form ultrafine mode particles by coagulation or agglomeration.12,17 The size of most ultrafine particles is about 0.12 μm. The PM0.38−2.5 component is mainly central-mode particles, which may be formed from fine residual ash particles 4302

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Energy & Fuels above, central-mode particles are primarily formed from fine residual ash particles coated with vaporized minerals by heterogeneous condensation. The increase of Si and Al concentrations in the PM0.38−2.5 fraction with increasing particle size provides strong support for this theory. In addition, further evidence for this mechanism of central-mode particle formation is the decrease in the concentrations of more volatile elements (Na, S, and Ca) with increasing particle size. As shown in Figure 6, the number concentrations of PM2.5 in flue gas produced by the boiler are more than ten million per cubic centimeter and the mass concentrations are about 300 mg/m3. The number and mass concentrations of PM2.5 increase with increasing boiler load. 3.2. Influence of Boiler Load on Generation Characteristics of PM0.38. The total number and mass concentrations of PM0.38 under different loads are shown in Figure 7, the

Figure 9. Proportions of particles on each channels in PM0.38 under different loads.

increased, but for the other channels, the number concentrations and the proportions of particles in PM0.38 increased with increasing load. This indicates that, when boiler load increased, the average particle size of the PM0.38 component tended to increase. The emission characteristics of PM0.38 were mainly influenced by the combustion atmosphere in the furnace. Because temperature and oxygen content in the furnace are the main parameters affecting PM emissions,24 data for these two parameters were collected from the boiler online monitoring system. The interior and outlet of the furnace are not conventional measuring points, so the flue gas temperature at the inlet of the low-temperature reheater was used to represent the temperature trends in the furnace and the flue gas oxygen content at the economizer outlet was used to represent the oxygen content in the furnace. The boiler monitoring system collected one data point every 20 s, thereby collecting 4320 data points for each parameter in a 24-h period. The changes of boiler load and flue gas temperature at the inlet of the low-temperature reheater are shown in Figure 10, and the changes of boiler load and oxygen content of

Figure 7. Total number and mass concentration of PM0.38 under different loads.

Figure 8. Number concentrations of each channels of PM0.38 under different loads.

number concentrations of PM0.38 particles on each channel under different loads are shown in Figure 8, and the proportions of particles on each channel in PM0.38 under different loads are shown in Figure 9. Figure 7 shows that both number and mass concentrations of PM0.38 increased as the boiler load increased: when the boiler load increased from 400 MW to 630 MW, the number concentration of PM0.38 increased by 10% (from 1.20 × 107/cm3 to 1.32 × 107/cm3) and the mass concentration increased by 72.8% (from 31.21 mg/m3 to 53.95 mg/m3). This indicates that when the boiler load increased, the increase in the mass concentration of PM0.38 was greater than the increase in the number concentration. As shown in Figure 8 and 9, the number concentration of PM0.38 mainly depends on particles in the channels representing diameters of 0.12 and 0.2 μm. For the channels for which the average particle diameter was less than or equal to 0.12 μm, the number concentrations and the proportions of particles in PM0.38 gradually reduced as the load

Figure 10. Relationship of boiler load and flue gas temperature.

the flue gas at the economizer outlet are shown in Figure 11. Figure 10 shows significant positive correlation between boiler load and flue gas temperature: when the boiler load increased from 350 to 660 MW, the temperature increased by about 160 °C. As shown in Figure 11, boiler load and oxygen content were negatively correlated: when the boiler load increased from 350 to 660 MW, the oxygen content decreased from 5% to about 3%. PM0.38 is mainly formed by minerals via a vaporization− condensation process. When the load increases, the temperature of furnace increases. Because higher temperatures are conducive to mineral vaporization,25,26 the formation of PM0.38 will be promoted. The oxygen content in the furnace decreases with increasing boiler load. The study of Timothy27 at a furnace 4303

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PM0.38 component, when the boiler load increased, the number and mass concentrations of PM0.38−2.5 increased, but the increase in mass concentration was smaller than the increase in number concentration. Figure 13 shows that the number concentrations of PM0.38−2.5 particles on each channel increased with increased boiler load. Figure 14 shows that all channels

Figure 11. Relationship of boiler load and oxygen content of flue gas.

temperature of 1700 K (1427 °C) indicated that the surface temperature of char increased from 2300 K (2027 °C) to 2800 K (2527 °C) when the oxygen concentration increased from 20% to 50%. As the oxygen content decreases, the extent of vaporization of minerals will decrease and the formation of PM0.38 will be suppressed. Combined with the results of this paper, the effect of temperature change caused by load change on the gasification of minerals was greater than the effect of change in oxygen content; therefore, as the load increased, the number and mass concentrations of PM0.38 increased. According to the results of this work, the increase in mass concentration of PM0.38 was greater than that of number concentration and the average particle size of PM0.38 tended to increase with increasing boiler load. Nanoparticles mainly grow by coagulation or agglomeration, which decrease the number of particles. As the flue gas temperature increased, the vaporization amount of minerals increased, i.e., the number of nanoparticles generated increased and the random motion of the nanoparticles became more violent, so collisions between particles increased. Because coagulation and agglomeration depend on collisions between particles,12 when the boiler load increases, the increase in the number concentration of PM0.38 is smaller than the increase in mass concentration and the average size of PM0.38 becomes larger. 3.3. Influence of Boiler Load on Generation Characteristics of PM 0.38−2.5. The total number and mass concentrations of PM0.38−2.5 under different loads are shown in Figure 12, the number concentrations of PM0.38−2.5 particles

Figure 13. Number concentrations of each channel of PM0.38−2.5 under different loads.

Figure 14. Proportions of particles on each channels in PM0.38−2.5 under different loads.

made a greater contribution to the number concentration of PM0.38, but the channel with an average particle size of 0.48 μm made the greatest contribution. For this channel, the proportion of particles increased with increasing load, but the proportions of these particles reduced in the other channels. This indicates that, when the boiler load increased, the average particle size of the PM0.38−2.5 component tended to decrease. The major portion of the PM0.38−2.5 particles forms from fine residual ash particles coated by condensate. Increases in the total mass and number concentrations of PM0.38−2.5 are therefore mainly due to the increase in fine residual ash particles. The concentrations of fine residual ash particles formed by melting and coalescence of inherent minerals in fine coal particles and fine external mineral particles are relatively stable, but the concentration of fine residual ash particles formed by fragmentation of coal and external minerals varies with changes in the combustion atmosphere in the furnace. As discussed in the preceding section, as the boiler load increases, the temperature in the furnace increases and the oxygen content decreases. Higher temperature increases the combustion rate, which is conducive to the crushing of coal and external minerals, thereby promoting the formation of PM0.38−2.5.28 Higher oxygen concentration makes more oxygen into char to react with carbon, so the porosity of the char will increase and crushing will be enhanced. Low oxygen concentration is therefore not conducive to the crushing of coal and the formation of

Figure 12. Total number and mass concentration of PM0.38−2.5 under different loads.

in each channel under different loads are shown in Figure 13, and the proportions of particles of each channel under different loads are shown in Figure 14. As shown in Figure 12, when the boiler load increased from 400 to 630 MW, the mass concentration of PM0.38−2.5 increased from 258.15 mg/m3 to 282.67 mg/m3, an increase of 9.5%, and the number concentration increased from 2.64 × 105/cm3 to 3.16 × 105/cm3, an increase of 19.96%. Similar to the behavior seen for the 4304

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load, attributed to the relatively higher number concentration of small fragments produced by crushing of the coal. (5) As the load increased, The PM0.38 particle size increased and PM0.38−2.5 particle size decreased, which means the proportion of particles between 0.1 and 1.0 μm got bigger and the PM2.5 removing efficiency of electrostatic precipitator got smaller. So for PM2.5 emission reduction, China needs to improve or develop new electrostatic dust removal technology or change electrostatic precipitator to fabric filter or hybrid electrostatic-fabric filter.

PM0.38−2.5 will be suppressed by dwindling oxygen content. In these experiments, the effect of temperature changes caused by load changes on the formation of PM0.38−2.5 was greater than the effect of changes in oxygen content: when the boiler load increased, the total mass and number concentrations of PM0.38−2.5 increased and, because this random crushing contributed to particles reporting to all channels in the 0.38−2.5 μm range, the number concentrations on each channel tended to increase. The increase in the 0.48 μm channel was much higher than that of the others, indicating that the average particle size of the PM0.38−2.5 component tended to decrease with increasing boiler load. Crushing not only produces more fine residual ash particles, but also decreases their size, so a larger proportion of particles in smaller size ranges will be generated by more intense crushing of coal and external minerals and the average particle size of the PM0.38−2.5 component will decrease.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-13274503358. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was sponsored by the National Science and Technology Support Project (2014BAA07B03) and the Science Foundation for Innovative Research Groups (51421063).

4. CONCLUSION In this paper, the generation characteristics of PM2.5 from a 660 MW supercritical pulverized coal boiler have been field tested under load conditions of 400, 550, 600, and 630 MW. The following conclusions are drawn. (1) The particle size distribution for the number concentration of PM2.5 generated by the boiler can be divided into two size fractions: PM0.38 and PM0.38−2.5. The number concentration of PM2.5 depends on ultrafine mode particles, i.e., PM0.38, and the mass concentration of PM2.5 depends on central-mode particles, i.e., PM0.38−2.5. There was a clear peak at 0.12 μm in PM0.38 for each load, but the number concentrations on each channel of PM0.38−2.5 showed a gentle downward trend. As boiler load increased, the total number and mass concentrations of PM2.5 increased. (2) Na, S, and Ca were enriched in the ultrafine particles and their contents in each channel decreased with increasing particle size. The Si, Al, and K concentrations in the centralmode particles were higher than those in the ultrafine mode. The Si and Al concentrations of the ultrafine mode particles were independent of particle size but increased with increasing particle size for central-mode particles. The Fe and Mg contents of PM2.5 were roughly independent of particle size. From the elemental deportment observed, it is proposed that centralmode particles are formed from fine residual ash particles and coated with vaporized minerals by heterogeneous condensation. (3) As the load increased, the total number and mass concentrations of PM0.38 tended to increase. The emission characteristics of PM0.38 were mainly influenced by the temperature and oxygen content in the furnace: the positive effect of increasing furnace temperature with increasing boiler load on the gasification of minerals was greater than the negative effect of decreasing oxygen content. The particle size of PM0.38 increased with increasing load because of increased coagulation and agglomeration in the furnace. (4) As the load increased, the total number and mass concentrations of the PM0.38−2.5 component and the number concentrations of particles on each channel of PM0.38−2.5 tended to increase. The emission characteristics of PM0.38−2.5 were also influenced by the temperature and oxygen content in the furnace: the positive effect of higher furnace temperature with increasing boiler load on the crushing of coal and external minerals was greater than the negative effect of decreasing oxygen content in the furnace. The PM0.38−2.5 particle size decreased with increasing

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