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Emission Characteristics of Particulate Matter from Two UltraLow Emission Coal-Fired Industrial Boilers in Xi’an, China Renhui Ruan, Xinwei Xu, Houzhang Tan, Sicong Zhang, Xuchao Lu, Peng Zhang, Ruiwu Han, and Xiaohe Xiong Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04069 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 4, 2019
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Emission Characteristics of Particulate Matter from Two Ultra-Low Emission Coal-Fired Industrial Boilers in Xi’an, China
5
Renhui Ruan1,2, Xinwei Xu1, Houzhang Tan1,2*, Sicong Zhang1, Xuchao Lu1, Peng Zhang1,
1 2 3
Ruiwu Han1, Xiaohe Xiong1
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1. Key Laboratory of Thermo-Fluid Science and Engineering of Ministry of Education,
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School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi, 710049,
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China 2. Research Institute of Xi’an Jiaotong University, Hangzhou, Zhejiang, 311215, China
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*
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Email:
[email protected] Corresponding author: Tel: +86-029-82668051
Fax: 029-82668703
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Abstract:
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backward compared to coal-fired power plants, enhancing serious PMs (particulate matters)
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pollution. The PM removal characteristics of APCDs from two industrial boilers (a CFB
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(circulating fluidized bed) boiler and a CGB (chain-grate boiler)) with ultra-low emission were
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studied in this research. PM was sampled at the inlet/outlet of FF (fabric filter), WFGD (wet flue
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gas desulfurization) and WESP (wet electrostatic precipitator) through the cyclone-filter sampling
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system. The morphology and water-soluble ions of PM were analyzed by SEM (scanning electron
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microscope) and IC (ion chromatography). The results show that the concentrations of PM1, PM2.5,
The APCDs (air pollution control devices) of industrial boilers in China are
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and PM10 (PM with aerodynamic diameter ≤ 1, 2.5, 10 μm, respectively) at the precipitator inlet of
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CFB are much higher than that of CGB. The PM removal efficiency of FF in the CFB boiler is
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98.12~99.56% while that of CGB is only 90.0~93.6%. WFGD can increase the alkalinity of PM
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and the combined PM removal efficiency of WFGD coupled with WESP is 46.75~62.77%. SO42-
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is the most abundant water-soluble anions in PM while the water-soluble cations are richest in
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Ca2+ and Na+. After ultra-low emission retrofit, the EF (emission factors) of PM10, PM2.5 and PM1
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are 0.028~0.033 kg/t, 0.025~0.028 kg/t and 0.014~0.017 kg/t, respectively. The percentage of
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PM2.5 in PM10 is 85.2~88.6% at WESP outlet. These results are significant to the understanding of
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emission characteristic for atmospheric fine particles from industrial boilers with ultra-low
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emission in China.
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1. Introduction
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Recent years, the frequent haze and dust in eastern China are harmful to human beings and
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the environment.1, 2 Source apportionment analysis reveal that coal combustion is one of the most
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essential sources of the primary and secondary particles in the atmosphere.3, 4 In 2017, China's
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total energy consumption was about 44.9 billion tons of coal equivalent, of which coal
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consumption accounted for 60.4%.5 Coal is mainly consumed by industrial boilers and coal-fired
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power plants which can release huge amounts of PM, SO2, NOx, VOCs and heavy metals6 and it is
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one of the main sources of atmospheric PM and SO2.7
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With the strengthening of the public environmental awareness, the emission standard of
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coal-fired power plants in China has been raised in the past five years and it is now the most strict
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standard around the world: PM#3>#2. This is mainly attributed to the different combustion technology of the three boilers
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and the fuel properties (ash content, fuel size, etc). The combustion temperature in #1 furnace is
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low and locates in the range of 800~900 °C which is not high enough for mineral gasification and
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molten, the fuel stays in the furnace for a long time, and the intensity of the gas-solid two-phase
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flow state is between that of CGB and PCUB. The collision among ash and fuel particles is the
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main way for the formation of fly ash. #2 furnace adopts the grate firing technology with the
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maximum combustion temperature of 900~1000 °C. The air required for combustion is blown
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from below the grate and passes through the fuel bed. The fuel is large in size of about 5~30 mm.
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The gas flow in the furnace is far less strong than that in #1 and # 3 so that the mineral particles
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with large size will remain on the chain grate, only some of the volatile matters and micron-sized
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particles can escape away from the grate. Thus the mass concentration of PM produced from #2
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furnace is much lower than that from #1 and #3. #3 adopts the tangential-firing technology with
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the average coal particle size of around 100 μm. The gas-solid two-phase flow is more violent than
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#1 and #2. The combustion temperature is about 1200~1300 °C, which is beneficial to mineral
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gasification and volatilization. The above conditions results in good carrying ability of fly ash by
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flue gas in #3 furnace, and the mass concentration of PM produced from #3 furnace is much
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higher than that from #2, especially for coarse particles like PM2.5 and PM10.
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3000 79.5 %
60
2000 47.3 % 1500
40
35.1 % 1139
1000
27.2 %
846
20
500
297 147
0 PM10
214 215
80
2500
117
8.8 % 6.0 %
213
40
51
PM2.5/PM10 and PM1/PM10 ratio %
2408
PM mass concentration mg/Nm3
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PM1
PM2.5
#1 mass of PM
#2 mass of PM
#3 mass of PM
#1 PM ratio
#2 PM ratio
#3 PM ratio
Figure 3. Mass concentration of PM10, PM2.5, PM1 at #a and PM2.5/PM10, PM1/PM10.
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Considering the ash content of fuel burned in #3 furnace is only 10.7% (Table 2) while that in
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#1 is 28.0%, it is reasonable that #1 furnace produce more PM than #3 furnace.27 The PM2.5/PM10,
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PM1/PM10 ratios at the outlet of #2 furnace are higher than that of #1 and #3, which is 79.5% and
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27.2% respectively. Meanwhile, the PM2.5/PM10, PM1/PM10 ratios at #1 and #3 are close to each
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other. These indicate that #2 furnace mainly produces fine particles with an aerodynamic diameter
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less than 2.5 μm. Cao et al.24 sampled PM from six industrial CGBs, and the PM2.5 mass
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concentration at the furnace outlet was in the range of 46.5~150.5 mg/Nm3, which was consistent
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with the result (117 mg/Nm3) of #2 furnace in our test.
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Figure 4 presents the quartz membranes sampling at #a. Figure 5 shows the microscopic morphology of PM on the quartz membranes in Figure 4. PM10
PM2.5
PM1
PM10
PM2.5
PM1
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PM10
PM2.5
PM1
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(a) #1a
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(b) #2a
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(c) #3a
Figure 4. Quartz membranes after sampling PM at the furnace outlet.
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The PM at #2a is black as shown in Figure 4(b), which indicates lots of unburned matters and
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low combustion efficiency. The PM10, PM2.5, PM1 of #1 is mostly irregular particles with sharp
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corners as shown in Figure 5(a), (d) and (g) respectively. The flame temperature of #1 furnace is
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lower than the melting point of most aluminosilicates. These particles are derived from the
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fragmentation during the collision among the fuel and ash particles.36 The PM10, PM2.5, PM1
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generated from #2 furnace are much small in diameter than that from #1 and #3. The EDS analysis
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indicated high carbon content in PM10, PM2.5, PM1, which is 71.1%, 83.2%, 88.8% respectively.
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Most of these nano-sized particles are soot produced by incomplete combustion, which is also
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consistent with the color of the quartz membranes in Figure 4(b). The PM10, PM2.5, and PM1 at
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#3a are mostly spherical particles. The EDS results indicate that the main components include Si,
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Al, Ca and S. The fly ash particles melt at high temperature and exhibit a uniform spherical
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morphology under the surface tension.
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(a) #1 PM10
5 µm
(b) #2 PM10
5 µm
(c) #3 PM10
5 µm
(d) #1 PM2.5
5 µm
(e) #2 PM2.5
5 µm
(f) #3 PM2.5
5 µm
(g) #1 PM1
5 µm
(h) #2 PM1
5 µm
(i) #3 PM1
5 µm
Figure 5. Morphology of PM10, PM2.5, PM1 sampled at the furnace outlet (#a).
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3.2 Influences of APCDs on PM Mass Concentration
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3.2.1 FF (fabric filter)
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The PM removal efficiencies of the precipitator are shown in Table 4. The removal efficiency
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of #1 FF varies in the range of 98.12% to 99.56% for PM10, PM2.5, and PM1. The PM removal
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efficiency of #2 FF is only 90.0~93.6%, which is much lower compared to #1. According to the
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design and selection criteria of FF, the dust concentration at the outlet of the precipitator is
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generally determined according to the local environmental emission standard. For different types
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of coal-fired industrial boiler, the designed value of dust concentration at the outlet of the FF is
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close to each other. In our test, the PM10 mass concentrations at the FF outlet of #1, #2 are 10.5
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mg/Nm3, 9.4 mg/Nm3. The PM mass concentration at #1a (precipitator inlet) is much higher than
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that of #2a, resulting in a lower PM removal efficiency for #2 FF.
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Table 4. PM Removal Efficiency by FF and LLT-ESP % #1 FF
#2 FF
#3 LLT-ESP
PM10
99.56
93.60
98.77
PM2.5
99.37
93.24
98.58
PM1
98.12
90.00
98.41
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FF is widely used in industrial boilers, and its filtration efficiency is usually higher than that
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of ESP. The ESP of #3 adopts three-phase power technology with five electric fields. The removal
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efficiencies of PM10, PM2.5, PM1 are all above 98%. However, the removal efficiencies of PM10
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and PM2.5 are lower than that of the FF of #1. The submicron particles are mainly removed by the
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dust cake on the surface of FF, and the resistance of the dust cake could account for 80~90% of the
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FF operating resistance.37 When the resistance of FF reaches a threshold, the cleaning operation is
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performed and the removal efficiency of dust temporarily decreases. The ESP usually uses
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mechanical vibration to remove the dust on the anode plate, causing the secondary dust which
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usually results in a temporary increase of mass concentration of the dust at the precipitator
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outlet.38
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3.2.2 WFGD (wet flue gas desulfurization) and WESP (wet electrostatic
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precipitator)
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WFGD not only effectively reduces SO2 but also affects the concentration of PM. As shown
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in Figure 6, the removal efficiencies of PM10, PM2.5, and PM1 of #1 WFGD are between 17.5%
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and 31.4%. The coarse particles are easier to be removed by WFGD. The circulating slurry
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droplets and ash particles with small size are difficult to be removed by the cleaning process of the
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circulating slurry and the demister.39 It is beneficial to install particle agglomerator between the
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precipitator outlet (also induced draft fan outlet) and the desulfurization scrubber inlet to promote
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the growth of particles, and then remove the coarse particles through the scrubber, thereby
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reducing the burden of ESP and WESP.40 The #3 WFGD scrubber adopted the structure of 4 layers
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sprayer with 3 stages demister, and the #1 WFGD scrubber is equipped with 4 layers sprayer with
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2 stages demister. After the flue gas passed through the #3 WFGD scrubber, the PM1 increased by
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58.7% in mass, while the #1 WFGD scrubber reduced the PM1 mass concentration. The removal
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efficiency of the WFGD scrubber on fine PM, especially PM1, is not only related to the
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performance of the demister but also the PM mass concentration at the scrubber inlet. The PM1
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mass concentration at #3 scrubber inlet was 0.97 mg/Nm3 while that at #1 scrubber inlet was 4
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mg/Nm3. When the PM mass concentration at the scrubber inlet is low, the entrainment of the
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micron-size circulating slurry droplets or micron-size insoluble particles in the slurry by flue gas
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could offset the PM removal performance of the scrubber, resulting in the increase of PM
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concentration.41 #3: PM10
52.47
#2: PM10 #1: PM10
62.77
31.43
#3: PM2.5
60.86
29.43
13.41
75.68
62.27
#2: PM2.5
60.76
60.76
#1: PM2.5
21.13
#3: PM1 -58.70 #2: PM1
50.71
29.58
88.60
29.9 56.25
56.25
#1: PM1 0
46.75
29.25
17.50
-60 -40
82.59
30.12
62.77
20
40
60
80
PM removal efficiency %
284 285
WFGD+WESP
WESP
WFGD
Figure 6. The removal efficiency of PM by WFGD and WESP.
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The #1 WESP can effectively remove PM10, PM2.5, PM1, and the removal efficiency varies in
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the range of 29.25% to 29.58% (relative to the PM concentration at the WFGD inlet). The removal
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efficiency of PM10, PM2.5, and PM1 by #3 WESP was higher, which is 30.12%, 62.27%, and 88.60%
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respectively. The PM removal performance is related to the operating voltage (also secondary DC
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voltage) of WESP. The operating voltage of #1 WESP is 23 kV while that for #3 WESP is 45 kV.
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The PM is easier to be charged and captured when the operating voltage is higher.42 Since the
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outlet of #2 WFGD has no measuring point, we only obtained the combined PM removal
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efficiency of WFGD coupled with WESP which is 56.25~62.77%.
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3.2.3 PM2.5/PM10 and PM1/PM10 ratios
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The PM2.5/PM10, PM1/PM10 ratios along the APCDs are presented in Figure 7. When the flue
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gas passes through the precipitator (FF or ESP), WFGD and WESP, the proportion of PM2.5 and
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PM1 in PM10 increase (except PM1/PM10 at the #3 WFGD outlet). It is illustrated that the coarse
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particles are much easier to be removed by APCDs than fine particles, resulting in an increasing
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proportion of fine particles. PM1/PM10 at the chimney inle of #1, #2 and #3 t are 51.1%, 50% and
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33.3% respectively, PM2.5/PM10 at the chimney inlet of #1, #2 and #3 are 85.2%, 88.6% and
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52.5% respectively. The final PM10 emissions from industrial boilers are mainly PM2.5.43 These
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particles are tend to enrich toxic and harmful substances and belongs to inhalable PM which
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should be further controlled in future.44
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100 Ratio of PM1/PM10 or PM2.5/PM10 %
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60
40
20
0
304 305 306
#a
#d
#c
#b
#1: PM1/PM10
#2: PM1/PM10
#3: PM1/PM10
#1: PM2.5/PM10
#2: PM2.5/PM10
#3: PM2.5/PM10
Figure 7. PM1/PM10 and PM2.5/PM10 along the APCDs.
3.2.4 EF (emission factor)
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EF is an important indicator for evaluating and quantifying the PM emission from coal
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combustion. It is related to the type of furnace, fuel properties, boiler capacity, operating load and
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the APCDs. The EF in this study is calculated based on the fuel mass according to equation (1).
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311 312
EF
C
PM
V flue gas
10
6
mcoal
(1)
CPM is the PM mass concentration of the flue gas at chimney inlet, mg/Nm3. Vflue gas is the volume flow rate of the flue gas, Nm3/h. mcoal is the coal feeding rate of the boiler, t/h.
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According to Table 5, the EF of PM10, PM2.5 and PM1 for #1 and #2 before APCDs is in the
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range of 1.168~19.264 kg/t, 0.934 ~9.112 kg/t, 0.320~1.704 kg/t respectively. CFB generates
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significantly much more PM than CGB. With the purification of APCDs, the EF of PM10, PM2.5
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and PM1 reduce to 0.028~0.033 kg/t, 0.025~0.028 kg/t and 0.014~0.017 kg/t. The difference of EF
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between #1 and #2 can be ignored after the purification.
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Table 5. EF of PM10, PM2.5, and PM1 before and after APCDs (kg/t) Unit
Before APCDs
After APCDs
PM10
PM2.5
PM1
PM10
PM2.5
PM1
#1
19.264±1.512
9.112±0.019
1.704±0.259
0.033±0.015
0.028±0.010
0.017±0.009
#2
1.168±0.196
0.934±0.232
0.320±0.073
0.028±0.012
0.025±0.009
0.014±0.005
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Figure 8 compared the EF of PM2.5 from the literature22-24, 45-47 with the EF from our test. The
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APCDs from literatures in Figure 8 are backward and the difference of EF is significant. After
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ultra-low emission retrofit, the EF of PM2.5 from our test is lower than that of most coal-fired
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industrial boilers from the literature. The pollutant removal technology applied in #1 and #2 unit
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of our study is the most advanced among the coal-fired industry boilers in current China, including
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SNCR/SCR+FF+WFGD+WESP. Zhang et al.25 test PM2.5 from CFB equipped with ESP and FF
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but without WFGD or WESP. The EF of PM2.5 varies from 6.58 mg/Nm3 to 29.17 mg/Nm3 with
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the average is 14.12 mg/Nm3. It is significantly higher than that in the unit we test which is
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3.1~3.5 mg/Nm3. Considering the contribution to reduce air pollutants, the pollutants emission
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characteristic of ultra-low emission coal-fired industrial boiler should be systematically studied.
1
PM2.5 Emission factor kg/t
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6 7
15
21 20 23
13 11 9 8 2,4,5
24 14
19
1
0.1
10 3
12
16,22 18 25 26
17
0.01
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