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Field measurements on the emission and removal of PM2.5 from coal-fired power stations: 2. studies on two 135 MW CFB boilers respectively equipped with electrostatic precipitator and hybrid electrostatic filter precipitator Xiaowei Liu, Yishu Xu, Bin Fan, Chen Lv, Minghou Xu, Siwei Pan, Kai Zhang, Li Li, and Xiangpeng Gao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00424 • Publication Date (Web): 11 May 2016 Downloaded from http://pubs.acs.org on May 16, 2016
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Field measurements on the emission and removal of PM2.5 from coal-fired power stations: 2. studies on two 135 MW CFB boilers respectively equipped with electrostatic precipitator and hybrid electrostatic filter precipitator Xiaowei Liu a, Yishu Xu a, Bin Fan a, Chen Lv a Minghou Xu a *, Siwei Pan b, Kai Zhang b, Li Li b, Xiangpeng Gao c a
State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Luoyu Road 1037, Wuhan 430074, China.
b
Electric Power Research Institute of Guangdong Power Grid Corporation, Guangzhou 510080, China c
Discipline of Electrical Engineering, Energy and Physics, School of Engineering and Information Technology, Murdoch University, 90 South Street, Murdoch, WA 6150, Australia.
*Corresponding Author. E-mail:
[email protected]; Tel: +86-27-87546631; Fax: +86-27-87545526
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ABSTRACT: This study reports the formation and emission characteristics of particulate matter (PM) with aerodynamic diameters less than 2.5 µm (PM2.5) from two coal-fired circulating fluidized bed (CFB) boilers (B1 and B2) respectively equipped with electrostatic precipitator (ESP) and hybrid electrostatic filter precipitator (EFP). PM and total fly ash samples are collected at the inlets and outlets of the ESP and the EFP via a low pressure impactor, a gravimetric impactor and/or a smoke analyser, and further characterized with X-ray fluorescence probe as well as field emission scanning electron microscopy with energy disperse X-ray analyzer. Results show that PM2.5 generated from the two CFB boilers are of uni-modal mass size distributions with the only size peak in the coarse mode. Different from PMs formed in pulverized coal-fired boilers, coarse mode PMs generated in CFB boilers are primarily irregular in shape and some small particles are observed to adhere on large ones due to the lower combustion temperature and the more vigorous collision and abrasion in CFB combustion. More PM2.5 are produced from CFB combustion as a result of enhanced fragmentation of minerals in coal and sorbent limestone compared with the conventional pulverized coal combustion. Yield of ultrafine PM is inhibited in CFB combustion due to the lower combustion temperature and fixation of volatile species by sorbent limestone, which lead to the absence of ultrafine modal peak in PM2.5. Downstream the boilers, most of PMs are removed in dust collectors with PM2.5 removal efficiencies of 98.90% and 99.96% for the ESP and the EFP respectively. And a significant difference appears in the size range of 0.1–2 µm, where the PM removal efficiency of the ESP is ~1.5% lower than that of the EFP.
KEYWORDS: Coal combustion; PM2.5; Circulating fluidized bed; Electrostatic precipitator; Hybrid electrostatic filter precipitator.
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1. Introduction PM2.5 [particulate matter (PM) with aerodynamic diameter of 10) were separated in the cyclone upstream the LPI. And PM10 (PMs less than 10 µm) were fractionated into 13 stages according particle size and collected on the aluminum foils in the LPI. Mass of the PMs was obtained by weighing the aluminum foils before and after the experiments with a microbalance (Sartorius MSA6.6S-0CE-DF). In the LPI-H sampling system, all of cyclone, LPI and the connecting pipes between them were heated to 130 °C to avoid the condensation of H2O and SOx. And in the LPI-D sampling system, a high dilution ratio of 64 was used to avoid the condensation of the gaseous species and the overload of LPI during the sampling. Detailed information on the two systems is described in Part I of this series. At least three parallel samplings were conducted at each condition and the average values were reported. DGI sampling system was implemented to collect enough PM samples for the morphology and composition analysis
14
. The DGI sampling system consists of a cyclone (cyclone-10), a high flow
Dekati gravimetric impactor (DGI), a dilution gas system and a vacuum pump. Different from the LPI sampling system, PM into DGI was fractionated into 5 stages (0.2–0.5 µm, 0.5–1 µm, 1–2.5 µm and 2.5–10 µm). After PM>10 were separated in the cyclone, a stream of 60 L/min dilution gas was added downstream of cyclone to make up a total stream of 70 L/min into the DGI. The sampling probe, cyclone and the pipes connecting them were also heated to 130 °C during the sampling. Total fly ash samples before the dust collectors were also collected through a smoke analyser (Laoying 3012H smoke/gas analyser) 16. During the fly ash sampling, flue gas was firstly isokinetically extracted into the sampling probe at the center of duct section and then the fly ash was collected by a 6 ACS Paragon Plus Environment
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glass fiber cartridge filter embedded in the probe. Both of the fractionated PMs and total fly ash samples were dried at 45 °C and saved for further analysis. Selected PM samples are observed under field emission scanning electron microscopy with energy disperse X-ray analyzer (FESEM-EDX, FEI Inc.) to obtain the particle morphology and mineral composition. Chemical composition of fractionated PM, total fly ash as well as the low temperature ash (LTA) were analyzed with the X-ray fluorescence (XRF, EDAX Inc.) probe.
3. Results and discussion 3.1. Particle size distributions (PSDs) of PMs emitted from the CFB boilers Fig. 2a depicts the particle size distributions (PSDs) of the PMs with aerodynamic diameter less than 10 µm (PM10) emitted from the CFB boilers (sampled at the inlet of dust collectors). Ash-based PSDs are also calculated and illustrated in Fig. 2c. Different from the PSDs of PMs from pulverized coal combustion, PSDs of the PM10 emitted from the two CFB boilers follow a uni-modal distribution, with only coarse mode peaked at ~2.5 µm (for B1) and ~5 µm (for B2). Most importantly, no obvious ultrafine mode peak were observed in the mass size distribution of PM2.5 from the CFB boilers. As summarized in Fig.3, the results above confirmed the uni-modal mass size distribution of the PMs from CFB boilers reported by Lind et al. 11 and Zhao et al. 13, 25. In comparison, the PSDs of PMs from pulverized coal combustion exhibit bimodal size distribution, with an additional ultrafine mode generally peak at ~0.1 µm (see Fig. 3). As reported, coarse modal PMs are mainly generated from the fragmentation and/or coalescence minerals in coal while the ultrafine modal PMs are mainly formed via the vaporization and nucleation of the mineral matter
2, 17-23
. The absence of ultrafine mode peak
implies a different PM2.5 formation process during the coal combustion in the CFB boilers. Morphology of PMs in size range of 0.5–1 µm (PM0.5-1), 0.1–0.2 µm (PM0.1–0.2) and PMs in 2.5– 10 µm (PM2.5–10) from boiler B2 are shown in Fig. 4 in comparison with the typical morphologies of the PMs from pulverized coal combustion (a 300 MW boiler in our unpublished studies). This sheds 7 ACS Paragon Plus Environment
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some light onto the PM formation mechanisms. As shown in Figs. 4a-4b, coarse modal PMs (PM0.5–1) from the CFB boiler are primarily in irregular shapes, which agreed well the findings of Zhao et al. 13 while is different from the common spherical particles in the coarse PMs generated from the pulverized coal boiler
2, 19, 24
. As well known, combustion temperatures in CFB boilers are typically in a range of
850–950 °C, which are 400–600 °C lower than those in pulverized-coal boilers
6, 7
. Under such
relatively low combustion temperatures, a majority of minerals in coal or their fragments are not molten during combustion and remain irregular in shape. However, ultrafine modal PMs (PM0.1–0.2) generated from the CFB boiler (B2) are in similar shape with those generated from the pulverized coal boiler. As can be seen in Figs. 4c-4d, PM0.1–0.2 produced in CFB boiler B2 are also spherical particles, confirming that ultrafine PMs in the CFB combustion are formed via the vaporization and nucleation of the mineral matter in coal, similar to that in the pulverized coal combustion 2, 17. The absence of ultrafine modal peak in the PMs from CFB combustion is supposed to be caused by the distinctive combustion conditions in the CFB boilers and this will be further discussed combining with the chemical composition in Section 3.3 below. 3.2. Yield of PMs emitted from the CFB boilers For comparison, the mass concentrations and the ash-based yields of PMs with aerodynamic diameters of
