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Investigations on the ash deposit formation of tubular air preheater in a coal-fired traveling grate boiler Xiaolu Chen, Qinxin Zhao, and Zhiyuan Liang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01964 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 8, 2017
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Investigations on the ash deposit formation of tubular air preheater in a coal-fired traveling grate boiler Xiaolu Chen, Qinxin Zhao*, Zhiyuan Liang (Key Laboratory of Thermo-Fluid Science and Engineering of MOE, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China) ABSTRACT A study was undertaken of the air preheater ash deposit in a 140-MW coal-fired traveling-grate boiler. Several ash samples were collected in the selected regions of the air preheater along with samples of the feed coal. The ash samples were examined using combination of ash X-ray fluorescence (XRF) and X-ray diffraction (XRD) analysis to determine the chemistry and mineralogy. The ash samples were also examined using scanning electron microscopy and energy dispersive spectrum (SEM-EDS) analysis to determine the microstructure and phase constitution. The analysis results showed that the sulfur content of ash deposits varied with the temperature of gas flue. The higher amounts of sulfur in ash deposits proved to be sulfates. It appeared that sulfuric acid was a driving factor in the formation of the deposition. The sulfates probably acted as cementing agent, giving the deposit its hardness and thickness. 1. Introduction Several researchers have studied ash deposits on boiler components and associated devices in coal-fired power plants [1-3]. However, fewer investigations were on the ash deposit in traveling-grate boilers. Traveling-grate boilers have been widely
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used in the combustion system for solid fuels in China. There are about 480,000 coal-fired industrial boilers in China, of which 95% are traveling-grate boilers
[4]
.
Although traveling-grate boilers have been used and tested over many years, there are still many problems, including low combustion efficiency and high pollutants emissions. Reducing the temperature of flue gas is an effective way to solve the problems. The air preheater is supposed to recover heat from high-temperature flue gas, which will enhance the thermal efficiency and reduce the temperature of flue gas. Trace SO3 is formed from SO2 during combustion and along the flue gas path. Gaseous sulfuric acid (H2SO4) will form from the reaction of sulfur trioxide (SO3) and water vapor when the flue gas is cooled down. When the temperature of flue gas falls below the H2SO4 dew point, gaseous H2SO4 starts to form an acid mist or condense on cold surfaces, resulting in low-temperature corrosion (LTC) or ash deposit [5]. A detailed mineralogical and chemical analysis was undertaken to understand the mechanism leading to the formation of ash deposition. In the study, some contributing factors have been examined for ash deposition such as sulfur content, black carbon from coal burning, and flue gas temperatures below the acid dew point [6]. The results will help to avoid blockages in the heat exchange elements of coal-fired traveling grate boilers. 2. Sampling and analytical procedures Two days after boiler shutdown, several ash samples were collected from an tube air preheater in a 140MW traveling grate boiler in Western China. The boiler has the largest installed capacity of existing heating boilers. A selective non-catalytic
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reduction (SNCR) denitrification system has been installed in this boiler for several months. The boiler has been in operation for about 2300 hours, and the excess air ratio (α) is 1.2. The district heating plant employs a tube preheater wherein hot flue gas flows through the tube, while cool, clean combustion air passes outside the tube. The structure of the boiler, the location and structure of air preheater are shown in Fig.1(a, b, c). Flue gas temperature of the air preheater inlet is about 140℃, while the temperature of preheater outlet varies from 90℃ to 120℃. The cold air is heated from about 10℃ to 30℃, and the wall temperature of the tubes can be as low as 50℃ in the air preheater. Half of the preheater tubes have been blocked by ash deposits. Hardened ash deposits were observed on the top cold side due to the low flue gas temperature, possibly as shown in Fig.1(d).
(a)
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(b)
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(c)
(d)
(e)
Fig.1 The structure of the boiler(a), the location(b) and structure(c) of an air preheater, photographs of hardened ash (d) and the layering in ash deposits (e) of the cold end
Five ash samples of the air preheater deposit were collected for analysis, which the corresponding regions of sample collection shown in Table 1. And the exact locations of ash samples are shown in Fig.2. Images of the ash samples were shown in Fig.3. The main flue gas parameters of the air preheater are listed in table 2.
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Fig.2 Locations of deposit samples Table 1 Locations and descriptions of deposit samples provided for analysis Sample location
Sample description
Sample id
Hot end, the inlet flue duct
Loose ash Deposit in the corner of inlet flue duct, the ash sample is black. Watery ash Deposit filling in the blocked tubes, the ash sample is black. Watery ash bottom Deposit located at the cold end of blocked tubes (as shown in fig.1d), under the hardened ash, the ash sample is grey green. Hardened ash above Deposit located at the cold end of blocked tubes, the outmost layer of the air preheater deposition, the ash sample is grey. Loose ash Deposit on the wall of air preheater exit, the ash sample is black.
A
Blocked tubes, in the blocked tubes Cold end, outlet of the blocked tubes Cold end, outlet of the blocked tubes Cold end, the exit flue duct
Sample A
Sample B
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B
C
D
E
Sample C
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Sample D
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Sample E Fig.3 Images of the ash samples
Table 2 Main flue gas parameters of the air preheater Parameters
Unit
Minimum
Maximum
Average
O2 SO2 NOx H2O Fly ash concentration
% mg/m3 mg/m3 % g/m3
5.1 1446.7 147.2 6.3 7.8
7.9 2133.3 171.4 8.5 11.7
6.5 1723.3 161.5 7.9 10.5
All of the ash samples were ground in an agate mortar and pestle to fine powders, preparing for XRF, XRD, and SEM-EDS analysis. Major element analysis of the ash samples was carried out by a Bruker SP PIONEER X-ray fluorescence instrument. The instrument was operated at 60kV and 150mA using Ru target and 4kW power, employing Be excitation source. The infrared spectroscopy of the synthesized samples was recorded using a Bruker Vertex 70 infrared spectrometer. The X-ray diffraction experiments were performed on a PANalytical X’pert MPD Pro employing Nifiltered Cu Kα radiation (λ=0.15406 nm). The X-ray tube was operated at 40 kV and 40 mA. The X-ray powder diffractogram was recorded at a scan rate of 2°/min in the range of 20°
