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Frontiers in Energy

ISSN 2095-1701

ISSN 2095-1698(Online)

CN 11-6017/TK

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Front. Energy    2021, Vol. 15 Issue (2) : 449-459    https://doi.org/10.1007/s11708-021-0734-3
RESEARCH ARTICLE
Compositional and structural study of ash deposits spatially distributed in superheaters of a large biomass-fired CFB boiler
Yishu XU1, Xiaowei LIU2(), Jiuxin QI2, Tianpeng ZHANG2, Minghou XU2, Fangfang FEI3, Dingqing LI3
1. State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China; School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
2. State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China
3. Guangdong Yudean Zhanjiang Biomass Power Plant, Zhanjiang 524300, China
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Abstract

Recognizing the nature and formation progress of the ash deposits is essential to resolve the deposition problem hindering the wide application of large-scale biomass-fired boilers. Therefore, the ash deposits in the superheaters of a 220 t/h biomass-fired CFB boiler were studied, including the platen (PS), the high-temperature (HTS), the upper and the lower low-temperature superheaters (LTS). The results showed that the deposits in the PSs and HTSs were thin (several millimeters) and compact, consisting of a yellow outer layer and snow-white inner layer near the tube surface. The deposits in the upper LTS appeared to be toughly sintered ceramic, while those in the lower LTS were composed of dispersive coarse ash particles with an unsintered surface. Detailed characterization of the cross-section and the initial layers in the deposits revealed that the dominating compositions in both the PSs and the HTSs were Cl and K (approximately 70%) in the form of KCl. Interestingly, the cross-section of the deposition in the upper LTS exhibited a unique lamellar structure with a major composition of Ca and S. The contents of Ca and Si increased from approximately 10% to approximately 60% in the deposits from the high temperature surfaces to the low temperature ones. It was concluded that the vaporized mineral matter such as KCl played the most important role in the deposition progress in the PS and the HTS. In addition, although the condensation of KCl in the LTSs also happened, the deposition of ash particles played a more important role.
Keywords ash deposition      biomass combustion      circulating fluidized bed      initial layer      structure analysis     
Corresponding Author(s): Xiaowei LIU   
Online First Date: 01 April 2021    Issue Date: 18 June 2021
 Cite this article:   
Yishu XU,Xiaowei LIU,Jiuxin QI, et al. Compositional and structural study of ash deposits spatially distributed in superheaters of a large biomass-fired CFB boiler[J]. Front. Energy, 2021, 15(2): 449-459.
 URL:  
https://academic.hep.com.cn/fie/EN/10.1007/s11708-021-0734-3
https://academic.hep.com.cn/fie/EN/Y2021/V15/I2/449
Fig.1  Sketch of experimental setup.
Items B1a) B2 B3 B4 B5
Proximate analysis/% Moisture 10.83±0.06 12.18±0.09 9.74±0.01 8.28±0.27 7.60±0.05
Volatile 70.80±1.10 55.52±1.71 65.93±1.05 72.36±0.51 69.15±0.66
Fixed carbon 14.74±1.04 12.33±1.07 15.52±1.18 17.89±1.32 15.45±0.63
Ash 3.63±0.00 19.97±0.73 8.81±0.13 1.47±1.08 7.80±0.08
Ultimate analysis/% C 44.73±0.16 37.69±0.01 41.58±0.57 48.58±0.08 42.94±0.66
H 5.88±0.05 4.84±0.08 5.23±0.00 6.13±0.04 5.75±0.04
N 0.25±0.00 0.39±0.01 0.53±0.01 0.52±0.01 1.02±0.01
S 0.03±0.02 0.03±0.01 0.03±0.01 0.05±0.02 0.08±0.03
Ob) 34.66±0.18 24.90±1.06 34.07±0.58 34.97±1.12 34.81±0.67
IC analysis S/(mg·g–1) 1.53±0.18 0.94±0.18 1.46±0.30 3.06±0.07 5.16±0.17
Cl/(mg·g–1) 0.88±0.22 0.39±0.08 0.70±0.15 0.30±0.13 0.55±0.10
Na/(mg·g–1) 0.10±0.03 0.70±0.04 0.91±0.01 0.05±0.02 0.06±0.03
K/(mg·g–1) 2.54±0.03 5.70±0.49 6.11±0.19 2.14±0.03 4.88±0.01
Ca/(mg·g–1) 0.45±0.18 12.15±1.96 19.23±0.82 0.00±0.00 1.45±0.50
Mg/(mg·g–1) 0.00±0.00 0.16±0.01 1.19±0.06 0.00±0.00 0.13±0.07
Fe/(mg·g–1) 0.84±0.07 3.00±0.45 0.57±0.14 0.15±0.05 1.00±0.01
Tab.1  Proximate and ultimate analysis of biomass (air-dried basis)
Biomass Na2O MgO Al2O3 SiO2 P2O5 SO3 Cl2O K2O CaO Fe2O3
B1 3.87±0.42 5.34±1.09 13.26±3.09 19.04±3.03 5.56±1.44 4.62±0.98 2.27±0.79 15.40±1.38 26.20±4.13 4.44±1.50
B2 2.76±1.10 1.96±0.71 10.09±2.01 56.29±5.14 2.96±0.78 0.78±0.34 2.60±0.74 4.97±1.15 15.63±3.61 1.95±0.76
B3 4.47±1.29 5.48±2.15 3.98±0.88 8.31±2.31 7.05±0.82 1.72±0.05 3.14±0.53 11.80±0.66 52.68±5.86 1.37±0.12
B4 1.83±0.69 7.65±1.01 3.35±0.25 10.05±3.37 5.33±0.37 12.08±0.64 2.51±1.16 21.14±2.57 33.70±2.74 2.36±0.56
B5 0.75±0.35 2.71±0.62 3.85±0.73 55.44±1.94 4.65±1.01 7.57±1.06 1.09±0.15 10.31±0.16 11.82±0.77 1.80±0.27
Tab.2  Ash analysis of the low temperature ash of the biomass
Fig.2  Morphology of ash deposits in PSs, HTSs, cyclone outlet, the upper and lower LTSs, and economizer (photographed in the direction of the flue gas).
Fig.3  Layered structure of ash deposits (Noting that the blue zone covers the resin background.)
Fig.4  XRD patterns of ash deposits at different superheaters and bulk fly ash.
Fig.5  Micromorphology and element distribution of inner layer of ash deposits (Red line indicates the line scanning route.)
Fig.6  Micromorphology and element distribution of cross section of ash deposits in upper LTSs.
Fig.7  Micromorphology and element distribution of cross section of ash deposits in lower LTSs.
Fig.8  Micromorphology and element distribution of inner layer of ash deposits in upper LTSs (Red line indicates the line scanning route.)
Fig.9  Spatially resolved deposit composition in PSs, HTSs, cyclone outlet, upper and lower LTSs, economizer, and flyash.
Fig.10  Variation of major crystal mineral components in ash deposits in different superheaters and bulk ash.
CFB Circulating fluidized bed
PS Platen superheater
HTS High-temperature superheater
LTS Low-temperature superheater
EPMA Electron probe microanalyzer
IC Ion chromatography
XRF X-ray fluorescence
XRD X-ray diffraction
  
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