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Ash formation and fouling during combustion of rice husk and its blends with a high alkali Xinjiang coal Jianqun Wu, Dunxi Yu, Xianpeng Zeng, Xin Yu, Jingkun Han, Chang Wen, and Ge Yu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02298 • Publication Date (Web): 17 Dec 2017 Downloaded from http://pubs.acs.org on December 18, 2017
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Ash formation and fouling during combustion of rice husk and its blends with a high alkali Xinjiang coal Jianqun Wu, Dunxi Yu*, Xianpeng Zeng, Xin Yu, Jingkun Han, Chang Wen and Ge Yu State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China
*Corresponding Author. E-mail:
[email protected]; Tel: +86-27-87545526; Fax: +86-27-87545526 Postal address: State Key Laboratory of Coal Combustion, Huazhong University of Science & Technology, 1037 Luoyu Road, Wuhan 430074, PR China.
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Abstract Limited data from fluidized bed combustion tests have shown that rice husk, a silicon-rich residual biomass, has the potential to be co-fired with coal while not inducing unacceptable ash-related problems. However, there is great concern regarding the behavior of rice husk ash under pulverized fuel combustion conditions, where the temperatures are much higher and expected to facilitate fuel interactions. This work, to the authors’ knowledge, is the first to investigate both ash formation and fouling behavior in rice husk firing and its co-firing with coal at a high temperature relevant to pulverized fuel combustion. A Chinese rice husk and a high alkali Xinjiang coal were selected. Combustion tests of individual fuels and their blends (with the share of rice husk being 10% and 20%, respectively) were performed at 1573 K on a laboratory drop tube furnace. Both bulk ash and fouling deposit samples were collected in each test. Techniques including X-ray diffraction (XRD), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS) and ash fusion temperature test were used for sample characterization. The results show that the rice husk ash generated at 1573 K is dominated by amorphous silica with only a trace amount of quartz. Partial melting of ash particles is observed and attributed to enhanced formation of potassium silicates, which were not found in fluidized bed combustion. Nevertheless, such differences do not seem to change the nonfouling nature of rice husk ash. The ash from co-firing rice husk and coal possesses similar crystalline structures to the coal ash, but extensive fuel interactions are observed. The high fouling tendency of the coal ash is greatly reduced by co-firing with rice husk. The effects of rice husk are both physical and chemical in nature. The presence of high-fusion-temperature, non-sticky rice husk ash in the deposits and the capture of fouling-inducing species from coal by rich husk ash are considered to play important roles in reducing ash fouling. The non-linear dependence of ash fouling tendency on the rice husk ratio, as found in fluidized bed combustion, is also observed. This work suggests that rice husk may also be co-fired with coal in pulverized fuel boilers while not causing significant ash-related problems, though further investigations are still necessary. Keywords Xinjiang coal, rice husk, co-firing, ash formation, ash fouling 2 ACS Paragon Plus Environment
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Introduction Concerns over the health and environmental effects of coal utilization1-5 have stimulated the use of biomass, the third largest and renewable energy resource in the world6,7. Co-firing biomass (a carbon-neutral renewable energy source) and coal has been identified as a promising technology to use bioenergy at large scales, which also has great potential in reducing SO2, NOx, CO2 and trace element emissions associated with coal utilization.8 However, despite its environmental benefits, co-firing biomass and coal is still facing a number of technical challenges such as fuel supply, handling, ash corrosion and deposition issues.9 Among them, ash deposition (including ash formation, fouling and slagging), mainly resulting from high alkali/alkaline earth metals (AAEMs) in biomass and their interactions with coal ash components, can significantly affect energy conversion, boiler performance and safety operation. Therefore, it has long been a primary concern and a subject of extensive research. Rice husk is an important residual biomass generated from rice milling industry. The world rice production in 2014 is approximately 740 million tons, while in China this value reaches about 200 million tons.10 If assuming that rice husk accounts for about 20 wt% of the rice grain on average11, its production can be tremendous. Rice husk contains energy comparable to lignite11, and has been recognized as a key renewable energy source especially in large rice producing countries like China. However, although the utilization of rice husk in small-scale boilers has a long history, it was not until the late 20th century that rice husk became an attractive fuel for large-scale combustors12. While there have been a number of studies on combustion characteristics and pollutant emissions in firing or co-firing rice husk13-15, very little work is available on ash-related issues especially at high temperatures (e.g. pulverized fuel combustion). Quite different from other herbaceous biomass fuels, rice husk has a high ash content (13-29%) and a very high silica content (87-97%) of the ash.16 In contrast, the contents of AAEMs can be much lower.17 These unique features are expected to clearly distinguish rice husk from other biomass fuels in ash behavior. However, this was not purposely investigated until 2005, when Skrifvars et al. published a series of two impressive papers18,19 on the fouling behavior of rice husk in fluidized-bed combustion. The study of fuel characteristics18 showed that, when ashed at 500 and 900 °C in air, rice husk produced large ash particles 3 ACS Paragon Plus Environment
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with almost identical structures to the fuel particles. The ash consisted almost purely of silica and no potassium silicates were observed, suggesting little interactions between potassium and silica during combustion. The initial deformation temperature of the rice husk ash was as high as 1480 °C. Consequently, no ash melting was expected under fluidized-bed combustion conditions (~850 °C). The Multi Fuel Fouling index suggested that rice husk was a non-fouling fuel. These were consistent with the subsequent work performed on an entrained-flow type of pilot furnace and a large-scale (157 MWth) bubbling fluidized-bed boiler19. The data collected from both facilities showed that no melting or deformation of rice husk ash could be observed, and the combustion of rice husk alone did not lead to any detectable fouling. When eucalyptus bark (a fouling biomass) was co-fired with rice husk, there were no clear chemical interactions between ashes from two fuels. However, a physical effect (“sand blasting”) caused by rice husk particles was observed and seemed to be able to alleviate fouling of eucalyptus bark ash. The non-fouling nature of rice husk ash was also observed in tests performed at a commercial 370 MWth CFB boiler20. These studies suggest that rice husk may be a good candidate for co-firing with fouling biomass fuels or low-rank coals. It can be noted that the above-mentioned work was primarily concentrated on the behavior of rice husk ash under fluidized-bed combustion conditions (1723
HT
1395
1425
1453
>1723
FT
1413
1567
1615
>1723
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List of Figure Captions Figure 1 DTF and deposition experimental system. Figure 2 Mineralogy of RH bulk ashes detected by XRD (Q: quartz). Figure 3 Mineralogy of all bulk ashes detected by XRD (Q: quartz, H: hematite, G: gehlenite). Figure 4 Bulk ash particle morphology and composition characterized by SEM/EDS (Composition data are reported as weight percent wt%). Figure 5 Rice husk particle morphology and composition characterized by SEM/EDS (A and C: Outer layer; B: Inner layer; Composition data are reported as weight percent wt%). Figure 6 Ash deposition propensities for different fuel combustion cases. Figure 7 Photographs of deposits on coupon for different fuel combustion cases. Figure 8 Deposit chemical composition for different fuel combustion cases.
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Figure 1 DTF and deposition experimental system.
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Figure 2 Mineralogy of RH bulk ashes detected by XRD (Q: quartz).
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Figure 3 Mineralogy of all bulk ashes detected by XRD (Q: quartz, H: hematite, G: gehlenite).
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(a) RH
(b) XJ
(c) XJ-10-RH
(d) XJ-20-RH Figure 4 Bulk ash particle morphology and composition characterized by SEM/EDS (Composition data are reported as weight percent wt%). 29 ACS Paragon Plus Environment
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Figure 5 Rice husk particle morphology and composition characterized by SEM/EDS (A and C: Outer layer; B: Inner layer; Composition data are reported as weight percent wt%).
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Figure 6 Ash deposition propensities for different fuel combustion cases.
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Figure 7 Photographs of deposits on coupon for different fuel combustion cases.
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Figure 8 Deposit chemical composition for different fuel combustion cases.
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Figure 1 DTF and deposition experimental system. 208x253mm (300 x 300 DPI)
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Figure 2 Mineralogy of RH bulk ashes detected by XRD (Q: quartz). 59x44mm (300 x 300 DPI)
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Figure 3 Mineralogy of all bulk ashes detected by XRD (Q: quartz, H: hematite, G: gehlenite). 65x53mm (300 x 300 DPI)
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Figure 4(a) Bulk ash particle morphology and composition characterized by SEM/EDS (Composition data are reported as weight percent wt%). 46x27mm (300 x 300 DPI)
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Figure 4(b) Bulk ash particle morphology and composition characterized by SEM/EDS (Composition data are reported as weight percent wt%). 45x26mm (300 x 300 DPI)
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Figure 4(c) Bulk ash particle morphology and composition characterized by SEM/EDS (Composition data are reported as weight percent wt%). 45x26mm (300 x 300 DPI)
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Figure 4(d) Bulk ash particle morphology and composition characterized by SEM/EDS (Composition data are reported as weight percent wt%). 45x26mm (300 x 300 DPI)
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Figure 5 Rice husk particle morphology and composition characterized by SEM/EDS (A and C: Outer layer; B: Inner layer; Composition data are reported as weight percent wt%). 160x100mm (300 x 300 DPI)
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Figure 6 Ash deposition propensities for different fuel combustion cases. 80x58mm (300 x 300 DPI)
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Figure 7 Photographs of deposits on coupon for different fuel combustion cases. 53x28mm (300 x 300 DPI)
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Figure 8 Deposit chemical composition for different fuel combustion cases. 28x20mm (300 x 300 DPI)
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