Compositional and structural study of ash deposits spatially distributed in superheaters of a large biomass-fired CFB boiler

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.

Key words： ash deposition; biomass combustion; circulating fluidized bed; initial layer; structure analysis

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant Nos. 51806075 and 51922045), and the Analytical and Testing Center of Huazhong University of Science and Technology.

Electronic Supplementary Material

Supplementary material is available in the online version of this article at https://doi.org/10.1007/s11708-021-0734-3 and is accessible for authorized users.

References

1 Demirbas A. Potential applications of renewable energy sources, biomass combustion problems in boiler power systems and combustion related environmental issues. Progress in Energy and Combustion Science, 2005, 31(2): 171–192

2 Saidur R, Abdelaziz E A, Demirbas A, A review on biomass as a fuel for boilers. Renewable & Sustainable Energy Reviews, 2011, 15(5): 2262–2289

3 Alam M T, Dai B, Wu X, A critical review of ash slagging mechanisms and viscosity measurement for low-rank coal and bio-slags. Frontiers in Energy, 2020, online, doi: 10.1007/s11708-020-0807-8

4 Khan A A, De Jong W, Jansens P J, Biomass combustion in fluidized bed boilers: potential problems and remedies. Fuel Processing Technology, 2009, 90(1): 21–50

5 Capablo J. Formation of alkali salt deposits in biomass combustion. Fuel Processing Technology, 2016, 153: 58–73

6 Niu Y, Tan H, Hui S. Ash-related issues during biomass combustion: alkali-induced slagging, silicate melt-induced slagging (ash fusion), agglomeration, corrosion, ash utilization, and related countermeasures. Progress in Energy and Combustion Science, 2016, 52: 1–61

7 Bryers R W. Fireside slagging, fouling, and high-temperature corrosion of heat-transfer surface due to impurities in steam-raising fuels. Progress in Energy and Combustion Science, 1996, 22(1): 29–120

8 Nielsen H P, Frandsen F J, Dam-Johansen K. Lab-scale investigations of high-temperature corrosion phenomena in straw-fired boilers. Energy & Fuels, 1999, 13(6): 1114–1121

9 Nielsen H P, Frandsen F J, Dam-Johansen K, The implications of chlorine-associated corrosion on the operation of biomass-fired boilers. Progress in Energy and Combustion Science, 2000, 26(3): 283–298

10 Nunes L, Matias J, Catalão J. Biomass combustion systems: a review on the physical and chemical properties of the ashes. Renewable & Sustainable Energy Reviews, 2016, 53: 235–242

11 Wang W, Wen C, Li C, Emission reduction of particulate matter from the combustion of biochar via thermal pre-treatment of torrefaction, slow pyrolysis or hydrothermal carbonisation and its co-combustion with pulverized coal. Fuel, 2019, 240: 278–288

12 Baxter L L, Miles T R, Miles T R Jr, The behavior of inorganic material in biomass-fired power boilers: field and laboratory experiences. Fuel Processing Technology, 1998, 54(1-3): 47–78

13 Knudsen J N, Jensen P A, Dam-Johansen K. Transformation and release to the gas phase of Cl, K, and S during combustion of annual biomass. Energy & Fuels, 2004, 18(5): 1385–1399

14 Xu Y, Liu X, Qi J, Characterization of fine particulate matter generated in a large woody biomass-firing circulating fluid bed boiler. Journal of the Energy Institute, 2021, 96: 11–18

15 Li B, Sun Z, Li Z, Post-flame gas-phase sulfation of potassium chloride. Combustion and Flame, 2013, 160(5): 959–969

16 Nielsen H P, Baxter L L, Sclippab G, Deposition of potassium salts on heat transfer surfaces in straw-fired boilers: a pilot-scale study. Fuel, 2000, 79(2): 131–139

17 Chen S, Li S, Marshall J S. Exponential scaling in early-stage agglomeration of adhesive particles in turbulence. Physical Review Fluids, 2019, 4(2): 024304

18 Niu Y, Zhu Y, Tan H, Experimental study on the coexistent dual slagging in biomass-fired furnaces: alkali- and silicate melt-induced slagging. Proceedings of the Combustion Institute, 2015, 35(2): 2405–2413

19 Miles T R, Miles T R Jr, Baxter L L, Boiler deposits from firing biomass fuels. Biomass and Bioenergy, 1996, 10(2–3): 125–138

20 Namkung H, Xu L, Lin K, Relationship between chemical components and coal ash deposition through the DTF experiments using real-time weight measurement system. Fuel Processing Technology, 2017, 158: 206–217

21 Aho M, Paakkinen K, Taipale R. Quality of deposits during grate combustion of corn stover and wood chip blends. Fuel, 2013, 104: 476–487

22 Garba M U, Ingham D B, Ma L, Prediction of potassium chloride sulfation and its effect on deposition in biomass-fired boilers. Energy & Fuels, 2012, 26(11): 6501–6508

23 Niu Y, Tan H, Wang X, Study on deposits on the surface, upstream, and downstream of bag filters in a 12 MW biomass-fired boiler. Energy & Fuels, 2010, 24(3): 2127–2132

24 Liu H, Tan H, Liu Y, Study of the layered structure of deposit in a biomass-fired boiler (case study). Energy & Fuels, 2011, 25(6): 2593–2600

25 Niu Y, Tan H, Ma L, Slagging characteristics on the superheaters of a 12 MW biomass-fired boiler. Energy & Fuels, 2010, 24(9): 5222–5227

26 Hansen L A, Nielsen H P, Frandsen F J, Influence of deposit formation on corrosion at a straw-fired boiler. Fuel Processing Technology, 2000, 64(1–3): 189–209

27 Huang Z, Deng L, Che D. Development and technical progress in large-scale circulating fluidized bed boiler in China. Frontiers in Energy, 2020 14(4): 699–714

28 Liu Z, Li J, Zhu M, Effect of oil shale semi-coke on deposit mineralogy and morphology in the flue path of a CFB burning Zhundong lignite. Frontiers in Energy, 2020, doi:10.1007/s11708-020-0668-1

29 Hupa M. Ash-related issues in fluidized-bed combustion of biomasses: recent research highlights. Energy & Fuels, 2012, 26(1): 4–14

30 Arjunwadkar A, Basu P, Acharya B. A review of some operation and maintenance issues of CFBC boilers. Applied Thermal Engineering, 2016, 102: 672–694

31 Scala F. Particle agglomeration during fluidized bed combustion: mechanisms, early detection and possible countermeasures. Fuel Processing Technology, 2018, 171: 31–38

32 Valmari T, Lind T M, Kauppinen E I, Field study on ash behavior during circulating fluidized-bed combustion of biomass. 2. Ash deposition and alkali vapor condensation. Energy & Fuels, 1999, 13(2): 390–395

33 Li L, Yu C, Huang F, Study on the deposits derived from a biomass circulating fluidized-bed boiler. Energy & Fuels, 2012, 26(9): 6008–6014

34 Sandberg J, Karlsson C, Fdhila R B A. 7-year long measurement period investigating the correlation of corrosion, deposit and fuel in a biomass fired circulated fluidized bed boiler. Applied Energy, 2011, 88(1): 99–110

35 Wei B, Tan H, Wang Y, Investigation of characteristics and formation mechanisms of deposits on different positions in full-scale boiler burning high alkali coal. Applied Thermal Engineering, 2017, 119: 449–458

36 Xu Y, Liu X, Wang H, Influences of in-furnace Kaolin addition on the formation and emission characteristics of PM_2.5 in a 1000 MW coal-fired power station. Environmental Science & Technology, 2018, 52(15): 8718–8724

37 Xu Y, Liu X, Zhang P, Role of chlorine in ultrafine particulate matter formation during the combustion of a blend of high-Cl coal and low-Cl coal. Fuel, 2016, 184: 185–191

38 Xu Y, Liu X, Zhang Y, A novel Ti-based sorbent for reducing ultrafine particulate matter formation during coal combustion. Fuel, 2017, 193: 72–80

39 Jenkins B, Baxter L L, Miles T R Jr, Combustion properties of biomass. Fuel Processing Technology, 1998, 54(1–3): 17–46

40 Wang Y, Tan H, Wang X, The condensation and thermodynamic characteristics of alkali compound vapors on wall during wheat straw combustion. Fuel, 2017, 187: 33–42

41 Christensen K A, Stenholm M, Livbjerg H. The formation of submicron aerosol particles, HCl and SO₂ in straw-fired boilers. Journal of Aerosol Science, 1998, 29(4): 421–444

42 Johansson L S, Leckner B, Tullin C L, Properties of particles in the fly ash of a biofuel-fired circulating fluidized bed (CFB) boiler. Energy & Fuels, 2008, 22(5): 3005–3015

43 Wang X, Xu Z, Wei B, The ash deposition mechanism in boilers burning Zhundong coal with high contents of sodium and calcium: a study from ash evaporating to condensing. Applied Thermal Engineering, 2015, 80: 150–159