Effect of temperature on Lu’an bituminous char structure evolution in pyrolysis and combustion

Abstract

In the process of pyrolysis and combustion of coal particles, coal structure evolution will be affected by the ash behavior, which will further affect the char reactivity, especially in the ash melting temperature zone. Lu’an bituminous char and ash samples were prepared at the N₂ and air atmospheres respectively across ash melting temperature. A scanning electron microscope (SEM) was used to observe the morphology of char and ash. The specific surface area (SSA) analyzer and thermogravimetric analyzer were respectively adopted to obtain the pore structure characteristics of the coal chars and combustion parameters. Besides, an X-ray diffractometer (XRD) was applied to investigate the graphitization degree of coal chars prepared at different pyrolysis temperatures. The SEM results indicated that the number density and physical dimension of ash spheres exuded from the char particles both gradually increased with the increasing temperature, thus the coalescence of ash spheres could be observed obviously above 1100°C. Some flocculent materials appeared on the surface of the char particles at 1300°C, and it could be speculated that β-Si₃N₄ was generated in the pyrolysis process under N₂. The SSA of the chars decreased with the increasing pyrolysis temperature. Inside the char particles, the micropore area and its proportion in the SSA also declined as the pyrolysis temperature increased. Furthermore, the constantly increasing pyrolysis temperature also caused the reactivity of char decrease, which is consistent with the results obtained by XRD. The higher combustion temperature resulted in the lower porosity and more fragments of the ash.

Keywords

bituminous char / pyrolysis / ash / structure evolution / reactivity

References

[1] BP. BP Statistical Review of World Energy, 2019, 45

[2] Teixeira P, Lopes H, Gulyurtlu I, Lapa N, Abelha P. Evaluation of slagging and fouling tendency during biomass co-firing with coal in a fluidized bed. Biomass and Bioenergy, 2012, 39: 192–203

[3] Williams A. Understanding slagging and fouling during PF combustion: Gordon Couch IEA Coal Research. Fuel, 1995, 74(10): 1541

[4] Benson S A, Harb J N. Fuel minerals, fouling and slagging. Energy & Fuels, 1993, 7(6): 743–745

[5] Seggiani M. Empirical correlations of the ash fusion temperatures and temperature of critical viscosity for coal and biomass ashes. Fuel, 1999, 78(9): 1121–1125

[6] Noda R, Naruse I, Ohtake K. Fundamentals on combustion and gasification behavior of coal particle trapped on molten slag layer. Journal of Chemical Engineering of Japan, 1996, 29(2): 235–241

[7] Wang X, Zhao D, He L, Chen Y, Aoki H, Miura T. Study on the mechanism and simulation method for wall burning process. Chinese Journal of Theoretical and Applied Mechanics, 2006, 38(4): 462–470

[8] Kong L, Bai J, Li W, Wen X, Liu X, Li X, Bai Z, Guo Z, Li H. The internal and external factor on coal ash slag viscosity at high temperatures, part 2: effect of residual carbon on slag viscosity. Fuel, 2015, 158: 976–982

[9] Ma Z, Iman F, Lu P, Sears R, Kong L, Rokanuzzaman A S, McCollor D P, Benson S A. A comprehensive slagging and fouling prediction tool for coal-fired boilers and its validation/application. Fuel Processing Technology, 2007, 88(11–12): 1035–1043

[10] Lackner M, Winter F, Agarwal A K. Handbook of Combustion, Vol. 4: Solid Fuels. Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA, 2010, 493–531

[11] Fryda L, Sobrino C, Cieplik M, van de Kamp W L. Study on ash deposition under oxyfuel combustion of coal/biomass blends. Fuel, 2010, 89(8): 1889–1902

[12] Akiyama K, Pak H, Tada T, Ueki Y, Yoshiie R, Naruse I. Ash deposition behavior of upgraded brown coal and bituminous coal. Energy & Fuels, 2010, 24(8): 4138–4143

[13] Wu X, Ji H, Dai B, Zhang L. Xinjiang lignite ash slagging and flowability under the weak reducing environment at 1300°C—a new method to quantify slag flow velocity and its correlation with slag properties. Fuel Processing Technology, 2018, 171: 173–182

[14] Dai B, Hoadley A, Zhang L. Characteristics of high temperature C–CO₂ gasification reactivity of Victorian brown coal char and its blends with high ash fusion temperature bituminous coal. Fuel, 2017, 202: 352–365

[15] Ma Z, Bai J, Li W, Bai Z, Kong L. Mineral transformation in char and its effect on coal char gasification reactivity at high temperatures, part 1: mineral transformation in char. Energy & Fuels, 2013, 27(8): 4545–4554

[16] Ma Z, Bai J, Bai Z, Kong L, Guo Z, Yan J, Li W. Mineral transformation in char and its effect on coal char gasification reactivity at high temperatures, part 2: char gasification. Energy & Fuels, 2014, 28(3): 1846–1853

[17] Bai J, Li W, Li C, Bai Z, Li B. Influences of minerals transformation on the reactivity of high temperature char gasification. Fuel Processing Technology, 2010, 91(4): 404–409

[18] Ma Z, Bai J, Wen X, Li X, Shi Y, Bai Z, Kong L, Guo Z, Yan J, Li W. Mineral transformation in char and its effect on coal char gasification reactivity at high temperatures part 3: carbon thermal reaction. Energy & Fuels, 2014, 28(5): 3066–3073

[19] Lin S Y, Hirato M, Horio M. The characteristics of coal char gasification at around ash melting temperature. Energy & Fuels, 1994, 8(3): 598–606

[20] Ding L, Zhou Z, Guo Q, Wang Y, Yu G. In situ analysis and mechanism study of char-ash/slag transition in pulverized coal gasification. Energy & Fuels, 2015, 29(6): 3532–3544

[21] Wu X, Zhang Z, Piao G, He X, Chen Y, Kobayashi N, Mori S, Itaya Y. Behavior of mineral matters in Chinese coal ash melting during char-CO₂ /H₂O gasification reaction. Energy & Fuels, 2009, 23(5): 2420–2428

[22] Liu H, Luo C, Toyota M, Uemiya S, Kojima T. Kinetics of CO₂/char gasification at elevated temperatures. Part II: clarification of mechanism through modelling and char characterization. Fuel Processing Technology, 2006, 87(9): 769–774

[23] Shen C, Lin W, Wu S, Tong X, Song W. Experimental study of combustion characteristics of bituminous char derived under mild pyrolysis conditions. Energy & Fuels, 2009, 23(11): 5322–5330

[24] Tang L, Hu S, Ni Y, Zhu X, Zhu Z. Effect of char marking temperature on coal char characteristics. Journal of East China University of Science and Technology, 2007, 33(2): 149–152

[25] Wang X, Xu W, Zhang L, Hu Z, Tan H, Xiong Y, Xu T. Char characteristics from the pyrolysis of straw, wood and coal at high temperatures. Journal of Biobased Materials and Bioenergy, 2013, 7(6): 675–683

[26] Xie K. Coal Structure and its Reactivity. Beijing: Science Press, 2002

[27] Speight J G. Handbook of Coal Analysis. 2nd ed. Hoboken: John Wiley & Sons, Inc, 2015

[28] Zhang H, Li X. Application of scanning electron microscope in coal and rock science. Journal of Chinese Electron Microscopy Society, 2004, 23(4): 467–467

[29] Cao J, Cong L. Discussion on the research and application of silicon nitride. Ceramics, 2008, 4(8): 34–36

[30] Lorenz H, Carrea E, Tamura M, Haas J. The role of char surface structure development in pulverized fuel combustion. Fuel, 2000, 79(10): 1161–1172

[31] Shi J Q, Durucan S. Gas storage and flow in coalbed reservoirs: implementation of a bidisperse pore model for gas diffusion in coal matrix. SPE Reservoir Evaluation & Engineering, 2005, 8(02): 169–175

[32] Clarkson C R, Wood J, Burgis S, Aquino S, Freeman M. Nanopore-structure analysis and permeability predictions for a tight gas siltstone reservoir by use of low-pressure adsorption and mercury-intrusion techniques. SPE Reservoir Evaluation & Engineering, 2012, 15(06): 648–661

[33] Wu J. Study on the characteristics of coal micro-pore and its relationship with hydrocarbon migration and reservoir. Science in China, 1993, 23(1): 77–84

[34] Zhang H, Yang S. Research into the nitrogen adsorption test of coal under low temperature. Journal of Shandong University of Science and Technology, 1993, 12(3): 245–249

[35] Gregg S J, Sing K S W, Salzberg H W. Adsorption surface area and porosity. Journal of the Electrochemical Society, 1967, 114(11): 279C

[36] Sing K S W. Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity. Pure and Applied Chemistry, 1985, 57(4): 603–619

[37] Li X, Ma B, Xu L, Hu Z, Wang X. Thermogravimetric analysis of the co-combustion of the blends with high ash coal and waste tyres. Thermochimica Acta, 2006, 441(1): 79–83

[38] Ma B G, Li X G, Xu L, Wang K, Wang X G. Investigation on catalyzed combustion of high ash coal by thermogravimetric analysis. Thermochimica Acta, 2006, 445(1): 19–22

[39] Feng B, Bhatia S K, Barry J C. Variation of the crystalline structure of coal char during gasification. Energy & Fuels, 2003, 17(3): 744–754

[40] Wu Y, Wu S, Gu J, Gao J. Differences in physical properties and CO₂ gasification reactivity between coal char and petroleum coke. Process Safety and Environmental Protection, 2009, 87(5): 323–330

[41] Lu L, Kong C, Sahajwalla V, Harris D. Char structural ordering during pyrolysis and combustion and its influence on char reactivity. Fuel, 2002, 81(9): 1215–1225

[42] Tang Z, Zhou G, Xiong J, Zhou Z. Measurement of graphitization degree of carbon-carbon composites by average X-ray diffraction angle method. The Chinese Journal of Nonferrous Metals, 2003, 13(6): 1435–1440

[43] Ding L, Gong Y, Wang Y, Wang F, Yu G. Characterisation of the morphological changes and interactions in char, slag and ash during CO₂ gasification of rice straw and lignite. Applied Energy, 2017, 195: 713–724

Acknowledgements

This work was financial supported by the National Natural Science Foundation of China (Grant No. 51576158).

RIGHTS & PERMISSIONS

2021 Higher Education Press