An experimental study on ignition of single coal particles at low oxygen concentrations
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RESEARCH ARTICLE
Wantao YANG, Yang ZHANG, Lilin HU, Junfu LYU, Hai ZHANG
An experimental study on ignition of single coal particles at low oxygen concentrations
© Higher Education Press 2020
Abstract An experimental study on the ignition of single coal particles at low oxygen concentrations (XO2 < 21%) was conducted using a tube furnace. The surface temperature (Ts) and the center temperature (Tc) of the coal particles were obtained from the images taken by an infrared camera and thermocouples respectively. The ignition processes were recorded by a high-speed camera at different XO2 values and furnace temperatures Tw. Compared with literature experimental data obtained at a high XO2 value, the ignition delay time ti decreases more rapidly as XO2 increases at the low XO2 region. The responses of Ts and Tc to the variation of XO2 are different: Ts decreases while Tc remains nearly constant with increasing XO2 at a low XO2 value. In addition, ti is less sensitive to Tw while the ignition temperature Ti is more sensitive to Tw at a low XO2 value than in air. Observations of the position of flame front evolution illustrate that the ignition of a coal particle may change from a homogeneous mode to a heterogeneous or combined ignition mode as XO2 decreases. At a low XO2 value, buoyancy plays a more significant role in sweeping away the released volatiles during the ignition process. Keywords coal particles, low oxygen concentration, ignition, ignition temperature, ignition modes
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Introduction
Coal ignition at a low oxygen concentration (XO2 < 21%) popularly exists in industrial applications. For example, in a coal-fired boiler, when the primary air flow carries coal particles into the furnace, it entrains the surrounding flue Received Oct. 11, 2019; accepted Feb. 26, 2020; online Sept. 10, 2020
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Wantao YANG, Yang ZHANG, Lilin HU, Junfu LYU, Hai ZHANG ( ) Key Laboratory for Thermal Science and Power Engineering of the Ministry of Education, Department of Energy and Power Engineering, Tsinghua University, Beijing 100084, China E-mail: [email protected]
gas, making the ignition of most coal particles happen in an environment with XO2 < 21% rather than in air (XO2 = 21%) [1]. Similarly, in the moderate or intense low-oxygen dilution (MILD) combustion, which is regarded as a new clean coal technology due to its advantages in NOx reduction and flame stability [2], coal ignition occurs in low XO2 conditions due to the strongly dilution of the flue gas [3]. Besides, in a circulating fluidized bed (CFB) boiler with a good fuel flexibility, high concentration mm-sized particles mix with secondary air in the dense zone, causing a low XO2 condition for combustion and SO2 absorption [4]. Therefore, understanding the ignition of coal particles in low XO2 conditions is essential for the coal-fired burner and combustor design, CFB boiler operation, and pollutant removal. The effect of oxygen concentration on coal ignition has been extensively studied before, but mostly under air or oxyfuel combustion conditions [5,6], and not mu
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