Ignition of single coal particle in a hot furnace under normal- and micro-gravity condition

Abstract

An experimental study on ignition and combustion of single particles was conducted at normal gravity (1-g) and microgravity (l-g) for three high volatile coals with initial diameter of 1.5 and 2.0 mm, respectively. The non-intrusive twin-color pyrometry method was used to retrieve the surface temperature of the coal particle through processing the images taken by a color CCD camera. At the same time, a mathematical model considering thermal conduction inside the coal particle was developed to simulate the ignition process.Both experiments and modeling found that ignition occurred homogeneously at the beginning and then heterogeneously for the testing coal particles burning at l-g. Experimental results confirmed that ignition temperature decreased with increasing volatile content and increasing particle size. However, contradicted to previous studies, this study found that for a given coal with certain particle size, ignition temperature was about 50-80 K lower at l-g than that at 1-g.The model predictions agreed well with the l-g experimental data on ignition temperature. The criterion that the temperature gradient in the space away from the particle surface equaled to zero was validated to determine the commence of homogeneous ignition. Thermal conduction inside the particle could have a noticeable effect for determining the ignition temperature. With the consideration of thermal conduction, the critical size for the phase transient from homogeneous to heterogeneous is about 700 lm at ambient temperature 1500 K and oxygen concentration 0.23.

Volatile Release and Ignition Behaviors of Single Coal Particles at Different Oxygen Concentrations Under Microgravity

“…The results were in good agreement with those reported (Zhu et al 2009), in which ignition temperatures were 50∼ 80 K lower at μ-g, but contrast to the experimental results reported by . The difference was attributed mainly to the different particle heating methods used in the studies.…”

“…1, with some modifications of the one used in our previous studies (Zhu et al 2009;Zhu et al 2011). The system consisted of a vertical, electrically heated ceramic tube furnace, a temperature controller, a gas supply, optical observation device and PLC controller.…”

“…Some of the selected coal particles were ground for proximate and ultimate analyses and the results were given in Table 1. Unlink the previous studies (Zhu et al 2009;Zhu et al 2011); a small hole of ∼ φ0.25 mm was drilled through the center of the particle. A fine bare-wired K type thermocouple (wire diameter φ0.125 mm, tip diameter φ 0.25 mm) run through the hole.…”

“…Under such a temperature, the coal particles with size of ∼ φ2.0 mm were subjected to a heating rate of 150-200 K/s, which was calculated in our previous studies [20,25]. To ensure ignition occurred in the drop time, the particle was sent into the furnace for preheating for 1.0-1.5 s before the drop (Tang et al 2010;Zhu et al 2009). During this period, validated by 1-g experiments, no volatile was released.…”

“…Thus, ignition of single coal particles occurs in an environment with quasi-onedimensional mass and heat transfer, thereby the associated experiments become ideal to discover fundamental phenomena, to explore the instinct mechanisms and to provide more accurate data for coal ignition/combustion (Katalambula et al 2000;Liu et al 2014;Gieras et al 1985;Gieras et al 1986;Wendt et al 1999;Tang et al 2010;Zhu et al 2009;Zhu et al 2011). For this reason, several μ-g experiments on coal ignition were conducted, mostly by utilising the drop towers Katalambula et al 2000;Gieras et al 1985;Gieras et al 1986;Wendt et al 1999;Tang et al 2010;Zhu et al 2009;Zhu et al 2011) and the effects of the flame transition (Gieras et al 1985;Gieras et al 1986), particle shape (Wendt et al 1999), coal type Davini et al 1996) and particle size Tang et al 2010) have been investigated. However, the existing experiments were mainly limited to air condition.…”