An experimental and theoretical investigation was performed to seek detailed ignition and combustion characteristics of a single coal water slurry (CWS) droplet exposed to hot gas stream. Also, the effect of the surface radiation was taken into account.
Experimentally, the ignition and combustion characteristics of CWS droplets were studied in the post-flame region generated by a flat flame burner. The gas stream temperature was varied between 950℃ and 1550℃ by adjusting inlet gas flow rates into the burner or mounting metal screen at the exit of burner. The gas stream temperature was measured with various diameter Pt/Pt-13\% Rh thermocouples to find the true ones by extrapolation. The gas stream velocity was measured with LDV system. The product gas concentrations were estimated by assuming the chemical equilibrium. The initial droplet diameter was measured by image analysis system. Individual droplet having an initial diameter of 1-3 mm was suspended around a 25 $\mu$m Pt/Pt-13\% Rh thermocouple and rapidly exposed to the hot gas stream by xy-traversing device. All the processes were operated by a personal computer equipped with a data acquisition board. The transient ignition processes were all recorded with a video camera and the thermocouple output. Based on both video recording and thermocouple output, the ignition delay time was determined either the first rapid temperature rise in time-temperature curves obtained by thermocouple output or onset of a visible flame in video recording.
Theoretically, a refined model considering the temporal temperature variation inside the droplet was devised to reveal the ignition and combustion phenomena of CWS droplet. When a droplet is introduced into the hot gas stream, the droplet would be heated by a convective heat transfer from the gas as well as a radiative heat transfer from metal screen or heating elements. Once the outer water layer in droplet reaches its boiling temperature, the evaporation begins. As the water is evaporated, the evaporation front of water regresses into the interior of the slurry droplet, leaving behind a porous shell near the droplet surface. And thus the inner sphere region of the binary mixture of coal and water is reduced. Therefore the energy transferred from the hot gas stream is not only used to heat up the rigid porous shell but also used to migrate the evaporation front inwards. At the same time the coal in porous shell starts to be pyrolyzed at a certain temperature, producing volatile gases. This results in the formation of pyrolysis front, behind which a pyrolyzed layer of char is left. The volatile gases generated at the pyrolysis front are diffusing out in the radially outgoing direction through pyrolyzed layer and leave the droplet surface to mix with the oxidizer. When this mixing layer is finally heated high enough, the mixture is ignited in the region near the droplet surface.
In this study, the governing equations based on the theoretical model were numerically solved and the results were corroborated in comparison with the experimental ones. The outer porous layer could be pyrolyzed while the evaporation front kept migrating inward to the center of droplet. Even before the binary mixture in inner region of the droplet was thoroughly evaporated, the volatile gases generated could be ignited in the vicinity of droplet surface. The ignition delay time was primarily comprised of both the heating time and mixing time. But the heating time, which is required for a droplet to be vaporized and then pyrolyzed, was found to be controlling one. The effect of surface radiation among droplet, surroundings, metal screen and heating elements was also considered. The difference of ignition delay between with/without metal screen or heating elements was found to be increased with increasing the ratio of radiative to convective heat transfer as well as droplet size. Therefore, it was noticeable that an inclusion of the surface radiation was imperative to predict more reasonable values of ignition delay.
