Experimental Study Of Soot Formation During The Combustion Of Coal And Biomass

Soot, a form of particulate pollution most often produced from biomassopen-burning as well as combustion with solid fuels like coal has a warming effect onthe atmosphere. In this thesis, a Hencken-type flat burner is specially designed anddeveloped as prototypical system for studying in situ soot formation during combustionof single solid particle. LII (Laser Induce Incandescence) and PLIF (Planar LaserInduced Fluorescence) are employed to study the soot volume fraction and distributionaround the particle in the combustion process. The effects of atmospheric temperature,oxygen content and relative gas velocity on soot formation are investigated.In the initial stage of combustion, the soot matter is concentrated in the two wingsof the diffusion flame of volatile matter, which is located at downstream of a fuel sphere.And then soot volume fraction in the center increases. The soot volume fraction in thecenter is higher than the two wings when the Reynolds number is larger than100. Thechanging of distribution is caused by the separation of fluid which leads to the mixingof volatile matter and high temperature gas.The total soot concentration, maximum soot concentration and conversion ofvolatile matter to soot are all in a descending sequence as coal, wood and straw. Resultsindicate that there are more hydrocarbons from the pyrolysis of coal, which are mucheasier to form soot.The effect of temperature on soot formation during combustion of fuel sphere isdependent on the gas-phase reaction rate after the release of volatile matter. Theformation rate and the oxidation rate both increases as the temperature increases. It isobserved that the conversion of volatile matter to soot decreases with increment oftemperature, because of enhanced oxidation of soot.The relative convective velocity doesn't have much effect on the releaserate of volatile matter. Both total soot concentration and peak soot concentrationincreases with the increasing flow velocity. And the conversion of volatilematter to soot also increases with it. It is implied that the increase of velocitymay form some vortices in the wake zone which will accelerate the mixing ofvolatile matter and high temperature gas, which is suitable for soot formation. When the oxygen content increases, the release rate of volatile matter willaccelerate, and the temperatures of both fuel particle and gas rise. The maximum sootconcentration at30%oxygen content increased to2.8times as much as that of10%oxygen content, but the increase speed will slow down when the oxygen contentexceeds20%. At this time, the soot oxidation rate increment exceeds the soot formationrate increment.The ejection of volatile matter during the combustion of coal particle leads to theincrease of flame radiation and soot concentration to twice as much as that when noejection occurs. The results indicate that the ejection of volatile matter is due to theconcentrated release of volatile matter via a narrow pass. The ejection changes thedistribution of volatile matter and temperature, and leads to a massive soot formationand increase of flame radiation.