Experimental studies on Soot and Particulate reduction in Heavy Fuel Oil Combustion

Abstract

Heavy fuel oil (HFO) is extensively used in industrial burners and marine engines. HFO droplet combustion gives rise to carbon particulates generated by pyrolysis, which are called cenospheres. There is a current hypothesis that ‘one HFO droplet generates one cenosphere’, though there is no strong experimental evidence in support of this hypothesis. The present research deals with experimental study of HFO droplet combustion and investigates effect of HFO spray characteristics on carbon particulate emission in a spray combustion environment. A new research burner is designed to study combustion characteristics of a sparse HFO spray in a controlled high temperature environment (800 K – 1300 K). A nebulizer system generates a sparse spray of HFO droplets which enables fundamental studies on droplet combustion. Initially, spray evaporation studies are performed with standard liquids such as n-decane and n-dodecane. The droplet evaporation rate constant Kevap data derived from experiments are found to be in good agreement with those from literature. The research burner is then utilized to study HFO spray combustion and particulate formation. A novel Laser-induced fluorescence (LIF) based optical technique is developed to optically image HFO droplets in the high temperature spray flame environment. Based on this data, fuel injection parameters are optimized to achieve spray characteristics with Sauter Mean Diameters (SMD) ranging from 24-μm to 53-μm. Soot measurements are carried out in the HFO spray flames using the Laser Induced Incandescence (LII) technique to identify soot formation and oxidation zones. Soot is observed to be produced at lower flame heights and subsequently oxidized along the spray flame height. It is observed that a reduction in SMD from 53-μm to 24-μm leads to a 58% reduction in soot 5 formation. To investigate the impact of HFO spray characteristics and environment temperature on droplet pyrolysis, the cenosphere sizes are measured using an aerodynamic particle sizer. With change in SMD from 53-μm to 24-μm, a drastic reduction (~90%) in cenosphere emission density (particles/cm3) is observed. It is also observed that higher temperatures ranging from 1073 K to 1223 K are favorable for cenosphere reduction. The morphological study of cenospheres indicates that these are nascent coke particles generated at the end of the droplet combustion phase. The results seem to indicate that the ‘one droplet generates one cenosphere’ theory is not applicable for smaller droplets. The data is further analyzed to establish the existence of a critical diameter of HFO, which undergoes complete combustion in the droplet combustion phase without generating a solid coke particle. In other words, if a HFO spray consists of droplets whose diameter is below this critical value, the particulate emissions can be nearly zero. From the experimental data, the critical droplet diameter is found to be in range of 18-μm to 23-μm for the medium grade of HFO used. Keywords: Heavy fuel oil (HFO), Spray, Droplets, Combustion, Particulate emission, Soot, Cenospheres, Laser Induced Incandescence, Laser Induced Fluorescence