The effect of coke and soot formation on the bio-oil combustion at high temperatures

Abstract

Bio-oil shows promising potential as an alternative fuel source. However, the formation of coke and soot during the combustion process hinders its use. This study investigates the formation mechanisms of primary coke, secondary coke, and soot during the initial stage of bio-oil combustion. The relationship between the physicochemical properties and the combustion characteristics of coke and soot is further analyzed to mitigate their adverse effect. It indicates that heavy aromatic compound polymerization plays a dominant role in primary coke formation, while graphitization and aromatic ring growth initially increase and then decrease with increasing temperature. Primary coke obtained at 1000 °C exhibits the highest degree of graphitization but small microscopic size, resulting in poor combustibility. Secondary coke is generated by the recombination reaction and its yield increases with temperature. The intensified reaction results in increased graphitization and decreased microscopic size, further leading to reduced combustibility due to its condensed structure. Soot generated above 1100 °C has low graphitization and combustibility due to short residence time. Based on the analysis, the temperature range around the feeding port is recommended to be controlled in 800–900 °C as it produces less coke and soot with better combustibility.