A review of the effects of hydrogen, carbon dioxide, and water vapor addition on soot formation in hydrocarbon flames

Addition of reactive or inert substances is one of the most effective and practical ways to control soot formation in combustion of hydrocarbon fuels. In this paper, the research progress on the effects of hydrogen, carbon dioxide, and water vapor addition on soot formation in hydrocarbon flames in the last few decades is systematically summarized. The summary shows that the number of studies on the effects of these three common diluents has increased dramatically in the last five years. Although the overall effects of all these three common diluents suppress soot formation, there is inconsistency with regard to the role of their chemical effects. The chemical effect of hydrogen (CE-H2) mainly acts on the soot nucleation process, followed by the soot surface growth and finally the soot oxidation process. CE-H2 seems significantly affected by the fuel type, oxygen concentration, and the ambient pressure. The chemical effect of carbon dioxide (CE-CO2) affects soot formation indirectly mainly through the reaction CO + OH ↔ CO2 + H. Some studies believe that CE-CO2 suppresses soot production by increasing the hydroxyl radical (OH) concentration, while other studies believe that it is primarily attributed to the decrease of the hydrogen radical (H) concentration. The reaction H2O + H ↔ H2 + OH plays a vital role in the chemical effect of water vapor (CE-H2O) addition on inhibiting soot formation. Most studies support the view that the chemical effect of water vapor mainly increases the OH concentration and suppresses soot formation by weakening the soot nucleation process. Moreover, we believe that reaction H2O + O ↔ OH + OH and phenylacetylene also play an essential effect on the CE-H2O.