M.Tech(Res) : Effect of hydrogen-enrichment on soot formation in laminar gaseous hydrocarbon flames

Gaseous and particulate pollutants pose a significant threat to human health and the environment, prompting regulatory action to address major sources of emissions. Soot is a key particulate pollutant. Recently, emission standards for commercial aeroengines have been revised, necessitating the mitigation of soot emissions. Investigating the soot formation process is a key step towards reducing emissions. Soot formation is a complex process that poses a challenge to the chemical kinetics community. Predicting soot is computationally expensive and challenging, requiring reliable reduced mechanisms for practical fuels. The primary obstacle is the lack of systematic data to develop and validate chemical kinetics models for soot prediction. Hydrogen (H2) is being explored as a means to decarbonize the automotive, aviation, and power generation sectors. However, implementing pure H2 in practical devices is difficult due to higher operating temperatures and flame speeds. Alternatively, H2 can be blended into traditional hydrocarbon fuels. The addition of H2 influences the combustion chemistry of hydrocarbon fuels, which consequently leads to changes in the composition of combustion products. The aviation industry uses practical fuels to form turbulent flames. However, the complexity of practical fuels and flow fields makes it difficult to predict the concentrations of combustion emissions. A systematic study of soot formation in laminar gaseous-fuel flames can aid in developing reduced soot reaction mechanisms and understanding the soot formation process. This work reports a database of soot concentrations for C1–C4 hydrocarbons (methane, ethane, propane, and butane) under laminar premixed and non-premixed conditions. Additionally, the influence of H2 blending on soot formation is examined for these fuels. The parameters, such as soot volume fraction (fv), distributions of soot precursors (PAH) and OH, and gas temperature, are measured using laser-based diagnostic techniques. The study of soot formation was performed on two different burner configurations: premixed and non-premixed. The premixed burner stabilized flames with φ = 2.3 were stabilized on the McKenna burner equipped with a stagnation plate. To ensure flame stability, a mixture of O2 and Ar was used as the oxidizer. The reactant flow rates for test cases are selected such that the carbon influx (Cin), C/O ratio, and O2 fraction in oxidizer are kept constant. The non-premixed flames were stabilized on a coflow burner. The flow conditions were selected to maintain a constant Cin, thereby isolating the influence of Cin on soot. For both flame configurations, H2 is added up to 40 % (by volume) to a base hydrocarbon fuel. H2 addition has three primary effects: thermal, dilution, and chemical. The chemical effect of H2 on soot is isolated using a reference flame, created by replacing H2 with helium. The comparison of fv with this reference flame allows for the quantification of the chemical effect of H2 on soot. The fv is measured for both premixed and non-premixed flames by using the laser-induced incandescence (LII) technique. The distribution of PAH is measured using the planar laser-induced fluorescence (PLIF) technique. Additionally, for non-premixed flames, the distributions of OH and the temperature field were measured using the PLIF technique. The elemental carbon-to-hydrogen ratio (C/H) governs the maturity of soot. The soot maturity changes with height above the burner (HAB), introducing a bias in LII measurements. The LII fluence curve trends with HAB in premixed flames are used to estimate the relative change in soot maturity. These trends along HAB are used to estimate relative changes in the optical properties of soot particles (E(m)). PAH are the precursors to soot formation. However, interpreting PAH-LIF (IPAH) trends is challenging due to the dependence of LIF on temperature and quenching by combustion products. In this work, an empirical approach is used to correct the IPAH in premixed flames for these dependencies. Additionally, the extinction signature in radial IPAH profiles is used to obtain absorption-based PAH concentration. This approach mitigates the bias in interpreting the PAH trends in premixed flames. Soot volume fraction (fv) increases monotonically with carbon number (C1 to C4) for alkanes in both laminar premixed and non-premixed flames. The total soot loading parameter is used to examine the overall sooting tendency. The soot loading decreases relative to neat flames with H2-enrichment for all fuels. The extent of suppression of soot formation by H2 addition is greater in premixed flames than in non-premixed flames. Cin is examined relative to CxHy/He flames. It was observed that Cin is strongly dependent on the type of fuel. H2 enrichment inhibits pyrolysis in ethylene (alkene) fuel, contributing to delayed soot onset relative to the helium reference flame. Conversely, H2 promotes (relative to helium) pyrolysis in non-premixed C1–C4 alkane flames, thereby enhancing soot. In premixed alkane flames, H2 suppresses soot in the inception-dominated region but enhances soot in growth-dominated regions. This contrasts with ethylene flame, where H2 reduces soot formation throughout HAB. These findings reveal the fuel-specific impact of H2 enrichment on soot formation, providing a systematic dataset to support the validation of chemical kinetics models and the design of low-emission combustion systems. The performance of the state-of-the-art soot reaction mechanism to predict fv is assessed against measurements. Additionally, chemical kinetics analysis is performed to examine the chemical effect of H2 on soot formation.

Speaker : Choudhari Aditya Sunil 

Research Supervisor :  Irfan Ahmed Mulla