Fundamental insights into the effect of blending hydrogen flames with sooting biofuels

A major challenge to using hydrogen as a carbon-free energy carrier to replace fossil fuels in high-temperature practical processes is the reduced thermal radiative transfer of hydrogen flames due to the absence of soot. To address the potential issue of low radiative heat transfer from hydrogen flames, the effect of blending highly sooting biofuels to turbulent nonpremixed hydrogen-based flames on flame luminosity and radiant heat flux enhancement is investigated in this paper. The sooting propensities of aromatic bio-oil surrogates (toluene, anisole and guaiacol) and monoterpene essential oil surrogates (eucalyptol and D-limonene) are evaluated. The Threshold Sooting Index (TSI) and Oxygen Extended Sooting Index (OESI) are calculated and compared with the data sets in the literature. The sooting propensities of the reported fuels are found to follow a decreasing order: aromatics > monoterpenes > alkenes > alkanes > aldehydes. The effect of chemical structure and functional group on sooting propensity tends to be more significant than the oxygen content and effective C/H ratio. Turbulent nonpremixed hydrogen–nitrogen (9:1 mol) jet flames are blended with 0.2 and 1 mol% (based on the molar concentration of hydrogen) vapourised biofuels. The results show that adding 0.2 and 1 mol% vapourised biofuels to the hydrogen-based flame non-linearly improves the flame luminosity and radiant fraction by 61%–253% and 2%–19%, respectively. Toluene is the most effective additive in luminosity and radiant fraction enhancement among all tested biofuels. Adding biofuels with an aromatic structure increases the radiant fraction by 10%–19%, which is more effective than cyclic monoterpenes (2%–9%). Adding non-oxygenated biofuels generally have larger increases in radiant fraction (9%–19%) than oxygenated biofuels (2%–10%).