Effect of Biofuel Blending on CombustiŁon Characteristics of Hydrogen Flames

Abstract

Hydrogen has gained recognition as a highly promising alternative fuel to replace fossil fuels, offering the potential of a renewable energy carrier and contributing to the mitigation of greenhouse gas emissions. One of the challenges associated with using hydrogen in combustion-based industrial processes is that such systems (e.g. furnaces and boilers) typically rely on radiative heat transfer, whereas hydrogen flames feature low radiant intensity. Adding soot-promoting additives such as biofuels, which can be renewably produced, to hydrogen flames may offer a potential approach to enhance the radiant intensity, making hydrogen more suitable for practical applications. However, neither the effectiveness of various biofuels on thermal radiation enhancement, nor the influence of biofuel addition on the combustion characteristics of biofuel/hydrogen flames were previously well understood. This research, therefore, fills this gap in the understanding of the combustion characteristics of biofuel/hydrogen flames and the influencing factors. To evaluate the relationship between the chemistry and the effectiveness of blending hydrogen flames with biofuels on combustion characteristics, toluene, anisole, and guaiacol are chosen as aromatic surrogates for bio-oils. Eucalyptol and D-limonene are recognised as monoterpenes chosen as surrogates for essential oils. The sooting propensity of these biofuel surrogates is evaluated by assessing laminar biofuel flames using the smoke point method, showing a decreasing order of toluene > anisole > guaiacol > D-limonene > eucalyptol. The results suggest that biofuels with an aromatic structure generally exhibit higher sooting propensities than monoterpenes, which is attributed to the higher degree of unsaturation in aromatic structures that favours PAH formation. Biofuels with lower bond dissociation enthalpy tend to be more effective in radiant intensity enhancement as a weaker C-H bond promotes hydrogen abstraction through the HACA mechanism. The efficacy of oxygenated fuels (i.e. anisole and eucalyptol) in enhancing radiant heat flux is lower compared with non-oxygenated fuels (i.e. toluene and D-limonene), as the oxygen content in oxygenated fuels aids in the oxidation of PAHs. The is increased by 2–22% through the addition of 0.2–1 mol% prevapourised and ultrasonically atomised biofuels. Toluene and anisole exhibit higher effectiveness in radiation enhancement than D-limonene and eucalyptol. These blended hydrogen flames illustrate enhanced blue colouration due to the promoted formation of carbonaceous radicals, whilst limited soot is observed. The corresponding flame luminosity exhibits an increase of 61– 293%. In comparison, adding 0.1–0.3 mol% biofuels via gas-assist atomisation alters the dominant colouration of the hydrogen flame to yellow, indicating that soot loading is significantly improved and sooting biofuel/hydrogen flames are achieved. The enhancement of flame luminosity and radiant fraction ranges from 30–500% and 2–15%, respectively. The results suggest that the method of biofuel introduction is an important influencing factor for soot formation in biofuel/hydrogen flames. The underlying mechanisms are analysed through spray characterisation using microscopic shadowgraphy. The results reveal that biofuel droplets create local fuel-rich conditions to enhance soot formation and hence radiation characteristics. Droplet size is a critical parameter in creating fuel-rich pockets as larger droplets generated by gas-assist atomisation further advance this effect. Blending hydrogen flames with biofuels for radiant intensity enhancement leads to an increase in the emission of nitrogen oxides. The computational investigation into the mechanisms of nitric oxide (NO) reveals that the increase in global NOx emissions is primarily ascribed to the enhanced subset of the thermal route: OH + N ⇌ H + NO, and the prompt route: CH + N2 ⇌ H + NCN. Based on this understanding, blending sprayed biofuels to hydrogen reduces thermal NOx because the enthalpy of vapourisation results in lower flame temperature. A larger temperature drop is observed in gas-assist atomised biofuel/hydrogen flames due to the enhanced radiative heat loss. The findings from this research establish an understanding of the efficacy and effectiveness of blending hydrogen flames with biofuels on radiant intensity enhancement and the corresponding impact on combustion characteristics. The analyses of influencing factors and their underlying mechanisms contribute to the adaptation of hydrogen to practical applications as an alternative energy carrier to mitigate greenhouse gas emissions.