Chemical Kinetics of Hydrogen-assisted Combustion of Kerosene

Abstract

To meet the goals of the Paris agreement,sustainable aviation fuels, such as hydrogen, need be adopted on a large scale within the coming decades. Hydrogen-assisted combustion of kerosene is investigated as an intermediate step in the transition towards sustainable aviation. This study addresses the gaps in the current understanding of hydrogen blending into kerosene with respect to chemical kinetics and emission of NOx, CO and un burnt hydrocarbons.  A perfectly-stirred chemical reactor model is developed to study the fundamental effects of hydrogen blending into kerosene. Additionally, to focus on a more practical application the single-reactor model is extended to a chemical reactor network which represents a lab-scale lean premixed prevaporized combustor setup supporting multi-fuel combustion. Combustion characteristics and emission profiles are carefully studied for different hydrogen blending fractions using the developed models, as well as the reaction pathways in oxidation of hydrocarbons.  Hydrogen blending is observed to increase the reactivity of kerosene, mainly due to increased availability of OH and H radicals, as well as O to a lesser extent. For 20% of kerosene mass substituted with H2, ignition delay time at 1200 K and 1 bard ecreases by 55%. Peak laminar flame speed increases by up to a factor of 2.2,while peak adiabatic flame temperature rises by 74 K. In rich burning conditions, H2 additions remove hydrocarbon reaction loops, which causes a strong decrease in un burnt hydrocarbon emissions and potentially soot. CO emissions can be greatly reduced from H2 additions at very lean conditions as overall burning rate is increased, which is corresponding to a reduction in lean extinction limit. This limit is reduced in the chemical reactor network combustor model by ∆φ=0.09 already for substitution of kerosene mass by 20%.  For φ>0.6, dissociation of CO2 is enhanced from thermal effects and causes emission of CO to rise with hydrogen blending when normalized to carbon in fuel. This effect is reflected in the combustion efficiency, which is improved with H2 blending only for very lean conditions,φ<0.6. The reaction pathways of aromatics are modified with H2 addition,which results in a chemical inhibition of the mechanism responsible for thema jority of un burnt hydrocarbon emissions at very lean conditions. Furthermore,formation of NOx is increased via the thermal path since flame temperatures rise. The prompt NOx route becomes more efficient due to promotion of CH formation, while the N2O and NNH paths are enhanced from increased radical availability. Despite the increased emission of NOx at constant φ, blending H2under constant combustion power and combustor mass flow leads to decreasedflame temperatures and reduces total emissions of NOx, CO and unburnt hydrocarbons.