A detailed chemical insights into the kinetics of diethyl ether enhancing ammonia combustion and the importance of NOx recycling mechanism

Abstract

In this work, we investigated the combustion characteristics of ammonia (NH₃) by blending it with various proportions of diethyl ether (DEE). We measured laminar flame speed of various NH₃/DEE blends (DEE, 10–40% by mole) using a constant volume spherical vessel at T_i = 298 K and P_i = 3 and 5 bar and Φ = 0.8–1.3. We developed a detailed kinetic model to describe the trends of the current and previously published experimental data. For the robustness of the model, we first developed a comprehensive diethyl ether kinetic mechanism to accurately characterize neat DEE oxidation behavior. We validated the kinetic model using a large pool of experimental data comprising shock tube, rapid compression machine, jet-stirred and flow reactors, freely propagating, and burner-stabilized premixed flames. The developed kinetic model performs remarkably in capturing the combustion behavior of pure DEE and NH₃. Importantly, our model captures the experimental data of laminar flame speed and ignition delay times of various NH₃/DEE blends over a wide range of conditions. We found that DEE is a promising candidate to promote the combustion characteristics of NH₃. A small portion of DEE (10%) enhances the laminar flame speed of NH₃ by a factor of 2 at P_i = 1 bar, T_i = 298 K, and Φ = 1.0. A further doubling of the DEE mole fraction to 20% did not enhance the laminar flame speed of NH₃ with the same propensity. At low temperatures, adding 5% DEE in NH₃ blend has significantly expedited the system reactivity by lowering the autoignition temperature. A further 5% increment of DEE (i.e., 10% DEE in NH₃) lowers the autoignition temperature by ∼120 K to achieve the same ignition delay time. The “NONO₂” looping mechanism predominantly drives such reactivity accelerating effect. Here, the reactions, NO + HO₂ = NO₂ + OH and NO₂ + H = NO + OH, appear to enhance the reactive radical pool by generating OH radicals. We observed that the HNO path is favored more with increasing DEE content which eventually liberates NO. Other key reactions in “NONO₂” looping mechanism are: CH₃ + NO₂ = CH₃O + NO, CH₃O₂ + NO = CH₃O + NO₂, C₂H₅ + NO₂ = C₂H₅O + NO, C₂H₅O₂ + NO = C₂H₅O + NO₂. In addition, CH₃ + NH₂(+M) = CH₃NH₂(+M) reaction is also one of the important cross-reactions which leads to the formation of HCN. Therefore, cross-reactions between the nitrogen and carbon family are crucial in accurately predicting autoignition timing. This work provides a detailed chemical insight into the NH₃ and DEE interaction, which could be applied to other fuel blends of NH₃. The kinetic model is also validated for several C₁C₃ fuels including their interaction with NOx.