Role of methane in ammonia combustion in air: From microscale to macroscale

Ammonia (NH₃) has gained increasing recognition as a carbon-free fuel. To enhance NH₃ combustion, reactive gases, like methane (CH₄), are usually added to the combustion system. In this work, the role of CH₄ in NH₃ combustion is systematically studied. A series of reactive force field molecular dynamic (ReaxFF MD) simulations are implemented to investigate effects of CH₄ addition on the consumption of NH₃ and the yields of nitrogen oxides (NO_x) from the atomic perspective: CH₄ accelerates the consumption of NH₃ by shortening the decomposition time of the first NH₃ molecule and increasing the translational kinetic energy of the system; CH₄ modifies the yield of NO_x by complicating the elementary reactions and introducing additional intermediates. The fuel ratio of CH₄ and NH₃ between 0.5 and 1 is suggested for a cleaner and enhanced NH₃ combustion. By summarising the findings from the latest publications and the present work, the role of CH₄ in NH₃ combustion is comprehensively analysed from the macroscale and microscale perspectives: CH₄ accelerates the progress of NH₃ combustion flame, activates chemical reactions, and aggravates NO_x emissions at a low CH₄ content. Taking the NH₃/CH₄ combustion as an example, this study provides an exclusive perspective to understand combustion phenomena from the microscale events to macroscale observations.