Lean combustion characteristics of hydrogen-nitrous oxide-ammonia mixtures in air

Abstract

We report experimental studies on the combustion of lean hydrogen-nitrous oxide-ammonia-air mixtures. This project is in support of the waste tank safety assessment for the storage facility at Hanford, WA. This study focuses on combustion limits, pressure histories, and flame speeds of flammable gas mixtures characteristic of this facility. A facility was constructed to study the lean combustion characteristics of mixtures containing ammonia. This facility consists of a gas-handling system, a remotely-controlled 400-liter pressure vessel, a vacuum system capable of pumping and disposing of the unburned ammonia, and a data acquisition system. A total of 208 experiments were conducted for this study. All experiments were performed using air as a primary oxidizer. Separate experiments were carried out for binary and ternary mixtures containing air, hydrogen, ammonia, hydrogen-nitrous oxide, ammonia-hydrogen, and ammonia-nitrous oxide. Other experiments examined quaternary mixtures of ammonia-nitrous oxide-hydrogen-air. Mixtures were burned under quiescent and turbulent conditions and, in some cases, with an inert substitute (nitrogen) for the nitrous oxide. The pressure and temperature were recorded in the tank throughout the combustion events. A schlieren video photography system was used to observe flame propagation and measure flame speeds. The final equilibrium pressures were also measured in the experiments. The results demonstrate that nitrous oxide is inert in lean mixtures of hydrogen-air-nitrous oxide and reactive in lean mixtures of ammonia-air-nitrous oxide. For mixtures of hydrogen-nitrous oxide-ammonia-air, the reactivity of the nitrous oxide depends on the hydrogen-ammonia ratio. A correlation between the adiabatic, constant-pressure, flame temperature of the mixture and the reactivity threshold of the nitrous oxide is proposed. It was found that the nitrous oxide began reacting when the adiabatic flame temperature was between 1100 and 1300 K. It was found that for flame temperatures above this threshold, presence of nitrous oxide affects the flammability limit, the pressure history, and the flame speed.