Luc Vandebroek

A numerical study of the influence of ammonia addition on the auto-ignition limits of methane/air mixtures

In this study the auto-ignition limit of ammonia/methane/air mixtures is calculated based upon a perfectly stirred reactor model with convective heat transfer. The results of four different reaction mechanisms are compared with existing experimental data at an initial temperature of 723 K with ammonia concentrations of 0-20 mol.% and methane concentrations of 2.5-10 mol.%. It is found that the calculation of the auto-ignition limit pressure at constant temperature leads to larger relative deviations between calculated and experimental results than the calculation of the auto-ignition temperature at constant pressure. In addition to the calculations, a reaction path analysis is performed to explain the observed lowering of the auto-ignition limit of methane/air mixtures by ammonia addition. It is found that this decrease is caused by the formation of NO and NO(2), which enhance the oxidation of methane at low temperatures.

T5-2 – Auto-ignition hazard of mixtures of ammonia, hydrogen, methane and air in a urea plant

Publisher Summary In many chemical processes, combustible gases and vapors at high pressures and high temperatures are present. To evaluate the auto-ignition hazard involved and to ensure the safe and optimal operation of these processes, it is important to know the auto-ignition temperature (AIT) of the gas mixtures. The AIT values found in literature are usually determined according to standard test methods in small vessels and at atmospheric pressure. AIT is not a constant but decreases with increasing pressures and increasing volumes. These AIT values are often not applicable to industrial environments. In addition, most available AIT data refer to single-component fuels, while information on multicomponent fuels is scarce. In the study described in the chapter, attention is focused on the auto-ignition hazard inside a urea plant. In the ammonia scrubber and the pool reactor of the plant, mixtures of ammonia, methane, hydrogen, and air are exposed to a temperature of 150°C and a pressure of 15 MPa. To evaluate these mixtures for their auto-ignition characteristics, the AIT of ammonia/air mixtures is determined experimentally for pressures up to 7500 kPa and for concentrations ranging from 20 to 80 mol%. The effect of limited methane and hydrogen additions was also investigated.