Towards the development of liquid ammonia/air spray combustion in a gas turbine-like combustor at moderately high pressure

This study is a numerical investigation of the potentials of liquid ammonia (LNH3) spray combustion in a gas turbine-like combustor at moderately high pressure. The energy costs for the vaporization of LNH3 and the compression of the fuel vapor in the gaseous ammonia (GNH3)/air combustion systems, as well as the capital cost for the equipment involved, can be mitigated by employing LNH3 spray combustion. Additionally, responsiveness to power demand can easily be controlled using direct liquid ammonia. In the first part of this study, the spray characteristics of pure LNH3 were extensively studied numerically and experimentally using a hollow cone nozzle to validate the numerical models and codes. In modeling LNH3 sprays at atmospheric pressure or moderately high ambient pressures, accounting for the flash (superheat) evaporation phenomenon in addition to equilibrium evaporation is very important. Measured and simulated data both show that LNH3 spray (both vapor and droplet phases) reached a minimum equilibrium temperature of 215 K (-60 degrees C) when LNH3 at a pressure of 0.9 MPa was injected into an environment at 0.1 MPa and 283 K. This minimum temperature is much lower than the saturation temperature of LNH3 of 240 K (-33 degrees C) at ambient pressure of 0.1 MPa due to occurrence of nonequilibrium flashing. The measured average LNH3 droplet size approximately varied from 15 - 20 mu m and the numerical results agreed with these measured values. In the second part of the study, numerical simulations of LNH3 spray combustion were carried out at a moderately high pressure of 0.3 MPa. First, the LNH3 spray was cofired with gaseous hydrogen (GH2) while maintaining approximately 50 %, 60 %, and 70 % of ammonia in the fuel by energy fraction (ENH3) based on the lower heating value at a moderately high pressure of 0.3 MPa. Finally, 100 % LNH3 spray flame was simulated. Preheated swirling airflow at 600 K was used to enhance the stability of the LNH3 spray flames, enabling stable combustion over a wide range of global equivalence ratios from 0.99 through 1.43. Moreover, the simulated emission characteristics of LNH3/GH2/air flames at ENH3 = 60 % were compared to those of GNH3/GH2/air flames (ENH3 = 60 %) at the same equivalence ratios. The NO and unburnt emissions of LNH3/GH2/air (ENH3 = 50 % - 100 %) flames and GNH3/GH2/air (ENH3 = 60 %) flames show a trade-off relation.