Flame stability and emissions characteristics of liquid ammonia spray co-fired with methane in a single stage swirl combustor

Direct combustion of liquid ammonia spray can reduce the cost and start-up time required for gas turbines fuelled with otherwise gaseous ammonia. This article reports the first successful study on liquid ammonia spray combustion in a novel swirl combustor with preheated air at 500 K. Particle Image Velocimetry (PIV) was employed to measure the non-reacting air flow field in the combustor while the liquid ammonia spray was visualized by Mie scattering techniques. Emissions from the flame were analysed using a Fourier Transform Infrared spectrometer. The present combustor encouraged a large central reverse flow, counter flowing with the spray. Hence, the preheated air enhanced the dispersion and evaporation of the liquid droplets, and thus promoted shorter spray heights. Stable combustion of liquid ammonia spray alone was achieved, while co-combustion with methane enhanced the combustion of the spray and reduced the flame height. For 70% ammonia fraction in the fuel by lower heating value, the flame was stable for global equivalence ratios, phi, between 0.66 and 1.37. The stability range narrowed as ammonia fraction and swirler mean inlet velocity increased. Emission analysis from the co-combustion shows that an optimum low emission phi exists at 1.06, thus rich-lean combustion can be employed in two-stage combustors to further control emissions from the spray flame when the primary zone phi is slightly rich. Numerical analysis indicates that the large latent heat of vaporization of liquid ammonia may contribute to a shift of the low emission phi towards leaner values.