Experimental Observation of Ammonia Blending and Cracking on Lean Blow-Out Behaviors in a Model Gas Turbine Combustor

Abstract With a large focus on decarbonization in recent years, the use of ammonia has been proposed as a possible carbon-free fuel for internal combustion engines. The use of ammonia presents technical challenges, since ammonia exhibits considerably low reactivity compared to hydrocarbon fuels, including slow laminar flame speed, low energy density, and significantly narrow flammability. To cope with low reactivity of ammonia combustion, two approaches have been explored; blending with hydrocarbon fuel and/or integrating with ammonia catalysis cracker that converts ammonia to hydrogen/nitrogen mixture. The objectives of this study are to investigate the impacts of ammonia blending on hydrocarbon fuel and to evaluate the required ammonia cracking fraction on the operability of gas turbine combustors. A model gas turbine combustor with optical access has been developed, where the operability limit, lean blow-out (LBO) behaviors, can be experimentally determined by controlling fuel mass flow rate at various air mass flow rates at 1 atm at a maximum of 10 kW of thermal power. As a representative fuel for hydrocarbon fuel, methane is utilized and the impacts of ammonia blending and/or cracking on LBO are compared to LBO behaviors of methane. Experimental results show significantly different LBO behaviors between pure methane and ammonia, a factor of 7 difference in terms of global equivalence ratio. Ammonia blending with methane renders an increase of effective flame volume, thus suppressing overall reactivity, consequently resulting in the increased global equivalence ratio at LBO, indicative of hindering the operability. The ammonia cracking is experimentally simulated by blending ammonia with hydrogen/nitrogen mixture (75/25% in mole). Similar LBO behaviors are observed, exhibiting hindered operability by reducing ammonia cracking fraction. While theoretical assessment through laminar flame speed estimation suggests ∼40% cracking required, the experimental results suggest that a comparable LBO behavior to pure methane can be achieved by cracking 25–30% of ammonia. To further characterize the flame stabilization modes associated with flow recirculation zone, high-speed imaging with power spectral density analysis is employed. Two distinct flame stabilizations modes are observed for ammonia cracking with flame anchoring initially located in the outer recirculation zone, however, as LBO is approached, flame anchoring transitions to the inner recirculation zone. The measured LBO behaviors and the transition of flame stabilization are further characterized through scaling analysis by considering the mass flow rates of fuel and air, laminar flame speed, and a representative rotational velocity in recirculation zone or mean fuel jet velocity.