Effect of Hydrogen and Ammonia Addition on Self-Excited Thermoacoustic Instabilities of Partially Premixed Methane/Air Flames

Abstract

The pressing need to realize carbon neutrality in gas turbine applications has recently led to a new wave of research on low-carbon energy alternatives, among which hydrogen and ammonia, as carbon-free fuels, have attracted great attention. This work studies the impact of hydrogen or ammonia addition on combustion instability characteristics of partially premixed methane/air flames. Bifurcation behaviors of the low-frequency thermoacoustic system (around 100 Hz) are experimentally identified and explored by decreasing global equivalence ratios (ϕ) from 0.7 to 0.45 at a fixed Reynolds number (Re = 10,000). Three different fuel cases are selected for comparative analysis, namely 100% methane (100M), 75%-25% (vol) methane-hydrogen (75M25H) and methane-ammonia (75M25A) blends. High-speed diagnostics and acoustic measurements are conducted to resolve different flame dynamics and thermoacoustic characteristics. The results show that co-firing methane with hydrogen or ammonia has significant impacts on thermoacoustic bifurcation behaviors and flame dynamics. For the 100M case, a clear mode switch occurs at ϕ = 0.63-0.61 from a low-amplitude limit cycle with a relatively higher frequency (Regime I) to a high-amplitude one with a relatively lower frequency (Regime II), while the mode transition point moves downward to ϕ = 0.59-0.52 with 25% hydrogen addition. Distinctively, no mode transition is observed with 25% ammonia addition, showing purely Regime II within the studied range of ϕ. Besides, the dominant frequency increases with hydrogen addition but decreases with ammonia addition. The three cases show roughly identical flame motions, exhibiting a rapid-extinction-slow-recovery process in the period of combustion oscillation. However, the flame intensity notably strengthens with hydrogen addition but weakens with ammonia addition and phase delay.