Experimental Investigation into the Behaviour of Methane-Hydrogen Blends and Exhaust Gas Recirculation in the Enclosed-Jet-In-Hot-Coflow Burner

Abstract

Hydrogen, as a carbon-free fuel, offers a promising solution to reduce CO2 emissions in pursuit of cleaner combustion technologies. However, its combustion poses challenges, including increased NOx production due to high adiabatic flame temperatures, along with risks of flashback and thermo-diffusive instabilities. One approach to addressing these challenges is Moderate or Intense Low-Oxygen Dilution (MILD) combustion, which significantly lowers NOx emissions by reducing reaction temperatures. Techniques like exhaust gas recirculation (EGR) can further support this process. Additionally, blending hydrogen with other fuels, such as ammonia and methane, improves flame stability, safety, and flame speed.

This thesis is part of the ACHIEVE project (Advancing the Combustion of Hydrogen-Ammonia Blends for Improved Emissions and Stability), funded by the European Union under the Horizon European Research and Innovation Actions in collaboration with the Clean Hydrogen Partnership. The project aims to advance Technology Readiness Level 4 technologies that reduce pollutants in unconventional hydrogen blends, such as mixtures of NH3, CH4, and H2. The combustion characteristics of these unconventional blends are studied numerically and experimentally in high-stability combustors, with a focus on MILD combustion and the effects of exhaust gas recirculation.

Aligned with these goals, the TU Delft Enclosed-Jet-in-Hot-Coflow (EJHC) burner, designed for MILD combustion, is used to experimentally investigate premixed and non-premixed methane-hydrogen and pure hydrogen flames. The research examines flame morphology, temperature, emissions profiles, and the effects of exhaust gas recirculation on NOx emissions. This serves as a foundational step for future research into ammonia-hydrogen blends. The burner features a central flame auto-ignited by a hot vitiated coflow produced by a secondary burner. Design modifications were implemented to improve coflow uniformity near the jet exit.

The research involved two experimental campaigns. The first, conducted under cold flow conditions using Particle Image Velocimetry (PIV), assessed coflow uniformity at different inlet velocities. The second campaign focused on reactive flow experiments, using thermocouples, chemiluminescence, and gas analyzers to study methane-air coflow and methane-hydrogen blends in premixed and non-premixed conditions. Hydrogen concentration was varied from 0% to 100%, with different power ratios between the central jet and coflow.
The study showed that using a perforated plate as a secondary burner enhanced coflow uniformity, with PIV results showing reduced radial variations of the vertical velocity component. Thermocouple measurements confirmed a uniform temperature profile.
Flame lift-off height and shape in both premixed and non-premixed conditions were influenced by jet velocity, laminar flame speed, and hydrogen content. Hydrogen addition reduced lift-off height. In premixed conditions, flames were compact and narrow, while in diffusion conditions, higher hydrogen content increased mixing with the coflow at greater radial distances. NO emissions remained stable in premixed cases but were affected by coflow composition in diffusion flames, with lower oxygen levels enhancing NO reburning.

This study provides a versatile system for investigating exhaust gas recirculation and hydrogen blends over a wide range of power and equivalence ratios. It lays the foundation for future studies on ammonia-hydrogen combustion.