Experimental study on the combustion of methane-hydrogen mixtures in a pilot-scale furnace

• Hydrogen blending reduces carbon dioxide emissions in a pilot furnace. • Nitrogen oxide emissions increase near stoichiometric hydrogen operation. • Lean combustion mitigates nitrogen oxide formation in hydrogen rich flames. • Optical diagnostics identify fuel dependent changes in flame structure. This study examines hydrogen as a low-emission alternative to natural gas in industrial furnace combustion, focusing on its well-known benefits of reducing carbon emissions and the technical challenges it presents. While decreasing carbon-based emissions with increased hydrogen content is well documented and serves as a primary motivation for its adoption, this work investigates the broader implications, particularly the increase in nitrogen oxide emissions, a significant contributor to air pollution, due to elevated combustion chamber temperatures. Experimental tests were conducted in a pilot-scale industrial furnace equipped with a burner operating at 42 kW, using pure methane, pure hydrogen, and various hydrogen-methane blends, over air-excess ratios ranging from 1.0 to 1.6. Temperature, heat transfer, pollutants, and radical-species emissions during combustion were measured using thermocouples, gas analyzers, spectroscopy, and optical imaging. Across the investigated air-excess range, carbon dioxide emissions decreased progressively by 10.5%, 18.8%, 45.3%, and 100% as the hydrogen content increased from 25% to 100% (relative to pure methane). In contrast, average nitrogen oxide emissions were maintained for a mixture of 25% of hydrogen, while they increased up to 28.5% for the blend with a 75% hydrogen content (relative to pure methane). Pure-hydrogen operation resulted in higher nitrogen oxide emissions, but these were partially mitigated by operating under lean conditions. Overall, hydrogen-enriched combustion supports decarbonization but can increase nitrogen oxide emissions, highlighting an important trade-off. Chemiluminescence analysis and visual diagnostics using RGB and Ultraviolet imaging further highlighted the qualitative differences between methane and hydrogen flames, with important implications for flame monitoring, real-time diagnosis of fuel composition, and safety in hydrogen-fired systems. These findings improve understanding of hydrogen’s role in industrial decarbonization and motivate the development of combustion strategies tailored to effectively control nitrogen oxide emissions.