Hydrogen combustion is seen as a promising carbon-free alternative for generating process heat in industrial processes requiring high temperatures and large amounts of energy, such as metal melting and reheating. The transition from natural gas to hydrogen in high-temperature furnaces raises concerns about heat transfer efficiency due to the altered flue gas composition. This study examines the influence of hydrogen enrichment of natural gas on thermal performance and emissions in a semi-industrial box furnace equipped with a flameless burner operating in oxyfuel and air-fuel modes at 200 kW. The burner is primarily designed for the aluminum industry, where oxyfuel mode is used for rapid melting and air-fuel mode during holding periods. Seven different NG/H2 fuel cases, ranging from natural gas to pure hydrogen, in both burner configurations, are compared experimentally in terms of temperature and energy distribution, gas emissivity, radiative heat flux, and NOx emissions. Additional heating trials on aluminum with hydrogen and natural gas are conducted to quantify the radiative and convective contributions using a combined experimental and numerical approach. Results show that under oxyfuel conditions, hydrogen addition has minimal impact on furnace temperature, flue gas losses, and radiative heat flux. In contrast, air-fuel operation exhibited continuous temperature rise and radiative heat flux, along with reduced flue gas losses. NOx emissions remained below 5 mg/MJ for oxyfuel and 12 mg/MJ for air-fuel combustion, with pure hydrogen achieving lower values than natural gas due to the elimination of prompt NOx. Aluminum heating trials revealed a 6 % improvement in heat flux with hydrogen under oxyfuel and 19 % under air-fuel conditions. These findings confirm that hydrogen can be effectively integrated into flameless combustion without compromising combustion performance, with oxyfuel technology offering greater potential for efficiency gains and NOx mitigation.
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