Upgrading kilns for structural clay production using green hydrogen as fuel

The ceramic industry, alongside other energy-intensive sectors such as steel and glass, is considered hard-to-abate due to its reliance on high-temperature processes, particularly for firing. Thermal energy demand is almost entirely supplied by fossil fuel combustion, which drives most greenhouse gas emissions in ceramic manufacturing. Decarbonization can be pursued through alternative fuels such as green hydrogen, applied either as a pure fuel or in co-firing with natural gas. This thesis investigates the feasibility of hydrogen adoption in industrial ceramic tunnel kilns for structural clay products, including bricks and roof tiles. A multi-step methodology combining computational fluid dynamics (CFD) simulations, laboratory-scale and semi-industrial experiments, and techno-economic assessments was applied. CFD showed that hydrogen stratification occurs immediately after injection. When the injection point is close to combustion equipment, static mixers are needed for homogeneity. In industrial retrofitting, pipeline geometry often ensures sufficient mixing, keeping the coefficient of variation below 2%. Experimental campaigns on a standard industrial burner demonstrated stable operation across the full substitution range, from natural gas to 100% hydrogen, with firing power between 20 and 80 kW. Hydrogen-rich flames anchored at the nozzle, while natural gas and low-hydrogen mixtures lifted off. No overheating, in-nozzle combustion, or damage was detected. The integration of flame ionization detectors (FID) and thermal conductivity detectors (TCD) for monitoring H2–NG combustion was explored. Optimized electrode positioning enabled reliable FID signals up to 95% hydrogen. TCD accurately quantified fuel composition from 0–100% H2. Combined with flow sensors, these low-cost tools support robust control loops regulating flame stoichiometry under dynamic blends. Industrial-scale firing tests confirmed that hydrogen did not affect ceramic quality. Mechanical and aesthetic properties were comparable to those obtained with natural gas under the same reference curve with a 1050 °C peak temperature, with only slight color shifts due to higher humidity and the absence of CO2. Steel reheating tests between 1100 and 1300 °C showed that hydrogen-rich fuels increased oxidation and oxide scale thickness but without compromising metallurgical integrity. These results confirm hydrogen’s technical viability as a substitute fuel. Semi-industrial tests (40–60 kW) on oxygen-enriched H2–NG combustion demonstrated stable flames across the entire range, shifting reaction zones and modifying structure. Raising O2 concentration from 21% to 50% reduced flue-gas heat losses by 27% for a 50% H2–NG blend. Hydrogen enrichment alone modestly increased NOx (1.5-fold), while oxygen enrichment of natural gas by 50% caused a much larger effect (20-fold). The techno-economic analysis assessed hydrogen substitution in terms of costs, CO2 emissions, and electrolytic oxygen valorization. Results show that while green hydrogen is currently more expensive than natural gas, competitiveness may be achieved by the mid-2030s through electrolysis scale-up and carbon pricing. Oxygen valorization could advance break-even by up to five years. Experiments confirmed that oxygen enrichment (25–29%) in 50% H2–NG blends yielded 22–30% fuel savings, reducing CO2 emissions and improving cost balance. Still, large-scale deployment in ceramics remains economically constrained until hydrogen prices drop substantially. In conclusion, this thesis shows that key technical barriers—pipeline mixing, burner compatibility, combustion monitoring, and product quality—can be overcome. Moreover, it demonstrates that beyond substitution, integrating oxygen enrichment can enhance combustion efficiency, lower fuel use, and accelerate the economic feasibility of hydrogen adoption in ceramics.