Advancing Hydrogen Development from 2015 to 2024 and Mitigating Noₓ Emissions from Hydrogen-Enriched Combustion for a Cleaner Energy Future

Review Advancing Hydrogen Development from 2015 to 2024 and Mitigating Noₓ Emissions from Hydrogen-Enriched Combustion for a Cleaner Energy Future Yi-Kai Chih 1,*, Shang-Rong Kuo 2, and Jing-Jie Wang 2 1 Department of Chemical and Materials Engineering, National University of Kaohsiung, Kaohsiung 811, Taiwan 2 Department of Greenergy, National University of Tainan, Tainan 701, Taiwan * Correspondence: [email protected] or [email protected] Received: 13 December 2024; Revised: 4 March 2025; Accepted: 13 March 2025; Published: 17 March 2025 Abstract: This study explores hydrogen energy’s transformative role in achieving net-zero greenhouse gas emissions, focusing on mitigating nitrogen oxides (NOx), a byproduct of hydrogen-enriched fuel combustion. Driven by rapid growth in hydrogen research from 2015 to 2024, it highlights hydrogen’s potential to address critical energy and environmental challenges. Hydrogen production is classified into thermolysis, biophotolysis, electrolysis, and photoelectrochemical processes, with distinct energy sources and outputs. Color codes denote hydrogen types: green (electrolysis using renewables), blue (carbon capture in natural gas reforming), gray (no carbon capture), pink (nuclear-powered), and turquoise (methane decomposition). By 2050, green hydrogen, aligned with decarbonization goals and declining costs, is expected to dominate the market, while blue hydrogen will act as a transitional source. The paper emphasizes the importance of hydrogen pricing, regional production cost disparities, and strategic investments to enhance low-emission hydrogen competitiveness. However, a major challenge is increased NOx emissions from higher combustion temperatures. This study reviews key mitigation strategies, including hydrogen-natural gas blending, staged combustion, exhaust gas recirculation (EGR), and post-combustion measures such as Selective Catalytic Reduction (SCR). Among these, EGR effectively lowers peak combustion temperatures, while staged combustion optimizes fuel-air mixing to minimize NOx formation. Additionally, SCR remains one of the most efficient post-combustion solutions, reducing NOx emissions by over 80% in various applications. This study demonstrates how these strategies can maximize hydrogen’s energy potential while minimizing environmental impacts.