Hydrogen–Natural Gas Blends in Combined Heat and Power Systems: A Comprehensive Review of Energy Performance, Emission Characteristics, and Integration Challenges

The decarbonization of energy systems has intensified interest in hydrogen-enriched natural gas (H2NG) as a transitional fuel for combined heat and power (CHP) units and micro-CHP systems. This review consolidates experimental and numerical studies that explore the energy, environmental, and techno-economic implications of H2NG blends in CHP applications. Research conducted over the last decade highlights that enriching natural gas with hydrogen extends the flammability limits, enhances combustion stability, and reduces CO2 and CO emissions, while maintaining or improving electrical efficiency. However, these benefits are accompanied by higher NOx formation under stoichiometric conditions, which can be mitigated by operating under lean-burn regimes. The review further examines hybrid solutions that integrate electrolyzers, photovoltaic systems, and oxygen-enriched combustion to improve system flexibility and sustainability. The findings consistently show that moderate hydrogen fractions (5–20% vol.) provide optimal trade-offs between efficiency gains and emission control, supporting the role of H2NG as an intermediate step toward fully hydrogen-powered CHP technologies. Technical challenges related to ignition control, thermal recovery efficiency, and infrastructure adaptation are also discussed, along with emerging strategies for techno-economic optimization. This comprehensive assessment contributes to understanding how hydrogen blending can accelerate the transition to low-carbon, distributed energy systems.