The thermal efficiency of a hydrogen-rich gas boiler is significantly affected by hydrogen purity, and its quantitative relationship requires comprehensive analysis from three dimensions: combustion characteristics, heat transfer process, and pollutant generation. As a high-calorific-value fuel, changes in hydrogen purity directly alter the combustion temperature, flame propagation speed, and radiation characteristics of the mixed gas, thus affecting the boiler's heat exchange efficiency and heat loss distribution.
The impact of hydrogen purity on combustion temperature is the foundation of quantitative analysis. When high-purity hydrogen burns, the flame temperature is higher than that of natural gas. This is because hydrogen has a high calorific value per unit mass and its combustion products are only water vapor, without low-radiative-value gases such as carbon dioxide. As the hydrogen blending ratio increases, the adiabatic combustion temperature of the mixed gas exhibits a non-linear upward trend, especially after the hydrogen-rich ratio exceeds a certain threshold, where the temperature increase intensifies. This high-temperature environment enhances radiative heat transfer within the furnace, causing the heated surfaces to absorb more heat, but it may also lead to localized overheating, increasing thermal stress on the heated surface materials, which may affect the equipment's lifespan in the long term. Furthermore, high temperatures accelerate the formation of nitrogen oxides, requiring emission control through combustion adjustment or exhaust gas treatment technologies. Changes in flame propagation speed are a key factor affecting combustion stability. The flame propagation speed of hydrogen is much higher than that of natural gas, and the combustion characteristics of the mixed gas significantly change after hydrogen blending. Under low load or low excess air coefficient conditions, high-purity hydrogen may lead to decreased flame stability, and even backfire or flameout, affecting combustion efficiency. To maintain stable combustion, it is necessary to adjust the burner structure or adopt staged combustion technology, such as optimizing the fuel-air mixing ratio through premixed burners, or using multi-stage air distribution to control the flame front position. While these adjustments can improve combustion stability, they may increase flow resistance, leading to increased flue gas heat loss, requiring a balance between thermal efficiency and safety.
Optimizing the heat transfer process is a core aspect of improving thermal efficiency. Hydrogen combustion products have a high water vapor content, with a significant latent heat. Traditional gas boilers mainly recover sensible heat from flue gas, while hydrogen-rich gas boilers can recover the latent heat of water vapor through condensing heat exchangers, allowing thermal efficiency to exceed the limits of traditional boilers. For example, at the same flue gas temperature, hydrogen-rich gas boilers achieve higher condensation heat recovery than natural gas boilers due to the release of latent heat of steam. However, excessively high hydrogen purity may lead to an increase in flue gas dew point temperature, exacerbating the risk of low-temperature corrosion. Therefore, corrosion-resistant materials or optimized heat exchange surface arrangements are necessary to balance heat recovery and equipment lifespan.
The indirect impact of pollutant generation characteristics on thermal efficiency cannot be ignored. Hydrogen combustion does not produce carbon dioxide, but nitrogen oxide emissions are affected by both flame temperature and excess air coefficient. When high-purity hydrogen is burned, improper control of the excess air coefficient can significantly increase nitrogen oxide generation.
To meet emission standards, low-NOx combustion technologies, such as flue gas recirculation or staged combustion, are required. While these technologies can reduce nitrogen oxide emissions, they may increase combustion resistance or lower combustion temperature, indirectly affecting thermal efficiency. Therefore, combustion parameters need to be optimized experimentally to achieve a synergistic improvement in environmental protection and efficiency.
In actual operation, the impact of hydrogen purity on thermal efficiency also needs to consider boiler load and fuel adaptability. Most existing gas-fired boilers are designed for natural gas, and their burners, heat exchange surfaces, and control systems may not be fully adapted to high-purity hydrogen. For example, high-purity hydrogen may cause excessively high burner outlet velocities, leading to vibration or noise; or changes in the heat transfer coefficient of the heat exchange surface may cause the flue gas temperature to deviate from the design value. Therefore, hydrogen-rich gas boilers require customized designs for hydrogen characteristics, such as optimizing the burner structure, using heat exchange surfaces with high thermal conductivity materials, or integrating intelligent control systems to fully release the efficiency potential of hydrogen combustion.
The thermal efficiency of hydrogen-rich gas boilers is significantly affected by hydrogen purity, and its quantification requires comprehensive evaluation using multiple dimensions of parameters, including combustion temperature, flame propagation speed, heat transfer process, and pollutant generation. In practical engineering, it is necessary to combine boiler design parameters, operating conditions, and environmental requirements, and optimize the hydrogen blending ratio and combustion control strategy through experiments and simulations to maximize thermal efficiency and ensure safe and stable operation.
