INVESTIGATION OF THE EFFECTS OF HYDROGEN-ENRICHED NATURAL GAS BLENDS AND PURE HYDROGEN ON THE PERFORMANCE OF A DOMESTIC CONDENSING BOILER

Following increased awareness about the deleterious effects of climate change on the environment, many corporations and institutions are making concerted efforts to reduce their greenhouse gas emissions through policy reviews and technological changes. One area of promise is the replacement of natural gas (NG) with hydrogen (H-2) for use in residentialand industrial sectors. Hydrogen can play a significant role in reducing emissions in heating applications within the built environment. In a net-zero emissions future where distributed combustion emissions must be largely eliminated, zero-carbon fuels and hydrogen solutions for heating are some of the most cost-effective and flexible paths that will facilitate the sector's energy transition. In the short term, hydrogen is currently being blended in small amounts into existing NG networks around the world. This has an impact on the performance of appliances designed for NG, as certain gas properties will change. In this study, the effects of hydrogen on a domestic condensing boiler designed for NG, was investigated. The combustion reaction as a function of hydrogen content and oxygen content was determined. Stoichiometric, lean, and incomplete combustion were considered. Incomplete combustion leads to the formation of polluting emissions, while lean combustion is used to mitigate this by using excess air. An analytical model was developed to determine the outputs, that are, theoretical condensable water mass, heating value ratio, saturation temperature, maximum saturation temperature, and condensation efficiency, which were the main outputs that were systematically analyzed. Results showed that water vapour in the exhaust gas had the most remarkable effect on properties, and in consequence, on the boiler's operation. Optimum efficiency occurred when the molar fraction of water vapour was maximized. This corresponded to the combustion of pure hydrogen at stoichiometric conditions. It was further observed that pollutant emissions formation was minimized with greater amounts of excess air via lean combustion, which also resulted in a smaller water vapour fraction. The addition of hydrogen also reduced emissions since the only combustion product of pure hydrogen is water. Hydrogen should be used in lean combustion with minimal excess air to optimize economic and environmental benefits. This guarantees greater performance over pure natural gas dispensed in the same conditions, while mitigating the formation of polluting emissions.