Experimental investigation of hydrogen-enriched natural gas combustion in a domestic natural gas compatible boiler

Reducing natural gas consumption, especially carbon emissions in boilers, which are widely used in residential heating, is one of the crucial goals. One of the applications for this purpose is the usage of sources with zero carbon, like hydrogen instead of carbon-based fuels. This way, adding hydrogen to existing natural gas networks for residential heating and cooking purposes seems feasible. Therefore, combustion of hydrogen-enriched natural gas in natural gas compatible domestic boilers used for residential heating will need to be examined in terms of flame structure and emissions. In this study, the safe combustion limits, flame characteristics and emissions of hydrogen-enriched natural gas in a natural gas compatible combi boiler used for domestic heating were investigated experimentally. In an experimental setup, natural gas-hydrogen mixtures with volumetric hydrogen ratios of 5 %, 10 %, 15 %, 20 %, 25 % and 30 % were burned. Firstly, a stable flame was observed at these hydrogen ratios. Then, the flame form and temperature were captured using video and thermal cameras, while flue gas emissions (CO, NOx) and temperatures were measured with a gas analyzer. As the hydrogen content in the mixture rose, the premixed flame length shortened and combustion chamber peak temperatures decreased. Furthermore, NO emissions reduced, NO2 emissions rose, and CO emissions increased slightly. It has been revealed that a 30 % hydrogen blend resulted in an 87 degrees C reduction in peak combustion chamber temperature compared to pure natural gas. It was observed that the temperatures of upper regions were lower in the combustion chamber due to the shortening of the flame length. Measurements from the combustion chamber showed that in these low-temperature zones, CO emissions were unable to oxidize, while NO emissions were converted to NO2 emissions. It has been concluded that the boiler design should be made for hydrogen operation due to changes in the combustion chamber temperature distribution of the boiler designed for natural gas. This study provides technical support on the reasons for flame shape and emission changes as hydrogen addition to natural gas increases in natural gas compatible boilers. It even includes a discussion on emissions improvement and the flame size-combustion chamber relationship.