Numerical investigation on hydrogen-enriched methane combustion in a domestic back-pressure boiler and non-premixed burner system from flame structure and pollutants aspect

In this paper, the non-premixed hydrogen-enriched methane-air combustion was investigated numerically with the use of a CFD code. In the first part of the study, the combustion experiments were performed in a back- pressure boiler using natural gas. The intake rate of fuel was kept constant as 45 Nm³/h while the coefficient for the air excess ratio was changed between 1.2 and 1.35. After the experiments, the numerical analyses were performed. The Fluent code was utilized as the simulation instrument. The eddy dissipation combustion model was selected to be used in the numerical analyses, since it is known that this combustion model can save computational time and fairly predict the combustion flame structure and emissions. Pure methane and natural gas were taken as fuels in the numerical analyses. The obtained results from the numerical analyses were validated with the experimental flue gas temperature and emission measurements. Then, the hydrogen-enrichment of pure methane fuel was investigated numerically in such a way that the boiler capacity (432 kW) was kept constant. The coefficient for the air excess ratio was 1.2 for all the considered combustion simulation cases. The hydrogen addition ratio was 25%, 50% and 75% by mass, respectively. The thermal NO emissions and temperature distributions in the combustion chamber were obtained according to the different hydrogen-enriched methane fuel combustion cases. In addition, the emissions contained in the flue gas together with the temperature values were calculated. The obtained results from the numerical studies indicate that the hydrogen-enrichment of methane reduces the carbon emissions, while it substantially augments the formation of the thermal NO emissions. The calculated thermal NO emission value in the flue gas is 147 ppm for the pure methane combustion case, and it is 566 ppm for the combustion case with 75% by mass of hydrogen addition ratio. Therefore, it is determined that hydrogen fuel is a pollutant from the thermal NO emission aspect for the considered enrichment ratios in the studied domestic boiler-burner system.