In this study, the influence of various factors on nitrogen oxides (NO_x) emissions of a low NO_x burner is investigated using a central composite design (CCD) approach to an experimental matrix in order to show the applicability of design of experiments methodology to the combustion field. Four factors have been analyzed in terms of their impact on NO_x formation: hydrogen fraction in the fuel (0%–15% mass fraction in hydrogen-enriched methane), amount of excess air (5%–30%), burner head position (20–25 mm from the burner throat) and secondary fuel fraction provided to the burner (0%–6%). The measurements were performed at a constant thermal load equal to 25 kW (calculated based on lower heating value). Response surface methodology and CCD were used to develop a second-degree polynomial regression model of the burner NO_x emissions. The significance of the tested factors over their respective ranges has been evaluated using the analysis of variance and by the consideration of the coefficients of the model equation. Results show that hydrogen addition to methane leads to increased NO_x emissions in comparison to emissions from pure methane combustion. Hydrogen content in a fuel is the strongest factor affecting NO_x emissions among all the factors tested. Lower NO_x formation because of increased excess air was observed when the burner was fuelled by pure methane, but this effect diminished for hydrogen-rich fuel mixtures. NO_x emissions were slightly reduced when the burner head was shifted closer to the burner outer tube, whereas a secondary fuel stream provided to the burner was found to have no impact on NO_x emissions over the investigated range of factors.

A promising candidate for CO₂ neutral power production is semiclosed oxyfuel combustion combined cycles (SCOC-CC). Two alternative SCOC-CCs have been investigated both with recirculation of the working fluid (WF) (CO₂ and H₂O) but with different H₂O content due to different conditions for condensation of water from the working fluid. The alternative with low moisture content in the recirculated working fluid has shown the highest thermodynamic potential and has been selected for further study. The necessity to use recirculated exhaust gas as the working fluid will make the design of the gas turbine quite different from a conventional gas turbine. For a combined cycle using a steam Rankine cycle as a bottoming cycle, it is vital that the temperature of the exhaust gas from the Brayton cycle is well-suited for steam generation that fits steam turbine live steam conditions. For oxyfuel gas turbines with a combustor outlet temperature of the same magnitude as conventional gas turbines, a much higher pressure ratio is required (close to twice the ratio as for a conventional gas turbine) in order to achieve a turbine outlet temperature suitable for combined cycle. Based on input from the optimized cycle calculations, a conceptual combustion system has been developed, where three different combustor feed streams can be controlled independently: the natural gas fuel, the oxidizer consisting mainly of oxygen plus some impurities, and the recirculated working fluid. This gives more flexibility compared to air-based gas turbines, but also introduces some design challenges. A key issue is how to maintain high combustion efficiency over the entire load range using as little oxidizer as possible and with emissions (NO_x, CO, unburnt hydrocarbons (UHC)) within given constraints. Other important challenges are related to combustion stability, heat transfer and cooling, and material integrity, all of which are much affected when going from air-based to oxygen-based gas turbine combustion. Matching with existing air-based burner and combustor designs has been done in order to use as much as possible of what is proven technology today. The selected stabilization concept, heat transfer evaluation, burner, and combustion chamber layout will be described. As a next step, the pilot burner will be tested both at atmospheric and high pressure conditions.