Retrofitting a standard oil-fired boiler to oxy-fuel mode: study on radiative and soot formation properties

Authored by: Mario Ditaranto, Morten Seljeskog & Inge Saanum, SINTEF Energy Research, Trondheim, Norway

A mail can be sent to the corresponding author at mario.ditaranto@sintef.no

In this paper, the effects of N₂ replacement by CO₂ on an industrial and laboratory flames is investigated. Thermal efficiency was found to monotonically increase with increasing O₂ concentration in the oxidizer. Soot formation and flame radiation were measured; the results suggest that the increase in radiative heat flux in O₂ enriched oxidizer systems is probably more dominated by temperature than soot formation.

Summary:

Oxy-fuel combustion is a recognized technology for capturing carbon dioxide (CO₂) from the power and industrial processes. Its retrofitting potential is an important characteristic for both the re-vamping of existing conventional boilers and the design of capture-ready plants. In this study, a light oil fired Central European Norm boiler has been retrofitted to oxy-fuel mode by simply replacing the burner blower by a premixing chamber for O₂ and CO₂ for simulating dry flue gas recirculation. Although the transition from air to oxy-fuel did not show major issues, the heat transfer and thermal efficiency of the boiler were affected. The thermal efficiency was found to monotonically increase with increasing O2concentration in the oxidizer, and was equal to that of air at 32-35% O₂ depending on operating equivalence ratio. Higher heat fluxes in the radiant section of the burner were also measured for all oxy-fuel cases investigated. Studies at laboratory scale and laminar conditions to analyse the influence of O₂ and CO₂ on both the radiative properties and soot formation have highlighted the global effects of the oxidizer composition. Where the radiant fraction and peak radiative heat flux increase as the O₂ concentration increased. The radiant fraction for a 35% O₂ mixture in CO₂ combustion was found to be equal to that of an air flame despite a difference in adiabatic flame temperature. Both the O₂ concentration and the replacement of N₂ by CO₂ were shown to have an influence on soot formation. The peak soot volume fraction increased with O₂ concentration until 70% O₂ in O₂ – CO₂ oxidizers, but was found to be lower than in oxygen enriched air flames at similar Reynolds number conditions reported in the literature, supporting the idea that the increase in radiative heat flux in O₂ – CO₂ systems is more dominated by temperature than soot formation.

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