Oxy-fuel combustion of methane in a swirl tubular flame burner under various oxygen contents: Operation limits and combustion instability

Oxy-fuel combustion of methane was conducted in a rapidly mixed tubular flame burner, as oxygen content (mole fraction) was varied from 0.25 to 1.0. The fuel and oxidizer were injected from individual slits, and the slit widths were varied to promote mixing. Temporal variation of flame structure and acoustic pressure fluctuations were simultaneously measured by using a high speed video camera and a microphone respectively to examine unsteady combustion. Detailed results show that when the oxygen mole fraction was below 0.5, a steady flame can be established in a wide range from lean to rich; above 0.6, combustion-driven oscillation occurred by increasing the equivalence ratio to around 1.0. By reducing fuel slit width to half of the oxidizer slit and assigning CO2 to both fuel and oxidizer slits to maintain the same, high injection velocities, steady tubular combustion range was much expanded. However, when the oxygen mole fraction exceeded 0.7, steady combustion evolved into periodic oscillations by gradually increasing equivalence ratio; around stoichiometry the flame became steady, and thereafter evolved into unsteady state in the rich condition. Based on pressure fluctuation measurements and frequency spectral analyses, high frequency (around 2000 Hz) combustion-driven oscillation was observed. This study shed lights on the operation limits of steady oxy-methane tubular combustion under various oxygen contents. For combustion under ultrahigh oxygen contents (0.7-1.0), the inlet width should be much thinner, and the fuel and oxidizer streams be maintained at the same, high injection velocities to achieve steady tubular flame around the stoichiometry.