To promote energy and environment security through combustion efficiency improvement as well as CO2 capture and sequestration (CCS), in this study oxygen enhanced combustion of methane has been investigated by using an inherently safe technique of rapidly mixed tubular flame combustion. As a new type of flame, the tubular flame has excellent flame characteristics such as negligible heat loss, aerodynamic stability and thermodynamic stability. Various applications have been proposed and demonstrated for determining the flammability limits, stabilizing a flame in a high speed flow, and obtaining a uniform and large-area laminar flame to heat iron slab or to reduce steel sheet surface. Especially, by individually injecting the fuel and the oxidizer into a cylindrical burner through four tangential slits hence, hence without flame flashback, the rapidly mixed tubular flame burner has been applied to analyze the characteristics of oxygen enhanced methane flame.
To make a fundamental investigation, methane oxygen combustion has been attempted under various oxygen mole fractions with nitrogen and carbon dioxide as the diluents respectively. At first, nitrogen was added to the oxygen stream, and the oxygen mole fraction in the oxidizer was increased from 0.21 to 1.0. A stable, laminar tubular flame can be obtained from lean to rich when the oxygen mole fraction is no more than 0.4. And the maximum adiabatic flame temperature reaches around 2700 K. To enhance the mixing of fuel and oxidizer, nitrogen was also added to the fuel inlet to increase the injection velocity of fuel stream. The results show that by assigning the nitrogen to both the fuel and oxygen inlets to approach the same injection velocity, the flames become more uniform and stable. However, the range of stable tubular flame in equivalence ratio remains almost the same.
Secondly, instead of nitrogen, carbon dioxide was used to dilute the methane/oxygen flames. Thus, the NOX emissions introduced by nitrogen will be greatly reduced, in addition, the main exhaust will be carbon dioxide and steam, which is beneficial for CCS. When carbon dioxide was only added into the oxygen stream, a stable tubular flame was obtained from 0.9 to 1.2 in equivalence ratio at the oxygen mole fraction of 0.21. With an increase of oxygen mole fraction, the stable tubular flame range enlarges in equivalence ratio, and up to the oxygen mole fraction of 0.50, stable tubular combustion could be achieved from lean to rich. By adding carbon dioxide to both the fuel and oxygen inlets to approach the same injection velocity, the upper limit of stable tubular flame increases much. Up to the oxygen mole fraction of 0.86, the stable combustion can be achieved at the stoichiometry, which gives a flame temperature around 3000 K.
To fully understand the flame characteristics above, the chemical effects of carbon dioxide are numerical analyzed in comparing with the nitrogen diluted flames using the CHEMKIN PREMIX code with the GRI kinetic mechanism.
