Experimental and numerical studies on combustion characteristics of N ₂ -diluted CH ₄ and O ₂ diffusion combustion in a packed bed

Abstract

Experimental and numerical studies were conducted to determine the combustion characteristics of gas diffusion combustion in a porous combustor packed with 2.5 mm or 3.5 mm alumina pellets, special attention being focused on the effect of packed bed height ( h ) on combustion, NO and CO emissions. The pollutant emission of diffusion filtration combustion is studied with different packed bed lengths in the range of 40 mm ≤ h ≤ 240 mm, fixed excess air ratio of 1.88 and fixed gas inlet velocity of 0.06 m s −1 . Results show that both immersed and surface flames coexist in the combustor. Although porous media enhance the mixing and diffusion processes, the diffusion flame shape is still observed from the side and top views of the combustor, and the diffusion filtration retains properties of diffusion combustion. The immersed flame is always observed with increase in h , whereas the height of surface flame decreases. The NO emission decreases sharply when h is increased from 40 mm to 120 mm. However, the NO emission decreases slightly when h > 120 mm. In the investigated range of h , it is shown that h has a significant influence on the CO emission, an increase in h leading to a constant increase in CO for the combustors packed with 2.5 mm or 3.5 mm pellets. The maximum CO emission is 662 ppm and the minimum value is 67 ppm. In the scope of this study, the temperature on the external wall of the combustor reaches 434–513°C.

The Effects of Multilayer Porous Media on the Efficiency of Filtration Combustion

“…This can be attributed to the combined effects of conduction and radiation heat transfer within the solid matrix, where the incoming reactants are preheated, combined with the increased surface area and decreased porosity of the downstream layer consequently yielding higher temperature. Figure 2 (a) and (b) show a conical flame outside of the porous zone, which is consistent to that of a diffusion flame as outlined in [14] meanwhile the lack of such flame in Figure 2 (c) can be attributed to the higher amount of methane present in the downstream region of the layered porous media and above in the free zone as seen in Figure 3. Furthermore, temperature increase in the porous zone with an additional layer (b) and further increase with height decrease of the higher porosity layer (c) indicates that the gas mixture velocity is increased, and the mixture is accelerated within the porous media.…”

“…Immersed packed bed flame characteristics were found to exhibit the outcome that of diffusion combustion, but experimental measurements showed wider high temperature zone and temperature distribution contradicted diffusion combustion characteristics. In a subsequent study Shi et al [14] conducted experimental and numerical study on DFC using a porous combustor with varying alumina pellet diameters, 2.5 and 3.5 mm. Results showed a coexisting immersed and surface flame within the combustor exhibiting diffusion combustion characteristics.…”