Hollow spheres as inert packed bed from lean to rich combustion in porous media

Abstract

• Hollow spheres were used in filtration combustion processes. • Lean and rich fuel-air filtration combustion was investigated. • Numerical and experimental comparison of solid and hollow spheres. • Hydrogen and syngas were produced at high temperatures. Hollow spheres as inert packed bed is an innovative technology with high potential in filtration combustion. Their lower specific density, geometry, and the radiation inside the sphere enhance the overall heat transfer of the sphere, and therefore, increase the effective conductivity of the packed bed, while the other thermo-physical properties remain unchanged. The present work investigates numerically and experimentally the effects of the packed bed made of hollow alumina spheres filled with air, from lean to rich premixed methane-air combustion within this porous media, with focus on the production of hydrogen and carbon monoxide species on rich conditions. The experimental part considers the equivalence ratio from 0.8 to 1.3. The numerical approach contemplates a one-dimensional model based on two-temperature approximation (gas and solid) with a four-step chemical reaction mechanism for homogeneous reactions at equivalence ratios between 0.4 and 1.6, for different spheres geometries. The results of experimental and numerical investigations are in good agreement. Both indicate that the use of hollow spheres packed bed (HS), in contrast to the packed bed made of the solid spheres (SS), shows higher temperatures and flame front velocities in all the equivalence ratio ranges. At the stoichiometric equivalence ratio, the numerical investigation in the HS case results in higher temperatures of 168 K and 21 K, in the solid and gas phases, respectively, in comparison with the SS case. The combustion products for different spheres geometries and equivalence ratios present the same trend and values for both HS and SS cases. The yields of carbon monoxide and hydrogen goes up to 37% and 42%, respectively.