Filtration gas combustion in a porous ceramic annular burner for thermoelectric power conversion

Abstract

A numerical study of the combustion of lean methane/air mixtures in a porous media burner is performed using a novelty geometry, cylindrical annular space. The combustion process takes place in the porous annular space located between two pipes, which are filled with alumina beads of 5.6 mm diameter (Al2O3) forming a porosity of 0.4. The outer tube diameter of 3.82 cm is isolated; meanwhile the inner tube of 2 cm in diameter is covered by a continuous set of thermoelectric elements (TEE) for transforming heat energy into electricity. To achieve and maintain the proper temperature gradient on TEE, convective heat losses are considered from the TEE. The respective heat transfer coefficient is variable and is in the range 800 < h < 1500 [W / m2]. The 2D mathematical model includes the energy equations for solid and gas phases, the momentum equations, the continuity equation, the fuel mass conservation, the perfect gas law and it is solved by Means of computational simulations in COMSOL Multiphysics. Computer simulations focus on the two-dimensional temperature analysis and displacement dynamics of the combustion front inside the reactor, depending on the values of the filtration velocity (0.1 < ug0 < 1.0, m/s) and the fuel equivalence ratio (0.06 < Φ < 0.5). The conditions that maximized the overall performance of the process of energy conversion are ug0 = 0.7 [m / s], Φ = 0.363 and h = 1500 [W/m2K], to obtain 2.05 [V] electrical potential, 21 [W] of electrical power and an overall efficiency of process η = 5.64%. The study shows that the cylindrical annular geometry can be used for converting the energy of combustion from lean gas mixtures into electricity, with a performance similar to the specified by manufacturers of TEEs.