This article presents a two-dimensional (2D) numerical study on hydrodynamics and thermal transport in the presence of superimposed thermal buoyancy in a rectangular channel with staggered ribs mounted on both walls. This study is carried out for the Newtonian fluid at moderate Reynolds number. The Reynolds number is considered within the range (50 <= Re <= 120) with constant Prandtl number Pr=0.7, and mixed convective heat transfer is studied for Richardson number range 0 <= Ri <= 2. The unsteady 2D governing equations are solved using the lattice Boltzmann method. The computational strategy is applied in various test cases and validated with the results reported in the literatures. The flow and heat transfer characteristics are analyzed with the streamlines and isotherm patterns at various Reynolds and Richardson numbers. The effects of superimposed thermal buoyancy on flow and isotherm patterns, drag coefficient, local Nusselt number distribution, and time- and surface-average Nusselt number are presented and discussed.