Gas-solid reactions have a very wide range of scales that significantly challenge computational models. This paper presents a first principles based rate equation theory that connects the various scales and couples the chemical reactions with the diffusion of the solid state ions. The theoretical models cross all the 'atom→grain→particle' size scales with efficient algorithms. The results show a discrete island growth mechanism of solid product on surfaces or interfaces. The rate equation theory is applied to CFD models of oxidation/reduction kinetics in chemical looping combustion, surface and interfacial phenomenon in thermal chemical energy storage, pollutant control, coal combustion and dual fluidized bed reactors.