Combustion of a single aluminum particle in a solid propellant flame is studied with complex chemistry and detailed molecular transport. The reacting flow around the droplet is treated as spherically symmetric. In a new droplet model, an average effect of the alumina cap is taken into account. The combustion model is validated with respect to experimental results and an axisymmetric simulation. The combustion of two particle classes is simulated with different surface reaction mechanisms. The effect of surface mechanism on the temporal evolution of particle parameters is investigated. The burning time and residue size are evaluated for the simulated cases.