Inrecent years, the lattice Boltzmann method (LBM) has been utilized to simulate various complex flow situations, including multiphase flows, microflows,bioflows, and convection-diffusion and heat transfer processes.Unlike the extensive efforts in LBM boundary methods for fluid flows, complex boundary conditions for the convection-diffusion and heat transfer processes have not been well addressed.In this work, a novel bounce-back scheme is proposed for LBM simulations of such processes with complex boundary conditions.Compared with existing boundary methods, our model can easily deal with boundaries with general geometries.Numerical results are compared with theoretical and numerical solutions, and a good agreement has been observed.This work might be useful in the flow and heat transfer studies of gun lunching safety and weapon efficiency.