In a rectangular fluidized bed combustor, the tracer gas is injected continuously into the bed from a point source at the center of the distributor plate. In this study, a general governing equation is formulated for tracer gas dispersion in the bed. An analytical solution is derived to estimate the dispersion coefficients, D-x and D-y, in a horizontal plane. The concentration profiles at different sampling heights with various gas velocities are plotted. Subsequently, to estimate the dispersion coefficients, surface fitting of the obtained analytical solution to the experimental data is performed. The dispersion coefficients obtained from this model are compared with those of a conventional model. Additionally, the effect of walls, bed height and gas injection rate on the dispersion coefficients in a horizontal plane is investigated, and the effect of distributor design on the dispersion coefficients in a horizontal plane is investigated with different tracer positions. It is found that D-x and D-y are nearly equivalent at a lower tracer gas ratio of the injected gas to the total gas flow rate. It is also demonstrated that the effect of bed height on D-x is minor. This model is also able to estimate the dispersion coefficients in the case of a multi-horizontal nozzle distributor. (C) 2017 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.