In this study, a nonstoichiometric single-phase chemical equilibrium model was developed to simulate atmospheric circulating fluidized bed (CFB) coal gasifiers in reference to the available experimental and simulation data. In the literature, since the single-phase model assumes complete conversion of the solid particles to gas phase, the predictions for the resulting products of the gasifiers considerably deviate from the measured product concentrations. Two different model modifications for carbon conversion and temperature to improve the model results were tested. Particularly, carbon conversion modification has been found to improve the predictive ability significantly. The main reasons of the inaccuracy in the tested cases as well as the performance of some other modifications (quasi-equilibrium approach, empirical correlation for carbon conversion, etc.) were also discussed in comparison with the literature data. An alternative mathematical procedure that simplifies the formulation and numerical implementation was suggested as well.