In recent years, gas mixtures are being used as alternative fuels in combustors. These gas mixtures are obtained by different methods. For instance, coal gasification and carbonization as coal have the largest reserves among fossil fuels. Gas mixtures obtained via coal gasification and carbonization are called water gas, generator gas, town gas and coke oven gas. These fuels contain various gases. As a result of this, heating values of fuels are also different. Therefore, combustion performances and emission characteristics of these fuels need to be investigated. In this study, combustion performances and emissions including CO, CO2 and NOX of water gas, generator gas, town gases, coke oven gas and methane were numerically investigated in a model gas turbine combustor. The numerical modelling of turbulent nonpremixed diffusion flames has been performed in this combustor. Mathematical models used in this study involved the k-epsilon model of turbulent flow, the PDF/mixture fraction model of nonpremixed combustion and P-1 radiation model. A CFD code ANSYS Fluent was used for all numerical investigations. Temperature distributions of axial and radial directions were determined. A NOX post-processor was used for the prediction of NOX emissions from the gas turbine combustor. Modelling was performed for 60kW thermal power and different equivalance ratios (i.e. Phi=0.91, Phi=0.77 and Phi=0.67). The studied type 1 model gas turbine combustor was modelled for Phi=0.91 equivalance ratio. Then, Other equivalance ratios were analysed for type 2 model gas turbine combustor. The effect of dilution air on combustion performances and emission characteristics was also investigated. It is concluded that the coke oven gas, the town gas I, town gas II and the water gas are appropriate for usage as alternative fuel, whereas the generator gas is not suitable for gas turbine combustors. Copyright (C) 2013 John Wiley & Sons, Ltd.