In this work, a series of calculations of a complete premixing injection gas burner with various air gap widths with ANSYS Fluent software was performed. A natural gas fired injection gas burner is used as a heat source as a part of a thermoelectric generator. Assuming that the gas-air mixture flow is one-dimensional and axisymmetric, the issues were solved in axisymmetric setting, taking into account the gas compressibility. In the performance of a task, a set of assumptions was made: k-epsilon realizable model was picked as turbulence model with scalable wall functions; the flow in the ejector, mixing chamber and diffuser is subsonic; the pressure of the injected air was set to atmospheric pressure; pressure at the inlet to the gas burner is subcritical (≤ 86 kPa for methane). Based on the calculations represented above, the dependence of the equivalence ratio on the air gap width was determined.