For thermal plasma synthesis of Si3N4 or SiC ultrafine powder when Si powder is used as a precursor, pneumatic injection and mixing of solid particle with high temperature DC plasma flow presents very important problem. Paper presents result of numerical simulation of two plasma reactor schemes: 1. multi torch system reactor simulated by central Si powder injection and coaxial convergent nitrogen plasma flow and 2. Classical plasma reactor with central nitrogen plasma flow and axisymmetric radial injection of Si powder. Results of the numerical simulation of the analyzed example indicate that classical plasma reactor is much more suitable for plasma synthesis of Si3N4 and SiC ultrafine powders if parameters of Si powder injection can be optimized and controlled. The main reasons are: 1. higher silicon evaporation efficiency and 2. lower mass fraction (lower particle concentration) of unevaporated Si particles in the exit mixture in case of classical plasma reactor.