During their pneumatic transport, powders accumulate electrostatic charge due to collisions with the walls of the pipe. This phenomenon is known as triboelectric charging and may cause hazardous spark discharges. For this reason, there is a strong interest in studying various options to constrain it. However, many aspects related to this phenomenon still remain poorly understood. In this paper we report on numerical studies of the role of the particle properties and flow rates to the buildup of electric charge. The turbulent flow of the carrier gas is treated numerically via Large Eddy Simulations (LES), whereas the motion of each particle is tracked individually in the Lagrangian framework. The governing equations are endowed with four-way coupling between particles and carrier gas, as well as dynamic models for the charge exchange during wall-particle and particle-particle collisions. According to our study, an increase of the particle diameter leads to higher charge exchange during wall-particle collisions and to higher average charge per particle. Also, the Young modulus of the powder appears to be the most important material property; as it increases the powder charge drops considerably. With regard to the powder mass-loading, it has minimal impact on the average charge per particle. Finally, our simulations predicted that at sufficiently low Reynolds numbers, reducing the inlet velocity can lead to an increase of the average particle charge due to the tendency of the particles to settle at the bottom wall.