  We consider a generic model with four gauge singlets which generates
successfully the right amount of baryon asymmetry through leptogenesis at the
TeV scale. It also reproduces left-handed neutrino masses compatible with
present data. In the present work, we perform a study of thermal production
mechanisms of the lightest right-handed neutrino $N_{1}$ responsible for the
lepton asymmetry taking into account all dominant processes (decays, $\Delta L
= 1$ and $\Delta L=2$ scatterings) by solving the full Boltzmann equations.
