  The measurement of the large-scale B-mode polarization in the cosmic
microwave background (CMB) is a fundamental goal of future CMB experiments.
However, because of unprecedented sensitivity, future CMB experiments will be
much more sensitive to any imperfect modelling of the Galactic foreground
polarization in the reconstruction of the primordial B-mode signal. We compare
the sensitivity to B-modes of different concepts of CMB satellite missions
(LiteBIRD, COrE, COrE+, PRISM, EPIC, PIXIE) in the presence of Galactic
foregrounds. In particular, we quantify the impact on the tensor-to-scalar
parameter of incorrect foreground modelling in the component separation
process. Using Bayesian fitting and Gibbs sampling, we perform the separation
of the CMB and Galactic foreground B-modes. The recovered CMB B-mode power
spectrum is used to compute the likelihood distribution of the tensor-to-scalar
ratio. We focus the analysis to the very large angular scales that can be
probed only by CMB space missions, i.e. the Reionization bump, where primordial
B-modes dominate over spurious B-modes induced by gravitational lensing. We
find that fitting a single modified blackbody component for thermal dust where
the "real" sky consists of two dust components strongly bias the estimation of
the tensor-to-scalar ratio by more than 5{\sigma} for the most sensitive
experiments. Neglecting in the parametric model the curvature of the
synchrotron spectral index may bias the estimated tensor-to-scalar ratio by
more than 1{\sigma}. For sensitive CMB experiments, omitting in the foreground
modelling a 1% polarized spinning dust component may induce a non-negligible
bias in the estimated tensor-to-scalar ratio.
