  Methanol (CH$_3$OH) is found to be abundant and widespread towards the
Central Molecular Zone, the inner few hundred parsecs of our Galaxy. Its origin
is, however, not fully understood. It was proposed that the high cosmic ray
ionisation rate in this region could lead to a more efficient non-thermal
desorption of this species formed on grain surfaces, but it would also mean
that this species is destroyed in a relatively short timescale. In a first
step, we run chemical models with a high cosmic ray ionisation rate and find
that this scenario can only reproduce the lowest abundances of methanol derived
in this region ($\sim$10$^{-9}$-10$^{-8}$). In a second step, we investigate
another scenario based on episodic explosions of grain mantles. We find a good
agreement between the predicted abundances of methanol and the observations. We
find that the dominant route for the formation of methanol is through
hydrogenation of CO on the grains followed by the desorption due to the grain
mantle explosion. The cyclic aspect of this model can explain the widespread
presence of methanol without requiring any additional mechanism. We also model
silicon monoxide (SiO), another species detected in several molecular clouds of
the Galactic Centre. An agreement is found with observations for a high
depletion of Si (Si/H $\sim$ 10$^{-8}$) with respect to the solar abundance.
