Alkyl halides are reduced by zinc and dil. $\ce{HCl}$ into alkanes $$\ce{ CH3Cl + H2 ->[Zn][dil\text{.} HCl]CH4 + HCl}$$
What I don't understand is the underlying mechanism behind this reaction, is it free radical or nucleophilic substitution. If it is the latter, then why isn't $\ce{CH3-CH3}$ a product? I would appreciate if someone could explain the entire mechanism behind this reaction.