How to shell a monoid

For a finitely generated submonoid Λ of N, we consider the minimal free resolution of a field k as a module over the monoid algebra k[Λ]. Interpreting the ranks of the free modules in the resolution as the homology of certain simplicial complexes associated to posets, we show how non-commutative Gröbner bases and the non-pure shellings of Björner and Wachs can be used to obtain information about the resolution. Complete results are obtained for monoids Λ which support posets in a certain sense.