There is a simple reduction from maximum flow with lower bounds to maximum flow:
http://www.cs.uiuc.edu/~jeffe/teaching/algorithms/2009/notes/18-maxflowext.pdf
The idea is that you saturate all edges. Then you are left with imbalanced nodes. You can use a max-flow algorithm to resolve the imbalance.
The construction works as follows: For every vertex v, define M(v) := sum of lower bounds on incoming edges - sum of lower bounds on outgoing edges. Remove all lower bounds and set capacity of each original edge to upper bound - lower bound (in your case, infinity).
Introduce a new super source S and a new super sink T. Add an edge (S, v) with upper capacity M(v) for every vertex v with M(v) >= 0. Add an edge (v, T) with upper capacity -M(v) for every vertex v with M(v) < 0.
Solve the resulting S-T maximum flow problem.
Finally, remove S and T and add the original lower bound to the flow on every original edge.