mass source and mass sink for multi-component
 

hi,everyone I am working with a 3d solid oxide fuel cell model.The flow in flow channel is multicomponent and I want to simulate a chemcal reaction at a 2d boundary. One component disappears and the other produces. but I find I can not set a such mass source and mass sink boundary condition at the same 2d boundary at the same time.what should i do? Any suggestion will welcome indeed. thanks frank