Whats the fastest way to grind up styrofoam?

im making a giant been bag couch and i need 35 cubic feet of eps beads, that stuff costs a lot for 1 cubic foot so i got a bunch of block of styrofoam from just companys that dont want it . ive been rubbing styrofoam against styrofoam and it works the best but its really slow, what is the fastest way to grind that stuff up back in the the bead form