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If two particles collide near the horizon of a rotating extremal black hole,under certain conditions the energy E_{c.m.} in the center-of-mass frame cangrow without limit (the so-called Banados-Silk-West effect). We considercollisions that produce two other particles. We show that for a generic dirty(surrounded by matter) black hole, there exist upper bounds on the energy andmass of product particles which can be detected at infinity. As a result, thepositive energy gain is possible but is quite modest. It mainly depends on twonumbers in which near-horizon behavior of the metric is encoded. The obtainedresults suggest astrophysical limits on the possibility of observation of theproducts of the collisional Banados-Silk-West effect. These results areconsistent with recent calculations for the Kerr metric, extending them togeneric dirty black holes. It is shown that for dirty black holes there aretypes of scenarios of energy extraction impossible in the Kerr case.

Energetics of particle collisions near dirty rotating extremal black holes: Banados-Silk-West effect versus Penrose process