I'm not familiar with pyrolysis, but from what I understand temperatures are normally kept well under the normal oxidation temp for aluminum. What color are the boulders and crystals? If it were alumina or other metal oxides I would expect them to be white (on the inside anyways, the outside might be covered with char from the plastic decomposition).
Also, what is the operating temperature of the fluidized bed reactor? I've operated similar reactors (completely different application) in the past with aluminum and know that at temperatures above around 550 C or a bit higher, aluminum gets soft and can begin to sinter. These sintered particles can tend to grow and grow into huge masses that eventually shut the reactor down.
