After Mildred turns him in, Montag and Beatty go to Montag's house. Beatty, knowing that Montag has been infected with curiosity about books, demands that he burn the house himself:
"I want you to do this job all by your lonesome, Montag. Not with kerosene and a match, but piecework, with a flamethrower. Your house, your clean-up."
(Bradbury, Fahrenheit 451, Google Books)
This is meant to force Montag to reject books and his own interest, to return to his work by ridding himself of the very thing that he is supposed to stand against. By using a flamethrower, Montag will be confronted with everything he had, and everything he threw away by keeping books; the personal connection will be more powerful than simply dousing the house in kerosene and lighting it with a match. After he is finished, he may be allowed to reenter society, but Montag's refusal to accept his role helps him to escape after burning the house.