The Flame-Thrower Schematic can be found in Chapter 6, at the end of the "school' area, after the giant necromorph attack on the gymnasium stage. After being attacked by the Crawlers and Pack in the gym, exit through the door on the right (if you are facing the bleachers). Follow the hallway down and to the left. The room on the right, where the woman was hugging the Crawler, will contain the schematic. Purchase the flame-thrower at the next store and start racking up kills. This weapon burns through fuel quickly so I recommend using it during encounters with multiple necromorphs. Always keep extra fuel canisters on reserve.