Abstract: A system and method remove undesirables from a gas produced by a combustion chamber. The system includes a first addition point, wherein treatment materials are added to the gas in a first addition temperature zone. The system also includes a second addition point, wherein treatment materials are added to the gas in a second addition temperature zone. The second addition point is downstream of the first addition point. The first addition temperature zone is at a higher temperature than the second addition temperature zone. The treatment materials remove undesirables from the gas.