Question 14 of 18: Three Principles of Incinerator Design

Question 14: Three Principles of Incinerator Design (6 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Complete, reliable destruction of a hazardous waste by incineration rests on three interdependent design principles, commonly summarized as the "three T's." (1) Temperature — the combustion chamber must be operated hot enough (typically 900–1,200°C for a hazardous-waste incinerator, higher for particularly refractory compounds) that the target compounds' thermal destruction reactions actually proceed to completion, rather than merely volatilizing the waste unburned. (2) (Residence) time — the combustion gases must remain in the high-temperature zone long enough (typically on the order of 1–2 seconds) for the destruction reactions, which are not instantaneous, to run to completion at that temperature. (3) Turbulence — the waste and combustion air must be adequately mixed so that every parcel of waste actually reaches the design temperature and has access to sufficient oxygen; without adequate turbulence, some fraction of the waste can bypass the hot zone through localized cool or oxygen-starved pockets. All three must be met simultaneously — adequate temperature and time are ineffective if turbulence allows a fraction of the waste stream to short-circuit the hot zone.