Question 12 of 12: The Three T's of Incineration

Question 12: The Three T's of Incineration (5 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.

The three T's — Temperature, Time and Turbulence — describe the combustion-chamber conditions needed to achieve complete oxidation of the waste stream and are important both for energy recovery efficiency and for minimizing products of incomplete combustion. Temperature must be sufficiently high — commonly a secondary-chamber minimum of roughly 850 °C (higher still where chlorinated wastes are present, to suppress dioxin/furan reformation) — to drive the combustion reactions to completion rather than quenching them partway. Time is the residence time the combustion gases spend at that temperature, commonly specified as a minimum of about 2 seconds in the secondary chamber, allowing the oxidation reactions enough time to actually go to completion rather than being swept out of the chamber unreacted.

Turbulence is the degree of mixing between combustion air and the waste-derived gases, typically promoted with overfire-air jets and chamber geometry; it ensures every parcel of gas is brought into intimate contact with sufficient oxygen, avoiding localized fuel-rich pockets that would otherwise survive even adequate time and temperature. All three must be satisfied simultaneously — high temperature with poor turbulence still leaves unmixed fuel-rich pockets, and good mixing without adequate residence time still ships partially reacted gas out the stack — because together, and only together, they minimize products of incomplete combustion (CO, dioxins/furans, unburned particulate-bound organics) and maximize the useful energy recovered per tonne of waste combusted.