Ceramics avoid dioxin reformation
Worcester-based Caldo Environmental Engineering has supplied a complete pollution control system employing ceramic filtration technology for a mixed general and clinical waste incinerator to be installed in the Middle East.The incinerator, built by Howden 3T's International, a member of the Charter plc group, is a two-stage semi-pyrolitic unit comprising an initial chamber, where thermal decomposition of the waste takes place, and a secondary chamber where combustion of the gases is completed. The gases leave the secondary chamber at temperatures up to 1,250°C and pass into the system.
The gases are cooled from the temperature of the secondary combustion chamber to 725°C by direct evaporation of water in a quench cooler. The water is injected into the gas stream from a high pressure nozzle incorporating compressed air atomisation. The semi-cooled gases then pass through a gas-to-air heat exchanger where the temperature is reduced to 420°C.
The acid gas sorbent powder is added to the gas as it enters the duct between the heat exchanger and the three ceramic filter vessels. The use of ceramic filters allows much higher gas temperatures than usual, giving higher removal efficiencies and better sorbent utilisation.
The filters operate at about 410°C. They reduce the particulate content of the gases, including the sorbent powder, to a value of <1mg/m3 - substantially below the most demanding of current regulations.
Acid gases and particulates are two of the building blocks needed for de novo dioxin formation. Removing these components from the gas stream at a temperature above the dioxin formation window - 250-400°C - ensures that there are no dioxins formed in the system. This is important because, if dioxins are formed they must be captured, normally using activated carbon, and disposed of.