Alkali/chloride release during refuse incineration on a grate: Full-scale experimental findings

Waste to energy (WtE) plants are utilised for the production of heat and electricity. However, due to corrosion at super heater surfaces a relatively low 25% of the waste lower heating value can with the present technology be converted to electricity. High contents of Cl, Na, K, Zn, Pb and S in waste cause relatively high super heater corrosion rates.

The Cl-content in waste is one of the key-factors for volatilisation of alkali and heavy metals in WtE plants. Little is known about the release of Cl, Na, K, Zn, Pb, and S along grate of waste incineration plants. The 26 t h(-1) WtE plant Vestforbraending unit S in Denmark was used for measurements of temperature, gas-concentration (O-2/CO/CO2), and sampling of gas phase Cl, Na, K, Pb, Zn, and S.

Unit 5 has 6 ports distributed along the 13 in long grate between 1.5-1.8 in above the grate. Five of these ports were used for measurements. Two aqueous absorption systems containing a solution of NH3 or a solution of H2O2/HNO3 were used to collect the gaseous samples. Tar was found to condense in the sampling system at the ports near the fuel inlet.

The experiments showed the majority of Cl, Na, and K to be volatilised during the early stages of combustion. The maximum release of Cl, Na, and K was measured in port 2 as 177 ppm(nu), 71 ppm(nu) and 44 ppm(nu) respectively. The maximum average gas temperature of 1140 was measured in port 3 compared to the temperatures at ports 2 and 4 of 816 and 551 respectively.

It has been suggested to use flue gas from the area of the grate near port 3 with a high temperature, that contains relatively low amounts of corrosive elements, and lead to a separate high temperature super heater and thus increase the electrical efficiency.