Convection pass
The convection pass of a boiler is characterised by the mode of energy transfer from the hot flue gas to water or steam which is primarily via convective heat transfer. On waste-to-energy boilers the flue gas temperature (at the entrance to the convection pass) is typically below 700°C, which makes it possible to have heating surfaces inside the flue gas, rather than surrounding the flue gas, as in a typical radiation pass.
Having heating surfaces inside the flue gas means that the heating surface area in a given space can be optimised. This means that the total size of the boiler can be optimised. The size of the plant building can therefore also be optimised. However, the burning of some fuels (such as waste) generates a high concentration of particles which are sticky at high temperatures. To avoid a blockage in the flue gas path, the flue gas temperature must thus be below a certain temperature, which limits the use of convective heating surfaces.
The heating surfaces in the convection pass are typically cleaned by means of soot blowers or a rapping device, depending on the arrangement of the heating surface. The convection pass is typically placed immediately before the economiser, which is also a convective heating surface.
