Compared to fossil fuels, biomass has a high concentration of alkali salts, and their removal from the product gas is a very important step in biomass gasification. Alkali salts will condense below 600 °C, which causes serious corrosion problems. If the temperature of the gas decreases below 600 °C, the alkali salts condense and can be separated in a cyclone or Alters. However, in some applications, gas cannot be cooled so alumosilicates, such as bauxite, kaolinite, bentonite, and naturally occurring zeolite, can be used for alkali removal at temperatures up to 700 °C [59].

10.5.1.1 Electrostatic Precipitators

Electrostatic precipitators (ESP) are used to remove fine solids and liquid droplets from gas stream; however, they are not very efficient in terms of tar removal at high temperature. In order to have an efficient tar removal, gas stream should be quenched before ESP. In wet ESP, gas is ionised upon passing between high voltage electrodes and a grounded electrode. The produced ions attach themselves to dust particles or tar and water droplets. The charged particles and droplets are attracted to the grounded electrode, flowing to the bottom of the ESP where they are collected.