Co-firing of sewage sludge with bark in a bench-scale bubbling fluidized bed - A study of deposits and emissions

It has been shown that addition of either sulfur and/or aluminosilicates such as kaolinite may reduce alkali induced deposit formation when firing biomass fuels. Sewage sludge is a fuel containing substantial amounts of sulfur and aluminosilicates, such as zeolites. In this work different amounts of sewage sludge (0, 2, 4, 6 and 8%_en) were co-fired with bark in a bench-scale BFB. SO₂ and HCl emissions were measured and deposits were sampled during 3 hrs with an air-cooled probe with a surface temperature of 500°C at two different locations with flue gas temperatures of 850°C and 650°C, respectively. The test results showed that an increase of the share of sewage sludge to the fuel mixture increased the formation of HCl and simultaneously decreased the Cl-content in the deposits. Usually this is considered to be a sign of sulfation of alkali chlorides. However, the increase of HCl can also be caused by Al-silicates capturing alkali, thus releasing Cl as HCl to the gas phase. Although, sulfur increased in the fuel input with an increased share of sewage sludge, this was not reflected in the gaseous emissions as may be expected. Up to 4%_en sewage sludge was fired together with bark without increasing the sulfur content in the emissions. At higher shares of sewage sludge the sulfur emissions increased linearly with an increase of sewage sludge. The amount of water soluble potassium fed into the boiler remained relatively constant in the different tests. This potassium is usually released as volatile salts. Nevertheless, the amount found in deposits decreased with an increase in sludge feeding. In this paper it was shown that interaction of potassium with Al-silicates in the bed is a probable cause for the decrease of potassium in the deposits, while both the sulfation of potassium chlorides and possibly also, the alkali capture by Al-silicates can weaken the deposition of Cl.