Impact of boiler load and limestone addition on SO₃ and corrosive cold-end deposits in a coal-fired CFB boiler

The risk of cold-end corrosion caused by two different phenomena was studied: the well-known sulfuric acid-induced corrosion and corrosion caused by hygroscopic fly ash deposits. Measurements were performed in a full-scale circulating fluidized bed boiler firing bituminous coal with high contents of sulfur and chlorine. The boiler was run both with and without limestone addition to reveal the effects of limestone on corrosion. Furthermore, the impact of boiler load on corrosion and deposit composition was studied. Corrosion probe, SO₃, and dew point measurements were performed up- and downstream of the electrostatic precipitator. Ash deposits were collected from the different sides of the corrosion probe and were analyzed. The formation of SO₃ was low in all cases (<0.1 ppm_v), which was connected to the relatively low furnace temperature in fluidized bed combustion and the efficient SO₃ capturing of the fly ash and limestone. The different operational parameters of the boiler had a significant impact on deposit composition and on expected corrosion risk. At full load and without limestone addition, the chlorine of the fuel stayed as gaseous HCl, whereas no Cl was found in the deposits. However, when limestone was added, corrosion was caused by the presence of deliquescent calcium chloride. At low load operation of the boiler, ammonium chloride was formed on the cold-end deposit probe. Ammonium chloride was formed via the reaction between HCl and NH₃ in the cooling flue gases. Laboratory studies with NH₄Cl was further conducted to assess its corrosivity.