Deposition of potassium salts on heat transfer surfaces in straw-fired boilers: a pilot-scale study

The fate of inorganic salts in straw-fired combustion systems was studied in Sandiaʹs Multifuel Combustor. Special attention was drawn to the deposition of potassium, chlorine, and sulphur and the sulphation of potassium chloride to potassium sulphate. The experiments included wheat straw-firing under different combustion conditions and with elevated sulphur levels. Investigations of deposit formation on a simulated superheater tube placed in cross flow with the gas revealed that the deposit was ellipse-shaped and had an inner layer of white condensed salts. The inner layer had a dendritic structure and consisted of potassium chloride and potassium sulphate. Part of the water-soluble potassium in the straw was transformed into potassium silicates and a mixed mineral of potassium, calcium, and silicon. A novel probe was designed to investigate condensation phenomena. SEM-analyses revealed that the structure of the potassium salts consisted of individual angular particles of primarily KCl (1–2 μm) and a sponge-like matrix of sub-micron particles consisting primarily of K2SO4, which may represent vapour condensate agglomerates. Mechanisms of deposit formation and the fate of chlorine, potassium, and sulphur in the system are discussed.