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 i n Sandia's Multifuel Combustor. Special attention was drawn to the depositi on of potassium, chlorine, and sulphur and the sulphation of potassium chlo ride to potassium sulphate. The experiments included wheat straw-firing und er different combustion conditions and with elevated sulphur levels. invest igations of deposit formation on a simulated superheater tube placed in cro ss flow with the gas revealed that the deposit was ellipse-shaped and had a n inner layer of white condensed salts. The inner layer had a dendritic str ucture and consisted of potassium chloride and potassium sulphate. Part of the water-soluble potassium in the straw was transformed into potassium sil icates and a mixed mineral of potassium, calcium, and silicon. A novel prob e was designed to investigate condensation phenomena. SEM-analyses revealed that the structure of the potassium salts consisted of individual angular particles of primarily KCI (1-2 mu m) and a sponge-like matrix of sub-micro n particles consisting primarily of K2SO4, which may represent vapour conde nsate agglomerates. Mechanisms of deposit formation and the fate of chlorin e, potassium, and sulphur in the system are discussed. (C) 1999 Elsevier Sc ience Ltd. All rights reserved.