Studies of Deposition on Cooled Tubes with Special Reference to Coal-Fired Boilers

Abstract

The present work is concerned with the formation of alkali-matrix deposits on superheater tubes of coal-fired boilers arising from the combustion of high alkali content coals. The main contribution to the problem of these deposits is through mass transfer studies. Four salts, namely sodium chloride, potassium chloride, sodium sulphate and calcium chloride were investigated in a model combustor containing a single tube. The rates of deposition to the surface of the tube and the morphology of the deposits were studied with surface temperature, concentration of vapour and mainstream velocity as parameters. Flow model studies in a wind tunnel showed that at the Reynolds numbers under investigation, a laminar boundary layer formed on the front of the tube separating to give a turbulent wake. The deposition rates of sodium chloride and potassium chloride were found to be predicted well by a heat/mass transfer analogy for the area beneath the laminar boundary layer. The nature of the two salts was found to be temperature dependent and the fusion temperatures were up to 200°C below their respective melting points. With sodium sulphate it was found that there was a 19% discrepancy between experimental and theoretical results calculated using a heat/mass transfer analogy for the area beneath the laminar boundary layer. The nature of the deposit was found to be dependent upon concentration as well as surface temperature. Fused deposits were obtained at temperatures up to 350°C below the melting point of the deposits. The observed condensation points for the three salts agreed well with previous workers' values. Deposition rates of the three salts in the turbulent wake were considerably less than was to be expected from a heat/mass transfer analogy. The results for calcium chloride indicated that it is unlikely to play any part in the formation of alkali-matrix deposits.