A New Method to Minimize High-Temperature Corrosion Resulting from Alkali Sulfate and Chloride Deposition in Combustion Systems. I. Tungsten Salts

Abstract

Based on a recently ascertained understanding of the parameters that control flame generated deposition of Na2SO4, NaCl, or Na2CO3 onto cooled surfaces immersed in flames, a new process has been developed to inhibit their formation. It is seen that no alkali sulfate is formed if tungsten salts are added to a flame containing an alkali (sodium or potassium), sulfur, and chlorine, when the tungsten-to-alkali gas-phase ratio is larger than about 2-fold on an atomic basis. Instead, the alkali exhibits a greater affinity for the tungsten and benign alkali polytungstates are deposited. The presence of chlorine appears to have a negligible effect on these systems. This modified behavior is further confirmed in experiments in which an Na2SO4 deposit is initially formed on a probe and then is seen to be fully converted by the addition of sufficient tungsten to the flame and overlaid by a similar tungstate growth. For sodium, the preferred polytungstates formed are Na2W2O7 and Na2W4O13, explaining to some degree the required W/Na addition ratios. Deposition appears to reflect the relative condensed phase thermodynamic stabilities of these salts and follows the order Na2W4O13 > Na2W2O7 > Na2WO4 > Na2SO4 > NaCl > Na2CO3 . In the case of potassium, behavior is very similar and the dominant polytungstate produced is K2W6O19, with some contribution from K2W3O10 . Conversions can occur in the direction of greater stability but are irreversible. The method is insensitive to fuel type, equivalence ratio, or general flame parameters. Deposits have been acquired on stainless steel and platinum clad probes operating in the temperature range 600 to 900 K. Analysis has utilized Fourier transform Raman spectroscopy and inductively coupled plasma atomic emission spectroscopy. An examination of potential interferences for the tungsten addition also has been made. This concerns whether any other element might have a greater affinity for the tungsten over that of the alkali. Calcium, strontium, and barium appear to be the most likely to fall into this potential category having well-defined and stable tungstates. Initial experiments with and without calcium addition do, in fact, show an interference with evidence of CaWO4 formation and a need for enhanced additions of tungsten. The process, outlined in detail herein, appears to be the first viable active solution for preventing Na2SO4- and NaCl-induced corrosion in a variety of combustion systems. Coupled to present techniques for reducing burned gas alkali concentrations, the method appears to be economically feasible.