Causes of Flue Gas Deposits and Corrosion in Modern Boiler Plants

Abstract

This paper is intended to be an amplification and continuation of the author's previous paper entitled “Causes of High Dew-point Temperatures in Boiler Flue Gases” read before the Institution in 1943, in which attention was drawn to the phenomenon of the catalytic production of sulphuric acid in flue gases by their passage over heated iron surfaces. At that time the full importance of these findings was not apparent, but evidence is now submitted to show that, in addition to causing deposits and corrosion by the deposition of acid in air heaters and economizers, this phenomenon is primarily responsible for the flue-gas deposits and corrosion in all parts of boiler plants. Experiments are described which show that when flue gases are passed over sand-blasted steel-surfaces which are maintained at the gradation in metal temperatures which can occur through a modern boiler plant, sulphur trioxide is produced at the high temperature surfaces, causing sulphuric acid to condense on the cooler surfaces. When the maximum surface temperature is moderated no appreciable formation of acid occurs. It is believed that the interaction of the sulphur trioxide in the generation zone, and the condensed acid in the cooler zone, with the various constituents of the fuel ash and the metal of the heating surface, can explain all the numerous deposit and corrosion effects which have been the subject of research work both here and abroad during recent years. The high surface temperatures which are necessary for the catalytic action to occur are largely due to the general advances in operating temperatures and pressures, and in some measure to “surface combustion” taking place on the tube surfaces, elevating their temperature above that which they would acquire if swept only by inert gases. Although the advance in steam and water temperatures is considered to be a primary factor in these difficulties, many other contributory factors, particularly the nature of the fuel, play an important part. Pulverized-fuel boiler-plants are least affected and, at the present time, the adoption of pulverized fuel appears to be the most effective way of dealing with the problem. Further research is necessary to devise practical means of prevention or alleviation in stoker and oil-fired plants, and it is thought that full consideration of the factors outlined in the paper may materially assist in this object.