Sulfur, the alkalies, and vanadium are the principal causes of corrosion when burning residual fuel.
Removal of these impurities generally is not economic, although many attempts have been made to improve residual fuels by water washing or by chemical treatment. Most effective of all schemes for decreasing corrosion has been to burn residual fuel with a minimum of excess air, preferably with less than 0.5 per cent oxygen remaining in the flue gas. Under such conditions, vanadium is not oxidized to form lowmelting compounds, and the formation of SO3 is essentially prevented. Additives, particularly of magnesia, also have been beneficial, both by chemical and physical actions.
Le soufre, les alcalis et le vanadium sont les principales causes de corrosion dans la combustion des fuels résiduels. I1 n'est généralement pas économique d'éliminer ces impuretés, bien que l'on ait tenté maintes fois d'améliorer les combustibles résiduels par lavage à l'eau ou par traitement chimique. La méthode la plus efficace pour abaisser la corrosion est encore de brûler le combustible résiduel avec un minimum d'excès d'air, de préférence avec moins de 0,5% d'oxygène restant dans les gaz de combustion. Dans de telles conditions, le vanadium n'est pas oxydé pour former des composés à bas point de fusion, et la formation de SO3 est largement évitée. L'action physique et chimique de certains additifs comme la magnésie est aussi très utile.
As with other fuels, the impurities in residual fuel oil released during combustion lead to many problems in central-station power plants. One of the most serious of these difficulties is the external corrosion of heatreceiving surfaces of boiler furnaces, mainly of superheaters and reheaters, and often of air heaters.
Another is the unpredictable decrease in heat transfer caused by deposits formed from ash in the fuel. Still another is the occasional plugging of gas passages.
Such problems have led to a great deal of effort over the past three decades in tracking down the causes of corrosion and deposits, and in devising means of coping with them. All these difficulties have not been eliminated as yet, but good advances are being made on several fronts.
In regard to corrosion, residual fuel is in a particularly favorable position at present compared with solid fuels because of the rapid advances made recently burning residual fuel with low excess air. Also, thanks to the low ash content of residual fuel, additives can be more effective on a weight basis than when the ash content is 100 times greater, as it is sometimes with solid fuels. Contrarywise, the aggressive impurities in liquid fuels, vanadium for example, lead to particularly troublesome problems. Although practical operating solutions with low excess air and with additives by WILLIAM T. REID, Battelle Memorial
