Prevention of airheater corrosion

Most sugar mill boilers are fitted with airheaters for improved combustion performance and efficiency matching to factory requirements. Irrespective of boiler design, capacity or operating conditions the vast majority of airheaters inevitably experience corrosion in specific sections, usually on the inside of the tubes at the cold air inlet. Full or partial tube replacement is expensive. More importantly, as the corrosion proceeds the output of the boiler will progressively be restricted due to air bypassing the furnace and convection bank and overloading the induced draft fan. In some cases, the output restriction can be severe enough to restrict factory operations and crushing rate. This paper describes measurements and computer modelling carried out on a sugar mill boiler with the aim of establishing the cause of airheater tube corrosion. The measurements show that even when average gas flow temperatures at the outlet of the airheater are well above the dew-point (below which water condensation and subsequent corrosion occurs), low tube metal temperatures can occur at the cold air side tubes of the airheater at all operating conditions. The computer modelling has shown that the major contributors to low tube metal temperatures and therefore dew-point corrosion are the poor gas and airflow distributions into the airheater. The information gained was used to carry out modifications to an airheater on a recently built boiler that was experiencing significant corrosion as well as tube blockages. The modifications at the gas inlet and air inlet were designed using the CFD computer package FURNACE. Results are presented which show that the modifications have significantly redl)ced the airheater corrosion and almost completely eliminated the occurrence of tube blockages.