Failure analysis on the economisers of a biomass fuel boiler

The premature failure of the economiser in a biomass fuel boiler was studied. On the internal oxide surface, three morphologies of iron oxides – aggregate oxide (Fe3O4), flower oxide (Fe3O4) and spherical oxide (FeO) have been discovered using ESEM/EDS. The oxidation and pitting occurred on the internal surface was trivial in comparison to the erosion–corrosion rate at outer surface of the economiser. The dominant abrasive SiO2 particles were partially cemented by the calcium compound particles in the biomass fly ash. The individual sharp abrasive SiO2 particles had been revealed with filtergram by dissolved the calcium compound particles in acid solution. The average size of SiO2 particles was approximately 100 μm. Some SiO2 particles were up to 500 μm. Study on the external scale found different oxide morphologies and the compositions, the flower-like oxide (Fe3O4) at the top and “mud-crack” FeO oxide at the flat sides. The wastage profile of ‘V-shaped thickness thinning’ can be explained by the mechanism proposed in this paper; the oxidation acting simultaneously with two-body abrasive wear by fly ash particle impingement.

► Three morphologies of iron oxides have been discovered in internal oxide scale.
► The sharp microscopic features of fly ash particles (SiO2) have been uncovered.
► The morphologies and compositions of the external oxide scale have been revealed.
► The detail mechanism of corrosion–erosion has been studied.
► The mechanism was oxidation and abrasive wear by fly ash particle impingement.