The influence of cow dung and mixed straw ashes on steel corrosion

Abstract

Presented paper identifies the possible corrosion problems related to animal waste incineration since it is a key issue for their use in the power sector. Cow dung as an example of animal-origin biomass is examined in terms of high-temperature corrosion and slagging hazards in comparison with commonly used mixed straw. Results of the laboratory-scale corrosion studies of pure ash deposits on two steel alloys: 16Mo3 and 10CrMo9–10 are reported. Selected steel types are commonly used in boiler construction for superheaters and waterwalls – parts of a boiler that are most exposed to the high-temperature corrosion risk. The corrosion experiments were carried out under oxidizing conditions at 460 °C, 510 °C and 560 °C for 168 h. To determine the corrosion rate the mass of steel samples was measured after 24, 72 and 168 h. For a better understanding of the corrosion process, a detailed chemical analysis of ashes was conducted together with SEM-EDS analysis of the corrosion products. Ash melting and deposition behavior of both ashes were predicted according to 11 indices. It was found that cow dung and mixed straw ash differ significantly in terms of quantitative chemical compositions and ash melting characteristics. Cow dung ash is characterized by a high concentration of alkali compounds (2.57 wt% Cl, 30.6 wt% CaO, 5.56 wt% K2O) together with high phosphorus content typical for animal manure. The presence of alkali chlorides in the cow dung ash deposits led to a build-up of chlorides on the steel surface, which caused accelerated corrosion. At 560 °C the corrosion rate was over 3 times higher for cow dung ash than for mixed straw ash. The corrosion occurred due to the formation of low-melting FeCl2 mixtures since chlorine- and iron-rich particles were found in the crevices and cracks within deposits. The results clearly demonstrated the high corrosive potential of animal-origin biomass and therefore indicate the potential problems related to its thermal conversion process. These adverse effects can be reduced by ash removal and maintaining the temperature of the heating surfaces below 560 °C.