CHARACTERIZATION OF PROBLEMATICS IN THE COLDER PARTS OF FLUE GAS DUCTS AT ÅC2 : Analysis of ashes and deposits resulting from combustion of residues in fixed grate furnace at a pulp and paper mill

In the colder flue gas parts of the steam producing residue furnace Ångcentral 2 (ÅC2) in Obbola, continuous repair work is carried out, especially in the economizer and the electrostatic precipitator (ESP). This is costly and causes some auxiliary oil combustion to maintain steam production. This project has been carried out with the aim of identifying potential damage mechanisms that may occur on these parts of the boiler due to the mix of fuels combusted. During the project, samples of the different fuel streams, ashes and deposits from both economizers and ESP were collected. Dust from the flue gases have also been collected using an impactor. The fuel was analyzed with the aid of an external laboratory and the material taken out of ÅC2 was analyzed with SEM-EDS and XRD at Umeå University. The results show that the fuel mainly consists of the ash-forming elements calcium (Ca), silicon (Si), aluminum (Al) and iron (Fe). The potentially corrosive substances sulfur (S) and chlorine (Cl) were also found in the incoming fuel, mainly with the reject and biosludge. There is a larger fraction S than Cl with a ratio of approximately 5:1. The deposits from the two positions in the flue gas system consist mainly of Ca and S with lower levels of K and Cl. Si, Fe and Al grains were also found in the surface layer. Within the deposits, Ca and S as well as K and Cl were often found spatially collocated. In the ashes, the proportion of K, S and Cl is more even with a large excess of Ca to a relatively even content in the ESP. In the fine mode fraction (particles <1µm) of the flue gas Cl is found together with alkali metals, whilst there also issufficient amounts of alkali remaining to form sulfates. In the coarse mode (particles >1µm) of the flue gas is dominated by Ca in both economizers and ESP. During this project, no clear indications have been found regarding the causes of wear in the colder flue gas ducts and equipment. Further investigation of the in- and output streams of ÅC2 are required to be able to draw more definitive conclusions, where this work can be used as a base.