Askans inverkan på beläggningstendensen i ett biobränsleeldat kraftvärmeverk och växtplatsens påverkan på askegenskaperna

Abstract: This report was written by Sofia Ericson and describes the degree thesis with the title: "Connections between the growing site of bio fuel, the chemical composition of bio fuel ash and the deposit growth in a bio fuel fired boiler." The purpose of this degree thesis was to avoid melted deposit growth in the bio fuel fired boiler at ENA Kraft in Enköping. Potassium and sodium are known to lower the melting point of bio fuel ashes and are therefore not wanted in the trees in higher amounts than necessary for the growth. Silicon is also known to be important for the deposit growth but it is not possible to say in general if higher or lower contents of silicon is better. The relation between silicon and other chemical substances is more important than the total amount of silicon. Literature studies on the subject resulted in a hypothesis saying that bio fuels from soils with high clay content are most likely to cause deposit growth in biomass fired boilers, because of their rich content of nouris hment such as for example potassium. To confirm or reject this hypothesis, samples of soil, bio fuel and deposits where gathered and analysed. AnalyCen in Lidköping performed the analysis of the fuel samples and the deposit samples. The soil samples were analysed by the Swedish University of Agricultural Sciences in Uppsala. The melting points of the fuel ashes were analysed at Termiska Processer in Nyköping by the student performing the degree thesis. The melting point analysis showed, all according to the hypothesis, that the fuel ash derived from the soil with high clay content melted at lower temperatures than the other ashes. Further analysis showed that the melted material contained mainly iron oxides and the fuel analyses showed that the melting sample ash had a higher content of iron oxides compared to the other sample ashes. This proves an unexpected effect of iron in bio fuel, but further soil analyses has to be done to explain the connection between the soil and the amount of iron in the trees. Three different means were used to explain the results of the fuel analyses, the Seger formula from the ceramic industry, ratios from the glass industry and phase diagrams for systems of two or three compounds. The Seger formula can be declared insufficient in predicting melting points of bio fuel ashes because it is not considering the forming of low temperature melting eutectics. The ratios used in this degree thesis are adjusted to suit boilers with fluid beds, and they should therefore be considered to be rather uncertain when used on ashes from grate fired boilers. The use of phase diagrams is a good way to make complicated systems of melting points foreseeable, but these diagrams are not fully developed and can therefore not be used in all situations.