Co-combustion of olive cake and high-sulfur lignites in a bubbling fluidized bed, investigation of control methods for agglomeration

Agglomeration of the bed material is an operational problem seen during the combustion of high alkali content biomasses in fluidized bed combustion systems. It is very important to determine agglomeration mechanisms and to identify control methods in terms of utilizing biomass resources in energy production. Alkali oxides in fuel ash can react with silica sand used as a bed material in fluidized bed systems so the formation of molten silicates at low temperature can occur. In order to prevent agglomeration, one of these two contents needs to be removed from the system or reduced in quantity. In this study, olive cake was used as a biomass because of its high alkali content. Three different control methods were investigated in order to prevent agglomeration. The control methods were; co-combustion of olive cake (50% by weight) with Turkish lignites (Tunçbilek, Çan), pre-combustion leaching of the olive cake and using limestone as an alternative bed material instead of silica-sand. The combustion tests were conducted in a laboratory scale bubbling fluidized bed combustion system having an inside diameter of 100 mm and a height of 1800 mm. At the end of each test, bottom ash samples and slag samples were subjected to X-ray fluorescence (XRF), Scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), Electron probe micro-analyzer (EPMA) and X-ray diffraction (XRD) analyses to determine elemental composition and phases of slag and bottom ashes. Agglomerated particles were detected at the bed temperature of 850 °C for the combustion of olive cake. Co-combustion tests were carried out without any operational problem. Low melting point eutectics tests were not determined in the bottom ashes of co-combustion tests. This can be explained by the high sulfur content of the lignites (Tunçbilek: 1.64%, Çan: 4.51%). Combustion of olive cake with high sulfur lignites hindered the formation of Potassium Silicate and subsequently prevented the agglomeration of the bottom ash. The potassium element was found in the form of KAlSiO4 in the bottom ash of the olive cake and lignite mixture tests. Co-combustion of olive cake and lignites with low alkali metal content also reduces the amount of alkali metal in the fuel mixture. This may be another reason for the agglomeration not to be seen. Removal of the alkaline content of the olive cake by the leaching process prevents silicate formation. A noticeable decrease in the rate of agglomeration was observed in the combustion of the leached olive cake. Using limestone as the bed material instead of silica is more effective than leached olive cake combustion tests in terms of reducing the amount of ash deposited on the inner surface of the fluidized bed. The use of limestone as a bed material caused the potassium observed in the form of Arcanite (K2SO4) phase in the bottom ash. It prevented the formation of alkaline silicates which cause agglomeration.