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Soot formation in industrial burners

Hırtıslı, İlkem
This thesis examines the problem of soot (also known as coke) formation in industrial burners by thermal methods and morphological characterization. The fuel gas soot (FG) was collected during maintenance stop(s) of a burner in which refinery fuel gas (RFG) is burned. The obtained sample is crushed in a mortar and sieved to 75-106 μm, to minimize particle diameter effects. Thermogravimetric analysis (TGA) is used to determine the combustion characteristic temperatures of the sample. In the TGA experiments which are carried out at non-isothermal conditions, four main characteristic temperatures were determined: the temperature at which devolatilization starts (Tv) ~630-640 °C, the temperature at which the sample ignites (Ti) ~700-720 °C, the highest reaction rate temperature (Tp) ~815-830 °C and the burnout temperature (Te) ~915-920 °C. Non-isothermal TGA trials allowed to find the target temperature (Tt) for the isothermal trials. For the evaluation of the sample content, a Fourier Transform Infrared Spectroscopy (FTIR) instrument connected to the TGA is used to examine the CO2, CO, H2O and SOx emissions. While CO2 and CO molecules exist from ignition until burnout, no signs of H2O and SOx molecules are observed. Considering the RFG composition, this is an unexpected situation and further evaluation was needed. The X-ray Diffraction (XRD) method is applied and graphite with 2H-crystal structure is observed. To provide further insight, Energy Dispersive X-Ray (EDX) analysis is applied and it is determined that the substance content is carbon. Because Scanning Electron Microscope (SEM) images were composed of non-crystalline carbon structures, which is in contradiction with the XRD results, RAMAN method is applied, and the sample content is found to be amorphous carbon. Therefore, the sample content is heterogeneous, but the dominant form is amorphous carbon. The morphology of the soot is also examined with SEM and High-Resolution Transmission Electron Microscopy (HRTEM) methods. SEM images show that the particles of various geometry and sizes exist in agglomerated forms. Observation of the surfaces of these structures revealed chain-like carbon structures. For further evaluation of morphology, HRTEM method is used and the mentioned carbon chains are examined closely. In this method, it is observed that the carbonaceous structure did not contain any graphene plane, but some crystalline nodes in amorphous matrix and deviations of interatomic distances in the order of mesoscopic scale (≃1 nm) exist.