Determining the bed settling rate in down-draft biomass gasifier using the radioisotope X-ray fluorescence - Measurement methodology

Studies of fuel settling in a fixed bed reactor are problematic in many areas. The operating temperature of the reactor (1200 degrees C in the burning zone) makes it impossible to do any measurements with the use of electronic sensors. The volume of fuel particles is decreasing during the gasification process, and this decrease is not constant in the whole height of the reactor, due to different speed of bed settling in each zone in the reactor. The particle volume change depends on temperature, heating rate, air ratio, and chemical reactions. The paper presents possibilities of using radiographic techniques to study fuel settling during the biomass gasification process. Radiography gives the possibility to register the movement of markers inserted into the reactor with fuel, and to follow this movement as the process proceeds. Adding a small number of tiny elements to the fuel does not affect the process nor the method itself, and therefore is not invasive. In order to determine the rate of fuel settling, experiments were carried out with the use of the RXRF (radioisotope X-ray fluorescence) technique. The radiation source (Se-75) was set on one side of the reactor, and the radiographic membrane on the other side. The analysis of the obtained X-ray photos enabled to determine the position and movement of the marker in all three directions, and to calculate the rate of bed settling.