Experimental investigation of the heat transfer coefficient to a horizontal tube submerged in bubbling fluidized bed

Abstract

In this thesis, the bed-to-tube heat transfer coefficient has been experimentally investigated in a lab scale fluidized bed to study the effect on varying the bed temperature, bed material, particle size and fluidization gas. Sand and ilmenite are the bed materials evaluated in this work and air, CO2, steam and flue gas are the tested fluidization gases. These have been tested at temperatures between 400-950 oC. The experimental bed-to-tube heat transfer coefficient have been compared to correlations, which are based on heat transfer theory, used for calculating the heat transfer coefficient in fluidized beds. Today, sand is the most common bed material for combustion of biomass fluidized beds. Ilmenite is of interest because of its oxygen carrier capacity, where it can transport oxygen from an air dense zone to a fuel dense zone in order to improve the fuel conversion and thereby reduce the emissions of CO, unburned hydrocarbons and NOX. Compared to sand, ilmenite has higher density, which is an interesting parameter to evaluate regarding its influence on the heat transfer. Because of the higher emissivity of the combustion gases CO2 and steam, radiative heat transfer rate can be higher compared to air, and a higher conductivity can be achieved especially from the steam. Therefore, investigating the effect of how flue gas contributes to the heat transfer rate is also interesting. The experiments were conducted in a small-scale fluidized bed reactor with a single tube immersed in the bed where water flows. The reactor were located in a furnace which heated up the reactor. The temperature was measured in the bed and in the inlet and exit of the water in the tube. The water flow and the fluidization gas velocity was measured as well. The experiments included investigation of the heat transfer coefficient for ilmenite and sand as bed material at four different particle size range: 90-150, 150-212, 212-250 and 250-355 μm, with the four fluidization gases: air, CO2, steam and flue gas, at temperatures of 400, 500, 600, 700, 800, and 950 oC. The result showed generally higher heat transfer rate for ilmenite compared to sand, where the smaller particle sizes resulted in a higher heat transfer. The bed that is fluidized by flue gas gave in average a 9.9 % higher heat transfer rate, compared to air as fluidization gas. In this study, the results does not show that CO2 and steam can contribute to a higher heat transfer rate.