Experimental investigation of the heat transfer coefficient to a horizontal tube submerged in bubbling fluidized bed
Typ
Examensarbete för masterexamen
Program
Sustainable energy systems (MPSES), MSc
Publicerad
2020
Författare
Lüsch, Sonja
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
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.
Beskrivning
Ämne/nyckelord
heat transfer coefficient , fluidized bed heat exchanger , oxygen carrier , ilmenite