A novel vortex-fluidized bed combustor with two combustion chambers for rice-husk fuel
Thanid Madhiyanon, Niwat Piriyarungroj, and Somchart Soponronnarit
pp. 875 - 893
Abstract
A novel vortexing-fluidized bed combustor (VFBC) using rice-husk as fuel was developed and
presented. The combined characteristics of vortex combustion and fluidized bed combustion are the main
features of the VFBC, which was designed to achieve high thermal capacity (MWth m-3), high thermal
efficiency and low diameter to height ratio. The VFBC comprises a vertical cylinder chamber and a conical
base, which provides a bed for incompletely combusted fuel. The overall dimensions are 1.10 m in height
and 0.40 m in diameter. To evaluate combustor performance, the specific feed rate of fuel and mass flow
rates of the primary, secondary, and tertiary air were varied independently of one another. The combustion
appeared into two zones characterized by different combustion behaviors, i.e. 1) vortext combustion above
the vortex ring and 2) fluidized bed combustion below the vortex ring. The fluidized bed zone has uniform temperature distributions across the cross-section of the combustor. The swirling of air above the vortex ring
and the vortex ring itself played important roles in preventing the escape of combustion particulates. Bottom
ash appeared as fine black and grey particles of ash, which ranged in size from 200 to 600 µm. Fluidization
could be initiated without the assistance of any inert material mixed into the bed. The experimental results
indicated that thermal efficiency did not depend on the secondary or tertiary airflows, but was significantly
influenced by the excess air resulting from the combined total of the three airflows. The introduction of the
tertiary airflow helped maintaining the temperature inside the combustor within acceptable levels. According to experimental conditions, i.e. a specific feed rate of 240 kg h-1m-3 and excess air (157%), it was found
that the VFBC could achieve an exit gas temperature of 1060ºC, thermal efficiency of 95%, and thermal
capacity of 0.91 MWth m-3. The amounts of CO2, CO, and O2 gases emitted were directly related to thermal
efficiency, and the amounts of CO and NOx were 50 and 380 ppm, respectively.
