Bench-Scale Fluidized Bed Combustor
|
The electrically heated Fluidized Bed Combustor (FBC) facility at the University of Utah is designed to thermally oxidize both wet and dry feed materials (biomass). The FBC facility includes a gas clean-up system for the removal of particulate matter and catalytic reduction of hazardous combustion byproducts to CO2 and water.
|
|
The FBC is fabricated from 15 cm (6 inch) schedule-10 stainless steel pipe with outer and inner diameters of 16.8 cm and 16.1 cm respectively, and an overall height of 183 cm. The fluidized bed facility is comprised of three main parts: a feed system, a combustor, and a flue gas clean-up system. A schematic representation of the unit appears below. The first zone is the plenum where preheated air expands below the distributor plate. The plenum is approximately 9 cm in.
The second zone is the lower bed which is operated in the bubbling bed mode. The zone contains 14 kg of zirconia bed material stabilized with calcium oxide corresponding to a fixed bed height of 8.7 inches and an expanded bed height of 10.7 inches. The average particle size is 300 microns with a particle density of 5740 kg/m3. The bed is targeted to operate at 800°C with an air preheat temperature of 400 to 600°C.
The final zone is the freeboard with a height of 80 cm. Combustion air is preheated by a 2.2 kW cylindrical heater in a stainless steel coil with a diameter of 1.3 cm and a length of 600 cm. The lower bed and freeboard sections are heated to the target temperature of 1000°C with three 1.1 kW cylindrical heaters. The fluidizing velocity is set at twice the minimum bubbling velocity. The clean-up system is comprised of three separate components. First, particulate matter (fly ash, entrained bed material, etc.) is removed from the gas stream using a high temperature ceramic filter developed by CeraMem Separations. The particulate filter is circular in cross-section and housed in a cylindrical metal casing and is approximately 38 cm in length by 18 cm in diameter with 2.4 x 104 cm2 of surface area. A single filter is sufficient to handle the flow rate from the present fluidized bed. The filter can be periodically regenerated using back pulsing. Particulate removed during back pulsing is collected in a bottom hopper. This hopper is removable and allows for particulate sampling when required. The flue gas, devoid of particulate, flows through a fixed bed catalytic reactor system that includes a reduction and an oxidation section. The beds are 48 cm in height with a diameter of 11 cm. The flue gas clean-up system has been designed to remove the primary undesired components produced during combustion. The final component in the gas clean-up system is the water condenser/knockout drum. The residual moisture in the gas leaving the condenser is captured in a bed of silica gel desiccant. The dry flue gas then enters the final clean-up absorber, a packed bed of one-eighth inch BPL activated carbon pellets, used to trap trace contaminants that have not been removed.
This system includes a flexibly coupled dual-screw design that is capable of maintaining very uniform feed rates for a wide variety of dry or moist biomass fuels and has proven to be completely reliable over the time available during this program. No failures or loss of performance were noted after over 100 hours of use. A wet feed system has also been developed during the course of the project and was tested at Johnson Space Center during the 1998 testing of the human rated test facility using actual human waste.
The feed system provides a steady flow of milled biomass to an inlet port located near the bottom of the fluidized bed. A drawing describing the location of this port and the arrangement of the critical elements of the remainder of the system is presented below. Preheated air is passed through a bubble cap distributor designed specifically for the bed. A 6" ID stainless steel cylinder serves to contain the unique zirconia bed material. The material properties of the zirconia and the low operating temperature of the bubbling bed (700 to 800°C have successfully eliminated the clumping and sticking that exists when this waste is processed under standard conditions. The flue gases, which contain several percent CO and hydrocarbons, then pass through a cyclone for the removal of the bulk of the particulate and a ceramic membrane filter for removal of extremely fine particulate. Multiple ports exist for the measurement of temperature, pressure, and gas composition. The unsteady release of volatile species from the fuel led to a corresponding variation in the temperature and composition of the flue gases that eventually enter the catalytic system. In order to improve the stability of the system a cooling loop was designed to improve the mixing in the bed and to improve the temperature control by providing a means to quickly remove heat.
