DESIGN AND CONSTRUCTION OF A CIRCULATING FLUIDIZED-BED COMBUSTION FACILITY FOR USE IN STUDYING THE THERMAL REMEDIATION OF WASTES

Fluidized bed combustion systems have been widely applied in the combu stion of solid fossil fuels, particularly by the power generation indu stry. Recently, attention has shifted from the conventional bubbling f luidized bed (BFB) to circulating fluidized bed (CFB) combustion syste ms. Inherent advantages of CFB combustion such as uniform temperatures , excellent mixing, high combustion efficiencies, and greater fuel fle xibility have generated interest in the feasibility of CFB combustion systems applied to the thermal remediation of contaminated soils and s ludges. Because it is often difficult to monitor and analyze the combu stion phenomena that occurs within a full scale fluidized bed system, the need exists for smaller scale research facilities which permit det ailed measurements of temperature, pressure, and chemical specie profi les. This article describes the design, construction, and operation of a pilot-scale fluidized bed facility developed to investigate the the rmal remediation characteristics of contaminated soils and sludges. Th e refractory-lined reactor measures 8 m in height and has an external diameter of 0.6 m. The facility can be operated as a BFB or CFB using a variety of solid fuels including low calorific or high moisture cont ent materials supplemented by natural gas introduced into the fluidize d bed through auxiliary fuel injectors. Maximum firing rate of the flu idized bed is approximately 300 kW. Under normal operating conditions, internal wall temperatures are maintained between 1150 and 1350 K ove r superficial velocities ranging from 0.5 to 4 m/s. Contaminated mater ial can be continuously fed into the fluidized bed or introduced as a single charge at three different locations. The facility is fully inst rumented to allow time-resolved measurements of gaseous pollutant spec ies, gas phase temperatures, and internal pressures. The facility has produced reproducible fluidization results which agree well with the w ork of other researchers. Minimum fluidization velocities (U-mf) rangi ng from 0.4 to 2.3 m/s were experimentally determined for various size s and types of material. Static wall pressure varied between 2.6 and 1 2.9 kPa along the length of the reactor over the range of superficial velocities. Superficial velocity was found to significantly influence the behavior of the axial pressure profiles, particularly in the slugg ing and turbulent regimes of operation. In addition to fluidization te sts, initial combustion tests were performed while burning natural gas and operating with an inert silica sand bed. Results indicate that co mbustion of natural gas occurred to only a limited extent within the b ed. The lowest CO2 and the highest CO concentrations (1.9% and 0.9%, r espectively) were found 0.5 m above the expanded bed surface. Maximum measured gas temperatures (1400 K) were also observed in this region. These results indicate that ignition occurred immediately above the be d surface and combustion proceeded in the freeboard section. Although significant quantities of NOx (45.0 ppm) and CO2 (7.2%) were formed fu rther downstream in the freeboard of the reactor, the combustion proce ss was found to be essentially complete before the entrance to the cyc lone.