Assessment of the rice husk combustion in a bubbling and atmospheric fluidized bed applying response surface methodology

Biomass is an attractive energy source that allows heat, power and gaseous or liquid fuel generation, from the thermochemical transformation process implemented (combustion, gasification and/or pyrolysis). Rice husk is a renewable energy source that presents disposal problems and is considered as an agricultural residual waste. Fluidized bed technology is an interesting and efficient mechanism in the thermochemical conversion of heterogeneous, low density and irregular/complex shape materials such as rice husk. In this sense, rice husk combustion in a bubbling and atmospheric fluidized bed reactor of 0.3 m diameter with expansion to 0.4 m in the freeboard zone, and 3m height was investigated. Experiment design-response surface methodology (RSM)-is used to evaluate the air excess and normal fluidizing velocity influence (independent and controlling variables), both in the combustion efficiency (carbon transformation), bed and freeboard temperature, and silica content in the ashes as well. Hot gases emissions (CO₂, CO and NO_x), crystallographic structure and morphology of the ash are also shown. A cold fluidization study is also presented. The values implemented in the equipment operation, air excess in the range of 40 to 125% and normal fluidization velocities (0.13 Nm/s-0.15 Nm/s) show that the values near to the lower limit, allow bed temperatures around 750°C with higher carbon transformation efficiencies around 98%, deteriorated the amorphous potential of silica present in the ash. An opposite behavior was evidenced at the upper limit of the air excess. The gases concentrations of the process showed a relative variability for each operating condition. In the CO case, showed variations between 200 and 3500 ppm (11% O₂ dry basis), according with the levels of air excess of 40 and 125% respectively. Also, NOx emission showed concentrations of 317 and 6.27 ppm (11% O₂ dry basis). The fluidization velocities (in the range evaluated) have not shown any significant influence on the characteristics of the rice husk ash. The low CO concentration in the hot gas produced by combustion, shows high potential in the industrial sector, for example in drying processes. The potential of this type of reactor was evidenced in variables such as speed, continuity and self-sufficiency energy of the process, comparatively to fixed beds which use external sources of energy and long reaction times for obtaining amorphous silica.