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The study of the effects of hydrodynamic behaviors on waste incineration
The primary object of investigation elucidates the relationship between the fluidized characteristics and pollutants during incineration. The work firstly emphasizes the effects of high temperature on minimum fluidized velocity, expanded bed rate, and attrition. Then, the effects of the mixing of multi substances on the fluidized behavior and generation of pollutants are also performed. Finally, the study focuses on the mechanism of formation of agglomeration and defluidization during incineration process and a simple model has been developed to simulate the defluidization time at various conditions.
At high temperature, the minimum fluidization velocity gradually reduces as temperature is increased, but above 800℃, the minimum fluidization velocity reverses with rising temperature. Comparing the result for combustion with those for non-combustion reveals that total weight of attrition during combustion is significantly higher than under the non-combustion condition, because the combustion heat and thermal shock increase attrition. According to index of pressure fluctuations, the concentration of organic pollutants does not increase with uniformity of fluidization decreasing. It may be that the explosion of the volatiles causes the pressure in the chamber to change suddenly and destroy the coalescent bubbles to form small bubbles again. The gas explosion possibly increases the efficiency of conversion of oxygen between bubbles and wastes, increasing the combustion efficiency and reducing the concentration of organic pollutants. The four particle size distributions could be divided into two groups by statistical analysis. The organic concentration of the Gaussian and narrow distributions are lower than that of the other distributions. Consequently, the bed materials should maintain narrow or Gaussian distributions to maintain a good combustion efficiency during incineration.
Alkali metals (Na) significantly affected the agglomeration of bed materials. Increasing the concentration of sodium caused a decrease of time to reach defluidization. However, the earth alkali metals (Mg, Ca) apparently inhibited the agglomeration generation, some species with high melting points, such as Mg2SiO4, MgO, CaO and Ca(OH)2 were generated, inhibiting agglomeration. For pollutants, the emission of organics (BTEXs and PAHs) increased with the operating time, as according to the generation of agglomerate, indicating that the size of the agglomerate gradually increased, reducing the efficiency of combustion. However, the concentrations of volatile metals (Cd, Pb and Cr) exhibited similar tendencies with operating time. However, the emission concentrations increased after defluidization, because the silica sand can not capture the heavy metals released from combusted waste in the fixed bed state. The increase in the temperature of the surface of the sand bed is important in promoting the emission level of heavy metal. Accordingly, the emissions of organics and heavy metals pollutants differed during agglomeration and defluidization. A simple model has been developed to describe the defluidization time at various conditions. When the exist ratio of Na in bed material is between 0.8 to 1, the prediction is agreed with experimental data and the error range is between -20% ~ +20%.
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