# Partition of Cu and Pb in a Two-Stage Fluidized-Bed Waste Gasification System
## 3 Results and Discussion
Hence, the Pb concentration in stage 1 bed material was lower than the Cu concentration and significantly decreased with increasing operating temperature. However, the Cu and Pb concentrations in stage 2 bed material were proportional to the operating temperature, suggesting that the greater the amount of heavy metals that escaped from stage 1 gasification, the greater the amount of heavy metals that would be entrapped in the stage 2 bed material. The comparison of heavy metal concentrations in fly ash between stage 1 and stage 2 under different operating temperatures are displayed in Figure 3. It can be seen that at a higher gasification temperature in stage 1, the content of Cu and Pb in fly ash both trended upward since high temperature facilitates the volatilization of heavy metals, leading to the escape of heavy metals deposited on fly ash through homogeneous nucleation and heterogeneous nucleation [22]. Therefore, a higher temperature will result in an increase in the heavy metal content of fly ash. As expected by their difference in volatility, according to the results, the order of the amount of the two heavy metals in fly ash was Pb \(>\) Cu. The amount of heavy metal that can escape is mainly related to the heavy metal's own volatility. According to the fly ash results shown in Figure 3, the Cu and Pb concentrations in fly ash were higher in stage 1 than in stage 2 when the temperature of stage 1 and 2 was 900 \({}^{\circ}\)C. This phenomenon is due to the increased emission of Cu and Pb in stage 1 when the temperature of stage 1 was up to 900 \({}^{\circ}\)C. The amount of Cu and Pb condensation on the fly ash in stage 1 was increased; therefore, the concentration of Cu and Pb in fly ash in stage 1 was also increased. On the other hand, the emission of Cu and Pb declined, and the amount of the Cu and Pb condensation on the fly ash in stage 1 decreased when the temperature of stage 1 was lower than that of stage 2. As a result, the concentration of Cu and Pb in fly ash in stage 1 was lower than that in stage 2. Influence of Different Equivalence Ratio Values on Heavy Metal Concentrations in Bed Material and Fly Ash  
Figure 4 shows a comparison of the heavy metal concentrations in the bed material and fly ash in stage 1 and stage 2 at different ER values. The results show that increasing the ER slightly increased the concentrations of heavy metals entrapped in stage 1 and stage 2 bed materials. The inference is that the increase in ER caused an increase in the amount of oxygen that entered the stage 1 gasifier, improving the decomposing effect on wastes. Thus, heavy metals in the waste had more contact with the bed material, increasing the adsorption of heavy metals by the bed material and causing an increase in heavy metal concentrations in the bed material. The distributions of different heavy metals in the bed material were similar to the results shown in Figure 3. Cu has the lowest volatility, so it was more concentrated in stage 1 bed material than in stage 2 bed material, whereas Pb displayed the opposite pattern. The change in the ER had an insignificant influence on the heavy metal distribution in fly ash, especially for Cu. This phenomenon is due to the enhanced efficiency of gasification when the ER is  
increased. In this condition, the waste was decomposed more completely and then converted to fine particles mixed with the bed material. Therefore, the emission of heavy metals decreased and resulted in the ER having an insignificant influence on their distribution in fly ash. Influence of Different Steam/Biomass Ratio Values on Heavy Metal Concentrations in Bed Material and Fly Ash  
Figure 5 shows the influence of different S/B ratios on the heavy metal concentrations in the bed material and fly ash of stage 1 and stage 2. The results suggest that the increase in the S/B led to an increase in heavy metal concentrations in stage 1 bed material and reduction in heavy metal concentrations in stage 1 fly ash.