# Partition of Cu and Pb in a Two-Stage Fluidized-Bed Waste Gasification System
## 3 Results and Discussion  
Figure 2 represents the syngas composition after the two-stage fluidized-bed gasification system. The operating temperatures of the stage 1 and stage 2 gasifiers were controlled at 800 \({}^{\circ}\)C and 900 \({}^{\circ}\)C at an ER of 0.3. The productivity of H\({}_{2}\) was increased after passing the stage 2 gasifier because of the reheated environment for the reaction of the syngas. As a result, the stage 2 gasifier increased the H\({}_{2}\) concentration in the syngas by 10-16 percentage points. Therefore, using the stage 2 gasifier can slightly improve the generation of H\({}_{2}\) in the syngas. In this study, in order to determine the influence of different operating temperatures on heavy metal entrapment in the bed material and heavy metal escape, the operating temperature of the stage 1 gasifier was changed from 700, 800, to 900 \({}^{\circ}\)C, while the temperature of the stage 2 gasifier was fixed at 900 \({}^{\circ}\)C. Figure 3 shows the changes in heavy metal concentrations in the bed material and fly ash  
Figure 2: Synthetic gas compositions in the two-stage fluidized bed gasifier. (Stage 1: 800 \({}^{\circ}\)C and Stage 2: 900 \({}^{\circ}\)C, ER = 0.3, S/B = 0). during gasification with different stage 1 operating temperatures. The heavy metal concentrations in the bed material were obtained by analyzing stage 1 and stage 2 bed materials after completing the experiment. Since the heavy metal concentrations in fly ash changed with different operating conditions and given the variations in different compositions of the gasification syngas, it was found that the sampling data after 14 min were relatively stable. Thus, the sampling result obtained at the 14 th minute of the experiment was used to represent the heavy metal concentrations in fly ash. Figure 3 also shows a comparison of the heavy metal concentrations in the bed materials between stage 1 and stage 2 under different operating temperatures. When the stage 1 operating temperature increased, the concentrations of entrapped heavy metals in stage 1 bed material decreased, causing an increase in heavy metal concentrations in the bed material of stage 2. The same trend was obtained for both heavy metals. As pointed out by [PERSON] et al. [PERSON] et al. (2018), when subjected to heat treatment, heavy metals escape as a result of three mechanisms--homogeneous nucleation, heterogeneous deposition,  
Figure 3: Heavy metal distributions at different operating temperatures in stage 1. (B: Bed materials, F: Fly ash; Stage 2: 900 \({}^{\circ}\)C, ER = 0.3, S/B = 0). and chemical adsorption--and they are mainly affected by the operating temperature [31; 32; 33]. Since Cu has the higher boiling point of the two heavy metals (boiling points: Cu 2562 \({}^{\circ}\)C and Pb 1749 \({}^{\circ}\)C), stage 1 bed material had the highest Cu concentration, indicating that most Cu did not escape with the syngas but was entrapped in stage 1 bed material. However, some Cu still escaped to stage 2 with the fly ash. So, stage 2 bed material not only provided the syngas with the opportunity for steam (which could be produced in stage 1) reforming or cracking to increase the H\({}_{2}\) yield but also entrapped the fly ash that escaped from stage 1. Thus, some heavy metals that escaped from stage 1 were entrapped in stage 2 bed material. According to studies conducted by [PERSON] et al. [34] and [PERSON] et al. [35] on heavy metal contaminant control using a fluidized-bed, the main mechanisms that control heavy metal escape are filtration and chemical adsorption by bed particles. Therefore, gaseous heavy metals or particles containing heavy metals entering the stage 2 fluidized-bed gasifier could be entrapped in stage 2 bed material because by these two mechanisms. According to [PERSON] and [PERSON] [36], who pointed out that the escape of a heavy metal is mainly affected by the metal's boiling point, heavy metals with relatively low boiling points are more volatile, so they escape at relatively high rates.