Effect of waste incineration and gasification processes on heavy metal distribution

• Heavy-metal concentration in fly ash increases with temperature in two methods.
• The content of heavy metals in fly ash decreases in the order Cd > Pb > Cr.
• Heavy-metal content of fly-ash gasification is 1/10–1/30 compared to incineration.
• Gasification has lower emission of metals and traps more metals in the bottom ash.

This study investigates the distribution of heavy metals during incineration and gasification. Because heavy metals (Cr, Pb, Cd) become volatile at high temperatures, their concentrations in fly ash increase as the operating temperature of incineration/gasification increases. By contrast, the efficiency of the bed-material mixing and the contact between the bed material and the heavy metals inside the fluidized bed increase when the operating-gas velocity increases; hence, the heavy-metal content in the bed material also increases. Our results show that the heavy-metal content in the fly ash increases with the bed-material particle size. The probability of contact between the heavy metals and the bed material is reduced at higher bed-material particle sizes; hence, the emission concentration of heavy metals increases. The heavy-metal concentration in the fly ash after gasification is approximately 1/10–1/30 of that after incineration. However, the proportion of the heavy metals trapped in the bottom ash after gasification is higher than that after incineration under any of the operating conditions, regardless of the volatility of the heavy metals. Overall, gasification lowers the emissions of heavy metals, but traps more heavy metals in the bottom ash than incineration.

According to the comparison between incineration and gasification processes in the emission concentration of heavy metals under different conditions, the concentration of heavy metals in fly ash after gasification is only about 1/10–1/30 of the concentration of heavy metals in fly ash after incineration. Additionally, based on the comparison between incineration and gasification processes in the total amount of heavy metals trapped in bottom ash, the proportion of heavy metals trapped in the bottom ash of the gasification process is higher than that of incineration under any operating conditions, and the heavy metals with different volatilities have the same result. Therefore, the result shows that the gasification process has lower emission of heavy metals, and traps more heavy metals in the bottom ash.Figure optionsDownload as PowerPoint slide