Metal volatility during plastic combustion

Abstract

The hazardous pollutants, such as heavy metals, nitrogen oxides, sulfur oxides, hydrogen chloride, and Dioxins, are emitted during incineration process. This study is intended to evaluate different factors affecting Pb or Cd emission during combustion of plastic powder and plastic membrane, of which plastics constitute 20% of solid wastes in Taipei. A laboratory scale incineration system was employed with a controller to adjust temperature, fuel velocity and other parameters. The major operation factors for heavy metal volatilization included combustion temperature and time. The results indicate that the combustion temperature exerts a significant effect on metal volatilization. For example, at the combustion time of 5 minutes at 600 °C, Pb emission is approximately 20% of the value initially present in the plastic powder, but it increases to 90% at 1000 °C. Also, metal emission increases with combustion time, until it reaches a plateau. Cd volatility after 5 minutes combustion is much higher than Pb at lower temperatures. The other factors evaluated include fuel gas flow (350–3600 mL/min), fuel N₂:O₂ ratio (4:1 and 1:4), fuel humidity (40 and 60 %), and the addition of Ca(OH)₂. Generally, these factors exhibit some degree of retardation/enhancement of metal emission. The X‐ray diffraction test indicates that the bottom ash consists of quartz (SiO₂), laurionite (PbCIOH), and calcite (CaCO₃). The EDAX(energy dispersive analysis of X‐ray) analysis further demonstrates a much higher ratio of CI:Ca:Pb present in the bottom ash.

Key words:

• Combustion
• metal emission
• plastics
• bottom ash
• lead chloride