Semi-Volatile Heavy Metal Emission Characteristics And Control During Waste Incineration

Waste incineration has become the main way of garbage disposal method in many countries for that it could be able to realize the reducing of volume and harmless better and to turn waste to energy. But waste incineration could produces serious secondary pollution, especially the heavy metal pollution which has restrict the development and application of waste incineration technology.The effects of inorganic chlorides (NaCl), organic chloride(PVC) and different forms of sulfur(S, Na2S, Na2SO4) on Pb and Cd emission during waste incineration were investigated using a tubular furnace. The results indicated that both inorganic chlorides and organic chlorine could effectively increase the volatility of Cd and Pb. However, the different emission characteristics of chlorine of NaCl and PVC lead to the mechanisms that influenced the emission behaviors of Cd and Pb were different from each other. Different forms of sulfur had different impacts on the emission of Cd and Pb. Sulfur could restrain the volatilization of Pb and Cd at low temperatures but had different effects at high temperatures, except Na2S. More Pb and Cd tended to be left over in the incineration bottom ash with the addition of kaolin and zeolite. Besides chemical adsorption, kaolin and zeolite also showed physics adsorption capacity while limestone had no obvious adsorption capacity.The distribution characteristic of lead and cadmium in PM10 was investigated in the fluidized bed incinerator, then kaolin was used as sorbent to control Pb and Cd emissions. The SEM/EDS/XRD was used to analysis the adsorption mechanism. More than 90% of particulate Pb and 85% of particulate Cd in flue gas existed in submicron particles. The addition of kaolin could effectively capture submicron Pb and Cd with the highest capture efficiency up to 80% and 50% respectively. The interactions of Pb and Cd with kaolin powder tend to induce a eutectic-melt at high temperature and produce PbO·Al2O3·2SiO2 and CdO·Al2O3·2SiO2 respectively. The eutectic-melt could shifts the metals in flue gas from the fine to coarse particles and the eutectic-melt increased with the rising temperature. The high temperature could induce excessive eutectic-melt and pore structure collapsing. Pb could promote the reaction of kaolin with Cd and Cd could restrain the excessive eutectic-melt of Pb when Pb and Cd combined. That means the capture of Pb and Cd is enhanced by each other when both metals are present.Based on minimization of Gibbs free energy, thermodynamic equilibrium calculations were performed to investigate the effect of incineration temperature, sulphur, chloride and sorbent on the partitioning of Pb and Cd. The results show that chloride tends to react with Pb and Cd to form chloride which can promote the volatilization of Pb and Cd during incineration. The addition of sulphur can depress the evaporation of Pb and Cd at low temperatures but not so much at high temperatures. The effects of sulphur and chloride can be depressed by Na which has much stronger affinity with chloride than Pb. The addition of CaO has a weak effect on the adsorption of Pb and Cd. SiO2 can effectively adsorb Pb and Cd, while Al2O3 only has limited effect on Cd.