Effects of CaO and CaCO₃ on Heavy Metal Capture in Bottom Ash during Municipal Solid Waste Combustion under a CO₂/O₂ Atmosphere

Abstract

Experiments were conducted to investigate the influence of CaO and CaCO3 on the capture efficiency of heavy metals during the combustion of municipal solid waste (MSW) in CO2/O2 and N2/O2 atmospheres. Six heavy metal elements were studied in this paper, including Pb, Cd, Cu, Cr, Ni, and Zn. On the basis of the results, the operating temperature has a great effect on the transformation of Pb and Cd, while the amount of Cu, Cr, and Ni in the bottom ash are affected only slightly by changes in the temperature and atmosphere. The volatilization of Cd, Cu, Cr, and Zn marginally decreased with the addition of CaCO3 or in a CO2/O2 atmosphere (oxy-fuel combustion). The X-ray diffraction results show that CaO and CaCO3 have the same ability to capture heavy metals under CO2/O2 and N2/O2 atmospheres as a result of the reaction of CaO with CO2. Thermodynamic equilibrium calculations indicate that CaO or CaCO3 could barely hinder the volatilization of heavy metals by chemical reactions with the heavy metals but might hinder their volatilization by their physical adsorption. The initial forms of the heavy metals in the MSW may be another factor that affects the volatilization of the heavy metals. The importance of these forms is indicated by these six heavy metals having similar enrichment behaviors in both the CO2/O2 and N2/O2 atmospheres with CaO or CaCO3 additives; the calculations predict that these heavy metals have the same main species, except for a slight change in the content of these species.

Behavior of Cd during Coal Combustion: An Overview

“…This is consistent with observations of Tang et al [78] adding CaO or CaCO 3 to MSW and evaluating Cd retention in bottom ash. At 700 • C-800 • C, some (moderate) capture of Cd in bottom ash was observed, whereas at 900 • C the release of Cd from bottom ash was even enhanced, due to experimental temperature exceeding the optimal temperature range of 500 • C-650 • C (releasing retained Cl back).…”

“…Results from recent studies indicate that the presence of PVC during the combustion/incineration of coal/waste with Cd increases its volatility and potential fractions released in emissions. Calcareous additives could suppress this unfavorable effect; but if added directly into the furnace, the temperature is usually too high for efficient retention of chlorination agents [43,52,68,78,79]. However, if limestone is applied within a semi-dry spray tower, good Cd retention results were achieved, which was documented by Chen et al [57].…”

“…Thermodynamic equilibrium calculations [97] concluded that distribution of Cd (and other HMs) in oxy-fuel combustion was nearly the same as in the case of air combustion. However, real incineration experiments with synthetic MSW [78] (flour, paper, sawdust, HDPE, texture, rubber, leather and Cd-acetate) in tubular furnace revealed certain differences in Cd volatility in CO 2 /O 2 and N 2 /O 2 atmospheres. In both atmospheres, the Cd residual rates at 1000 • C were almost zero; at 900 • C they were nearly the same-ca.…”

“…Not only % O 2 , but also the composition of "the rest" of the combustion/incineration atmosphere can play an important role [78,96], which is studied namely due to oxy-fuel combustion technology where CO 2 can be captured due to recycling of the flue gas and oxygen is used instead of air (thereby increasing CO 2 content up to 95%) [96]. Thus, instead of N 2 /O 2 atmosphere, CO 2 /O 2 is used in oxy-fuel combustion.…”

Behavior of Pb During Coal Combustion: An Overview

“…Tang et al [53] reported that adding 10% CaO or CaCO 3 to synthetic MSW (flour, paper, wood sawdust, high-density polyethylene (HDPE), textiles, rubber and leather) did not show any increase in the Pb residual rate at 700 • C (i.e., no effect on Pb retention in ash); at 800-1000 • C the residual rate even slightly decreased, even if chlorine was present in these wastes. The results are still consistent with the conclusions of Wang et al [31], stating that the efficient retention temperature of CaO was below 700 • C (at higher temperatures, CaCl 2 can release chlorine back for Pb chlorination).…”