The removal of an airborne low-volatility heavy metal from exhaust gases through condensation onto sorbent particles
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The removal of a low volatility heavy metal through condensation was investigated experimentally. The unfavorable volatilization behavior of a low volatility metal was altered using chlorine gas and an inorganic sulfur compound. Using such compounds the speciation of the heavy metal during combustion was modified, which caused a different volatilization and condensation behavior. Condensation sites included predistributed sorbent and soot particles. In addition to condensation rate, other processes, such as nucleation and coagulation influencing the particle size distribution and capture efficiency were investigated experimentally using particle size segregation, atomic absorption analysis, atomic emission analysis, nitrogen porosimetry, X-ray diffraction analysis and scanning electron microscopy. Using these techniques, a set of parameters promoting condensation capture was developed. Parameters under investigation were temperature, rate of temperature drop, equivalence ratio, concentrations of heavy metal, sorbent, chlorine gas and sulfur. After investigating the condensation behavior experimentally, a numerical model BAEROSOL was developed. BAEROSOL contains only condensation, nucleation and Brownian coagulation, neglecting the impact of other physical and chemical particle evolution processes. The experimental findings were compared to the model. The model BAEROSOL was used to investigate the pertinent mechanisms. The model proved to be a useful tool to estimate the general trend of the particle size evolution and its dependence on the investigated parameters. Without chlorine, typical values for the recovered fraction of the low volatility heavy metal ranged from 15% to 25%. Chlorine proved to be a suitable agent to improve the particle size distribution and the overall capture efficiency. Efficiencies could be increased up to 73% in the presence of chlorine. Sulfur did not exhibit a positive influence on the particle size distribution or capture efficiency.