The Influence of Injected HCl and SO₂ on the Behavior of Trace Elements during Wood-Bark Combustion

Abstract

The release of toxic trace elements from the combustion of fuels in power plants is an environmental issue of increasing concern. Trace element release mechanisms are known to be complex and may depend on the levels of chlorine and sulfur in the fuel burned. This dependency has been investigated experimentally by injecting first hydrogen chlorine, and then sulfur dioxide, into a suspension-firing reactor burning wood-bark under simulated fluidized bed combustion conditions. Data interpretation has been aided by parallel theoretical studies, using a thermodynamic equilibrium model based on the principle of Gibbs free energy minimization. The influence of chlorine and sulfur was confirmed for some, but not all, of the 10 trace elements studied. Injection of HCl served to increase the emission of Cd, but to reduce emission of Cu, Mn, Zn, and possibly Cr and Ni. Injection of SO2 served to increase emissions of Cd, but reduced emissions of As and Hg. The thermodynamic model proved valuable for interpreting the experimental data. However, evidence suggests that predictions not validated by measurement should be treated with caution, given the complexity of the systems being studied.

Trace elements in co-combustion of solid recovered fuel and coal

“…However, in the case with 4 wt.% PVC addition, a considerably higher RE factor of Cd is obtained, which deviates from the tendency predicted by other experiments. The influence of Cl-based additives may be related with the formation of CdCl 2 , which is a volatile and thermodynamically favorable compound when chlorine is present in the flue gas [3,19]. Although most of the thermodynamic calculations predict that Cd would be totally volatilized (mainly as gaseous Cd) at combustion temperatures [3,19], retention of Cd at high temperature is observed [19], possibly related with the association form of Cd in the fuel and the limitation on the reaction time and/or mixing in a combustor.…”

“…On the other hand, the vaporization behavior of trace elements is also influenced by parameters such as combustion temperature, transport/ mixing phenomena, oxidizing/reducing condition, and the presence of gaseous species such as HCl and SO 2 [3]. In most cases, the presence of gaseous chlorine may shift the distribution of trace elements such as Pb and Cd towards more volatile chlorides, thus promoting the vaporization [1,3,19]. However, the presence of SO 2 may result in the formation of sulphates with relatively low vapor pressure [19].…”

“…In most cases, the presence of gaseous chlorine may shift the distribution of trace elements such as Pb and Cd towards more volatile chlorides, thus promoting the vaporization [1,3,19]. However, the presence of SO 2 may result in the formation of sulphates with relatively low vapor pressure [19].…”

“…After being vaporized from the fuel, the trace elements may undergo reactions both with the ash particles and the gaseous species in the flue gas [19,20]. The reactions between the vaporized trace elements and the ash particles constitute an important mechanism for the retention of trace elements in large fly ash particles that are easier to be captured by the air pollution control systems [1,2].…”

“…In addition, the effect of additives such as NaCl, PVC, ammonium sulphate, and kaolinite on the behavior of trace elements was investigated. These additives were selected since they may have notable effect on the partitioning of trace elements [19,20]. Therefore the results from the present work could provide suggestions for trace element emission control in co-combustion of coal and SRF, through a systematic assessment of the behavior of trace elements at different fuel conditions.…”