Interaction of the Behavior of Lead and Zinc With Alkalis in Fluidized Bed Combustion or Gasification of Waste Derived Fuels

Abstract

Combustion of waste derived fuels in fluidized bed boilers may result in fly ashes containing increased amounts of lead and zinc, besides the common alkali and alkaline earth metal compounds. Although the absolute concentrations of lead and zinc may be relatively low, around 1%, in the bulk ash, they may induce unwanted effects in combustors, partly due to their significant enrichment in the fly ash. First, lead and zinc in fly ashes may lead to unwanted heavy metal emissions. Further, they can also alter the behavior of the fly ash and cause it to become sticky and possibly corrosive. This paper discusses the mechanism of volatilization of lead and zinc and stickiness properties of their fly ash compounds under different conditions, based on data from a FBC gasifier using waste fuels with significant amounts of lead and zinc. Advanced thermochemical calculations using the data bases developed at A˚bo Akademi show that both lead and zinc can form volatile compounds and thus be strongly enriched in the fly ash. They can be volatilized as elemental gases, Pb(g) and Zn(g), or they can form gaseous chlorides, PbCl2(g) and ZnCl2(g). But they can also form non-volatile oxides. Thus their behavior is very dependent on the combustion conditions, particularly on the availability of chlorine. This way there is also a direct coupling of the volatilization behavior of lead and zinc with the chemistry of the alkali metals and calcium, all of which govern the availability of chlorine. Simplified thermochemical diagrams are shown to explain the complex interaction of the lead and zinc chemistry with the rest of the flue gas and fly ash chemistry. The thermochemical data can be used to explain the practical results from full scale boilers.

Thermal Stability of Zinc Compounds

“…Previous studies indicate that Zn from the fuel can accumulate in the bed material, in the ash, participate in the formation of deposits in boilers, , and also increase the deposit formation rate . Ljungdahl and Zintl showed both experimentally and with thermodynamic equilibrium calculations that Zn present in the bed material of a fluidized bed can cause severe agglomeration of the bed if the stoichiometric conditions are changed after a long period of oxidizing to reducing.…”

Effects of Reduced Bed Temperature in Laboratory- and Full-Scale Fluidized-Bed Boilers: Particle, Deposit, and Ash Chemistry

“…During combustion, zinc is released and readily available to form gaseous products in the flue gas, and previous studies indicate that zinc readily forms zinc chloride, which melts at 318 °C . Findings of ZnCl 2 in fireside deposits in municipal solid waste (MSW) boilers have led to the general belief that ZnCl 2 might play a role in the corrosion of heat-transfer tubes in these boilers owing to its low melting temperature and high corrosivity when molten. − …”

Ash Deposition on Heat Transfer Tubes during Combustion of Demolition Wood

“…Backman et al showed by equilibrium calculations that during oxidizing conditions the presence of alkali (but not sulfur) can affect both the amount of zinc volatilization and the chemical form (ZnO or ZnCl 2 ) in the flue gas. This effect arises because KCl and NaCl are more stable than ZnCl 2 .…”

“…Adding sulfur to the calculations will increase the available chlorine as HCl (see Reaction 4) leading to formation of ZnCl 2 . Using the full element mechanism in the calculations (Backman et al) results only in a minor production of ZnCl 2 due to formation of other complex compounds that are not volatile.…”