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.
Energy recovery of used materials can be performed as mixed municipal solid waste (MSW) incineration or as fuel recovery for co-combustion. Recovered fuels are refuse-derived fuel (RDF), which is mechanically separated and processed from MSW, and packaging-derived fuel (PDF), which is the source-separated, processed, dry combustable part of MSW.
The rising use of agricultural residues (agros) aggravates some of the well-known challenges of biomass combustion in plants, such as agglomeration, fouling, and corrosion (AFC). Several countermeasures have been devised to contain AFC problems in biomass plants, some of which are broadly effective but somewhat costly, while others may be ineffective or harmful if deployed on unsuitable agros. The use of additives often falls in the first category, having broad applicability, high efficiency, high reliability, but high procurement costs. However, ash discarded from combustion plants firing coal can be a convenient exception, because its cost is negligible or even negative. Ash coming from pulverized fuel plants firing coal (PC-ash) was tested in a bench-scale reactor and a 1 MWth circulating fluidized-bed (CFB) pilot to assess its beneficial effects against agglomeration when firing the most demanding agro biomass. PC-ash was procured from different plants based on different PC technologies and firing diverse coals. Process conditions, boiler design, and coal type influenced dramatically the performance of PC-ash, but even the worst PC-ash still improved the resistance of agglomeration in CFBs by a factor of 2, as compared to the sand regularly used as bed material. Such performance resulted from the synergy of physical and chemical interactions between ash-forming elements and PC-ash. Contrary to many other countermeasures and additives, the beneficial effects of PC-ash against agglomeration did not unveil major drawbacks on fouling and corrosion. Because fuel alkalis are captured by the PC-ash and chlorine is released as HCl, both fouling and corrosion can be kept at bay. Nonetheless, proper management of the PC-ash is imperative because fine fractions increase dust loading in the backpass and can increase fouling if uncontrolled. This paper summarizes the key observations from PC-ash testing and consolidates the role of PC-ash against AFC. Chemical/physical mechanisms are proposed and verified against test results.
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