Role of Pulverized Coal Ash against Agglomeration, Fouling, and Corrosion in Circulating Fluidized-Bed Boilers Firing Challenging Biomass

摘要

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.