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

Abstract

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.

The role of limestone during fluidized bed oxy-combustion of coal and biomass

“…Visual inspection confirmed that none of the experiments presented agglomerated particles, despite the 30% of corn stover in the blend and high temperature (925º C) during some experiments. The presence of calcite and some refractory elements from lignite ash contributes to the avoidance of agglomeration [27].…”

“…The co-combustion of coal and residual biomass has been found to decrease agglomeration and corrosion risks compared to firing biomass alone [25][26][27]. The effectiveness of co-firing in preventing deposition of alkali chlorides is due to sulphur in coal since SO 2 and SO 3 react with alkali chlorides, yielding alkali sulphates (R.1, R.2):…”

Effects of Coal Bottom Ash on Deposit in a Full-Scale Biomass-Fired Circulating Fluidized Bed Boiler

“…The water-insoluble K concentration was much higher than that in OBA, which suggests that CBA can capture K mainly through chemical capture. It is widely known that Si and Al in CBA can react with KCl through the following reactions: ,− …”

“…Unfortunately, natural biomass is highly rich in potassium (K) and chlorine (Cl) while depleted in aluminum (Al), sulfur (S), and silicon (Si) in comparison with coal, especially when it is obtained from agroforestry residues such as straw and bark. , During biomass combustion, these properties increase the risk of deposition and high-temperature corrosion (HTC) of the superheater, which becomes more serious when trying to increase the electricity production efficiency in biomass-fired CFB. ,− The consequences of damaging the superheater include unscheduled shutdowns, prolonged downtime, increased maintenance costs, and more frequent replacement of superheater tubes. , …”

“…The employment of additives is regarded as an effective measure to destroy alkali chlorides before their deposition during biomass combustion. ,− Among many kinds of additives, coal ash (both bottom ash and fly ash) obtained from a pulverized coal-fired power plant, rich in Si and Al, is considered a low-cost aluminosilicates-based additive, which has proven to be highly effective in the capture of potassium in a lab-scale experiment and to alleviate deposition problems in a full-scale wood suspension-firing boiler . However, detailed investigations on the impact of coal bottom ash (CBA) on a full-scale biomass-fired CFB plant have rarely been carried out to date.…”