Investigation of Ash Deposition Dynamic Process in an Industrial Biomass CFB Boiler Burning High-Alkali and Low-Chlorine Fuel

Abstract

The circulating fluidized bed (CFB) boiler is a mainstream technology of biomass combustion generation in China. The high flue gas flow rate and relatively low combustion temperature of CFB make the deposition process different from that of a grate furnace. The dynamic deposition process of biomass ash needs further research, especially in industrial CFB boilers. In this study, a temperature-controlled ash deposit probe was used to sample the deposits in a 12 MW CFB boiler. Through the analysis of multiple deposit samples with different deposition times, the changes in micromorphology and chemical composition of the deposits in each deposition stage can be observed more distinctively. The initial deposits mainly consist of particles smaller than 2 μm, caused by thermophoretic deposition. The second stage is the condensation of alkali metal. Different from the condensation of KCl reported by most previous literatures, KOH is found in deposits in place of KCl. Then, it reacts with SO2, O2 and H2O to form K2SO4. In the third stage, the higher outer layer temperature of deposits reduces the condensation rate of KOH significantly. Meanwhile, the rougher surface of deposits allowed more calcium salts in fly ash to deposit through inertial impact. Thus, the elemental composition of deposits surface shows an overall trend of K decreasing and Ca increasing.

Aluminosilicate Clay Minerals: Kaolin, Bentonite, and Halloysite as Fuel Additives for Thermal Conversion of Biomass and Waste

“…Chlorine and alkalis tend to merge with silica, which occurs in the ash mainly as silica (SiO 2 ), and reduce the fluid temperatures from approximately 1700 • C (melting point of SiO 2 ) to approximately 750 • C. As a result of such a process, the AFTs of biomass and waste are usually lower than those of typical coals; for instance, the initial deformation temperature of biomass ash can be as low as 700 • C [21]. Such low melting tendencies lead directly to issues such as formation of deposits on heating surfaces of a boiler, slagging, fouling, high-temperature corrosion, bed agglomeration (defluidization) in circulating fluidized bed (CFB), boilers, and increased particulate matter (PM) emission [22][23][24][25][26][27][28][29], as clearly indicated by both experimental research and industrial experience [30][31][32][33]. These issues are not specific to biomass and waste but are likely to occur during the combustion of low-rank coals, as well.…”

Simulation of Two-Phase Flow and Syngas Generation in Biomass Gasifier Based on Two-Fluid Model

“…Lathouwers and Bellan introduced chemical reactions and heat and mass transfer into this method and studied biomass pyrolysis in fluidized beds [8,9]. In recent years, application of the two-fluid model (TFM) based on particle dynamics theory in the coupling of dense gas-solid fluidization and chemical reaction has been widely studied [10][11][12][13].…”

Morphological and Chemical Differences within Superheater Deposits from Different Locations of a Black Liquor Recovery Boiler

“…Studies utilising full-scale deposits focused mainly on deposit sintering [14,15], or the initial deposit build-up by carrying out short-term probe measurements [16,17]. However, a few examples indicating deposit ageing to be also of relevance for actual ash deposits in steam power boilers were found in the literature [18,19].…”