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.
Highlights
Compared with fossil energy, biomass has the characteristics of zero CO2 emission
In order to study the mechanism of deposition on a high-temperature superheater of a circulating fluidized bed (CFB) boiler burning biomass with high-alkali metal content, the experiment was carried out in a CFB boiler at a biomass power plant (2 × 12 MW) in Jiangsu province, China
A sketch is given in 820 ◦ C, minimizing the migration of alkali metals into the gas phase, inhibiting deposition and high-temperature corrosion on the heating surface
Summary
Biomass has the characteristics of zero CO2 emission. Biomass direct combustion technology for power generation, as a relatively mature large-scale biomass utilization technology, has been widely used in China. CFB boilers have a wide combustion adaptability, low combustion temperature and good gas–solid mixing characteristics in the bed, making them a very suitable combustion technology for biomass. This technology has taken up a large proportion in biomass power generation projects in recent years in China [1]. K element in biomass fuel is released to the gas phase in the form of KCl, KOH and K2 SO4 [4,5,6,7,8,9,10] These potassium-containing compounds cause severe slagging due to their low melting temperature [11,12,13,14,15,16]
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