Fly Ash Deposition during Oxy-fuel Combustion in a Bench-Scale Fluidized-Bed Combustor

Abstract

Ash deposition on heat-exchanger surfaces in boiler systems can cause numerous problems, including slagging, fouling, and corrosion. These deleterious processes can be compounded if the boiler combustion process is changed from air to oxy-fuel. In this paper, fly ash deposition characteristics under both air and oxy-fuel combustion conditions were investigated using a bench-scale fluidized-bed combustor (FBC) based on measurements of ash deposition rates via a temperature-controlled probe. Three different combustion atmospheres were studied, and results demonstrated that, under similar combustion temperature profiles and equivalent fluidization velocities, the deposition rate increased when transitioning from combustion atmospheres consisting of 21% O 2 /79% CO 2 to air to 30% O 2 /70% CO 2 . To determine the primary factors associated with the observed variations in deposition rates, the chemical compositions and micromorphologies of ash and fly ash deposits were analyzed by inductively coupled plasma−atomic emission spectrometry (ICP−AES) and scanning electron microscopy (SEM). Particulate matter with aerodynamic diameters less than 10 μm (PM 10 ) was measured by an electrical low-pressure impactor (ELPI), and the particle size distributions (PSDs) and carbon contents of the collected filter ash were also ascertained. The results indicate that the higher deposition propensity associated with a 30% O 2 /70% CO 2 atmosphere can be largely attributed to a wider PSD rather than any changes in the chemical compositions of the fly ash or deposited ash, in which there are no obvious differences between air and oxy-fuel combustion. In addition, the slightly higher concentration of fine particles produced under this atmosphere also promotes the deposition of fly ash.

A review of oxy-fuel combustion in fluidized bed reactors

“…Ash deposition on heat exchanger surfaces of boiler systems might cause several problems, such as slagging, fouling and/or corrosion. For that reason, Zheng et al at the Harbin Institute of Technology (China) studied the fly ash deposition characteristics under both air and oxy‐fuel combustion conditions in a bench scale FBC unit. This study demonstrated a greater tendency for fly ash deposition during oxy‐FB combustion than for air‐firing atmosphere.…”

“…Zheng et al conducted ash deposition experiments in a bench scale FBC under both oxy‐fuel and air combustion conditions. The results showed a higher tendency for fly ash deposition during oxy‐CFB combustion than for air‐firing mode.…”

A Comprehensive Review of Ash Issues in Oxyfuel Combustion of Coal and Biomass: Mineral Matter Transformation, Ash Formation, and Deposition

“…The Harbin Institute of Technology (HIT) research group has conducted several tests on ash deposition under oxyfuel combustion conditions on a bench-scale fluidized-bed reactor. When an anthracite was considered, the ash deposition rate increased in the order of 21% O 2 /79% CO 2 < air <30% O 2 /70% CO 2 . The significantly higher deposition rate in 30% O 2 /70% CO 2 was attributed to the higher production of PM 10 and coarser particle size of the fly ash, compared with the other two combustion cases.…”

A novel method used to study growth of ash deposition and in situ measurement of effective thermal conductivity of ash deposit

“…The details about controlling probe surface temperature can be found in Refs. and . The temperature controlling system is the same as that of the previous ash deposit probe.…”