Avoiding the sintering of coal fired shallow fluidized beds

Afilaka, Daniel T. (2016) Avoiding the sintering of coal fired shallow fluidized beds. EngD thesis, University of Nottingham.

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Fluidised bed combustion (FBC) has been identified as one of the best technologies available for lump coal combustion. A major drawback during prolonged operation of FBC systems particularly bubbling fluidised bed (BFB) systems is sintering and agglomerate formation of bed material that affects performance efficiency and reliability in industrial applications as exemplified at Associated British Sugar (AB Sugar). The mechanisms responsible for sintering and agglomerate formation in this type of system need to be understood, to promote continual use of this technology for efficient coal utilisation.

The first set of investigations focused on agglomeration properties of bed material (Garside 14/25 sand) used in Industrial FBC at AB Sugar. Bed material was calcinated between 800 and 1200°C in a high temperature furnace in the absence and presence of coal (three types of bituminous coals) or coal ash. Results showed sintering and agglomerate formation of bed material can occur in the absence of coal or coal ash at a calcination temperature near 1200°C. Addition of coal or coal ash further promotes sintering and agglomerate formation at 1000°C. Combustion stages appears to influence surface morphology, chemistry and mechanisms of agglomerated bed material based on similarities observed in the agglomerated bed material formed from calcination of Garside 14/25 sand bed material mixed with coal, and those formed in industrial scale FBC during combustion of lump coal.

The second set of investigations used two different lump bituminous coals classified as washed (undergone washing process to remove mud/shale stone) or unwashed (still containing the mud/shale) from the same mine (Blyth, typically referred to as Blyth coals) as those used in the AB Sugar industrial FBC. Combustion of washed and unwashed Blyth lump coals (9 to 19 mm particle size) was investigated in a 30 kW pilot scale bubbling fluidised bed combustor (PSBFBC) during normal combustion and crash stop combustion runs. This simulated conditions in the AB Sugar Industrial FBC system with a thermal rating of approximately 30 MW, which uses larger coal particle size of 12 to 25 mm.

Results reveal unwashed Blyth lump coal in the PSBFBC and industrial FBC causes some sintering and agglomerate formation of the bed material over short operation periods of 52 and 240 hours respectively, which was not observed in the washed Blyth coal system over a similar operating period. Observed sintering and agglomeration formation in unwashed Blyth coal is mainly attributed to accumulation of mud/shale stones in the bed, which would have been mostly removed by the washing process. The crash stop combustion run, done to simulate the fan trip scenario in the industrial FBC system, promoted sintering and agglomerate formation in the PSBFBC, possibly due to the 30 to 50°C temperature rise in the bed when fluidised air was stopped.

Continuous deposition and increasing concentration of mud/shale stones in the bed affects the localised temperature as well as the fluidising properties and quality, eventually promoting sintering and agglomerate formation. PSBFBC bed height, bed material particle size and measured pressure drop also increase with increasing operating time and mud/shale stones deposition in the bed.

Deposition of coal ash to the surface of the bed material (sand) in the PSBFBC was analysed by the use of SEM-EDX and XRF. The deposition of ash to the surface of PSBFBC bed material sand increases as the operation times increases, as identified by increasing concentration of Al, K and Ca on fluidised bed sand particle surfaces in their stable oxide forms of Al2O3, K2O and CaO respectively.

Item Type: Thesis (University of Nottingham only) (EngD)
Supervisors: Liu, H.
Eastwick, C.N.
Keywords: Sintering, Agglomeration,Lump Coal,Bubbling Fluidised Bed
Subjects: T Technology > TN Mining engineering. Metallurgy
Faculties/Schools: UK Campuses > Faculty of Engineering > Department of Chemical and Environmental Engineering
Item ID: 33534
Depositing User: Afilaka, Daniel
Date Deposited: 27 Jul 2016 14:55
Last Modified: 17 Sep 2016 15:20
URI: http://eprints.nottingham.ac.uk/id/eprint/33534

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