A critical review of ash slagging mechanisms and viscosity measurement for low-rank coal and bio-slags
Md Tanvir ALAM1, Baiqian DAI1, Xiaojiang WU2, Andrew HOADLEY1, Lian ZHANG1()
1. Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia 2. R&D Division, Shanghai Boiler Works Co. Ltd., Shanghai 200245, China
Gasification or combustion of coal and biomass is the most important form of power generation today. However, the use of coal/biomass at high temperatures has an inherent problem related to the ash generated. The formation of ash leads to a problematic phenomenon called slagging. Slagging is the accumulation of molten ash on the walls of the furnace, gasifier, or boiler and is detrimental as it reduces the heat transfer rate, and the combustion/gasification rate of unburnt carbon, causes mechanical failure, high-temperature corrosion and on occasions, superheater explosions. To improve the gasifier/combustor facility, it is very important to understand the key ash properties, slag characteristics, viscosity and critical viscosity temperature. This paper reviews the content, compositions, and melting characteristics of ashes in differently ranked coal and biomass, and discusses the formation mechanism, characteristics, and structure of slag. In particular, this paper focuses on low-rank coal and biomass that have been receiving increased attention recently. Besides, it reviews the available methodologies and formulae for slag viscosity measurement/prediction and summarizes the current limitations and potential applications. Moreover, it discusses the slagging behavior of different ranks of coal and biomass by examining the applicability of the current viscosity measurement methods to these fuels, and the viscosity prediction models and factors that affect the slag viscosity. This review shows that the existing viscosity models and slagging indices can only satisfactorily predict the viscosity and slagging propensity of high-rank coals but cannot predict the slagging propensity and slag viscosity of low-rank coal, and especially biomass ashes, even if they are limited to a particular composition only. Thus, there is a critical need for the development of an index, or a model or even a measurement method, which can predict/measure the slagging propensity and slag viscosity correctly for all low-rank coal and biomass ashes.
. [J]. Frontiers in Energy, 2021, 15(1): 46-67.
Md Tanvir ALAM, Baiqian DAI, Xiaojiang WU, Andrew HOADLEY, Lian ZHANG. A critical review of ash slagging mechanisms and viscosity measurement for low-rank coal and bio-slags. Front. Energy, 2021, 15(1): 46-67.
By measuring the torque of a rotating plate; By measuring the sample height and time for parallel plate By measuring the rate of penetration in indentation
102 –1011
Impractical for high temperature
Rotating crucible
By measuring torque on static bob
10–2 –101
The exceptionally exact vertical arrangement required
Rotating bob
By measuring torque on the bob
100–102
Using flexible joint alignment problems can be resolved
Falling body
By measuring time for bob to fall (or drag) through a known distance
100.5 –105
Need a broad zone of uniform temperature
Oscillating
By measuring log decrement of the amplitude of twisting
10–4 –10–1
Applicable for depolymerised slag with low viscosity
IP
By measuring slag travel length
1.5–6
Inclination range is restricted between 9°–23°
M-IP
By measuring slag travel length
1–17.9
Restricted to an upper temperature of 1400°C
Tab.2
Models
Applicability
Correlation
Remarks
Reference
Urbain
Various
Authentic for specific compositions and temperature category
[111]
Modified Urbain
Coal
Accurate for ash within the four component system (CaO-Al2O3-SiO2-FeO)
[112]
Riboud
Mould powder
Unable to differentiate between different cations; accurate for SiO2-CaO-Al2O3-CaF-Na2O system
[113]
NPL
Industrial slags and mould fluxes
Optical basicity data requires for accurate prediction
[114]
Iida
Mould fluxes and metallurgical slags
For accurate prediction basicity index value required; calibration of experimental data is necessary
[116]
KTH
Metallurgical slags
Coefficients are not available
[117]
Mills
Coal, mould powder, non-ferrous slag and blast furnace slag
Viscosity range is limited between 1.5–6 Pa?s; inclination range is restricted between 9–23
[109]
Dai
Coal
Restricted to an upper temperature of 1400°C and a viscosity limit of 17.9 Pa?s
[108]
Tab.3
Fig.6
Fig.7
Fig.8
Fig.9
Fig.10
Fig.11
Fig.12
Tab.4
Fig.13
Fig.14
Fig.15
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