A critical review of ash slagging mechanisms and viscosity measurement for low-rank coal and bio-slags

doi:10.1007/s11708-020-0807-8

Front. Energy

2021, Vol. 15

Issue (1) : 46-67 https://doi.org/10.1007/s11708-020-0807-8

REVIEW ARTICLE

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

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Abstract

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.

Keywords slag viscosity biomass low-rank coal combustion gasification

Corresponding Author(s): Lian ZHANG

Online First Date: 20 April 2020 Issue Date: 19 March 2021

Cite this article:

Md Tanvir ALAM,Baiqian DAI,Xiaojiang WU, et al. A critical review of ash slagging mechanisms and viscosity measurement for low-rank coal and bio-slags[J]. Front. Energy, 2021, 15(1): 46-67.

URL:

https://academic.hep.com.cn/fie/EN/10.1007/s11708-020-0807-8
https://academic.hep.com.cn/fie/EN/Y2021/V15/I1/46

Fig.1 Relationship between ash and volatile content for different rank coal and biomass (adapted from Ref. [⁴⁶] with modification. Note that d denote dry basis, and VM^daf denote volatile matter content on the dry and ash free basis).

Tab.1 Average elemental compositions of different biomass and coal ashes based on high-temperature ash analyses

Fig.2 Mean contents of the principal oxides found in biomass ash in a downward trend (Note that WWB stands for wood and woody biomass; HAG for herbaceous and agricultural grass; HAS for herbaceous and agricultural straw; HAR for herbaceous and agricultural residue; AB for animal biomass, and CB for contaminated biomass.)

Fig.3 Formation mechanism of slagging and agglomeration during biomass combustion (adapted from Ref. [²¹] with permission).

Fig.4 Slag formation and deposition inside a gasifier (adapted from Ref. [⁹²] with permission.)

Fig.5 Schematic drawing (adapted from Ref. [¹⁰⁴]. Note that here pink sphere denotes non-bridging oxygen.) (a) SiO₂ structure showing 3-D characteristics; (b) silicate chain with bridging, non-bridging and free oxygen; (c) Al³⁺ incorporation in silicate chain.

Tab.2 Available viscosity measuring methods for slags, fluxes and glasses

Models	Applicability	Correlation	Remarks	Reference
Urbain	Various	$η = A A e E A R T$	Authentic for specific compositions and temperature category	[¹¹¹]
Modified Urbain	Coal	$η = A T e 1000 B T$	Accurate for ash within the four component system (CaO-Al₂O₃-SiO₂-FeO)	[¹¹²]
Riboud	Mould powder	$η = A T e B T$	Unable to differentiate between different cations; accurate for SiO₂-CaO-Al₂O₃-CaF-Na₂O system	[¹¹³]
NPL	Industrial slags and mould fluxes	$η = A e B T$	Optical basicity data requires for accurate prediction	[¹¹⁴]
Iida	Mould fluxes and metallurgical slags	$η = A n 0 e E B i$	For accurate prediction basicity index value required; calibration of experimental data is necessary	[¹¹⁶]
KTH	Metallurgical slags	$η = h N A ρ M A A e G R T$	Coefficients are not available	[¹¹⁷]
Mills	Coal, mould powder, non-ferrous slag and blast furnace slag	$log 10 ? η = log 10 ? A + B T$	Viscosity range is limited between 1.5–6 Pa?s; inclination range is restricted between 9–23	[¹⁰⁹]
Dai	Coal	$ln (η) = 1 n ln (cos β) − ln (v z) + ln {(ρ g) 1 n δ 1 n + 1 (1 n + 2)}$	Restricted to an upper temperature of 1400°C and a viscosity limit of 17.9 Pa?s	[¹⁰⁸]

Tab.3 Available slag viscosity prediction models

Fig.6 Comparison of viscosity models for Australian bituminous coal slag (adapted from Ref. [¹²²] with permission. Note that slag 9 has a composition of SiO₂ 48.1 wt%, Al₂O₃ 25.3 wt%, CaO 25.4 wt%, and FeO 1.2 wt%. Slag 38 consists of SiO₂ 50.1 wt%, Al₂O₃ 30.6 wt%, CaO 15.5 wt%, and FeO 3.8 wt%. Slag 59 has a composition of SiO₂ 53.9 wt%, Al₂O₃ 29.4 wt%, CaO 11.3 wt%, and FeO 5.5 wt%. Slag 75 is made up of SiO₂ 53.8 wt%, Al₂O₃ 25.6 wt%, CaO 11.5 wt%, and FeO 9.1 wt%. Model 1 denotes Urbain model; Model 2 denotes synthetic slag SAC (SiO₂-Al₂O₃-CaO) model; Model 3 denotes coal ash slag model for<2.5 wt% FeO; Model 4 denotes synthetic slag SACF (SiO₂-Al₂O₃-CaO-FeO) model for 5 wt% FeO; Model 5 denotes coal ash slag SACF model for 2.5 wt%–5 wt% FeO; Model 6 denotes coal ash slag SACF model for 5 wt%–7.5 wt% FeO; Model 7 denotes synthetic slag SACF model for 10 wt% FeO; Model 8 denotes coal ash slag SACF model for 7.5 wt%–10 wt% FeO.)

Fig.7 Slag viscosity measured at distinct cooling rates (adapted from Ref. [¹²⁰] with permission.)

Fig.8 For ZZ and DT anthracite coal T_cv at the different cooling rate (adapted from Ref. [¹²⁰] with permission. Note that ZZ stands for Zhaozhuang and DT for Datong.)

Fig.9 For standard coal ash sample (adapted from Ref. [¹⁰⁸] with permission.)

Fig.10 Ash slag pictures of Xinjiang Uygur Autonomous Region at 1300°C–1400°C at the exposure time of 40 min for A–C and 2 h for D and E, for the inclined angle of 25° (adapted from Ref. [¹³⁰] with permission).

Fig.11 Xinjiang lignite ash viscosity of Xinjiang Uygur Autonomous Region obtained at 1400°C from different models (adapted from Ref. [¹³⁰] with permission. Note that M-IP denotes modified incline plane, Rib denotes Riboud model, Urb denotes Urbain model, Mill denotes Mill model, Iida denotes Iida model, For denotes Forsbacka model, M-Urb denotes modified Urbain model, OB denotes optical basicity model, Fact denotes Factsage, and Wu denotes Wu model).

Fig.12 Viscosity as a function of the volume fraction of crystals (adapted from Ref. [¹³⁶] with permission).

Tab.4 Slagging propensity of different biomass in the different slagging indexes

Fig.13 Correlations among different slagging indices (adapted from Ref. [³] with permission).

Fig.14 Effect of SiO₂ concentrations on the viscosities (adapted from Ref. [¹⁴⁷] with permission).

Fig.15 Comparisons between the measured and predicted viscosities from different slag models (adapted from Ref. [¹⁴⁷] with permission. Note that experimental readings are shown as Present).

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