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Title: A Brief Review of Viscosity Models for Slag in Coal Gasification

Abstract

Many researchers have defined the phenomenon of 'slagging' as the deposition of ash in the radiative section of a boiler, while 'fouling' refers to the deposition of ash in the convective-pass region. Among the important parameters affecting ash deposition that need to be studied are ash chemistry, its transport, deposit growth, and strength development; removability of the ash deposit; heat transfer mechanisms; and the mode of operation for boilers. The heat transfer at the walls of a combustor depends on many parameters including ash deposition. This depends on the processes or parameters controlling the impact efficiency and the sticking efficiency. For a slagging combustor or furnace, however, the temperatures are so high that much of the coal particles are melted and the molten layer, in turn, captures more particles as it flows. The main problems with ash deposition are reduced heat transfer in the boiler and corrosion of the tubes. Common ways of dealing with these issues are soot blowing and wall blowing on a routine basis; however, unexpected or uncontrolled depositions can also complicate the situation, and there are always locations inaccessible to the use of such techniques. Studies have indicated that slag viscosity must be within a certainmore » range of temperatures for tapping and the membrane wall to be accessible, for example, between 1300 C and 1500 C, the viscosity is approximately 25 Pa {center_dot} s. As the operating temperature decreases, the slag cools and solid crystals begin to form. In such cases the slag should be regarded as a non-Newtonian suspension, consisting of liquid silicate and crystals. A better understanding of the rheological properties of the slag, such as yield stress and shear-thinning, are critical in determining the optimum operating conditions. To develop an accurate heat transfer model in any type of coal combustion or gasification process, the heat transfer and to some extent the rheological properties of ash and slag, especially in high-temperature environments need to be understood and properly modeled. The viscosity of slag and the thermal conductivity of ash deposits are among two of the most important constitutive parameters that need to be studied. The accurate formulation or representations of the (transport) properties of coal (and biomass for co-firing cases) present a special challenge of modeling efforts in computational fluid dynamics applications. In this report, we first provide a brief review of the various approaches taken by different researchers in formulating or obtaining a slag viscosity model. In general, these models are based on experiments. Since slag behaves as a non-linear fluid, we discuss the constitutive modeling of slag and the important parameters that must be studied.« less

Authors:
;
Publication Date:
Research Org.:
National Energy Technology Lab. (NETL), Pittsburgh, PA, and Morgantown, WV (United States). In-house Research
Sponsoring Org.:
USDOE Office of Fossil Energy (FE)
OSTI Identifier:
1036735
Report Number(s):
NETL-PUB-232
TRN: US201207%%11
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
01 COAL, LIGNITE, AND PEAT; BIOMASS; BOILERS; CHEMISTRY; COAL; COAL GASIFICATION; COMBUSTION; COMBUSTORS; COMPUTERIZED SIMULATION; CORROSION; DEPOSITION; FLUID MECHANICS; FOULING; GASIFICATION; HEAT TRANSFER; MEMBRANES; SILICATES; SLAGS; SOOT; THERMAL CONDUCTIVITY; VISCOSITY

Citation Formats

Massoudi, Mehrdad, and Wang, Ping. A Brief Review of Viscosity Models for Slag in Coal Gasification. United States: N. p., 2011. Web. doi:10.2172/1036735.
Massoudi, Mehrdad, & Wang, Ping. A Brief Review of Viscosity Models for Slag in Coal Gasification. United States. doi:10.2172/1036735.
Massoudi, Mehrdad, and Wang, Ping. Tue . "A Brief Review of Viscosity Models for Slag in Coal Gasification". United States. doi:10.2172/1036735. https://www.osti.gov/servlets/purl/1036735.
@article{osti_1036735,
title = {A Brief Review of Viscosity Models for Slag in Coal Gasification},
author = {Massoudi, Mehrdad and Wang, Ping},
abstractNote = {Many researchers have defined the phenomenon of 'slagging' as the deposition of ash in the radiative section of a boiler, while 'fouling' refers to the deposition of ash in the convective-pass region. Among the important parameters affecting ash deposition that need to be studied are ash chemistry, its transport, deposit growth, and strength development; removability of the ash deposit; heat transfer mechanisms; and the mode of operation for boilers. The heat transfer at the walls of a combustor depends on many parameters including ash deposition. This depends on the processes or parameters controlling the impact efficiency and the sticking efficiency. For a slagging combustor or furnace, however, the temperatures are so high that much of the coal particles are melted and the molten layer, in turn, captures more particles as it flows. The main problems with ash deposition are reduced heat transfer in the boiler and corrosion of the tubes. Common ways of dealing with these issues are soot blowing and wall blowing on a routine basis; however, unexpected or uncontrolled depositions can also complicate the situation, and there are always locations inaccessible to the use of such techniques. Studies have indicated that slag viscosity must be within a certain range of temperatures for tapping and the membrane wall to be accessible, for example, between 1300 C and 1500 C, the viscosity is approximately 25 Pa {center_dot} s. As the operating temperature decreases, the slag cools and solid crystals begin to form. In such cases the slag should be regarded as a non-Newtonian suspension, consisting of liquid silicate and crystals. A better understanding of the rheological properties of the slag, such as yield stress and shear-thinning, are critical in determining the optimum operating conditions. To develop an accurate heat transfer model in any type of coal combustion or gasification process, the heat transfer and to some extent the rheological properties of ash and slag, especially in high-temperature environments need to be understood and properly modeled. The viscosity of slag and the thermal conductivity of ash deposits are among two of the most important constitutive parameters that need to be studied. The accurate formulation or representations of the (transport) properties of coal (and biomass for co-firing cases) present a special challenge of modeling efforts in computational fluid dynamics applications. In this report, we first provide a brief review of the various approaches taken by different researchers in formulating or obtaining a slag viscosity model. In general, these models are based on experiments. Since slag behaves as a non-linear fluid, we discuss the constitutive modeling of slag and the important parameters that must be studied.},
doi = {10.2172/1036735},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Tue Nov 01 00:00:00 EDT 2011},
month = {Tue Nov 01 00:00:00 EDT 2011}
}

Technical Report:

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  • A rotating cylinder apparatus has been designed and constructed for measuring the viscosity of molten slags under steam pressure in the range 15 to 300 psi and up to about 1400/sup 0/C. Two synthetic coal slags, one high in alkali and the other lower in alkali but higher in calcia were studied with this apparatus. In the high alkali slag equilibrium was probably not achieved and the viscosity appeared to increase with time. In the low alkali slag although the viscosity appeared to be increased with increasing steam pressure the ambient and higher pressure curves appear to tend to coalescemore » with increasing temperature.« less
  • The primary objective of this study was to characterize leachate quality for representative coal gasification solid residues in an environment that represented one of the disposal practices that TVA uses. An additional objective was to compare the leachate quality for solid residues produced by gasification of different types of coal. The approach taken was to investigate the chemical characteristics of waters in contact with coal gasification slags disposed in small surface impoundments (field test cells). Slags from gasification of two coals were placed in lined field test cells. One feed coal was a low sulfur (0.4%) western bituminous coal producedmore » by Southern Utah Fuel Company (SUFCO). The other feed was Illinois No. 6 coal, a medium sulfur (2.8%), midwestern bituminous coal. 9 refs., 17 figs., 19 tabs.« less
  • Bench scale evaluation tests were made on candidate refractory materials considered for use within Combustion Engineering's atmospheric pressure, entrained bed, 5-ton-per-hour coal gasification Process Development Unit (PDU). Bench-scale dynamic slag tests, found to be comparative within a given test, produced relative wastage rates of 1 x 10/sup -3/ in./sup 3//in./sup 2/ hr for the best alumina--chromia refractories to 225 in./sup 3//in./sup 2/ hr for a 70 percent alumina firebrick. Alumina--chromia refractories with all types of bond schemes did well; fusion cast refractories gave best results. Basic refractories (magnesio--chromite) showed good corrosion resistance, however, uncombined magnesia was dissolved by the coalmore » ash slag. Tests showed that the simulated product gas (without the presence of slag), did not degrade the physical integrity of phosphate-bonded, high-alumina refractories after exposure at 1700 to 2200/sup 0/F for 100 hr nor at 2600/sup 0/F for 50 hr despite phosphorus loss. Tests with dry (dew point approximately -60/sup 0/F) 8 percent hydrogen-92 percent argon showed phosphorus was lost by reduction of P/sub 2/O/sub 5/ and volatilization of elemental phosphorus at the test temperatures with subsequent deposition of the elemental phosphorus in cool components down stream. Similar tests with wet (dew point approximately +85/sup 0/F) 8 percent hydrogen-92 percent argon also caused a phosphorus loss with condensation of phosphoric acid upon cooling. Recommendations were made that the findings of this study be correlated with results obtained from a post operative examination of the refractories exposed to the actual environment established in the PDU. This correlation would aid in establishing future laboratory procedures for selecting refractories.« less
  • The research activities performed in this quarter (reporting period: 10/01/97- 12/31/97) are summarized as follows: The activities concentrated on: . Partial analysis of the multi pulse test results . Data analysis and comparison with that of the single pulse test data