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Research Papers: Gas Turbines: Coal, Biomass, and Alternative Fuels

Deposition of Corrosive Alkali Salt Vapors on the Blades of Gas Turbines Fueled by Coal-Derived Syngases

[+] Author and Article Information
John B. Young

Hopkinson Laboratory,
Cambridge University Engineering Department,
Cambridge CB2 1PZ, UK
e-mail: jby@eng.cam.ac.uk

Richard J. Tabberer

E.ON UK plc,
Westwood Way,
Coventry CV4 8LG, UK

John E. Fackrell

Technology Centre,
E.ON New Build & Technology Ltd.,
Ratcliffe-on-Soar,
Nottinghamshire NG11 0EE, UK

1Corresponding author.

2Retired.

Contributed by the Coal, Biomass and Alternate Fuels Committee of ASME for publication in the Journal of Engineering for Gas Turbines and Power. Manuscript received March 25, 2013; final manuscript received June 12, 2013; published online August 21, 2013. Assoc. Editor: Paolo Chiesa.

J. Eng. Gas Turbines Power 135(9), 091401 (Aug 21, 2013) (13 pages) Paper No: GTP-13-1088; doi: 10.1115/1.4024948 History: Received March 25, 2013; Revised June 12, 2013

Many proposed clean coal technologies for power generation couple a gasification process with a gas turbine combined cycle unit. In the gasifier, the coal is converted into a syngas which is then cleaned and fired before entering the turbine. A problem is that coal-derived syngases may contain alkali metal impurities that combine with the sulfur and chlorine from the coal to form salts that deposit on the turbine blades, causing corrosion. This paper describes a new model, applicable to most types of coal, for predicting the dewpoint temperatures and deposition rates of these sodium and potassium salts. When chlorine is present the main alkali species in the mainstream gas flow are the chlorides; but when chlorine is absent, the superoxides dominate. However, because the high-pressure turbine blades are film-cooled, they are at much lower temperatures than the mainstream gas flow and analysis then shows that the deposit is composed almost entirely of the sulfates in either liquid or solid form. This is true whether or not chlorine is present. Detailed calculations using the new model to predict the alkali salt deposition rates on three stages of an example utility turbine are presented. The calculations show how the dewpoint temperatures and deposition rates vary with the gas-phase chlorine and sulfur levels as well as with the concentrations of sodium and potassium. It is shown that the locations where corrosion is to be expected vary considerably with the type of coal and the levels of impurities present.

Copyright © 2013 by ASME
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References

Minchener, A. J., 2005, “Coal Gasification for Advanced Power Generation,” Fuel, 84, pp. 2222–2235. [CrossRef]
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Fackrell, J. E., Tabberer, R. J., Young, J. B., and Fantom, I. R., 1994, “Modelling Alkali Salt Vapour Deposition in the British Coal Topping Cycle System,” Proceedings of the ASME Gas Turbine and Aeroengine Congress, The Hague, The Netherlands, June 13–16, Paper No. 94-GT-177.
Clark, R. K., Arnold, M., Fackrell, J. E., Mordecai, M., and Dawes, S. G., 1991, “The Grimethorpe Topping Cycle Project,” FBC Technology and the Environmental Challenge, Adam Hilger, Bristol, UK, pp. 353–362.
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Dessureault, Y., Sangster, J., and Pelton, A. D., 1990, “Coupled Phase Diagram and Thermodynamic Analysis of the Nine Common-Ion Binary Systems Involving the Carbonates and Sulphates of Lithium, Sodium and Potassium,” J. Electrochem. Soc., 137, pp. 2941–2950. [CrossRef]
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Figures

Grahic Jump Location
Fig. 2

Variations with reciprocal temperature of the gas-phase equilibrium constants for the chemical equilibria of Eqs. (1)–(3), calculated from Eq. (A2)

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Fig. 3

Partitioning of the alkali species in the mainstream flow of a hypothetical turbine expansion from 1800 K and 15 bar with polytropic efficiency 0.9. Full equilibrium calculations based on XO2 = XH2O = 0.1, XSO2 = 10-4, XNa = XK = 4×10-8; (a) chlorine free coal with XHCl = 0, (b) high chlorine coal with XHCl = 10−4.

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Fig. 5

Variation with mole fraction of the activity coefficients for mixtures of Na2SO4 and K2SO4 calculated from Eqs. (D2) at three temperatures; (a) liquid solution, (b) solid solution

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Fig. 4

Saturated vapor pressures for (a) Na2SO4 and NaCl, and (b) K2SO4 and KCl, calculated from Eq. (C2). The SVP over liquid has been extended below the melting temperature into the supercooled region.

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Fig. 1

Layout diagram of the proposed MHI 250 MW coal gasification combined cycle plant

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Fig. 10

High chlorine coal. Deposition rates on the first stage stator of the example turbine: (a) original model without superoxides, (b) new model with superoxides.

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Fig. 9

Flow calculations for the first stage stator of the example turbine: (a) velocity distribution outside the boundary layer, (b) surface temperature distribution

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Fig. 8

High chlorine coal. Deposition rates at the leading edges of the first three stages of the example turbine as a function of blade surface temperature: (a) original model without superoxides, (b) new model with superoxides.

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Fig. 11

Low chlorine coal. Deposition rates on the leading edges of the first three stages of the example turbine as a function of blade surface temperature: new model with superoxides.

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Fig. 12

Low chlorine coal. Deposition rates on the first stage stator of the example turbine: new model with superoxides.

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Fig. 13

First stage stator leading edge deposition rates for varying chlorine levels

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Fig. 14

First stage stator leading edge deposition rates for varying sulfur levels

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Fig. 15

First stage stator leading edge deposition rates for varying alkali salt levels: (a) 0 ppmv HCl, (b) 300 ppmv HCl

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Fig. 6

Computed equilibrium phase diagram for a mixture of Na2SO4 and K2SO4 together with a compilation of experimental data points from Dessureault et al. [9]

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Fig. 7

Variation with temperature of the mole fraction of MCl in a liquid or solid mixture of M2SO4 and MCl. Calculations based on XO2 = XH2O = 0.1, XSO2 = XHCl = 10-4, γMSu = γMCl = 1, and p = 10 bar.

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