Erosion-corrosion modelling of gas turbine materials for coal-fired combined cycle power generation

WEAR Wear186187 (1995)247-255

Erosion-corrosion

modelling of gas turbine materials for coal-fired combined cycle power generation

N.J. Simms a, J.E. Oakey a, D.J. Stephenson b, P.J. Smith b, J.R. Nicholls b ’British Coal Corporation, Coal Technology Development Division, Stoke Orchard, Cheltenham, Glos. GL52 4ZG, UK b Cranfeld Universiry, School of Industrial and Manufacturing Studies, Cranjield, Bedford MK43 OAL, UK

Abstract The development of coal-fired combined cycle power generation systems is receiving considerable worldwide interest. The successful development and commercialisation of these new systems require that all the component parts are manufactured from appropriate materials and that these materials give predictable in-service performance. Corrosion and erosion-corrosion, resulting from coal derived particulates, deposition and gaseous species, have been identified as potential life limiting factors for these systems. Models to predict these modes of materials degradation are under active development. This paper outlines the development and testing of models suitable for use in gas turbine environments. The complexity of the corrosion processes means that an empirical approach to model development is required whereas a more mechanistic approach can be applied to erosion processes. For hot corrosion conditions, statistically based corrosion models have been produced using laboratory tests for two coatings and a base alloy at typical type I and type II hot corrosion temperatures (900 and 700 “C) . These models use the parameters of alkali sulphate deposition flux and SO, partial pressure (at each temperature and for set HCl partial pressures), to predict the rate of the most likely localised damage associated with hot corrosion reactions. For erosion-corrosion modelling, a series of laboratory tests have been carried out to investigate erosion behaviour in corrosive conditions appropriate to coal-fired gas turbines. Materials performance data have been obtained from samples located in the hot gas path of the Grimethorpe PFBC pilot plant, under well characterised conditions, for testing the corrosion and erosion-corrosion models. The models successfully predict the materials damage observed in the pilot plant environments. Keywords:

Erosion; Corrosion; Gas turbines; Coal-firedpower generation

1. Introduction

The development of coal-fired combined cycle power generation systems is receiving considerable worldwide interest [ l-31. These systems, utilising both steam and gas turbines, offer many advantages over conventional coal-fired power generation systems, which include increased efficiency of electricity production and lower environmental emissions (specifically CO,, SO, and NO,). The choice of a particular combined cycle power system also depends on many economic factors, e.g. the capital costs of any plant, maintenance and operating costs [ 2,3]. The influence of materials on the development of these systems can be considerable as it is necessary that components have adequate lifetimes in their required operational environments. Thus, materials limitations can put constraints on the range of operating conditions for some system components. The successful development 0043-l&-%8/95/$09.50 0 1995Elsevier Science S.A. All rights reserved SSD10043-1648(95)07167-9

and commercialisation of these new systems require that all the component parts are manufactured from appropriate materials and that these materials give predictable in-service performance. The performance of the gas turbine is critical to the efficiency and availability of all these coal-fired combined cycle power systems. The gas turbines which are currently being used, and are proposed for use in the future, in such coalfired systems are those that have been developed for use with natural gas or oil firing. However, the use of coal derived gases in such turbines presents a number of different potential materials problems as a result of the different hot gas environments produced. The presence of particles may cause erosion or deposition, and gaseous species (e.g. SO,, alkali compounds, HCl) may cause deposition and/or enhance corrosion [ 4-81. Synergistic effects between these various degradation processes are also likely under gas turbine operating

N.J. Simms et al. /Wear IR6-187 (1995) 247-255

248

conditions. These various potential degradation modes may be life limiting for gas turbine hot gas path components (e.g. combustion chamber, vanes and blades), rather than creep and fatigue processes. Thus, hot corrosion and erosion-corrosion models to predict the lives of candidate gas turbine hot gas path materials in realistic environments for a gas turbine operating on coal-derived gases are necessary to assess the potential lives of such components, and any changes to these environments which would significantly extend (or shorten) these lives. This paper describes an approach to the development of models to predict hot corrosion and erosion-corrosion in environments appropriate for gas turbines operating on coalderived gases. This work was carried out as part of the Grimethorpe Topping Cycle Project [ 9,101 (a partnership between British Coal, GEC-Alsthom, PowerGen and the UK Department of Trade and Industry) and as part of an EC JOULE programme [ 111.

terns, the gas turbine entry temperature is limited to = 800900 “C by the gas exit temperature from the fluidised bed. Up to = 90% of the sulphur present in the coal may be removed by the addition of limestone, or dolomite, to the fluidised bed.

2. Background

Partial gasiJication.khar combustion systems [2,7,9,10/ In these combined cycle systems, air blown entrained or fluidised bed coal gasifiers are operated to achieve only partial conversion of the coal to fuel gas (e.g. 75% conversion). This fuel gas is then passed through a hot dry gas cleaning process before being burnt and used to drive a gas turbine. Steam is produced in heat exchangers located upstream of the gas cleaning process and downstream of the gas turbine. The unburnt coal/char is used to fire a PFBC or CFBC, with the heat produced being used to generate more steam: if a pressurised combustor is used, then the gases produced can be used in the gas turbine after cleaning in a hot gas filter unit. In such combined cycle systems, it is possible to utilise advanced industrial gas turbines with TET of 1260 “C. Such systems, variously called topping cycles, air blown gasification cycles, second generation PFBCs, etc, have been proposed by, for example, British Coal, Foster Wheeler and ABB (e.g. Fig. 3).

2.1. Gas turbine environments

and materials

There are a number of different coal-fired combined cycle power systems which are currently under development. Detailed reviews of these various systems are beyond the [lscope of this paper, but are available elsewhere 3,9,10,12]. The main types of proposed systems areas follows. 2.1.1. Pressurisedjkidised

bed combustion

(PFBC) systems

[l-3,5,71 In PFBC systems, coal is combusted in a bubbling pressurised fluidised bed to produce hot gases which are passed through a particle removal system (a series of cyclones or a hot gas filter [ 2,7,13] ) to drive a gas turbine. A steam turbine is driven by steam produced in heat exchangers located in the fluidised bed and downstream of the gas turbine. A schematic diagram of a PFBC systems is given in Fig. 1. In these sys-

‘Tf=Fig.

jw

1.Pressurised fluidised bed combustion power system

2.1.2. Integrated

gas$cation

combined

cycles (IGCC) [I-

3,7,121 These are based on coal gasification processes in which coal is gasified under pressure (e.g. oxygen blown entrained, static or fluidised bed gasifiers) and the gases produced are cooled and cleaned before being burnt and passing through a gas turbine. A steam turbine is driven by steam produced in a heat exchanger located downstream of the gas turbine and also, in some systems, by a heat exchanger used to cool the gasifier product gases. A schematic diagram of such a generalised IGCC system is given in Fig. 2. IGCC systems can use advanced industrial gas turbines with turbine entry temperatures (TET) of 1260 “C (gas inlet temperatures of 1380°C).

The compositions of the gases entering the gas turbine in these systems can differ considerably depending on the

Fig. 2. Generalised

integrated gasification

combined cycle power system.

N.J. Simms et al. /Wear

186-187 (1995) 247-255

249

monly used industrial gas turbine alloy, IN738LC, and the coatings RT22 (a platinum aluminide coating) and ATD2 (a plasma sprayed CoCrAlY coating). (Other materials were included in the test programmes where appropriate, though the performances of these materials are not reported in this paper).

2.2. Approach to corrosion modelling

Fig. 3. Partial gasification/combustionpower system. source of the gases (e.g. combustor or gasifier and type of gasifier) and gas cleaning processes employed (e.g. cyclone or barrier filtration using ceramic candles in PFBC; tradition-

ally wet gas cleaning or more recently barrier filtration and possibly hot gas cleaning in gasification systems). However, for the purposes of developing an erosion-corrosion model for gas turbine materials, an envelope of possible operating environments was defined after assessing a range of different possible cycles. Ranges of exposure conditions are given in Table 1. However, this method of presentation is an over simplification of the conditions which can be experienced in practice, in which the magnitude of many variables are linked, for example partial pressures of SO, and HCl are linked by the total local gas pressure, and depositions rates are dependent on the surface metal temperatures. Attention to such linkages considerably reduces the range of the potential exposure conditions, and these were used in planning the test programme carried out. Further limits are found when considering particular power systems: for example, the gas turbine on a simple PFBC system only has an entry temperature of up to = 850 “C, and so uncooled turbine blades can be used throughout the turbine; whereas the other systems can use gas turbines with gas entry temperatures of up to = 1380 “C, where extensive aerofoil cooling is necessary. However, for this study the scope of the models were not limited by such considerations, so that generalised models could be produced. It should be noted that to use the models of materials performance, process data are needed to allow the exposure conditions to be fully described. All the required data may be obtained from well instrumented pilot plants, but are unlikely to be completely available for utility scale plants. However, knowledge of the sensitivity of materials damage to changes in the different exposure parameters from the models will help to minimise the effect of this possible practical limitation, especially when combined with a thorough knowledge of the interactions of the process parameters. This paper reports the performance of three materials which were included throughout the test programme: a com-

The approach to corrosion modelling adopted in this study was to identify the exposure parameters of most interest to coal-fired gas turbine (sodium and potassium sulphate deposits mixed with coal ashes on the surfaces of metals in SO, and HCl containing atmospheres-Table 1) and then to carry out a series of laboratory tests to systematically investigate the effect of these exposure parameters on candidate materials. To assess the extent of corrosion damage, a statistical approach was adopted. Traditionally high temperature corrosion damage measurements have produced estimates of the mean corrosion damage [ 141. Thus, corrosion life assessments have often underestimated life limiting corrosive damage (the deepest areas of corrosive damage, which allow localised overload or fatigue initiation). The corrosion life prediction models derived in this work are based on a statistical approach which uses the extreme corrosion data, e.g. the pit depths for type II hot corrosion. This approach to assessing high temperature materials damage has been investigated at Cranfield University for several years e.g. [ 14-201. To satisfy the requirements of this statistical approach, the corrosion damage data set must consist of sufficient randomly distributed data points [ 211. Thus, during this study a very large data set has been gathered of material performances under well characterised exposure conditions. For the purposes of the corrosion modelling these data sets have been evaluated using an extreme value statistical approach: this method involves ordering the sets of materials damage data and then using the Gumbel type I model of maxima to analyse the extreme data points [ 141. The characteristic extreme value parameters obtained are used to generate the overall corrosion Table 1 Range of steady state exposureconditions in gas turbine hot gas path Parameter

Range

Gas temperature (“C) Gas pressure (bar) Metal temperature (“C) Gas conditions

550-1380 l-14 550-900 Oxidising p(S0,) <3000 p,bar p(HCI) < 3000 p,bar Alkali sulphate < 1000 Ash < 1000
Deposition rates (p,g cm-* h-‘) Particle sizes Particle loading (mg m- 3, Particle velocity (m s- ’) Impact angles (deg)

2.50

N.J. Simms et al. /Wear 186187 (1995) 247-255

models by curve fitting with respect to exposure parameters. The data sets gathered also allow average (median) values for corrosion damage to be found, which can be compared with these extreme values. 2.3. Approach to erosion-corrosion modelling The approach to erosion-corrosion modelling in this study has been based on an assessment of possible scenarios of erosion damage: particles impacting on a surface causing, for example, the removal of all or part of the corrosion scale (and/or deposit), or removal of scale and underlying metal, etc. These scenarios are covered well by a computer model of erosion-corrosion processes which has been under development for many years at Cranfield University e.g. [ 22-251. In this model, the continuum of possible scenarios is divided into three distinct erosion regimes: the ratio between the thickness z of the scale and the radius a of contact on the scale-metal surface, has been identified as the critical parameter dictating the transition between the three erosion regimes. With increasing z/a ratio, these are the “substrate dominated”, the “scale modified” and the “scale dominated” regimes. Appropriate mathematical procedures to describe and simulate the mechanisms characterising each regime have been produced. These procedures encompass specific decision criteria and correlations in order to give a damage morphology and damage volume for each regime. The model uses Monte Carlo methods to simulate the erosion processes of materials at high temperature (i.e. which are simultaneously corroding). This model predicts the erosion behaviour of a typical composite system consisting of a brittle surface scale on a more ductile substrate. Thus, the required input parameters for the model include the properties of particle, alloy substrate and scale, as well as environmental variables. The model adopts a log-normal distribution of erodent particle sizes, from which the selection of the particle size and the time increment between each impact are determined on a random basis, using a set particle loading. The knowledge of the particle size allows the particle velocity to be calculated. The model then selects the most appropriate of the three distinct erosion regimes. When the total exposure time is reached, totaI mass loss due to scale and substrate removal is then calculated leading to the determination of dimensionless erosion rates and material loss values.

3. General experimental methodology In order to produce the materials performance data required for the generation and testing of the corrosion and erosioncorrosion models, a common approach was adopted for all the laboratory and pilot plant test work. This involved precision contact metrology of all samples before exposure (before and after coating if necessary), exposure to well characterised conditions (with intermediate shutdowns being

used to give weight change data if possible), post-exposure surface examinations and post-exposure metrology on polished cross-sections through samples. All the metrology work has involved the use of reference samples and calibrations wherever necessary to ensure that all systematic errors are eliminated. Post exposure surface examinations have been carried out using X-ray diffraction, as well as scanning electron microscopy coupled with energy-dispersive X-ray analysis, as appropriate to identify corrosion products/deposits and their morphologies. The metrology procedures took into account the expected morphologies and magnitudes of damage under the test conditions, as well as the requirement to generate statistically valid data [ 261. In the case of the corrosion models, the laboratory generated materials performance data were used to produce the models, whilst the pilot plant data were used to test these models. Data from both sources were subject to the same statistical analysis methods. For the more developed mechanistic erosion-corrosion models, the laboratory data were used to extend and test the models, whilst the pilot plant data were used to test the models.

4. Experimental programmes, results and discussion 4.1. Hot corrosion tests and model development

Mathematical models of hot corrosion were derived from basic corrosion data gathered in a series of 500 h laboratory furnace tests, which used realistic simulated environments. In light of previous experience, the major variables were the flux of alkali sulphate deposit, partial pressure of SO, and temperature (at a fixed HCl partial pressure). The flux of deposit was achieved by recoating the corroding test samples after every 20 h exposure; the gas compositions were controlled by using pre-blended gas mixtures in controlled atmosphere autoclaves. These tests are described in more detail in [ 271. The time dependences of the corrosion kinetics were examined for all the exposure conditions tested using the most extreme data points and also the median data points. In most cases the data showed a linear dependence of corrosion rate on time. In some cases (high alkali deposition rates, high SO,), however, no clear distinction between linear and parabolic kinetics could be drawn. The literature suggests that parabolic kinetics are often observed in more traditional laboratory hot corrosion studies. However, under more realistic burner rig conditions the corrosion rates observed are usually linearly dependent on the rate of arrival of contaminants (contaminant flux rate controlled). The present study supports the flux rate control observed under burner rig conditions. For all materials at 700 “C, the dependence of corrosion rate on alkali sulphate deposition flux was found to be sigmoidal and with three distinct regimes of behaviour (Fig. 4).

N.J. Simms etal. /Wear 186-187 (1995) 247-255

1.1

0.3

1

3

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/

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10

30

loo

300

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Deposition Flux tpg/cmUhour) Fig. 4. Example of variation of corrosion damage of IN738LC with alkali sulphate flux.

The corrosion rates were found to be strongly material dependent, as expected. In the low alkali sulphate deposition regime the corrosion rates for all materials were low. As the alkali sulphate deposition flux was increased the corrosion proceeded at a much faster rate in each of the materials, but both the onset of this second regime and the changes in corrosion rates were material dependent. In this regime, the corrosion damage showed a linear dependence on alkali sulphate deposition flux. At higher alkali sulphate deposition fluxes, the corrosion product and “deposit” were observed to have resulted in extremely thick surface scales on all materials. The effect of this was to gradually reduce the rates of corrosion on each material as progressively heavier alkali sulphate loadings were applied. This effectively creates a “buried in ash” scenario where the ambient gaseous atmosphere is separated from the alloy surface and this prevents easy diffusion of SO&O3 to maintain the corrosion reaction. For the materials tested, an increase in SO, partial pressure resulted in an increase in the corrosion rate, but the magnitude of this change varied with material. The corrosion rate of ATD2 coated IN738LC was the least affected by changes in SO, partial pressure. Mathematical models were deduced by fitting equations to the data generated for the behaviour of IN738LC, RT22 coated IN738LC and ATD2 coated IN738LC at both 700 “C and 900 “C in the lower alkali sulphate deposition ranges. The equations were found to have the following form (for a set temperature and HCl partial pressure) : Corrosion rate = (a + bx + c$) + (d + ex +$?)

log ( SO,)

where x is the alkali sulphate deposition flux, SOZ is a partial pressure and u-f are constants. 4.2. High temperature erosion tests and model development These studies utilised three test facilities: a high velocity gas gun facility permitted erosion-corrosion data to be obtained from a continuous stream of particles at velocities

251

up to 600 m s- ’; a centrifugal erosion-corrosionrig was used to generate multi-impact and continuous erosion data over the velocity range l-200 m s- ‘; single impact tests were used to investigate damage mechanisms and morphologies. These tests are described in more detail in [ 28,291. The results of these studies showed that both the uncoated and coated alloys investigated all exhibited similar trends in erosion behaviour, with surface recession rates determined principally by the surface mechanical properties, the rate of surface oxide formation and the impact dynamics; the first two of these factors being temperature dependent. The results can be summarised as follows. Effect of time: the longer term exposures in these tests (300 h) demonstrated that the erosion mechanism and behaviour under steady state conditions did not change with time nor with the addition of SOa. Effect of impact angle: the dimensionless erosion rates were similar for many materials, gradually increasing at low angles to a constant value between 45” and 90”. This trend is typical of the behaviour exhibited by alloys which possess high strength or low ductility, with low work hardening exponents (typically less than 0.1) . Although the change in erosion rate with impact angle is more typical of a brittle target, SEM observations of eroded surfaces confirmed that plastic deformation was the primary mechanism of material removal. It should be noted that the erosion conditions were chosen to obtain substrate dominated steady state material removal. Diffusion coatings will be of benefit under scale modified conditions where their improved oxidation-corrosion resistance is the dominant factor and their lower strength is of less importance. Effect of particle velocity: velocity exponents generally varied between 2 and 3; typical values for the ductile erosion of metallic materials (Fig. 5) . Effect of particle size: erosion damage was found to increase with particle size. Effect of particle loading: after 300 h exposure, the relative levels of metal loss demonstrate the significant difference in erosion behaviour between IN738LC, ATD2 and RT22, particularly at high particle loadings (Fig. 6). For the highest particle loadings the substrate dominated regime applies and the difference in intrinsic properties between the alloys/coatings results in a considerable variation in erosion rate. However, as the particle loadings are reduced, the probability of impingement at a given impact site is reduced and the erosion mechanism shifts to the scale modified regime. 4.3. Grimethorpe PFBCpilotplant

exposure studies

The Grimethorpe PFBC pilot plant, at the time of these studies, consisted of a pressurised fluidised bed combustor, the hot gases of which passed through a ceramic filtration unit, a topping combustor and a single stage gas turbine (Fig. 7). In addition, there was a separate sidestreamcascade of aerofoils. Locations were selected in the hot gas path of

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different. On cylindrical samples, the upstream faces were generally bare and showed signs of particle impacts (e.g. craters). Along the sides of these samples there was an abrupt change to a surface covered with fine deposited particles. On many of the probes a distinct alkali sulphate dewpoint was observed, with lower temperature surfaces showing several morphologies of sodium-potassium sulphates. During statistical data analysis care was taken to analyse these zones separately and so gain quantitative information on all the different types of materials damage observed. The metrology data revealed a wide range of behaviour, from minimal damage to regions of total coating penetration for some coatings. However, many sections examined showed only a few localised areas of extreme corrosion damage, with minimal damage to the remainder of the metal surface. In cases where there are areas of extreme damage then extreme value statistical methods were used to define characteristic damage parameters, as for the laboratory hot corrosion modelling studies. Similar methods were successfully used as part of the GTCP programme to analyse the performance of vanes/ blades, and to develop links between the performances of materials in cylindrical samples and vanes/blades.

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Fig. 6. Effect of particle loading on erosion damage.

this plant to allow gas turbine materials to be exposed in realistic gas conditions and also in related environments, i.e. with only limited changes in exposure conditions (e.g. high/ low dust loading) to provide test data for the materials performance models. The materials were exposed on a series of specially designed cooled and uncooled probes to allow a range of gas-metal temperatures to be studied. During the course of these exposures, the following exposure parameters were monitored: gas composition ( SOZ, SO3 and HCl) ; gas temperature and pressure; dust loading and particle size distribution; deposition rates (fluxes) and deposit composition; alkali compound dewpoints; and metal temperatures. Various zones could be defined around samples where exposure conditions/materials behaviour had been distinctly

In order to have confidence in using any mathematical model predicting materials performance, it is necessary to test the model against as much data as possible, obtained under well characterised and realistic exposure conditions. It is also necessary to recognise the limitations of the models in terms of the exposure conditions for which they have been produced and the conditions for which they have been tested, particularly for empirically derived models. For the hot corrosion models derived from the laboratory data, the best available tests of the models are against the pilot plant data generated in this programme. Fig. 8 illustrates this test for two of the materials tested in a selection of the gas turbine environments generated in the pilot plant (also included in this figure are results from the laboratory test

NJ. Simms et al. /Wear 186187

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programmes). It should be noted that the limited scatter of the pilot plant data presented in Fig. 8 is a combination of the uncertainties in determining the exposure parameters in the pilot plant, limitations in the actual model and measurement errors (which increase in significance at the low damage rates). Thus, overall agreement between the pilot plant results

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and this first generation corrosion model is good. The erosion-corrosion model, was found to be consistent with the pilot plant observations, but having a more mechanistic basis, could also be tested against the extensive laboratory data (Fig. 9). These figures both illustrate the good fit of the models to appropriate pilot plant and laboratory data, and give confidence in applying the models to predict materials behaviour in environments anticipated in advanced coal fired power systems. The models which have been derived for hot corrosion and erosion-corrosion conditions enable the effect of process conditions on materials corrosion behaviour to be assessed critically, and the implications for component lives evaluated quantitatively. For hot corrosion effects alone, the process conditions which may be assessed are the partial pressures of SO,, alkali sulphate deposition flux and metal temperature. Fig. 10 illustrates predictions for corrosion damage for IN738LC at 700 “C (i.e. under Type II hot corrosion conditions) as a function of alkali sulphate deposition flux at three levels of SO2 partial pressures. For erosion-corrosion effects, the process conditions which can be assessed are particle size, particle loading, impact angle, impact velocity and metal temperature. Fig. 11 illustrates the metal loss predictions for IN738LC as a function of impact velocity and particle size under one particular set of exposure conditions (Note the use of constant particle flux in this plot means that the number of impacting particles is much higher for the small particle sizes). Similar erosion+orrosion maps may be produced using other variables in the model. The use of such maps and damage sensitivities allow the worst combinations of these process conditions and metal temperatures to be identified. In practice, these process con-

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of materials in selected environments appropriate to coalfired combined cycle power generating systems. The predictions of the corrosion and erosion-corrosion models and results of materials assessment studies in the Grimethorpe PFBC pilot plant were found to be in good agreement, giving confidence in the model predictions,

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The work reported in this paper was partly funded by the EC JOULE programme (through contract JOUF-0022). The materials studies carried out in the Grimethorpe pilot plant were also partly funded by the Grimethorpe Topping Cycle Project, and were carried out in conjunction with other parts of this project, whose contribution is gratefully acknowledged. The Grimethorpe Topping Cycle Project was a partnership between British Coal, GEC-Alsthom, PowerGen and the UK Department of Trade and Industry. The views expressed in this paper are those of the authors and not necessarily of those of British Coal nor Cranfield University.

References [ 1] A.D. Dainton, J.S. Harrison and J. Holmes, Advanced cycles for coalfired power generation, Proc. 8th Int. Conf on Coal Research, Tokyo, Japan, Oct. 1988, ICCR, Tokyo, 1988, pp. 88-102. [2] W. Schlachter and G.H. Gessinger, Innovation in power engineering: Role of materials, in E. Bachelet et al. (eds.), Proc. Conf High Temperature Materials for Power Engineering 1990, Liege. Belgium, Sept. 1990, Kluwer, 1990, pp. l-24.

1.

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Particle size (urn) Fig. 11. Example of map from erosion/corrosion model for M738LC at 700 “C (particle flux of 0.1 mg mm -Zh-‘ofSiO~for1000hat700”Cand 90”impact angle) [29].

ditions need to be linked to process variables which may be controlled: operating temperatures and pressures, coal and limestone feed rates, coal composition, etc. However, this linkage is beyond the scope of this paper, though has to some extent been investigated during the Grimethorpe Topping Cycle Project. 6. Conclusions Models have been successfully developed from laboratory studies for the hot corrosion and erosion-corrosion behaviour

[3] S.G. Dawes, C.J. Bower, C. Henderson, D. Brown and J.A.C. Hyde, Options for advanced power generation from coal, Proc. Conf Power Generation and the Environment, Inst. Mech. Engineers, London, November 1990, p. 123. [4] J.B. Marriott, M. Van de Voorde and W. Betteridge, Coal conversion processes and their materials requirements, EUR 9182 EN (EEC 1984). [5] J. Stringer, Proc. EPRI Seminar on Fluidised-Bed Combustor Technology for Utility Applications, Palo Alto, CA, May 1988, EPRI, Palo Alto, CA, 1988. [6] B.1. Davidson, D.B. Meadowcroft and J. Stringer, High temperature alloy requirements for coal fired combined cycles, in W. Betz et al. (eds.), Proc. Conf High Temperature Alloys for Gas Turbines and Other Applications, Liege, Belgium, Oct. 1986, Reidel, Dordrecht, 1986, pp. 219-244. [7] N.J. Simms, J.E. Oakey and M.A. Smith, Materials issues for coalfired combined cycle power plants, Proc. Conf Materials for Combined Cycle Power Plant, Shefield. UK, June 1991, Institute of Metals, London, pp. 229-235. [ 81 D.B. Meadowcroft and J. Stringer, Corrosion in coal-fired gas turbines, Proc. Conf High Temperature Corrosion of Superalloys, London, UK, Feb. 1986, in Mater. Sci. Technol., 3 (7) (1987) 562-570. [9] R.K. Clark, M. Arnold, J.B. Fackrell, M. Mordecai and S.G. Dawes, The Grimethorpe Topping Cycle Project, Proc. Conf FBC Technology and the Environmental Challenge (5th Int. Conf on Fluidised Combustion), Inst. Energy, London, UK, Dec. 1991, Adam Hilger,

Bristol, pp 353-362.

NJ. Simms et al. /Wear 186-187 (1995) 247-255

[lo] A.J. Minchener, P.J.I. Cross, M.A. Smith, J.J. Gale and S.G. Dawes, Advanced clean coal technology for power generation, Proc. Co@ FBC Technology and ihe Environmental Challenge (5th International Conference on Fluidised Combusrion), Inst. Energy, London, UK, Dec. 1991, Adam Hilger, Bristol, pp 331-341. [ 111 EC JOULE Project, Erosion/Corrosion of Advanced Materials for Coal-fired Combined Cycle Power Generation, JOUF-0022. [ 121 Coal gasification systems, Modern Power Syst., 22 (6) ( 1992) 5357. [13] G.P. Reed, J.E. Oakey and N.J. Simms, Grimethorpe High-

Temperature/High-Pressure Gas Filter Experimental Programme, EPRI Rep. TR-100499, Vol. 4, 1992. [14] J.R. Nicholls and P. Hancock, The analysis of oxidation and hot corrosion data-a statistical approach, in R.A. Rapp (ed.), Proc. Conj: High Temperature Corrosion, San Diego, CA. March 1981, NACE, Houston, TX, 1983, pp 198-210. [ 151 J.R. Nicholls and D.J. Stephenson, A life prediction model for coatings based on the statistical analysis of hot salt corrosion performance, Corros. Sci., 33 (8) (1992) 1313-1325. [ 161 J.E. Strutt, J.R. Nicholls and B. Barbier, The prediction of corrosion by statistical analysis of corrosion profiles, Corros. Sci., 25 (5) ( 1985) 305-316. [ 173 J.R. Nicholls and D.A. Triner, The influence of fuel compositions on lives of current valve materials-parametric equations for valve life predictions, in Diesel Engine Combustion Chamber Materials for Heavy Fuel Operation, Institute of Marine Engineers, 1990, pp. 121130. [18] J.R. Nicholls, High temperature surface engineering, accepted for publication in Sur$ Coat Technol. [ 191 J.R. Nicholls and P. Hancock, Prediction of high temperature corrosion performance using statistical analysis techniques, in J.E. Strutt and J.R. Nicholls (eds.), Plant Corrosion: Prediction of Materials Performance, Ellis Horwood, 1987, pp. 257-274.

255

[20] J.R. Nicholls and D.J. Stephenson, Hot corrosion tests on candidate diesel valve materials, in Materials for Diesel Engines, Combustion Chamber Materials for Heavy Fuel Operations, Institute of Marine Engineers, 1990, pp. 4760. [211 K.V. Bury, Statistical Models in Applied Science, Wiley, New York, 1975. [22] D.J. Stephenson and J.R. Nicholls, Modelling erosive wear, in R.P.M. Procter (ed.), Proc. Conf on Advances in Corrosion and Prorecrion, Manchester, UK, July 1992, in Corros. Sci. 35 (5-8) (1993) 10151026. [231 D.J. Stephenson and J.R. Nicholls, Modelling high temperature

erosion, in J.E. Strutt and J.R. Nicholls (eds.), Planr Corrosion: Prediction of Materials Performance, Ellis Horwood, 1987, pp. 313324. [241 P. Hancock, J.R. Nicholls and D.J. Stephenson, Surf: Coat Technol., 32 (1987) 285. 1251 D.J. Stephenson, Corros. Sci., 29, (1989) 647. [261 N.J. Simms, J.E. Oakey, J Banks and S.R.J. Saunders, Valid

measurement of metal wastage: application in operational plants, to be published. [271 J.R. Nicholls, P.J. Smith and J.E. Oakey, Prediction of hot salt corrosion within utility gas turbines, in D. Coutsouradis et al. (eds.), Proc. Conf: Materials for Advanced Power Engineering,

Li.?ge,

Belgium, Oct. 1994, Kluwer, 1994, pp. 1273-1289. [281 J.R. Nicholls and D.J. Stephenson, Monte Carlo modelling of erosion

processes, Proc. Con& Erosion by Liquid and Solid Impacr, Cambridge, UK, September 1994, in Wear, 186-187 (1995) 64-17. t291 D.J. Stephenson and J.R. Nicholls, Modelling the influence of surface oxidation on high temperature erosion, Proc. Con& Erosion by Liquid and Solid Impact, Cambridge, UK, September 1994, in Wear, 186 187(1995)284-290.