Interactions between coal-ash and burner quarls. Part 1: Characteristics

of burner refractories and deposits taken from utility boilersq

Jon Wellsa,*, Gerry Rileyb,1, Jim Williamsona

aDepartment of Materials, Imperial College, London SW7 2BP, UKbInnogy Holdings plc, Windmill Hill Business Park, Swindon, Wiltshire SN5 6PB, UK

Received 23 October 2002; revised 10 January 2003; accepted 28 January 2003; available online 10 June 2003

Abstract

Samples of refractory materials with adhering coal-ash deposits were collected from the burner quarl regions of boilers at two power

stations. The degree of slag/refractory interaction was evaluated from polished cross-sections. Silicon carbide refractory materials showed

better resistance than aluminosilicate materials to slag chemical attack and penetration. Quarl design had also played a significant role with

less attack to quarls with reduced surface areas filled with rammable refractories of reduced thickness. Since a rammable refractory is cured

in situ in a boiler, the refractory closest to the water tubes will not fully cure leading to a reduction in thermal conductivity.

q 2003 Elsevier Ltd. All rights reserved.

Keywords: Slag/refractory interactions; Burner quarls

1. Introduction

The use of refractory materials in the combustion

chamber of a pulverised coal fired boiler is generally

limited to the burner quarl region. Modern quarls usually

consist of a series of shaped refractory tiles with a rammable

refractory used to fill the gaps between the tiles and the

membrane walls. The refractories chosen should show good

resistance to high temperatures, dimensional stability,

abrasion and erosion resistance, good thermal shock

properties and a resistance to chemical attack from adhering

coal-ash slags. Silicon carbide (SiC) is now the preferred

material. The higher thermal conductivity shown by SiC

gives rise to lower surface temperatures, thus reducing the

likelihood of slag interactions. The difference in thermal

expansion between SiC and aluminosilicate slag encourages

crack propagation and thermal shedding.

Nevertheless, ash deposition and the growth of deposits

is frequently encountered in units fitted with low NOx

burners. The quarls and near burner region (NBR) are

surfaces that cannot be cleaned by soot blowing operations.

Thus, deposits can grow with time leading to the formation

of what are generally known as ‘burner eyebrows’,

i.e. deposits shaped by the turbulent passage of the flue

gases. One consequence of this is a change in the

aerodynamics in the combustion zone and an increase in

NOx formation. Growth of ‘eyebrows’ can then lead to

‘eyebrow bridging’ with the accumulation of massive

amounts of slag, which if detached can cause damage to

the water tubes and ash hopper bridging. The phenomenon

has been well described by Raask [1].

1.1. Types of refractory used for quarls

SiC tiles consist of coarse SiC grains with a bonding

phase containing smaller SiC particles. The bonding phase

is either an aluminosilicate based material or silicon nitride.

Much attention is paid during the manufacture of the tiles to

reduce the porosity into which liquid slags can penetrate.

While nitride bonded tiles appear to offer superior

mechanical and thermal properties, utility experience

suggests that over a period of years thermal cracking of

these tiles occurs more readily than those with an

aluminosilicate bond.

0016-2361/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S0016-2361(03)00162-5

Fuel 82 (2003) 1859–1865

www.fuelfirst.com

q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com

1 Tel.: þ44-1793-896-298; fax: þ44-(0)-1793-906-251.

* Corresponding author. Tel.: þ44-20-7589-5111x56758; fax: þ44-20-

7594-6748. Present address: Innogy One, Platlife and Integrity Group,

Windmill Hill Business Park, Swindon, Wiltshire, SN56PB, UK.

E-mail addresses: janathan.wells@innogy.com (J. Wells), gerry.riley@

innogy.com (G. Riley).

Rammable refractories are used to fill the gaps between

the tiles and the water tubes. These may be aluminosilicate

or SiC based. Since rammable refractories have to be cured

‘in situ’, they inevitably have a higher porosity than the tile

materials. Inspections of the burner quarls during boiler

outages indicate that deposits form initially on the

rammable materials, but may then subsequently extend

over the surface of the tiles.

This study has been undertaken to provide a further

insight into the nature of refractory/slag interactions, in

the hope that an improved understanding of the

phenomenon could be used in burner design and in the

optimisation of the refractories to be used, thus

minimising the need for unscheduled boiler outages for

quarl maintenance.

2. Experimental

Samples of refractory materials with adhering slag layers

were manually collected from boilers at two power stations.

Power station A was found to have only minor slagging in

the quarl region of the boiler, whilst at power station B,

major eyebrow deposits had built up during operation.

Differences between the boilers included the size of the

burners, the distance between the burners, the size and

geometry of the quarl opening and tiles, the arrangement of

water tubes around the quarl, and the type of refractory

materials used. Fig. 1(a) and (b) shows the burner quarls at

power stations A and B, respectively. The quarl design at

power station A provided less surface area of rammable

refractory exposed to impingement from ash particles than

that at power station B.

At power station A, samples were collected before

cleaning and repair work started. In areas where deposits

were found adhering to refractory quarl materials, pieces of

refractory with the deposits still attached were chiselled

from the surface, having noted their position relative to the

quarl opening. The quarl tiles were found to be nitride

bonded SiC, and a clay bonded SiC rammable refractory

was found in all gaps around the quarl.

At power station B cleaning of the quarls had

commenced prior to the inspection. Large pieces of

refractory with thick slag layers were collected from the

ash hopper. The position of these samples relative to the

quarl opening could not be determined, but these samples

provide examples of the extent of slag/refractory

interactions. Other samples were taken from the quarls,

and these could be linked to positions relative to the quarl

opening. Three different types of rammable refractory were

found in gaps in the quarl region of this boiler, namely a

clay bonded SiC rammable, a clay bonded aluminosilicate

rammable and a phosphate bonded aluminosilicate

rammable. The quarl tiles were all clay bonded SiC.

Large eyebrows were reported to have formed on all the

rammable refractories.

2.1. SEM characterisation

Cross-sections from samples were cut using a slow-speed

diamond saw, sections set in cold setting resin and then

polished with diamond sprays to a 0.25 mm finish. A thin,

evaporated carbon coating was applied to eliminate surface

charging in the electron beam.

SEM analysis was performed using a JEOL 6400 SEM

fitted with a NORAN light element detector and a Voyager 4

analysis system. Chemical analysis of areas and individual

phases were performed by EDS with Proza corrections to

give quantitative results.

2.2. In situ curing of rammable refractories

Some of the slag/refractory samples collected from the

power stations had a layer of fine ash particles between

the refractory and the water tube suggesting there had been

an air gap. In other samples, the refractory closest to the

water tubes was crumbly in comparison to the bulk of

the refractory. To fully cure the rammable refractory

requires a temperature of ,830 8C. However, the refractory

is close to the water pipes, which will be maintained at

Fig. 1. Differences in refractory quarl designs at (a) power station A, and (b)

power station B.

J. Wells et al. / Fuel 82 (2003) 1859–18651860

,450 8C. Therefore, it is unclear exactly what happens to a

rammable refractory as it cures in situ. The question is, does

it shrink away from the water pipes leaving a gap, or does it

just not cure properly adjacent to the water pipes? To answer

these questions some aluminosilicate rammable refractory

was mounted on a probe such that the back surface of the

refractory was maintained at 450 8C. The probe was then

inserted into the side of the Innogy 0.5 MW Combustion

Test Facility (CTF) and held there for 3 days. A schematic

representation of the probe is shown in Fig. 2(a).

The temperature in the combustion zone was ,1600 8C.

The refractory was then examined for signs of shrinkage,

or problems with curing.

2.3. Thermal conductivity models and measurements

The nature of slag that forms on a refractory is believed

to depend upon the surface temperature of the refractory,

which in turn is believed to depend upon the thermal

conductivity of the refractory. SiC has a far higher thermal

conductivity than an aluminosilicate refractory and using

SiC in a boiler should therefore lower the surface

temperature of the quarl. A simple one-dimensional heat

transfer model was used to predict the surface temperature

of different refractories under boiler conditions. The model

assumed a combustion zone temperature of 1600 8C,

an emissivity of 0.85 for all refractories and thermal

conductivities of 1.55 W/mK for an aluminosilicate

refractory [2] and 15 W/mK for a SiC refractory [3].

The effect that the degree of contact between the

refractory and water tubes on the surface temperature of

the refractories was predicted using the heat transfer model.

The effect of air gaps, and the degree to which the material

has been cured, on the thermal conductivity of a clay bonded

SiC rammable refractory were made using a Lee’s Disk

apparatus with variable copper spacers to give known air

gaps between the refractories and the copper disks.

3. Results and discussion

3.1. Power station samples

Only minor deposits had formed in the quarl region of the

boiler at power station A. On both the clay bonded SiC

rammable refractory and the nitride bonded SiC tiles the

deposits were light brown in colour, dusty and friable,

and easily removed by hand. Below the friable deposits

were thin layers of a more strongly adhering slag.

A backscattered electron (BSE) image of a cross-section

through the interface between a friable deposit and a piece

of clay bonded SiC rammable refractory taken from

between two water tubes at the bottom centre of a quarl is

shown in Fig. 3(a). This shows that ash particles arriving at

the quarl surface had infiltrated crevices and voids in the

surface of the refractory, but had not chemically interacted.

Fig. 3(b) shows a BSE image of a polished cross-section

of another area of deposit/refractory interface, also for a

piece of clay bonded SiC rammable refractory taken from

the bottom centre of a quarl. In this sample a thin layer of

consolidated deposit has wetted the surface of the

refractory, but there has been no interaction between the

SiC grains and the deposit. However, EDS chemical

analysis has shown there to be minor interaction between

the clay bonding material and the deposit, with Fe species in

the deposit diffusing into the refractory.

Examination of deposits on the nitride bonded SiC tiles

showed limited wetting of the surface of the tiles as shown

in Fig. 3(c). Where wetting had occurred there was no

interaction between the deposit and the SiC grains, but there

was minor interaction between the deposit and the nitride

bonding phase, which appeared to have oxidised during

service. Instances were found where a zircon wash (ZrSiO4),

applied to the surface of the quarl tiles to give protection

against attack from vanadium residues in oils used during

start up of the boiler, had not spalled off during the time in

service (normally thought to be a few days after start-up).

The zircon layer had acted as a bond phase between the

deposit and the nitride phase, having chemically reacted

with both.

At power station B four different refractory materials

were identified, namely a clay bonded SiC rammable, a clay

bonded aluminosilicate rammable, a phosphate bonded

Fig. 2. (a) A schematic representation of a cross-section of the

aluminosilicate rammable refractory mounted on a steam cooled probe

inserted in the Innogy combustion test facility; (b) sample of the refractory

after it was removed from the probe.

J. Wells et al. / Fuel 82 (2003) 1859–1865 1861

aluminosilicate rammable, and clay bonded SiC tiles.

Dense, consolidated coal-ash deposits that had clearly

been fluid were found attached to the surface of all these

refractories, with the degree of slag/refractory chemical

interaction depending on the type of refractory.

The aluminosilicate rammables comprised of a mixture

of low-grade alumina and mullite grains bonded together by

a finer dispersion of smaller alumina grains mixed with

clays or phosphate alumina cement. A BSE image of a

cross-section of the slag/refractory interface for a clay

bonded aluminosilicate refractory collected from the ash

hopper after quarl cleaning is shown in Fig. 4(a). There are

five distinct layers to this sample. At the outer surface there

is a glassy slag, ,1 mm thick, containing iron oxide

dendrites. There is a crack below this at the original

interface of the aluminosilicate refractory. Directly below

the slag is a region 2–3 mm in depth where slag has fully

penetrated the material and most coarse aggregates have

gone into solution. Over the next 4–6 mm the slag has fully

penetrated the clay bonding material but most of the

aggregate is still visible, although with rounded corners.

Between this layer and the unaffected refractory there is

a layer 2–3 mm thick, where partial interaction of the slag

Fig. 3. BS SEM images of polished cross-sections of the slag/refractory

samples removed from power station A; (a) clay bonded SiC rammable

refractory, (b) second sample of clay bonded SiC rammable refractory, and

(c) silicon nitride bonded SiC tile.

Fig. 4. BS SEM images of polished cross-sections of the slag/refractory

samples removed from power station B, (a) clay bonded aluminosilicate

rammable refractory, (b) clay bonded SiC rammable refractory, and (c) clay

bonded SiC tile.

J. Wells et al. / Fuel 82 (2003) 1859–18651862

with the clay bonding material has occurred but the

aggregates are unaffected. The total interaction between

the slag and refractory occurs to a depth of 8–12 mm.

Similar interactions were found for slags adhering to the

phosphate bonded aluminosilicate rammable refractory.

Clay bonded SiC rammable refractories from power

station B showed very little interaction with the slag layer,

as observable in Fig. 4(b), a cross-section of refractory with

attached slag taken from above water tubes to the top left of

the quarl. In this case the deposit adhering to the refractory

consists of two distinct layers, a highly porous friable layer

adjacent to the SiC followed by a well-fused layer with

some porosity. The slag has adhered through two

mechanisms. Firstly, the ash arriving at the surface has

infiltrated crevices and voids in the refractory surface giving

an unconsolidated mechanical bond. Secondly, where the

slag has wetted the surface of the refractory there are minor

interactions with the clay bonding material to a maximum

depth of ,0.2 mm, though there are no interactions

between the SiC grains and the slag. Another feature of

the area where the slag has wetted the surface of the

refractory is the presence of what appear to be gas bubbles

in the slag adjacent to the refractory surface.

The interaction between the clay bonded SiC quarl tiles

and the slag is minimal as can be seen in Fig. 4(c),

a cross-section through the slag/tile interface for a quarl tile

with a glassy, red/brown coloured adhering slag. The contact

between the slag and tile is minimal due to the formation of

large gas bubbles at the tile surface. The bulk of the slag also

contains gas bubbles. Where there is contact between the

slag and the tile there appears to be no chemical interaction

between the SiC grains and the slag and only minor

interactions with the clay bonding phase. The main

mechanism of adhesion of the slag appears to be

mechanical, where a liquid slag has flowed into voids and

crevices in the surface of the tile.

The deposits in the burner region of power station A were

dusty and friable, whilst the deposits from power station B

were well consolidated and had clearly been fluid.

The average chemical composition of the quarl region

deposits from power stations A and B are shown in Table 1.

Whilst the chemistry of the quarl deposits from power

station B shows higher Fe2O3 and CaO concentrations

(both fluxing agents) than for the deposits from power

station A, it is not believed these are high enough to fully

account for the difference in nature between the deposits.

It is more likely that the SiC refractories, with higher

thermal conductivities, have lower surface temperatures

making it less likely that the slag can consolidate. The slag

on the SiC rammable refractory in power station B is more

consolidated than in power station A, which is probably due

to the refractory being thicker in power station B. The high

penetration of slag into the aluminosilicate rammable

refractories could also be due to the aluminosilicate

materials having a higher surface temperature due to their

lower thermal conductivity.

A second theory may explain why the SiC based

refractories are more slag resistant than the aluminosilicate

based refractories, and that is that the SiC grains slowly

oxidise leading to CO2 being evolved from the surface of the

refractory. This would account for the high level of porosity

at the slag/refractory interface found with the SiC based

refractories. This feature will be discussed further in Part 2

of this paper [4].

At power station B it was found that eyebrow deposits

occurred as readily on the SiC refractories as on the

aluminosilicate rammables. This suggests that even though

the level of interaction is significantly less with SiC based

materials, any bond between slag and refractory (even if it is

solely a mechanical bond) will be sufficient to initiate major

deposit formation. Deposits might, however, be more easily

removed from SiC refractories during thermal cycling,

for instance due to the lower interaction depths.

However, such a conclusion cannot be made from SEM

analysis alone. At power station B the eyebrow problem is

independent of the refractory used, and growth is related to

the amount of ash arriving at the quarl surface. This is most

likely to be a function of burner and quarl geometry, and the

nature of the coal used.

3.2. Curing of a rammable refractory

The aluminosilicate rammable refractory on the cooled

probe that was cured in the Innogy CTF was ,15 cm thick.

This was typical of the amount of rammable refractory used

at power station B. After the three day curing, the sample

was removed from the end of the probe. During curing the

refractory changes colour from white to brown. The photo in

Fig. 2(b) shows that where the refractory has been in contact

with the cooled probed there is a layer, ,1 cm thick, that has

not fully cured. This layer of uncured refractory may

significantly influence the surface temperature of the

rammable refractory, as will now be discussed.

Table 1

The average chemical composition (wt%) of quarl region deposits from power stations A and B

SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O TiO2 P2O5 SO3

Power station A 54.7 26.8 6.9 5.1 1.5 0.8 1.8 1.7 0.2 0.3

Power station B 56.8 24.5 8.5 9.0 1.5 0.6 1.2 0.8 0.2 0.0

J. Wells et al. / Fuel 82 (2003) 1859–1865 1863

3.3. Thermal conductivity models and measurements

It had been noted that for some of the samples removed

from the power stations there was a layer of fine ash between

the rammable refractory and the water tubes. Initially there

must have been an air gap for ash to have arrived here. It has

also been shown that if there is no air gap between the

rammable refractory and the water tubes, then the refractory

will not fully cure. A Lee’s disk apparatus was used to

assess the impact that such installation problems would

have on the overall thermal conductivity of a rammable

refractory.

The Lee’s disk experiments used a disk of SiC refractory

5 cm in diameter and 0.6 cm thick. The SiC disk was cured

at 1000 8C to ensure all refractory bonds had formed.

Thermal conductivities were measured for the disk alone,

for the disk with a ,0.5 mm air gap, and for the disk with a

,1 mm ash filled gap. In addition to this, the thermal

conductivity of a disk of dried but uncured SiC refractory,

5 cm in diameter and 0.75 cm thick, was measured.

The thermal conductivity as a function of temperature for

the conditions outlined above is shown in Fig. 5.

It is clear that the SiC refractory with an air gap, the SiC

refractory with an ash filled gap, and the uncured SiC

refractory all have substantially lower thermal conduc-

tivities than for the SiC rammable refractory alone.

To evaluate the impact this might have on the surface

temperature of a rammable refractory as a function of

Fig. 5. Thermal conductivity of SiC rammable in cured and uncured states, cured with a 0.5 mm air gap, and cured with a 1 mm ash-filled gap.

Fig. 6. Calculated changes in surface temperature of a SiC and an aluminosilicate rammable with thickness of refractory, also showing the effects of an ash

filled gap between the refractory and the water tube.

J. Wells et al. / Fuel 82 (2003) 1859–18651864

refractory thickness, the one-dimensional heat transfer

model was used to calculate the surface temperatures.

The model was applied to two different sets of

conditions. Under the first set of conditions the refractory

layer was placed directly in contact with a water tube at

527 8C, whilst under the second set of conditions a 1 mm

gap was inserted between the refractory and the water tube.

A thermal conductivity of 0.5 W/mK was attributed to this

gap (Wain [5]) to simulate a 50% porous aluminosilicate

layer as might be expected for an ash filled gap. The results

of the calculations for both sets of conditions are shown

graphically in Fig. 6. Even a small air gap filled with

pfa particles will increase surface temperatures by 200 8C

or more.

SiC was initially believed to be successful as a quarl

refractory material due to its high thermal conductivity and

lower surface temperatures, thus reducing the likelihood of

consolidation of ash particles on the quarl surface.

This study suggests that given the thickness of refractory

needed in some quarl designs, and given inevitable

irregularities in installation, the benefit in reduced surface

temperatures when using SiC may be minimal.

4. Summary

Quarl refractories with adhering slag layers from two

different power stations have been characterised using SEM

microstructure and chemical analysis. The slags have been

observed to penetrate deeply into aluminosilicate based

refractories creating a strong chemical slag/refractory bond,

while for SiC based refractories the slag remains on the

surface with only minimal interactions, the slag/refractory

bond being predominantly mechanical where ash particles

have infiltrated voids and crevices. Lower surface

temperatures could account for the improved slag resistance

of the SiC as it has a higher thermal conductivity than for

aluminosilicate-based materials. However, it has been

shown that it is unlikely that a perfect contact can be

made between rammable refractory and water tubes,

since rammable refractory in direct contact with a water

tube will not cure fully. The effect of this would be a

reduction in the difference in surface temperature between

using SiC based refractory as opposed to aluminosilicate

based refractory, especially when some quarl designs

require a refractory thickness in excess of 15 cm.

Hence, while SiC based refractories should be used in

preference to aluminosilicate based refractories in the quarl

region of utility boilers, the quarl should be designed to

minimise the thickness of refractory required.

Acknowledgements

This work was performed with a grant from the ECSC

(7220-PR-048). The authors are grateful to Innogy Holdings

plc for the permission to publish this paper.

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[5] Wain SE, Livingston WR, Sanyal A, Williamson J. In: Benson SA,

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J. Wells et al. / Fuel 82 (2003) 1859–1865 1865