Interactions between coal-ash and burner quarls. Part 2: resistance of

different refractory materials to slag attack in a combustion test facilityq

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 11 June 2003

Abstract

The interactions between coal-ash and a variety of refractory materials with a potential as refractory quarls have been studied in a 0.5 MW

combustion test facility. A Clay-bonded SiC refractory has been found to react least with coal-ash, but not as a result of its lower surface

temperatures. It is suggested that oxidation of the SiC leads to CO2 evolution from the surface of the refractory, hindering coal-ash

penetration into the refractory. A technique for modifying the surface of rammable refractory materials, to reduce the adhesion strength of

coal-ash deposits, is proposed. Initial results indicate that the technique can be successful and could easily be applied in a utility boiler.

q 2003 Elsevier Ltd. All rights reserved.

Keywords: Coal ash/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. The refractory materials used are either

shaped tiles that line the burner opening, or rammable

refractories that fill the gaps between water pipes [1]. The

quarls and near burner region of a boiler are areas that cannot

be cleaned by soot blowing operations. Thus, any deposits

grow with time leading to the formation of what are known as

‘burner eyebrows’, i.e. deposits shaped by the turbulent

passage of the flue gases. These deposits can change the

aerodynamics in the burner zone, leading to an increase in

NOx emissions [2]. The growth of burner eyebrows is

believed to initiate on the rammable refractory materials.

Hence, the best way to reduce the formation of burner deposits

would be to arrange the water pipes around the burner opening

so as to minimise the amount of exposed rammable refractory.

However, designs that completely eliminate the use of

rammable refractories have not yet been produced.

In practice, industrial boiler operators have tried to

minimise burner quarl slagging by replacing aluminosilicate

refractories with SiC based materials. SiC has a high

thermal conductivity, which should give rise to lower

surface temperatures, reducing the likelihood of slag

adherence. Changing the material used in an existing boiler

design would be far more cost effective than changing the

arrangement of the water tubes. In certain cases, the use of

SiC has been successful.

There remains the hope that further improvements to the

materials used in the quarl region of utility boilers could

offer an effective method for reducing quarl slagging

problems. Coal-ash/refractory interactions in samples

taken from power station boilers have been reported [1].

This paper presents the results of inserting commonly used

and modified refractory materials into a test quarl in the

Innogy 0.5 MW combustion test facility (CTF).

2. Experimental

The CTF is a water-cooled refractory lined combustion

chamber followed by a simulated superheater zone for

studying slagging and an exhaust zone for simulating fouling.

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

doi:10.1016/S0016-2361(03)00164-9

Fuel 82 (2003) 1867–1873

www.fuelfirst.com

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

1 Tel.: þ44-1793-896-298; fax: 44-1793-906-251.

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

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

Windmill Hill Business Park, Swindon, Wiltshire, SN5 6PB, UK.

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

innogy.com (G. Riley).

The facility is designed to replicate the conditions in a full-

scale pulverised coal boiler. The coal is injected through a

0.5 MW low NOx burner, which is fitted to a scaled down

refractory quarl. Deposits can build up in the quarl region of

the CTF in the same way as in a full-sized boiler.

For this study, test quarls were manufactured with spaces

for four different refractory inserts. The spaces for the inserts

gave a 50 mm by 100 mm surface area and were 50 mm deep.

A total of 10 different insert materials were tested in three

separate trials. Each trial lasted three weeks, with three days

in each week burning the test coal, the firing up being fuelled

with natural gas. Pictures of a test quarl before and after a trial

are shown in Fig. 1(a) and (b), respectively. Two trials were

conducted burning a bituminous coal A, and one trial with

bituminous coal B. Both coals are typical UK power station

coals. The surface temperature of the quarl was measured

with an optical pyrometer during one of the trials, and was

found to be approximately 1500 8C. This surface temperature

was the same for all the inserts, which is understandable

given that there was no back cooling of the quarl.

The refractory materials tested during the trials are given in Table 1. In addition to the test inserts, the interaction between

coal-ash and the material of the test quarl was also examined.

The quarl, and any rammable materials used as inserts,

were cured in situ during the initial firing of the CTF on

natural gas.

2.1. Characterisation of the samples

Following each of the trials the inserts were chiselled out

from the quarl. Cross-sections were cut from each sample

using a slow-speed diamond saw to reveal the slag/refrac-

tory interface. The sections were 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 the surface to

eliminate surface charging in the 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 was performed by EDS with Proza corrections to

give quantitative results.

3. Results and discussion

Average chemical compositions of the slags from coals

A and B that were found on the test inserts are shown in

Table 2. Although the two compositions are very similar,

the slag from coal B was found to be slightly more corrosive

than the slag from coal A. If the different test inserts were to

be ranked according to their resistance to chemical reaction

with the coal slags, the order of ranking would be the same

for both coals. The refractory materials have been ranked in

order of resistance to slag penetration and attack in Table 1.

The results have been split into four categories; namely,

pre-processed refractory materials, rammable/plastic

Fig. 1. (a) Shows four refractory materials fitted into a test quarl prior to

firing in the CTF; the samples are: 1, clay bonded SiC tile; 2, MgO plastic

refractory; 3, aluminosilicate rammable; 4, SiC rammable, and (b) the quarl

with the inserts in the same positions as for (a) but after a three-week trial

burning coal A.

Table 1

Refractory materials used for determining coal-ash burner quarl inter-

actions. Materials listed in order of resistance to slag attack and penetration

Rank Sample Coal

1 SiC rammable refractory with 400 mesh

SiC grains pressed into the surface

and smoothed with a palate knife

B

2 Zircon Wash coatinga A

3 Clay bonded SiC tile A and B

4 Nitride bonded SiC preformed refractory A

5 SiC rammable refractory with 100 mesh

SiC grains pressed into the surface

and smoothed with a palate knife

B

6 Clay bonded SiC rammable refractory A and B

7 Mullite tile A

8 Aluminosilicate rammable refractory with 100 mesh

SiC grains pressed into the surface

and smoothed with a palate knife

B

9 Aluminosilicate rammable refractory A and B

10 Magnesia plastic refractory A

a Effective for the detachment of just one coating of slag.

J. Wells et al. / Fuel 82 (2003) 1867–18731868

refractory materials, coatings, and surface modified ram-

mable refractories. Following this, general conclusions have

been made.

3.1. Pre-processed refractory materials

Pre-processed refractory materials are materials that

were formed into a required shape before being fitted into

the test quarl. An ultrasonic assisted compaction of the

ceramic particles will have been included in the forming

process to reduce the porosity. The test inserts that fell into

this category were the clay bonded SiC tile, the nitride

bonded SiC (from a tile and flame holder), the mullite tile,

and the refractory used to fabricate the test quarl itself.

Backscattered SEM images of polished cross-sections of

these materials after the trials are shown in Fig. 2(a)–(e),

respectively.

The clay bonded SiC tile (Fig. 2(a)), taken from a trial

burning coal A, has a thin layer of slag on the surface. The

slag has been fluid and has wetted the surface of the

refractory flowing into open pores. The slag has not

chemically reacted with the SiC grains and there has been

negligible interaction between the slag and the clay-bonding

phase. The slag is effectively adhering to the refractory

through mechanical adhesion.

There were more slag/refractory interactions with the

silicon nitride bonded SiC (Fig. 2(b) and (c)). The SiC

grains themselves showed no chemical interaction with the

slag, but the silicon nitride bonding phase had chemically

reacted. For the insert from the silicon nitride bonded SiC

tile, the bonding phase had oxidised near the surface, and Fe

had diffused from the slag into this oxidised layer. The outer

surface of the insert from the flame holder was deficient in

SiC grains (Fig. 2(c)) and had a 6 mm thick coating of silica

at the slag/refractory interface. There was little oxidation of

the silicon nitride below this, but there was evidence of Fe

having diffused from the slag. Slag is chemically bonded to

the silicon nitride bonded SiC refractory materials, as well

as being mechanically anchored where the slag has flowed

into open porosity at the refractory surface.

The mullite tile consisted of mullite and alumina grains

bonded together with a phase consisting largely of fine

alumina. Fig. 2(d) shows coal-ash has penetrated into the

mullite tile and consolidated the outer surface giving a lower

porosity than the bulk of the material. There is a thin layer of

slag on the insert suggesting the coal-ash has arrived at the

refractory surface faster than it could penetrate the surface.

There is a strong chemical bond between the slag and

the mullite tile suggesting that quarl tiles should not be

made from mullite.

The test quarl itself was made from pure alumina grains

bonded with a CaO–Al2O3 high-alumina cement. The coal-

ash arriving at the surface of the quarl has penetrated

,4 mm into the cement phase, consolidating the refractory.

The high degree of chemical interaction between the slag

and refractory for this insert suggests that high-alumina

cement would be unsatisfactory as a quarl material.

The clay-bonded SiC tile was clearly the most slag

resistant of the pre-processed refractory materials tested.

SiC is used in utility boilers since the higher thermal

conductivity should give a lower surface temperature than

other refractory materials. However, in the CTF trial the

surface temperature was the same for all the inserts.

Table 2

The average composition (wt.%) of the slags found on the surface of the test quarls using coal A and coal B

Sample SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O TiO2 P2O5

Coal A 63.0 20.2 7.6 3.4 1.6 1.2 1.8 1.1 –

Coal B 57.9 24.5 6.9 4.1 2.3 0.9 2.1 1.0 0.4

Fig. 2. BS SEM images of polished cross-sections of the pre-processed

refractory inserts (a) clay bonded SiC tile, (b) nitride bonded SiC tile, (c)

nitride bonded SiC flame holder, (d) mullite tile, and (e) the test quarl

material.

J. Wells et al. / Fuel 82 (2003) 1867–1873 1869

The mechanism preventing slag from chemically reacting

with the clay bonded SiC refractory must therefore be

independent of thermal conductivity. Oxidation of the SiC

grains could be a possible mechanism for reducing

slag/refractory interactions.

The slag coating the clay bonded SiC insert contained

many gas pores at the slag/refractory interface. This was not

found in the slag on the other refractory materials. These

pores appear to be limiting the area of contact between the

slag and the refractory. At the combustion temperatures in

the CTF SiC grains in the refractory could be oxidising,

especially given that the coal will have had inherent

moisture, and that water vapour is known to increase

oxidation of SiC [3]. It is therefore likely that CO2 gas

produced by the oxidation of the SiC grains will have

evolved from the surface of the refractory. This slow

evolution of gas from the surface of the refractory could

hinder coal-ash bonding to the refractory surface.

3.2. Rammable/plastic refractory materials

Rammable/plastic refractory materials are pliant and are

pressed into gaps between the quarl tiles and the water

tubes. They only cure when heated to above 830 8C [1]. The

inserts used in the trials that fell into this category were the

SiC rammable refractory, the aluminosilicate rammable

refractory and the magnesia plastic refractory. Backscat-

tered SEM images of polished cross-sections of these

materials after the trials are shown in Fig. 3(a)–(c).

The SiC rammable, like the clay bonded SiC tile, consists

of SiC grains with an aluminosilicate clay bonding phase,

but with a higher level of porosity, and hence a lower

thermal conductivity. The slag on the SiC rammable

refractory has wetted the surface of both the bonding

phase and the SiC grains (Fig. 3(a)), but there has been no

chemical reaction between the SiC grains and the slag and

only minor interaction between the bonding phase and the

slag. Hence, the slag is only adhering to the SiC rammable

refractory by mechanical bonding where the slag has flowed

into open porosity on the surface of the refractory.

The aluminosilicate refractory consists of alumina grains

bonded together with an Al2O3–P2O5 phase. Coal-ash

arriving at the surface of the aluminosilicate rammable

refractory has penetrated some 2 mm into the bonding phase

(Fig. 3(b)) giving a strong chemical bond between the slag

and refractory. Some of the smaller alumina grains near the

slag/refractory interface have started to dissolve in the slag.

A slag layer has then built up on top of the consolidated, slag

saturated, refractory surface.

The magnesia plastic refractory consists of magnesia and

alumina grains bonded together with an aluminosilicate clay

phase. Coal-ash arriving at the refractory surface has been

totally absorbed into the refractory consolidating the top

1.5 mm of the insert (Fig. 3(c)). Below this the material is

crumbly, porous and full of internal cracking. This is

probably due to the high thermal expansion coefficient of

Fig. 3. BS SEM images of polished cross-sections of the rammable/plastic

refractory inserts (a) SiC rammable, (b) aluminosilicate rammable, and (c)

magnesia plastic refractory.

J. Wells et al. / Fuel 82 (2003) 1867–18731870

the magnesia grains giving the MgO a poor thermal shock

resistance. Such a material would be a poor choice for the

burner quarl region of a utility boiler.

The clay bonded SiC based refractory has proved to be

the most effective of the rammable/plastic refractory

materials for resisting chemical reaction with coal slag.

The slag on the SiC rammable refractory contained many

pores at the slag/refractory interface, whereas the slags on

the other rammable/plastic refractory materials, although

porous, contained less pores connected to the refractory

surface. These observations support the proposed mechan-

ism that CO2 from the oxidation of the SiC grains might be

evolved from the surface of the SiC based refractories

hindering ash/refractory contact.

3.3. Coatings

During a trial burning coal A, half the SiC tile and mullite

tile inserts were coated with zircon wash. Zircon wash is a

slurry of zircon (ZrSiO4), quartz and ball clays that can be

painted on to the surface of refractory materials to protect

them against chemical attack from slags. It is commonly

used in pulverised coal fired boilers to protect the quarl tiles

from chemical attack from vanadium residues during fire-up

of the boiler on oil. Although a zircon wash is commonly

used, the reasons it is effective are less obvious. Back-

scattered SEM images of polished cross-sections of the

inserts coated with zircon wash after the trials are shown in

Fig. 4(a) and (b).

A thicker coating of the zircon wash was applied to the

mullite tile than the SiC tile. With the thinner coating

(Fig. 4(a)), slag had penetrated through the coating and

filled pores and voids in the surface of the SiC refractory.

Given this degree of penetration the zircon wash would not

be able to spall off during service. With the thicker coating,

slag had only penetrated part way into the zircon layer (Fig.

4(b)). At the same time, the slag then started to build up on

top of the coating. A line of voids in the zircon wash would

allow the coating to spall off through the effects of gravity or

thermal shock.

As long as the zircon wash coating is above a minimum

thickness then the coating should be effective for the

removal of at least one burner region deposit. After the

initial deposit has formed and fallen off then the surface of

the quarl refractory would be exposed to coal ash. An exact

figure of the minimum thickness cannot be calculated from

this work; however, a 200 mm thick Zircon Wash layer did

not seem to have been thick enough, whilst a 400 mm thick

coating appeared sufficient.

Zircon Wash might only be effective for one burner

region deposit to form and fall off before exposing the

refractory beneath. However, If the depth to which slag can

penetrate into a coating could be minimised before the

deposit starts to build up, then the coating may be useful for

two or three burner region deposits to form and fall off

before the underlying refractory is exposed.

3.4. Surface modified rammable refractories

Rammable refractory materials were modified for two

purposes, to reduce the amount of open porosity at the

surface of the refractory making surfaces as smooth as

possible, and to increase the proportion of SiC grains on the

surface of the refractory. The modified rammable refractory

inserts were: an aluminosilicate rammable refractory with

100 mesh SiC grains pressed into the surface, a SiC

rammable refractory with 100 mesh SiC grains pressed into

the surface, and a SiC refractory with 400 mesh SiC grains

pressed into the surface. Backscattered SEM images of

polished cross-sections of these materials after the trials are

shown in Fig. 5(a)–(c).

The trial with the surface modified rammable refractory

inserts was conducted with coal B. The SiC grains added to

the rammable refractory reacted with the slag from coal B to

form iron silicide. Iron silicide would only be expected

to form under reducing conditions, and this suggests

Fig. 4. BS SEM images of polished cross-sections of zircon wash coatings

on inserts of (a) clay bonded SiC tile, and (b) mullite tile.

J. Wells et al. / Fuel 82 (2003) 1867–1873 1871

that locally reducing conditions may somehow have been

present in some parts of the system. Iron silicide was not

formed when the SiC grains in the clay bonded SiC tile and

the unmodified SiC rammable were in contact with the same

slag, a scenario that one would have expected under

equilibrium conditions.

The aluminosilicate rammable refractory with the 100

mesh SiC grains pressed into the surface did react with the

slag. However, the slag only penetrated to a depth of

about 400 mm into the alumino-phosphate bonding phase

(Fig. 5(a)) as compared to 1.5 mm for the unmodified

aluminosilicate rammable refractory (Fig. 3(a)). There are

more pores at the slag/refractory interface for the modified

aluminosilicate rammable refractory. This supports the idea

that oxidation of the SiC grains is again hindering the slag/

refractory interactions, though gas may also have been

evolved due to the SiC grains reacting to form iron silicide.

It is impossible to determine exactly which effect is

dominating.

Pressing the 100 mesh SiC grains into the surface of the

SiC rammable refractory did not show any improvement in

reducing the slag/refractory adhesion strength compared to

the unmodified SiC rammable refractory. The added 100

mesh SiC grains again reacted with the slag to form iron

silicide. Fig. 5(b) shows a pore at the slag/refractory

interface with a tail between the 100 mesh SiC grains. The

shape of the pore suggests that gas evolved from the surface

of the refractory and passed between the 100 mesh SiC

grains until it could expand in the fluid slag, suggesting

oxidation of the SiC grains may be the controlling

mechanism in reducing slag/refractory interactions.

Coating the surface of the SiC rammable refractory with

400 mesh SiC grains reduced the slag/refractory adhesion

causing the slag to easily break from the surface when the

insert was removed from the quarl. Three possible reasons

exist for this reduction in slag/refractory adhesion strength,

namely, there was very little open porosity in the surface of

the insert, the 400 mesh SiC grains reacted with the slag to

form iron silicide, and the SiC grains could have oxidised

leading to CO2 being evolved from the surface. The exact

mechanism cannot be determined from this study.

The SiC rammable with 400 mesh SiC grains pressed

into the surface was the most effective treatment found to

reduce the slag/refractory interactions. The slag/refractory

adhesion strength had clearly been reduced which means

that a deposit formed might break off under it’s own weight,

or be removed by thermal cycling. Pressing 400 mesh SiC

grains into SiC rammable refractory and then smoothing the

surface could easily be achieved in a utility boiler during a

scheduled outage, at very little cost.

4. Conclusions

SiC based refractory materials have been shown to be

more resistant to chemical attack by coal ash slags than

other refractory materials. Specifically, SiC based refrac-

tories should be used as the refractory of preference in

coal fired boilers compared to phosphate bonded alumino-

silicate refractory materials, mullite, clay bonded magnesia

Fig. 5. BS SEM images of polished cross-sections of the surface modified

rammable refractory inserts (a) aluminosilicate rammable with 100 mesh

SiC grains pressed into the surface, (b) SiC rammable with 100 mesh SiC

grains pressed into the surface, and (c) SiC rammable with 400 mesh SiC

grains pressed into the surface.

J. Wells et al. / Fuel 82 (2003) 1867–18731872

refractories or a refractory containing high-alumina cement.

Clay bonded SiC refractories appear to be more chemically

resistant to coal slag than nitride bonded materials.

SiC refractory materials are said to be resistant to

chemical attack from coal slags on account of their higher

thermal conductivity and hence lower surface temperatures.

In the CTF trials, the surface temperature of all inserts was

the same, hence the thermal conductivity cannot have

effected the reactivity of the refractory. Slag layers on SiC

based refractory materials tend to have a higher level of

porosity at the slag/refractory interface. This porosity seems

to be due to CO2 being evolved from the surface of the

refractory as a result of the SiC grains slowly oxidising.

Evolution of CO2 from the surface of such a material would

make it difficult for coal ash to bond to the surface and

chemically interact.

Pressing SiC grains into the surface of an aluminosilicate

rammable refractory and then smoothing the surface

reduced the degree of slag/refractory interaction. Adding

400 mesh SiC grains into the surface of a SiC rammable

refractory, and then smoothing the surface, reduced the

slag/refractory adhesion strength.

Acknowledgements

This work was performed with a grant from the ECSC

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

plc for conducting the CTF trials, and for the permission to

publish this paper.

References

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burner quarls. Part 1: characteristics of burner refractories and deposits

taken from utility boilers. FUEL; this issue.

[2] Raask E. Mineral impurities in coal combustion: behaviour, problems,

and remedial measures. New York: Hemisphere Publishing Corpor-

ation; 1985. pp. 243–8.

[3] Lea AC. Trans Brit Ceramic Soc 1941;40(4):93–118.

J. Wells et al. / Fuel 82 (2003) 1867–1873 1873