Factors affecting coal particle ignition under oxyfuel combustion atmospheres

C.K. Man ⇑,

⇑ Corresponding author. Present address: Pittsburgh Research Laboratory,

National Institute for Occupational Safety and Health, PO Box 18070, Pittsburgh,

PA 15236, USA. Tel.: +1 412 386 6628; fax: +1 412 386 6595.

E-mail address: cman@cdc.gov (C.K. Man).

J.R. Gibbins

Department of Mechanical Engineering, Imperial College London, Exhibition Road, London SW7 2AZ, UK

abstract

A set of 13 coals of different rank has been tested for ignition propensity in a 20 L explosion chamber

simulating oxyfuel combustion gas conditions. Their char residues were also analysed thermogravimet

rically. The effects of coal type, coal concentration (from 100 to 600 g/m3

), O2 in CO2 atmospheres (up to

40% v/v) and particle size were investigated.

The higher rank coals were significantly more difficult to ignite and mostly required higher energy

chemical igniters (1000 or 2500 J) whereas the lower rank coals could be ignited with a 500 J igniter even

at low coal dust concentrations.

The minimum explosibility limit/ignition concentration in air varied slightly around a value of

200 g/m3

, a little higher for low volatile coals and a little lower for high volatile coals.

The ignition limit changed significantly, however, with O2 concentration in CO2, where coals required

more oxygen to ignite. Most coals failed to ignite at all in 21% v/v O2 in CO2, but an increase to 30 or

35% v/v O2 gave ignition patterns similar to those in air. In addition, the minimum ignition concentration

decreased with increase in O2. However, a further increase to 40% v/v O2 did not generally affect the

minimum ignition concentration.

Particle size had a non linear effect on coal ignition. The fine particles (<53 lm) behaved almost iden

tical to the whole coal. However, the larger size fraction (>53 lm) was generally more difficult to ignite

and exhibited a much lower weight loss.

1. Introduction

Oxyfuel combustion, also known as oxyfiring, of pulverised fuel

(PF also known as pulverised coal) in relatively conventional

supercritical boilers has been estimated potentially to be able to

achieve cost and performance levels with CO2 capture that are

comparable to those for alternative pre combustion (using inte

grated gasifier combined cycle IGCC) and post combustion

(amine) options [1 3]. Currently the largest oxyfuel trials are at

around 30 40 MW thermal scale, at Schwarze Pumpe in Germany

[4] and Renfrew in Scotland [5]. The latter work is, however, devel

oping a single burner which in multiple units could be used in any

size of plant.

In oxyfuel combustion systems based on air fired plant recycled

flue gas, consisting mainly of CO2 and water vapour, is used to

replace nitrogen from the air and to moderate flame temperatures

[6,7]. Under oxyfuel combustion conditions formation of both SOx

and NOx has been reported to be reduced [8,9]. In addition, high

levels of SOx and NOx removal may be possible by relatively sim

ple processing as the CO2 is compressed for transport and storage

[10].

While oxyfuel burners have similarities to conventional air

burners in some areas, they differ significantly in others such as

different levels of O2 will be encountered in primary and secondary

gas streams. In particular, it is possible that oxygen would not be

added to the primary flue gas recycle stream passing through the

mills for safety reasons (i.e. the certainty of serious explosions if

oxygen levels were accidentally elevated in the mills), or only be

added up to air concentrations (i.e. 21% v/v). In contrast, in conven

tional bituminous coal burners the incoming fuel is ignited using

oxygen pre mixed with the fuel in the primary air. In oxyfuel burn

ers with recycled flue gases as primary ‘air’, oxygen must be mixed

with the fuel after injection giving potentially quite different

combustion patterns. The recycled flue gases replacing the nitro

gen in air would also contain much higher levels of CO2 and water

vapour, both triatomic gases with significantly higher heat capaci

ties than diatomic nitrogen, and possibly also higher dust loadings.

The ability to assess the ignition behaviour of pulverised coal

under oxyfuel combustion conditions, and to predict the behaviour

of different coal types, is therefore important for oxyfuel burner

development and design, and also for oxyfuel plant safety
considerations related to PF handling. This paper examines the ef

fect of coal type and oxygen/CO2 concentrations on ignition in a

laboratory apparatus and an analysis of the properties of the

residual char. The principal objectives of the study were to estab

lish ignition patterns comparable to air and to investigate the effect

on coal rank.

2. Experimental

2.1. Coals

Thirteen coals were used in this study covering a wide range of

coal rank. Six were from North America (USA) and the remainder

from China (CN), the UK, South Africa (SA) and Vietnam (VNM). All

coal samples were supplied in pulverised form,typically 75%passing

through a 75 lm screen. Table 1 shows the basic coal characteristics.

The effect of particle size was investigated using three of the

coals (Freeport, Pittsburgh and Sewell). Samples were sieved by

hand at 53 lm to give large and small particle fractions. This cut

off size was chosen because it gave roughly an equal 50:50 split

in the pulverised fuel, which is normally ground to 75 80% below

75 lm. In addition, particle size distributions for the coals were

determined using a Gilsonic Autosiever TM. The particle size distri

butions, i.e. grind quality for all the coals are illustrated in Fig. 1.

Fig. 1. Cumulative particle size distributions.

2.2. Coal ignition apparatus and test procedures

The Pittsburgh Research Laboratory (PRL) 20 L ignition chamber

(Fig. 2) has been described in detail elsewhere [11 13]. It is almost

spherical in shape and made of stainless steel with a pressure rat

ing of 21 bar (g). The top of the chamber is hinged and opens across

the whole chamber diameter, thus allowing easy access to the inte

rior for sample loading and cleaning after each test. For tests it is

secured to the main body with six bolts. A strain gauge pressure

transducer is used to measure the explosion pressure and rate of

pressure rise. As this measures absolute pressures, it can also be

used to monitor the evacuation of the chamber prior to the addi

tion and dispersion of the test gas. The sapphire windows serve

as viewports and permit temperature measurements using infra

red pyrometers (not used in these tests). Pressure data (and tem

perature if required) can be sampled at a maximum rate of 9 kHz

using a desktop computer. An in house computer program allows

the data to be processed, printed and stored.

Fig. 2. The 20-L ignition apparatus.

Initial ignition/deflagration tests were carried out in air using

whole PF samples. The pulverised coal dust samples were carefully

weighed and placed on the dispersion nozzle at the bottom of the

chamber (Fig. 3). The coal concentration for any test (in g/m3

) is

equal to the mass divided by the chamber volume. After the coal

and igniter have been placed in position, the lid was bolted

securely and the chamber partially evacuated to 13.8 kPa (2 psi).

Table 1

List of coals and properties (n/a = not analyzed, VM = volatile matter, GCV = gross calorific value).

Coal name Coal origin Moisture VM FC Ash VM GCV C H S Cl N O

%ar %ar %ar %ar %daf MJ/kg %daf %daf %daf %daf %daf %daf

Chang Chun CN 0.6 12.2 72.7 14.5 14.4 30.1 90.1 4.3 0.4 0.0 1.7 3.4

Hongai VNM 0.7 6.5 68.4 24.4 8.7 26.1 93.3 3.5 0.7 0.0 0.9 1.5

Kellingley UK 3.3 30.3 50.9 15.5 37.3 28.0 83.1 5.3 2.3 0.4 1.8 7.2

Kleinlopje SA 3.6 21.6 56.2 18.6 27.8 25.8 83.9 5.0 0.4 0.0 1.7 9.0

Qiyi CN 0.5 13.0 67.8 18.7 16.1 28.4 89.1 4.5 0.3 0.5 1.4 4.2

Tower UK 0.5 9.1 84.1 6.3 9.8 33.4 91.8 3.9 0.9 0.0 1.3 1.9

WFD UK 0.7 8.4 84.6 6.3 9.0 33.4 92.5 3.8 0.9 0.0 1.3 1.4

Freeport USA 0.6 22.8 69.1 7.5 24.8 n/a 88.0 4.8 1.2 0.0 1.5 4.5

Pittsburgh USA 1.8 35.4 57.5 5.3 38.5 n/a 84.0 5.5 1.2 0.0 1.7 7.7

Pocahontas USA 0.9 18.9 74.8 5.4 20.2 n/a 90.5 4.1 0.9 0.0 1.2 3.2

PRB USA 11.3 37.5 45.4 5.9 45.2 n/a 74.2 4.8 0.3 0.0 1.1 19.7

Sewell USA 1.4 29.7 62.0 6.9 32.4 n/a 86.1 4.8 0.8 0.0 1.7 6.6

Utah USA 2.2 38.5 52.9 6.4 42.1 n/a 81.7 5.5 0.8 0.0 1.9 10.2
Fig. 3. Inside of ignition chamber with coal and igniter in position.

15 

10 

5 

0 

0 5 10 15 

raw data 

best fit 

Coal loaded, g 

Fig. 4. Coal loading and collection correlation.

Coal recovered, g 

c

Weight loss (wt% daf) Pressure ratio

Q-factor

0 100 200 300 400 500 600 0 100 200 300 400 500 600 

Concentration (g/m3) Concentration (g/m3) 

100 Chang Chun 500J air 80 21%O2-CO2 

30%O2-CO2 1000J 

60 35%O2-CO2 

40%O2-CO2 2500J 40 

Effect of coal concentration on 20 a) pressure ratio 

b) Q-factor 0 

0 100 200 300 400 500 600 c) weight loss 

Concentration (g/m3) 

Fig. 5. Chang Chun coal – effect of coal loading.

0 

2 

4 

6 

8 

10 

0 

2 

4 

6 

a b 

Weight loss (wt% daf)

Q-factor

Pressure ratio

a 

c 

b 

10 6 

8 

4 6 

4 2 

2 

0 0 

0 100 200 300 400 500 600 0 100 200 300 400 500 600 

Concentration (g/m3) Concentration (g/m3) 

100 Freeport 500J air 80 21%O2-CO2 

30%O2-CO2 1000J 

60 35%O2-CO2 

40%O2-CO2 2500J

40 

Effect of coal concentration on 20 a) pressure ratio 

b) Q-factor 0 

c) weight loss 0	 100 200 300 400 500 600
 

Concentration (g/m3)
 

Fig. 6. Freeport coal – effect of coal loading.
10 

8 

6 

4 

2 

0 

0 100 200 300 400 500 600 

Concentration (g/m3) 

a 

100 

80 

60 

40 

20 

0 

c 

0	 100 200 300 400 500 600 

Concentration (g/m3) 

Weight loss (wt% daf)

Q-factor

Pressure ratio

6 

4 

2 

0 

b 

0 100 200 300 400 500 600 

Concentration (g/m3) 

500J Hongai
 

air
 

35%O2-CO2
 1000J 

40%O2-CO2 

2500J 

Effect of coal concentration on 

a) pressure ratio 

b) Q-factor 

c) weight loss 

Fig. 7. Hongai coal – effect of coal loading.

Concentration (g/m3)

0 100 200 300 400 500 600 

Concentration (g/m3) 

0 

2 

4 

6

Q-factor

Kellingley 

air 

35%O2-CO2 

40%O2-CO2 

Effect of coal concentration on 

a) pressure ratio 

b) Q-factor 

c) weight loss 

c 

b 

1000J 

2500J 

500J

Weight loss (wt% daf) Pressure ratio

10 

8 

6 

4 

2 

0 

0 100 200 300 400 500 600 

a 

100 

80 

60 

40 

20 

0 

0 100 200 300 400 500 600 

Concentration (g/m3) 

Fig. 8. Kellingley coal – effect of coal loading.

A short blast of dry air (0.3 s at 758.4 kPa (110 psi) from a 16 L re

serve tank) was use to disperse the dust and raise the chamber

pressure to about 1 bar (a). After an additional delay of 0.1 s the

igniter was activated electrically.

Ignition tests were also carried out in 21, 30, 35 and 40% v/v O2

in CO2. For tests using O2/CO2 mixtures, pre mixed cylinders of the

appropriate concentrations were obtained. The switch from using

air to other gases required a few small modifications to the normal

ignition chamber test procedure. The reserve tank was flushed out

several times before being filled with the gas mixture used to dis

perse the coal. In addition, the chamber was evacuated totally be

fore it was back filled to 13.8 kPa (2 psi) with the gas mixture

followed by dispersion and ignition.

2.3. Igniters

The pyrotechnic igniters were manufactured by Fr. Sobbe,

Germany and consist of 40 wt% zirconium, 30 wt% barium nitrate

and 30 wt% barium peroxide. These generate a large number of

hot particles with minimal increase in gas volume through the fol

lowing reaction:

3Zr þ ð Þ þ ! þ þ Ba NO3 2 BaO2 2BaO 3ZrO2 N2

The igniters are activated electrically with an internal fuse wire

with a delay of about 0.01 s. A 2500 J igniter can be considered to

be equivalent to about 20 matches all ignited at once.

2.4. Weight loss measurements in ignition tests

Weight loss i.e. devolatilisation from the ignition process can be

calculated in two ways. One is by the ‘ash tracer’ method, which

assumes the ash material from the coal to be inert.

Ash tracer weight loss; wt%db 100 x ð1 coal ash=char ashÞ

However, this method is often unreliable [14,15], especially for

coals with low ash contents where the error bars are very wide.

Igniter residue material, barium oxide and zirconium oxide, is also
not readily distinguished from the coal ash in the samples collected

from ignition tests.

An alternative collection of the char residue for direct weighing

to derive the weight loss was investigated. It is still necessary to

correct for the igniter residue, but this can be done by simple

chemical stoichiometry; a 500 J igniter contains 120 mg of reac

tants and larger igniters pro rata. In addition, since it is not possible

to collect absolutely all of the material, a collection factor, essen

tially a collection efficiency, was found to be necessary. A formula

for this was established by carrying out ‘blank’ runs where coal

samples were dispersed in the 20 L chamber but without any ignit

ers using about 2, 4, 8 and 12 g of coal which is equivalent to dust

concentrations of 100, 200, 400 and 600 g/m3 respectively. A good

correlation was obtained between the coal loaded and the coal col

lected (Fig. 4) and a linear collection factor correction equation was

derived, as shown below:

Corrected mass recovered mass collected x 1:029 þ 0:458 g

igniter residue

From this, the weight loss from the ignition experiments can easily

be obtained by the equation below.

Weight loss; wt%ad

100 1x ðcorrected mass recovered=coal loaded Þ

Finally, this can be converted to a dry as free basis by the following

equation:

100 x weight lossðwt%adÞ moisture Weight loss; wt%daf 100 ðcoal ash þ moistureÞ

2.5. Char sample TGA tests, residual volatile matter and Q factor

calculations and interpretation

All the residual char samples from the ignition tests were ana

lysed in a Mettler Toledo TGA/SDTA851e thermogravimetric ana

lyser to give micro proximate data [16]. Samples (typically

10 mg) were heated in a 75 ll alumina crucible inside a TGA fur

nace with a flow of high purity nitrogen to 105 °C then to 900 °C

0 100 200 300 400 500 600 

Concentration (g/m3) 

0 

2 

4 

6

Q-factor

Pressure ratio

Kleinkopje 

air 

35%O2-CO2 

40%O2-CO2 

Effect of coal concentration on 

a) pressure ratio 

b) Q-factor 

c) weight loss 

1000J 

2500J 

500J 

Fig. 9. Kleinkopje coal – effect of coal loading.

10 

8 

6 

4 

2 

0 

a 

0 100 200 300 400 500 600 

Concentration (g/m3) 

100 

80 

60 

40 

20 

Weight loss (wt% daf)

0

c 

0	 100 200 300 400 500 600 

Concentration (g/m3) 

b 

Q-factor

Pressure ratio Weight loss (wt% daf)

10 

8 

6 

4 

2 

0 

0 100 200 300 400 500 600 

a 

100 

80 

60 

40 

20 

0 

0 100 200 300 400 500 600 

Concentration (g/m3) 

Concentration (g/m3) 

c 

6 

4 

2 

0 

b 

0 100 200 300 400 500 600 

Concentration (g/m3) 

Pittsburgh 500J air
 

21%O2-CO2
 

30%O2-CO2
 1000J 

35%O2-CO2
 

40%O2-CO2
 2500J 

Effect of coal concentration on 

a) pressure ratio 

b) Q-factor 

c) weight loss 

Fig. 10. Pittsburgh coal – effect of coal loading.
(at 30 °C/min), to give values for moisture and volatile matter con

tent respectively. Finally, introduction of air resulted in 100% burn

off and allowed the fixed carbon and ash values also to be

calculated.

Micro proximate ash contents can be used to estimate weight

loss during the ignition test but, as discussed above, this appears

to be a less reliable method than direct weighing. Residual volatile

matter contents can be used to estimate the extent to which the

potential volatile release from the coal has been completed.The

Q factor is the ratio of coal volatile matter release, which repre

sents the efficiency of volatile release compared to standard prox

imate analysis [17,18].

weight loss from devolatilisationð%daf coalÞ Q factor Coal VMð%dafÞ Char VMð%daf of original coalÞ

This should not be confused with the R factor which is simply the

ratio of measured volatiles divided by the proximate volatiles

[17,19].

weight loss from devolatilisationð%daf coalÞ R factor Coal VMð%dafÞ

3. Results and discussion

The relationship between coal loading on ignition and the effect

of air and oxygen concentration in CO2 for all 13 coals are shown in

Figs. 5 17. A pressure ratio (PR) of two and above i.e. a doubling of

the pressure inside the chamber can be regarded as a positive igni

tion test. It was possible to ignite all coals in the 20 L chamber

Weight loss (wt% daf)

Q-factor

Pressure ratio

10
 

8
 

6
 

4
 

2
 

0
 

a 

0 100 200 300 400 500 600 

Concentration (g/m3) 

100 

80 

60 

40 

20 

0 

c 

0	 100 200 300 400 500 600 

Concentration (g/m3) 

Fig. 11. Pocahontas coal – effect of coal loading.

6 

4 

2 

0 

b 

0	 100 200 300 400 500 600
 

Concentration (g/m3)
 

Pocahontas 500J air 

21%O2-CO2 

30%O2-CO2 1000J 

35%O2-CO2 

40%O2-CO2 2500J 

Effect of coal concentration on 


a) pressure ratio
 

b) Q-factor
 

c) weight loss
 

Weight loss (wt% daf)

Q-factor

Pressure ratio

10 

8 

6 

4 

2 

0 

0 100 200 300 400 500 600 

a 

100 

80 

60 

40 

20 

0 

Concentration (g/m3) 

c 

0 100 200 300 400 500 600 

Concentration (g/m3) 

6 

4 

2 

0 

b 

0 100 200 300 400 500 600 

Concentration (g/m3) 

PRB 

air 500J 

21%O2-CO2 

30%O2-CO2 1000J 

35%O2-CO2 

40%O2-CO2 2500J 

Effect of coal concentration on 

a) pressure ratio 

b) Q-factor 

c) weight loss 

Fig. 12. PRB coal – effect of coal loading.
using air as the dispersal medium, with the exception of Hongai

coal (Fig. 7) which did not ignite even with the 2500 J igniter.

The higher rank coals were significantly more difficult to ignite,

as one might expect [20], but most ignited in air using the higher

energy chemical igniters (1000 or 2500 J) whereas the lower rank

coals could be ignited with a 500 J igniter even at low coal dust

concentrations.

The minimum explosibility limit (MEL)/ignition concentration

in air varied slightly for each coal. This was mostly around the

200 g/m3 level, a little higher for low volatile coals and a little low

er for high volatile coals. The ignition limit, on the other hand,

changed significantly with O2 concentration.

Only the high volatile coals Pittsburgh (Fig. 10), PRB (Fig. 12),

Sewell (Fig. 14) and Utah (Fig. 16) ignited in 21% O2/CO2 and all re

quired the large 2500 J igniters (the Kellingley coal was not tested

under these conditions). However, an increase in O2/CO2 levels to

30 or 35% gave ignition patterns similar to that carried out in air

for all coals. In addition, the minimum ignition concentration de

creased by about 100 g/m3 with these higher O2 levels compared

to the corresponding runs in air. Interestingly, further increase in

O2/CO2 from 35 to 40% did not have much additional effect on

the minimum ignition concentration.

Above the minimum ignition concentration the peak PR for any

given oxygen concentration had a tendency to only increase

slightly if at all and this appears to be independent of coal rank.

However, there appeared to be a slight increase in PR with increas

ing oxygen concentration.

The char residues from the ignition tests showed a wide range

of Q factors. Low rank, high volatile coals such as Pittsburgh

(Fig. 10), PRB (Fig. 12) and Utah (Fig. 16) have low Q factors, typi

cally around 2. In addition, this value appears to be almost con

stant for all tests for these coals, regardless of coal loading and

oxygen concentration. A value of 2 and the invariance suggests that

mainly devolatilisation is taking place.

10
 

8
 

6
 

4
 

2
 

0
 

a 

0 100 200 300 400 500 600 

Concentration (g/m3) 

100 

80 

60 

40 

20 

0 

c 

0	 100 200 300 400 500 600 

Concentration (g/m3) 

Fig. 13. Qiyi coal – effect of coal loading.

Weight loss (wt% daf)

0 100 200 300 400 500 600 

Concentration (g/m3) 

0 

2 

4 

6

Q-factor

Pressure ratio

Qiyi 

air 

21%O2-CO2 

30%O2-CO2 

35%O2-CO2 

40%O2-CO2 

Effect of coal concentration on 

a) pressure ratio 

b) Q-factor 

c) weight loss 

b 

1000J 

2500J 

500J 

10 

8 

6 

4 

2 

0 

0 100 200 300 400 500 600 

a 

100 

80 

60 

40 

20 

0 

Concentration (g/m3) 

c 

0 100 200 300 400 500 600 

Concentration (g/m3) 

6 

4 

2 

0 

b 

0 100 200 300 400 500 600 

Concentration (g/m3) 

Sewell 500J air
 

21%O2-CO2
 

30%O2-CO2
 1000J 

35%O2-CO2
 

40%O2-CO2
 2500J 

Effect of coal concentration on 

a) pressure ratio 

b) Q-factor 

Weight loss (wt% daf)

c) weight loss

Q-factor

Pressure ratio

Fig. 14. Sewell coal – effect of coal loading.
The medium rank coals Chang Chun (Fig. 5), Pocahontas

(Fig. 11), and Qiyi (Fig. 13) exhibited higher Q factors which range

from about 3 to 5 and are again in a narrow band, but these show a

definite negative slope with coal loading. The high rank low vola

tile coals Tower (Fig. 15) and WFD (Fig. 17) have very high Q fac

tors which spread from below 5 to over 10. Again there is a

negative slope with coal loading. For the Tower coal there appears

to be an additional trend of increase in Q factor with oxygen

concentration.

The weight loss results show a similar trend to the pressure

ratio data (Figs. 5 17). There is sharp increase in weight loss at

the minimum explosibility limit but this often levels off or falls

slightly with increase in coal loading.

There appears to be a maximum weight loss of around

40 50%daf for many of the coals. This is certainly not unexpected

or unusual for high volatile coals such as Pittsburgh (Fig. 10),

Sewell (Fig. 14) and Utah (Fig. 16). This equates to R factors of about

1.3, which has been confirmed in other studies [19] as being a rea

sonable increase over proximate VM yields for rapid heating. How

ever, the low volatile coals such as Chang Chun (Fig. 5) and Tower

(Fig. 15) give almost the same amount of weight loss, typically

around 30 40%daf, which appear high relative to their proximate

volatile matter contents. For these coals, the R factors are about 3.

There is a general trend for weight loss to increase with increase

in oxygen concentration. For some coals, weight loss in 40% O2/CO2

can be as much as 20% higher than the corresponding experiment

carried out in air (i.e. constant coal loading and igniter strength).

One possible explanation for this extraordinarily high weight

loss is heterogeneous combustion in addition to devolatilisation.

Although the 20 L chamber only attains temperatures suitable

for heterogeneous combustion to take place (approxi

mately >400 500 °C) for a short period of time (estimated to be

about 1 s), a small amount of char combustion cannot be ruled

out. Perhaps even more surprising is that this occurs across the

10
 

8
 

6
 

4
 

2
 

0
 

a 

0 100 200 300 400 500 600 

Concentration (g/m3) 

100 

80 

60 

40 

20 

0 

c 

0	 100 200 300 400 500 600 

Concentration (g/m3) 

Fig. 15. Tower coal – effect of coal loading.

Weight loss (wt% daf)

0 100 200 300 400 500 600 

Concentration (g/m3) 

4 

6 

8 

10

Q-factor

Pressure ratio

Tower 

air 

21%O2-CO2 

30%O2-CO2 

35%O2-CO2 

40%O2-CO2 

Effect of coal concentration on 

a) pressure ratio 

b) Q-factor 

c) weight loss 

b 

1000J 

2500J 

500J 

10 

8 

6 

4 

2 

0 

0 100 200 300 400 500 600 

a 

100 

80 

60 

40 

20 

0 

Concentration (g/m3) 

c 

0 100 200 300 400 500 600 

Concentration (g/m3) 

Weight loss (wt% daf)

Q-factor

Pressure ratio

6 

4 

2 

0 

b 

0 100 200 300 400 500 600 

Concentration (g/m3) 

Utah 

air 500J 

21%O2-CO2 

30%O2-CO2 1000J 

35%O2-CO2 

40%O2-CO2 2500J 

Effect of coal concentration on 

a) pressure ratio 

b) Q-factor 

c) weight loss 

Fig. 16. Utah coal – effect of coal loading.
Pressure ratio Weight loss (wt% daf)

0	 100 200 300 400 500 600 0 100 200 300 400 500 600 

Concentration (g/m3) Concentration (g/m3) 

100 WFD 500J

c air 80 21%O2-CO2 

30%O2-CO2 1000J 

60 35%O2-CO2 

40%O2-CO2 2500J

40 

Effect of coal concentration on 20 a) pressure ratio 

b) Q-factor 0 

0 100 200 300 400 500 600 c) weight loss 

Concentration (g/m3) 

Fig. 17. WFD coal – effect of coal loading. Q-factor

6 

8 

10 

12

0 

2 

4 

6 

8 

10 

a b 

Coal Oxygen concentration (% v/v)

21 in N2 30 in CO2 35 in CO2 40 in CO2

Chang Chun

Freeport

Hongai

Kellingley

Kleinlopje

Pittsburgh

Pocahontas

PRB

Qiyi

Sewell

Tower

Utah

WFD

d

Table 2

Ignition ‘‘goes” under different oxygen concentrations ( = no ignition, = 50%

ignition, = 100% ignition, empty space means no data).

set of coals tested, even the low volatile coals which are generally

considered to be less reactive.

Comparing the weight loss data with oxygen concentration, it

appears that the ignition pattern for tests carried out in air are sim

ilar to those done in the 30 or 35% O2/CO2 mixtures depending on

the coal. The low volatile coals appear to need a slightly higher le

vel of oxygen i.e. 35% to match the data in air whereas the higher

volatile coals in general only ‘need’ about 30% oxygen. This ‘‘ignit

ability” data has been summarized in Table 2 which shows this

trend more clearly.

Low volatile coals are known to be difficult to ignite. Therefore,

the five low volatile coals have been ranked in order of ignitability

from the ignition experiments carried out in air. To minimize

external influences, the data chosen consisted of tests using a com

mon strength of igniter (1000 J). Hongai (Fig. 7) did not ignite with

the 2500 J igniter so it was not tested with the 1000 J igniter. It has

been included in this comparative study for completeness. All fac

tors were considered, including pressure ratio, minimum explosive

limit, and weight loss. Table 3 shows the five coals listed in order of

ease of ignition (with the easiest first).

The order generally follows coal rank with the highest volatile

matter content coal being the easiest to ignite. Chang Chun appears

to be the easiest coal to ignite with fairly high PR values and a MEL

of about 300 g/m3

. Tower was a close second using these criteria.

Table 3

Low volatile coal ignitability ranking.

Coal ignitability TGA prox Wt.%daf TGA volatile released at, °C

(top = most easy) VM%daf 500 550 600

Chang Chun 15.5 2.49 5.47 8

Tower 11.0 2.01 3.01 4.4

Qiyi 16.1 2.9 6.29 8.95

WFD 9.1 0.77 1.55 2.92

Hongai 8.3 2.33 3.29 4.05	 

Table 4

Coal ranking by ignition index.

Coal ignitability 

(top = most easy)

TGA prox 

VM%daf

Ignition index for two igniter energies

1000 J 2500 J Mean

ChangChun 15.5 243 306 274

Tower 11.0 300 243 271

Qiyi 16.1 211 232 221

WFD 9.1 176 245 210

Hongai 8.3 0 0 0
The ignition at 200 g/m3 may be an anomaly because it was not

repeatable using the 2500 J igniter. Therefore, its MEL was roughly

the same as Chang Chun at about 300 g/m3

.

Tower was judged to be better than Qiyi in ignition, despite

having a lower volatile matter content from the TGA results (which

might have suggested the opposite). Again this assessment was

based on the respective PR, weight loss and minimum explosive

limits, which mostly places Tower to be easier to ignite of the

two coals. The MEL for both Qiyi and WFD is about 400 g/m3

. How

ever WFD gave lower PR values and it did not ignite at 600 g/m3

with the 1000 J igniters. This further weakened its ignition propen

sity ranking. In general, WFD required the 2500 J igniters to give a

successful ignition in air. As mentioned before, Hongai did not

ignite even with 2500 J igniters. This is attributed to its low volatile

content and relatively coarser grind (about 72% below 75 lm,

Fig. 1) a poor combination of factors.

The partial TGA volatile release values at three different tem

peratures shown in Table 3 have been added to evaluate whether

the amount of volatiles released at relatively low temperatures

had an effect on the coals ignitability. The general trend appears

similar to the overall volatile matter content.

An attempt to quantify this coal ignition ranking has been made

by introducing an ignition index [21]. This is intended to make the

ignition ranking process less subjective.

Ignition indexðin airÞ

maximum weight loss from ignitionðwt%dafÞ x PR

The results are shown in Table 4. Interestingly the order has not

changed but this way of ranking the coals does have the advantage

of allowing coals to be both compared and contrasted from an igni

tion point of view. From the mean values, two of the coals, Chang

Chun and Tower, appear to have similar ignitability. Two other

coals Qiyi and WFD also appear to be comparable in ignitability

from this evaluation method.

Particle size appears to have a small effect on coal ignition.

Overall, the <53 lm fraction behaved very similar to the whole

coal, i.e. there was no additional ‘fines’ effect with this sample. This

might suggest that the ignition process is almost insensitive to par

ticle size as the weight losses are very close between the whole

coal and the <53 lm fraction for the three coals studied (Figs.

18 20). However, the large size fractions (>53 lm) of a coal were

generally slightly more difficult to ignite and always resulted in a

significantly lower weight loss compared to its corresponding Wt loss, %daf

70 

60 

50 

40 

30 

20 

10 

0 

Ignition in air, igniter J 

Freeport >53μm coal (1000J) 

Freeport <53μm coal (1000J) 

whole coal 500J 

whole coal 1000J 

100 200 300 400 500 600 

Coal concentration, g/m3 

Fig. 18. Effect of particle size on weight loss for Freeport coal.

Wt loss, %daf

70 

60 

50 

40 

30 

20 

10 

0

100 200 300 400 500 600 

Ignition in air, igniter J 

Pittsburgh >53μm coal (500J) 

Pittsburgh <53μm coal (500J) 

whole coal 500J 

Coal concentration, g/m3 

Fig. 19. Effect of particle size on weight loss for Pittsburgh coal.

100 200 300 400 500 600 

Coal concentration, g/m3 

0 

10 

20 

30 

40 

50 

60 

70 Ignition in air, igniter J 

Sewell >53μm coal (500J) 

Sewell <53μm coal (500J) 

whole coal 500J 

Wt loss, %daf

Fig. 20. Effect of particle size on weight loss for Sewell coal.

small size fraction (<53 lm). As with the whole coal ignition tests,

the weight loss peaks at the minimum explosibility level (around

200 g/m3 for two of the set of three coals examined) and gradually

decreases with increase in coal loading. Pittsburgh was slightly dif

ferent (Fig. 19). Although the <53 lm fraction had a MEL level of

about 200 g/m3 like the others, the large particle size fraction

was more difficult to ignite and the explosibility limit was

increased to about 300 400 g/m3

.

4. Conclusion

The PRL ignition apparatus can be used to assess and rank coals

in order of ease of ignition in air and in O2/CO2 mixtures from 21 to

40% v/v. The ignition order generally followed coal rank. There

were relatively smaller differences than expected between the dif

ferent coals at higher O2 levels since the low volatile coals showed

higher than expected weight losses.

Almost all the coals ignited in O2 in CO2 at some point. The con

centration of O2 in CO2 that gave a similar ignition comparable to

that in air was established to be between 30 and 35%. This is con

sistent with recent data reported by CANMET, which concluded

that the heat flux from oxyfuel experiments between 28 and 35%

O2 in CO2 was comparable to that carried out in air [22]. Few coals
ignited in 21% O2 in CO2 even with the large 2500 J igniter. This has

important safety implications for coal mills in an oxyfuel power

plant, since it could be dangerous if coal ignited inside the mills

or in the adjacent pipework.

Particle size had a small but nevertheless notable effect on coal

ignition with the finer particles behaving almost identical to the

bulk coal but ‘large’ particles (>53 lm) tended to be more difficult

to ignite.

Acknowledgements

This project was funded by BCURA with industrial support from

Mitsui Babcock Energy Limited (now Doosan Babcock Energy Lim

ited). The authors thank Gregory Green of NIOSH for providing the

particle size data.

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