A comparison between ignition behaviours
 

of 7 different UK and World-Traded coals in air,
 

and in a mixture of oxygen and carbon dioxide gases
 

representative of oxy-combustion conditions
 

Jon Gibbins1 and Niall McGlashan1 Mark Flower1*, 

1

Energy Technology for Sustainable Development Group, 


Mechanical Engineering Department, Imperial College London, London, UK
 

Chi Man2†, 

2

National Institute for Occupational Safety and Health, 


Pittsburgh Research Laboratory, Pittsburgh, PA 15236, USA
 

† Disclaimer: "The findings and conclusions in this publication have not been formally disseminated by 

NIOSH and should not be construed to represent any agency determination or policy." 

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*Mark.Flower02@Imperial.ac.uk, +44 (0)2075 941 618 

1. Abstract 

A 20L ignition test chamber has been used to test suspensions of 7 UK and world traded 

coals in air and O2/CO2 mixtures typical of oxy-combustion conditions. The coals varied 

in rank from sub-bituminous to bituminous and were tested in varying concentrations 

from the ignition limit to 400g/m3

. Following each successful test the combustion 

residue was collected, weighed and analysed within a thermogravimetric analyser, 

allowing Q factors to be estimated. 

The ignition limit varied slightly in air, but was mostly around the 200g/m3 level. With 

the exception of Coal C, which was much harder to ignite, it correlated roughly with coal 

rank. The ignition limit changed significantly with O2 concentration when in mixed 

O2/CO2 gases. Only a few high volatile coals ignited in 21% O2/CO2 v/v, and then only 

with a 2500 J, rather than a 1000J, igniter. 

An increase in O2/CO2 levels to 30 or 35% gave ignition patterns similar to those carried 

out in air with a further increase to 40% having little additional effect. In addition the 

minimum ignition concentration decreased with increase in O2. 

Heterogeneous combustion or gasification of the coal by CO2 appears to be confirmed by 

comparing weight loss results for air and O2/CO2 mixtures respectively for an equivalent 

peak pressure rise. 

2. Introduction 

Oxy-combustion consists of the combustion of coal in either oxygen or more likely 

O2/CO2 mixtures. This produces a flue gas with high levels of CO2 and low levels of 

inert gases, particularly nitrogen. Oxyfuel combustion therefore reduces the cost and 

energy penalty inherent in actually capturing CO2; however, there is a trade off due to the 
requirement for an air separation unit to generate the necessary oxygen. Oxyfuel is 

currently less well-developed than IGCC and post combustion capture, with oxyfuel 

burner design a key factor affecting the technical feasibility of the process. 

The recycling and replacement of the oxidant stream’s atmospheric nitrogen with flue 

gases will have a major effect upon combustion. Diatomic nitrogen has a lower molar 

heat capacity than the mainly triatomic flue gas. This is the primary reason for the higher 

O2 concentrations shown by this study to be necessary. The ability to accurately model 

the ignition behaviour of pulverised coal, and to predict the behaviour of different coal 

types, will be critical for oxyfuel burner and development. 

3. Samples and experimental programme 

Seven coals from a range of sources, but mostly typical of those burned in UK power 

stations have been tested in a 20 litre ignition chamber. The set of coals covered those 

with volatile matter contents ranging from below 30 to ca. 50 wt% daf – spanning the 

range of bituminous to sub-bituminous coals. These coals and their proximate analysis 

are listed in Table 1.

Table 1 - Proximate analysis of the coal samples in rank order 

Coal Moisture VM FC Ash VM FC

(%ar) (%db) (%db) (%db) (%daf) (%daf) 

A 7.4 44.4 45.9 9.7 49.1 50.9 

B 3.3 36.7 51.9 11.4 41.5 58.5 

C 2.6 33.0 53.1 13.9 38.3 61.7 

D 1.7 32.7 52.7 14.6 38.3 61.7 

E 1.6 30.3 49.4 20.3 38.0 62.0 

F - 34.7 60.2 5.1 36.6 63.4 

G 1.7 24.8 57.5 17.8 30.1 69.9 

 All coal samples were supplied in pulverised form. 

The following were investigated: 

•	 Effect of coal rank, 

•	 Effect of coal concentration (from the ignition limit to 400g/m3 in O2/CO2 gases, 

and the ignition limit to 600g/m3 in air), 

•	 Effect of oxygen concentration (air, 21.5, 26.5, 30.2, 34.9 and 40.0% v/v O2 in 

CO2 were studied for all coals except Coal F, which was only studied in air) 

The 20 litre ignition chamber has been described previously [Cashdollar, K.L. (1996)]. 

Briefly, the chamber is near spherical with a hinged top, which is bolted down during 

experiments (Figure 1a). The coal dust is weighed and placed on the dispersion nozzle at 

the bottom of the chamber. An electrically fired igniter is connected to the control 

system and the lid closed. 

For tests in air the chamber was partially evacuated to ca. 0.14 bar a before a short blast 

of dry air 0.3 s at 7.6 bar(g) dispersed the dust and raised the chamber pressure to about 1 

bar(a). After an additional delay of 0.1 s the igniter was fired. Tests in the various O2 in 

CO2 mixtures were almost identical, except that prior to ignition the chamber was 

completely evacuated before being back filled to ca. 0.14 bar(a) with the chosen gas. 

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a 

a b 

Figure 1 - NIOSH 20 litre ignition chamber a) outside b) inside (with coal and igniter) 

A desktop computer sampled a strain gauge (which measures the pressure within the 

chamber) at a maximum rate of 9 kHz throughout the firing process. This allows 

calculation of the peak pressure ratio and rate of pressure rise. 

The chemical igniters were manufactured by Fr. Sobbe, Germany. When activated a 

large number of hot particles are generated with little increase in gas volume. The 

majority of tests, using both air and O2/CO2 atmospheres, were conducted using 1000J 

igniters. A series of additional tests were conducted with all coals except Coal F using 

2500J igniters in 21.3% v/v O2 in CO2. The only difference between the different igniters 

was the mass of chemical charge inside the capsule unit. 

After the ignition experiments were completed the char residues were collected, weighed 

and stored. Micro-proximate analysis was then undertaken for each char sample at 

Imperial College with a Mettler-Toledo TGA/SDTA851e thermogravimetric analyser. 

Between 10 and 30mg of each sample was placed in a 75μl alumina crucible, before 

being heated under nitrogen. The samples were first heated to 105 ̊C, before volatile 

content was measured by further heating the sample (at 20K/min) to 900 ̊C. After 45 

minutes, air was introduced to burn off the fixed carbon leaving just the ash and igniter 

residue. 

Weight loss from the ignition process was calculated by collecting the sample and 

comparing with initial mass – as previously completed by Man et al. (2007). This 

method still has a number of problems, including accounting for the igniter residues and 

the need to determine collection efficiency. A vacuum cleaner filter was designed to aid 

collection. Non-woven bags were used to collect the sample, with the filter being 

weighed prior to, and after, utilisation. This dramatically increased the speed of 

experimentation. 

The collection efficiency was established by dispersing and then collecting coal without 

igniters at concentrations varying from 100 – 600 g/m3

. Separately igniters were fired on 

an empty chamber before coal was loaded and dispersed. Collection efficiency was then 

measured. The results were pooled and found to be strongly linear (and nearly identical) 

with a coefficient of determination about a correction equation (eqn. 1) of 0.999. 

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Corrected mass =1.035 ×mass collected +0.2957 (1) 

 

Weight loss, assumed to be from devolatilisation, can then be readily calculated to give a 

dry ash free value. 

The Q factor can then be evaluated. It is known to be a multiplier representing the 

greater efficiency of volatile production/release during rapid heating compared to the 

standard proximate analysis [Gibbins et al. (1993)]. It is defined in eqn. 2. 

 

weight loss from devolatilisation (wt %daf ) Q factor = (2) Coal VM (wt %daf ) −Char VM (wt %daf of original coal )

4. Results and discussion 

Whilst the higher rank coals were more difficult to ignite, all ignited in air using a 

chemical igniter of 1000 J. The minimum explosibility limit (MEL) in air varied, but was 

around 200g/m3

. In this study ignition is defined as: a pressure ratio, PR> 2 and pressure 

rise rate, Kst > 1.5 bar m/s (where PR is the maximum pressure divided by initial ignition 

pressure, and Kst is the rate of pressure rise normalised to the explosion chamber 

volume). 

The ignition limit, on the other hand, changed significantly with O2 concentration. Only a 

few of the high volatile coals ignited in 21% O2/CO2 and all of these required a 2500 J 

igniter. However, an increase in O2/CO2 levels to 30 or 35% gave ignition patterns 

similar to those when air was the oxidant. In addition, the minimum ignition 

concentration decreased by about 100g/m3 with increase in O2 compared to the 

corresponding runs in air. Interestingly, further increase in O2/CO2 from 35 to 40% did 

not significantly change the minimum ignition concentration. Above the minimum 

ignition concentration, the peak PR for any given O2 concentration had a tendency to 

form a plateau independent of coal rank. 

The devolatilisation results show a similar trend to the pressure ratio data. There is sharp 

increase in weight loss at the minimum explosibility limit that appears broadly 

independent of, or falls slightly with increase in coal loading. In air there appears to be a 

maximum weight loss of around 50 to 60% daf for most of the coals. However, there is a 

general trend for weight loss to increase with increase in oxygen concentration. For some 

coals, the 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). 

Comparing the weight loss data with oxygen concentration, it appears that the ignition 

pattern carried out in air are similar to those done in the ca. 30% O2/CO2 mixtures 

depending on the coal. The low volatile coals appear to need a slightly higher amount of 

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oxygen (up to 35%) to match the data in air whereas the higher volatile coals in general 

only ‘need’ about 30% oxygen. However, this trend is difficult to discern with certainty. 

The char residues from the ignition tests exhibited a fairly narrow range of apparent Qfactors (apparent, as this assumes all the weight loss was in fact due to devolatilisation 

alone). The lower rank coals (such as Coal A and B) have low Q-factors typically around 

2. The higher rank coals (Coal C, D, E and G) exhibit slightly higher Q-factors, which 

range from about 2.5 to 3.5. All of the coals show a Q-factor with a definite negative 

slope with coal loading (shown for Coal F and G in Figure 3), though this effect is more 

pronounced in the higher rank coals. 

The effect of O2 concentration is far from clear but there does appear to be a trend of 

increasing Q-factor with higher oxygen concentration. It is unclear from the data how 

this trend is affected by variation in coal rank. The Q factor is known not to be affected 

by the extent of weight loss in situations where only devolatilisation is taking place. 

Heating rates (which would also affect Q factor) are likely to be similar in all cases, and 

as such it appears that the actual Q factor for the coals in these experiments is at, or lower 

than, the minimum value observed (allowing for experimental scatter). The recorded 

higher apparent Q factors then indicate heterogeneous combustion of some of the sample, 

proceeding in parallel with devolatilisation of other particles in the sample (and possibly, 

but not necessarily, in parallel with devolatilisation of the heterogeneously-combusted 

particles). 

Although the 20-L chamber only attains temperatures suitable for heterogeneous 

combustion to take place for a short period of time, a small amount of char combustion 

cannot be ruled out given the significantly enhanced oxidation potential of an O2/CO2 

atmosphere compared to an O2/N2 atmosphere. 

To visualise the effect of oxygen on coal ignition, the data has been plotted (Figure 2) so 

that the effect of coal loading and rank is removed and the O2 / CO2 data is shown 

relative to that carried out in air. 

Table 2 shows the oxygen concentration for performance similar to air. It also illustrates 

how similar the coals were in ignition behaviour. 

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Table 2 - Level of CO 2 for ignition equivalent to air 

Coal TGA VM (%daf) % O 2 in CO2 equivalent to air 

A 44.4 30 

B 33.7 25 to 30 

C 32.6 35 to 40 

D 30.8 30 to 35 

E 31.8 30 to 35 

G 25.8 30 to 35 
 

(Rp

)

 (Rp)

 

 

 

 

7 

8 

80 

3.5 

21.6% 

26.5% 

30.2% 

34.9% 

40.0% 

1 2 3 4 5 6 7 8 

26.5% 

30.2% 

34.9% 

40.0% 

Q-Factor O 2 

/CO 

2 

Pressure Ratio O /CO (PR) 2 2 

6 3

5 

4 

3 

2.5 

2 

2 

1 1.5 

1.5 2 2.5 3 3.5 

a) Pressure Ratio Air (PR) b) Q-Factor Air 

100 30 

26.5% 

30.2% 

34.9% 

40.0% 

21.3% 

26.5% 

30.2% 

34.9% 

40.0% 

25 

Weight loss O /CO (wt%daf) 2 2 

Kst O /CO (bar.m/s) 2 2 

20 

15 

60 

40 

10 

5 

20 

0 0 

0 20 40 60 80 100 0 5 10 15 20 25 30 

c) Weight loss Air (wt%daf) d) Kst Air (bar.m/s) 

Figure 2 - Effect of oxygen concentration on ignition - cumulative results for all coals – 

a) pressure ratio, b) Q-factor, c) weight loss and d) Kst 

An ignition index has been applied (eqn. 3) [Man et al. (2007)], in an attempt to make the 

process less subjective. 

Ignition index (in air) = Max weight loss from ignition (wt %ad )× PR (3) 

The results are shown in Table 3, where higher rank coals clearly have a higher ignition 

index. The index also rises with O2 concentration, which is consistent with the expected 

and observed rise in explosibility with O2 levels. 

The one anomaly apparent is Coal C which apparently requires at least 34.9% O2/CO2 to 

give satisfactory ignition. Apart from this a clear trend of increasing explosibility with 

falling rank is seen. 

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Table 3 - Coal ranking by ignition index
 

Coal 

A 

F 

B 

C 

D 

E 

G 

Ignition Index TGA VM (%daf) Air 26.5% O2 30.2% O2 34.9% O2 40.0% O2 

44.4 371.3 294.5 353.0 394.1 377.5 

36.6 293.6 - - - -

33.7 285.7 294.8 285.0 438.5473.5 

32.6 182.9 no ignition 193.2 277.9 294.6 

30.8 270.6 309.8 360.9 377.2 485.1 

31.8 291.2 234.8 302.3 380.0 543.3 

25.8 258.3 no ignition 215.6 327.0 407.2 

The effect of coal particle size has also been examined for Coals F and G. The coals 

were sieved into 6 different fractions, split around 38μm, 53μm and 75μm. It is 

recognised that this process may have caused selective maceral enrichment (more likely 

for Coal G), although this has not been studied. The samples were tested in air at 

different concentrations. Evaluated Q-factors are shown in Figure 3. 

Figure 3 – Effect of particle size on Q-factor of a) Coal F and b) Coal G 

The results show that for both coals, whilst the pressure ratio, Kst and weight loss 

increase with the increasing percentage fines used and decrease with increasing sieve size 

the Q-factor does not appear to be greatly affected. Rather the Q-factor appears to 

depend more upon the coal concentration – indeed heterogeneous combustion appears to 

be taking place more at low coal concentrations in the Coal G case. 

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5. Conclusions 

From a large number of ignition experiments it is possible to rank the coals in order of 

ease of ignition and to establish the effect of O2 in CO2 at various concentrations from 21 

to 40% v/v. The ignition order followed the general trend of rising explosibility with 

falling rank as would be expected. The differences between the coals at higher O2 levels 

were relatively small, although the low volatile coals showed higher than expected weight 

losses. An increased Q-factor is also seen where the concentration of O2 in CO2 increases 

above 21% - although there appears to be little evidence that the Q-factor continues to 

increase with increasing oxygen concentration. The Q-factor is also not seen to 

significantly vary with particle size. 

The concentration of O2 in CO2, which gave a similar ignition comparable to that in air, 

was established to be between 25 and 35%. This is a slightly broader range than reported 

by Tana et al. (2006) - 28% to 35% who used heat flux data; and by Man et al. (2007), 

30% to 35%, whom conducted similar tests with high rank coals. 

None of the coals ignited in 21% O2 in CO2 using a 1000J igniter, and only four of the six 

tested ignited using a 2500J igniter. This means the risk of explosion in mills and 

associated pipework is less than for air if the O2 concentration in the primary stream is 

consistently maintained at a value no higher than that for air (i.e. ~ 21% v/v). 

6. Acknowledgements 

The activities were completed under Phase 1 of OxyCoal-UK. This was supported by the 

Engineering and Physical Sciences Research Council (EPSRC), whose funding is 

gratefully acknowledged. 

The authors gratefully acknowledge the assistance of Mr. Greg Green and Mr. Jon Jobko 

from the CDC/NIOSH Pittsburgh Research Laboratory. 

7. References 

Cashdollar, K.L. Coal dust explosibility. Journal of Loss Prevention in the Process 

Industries 9 (1996), 65-76. 

Gibbins, J.R., Man, C.K., Pendlebury, K.J. Determination of rapid heating volatile matter 

contents as a routine test. Combustion Science and Technology 93:1 (1993) 349-61. 

Man, C.K., Gibbins, J.R., Cashdollar, K.L. Effect of coal type and oxyfuel combustion 

parameters on pulverised fuel ignition. International conference on coal science and 

Technology, 28th-31st August 2007, Nottingham, United Kingdom. 

Tana, Y., Croiset, E., Douglas, M.A., Thambimuthu, K.V. Combustion characteristics of 

coal in a mixture of oxygen and recycled flue gas. Fuel 85 (2006) 507-12. 

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