Effect of Coal Type and Oxyfuel Combustion Parameters 

on Pulverised Fuel Ignition 

C.K. Man1 J.R. Gibbins 1 and K.L. Cashdollar2 

1

 Dept. of Mechanical Engineering, Imperial College, London SW7 2AZ, England.
 2 National Institute for Occupational Safety and Health, 

Pittsburgh Research Laboratory, Pittsburgh, PA 15236, USA.
 

Abstract 

Oxyfuel combustion is currently one of the principal technological options being considered for 

capturing carbon dioxide from pulverised coal power plants. In oxyfuel combustion, coal is burnt in a 

mixture of recycled flue gases and added oxygen instead of in air. In principle this yields a flue gas 

containing only carbon dioxide, water vapour and small amounts other impurities. Oxyfuel combustion 

is superficially similar to combustion in air, but there are inevitable differences. 

This paper describes ignition tests on pulverised coal suspensions using a suite of coals of different 

rank and from different countries of origin and over a range of oxygen and coal concentrations of 

interest for oxyfuel combustion. The ignition tests were carried out using the National Institute for 

Occupational Safety and Health (NIOSH) 20-litre explosion chamber. The purpose was to determine 

how easily the various coals ignite and burn in mixtures of O2 and CO2 compared to their combustion 

in air. The residual char volatile and ash contents for the ignition residues were measured using a 

thermogravimetric analyser (TGA), allowing Q factors (i.e., enhancement in apparent volatile yield due 

to higher heating rates and/or heterogeneous reactions) to be determined and compared. 

An ignition index has been derived from the explosion and char data. This index has been used to rank 

low volatile coals for ignition propensity in air. The results show that to obtain ignition and combustion 

comparable to that in air, the oxyfuel O2-CO2 gas mixture must contain over 30% O2. 

Keywords 

CO2 capture, combustion, pyrolysis, oxyfuel. 

INTRODUCTION 

In oxyfuel combustion, coal is burnt in a mixture of recycled flue gases and added oxygen instead of 

in air, i.e., it is almost totally absent of nitrogen. In principle this yields a flue gas containing only 

carbon dioxide, water vapour and smaller amounts of NOx, SOx and other impurities, which could 

be sent directly to storage and subsequent sequestration in aquifers after the water vapour has been 

condensed. 

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." 
 

 

TABLE 1 TGA microproximate analysis of the coal samples (in VM order). 

Coal Country of Moisture VM (% db) Ash (% db) FC (% db) VM (% daf) 

origin (% ad) 

NIOSH coal 

Pocahontas USA 0.33 18.6 6.2 75.2 19.9 

MBEL coals 

Qiyi China 0.45 13.5 16.1 70.4 16.1 

Chang Chun China 0.44 13.1 15.4 71.5 15.5 

Tower UK 0.90 10.4 5.7 83.9 11.0 

WFD UK 0.43 8.5 6.7 84.8 9.1 

Hongai Vietnam 0.88 6.4 23.5 70.1 8.3 

Note: ad =air dried, db=dry basis, daf=dry ash free 

Oxyfuel combustion is superficially similar to combustion in air, but there are inevitable differences. 

The higher specific heat capacity of carbon dioxide compared to that of air is an important factor. 

Because the CO2 would absorb more heat, combustion of coals in an O2-CO2 mixture may lead to 

lower flame temperatures and hence lower ignitability compared with combustion in air at the same 

oxygen concentration (i.e. ~21% molar). 

This paper reports the results of joint experimental research by Imperial College of London, England 

and NIOSH of Pittsburgh, USA on the ignitability of various coals dispersed in mixtures of oxygen 

and carbon dioxide. 

EXPERIMENTAL 

Coal ignition studies were carried out on six coal samples. The TGA microproximate analysis data 

are summarized in Table 1. Microproximate analysis is similar to standard proximate analysis but 

only requires ~10 mg of coal compared to ~1 g for standard analysis (Ottaway, 1982). The coals 

were selected based on rank, availability, and commercial importance. These included one US 

bituminous coal (from NIOSH) with volatile matter (VM) content of about 18 wt% dry basis and 

five low volatile coals (VM roughly between 6 and 13 wt%db) supplied by Doosan Babcock 

Energy Limited (formerly Mitsui Babcock Energy Ltd., MBEL). The ignition test program included 

the following parameters: 

- effect of coal type, 

- effect of coal dust concentration from 100 to 600 g/m3 , and 

- effect of gas atmosphere (air and gas mixtures from 21-40% O2 v/v , balance CO2). 

Ignition tests were carried out in the NIOSH 20-litre explosion chamber (Figure 1). Details of the 

apparatus have been described elsewhere (Cashdollar, 1996, 2000). 

The NIOSH 20-litre ignition chamber is near-spherical in shape and made of stainless steel with a 

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

diameter thus allowing easy access to the interior for sample loading and cleaning after each test (as 

shown in Figure 1). The hinged top is attached 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 
transducer measures absolute pressures, it can also be used to monitor the evacuation of the 

chamber prior to the addition and dispersion of the test gas. Three sapphire windows serve as 

viewports. Pressure data were sampled at a rate of 2 kHz using a desktop computer. An in-house 

computer program allows the data to be processed, printed and stored. 

The coal dusts, previously dried overnight in a desiccator, were ignited with SobbeTM (Dortmund, 

Germany) chemical igniters. These generate a large number of hot particles but with little or no gas. 

The igniters were activated electrically with an internal fuse wire. Attempts were made to ignite each 

of the coal samples using a small 500 J igniter whenever possible. However, if it did not ignite, then a 

larger 1000 J igniter was used and, if necessary, a 2500 J igniter. After each successful ignition, the 

resultant char residues were recovered for analysis and weighed to establish mass loss during 

ignition. Weight loss or devolatilisation from the ignition process can be calculated in two ways. One 

is by ‘ash tracer’, which assumes the ash material from the coal to be inert (Equation 1). 

Ash tracer weight loss: wt%db = 100 x ( 1 – coal ash / char ash ) [1] 

However, this method is often unreliable – especially for coals with low ash contents where the error 

bars can be very wide. The ash also contains igniter residue material – such as barium oxide and 

zirconium oxide. Therefore, it was decided to use direct weighing to derive the weight loss values, 

i.e., simply weighing how much char is produced. This method must correct for the presence of 

igniter residues. In addition, since it is not possible to collect 100% of the material, a correction 

factor or collection efficiency has to be determined. This was established by carrying out ‘blank’ 

runs where coal samples were dispersed in the 20-litre chamber but not ignited at 100, 200, 400 

and 600 g/m3 loadings. A good correlation was obtained and a linear correction equation derived 

(Equation 2). The collection efficiency was typically about 85%. The amount of igniter residue was 

calculated by simple chemical stoichiometry given the fact that a 500 J igniter contains 120 mg of 

reactants. Larger igniters were scaled up pro rata. 

 

FIGURE 1: Drawing and photograph of NIOSH 20-L explosion chamber. 

Corrected char mass = mass char collected x 1.029 + 0.458 – igniter residue [2] 
 

 

 

 

 

From this, the weight loss from the ignition and combustion in the 20-L chamber can easily be 

calculated by Equation 3. 

Weight loss, wt% ad = 100 x ( 1 – corrected char recovered / coal loaded ) [3] 

Finally, this value can be converted to a dry-ash-free basis by Equation 4. 

Weight loss, wt% daf = 100 x weight loss (wt% ad) - %moisture [4] 

100 - ( %coal ash + %moisture ) 

Char samples from the ignition tests were analysed in a Mettler-Toledo TGA/SDTA851e 

thermogravimetric analyser to give micro-proximate data (Ottaway, 1982). Samples (typically 10 

mg) were heated in a 75 μL alumina crucible inside a TGA furnace with a flow of high purity nitrogen 

to 105°C then to 900°C (at 30°C/min), which gives respective values for moisture and volatile 

matter content. Finally, introduction of air resulted in 100% burn-off, allowing the fixed carbon and 

ash values also to be calculated. This procedure is similar to microproximate analysis of the original 

coal samples. 

The Q-factor (Equation 5) represents the enhancement in volatile release under actual heating 

conditions compared to the release under proximate analysis heating (and volatiles transport) 

conditions. 

Q-factor = weight loss from volatile release (%daf coal) [5]

 Coal VM (%daf) – Char VM (%daf of original coal) 

This should not be confused with the R-factor (Equation 6), which is simply the quotient of the 

measured weight loss and the coal volatile matter (Kimber, 1967). 

R-factor = weight loss from devolatilisation (%daf coal) [6]

 Coal VM (%daf) 

RESULTS 

The results showed it was possible to ignite all the coals in air with the exception of one low volatile 

coal, Hongai. This coal could not be ignited even with the 2500 J igniter. This coal had the lowest 

volatile matter content (6.4%db) and highest ash content (23.5 wt% db) of all the coals tested and 

this probably had an effect on its ignition propensity. The higher rank (lower volatility) coals were 

significantly more difficult to ignite, as one might expect, but most ignited in air using the higher 

energy pyrotechnic chemical igniters (2500J) whereas the lower rank coals could be ignited with the 

smaller igniters even at low coal dust concentrations. Note that for simplicity reasons, successful 

ignitions using only the smallest igniters have been shown. 
Data for two of the coals tested, Pocahontas and Tower, have been summarised in Figures 2 and 3, 

respectively. Peak pressure ratio (PR) is simply the ratio of the peak absolute pressure developed 

during combustion divided by initial (atmospheric) absolute pressure. A PR >2 was taken to 

indicate a positive ignition test (Cashdollar, 1996). Figures 2a and 3a show the peak explosion 

pressure ratios from the 20-litre chamber. The Q-factors (Figures 2b and 3b) were derived using 

Equation 5, and the corrected weight losses (Figures 2c and 3c) were calculated using Equation 4. 

Note that 21%O2-CO2 in the key refers to an O2 and CO2 gas mixture containing 21% O2. Note 

also that the shape of the data points refers to the igniter energy used, with triangles for 500 J, 

diamonds for 1000 J, and squares for 2500 J. 

0 100 200 300 400 500 600 

Concentration (g/m^3) 

0 

20 

40 

60 

80 

Weight loss (wt% daf) 100

0 100 200 300 400 500 600 

Concentration (g/m^3) 

0 

2 

4 

6

Q-factor

0 100 200 300 400 500 600 

Concentration (g/m^3) 

0 

2 

4 

6 

8 

10

Pressure ratio

air 

21%O2-CO2 

30%O2-CO2 

35%O2-CO2 

40%O2-CO2 

a 

c 

b 

1000J 

2500J 

Ignition atmosphere, igniter energy 

500J 

FIGURE 2: Effect of coal concentration and atmosphere type on Pocahontas coal ignition for a) pressure ratio, b) 

Q-factor and c) weight loss. (Q-factors shown only for those runs that achieved ignition, i.e. PR>2) 
0 100 200 300 400 500 600 

Concentration (g/m^3) 

0 

20 

40 

60 

80 

Weight loss (wt% daf) 100

0 100 200 300 400 500 600 

Concentration (g/m^3) 

4 

6 

8 

10

Q-factor

0 100 200 300 400 500 600 

Concentration (g/m^3) 

0 

2 

4 

6 

8 

10

Pressure ratio

air 

21%O2-CO2 

30%O2-CO2 

35%O2-CO2 

40%O2-CO2 

a 

c 

b 

1000J 

2500J 

Ignition atmosphere, igniter energy 

500J 

FIGURE 3: Effect of coal concentration and atmosphere type on Tower coal ignition for a) pressure ratio, b) Qfactor and c) weight loss. (Q-factors shown only for those runs that achieved ignition, i.e. PR>2) 
10 10 

Pressure ratio8 8 Weight loss (wt% daf)

a 

Coal, VM (wt% daf) 

Q-factor

Qiyi, 16.1 

Chang Chun, 15.5 

Tower, 11.0 

WFD, 9.1 

Hongai, 8.3

b 

6 6 

4 4 

2 2 

0 0 

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

Concentration (g/m^3) Concentration (g/m^3) 

100 

80 

60 

40 

20 

0 

c Igniter energy 

0 100 200 300 400 500 600 

Concentration (g/m^3) 

1000J 

2500J 

FIGURE 4: Comparison of coal ignition in air for 5 low volatile coals looking at the effect of coal concentration 

on a) pressure ratio, b) Q-factor and c) weight loss. (Q-factors shown only for those runs that achieved ignition, 

i.e. PR>2) 

The ignitability of the various coals in air is shown in Figure 4. The Hongai coal has the lowest 

volatility and can not be ignited with the higher energy 2500 J igniter at any of the concentrations 

tested. The next higher volatile coal, WFD, can be ignited with 2500 J but not with 1000 J. The 

three highest volatile coals in Figure 4 could be ignited with the 1000 J igniter. 

All of the coals were tested in the various O2-CO2 oxyfuel gas mixtures, producing data in a similar 

form as those in Figures 2 and 3. None of the coals listed in Table 1 ignited at 21% O2 in the 

oxyfuel mixture. At an O2 level of 30 or 35% in the oxyfuel mixture, the Pocahontas, Qiyi, Chang 

Chun, and Tower coals ignited with either the 1000 or 2500 J igniter. The WFD coal only ignited in 

the 30%O2 oxyfuel mixture at 600 g/m3 with a 2500 J igniter. It was possible to ignite the WFD 

coal in 35 and 40% O2 oxyfuel gas mixtures using the 1000 J igniters at 300-600 g/m3. The Hongai 

coal could not be ignited even with the 2500 J igniters at 40% O2 in the oxyfuel mixture. 

To reduce the subjectivity in assessing the coals in terms of ignition, a numerical ‘ease of ignition’ 

index (Equation 7) is proposed in order to rank the low volatile coals (which are known to be 

difficult to ignite) for their ability to ignite in air. In Equation 7, the “maximum weight loss” is 

maximum of the weight loss calculated from Equation 4 over the range of dust concentrations tested. 

The “maximum pressure ratio” is the highest explosion pressure ratio in air over the range of 

concentrations tested. 

Ignition index = max. weight loss (wt% daf) x max. pressure ratio [7] 

The ordering of the coals using this index for the two igniter energies is shown in Table 2, with 

highest ignition index at the top. Some of the variations, such as the difference between the 1000 
 

TABLE 2 Coal ranking by ignition index 

Coal ignitability TGA Proximate Ignition index for two igniter energies 

VM %daf 1000J 2500J Mean 

Chang Chun 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 

and 2500 J data for the Tower coal, were due to the limited number of tests. The ease of ignition 

index order roughly correlates with coal rank order, but with one or two important exceptions. 

From the mean values, Qiyi and WFD coals appear to be comparable in ignitability (in air) from this 

evaluation method despite having quite different volatile matter contents. This trend matches some 

industrial experience of firing low volatile coals in wall-fired burners (Cameron, 2005). 

CONCLUSIONS 

A number of low volatile coals have been tested and ranked for ease of ignition in air and in an 

oxyfuel mixture of O 2 in CO2 at various concentrations ranging from 21 to 40% O2. The ignition 

order in air generally followed coal rank but an alternative index is suggested based on the product 

of maximum weight loss and explosion pressure. This appears to correspond better with observed 

ignition behaviour in actual burner trials. There were relatively small differences between the various 

coals at the 40% O 2 level. The low volatile coals showed higher than expected weight losses 

probably due to some heterogeneous combustion occurring in the test chamber. Therefore the 

weight loss values shown are not exclusively from devolatilisation processes. 

Apart from Hongai, which did not ignite, all the coals ignited in O2 in CO2 at some point. The 

concentration of O2 in CO2, which gave ignition comparable to that in air, was established to be 

between 30 to 35% v/v. This is consistent with data reported by Tan (2006), who concluded that 

the heat flux from oxyfuel experiments between 28 and 35% v/v O2 in CO2 was comparable to that 

carried out in air. Few coals ignited in 21% v/v O2 in CO2 even with the large 2500J igniter. 

Acknowledgements 

CKM and JRG would like to thank the British Coal Utilisation Research Association and the UK Department of 

Trade and Industry for funding this research project. The authors would also like to thank Doosan Babcock 

Energy Limited (formerly Mitsui Babcock Energy Limited) for supplying some of the coal samples. 

 

References 

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