lable at ScienceDirect

Applied Thermal Engineering 66 (2014) 35e42

Contents lists avai

Applied Thermal Engineering

journal homepage: www.elsevier .com/locate/apthermeng

Experimental studies of single particle combustion in air and differentoxy-fuel atmospheres

Ewa Marek*, Bartosz �SwiatkowskiDepartment of Thermal Processes, Institute of Power Engineering, Augustowka 36, 02-981 Warsaw, Poland

h i g h l i g h t s

� Particle temperature during combustion was lower in O2/CO2 than in O2/N2 mixture.� Greater temperature differences were observed for coal then for char particles.� CO2 hindered volatiles release and inhibited particle swelling during combustion.� Presence of H2O in oxy-fuel atmosphere increased temperature of combusted particle.

a r t i c l e i n f o

Article history:Received 31 August 2013Accepted 30 January 2014Available online 8 February 2014

Keywords:Single particleOxy-fuelCoalWater additionCombustion

* Corresponding author. Tel./fax: þ48 22 642 8378E-mail addresses: ewa.marek@ien.com.pl (E. Mare

com.pl (B. �Swiatkowski).

http://dx.doi.org/10.1016/j.applthermaleng.2014.01.071359-4311/� 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

In this work, direct observation of char and coal single particle combustion in different gases mixtureshas been performed. Investigation focused on the influence of atmosphere composition on combustionprocess and especially on the comparison between combustion in air-like versus oxy-fuel dry and oxy-fuel wet conditions. For these tests, particles from Pittsburgh coal and South African Coal were preparedmanually to cubical shape (approximately 2 mm and 4 mg). To investigate fuel type influence on oxy-fuelcombustion, some tests were also conducted for Polish lignite coal from Turów mine. Experiments werecarried out in a laboratory setup consisted of an electrically heated horizontal tube operated at 1223 Kwith observation windows for high speed video recording (1000 frames per second). During the ex-periments, particle internal temperature was measured to obtain comprehensive temperatureetimehistory profile. Results revealed that particles burned at higher temperatures in high water vapourcontent mixtures than in dry O2/CO2 mixture. This behaviour was attributed to lower molar specificheat of water than of CO2 and four times higher reaction rate for chareH2O gasification reaction thanchareCO2 reaction. Also visible dynamic of combustion process recorded with the high speed cameradiffers for experiments carried with water vapour addition.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Oxy-fuel combustion is a technology introduced with aim tohelp reduce CO2 emission, which is especially urgent in recenttimes when demand for coal is still growing. In Poland, wheremorethan 90% of electricity is generated from coal, the oxy-fuel tech-nology with possible option of boilers’ retrofitting, seems to be anespecially attractive variant for CO2 mitigation. However, oxy-fueltechnology is only at pilot-scale and the knowledge of combus-tion mechanisms in changed atmosphere can be still perceived asinsufficient.

.k), bartosz.swiatkowski@ien.

0

Exhaust gas from oxy-fuel combustion contains mostly CO2 anH2O. Part of produced flue gas must be recycled to maintain properheat exchange and safe operation within the boiler. Whether therecycled stream is dried or contains a significant amount of water isthe matter of later optimization of combustion process as well astechnical andeconomic analysis. But lately an agree is emerging, thatat least some amount of water in recycled flue gases is inevitable[1,2]. So far a lot of effortwasundertaken to investigate the differencebetween air and dry oxy-fuel combustion [2,3]. But it should beremembered, that H2O as well as CO2 can participate in char gasifi-cation reactions and from that point of view, possible interaction ofH2O in oxy-fuel combustion process should be better understood.

Char gasification reactions can significantly compete withcombustion reactions but only under specific conditions. Those arehigh temperature and/or low oxygen concentration in gas mixture.In comparison to O2echar reaction, gasification either with CO2 or

Fig. 1. Dominant reactions in char oxidation and gasification experiments (adaptedfrom Chen et al. [3]).

E. Marek, B. �Swiatkowski / Applied Thermal Engineering 66 (2014) 35e4236

H2O is much slower and requires a lot of external energy to takeplace. Thereby, if amount of oxygen molecules near char surface issufficient, quick and exothermic combustion reaction is alwayspromoted. On the other hand, when gas temperature is highenough, O2echar oxidation is too quick for sufficient supply ofoxygen molecules and reaction becomes limited by O2 diffusion(both internal and external). In this case, gasification can be pro-moted, because char surface is still surrounded by plenty CO2 andH2O molecules. Fig. 1 (adapted from Chen et al. [3]) presents thediagram that summarizes the conclusions from char oxidation andgasification experiments found in the literature. Diagram showsthree temperatureeO2 concentration dependent regions, amongwhich Regions B and C represent conditions in which gasificationreactions are expected to noticeably contribute to char consump-tion, whereas in Region A only combustion reaction was foundsignificant. Boundaries imposed on the regions are qualitativeillustration only and are not conclusive, as emphasized by Chenet al. [3].

The questions that remain interesting are how and whichgasification reaction influences char consumption more. WhileCO2echar reaction was widely investigated in oxy-fuel combustionconditions, only a little effort was taken to study influence of var-iable steam concentration in oxy-atmosphere on parameters ofcombustion process [1].

Fig. 2. Schematic diagram of Single Particle Combustion stand (SPC stand). 1. Coal/charparticle, 2. Thermocouple, 3. Oil shield tube, 4. Reactor, 5. Gas inlet, 6. High speedcamera, 7. Quartz window, 8. Gas outlet.

The aim of this work is to experimentally study and compare thebehaviour of coal and char particles during high temperaturecombustion in 21 and 35% oxygen concentrations with differentconcentrations of CO2, H2O and N2, introduced as the diluent gases.

2. Experimental setup

2.1. Single particle combustion stand description

Research stand for ignition and combustion of single particleallows carrying out experiments of quick fuel-particle combustionin controlled temperature and demanded gas mixtures. A sche-matic idea of Single Particle Combustion stand (SPC stand) is shownin Fig. 2. The main part of the rig is the reactor zone, which basicallyis a horizontal furnace, electrically heated up to 1000 �C (4), withobservation windows at both ends. At the quarter of reactor lengthfrom quartz windows (7) are located two thermocouples used forheating control and setting of experimental temperature.

Tip of a 0.5 mm thermocouple (2) was inserted into the holedrilled in the coal particle (1) and with the thermocouple as asupport, the particle was then inserted into the movable oil-cooledshield tube (3) inside the reactor zone. This tube, located at thevertical axis of furnace, created cool space inside the reactor andprotected particle prior to the experiment’s beginning. Whendemanded temperature conditions were stable, shield lock wasreleased resulting in quick removal of the screen-tube from thereactor zone. Since that moment, investigated particle was exposedto the high temperature and oxidizing gases and this was consid-ered the beginning of experiment. Ignition and combustion of fuelparticlewas recorded by high speed camera (6), Phantomv310withapplied recording speed of 1000 fps (frames per second). Camerawas activated simultaneously with shield lock release.

Temperature of particle when placed into the cool shield tubebefore the experiment was about 110 �C. Thus when the shieldblockadewas released, experiment startedwith particle heating up.

Experiments were carried out with different gas mixtures (O2,N2, CO2, H2O, air), slightly above atmospheric pressure to preventair leakage to the reactor zone. Gases from cylinders (O2, N2, CO2,air) passed through an electric pre-heater (not shown) where waterwas vaporized and all gaseous components were mixed and pre-heated before entering the reactor (5). Water was supplied to thepre-heater by a peristaltic pump while gases flows were setup withflowmeters. After passing the reactor zone (8), gas mixture reacheda FTIR analyser which provided additional control of mixturecomposition.

2.2. Experimental conditions

The experimental matrix is presented in Table 1. Basically, ex-periments were carried out for both coal and char particles in950 �C with different atmosphere compositions (temperature wasconstrained due to capabilities of the heaters). Composed oxy-fuelatmosphere had different physical properties than air. Table 2summarizes properties of gases used in this work, at experi-mental temperature. Gas mixture flow at reactor inlet was always5 dm3/min thus the laminar flow of gas did not disturb volatilesrelease and burning.

For such experimental conditions the average heating rate ofparticle was about 200 K/s. Although that slow heating rate is notcomparable with industrial PC combustion (104e106 K/s), it allowsto investigate sequential combustion of particle, which is essentialfor our studies.

From 3 to 5 coal or char particles were combusted for everyexperimental setup. For lignite coal tests were limited to one seriesand coal particles only. Totally, about 110 experiments were

Table 1Experimental matrix for experiments with coal and char particles at reactor tem-perature 950 �C.

Particle type Atmosphere composition [%]

O2 N2 CO2 H2Ovapour

Char/coal 21 79 0 0Char/coal 35 65 0 0Char/coal 21 0 79 0Char/coal 35 0 65 0Char/coal 21 0 64 15Char/coal 21 0 54 25Char/coal 21 0 44 35

Fig. 3. Coal particles used for experiments.

E. Marek, B. �Swiatkowski / Applied Thermal Engineering 66 (2014) 35e42 37

performed. For every tested particle, video recording of particlecombustion as well as particle internal temperature were obtained.Signal from 0.5 mm thermocouple was collected every 10 ms toreceive a comprehensive ‘particle internal temperature-time historyprofile’.

Char preparationwas conducted in the SPC stand at 950 �C withthe nitrogen flow of 5e8 dm3/min. Single particle was put on top ofa thermocouple and then introduce to the reactor with the oil-shield tube closed. When the shield lock was released, the parti-cle heating started, the increase of particle internal temperaturewas observed and species like CO2, CO and CH4 in exhaust gas werenoticed. After the extinction of volatile products, the newly createdchar was kept inside reactor for some time to assure completion ofthe degassing process. Then, the screen tube was once again low-ered and locked. Summarizing, particles were exposed in the hotfurnace during this procedure for 2 min. Since the SPC stand wasworking at overpressure, nitrogen flowing through reactor was alsopresent inside the shielding tube after its lowering and in that at-mosphere devolatilized coal was cooled to temperature approxi-mately 120 �C. Due to this action no oxygen should have beenabsorbed on particle surface before char combustion in experiment.

3. Methodology

3.1. Particle preparation

For the purpose of this study, particles from two bituminouscoals: Pittsburgh and South African Coal were prepared manually.At first, fuels were dried for 24 h and the big coal lumps werecrushed with the hammer. Then, in resulting smaller pieces(w1 cm) a hole was drilled with 0.5 mm drill. Finally, particles wereshaped with diamond friction disks into approximately cuboidalsolids, with an average size of w2 mm and weight of 4 mg (Fig. 3).To compare the influence of the fuel rank on oxy-fuel combustion,few additional experiments were also conducted for Polish lignitecoal: Turów. Particles from this fuel were prepared in the samewayas for bituminous coals, but weight of a typical lignite coal particlewith a size of w2 mm, was 1.5e2 mg due to density differences.Results of proximate and ultimate analysis of fuels used in thisstudy are shown in Table 3.

Combustion of a relatively big particle cannot be directlycompared to the combustion of PC in industrial units since some

Table 2Properties of CO2 and H2O at 950 �C and 1 atm with reference to N2 (from Aspen Proper

Property CO2

Density kg/m3 0.438Specific heat capacity kJ/kmol K 56.60Thermal conductivity W/m K 0.080Mass diffusivitya (binary diffusion of O2 in X) m2/s 1.7E-04Absorptivity/emissivity >0

a From Ref. [8].

processes depend on the particle size, i.e. ignition mechanism,fragmentation, etc. Nonetheless, experiment with the use of 2 mmparticle can provide more specific insight, especially when coupledwith particle temperature profile measurements. Hence it can be ofuse for a development and validation of mathematical models.

3.2. Particle combustion

Coal particle was first inserted into the shield-tube and whendemanded atmosphere in the reactor was obtained, the shield lockwas released and the particle was exposed. Combustion wasconsidered complete when the temperature measured with thethin thermocouple dropped to the ambient temperaturewhich wasthe reactor operating temperature and when there were no morevisible changes happening within observed particle.

Similarly, char particle combustion was conducted. After parti-cle devolatilization and proper cooling inside the oil-shield tube,the atmosphere was switched from nitrogen to oxidizing mixtureand then combustionwas conducted in the samemanner as for coalparticles. Degassed chars were never removed from reactor beforecombustion experiment.

4. Results and discussion

4.1. Visual observations of the combustion

When coal-particle heats up, volatile matter are released and atthe temperature of volatiles ignition the gaseous yellow flame ap-pears which indicates the combustion process. This phenomenoncorresponds to the homogeneous ignition and takes place awayfrom the coal surface. Once the volatile flame extinguishes, thedevolatilized particle starts to combust and that is the second stageof coal combustion. Sometimes both stages can happen at the sametime, when the volatile flame is still present but particle surfacealso starts to glow. This is joint heteroehomogeneous combustionand usually can be observed for pulverized coal particles.

Volatile-free char particles ignite and combust only in hetero-geneous mechanism, like the described second stage of coal com-bustion, where reactions take place between gaseous oxygen andthe particle surface. The visible sign of the progressing char com-bustion process is particle glowing. At first vertices and edges startto glow, then combustion progresses over the entire visible surface.

In Figs. 4e6 pictures of combustion of single particles are pre-sented (photos are selected from high speed recordings). Firstpicture in row (0 ms) was taken at the moment of ignitionwhich inthis study was intended as the first visible sign of combustion. It is

ties database).

H2O N2 O2 Ratio CO2/N2 Ratio H2O/N2

0.179 0.279 0.319 1.57 0.6444.04 33.83 35.75 1.67 1.300.127 0.079 0.086 1.00 1.612.8E-04 2.2E-04 e 0.79 1.29>0 0 0 e e

Table 3Properties of the investigated coals.

Fuel Pittsburgh No8 South African Coal Turów

Proximate analysis as received (on a dry basis)Moisture (%) 2.2 2.7 12.9Volatile matter (%) 31.5 25.3 48.9Fixed carbon (%) 53.0 57.7 33.4Ash (%) 13.3 (13.6) 14.3 (14.7) 4.8 (5.5)LCV (MJ/kg) 28.35 26.01 22.86Ultimate analysis (on a dry basis)Carbon (%) 74.2 71.4 66.3Hydrogen (%) 4.8 4.0 5.8Oxygen (%) (by diff.) 5.3 7.8 24.3Nitrogen (%) 1.3 1.6 0.6Sulphur (%) 0.89 0.63 0.55

E. Marek, B. �Swiatkowski / Applied Thermal Engineering 66 (2014) 35e4238

worth noticing, that the particle surface between homogeneousand heterogeneous combustion was relatively dark which assuresthat stages of particle combustion did not overleap and took placeone after another. Visual observations from experiments of char-particle combustion are herein not presented because of identicalnature as the heterogeneous stage of coal-particle combustion.

4.1.1. Bituminous coalsPittsburgh and SAC coal-particles combusted in similar way

(Figs. 4 and 5). When experiments were conducted with 35% H2Oaddition, the particle ignited and slowly developed volatile flame,at first visible only near the solid surface. After 100 ms, the particlewas surrounded by a translucent flame (the particle was stillvisible) which front started to move away from the surface. At700 ms the flame was very high and looked fully developed, butwas weakly luminous which indicated that only a little amount oftars and soot appeared within this stage of combustion. At

Fig. 4. Pictures from high-speed recording of single particle combustion of Pittsburgh coexperiment with 35% H2O). Numbers under frames indicate time-scale of particle combust

approximately 1000 ms, more rapid burning of the particle began.Volatiles streams were escaping from the particle very quickly, indifferent places, sometimes getting far away from the solid andinstantaneously combust, which was seen as small explosions. Jetsof volatiles created some kind of escaping routes within the parti-cle, where almost all remaining volatiles flowed later on. A flamefrom quickly escaping volatiles was very bright which indicatedthat soot and tars were main burning species. This effective com-bustion took place until the flame extinguished and the first stageof combustion was over, which for presented particles happenedapproximately 2.5 s after the ignition. Then, the dark particle sur-face started to glow, which indicated that the char combustion wasprogressing. The temperature of particle was still rising but at thetime when the whole particle was already glowing, the tempera-ture reached maximum and maintained around this value for fewseconds of char stable combustion (Fig. 7). As heterogeneouscombustion was progressing, particle was being “used up”, its sizevisually reduced until only small remain of ash was present. At thismoment the particle glow faded away and the temperature starteddecreasing till it reached the temperature of surrounding.

Combustion in air and 21% O2eCO2 mixture was similar to theprocess described above. At first a volatile flame appeared and afterits extinguishing, char combustion proceeded. In nitrogen diluentmixtures, the volatile matter was often released in a jet form andcombusted very brightly. In O2/CO2 atmosphere the flamewasmoretranslucent, which means that less tar and soot combusted. Alsovolatiles flow was more obstructed by thicken surrounding and thelong flame tail was created (upwards or downwards). What wascharacteristic for experiments carried out in air, was that at the endof the volatile’s combustion, significant changeswithin solid particleoccurred. A lot of aflyashwas released and theparticlewas swelling.This means that the pressure inside the particle was very high andaltered particle internal structure. Explanation for this phenomenon

al in different atmospheres (all experiments without water vapour addition and oneion, in ms. Zero represents the beginning of combustion (first visible sign).

Fig. 5. Pictures from high-speed recording of single particle combustion of South African Coal in different atmospheres (all experiments without water vapour addition and oneexperiment with 35% H2O). Numbers under frames indicate time-scale of particle combustion, in ms. Zero represents the beginning of combustion (first visible sign).

Fig. 6. Pictures from high-speed recording of single particle combustion of Turów coal in different atmospheres (all experiments without water vapour addition and one experimentwith 35% H2O). Numbers under frames indicate time-scale of particle combustion, in ms. Zero represents the beginning of combustion (first visible sign).

E. Marek, B. �Swiatkowski / Applied Thermal Engineering 66 (2014) 35e42 39

Fig. 7. Sample SAC char-particles temperature profiles during combustion in differentatmosphere compositions.

E. Marek, B. �Swiatkowski / Applied Thermal Engineering 66 (2014) 35e4240

is probably connected with the higher flame and particle tempera-ture in nitrogen diluent atmosphere than in CO2 atmosphere ex-periments (Fig. 8). Higher particle temperature acceleratesdevolatilizationwhich increases pressure of volatiles trapped in theparticle even more and causes the particle swelling. On the otherhand, CO2 presencewas found inhibiting to the particle swelling butbeside the influence of lower particle temperature, mechanism ofthat inhibiting behaviour is not fully explained. Borrego and Alvarezconcluded that CO2 may participate in the surface process of cross-linking which they believe reduces the swelling [5]. On the con-trarye in increased oxygen atmosphere, the swelling behaviourwasvisible for both diluent gases. Then also in CO2 atmosphere, thepressure within the particle was very high because combustion inenriched O2 atmosphere in both diluent gases was quicker and tookplace closer to surface, resulting in faster particle heating.

When comparing the combustion behaviour of particles in ni-trogen, carbon dioxide andmixture ofwater vapour/carbon dioxide,the last one looked similar both to the combustion in nitrogen andcombustion in CO2. The conclusion that can be drown is that highamount of water vapour addition cancelled some part of CO2 in-fluence on the combustion. Experiments with lower amount ofwater addition (herein not shown) visually looked similar to ex-periments carried in O2/CO2 atmosphere only. But 35% of water

Fig. 8. Particles average temperatures during combustion in N2 and CO2 diluentatmospheres.

vapour content caused significant change to physical properties ofoxidizing O2/CO2 mixture. In addition, visual similarity of combus-tion phenomena in N2 and H2O/CO2, indicates that in pursuingoverall similarity between oxy-fuel and conventional combustion,recirculation of wet exhaust gases should be promoted. Slightlyhigher temperature during combustion in mixture with H2O addi-tion, also means that the higher flue gas recycle ratio will be neededtomatch the temperature inside the retrofitted boiler in casewherewet recirculation is considered. Also the higher recycle ratio ispreferential because of better velocity distribution inside boiler andbetter convective heat transfer. The same observation regardingwetflue gas recirculation arises from Wall et al. [6] theoretical calcula-tions and was pointed out in review by Toftegaard et al. [2].

4.1.2. Lignite coalIn general, for lignite coal, the homogeneous stage of particle

combustion was quicker than for bituminous coals (Fig. 6). Flamewas fully developed within 20e60 ms after ignition in air andmixtures containing high amount of water vapour. In O2/CO2without H2O addition, particle surrounding by flame took longer, atleast 60 ms. Once again in nitrogen diluent mixtures, a lot of fly ashwas released which at the pictures looks like sparks at front of theflame. The same occurrence presented itself when particle burnedin 35% H2Oe44% CO2e21% O2 mixture. Bright luminosity of theflame was observed in every experimental conditions, but withinmixtures containing 21% of oxygen the particle was still visible,while in the higher oxygen concentration was veiled by the flame.In 35% O2eN2 atmosphere flame was present at the beginning ofvolatiles combustion, while in CO2 mixture its occurrence tookplace after 700 ms from ignition and lasted till flame extinguished.Very interesting phenomenon was observed in this atmosphereduring the next stage of combustion. When particle started to glow,indicating char combustion, also gaseous particle surroundingstarted to glow. Luminescent areola lasted to the end of particlecombustion. While char contains no more of volatile matter, thisglow should be attributed only to reactions engaging gaseousspecies that are formed during incomplete heterogeneous com-bustion or gasification reaction.

4.2. Temperatureetime history profiles

Based on measurements from 0.5 mm thermocouple, tempera-tureetime history profiles for single particles combustion were

Fig. 9. Particles average temperatures during combustion in oxy-fuel atmosphere inregards to H2O content.

E. Marek, B. �Swiatkowski / Applied Thermal Engineering 66 (2014) 35e42 41

obtained. Fig. 7 presents sample temperature profiles for typical SACchar particles in every experimental conditions. For all combustedparticles, the average temperatures were obtained, taking theaverage temperature from 5 or 2.5 s of particle stable combustion forbituminous coals and lignite coal respectively. In case of Turów coal,the shorter time was include to the calculation, because particles oflower rank coal were more reactive thus combustion was quickerand the stable part of it lasted less than the stable part of bituminouscombustion. Figs. 8 and 9 show comparison of the average particletemperature during experiments in all investigated oxidizer com-positions Results are divided for more clarity into two diagrams: thefirst one compiles experiments in oxy-fuel and nitrogen diluentatmospheres while the second one presents data for oxy-fuel con-ditions only, in relation to the amount of water vapour addition.

The highest temperatures during particle combustion, up to1320 �C were noticed for every fuel in experiments conducted in35% O2/65% N2 mixture. When nitrogen was switched for CO2,particle temperature was lower, but difference was greater in caseof coal particle combustion than in char particles combustion. Theonly distinction between coal and char particle combustion isdevolatilization stage and volatile matter combustion. So it may beassumed that the presence of CO2 hinders volatile matter releaseand influences its combustion, resulting in lower combustiontemperature.

Similar results were observed for lower oxygen concentration(21% O2). When experimentswere carried in air, temperature curvesreached 1220 �C. When again nitrogen was replaced with 79% CO2,temperatures of burning bituminous particles were usually thelowest temperatures from all profiles obtained in experiments(beside Pittsburgh char particles). In case of experiments with chars,the lower particle temperature when combusted in O2/CO2 mixturethan in O2/N2 mixture can be explained by changes in heteroge-neous reactions. It can be caused either by more difficult O2 diffu-sion through CO2 molecules or by the gasification reaction betweensolid carbon and CO2. The Boudouard reactionmay contribute to thechar consumption but is strongly endothermic and demands 172 kJenergy for every reacted mol of solid C [1]. Because of this endo-thermic character, the gasification should lower the particle tem-perature which is consistent with presented results. On the otherhand, limited O2 diffusion could also not be excluded, becauseexperimental conditions chosen for this study fall on the borderbetween region A and B, where in region B diffusion control ofcombustion is present as well as gasification (see Fig. 1).

When particles burned with the water vapour addition, theirtemperature profiles were close to each other for every tested H2Oconcentration. Only in case of Pittsburgh char and Turów coalparticles all temperatures measured within H2O enriched experi-ments were almost identical. For the rest of tested fuels, tempera-tures did not vary significantly, but were the highest for the largestwater content in oxidizer and the lowest in the dry oxy-fuel con-ditions. Gasification reaction that involves H2O is less endothermicthan the reaction of carbon and CO2. The amount of energy neededfor the H2Oechar reaction equals 131 kJ per mol of C solid [7],which is approximately 24% less than amount of energy necessaryfor the CO2echar reaction. For the H2Oechar reaction also theactivation energy is lower (230 kJ/mol for H2Oechar reactionversus 250 kJ/mol for the CO2echar reaction [1]) and this meansthat the H2O gasification reaction is more promoted. What followsfrom above facts is that when the H2Oechar gasification occurs, theparticle temperature should be expected to be higher than whenthe CO2echar reaction takes place, which can explain results pre-sented herein.

One should remember that when the water vapour addition inthe mixture was increased, at the same time CO2 concentrationdecreased, due to fixed O2 fraction. Observed temperature

differences can be attributed to the four times higher reaction ratefor the H2Oechar gasification than the CO2echar gasification [4] aswell as to H2O lower than CO2 molar specific heat (Table 2).

Despite the fact that lignite coal particles weighted less thanbituminous coal particles, temperatures obtained during lignitecombustion were only slightly lower than in case of higher rankcoal combustion. In atmospheres with 35% O2 and in air, the tem-perature of Turów particles almost equalled Pittsburgh char parti-cles. On the other hand, in 21% O2 oxy-fuel conditions with andwithout H2O addition, the lignite particle temperature was alwaysthe same (around 1090 �C) and was the lowest temperature of allparticles. The temperature of lignite combustion was not sensitiveto water vapour presence in oxidizer, but more tests should beperformed for this type of coal to confirm these results.

It was pointed out by Chen et al. [3], that the coal type can be animportant factor that also should be taken into consideration if thetemperature under oxy-fuel conditions should match the temper-ature in conventional combustion. Results presented herein indi-cate that N2 replacement with CO2 in the experimental mixturelowered lignite temperature less than bituminous coals tempera-ture (temperature reduction around 40 �C for Turów, and around60 �C for bituminous). This would suggest that the higher flue gasrecycle ratio is necessary in case of the lower rank lignite coalcombustion than in higher rank coals but again further in-vestigations are required to confirm these findings.

5. Conclusions

Single particle combustionwith 2mm particles as tested objectscan provide good insight into fundamental knowledge of com-bustion and ignition. Introduced SPC stand was used to investigatecoal and char particles combustion in air and under oxy-fuel con-ditions, with and without additional water vapour content. Con-clusions from this study may be summarized as follows:

� Particle in O2/CO2 mixture burned with lower temperature thanin N2 diluent atmosphere (beside Pittsburgh char particles). Thehighest temperature difference (70 �C) was observed for bitu-minous coal experiments. Water vapour addition in oxy-fuelatmosphere increased particle temperature during combustionin case of Pittsburgh coal, SAC char and SAC coal particles. Thisbehaviour can be attributed both to H2O lower than CO2 molarspecific heat and more promoted, less energy demanding H2Ogasification reaction.

� Visual similarity was observed between particle combustion inair and in 35% H2Oe44% CO2e21% O2 mixture.

� The halo effect observed during lignite coal particle combustionin high oxygen containing oxy-fuel environment is interestingand cannot be definitely explained. Whether there were gasifi-cation reactions involved and CO combustion took place awayfrom particle surface, should be carefully considered and moreextensively tested (for example with the use of more sophisti-cated optical and spectroscopic methods). Observed occurrenceof intensive glow around lignite char and interpretation of thisphenomenon should be considered as a subject of open question.

Acknowledgements

Presented research regarding bituminous coals was funded bythe European Commission 7th FP through RELCOM project, No.268191. Lignite coal investigation was sponsored by NationalResearch Development Centre through Strategic Program, GrantNo. SP/E/2/666420/10. Also contribution fromDr Jaros1awHercog isappreciated.

E. Marek, B. �Swiatkowski / Applied Thermal Engineering 66 (2014) 35e4242

References

[1] E.S. Hecht, C.R. Shaddix, M. Geier, A. Molina, B.S. Haynes, Effect of CO2 andsteam gasification reactions on the oxy-combustion of pulverized coal char,Combust. Flame 159 (2012) 3437e3447.

[2] M.B. Toftegaard, J. Brix, P.A. Jensen, P. Glarborg, A.D. Jensen, Oxy-fuel combus-tion of solid fuels, Prog. Energy Combust. Sci. 36 (2010) 581e625.

[3] L. Chen, S.Z. Yong, A.F. Ghoniem, Oxy-fuel combustion of pulverized coal:characterization, fundamentals, stabilization and CFD modeling, Prog. EnergyCombust. Sci. 38 (2012) 156e214.

[4] M.F. Irfan, M.R. Usman, K. Kusakabe, Coal gasification in CO2 atmosphere and itskinetics since 1948: a brief review, Energy 36 (2011) 12e40.

[5] A.G. Borrego, D. Alvarez, Comparison of chars obtained under oxy-fuel andconventional pulverized coal combustion atmospheres, Energy Fuels 21 (2007)3171e3179.

[6] T. Wall, Y. Liu, C. Spero, L. Elliott, S. Khare, R. Rathnam, et al., An overview onoxyfuel coal combustiondstate of the art research and technology develop-ment, Chem. Eng. Res. Des. 87 (2009) 1003e1016.

[7] E.S. Hecht, C.R. Shaddix, A. Molina, B.S. Haynes, Effect of CO2 gasification re-action on oxy-combustion of pulverized coal char, Proc. Combust. Inst. 33(2011) 1699e1706.

[8] B.E. Poling, J.M. Prausnitz, J.P. O’Connell, The Properties of Gases and Liquids,McGraw-Hill, 2001.