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Experimental studies of single particle combustion in air and different oxy-fuel atmospheres
Applied Thermal Engineering 66 (2014) 35e42
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Experimental studies of single particle combustion in air and different oxy-fuel atmospheres ˛tkowski Ewa Marek*, Bartosz Swia Department 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
a b s t r a c t
Article history: Received 31 August 2013 Accepted 30 January 2014 Available online 8 February 2014
In this work, direct observation of char and coal single particle combustion in different gases mixtures has been performed. Investigation focused on the influence of atmosphere composition on combustion process and especially on the comparison between combustion in air-like versus oxy-fuel dry and oxyfuel wet conditions. For these tests, particles from Pittsburgh coal and South African Coal were prepared manually to cubical shape (approximately 2 mm and 4 mg). To investigate fuel type influence on oxy-fuel combustion, some tests were also conducted for Polish lignite coal from Turów mine. Experiments were carried out in a laboratory setup consisted of an electrically heated horizontal tube operated at 1223 K with observation windows for high speed video recording (1000 frames per second). During the experiments, particle internal temperature was measured to obtain comprehensive temperatureetime history profile. Results revealed that particles burned at higher temperatures in high water vapour content mixtures than in dry O2/CO2 mixture. This behaviour was attributed to lower molar specific heat of water than of CO2 and four times higher reaction rate for chareH2O gasification reaction than chareCO2 reaction. Also visible dynamic of combustion process recorded with the high speed camera differs for experiments carried with water vapour addition. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Single particle Oxy-fuel Coal Water addition Combustion
1. Introduction Oxy-fuel combustion is a technology introduced with aim to help reduce CO2 emission, which is especially urgent in recent times when demand for coal is still growing. In Poland, where more than 90% of electricity is generated from coal, the oxy-fuel technology with possible option of boilers’ retrofitting, seems to be an especially attractive variant for CO2 mitigation. However, oxy-fuel technology is only at pilot-scale and the knowledge of combustion mechanisms in changed atmosphere can be still perceived as insufficient.
* Corresponding author. Tel./fax: þ48 22 642 8378. E-mail addresses: [email protected] (E. Marek), bartosz.swiatkowski@ien. ˛ tkowski). com.pl (B. Swia http://dx.doi.org/10.1016/j.applthermaleng.2014.01.070 1359-4311/Ó 2014 Elsevier Ltd. All rights reserved.
Exhaust gas from oxy-fuel combustion contains mostly CO2 an H2O. Part of produced flue gas must be recycled to maintain proper heat exchange and safe operation within the boiler. Whether the recycled stream is dried or contains a significant amount of water is the matter of later optimization of combustion process as well as technical and economic analysis. But lately an agree is emerging, that at least some amount of water in recycled flue gases is inevitable [1,2]. So far a lot of effort was undertaken to investigate the difference between air and dry oxy-fuel combustion [2,3]. But it should be remembered, that H2O as well as CO2 can participate in char gasification reactions and from that point of view, possible interaction of H2O in oxy-fuel combustion process should be better understood. Char gasification reactions can significantly compete with combustion reactions but only under specific conditions. Those are high temperature and/or low oxygen concentration in gas mixture. In comparison to O2echar reaction, gasification either with CO2 or
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The aim of this work is to experimentally study and compare the behaviour of coal and char particles during high temperature combustion in 21 and 35% oxygen concentrations with different concentrations of CO2, H2O and N2, introduced as the diluent gases. 2. Experimental setup 2.1. Single particle combustion stand description
Fig. 1. Dominant reactions in char oxidation and gasification experiments (adapted from Chen et al. [3]).
H2O is much slower and requires a lot of external energy to take place. Thereby, if amount of oxygen molecules near char surface is sufficient, quick and exothermic combustion reaction is always promoted. On the other hand, when gas temperature is high enough, O2echar oxidation is too quick for sufficient supply of oxygen molecules and reaction becomes limited by O2 diffusion (both internal and external). In this case, gasification can be promoted, because char surface is still surrounded by plenty CO2 and H2O molecules. Fig. 1 (adapted from Chen et al. [3]) presents the diagram that summarizes the conclusions from char oxidation and gasification experiments found in the literature. Diagram shows three temperatureeO2 concentration dependent regions, among which Regions B and C represent conditions in which gasification reactions are expected to noticeably contribute to char consumption, whereas in Region A only combustion reaction was found significant. Boundaries imposed on the regions are qualitative illustration only and are not conclusive, as emphasized by Chen et al. [3]. The questions that remain interesting are how and which gasification reaction influences char consumption more. While CO2echar reaction was widely investigated in oxy-fuel combustion conditions, only a little effort was taken to study influence of variable steam concentration in oxy-atmosphere on parameters of combustion process [1].
Research stand for ignition and combustion of single particle allows carrying out experiments of quick fuel-particle combustion in controlled temperature and demanded gas mixtures. A schematic idea of Single Particle Combustion stand (SPC stand) is shown in Fig. 2. The main part of the rig is the reactor zone, which basically is a horizontal furnace, electrically heated up to 1000 C (4), with observation windows at both ends. At the quarter of reactor length from quartz windows (7) are located two thermocouples used for heating control and setting of experimental temperature. Tip of a 0.5 mm thermocouple (2) was inserted into the hole drilled in the coal particle (1) and with the thermocouple as a support, the particle was then inserted into the movable oil-cooled shield tube (3) inside the reactor zone. This tube, located at the vertical axis of furnace, created cool space inside the reactor and protected particle prior to the experiment’s beginning. When demanded temperature conditions were stable, shield lock was released resulting in quick removal of the screen-tube from the reactor zone. Since that moment, investigated particle was exposed to the high temperature and oxidizing gases and this was considered the beginning of experiment. Ignition and combustion of fuel particle was recorded by high speed camera (6), Phantom v310 with applied recording speed of 1000 fps (frames per second). Camera was activated simultaneously with shield lock release. Temperature of particle when placed into the cool shield tube before the experiment was about 110 C. Thus when the shield blockade was released, experiment started with particle heating up. Experiments were carried out with different gas mixtures (O2, N2, CO2, H2O, air), slightly above atmospheric pressure to prevent air leakage to the reactor zone. Gases from cylinders (O2, N2, CO2, air) passed through an electric pre-heater (not shown) where water was vaporized and all gaseous components were mixed and preheated before entering the reactor (5). Water was supplied to the pre-heater by a peristaltic pump while gases flows were setup with flow meters. After passing the reactor zone (8), gas mixture reached a FTIR analyser which provided additional control of mixture composition. 2.2. Experimental conditions
Fig. 2. Schematic diagram of Single Particle Combustion stand (SPC stand). 1. Coal/char particle, 2. Thermocouple, 3. Oil shield tube, 4. Reactor, 5. Gas inlet, 6. High speed camera, 7. Quartz window, 8. Gas outlet.
The experimental matrix is presented in Table 1. Basically, experiments were carried out for both coal and char particles in 950 C with different atmosphere compositions (temperature was constrained due to capabilities of the heaters). Composed oxy-fuel atmosphere had different physical properties than air. Table 2 summarizes properties of gases used in this work, at experimental temperature. Gas mixture flow at reactor inlet was always 5 dm3/min thus the laminar flow of gas did not disturb volatiles release and burning. For such experimental conditions the average heating rate of particle was about 200 K/s. Although that slow heating rate is not comparable with industrial PC combustion (104e106 K/s), it allows to investigate sequential combustion of particle, which is essential for our studies. From 3 to 5 coal or char particles were combusted for every experimental setup. For lignite coal tests were limited to one series and coal particles only. Totally, about 110 experiments were
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37
Table 1 Experimental matrix for experiments with coal and char particles at reactor temperature 950 C. Particle type
Char/coal Char/coal Char/coal Char/coal Char/coal Char/coal Char/coal
Atmosphere composition [%] O2
N2
CO2
H2Ovapour
21 35 21 35 21 21 21
79 65 0 0 0 0 0
0 0 79 65 64 54 44
0 0 0 0 15 25 35
performed. For every tested particle, video recording of particle combustion as well as particle internal temperature were obtained. Signal from 0.5 mm thermocouple was collected every 10 ms to receive a comprehensive ‘particle internal temperature-time history profile’. Char preparation was conducted in the SPC stand at 950 C with the nitrogen flow of 5e8 dm3/min. Single particle was put on top of a thermocouple and then introduce to the reactor with the oilshield tube closed. When the shield lock was released, the particle heating started, the increase of particle internal temperature was observed and species like CO2, CO and CH4 in exhaust gas were noticed. After the extinction of volatile products, the newly created char was kept inside reactor for some time to assure completion of the degassing process. Then, the screen tube was once again lowered and locked. Summarizing, particles were exposed in the hot furnace during this procedure for 2 min. Since the SPC stand was working at overpressure, nitrogen flowing through reactor was also present inside the shielding tube after its lowering and in that atmosphere devolatilized coal was cooled to temperature approximately 120 C. Due to this action no oxygen should have been absorbed on particle surface before char combustion in experiment. 3. Methodology 3.1. Particle preparation For the purpose of this study, particles from two bituminous coals: Pittsburgh and South African Coal were prepared manually. At first, fuels were dried for 24 h and the big coal lumps were crushed with the hammer. Then, in resulting smaller pieces (w1 cm) a hole was drilled with 0.5 mm drill. Finally, particles were shaped with diamond friction disks into approximately cuboidal solids, 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 lignite coal: Turów. Particles from this fuel were prepared in the same way as for bituminous coals, but weight of a typical lignite coal particle with a size of w2 mm, was 1.5e2 mg due to density differences. Results of proximate and ultimate analysis of fuels used in this study are shown in Table 3. Combustion of a relatively big particle cannot be directly compared to the combustion of PC in industrial units since some
Fig. 3. Coal particles used for experiments.
processes depend on the particle size, i.e. ignition mechanism, fragmentation, etc. Nonetheless, experiment with the use of 2 mm particle can provide more specific insight, especially when coupled with particle temperature profile measurements. Hence it can be of use for a development and validation of mathematical models. 3.2. Particle combustion Coal particle was first inserted into the shield-tube and when demanded atmosphere in the reactor was obtained, the shield lock was released and the particle was exposed. Combustion was considered complete when the temperature measured with the thin thermocouple dropped to the ambient temperature which was the reactor operating temperature and when there were no more visible changes happening within observed particle. Similarly, char particle combustion was conducted. After particle devolatilization and proper cooling inside the oil-shield tube, the atmosphere was switched from nitrogen to oxidizing mixture and then combustion was conducted in the same manner as for coal particles. Degassed chars were never removed from reactor before combustion experiment. 4. Results and discussion 4.1. Visual observations of the combustion When coal-particle heats up, volatile matter are released and at the temperature of volatiles ignition the gaseous yellow flame appears which indicates the combustion process. This phenomenon corresponds to the homogeneous ignition and takes place away from the coal surface. Once the volatile flame extinguishes, the devolatilized particle starts to combust and that is the second stage of coal combustion. Sometimes both stages can happen at the same time, when the volatile flame is still present but particle surface also starts to glow. This is joint heteroehomogeneous combustion and usually can be observed for pulverized coal particles. Volatile-free char particles ignite and combust only in heterogeneous mechanism, like the described second stage of coal combustion, where reactions take place between gaseous oxygen and the particle surface. The visible sign of the progressing char combustion process is particle glowing. At first vertices and edges start to glow, then combustion progresses over the entire visible surface. In Figs. 4e6 pictures of combustion of single particles are presented (photos are selected from high speed recordings). First picture in row (0 ms) was taken at the moment of ignition which in this study was intended as the first visible sign of combustion. It is
Table 2 Properties of CO2 and H2O at 950 C and 1 atm with reference to N2 (from Aspen Properties database). Property Density Specific heat capacity Thermal conductivity Mass diffusivitya (binary diffusion of O2 in X) Absorptivity/emissivity a
From Ref. [8].
kg/m3 kJ/kmol K W/m K m2/s
CO2
H2O
N2
O2
Ratio CO2/N2
Ratio H2O/N2
0.438 56.60 0.080 1.7E-04 >0
0.179 44.04 0.127 2.8E-04 >0
0.279 33.83 0.079 2.2E-04 0
0.319 35.75 0.086 e 0
1.57 1.67 1.00 0.79 e
0.64 1.30 1.61 1.29 e
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38 Table 3 Properties of the investigated coals. Fuel
Pittsburgh No8
South African Coal
Proximate analysis as received (on a dry basis) Moisture (%) 2.2 2.7 Volatile matter (%) 31.5 25.3 Fixed carbon (%) 53.0 57.7 Ash (%) 13.3 (13.6) 14.3 (14.7) LCV (MJ/kg) 28.35 26.01 Ultimate analysis (on a dry basis) Carbon (%) 74.2 71.4 Hydrogen (%) 4.8 4.0 Oxygen (%) (by diff.) 5.3 7.8 Nitrogen (%) 1.3 1.6 Sulphur (%) 0.89 0.63
Turów 12.9 48.9 33.4 4.8 (5.5) 22.86 66.3 5.8 24.3 0.6 0.55
worth noticing, that the particle surface between homogeneous and heterogeneous combustion was relatively dark which assures that stages of particle combustion did not overleap and took place one after another. Visual observations from experiments of charparticle combustion are herein not presented because of identical nature as the heterogeneous stage of coal-particle combustion. 4.1.1. Bituminous coals Pittsburgh and SAC coal-particles combusted in similar way (Figs. 4 and 5). When experiments were conducted with 35% H2O addition, the particle ignited and slowly developed volatile flame, at first visible only near the solid surface. After 100 ms, the particle was surrounded by a translucent flame (the particle was still visible) which front started to move away from the surface. At 700 ms the flame was very high and looked fully developed, but was weakly luminous which indicated that only a little amount of tars and soot appeared within this stage of combustion. At
approximately 1000 ms, more rapid burning of the particle began. Volatiles streams were escaping from the particle very quickly, in different places, sometimes getting far away from the solid and instantaneously combust, which was seen as small explosions. Jets of volatiles created some kind of escaping routes within the particle, where almost all remaining volatiles flowed later on. A flame from quickly escaping volatiles was very bright which indicated that soot and tars were main burning species. This effective combustion took place until the flame extinguished and the first stage of combustion was over, which for presented particles happened approximately 2.5 s after the ignition. Then, the dark particle surface started to glow, which indicated that the char combustion was progressing. The temperature of particle was still rising but at the time when the whole particle was already glowing, the temperature reached maximum and maintained around this value for few seconds of char stable combustion (Fig. 7). As heterogeneous combustion was progressing, particle was being “used up”, its size visually reduced until only small remain of ash was present. At this moment the particle glow faded away and the temperature started decreasing till it reached the temperature of surrounding. Combustion in air and 21% O2eCO2 mixture was similar to the process described above. At first a volatile flame appeared and after its extinguishing, char combustion proceeded. In nitrogen diluent mixtures, the volatile matter was often released in a jet form and combusted very brightly. In O2/CO2 atmosphere the flame was more translucent, which means that less tar and soot combusted. Also volatiles flow was more obstructed by thicken surrounding and the long flame tail was created (upwards or downwards). What was characteristic for experiments carried out in air, was that at the end of the volatile’s combustion, significant changes within solid particle occurred. A lot of a fly ash was released and the particle was swelling. This means that the pressure inside the particle was very high and altered particle internal structure. Explanation for this phenomenon
Fig. 4. Pictures from high-speed recording of single particle combustion of Pittsburgh coal in different atmospheres (all experiments without water vapour addition and one experiment with 35% H2O). Numbers under frames indicate time-scale of particle combustion, in ms. Zero represents the beginning of combustion (first visible sign).
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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 one experiment 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 experiment with 35% H2O). Numbers under frames indicate time-scale of particle combustion, in ms. Zero represents the beginning of combustion (first visible sign).
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vapour content caused significant change to physical properties of oxidizing O2/CO2 mixture. In addition, visual similarity of combustion phenomena in N2 and H2O/CO2, indicates that in pursuing overall similarity between oxy-fuel and conventional combustion, recirculation of wet exhaust gases should be promoted. Slightly higher temperature during combustion in mixture with H2O addition, also means that the higher flue gas recycle ratio will be needed to match the temperature inside the retrofitted boiler in case where wet recirculation is considered. Also the higher recycle ratio is preferential because of better velocity distribution inside boiler and better convective heat transfer. The same observation regarding wet flue gas recirculation arises from Wall et al. [6] theoretical calculations and was pointed out in review by Toftegaard et al. [2].
Fig. 7. Sample SAC char-particles temperature profiles during combustion in different atmosphere compositions.
is probably connected with the higher flame and particle temperature in nitrogen diluent atmosphere than in CO2 atmosphere experiments (Fig. 8). Higher particle temperature accelerates devolatilization which increases pressure of volatiles trapped in the particle even more and causes the particle swelling. On the other hand, CO2 presence was found inhibiting to the particle swelling but beside the influence of lower particle temperature, mechanism of that inhibiting behaviour is not fully explained. Borrego and Alvarez concluded that CO2 may participate in the surface process of crosslinking which they believe reduces the swelling [5]. On the contrary e in increased oxygen atmosphere, the swelling behaviour was visible for both diluent gases. Then also in CO2 atmosphere, the pressure within the particle was very high because combustion in enriched O2 atmosphere in both diluent gases was quicker and took place closer to surface, resulting in faster particle heating. When comparing the combustion behaviour of particles in nitrogen, carbon dioxide and mixture of water vapour/carbon dioxide, the last one looked similar both to the combustion in nitrogen and combustion in CO2. The conclusion that can be drown is that high amount of water vapour addition cancelled some part of CO2 influence on the combustion. Experiments with lower amount of water addition (herein not shown) visually looked similar to experiments carried in O2/CO2 atmosphere only. But 35% of water
Fig. 8. Particles average temperatures during combustion in N2 and CO2 diluent atmospheres.
4.1.2. Lignite coal In general, for lignite coal, the homogeneous stage of particle combustion was quicker than for bituminous coals (Fig. 6). Flame was fully developed within 20e60 ms after ignition in air and mixtures containing high amount of water vapour. In O2/CO2 without H2O addition, particle surrounding by flame took longer, at least 60 ms. Once again in nitrogen diluent mixtures, a lot of fly ash was released which at the pictures looks like sparks at front of the flame. The same occurrence presented itself when particle burned in 35% H2Oe44% CO2e21% O2 mixture. Bright luminosity of the flame was observed in every experimental conditions, but within mixtures 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 of volatiles combustion, while in CO2 mixture its occurrence took place after 700 ms from ignition and lasted till flame extinguished. Very interesting phenomenon was observed in this atmosphere during the next stage of combustion. When particle started to glow, indicating char combustion, also gaseous particle surrounding started to glow. Luminescent areola lasted to the end of particle combustion. While char contains no more of volatile matter, this glow should be attributed only to reactions engaging gaseous species that are formed during incomplete heterogeneous combustion or gasification reaction. 4.2. Temperatureetime history profiles Based on measurements from 0.5 mm thermocouple, temperatureetime history profiles for single particles combustion were
Fig. 9. Particles average temperatures during combustion in oxy-fuel atmosphere in regards to H2O content.
E. Marek, B. Swia˛ tkowski / Applied Thermal Engineering 66 (2014) 35e42
obtained. Fig. 7 presents sample temperature profiles for typical SAC char particles in every experimental conditions. For all combusted particles, the average temperatures were obtained, taking the average temperature from 5 or 2.5 s of particle stable combustion for bituminous coals and lignite coal respectively. In case of Turów coal, the shorter time was include to the calculation, because particles of lower rank coal were more reactive thus combustion was quicker and the stable part of it lasted less than the stable part of bituminous combustion. Figs. 8 and 9 show comparison of the average particle temperature during experiments in all investigated oxidizer compositions Results are divided for more clarity into two diagrams: the first one compiles experiments in oxy-fuel and nitrogen diluent atmospheres while the second one presents data for oxy-fuel conditions only, in relation to the amount of water vapour addition. The highest temperatures during particle combustion, up to 1320 C were noticed for every fuel in experiments conducted in 35% O2/65% N2 mixture. When nitrogen was switched for CO2, particle temperature was lower, but difference was greater in case of coal particle combustion than in char particles combustion. The only distinction between coal and char particle combustion is devolatilization stage and volatile matter combustion. So it may be assumed that the presence of CO2 hinders volatile matter release and influences its combustion, resulting in lower combustion temperature. Similar results were observed for lower oxygen concentration (21% O2). When experiments were carried in air, temperature curves reached 1220 C. When again nitrogen was replaced with 79% CO2, temperatures of burning bituminous particles were usually the lowest 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 mixture than in O2/N2 mixture can be explained by changes in heterogeneous reactions. It can be caused either by more difficult O2 diffusion through CO2 molecules or by the gasification reaction between solid carbon and CO2. The Boudouard reaction may contribute to the char consumption but is strongly endothermic and demands 172 kJ energy for every reacted mol of solid C [1]. Because of this endothermic character, the gasification should lower the particle temperature which is consistent with presented results. On the other hand, limited O2 diffusion could also not be excluded, because experimental conditions chosen for this study fall on the border between region A and B, where in region B diffusion control of combustion is present as well as gasification (see Fig. 1). When particles burned with the water vapour addition, their temperature profiles were close to each other for every tested H2O concentration. Only in case of Pittsburgh char and Turów coal particles all temperatures measured within H2O enriched experiments were almost identical. For the rest of tested fuels, temperatures did not vary significantly, but were the highest for the largest water content in oxidizer and the lowest in the dry oxy-fuel conditions. Gasification reaction that involves H2O is less endothermic than the reaction of carbon and CO2. The amount of energy needed for the H2Oechar reaction equals 131 kJ per mol of C solid [7], which is approximately 24% less than amount of energy necessary for the CO2echar reaction. For the H2Oechar reaction also the activation energy is lower (230 kJ/mol for H2Oechar reaction versus 250 kJ/mol for the CO2echar reaction [1]) and this means that the H2O gasification reaction is more promoted. What follows from above facts is that when the H2Oechar gasification occurs, the particle temperature should be expected to be higher than when the CO2echar reaction takes place, which can explain results presented herein. One should remember that when the water vapour addition in the mixture was increased, at the same time CO2 concentration decreased, due to fixed O2 fraction. Observed temperature
41
differences can be attributed to the four times higher reaction rate for the H2Oechar gasification than the CO2echar gasification [4] as well as to H2O lower than CO2 molar specific heat (Table 2). Despite the fact that lignite coal particles weighted less than bituminous coal particles, temperatures obtained during lignite combustion were only slightly lower than in case of higher rank coal combustion. In atmospheres with 35% O2 and in air, the temperature of Turów particles almost equalled Pittsburgh char particles. On the other hand, in 21% O2 oxy-fuel conditions with and without H2O addition, the lignite particle temperature was always the same (around 1090 C) and was the lowest temperature of all particles. The temperature of lignite combustion was not sensitive to water vapour presence in oxidizer, but more tests should be performed for this type of coal to confirm these results. It was pointed out by Chen et al. [3], that the coal type can be an important factor that also should be taken into consideration if the temperature under oxy-fuel conditions should match the temperature in conventional combustion. Results presented herein indicate that N2 replacement with CO2 in the experimental mixture lowered lignite temperature less than bituminous coals temperature (temperature reduction around 40 C for Turów, and around 60 C for bituminous). This would suggest that the higher flue gas recycle ratio is necessary in case of the lower rank lignite coal combustion than in higher rank coals but again further investigations are required to confirm these findings. 5. Conclusions Single particle combustion with 2 mm particles as tested objects can provide good insight into fundamental knowledge of combustion and ignition. Introduced SPC stand was used to investigate coal and char particles combustion in air and under oxy-fuel conditions, with and without additional water vapour content. Conclusions from this study may be summarized as follows: Particle in O2/CO2 mixture burned with lower temperature than in N2 diluent atmosphere (beside Pittsburgh char particles). The highest temperature difference (70 C) was observed for bituminous coal experiments. Water vapour addition in oxy-fuel atmosphere increased particle temperature during combustion in case of Pittsburgh coal, SAC char and SAC coal particles. This behaviour can be attributed both to H2O lower than CO2 molar specific heat and more promoted, less energy demanding H2O gasification reaction. Visual similarity was observed between particle combustion in air and in 35% H2Oe44% CO2e21% O2 mixture. The halo effect observed during lignite coal particle combustion in high oxygen containing oxy-fuel environment is interesting and cannot be definitely explained. Whether there were gasification reactions involved and CO combustion took place away from particle surface, should be carefully considered and more extensively tested (for example with the use of more sophisticated optical and spectroscopic methods). Observed occurrence of intensive glow around lignite char and interpretation of this phenomenon should be considered as a subject of open question. Acknowledgements Presented research regarding bituminous coals was funded by the European Commission 7th FP through RELCOM project, No. 268191. Lignite coal investigation was sponsored by National Research Development Centre through Strategic Program, Grant No. SP/E/2/666420/10. Also contribution from Dr Jaros1aw Hercog is appreciated.
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References [1] E.S. Hecht, C.R. Shaddix, M. Geier, A. Molina, B.S. Haynes, Effect of CO2 and steam 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 combustion 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. Energy Combust. Sci. 38 (2012) 156e214. [4] M.F. Irfan, M.R. Usman, K. Kusakabe, Coal gasification in CO2 atmosphere and its kinetics since 1948: a brief review, Energy 36 (2011) 12e40.
[5] A.G. Borrego, D. Alvarez, Comparison of chars obtained under oxy-fuel and conventional 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 on oxyfuel coal combustiondstate of the art research and technology development, Chem. Eng. Res. Des. 87 (2009) 1003e1016. [7] E.S. Hecht, C.R. Shaddix, A. Molina, B.S. Haynes, Effect of CO2 gasification reaction 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.
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Experimental studies of single particle combustion in air and different oxy-fuel atmospheres ˛tkowski Ewa Marek*, Bartosz Swia Department 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
a b s t r a c t
Article history: Received 31 August 2013 Accepted 30 January 2014 Available online 8 February 2014
In this work, direct observation of char and coal single particle combustion in different gases mixtures has been performed. Investigation focused on the influence of atmosphere composition on combustion process and especially on the comparison between combustion in air-like versus oxy-fuel dry and oxyfuel wet conditions. For these tests, particles from Pittsburgh coal and South African Coal were prepared manually to cubical shape (approximately 2 mm and 4 mg). To investigate fuel type influence on oxy-fuel combustion, some tests were also conducted for Polish lignite coal from Turów mine. Experiments were carried out in a laboratory setup consisted of an electrically heated horizontal tube operated at 1223 K with observation windows for high speed video recording (1000 frames per second). During the experiments, particle internal temperature was measured to obtain comprehensive temperatureetime history profile. Results revealed that particles burned at higher temperatures in high water vapour content mixtures than in dry O2/CO2 mixture. This behaviour was attributed to lower molar specific heat of water than of CO2 and four times higher reaction rate for chareH2O gasification reaction than chareCO2 reaction. Also visible dynamic of combustion process recorded with the high speed camera differs for experiments carried with water vapour addition. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Single particle Oxy-fuel Coal Water addition Combustion
1. Introduction Oxy-fuel combustion is a technology introduced with aim to help reduce CO2 emission, which is especially urgent in recent times when demand for coal is still growing. In Poland, where more than 90% of electricity is generated from coal, the oxy-fuel technology with possible option of boilers’ retrofitting, seems to be an especially attractive variant for CO2 mitigation. However, oxy-fuel technology is only at pilot-scale and the knowledge of combustion mechanisms in changed atmosphere can be still perceived as insufficient.
* Corresponding author. Tel./fax: þ48 22 642 8378. E-mail addresses: [email protected] (E. Marek), bartosz.swiatkowski@ien. ˛ tkowski). com.pl (B. Swia http://dx.doi.org/10.1016/j.applthermaleng.2014.01.070 1359-4311/Ó 2014 Elsevier Ltd. All rights reserved.
Exhaust gas from oxy-fuel combustion contains mostly CO2 an H2O. Part of produced flue gas must be recycled to maintain proper heat exchange and safe operation within the boiler. Whether the recycled stream is dried or contains a significant amount of water is the matter of later optimization of combustion process as well as technical and economic analysis. But lately an agree is emerging, that at least some amount of water in recycled flue gases is inevitable [1,2]. So far a lot of effort was undertaken to investigate the difference between air and dry oxy-fuel combustion [2,3]. But it should be remembered, that H2O as well as CO2 can participate in char gasification reactions and from that point of view, possible interaction of H2O in oxy-fuel combustion process should be better understood. Char gasification reactions can significantly compete with combustion reactions but only under specific conditions. Those are high temperature and/or low oxygen concentration in gas mixture. In comparison to O2echar reaction, gasification either with CO2 or
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The aim of this work is to experimentally study and compare the behaviour of coal and char particles during high temperature combustion in 21 and 35% oxygen concentrations with different concentrations of CO2, H2O and N2, introduced as the diluent gases. 2. Experimental setup 2.1. Single particle combustion stand description
Fig. 1. Dominant reactions in char oxidation and gasification experiments (adapted from Chen et al. [3]).
H2O is much slower and requires a lot of external energy to take place. Thereby, if amount of oxygen molecules near char surface is sufficient, quick and exothermic combustion reaction is always promoted. On the other hand, when gas temperature is high enough, O2echar oxidation is too quick for sufficient supply of oxygen molecules and reaction becomes limited by O2 diffusion (both internal and external). In this case, gasification can be promoted, because char surface is still surrounded by plenty CO2 and H2O molecules. Fig. 1 (adapted from Chen et al. [3]) presents the diagram that summarizes the conclusions from char oxidation and gasification experiments found in the literature. Diagram shows three temperatureeO2 concentration dependent regions, among which Regions B and C represent conditions in which gasification reactions are expected to noticeably contribute to char consumption, whereas in Region A only combustion reaction was found significant. Boundaries imposed on the regions are qualitative illustration only and are not conclusive, as emphasized by Chen et al. [3]. The questions that remain interesting are how and which gasification reaction influences char consumption more. While CO2echar reaction was widely investigated in oxy-fuel combustion conditions, only a little effort was taken to study influence of variable steam concentration in oxy-atmosphere on parameters of combustion process [1].
Research stand for ignition and combustion of single particle allows carrying out experiments of quick fuel-particle combustion in controlled temperature and demanded gas mixtures. A schematic idea of Single Particle Combustion stand (SPC stand) is shown in Fig. 2. The main part of the rig is the reactor zone, which basically is a horizontal furnace, electrically heated up to 1000 C (4), with observation windows at both ends. At the quarter of reactor length from quartz windows (7) are located two thermocouples used for heating control and setting of experimental temperature. Tip of a 0.5 mm thermocouple (2) was inserted into the hole drilled in the coal particle (1) and with the thermocouple as a support, the particle was then inserted into the movable oil-cooled shield tube (3) inside the reactor zone. This tube, located at the vertical axis of furnace, created cool space inside the reactor and protected particle prior to the experiment’s beginning. When demanded temperature conditions were stable, shield lock was released resulting in quick removal of the screen-tube from the reactor zone. Since that moment, investigated particle was exposed to the high temperature and oxidizing gases and this was considered the beginning of experiment. Ignition and combustion of fuel particle was recorded by high speed camera (6), Phantom v310 with applied recording speed of 1000 fps (frames per second). Camera was activated simultaneously with shield lock release. Temperature of particle when placed into the cool shield tube before the experiment was about 110 C. Thus when the shield blockade was released, experiment started with particle heating up. Experiments were carried out with different gas mixtures (O2, N2, CO2, H2O, air), slightly above atmospheric pressure to prevent air leakage to the reactor zone. Gases from cylinders (O2, N2, CO2, air) passed through an electric pre-heater (not shown) where water was vaporized and all gaseous components were mixed and preheated before entering the reactor (5). Water was supplied to the pre-heater by a peristaltic pump while gases flows were setup with flow meters. After passing the reactor zone (8), gas mixture reached a FTIR analyser which provided additional control of mixture composition. 2.2. Experimental conditions
Fig. 2. Schematic diagram of Single Particle Combustion stand (SPC stand). 1. Coal/char particle, 2. Thermocouple, 3. Oil shield tube, 4. Reactor, 5. Gas inlet, 6. High speed camera, 7. Quartz window, 8. Gas outlet.
The experimental matrix is presented in Table 1. Basically, experiments were carried out for both coal and char particles in 950 C with different atmosphere compositions (temperature was constrained due to capabilities of the heaters). Composed oxy-fuel atmosphere had different physical properties than air. Table 2 summarizes properties of gases used in this work, at experimental temperature. Gas mixture flow at reactor inlet was always 5 dm3/min thus the laminar flow of gas did not disturb volatiles release and burning. For such experimental conditions the average heating rate of particle was about 200 K/s. Although that slow heating rate is not comparable with industrial PC combustion (104e106 K/s), it allows to investigate sequential combustion of particle, which is essential for our studies. From 3 to 5 coal or char particles were combusted for every experimental setup. For lignite coal tests were limited to one series and coal particles only. Totally, about 110 experiments were
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Table 1 Experimental matrix for experiments with coal and char particles at reactor temperature 950 C. Particle type
Char/coal Char/coal Char/coal Char/coal Char/coal Char/coal Char/coal
Atmosphere composition [%] O2
N2
CO2
H2Ovapour
21 35 21 35 21 21 21
79 65 0 0 0 0 0
0 0 79 65 64 54 44
0 0 0 0 15 25 35
performed. For every tested particle, video recording of particle combustion as well as particle internal temperature were obtained. Signal from 0.5 mm thermocouple was collected every 10 ms to receive a comprehensive ‘particle internal temperature-time history profile’. Char preparation was conducted in the SPC stand at 950 C with the nitrogen flow of 5e8 dm3/min. Single particle was put on top of a thermocouple and then introduce to the reactor with the oilshield tube closed. When the shield lock was released, the particle heating started, the increase of particle internal temperature was observed and species like CO2, CO and CH4 in exhaust gas were noticed. After the extinction of volatile products, the newly created char was kept inside reactor for some time to assure completion of the degassing process. Then, the screen tube was once again lowered and locked. Summarizing, particles were exposed in the hot furnace during this procedure for 2 min. Since the SPC stand was working at overpressure, nitrogen flowing through reactor was also present inside the shielding tube after its lowering and in that atmosphere devolatilized coal was cooled to temperature approximately 120 C. Due to this action no oxygen should have been absorbed on particle surface before char combustion in experiment. 3. Methodology 3.1. Particle preparation For the purpose of this study, particles from two bituminous coals: Pittsburgh and South African Coal were prepared manually. At first, fuels were dried for 24 h and the big coal lumps were crushed with the hammer. Then, in resulting smaller pieces (w1 cm) a hole was drilled with 0.5 mm drill. Finally, particles were shaped with diamond friction disks into approximately cuboidal solids, 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 lignite coal: Turów. Particles from this fuel were prepared in the same way as for bituminous coals, but weight of a typical lignite coal particle with a size of w2 mm, was 1.5e2 mg due to density differences. Results of proximate and ultimate analysis of fuels used in this study are shown in Table 3. Combustion of a relatively big particle cannot be directly compared to the combustion of PC in industrial units since some
Fig. 3. Coal particles used for experiments.
processes depend on the particle size, i.e. ignition mechanism, fragmentation, etc. Nonetheless, experiment with the use of 2 mm particle can provide more specific insight, especially when coupled with particle temperature profile measurements. Hence it can be of use for a development and validation of mathematical models. 3.2. Particle combustion Coal particle was first inserted into the shield-tube and when demanded atmosphere in the reactor was obtained, the shield lock was released and the particle was exposed. Combustion was considered complete when the temperature measured with the thin thermocouple dropped to the ambient temperature which was the reactor operating temperature and when there were no more visible changes happening within observed particle. Similarly, char particle combustion was conducted. After particle devolatilization and proper cooling inside the oil-shield tube, the atmosphere was switched from nitrogen to oxidizing mixture and then combustion was conducted in the same manner as for coal particles. Degassed chars were never removed from reactor before combustion experiment. 4. Results and discussion 4.1. Visual observations of the combustion When coal-particle heats up, volatile matter are released and at the temperature of volatiles ignition the gaseous yellow flame appears which indicates the combustion process. This phenomenon corresponds to the homogeneous ignition and takes place away from the coal surface. Once the volatile flame extinguishes, the devolatilized particle starts to combust and that is the second stage of coal combustion. Sometimes both stages can happen at the same time, when the volatile flame is still present but particle surface also starts to glow. This is joint heteroehomogeneous combustion and usually can be observed for pulverized coal particles. Volatile-free char particles ignite and combust only in heterogeneous mechanism, like the described second stage of coal combustion, where reactions take place between gaseous oxygen and the particle surface. The visible sign of the progressing char combustion process is particle glowing. At first vertices and edges start to glow, then combustion progresses over the entire visible surface. In Figs. 4e6 pictures of combustion of single particles are presented (photos are selected from high speed recordings). First picture in row (0 ms) was taken at the moment of ignition which in this study was intended as the first visible sign of combustion. It is
Table 2 Properties of CO2 and H2O at 950 C and 1 atm with reference to N2 (from Aspen Properties database). Property Density Specific heat capacity Thermal conductivity Mass diffusivitya (binary diffusion of O2 in X) Absorptivity/emissivity a
From Ref. [8].
kg/m3 kJ/kmol K W/m K m2/s
CO2
H2O
N2
O2
Ratio CO2/N2
Ratio H2O/N2
0.438 56.60 0.080 1.7E-04 >0
0.179 44.04 0.127 2.8E-04 >0
0.279 33.83 0.079 2.2E-04 0
0.319 35.75 0.086 e 0
1.57 1.67 1.00 0.79 e
0.64 1.30 1.61 1.29 e
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38 Table 3 Properties of the investigated coals. Fuel
Pittsburgh No8
South African Coal
Proximate analysis as received (on a dry basis) Moisture (%) 2.2 2.7 Volatile matter (%) 31.5 25.3 Fixed carbon (%) 53.0 57.7 Ash (%) 13.3 (13.6) 14.3 (14.7) LCV (MJ/kg) 28.35 26.01 Ultimate analysis (on a dry basis) Carbon (%) 74.2 71.4 Hydrogen (%) 4.8 4.0 Oxygen (%) (by diff.) 5.3 7.8 Nitrogen (%) 1.3 1.6 Sulphur (%) 0.89 0.63
Turów 12.9 48.9 33.4 4.8 (5.5) 22.86 66.3 5.8 24.3 0.6 0.55
worth noticing, that the particle surface between homogeneous and heterogeneous combustion was relatively dark which assures that stages of particle combustion did not overleap and took place one after another. Visual observations from experiments of charparticle combustion are herein not presented because of identical nature as the heterogeneous stage of coal-particle combustion. 4.1.1. Bituminous coals Pittsburgh and SAC coal-particles combusted in similar way (Figs. 4 and 5). When experiments were conducted with 35% H2O addition, the particle ignited and slowly developed volatile flame, at first visible only near the solid surface. After 100 ms, the particle was surrounded by a translucent flame (the particle was still visible) which front started to move away from the surface. At 700 ms the flame was very high and looked fully developed, but was weakly luminous which indicated that only a little amount of tars and soot appeared within this stage of combustion. At
approximately 1000 ms, more rapid burning of the particle began. Volatiles streams were escaping from the particle very quickly, in different places, sometimes getting far away from the solid and instantaneously combust, which was seen as small explosions. Jets of volatiles created some kind of escaping routes within the particle, where almost all remaining volatiles flowed later on. A flame from quickly escaping volatiles was very bright which indicated that soot and tars were main burning species. This effective combustion took place until the flame extinguished and the first stage of combustion was over, which for presented particles happened approximately 2.5 s after the ignition. Then, the dark particle surface started to glow, which indicated that the char combustion was progressing. The temperature of particle was still rising but at the time when the whole particle was already glowing, the temperature reached maximum and maintained around this value for few seconds of char stable combustion (Fig. 7). As heterogeneous combustion was progressing, particle was being “used up”, its size visually reduced until only small remain of ash was present. At this moment the particle glow faded away and the temperature started decreasing till it reached the temperature of surrounding. Combustion in air and 21% O2eCO2 mixture was similar to the process described above. At first a volatile flame appeared and after its extinguishing, char combustion proceeded. In nitrogen diluent mixtures, the volatile matter was often released in a jet form and combusted very brightly. In O2/CO2 atmosphere the flame was more translucent, which means that less tar and soot combusted. Also volatiles flow was more obstructed by thicken surrounding and the long flame tail was created (upwards or downwards). What was characteristic for experiments carried out in air, was that at the end of the volatile’s combustion, significant changes within solid particle occurred. A lot of a fly ash was released and the particle was swelling. This means that the pressure inside the particle was very high and altered particle internal structure. Explanation for this phenomenon
Fig. 4. Pictures from high-speed recording of single particle combustion of Pittsburgh coal in different atmospheres (all experiments without water vapour addition and one experiment with 35% H2O). Numbers under frames indicate time-scale of particle combustion, in ms. Zero represents the beginning of combustion (first visible sign).
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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 one experiment 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 experiment with 35% H2O). Numbers under frames indicate time-scale of particle combustion, in ms. Zero represents the beginning of combustion (first visible sign).
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vapour content caused significant change to physical properties of oxidizing O2/CO2 mixture. In addition, visual similarity of combustion phenomena in N2 and H2O/CO2, indicates that in pursuing overall similarity between oxy-fuel and conventional combustion, recirculation of wet exhaust gases should be promoted. Slightly higher temperature during combustion in mixture with H2O addition, also means that the higher flue gas recycle ratio will be needed to match the temperature inside the retrofitted boiler in case where wet recirculation is considered. Also the higher recycle ratio is preferential because of better velocity distribution inside boiler and better convective heat transfer. The same observation regarding wet flue gas recirculation arises from Wall et al. [6] theoretical calculations and was pointed out in review by Toftegaard et al. [2].
Fig. 7. Sample SAC char-particles temperature profiles during combustion in different atmosphere compositions.
is probably connected with the higher flame and particle temperature in nitrogen diluent atmosphere than in CO2 atmosphere experiments (Fig. 8). Higher particle temperature accelerates devolatilization which increases pressure of volatiles trapped in the particle even more and causes the particle swelling. On the other hand, CO2 presence was found inhibiting to the particle swelling but beside the influence of lower particle temperature, mechanism of that inhibiting behaviour is not fully explained. Borrego and Alvarez concluded that CO2 may participate in the surface process of crosslinking which they believe reduces the swelling [5]. On the contrary e in increased oxygen atmosphere, the swelling behaviour was visible for both diluent gases. Then also in CO2 atmosphere, the pressure within the particle was very high because combustion in enriched O2 atmosphere in both diluent gases was quicker and took place closer to surface, resulting in faster particle heating. When comparing the combustion behaviour of particles in nitrogen, carbon dioxide and mixture of water vapour/carbon dioxide, the last one looked similar both to the combustion in nitrogen and combustion in CO2. The conclusion that can be drown is that high amount of water vapour addition cancelled some part of CO2 influence on the combustion. Experiments with lower amount of water addition (herein not shown) visually looked similar to experiments carried in O2/CO2 atmosphere only. But 35% of water
Fig. 8. Particles average temperatures during combustion in N2 and CO2 diluent atmospheres.
4.1.2. Lignite coal In general, for lignite coal, the homogeneous stage of particle combustion was quicker than for bituminous coals (Fig. 6). Flame was fully developed within 20e60 ms after ignition in air and mixtures containing high amount of water vapour. In O2/CO2 without H2O addition, particle surrounding by flame took longer, at least 60 ms. Once again in nitrogen diluent mixtures, a lot of fly ash was released which at the pictures looks like sparks at front of the flame. The same occurrence presented itself when particle burned in 35% H2Oe44% CO2e21% O2 mixture. Bright luminosity of the flame was observed in every experimental conditions, but within mixtures 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 of volatiles combustion, while in CO2 mixture its occurrence took place after 700 ms from ignition and lasted till flame extinguished. Very interesting phenomenon was observed in this atmosphere during the next stage of combustion. When particle started to glow, indicating char combustion, also gaseous particle surrounding started to glow. Luminescent areola lasted to the end of particle combustion. While char contains no more of volatile matter, this glow should be attributed only to reactions engaging gaseous species that are formed during incomplete heterogeneous combustion or gasification reaction. 4.2. Temperatureetime history profiles Based on measurements from 0.5 mm thermocouple, temperatureetime history profiles for single particles combustion were
Fig. 9. Particles average temperatures during combustion in oxy-fuel atmosphere in regards to H2O content.
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obtained. Fig. 7 presents sample temperature profiles for typical SAC char particles in every experimental conditions. For all combusted particles, the average temperatures were obtained, taking the average temperature from 5 or 2.5 s of particle stable combustion for bituminous coals and lignite coal respectively. In case of Turów coal, the shorter time was include to the calculation, because particles of lower rank coal were more reactive thus combustion was quicker and the stable part of it lasted less than the stable part of bituminous combustion. Figs. 8 and 9 show comparison of the average particle temperature during experiments in all investigated oxidizer compositions Results are divided for more clarity into two diagrams: the first one compiles experiments in oxy-fuel and nitrogen diluent atmospheres while the second one presents data for oxy-fuel conditions only, in relation to the amount of water vapour addition. The highest temperatures during particle combustion, up to 1320 C were noticed for every fuel in experiments conducted in 35% O2/65% N2 mixture. When nitrogen was switched for CO2, particle temperature was lower, but difference was greater in case of coal particle combustion than in char particles combustion. The only distinction between coal and char particle combustion is devolatilization stage and volatile matter combustion. So it may be assumed that the presence of CO2 hinders volatile matter release and influences its combustion, resulting in lower combustion temperature. Similar results were observed for lower oxygen concentration (21% O2). When experiments were carried in air, temperature curves reached 1220 C. When again nitrogen was replaced with 79% CO2, temperatures of burning bituminous particles were usually the lowest 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 mixture than in O2/N2 mixture can be explained by changes in heterogeneous reactions. It can be caused either by more difficult O2 diffusion through CO2 molecules or by the gasification reaction between solid carbon and CO2. The Boudouard reaction may contribute to the char consumption but is strongly endothermic and demands 172 kJ energy for every reacted mol of solid C [1]. Because of this endothermic character, the gasification should lower the particle temperature which is consistent with presented results. On the other hand, limited O2 diffusion could also not be excluded, because experimental conditions chosen for this study fall on the border between region A and B, where in region B diffusion control of combustion is present as well as gasification (see Fig. 1). When particles burned with the water vapour addition, their temperature profiles were close to each other for every tested H2O concentration. Only in case of Pittsburgh char and Turów coal particles all temperatures measured within H2O enriched experiments were almost identical. For the rest of tested fuels, temperatures did not vary significantly, but were the highest for the largest water content in oxidizer and the lowest in the dry oxy-fuel conditions. Gasification reaction that involves H2O is less endothermic than the reaction of carbon and CO2. The amount of energy needed for the H2Oechar reaction equals 131 kJ per mol of C solid [7], which is approximately 24% less than amount of energy necessary for the CO2echar reaction. For the H2Oechar reaction also the activation energy is lower (230 kJ/mol for H2Oechar reaction versus 250 kJ/mol for the CO2echar reaction [1]) and this means that the H2O gasification reaction is more promoted. What follows from above facts is that when the H2Oechar gasification occurs, the particle temperature should be expected to be higher than when the CO2echar reaction takes place, which can explain results presented herein. One should remember that when the water vapour addition in the mixture was increased, at the same time CO2 concentration decreased, due to fixed O2 fraction. Observed temperature
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differences can be attributed to the four times higher reaction rate for the H2Oechar gasification than the CO2echar gasification [4] as well as to H2O lower than CO2 molar specific heat (Table 2). Despite the fact that lignite coal particles weighted less than bituminous coal particles, temperatures obtained during lignite combustion were only slightly lower than in case of higher rank coal combustion. In atmospheres with 35% O2 and in air, the temperature of Turów particles almost equalled Pittsburgh char particles. On the other hand, in 21% O2 oxy-fuel conditions with and without H2O addition, the lignite particle temperature was always the same (around 1090 C) and was the lowest temperature of all particles. The temperature of lignite combustion was not sensitive to water vapour presence in oxidizer, but more tests should be performed for this type of coal to confirm these results. It was pointed out by Chen et al. [3], that the coal type can be an important factor that also should be taken into consideration if the temperature under oxy-fuel conditions should match the temperature in conventional combustion. Results presented herein indicate that N2 replacement with CO2 in the experimental mixture lowered lignite temperature less than bituminous coals temperature (temperature reduction around 40 C for Turów, and around 60 C for bituminous). This would suggest that the higher flue gas recycle ratio is necessary in case of the lower rank lignite coal combustion than in higher rank coals but again further investigations are required to confirm these findings. 5. Conclusions Single particle combustion with 2 mm particles as tested objects can provide good insight into fundamental knowledge of combustion and ignition. Introduced SPC stand was used to investigate coal and char particles combustion in air and under oxy-fuel conditions, with and without additional water vapour content. Conclusions from this study may be summarized as follows: Particle in O2/CO2 mixture burned with lower temperature than in N2 diluent atmosphere (beside Pittsburgh char particles). The highest temperature difference (70 C) was observed for bituminous coal experiments. Water vapour addition in oxy-fuel atmosphere increased particle temperature during combustion in case of Pittsburgh coal, SAC char and SAC coal particles. This behaviour can be attributed both to H2O lower than CO2 molar specific heat and more promoted, less energy demanding H2O gasification reaction. Visual similarity was observed between particle combustion in air and in 35% H2Oe44% CO2e21% O2 mixture. The halo effect observed during lignite coal particle combustion in high oxygen containing oxy-fuel environment is interesting and cannot be definitely explained. Whether there were gasification reactions involved and CO combustion took place away from particle surface, should be carefully considered and more extensively tested (for example with the use of more sophisticated optical and spectroscopic methods). Observed occurrence of intensive glow around lignite char and interpretation of this phenomenon should be considered as a subject of open question. Acknowledgements Presented research regarding bituminous coals was funded by the European Commission 7th FP through RELCOM project, No. 268191. Lignite coal investigation was sponsored by National Research Development Centre through Strategic Program, Grant No. SP/E/2/666420/10. Also contribution from Dr Jaros1aw Hercog is appreciated.
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