Energy Fuels2010,24,4803–4811:DOI:10.1021/ef100314kPublished on Web08/31/2010Experimental Investigation of the Combustion of Bituminous Coal in Air and O2/CO2 Mixtures:1.Particle Imaging of the Combustion of Coal and Char Lian Zhang,*,†Eleanor Binner,†Luguang Chen,†Yu Qiao,†Chun-Zhu Li,†,‡Sankar Bhattacharya,†and Yoshihiko Ninomiya§†Department of Chemical Engineering,Monash University,GPO Box36,Clayton Campus,Victoria3800,Australia,‡Curtin Centre for Advanced Energy Science and Engineering,Curtin University of Technology,WA6102,GPO Box U1987, Perth,WA6845,Australia,and§Department of Applied Chemistry,Chubu University,1200Matsumoto-Cho,487-8501,Kasugai,Aichi,JapanReceived January14,2010.Revised Manuscript Received August18,2010Combustion of a low-volatile bituminous coal in air versus two O2/CO2mixtures(21/79and27/73,v/v)was conducted at two furnace temperatures of800and1000°C in a lab-scale drop tube furnace(DTF).Through in situ photographic observation and measurement of overall coal burnout rate,CO emissionprofile,and unburnt char properties,a variety of distinct phenomena relating to oxy-fuel combustion hasbeen revealed.Consistent with the literature,the significant thermal effect of CO2due to its large productof C p F(specific heat capacity and density)relative to that of N2retarded volatile ignition in the two O2/CO2mixtures.As a result,the volatiles released in O2/CO2remained as a thick protective sheath on char surfacefor a relatively long duration,which mainly converted into CO through partial oxidation in21%O2/79%CO2.Increasing the O2fraction to27%in CO2triggered the ignition/oxidation of the unburnt volatilesonce their concentrations were critically accumulated on char surface in a relatively low position in theDTF.Char oxidation behavior in the late stages of the DTF was also greatly changed under oxy-fuelconditions.Due to an insufficient O2in char particle vicinity,the partial oxidation and even gasification ofchar to CO were favored during oxy-firing,which yielded less enthalpy heat and hence lowered charparticle temperature substantially.Char consumption rate was,however,affected little or even slightlyincreased.A detailed mathematical modeling is required to quantitatively clarify the oxidation behavior ofcoal char in the presence of the abundant CO2in the DTF.IntroductionCoal combustion is one of the major sources for power generation,providing approximately37%of the electricity requirement in the world.1Its greenhouse gas emissions, particularly of carbon dioxide(CO2),however,have been facing stringent regulations with respect to the climate change. Efforts must be made to reduce and eventually eliminate CO2 emission in the short/medium term,thereby maintaining a sustainable utilization of coal in the carbon-constrained future.Oxy-fuel combustion is a process of burning coal in a gas stream of oxygen(O2)mixed with recycled flue gas(RFG), generating a CO2-rich flue gas that is potentially subjected to direct sequestration/storage with minimal treatment.1,2Ex-tensive studies in both pilot-plant and lab scales have pointed out the pronounced influence of gas composition(air versus O2/CO2)on coal combustion performance.The heat transfer and temperature distribution in a furnace are greatly affected by the large specific heat capacity of CO2.1,3,4Coal ignition is delayed in O2/CO2in comparison to in O2/N2with the same O2concentration.To match the flame/particle temperature in air,a large amount of O2in CO2,typically around30%,is required.1Coal conversion rate,char properties,and reactivity are also affected by the replacement of air with an O2/CO2 mixture.The influence of bulk gas,however,varies greatly with coal property and combustion facility/condition.At a given O2concentration,coal burnout rate in O2/CO2is slower than in O2/N2.5,6This is not unexpected as a lower particle/ flame temperature exists in O2/CO2.A slow transfer of O2in CO2(20%less than in N2)also greatly retards the char-O2 oxidation reaction on the condition that this reaction is controlled by O2diffusion through an external gas boundary layer.7The endothermic char-CO2gasification reaction,as most likely occurring at high temperatures,8,9further makes oxy-fuel combustion complex.Knowledge for oxy-fuel combustion is still scarce.One major reason is that coal combustion is a very complex process governed by transient phenomena and a series of chemical*To whom correspondence should be addressed.Phone:þ61-3-9905-2592.Fax:þ61-3-9905-5685.E-mail:lian.zhang@.(1)Buhre,B.J.P.;Elliott,L.K.;Sheng,C.D.;Gupta,R.P.;Wall, T.F.Prog.Energy Combust.Sci.2005,31(4),283–307.(2)Molina,A.;Shaddix,bust.Inst.2007,31,1905–1912.(3)Kakaras,E.;Koumanakos,A.;Doukelis,A.;Giannakopoulos,D.;Vorrias,I.Fuel2007,86,2144–2150.(4)Khare,S.P.;Wall,T.F.;Farida,A.Z.;Liu,Y.;Moghtaderi,B.; Gupta,R.P.Fuel2008,87,1042–1049.(5)Bejarano,P.A.;Levendis,bust.Flame2000,153,270–287.(6)Liu,H.;Zailani,R.;Gibbs,B.M.Fuel2005,84,833–840.(7)Shaddix,C.R.;Molina,A.Proceeding of the5th joint meeting of the US sections of the Combustion Institute,San Diego,CA,USA,March 25-28,2007;Paper G24.(8)Rathnam,R.K.;Elliott,L.K.;Wall,T.F.;Liu,Y.;Moghtaderi,B.Fuel Process.Technol.2009,90,797–802.(9)Shaddix,C.R.;Murphy,J.J.Proceedings of the20th Pittsburgh Coal Conference,Pittsburgh,USA,Sept15-19,2003;CD-ROM.reactions occurring at each subprocess.The role of CO2in each process has not been clarified yet.Most of the works to date mainly focused on heat transfer,4,10,11global conversion rate of coal/char,6,12and pollutant emissions.6,13Little is known about the details of each subprocess.One can imagine that,once the ignition of the volatiles is delayed with shifting bulk gas from air to an O2/CO2mixture,all the subsequent subprocesses would be delayed and altered accordingly.In this regard,a state-of-the-art high-speed camera with a maxi-mum shutter speed of2000frames per second(fps)was employed to in situ record the dynamic oxidation of coal in a transparent quartz DTF.In addition to the previous investi-gation of particle velocity with the use of high-speed camera,14 this study focused on photographing transient phenomena to examine in situ the oxidation dynamics of the burning of volatiles and char pared to the imaging system used in the literature,15,16the high-speed camera employed here is advanced enough ensuring the resolution of coal combustion sequence down to2ms.The luminosity(i.e., brightness)of discernible spots in the camera’s field of view (FOV)was used as a sign of the oxidation intensity of volatiles and char.Measurement of coal conversion rate,char proper-ties,and CO emissions was also carried out to clarify coalcombustion behavior in O2/CO2.By augmenting a companion paper on ash formation in air versus O2/CO2,17this study aims to provide further evidence to promote the understanding on the role of CO2on the combustion of bituminous coal and hence shed new lights into the retrofitting of existing power generation plants with oxy-firing technology.Experimental SectionCoal Property.A bituminous coal from China was tested for the combustion experiments.It was pulverized to106-153μm and air-dried prior to use.As shown in Table1,the coal sample tested contains24.4wt%volatile matter(VM)and a large quantity of ash(27.8%on dry mass basis).The char generated from coal pyrolysis in N2at the furnace temperature of1000°C was also tested.Coal pyrolysis was conducted in a lab-scale DTF,as will be explained in detail later.All the volatile matter was removed during coal pyrolysis,resulting in a char yield of 55.2wt%on the dry-and-ash-free(daf)basis.Coal Combustion Facility.An electrically heated DTF coupled with a transparent quartz reactor with a length of 2000mm and two cylindrical chambers was employed for coal combustion.14,18Coal at a feeding rate of∼0.5g/min was entrained by1.0L/min cold primary gas into the top of the inner chamber(50mm in diameter)of the quartz reactor.The majority of the gas used for combustion,namely secondary gas, was introduced at9.0L/min from the bottom of the outer chamber(80mm in diameter).It was heated to furnace tem-perature before passing through a quartz frit and mixing with coal and primary gas at the top of the inner tube of the reactor.Due to this unique configuration,a very uniform gas tempera-ture is guaranteed in most of the whole reactor.14The same temperature profile was confirmed between air and the two O2/CO2mixtures at a given furnace temperature.Three water-cooled coal injectors with a length of0,600,and 1200mm protruding into the inner chamber of the quartz reactor were employed for coal combustion at a reactor distance of1800,1200,and600mm,respectively.Coal particle residencetime for a reaction distance of600mm is estimated to be∼1s.14 Therefore,the longest residence time for coal particle in DTF is ∼3s,which is similar to the industrial scale.A flask and a thimble filter with a cutoff size of∼0.5μm were installed atabout100mm downstream the reactor to collect particles,which were also continuously quenched by water and dry ice at an estimated quench rate of∼5000°C3s-1.This rate is comparable to that usually achieved by a water-cooled nitrogen-quenched suction probe in the conventional DTF reaction facility.19,20It is high enough to prevent any secondary reactions of char and ashes.This sampling method is advantageous in terms of sample collection and coal conversion determination.Conventionally, only a portion of coal combustion products is iso-kinetically sucked by a sampling probe,which is then used for coal burnout calculation based on the ash-tracer method.21The accuracy of this method,however,depends on a number of assumptions such as that the particles collected are representative and that ash recovery is100%when compared with the original mineral matter content in raw coal.These assumptions are unrealistic when the vaporisation of metals is prominent,especially during low-rank coal combustion.The unburnt char yield was calcu-lated on the carbon balance basis:Char yieldð%,dafÞ¼M char-M ash-in-charcoal-M ash-in-coalÂ100ð1ÞThe symbols M char and M coal denote the mass of char collected and coal fed during an experiment,respectively.M ash-in-char and M ash-in-coal are the mass of ash in the char and coal respectively, which were determined by burning char and coal at a heating rate of<10°C/min up to800°C in a muffle furnace.The ash Table1.Properties of the Coal Sample Tested HereProximate Analysis,wt%,As Receivedmoisture 3.8volatile matter(VM)24.4 fixed carbon(FC)44.0 ash27.8Ultimate analysis,wt%dafcarbon76.0 hydrogen 5.2nitrogen 1.2oxygenþsulfur(by diff.)17.6Ash Composition,wt%SiO241.3 Al2O330.4 Fe2O3 5.7CaO 2.4MgO0.5TiO2 2.4Na2O0.1K2O0.8SO315.3 P2O50.4Cl0.2ZnO0.02(10)Andersson,K.;Johansson,R.;Hj€a rstam,S.;Johnsson, F.; Leckner,B.Exp.Thermal Fluid Sci.2008,33,67–76.(11)Bejarano,P.A.;Levendis,bust.Flame2008,153,270–287.(12)Liu,H.;Zailani,R.;Gibbs,B.M.Fuel2005,84,2109–2115.(13)Andersson,K.;Normann,F.;Johnsson,F.;Leckner,B.Ind.Eng. Chem.Res.2008,47(6),1835–1845.(14)Zhang,L.;Binner,E.;Qiao,Y.;Li,C.-Z.Energy Fuels2010,24, 29–37.(15)Shaddix,C.R.;Molina,b.Inst.2009,32,2091–2098.(16)McLean,W.J.;Hardesty,D.R.;Pohl,b.Inst.1981,8,1239–1248.(17)Zhang,L.;Jiao,F.;Chen,L.;Binner,E.;Bhattacharya,S.; Ninomiya,Y;Li,C.-Z.Fuel,2009,submitted.(18)Zhang,L.;Binner,E.;Qiao,Y.;Li,C.-Z.Fuel2010,89,2703–2712.(19)Zhang,L.;Sato,A.;Ninomiya,Y.Fuel2002,81,1499–1508.(20)Borrego,A.G.;Alvarez,D.Energy Fuels2007,21,3171–3179.(21)Khan,N.;Dollimorey, D.;Alexander,K.;Wilburn, F.W. Thermochim.Acta2001,367-368,321–333.loss in this process is minor as the sulfur and chlorine contents inchar/ash samples are less than3wt%on the ash mass basis.No carbonates were detected by X-ray diffraction(XRD)either.17Combustion experiments were conducted at a high percen-tage excess O2(defined as the O2supplied in excess of that required for stoichiometric combustion of coal1)of10%at the O2fraction of21%in a diluent gas.Bulk gas flow rate was kept constant throughout this study,that is,10L/min in total, whereas its composition varied from air to21%O2/79%CO2 and27%O2/73%CO2.The once-through CO2rather than RFG was used.The first O2/CO2mixture bears the same O2 fraction with that in air.The diffusion rate of O2within it can be 20%less than in N2.7An O2/CO2ratio of27/73increases the O2 diffusion rate to a level close to that in air.For comparison of the combustion of this coal and an air-dried Victorian brown coal containing12.5wt%moisture,18∼0.1g/min water was also fed into the reactor,yielding a steam concentration of about1.2% (v)in bulk gas.Three replicates were made for each combustion condition.A standard error less than10%was confirmed for coal conversion and ash recovery.Flue gas composition was also monitored online by a Servo-mex4900CO/CO2analyzer installed downstream of the reactor. The intrinsic error(accuracy)of CO is<0.1%of its measure-ment range(1.0%,v),equaling e10ppm(v)in flue gas.The CO detector was precalibrated by standard gases in each run.Char Characterization.The specific surface areas of char samples were determined using CO2at0°C through adsorption in a Micromeritics ASAP2020instrument.Chars were degassed at about350°C overnight under vacuum prior to gas adsorption experiments to eliminate moisture or condensed volatiles.CO2 adsorption isotherms were performed at0°C up to a pressure of 0.035Torr.The Dubinin-Radushkevich(D-R)equation was used for surface area calculation on the daf basis,assuming a surface area of∼0.8m2g-1for the ash.20High-Speed Camera Observation and Image Processing.Ob-servation ports with a diameter of20mm are located at300mm apart along the furnace for in situ optical diagnostics.14Each of the levels has two ports arranged as opposite collinear pairs to eliminate the forward scattering effect of the furnace wall.For the observation of any distances between two adjacent ports, coal injectors of various lengths were employed.For instance,a coal injector of200mm enables us to observe a distance of100 mm through the first observation port.By this method,coal combustion characteristics were resolved to50mm.A high-speed camera(MotionPro Y-3)with a high sensitivity of3000ASA monochrome was employed for phenomenon ob-servation,which was mounted with a105mm macro lens and an antiblooming CMOS sensor.The camera was aligned horizon-tally along the observation ports,focusing on the centerline of the reactor where the particle density is the highest.The resulting FOV and pixel resolution are2Â2cm and∼100μm square, respectively.Since the alignment and tuning of camera are very time-consuming,the observation of different distances was made by varying coal injector length rather than adjusting the camera’s height,particularly when the distances are located between two adjacent ports on the reactor wall.This also avoided the aberration of imaging focus caused by the frequent reallocation of camera.Moreover,one has to bear in mind that ash deposit on the reactor wall can blur the images.This problem was minimized by employing a new reactor once the image captured by high-speed camera was found to be blurred. Ash deposition is also one major concern for us to choose a low coal feeding rate throughout our studies using DTF. Camera’s shutter speed was set to500fps in this study;the sensor’s gain wasþ6dB and the exposure time was980μm.No extra illumination was provided,the light source simply being the hot furnace and emitting particles and volatiles.Calibration of camera shutter speed was conducted periodically by internal and external methods.The internal method is a built-in proce-dure in the camera,whereas the external method involves the use of high-speed camera to record a running electric counter with a time resolution of1ms.Each distance/stage was recorded at least three times,yielding 3000pictures with an interval of2ms.Image processing for the properties of discernible spots and volatiles cloud/flame was conducted through the use of Image-Pro6.2program,which is able to simultaneously count and measure size(pixel numbers) and luminosity of multiple objects in a photograph.The spot size measured here is indeed the track of the spot travelled in an interval of2ms during photography.Luminosity represents a level of grayness or brightness,ranging in value from0denot-ing completely black to255for a completely white object in an8-bit gray scale image,which,though not confirmed if pro-portional to thermal radiation,apparently provides direct evi-dence on the oxidation intensity of char particles and volatile cloud.The brighter the particle is,the more intense its ignition/ oxidation is.Results and DiscussionCoal Burnout Rate and Char Conversion.Coal burnoutrates in Figure1demonstrate the importance of bulk gascomposition.At the furnace temperature of800°C(panel a),coal conversion in air reached∼50%daf at600mm,which is ∼14%higher than the inherent VM yield in coal.In contrast, only∼35wt%daf of coal was consumed at600mm in21%O2/79%CO2.As such a low value is equivalent to the yield ofVM,it is indicative that the oxidation of VM is the principalphenomena occurring before600mm in this gas atmosphere.Little of the fixed carbon(FC)or char was consumed at thisstage.Increasing the O2fraction in CO2to27%improvedcoal conversion at600mm to∼72%,suggesting that besidesvolatiles,a large fraction of char was also consumed in thisgas atmosphere,the extent of which is clearly higher than in air. Figure1.Coal conversion rate as a function of reaction distance for the furnace temperatures of800°C(a)and1000°C(b).In this regard,it is reliable to conclude that a 27%O 2fraction in CO 2is sufficient to exceed air in terms of coal conversion.The gap of coal conversion among three gases became smaller and even negligible with the increase in reaction distance (i.e.,coal residence time).Irrespective of bulk gas composition,the overall coal combustion rate was significantly improved when the fur-nace temperature was increased to 1000°C.The difference of coal conversion among three bulk gases was also greatly reduced.As evidenced in Figure 1b,an unburnt char yield of ∼10%was achieved at 600mm in air,demonstrating an extremely intense oxidation of both volatiles and char.Replacing air by 21%O 2/79%CO 2still played a negative role in coal conversion,as evidenced by the ∼20%unburnt coal at 600mm in this gas atmosphere.Increasing the O 2fraction in CO 2to 27%reduced the unburnt coal content to ∼5wt %at 600mm,further demonstrating the intense coal combustion in this gas atmosphere.With the reactor distance increasing to 1200mm,nearly all the carbonaceous materials in coal were burnt out irrespective of bulk gas composition.Since the oxidation of char is the principal phenomenon occurring from 600mm onward,char conversion rates at three distances/stages,that is,0-600,600-1200,and 1200-1800mm,were further extracted from Figure 1.Char con-version at the first stage was determined byChar conversion,%¼FC -UC 600FCÂ100ð2ÞWhere the symbol UC 600denotes the mass of the unburnt carbon collected at 600mm,with a unit of wt %on the daf basis of raw coal.Regarding char conversion in the latter two stages,it was determined byChar conversion,%¼UC i -UC i þ600FCÂ100ð3ÞWhere the subscript “i ”denotes 600or 1200mm,and “i þ600”denotes a distance 600mm downward of i .As illustrated in Figure 2,at a low furnace temperature of 800°C,char conversion in air progressed steadily withapproximately 30,40,and 20%consumed respectively from the first stage through to the third stage.Little char was consumed at the first stage in 21%O 2/79%CO 2,in contrast to the fastest conversion of char before 600mm in 27%O 2balanced with CO 2.For the middle stage (600-1200mm),the char oxidation rate is,however,the largest in 21%O 2/79%CO 2.Apparently,even though a noticeable delay occurred for coal oxidation at the initial stage,the char attained in the oxy-firing environment exhibits good reactivity in terms of mass conversion in the late stages.Char oxidation before 600mm was greatly intensified in either gas environment at the furnace temperature of 1000°C.Same as at 800°C,a descending sequence of 27%O 2/73%CO 2>air >21%O 2/79%CO 2was observed at the first stage.Moreover,it is further confirmed that char oxidation rate at the middle stage is the largest in 21%O 2/79%CO 2.Irrespective of gas environment,the majority of the char was consumed before 1200mm,thus little of it remained unburnt at the last stage.Coal Combustion Sequence.Oxidation of volatiles played an important role at the initial stage (0-600mm)during coal combustion.In this regard,the combustion sequence of raw coal before 600mm was recorded by the high-speed camera at several distances,including coal pyrolysis,volatile ignition,and oxidation.Typical photographs for coal combustion in air are shown in Figure 3.When coal particles were injected into hot air at the furnace temperature of 800°C (panel a),they initially underwent heating before 200mm,thus most of the particles were invisible or weakly luminous in the camera’s FOV.Round luminous spots were observed with the reaction length extend-ing to 300mm,indicative of the release and ignition of volatiles.The round shape was attributed to the ejection of both gaseous and tarry volatiles out of coal particles.Particularly,the tarry volatiles are dominant in the products of bituminous coal pyrolysis,which preferentially remained on particle surface due to the viscous properties.22,23Upon meeting oxygen,the evolved volatiles rapidly ignited from 300mm onward,the oxidation of which progressed steadily and was intensified with the reactor length down to 400mm.Accordingly,a large quantity of luminous spots attached with volatile flame were observed in camera’s FOV.The volatile oxidation was nearly complete at 600mm,hence only a few of sparkling spots were observed at a long distance.Irrespective of reaction distance,coal combustion inten-sity is greatly improved with furnace temperature increasing to 1000°C,as expected.As illustrated in panel b of Figure 3,coal ignition in air occurred before 150mm,relative to 300mm for 800°C,yielding a large quantity of strongly incan-descent spots.The oxidation of volatiles was further intensi-fied at 250-300mm,as evidenced by the presence of vast luminous spots with long volatile trails in the camera’s FOV.The oxidation of volatiles was complete by 400mm,leaving abundant elongated spots with weak luminosity.Char oxi-dation should commence at this step or between 300and 400mm,as the volatile tails disappeared.Most of the char was also consumed before 600mm,thus leaving very limited discernible spots at 600mm,in agreement with a low char yield (∼10%in Figure 1b)achieved at this stage.A Similar coal combustion sequence was observed in the two O 2/CO 2mixtures,that is,coal pyrolysis occurred initially,Figure 2.Char conversion as a function of reaction distance.Panels a and b depict the furnace temperatures of 800and 1000°C,respectively.(22)Chen,J.C.;Taniguchi,M.;Ito,K.Fuel 1995,74(3),323–330.(23)Ponzio,A.;Senthoorselvan,S.;Yang,W.;Blasiak,W.;Eriksson,O.Fuel 2008,87,974–987.followed by volatiles ignition,volatiles oxidation,and char oxidation.Moreover,due to a prior heat-up of the secondary gas and a similar thermal conductivity between N 2and CO 2,2,20the initial heat-up profile of coal before ignition was plausibly the same in the three bulk gases.2A similar onset time for coal devolatilization is also expected in the three bulk gases.However,due to the distinct properties of CO 2,the distinguishable ignition and oxidation behaviors in O 2/CO 2were revealed by high-speed camera photography.As indicated by the typical multiple photographs for a short distance of 150mm in Figure 4,the round spot highlighted by a circle in air was weakly incandescent at the beginning of photography.It ignited quickly in 4ms,and consequently emitted strong radiation around particles in the next photo-graph.The release of volatiles and their ignition continuedsteadily due to the thermal feedback from volatiles oxida-tion.The luminous spot in camera’s FOV thus gradually grew in size.Coal ignition was greatly delayed with the bulk gas shifting from air to 21%O 2/79%CO 2.As indicated,the volatiles evolved were mostly accumulated on char surface,which were even difficult to ignite in 12ms before leaving camera’s FOV.This is attributed to the distinct properties of CO 2.Physically,the combination of heat capacity (C p on molar basis)and density (F )of a gas gives a measure of the thermal sink for any heat that is chemically released.2Therefore,the ignition of volatiles was delayed,and the oxidation of volatiles was elongated in this gas,as the product of C p F of CO 2is around 1.7times that of N 2.Due to this reason,the unburnt volatiles were accumulated as thick cloud as a protective sheath on char particle surface,which signifi-cantly blurred the spots in camera’s FOV,and also doubled the average size of the discernible spots from 6.0pixel numbers in air to 13.1in 21%O 2/79%CO 2(as suggested by statistically measuring through Image-Pro 6.1,data not shown here).Increasing O 2fraction in CO 2to 27%is beneficial for volatile ignition,as the product of C p F of the bulk gas is reduced.It is however still insufficient to match the product of C p F of air.As suggested by the spots in circle for 27%O 2/73%CO 2in Figure 4,the volatiles released were only partly ignited in 4ms,most of which still remained unburnt (and thus gray in the camera’s FOV)until 12ms and greatly blurred char particles in the camera’s FOV.Apparently,improving the O 2fraction in CO 2to a higher level such as 30-35%is essential.5The optical intensities of the discernible spots at three different stages for the furnace temperature of 1000°C were further statistically processed and are shown in Figure 5.Note that 300-1000individual spots at a distance were counted for the average optical intensity and standard deviation.The error bar denotes twice the standard devia-tion.In a given gas atmosphere the largest intensity was observed at 250mm,responding to intense volatiles oxida-tion providing vast thermal feedback to coal particles.The low intensities at 150and 600mm account for coal devola-tilsation/ignition and char oxidation,respectively.In both cases fewer of the evolved volatiles were oxidized.This measurement is consistent with the photographs illustrated in Figures 3and 4.Figure 3.Air combustion sequences at 800°C (a)and 1000°C (b).Photos 1-4in panel a,for 800°C,were taken at the distances of 200,300,400,and 600mm,respectively.Photos 1-4in panel b,for 1000°C,were taken at 150,300,400,and 600mm,respectively,cited from ref 14.Figure 4.Dynamic information for the release of volatiles and ignition at the reactor distance of 150mm and the furnace tempera-ture of 1000°C.The bulk gas composition affected coal particle luminosity differently at a different step.For both coal pyrolysis at 150mm and char oxidation at 600mm,the average intensity of coal particles in the two O 2/CO 2mixtures was reduced to less than half of those in air,indicating the substantially large thermal effect of CO 2at these two stages.However,at 250mm for an intense oxidation of volatiles,the optical intensity of coal particles decreased in a sequence of 27%O 2/73%CO 2>air >21%O 2/79%CO 2,which is inconsistent with other distances,but in agreement with the sequence of coal burnout rate in Figures 1and 2.Furthermore,as the luminosity of coal particles in 27%O 2/73%CO 2was in-creased by nearly 20%compared to air (according to the difference of the average grayness between 27%O 2/73%CO 2and air,175versus 150),it is indicative that the volatiles evolved during coal pyrolysis was accumulated and oxidized until 250mm in 27%O 2/73%CO 2,relative to the intense ignition and oxidation of volatiles in 150-200mm in air.Volatiles were rarely ignited/oxidized before 250mm in 21%O 2/79%CO 2,although more of the unburnt volatiles were accumulated on the char surface.This should be attributed to the lower O 2concentration on coal particle surface.The slow diffusion of O 2in CO 2greatly restricted its local quantity on coal particle surface.Conversion of the unburnt volatiles before 600mm in O 2/CO 2is noteworthy.As few of them were ignited at the low furnace temperature,800°C,the volatiles evolved could decompose into gaseous species and/or light hydrocarbons through partial oxidation.The emissions of CO as a function of reaction distance in Figure 6partly proved this hypothesis.As illustrated in panel a,for each gas atmosphere,a large amount of CO was emitted at 600mm.At the furnace temperature of 800°C,approximately 8.0wt %daf of CO was emitted at 600mm in 21%O 2/79%CO 2,which was entirely contributed from volatiles decomposition,as little char was oxidized at this distance.Increasing the O 2fraction in CO 2enhanced the diffusion rate of O 2.Therefore,the amount of unburnt CO dropped to less than 4wt %daf at 600mm.Less CO was emitted at 600mm in air,suggesting its rapid mass transfer and oxidation into CO 2in nitrogen.This reaction might happen concurrently with coal oxidation to CO,rather than slowly in the oxy-fuel cases.Moreover,it is clear that,irrespective of the bulk gas composition,the COgenerated from volatiles oxidation before 600mm was rapidly oxidized into CO 2with the reactor distance increas-ing to 1200mm onward.Increasing furnace temperature to 1000°C further increased the oxidation rate of CO into CO 2.As shown in panel b,the CO content emitted at 600mm in 21%O 2/79%CO 2only accounts for 0.9wt %daf,relative to 0.7and 0.4wt %daf emitted in the 27%O 2/73%CO 2and air,respectively.Char Oxidation in Air versus O 2/CO 2Mixtures.Char oxidation is the principal phenomenon occurring from 600mm onward.Its observation with high-speed camera was not made in the raw coal case due to the unavailability of coal injectors to access any distance between 600mm and 1200mm.In contrast,a char sample generated by coal pyrolysis in N 2at 600mm and 1000°C was fed into DTF for photography.High-speed camera observations were made at several distances before 600mm,which was anticipated to provide information about the combustion of raw coal between 600and 1200mm.The furnace temperature of 1000°C and two bulk gases (air vs 21%O 2/79%CO 2)were tested.Char ignition occurred slowly compared to volatiles.There are two major types of luminous spots observed during char ignition,as illustrated in Figure 7for a reaction distance of 200mm from coal injector in air.The round spot (see panel a)denotes porous char particles formed from coal swelling in inert N 2,which was dim at the beginning of photography,gradually turning bright due to ignition in 24ms.With time further increasing to 34ms,the round spot became smaller and dimer again,indicating the completion of char oxidation.The rod-like spot (see panel b)indicatesFigure 5.Statistic comparison of the optical intensity of luminous spots observed at different stages in three bulk gases at the furnace temperature of 1000°C.Figure 6.CO emission versus particle residence time at the furnace temperatures of 800°C (a)and 1000°C(b).。
