Application of TG–FTIR to the determination

Application of TG–FTIR to the determination of organic oxygenand its speciation in the Argonne premium coal samplesJ.A.MacPhee *,J.-P.Charland,L.GirouxCANMET Energy Technology Centre-Ottawa,1Haanel Drive,Ottawa,Ontario,Canada K1A 1M1Received 1April 2005;accepted 1October 2005AbstractDuring rapid pyrolysis of coal,TG–FTIR (thermogravimetry –Fourier transform infrared)technique can be effectively used to simultaneously detect and measure the three main O-containing gases,namely H 2O,CO and CO 2.Their sum corresponds to the quantitative amount of oxygen in the coal and is,in general,inherently more accurate than the F by-difference _values.In this paper,we first attempt to relate the F by-difference _values for %O reported for the Argonne premium coal samples (lignite to bituminous rank)(Argonne Users Handbook)to those determined from a TG–FTIR examination of the pyrolysis gases evolved.Another objective of the work is to relate the pyrolysis gases (H 2O,CO and CO 2)evolved to oxygen-containing functional groups found in coals as well as the evolution of these functional groups as a function of rank.Correlations are also developed between the TG–FTIR oxygen values and other parameters determined for the Argonne Premium Coals.In particular,comparisons of our results using TG–FTIR with analyses carried out by other workers on functional group analysis of acidic groups are considered.D 2005Elsevier B.V .All rights reserved.Keywords:Argonne premium coals;Oxygen content;TG –FTIR;Oxygen functional groups1.IntroductionKnowledge of the amount of organic oxygen in a coal sample is important for many reasons.In coal conversion processes,it has a bearing on the process parameters necessary to obtain high conversion and determines the difficulty of upgrading liquid products to stabilized chemical entities.For coking coals,it may indicate undesirable weathering that will have a negative impact on coke bustion techniques are commonly used in the analysis of coal and organic compounds in general for the determination of elemental hydrogen,carbon and nitrogen according to ASTM D3176-89[1].For oxygen,the method in common practice involves the determination F by-difference _from directly determined values for moisture,ash,sulfur,hydrogen,carbon and nitrogen.In spite of the inherent errors of this approach,which may be significant,it must be recognized that,in many cases,oxygen values obtained F by-difference _are adequate;for others,such as studies of coal weathering,more accurate values are required.We have shown recently [2]that the nature of oxygen functional groups is important in assessing the degree of oxidation of a coal.In many cases it is impossible to know whether a particular coal has suffered oxidation purely from the F by-difference _oxygen value because the overall oxygen content of the sample has not changed in a manner that is detectable by this approach.Some years ago,a pyrolysis technique was developed for the direct determination of oxygen using the Perkin Elmer 240Microanalyser [3].This technique was based on earlier work,which showed that all of the organic oxygen in coal could be liberated by pyrolysis as H 2O,CO and CO 2.Culmo showed that the passage of the pyrolysis gases over hot activated carbon converts all the oxygen to CO.Subsequent passage of the CO over cupric oxide forms CO 2,which can be identified quantitatively by a suitable detector.The pyrolysis technique has been developed by other groups as well [4,5].For a number of years,we have used this technique routinely in our work on coal oxidation where small changes in oxygen content are important [6,7].However,although we have found this technique both useful and reliable,it does not provide information about the chemical speciation of0378-3820/$-see front matter D 2005Elsevier B.V .All rights reserved.doi:10.1016/j.fuproc.2005.10.004*Corresponding author.E-mail address:tmacphee@nrcan.gc.ca (J.A.MacPhee).Fuel Processing Technology 87(2006)335–341oxygen in coal.This has led us to consider the use of thermogravimetry coupled to gas analysis by infrared spectroscopy (TG–FTIR)to measure organic oxygen in coal directly.Although this technique,developed by Solomon et al.[8],has been extensively used by our group [9–11]and others,it appears not to have been considered for this particular purpose.In this work,we report the application of TG–FTIR under pyrolysis conditions to measure total organic oxygen and examine oxygen speciation in the Argonne premium coal samples.2.ExperimentalThe technique has been described elsewhere in some detail [9].Here follows a brief description.TG–FTIR —as mentioned above,the TG–FTIR technique has been developed and used extensively by Solomon et al.[8].In the current configuration of the system,a Bomem TG/Plus TGA/FTIR spectrometer consisting of a DuPont 951TGA,multipass gas cell,Michelson 110FTIR and microcomputer was used.The system can simultaneously control furnace tempera-ture,record weight loss and capture IR spectra for up to 20evolving species in real time.Typically,a 25–35mg sample was continuously weighed in the TG/Plus system while heated using the following temper-ature program:a drying step at 150-C followed by pyrolysis in helium with a temperature ramp from 150to 900-C at 30-C/min.Gases and volatiles were entrained in a gas cell by a helium stream where the rate and amount of the IR-active species were quantitatively measured.During the thermal ramp program,infrared spectra are collected every 30s.On-line analysis and post process manipulations of the data were performed by the TG/Plus and SpectraCalc software packages.In general,the spectra obtained from the coal samples showed absorption bands for tars,CH 4,C 2H 4,NH 3,CO,CO 2,COS,SO 2and H 2O.The TG/Plus quantitative analysisprogramT,°CT ,°CW t %/m i nabTime, minW t %/m i nFig.1.Illinois #6coal:(a)evolution of H 2O,CO,CO 2and thermocouple temperature vs.time;(b)evolution of H 2O,CO and CO 2vs.temperature.J.A.Macphee et al./Fuel Processing Technology 87(2006)335–341336employs a database of calibrated IR spectra of gases/volatiles to obtain their relative amounts.All proximate and ultimate analyses were carried out according to ASTM D3176-89[1].3.Results and discussionPlots of the H 2O,CO and CO 2evolved during pyrolysis are used to estimate the total organic oxygen in a coal sample.Typical plots for Illinois #6and Beulah-Zap are given in Figs.1a,b and 2a,b ,respectively.Integration of these curves yield the wt.%of the various oxygen containing gases evolved from the coal from which the contribution to the total oxygen content is calculated.In Fig.1b,there is a sharp peak in the CO 2evolution curve at 765-C that corresponds to calcite decomposition in the Illinois #6coal [8,12].The shape of the water evolution curves for Illinois #6and Beulah-Zap is important in the present context.The curve for the former indicates no water evolution below¨300-C and is a fairly simple bell-shaped curve consisting of two or more major components,whereas the curve for the latter indicates small yet important amounts ofwaterT, °CT , °CW t %/m i nabTime, minW t %/m i nFig.2.Beulah-Zap lignite:(a)evolution of H 2O,CO,CO 2and thermocouple temperature vs.time;(b)evolution of H 2O,CO and CO 2vs.temperature.Table 1Analysis of Beulah-Zap and Wyodak coals (CETC-Ottawa)(wt.%,as analysed)Analysis Coal Beulah-Zap Wyodak Proximate Moisture 17.1716.39Ash7.977.42VM+FC74.8676.19Ultimate C 53.8856.40H 3.50 4.01N 0.830.89S0.670.57O (F by-difference _)15.98(12.32a )14.32(11.68a )aArgonne Users Handbook.J.A.Macphee et al./Fuel Processing Technology 87(2006)335–341337evolution at surprisingly low temperatures,starting at ¨120-C.How we interpret this phenomenon,this low-temperature water,is of some significance to this work and to the analysis of lignites in general.The question of whether this water is bulk water or pyrolysis water has to be considered since it determines whether this water originates as organic oxygen or not.We will indicate our interpretation of this below in some detail.In previous work that considered bituminous and sub-bituminous coals [7,9,10]but not lignites,the estimation of total oxygen via TG–FTIR was rather straightforward with total oxygen values close to our in-house by-difference values.In the present work,six of the eight Argonne samples behaved as expected but difficulties were encountered with Wyodak (sub-bituminous)and Beulah-Zap (lignite).For the latter two coals,the TG–FTIR values were several percentage points higher than the dry F by-difference _values reported in the Argonne Users Handbook,V orres [12].To address this problem,we first had Wyodak and Beulah-Zap analysed by our own characterization laboratory.Prior to analysis,the two coals were dried at ambient temperature overnight.We found that pre-drying of the sample (under nitrogen)was necessary to obtain reliable sample weights for analysis since both samples in their pristine state from the vialshad ¨30%moisture.Allowing the moisture to stabilize at ambient conditions made the analysis much simpler.Results are reported in Table 1where some differences with the previously reported results are noted.In particular,the carbon values from our laboratory are ¨1%lower that the reported values making the O F by-difference _values higher by the same amount.These new values were still lower than the %O values determined by TG–FTIR.In an attempt to explain this occurrence,the TG–FTIR behaviour of Beulah-Zap pre-dried at 75-C and 105-C for 24h in a nitrogen atmosphere was examined.The water evolution curves for the un-dried and pre-dried coals are shown in Fig.3.It is clear that even at 75-C the water evolution peak at ¨150-C is drastically reduced,suggesting that this is bulk water which is not removed in conventional drying at 105-C and consequently should not be considered as contributing to the overall organic oxygen of the sample.As a consequence of this finding,the water evolution peaks for both Beulah-Zap and Wyodak were integrated manually so as to eliminate this low-temperature contribution to the overall water evolution.The work of Bartholomew et al.[13]on measurements of surface and pore properties of Argonne National Laboratory and Pittsburgh Energy Technology Center coals found thatT, °CW t %/m i nFig.3.Beulah-Zap lignite —effect of heat treatment on H 2O evolution vs.temperature.Table 2Argonne premium coals —TG –FTIR data CoalRank H 2O(wt.%;db a )CO(wt.%;db)CO 2(wt.%;db)O(wt.%;db)H 2O(mmol/g;db)CO(mmol/g;db)CO 2(mmol/g;db)O(mmol/g;db)Pocahontas #3lvb 1.15 1.990.70 2.670.640.710.16 1.67Upper Freeport mvb 3.12 2.09 1.14 4.83 1.740.750.26 3.02Pittsburgh #8hvAb 4.09 2.70 1.42 6.25 2.270.970.32 3.91Lewis-Stockton hvAb 4.65 3.05 1.39 6.92 2.58 1.090.32 4.33Illinois #6hvCb 5.58 3.69 1.828.48 3.10 1.320.41 5.30Blind Canyon hvBb 6.00 4.67 2.009.47 3.33 1.670.45 5.92Wyodak subb C 9.978.42 5.5317.73 5.54 3.01 1.2611.08Beulah-Zaplig A11.239.007.4120.566.243.221.6812.85aDry basis.J.A.Macphee et al./Fuel Processing Technology 87(2006)335–341338most of the internal surface area of Wyodak and Beulah-Zap coals consists of micropores having diameters less than 1nm.Also,the low pore volumes reported by Bartholomew et al.[13]for these coals lends support to our attribution of the shoulder peak of the H 2O evolution profile of Beulah-Zap in Fig.3to bulk water.In light of what has been stated above concerning the data treatment,the amounts of H 2O,CO and CO 2and the corresponding total organic oxygen evolved from the Argonne Premium Coals using TG–FTIR are given in Table 2.The total organic oxygen values obtained for the Argonne coals by TG–FTIR and F by-difference _are given in Table 3for comparison purposes.These values are plotted in Fig.4where the least squares line is constrained to pass through zero.It should be mentioned that the total organic oxygen measured via TG–FTIR and presented in this paper refers strictly to organic oxygen,although four of the eight Argonne coals,namely Pocahontas #3,Illinois #6,Blind Canyon and Beulah-Zap,were found to contain inorganic oxygen from the decomposition of calcite and also siderite in the case of Pocahontas #3.The fraction of inorganic oxygen present in these four coals was obtained through CO 2profile resolution and was based both on the evolution temperature for CO 2from decomposition of pure siderite and calcite via TG–FTIR [9,10].In other words,the amount of organic CO 2listed in Table 2for the Argonne coals identified ascontaining carbonates was obtained by subtracting the inorganic CO 2contribution from the total CO 2measured.3.1.Oxygen distribution as a function of rankThe contributions from H 2O,CO and CO 2to organic oxygen in the coals are shown in Table 4and graphically in Fig.5.As expected,there is a general monotonic increase in all quantities with decreasing carbon content.Extensive work on oxygen functional analysis has been reported and discussed in Van Krevelen [14].The shapes of curves of functional group content versus carbon content provided in that reference are concave downward while those shown in Fig.5are all concave upward.The results cited in Van Krevelen are from a variety of sources and make use of a variety of chemical techniques.Van Krevelen points out that,except for peat and brown coal,the sum of the functional group oxygen and the total oxygen in the coal are in agreement.Consequently,it is doubtful that ‘‘unreactive’’oxygen really exists.In the present work,it has been shown that all the oxygen in the coal sample is expelled during pyrolysis and measured by the amounts of the three gases,H 2O,CO and CO 2.The normalized organic oxygen content occurring in the three O-containing pyrolysis gases is given in Table 5where it is seen,with the exception of Pocahontas #3,the highest rank coal in the series,that there is a remarkable similarity of the various contributions as noted previously [9,10].Table 3Comparison of oxygen concentrations —Argonne and TG –FTIR CoalOArgonne (wt.%;db)TG –FTIR (wt.%;db)Pocahontas #3 2.36a 2.67Upper Freeport 4.84a 4.83Pittsburgh #8 6.74a 6.25Lewis-Stockton 7.79a 6.92Illinois #68.65a 8.48Blind Canyon 10.79a 9.47Wyodak 17.13b 17.73Beulah-Zap19.29b20.56a Argonne Users Handbook.bDetermined at CANMET Energy TechnologyCentre-Ottawa.O (Argonne, by difference), wt% (db)O (T G -F T I R ), w t % (d b )parison of the TG –FTIR organic O (wt.%,db)with the Argonne by-difference values.Table 4Argonne premium coals —contributions to overall O content,wt.%(db)CoalO (H 2O)O (CO)O (CO 2)O total (FTIR)Pocahontas #3 1.02 1.140.51 2.67Upper Freeport 2.79 1.200.84 4.83Pittsburgh #8 3.66 1.55 1.04 6.25Lewis-Stockton 4.15 1.75 1.02 6.92Illinois #6 5.01 2.13 1.348.48Blind Canyon 5.34 2.67 1.469.47Wyodak 8.88 4.82 4.0317.73Beulah-Zap10.005.165.4020.56C (Argonne), wt% (dmmf)O (T G -F T I R ), w t % (d b )Fig.5.O (TG–FTIR)vs.C (Argonne).J.A.Macphee et al./Fuel Processing Technology 87(2006)335–3413393.2.Oxygen speciationThe concentrations of –OH and –CO 2H functional groups occurring in the Argonne Premium Coals have been measured by Aida et al.[15]using a chemical method.His method involves the reaction of carboxyl functional groups in coal with n -Bu 4NBH 3in pyridine.The amount of hydrogen evolved is assumed to be equivalent to the carboxyl concentration in the coal.Carboxylate groups are not detected by this method but,after acid washing,the total carboxyl content was measured to yield carboxyl and carboxylate amounts in the original coal.The underlying assumption is that this reagent (n -Bu 4NBH 3)reacts only with carboxyl groups and not with phenolic –OH groups.Corroborating evidence from solid-state 13C NMR supports the assumption.A further reagent,LiBH 3in pyridine,which reacts with both carboxyl and phenolic –OH generating hydrogen,was used to measure the concentrations of these two functional groups together.The concentration of phenolic –OH is then calculated from the difference between total acidic groups and the total carboxyl group concentrations.A direct comparison between the pyrolysis data of our work and Aida’s functional group analysis is therefore possible.We first consider the relationship between pyrolysis CO 2and total carboxyl plus carboxylate measured chemically by this author shown in Fig.6.It is evident that the pyrolysis CO 2is equivalent to the –CO 2H(M)measured by Aida.From a chemical point of view,this is reasonable since it is difficult to envisage the production of CO 2from any other functional group as pointed out by Schafer [4]for Australian brown coal decomposition.Fig.6therefore constitutes a validation of boththe Aida and the pyrolysis (TG–FTIR)approaches to the measurement of carboxyl groups in coal.It is worth pointing out that the curves for the evolution of CO 2as a function of temperature are not simple,indicating more than one compo-nent.It is possible that carboxyl and carboxylate decompose to produce CO 2in different temperature ranges and that the relevant information may be extracted from the pyrolysis curves by deconvolution.This point will be examined in future work.The situation for acidic –OH groups is somewhat different.A plot of pyrolysis H 2O as a function of –OH groups from Aida’s work indicates that there is more water evolved than can be accounted for by his estimation of phenolic –OH groups (Fig.7).There exists a large body of work on phenolic –OH groups in coal estimated by a variety of techniques that was reviewed recently [16].This author sums up the literature data on phenolic –OH groups by means of the following equation:ln O ph ÀÁ¼130:3909þ96:124ln c C ðÞÀ134:133c C r ¼0:859ðÞwhere c C is the fractional carbon content on a dry,ash-free basis.A comparison of phenolic –OH content generated by the above equation and the experimental work of Aida for the Argonne coals indicates that the latter are somewhat lower than the ‘‘consensus’’values assembled by Gagarin [16].Some ofTable 5Argonne premium coals —normalised organic O wt.%CoalO (H 2O)O (CO)O (CO 2)Pocahontas #3384319Upper Freeport 582517Pittsburgh #8582517Lewis-Stockton 602515Illinois #6592516Blind Canyon 572815Wyodak 502723Beulah-Zap492526COOH+COOM (Aida), mmol/g (db)C O 2 (T G -F T I R ), m m o l /g (d b )Fig.6.CO 2(TG –FTIR)vs.COOH+COOM functionality (Aida et al.[15]).Ar-OH (Aida), mmol/g (db)H 2O (T G -F T I R ), m m o l /g (d b )Fig.7.H 2O (TG –FTIR)vs.Ar –OH functionality (Aida et al.[15]).Ar-OH (van Krevelen), mmol/g (db)H 2O (T G -F T I R ), m m o l /g (d b )Fig.8.H 2O (TG–FTIR)vs.Ar–OH functionality (Van Krevelen [14]).J.A.Macphee et al./Fuel Processing Technology 87(2006)335–341340the work dates from Van Krevelen’s group in the1950s [17,14].It involved measurement of phenolic–OH groups by acetylation with acetic anhydride in pyridine for a relatively wide range of coal rank.From that work,it is possible to estimate the phenolic–OH content for the Argonne coals from the appropriate carbon content.A comparison of the pyrolysis H2O content of the Argonne coals with the phenolic–OH from van Krevelen’s work is given in Fig.8.The correlation between these two values is excellent and,perhaps what is more significant,the slope is close to unity and the intercept of the least squares line is close to zero.This leaves us with the intriguing conclusion that the pyrolysis water originates mainly as phenolic–OH,although it is difficult to see how this can be the case since it is likely that the reaction mechanisms producing pyrolysis water are complex.4.Observations and conclusions1.TG–FTIR can be reliably used for the determination of theorganic oxygen content of coals in general as indicated by the results reported here for Argonne premium coals.2.Low rank coals contain water that is difficult to remove at105-C.3.The normalized amounts of oxygen found in the gaseousspecies H2O,CO and CO2remain remarkably constant with changes in rank.4.Pyrolysis CO2corresponds to the CO2H(M)functionalgroup in coal.5.The quantity of pyrolysis H2O is accounted for almostentirely by–OH functional groups(using the experimental results of Van Krevelen)indicating this functional group as its principal source.AcknowledgementThe authors would like to thank the Canadian Carbonization Research Association for support of this work.References[1]ASTM Standards,V olume05.06,Gaseous Fuels;Coal and Coke,ASTMInternational,100Barr Harbor Drive,PO Box C700,W.Conshohocken, PA19428-2959,USA,2004.[2]J.A.MacPhee,L.Giroux,J.-P.Charland,J.F.Gransden,J.T.Price,Detection of natural oxidation of coking coal by TG–FTIR—mechanistic implication,Fuel83(2004)1855–1860.[3]R.Culmo,Microdetermination of oxygen in organic compounds with anautomatic elemental analyzer,Microchimica Acta(1968)811–815. [4]H.N.S.Schafer,Pyrolysis of brown coals:2.Decomposition of acidicgroups on heating in the range100–900-C,Fuel58(1979)673–679.[5]B.Zischka,H.Stemming,Oxygen determination in solid fuels,ErdoelKohle,Erdgas,Petrochem38(5)(1985)221–224.[6]J.A.MacPhee,W.P.Hutny,L.Giroux,J.-P.Charland,J.T.Price,Characterization of coal weathering/oxidation by TGA–FTIR,Proc.Int.Conf.on Coal Science,Banff,Sept.12–19,1993,pp.590–593.[7]J.A.MacPhee,L.Giroux,J.-P.Charland,J.T.Price,A study of theoxidation pathways of coking coal by thermogravimetric Fourier transform infrared spectroscopy(TG–FTIR),Proc.Int.Conf.on Coal Science,San Francisco,2001.[8]P.R.Solomon,M.A.Serio,R.M.Carangelo,R.Bassilakis,D.Gravel,M.Baillargeon,F.Baudais,G.Vail,Analysis of the Argonne premium coal samples by thermogravimetric Fourier transform infrared spectroscopy, Energy and Fuels4(3)(1990)319–333.[9]J.-P.Charland,J.A.MacPhee,L.Giroux,J.T.Price,M.A.Khan,Application of TG–FTIR to the determination of oxygen content of coals,Fuel Processing Technology81(2003)211–221.[10]J.A.MacPhee,J.-P.Charland,L.Giroux,J.T.Price,W.P.Hutny,M.A.Khan,Determination of organic oxygen in coals with application to the study of coal weathering,ACS Division of Fuel Chemistry Preprints46(1)(2001)340–342.[11]J.A.MacPhee,L.Giroux,J.-P.Charland,R.Sakurovs,J.T.Price,Interactions between coals in binary blends during heating as studied by thermal rheology,PMRTA,TG–FTIR and optical microscopy,Proc.18th Ann.Pittsburgh Coal Conf.,Newcastle Australia,Dec.4–7,2001. [12]K.S.V orres,Users Handbook for the Argonne Premium Coal SampleProgram:accessed December9,2003at URL http://www.anl.gov/PCS/pcshome.html,1993.[13]C.H.Bartholomew,W.E.White,D.Thornock,W.F.Wells,W.C.Hecker,L.D.Smoot,D.M.Smith,F.L.Williams,Surface and pore properties of ANL and PETC coals preprints,Fuel Chemistry Division,American Chemical Society33(3)(1988)24–31.[14]D.W.Van Krevelen,Coal:Typology,Physics,Chemistry,Constitution,Third ed R,Elsevier,Amsterdam,1993.[15]A.Aida,A.Nishisu,M.Yoneda,T.Yoshinaga,Y.Tsutsumi,I.Yamanishi,T.Yoshida,Chemical determination of oxygen-containing functionality in Argonne premium coal samples,Fuel Chemistry Division Preprints46(1) (2001)325–327.[16]S.G.Gagarin,Forms of Oxygen in Coal Organic Matter(Review)Cokeand Chemistry,No.10,2001.[17]L.Blom,L.Edelhausen,D.W.van Krevelen,Chemical structure andproperties of coal:XVIII.Oxygen groups in coal and related products, Fuel36(1957)135–153.J.A.Macphee et al./Fuel Processing Technology87(2006)335–341341。