Characterization of Coking Coals Using Solvent Extraction at 350 ˚C∗

CHENG Xiaoai,JIAN Yiming,Inamullah Mian,ZHONG Mei,LIU Jingmei

(Key Laboratory of Coal Clean Conversion and Chemical Process Autonomous Region,College of Chemistry and Chemical Engineering,Xinjiang University,Urumqi Xinjiang 830046,China)

Abstract: The commonly used coal analyses such as ultimate,proximate analyses etc.have no clear correlation with conventional characteristic parameters of coking coal,e.g.maximum fluidity(MF)and maximum dilatation(MD).Therefore,it has been desired to develop a new coal characterization method from which properties of coking coal can be reasonably predicted.The paper presents,eleven slightly or strongly coking coal were separated into three fractions with different molecular weight size without decomposing the coals by 1-methylnaphthalene(1-MN)extraction at 350˚C.For the detailed assistance,the samples were analyzed through,ultimate analysis,thermo-gravimetric analysis(TGA),thermomechanical analysis(TMA),and molecular weight distribution(MWD)measurement for the parent coal and fractions.The possibility of the proposed solvent extraction was a new method for the characterization of coking coal.The results showed that fractions of the extraction yields were significantly different among the coals,and the distribution orders of oxygen between three fractions were also different among the coals,depends on the coal type.However,the differences in the properties of the same fractions obtained from various coals,e.g.pyrolysis,melting and thermoplastic behavior,MWD,were much smaller than the differences in the properties among the three fractions.The regression analysis showed that the significant correlations among the extraction yields,property parameters of fractions or parent coals,and characteristic parameters of coking coal indeed exist.Therefore,the proposed extraction as a new characterization method to studied coking coal.

Key words:degradative solvent extraction；coking coal；maximum fluidity；maximum dilatation

0 Introduction

In order to cover rapid increasing of worldwide need for metallurgical coke and meet the high quality requirements of the coke for blast furnace technology,great effort have been made by researchers to find out new ways for the extension of cokeable coals.The prediction and evaluation of coking coal characteristics depend on the basic parameters of coal play an important role in the work of extending range of cokeable coals[1,2].The large amount of work has been done by researchers to study the properties of coking coal based on the coal rank,such as a petrographic composition of coals,etc,as summarized by Varma[3].However,the conventional characteristic parameters of coking coal,e.g.Log MF and MD,as the important indices for evaluation of coking coal property and predicting the coke quality,have no clear correlations with the commonly-used coal analyses,e.g.ultimate and proximate analyses,and petrographic composition of coals.Therefore,it has been desired to develop a new coal characterization method from which MF and MD can be reasonably predicted,as well as coke property is expected to be predicted by the new coal characterization methods.

The relationship between the properties of coking coal and the extraction yield of the coking coal by using organic solvent have been studied by researchers[4−6].Takanohashi et al.[7]extracted various heat-pretreatment coking coals using CS2-NMP as solvent at room temperature and found the extraction yield increased greatly for the coal with the pretreatment at the softening and fluidity temperature.However,the extraction was performed after the pretreated coal cooled to room temperature.So,it cannot directly estimate the total amount of solvent-solubilized component at the softening and fluidity temperature.Ouchi et al.[8]extracted four bituminous coals using quinoline as solvent at 350˚C.The acceptable correlation was found between the log MF values of the parent coals and the extraction yield with the correction for inert content.The authors have succeeded in separating bituminous coals into several fractions having different molecular weight without decomposing coals by using the non-polar solvent extraction at below 350˚C[9−12],and it was shown that the extraction method was effective in characterizing coals.In this work,the proposed extraction method was applied to fractionate eleven slightly or strongly coking coals.Base on the properties of analyses,e.g.MWD measurement,elemental and thermal analysis.The possibility of the proposed method as a new characterization method to examined coking coal.

1 Experimental

1.1 Coal sample

Four slightly coking coals(A,C,D,K)and seven strongly coking coals(B,E,F,G,H,I,J)were studied in this work.The coals were named from A to K based on the increase of carbon content of the coals,and the properties of the coals are given in Table 1.

Table1 Properties of coal samples and their cokes

1.2 Experimental procedure

The flow type extractor was applied to fractionate the coals,and the procedure of the extraction method has been described in detail in our previous papers[11,12].1-methylnaphthalene was used as an extraction solvent and the flow rate was kept at 2mL/min.The extractor was heated at a heating rate of 10 ˚C/min to 350 ˚C,where it was kept for 90 min.The extraction pressure was regulated at 10MPa during the whole extraction process.Part of the extracts came out from the extractor with the solvent precipitated at room temperature,which was collected through filters which are called deposit.The other part of the extracts which was still solubilized in the solvent at room temperature was collected in a separation trap after the filter which is known as soluble.Residue is the undissolved part at 350˚C.The produced gaseous products were collected in a gas bag and analyzed using gas chromatograph.The solvent containing Soluble was evaporated under reduced pressure at 150˚C to recover Soluble as solid.The Soluble,Deposit and Residue were weighed after dried in vacuo at 150˚C for 5 h to estimate their yields.

1.3 Analyses of fractions

The obtained fractions were characterized through various analyses.Ultimate analysis of the solid fraction was performed using a CHN analyzer(Yanaco,CHN-500).The lasers desorption/ionization time-of-flight mass spectrometry(MALDI-TOFMS;Shimadzu/Kratos KOMPACT-MALDI-II)was used to estimate the MWD of the fractions obtained and the parent coal.The MALDI spectrometer is equipped with an N2UV laser of 337 nm in wavelength and its acceleration voltage could be chosen as either 5 kV(low mode)or 20 kV(high mode).The laser power was carefully selected to be small enough not to decompose the sample.Weight decreasing curve of the fraction was measured using a thermo-balance type reactor(Shimadzu;TGA50)during a heating rate of 10˚C/min up to 900 ˚C in a nitrogen atmosphere.Under the same heating conditions,the melting and thermo plastic behaviors of the fractions were examined using a thermomechanical analyzer(Shimadzu;TMA50)in which the displacement depth of a rod into the solid fraction was continuously monitored.The solid fraction was placed in a pan(5.2 mm I.D.and 6.0 mm high)at the height of 1 mm.The rod of 4.3 mm in diameter was loaded with a constant load of 0.098 N for the measurement.

2 Results and Discussion

2.1 Extraction yield

The extraction yields of fractions were shown in Fig 1.The results showed that the mass balance of each run was sound and the gas yields were negligibly small(lower than 0.1 wt.%)for all the eleven coals,indicating that each of the coals was almost not decomposed during the extraction process.Extraction yields(sum of the Soluble and Deposit yields)were between 50wt%to 67wt.%for the coals from A to H whose carbon content were less than 89wt.%,whereas the extraction yields were between 22wt.%to 35wt.%for the coals from I to K whose carbon content were over 89 wt.%.So the extraction yields tended to decrease with the increase of carbon content of the parent coals.Furthermore,the extraction yield roughly increased with the increase of the Log MF values,and this was basically consistent with Ouchi‘study[8],although we used a different solvents extraction from Ouchi‘study.The correlation between the extraction yield and Log MF would be further investigated in the last part of the paper.The correlation between TD and extraction yield did not show directly.The Soluble yields ranged from 17 wt.%for G to 34 wt.%for E,and the Deposit yields ranged from 5 wt.%for K to 40 wt.%for A.Thus,the yield of each fraction was significantly different depending on the coals type.

Fig 1 Yield of each fraction obtained through extraction using 1-methylnaphthalne

2.2 Ultimate and proximate analysis of fractions and parent coal

Ultimate and proximate analyses of the fractions were listed in Table 2,and the elemental and proximate analyses results of their parent coals are also included in the table for comparison.The results of ultimate analysis showed that the hydrogen and carbon content of soluble was the largest among the three fractions,except K.On the contrary,the hydrogen and carbon content of Residue was the smallest among the three fractions,except K.Thus,the distribution order of hydrogen and carbon among the three fractions were very similar,independent of the coal type.However,the oxygen content of the Residue obtained from A,B,C,D,E,G and J,and the oxygen content of the Soluble obtained from F,H,I and K,were the largest among the three fractions.Thus,the distribution orders of oxygen among the three fractions were different,depending on coal type.It can be explained by the difference in the oxygen exiting state in the parent coals.the oxygen containing functional group could be mostly concentrated in a soluble,but heterocyclic aromatic compounds containing oxygen could mostly concentrate in Residue during the extraction process,which was already observed in a previous study[13,14].Carbon and hydrogen balances in Residue,Deposit and Soluble were checked in Fig 2 and Fig 3.The balances were fairly sound even for hydrogen which accurate quantification is generally more difficult to obtain than the carbons.The good balance of hydrogen and carbon indicated that no appreciable chemical interaction between the coal molecules and the solvent occurred during the extraction process.Furthermore,during this process hydrogen transfer did not take place between the coal molecules and solvent.

Fig 2 Carbon(C)balances of fractions through extraction using 1-methylnaphthalne

Fig 3 Hydrogen(H)balances of fractions through extraction using 1-methylnaphthalne

2.3 Molecular weight distribution of fractions and parent coals

Fig 4 showed MWDs of the fractions and their parent coals.The MWDs were rather broad for all the parent coals,ranging from 200 to 3 000.However,the MWDs of Soluble and Deposit merely ranged from 200 to 500 and from 400 to 1 000 respectively.The rest of the compounds in the parent coal were recovered as Residue.The same fractions obtained from various coals have quite similar MWDs,almost independent on the coal types.Thus,the extraction method can separate the coals into three fractions having different molecular weights,independent of the raw coal types.

Fig 4 MWD of each fraction obtained through extraction using 1-methylnaphthalne

Table2 Ultimate and proximate analyses of fractions and parent coals

2.4 Thermal analyses of fractions and parent coals

Thermal analysis can directly indicate the pyrolysis,melting and thermo plastic behavior of fractions and raw coals,which are the important properties of coking coal.Thermalgravimetric analyses were performed to examine the pyrolysis behavior of the fractions and the parent coals,and the weight decreasing curves were shown in Fig 5.The decrease in the weight of Soluble obtained from all the eleven coals started at below 250˚C,which was lower than the starting temperature of decomposition.Thus,part of the weight loss of Soluble was due to the vaporization of low molecular compounds.On the other hand,weight decreasing of Deposit and Residue started at around 400˚C for all the eleven coals,indicating that weight loss was due to the pyrolysis.The volatile matter(weight loss in the whole pyrolysis process)of Soluble ranged from 33 wt.%to 57 wt.%,as shown in Table 2.However,the volatile matter of Deposit and Residue merely ranged from 17 wt.%to 33 wt.%and 13 wt.%to 26 wt.%respectively.Fig 5 also showed that the fractions,obtained from the parent coals having a larger amount of volatile matter compared with other raw coals,tended to have a larger amount of volatile matter compared with the same fractions obtained from the other raw coals.However,the difference in the amount of volatile matter among the same fractions obtained from various coals was much smaller than the difference among the three fractions.Therefore,the same fractions obtained from various coals have similar pyrolysis behavior,compared with the three fractions obtained from the same coals.

Fig 5 Comparison of thermogravimetric curves of the fractions.(10˚C/min,in N2)

Thermo-mechanical analyses were performed to examine the melting and thermo plastic behaviors of their fractions and the parent coals.This analysis continuously monitors the displacement of the rod into a sample bed during heating.When the sample melts completely,the rod displaces the sample and reaches the bottom of the sample pan.Fig 6 compared the thermomechanical curves of the fractions and the parent coals.Deposit and Soluble were judged to melt at below 400 ˚C and 200 ˚C respectively.Because Fig 5 showed that the pyrolysis did not start at the temperature where the thermomechanical curves started to decrease.Almost all of the Soluble melted completely before 300˚C,but Deposit did not melt completely up to 600˚C.On the contrary,Residues did not melt at all during the whole process of thermo-mechanical analysis.Therefore,the difference in melting and thermoplastic behavior among the same fractions obtained from various coals was much smaller than the difference among the three fractions,was in the case of pyrolysis behavior of the fractions.In summary,the difference in the thermal behavior among the same fractions obtained from various coals was much smaller than the difference among the three fractions.

Fig 6 Comparison of thermomechanical curves of the fractions.(10˚C/min,in N2)

2.5 Regression analysis

Fong,et al[15]succeed in predicting the plastic period of bituminous coal at different holding temperature using a mathematical model for a viscosity of the solid-liquid suspension.In this work,it was found that the extraction yield roughly increased with the increase of the Log MF values.Furthermore,the TMA results show that Soluble and Deposit were fusible upon heating but Residue was not fusible.That means(YS+YD)/YRis the ratio of the liquid state part to the solid state part of the coal upon heating.Here,YS,YD,andYRrespectively represent the yields of Soluble,Deposit,and Residue.Thus,the correlation between(YS+YD)/YRand Log MF were firstly investigated and their relationship was shown in Fig 7.And linear regression was performed and the regression equation as given below

The regression results indicated that the correlations between(YD+YS)/YRand Log MF indeed exit.The Log MF increased with the increase of extraction yield.In other words,the Log MF increased with the increase of the ratio of fusible part to infusible part of the raw coals.

Unfortunately,the correlation between MD and extraction yield was not obtained.It might because that the plastic behavior of coal is affected by various coal basic properties and the value of MD can be not perfectly related monotonously with any single characteristic parameter.The effect of oxygen on the plastic properties of coal has been studied widely in previous work[16−21].Ignasiak[18]found the carboxyl groups in coal did not appreciably affect the plasticity of coal.However,even a small amount of the ether-type crosslinks bonds can stiffen the whole coal molecular structure and decrease the thermoplastic properties of the coal,especially the dilatation.This kind of bonds in coal was considerable thermal stable at below 350˚C[17−19].Table 2 showed that the distribution orders of oxygen among the three fractions were different,depending on the existing state of oxygen in the parent coals.Thus,the difference of oxygen content in the three fractions might affect the dilatation behavior of coking coal.Therefore,multiple regressions of MD against extraction yield(YD+YS)andOD+Swas performed and described by regression equation(2).Here,OD+Srepresent oxygen content of extracts(Deposit+Soluble)on the basis of coal(d.a.f).The correlation among TD estimated by equation(2),(YD+YS)andOD+Swere shown in Fig 8.The comparison between experimental and calculated values of MD was shown in Fig 9.Regression results indicated there was a significant correlation among MD,(YD+YS)andOD+S.The value of MD increased with the increase of extraction yield or the decrease ofOD+Swithin the research range of(YD+YS)from 20 wt.%to 65 wt.%andOD+Sfrom 0 wt.%to 3.5 wt.%.

Fig 7 Correlation between(YD+YS)/YR and Log MF

Fig 8 Correlation among MD,YD+YS,and OD+S

Fig 9 Comparison between experimental and calculated values of MD

3 Conclusions

Eleven coking coals were fractionated into three fractions by using 1-MN extraction at 350˚C without decomposing the coals.The yield of each fraction was significantly different depending on the type of coals.However,the difference in properties,including MWD,pyrolysis,melting and thermoplastic behavior among the same fractions obtained from various coals was much smaller than the difference among the three fractions.

The significant correlation between Log MF and(YS+YD)/YR,among the MD,extraction yield(YS+YD)and oxygen content of extracts(OD+S)were obtained by the regression analysis.The value of Log MF increased with the increase of(YS+YD)/YR.The value of MD increased with the increase of extraction yield or the decrease ofOD+S.Therefore,we conclude that the possibility of the proposed extraction method as a new characterization method for coking coal.