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Effect of pre-oxidation at low temperature on the carbonization behavior of coal

Effect of pre-oxidation at low temperature on the

carbonization behavior of coal q

Nakorn Worasuwannarak,Hiroyuki Nakagawa,Kouichi Miura *

Department of Chemical Engineering,Kyoto University,Kyoto 606-8501,Japan

Received 10May 2001;revised 1August 2001;accepted 25October 2001;available online 10April 2002

Abstract

Several coals were pre-oxidized by air at 250,270and 3008C to different levels,then the carbonization behaviors of the pre-oxidized coals were systematically examined.The gas formation rates and weight change during the carbonization were measured continuously using a mass spectrometer and a thermogravimetric analyzer,respectively.The changes in the functional groups during the carbonization were measured by an in situ FTIR technique.The micropore distributions of the pre-oxidized coals were measured before and after the carboniza-tion.From these measurements the effect of pre-oxidation on the carbonization of coal was examined in detail.q 2002Elsevier Science Ltd.All rights reserved.

Keywords :Chemical adsorption of oxygen;Oxidation of coal;Carbonization of coal

1.Introduction

It is well known that caking coal softens,melts,and re-solidi?es to form coke when heated in the absence of air.These properties peculiar to the caking coal are advan-tageous for the coke production,but they are well known to be affected by the pre-oxidation of the coal.Weathering of coal during storage,that is a kind of oxidation,for example,is known to affect signi?cantly the property of coke produced through carbonization [1±6].On the other hand,the caking properties are sometimes disadvantageous for other coal utilization processes such as gasi?cation,combustion,and production of activated carbon.Pre-oxidation of coal is utilized to reduce the caking properties for these processes.So,it is very important to examine the effect of pre-oxidation on the carbonization behavior of coal.

Many studies have been performed to examine the effect of oxidation and/or weathering of coal on the properties of coal and carbonized coal.The following changes have been reported to occur as the results of oxidation and/or weather-ing of coal.The solvent extractability is decreased [7],the softening and swelling tendencies are reduced [8,9],and the yield of total volatiles during the carbonization is decreased [10].Lopez et al.[11]found that the decrease in the solvent

extractability is caused by a reduction of the transferable hydrogen during the oxidation.Mahajan et al.[9]have reported that pre-oxidation decreased the weight loss during pyrolysis in N 2but enhanced the gasi?cation reactivity of char in air at 4708C.However,little work has been done to examine the changes during the carbonization process such as gas formation rates,changes in functional groups of the pre-oxidized coals.

Since coal is a highly heterogeneous substance,the low temperature oxidation of coal would be a complex process.Painter et al.[12,13]reported that air oxidation of coal under atmospheric pressure proceeds selectively:the ben-zylic positions,being the most reactive,are ?rst oxidized to produce carbonyl groups and carboxylic acids.The authors have recently examined the oxidation process in detail by using in situ FTIR technique and following results have been obtained [14]:the aliphatic groups of coal were ?rst oxidized to form aldehydes,and then the aldehydes were oxidized further to form carboxylic groups and esters;an-hydrides were also formed from the carboxylic groups and the aldehydes.These signi?cant changes caused by the pre-oxidation were well expected to affect the carbonization behavior of coal.

In this study the carbonization behavior of three coals pre-oxidized to different degrees was systematically exam-ined through the measurements of changes in weights,gas formation rates,XRD patterns,and the changes of func-tional groups by using a thermogravimetric analyzer

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*Corresponding author.Tel.:181-75-753-5578;fax:181-75-753-5909.E-mail address:miura@cheme.kyoto-u.ac.jp (K.Miura).q

Published ?rst on the web via Fuel?https://www.wendangku.net/doc/4b2480121.html,Dhttp://www.fuel?https://www.wendangku.net/doc/4b2480121.html,

(TGA),a mass spectrometer(MS),an XRD diffractometer, and an in situ FTIR spectrometer,respectively.The effect of the pre-oxidation on the development of the micropore structure was also examined in detail.On the basis of these measurements,the carbonization mechanism of the coal and the pre-oxidized coal was examined.

2.Experimental

2.1.Coal samples

Three bituminous coals,Cerrejon(abbreviated to CR), Pittsburg#8(PITT),and Goonyella(GO),were used as coal samples.Their ultimate analyses are given in Table 1.The coal samples were ground into?ne particles of less than74m m in diameter,and dried in vacuo at708C for24h before the experiments.

2.2.Pre-oxidation of the coals

A differential type reactor was used to prepare the pre-oxidized coals.About50mg of coal,placed in a reactor made of quartz tube(8mm in ID and30cm in length), were heated from room temperature to250,270,300or 3308C at the rate of5K/min in air stream diluted by nitro-gen(11%oxygen)to prepare the pre-oxidized coals of different oxidation degrees.

2.3.Carbonization experiments

A differential type reactor connected to a mass spectro-meter(MS)was used for continuous analysis of the product gas during the carbonization of the pre-oxidized coals.About20mg of coal,placed in a same reactor as used for pre-oxidation were heated from room temperature to9008C at the rate of10K/min in a helium stream and held at9008C for10min.A part from the product gas stream was intro-duced directly to a mass spectrometer(Nichiden Anerva, AQA100R)by which signals for mass numbers of2,15, 18,28,and44were continuously detected to estimate the concentrations of H2,CH4,H2O,CO,and CO2in the product gas.The weight change during the carbonization reaction was measured by use of a thermogravimetric analyzer (Shimadzu TGA-50).About5mg of sample was used for this purpose.All the results were represented on the basis of 1kg of dried raw coal.

2.4.Measurements of the changes in properties of the pre-oxidized coals during the carbonization

To investigate the change of the functional groups through the carbonization,in situ FTIR analysis was employed.All the IR spectra were measured at4cm21resolutions on a FTIR spectrometer(JEOL,JIR-WINSPEC50)with a microscope (JEOL,MAV204)and a hot stage(JEOL,CS-30).About 0.5mg of pre-oxidized neat coal particles were heated up to 5508C at the rate of5K/min in a highly pure nitrogen stream. Diffuse reˉectance spectra were collected by acquisition of 100scans at every208C of intervals,and they were converted to the K±M(Kubelka±M unk)function.

The raw coal and the pre-oxidized coals after carboniza-tion at9008C were characterized by a X-ray diffractometer (Shimadzu,XD-610)to examine the effect of pre-oxidation on the development of graphite-like structure during the carbonization.

2.5.Measurement of micropore volume distributions The pre-oxidized coals before and after carbonization were characterized by the accessible micropore volumes measured by the so-called molecular probe technique[15]. Adsorption isotherms of four different gases(CO2,C2H6,n-C4H10and iso-C4H10)measured at258C using an automated adsorption apparatus(Bell Japan Inc.,BELSORP28)were analyzed by the Dubinin±Astakhov equation[16]with n 2to obtain the limiting micropore volume,which is corre-sponding to the total volume of micropore whose diameter is larger than the minimum dimension of each gas.The minimum dimensions of CO2,C2H6,n-C4H10,and iso-C4H10are0.33,0.40,0.43,and0.50nm,respectively.Accu-mulated pore volume distribution curves were constructed by plotting the limiting micropore volumes against the mini-mum dimensions of the gases.

3.Results and discussions

3.1.Changes of the coal sample during the pre-oxidation Fig.1(a)shows the change in the weight during the

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Table1

The yields and ultimate analyses of the raw coal and the pre-oxidized coals

for the three coals

Sample Ultimate analyses(wt%daf)Yield

(kg/kg raw coal)

C H N S O(diff.)

Raw79.8 5.1 1.80.612.7±

PO-25078.8 4.4 1.40.614.8 1.01

PO-27078.2 4.0 1.40.615.8 1.02

PO-30076.7 3.5 1.40.617.8 1.01

PO-33073.4 2.9 1.40.621.70.98

PITT

Raw83.2 5.3 1.60.99.0±

PO-25082.8 5.0 1.60.99.7 1.01

PO-27080.9 4.7 1.50.912.0 1.02

PO-30078.6 4.1 1.50.914.9 1.03

PO-33076.5 3.5 1.50.917.6 1.02

Raw89.1 5.0 1.00.6 4.3±

PO-25088.6 4.9 1.00.6 4.9 1.01

PO-27087.9 4.8 1.00.6 5.7 1.01

PO-30086.8 4.5 1.00.67.1 1.02

PO-33084.7 4.10.90.69.7 1.03

pre-oxidation of the three coals.The behaviors of weight change are signi?cantly different among the coals.The weight started to increase at ca.1508C and reached maxima at 1.02kg/kg coal around 2808C for CR,and at 1.03kg/kg coal around 3108C for PITT,then decreased rapidly.On the other hand,the weight increased monotonously up to 3508C for GO.The yields of the pre-oxidized coal at 3508C were 0.95,1.01,and 1.03kg/kg coal for CR,PITT,and GO,respectively.Fig.1(b)±(d)shows the changes in gas formation rates during the pre-oxidation.Only H 2O,CO and CO 2were produced during the course of pre-oxidation.Neither sulfur containing gases nor nitrogen containing gases was detected.Water was the main gas product for all the coals.Formation of these gaseous products causes the decrease in the weight of coal by consuming carbon and hydrogen.On the other hand,incorporation of oxygen in the coal contributes to the increase in the weight.The weight change curves in Fig.1(a)are the results of these two competing effects.Then,by utilizing the change in weight and gas formation rates during the pre-oxidation and the elemental analysis of the raw coal,we could calculate the change of the oxygen content of the coal sample as follows:the amount of carbon and hydrogen removed from the coal during the oxidation are obtained from the measurements of the gas formation rates.So the sum of the weights of carbon,hydrogen,sulfur and nitrogen in the coal can be calculated continuously from these results and the contents of the elements in the starting sample.The difference between the weight of the coal and the sum of weights of carbon,hydrogen,sulfur and nitrogen in the coal corresponds to the weight of oxygen in the coal.Thus the changes in the oxygen contents could be continuously calculated against temperature as shown in Fig.1(e).The oxygen content increased steadily with increasing temperature for all the coals,and it reached 0.212,0.205and 0.118kg/kg coal for CR,PITT and GO,respectively,at 3508C.

To prepare the coals pre-oxidized to different degrees,the coals were heated to ?nal temperature of 250,270,300,and 3308C in a differential type reactor as stated in Section 2.The pre-oxidized coals (PO coals)were abbreviated by attaching the ?nal temperatures after PO as PO-250,PO-270,PO-300,and PO-330.Table 1lists the yields and the ultimate analyses of the PO coals for the three coals.The carbon and hydrogen contents decreased while the oxygen content increased with the progress of the pre-oxidation.For CR,for example,the hydrogen content of PO-330was only 2.9%.Next,to examine how the change in the elemental composition reˉects the structures of the PO coals,the changes in the amounts of functional groups of CR coal during the pre-oxidation were measured by the in situ FTIR technique.Fig.2shows the in situ FTIR spectra ranging from 800to 4000cm 21.With increasing tempera-ture,signi?cant changes occurred in OH absorption band region (2200±3700cm 21),C y O absorption band region (1500±1900cm 21),and aliphatic C±H absorption bands

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Fig.1.Change in weight,gas formation rates and O content of the coal samples during the

pre-oxidation.

Fig.2.In situ FTIR spectra measured during the pre-oxidation for CR coal.

(2920and 2840cm 21).These changes clearly show that net effect of oxidation is the formation of oxygen functional groups by the consumption of OH and aliphatic C±H groups.To examine the changes quantitatively,the spectra ranging from 2200to 3600cm 21and those ranging from 1500to 1900cm 21were,respectively,curve-resolved to nine and nine peaks by a curve-?tting method [17]as typi-cally shown for PO-330in Fig.3.Then the relative changes of the amounts of OH 1COOH groups (3516,3400,3280,3150,2940,and 2650cm 21),aliphatic C±H groups (2920and 2840cm 21),aromatic C±H groups (3050cm 21),COOH groups (2650cm 21),carbonyl groups (1700cm 21),aldehydes (1740cm 21),esters (1760cm 21),and anhydrides (1800and 1840cm 21)were calculated,and their values are shown for the PO coals in Table 2.All the amounts were normalized by the initial values in the raw coal except the amounts of esters and anhydrides,which were normalized by the value of esters in PO-250,and the value of anhydrides in PO-300,respectively.The amount of aliphatic C±H decreased signi?cantly with the progress of oxidation and reached 0.21for PO-330prepared from CR.For PITT,and GO,it reached 0.87,and 0.93,respectively,for PO-330.On the other hand,the amount of aromatic C±H was virtually unchanged for all the coals.In parallel with the decrease of the aliphatic C±H the amount of carbonyl groups increased signi?cantly,and reached,for example,2.12for PO-300prepared from CR.The decrease of the amount to 1.85for PO-330is judged to be due to the decomposition of the formed carbonyl groups.For CR,the amount of aldehydes increased with the progress of oxidation and reached maxi-mum at around 2708C then decreased.On the other hand,the amounts of carbonyl groups and aldehydes increased monotonously up to 3308C for PITT and GO.Esters were formed at above 2508C for all the coals.The amount of

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1480Fig.3.The results of peak division by a curve-?tting method for PO-330prepared from CR coal.

Table 2

The relative amounts of each functional group in the raw coal and the pre-oxidized coals for the three coals Sample Aliphatic C±H Aromatic C±H OH 1COOH COOH Carbonyl Aldehyde Ester Anhydride CR Raw 1.00 1.00 1.00 1.00 1.00 1.00±±PO-2500.85 1.000.970.89 1.72 1.30 1.00±PO-2700.73 1.000.920.86 1.97 1.30 1.84±PO-3000.48 1.000.880.77 2.120.93 3.57 1.00PO-3300.21 1.000.870.73 1.850.43 4.65 2.81PITT Raw 1.00 1.00 1.00 1.00 1.00 1.00±±PO-2500.95 1.000.960.88 3.84 2.47 1.00±PO-2700.93 1.000.950.80 5.51 3.12 2.85±PO-3000.90 1.000.940.78 6.95 4.30 4.96±PO-3300.87 1.000.940.757.54 6.50 6.98±GO Raw 1.00 1.00 1.00 1.00 1.00 1.00±±PO-2500.99 1.000.970.96 2.39 1.84 1.00±PO-2700.98 1.000.960.93 2.87 2.45 1.19±PO-3000.95 1.000.960.91 3.29 2.76 1.60±PO-330

0.93

1.00

0.95

0.89

4.13

3.46

2.24

esters increased signi?cantly with the progress of oxidation and reached4.65for PO-330.Anhydrides were formed at above3008C for CR,but none was observed for PITT and GO.The amount of anhydrides increased signi?cantly with the progress of oxidation and reached2.81for PO-330 prepared from CR.These results indicate that the aliphatic groups were selectively oxidized to form aldehydes.Then the aldehydes were oxidized to form carboxylic groups, carbonyl groups,and esters.Anhydrides were also formed from the carboxylic groups and aldehydes.The aromatic C±H groups were not oxidized at all at temperatures below 3308C.

3.2.Effect of the pre-oxidation on the carbonization behaviors of the coal

3.2.1.Weight decreasing behavior

We could prepare the pre-oxidized coals with different degrees of oxidation as shown earlier.Next,we examined the effect of the pre-oxidation on the carbonization beha-viors of the coals.This was studied from the viewpoint of the weight changes and gas formation rates during the carbonization.The pre-oxidized coals,PO-250,PO-270, and PO-300,were heated from room temperature to 9008C at10K/min of rate in a nitrogen stream to be carbon-ized.Fig.4compares the weight changes during the carbon-ization between the pre-oxidized coals and the raw coals. The weight of the pre-oxidized coal is normalized by the weight of the raw coal.Therefore,the weight change curve starts from the yield of the PO coal for all the coals.The weight decreasing pro?les of the PO coals were signi?cantly different from those of the raw coals.The weight of the pre-oxidized coals decreased more gradually than the raw coals, especially for PITT coal,and the yields of the carbonized coals increased with the progress of the pre-oxidation for all the coals.For PITT coal,the carbonization yield of PO-300 at9008C was larger than that of the raw coal by0.12kg/kg coal.These results clearly show that the pre-oxidation affects signi?cantly the carbonization behaviors of the coal.Table3shows the yields and the ultimate analyses of the raw coal and the pre-oxidized coals after carboniza-tion at9008C for CR.The yield increased with the increase of the degree of pre-oxidation,had a maximum at270±3008C,then decreased.On the other hand,the ultimate analyses of the carbonized coals were not signi?cantly different among the pre-oxidized coals and the raw coal. Therefore,it is rather dif?cult to examine the effect of the pre-oxidation on the carbonization behaviors from only the ultimate analyses.

3.2.2.Gas formation rates

Fig.5compares the formation rates of H2,CH4,H2O,CO, CO2and tar of the three pre-oxidized coals with those of the raw coal for CR.The formation pro?le of H2was little affected by the pre-oxidation:the formation of H2started from ca.5008C and had a maximum at around7508C then decreased rapidly.The formation rate of CH4decreased considerably with the progress of pre-oxidation.Since CH4is formed from the decomposition of aliphatic groups in coal and aliphatic groups were oxidized during the pre-oxidation,the decrease of the formation rate of CH4is judged to be due to the decrease of the aliphatic groups during the pre-oxidation.Formation of H2O started from ca.1508C for the raw coal and from ca.2508C for the

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Fig.4.Change in weight of the pre-oxidized coals and the raw coals during the carbonization.Table3

Yields and ultimate analyses of the raw coal and the pre-oxidized coals after carbonization at9008C for CR

Sample Ultimate analyses(wt%daf)Yield

(kg/kg raw coal)

C H N O1S(diff.)

Raw91.70.7 1.6 6.00.60

PO-25091.50.7 1.8 6.00.62

PO-27092.40.6 1.6 5.40.65

PO-30092.00.7 1.5 5.80.65

PO-33092.00.7 1.6 5.70.62

pre-oxidized coals,but the formation pro?le was little different among the samples at above 3508C.The formation rates of CO and CO 2increased signi?cantly with the progress of pre-oxidation.This is due to the increase of the carbonyl groups,esters,and anhydrides in the coal during the pre-oxidation (Table 2).The formation of tar was signi?cantly suppressed by the pre-oxidation.The amount of tar formed was only 0.12kg/kg coal for PO-330whereas that was 0.21kg/kg coal for the raw coal.3.2.3.Change in functional groups

In this section we examined the changes in functional groups of the coals and the pre-oxidized coals by utilizing the in situ FTIR analysis.Fig.6shows the change in the in situ FTIR spectra (800±4000cm 21)of the raw coal and PO-300for CR with the increase of temperature from room temperature to 5508C in a highly pure nitrogen stream.To examine the change of functional groups during the carbonization,these spectra were curve-resolved as were done for the pre-oxidized coals,and the amount of each functional group was calculated.Fig.7compares the change in the amounts of several functional groups during the carbonization between the raw coal and PO-300.All the amounts are normalized by the values of the raw coal.The amount of aliphatic C±H decreased at 350±5008C for both the raw coal and PO-300.The amount of aromatic C±H,on the other hand,increased at above 3008C for both the raw coal and PO-300.For PO-300,it reached 1.6as large as the initial amount at 4708C.This change,which was well associated with the small tar yield during the carbonization of PO-300(Fig.5(f)),indicates that the pre-oxidation enhances the development of aromatic structure.The change in the amounts of OH 1COOH groups was well related to the H 2O formation pro?les shown in Fig.5(b):they are different at only below 3008C between the raw coal and PO-300.The difference in the decreasing behaviors of the carbonyl groups between the raw coal and PO-300was well related to the difference in the formation pro?les of CO 2and CO between the two samples.These results indi-cate that the in situ FTIR analysis is a powerful technique for examining the carbonization behavior of the coal and the pre-oxidized coal.It was clearly shown that most of alipha-tic C±H,COOH,carbonyl and OH groups are consumed by 5508C for both the raw coal and PO-300.The only distinct difference in functional groups between the raw coal and PO-300at around 5008C is that the amount of aromatic C±H group of PO-300is larger than that of the raw coal.This difference well reˉects the difference in the carbonization yields between the raw coal and PO-300(Fig.4).These results and the ultimate analyses given in Table 3conclude

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1482Fig.5.Change in formation rates of H 2,CH 4,H 2O,CO,CO 2and tar during the carbonization for CR coal.

that the apparent effect of the pre-oxidation on the carboni-zation behavior is the enhancement of the development of aromatic structure,leading to the increase in the yield.It was suggested that the cross-linking reactions of the oxygen functional groups formed during the pre-oxidation contrib-uted to the development of the aromatic structure.

3.2.

4.X-ray diffraction patterns

Fig.8shows the X-ray diffraction patterns of the raw coal and the pre-oxidized coals after carbonization at9008C for CR.The X-ray diffraction patterns were not signi?cantly different among the pre-oxidized coals and the raw coal.

Therefore,it is rather dif?cult to examine the effect of the pre-oxidation on the carbonization behavior from the X-ray diffraction patterns.

3.2.5.Micropore volume distributions

Finally,the effect of the pre-oxidation on the carboni-zation of coal was examined from the viewpoint of micro-pore structures of the coal.Fig.9shows how pore volume distributions of the raw coal and the pre-oxidized coals change through the carbonization for CR,where the accu-mulated pore volumes on the raw coal basis are plotted against the minimum dimensions of the probe molecules used.Before the carbonization the total micropore volumes of the pre-oxidized coals were larger than the total micropore volume of the raw coal and were depen-dent on the degrees of the pre-oxidation.After carboniza-tion at7008C the distributions of micropore volume were almost same for all the pre-oxidized coals and they were broader than the distribution of the raw coal.After carbon-ization at9008C the total micropore volumes were almost the same for all the pre-oxidized coals and were almost the same as those of the samples carbonized at7008C.On the other hand,the micropore distributions became broader with the progress of pre-oxidation.Thus,it was found that the pre-oxidation perceptibly affects the development of the micropores during the carbonization of coal.This was judged to be the result of the enhanced cross-linking reactions of the oxygen functional groups introduced during the pre-oxidation.

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Fig.6.In situ FTIR spectra of the raw coal and PO-300for CR coal during the

carbonization.

Fig.7.Change in the amount of the functional groups of the coal during the

carbonization for CR coal.

4.Conclusions

Three bituminous coals were pre-oxidized in air stream diluted by nitrogen (11%oxygen)to different degrees of oxidation.The changes in weight,gas formation rates,amount of functional groups during the pre-oxidation were measured continuously by a thermogravimetric analyzer,a mass spectrometer,and in situ FTIR spectrometry,respec-tively.It was found that aliphatic groups were selectively oxidized to form aldehydes.Then the aldehydes were oxidized to carboxylic groups,carbonyl groups,and esters during the pre-oxidation.Anhydrides were also formed from the carboxylic groups and aldehydes.The oxygen content in the pre-oxidized coals increased with the increase of temperature for all the coals.The effects of pre-oxidation on the carbonization behavior of the coals were examined from several aspects.It was found that the yield of the carbonized coal increased with the progress of the pre-oxidation.During the carbonization,the formation rates of CO and CO 2increased while the formation rates of CH 4and tar decreased with the increasing degree of the pre-oxidation.These results were brought about by the increase in carbonyl groups and the decrease in aliphatic groups during the pre-oxidation.The total micropore volume increased signi?cantly through the pre-oxidation.After the carbonization,the distribution of the micropore became broader with the progress of pre-oxidation.The changes in the carbonization yields and the micropore distributions were judged to be brought about by the enhancement of the cross-linking reactions of the oxygen functional groups introduced during the pre-oxidation stage.References

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1484Fig.8.X-ray diffraction patterns of the raw coal and the pre-oxidized coals after carbonization for CR

coal.

Fig.9.Accumulated micropore volume distributions of the raw coal and the pre-oxidized coals before and after carbonization for CR coal.