Low-temperature pyrolysis of coals with different coking pressure characteristics

Low-temperature pyrolysis of coals with different coking

pressure characteristics

M.D.Casal,C.S.Canga,M.A.D?′ez,R.Alvarez,C.Barriocanal*

Instituto Nacional del Carbo′n INCAR-CSIC,Apartado73,33080Oviedo,Spain

Received7June2004;accepted22October2004

Available online17March2005

Abstract

Low-temperature pyrolysis by means of the Gray–King test was carried out with ten coals of different coking pressure characteristics.The ?nal pyrolysis temperature was selected taking into account the beginning and end of the coking pressure development.The soluble fraction in dicloromethane obtained from the tars was analysed by gas chromatography using?ame ionization detection(GC–FID)and high performance liquid chromatography(HPLC).The coking pressure generated by the coals was assessed by means of a movable wall oven.Furthermore,the contraction/expansion of the coals during thermal treatment was measured by using the Koppers-INCAR and sole heated oven tests.The primary tar composition was studied in relation with contraction/expansion and the coking pressure generated by the coals.Although not conclusive,there is strong evidence to suggest that the relative amount of heavy compounds in the primary tars is related to the coking pressure generated by the coals studied.

#2005Elsevier B.V.All rights reserved.

Keywords:Coal;Pyrolysis;Coking pressure;Primary tars

1.Introduction

The carbonization of coal to produce metallurgical coke for use in a blast furnace is carried out in horizontal slot-like ovens,which are indirectly heated through the side walls. During this process,the volatile matter that evolves as a consequence of the thermal decomposition of the coal,either moves towards the coolest part of the oven where it condenses on the unreacted coal,or towards the wall, passing through the different layers of material already formed,i.e.semicoke and coke,and eventually resulting in the development of coking pressure[1–3].In the case of some high-rank coals,commonly referred to as dangerous, the generation of excessive coking pressure during the coking process may cause the bricks in the side walls of the oven to deteriorate,thereby shortening the life span of the oven[4,5].For this reason,the coke industry and coal producers need to be able to forecast and assess the danger of coal,to keep the coking pressure below certain limits in order to prolong the coke oven life.

A number of studies related to the coking pressure developed during the carbonization process have been carried out.Some of them have focussed on the mechanism of the generation of coking pressure[3–13],while others have tried to?nd reliable methods of identifying dangerous coking coals[2,11,14,15].From all of these works it is clear that the type of coal has a profound in?uence on coking pressure.However,the release of volatile matter from the coal during the plastic stage also plays a signi?cant role in the generation of coking pressure and the degree of coal dangerousness.

Several studies have examined coal pyrolysis and the volatile matter evolved during this process[16–23].Such works have taken into account the effects of heating rate, particle size,coal type,macromolecular structure in the plastic stage,temperature and pressure on the different products obtained from the pyrolysis(solid and liquid residue).

Most of these authors studied the behaviour of different coking coals but failed to take into account that coals with

https://www.360docs.net/doc/753298626.html,/locate/jaap

J.Anal.Appl.Pyrolysis74(2005)96–103

*Corresponding author.

E-mail address:carmenbr@incar.csic.es(C.Barriocanal).

0165-2370/$–see front matter#2005Elsevier B.V.All rights reserved.

doi:10.1016/j.jaap.2004.10.012

similar properties as measured by means of conventional tests can show different behaviours in terms of coking pressure generation.

In addition to these studies,several research works have also investigated the relationship between dangerous/safe coals and the devolatilisation that takes place during the coking process[9–13].

Koch et al.[9–11]concentrated their work on the migration of volatile matter through the different layers of material formed in the oven.Their research supported by samples extracted with N-methyl-2-pyrrolidinone reported that for dangerous coals the volatile material migrates mainly from the plastic layer to the cold side of the oven, impregnating the remaining coal.On the other hand,the volatile compounds of the safe coals mainly migrated through the semicoke and coke.

Other works dealing with devolatilisation[12,13]by means of pyrolysis-?eld ionization mass spectrometry(Py-FIMS)have revealed differences in the pyrolysis behaviour of coals that generate different coking pressures.From the results it was concluded that coking pressure is related to the yield of pyrolysis products generated in a given temperature range and to the individual composition of the thermal degradation products produced by the coal.

The objective of this paper was?rstly to correlate the variations in the composition of the primary tars from coals used by the steel industry in coke manufacture with parameters that re?ect the contraction/expansion of coals and with the wall pressures developed during pilot-scale coking,and secondly to shed some light on the mechanism of the generation of coking pressure.

2.Experimental

2.1.Materials

Ten bituminous coals were chosen as showing different coking pressures in a pilot coke oven.The main characteristics of the coals used are presented in Table1. Proximate analyses were performed following standard procedures ISO562and ISO1171for volatile matter and ash content,respectively.

2.2.Low-temperature pyrolysis

Pyrolysis tests according the Gray–King standard procedure(ISO502)modi?ed by Sua′rez-Ruiz et al.[24]were conducted on the ten single coals.For each pyrolysis experiment,a sample of6–8g with a size<3mm was placed in a quartz retort,which was then heated in a horizontal electrical oven at58C/min to a?nal temperature of450and5508C in the atmosphere of evolving gases.After pyrolysis,the liquid products were collected using an ice-cooled trap and recovered by solvent extraction with dichloromethane.The yields of coke,gas and liquid products(primary tar)were calculated relative to the starting material on a dry basis.

2.3.Primary tar characterization

GC analysis was performed on a Hewlett-Packard Model 5890Series II gas chromatograph equipped with a?ame ionization detector(FID).Separations were carried out in a fused-silica capillary column of length25m and i.d.

0.22mm(Quadrex,New Haven,CT,USA)coated with OV-1701stationary phase(McReynolds polarity=789). The temperature was programmed from50to2908C at a rate of48C/min.The initial and?nal temperatures were maintained for5and10min,respectively.Helium was used as the carrier gas at a?ow rate of1ml/min and a split ratio of 1:42.The detector and injector temperatures were350and 3008C,respectively,and the volume of sample injected was 1m l.

The compounds were identi?ed by means of GC–MS using a Finnigan Mat GCQ instrument equipped with the same fused-silica capillary column and similar conditions.

An HPLC analysis of the aromatic compounds present in the primary tar from the coals was carried out using a Hewlett-Packard HP1100system incorporating two PLGel columns(300mm length?7.5mm i.d.)packed with poly(styrene/divinylbenzene)of different nominal pore size (500and100A?,respectively)and connected in series.The mobile phase was dichloromethane/methanol(9:1,v/v)and the?ow rate1ml/min.The injection volume was1m l.Four major regions can be de?ned,the?rst three regions corresponding to cata-condensed PACs,while the last one corresponds to peri-condensed PACs.The method used [25,26]was based on a previous work reported by La?eur and Wornat[27,28]that allows the separation of cata-and peri-condensed PACs.

The?rst region named cata1is mainly characterized by heteroaromatics and compounds substituted with alkyl,aryl and heteroatomic(OH,N,Ar–O–Ar,etc.)groups.The second region(cata2)mainly includes compounds that have alkyl and aryl substitution,naphthenic groups and those

M.D.Casal et al./J.Anal.Appl.Pyrolysis74(2005)96–10397 Table1

Proximate analyses of the single coals studied

Coal P RF RM LU BC RV PD K PB W Moisture(wt.%) 3.3 2.28.5 2.0 2.4 3.1 2.6 3.8 3.1 2.6 Ash(wt.%db) 6.17.0 6.09.38.29.710.1 4.7 5.0 5.0 V.M.(wt.%db)32.832.131.325.623.722.920.419.419.317.9 Sulphur(wt.%db)0.750.990.770.240.690.510.580.720.750.72

groups classi ?ed as hydroaromatic compounds.The next region (cata3)is mainly characterized by unsubstituted PACs,while the last region corresponds to peri-condensed PACs.The areas of the above regions were considered for quantitative analysis.

2.4.Semicoke contraction measurement

Laboratory scale experiments were performed using the Koppers-INCAR expansion/contraction test (KI),developed and patented by INCAR [14].Brie ?y,80g of coal sample,ground to <1mm size,was placed inside a stainless steel crucible,with a bulk density of 820kg/m 3.The crucible was placed in a sole heated oven at 9008C.A constant pressure of 0.89kg/cm 2was exerted on the charge for 2h.The change in charge volume compared to the initial state of the coal sample was recorded on a graph.

In addition,the sole heated oven (SHO)test was carried out according to ASTM D 2014-85speci ?cations.A coal mass between 4.25and 5.30kg,crushed to no less than 70%and no more than 85%below 3.35mm,with 1%moisture,was enclosed in the carbonisation chamber at a bulk density of 881kg/m 3.A constant force of 15.2kPa was applied on the top charge.Carbonisation temperature rose from 5508C on the sole to 9508C.The test was considered to be completed when the temperature of the top surface of the coal was 5008C.

2.5.Coking pressure measurement

Semi-pilot-scale carbonisations were carried out in a movable wall oven of about 15kg capacity (MWO15).The dimensions of the oven are 15cm W ?25cm L ?79cm H .A load cell is mounted on the movable wall to measure the force exerted on the wall during carbonisation.Coals were charged in the oven and carbonised at 10108C for 2h and 45min.The different carbonisation experiments were performed with a similar dry bulk density to avoid any distortion of the results.

3.Results and discussion

The coals studied are among those normally available on the international market,and used by the Steel industry in their blends to produce metallurgical coke.Proximate analyses of the coals are presented in Table 1.All of them are bituminous coals with a volatile matter content ranging between 17.9and 32.8wt.%.3.1.Low-temperature pyrolysis test

Taking into account the fact that the temperature at which the value of maximum wall pressure is reached is about 4508C (coinciding with the carbonization stage when the rapid evolution of volatile matter takes place)and the fact

that the end of the wall pressure peak occurs at about 5508C (when practically all of the condensable products have been obtained),it was decided to carry out the pyrolysis tests at these two temperatures.

The tar,coke and gas yields for the pyrolysis tests at 450and 5508C on the single coals are shown in Fig.1.The gas yield has been calculated by difference.The coke yield is directly related to the volatile matter content of the coal and is higher than the tar or gas yields.For all the coals,the major pyrolysis product was solid 90–98wt.%on a dry basis (db)at 4508C and 77–92wt.%db at 5508C.The tar yields decreased with increasing coal rank from 8.9to 0.9wt.%db and from 15.8to 5.2wt.%db at 450and 5508C,respectively.

3.2.GC –MS and GC –FID analysis of low-temperature tars Most coal structures consist of structural units (e.g.aromatic and hydroaromatic clusters linked by alkyl or etheric bridges)joined together by either weak or strong linkages to form a three-dimensional macromolecular network.During pyrolysis,the weakest bridges break,producing molecular fragments that will be released as tar.It is known that the lower the rank of the coals,the more dissimilar the evolved tars are to the parent coal [29].However,primary tars are considered to be structurally related to parent coals as they contain structures and functional groups that are present in the original coal [30].The main compounds identi ?ed by means of GC –MS were:phenol,naphthalene,?uorene,phenanthrene,methyl,dimethyl and trimethyl derivatives,benzozo ?uorene and aliphatic compounds ranging from decane (C10)to

M.D.Casal et al./J.Anal.Appl.Pyrolysis 74(2005)96–103

98Fig.1.Yields of coke (),tar ()and gas ()obtained for the coals tested at 4508C (a)and 5508C (b).

triacontane(C30).An example of the chromatograms of the primary tars obtained at4508C for2out of the10coals studied shows that the pro?les of the primary tars are qualitatively similar(Fig.2),although there are some variations with the rank of the coal,i.e.a clear series of n-alkanes from C10to C30is present in the primary tar from the high-volatile coals.The greater amount of aliphatic species found in the lower rank coals has been associated with the lower degree of coali?cation of the precursor[29].

In order to obtain information on the relationship between the primary tar composition and the dangerousness of coals, two chromatographic regions were de?ned.The?rst region (C11–C19)included the low-and medium-volatile com-pounds which eluted between n-alkanes,undecane (MW=156,bp=1968C)and nonadecane(MW=268, bp=329.98C)and the second region,>C19,was composed of the heaviest compounds ranging from C19to the end of the chromatogram.The percentage of each region present in all the primary tars obtained at450and5508C is shown in Fig.3.

The percentages of each region for the same coal at450 and5508C are similar(Table2).However,the percentage corresponding to the compounds with a low-and medium-boiling point(C11–C19)is slightly higher for the tars obtained at5508C with the exception of coals Lu and Rv. The general trend for the rank of the parent coal is an increase in the amount of heavier compounds in the primary tars(>C19region),and a consequent decrease in low-and medium-volatile compounds(C11–C19region).These results are in agreement with those found by other authors [30]which showed that tar from high-rank bituminous coals present an increase in aromatic hydrogen and carbon content and in aromaticity.However,it should be noted that the dangerous coal BC with a volatile matter content similar to coal RV(VM,23.7wt.%versus22.9wt.%)presents a percentage of the>C19region similar to that of coal K, especially at4508C(39.8and40.3wt.%).That is,it exhibits a higher amount of the>C19fraction than one would expect from its rank.As will be seen later this behaviour of coal BC that resembles that of lower volatile coals,might explain the high coking pressure generated by this coal.The same applies to coal PD which has a similar volatile matter content to coal K(20.4wt.%versus19.4wt.%)and a percentage of the>C19region similar to that of coal RV, especially at5508C(Table2).

M.D.Casal et al./J.Anal.Appl.Pyrolysis74(2005)96–103

99

Fig.2.GC chromatograms of the tars obtained at4508C from coals P and W.Numbers in brackets correspond to the identi?cation of the peaks.C11 (1);C12(2);phenol(3);o-cresol(4);C13(5);p-and m-cresol(6); dimethylphenol(7);2-methylnaphtalene(8);1-methylnaphtalene(9); C14(10);dimethylnaphtalenes(11);C16(12);trimethylnaphtalenes (13);C17(14);?uorene(15);methyl?uorene(16);C19(17);phenanthrene (18);C20(19);methylanthracene–phenanthrene(20);C21(21);dimethyl-phenanthrenes(22);C22(23);C23(24);benzo(a)?uorene(25);C24(26); benzo(b)?uorene(27);C25(28);methylbenzo?uorenes(29);C26(30);C27 (31);C28

(32).Fig.3.Percentage of the C11–C19region ()and the>C19region ()for all the coals studied.

Table2

GC–FID data of the primary tars obtained from the single coals at450and5508C

P RF RM LU BC RV PD K PB W 4508C

C11–C19a(%)72.573.271.571.160.266.059.758.656.854.2 >C19b(%)27.526.828.528.939.834.040.341.443.245.8 5508C

C11–C19a(%)73.274.871.769.463.565.466.461.660.256.4 >C19b(%)26.825.228.330.636.534.633.638.439.843.6

a Percentage of the high-and medium-volatile compounds present in the primary tar.

b Percentage of the low-volatile compounds present in the primary tar.

3.3.HPLC analysis of low-temperature tars

An example of the HPLC chromatograms of the primary tars obtained at 5508C is shown in Fig.4.No qualitative differences are observed when the pro ?les of the chromato-grams of the tars resultant from the pyrolysis of different coals are compared (Fig.4).All the pro ?les are mainly centred on the cata1and cata2region,indicating the presence of heterocyclic and aromatic compounds sub-stituted with alkyl or aryl groups.These fractions (cata1and cata2)represent more than 78%of the total amount,the number of peri -condensed PACs being relatively low and not exceeding 6.5%(Table 3).The composition of the tars obtained at 450and 5508C is similar,although the amount of cata1and peri -condensed compounds increases slightly with temperature (Fig.5).The comparison of the primary

tars from coals of different rank clearly shows that,as the volatile matter of the coal diminishes,the amount of compounds containing heteroatoms in the tar (cata1)diminishes.On the other hand,the number of compounds containing alkyl or aryl substituents (cata2)increases as do the planar cata -condensed (cata3)and peri -condensed PAC ’s.

3.4.Relationship between low-temperature tar composition and coking pressure

Coking pressure is an important parameter when considering coals for their use as raw materials by the iron and steel industry.A simple and complete description of the mechanism of coking pressure generation is not yet available.The present work tries to establish the link

M.D.Casal et al./J.Anal.Appl.Pyrolysis 74(2005)96–103

100Fig.4.HPLC pro ?les of primary tars from coals P and W.

Table 3

Percentage of the different types of PACs found in the tars by means of HPLC

P

RF RM LU BC RV PD K PB W 4508C

cata1a (%)50.849.749.042.340.743.235.828.729.120.9cata2b (%)38.339.440.044.546.144.950.453.752.558.4cata3c (%)8.58.68.610.410.29.311.114.114.717.0Peri d (%) 2.4 2.3 2.4 2.8 3.0 2.6 2.7 3.5 3.7 3.75508C

cata1a (%)51.554.852.947.944.548.643.936.034.729.4cata2b (%)36.335.336.639.542.039.142.146.046.449.1cata3c (%)8.87.27.68.89.38.69.912.412.915.1Peri d (%)

3.4

2.7

2.9

3.8

4.2

3.7

4.1

5.6

6.0

6.4

a Heteroaromatics,compounds with alkyl and aryl substitution,hydroaromatics.

b PACs with alkyl and aryl substitution.

c Unsubstitute

d PACs.d

Peri -condensed

PACs.

Fig.5.Percentage of cata1();cata2();cata3();and peri regions ()for all the coals studied.

between the composition of the gas generated during the process and the coking pressure generated by a series of coals.

In order to distinguish between dangerous from non-dangerous coals during coking,expansion/contraction and movable wall oven tests are usually carried out by the coking industry.In the present work,the laboratory Koppers-INCAR and the ASTM-SHO tests were used to assess contraction/expansion.In addition,the coals were carbo-nised in a movable wall oven to measure the coking pressure generated during the process (Table 4).A coal can be considered dangerous during coking if:(i)the Koppers-INCAR contraction is lower than 10mm [14,31,32];(ii)in the SHO test the coal shows an expansion value greater than 2%;and (iii)the coking pressure measured in the MWO15is higher than 10kN/m 2(bulk density =700kg/m 3db).According to these criteria,coals P,RF,RM,LU and RV can be considered as not dangerous,coal PD would be at the limit,while coals BC,PB,K and W can be considered as dangerous coals.

It is well known that it is not possible to distinguish between dangerous and safe coals on the basis of their rank.However,most low-volatile coals are dangerous in coking,while high-volatile coals generate a very low coking pressure.Nevertheless,it is important to bear in mind that coals with a similar volatile matter content can behave differently as regards coking pressure generation,i.e.coal BC (VM =23.7wt.%and KI contraction =à5mm)and coal RV (VM =22.9wt.%and KI contraction =à20mm)or coal K (VM =19.4wt.%and KI contraction =à1mm)and coal PD (VM =20.4wt.%and KI contrac-tion =à15mm).The amount of volatiles and their rate of evolution during the process seem to be irrelevant in elucidating the behaviour of a coal during coking.High-volatile coals that produce a greater amount of volatiles and at a greater rate than low-volatile coals do not generate any signi ?cant pressure during the process.It appears that the composition of the volatile products that pass through the different layers formed during the process plays a more important role [11–13].

The composition of the primary tars generated during the low-temperature pyrolysis of the coals was related to the coking pressure generated by the coals included in the

present study.In this way,the percentage of the >C19region was related to the contraction/expansion determined by means of the KI and SHO tests and the coking pressure generated in the MWO15(Fig.6).It was found that the amount of high-boiling point compounds in the low temperature tar (450and 5508C)increases with decreasing contraction as measured in the KI and SHO tests.Similarly,the higher the coking pressure in the movable wall oven (MWO15)the higher the amount of the >C19fraction in the tar.Two regions that discriminate between dangerous and safe coals as a function of the percentage of heavy compounds in the tar can be de ?ned for the two temperatures tested.Dangerous coals are characterised by a percentage of the >C19fraction higher than 40%for tars produced at

M.D.Casal et al./J.Anal.Appl.Pyrolysis 74(2005)96–103

101

Table 4

Expansion/contraction and coking pressure data of the coals selected Coal

P RF RM LU BC RV PD K PB W Koppers-INCAR test Contraction (mm)à27à30à26à19à5à20à15à7à1+7Expansion (mm)00000000+7+15Sole heated oven

Expansion/contraction (%)à11.9à20.8à10.7à11.6+7.3à5.0+2.4+9.8+18+9.3MWO15

Bulk density (kg/m 3db)714714686700705689723731680692Coking pressure (kN/m 2)

1.6

3.0

4.8

5.5

12.2

2.1

9.4

27.6

38.7

32.0

Fig.6.Relationship between the GC –FID analysis of the primary tars obtained at 450and 5508C and semicoke contraction and coking pressure.

4508C and 35%for tars produced at 5508C.In other words,regardless of coal rank,the higher the amount of heavy compounds in the primary tar,the greater the danger presented by the coal.The higher boiling point of the compounds present in the tars generated by dangerous coals would increase viscosity and the possibility of pores being totally or partially blocked,which in turn would result in an increase in pressure.The results are in agreement with what is already common knowledge [1,13,32]that co-carboniza-tion of a coal with coal tar pitch gives rise to an increase in coking pressure.Even a low level of coal tar pitch addition can convert a safe coal into a dangerous one [32].This effect might be related to the high amount of compounds in a coal tar pitch that volatilise at high temperature.This in turn,would increase the amount of high-boiling point compounds in the gas generated during devolatilization.Furthermore,the wall pressure that develops during coal pyrolysis is related to the impregnation of non-transformed coal by re-condensed tars [11]and is favoured by the presence of high-molecular weight compounds of a low vapour pressure in the reaction media.

The composition of the low-temperature tars as determined by HPLC was related to the expansion/contraction mechanism of the coals and to the wall pressure (Fig.7).The percentage of peri -condensed PAHs for the tars produced at 450and 5508C increases with increasing coal dangerousness.As in the case of the GC analysis,tar from dangerous coals is related to a greater amount of high-molecular weight compounds.Tars produced at 4508C from

safe coals are characterized by a percentage of peri -condensed compounds lower than 2.8%,while for tars produced at 5508C the limit value between the two regions (dangerous and safe)is 4.1%.In addition,cata1type compounds decrease in number while cata2and cata3type compounds increase (Table 3).The comparison between the composition of the tars obtained at the two temperatures could provide an idea of the reactions that take place during the interval and the type of compounds that are related to the development of higher coking pressures.Tar produced from safe coals at the two temperatures tested is formed by cata1>cata2>cata3>peri type compounds,while for dangerous coals the order of decreasing percentage of compounds is cata2>cata1>cata3>peri (Table 3).Furthermore,the sum of the cata1and cata2type compounds is higher for the safe coals than for the dangerous coals.In other words,the reactivity of the compounds,which are more abundant in the tar from safe coals,is higher than that of the compounds present in the dangerous coals.This would seem to suggest that the components present in tars from safe coals react quickly and therefore abandon the gas phase causing a decrease in vapour pressure.In the 450and 5508C temperature range,the reactions that take place involve an increase in the amount of cata1and peri type compounds,together with a decrease in the percentage of the cata2and cata3type compounds present in the tars (Table 3)obtained from the coals included in the present study.However,the rise is considerably higher for the dangerous coals than for the safe ones.

The composition of the gas generated during coal decomposition in the coking process seems to be related to the coking pressure exerted in the process.It seems that an abundance of heavy compounds in the primary tar is responsible for the low contraction and the generation of a high coking pressure.Furthermore the presence of highly reactive species in the primary tar would contribute to relieving vapour pressure.

Acknowledgements

The authors thank the European Coal and Steel Community (ECSC)(project 7220-PR/069)and Ministerio de Ciencia y Tecnolog ?′a (MCYT)(project PPQ2001-1450)for ?nancial support.

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