P et r oleu m cokes a r e widely u sed in cem en t pla n t s for en er gy gen er a t ion . Beca u se of t h eir ver y low vola t ile con t en t , t h e ph ysicoch em ica l pr ocesses occu r r in g du r in g t h eir com bu st ion sh ou ld be m a in ly con t r olled by h et er ogen eou s r ea ct ion s. Th e r ea ct ivit ies of som e pet r oleu m cokes t owa r d NO bet ween 800 a n d 1000 °C h a ve been ch a r a ct er ized t h r ou gh exper im en t s in a n en t r a in ed flow r ea ct or . Th ese ca r bon a ceou s m a t er ia ls exh ibit ver y h igh r ea ct ivit ies t owa r d NO. Th eir ca pa cit y t o r edu ce NO con cen t r a t ion in t h e su r r ou n din g ga s is sign ifica n t , so t h a t t h is r ea ct ion ca n h a ve a sign ifica n t im pa ct in t er m s of NO em ission s r esu lt in g fr om t h eir com bu st ion . Th e in flu en ce of su r fa ce a r ea a n d of t h e h ea vy m et a l con t en t on su r fa ce r ea ct ivit y wa s t est ed. As it wa s t h e ca se for ca r bon -oxygen r ea ct ion , t h e su r fa ce m ea su r ed by m er cu r y por osim et r y seem s t o be a good pa r a m et er t o ca lcu la t e su r fa ce r ea ct ion r a t e. No cor r ela t ion wa s fou n d bet ween ca lciu m or ir on con t en t a n d NO-ca r bon r ea ct ion r a t e. As for t h e r ea ct ion of pet r oleu m coke wit h oxygen , va n a diu m a ppea r s t o h a ve a ca t a lyt ic effect on t h e r edu ct ion r ea ct ion of NO wit h ca r bon .

In tro d u c tio n

P et r oleu m coke, a r esidu e of t h e pet r oleu m r efin in g in du st r y, is u sed t oda y in la r ge qu a n t it ies a s a sou r ce of en er gy. Th is solid fu el h a s t h e a dva n t a ge of bein g ea sy t o gr in d, of con t a in in g a h igh ca lor ific va lu e (bet ween 32.5 a n d 34.5 MJ kg -1 ), a n d of bein g ch ea p in the market place. For cement plant precalciner applicat ion s, it is gr ou n d t o a ppr oxim a t ely 50% < 90 µm, and u sed a s a coa l r epla cem en t fu el. It h a s a lso been u sed in flu idized bed com bu st or s, gen er a lly co-fir ed wit h coa l. 1,2 Th e pr eviou s yea r s of u t iliza t ion h a ve iden t ified a number of specificities in their utilization, particularly in t er m s of pr ocess con t r ol a n d of pollu t a n t for m a t ion . Ver y lit t le lit er a t u r e h a s been devot ed t o t h ese fu els, m a kin g it difficu lt t o in t er pr et t h e differ en ces obser ved in t h eir beh a vior a s com pa r ed wit h coa l.

Beca use of the low vola tile ma tter content of this type of fu el, t h e im por t a n ce of h et er ogen eou s r ea ct ion s sh ou ld be en h a n ced bot h for t h e oxida t ion of t h e ca rbona ceous ma teria l or for the forma tion of polluta nts. Th e wide va r ia t ion of r ea ct ivit ies of pet r oleu m cokes t owa r d oxygen h a s a lr ea dy been dem on st r a t ed in a previous paper; 3 it has been shown that these variations ca n be cor r ela t ed wit h t h e ca t a lyt ic effect s of va n a diu m . Th e em ission s of n it r ogen oxides a t t h e exh a u st of a com bu st or a r e t h e r esu lt of a ba la n ce bet ween t h e for m a t ion of NO a n d it s r edu ct ion by h om ogen eou s a n d h et er ogen eou s r ea ct ion s. Th er efor e, t h e h et er ogen eou s r edu ct ion of NO on pet r oleu m coke is expect ed t o in fluen ce t h e n it r ogen oxide level in t h e exh a u st , a n d t h e det er m in a t ion of it s r ea ct ivit y t owa r d NO is of in t er est .

Th e r edu ct ion of n it r ogen m on oxide by differ en t sor t s of ca r bon a ceou s solids h a s been t h e a im of m a n y st u dies since the 1980's, beca use this phenomenon is importa nt in flu idized bed com bu st ion . 4 Despit e t h is la r ge a m ou n t of pu blish ed m a t er ia l, n u m er ou s u n cer t a in t ies st ill remain. The mechanism of the reaction itself is not fully u n der st ood a lt h ou gh m a n y a t t em pt s h a ve been pr esented. 5,6 Because of this lack of established mechanism, m ost a u t h or s h a ve t r ied t o fit t h eir exper im en t a l da t a wit h globa l kin et ic la ws. Bu t , a s it h a s been sh own by Aa r n a a n d Su u ber g in t h eir r eview, 7 la r ge va r ia t ion s a r e obser ved bet ween pu blish ed da t a , wh a t ever t h e temperature.

* Cor r espon din g a u t h or . Tel: (33)3 89 32 76 55. F a x: (33)3 89 32 76 61. E -m a il: R.Ga diou @u h a .fr .

† E cole des Min es d'Albi-Ca r m a u x. ‡ La bor a t oir e de Gest ion des Risqu es et E n vir on n em en t . § Tel: (33)5 63 49 31 27. F a x: (33)5 63 49 32 43. E -m a il: sa lva dor @ en st im a c.fr .

(1) An t h on y, E . J . F lu idized bed com bu st ion of a lt er n a t ive solid fu els: st a t u s, su ccesses a n d pr oblem s of t h e t ech n ology. Prog. E n ergy Com bu st. S ci. 1995, 25, 239-268. (2) An t h on y, E . J .; Ir iba r n e, A. P .; Ta lbot , R.; J ia , L.; Gr a n a t st ein , D. L. Fouling in a 160 MWe FBC boiler firing coa l a nd petroleum coke. Fu el 2001, 80, 1009-1014.

(3) Sa lva dor , S.; Com m a n dr e ´, J . M.; St a n m or e, B. Rea ct ion r a t e for t h e oxida t ion of h igh ly su lph u r ised pet r oleu m cokes: t h e in flu en ce of t h er m ogr a vim et r ic con dit ion s a n d som e coke pr oper t ies. Fu el 2003, 82, 715-720.

(4) J oh n sson , J . E . F or m a t ion a n d r edu ct ion of n it r ogen oxides in flu idized bed com bu st ion . Fu el 1994, 73 (9), 1398-1415.

(5) Ch a n , L. K.; Sa r ofim , A. F .; Beer , J . M. Kin et ics of t h e NOcarbon reaction at fluidized bed combustion conditions. Com bust. Flam e 1983, 52, 37-45. (6) De Soet e, G. G. H et er ogen eou s N2O a n d NO for m a t ion fr om bou n d n it r ogen a t om s du r in g coa l ch a r com bu st ion . 23rd S ym posiu m (In tern ation al) on Com bu stion ; The Combustion Institute: Pittsburgh, 1990; pp 1257-1264. (7) Aa r n a , I.; Su u ber g, E . M. A r eview of t h e kin et ics of t h e n it r ic oxide-ca r bon r ea ct ion . Fu el 1997, 76 (6), 475-491.

To expla in t h ese differ en ces, t h e ch em ica l st r u ct u r e a n d t h e t ext u r a l pr oper t ies of t h e solid a r e t h e m ost oft en in vest iga t ed pa r a m et er s. Rodr igu ez-Mir a sol et a l. 8 st u died t h e in flu en ce of t h e r a n k of t h e pa r en t coa l on t h e r ea ct ivit y of t h e ch a r t owa r d NO in a sm a ll sca le fixed bed. H e obser ved t h a t lign it e ch a r s a r e m or e r ea ct ive t h a n ch a r s obt a in ed fr om h igh r a n k coa ls; t h e gr a ph it ic st r u ct u r e of t h e ch a r is t h er efor e a n im por t a n t pa r a m et er in t h e det er m in a t ion of t h e r ea ct ivit y. A st u dy by Zeven h oven a n d H u pa 9 on a la r ger n u m ber of ch a r s led t o t h e sa m e con clu sion . Never t h eless, t h ese r esu lt s m a y be m a in ly a con sequ en ce of t h e differ en ces in t ext u r e a n d ch em ica l com posit ion of t h e sa m ples. A m or e pr ecise st u dy of t h is poin t h a s been m a de by Gu lyu r t lu et a l. wh o u sed ch a r pr epa r ed a t differ en t s tempera tures from the sa me ra w coa l. 10 Th e a u t h or s obser ved t h a t t h e h igh er t h e pr epa r a t ion t em per a t u r e, t h e m or e r ea ct ive t h e sa m ple is. Th is r esu lt cou ld be expla in ed by a n in cr ea se of su r fa ce a r ea a n d a lso by t h e gr a ph it iza t ion of t h e ca r bon st r u ct u r e.

To t a ke in t o a ccou n t t h e in flu en ce of t h e t ext u r e of t h e sa m ples, m ost a u t h or s ca lcu la t ed t h e specific r ea ct ion r a t e by dividin g t h e globa l r a t e of r ea ct ion by a su r fa ce a r ea . Th is la st pa r a m et er is eit h er t h e CO 2 a dsor pt ion su r fa ce or t h e N 2 a dsor pt ion su r fa ce compu t ed by t h e BE T equ a t ion . Sh im izu u sed t h e for m er su r fa ce a n d fou n d a con st a n t va lu e of t h e specific r ea ct ion r a t e for n in e sa m ples wh ich h a d su r fa ces ranging between 250 and 900 m 2 /g. 11 Illia n -Gom ez et al. 12 obser ved a lso a lin ea r cor r ela t ion bet ween t h e r ea ct ion r a t e of ca r bon sa m ples a t 873 K a n d t h e BE T su r fa ce a r ea . Never t h eless, ot h er st u dies sh owed t h a t t h e specific r ea ct ion r a t e pr esen t s la r ge va r ia t ion s a s a function of the chemica l composition of the ca rbona ceous sa m ples.

Wit h a n exper im en t a l set u p sim ila r t o Rodr igu ez-Mir a sol, Ch a n et a l. 5 exa m in ed t h e in flu en ce of m in er a l m a t t er , a n d obser ved t h a t t h e dem in er a liza t ion of a lign it e ch a r lea ds t o a decr ea se in it s r ea ct ivit y wit h NO by a n or der of m a gn it u de. As for t h e ca r bon -oxygen r ea ct ion , m a n y elem en t s ca n h a ve a ca t a lyt ic effect on the NO-ch a r a n d t h e N 2 O-ch a r r ea ct ion s. Ch a n et a l. 5 a n d Zeven h oven a n d H u pa 9 obser ved t h a t t h e ca lciu m con t en t of t h e m ost r ea ct ive ch a r , i.e., t h e lign it e ch a r , wa s h igh . Illa n -Gom ez et a l. st u died t h e in flu en ce of m in er a l m a t t er on t h e r a t e of NO r edu ct ion over ch a r s. Th ey obser ved t h a t wh a t ever t h e t em per a t u r e, t h e a ddit ion of a m et a l su ch a s F e, Cu , or Ni decr ea ses t h e a ct iva t ion en er gy of t h e r ea ct ion . 13,14 Th e ca t a lyt ic activity of potassium and calcium was also observed. 15,16 Som e a u t h or s t r ied t o r em ove t h e m in er a l m a t t er of ch a r s in or der t o qu a n t ify t h e decr ea se in t h e r ea ct ion rate. [17][18][19] The main problem is that the demineralization is n ever com plet e, so t h a t a ca t a lyt ic a ct ivit y r em a in s; a n ot h er pr oblem is t h a t t h e pr epa r a t ion of t h e sa m ple ch a n ges t h e t ext u r a l pr oper t ies of t h e ca r bon a ceou s solid. Despit e t h ese difficu lt ies, a decr ea se of t h e r a t e of t h e r edu ct ion of NO on t h e ch a r su r fa ce wa s obser ved in a ll ca ses.

In t h is wor k, we h a ve ch a r a ct er ized t h e kin et ics of t h e r ea ct ion of NO r edu ct ion on t h e ch a r su r fa ce of a n u m ber of pet r oleu m cokes bet ween 800 a n d 1000 °C. This temperature range is representative either for FBC com bu st ion of pet r oleu m coke or for t h e pr eca lcin er of cem en t pla n t s. As we will dem on st r a t e, t h is r ea ct ion h a s a sign ifica n t im pa ct on t h e globa l em ission s of NO r esu lt in g fr om t h e com bu st ion of t h e pet cokes.

Ex p e rim e n ta l S e c tio n 2.1. Ex p e rim e n ta l S e tu p .

To ch a r a ct er ize t h e r ea ct ion kin et ics in con dit ion s close t o t h ose of a flu idized bed or of a n en t r a in ed flow su ch a s a cem en t pla n t pr eca lcin er , a set of exper im en t s wa s ca r r ied ou t in a la bor a t or y sca le en t r a in ed flow r ea ct or (E F R). Alt h ou gh m a n y differ en t exper im en t a l devices may be used to obtain heterogeneous reactions kinetics su ch a s a t h er m ogr a vim et r ic a ppa r a t u s or fixed beds, 6,8,20 the E F R h a s been widely u sed beca u se it a llows t h e disper sion of pa r t icles in a ga s flow. Th er efor e t h e diffu sion lim it a t ion s obser ved in fixed beds ca n be a voided. An ot h er a dva n t a ge is t h a t h igh t em per a t u r es m a y be r ea ch ed. 21,22 Th e r ea ct or design h a s a lr ea dy been det a iled elsewh er e. 3,23 Th e a ppa r a t u s sh own in F igu r e 1 con sist s of a n 80 m m i.d. a n d 1 m lon g isot h er m a l fu sed silica r ea ct or fed wit h a n elect r ica lly pr eh ea t ed ga s flow. A st r ea m of powder ed fu el pa r t icles wa s in ject ed t h r ou gh a wa t er -cooled feedin g pr obe.

Ca r e wa s t a ken t o obt a in a good disper sion of t h e pa r t icles on a cr oss sect ion of t h e r ea ct or by pla cin g a disper sion dom e 1 m m down st r ea m fr om t h e t ip of t h e feedin g pr obe.

Th e ga s wa s sa m pled a ft er a con t r olled r esiden ce t im e t h r ou gh a wa t er -cooled n it r ogen -qu en ch ed pr obe. It wa s t h en analyzed; NO, CO, and CO2 wer e qu a n t ified by on -lin e NDIR (8) Rodr igu ez-Mir a sol, J .; Oom s, A. C.; P els, J . R.; Ka pt eijn , F .; Mou lijn , J . A. NO a n d N2O decom posit ion over coa l ch a r a t flu idizedbed com bu st ion con dit ion s. Com bu st. Flam e 1994, 99, 499-505.

(9) Zeven h oven , R.; H u pa , M. Th e r ea ct ivit y of ch a r s fr om coa l, pea t , and wood toward NO, with and without CO. Fu el 1998, 77 (11), 1169-1176. (10) Gu lyu r t lu , I.; E spa r t eir o, H .; Ca br it a l, I. N2O formation during flu dized bed com bu st ion of ch a r s. Fu el 1994, 73 (7), 1098-1102.

(11) Sh im izu , T.; Sa za wa , Y.; Adsch ir i, T.; F u r u sa wa , T. Con ver sion of ch a r -bou n d n it r ogen t o n it r ic oxide du r in g com bu st ion . Fu el 1992, 71, 361.

(12) Illa n -Gom ez, M. J .; Lin a r es-Sola n o, A.; Sa lin a s-Ma r t in ez de Lecea , C.; Ca lo, J . M. NO r edu ct ion by a ct iva t ed ca r bon s. 1. Th e r ole of ca r bon por osit y a n d su r fa ce a r ea . E n ergy Fu els 1993, 7, 146-154.

(13) Illa n -Gom ez, M. J . a n a lyzer s, a n d ga seou s h ydr oca r bon s wer e m ea su r ed wit h a fla m e ion iza t ion a n a lyzer .

Sam ple s Ch aracte rization .

Eight samples were used in t h is wor k, six pet r oleu m cokes, a bit u m in ou s coa l a n d a n a ct iva t ed ca r bon . Th e pet cokes u sed in t h is st u dy wer e sa m pled a t differ en t in du st r ia l sit es; n o da t a con cer n in g t h eir or igin wa s a va ila ble. Ta ble 1 gives t h e r esu lt s of t h e pr oxim a t e a n d u lt im a t e a n a lyses. Th ey sh ow ver y sim ila r com posit ion s in t er m s of t h e per cen t a ges of C a n d H . As expect ed for h ea vy pet r oleu m fu els, t h e n a t u r a l su lfu r con t en t s a r e h igh , a n d range between 4% and 7.5%. The fuel nitrogen concentrations a r e sim ila r for a ll of t h e pet cokes. Th e pr oxim a t e a n a lysis in dica t es low vola t ile m a t t er a n d low a sh con t en t . Th e a im of t h is wor k wa s t o ext r a ct kin et ic da t a for a n u m ber of pet cokes a n d t o st u dy t h e in flu en ce of t h e n a t u r e of t h e sa m ple. Th en , all the chars were prepared at the same temperature and their reactivity was studied as a function of temperature. The chars wer e obt a in ed by pyr olyzin g t h e fu el sa m ples in t h e E F R a t 900 °C. Th e bit u m in ou s coa l (Iva n Coa l) wa s u sed a s a reference to a classical fuel, and the activated carbon (AC) was r et a in ed a s a h igh BE T a r ea ca r bon for con sider a t ion s of r ea ct ive su r fa ce, a s discu ssed fu r t h er . Th e a n a lyses of t h ese t wo fu els a r e a lso given in Ta ble 1.

P r ior t o a ll a n a lysis a n d exper im en t s, a ll t h e fu els wer e gr ou n d t o less t h a n 200 µm , dr ied, a n d t h e fr a ct ion 30-100 µm wa s select ed by pn eu m a t ic sievin g. P a r t icles of less t h a n 30 µm wer e r em oved t o en a ble r egu la r feedin g of t h e fu el du r in g E F R exper im en t s.

To obt a in a good ch a r a ct er iza t ion of t h e t ext u r e of t h e ch a r samples, the surface areas were measured using two methods:

• n it r ogen a dsor pt ion a t 77 K wit h t h e BE T equ a t ion ,

• m er cu r y por osim et r y t o qu a n t ify t h e su r fa ce a r ea of t h e m esopor es a n d m a cr opor es of t h e pa r t icles. Th is m et h od wa s u sed t o m ea su r e t h e su r fa ce a r ea of por es la r ger t h a n 20 n m (H g20).

Th e su r fa ce H g20 wa s h er e der ived fr om m er cu r y por osimet r y t est s, u sin g t h e st a n da r d por e m odel pr oposed by Sa t t erfield, 27 in wh ich t h e por e dia m et er is ca lcu la t ed fr om t h e por osit y ϵ, t h e pa r t icle den sit y Fp, a n d t h e por e su r fa ce a r ea S a s follows: 26 Th e a dsor pt ion of CO2 a t 293 K a lso a llows t h e det er m in at ion of a su r fa ce a r ea wit h t h e h elp of m odels su ch a s t h e Du bin in -P ola n yi equ a t ion . Th e pr oblem is t h a t in m a n y exper im en t s, t h e a ccess of t h e ga seou s r ea ct a n t t o m icr opor es is difficu lt . As a r esu lt , t h is m et h od wa s n ot u sed in t h is wor k. Th e por osit y wa s obt a in ed by h eliu m pycn om et r y. Ta ble 1 presents the results of this cha ra cteriza tion. The va lues of the su r fa ce a r ea s of t h e pet cokes a r e ver y low com pa r ed, for in st a n ce, t o t h a t of t h e coa l, wh ich t ypica lly exh ibit s BE T a r ea of sever a l t en s or h u n dr eds of m 2 g -1 . It sh ou ld be n ot ed t h a t t h e pet coke sa m ples exh ibit lower su r fa ce a r ea t h a n m a n y sa m ples u sed in pr eviou sly pu blish ed st u dies. 5,8,11 Th e con cen t r a t ion s of 7 elem en t s kn own for t h eir ca t a lyt ic activity in carbon reactions were analyzed: vanadium, copper, m olybden u m , ir on , pot a ssiu m , ca lciu m , a n d m a n ga n ese; t h e r esu lt s a r e r epor t ed in Ta ble 2. As in h ea vy fu el oils, la r ge a m ou n t s of va n a diu m a r e fou n d. Th is elem en t is pr esen t in quantities as large as 0.26% of the dry mass in Asland petcoke. Th e ot h er m et a ls a r e m u ch less con cen t r a t ed, a pa r t fr om ir on in St Con st a n t (0.17%) a n d in Lily (0.061%). a Th e BE T, t h e H g20 su r fa ce a r ea , a n d t h e por osit y ϵ a r e t h ose of t h e ch a r s.

d p ) 4ϵ F p S
exper im en t s. Th e ch a r s wer e pr eviou sly pr epa r ed by pa ssin g t h e fu els in 4.16% O2 in n it r ogen t h r ou gh t h e E F R a t 900 °C. Th e exper im en t s t o ch a r a ct er ize t h e ch a r s r ea ct ivit ies t owa r d NO wer e ca r r ied ou t a t t h r ee t em per a t u r ess800, 900, a n d 1000 °C, so t h a t a n a ct iva t ion en er gy ca n be det er m in ed. Th e pa r t icles feed flowr a t e wa s 1.5 g/m in . Th e ga s feed flow wa s a m ixt u r e of 955 ppm of NO in n it r ogen , a n d t h e flow r a t e wa s 10 L/m in (NTP ). Un der t h ese exper im en t a l con dit ion s, t h e r esiden ce t im e r a n ged bet ween 3.25 a n d 3.6 s, depen din g on t em per a t u r e. If we a ssu m e a h om ogen eou s disper sion of t h e pa r t icles in t h e volu m e of t h e r ea ct or , t h e solid con cen t r a t ion s in t h e t u bu la r r ea ct or a r e equ a l t o 38, 35, a n d 32 g/m 3 for exper im en t s a t 800, 900, a n d 1000 °C, r espect ively.

We report in Table 3 the NO reduction yield for the different sa m ples a s a fu n ct ion of t em per a t u r e. Th e decr ea se of NO con cen t r a t ion bet ween in ject ion a n d t h e bot t om of t h e r ea ct or va r y bet ween 11 ppm v for Lily pet coke ch a r a t 800 °C a n d 920 ppm v for t h e coa l ch a r a t 1000 °C. It is obviou s fr om t h ese r esu lt s t h a t t h e qu a n t it y of NO t h a t ca n be r edu ced on petroleum coke, in the presence of an NO concentration similar t o t h a t in a n in du st r ia l fu r n a ce, is ver y la r ge. In a gr eem en t wit h pr eviou s st u dies in t h is t em per a t u r e r a n ge, t h e in flu en ce of t em per a t u r e is im por t a n t . F or exa m ple, Ch a n et a l. 5 reported a ctiva tion energies la rger tha n 200 kJ /mol for gr a ph it e a n d ch a r sa m ples.

F or a ll exper im en t s, t h e con cen t r a t ion of h ydr oca r bon s a t the exhaust of the reactor was below 20 ppmv. Small amounts of CO wer e det ect ed wh ich cou ld be r ela t ed m a in ly t o t h e r ea ct ion 2NO + 2C f 2CO + N2. In m ost ca ses, t h e CO m ea su r ed wa s h igh er t h a n t h e NO con su m ed; t h e excess of CO r a n ged bet ween 0 a n d 500 ppm v for a ll exper im en t s. Th is excess ca n be a t t r ibu t ed t o t h e desor pt ion of CO fr om t h e oxygen a dsor bed on t h e ch a r .

Th er e h a ve been n u m er ou s st u dies in wh ich t h e en h a n cem en t of t h e NO r edu ct ion r a t e by CO wa s r epor t ed. 5,18 Th is lea ds t o difficu lt ies in t h e kin et ic in t er pr et a t ion of t h e exper im en t a l r esu lt s sin ce CO is a pr odu ct a n d a r ea ct a n t . 7 Two expla n a t ion s h a ve been pr esen t ed t o expla in t h is beh a vior :

• the stripping of surface oxides by CO through the reaction CO + C(O) f CO2 + C*. Th is pr ocess ca n be sign ifica n t a t low t em per a t u r e wh en t h e desor pt ion of su r fa ce oxides is slow,

• t h e ca t a lysis of t h e r ea ct ion NO + CO f 0.5N2 + CO2 by va r iou s su bst a n ces su ch a s t h e ch a r it self.

It m u st be n ot iced t h a t , in ot h er st u dies, it h a s been obser ved t h a t t h e a ddit ion of low con cen t r a t ion s of CO (i.e., below 700 ppm v) does n ot lea d t o a n in cr ea se of t h e ch a r -NO r ea ct ion r a t e. 4 Th e or der of t h e r ea ct ion wit h r espect t o CO is a lso n ot kn own . Th e st u dy of t h is com plex ph en om en on wa s ou t side t h e scope of t h is wor k, a n d it wa s decided t o a n a lyze t h e exper im en t a l r esu lt s in a cla ssica l wa y wit h a sin gle fir stor der kin et ic la w.

Th e con cen t r a t ion of CO2 wa s n ot sign ifica n t . Th is is con sist en t wit h pr eviou sly pu blish ed st u dies in wh ich it h a s been sh own t h a t t h e CO/CO2 ratio is very high in this temperature range. 7 Th e low a m ou n t of ca r bon oxides a n d h ydr oca r bon s sh ows a lso t h a t t h er e is n o sign ifica n t fu r t h er devola t iliza t ion of t h e ch a r s du r in g t h e exper im en t s in t h e EFR.

As expect ed, t h e coa l ch a r a n d t h e a ct iva t ed ca r bon lea d t o t h e h igh est r ea ct ion yield; t h is ca n be r ela t ed t o t h e h igh su r fa ce a r ea s of t h ese t wo sa m ples. Th e elem en t a l a n d pr oxim a t e a n a lyses of t h e six pet cokes a r e sim ila r , so t h a t on e cou ld expect t o obser ve t h e in flu en ce of t h e t ext u r e of t h e sa m ples. Am on g t h e pet r oleu m coke sa m ples, H a vr e CDP , wh ich exh ibit s t h e h igh est su r fa ce a r ea , does n ot lea d t o t h e h igh est r edu ct ion yield. Th er efor e, ot h er pa r a m et er s su ch a s ca t a lyt ic elem en t s m u st in flu en ce t h e r ea ct ivit y.

D is c u s s io n

Th e r a t e con st a n t s wer e det er m in ed by fit t in g t h e exper im en t a l r esu lt s wit h t h ose fr om a m odel. As poin t ed ou t by Aa r n a a n d Su u ber g in t h eir r eview, 7 many published results use the mass of NO divided per m a ss of ch a r per u n it t im e per u n it pr essu r e of NO t o define their ra te consta nt. In this work we use a simila r defin it ion for t h e ch em ica l r ea ct ion r a t e con st a n t s, bu t we t a ke in t o a ccou n t t h e su r fa ce a r ea of t h e sa m ples. Th er efor e, t h e r ea ct ion r a t e wa s expr essed a s follows: wit h wh er e [NO] is t h e m ola r con cen t r a t ion of NO in t h e gas and C c is the concentration of carbonaceous material in t h e ga s. η NO is t h e effect iven ess fa ct or , a s ca lcu la t ed fr om t h e Th iele m odu lu s for a r ea ct ion or der equ a l t o 1. Som e a u t h or s h a ve obser ved r ea ct ion or der s t owa r d NO wh ich r a n ge bet ween 0.2 a n d 0.8 for t h is r edu ct ion r ea ct ion . 8,28 Never t h eless, m ost st u dies u sed a r ea ct ion or der of 1, wh a t ever t h e exper im en t a l set u p u sed t o obt a in t h e r esu lt s. 20,22 To fit exper im en t a l da t a , a m a ss ba la n ce wa s don e between the entry and the exhaust of the entrained flow r ea ct or . Th e m a in a ssu m pt ion s of t h is m odel a r e t h e followin g:

• t h e zon e close t o t h e a xis is a plu g flow r ea ct or ,

• t h e disper sion of t h e pa r t icles in t h e ga s is h om ogen eou s,

• t h e pa r t icles velocit y is equ a l t o t h e ga s velocit y. Th e det a ils of t h e ca lcu la t ion s a r e given elsewh er e. 26 An it er a t ive com pu t a t ion wa s u sed t o obt a in t h e effect iven ess fa ct or η NO and the Arrhenius parameters for ea ch sa m ple. Th e plot s in F igu r e 2 a n d F igu r e 3 a llow t h e comparison between the reactivities of the different petcoke ch a r s a n d t h a t of t h e coa l ch a r . Sh im izu et a l. wer e a ble to find a correlation between CO 2 surface area and NO-(28) Sch u ler , J .; Ba u m a n n H .; Klein , J . Kin et ics of NO r edu ct ion by coa l ch a r . Con f. (In t.) Coal S ci. 1987, 857-860. a Th e in it ia l NO con cen t r a t ion wa s 955 ppm v.

Ta ble 2. An a ly s is o f F iv e

dm NO dt ) -k m c S sp η NO P NO k ) A e (-E /R T )
ch a r r ea ct ivit y, 11 bu t t h is wa s n ot possible in som e ot h er s st u dies. 8 Aa r n a a n d Su u ber g 7 h a ve pr oposed a syn t h esis of t h e r ea ct ion r a t es, specified on BE T a r ea s, der ived by a n u m ber of a u t h or s. Th eir cor r ela t ion wa s con ver t ed t o t h e sa m e u n it s a s t h e on es u sed in t h is wor k, a n d plot t ed in F igu r es 2 a n d 3. Th e spr ea d in t h e r ea ct ion r a t es is m u ch m or e im por t a n t in F igu r e 2, on BE T a r ea ba se, t h a n in F igu r e 3, wh ich wa s plot t ed on Hg 20 ba se. Ra dovic et a l. 24 expr essed a s ea r ly a s 1983 t h a t BE T a r ea is a n in a dequ a t e ba sis for t h e expr ession of reactivity. Aarna and Suuberg 25 showed that micropores h a r dly pa r t icipa t e in t h e r ea ct ion bet ween oxygen a n d ca r bon . In a r ecen t wor k by Sa lva dor et a l., 3 it wa s sh own t h a t t h e sca t t er in t h e r ea ct ivit ies of ca r bon s of va r iou s for m s wa s m u ch sm a ller wh en t h e su r fa ce of t h e por es la r ger t h a n 20 n m (H g 20 ) was used to express t h e r ea ct ive su r fa ce, in st ea d of t h e t ot a l por e su r fa ce. Th is t en ds t o con fir m t h a t m icr opor es sm a ller t h a n 20 n m do n ot pa r t icipa t e in t h e r ea ct ion . A st u dy on t h e reactivity of carbons issuing from different origins with NO lea d Com m a n dr e et a l. 26 t o t h e sa m e con clu sion .

It a ppea r s t o be t h e ca se t h a t exclu din g t h e su r fa ce a r ea of t h e sm a ll por es (i.e., wit h dia m et er s lower t h a n 20 n m ) fr om t h e r ea ct ive su r fa ce r econ ciles t h e da t a from various forms of carbon; and thus Hg 20 surface area is likely t o be a bet t er in dica t or of t h e r ea l r ea ct ive surfa ce tha n BET. We ha ve a lrea dy reported this result for ca r bon oxida t ion in a pr eviou s wor k. 3 Petcokes again exh ibit ver y h igh r ea ct ivit ies a s com pa r ed wit h r esu lt s fr om t h e lit er a t u r e. 7 Most a u t h or s h a ve obser ved t h a t t h e a ct iva t ion en er gy in cr ea ses in va lu e a t t em per a t u r es bet ween 800 a n d 1000 K. Alt h ou gh t h er e a r e la r ge va r ia t ion s in t h e m ea su r ed a ct iva t ion en er gies, t h e va lu es gen er a lly m ea su r ed ch a n ge fr om 75 kJ /m ol below 950 K t o 180 kJ /mol a bove this tempera ture. 5,20,29 Th e low a ct iva t ion en er gy obser ved a t low t em per a t u r e h a s been r ela t ed to the limitation by the desorption of reaction products; a limitation which disappears as the temperature in cr ea ses. Th e r a t e of t h e h igh t em per a t u r e r ea ct ion is therefore controlled by the dissociative adsorption of NO on t h e ca r bon su r fa ce. As sh own in Ta ble 4, we obser ved la r ge va r ia t ion s of t h e a ct iva t ion en er gies of t h e differen t sa m ples t est ed in t h is st u dy, a s t h e va lu es obt a in ed wit h t h e six pet cokes r a n ge fr om 115 t o 240 kJ /m ol. Th ese va lu es a r e in good a gr eem en t wit h pr eviou sly pu blish ed da t a . It m u st be n ot iced t h a t t h e t h r ee sa m ples wh ich lea d t o a iva t ion s en er gies close t o 235 kJ /mol are the ones which have a low vanadium content. Th is va lu e is in t h e h igh est r a n ge of a ct iva t ion en er gies fou n d in t h e lit er a t u r e, 5,15,29 and may be related to the n on -ca t a lyzed r ea ct ion of NO wit h ca r bon .

The catalytic effect of metals on the reaction of C with NO ca n be est im a t ed fr om F igu r es 4 a n d 5 wh er e t h e metal concentration was plotted versus the reaction rate at 1200 K, using Hg 20 a s t h e a r ea ba se. F igu r e 5 sh ows t h a t a clea r cor r ela t ion a ppea r s bet ween t h e va n a diu m con t en t of t h e pet cokes a n d r ea ct ivit y t owa r d NO. Th e cor r ela t ion fa ct or for t h e lin ea r lea st -squ a r es est im a t ion is r 2 ) 0.72. Th is sh ows t h a t va n a diu m ca n h a ve a ca t a lyt ic effect even in t h e ca se of a r a pid r ea ct ion . Th e r a t io bet ween t h e r ea ct ion r a t e of t h e m or e r ea ct ive pet coke a n d t h a t of t h e less r ea ct ive on e is 3.5. Th e t wo ot h er elem en t s wh ich a r e pr esen t ed in t h is figu r e a r e ir on a n d ca lciu m ; it ca n be obser ved t h a t t h er e is n o clea r cor r ela t ion bet ween r ea ct ion r a t e a n d t h e con cent r a t ion s of t h ese t wo elem en t s. As poin t ed ou t in t h e In t r odu ct ion , Ca h a s oft en been pr esen t ed a s a n a ct ive ca t a lyst for r edu ct ion r ea ct ion s. 9,17,19 It m u st be n ot iced t h a t t h e con cen t r a t ion of Ca in pet coke is lower t h a n t h a t of t h e ch a r sa m ples u sed in m ost st u dies. Th e con cen t r a t ion of ir on in pet coke is h igh a n d a n in flu en ce of t h is m et a l on t h e NO-h a r r ea ct ion r a t e cou ld be expect ed. 30 Never t h eless, t h is ca t a lyt ic a ct ivit y of ir on Figu re 2. Ar r h en iu s r epr esen t a t ion for t h e r ea ct ion r a t e of ca r bon t owa r d NO, specified on BE T a r ea .

Figu re 3. Ar r h en iu s r epr esen t a t ion for t h e r ea ct ion r a t e of ca r bon t owa r d NO, specified on H g20 area. wa s n ot obser ved in som e ot h er st u dies, 9 and Illian-Gom ez et a l. 13 obser ved a h igh a ct ivit y of F e on ly a t low t em per a t u r e (i.e., below 700 K). F igu r e 5 pr esen t s t h e fou r m et a ls wh ich a r e pr esen t in low con cen t r a t ion in pet r oleu m cokes. Alt h ou gh pot a ssiu m h a s oft en been fou n d a s a n a ct ive elem en t wh a t ever t h e t em per a t u r e, 15 it is obviou s t h a t it s low con cen t r a t ion lea ds t o a n egligible in flu en ce on ch a r r ea ct ivit y com pa r ed t o t h a t of va n a diu m . Th e sa m e explanation can be applied for copper, molybdenum, and manganese.

Th e ca lcu la t ion of t h e pa r t icle-r ela t ed effect iven ess fa ct or (followin g t h e m odel of Com m a n dr e et a l. 26 ) leads to values between 0.7 and 1 if BET surface area is used. As it is pr esen t ed in Ta ble 4, if H g 20 surface area is used, η NO is a bove 0.88 for a ll t h e sa m ples wh a t ever t h e exper im en t a l t em per a t u r e. Th is sh ows t h a t NO m a ss t r a n sfer in t h e por ou s st r u ct u r e of t h e ch a r s does n ot con t r ol t h e r ea ct ion yield. Th is expla in s wh y t h e ca t alytic effect of vanadium on the reaction is clearly evident h er e. Th is wa s n ot t h e ca se for t h e r ea ct ion of C wit h O 2 fr om a ir du r in g ou r TG exper im en t s, wh er e oxygen transfer was apparently limiting the combustion velocit y. 3

Co n c lu s io n

A st u dy of t h e r ea ct ion of NO on pet r oleu m cokes h a s been ca r r ied ou t in a n en t r a in ed flow r ea ct or . It wa s obser ved t h a t pet cokes a r e ver y r ea ct ive for m s of ca r bon in t h e h et er ogen eou s r ea ct ion of ca r bon wit h NO, r esu lt in g in NO con cen t r a t ion deplet ion s of h u n dr eds of ppm v a ft er a r esiden ce t im e of less t h a n 1.5 s a t temperatures in the range 800-1000 °C.

F or t h e h et er ogen eou s r ea ct ion con sider ed, we h a ve sh own t h a t exclu din g t h e su r fa ce of t h e por es sm a ller t h a n sever a l t en s of n a n om et er s fr om t h e t ot a l por e su r fa ce lea d t o m or e u n ifor m va lu es for t h e r ea ct ion r a t es of ch a r s fr om differ en t or igin s. Th is t en ds t o con fir m t h e a ssu m pt ion t h a t sm a ll por es do n ot pa r t icipa t e in t h ese h et er ogen eou s r ea ct ion s. We t h in k t h a t t h e su r fa ce a r ea of t h e por es la r ger t h a n 20 n m wh ich is ca lcu la t ed fr om m er cu r y por osim et r y m ea su r em en t s is a better estimate of the reactive surface than the total por e a r ea det er m in ed by cla ssica l BE T t est s or by CO 2 a dsor pt ion exper im en t s.

A r a t io of 3.5 is in dica t ed bet ween t h e r ea ct ion r a t es a t 1200 K of t h e m or e r ea ct ive a n d t h e less r ea ct ive pet coke ch a r s. Seven elem en t s (va n a diu m , copper , molybdenum, iron, potassium, calcium, and manganese) wh ich a r e kn own a s ca t a lyst s for t h is sor t of r ea ct ion wer e a n a lyzed t o expla in t h e differ en t r ea ct ivit ies of petcokes. Although petroleum cokes are low-ash-content fu els, t h e a n a lysis of t h ese seven elem en t s in dica t ed va n a diu m con t en t s a s h igh a s 0.26%. Am on g t h ese elem en t s, va n a diu m a ppea r s t o be r espon sible for t h e high variation of reactivities which are observed for the pet coke ch a r s, a n d a clea r cor r ela t ion cou ld be fou n d bet ween t h e va n a diu m con t en t of t h e sa m ple a n d it s r ea ct ivit y t owa r d NO.

Figu re 4. V, F e, a n d Ca con t en t of t h e pet cokes ver su s t h eir r ea ct ion r a t e t owa r d NO a t 1200 K. Th e lin e is a lin ea r cor r ela t ion for va n a diu m . Figu re 5. Tr a ce m et a ls con t en t of t h e pet cokes ver su s t h eir r ea ct ion r a t e t owa r d NO a t 1200 K.

The

  Ca ta ly tic Effe c t o f Va n a d iu m o n th e Re a c tiv ity o f P e tro le u m Co k e s w ith N O S. Sa lva dor , †, § J .-M. Com m a n dr e ´, † B. R. St a n m or e, † a n d R. Ga diou * , ‡ E cole d es M in es d 'Albi-Carm au x, L aboratoire d e Ge ´n ie d e Proce ´de ´s d es S olid es Divise ´s, UM R CN R S 2392, Cam pu s J arlard , rou te d e T eillet, 81013 Albi CT ced ex 09, Fran ce, an d L aboratoire d e Gestion d es R isqu es et E n viron n em en t, 25, ru e d e Ch em n itz, 68200 M u lh ou se, Fran ce

2. 3 .

 3 Re a c tio n o f Ch a rs w ith N O in th e EF R. Th e r ea ct ivit y t owa r d NO of ch a r s h a s been det er m in ed fr om E F R (24) Ra dovic, L. R.; Wa lker , P . L.; J en kin s, R. G.

Figu re 1 .

 1 Figu re 1. Entrained flow reactor: (1) Conveyor belt/vibrating feeder /eject or ; (2) pn eu m a t ic t r a n spor t ; (3) elect r ica l pr eh ea t er ; (4) flow m et er s; (5) wa t er -cooled feedin g pr obe; (6) disper sion dom e; (7) t h r ee-zon e elect r ica l fu r n a ce; (8) 80 m m i.d. qua rtz tube rea ctor; (9) cyclone collector; (10) exha ust fa n; (11) wa t er -cooled n it r ogen -qu en ch ed sa m plin g pr obe; (12) pa r t icle collect or (filt er ); (13) sa m plin g pu m p; (14) ga s con dit ion er ; (15) ga s a n a lyzer s (in fr a r ed/pa r a m a gn et ic); M, m a ss flow m et er s a n d con t r oller s; N2, n it r ogen ; C.A., com pr essed a ir ; W, wa t er (pr obes coolin g).

(

  29) F u r u sa wa , T.; Tsu n oda , M.; Tsu jim u r a , M.; Adsch ir i, T. Nit r ic oxide r edu ct ion by ch a r a n d ca r bon m on oxide. Fu el 1985, 64 (9), 1306-1309. (30) Allen , D.; H a yh u r st , A. N. FB C T ech n ology an d th e E n vironm en tal Ch allen ge; Ada m H ilger : Br ist ol, U.K., 1991.

Ta ble 4 .

 4 Ac tiv a tio n En e rg ie s a n d Effe c tiv e n e s s F a c to r ηNO fo r N O Re d u c tio n (c o m p u te d w ith Hg 20 su rface are a)

  Gom ez, M. J .; Lin a r es-Sola n o, A.; Sa lin a s-Ma r t in ez de Lecea , C. NO Redu ct ion by Act iva t ed Ca r bon s. 6. Ca t a lysis by t r a n sit ion m et a ls. E n ergy Fu els 1995, 9, 976-983. Martinez de Lecea, C. NO Reduction by Activated Carbons. 2. Catalytic E ffect of P ot a ssiu m . E n ergy Fu els 1995, 9, 97-103. Martinez de Lecea, C. NO Reduction by Activated Carbons. 3. Catalysis by Ca lciu m . E n ergy Fu els 1995, 9, 112-118. (17) Gu o, F .; H ecker , W. C. E ffect s of Ca O a n d bu r n ou t on t h e kin et ics of r edu ct ion by Beu la h za p ch a r . 26th S ym posiu m (In tern ation al) on Com bu stion ; Th e Com bu st ion In st it u t e: P it t sbu r gh , 1996;

	pp 2251-2257.
	(18) Aa r n a , I.; Su u ber g, E . M. Th e r ole of ca r bon m on oxide in t h e
	NO-ca r bon r ea ct ion . E n ergy Fu els 1999, 13, 1145-1153.
	(19) P et er , M. R eactivite d es oxyd es d 'azote form es lors d e la
	com bu stion d e ch arbon en lit flu id ise circu lan t. Ph.D. Thesis, Univer-
	sit y of Mu lh ou se, 2000.
	(20) Su u ber g, E . M.; Ten g, H .; Ca lo, J . M. St u dies on t h e kin et ics
	a n d m ech a n ism of t h e r ea ct ion of NO wit h ca r bon . 23rd S ym posiu m
	(In tern ation al) on Com bu stion ; The Combustion Institute: Pittsburgh,
	1990; pp 1199-1205.
	(21) Levy, J . M.; Ch a n , L. K.; Sa r ofim , A. F .; Beer , J . M. NO-ch a r

; Lin a r es-Sola n o, A.; Ra dovic, L. R.; Sa lin a s-Martinez de Lecea, C. NO Reduction by Activated Carbons. 5. Catalytic E ffect of Ir on . E n ergy Fu els 1995, 9, 540-548. (14) Illa n -(15) Illa n -Gom ez, M. J .; Lin a r es-Sola n o, A.; Ra dovic, L. R.; Sa lin a s-(16) Illa n -Gom ez, M. J .; Lin a r es-Sola n o, A.; Ra dovic, L. R.; Sa lin a sr ea ct ion s a t pu lver ized coa l fla m e t em per a t u r es. 18th S ym posiu m (In tern ation al) on Com bu stion ; The Combustion Institute: Pittsburgh, 1980; pp 111-120. (22) Son g, Y. H .; Beer , J . M.; Sa r ofim , A. F . Redu ct ion of Nit r ic Oxide by Coal Char at Temperatures of 1250-1750 K. Com bust. S ci. T echnol. 1981, 25, 237-240. (23) Va n de St een e, L.; Sa lva dor , S.; Ch a r n a y, G. Con t r ollin g powdered fuel combustion a t low tempera ture in a new entra ined flow r ea ct or . Com bu st. S ci. T ech n ol. 2000, 159, 255-279.

Ta ble 1. U ltim a te a n d P ro x im a te An a ly s is a n d Lo w e r Ca lo rific Va lu e o f th e S a m p le s a

			u lt im a t e a n a lysis (% r a w)		pr oxim a t e a n a lysis (% r a w)		su r fa ce a n a lysis
	sa m ple	C	H	O	S	N	VM	a sh	LCV (kJ /g) BE T (m 2 /g) H g20 (m 2 /g)	ϵ
	pet coke Lily	87.8 3.51	1.1 7.47 1.61	10.5	0.33	34.45	4.5	1.77	0.57
	pet coke H a vr e CDP	87.9 3.51	1.1 7.08 1.51	10.1	0.38	34.35	10.0	2.20	0.57
	pet coke Asla n d	89.4 3.60		4.02 1.88	10.1	0.47	34.50	5.3	1.82	0.59
	pet coke Villa u en ga s 88.4 3.70		6.16 1.56	11.0	0.12	34.35	6.2	2.00	0.65
	pet coke St Con st a n t 88.5 1.94		6.00 1.63	5.2	0.50	32.50	6.3	1.33	0.54
	pet coke Niebla	85.8 3.50		5.85 1.70	11.1	3.35	33.40	4.8	1.80	0.66
	coa l Iva n	67.0 4.54 11.3 0.24 1.85	30.7	13.23	25.95	64.0	8.30	0.73
	a ct iva t ed ca r bon	85.9 0.60		0.50 0.11		2.70		890.0	12.89	0.78
											Fuel 1983, 62, 849-
							857.

(25) Aa r n a , I.; Su u ber g, E . M. 27th S ym posiu m (In tern ation al) on Com bu stion ; Th e Com bu st ion In st it u t e: P it t sbu r gh , 1998; pp 2933-2939. (26) Com m a n dr e ´, J . M.; St a n m or e, B.; Sa lva dor , S. Th e H igh Tem per a t u r e Rea ct ion of Ca r bon wit h Nit r ic Oxide. Com bu st. Flam e 2002, 128, 211-216. (27) Sa t t er field, C. N. M ass T ran sfer in H eterogen eou s Catalysis; Kr ieger P u blish in g Com pa n y: 1981.

Me ta ls , K, a n d Ca in th e P e tcoke s (ppm raw )

	sa m ple	V	Cu	Mo	F e	K	Mn		Ca
	Lily	507	9.1	7.0	615	45	5.1		220
	Havre CDP	819			6				180
	Asla n d	2570	1.9	1.4	93	10	0.5		20
	Villa u en ga s	1390	3.8	17.3	20	17	2.6		100
	St Con st a n t	899	6.1	26.3	1740	15	17.1		150
	Niebla	1830	3.1	16.6	381	57	6.1	2190
	Ta ble 3. Re d u c tio n Yie ld s o f N O by th e D iffe re n t Ch a rs
	du rin g EFR Expe rim e n ts (in m ol %) a
	sa m ple			800 °C	900 °C	1000	°C
	pet coke Lily			1.2		15.7		59.9
	pet coke H a vr e CDP		6.3		15.2		58.8
	pet coke Asla n d		11.1		35.0		74.7
	pet coke Villa u en ga s		23.0		42.4		77.4
	pet coke St Con st a n t		3.8		18.4		63.4
	pet coke Niebla		9.4		42.4		74.6
	coa l Iva n			57.6		84.7		96.3
	a ct iva t ed ca r bon		27.2		44.8		69.1


F Mermoud
email: mermoud@cirad.fr

S Salvador

L Van De Steene

F Golfier

Influence of the pyrolysis heating rate on the steam gasification rate of large wood char particles

Keywords: Biomass, Gasification reactivity, Pyrolysis conditions

This work was carried out in order to quantify the impact of the pyrolysis heating rate both on the properties of the residual charcoal and on the behaviour during gasification by H 2 O of the charcoal. The experiments were conducted on 10 mm diameter beech wood spheres, pyrolysed at atmospheric pressure under heating rates covering the range from very slow, 2.6 K min K1 , to very rapid, over 900 K min K1 , i.e. the highest value that can be reached. When charcoal is submitted to gasification at 20% H 2 O i nN 2 at 1200 K, the ratio of the times for complete conversion reach 2.6. Such a difference is considerable as far as an industrial application is concerned. The initial properties of the charcoal such as apparent density, porosity, and pore surface area obtained by N 2 or Ar adsorption were measured in order to explain the differences in gasification kinetics within the charcoal. The charcoal particles exhibit densities as different as 219-511 kg m K3 and porosities between 87 and 70% for charcoal prepared at 900 and 2.6 K min K1 respectively. The specific surface area is higher than 600 m 2 g K1 for three charcoals. Influence of ash content of the initial charcoals, at 1.6-2.7%, is also regarded with particular attention to explain the observed differences in gasification kinetics.

Introduction

The conversion of solid biomass products into high calorific value gases is one of the most interesting ways to convert solar energy into mechanical and electrical energy. A variety of processes have been developed over the past decades to achieve this conversion, termed gasification. Fixed bed reactors, fluidised bed reactors or flash processes are the most common techniques.

Whatever be the process used, it is admitted that the biomass will go through at least the following transformations: at temperatures between 300 and 500 K, the biomass is dried; when the temperature reaches 600 K approximately, thermal cracking occurs, during which volatile matters (VM) are released and a solid carbonaceous residue, called charcoal, is formed, this transformation is termed pyrolysis or devolatilisation; finally, gasification of the solid charcoal residue, involving heterogeneous reactions, occurs in an oxidising atmosphere such as O 2 , H 2 O or CO 2 , at higher temperatures, typically 1000-1600 K.

Gasification by H 2 O is regarded with particular interest for two reasons. Firstly, the gases formed, H 2 and CO, present a high calorific value compared to CO 2 and CO formed during combustion with O 2 , or CO formed during CO 2 gasification. Secondly, the reaction is much more rapid using H 2 O rather than CO 2 as an oxidant. For instance, in the case of wood char, reaction is 2-5 times more rapid according respectively to [START_REF] Van Den Aarsen | Wood pyrolysis and carbon dioxide char gasification kinetics in a fluidized bed[END_REF][START_REF] Rensfelt | Basic gasification studies for development of biomass medium-btu gasification processes[END_REF]; for coal, it is 2-3 times more rapid [START_REF] Harris | [END_REF][START_REF] Walker | Advances in catalysis[END_REF].

Operating conditions during pyrolysis-such as heating rate (HR, in K min K1 ), final temperature, residence time, pressure or size of the particles-are known to have an influence on the morphology of the charcoal obtained, as well as on its reactivity during gasification.

Kumar and Gupta [START_REF] Kumar | [END_REF] observed that an increase of the final pyrolysis temperature-from 1073 to 1473 K-decreases charcoal reactivity to CO 2 gasification. This suggests that a higher pyrolysis temperature enables a better structural ordering of the carbon matrix, with a lower concentration of active sites available for reaction.

The soaking time at final pyrolysis temperature is known to have a negative influence on the reactivity of the charcoal. This phenomenon is called thermal annealing or thermal deactivation in the sequel of the paper. Walker [START_REF] Walker | Char properties and gasification[END_REF], considering coal gasification in air, explained thermal annealing by a phenomenon of sintering of the minerals present in the charcoal. Dispersion of the minerals in the charcoal, which act as a catalyst for the reaction, is then decreased and the catalyst activity is lowered.

Roberts et al. [START_REF] Roberts | [END_REF] studied the effect of pressure-from 1 to 15 atm-during pyrolysis of several coals on the reactivity of charcoal with CO 2 and H 2 O, and observed that a higher pyrolysis pressure yields a more reactive charcoal, with a larger surface area. They suggested that a high pressure could produce large void areas in the charcoal, increasing the accessible porosity for the reaction and then the charcoal reactivity.

The size of the wood particles is a determining parameter throughout the pyrolysis process. At first, due to heat transfer limitations, the heating rate at various positions along a radius inside a particle is different. Secondly, the residence time of evolved volatile matters and tars depends on the radius of the particle; the deposition of pyrolytic carbon is affected by this parameter.

It is clearly established today that pyrolysis HR has an impact on the total volatile matter yield, its chemical composition [START_REF] Van De Steene | Rice husk, straw and bark behaviour during combustion, pyrolysis and gasification: fundamental study[END_REF][START_REF] Zanzi | [END_REF][10][11][12] and on the structural properties of the charcoal produced. Numerous authors [START_REF] Kumar | [END_REF][START_REF] Walker | Char properties and gasification[END_REF][START_REF] Roberts | [END_REF]11,13,[START_REF] Aarna | Changes in reactive surface area and porosity during char oxidation[END_REF] agree that the pyrolysis HR of the initial biomass significantly influences the reactivity of the charcoal during its gasification. Higher HR were found to provide more reactive charcoals during gasification by CO 2 [START_REF] Kumar | [END_REF][START_REF] Roberts | [END_REF]13] or H 2 O [START_REF] Roberts | [END_REF]11]. Even if most of the authors attribute this behaviour to structural properties of charcoal, explanation remains subject to discussion. Kumar and Gupta [START_REF] Kumar | [END_REF] explain that at high HR conditions, both a lower amount of deposit of pyrolytic carbon-which retards reactivity-and a carbon matrix with defective microcrystallites-which provide a higher concentration of active site-are responsible for the higher reactivity of charcoal during gasification.

It is today acknowledged that a high pyrolysis HR yields more VM than a low pyrolysis HR, and that these VM mainly consist of heavy compounds, with a high C/H ratio. Indeed, a high HR leads to a rapid formation of VM causing a rapid increase of pressure in the wood particle and an explosive release of gases. On the contrary, a low HR enables an extended residence time of the volatiles in the particle, increasing the occurrence of repolymerisation of the volatiles inside the charcoal. These secondary reactions take place upon contact with the charcoal and generate lighter compoundswith a lower C/H ratio, re-forming charcoal at the surface of the solid. The char yield is then increased compared to a rapid pyrolysis.

Despite the fact that charcoals are all essentially made of carbon, the reactivity during gasification of charcoals from different origins can vary in a range of several orders of magnitude. Numerous authors are looking for an explanation in terms of a reactive surface. For Roberts et al. [START_REF] Roberts | [END_REF], reaction of charcoals with CO 2 /H 2 O uses and develops the micropore surface area, unlike char-O 2 reaction. On the contrary, Fushimi et al. [11] explained that macropores, because of their accessibility to the reactants, are supposed to provide more active sites than micropores, which seem to hardly participate in the reaction; the difference in reactivities observed is explained by the difference in macropore surface area. This conclusion is in accordance with work by Cetin et al. [13], who showed that high HR charcoals mainly consist of macropores. However, the determination of a reactive surface remains a difficult task. The values obtained for the total pore surface area sometimes depends on the technique used, such as N 2 , Ar or CO 2 adsorption or Hg intrusion, which raises here numerous questions. An attempt is made later in this work to identify a reactive surface area that can be correlated with the reactivity of the charcoals. The gasification process is known to be very complex since lots of phenomena take place together and also because fracturation can occur, possibly increasing the reactivity by facilitating the diffusion of the reactants inside the particle.

In many processes, the transformations of pyrolysis and gasification take place inside the same reactor. In other processes, called staged gasifiers, pyrolysis is realised in a separate reactor. This enables better control of the process, giving the option of acting separately on the two transformations. The impact of the pyrolysis HR on the gasification kinetics is especially important for the design of staged gasifiers. Gasification of charcoals by CO 2 was widely studied in previous works [START_REF] Van Den Aarsen | Wood pyrolysis and carbon dioxide char gasification kinetics in a fluidized bed[END_REF][START_REF] Kumar | [END_REF][START_REF] Roberts | [END_REF]13,[START_REF] Dasappa | [END_REF][16][17][18][19], but only a few authors worked on steam gasification [11,12,17,20] of process size particles pyrolysis, and the impact of this treatment on the following gasification by H 2 O. In other words, an attempt is made to answer to the following questions:

(i) what are the structural properties of the charcoals obtained after pyrolysis in different conditions of HR? (ii) what is the impact of the pyrolysis HR on the behaviour of the charcoal when submitted to gasification by H 2 O? (iii) is it possible to link measured properties of a charcoal to its behaviour during gasification?

2. Pyrolysis at different heating rates and properties of the charcoals

Pyrolysis experiments

Wood is a good material to represent biomass products. In an industrial fixed bed, the size of the wood chips is typically up to several centimetres. The smaller dimension is typically around 1 cm; this characteristic size was retained for the present work. Beech wood spheres calibrated at a 10.2G 0.1 mm diameter were used in order to eliminate the potential effect on the results of changes in the geometry of the particles from one experiment to another. The proximate and ultimate analysis on dry basis of the initial wood, reported in Table 1, were measured in compliance with standards NF-M03-003 and NF-B55-101. The volatile matter percentage is as high as 82%, and the ash content of the wood is low: 0.38%.

In this work, the choice for the range of studied HR was realised as follows. The lower value for the HR-2.6 K min K1 -is very low compared to an industrial process, corresponding to heating to 773 K in more than 3 h. The value of 900 K min K1 is the most rapid heating that can be operated on such particles, as explained below. Such high values can be encountered in an industrial process in the case where a particle enters into contact with a heated wall or is in a fluidised bed. The intermediate value of 12 K min K1 corresponds to heating to 773 K within approximately 40 min; it is close to typical values in an industrial process. The extremely high HR such as 10 5 K s K1 , commonly studied, were not taken into account in this study, because not reachable with particles greater than about 0.1 mm diameter [21].

Pyrolysis at 2.6 and 12 K min K1 was carried out in a muffle furnace; the wood particles were placed in a refractory steel box swept with nitrogen. The temperature was increased at the controlled HR from room temperature to 1073 K. The furnace was kept 1 at this temperature and then stopped before cooling the sample with nitrogen. It is likely that complete pyrolysis was achieved before the temperature of 1073 K was reached [16], as observed in the preparation of our charcoal samples.

The high HR was achieved using the 'macro-TG' apparatus described further (cf. Fig. 1), with N 2 as the atmosphere gas. The reactor and atmosphere gas were heated at 1200 K; when the temperature was stabilised, seven wood particles, not in contact, held on a flat basket consisting of a refractory steel grid-were lifted and hanged on the precision load cell. The evolution of the normalised mass of the seven particles showed that almost complete pyrolysis was achieved within 1 min. For this reason, we considered that the global HR of the particles, that were probably not isothermal, was greater than 900 K min K1 . The wood particles were kept under nitrogen during 8 min in order to assure complete pyrolysis before feeding steam and beginning the gasification of the charcoal particles.

Characterisation of the charcoals

Size and mass of the charcoal particles obtained after pyrolysis at HR of 2.6, 12 and 900 K min K1 were precisely measured; their apparent densities were then calculated (cf. Table 2). We observed that the original wood spheres got an ovoid shape after pyrolysis. The projected surround in the three directions was an ellipse, resulting from the non-isotropic properties of wood. The equivalent sphere diameter was calculated as the average of three different dimensions (cf. Table 2). It is interesting to note that the size of the particles is similar for pyrolysis at 2.6 and 12 K min K1 ; it is a bit higher in the case of high HR. It can be noted that the charcoal particles volume is approximately three times smaller than that of the initial particles. Nevertheless, no cracks could be observed at the surface of the charcoal particles.

Very important differences in the charcoal apparent densities r apparent were observed: 511 kg m K3 for charcoal obtained at 2.6 K min K1 , instead of 219 kg m K3 for the one obtained at 900 K min K1 (cf. Table 2 and Fig. 2). High HR lead to low density charcoals, as it was observed by previous authors [START_REF] Zanzi | [END_REF][10][11][12]. The ratio of apparent densities is here as high as 2.3.

Volatile matter (VM) yield during pyrolysis has been calculated for the 3 HR conditions and compared with standard tests (cf. Table 1). Briefly, a standard test consists of putting rapidly between 1 and 2 g of wood in a 1223 K furnace for a duration of 1 h. The influence of the HR during pyrolysis on the VM yield is shown in Fig. 2. It can be seen that if a log scale is adopted for the HR axis, the charcoal particle apparent density has an almost linear evolution. This result can have practical applications.

The present results can also be expressed in terms of the porosity of the particles. The density of the solid phase r solid -carbon and ash-was measured using helium pycnometry for the three charcoals obtained at different HR. A similar value of 1700 kg m K3 has been found. This enables calculation of the porosity of the particles as indicated in Table 2:

3 Z 1K r apparent r solid (1)
The porosity of the charcoals lie from 70 to 87% for the charcoals prepared at 2.6 and 900 K min K1 , respectively.

The total pore surface area of the three samples were measured using both N 2 and Ar adsorption/desorption analysis. Mercury porosimetry was used to determine the pore size distribution above 3 nm.

N 2 or Ar adsorption/desorption analysis

The pore size distribution of micropores (!2 nm) and mesopores (between 2 and 50 nm) were characterised using N 2 as the adsorbed gas, on an ASAP 2010 apparatus. Many authors [13,[START_REF] Aarna | Changes in reactive surface area and porosity during char oxidation[END_REF] stated that diffusion of N 2 in micropores is very slow and equilibrium is difficult to reach, explaining the problems encountered to measure surface areas in microporous samples with N 2 as adsorptive. CO 2 or Ar are generally preferred to N 2 in the case of microporous structures: analysis was also carried out using Ar. The total pore surface area of the charcoal particles was then calculated from these standard tests by the Langmuir method, classically used for microporous samples. The values are reported in Table 3.

For both N 2 and Ar analysis, isotherm curves for low and medium HR charcoals are comparable. As a result, Langmuir surface area values are very close: 637 m 2 g K1 (N 2 analysis) and 617 m 2 g K1 (Ar analysis) for low HR charcoal, and 605 m 2 g K1 (N 2 analysis) and 620 m 2 g K1 (Ar analysis) for medium HR charcoal. The isotherm shape for Ar analysis reveals a totally microporous surface for the two samples whereas analysis with N 2 exhibits a low mesoporosity, calculated by BJH method applied on the desorption curves, at 73 and 72 m 2 g K1 for the low and medium HR respectively. For charcoal prepared at the HR of 900 K min K1 , N 2 adsorption/desorption tests results were not repeatable, and could vary between 22 and 64 m 2 g K1 . Test duration was very long because equilibrium could not be reached. Ar analysis was then performed several times, but the same problem was encountered. It is commonly observed that biomass charcoals, unlike coals, are very difficult to analyse using the adsorption/desorption technique.

Mercury porosimetry analysis

Mercury porosimetry technique enables us to quantify the volume and then to calculate the surface area of the pores from macroscopic size down to 3 nm, by injecting mercury into the charcoal under increasing pressure, from 0 to 4000 bar. The cumulative pore volume of three charcoals were measured using AUTOPORE II 9220 apparatus; the test was repeated for the two samples prepared under two extreme conditions, i.e. 2.6 and 900 K min K1 (cf. Fig. 3). The corresponding surface area distribution can be calculated from the pore volume data. The surface area of the pores larger than 3 nm were determined from these data, and reported in Table 4.

As indicated by the plots in Fig. 4, the surface area of the pores larger than 100 nm is very small. It is important to note that in all experiments it was observed that when decreasing the pressure, mercury was not extruded from the sample as it should be in a dedicated experiment. The most likely explanation put forward by previous authors [16] is that the charcoal structure was broken during the increase of pressure and mercury was not extruded during the pressure decrease. Therefore, the values reported here are probably affected by a large degree of uncertainty, despite the good repeatability of the measures.

The porosities measured by mercury porosimetry-that exclude the pores smaller than 3 nm-confirm the values calculated from the apparent densities of charcoals (cf. Table 4): the first are approximately 3% smaller than the last.

Scanning electron microscopy (SEM)

An attempt was made to observe differences in the structure of different charcoals, as did several authors [11,13,16,22] using SEM. Our SEM observations-an example is given in Fig. 5-revealed a very non-uniform structure, making it very difficult to compare different charcoals, and even different zones inside the same charcoal particle. Moreover, using the SEM technique, it was not possible to observe the actual reactive surface, i.e. at nanometer scale. As a consequence, even qualitative analysis was difficult using SEM.

Gasification of the charcoals

Experimental set-up

The charcoals obtained at three different HR were gasified by steam in a single particle situation, using a specially designed macro-TG reactor. The general principle consists of holding the particle inside a reactor swept by the oxidising agent H 2 O in a N 2 atmosphere gas, at a controlled temperature and at atmospheric pressure. The weight of the sample is continuously monitored in order to observe the conversion of the charcoal. The experimental set-up is presented in Fig. 1.

The reactor is a 2 m long quartz reactor heated electrically (1). The atmosphere is generated by a N 2 flow controlled by a mass flowmeter/controller (M) and a liquid water flow controlled by a rotameter [START_REF] Roberts | [END_REF] and directed to a steam generator (5), whose output temperature is controlled at 473 K. The gas mixture is then fed to two coiled tubes (4) placed inside the reactor to preheat the atmosphere gas. The gas flow rate was high enough so that no limitations by external mass (H 2 O) transfer occurred. This was tested by varying the gas flow rate: this had no impact on the gasification rate. The precision load cell (10) holds a nickel hanging wire and basket [START_REF] Zanzi | [END_REF] in which particles are placed.

One difficulty encountered when constructing the macro-TG was to control the gas flow through the holding wire aperture at the top of the reactor. Indeed, steam should not escape from the reactor, to be sure of the molar fraction of H 2 O in the reactor atmosphere gas and to prevent water condensation. Neither should air enter the reactor, to avoid a reaction between carbon and oxygen. A 4 N L min K1 flow of N 2 was then fed through a small tube (8) to the vertical tube surrounding the hanging wire in order to create a counter flow. The flow rate of this additional N 2 actually entering the reactor was measured accurately by the tracer gas method at 0.91 L min K1 STP and taken into account in the composition of the atmosphere gas.

Experimental procedure

The reactor was first heated under N 2 atmosphere at the operating temperature, 1200 K, until a stable regime was achieved. The basket was then lifted from the bottom of the reactor, and hung on the load cell. A constant mass achieved under N 2 ensured that adsorbed gas at the surface of the charcoal (H 2 O, hydrocarbons) was released before operating the gasification by H 2 O. The water flow was then established, producing an atmosphere of 20% mole fraction of H 2 O in N 2 .

The mass of the sample progressively decreased until a constant mass-that of ashes-was achieved to conclude a test.

The experiments were made with seven particles together. Using several particles rather than a single one presented two advantages: (i) it enabled results to be obtained from an average of seven particles. Indeed, it is known that disparities between different biomass particles exist; (ii) it increased the signal/noise ratio of the load cell by increasing the total mass of the sample: the weight of a single charcoal particle was very small (!100 mg). The seven particles were placed in a horizontal plane and were not in contact together, so that no interaction between particles occurred.

Results and discussion

The conversion X during steam gasification was calculated following Eq. ( 2). The evolution of X versus time for all the experiments is plotted in Fig. 6 X Z m init Km m init Km ash (2)

The repeatability of the experiments was checked by repeating each test several times: 6, 2 and 3 times for the charcoals prepared at 2.6, 12 and 900 K min K1 respectively. The results in Fig. 6 show that a good repeatability was achieved, considering that biomass is used here and that the size of the particles is important. Some dispersion still existsa deviation of about 10% for six tests is observed-showing that repeating all experiments is necessary.

The examination of the results first indicates that the general shape of the conversion curves versus time is globally almost linear. Previous authors [START_REF] Kumar | [END_REF]12,[START_REF] Dasappa | [END_REF]19] also observed a similar pattern, but with CO 2 as the oxidant.

The results indicate clear differences in the reaction rates for the three charcoals. The linear shape of the curves enables us to extrapolate the curves to XZ1 in order to determine a time for complete gasification, as shown in Fig. 6. The times observed for complete gasification are 1880 s (31 min 20 s), 1320 s Fig. 6. Gasification progress under 20% H 2 O at 1200 K of charcoal particles pyrolysed at 2.6, 12 and 900 K min K1 . (22 min), and 720 s (12 min) for the charcoals prepared at 2.6, 12 and 900 K min K1 respectively. Such differences are significant if one considers an industrial process. They indicate that depending on the pyrolysis process used upstream of the gasifier, the gasification duration can vary in a ratio of 1-2.6 for similar operating gasification conditions. As a reference, and regarding classical values for the activation energy of the charcoal gasification by H 2 O reaction, the gasification temperature should be increased by almost 100 K to gasify the charcoal prepared at 2.6 K min K1 in the same duration as the charcoal prepared at 900 K min K1 . In the following sections we try to provide explanations for such differences in the behaviour of the different charcoals during gasification.

The differences in the gasification rates observed here may be due to limitations of the reaction by heat or mass transfer inside the particles. In order to eliminate this possibility, additional experiments were carried out using smaller particles, obtained from char spheres broken into four or five pieces. In the case of char pyrolysed at the higher HR, i.e. with the lower apparent density-219 kg m K3 , no difference in gasification rate could be observed between broken and entire particles, indicating that no internal transfers limitation occur. In the case of char pyrolysed at the lower HR, i.e. with the higher apparent density, 511 kg m K3 , the broken particles exhibit a time for complete conversion that is 10% lower than the entire particles, showing that the gasification is only slightly limited by internal transfers. A previous modelling study [START_REF] Golfier | Modeling of char gasification at particle scale: how to select the best assumptions in the scope of fixed bed modeling[END_REF] leads to the same conclusion. However, this limitation is not significant enough to explain the large difference in gasification times of chars obtained at low and high HR.

The explanation has to be found in the characteristics of the chars prepared at three different HR.

A first explanation for the high reactivity of the high HR charcoal could be that it did not suffer from thermal annealing, or thermal deactivation, during the pyrolysis stage. Indeed, the soaking time under N 2 at the final temperature of 1200 K was much smaller than in the case of two other charcoals: 8 min instead of 1 h. In order to clarify this point, specific experiments were carried out at high HR but this time with a soaking time of 1 h. No difference in gasification rate was observed between the high HR charcoals with soaking times of 8 min or 1 h. It can be concluded that the beech charcoal does not undergo thermal annealing at a temperature of 1200 K in a duration of 1 h.

In the following sections, the experimental results are confronted to different kinetic models in order to explain the differences in reactivity of the three charcoals.

Homogeneous model

The most classical model for dealing with the reaction of a porous carbonaceous medium with an oxidant gas is the so called homogeneous model

dm dt ZKk app m (3)
in which the apparent kinetics constant k app (s K1 ), currently called apparent reactivity, is a function of temperature and concentration in the gas phase of the species involved in the reactions. Different expressions for k can be found in the literature [START_REF] Chornet | [END_REF][25][26]. These expressions were developed to describe both the increase of gasification rate with an increase of the oxidant gas concentration-here H 2 O-and the inhibition of the gasification process by H 2 produced by the reaction. This expression can be used if no limitation by heat/ mass transfers inside the particle occurs. Since in our experiments the temperature and the water vapour content were similar throughout the whole gasification process, the apparent reactivity should be a constant for all experiments.

The plots in Fig. 7 clearly indicate that it is not the case here.

From the beginning of the experiments, clear differences appear between the apparent reactivities of the different charcoals. When the gasification reaction progresses, a very important increase in the apparent reactivity is observed, leading to values 10-20 times higher than at the beginning of the experiments. The homogeneous model is therefore not suitable for describing the situation.

Surface related model

Differences in the reactive surface can be an explanation for the differences in the apparent reactivities of the three charcoals, as suggested by numerous authors [START_REF] Roberts | [END_REF]13,16,18,[27][28][START_REF] Klose | Fuel, Corrected proof[END_REF].

It is admitted today that in heterogeneous reactions such as C-H 2 O, C-CO 2 , or C-O 2 , the apparent reactivity is highly influenced by the reactive surface. As a consequence, Eq. ( 3) can be expressed as

dm dt ZKks int S r m (4)
where ks int (g s K1 m K2 ) is the intrinsic reactivity that should be constant since all experiments were carried out in the same conditions. The determination of the reactive surface S r (m 2 g K1 ) is a subject for discussion. Beyond the fact that the initial value measurement is still questionable as presented before, its evolution along the gasification process remains Fig. 7. Apparent reactivity versus conversion of the three charcoals prepared at 2.6, 12 and 900 K min K1 during steam gasification under 20% H 2 O at 1200 K.

difficult to describe, and makes modelling of gasification by H 2 O very complex. Several works [13,16,18,22,27] have dealt with the increase of char reactivity during conversion. It seems to develop with an increase of the reactive surface area. They stated that the reaction opens up the pore volume by enlarging the connections between the pores. The surface area available for reaction is then increased by making new pores accessible to the reactants. Moreover, numerous authors have pointed out that fracturation of the particle occurred during the process, creating additional reactive surface together with facilitating the diffusion of the reactant and product gases inside the core of the particles. This phenomenon has been observed in our experiments as illustrated in Fig. 8, where fracturation clearly appears after only 20% of conversion.

The estimation of the charcoal pore surface area that actually participates in heterogeneous carbon reactions was extensively discussed in various research works [START_REF] Aarna | Changes in reactive surface area and porosity during char oxidation[END_REF]16,28,[START_REF] Commandre | [END_REF]31] in the case of the reactions of oxidation by O 2 , gasification by H 2 O or CO 2 and the reduction of NO at the surface of a porous carbon. They came to the conclusion that the micropores (!2 nm) probably do not participate in the reaction, and that the surface developed by the macropores and the mesopores only is a better indicator for the reactive surface than the total pore surface area measured by adsorption/ desorption, in which the contribution of the micropore surface area is important. This is investigated below.

Beginning of the gasification reaction

In a first approach, we will refer to the very beginning of the gasification experiments, i.e. at XZ0.05, when the gasification reaction has just started, but the particles are not fractured. At this time, the reactive surfaces are known from the measurements on the initial charcoals.

The total pore surface area is classically used as a reference reactive surface. From (3) and ( 4), apparent reactivity can be written:

k app Z ks int S r (5)
If the total surface area is a good indicator for the reactive surface, then k app should be proportional to this surface. The plot of the total pore surface area measured by N 2 adsorption/desorption versus the initial apparent reactivity of the charcoals (at XZ0.05), shown in Fig. 9, does not indicate a correlation with the expected positive evolution. The total pore surface area is therefore not a good indicator for the reactive surface.

We plotted in Fig. 9 the evolution of the surface area of pores larger than 3 nm, measured by Hg intrusion. Their evolution with the initial apparent reactivity of the charcoals is closer to a linear shape. The mesopore and macropore surface area seems to be a better indicator for the reactive surface than the total surface area, but is not sufficient to predict the charcoal reactivity.

Progress of the reaction

When the gasification reaction progresses, very important increases in the reactivities are observed, leading to values as high as 10-20 times greater than at the beginning. If the total pore surface area was representative of the reactive surface S r , according to Eq. ( 5), an increase in the same proportion of the total pore surface area along the gasification reaction could be an explanation. But results from the literature [13,16,18,22,27] do not indicate such high increases in the char reactive surface during the gasification process: this tends to show again that this surface is not the best parameter for explaining the evolution of charcoal gasification reactivity.

Ash related model

The catalytic effect of the ashes in the gasification of coals has been widely studied in the literature [32][33][34][35], as coals present very high ash content compared to biomass chars. However, previous authors focused on the role of ashes in the gasification of biomass chars [20,36]. In order to investigate this point, results will be confronted to a model of the following type dm dt ZKka int ½Ashm

where [Ash] is the ash mass fraction in the carbon matrix, and ka int (g char g ash K1 s K1 ) is the intrinsic reactivity related to the ash fraction. Eq. ( 3) and ( 6) lead to:

k app Z ka int ½Ash (7)

Beginning of the gasification reaction

If the model in Eq. ( 6) is adapted, then a linear correlation between the apparent reactivities and the ash fraction in the charcoals should be observed. We plotted in Fig. 10 the values of k app of three charcoals at the beginning of the reaction versus their content in ash. The plot indicates a clear linear increase.

Progress of the reaction

We have plotted in Fig. 11 the evolution of ka int along the gasification process. If this model was an exact representation of the gasification process, the ka int values should be constant along the process and similar for the three charcoals. One can notice that they are not equal, but they appear not to change in a large range along the gasification process: the ratio between the higher and the smaller values for three charcoals is less than two instead of 10-20 when using the homogeneous model.

This model is much more accurate than the homogeneous model for describing the gasification reaction. Summarising these results it can be thought that the ash percentage has an effect on the gasification reaction. This hypothesis is supported by the ash content of the initial beech wood, reported in Table 5. The analysis, performed by Inductive Coupling Plasma (ICP), reveals a great quantity of calcium and potassium in the ash (45.5 and 38% of the total mass of ash respectively). Calcium and potassium are known to have a catalytic effect on the gasification of chars and coals [20,32,33].

Nevertheless, the effect of ash alone is not sufficient to explain the reactivity of the different charcoals to water vapour. It is probable that the gasification reaction is both influenced by the reactive surface, which is not easy to identify, and by the amount of ash. At this stage, it was not possible to distinguish the two effects. Indeed, regarding the difficulties encountered in measuring reactive surfaces as discussed before, measurements of reactive surfaces along the reaction were not undertaken.

Conclusion

Changing the heating rate during the pyrolysis of a 1 cm characteristic size beech wood particle from very low-2.6 K min K1 -to very high-900 K min K1 -yields charcoals with drastically different properties. At the lower heating rate, the charcoal is as dense as 511 kg m K3 and its total pore surface area is as high as 637 m 2 g K1 , while at the higher heating rate, the charcoal density is 219 kg m K3 only. The apparent density follows a linear evolution versus the log of the heating rate. The volume reduction from the initial 10.2 mm diameter wood spherical particles to the ovoid charcoal particles is nevertheless similar in the three cases of heating rate, with a ratio of approximately three between the volume of the wood particles and the volume of the charcoal particles. The difference in apparent densities is therefore due to the differences in the volatile matter yield.

A new macro-TG experiment was developed, and used to submit the charcoal particles to gasification under 20% H 2 O in N 2 at a temperature of 1200 K. A good repeatability in the experiments was achieved; we demonstrate that repeating all experiments is nevertheless necessary. The charcoal obtained at different heating rates exhibit very different gasification kinetics. The gasification rate of the charcoal with a high density-i.e. prepared at a low heating rate-is 2.6 times lower than that of the low density charcoal-i.e. prepared at a high heating rate. Such differences have a strong impact on the design of an industrial process. The results of this work are likely to be useful for the optimisation of industrial staged gasifiers.

This work also demonstrates that the lower reactivity of the charcoal produced under low or medium HR is not due to thermal deactivation during pyrolysis: maintaining a charcoal obtained at a high heating rate for 1 h at 1200 K does not reduce its reactivity towards H 2 O.

An attempt was made to find a correlation between a reactive surface and the reactivity of the charcoals during gasification by H 2 O. The total pore surface, whatever measured by N 2 or Ar adsorption, is not a good indicator for the reactive surface. This can be explained by the fact demonstrated by previous authors that micropores-which do not actively participate to the gasification reactionrepresent an important part of the total surface area. The mesopore plus macropore surface area seems to be a better indicator for the reactive surface during gasification, but cannot be precisely correlated to the reactivity of the charcoals.

The present results indicate that the amount of ash in the charcoals probably plays an important role in the gasification process. Indeed, the initial apparent reactivities of the charcoals are proportional to the initial ash content of the charcoals. Moreover, throughout the gasification process, the reactivity of the charcoals related to their ash content appears to change by a ratio of 2 only, whereas neglecting this effect-considering an homogeneous model-leads to ratios of 10-20. It is probable that both reactive surface evolution and ash content evolution play a role in the gasification by H 2 O. It was not possible in this work to differentiate between the two effects.

Fig. 5 .

 5 Fig. 5. SEM picture of the initial char.

Fig. 8 .

 8 Fig. 8. Macroscopic evolution of a gasifying particle (0, 20, 50, 70% conversion).

Fig. 9 .

 9 Fig.9. Total pore surface area ($) and surface area of the pores larger than 3 nm (,) versus the apparent reactivity of the three charcoals at XZ0.05.

Fig. 11 .Fig. 10 .

 1110 Fig.11. Evolution of the intrinsic reactivity related to ash fraction along the gasification reaction of the three charcoals.

Table 1

 1 Analysis of the initial beech wood samples

	Proximate analysis (wt%)
	Ash	0.38
	Volatile matters	82.45
	Fixed carbon (by difference)	17.17
	Ultimate analysis (wt%)
	C	44.16
	H	5.48

O (by difference) 50.36 Fig. 1. Macro-TG experimental apparatus.

Table 2

 2 Properties of the initial wood and of the charcoals obtained by pyrolysis at 2.6, 12 and 900 K min K1

	Heating rate (K min K1 )		2.6	12	900
	Initial beech wood
	Diameter of a particle	mm	10.20	10.20	10.21
	Weight of a particle (dry basis)	g	0.3752 0.3699 0.3761
	Apparent density (dry basis)	kg m K3 664	666	675
	Charcoals
	Equivalent diameter of a particle	mm	6.99	6.92	7.76
	Weight of a particle	g	0.0912 0.0714 0.0535
	Apparent density	kg m K3 511	449	219
	Porosity	%	70	74	87
	VM yield	wt%	68	73	78

Table 3

 3 Surface area of the three charcoals measured by adsorption/desorption technique with N 2 and Ar on an ASAP 2010 apparatus

	Sample	Ar (m 2 g K1 )	N 2 (m 2 g K1 )
		Langmuir	Langmuir	BJH
				(mesopores)
	Low HR (2.6 K min K1 )	617	637	73
	medium HR (12 K min K1 )	620	605	72

Fig.

3

. Cumulative pore volume versus pore diameter resulting from mercury porosimetry analysis for the three charcoals. Fig.

4

. Cumulative pore surface area versus pore diameter resulting from mercury porosimetry analysis for the charcoals prepared at 2.6 (!), 12 (B) and 900 (,) K min K1 .

Table 4

 4 Porosity, pore volume and surface area of three charcoals measured by mercury porosimetry on an AUTOPORE II 9220 apparatus

	Sample	Calculated	Measured	Pore	Pore surface
		porosity	porosity	volume	area
		(%)	(%)	(ml g K1 )	(m 2 g K1 )
	Low HR	70	65	1.203	106
	(2.6 K min K1 )
	Medium HR	74	71	1.460	127
	(12 K min K1 )
	High HR	87	85	3.470	120
	(900 K min K1 )

Table 5

 5 Ash content of the initial beech wood

		10 K3 wt% in the wood	wt% in the ash
	Ca	97	45.5
	K	81	38.0
	Mg	19	8.9
	P	6.1	2.9
	Mn	4.95	2.3
	Na	3.11	1.5
	Fe	1.64	0.8
	Zn	0.37	0.2
	Cu	0.21	0.1

Acknowledgements

The research is partly funded by the European Commission in the framework of 5th Programme, Lift off project, contract NNE5/2001/704.