Changes in char reactivity due to char-oxygen and char-steam reactions using Victorian brown coal in a fixed-bed reactor☆

Shu Zhang *,Yonggang Luo ,Chunzhu Li,3,Yonggang Wang

1 Department of Chemical Engineering,Monash University,Clayton,VIC 3800,Australia

2 School of Chemical and Environmental Engineering,China University of Mining and Technology,Beijing 100083,China

3 Fuels and Energy Technology Institute,Curtin University of Technology,GPO Box U1987,Perth,WA 6845,Australia

Keywords:Brown coal Gasifying agent Char reactivity Fixed-bed reactor

ABSTRACT This study was to examine the influence of reactions of char-O2 and char-steam on the char reactivity evolution.A newly-designed fixed-bed reactor was used to conduct gasification experiments using Victorian brown coal at 800°C.The chars prepared from the gasification experiments were then collected and subjected to reactivity characterisation(ex-situ reactivity)using TGA(thermogravimetric analyser)in air.The results indicate that the char reactivity from TGA was generally high when the char experienced intensive gasification reactions in 0.3%O2 in the fixed-bed reactor.The addition of steam in to the gasification not only enhanced the char conversion significantly but also reduced the char reactivity dramatically.The curve shapes of the char reactivity with involvement of steam were very different from that with O2 gasification,implying the importance of gasifying agents to char properties.

1.Introduction

Coalgasification application could be tracked back to early 19th century[1],and has been again becoming an important focus since 1970s as its product gases can be used for electricity generation,valuable chemical production and especially for liquid fuel synthesis[2-8].Gasification science and technology has been well developed in last decades,but mainly for high rank coal.Compared to high rank coal,low rank coal(i.e.brown coal)generally features high contents of ash,oxygen and water with porous structure and high reactivity.

cience about Victorian brown coal gasification has been intensively studied,particularly by Monash University in Australia[9-21].For example,it was found that volatile-char interaction was one of the most important factors affecting almost every aspect of gasification using Victorian brown coal[3,14-21].The results also implied that gasifying agents(steam,oxygen and CO2)might react with Victorian brown coal on different reactive sites in a fluidised-bed reactor[15,22,23].In a fluidised-bed,the heating rate of coal particles could reach about 103-104K·s?1[24,25].Heating rate is surely a critical factor for any thermal conversion technology.High heating rate applied to coal particles will result in quick cracking of coal structure,forming more volatile materials.Also,the left solid(char)from high heating rate would feature different porosity,different aromatization and so on from low heating rate.The differences in physico-chemical property of char can fundamentally affect the subsequent gasification reactions between char and gasifying agents,even the reaction pathway.In industrial gasifiers,the coal particles are mostly fed into the hot gas reaction zone in the absence of fluidised agents.Indeed,the reaction conditions(such as the heating rates)of coal in a hot gas atmosphere could be well simulated in a fixed-bed reactor.Furthermore,an industrial gasifier could conceptually be divided into a few reaction zones(gasification zone,combustion zone,etc.).The slow reaction zone in a gasifier normally refers to the gasification reaction region where oxygen concentrations may vary from a few to less than 1%.Understanding of char reactivity evolution in an environment of low oxygen content is very crucial for improving gasification technology.As oxygen and steam often co-exist in a practical gasifier,the influence of steam on char-oxygen reaction may also be fundamentally important regarding gasification efficiency.

The gasification efficiency at different reaction conditions could be directly measured by coal converting rates.To better understand the difference in gasification rate of coal,the chars generated at certain reaction conditions need to be further analysed.One of the most useful techniques was the isothermal reactivity measurement of chars in oxidising environments using TGA at low temperatures(such as 400°C)[10,15,17,20,22].At the low temperatures,the carbonaceous material in chars would be consumed gradually.Different reaction rates at different stages of char conversion in TGA can suggest some detailed information ofchar properties(i.e.carbon structure,AAEM status,etc.).Indeed,some researchers have even selected lower temperature than 400°C for the reactivity measurement because the chars prepared under some conditions were very reactive[26].The very reactive chars would be immediately combusted instead of slowly oxidised when oxygen was introduced into the reaction zone[22,26].If combustion take place,it would be difficult to compare the difference in charreactivity as the exothermal reaction could cause the sudden increase in temperature.For the chars prepared from this study,400°C was working well for conducting the reactivity measurements.

Therefore,the purpose of this work is to study coal conversions during the gasification in oxygen and steam,as well as the resultant char's reactivity.The result shows that the char-oxygen and char-steam reactions in the fixed bed reactor have induced significant changes in ex-situ char reactivity characterised by TGA.The property of chars prepared in this study was very different from that of chars prepared from a fluidised-bed reactor previously used,and the comparisons was made in this paper.

2.Experimental

2.1.Coal sample

Victorian brown coal which contains～60%(by mass)moisture was firstly dried at very low temperatures(＜35 °C).The dried coal containing about 10%moisture was then milled and sieved to obtain particle sizes between 106 and 150 μm[9].This pre-treated coal sample is hereafter termed as raw coal with properties:C,70.40;H,5.40;N,0.62;S,0.28;Cl,0.10;O,23.20(by difference)and volatile matter,52.20%(by mass)(daf)together with an ash yield of 1.10%(by mass)(db).

2.2.Gasification experiments

The fixed-bed quartz reactor as shown in Fig.1 consists mainly of four parts:inject tube, fixed-bed 1, fixed-bed 2 and steam tube.To start experiments,they were firstly assembled and also connected with accessories used for gas supply(pure argon,0.3%O2in argon and/or 15%steam in argon)for feeding coal particles.The total gas flow into the reactor was always 2 L·min?1.The reactor was then put into an electrically-heated furnace heating up to the described temperature(800 °C).The coal powder(106-150 μm)in a feeder was then fed into the reactor from the top(injecttube)at a planned feeding rate(25,50,75 or 100 mg·min?1),feeding time(10,20,30,40,50 or 60 min)and holding time(0,10,20,30 or 40 min)for each experiment.Different reaction extents(coal conversion)were realised by varying feeding rate,feeding time and holding time.The char particles formed from gasification/pyrolysis would stay on the first frit( fixe-bed 1)inside the reactor and forma charbed.The gasifying agent would pass through the char bed and react with char.The product gas went out of the reactor from the bottom of reactor.At the end of each experiment,the reactor was taken out of reactor and cooled in air.The chars were then collected for further analysis after cooling down.

2.3.Ex-situ char reactivity measurement

The reactivity of chars prepared from the gasification experiments in the fixed-bed reactor was measured at400°Cin air(21%O2in nitrogen)using a Perkin Elmer Pyris 1 thermogravimetric analyser(TGA)following the procedure outlined previously[27].Brie fly,an empty platinum crucible was firstly loaded up and the balance was tarred with the empty platinum crucible.About 4 mg of char sample was placed in the platinum crucible and heated in pure nitrogen(99.999%)atmosphere in the TGA to 105°C to remove the moisture from the char.The temperature was then increased at 50 K·min?1to 400 °C.After 2 min at 400°C,the atmosphere was switched from nitrogen to air to commence data collection.400°C was chosen as the isothermal oxidation temperature in this study in order to minimise the changes in char structure due to thermal annealing and avoid the possible ignition.The specific reactivity(R)of the char was calculated using the equation:

where,W is the mass(dry-ash-free basis)of the charatany given time t.After the mass of the char sample became constant,the temperature was further increased at 50 K·min?1to 600 °C for an additional 30 min to ensure the complete combustion of any carbonaceous material possibly remaining in the char[15,17,27].The reason for choosing 600°C rather than higher temperatures was due to that higher temperatures could possibly lead to the volatilisation of some inorganic matters(i.e.AAEM,alkali and alkaline earth metallic metal species)[9].The final mass was taken as the mass of ash in the char sample.

Fig.1.Schematic diagram of fixed-bed reactor used in this study.

3.Results and Discussion

3.1.Effects of feeding rate and feeding time on char conversion and reactivity

Fig.2 shows the effect of coal feeding time and feeding rate on coal conversions during gasification in 0.3%O2at800 °C.The“coal conversion”in this study was calculated by the equation of 100%·(Wcoal?Wchar)/Wcoalon dry basis.The mass of char was obtained by weighing the reactor before and after the experiment.Fig.2(a)shows the slightly increasing coal conversion with increasing feeding time while the increase in feeding rates has resulted in a slow decrease in coal conversion in Fig.2(b).For a given feeding rate of 100 mg·min?1,increasing feeding time means the longer average reaction time between coal and O2,thus causing high coal conversion.The increase in coal feeding rates resulted in high concentration of volatiles inside the reaction zone.The radicals(especially H radicals)from interactions between volatiles and coal could occupy reactive sites on char surface,which was believed to be the main reason for inhibiting the char conversion during the gasification in steam[14-17].For the gasification with O2as gasifying agent in this study,the actual role of the volatile-char interaction on char conversion will be further discussed later.Besides,it is reasonably envisaged that the high concentration of volatiles from feeding rates could further dilute the O2concentration and decrease its partial pressure,which should contribute to the gradual reduction in coal conversion with increasing coal feeding rates.

Fig.2.Effects of feeding time(a)and feeding rate(b)on coal conversion during the gasification in 0.3%O2 using the fixed-bed reactor at 800°C.

Fig.3 shows the ex-situ reactivity in air at 400°C for chars having experienced gasification in 0.3%O2with different feeding times and feeding rates above.Broadly,the ex-situ char reactivity increased with increasing feeding time and kept nearly constant with varied feeding rates as shown in Figs.3(a)and(b).For Fig.3(a),the change in ex-situ reactivity is in essence related to the difference in the average of coal/char reaction time with O2in the fixed-bed reactor.The gasification reactions in the quartz reactor at 800°C could possibly lead to the rearrangement of inherent catalytic species(i.e.Na and K)and more metallic species migrated to the surface of char with longer feeding(reaction)time[28,29].The dispersion of abundant metal species on the char surface may explain the high ex-situ reactivity of chars with long feeding time.

The little change in ex-situ char reactivity with increasing feeding rate(increasing extents of volatile-char interactions)in Fig.3(b)suggests that the potential effect of volatile-char interactions on the char reactivity during gasification at low concentration of O2in the quartz reactor was smaller compared to the effect of volatile-char interactions on char property during gasification in steam[14].Indeed,the effect of volatile-char interactions on ex-situ char reactivity during gasification in steam was very obvious and has drastically resulted in decreasing char reactivity with increasing feeding time and feeding rate[14].This,in turn,indicates that the low char conversion in Fig.2(b)is not mainly attributed to the volatile-char interactions,and the consumption of O2by the volatiles/radicals should be mainly responsible for the low coal conversion with increasing feeding rate.

Fig.3.Effects of feeding time(a)and feeding rate(b)during the gasification in 0.3%O2 at 800 °C on ex-situ char reactivity measured by TGA in air at 400 °C.

3.2.Effects of holding time on char conversion and reactivity

Fig.4 exhibits the change of coal conversion as a function of holding time during gasification in 0.3%O2for the given feeding rate(100 mg·min?1)and feeding time(20 min).The coal conversion gradually increases with increasing holding time from 0 to 40 min.As expected,the coal is continuously converted or gasified as O2is continuously supplied.During the holding time,the coal particles are stopped to be fed into the reactor,which means that all the O2flowing into the reactor would react directly to the char particles being free from the interruption of volatiles.The slow converting rate is due to the very low O2concentration and also relatively low gasification temperature of 800°C.

Fig.4.Effects of holding time on coal conversion during the gasification in 0.3%O2 using the fixed-bed reactor at 800°C.

Fig.5.Effects of holding time during the gasification in 0.3%O2 at800°C on the ex-situ char reactivity measured by TGA in air at 400°C.

The changes in ex-situ reactivity for the chars with different holding times in the fixed-bed reactor are shown in Fig.5.The specific reactivity measured by TGA increases with increasing holding time in the fixed bed reactor from 0 to 40 min in 0.3%O2at 800°C.As explained for Fig.3(a),the longer holding time probably has also allowed more inherent catalytic metal species to accumulate on the char surface[28,29],thus featuring high reactivity.Additionally,the increase in coal conversion with increasing holding time indicates the slow increase in Na/K content in the resultant chars[15].The change in the trend/shapes of ex-situ char reactivity due to the holding time during gasification in O2in the fixed-bed reactor is quite different from that of chars prepared from a fluidised-bed reactor under the similar experimental conditions[22].The increase in char reactivity with increasing holding time in Fig.5 is faster than that from the fluidised-bed reactor(our previous study).More importantly,the curve shapes of char reactivity prepared from the fixed-bed reactor in Fig.5 are more waved than that of char reactivity from the fluidised-bed reactor[22].Compared to the heating rates(103-104K·s?1)in the fluidised-bed reactor[24,25],the relatively low heating rates for coal particles in this fixed-bed reactor have left more reactive char before holding time started as the thermal cracking reactions with low heating rate would be less vicious and more vulnerable char structure could remain.The waved shape of reactivity curves in this study could reflect the uneven carbon structure consisting of small and large aromatic ring systems[26].The small aromatic ring systems(3-5 fused rings)would be preferentially reacted in TGA,corresponding to the first peak of reactivity curves.The metallic species originally coupling with the small aromatic ring systems would then have to absorb on the sites of large aromatic ring systems(no less than 6 fused rings)[26].The consumption of large aromatic ring systems catalysed by the inherent catalysts has generated the second peak of reactivity curves.Thus,the difference in the trend and shapes of reactivity curves are largely resulted from the difference in char structure and effect of char structure on the distribution of AAEM in char.

3.3.Effects of steam presence on char conversion and char reactivity

The net coal conversions in Fig.6 are calculated using total coal conversion in each condition minus the coal conversion(56%)from the pyrolysis in pure argon,which was trying to show the net gasification effect by steam/O2on coal consumption by taking the coal conversion(due to the thermal cracking)as zero.

Fig.6.Netcoalconversion(based on pyrolysis condition)versus gasifying agents at800°C.Feeding time:40 min,feeding rate:25 mg·min?1.

From Fig.6,the influence of steam introduction on the net coal conversion during 0.3%O2gasification could be observed.The additional coal conversion upon the introduction of steam is as large as 22.1%.The gasification experiment only in the presence of 15%steam without O2results in 19.8%net coal conversion.The comparison between 22.1%and 19.8%implicates that the synergistic effect of O2and steam on coal gasification might exist,but not significant for the examined condition.

The char reactivity for the chars from the reactions in pure argon,0.3%O2and the mixture of O2and steam in the fixed-bed reactor was characterised by TGA.The results of reactivity measurements are shown in Fig.7.The reactivity curve shapes from pure argon and 0.3%O2are very close,implying that the char property in the meaning of char's sensitivity to oxygen is very similar.In other words,the char consumption by O2is less selective and the distribution of aromatic ring systems is not largely changed by gasification at the very low concentration of O2[22].However,the reactivity of char produced from gasification with O2is much higher(especially at the later stage of char conversion)than that of char prepared from the gasification with O2and steam together.This strongly demonstrates that the reaction between char and steam is very selective and steam has preferentially consumed certain char structural units.Different gasifying agents could produce chars with different properties due to the different reaction pathways.

Fig.7.Effects of steam introduction during the gasification in 0.3%O2 at 800°C on ex-situ char reactivity measured by TGA in air at 400°C.