Fuel 87 (2008) 2972–2976

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Characterization of unburned carbon in bagasse fly ash

Vidya S. Batra a,*, Sigita Urbonaite b, Gunnar Svensson b

a The Energy and Resources Institute, India Habitat Centre, Lodhi Road, New Delhi 110003, Indiab Division of Structural Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden

a r t i c l e i n f o

Article history:Received 28 September 2007Received in revised form 16 April 2008Accepted 16 April 2008Available online 13 May 2008

Keywords:Bagasse ashUnburned carbonSurface area

0016-2361/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.fuel.2008.04.010

* Corresponding author. Tel.: +91 11 2468 2100/211E-mail address: vidyasb@teri.res.in (V.S. Batra).

a b s t r a c t

Bagasse is one of the important biomass sources, which is used as a fuel in the sugar industry in India. Asa result, large quantities of fly ash are generated and create a serious disposal problem. This is furtheraggravated by the presence of unburned bagasse mainly as carbonised fibre. In this study, the unburnedcarbon in bagasse fly ash is characterized by thermal analysis, electron microscopy and adsorption. Thecarbon particles can be separated from oxide fraction of fly ash by floating it in water. This processincreases the loss on ignition from 20–30% to 80%. N2 adsorption measurements give BET surface areasof �200 m2/g for the separated carbons. Analysis of the isotherms indicates a large fraction of pores inthe size range of 10–12 Å. Scanning and transmission electron microscopy studies show that theunburned carbon is amorphous and the morphology retains the cellular characteristics of the parentbagasse fibres.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Bagasse is the fibrous residue from sugarcane juice extractionand is one of the most valuable by-products in Indian sugar mills.Bagasse has a net calorific value of around 8000 kJ/kg, with a mois-ture content of around 50 wt% and ash content in the range4–5 wt%. It is therefore utilised as a fuel in boilers in the sugar millsto generate steam and electricity. The efficiency of boilers used insugar mills is typically 60–70% [1,2]. Higher efficiency can be ob-tained by using fluidised bed combustion. However, it is difficultto fluidise bagasse since it is fibrous, has low density, containsmoisture and requires an inert fluidising solid. Thus, the fly ashcontains high unburned carbon amounts and poses a disposalproblem. Its utilization as an adsorbent as well as an additive incement and concrete has been examined [3–6]. However, the highcarbon content is an obstacle for its use in concrete. Therefore, itwould be valuable if the unburned carbon in bagasse fly ash couldbe separated and used for other applications.

The nature of carbon formed from biomass depends on the ther-mochemical conditions during conversion as well as the nature ofparent biomass. Bagasse is made up of fibre bundles as well as ves-sels, parenchyma and epidermal cells [7]. Bagasse fibres mainlyconsist of cellulose, lignin and pentosane [8]. Several studies haveexamined the pyrolysis and combustion behaviour of bagasse. Ingeneral, the first stage of pyrolysis leads to rapid mass loss dueto volatilisation of cellulose. This is followed by a slower rate of lig-nin decomposition [9]. Bagasse carbonisation in argon atmosphere

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1; fax: +91 2468 2144/2145.

was studied and it was found that shrinking and formation of sur-face pores occurred during volatilisation [10]. Other studies havefound that the nature of char maintains the original cellular struc-ture at low heating rates [11,12]. Under extreme conditions (rapidheating, high pressure) melting of cell structure was observed.However, it was observed that bagasse was less susceptible tomelting compared to other biomass such as pine and eucalyptus[11]. Carbonisation of bagasse for charcoal production led to char-coal fraction with less than 15% ash. This has potential for use inhousehold applications after briquetting [13]. Thermochemicalbehaviour of bagasse pellets for use in gasification has been stud-ied. It was found that the char surface areas were higher whensmall amounts of oxygen was introduced with nitrogen duringpyrolysis (e.g. 2.7 m2/g at 5% oxygen and 101.6 m2/g at 15% oxy-gen) [14].

Many studies have prepared activated carbon from bagassewith reported surface areas in the range 500–1947 m2/g [15–19].Char from bagasse pyrolysed under vacuum with surface area of529 m2/g could be activated with steam to a higher surface areaof 1947 m2/g. While unactivated char had mostly micropores, theactivated sample had both micropores (with a maximum of thepore width at 12 Å) and mesopores (around 30 Å) [20]. Other stud-ies have also reported a combination of micro and mesopores. Themicropore content was higher at lower burn off [19].

There have been several studies discussing the removal of un-burned carbon from fly ash but these have mostly concerned flyash from coal. In one study, carbon concentrates with an LOI (losson ignition) of 67–80% value was prepared by employing varioustechniques. These included gravity separation, electrostatic separa-tion, froth flotation, and magnetic separation [21]. Chemical

0

20

40

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80

100

120

0 100 200 300 400 500 600 700 800 900 1000

Temperature (°C)

Wei

ght (

%) 1-B, 2-A

2-B

2-C

2-D

1-A

1-D

Fig. 1. Weight change as a function of temperature for samples from unit 1 (solidlines, 1-A – as received, 1-B – 150–425 lm fraction, 1-D – floating fraction) and unit2 (grey lines 2-A – as received, 2-B – 150–425 lm fraction, 2-C – fraction larger than425 lm, 2-D – floating fraction).

V.S. Batra et al. / Fuel 87 (2008) 2972–2976 2973

analysis done on the separated carbons indicated them to haveproperties similar to activated carbon. In another study, threemicroscopically identifiable forms of unburned carbon were sepa-rated using density gradient centrifugation (DGC) [22] The threecarbon types showed different values of density, nitrogen content,oxygen content, surface area and mesoporous volume. This wasalso reflected in the differences in their adsorption properties.

In this report, we present the separation of unburned carbon inbagasse fly ash by floating or sieving and its characterization interms of surface area, pore size distribution, thermal behaviour,morphology and composition.

2. Experiment

Bagasse ash from two different sugar mills in India was used forthe study (unit 1 and unit 2). Different fractions were obtainedfrom the fly ash and analysed as described below.

2.1. Ash segregation

To obtain ash with different carbon contents, the as receivedash was separated in fractions by sieving or floatation in water.The following fractions were used for subsequent analysis:

� As received ash.� Sieving fraction greater than 425 lm.� Sieving fraction between 150 and 425 lm.� Fraction floating in water.

In case of unit 1, sufficient amount of particles greater than425 lm could not be obtained and this fraction therefore was notconsidered in the analysis.

2.2. Ash characterization

Each of the segregated fractions was analysed in a thermalgravimetric analyser (Perkin Elmer 519 N). The samples wereplaced in a platinum crucible and heated from 20 to 900 �C in airat the rate of 10 �C/min and the weight remaining in the cruciblewas measured as a function of temperature.

Surface area was determined using nitrogen adsorption method(Micromeritics ASAP 2020). The samples were degassed at 200 �Cfor 10 h in vacuum prior to analysis. The nitrogen adsorption wascarried out at 77 K and BET [23], DR [24] and DFT [25] methodswere used to determine the apparent surface area, pore volumeand pore size distributions.

Scanning electron microscopy (SEM) studies where performedwith ‘JEOL JSM7000’ microscope equipped with a ‘Oxford INCAxSIGHT’ energy dispersive detector (EDS). Transmission electronmicroscopy (TEM) studies were conducted with ‘JEOL JEM2000FX’microscope equipped with a ‘LINK 200QX EDS’ analyser. The sam-ples were carbon coated prior to the analysis in the SEM. For theTEM studies, the powders were gently crushed in an agate mortar,dispersed in butanol using a sonicator. Thereafter, a drop of thesuspension was placed on a holey carbon film supported by a cop-per grid.

3. Results

3.1. Thermal analysis

The weight changes upon thermal gravimetric analysis for thedifferent fractions from the two units are shown in Fig. 1. For flyash from unit 1 the weight loss takes place mainly between 300and 500 �C for the samples with low carbon content; i.e. as

received and 150–425 lm fractions. The total weight lost is inthe range 25–35 wt%. For the floating fraction, the weight loss oc-curs over a wider temperature range (300–600 �C). The totalweight loss was around �75%. Fly ash from unit 2 shows weightloss over a narrower temperature range (from 450 �C to 600 �C)for all fractions, independent of carbon content. However, the asreceived fly ash and the fraction between 425 and 150 lm exhibita weight loss similar (20–25 wt%) to the same fraction from unit 1.The weight losses for the fractions floated in water or with parti-cles larger than 425 lm, in contrast, are higher (85 wt%). Thus,the as received fraction and 150–425 lm fraction are similar andfraction greater than 425 lm is similar to the floating fraction.Therefore, only the as received fraction and floating fractions wereconsidered for subsequent analysis.

The fraction greater than 425 lm and fraction between 150 and425 lm constitute around 15 wt% and 20 wt% of the total fly ashrespectively. The floating fraction amount was less since part ofthe floating fraction mixed with the settled fraction duringrecovery.

3.2. Surface area analysis

The surface area, pore volume and pore size of the as receivedand floating fractions from unit 1 and unit 2 are summarised in Ta-ble 1. In both cases, it can be observed that the floating fraction hasa higher surface area compared to that of the as received fraction;this is true for both BET surface area and micropore surface area.The BJH adsorption pore width is, however, in the same range forall the samples (28–38 Å).

DFT analysis of the adsorption data showed a complex pore sizedistribution. Pore widths in the 28–38 Å range were observed forall samples matching the BJH pore width. In addition to thesepeaks, the floating fractions showed maxima in pore width distri-bution between 10 and 12 Å.

3.3. Morphology

Morphologies of the as received fractions from unit 1 and unit 2had similar appearances in SEM. An image of the latter is shown inFig. 2. The size and shape of the particles vary; however, they canbe sorted into three main groups – prismatic (A), spherical (markedB) and fibrous (C). The prismatic (A) particles consist mainly of Siand O. The spherical ones (B) in contain Si and O as well as Na,

Table 1Characteristics of as received and float separated fly ash from bagasse burned in twodifferent boilers

Unit 1 asreceived

Unit 1floatingfraction

Unit 2asreceived

Unit 2floatingfraction

Surface area (m2/g)BET surface area 64 173 98 288t-Plot micropore area 26 120 78 241t-Plot external surface area 37 40 19 48

Pore volume (cm3/g)Single point adsorption total

pore volume < 870.8936 Åat p/p� = 0.978352185

0.064 0.092 0.054 0.145

D–R microporous volume 0.039 0.077 0.047 0.130

Pore diameter (Å)BJH desorption average pore

width (4 V/A) (Å)38 33 34 28

Fig. 2. Scanning electron image of as received fraction from unit 2. EDS-analysisshow that main constituents of the particles marked A is silicon oxide; B besidessilicon and oxygen typically also contains potassium, magnesium, calcium andaluminium; C is carbon.

Fig. 3. Scanning electron image of floating fraction of fly ash from unit 1. Theparticle B is carbon (EDS-results) containing pyrolysed bagasse where the cellstructure is shown, while particle A is remains of epidermal layer containing siliconoxide (EDS-results).

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K, Al, Mg, Ca. They also occasionally contained very small amountsof P as well as Fe. The fibrous ones (C) consist of only carbon. Theseresults are consistent with XRF analysis of bagasse fly ash [26].

An image of the floated fraction from unit 1 is shown in Fig. 3.The major constituent of this fraction is pure carbon (EDS-result)as fibres (marked B in Fig. 3). On one of the carbon fibres, a cover-ing is observed (marked A in the figure). This covering materialconsists of Si and O according to the EDS-analysis. At higher mag-nification segments with curved boundaries and discreet dumbbellshaped particles can be seen, as shown in Fig. 4. A closer inspectionof the dumbbells reveals the presence of fine pores (insert in Fig.4). The cell structure of the carbon fibres containing numerous pitsin the cell walls is clearly seen in Fig. 5. At large magnification thefine particulate structure of the pit area is clearly seen (insert inFig. 6).

The transmission electron microscopy image of the floated car-bon from unit 1 confirms the result from the SEM study. In additionit shows the amorphous nature of the carbon and the presence ofsmall particles of oxides as shown in Fig. 7.

Fig. 4. Scanning electron image of the silicon oxide (EDS-analysis) containing epi-dermal layer of bagasse fibres from unit 1. An enlargement of the squared area onthe dumbbell reveals its porous nature.

Fig. 5. Scanning electron image of pyrolysed bagasse from floating fraction of unit2. The images show the cell structure of the bagasse with pits from the plasmo-desmata intercellular channels in the cell walls. In the centre of the image a brokenpart of the cell structure is shown.

Fig. 6. Scanning electron image of the cell wall with a plasmodesmata channelobserved in a particle from the floating fraction from unit 2. An enlargement of theenlighten square in the image is inserted in the lower right corner. The cell wallsseem to consist of small plates being some tenths of nano meter in size. The spacingbetween these is in the range of a few nano meters.

Fig. 7. TEM image of carbon from unit 2.

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4. Discussions

This study confirms that bagasse fly ash comprises of more than25 wt% unburned carbon, which is present as fibres and can be sep-arated by sieving or floatation techniques. The unburned carbonretains the morphology of parent bagasse, which contains pittedcells [12]. There are no signs of graphitisation in the electron dif-fraction patterns or TEM images. This shows that conditions inthe sugar mill boilers were not extreme enough to cause disinte-gration of cell structure during pyrolysis of bagasse.

The outer layer containing Si and O, covering some of the car-bon fibres seems to be the source of SiO2 in bagasse fly ash. Thestructure of the covering (Fig. 4) matches that of the epidermislayer of bagasse [27]. The epidermis layer consists of long rectan-gular cells and short cells comprising cork and silica cells. Thedumbbell shapes particles appear to be the cork/silica cells. Thelong cells have curved cell walls, which is also seen in the coveringin Fig. 4. The pits in the carbon cell walls (Fig. 5) are remains of theplasmodesmata, which are microscopic channels between the cellsin the living stock.

Fly ash in addition to carbon fibres and SiO2 also has oxides con-taining other metals. The mixed oxide appears to have melted andresolidified forming spheres with entrapped air (particle marked Bin Fig. 2). Silica appears in primatic particles or retains features ofepidermal layer. Similar morphology has been observed in otherbagasse fly ash powders [6].

SEM and TEM studies show that both SiO2 and carbon fibre havea fine structure, the latter having a finer one. This could be associ-ated with pore size of 30 Å observed in adsorption studies. The un-burned carbon has significant surface area in the range of hundredsof square meters per gram and micropores with pore size maximaaround 9–12 Å. This is similar to bagasse char obtained in otherstudies [20]. It has also been reported that chars generated atlow heating rates have mainly micropores whereas chars whichundergo melting have mainly macropores [11]. The unburned car-bon in this case has retained the original structure and thus hasmainly micropores.

The high carbon content in the floated fraction makes it suitablefor use as fuel after briquetting [11]. It would also be worthwhile tocheck the reactivity of pelletised unburned carbon for use as gas-ifier feed material. The high surface area with micropores makesit attractive as a precursor for activated carbon. In some mills, inan effort to improve boiler efficiency, the high carbon containingfly ash is recirculated. If the fly ash is sieved, the high carbon con-taining coarse fraction can be separated and recirculated in theboilers.

Industrially, the carbon separation can be carried out usingindustrial scale sieve shakers for removal of the coarse carbon richfraction. The separation by floatation can be scaled up by elutria-tion and the water consumption can be reduced by recycling thewater. The main component in bagasse fly ash is silica in the formof quartz and no water soluble materials are expected. Therefore,separation by water is not anticipated to be a problem. Due to thisproperty of bagasse fly ash, many studies have also used it as anadsorbent for removal of heavy metals and dyes from water.

The unburned carbon recovery by sieving is around 60%. It is ex-pected that floatation in a continuous mode in an industrial scaleusing techniques like elutriation will yield a higher recovery. Theamount of bagasse generated in India is around 68 million tonnesper annum. Assuming an ash content of 4% in bagasse and un-burned carbon content of 20% in fly ash, with a recovery of 60%,the amount of carbon that can be obtained is 0.3 million tonnesper year. This is almost ten times the demand for activated carbon(0.04 million tonnes per annum). For a single mill generatingaround 0.27 million tonnes per annum of bagasse, the carbon thatcan be recovered is more than 1000 tonnes per annum. The acti-vated carbon manufacturers in India have capacities of around 60tonnes per annum and a sugar mill can supply to 10–12 manufac-turers. Thus, the carbon that can be recovered is sufficient for mul-tiple applications and therefore, the separation of unburned carbonusing simple techniques like sieving or elutriation can lead to eco-nomic benefits.

5. Conclusions

This study shows that it is straightforward to separate un-burned carbon from bagasse fly ash using floating or sieving meth-ods. The unburned carbon consists of pyrolysed fibres, with surfaceareas of a few hundreds of square meters per gram. The unburnedcarbon has a combination of meso and micropores. These proper-ties of unburned carbon make it suitable as a low cost precursorfor the production of activated carbon by means of steam activa-tion. The high carbon content makes them suitable for use ashousehold fuel or gasifier feed after briquetting or pelletising.Industrially, the separation can be carried out by sieving or

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elutriation. The amount of unburned carbon that can be separatedfrom Indian sugar mills is sufficient for multiple applications.

Acknowledgements

The Swedish Research Council is acknowledged for funding theproject as part of which this work was carried out. The authors alsothank Dr. Kjell Jansson for assistance with the SEM studies.

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