ARTICLE IN PRESS

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doi:10.1016/j.bi

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Biomass and Bioenergy 30 (2006) 144–152

www.elsevier.com/locate/biombioe

Development of activated carbon pore structure via physical andchemical activation of biomass fibre waste

Paul T. Williams�, Anton R. Reed

Energy & Resources Research Institute, Houldsworth Building, University of Leeds, Leeds LS2 9JT, UK

Received 10 December 2004; received in revised form 15 November 2005; accepted 15 November 2005

Available online 27 December 2005

Abstract

Biomass waste in the form of biomass flax fibre, produced as a by-product of the textile industry was processed via both physical and

chemical activation to produce activated carbons. The surface area of the physically activated carbons were up to 840m2 g�1 and the

carbons were of mesoporous structure. Chemical activation using zinc chloride produced high surface area activated carbons up to

2400m2 g�1 and the pore size distribution was mainly microporous. However, the process conditions of temperature and zinc chloride

concentration could be used to manipulate the surface area and porosity of the carbons to produce microporous, mesoporous and mixed

microporous/mesoporous activated carbons. The physically activated carbons were found to be a mixture of Type I and Type IV carbons

and the chemically activated carbons were found to be mainly Type I carbons. The development of surface morphology of physically and

chemically activated carbons observed via scanning electron microscopy showed that physical activation produced activated carbons

with a nodular and pitted surface morphology whereas activated carbons produced through chemical activation had a smooth surface

morphology. Transmission electron microscopy analysis could identify mesopore structures in the physically activated carbon and

microporous structures in the chemically activated carbons.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Pyrolysis; Activated carbon; Waste disposal

1. Introduction

Biomass wastes have an enormous potential for therecovery of energy and valuable materials. Processingbiomass wastes has the added advantage that the waste,which may be derived from an industrial or agriculturalprocess, has to be treated and disposed of with consequentadded costs. Therefore, if treatment routes can beidentified, which produce useful end products from thatwaste, the economic input from the end products wouldoff-set the costs of treatment and disposal. The textileindustry uses a variety of materials derived from naturalbiomass fibres in the production of a wide range ofcommercial, industrial and engineering products. Forexample, natural fibres are used in such applications asthe production of yarns, sheeting, twine, industrial andmarine rope, nets, sailcloth and canvas [1]. The natural

e front matter r 2005 Elsevier Ltd. All rights reserved.

ombioe.2005.11.006

ing author. Tel.: +441133432504; fax; +441132467310.

ess: p.t.williams@leeds.ac.uk (P.T. Williams).

fibres are obtained by removal from different parts of theplant. However, the commercial process involves highwastage rates of the natural fibre, often exceeding 50wt%,and thereby generating large tonnages of such wastesthroughout the world. Flax is a typical example of anatural fibre with an increasing use due to the higherstrength of the fibres. The plant is also grown for its seed,which is used to produce linseed oil. There is increasing useof flax throughout the European Union since it has lessenvironmental impact than the production of cotton, themain alternative, and that there is a general move awayfrom man-made fibres back to natural fibres. A particularadvantage of textile waste is that the biomass material hasa fibrous characteristic enabling the fibres to be processedvia entanglement, layering and needling to produce amatting material. To realise the potential of such a processthe final product activated carbon matting should havesurface areas comparable with those produced fromconventional sources, but also retain the strength andflexibility characteristics of the original biomass non-woven

ARTICLE IN PRESSP.T. Williams, A.R. Reed / Biomass and Bioenergy 30 (2006) 144–152 145

matting. Such a matting then has the potential to be useddirectly in pollution abatement applications rather thanhaving to be further processed.

Activated carbons are used extensively in industrialpurification and chemical recovery operations. They areparticularly advantageous because of their high internalsurface areas and active surfaces. In general, higher surfaceareas result in higher adsorption capacities. In addition, thepore characteristics of activated carbons are important indetermining the particular application of the carbon.Activated carbons may be produced through physical orchemical activation. For example, physical activation viapyrolysis to produce a char followed by steam gasification,alternatively, the biomass may be treated with a chemical,for example, zinc chloride followed by thermal treatmentvia pyrolysis to produce an activated carbon. Physical orchemical activation of the non-woven matting undercareful process conditions can produce a non-wovenmatting composed of activated carbon.

In this paper, flax waste material has been manufacturedinto a non-woven, pre-formed matting material viaentanglement, layering and needling and either physicallyor chemically activated under a variety of process condi-tions to produce activated carbons. The surface area,nitrogen adsorption and hence porosity characteristics ofthe activated carbons have been determined in relation toprocess conditions. In addition, both scanning electronmicroscopy (SEM) and transmission electron microscopy(TEM) have been used to visually observe the differences inthe development of the porosities of the derived activatedcarbons.

Thermocouples

Furnace

Nitrogencarrier gas

2. Materials and methods

2.1. Biomass waste

Table 1 shows the characteristics of the flax biomass fibrewaste. The flax had a high cellulose and hemicellulosecontent and low lignin content. Typical of biomass, theoxygen content and volatile contents were high and ashcontent low. The production of a non-woven matting fromthe fibrous flax waste involved a sequential processconsisting of a dry laid carded method, needle punchbonding and calender bonding. Dry laid carding involves

Table 1

Characteristics of the flax biomass fibre waste

Property Flax (wt%) Property Flax (wt%)

Carbon 43.3 Moisture 7.4

Hydrogen 6.5 Ash 1.8

Oxygen 50.2 Volatiles 75.2

Fixed carbon 15.6

Cellulose 56.5

Hemicellulose 15.4 Calorific value 17.2

Lignin 2.5 (MJkg�1)

the use of rotating cylinders covered in wires or teeth thatarrange the fibres into parallel arrays. Fibre lengths of upto 150mm are used and a uniform thickness of 8mm wasproduced. Needle punch bonding gives extra strength tothe non-woven matting and involves the punching ofbarbed needles through the material, hooking tufts of fibrethrough it and adding to the stability of the mattingmaterial. The non-woven matting is pulled out by drawrolls. The final process step of calender bonding involvesstabilisation of the non-woven matting via heat andpressure where the fibres tend to fuse together producinga more stable structure. The final product was a roll ofbiomass flax fibre matting, approximately 1m in width, 5min length and 5mm in thickness.

2.2. Physical processing of textile waste

Physical processing of the flax non-woven mattinginvolved a two stage process of pyrolysis and gasification.Pyrolysis consisted of slow heating in a stainless steel, fixedbed batch pyrolysis unit (Fig. 1). The reactor wasconstructed of stainless steel of 65mm diameter and200mm length, and was heated by an electrical ringfurnace. The reactor was continuously purged withnitrogen at a fixed metered flow rate. A sample of 50 gwas placed on a support in the centre of the hot zone of thereactor and heated at a controlled heating rate of2 1Cmin�1 to the final temperature and held at thattemperature for 2 h. The condensable liquid product wascondensed in a series of stainless steel and glass condensersto trap the liquid phase, followed by a water bubble trap.The residual char was removed from the reactor at the endof each experiment, weighed and stored under nitrogenprior to further analysis and activation. Non-condensablegases were sampled using gas syringes throughout theexperiments and were analysed off-line by packed column

CO2/acetonecondensers

Bubbletrap

Gas samplingpoint

Biomass orchemicallyactivatedbiomass

Fig. 1. Schematic diagram of the pyrolysis reactor.

ARTICLE IN PRESSP.T. Williams, A.R. Reed / Biomass and Bioenergy 30 (2006) 144–152146

gas chromatography and used for the determination ofmass closure, as reported earlier [2,3].

The char derived from the pyrolysis of the flax fibre non-woven matting was activated using the pyrolysis reactor,but modified for the activation experiments (Fig. 2). Theincoming activating agent was steam/nitrogen. The reactorsystem was therefore of static-bed batch design. Prior toactivation the char was dried in an oven at 105 1C. A 5 gsample of the pyrolysis char was placed in the reactor andnitrogen was passed through the unit in order to purge airfrom the system. The sample was heated at 4 1Cmin�1 tothe final temperature of 800 1C. After the sample tempera-ture had stabilised, steam was introduced to the nitrogengas flow, mixing in the distributor tube before entering thereactor. The steam was generated via injection of deionisedwater into a stainless steel reactor tube containing stainlesssteel packing material and heated to 200 1C. The activatingagent/nitrogen ratio was 25%:75% and the activatingagent molar flow rate per unit weight of char was0.02678mol g�1 h�1.

The degree of carbon loss or burn-off (BO) wascalculated from

Burn-off ¼ ðw1 � w2Þ=w� 100wt%ðdafÞ,

where w1 is the initial char mass on a dry, ash free (daf)basis, g, w2 the mass of char after activation, daf basis, g.

2.3. Chemical processing of flax biomass fibre

Chemical processing of the flax to produce activatedcarbons involved a two-stage process of dehydrationfollowed by pyrolysis. Chemical activation of the flax andhemp non-woven matting was carried out using zincchloride. There are a number of different chemicals which

Thermocouples

Furnace

Char

Steam generatingfurnace

Syringe waterpump

Bubble trap

Gas samplingpoint

N2

Fig. 2. Schematic diagram of the char activation reactor.

have been used either commercially or in bench scalestudies and include, in addition to zinc chloride, potassiumhydroxide, phosphoric acid, ammonium chloride, hydro-chloric acid, sodium hydroxide, etc. [4–7]. The chemicalactivating agents act by dehydration of the sample duringthe chemical treatment stage and inhibit the formation oftar and volatiles and thereby increase the char yield duringthe second, pyrolysis stage.The flax fibre, non-woven matting was cut to rectangular

pieces of 50 g mass and mixed and gently kneaded with theactivating agent for several hours to ensure completeadsorption of the zinc chloride solution by the biomasssample. Zinc chloride solutions were used at impregnationratios of 0.2:1, 1:1 and 2:1 of weight of zinc chloride:weight of biomass (referred to as 20, 100 and 200wt%loading in the discussion). The sample was then dried in amoisture oven at 105 1C. The concentrations of thechemicals used for activation are typical of those used forthe commercial production of activated carbons bychemical activation. The method used was adapted fromprevious reported methods [8–10]. The treated flax fibre,non-woven matting samples were then pyrolysed in thestatic batch reactor. The reactor was continuously purgedwith nitrogen and was heated at a controlled heating rateof 2 1Cmin�1 to the final temperature of either 450, 550 or650 1C. After cooling in nitrogen overnight, the productcarbons were removed from the reactor at the end of eachexperiment and weighed. The carbons were thoroughlywashed with solutions of hydrochloric acid (0.5N), hotwater and finally cold distilled water to remove anyresidual organic and mineral matter. The activated carbonsamples were then dried at 105 1C prior to analysis.

2.4. Porosity and surface area determination

The surface area and pore size distribution of theproduct activated carbons was determined using a Micro-meritics ASAP 2000 system. The system determinesthe volumetric adsorption and desorption of nitrogen bythe activated carbons. The system provides data for thedetermination of the monolayer adsorbed amount, appar-ent specific surface area, pore volume and pore sizedistribution. The activated carbons, of sample massbetween 0.04 and 1.0 g were outgassed for 5 h undervacuum at a temperature of 300 1C prior to analysis toremove any adsorbed species. The adsorption and deso-rption of nitrogen under different partial pressures ofnitrogen was then determined. The calculation of surfacearea, microporosity, mesoporosity and pore volume werecarried out according to standard methodologies [11].

2.5. Electron microscopy

The surface characteristics of the activated carbons wereanalysed using a Cambridge S360 SEM. The activatedcarbon fibres were mounted on an aluminium stub andcoated with a thin layer of gold. The Cambridge S360

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550C -800°C

200

400

600

V (

cc/g

)

41.8% Burn Off58.7% Burn Off73.1% Burn Off

P.T. Williams, A.R. Reed / Biomass and Bioenergy 30 (2006) 144–152 147

microscope used an accelerating voltage of 10 kV at aworking distance of 18mm. The activated carbons werealso examined using TEM. A Philips CM200 FEGTEMwas used to attain high magnification brightfield images ofthe activated carbon samples. The instrument was operatedat an accelerating voltage of 200 kV. The samples wereground to a fine powder and mounted on a thin carbon filmsupported by a copper grid.

00 0.2 0.4 0.6 0.8 1

P/Po

Fig. 4. Nitrogen adsorption–desorption isotherms for physically activated

carbon 550 1C char in relation to carbon burn-off.

650C –800°C

0

200

400

600

0 0.2 0.4 0.6 0.8 1P/Po

56.1% Burn Off64.8% Burn Off72.5% Burn Off

V (c

c/g

)

Fig. 5. Nitrogen adsorption–desorption isotherms for physically activated

carbon 650 1C char in relation to carbon burn-off.

3. Results and discussion

3.1. Physically activated carbons

3.1.1. Pore structure and surface area of the physically

activated carbons

The flax chars produced at the pyrolysis temperatures of450, 550 and 650 1C were activated using steam gasificationat 800 1C. The chars generated at each pyrolysis tempera-ture were subject to various increasing degrees of BO whereincreasing BO was achieved by increasing the reaction timeat the activation temperature. The analysis of the nitrogenadsorption and desoprtion of the product activatedcarbons are shown in Figs. 3–5. An assessment of thecharacteristics of the activated carbons can be determinedfrom the adsorption and desorption isotherms produced.These can be compared to the standard classification ofisotherms [12]. Type I isotherms occur due to enhancedadsorbent–adsorbate interactions in micropores of mole-cular dimensions. The narrow range of relative pressureneeded to achieve the plateau gives an indication into thelimited pore size range. Type II isotherm indicates theformation of an adsorbed layer whose thickness continu-ally increases with increasing relative pressure. These areobtained with non-porous or macroporous adsorbents withthe complete reversal of the isotherms being possible. TheType III isotherm is indicative of weak adsorbent–adsor-bate interactions. Type IV isotherms are closely related tothe Type II but exhibits a hysteresis loop, which is usuallyassociated with the filling and emptying of mesopores bycapillary condensation. A Type V isotherm like the Type

450C-800°C

0

200

400

600

0 0.2 0.4 0.6 0.8 1

P/Po

57.4% Burn Off71% Burn Off68% Burn Off

V (

cc/g

)

Fig. 3. Nitrogen adsorption–desorption isotherms for physically activated

carbon 450 1C char in relation to carbon burn-off.

III demonstrates weak adsorbate–adsorbent interactions,but with the hysteresis loop there is a mechanism of porefilling and emptying. Type VI isotherms are associated withlayer-by-layer adsorption on a highly uniform surface, theshape being system and temperature dependent.The nitrogen adsorption and desorption isotherms for

the activated carbons shown in Figs. 3–5 for differentdegrees of carbon BO, suggests that the physicallyactivated carbons produced adsoprtion–desorption iso-therms characteristic of Type I and Type IV isotherms. Theisotherms range in general from Type I isotherms at lowdegrees of carbon BO to Type IV isotherms as thepercentage of BO is increased. The physically activatedcarbons demonstrated a microporous structure at lowdegrees of carbon BO but developed a mesoporousstructure at higher degrees of BO due to pore widening.The isotherms may also be analysed in terms of their

adsoprtion–desorption hysteresis loops [12]. Hysteresisloops appear in the multi-layer region of the physisorptionisotherm and are considered to be associated with capillarycondensation. Most solids give reproducible hysteresisimplying the existence of a well-defined metastable state.

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Fig. 6. Scanning electron micrograph of physically activated carbon.

Fig. 7. Scanning electron micrograph of physically activated carbon.

P.T. Williams, A.R. Reed / Biomass and Bioenergy 30 (2006) 144–152148

The standard classification [12] contains four general types,H1–H4. Type H1 hysteresis loops are given by adsorbentswith a narrow distribution of uniform pores (open-endedtubular pores). Type H2 hysteresis loops are caused bycomplex pore structures, which are characterised byinterconnected networks of pores of different size andshape. Type H3 hysteresis loops are usually given byaggregates of platey particles or adsorbents containing slitshaped pores. Type H4 hysteresis loops are also formed byslit shaped pores and are characteristic of activatedcarbons. The hysteresis loops of H3 and H4 shape oftendo not close until the pressure is at or very close to thesaturation pressure.

The style of hysteresis loops found for the activatedcarbons produced by physical activation and shown inFigs. 3–5 are characteristic of mesoporous-activatedcarbons. In addition, the hysteresis loops indicate thatthe pores are slit shaped or that the carbons are made up ofplate-like material, perhaps graphine layers between whichare the pores.

The chars generated at 450, 550 and 650 were all heatedto 800 1C prior to the introduction of steam for theactivation stage of physical activation. This gasification oractivation stage of the physical activation process involvedheating each of the char samples to 800 1C in nitrogen priorto steam activation. This preliminary heating processwould serve to further pyrolyse the char, consequently,the influence of the original biomass pyrolysis temperaturewhich produced the char would not be expected to besignificant. However, there were small but significantdifferences between the adsorption–desorption hysteresisloops for the activated carbons produced from charsgenerated at different pyrolysis temperatures (Figs. 3–5).

Analysis of the physically activated carbons for theirsurface areas showed that the maximum surface areaachieved, at the maximum degree of carbon BO was840m2 g�1 for the physically activated carbons. The surfaceareas of the activated carbons produced after steamactivation where very much higher than the char precursorproduced from biomass pyrolysis. The surface areas of thechars produced in the first stage of physical activation viapyrolysis of the biomass were 5, 13 and 28m2 g�1 for thechars produced at 450, 550 and 650 1C, respectively.

3.1.2. Electron microscopic examination of the physically

activated carbons

The activated carbons were examined by SEM andTEM. TEM can produce significantly higher magnifica-tions than SEM. SEM micrographs of the physicallyactivated carbons are shown in Figs. 6 and 7. Fig. 8 showsa TEM of the physically activated carbons. Fig. 6 showsthat the surface characteristics of the original biomass fibreare not significantly altered by the process of physicalactivation. Fig. 7 represents a higher magnification imageof the activated carbon and demonstrates a cracked andpitted surface morphology. For the characterisation ofactivated carbons with a high degree of porosity created by

an internal pore structure, it is often useful to be able tocharacterise the carbon in relation to its pore sizedistribution; micropores, of pore width 2 nm or less,mesopores, of pore width 2–50 nm and macropores, ofpore width greater than 50 nm. This classification has beenadopted by the International Union of Pure and AppliedChemistry [12]. This pore width refers to the distancebetween the walls of slit-shaped pores or the radius ofcylindrical-shaped pores. The differentiation between themicro and mesopore range is based on the behaviour ofadsorbate molecules within the pores. The mesopores canbe defined by the hysteresis loops during adsorption anddesorption at relatively high relative pressures. The divisionbetween mesopores and macropores represent the practicallimiting width for the analysis of pore size distributions by

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Fig. 8. Transmission electron micrograph of physically activated carbon.

0

50

100

150

200

250

300

0 0.2 0.4 0.6 0.8 1

P/Po

V (c

c/g

)

450°C550°C650°C

ZnCl2 20%

Fig. 9. Nitrogen adsorption–desorption isotherms for chemically acti-

vated carbon, 20% zinc chloride in relation to temperature.

0

200

400

600

0 0.2 0.4 0.6 0.8 1P/Po

450°C550°C650°C

V (c

c/g

)

ZnCl2100%

Fig. 10. Nitrogen adsorption–desorption isotherms for chemically acti-

vated carbon, 100% zinc chloride in relation to temperature.

0

150

300

450

600

750

900

0 0.2 0.4 0.6 0.8 1P/Po

ZnCl2200%

V (c

c/g

)

450°C550°C650°C

Fig. 11. Nitrogen adsorption–desorption isotherms for chemically acti-

vated carbon, 200% zinc chloride in relation to temperature.

P.T. Williams, A.R. Reed / Biomass and Bioenergy 30 (2006) 144–152 149

isotherm analysis. Nitrogen adsorption isotherms of theactivated carbons derived from physical activation showedthat they were mainly mesoporous (Figs. 3–5). The smallcracks visible in Fig. 7 are in the range to view mesopores.The mesoporosity is developed from the micropores as thedegree of carbon BO, which occurs during the physicalactivation gasification phase, results in loss of carbon. Thisloss of carbon results in pore widening and the develop-ment of mesoporosity.

Fig. 8 represents a TEM image of the flax physicallyactivated carbons. The TEM micrograph of Fig. 8 is at amuch higher magnification than the SEM micrographs ofFigs. 6 and 7. Fig. 8 shows that mesoporosity is clearlydeveloped and imaged. The mesopores are mainly between5 and 20 nm in size and demonstrate the extent to whichthese pores make up the surface of the carbon. The poresare rounded and not necessarily the slit shaped pores thathave been assigned to activated carbons.

3.2. Chemically activated carbons

3.2.1. Pore structure and surface area of the chemically

activated carbons

The activated carbons produced from the chemicallyactivated flax, in relation to the concentration of zincchloride were analysed by nitrogen adsorption to determinetheir porosity and surface area characteristics. Figs. 9–11show the nitrogen adsorption–desorption isotherms for thechemically activated zinc chloride carbons produced withzinc chloride concentrations from 20%, 100% and 200%concentration, respectively. The chemically activated car-bons produced Type I isotherms only. The adsorption–de-sorption hysteresis loops for the chemically activated

carbons were small but indicate the presence of slit shapedpores or the presence of plate-like material.The surface area analysis of the chemically activated

carbons were significantly higher than the physicallyactivated carbons. For the activated carbon produced with20% zinc chloride, the maximum surface area of

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Fig. 13. Scanning electron micrograph of chemically activated carbon.

P.T. Williams, A.R. Reed / Biomass and Bioenergy 30 (2006) 144–152150

700m�2 g�1, was obtained at 550 1C final pyrolysistemperature. At 100% zinc chloride addition, the max-imum surface area was obtained at the final pyrolysistemperature of 450 1C and was 1600m2 g�1. At 200% zincchloride, the maximum surface area was obtained at 450 1Cpyrolysis temperature and was 2400m2 g�1. Such highsurface areas are comparable commercially producedactivated carbons which typically range from 500 to2000m2 g�1 [11,13].

3.2.2. Electron microscopic examination of the chemically

activated carbons

Figs. 12 and 13 show the SEM images of chemicallyactivated carbon and Fig. 14 a TEM image of chemicallyactivated carbon derived from flax. Fig. 12 shows that thechemical activation process does not significantly affect thesurface morphology of the original biomass. Fig. 13 showsa higher magnification of the surface using SEM. Thesurface is smooth and distinctly different from the pittedand cracked surface of the physically activated carbons.Even at higher magnifications, the surface of the chemicallyactivated carbon was smooth. The nitrogen adsorptiondata, shown in Figs. 9–11 derived from chemicallyactivation showed that the activated carbons producedwere mainly microporous. The pores would not be visibleat the magnification of the SEM, since micropores arecharacterised in the region of equal to or less than 2 nm.

TEM enables much higher magnification of the productactivated carbons to be carried out. Fig. 14 shows the TEMof the chemically activated carbon derived from flax. TheTEM image is distinctly different from the physicallyactivated carbon image shown in Fig. 8, where there wasclear mesoporosity visible. The predominant microporosityof the chemically activated carbon gives a homogeneousmorphology viewed through the transmission electronmicroscope.

Fig. 12. Scanning electron micrograph of chemically activated carbon.

Fig. 14. Transmission electron micrograph of chemically activated

carbon.

The different pore structures of the activated carbonsderived from either physical or chemical activation aredependent on the process conditions and different mechan-isms of reaction which produce the activated carbons. Theproduction of the surface area and porosity characteristicsof an activated carbon produced by physical activation areinfluenced by the nature of the original biomass feedstockand the process conditions of pyrolysis and activation. Thepyrolysis process involves the initial softening of thematerial and the release of volatile matter, then hardening

ARTICLE IN PRESSP.T. Williams, A.R. Reed / Biomass and Bioenergy 30 (2006) 144–152 151

and shrinkage of the char. During this initial stage, thepore structure can be influenced by the bubbling of gasesthrough the material. Feedstocks with a high volatilematter content tend to produce chars with a high porevolume and a lower proportion of micropores. In addition,the temperature of pyrolysis and the heating rate alsoinfluence the development of pores and surface area for thechars produced. The subsequent activation process willalso influence the final surface area and pore sizedistribution of the resultant activated carbons. It isgenerally agreed that, activation proceeds by microporeformation, followed by pore enlargement, as the steamreacts with the carbon, as has been found by other workers[14–16]. Various stages in the pore development process inrelation to the degree of BO have been suggested [17,18].These include the opening of previously inaccessible poresthrough the removal of tars and disorganised carbon,which is suggested to be complete after about 10wt% BO[18]. New pores are created by selective activation whichtakes place within the first 50wt% BO. Thereafter, thewidening of the developed porosity occurs. Wigmans [17]has also reported that micropore volume reaches amaximum at about 50–60wt% BO and then decreasesdue to the effect of pore widening. It has also been reportedthat the development of microporosity is also dependent onthe original precursor feedstock material [19]. In addition,it has been shown that the temperature of activation andthe composition of the activating agent all have asignificant influence on surface area of activated carbons[11,13,14].

The initial stages of chemical activation where theactivating agent, in this case, zinc chloride, is mixed withthe biomass, is believed to involve chemical attack of thecellulosic structure of the biomass [14]. As such, thecomposition of the biomass and the type and concentrationof the activating agent would influence the final productactivated carbon. During the second-stage pyrolysis adehydration reaction occurs where water is eliminated andcross-linking reactions of the carbon, hydrogen and oxygenin the biomass structure increase the aromatisation of thechar product. It is suggested that the cross linking inhibitsthe shrinkage of the structure during pyrolysis aiding in thecreation of the porous structure related to chemicallyactivated carbons [14]. Volatile products are bound into thecarbon structure by the cross linking reactions beforereaching pyrolysis temperatures. During the pyrolysisprocess the volatile and tar product yield is reduced witha consequent increase in char yield. The temperature ofpyrolysis, heating rate and residence time at the pyrolysistemperature are important process parameters in determin-ing the final surface area and pore size characteristics of theactivated carbon. The final process of water or acidwashing the carbon exposes the porous carbon structure.

The use of activated carbons for the remediation ofpolluted gas and liquid waste streams, gas storageapplications, etc., requires not only high surface areas tomaximise the capture of pollutants, but also the pore size

characteristics are significant in determining the size of thepolluting molecules that can be retained by the carbon. Forexample, if the activated carbon has a mainly microporousstructure, then large molecules cannot enter the internalsurface of the carbon and be adsorbed. The classification ofpore size in activated carbons is that micropores are of porewidtho2 nm, mesopores of pore width 2–50 nm andmacropores of pore width450 nm. For physical activationit was shown that higher degrees of BO (carbon loss due toreaction with steam) produced the higher surface areas.However, the BO represents a significant loss of the finalproduct activated carbon. For example, to produce thehigher surface areas resulted in losses of the originalbiomass of over 80wt%, that is, a final product mass of lessthan 20% of the original biomass [2]. Physical activation offlax was also shown to produce mainly microporouscarbons at lower degrees of BO and more mesoporouscarbons at higher degrees of BO. Corresponding to thedevelopment of the porosity and surface area of thecarbons by reaction of the carbon with the steam activatingagent widening the micropores to produce mesopores. Forchemical activation with zinc chloride, the results haveshown that a much higher mass of product activatedcarbon was produced, typically between 30 and 40wt% ofthe original biomass matting material [3]. In addition,significantly higher surface areas of the product activatedcarbon are produced, in some cases over 2400m2 g�1. Thepore size characteristics of the chemically activated carbonwere also influenced by the activation process, but weremainly microporous in nature.The original biomass material used for the production of

the activated carbons by either physical or chemicalactivation were fibrous biomass. The flax biomass materialwas woven to produce a biomass matting prior to theactivation process. The final product activated carbon wastherefore a non-woven activated carbon matting. Thephysically activated carbons became very brittle at thehigher degrees of carbon BO. However, the chemicallyactivated carbons retained their original non-wovenmorphology and retained their original flexibility.

4. Conclusions

Biomass waste in the form of fibrous flax textile wastehas been physically and chemically activated to produceactivated carbons. The surface area of the physicallyactivated carbons were up to 840m2 g�1 and the poreswere mainly mesoporous in structure. Chemical activationusing zinc chloride produced high surface area activatedcarbons up to 2400m2 g�1 and the pore size distributionwas mainly microporous. However, the process conditionsof physical or chemical activation could be used tomanipulate the surface area and porosity of the carbonsto produce microporous, mesoporous and mixed micro-porous/mesoporous activated carbons. The physicallyactivated carbons were found to be a mixture of Type Iand Type IV carbons and the chemically activated carbons

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were found to be mainly Type I carbons according to thestandard system of nomenclature. The development ofsurface morphology of physically and chemically activatedcarbons observed via SEM showed that physical activationproduced activated carbons with a nodular and pittedsurface morphology whereas activated carbons producedthrough chemical activation had a smooth surface mor-phology. High magnification TEM analysis of the activatedcarbons identified mesopore structures in the physicallyactivated carbon and microporous structures in thechemically activated carbons.

Acknowledgements

The authors gratefully acknowledge the support of theUK Engineering and Physical Sciences Research Council.

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