M.. Prauchner et al. Carbon 43(2005)591-597 mixtures [13], gas storage adsorbents [14], and water are peculiar features which make biopitches behave far fying and filtering ele 5-17 differently from fossil pitches [22, 23] In Brazil, a different type of pitch, derived from bio- The isotropic character and low carbon yield (<50%) mass, is generated in large scale as a by-product of the of eucalyptus tar pitches allows them to develop easily charcoal-making industry. In charcoal production, the microporosity during carbonization. Even without any volatiles released during wood pyrolysis can be recov- additional activation process, eucalyptus tar pitch-based ered by condensation to give rise to an oily liquid called carbons obtained at 800C have BET microporous sur- wood tar. This is separated by decanting to give rise to face areas over 200m/g [24]. Although these values are an aqueous fraction(so-called pyroligneous acid)and still low if compared to those typical for activated car an organic fraction (insoluble tar), which corresponds bons, in the range 700-2000m1g [27, 28]. the pores to around 35% and 7% of the initial mass of wood, formed during carbonization will certainly facilitate respectively. The insoluble tar can be distilled to sepa he diffusion and attack of oxidizing agents during a rate fractions used as flavors, fragrances and sources subsequent activation process [29]. In this context, and of fine chemical products [18, 19]. A heavier fraction, taking into account the current interest in the filamen the so-called wood tar pitch, is then obtained as a distil- tary form of activated carbons, which presents higher lation residue(about 50% in mass). adsorption and desorption rates and can be easily pro- t Most part of the Brazilian charcoal production uses cessed into sheets, felts and cloths, the forms preferred ood from planted eucalyptus forests [20]. This is an for manufacturing filtering elements, eucalyptus tar important and environmentally friendly activity because pitches have been studied as precursors of GPCf. The biomass is a renewable energy source which, on the con- present paper reports on pitch processing and the char- trary that fossil fuels, gives rise to a positive balance of acterization of the fibers obtained CO2 fixation and O2 release and to a comparatively low level of emission of toxic gases such as SO2, NO2 and No [21]. In turn, developing applications for wood tar 2. Experimental fractions is important as a means to stimulate tar recov- ering in the industrys chimneys, therefore preventing 2. 1. Raw materials the release of a large volume of pollutants into the atmo phere, and to aggregate revenue to the charcoal-making The precursor pitch used in the present work, the so- industry. In this context, our research group has devel- called crude pitch, was obtained by vacuum distillation oped work aiming to characterize eucalyptus tar pitch of eucalyptus tar in a pilot batch plant. Tar was ob- and investigate its potential uses [22-26 tained by condensation of the volatile released during Previous studies demonstrated that eucalyptus tar the slow pyrolysis (500C, 12-14 C/h)of eucalyptus pitch presents a macromolecular structure constituted wood in industrial masonry ovens. The distillation cut mainly of phenolic, guaiacyl, and siringyl units(Fig. 1) temperature was 180C at 30-38 mmHg and pitch yield resulting from lignin degradation during wood pyroly- was about 50%(w/w) sis. Their low aromaticity(60-70%), high O/C atomic In order to increase the softening point of the crude ratios(0.20-0.27%), and large molar mass distribution pitch(76 1C), about 400g was heat-treated in a 1000- mL Kettle vessel connected to a vigreaux column. Pitch homogenization was achieved through a mechanical H/OCH stirrer. Different temperatures and treatment times, in HaCO/H the range of 190-250C and 1-8h, respectively, were used [22, 2 2.2. Carbon fiber preparation (CH2)n The pitches were converted into the filamentary form H3 CO/H OCHvH by melt spinning. The melted pitches were extruded der nitrogen pressure through a circular-shaped spin- ning nozzle (diameter =0. 5mm; length over diameter (L/D)=4.4)using a laboratory-scale monofilament apparatus. After being condensed by cooling, the fibers were wound onto a winding bobbin. This step involved adjusting the spinning temperature, pressure and rate The as-spun fibers, also called green fibers, were stabi- ig. 1. Model structure illustrating the main functional groups lized by oxidative thermal treatments. For that, the presents in eucalyptus tar pitches fibers were cut into M6-in. bundles weighing 1.5-2gmixtures [13], gas storage adsorbents [14], and water purifying and filtering elements [15–17]. In Brazil, a different type of pitch, derived from bio￾mass, is generated in large scale as a by-product of the charcoal-making industry. In charcoal production, the volatiles released during wood pyrolysis can be recov￾ered by condensation to give rise to an oily liquid called wood tar. This is separated by decanting to give rise to an aqueous fraction (so-called pyroligneous acid) and an organic fraction (insoluble tar), which corresponds to around 35% and 7% of the initial mass of wood, respectively. The insoluble tar can be distilled to sepa￾rate fractions used as flavors, fragrances and sources of fine chemical products [18,19]. A heavier fraction, the so-called wood tar pitch, is then obtained as a distil￾lation residue (about 50% in mass). Most part of the Brazilian charcoal production uses wood from planted eucalyptus forests [20]. This is an important and environmentally friendly activity because biomass is a renewable energy source which, on the con￾trary that fossil fuels, gives rise to a positive balance of CO2 fixation and O2 release and to a comparatively low level of emission of toxic gases such as SO2, NO2 and NO [21]. In turn, developing applications for wood tar fractions is important as a means to stimulate tar recov￾ering in the industry
s chimneys, therefore preventing the release of a large volume of pollutants into the atmo￾sphere, and to aggregate revenue to the charcoal-making industry. In this context, our research group has devel￾oped work aiming to characterize eucalyptus tar pitch and investigate its potential uses [22–26]. Previous studies demonstrated that eucalyptus tar pitch presents a macromolecular structure constituted mainly of phenolic, guaiacyl, and siringyl units (Fig. 1) resulting from lignin degradation during wood pyroly￾sis. Their low aromaticity (60–70%), high O/C atomic ratios (0.20–0.27%), and large molar mass distribution are peculiar features which make biopitches behave far differently from fossil pitches [22,23]. The isotropic character and low carbon yield (<50%) of eucalyptus tar pitches allows them to develop easily microporosity during carbonization. Even without any additional activation process, eucalyptus tar pitch-based carbons obtained at 800C have BET microporous sur￾face areas over 200m2 /g [24]. Although these values are still low if compared to those typical for activated car￾bons, in the range 700–2000m2 /g [27,28], the pores formed during carbonization will certainly facilitate the diffusion and attack of oxidizing agents during a subsequent activation process [29]. In this context, and taking into account the current interest in the filamen￾tary form of activated carbons, which presents higher adsorption and desorption rates and can be easily pro￾cessed into sheets, felts and cloths, the forms preferred for manufacturing filtering elements, eucalyptus tar pitches have been studied as precursors of GPCF. The present paper reports on pitch processing and the char￾acterization of the fibers obtained. 2. Experimental 2.1. Raw materials The precursor pitch used in the present work, the so￾called crude pitch, was obtained by vacuum distillation of eucalyptus tar in a pilot batch plant. Tar was ob￾tained by condensation of the volatile released during the slow pyrolysis (500C; 12–14C/h) of eucalyptus wood in industrial masonry ovens. The distillation cut temperature was 180C at 30–38mmHg and pitch yield was about 50% (w/w). In order to increase the softening point of the crude pitch (76.1C), about 400 g was heat-treated in a 1000- mL Kettle vessel connected to a vigreaux column. Pitch homogenization was achieved through a mechanical stirrer. Different temperatures and treatment times, in the range of 190–250C and 1–8 h, respectively, were used [22,23]. 2.2. Carbon fiber preparation The pitches were converted into the filamentary form by melt spinning. The melted pitches were extruded un￾der nitrogen pressure through a circular-shaped spin￾ning nozzle (diameter = 0.5mm; length over diameter (L/D) = 4.4) using a laboratory-scale monofilament apparatus. After being condensed by cooling, the fibers were wound onto a winding bobbin. This step involved adjusting the spinning temperature, pressure and rate. The as-spun fibers, also called green fibers, were stabi￾lized by oxidative thermal treatments. For that, the fibers were cut into 6-in. bundles weighing 1.5–2 g. OH H3CO/H O H/OCH3 (CH2)n (CH2)n (CH2)n O H3CO/H OCH3/H O H n = 0, 1, 2, 3 H (CH2)n Fig. 1. Model structure illustrating the main functional groups presents in eucalyptus tar pitches. 592 M.J. Prauchner et al. / Carbon 43 (2005) 591–597