Structural evolution of Eucalyptus tar pitch-based carbons during carbonization
Introduction
Brazil is one of the few countries where forest biomass has always played an important role as an energy source because the tropical climate favors forest growth. In 2000, about 6.5×106 tons of charcoal was consumed in Brazil, 88% of which were destined to the siderurgical industry. [1] Charcoal is produced by pyrolysis of wood at final temperatures on the order of about 400–500 °C. This treatment is followed by intense release of gases and volatiles generated by wood decomposition. Volatiles can be recovered by condensation, giving rise to a liquid called wood tar. This tar can be 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.
For 2000, 72% of the Brazilian charcoal production used planted Eucalyptus forests [1]. The use of charcoal produced from planted forests presents advantages when compared to the use of metallurgical coke: biomass is a renewable energy source; charcoal minimizes the environmental impact arising from the siderurgical activity because it does not affect the natural carbon cycle and gives rise to a comparatively low level of emission of toxic gases such as SO2, NO2 and NO [2].
However, the use of charcoal has faced some serious drawbacks. Due to the low price for charcoal and the relatively high costs for forestation, charcoal from planted forests may have an incompatible cost. In addition, the low wood tar aggregated value has discouraged its recovery, so that large amounts of volatiles have been released into the atmosphere. The way envisaged to overcome these problems is to develop nobler applications for wood tar, in order to increase its aggregated value and develop it as a revenue source for the charcoal manufacturing industry. In this way, it is possible to stimulate tar recovery at industry chimneys and balance forestation expenses and charcoal price.
Therefore, a new research line called “Vegetal Carbochemisty” was initiated in Brazil during the early 1990s. In this respect, Eucalyptus tar has been distilled aiming to separate fractions used as flavors, fragrances and source of fine chemical products [3], [4]. A heavier fraction, Eucalyptus tar pitch, is generated as a tar distillation residue (about 50% mass). Taking into account the high Brazilian potential for producing this kind of pitch and its lack of applications, our research group is developing works aiming to make possible the use of biopitches as precursors of advanced carbon materials such as isotropic carbon fibers, bioelectrodes and activated carbons.
Previous studies demonstrated that Eucalyptus tar pitches have a macromolecular structure constituted mainly of phenolic, guaiacylic, and siringylic units resulting from lignin degradation during wood pyrolysis. Remarkable features are their low aromaticities (60–70%) and high O/C ratios (0.20–0.27%), which lead them a high reactivity. In addition, they present large molar mass distribution, purely viscous and Newtonian behaviors, low ash contents (about 1%), and isotropic character [5], [6], [7].
In this work, structural changes occurring during carbonization of Eucalyptus tar pitch up to 2300 °C are investigated step-by-step. An improved understanding about this subject is important to make it possible to plan and control the production of hard carbon better and to improve the property of the end-products. The results presented are based mainly on X-rays diffraction and infrared analyses, besides weight loss, helium density, BET surface area, and BJH pore volume measurements.
Section snippets
Materials
A precursor Eucalyptus tar pitch was produced as follows. Wood chips of planted Eucalyptus forests (Minas Gerais, Brazil) were submitted to slow pyrolysis in industrial masonry ovens with a maximum pyrolysis temperature of about 500 °C (12–14 °C/h). The smokes were washed and condensed to produce Eucalyptus tar. This tar was vacuum distillated in a 3000-L boiler coupled to a fractionation column of four drilled plates. The cut temperature was 180 °C at 30–38 mmHg. The distillation time was 8 h and
Results and discussions
Some properties of the pitch used as the starting material for the carbonization studies (P8250) are listed in Table 1.
Conclusions
The whole of the results discussed in the present work shows that Eucalyptus tar pitch processing toward carbon materials can be basically divided in three steps:
(1) Up to around 600 °C the material presents a much disordered structure. The main event to take place in this temperature range is the release of aliphatic side chains and volatiles, which permits a gradual nearing of the aromatic rings and a consequent density increase. The surface area is very reduced due to the absence of micro and
Acknowledgements
The authors thank Fundação de Amparo a Pesquisa do Estado de Minas Gerais (FAPEMIG) for financial support and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for fellowship grant.
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