J.J. Rodríguez

Predicting heating values of lignocellulosics and carbonaceous materials from proximate analysis

A simple equation based on proximate analysis (volatile matter and fixed carbon contents) is presented which allows calculation of the higher heating value of lignocellulosics as well as the charcoals resulting from their carbonization. The equation has been tested with different lignocellulosic wastes and chars obtained from carbonization at different temperatures. Deviations from the experimental heating values fall in most cases below 2%. A comparison is presented with some other equations from the literature based on proximate, ultimate and chemical analysis data. As a general conclusion the equation proposed in this paper leads to comparable and in many cases more accurate predictions of heating values and has the advantage of being applicable to a wide range of carbonaceous materials, requiring only a simple, rapid and cheap proximate analysis of the samples.

Preparation and characterization of activated carbons from eucalyptus kraft lignin

Abstract Preparation of activated carbons from eucalyptus kraft lignin has been investigated. A pretreatment method has been established to avoid partial fusion and swelling in the carbonization stage. Carbonization has been studied at different temperatures and the structure of the microporous chars has been characterized. Activated carbons have been prepared from CO2 partial gasification of chars obtained at 823 and 1073 K. Both chars show a comparable behavior regarding to the evolution of porous structure. Activation increases both total and narrow microporosity and develops a substantial mesoporosity. At high burnoff levels, macroporosity becomes also significant. BET surface areas in the vicinity of 1,300–1,400 m2/g have been achieved at burnoff levels around 70–75% which correspond to overall carbon to lignin yields of about 10%–11% (d.a.f. basis).

On the kinetics of thermal decomposition of wood and wood components

Abstract A kinetic analysis of the thermal decomposition of eucalyptus sawdust, cellulose and eucalyptus kraft lignin has been accomplished from temperature-programmed reaction experiments, using two different approaches: the first assumes an overall reaction model, whereas in the second, pyrolysis is viewed as a process consisting of multiple reactions in parallel and a distribution of activation energies is derived. The results obtained reveal that the simple first approach, which leads to overall values of the apparent kinetic parameters allows a fairly good reproduction of the experimental curves, although in the case of wood a two-stage analysis was necessary. No improvement was noticed with the use of activation energy distribution functions resulting from the multiple reaction treatment, except for cellulose and lignin at high conversion, whereas the steep region of the weight-loss curve for cellulose is better described with the overall reaction model.

CO2 gasification of eucalyptus wood chars

Chars obtained from Eucalyptus grandis sawdust at different carbonization temperatures were gasified with CO2 in isothermal and non-isothermal t.g. experiments. At low and intermediate conversion values the reactivity can be reasonably well explained in terms of the development of surface area as gasification proceeds. At high conversion values a steep increase in reactivity is observed which can be attributed to the increasing catalytic effect of the metallic constituents (mainly Na and K) of the inorganic matter present in the chars. Activation energies in the range 230–261 kJ mol−1 are obtained.

Activated carbons from Uruguayan eucalyptus wood

Activated carbons were prepared from eucalyptus wood chars and the results of CO2, CO2-O2 and steam activation were compared. The carbonization step gave rise to a narrow micropore structure and a highly developed macroporosity which increased slightly upon CO2 activation and significantly upon steam activation. This last process led also to a widening of micropore size distribution and developed the mesoporosity more than CO2 activation did. The presence of O2 accompanying CO2 in the activating gas increased the micro- and macroporosity of the carbons. No net destruction of microporosity was observed even at high burnoff levels and with as much as 5 vol. % O2 in the activating gas.

High-temperature carbons from kraft lignin

High-temperature carbons have been prepared from kraft lignin on thermal treatment up to 3073 K. The structure of these high-temperature carbons has been studied by X-ray diffraction and Raman spectroscopy. The interlayer spacing obtained from XRD approaches the d002 value of graphite as the heat treatment temperature is increased. The (002) diffraction peak of the higher temperature carbons shows a modulated profile, which has been attributed to the presence of both graphitic or highly ordered carbon, and turbostratic or less ordered carbon. The average thickness of graphite-like crystallites evaluated from XRD increases with increasing heat treatment temperature. Raman spectra confirm the progressive structural ordering as treatment temperature increases. The E2g line-width decreases and its frequency shifts to a value close to the 1582 cm−1 of graphite. At the same time a substantial decrease in the intensity of the band in the 1350 cm−1 region can be observed, indicating a decreasing proportion of imperfect carbon. The evolution of the 2700 cm−1 line-width indicates a progressive onset of three-dimensional order as the heat treatment temperature increases. The O2-gasification X-T curves of the high-temperature carbons show an increasing oxidation resistance with increasing heat treatment temperature, in agreement with a higher structural ordering and a lower surface area. The inorganic impurities of the precursor (mainly Na) seem to enhance the onset of structural ordering, and the oxidation resistance of the 3073 K carbon prepared from high-ash lignin proved to be similar to that of graphite SP-1.

CO2-reactivity of eucalyptus kraft lignin chars

CO2-gasification of four chars prepared from slow pyrolysis of eucalyptus kraft lignin at temperatures ranging from 823 to 1673 K has been studied by isothermal thermogravimetric experiments. The reactivity versus conversion curves at temperatures between 1023 and 1223 K have been obtained. A substantial decrease in reactivity results from increasing the severity of pyrolysis conditions which can be attributed to a decrease of active site concentration for both catalysed and uncatalysed reaction. The inorganic impurities of the chars, primarily Na, showed an important catalytic effect at comparatively low Na/C atomic ratios, most probably due to a fine dispersion throughout the carbon matrix related to the origin of the starting material. Both catalysed and uncatalysed gasification can be well represented by an apparent activation energy value of 230 ± 15 kJ/mol.

Thermal decomposition of wood in oxidizing atmosphere. A kinetic study from non-isothermal TG experiments

The kinetics of thermal decomposition of four wood species in oxygen-bearing atmospheres of 5, 10 and 20% molar O2 concentrations have been studied from temperature-programmed experiments carried out at 5, 10 and 20 K min−1 heating rate. Devolatilization as well as combustion of the reamining solid have been considered to analyze the weight loss curves. The homogeneous volume reaction (VR) model has been used to describe devolatilization, whereas for solid combustion the grain model has been also checked. A two-stage approach has been used to fit the conversion-time curves and to derive the corresponding apparent kinetic parameters. The VR/VR (pyrolysis/combustion) combination provided a better description of the experimental α−t curves than the VR/grain combination. Holm oak and cork oak showed very close reactivities, whereas some differences were observed for aleppo pine and eucalyptus.

POWDERED ACTIVATED CARBONS FROM PINUS CARIBAEA SAWDUST

Activated carbons with different porous structures have been prepared from Pinus caribaea sawdust through the use of CO2 and steam as activating agents. The evolution of the Brunauer-Emmett-Teller (BET) surface area upon activation becomes fairly similar in both cases, and values above 1000 m2/g can be reached at high burn-off levels. Activation with steam produces a more developed porous structure, with a substantially higher contribution of mesoporosity, than does activation with CO2. Increasing the activation temperature leads in both cases to a wide pore-size distribution. The presence of a well-developed mesoporosity makes the resulting products good candidates for adsorbents for water and waste-water treatment. For these purposes, the characteristics of these activated carbons are comparable to those used as commercial adsorbents in those fields, showing in the case of the steam-activated carbons, a somewhat higher mesopore volume than many of the commercial products while maintaining similar surface area values to them. The relative simplicity of the process makes the production of steam-activated carbons a feasible and economically valuable alternative for sawmill wastes.

Activated Carbons from Eucalyptus Wood. Influence of the Carbonization Temperature

Abstract CO2 activation of eucalyptus wood chars obtained at 400, 600, and 800°C has been studied. The carbonization stage develops an incipient microporosity. The micropore volume and width increase with increasing temperature. Partial gasification of these chars with CO2 allows one to obtain activated carbons which show differences in micropore size distribution depending on the temperature at which the starting char was obtained. No significant effect on mesoporosity development was noticed whereas lower macropore volumes were obtained with activated carbons resulting from the char prepared at the highest temperature (800°C).