POTENTIALITIES OF BIOCHARS FROM DIFFERENT BIOMASSES FOR CLIMATE CHANGE ABATEMENT BY CARBON CAPTURE AND SOIL AMELIORATION

A. L. CUKIERMAN; P. BONELLI

Advances in Environmental Research

Nova Science Publishers, Inc

Lugar: Hauppauge; Año: 2015; p. 57 - 80

Incorporation of the enriched carbon solid product arising from biomass pyrolysis, generically known as biochar, into soil has been proposed and increasingly investigated as a means of abating climate change by carbon sequestration, simultaneously improving soil quality and protecting natural resources. Selection of a particular biochar for this strategy depends on the biomass and pyrolysis conditions used for production. In this direction, the present chapter explores the biochar generated from slow pyrolysis of some lignocellulosic biomasses to evaluate its potential addition into soil in an attempt to contribute to mitigate environmental impacts of climate change. The biomasses investigated comprise: sawdust and bark from poplar (Populus deltoide) wood, sawdust from a native hardwood (Aspidosperma q-b), and woody shells from hazelnut (Corylus avellana). Biochars are obtained from bench-scale pyrolysis experiments for the four biomasses operating at 850 °C and 1 h reaction time. Additional biochars are prepared from the shells at 350 °C and 600 °C, keeping the same reaction time, and from the hardwood at 350 °C and 3 h. Yields in the range 20-39 wt% are obtained depending on the severity of the pyrolysis conditions and the parent biomass. The highest yields correspond to the biochar obtained at the lower temperatures and to those derived from the shells, which possess the greatest lignin content among the biomasses. Pyrolytic behavior of the biomasses with the process course is also investigated by thermogravimetric analysis from ambient temperature up to 950 °C. The residual solid weight fractions at 950 °C are in line with the trend found for the yields. Chemical and textural properties of the biochar are determined by a set of complementary techniques, that include proximate and ultimate analyses as well as physical adsorption measurements of N2 (- 196 ºC) and CO2 (25 °C). Potentialities of the biochars for carbon capture and soil amelioration are comparatively examined considering different predictors based on their main properties. Among the predictors used to estimate biochar stability, the volatile matter content indicates that the biochars prepared from the shells and Populus wood sawdust at 850 °C with the lowest contents of volatiles will potentially show the highest stability. Based on other predictor which considers that biochars with O/C atomic ratios lower than 0.2 are expected to present a very prolonged half-life, all the samples (O/C ratios: 4x10-3 ?1.6x10-1) should be highly stable. A Van Krevelen diagram, i.e. representation of H/C vs O/C ratios, as well as the application of principal component analysis to all the chemical characteristics and biochars agree to show that the biochars may be assembled into two well distinguishable groups: one formed by the biochars obtained at 600 °C and 850 °C, with lower atomic ratios, and a second group that includes those produced from the shells and the hardwood at 350 °C. The biochars of the first group are expected to present a higher stability and a greater degree of aromaticity, which is also in direct relation to higher stabilities in soil. A correlation based on reported data is also developed to estimate carbon loss of biochar after 100 years in terms of the O/C ratio. Correlation predictions for the present biochars are in general consistent with the ones inferred from the Van Krevelen diagram. On the other hand, most biochars show low BET surface areas (~ 1?21 m2 g-1) with the exception of that derived from the shells at 600 °C, that exhibits a pronouncedly higher BET area (275 m2 g-1). Since biochar characteristics rendering beneficial impact on soil are related to large BET areas, only the latter biochar appears as an appropriate candidate. Besides, the shells-derived biochar at 850 °C is found to adsorb the greatest CO2 volume at equilibrium (46 cm3 g-1 at 100 MPa). Accordingly, the results point to the hazelnut shells as the most suitable biomass for the sustainable production of highly stable biochars with adequate properties, which might be conveniently tuned by varying the pyrolysis temperature, for CO2 sequestration and soil amelioration.