Using analytical pyrolysis and scanning electron microscopy to evaluate charcoal formation of four wood taxa from the caatinga of north-east Brazil

Abstract

• People in north-east Brazil mostly rely on fuelwood and charcoal for domestic energy consumption. Traditionally, four local wood taxa (Mimosa tenuiflora, Mimosa ophthalmocentra, Croton sonderianus and Cenostigma pyramidale) from the caatinga have been selected for this purpose. As the final quality of charcoal is directly related to the charring conditions, as well as the chemical composition of the raw materials and the behaviour of the lignocellulosic matrix during the charring process, the aim of this study was to investigate how far differences in these parameters exert a major control on the charcoal formation, in order to give a molecular insight as to why these particular wood species might be preferred for high-quality charcoal in the perspective of sustainable management and harvesting regimes. Pyrolysis gas chromatography mass spectrometry (Py-GC-MS) and scanning electron microscopy (SEM) were used to study the chemical composition and anatomical changes of the wood samples before and after charring experiments were carried out at 400 and 600 °C. The charring conditions were also reproduced in the controlled environment of the pyrolysis chamber and evolved gas analysis mass spectrometry (EGA-MS) was used for the first time to monitor the evolution of the thermal degradation products. The thermal behaviour of the wood components was different compared to other wood taxa for which comparable data are available. These differences were partially related to the particular nature of the carbohydrate-lignin interactions, to a relatively low level of lignin condensation, and to the substantial presence of gums or resins, which formed charred deposits. These characteristics, often neglected in charcoal analysis, imply that the wood components (especially cellulose) are released more slowly during the charring process, and this translates into the ability of these wood species to retain the main features of their anatomical structure even when exposed to high temperature for a relatively long time. The anatomical alterations observed, such as the loss of the middle lamella, the disappearance of the pit membranes, and the homogenisation of cell walls are ultimately in agreement with the formation of a particularly recalcitrant carbonaceous matrix upon charring at relatively low temperature (400 °C for 2 hours). This study presents the first molecular characterisation of these wood species and some methodological advancement on the use of EGA analysis using isothermal conditions is provided. The role of carbohydrate-lignin interactions and non-lignocellulosic wood components in charcoal production is discussed and the importance of combining chemical data and anatomical observations in wood research is underlined.