Thermochemical conversion of cashew nut shells, an extractives-rich biomass

Source Agritrop Cirad (agritrop.cirad.fr) International audience Introduction - Cashew nut shells (CNS) are non-edible agricultural residues produced by cashew nut shelling units, found abundantly in West Africa. The valorization of these shells is currently problematic due to the fact that they contain up to 40% of acidic extractives, called Cashew Nut Shell Liquid (CNSL). Consequently, CNS tend to accumulate near the grounds of the shelling units, contributing to a decline in both air and soil quality within the environment (Balmes, 2019; de la Sota et al., 2018). CNSL is mainly composed of anacardic acids, cardanols, cardols (ACC) and traces of 2-methylcardols (Gandhi et al., 2013). When converted through gasification, this high extractives content is the source of tars in the syngas, which is known to be problematic for useful energy production such as electricity or biofuels. This work is part of a development project relating to the energy recovery of cashew nut shells in West Africa via gasification processes. In this context, the challenge of the research carried out at CIRAD is the implementation of bioenergy processes adapted to this type of specific resources rich in organic extractives. The scientific objective of this study is the understanding of the behavior of CNSL during pyrolysis as well as thermal/catalytic cracking, and reforming that take place in a gasification process. For this, an experimental study supported by specific lab reactors and innovative analytical methods was carried out. Scientific innovation and relevance - This research delves into a better understanding on CNS and their conversion during different gasification stages, providing insights into the fundamental mechanisms and reaction kinetics involved. The main scientific innovation is related to the specificity of this biomass and the presence of significant quantities of extractives, for which limited attention was given to its thermochemical conversion. The application of numerous analytical methods on the identification of condensates (ACC content, tar, water, non-condensable gases), that allows quantification of up to 76% of condensates is a strong point of the study. Moreover, these works provide more general insights to extractives conversion in gasification processes. This study encompasses environmental sustainability, energy production and waste production. Despite the fact that CNS possess higher heating value (26MJ/kg) (Melzer et al., 2013) than wood, they are rarely used to produce energy. In Africa, small quantities of CNS are burned for heat or steam production, but the main portion is simply discarded. This research not only contributes to the academic understanding of this specific biomass conversion but also possesses substantial potential for practical applications in the renewable energy sector. Results and Conclusions - The results and conclusions of this study are presented in the Explanatory Pages.