Energy recovery from macadamia husk via pyrolysis: a bibliometric review using the ProKnow-C method

Abstract

The growing demand for renewable energy sources drives the search for efficient methods to valorize biomass waste, such as macadamia shell, an agro-industrial byproduct with underutilized energy potential. This study aimed to systematically compare pyrolysis and hydrothermal carbonization (HTC) as thermochemical conversion routes for macadamia shell, evaluating the properties of the solid products and their potential as biofuels. For this purpose, a bibliographic review methodology based on the ProKnow-C method was employed, allowing for the selection and in-depth analysis of a portfolio of ten relevant scientific articles.

The analysis results revealed that, although pyrolysis is effective in carbon concentration and increasing the higher heating value (HHV) of biochar, it tends to raise the ash content. In contrast, HTC proved to be particularly advantageous in significantly reducing ash content, producing a cleaner material, and retaining a larger fraction of the biomass's original energy, resulting in superior global energy efficiencies. Comparative analysis of combustion behavior indicated that pyrolytic biochar exhibits greater thermal stability, while hydrochar displays a two-stage combustion profile with higher initial energy release rates. It is concluded that the choice between pyrolysis and HTC for macadamia shell depends on the specific requirements of the final application, whether for a high-energy-density biofuel or a low-ash product. This work contributes to understanding the advantages and limitations of each process, providing insights for optimizing the energy valorization of this residue.