Enhancing the quality of products from slow pyrolysis of an agro-industrial biomass waste with natural mineral additives

Given the increasing demand for sustainable energy sources and the need to revalorize agro-industrial waste, this work presents the slow pyrolysis of a widely available agro-industrial biomass waste (pecan nut shells) in the presence of natural mineral additives such as dolomite, natural zeolite (clinoptilolite), and limestone (calcium carbonate), with a focus on elucidating the impact of the natural additives on increasing the quality of the pyrolysis products. In addition, the effect of a simple additive pretreatment (calcination) has been evaluated to develop an insight into its influence on the pyrolysis reaction. Slow pyrolysis of pecan nut shells was conducted using a lab-scale batch reactor, from room temperature to 600 degrees C with a heating rate of 10 degrees C/min; the resulting products were collected and characterized with several instruments such as GC-MS and GC-TCD. Additive calcination caused chemical phase changes, a significant loss of total acid sites, reduction of surface area (except for calcined limestone), and elimination of micropores. A remarkable increase in the condensate product yield between 42% and 114% was observed for most tests where additives were used. Further investigation using GCMS revealed that guaiacol and dimethoxyphenol were the primary products in the liquid phase, which underwent more pronounced deoxygenation when calcined additives were present in the pyrolysis tests, favoring the generation of organic derivatives (such as phenol, cresol, and xylenol) along with other molecules with low oxygen content. Calcined limestone showed the highest solid yield compared with the rest of the additives at the expense of liquid production. Furthermore, the presence of additives favored higher content of H 2 , CH 4 , CO and CO 2 in the gas products compared with the control test; the calcined limestone additive promoted the highest hydrogen, methane and carbon monoxide concentrations in the gas due to CO 2 adsorption and mineralization, along with the enhancement of other reactions such as bio-oil decarbonylation, catalytic cracking and the Boudouard reaction. In turn, the H 2 /CO ratio obtained with this additive was also the highest, with a value of 1, making it a promising candidate for syngas-production applications. Overall, the present study contributes valuable insight into the utilization of pecan nut shells to produce fuel precursor molecules and other valueadded products, emphasizing the impact of mineral additives on the pyrolysis outcome. Our findings suggest a potential alternative for the revalorization of overlooked agroindustrial byproducts, contributing to the development of more sustainable energy sources.