How does pyrolysis temperature influence the physicochemical properties of biochar from Coffea canephora husk?

Abstract

Background: Coffee cultivation is one of the most important agricultural activities in Brazil and generates large amounts of waste during fruit processing. Approximately 93% of the coffee fruit volume is discarded, with the husk being one of the main by-products. Inadequate disposal may cause environmental impacts, whereas its valorization through pyrolysis can provide a sustainable and value-added alternative. This study evaluated the effect of pyrolysis temperature (400, 600, and 800 °C) on the physicochemical properties of biochar produced from coffee husk, aiming at its use as a soil conditioner.

Results: The biomass was subjected to controlled pyrolysis in a muffle furnace at a heating rate of 5 °C.min⁻¹ and a residence time of one hour after reaching the final temperature. The resulting biochar was characterized for co-product yield, bulk density, chemical composition (ash, volatiles, and fixed carbon), and pH. Increasing the pyrolysis temperature reduced biochar yield and density, while increasing fixed carbon content. Fixed carbon increased from 67% at 400 °C to 77% at 800 °C, whereas volatile content decreased from 25% to 10%. The pH decreased up to 600 °C and increased at 800 °C.

Conclusion: Pyrolysis temperature strongly influences biochar properties, requiring a balance between yield and quality according to the intended application. Although all evaluated temperatures were technically feasible, biochar produced at 800 °C showed the greatest potential for long-term application in tropical acidic soils.