Global coffee production generates millions of tons of agricultural residue annually, with processing byproducts accounting for approximately 93 percent of total fruit weight. Coffee husks represent the primary fraction of this residue stream, yielding roughly 10 million tons globally that are frequently discarded without adequate treatment. Improper disposal of raw husks poses environmental risks, including soil contamination, water pollution, and greenhouse gas emissions. However, coffee husks possess a rich lignocellulosic composition containing lignin, cellulose, and hemicellulose, making them prime feedstocks for thermochemical conversion. Valorizing coffee husks through pyrolysis converts agricultural liabilities into functional carbonaceous materials that improve soil health and support sustainable farming practices.

The physical and chemical performance of biochar depends directly on the thermal conditions used during its production. Subjecting Coffea canephora husks to controlled pyrolysis across varying thermal points induces major structural transformations. Raw coffee husk biomassBiomass is a complex biological organic or non-organic solid product derived from living or recently living organism and available naturally. Various types of wastes such as animal manure, waste paper, sludge and many industrial wastes are also treated as biomass because like natural biomass these More displays a compact, fibrous structure with low bulk density and volatile content exceeding 67 percent. As heating progresses from 400 to 800 degrees Celsius, volatile components progressively evaporate while non-condensable gas yields rise from 22 percent to over 32 percent. This devolatilization process drives continuous carbonization, shrinking solid biochar yield from 43 percent down to 32 percent as the carbonaceous skeleton becomes increasingly concentrated.

Modulating the processing temperature creates distinct trade-offs between final mass yield, physical density, and chemical composition. Biochar produced at 400 degrees Celsius retains a higher bulk density of 293 kilograms per cubic meter alongside a volatile content of 25 percent. These unreacted volatile components provide easily assimilable carbon that stimulates soil microbial activity and facilitates rapid nutrient mineralization. In contrast, raising the temperature to 800 degrees Celsius reduces bulk density to 249 kilograms per cubic meter while increasing fixed carbon content from 67 percent up to 77 percent. High-temperature treatment also concentrates inorganic minerals, elevating total ashAsh is the non-combustible inorganic residue that remains after organic matter, like wood or biomass, is completely burned. It consists mainly of minerals and is different from biochar, which is produced through incomplete combustion.   Ash Ash is the residue that remains after the complete More content to nearly 12 percent.

Microscopic examination confirms that thermal processing restructures the physical internal matrix of the biomass. Raw husks feature smooth, non-porous surfaces that offer limited reactive surface area. Pyrolysis at 400 degrees Celsius initiates surface breakdown and lamellae formation, while heating to 600 and 800 degrees Celsius develops a fully interconnected network of macropores with a characteristic honeycomb morphology. This porous network enhances fluid storage and provides shelter for beneficial soil microorganisms. Furthermore, biochar pHpH is a measure of how acidic or alkaline a substance is. A pH of 7 is neutral, while lower pH values indicate acidity and higher values indicate alkalinity. Biochars are normally alkaline and can influence soil pH, often increasing it, which can be beneficial More shifts non-linearly with temperature, dropping slightly to 9.75 at 600 degrees Celsius before rising to an alkaline peak of 10.8 at 800 degrees Celsius as basic mineral oxides concentrate within the matrix.

These experimental insights enable the custom design of biochar for targeted land management strategies. Material pyrolyzed at 400 degrees Celsius is ideal for sandy or nutrient-poor soils requiring immediate fertility boosts due to its higher density and accessible volatile matterVolatile matter refers to the organic compounds that are released as gases during the pyrolysis process. These compounds can include methane, hydrogen, and carbon monoxide, which can be captured and used as fuel or further processed into other valuable products.   More. Conversely, biochar created at 800 degrees Celsius exhibits superior chemical stability, strong recalcitrance against microbial breakdown, and high alkalinity. These characteristics make 800-degree biochar the most effective option for correcting soil acidity, enhancing cation exchange capacity, and achieving long-term carbon sequestration in weathered tropical agricultural soils.

Source: Pimentel, L. M., Delatorre, F. M., Cupertino, G. F. M., Manjate, M. J., Silva, Á. M. d., Oliveira, T. R., Cruz, B. d. S., Protásio, T. d. P., & Dias Júnior, A. F. (2026). How does pyrolysis temperature influence the physicochemical properties of biochar from Coffea canephora husk? CERNE, 32, e103665.