The distinction is central to evaluating biochar as a carbon-storage material. Pyrolysis redistributes carbon among solid biochar, bio-oil and gases, including carbon dioxide, carbon monoxide and hydrocarbons. A higher-temperature char can contain a greater fraction of carbon because volatile compounds have been removed, yet the absolute amount of carbon in the recovered solid can be lower when much of the feedstock has decomposed.

In the experiment, material produced at 420 °C had the highest measured carbon concentration, 46.080%, but the 380 °C biochar retained more carbon because it weighed 60.108 g rather than 40.153 g. The study therefore distinguishes carbon concentration, which describes the composition of the recovered solid, from total carbon retention, which depends on both that concentration and the mass of biochar produced. The researchers obtained rice husk from a local agricultural biomass vendor.

Before pyrolysis, the material was visually inspected to remove impurities, gently washed with distilled water, dried in an oven at 110 °C for 24 hours until it reached a consistent weight, and stored in a pre-weighed vacuum desiccator. It was then mechanically ground and sieved to standardize particle size. No chemical activation or compositional modification was applied during pretreatment.

These steps were intended to reduce moisture and contamination and make the thermal comparison reproducible. The baseline sample, designated BM001, consequently represented standardized, pretreated rice husk rather than the material exactly as received from the vendor. Biochar was produced in a laboratory-scale fixed-bed reactor at three temperatures while continuous nitrogen flowed through the system at 1.5 L/min and the pressure remained at 1 bar.

The heating rates were 2.917 °C/min, 3.083 °C/min and 3.250 °C/min for the 380, 400 and 420 °C treatments, respectively, and every treatment had a two-hour residence time. Pyrolysis was performed independently in triplicate at each temperature, allowing biochar yield to be calculated for three runs under each condition. A representative sample from each temperature was then selected for analytical characterization.

Per 100 g of initial dry biomass, mean biochar yield fell from 60.108 g at 380 °C to 43.236 g at 400 °C and 40.153 g at 420 °C. The decline in solid yield was statistically significant. One-way analysis of variance found an effect of pyrolysis temperature on yield, with F(2,6) = 4535.05 and p < 0.001.

Tukey’s honestly significant difference test found that all three pairwise comparisons were significant at p < 0.001: 380 versus 400 °C, 380 versus 420 °C, and 400 versus 420 °C. The measured trend is consistent with greater thermal decomposition at higher temperatures, as dehydration and devolatilization release gases, vapours and condensable products and leave less solid residue. This loss of feedstock mass was the principal reason that the higher-temperature biochars retained less total carbon despite containing a larger carbon fraction. Powder X-ray diffraction revealed major structural changes during thermal conversion.

The cellulose-associated feature visible in the untreated rice husk disappeared from the biochar patterns, consistent with degradation of the native cellulose structure. At the same time, the initially sharper silica-rich diffraction feature became broader and more diffuse, indicating a greater contribution from poorly ordered or amorphous silica together with disordered carbonaceous material. The area-based Crystallinity Index was 67.27% for the biomass, rose to 88.37% for BC380, and then declined to 86.85% for BC400 and 83.18% for BC420.

The authors cautioned that this comparative index is not a direct measurement of cellulose or graphitic crystallinity. Its increase after pyrolysis primarily reflects removal of amorphous cellulose and a change in the relative contributions of ordered and disordered components, not formation of a uniformly crystalline carbon matrix. Elemental analysis documented progressive enrichment and chemical transformation within the surviving solid.

Carbon concentration increased from 39.617% in BM001 to 42.250% in BC380, 43.064% in BC400 and 46.080% in BC420. From the pretreated biomass to the highest-temperature biochar, the hydrogen-to-carbon atomic ratio declined from 0.94 to 0.76, while the oxygen-to-carbon ratio fell from 0.98 to 0.79. The authors interpret these trends as evidence of deoxygenation, loss of hydrogen- and oxygen-rich constituents, and greater aromatic condensation at elevated temperatures.

Declining H/C and O/C ratios are associated with increasing carbon recalcitrance, but the experiment measured chemical and structural indicators rather than directly demonstrating how long the retained carbon would persist under field conditions. Proximate analysis showed substantial removal of unstable material during heating. Moisture decreased from 9.08% in the pretreated biomass to 2.28% in BC380, while volatile matter fell from 61.32% in the biomass to 23.26% in BC380, 17.04% in BC400 and 16.55% in BC420.

Ash increased from 12.68% in BM001 to 50.00%, 55.72% and 56.74% in the three biochars. The authors attributed this increase to concentration of thermally stable, silica-rich minerals as volatile organic constituents were lost, rather than to the generation of new inorganic matter. Fixed carbon rose from 16.92% in the biomass to 24.46% at 380 °C, then measured 22.54% at 400 °C and 22.79% at 420 °C. The authors described the slight reduction between the two higher-temperature samples as an apparent anomaly that might reflect mathematical, instrumental or handling variation.

Converting the retained solid carbon into carbon dioxide equivalent produced the same overall ranking as the absolute carbon measurements. BC380 retained 93.127 g of carbon dioxide equivalent per 100 g of biomass, BC400 retained 68.275 g and BC420 retained 67.846 g, while the original 100 g of BM001 contained a calculated 145.276 g of carbon dioxide equivalent. These values describe carbon remaining in the biochar under a mass-based calculation; they are not verified net removals of carbon dioxide from the atmosphere.

The study did not quantify carbon in bio-oil or gaseous products and did not include process-energy inputs or emissions from feedstock handling and transportation. It also did not directly test the long-term fate of retained carbon under an end-use environment. A complete assessment of net climate benefit would require a whole-system carbon balance covering solid, liquid and gaseous products, process inputs and the persistence of biochar-derived carbon over time.

Within the 380–420 °C range tested, the results reveal a trade-off between the carbon concentration and the total quantity of carbon in the recovered biochar. Pyrolysis at 380 °C yielded more biochar and therefore preserved more carbon in absolute terms, even though its carbon concentration and degree of carbonization were lower. The study does not establish that 380 °C is universally optimal; its evidence applies to the tested feedstock, reactor conditions, heating rates, two-hour residence time and temperature range.

Its central finding is that elemental concentration alone is an incomplete measure of biochar carbon storage: total retention depends on the composition and yield of the solid, while claims of atmospheric carbon removal require additional measurements of coproducts, process emissions and long-term carbon stability.

Subject of Research: Laboratory assessment of how pyrolysis temperature affects rice-husk biochar properties and carbon retention.

Article Title: Pyrolysis temperature-induced changes in physicochemical properties of rice husk-derived biochar: an assessment for CO2 retention potential

Article References: Singh, R. K., Wadhwa, V., & Nayak, N. P. (2026). Pyrolysis temperature-induced changes in physicochemical properties of rice husk-derived biochar: an assessment for CO2 retention potential. Frontiers in Earth Science, 14, Article 1894009. https://doi.org/10.3389/feart.2026.1894009

Image Credits: AI Generated

DOI: 10.3389/feart.2026.1894009

Keywords: rice husk, biochar, pyrolysis, carbon retention, carbon dioxide equivalent, biochar yield, aromatic carbon, rice-husk silica, agricultural waste, temperature-induced, changes, physicochemical