Comparisons of Biochar Properties from Wood Material and Crop Residues at Different Temperatures and Residence Times

TLDR

Biochar is increasingly used for carbon sequestration and soil improvement. The study examined how feedstock type, pyrolysis temperature, and residence time influence biochar properties by pyrolyzing bamboo, elm, and five crop residues at 500 or 700 °C for 4, 8, or 16 h under oxygen‑limited conditions. Two wood‑based and five crop‑residue biochars were produced by the described pyrolysis regimes, and their physicochemical properties were measured. Higher temperature raised ash, BET surface area, pH, and total P and Ca while lowering yield, CEC, total acid, and total N; longer residence time increased ash and BET surface area but reduced yield; FTIR showed more aromatic structures at higher temperatures; straw‑based biochars consistently exhibited higher ash, CEC, nutrient contents, yield, pH, and acid content, and their BET surface area at 700 °C matched that of wood‑based biochars, indicating that crop‑straw biochars may be more effective for soil fertility and carbon sequestration in China.

Abstract

Biochar has been increasingly used as a method for C sequestration and soil improvement. To understand how feedstock and pyrolysis conditions affect biochar characteristics, we investigated two wood-based biochars (bamboo and elm) and five crop-residue-based biochars (wheat straw, rice straw, maize straw, rice husk, and coconut shell), which were pyrolyzed at 500 or 700 °C and remained at that temperature for 4, 8, and 16 h under oxygen-limited conditions. For a given feedstock, increasing pyrolysis temperature from 500 to 700 °C resulted in increases in ash content, BET surface area, pH, and total P and Ca contents (P < 0.05) and decreases in yield, cation exchange capacity (CEC), total acid, and total N (P < 0.01). Prolonging residence time (from 4 to 8 or 16 h), the BET surface area and ash content of biochars increased (P < 0.05), whereas the yield decreased (P < 0.01). Fourier-transform infrared spectroscopy (FTIR) analysis showed that more recalcitrant and aromatic structures were formed in the biochars with increased temperature. The three straw-based biochars consistently exhibited far greater ash percentage (14.5–40.3 wt %), CEC (14.1–34.8 cmol kg–1), and the contents of total N (0.24–2.81 wt %), P (0.60–8.41 wt %), Ca (0.63–1.48 wt %), and Mg (0.24–0.63 wt %) and generally had higher yield (19.0–37.6 wt %), pH (9.2–11.1), and contents of total acid (0.15–0.53 mmol g–1), C (41.7–55.1 wt %), Na (0.27–6.72 wt %), and K (6.56–28.1 wt %) than the two wood-based biochars. The BET surface area of straw-based biochars with 700 °C pyrolysis temperature could be mostly as high as 112–378 m2 g–1, a comparable level with that of wood-based biochars. Despite the high variability in biochar properties, these results demonstrate that biochars from crop straw may be more effective and desirable for improving soil fertility and C sequestration in Chinese vast soils.