Design constraints governing household-scale rice-straw biochar systems: The role of feedstock densification and particle-size selection

Household-scale biochar systems are increasingly promoted for agricultural waste management and decentralized environmental applications; however, their design is often constrained by feedstock handling, material stability, and process simplicity rather than maximum functional performance. This study examines key design constraints in household-scale rice-straw biochar production, focusing on feedstock densification and particle-size selection under practical conditions. Rice straw was pyrolyzed in a household drum kiln with and without mechanical compression. Biochar yield, physicochemical properties, mechanical stability, and adsorption behavior were evaluated. Uncompressed rice-straw biochar showed poor structural integrity and fragmented during handling, preventing reproducible adsorption measurements. In contrast, compressed rice-straw biochar (BCRS) exhibited markedly improved yield and stability. At 400 °C for 2 h, BCRS achieved a mass yield of 66.7 ± 1.5%, compared with 31.6 ± 0.9% for uncompressed straw. Methylene blue adsorption showed clear particle-size dependence: finer particles enabled faster uptake, whereas coarser particles exhibited higher apparent capacity under batch conditions. These findings indicate that handling feasibility and material stability are primary design constraints, while adsorption performance serves as a secondary screening criterion in decentralized systems. • Compression improves carbon retention and yields stable, compact biochar. • BCRS at 400–500 °C for 2–3 h balances yield and structural quality. • Smaller BCRS particles adsorb faster; larger ones offer higher capacity. • BCRS is a low-cost sorbent for water treatment.