Net CO2 removal in Korean orchard soils using prunings-derived biochar: effects of pyrolysis type and CH4 emissions in biochar production stage

다국어 초록 (Multilingual Abstract)

This study evaluated the net carbon dioxide (CO2) removal (NCR) potential of a local circular system utilizing orchard-pruning residues for biochar production. The life-cycle system comprised six stages: residue collection and natural drying, chipping, transport of chipped residues, local biochar production, biochar transport, and biochar application to orchards. The analysis focused on the biochar production stage, comparing pyrolysis types (TLUD and rotary kiln) and methane (CH4) emission factors (combustion and feedstock moisture). The amount of orchard-pruning residues was estimated for each city and county based on fruit orchard area in 2022, fruit production, and biomass conversion factors. To determine the optimal temperature, pruning residues from apple, pear, and grape orchards were pyrolyzed at 400, 500, and 600°C.
Based on the molar H/Corg ratio (a carbon stability indicator) and biochar yield, 500°C was identified as optimal. At this temperature, long-term (>100 years) CO2 sequestration was estimated to range from 0.66 to 0.72 t CO2-eq t-1 dry feedstock. NCR was calculated by subtracting life-cycle greenhouse-gas (GHG) emissions, derived from literature values, from the CO2 sequestration potential via biochar. National NCR of local biochar circular systems, assuming 10% utilization of potential orchard-pruning residues (apple, pear, grape) was estimated as follows: TLUD with CH4 combustion 43 kt CO2-eq > TLUD without CH4 combustion using dry feedstock (<15% moisture) 41 kt CO2-eq > rotary kiln with CH4 combustion 38 kt CO2-eq > rotary kiln without CH4 combustion 34 kt CO2-eq > TLUD without CH4 combustion using average-moisture feedstock 19 kt CO2-eq. The findings suggest that effective NCR in orchard-pruning biochar local cycles requires sufficient feedstock drying and pyrolysis systems equipped with CH4-oxidizing afterburners, particularly stationary rotary kilns or well-operated TLUD units. However, when natural drying cannot reduce feedstock moisture below 15%, forced drying may introduce additional GHG emissions from energy consumption, thereby reducing the overall NCR efficiency.