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

[Geoengineering News]

https://www.dbpia.co.kr/journal/articleDetail?nodeId=NODE12739635

Authors: Kyung-Hwa Han, Min-Hyo Lim 

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.

Source: DBpia