Herbal biochar preparation and its application as soil amendment: Is such disposal of traditional Chinese medicine residue cost-efficient towards carbon neutrality?

Herbal biochar from traditional Chinese medicine residue (TCMR) and related carbon-negative utilization could contribute to the realization of carbon-neutral disposal process of TCMR. For this purpose, herbal biochar preparation by low-temperature pyrolysis and its application as soil amendment is designed, and the integrated effects are assessed by a combined techno-economic, exergy, thermoeconomic and life cycle analyses based on the accounting of material flow, energy flow and environmental flow. Firstly, the economic benefits (i.e., net present value, internal rate of return and payback period) of the system is analyzed by combination of cost-benefit method and sensitivity analysis. Secondly, exergy analysis determines exergy destruction, exergy destruction ratio, exergy destruction coefficient, energy efficiency and exergy efficiency of pyrolysis system. Furthermore, by combining techno-economic and exergy analyses, exergy destruction cost and exergy economic coefficient are calculated to optimize pyrolysis system. Thirdly, the CO2 mitigation cost, carbon neutrality potential and environmental impact are evaluated via combining life cycle assessment with techno-economic and exergy analyses. The results indicate that the higher pyrolysis temperature shows preferable exergy efficiency, energy efficiency and thermodynamic performance. From 400 to 600 °C, herbal biochar preparation and its application can improve economic benefits by 12.9–15%. At 550 °C, such TCMR disposal process can reduce environmental impact to −8651.65 mPts, and the GHG emissions and CO2 mitigation cost of the life cycle process are 395.1 kg CO2-eq/t feedstock and 63.4 CNY/t CO2-eq, respectively. In general, herbal biochar preparation by low-temperature pyrolysis and related application as soil amendment can realize the carbon-negative disposal of TCMR by an economical and efficient way.