Preliminary Techno–Economic Assessment of Carbonization Methods and Comparison of Napier Grass and Sewage Sludge Biochar for Soil Fertility Improvement

The handling of sewage treatment plant (STP) sludge represents an environmental challenge and provides an opportunity for resource recovery within a circular economy paradigm. This study evaluates the cost‐effectiveness of carbonization techniques under hydrothermal and non‐hydrothermal environments (150°C–500°C) to identify economically viable methods for biochar production. Pyrolysis was identified as the most cost‐effective method, with pyrochar demonstrating the lowest energy consumption (2639.91 kWh/kg char) and variable cost (26.11 INR/batch). The study involved the conversion of STP sludge into biochar, subsequently compared to biochar sourced from Napier grass (NG) for soil applications. Biochar produced at 300°C and 500°C was assessed for yield, composition, and soil fertility over a 30‐day incubation period. Biochar produced from NG showed increased volatile solids and reduced ash content, whereas STP‐derived biochar displayed substantial ash fractions, peaking at 92.09%. Soil incubation results indicated increased nutrient availability with NG‐derived biochar (NPK: ∼170–250, 0.04–0.06, 120–150 ppm) compared to STP‐derived biochar (∼100–200, 0.04–0.05, 50–100 ppm). The feedstock and processing conditions significantly influenced biochar performance. The proposed cost‐based screening method enables scalable decision‐making, with NG‐derived biochar showing promise for soil enhancement, while the feasibility of STP‐derived biochar requires further investigation.