Valorization of Municipal Tree Leaf Waste via Mobile Pyrolysis: Techno-Economic and Environmental Feasibility Assessment

Abstract

A case study of municipal tree leaf waste valorization is presented for Jubail Industrial City, Saudi Arabia. The current disposal practice primarily relies on landfilling, resulting in resource loss and greenhouse gas emissions. This study evaluated the technical, economic, and environmental feasibility of a mobile pyrolysis station for valorizing municipal tree leaf waste. Laboratory slow pyrolysis experiments were conducted at 350 °C for 2 h, producing 64.67% biochar, 6.67% bio-oil, and 28.67% non-condensable gases by weight. These findings guided the conceptual design of a truck-mounted mobile pyrolysis station integrating mass and energy balances, equipment sizing, process engineering, safety, techno-economic, and environmental evaluations. At an annual capacity of 1252.8 t of wet leaf biomass, the station produces approximately 622.4 t of biochar, 64.2 t of bio-oil, and 275.9 t of non-condensable gases. The total capital investment was estimated at USD 484,522. Three economic scenarios were evaluated. Under the high-price biochar scenario, using a selling price of USD 2.15 kg−1, the initial full-capacity annual revenue was USD 1.389 million, with a 10-year NPV of USD 6.19 million, an IRR of 155.1%, and payback of 0.81 years. Under the low-price biochar scenario, using a selling price of USD 1.075 kg−1, the NPV decreased to USD 2.11 million, the IRR decreased to 67.1%, and payback increased to 1.88 years. With the same low biochar price and a 50% increase in OPEX, the NPV remained positive at USD 1.22 million, with an IRR of 44.6% and payback of 2.64 years. The environmental assessment evaluated two biochar carbon storage cases. Using the full calculated carbon storage credit, the net annual environmental benefit was 7036.79 t CO2e, whereas a conservative 50% carbon storage sensitivity scenario provided a benefit of 6489.96 t CO2e compared with current landfill disposal practices. These findings support mobile pyrolysis as a technically feasible, economically resilient, and environmentally sustainable strategy for municipal tree leaf waste valorization in Saudi Arabia.