Seeking the synergistic potential of biochar integration in municipal composting plants for techno-economic and environmental leverage

[Display omitted] •Co-composting offers sustainable waste treatment and enables carbon storage.•An innovative hybridization of composting and gasification plants is proposed.•In-situ gasification power plant and/or biochar trading scenarios are considered.•The best scenario is evaluated on economic, environmental and technological aspects.•Carbon credits and avoided gas emissions are part of the environmental assessment. This work comprehensively investigates the technological, economic, and environmental implications of biochar application to a municipal composting facility through various combinations of in situ gasification or biochar trading. The aim is to find the optimal solution to minimize environmental impacts and to maximize resource utilization over the year. The study hybridizes an existing composting facility using various gasification plants chosen in between the commercial SynCraft model, resulting in a pioneering study where a gasifier is integrated in a composting plant. The techno-economic feasibility of each scenario is obtained calculating the payback time, estimating the annual biomass demand, evaluating the seasonal variability of biomass, and setting as a key-point the amount of produced electrical energy and gasification biochar. The scenarios differ in the installed gasifier sizing (500 kW, 1 MW and 1.7 MW) and the supplementary amount of biochar that may be purchased, applied at a 3 % w/w rate on the treated organic mix. Economic results reveal that a purchase of all necessary biochar is infeasible; therefore, the installation of a gasifier for biochar self-production should be always considered. For the 500-kW gasifier a 25-year payback (weighted average cost of capital = 10 %) that reduces to 14 or 12 years for the 1 MW gasifier, respectively with and without purchase of supplementary biochar. The 1.7 MW gasifier is the most cost-effective option, but it requires high initial investments. Additionally, this work studies both the carbon dioxide equivalent storage, thanks to the biochar utilization, quantifying the corresponding Carbon Credits in 3652 credits for the best scenario (through VERRA method), and the reduction of methane emissions during composting that are abated by nearly half in the best scenarios. Aiding biochar into composting operations emerges as a promising way for waste management optimization, balancing economic viability and environmental sustainability. On the other hand, some operational issues are identif