The traditional ammonia production route involves a Steam Methane Reforming (SMR) process followed by the Haber–Bosch process. In SMR, natural gas undergoes reforming with steam and air in two stages to generate syngas (H₂ + N₂). The air also supplies the required nitrogen. This syngas then passes through a Water-Gas Shift (WGS) reactor to increase hydrogen content, followed by CO₂ removal via an Acid Gas Removal (AGR) system. Finally, catalytic methanation is employed to remove oxidizing species and achieve the precise 3:1 H₂:N₂ ratio needed for the Haber–Bosch process. However, this configuration is highly energy-intensive and emits large quantities of CO₂.
This study presents a sustainable alternative using biomass-based Chemical Looping (CL) integrated directly with ammonia synthesis. CL is a fuel-flexible, clean energy technology that leverages redox cycles of metal oxide oxygen carrier (OC) particles to produce high-purity hydrogen and sequestration-ready CO₂. A three-reactor CL system is developed, using biomass as the feedstock to generate a direct 3:1 H₂:N₂ mixture suitable for the Haber–Bosch process.
In the proposed setup, biomass reacts with iron-based OCs in the reducer reactor, producing a pure stream of CO₂ while reducing the OCs. These reduced particles are transferred to the oxidizer reactor, where partial oxidation is achieved using steam and nitrogen-rich air. The partially oxidized OCs are then conveyed to the combustor reactor via an L-valve and fully oxidized using air. The air exiting the combustor is nitrogen-rich (with 2–5% oxygen) and is partially recycled to the oxidizer to maintain the target 3:1 H₂:N₂ ratio. The OCs exhibit strong durability and low attrition (<0.02%) over 3000 cycles, supporting long-term operation.
The system is simulated in Aspen Plus to analyze the sensitivity toward key process parameters and to optimize reactor conditions. A scaled-up version of the process is compared with the SMR-based conventional method in terms of energy efficiency and economic performance. Experimental validation is carried out in a 2.5 kWₜₕ lab-scale moving bed CL reactor. The reducer reactor is designed for middle fuel injection, establishing counter-current contact for optimal conversion of both biomass char and volatiles. Results indicate complete biomass conversion and generation of CO₂ with 100% dry purity. The reduced OCs are oxidized in a second moving bed reactor, achieving steam conversion rates as high as 60% and complete oxygen depletion, resulting in a pure H₂ + N₂ output.
The near-thermodynamic conversions observed in both the reducer and oxidizer reactors validate the feasibility of this integrated process. A detailed techno-economic analysis compares the Levelized Cost of Product (LCOP) and Minimum Selling Price (MSP) for both conventional and CL-based ammonia production routes. The LCOP for both processes is found to be close to $270/ton of NH₃. However, while the SMR route yields blue ammonia, the CL route offers green ammonia with significantly lower environmental impact. After accounting for carbon credits, the CL-based route shows up to 50% lower MSP compared to the SMR-based process; even without credits, it achieves a 2.5% reduction in MSP.
Key advantages of the CL process include the use of carbon-neutral biomass, inherent generation of sequestration-ready CO₂, autothermal operation, and elimination of multiple complex units (SMR, WGS, methanation, AGR, and post-combustion CO₂ capture). The system’s autothermal nature significantly reduces external energy requirements, while also eliminating the need for standalone air separation by integrating it inherently within the loop. Additionally, the pure CO₂ stream from the reducer can be directly utilized in urea production, further enhancing the process's industrial relevance and sustainability.
This work demonstrates that integrating biomass chemical looping with ammonia synthesis can deliver a technically viable and economically competitive pathway to decarbonize one of the most emission-intensive chemical industries.