Solar-Driven CO₂ Methanation at Industrial Interfaces: Technology Readiness, Process Integration, and Techno-Economics

Solar-driven CO₂ methanation is being investigated as a pathway for carbon utilization and renewable-energy storage, yet deployment at industrial point sources, including ammonia–urea complexes, remains constrained by heterogeneous reporting, uncertain feedstock availability, heat-management requirements, and boundary-dependent process economics. This structured critical narrative review evaluates thermocatalytic Power-to-Methane, H₂-fed photothermal and light-assisted methanation, and direct photocatalytic CO₂-to-CH₄ conversion across catalyst, reactor, and plant scales. A domain-specific evidence-grading framework is applied to distinguish comparatively well-supported mechanistic conclusions from studies with incomplete local thermometry, matched-temperature dark controls, photon-flux calibration, isotope tracing, or material and charge balances. The review further examines the stoichiometric and operational constraints governing residual process-CO₂ availability at ammonia–urea sites and assesses how renewable-H₂ supply, buffer storage, gas conditioning, heat recovery, and product specifications affect process integration. An illustrative industrial screening case for a 2000 t d⁻¹ NH₃/2000 t d⁻¹ urea complex yields a nominal residual of 472.4 t d⁻¹ CO₂ before operational deductions, corresponding to 86.5 t d⁻¹ H₂ demand and an approximately 198 MW electrolyzer load at 55 kWh kg⁻¹ H₂. Published techno-economic and life-cycle results are interpreted within their reported functional units and system boundaries rather than directly rank-ordered. The evidence base is strongest for conventional thermocatalytic methanation at pilot and industrial scale, whereas photothermal and direct photocatalytic routes retain substantial diagnostic, transport, durability, and scale-up uncertainties. Commercial feasibility depends not only on nominal CO₂ availability but also on the economically and environmentally justified conversion fraction, renewable-electricity cost, electrolyzer utilization, reactor durability, methane losses, product-conditioning requirements, and heat and co-product credits. Accordingly, complete methanation of a nominal residual CO₂ stream should not be adopted as a design objective without site-specific marginal techno-economic and life-cycle evaluation. A five-gate framework is proposed to guide progression from mechanistic credibility and reproducible performance to module qualification and site-specific deployment.