Prospects of solar-powered nitrogenous fertilizers

Nitrogenous fertilizer is integral to the food system for better yield of crops, and urea is the most common one. It requires ammonia as the primary reactant, while ammonia requires hydrogen for its production. Synthesis of these products is based on fossil fuels and is very carbon intensive, and pose many environmental threats. To reduce these, this review aims at understanding major solar energy-based pathways for three scenarios of producing solar fertilizers; producing urea using ammonia synthesized via solar hydrogen through water using thermolysis, photocatalytic, thermochemical, or electrolysis processes; using solar-powered ammonia using nitrogen and air/water by thermochemical, electrochemical, or photoelectrochemical methods; or directly producing urea using solar energy via photocatalytic and electrochemical processes. The potential of solar fertilizers, along with their advantages and disadvantages in agriculture, have also been highlighted. Besides this, an estimate of the land required (percent) to produce urea for all the scenarios using solar energy has been carried out for India with typical values of 0.119-0.130% for hydrogen production, 0.003-0.010% for ammonia and least for urea production. The review of techno-economic analysis and life cycle assessment for different hydrogen and ammonia production methods has been presented, and a comparative life cycle assessment study for certain hydrogen, ammonia, and nitrogenous fertilizer production methods using GaBi software was undertaken. Global warming potential, acidification potential, and eutrophication potential for 1 kg solar urea production were found as 0.092 kg CO2 equivalent, 0.014 mol of H+, and 1.869x10-6 kg phosphate equivalent.