Comparative analysis of eight urea-electricity-heat-cooling multi-generation systems: Energy, exergy, economic, and environmental perspectives

The Haber-Bosch process for urea production is known for its significant carbon emissions. While the chemical looping ammonia generation method presents a potential alternative, its technical and economic feasibility still requires further investigation. This paper conducts a comparative analysis of eight renewable energy-driven urea-electricity-heat-cooling multi-generation systems by developing their energy, exergy, economic, and environmental evaluation models. Considering the fluctuation in renewable energy resources, an annual evaluation of the different systems is conducted based on a monthly time scale. The results indicate that systems based on the Haber-Bosch process have higher average energy and exergy efficiencies of 60-80 % and 40-50 %, respectively. For the chemical looping ammonia generation process, the respective efficiencies are 50-70 % and 30-40 %. Additionally, the system integrating the Haber-Bosch reaction, reduction reactor, and solid oxide fuel cell yields the highest energy and exergy efficiencies among the eight systems, at 68.2 % and 45.78 %, respectively. However, it is inferior to the multi-generation system integrating the chemical looping ammonia generation process in terms of economic and environmental performance. In addition, the system integrating the chemical looping ammonia generation process and solid oxide fuel cell has the shortest discounted payback period of 8 years, with annual revenue and net present value of 32.63 and 11.66 MUSD, respectively. While the operating expenditures of the Haber-Bosch (12.6-15.7 MUSD) are lower than those of the chemical looping ammonia generation process (14.0-17.5 MUSD), its capital expenditures (382.0-410.8 MUSD) are significantly higher than those of the chemical looping ammonia generation process (302.4-307.3 MUSD). Furthermore, the conducted environmental evaluation shows that the multi-generation system integrating the chemical looping ammonia generation process and reduction reactor exhibits lower overall carbon emissions (0.013 t CO2/t urea) compared to the state-of-the-art method of oxy-fuel coal-fired power plant for urea synthesis.