The nitrogen and carbon footprints of ammonia synthesis in China based on life cycle assessment

Abstract

The global nitrogen (N) cycle has emerged as an earth system process with more serious artificial disruption than climate change. Artificially synthesized reactive nitrogen (Nr) already accounts for nearly 50% of the total Nr in the earth system. The massive anthropogenic conversion of inert nitrogen (N₂) to Nr is a major driver of imbalance and disruption of the earth's N cycle, where the artificial ammonia (NH₃) synthesis process is the main trigger. Existing studies on life cycle environmental impacts of ammonia synthesis mainly focused on the greenhouse effect but lacked or underestimated the interference with the nitrogen cycle due to currently incomplete nitrogen footprint frameworks. In addition, the comprehensive evaluation of the nexus between nitrogen and carbon footprint of NH₃ synthesis systems is also insufficient. Attempting to solve the above-mentioned problems, life cycle assessment models of seven ammonia synthesis systems were established considering different raw material pathways and production technologies under China's context, assisted by the Brightway2 platform. The general framework of nitrogen footprint accounting (GFNFA) that was established by the authors previously was employed to assess the ammonia synthesis on nitrogen footprint covered all ecosphere. The performance and hotspots of the system nitrogen footprint, carbon footprint (CF) and nitrogen-carbon nexus were then systematically quantified and analyzed. Results indicated that electrolysis-based ammonia powered by renewable and nuclear energy had the lowest Nr emission (0.499-1.148 kg Nr/t NH₃) and carbon emission (592.822-1045.494 kg CO₂-eq/t NH₃). Among the seven ammonia synthesis systems investigated, biomass-based ammonia had the largest Nr emission and system nitrogen accumulation, and it converts the most N₂ to Nr per ton ammonia produced, due to the extensive resources consumption and emissions during straw growth and direct Nr emission in gasification process. Thus, it caused the most significant disturbance to the earth's nitrogen cycle. The nexus between nitrogen and carbon footprints was revealed that the system's energy consumption was found to be a common driver through hotspots and contribution analysis. NH₃ synthesis efficiency was the most determining factor in the system's Nr and carbon emissions. With a 15% increase in synthesis efficiency, nitrogen and carbon footprints can be reduced by more than 12.5%. This study can help researchers better understand the life cycle impacts of ammonia synthesis systems on earth's nitrogen and carbon cycle from multidisciplinary ecological origins.

Keywords: Ammonia synthesis; Carbon footprint; Ecological indicators nexus; Life cycle assessment; Nitrogen footprint.