Exergy efficiency of ammonia production

Abstract

The chemical industry is responsible for roughly 6 % of global primary energy consumption and global greenhouse gas emissions, as well as a key fossil-fuel user (Cullen and Allwood 2010). Material and energy efficiency are the focus of industrial and policy decision-makers but are often pursued as separate strategies. This paper applies exergy analysis to reconcile material and energy flows into a single unit and the separate efficiencies into an integrated metric and take into account the quality of the flows. The analysis was performed on simulated data for an ammonia production site based on Steam Methane Reforming (SMR) to produce syngas from natural gas. The combustion zone of the reformer in the SMR plant is modelled separately from the reaction zone, facilitating the allocation of exergy destruction to the separate sections. Exergy flow maps are constructed using Sankey diagrams to illustrate the resource (material and energy) flows around the site and the principal sources of exergy loss and destruction. Exergy efficiencies and exergy destruction values are calculated for every plant. A more detailed analysis of the SMR plant is conducted that takes into account transit exergy (untransformed exergy). Losses in the SMR plant are finally allocated to the specific mechanism of loss (for instance combustion and heat exchange) to guide future improvements. The analysis yields specific results for the ammonia and syngas case study but the methodology is applicable to any chemical production site with minimal adjustments.