Energy integration optimisation of chilled ammonia process for CO2 capture

Within the global challenge of climate change mitigation, CO2 capture and storage is regarded as a potential option to reduce the CO2 emissions in power plants. The most common technology to capture CO2 is chemical absorption with amines, requiring however a significant amount of energy for solvent regeneration and CO2 compression and therefore penalizing the performance of the electricity production. The energy efficiency is reduced by up to 10%-points and the production costs increased by around one third. To reduce the energy and cost penalty of CO2 capture, the recently developed chilled ammonia process is studied here as a promising alternative. It is focused especially on process integration aspects to improve the competitiveness of power plants with CO2 capture. By applying process integration techniques based on the pinch analysis concept, the process energy requirement is identified and the maximal heat recovery and the optimal utility integration are computed. The optimal process integration is computed by solving the heat cascade model of the process. A detailed analysis of the energy integration results, in the form of composite curves expressed in Carnot axis, allows to identify the major exergy losses and to propose process modifications to reduce the losses and improve the performance. It appears that a refrigeration cycle using a water-ammonia mixture as a refrigerant can satisfy the cooling need with almost no exergy losses. It is highlighted that by improving the absorber and refrigeration integration, the competiveness of an NGCC plant with 90% CO2 capture by chilled ammonia can be enhanced. The energy efficiency can be increased from 47% to 52%, which leads to a production cost decrease of 15%.