Modeling of post-combustion carbon capture in power plants and process industries - partial capture and load following conditions
Licentiate thesis, 2015
Carbon capture and storage (CCS) is recognized as a potent technology for the mitigation of global warming, since it can significantly reduce CO2 emission from large point source emitters. Post-combustion CO2 capture using chemical absorption is a versatile end-of-pipe technology that can be implemented at all point sources of CO2, such as a power production facility or industrial plant. The increasing capacities of intermittent renewable sources in the energy system, such as wind and solar power, mean that fossil-fueled power generation faces a two-fold challenge: to supply regulating power and other ancillary services with increased flexibility while operating with near-zero emissions of CO2. Industrial plants generally operate under more stable conditions, and the main challenge for energy-intensive industries is to meet their long-term CO2 emission reduction targets, given that the possibilities for fuel substitution and further process optimization are limited in many cases and thus CCS will be required to meet this challenge.
The overall aim of this work was to evaluate operational strategies for efficient application of post-combustion CO2 capture to reduce emissions of CO2 from fossil fuel combustion, while meeting the demands of an energy system with large shares of renewable energy. This involved a steady-state investigation of CO2 capture (with the focus on partial capture of CO2) in three separate industrial applications: a pulp mill; an aluminum mill; and an oil refinery. Furthermore, CO2 capture in load-following power generation under transient conditions was investigated.
It is shown that the concentration of CO2 in the flue gas has a strong effect on capture process performance and that the design of the absorber temperature profile becomes increasingly important at high concentrations of CO2during absorption based on monoethanolamine (MEA), which, compared to ammonia, has high reactivity and a high heat of reaction. This is further illustrated by the three industrial case studies performed in this work, in which the flow conditions and exhaust composition differed significantly between the industrial plants. The results suggest that it may be unfeasible to aim for a high overall capture rate at plant sites because they usually consist of several CO2 sources, some of which may not be suitable for CO2 capture. Opportunities for waste heat utilization also vary considerably across the industrial plants, which further emphasizes the importance of case-specific studies.
The evaluation of the impacts of transient power plant operation on the capture unit included two load-change scenarios: the transitions to part-load and peak-load operations, respectively, from design conditions of full load. Simulations of the load-variation scenarios reveal that the implementation of active control strategies improves capture system performance with respect to transition rate, capture efficiency, and the heat requirement, for both part-load and peak-load operations. Integration of the capture process results in decreases in electric efficiency of around 9 percentage points at full load and in the range of 5–12 percentage points for off-design conditions, i.e., under part-load and peak-load conditions, respectively.