Carbon Dioxide Capture from Flue Gas

Abstract

One of the main global challenges in the years to come is to reduce the CO2 emissions in view of the apparent contribution to global warming. Carbon dioxide capture, transport, and storage (CCS) from fossil fuel fired power plants is drawing increased interest as an intermediate solution towards sustainable energy systems in the long term. However, CCS is still facing some challenges, such as large scale implementation requires high energy demands and leads to high cost. Innovation and optimization of the capture process is needed to reduce the energy requirement and to minimize the investment cost in order to make CCS viable for application in the near future. The CO2 post-combustion capture based on the absorption/desorption process with monoethanolamine (MEA) solutions, is considered as the state-of-art technology. In this thesis, the MEA process has been defined as the reference case for the purposes of comparison and benchmarking. By analysing the MEA reference case, it can be concluded that this is an energy intensive process due to the regeneration energy of the MEA solution (4 GJ/tonne CO2). For this conventional process, major energy savings can be realized by optimizing the lean solvent loading, the amine solvent concentration, as well as the stripper operating pressure. A minimum thermal energy requirement of 3.0 GJ/tonne CO2 can be obtained using a 40 wt. % MEA solution and a stripper operating pressure of 210 kPa. Significant energy and cost savings can be achieved by increasing the MEA concentration in the absorption solution. It is, however, still to be investigated if high MEA concentrations can be used due to possible corrosion and solvent degradation. Increasing the temperature (operating pressure) in the stripper will lead to a higher efficiency of the regeneration and will reduce thermal energy requirement. Moreover, a high operating pressure will reduce the cost and the energy needed for CO2 compression. The economic baseline for CO2 post-combustion capture using MEA is defined using 600 MWe coal fired power plant as a reference case and assuming 2005 as the reference year, 8% discount factor and 25 years as a project life. The process modelling results are used for providing the required input to the economic modelling. The economic evaluation for the MEA conventional process has shown that this process will lead to a cost of ~40 €/tonne CO2 avoided. Using the baseline techno-economic evaluation as a starting point, a parameter study for the conventional CO2 post-combustion capture process is performed. The main operating variables considered in this study were the MEA solvent concentration, the CO2 removal percentage, the solvent lean loading, and the stripper operating pressure. The economic results show a minimum CO2 avoided cost of 33 €/tonne CO2 with an optimized process conditions of 0.3 mol CO2/mol MEA lean solvent loading, using a 40 wt. % MEA solution and a stripper operating pressure of 210 kPa. This translates to 53 €/MWh cost of electricity, compared to 31 €/MWh for the power plant without capture. The difference in costs per tonne CO2 avoided is small for CO2 removal in the range between 80% and 95%. The CO2 post-combustion capture process overall performance is evaluated using pilot plant experimental results. Two different modelling approaches (equilibrium-stage and rate-based) are validated and compared using these large-scale pilot plant data. Equilibrium-stage and rate-based models are implemented using the commercial Aspen plus simulation tool. The study indicates that there are no major differences between the two modelling approaches in predicting the overall capture process behaviour for this pilot plant case (e.g. regeneration energy requirement, CO2 removal % and solvent rich loading). Hence an equilibrium-stage model was preferred as the basis for over-all process modelling and benchmarking different capture solvents in view of its lesser complexity. The rate-based model, however, did yield more accurate predictions of the temperature profiles and mass transfer inside the columns. As a result, for a detailed process design or understanding of the mass and energy profiles in the absorber and stripper columns, the rate-based approach should be applied. The Hypogen concept (electricity generation with co-production of hydrogen) is considered one of the future energy options. This option will facilitate the use of a clean source of energy (hydrogen) for purposes like transportation and heating. This concept is based on the use of syngas for power production with CO2 post-combustion capture incorporating the possibility of co-production of hydrogen (5-10% of the total syngas). In this concept, hydrogen is produced and purified in two different methods. The first method is based on increasing hydrogen content using the water gas shift reaction, followed by the separation of hydrogen from CO2 using a high-pressure absorber. This absorber column is integrated with the ambient post-combustion capture process. The second method is based on the separation of hydrogen from syngas using polymeric membranes. In both options, the hydrogen will be further purified using a pressure swing adsorption system. Both options are feasible with an overall CO2 capture cost comparable to the conventional post-combustion capture process. However, there are some limitations in the hydrogen purity using polymeric membranes. The advantage of the high-pressure absorber is more obvious if an advanced solvent, like the sterically hindered 2-amino-2-methyl-1-propanol (AMP), is used instead of a conventional solvent like MEA. Increasing the CO2 content in the flue gas is investigated by recycling the flue gas over the gas turbine. The flue gas recycle is beneficial for the overall capture process behaviour. The total flue gas flow rate is reduced with increasing flue gas recycle ratio. This reduction in the flue gas flow rate results in a smaller absorber column. The capital investment, the cost of electricity and cost of CO2 avoided are reduced with increasing the flue gas recycle ratio. There is a marginal effect of the flue gas recycle on the solvent regeneration energy using the conventional MEA solvent. This is due to the limitation in MEA solvent capacity. Moreover, the effect of the flue gas recycle on the energy requirement and the overall cost is more significant using a different solvent with higher loading capacity (e.g. AMP). As has been observed out of the MEA conventional process analysis, the desorption energy requirement is a significant burden for large-scale applications. To overcome the high-energy demand and to increase the operational flexibility, a new process concept is investigated. This process concept is based on dividing the CO2 capture process into a bulk removal step and a deep removal step using two different solvent/systems. This two-step concept is evaluated for two different cases. Both cases are based on the use of MEA in the first step. In the second step, either AMP solution or coal/activated carbon is used for the removal of the remaining CO2. The results show that the removal of CO2 using coal or activated carbon is not advantageous due to the large quantity of coal/activated carbon needed. On the hand, the use of the two-chemical solvent has shown potential for possible process improvement. The overall energy requirements for the two-solvent concept can be reduced by 16 % as compared to the MEA reference case. Due to the higher capital costs, the overall cost of carbon dioxide avoided in the 2-step concept increases by 13 %. Still, increasing the capture process flexibility can be an advantage of the 2-step concept. This flexibility allows the application of different operating conditions and/or process systems in the different absorption-desorption units. One of the benefits can be the use of waste heat for regeneration, by operating one of the desorbers at lower temperature. From the analysis of the post-combustion capture process that has been done in this thesis, it is evident that to achieve significant reduction of the capture process cost, multiple process parameters need to be improved. For future development of the CO2 post-combustion capture process, it would be beneficial to direct the solvent development research towards solvents systems, which have lower reaction enthalpy and higher capacity. A significant improvement can be obtained by the development of solvent systems where the solvent is regenerated at higher pressure. In addition, smart process improvement and integration are required to achieve a reasonable cost reduction. Flue gas recycle over the gas turbine can contribute by reducing the overall capital investment. Splitting the capture process and/or combining it with co-production of hydrogen can be an extra economic parameter in the overall process optimization. It can be expected that by improving the process design and the solvent, implementation of post combustion capture on larger scale will be possible in the near future.