Thermodynamic analysis of CO2 capture processes for power plants

Abstract

This thesis work presents an evaluation of various processes for reducing CO2 emissions from natural-gas-fired combined cycle (NGCC) power plants. The scope of the thesis is to focus mainly on post-combustion chemical absorption for NGCC. For the post-combustion capture plant, an important interface is the steam extraction from the steam turbine in order to supply the heat for solvent regeneration. The steam extraction imposes a power production penalty. The thesis includes analysis and comparison between several chemical absorption processes configurations integrated with NGCC.

The objectives of the present work were to use thermodynamic analysis on various chemical absorption process configurations to evaluate, quantify and justify improved design of NGCC with post-combustion CO2 capture. The thermodynamic evaluation of the processes gave insight to the detailed distribution of process irreversibilities and supports the state-of-the-art process configuration with the lowest energy penalty due to addition of CO2 capture to the power plant.

The reference power plant without CO2 capture has a power production of 384 MW and a net electric efficiency of 56.4% (LHV) with CO2 emissions of ≈ 362 g CO2/ net kWh electricity. The power plant design was carried out using the computational tool GTPRO. The aim of the CO2 capture plant was to remove 90% of the CO2 emissions present in the flue gas. To assess and analyse the various chemical absorption process configurations, the UniSim Design software was used, which contains the Amines Property Package. This special property package has been designed to aid the modelling of alkanolamine treating units in which CO2 is removed from gaseous streams. The downstream compression of the captured CO2 was also simulated using UniSim Design.

The investigated process configurations were comprised of chemical absorption process with absorber inter-cooling, split-flow process and lean vapour recompression (LVR) process. Several design parameters were modified for each of the process configurations to achieve low energy consumption and consequently low work demand. The inter-cooling of the absorber column led to increased solvent rich loading. Consequently, the solvent circulation rate and reboiler energy requirement was decreased. In the split-flow configuration, due to splitting of the rich solvent into two streams, the amount of rich solvent entering the bottom section of the stripper was reduced. Therefore, less reboiler energy was required to remove CO2 from the solvent to reach the same solvent lean loading as of the reference chemical absorption process. In the configuration with lean vapour recompression (LVR), the lean solvent stream was utilised as a low temperature heat source in order to add exergy input in the form of steam to the stripper column and thus reduce the reboiler duty. The reboiler duty for the CO2 capture was decreased from 3.74 MJ/kgCO2 in the reference chemical absorption process to 2.71 MJ/kgCO2 for the case of LVR with absorber inter-cooling. The net electric efficiency of the reference process with CO2 capture was calculated to 49.5% (LHV). With the improved process design, the highest net power plant efficiency was calculated to 50.2 % (LHV) for the case of LVR with absorber inter-cooling.

Moreover, exergy analysis was performed to identify the irreversibilities associated with the integration of power plant with various CO2 capture and compression processes. Particularly, the second law of thermodynamics was used as a tool to evaluate and quantify the reduction of energy penalty associated with CO2 capture for each process modification. Defining the work input for a theoretical reversible CO2 capture process as the minimum required work was functional step in characterising the difference of the work input of theoretical reversible processes and the real irreversible processes. Exergy efficiency of the reference chemical absorption process was calculated to 21.3 % versus 25 % for the case of LVR with absorber inter-cooling. Through exergy balance for every CO2 capture process configuration, the exchange of exergy content of material and energy streams was assessed.

Using the combination of power plant efficiency and exergy analysis as tools, a pre-combustion reforming combined cycle (IRCC) process with chemical absorption CO2 capture process was investigated. A rational efficiency of 43.8% was achieved, which indicates the share of input exergy utilised for work production by the power cycle in addition to the exergy of the pure compressed CO2 stream. The highest amount of irreversibility was contributed by the gas turbine and mainly by the combustor. The irreversibility which is inherent in the combustion process corresponded to a large fraction of original exergy of the fuel. This could be partially compensated by increase the preheating of the fuel supplied to the combustor. Also preheating the inlet streams to auto-thermal reactor (ATR) was found advantageous in decreasing the ATR irreversibilities.