(681d) Optimization of the Various Modes of Flexible Operation for Post-Combustion CO2 Capture Plant

Adding a post-combustion CO₂ capture plant to a power plant comes with a substantial energy penalty as significant amounts of steam must be used to strip off the absorbed CO₂ and regenerate the solvent. This leads to a decrease in the power output of the plant, typically by ~30 %. One option to mitigate the adverse effect of power plant output loss is to operate the CO₂ capture plant in flexible modes in response to the varying electricity market price.  This way the overall profit can be improved while meeting the peak electricity demand. Flexible operation can be done, for example, by reducing the capture level (through CO₂ venting, by-passing a part of flue gas, CO₂ capture level reduction, etc.), or by reducing the amount of regenerated spent solvent on a periodic or temporary basis, i.e., storing the CO₂ rich solvent in a tank for a time while ensuring a constant CO₂ capture level by using the already regenerated solvent stored in another tank. The objective of this work is to simulate, optimize, and analyze the economics of the various modes of flexible operation including the CO₂ capture level reduction, solvent storage, and the both options simultaneously. An equilibrium based model for complete absorption/regeneration system has been used and its optimizations are performed in the gPROMS interface.

There are three novel features in this work when compared to the previous works [1-5]:  1) A more detailed, accurate process flow sheet model is used for the analysis and optimization rather than first-order approximations more suitable for quick screening purposes. 2) Many previous works treated captured CO₂ as products of market value. They then posed the problem as optimizing the flexible operation in light of varying energy price and CO₂ market price. In contrast, we consider penalties on the CO₂ emission. We assume that captured CO₂ holds no market value as the market for CO₂ is likely to be highly limited in view of the huge CO₂ quantity once CO₂ capture becomes ubiquitous. 3) Both options of dynamically varying the capture level and storing the solvent (with varying storage capacities and different input energy cost profiles) are considered simultaneously. Therefore, the simultaneous operation can have advantages in optimizing both capture reduction level and regeneration level for the solvent storage.  The Simultaneous mode of the flexible operation has been shown to offer the greatest advantage over the inflexible operation. 

References

1.            Cohen, S.M., et al., Comparing post-combustion CO 2 capture operation at retrofitted coal-fired power plants in the Texas and Great Britain electric grids. Environmental Research Letters, 2011. 6(2): p. 024001.

2.            Cohen, S.M., G.T. Rochelle, and M.E. Webber, Optimizing post-combustion CO2 capture in response to volatile electricity prices. International Journal of Greenhouse Gas Control, 2012. 8(0): p. 180-195.

3.            Wiley, D.E., M.T. Ho, and L. Donde, Technical and economic opportunities for flexible CO2 capture at Australian black coal fired power plants. Energy Procedia, 2011. 4(0): p. 1893-1900.

4.            Cohen, S.M., G.T. Rochelle, and M.E. Webber, Optimal operation of flexible post-combustion CO2 capture in response to volatile electricity prices. Energy Procedia, 2011. 4(0): p. 2604-2611.

5.            Chalmers, H., et al., Valuing flexible operation of power plants with CO2 capture. Energy Procedia, 2009. 1(1): p. 4289-4296.