https://doi.org/10.1016/j.ijggc.2015.12.009
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Before the capture plant decoupling scenario is initiated, the plant is operating at steady state baseload. The flow rates of steam and gas are reduced to zero at t = 0 min and t = 1 min, respectively, with the capture rate dropping to zero shortly thereafter, at t = 2.5 min, since gas no longer passes through the absorber. The manual valve maintaining pressure in the desorber is released at t = 8 min to prevent creation of vacuum in the desorber. The flow of liquid is turned down in two steps, to 75% of baseload at t = 9 min, then 50% at t = 14 min. Solvent flow is maintained at 50%, reproducing likely operating conditions where power for solvent circulation is reduced to a minimum to maximise electricity power output. Maintaining a minimum solvent flow rate also allows, if necessary, for fast startup of the capture plant than turning off the solvent pumps. This lower solvent flow rate, combined with a constant solvent cooling capacity, ensures that a substantial reserve of solvent capacity is available so that capture rate can rapidly return to normal levels upon reintroduction of flue gas to the absorber column.
Lean solvent loading measured via titration at the desorber outlet appears to rise between t = 1 min and t = 6 min, before dropping once again at t = 11 min. The rise in lean loading around t = 6 min is not observed in the continuous measurement, which indicates that this sample may be unrepresentative of plant trends, as discussed at the end of Section 5.1. The liquid flow rate is still operating at 100% until t = 9 min, and by the time the sample at t = 11 is taken, solvent which has not been exposed to CO2 in the absorber begins to appear at the desorber outlet. This is consistent with the circulation time between the top of the absorber sump and top of the desorber sump at L = 100% being approx. 8 min in total, allowing 6 min from absorber outlet to desorber, plus an additional 2 min to pass through the desorber sump.
After the flow of gas has been shut down, the absorber temperature bulge decreases in magnitude and moves down the column as hot solvent in the upper region of the absorber flows down the packed bed, being replaced by colder solvent as liquid (Fig. 11a). The absorber is insulated and requires some time to cool down, as heat is transferred from the packing to the incoming cool solvent.
Assuming that the lean titration measurement at t = 6 min is anomalous and unrepresentative of true plant trends, lean loading at the desorber outlet sampling port decreases from 0.202 mol/mol to 0.174 mol/mol from t = 0 min until around t = 50 min (Fig. 10b) as residual heat in the desorber continues to liberate small amounts of CO2, from solvent not exposed to CO2 in the absorber.
Fig. 10. (a) Gas, liquid and steam flow rates as percentage of previously-defined baseload operation, power output maximisation by capture plant decoupling scenario. (b) Rich and lean solvent CO2 loading at sampling ports, desorber and reboiler temperatures, power output maximisation by capture plant decoupling scenario. (c) Lean and rich solvent loading titration measurements, shifted to time of absorber inlet entry and outlet exit respectively, continuous lean loading measurement, CO2 capture rate, power output maximisation by capture plant decoupling scenario. (d) Predicted real-time solvent capacity and CO2 capture rate, power output maximisation by capture plant decoupling scenario. (For interpretation of the references to color in the text, the reader is referred to the web version of the article.)
Lean loading titration measurements taken from t = 0 min are shifted forward to the time of entry of the corresponding packet of solvent to the absorber inlet (blue markers) in Fig. 10c. The continuous online solvent measurement shows good agreement with the titration data until t = 85 min. Based on the liquid flow rate and volume of liquid in the pipework between the lean solvent sampling port and absorber inlet, the lean loading value of 0.204 mol/mol at t = 1 min (Fig. 10b), is predicted to reach the absorber inlet at t = 54 min (Fig. 10c). This appears very close to the rich loading value of 0.213 mol/mol, corresponding to the time of exit of the absorber outlet at t = 54 min. Given that there is no CO2 flow into the absorber between t = 1 min and t = 54 min, this indicates that the time-shifting method provides a moderately accurate estimation of solvent circulation time of 53 min. This is also roughly consistent with the estimated circulation time of 63 min at 50% of baseload flow rate shown in Table 3, given than titration measurements are taken every 5 min and that the solvent flow rate does not reach 50% of baseload for the first 15 min.
The temperature profile of the absorber (Fig. 11a) decreases in magnitude after t = 0 min, as the exothermic absorption of CO2 stops after the flow rate reaches zero. The absorber temperature bulge exhibits a similar response to that observed during simulated gas turbine shutdown, decreasing in magnitude and moving down the packing height as hot solvent flows down the packed bed and is replaced by slow flowing incoming cooler solvent. By t = 15 min there is no observable bulge, and the temperature across the entirety of the packed bed decreases until flue gas is reintroduced.
Fig. 11. (a) Evolution of absorber temperature profile during power output maximisation by capture plant decoupling scenario – temperature decrease. (b) Evolution of absorber temperature profile during power output maximisation by capture plant decoupling scenario – temperature increase.
The desorber manual release valve is closed and pressure begins to rise at t = 55 min. At t = 60 min, the bypass event ends and the flow rates of gas and steam are ramped back up to 100%.
Rich loading increases rapidly upon reintroduction of CO2 to the absorber at t = 62 min. The capture rate is initially higher than the baseload value as a batch of cool solvent promotes CO2 absorption, and this is reflected in the rich loading titration measurements of 0.316, 0.313, 0.306 mol/mol at t = 72, 77 and 87 min (respectively Fig. 10b). The lean loading at the desorber outlet follows a similar trend once the desorber has reached operational temperature, with the circulation time from the absorber sump to the desorber sump being around 6 min at 100% baseload solvent flow rate. A temperature bulge is observed in the absorber at t = 65 min, 3 min after the reintroduction of gas flow, and is fully established by t = 95 min.
Fig. 10c shows the lean and rich solvent measurements time-shifted to the absorber inlet and outlet. It should be noted that the time-shifting method detailed in Section 3.4 contains a level of uncertainty due to the hydraulic response of the plant, and in this case predicts an increase in solvent loading at the base of the absorber packing at t = 61 min, 60 s before the flow of gas is reintroduced. A batch of richer lean solvent which is predicted to reach the inlet between t = 85 and t = 99 min appears not to be detected by the online solvent sensor, and has no real effect on the capture rate. It is possible that there is sufficient mixing with leaner solvent between the sampling port and the absorber inlet, that the increase in loading is dampened. It is also possible that the titration measurement at t = 72 min (Fig. 10b)/t = 99 min (Fig. 10c) is unrepresentative of plant conditions, as discussed in Section 5.1.
The time shifting method is used to show the solvent working capacity in the absorber, in mol/mol (Fig. 10d), and how it correlates to capture level. Solvent working capacity is calculated as described in Section 3.4.
In this scenario there is no observable reduction in capture rate upon reintroduction of flue gas. However, it is worth noting that the effect on capture rate upon reintroduction of flue gas will be highly dependent on the length of the shutdown operation, the circulation time, total solvent inventory and extent of mixing in the liquid loop for each individual capture plant.
Potential detrimental effects on capture rate upon restart could be mitigated by using interim solvent storage. This involves holding a batch of lean solvent in reserve so it can be fed to the absorber while the original solvent is being regenerated. The stored lean solvent can also be used to replace the original which is diverted to a rich solvent storage tank for future regeneration when energy selling price is low.
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