https://doi.org/10.1016/j.ijggc.2015.12.009
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Although similar to the previous scenario, a significant difference is that flue gas is continuously fed to the absorber column during reboiler steam shutdown. Steam flow to the reboiler is reduced to zero at t = 0 min, followed by the opening of the desorber pressure release valve at t = 1 min. Solvent flow rate is turned down to 75%, then 50% at t = 8 min and t = 15 min respectively. The capture rate begins to drop at t = 8 min after the initial decrease in solvent flow, but does not decrease sharply until t = 17 min.
Lean loading at the desorber outlet appears to fluctuate between t = 6 and t = 31 min, based on the titration measurements taken. When the flow rate of solvent is reduced rapidly, the PID control system may fully close the desorber outlet valve for several minutes in an attempt to maintain desorber sump level. Solvent which has not been exposed to steam flow will have ample opportunity to mix with fully regenerated solvent in the desorber sump. This mixing effect makes it very difficult to predict what will happen to the lean loading at the desorber outlet, although based on the results from the online solvent sensor, it is possible that the titration measurements at t = 11 and t = 21 min are abnormal or unrepresentative of the lean solvent loading which will be seen by the solvent sensor, due to mixing effects in the main solvent tank.
In this and the final dynamic scenario (Section 5.5), the online solvent sensor is relocated to just downstream of the main solvent pump. Interestingly, when the continuous measurement output and titration values are shifted to the time at which each packet of solvent would reach the absorber inlet (Fig. 12c), this fluctuating lean loading behaviour is not observed. This may suggest that the fluctuations in lean solvent loading at the desorber are dampened due to mixing in the solvent tank, and will not have a considerable impact on the lean loading at the absorber inlet.
Fig. 12. (a) Gas, liquid and steam flow rates as percentage of previously-defined baseload operation, for power output maximisation by reboiler steam decoupling scenario. (b) Rich and lean solvent CO2 loading at sample points, desorber and reboiler temperatures, power output maximisation by reboiler steam decoupling scenario. (c) Rich and lean solvent loading bench measurements, time-shifted to absorber inlet and outlet respectively, continuous lean loading measurement, CO2 capture rate, power output maximisation by steam decoupling scenario. (d) Predicted real-time solvent capacity and CO2 capture rate, power output maximisation by steam decoupling scenario.
The general trend in rich loading at the absorber outlet before the flow rate of liquid is ramped back up to 100% at t = 53 min is an increasing one (Fig. 12b). This is a direct result of the decreased L/G flow ratio, down from 2.86 at baseload to 1.43 from t = 17 min, and the increased liquid residence time on the packing, which results in additional CO2 being absorbed per unit of solvent flowing into the column. Although the rich solvent loading at the bottom of the absorber increases, the liquid flow rate is low enough that the amount of CO2 captured is reduced overall. The absorber temperature bulge decreases in magnitude (Fig. 13a), consistent with the decrease in capture rate, and moves upwards. The upwards shift may be due to diminished working capacity, capture rate and L/G ratio, which results in higher concentrations of CO2 in the upper regions of the absorber. The solvent becomes loaded with CO2 more rapidly upon entry, decreasing the driving force for CO2 absorption as it flows down the packed bed. Higher CO2 concentration in the upper regions of the packing also results in a higher driving force for CO2 absorption on the gas-side.
Fig. 13. (a) Evolution of absorber temperature profile, power output maximisation by reboiler steam decoupling scenario – temperature decrease. (b) Evolution of absorber temperature profile, power output maximisation by reboiler steam decoupling scenario – temperature increase.
The desorber pressure release valve is closed and liquid flow ramped back up at t = 52 and 53 min, respectively, in anticipation of the plant returning to normal operating conditions. Steam flow is ramped back up to 100% at t = 60 min. In total, the steam shutdown operation lasts 60 min, a plausible duration for a bypass in response to an evening peak in electricity prices. Lean loading at the desorber outlet decreases between t = 71 and t = 76 min, once the desorber reaches operational temperature (Fig. 12b). The estimated real-time solvent loading capacity (Fig. 12d) appears to follow the capture rate for the majority of the scenario, except for two points at t = 54.07 and 61.08. This may be due to the fact that the lean loading titration measurements, which the calculations are based on, are erroneous or that the CO2 flow rate into the absorber is not entirely stable during this time, with the mass flow controller aiming to maintain a steady CO2 concentration at the absorber inlet while the capture rate and concentration of CO2 in the recycled flue gas stream is highly variable.
Capture rate is observed to decrease between t = 75 and t = 95 min, corresponding to the time at which solvent not fully regenerated in the desorber is predicted to reach the absorber inlet (Fig. 12c).
There is good agreement around t = 95 min between solvent working capacity and capture rate, yet, in Fig. 12c, the lean loading value of 0.312 mol/mol, time shifted to t = 95 min, is close to the rich loading value of 0.348 mol/mol while a capture rate of >75% is achieved. This is considerably higher than the baseload lean loading value of 0.232 mol MEA/mol CO2 (Section 3.1) As mentioned in Section 3.4, the time shifting method is not able to account for mixing effects, since it relies on the assumption of plug flow. This illustrates that it is likely that mixing effects in the solvent tank, located between the lean and the rich sampling points, play an important role to maintain solvent working capacity and capture rate around 75–80% for 20–25 min from t = 75 min onwards.
The batch of richer lean solvent passing through the absorber, indicated by the online measurement lean loading readings in Fig. 12c at around t = 95 min, is then followed by a rise in capture rate before the plant returns to steady state baseload operation. The absorber temperature bulge reflects the evolution of the capture rate, increasing in magnitude and moving down the packing height after t = 60 min (Fig. 13b). The temperature profile decreases slightly around t = 100 min, coinciding with the drop in capture rate, before increasing as the plant reaches steady state operation at t = 120 min. This decrease in capture rate observed 20 min after the supply of thermal energy to the reboiler is resumed could be avoided by using interim solvent storage.
A thorough understanding of plant circulation times, combined with continuous monitoring of solvent loading at the liquid inlet and outlet of the absorber would allow operators to anticipate this decrease in capture rate and compensate accordingly by anticipating to adjust solvent working capacity.
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