https://doi.org/10.1016/j.ijggc.2015.12.009
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Due to the nature of the shutdown sequence, which involves a continued circulation of solvent at 30% of baseload in order to lower solvent temperature more rapidly, and consequently an equilibration of solvent loadings due to mixing, the startup sequence is initiated with lean loading and rich loading, respectively, higher and lower than baseload values at baseload conditions.
Gas flow is initiated at t = 0 and increased at a rate of 5.7% of baseload/min. Once the flow of gas reaches 30%, at approx. t = 5 min 15 s, the flow rate of liquid commences ramping at 5.7%/min in order to maintain a constant L/G flow ratio.
As suggested by the models of Ceccarelli et al. (2014), the CO2 capture rate (Fig. 8c) is initially higher than that of baseload operation between t = 0 and approximately t = 5 min, as the gas flow rate is ramped up to reach the operating liquid to gas ratio of the absorber. With the holdup on the packing already established, a cool solvent also results in a higher driving force for CO2 absorption. Once the target liquid/gas flow ratio is established at t = 5 min and the absorber temperature profile increases in magnitude (Fig. 9a), the capture rate begins to drop due to the lean solvent loading becoming gradually higher than at baseload operation, resulting in a lower driving force for the absorption of CO2 (Fig. 8c).
Fig. 8. (a) Gas, liquid and steam flow rates as percentage of previously-defined baseload operation, gas turbine startup scenario. (b) Rich and lean solvent CO2 loading at sampling ports, desorber and reboiler temperatures, gas turbine startup 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, gas turbine startup scenario. (d) Predicted real-time solvent capacity and CO2 capture rate, gas turbine startup scenario.
Fig. 9. (a) Evolution of absorber temperature profile, t = 0 min – t = 30 min, gas turbine startup scenario. (b) Evolution of absorber temperature profile, t = 50 min – t = 80 min, gas turbine startup scenario. (c) Evolution of absorber temperature profile, t = 90 min – t = 140 min, gas turbine startup scenario.
At t = 11 min, ramp rates of both liquid and gas flow rates are decreased to approx. 2.45%/min and retained at this value until both reach 100% of baseload, at around t = 26 min. Steam flow rate is ramped at a rate of 18.0%/min from t = 15 to t = 18 min, before the ramp rate is decreased to approx. 2.55%/min. The manual valve controlling desorber pressure is closed at t = 17 min. Although the flow of steam to the reboiler is established and stabilised at 100% of baseload by t = 35 min, the steam flow to the rich solvent heater (which is used to simulate the effect of the cross-flow heat exchanger) is not initiated until t = 60 min (Fig. 8b), due to an error by the plant operator. The capture rate continues to decrease until solvent which has been exposed to standard desorber operation circulates through the liquid tank and pipework to the absorber inlet.
This was not identified until after the conclusion of the test campaign, but the data is included since valuable information can still be gained from this scenario.
Until t = 60 min, a significant fraction of the thermal input from the reboiler contributes to increasing the temperature of incoming solvent to the desorber by an additional 50–60 °C compared to baseload operation, and energy which could otherwise be used to strip CO2 is lost as sensible heat.
As a consequence, the lean loading gradually increases to around 0.36 mol CO2/mol MEA which compromises the driving force and solvent capacity in the absorber column, as shown in Fig. 8d. This results in a gradual drop in capture level down to around 65% from t = 65 min to t = 90 min.
Interestingly, this effect is somewhat less pronounced than what would be expected for an absorber configuration optimised for natural gas equivalent CO2 concentrations, since the pilot plant facility is operated with a lower rich loading. As a result there is additional margin for the rich loading to increase before a pinch in absorption is reached at the bottom of the absorber.
The decrease in absorber temperature profile (Fig. 9b) can be directly linked to this decrease in capture rate, both of which reach a minimum at t = 80 min. Leaner solvent which has been exposed to baseload desorber and reboiler operation is returned to the absorber after t = 80 min (Fig. 8c and d), resulting in a higher driving force for CO2 absorption and a steady increase in capture rate (Fig. 8c and d) and absorber temperature (Fig. 9c). The capture rate, predicted solvent capacity and continuous lean solvent loading measurement all suggest a steady decrease in solvent lean loading at the absorber inlet after t = 80 min, so it is believed that the bench lean loading measurement of 0.3787 at t = 113 min (Fig. 8c) is the result of an abnormal or unrepresentative titration measurement. Triplicate analysis of 14 samples taken over the course of the test campaign resulted in an estimated uncertainty of 5.2%, suggesting that the source of the error most likely lies within the sample, and not the titration method. The sample may be an example of abnormal “noise” and therefore unrepresentative of overall change, or the result of unknown mixing effects in the desorber sump. Several similar measurements are encountered throughout the test campaign, and unfortunately it is not possible to know definitively the source of the abnormal measurement without more regular sampling.
The outcomes from this experiment suggest that a decrease in capture rate following startup is inevitable as plant parameters stabilise, in cases where some solvent flow rate has been maintained between the shutdown and startup to use the cooling capacity available. If maintaining a high capture rate is of critical importance, interim solvent storage has been proposed as a method to increase capture flexibility, by ensuring that a large separate reserve of lean solvent is available until the plant is stabilised at baseload (Lucquiaud et al., 2014).
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