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Gas turbine shutdown results and discussion

https://doi.org/10.1016/j.ijggc.2015.12.009

The shutdown sequence is initiated at t = 0 min as shown in Fig. 6a. Steam flow rate is ramped down to zero over 10 min at a rate of 10% of baseload/min, as is representative of state-of the-art gas turbine operation. Liquid and gas flow rates are ramped down simultaneously at a rate of 2.5%/min for a period of 16 min. At t = 16 min, the shutdown rate of liquid and gas was changed to 7.5% of baseload/min. At t = 20 min, liquid and gas flow rates both reach 30% of baseload. The liquid flow rate is held at this value, while the gas flow is further reduced at a rate of 15%/min until it reaches zero. Gas flow rate reaches zero at t = 22 min, while liquid flow is maintained at 30% of baseload until the absorber temperature bulge has subsided (Fig. 7).

Fig. 6. (a) Gas, liquid and steam flow rates as percentage of previously-defined baseload operation, gas turbine shutdown sequence. (b) Rich and lean solvent CO2 loading, CO2 capture rate, desorber and reboiler temperatures, gas turbine shutdown sequence.

Fig. 7. Evolution of absorber temperature profile during gas turbine shutdown operation.

The solvent lean loading at the desorber outlet begins to rise sometime between t = 5 min and t = 10 min due to the reduction in steam flow to the reboiler. Assuming plug flow and no mixing in the solvent tank, the more CO2-rich “packet” of solvent sampled at t = 10 min does not reach the absorber inlet before the gas flow rate reaches zero, using the estimated solvent circulation times as described in Section 3.4. Lean loading at the desorber outlet increases throughout the shutdown operation, but has no significant effect on capture rate as this “richer” lean solvent is not predicted to reach the absorber inlet before of the flow rate of gas reaches zero.

Capture rate is observed to increase from 90% to 97.5% as gas and liquid flow is reduced (Fig. 6b). Since the solvent flow rate is gradually ramped down, the liquid holdup in the absorber requires additional time (compared to baseload) to flow over the packing and into the absorber sump. Although the L/G flow ratio is controlled to remain constant, the flow rate of gas passing through the absorber decreases more rapidly than the liquid holdup on the packing it comes into contact with, so a greater proportion of the CO2 in the flue gas may be absorbed as the gas flow rate approaches and eventually drops to zero. Furthermore, the increased residence time of liquid and gas in the absorber column could result in this increased capture rate.

Although capture rate increases, the temperature profile of the absorber (Fig. 7) decreases in magnitude, as the absolute rate of exothermic CO2 absorption decreases as the gas flow rate approaches zero. After the flow of gas is stopped at t = 20 min, the location of the temperature bulge decreases in magnitude and moves down the packing height. Fig. 7 illustrates the evolution of the absorber temperature profile as hot solvent flows down the packed bed and into the absorber sump, with the packing hold up being replaced by slow flowing, semi-stagnant cooler solvent.

Although no exothermic absorption reaction is taking place, residual heat in the metal elements of the absorber column combined with additional insulation to prevent heat loss, result in the absorber requiring some time to cool down.

It should be noted that the values of temperature at a height of 7.1 m in Fig. 7, and all other figures which represent the temperature profile, refer to a temperature measured directly before the absorber inlet. Heat transfer between the gas flow rate and solvent inventory in liquid distributors can, in practice, be neglected to a first order approximation.

It is important to note that the rich and lean loading values, shown in Fig. 6b, are not representative of the solvent loading at the absorber inlet and outlet at the reported time, and instead represent the loadings at the outlet of the desorber (lean) and absorber (rich) sumps, from where solvent samples were manually taken. In other dynamic scenarios with a longer duration, it is possible to estimate the circulation time required for a “packet” of solvent to reach the absorber inlet and outlet from each sampling port and hence, the real-time solvent working capacity, as explained previously in Section 3.4.

The time shifting method predicts that only the first lean solvent “packet” sampled at t = 0 will reach the absorber inlet before the end of this scenario. Subsequent lean solvent samples were not given sufficient time to reach the absorber inlet. To observe the increase in solvent loading at the rich loading sampling port, the solvent could have be allowed to circulate for a longer period of time, with lean and rich samples taken at more regular intervals in order to observe the full convergence of lean and rich loading as the solvent continues to circulate.

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