Effect of the Capture Rate

The effect of the capture rate in the specific total cost was tested for the values of 70%, 80%

and 99%. The results obtained after optimisation are shown in Table A2.6, Table A2.7 and Table A2.8, being compared with those obtained from the initial optimisation.

Considering that the capture rate has a major effect in the reboilers total heat duty, the number of required stripping trains and reboilers in each train change in each optimisation. This way, for a capture rate of 70 and 80% 2 stripping trains containing 3 reboilers each are required, while for a capture rate of 99, 3 stripping trains are required, also associated with 3 reboilers each.

Figure 7.14 captures of the optimal specific total cost for the four capture rates considered. It is possible to observe that for capture rates below 90% there is a reduced cost variation, which at 70% is only 2% lower than the initial optimisation scenario. On the other hand, by increasing the capture rate from 90 to 99% there is an increase in the optimal cost of 28%.

Figure 7.14 – Variation of the optimal specific total cost with the imposed capture rate.

Based on the optimal values obtained, it is possible to demonstrate that the increase in the capture rate is achieved through the increase in the absorber’s height and in the lean solvent flow rate, which is accompanied by a small decrease in the optimal lean loading. This trend can be observed in Figure 7.15, for capture rates between 70% and 90%.

20 30 40 50

70 80 90 99

Specific total cost (€/tCO2)

Capture rate (%)

Figure 7.15 – Variation of the optimal lean solvent flow rate (on the right) and loading (on the left) with the imposed capture rate.

On the other hand, the increase of the capture rate to 99% leads to the lean solvent flow rate reduction, when compared with the 90% case, through the reduction of the optimal lean loading from 0.200 to 0.111 molCO2/molMEA. With the approximation to the physical limit, the costs of increasing the solvent flow rate would surpass the costs of keeping a low CO2 recovery rate, that is, a higher lean loading.

Looking into the lean solvent temperature at the cooler’s outlet (Table A2.6 to Table A2.8), it is possible to see that with the increasing capture rate this temperature is decreased. In the cases of a capture rate of 70 and 80% the temperature difference across the coolers tends to the imposed limit of 0, once again showing that this equipment might not be required.

Considering that the higher the capture rate the more heat is released in the absorber, for lower capture rates the temperature in the absorber tends to be lower and solvent cooling loses relevance, since the flue gas temperature is constant. On the other hand, for higher capture rates, such as 99%, the temperature in the absorber tends to increase. Taking into account that the optimal flow rate in this case is 20% lower than in the 90% capture rate case, this heating effect is enhanced, as can be verified by looking to the absorber temperature profiles shown in Figure 7.16. Since the absorption efficiency tends to decrease at high temperatures, for high capture rates the lean solvent cooling gains relevance.

Therefore, the lean solvent coolers’ outlet temperature shows a decrease with the increase of the capture rate.

600 800 1000 1200

70 80 90 99

Lean solvent flow rate (kg/s)

Capture rate (%)

0 0.05 0.1 0.15 0.2 0.25

70 80 90 99

Lean loading (molMEA/molCO2)

Capture rate (%)

64

Figure 7.16 – Axial temperature profile of the vapour phase (on the left) and liquid phase (on the right) in the absorber for the optimised cases with capture rates (CR) of 70, 80, 90 and 99%.

A reduction in the optimal rich solvent temperature at the lean-rich heat exchanger outlet is also observed. The reduction is associated with a major change in the strippers’ operating conditions. This can be observed in Figure 7.17, that shows the optimal temperature profiles in both liquid and vapour phases for each optimisation.

Figure 7.17 – Axial temperature profile of the vapour phase (on the left) and liquid phase (on the right) in the stripper for the optimised cases with capture rates (CR) of 70, 80, 90 and 99%.

Despite the lower inlet temperature of the rich solvent, the bulk temperature in both phases is kept at a higher value across the column. This can be explained by the reduction of 20% in the solvent flow rate, associated with an increase of 45% in the vapour flow rate exiting the reboiler (at the same temperature), which reduces the temperature variation in the vapour phase.

Figure 7.17 also shows that for the 99% capture rate case, the temperature difference between the strippers’ liquid inlet and vapour outlet changes. While for lower capture rates the liquid inlet optimal temperature is higher than the vapour outlet temperature, in the 99% capture rate case, the liquid

310 315 320 325 330 335

0 0.5 1

Temeprature (K)

Relative position to the absorber top CR = 70% CR = 80%

CR = 90% CR = 99%

300 305 310 315 320 325 330 335

0 0.5 1

Temeprature (K)

Relative position to the absorber top

CR = 70% CR = 80%

CR = 90% CR = 99%

370 375 380 385 390 395

0 0.5 1

Temeprature (K)

Relative position to the stripper top

CR = 70% CR = 80%

CR = 90% CR = 99%

330 345 360 375 390 405

0 0.5 1

Temeprature (K)

Relative position to the stripper top

CR = 70% CR = 80%

CR = 90% CR = 99%

stream enters the column at a lower temperature. This way, instead of an initial increase in the CO2 flux from the liquid to the vapour phase, there is an increase of the flux in the opposite direction, as seen in Figure 7.18. In this case, the reduced temperature of the liquid phase leads to the partial dissolution of the vapour phase, thus leading to an initial CO2 absorption into the liquid phase. Nevertheless, due to this transfer to the liquid phase, this phase’s temperature is increased leading to a major increase in the CO2 flux to the vapour phase.

Figure 7.18 – CO2 molar flux from the gas to the liquid phase across the stripper, for the optimised cases with capture rates (CR) of 70, 80, 90 and 99%.

Considering this, in the 99% capture rate case, the temperature profile in the stripper is defined by the reboiler temperature and the heat released in the initial absorption and is not particularly affected by the rich solvent temperature, which can be decreased to the optimal value of 346.52 K.

Nevertheless, the CO2 recovered in the stripping section at a capture rate of 99% is increased from 56% (optimal value at 90% capture rate) to 76%. This is due to an increasing in the columns’

height and in the vapour flow rate exiting the reboilers and consequently in the steam consumption, thus greatly increasing the sOPEX.

Besides the steam consumption, the electricity annual consumption also tends to increase with the increasing capture rate, either due to the flow rate increase or to the increase in the column’s height.

The cooling water consumption is also increased, since the flow rate entering the condenser is higher for higher capture rates. In the case of 99% capture the cooling consumption is further increased, due to the reduction in the lean solvent temperature after the coolers.

Observing the variation in the sCAPEX for lower capture rates (Figure 7.19), it is possible to conclude that its value does not change considerably, suggesting an increase, which leads to a minor increase in the optimal specific total cost between the capture rates of 80 and 70%. This way, it is possible to conclude that for lower capture rates the reduction in the captured amount of CO2 may surpass the reduction in the total cost, thus increasing the cost per tonne of captured CO2.

-0.01 -0.008 -0.006 -0.004 -0.002 0 0.002

0 0.2 0.4 0.6 0.8 1

CO2molar flux (kmol.m-2.s-1)

Relative position to the stripper top

CR = 70% CR = 80% CR = 90% CR = 99%

66

Figure 7.19 – sCAPEX percentage variation with the imposed capture rate (90% capture rate considered 100%).