https://doi.org/10.1016/j.ijggc.2015.12.009
“The purpose of the test campaign is to implement dynamic post-combustion capture scenarios using a test facility configuration which approximates a PCC pilot plant. A reference “baseload” operation scenario, which all other scenarios are carried out in comparison to, is first determined.
Thirty percent (w/w) MEA solution is used as a liquid absorbent, since it is commonly used as a benchmark for pilot-scale facilities worldwide (Fitzgerald et al., 2014, Hamborg et al., 2014, Artanto et al., 2012).
A desired baseload capture rate of 90% is selected in order to remain consistent with other pilot-scale studies (Rabensteiner et al., 2014, Carey et al., 2013, Mejdell et al., 2011).
Previous work on natural gas flue gas capture (Ystad et al., 2012) indicates that for 30% (w/w) MEA, the liquid to gas flow ratio (L/G) which results in the lowest energy expenditure per ton CO2 captured is approx. 1.5l solvent per m3 gas. Flue gas flow rate is kept constant at 115 m3/h. With further addition of CO2 at a rate of approx. 8.1 kg/h and an inlet CO2 concentration of 4.3% (v/v) (moist), an f-factor of 1.85 Pa0.5 is obtained. Liquid flow rate is varied to obtain L/G ratios between 2.4 and 3.6 l/N m3. For each ratio, at a desorber pressure of 0.8 barg, the flow rate of steam to the reboiler at 4 barg is increased until a CO2 capture rate of 90% is achieved. A minimum reboiler duty of 3.96 GJ/tCO2 for this plant is observed at an L/G ratio of 2.858 l/m3. Typical reboiler duty values for NGCC base case studies have been reported between 3.4 and 4.04 GJ/tCO2 (Jordal et al., 2012).
It should be noted that the lean loading is low in comparison to what would be expected in a capture plant which is purpose-built and optimised for NGCC flue gas, due to a lack of packing height in the absorber compared to current modelling studies (Biliyok et al., 2014, Rabensteiner et al., 2014). In order to compensate and achieve 90% capture, a lower lean loading is used to enhance the driving force for CO2 absorption and the liquid flow rate is increased. This results in a similarly low rich loading, with the effect being exacerbated as the increased liquid flow rate results in a smaller difference in loading between the liquid inlet and outlet. An alternative strategy would be to accept a lower CO2 capture rate while maintaining more representative values of L/G ratio, lean loading and rich loading, but due to equipment limitations caused by the turndown ratio of the liquid pump and steam inlet valve, further reductions in flow rate would result in considerable flow instability, especially during dynamic scenarios which require additional turndown. Increasing the gas flow rate is also not an option, as the blower has a maximum flow rate of around 130 N m3/h due to the pressure drop of the packing and liquid holdup.
Additionally, the energy input to the absorber and desorber solvent trim heaters is not accounted for, due to the absence of steam flow rate measurement to each heat exchanger. It is possible that the reboiler inlet liquid temperature is unrealistically high, so less energy is required to provide sensible heat, and if a cross-heat exchanger were installed a higher reboiler duty would be observed. This seems likely, as the figure of 3.96 GJ/tCO2 is still within the reported range of typical values, with a liquid flow rate which is much higher than optimal.
A full list of conditions is summarised in Table 2.
To allow for comparison with real plant ramping events and future pilot plant campaigns, all flow rates and ramp rates in the dynamic scenarios investigated are subsequently referred to as percentages of this baseload condition.
Due to the low loadings, the solvent retains unused capacity of approximately 0.155 mol CO2/mol free amine at the bottom of the absorber. This results in a significant driving force in this location during baseload operation. During dynamic scenarios which involve shutdown or reduction of stripping steam, this additional capacity will allow the plant to continue to capture CO2 for longer periods of time than if the plant were operated with a rich loading which is closer to equilibrium with the inlet gas CO2 concentration.”