PCC – CARER: Post-Combustion Capture – Cost and Residual Emission Reduction (Flexible Funding 2021)

Work on this Flexible Funding 2021 project so far has involved two main components:

1. Pilot scale testing on the TERC amine capture plant (ACP) to explore plant parameters required to obtain very high capture levels, 95% and above
2. Development of a lab-scale reclaiming capability

In the first ACP trials, for elevated temperature solvent reclaiming, the ACP experienced a heater failure, although within its rated operating envelope.  The lost time while this was repaired caused severe pressure on the overall ACP testing schedule, covering a number of projects, which was already tight because of a flood in 2019 and delays in reinstating the plant due to COVID.  As a result, subsequent tests to investigate absorber performance were delayed until November 2022, and access to install and test temporary solvent storage has not been feasible to date.

Nonetheless, work on the ACP has shown that high levels of CO₂ capture can indeed be achieved even with the modest amount of absorber packing (12m) available and that the ACP reclaimer can deliver solvent with very low lean loadings (~0.1 molCO₂/molMEA), although not at high enough pressures and temperatures to test modelling predictions of low specific reboiler duties.  Techniques to operate at high capture levels have been explored and the limits on achievable gas and solvent flow rates for operation under gas turbine conditions have been defined – the plant was originally designed for operation on coal flue gases.

Figure 1: TERC amine capture plant (ACP) – the two absorber columns are on the left

Two 0.25 m diameter absorber vessels are installed in series to increase residence time and contact between liquid and gas.  Each of the absorbers is equipped with two beds of Flexipac 350X structured packing, 3m each. In further discussions they are viewed as a single absorber with a total of 12m of packing.

The objective of the main test campaign undertaken was to investigate whether high capture levels, 95% and above, and high rich loadings (~0.45 molCO₂/molMEA) could be obtained with the 12m of packing in the ACP absorber, using a combination of low lean loadings and low liquid-to-gas (L/G) ratios.  Artificial flue gas mixtures of CO₂ in air were used.

Results for the nine tests undertaken are shown in the table below.  High capture levels (>95-99%) were measured using gas concentration measurements, giving a good confidence, at lean loadings up to around 0.15 molCO₂/molMEA.  Where L/G ratios allowed, rich loadings of 0.45 molCO₂/molMEA or higher were measured.  Based on previous modelling work (see UKCCSRC Co-Cap project) lean loadings in the range shown below are expected to require little or no additional reboiler heat per tonne of CO₂ captured, provided a sufficiently-high stripper pressure is used to suppress excessive steam production.

Table 1: Steady-state test points from high-capture test campaign

Measured absorber temperature probe readings (an uncontrollable combination of gas and liquid temperatures) and flue gas CO₂ concentrations at the midpoint (see Figure 2 below) show the upper absorber has little to do; by the time the flue gas reaches it, CO₂ concentrations are already very low.

Figure 2: Absorber temperature profiles and measured CO₂ concentrations for selected runs with liquid and gas flows approximately matched

A key lesson for commercial plant operation at high capture levels is that they may benefit from using lean and rich solvent storage all the time (i.e. not just for start/stop), to give independent operation of absorber and stripper and thus allow precise solvent flow control to meet time-specific requirements of each unit, for the reasons noted below:

Absorber:

• important to have the L/G ratio in the absorber neither too low, to get the capture level required,
• nor too high, to get the highest possible rich loading

Stripper/reboiler:

• liquid flow to the stripper no higher than the energy available can strip to the required lean loading (otherwise high capture impossible)
• and rich flow not too low either if the lean loading then goes beyond the specific reboiler duty (SRD)/loading inflection point at that pressure (see UKCCSRC Co-Cap modelling for a more detailed explanation of the inflection point in SRD vs lean loading)
• Some flexibility in lean loading if above the inflection point and higher than required for the capture level, but at the expense of higher reboiler temperatures and extra packing in the absorber (for periods when lean loading is lower than the inflection value).

Approximate quantification of the consequences of solvent flow (based on UKCCSRC Co-Cap modelling) are as follows:

1. Being short on solvent by 1% will decrease capture by about 1 percentage point, although the rich loading may increase slightly (but depends on packing height) and hence SRD may decrease slightly.
2. Being high on solvent flow by 1% for e.g. a lean of 0.15 and a rich of 0.45 molCO₂/molMEA will decrease rich loading by roughly 0.35/100 = 0.0035, which would correspond to 0.035 GJ/tCO2 or 1% of 3.5 GJ/tCO₂ total SRD.

Possible control options that commercial plants using solvent storage might use to match solvent flows precisely to absorber and stripper conditions are summarized below:

Absorber:

• Rapid assessment of capture level possible based on gas measurements
• If capture level is too low then can increase solvent flow up to limit set by lean loading and packing height
• Max lean flow needs to be limited to achieve high rich loading, ideally would have rapid rich loading assessment and not go below limiting rich loading value; could also have upper lean solvent flow limit based on inlet CO₂ and total flue gas flow, with matrix of values set by experience
• The value of marginal increase/decrease in capture level will change with effective electricity and carbon price – but may be market distortions due to DPA terms

Stripper/reboiler:

• In principle could use reboiler temperature at a given stripper pressure to indicate lean loading achieved, but may not be precise enough; otherwise need to measure lean loading directly or estimate from solvent flow and CO₂ flow
• If lean loading is too high, increase steam or decrease liquid flow to the stripper and vice versa
• Could also make heat input a function of solvent flow and/or CO₂ flow, or limit solvent flow based on heat available, with ratios based on experience

Overall, absorber and stripper must match flows but with buffering from storage to avoid disturbances propagating and long delays in adjusting to changed flue gas inlet conditions.  There ought to be economic incentives to time-shift the capture energy penalty as well (i.e. deliberately delay in replenishing the lean solvent store to a period when the value of the electricity output penalty is as low as possible), within the hardware limits of the plant.  This is an obvious job for an ‘Efficiency Engineer’ on a large power/PCC plant!

The proposed future PCC-CARER work using industry funding is an investigation of absorber performance under conditions where high capture levels can be achieved, with comprehensive data collection to give a more detailed understanding of ACP performance and to inform future testing and commercial plant operating principles.

The reclaimer work is being taken forward by a new PhD student, Marcin Pokora, supervised by co-investigator Abby Samson.  Starting in a new laboratory, an atmospheric, or reduced, pressure reclaimer system has been set up and is nearly ready to use (see Figure 3).  Dr Samson is also supervising an external student at the University of Lincoln who is building a stainless steel reclaimer rig capable of operating at elevated pressures. Since both of these students are at a relatively early stage in their studies, further results are not available at this time, but scope exists for a sustained research programme on the relationship between reclaimer operation and effectiveness.

Figure 3: Glassware laboratory reclaimer – atmospheric and sub-atmospheric testing

Jon Gibbins, Muhammad Akram, Daniel Mullen, Marcin Pokora, Mohamed Pourkashanian, Abby Samson (University of Sheffield)