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Post-Combustion CO, Capture

In contrast with from upstream decarbonation and pre-combustion, post-combustion capture (PCC) consists of removing CO, from flue after NG combustion. The downmost step in CO, management in the NG to energy value-chain, PCC is applied to fuel-fired power plants that use air as an oxidant, resulting in CO, diluted in nitrogen and CO, in the 12-15%mol range (Olajire, 2010), affecting capture performance. One of the main PCC challenges is the large energy penalty posed to power generation, representing up to 65-80% of the cost for CCS (D'Allesandro et al., 2010). Adding PCC to fossil fuel-fired power plants decreases the net power production efficiency and increases capital expenditure (Araujo and de Medeiros, 2017). Effective PCC requires CO, recovery with concentrations > 95.5%mol and density = 900 kg/m3 for feasible transportation and storage. The technologies addressed for CO,-rich NG decarbonation also apply in a PCC context. While maintaining their general characteristics, low CO, fugacity in the flue-gas, with high nitrogen and oxygen content, huge flowrates at low pressure creates particularities affecting technical parameters and efficacy (Araujo and de Medeiros, 2017). PCC is a context different from NG decarbonation, resulting in a distinct maturity level, i.e., certain commercially proven technologies in NG are not yet used for flue gases. In the PCC context, employing CA/PA is the most mature and cost- effective CO, capture method (Boot-Handford et ah, 2014).

CA/PA

The low CO, fugacity in flue-gas results in CA being preferred over PA. CA performance stays much the same when compared to decarbonation CA, with MEA, MDEA and DEA being the most used solvents and regeneration heat requirements the main drawback. A PCC specific singularity is the increased amine degradation and equipment corrosion due to high O,, SOx and NOx presence. Consequently, higher amine

Technology

CA

PA

ADS

MP

GLMC

CD

SS

HYB

Commercially

Available

Yes

Yes

No

Yes

No

Yes

Yes

Only CD+MP

TRL

7

7

3

7

2

7

5

2-7

Energy

Requirements

High

High

Medium

Low

Medium

High

Low

Low

Ease of Operation

Medium

Medium

High

High

Low

Low

Medium

Medium

Flexibility degree

Low

Low

Medium

High

High

Medium-High

Medium

High

Best СОг Concentration

Low

High

High

High

Medium

Medium-High

Medium-High

All

Interests

Maturity

Selectivity

Maturity Effectiveness at high pressures

No Emissions Ease of Operation

Flexibility Low energy No emissions Modularity

Modularity

Selectivity

High removal High product pressure

Compactness High CO, removal

Flexibility Potential for reduced cost

Gaps

Solvent degradation Environmental performance Pressure recovery Energy efficiency

CO, selectivity Slow

absorption

CO, selectivity Cost effectiveness Cost of adsorbent Type of

Regeneration (PSA, TSA, ESA)

Selectivity/ permeation Stagnation Efficiency for low pressure

Pressure recovery

Long Term Performance Type of solvent Mass transfer rate Membrane wetting

Energy

Consumption

Size

Cost

Ongoing tests Ease of operation

Real scenario tests Best technology combination

Table 4. Largest CCS projects in the power sector. Source: Koytsoumpa et al (2018).

Project

name

Boundary dam carbon CCS project-the Saskatchewan

Petra Nova carbon capture project

Sinopec Shengli power plant CCS project

Location

Saskatchewan, Canada

Texas, US

Shengli power plant, Dongying, Shangdong Provmce, Chuia

Industry

Power generation (lignite/brown coal)

Power generation - pulverized coal boiler

Power generation - pulverized coal boiler

coi

Capacity

1 Mt/yr

1.4 Mt/yr

40,000 t CO,/yr Scale-up to: 1 Mt/yr

Capture

Process

PCC Amme Shell Global, Cansolv technology

PCC KM-CDR amine scrubbing CO, developed by MHI and KEPCO

PCC SINOPEC

makeup rates are required, increasing operational costs (Araujo and de Medeiros. 2017). Table 4 contains the three largest CCS projects in the power sector employing absorption PCC.

CD

CD occurs the same way in PCC as it does in NG, but separation is no longer from CH4 but rather from N,. Since N. and CO, liquefactions points are substantially different, separation via CD is technically possible. The main issue is the high heat consumption. Leung et al. (2014) state that flue-gas is usually obtained at high temperatures (over 600 °C in some cases) and, to reach cryogenic temperatures, would require impossibly high cooling CD. However, Leung neglected that HRSG (heat recovery and steam generation) is used to produce steam, which drives steam turbines, reducing exhaust gas temperature. A critical aspect for CD in PCC is that the operational pressure should be greater than the triple-point pressure of CO, (5.2 bar) in order to allow existence of liquid CO, for distillation, entailing compression of flue-gas. CD in PCC is limited to niche applications, in which colder flue-gas is obtained and the resulting energy penalty would be low. In addition, some of the main advantages of CD, such as high CO, selectivity and pressurized products, remain relevant only for the CO, product, while N, losses are irrelevant economically, opposed CH4 losses in NG processing.

ADS

ADS has attracted great attention in PCC literature due to its heat reduction potential (Wang et al., 2011). Additionally, ADS materials are more resistant to poisoning than membranes and could minimize footprint when compared to traditional absorption. Aaron and Tsouris (2005) indicate two drawbacks that make ADS currently unfavorable to treat flue-gas. Fust, ADS cannot easily handle high contents of CO,, with optimal conditions being between 0.04% and 1.5%mol. Most carbon-fired power plants have higher contents of CO, in flue-gas such as = 15%v/v (Li et al., 2003). The second reason is that available sorbents are not selective enough for CO, separation from flue-gases.

GLMC

Compared to CA, one of the most noticeable advantages of GLMC is its high interfacial area, which can significantly reduce equipment size and, thus, lead to process intensification (Zhao et al., 2016). GLMC can offer up to 30 times more interfacial area than conventional packed tower gas absorbers, reducing absorber size tenfold. PCC GLMC is comparable to decarbonation GLMC, maintaining much of its benefits and drawbacks without major changes. Despite its great potential in PCC, GLMC is yet to prove its economic viability in the long term.

Table 5. Suggested TRL for CO, capture technologies m the PCC context.

TRL

Development stage completed

Technology for CO, capture from exhaust gas

0

Unproven Concept

-

1

Proven Concept

Supersonic Separator (SS)

2

Validated Concept

-

3

Prototype Teste

Cryogenic Distillation (CD); Adsorption (ADS)

4

Envu-onment Tested

Membrane Penneation (MP); Gas-Liquid Membrane Contactor (GLMC)

5

System Tested

-

6

System Installed

Cheimcal and Physical Absorption (CAPA)

7

Field Proven

-

SS

SS was proposed as a flue-gas treatment technology by Hammer et al. (2014). To assess the operational performance, a Laval nozzle model was implemented and successfully integrated in a steady-state process flowsheet simulator. The model includes equilibrium thermodynamics describing freeze-out of diy-ice from a gas mixture containing CO,. A temperature/pressure-based optimization showed that supersonic expansion is a viable strategy for CO, capture from flue-gas of offshore gas turbines. However, SS in the PCC context is still in its incipient phase, with many proof-of-concept studies and few experimental results. Thus, more tests, including full viability and economic analysis, must be performed before SS can be considered a feasible PCC technology. Based on the literature review, Table 5 shows the suggested TRL values for each of the covered technologies in a PCC context.

Offshore Power Generation

When processing CO,-rich NG at ultra-deepwater offshore rigs, in both NG to onshore facilities and CO, injection wells for EOR. transportation is required (refer to Figure 2). An alternative to bypass the complex subsea logistics network consists in adopting floating power-plants. Often, a simple gas cycle is preferred for power generation on platforms, due to its potentially smaller footprint and weight (Bimuller and Nord, 2015; Flatebo. 2012). On the other hand. NG combined cycle (NGCC) shows higher efficiencies and lower carbon emissions (Folgesvold et al., 2017; Song et al., 2017b; Nord and Bolland, 2013). To further reduce emissions, NGCC can be associated with PCC to directly capture CO,. The generated power is transported through subsea cables (HYDC) to onshore facilities. The concept of transporting electricity instead of gas is known as Gas-to-Wire (GTW) (Watanabe et al., 2006).

Advances in turbo-shafts enable the firing of NG containing up to = 20%mol CO, with satisfactory performance in energy generation (Ariuelli et al., 2017). GTW becomes an attractive solution to process CO,-rich NG in ultra-deepvaters. GTW also bypasses traditional bottlenecks of high GOR by reducing logistics costs and providing additional revenue, namely powder sent to grid and in situ CO,-EOR. Furthermore, electricity has a more stable price than volatile oil products.

Hetland et al. (2009) evaluated the technical feasibility of a GTW-CCS system producing 580 MW of power. A traditional North Sea NG feed was fired in a four-turbine NGCC arrangement followed by an MEA CA-PCC. Results shewed that offshore skids could withstand both an NGCC and a PCC plant while producing powder, but further attention should be given to CO, transportation and water balance. Kvamsdal et al. (2010) expanded the proposed GTW by implor ing the systems water balance and including platform tilting effects on PCC. Optimal PCC design was able to capture 90% of the CO, in flue-gas, while producing 450 MW, noting that the addition of PCC inclined a 9% power loss when compared to onshore NGCCs.

More recently, Roussaualy et al, (2019) studied how rig electrification (i.e., using onshore electricity) w'ould affect a PCC-MEA NGCC. Results showed that full electrification wrould attain best economic/ environmental results, reducing the electricity price from 178/258 USD/MWh to 95 USD/MWh. Nguyen et al. (2016), despite not directly studying GTW-CCS, generated a multi-objective optimization framework to analyze offshore rig performance facing increasing carbon taxes that also concluded that full electrification is the most beneficial solution. In addition, results showed aqueous amine-based CA is the best performing PCC technology for offshore rigs and that, for maximum emission reduction and profit, capture should occur with simultaneous post and pre-combustion (Nguyen et al., 2016).

In summary, with CO,-rich NG in ultra-deepwaters and high GOR—as found in Brazilian pre-salt reservoirs—an innovative and promising technological solution consists in removing just enough CO, to meet NG-fired turbine standards. With turbines receiving undertreated NG, the task effort of NG upgrading is minimized, using only a CO, bulk-removal process, transforming the unpolished gas into power and capturing the unseparated CO, from the flue-gas. Hence, the floating NGCC power plant includes decarbonation and post-combustion capture, maintaining economic viability and drastically improving environmental performance. Literature favors absorption-based technologies, with a focus in CA, with high TRL and good performance in both NG and flue-gas CCS. CA is easily adapted to offshore skids and, if optimized, can have a low footprint maintaining viability though platform tilting and its effects on PCC must be considered. Additionally, since CA usually requires aqueous solvents to be used, special attention must be given to the water balance, as process water make-up is an issue in offshore conditions.

Further attention should also be given to other potential CCS technologies to determine whether performance improvements can be attained. As an example, MP could promote further footprint reduction while maintaining adequate results as high CO, selectivity is not uecessaiy since NG will be binned. Furthermore, technological improvements both in CO, capture and NG processing will greatly affect results and potentially improve performance (Nguyen et al.. 2016).

 
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