Current status and future development of solvent-based carbon capture

Solvent-based carbon capture is the most commercially-ready technology for economically and sustainably reaching carbon emission reduction targets in the power sector. Globally, the technology has been deployed to deal with flue gases from large scale power plants and different carbon-intensive industries. The success of the technology is due to significant R&D activities on the process development and decades of industrial experience on acid gas removal processes from gaseous mixtures. In this paper, current status of PCC based on chemical absorption—commercial deployment and demonstration projects, analysis of different solvents and process configurations—is reviewed. Although some successes have been recorded in developing this technology, its commercialization has been generally slow as evidenced in the cancellation of high profile projects across the world. This is partly due to the huge cost burden of the technology and unpredictable government policies. Different research directions, namely new process development involving process intensification, new solvent development and a combination of both, are discussed in this paper as possible pathways for reducing the huge cost of the technology.


Introduction
Carbon capture and storage (CCS) is considered the most sustainable and economic option for cutting down CO 2 emissions from large stationary sources such as coal-fired power plants and other carbon-intensive industries (e.g. refineries, steelworks, cement plants) due to the trilemma of ensuring clean, secure and affordable energy sources (IPCC 2014). CCS technology involves capturing CO 2 from these sources and transporting them to underground storage sites, namely saline aquifer and depleted oil and gas reserves, where they are either stored permanently and prevented from entering the atmosphere or used for enhanced oil recovery (EOR) purposes (IPCC 2014). Without CCS, cost of CO 2 emission reduction in these sectors may be up to 70% more (CCSA 2011). CCS can be implemented using different approaches, namely post-combustion (PCC), pre-combustion and oxyfuel capture (Wang et al. 2011). In the different approaches, there are different processes for separating CO 2 from gas mixtures such as chemical absorption, physical absorption, adsorption and membrane separation. Other emerging processes such as chemical looping (Olaleye and Wang 2014) and calcium looping (Blamey et al. 2010) also have good potentials. Implementing CCS through PCC based on chemical absorption ( Fig. 1) offers some benefits compared to other processes (IEAGHG 2014). These include reliance on established technologies and capacity to be retrofitted to existing power plants/industrial plants with minimal modifications. PCC processes based on chemical absorption (with conventional amine solvents) is also currently at a technology readiness level (TRL) of 6-8 (TRL 6-fully integrated pilot tested in a relevant environment, TRL 7-subscale demonstration, fully functional prototype, TRL8-commercial demonstration) (IEAGHG 2014). Consequently, many first generation CCS projects are expected to be implemented through PCC based on chemical absorption (Wang et al. 2011). A detailed description of the process is given in Wang et al. (2011).
Although several CCS projects using PCC based on chemical absorption have been completed in the past, the capital and operating cost of the process remains unacceptably high and substantial research efforts have been devoted to address this need. The aim of this paper is to provide an update on current and predicted future research and development (R&D) activities on PCC based on chemical absorption. These include pilot plant testing, demonstration project overview, and other commercial activities, assessment of the process configurations (flowsheet development) and different solvents used in the process. These discussions set this paper apart from other related reviews such as Wang et al. (2011) andBoot-Handford et al. (2014).

Process configurations
Alternative process configurations have been developed by adding extra equipments (e.g. heat exchangers, compressors, flash drum etc.) to the conventional process (Fig. 1). Typical examples include configurations involving absorber inter-cooling, multi-pressure stripping and split-flow  (Fisher et al. 2007;Ahn et al. 2013;Boot-Handford et al. 2014). Thermodynamic analysis of these configurations with 30 wt% monoethanolamine (MEA) solvent shows that they are more energy efficient than the conventional configuration (see Table 1). However, due to the extra instrumentation/equipment, their capital costs will be predictably higher. The Boundary Dam commercial PCC plant in Canada incorporates an absorber inter-cooler configuration (IEAGHG 2015).

Solvents
MEA solution (30 wt% or less MEA) is generally considered a benchmark solvent for PCC based on chemical absorption process. The oldest commercial PCC processes, Kerr-McGee/ABB Lummus Crest process and Fluor Daniel's Econamine FG process, use 20 wt% and 30 wt% MEA solutions as solvents respectively (Rao et al. 2004). MEA has rapid kinetics but requires high regeneration energy (in the range of 3.2-4.2 GJ/tonne CO 2 ); the host power plant could be de-rated by more than one-third of its capacity when integrated to a PCC plant with MEA solution as solvent (Fisher et al. 2007). CO 2 loaded MEA solution is also very corrosive and degrades rapidly. MEA solution also entails high solvent circulation rate which leads to large equipment sizes and high energy consumption.
These drawbacks of MEA has been addressed through development of new solvents which include mixed amine solvents such as mixtures of MEA and MDEA, AMP and PZ among others (Dubois and Thomas 2012), ammoniabased solvents (Darde et al. 2010), amino acid solvent (Brouwer et al. 2005), biphasic solvents (Raynal et al. 2011) and ionic liquid-based solvents (Boot-Handford et al. 2014;Zacchello et al. 2016). The new solvents have shown great potential. For instance, biphasic solvents require about 50% less regeneration energy and have about four times cyclic loading capacity compared to MEA (Zhang et al. 2013). Existing commercial PCC processes (see Table 3) use solvents formulated with these new solvents.
Notwithstanding the successes with solvent development, analysis of PCC process using improved solvents such as improved conventional solvents (i.e. mixed amine), precipitating solvents (i.e. amino acid) and biphasic solvents among others shows that levelized cost of electricity (LCOE) for a power plant integrated with the PCC process will only reduce by less than 20% compared to a scenario with MEA solvent (see Fig. 3). The solvents that have shown the greatest potentials namely biphasic solvents are still at development phase with TRL of 4.  Table 2). The demonstration plants integrated to the power plants via flue gas slipstream have resulted in the development of commercial PCC processes (Table 3). Another major milestone in PCC development is the European CO 2 Test Centre Mongstad (TCM) in Norway. TCM is a specialist centre for testing different PCC technologies, namely Chilled Ammonia and Amine-based PCC processes (TCM 2016). TCM was developed by a consortium involving Gassnova, Statoil, Sasol and Shell, and is estimated to have cost about US$1.02 billion. Several PCC technologies based on chemical absorption have been tested successfully at TCM, namely Alstom's Chilled Ammonia Process (CAP TM ), Aker's Clean Carbon, Shell's CanSolv TM and Siemens' PostCap TM among others.
A generic independent verification protocol (IVP) developed by Electric Power Research Institute (EPRI) has been used as an independent benchmark for assessing the

Model development
Dynamic models of stand-alone absorber (Posch and Haider 2013;Kvamsdal et al. 2009;Kvamsdal and Rochelle 2008;Khan et al. 2011;Lawal et al. 2009a) and stand-alone stripper (Lawal et al. 2009b;Zaii et al. 2009) which are the main components of the PCC process based on chemical absorption are available in literature. The dynamic models of the complete process including the absorber and stripper are also available (Lawal et al. 2010;Harun et al. 2011;Gáspár and Cormoş 2011;MacDowell et al. 2013;Flø et al. 2015). Two-film theory is used in most of the models to represent rate-based mass transfer. Some papers such as Posch and Haider (2013) used equilibrium-based approach which involves approximate mass transfer calculations. Comparative assessment of rate-based and equilibriumbased models of the process in Lawal et al. (2009a, b) showed that rate-based models give better predictions. Reaction kinetics are neglected in some of the models (Lawal et al. 2009a, b;Zaii et al. 2009;Lawal et al. 2010;Biliyok et al. 2012;MacDowell et al. 2013). This is based on the assumption that the reactions have rapid kinetics and are able to attain equilibrium. The assumption is valid for cases involving solvents with rapid kinetics such as MEA (Kenig et al. 2001). Reaction kinetics have also been described more accurately by introducing an enhancement factor (Kvamsdal and Rochelle 2008;Kvamsdal et al. 2009;Gáspár and Cormoş 2011;Harun et al. 2011;Khan et al. 2011;Flø 2015) or using actual reaction kinetics model (Aboudheir et al. 2003;Posch and Haider, 2013).
Dynamic models by Lawal et al. (2009a, b), Lawal et al. (2010), Gáspár and Cormoş (2011) and MacDowell et al. (2013) were validated at steady state conditions only over limited conditions at pilot scale due to lack of experimental data for detailed validations as at the time of their publications (Chikukwa et al. 2012). Validation of dynamic models under state steady state and dynamic conditions have also been attempted by Biliyok et al. (2012). The validation results showed good agreement.
The models were used to study the sensitivities of key process variables (e.g. capture level, solvent loading) at different operating conditions under steady state and dynamic scenario and phenomena such as temperature bulge in the absorber (Kvamsdal and Rochelle 2008). More extensive pilot PCC plant data logs (steady state and dynamic) acquired from different PCC demonstration plants namely Brindisi CO 2 capture pilot plant (Italy) and TCM (Norway) are now available (Flø 2015). Brindisi and TCM are significantly large scale compared to the pilot PCC plant at University of Texas, Austin (Dugas 2006), the capture facility at NTNU laboratory in Gløshaugen, Trondheim and the capture facility at SINTEF laboratory in Tiller, Trondheim among others where data used for validating most models were obtained (Lawal et al. 2010;Biliyok et al. 2012;Flø et al. 2015). The new data have been used to validate dynamic models by Flø et al. (2015) under steady state and dynamic conditions.

Commercial tools for model development
Available commercial tools for developing PCC models include Aspen Plus/Custom Modeller developed by Aspen Technology Inc., USA (Zhang et al. 2009) and gCCS developed by a consortium headed by PSE Ltd, UK (Rodríguez et al. 2014). Both applications are CAPE-OPEN compliant and supports rigorous thermodynamic models, namely eNRTL and SAFT-VR among others. The gCCS platform also supports modelling and simulation of all components of the CCS chain (i.e. power plant, solventbased capture plant, CO 2 compression and pipeline transport, and underground storage). It was selected to be used for the Front-End Engineering (FEED) study of the planned commercial-scale Peterhead CCS in Scotland (PSE 2014) which is now suspended indefinitely (BBC 2015).

Commercial deployment
Commercial CO 2 absorption/stripping plants within CCS context are widely deployed in industries; Sleipner CCS Norway, In Salah CCS Algeria, Snøhvit CCS Norway (Eiken et al. 2010;Ringrose et al. 2013) and more recently, Gorgon CCS Australia and Quest CCS Canada (Shell 2015). However, in power plants, Boundary Dam CCS Canada is the only operational CCS project that is based on chemical absorption process (Stéphenne 2014;. The plant was built at a total cost of US$1.3 Billion to capture about one million tonnes of CO 2 per annum using Shell's CanSolv Ò PCC process from a re-built 139 MWe (gross) coal-fired power plant. Based on expected revenue from sales of CO 2 , sulphuric acid and fly ash sales, SaskPower claims that the LCOE of the host power plant is comparable to that of a Natural Gas Combined Cycle (NGCC) Power Plant (Daverne 2012; Clark and Herzog 2014). There have been mixed reports about the success of the Boundary Dam project. SaskPower claims that the plant achieves up to 90% capture level when operational (SaskPower 2015). Media reports suggest otherwise claiming that the plant only achieves about 45% capture level (ENDCOAL 2015;Reneweconomy 2015). Regular mechanical failures have also been reported (Power 2015) and this limited the plant availability to about 40% during some period (Reneweconomy 2015). During this period, SaskPower could not deliver on their CO 2 supply agreement with Cenovus Energy. Recently, SaskPower confirmed nearly 100% availability for the months of Dec. 2015 and Jan. 2016. It projected an average 85% availability for the next year (Estevan 2016).
Other power plant-based carbon capture projects across the world such as Petra-Nova CCS, Texas (to be launched later in 2017) are beset with challenges ranging from unfavourable government policies, lack of economic incentives and huge capital cost.
6 Future research directions 6.1 New process Currently, the process comprises of large absorber and stripper packed beds which contribute significantly to plant footprint, capital and operating cost. Through process intensification (PI), the size of the absorber and strippers can be reduced significantly (Reay et al. 2013;Wang et al. 2015). In PI equipments such as rotating packed beds (RPBs), the liquid and gas flows are subjected to intense centrifugal acceleration which is many times the gravitational acceleration in conventional packed beds. This allows higher flooding rate and lower interfacial mass transfer resistance resulting in significant reduction in the packed bed sizes. Recent studies have demonstrated prospects of replacing conventional packed beds in PCC with RPB (Agarwal et al. 2010;Joel et al. 2014;Thiels et al. 2016). Agarwal et al. (2010) and Joel et al. (2014) reported 7 and 12 times packing volume reduction respectively for separate cases involving replacement of conventional packed bed with RPB for Absorbers in PCC based on chemical absorption. RPBs have been demonstrated successfully in industry for natural gas desulphurization applications (Fig. 4) (Qian et al. 2012). However, application of RPBs in PCC is still at an early stage of development with many issues, namely scale-up, flooding limit, operating performance and pressure drop, that are yet to be properly understood. This is the focus of an engineering and physical sciences research council (EPSRC) funded research consortium in the UK (EPSRC 2014). Other new designs include spinning disc (EPSRC 2014) and microwave technologies for solvent regeneration. The spinning disc technology involves an RPB and reboiler rotated on the same shaft. This design is currently developed in a UK EPSRC funded project (EPSRC 2014). It's a first-of-itskind design and presents a lot of structural and process design challenges. Microwave technology on the other hand, involves using microwave heating for solvent regeneration instead of steam. Industrial microwave heating is already a commercial technology (AMT 2016). However, for solvent regeneration, no previous study has been reported. New challenges expected include thermophysical characterisation of the system among others.

New solvents
New solvents with higher CO 2 loading capacity and lower regeneration energy could significantly reduce the CAPEX and OPEX of PCC process. Emerging solvents with this potential could be classified as follows: • Precipitating solvents: An example include amino acid salts such as potassium taurate among others. Technoeconomic analysis of the DECAB process (Versteeg et al. 2003), a patented PCC process with aqueous amino acid salt solvent, indicated that the capital and operating cost is about half that of a similar capacity MEA process (Brouwer et al. 2005). • Biphasic solvents: These solvents under regulated conditions undergoes liquid-liquid phase separation to give a CO 2 lean and rich phases respectively (Raynal et al. 2011). Analysis of DMX TM process which uses biphasic solvents shows about 50% less regeneration energy and lower reboiler temperature compared to the MEA process.
These solvents are largely at an early stage development with TRL of 4 and not yet substantially proven for commercial deployment in PCC applications. There are also issues with them such as dealing with precipitates in the absorber for amino acid salts (Lerche 2012) and regulating phase change behaviour for biphasic solvents (Zhang et al. 2013) which are yet to be properly understood.

Combination of new solvents and process
Emerging solvents are generally viscous and are difficult to handle efficiently in conventional packed bed designs. New packed designs must therefore be developed for these solvents. In a new project (started in Oct. 2016), ROLIN-CAP (CORDIS 2016), a consortium of 12 partners funded by EU seeks to develop specialized RPBs for biphasic solvents. Biphasic solvents have shown low regeneration energy requirement of about 2.4 GJ/ton of CO 2 compared to about 4 GJ/ton of CO 2 for MEA (Raynal et al. 2011). RPBs on the other hand have also shown good potential in replacing conventional packed beds.

Conclusions
PCC based on chemical absorption is a near term technical option for commercial CCS deployment. The technology has been widely validated through pilot plant tests and different aspects of the technology have been investigated through modelling and simulation. Commercial products for modelling and simulation of such processes are now available. The technology can now be purchased off-the-shelf from different vendors namely Shell, Siemens, FLOUR and Alstom. The technology is already widely deployed in industry but only one large scale commercial carbon capture plant is operating in the power sector. Regardless, the technology CAPEX and OPEX remains unacceptably high. Research in the past decade in this area have been targeted at making the technology more economically-attractive so as to drive its development and deployment. Predicted pathways for research include developing new processes based on PI technology, developing new solvents including precipitating and biphasic solvents and a combination of new processes and new solvents such as the ROLINCAP approach.

References
Aboudheir A, Tontiwachwuthikul P, Chakma A, Idem R (2003) Kinetics of the reactive absorption of carbon dioxide in high Fig. 4 Commercial demonstration of RPB for desulfurization (Qian et al. 2012) Current status and future development of solvent-based carbon capture 11