Reduction potential of the energy penalty for CO₂ capture in CCS

Abstract

CO₂ capture and storage (CCS) has been acknowledged as an essential part of a portfolio of technologies that are required to achieve cost-effective long-term CO₂ mitigation. However, the development progress of CCS technologies is far behind the targets set by roadmaps, and engineering practices do not lead to commercial deployment. One of the crucial reasons for this delay lies in the unaffordable penalty caused by CO₂ capture, even though the technology has been commonly recognized as achievable. From the aspects of separation and capture technology innovation, the potential and promising direction for solving this problem were analyzed, and correspondingly, the possible path for deployment of CCS in China was discussed. Under the carbon neutral target recently proposed by the Chinese government, the role of CCS and the key milestones for deployment were indicated.

Graphical abstract

Keywords

CO₂ capture and storage (CCS) / CO₂ separation / energy penalty

1 Introduction

Being “net-zero” or carbon neutral, has made the spotlight in the past several years as the most commonly accepted target for climate change mitigation. The number of countries, which had already reached 127 by the end of 2020, that made net-zero pledges, has been increasing. Indeed, a carbon neutral target leaves fossil fuel dependent countries with no other choices than decarbonize the fossil fuel. Among all mitigation options, only CO₂ capture and storage (CCS) provides a solution for using fossil fuels in a low carbon way. Accordingly, the role of CCS is specifically clarified as the only technology that contributes to both reducing emissions in key sectors directly and removing CO₂ from the atmosphere (so-called “negative emissions”, including bioenergy with carbon capture and storage (BECCS) and direct air capture (DAC) to counterbalance emissions that cannot be avoided [1].

CCS has attracted extraordinary attention in the last two decades. Positive research and development programs of CCS had been supported and dozens of roadmaps envisioning the development and deployment of CCS had been issued by different organizations and countries, including the International Energy Agency (IEA) (2009 and 2013) [2,3], the Department of Energy (DOE) and the National Energy Technology Laboratory (NETL) of the US (2002 and 2010) [4,5], the Department of Energy and Climate Change (DECC) of the UK (2012) [6], Norway (2015) [7], Canada (2006) [8] and the National Development and Reform Commission (NDRC) and the Ministry of Science and Technology (MoST) of China (2015, 2013, and 2019) [9–11], etc. Supporting policies, such as the well-known 45Q sponsored by the US government from 2018, provide tax incentives for CCS and are expected to promote the capture and storage of 49 million tons of CO₂ by 2030. Generally, ever since being officially recognized in 1995, CCS has taken on great expectations to make a significant contribution to CO₂ emission reduction [12,13].

However, for a long time, CCS had been considered an achievable technology because most of its component technologies have many years of industrial experience. For example, since the 1960s, the CO₂ separation technology has been widely used in NH₃ and methanol production. CO₂ geological storage, such as the enhancement of oil recovery, has been successfully commercialized in many onshore and offshore oil fields since the 1970s [14–17]. Consequently, CCS has been commonly recognized as a mature technology, and its large-scale demonstration has been set as the priority task in many roadmaps issued in the last two decades [2,3].

Contrary to the high expectations and maturity recognition, the progress of CCS worldwide is far behind. The comparisons of 2009 and 2013 targets in the CCS roadmaps of IEA and the latest situations in 2020 are summarized in Tab.1. As indicated, the progress of demonstration deployment of CCS as of 2020 is still lagging behind the 2013 target (only 20 large-scale demonstration projects were constructed), even though the 2013 target had already significantly shrank compared to the ambitious target in 2009. Additionally, the storage capacity and financial investment did not meet the expectations. The reasons for this delay are of multi-aspects and rather complicated, which may include the high investment and operation cost of CO₂ capture, concerns about the safety of CO₂ storage, gaps in legality and regulations, unsmooth coordination between project partners, limited public understanding, and negative attitudes toward fossil fuels [18–20]. Among all these causes, a rather high CO₂ capture cost of approximately 45–140 $/t CO₂ is the primary and direct one, while behind the cost, high extra energy consumption caused by CO₂ capture is the critical technical gap.

Fig.1 illustrates the efficiency involvement of power generation in the last century to help understand the energy penalty imposed by CO₂ capture. Through cycle upgrades such as increasing the initial temperature and pressure of the cycle and introducing reheating and regeneration, two dominant power generation technologies (coal fired power plants and natural gas fuelled combined cycle) went through an efficiency evolution, where the former increased from 17% to 45% and the latter as high as 60% [21–23]. At the current efficiency level, approximately 1.0 kg CO₂ will be emitted for 1 kW·h power generated, which means that nearly 2 million tons of CO₂ should be captured per year for a typical 300 MW coal fired power plant with a 90% recovery ratio. Correspondingly, at the current technical level, 8–14 points will be paid in a power plant adopting post-combustion capture [24–26], 6–10 points in pre-combustion capture [27–29] and approximately 8–12 points in oxy-fuel combustion [30,31]. It is apparent that the power efficiency will suffer a dramatic drop due to the extra energy consumption if CO₂ capture is adopted, which nearly sacrifices the efforts of a half century by the power industry.

The rather high energy consumption and cost had already been verified by most CO₂ capture demo projects, as shown in Tab.2, and damage had occurred [32,33]. A rather high cost was the leading cause for suspending Kemper County facilities in 2017, which was well known as the first and largest pre-combustion capture demonstration project in the world [34]. The only existing large-scale CCS demonstration projects in the power sector (Petronova project and Boundary dam project) are trapped by the high capture cost, which can only be operated relying on the subsidies from public funding and the benefit from EOR. This is also why the operation of CO₂ capture in Petronova ceased and the facilities mothballed due to low prices of oil in early 2020 [35]. More seriously, considering the need for the construction of hundreds of CCS operations worldwide in the 2020s, rising to thousands in the 2030s and beyond, to capture, transport and store over 8 Gt of CO₂ per year by 2050, these scenarios imply extra energy consumption of more than 1.0 Gt standard coal per year, which is apparently hard to accept under the current conditions. Unaffordable energy consumption and cost due to CO₂ capture undermined the confidences of key stakeholders of CCS and blocked the path from engineering practices to the commercial deployment of CCS. Cutting down the energy consumption and cost of the CO₂ capture process may be the only way to rescue CCS from the current dilemma.

This paper aims to point out the dilemma, lessons and opportunities for CCS development and deployment, and provide a promising direction for energy saving of CO₂ capture. By pointing out the time window of CCS technology, the possible future development path of CCS technology will be given.

2 Methods

2.1 Equivalent work

The boundary of CO₂ separation process in this study is considered from the emission sources to pure CO₂ without compression. Considering the diversity of energy consumption forms according to various technical routes, the value and unit for each type of energy, such as heat for reboiler or cold energy for refrigeration (GJ/t CO₂ (Q)) and work for pump and compressors (MJ/t CO₂ (W)), have to be unified before evaluation and comparison. The reboiler duty in chemical absorption process was converted to equivalent work (W_eq) according to Eq. (1) given by a Carnot cycle efficiency with a turbine cycle efficiency of 75% [30,36,37]:

(1)W_{\mathrm{e}\mathrm{q}}=0.75Q_{\text{re}}\left(1-\frac{T_{\mathrm{L}}}{T_{\mathrm{H}}}\right).

Equation (1) indicates the work that the steam extracted from the turbine could have done in the turbine, and 0.75 is the average turbine efficiency. Thus, Eq. (1) is applicable to the scenario for meeting the duty of the reboiler by extracting steam from the turbine.

2.2 Exergy efficiency

From the thermodynamic viewpoint, exergy efficiency is used as the criterion assessing the separation process, which is defined as the ratio between ideal work and exergy corresponding to the actual energy consumption, as illustrated by Eq. (2).

(2){\eta }_{\mathrm{s}}=\frac{W_{min}}{\sum W+\sum Q\eta }.

The ideal work, represented by W_min, is the thermodynamic minimum exergy required to separate CO₂ from the emission source. The denominator is the exergy corresponding to the actual energy consumption, which is generally composed of work and heat that depend on the separation technical routes. Equation (2) is applicable in almost all CO₂ separation technologies currently.

3 Opportunities and potential of CO₂ capture

The concepts and relationships of separation and capture are illustrated in Fig.2. CO₂ separation is a process index as the kernel course of the CO₂ capture technology, which specifically refers to the separation of CO₂ from mixed streams. As illustrated by the red boundary in Fig.2, chemical absorption for post-combustion capture is a representative separation process. Comparatively, CO₂ capture is a systematic technology that not only includes the separation process but also includes the integration between the separation and the emission source.

As the kernel process of CO₂ capture, the separation technology is undoubtedly the focus in the field of reducing energy consumption, whose aim is to selectively separate CO₂ from mixed gases. Separation technologies can be grouped into two categories according to the principle: equilibrium separation process (ESP), and rate separation process (RSP). ESP is related to phase equilibrium, in which insoluble phases are created in or added to emission sources and reach phase equilibrium with different components, such as absorption and adsorption with liquid or solid mass separator additives (MSA) and energy separator additives (ESA), while cryogenics realizes phase separation only by ESA (heat or work). RSP relies on the differences in mass transfer rates between various components in the same phase, such as membrane separation [38].

For most separation technologies, there are two ways to reduce the energy consumption: exploring new separation media [39–44], such as absorbents with lower reaction heat, and adsorbents with higher adsorption capacity, or membranes with higher selectivity, and upgrading the separation process [45–47], such as absorber intercooling, multi-pressure stripping, and combination of various separation techniques.

Chemical absorption is known to be relatively mature, as it has reached TRL 9 compared to the lower TRLs of other technologies, which has been widely adopted by early CCS demonstration projects, especially for emission sources with low CO₂ concentrations, such as flue gas from power plants.

The data of chemical absorption in the literature were collected to show the performance and energy saving potential of the CO₂ separation process. In this study, the heat duty of reboiler is focused and its internal causes has been investigated because it accounts for more than 99% of the energy consumption in the chemical absorption process. Moreover, the heat required to regenerate solution in the reboiler can be divided into three parts [45,48–50]: desorption reaction heat Q_des, sensible heat for heating rich solution Q_sen, and evaporation heat for producing stripping steam Q_vap. In Fig.3, the purple pentacle icons represent a variety of novel absorbents, which shows a dramatically decline in η_s within a small range of CO₂ concentration. In addition, the highest point in Fig.3 refers to a non-aqueous solvent in Ref. [39], whose separation efficiency is much higher than 30% MEA at the same entrance CO₂ concentration. The reason comes from the better characteristics of such novel absorbents with a lower reaction heat, a lower C_p, a larger CO₂ loading capacity, and less water contained, which is what chemical researchers struggle to achieve. According to the data referenced in Fig.3, exploring novel absorbents shows a greater potential in energy saving than upgrading the separation process, even though these two aspects of improvement are not simply additive, but closely interactive.

With the innovation of absorptions, such as amine blends, multi-phase absorbents, non-aqueous solution, and ionic liquid, the duty of the reboiler of chemical absorption was significantly reduced from higher than 3.5 GJ/t CO₂ to lower than 2.5 GJ/t CO₂, approaching a 50% separation efficiency. According to the latest information from demonstration projects such as Guohua Jingjie projects (150 thousand t/a in Shaanxi Province, construction completed in Jan. 2021), 2.4 GJ/t CO₂ had already been achieved in industrial conditions. In addition, in the well-known TCM center, 2.6 GJ/t CO₂ had already been announced on amino acid salt absorbents by the end of 2020. In the decade that follows, improving the efficiency will continue to be the priority task of the separation technology, and 2.0 GJ/t CO₂ may be the next target.

Removing CO₂ in concentrated form from emission sources is termed CO₂ capture. Although the concepts of CO₂ separation and CO₂ capture are usually not to be distinguished in many conditions, the latter is much more comprehensive than the former: in contrast to the concept of separation as a process, CO₂ capture can be considered a systematic technology that can generate power (or other products for industrial emission sources) from fossil fuel and simultaneously produce concentrated CO₂. Correspondingly, the energy penalty for CO₂ capture is generally defined as the decline of system efficiency due to CO₂ production, which ranges from 6 points to 14 points in different technical routes, as indicated in Fig.1. It is understandable that the energy penalty of CO₂ capture may not only be caused by energy directly consumed by CO₂ separation but also related to the deterioration of energy utilization associated with adding CO₂ production.

Post-combustion, pre-combustion, and oxy-fuel combustion are currently three main capture technical routes, which seemingly indicate the orders of combustion and separation but essentially adopt different fuel conversion and CO₂ generation paths. With a CO₂ separation process attached downstream of power generation, the post-combustion system keeps the combustion and CO₂ generation processes unchanged. Comparatively, the pre-combustion and oxy-fuel combustion technology integrates CO₂ production upstream of power generation and changes direct burning with air into indirect combustion (gasification, decarbonization, and then combustion) or burning with pure oxygen. The superficial motivation for changing the fuel conversion path is to increase the concentration of CO₂ at its generating point, which is approximately 30% for pre-combustion and as high as 90% for oxy-fuel combustion. From a thermodynamic viewpoint, the minimum work for CO₂ separation (W_min in Eq. (3)) can be expressed as a function of CO₂ concentration X of the emission source and recovery ratio K [51]. As illustrated by Fig.4, W_min declines along with the increment of X with a fixed K, and in Fig.4, this reduction will be enhanced (whose enlargement factor equals the reciprocal separation efficiency, as indicated by Eq. (3)) and imposed on the energy consumption of separation. As can be seen from Fig.4, with post-combustion as the reference, the theoretical energy consumption will be reduced by 47% for pre-combustion capture and nearly 86% for oxy-fuel combustion.

(3)\begin{array}{rl}{W}_{min}=& R{T}_0\\ & \cdot \frac{X(1-K)\ln [X(1-K)]-(1-XK)\ln (1-XK)-X\ln (X)}{XK}\\ =& f(X,K).\end{array}

In addition to the intention to reduce W_min, the deeper principle supporting the exploration of new fuel conversion paths lies in the potential of chemical energy utilization. In addition to the process generating CO₂, fuel conversion is also the breakpoint for improving chemical energy utilization because it has the largest exergy destruction (nearly 1/3 of the availability of fuel in traditional combustion) in power systems. This close tie inspires the possibility of finding a new fuel conversion path with less exergy destruction and at the same time generating concentrated CO₂. On the one hand, a higher CO₂ concentration can promise a lower or even a near zero energy consumption in separation. On the other hand, a better chemical energy utilization can compensate for the efficiency reduction due to CO₂ separation. Exploration of fuel conversion paths with coupling effects, the so-called “source control technology”, provides a new dimension for reducing the penalty of the CO₂ capture technology in addition to improving the efficiency of the CO₂ separation technology.

In the direction of the source control technology, chemical-looping combustion (CLC) and some new emerging technologies have demonstrated attractive performances [52,53]. In the 1990s, Chinese scholars took the lead in discovering the new phenomenon of high-concentration CO₂ enrichment in CLC [54]. As a representative source control technology, flameless CLC (Fig.5(a)) divides direct combustion into an oxidation reaction and a reduction reaction to form a looping. CLC is a typical process realizing CO₂ capture during the energy conversion process without any CO₂ separation energy penalty. The flow sheet of the chemical-looping process is shown in Fig.4(a). During the CLC process, a metal oxide acts as the oxygen carrier and transports oxygen from the air to the combustion process of the hydrocarbon fuel. After that, the metal oxide is reduced to metal in the fuel reactor and re-oxidized into metal oxide in the air reactor. As the fuel contacts with air indirectly, CO₂ is not diluted by air, and high purity CO₂ is obtained via the condensation of the exhaust gas of the fuel reactor with a low energy penalty in CO₂ separation. In this fuel conversion path, by avoiding the direct contact between fuel and air, CO₂ is generated in conditions that contain only H₂O and solid reactants, which promises that CO₂ can be separated without energy consumption. Meanwhile, the exergy destruction of chemical combustion can be reduced by more than 10% compared to traditional direct combustion [55]. Correspondingly, CLC can capture CO₂ with a near zero energy penalty, whose performance has already been commonly recognized [55–58].

Another interesting case of the source control technology is a polygeneration system adopting postsynthesis CO₂ capture (Fig.5(b)). After gasification and purification, the syngas is sent to the methanol synthesis unit, and a part of the unreacted gas at the outlet of the synthesis unit is recycled back to re-enter the synthesis reaction. The remaining unreacted gas is shifted, decarbonized and delivered to the combined cycle as the fuel for power generation. The black point represents CO_x, the blue one represents H₂, and the red one represents methanol. The relative number of different particles simply expresses their concentration in the stream. This coal-based system produces methanol upstream and generates power downstream. Different from the traditional process producing methanol from coal, methanol synthesis in a polygeneration system does not need a shift reaction to adjust the CO/H₂ of syngas before synthesis, and only a part of the unreacted gas is recycled back to re-enter the synthesis reaction. The remaining unreacted gas is shifted, decarbonized and delivered to the combined cycle as the fuel for power generation. Instead of pursuing a conversion as high as possible between syngas and the product, the polygeneration system adopts a partial conversion scheme and postsynthesis CO₂ capture (also pre-combustion capture) and achieves dual positive effects on CO₂ capture and energy utilization. On the one hand, the carbon components (CO and CO₂) will be concentrated in the unreacted gas because methanol synthesis takes 3 times more H than C, while the syngas from coal generally contains much more C than H. Correspondingly, the CO₂ separation process can obtain shifted syngas with more than 50% CO₂, whose energy consumption can be saved by 30%–40% compared to post-combustion capture. On the other hand, by avoiding the pursue of the total chemical conversion and a better energy integration between chemical production and power generation, the energy utilization of the system will be significantly improved. As a result, the efficiency of the polygeneration system with a CO₂ recovery of 72% is even superior to that of the IGCC system without CO₂ capture [59–62].

There are already small and medium-sized demonstrations of CLC in the US, Europe, and Asia. The largest coal direct combustion CLC pilot is 250 MW_th at Ohio State University in the US [63]. In addition, the polygeneration system has also been successfully demonstrated in the US and China. The coal-based CCS demonstration project from DOE co-products electricity and hydrogen, which has good economic benefits.

It is difficult to compare the significance of capture innovation and separation improvement, but it is undoubted that they are complementary to each other. Fig.6 projects the potential of energy savings for both efficiency improvement of separation technologies and systematic innovation of the capture technology. Typically, the penalty for the post-combustion capture technology will be decreased from 8–14 to 6–9 point loss, mainly benefiting from the separation technology (reboiler duty dropping to approximately 2 GJ/t CO₂). Pre-combustion, polygeneration, oxygen-fuel combustion, and CLC will benefit from improvements in separation upgrading and capture innovation, and the efficiency penalty may be reduced by 2–4 percentage points. The new generation of the CO₂ capture technologies such as source control may approach a zero penalty, which means solving the problem thoroughly.

4 A narrowing window: The last chance from carbon neutrality

It is difficult to identify the exact turning point for CCS, but it may be between Copenhagen (2009) and Paris (2015). In the past 5 years, CCS has obviously lost attention and expectations when compared to the progress of renewable energy. Even worse, CCS has to confront negative policies aimed at coal. Struggling to reach the goal set by the Paris Agreement, an increasing number of policies phasing out coal have been proposed. According to a report of E3G, 52% of Organization for Economic Co-operation and Development (OECD) and EU28 coal capacity has either closed already or is scheduled to close by 2030 [64]. Coupling with coal tightly, the space for CCS is sharply squeezed along with the steps of phasing out coal. It can be said that the window for CCS worldwide deployment is narrowing. The progress has to catch up, or else, CCS may be abandoned, even if much more energy, environment, and economic penalties will have to be paid in a mitigation scenario in the absence of CCS.

China has a little different situation from others. The Chinese government had announced their carbon neutrality before 2060 in the 75th session of the UN General Assembly (22 September, 2020, New York). It is commonly recognized that the share of coal will be significantly reduced, especially after the surprising growth of renewable energy in the last decade. However, given the huge amount of energy consumption, high reliance on coal, limited availability of oil and gas, defects of stability and continuity of renewable energies, and concerns about the security of energy supplies, it is hard to image that coal will be totally abandoned before 2060, which may still act as an important supplier to maintain the stability and security of China’s enormous energy system. Consequently, a carbon neutral target and the necessity of coal will lead to a certain scenario in which the role of CCS has to be acknowledged in the technology package for reaching the multiple targets of energy security, efficiency, cleanliness and low carbon emissions. Carbon neutrality provides a new (maybe also the last) chance for CCS.

The “Roadmap for CCS Demonstration and Deployment in the People’s Republic of China” was issued in COP21 of Paris by November 2015, which illustrates the strategy for CCS development in China [9]. Using the original graph of the roadmap as the background, Fig.7 updates the progress of CO₂ capture technologies in the past 5 years. The latest information has been updated to illustrate the progress of CCS in the last five years as shown in Fig.7: (A) for demo progress of the 1st generation of the CO₂ capture technology. By the end of 2020, more than 10 CO₂ capture demonstration projects had been constructed and operated (some projects are timely) in China, covering almost all representative technical options, including post-combustion (coal fueled and natural gas fueled), pre-combustion, and oxy-fuel, whose avoidance costs ranged from 44 to 66 $/t CO₂. (B) Meanwhile, although the cost of the first large-scale CCS project (Boundary Dam Project) is around 140 $/t CO₂, as high as a cost reduction of 67% will be anticipated (around 60 $/t CO₂) in the second demonstration project (Shand Power Station) according to the pre-feasibility study of Saskpower [65]; (C) for innovation of new generation of the low energy penalty capture technology. A high priority on the innovation of the CCS technology was placed by MoST of China, who supported 11 projects on the development of a new generation of the CCS technology in the National Key Research Program. These projects include three on source control technologies and two on separation technologies, accounting for more than 20 million dollars of public funding. In the 14th Five-Year Plan issued in March 2021 by the Chinese government, CCUS is identified as one of the key tasks of Green Transition, which means that support will be enhanced, and at three large-scale full chain CCUS projects will be implemented in the subsequent years [66].

In the original roadmap, two core elements, industrialization of the first-generation CO₂ capture technology and innovation of the second-generation technologies with lower penalties, piece the possible path for CCS in China. The avoidance cost of the first-generation CO₂ capture technology, mainly post-combustion capture, is anticipated to be reduced to around 40 $/t CO₂ by 2030, mainly driven by the improvement of separation technologies and accumulation of engineering experience, etc. Parallel to the industrialization of the first-generation technology, the second-generation technologies may reach the same cost level and take baton from the first-generation technology by 2040. However, given the narrowing window for CCS, this dual-track approach has to be accelerated. The cross point, which represents the relay between the first-generation and the second-generation technologies, should be advanced from 2040 to 2030, which can pave the way for a wider deployment of cost-competitive CCS. By 2035, the capture cost reductions achieved through the application of the second-generation CO₂ capture technologies and carbon pricing may reach a level that will trigger the wider commercial application of CCS.

The first- and second-generation capture technologies will play an individual important role in different periods in the future. Post-combustion capture is a key technology for the retrofit of existing power plants. Considering that the service life of most thermal power plants in China is relatively short, there will be a large retrofitting period for existing power plants to recover the sunk cost after the decline of the post-combustion capture cost. Then, a new generation of low-cost capture technology will take over and gradually replace the post-combustion capture power plant after the decommissioning of the retrofitted power plant, contributing a certain amount of emission reduction under the carbon-neutral scenario.

5 Conclusions: Lessons, opportunities, and challenges

Technically achievable should not be taken for granted as cost affordable, and unaffordable penalty is the critical gap deterring the deployment of CCS. Compared to copying more projects before the penalty is reduced (or the climate pressure increases), it is more important to learn experiences and lessons from existing demonstration projects.

Improvement of separation can provide a distinctive potential for energy savings, whereas CO₂ capture can provide systematic dimensions beyond the scope of the separation technology. By concentrating CO₂ in the source and improving chemical energy utilization, exploring new fuel conversion paths provides a promising direction for reducing the energy penalty for CO₂ capture. With joint force from innovation of capture technologies and improvement of separation efficiency, zero penalty for CO₂ capture may become reachable.

The carbon neutral scenario may be the last chance for CCS to obtain a foothold in the future emissions-reduction portfolio. Parallel to the innovation of a new generation of CO₂ capture technology, the improvement of the first-generation technology through learning from industrialization has equal importance. The smooth relay between two generations of technology should be foreseeable by 2030.

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