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Carbon capture: Energy wasting technologies or the MCFCs challenge?
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Carbon capture: Energy wasting technologies or the MCFCs challenge? L. Caprile*, B. Passalacqua, A. Torazza Ansaldo Fuel Cells S.p.A., Genoa, Italy
article info
abstract
Article history:
Carbon dioxide is more and more pointed out as one of the factors mostly responsible of
Received 26 February 2010
climate changes. As a consequence the reduction of CO2 emissions, especially in the energy
Received in revised form
generation field, is becoming a worldwide must.
2 October 2010
This paper presents an overview on the main issues that are expected to affect, from this
Accepted 10 October 2010
standpoint, power generation scenario and a spur for a critical comparison among ways to
Available online 12 November 2010
capture CO2 by proposing new aspects to be considered within the evaluation criteria. In particular attention is drawn on the fully innovative opportunities that are offered by
Keywords:
Molten Carbonate Fuel Cells (MCFCs) as a unique option suitable to effectively combine
GHG
carbon capture from thermal plants and typical benefits of hydrogen and fuel cell power
CCS
generation.
Post-combustion
As an example, such a new option is compared with one of the most common technologies
MCFC
forecast for reducing carbon dioxide emissions and relevant results are shortly presented.
CO2
Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Emission
1.
Introduction
Although some opinions are not in agreement with the heavy weight generally given to the emission of the CO2 on the climate changes, more and more attention is being paid worldwide to avoid further, uncontrolled increases of the content of this gas in the atmosphere. In fact, as well known, the average earth temperature is affected by some atmospheric gas, the so-called GHGs (GreenHouse Gas), amongst which CO2 is considered a key one, for both its diffusion and its “life” in the atmosphere, as reported in the following chapter. Really the increase of average temperature on the earth could be due, besides the effect of the CO2, also to a natural cyclic phenomenon. In any case, since the beginning of the industrial era, all human activities are emitting into the atmosphere great and great quantity of the GHG, in particular in these latest decades.
In this scenario, the Kyoto Protocol established an agreement, in force until 2012, in order to reduce the emissions of the GHG, compared with that ones of the 1990. Subsequent expected international agreements are under negotiation, but arriving at a satisfactory compromise among various divergent interests is quite difficult, also because of their dramatic impact on the energy scenario.
2.
GreenHouse Gas
Some GHGs can be produced both by natural processes and human activities (i.e. CO2, CH4, NOx), other ones only by industrial processes (i.e. CFC e chlorofluorocarbons). The Table 1 shows, for the main GHGs, the percentage of distribution in the atmosphere, the compared greenhouse effect, the average life and the annual trend of increase.
* Corresponding author. E-mail address: [email protected] (L. Caprile). 0360-3199/$ e see front matter Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.10.028
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Table 1 e Reference data for the main GHGs. GHG
CO2 CH4 NOx CFC
Relative Compared Life in the Annual distribution in greenhouse atmosphere increase the atmosphere effect [year] [%] [%] 76 13 6 5
1 20 290 1700e12000
50e200 10e15 110e130 1e264
0.4 0.6 0.25 0e5a
a Application of the Montreal Protocol on Substances That Deplete the Ozone Layer.
In that Table it is not considered another important GHG: the aqueous vapour, because steam is a natural component of the atmosphere and it is mainly due to meteorological effects, even if it is affected also by the evolution of the other GHGs. Due to its prevalent distribution in the atmosphere and its long life, CO2 is considered as a reference for defining the greenhouse effect of the other gas. As regards, to be noted that the gas, other than CO2, have a significant effect, even if their content in the atmosphere is lower [1]. In short, taking into account the other main GHGs: CH4 e Methane is mainly produced by natural processes of anaerobic decomposition, in the rice-fields and other agricultural practices, or it is released from the landfill of municipal waste, from the coal mines and, of course, from natural gas fields. NOx e The sources of nitrogen oxides are from both natural processes and human activities. Some industrial processes, such as combustion (power generation), waste incineration, production and use of fertilizers, give a strong contribute to the emission of this GHG. CFC e The chlorofluorocarbons are the worst pollutants in terms of greenhouse effect and depletion of ozone; for this reason the production and the use of this family of compounds are prohibited in several countries, at least in the most developed ones (Montreal Protocol). Completely different is the problem about the production and the emission of the CO2: in fact CO2 is a natural compound of the atmosphere and its concentration follows mainly the biologic cycles (absorption from the forest), volcanic events, but, since the beginning of the so-called industrial era, its emission due to human activities is raising, accordingly with the development of the industry and the growth of the energy that is spent for the civil use, including the transport of people and goods.
Table 2 e CO2 emission versus kind of human activity. CO2 emission sources Power generation Transportation Industrial activities Civil and commercial activities
Estimated percentage [%] 30e40 20e30 15e25 15e20
The Table 2 shows the estimated percentage of CO2 emissions for the different human activities. The data of this Table refer to years 2007e2008 [2]. The expectation and the hope are for a reduction of the GHG emission, and in particular for CO2, in the near future, considering the Agreements subsequent the Kyoto Protocol, presently under negotiation. As clearly shown in the Table 2, the power generation plays an important role in the emission of the CO2, taking into account that nowadays the plants for power generation are mainly fed with fossil fuels. Globally in the world, about 10 billions ton/year of CO2 are emitted today by power generation plants [3]. Although there is an effort aimed to reduce the use of fossil fuels in the power plants and to increase the energy generation from natural resources (i.e. sunlight, wind, etc.) and from biomass, the fossil fuels, and amongst these coal, will be also for the future the primary sources of the energy worldwide. In particular, coal is available in large quantities, usually at costs lower and more constant than oil or natural gas. So, in the scenario with an increasing of the energy needs and more stringent rules for CO2 emission, the capture and storage of CO2 (CCS - Carbon Capture and Storage) will be a must, in order to achieve the ambitious objectives of Kyoto Protocol and subsequent Agreements, to mitigate the climate change and the global warming. Fortunately, the capture of CO2 from power generation is easier than that from other small or distributed sources, because the technologies today under industrial development will be mostly applicable only to large plants.
3.
CO2 capture technologies
In this chapter a short review of the technologies for CO2 capture that can be applied to power generation plants, fed by solid or gaseous fuels, is presented. Generally speaking, the specific emission of CO2 is, for kind of fuel and for generated electric energy, an inverse function of the overall efficiency of the plant while the emission of the other GHGs (e.g. NOx) is strongly dependent on the process and technology adopted for their abatement. That all granted, even though the trend is an increasing of both generic energy saving and the efficiency of the several power generation systems as well as of the end-user devices, it is hard to think that only these measures can reduce the CO2 emissions in accordance to the targets. Therefore, the CCS technologies appear to be the only way to achieve the stringent requirements established on the Kyoto Protocol and, all the more reason, the more challenging ones expected from ongoing negotiations. The CCS technologies, more or less mature from an industrial point of view, can be classified in three main families [4], [5]: Oxy-combustion e This technology is very easy to be described: the comburent agent is not air, but quite pure oxygen (about 95%). In that manner the flue gas is mainly composed by CO2 and H2O and so the separation of the concentrated CO2 to be delivered to the storage is quite easy.
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Much less easy are the plant design and the manufacture of the combustors and/or the combustion chambers, because the flame temperature could reach values too high: usually a suitable flue gas recirculation is to be provided. To be noted that this technology requires the production of pure oxygen, with a loss of efficiency of at least 9e10 percentage points for the whole power generation plant. Moreover a significant technological jump is necessary, considered how large is the needed scale up from the most advanced state of art experiences on oxy-combustors (30e40 MWth maximum until now). Pre-combustion capture e This technology solves the problem to separate “carbon”, namely CO2, before the combustion; of course, it is applicable to well defined fuel processes generating syngas. Three main steps can be envisaged: Gasification of a fossil fuel (methane, petroleum residual products, coal) to produce syngas (main compounds: hydrogen and carbon monoxide) Shift reaction to transform CO into CO2 Separation of the CO2 from the gaseous stream, by means, e.g. of suitable membranes, in order to obtain hydrogen, as clean fuel. Also the application of this technology involves a loss of efficiency of at least 8e10 percentage points. Post-combustion capture e The post-combustion capture consists in the separation of the diluted CO2 from “normal” flue gas, in which the nitrogen of the air, used as comburent, is present. As a matter of fact, this separation can be realised with different technologies; for example: Absorption of the CO2 by means of a solvent (typically aqueous solutions of amines) at low temperature and subsequent desorption of the CO2 at high temperature with recovery of the solvent to be reused in the absorption phase. Adsorption of the CO2 on solid matter and post-recovery of the CO2 in a concentrated stream. In this family it can be mentioned a process (under development), based on calcium cycle: CO2 reacts with lime to form limestone; the subsequent calcination produces again lime and allows the recovery of the CO2. The absorption with solvents (amines) is considered more mature than the other ones, just because since long-time it is common in various chemical and petrochemical processes. Unfortunately its transfer to the capture of CO2 from combustion flue gases has been not found simple, because the different operative context significantly increases the energy penalty and opens very serious environmental issues. So new solvents are in a development phase to optimise the process, reducing solvent losses, and improving absorption/desorption efficiency. In this case the cost of the energy produced is estimable at least up to 30e40% more than without CCS. In fact all these processes need significant quantities of energy and penalise the power generation (now the power lost could be 15e30%, corresponding to a loss of
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8e13 percentage points on efficiency, while in perspective it could be limited to 8e15%, corresponding to a loss of 4e8 percentage points on efficiency). To be noted that the oxy-combustion and pre-combustion, when mature and commercially available, will be applicable without major changes only to new plants, while the postcombustion can be applied also to existing plants (retrofitting), without essential modifications, apart from the plant depowering, as above mentioned. Some European Countries are adopting directives that require to provide, for new plants, suitable spaces and features to allow installing future CCS systems (the so-called “plants CCS ready”), typically post-combustion systems. Other technologies, that need a strong development to demonstrate their viability, are, for example, the following: Polymeric or composite membranes, assembled into compact and flexible systems, separating CO2 from gaseous streams at various temperatures and pressure. Their disadvantage is that contaminants can negatively affect the performance. Liquefaction of CO2, after removing water and impurities, obtaining a high purity of this compound, suitable especially for food use. A disadvantage is the great energy need. As a conclusion, all the above mentioned technologies can be classified “passive”, because they treat fuel or flue gas, “wasting” large amount of the energy generated by the main plant. At present it results into a dramatic penalty of power, reaching also up to 30%. Only in a long-term perspective such a penalty is expected to be limited to about 10%, which anyway will represent a large amount of wasted energy.
4.
“Active CCS technologies”
Amongst the post-combustion options, very interesting is the application of the MCFCs (Molten Carbonate Fuel Cells), alone or with the integration of suitable ancillary devices (e.g. membranes). Generally speaking, the fuel cells have great advantages: higher efficiency, compared with same size other plants, with no impact on environment, because they do not generate NOx, a pollutant and a GHG. In particular, MCFCs have another unique advantage: they concentrate CO2, at the same time generating electric energy instead of wasting it [6], [7], [8], [9]. So, MCFC can be classified as an “active” CCS technology. For a better comprehension of the application of the MCFC as a CCS system, Fig. 1 clearly shows how MCFCs generate electric energy. As shown in that figure, the working principle implies the transfer of a relevant quantity of CO2 from the cathode side to anode side of the cell, operating there its concentration. In fact, this kind of cell needs CO2 at cathode side, because the electric current travels through the electrolyte thanks to the ions CO¼ 3, that form from diluted CO2, present in the oxidising stream (i.e. flue gas), and restore CO2 at the anode side. So, the diluted CO2
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Fig. 1 e Working scheme of MCFC.
of the flue gas is concentrated in the anodic exhaust, by means of the intrinsic mechanism of generation of electric energy. Fig. 2 and Fig. 3 schematically show an application of MCFC to flue gas, in combination with separation membranes. This latter device could be replaced with other separation systems (e.g. PSA e Pressure Swing Adsorption or cryogenic techniques). The retrofitting shown can be thought in principle for CC (Combined Cycle) (see in Fig. 4 a possible simplified plant scheme for this application) or boilers (e.g. pulverised coal), while the fuel feeding the MCFC can be methane or biogas (e.g. from anaerobic digestion of sewage). Using “standard” MCFC, it is possible to concentrate CO2 in the anodic exhaust up to 60% (weight), while the cathodic exhaust benefit of a CO2 reduction in the range of 50e60%. Other important advantages of this solution are:
monoxide so that its subsequent separation is made much easier; If membrane separation devices are used for the final separation, the gas flow rate to be treated is very lower than that one coming directly from combustors or boilers, in which the CO2 is diluted (3e15%) and a large quantity of nitrogen, oxygen and steam is present; About 50e55% of CO2 of the flue gas from the main plant is removed and transferred to the anodic side of the MCFC; All CO2 generated inside the MCFC is completely “selfseparated”, regardless the primary fuel feeding the cells; The percentage of CO2 captured by this way can reach a value of 70%, taking into account the overall electric power generated.
In the anodic stream CO2 is mixed only with water, small quantity of hydrogen and traces of methane and carbon
Fig. 2 e Post-combustion CCS with MCFC.
Fig. 3 e CCS Retrofitting with MCFC.
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Fig. 4 e Simplified plant scheme for MCFC-CCS applied to NGCC.
5.
Comparison amongst CCS technologies
The comparison amongst CCS technologies, in particular between “active” and “passive” technologies, is not easy, because many factors can affect this calculation: the base should be the quantity of CO2 “avoided” in ratio to the “net” energy produced by the power generation systems. To better explain such a concept, a short comparison between the application of a passive system, having the performance typical for the amine absorption/desorption cycle, and MCFC based systems is presented with reference to a grid portion (“node”) having a given power capacity, both without and with CCS. Example 5.1: As first example is here considered a “node” having a power capacity of 535 MW, coming from an NGCC (Natural Gas Combined Cycle, 56% efficiency and therefore with a specific CO2 emission of 337 kg/MWh) as for 400 MW and from grid (37% mean efficiency, with a mix of fuel determining a mean specific emission of 510 kg/MWh) as for the remaining 135 MW. Fig. 5 a, b, c show the radical difference between passive and active approach in terms of required/delivered power and captured/released CO2 by comparing three cases: a) no CCS, the base reference; b) CCS by amine system; c) CCS by MCFC system. Main hypothesis and boundary conditions for that comparison are shortly summarized in Table 3 while Table 4 shows major results. In this example it was assumed an efficiency of 35% for the whole MCFC-CCS system (i.e. for the MCFC autonomous power generator integrated by the tool treating its anode
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Fig. 5 e (a) Grid node without CCS. (b) Grid node with CCS by passive system (e.g. amine). (c) Grid node with CCS by active system (MCFC).
exhaust, in order to generate a CO2 stream fulfilling requirements for subsequent sequestration). A graphic visualisation of the above results is presented in Fig. 6 a, b. By the above results, “normalised” data can be easily drawn, i.e. data referred to the unitary MWh generated at a node 1 MW sized. The assumptions of such example can be written as follows: without CCS, the overall energy 1 MWh comes as 0.75 MWh at site þ 0.25 MWh from grid with a passive CCS: having at site a 90% CO2 capture and an energy loss of 15%, the overall energy 1 MWh comes as 0.64 MWh at site þ 0.36 MWh from grid with an active CCS: having 50% CO2 capture at site, the overall energy 1 MWh is produced exclusively at site (0.75 MWh by NGCC þ 0.25 MWh by MCFC) CO2 specific emissions: natural gas combined cycle 337 kg/MWh; grid 510 kg/MWh, MCFC 0 kg/MWh( with reference to C content of the fuel supplied to MCFC)
Table 3 e Hypothesis and boundary conditions for the comparison. Main plant: 400 MWe Natural Gas Combined Cycle (NGCC) NGCC Rated Power [MW] CO2 released by NGCC Plant [kg/MWh] Integration from the grid [MW] CO2 release due to integration from the grid [kg/MWh] “Equivalent” CO2 released at the “node” [kg/MWh] CO2 separation by passive system [%] Required power by passive system (15%) [MW] CO2 separation by MCFC system [%] MCFC power/separated CO2 [MW/(T/h)]
400 337 135 510 381 90 60 50 2
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Table 4 e Comparison between passive and active CCS. Main plant: 400 MW Natural Gas Combined Cycle (NGCC) within a 535 MW grid node Reference: 535 MW grid node Total released CO2 [T/h] Specific emissions [kg/MWh] Reduction of released CO2 [%] Equivalent efficiency of grid node [%]
Without CCS 204 381 0 49.6
Passive CCS 113 211 45 43.3
MCFC CCS 67 125 67 48.6
According to these assumptions, Table 5 shows the quantity of CO2 released to produce 1 MWh. Example 5.2: As another example, the comparison among the different CCS approach is made, taking into account a boiler fed with pulverized coal. In this case a “node” having a power capacity of 492 MW, coming from a coal plant, (with 40% efficiency and therefore with a specific CO2 emission of 850 kg/MWh) as for 320 MW and from grid (40% mean efficiency, with a mix of fuel determining a mean specific CO2 emission of 450 kg/MWh) as for the remaining 172 MW. Tables 6 and 7 summarize assumptions and main results.
Table 5 e CCS “hosted” by a Natural Gas Combined Cycle plant (NGCC) of the grid portion. 1 MWh at the node (starting point: without CCS 0.75 MWh NGCC þ 0.25 MWh grid) CCS approach No CCS Passive CCS Active CCS
Releases of CO2 Releases of CO2 Overall CO2 at site elsewhere releases 253 kg 25 kg 126 kg
128 kg (128 þ 57) kg e
381 kg 210 kg 126 kg
Also in this example it was assumed an efficiency of 35% for the whole MCFC-CCS system (i.e. for the MCFC autonomous power generator integrated by the tool treating its anode exhaust, in order to generate a CO2 stream fulfilling requirements for subsequent storage). A graphic visualisation of the above results is presented in Fig. 7 a, b. Also in this example, by the above results, “normalised” data can be easily drawn, i.e. data referred to the unitary MWh generated at a node 1 MW sized. The assumptions of such example can be written as follows: without CCS, the overall energy 1 MWh comes as 0.65 MWh at site þ 0.35 MWh from grid with a passive CCS, having at site a 90% CO2 capture and an energy loss of 15%, the overall energy 1 MWh comes as 0.55 MWh at site þ 0.45 MWh from grid with an active CCS: having 60% CO2 capture at site, the overall energy 1 MWh is produced exclusively at site (0.65 MWh by NGCC þ 0.35 MWh by MCFC) CO2 specific emissions: coal plant 850 kg/MWh; grid 450 kg/ MWh, MCFC 0 kg/MWh (with reference to C content of the fuel supplied to MCFC). According to these assumptions Table 8 shows the quantity of CO2 released to produce 1 MWh. Passive CCS have CO2 capture significantly higher at site level but a proper analysis, taking into account the effects at grid level, turns completely upside the situation, as shown by the schematic above examples (carried out with conservative assumptions about both the fraction of CO2 captured in the active CCS option and the specific CO2 emissions from grid). The above reported considerations are valid also for other kind of plants (e.g. boilers fed with oil or natural gas). Of
Table 6 e Hypothesis and boundary conditions for the comparison. Main plant: 320 MW Coal Plant within a 492 MW grid node
Fig. 6 e (a) Comparison of results for the three cases: without CCS, passive CCS and MCFC-CCS - Total released CO2 [T/h] and Specific emissions [kg/MWh]. (b) Comparison of results for the three cases: without CCS, passive CCS and MCFC-CCS - Reduction of released CO2 [%] and Equivalent efficiency of grid node [%].
CC Rated Power [MW] CO2 released by coal Plant [kg/MWh] Integration from the grid [MW] CO2 release due to integration from the grid kg/MWh “Equivalent” CO2 released at the “node” [kg/MWh] CO2 separation by passive system [%] Required power by passive system (15%) [MW] CO2 separation by MCFC system [%] MCFC power/separated CO2 [MW/(T/h)]
320 850 172 450 710 90 48 60 1
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Table 7 e Comparison between passive and active CCS.
Table 8 e CCS “hosted” by a coal plant of the grid portion.
Main plant: 320 MW Coal Plant within a 492 MW grid node
1 MWh at the node (without CCS 0.65 MWh by coal boiler þ 0.35 MWh by grid)
Reference: 492 MW grid node Total released CO2 [T/h] Specific emissions [kg/MWh] Reduction of released CO2 [%] Equivalent efficiency of grid node [%]
Without CCS
Passive CCS
MCFC CCS
349 710 0 40
126 256 63.9 36.4
108 221 68.9 38.1
course, some parameters are to be changed (e.g. the ratio CO2 captured against power generated) but in any case the comparison is always favourable to MCFC systems [10].
6. MCFC systems e perspectives and challenges MCFC technology was till now developed, above all, for application to distributed power generation and their well known capacity of separating the CO2 while producing power, seems not to be completely exploited until now. From this point of view, MCFCs can offer new possibility of application and great advantages, in comparison with other
Fig. 7 e (a) Comparison of results for the three cases: without CCS, passive CCS and MCFC-CCS - Total released CO2 [T/h] and Specific emissions [kg/MWh]. (b) Comparison of results for the three cases: without CCS, passive CCS and MCFC-CCS - Reduction of released CO2 [%] and Equivalent efficiency of grid node [%].
CCS approach No CCS Passive CCS Active CCS
Releases of CO2 Releases of CO2 Overall CO2 at site elsewhere releases 552 kg 55 kg 221 kg
157 kg (157 þ 44) kg e
709 kg 256 kg 221 kg
competitive CCS systems thus opening new perspectives and challenges for that technology. As above described, the need for introducing CCS into existing or new plants will become a must in the next two decades and will offer great opportunities to all CCS technologies, MCFC included. At this moment, none of the technologies potentially applicable is commercially available, because of not sufficient technological maturity or high costs, or both reasons. Market dimensions and subsequent turnover are to be considered only as rough reference for future perspectives and development actions. It is reasonable think that for all the existing plants (today emitting about 10 billions of tons of CO2 per year) and for underway “CCS ready” plants (a significant amount of 5 billions of tons per year of new CO2 emissions expected within 2030) only post-combustion technologies will be really applicable. A reasonable hypothesis is that, as a target for 2030, a reduction of CO2 emission of about 30% will be required, corresponding to about 4e5 billions of tons of CO2 per year. This means that an intervention on plants having an overall power of at least 800 GW should be considered. These figures could be assumed as a reasonable dimension of a potential market for CCS. The perspectives of acquiring a share of this market by MCFC systems justify economical public supports and important investments for accelerating the development of several components and equipment, in order to make the MCFCs fully suitable for this application. In particular, efforts have to be made for developing an efficient
Fig. 8 e Cell performance versus CO2 content at cathode inlet.
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Fig. 9 e Performance of a full scale MCFC stack with a CO2 content 3.9% molar at the cathode inlet.
deep clean-up of the flue gas delivered to MCFC. Moreover, the size of MCFC should be at least one order bigger than that one planned for distributed power generation even if this issue is mitigated because of the MCFC modularity. All that represents a hard challenge for the developers of this technology but, on the other hand, it offers a unique opportunity for the manufacturers, because the large volume of production required by MCFC-CCS applications, unthinkable for distributed power generation only, is expected to allow a dramatic cost reduction.
7.
First experimental results
As a consequence of the above scenario, Ansaldo Fuel Cells is now orienting its MCFC development to CO2 capture. About 10 years ago, tests on single cells confirmed the actual MCFC capability of satisfactory working (Fig. 8) also with cathode gas having the typical composition of a gas turbine exhausts, i.e. a mixture with CO2 contents largely lower than standard ones [8]. Most recently similar tests have been performed under pressurised conditions on a full scale stack having a rated power of 125 kW with a CO2 content at the cathode inlet pushed down up to 3.9%. Under these operating conditions a CO2 utilisation of 55,7% has been obtained at the maximum current (Fig. 9). These results, reaching a current density of over 1200 A/m2, fully confirmed the previous ones on single cell.
8.
Conclusions
The reduction of GHG emissions and, amongst these, of CO2, leads to the foreground most problems arising from energy scenario which solution, even if though technically possible,
has strong economic and technical impacts. The trend is an increase of CO2 emissions. Taking into account the increasing energy demand, the construction of new power generation plants fed with fossil fuels, in particular coal, will be a must for the next decades so that CO2 emissions will have an unavoidable “natural” increasing trend. The only way for effectively contrasting such a process is a wide application of CCS even if none of the technologies under development will be really applicable in the near future and some of them will still remain in experimental phase by demo plants of quite small size. Geo-political reasons and concerns about the economical impact are very influential on the decision process of adoption of international agreements for a real reduction of CO2 emissions and wasted energy associated with the application of “passive” CCS technologies makes more difficult reaching an effective agreement. In this very complex scenario, the MCFCs have a great potentiality, because they can combine power production with the concentration of CO2. Their feature for an “active” CO2 capture is then potentially suitable for solving the conflict between the increasing energy demand and the need for a dramatic reduction of the CO2 emissions. At a preliminary look at the market perspectives, even rough, the MCFCs appear to be very interesting, from an industrial point of view, as a very attractive way for contributing at solving a global environmental problem, also in comparison with other CCS technologies. For these reasons, the need of giving rapid and real answers to GHG reduction suggests to accelerate the development of the technology of the MCFC-CCS systems, in order to offer an alternative to other CCS technologies and also to present, as a fallout, improved MCFCs to the market of distributed power generation. First experimental results also on full scale MCFC stacks demonstrate that, through proper development, the MCFCCCS can be a viable solution.
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Acknowledgements The authors wish to thank Dr. Angelo Airaghi, President of Ansaldo Fuel Cells, for his encouragement and support of these studies.
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[4] Domenichini R, Cotone P. Cattura e sequestrazione della CO2. In: Convegno ATI “Processi di combustione” Milano; 31 Marche1 April 2009. [5] Barbucci P. The ENEL’s CCS projects. In: Convegno ATI “Il Controllo della CO2 negli impianti di produzione energia” Milano; Luglio 2008. [6] M. Farooque. Integrated high efficiency fossil fuel power plant/fuel cell system with CO2 emission abatement Patent US7,396,603 B2. [7] Sugiura K, Takei K, Tanimoto K, Miyazaki Y. The carbon dioxide concentrator by using MCFC. Journal of Power Sources 2003;118:218e27. [8] Amorelli A, Wilkinson MB, Bedont P, Capobianco P, Marcenaro B, Parodi F, et al. An experimental investigation into the use of molten carbonate fuel cells to capture CO2 from gas turbine exhaust gases. Energy JulyeAugust 2004;29(9e10):1279e84. [9] Campanari S, Chiesa P, Manzolini G. CO2 capture from combined cycles integrated with molten carbonate fuel cells. International Journal of Greenhouse Gas Control May 2010; 4(3):441e51. [10] Milewski J, Lewandowski J, Miller A. Reducing CO2 emissions from a coal fired power plant by using a molten carbonate fuel cell. Berlin, Germany: ASME Turbo Expo; 2008. 9e13 June.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Carbon capture: Energy wasting technologies or the MCFCs challenge? L. Caprile*, B. Passalacqua, A. Torazza Ansaldo Fuel Cells S.p.A., Genoa, Italy
article info
abstract
Article history:
Carbon dioxide is more and more pointed out as one of the factors mostly responsible of
Received 26 February 2010
climate changes. As a consequence the reduction of CO2 emissions, especially in the energy
Received in revised form
generation field, is becoming a worldwide must.
2 October 2010
This paper presents an overview on the main issues that are expected to affect, from this
Accepted 10 October 2010
standpoint, power generation scenario and a spur for a critical comparison among ways to
Available online 12 November 2010
capture CO2 by proposing new aspects to be considered within the evaluation criteria. In particular attention is drawn on the fully innovative opportunities that are offered by
Keywords:
Molten Carbonate Fuel Cells (MCFCs) as a unique option suitable to effectively combine
GHG
carbon capture from thermal plants and typical benefits of hydrogen and fuel cell power
CCS
generation.
Post-combustion
As an example, such a new option is compared with one of the most common technologies
MCFC
forecast for reducing carbon dioxide emissions and relevant results are shortly presented.
CO2
Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Emission
1.
Introduction
Although some opinions are not in agreement with the heavy weight generally given to the emission of the CO2 on the climate changes, more and more attention is being paid worldwide to avoid further, uncontrolled increases of the content of this gas in the atmosphere. In fact, as well known, the average earth temperature is affected by some atmospheric gas, the so-called GHGs (GreenHouse Gas), amongst which CO2 is considered a key one, for both its diffusion and its “life” in the atmosphere, as reported in the following chapter. Really the increase of average temperature on the earth could be due, besides the effect of the CO2, also to a natural cyclic phenomenon. In any case, since the beginning of the industrial era, all human activities are emitting into the atmosphere great and great quantity of the GHG, in particular in these latest decades.
In this scenario, the Kyoto Protocol established an agreement, in force until 2012, in order to reduce the emissions of the GHG, compared with that ones of the 1990. Subsequent expected international agreements are under negotiation, but arriving at a satisfactory compromise among various divergent interests is quite difficult, also because of their dramatic impact on the energy scenario.
2.
GreenHouse Gas
Some GHGs can be produced both by natural processes and human activities (i.e. CO2, CH4, NOx), other ones only by industrial processes (i.e. CFC e chlorofluorocarbons). The Table 1 shows, for the main GHGs, the percentage of distribution in the atmosphere, the compared greenhouse effect, the average life and the annual trend of increase.
* Corresponding author. E-mail address: [email protected] (L. Caprile). 0360-3199/$ e see front matter Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.10.028
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Table 1 e Reference data for the main GHGs. GHG
CO2 CH4 NOx CFC
Relative Compared Life in the Annual distribution in greenhouse atmosphere increase the atmosphere effect [year] [%] [%] 76 13 6 5
1 20 290 1700e12000
50e200 10e15 110e130 1e264
0.4 0.6 0.25 0e5a
a Application of the Montreal Protocol on Substances That Deplete the Ozone Layer.
In that Table it is not considered another important GHG: the aqueous vapour, because steam is a natural component of the atmosphere and it is mainly due to meteorological effects, even if it is affected also by the evolution of the other GHGs. Due to its prevalent distribution in the atmosphere and its long life, CO2 is considered as a reference for defining the greenhouse effect of the other gas. As regards, to be noted that the gas, other than CO2, have a significant effect, even if their content in the atmosphere is lower [1]. In short, taking into account the other main GHGs: CH4 e Methane is mainly produced by natural processes of anaerobic decomposition, in the rice-fields and other agricultural practices, or it is released from the landfill of municipal waste, from the coal mines and, of course, from natural gas fields. NOx e The sources of nitrogen oxides are from both natural processes and human activities. Some industrial processes, such as combustion (power generation), waste incineration, production and use of fertilizers, give a strong contribute to the emission of this GHG. CFC e The chlorofluorocarbons are the worst pollutants in terms of greenhouse effect and depletion of ozone; for this reason the production and the use of this family of compounds are prohibited in several countries, at least in the most developed ones (Montreal Protocol). Completely different is the problem about the production and the emission of the CO2: in fact CO2 is a natural compound of the atmosphere and its concentration follows mainly the biologic cycles (absorption from the forest), volcanic events, but, since the beginning of the so-called industrial era, its emission due to human activities is raising, accordingly with the development of the industry and the growth of the energy that is spent for the civil use, including the transport of people and goods.
Table 2 e CO2 emission versus kind of human activity. CO2 emission sources Power generation Transportation Industrial activities Civil and commercial activities
Estimated percentage [%] 30e40 20e30 15e25 15e20
The Table 2 shows the estimated percentage of CO2 emissions for the different human activities. The data of this Table refer to years 2007e2008 [2]. The expectation and the hope are for a reduction of the GHG emission, and in particular for CO2, in the near future, considering the Agreements subsequent the Kyoto Protocol, presently under negotiation. As clearly shown in the Table 2, the power generation plays an important role in the emission of the CO2, taking into account that nowadays the plants for power generation are mainly fed with fossil fuels. Globally in the world, about 10 billions ton/year of CO2 are emitted today by power generation plants [3]. Although there is an effort aimed to reduce the use of fossil fuels in the power plants and to increase the energy generation from natural resources (i.e. sunlight, wind, etc.) and from biomass, the fossil fuels, and amongst these coal, will be also for the future the primary sources of the energy worldwide. In particular, coal is available in large quantities, usually at costs lower and more constant than oil or natural gas. So, in the scenario with an increasing of the energy needs and more stringent rules for CO2 emission, the capture and storage of CO2 (CCS - Carbon Capture and Storage) will be a must, in order to achieve the ambitious objectives of Kyoto Protocol and subsequent Agreements, to mitigate the climate change and the global warming. Fortunately, the capture of CO2 from power generation is easier than that from other small or distributed sources, because the technologies today under industrial development will be mostly applicable only to large plants.
3.
CO2 capture technologies
In this chapter a short review of the technologies for CO2 capture that can be applied to power generation plants, fed by solid or gaseous fuels, is presented. Generally speaking, the specific emission of CO2 is, for kind of fuel and for generated electric energy, an inverse function of the overall efficiency of the plant while the emission of the other GHGs (e.g. NOx) is strongly dependent on the process and technology adopted for their abatement. That all granted, even though the trend is an increasing of both generic energy saving and the efficiency of the several power generation systems as well as of the end-user devices, it is hard to think that only these measures can reduce the CO2 emissions in accordance to the targets. Therefore, the CCS technologies appear to be the only way to achieve the stringent requirements established on the Kyoto Protocol and, all the more reason, the more challenging ones expected from ongoing negotiations. The CCS technologies, more or less mature from an industrial point of view, can be classified in three main families [4], [5]: Oxy-combustion e This technology is very easy to be described: the comburent agent is not air, but quite pure oxygen (about 95%). In that manner the flue gas is mainly composed by CO2 and H2O and so the separation of the concentrated CO2 to be delivered to the storage is quite easy.
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Much less easy are the plant design and the manufacture of the combustors and/or the combustion chambers, because the flame temperature could reach values too high: usually a suitable flue gas recirculation is to be provided. To be noted that this technology requires the production of pure oxygen, with a loss of efficiency of at least 9e10 percentage points for the whole power generation plant. Moreover a significant technological jump is necessary, considered how large is the needed scale up from the most advanced state of art experiences on oxy-combustors (30e40 MWth maximum until now). Pre-combustion capture e This technology solves the problem to separate “carbon”, namely CO2, before the combustion; of course, it is applicable to well defined fuel processes generating syngas. Three main steps can be envisaged: Gasification of a fossil fuel (methane, petroleum residual products, coal) to produce syngas (main compounds: hydrogen and carbon monoxide) Shift reaction to transform CO into CO2 Separation of the CO2 from the gaseous stream, by means, e.g. of suitable membranes, in order to obtain hydrogen, as clean fuel. Also the application of this technology involves a loss of efficiency of at least 8e10 percentage points. Post-combustion capture e The post-combustion capture consists in the separation of the diluted CO2 from “normal” flue gas, in which the nitrogen of the air, used as comburent, is present. As a matter of fact, this separation can be realised with different technologies; for example: Absorption of the CO2 by means of a solvent (typically aqueous solutions of amines) at low temperature and subsequent desorption of the CO2 at high temperature with recovery of the solvent to be reused in the absorption phase. Adsorption of the CO2 on solid matter and post-recovery of the CO2 in a concentrated stream. In this family it can be mentioned a process (under development), based on calcium cycle: CO2 reacts with lime to form limestone; the subsequent calcination produces again lime and allows the recovery of the CO2. The absorption with solvents (amines) is considered more mature than the other ones, just because since long-time it is common in various chemical and petrochemical processes. Unfortunately its transfer to the capture of CO2 from combustion flue gases has been not found simple, because the different operative context significantly increases the energy penalty and opens very serious environmental issues. So new solvents are in a development phase to optimise the process, reducing solvent losses, and improving absorption/desorption efficiency. In this case the cost of the energy produced is estimable at least up to 30e40% more than without CCS. In fact all these processes need significant quantities of energy and penalise the power generation (now the power lost could be 15e30%, corresponding to a loss of
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8e13 percentage points on efficiency, while in perspective it could be limited to 8e15%, corresponding to a loss of 4e8 percentage points on efficiency). To be noted that the oxy-combustion and pre-combustion, when mature and commercially available, will be applicable without major changes only to new plants, while the postcombustion can be applied also to existing plants (retrofitting), without essential modifications, apart from the plant depowering, as above mentioned. Some European Countries are adopting directives that require to provide, for new plants, suitable spaces and features to allow installing future CCS systems (the so-called “plants CCS ready”), typically post-combustion systems. Other technologies, that need a strong development to demonstrate their viability, are, for example, the following: Polymeric or composite membranes, assembled into compact and flexible systems, separating CO2 from gaseous streams at various temperatures and pressure. Their disadvantage is that contaminants can negatively affect the performance. Liquefaction of CO2, after removing water and impurities, obtaining a high purity of this compound, suitable especially for food use. A disadvantage is the great energy need. As a conclusion, all the above mentioned technologies can be classified “passive”, because they treat fuel or flue gas, “wasting” large amount of the energy generated by the main plant. At present it results into a dramatic penalty of power, reaching also up to 30%. Only in a long-term perspective such a penalty is expected to be limited to about 10%, which anyway will represent a large amount of wasted energy.
4.
“Active CCS technologies”
Amongst the post-combustion options, very interesting is the application of the MCFCs (Molten Carbonate Fuel Cells), alone or with the integration of suitable ancillary devices (e.g. membranes). Generally speaking, the fuel cells have great advantages: higher efficiency, compared with same size other plants, with no impact on environment, because they do not generate NOx, a pollutant and a GHG. In particular, MCFCs have another unique advantage: they concentrate CO2, at the same time generating electric energy instead of wasting it [6], [7], [8], [9]. So, MCFC can be classified as an “active” CCS technology. For a better comprehension of the application of the MCFC as a CCS system, Fig. 1 clearly shows how MCFCs generate electric energy. As shown in that figure, the working principle implies the transfer of a relevant quantity of CO2 from the cathode side to anode side of the cell, operating there its concentration. In fact, this kind of cell needs CO2 at cathode side, because the electric current travels through the electrolyte thanks to the ions CO¼ 3, that form from diluted CO2, present in the oxidising stream (i.e. flue gas), and restore CO2 at the anode side. So, the diluted CO2
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Fig. 1 e Working scheme of MCFC.
of the flue gas is concentrated in the anodic exhaust, by means of the intrinsic mechanism of generation of electric energy. Fig. 2 and Fig. 3 schematically show an application of MCFC to flue gas, in combination with separation membranes. This latter device could be replaced with other separation systems (e.g. PSA e Pressure Swing Adsorption or cryogenic techniques). The retrofitting shown can be thought in principle for CC (Combined Cycle) (see in Fig. 4 a possible simplified plant scheme for this application) or boilers (e.g. pulverised coal), while the fuel feeding the MCFC can be methane or biogas (e.g. from anaerobic digestion of sewage). Using “standard” MCFC, it is possible to concentrate CO2 in the anodic exhaust up to 60% (weight), while the cathodic exhaust benefit of a CO2 reduction in the range of 50e60%. Other important advantages of this solution are:
monoxide so that its subsequent separation is made much easier; If membrane separation devices are used for the final separation, the gas flow rate to be treated is very lower than that one coming directly from combustors or boilers, in which the CO2 is diluted (3e15%) and a large quantity of nitrogen, oxygen and steam is present; About 50e55% of CO2 of the flue gas from the main plant is removed and transferred to the anodic side of the MCFC; All CO2 generated inside the MCFC is completely “selfseparated”, regardless the primary fuel feeding the cells; The percentage of CO2 captured by this way can reach a value of 70%, taking into account the overall electric power generated.
In the anodic stream CO2 is mixed only with water, small quantity of hydrogen and traces of methane and carbon
Fig. 2 e Post-combustion CCS with MCFC.
Fig. 3 e CCS Retrofitting with MCFC.
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Fig. 4 e Simplified plant scheme for MCFC-CCS applied to NGCC.
5.
Comparison amongst CCS technologies
The comparison amongst CCS technologies, in particular between “active” and “passive” technologies, is not easy, because many factors can affect this calculation: the base should be the quantity of CO2 “avoided” in ratio to the “net” energy produced by the power generation systems. To better explain such a concept, a short comparison between the application of a passive system, having the performance typical for the amine absorption/desorption cycle, and MCFC based systems is presented with reference to a grid portion (“node”) having a given power capacity, both without and with CCS. Example 5.1: As first example is here considered a “node” having a power capacity of 535 MW, coming from an NGCC (Natural Gas Combined Cycle, 56% efficiency and therefore with a specific CO2 emission of 337 kg/MWh) as for 400 MW and from grid (37% mean efficiency, with a mix of fuel determining a mean specific emission of 510 kg/MWh) as for the remaining 135 MW. Fig. 5 a, b, c show the radical difference between passive and active approach in terms of required/delivered power and captured/released CO2 by comparing three cases: a) no CCS, the base reference; b) CCS by amine system; c) CCS by MCFC system. Main hypothesis and boundary conditions for that comparison are shortly summarized in Table 3 while Table 4 shows major results. In this example it was assumed an efficiency of 35% for the whole MCFC-CCS system (i.e. for the MCFC autonomous power generator integrated by the tool treating its anode
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Fig. 5 e (a) Grid node without CCS. (b) Grid node with CCS by passive system (e.g. amine). (c) Grid node with CCS by active system (MCFC).
exhaust, in order to generate a CO2 stream fulfilling requirements for subsequent sequestration). A graphic visualisation of the above results is presented in Fig. 6 a, b. By the above results, “normalised” data can be easily drawn, i.e. data referred to the unitary MWh generated at a node 1 MW sized. The assumptions of such example can be written as follows: without CCS, the overall energy 1 MWh comes as 0.75 MWh at site þ 0.25 MWh from grid with a passive CCS: having at site a 90% CO2 capture and an energy loss of 15%, the overall energy 1 MWh comes as 0.64 MWh at site þ 0.36 MWh from grid with an active CCS: having 50% CO2 capture at site, the overall energy 1 MWh is produced exclusively at site (0.75 MWh by NGCC þ 0.25 MWh by MCFC) CO2 specific emissions: natural gas combined cycle 337 kg/MWh; grid 510 kg/MWh, MCFC 0 kg/MWh( with reference to C content of the fuel supplied to MCFC)
Table 3 e Hypothesis and boundary conditions for the comparison. Main plant: 400 MWe Natural Gas Combined Cycle (NGCC) NGCC Rated Power [MW] CO2 released by NGCC Plant [kg/MWh] Integration from the grid [MW] CO2 release due to integration from the grid [kg/MWh] “Equivalent” CO2 released at the “node” [kg/MWh] CO2 separation by passive system [%] Required power by passive system (15%) [MW] CO2 separation by MCFC system [%] MCFC power/separated CO2 [MW/(T/h)]
400 337 135 510 381 90 60 50 2
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Table 4 e Comparison between passive and active CCS. Main plant: 400 MW Natural Gas Combined Cycle (NGCC) within a 535 MW grid node Reference: 535 MW grid node Total released CO2 [T/h] Specific emissions [kg/MWh] Reduction of released CO2 [%] Equivalent efficiency of grid node [%]
Without CCS 204 381 0 49.6
Passive CCS 113 211 45 43.3
MCFC CCS 67 125 67 48.6
According to these assumptions, Table 5 shows the quantity of CO2 released to produce 1 MWh. Example 5.2: As another example, the comparison among the different CCS approach is made, taking into account a boiler fed with pulverized coal. In this case a “node” having a power capacity of 492 MW, coming from a coal plant, (with 40% efficiency and therefore with a specific CO2 emission of 850 kg/MWh) as for 320 MW and from grid (40% mean efficiency, with a mix of fuel determining a mean specific CO2 emission of 450 kg/MWh) as for the remaining 172 MW. Tables 6 and 7 summarize assumptions and main results.
Table 5 e CCS “hosted” by a Natural Gas Combined Cycle plant (NGCC) of the grid portion. 1 MWh at the node (starting point: without CCS 0.75 MWh NGCC þ 0.25 MWh grid) CCS approach No CCS Passive CCS Active CCS
Releases of CO2 Releases of CO2 Overall CO2 at site elsewhere releases 253 kg 25 kg 126 kg
128 kg (128 þ 57) kg e
381 kg 210 kg 126 kg
Also in this example it was assumed an efficiency of 35% for the whole MCFC-CCS system (i.e. for the MCFC autonomous power generator integrated by the tool treating its anode exhaust, in order to generate a CO2 stream fulfilling requirements for subsequent storage). A graphic visualisation of the above results is presented in Fig. 7 a, b. Also in this example, by the above results, “normalised” data can be easily drawn, i.e. data referred to the unitary MWh generated at a node 1 MW sized. The assumptions of such example can be written as follows: without CCS, the overall energy 1 MWh comes as 0.65 MWh at site þ 0.35 MWh from grid with a passive CCS, having at site a 90% CO2 capture and an energy loss of 15%, the overall energy 1 MWh comes as 0.55 MWh at site þ 0.45 MWh from grid with an active CCS: having 60% CO2 capture at site, the overall energy 1 MWh is produced exclusively at site (0.65 MWh by NGCC þ 0.35 MWh by MCFC) CO2 specific emissions: coal plant 850 kg/MWh; grid 450 kg/ MWh, MCFC 0 kg/MWh (with reference to C content of the fuel supplied to MCFC). According to these assumptions Table 8 shows the quantity of CO2 released to produce 1 MWh. Passive CCS have CO2 capture significantly higher at site level but a proper analysis, taking into account the effects at grid level, turns completely upside the situation, as shown by the schematic above examples (carried out with conservative assumptions about both the fraction of CO2 captured in the active CCS option and the specific CO2 emissions from grid). The above reported considerations are valid also for other kind of plants (e.g. boilers fed with oil or natural gas). Of
Table 6 e Hypothesis and boundary conditions for the comparison. Main plant: 320 MW Coal Plant within a 492 MW grid node
Fig. 6 e (a) Comparison of results for the three cases: without CCS, passive CCS and MCFC-CCS - Total released CO2 [T/h] and Specific emissions [kg/MWh]. (b) Comparison of results for the three cases: without CCS, passive CCS and MCFC-CCS - Reduction of released CO2 [%] and Equivalent efficiency of grid node [%].
CC Rated Power [MW] CO2 released by coal Plant [kg/MWh] Integration from the grid [MW] CO2 release due to integration from the grid kg/MWh “Equivalent” CO2 released at the “node” [kg/MWh] CO2 separation by passive system [%] Required power by passive system (15%) [MW] CO2 separation by MCFC system [%] MCFC power/separated CO2 [MW/(T/h)]
320 850 172 450 710 90 48 60 1
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Table 7 e Comparison between passive and active CCS.
Table 8 e CCS “hosted” by a coal plant of the grid portion.
Main plant: 320 MW Coal Plant within a 492 MW grid node
1 MWh at the node (without CCS 0.65 MWh by coal boiler þ 0.35 MWh by grid)
Reference: 492 MW grid node Total released CO2 [T/h] Specific emissions [kg/MWh] Reduction of released CO2 [%] Equivalent efficiency of grid node [%]
Without CCS
Passive CCS
MCFC CCS
349 710 0 40
126 256 63.9 36.4
108 221 68.9 38.1
course, some parameters are to be changed (e.g. the ratio CO2 captured against power generated) but in any case the comparison is always favourable to MCFC systems [10].
6. MCFC systems e perspectives and challenges MCFC technology was till now developed, above all, for application to distributed power generation and their well known capacity of separating the CO2 while producing power, seems not to be completely exploited until now. From this point of view, MCFCs can offer new possibility of application and great advantages, in comparison with other
Fig. 7 e (a) Comparison of results for the three cases: without CCS, passive CCS and MCFC-CCS - Total released CO2 [T/h] and Specific emissions [kg/MWh]. (b) Comparison of results for the three cases: without CCS, passive CCS and MCFC-CCS - Reduction of released CO2 [%] and Equivalent efficiency of grid node [%].
CCS approach No CCS Passive CCS Active CCS
Releases of CO2 Releases of CO2 Overall CO2 at site elsewhere releases 552 kg 55 kg 221 kg
157 kg (157 þ 44) kg e
709 kg 256 kg 221 kg
competitive CCS systems thus opening new perspectives and challenges for that technology. As above described, the need for introducing CCS into existing or new plants will become a must in the next two decades and will offer great opportunities to all CCS technologies, MCFC included. At this moment, none of the technologies potentially applicable is commercially available, because of not sufficient technological maturity or high costs, or both reasons. Market dimensions and subsequent turnover are to be considered only as rough reference for future perspectives and development actions. It is reasonable think that for all the existing plants (today emitting about 10 billions of tons of CO2 per year) and for underway “CCS ready” plants (a significant amount of 5 billions of tons per year of new CO2 emissions expected within 2030) only post-combustion technologies will be really applicable. A reasonable hypothesis is that, as a target for 2030, a reduction of CO2 emission of about 30% will be required, corresponding to about 4e5 billions of tons of CO2 per year. This means that an intervention on plants having an overall power of at least 800 GW should be considered. These figures could be assumed as a reasonable dimension of a potential market for CCS. The perspectives of acquiring a share of this market by MCFC systems justify economical public supports and important investments for accelerating the development of several components and equipment, in order to make the MCFCs fully suitable for this application. In particular, efforts have to be made for developing an efficient
Fig. 8 e Cell performance versus CO2 content at cathode inlet.
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Fig. 9 e Performance of a full scale MCFC stack with a CO2 content 3.9% molar at the cathode inlet.
deep clean-up of the flue gas delivered to MCFC. Moreover, the size of MCFC should be at least one order bigger than that one planned for distributed power generation even if this issue is mitigated because of the MCFC modularity. All that represents a hard challenge for the developers of this technology but, on the other hand, it offers a unique opportunity for the manufacturers, because the large volume of production required by MCFC-CCS applications, unthinkable for distributed power generation only, is expected to allow a dramatic cost reduction.
7.
First experimental results
As a consequence of the above scenario, Ansaldo Fuel Cells is now orienting its MCFC development to CO2 capture. About 10 years ago, tests on single cells confirmed the actual MCFC capability of satisfactory working (Fig. 8) also with cathode gas having the typical composition of a gas turbine exhausts, i.e. a mixture with CO2 contents largely lower than standard ones [8]. Most recently similar tests have been performed under pressurised conditions on a full scale stack having a rated power of 125 kW with a CO2 content at the cathode inlet pushed down up to 3.9%. Under these operating conditions a CO2 utilisation of 55,7% has been obtained at the maximum current (Fig. 9). These results, reaching a current density of over 1200 A/m2, fully confirmed the previous ones on single cell.
8.
Conclusions
The reduction of GHG emissions and, amongst these, of CO2, leads to the foreground most problems arising from energy scenario which solution, even if though technically possible,
has strong economic and technical impacts. The trend is an increase of CO2 emissions. Taking into account the increasing energy demand, the construction of new power generation plants fed with fossil fuels, in particular coal, will be a must for the next decades so that CO2 emissions will have an unavoidable “natural” increasing trend. The only way for effectively contrasting such a process is a wide application of CCS even if none of the technologies under development will be really applicable in the near future and some of them will still remain in experimental phase by demo plants of quite small size. Geo-political reasons and concerns about the economical impact are very influential on the decision process of adoption of international agreements for a real reduction of CO2 emissions and wasted energy associated with the application of “passive” CCS technologies makes more difficult reaching an effective agreement. In this very complex scenario, the MCFCs have a great potentiality, because they can combine power production with the concentration of CO2. Their feature for an “active” CO2 capture is then potentially suitable for solving the conflict between the increasing energy demand and the need for a dramatic reduction of the CO2 emissions. At a preliminary look at the market perspectives, even rough, the MCFCs appear to be very interesting, from an industrial point of view, as a very attractive way for contributing at solving a global environmental problem, also in comparison with other CCS technologies. For these reasons, the need of giving rapid and real answers to GHG reduction suggests to accelerate the development of the technology of the MCFC-CCS systems, in order to offer an alternative to other CCS technologies and also to present, as a fallout, improved MCFCs to the market of distributed power generation. First experimental results also on full scale MCFC stacks demonstrate that, through proper development, the MCFCCCS can be a viable solution.
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Acknowledgements The authors wish to thank Dr. Angelo Airaghi, President of Ansaldo Fuel Cells, for his encouragement and support of these studies.
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