Thermodynamic assessment of CO2 to carbon nanofiber transformation for carbon sequestration in a combined cycle gas or a coal power plant Academic research paper on "Nano-technology"

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Energy Conversion and Management

journal homepage: www.elsevier.com/locate/enconman

Energy

Conversion IManagement

Thermodynamic assessment of C02 to carbon nanofiber transformation ■. for carbon sequestration in a combined cycle gas or a coal power plant

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Jason Laua, Gangotri Deyb, Stuart Licht

aDepartment of Chemistry, The George Washington University, Washington, D.C. 20052, United States

b Institute for Massively Parallel Applications and Computing Technology (IMPACT), The George Washington University, Washington, D.C. 20052, United States

ARTICLE INFO

Article history:

Received 10 April 2016

Received in revised form 1 June 2016

Accepted 3 June 2016

Keywords:

Carbon nanofibers

CO2 utilization

Molten carbonates

Thermodynamic analysis

Solar thermal electrochemical process

CO2 sequestration

ABSTRACT

Molten carbonate electrolyzers offer a pathway to capture emitted CO2 from the flue gas of the power plants and transform this greenhouse gas emission at low energy and high yield instead into a specific, value added, hollow carbon nanofiber product, carbon nanotubes. The present day value of the carbon nanotubes product is ~10,000 that of proposed, or in place, current carbon tax costs of $30 per ton, strongly incentivizing carbon dioxide removal. The recent progress in high-temperature molten carbonate electrolysis systems for carbon dioxide utilization and the impact these advances have on developing a CO2-free fossil fuel power plant for electricity generation is presented. A thermodynamic model analysis is presented for a molten Li2CO3 electrolysis system incorporated within a combined cycle (CC) natural gas power plant to produce carbon nanofibers (CNF) and oxygen. Such a CC CNF plant system is shown to require 219 kJ to convert one mole of CO2 to carbon, and generates electricity at higher efficiency due to pure oxygen looped back to the gas turbine input from the CO2 splitting, with the added advantages that (i) the CC CNF plant emits no CO2 and (ii) all CO2 is converted to value added carbon nanotubes useful for strong, lightweight construction, batteries and nanoelectronics. Converting to power and ton units, per metric ton of methane fuel consumed the CC CNF plant is thermodynamically assessed to produce 8350 kWh of electricity and 0.75 ton of CNT and emits no CO2, while the CC plant produces 9090 kW h of electricity and emits 2.74 ton CO2. The required energy balance for a carbon nanotube production from an analogous coal power plant consumes a larger fraction of the coal energy, and encourages co-generation with renewable electric energy.

© 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CCBY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4XI/).

1. Introduction

In 2014, the United States produced 5209 million metric tons of CO2 from fossil fuel combustion. Fossil fuel combustion is the single largest source of greenhouse gas emissions in the United States, and this accounts for more than the rest of the sources combined [1a]. The largest fraction of this fossil fuel combustion is for electricity generation at stationary power plants. If these power plants are allowed to continue to operate as they have done in the past, the amount of CO2 in the atmosphere will continue to increase, driving ongoing climate change around the world. In 2013 the renewable energy share of power generation is 22% [1b]. Any carbon based fuel sources will produce CO2 during their combustion, which means that a carbon dioxide sequestration and/or transformation technology must be added

* Corresponding author. E-mail address: slicht@gwu.edu (S. Licht).

to these power plants if CO2 concentration in the atmosphere is decreased.

A promising technology for carbon dioxide transformation is the electrolytic synthesis of carbon and carbon nanofibers (CNF) from captured carbon dioxide in molten lithium carbonate [2-9]. In this process, voltage is applied to split carbon dioxide in an electrolysis chamber on a nickel anode and a galvanized steel cathode into pure oxygen gas and a solid carbon product. A low energy, high efficiency process when conducted in lithiated molten carbonate electrolytes, this electrolysis reaction offers a pathway to transform the greenhouse gas into a high value commodity [2-10]. The carbonate electrolyte is not consumed and the net reaction is CO2 splitting into carbon and O2, and as presented here using pure Li2CO3 as the carbonate electrolyte:

Dissolution : CO2(gas) + Li2O(soluble) ! Li2CO3(molten)

Electrolysis : Li2CO3(molten) ! C(solid) + Li2O (soluble) + O2(gas)

http://dx.doi.org/10.1016/j.enconman.2016.06.007 0196-8904/© 2016 The Authors. Published by Elsevier Ltd.

This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Net: CO2(gas)! C(solid) +O2(gas) (1)

As shown in Fig. 1, CO2 directly from the atmosphere or the CO2 emission from a combined cycle (CC) natural gas power plant is electrolyzed to produce a variety of value carbon nanomaterials that have a range of uses. The morphology of the carbon product can vary based on diffusions controls of natural abundance CO2 or C13 (Fig. 1(b) and (c)). High concentrations of lithium oxide can produce a tangled morphology (Fig. 1(d)), while no additional concentration of lithium oxide produces straight nanotubes (Fig. 1(f)) [2]. The use natural abundance CO2, electrolysis current control and the addition of small quantities of nickel to act as nucleating agents leads to high yield of a particularly valuable form of hollow carbon nanofibers, carbon nanotubes. Due to their superior strength, conductivity, flexibility, and durability CNFs have a variety of applications including in nanoelectronics, in Li-ion batteries, as a material for solar thermal conversion [11], and as a principal component in the light-weighting of infrastructure construction materials, transportation (air, land, sea) vehicles, consumer electronics, wind turbine blades, and athletic equipment.

One of the principal global emission sources of the greenhouse gas carbon dioxide is from electrical power plants fueled by fossil fuels. Presently, the most efficient of these plants in widespread use are combined cycle natural gas plants, which nevertheless emit massive amounts of CO2. Fig. 2 presents the energy distribution of a conventional combined cycle power plant [12]. It is evident that there are several major energy efficiency losses. Recognition of these losses offer opportunities for improvement and for rerouting those energies to simultaneous sequestration, or transformation of CO2. For example, when a CO2 to CNF electrolyzer is added to a CC power plant, pure oxygen is generated, which can be used to enrich the oxygen content of the air used for combustion. Enriched oxygen combustion allows for the combustion chamber to reach higher temperatures and combustion efficiencies, improving thermal energy efficiencies of the gas turbine as well as the steam turbine. Additionally, the system decreases stack heat losses based on the diminished volume of N2 in the combustion process compared to conventional CC power plants. The electrolyzer can form a high yield of a specific form of carbon nanofiber, carbon nanotubes. Carbon nanotubes (90% industrial grade) are currently valued at

Fig. 1. Scheme for the electrolytic synthesis of carbon nanostructures: (a) source of CO2 as dissolved air or smoke stack concentrations of CO2, (b and c) demonstrate diffusion controls on formation of carbon nanotubes or nanofibers, high oxide concentration produce tangled morphogies (d) while low concentrations produce straight nanotubes (f), and (e) nickel nucleation sites, the bright spots, as identified by EDS. Lower panel: The carbon nanofiber and nanotubes provide high conductivity and superior carbon composite lightweight structural materials for jets, bridges, wind turbines, and electric vehicle bodies and batteries.

Fig. 2. Diagram of energy distribution in a conventional CC power plant reprinted with permission from Beér [12]. Copyright Elsevier Ltd. 2006.

~$300,000 per ton [12]. This value is 10,000 times the estimated current cost of carbon tax credits [13].

The cost of STEP (Solar Thermal Electrochemical Process) carbon nanofibers and carbon nanotubes produced by molten carbonate electrolysis can be estimated with a generic electric supply, and does not include the additional efficiency advantages of the previous solar thermal and solar electric supply [2-10,14-16]. Electrolysis costs to produce CNFs, or CNTs, will be similar to infrastructure costs associated with chlor-alkali and aluminum industries, which are the major traditional electrochemical industries. The electrical energy costs are low, requiring 0.9-1.4 V, with a high columbic efficiency of 80-100% for the four electrons required to reduce CO2 to CNF or CNT [3]. The energy requirements for carbon nanotube growth with various nucleation metals has been explored using first principal Density Functional Theory using quantum espresso packaged with a generalized gradient approximation with Predew-Burke-Ernzerhof exchange-correlation functional using PAW pseudopotentials to identify the most energy efficient pathway for CNT growth, shown in Fig. 3 [9,17]. A CNT adsorbed onto a Ni cluster has the lowest energy state as a result of the strong metal-carbon adhesion bonds formed, while Zn has the highest energy state. A mixture of Ni/Zn metal cluster was found to have the optimized energy per atom ratio required for proper carbon nanotube growth [9]. Energy minimization for the growth of CNT on Ni/Zn cluster, as opposed to Zn alone is presented in Ref. [9].

Using an electricity cost of $0.10 per kW h, the electrosynthesis of CO2 to CNF suggests an equivalent $800 to $1600 per metric ton CNF. Lithium carbonate is not consumed during the CO2 electrolysis and at today's costs of $6000 per ton, as amortized over ten year's usage, the electrolyte adds an additional $140 per metric ton of CNF. An alternative less expensive mix of sodium and lithium carbonate is also an effective electrolyte [8]. Additional costs such as nickel and steel electrodes and ancillary equipment is low due to the low wear on the system, and is expected to add a combined upper limit cost of $2000 per metric ton of CNF. These costs compared to today's conventional chemical vapor deposition or electrospun production costs of ~$25,000 per ton of CNF and $200,000-400,000 per metric ton of industrial grade (90% purity) CNTs [4,18]. The low costs of C2CNT (carbon dioxide to carbon nan-otube) production opens high revenue windows and provides a

Geometry

Fig. 3. Theoretical energy of CNT formation with adsorbed Ni or Zn cluster [9].

significant incentive for CO2 removal, while providing an impetus for CNF and CNT market growth through a decrease in cost. This study will explore the thermodynamic feasibility of transforming the exhaust CO2 from a combined-cycle natural gas power plant into a stable, valuable carbon nanofiber product using a lithiated carbonate high temperature electrolysis system.

2. Systems description

2.1. Conventional combined cycle gas turbine power plant

Advanced gas turbine technology stands at the forefront of fossil fuel electricity generation as a result of its high efficiency, fast load-response times, and abundance of fuel (methane). A combined cycle gas turbine power plant utilizes two heat engine forms to convert heat energy into mechanical energy, which is then transformed into electrical energy. An example of a conventional combined cycle plant, that utilizes a Brayton combustion cycle system to convert heat to mechanical energy, followed by a second steam turbine that uses a Rankine cycle system to convert the residual heat to mechanical energy, is shown in Fig. 4. While the basic scheme of the plant remains the same, the fuel to electrical efficiency vary between 50 and 60% with newer plants incorporating more efficient technologies [19-23]. These efficiencies can be further improved with new turbine technologies and higher combustion efficiencies. In a conventional, contemporary CC gas power plant, ~38% and ~21% of the available fuel enthalpy are converted to electricity respectively by the gas combustion and steam turbines, for a combined enthalpy to electricity efficiency of ~59%; heat is lost in the steam condenser (~30%), stack (~10%), and through radiative losses (~1%) [5]. Typical pipeline quality natural gas is ~93% Methane (CH4), ~3% Ethane (C2H6), 0.7% Propane (C3H8),0.4% n-Butane (C4H10), ~1% Carbon dioxide (CO2), and ~1-2% Nitrogen (N2) [24,25]. The natural gas mixture burned at these plants can vary based on the specific source of natural gas used as well as the specific requirements of the power plant.

2.2. Carbon nanofiber combined cycle power plant

A conventional CC power plant, offers synergistic opportunities for efficient CO2 mitigation in conjunction with an added high temperature electrolysis component to split CO2. A CC power plant generates a source of hot CO2 that can be sparged directly into a

Fig. 4. Schematic of a conventional combined cycle power plant which utilizes a gas turbine to convert heat to mechanical energy, followed by a second steam turbine to convert the residual heat to mechanical energy [26].

molten carbonate electrolyte, rather than emitted into the atmosphere as a greenhouse gas. This CO2 can then be electrolyzed into carbon nanofibers, while remaining other flue gas products, such as nitrogen and steam are insoluble in the molten electrolyte, and can be passed through a heat exchanger to create the steam that will be used for the second part of the combined cycle. This carbon nanofiber combined cycle (CNF CC) power plant is illustrated in Fig. 5. The CNF-CC plant adds an electrolysis chamber absorbing hot CO2 from the gas turbine exhaust, which is converted to carbon nanofibers and pure O2. The pure oxygen produced through electrolysis is cycled back to the combustion chamber to improve efficiency, while the recovered heat is passed on to the steam turbine and the entire process produce valuable carbon nanofiber instead of releasing CO2. The carbon nanofibers are easily recovered and are more valuable than carbon dioxide. A benefit of this system is the ability for the carbon dioxide to be captured without precon-centration, and sequestered and transformed at high temperature. Conventional absorption CCS technologies require an absorption material that captures the CO2 at a lower temperature and must be heated to release the CO2 and regenerate the material. A molten carbonate electrolyte can utilize metal oxides such as lithium, sodium, potassium, calcium, or barium oxide to chemically react with CO2 to form a carbonate. This carbonate is than electrolyzed to produce a net reaction shown in Eq. (1).

The advantage of using an oxide CO2 absorber is that both the oxide as well as the CO2 are able to be kept at high temperature, preventing the need for thermal cycling as commonly performed with ionic liquid, amines, and other CCS technologies [27-34].

The configuration of a CNF CC power plant can utilize the pure oxygen produced by the electrolysis, the anodic product of the carbon dioxide splitting, to improve the efficiency of the combustion include happen for a variety of reasons, ranging from a decrease in partial combustion as more oxidant is present, the reduction of NOx present as less nitrogen is available, and the higher temperatures reached which allow for improved thermodynamic efficiency [35-40]. This transition to an oxy-fuel combustion process reduces the impurities in the exhaust gas, which will prevent side reaction from occurring in the electrolysis chamber. An enhanced temperature will allow for a decrease in the electrolysis potential for the electrosynthesis of carbon nanofibers as previously reported, allowing for additional temperature benefits [2-9]. This increased temperature can impose additional material requirements, but has been successful in CC plants utilizing (expensive) cryogenically isolated pure oxygen as the combustion feed gas, and an optimization study will need to be performed to ensure the proper mixture of oxygen and air is used to ensure maximum performance with cost-effective materials.

In order to demonstrate the synergy of a CNF CC power plant in comparison to alternative carbon sequestration technology, we will be the CNF CC power plant using thermodynamic calculations. This assessment will compare the energy production of a typical CC power plant burning pure methane with no sequestration, with conventional carbon capture sequestration, and with carbon nano-fiber production. Using thermodynamic calculations based on the fundamental characteristics of the materials involved, the energy costs of the electrolysis reaction will be analyzed, while exploring

Fig. 5. Schematic of a proposed carbon nanofiber combined cycle power plant [26].

the energy advantages as a result of the coupling with a combined cycle power plant.

3. Thermodynamic analysis of a CNF CC power plant energy balance

3.1. The combustion of methane

The first section of the thermodynamic assessment will focus on the energy output of a combined cycle power plant, and makes the reasonable assumption that the natural gas fuel may be idealized as shown in Eq. (2):

CH4(g) + 2O2(g) ! CO2(g) + 2H2O(g) + heat (2)

This methane combustion reaction has an enthalpy of 890 kJ/mol at 25 °C from the thermodynamic data available through the NIST Webbook and NIST-JANAF Thermochemical Tables [41,42]. Of this 890 kJ/mol at 59% efficiency, the CC power plant generates 525 kJ of electricity per mol of methane oxidized.

3.2. The CO2 to CNF transformation

A variety of carbonate electrolytes may be used in the CO2 electrolysis. We have shown that pure Li2CO3 yields a particularly high yield (85-100%) of carbon nanotubes at low energy, and hence Li2CO3 is used in this thermodynamic assessment. This electrolyte has a melting point of 723 °C and has been shown experimentally to require the lower electrolysis potential for the process than potassium or sodium carbonate electrolytes, which have a higher melting point [2-10]. Various eutectic mixtures

using mixed sodium or potassium carbonates have been found to have a lower melting point that pure Li2CO3, however, we have observed that they are more corrosive to the oxygen anode, although mixed electrolytes can lead to lower systems costs. While the Li2CO3 is less prevalent and more costly than Na2CO3 or K2CO3, the amortized lithium carbonate is not high as the electrolyte is not consumed in the CNF production process. As will be shown in this study, a number of heat sources internal to the CNF CC plant supply excess of the thermal energy required for the molten Li2CO3 electrolyte, such as the excess heat of the CO2 from combustion, heat generated by the dissolution and reaction of the CO2 with oxide in the electrolysis chamber to form carbonate, as well as the heat exchange of the electrolysis products during recovery. An additional fraction of required energy is supplied by the applied electrolysis potential to drive the electrolysis process to produce the desired carbon nanofiber products.

3.2.1. Heat released in the carbonate dissolution of hot CO2

In the CC CNF plant, the first step of the CO2 to CNF technology utilizes the electrochemically produced metal oxide, in this case Li2O, to react with the CO2 from the combustion process to produce Li2CO3. This process allows for the removal of the CO2 from hot exhaust gas without the need for cooling, as the capture process produces more energy at higher CO2 temperatures shown in Fig. 6. The capture process plays an additional role of regenerating the Li2CO3 that was lost from the electrolyte during the electrolytic formation of carbon nanofiber. The reaction for this capture process is shown in Eq. (3).

Li2O + CO2 ! Li2CO3 (3)

Fig. 6. The enthalpy for the regeneration of Li2CO3 from Li2O and CO2 from the temperature variation of the enthalpy CO2, while the temperature of Li2CO3 and Li2O is held constant at the electrolysis temperature of 750 °C Calculated using the individual thermochemical species date in references [41,42].

Fig. 6 presents the variation of enthalpy calculated from the thermochemical enthalpies of the individual species, for this lithium carbonate formation reaction. As the temperature of the CO2 increases, the energy gains for the process increases as well. This suggests that the CO2 capture should occur at the highest possible temperature, which synergizes well with the increased temperature of burning methane in an oxygen-rich environment. The enthalpy of the solid carbon product and produced O2 gas at increasing temperature is compared to the enthalpy of the carbon and O2 gas when removed from the system at 25 °C. This study assesses the process at the temperature of 750 °C, which is the experimentally observed temperature for high coulombic efficiency production of CNFs [2-9].

As seen in Fig. 6, the heat generated by CO2 dissolution reaction is -158 kJ/mol of CO2 captured, at 750 °C. This heat is released back into the system. Additionally, this CO2 capture process decreases the heat lost through the stack of the plant, as rather than being emitted, the CO2 is kept within the system. In a conventional CC power plant, CO2 accounts for approximately 4-9% of the stack emissions by volume depending if the flue gas is recycled [43]. As shown in Fig. 2, the stack emissions of contemporary CC plants account for ~10% of the total energy of the combustion reaction and ~5% of those stack emissions by volume are from the CO2. However, due to its higher molecular weight, greater kinetic energy, and the great heat capacity of CO2 compare to N2 (N2 dominates the composition of contemporary CC stack emissions), the proportional heat loss by CO2 is greater in the flue gas. We estimate the percent of the available methane combustion enthalpy lost by the CO2 stack emission from the relative heat capacities of CO2 and N2 at the stack temperature of 106 °C, which equals:

(890 kJ) mol

(40.55 kJ/mol of CO2) 29.55 kJ/mol N2

= -6.1 kJ/mol (4)

From Eq. (4), we can estimate that a total of 6.9% of the total thermal capacity of the stack emissions is from the CO2 that is not released, meaning that 0.69% of the entire combustion energy is additionally saved in the system. This energy accounts for 6 kJ per mol of methane combusted.

3.3. Heat consumed during the transformation of CO2 to value-added products

The CO2 transformation to products in molten carbonates is complex and involves the formation of amorphous carbon (at

low temperature and without transition metal nucleation), carbon nanoparticles (at intermediate temperature and in the presence of transition metal nucleating agents) or carbon monoxide at higher temperature. While the CO2 transformation by electrolysis is complex, at low to moderate temperatures (up to ~800 °C) the global lithium carbonate electrolysis process in the absence of added carbon dioxide is straightforward; with application of 4 Faraday (1 F araday = 1 mole of electrons = 96,485 C = 96,485 As), one mole of solid carbon (here carbon nanofiber), one mole of Li2O, and 1 mol of oxygen product is generated for each lithium carbonate consumed, and in the presence of CO2, the generated Li2O regenerates the Li2CO3 consumed electrolyte. At ~750 °C in the presence of transition metal nucleating agents, the yield for the production of carbon nanofibers is high (approaching 100% columbic efficiency) and the required electrolysis potential is generally low (~1 V) and falls with increasing concentration of added lithium oxide [2,44-46].

L12CO3 ! L12O + C(CNF) + 02(g)

750 °C is the typical temperature used to generate carbon nano-fibers by electrolytes. At this temperature the coulombic efficiency for carbon nanofiber synthesis can approach 100% (that is each four electrons generates a zero valent carbon from), while at increasing greater temperature the process shifts toward the production of CO, rather than carbon [44-46]. This model does not take into account the small solvation energy of the Li2O, which was found to be less than 7.8 kJ/mol at 750 °C [3]. A more comprehensive understanding of the effect of temperature on solvation energy is required before it can be incorporated. The enthalpy of this reaction at 750 °C is 553 kJ/mol for every mol of carbon nanofibers produced. This is the energy required to form the Eq. (4) products without cooling. The electrolysis potential varies with the free energy, rather than the enthalpy, as shown in Fig. 7.

3.4. Recycled heat extracted from electrolysis products

These oxygen and CNF products generated by Eq. (5), while formed at high temperature (750 °C), will eventually be used at room temperature, for combustion (O2) or as a finished, value added product (carbon nanofibers). With proper heat capture technology a significant fraction of these product's heat can be recaptured back to the steam turbine component of the combined cycle plant to improve efficiency. The heat released from each

Fig. 7. The calculated Gibb's free energy for the formation of C and CO from CO2 or Li2CO3 using a 4-electron pathway for the formation of C and a 2-electron pathway for the formation of CO [41,42].

product as a function of temperature is included in Fig. 6. The enthalpy of the O2 from 750 °C to 25 °C is an energy savings of -24 kJ/mol of O2. The enthalpy of the carbon from 750 °C to 25 °C is an energy savings of -12 kJ/mol of carbon, for a combined savings of -36 kJ/mol of product.

3.5. Heat added through improved oxy compared to air combustion

Fossil fuel power plant energy efficiency can improve by 30-54% by enriched oxygen mixtures, rather than pure air, during the fuel combustion [5,35,47,48]. These improvements can come in improved heat transfer properties from the increased temperature of combustion and the improved heat transfer properties of the flue gas with the decrease of N2 present. Hurley et al. propose that a system using a high temperature heat exchanger that is oxygen blown instead of air blown can produce a 50% or greater increase in the heat recovery rate for the system [49]. Efficiency gains can also be found from the increase in overall combustion efficiency as well as an increase in heating rate of the combustion system. A natural gas combustion with 30% oxygen-enriched air process sees a 53.6% decrease in fuel consumption during heating as well as a 26.1% decrease in fuel consumption during constant furnace-temperature combustion, compared to unenriched 21% oxygen combustions [47]. Furthermore, at even high enriched oxygen levels, there is a continued improvement of combustion efficiency as Belohradsky shows an increase from 60% to 78% to 82% combustion efficiency when using 21% O2, 38% O2 and 46% O2 respectively [35].

The challenge for current implementation of oxygen enriched combustion process is the energy required to purify the oxygen, for example via cryogenic liquefaction and extraction, the process consumes and loses the substantial majority of the energy gained by improved efficiency, and imposes satellite oxygen purification infrastructure costs. An additional disadvantage is that the higher temperature of pure oxygen combustion poses a material challenge, and hence a mix of air and pure oxygen to improve efficiency had been preferred [4]. Interestingly, the CC CNF plant provides the oxygen efficiency gains without these drawbacks. The electrolysis chamber generates one mole of pure oxygen for each mole of methane consumed, which, as per Fig. 5, may be blended with air to provide the two moles of O2 per CH4 combustion required in Eq. (2). Without the energy loss of other oxy fossil fuel processes, we conservatively estimate an absolute efficiency gain of 15%, that is from 59% for the CC power plant to 74% for the CC CNF power plant. This is equivalent to an extractable energy gain from the methane of 0.15 / -890 = -134 kJ/mol, that is 659 kJ/mol for the CC CNF, rather than 535 kJ/mol for the CC power plant.

4. Global enthalpy of CO2 to CNF electrolysis addition to CC power

Li2Ü(750 °C) + CÜ2(g)(750 °C) ! Li2CÜ3(i)(750 °C) DH = -158 kJ/mol

Li2CÜ3(i)(750 °C) ! Li2Ü(750 °C) + C(cnf)(750 °C) + Ü2(g)(750 °C) DH = 553 kJ/mol

C(cnf)(750 °C) + Ü2(g)(750 °C) ! C(cnf)(25 °C) + Ü2(g)(25 °C) DH = -36 kJ/mol

subtotal : CÜ2(g)(750 °C) ! C(cnf)(25 °C) + Ü2(g)(25 °C) DH = 359 kJ/mol

Prevented Stack Emission Loss: 6.9% CO2 of 10% Stack Emission AH = -6 kJ/mol

O2 combust enhancement: CH4 + O2(pure) + O2(air) ! CO2

+ H2O AH = -134 kJ/mol

Total enthalpy CO2 to CNF electrolysis in power plant AH = 219 kJ/mol (6)

5. CC CNF plant equivalent potential, power and CNT produced

The thermoneutral potential is the voltage equivalent to the enthalpy associated with the Eq. (1) enthalpy of the n = 4 electron methane combustion is given using 1 mol of electrons = 1 Faraday = 96,485 C = 96.485 V-1 kJ/mol:

E(volts, methane combustion) = AH/n = 890 kJ/mole/(4 * Faraday) = 2.31 V (7)

In conventional CC power plants, at 59% heat to electricity conversion efficiency, this is equivalent to the plant operating at 0.59 / 2.31 V= 1.36 V per equivalent of methane combustion. The modeled CC CNF power plant operating at 79% conversion efficiency is equivalent to this plant operating at 0.579 / 2.31 V = 1.82 V per equivalent of methane combustion.

The calculated total additional enthalpy consumed by the CC CNF power plant in the production of carbon nanofibers, 219 kJ/mol from Eq. (6), is equivalent to a thermoneutral potential of:

E(volts, CNF plant production) = 219 kJ/mole/(4 * Faraday) = 0.57 V (8)

Hence, this assessment yields that per equivalent methane the plant will generate a net of (1.82 - 0.57) = 1.25 V; that is 482 kJ of electricity and 12 g of carbon nanotubes per mol of methane oxidized. This compares to 1.36 V (525 kJ of electricity and 44 g of CO2 per mol of methane oxidized) for the conventional CC power plant with the added advantages that (i) in the CC CNF plant no CO2 is emitted and (ii) all CO2 is converted to value added carbon nan-otubes. Converting to power and ton units, per metric ton of methane fuel consumed the CC CNF plant produces 8350 kWh of electricity and 0.75 ton of CNT and emits no CO2, while the CC plant produces 9090 kW h of electricity, no CNT and emits 2.74 ton CO2. At an estimated $0.1 per kW h and $300 K per ton CNT, per ton of methane fuel (~$100) the CC CNF plant produces $835 of electricity and $225,000 of CNTs, while the CC plant produces $909 of electricity, no CNT and emits 2.74 ton CO2. Even if the market price of CNTs were to fall by two orders of magnitude with increasing supply, the CC CNF still provides a strong value incentive (CNT product is higher than the electricity product value) for carbon dioxide removal.

The required voltage to drive electrolysis between electrodes in the electrolysis chamber is constrained by the free energy, AG, rather than the enthalpy, AH, of the specific electrode reactions and isolation of the electrode specific reactions varies with temperature, electrolyte, and electrolyte additives, such as Li2O which decreases the required voltage [3]. The surplus energy of applied voltages of greater than the 0.57 V of Eq. (8), will be recycled from the electrolysis chamber as heat back into the steam turbine via the heat exchange loop evident in Fig. 5. The required electrolysis potential is generally low (~1 V), and increases with the overpotential necessary to drive CNF production at higher rate (the applied potential increases with electrode current density).

5.1. Coal, rather than natural gas, to CNF transformation

The vast majority of coal power plants combust pulverized coal to produce high temperature, which evaporates to produce pressurized steam that drives turbines to generate electricity. The exhaust of the combustion is generally cleansed of the majority of sulfur, heavy metal and particulates and the remaining flue gas exhaust, which contains a high carbon dioxide content (along with nitrogen and water vapor) and is emitted directly to the atmosphere. Components of a convention coal power plant are illustrated in the top portion of Fig. 8 [50]. Conventional coal electric power stations emit massive amounts of carbon dioxide to the atmosphere, which comprises a substantial fraction of the total greenhouse gases. Exhaust flue gas volume composition varies with plant construction. The flue gas volume is ~323 m3/GJ from coal power plants. The flue gas contains a majority of nitrogen, water vapor, and generally 8-9% (between ~4 and ~13% of CO2 depending on the type of burner and load being used [51]). Additional infrastructure is included to scrub the flue gases of sulfur, nitrous oxides and heavy metals. Coal is principally carbon and moisture. More specifically for the coals lignite contains 45-55% carbon and up to 55% moisture, bituminous coal contains 60-82% carbon, while anthracite is 93-95% carbon. The three respectively have heat contents of ~24-30 kJ/g, 33-40 kJ/g, and 37-39 kJ/g [52].

There exist a few integrated (coal) gasification combined cycle (IGCC) power plants, which burn the coal with purified oxygen, rather than air, and can gasify the coal to hydrogen or syngas. An IGCC is illustrated on the bottom panel of Fig. 8 [53]. These IGCC plants have higher energy conversion efficiency (~50% compared to ~35% for traditional). IGCC plants have the potential to reach substantially higher efficiency when the energy penalty to form the required pure oxygen is circumvented, such as via the in-situ electrolysis of CO2 studied herein. Oxy-fuel, rather than air, coal plants have several advantages and energy efficiency is higher due to the higher temperatures achieved with a higher O2 combustion and because heat stack losses are less, as the emitted gas volume is significantly smaller with less N2 from air. Oxy-fuel coal plants generate a more concentrated carbon dioxide emission than simple coal combustion for heat, which have been explored as potential carbon sequestration opportunities for coal plants. Here, by integrating the coal plant with the STEP CNF plant, pure oxygen from the electrolysis is available to improve coal combustion efficiency without the need for the cryogenic production of pure oxygen, there is no carbon dioxide emission, heat is retained in the combustion chamber, as there is little excess stack as to exhaust, and a useful, stable product CNFs (and in particular, their more valuable form as carbon nanotubes) are produced.

The top corner (panel A) of Fig. 9 simplifies the conventional CO2-emitting coal power plant illustrated in the top of Fig. 8, by reducing the plant to its simple functional components. Panels B-D in Fig. 9 present non-CO2 emitting coal combustion plants. Each incorporates an electrolysis chamber, which splits hot CO2 from the coal combustion into a CNF product and into pure oxygen which is improve the coal combustion. B is a coal CNF plant. C is a coal CNF & electricity plant, and D is a highest efficiency, STEP coal CNF & electricity plant that utilizes the full spectrum of sunlight to energize the electrical and CNF production.

The thermodynamic enthalpy available per CO2 emitted from carbon combustion (-394 kJ/mole) is less than that from methane (-890 kJ/mole) combustion. Coal, compared to natural gas, combustion will generate less excess heat per transformable CO2 to drive CNF and CNT production product is retained within the system at (the electrolysis chamber temperature of) 750 °C, and hence under normal circumstances while a CC CNF plant can co-produce electricity and CNFs, a coal CNF plant (Fig. 9(b)) will produce only value-added carbon nanotubes, and not surplus electricity.

However, the coal CNF plant is energetically efficient, due to the low electrolysis voltage and high coulombic efficiency observed for the CO2 to CNF transformation, and is further enhanced given the opportunities of the CO2 electrolysis chamber to retain heat within the coal CNF plant, to use the electrolysis generated pure oxygen to increase the coal combustion efficiency, and to use the steam condenser heat loss to improve the product extraction and heat balance of the electrolysis chamber.

Fig. 9(c) and (d) explores coal power plants that simultaneously produce electricity and CNFs without CO2 emission, by supplementing the coal generated electricity with a non-CO2 electrical source. Plant Fig. 9(c) is a renewable/coal plant produces both electricity and CNFs, and uses non-CO2 emitting renewable (or nuclear) power, rather than electricity from the coal plant, to drive the electrolysis. Note, that in Fig. 9(d), the STEP/coal plant has the highest efficiency. The plant uses the full spectrum of sunlight, and produces both electricity and CNFs; concentrated sunlight is split into two band, the infrared band heats the electrolysis chamber and directs excess heat into the coal combustion chamber, and the visible drives photovoltaic power of the electrolysis chamber. Higher efficiency is achieved by using the full spectrum of sunlight. As we have previously demonstrated, visible sunlight provides efficient electrical power and the thermal sunlight provides supplemental heating of STEP electrolyses [44-46,54,55]. For example, conventional photovoltaics use super-bandgap (visible) radiation, and cannot use sub-band (thermal energy) as it is not sufficient to drive electron/hole separation. For example a conventional efficient (40% solar to electrical) concentrator photovoltaic, can achieve over 50% solar energy efficiencies [44] when the excess thermal energy is directed to heat the chemical and electrical reactions here. The material in solar thermal conversion has an important role to increase the efficiency of STEP. CNT is one of the materials which efficiently absorb the full spectrum [56].

6. CC CNF thermodynamic assessment refinements

With scale-up of the CO2 to CNF process ongoing refinements will be evident. As this refinement occurs, both enhancements and losses to the total enthalpy estimates of Eq. (6) can be expected. The global efficiency of the CC CNF power plant can be expected to improve. Today, a large heat loss (30% in Fig. 2) is associated with condensation of steam in the steam turbine loop. Future refinements may use, rather than lose, a portion of this heat for example in the CNF extraction process, and developments of this extraction process are of interest. One potential improvement for the capture of this lost heat is the ability for a direct heat transfer to be employed for the addition of post-combustion H2O to be added to the water used for the Rankine cycle. As more oxygen-enriched air is used for combustion, the higher the concentration of water will be in the exhaust gas allowing for larger gains for a direct heat transfer method opposed to the current indirect heat transfer method used now. Conversely, there are enthalpy losses not accounted for to be added in further refinements such as the (i) less than ideal heat exchanger losses from the CNF and O2 electrolysis products back into the and (ii) heat radiative losses from the electrolysis chamber.

7. Comparison with conventional carbon extraction technology

Carbon dioxide removal technology has used been employed at pilot plants using an amine-solution absorption trap with 90% CO2 capture rates. These plants have an average energy efficiency loss of 7%, although combined losses have been estimated as high as 30% in conjunction with CC power plant [57]. The subsequent challenge of an effective storage of that extracted CO2 was not effectively

Fig. 8. Conventional (top) and integrated gasification combined cycle (bottom) coal power plants [50,53]. Key for conventional power plant: 1. Cooling tower, 2. Cooling water pump, 3. Transmission line (3-phase), 4. Step-up transformer (3-phase), 5. Electrical generator (3-phase), 6. Low pressure steam turbine, 7. Condensate pump, 8. Surface condenser, 9. Intermediate pressure steam turbine, 10. Steam Control valve, 11. High pressure steam turbine, 12. Deaerator, 13. Feedwater heater, 14. Coal conveyor, 15. Coal hopper, 16. Coal pulverizer, 17. Boiler steam drum, 18. Bottom ash hopper, 19. Superheater, 20. Forced draught (draft) fan, 21. Reheater 22. Combustion air intake, 23. Economiser, 24. Air preheater, 25. Precipitator, 26. Induced draught (draft) fan, 27. Flue-gas stack.

addressed, with one principle course of action to pump the extracted CO2 to release more fossil fuels. This may raise, rather than lower, the carbon footprint, and the characterization of concentrating carbon dioxide using amines as carbon capture

sequestration may be a misnomer. Note the CC CNF does not require a CO2 pre-concentration step to eliminate the CO2 emission.

One benefit of the integrated CC CNF power plant assessed in this study, in addition to CO2 emission elimination and the

Fig. 9. Various plant configurations to transform carbon dioxide emissions from coal combustion into carbon nanofibers/nanotubes using various renewable energy sources.

incentivized transformation into a value-added carbon nanotube product, is the ability to absorb the carbon dioxide at a high temperature instead of at a low temperature as performed with conventional CCS technology. This additional enthalpy savings is shown in Eq. (9).

CO2(a,ptured)(25 °C)! CO2(Mptured)(750 °C) DH = -35 kJ/mol (9)

A summary of all the energy savings compared to that of a conventional CCS include the energy gained from the reaction of lithium oxide with carbon dioxide to form lithium carbonate, the energy from cooling both the carbon and oxygen products, the energy saved from preventing emissions of CO2, and the energy gained in the system from capturing hot CO2 instead of cold CO2. These savings reduce the cost of the electrolysis process to 312 kJ/mol, allowing for the production of one mol of carbon nanofibers and 213 kJ of additional energy for each mol of CO2 transformed. This energy output is further improved to 357 kJ/mol of CO2 transformed when the 15% efficiency benefit is added from the enriched-oxygen combustion.

An additional benefit, not measurable in dollars or kilojoules, of the CC CNF plant and/or coal to CNF transformation is that more readily available CNF will positively impact the automobile, airline, trucking, shipping, etc. industries; especially with respect to the reduced weight (and therefore reduced fuel consumption and emissions), increased strength and lifespan of the products (which will reduce the manufacturing use of fossil fuels), etc.

8. Conclusion

An assessment of the energy balance of a combined cycle gas plant, which additionally incorporates a molten electrolysis unit to transform CO2 to carbon nanotubes has been conducted. The assessment indicates that the electricity and carbon nanotubes can be co-generated and that the CO2 emission eliminated. The CC and electrolysis components act in a complementary/synergistic manner. The hot product from the gas turbine is an excellent reac-tant for the molten electrolysis, analyzed with a lithium carbonate electrolyte. Energy savings include the energy gained from the

reaction of lithium oxide with carbon dioxide to form lithium carbonate, the energy from cooling both the carbon and oxygen products, the energy saved from preventing emissions of CO2, and the energy gained in the system from capturing hot CO2 instead of cold CO2. This energy output is further improved as the secondary product of the electrolysis product (in addition to carbon nanotubes) of pure oxygen is looped back into the CC gas combustion improving the CC efficiency with an enriched oxy-fuel mixture for increased efficiency.

Fossil fuel combustion is the largest anthropogenic source of CO2 emissions and until renewable energy technologies are fully deployed, continues to play a significant role in the energy portfolio of the world. A combined cycle, CC, natural gas power plant is poised to play a continuing role in energy production as a result of the abundance of the fuel source and the high efficiencies of the systems. However, these power plants require some type of CO2 capture and sequestration to mitigate climate change. One such system is an alternative carbon nanofiber combined cycle, CC CNF, plant. This assessment yields that per equivalent methane the CC CNF plant will generate a net of (1.82 - 0.57) = 1.25 V. This compares to 1.36 V for the conventional CC power plant with the added advantages that (i) in the CC CNF plant no CO2 is emitted and (ii) all CO2 is converted to value added carbon nanotubes. The CC CNF adds a high-temperature molten Li2CO3 electrolyzer to a CC power plant design to remove exhaust CO2 and transform it into a valuable carbon nanotube product. The present day value of the carbon nanotubes product is ~10,000 times that of proposed, or in place, current carbon tax costs of $30 per ton, strongly incentivizing carbon dioxide removal and spur new public/private investment in this process to convert CO2 into carbon nanotubes globally.

Acknowledgement

This work was partially funded by the United States National Science Foundation under grant number 1505830. The authors are grateful to Jiawen Ren for help with electron microscopy associated with Fig. 1.

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