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FUEL PRODUCTION SYSTEMS FOR REMOTE AREAS VIA AN ALUMINUM ENERGY VECTOR Eric Morgan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01482 • Publication Date (Web): 29 Jul 2018 Downloaded from http://pubs.acs.org on August 2, 2018

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Energy & Fuels

FUEL PRODUCTION SYSTEMS FOR REMOTE AREAS VIA AN ALUMINUM ENERGY VECTOR Eric R. Morgan MIT Lincoln Laboratory, Lexington, MA 02420 E-mail: [email protected]

Abstract Autonomous fuel synthesis in remote locations remains the Holy Grail of fuel delivery logistics. The burdened cost of delivering fuel to remote locations is often significantly more expensive than the purchase price. Here we show that newly developed solid aluminum metal fuel is suited for remote production of liquid diesel fuels. On a volumetric basis, aluminum has more than twice the energy of diesel fuel, making it a superb structural energy vector for remote applications. Once aluminum is treated with gallium, water of nearly any purity is used to rapidly oxidize the aluminum metal which spontaneously evolves hydrogen and heat, in roughly equal energetic quantities. The benign byproduct of the reaction could, in theory, be taken to an off site facility and recycled back into aluminum using standard smelting processes, or left on site as a high-value waste. The hydrogen can easily be used as a feedstock for diesel fuel, via the Fischer-Tropsch (FT) reaction mechanisms, while the heat can be leveraged DISTRIBUTION STATEMENT A. Approved for public release: distribution unlimited. This material is based upon work supported under Air Force Contract No. FA8721-05-C-0002 and/or FA8702-15-D-0001. Any opinions, findings, conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the U.S. Air Force.

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for other processes, including synthesis gas compression. We show that as long as a carbon source such as diesel fuel is already present, additional diesel can be made by recovering and recycling the CO2 in the diesel exhaust. The amount of new diesel that can be made is directly related to the fraction of available CO2 that is recovered, with 100% recovery being equivalent to doubling the diesel fuel. The volume of aluminum required to accomplish this is lower than simply bringing twice as much diesel, and results in a 50% increase in volumetric energy density. That is, 50% fewer fuel convoys would be required for fuel delivery. Moreover, aluminum has the potential to be exploited as a structural fuel that can be used as pallets, containers, etc, before being consumed to produce diesel. Furthermore, FT diesel production via aluminum and CO2 can be achieved without sacrificing electrical power generation.

Introduction This work proposes solid aluminum metal as an energy vector for various domains including the military, humanitarian aid, or remote communities. Prized for its favorable strength-toweight ratio as well as its ductility, aluminum also has extremely high energy density and can serve as a non-toxic fuel for energy storage and transmission, via truck, freight, or air. When activated with gallium and indium and reacted with water, aluminum spontaneously generates both heat and hydrogen (H2 ) on demand. 1–4 The heat and H2 can then be used to directly produce electricity via fuel cells, Stirling engines or thermoelectric generators, or to synthesize fuels using the hydrogen as a chemical feedstock.

Synthesizing liquid fuel in situ, and on demand, using local resources and fuel synthesis devices has been the Holy Grail for the Department of Defense (DoD) and for remote communities for decades. Traditionally, such systems would require a source of power, such as nuclear reactors, 5–8 wind turbines, 9 or ocean thermal energy conversion 10 in order to produce a liquid commodity. Another possibility is to utilize solid fuels such as aluminum as an

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energy vector in order to deliver energy and hydrogen to fuel synthesis reactors. Aluminum has more than twice the volumetric energy content of all traditional liquid fuels. The energy in aluminum is ultimately derived from the Hall-Héroult smelting process that converts aluminum oxide into relatively pure aluminum metal. Utilizing solid aluminum as an energy vector enables the concept of a structural fuel element; the fuel itself is multi-functional.

Employing aluminum as an energy vector under adverse conditions, such as war zones or disaster areas, reduces the burden of carrying volatile liquids while enabling the use of embedded or ‘structural fuel’ concepts. Aluminum is a ubiquitous material that boasts a worldwide production and distribution network, so handling, delivery, and familiarity are already established. Aluminum fuel presents the opportunity to design a delivery system focused on a solid material, rather than a liquid fuel. To the author’s knowledge, no such structural fuel exists beyond wood and other, similar, waste streams like plastic. Containers, pallets, or other common structures could be fabricated with aluminum and then used as fuel when advantageous. Considerations regarding the size, shape, and utility of the aluminum can create a new paradigm of energy delivery to harsh environments.

Remote fuel synthesis units have different hydrogen transport constraints than traditional fuel cell systems. First, the balance-of-plant (BOP) is less of a concern in stationary systems than it is in mobile platforms (vehicles) because stationary systems require upfront equipment delivery and installation whereas mobile systems are tasked with moving everything. Thus, a stationary fuel production plant can be installed and commissioned separately from the initial energy delivery. Only the energy vector must be mobilized once the fixed assets are installed. The most energy dense vector is then chosen to minimize the transportation costs. Second, fixed installations have the opportunity to leverage local resources as feedstocks. For example, aluminum systems require water of nearly any purity which, in many places, can be locally sourced either from natural waterways, or host nations.

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Aluminum Background Aluminum is the most common metal in the Earth’s crust, making up about 8% of the crust by mass. Due to its high reactivity, it is almost never found in pure form, instead it typically combines with other minerals to form raw ores such as bauxite. Aluminum’s unique combination of low density, corrosion resistance, malleability, ductility, and strength make it a desirable engineering material for use in aerospace, transportation, and many other disciplines. Aluminum has one of the highest volumetric chemical energy densities of all solids, behind beryllium and boron; it is easily transportable, and non toxic. 11 Once aluminum is produced, it remains stable and unreactive due to a passivation layer of aluminum oxide (Al2 O3 ) on the aluminum surface. 12,13 The removal of the Al2 O3 layer allows the aluminum metal to be oxidized which occurs rapidly once initiated. A eutectic of gallium and indium has been shown to remove the passivation layer on aluminum, 1–4 effectively unlocking the stored energy content. Once the aluminum is activated, it can be corroded (oxidized) by water according to three global chemical reactions:

2 Al + 6H2 O ⇀ 2 Al(OH)3 + 3 H2 (∆H ◦ = −836kJ)

(1)

2 Al + 4 H2 O ⇀ 2 AlO(OH) + 3 H2 (∆H ◦ = −826kJ)

(2)

2 Al + 3H2 O ⇀ Al2 O3 + 3 H2 (∆H ◦ = −820kJ)

(3)

Each reaction requires 2 moles (54 grams) of aluminum to produce 3 moles (6 grams) of H2 . The reactions are also strongly exothermic, releasing approximately as much heat content as hydrogen chemical energy content. A Department of Energy (DOE) report on aluminum fuels 11 states that Equation 1 is favorable below 280◦ C and produces bayerite and H2 ; Equation

4


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2 is favorable from 280–480◦ C and produces boehmite and H2 ; and Equation 3 is favorable above 480◦ C and produces aluminum oxide and H2 . All of the byproducts of the reactions are non-toxic with the H2 being useful for electricity generation via fuel cells, or fuel synthesis. The eutectic is not consumed in the reaction, and can be recovered using a simple centrifuge since it is far more dense than any of the reactants or products.

As the reaction temperature increases, the water demand decreases, eventually becoming half of the original value at temperatures above 480◦ C. Equations 1 and 3 indicate that aluminum hydroxide is calcined to form water and aluminum oxide and water spontaneously at high temperatures:

2 Al(OH)3 ⇀ Al2 O3 + 3 H2 O

(4)

While water could be saved in this manner by designing two reactors in series, one reactor for Equation 1, and a second reactor for Equation 4, a single, high temperature reactor offers an elegant means of reducing total water consumption while simplifying the system design. Moreover, the aluminum mineral waste product is reduced by 33% and can be smelted directly using the Hall-Héroult process to regenerate aluminum metal. Doing so would require transport of the Al2 O3 to an off-site smelting location.

The three equations presented vide supra can also be cast in terms of mass, which is useful for comparing the energetics of aluminum to other fuels:

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54gAl + 108gH2 O ⇀ 156gAl(OH)3 + 6gH2

(5)

54gAl + 72gH2 O ⇀ 120gAlO(OH) + 6gH2

(6)

54gAl + 54gH2 O ⇀ 102gAl2 O3 + 6gH2

(7)

Aluminum has a specific density of 2.7, so the nominal energy density can be plotted against other common liquid fuels, as shown in Figure 1. To give a sense of the scale of industrial production, the figure also shows the relative worldwide production of the fuels, and indicates which ones are synthetic – assembled by humans – versus distilled or purified. Aluminum is an outlier in terms of volumetric energy density, having more than twice the energy density of diesel fuel. Moreover, aluminum is the only solid (at STP) on the chart, which opens a new paradigm of fuel transportation and handling. While liquid hydrogen is an outlier on the gravimetric axis, it is only useful for mass constrained, high power applications such as rockets.

Aluminum Production All aluminum metal is ultimately derived from bauxite deposits, though the aluminum market includes both primary metal and secondary metal classifications. Primary aluminum is produced directly from ore; secondary aluminum is produced from recycled scrap aluminum. Primary aluminum production entails mining bauxite ore from geological deposits, refining the ore to form alumina (Al2 O3 ), and then smelting the alumina to form aluminum. On average, about 5.1 kilograms of bauxite produces 1.93 kilograms of alumina which in turn produces 1 kilogram of aluminum metal. 14 The production of aluminum is highly energy intensive, and involves two main processing steps: the Bayer Process which converts raw bauxite ore to alumina (Al2 O3 ), and the Hall-Héroult Process which converts alumina to aluminum metal. Overall, the Bayer process requires about 24 6


GJ/tonne

of aluminum, while

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VOLUMETRIC AND GRAVIMETRIC ENERGY DENSITIES FOR ENERGY STORAGE MEDIA 120

LIQUID HYDROGEN

100 GRAVIMETRIC DENSITY (MJ/kg)

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Energy & Fuels

BUBBLE SIZE ∝ WORLDWIDE PRODUCTION SYNTHETIC: CARBON FREE:

80 GASOLINE

60 DIESEL LPG

40

ALUMINUM

ETHANOL

20

AMMONIA

METHANOL

BIODIESEL

0 0

10

20

30

40

50

60

VOLUMETRIC DENSITY (MJ/L)

Figure 1: Energy densities of fuels.

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70

80

90

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the Hall-Héroult Process requires 54

GJ/tonne

of aluminum, mostly in the form of electricity.

The simplified schematic of the energy flows in a bauxite to aluminum process is shown in Figure 2. The molten metal aluminum output from the Hall-Héroult process is then cast into ingots for the metals markets. Note that Figure 2 implies that a ‘closed loop’ aluminum energy storage is possible. The overall storage efficiency would be on the order of 50%, which considers the energy stored in the aluminum divided be the energy for both the HallHéroult process and the calcine step. In theory, aluminum could be a sustainable energy vector if alternative energy is used to for the energy input. However, to use aluminum as an energy vector for remote locations the aluminum hydroxide or alumina waste would need to be shipped to a suitable smelting facility, which might be impractical under some conditions.

Secondary aluminum is produced via recycling scrap aluminum which is sorted, melted and recast. Secondary aluminum production requires only about 6% of the energy required to produce primary aluminum. 15 There are several grades of recycled aluminum including industrial scrap and post-consumer scrap. Industrial scrap includes the waste generated from manufacturing processes while post-consumer scrap includes beverage cans, construction materials, automotive parts, etc.

Aluminum as a Hydrogen Vector Hydrocarbon synthesis requires only two feedstocks: hydrogen and carbon. Typically, the carbon is derived from natural gas or coal, but can also be derived from carbon dioxide which is present in the atmosphere and in engine exhaust. On a molar basis, hydrogen is the most energetic of the chemical feedstocks required for hydrocarbon synthesis, and is therefore the bottleneck in remote production of hydrocarbons. Traditionally, hydrogen in remote systems was derived from water via electrolysis because other feedstocks, such as natural gas, are not available. Aluminum based water splitting is an attractive alternative to expensive 8


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Energy & Fuels

Bauxite

Bayer Process

H2 O

54 GJ/t Al

24 GJ/t Al Al2O3

HallHéroult Process

Al

Al-Ga Reactor

31 GJ/t Al

Al(OH)3 Al2O3

Calcine

3.4 GJ/t Al

Figure 2: Bauxite ore to aluminum metal with recycle. electrolysis. Yet, while aluminum has been considered by the DOE as a hydrogen vector, 11 it was only considered for vehicles, and not as a hydrogen vector for general applications. Aluminum has merit in situations where the water is not carried, including marine applications, or fixed applications with local water supplies. The hydrogen density of an aluminum/water reaction depends on which chemical reaction (Equations 1-3) is applied since the required water differs by a factor of two. Moreover, for fuel cell systems the recovery of water vapor with a water trap can increase the hydrogen storage density in aluminum systems. This is unique to aluminum because the water itself acts as an oxidizer whereas in conventional fuel cell systems the water is a byproduct.

To determine the value of recycling water, we can sweep the amount of recycled water from 0% to 100% in increments of 25% for each of the three chemical equations to determine the theoretical hydrogen storage density for aluminum. The results are shown in Figure 3 where the upper-rightmost points for each of the three chemical equations depict 100% recycled water which is therefore not present in the energy density calculation. Remote fuel file:///P:/ACS/Bauxite_to_Al_PPTv2.svg 4/25/2018 synthesis would likely leverage Equation 3 to minimize water usage, so the H2 density would

be near the theoretical maximum. 9


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Figure 3: Theoretical maximum H2 storage density.

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The pattern is evident: high temperature aluminum-water reactions that recycle some water can offer very high hydrogen storage densities. The DOE 2020 targets for H2 storage in light vehicles are 45 g

H2/kgsys

(4.5 weight %) and 30 g

H2/Lsys

so there are opportunities

for aluminum systems to meet that goal. 16

Hydrogen Production with Aluminum and Water Aluminum/water reactions have been investigated since at least the 1960s when salts were used to remove the oxide layer from aluminum. 17 Work continued in the 1970s with mercury/aluminum amalgams 18 and the 1980s with gallium/aluminum eutectics. 19 More recently, additional research has been conducted on aluminum/water reactions 1–4,20–23 showing that a wide variety of methods can be employed to ‘unlock’ the energy content in aluminum. However, some methods use significant amounts of eutectic 2,20 while other methods use dangerous powders. 22,23 Another, elegant, solution was found that uses low concentrations of gallium/indium eutectic (≈ 2%) and easily treats bulk aluminum 1,4 which can then be directly oxidized with water. Aluminum treated in this way can be used as an on-demand hydrogen generator with the hydrogen production being controlled by the water feed rate, the aluminum feed rate, or the reaction temperature. The supply of hydrogen can be easily ramped up in a matter of seconds or minutes; it can be stopped just as easily. In that sense, H2 generation using aluminum/water is similar to water electrolysis in terms of being an on-demand hydrogen generator. However, an aluminum/water H2 generator would be two orders of magnitude smaller than an equivalent alkaline electrolyzer, and wouldn’t require any external power generation.

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Diesel versus Aluminum The main competitor for aluminum is diesel fuel which is used almost exclusively in DoD applications. 24 Even though diesel fuel is ubiquitous in the US military, it does have drawbacks. Notably, diesel is a volatile liquid that must be defended from attack on the battlefield. For example, when being delivered to DoD Forward Operating Bases (FOBs) diesel fuel protection costs can be as high as $100 per gallon at the tactical edge. 25 Thus, using traditional energy efficiency or energy density metrics as a head-to-head comparison between diesel fuel and aluminum misses some of the story. In that context, cost, logistics, and safety could be considered more important factors than energy density. That being said, aluminum has more than double the volumetric energy content over diesel fuel and produces high grade heat and hydrogen upon reaction. In terms of energetics, assuming the hydrogen is 45% (37.7 MJ/L) of the energy content of the aluminum, and that a hydrogen to liquid fuel process is 50% efficient, about 22.5% (18.86 MJ/L) of the aluminum could be used to produce a liquid fuel. While this is about half of the energy content of diesel fuel (≈35

MJ/L)

the remaining

heat could be used for water purification, power generation, or space heating.

Remote Fuel Synthesis Remote fuel synthesis units have different hydrogen transport constraints than traditional fuel cell systems. First, the balance-of-plant (BOP) is less of a concern in stationary systems (processing plants) than it is in mobile systems (vehicles) because stationary systems require upfront equipment delivery and installation whereas mobile systems are tasked with moving everything. Thus, a stationary fuel production plant can be installed and commissioned separately from the initial energy delivery. Only the energy must be mobilized once the fixed assets are installed. The most energy dense vector is then chosen so that transportation costs are minimized. Second, fixed installations have the opportunity to leverage local resources as feedstocks. For example, aluminum systems require water of nearly any purity

12


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Energy & Fuels

which, in many places, can be locally sourced either from natural waterways, or host nations.

The DoD has investigated remote fuel synthesis in the past, but abandoned work due to lack of compact non-nuclear power equipment. 26 Moreover, RAND has concluded that remote fuel synthesis is impractical due to lack of carbon availability. 24 However, carbon is routinely shipped into the theater in the form of diesel fuel. Carbon could therefore be recovered from a stationary generator exhaust stream and used to produce additional diesel; the carbon would be recycled and synthetic fuel production would be directly related to the ability to recapture carbon. Carbon capture is a mature technology 27,28 that is used in power plants and other industrial settings. Moreover, the energy input to capture carbon can be heat which is ideally suited to aluminum/water reactions.

In order to synthesize a meaningful amount of liquid fuel, say 1000 kg of diesel fuel per day (about 320 gallons per day), using traditional electrolytic approaches roughly 1MW of constant power is required, assuming a 50% efficient power to fuel process. To achieve that average power level, the best solar photovoltaic (PV) sites would require approximately 6.6MW of installed nameplate capacity; energy storage would still be required to smooth out the peaks and valleys. Similar analysis shows that about 2.5MW of wind capacity are needed to supply that average power. Assuming that the wind or solar could provide the necessary power, the next challenge would be generating the hydrogen. Hydrogen can easily be produced from electricity and water using electrolyzers. However, in order to provide a sufficient hydrogen stream of hydrogen for 1000 kg/day of diesel fuel production – 160

kgH2/day

– large electrolyzers are needed. Electrolyzers in the MW class are nominally larger than typical military equipment. Commercial 2.3 MW electrolyzers, for example, have a footprint of 20 meters by 15 meters and a height of 7 meters. Without considering the power supplies (e.g., solar PV, wind), the power density of such a device is 1MW/900m3 . By contrast, based on data in 1 a scaled aluminum reactor would have a power density of 1MW/2m3 , without requiring

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any power generation equipment. Such a power density warrants a re-analysis of remote fuel synthesis.

Fuel Synthesis Candidates Liquid fuels for transportation fall into two broad categories, the gasoline category and the distillate category. These two categories correspond to the two major types of ground transportation engines, internal combustion (IC) and compression ignition (CI), respectively. In general, the gasoline-based fuels are highly volatile, with a higher flash point and vapor pressure than distillates. Due to the lower volatility of distillates, they are preferred over gasoline for many remote locations. In the United States, the DoD fuel portfolio, for example, was comprised of about 75% JP-8 and JP-5, and about 13.6% naval distillate fuel; only 1.12% was gasoline or gasohol, which were used solely for non-combat vehicles. 24 The following include some of the important fuel characteristics that must be met by synthetic fuels for DoD applications: 24,29 1. Chemical composition 2. Thermal stability 3. Thermal properties including freezing point and viscosity 4. Lubricity 5. Combustion properties. Fischer–Tropsch (FT) synthesis was developed in the 1920s by the German scientists Franz Fischer and Hans Tropsch. FT has primarily been employed to produce hydrocarbons for isolated nations including Germany during World War II, and South Africa during apartheid. 30,31 FT synthesis is a versatile means of synthesizing liquid fuels from a variety of feedstocks, including coal, natural gas, biomass, and other carbonaceous sources. 32 In all 14


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Energy & Fuels

cases, a synthesis gas composed of hydrogen and carbon monoxide or carbon dioxide, or both must be produced so that the catalytic reaction to produce hydrocarbons can proceed. FT synthesis converts the hydrogen and carbon feedstock into hydrocarbons of differing molecular weights according to:

(2n + 1) H2 + nCO ⇀ Cn H(2n+2) + nH2 O

(8)

where n is a positive integer. The FT process produces a spectrum of hydrocarbon chain lengths most using the building block −CH2 − for chain growth. Figure 4 illustrates the chain growth process, which starts with methane (CH4 ) and proceeds through higher order hydrocarbons. To produce a majority of diesel fuel, the FT reactor conditions are set such that n is greater than 5, typically so that n = 12. 32 By using low temperature FT reactors, the diesel selectivity can be as high as 80%, with 15% naphtha, and 5% C1 −C4 gas. 33 A schematic for FT chain growth is shown in Figure 4. CH2

+H2

CH4

α +CH2 C2 H4

d

C2 H4

+H2

C2 H6

α +CH2 C3 H6

d

C3 H6

+H2

C3 H8

α +CH2 etc Figure 4: Chain growth during Fischer–Tropsch synthesis. 33

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FT from Aluminum In order to analyze how diesel fuel can be produced with aluminum we first combine Equations 1 and 8 to form:

50Al + 114H2 O + 36CO ⇀ 50 Al(OH)3 + 3C12 H26

(9)

This reaction is strongly exothermic, combining both the exothermicity of the aluminum reaction and the FT reaction. However, since remote areas will likely not have access to CO from the reformation of methane, or natural gas in general, we must assume that the carbon source is CO2 . In order to produce the synthesis gas necessary for FT diesel, the reverse water shift (rWGS) reaction is employed to generate CO from CO2 and H2 :

CO2 + 3 H2 CO + H2 O (∆H ◦ = 41kJ)

(10)

Combining Equations 8 and 10, clearing fractions and converting to a mass basis gives an idealized view of how carbon capture technologies and aluminum-water reactions can be combined to form diesel fuel:

74 Al + 150H2 O + 36 CO2 ⇀ 74 Al(OH)3 + 3 C12 H26

(11)

1998gAl + 2700gH2 O + 1584gCO2 ⇀ 5772gAl(OH)3 + 510gC12 H26

(12)

Operating at a higher temperature and pressure, according to Equations 3 and 12, reduces water consumption and generates alumina:

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Energy & Fuels

1998gAl + 702gH2 O + 1584gCO2 ⇀ 3774gAl2 O3 + 510gC12 H26

(13)

Both Equations 12 and 13 indicate that the ratio of aluminum mass to produced diesel fuel mass is roughly 4:1. If the exothermic reaction energy of only the aluminum reaction is considered, then about 31 MJ is also available for other processes. A process flow diagram (PFD) based on Equation 13 is presented in Figure 5. The PFD has five main sections: the aluminum-water reactor, the reverse water gas shift (rWGS) reactor, the CO2 scrubber, the Fischer–Tropsch reactor, and the diesel engine. The steady state flow rates are given based on the stoichiometry already presented. We further assume that the reactors are 100% efficient and that all ‘new’ carbon comes from the fresh diesel feed.

648g

FT H2O

648g

702g

rWGS H2O

New H2O

FT Reactor CO H2 Mixer

Water Mixer 150g

H2O

1158g Syngas

Stream 1 H2

Aluminum Water RX H2

1008g CO

H2 Split Stream 2 H2

Al H2O Mix

New Diesel

72g

FT Products

Al RX Mix Aluminum

CO H2O Split CO2

1998g

H2 CO2 Mix

1584g

Al2O3

CO2 H2 Mix

Heat to rWGS CO H2O

31MJ Heat

3774g

Diesel

rWGS

FT Water Diesel Split 510g

2MJ

CO2 Stripping

Diesel Tank

Waste Heat to Heat Engine 30MJ

CO2 Lean Exhaust

36

Power

Diesel Exhaust

CI Engine

Figure 5: Process flow diagram for aluminum FT.

Based on this PFD we can evaluate how an aluminum-water system would benefit a remote area that exclusively uses diesel fuel. First, we assume that the supply chain to the 17


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remote area is volume constrained, rather than mass constrained. That is, we assume that existing trucks can be used, but loaded with more dense cargo. Then, for each unit volume (in this case, a gallon) we can determine what the existing diesel system provides, versus what the aluminum system would provide. We begin by recognizing that diesel is itself a carbon vector and can therefore help produce FT diesel. This assumption is only valid for diesel that is burned in stationary systems such as generators where CO2 can easily be recovered. Then, based on the recovery rate of the CO2 from the exhaust stream, the required mass of aluminum can be calculated using Equation 13. We also consider that the rWGS reaction requires hydrogen to produce CO for the synthesis gas, which generates water as a byproduct, and further adds to the aluminum requirement. Once the mass of aluminum is calculated, the heat generation can be found to be equal to the enthalpy released. The synthesis gas flows into the FT reactor and produces diesel fuel, water, and more heat. The heat from the FT synthesis and the heat required for the rWGS reaction are roughly equivalent, and heat integration between the two would cancel out and alleviate any external heat requirements. Finally, we find that the system produces diesel fuel and heat from diesel fuel, aluminum and water. If we then assume a 75% carbon recovery rate from the exhaust and sweep the daily diesel consumption, we can determine the amount of fielded aluminum, the amount of FT diesel produced, and the electrical power potential of the aluminum and diesel fuel. The results are shown in Figure 6. The figure can be interpreted as follows. A given amount of diesel fuel is capable of generating electricity, as it normally does. Then Equation 13 is used to determine, based on the available carbon dioxide in the exhaust, the volume of aluminum required to produce FT diesel. The aluminum also produces heat, which can be used, via a Stirling engine, to produce electrical power. The sum of the diesel generator output and the Stirling engine is reported as ‘Total Power’. For example, at 1000 gallons per day (GPD) of diesel consumption, about 1100 GPD of aluminum would be needed to produce 750 GPD of FT diesel, which would yield 900kW of power.

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Figure6:6:Diesel Diesel and and aluminum aluminum tradeoffs. Figure tradeoffs.

In order to compare the aluminum-diesel system to a traditional diesel system, it is useful to put everything on a per volume basis. To do this we sweep the CO2 recovery fraction, determine how much aluminum is needed to react the recovered carbon, and then normalize the sum of the aluminum and diesel to one ‘blended’ gallon. Moreover, the expected power for each blended gallon is given as the aluminum fraction, the diesel fraction, and the total. We find that a high carbon recovery fraction is exceptionally valuable for several reasons. First, the amount of FT diesel that can be produced equals the amount of fielded diesel when the recovery ratio is unity: for every gallon of diesel brought to the field, another gallon is made. Second, the FT diesel can be produced without sacrificing the total power generation, which is essentially constant across all recovery rates. The reason is that on a per volume basis, aluminum has the same heat content as diesel fuel when the hydrogen energy and the parasitic energy loads are removed. A Stirling engine and a diesel engine are comparable in efficiency at the microgrid scale sizes. Thus, it makes the most sense to bring as much Figure 7: Volumetric energy density of aluminum diesel blends.

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aluminum as possible out to the field along with diesel in order to maximize the delivered energy density. Figure 7 illustrates that at high recovery rates of CO2 , the combination of aluminum and diesel has an energy density that is 75% higher than a diesel only system. Aluminum and CO2 recovery can reduce the number of convoys by more than 40%. That is, as the recovered fraction of CO2 increases, the aluminum system becomes more and more valuable because the CO2 enables the direct production of distillate fuels from aluminum. Figure 6: Diesel and aluminum tradeoffs.

This is conceptually simple, the aluminum system has more than twice as much energy as diesel; the hydrogen is fully utilized, and the heat is used at about the same rate as a diesel engine.

Figure diesel blends. blends. Figure7:7:Volumetric Volumetricenergy energy density density of of aluminum aluminum diesel

Process Energy Requirements 11 Thermodynamically, converting from one form of energy to another results in inherent energy loss. The tradeoff is that the resulting fuel is energetically dense, which can be valuable for volume or mass constrained energy conversion systems like vehicles, or aircraft. This 20


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Energy & Fuels

tradeoffs. Figure 8: Fuel tradeoffs. subsection detailsDioxide some of the energy requirements for fuel synthesis via Fischer–Tropsch 4.2.1. Carbon

The energy estimates for separating a one stream from a mixture of streams, reactions. regardless of the material, is given by:

Carbon Dioxide

Wrev = −Ru To

X

Ni ln yi

(14)

thisestimate context, canseparating be captured theofexhaust streams fossil The In energy forcarbon completely a purefrom stream a component from aofmixture fuel fired engines and recycled back into synthetic fuels [21]. The concentration of regardless of the(12%) material, is given by: of components, CO2 in exhaust streams is approximately 300 times higher than than

in the atmosphere (0.04%), but according to equation 14, the minimum work of separation decreases by only a factor of three. However, according to the Sherwood plot which relates the dilution of a∑ substance to the cost of extraction, Wrev =capture −Ru To areNprohibitively (14) the logistics of ambient air carbon expensive compared i ln yi with carbon capture from exhaust streams [22]. The extraction of 99% of CO2 from an exhaust stream containing 12% CO2 requires a minimum work input of Ru and To have their usual meanings; Ni is the number of moles, and yi is the mole 7 kJHere, /molCO 2 . According to [22], the Second Law efficiency of carbon scrubbing is in theof20–25% range, thatcarbon the actual is closer to streams 28–35 fraction, component i. Inmeaning this context, can be energy capturedinput from the exhaust kJ/molCO . As Lackner points out, the energy to accomplish the separation from 2 of fossil fuel fired engines and recycled back into synthetic fuels. 34 The concentration of CO2 conventional scrubbers could come from the ‘free energy’ of low grade heat [23], which could be from either the exothermic aluminum-water reaction, or the exothermic FT reaction. 21 ACS Paragon Plus Environment

The production of diesel fuel requires several steps that must be performed

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in exhaust streams (12%) is approximately 300 times higher than than in the atmosphere (0.04%), but according to Equation 14, the minimum work of separation decreases by only a factor of three. However, according to the Sherwood plot which relates the dilution of a substance to the cost of extraction, the logistics of ambient air carbon capture are prohibitively expensive compared with carbon capture from exhaust streams. 28 The extraction of 99% of CO2 from an exhaust stream containing 12% CO2 requires a minimum work input of 7 kJ/molCO2 . According to House, et al., 28 the Second Law efficiency of carbon scrubbing is in the 20–25% range, meaning that the actual energy input is closer to 28–35

kJ/molCO

2

.

As Lackner points out, the energy to accomplish the separation from conventional scrubbers could come from the ‘free energy’ of low grade heat, 27 which could be from either the exothermic aluminum-water reaction, or the exothermic FT reaction.

The production of diesel fuel requires several steps that must be performed in sequence before the diesel is synthesized. First, high purity hydrogen and carbon dioxide feedstocks are needed to form the synthesis gas, which must be pressurized to about 40 bar. 35,36 The isentropic energy requirements for compressing a gas is given by

wcomp

Since the

H2/CO

kRT1 = k−1

[(

P2 P1

) (k−1) k

] −1

(15)

ratio is roughly 3 for the synthesis gas, 36 the partial pressure of CO and

H2 are 10 and 30 bar, respectively for a 40 bar syngas. This equates to 172 kJ/kg for CO and 6392

kJ/kg

for H2 . Assuming that the FT process has a 3:1 ratio of

H2/CO

at the inlet, the

total energy requirement would be 4837 kJ/kgsyngas . While the energy for such compression is typically supplied by electricity or steam driven turbines there is an opportunity to leverage the heat of reaction itself by containing the reaction. Doing so would eliminate the costly mechanical compressor and driver which would simplify the system while increasing its reli-

22


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ability. This can be done at the expense of making the pressure vessel walls slightly thicker. This same scheme is employed in high pressure electrolyzers, for which it is shown that the compression energy is the same for compressors and for reaction containment. 37

Another heat requirement is compensating for the endothermic reaction to produce CO from CO2 and H2 . The so-called reverse water gas shift reaction requires 41 kJ/mol for input heat in order to drive the reaction forward. Thus, for every mole of CO2 we require 28–35 kJ/molCO

2

for separation from the exhaust stream, another 7.5

10 bar, and 40

kJ/mol CO 2

kJ/molCO

2

for compression to

for the rWGS reaction. If we consider the inherent compression

efficiency to be only 50%, then the compression equates to 15 kJ/mol CO2 . The total energy requirement for CO2 is then 85–90 kJ/molCO2 , or 1930–2045 kJ/kgCO2 .

While other energy requirements in the aluminum to diesel process exist, including gallium separation from the slurry and post-processing diesel, they are minor compared to those discussed above. The question then becomes: is it possible to utilize only the waste heat from the aluminum reaction to run all of the required processes? We have already established that 2 kilograms of aluminum could produce 1/2 L of diesel fuel. Since it can be further shown that 1/2 L of diesel fuel contains 0.057 kilograms of H2 and 0.358 kilograms of C (or 1.312 kilograms CO2 equivalent), the total energy requirement for 1/2 L of diesel is about 1100 kJ. According to Equation 12 the production of 1/2 L of diesel requires 2 kg of aluminum. The heat of reaction associated with this input equates to 31 MJ of liberated heat, or 30 times the requirements for FT synthesis. Moreover, this does not include the exothermic reaction of the FT synthesis itself. It is clear through this analysis that even considering Carnot limits, efficiency losses, etc., a 3% efficiency to convert aluminum heat to useful work is entirely attainable. The low grade heat could also be used for other purposes, including power generation via a Stirling engine, distillation or purification of water, space heating, etc. The concept is similar to cogeneration in building and power systems design.

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Way Ahead Aluminum fuel is an emerging technology with the potential to disrupt several sectors including remote fuel synthesis. Aluminum is unique in that it produces not one, but two, separate streams of energy; traditional fuels typically produce only heat or electricity. Remote fuel synthesis is uniquely suited to leveraging water-aluminum reactions because synthesis needs both hydrogen and heat in the subprocesses. Aluminum-water reactors can be scaled to match any required hydrogen output while also providing high grade and low grade heat for other processes and electricity generation. In that regard, aluminum-based fuel synthesis is similar to combined heat and power systems for buildings where electricity and heat loads are proximate. Remote fuel synthesis can benefit from aluminum’s non-volatile, non-toxic, highly energy dense properties. Aluminum coupled with CO2 capture from diesel generators can provide the necessary feedstocks to produce high quality, zero sulfur diesel fuel on site. In fact, doing so would not only yield liquid diesel, it would likely provide other valuable hydrocarbons, and high quality heat from the FT process as well.

Acknowledgements Many thanks to Jean Sack at LL for fruitful discussions on aluminum as an energy carrier. Thanks also to Doug Hart and Jonathan Slocum who have advanced aluminum technology to where it is today. This work was supported by the LL Energy Initiative, led by Bill Ross and Nick Judson who saw the potential for aluminum to be a unique fuel across many domains.

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