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Effective Use of Renewable Electricity for Making Renewable Fuels and Chemicals Robert S. Weber ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04143 • Publication Date (Web): 14 Dec 2018 Downloaded from http://pubs.acs.org on December 14, 2018
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Effective Use of Renewable Electricity for Making Renewable Fuels and Chemicals Robert S. Weber* Pacific Northwest National Laboratory, PO Box 999 MS-IN K2-12, Richland, WA 99362 Keywords: electrocatalysis, process economics
Abstract If the electrochemical upgrading of CO2 is directed at products with the market sizes and environmental footprints of fuels then even very inexpensive, renewable electricity may not be sufficient to permit economic operation. If instead, the upgrading is directed at higher value products, then the size of the markets will likely be too small to offer a significant environmental benefit. An alternative approach would be to use the renewable electricity to ameliorate environmental burdens other than CO2, for example, wet or solid wastes that would otherwise be disposed in a landfill. In all cases there are interesting opportunities to pursue and deploy research on heterogeneous catalysis.
This Viewpoint starts with the implications of success, that electrocatalysts can be devised to efficiently convert waste carbon, such as CO2, sludges, manures, food waste, aquatic crops and municipal solid waste, into higher valued fuels and chemicals. That assumption permits focusing on the choice of endpoints—the reactants and products—and on the energy budget required to make a significant difference in the environmental burden of those wastes. I start with consideration of the conversion of CO2 and then switch to the conversion of other reactants that require the addition of comparatively less energy. The up-valuing of the other reactants does, however, necessitate removal of impurities, which may open new targets for research in electrocatalysis. The input energy for the electrochemical reduction of CO2 must include the cell potential required to satisfy the thermodynamics of the reaction1-5 (Table 1) plus that required to surmount the apparent activation barrier for the reaction (the overpotential). Weber, Electrocatalysis for effective use of renewable electricity ACS Paragon Plus Environment
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Table 1. Reactions that convert CO2 into higher valued products; the listed potentials are those that correspond to the free energy changes: E°=ΔG°/nF. Reaction
n
E°/Va
ΔG°/kJ mol-1
CO2 ⇌ CO + 0.5 O2
2
-1.33
257
CO2 + H2O ⇌ HCOOH + 0.5 O2
2
-1.43
276
CO2 + 2 H2O ⇌ CH3OH + 1.5 O2
6
-1.21
702
CO2 + 2 H2O ⇌ CH4 + 2 O2
8
-1.06
818
2 CO2 + 3 H2O ⇌ C2H5OH + 3 O2
12
-1.14
1320
2 CO2 + 2 H2O ⇌ C2H4 + 3 O2
12
-1.15
1331
3 CO2 + 4 H2O ⇌ C3H7OH + 4.5 O2
18
-1.13
1960
a) Cell potentials and free energies have been calculated from half-cell potentials3 extrapolated to pH 7; the free energy is per mole of the indicated product. The half-cell potential for the conversion of CO2 to propanol was calculated from the free energy of formation of the alcohol.6 The overpotential represents a kinetic barrier,7 it is not directly related to the entropy or thermoneutrality of the reaction as has been averred.5 The overpotential must be evaluated at a given current, because the cell voltage will increase as the cell current is increased to drive the reaction and all the other, associated rate processes. A minimum practicable current would be one that allows the electrochemical cell to produce product at an economically viable rate of production. Many industrial processes produce roughly 1 mol of product per cubic meter of reactor per second.8 For a 4-electron reaction running in an electrolysis stack whose cells each have a thickness (repeat distance) of, say, 4 mm, that productivity corresponds to a current density of 150 mA cm-2: 1 molproduct m-3 s-1 × 4 × 96485 C/molproduct × 0.004 m/cell = 154 mA cm-2 That crudely estimated minimum current density is gratifyingly close to the value of 250 mA cm–2 calculated on the basis of a more complete technoeconomic analysis.9 Even at the much lower current densities (ca. 5 mA cm-2) at which the rates of those reactions have been reported, the overpotentials for just the cathodic reactions exceed 1 V (Table 2). The potential applied across the whole reactor will also include the barrier of the anode reaction and for transport across the cell separator. For reference, consider that modern hydrogen PEM fuel cells10 can operate at current densities >1 A cm-2 at cell overpotentials of about 0.6 V. On the
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other hand, for formic acid, one group11 found that the cell voltage required to achieve the minimum practicable current density is 3.5 V, which corresponds to a cell overpotential of more than 2 V (=-3.5V + 1.43V). Table 2. Estimates of overpotentials for some of the half reactions shown in Table 1: Metal Ered/Va vs nhe at 5 mA cm-2
Reaction CO2 + 2e– ⇌ CO + 0.5 O2
+
–
CO2 + 2H + 2e ⇌ HCOOH
+
–
CO2 + 8H + 8e ⇌ CH4 + 2H2O
η/Vb
Au
-1.14
-1.04
Ag
-1.37
-1.27
Zn
-1.54
-1.44
Sn
-1.48
-1.28
Hg
-1.51
-1.31
In
-1.55
-1.35
Pb
-1.63
-1.43
Cu
-1.44
-1.61
Ecell/V (150 mA cm-2)
-3.511
a) from , extrapolated to pH 7; b) η = Ered–1.23–E° 3
Those estimates for the overpotential do not account for resistances at the anode or transport resistances. Consistent with the inherent optimism of this Viewpoint, let us assume that an electrolysis cell for converting CO2 to useful products can be operated at a practicable current density at a cell overpotential, ηcell, just comparable to that of a modern PEM fuel cell, namely ~0.6 V. As a very crude approximation to a full technoeconomic analysis,2 it is instructive to compare the costs of the input energy and the market prices of the products produced by the reactions listed above (Table 3). In the absence of subsidies, carbon tax, or mandates, economically viable processes must spend less on the input energy than the selling price of the product (i.e., Energy cost/Market price < 1). This crude approximation also ignores capital expenses, other operating expenses, anticipated profits and the value of any co-products, e.g., H2. As a very crude approximation to an environmental analysis, Table 3 also shows the fraction of the annual CO2 emissions in the US14 that would be mitigated by producing the annual consumption in the US of each product according to the reactions listed in Table 1, For example, the world produces 36 GT of CO2 per year15 (= 8.2×1014 mol/y) and produces about 77 MT of ethanol16 (= 1.7×1012 mol/y × 2 mol CO2/mol ethanol) so producing all of the ethanol by electrochemical conversion of CO2 could beneficiate about 0.4% of the world CO2 production.
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Table 3. Approximate technoeconomic and environmental analyses of converting CO2 into commodity chemicals, using renewable (i.e., CO2-free) electricity. Product
n
CO
2
Operating potential/V Electricity Cost/¢ mol-1 𝐸𝑛𝑒𝑟𝑔𝑦 𝑐𝑜𝑠𝑡 % CO2 (Ecell = E° + ηcell) (at 2.5 ¢/kWh)a Market priceb/¢ mol-1 𝑀𝑎𝑟𝑘𝑒𝑡 𝑝𝑟𝑖𝑐𝑒 mitigationc -1.94
0.26
0. 8717,18 19,20
HCOOH
2
-2.03
0.27
2.3
CH3OH
6
-1.81
0. 73
1.1721,22 23,24
CH4
8
-1.66
0.89
0.24
C2H5OH
8
-1.75
1.4
0.2025
C2H4
26,27
12
-1.77
1.4
1.6
C3H7OH 18
-1.74
2.1
7.5028,29
0.30
0.007
0.12
0.003
0.62
0.29
3.7
20.0
0.70
0.4
0.89
1.4
0.28
0.01
a) Electricity cost is representative of recent power purchase agreements surveyed by the US EPA30. b) Market prices and volumes were estimated from the indicated, publicly available reports, extrapolated as needed using producer price indices31 from the time when the report was created. c) CO2 mitigation was calculated by dividing the amount of each product consumed annually in the world by the annual global emissions of CO2 multiplied by the stoichiometric ratio of the reactions in Table 1. For four of the products (methanol, methane, ethanol and ethylene) the cost of the electricity required to convert CO2 approaches (or exceeds) the current market price of the product, leaving little (or no) budget available for capital expenses and other operating expenses. For the other products (CO, formic acid and propanol), there appears to be a margin for profit, under the very conservative consideration of only those products whose inputs cost less than half the market price of the existing products. Those inferences are, roughly, in line with those obtained from more detailed technoeconomic analysis cited above.2 Moreover, it should be mentioned that switching an incumbent product for even an ostensibly identical substitute typically is subject to an elasticity of substitution, which is about 1.4 for commodity chemicals.32 The table also shows that saturating the market with CO2-derived versions of the products, that are possibly profitable (in the absence of subsidies, taxes or mandates), would ameliorate, in total, less than 0.02% of the CO2 produced each year in the world.
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To further illustrate the cost of an enterprise that would mitigate a large amount of CO2, consider converting the emissions associated with ethanol fermentation at a volume equal to the 2022 target of 16 billion gallons per year of cellulosic biofuel required in the US by the Energy Independence and Security Act of 2007.33 The production of each gallon of ethanol produces 2.7 kg of CO2 (61.4 mol).16 The annual target production rate of ethanol corresponds to 31.2 kmol/s of CO2. At the optimistic cell potentials used in Table 3, converting that stream of CO2 requires a noticeable fraction of the electricity34 produced from hydropower, wind power or solar-powered photovoltaics consumed annually in the US (Table 4). The upgrading requires only a small fraction of the US consumption of water, which is assumed to be the source of the hydrogen employed in the upgrading reaction. Table 4. Power required to make to each of the indicated products from the CO2 produced in the fermentation of the ethanol required to meet 2022 US target of 16 billion gallons/year, compared to renewable power generated34 and the water consumed35 in the US each year
Product CO HCOOH CH3OH CH4 C2H5OH C2H4 C3H7OH
Required power to Fraction of US convert CO2 ex consumption of fermentation/GW hydroelectricity 12 12 33 40 63 64 94
12% 12% 33% 40% 32% 32% 31%
Fraction of US consumption of windgenerated electricity
Fraction of US consumption of solargenerated electricity
12% 13% 35% 42% 33% 34% 33%
57% 60% 161% 196% 155% 157% 154%
Fraction of US consumption of water 0% 0.004% 0.008% 0.008% 0.003% 0.004% 0.003%
The availability of renewable energy from those sources is increasing steadily, and the price of renewable electricity price is steadily dropping,30 so it is conceivable to envisage such an application. However, another way to beneficially use the renewable power is to target environmentally-burdensome feedstocks that require less energy to reduce than CO2 and/or to target products with even higher prices than those listed above (albeit with reciprocally smaller markets). Manure, sewage sludge, food waste are feedstocks that constitute an environmental burden and that are less oxidized than CO2. In particular, my colleagues have shown that hydrothermal liquefaction of the organic fraction of those feedstocks36-38 produces a bio-oil with heating value ≳80% that of fractions of petroleum. Electrochemical upgrading of that bio-oil promises a Weber, Electrocatalysis for effective use of renewable electricity ACS Paragon Plus Environment
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renewable fuel that requires a smaller energy input than does a fuel based on upgrading of CO2.39,40 Moreover, the products contain polyvalent functional groups, which invite their consideration as intermediates for conversion into chemicals and materials whose market value is higher than that of fuels.38 The bio-oils prepared from these feedstocks contain nitrogen and sulfur compounds, which can also be treated electrochemically.41,42 Another example of the first aspect—producing fuels from a feedstock that is less oxidized than CO2—is the electrochemical oxidative coupling of levulinic acid followed by the electrochemical reduction of the resulting diketone:43 The electrolysis runs at a practicable rate (~200 mA/cm2)43 at an overall cell potential of about -3.5 V (ignoring effects of concentration and temperature). At the assumed, optimistic price for renewable electricity (0.025$/kWh), the input energy for this conversion would cost 0.61 $/mol, of which only 0.019 $/mol is for the electrical energy; the covers the cost of the levulinic acid44 of about 5 $/kg. The wholesale price of gasoline in the US45 is about 2 $/gal or about 0.086 $/mol, if it consisted of octane. In this case, making a fuel would not appear to be a profitable application for an electrochemical conversion.
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Figure 1. Conversion via electrochemistry of a biomass-derived intermediate to a component of a liquid fuel, after43.
An example of the second aspect—producing higher value products than fuels—is the electrochemical reduction of hydroxymethylfurfural to make 2,5-hexanedione,46 which is noxious47 but finds use as a fine chemical.48
The electrochemical reduction of HMF proceeds46 at a practicable rate at an applied potential of –0.89V vs RHE, i.e., at an overall cell potential of ca -2.1 V if the anode were used to oxidize water. Therefore, the cost of the input energy for just the chemical transformation would be 810 kJ/mol × 0.025 $/kWh = 0.0056 $/mol. The market price of the hexanedione49 is about 8.9 $/mol (78 $/kg * 0.114 kg/mol). In this case, clearly the cost of the input energy plus the cost of the feedstock, hydroxymethylfurfural (~1 $/kg = 0.11 $/mol)50 is a small fraction of the market price (0.12/8.9 = 0.01). However, because 2,5-hexanedione serves as a precursor to a fine chemical, not as a commodity chemical, it has a small market (
