Direct Carbon Conversion: Review of Production and Electrochemical Conversion of Reactive Carbons, Economics and Potential Impact on the Carbon Cycle
Concerns over global warning have motivated the search for more efficient technologies for electric power generation from fossil fuels. Today, 90% of electric power is produced from coal, petroleum or natural gas. Higher efficiency reduces the carbon dioxide emissions per unit of electric energy. Exercising an option of deep geologic or ocean sequestration for the CO{sub 2} byproduct would reduce emissions further and partially forestall global warming. We introduce an innovative concept for conversion of fossil fuels to electricity at efficiencies in the range of 70-85% (based on standard enthalpy of the combustion reaction). These levels exceed the performance of common utility plants by up to a factor of two. These levels are also in excess of the efficiencies of combined cycle plants and of advanced fuel cells now operated on the pilot scale. The core of the concept is direct carbon conversion a process that is similar to that a fuel cell but differs in that synthesized forms of carbon, not hydrogen, are used as fuel. The cell sustains the reaction, C + O{sub 2} = CO{sub 2} (E {approx} 1.0 V, T = 800 C). The fuel is in the form of fine particulates ({approx}100 nm) distributed by entrainment in a flow of CO{sub 2} to the cells to form a slurry of carbon in the melt. The byproduct stream of CO{sub 2} is pure. It affords the option of sequestration without additional separation costs, or can be reused in secondary oil or gas recovery. Our experimental program has discovered carbon materials with orders of magnitude spreads in anode reactivity reflected in cell power density. One class of materials yields energy at about 1 kW/m{sup 2} sufficiently high to make practical the use of the cell in electric utility applications. The carbons used in such cells are highly disordered on the nanometer scale (2-30 nm), relative to graphite. Such disordered or turbostratic carbons can be produced by controlled pyrolysis (thermal decomposition) of hydrocarbons extracted from coal, petroleum or natural gas. For coal and lignite, such hydrocarbons may be produced by cyclic hydrogenation (hydropyrolysis), with the recycle of the hydrogen intermediate following pyrolysis. Starting with common CH{sub x} feedstock for carbon black manufacture, the ash entrained into the carbon (<0.03%) does not jeopardize cell life or enter into the economic estimates for power generation. The value of carbon (relative to hydrogen) as an electrochemical fuel derives from thermodynamic aspects of the C/O{sub 2} reaction. First, the entropy change of the C/O{sub 2} reaction is nearly zero, allowing theoretical efficiencies ({Delta}G(T)/{Delta}H{sub i298}) of 100% (cf. H{sub 2}/O{sub 2} theoretical efficiency of 70%). Second, the thermodynamic activity of the carbon fuel and the CO{sub 2} product are spatially and temporally invariant. This allows 100% utilization of the carbon fuel in single pass (cf. hydrogen utilizations of 75-85%). The carbodmelt slurry is non-explosive at operating temperatures. The total energy efficiency for the C/O{sub 2} is roughly 80% for cell operation at practical rates. In summary, what gives this route its fundamental advantage in energy conversion is that it derives the greatest possible fraction of energy of the fossil resource from an electrochemical reaction (C+O{sub 2} = CO{sub 2}) that is comparatively simple to operate at efficiencies of 80%, in a single-pass cell configuration without bottoming turbine cycles.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- US Department of Energy (US)
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 15007473
- Report Number(s):
- UCRL-ID-141818; TRN: US200417%%200
- Resource Relation:
- Other Information: PBD: 12 Dec 2000
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
29 ENERGY PLANNING
POLICY AND ECONOMY
30 DIRECT ENERGY CONVERSION
54 ENVIRONMENTAL SCIENCES
CARBON
CARBON BLACK
CARBON CYCLE
CARBON DIOXIDE
COMBINED CYCLES
ECONOMICS
ELECTRIC POWER
ENERGY CONVERSION
ENERGY EFFICIENCY
FOSSIL FUELS
FUEL CELLS
GREENHOUSE EFFECT
NATURAL GAS
POWER DENSITY
POWER GENERATION
PRODUCTION
THERMODYNAMIC ACTIVITY