Abstract
Modes of power plant operation are presented which remove the greenhouse carbon dioxide from fossil fuel plant power station exhausts and transform the carbon dioxide into a valuable carbon nanofiber product. The first mode uses the emissions from a natural gas CC power plant to provide hot CO2 to a molten electrolysis chamber which generates both carbon nanofiber and oxygen. The valuable carbon nanofiber product is removed, heat from the carbon nanofiber and oxygen products is transferred into heating steam for the steam turbine, and the pure oxygen is blended into the air inlet to allow the gas turbine to operate at higher temperature and higher efficiencies. A second mode converts a conventional coal power plant to a STEP coal CNF power plant by directing the hot carbon dioxide combustion emission into carbon nanofiber production electrolysis chamber, and transforming the carbon dioxide to carbon nanofibers with the use of renewable or nuclear energy. Other intermediate modes of fossil fuel carbon nanofiber electric power plants with partial solar input are also evident, as well as a simplified, smaller version (for heating/cooking) rather than electrical production.
The Natural Gas Carbon Nanofiber Combined Gas Steam Cycle
Introduction
Fossil fuels comprise approximately 80% of global energy sources and are nonrenewable. When oxidized as fuels, they emit the greenhouse gas CO2, which has risen to an atmospheric concentration of over 400 ppm during the industrial age and contributes to climate change. A recent report on the future of carbon dioxide emission forecasts that from 2010 to 2060 approximately 496 gigatons of CO2 will be generated by fossil fuel combustion.1 Pathways to avoid this emission are sought (i) to avoid the substantial greenhouse gas climate change consequences and (ii) to preserve this carbon in a compact, energetic form as a resource for the future. We have been developing a high solar energy conversion process, STEP (the solar thermal electrochemical process), which drive the conversion of CO2, and also drives the syntheses of a variety of societal staples without CO2 emissions.2-9 In this study we probe the cost effective conversion of fossil fuel greenhouse gas emissions to value added commodities.
Recently, we introduced the direct, efficient STEP conversion of atmospheric CO2 to carbon nanofibers. The carbon nanofibers are a valuable commodity due to their unusually high strength, electronic and thermal conductivity and have a wide range of high end applications in construction, transportation, medical, electronics and sports fields. Under the appropriate electrolysis conditions (inexpensive anodes (such as nickel) and cathodes (such as galvanized steel), controlled composition molten carbonate oxide electrolytes, and transition metal growth points, and controlled current densities), carbon dioxide is split into carbon nanofibers and oxygen gas:10,11
CO2 → C(carbon nanofiber) + O2 (1)
The chemical and electrochemical conditions of the CO2 transformation are readily controlled, and as shown in Figure 1, under the correct conditions can generate either straight or tangled carbon nanofibers and a carbon nanotubes subset of carbon nanofibers, carbon nanotubes.10,11 The electrochemical energy required to form the carbon tubes is as low as 0.8V and carbon nanofibers can be formed at a high rate (rates of 0.1 to 1 amps per cm-2 of electrode) at electrochemical potentials over 1V.12
In this study we develop processes for the transformation of carbon dioxide to carbon nanofibers from more concentrated CO2 sources, industrial flue gas, and in particular to remove the carbon dioxide from fossil fuel electric plants while producing the valuable carbon nanofiber product. This provides less of a challenge than the previously demonstrated direct removal of atmospheric carbon dioxide, which is dilute (0.04%), as the flue gas is already hot, and several hundred fold more concentrated in CO2. CO2 is readily soluble in the molten carbonate oxide electrolytes, while nitrogen and water vapor are not. The undissolved gases (N2 and H2) exhaust from the electrolyzer with the CO2 removed (and transformed to carbon nanofibers). If not removed in pre-exhuast states, lower levels of impurities, such as sulfur gases are reduced to sulfur in the electrolysis chamber13 and do not inhibit or poison the electrolysis process. The dissolution of carbon dioxide in molten carbonates occurs via reaction with the dissolved oxide, for example in the case of lithium carbonate:
CO2 (gas) + Li2O(dissolved) → Li2CO3 (molten) (3)
Electrolysis with 4 electrons per molecule yields:
Li2CO3 → C(carbon nanofiber) + Li2O + O2 (4)
Note, that the net reaction of equations 3 and 4 equals equation 1.
Figure 1. Carbon dioxide transformed into uniform straight (left) and tangled (middle) carbon nanofibers10 and carbon nanotubes11 (right).
In this study we probe efficient designs for the conversion of (i) CC (combined cycle) natural gas electric power plants and (2) coal plants to (1) CC CNF (combined cycle carbon nanofiber) and (2) STEP coal CNF (STEP Coal carbon nanofiber) electric power plants.
1. Natural Gas Combined Cycle Carbon Nanofiber Power Plant
1a. Natural gas fuels
1a. Natural gas fuels
The composition of natural gas used to drive combined cycle (CC) power plants varies widely, but generally contains approximately 95% methane (CH4), 3% ethane (C2H6), 1% nitrogen, 0.5% carbon dioxide (CO2), 0.2% propane (C3H8), and less than 0.1% of other organics (primarily alkanes), oxygen (O2) and sulfur compounds, although the ratio of methane to other organics can vary widely.14
1b. Exhaust from conventional CC gas electric power stations.
Conventional coal CC (gas/steam turbine combined cycle) electric power stations emit massive amounts of carbon dioxide to the atmosphere. Combined cycle gas power plants energetically operate more efficiently (~55%) than coal power plants (~35%) and exhaust less carbon dioxide per watt of power. Exhaust flue gas volume composition varies with plant construction. The flue gas volume is ~295 m3/GJ respectively from gas CC power plants. The gas contains a majority of nitrogen, water vapor, and ~9% or ~14% CO2 in the flue gas respectively from gas or coal power plants.15 Depending on the source and purity of the natural gas, additional infrastructure can be included to scrub the flue gases of sulfur, nitrous oxides and heavy metals.
1c. Conventional Combined cycle CC Gas/Steam Power plant
The left side of Figure 1 illustrates the action of a CC electrical power plant which emits a flue gas that contains ~9% carbon dioxide (compare to 0.04% CO2 in ambient air). As illustrated, the conventional CC plant increases the fuel to electricity efficiency compared to single cycle electrical power plants by directing the exhaust heat emissions from a gas turbine (the Brayton thermodynamic cycle to generate electricity) to boil water to power a steam turbine (the Rankine thermodynamic cycle to generate electricity).
In this CC power plant, the heat available from regular (25°C) combustion is:
CH4 + 2O2 → CO2 + 2H2O 889 kJ/mole (5)
and approximately 55% of this heat is converted to combined electric power by the gas (top) and steam (bottom) generators of Figure 2 left.
Figure 2. The action of a conventional CC power plant which has an exhaust with CO2 (left) compared to the introduced CNF CC power plant (left) which has carbon dioxide removed from the exhaust gas (right) and instead produces a valuable carbon product. Left: The conventional gas/steam combined cycle CC power plant is illustrated as modified from reference 16. Right: The CC CNF power plant in this study, illustrating the CNF (including carbon nanofibers or carbon nanotube) product, the electrolysis pure oxygen cycled back into the gas combustion increasing efficiency, the recovered heat returned to the steam turbine, and the carbon dioxide removed from the exhaust gas.
1d. CC CNF Power Plant: The Natural Gas Carbon Nanofiber Combined Gas Steam Cycle
As shown on the right side of Figure 2, the Gas Combined Cycle Carbon Nanofiber or CNF CC plant product, in addition to electricity, simultaneously produces a valuable carbon nanofiber product. Unlike the conventional CC on the left side of the figure, CO2 is removed from the exhaust of the CNF CC power plant on the right. As shown, the hot CO2, N2 and H2O CC emission is instead bubbled into molten carbonate where only the CO2 dissolves. The remaining gases (with the carbon dioxide removed) become the exhaust gas (after heat recovery). The dissolved carbon dioxide is split by electrolysis into oxygen gas at the anode, which is fed (after heat recovery) back into the gas turbine and carbon (CNF) at the cathode. The CNF product is tuned for strength, diameter, length, vacancy, geometry, and electrical or thermal conductivity by the specific molten salt electrochemistry employed. The CNF product may be removed periodically or as a constant throughput). The recovered heat boils water to power a steam turbine to also generate electricity. Heat is returned to the steam generator chamber using (i) heat recovered from the electrolysis (pure oxygen and carbon nanofiber) products, and (ii) from the carbon dioxide removed electrolysis exhaust.
The CNF CC natural gas to electricity plant efficiency is increased by the “free” pure oxygen generated during the carbon nanofiber electrolysis and is offset by the energy required to drive the electrolysis. The minimum energy required to drive the electrolysis is very low (<< 1 V12) and increases with production rate. This is recovered many fold in value by the increased value of the CNF product compared to the cost of the natural gas consumed. In 1 atm 20°C air, a methane flame temperature varies from 900 to 1,500 °C, and the temperature increases with combustion in pure O2, rather than air.
Oxy-fuel combustion has been used to increase fuel efficiency (and increase CO2 concentration for conventional sequestration) in IGCC electric power plants (section 2). Here this pure O2 is utilized without the energy loss of the cryogenic “cold box” of the bottom panel of Figure 3, or other forms of oxygen separation from air. Increased efficiency is balanced with materials constraints of too high a temperature by blending O2 concentrations higher than that in air, but lower than pure O2. In the new CNF CC design presented here, to a first order approximation the increased efficiency from the blended pure oxygen offsets the consumed electrolysis, such that the CNF CC power plant retains at a minimum the approximate electrical energy efficiency as a conventional coal power plant (> 35%) with further increases in efficiency as the CNF CC system advances.
2. Coal Nanofiber Power Plant
2a. Coal fuel
2a. Coal fuel
Coal is principally carbon and moisture, and natural gas is principally composed of methane. More specifically for the coals lignite contains 24-35% carbon and up to 66% moisture, bituminous coal contains 60-80% carbon, while anthracite is 92 to 98% carbon. The three respectively have heat contents of ~15, 24 to 35, and 36 kJ/g.17
2b. Exhaust from conventional coal power stations
Conventional coal electric power stations emit massive amounts of carbon dioxide to the atmosphere. 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 ~14% CO2.15 Additional infrastructure is included to scrub the flue gases of sulfur, nitrous oxides and heavy metals.
2c. Coal Power plants
The vast majority of coal power plants simply combust pulverized coal to produce steam, which drives turbines to generate electricity. The exhaust of the combustion is generally cleansed of the majority of sulfur, nitrogen, heavy metal and particulates and the remaining flue gas exhaust, which contains a high carbon dioxide content (along with nitrogen and water vapor) is emitted directly to the atmosphere.6
There are a relatively few number of integrated (coal) gasification combined cycle (IGCC) power plants, which gasify the coal to hydrogen. An IGCC is illustrated on the bottom panel of Figure 2.7 The heat from combustion of the gas produced from the coal drives a turbine(s), while heat recovered from this combustion, as well heat as from the coal gasification process, boils water to drive a second set of turbines. These IGCC plants higher energy conversion efficiency (~50% compared to ~35% for traditional), but have higher capital costs related to the increased complexity and higher grade materials need of the increased temperature combustion. The coal gasification generates a more concentrated carbon dioxide emission than simple coal combustion for heat. Coal gasification proceeds from coal combustion with a lean oxygen mix, to carbon monoxide (CO; with CO2), to syngas (CO + H2; with CO2), and/or to H2 (via the water shift reaction, CO +H2O → CO2 + H2).8
Figure 3. Illustrative examples of coal power plants. Conventional (top)6 and integrated gasification combined cycle (IGCC, bottom)8 coal power plants. 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.
2d. Alternate modes of introducing added heat in thermal and solar heat to coal plants
In a manner similar to that described here for the conversion of natural gas CC power plants to CC CNF power plants, the IGCC can be converted to IGCC CNF power plants, with the additional opportunity that the initial lean oxygen coal gasification step prefers an enriched oxygen (nitrogen deplete) source such as is available from the CNF electrolysis anode product output. Also, in a manner similar to that described for the conversion of CC power plants to CC CNFplants, there are several points in a conventional, pulverized coal electric power plant (as illustrated on the top panel of Figure 3), in which heat may be extracted for addition of an CO2 to CNF electrolysis chamber. These permutations to both the conventional coal and IGCC power plants are evident from Figure 1, and will not be detailed in this study. Instead the design of the conventional coal power plants will be extended, rather than substantially modified, to produce carbon nanofibers using solar energy.
Solar thermal energy can be incorporated at several points in the heat exchange processes in the top of Figure 2. However, rather than this, an expeditious path to incorporate solar energy and remove the CO2 from the coal plant exhaust is accomplished without substantially changing the existing coal plant infrastructure by direct coupling of the solar process to the existing coal plant exhaust. Figure 4 summarizes key components of a coal power plant from the top of Figure 3, including the combustion of the pulverized coal fuel with air, the extraction of combustion heat to drive a generator and the exhaust emitted to the atmosphere containing nitrogen, carbon dioxide and water vapor.
Figure 4. Simplified illustration of a coal power plant.
Figure 5 illustrates various paths by which STEP (the solar thermal electrochemical process) can utilize solar thermal energy, including on the left side, by exchange of excess heat from a photovoltaic to the STEP reactants. A second optic mode is shown in the middle panel via a solar tower in which mirrors, mounted on heliostats to track the sun, reflect and concentrate the sunlight at a central point, where upon it is split into the visible concentrated sunlight (to drive concentrator photovoltaics providing electronic charge to the electrolyzer) and thermal sunlight (providing heat to the heat the electrolyzer). Finally a third optic mode is accomplished via short path length solar concentrators such as parabolic mirrors, or convex or Fresnel lenses with the light split into infrared and visible components via a dielectric filter (also referred to as a hot/cold mirror). The wavelength at which light is reflected or transmitted through the filter is tuned by the dielectric.9 The right side design will be used to symbolize the various alternative STEP optic modes in Figure 6.
Figure 5. There are various optical modes of solar concentration available to heat the electrolyte in STEP including via excess heat transferred from the photovoltaic (left) or via splitting concentrated sunlight into separate solar visible and solar IR (thermal) energy via long (middle) or short (right) focal length optics.
2e. The STEP Coal/CNF Power Plant
Figure 6 illustrates a coal electric plant in which carbon dioxide has been removed from the exhaust, and provides a carbon source to generate a value-added carbon nanofiber product using the solar thermal electrochemical process. In this STEP coal/CNF the heat is extracted from the oxygen product and CNF products to decrease the heat needed in the electrolysis chamber, and pure oxygen product is mixed with the air inlet to increase efficiency of the coal combustion and electric power generation. Specifically, two modes of STEP are illustrated in the figure and both use concentrated sunlight. In STEP, sunlight is split into its visible and infrared spectra. The visible sunlight drives an efficient concentrator photovoltaic (or solar cell), while the thermal (infrared) sunlight heats the electrolysis chambers.9 directed to heat the electrolysis chamber, and any form of renewable or nuclear electric energy is drive the electrolysis transforming CO2 to CNF (and O2). In Hy-STEP (hybrid-STEP), all sunlight is directed to heat the electrolysis chamber, and any form of renewable or nuclear electric energy is drive the electrolysis transforming CO2 to CNF (and O2).10,11,12
Figure 6. Two modes of STEP CNF in which the coal power plant CO2 emission is transformed into a valued added carbon nanofiber commodity. In STEP coal/CNF (top panel), concentrated sunlight is split into two band, the infrared band heats the electrolysis chamber, and the visible all the concentrated sunlight is directed to heat the electrolysis chamber, and driven by any form of renewable or nuclear generated electricity.
3. Conclusions
Valuable carbon nanofiber, CNF, products replace the CO2 emission of power plants. The synthesis of CNFs has been of increasing interest, with applications ranging from capacitors and nanoelectronics, Li-ion batteries and electrocatalysts to the principal component of lightweight, high strength building materials.18,19 CNFs formed from CO2, can contribute to lower greenhouse gases for example by consuming, rather than emitting CO2, and by providing a carbon composite material that can be used as an alternative to steel, aluminum, and cement whose productions are associated with massive CO2 emissions.4-8 Carbon composites will further decrease emissions by facilitating both wind turbines and lightweight, low-carbon-footprint transportation.20
Modes of power plant operation are presented which remove the greenhouse carbon dioxide from fossil fuel plant power station exhausts and transform the carbon dioxide into a valuable carbon nanofiber product. The first mode uses the emissions from a natural gas CC power plant to provide hot CO2 to a molten electrolysis chamber which generates both carbon nanofiber and oxygen. The valuable carbon nanofiber product is removed, heat from the carbon nanofiber and oxygen products is transferred into heating steam for the steam turbine, and the pure oxygen is blended into the air inlet to allow the gas turbine to operate at higher temperature and higher efficiencies. A second mode converts a conventional coal power plant to a STEP coal CNF power plant by directing the hot carbon dioxide combustion emission into carbon nanofiber production electrolysis chamber, and transforming the carbon dioxide to carbon nanofibers with the use of renewable or nuclear energy. Intermediate modes in which the sunlight is split to generated both electrical (from visible) and thermal (from infrared) sunlight is shown as well as a mode in (Hy-STEP) in which all sunlight is directed to heat the electrolysis chamber. Other intermediate modes of fossil fuel carbon nanofiber electric power plants with partial solar input are also evident. A simplified, smaller version (not illustrated, for heating/cooking) rather than electrical production, would place the carbon nanofiber production electrolysis chamber in the exhaust path of a fossil or biofuel stove, with renewable energy to generate electricity for the electrolysis and returning the hot oxygen to the fuel chamber to facilitate higher efficiencies and more complete combustions.
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