Since we've lately aroused some ire with our demeaning of the deceptive concept of Geologic Sequestration as a means to "abate" our Carbon Dioxide "problem", we thought everyone might be interested in a what a US Government Geologist thinks we coal people ought to do with our offensive gas.
The revealing excerpt, containing some preamble, comment interspersed and appended, with the full document attached and available via the enclosed link:
"Reductive Sequestration of Carbon Dioxide
T. Mill
SRI
333 Ravenswood
Menlo Park, CA 94025
333 Ravenswood
Menlo Park, CA 94025
D. Ross
U.S. Geological Survey, Bldg 15 MS 999
345 Middlefield Rd.
Menlo Park, CA 94025
The United States currently meets 80% of its energy needs by burning fossil fuels to form CO2. The combustion-based production of CO2 has evolved into a major environmental challenge that extends beyond national borders and the issue has become as politically charged as it is technologically demanding.
(A lot of political charges are involved, it would seem - even more than immediately meet the eye, based on our recent experiences. - JtM)
Whereas CO2 levels in the atmosphere had remained stable over the 10,000 years preceeding the industrial revolution, that event initiated rapid growth in CO2 levels over the past 150 years (Stevens, 2000). The resulting accelerating accumulation of CO2 in the troposphere is increasingly linked to global climate-warming, with projections of continued warming in the absence of resolute changes in CO2 management (Revkin, 2000).
The worldwide effects of warming on forestry, fresh water supplies, farming, coastal stability, and human health could be enormous. In response to these threats, the 1997 Kyoto Treaty on Global Warming was initiated in a major world-wide effort to curtail CO2 emissions. A major feature of this activity involves separation, collection and storage of a significant fraction of the 6-billion tons of CO2 currently produced worldwide each year. The annual U.S. production is about a third of that value, and sites considered for storing U.S. produced CO2 include depleted gas reservoirs, deep saline aquifers, depleted oil reservoirs, coal beds, and the deep ocean (Noserale, 1999).
(After those obligatory genuflections to speculative fears, the authors start getting to the meat of it. The following concepts need far more serious public consideration than they have so far received. - JtM)
Although accumulating the captured gas in vast reservoirs seems a rational approach to the problem, a second, potentially more rewarding route is suggested here. Because the nominal 2 billion tons of CO2 the U.S. produces annually represents the energy content of about 11 million barrels of oil per day, or roughly the U.S. daily import (Feld, 2000), we propose that CO2 be considered a renewable resource. We propose specifically that it be recycled back to fuel by employing water as the hydrogen source, and the reductive chemical energy available from sunlight driven electron/hole production, or thermal reduction using abundant Fe[II]-containing minerals.
(Note how carefully they approach this topic: "Although accumulating the captured gas in vast reservoirs seems a rational approach to the problem, a second, potentially more rewarding route is suggested here." Really. Does attempting to accumulate "the captured gas in vast reservoirs", at some great expense, really seem "a rational approach" to anyone? Especially since, as we've earlier documented, those "vast reservoirs" are pretty darned leaky? - JtM)
If the process (of converting CO2 to hydrocarbons - JtM) can be successfully applied, it would not only satisfy concerns tied to global warming, but would also eliminate some fraction of the nation’s daily dependence on imported oil. This dependence is now a matter of renewed concern and uneasiness with impact on both national security and the economy (Ebel, 2000; New York Times, 2001).
(Understatement seems these authors' style. - JtM)
The proposed process involves reductive capture of CO2 in a two-part scheme related in part to the Fischer Tropsch process, and employing common minerals with sufficient reduction and/or photoreduction potential to convert CO2 to formate and/or methanol (Yoneyama, 1997) and then to fuel-valued products. Thermochemical calculations show the overall scheme to be highly exothermic, and thus self-sustaining with the proper process design.
(Note: Their concept is "related in part to the Fischer Tropsch process" wherein, as all our readers should by now know, gasses, including CO2, are generated from coal, and then condensed, via catalysis, into hydrocarbon liquids. And, they point out a fact that we have earlier documented for you: A portion of the reaction process of converting Carbon Dioxide into useful hydrocarbons is "highly exothermic, and thus self-sustaining. The need for external energy inputs to drive the conversion is reduced, with corresponding reductions, we are led to presume, in the overall cost. - JtM)
Objectives
This paper explores the photochemical and thermochemical parameters associated with reduction of CO2 to C1 products and their subsequent conversion to alkanes and proposes a program to assess the key kinetic steps required to effect reduction to methane and higher alkanes. With that information, the overall feasibility of using this process for sequestration of CO2 can be evaluated relative to other CO2 sequestration
processes.
(For emphasis: These authors are presenting details on the efficient "reduction of CO2 to C1 products and their subsequent conversion to alkanes". Easily accessible web references will tell you that gasoline is a mixture of hydrocarbons, and most of those hydrocarbons are "alkanes". These authors are telling us, without, we suspect deliberately using the word, that we can, through efficient catalytic processes, convert Carbon Dioxide into Gasoline. - JtM)
Carbon dioxide and its aqueous counterparts, bicarbonate and carbonate, are inherently highly stable.
Recently, however, the possibility of large-scale, process-level CO2 reduction has become more viable following two separate published accounts describing the reduction of CO2 with common minerals. The first is a report by McCollom (2000) who discussed experiments in hydrothermal media with dissolved CO2 and the mineral olivine, which showed that bicarbonate ion was immediately reduced to formate at 300°C and 350 bar. These conditions are at pressures above the water saturation curve and therefore there was no headspace in the reactor. McCollom suggested that absence of headspace stopped the reduction at formate, since further reduction to alkanes is highly favored thermodynamically.
McCollom et al. (1999) confirmed this speculation in other work using a system purposefully containing a headspace, in which reduction of aqueous formate to a broad array of hydrogen-rich Fischer Tropsch-like products in the C2 - C35 range, including alkanes, alkenes, and oxygenated products, readily took place at 175°C. In the absence of headspace, formic acid is in solution, where it is mostly ionized to the stable formate ion. When headspace is available, further reduction occurs can in the gas phase through the molecular acid, most probably on the reactor surfaces.
("Formate", made from CO2, can be chemically reduced into "hydrogen-rich Fischer Tropsch-like products", just as coal can be processed via Fischer-Tropsch technology, as we've more than thoroughly documented, to manufacture liquid hydrocarbon fuels. - JtM)
The second account of a mineral-based conversion describes sunlight-driven production of formic acid from CO2 at ambient temperatures. In this case, Ohta et al. (2000) worked with a selection of common Iron containing silicate rocks (amphibolite, granite, gneiss, shale) which were powdered and suspended in CO2-saturated water. When the mixtures were irradiated by sunlight at ambient temperatures, formic acid was formed at a rate of several µgm/m2 over 18 hours. No evidence was found for further reduction to other acids or hydrocarbons. Ohta et al. report no effort, however, to identify the specific metal oxide(s) responsible for reduction of CO2, nor to determine the photoefficiency of the process.
(Note: Some minerals can catalyze the conversion of CO2 into formic acid at "ambient temperature", that is, with little additional energy input. The formic acid, formate, as reported above, can then be converted into "alkanes" - the primary constituents of gasoline. - JtM)
Photoreduction of CO2 on irradiated semiconductor surfaces has been widely reported to give a range of C1 and C2 products, including CO, formate, methanol, methane, formaldehyde, oxalic acid and glyoxal (Yoneyama, 1997). CO2 photoreductions are observed on a variety of metal oxides, including WO3, TiO2, ZnO, as well as on GaP. ZnS and CdS. Reductions are believed to result from photopromotion of hole/electron pairs in the oxide/sufide conduction bands, capture of electrons by CO2 and hole oxidation of water or some added reducing agent. Photoefficiencies for CO2 reduction appear to range from less than 1% to 23% on certain quantized oxide particles (Yoneyama, 1997; Inoue et al, 1990). The high efficiencies also appear to require large band gaps, thus reducing efficient use of the full spectrum of sunlight.
The existing literature on CO2 reductions shows that the reactions do work and can be modified to produce varying proportions of C1 and C2 products. However, efficiencies vary widely and the factors controlling selectivity and band gap energies are not well understood. Nor is it clear how the work of Ohta et al. with ferrous minerals relates to the semiconductor oxide processes. Despite these important gaps in understanding these two reduction processes, we propose a scheme to create fuel-valued products from process CO2. ... The hydrogen source is water in the feed mixture.
(As we've reported from other research: Hydrogen is needed for hydrogenation, and the "hydrogen source is water in the feed mixture", a point we've made several times in the earlier citations. - JtM)
Conversions of CO2 are highly exothermic, with the production of methane being the most heat-yielding. Thus, a self-sustaining operation should be attainable with the proper process design.
(Again, "a self-sustaining operation" to recycle CO2 into valuable hydrocarbons, because the reaction sequence is "highly exothermic", "should be attainable". - JtM)
In summary, it appears that the practical sequestration of CO2 through conversion to formic acid and fuel-valued materials is feasible. The sensible utilization of the concept will depend on the balance between the fuel value provided by reduction of CO2 and the energy requirements for reduction. The energy requirements are directly tied to the basic thermochemistry and kinetics of the individual steps in the sequence, including the two thermal and one photochemical step. The program proposed here is designed to develop the necessary kinetic and photochemical parameters to evaluate the commercial potential of CO2 recycling.
Because of the fundamental nature of the study, the results should also serve as a significant contribution to the chemical literature.
Because of the fundamental nature of the study, the results should also serve as a significant contribution to the chemical literature.
A research program to evaluate key kinetic and photochemical steps will be conducted over 24 months in which work will be conducted concurrently on the photo- and thermal-reduction processes to allow a direct comparison of their features. Several of the minerals described by Ohta et al. (2000) will be used with near uv light for the initial measurements of CO2 conversion to formate to confirm their observations. Purified semiconductor oxide components of the minerals will be examined under the same conditions of light intensity, wavelength and particle size to identify the most likely mineral constituents responsible for CO2
reduction to formate. Band gaps and photoefficiencies of several minerals for reduction of CO2 will be measured as a function of particle size and the results used to optimize a photoreactor design with practical value.
reduction to formate. Band gaps and photoefficiencies of several minerals for reduction of CO2 will be measured as a function of particle size and the results used to optimize a photoreactor design with practical value.
The ultimate product of the program will be a detailed understanding of the reaction fundamentals of the multistep process. The database will be suitable for a sound appraisal of the conversion concept, and an evaluation of the prospect of advancing to a process development study."
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Following are a selection from the authors' reference list. Brief review suggests there might be some interesting reading available for anyone genuinely interested in a productive and profitable alternative to the geologic sequestration squandering of a potentially valuable by-product of our coal use industries.
References
Berndt, M. E., D. E. Allen, and W. E. Seyfried, Jr. (1996). Reduction of CO2 during serpentinization of olivine at 300°C and 500 bar. Geology 24, 351-354.
Ebel, R. E. (2000). “Comments on the economic and security implications of recent developments in the world oil market.” Presented before the United States Senate Committee on Governmental Affairs, March 24.
Inoue, H., T. Torimoto, T. Sakata, H. Mori, and H. Yoneyama (1990). Photoreduction of carbon dioxide on quantized zinc sulfide. Chem. Lett. 1483-1486.
McCollom, T. M. (2000). Reduction of aqueous CO2 to organic compounds at hydrothermal conditions: Does it work? Presented at the First Astrobiology Science Conference, Ames Research Center, Mountain View, California, April 2-5.
McCollom, T. M., G. Ritter, and B.R.T. Simoneit (1999). Lipid synthesis under hydrothermal conditions by Fischer-Tropsch-type reactions. Origins of Life and Evol. of the Biosphere 29, 153-166.
Ohta, K., H. Ogawa, and T. Mizuno (2000). Abiological formation of formic acid on rocks in nature. Applied Geochemistry 15, 91-95.
Yoneyama, H. (1997). Photoreduction of carbon dioxide on quantized semiconductor nano particles in solution. Catalysis Today 39: 169-175.