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In a number of previous reports, we've documented that Oxygen, concentrated to at least some extent, as opposed to unpurified air, is preferred for use in gasification procedures intended to produce, primarily, a blend of Carbon Monoxide and Hydrogen, a hydrocarbon synthesis gas, from Coal.
The use of purified Oxygen efficiently promotes exothermic reactions with Coal which enable the inclusion of other co-reactants, such as Steam and/or Carbon Dioxide, whose breakdown into their elemental constituents consumes a great deal of thermal energy; and, it does so without promoting or enabling the wasteful formation of Nitrogen Oxide pollutants, as would be the case if unpurified air were to be used.
A fairly early example of such technology, which doe a fair job of explaining those alternatives, can be found in our report of:
Pittsburgh 1942 Coal Gasification Utilizes CO2 | Research & Development; concerning: "United States Patent 2,302,156 - Process and Apparatus for the Production of Useful Fuel Gas; 1942; Koppers Company, Pittsburgh, PA; Abstract: This invention relates to the production of fuel gas or high heating power out of dusty of finely granular fuels, such as black or brown coal, or coke or semi-coke made therefrom, the fuel being converted at a high temperature with air (oxygen), steam and carbon dioxide whereby a gas is produced which is rich in hydrogen and carbon monoxide."
In their full Disclosure of "United States Patent 2,302,156", Koppers does describe how the processes of using of Air or Oxygen in Coal gasification can differ from each other, indicating especially how the use of Oxygen to drive a process of Coal gasification can enable the inclusion not only of Steam, but, perhaps intriguingly, of Carbon Dioxide as well into the mix of initial reactants.
Later developments, as we've reported recently in:
Exxon Oxygen Donor Coal Gasification | Research & Development; concerning: "United States Patent 4,309,198 - Converting Liquid and/or Solid Fuel to a Substantially Inerts-free Gas; 1982; Exxon Research and Engineering Company, New Jersey; Abstract: The invention relates to the conversion of fuel (solid and/or liquid) to reducing and/or synthesis gas by contacting the fuel in a fluidized conversion bed with a solid oxygen donor (e.g. CaSO4 - Calcium Sulfate) at a fuel conversion temperature ... in the presence of at least one gas/vapor phase substance such as H2 and/or H2O and/or CO and/or CO2 which serves to promote and or mediate the transfer of oxygen from the donor to the fuel and preferably in the absence of non-reactive gases (e.g. Nitrogen) whereby the fuel is converted to a reducing and/or synthesis gas";
demonstrate that an "oxygen donor" substance, instead of purified Oxygen, can be utilized to achieve very nearly the same effect, with the added benefit of restricting to a certain extent the availability of the Oxygen, and thus as well restricting to a certain extent the co-generation of Carbon Dioxide.
And, in another report:
Oklahoma Oxygen Donor Coal Gasification | Research & Development; concerning: "United States Patent 4,496,370 - Zinc Oxide-Char Gasification Process; 1985; Assignee: Phillips Petroleum Company, Oklahoma; Abstract: In the gasification of char with zinc oxide, the improvement which comprises reacting the off-gas stream of gaseous zinc and carbon monoxide with steam at elevated temperatures in a second reactor means thereby oxidizing the zinc to zinc oxide and yielding a second gaseous stream containing carbon monoxide and hydrogen";
we learned that Zinc Oxide could be used as an Oxygen transfer substance, which, in the course of being regenerated, through reactions between the reduced Zinc metal, after it had donated it's Oxygen, and Water, synergistically produces Hydrogen as a byproduct, which could be added to the Coal-derived synthesis gas to improve or adjust the ratios of Carbon Monoxide and Hydrogen in that syngas.
Herein, from scientists in a somewhat unlikely trio of institutions, we submit even further confirmation of the Phillips technology disclosed in the above "US Patent 4,496,370", wherein the use of Zinc Oxide as an Oxygen donor enables improved economies in the coordinated production of Carbon Monoxide from Coal and Hydrogen from Water, which gases can then be combined in any desired ratios for further catalytic processing to synthesize specific types of hydrocarbons.
Comment follows excerpts from:
"Coal Gasification Using the ZnO/Zn Redox System
Energy & Fuels; 1996
Masamichi Tsuji, et. al.
Research Center for Carbon Recycling and Utilization, Tokyo Institute of Technology, Japan; Laboratory for Energy and Process Technology, Paul Scherrer Institute, Switzerland; (and,) Mechanical Engineering Department, Valparaiso University, Indiana
A two-step thermochemical process is proposed for converting coal to high-quality synthesis gas.
In the first, high temperature, endothermic step, coal is reacted with zinc oxide to form metallic zinc and an H2-CO gas mixture. In the second, low temperature, exothermic step, zinc is used for splitting water and producing hydrogen and zinc oxide.
The hydrogen is employed to enrich and adjust the synthesis gas mixture obtained in the first step, while the zinc oxide is recycled to the first step. Experimental studies have shown a more effective chemical conversion obtained via the proposed two-step scheme as compared to that obtained via the conventional single-step direct steam gasification.
CO formation was more favorable with the coal-ZnO redox reaction than with the coal-H2O reaction ... .
This highly endothermic reaction could be conducted using concentrated solar radiation as the energy source of high-temperature process heat, allowing for a combined utilization of coal and solar energy, and consequently for a significant reduction of the CO2 emissions derived from the combustion of coal.
(For more on the above and related potentials, see, for just two examples:
USDOE Hydrogasifies Coal with Solar Power | Research & Development; concerning: "United States Patent 4,415,339 - Solar Coal Gasification Reactor; 1983; The USA, as represented by the Department of Energy; Abstract: Coal (or other carbonaceous matter, such as biomass) is converted into a duct gas that is substantially free from hydrocarbons. The coal is fed into a solar reactor, and solar energy is directed into the reactor onto coal ... . Steam, injected into the reactor adjacent to the gasification zone, reacts with char to generate product gases. A method of producing a substantially hydrocarbon-free product gas with a solar reactor from a carbonaceous-material feed ... (and) wherein the feed is coal and includes water, thereby generating steam from said water (and) injecting at least one reactive gas selected from the group consisting of steam, CO2, H2 and CH4 into said solar reactor"; and:
Switzerland Hydrogasifies Coal with Solar Power | Research & Development | News; concerning: "United States Patent 4,149,856 - Producing a Gaseous Fuel by Means of Solar Energy; 1979; Inventor: Willy Keller, Switzerland; Abstract: A gaseous fuel is produced, utilizing thermolysis and a water-gas reaction, by heating a piece of carbon through exposure to reflector-focused solar radiation and contacting it with steam".)
Introduction: The gasification of coal and carbonaceous materials has been widely studied and is presently practiced at an industrial scale. The conversion of coal to synthesis gas ... provides a chemical pathway for the production of synthetic fuels and commodity organic chemicals.
Coal gasification is a high-temperature energy-intensive process and coal itself is burned internally in the gasifier to supply the process heat. Off-gases usually contain substantial amounts of CO2, which, depending on their final end use, may require further energy-consuming separation systems. Catalysts can improve the kinetics and chemical conversion and reduce the operating temperature requirements, but their use is subjected to the feasibility of recovering them from the remaining ash.
(Such recovery of Coal hydro-gasification catalysts from the resulting ash is certainly feasible, as we reported, for one example, in:
Exxon Recovers and Recycles Coal Conversion Catalyst | Research & Development; concerning: "United States Patent 4,157,246 - Hydrothermal Alkali Metal Catalyst Recovery Process; 1979; Exxon Research and Engineering Company; Abstract: In a coal gasification operation or similar conversion process carried out in the presence of an alkali metal-containing catalyst wherein solid particles containing alkali metal residues are produced, alkali metal constituents are recovered ... in the form of water soluble alkali metal formates by treating the particles with a calcium or magnesium-containing compound in the presence of water ... and in the presence of added carbon monoxide.The resultant aqueous solution containing water soluble alkali metal formates is then ... recycled to the gasification process where the alkali metal formates serve as at least a portion of the alkali metal constituents which comprise the alkali metal-containing catalyst. This process permits increased recovery of alkali metal constituents, thereby decreasing the overall cost of the gasification process by reducing the amount of makeup alkali metal compounds necessary. Government Interests: The government of the United States of America has rights in this invention pursuant to Contract No. E(49-18)-2369 awarded by the U.S. Energy Research and Development Administration";
and, more documentation of that fact will follow in future reports. Further, though, since the catalysts, as per the Exxon process, can be recovered with "added carbon monoxide", we remind you that the minimized amount of any Carbon Dioxide that is co-generated in these Oxygen donor gasification processes, as exemplified by our subject herein, "Coal Gasification Using the ZnO/Zn Redox System", can be recovered and reacted with more hot Coal, as in:
Germany 98% Pure Carbon Monoxide from Coal, CO2 and O2 | Research & Development; concering: "Carbon Monoxide from Coke, Carbon Dioxide and Oxygen; 1986; Lurgi GmbH, Frankfurt (Germany); Abstract: Many valuable organic chemicals-both as intermediate or final products-can be made from high purity carbon monoxide (and) to provide a source of inexpensive CO ... a very attractive new scheme has been developed. According to this concept merely two process steps are required to convert coke to high purity CO. The purpose of the first process step is to gasify coke using a mixture of CO2 and O2 as gasification agent while the second one serves to remove sulfur compounds and residual CO2. (The) produced gas shows a CO concentration of 97 to 98.5% by volume. This CO level makes for an excellent feedstock for most syntheses"
and be made through such reactions to yield the needed Carbon Monoxide for that catalyst recovery.)
Nonetheless, environmental considerations favor converting coal into cleaner fluid fuels that have higher
H/C ratios, provided the process heat required for the transformation is supplied by a clean renewable energy source, e.g., solar energy. In industry, steam and/or air is usually the oxidant in the gasification of coal. Alternatively, one could use a metal oxide as the donor of oxygen. This approach is formally equivalent to combining two processes, viz., the gasification of coal for the production of syngas and the reduction of metal oxides for the production of metals
CO2 emissions could be completely avoided if coal were used exclusively as the reactant ... and process
heat were supplied by solar energy. The use of concentrated solar radiation for extracting Zn from ZnO, either thermochemically or thermoelectrochemically, has been demonstrated ... .
Steam gasification and pyrolysis of coal using solar energy has also been experimentally investigated.
(As per our above reference to "United States Patent 4,415,339 - Solar Coal Gasification Reactor", such potentials have been far more than just "experimentally investigated".)
The direct irradiation of solid reactants by high solar flux intensities provides effective heat transfer and energy absorption at the reaction site. These previous experiences indicate that the process technology for the proposed reaction can be developed.
Conclusion: The coal/ZnO redox system offers the possibility of combining the gasification of coal to produce syngas, using ZnO as the oxidant, with the reduction of ZnO to produce Zn, using coal as the reducing agent. When ZnO is the oxidant, CO evolution is higher than when H2O is the oxidant.
CO2 formation was negligible, eliminating the need for gas separation.
Metallic zinc can be further used to split water and form H2 for enriching the syngas mixture, while the regenerated ZnO is recycled. The use of solar energy for replacing coal as the source of high-temperature process heat for conducting the combined coal gasification/ZnO reduction will avoid the CO2 emissions
released in this process."
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The point being, in the final excerpted paragraph, that Coal could be "the source of ... process heat", although it's auxiliary use for such a purpose in this technology could result in some net co-production of CO2 from the total Oxygen-donor system, even though "CO2 formation" otherwise would be "negligible".
And, as noted in our citation of "Carbon Monoxide from Coke, Carbon Dioxide and Oxygen", we do have some creative options available for the productive, and even, relative to our subject "Coal Gasification Using the ZnO/Zn Redox System", synergistic utilization of any Carbon Dioxide that might be co-produced.
Other options, as seen in:
Standard Oil Electrolyzes CO2 to Carbon Monoxide | Research & Development; concerning: "US Patent 4,668,349 - Electrocatalytic Reduction of CO2 by Square Planar Transition Metal Complexes; 1987; The Standard Oil Company; A process for the electrocatalytic reduction of carbon dioxide to carbon monoxide".
for efficiently converting any minor amount of residual Carbon Dioxide into even more of the desired Carbon Monoxide, also exist.
And, if an excess of Carbon Monoxide should result from all of the combined and related Coal and Carbon Dioxide conversion technologies represented by the foregoing examples of available technology, then we can, as in:
Standard Oil Carbon Monoxide + Water = Gasoline | Research & Development; concerning: "United States Patent 4,559,363 - Process for Reacting Carbon Monoxide and Water; 1985; Abstract: A process for reacting carbon monoxide and water in the presence of a cadmium-containing catalyst ... for the production of hydrocarbons";
simply mix that excess Carbon Monoxide with Water, and thereby make more of the "hydrocarbons" we all seem so desperately to desire.