Catalyst – solid sorbent – or support therefor: product or process – Solid sorbent
Reexamination Certificate
2000-01-27
2004-02-10
Silverman, Stanley S. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Solid sorbent
Reexamination Certificate
active
06689714
ABSTRACT:
BACKGROUND OF THE INVENTION
Most coal as it occurs in nature contains some sulfur which is converted into gaseous compounds when the coal is either burned or gasified. If coal is burned with excess air, most of the sulfur is converted to sulfur dioxide (SO
2
). If coal is gasified by reaction with steam and a limited amount of oxygen, the sulfur is largely converted to hydrogen sulfide (H
2
S) and carbonyl sulfide (COS). Some coal liquefaction processes also produce hydrogen sulfide as a by-product. In all of these cases a hot, multicomponent gas stream is produced which needs to be desulfurized for the purpose of controlling environmental pollution. Flue gas produced by coal combustion generally is a mixture of nitrogen, carbon dioxide, water vapor, oxygen, and sulfur dioxide with the latter being present in a concentration less than 0.1 vol. %. The product of coal gasification is usually a mixture of hydrogen, carbon monoxide, carbon dioxide, water vapor, nitrogen, hydrogen sulfide, and carbonyl sulfide. Again, the sulfur compounds are present in small concentrations.
Numerous methods have been proposed for removing the aforementioned sulfur compounds from gas streams, and several of the methods are in current use. One widely used method for desulfurizing flue gas involves scrubbing the gas with an aqueous suspension of limestone particles which react with sulfur dioxide to produce calcium sulfite and/or calcium sulfate. A waste product is produced in the form of a wet sludge which is difficult to dewater and to dispose. Consequently, the sludge is impounded and stored ad infinitum. Furthermore, this method imposes an energy penalty since the flue gas is cooled for wet scrubbing and subsequently reheated for stack disposal.
Another method for desulfurizing coal combustion gases involves contacting the products of combustion with limestone particles in such a way that a dry, granular waste by-product is produced which is a mixture of calcium sulfate and unreacted lime. Here too, the material presents a waste disposal problem.
Limestone has also been proposed for removing hydrogen sulfide and carbonyl sulfide from the fuel gas produced by gasifying coal. In one system, which is becoming commercialized, limestone particles are added to a fluidized bed gasifier where they react with the sulfurous gases to form calcium sulfide. The calcium sulfide particles are treated subsequently in another fluidized bed reactor with air to convert the calcium sulfide into calcium sulfate for disposal.
In all of these methods the waste is difficult to reclaim and reuse. Therefore, the methods consume prodigious quantities of limestone and generate tremendous amounts of waste for disposal.
Lime (CaO) which is derived from the decomposition of limestone(CaCO
3
) is an excellent sorbent for hot gas cleanup. However, in order to employ lime as a regenerable sorbent, it needs to be strengthened to reduce its friability. Structural based modifiers have been used to try to achieve this.
Alumina has been used as a CaO carrier. Snyder et al. (Snyder, R. B. et al. “Synthetic Sorbents for Removal of Sulfur Dioxide in Fluidized-Bed Coal Combustors,” ANL/CEN/Fe-77-1, Argonne National Laboratory, Argonne, Ill., June 1977; Snyder, R. B. et al. “Synthetic SO
2
Sorbents for Fluidized-Bed Coal Combustors,”
J. Air Poll. Control Assoc.,
27, pp. 975-981, 1977) introduced CaO into porous alumina pellets by refluxing the substrate in a calcium nitrate solution. Via this method up to 15% CaO was impregnated into the carrier. Wolff (Wolff, H. E. P. Regenerative Sulfur Capture in Fluidized Bed Combustion of Coal: A Fixed Bed Sorption Study. Ph.D. Dissertation,
Delft University of Technology, Delft,
1991, pp. 1-177) applied a different method to arrive at a similar product. In their work, the alumina and CaO were combined in-sito via a sol-gel technique. They produced a sorbent formulation that contained approximately 6% calcium. Although sorbents fabricated using these two methods produce extremely strong pellets, the preparation methods are expensive and adsorption capacity in terms of weight gain was too low for economical use (Wolff, 1991).
Several zinc-based sorbents have been proposed for desulfurizing hot coal gas. While the materials have a strong affinity for hydrogen sulfide and carbonyl sulfide at high temperature and can be regenerated, they are expensive and decompose at 700° C. and above.
An example of a specific process requiring hot-gas desulfurization is integrated coal gasification combined-cycle power generating systems. Though plants that employ the integrated gasification combined-cycle (IGCC) system provide an efficient means of generating electrical power, the power generating systems call for a sorbent capable of removing H
2
S and COS from coal gas at near gasifier operating temperature which can be 1255° K. (1800° F.) or more. The gaseous contaminants, mainly H
2
S, need to be reduced to less than 100 ppm prior to the coal gas entering the gas turbine (Gasper-Galvin et al. Zeolite-Supported Metal Oxide Sorbents for Hot-Gas Desulfurization.
Ind. Eng. Chem. Res.
1998, 37 (No. 10), pp. 4157-4166). To maximize the efficiency of an IGCC plant, an adsorbent material capable of removing these contaminants at exit conditions of the gasifier (>900° C.) is preferable. Among various materials which have been proposed for this service, limestone offers several advantages including low cost and widespread availability. Moreover, after limestone is calcined, the resulting CaO in theory can capture 95% or more of the sulfurous species in coal gas when applied within a temperature range of 1070 to 1570° K. (1470 to 2370° F.) (Westmoreland, P. R. and Harrison, D. P. “Evaluation of Candidate Solids for High-Temperature Desulfurization of Low-Btu Gases,”
Environmental Science and Technology,
10, pp. 659-661, 1976). However, lime is soft and friable, and the spent sorbent in the form of CaS is not easily regenerated. Therefore, it has been widely regarded as a material to be used once and then discarded. Unfortunately, materials containing CaS cannot be placed directly in a landfill where they will react slowly with moisture and CO
2
under ambient conditions to form H
2
S.
These problems are not insurmountable. The problem of sorbent regeneration may be overcome, for example, by a new process which converts CaS to CaO by alternately oxidizing and reducing the material (Jagtap, S. B. and Wheelock, T. D., “Regeneration of Sulfided Calcium-Based Sorbents by a Cyclic Process,”
Energy & Fuels,
10, pp. 821-827, 1996; Wheelock, T. D., “Cyclic Processes for Oxidation of Calcium Sulfide, U.S. Pat. No. 5,433,939, Jul. 18, 1995; Wheelock, T. D., ” Cyclic Process for Oxidation of Calcium Sulfide, U.S. Pat. No. 5,653,955, Aug. 5, 1997). The poor physical properties may be overcome by combining lime with a stronger material to create a composite structure which retains the chemical reactivity of lime and the strength of the second material. Previous investigations have employed the following general methods for producing a calcium-based composite: (1) infusion of a strong inert porous substrate with a calcium compound, (2) pelletization of a powder mixture followed by partial sintering, and (3) a sol-gel technique.
Pelletization provides a cheaper means of manufacturing a sorbent. The traditional sorbent preparation method is to combine CaO with a binder in a mixture. A patent by Voss entitled “Limestone-based sorbent agglomerates for removal of sulfur compounds in hot gases and methods of making”, U.S. Pat. 4,316,813, issued Feb. 23, 1982, described a method for preparing an attrition resistant, highly reactive limestone-based sorbent which involves binding limestone particles with a material such as attapulgite clay or Portland cement. Fine particles of limestone and binder are dry-blended, and then water is added to form a paste which is subsequently agglomerated with a pin mixer or pug mill. The agglomerates are subsequently dried and calcined to produce a sorbent for hot sulfurous gases.
The possibility of u
Akiti, Jr. Tetteh T.
Wheelock Thomas D.
Iowa State University & Research Foundation, Inc.
Johnson Edward M
McKee Voorhees & Sease, P.L.C.
Silverman Stanley S.
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