FINAL TECHNICAL REPORT

September 1, 1996, through August 31, 1997

Project Title:  PREPARATION OF NOVEL SORBENTS FROM ILLINOIS COAL FOR HOT GAS CLEANUP

ICCI Project Number: 96-1/2.2A-5

Principal Investigator: Mark P. Cal, Illinois State Geological Survey (ISGS)

Other Investigators: Brooks W. Strickler, University of Illinois at Urbana-Champaign (UIUC); Anthony A. Lizzio, ISGS; Santosh K. Gangwal, Research Triangle Institute; John M. Lytle, ISGS; Mark J. Rood, UIUC

Project Manager: Ron Carty, ICCI

ABSTRACT

The overall objective of this research was to produce a carbon-based sorbent from Illinois bituminous coal capable of efficiently removing H2S from hot coal gas streams. The goal was to produce a regenerable carbon-based sorbent having a H2S adsorption capacity and fixed-bed breakthrough time comparable to current leading sorbents, but having a lower sorbent and/or process cost. Although, the use of carbon for hot gas cleanup has significant potential, the possibility of using carbon-based materials to remove sulfur containing gases from the products of coal gasification has been overlooked as other sorbents such as zinc ferrite, zinc titanate and copper oxides have been extensively studied. Carbon has several advantages compared to metal-based hot gas cleanup sorbents: 1) the harsh coal gas environment should not affect the properties of the carbon during operation (carbon will not gasify in a reducing atmosphere at temperatures less than 800C), 2) carbon, itself, adsorbs large quantities of H2S, meaning that it could be used as an active support for metals such as copper and zinc which also adsorb H2S; most metal-based sorbents have an inert support matrix, sometimes constituting up to 60% of the mass of the sorbent, 3) carbon will not chemically spall unlike metal-based sorbents and it is more physically stable, since there is little or no volumetric change in a carbon-based sorbent due to sulfur loading, and 4) coal, which is used to make the carbon-based sorbents, is a very inexpensive starting material.

Results of this project have shown that carbon-based sorbents are capable of adsorbing H2S at sulfur loadings of up to 20 weight percent sulfur from a simulated coal gas stream. Fixed-bed breakthrough curves were obtained for many types of carbon-based sorbents using simulated coal gas streams containing 0.5% H2S, and varying concentrations of CO, CO2, H2, H2O, and N2 at a temperature of 550C, pressures ranging from 1 to 10 atm, and a space velocity of ~2000 h-1. Breakthrough times to 200 ppmv effluent H2S concentration ranged from 10 min to 450 min, depending on the carbon sorbent used and the influent gas composition. Breakthrough times for some of the carbon sorbents were comparable to some metal-based sorbents. Many methods of carbon sorbent regeneration were investigated and two methods showed much promise for regenerating the carbon sorbents. Adsorption/regeneration cycle tests were also performed on the most promising sorbents to determine the effect of regeneration on subsequent adsorption cycles. Results of adsorption and regeneration experiments are encouraging and show that carbon may be a viable hot gas cleanup sorbent.

Pages 1-26 contain proprietary information.

EXECUTIVE SUMMARY

Background

Integrated Gasification Combined Cycle (IGCC) power systems are emerging as the most promising technology to convert high sulfur coal into electricity. Illinois Basin coal is a proven feedstock for IGCC, e.g., the Destec process in Terre Haute, IN. Hot gas cleanup for desulfurization is needed to accelerate the successful demonstration and commercialization of advanced coal gasification systems worldwide. In IGCC processes, hydrogen sulfide is removed from the coal gas before it enters the turbine. To achieve maximum efficiency in IGCC systems, H2S should be removed from the fuel gas while hot. Although H2S can be removed quite effectively by cooling the hot gases to temperatures less than 100C, removal of H2S at 300-800C can lead to significant increases (up to 3%) in overall thermal efficiency. More efficient IGCC processes expand markets for hot gas cleanup sorbents and for high sulfur Illinois coal.

Numerous metals and mixed metal compounds have been studied as possible desulfurization sorbents. Current leading sorbents include zinc titanate and Z-Sorb (a proprietary zinc-oxide-based sorbent). However, not only are these sorbents expensive (up to $7-15/lb), they are also prone to chemical and/or physical degradation during cycling. Zinc titanate suffers from spalling due to formation of sulfide and sulfate which have 2 to 3 times higher molar volume than the oxide. Z-Sorb degrades in the presence of steam present in coal gas. Both sorbents sinter during regeneration and typically their reactivity drops by around 50 percent in just 50 cycles.

One may conclude by reviewing the literature on hot gas cleanup sorbents that while research on sorbent materials has been extensive, continuing efforts are important because these sorbents are not yet used commercially in coal gasification processes, and there is still time for the development of improved sorbents. Another reason to encourage research in this area is that not all gasification systems are alike, and because of these differences in operating conditions and requirements, it is likely that more than one type of sorbent will be needed to satisfy the market. Every sorbent examined to date has had at least one major deficiency that prevents its widespread use. The fact that there is still ample opportunity to develop new types of sorbents for hot gas cleanup provides incentive for research on new types of materials.

One material that should be examined in more detail is carbon. At first glance, carbon may appear unfit for coal gas environments, however, it is well known that carbon will not react (gasify) to any appreciable extent with the gases found in a reducing atmosphere (CO, H2O, H2) at temperatures less than 800C. A well documented advantage of carbon sorbents, e.g., carbon molecular sieves (CMS), over other materials used in gas separation, e.g., zeolites, is its exceptional resistance to chemical attack in harsh process environments. Carbon molecular sieves also can be made, e.g., by pelletization, to withstand attrition for long periods of time, and often last for more than ten years in air separation plants. In a pressure swing adsorption process, the CMS is exposed to severe changes in pressure as well as temperature, and with typical cycle times of two minutes or less, thousands of adsorption/desorption cycles must be performed before the sorbent is replaced.