FINAL TECHNICAL REPORT

September 1, 1995, through August 31, 1996

Project Title: TESTING AND TECHNOLOGY TRANSFER FOR ZINC TITANATE SORBENT IN A TITANIA MATRIX

DOE Cooperative AGreement Number:  DE-FC22-92PC92521 (Year 4)

ICCI Project Number: 95-1/2.2A-1M

Principal Investigator: James H. Swisher, E&A Associates

Other Investigators: Raghubir P. Gupta, Research Triangle Institute

Project Manager: Daniel D. Banerjee, ICCI

ABSTRACT

For new coal gasification systems, zinc-based sorbents are bering developed to remove sulfur from the hot product gas prior to its use in combines-cycle gas turbines and high-temperature fuel cells.  In general, the properties of these sorbents are considered to be very attractive, but there are still concerns about degradation of mechanical properties and sulfur capacity over many sulfidation-regeneration cycles. It is believed that containing zinc titanate in a matrix of excess titania could solve both problems, which is the objective of this project.  Four years of research have now been completed on zinc titanate sorbents.  This year several 5-lb batches of material were prepared with pilot plant equipment by United Catalysts, Inc. (UCI).  Characterization and testing of the material was done primarily by the Research Triangle Institute (RTI).  Preliminary evaluation led to the selection of two formulations for multicycle testing in a fixed-bed reactor.  The first of these, designated ICCI-1, was subjected to 20 sulfidation-regeneration cycles. Its chemical reactivity was found to be excellent, but its decrepitation resistance was only fair. Formulation ICCI-2, which had a higher initial crush strength, was tested for 50 cycles.  While its reactivity was slightly less than that of ICCI-1, it was better in decrepitation resistance and therefore had a better combination of properties.  Seven tons of the material are now being prepared under a separate ICCI contract, and plans are under way for a pilot plant test at General Electric Company's moving-bed test facility in Fall 1996.

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Restricted distribution pursuant to 35USC205.  Invention disclosed within:  Technical data appear on pages 3 to 12.  Administration of Patent Rights Clauses-US DOE Docket No. S-86,835.

EXECUTIVE SUMMARY

The timing on availability of hot-gas cleanup technology for two integrated gasification combines cycle (IGCC) plants scheduled to start up in the next several months is now critical.  Unfortunately, some nagging problems remain with both sulfur sorbent and particle filter technology that may result in the use of cold- rather than hot-gas cleanup in these plants.  With sulfur sorbents, the main problems are with mechanical property degradation and/or loss of sulfur capacity over many sulfidation-regeneration cycles.  The sorbents receiving the most attention are all zinc based.  They include various zinc titanate formulations and proprietary materials developed by the U.S. Department of Energy/Morgantown Energy Technology Center (DOE/METC) staff and the Phillips Petroleum Company.

The most important outcome of prior research done by the principal investigator and coworkers on zinc titanate sorbents (now in its fourth year) was the development of a super-strong formulation that contained zinc titanate in a structural skeleton of excess titania.  Its starting composition was 33.5 wt% ZnO and 66.5 wt% TiO₂, and it was prepared from 2-um powders fired for 3 hours at 1,000^oC.  The resulting crush strength was more than six times that of zinc titanate sorbents prepared by other investigators.  In 10-cycle fixed- and fluidized-bed tests carried out at Research Triangle Institute (RTI), its chemical reactivity was good after the first few cycles.  An activation pretreatment was developed to improve initial performance.

This year's project was a continuation of work on the same sorbent.  The material was prepared by United Catalysts, Inc. (UCI), using a proprietary process for extrusion and rounding of pellets.  Sorbent characterization at various stages of testing and multiple cycle fixed-bed tests were carried out at RTI.  The test conditions were selected to mimic conditions in the General Electric's (GE's) moving-bed hot-gas desulfurization system to be used in the TECO Clean Coal Technology Demonstration Project.

The TECO plant in Lakeland, Florida, will use 1,900 tons/day of bituminous coal.  In 1993, DOE projected that IGCC plants may produce as much as 130 GW of electricity in the year 2040.  This power estimate corresponds to 1.17 million tons/day of coal consumption, much of which would be high sulfur bituminous coal.  If the sorbent under development here were used in all these plants, approximately 150,000 tons/year of the sorbent would be needed.

On this year's project, several 5-lb batches of sorbent pellets were prepared by UCI for evaluation.  Originally, only two batches were to be made, but more effort was needed than anticipated to produce pellets with the desired size, shape, bulk density, and crush strength.  This difficulty was compounded by problems with raw materials procurement.

In March, a formulation designated as ICCI-1 was selected for a multicycle fixed-bed test.  It had a crush strength of 27 lb/pellet and performed well in thermogravimetric analysis (TGA) tests for chemical reactivity.  The fixed-bed test was interrupted at the end of the 21st (TGA) tests for chemical reactivity. The fixed-bed test was interrupted at the end of the 21st sulfidation to analyze the data and to examine the pellets for signs of degradation.  The H₂S breakthrough data were very good, with no indication of reactivity loss as a function of cycle number.  Pellets taken from the gas outlet end of the reactor looked the same as the starting number.  Pellets taken from the gas outlet end of the reactor looked the same as the starting material.  Near the gas inlet, however, surface cracks were observed on some of the pellets.  Although relatively minor, this damage was not expected because hand-made pellets tested previously did not exhibit any cracking.  With the concurrence of Illinois Clean Coal Institute (ICCI), DOE, and GE program managers, it was decided to prepare a stronger batch of pellets and restart the multicycle test.

In April, Formulation ICCI-2 was prepared and another fixed-bed test was started.  The material had an average crush strength of 30.4 lb/pellet.  An ASTM attrition loss test gave a value of 1.9 percent on the fresh material.  The fixed-bed test was interrupted this time after the 20th sulfidation.  The H₂S breakthrough times showed steady improvement over the first 10 cycles, a slight loss up to cycle 15, then steady behavior over the last five cycles. Pellets taken from the inlet end of the reactor contained only a few surface cracks, and they were barely noticeable.  Thus, Formulation ICCI-2 exhibited a better compromise than ICCI-1 between mechanical properties and chemical reactivity.  The investigators believe that further improvements can be made in the future with straightforward effort.

A decision was made to continue the test on ICCI-2 for an additional 30 cycles.  Funding for the continuation was provided by DOE/METC, so a separate report on this task is being prepared by RTI.  The investigators believe that the sorbent met all of GE's 50-cycle qualification test requirements.  The only complication was that a measurable loss of reactivity occurred during a 10-week storage period between the first 20 and last 30 cycles.  The material was stored in the sulfided state, so it was suspected that enough zinc sulfate formed during storage to constrict some of the pores in the sorbent.  Support for this hypothesis was obtained when a chemical analysis showed the presence of sulfate, and a TGA experiment showed that the lost reactivity could be recovered by heating to 800^oC in helium.  A calculation showed that zinc sulfate should decompose under these conditions.

In the technology transfer task, dozens of organizations were contracted to acquaint potential manufacturers and users of the sorbent with its properties and potential.  One important outcome was that a titanium dioxide vendor produced a new grade of material for our application.  Another important event was a presentation at the advanced coal-fired power systems '96 review meeting held at DOE/METC in July 1996.

Under a new ICCI contract, 7 tons of the sorbent are now being produced for testing by GE in its moving-bed pilot plant in Fall 1996.  A patent application will be filed in the near future.

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Restricted distribution pursuant to 35USC205.  Invention disclosed within:  Technical data appear on pages 3 to 12.  Administration of Patent Rights Clauses-US DOE Docket No. S-83,835.