FINAL TECHNICAL REPORT

September 1, 1996, through August 31, 1997

Project Title: COPPER-BASED SORBENTS FOR HOT GAS DESULFURIZATION SYSTEMS

ICCI Project Number: 96-1/2.2A-3

Principal Investigator: Javad Abbasian, Institute of Gas Technology

Other Investigators: Rachid B. Slimane, James R. Wangerow, and Minoo K. Zarnegar, Institute of Gas Technology

Project Manager: Ronald H. Carty, ICCI

ABSTRACT

The overall objective of this study was to produce highly reactive and attrition resistant copper-based sorbents for removal of hydrogen sulfide from coal gas and to evaluate the effectiveness of the superior sorbent formulations in multi-cycle tests under process conditions closely simulating those of the Clean Coal Technology IGCC Demonstration Programs.

To achieve this objective, several formulations of copper-based sorbent pellets and granules were prepared. These sorbent formulations were evaluated for their attrition resistance and crush strength as well as their desulfurization and regeneration capabilities.

During this study, a method was established for the preparation of copper chromite powders that are easily amenable to pelletization. Copper chromite sorbents in the form of spherical pellets, cylindrical tablets, and granules were prepared. Based on packed-bed reactor testing and crush strength evaluation, these sorbents were found to be suitable for moving-bed (pellets and tablets) and fluidized-bed (granules) desulfurization applications at 600C. However, these copper chromite sorbents required an activation step to enhance their reactivity at 600C and were found to have unacceptably low reactivities for fuel gas desulfurization at temperatures below 500C.

A new class of attrition-resistant copper-based sorbents in the form of pellets and granules was prepared for lower temperature application. This new class of copper-based sorbents was evaluated at 350 to 450C and was found to have a far superior performance in this lower temperature application, achieving an average effective sulfur capacity of 9.3 g S/100 g sorbent at 450C and about 6.3 g S/100 g sorbent at 350C, with a pre-breakthrough H2S concentration in the cleaned fuel gas of below 1 ppmv.

EXECUTIVE SUMMARY

Integrated Gasification Combined-Cycle (IGCC) Power Plants and Integrated Gasification Fuel Cell (IGFC) Power Generation Technologies are among the leading contenders for coal conversion. Coal gas desulfurization to sufficiently low levels at elevated temperatures (i.e., T > 350) is now recognized as crucial to efficient and economic coal utilization in advanced IGCC and IGFC power generation processes. The implementation of hot coal gas desulfurization heavily relies on the development of regenerable sorbent materials which can efficiently remove H2S (from several thousand ppmv levels down to a few ppmv) over many cycles of sulfidation/regeneration. Structural stability and good mechanical strength are additional desired features of the sorbents.

The zinc-based sorbents, such as zinc titanate, have been shown to suffer from zinc volatilization at elevated temperatures (i.e., T > 550C), leading to sorbent deterioration, increasing sorbent replacement costs, and the overall cost of hot gas cleanup. Copper-based sorbents, because of the high melting point of the metal, do not suffer from this problem.

A novel copper-chromite sorbent has been developed in a recent ICCI-funded project for hot coal gas desulfurization in a fluidized-bed. The results obtained in this previous study indicated that the copper-chromite sorbent produced in granular form (i.e., CuCr-29), had a much higher attrition resistance compared to a commercial granular zinc titanate sorbent (i.e., UCI-4169), as well as excellent desulfurization efficiency. Furthermore, unlike most zinc titanate sorbents, the reactivity of IGT's CuCr-29 copper-chromite sorbent gradually and consistently improved during the 20 cycles tested.

This study focused on the production of highly reactive and attrition resistant copper-based sorbents and on the evaluation of the effectiveness of the superior sorbent formulations in multi-cycle tests, under process conditions closely simulating those of the Clean Coal Technology IGCC Demonstration Programs.

In this program, a number of formulations of attrition resistant copper-based sorbent pellets (dp = 2 - 5 mm) as well as granules (dp = 0.1- 0.3 mm) were produced. Initially, the starting materials and sorbent composition were comparable to those identified in the previous ICCI-funded project. However, because the goal of this project was also directed toward moving-bed technology, the sorbent composition was modified to suit the desired applications. Such modifications included additives (to enhance and/or maintain porosity) and thermal treatment history (i.e., induration temperature and time). In addition, to minimize attrition losses and achieve optimum packing efficiency, a special pelletization technique was used to produce durable and highly reactive spherical sorbent pellets.

A method was established for the preparation of copper-based powders that are easily amenable to pelletization. Copper-based sorbents in the form of cylindrical tablets, spherical pellets, and granules were prepared. The average particle diameters of the spherical pellets and the granules were 0.35 cm and 0.02 cm, respectively. The crush strengths of the pellet formulations were determined in accordance with ASTM procedures and were found to exceed the requirement for moving-bed applications.

A selected number of copper chromite sorbents were subjected to multi-cycle testing in the ambient pressure packed-bed unit to demonstrate their durability and regenerability. These tests were conducted at a space velocity of 2000 hr-1 in the sulfidation temperature range of 600 to 650C and at a regeneration temperature of 750C. The results obtained indicate that all selected formulations of copper chromite sorbents are sufficiently reactive toward H2S at 600C. The pre-breakthrough H2S concentration in the cleaned gas was less than 5 ppmv with the granular formulations, and below 10 ppmv with the pellets, qualifying the granular formulations for fluidized-bed and the pellet formulations for moving-bed desulfurization of fuel gases at elevated temperatures. However, these copper chromite sorbent formulations were found to require an activation step to enhance their reactivities toward H2S.

The suitability of two copper chromite sorbents (i.e., CuCr-29 and CuCr-41G) was also assessed for application in the lower temperature range of 350 to 450C. These copper chromite sorbents, which have been developed for the temperature range of 550 to 650C, were found to have unacceptably low reactivities for fuel gas desulfurization at lower temperatures, even following activation.

A new class of attrition-resistant copper-based sorbents in the form of pellets and granules was prepared and evaluated in a packed-bed reactor. One of these formulations (i.e., IGTSS-145) was sulfided repeatedly at 600C and regenerated at 725C, and was found to perform remarkably well without requiring activation, unlike the copper chromite sorbents. This new class of copper-based sorbents was also evaluated at 350 to 450C and several formulations were found to have a far superior performance in this lower temperature application. The best sorbent formulation (i.e., IGTSS-179) had an average effective sulfur capacity of 9.3 g S/100 g sorbent at 450C and about 6.3 g S/100 g sorbent at 350C, with a pre-breakthrough H2S concentration in the cleaned fuel gas of below 1 ppmv.

The reproducibility of the sorbent preparation technique developed at IGT was confirmed by making two different batches of the IGTSS-179 sorbent followed by testing at 450C. The results obtained also served to corroborate the fact that the IGTSS-179 copper based sorbent does not require an activation step to enhance its reactivity toward H2S at 450 or 350C.

Multi-cycle testing of two formulations of the new class of copper-based sorbents determined the effectiveness of one sorbent formulation (i.e., IGTSS-145) for desulfurization applications at 450C and the effectiveness of another sorbent formulation (i.e., IGTSS-179) for those applications at 350 to 450C.