FINAL TECHNICAL REPORT

September 1, 1995, through August 31, 1996

Project Title: DEVELOPMENT OF REGENERABLE COPPER-BASED SORBENTS FOR HOT GAS CLEANUP

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

ICCI Project Number: 95-1/2.2A-5M

Principal Investigator: Javad Abbasian, Institute of Gas Technology

Other Investigators: Rachid B. Slimane and James R. Wangerow

Institute of Gas Technology

Project Manager: Daniel D. Banerjee, ICCI

ABSTRACT

The overall objective of this study was to determine the effectiveness of the copper-chromite sorbent (developed in previous ICCI-funded projects) for longer duration application under optimum conditions in the temperature range of 550-650C to minimize sorbent reduction and degradation during the cyclic process.

To achieve this objective, a large number of different formulations of copper chromite based sorbents were prepared in the form of granules (i.e., dp = 0.02-0.08 cm) and pellets (i.e., dp = 1-5 mm). Three (3) formulations of attrition resistant granules of the copper chromite sorbent (i.e., CuCr-10, CuCr-21, and CuCr-29) as well as one (1) copper chromite sorbent in pellet form (i.e., CuCr-36) were selected for cyclic desulfurization tests. The desulfurization and regeneration capabilities of the selected formulations as well as the effects of operating parameters were determined, to identify the "best" sorbent formulation and the optimum operating conditions. The durability of the "best" sorbent formulation was determined in "long-term" muticycle tests conducted at the optimum operating conditions. The attrition resistance of the selected formulations were determined and compared with those of other sorbents, including a limestone, a dolomite, and a commercial zinc titanate sorbent.

The results obtained in this study indicate that, the CuCr-29 sorbent has excellent attrition resistance and desulfurization performance, which are far superior to the commercial zinc titanate sorbents. The optimum desulfurization temperature in terms of sorbent efficiency and utilization appears to be about 600C. Sorbent regeneration at 750C ensured complete conversion of the copper sulfide to oxide without sulfate formation or reactivity deterioration in subsequent cycles. The sulfidation reactivity and sulfur capacity of the CuCr-29 sorbent consistently improved during the first 25 cycles. The CuCr-36 sorbent pellets also exhibited excellent desulfurization performance making this formulation a good candidate for moving-bed application.

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>350C) 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 relies heavily on the development of regenerable sorbent materials which can efficiently reduce H₂S from several thousand ppm 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.

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 higher melting point of the metal, do not suffer from this problem.

A novel copper chromite sorbent has been developed under an earlier ICCI-funded project for much higher temperature application (750-850C). Although excellent desulfurization efficiency has been achieved with this sorbent at these extreme conditions, the results indicate that the sorbent will undergo reduction during sulfidation stage, reducing the sorbent capacity for high desulfurization efficiency. The results of the previous study also indicated that the rate of reduction of the copper chromite sorbent is significantly lower below 650C, suggesting that to avoid loss of desulfurization efficiency, the hot gas cleanup application with this sorbent should be limited to 650C.

This study focused on the evaluation of the regenerable novel copper-chromite sorbents for hot gas cleanup application at 550 to 650C in long duration cyclic tests to provide the data necessary for direct comparison with zinc titanate sorbents.

A large number of copper chromite sorbent formulations were produced in granules (i.e., diameter = 0.02 to 0.08 cm) as well as pellet form (i.e., diameter = 0.1 to 0.5 cm). These sorbent formulations were first screened to select the formulation with acceptable crush strength. Three (3) formulations of attrition resistant granules of the copper chromite sorbent (i.e., CuCr-10, CuCr-21, and CuCr-29) as well as one (1) copper chromite sorbent in pellet form (i.e., CuCr-36) were selected for cyclic desulfurization tests. The selected formulations were tested in the packed-bed reactor in the sulfidation temperature range of 550C to 650C. The parametric studies included the effect of sulfidation temperature and sorbent pre-reduction on the desulfurization performance of the sorbents. The attrition resistance of the selected formulations were determined and compared with those of other sorbents, including a limestone, a dolomite, and a commercial zinc titanate sorbent. The best sorbent formulation and operating conditions identified in the parametric tests were used to determine the sorbent durability in long-term tests in the high pressure fluidized bed reactor (HPTR) unit.

The results obtained in this project, indicate that the CuCr-29 sorbent has excellent attrition resistance, which is far superior to that of the commercial UCI-4169 zinc titanate sorbent. Furthermore, the CuCr-29 sorbent has superior desulfurization performance, achieving less than 5 ppm H₂S concentration in the cleaned fuel gas. The optimum desulfurization temperature in terms of sorbent efficiency (terminal H₂S levels in the cleaned fuel gas) and utilization (sulfur capacity at breakthrough or effective capacity) appears to be about 600C. Pre-reduction did not appear to affect the performance of the CuCr-29 sorbent. Sorbent regeneration at 750C ensured complete conversion of the copper sulfide to oxide without sulfate formation or reactivity deterioration in subsequent cycles. The sulfidation reactivity and sulfur capacity of the CuCr-29 sorbent consistently improved during the first 25 cycles. The CuCr-36 sorbent pellets also exhibited excellent desulfurization performance making this formulation a good candidate for moving-bed application.