Development of Improved Sorbents for the Moving-Bed Copper Oxide Process

FINAL TECHNICAL REPORT

November 1, 1998, through October 31, 1999

Project Title: DEVELOPMENT OF IMPROVED SORBENTS FOR THE MOVING -BED COPPER OXIDE PROCESS

ICCI Project Number: 98-1/1.1C-2

Principal Investigator: Javad Abbasian, Institute of Gas Technology

Other Investigators: Rachid B. Slimane, Perin A. Cengiz, and Brett E. Williams, Institute of Gas Technology

Project Manager: Ronald H. Carty, ICCI

ABSTRACT

The overall objective of this program was to develop new and improved regenerable copper-based sorbents for removal of SO₂ and NO_x from flue gas. The sorbents developed in this program were geared towards application in the Copper Oxide Bed Regenerable Absorber (COBRA) process, which is being developed under the joint sponsorship of DCCA and the U.S. Department of energy (DOE). The targeted areas of sorbent improvement included higher reactivity and effective capacity, and better attrition resistance and durability.

To achieve this objective, different formulations of copper-based sorbents in pellet form were prepared. The new sorbents were evaluated for their physical and chemical properties as well as their desulfurization and regeneration reactivities. The alumina-supported copper-based (ALCOA) sorbent, that has been used in the pilot plant scale testing of the COBRA process, was used as the baseline sorbent for comparison with the new sorbents.

In this project, the baseline sorbent was evaluated in packed-bed experiments. Parametric studies were carried out to determine the effects of operating parameters on the performance of the sorbent. Long term durability of the baseline sorbent was assessed. Physical and chemical properties of the baseline sorbent were also determined.

A number of new sorbent formulations were prepared by incorporating and impregnating copper into an alumina support prepared using a modified sol-gel technique. These sol-gel sorbents have significantly higher crush strength and similar surface area compared to the baseline sorbent. However, the reactivities of these sorbents, in the pellet form, were somewhat lower than that of the baseline sorbent. To improve the performance of the new sorbents, the chemical composition and the preparation technique were modified. The results indicate that, although the reactivity of the sorbents improved by these modifications, the crush strength of the sorbent generally decreased. Therefore, additional work is needed to optimize the sorbent composition and preparation technique to further improve the performance of the sorbent.

EXECUTIVE SUMMARY

In the Clean Air Act Amendments (CAAA) of 1990, legislation was introduced requiring electric utilities to adopt available technology for removal of pollutant gases and particulates from coal combustion flue gases so that the increased use of coal is done in an environmentally acceptable manner.

During coal combustion in the pulverized coal combustion process sulfur in coal is released in the form of sulfur dioxide (SO₂) in the flue gas and a small fraction of nitrogen in the form of NO₂ and NO, commonly referred to as NO_x. The SO₂ and NO_x emissions are very damaging to the environment because they combine with the water vapor in the air and deposit as acid rain. The threat from acid rain is more of a concern in Illinois where over 90% of the high sulfur coal mined is consumed by electric utilities that are based on pulverized coal combustion and, only a very small fraction of the coal-based power plants in Illinois are currently equipped with Flue Gas Desulfurization (FGD) processes.

The development of the Copper Oxide Bed Regenerable Absorber (COBRA) process, which is based on moving-bed crossflow reactor design for the combined removal of SO₂, NO_x, and particulates, has been pursued in conjunction with the use of Illinois coal. Given the strict limits on SO₂ emissions (1.2 lbs of SO₂ per million Btu by the year 2000), the high sulfur content of Illinois coal, and the growing concern with the disposal of solid residues from conventional flue gas desulfurization (FGD), the selection of the COBRA technology as one of the most promising technologies to meet CAAA emission standards represents a strategic choice for the Illinois coal research and development program.

Development of the COBRA flue gas cleanup process is well on its way toward commercialization by Sargent & Lundy under the joint sponsorship of DCCA/OCDM and the U.S. DOE as part of the combustion 2000 program. This process has been demonstrated at the nominal half-megawatt scale at the Illinois Coal Development Park in Carterville, Illinois.

The overall objective of this program was to develop new and improved regenerable copper-based sorbents for removal of SO₂ and NO_x from flue gas. The sorbents developed in this program are geared towards application in the COBRA process. The targeted areas of sorbent improvement included higher reactivity and effective capacity, lower regeneration temperature, and better attrition resistance and durability.

To achieve this objective, a number of copper-based sorbents in the form of pellets were prepared. The parameters considered during sorbent preparation included chemical composition as well as physical properties, such as surface area, porosity, and pore size distribution.

The new sorbent formulations were evaluated for their attrition resistance, crush strength, SO₂ removal efficiency and effective sulfur capacity, and regeneration capability. The alumina-supported copper-based sorbent (produced by ALCOA), that has been used in the pilot plant scale testing of the process at the Illinois Coal Development Park, was used as the baseline sorbent to quantify the improvements achieved in this program.

To establish a baseline for comparison of the improved sorbents developed in this program, the baseline sorbent was evaluated for its sulfation performance, regenerability, long term durability, as well as physical and chemical characteristics. The results of these tests indicate that, at the baseline condition used in this project, the effective capacity of the sorbent is about 3% and that, a temperature change of ±100°F can affect the effective sorbent capacity by up to ±20%. The results also indicate that the effective capacity of the sorbent generally improves with increasing regeneration temperature. An increase in regeneration temperature of 100°F improves effective capacity by 20% and a decrease in temperature of 100°F results in 40% decrease in effective capacity. Based on the results of multi-cycle durability tests conducted with the baseline sorbent, it appears that the effective sulfur capacity of this sorbent gradually decreases by about 10% after 20 sulfation/regeneration cycles. Furthermore, the results suggest that adsorption and/or formation of aluminum sulfate are probably contributing to SO₂ sorption during the sulfation stage.

Preparation of improved sorbents (TASK 2) was initiated by channeling initial efforts towards employing alternative sorbent synthesis techniques that have been shown to produce mechanically strong porous solids. A number of batches of alumina support materials were prepared by modified sol-gel techniques. The specific surface areas, copper contents as well as the crush strength of the new sorbents were determined along with those of the baseline sorbent. The results indicate that the crush strength of the alumina support materials produced by various sol-gel techniques is about 7 times higher than that of the ALCOA alumina support, while the crush strength of the copper-based sorbents produced by sol-gel techniques is about 5 times higher than that of the ALCOA sorbent. The results also indicate that while the overall BET surface areas of these sorbents are comparable, a significantly higher fraction of the surface area of the new sorbents is attributed to mesopores (i.e., larger than 50 Å). However, the reactivities of these sorbents in the pellet form (i.e., d=3 mm) were somewhat lower than that of the baseline sorbent, while in the granular form (i.e., d = 0.5-0.8 mm), these sorbents exhibited higher reactivities than that of the baseline sorbent.

Scanning Electron Microscopy (SEM) analyses revealed that ALCOA sorbent has much higher macroporosity compared to these new sorbents, which may explain the lower reactivity of the new sorbents despite their favorable active metal content and surface area. To improve the performance of the new sorbents, the chemical composition and the preparation technique were modified. The results indicate that although, the reactivity of the sorbents improved by these modifications, the crush strength of the sorbent generally decreased. Therefore, additional work is needed to optimize the sorbent composition and preparation technique to further improve the performance of the sorbent.