FINAL TECHNICAL REPORT

September 1, 1996, through August 31, 1997

Project Title: UTILIZATION OF WASTE METAL OXIDES FOR COAL GAS DESULFURIZATION

ICCI Project Number: 96-1/2.2A-2

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 objective of this study was to evaluate metal oxide-containing waste materials such as iron oxide dust from iron and steel making plants, zinc oxide waste from smelting operations, as well as iron-containing Illinois coal ash samples, for their suitability as hot coal gas desulfurization sorbents in integrated gasification/combined cycle (IGCC) systems.

To achieve these objectives, several metal oxide waste materials were procured and their reactivities toward hydrogen sulfide (H2S) were determined. The sulfur producing capabilities of the sulfided iron oxide-containing waste materials during regeneration with SO2 were also investigated. The best waste material as well as the optimum operating conditions for sulfidation and regeneration was identified and the durability of the selected sorbent at the optimum operating conditions for sulfidation and regeneration was determined in extended multiple sulfidation/regeneration tests.

Four metal oxide waste materials from metal processing operations in the Chicago area and one Illinois coal bottom ash sample were procured and evaluated for their desulfurization potential. A special low-cost sorbent preparation technique was applied to produce preliminary sorbent formulations in the form of attrition-resistant granules for further evaluation. The results obtained in this program indicate that the iron oxide waste and the zinc oxide waste material (in both as-received and granulated forms) are very effective in removing H2S from coal gases to sufficiently low levels in the temperature range of 400 to 500C and are readily regenerable with oxygen in the temperature range of 500 to 700C. The results also indicate that regeneration of the sulfided iron oxide waste with SO2 is quite feasible at temperatures above 700C.

The result of the pPreliminary economic assessment performed in this project indicates that the iron oxide waste materials should be regarded as viable low cost alternativess to the leading zinc titanate sorbents. However, further evaluation of sorbents prepared based on the iron oxide waste-based sorbentsmaterials is necessary to confirm the long term durability and economical advantages of these sorbents.

EXECUTIVE SUMMARY

Integrated Gasification/Combined Cycle (IGCC) processes for power generation offer the potential of both lower cost power and lower emissions than conventional power plants with flue gas desulfurization (FGD). IGCC also presents an opportunity to increase the market share of high sulfur Illinois coal, if the contaminants in the fuel gas from gasifying the coal, especially sulfur, can be controlled in a cost effective manner.

Calcium-based sorbents (limestone and dolomite) and zinc titanate are the leading candidates for use in high temperature fuel gas desulfurization in IGCC systems. Although quite promising results have been obtained for hot gas desulfurization of coal gases with these sorbent systems, they have a number of limitations. Calcium-based sorbents are generally used as once-through in-bed desulfurization sorbents. Because of thermodynamic equilibrium limitations, calcium based sorbents cannot reduce the H2S content of the coal gas to the levels required for the gas turbine, necessitating a second "polishing" sorbent such as zinc titanate. Furthermore, the large quantities of spent sorbents produced with limestone and dolomite contain calcium sulfide that requires a further stabilization step to convert calcium sulfide to environmentally acceptable material for landfill disposal. For the zinc titanate sorbent systems, significant declines in sorbent reactivity have been reported in successive sulfidation/regeneration cycles leading to increased sorbent replacement costs. In addition, the sorbent replacement cost significantly increases if the sorbent is used for bulk desulfurization, requiring significantly more durable sorbents compared to the polishing mode of operation. Another limitation is that zinc titanate produces dilute SO2-containing gas streams upon regeneration with O2, which require an additional processing step for conversion of SO2 to H2S, elemental sulfur, sulfuric acid, or by recycling SO2 to the gasifier for further reaction with fresh limestone/dolomite.

Because of these limitations, two-sorbent hot gas cleanup systems have been considered in several Clean Coal Technology IGCC Demonstration Programs that consist of a bulk sorbent (i.e., dolomite) and a polishing sorbent such as zinc titanate. Low-cost (or essentially free) metal oxide-containing wastes, that are reactive toward H2S and can be regenerated to produce elemental sulfur, offer an attractive alternative to the current leading candidate sorbents.

In this investigation, several metal oxide-containing waste materials from metal smelting and refining operations such as basic oxygen furnace (BOF), as well as high iron-containing Illinois coal ash samples were evaluated for their suitability as hot coal fuel gas desulfurization sorbents in IGCC systems. This systematic study was designed to evaluate these materials for their H2S removal efficiency, sulfur capacity, and sulfur producing capabilities during regeneration, by reaction with SO2. The information generated in this study will help determine the superior metal oxide waste material and operating conditions for desulfurization of hot coal gases and for production of elemental sulfur.

Because of the small particle size of the metal oxide waste materials obtained from metal processing operations, these materials in their as-received condition, were not suitable candidates for the leading hot gas cleanup processes currently being developed for the Clean Coal Technology Demonstration Programs. Therefore, in this program a special low-cost sorbent preparation technique was applied to produce preliminary sorbent formulations in the form of attrition-resistant granules for further evaluation.

Four metal oxide waste materials and a sample of bottom ash were procured. The metal oxide wastes consisted of a tin oxide waste (Sn1W) from AMG Resources Corp., a mixed metal oxide (primarily zinc oxide) waste (Zn1W) from North Chicago Refining Co., an iron oxide waste (Fe1W) from Acme Steel Corp., and a zinc oxide waste (Zn2W) from the Imperial Smelting Company. The bottom ash sample was procured from the E. D. Edwards power station of the Central Illinois Light Company. The potential usefulness of these waste materials as regenerable sulfur sorbents was evaluated in this research program.

The results obtained in this program indicate that the iron oxide waste material (Fe1W) and the zinc oxide waste material (Zn2W) are very reactive toward H2S at 500C, achieving effective sulfur capacities approximating 0.22, and 0.18 g S/g waste, respectively. The mixed metal oxide waste material (Zn1W), however, appears to contain some sulfur compounds that decompose at high temperature, releasing SO2, suggesting that a pretreatment step may be required prior to sulfidation to prevent SO2release during sulfidation.

The maximum weight gain by the bottom ash sample is less than 2%, making it unsuitable as a stand alone sorbent. The results also indicate that the reactivity of the iron oxide waste material (i.e., Fe1W) is much higher than those of other oxides at 400C. The reactivity and the sulfur capacity of the Zn1W zinc oxide waste material significantly increases with increasing temperatures, while the effect of temperature on the Zn2W waste material appears to be minimal. The tin oxide waste (Sn1W) material appears to be less effective than the Fe1W material.

A special low-cost sorbent preparation technique was applied to produce preliminary sorbent formulations in the form of attrition-resistant granules for further evaluation. These preliminary sorbent formulations were found to be very effective in removing H2S from coal gases to sufficiently low levels at 500C.

Among the waste materials evaluated, the sorbents based on iron oxide waste materials, which areprepared based on the iron oxide waste materials, which are the most abundant and the least expensive waste materialss, were the most reactive and had the highest sulfur absorption capacity. The regeneration of the iron oxide waste materials could be carried out over a lower and a broader temperature range. The results also indicate that the reactivity of sorbents based on the iron oxide waste materialss improves at least during the initial stages of the cyclic process. Furthermore, the iron oxide waste material could also be regenerated with SO2 to directly convert the iron sulfide to elemental sulfur. Although the result of thepreliminary economic assessment performed in this project indicates that the iron oxide waste materials should be regarded as viable low cost alternativess to the leading zinc titanate sorbents, additional testing of sorbents prepared based on the iron oxide waste-based sorbents materials is necessary to confirm the long term durability and economical advantages of these sorbents.