Advanced Hot Gas Desulfurization Sorbents

Integrated gasification combined cycle (IGCC) power systems are being advanced worldwide for generating electricity from coal due to their superior environmental performance, economics, and efficiency in comparison to conventional coal-based power plants. Hot gas cleanup offers the potential for higher plant thermal efficiencies and lower cost. A key subsystem of hot-gas cleanup is hot-gas desulfurization using regenerable sorbents. Sorbents based on zinc oxide are currently the leading candidates and are being developed for moving- and fluidized- bed reactor applications. Zinc oxide sorbents can effectively reduce the H2S in coal gas to around 10 ppm levels and can be regenerated for multicycle operation. However, all current first-generation leading sorbents undergo significant loss of reactivity with cycling, as much as 50% or greater loss in only 25-50 cycles. Stability of the hot-gas desulfurization sorbent over 100's of cycles is essential for improved IGCC economics over conventional power plants. This project aims to develop hot-gas cleanup sorbents for relatively lower temperature applications, 343 to 538°C with emphasis on the temperature range from 400 to 500°. Recent economic evaluations have indicated that the thermal efficiency of IGCC systems increases rapidly with the temperature of hot-gas cleanup up to 350°C and then very slowly as the temperature is increased further. This suggests that the temperature severity of the hot-gas cleanup devices can be reduced without significant loss of thermal efficiency. The objective of this study is to develop attrition-resistant advanced hot-gas desulfurization sorbents which show stable and high sulfidation reactivity at 343°C (650°F) to 538°C(1OOO°F) and regenerability at lower temperatures than leading first generation sorbents.

The objective of this project was to develop hot-gas desulfurization sorbent formulations for relatively lower temperature application, with emphasis on the temperature range from 343--538 C. The candidate sorbents include highly dispersed mixed metal oxides of zinc, iron, copper, cobalt, nickel and molybdenum. The specific objective was to develop suitable sorbents, that would have high and stable surface area and are sufficiently reactive and regenerable at the relatively lower temperatures of interest in this work. Stability of surface area during regeneration was achieved by adding stabilizers. To prevent sulfation, catalyst additives that promote the light-off of the regeneration reaction at lower temperature was considered. Another objective of this study was to develop attrition-resistant advanced hot-gas desulfurization sorbents which show stable and high sulfidation reactivity at 343 to 538 C and regenerability at lower temperatures than leading first generation sorbents.

The overall objective of this project is to develop regenerable sorbents for hot gas desulfurization in IGCC systems. The specific objective of the project is to develop durable advanced sorbents that demonstrate a strong resistance to attrition and chemical deactivation, and high activity at temperatures as low as 343 C (650 F). A number of formulations will be prepared and screened in a 1/2-inch fixed bed reactor at high pressure (1 to 20 atm) and high temperatures using simulated coal-derived fuel-gases. Screening criteria will include, chemical reactivity, stability, and regenerability over the temperature range of 343 C to 650 C. After initial screening, at least 3 promising formulations will be tested for 25-30 cycles of absorption and regeneration. One of the superior formulations with the best cyclic performance will be selected for investigating scale up parameters. The scaled-up formulation will be tested for long term durability and chemical reactivity.

Economic and environmental requirements for advanced power generating systems demand the removal of corrosive and other sulfurous compounds from hot coal gas. After a brief account of the world energy resources and an overview of clean coal technologies, a review of regenerable metal oxide sorbents for cleaning the hot gas is provided. Zinc oxide, copper oxide, calcium oxide, manganese oxide based as well as supported and mixed metal oxide sorbents are treated. Performance analysis of these sorbents, effects of various parameters on the desulfurization efficiency, kinetics of sulfidation and regeneration reactions, sulfiding and regeneration mechanisms are discussed. Two chapters present recent results in the direct production of elemental sulfur from regeneration or SO2-rich gases.

The overall objective of this project is to develop regenerable sorbents for hot gas desulfurization in IGCC systems. The specific objective of the project is to develop durable advanced sorbents that demonstrate a strong resistance to attrition and chemical deactivation, and high activity at temperatures as low as 343°C(650°F). A number of formulations will be prepared and screened in a 1/2-inch fixed bed reactor at high pressure (1 to 20 atm) and high temperatures using simulated coal-derived fuel-gases. Screening criteria will include, chemical reactivity, stability, and regenerability over the temperature range of 343°C to 650°C. After initial screening, at least 3 promising formulations will be tested for 25-30 cycles of absorption and regeneration. One of the superior formulations with the best cyclic performance will be selected for investigating scale up parameters, The scaled-up formulation will be tested for long term durability and chemical reactivity.

The objectives of this project are to develop hot-gas cleanup sorbents for relatively lower temperature application, with emphasis on the temperature range from 343-538°C. The candidate sorbents include highly dispersed mixed metal oxides of zinc, iron, copper, cobalt and molybdenum. The specific objective in the successful preparation of H2S absorbents will be to generate as high and as stable a surface area as possible, in order to develop suitable sorbent, that are sufficiently reactive and regenerable at the relatively lower temperatures of interest in this work. A number of formulations will be prepared and screened for testing in a 1/2-inch fixed bed reactor at high pressure (1 to 20 atm) and high temperatures using simulated coal-derived fuel-gases. Screening criteria will include, chemical reactivity, stability, and regenerability over the temperature range of 343°C (650°F) to 538°C (1000°F). Each formulation will be tested for up to 5 cycles of absorption and regeneration. To prevent sulfation, catalyst additives will be investigated, which would promote a lower ignition of the regeneration. Selected superior formulation will be tested for long term (up to least 30 cycles) durability and chemical reactivity in the reactor.

The overall objective of this project is to develop regenerable sorbents for hot gas desulfurization in IGCC systems. The specific objective of the project is to develop durable advanced sorbents that demonstrate a strong resistance to attrition and chemical deactivation, and high activity at temperatures as low as 343 °C(650°F). A number of formulations will be prepared and screened in a 1/2-inch fixed bed reactor at high pressure (1 to 20 atm) and high temperatures using simulated coal-derived fuel-gases. Screening criteria will include, chemical reactivity, stability, and regenerability over the temperature range of 343°C to 650°C. After initial screening, at least 3 promising formulations will be tested for 25-30 cycles of absorption and regeneration. One of the superior formulations with the best cyclic performance will be selected for investigating scale up parameters. The scaled-up formulation will be tested for long term durability and chemical reactivity. Accomplishments for this period are presented for the following tasks: optimization of preparation; investigation of scale-up; and preparation of 100 lb batch.

This topical report de-scribes the results of Phase 1 research performed during the first six months of a three-year contract to study the feasibility of the direct production of elemental sulfur during the regeneration of high temperature desulfurization sorbents. Much effort has gone into the development of a high-temperature meal oxide sorbent process for removal of H2S from the coal gas. A number of sorbents based upon metals such as zinc, iron, manganese and others have been studied. In order for high temperature desulfurization to be economical it is necessary that the sorbents be regenerated to permit multicycle operation. Current methods of sorbent regeneration involve oxidation of the metal sulfide to reform the metal oxide and free the sulfur as SO2. An alternate regeneration process in which the sulfur is liberated in elemental form is preferable. The overall objective of the current research is to study simpler and economically superior processing of known sorbents capable of producing elemental sulfur during regeneration. This topical report summarizes the first steps of this effort. A literature search has been completed to identify possible regeneration concepts and to collect relevant thermodynamic, kinetic, and process data. Three concepts involving reaction with SO2, partial oxidation using an O2 - H2O mixture, and steam regeneration have been identified. The first two concepts result in the direct production of elemental sulfur while H2S is the product of steam regeneration. This concept is of potential interest, however, since existing Claus technology can be used to convert H2S to elemental sulfur. Following the literature search, a thermodynamic analysis, based upon free-energy minimization was carried out to evaluate candidate sorbents for possible use with the three regeneration concepts.

Advanced power generation systems such as the integrated gasification combined cycle power generators and the molten carbonate fuel cells have stringent fuel gas desulfurization requirements and process economics dictates that this desulfurization be performed near the temperature of the gasification off-gas. The most advanced hot gas desulfurization technology today is based on the zinc ferrite sorbent which has several shortcomings such as zinc loss by evaporation, and incomplete regeneration due to sulfate formation. The objective of this study is to develop an improved sorbent which can reduce H2S levels to 1 ppmv or less, which can stabilize zinc, and produce economically recoverable amounts of elemental sulfur during regeneration. For this purpose, the desulfurization performance.of sorbents prepared by the addition of various amounts of V205 to the zinc ferrite sorbent is investigated. Preliminary experiments show that the sorbent containing about 4.8 mass % vanadium shows a superior desulfurization performance compared to zinc ferrite. Addition of vanadium suppresses residual sulfate formation and possibly zinc evaporation. significant quantities of elemental sulfur were observed after the regeneration of vanadium containing sorbents.