Development Of A Hot Gas Desulfurization System For Igcc Applications

Integrated gasification combined cycle (IGCC) power plants are being advanced worldwide to produce electricity from coal because of their superior environmental performance, economics, and efficiency in comparison to conventional coal-based power plants. One key component of an advanced IGCC power plant is a hot-gas desulfurization system employing regenerable sorbents. To carry out hot-gas desulfurization in a fluidized-bed reactor, it is necessary that the sorbents have high attrition resistance, while still maintaining high chemical reactivity and sulfur absorption capacity. Also, efficient processes are needed for the treatment of SO2-containing regeneration off-gas to produce environmentally benign waste or useful byproducts. A series of durable zinc titanate sorbents were formulated and tested in a bench-scale fluidized-bed reactor system. Reactive sorbents were developed with addition resistance comparable to fluid-bed cracking (FCC) catalysts used in petroleum refineries. In addition, progress continues on the development of the Direct Sulfur Recovery Process (DSRP) for converting SO2 in the regeneration off-gas to elemental sulfur. Plans are under way to test these bench-scale systems at gasifier sites with coal gas. This paper describes the status and future plans for the demonstration of these technologies.

Advanced integrated gasification combined cycle (IGCC) power plants nearing completion, such as Sierra-Pacific, employ a circulating fluidized-bed (transport) reactor hot-gas desulfurization (HGD) process that uses 70-180 [mu]m average particle size (aps) zinc-based mixed-metal oxide sorbent for removing H2S from coal gas down to less than 20 ppmv. The sorbent undergoes cycles of absorption (sulfidation) and air regeneration. The key barrier issues associated with a fluidized-bed HGD process are chemical degradation, physical attrition, high regeneration light-off (initiation) temperature, and high cost of the sorbent. Another inherent complication in all air-regeneration-based HGD processes is the disposal of the problematic dilute SO2 containing regeneration tail-gas. Direct Sulfur Recovery Process (DSRP), a leading first generation technology, efficiently reduces this SO2 to desirable elemental sulfur, but requires the use of 1-3 % of the coal gas, thus resulting in an energy penalty to the plant. Advanced second-generation processes are under development that can reduce this energy penalty by modifying the sorbent so that it could be directly regenerated to elemental sulfur. The objective of this research is to support the near and long term DOE efforts to commercialize the IGCC-HGD process technology. Specifically we aim to develop: optimized low-cost sorbent materials with 70-80 [mu]m average aps meeting all Sierra specs; attrition resistant sorbents with 170 [mu]m aps that allow greater flexibility in the choice of the type of fluidized-bed reactor e.g. they allow increased throughput in a bubbling-bed reactor; and modified fluidizable sorbent materials that can be regenerated to produce elemental sulfur directly with minimal or no use of coal gas The effort during the reporting period has been devoted to development of an advanced hot-gas process that can eliminate the problematic SO2 tail gas and yield elemental sulfur directly using a sorbent containing a combination of zinc and iron oxides.

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.

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.

801The Morgantown Energy Technology Center (METC) has designed and is currently constructing an on-site, hot gas desulfurization (HGD) Process Development Unit (PDU). The PDU is designed to use regenerable solid metal oxide sorbents that absorb hydrogen sulfide from high-temperature, high-pressure simulated coal-gasification fuel gas that is generated by a METC-designed syngas generator. The simulated coal gas is a mixture of partially combusted natural gas, water, carbon dioxide and hydrogen sulfide. PDU process conditions will be representative of anticipated commercial applications in terms of temperatures, pressures, compositions, velocities, and sorbent cycling. The PDU supports the Integrated Gasification Combined Cycle (IGCC) mission at METC by providing a test bed for development of IGCC cleanup systems that offer low capital cost, operating costs, and costs of electricity. METC intends to develop additional industrial involvement opportunities as the project progresses towards operations. Objectives The primary objectives of the PDU are to: (1) fill the gap between small-scale testing and large-scale demonstration projects by providing a cost effective test site for transport and fluid-bed desulfurization reactor and sorbent development, (2) demonstrate sorbent suitability over a wide range of parameters and (3) generate significant information on process control for transport and fluidized bed based desulfurization. PDU data is expected to be used to optimize process performance by expanding the experience for larger-scale demonstration projects, such as Sierra Pacific Power Company's Clean Coal Technology project.

Designs for advanced integrated gasification combined cycle (IGCC) power systems call for desulfurization of carbonaceous fuel-derived synthesis gas (syngas) using regenerable sorbents at high-temperature, high pressure (HTHP) conditions. Regeneration of the sulfided sorbent using an oxygen-containing gas stream or air results in a sulfur dioxide (SO2)-containing offgas at HTHP conditions. The patented Direct Sulfur Recovery Process (DSRP) developed by RTI with support from the National Energy Technology Laboratory (NETL) and its precursor organizations [Federal Energy Technology Center (FETC) and Morgantown Energy Technology Center (METC)] efficiently converts the SO2 in this offgas to elemental sulfur. Under development since 1988, the original work was conducted in a laboratory with simulated laboratory gas mixtures. The Direct Sulfur Recovery Process is a catalytic reduction process for efficiently converting to elemental sulfur up to 98% or more of the sulfur dioxide (SO2) contained in the regeneration offgas streams produced in advanced integrated gasification combined cycle (IGCC) power systems. The DSRP reacts the regeneration offgas with a small slipstream of syngas to effect the desired reduction. In this project, the DSRP was demonstrated with actual coal-derived syngas (as opposed to the simulated laboratory mixtures used in previous projects for the original development work) in 75-mm (3-in) and 125-mm (5-in) fixed- and fluid-bed reactors. This report focuses primarily on the slipstream testing of a skid-mounted DSRP field-test unit that utilized the 125 mm (5-in) fluid-bed reactor. This slipstream testing was conducted at the US Department of Energy's (DOE's) Power System Development Facility (PSDF) in Wilsonville, Alabama in conjunction with their coal gasification tests. The earlier work with 75 mm (3-in) reactors has been previously reported in detail. Thus, only the highlights of this earlier work will be reported in the main body of this report.

The U.S. Department of Energy (DOE), Morgantown Energy Technology Center (METC), is sponsoring research in advanced methods for controlling contaminants in hot coal gasifier gas (coal gas) streams of integrated gasification combined-cycle (IGCC) power systems. Through bench-scale development, both fluidized-bed zinc titanate and Direct Sulfur Recovery Process (DSRP) technologies have been shown to be technically and economically attractive. In the zinc titanate approach, sulfur dioxide is the produced and must be disposed of in an environmentally sound manner. In the DSRP, elemental sulfur is the catalytic product.

The objective of this work is to further the development of zinc titanate fluidized-bed desulfurization (ZTFBD), and the Direct Sulfur Recovery Process (DSRP) for hot gas cleanup of coal gas used in integrated gasification combined-cycle (IGCC) power generation systems. Results are described.