Desulfurization Of Hot Coal Gas In A Fluidized Bed With Regenerable Zinc Titanate 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.

Research Triangle Institute (RTI) under contract to the US Department of Energy (DOE), Morgantown energy Technology Center has recently completed a long-term test consisting of 100 sulfidation-regeneration cycles on a zinc titanate material intended for use as a high-temperature, regenerable sorbent to desulfurize coal-derived gas. The primary motivation for this development is to generate a more economical, environmentally superior, and reliable process to purify the product gas of coal gasifiers for use in gas turbines and fuel cells. This zinc titanate formulation (designated as ZT-4 and containing Zn-to-Ti in a molar ratio of 1.5) exhibited the best overall performance in terms of chemical reactivity, sulfur capacity, regenerability, structural properties and, most importantly, the attrition resistance based on multicycle testing of a number of sorbent formulations in a bench scale fluidized-bed reactor. The conditions in the test were -- desulfurization temperature: 750C (1382F); pressure: 1.52 MPa (220 psia); coal gas: simulated Texaco entrained-bed oxygen-blown gasifier gas containing 12,000 ppmv of H2S; superficial gas velocity: 15 cm/s (0.49 ft/s). The ZT-4 sorbent used in this test was prepared using a granulation technique and 500 g of the sorbent in the 100 to 300 microns particle diameter range were used in a 5.1-cm (2-inch) i.d. stainless steel reactor.

The objectives of this project are to identify and demonstrate methods for enhancing long-term chemical reactivity and attrition resistance of zinc ferrite and zinc titanate sorbents to be employed for desulfurization of hot coal-derived gases in a high-temperature, high-pressure (HTHP) fluid-bed reactor. The sorbent formulation specified for study during the base period of this project was zinc ferrite. Zinc titanate sorbents are being studied under two options to the base contract. Specific objectives of the zinc titanate sorbent development work are the following: The effect of following process variables was investigated o the performance of zinc titanate sorbents: Method of sorbent preparation, Composition of fuel gas, Zn to Ti ratio of the sorbent, Sulfidation temperature, and Superficial gas velocity. The effect of first three variables has been covered in RTI's 1991 paper (Gupta and Gangwal, 1991b), while the effect of temperature and superficial gas velocity is described here.

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.

Previous work established the technical feasibility of desulfurizing the hot product gases of coal gasification with fixed beds of a regenerable zinc ferrite sorbent. This process, intended for integration with coal gasifiers and gas turbines, has been tested and studied in considerable detail in a process development unit. Though possessing the advantages of high-sulfur absorption at low-sulfur breakthrough and the lack of sorbent attrition characteristic of a stationary bed, fixed beds also have inherent disadvantages: susceptibility to plugging by particles and a large diluent requirement during regeneration to control the reaction zone temperature. Therefore, METC conducted a scoping laboratory test program to determine the desulfurizing capability of fluid beds of zinc ferrite. Results from this program are presented. The results generally demonstrated that fluid beds of zinc ferrite have the potential to lower the H2S level in hot gas from 10,000 to 10 ppmv. To achieve this at a high-sorbent sulfur loading would require two fluid-bed stages. Sorbent attrition appears to be acceptably low. Planned future activities include tests at high pressure with both simulated gas and in a gasifier sidestream.

To extend the operating temperature range and further improve the durability of fluidizable sorbents, zinc titanate, another leading regenerable sorbent, was selected for development in the later part of this project. A number of zinc titanate formulations were prepared in the 50 to 300 [mu]m range using granulation and spray drying methods. Important sorbent preparation variables investigated included zinc to titanium ratio, binder type, binder amount, and various chemical additives such as cobalt and molybdenum. A number of sorbents selected on the basis of screening tests were subjected to bench-scale testing for 10 cycles at high temperature, high pressure (HTHP) conditions using the reactor system designed and constructed during the base program. This reactor system is capable of operation either as a 2.0 in. or 3.0 in. I.D. bubbling bed and is rated up to 20 atm operation at 871[degrees]C. Bench-scale testing variables included sorbent type, temperature (550 to 750[degrees]C), gas type (KRW or Texaco gasifier gas), steam content of coal gas, and fluidizing gas velocity (6 to 15 cm/s). The sorbents prepared by spray drying showed poor performance in terms of attrition resistance and chemical reactivity. On the other hand, the granulation method proved to be very successful. For example, a highly attrition-resistant zinc titanate formulation, ZT-4, prepared by granulation exhibited virtually no zinc loss and demonstrated a constant high reactivity and sulfur capacity over 10 cycles, i.e., approximately a 60 percent capacity utilization, with Texaco gas at 750[degrees]C, 15 cm/s fluidizing velocity and 15 atm pressure. The commercial potential of the granulation method for zinc titanate manufacture was demonstrated by preparing two 80 lb batches of sorbent with zinc to titanium mol ratios of 0.8 and 1.5.

Fuel Cells: Technologies for Fuel Processing provides an overview of the most important aspects of fuel reforming to the generally interested reader, researcher, technologist, teacher, student, or engineer. The topics covered include all aspects of fuel reforming: fundamental chemistry, different modes of reforming, catalysts, catalyst deactivation, fuel desulfurization, reaction engineering, novel reforming concepts, thermodynamics, heat and mass transfer issues, system design, and recent research and development. While no attempt is made to describe the fuel cell itself, there is sufficient description of the fuel cell to show how it affects the fuel reformer. By focusing on the fundamentals, this book aims to be a source of information now and in the future. By avoiding time-sensitive information/analysis (e.g., economics) it serves as a single source of information for scientists and engineers in fuel processing technology. The material is presented in such a way that this book will serve as a reference for graduate level courses, fuel cell developers, and fuel cell researchers. Chapters written by experts in each area Extensive bibliography supporting each chapter Detailed index Up-to-date diagrams and full colour illustrations