Rachid B. Slimane

Progress in carbon dioxide separation and capture: A review

This article reviews the progress made in CO2 separation and capture research and engineering. Various technologies, such as absorption, adsorption, and membrane separation, are thoroughly discussed. New concepts such as chemical-looping combustion and hydrate-based separation are also introduced briefly. Future directions are suggested. Sequestration methods, such as forestation, ocean fertilization and mineral carbonation techniques are also covered. Underground injection and direct ocean dump are not covered.

Regenerable mixed metal oxide sorbents for coal gas desulfurization at moderate temperatures

Abstract This paper reports on research conducted at the Institute of Gas Technology (IGT) for the development of durable metal oxide-based sorbents for fluidized-bed desulfurization of coal-derived fuel gases in the moderate temperature range of 350–550°C, which is currently of industrial interest. This study has systematically considered copper (Cu), iron (Fe), manganese (Mn), and zinc (Zn) oxides as bases for developing regenerable sorbents. The sorbent formulations prepared, their sulfidation performance and regenerability, and the physical and chemical properties of a select group of sorbents are presented and discussed. The results from multi-cycle evaluation tests of a few sorbents in a packed-bed reactor are also presented. The results of attrition resistance tests carried out according to the ASTM D 5757-95 method are also presented and their implications discussed in detail. Sorbents based on copper oxide were found to possess the best combination of high attrition resistance and sulfidation reactivity, sulfur removal efficiency, and pre-breakthrough conversion in the moderate temperature range of 350–550°C. Encouraging results were also obtained with sorbents based on manganese oxide; however, their regeneration necessitates very high temperatures that cannot be accommodated by current desulfurization systems requiring regeneration ignition temperatures of approximately 550°C. No sorbent based on iron oxide was found to have sufficient reactivity in the moderate temperature range.

Utilization of metal oxide-containing waste materials for hot coal gas desulfurization

Four metal oxide waste materials from metal processing operations and one coal bottom ash sample were procured and their reactivities toward hydrogen sulfide (H2S) were evaluated in the temperature range of 400°C to 600°C. A low-cost sorbent pelletization/granulation technique was applied to produce preliminary sorbent formulations in the form of attrition-resistant granules that were also evaluated. The results indicate that sorbents based on an iron oxide waste material, in the as-received as well as processed form, were the most reactive and exhibited the highest effective capacities for sulfur. The regeneration of these sorbents could be carried out over a relatively moderate temperature range, suggesting that the iron oxide waste material might be a viable candidate for the development of low-cost regenerable sorbents for H2S removal from hot coal gases under conditions of current practical interest.

ZnO-based regenerable sulfur sorbents for fluid-bed/transport reactor applications

Introduction High-temperature desulfurization of coal-derived fuel gases is an essential process in emerging power generation technologies, such as the Integrated Gasification Combined Cycle (IGCC), aiming to improve both the efficiency and environmental performance of power generation from coal. Hot gas desulfurization may be accomplished by using solid sorbents such as oxides of those metals that form stable sulfides. The effectiveness of a desulfurizing sorbent in treating such gases is related to the predicted equilibrium partial pressure of hydrogen sulfide (H2S), which will be present in a combination of the reduced form of the sulfide and oxide phases.

Development of durable and reactive regenerable sorbents for high temperature flue gas desulphurisation

One of the emerging technologies for combined SO2 and NOx removal from flue gases is the copper oxide process, which is based on the use of a regenerable sorbent. Sorbent properties such as SO2 sorption capacity, reactivity, crush strength, and long-term durability have significant impact on the overall process cost. In this study, a number of sorbents were prepared by using various modifications of the sol-gel technique. Compared to the commercially available sorbent used for evaluation of the process, sorbents prepared by wet impregnation of sol-gel alumina exhibited comparable sulphur capacity and about seven times higher crush strength, while those prepared by incorporation of copper in the sol resulted in three times higher sulphur capacity and 55% higher crush strength. Significant improvement in long-term durability was also achieved with these sorbents. Preliminary economic evaluation indicates that these new sorbents have the potential to reduce the projected levelised process cost down to 3.17 mil/kWh, which is lower than the cost of current SO2 emission allowance.

Development of Highly Durable and Reactive Regenerable Magnesium-Based Sorbents for CO2 Separation in Coal Gasification Process

The specific objective of this project was to develop physically durable and chemically regenerable MgO-based sorbents that can remove carbon dioxide from raw coal gas at operating condition prevailing in IGCC processes. A total of sixty two (62) different sorbents were prepared in this project. The sorbents were prepared either by various sol-gel techniques (22 formulations) or modification of dolomite (40 formulations). The sorbents were prepared in the form of pellets and in granular forms. The solgel based sorbents had very high physical strength, relatively high surface area, and very low average pore diameter. The magnesium content of the sorbents was estimated to be 4-6 % w/w. To improve the reactivity of the sorbents toward CO{sub 2}, The sorbents were impregnated with potassium salts. The potassium content of the sorbents was about 5%. The dolomite-based sorbents were prepared by calcination of dolomite at various temperature and calcination environment (CO{sub 2} partial pressure and moisture). Potassium carbonate was added to the half-calcined dolomite through wet impregnation method. The estimated potassium content of the impregnated sorbents was in the range of 1-6% w/w. In general, the modified dolomite sorbents have significantly higher magnesium content, larger pore diameter and lower surface area, resulting in significantly higher reactivity compared to the sol-gel sorbents. The reactivities of a number of sorbents toward CO{sub 2} were determined in a Thermogravimetric Analyzer (TGA) unit. The results indicated that at the low CO{sub 2} partial pressures (i.e., 1 atm), the reactivities of the sorbents toward CO{sub 2} are very low. At elevated pressures (i.e., CO{sub 2} partial pressure of 10 bar) the maximum conversion of MgO obtained with the sol-gel based sorbents was about 5%, which corresponds to a maximum CO{sub 2} absorption capacity of less than 1%. The overall capacity of modified dolomite sorbents were at least one order of magnitude higher than those of the sol-gel based sorbents. The results of the tests conducted with various dolomite-based sorbent indicate that the reactivity of the modified dolomite sorbent increases with increasing potassium concentration, while higher calcination temperature adversely affects the sorbent reactivity. Furthermore, the results indicate that as long as the absorption temperature is well below the equilibrium temperature, the reactivity of the sorbent improves with increasing temperature (350-425 C). As the temperature approaches the equilibrium temperature, because of the significant increase in the rate of reverse (i.e., regeneration) reaction, the rate of CO{sub 2} absorption decreases. The results of cyclic tests show that the reactivity of the sorbent gradually decreases in the cyclic process. To improve long-term durability (i.e., reactivity and capacity) of the sorbent, the sorbent was periodically re-impregnated with potassium additive and calcined. The results indicate that, in general, re-treatment improves the performance of the sorbent, and that, the extent of improvement gradually decreases in the cyclic process. The presence of steam significantly enhances the sorbent reactivity and significantly decreases the rate of decline in sorbent deactivation in the cyclic process.

ADVANCED SORBENT DEVELOPMENT PROGRAM DEVELOPMENT OF SORBENTS FOR MOVING-BED AND FLUIDIZED-BED APPLICATIONS

The integrated gasification combined cycle (IGCC) power system using high-temperature coal gas cleanup is one of the most promising advanced technologies for the production of electric power from coal in an environmentally acceptable manner. Unlike conventional low-temperature cleanup systems that require costly heat exchangers, high-temperature coal gas cleanup systems can be operated near 482-538 C (900-1000 F) or higher, conditions that are a closer match with the gasifier and turbine components in the IGCC system, thus resulting is a more efficient overall system. GE is developing a moving-bed, high-temperature desulfurization system for the IGCC power cycle in which zinc-based regenerable sorbents are currently being used as desulfurization sorbents. Zinc titanate and other proprietary zinc-based oxides are being considered as sorbents for use in the Clean Coal Technology Demonstration Program at Tampa Electric Co.s (TECo) Polk Power Station. Under cold startup conditions at TECo, desulfurization and regeneration may be carried out at temperatures as low as 343 C (650 F), hence a versatile sorbent is desirable to perform over this wide temperature range. A key to success in the development of high-temperature desulfurization systems is the matching of sorbent properties for the selected process operating conditions, namely, sustainable desulfurization kinetics, high sulfur capacity, and mechanical durability over multiple cycles. Additionally, the sulfur species produced during regeneration of the sorbent must be in a form compatible with sulfur recovery systems, such as sulfuric acid or elemental sulfur processes. The overall objective of this program is to develop regenerable sorbents for hydrogen sulfide removal from coal-derived fuel gases in the temperature range 343-538 C (650-1000 F). Two categories of reactor configurations are being considered: moving-bed reactors and fluidized-bed (bubbling and circulating) reactors. In addition, a cost assessment and a market plan for large-scale fabrication of sorbents were developed. As an optional task, long-term bench-scale tests of the best moving-bed sorbents were conducted. Starting from thermodynamic calculations, several metal oxides were identified for potential use as hot gas cleanup sorbents using constructed phase stability diagrams and laboratory screening of various mixed-metal oxide formulations. Modified zinc titanates and other proprietary metal oxide formulations were evaluated at the bench scale and many of them found to be acceptable for operation in the target desulfurization temperature range of 370 C (700 F) to 538 C (1000 F) and regeneration temperatures up to 760 C (1400 F). Further work is still needed to reduce the batch-to-batch repeatability in the fabrication of modified zinc titanates for larger scale applications. The information presented in this Volume 1 report contains the results of moving-bed sorbent development at General Electrics Corporate Research and Development (GE-CRD). A separate Volume 2 report contains the results of the subcontract on fluidized-bed sorbent development at the Institute of Gas Technology (IGT).

Production of Hydrogen by Superadiabatic Decomposition of Hydrogen Sulfide - Final Technical Report for the Period June 1, 1999 - September 30, 2000

The Gas Technology Institute, in collaboration with the University of Illinois at Chicago and industrial partners, including UOP, has been developing an innovative noncatalytic, thermochemical process for the production of hydrogen and elemental sulfur from hydrogen sulfide in H2S-containing waste gases. The key feature of this process is the superadiabatic reactor, where partial oxidation of H2S in the feed gas is carried out in a cylindrical vessel packed with a porous ceramic medium with a high thermal capacity. The intensive heat exchange between the filtrating and burning gas mixture, and the porous medium through the highly developed internal surfaces permits the accumulation of combustion energy in the solid matrix. As a result, flame temperatures can be significantly higher than the adiabatic temperature for the mixture. This process has potential to produce economically viable quantities of hydrogen through the superadiabatic partial oxidation of H2S in the feed at very high temperatures, which can be achieved without the input of external energy, and with no additional carbon dioxide (CO2) emissions. GTI has envisioned a process comprising the superadiabatic H2S decomposition reactor, product/byproduct separation schemes, hydrogen purification, and tail gas cleanup. With funding from the U.S. Department of Energy, GTI, and UIC, work has so far concentrated mainly on the superadiabatic reactor, and has comprised computational modeling and experimental studies to demonstrate the technical and economical feasibility of the superadiabatic H2S decomposition concept, using H2S-N2-O2 gas mixtures. Theoretical (numerical modeling) studies at UIC and collaborative experimental investigations by GTI and UIC researchers on the generation of hydrogen-rich gases from hydrocarbons via the superadiabatic partial oxidation have shown the high potential of this approach. It has been shown that stable self-sustained flames could be generated using H2S-containing gases as a feedstock in the range of equivalence ratios from 2 to 5 with hydrogen output at about 20%. The performed experimental and numerical studies analyzed chemical and thermal structures of the H2S-containing gases/air flames stabilized in an inert porous medium. The agreement between the groundwork experimental data developed to-date and modeling predictions is quite reasonable. To carry out a rigorous process evaluation, GTI has designed and constructed a state-of-the-art superadiabatic H2S decomposition reactor system. This reactor is currently being operated to demonstrate the technical feasibility of the superadiabatic decomposition process, to evaluate the agreement between modeling predictions and experimental results, and to reassess the economic potential of the process.