Santosh K. Gangwal

Hot-gas cleanup—sulfur recovery technical, environmental, and economic issues

Abstract Advanced high-efficiency integrated gasification combined cycle (IGCC) power systems employing hot-gas cleanup are being developed to produce electric power from coal. Hot-gas cleanup consists of hot particulate removal and hot-gas desulfurization (HGD) technologies that match or nearly match the temperature and pressure of the gasifier and turbine generator. HGD is carried out using solid regenerable metal oxide sorbents that can remove the sulfur down to low parts per million and can be regenerated with air for multicycle operation. Economic studies have shown that HGD results in lower capital and operating costs than conventional cold gas desulfurization. Development of efficient sorbent desulfurization-regeneration reactor and tailgas treatment subsystems properly integrated with the overall IGCC systems is a key requirement for successful commercialization of HGD.

Carbon membranes for gas separation: Developmental studies

Abstract Carbon membranes with 0.2 and 1.0 μm pore sizes are commercially available for liquid microfiltration applications. These membranes may be modified for gas separation applications by providing a gas separation layer with pores in the 1 to 10 nm range. With such pores, gases are separated by Knudsen diffusion with an individual gas species permeation rate inversely proportional to the ratio of the square root of the molecular weight of the permeating species. This paper describes some of the techniques used for depositing a suitable layer starting with various organic polymeric precursors. The in situ polymerization technique was found to be the most promising, and pure component tests with membrane samples prepared with this technique indicated Knudsen diffusion behaviour. The gas separation factors obtained by mixed-gas permeation tests were found to depend strongly on gas temperature and pressure indicating significant viscous flow at high-pressure conditions.

A novel periodic reactor for scrubbing SO2 from industrial stack gases

Abstract A laboratory-scale investigation of SO 2 scrubbing was carried out in two parts: (1) screening of catalysts, and (2) continuous runs in a 50 mm (diameter) × 190 mm trickle bed using periodic interruption of liquid flow. The screening study disclosed that impregnating activated carbons with platinum substantially increased the activity for SO 2 oxidation. However, the continuous runs indicated that 95% SO 2 removal from simulated stack gas can be achieved with either an activated carbon or with one impregnated with Pt. Scrubbing at 80°C removes much more SO 2 from the gas phase than at room temperature and converts SO 2 to H 2 SO 4 . Cycle periods should exceed 30 min and the duration of liquid flow through the trickle bed should be greater than 3 min. Projecting results to an industrial scale indicates that the impregnated carbon should be chosen if a high by-product sulfuric acid strength is needed. For installations handling large gas volumes, activated carbon is the preferred catalyst and only weak sulfuric acid can be produced. The advantage of periodic flow interruption appears to be higher by product acid strength and lower pressure drop in the scrubbing operation.

Application of a periodically operated trickle bed to sulfur removal from stack gas

Current technology in the area of a regenerative processes for the removal of SO2 from stack gases is too expensive. As an alternative, during the 1970s, work was done on the effectiveness of activated carbon operating in a trickle bed reactor to remove SO2 by oxidizing it to form sulfuric acid. Recent experiments using pulsed liquid flow in a trickle bed reactor have demonstrated increased average reaction rates and higher acid concentrations than can be achieved in steady-state processes.

Reactivity of Metal Oxide Sorbents for Removal of Sulfur Compounds from Coal Gases at High Temperature and Pressure

Abstract Hot-gas desulfurization for the integrated gasification combined cycle (IGCC) process has been investigated to effectively remove hydrogen sulfide with various metal oxide sorbents at high temperatures and pressures. Metal oxide sorbents such as zinc titanate oxide, zinc ferrite oxide, copper oxide, manganese oxide, and calcium oxide were found to be promising sorbents in comparison with other removal methods such as membrane separation and reactive membrane separation. The removal reaction of H2S from coal gas mixtures with zinc titanate oxide sorbents was conducted in a batch reactor. The main objectives of this research are to formulate promising metal oxide sorbents for removal of hydrogen sulfide from coal gas mixtures, to compare reactivity of a formulated sorbent with a sorbent supplied by the Research Triangle Institute at high temperatures and pressures, and to determine effects of concentrations of moisture contained in coal gas mixtures, and to determine effects of concentrations of mo...

Reactivity of sorbents with hot hydrogen sulfide in the presence of moisture and hydrogen

Hot-gas desulfurization for the integrated gasification combined cycle (IGCC) process has been investigated by many researchers to remove effectively hydrogen sulfide (H2S) with various metal oxide sorbents at elevated temperatures. Various metal oxide sorbents are formulated with metal oxides such as Fe, Co, Zn, and Ti. In this article, reactivity of AHI-1 sorbent, obtained from the Research Triangle Institute (RTI), was investigated. Initial reactivity of AHI-1 sorbent with hydrogen sulfide was studied in the presence of various amounts of moisture and hydrogen at various reaction temperatures. AHI-1 sorbent consists of 20-w% Fe2O3, 10-w% ZnO, and 70-w% spent fluid cracking catalyst (FCC). The objectives of this research were to study initial reaction kinetics for the AHI-1 sorbent–hydrogen sulfide heterogeneous reaction system, to investigate effects of concentrations of hydrogen sulfide, hydrogen, and moisture on dynamic absorption of H2S into the sorbent, to understand effects of space time of reaction gas mixtures on initial reaction kinetics of the sorbent–hydrogen sulfide system, and to evaluate effects of temperature and sorbent amounts on dynamic absorption of H2S into the sorbent. Experimental data on initial reaction kinetics of hydrogen sulfide with the metal oxide sorbent were obtained with a 0.83-cm3 differential reactor. The sorbent in the form of 130-μm particles was reacted with 1000 to 4000 ppm hydrogen sulfide at 450 to 600°C. The range of space time of reaction gas mixtures is 0.03 to 0.09 s. The range of reaction duration is 4 to 14,400 s.

ADVANCED SULFUR CONTROL CONCEPTS

Conventional sulfur removal in integrated gasification combined cycle (IGCC) power plants involves numerous steps: COS (carbonyl sulfide) hydrolysis, amine scrubbing/regeneration, Claus process, and tail-gas treatment. Advanced sulfur removal in IGCC systems involves typically the use of zinc oxide-based sorbents. The sulfides sorbent is regenerated using dilute air to produce a dilute SO{sub 2} (sulfur dioxide) tail gas. Under previous contracts the highly effective first generation Direct Sulfur Recovery Process (DSRP) for catalytic reduction of this SO{sub 2} tail gas to elemental sulfur was developed. This process is currently undergoing field-testing. In this project, advanced concepts were evaluated to reduce the number of unit operations in sulfur removal and recovery. Substantial effort was directed towards developing sorbents that could be directly regenerated to elemental sulfur in an Advanced Hot Gas Process (AHGP). Development of this process has been described in detail in Appendices A-F. RTI began the development of the Single-step Sulfur Recovery Process (SSRP) to eliminate the use of sorbents and multiple reactors in sulfur removal and recovery. This process showed promising preliminary results and thus further process development of AHGP was abandoned in favor of SSRP. The SSRP is a direct Claus process that consists of injecting SO{sub 2} directly into the quenched coal gas from a coal gasifier, and reacting the H{sub 2}S-SO{sub 2} mixture over a selective catalyst to both remove and recover sulfur in a single step. The process is conducted at gasifier pressure and 125 to 160 C. The proposed commercial embodiment of the SSRP involves a liquid phase of molten sulfur with dispersed catalyst in a slurry bubble-column reactor (SBCR).

Catalytic Ammonia Decomposition for Coal-Derived Fuel Gases

The objective of this study is to develop and demonstrate catalytic approaches for decomposing a significant percentage (up to 90 percent) of the NH{sub 3} present in fuel gas to N{sub 2} and H{sub 2} at elevated temperatures (550 to 900{degrees}C). The NH{sub 3} concentration considered in this study was {similar_to}1,800 to 2,000 ppmv, which is typical of oxygen-blown, entrained-flow gasifiers such as the Texaco coal gasifier being employed at the TECO Clean Coal Technology Demonstration plant. Catalysts containing Ni, Co, Mo, and W were candidates for the study. Before undertaking any experiments, a detailed thermodynamic evaluation was conducted to determine the concentration of NH{sub 3} in equilibrium with the Texaco gasifier coal gas. Thermodynamic evaluations were also performed to evaluate the stability of the catalytic phases (for the various catalysts under consideration) under NH3 decomposition conditions to be used in this study. Two catalytic approaches for decomposing NH{sub 3} have been experimentally evaluated. The first approach evaluated during the early phases of this project involved the screening of catalysts that could be combined with the hot-gas desulfurization sorbents (e.g., zinc titanate) for simultaneous NH{sub 3} and H{sub 2}S removal. In a commercial system, this approach would reduce capital costs by eliminating a process step. The second approach evaluated was high-temperature catalytic decomposition at 800 to 900{degrees} C. In a commercial hot-gas cleanup system this could be carried out after radiative cooling of the gas to 800 to 900{degrees}C but up stream of the convective cooler, the hot particulate filter, and the hot-gas desulfurization reactor. Both approaches were tested in the presence of up to 7,500 ppmv H{sub 2}S in simulated fuel gas or actual fuel gas from a coal gasifier.

Bench-Scale Development of Fluidized-Bed Spray-Dried Sorbents

Successful development of regenerable mixed-metal oxide sorbents for removal of reduced sulfur species (such as H{sub 2}S and COS) from coal-derived fuel gas streams at high=temperature, high-pressure (HTHP) conditions is a key to commercialization of the integrated-gasification-combined-cycle (IGCC) power systems. Among the various available coal-to-electricity pathways, IGCC power plants have the most potential with high thermal efficiency, simple system configuration, low emissions of SO{sub 2}, NO{sub x} and other contaminants, modular design, and low capital cost. Due to these advantages, the power plants of the 21st century are projected to utilize IGCC technology worldwide. Sorbents developed for sulfur removal are primarily zinc oxide-based inorganic materials, because of their ability to reduce fuel gas sulfur level to a few parts-per-million (ppm). This project extends the prior work on the development of fluidizable zinc titanate particles using a spray-drying technique to impart high reactivity and attrition resistance. Specific objectives are to develop highly reactive and attrition-resistant zinc titanate sorbents in 40- to 150-{mu}m particle size range for transport reactor applications using semicommercial- to full commercial-scale spray dryers, to transfer sorbent production technology to private sector, and to provide technical support for Sierra Pacific`s Clean Coal Technology Demonstration plant and METC`s hot-gas desulfurization process development unit (PDU), both employing a transport reactor system.

CHAIN-LIMITING OPERATION OF FISCHER-TROPSCH REACTOR

The use of pulsing to limit the chain growth of the hydrocarbon products of the Fischer-Tropsch (FT) synthesis in order to maximize the yield of diesel-range (C{sub 10}-C{sub 20}) products was examined on three high-chain-growth-probability ({alpha} {ge} 0.9) FT catalysts. On a Co-ZrO{sub 2}/SiO{sub 2} FT synthesis catalyst the application of H{sub 2} pulsing causes significant increase in CO conversion, and only an instantaneous increase in undesirable selectivity to CH{sub 4}. Increasing the frequency of H{sub 2} pulsing enhances the selectivity to C{sub 10}-C{sub 20} compounds but the chain-growth probability {alpha} remains essentially unaffected. Increasing the duration of H{sub 2} pulsing results in enhancing the maximum obtained CO conversion and the instantaneous selectivity to CH{sub 4}. An optimum set of H{sub 2} pulse parameters (pulse frequency and duration) is required for maximizing the yield of desirable diesel-range C{sub 10}-C{sub 20} products. On a high-{alpha} Fe/K/Cu/SiO{sub 2} FT synthesis catalyst H{sub 2} pulsing enhances the yield of C{sub 10}-C{sub 20} but at the same time decreases the catalyst activity (CO conversion) and increases the selectivity to CH{sub 4}. On the other hand, pulsing with CO also increases the yield of C{sub 10}-C{sub 20} but has no impact on the selectivity to CH{sub 4} or CO{sub 2} and decreases catalytic activity only moderately. In contrast to these catalysts, H{sub 2} pulsing on a high-{alpha} Ru/alumina FT synthesis catalyst has only minimal effect on activity and product distribution, showing enhanced activity towards methanation and water-gas-shift at the expense of FT synthesis. However, these observations are based on experiments performed at a significantly lower reaction pressure (ca. 26 atm) and higher reaction temperature (210-250 C) than those commonly used for supported-Ru FT catalysts (typically 100-1000 atm, 160-170 C).