Desulfurization Sorbents for Transport-Bed Applications

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. The specific objectives are: (1) To develop highly reactive and attrition-resistant zinc titanate sorbents in 40- to 150-{micro}m particle size range for transport reactor applications; (2) To transfer sorbent production technology to private sector; and (3) To provide technical support to Sierra Pacific Clean Coal Technology Demonstration plant and FETC's Hot-Gas Desulfurization Process Development Unit (PDU), both employing a transport reactor system

Carbon Dioxide Capture From Flue Gas Using Dry Regenerable Sorbents Quarterly Report

This report describes research conducted between October 1, 2004 and December 31, 2004 on the use of dry regenerable sorbents for removal of carbon dioxide from flue gas. Two supported sorbents were tested in a bench scale fluidized bed reactor system. The sorbents were prepared by impregnation of sodium carbonate on to an inert support at a commercial catalyst manufacturing facility. One sorbent, tested through five cycles of carbon dioxide sorption in an atmosphere of 3% water vapor and 0.8 to 3% carbon dioxide showed consistent reactivity with sodium carbonate utilization of 7 to 14%. A second, similarly prepared material, showed comparable reactivity in one cycle of testing. Batches of 5 other materials were prepared in laboratory scale quantities (primarily by spray drying). These materials generally have significantly greater surface areas than calcined sodium bicarbonate. Small scale testing showed no significant adsorption of mercury on representative carbon dioxide sorbent materials under expected flue gas conditions

Carbon Dioxide Capture From Flue Gas Using Dry Regenerable Sorbents Quarterly Report

The objective of this project is to develop a simple and inexpensive process to separate CO{sub 2} as an essentially pure stream from a fossil fuel combustion system using a regenerable sorbent. The sorbents being investigated in this project are primarily alkali carbonates, and particularly sodium carbonate and potassium carbonate, which are converted to bicarbonates or intermediate salts through reaction with carbon dioxide and water vapor. Bicarbonates are regenerated to carbonates when heated, producing a nearly pure CO{sub 2} stream after condensation of water vapor. This quarter, electrobalance tests suggested that high calcination temperatures decrease the activity of sodium bicarbonate Grade 1 (SBC No.1) during subsequent carbonation cycles, but there is little or no progressive decrease in activity in successive cycles. SBC No.1 appears to be more active than SBC No.3. As expected, the presence of SO{sub 2} in simulated flue gas results in a progressive loss of sorbent capacity with increasing cycles. This is most likely due to an irreversible reaction to produce Na{sub 2}SO{sub 3}. This compound appears to be stable at calcination temperatures as high as 200 C. Tests of 40% supported potassium carbonate sorbent and plain support material suggest that some of the activity observed in tests of the supported sorbent may be due to adsorption by the support material rather than to carbonation of the sorbent

Carbon Dioxide Capture from Flue Gas Using Dry Regenerable Sorbents

Laboratory studies were conducted to investigate dry, regenerable, alkali carbonate-based sorbents for the capture of CO{sub 2} from power plant flue gas. Electrobalance, fixed-bed and fluid-bed reactors were used to examine both the CO{sub 2} capture and sorbent regeneration phases of the process. Sodium carbonate-based sorbents (calcined sodium bicarbonate and calcined trona) were the primary focus of the testing. Supported sodium carbonate and potassium carbonate sorbents were also tested. Sodium carbonate reacts with CO{sub 2} and water vapor contained in flue gas at temperatures between 60 and 80 C to form sodium bicarbonate, or an intermediate salt (Wegscheider's salt). Thermal regeneration of this sorbent produces an off-gas containing equal molar quantities of CO{sub 2} and H{sub 2}O. The low temperature range in which the carbonation reaction takes place is suited to treatment of coal-derived flue gases following wet flue gas desulfurization processes, but limits the concentration of water vapor which is an essential reactant in the carbonation reaction. Sorbent regeneration in an atmosphere of CO{sub 2} and water vapor can be carried out at a temperature of 160 C or higher. Pure CO{sub 2} suitable for use or sequestration is available after condensation of the H{sub 2}O. Flue gas contaminants such as SO{sub 2} react irreversibly with the sorbent so that upstream desulfurization will be required when sulfur-containing fossil fuels are used. Approximately 90% CO{sub 2} capture from a simulated flue gas was achieved during the early stages of fixed-bed reactor tests using a nominal carbonation temperature of 60 C. Effectively complete sorbent carbonation is possible when the fixed-bed test is carried out to completion. No decrease in sorbent activity was noted in a 15-cycle test using the above carbonation conditions coupled with regeneration in pure CO{sub 2} at 160 C. Fluidized-bed reactor tests of up to five cycles were conducted. Carbonation of sodium carbonate in these tests is initially very rapid and high degrees of removal are possible. The exothermic nature of the carbonation reaction resulted in a rise in bed temperature and subsequent decline in removal rate. Good temperature control, possibly through addition of supplemental water and evaporative cooling, appears to be the key to getting consistent carbon dioxide removal in a full-scale reactor system. The tendency of the alkali carbonate sorbents to cake on contact with liquid water complicates laboratory investigations as well as the design of larger scale systems. Also their low attrition resistance appears unsuitable for their use in dilute-phase transport reactor systems. Sodium and potassium carbonate have been incorporated in ceramic supports to obtain greater surface area and attrition resistance, using a laboratory spray dryer. The caking tendency is reduced and attrition resistance increased by supporting the sorbent. Supported sorbents with loading of up to 40 wt% sodium and potassium carbonate have been prepared and tested. These materials may improve the feasibility of large-scale CO{sub 2} capture systems based on short residence time dilute-phase transport reactor systems

Durable Zinc Oxide-Based Regenerable Sorbents for Desulfurization of Syngas in a Fixed-Bed Reactor

A fixed-bed regenerable desulfurization sorbent, identified as RVS-land developed by researchers at the U.S. Department of Energy's National Energy Technology Laboratory, was awarded the R&D 100 award in 2000 and is currently offered as a commercial product by Sued-Chemie Inc. An extensive testing program for this sorbent was undertaken which included tests at a wide range of temperatures, pressures and gas compositions both simulated and generated in an actual gasifier for sulfidation and regeneration. This testing has demonstrated that during these desulfurization tests, the RVS-1 sorbent maintained an effluent H2S concentration of <5 ppmv at temperatures from 260 to 600 C (500-1100 F) and pressures of 203-2026 kPa(2 to 20 atm) with a feed containing 1.2 vol% H{sub 2}S. The types of syngas tested ranged from an oxygen-blown Texaco gasifier to biomass-generated syngas. The RVS-1 sorbent has high crush strength and attrition resistance, which, unlike past sorbent formulations, does not decrease with extended testing at actual at operating conditions. The sulfur capacity of the sorbent is roughly 17 to 20 wt.% and also remains constant during extended testing (>25 cycles). In addition to H{sub 2}S, the RVS-1 sorbent has also demonstrated the ability to remove dimethyl sulfide and carbonyl sulfide from syngas. During regeneration, the RVS-1 sorbent has been regenerated with dilute oxygen streams (1 to 7 vol% O{sub 2}) at temperatures as low as 370 C (700 F) and pressures of 304-709 kPa(3 to 7 atm). Although regeneration can be initiated at 370 C (700 F), regeneration temperatures in excess of 538 C (1000 F) were found to be optimal. The presence of steam, carbon dioxide or sulfur dioxide (up to 6 vol%) did not have any visible effect on regeneration or sorbent performance during either sulfidation or regeneration. A number of commercial tests involving RVS-1 have been either conducted or are planned in the near future. The RVS-1 sorbent has been tested by Epyx, Aspen Systems and McDermott Technology (MTI), Inc for desulfurization of syngas produced by reforming of hydrocarbon liquid feedstocks for fuel cell applications. The RVS-1 sorbent was selected by MTI over other candidate sorbents for demonstration testing in their 500-kW ship service fuel cell program. It was also possible to obtain sulfur levels in the ppbv range with the modified RVS-1 sorbent

Enhanced durability of high-temperature desulfurization sorbents for moving-bed applications

Chemical reactivity was determined at GECRD by measuring sorbent sulfur loading (defined as grams of sulfur absorbed per 100 g of fresh sorbent) in fresh and in cycled samples from a bench-scale reactor. Only formulations that exhibited a good balance of chemical and mechanical performance as fresh pellets were selected for further cyclic testing in the benchscale reactor system. Details of the bench-scale reactor and procedures have been given before (Ayala, 1991). The important aspect of the benchscale testing is that both absorption and regeneration were conducted in a packed-bed reactor simulating the time/temperature environment to which the sorbent would be exposed in a typical cycle of the full-scale moving-bed system. Absorption was carried out at 1000[degrees]F using any of three gas compositions, all having a deliberately high H[sub 2]S concentration (1 %) to accelerate testing. The oxidative regeneration was carried out between 1000 and 1250[degrees]F and 1--21% oxygen during the early phases of regeneration, and at 1400[degrees]F during the final phase simulating the temperature rise of the sorbent bed. Sixteen zinc titanate formulations were prepared as cylindrical extrudates. For all formulations, the calcination time was held constant at 2 hours. The following results were obtained: Formulations containing a 0.8 Zn:Ti ratio produced mixtures of several stoichiometric titanates: Zn[sub 2]Ti[sub 3]O[sub 8], ZnTiO[sub 3], and Zn[sub 2]TiO[sub 4], with the relative amount of each depending on temperature. Formulations containing a 2.0 Zn:Ti ratio exhibited exclusively the Zn[sub 2]TiO[sub 4] structure. The higher calcination temperature of 1800[degrees]F significantly reduced the porosity available for chemical reactivity, while the lower calcination temperature of 1400[degrees]F produced, in some cases, formulations with traces of residual unreacted zinc oxide and anatase titanium dioxide

An attrition-resistant zinc titanate sorbent for sulfur; [Quarterly] report, September 1--November 30, 1993

In the continuing search for good sorbent materials to remove sulfur from hot, coal-derived gases, zinc titanate sorbents have shown great promise. The objective of this project is to extend the effort started last year on increasing the strength and durability of zinc titanate sorbents with little or no loss in chemical reactivity. The principle is to contain Zn{sub 2}TiO{sub 4} in a structural matrix of excess TiO{sub 2}. A fluidized bed test was completed this quarter on the first of two promising sorbent formulations. The chemical reactivity in the form of breakthrough curves became progressively better over ten sulfidation-regeneration cycles. While the chemical reactivity was very good, the attrition resistance was only fair, due presumably to the conversion of Zn{sub 2}TiO{sub 4} to ZnTiO{sub 3} during cycling. The next formulation to be tested is expected to be better in attrition resistance