Brian S. Turk

Capture of carbon dioxide from flue gas using solid regenerable sorbents

Carbon dioxide emissions from the combustion of fossil fuels are a significant factor in global climate change. Large stationary sources such as coal-fired electric generating plants are likely to be the most cost-effective targets for carbon dioxide capture. At present, liquid amine-based scrubbing systems are the only processes available for this application. Processes based on regenerable solids that absorb carbon dioxide from flue gas and release it in concentrated form have the potential to be less expensive to operate. This paper summarises the results of studies conducted at RTI and Louisiana State University (LSU) to investigate the feasibility of using sodium or potassium carbonate as a sorbent. Upon reaction with carbon dioxide and water (also present in flue gas), this material is converted to sodium or potassium bicarbonate. Upon heating (ideally with low grade heat from the generating plant), carbon dioxide and water vapour are released and the solid carbonate can be reused. Work to date has focused on thermogravimetry (TG) and bench scale fluidised-bed testing, as well as characterisation of materials and thermodynamic and kinetic analyses. TG studies with sodium carbonate have indicated that the sorption reaction takes place rapidly at approximately 60°C and that the sorbent can be regenerated at temperatures less than 120°C. A five-cycle test conducted in a bench scale fluid bed reactor system indicated that the sorbent could be regenerated and reused. The process implications of compound salts and hydrates in the sodium carbonate system on the useful capacity of the sorbent and heat removal requirements were also investigated.

Carbon Dioxide Capture from Flue Gas Using Dry, Regenerable Sorbents

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.

Moving Granular Bed Filter Development Program

For coal-fired power plants utilizing a gas turbine, the removal of ash particles is necessary to protect the turbine and to meet emission standards. Advantages are also evident for a filter system that can remove other coal-derived contaminants such as alkali, halogens, and ammonia. With most particulates and other contaminants removed, erosion and corrosion of turbine materials, as well as deposition of particles within the turbine, are reduced to acceptable levels. The granular bed filter is suitable for this task in a pressurized gasification or combustion environment. The objective of the base contract was to develop conceptual designs of moving granular bed filter (GBF) and ceramic candle filter technologies for control of particles from integrated gasification combined cycle (IGCC), pressurized fluidized-bed combustion (PFBC), and direct coal-fueled turbine (DCFT) systems. The results of this study showed that the GBF design compared favorably with the candle filter. Three program options followed the base contract. The objective of Option I, Component Testing, was to identify and resolve technical issues regarding GBF development for IGCC and PFBC environments. This program was recently completed. The objective of Option II, Filter Proof Tests, is to test and evaluate the moving GBF system at a government-furnished hot-gas cleanup test facility. This facility is located at Southern Company Services (SCS), Inc., Wilsonville, Alabama. The objective of Option III, Multicontaminant Control Using a GBF, is to develop a chemically reactive filter material that will remove particulates plus one or more of the following coal-derived contaminants: alkali, halogens, and ammonia.

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.

Mechanistic study of CO formation from CO2 using a mixed-metal oxide of tin, iron, and aluminum

A mechanistic study has been performed to show that a reduced mixed metal oxide derived from tin, iron, and aluminum oxides can remove oxygen from carbon dioxide. Thermogravimetric analysis confirms that reduction of the mixed-metal oxide likely involves the reduction of SnO2and Fe2O3 phases. The reduced mixed-metal oxide can remove oxygen from carbon dioxide and this is shown using isotopically labelled C18O2 and mass spectroscopy. The 18O-labelled mixed-metal oxide can transfer the abstracted oxygen to a different carbonaceous compound, in this case carbon monoxide. Oxygen is readily exchanged in the mixed-metal oxide. Under both oxidizing and reducing conditions 18O is exchanged with unlabelled O resulting in the observation of all isotopomers.

CARBON DIOXIDE CAPTURE FROM FLUE GAS USING DRY REGENERABLE SORBENTS

Electrobalance studies of calcination and carbonation of sodium bicarbonate materials were conducted at Louisiana State University. Calcination in an inert atmosphere was rapid and complete at 120 C. Carbonation was temperature dependent, and both the initial rate and the extent of reaction were found to decrease as temperature was increased between 60 and 80 C. A fluidization test apparatus was constructed at RTI and two sodium bicarbonate materials were fluidized in dry nitrogen at 22 C. The bed was completely fluidized at between 9 and 11 in. of water pressure drop. Kinetic rate expression derivations and thermodynamic calculations were conducted at RTI. Based on literature data, a simple reaction rate expression, which is zero order in carbon dioxide and water, was found to provide the best fit against reciprocal temperature. Simulations based on process thermodynamics suggested that approximately 26 percent of the carbon dioxide in flue gas could be recovered using waste heat available at 240 C.