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Show Calcium-based sorbents such as calcium hydroxide (hydrated lime) will generally provide the most cost-effective approach for SNRB applications in the eastern United States. These sorbents require baghouse operating temperatures near 850·F for optimal SNRB SOx removal performance. Sodium-based sorbents such as sodium bicarbonate (NaHC03) may the preferred approach for the application of SNRB in the western U.S. where natural deposits of these materials occur. Baghouse operating temperatures in the range of 500· - 80CtF will be used for these systems. If maximum NOx removal performance (90+%) is needed, the lower operating temperature of the SNRB baghouse for sodium applications may result in the need to use a promoted SCR catalyst for NOx removal in these systems. The SOx-NOx-Rox Box process has several potential benefits: • One Major Component. Capital cost and space requirements are reduced by performing the SOx, NOx' and particulate removal operations in a single piece of equipment. • Longer Catalyst Life. SOx and particulates are removed from the flue gas stream upstream of the SCR catalyst, minimizing catalyst poisoning, pluggage, and erosion concerns. • Dry Materials Handling. Reagent preparation, handling, and disposal costs are minimized because both the fresh sorbent and waste streams are dry. • "Unpromoted Catalyst." The SCR catalyst used for calcium-based SNRB systems can be an unpromoted zeolite material. This avoids the potential hazardous waste disposal concerns associated with promoted catalysts containing metals such as vanadium. • No Flue Gas Reheat Required. NOx removal upstream of the boiler air heater eliminates the need for flue gas reheat for optimal NOx removal performance, as is required in many conventional SCR systems. • Increased Boiler Thermal Efficiency. The SNRB process is one of the few SOx removal processes offering the potential for a decrease in plant net heat rate due to the removal of S03 upstream of the air heater - virtually eliminating acid dew point concerns in the combustion air preheater. PROCESS DEVELOPMENT WORK AT B&W Development of the SNRB process began at B&W in 1979 when a series of laboratory screening tests was performed to evaluate the applicability of a variety of materials to the catalytic reduction of NOx' Materials such as fly ash, transition metals, and a Norton Company zeolite catalyst were evaluated. Development work then proceeded through a series of pilot-scale test programs conducted in baghouses ranging in size from 350 ft3/minute to 3000 ft3/ 3 minute. It was at this point that 3M's NextelTht ceramic fiber was identified as a potential material for fabrication of the high temperature bags. Nextel can be used on a continuous basis at temperatures up to 1400·F, with brief temperature excursions up to 2200·F. These in-house development programs eventually led to two OCDO-sponsored testing programs wherein the process concept was further refined, and preliminary performance data was obtained( 1). The successful early pilot test results were used to pursue funding under the DOE Innovative Clean Coal Technology Development Program. In 1989, a proposal was accepted by DOE and OCDO for demonstration of the technology on a larger, intermediate-scale pilot facility. Additional laboratory pilot testing under this program has been completed in support of the design of a 5 MW e demonstration facility<2,3). A preliminary economic analysis based on the laboratory pilot results has been completed to help focus the testing activity in the demonstration facility. Erection of the demonstration facility is nearing completion. The facility will be operated to assess the commercial readiness of the SNRB technology. LABORA TORY PILOT TEST RESULTS In the early test programs, it was not possible to evaluate a full-size, integrated bag/catalyst arrangemenl. The bags used in the 3000 ft3/minute baghouse were 4 inches in diameter and 10 feet long, whereas a commercial bag is more likely to be 6 inches in diameter and 20 feet long. The catalyst was also located in the exhaust plenum of the baghouse, as opposed to being integrated into each filter bag assembly. Since both of these design features could have a significant impact on bag c1eanability, the inability to assess these factors was a serious limitation of the early pilot test programs. Further, the effect on SOx removal perfonnance of sorbent injection into or upstream of the boiler economizer could not be fully evaluated. In a typical commercial, calcium- based SNRB application, the sorbent is injected ahead of, or into, the boiler economizer. While passing through the economizer the flue gases. and hence the sorbent, experience a sharp drop in temperature as heat is extracted. It was anticipated that the time-temperature history of the sorbent could have a significant impact on the SOx removal process, but the equipment used for the earlier tests did not lend itself to a comprehensive evaluation of these effects. Therefore. while these earlier tests provided encouraging results. it was concluded that additional SNRB laboratory pilot tests were necessary before the 5 MW e field demonstration facility could be properly designed. The primary objective of the most recent series of SNRB laboratory pilot tests was to develop design, operating, and performance specifications for the 5 MW e field demonstration facility. The major issues addressed included the: |
