IMPROVED CORROSION RESISTANCE FOR ALUMINA REFRACTORY

In order to increase the efficiency of advanced coal-fired power systems, higher working fluid temperatures must be reached. Some system surfaces will have to be protected by covering them with corrosion-resistant refractories. Corrosion is the degradation of the material surfaces or grain boundaries by chemical reactions with melts, liquids, or gases causing loss of material and, consequently, a decrease in the strength of the structure. In order to develop methods of reducing corrosion, the microstructure that is attacked must be identified along with the mechanism and rates of attack. Earlier tests with several commercially available high-temperature castable refractories showed that the fused-alumina aggregate grains within the materials had the highest corrosion resistance of any of the castable materials. However, the cement holding the grains was easily attacked. Therefore, to improve the corrosion resistance and thermomechanical properties of alumina-based refractories, we attempted to change the cement to a more corrosion- and erosion-resistant bonding material through the addition of rare-earth oxides (REO). Phase diagrams were used to identify stable high-melting-temperature materials within the lanthanide-alumina series that could modify the bonding phase of the alumina-based refractory. Two mechanisms of reducing corrosion were investigated. One was the formation of corrosion-resistant layers within the refractory. The other was increased sintering to increase strength and seal continuous pores that would reduce slag penetration. Garnets (Re{sub 3}Al{sub 5}O{sub 12}) and perovskites (ReAl{sub 2}O{sub 3}), where Re is the REO, are two of the stable high-melting-temperature materials identified that were believed could be formed in the refractory matrix to help reduce corrosion rates. For the base refractory, Plicast 99 made by Plibrico was chosen. It is a 99% alumina castable composed of fused alumina aggregate and a cement made primarily from Alphabond 100, produced by Alcoa. The initial work involved designing a test matrix to study the effects of selected REOs on the corrosion resistance of the refractory. Three different processing methods were employed for fabricating the test samples. These included bulk mixing, impregnation, and surface coatings. Two different corrosion test methods were used to test the mixtures. The first was the static cup test that was used to screen the samples for the second corrosion test which used flowing slag. In addition to the corrosion tests, three-point modulus-of-rupture (MOR) tests were performed using the standard American Society for Testing and Materials (ASTM) C133 procedure to determine if the addition of an REO improved the strength of the refractory. A strength increase would show that the refractory was more resistant to erosion and also that sintering had occurred, which would imply a reduction in porosity

IMPROVED CORROSION RESISTANCE OF ALUMINA REFRACTORIES

The initial objective of this project was to do a literature search to define the problems of refractory selection in the metals and glass industries. The problems fall into three categories: Economic--What do the major problems cost the industries financially? Operational--How do the major problems affect production efficiency and impact the environment? and Scientific--What are the chemical and physical mechanisms that cause the problems to occur? This report presents a summary of these problems. It was used to determine the areas in which the EERC can provide the most assistance through bench-scale and laboratory testing. The final objective of this project was to design and build a bench-scale high-temperature controlled atmosphere dynamic corrosion application furnace (CADCAF). The furnace will be used to evaluate refractory test samples in the presence of flowing corrodents for extended periods, to temperatures of 1600 C under controlled atmospheres. Corrodents will include molten slag, steel, and glass. This test should prove useful for the glass and steel industries when faced with the decision of choosing the best refractory for flowing corrodent conditions

High-Temperature Heat Exchanger Testing in a Pilot-Scale Slagging Furnace System

The University of North Dakota Energy & Environmental Research Center (EERC), in partnership with United Technologies Research Center (UTRC) under a U.S. Department of Energy (DOE) contract, has designed, constructed, and operated a 3.0-million Btu/hr (3.2 x 10{sup 6} kJ/hr) slagging furnace system (SFS). Successful operation has demonstrated that the SFS meets design objectives and is well suited for testing very high-temperature heat exchanger concepts. Test results have shown that a high-temperature radiant air heater (RAH) panel designed and constructed by UTRC and used in the SFS can produce a 2000 F (1094 C) process air stream. To support the pilot-scale work, the EERC has also constructed laboratory- and bench-scale equipment which was used to determine the corrosion resistance of refractory and structural materials and develop methods to improve corrosion resistance. DOE projects that from 1995 to 2015, worldwide use of electricity will double to approach 20 trillion kilowatt hours. This growth comes during a time of concern over global warming, thought by many policy makers to be caused primarily by increases from coal-fired boilers in carbon dioxide (CO{sub 2}) emissions through the use of fossil fuels. Assuming limits on CO{sub 2} emissions from coal-fired boilers are imposed in the future, the most economical CO{sub 2} mitigation option may be efficiency improvements. Unless efficiency improvements are made in coal-fired power plants, utilities may be forced to turn to more expensive fuels or buy CO{sub 2} credits. One way to improve the efficiency of a coal-fired power plant is to use a combined cycle involving a typical steam cycle along with an indirectly fired turbine cycle using very high-temperature but low-pressure air as the working fluid. At the heart of an indirectly fired turbine combined-cycle power system are very high-temperature heat exchangers that can produce clean air at up to 2600 F (1427 C) and 250 psi (17 bar) to turn an aeroderivative turbine. The overall system design can be very similar to that of a typical pulverized coal-fired boiler system, except that ceramics and alloys are used to carry the very high-temperature air rather than steam. This design makes the combined-cycle system especially suitable as a boiler-repowering technology. With the use of a gas-fired duct heater, efficiencies of 55% can be achieved, leading to reductions in CO{sub 2} emissions of 40% as compared to today's coal-fired systems. On the basis of work completed to date, the high-temperature advanced furnace (HITAF) concept appears to offer a higher-efficiency technology option for coal-fired power generation systems than conventional pulverized coal firing. Concept analyses have demonstrated the ability to achieve program objectives for emissions (10% of New Source Performance Standards, i.e., 0.003 lb/MMBtu of particulate), efficiency (47%-55%), and cost of electricity (10%-25% below today's cost). Higher-efficiency technology options for new plants as well as repowering are important to the power generation industry in order to conserve valuable fossil fuel resources, reduce the quantity of pollutants (air and water) and solid wastes generated per MW, and reduce the cost of power production in a deregulated industry. Possibly more important than their potential application in a new high-temperature power system, the RAH panel and convective air heater tube bank are potential retrofit technology options for existing coal-fired boilers to improve plant efficiencies. Therefore, further development of these process air-based high-temperature heat exchangers and their potential for commercial application is directly applicable to the development of enabling technologies in support of the Vision 21 program objectives. The objective of the work documented in this report was to improve the performance of the UTRC high-temperature heat exchanger, demonstrate the fuel flexibility of the slagging combustor, and test methods for reducing corrosion of brick and castable refractory in such combustion environments. Specific technical issues of interest included measuring the effects of coatings on heat transfer in the RAH, determining the general impact of firing a lower-iron bituminous coal on the operation of the RAH panel and SFS, and the development of ways to treat slag and refractories to decrease corrosion rates