EERC pilot-scale CFBC evaluation facility Project CFB test results

Project CFB was initiated at the University of North Dakota Energy and Environmental Research Center (EERC) in May 1988. Specific goals of the project were to (1) construct a circulating fluidized-bed combustor (CFBC) facility representative of the major boiler vendors' designs with the capability of producing scalable data, (2) develop a database for use in making future evaluations of CFBC technology, and (3) provide a facility for evaluating fuels, free of vendor bias for use in the - energy industry. Five coals were test-burned in the 1-MWth unit: North Dakota and Asian lignites, a Wyoming subbituminous, and Colorado and Pennsylvania bituminous coats. A total of 54 steady-state test periods were conducted, with the key test parameters being the average combustor temperature, excess air, superficial gas velocity, calcium-to-sulfur molar ratio, and the primary air-to-secondary air split. The capture for a coal fired in a CFBC is primarily dependent upon the total alkali-to-sulfur ratio. The required alkali-to ratio for 90% sulfur retention ranged from 1.4 to 4.9, depending upon coal type. While an alkali-to-ratio of 4.9 was required to meet 90% sulfur retention for the Salt Creek coal versus 1.4 for the Asian lignite, the total amount of sorbent addition required is much less for the Salt Creek coal, 4.2 pound sorbent per million Btu coal input, versus 62 pound/million Btu for the Asian lignite. The bituminous coals tested show optimal capture at combustor temperatures of approximately 1550[degree]F, with low-rank coals having optimal sulfur capture approximately 100[degree]F lower

Subtask 3.12 - Oxygen-Fired CFBC

Traditionally, air is used as the source of oxygen for firing a combustion system. A fully oxygen-fired circulating fluidized-bed combustion (CFBC) system would result in the production of a flue gas stream consisting of mostly carbon dioxide and water. The concentrated carbon dioxide stream would be available for sequestering or other purposes. Temperatures in an oxygen-blown CFBC system would be controlled by a combination of flue gas recycle, solids recirculation, and by appropriately sizing and locating the amount of heat-transfer surface required. Flue gas recycle provides the additional gas required for adequate fluidization and circulation of solids replacing the nitrogen that would be present in an air-blown system. The amount of flue gas recycle will determine how much of the remaining heat from the coal combustion will have to be removed. If the amount of flue gas recycle required by increasing solids recirculation and oxygen staging is limited, introducing the pure oxygen at multiple locations in the combustor to result in a more even temperature profile should result in a more compact system, thus reducing initial capital costs for construction. The overall efficiency of the process should be greater than that of an air-blown system since less fuel is required for the creation of the same amount of energy. The Energy & Environmental Research Center (EERC) is in a unique position to advance this technology. It has a world-class CFBC pilot plant, has experience with firing a wide range of fuels in our air-fired CFBC pilot plant, has prior experience with oxygen-firing a slagging furnace system in a pulverized coal-fired mode with a bituminous coal, and has all of the components required for oxygen-firing right next to the CFBC pilot plant already in place. An engineering study was performed to identify methods, an overall appropriate configuration, and an operating strategy for a fully oxygen-fired CFBC pilot plant by: (1) developing a plan to optimize the amount of flue gas recycle required to control bed temperature; (2) determining how to best utilize the heat-transfer surfaces in a CFBC pilot plant, such as increased external bed surface, to more beneficially use increased solids recycle; (3) identifying appropriate methods for staged addition of the oxygen/recycled flue gas streams to maintain a good temperature profile in the combustor; and (4) determining the level of effort required to convert the EERC CFBC pilot plant to one that is oxygen-blown. The EERC additionally used its experience to consider what would be required for firing a wide range of fuels from biomass to low-rank coals to high-rank coals and petroleum cokes. These proposed modifications will be performed based upon obtaining the required funding

JV Task 108 - Circulating Fluidized-Bed Combustion and Combustion Testing of Turkish Tufanbeyli Coal

Two combustion tests were performed at the Energy & Environmental Research Center (EERC) using Tufanbeyli coal from Turkey. The tests were performed in a circulating fluidized-bed combustor (CFBC) and a pulverized coal-fired furnace, referred to as the combustion test facility (CTF). One of the goals of the project was to determine the type of furnace best suited to this coal. The coal is high in moisture, ash, and sulfur and has a low heating value. Both the moisture and the sulfur proved problematic for the CTF tests. The fuel had to be dried to less than 37% moisture before it could be pulverized and further dried to about 25% moisture to allow more uniform feeding into the combustor. During some tests, water was injected into the furnace to simulate the level of flue gas moisture had the fuel been fed without drying. A spray dryer was used downstream of the baghouse to remove sufficient sulfur to meet the EERC emission standards permitted by the North Dakota Department of Health. In addition to a test matrix varying excess air, burner swirl, and load, two longer-term tests were performed to evaluate the fouling potential of the coal at two different temperatures. At the lower temperature (1051 C), very little ash was deposited on the probes, but deposition did occur on the walls upstream of the probe bank, forcing an early end to the test after 2 hours and 40 minutes of testing. At the higher temperature (1116 C), ash deposition on the probes was significant, resulting in termination of the test after only 40 minutes. The same coal was burned in the CFBC, but because the CFBC uses a larger size of material, it was able to feed this coal at a higher moisture content (average of 40.1%) compared to the CTF (ranging from 24.2% to 26.9%). Sulfur control was achieved with the addition of limestone to the bed, although the high calcium-to-sulfur rate required to reduce SO{sub 2} emissions resulted in heat loss (through limestone calcination) and additional ash handling. A more efficient downstream sulfur scrubber capable of operation at a much lower Ca/S ratio would result in significantly higher boiler efficiency for this coal. At the operating temperature of a typical CFBC, bed agglomeration and convective pass fouling are not likely to be significant problems with this fuel. Compared to pulverized coal-firing, CFBC technology is clearly the better choice for this fuel. It provides more efficient sulfur capture, lower NO{sub x} emissions, better solids-handling capability, and can utilize a wetter feedstock, requiring less crushing and sizing. The lower operating temperature of CFBC boilers (820 C) reduces the risk of fouling and agglomeration. Care must be taken to minimize heat loss in the system to accommodate the low heating value of the coal

JV Task - 129 Advanced Conversion Test - Bulgarian Lignite

The objectives of this Energy & Environmental Research Center (EERC) project were to evaluate Bulgarian lignite performance under both fluid-bed combustion and gasification conditions and provide a recommendation as to which technology would be the most technically feasible for the particular feedstock and also identify any potential operating issues (such as bed agglomeration, etc.) that may limit the applicability of a potential coal conversion technology. Gasification tests were run at the EERC in the 100-400-kg/hr transport reactor development unit (TRDU) on a 50-tonne sample of lignite supplied by the Bulgarian Lignite Power Project. The quality of the test sample was inferior to any coal previously tested in this unit, containing 50% ash at 26.7% moisture and having a higher heating value of 5043 kJ/kg after partial drying in preparation for testing. The tentative conclusion reached on the basis of tests in the TRDU is that oxygen-blown gasification of this high-ash Bulgarian lignite sample using the Kellogg, Brown, and Root (KBR) transport gasifier technology would not provide a syngas suitable for directly firing a gas turbine. After correcting for test conditions specific to the pilot-scale TRDU, including an unavoidably high heat loss and nitrogen dilution by transport air, the best-case heating value for oxygen-blown operation was estimated to be 3316 kJ/m{sup 3} for a commercial KRB transport gasifier. This heating value is about 80% of the minimum required for firing a gas turbine. Removing 50% of the carbon dioxide from the syngas would increase the heating value to 4583 kJ/m{sup 3}, i.e., to about 110% of the minimum requirement, and 95% removal would provide a heating value of 7080 kJ/m{sup 3}. Supplemental firing of natural gas would also allow the integrated gasification combined cycle (IGCC) technology to be utilized without having to remove CO{sub 2}. If removal of all nitrogen from the input gas streams such as the coal transport air were achieved, a heating value very close to that needed to fire a gas turbine would be achieved; however, some operational issues associated with utilizing recycled syngas or carbon dioxide as the transport gas would also have to be resolved. Use of a coal with a quality similar to the core samples provided earlier in the test program would also improve the gasifier performance. Low cold-gas efficiencies on the order of 20% calculated for oxygen-blown tests resulted in part from specific difficulties experienced in trying to operate the pilot-scale TRDU on this very high-ash lignite. These low levels of efficiency are not believed to be representative of what could be achieved in a commercial KRB transport gasifier. Combustion tests were also performed in the EERC's circulating fluidized-bed combustor (CFBC) to evaluate this alternative technology for use of this fuel. It was demonstrated that this fuel does have sufficient heating value to sustain combustion, even without coal drying; however, it will be challenging to economically extract sufficient energy for the generation of steam for electrical generation. The boiler efficiency for the dried coal was 73.5% at 85% sulfur capture (21.4% moisture) compared to 55.3% at 85% sulfur capture (40% moisture). Improved boiler efficiencies for this coal will be possible operating a system more specifically designed to maximize heat extraction from the ash streams for this high-ash fuel. Drying of the coal to approximately 25% moisture probably would be recommended for either power system. Fuel moisture also has a large impact on fuel feedability. Pressurized gasifiers generally like drier fuels than systems operating at ambient pressures. The commercially recommended feedstock moisture for a pressurized transport reactor gasifier is 25% moisture. Maximum moisture content for a CFB system could be approximately 40% moisture as has been demonstrated on the Alstom CFB operating on Mississippi lignite. A preliminary economic evaluation for CO{sub 2} was performed on the alternatives of (1) precombustion separation of CO{sub 2} in an IGCC using the KBR transport gasifier and (2) postcombustion CO{sub 2} capture using a CFBC. It appears that the capture of CO{sub 2} from the high-pressure IGCC precombustion system would be less costly than from the low-pressure postcombustion CFBC system by a factor of 1.5, although the cost difference is not directly comparable because of the model input being limited to a higher coal quality than the Bulgarian lignite. While the decision to pursue precombustion removal of carbon dioxide has been technically proven with the Rectisol{reg_sign} and Selexol{trademark} processes, General Electric and Siemens have not sold any gas turbine systems running on the high-hydrogen syngas. They have successfully demonstrated a gas turbine on syngases containing up to 95% hydrogen. The technological hurdles should not be too difficult given this experience in the gas turbine industry

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