COMBUSTION STUDIES OF NATURAL GAS AND SYN-GAS WITH HUMID AIR

Transcription

1 COMBUSTION STUDIES OF NATURAL GAS AND SYN-GAS WITH HUMID AIR Abstract Dr. Michael Nakhamkin Eric Swensen Hubert Paprotna Energy Storage and Power Consultants 200 Central Avenue Mountainside, New Jersey Paul Yankowich Nick Marchionna Textron Lycoming Turbine Engines 550 Main Street Stratford, Connecticut Dr. Arthur Cohn Electric Power Research Institute 3412 Hillview Avenue Palo Alto, California This paper presents the results of a humid air combustor study sponsored by the Electric Power Research Institute (EPRI) and conducted jointly by Energy Storage and Power Consultants (ESPC), Textron Lycoming, and Aero & Industrial Technology (AIT). The goal of this study was to determine the effects of high combustor inlet air/gas humidity on the combustion of methane and coal gasification fuel. The following key performance characteristics were recorded and analyzed over a range of inlet humidity levels: Flame stability. Combustion efficiency and emissions. Combustor liner wall temperatures. A primary objective of this investigation was to determine the maximum levels of humidification that can be achieved, while maintaining stable combustor operation and coincident high combustion efficiencies equivalent to those of "dry" combustors (equal to or greater than 99.9%). 1

2 Introduction In a simple cycle combustion turbine (CT) plant about half the power developed in the CT expander is used for driving the compressor, with only the remainder available for electric power generation to the grid. The replacement of a portion of the air entering the combustors with moisture would reduce the load of the compressor and results in an increase in power delivered to the grid. Among the advanced power generation concepts utilizing the humid air principle are the Humid Air Turbine (HAT) and Compressed Air Energy Storage with Humidification (CASH) concepts. In both HAT and CASH, natural gas or methane is burned in the combustors. When a coal "Gasification" plant is "Integrated" with a HAT or CASH plant, the concepts are renamed as IGHAT or IGCASH. These concepts, classified as humid air cycles, are identified by their use of a saturator which adds moisture to the motive air stream entering the combustors. For additional information regarding the humid air cycles the reader is encouraged to review the referenced publications. The levels of moisture entering the combustors in the thermodynamically and economically optimized humid air cycles, as shown in Table 1, are considerably higher than the levels presently occurring in conventional combustion turbine and CAES cycles. This causes a valid concern regarding combustion stability, efficiency and emissions of a combustor operating in the high moisture environment. Additionally, for the IGHAT and IGCASH cycles, the combustor design is affected by both the different combustion characteristics of the coal gas fuel (composed primarily of Co, H2 and C02) and high moisture levels in the air which should require a combustor design that significantly deviates from conventional gas turbine experience. Table I Humid Air Cycles Combustor Inlet Conditions Cycle HAT IGHAT CASH LP Comb CASH HP Comb IGCASH LP Comb IGCASH HP Comb Combustor Inlet Temperature, F Combustor Inlet ~350 ~ Pressure, psia Combustor Inlet Moisture, %H2O/dry air To address these concerns Textron Lycoming Turbine Engine Division, Textron, Inc. and Aero & Industrial Technology Ltd. (AIT), Combustion Technology Center, were subcontracted by ESPC to conduct a two-phase combustor test program with both methane and coal gasification fuel. AIT performed the humid air test with the methane fuel only and Textron Lycoming conducted the tests with both methane (a relatively complete program) and coal gasification fuel (limited tests). Due to similar test results, only the combustor tests conducted by Textron are presented in this paper. A more 2

3 detailed presentation of the humid air combustor study with the additional AIT tests are provided in Reference 1, to be published by EPRI by early Test Program The test conditions selected for the humid air combustor tests were chosen to simulate the combustor inlet conditions of the humid air cycles and to gain an understanding of the major parameters affecting key combustor operating characteristics. In addition to combustor operational parameters, the combustors physical characteristics were modified in order to arrive at potential future combustor designs. The scope of the test program was constrained by limited funds and laboratory capabilities. The first series of tests were aimed at simulating the methane fired combustor operation typical for the HAT or CASH concepts. The second abbreviated series of tests were aimed at simulating the coal gas fired combustor operation typical for the IGHAT or IGCASH concepts. These tests were conducted over a range of combustor inlet moisture levels. At each moisture level, the combustor exhaust emissions data were taken to determine the combustion efficiency and emissions. Major procedural considerations for the tests were as follows: Preliminary analytical studies set target H20/Air by determining the maximum theoretical moisture level attainable to maintain stable operation and achieve high combustion efficiencies. Initial investigations determined the importance of residence time on humid air combustion. The residence time was accounted for by the loading factor: Loading Parameter = 3.72 * W pz [pps] P [atm] * Vpz [ft3] * * T3 { K] W PZ = (W AIR + W STEAM )*(%PZ Flow) + W FUEL Vpz = Primary Zone Volume Test Rig and Facilities Textron Lycoming has modem high pressure test facilities dedicated to combustor research and development. The test facility capabilities were expanded to handle the specified test requirements. Schematics of the combustor rig and the combustor test configuration are provided in Figures 1 and 2. 3

4 Test Combustor Textron used its single injector combustor research rig for testing a standard five-inch diameter TF15 can combustor with a single fuel injector, see Figure 2. The liner is a derivative of the successful AGTI500 production combustor liner with over ten years of field experience in the Army MIAI main battle tank. The dual-fuel TF15 burner is designed for industrial applications with liquid/gaseous fuel capabilities and tested with methane and diesel fuels. Fuel supply Gaseous fuel was supplied from compressed gas trailers (2400 psia), rented and delivered to the Textron facility. Three multi-tube compressed gas trailers were required containing: Methane (CH4). Carbon monoxide (CO). Hydrogen (H2). In addition, a tank of liquid carbon dioxide (CO2) and a C02 pump and vaporizer were required for the coal gasification fuel testing. Simulation of Various Moisture Levels The high humidification levels desired in the combustor were simulated by injecting high pressure steam into the pressurized exhaust from the facility high pressure compressor system. The injection point was located over 200 feet upstream of the test combustor and mixing was promoted as the air and steam pass through a tubular waste heat exchanger and multi-element electric preheater. These units also function to preheat and control the inlet temperature to the test combustor. Results of the Humid Air Combustor Tests Fired on Methane Initial Combustor Design - CFG "A The first aero configuration tested was similar to the aero configuration of the Textron Lycoming TF15 combustor. Still some modifications were made to reflect the test specification. The dilution zone wall jets and most of the dilution zone wall cooling were blocked to eliminate the quenching effect of the air in the primary zone not directly involved in the combustion process, and to provide additional residence time for CO burnout. Figure 3 shows a cross-section of the Cfg "A" liner and zonal air distribution. The primary zone was defined as the volume between the fuel injector and the first row of wall jets. Test Results - CFG "A " Combustor Fired on Methane. Liner Cfg "A" was tested with methane fuel and dry air with gradually increasing the levels of humidification. Throughout the testing the combustor inlet temperature and pressure were set at inlet temperature (T3)=725 F, and inlet pressure (P3)=215 psia. 4

5 Cfg "A" encountered combustion instabilities above 15% H 2 O /Air (mass ratio) with blowout occurring at approximately 18%. The maximum percent moisture obtained with good stability and efficiency greater than 99.9% was only 13.5% H 2 O /Air (mass). Increased moisture levels could not be sustained with stable operation, so testing was suspended, and the data reduced and analyzed to assess these results. Conclusions of CFG "A" Combustor Testing. Test results for the baseline aero configuration (Cfg "A") burning methane indicated that the primary combustion zone was undersized and too heavily loaded to achieve the humidity levels in the HAT and CASH cycles (15 to 45% H 2 O /Air). Facility limitations, such as accuracy of flow measurement and low pressure drop across the liner, prevented running this configuration at the very low airflows required to further reduce the loading factor. ESPC and Textron therefore decided to modify the combustor aero configuration to reduce primary zone loading. Tests performed with the modified combustor design, identified as Cfg "B", are summarized below. Modified Combustor Aero Design - CFG "B" This modified aero configuration CFG "B" is shown in Figure 4. The modifications to the Cfg "A" combustor aero design included blocking the first row of wall jets and opening the dilution jets. The increased primary mixing zone volume and reduced mass flow provided more residence time to complete hydrocarbon reactions. The dilution air was added back in to reduce exhaust gas temperatures to protect the test instrumentation. Test Results - CFG "B" Combustor Fired on Methane Liner Cfg "B" tests were conducted at an inlet pressures of 200 to 215 psia, and at inlet temperatures of approximately 725 F, 840 F and 950 F. Figures 5 and 6, show the relationship between moisture levels, loading parameters and combustion efficiency with T3=840 F and 950 F, respectively. These figures supports the premise that reduced loading is a key to achieving high levels of humidification with good stability and high combustion efficiency. Combustor instabilities were encountered at T3=840 F with conditions approaching 30% H 2 O /Air, with blowout occurring at 34%. At 950 F, the unstable region is moved slightly to higher moisture levels. The maximum moisture obtained with good stability and combustion efficiency greater than 99.9% was 20%H 2 O/Air at T3=840 F and 25% at T3=950 F. The NOx emissions results are shown in Figure 7. This data shows the expected decrease in NOx emissions at higher H 2 O /Air levels. The graph also shows that an increase in inlet temperature increases NOx emissions. As indicated in Figure 8, CO emissions begin to increase rapidly above a certain level of % H 2 O /Air, and this limit was strongly influenced by loading. This graph also indicates 5

6 that an increase in H 2 O /Air or a decrease in inlet temperature will increase CO emissions. These trends are also seen in the HC emissions results shown in Figure 9. Conclusions of CFG "B" Combustor Testing. Cfg "B" was successful in obtaining the lower range of humidity levels typical for the HAT and CASH cycles (15 to 25% H 2 O /Air) with high efficiency and flame stability. To cover the full operating range for these cycles, ESPC and Textron made additional modifications to the Cfg "B" combustor and further tests were performed. The modified Cfg "B" combustor was designated as Cfg "C" and the test results are summarized below. Totally Front-end Mixed Aero Design - CFG "C" The third aero configuration tested with methane attempted to increase the primary zone to the maximum extent along with reducing the primary zone loading. All the wall jets upstream of the dilution zone were taped and all front-end mixing and stabilization was controlled by the swirling flow field. The dilution wall jets were relocated about three inches downstream of the original holes and enlarged to achieve the airflow split defined in Figure 10. The logic behind this approach was that the high water vapor content in the burning zone would reduce the peak gas temperature, resulting in slower CO reaction rates. Front loading the combustor with all the oxygen and increasing the residence time in the combustor were expected to increase the combustion efficiency at high % H 2 O /Air ratios. Test Results - CFG "C" Combustor Fired on Methane. Liner Cfg "C" tests were conducted at an inlet pressure of 200 psia, and at inlet temperatures (T3) of about 840 F and 950 F. The test data for Cfg "C" indicated good efficiency and stability up to 34% H 2 O /Air, but the operating range was very limited. Blowout occurred at about 40%. Cfg. "C" achieved the highest humidification levels of the three designs tested with methane of 28% H 2 O /Air at T3=950 F, 24% H 2 O /Air at T3=840 F, and 18% H 2 O /Air at T3=725 F with high efficiency and reasonable stability. Conclusions of CFG "C" Combustor Testing. The Cfg "C" values were in the mid range of humidity level for the HAT and CASH cycles, but still well below the maximum value of approximately 45% H 2 O /Air. The methane testing ended here and additional modifications to the combustor design were not investigated due to funding limitations. 6

7 Conclusions of the Humid Air Combustor Tests Fired on Methane The maximum % H 2 O /Air attainable was limited by the flammability characteristics of the fuel. Combustor stability and flammability (blowout) limits expand as inlet pressure and temperature is increased, and are a function of the % diluents (% humidity) added to the air. Increasing pressure, however, only affects the rich blowout limit. The combustor test study was successful in achieving the mid range of moisture levels of the HAT and CASH cycles with high efficiency and flame stability. These maximum levels of 28% H 2 O /Air at T3=950 F, 24% H 2 O /Air at T3=840 F, and 18% H 2 O /Air at T3=725 F These tests conducted were conducted on a very small test combustor, designed for dry air, with a high loading factor which adversely affected the combustion characteristics. The maximum H 2 O /Air levels obtained in this study could be further increased with additional testing utilizing the lessons learned from this study. Key combustor operational parameters and their effect on emissions and efficiency were determined. Combustion efficiency can be improved by: o Increasing combustor primary zone volume (residence time). o Increasing combustor inlet temperature. o Designing for a primary combustion zone stoichiometry equal to one (Ф PZ =1) to achieve the peak flame temperature in the combustor. While these factors were also significant to the production of NOx, the addition of moderate levels of humidity, 15% H 2 O /Air, substantially reduce NOx emissions. AIT achieved similar results by decreasing the combustor loading factor with a pilot low NOx combustor. The maximum moisture achieved by AIT was 27% H 2 O Air (mass) at an inlet temperature of 850 F. Results of the Humid Air Combustor Tests Fired on Coal Gasification Fuel Since the Cfg. "C" aero configuration achieved the highest stable humidification levels of the three designs tested with methane it was used for the coal gas with humidification combustor testing. The effective area of the fuel injector nozzle was increased for the coal gas fuel testing to compensate for the low-btu content of the fuel and the increase in volumetric flow. 7

8 Test Results - CFG "C" Combustor Fired on Coal Gasification Fuel Liner Cfg "C" was tested with a simulated coal gas fuel, CO=52.0%, H2=36.0%, C0 2 =12.0% by volume. Tests were conducted at an inlet pressure of 200 psia and at inlet temperatures of approximately 840 F and 950 F. Although the results from burning methane in this aero configuration were good, the coal gas fuel exhibited very different characteristics. The data suggests that the primary burning zone might have been fuel rich and poorly mixed, and that additional volume and residence time is required to burn this low-btu fuel. In terms of stability, the coal gas exhibited a superior range of stable combustion, up to 90% H 2 O /Air, noticeably better than methane. The combustor efficiencies at these high moisture levels, as indicated in Figure 11, were lower than desired. The mid range moisture levels, which are applicable to the IGHAT and IGCASH cycles, achieved higher efficiencies and can be further improve with additional modifications to the combustor design. Due to limited funding, additional tests with a modified combustor design was not conducted. Figure 12 shows the effect of increasing H 2 O /Air levels on NOx emissions for Cfg "C" fired on coal gasification fuel. The results show that significant reductions in NOx emissions were attainable (less than 3 ppm 15%O 2 ) at the higher moisture levels. The graph also shows a slight increase in NOx emissions as inlet temperature increases. Similarly as in the methane tests, CO emissions were seen to increase with increasing H 2 O /Air levels and decrease with increasing combustor inlet temperature as shown in Figure 13. Conclusions of the Humid Air Combustor Tests Fired on Coal Gasification Fuel Due to higher flammability limits for hydrogen and carbon monoxide, coal gas fuel burned with much higher percentages of inert than methane. High levels of humidification with stable combustion were achieved with coal gas fuel (stable combustion was achieved with up to 90% H 2 O /Air mass.) However, combustion efficiencies achieved for the aero configuration tested did not meet the expected goals. The combustion characteristics of the coal gas fuel are significantly different than methane, and require a different design approach for the combustor to achieve high combustion efficiency. Due to limited funding, further testing required to increase combustion efficiency was not performed. The results indicate that with a different combustor design the desired humidification levels in the IGHAT and IGCASH cycles should be attainable. 8

9 Recommendations for Future Testing Conduct additional testing to define the stability (flammability) limits at high temperature and pressure. Investigate methods to increase stability limits for methane (natural gas) i.e. mixing hydrogen with methane, Mixing hydrogen with methane may have the potential to expand the flammability limits, and improve stability at higher levels of humidity. Investigate alternate fuel injection and stabilization methods for maximizing useful combustor volume. Effective utilization of the available primary combustion zone volume was not realized with the coal gas fuel. Alternate methods include: o Fuel staging o Multiple air swirlers o Fuel/air premixing References 1. Combustion Studies of Natural Gas and Syn-Gas with Humid Air. Electric Power Research Institute, Palo Alto, CA: To be published TR Additional References 1. Nakhamkin, M. et. al., Technical and Economic Characteristics of Compressed Air Energy Storage Concepts with Air Humidification. presented at the Second International CAES Conference, San Francisco, CA. (July 1992). 2. The Compressed-Air Storage with Humidification (CASH) Coal Gasification Power Plant Electric Power Research Institute, Palo Alto, CA: November Report TR A Comparison of Humid Air Turbine (HAT) Cycle and Combined-Cycle Power Plants. Electric Power Research Institute, Palo Alto, CA: March Report IE Nakhamkin, M. et. al., Compressed Air Energy Storage: Survey of Advanced CAES Development." ASME Paper 91-JPGC-NE-26, presented at the International Power Generation Conference, (October 1991). 5. Nakhamkin, M. et. al., Compressed Air Energy Storage Thermal Performance Improvements." ASME Paper 93-JPGC-GT-2, presented at the International Joint Power Generation Conference, Kansas City, Missouri (October 1993). 6. Application of Air Saturation to Integrated Coal Gasification/CAES Power Plants. Electric Power Research Institute, Palo Alto, CA: March Internal report for Project RP

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