Tohru Hisamatsu

Structural Design and High-Pressure Test of a Ceramic Combustor for 1500°C Class Industrial Gas Turbine

The development of a high-temperature gas turbine is being carried out to improve the thermal efficiency on IGCC (Integrated coal Gasification Combined Cycle power plant), which is expected to be the thermal power plant of the future. A ceramic combustor for a 1,500 C, 20 MW class industrial gas turbine was developed and tested. This combustor has a hybrid ceramic/metal structure. To improve the durability of the combustor, the ceramic parts were made of silicon carbide (SiC), which has excellent oxidation resistance under high-temperature conditions as compared to silicon nitride (Si{sub 3}N{sub 4}), although the fracture toughness of SiC is lower than that of Si{sub 3}N{sub 4}. Structural improvements to allow the use of materials with low fracture toughness were made to the fastening structure of the ceramic parts. Also, the combustion design of the combustor was improved. Combustor tests using low-Btu gaseous fuel of a composition that simulated coal gas were carried out under high pressure. The test results demonstrated that the structural improvements were effective because the ceramic parts exhibited no damage even in the fuel cutoff tests from rated load conditions. It also indicated that the combustion efficiency was almost 100% even under part-load conditions.

Recession Rate Prediction for Ceramic Materials in Combustion Gas Flow

The influence of various basic factors of combustion gas flow conditions on the recession rate of silicon nitride, silicon carbide, and alumina has been experimentally clarified, and the recession rate equation is deduced using the dependence of influential factors on the recession rate and the mass transfer theory. The exposure tests are performed under various gas flow conditions (T = 1100–1500 °C, P = 0.3–0.7 MPa, V = 40–250 m/s, PH20 = 30–120 kPa, PO2 = 20–45 kPa). Recession rates mainly depend on water-vapor partial pressure, pressure, gas temperature, and Reynolds number in the gas flow conditions inside the specimen holder. The dependent on oxygen partial pressure is extremely low for silicon nitride and silicon carbide. The recession rates of silicon nitride, silicon carbide, and alumina in combustion gas flow are expressed in the form exp(−E/RT)·(PH20 )n · Re0–8 /P, and the predicted recession rates of silicon nitride shows good agreement with reported exposure test results under gas turbine conditions.Copyright

Oxidation Behavior of Ceramics for Gas Turbines in Combustion Gas Flow at 1500°C

In order to evaluate the durability of silicon-carbides (SiC) and silicon-nitrides (Si3N4), we studied the oxidation behavior of SiC and Si3N4 in 1500°C combustion gas flow. We found that the exposure to the combustion gas flow resulted in the weight losses of those ceramics due to the partial disappearance of the oxidized surface layer.We investigated the effects of sintering aids and high speed gas flow as possible factors for the disappearance of the oxide layer. Two kinds of SiC, without sintering aids and sintered with B4C, were used as test specimens. After the exposure to combustion gas flow conditions of 1500°C, 150m/s, 0.18MPa, the weight loss rate and thickness of the oxide layer were quite the same for each specimen of SiC. The existence of sintering aids did not have any effect on the disappearance of the oxide layer. To investigate the effect of gas flow, we set each specimen in a tube made of SiC to protect it from the gas flow. The tube had two holes each acting both as inlet and exhaust vents. Consequently, the oxide layer formed thickly. But at the spots on the specimen facing the holes, the oxide layer was thin. Hollows occurred on the specimen of SiC at these spots. It seems that the existence of gas flow is a very important factor in the disappearance of the oxide layer.Alumina (Al2O3) and zirconia (ZrO2) as oxide ceramics were exposed to the combustion gas flow. The weight of these also decreased. There is a possibility that the weight loss of ceramics in combustion gas flow is caused by degradation of oxide layer on their surface from erosion and hot corrosion due to some oxide scales coming from the test equipment.Copyright

Development of Low NOx Combustion Technology in Medium-Btu Fueled 1300°C-Class Gas Turbine Combustor in an Integrated Coal Gasification Combined Cycle

The development of integrated coal gasification combined cycle (IGCC) systems ensures higher thermal efficiency and environmentally sound options for supplying future coal utilizing power generation needs. The Japanese government and electric power industries in Japan promoted research and development of an IGCC system using an air-blown entrained-flow coal gasifier. On the other hand, Europe and the United States are now developing the oxygen-blown IGCC demonstration plants. Gasified coal fuel produced in an oxygen-blown entrained-flow coal gasifier has a calorific value of 8-13 MJ/m 3 which is only 1/5-1/2 that of natural gas. However, the flame temperature of medium-Btu gasified coal fuel is higher than that of natural gas and so NO x production from nitrogen fixation is expected to increase significantly. In the oxygen-blown IGCC, a surplus nitrogen produced in the air-separation unit (ASU) is premixed with gasified coal fuel (medium-Btu fuel) and injected into the combustor, to reduce thermal-NO x production and to recover the power used for the ASU. In this case, the power to compress nitrogen increases. Low NO x emission technology which is capable of decreasing the power to compress nitrogen is a significant advance in gas turbine development with an oxygen-blown IGCC system. Analyses confirmed that the thermal efficiency of the plant improved by approximately 0.3% (absolute) by means of nitrogen direct injection into the combustor, compared with a case where nitrogen is premixed with gasified coal fuel before injection into the combustor. In this study, based on the fundamental test results using a small diffusion burner and a model combustor, we designed the combustor in which the nitrogen injection nozzles arranged on the burner were combined with the lean combustion technique for low-NO x emission. In this way, we could reduce the high-temperature region, where originated the thermal-NO x production, near the burner positively. And then, a combustor with a swirling nitrogen injection function used for a gas turbine, was designed and constructed, and its performance was evaluated under pressurized conditions of actual operations using a simulated gasified coal fuel. From the combustion test results, the thermal-NO x emission decreased under 11 ppm (corrected at 16% O 2 ), combustion efficiency was higher than 99.9% at any gas turbine load. Moreover, there was different effects of pressure on thermal-NO x . emission in medium-Btu fuel fired combustor from the case of a natural gas fired combustor.

Strength Design and Reliability Evaluation of a Hybrid Ceramic Stator Vane for Industrial Gas Turbines

Employing ceramic materials for the critical components of industrial gas turbines is anticipated to improve the thermal efficiency of power plants. The authors developed a first-stage stator vane for a 1,300 C class, 20-MW industrial gas turbine. This stator vane has a hybrid ceramic/metal structure, to increase the strength reliability of brittle ceramic parts, and to reduce the amount of cooling air needed for metal parts as well. The strength design results of a ceramic main part are described. Strength reliability evaluation results are also provided based on a cascade test using combustion gas under actual gas turbine running conditions.

Exposure Test Results of Lu2Si2O7 in Combustion Gas Flow at High Temperature and High Speed

In order to understand recession behavior and the amount of recession of Lu2 Si2 O7 in the combustion gas flow, sintered Lu2 Si2 O7 specimens were manufactured by hot pressing and exposed under various combustion gas flow conditions (T = 1300–1500 °C, P = 0.3 MPa, V = 150 m/s, PH2O = 27–69 kPa, t = 10h). After the exposure tests, etch pits, which are assumed to form due to volatilization of SiO2 in the grain boundary phase, were observed at the surface of specimen. The amount of Lu2 SiO5 phase at the surface of specimen increased with the increase of gas temperature or water vapor partial pressure. A corresponding decrease in the amount of Lu2 Si2 O7 phase was observed. Furthermore, by using the average weight loss rate for exposure times of ten hours, the influence of gas temperature and water vapor partial pressure on weight loss rate was examined, and the amount of recession under gas turbine conditions was calculated.Copyright

Development of Silicon Nitride Components for Gas Turbine

Silicon nitride is one of the most practical candidates for ceramic gas turbines. The SN282 is silicon nitride material developed by Kyocera for gas turbines. Several new technologies have been developed to achieve materialization of ceramic gas turbines, such as material, fabrication process, evaluation / analysis technology. Recent technology is focused on recession of silicon-based ceramics under combustion gas. Environmental Barrier Coatings (EBCs) are developed to suppress these recession. We have found rare-earth element silicate and yttrium stabilized zirconium oxide (YSZ) have high corrosion resistance to the combustion gas. These materials were applied to the ceramic gas turbine components. The components with EBCs were evaluated in the actual engine tests. We have confirmed that the EBCs effectively work for the recession resistance.

Degradation of Silicon Carbide in Combustion Gas Flow at High-Temperature and Speed

Effects of various basic factors of combustion gas flow conditions on degradation behaviors of silicon carbide have been experimentally determined. The exposure tests were performed for widely varied experimental parameters of the gas temperatures (T = 900–1500°C), pressure (P = 0.3–0.8MPa), gas flow rate (V = 50–250m/s), water vapor partial pressure (PH2O = 32–82kPa) and oxygen partial pressure (PO2 = 24–44kPa). Degradation behaviors of silicon carbide were expressed as the weight loss of the substrate. The weight loss rate depends on the water vapor partial pressure remarkably. The effect of the oxygen partial pressure on the weight loss was smaller than that of the water vapor partial pressure, and the weight loss decreased with the increase of the oxygen partial pressure. Considering the effects of partial pressures of oxygen and water vapor, the gas temperature and the pressure didn’t have much effect on the weight loss. The weight loss depends on the gas flow rate, the increase rate of the weight loss for the gas flow rate becomes small with the gas flow rate. Consequently, the water vapor partial pressure, the oxygen partial pressure, the gas temperature, the pressure and the gas flow rate dependence of the weight loss rate is expressed as PH2O1.9 V0.6 P0.3 / PO20.6.Copyright

A Study of Low NOx Combustion in Medium-Btu Fueled 1300 °C-Class Gas Turbine Combustor in IGCC

The development of integrated coal gasification combined cycle (IGCC) systems ensures cost-effective and environmentally sound options for supplying future coal utilizing power generation needs. The Japanese government and the electric power industries in Japan promoted research and development of an IGCC system using an air-blown entrained-flow coal gasifier. We worked on developing a low-Btu fueled gas turbine combustor to improve the thermal efficiency of the IGCC by raising the inlet-gas temperature of gas turbine.On the other hand, Europe and the United States are now developing the oxygen-blown IGCC demonstration plants. Coal gasified fuel produced in an oxygen-blown entrained-flow coal gasifier, has a calorific value of 8.6MJ/m 3 which is one fifth that of natural gas. However, the adiabatic flame temperature of oxygen-blown medium-Btu coal gaseous fuel is higher than that of natural gas and so NOx production from nitrogen fixation is expected to increase significantly. In the oxygen-blown IGCC system, a surplus nitrogen in quantity is produced in the oxygen-production unit. When nitrogen premixed with coal gasified fuel is injected into the combustor, the power to compress nitrogen increases. A low NOx combustion technology which is capable of decreasing the power to compress nitrogen is a significant advance in gas turbine development with an oxygen-blown IGCC system. We have started to develop a low NOx combustion technology using medium-Btu coal gasified fuel produced in the oxygen-blown IGCC process.In this paper, the effect of nitrogen injected directly into the combustor on the thermal efficiency of the plant is discussed. A 1300 °C-class gas turbine combustor with a swirling nitrogen injection function designed with a stable and low NOx combustion technology was constructed and the performance of this combustor was evaluated under atmospheric pressure conditions. Analyses confirmed that the thermal efficiency of the plant improved by 0.2 percent (absolute), compared with a case where nitrogen is premixed with coal gasified fuel before injection into the combustor. Moreover, this new technique which injects nitrogen directly into the high temperature region in the combustor results in a significant reduction in NOx production from nitrogen fixation. We estimate that CO emission concentration decreases to a significant level under high pressure conditions, while CO emission concentration in contrast to NOx emission rises sharply with increases in quantity of nitrogen injected into the combustor.Copyright

Study of Medium-Btu Fueled Gas Turbine Combustion Technology for Reducing Both Fuel-NOx and Thermal-NOx Emissions in Oxygen-Blown IGCC

In order to improve the thermal efficiency of the oxygen-blown IGCC (Integrated Gasification Combined Cycle) for stricter environmental standards and cost-effective option, it is necessary to adopt the hot/dry gas cleaning system. In this system, the flame temperature of medium-btu gasified fuel is higher and so NOx production from nitrogen fixation is expected to increase significantly. Also the gasified fuel contains fuel nitrogen, such as ammonia, in the case of employing the hot/dry gas cleaning system. This ammonia is easily oxidized into fuel-NOx in the combustor. For contribution to the protection of the environment and low cost operations of all kinds of oxygen-blown IGCC, low NOx combustion technology for reducing both the fuel-NOx and thermal-NOx emission has to be developed. In this paper, we clarified effectiveness of applying both the two-stage combustion and the nitrogen injection, and the useful engineering guidelines for the low-NOx combustor design of oxygen-blown gasified, medium-btu fuels. Main results obtained are as follows: (1) Based on the fundamental combustion tests using the small diffusion burner, we clarified that equivalence ratio at the primary combustion zone has to be adjusted due to the fuel conditions, such as methane concentration, CO/H2 molar ratio, and calorific values of gasified fuels in the case of the two-stage combustion method for reducing fuel-NOx emission. (2) From the combustion tests of the medium-btu fueled combustor the two-stage combustion with nitrogen direct injection into the combustor results in reduction of NOx emission to 80ppm (corrected at 16% O2) or less, the conversion rate of ammonia to NOx was 35% under the gas turbine operational conditions for IGCC in the case where fuel contains 3% of methane and 2135ppm of ammonia. By means of nitrogen direct injection, the thermal efficiency of the plant improved by approximately 0.3 percent (absolute), compared with a case where nitrogen is premixed with gasified fuel. The CO emission concentration decreased drastically, as low as 20ppm, or combustion efficiency was kept higher than 99.9%. Furthermore, based on the fundamental combustion tests’ results, the ammonia conversion rate is expected to decrease to 16% and NOx emission to 26ppm in the case of gasified fuel that contains 0.1% methane and 500ppm of ammonia. From the above results, it is clarified that two-stage combustion method with nitrogen injection is very effective for reducing both the fuel-NOx and thermal-NOx emissions at once in IGCC and it shows the bright prospects for low NOx and stable combustion technology of the medium-btu fuel.© 2002 ASME