Modelling the Air-Cooled Gas Turbine: Part 1 — General Thermodynamics

This paper is Part I of a study concerned with developing a formal framework for modelling air-cooled gas turbine cycles and deals with basic thermodynamic issues. Such cycles involve gas mixtures with varying composition which must be modelled realistically. A possible approach is to define just two components, air and gas, the latter being the products of stoichiometric combustion of the fuel with air. If these components can be represented as ideal gases, the entropy increase due to compositional mixing, although a true exergy loss, can be ignored for the purpose of performance prediction. This provides considerable simplification. Consideration of three idealised simple cycles shows that the introduction of cooling with an associated thermal mixing loss does not necessarily result in a loss of cycle efficiency. This is no longer true when real gas properties and turbomachinery losses are included. The analysis clarifies the role of the cooling losses and shows the importance of assessing performance in the context of the complete cycle. There is a strong case for representing the cooling losses in terms of irreversible entropy production as this provides a formalised framework, clarifies the modelling difficulties and aids physical interpretation. Results are presented which show the effects on performance of varying cooling flowrates and cooling losses. A comparison between simple and reheat cycles highlights the rôle of the thermal mixing loss. Detailed modelling of the heat transfer and cooling losses is discussed in Part II of this paper.

Development of Dry Low-NOx Combustor for 300 kW Class Gas Turbine Applied to Co-Generation Systems

A 300 kWe class gas turbine which has a two-shaft and simple-cycle has been developed to apply to co-generation systems. The gas turbine engine is operated in the range of about 30% partial load to 100% load. The gas turbine combustor requires a wide range of stable operations and low NOx characteristics. A double staged lean premixed combustor, which has a primary combustion duct made of Si3N4 ceramics, was developed to meet NOx regulations of less than 80 ppm (corrected at 0% oxygen). The gas turbine with the combustor has demonstrated superior low-emission performance of around 40 ppm (corrected at 0% oxygen) of NOx, and more than 99.5% of combustion efficiency between 30% and 100% of engine load. Endurance testing has demonstrated stable high combustion performance over 3,000 hours in spite of a wide compressor inlet air temperature (CIT) range of 5 to 35 degree C.. While increasing the gas generator turbine speed, the flow rate of primary fuel was controlled to hold a constant equivalence ratio of around 0.5 in the CIT range of more than 15 C. The output power was also decreased while increasing the CIT, in order to keep a constant temperature at the turbine inlet. The NOx decreases in the CIT range of more than 15 C. On the other hand, the NOx increases in the CIT range of less than 15 C when the output power was kept a constant maximum power. As a result, NOx emission has a peak value of about 40 ppm at 15 C.

A Thermodynamic Analysis of Tubular SOFC Based Hybrid Systems

The goals of a research program recently completed at the University of California, Irvine were to develop analysis strategy for Solid Oxide Fuel Cell (SOFC) based systems, to apply the analysis strategy to tubular SOFC hybrid systems and to identify promising hybrid configurations. A pressurized tubular SOFC combined with an intercooled-reheat gas turbine (SureCell™ cycle) is chosen as the Base Cycle over which improvements are sought. The humid air turbine (HAT) cycle features are incorporated to the Base Cycle resulting in the SOFC-HAT hybrid cycle which shows an efficiency of 69.05% while the Base Cycle has an efficiency of 66.23%. Exergy analysis identified the superior efficiency performance of the SOFC component. Therefore, an additional cycle variation added a second SOFC component followed by a low pressure combustor in place of the reheat combustor of the gas turbine of the SOFC-HAT hybrid. The resulting Dual SOFC-HAT hybrid has a thermal efficiency of 75.98%. The Single SOFC-HAT hybrid gives the lowest cost of electricity (3.54¢/kW-hr) while the Dual SOFC-HAT hybrid has the highest cost of electricity (4.02¢/kW-hr) among the three cycles with natural gas priced at $3/GJ. The Dual SOFC-HAT hybrid plant cost is calculated to be significantly higher because the fraction of power produced by the SOFC(s) is significantly higher than that in the other cases on the basis of $1100/kw initial cost for the SOFC. The Dual SOFC-HAT hybrid can only be justified in favor of the Single SOFC-HAT hybrid when price of natural gas is greater than $14/GJ or if a severe carbon tax on the order of $180/ton of CO2 is imposed while natural gas price remains at $3/GJ.

Materials Compatibility With Pyrolysis Biofuel

The conversion of biomass such as wood and wood byproducts via pyrolysis into a liquid fuel is important in maximizing the use of material resources and in providing alternative and renewable sources of energy. Pyrolysis oils (or biofuels) have good combustion characteristics but are compositionally different from conventional diesel fuels. This difference requires that materials in contact with the biofuel be tested for compatibility. Three types of biofuels were tested for compatibility with a variety of polymeric materials and metal alloys. The test temperatures were set at 80°C to represent aggressive field usage conditions. The tests were conducted using coupons, which were fully immersed in the fluid for periods up to 15 days. These tests revealed that the metals 304L stainless steel, 316L stainless steel, 430 stainless steel and 20M04 stainless steels had corrosion rates of less than 0.007 mm/y and are suitable for use with the oils tested. A non-traditional low chromium alloy steel, MASH, was also examined and was found to be highly susceptible to all fuels at the high temperature tested and corroded at rates up to 3.7 mm/y. At room temperature, the alloy showed good resistance with a corrosion rate less than <0.009 mm/y. The polymeric materials showed a wide range of properties in the oils tested. Non-elastomers such as polytetrafluoroethylene, polypropylene and high-density polyethylene in general showed little swelling or staining in the oils. The elastomeric materials were much more susceptible to swelling, weight gain and change of surface properties. The attack on elastomeric materials was quite rapid with significant volume expansion seen within 24 hours. Viton, Buna-N and EPDM had volume changes up to 100% during a 10-day test and were not considered suitable seal materials for these oils. Multiple day tests for the low alloy steel at 80°C revealed that the corrosion attack was linear in nature leaving a corrosion scale, which slowed but did not prevent further attack. Details of the material degradation will be discussed.

Development of a Variable Fuel Placement Airblast Atomiser

This paper presents the progress made on the development of a dual spray, direct injection airblast fuel nozzle capable of variable fuel placement. It is anticipated that by varying the fuel placement within the confines of a combustion chamber it will be possible to control localised flame ‘Fuel Air Ratio’ and thus extend both stability and emissions performance in respect of engine power range. The extension of combustion stability is particularly desirable to high pressure, temperature and turndown ratio aero engines where the ratio between maximum and flight idle fuel flows is extreme. Atomiser aerodynamics have been developed that produce two different airflow re-circulating regions within the combustor. A concentric fuel filmer feeds each of these regions. By staging the fuel into each flame re-circulation zone the variation of local ‘Fuel Air Ratio’ can be more accurately controlled. A combination of bench testing and CFD has been used to analyse and manipulate airflow distribution between swirlers to form the two distinct flame regions. The work is ultimately concerned with the rationalisation of airflow distribution and fuel placement to best fit the operational envelope of the engine. The variable placement fuel injector features three or more air swirlers (inner swirler, middle swirler and dome swirler) and two ‘airblast’ fuel filmers (pilot and main). The paper describes the progress made with a number of fuel injector configurations.

Steam–Gas Condensing Turbine System for Power and Heat Generation

This study deals with new internal combustion turbine power systems in which a steam-gas mixture is the working medium. Heat is delivered to the system by injecting gaseous fuel and steam into the combustion chamber. Unlike in STIG systems, the fluid expansion in the turbine is much deeper (much below the atmospheric pressure) and the exhaust gas is cooled in a heat exchanger-condenser in such a manner that a significant amount of water can be recovered. The non-condensing gases (CO2 + N2 + rest of O2) from the exhaust fluid are compressed, after additional cooling, and discharged into the atmosphere. If a cheap or waste fuel is available, the steam to be injected into the combustor can be produced in a waste fuel-burning boiler or in conventional coal boiler. In this case the heat exchanger between the turbine and condenser can deliver significant amounts of useful (process or district) heat or / and preheated feedwater for the boiler. The efficiency analysis of this new energy system shows a growth by more than 10 percent points in comparison with the conventional STIG engine, at the same pressure ratio and turbine inlet temperature.

Thermodynamic Analysis of Hydrogen Combustion Turbine Cycles

The “International Clean Energy System Technology Utilizing Hydrogen (World Energy Network)”: WE-NET is a research program directed at the development of the technologies needed build a hydrogen-based energy conversion system. It proposes to set up a world energy network to convert renewable energy, such as hydropower and solar energy, into a secondary and transportable form to supply the demand centers, and to make possible the utilization of existing power generation, transportation, town gas, etc. Within the framework of this program Mitsubishi Heavy Industries, Hitachi and Westinghouse Power Corporation are working to develop an hydrogen-fueled combustion turbine system designed to meet the goals set by the WE-NET Program. The hydrogen–fueled power generation cycle will be able to satisfy the requirements of an efficiency based on the lower heating value higher than 70% and of reliability, availability and maintainability equivalent to current base-loaded natural gas-fired combined cycle. The use of hydrogen will eliminate emissions of CO2 and SOx and significantly reduce those of NOx. This paper presents a thermodynamic analysis of some concepts of hydrogen fuelled cycles which have been studied in the WE-NET program and makes a comparison of their performance.

Intermediate Temperature SOFC in Gas Turbine Cycles

Operational temperature around 800°C is desirable for solid oxide fuel cells (SOFC) due to alleviation of many serious problems, associated with high temperature, i.e., high degradation rate and cost of balance of plant components along with the need for expensive ceramic interconnect. This paper is concerned with the performance of hybrid cycles employing the intermedium temperature SOFC and a gas turbine. The calculations are performed with Aspen Plus® for a system in a size of 500 kW, using methane as fuel. The simulation tool is completed by a mathematical model of the fuel cell. Cell geometry is chosen to represent the type of cells developed at Risø National Laboratory. For the stand alone SOFC, introduction of the metallic interconnect gave an overall performance improvement. A maximum electric efficiency of more than 70% for the system was calculated at low pressure ratios.

Combustion Test Results of an Uncooled Combustor With Ceramic Matrix Composite Liner

A reverse-flow annular combustor with its casing diameter of 400 mm was developed using an uncooled liner made of three-dimensional-woven ceramic-matrix composite. The combustor was tested using the TRDI high-pressure combustor test facility at the combustor maximum inlet and exit temperature of 723K and 1623K respectively. Although both the material and combustion characteristics were evaluated in the test, this report focused on the combustion performance. As the results of the test, the high combustion efficiency and high heat release ratio of 99.9% and 1032 W/m3/Pa were obtained at the design point. The latter figure is approximately twice as high as that of existing reverse–flow annular combustors. Pattern factor was sufficiently low and was less than 0.1. Surface temperatures of the liner wall were confirmed to be higher than the limit of the combustor made of existing heat-resistant metallic materials.

Pre-Integrated Non-Equilibrium Combustion-Response Mapping for Gas-Turbine Emissions

In gas-turbine combustion the gas-dynamic and chemical-energy-release mechanisms have comparable time scales, so that equilibrium chemistry is inadequate for predicting species formation (emissions). In current practice either equilibrium chemical reactions are coupled with experimentally derived empirical equations, or time-consuming computations are used. Coupling non-equilibrium chemistry, fluid-dynamic, and initial- and boundary-condition equations results in large sets of numerically stiff equations; and their time integration demands enormous computational resources. The response modeling approach has been used successfully for large reaction sets. This paper makes two new contributions. First it shows how pre-integration of the heat-release maps eliminates the stiffness of the equations. This is a new modification to the response mapping approach, and it performs satisfactorily for non-diffusion systems. Second the theoretical framework is further extended to predict species formation in cases with diffusion, which is applicable to gas-turbine combustion systems and others. The methodology to implement this approach to reacting systems, and to gas turbine combustion, is presented. The benefits over other reaction-mapping techniques are discussed.