By Gerrill Griffith

 

GE’s 7HA gas turbine. Photo courtesy: GE Power

Inventors and scientists have been fascinated by the concept of turbine engines to create power even before Leonardo di Vinci sketched an early turbine idea in one of his famous notebooks. But, it has only been in the last 80 years that the electricity-generating potential of turbines has been realized. Now, with demands for energy rising along with calls for reduced greenhouse gas emissions, the need for cleaner more efficient next generation turbine technology is critical. With a robust research portfolio, productive partnerships, and a mandate to increase power-producing efficiencies and improve the environment for future generations, the National Energy Technology Laboratory (NETL) is shepherding innovations for improved gas turbine productivity.

The goals of turbine research at NETL and the U.S. Department of Energy (DOE) are to reduce the cost of electricity, reduce emissions, and increase turbine efficiencies. NETL’s Advanced Turbines Program addresses those goals by conducting its own intensive research while partnering with other researchers in the private sector to develop technologies that will accelerate turbine performance and efficiency beyond current state-of-the-art and reduce the risk to market for novel and advanced turbine-based power cycles. The Laboratory pursues a wide range of turbine research in five specific areas in pursuit of its goals.

Hydrogen Turbines

NETL conducts research under a DOE-sponsored a program for developing hydrogen-fueled gas turbine technology for coal-based integrated gasification combined cycle (IGCC) power generation to improve efficiency, reduce emissions, lower costs, and allow for carbon capture, utilization, and storage (CCUS). DOE expanded program applicability to industries such as refineries and steel mills. Recent funding has been used to facilitate a set of gas turbine technology advancements that will improve the efficiency, emissions, and cost performance of turbines with industrial CCUS. Efforts supporting industrial technology acceleration, application, and adaptation will also benefit the advanced hydrogen turbine development and existing machines in typical utility applications.

Turbine systems and components targeted for improvement include combustor technology, materials research, enhanced cooling technology, and coatings development. These technologies are considered key components of hydrogen turbines, which, along with other advanced energy system technologies, will combine to develop next generation of high efficiency coal-based power systems.

The Hydrogen Turbine technology area is showing that the U.S. can operate on coal-based hydrogen fuel power, increase combined cycle efficiency over the baseline, and reduce carbon dioxide and other emissions.

The Hydrogen Turbine Program is focused on further advancements to turbine technology to attain the ultimate performance targets for IGCC power plants with CCUS. NETL intends to demonstrate:

• Hydrogen-fueled turbines with 3-5 percentage improvement in combined cycle efficiency (total above baseline)
• Competitive cost of electricity for near-zero emission systems
• Hydrogen-fueled IGCC with 2 ppm NOx at the power plant exhaust

Advanced Combustion Turbines

The Advanced Combustion Turbines for combined cycle applications area is focused on components and combustion systems for advanced combustion turbines in combined cycle operation that can achieve greater than 65 percent combined cycle efficiency (LHV, natural gas benchmark) and support load following capabilities to meet the demand of a modern grid. To achieve this target, emphasis is placed on advanced turbine concepts that are fueled with natural gas and coal derived fuels, including hydrogen and syngas, and higher firing temperatures (3,100 F).

Component R&D is being conducted that will allow higher turbine inlet temperatures, manage cooling requirements, minimize leakage, advance compressor and expander aerodynamics, advance the performance of high-temperature load following combustion systems with low emissions of criteria pollutants including oxides of nitrogen (NOx), and overall lead to improved efficiency of the gas turbine machine in a combined cycle application. Projects in this topic area include research on pressure gain combustion systems, ceramic matrix composite components, and advanced turbine configurations for improved cooling and efficiency.

Pressure gain combustion (PGC) has the potential to significantly improve combined cycle performance when integrated with combustion gas turbines. While conventional gas turbine engines undergo steady, subsonic combustion, resulting in a total pressure loss, PGC uses multiple physical phenomena, including resonant pulsed combustion, constant volume combustion, or detonation, to affect a rise in effective pressure across the combustor while consuming the same amount of fuel as the constant pressure combustor. The methodology resulting in a pressure-gain across the combustor relies on the Humphrey (or Atkinson) cycle, and is seen to have great potential as a means of achieving higher efficiency in gas turbine power systems, potentially reaching 4-6 percent for simple cycle systems and 2-4 percent in combined cycle systems. Potential technical challenges include fuel injection, fuel and air mixing, backflow prevention, detonation initiation, wave directionality, maintaining a pressure gain, controlling emissions of NOx and CO, as well as unsteady heat transfer and cooling flow challenges resulting from integration with the turbine hot gas path expansion components.

The University Turbine Systems Research Program (UTSR)

NETL’s UTSR Program addresses scientific research to develop and transition advanced turbines and turbine-based systems that will operate cleanly and efficiently when fueled with coal-derived synthesis gas (syngas) and hydrogen fuels. This research focuses on the areas of combustion, aerodynamics/heat transfer, and materials.

UTSR also offers a Gas Turbine Industrial Fellowship program to recruit qualified university research students. This fellowship brings highly trained student researchers from the university to industrial gas turbine manufacturing environments. The UTSR Fellowship experience often results in the employment of highly trained professionals in the gas turbine industry working to continue the advancement of gas turbine technology.

The UTSR Program has evolved over time in response to power generation markets and DOE objectives. Evolution of objectives has involved a transition from turbines operating on natural gas to coal derived syngas to very high hydrogen fuels derived from syngas. This fuel flexibility will also allow gas turbines to be used in integrated gasification combined cycle (IGCC) applications that are configured to capture carbon dioxide (CO2). The transition requires the development of low-emission turbine combustion technologies for this variety of fuels, improved turbine hot section flow path aero/heat transfer methods, and durable, low-cost materials for the stressing environment.

Supercritical CO2 Based Power Cycles

The Advanced Turbines Program at NETL conducts R&D for directly and indirectly heated supercritical carbon dioxide (sCO2) based power cycles for fossil fuel applications. The focus is on components for indirectly heated fossil fuel power cycles with turbine inlet temperature in the range of 1300 – 1400 °F (700 – 760 °C) and oxy-fuel combustion for directly heated supercritical CO2 based power cycles.

The sCO2 power cycle operates in a manner similar to other turbine cycles, but it uses CO2 as the working fluid in the turbomachinery. The cycle is operated above the critical point of CO2 so that it does not change phases (from liquid to gas), but rather undergoes drastic density changes over small ranges of temperature and pressure. This allows a large amount of energy to be extracted at high temperature from equipment that is relatively small in size. At the same power rating, sCO2 turbines will have a nominal gas path diameter significantly smaller than utility scale combustion turbines or steam turbines.

The cycle envisioned for the first fossil-based indirectly heated application is a non-condensing closed-loop recompression Brayton cycle with heat addition and rejection on either side of the expander. In this cycle, the CO2 is heated indirectly from a heat source through a heat exchanger, not unlike the way steam would be heated in a conventional boiler. Energy is extracted from the CO2 as it is expanded in the turbine. Remaining heat is extracted in one or more highly efficient heat recuperators to preheat the CO2 going back to the main heat source. These recuperators help increase the overall efficiency of the cycle by limiting heat rejection from the cycle.

Research is continuing in the areas discussed above. For example, NETL is teamed with multiple private sector companies to further develop innovative technologies for advanced turbine components and sCO2 power cycles by selecting six projects to receive more than $30 million of research funding for up to 42 months.

In a combined power cycle, a gas turbine generates electricity and its waste heat is used to make steam that generates additional electricity via a steam turbine. Meanwhile, sCO2is a supercritical fluid state of carbon dioxide exhibiting properties of both a gas and a liquid. sCO2 is also non-toxic and non-flammable. Using sCO2 in power turbines is attractive compared to steam because of its thermal stability, allowing for higher power outputs in a smaller package. The goal of the NETL-announced research projects is to increase the efficiency of combustion turbines and sCO2 power cycles for use in new power generation facilities.

The six projects are:

• Ceramic Matrix Composite Advanced Transition for 65 Percent Combined Cycle Efficiency-Siemens Energy, Inc., working with COI Ceramics and Florida Turbine Technologies will further develop a ceramic matrix composite design for Siemens’ Advanced Transition combustor in support of 65 percent efficient gas turbine combined cycles.
• Cooled High-Temperature Ceramic Matrix Composite Nozzles for Gas Turbines for 65 Percent Efficiency-GE Power, working with GE Global Research and Clemson University, will further develop high-temperature ceramic matrix composite turbine nozzles as an innovative component that will contribute to the DOE goal for advanced gas turbines.
• Advanced Multi-Tube Mixer Combustion for 65 Percent Efficiency-GE Power, partnering with GE Global Research, will apply an advanced version of GE’s Micro Mixer multi-tube fuel-air pre-mixer with an advanced version of Axial Fuel Staging to enable turbine inlet temperatures in excess of 3,200 °F.
• Rotating Detonation Combustion for Gas Turbines-Aerojet Rocketdyne, in partnership with the Southwest Research Institute, Purdue University, the University Alabama, the University of Michigan, the University of Central Florida, and Duke Energy will develop and demonstrate an air-breathing rotating detonation engine combustion system for power generating gas turbines.
• High-Inlet Temperature Combustor for Direct-Fired Supercritical Oxy-combustion-Southwest Research Institute, in partnership with Thar Energy, LLC; GE Global Research; Georgia Tech; and the University of Central Florida will demonstrate a directly heated 1 MWth oxy-combustion sCO2 cycle to advance the state-of-the-art fossil-fired sCO2 power cycles.
• Development of Low-Leakage Seals for Utility Scale sCO2 Turbines-GE Global Research, in partnership with Southwest Research Institute, will develop turbine end seals and inter-stage seals for utility-scale sCO2 power cycles to achieve a field-trial-ready design. The majority of the work is focused on maturing turbine end seals by testing them in new and existing facilities at increasing pressures, temperatures, and seal sizes in both air and sCO2 environments.

In 2014, DOE funded 11 Phase I projects for research on advanced turbine components for combined cycle and supercritical based power cycle applications. The new Phase II awards listed above show the most promise in reaching the DOE goals.

Advanced Turbine Research

There are three major areas involved in NETL’s Advanced Turbine Research.

Aerodynamics/Heat Transfer-Project goals of the aero-thermo-mechanical design sector are to assess the unique operation conditions associated with hydrogen turbines and investigate design improvements for addressing these unique design spaces. Efforts are focused on reducing cooling flows, reducing sealing and leakage flow rates, reducing rotating airfoil count, increasing expansion stage areas, and increasing airfoil length. These efforts are intended to develop machines that are more efficient with a higher power output.

Combustion

The combustion program goal is to design and develop the combustion portion of the turbine leveraging the best current and advanced technologies to meet strategic system-level goals of an advanced syngas or hydrogen fueled gas turbine. Efforts are focused on the measurement and assessment of the fundamental properties of hydrogen combustion and the use of these properties to design and develop low-NOx (oxides of nitrogen) combustion systems. Several combustion technologies are under evaluation, including axial fuel staging, diffusion, hybrid forms of premixed and diffusion, and pressure gain combustion.

Materials: Thermal Barrier Coatings-The goal of the projects in the materials sector is to assess and develop thermal barrier coatings (TBCs) that can provide the performance and durability required for use in syngas- and hydrogen-fueled advanced gas turbines. Efforts are focused on identifying candidate TBC architectures and material compositions with the proper thermal, mechanical, and chemical properties for use in reducing heat flux to combustor transition pieces, stationary nozzles, and rotating airfoils. Advanced TBC and bond coat architectures are being developed to improve durability and thermal performance in the harsh environment found in the IGCC gas turbine.

Recent research in this area supported by NETL has led to a discovery that could significantly increase the efficiency of turbines in fossil fuel electricity generation. This breakthrough could reduce CO2 emissions from power plants and help drive the clean energy economy in the U.S.

When gas turbines operate at high temperatures, they use less fuel, operate more efficiently, and enable carbon capture technologies to more effectively reduce greenhouse gas emissions. The problem is that the thermal barrier coatings protect the turbines from high heat degrade and fail when they’re exposed to temperatures that exceed 1,200°C, which is required for more efficient operations and greenhouse-gas capture.

Working under an NETL-sponsored Small Business Technology Transfer project, researchers from HiFunda LLC and the University of Connecticut successfully demonstrated that an oxide called yttrium aluminum garnet (YAG) deposited by the relatively new process “solution precursor plasma spray” (SPPS) provides a thermal barrier coating with the potential for use at 1,500 °C. That’s a 300 °C temperature advantage compared to current state-of-the-art air plasma-sprayed thermal barrier coatings.

Nine major industrial partners are now testing this technology and evaluating the process in production facilities. In addition, a new spin-off company-Solution Spray Technologies LLC, a Delaware company with operations in Connecticut-has been created to be a thermal barrier coating service provider for this new technology.

If adopted throughout the gas turbine industry, this technology could significantly increase turbine efficiency and reduce overall fuel consumption. It may also enable development of technologies for next-generation, high-temperature, high-efficiency systems that could lay the ground work for more effective carbon capture in power plants – and help to drive the clean energy economy.

NETL’s ongoing research on gas turbines is emblematic of the Laboratory’s overall mission to discover, integrate, and mature technology solutions to enhance the nation’s energy foundation and protect the environment for future generations.

Author

Gerrill Griffith is a contract technical writer for the National Energy Technology Laboratory.