Controlling combustion in gas turbines

New simulation methods were developed using Titan

Credit: Oak Ridge National Laboratory

Oak Ridge National Laboratory is the largest US Department of Energy laboratory and is supporting efforts to increase the efficiency of combined-cycle power plants. Currently, state-of-the-art CCPP’s have net thermal efficiencies of around 61-62 per cent. The objective is to increase that efficiency to 65 per cent, writes David Flin

The use of natural gas for electricity generation continues to grow, as a result of historically low prices, a boom in production, and lower CO2 emission levels compared to coal-fired power plants.

As a result, gas turbines are growing in significance as a source of electricity supply, with their use in conjunction with intermittent renewables such as wind and solar to balance supply and demand.

GE considers gas-fired power generation to be a key growth sector of its business, and a practical step toward reducing global greenhouse gas emissions by replacing coal-fired generation.

The US Department of Energy (DOE) is, through its National Energy Technology Laboratory, supporting efforts to further develop innovative technologies for advanced gas turbine components, with the intention of increasing combined-cycle efficiencies up to 65 per cent.

In a research report by Oak Ridge National Laboratory (ORNL), ‘Better Combustion for Gas Turbines’, Joe Citeno, combustion engineering manager for GE Power, says: “Advanced gas turbine technology gives customers one of the lowest installed costs per kW. We see it as a staple for increased power generation around the world.”

GE’s H-class gas turbines are currently the world’s largest and most efficient gas turbines, capable of enabling 62 per cent combined cycle efficiency. The company is seeking ways to improve the performance of these turbines. A 1 per cent increase in efficiency in a 1 GW power plant saves 17,000 tonnes of CO2emissions per year.

At higher inlet temperatures, gas turbines are more efficient and produce higher outputs, but they also produce more NOx emissions. To reduce emissions, combustion technology, such as GE’s Dry Low NOx (DLN), mixes fuel with air before burning it in the combustor. Also in the ORNL report, Jin Yan, Manager of the computational combustion lab at GE’s Global Research Centre, says: “When the fuel and air are nearly perfectly mixed, you have the lowest emissions.”

In an H-class gas turbine, combustion takes place at high pressures and temperatures within chambers 2 metres in length. These turbines have a ring of 12 or 16 combustors, each capable of burning nearly three tonnes of fuel and air mixture at firing temperatures in excess of 1500à‚°C. The extreme conditions make it one of the most difficult processes to test.

By mixing air with the fuel, the firing temperature can be increased, resulting in the potential for increased efficiency, while preventing NOx levels from increasing.

However, mixing air and fuel can lead to swirling, which in turn can result in flame instability from large pressure, velocity and heat release fluctuations.

An unstable flame can also result in acoustic pulsations, which can affect turbine performance and, in the most extreme conditions, can wear out machinery in minutes. Because of this, understanding the cause of a pulsation and predicting whether it might affect future products is important.

In 2014, one such pulsation caught the attention of researchers during a full-scale test of a gas turbine. The test revealed a combustion instability that had not been observed during combustor development testing.

GE determined that the instability levels were acceptable for sustained operation, and would not affect gas turbine performance. However, GE researchers wanted to understand the cause of the instability to help them predict how pulsations could manifest in future designs.

GE suspected that the pulsations stemmed from an interaction between adjacent combustors, but had no physical test capable of confirming this hypothesis.

Because of facility airflow limits, GE is able to test only one combustor at a time. Even if GE could test multiple combustors, access-visibility and camera technology currently limit the researchers’ ability to understand and visualize the causes of high-frequency flame instabilities. As a result, GE decided to use high-fidelity modelling and simulation to reveal what the physical tests could not.

The company asked its team of computational scientists, led by Yan, to see if it could reproduce the instability virtually, using high-performance computers. GE also asked Yan’s team to use the resulting model to determine whether the pulsations might manifest in a new GE engine incorporating DOE-funded technology.

GE challenged Yan’s team, in collaboration with the software company Cascade Technologies, to deliver these results before the engine was tested, to demonstrate its predictive capability.

Oak Ridge National Laboratory

Credit: ORNL

Yan says: “We didn’t know if we could do it. First, we needed to replicate the instability that appeared in the 2014 test. This required modelling multiple combustors, something we had never done. Then we needed to predict through simulation whether that instability would appear in the new turbine design, and at what level.”

Enhanced modelling and simulation capabilities held the potential to dramatically accelerate future product development cycles, and could provide GE with new insights into turbine performance earlier in the design process, instead of after testing physical prototypes.

However, GE faced another hurdle. To meet the challenging time frame, Yan and his team needed computing power that was greatly in excess of that available through the company’s internal capabilities.

As a result, in the spring of 2015, GE turned to the Oak Ridge Leadership Computing Facility (OLCF) for help. Through the OLCF’s Accelerating Competitiveness through Computational Excellence (ACCEL) industrial partnerships programme, Yan’s team received a Director’s Discretionary allowance on Titan, a Cray XK7 system capable of 27 petaflops, or 27 quadrillion calculations per second.

Yan’s team worked closely with Cascade Technologies to scale up Cascade’s CHARLES code, a flow solver for large eddy simulation, a model based on the Navier-Stokes fluid flow equations. The code is capable of capturing the high-speed mixing and complex geometries of air and fuel during combustion.

Air and fuel mix violently during turbulent combustion. The mixing needed to ignite fuel and sustain its burning is governed by the Navier-Stokes fluid dynamics equations, with large swirls generating smaller swirls. The multiple scales of swirls pose a challenge to the computation of these equations simulating turbulent combustion. A burning flame can manifest chemical properties on small scales ranging from billionths to thousandths of a metre.

Using CHARLES’s massively parallel grid generation capabilities – a new software feature developed by Cascade – Yan’s team produced a fine-mesh grid composed of nearly one billion cells. Each cell captured microsecond-scale snapshots of the air-fuel mix during turbulent combustion, including particle diffusion, chemical reactions, heat transfer and energy exchange.

Yan’s team was able to analyze the simulation data and view the flame structure in high definition. By early summer, the team had made enough progress to view the results: the first ever multi-combustor dynamic instability simulation of a GE gas turbine. Yan says: “It was a breakthrough for us. We successfully developed a model that was able to repeat what we observed in the 2014 test.”

GE’s H-class gas turbine

Credit: Oak Ridge National Laboratory

The new capability gave GE’s researchers a clearer picture of the instability and its causes that could not otherwise be obtained. Beyond reproducing the instability, the advanced model allowed the team to slow down, zoom in, and observe combustion physics at the sub-millisecond level, something impossible to match using empirical methods.

Citeno says,”These simulations are actually more than an experiment. They provide new insights which, combined with human creativity, allow for opportunities to improve designs within the practical product cycle.”

Applying the new simulation methods to the 2015 gas turbine, the team predicted a low instability level in the latest design that was acceptable for operation, and would not affect performance. These results were confirmed during the full-scale gas turbine test, validating the predictive accuracy of the new simulation methods developed on Titan.

GE then began using these new simulation methods to evaluate gas turbines. These new methods give GE the ability to better integrate simulation directly into its product design cycle. Yan notes: “We can now look at all kinds of ideas that we never thought about before. The number of designs we can evaluate has grown substantially.”

Citeno says that, in combination with other aspects of gas turbine design, the projected end result will be a full percentage point gain in efficiency. This is an important step towards the DOE’s goal of achieving 65 per cent combined-cycle efficiency.

GE’s experience with these computing resources and expertise has enabled it to understand and evaluate the value of larger-scale high-performance computing, supporting the case for future investment in in-house capabilities. Citeno says: “Access to OLCF systems allows us to see what is possible, and to de-risk our internal computing investment decisions. We can show concrete examples to our leadership of how advanced modelling and simulation is driving new product development instead of hypothetical charts.”

As a result of the successful use of Titan, GE is continuing to develop its combustion simulation capabilities under a 2016 allocation awarded through the DOE Office of Advanced Scientific Computing Research (ASCR) Leadership Computing Challenge, or ALCC, programme. As part of the project, Cascade is continuing to enhance its CHARLES code so that it can take advantage of Titan’s GPU accelerators.

Citeno concludes: “A year ago, these were gleam-in-the-eye calculations. We wouldn’t do them because we couldn’t do them in a reasonable time frame to affect product design. Titan collapsed that, compressing our learning cycle by a factor of 10-plus and giving us answers in a month that would have taken a year with our own resources.”

Previously, analyzing flame instability in gas turbine combustors was limited to consideration of a single combustor, and it was not possible to determine swirling interactions brought about by multiple combustors. Using the Titan supercomputer has enabled modelling of multiple combustors to predict instability conditions, and the greater understanding of the flow mechanics allows flame optimization.

Reaching 65 per cent combined-cycle efficiency

The US DOE’s National Energy Technology Laboratory has selected six Phase II projects to further develop innovative technologies for advanced gas turbine components and supercritical CO2 power cycles, with the intention of enabling combined cycle efficiencies of 65 per cent to be achieved. These six projects will receive a total of $30 million in research funding from the DOE over the next 3.5 years.

The use of CO2 in supercritical state is an emerging alternative to using steam as the working fluid in a power cycle. Supercritical CO2 power cycles have gained significant interest in power generation applications, because the thermophysical properties allow for higher power outputs in a smaller package. The reduced size increases efficiency and potentially reduces the cost of electricity.

The six projects selected for Phase II funding are:

Rotating Detonation Combustion for Gas Turbines: Aerojet Rocketdyne, in partnership with the Southwest Research Institute, Purdue University, the University of 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.

Development of Low-Leakage Seals for Utility-Scale supercritical CO2 turbines: GE Global Research, in partnership with Southwest Research Institute, will develop turbine end seals and inter-stage seals for utility-scale supercritical CO2 power cycle to achieve a field trial-ready design.

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.

Advanced Multi-Tube Mixer Combustion for 65 per cent efficiency: GE Power, partnering with GE Global Research, will apply an advanced version of GE’s Micro Mixer combustion technology to enable turbine inlet temperatures in excess of 1700à‚°C while minimizing NOx emissions.

Ceramic Matrix Composite Advanced Transition for 65 per cent combined cycle efficiency: Siemens Energy, 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 per cent efficient combined cycles.

High-Inlet Temperature Combustor for Direct-Fired Supercritical Oxy-combustion: Southwest Research Institute, in partnership with Thar Energy, GE Global Research, Georgia Tech, and the University of Central Florida, will demonstrate a directly-heated supercritical CO2 oxy-combustor for an advanced fossil-fired supercritical CO2 power cycle.

David Flin is a freelance journalist with a focus on the energy sector.

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