Taking DLN gas turbine hydrogen blending to the next level

(Source: Georgia Power.)

By Bobby Noble, Jim Harper, Mike Gagliano and Rob Steele

Climate change, energy independence, renewable power, the hydrogen economy, etc., are all terms used in recent years (or even decades) to describe the most important challenges of society today.

While arguments are made over the proper direction to face these challenges, it is important to estimate the potential of candidate technologies for reaching these goals before society commits completely to them. While it is popular to look for a “quick-fix”, solutions almost always require an assortment of varied technologies implemented throughout the entire energy value chain, involving millions of people and costing hundreds of billions (if not trillions) of dollars. These solutions are also likely to involve technologies that require incremental steps to gain public acceptance and/or prove parts of the process are feasible.

Combustion or “gas” turbines are highly valuable power generation and heating commodities. In many gas turbines, natural gas (NG) is the fuel of use. NG is generally a low cost and relatively high energy density fuel with a lower carbon content then coal or liquid fuels. This leads to reduced carbon emissions relative to those fossil fuels. However, while burning NG, carbon dioxide (CO₂) remains a primary constituent of the exhaust emissions. Therefore, shutting down gas turbines or re-envisioning them for a low-carbon future is a popular research, discussion and development topic. One such proposed re-configuration is blending Hydrogen (H₂) with the NG fuel stream to further reduce CO₂ emissions.

The reduction in CO₂ emissions trends generally with the mass ratio of H₂ in the fuel, such that burning 100% H₂ reduces the CO₂ emissions by 100%. Note the reduction of CO₂ is non-linear with volume % of H₂ in the fuel, with the largest reductions being realized at the higher H₂ volumetric percentages (20% by vol is 3% by mass and 80% by volume is 33% by mass).

OEMs have been working for many years now to prove out or upgrade their combustion systems for higher H₂ content for which they were originally designed. Mitsubishi Power has been doing this same work for its M501G advanced class gas turbine.

Overall, this case study is focused on the H₂/NG blending and burning project conducted at Southern Company/Georgia Power’s Plant McDonough in the Atlanta, GA metro area. The unit is a Mitsubishi Power M501G gas turbine with a nameplate load of 265MW at baseload and a dry, low NO_x (DLN) combustion system.

Getting to a H₂ economy which provides H₂ for fueling gas turbines for low- or no-carbon emissions will require many technological advancements. However, this fact should not dissuade the reader. These advancements appear achievable as several OEMs and 3^rd party vendors are working on viable options. Included here, are details of the work of Mitsubishi Power, EPRI, and Southern Company who have teamed together to advance the technology further. The value of this project cannot be overstated. This allows the industry and society to view H₂ as a potentially viable fuel for modern turbines to reduce CO₂ emissions, CO emissions and allow further flexibility of operation of gas turbines with no, or minimal, changes to NOx emissions compared to traditional NG gas turbines.

Hydrogen impacts on gas turbines

The impacts of H₂, whether in blends or pure form, on gas turbines is varied and often misunderstood. Recent reports have been issued, or are in progress, to provide more information regarding the influence of H₂ containing fuels on emissions, performance, durability, and service life[1]^,[2],[3]. Albeit details can be found with these and other additional sources, it is important to baseline some considerations when contemplating H₂ inclusion.

Figure 1: Stoichiometric Property comparisons between NG and H₂.

Flame Speed

As indicted in Figure 1, the flame speed of H₂ is much faster than NG. Higher flame speeds require design variation in the DLN combustion systems, which is a primary driver for current gas turbine DLN technology %H₂ blend limitations². The prominent way to include H₂ in NG fueling up to the allowable % by volume is through control of fuel to air ratios in the combustor. To push further, total redesigns are needed, which typically aim to increase axial flow velocity and/or staging combustion to combat the wide flame speed ranges between NG and H₂.

Heating Value

Heating value is a measure of the amount of energy contained in a specific volume (or mass) of fuel, and the heating value of H₂ is significantly different than that of NG. It is the impact of the low density of H₂ that makes the comparison with NG heating values a topic which can lead to incorrect conclusions. Figure 1 compares the heating values of H₂ and NG in mass and volume bases.

Performance and Emissions

An increase of H₂ content in the fuel gas will result in a performance and efficiency impact (in a positive direction) on the gas turbine; however, not due to volumetric- or mass-based heating value impacts. The impact is related to the exchange of CO₂ emissions for H₂O emissions with H₂. As CO₂ decreases in the exhaust, the concentrations of other constituents, namely oxygen and water, increase. Figure 2 shows the general trend of exhaust products with increasing H₂.

Figure 2: Exhaust constituents relative to increase in H2 fuel percent.

The change in exhaust products results in an exhaust with a higher energy content relative to its temperature (specific heat, c_p). This results in more work for the same temperature and/or higher efficiency of the gas turbine, depending on the type of control used. This nominal trend in efficiency, performance, and CO₂ reduction with increasing H₂ is shown in Figure 3. Note that this is shown for a nominal turbine and specific benefits will be model specific.

Figure 3: CO₂, performance, and efficiency typical impacts with H₂ percent increase (nominal)

KEY takeaways

Major project team members

System design modifications

A temporary blending system was added to the NG supply to introduce specific concentrations of H₂ gas to the fuel gas sufficiently upstream of the gas turbine to ensure adequate mixing, as shown in Figure 4.

Safety and code compliance

The importance of safety during the design and execution phases of the project cannot be overstated. All team members served critical roles in ensuring that all applicable design codes and regulations were reviewed and complied with. Systems constructed specifically for hydrogen service were designed or modified in accordance with current ASME B31.12 piping and pipeline code and other relevant standards and/or recommended practices. Functional testing, final inspection, and review of quality documents was performed in person by qualified personnel from each member organization. While the project itself was intended to operate for only for the duration of the test program, the systems were designed to comply with industry codes and standards applicable to permanent installations.

Summary and conclusions

Overall, the Georgia Power, Southern Company, EPRI, and Mitsubishi Power consortium successfully operated a M501G with H₂ blending up to ~20.9% by volume. The results of the preparation and testing execution exhibit the feasibility of utilizing H₂ on-site with an existing DLN gas turbine asset while maintaining emissions compliance. This project constitutes the first of a kind in blending large volume flows of H₂ in an advanced, high efficiency gas turbine operating in combined cycle mode. Testing results show the promise that H₂ blending holds in the Energy Transformation to a low- to zero-carbon future.

About the authors:

Bobby Noble is the program manager of Gas Turbine R&D at EPRI and a Fellow of American Society of Mechanical Engineers. He is a key global leader in gas turbine combustion and diagnostics, authoring a number of EPRI reports and conference papers on GT hydrogen combustion.

Jim Harper is a Principle Technical Leader in the Gas Turbine programs at EPRI and has extensive Gas Turbine design, control, testing and fleet experience. In addition to Gas Turbines he has automotive and EV battery thermal systems design experience. He has authored over 10 patents in Gas Turbine Design and Gas Turbine as well as EV Control architectures.

Michael Gagliano is a Technical Executive at EPRI executing material-based research for the Low Carbon Resources Initiative. He has a Ph.D in Materials Science and Engineering and has expertise in boiler materials, low and high temperature degradation mechanisms, and metallurgical failure analysis.

Dr. Robert Steele is a Technical Executive for Gas Turbine Advanced Components and Technologies at EPRI and has 35 years of experience in gas turbine combustion research, development and testing, and electric power generation industry technologies including carbon capture, compression, and sequestration.

[1] Douglas, C. M., Shaw, S. L., Martz, T. D., Steele, R. C., Noble, D. R., Emerson, B. L., and Lieuwen, T. C. (July 28, 2022). “Pollutant Emissions Reporting and Performance Considerations for Hydrogen–Hydrocarbon Fuels in Gas Turbines.” ASME. J. Eng. Gas Turbines Power. September 2022; 144(9): 091003. https://doi.org/10.1115/1.4054949

[2] Noble, D., Wu, D., Emerson, B., Sheppard, S., Lieuwen, T., and Angello, L. (February 9, 2021). “Assessment of Current Capabilities and Near-Term Availability of Hydrogen-Fired Gas Turbines Considering a Low-Carbon Future.” ASME. J. Eng. Gas Turbines Power. April 2021; 143(4): 041002. https://doi.org/10.1115/1.4049346

[3] Emerson, B., Lieuwen, T., Noble, B., and Espinoza, N. “Hydrogen substitution for natural gas in turbines: Opportunities, issues, and challenges,” Power Engineering, June 18, 2021. https://www.power-eng.com/gas/hydrogen-substitution-for-natural-gas-in-turbines-opportunities-issues-and-challenges/#gref