HYDROGEN TURBOMACHINERY Readying Pipeline Compressor Stations for 100% Hydrogen by PETER ADAM, RALF BODE, & MARKUS GROISSBOECK

SPECIAL REPORT: HYDROGEN

HYDROGEN TURBOMACHINERY Readying Pipeline Compressor Stations For 100% Hydrogen BY PETER ADAM, RALF BODE, & MARKUS GROISSBOECK

lue hydrogen, and more long-term, green The compression of hydrogen with turbo- hydrogen (i.e., hydrogen produced via compressors is more complicated. Although cen- Brenewable-powered water electrolysis), trifugal compressors for hydrogen recycle service holds enormous potential in helping the world have been used in downstream and petrochem- achieve its decarbonization goals. In particular, ical applications for decades, their efficiency is green hydrogen promises to bring flexibility lower than that of reciprocating compressors. and dispatchability to emissions-free power For a given impeller tip speed in a turbo- from intermittent sources like solar and wind. compressor, the pressure increase is directly Two central building blocks are needed to proportional to the molecular weight of the gas. make this reality: Hydrogen’s molecular weight is approximately 1) A sufficient source of blue/green hydro- 1/16th of methane, which means that achieving Figure 1: Sulfide-stress-cracked gen production a comparable pressure ratio to an existing nat- impeller from a plant where the 2) A needs-based storage and transportation ural gas line would require much higher impel- operator failed to specify network that can reliably and cost-effec- ler tip speeds or a much higher number of com- hydrogen sulfide in the gas. tively supply hydrogen to end-users. pressor stages in several compressor casings. Given the high costs and complexity associ- Impeller mechanical strength limits are ated with developing new transportation infra- directly correlated with tip speed. The maxi- structure, many pipeline operators and regula- mum allowable tip speed of the impeller varies tory bodies are asking the question, “How depending on the material used. Typically, these much effort would be required to repurpose material strength limitations are not a concern existing natural gas infrastructure to accommo- when designing compressors for service with air, date hydrogen?” CO2, or natural gas. In the case of low weight Let’s look at some specific changes required gas compositions, like hydrogen, however, they to make pipeline assets ready for hydrogen can be approached. Therefore, the design of a operation, focusing on the rotating equipment compressor for hydrogen operation is not dic- within compression stations. tated by aerodynamic limits, so much as it is by the impellers’ mechanical strength limits. HYDROGEN COMPRESSION Extensive studies on the design of blades Contrary to popular belief, the effective trans- and impeller geometry have shown that when port energy density of hydrogen in an existing high-strength titanium alloys are used, these gas pipeline is only slightly lower than that of stress levels can be reduced to allow for pressure natural gas. Therefore, the switch from natural ratios of up to 1.45:1 per stage. Therefore, a six- gas to hydrogen would have little impact on a stage machine with a total pressure ratio of 4:1 pipeline’s capacity to transport energy. However, with 100% hydrogen is technically possible. It the much lower molecular weight and heating can be assumed that the commercial availabil- value of hydrogen relative to natural gas have ity of these machines will increase in the com- implications on the type and design of rotating ing years when the market demands them. equipment used in compression stations. Overall, the extent to which compression For pipelines transporting 100% hydrogen, equipment will need to be adapted will depend reciprocating compressors are currently the most on the pipeline’s hydrogen content. If the admix- economical solution. With reciprocating com- ture is less than 10% hydrogen, the compressor pressors, the gas is efficiently compressed in the can be operated without any significant changes. cylinders. By increasing the number of cylinders When the admixture is under 40% hydro- and drive power, as well as a parallel arrange- gen, the compressor housing can be main- ment of compressors, a viable transport capacity tained, but the impellers and feedback stages of up to 750,000 Nm³/h can be achieved. and gears require adjustment. For pipelines

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Figure 2: Hydrogen content possible in various Siemens Energy models. with greater than 40% hydrogen content, the nium impellers require reliable surface coatings. entire compressor must be replaced. Design changes can be made to reduce the likelihood of HIC in compressor impellers, for MATERIAL CONSIDERATIONS example, better interstage cooling. Ultimately, The switch from natural gas to hydrogen also however, better alloys must be developed with warrants consideration of the materials used for greater component reliability. rotating equipment and the physical pipelines. A test method has been developed to sim- Hydrogen embrittlement, or hydrogen-induced ulate high-speed impeller conditions in a tur- cracking (HIC), is a type of deterioration that bocompressor operating in pure hydrogen ser- occurs when atomic hydrogen diffuses into an vice. The technique will be presented at a alloy (see Figure 1). Depending on the steel technical conference late in 2020. It challenges grade and the operating conditions of the pipe- a historical specification limit within the oil line, this reduction in toughness can lead to the and gas production work. The goal is to pro- growth of existing crack-like defects, thus reduc- vide guidance on a new NACE standard for ing the service life of the line or component. testing HIC and identify fit-for-service envi- Although HIC can occur in pipeline walls, ronments, much like the limits for stress cor- it is unlikely as there are rarely dynamic pres- rosion cracking (SCC) in NACE MR0175 sure fluctuations during regular operation, and (Figure 3). no atomic hydrogen is produced during transport. It is generally accepted that both are TURBINE ADAPTATION required for the phenomenon to occur. HIC, Gas turbines that drive compressors draw their however, is a concern in impellers, which are drive energy directly from the line. They must be subject to stress due to high speeds. adapted accordingly to the hydrogen admixture. While the titanium alloys used for high- The combustion characteristics of hydrogen speed impellers offer excellent strength, they are differ from natural gas and other hydrocarbon subject to hydrogen embrittlement and HIC in fuels. This poses challenges for the design of hot the presence of high concentrations of hydrogen. gas path components. It is a challenge to control To avoid these deteriorating mechanisms, tita- the flame, maintain combustion system integ-

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rity, and reach the desired level of emissions. Many gas turbines are already capable of operating on high percentages of hydrogen fuel, some at 100% hydrogen (Figure 2). The capa- bility of each unit depends on multiple factors, one being the type of combustion system the turbine utilizes, either dry-low emissions (DLE) or wet-low emissions (WLE). Existing turbines can be upgraded to allow increasing hydrogen content in the fuel or even work with 100% hydrogen in the future. In gas turbines with DLE combustion systems, fuel and air are mixed before combustion to control flame temperature. This, in turn, controls the rate of emissions. The relative pro- portions of fuel and air are one of the driving factors for NOx and flame stability. Hydrogen’s higher reactivity poses challenges for the mixing technology in DLE systems, including: • Higher flame speeds increase the risk of the Figure 3: Siemens Energy proposes a test method where corrosion-resistant autoclaves are required to perform the tests on pre-stressed specimens at elevated flame burning closer to the injection points, pressures and temperatures. traveling back into mixing passages, or burning too close to liner walls. This risk increases as hydrogen content rises and with increasing changes in the balance of volumetric flow combustion inlet and flame temperature. between the compressor and turbine. This issue • Hydrogen’s lower auto-ignition delay com- can typically be managed by making modifica- pared to methane increases the likelihood tions to the compressor or turbine. of igniting the fuel in the mixing passages. • Changes to thermoacoustic noise patterns THE ROAD AHEAD because of the different flame heat release A building block of a low-carbon energy system distribution can lower component life. is a needs-based storage/transportation network DLE combustion systems are available using to supply hydrogen to end-users. In parts of swirl stabilized flames combined with lean pre- Europe, establishing a hydrogen infrastructure mixing that can achieve low NOx emissions may be possible with little effort. The pipeline without diluting the fuel. Hardware and control networks can gradually be converted to hydro- system changes are also required for higher gen operation with an investment of an esti- hydrogen fuel content to allow systems to operate mated 10-15% of the cost of new construction. ■ safely, meet NOx emissions limits, and manage varying fuel compositions. A goal has been set of making industrial turbines with DLE capable of Peter Adam is Vice President burning 100% hydrogen by 2023. Business Development for Oil Non-DLE technology uses diffusion flames & Gas for the Siemens Energy or partially premixed flames. It can handle Industrial Applications variability in fuel composition. On certain non- business. He is also head of sustainable and hydrogen DLE models, 100% hydrogen is already possi- business development. ble. A disadvantage is that diffusion flames require dilution to control NOx emissions. Dr. Ralf Bode is Manager of Higher flame temperatures mean higher NOx Core Technology teams for emissions without abatement. Dilution is aerodynamics, mechanics, rotor- achieved by the introduction of nitrogen, steam, dynamics, and materials for R&D or water into the flame: for large turbocompressors in Nitrogen dilution is often economical in Germany and the U.S. power plant applications as it is a byproduct of gasification. Steam is also available in com- Markus Groissböck is Portfolio bined cycle applications. However, this is not Marketing Manager and Energy the case in compression stations, with the tur- System Designer for Decarbon- bine in mechanical drive. Additionally, for sin- ization Solutions in Germany. gle-shaft turbines, surge margin can be a chal- For more information, visit lenge with diluted high-hydrogen fuels due to Siemens-energy.com

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