OR WAIT null SECS
Reciprocating and centrifugal compressors are viable options for hydrogen compression with enhanced and adapted sealing methods, alternative valve selections, and additional staging.
The use of hydrogen as a decarbonized fuel and for long-term energy storage involves various types of hydrogen-based solutions. These solutions must be capable of delivering clean power via hydrogen fuel for the production, transport, storage, and use of hydrogen, with lower overall emissions than alternative decarbonized energy-storage technologies. Thermochemical technologies, including hydrogen as energy storage, provide the only long durations required for seasonal and daily fluctuations in renewables.
THE HYDROGEN VALUE CHAIN
Hydrogen is used in a power-to-gas (P2G) approach as energy storage for excess renewable power that can be stored at high pressures and transported via pressurized pipelines, typically up to 1,440 psi. However, compressing hydro-gen is power-intensive due to its energy density being very low on a volumetric basis. Compared to natural gas pipelines, for equivalent energy volumes, hydrogen compression will likely require 5 - 6 times the power (FIGURE 1)1. This makes efficiency in design and the overall footprint requirement key aspects when evaluating various compressor technologies.
In this value chain, hydrogen compression will be needed for a range of applications, including pipeline transportation, production, storage, transport, fuel boosting, and use at gas-fired power plants, as well as vehicle/aircraft fueling stations. Providing hydrogen-compres-sion solutions for such a broad range of target applications challenges the technologies that are currently available, giving rise to critical questions on how to adapt compression technologies to meet the wide variety of needs.
In the short term, generating hydrogen from fossil fuels with carbon capture and sequestration (CCS) offers lower costs and production closer to the point of use. This pathway to power production from hydrogen-blended gas will likely require less pipeline compression and possibly lower delivery pressures. In the case of gas-fired power plants, volume flows will be scaled to the size of the existing gas turbine and natural gas combined-cycle (NGCC) power plants, so compression technology will need to match the power-plant delivery flow rates. For the transportation sector, smaller modular compressors will be required for vehicle fueling stations. The CO₂ compression requirements for this pathway will be significant and require additional CO₂ pipelines and compression.2
While different types of compressors are used in industrial applications and vehicle refueling stations, the larger power applications for storage and pipelines realistically limit the selection to reciprocating and centrifugal compressors. Both have precedence in this area as well as relevant designs, with high reliability in refinery/plastic production processes, albeit at lower ratios and lower overall discharge pressures.
Compression requirements are largely similar from production to transport, requiring booster units to compress into the pipeline at ratios of 2.5 - 3.0 and recompression in the pipeline at ratios of 1.2 - 1.3. Compression ratios for storage applications will be much higher, up to 10.0. Fuel-line-booster compression may require ratios of 2.5 - 3.5 and be most relevant to hydrogen production at the point of use, i.e., the gas turbine power plant.
Due to hydrogen’s low energy volume density compared to pipeline-quality natural gas volume density, in blending hydrogen with natural gas, appreciable concentrations of hydrogen will be needed to significantly decrease carbon emissions. The Btu/min flow to the turbine must remain constant for the 50 MW combustion gas turbine system shown in FIGURE 2 for a blended fuel stream, which means that the natural gas flow falls off non-linearly with increasing hydrogen content. Operators will need to blend more than 50% hydrogen to start making an appreciable difference in carbon emissions, and many gas turbine manufacturers are already investigating means of operating at 100% hydrogen fuel.
The challenge with hydrogen compression is in the large flow rates and the variety of pressure ratios required for the decarbonized energy market compared to compressors used in refinery applications. The higher compression ratios required for the transportation/power sector will require design adaptations for both reciprocating and centrifugal compressors, including enhanced sealing methods, alternative valve selections, and additional staging.
FIGURE 3 displays some of these requirements relative to the head/flow capabilities of existing reciprocating and barrel-style multistage centrifugal compressors for a range of hydrogen flow conditions corresponding to transported fuel energy content from 6-25 GW (500-2,150 MMSCFD natural gas), which correspond to a range of pipeline sizes analyzed in Allison, et al., 2021.3
The two lower-left solid rectangles show a comparison of natural gas versus hydrogen compressors for transport at pipeline stations, where compression requirements increase for hydrogen due to the lower volumetric energy density of hydrogen and the thermodynamic properties of hydrogen that require more power to achieve the same pressure ratio when compared with natural gas. The upper-right solid rectangle illustrates compression requirements for boosting hydrogen from its production pressures of 20 - 25 bar to pipeline pressures of 85 - 100 bar. The dashed rectangles show the capabilities of existing technologies for hydrogen compressors. In general, reciprocating compressors may struggle with meeting flow requirements for hydrogen compression, whereas centrifugal compressors will struggle to meet the head requirement for hydrogen compression in a single unit.
STORAGE PRESSURES AND TEMPERATURES
In addition, to harness excess renewable power in P2G applications, hydrogen compressors for storage will need to likely deliver at 3,000 psi and above (FIGURE 4)4 for injection at the top end of the reservoir.
Initial boosters to take hydrogen from production pressure to pipeline pressures (atmospheric to 100 psi) will require hydrogen compressors capable of a high number of multiple stages due to significant head requirements. Hydrogen compressors will need to move to designs at higher rated motor and engine drivers, likely 10 MW and above, and compression design variability for a range of pressure ratios. The compounded effect of low energy density by volume and high head due to its high specific heat makes hydrogen compression challenging and power-intensive.
Managing the higher head for the larger pressure ratios associated with hydrogen production and pipeline boosters is a key design parameter. Intercooling between stages is crucial to protecting internal components and keeping power consumption low but is complicated by the need to maintain a semi-compact footprint for reciprocating compressor pulsation bottles, lube oil systems, and intercoolers and their associated piping.5
The following tables (TABLES 1, 2, and 3) compare the advantages and disadvantages of the various compression options for hydrogen.
The various hydrogen technologies that are currently available each have distinct applications where the compressor choice may be more apparent. However, in many applications, an overlap of competing technologies can be expected. It is in the best interest of operators to compare these technologies for each application space and to evaluate details of the compressor offerings such as overall physical size, accessibility for operations, capex/opex cost, manufacturer experience, and durability of each technology.
Operators should also consider how the technology accommodates the following factors for future hydrogen compression applications:
Reciprocating and centrifugal compressors are viable options for hydrogen compression with enhanced and adapted sealing methods, alternative valve selections, and additional staging. Reciprocating compressors are available for hydrogen service in both low- and high-speed options but will struggle to meet larger volume flow requirements for some applications. Centrifugal compressors for pipeline and storage applications will be capable of higher volumetric flow rates but will need to meet the higher head requirements by using adaptable multi-body arrangements. Finally, centrifugal compressor designers are working to achieve higher tip speeds to overcome the high head required in hydrogen service with fewer compressor stages.6
Klaus Brun, Ph.D., is the Director of R&D at Elliott Group; Timothy C. Allison, Ph.D., is the Director of R&D at Southwest Research Institute (SwRI); Marybeth McBain is the Principal Rotating Machinery Engineer at Venture Global LNG; Stephen Ross is the Manager of Compressor Development at Elliott Group; and Eugene L. Broerman III is the Principal Engineer of Fluid Machinery Systems at SwRI.