Special Focus: Pathways for Sustainable Hydrogen
M. BRÜCHER, Siemens Energy, Cologne, Germany
Green hydrogen (H2) is poised to play a crucial role in the energy transition by opening new decarbonization pathways, particularly in hard-to-abate industries, such as marine shipping and transport, chemicals, steel and cement. However, an efficient and reliable transport network is needed to realize the full potential of H2 as a clean energy carrier.
Chemically combining green H2 with other constituents to create liquid eFuels, such as eAmmonia (NH3) and eMethanol, are essential to solving the transport puzzle and extending the reach of renewable energy sources. Along with pipelines, eFuels are the most efficient method for bulk storage and H2 transportation, particularly over long distances. This article discusses the potential of NH3 as a clean energy carrier and the critical role compressors play in its production (FIG. 1).
NH3 as a clean energy carrier. While 70%–80% of all ammonia produced is used for fertilizers, it has several other applications relevant to the energy transition, including carrying clean H2. NH3 has a high volumetric H2 density (17.6 wt%) and a heating value 1.5x greater than pure liquid H2. It is also very stable (i.e., high auto-ignition temperature and low condensation pressure) and requires less energy to liquefy than pure H2. These properties, coupled with well-established transport infrastructure for NH3 (e.g., rails, trucks and pipelines), make it an ideal H2 transport vector and long-duration energy storage medium.
H2 and nitrogen can be extracted from NH3 via established cracking processes. The nitrogen can be disposed of or potentially compressed and sold for use in other industrial processes, creating opportunities for additional streams of revenue for H2 end users.
Another NH3 application is in the marine sector, where it is being explored as a sustainable alternative to fuel oil. Direct use is advantageous because it prevents losses from cracking. Research and development are ongoing for NH3 blends and direct NH3 combustion. As a combustion fuel, NH3 opens many decarbonization pathways, particularly in the power generation, mobility and chemical sectors.
One significant advantage NH3 possesses over other carbon-based eFuels is the feedstock cost of nitrogen compared to carbon dioxide (CO2). While biogenic and industrial waste streams are sources of low-cost CO2 feedstock today, direct air capture (DAC) is expected to be required for future large-scale, carbon-based eFuel production. While DAC is maturing, capturing atmospheric nitrogen is more efficient than capturing CO2 because it exists at a much higher concentration (78% for nitrogen compared to <0.5% for CO2).
Compressor solutions for clean NH3 applications. Compressors play a crucial role in NH3 production and directly impact the boundary conditions of many plants and processes across the NH3 value chain. Compressors are needed to produce, transport and store clean NH3 and its base constituents (H2 and nitrogen). CO2 compressors are also integral to the carbon capture systems that produce blue NH3 through traditional reforming processes.
The author’s company has more than 2,500 reciprocating compressor units (more than 2.5-MM horsepower) installed in H2 applications and pipeline services worldwide, along with more than 450 process reciprocating units operating in NH3 syngas service. The company also has a large installed base for H2 and NH3 turbocompressors, along with integrally geared compressors for main air separation units and refrigerant compressors for NH3 liquefaction (FIG. 2).
One example is using hybrid (turbo and reciprocating compressors) to enable increased turndown and flow flexibility. This configuration can be particularly beneficial for green H2 facilities, where production rates will vary depending on how much electricity is supplied by intermittent sources (e.g., wind and solar). In such cases, efficiently meeting pressure and flow requirements can be challenging. A hybrid compressor solution offers a solution to this problem.
Making NH3 production more sustainable. There are methods to produce traditional NH3 more sustainably, for example, through the electrification of compressor drives and waste heat recovery.
The synthesis process for NH3 is carbon-intensive and involves chemically combining H2 and nitrogen under high temperature and pressure in a reactor (i.e., the Haber-Bosch process). During the steam methane reforming of natural gas or gasification of coal, a substantial portion of the total energy input is consumed to produce H2. The rest is consumed as process energy, mainly for generating heat to initiate the chemical reaction for synthesis.
Using high-temperature heat pumps and/or mechanical vapor recompression cycles with steam compression enables operators to recover waste heat that would otherwise be lost to the atmosphere. The thermal energy can then be used to meet process heating requirements. Specific examples where waste heat recovery can be beneficial are in upstream electrolyzer plants.
When using an advanced amine system, 60%–80% of the required steam and thermal energy can be provided through waste heat recovery while only requiring between one-quarter and one-sixth of the heat as additional mechanical power for the compressor, making it competitive in capital expenditure (CAPEX) and footprint. This results in a coefficient of performance between 4.0–6.0, potentially reducing plant operational expenditure (OPEX) depending on the alternative heat source and value of heat vs. mechanical or electrical energy. Initial calculations show that this innovative waste heat recovery method could reduce the specific energy demand of captured CO2/t by up to 1.3 gigajoule/t.
Applying the heat recovered is not limited to the amine system’s steam production. However, the synergies are obvious, as most carbon capture processes apply amine systems for sequestrating the CO2 from the flue gas, which requires significant amounts of low-pressure steam. Depending on the plant’s location, opportunities may also exist to use the heat for other purposes, including district heating.
Collaboration is essential. These measures (and others) can yield immediate emissions reductions from NH3 production. However, more than technology alone is required to drive a successful energy transition. Collaboration and co-creation are also essential. Producers and technology providers must come together to utilize the full breadth of everyone’s experience, expertise and resources to drive innovation. Strong partnerships and new ways of working are necessary to make the clean H2 and NH3 economies a reality.H2T
About the author
DR. MARCUS BRÜCHER is the Senior Vice President of Compression for Siemens Energy. With more than 20 yr of experience in senior roles, his professional background has primarily been in manufacturing and project execution of turbomachinery equipment for oil and gas power generation industries. Brücher earned his BS in mechanical engineering and PhD in production technology from the Technical University of Berlin.