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.