M. Brücher, Siemens Energy, Duisburg, Germany
Green hydrogen (H2) is poised to play a crucial role in the energy transition by opening new decarbonization pathways, particularly in energy-intensive industries (marine shipping and transport, chemicals, steel, cement, etc.). 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 (CH3OH), is essential to solving the transport puzzle and extending the reach of renewable energy sources. Along with pipelines, eFuels are presently the most efficient method for bulk storage and H2 transportation, particularly over long distances.
This article discusses the potential of ammonia as a clean energy carrier and the critical role compressors like the one shown in FIG. 1 play in its production.
Ammonia as a clean energy carrier. While 70%–80% of all ammonia produced today is used for fertilizers, it has several other applications relevant to the energy transition, including as a carrier of clean H2.
Ammonia 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 the fact that transport infrastructure for ammonia is already well-established (e.g., rail, truck, pipeline), make it an ideal H2 transport vector and long-duration energy storage medium.
H2 and nitrogen can be extracted from ammonia 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 application for ammonia is in the marine sector, where it is being explored as a sustainable alternative to fuel oil. Direct use is advantageous as it avoids any losses from cracking. Research and development activities into the combustion of ammonia blends—and even direct ammonia combustion—are ongoing. As a combustion fuel, ammonia opens many decarbonization pathways, particularly in the power generation, mobility and chemical sectors.
One significant advantage ammonia possesses over other carbon-based eFuels is the feedstock cost of nitrogen compared to 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 large-scale, carbon-based eFuel production in the future. 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 ammonia applications. Compressors play a crucial role in ammonia production (FIG. 2) and directly impact the boundary conditions of many plants and processes across the ammonia value chain. Compressors are required to produce, transport and store clean ammonia and its base constituents (H2 and nitrogen). CO2 compressors are also integral to the carbon capture systems that produce blue ammonia via traditional reforming processes.
The author’s company provides compression solutions that cover the entire spectrum of applications in the ammonia, H2, CO2 and air separation markets. The company has more than 2,500 reciprocating compressor units (> 2.5 MM hp) installed in H2 applications and pipeline service worldwide, along with > 450 process reciprocating units currently operating in ammonia syngas service. It also has experience and a large installed base for H2 and ammonia turbocompressors, along with integrally geared compressors (IGCs) for main air separation units (ASUs) and refrigerant compressors for ammonia liquefaction.
The company is collaborating with its customers in the ammonia and H2 markets to help them better understand how its compression packages can optimize energy efficiency and power consumption.
One specific example is using hybrid (turbo + recip 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, such as 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 ammonia production more sustainable. Traditional ammonia production can be made more sustainable (e.g., through the electrification of compressor drives and waste heat recovery).
The synthesis process for ammonia is very carbon-intensive and involves chemically combining H2 and nitrogen under high temperature and pressure in a reactor (i.e., the Haber-Bosch process). A substantial portion of the total energy input is consumed by the production of H2 during steam methane reforming (SMR) of natural gas or gasification of coal. 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 electrolyzer plants upstream. The author’s company is working on novel concepts to recover waste heat from CO2 compressors to enhance the efficiency of carbon capture systems.
Exemplary for an amine system, 60%–80% of the required steam and thermal energy can be provided through waste heat recovery while only requiring between 16% and 25% of that heat as additional mechanical power for the compressor at comparable capital expenditure (CAPEX) and footprint. This results in a coefficient of performance between 4 and 6, potentially reducing plant operational expenditure (OPEX), depending on the alternative heat source and the value of heat vs. mechanical or electrical energy. Initial calculations show that this innovative waste heat recovery method could reduce the specific energy demand per ton of CO2 captured by up to 1.3 GJ/t.
The application of the heat recovered is not limited to steam production of amine systems. 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 ammonia production. However, more than technology alone is needed to drive a successful energy transition. Collaboration and co-creation are also essential. Producers and technology providers must collaborate to utilize the full breadth of everyone’s experience, expertise and resources to drive innovation. Strong partnerships and new ways of working will ultimately be necessary to make the clean H2 and ammonia economies a reality. HP
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 principally been in manufacturing and project execution of turbomachinery equipment for oil and gas power generation industries. Dr. Brücher earned his mechanical engineering degree and his doctorate in production technology from Technische Universität Berlin.