Design Considerations on Reciprocating Compressors for Green Hydrogen Production Plants
Without compressors to move light hydrogen molecules around, there is simply no hydrogen value chain. The energy transition will require an immense amount of clean hydrogen applied to a variety of old and new applications. A significant increase in demand for hydrogen compressors is expected in the coming years. Most of this demand will be met by positive displacement compressors, among them the piston reciprocating type.
Green hydrogen production plants will play a significant role in the energy transition by providing clean hydrogen without the need to install expensive carbon capture systems. However, to achieve cost parity with fossil fuel-based clean hydrogen, these plants need to be built at an optimum cost and operate very efficiently throughout the year. The natural intermittency of renewable power generation introduces another challenge to this scenario. Therefore, the three pillars needing to be addressed by any green hydrogen developer are:
The balance of plant equipment, including reciprocating compressors, needs to be designed and operated with the same focus on these three pillars to achieve the low-cost production target.
At first, it seems strange to an experienced user to discuss compressor technology selection for a hydrogen application. Most hydrogen compressors operating today are positive displacement machines, primarily the reciprocating type, due to a high efficiency and high-pressure ratio on low-molecular weight gases. There are thousands of hydrogen reciprocating compressors operating efficiently today at refineries, petrochemical and chemical facilities, and industrial complexes. These compressors are as low as 10 hp (7.5 kW) and as big as 20,000 hp (14,920 kW).
However, reciprocating compressors alone may not be a perfect choice on applications that combine very high flow and very low suction pressure. A typical example is a large green hydrogen production plant (above 45 temperature programmed desorption [TPD]) using alkaline electrolyzers. In this scenario, several first-stage cylinders are required to provide enough displacement to meet the required volumetric flow. Since most original equipment manufacturers (OEMs) limit their design to 10-throws on each frame, multiple units would be required. This would considerably increase capital expenditures and operating expenses for the reciprocating only solution.
A solution to this challenge is to deploy a turbo compressor (integrally geared or single-shaft type) acting as a booster to the reciprocating compressor. This booster will increase pressure to around 43.5 to 87 psi (3 to 6 bar), therefore eliminating first- and/or second-stage cylinders on the reciprocating compressor and reducing overall footprint, total installed cost, and operational cost.
This sophisticated arrangement is commonly referred to as a “hybrid solution” in the compression industry and requires a collaborative approach during the design phase between the engineering, procurement, and construction (EPC) team, OEM, and end user.
Arrangement is defined as the quantity of compressors necessary to meet the required total flow. Common arrangements are 1 x 100%, 2 x 100%, 2 x 50% or even 3 x 33%. There is a balance between the required redundancy at the site and the overall budget available for the project to procure and install the compressors. Large reciprocating compressors will have concrete block foundations that typically require a significant amount of work to be done at the site. The frame and cylinders will need to be assembled. The motor will ship separately and be installed and coupled to the compressor. All the interconnecting piping, wiring, and tubing between the compressor and its auxiliary systems is done at the site. These activities require a significant amount of skilled labor. Skid packaged units will minimize the work done at the site, since most of these activities can be done at a packager.
The number of compressors installed will define the redundancy available at the plant. A 2 x 100% arrangement includes a spare unit, providing full redundancy and guaranteeing uninterrupted plant operation. A 2 x 50% arrangement with smaller individual units provides reduced total installed cost while providing some redundancy during unexpected shutdowns. Large production plants may require 3 x 33% arrangements, providing a high level of redundancy but at a higher total installed cost.
The selected arrangement also has a significant impact on the ability of the compression system to operate at partial load conditions.
Due to its positive displacement principle, reciprocating compressors can perform at partial load operation while keeping efficiency almost constant. This is one of the most significant advantages of this compressor technology, and critical for green hydrogen production plants expecting to run at partial load conditions for extended periods of time due to typical renewable power generation profiles.
The lack of the right capacity control method will inevitably lead to unnecessary gas recycling that increases overall power consumption and greenhouse gas emissions. Alternatively, the right capacity control method will allow efficient operation and optimum power consumption without impacting plant uptime.
A comprehensive capacity control suite is available to operators today, including a range of solutions going from simple pneumatic actuated valve unloaders to complex stepless capacity control systems using hydraulic actuators. To define the best solution for each project, the following factors need to be considered:
Based on this information, the compressor OEM can provide a recommendation of the best capacity control method for each project, allowing operators to achieve an optimum balance between initial acquisition cost and overall energy consumption during the lifetime of the compressor.
The machine monitoring system (MMS) is fundamental to guarantee long operational uptime. It is also one of the most overlooked features during the design phase of a reciprocating compressor. The main objective of an MMS is to provide critical, real-time information about the compressor condition to allow operators to identify potential malfunction or underperformance of main components. Operators can use that information to take preventative action, plan corrections, and avoid unexpected shutdowns.
A modern MMS offers a wide range of functionalities and can be designed to meet any specific project requirement. Basic functions include frame vibration, piston rod drop systems, and process gas pressure and temperature monitoring. Advanced monitoring functions include cylinder dynamic pressure, crosshead vibration, and valve temperature. Monitoring of each parameter will have different objectives. For example, individual valve temperature monitoring will help with fast identification of potential valve malfunction. Piston rod drop monitoring allows tracking of piston rings wear rate and performance. The selection of the right monitoring parameters for each application needs to be a balance between the monitoring capabilities desired and the budget available in the project. Once again, OEMs can provide valuable input to support this decision process.
Many OEMs today offer remote monitoring services to operators. Through a secured internet connection, the OEM can access historic and real-time data from the compressor to provide diagnostics and recommend a potential intervention. This service is particularly valuable for operators that don’t have a highly skilled crew in reciprocating compressor operation or plan to operate the plant remotely.
The MMS requires upfront engineering efforts and additional acquisition costs, but the payback can be very short if the calculation includes potential loss of production due to unexpected shutdown. Many operators define the entire plant uptime based on reciprocating compressor uptime. An extra 1% or 2% uptime on a large hydrogen production facility represents hundreds of thousands of dollars in additional hydrogen produced.
Many new applications for hydrogen associated with energy transition projects, i.e. as a fuel for fuel cells, have stringent oil contamination limits in the process gas. Due to the inherent characteristic of low energy density by mass of the hydrogen gas, very high pressures are required on these applications to meet the required energy content. Therefore, the compression industry is seeing an increased demand for non-lube, high-pressure hydrogen compressors with typical discharge pressures between 2000 and 3000 psig (137.9 and 206.8 barg).
It is important to emphasize this demand didn’t exist a few years ago, as historically the threshold for non-lubricated reciprocating compressors was around 1500 psig (103.4 barg). The industry is quickly evolving to meet this new demand with developments in rings materials and design upgrades.
The objective of cylinder lubrication is to create an oil film around the cylinder liner, reducing friction between the component and the piston rings and guide rings, therefore reducing the wear rate and increasing compressor uptime. Industry experience indicates that correct cylinder lubrication has a positive impact on compressor uptime and will always have a longer uptime than the equivalent non-lube compressor.
Non-lubricated compressors have special materials on piston rings and guide rings (generally polytetrafluoroethylene [PTFE] or polyetheretherketone [PEEK] proprietary alloys) that provide adequate resistance to friction while guaranteeing an acceptable wear rate without the oil film protection. Other considerations like reduced piston speed and reduced piston weight are proven to be efficient to reduce load on rings and increase their expected lifetime.
Alternatively, operators looking to take advantage of the longer uptime of lubricated compressors, or operators with applications with a final discharge pressure above 3000 psig, can choose to install high-efficiency oil removal systems downstream from the compressor. These systems are available today, have demonstrated field-proven experience, and can handle large volumes of flow and high pressures while removing oil on the hydrogen stream up to 1 parts per billion.
Hydrogen is not new to reciprocating compressors. For more than 70 years, many companies in several different markets have been choosing reciprocating compressors for their hydrogen compression needs. This trend is expected to remain true in a clean energy future, due to the unique characteristics of this machine.
The energy transition brings new opportunities and challenges for this reliable workhorse of the compression industry. Luckily, most of these challenges can be solved with existing technology available to operators today.
Many decisions made during the design phase of a reciprocating compressor can have a significant and lasting impact on the overall installed cost, the ability of the compressor to achieve its operational uptime target, and its ability to operate efficiently at partial loads. It is fundamental for the hydrogen industry to be familiar with the main design considerations on reciprocating compressors for green hydrogen production plants. This article provides the necessary information to allow EPC’s and operators to make informed decisions on their projects.
Luiz Soriano has more than 20 years of experience in the energy industry in various leadership roles in quality, manufacturing, applications, and sales, including more than 12 years of experience in the design of reciprocating compression solutions. During this time, he successfully developed multiple hydrogen compressors for different applications like liquefaction, underground storage, pipeline and renewable fuels, among others. He is currently key account manager at Neuman & Esser, where he is focused on development of compression and electrolyzer solutions across the entire hydrogen value chain. Soriano is a mechanical engineer with an MBA in Business Administration. The author would like to thank Electric Hydrogen for the inputs provided to this article.