S. Diezinger, Siemens Energy, Erlangen, Germany
As countries responsible for 75% of greenhouse gas (GHG) emissions set net-zero targets, oil and gas, chemicals and other energy-intensive process industries are under increasing pressure to decarbonize. While clean hydrogen (H2) and carbon capture, utilization and storage (CCUS) are gaining attention and will play a vital role in the coming years, electrification represents one of the most effective strategies for reducing fossil fuel consumption and associated emissions. It will become even more attractive over the next decade as the share of renewables within the overall energy system increases and electricity costs decrease.
Electric drives have been used for > 150 yr and are now available in up to 80-MW unit sizes. The electrification of process heat is also becoming more economical, with electric heaters and high-temperature heat pumps showing success in a wide range of facilities. However, the electrification of large process facilities, such as liquefied natural gas (LNG) plants, refineries and chemical complexes, comes with inherent challenges related to grid integration and energy management.
This article discusses current opportunities for decarbonizing process facilities through electrification and outlines solutions to ensure stable power and grid compliance.
Drive electrification. Gas and steam turbines are prevalent in many process applications today. However, their use as a mechanical driver for compressors and pumps is increasingly at odds with operators’ need (and desire) to decarbonize.
In recent years, the author’s company has received several customer requests to convert legacy gas and/or steam turbines to electric motor drives. The typical efficiency of steam and gas turbines ranges from 30%–40%. Conversely, state-of-the-art electric motors offer efficiencies of up to 96%. In addition, motors are more flexible in operation, require less maintenance, have higher availability and generate no onsite emissions. For context, replacing a 5-MW steam turbine with an electric motor drive can save around 25,000 tpy of carbon dioxide (CO2)—this assumes 100% carbon-free electricity.
Several different configurations of electric drives are possible depending on the facility’s requirements. For example, a gearbox and two couplings are required for high-speed compressor applications with a low-speed motor and associated variable frequency drive (VFD). No gearboxes are necessary with a high-speed VFD combined with a high-speed electric motor, and direct coupling with a compressor or pump is possible.
Hybrid drives are also possible and may be more advantageous in certain situations. Using a mechanical drive and electric motor on the same shaft line enables one (or both) machines to produce the required horsepower to drive the compressor. This allows greater reliability, flexibility, operational expenditure optimization and emissions reductions.
Hybrid drives are not new technology; they have been applied for decades (FIG. 1). The author’s company has installed several hybrid drives in the hydrocarbon processing sector, such as in purified terephthalic acid (PTA) and ammonia plants. The PTA process is exothermic and typically utilizes a steam turbine, one or two hot gas expanders and a motor/generator. The motor starts the process by driving the compression train to compress air to the required pressure for the reactor. Once the process produces steam and offgas, the electrical machine is switched to generator mode, and power is exported to the grid while the steam turbine and the expanders drive the compressor. During the switching process, the train operation is uninterrupted.
In turbo-compressors used for midstream applications, an electric motor is installed on one side of the compressor shaft end and the gas turbine on the other. The type of motor used, and the operation of the compression train, determine the need for clutches and gearboxes. The motor is installed between the gas turbine and the compressor for centrifugal compressors with axial inlets, which only have one end for connecting a driver.
Carrying out a drive conversion project economically and maximizing the use of existing plant equipment require a great deal of expertise and detailed planning. All relevant mechanical, structural and electrical interfaces must be analyzed to confirm technical feasibility. The project usually takes around 18 mos to complete. The installation and commissioning process can be completed in roughly 3 wk–4 wk. This work is typically conducted during a scheduled maintenance outage to avoid extended production downtimes and associated financial losses. There are examples where exchanges have been completed in a shorter window without significantly disrupting production schedules.
The author’s company recently completed a large drive conversion project for an integrated refining and petrochemical facility in Western Europe. The project involved replacing the plant’s steam turbine with a low-frequency electric motor powered 100% by renewable energy. In the feasibility study (conducted by the author’s company), the effects on the electrical grid, foundation, process safety and steam balance were examined in detail. The project resulted in a reduction of approximately 25,000 tpy of CO2 from the plant.
Decarbonizing heat and steam production. Process heat makes up approximately two-thirds of industrial energy demand and nearly 20% of global energy consumption. It also constitutes a substantial portion of direct CO2 emissions from industry, as most heat is generated via fossil-fired equipment.
Decarbonizing heat is one of five fields the author’s company has identified as critical to a successful energy transition. Over the years, several technologies (FIG. 2) have been developed to enable heat and steam electrification across a wide temperature range (100°C–1,000°C).
Heat pumps are a proven technology that has been used for decades. Most of the installed base of heat pumps today are classified as low-temperature (e.g., temperatures up to 100°C). These are typically used for district heating applications.
High-temperature heat pumps (HTHPs) are more applicable for the process industries and can achieve temperature levels up to 150°C at a pressure of 5 bar (steam). Even higher temperatures and pressures (~300°C) are possible by combining HTHPs with other technologies, such as steam compressors. Mechanical vapor recompression (MVR) cycles are an alternative to classical heat pumps for recovering waste heat to produce low-, medium- or high-pressure steam.
For processes that require highly elevated temperatures (> 500°C), electric heaters are the most viable alternative to fossil-fired boilers. The author’s company is now working to improve the efficiency and scalability of multiple electric heating technologies. Demonstration projects have achieved fluid temperatures approaching 600°C, which is well within the range of the thermal requirements for a broad range of processes across the oil, gas and chemical sectors. Full-scale demonstration projects exceeding 650°C are planned within the next year.
Other innovative technologies have also shown promise in electrifying heat and steam production. For example, the author’s company has demonstrated a novel type of turbomachine employing supersonic gas dynamics and shock waves to directly heat a gas mixture. The turbo heater can be driven by an electric motor (or a gas turbine operating on H2) and does not require the direct burning of fossil fuels or the mediation of a heat exchanger. A megawatt (MW)-scale turbo heater demonstrator unit has already been tested. A variant capable of achieving temperatures > 1,000°C is in the preliminary stages of development.
Addressing electrification challenges. Electrification cannot be viewed as a simple equipment swap or changeout, particularly in the case of large drives for compressors or electric heaters. In most cases, transmission and distribution infrastructure are required to feed power from the generation source (i.e., self-generated or utility) to the plant. This includes transmission lines, step-up and step-down distribution transformers, high-voltage switchgear, compensators, and remotely monitored and controlled protection devices and/or relays.
The type of electrical source and its distance from the plant will influence the design of the electrical system. Developing an optimized solution requires evaluating many variables across the entire electromechanical system, including the available footprint, torsional analysis, vibrations, the foundation and available power.
One concern that must be addressed early is harmonic distortions in the electrical system caused by the non-linear loads from large VSDs or heaters. Failure to understand the impact of transient scenarios can lead to disruptions in the electrical supply if mitigation measures are not implemented.
As industrial power loads have increased in recent years, utilities (i.e., grid operators) are placing greater responsibilities on operators to maintain power quality and power factors. Enforcement will likely become even more stringent in the coming years, as large-scale process electrification continues.
Performing grid studies and dynamic modeling is an essential step in predicting how electrical systems will behave under the various operating conditions of the plant, including during faults and transient scenarios. The objective is to develop a digital twin of the plant’s electrical infrastructure to be tested and optimized before physical implementation. Grid studies involve analyzing the entire electrical supply system—from the generation source to the facility’s consumption point. Studies typically encompass a steady-state (i.e., static) analysis of the network, and for complex distribution systems, dynamic modeling and simulation.
After the system’s behavior is understood, technologies for addressing active and reactive power imbalances can be selected [e.g., static var compensators, static synchronous compensators (STATCOM) (FIG. 3), passive or active filters].
The plant’s power management system (PMS) also plays a critical role in ensuring electrical system stability. It serves several vital functions, including generation management (i.e., fast switching between various sources) and load-shedding. These functions balance power generation and the electrical load consumed.
Effective power management is a prerequisite for capitalizing on intermittent sources like wind and solar. The PMS can help define peak load shifting and forecast demand in combination with weather forecasts and other digital technologies. Battery energy storage can replace one or more conventional generating units while maintaining N+1 redundancy.
Given the complexity of electrification projects, it is advantageous for operators to engage with partners who can provide a substantial portion of the equipment scope for the drive train and electrical system. Doing so minimizes interface and execution risks and facilitates the development of a robust electromechanical system design capable of ensuring stable power during all scenarios, including steady-state operation, startup/shutdown and upset events.
Future possibilities using green H2. For many process facilities, particularly in the hydrocarbon processing sector, a sizable portion of GHG emissions is represented by the consumption of H2 produced via the steam methane reforming (SMR) or autothermal reforming (ATR) of natural gas. Replacing this gray H2 with clean (preferably green) H2 opens a pathway for significant carbon footprint reductions.
Several plants worldwide have already begun implementing plans to replace some of their SMR/ATR capacity with electrolyzers. Depending on the required H2 volumes, the size of electrolysis plants can reach gigawatt-scale. Powering these plants presents many of the same challenges as electrifying large drives and heaters when it comes to ensuring power stability and quality, especially if the source of generation is intermittent.
As the energy transition progresses and companies become more incentivized to reduce emissions via credits and/or carbon taxes, use cases for green H2 will expand. The path to a net-zero process facility is increasingly viable with the electrification of drives and decarbonized heat production. HP
REFERENCES
Stefan Diezinger is the Vice President of Industrial Solutions Sales for the EU&AF region at Siemens Energy, where he provides digital and integrated solutions for electrification and decarbonization of various industries. Dr. Diezinger earned an MS degree and a PhD in process engineering from FAU Erlangen-Nürnberg, and a Bch degree in business administration from the University of California at Berkley.