Power grids will have to become more flexible to support the energy transition. Linde has enhanced its air separation units to meet this adaptability challenge. The new FLEXASU® plant in Vejle, Denmark, has been operational since August 2020 and is currently producing liquid oxygen and nitrogen. The plant uses wind energy, in particular ramping up production when an excess supply is available to help balance the energy grid.
Denmark is harnessing the power of the wind with huge success. As far back as 1985, the country began to increase government subsidies for wind turbines and pull away from nuclear energy. It is generally assumed that Denmark will achieve its goal of obtaining 100 percent of its energy from
carbon-neutral sources by 2028. Even so, Denmark still faces the same challenges as other countries in its efforts to increase the share of renewables in its energy mix. Sources such as wind are prone to wide fluctuations and this can compromise grid stability. Industry can help balance these fluctuations by building more flexibility into their process plants. In other words, drawing more electricity during times when supply exceeds demand – which happens when the wind is feeding a high flow of energy into the grid. Which is easier said than done. Daily operations and production processes at industrial plants are typically designed for continuous output. Load changes are rare in such settings, and are mainly limited to maintenance windows. To successfully transition to more sustainable sources of energy, however, production plants will have to adapt to load changes on a daily basis. This means that more flexibility will have to be built in from the ground up – something that Linde has managed to achieve with its new generation of air separation units (ASU).
Industry can help balance fluctuations by building more flexibility into their process plants.
Building flexibility and sustainability into industrial production
One of these new-generation plants has been operational in the southern Danish town of Vejle since August 2020. “The Vejle plant has brought our
innovative concept for flexible air separation, known as FLEXASU® to life,” says Frank Stumpf-Marquardt, execution project manager at Linde Engineering. “The air separation unit in Vejle adjusts its output of liquid oxygen and nitrogen to the external boundary conditions.” Specifically, this means that the Danish plant front-loads its production during times when wind energy is in surplus and therefore cheaper. It can then fall back on these stored reserves and turn down production when energy is less abundant and consequently more expensive. This intelligent alignment is called demand-side management (DSM). As well as relieving the energy grid, this approach also reduces costs for the operator – with Linde Gas being the beneficiary in Vejle. “Not only does demand-side management with FLEXASU relieve the energy grid by tapping into excess electricity generated by strong winds, it can also create grid-side financial incentives to the benefit of the plant operator,” explains Stumpf-Marquardt.
ASUs that can handle 10,000 pressure load changes
From the outside, there are no clues as to what makes this ASU “flex”ible. The answer lies in a closer look at the inner workings. Linde engineers gradually adapted a large number of the plant’s components, including heat exchangers, compressors, turbines and cooling systems for greater agility. “The components in Vejle have now been designed and certified to withstand around 10,000 pressure load changes during their 25-year service lifetime. We are talking here about a pressure range from 70 to 6 bar,” underlines the Linde engineer. “This is like repeatedly bending a piece of metal in one direction and then in the other. The materials therefore have to withstand the huge forces to which they are subjected.” All of this had to be properly certified and documented as well – not just by Linde but also by the subcontractors. “The lifecycle documentation is legally binding and is regularly verified. Endurance over the full lifecycle is a really important design criterion – one that all players are aware of and take very seriously,” according to Stumpf-Marquardt.
From the outside, there are no clues as to what makes this ASU “flex"ible.
The new dynamic demand-side management of the plant in Vejle is partly attributable to the design fine-tuning measures outlined above. The combination of all of these changes allows more flexible operation of the air separation unit. But that is not all. The coldbox also plays a key role in maximizing efficiency and adaptability. “Renowned worldwide for its manufacturing excellence, our Schalchen site produces coldboxes offering unparalleled quality, robustness and adaptability so the entire system can be designed to operate in lockstep with the cyclical fluctuations in electricity prices,” explains Stumpf-Marquardt. At the heart of an air separation unit, the coldbox sucks in air, purifies it and cools it to a temperature of approximately minus 190 degrees Celsius. This has the effect of liquefying the constituents of air, the main ones being oxygen and nitrogen, which are then separated by the process of distillation.
Modifications to the instrumentation and control technology also played a role in increasing the ASU efficiency and flexibility.
Output of liquefied gases increases three-fold
A further challenge when it came to designing the ASU in Denmark was meeting the purity requirements for medical oxygen. “This was something we had to bear in mind when selecting materials,” explains the Linde expert. “For example, it is imperative that only specially approved and qualified materials are used at the point where liquid oxygen exits the coldbox.
Modifications to the instrumentation and control technology also played a role in increasing the ASU efficiency and flexibility. The plant is operated from a Remote Operations Center in Avesta, Sweden. There is still a small team on site in Vejle, however, to perform tasks like maintenance and to handle gas logistics. The FLEXASU has trebled Denmark’s gas volumes for the merchant market. The new plant has a daily output of 470 tons of industrial and liquid oxygen, medical oxygen and liquid nitrogen – enough for Linde Gas to meet the entire demand of the Danish market. This has dramatically reduced the transport emissions previously released to source these products from Sweden and Germany.
Teamwork and technology combine for success
“The plant in Vejle has demonstrated the market maturity of our FLEXASU concept. This is a notable success for all of us and it further advances our corporate objective of investing in sustainable technologies,” claims project manager Stumpf-Marquardt. “A key success factor on this project was our highly committed team, who helped to ensure that the plant went on stream on schedule despite the pandemic and the resulting travel restrictions and occasional staffing shortages on site.” As if that were not enough, the plant notches up a further success for Linde. It is actually performing better today than predicted by the initial calculations. Not only does this FLEXASU landmark concept help balance the grid, it is also driving the industry transition towards greater energy efficiency and sustainability.