New Life for Exhaust Steam

Recently, China's first hundred-tonne exhaust steam ejector completed its first operation cycle. The ejector recovers the large amount of waste heat generated by thermal power to use for heating in cities.

Starting from the bottom

Exhaust steam refers to steam that has already performed work and is discharged from equipment such as steam turbines. An exhaust steam ejector uses turbine extraction steam as the motive steam to recover exhaust steam that would otherwise be discharged.

The recovered steam heats the circulating water in the district heating network. The heated water is then delivered to end users without consuming any additional energy.

A research team from Taiyuan University of Technology (TYUT) in north China began to tackle the technological barriers of developing a hundred-tonne exhaust steam ejector in 2020. In the initial stage, the team conducted theoretical research using simulation models. But the first prototype's performance was far from satisfactory.

Zhang Xiaoxian, a member of the research team, said the ejector in the research paper was too small, like trying to build a heavy-duty truck with a toy car model. The errors became huge after the parameters were enlarged several hundred times.

The team turned to the underlying principles. To quantify the ejector's entrainment capability, they broke it down into three key sections — the nozzle, mixing section and diffuser — and carried out more than 10,000 computational fluid dynamics simulations.

They adjusted details such as the size and angle of each part like adjusting a microscope, and figured out that even changing 0.1 mm of the nozzle's diameter would influence the ejector's entrainment capability.

Drawing on data from more than 10,000 simulations, they derived an equation for calculating the motive steam entrainment ratio and established a full-scale design methodology for adjustable high-temperature, high-pressure steam ejectors.

Adjusting the cone for flexibility

Combined heat and power units operate under highly variable conditions, with heat demand fluctuating significantly between day and night. As a result, the volume of exhaust steam can rise and fall dramatically.

This severely tests the exhaust steam ejector's ability to adapt. Regardless of changes in load or operating conditions, it must precisely regulate steam flow while maintaining stable entrainment. The design and geometry of its regulating cone are critical to achieving this capability.

Initially, the team adopted the conventional straight-cone design that had been used across the industry for years, Ma Suxia, leader of the research team and a professor at TYUT, said. But it was a crude way of regulation — like an old-fashioned faucet. Open it too far and it overdrives the ejector; close it too much and it provides insufficient entrainment.

When the thermal power plant experiences an extreme cold snap or during unit startup or shutdown, the equipment is highly susceptible to flow instability and ejector failure.

The team decided to rethink the regulating cone. After dozens of simulation iterations, multiple rounds of prototype redesigns, and hundreds of comparative tests under different operating conditions, they finalized a new streamlined curved regulating cone.

Compared with the conventional straight-cone design, the new cone offers a smoother flow transition and has a more efficient flow-passage geometry. With only a slight axial movement, it can precisely regulate the motive steam flow rate within milliseconds, enabling stable operation under varying loads, extreme weather and other demanding operating conditions.

Sealing for safety

As the only movable core component of the equipment, the adjustable cone has many weak points. Due to frequent axial movement and drastic changes in operating conditions, steam leaks are highly likely to occur at the clearances between moving and stationary components. This could lead to energy waste and equipment damage, or even accidents that would impact the electricity and heat usage in the entire region.

In the laboratory, experiments can be done by simply sealing the weak points, but the weaknesses will expose themselves easily when large engineering equipment is put into operation in a thermal power plant, Ma said.

To guarantee electricity and heat supply in cities, combined heat and power units have to operate 24/7 all year round with very little maintenance. Thus, the sealing system cannot have even the slightest design oversight or manufacturing defect.

To solve this problem, the team learned from factories that have practical production experience and explored sealing materials and structures.

A solution was finalized. A special supporting structure would be added to the adjustable cone to prevent it from deforming or breaking under high pressure. Graphene is used to design a multiple sealing structure that fits perfectly while ensuring smooth, jam-free adjustment.

In extreme-condition tests evaluating the sealing structure's stability and durability, the ejector completed the entire test without steam leakage, component jamming or noticeable deformation. The regulating cone moved smoothly throughout its full travel, maintaining stable sealing performance.

If the technology is widely adopted across China's thermal power sector, it could reduce carbon emissions by more than 100 million tonnes.

Source: Science and Technology Daily