1. Abstract

In the era of renewable energy, power operators need to address the intermittency which is a result from the natural environemtn.et. As a result, it is an important task to stabilize the power grid by complying with total energy supply and demand in this era of renewable energy.

From now on, steam power plants will be required to have a high load followability not only as a base load power source but also as an ancillary power source. However large boilers for steam power have structures with large thermal inertia. It is an important point to take measures to stabilize the temperature (flow balance) of the water wall tubes in the furnace to improve the agility.

Our new USC boiler was developed to solve the trade-off dilemma be-tween the flow stability of the furnace wall tubes and the cooling performance in the tubes by the new perspective of low mass velocity design to achieve flexible load following. Large capacity boilers will be expected to be one of the most important ancillary power sources, since it has a big ability of adjustment output (so-called ΔkW).

2. Technical Contents

(1) Challenges for achieving high load followability
There are flames in the boiler, surrounded by a water wall with a height of several tens of meters and width of a few tens of meters (Fig.1). The water wall consists of hundreds of vertical tubes and fins welded together resulting in a structure with a very large thermal inertia. There-fore, when there is a fast load change, a very large thermal stress is ap-plied to the water wall by the temperature increase which is caused by the degraded flow stability. It may create operational due to the de-formation or damage.

(2) Improved load tracking by natural circulation characteristics
In order to stabilize the temperature of the furnace wall tube against high-speed load changes, a sufficient amount of boiler water should be selectively supplied to the tube having a high heat absorption. Therefore, the focus should be on strengthening the “natural circulation character-istics” that automatically distributes the appropriate flow rate according to the heat absorption to the tubes connected by the header. For this purpose, it is effective to reduce the mass velocity in the tube and reduce the friction loss. However, the furnace tube of the USC boiler is under a very high heat flux condition of 600 kW / m2 as a maximum at the varia-ble pressure (9 to 30 MPa) operation. Therefore, the risks of the heat transfer deterioration at both supercritical pressure and subcritical pressure should be considered and measured when we apply low mass velocity design. In addition, it is also necessary to surely prevent the occurrence of the other type of flow instability such as density wave oscillation.

(3) Development of high-performance heat transfer tube for low-mass velocity operation
In this development, we succeeded in suppressing various factors heat transfer deterioration that occur when the mass velocity is reduced by using a ribbed tube with a spiral groove on the inner surface of the tube and optimizing the groove shape. In particular, by combining precise CFD analysis of the supercritical pressure fluid and verification by one of the world’s leading large-scale test facilities, a high heat transfer perfor-mance ribbed tube shape with excellent cooling capacity at both super-critical pressure and subcritical pressure was finally obtained. In addi-tion, since the shape and dimensions are very important when develop-ing ribbed tubes, the manufacturing quality of the ribbed tube was sta-bilized with the cooperation of steel tube producer from the viewpoint of quality and ease of manufacture.

3. Conclusion

By this development, a load change rate of the boiler, which had been operated at a load change rate of 1 – 3%/min, was increased to 4%/min, and it was shown that it could be increased to a maximum of 10%/min with other countermeasures. From now on, taking advantages of the features of the USC boiler, which has a low mass velocity design with remarkably improved load followability, low cost steam power plants will be one of the important ancillary power sources, able to accept the continuous increase of renewable energy, coordinating with the charac-teristics of nuclear and GTCC. We are preparing for the achievement of a net zero carbon society by ensuring energy security and realizing a re-duction in total CO2 emissions. We believe this technology will work well for the upcoming the era of renewable energy.

Fig.1 Structure of USC Boiler

Fig.2 CFD Analysis and Large Scale Test Facility

Kazuhiro Domoto*1
Hiroyuki Nakaharai*2
Kenjiro Yamamoto*1
Munehiro Kakimi*1
Yoshinori Yamasaki*3

*1Member，Mitsubishi Power, Ltd.
(〒220-8401 3-3-1, Minatomirai, Nishi-Ward,Yokohama City,Kanagawa)
*2Member，Mitsubishi Heavy Industries, Ltd.
(〒851-0392 5-717-1,Fukahori machi, Nagasaki City, Nagasaki)
*3Member, Mitsubishi Power(Philippines)Inc.
(AG&P Special Economic Zone Brgy. San Roque, Bauan, 4201, Batangas, Philippines)