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34 www.coalage.com June 2017 power plant technology Conventional Power Stations Become More Flexible and Safe by hans christian schröder Power producers are increasingly faced with the need to respond to fluctuating amounts of competitive sources being fed into the grid. For them to be able to do so, technical changes must be implement- ed in the operation of large-scale existing power stations. The change to "single-mill" operation at a power station demonstrates that even older power plants can be oper- ated at low partial loads. Last December demonstrated the degree of flexibility that is possible in the operation of conventional power stations today. A storm front caused the wind farms in northern Germany to feed exces- sive amounts of electricity into the grid. As a consequence, the power output of many conventional power stations had to be reduced by up to 40%. Through these measures, conventional power stations provide the level of flexibility required to ensure permanent load balancing. And, they have been doing so with great suc- cess. But how are these changes realized in practice? And what legal requirements must be observed in terms of licensing and approval? Startup Operation Becomes the Standard Mode of Operation While new power stations are to some extent designed for particularly flexible operations, the operational equipment of many existing plants needs to be aligned. Noticeable in this context is that many older plants are particularly well-suit- ed for this task, due to their more robust construction and often ideal design and processes. Single-mill operation is one possible solution. However, this approach depends strongly on the design of the combustion chamber and the implement- ed firing system. Tangentially fired burn- ers, also known as the "EVT method," are particularly well-suited for this purpose, as their combustion chambers can be re- garded as a complete burner unit. Load flexibility primarily means oper- ating the plants at lower part loads. In the past, this load condition was referred to as "startup operation," and was necessary to synchronize the generator. In a startup operation, auxiliary firing (oil or gas burn- ers) was used to bring the plants to a pre- defined minimum thermal output. Coal mills were then successively activated one by one. The auxiliary firing was shut off on activation of the second mill, when the defined thermal output was reached using pulverized coal. In most plants with a pre- A power plant relying on single-mill operation could be run at a higher load range or could accept a higher load than in a two-mill operation.