Are Advanced Large Gas Turbine Ready for Use in Captive Applications?
Industrial facilities are applications where the majority of electricity and steam productions are devoted to internal consumption rather than being exported. Oil refineries, smelters, and chemical and desalinization plants are typical examples. In the case of aluminum smelters, the vast quantities of electricity required by the process cannot be supplied by utilities from the grid and therefore cogeneration is required. Since power and/or steam supply interruptions might have catastrophic effects on the facility processes, the paramount requirement for “dedicated power plants’ is availability and reliability rather than high performance. It is common to find older D and E gas turbines used as the prime mover. In recent years, however, advanced gas turbines (GTs) successfully demonstrated close to 60% efficiency in combined cycle applications. The G and H technology classes, using steam to perform GT cooling duty, accumulated thousands of operating hours. Many improvements from G and H were also implemented into FX Class (latest variants of the air-cooled F technology class). Firstly, the paper addresses the strategies for incorporation of advanced GTs in captive applications where the equipment must cope with rapid changes in power demand, such as load swings, load rejection, harmonic currents, etc. Further, it examines a variety of designs, where there is a high and low process steam demand for process. The discussion encompasses plant optimization aiming at a high level of redundancy: multi-shaft arrangements, common steam headers, and heavy supplementary firing. The selection of an optimum steam turbine (ST) is also discussed, including steam extraction locations and the ability to operate efficiently with steam extraction on and off. Issues dealing with steam purity requirements and water treatment sizing will also be addressed. Since the amount and quality of the condensate return vary substantially, maintaining the water chemistry is essential. In continuation, the article will describe the challenges for the control system design and in particular, the requirements to maintain tight process conditions during transients. Finally, the paper will present the experience of an engineering, procurement, and construction (EPC) contractor for several “captive applications” projects.