Efficiently generate steam from cogeneration plants

As cogeneration gets more popular, some plants have two choices of equipment for generating steam. Plant engineers need to have a decision chart to split the duty efficiently between (oil-fired or gas-fired) steam generators (SGs) and heat recovery steam generators (HRSGs) using the exhaust from gas turbines. Underlying the dilemma is that the load-versus-efficiency characteristics of both types of equipment are different. When the limitations of each type of equipment and its capability are considered, analysis can come up with several selection possibilities. It is almost always more efficient to generate steam in an HRSG (designed for firing) as compared with conventional steam generators. However, other aspects, such as maintenance, availability of personnel, equipment limitations and operating costs, should also be considered before making a final decision. Loading each type of equipment differently also affects the overall efficiency or the fuel consumption. This article describes the performance aspects of representative steam generators and gas turbine HRSGs and suggests how plant engineers can generate steam efficiently. It also illustrates how to construct a decision chart for a typical installation. The equipment was picked arbitrarily to show the method. The natural gas fired steam generator has a maximum capacity of 100,000 lb/h,more » 400-psig saturated steam, and the gas-turbine-exhaust HRSG has the same capacity. It is designed for supplementary firing with natural gas.« less