Technical and economic analysis of steam-injected gas-turbine cogeneration

Industrial cogeneration is gaining popularity as an energy and money saving alternative to separate steam and electricity generation. Among cogeneration technologies, gas‐turbine systems are attractive largely because of their lower capital cost and high thermodynamic efficiency. However, at industrial plants where steam and electricity loads vary daily, seasonally, or unpredictably, the economics of conventional gas turbines are often unfavorable due to low capacity utilization.Steam‐injected gas‐turbine cogeneration overcomes the part‐load problem by providing for excess steam to be injected back into the turbine to raise electrical output and generating efficiency. Under provisions of the Public Utilities Regulatory Policies Act, any excess electricity can be sold to the local grid at the prevailing avoided cost of electricity. Steam‐injected gas‐turbine cogeneration can result in a consistently high rate of return on investment over a wide range of variation in process steam loads. Moreover, this technology can also give rise to greater annual electricity production and fuel savings per unit of process steam generated, compared to simple‐cycle cogeneration, making the technology attractive from the perspective of society, as well as that of the user.Steam‐injected gas‐turbines may soon find applications in electric utility base‐load generation, as well, since it appears that electrical generating efficiencies in excess of 50% can be obtained from turbines producing of the order of 100 MW of electricity at a fully‐installed capital cost as low as $500/kW.