A gas turbine cogeneration plant produces power and process steam. Under the PURPA law, surplus electric power can be sold to the local utility. Since process steam generally cannot be exported, it is better to have an excess of power than an excess of steam.
Because of low rates offered for surplus power, or for other possible reasons, an owner may not wish to sell power, so it may be necessary to operate at a power-to-steam ratio that does not match the outputs of a gas turbine with a simple heat recovery boiler. If more steam is needed, supplementary firing may be included in the heat recovery boiler.
If the need is for more power, a back pressure steam turbine can be included. This reduces the steam output by requiring higher steam pressure. Further power increase and steam reduction can be obtained with a condensing steam turbine. If neither the full steam output nor additional power is required, capital cost can be reduced by inclusion of a smaller, less-efficient heat recovery boiler.
This paper compares these means of adjusting the power and steam outputs of a gas turbine cogeneration system to obtain the most cost effective system.