Operation of Large Industrial Gas Turbines During Periods of Low Electricity Demands

When grid power demand is low or the power price is negative, running a gas turbine at Full Speed No Load (FSNL) provides a way to avoid producing power, while maintaining the ability to both generate process steam for industrial purposes in a Heat Recovery Steam Generator (HRSG) and to respond quickly to sudden surges in grid power demand. In this paper, turbine thermodynamics of FSNL, closure of inlet guide vanes (IGV), and compressor bleed rates are analysed for General Electric’s 7EA gas turbine using the software Thermoflow. This study shows that FSNL can be achieved only by bleeding of compressor mass flow through two available bleeds, one in the front section of the compressor, and one in the back section of the turbine. This paper shows that bleeding from bleed 1 will lower compressor work, saving fuel, but it will also drop the exhaust temperature too low, preventing steam production. With extraction from bleed 2, the exhaust temperature remains high. Either way, the exhaust energy is reduced due to reduction of mass flow, but steam generation in reduced amount is possible. The extracted mass flow is choked and limited by the critical section in the flow path of the compressor bleed. In this study, the required minimal size of the slot in bleed 2 is identified for a range of IGV positions.