Abstract
Post-combustion capture from power plants may offer an opportunity for power generation where the CO2 capture unit can be shut down in periods, in order to respond to short periods of high electric power demand in the power grid. As the CO2 capture unit is shut down, the power output from the power plant can be increased due to a temporary stop in low-pressure (LP) steam extraction for solvent regeneration, i.e. an increase of steam flow through the LP turbine section.
In the case of natural-gas fired combined cycles, there is (usually) only one LP double flow steam turbine, and hence there is no flexibility in terms of possibility to shut down an LP turbine.
If an NGCC is to be designed so that it is able to handle operation without CO2 capture as well as operation at full load and part load with CO2 capture and solvent regeneration, this would put high demands on LP steam turbine design, since varying LP turbine flow will change the last-stage loading and may turn the stage into "turn-up" (or recompression) mode, as described below.
The pressure at a certain point within the turbine is fairly proportional to the mass flow passing downstream. Hence, one can show that the pressure ratio over any turbine stage except the last one is constant since the pressure before and after is proportional to the flow. The condenser pressure, however, is driven by the LMTD and HTC-values and the last LP turbine stage therefore becomes too lightly loaded when the LP turbine is operating at part load. This “turn-up" mode is a direct result of light loading and an associated imbalance in the radial force field. The resulting effect is that the stream-lines are packed towards the outer annulus, leaving a strong recirculation zone near the hub region. This recirculation zone generates large amounts of heat since the rotor feeds energy into the “trapped” steam, which inevitably calls for cooling of the last turbine stage when steam flows are low.
All OEMs have certain LP turbine exhaust size series with certain end blade lengths, available and the optimum when designing a LP turbine is to choose a series that gives an exhaust loss in the order of 30 kJ/kg of steam. This level will result in a suitable balance between the direct exhaust loss and the need for low load cooling. A very large exhaust size will most likely provide a minimum design exhaust loss but not necessarily be optimum at low load, since the need for last turbine stage cooling would increase. A small exhaust size, that fits better with a steady-state operation with steam extraction for 90% CO2 capture, would reduce the need for cooling, but would give large exhaust losses when operating the plant without CO2 capture. Altogether, the LP turbine for an NGCC with post-combustion CO2 capture should be designed for operation in both modes – with a balance between full condensing mode exhaust loss and turn-up at CO2 capture mode.
With the combination of established technology and modern 2D and 3D design tools, it is shown how a modern LP steam turbine could be designed in a way that allows for large variations in the steam flow, and thus flexibility in the CO2 capture over time from an NGCC. Furthermore, the paper will provide the knowledge and typical values of input data that are required in order to enable the process calculations of these flexible NGCC power plants with post-combustion capture of CO2. This, in turn, enables quantification of what the efficiency penalty will be for an NGCC designed for operating both with and without post-combustion capture, compared to power plants that are optimized for operating either without or with CO2 capture.
In the case of natural-gas fired combined cycles, there is (usually) only one LP double flow steam turbine, and hence there is no flexibility in terms of possibility to shut down an LP turbine.
If an NGCC is to be designed so that it is able to handle operation without CO2 capture as well as operation at full load and part load with CO2 capture and solvent regeneration, this would put high demands on LP steam turbine design, since varying LP turbine flow will change the last-stage loading and may turn the stage into "turn-up" (or recompression) mode, as described below.
The pressure at a certain point within the turbine is fairly proportional to the mass flow passing downstream. Hence, one can show that the pressure ratio over any turbine stage except the last one is constant since the pressure before and after is proportional to the flow. The condenser pressure, however, is driven by the LMTD and HTC-values and the last LP turbine stage therefore becomes too lightly loaded when the LP turbine is operating at part load. This “turn-up" mode is a direct result of light loading and an associated imbalance in the radial force field. The resulting effect is that the stream-lines are packed towards the outer annulus, leaving a strong recirculation zone near the hub region. This recirculation zone generates large amounts of heat since the rotor feeds energy into the “trapped” steam, which inevitably calls for cooling of the last turbine stage when steam flows are low.
All OEMs have certain LP turbine exhaust size series with certain end blade lengths, available and the optimum when designing a LP turbine is to choose a series that gives an exhaust loss in the order of 30 kJ/kg of steam. This level will result in a suitable balance between the direct exhaust loss and the need for low load cooling. A very large exhaust size will most likely provide a minimum design exhaust loss but not necessarily be optimum at low load, since the need for last turbine stage cooling would increase. A small exhaust size, that fits better with a steady-state operation with steam extraction for 90% CO2 capture, would reduce the need for cooling, but would give large exhaust losses when operating the plant without CO2 capture. Altogether, the LP turbine for an NGCC with post-combustion CO2 capture should be designed for operation in both modes – with a balance between full condensing mode exhaust loss and turn-up at CO2 capture mode.
With the combination of established technology and modern 2D and 3D design tools, it is shown how a modern LP steam turbine could be designed in a way that allows for large variations in the steam flow, and thus flexibility in the CO2 capture over time from an NGCC. Furthermore, the paper will provide the knowledge and typical values of input data that are required in order to enable the process calculations of these flexible NGCC power plants with post-combustion capture of CO2. This, in turn, enables quantification of what the efficiency penalty will be for an NGCC designed for operating both with and without post-combustion capture, compared to power plants that are optimized for operating either without or with CO2 capture.
