HRSG User's Group: Improving steam-plant reliability, durability and profitability    

Next-Generation, Fast-Start HRSGs

Marlon Farquharson, Siemens Heat Transfer Technology

Today’s heat-recovery steam generators (HRSGs) are suffering reliability problems, mainly because they’re being operated in the cycling mode, instead of the baseload mode for which they were designed. The good news is, HRSG suppliers are adapting to the market demands and are designing the next generation of HRSGs to have cycling-friendly—also called fast-start—features.

One of the market trends that is forcing HRSGs into the cycling mode is the mandate for more renewable-energy plants. Why does that affect HRSGs? Because a typical daily electricity-demand curve shows a morning peak and an evening peak. However, these peaks in demand don’t correspond with the output of renewable energy sources, which fluctuate based on natural changes in the winds, and the sun’s position in the sky (Figure 1). 

Figure 1: Combined-cycle plants must operate in the cycling mode to match the fluctuating demand-curve. Source: Siemens Heat Transfer Technology

This intermittent and stochastic nature of Renewable energy creates an imbalance in the electricity grid, which must be stabilized by other powerplants—a rigorous job that falls primarily on the shoulders of gas-fired combined cycles. To fill the Renewables’ shortfall and maintain grid-balance, combined-cycle plants need to:
            1. Be very flexible to keep up with the fluctuating demand-curve.
            2. Be able to withstand daily starts and stops.
            3. Be able to ramp-up and throttle-down to meet demand as quickly as possible.

In the coming years, renewable-energy plants are expected to supply even more electricity to the grid, so tomorrow’s combined-cycle plants must be designed to be even better at these three needs than today’s combined cycles (Figure 2).

Figure 2: 2020 expected CaISO duck curve. Courtesy: Siemens Heat Transfer Technology

With this goal in mind, Siemens Heat Transfer Technology—the Siemens division formed from the acquisition of NEM Energy—is developing the DrumPlus™ HRSG.

The DrumPlus™ HRSG combines the fast-start characteristics of a once-through steam generator, with the benefits of the conventional drum-type HRSG—namely, greater operator familiarity with the water-chemistry regime, and better tolerance of chemistry upsets. In the DrumPlus™ HRSG, the high-pressure steam drum’s diameter and wall thickness are minimized by splitting up the important function of water-steam separation into two stages, with the second stage occurring in bottles outside the steam drum (Figure 3).

Figure 3: The second stage of water-steam separation occurs in bottles outside the drum, in Siemens’ new drum-plus technology. Courtesy: Siemens Heat Transfer Technology

Another unique feature of the DrumPlus™ HRSG is that the two exterior Downcomers in today’s Evaporator section have been replaced by interior Downcomers, which further improves flexibility, and lowers thermal stresses. The DrumPlus™ HRSG is already in commercial operation at the El Segundo combined-cycle project in California and is under construction at nine other combined-cycle projects worldwide. According to Siemens, this design will increase the service life of the HRSG by as much as seven times, compared to today’s typical, high-pressure drum-type HRSG.

In addition to these engineering benefits, there are economic and environmental benefits of the DrumPlus™ HRSG. Specifically, it will yield greater electricity sales and fewer air emissions, simply because it will complete each startup faster. For instance, during the first 40 minutes of startup with today’s HRSG, the typical F-class plant generates approximately 20 MWh. In comparison, during the first 40 minutes of startup with a DrumPlus™ HRSG, the typical F-class plant will generate approximately 116MWh—nearly six times more! The faster startup will also warm up the selective catalytic reduction (SCR) system faster, so it will begin reducing NOx emissions sooner. Siemens projects that NOx emissions from tomorrow’s F-class plant will be reduced by 58 percent, compared to today’s plant (Figure 4).

Figure 4. Benefits of fast-start DrumPlusTMHRSG Courtesy: Siemens Heat Transfer Technology

The future is unpredictable, of course, but the mandate for renewables seems here to stay and probably will grow stronger. As a result, tomorrow’s combined-cycle plants will need fast-start HRSGs—such as the Siemens DrumPlus™ design—if they are going to remain competitive, and productive, for many years to come.