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Optimized Cooling System Operation for a Medium Sized Coal-fired Steam Plant

Whereas in STEAM PRO you often run many cases with different configurations or different design assumptions to optimize a design for a possible project, in STEAM MASTER, you often want to run many different operational scenarios using a single plant configuration.  This sample focuses on using a single plant model to compute expected performance under different ambient situations.

This is a model of a 250 MW single reheat steam plant with a pulverized coal boiler, and a cooling tower servicing the wet condenser.  The plant was designed for summer ambient conditions in Manila, Philippines where the ASHRAE 0.4% evaporation ambient is 32.8 °C mean dry bulb with a 28.4 °C mean coincident wet bulb.

Thermoflow’s E-LINK utility lets you run batches of cases from MS Excel.  This model is run in E-LINK at various conditions to develop the results described below.

This chart shows baseload plant power when the cooling system, designed at 32.8 °C at 72% relative humidity (28.4 °C wet bulb), is operated at different conditions. Typically, operators have cooling tower operating guidelines that are developed using approximate methods, or rules of thumb related to tower basin temperature limits. This can lead the operators to miss revenue opportunities when the plant could safely export more power at the same fuel flow. These missed opportunities can add up to tens of thousands of dollars per year of forgone revenue.

Here, STEAM MASTER is used to simulate plant operation at different site weather conditions representing seasons of the year. The plant model is run at baseload, with different number of cooling tower cells running. This mimics the choices the operators have to select number of running tower cells. The question is: what is the best way to run the cooling tower?  The answer is the STEAM MASTER prediction of highest net output at fixed fuel consumption for each ambient wet bulb temperature.  These points are encircled in green in the above chart.  The results can be summarized for easy lookup as follows.

Ambient Wet Bulb Temperature, °C Optimum Number of Running Cooling Tower Cells
7 4
19 6
24 8
30 11

Some operators equate minimum condenser pressure with optimum operation. However, sometimes lower condenser pressures acts to reduce plant net power due to increased leaving losses and/or higher auxiliary loads. The only way to determine how to optimize cooling system operation is by collectively considering these effects, as is done in STEAM MASTER. The above plot shows the condenser pressure achieved at various wet bulb temperatures with different numbers of cooling tower fans running. At each ambient, the condenser pressure is clearly reduced by running more cells, as expected. However, the green ovals encircle the condition that maximizes net power, at fixed fuel consumption. None of these optima are associated with minimum achievable condenser pressure.

Consider the coolest case where the wet bulb is 7 °C. If we assume power is worth 35 $/MWhr, then the difference in revenue between optimum number of cells, and minimum condenser pressure is 2.24 MW, or 78 $/hour. If one assumes that ambient condition occurs 5% of the year, and the plant runs at baseload for 8000 hours/year, the difference between optimized operation and operation at minimum condenser pressure is over 31,000 $/year.