Gas Turbine 10 Exhaust Stack Heat Recovery System

Photo of Derek Bass, Chris Lam, Vian Nguyen Students: Derek Bass, Chris Lam, Vian Nguyen

Sponsor: University of Texas Utilities and Energy Management

Date: Spring 2011

Requirements:
The main requirement of the project was a four year payback by offsetting natural gas usage or electricity usage rated at $6.50 per MMBTU and $0.07 per kWh, respectively. The major constraints of the project were no back pressure in the exhaust stack, space constraints, and corrosion issues. Additional back pressure in the exhaust stack was not allowable because it decreases the overall efficiency of the gas turbine. Space is limited within the power plant building and placing equipment on the roof introduces weight concerns due to the low load bearing design on the roof. The possibility of corrosion from sulfur in the flue gas and chlorides in the tower drift from the cooling towers were considered in the material selection.

Problem:
The University of Texas at Austin operates its own natural gas power plant to supply the campus' electrical, steam, and chilled water needs. The power plant is well equipped to handle campus energy needs as it has a maximum capacity of 137 Megawatts, with a historic peak demand of 67 Megawatts. The University of Texas at Austin recently installed and commenced operation of a new GE LM2500 G4+ gas turbine in March, 2010. The new gas turbine replaced an older, less efficient gas turbine and only runs during the winter season. Although the new turbine is more efficient, an unusually high exhaust stack temperature is observed during operation, at approximately 315°F. Gas turbines with an operating history on campus have normally exhibited exhaust stack temperatures of 250°F to 260°F. The project scope entailed recovering the excess waste heat from the exhaust stack, identifying an energy stream to use the recovered excess waste heat, and designing a viable solution proven by a technical and economic analysis.

Solution:
The final design of the exhaust stack heat recovery system entailed removing excess flue gases with a fan from the side of the exhaust stack and using it to heat condensate water. This layout avoids backpressure within the stack. The fan is placed downstream of the heat exchanger because the temperature of the flue gas is lower at that point. The heat exchanger itself is placed downstream of the condensate preheater and upstream of the deaerator. The deaerator flashes condensate with steam to remove dissolved gases in the water. Heating the condensate with the exhaust gases allows the heat recovery system to offset the amount of natural gas used to make steam for the deaerator. The heat exchanger will be placed on the roof of the power plant due to the space constraints, but in an area with load bearing structural supports. When selecting the heat exchanger, corrosion concerns were considered, and a measures were taken to prevent the possibility of corrosion arising from sulfur, although minimal, with a Heresite coating in the heat exchanger. The initial capital investment of the proposed system is estimated to be $58,170 with a calculated $75,000 annual savings, resulting in a payback period of less than one year.

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