Mighty River Power operate a 122MW natural gas-fuelled, co-generation station by the name of Southdown. Southdown employs two gas turbines which exhaust into Heat Recovery Steam Generators (HRSG). The HRSG then produces steam to power a steam turbine in a co-generation process.
The ability of the station's turbines to turn the natural gas fuel into useful energy is heavily influenced by the pressure losses downstream in the exhaust ducting. In an effort to improve the efficiency of this process, design modifications to the exhaust duct were proposed. However, the cost of the proposed design alterations verses the benefits from improved efficiencies needed to be quantified.
CFD fluid velocities in the
existing steam duct
Using a simulation technique called Computational Fluid Dynamics (CFD), the existing and proposed systems were modelled and the system pressure drops calculated. CFD is a simulation tool used extensively in process and utility industries to predict pressure, temperature and velocity distributions in fluids. In this case, CFD was required to provide a realistic prediction of the pressure losses for both existing and proposed designs so that the benefit could be quantified.
The first phase of the study was to match experimental measurements of pressure and temperature in the exhaust duct and use this to gauge the level of accuracy of the simulation.
Interestingly, the CFD result was able to explain several observations in the experimental data that were not immediately apparent and provided an accurate estimate of the current system losses.
The second phase of the study brief involved simulation of proposed duct modifications to determine predicted efficiency gains in the system while optimising the design to ensure optimum flow performance. This efficiency gain provided a monetary value that could be used for the return on investment (ROI) calculation of the planned upgrade.
In this study CFD provided the following benefits to Mighty River Power:
- Quantitative ROI numbers for go/no-go decision on capital investment
- Reduced the risk that proposed designs would not perform to requirements
- Explanation of key physical observations in existing ducts
- Improved understand of the existing and proposed steam ducts
To learn more about Computational Fluid Dynamics (CFD) click here.