2010

Innovative Waste Heat Recovery Systems and Improvements with Advanced Turbomachinery

CO2 Capture and Power Generation

It is interesting to consider the “advantage” that this fluid has in being a fluid that is almost universally considered to be best kept contained, rather than released into the atmosphere. There is considerable prospect in having carbon dioxide as the working fluid in a waste heat recovery system. We could contemplate that the CO2 containment vessel be the closed piping of a power-producing cycle that generates as much power from the exhaust gas of a fossil-fueled power plant as is needed to drive compressors for later sequestration of the CO2. The DOE has been sponsoring feasibility studies in this field for many years; in particular, cycles that operate in the supercritical region. In the supercritical region, the fluid can sometimes behave as a liquid and then suddenly as a vapor, depending on the small changes in temperature or pressure.

The capture of CO2 from the exhaust products of a conventional fossil-fueled (particularly coal) power generation facility has been given increased interest in the advent of recent “Cap-and-Trade” legislation. Research has been funded by DOE’s National Energy Technology Laboratory (NETL) for methods of capturing and sequestering CO2. Several of these studies have promoted the integration of CO2 capture with power generation using coal as the primary fossil fuel. For example, a project funded by NETL (Advanced CO2 Cycle Power Generation) for Foster Wheeler North America, focuses on a cycle that combines power generation using fluidized bed technology and coal to syn-gas production to generate power while also capturing 100% of the CO2. This cycle has the advantage of not requiring amines to absorb the CO2 from the exhaust products of coal combustion. An additional CO2 sequestration and solid-fueled power generation system, developed by Concepts NREC, reduces CO2 emissions while combusting coal or biomass. Clearly prominent once again is the need for advanced turbomachinery that can improve the power production and thus increase the cost-effectiveness for the user.

A common thread exists through these advanced, but very feasible, waste heat recovery systems, and truly the heart of the engineering challenge is the analysis, design, and fabrication of the necessary turbomachinery. For power generation, this includes turbines, compressors, and pumps. The turbomachinery can be axial or radial, impulse or reactionary type. The choice depends upon the fluid operating pressure ratios and volume flow rates, and these in turn depend on the temperature of the available waste heat, the temperature of the available cooling source, and the power required. The advances in machining technology include CAM software that can transform detailed engineering drawings of the turbine rotor almost immediately into machine code that can guide the 5-axis machining of the entire turbine or compressor impeller. The turbomachinery specialty software, combined with CFD technology, has enabled even the most sweeping or contoured, shrouded turbine or compressor blade to be manufactured without compromising the thermo-fluids structural engineering analysis for the blade shape in order to achieve maximum power recovery efficiency.

The common technical challenge that must be met for each of these turbomachines, in addition to the expert thermo-fluid analysis and design, is the proper selection of compatible materials between the working fluid of choice and the materials of construction for the high-speed turbine or compressor. While it is desirable to increase the operating limits of the rotor tip speeds to reduce the number of stages for a particular application (and thus reduce complexity and cost of the unit), the rotor materials must be chosen for their strength to survive the enormous centrifugal stresses, while also maintaining compatibility with the working fluid. Thus, the proper application of CFD and FEA in the design of the machine is necessary.

This article was written by Frank Di Bella, Large-Product Development Program Manager at Concepts NREC in Woburn, MA. For more information, visit http://info.hotims.com/28053-121.