Direct power extraction with oxy-combustion: An overview of magnetohydrodynamic research activities at the netl-Regional University Alliance (RUA)

In conventional oxy-fuel power generation scenarios, oxy-fuel combustion provides no significant advantage other than to simplify CO₂ capture. So in terms of power production and efficiency, the energy and costs required to produce that oxygen are a burden. However, the high temperatures possible with oxy-fuel combustion can enable direct electric power extraction from high-temperature electrically-conductive gases using magnetohydrodynamic (MHD) principles, which would then be followed by a steam cycle also producing electricity. The combined system would produce a high CO₂ exhaust stream - yet with efficiency that may exceed today's best coal power systems. The concept of adding an MHD topping unit to a coal fired power plant in order to directly extract electrical power is not new, and significant effort was made in this direction from about 1973 to 1993. During this time period, it was shown the MHD concept worked in the sense that power was generated, but ultimately development was discontinued due to the high cost of designing, constructing, and operating a complete MHD-steam plant. Additionally, there were a number of technical challenges associated with the technology. Some specific issues cited for coal MHD were slag removal problems, MHD channel operation problems, and cost effectiveness of seed utilization. In this paper, we revisit the use of MHD technology in the context of using it with oxy-combustion to enable cost effective carbon capture. Ongoing research activities within the National Energy Technology Laboratory - Regional University Alliance (NETL-RUA) to address legacy MHD power challenges, and apply new computational tools to MHD power systems are presented. Much has changed since earlier MHD studies: oxygen supplies have become less expensive (because of interest in oxy-fuel for CO₂ control). Superconducting magnets have improved substantially. Perhaps the most dramatic technological improvement since previous MHD efforts is in the area of computational modeling. Today, we have three dimensional multi-physics models that could be utilized to design a more effective system (combustor and generator). To begin addressing the combustion issue, current work applies transported probability density function methods to solve the high-temperature combustion problem. An MHD generator model, which considers fluid dynamics and heat transfer, as well as relevant MHD equations involved in the process, is presented. The modeling efforts also address issues in using wall functions to bridge the laminar sublayer to the fully turbulent boundary layer when Lorentz force is dominant or equally important as compared to other forces.