From its peak use in 2007, America’s coal consumption has fallen over 44%. Since 2010, around 630 coal-fired plants across 43 states (nearly 40 percent of U.S. coal-powered facilities) have either closed or announced their intentions to do so, according to data by the American Coalition for Clean Coal Electricity. Renewable fuel sources such as solar and wind are becoming available and affordable, spelling the demise of coal as a power source in the United States.
As ambitious and motivated as energy companies are to save costs and save the planet, shuttering coal-fired plants takes time. While new renewable energy comes online, coal power is still in the mix, going through what the industry terms as “cycling”. On again, off again, providing variable amounts of electricity to the grid, one ingredient in a recipe of power sources. While this might sound like a winning combination, there is a problem: coal-fired equipment and facilities weren’t design to “cycle” and doing so puts a tremendous strain on infrastructure, particularly steam headers.
Dr. Ahmed Megri, an N.C. A&T associate professor in the Department of Civil, Architectural and Environmental Engineering in the College of Engineering, is trying to buy the coal industry some time with a $400,000 research contract through the U.S. Department of Energy.
To review, coal-fired power plants produce electricity by burning coal in a steam generator, transforming water into high pressure, high temperature steam. The steam header receives this steam from a series of boilers, holding it under pressure. As it is released, the steam flows through a series of turbines which drive an electrical generator to produce electricity. The exhaust steam from the turbines is cooled, condensed back into water and finally returned to the steam generator to start the process once again. As the steam is being held under pressure in the steam header, damage can occur in the form of cracks, leading to leaks and eventually failure. When a steam header fails, it costs a power plant about $2.5 million in repairs and downtime. Finding a way to avoid steam header failures is the aim of Megri’s work.
Megri believes fatigue damage of steam headers under cyclic loading can be mitigated by using an advanced nickel-based alloy called INCONEL® ALLOY 740H®. Nicknamed IN740H, this novel alloy displays fantastic mechanical properties which make it an excellent candidate for use in power plants: it is stronger than presently-used steel, you can use less of it which means less welding, smaller surface areas, fewer incidents of cracking, lower maintenance and lower human labor costs.
Steam header photo courtesy of the Electric Power Research Institute
Megri, in collaboration with the University of North Carolina at Charlotte, is designing a replacement header for an existing coal-fired power plant using IN740H and conducting advanced computational studies to investigate the accumulating fatigue damages under cyclic loading. Research involving computational fluid dynamics along with machine learning will be used to obtain an accurate description of the temporal and spatial distribution of the heat transfer coefficient in the header.
The heat transfer coefficient will then be used in a thermo–mechanical finite element analysis of the header to obtain the stress distribution in the header. To conduct the stress analysis, Megri will use the cyclic experimental data of IN740H to develop a viscoplastic material (a viscoplastic material has a yield stress under which it will not deform) model for IN740H.
The stress distribution obtained using finite element analysis will be used to assess the fatigue damage occurring during each loading cycle in the steam header. This will allow Megri to obtain an accurate estimate of the lifetime of steam headers fabricated from IN740H. Having this estimate will allow coal-fired power plants to effectively measure the return on investment of replacing old, at-risk steel steam headers with new ones made from the new, novel alloy.