LabNotes - March 2014

Underground Microbes and NETL Energy Research

The Earth’s subsurface is an ecosystem rich in microbes.  It is also the globe’s source of fossil fuels and houses huge volumes of pore space for the storage of CO2 generated by power plants.  CO2 storage and fossil fuel production have the potential to both affect and be affected by microbial populations.  NETL is investigating the interactions of microbes with substances commonly injected into the subsurface – CO2 used to enhance oil production, and fluids used for shale gas development.  The goal is to learn more about how the subsurface changes when exposed to these substances, and how the microbes themselves participate in or are impacted by those changes.

The toughness of microbes is staggering. They “live in the deep subsurface, on the surface, in the oceans, in lakes, in rivers, in our guts, in high salinity, in low salinity, and in almost all climates from zero to 121 degrees Celsius,” says Kelley Rabjohns a postgraduate Oak Ridge Institute of Science and Technology (ORISE) intern.  Microbes’ homes even include underground carbon sequestration sites. Until now, few publications have examined the microbial communities found in sequestration sites and how those communities respond when supercritical CO2 invades their surroundings.  Rabjohns, working on carbon storage, is looking to change that. 

Figure 1. Temperatures of carbon capture and storage sites in North America. Most of the sites are at temperatures capable of supporting microbial life.

Rabjohns used data from NETL’s Carbon Capture, Utilization, nd Storage (CCUS) database, NATCARB, to make a map that lays the temperatures of active CCUS sites over a geothermal heat flow map of the United States. She considered the temperatures and pressures found at active CCUS sites to determine which ones were likely hosts to microbes such as bacteria and archaea (single-celled organisms similar in size to bacteria but differing from them in molecular organization). Only three sites out of 94 had temperatures too high to be hospitable to microbes. The others could suit a variety of microbes, and those microbes could have distinct effects on CO2 behavior.  Likewise, CO2 could affect the microbial ecosystems.  Rabjohns gives one example of how microbes and CO2 can interact: archaea use inorganic carbon, such as CO2, to produce organic compounds for biosynthesis; not only does this process cause the microbial community to grow, but it also helps keep sequestered CO2 fixed in place.

It is these types of interactions that make Rabjohns’ research so intriguing. Her work affirms that species thriving under the surface of the earth can influence, and be influenced by supercritical CO2. “Microbes are an active part of the subsurface,” says Rabjohns, “and should be considered when thinking about the effects of CO2 sequestration.”

Figure 2. Clusters of pyrite crystals in the Marcellus shale before gas production. After injection, the shale will be analyzed to evaluate any changes in the minerals or microbes.

A second ORISE intern, Jessie Wishart, is studying microbes in a different environment – shale gas formations.  As part of the Unconventional Resources research team, she is defining the characteristics of the shale gas formation before production begins, and outlining the parameters that permit or discourage microbial colonization.  In addition to temperature, Wishart found that porosity, the presence of fractures in the rocks, and the salinity of fluids in the rocks are also important.  She is developing a protocol for isolating and analyzing microbes from flowback fluids (fluids injected into the shale gas formation that return to the surface), and characterizing the minerals in the pre-injection shale. 

The research team will inoculate the shales with injection fluids in the lab, exposing them to oxidizing and reducing conditions, and Wishart’s work will serve as a baseline for understanding the changes that occur in the rock and the fluid during those tests.  For now, her results show that some microbes may flourish in the flowback fluids, further evidence of microbial adaptability and a happy outcome for the microbes themselves.

Contact: Circe Verba, 541-918-4437


Advanced Alloy Treatments for Demanding Applications

 

Figure 1. NETL researchers pouring molten metal.

One might think that all the materials needed to support our modern way of life have already been invented, and that there is little room for improvement, but one would be wrong to think that!  The industries that produce our power, fly us from one side of the country to the other, and build our bridges are constantly changing; and new materials are needed to increase the life span of infrastructure, to meet new performance requirements, and to lower costs.   NETL scientists in the field of alloy development work with, and contribute to, these industries under the Advanced Combustion research program, to help them meet their changing needs.

The process of producing a metal part for an industrial application doesn’t generally end when the part is cast into its final, solid shape.  Instead, the part frequently undergoes heat treatment, a process that changes the part’s microstructure and gives it greater strength or corrosion resistance.  When casting alloy parts, which are made of mixtures of two or more elements, the elements can very easily segregate from the melt in the solidified product.  The result is that on a micro scale, some areas of the microstructure of the casting are enriched in one or several elements while other nearby regions may be deficient in these and enriched in other elements.  Homogenization heat treatments can eliminate these segregated regions by redistributing those elements via diffusion in a more random manner.  Poorly homogenized alloys may show signs of isolated corrosion product quickly – or in the case of a boiler, for example – crack in highly stressed regions during normal use, leading to costly repair or replacement during normal inspection outages.

During homogenization heat treatment, alloys are held at a particular temperature for long periods of time just below the temperature where melting would occur for that alloy’s composition. Temperatures that are well-known for simple alloys or ones that are mature in the industry, but less easy to determine for complex advanced alloys used for heat-resistant applications in aerospace and power. 

Figure 2. A non-homogenized as-cast microstructure is shown on the left. An equivalent region of microstructure after homogenization is shown on the right.

Conventional trial-and-error methods of developing homogenization heat treatment processes are time-consuming and equipment specific (e.g., heating or cooling rate used in differential thermal analysis techniques) and dependent on subsequent microstructure inspection. The process to eventually determine a multi-step homogenization heat treatment may require weeks or months of effort and still result in an unsatisfactory result.  NETL researchers, on the other hand, have developed and tested a materials-based computational approach for developing homogenization heat treatments that eliminates the Edisonian approach.

The NETL-developed method starts with the alloy chemistry then combines thermodynamic and kinetic information with microstructural measurements from the as-solidified casting to design a material- and casting-specific homogenization heat treatment.  The heat treatment is next fully simulated using ThermoCalc© and DICTRA© software. The NETL’s casting is then heat treated according to the simulation. 

NETL has applied for several patents based on this approach, and is continuing to develop and test it for multiple types of heat-resistant alloys, including single crystal nickel-base alloys, high-performance solution- and gamma prime-strengthened nickel-base alloys, as well as for more conventional austenitic, martensitic and ferritic steels. 

Industrial partners such as GE, a manufacturer of turbine components for power generation, are helping to scale the process from lab-scale to commercial through NETL’s ongoing support of the 1400F Steam Turbine Consortium.  Power plants that operate at higher temperatures produce more electricity and less carbon dioxide per ton of coal, but also need to be constructed of materials that can withstand those higher temperatures, making this approach all the more important.  NETL’s computational homogenization heat treatment method can help them achieve their goals in less time and at lower cost.

Contact: Jeff Hawk, 541-918-4404

 
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