NETL researchers are studying supercritical CO₂ power cycles to improve thermal efficiency and alleviate adverse environmental impacts of using fossil fuels to generate power—work they hope will someday result in zero emissions and record-breaking efficiencies. This work features a special type of combustion known as oxyfuel combustion (or oxycombustion), in which oxygen rather than ambient air is used to combust fuel. The resulting flue gas is composed of highly concentrated, or supercritical, CO₂.

Supercritical CO₂ power cycles operate similarly to other turbine cycles, but they use CO₂ as the working fluid in the turbomachinery. In its supercritical state, the CO₂ remains liquid-like rather than gas-like. One advantage of this approach is that a large amount of energy can be extracted from relatively small-scale equipment.  In addition, because supercritical CO₂ power cycles operate at very high pressures, they operate more efficiently, which means more energy can be created from less fuel and with a reduced cost. Integrating oxycombustion into supercritical CO₂ power cycles offers an added benefit of avoiding the need for expensive carbon capture systems that hinder efficiency and drive up the cost of energy.

To help advance this technology to market, NETL researchers are investigating the science behind oxycombustors. Because supercritical CO₂ power cycles require higher pressures than traditional power generation systems, the physics, chemistry, materials, and components do not behave as they would in conventional systems. Therefore, designing oxycombustors is pioneering territory that requires new information.

“Supercritical CO₂ oxycombustors operate at pressures that are well beyond our current design experience,” said Dr. Peter Strakey of NETL’s Thermal Sciences Team. “Such high pressures affect combustion dynamics and heat transfer to name a few, and affect how the combustor will operate. Understanding how these processes function under the high pressures of supercritical CO₂ power cycles is necessary for the design process. One method to study those processes is through the use of computational fluid dynamics.”

Computational fluid dynamics (CFD) is a type of computer modeling that allows researchers to simulate the physical and chemical processes occurring in a complex environment such as a combustor. In an oxycombustor, this includes factors such as heat release, which occurs in a different manner for oxygen and CO₂ than it does for ambient air and so affects the combustion dynamics and other processes differently. 

“Right now, we’re developing validation data to prove the models we’re using will be accurate for oxycombustors,” Strakey explained. “We know how the chemistry and physics behave at pressures of 30 bar, but we need to find out how they will behave at pressures of 300 bar.”

CFD simulations, such as the example in the illustration above, show researchers how the fuel, oxygen and CO₂ mix and react in a combustor to produce heat. The simulations can help guide the combustor designer in how to optimize injector mixing, combustor wall cooling, flame ignition and control combustion dynamics.

The work is providing a detailed analysis of oxycombustion systems, which is breakthrough work because no industrial-scale demonstration projects or even lab-scale data are currently available for study. The data from this CFD work will be essential for designing oxycombustors with optimized performance—a critical aspect because the economics of supercritical CO₂ power cycles rely on a highly efficient, optimized combustor.

Ultimately NETL research on oxycombustion is ushering in power generation that is highly efficient with near zero emissions, enhancing our nation’s energy foundation and protecting the environment for future generations.