NETL Develops Sensor and Control Technologies

Detecting Gas Composition Rapidly and Accurately

Machines and devices that function automatically to keep us safe and provide convenience are characteristic of modern life. For example, our home heating, ventilation and cooling systems have temperature sensors that know if the room is at the user’s desired level of comfort and then cause something like a furnace, heat pump, air conditioner, or blower to turn on or off. The sensor and the resulting control signal comprise a sensor and control loop. Our automobiles and their sub-components use a myriad of sensor and control loops to keep them running safely and efficiently - for example, anti-lock braking systems, air bags, oxygen sensors, and throttle sensors. Similarly, NETL engineers and scientists are working on ways to make energy systems safer and more efficient through the use of novel sensors and control loops that provide more information to the operators and afford a higher level of control.

As natural gas plants become more competitive to build and operate, we will increasingly use natural gas to generate electricity. . We may think of natural gas as a uniform substance, but as a product of nature, it has a significant degree of variability. Natural gas providers take care to remove many of the undesirable components like carbon dioxide, water and nitrogen. But the resulting fuel, composed primarily of methane-- a hydrocarbon molecule with one carbon atom--may also contain varying amounts of hydrocarbon with two or three carbon atoms. The combustion properties of these various constituents vary and can affect BTU content, flame speed, and the need for dilution gases. As the composition of the fuel supply strays from a standard composition for which, say, a gas turbine was designed, adjustments to the turbine operating conditions must be made nearly instantaneously to keep the combustion turbine running at peak efficiency and minimizing emissions.

Laser lab demonstrating optical wave guide.

NETL researchers have devised a gas sensor capable of accurate and continuous readout of the relative amounts of all major fuel-gas components including hydrogen, carbon monoxide, carbon dioxide, oxygen, nitrogen, methane, ethane and propane. The sensor is based on Raman spectroscopy and has been developed to utilize low-power lasers and low-resolution spectrometers and detectors to give readouts in one second or less. The NETL technique uses novel, internally mirrored, hollow core optical fibers that serve as the sampling device as well as the conduit for optical signals to and from the sample. By trapping the gas of interest in a confined space, the optical signal is magnified by a factor of 100 or more over the signal acquired in free space, resulting in measurement errors less than 0.3%. The combination of speed, accuracy, and sensitivity to multiple species makes the gas detection system well-suited for improving control of natural gas-fired turbines based on real-time measurement of the input-fuel composition.

This sensor will greatly benefit the power industry, as well as other industries utilizing gaseous input or output streams by enabling smarter control to increase process efficiency and reduce emissions.

The ability to provide knowledge about fuel composition not only helps with efficient operation of existing systems, but can open the door to “opportunity fuels” such as biogas and landfill gases that also have significant variation in quality requiring operators often to use natural gas as a backup. The new sensing technique may also be applicable to monitoring effluent gases for a variety of processes and industries.

Ultrasensitive Gas Sensing Using Novel Composite Materials

The ability to detect contaminant gases at extremely low levels is important in many applications including the petro-chemical industry. For example, hydrogen sulfide is a common impurity found in natural gas that is detrimental to many processes and devices. Sensors that can detect impurities such as hydrogen sulfide to extremely low levels would be valuable in many applications.

Gold has been used in chemical sensors for decades because of its chemical inertness and high conductivity, which changes upon adsorption of different molecules. Hydrogen sulfide, for example, has been detected using gold thin films, gold nanoparticles, and most recently gold nanoparticle-decorated carbon nanotubes. Despite the excellent sensitivity achieved by gold nanoparticles for hydrogen sulfide, there has been minimal advancement in developing hydrogen sulfide sensors based on gold nanowires, wires being considered the ideal sensor architecture.

Illustration of the nanowelding process.

University of Pittsburgh researchers collaborating with NETL researchers recently published in the Journal of the American Chemical Society their efforts to synthesize gold nanowires and understand the mechanisms that make the technique work. Using a functionalized single-wall nanotube as a template, the researchers found that stabilized gold nanoparticles would self-assemble on the template. The assembled gold nanoparticles were then nanowelded into gold nanowires. The NETL researchers used density functional theory calculations to understand the underlying mechanism of the entire self-assembly and nanowelding processes.

In a complementary effort, the mechanisms that describe the observed growth of the gold nanowires, composed of an ensemble of adsorption/desorption, shuttling and nanowelding, have also been deduced. The elementary steps of this mechanism have been demonstrated through first principles density functional theory calculations, validated with X-ray diffraction and transmission electron microscopy measurements.

Tests to determine the composite gold nanowires-carbon nanotube material’s effectiveness in sensing applications found an ultrasensitivity to hydrogen at both the part per billion (ppb) and part per million (ppm) levels and demonstrated a detection limit as low as 5 ppb at room temperature. Such a hybrid gold nanowire on carbon nanotube material, and the associated device, has potential application as a portable detection sensor in very diverse areas ranging from the natural gas industry to personal safety and personal healthcare.

In a recent article in Chemical and Engineering News by Reginald Penner, a chemist at the University of California, Irvine, described the researchers’ efforts as “Strikingly original. ”

Sensing in Extreme Environments

Improved sensors and controls can play a significant role in increasing efficiency and reducing emissions in existing coal-fired power plants and enabling future technologies for utility-scale power generation such as coal gasification, solid oxide fuel cells (SOFCs), gas turbines, and advanced boiler systems. Gas sensors capable of operation temperatures up to 500°C are valuable for determining the chemical composition of gas streams taken from sampling ports or exhausts. However, more robust sensors capable of higher temperature operation (approximately 500°C -1600°C) in extreme environments are an enabling technology for embedded sensing at the highest value locations from a process control perspective. Optical-based sensing platforms are of increasing interest for such applications and they often require the development of advanced optical materials with an optimal response to particular chemical species of interest and long-term stability at extreme operating temperatures.

Temperature and material property modeling of optical absorption cross section for gold/titanium dioxide.

NETL is currently developing novel optical materials for such applications. Titanium dioxide/gold nanocomposite thin films have been demonstrated as potential detectors of hydrogen at temperatures relevant to solid oxide fuel cells.

NETL efforts have resulted in a model for explaining the preferential occupation of gold nanoparticles at specific types of sites in the titanium dioxide matrix including grain boundaries, twin boundaries, and triple point junctions. NETL researchers have also identified unique crystallographic orientation relationships between gold and titanium dioxide. They demonstrated the potential for using these nanocomposites as embedded sensors in solid oxide fuel cells when they observed a rapid and reversible optical absorption response at temperatures as high as 850°C when switching between oxidizing and reducing gases.