Development and characterization of an impedance-based carbon sensor for the detection of catalyst coking in steam-methane and dry-methane reforming systems
A carbon sensor designed to detect the early stages of catalyst coking in methane reforming systems has been developed. The sensor is similar to the one developed by Wheeler et al., but it has been redesigned to reduce unwanted gas-phase response and increase coking response in methane reforming environments. The sensor comprises of a Wheatstone bridge with two catalytic, non-percolating cermet resistive elements printed using an inkjet printer. As carbon begins to grow on the catalytic surface, the electrical conductivity of the catalytic material increases, which results in a change in the continuously monitored bridge output voltage. By replacing SLT (strontium-doped lanthanum titanate) as the non-catalytic half of the Wheatstone bridge, gas phase response is reduced. In the existing design, all four of the bridge elements are YSZ (yttria-stabilized zirconia) based and voltage change due to gas-phase composition is reduced by a factor of 10 from a SLT-based sensor. Steam and dry reforming tests were conducted at 600 °C with low steam-to-carbon and CO2-to-carbon feed ratios to promote coking. The sensor showed strong response (on the order of several hundred millivolts) to carbon formation on the surface under both of the reforming environments studied. Field-emission scanning electron microscope (FESEM) imaging showed surface catalyst (Ni) particles encapsulated in films of carbon. Sensors were regenerated using steam and then demonstrated similar, but smaller, responses to the same coking conditions. Investigation of regenerated sensor surfaces showed the presence of fewer catalyst sites compared to fresh sensors, indicating that the loss of nickel particles leads to a degradation in the sensor response.
coke formation, methane reforming, carbon sensor, Wheatstone bridge, inkjet printing
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