Simulation of a thermoelectric gas sensor that determines hydrocarbon concentrations in exhausts and the light-off temperature of catalyst materials

Abstract

Abstract. Catalyst materials can be characterized with a thermoelectric gas sensor. Screen-printed thermopiles measure the temperature difference between an inert part of the planar sensor and a part that is coated with the catalyst material to be analyzed. If the overall sensor temperature is modulated, the catalytic activity of the material can be varied. Exothermic reactions that occur at the catalyst layer cause a temperature increase that can then be measured as a sensor voltage due to the Seebeck coefficient of the thermopiles. This mechanism can also be employed at stationary conditions at constant sensor temperature to measure gas concentrations. Then, the sensor signal changes linearly with the analyte concentration. Many variables influence the sensing performance, for example, the offset voltage due to asymmetric inflow and the resulting inhomogeneous temperature distributions are an issue. For even better understanding of the whole sensing principle, it is simulated in this study by a 3-D finite element model. By coupling all influencing physical effects (fluid flow, gas diffusion, heat transfer, chemical reactions, and electrical properties) a model was set up that is able to mirror the sensor behavior precisely, as the comparison with experimental data shows. A challenging task was to mesh the geometry due to scaling problems regarding the resolution of the thin catalyst layer in the much larger gas tube. Therefore, a coupling of a 3-D and a 1-D geometry is shown. This enables to calculate the overall temperature distribution, fluid flow, and gas concentration distribution in the 3-D model, while a very accurate calculation of the chemical reactions is possible in a 1-D dimension. This work does not only give insight into the results at stationary conditions for varying feed gas concentrations and used substrate materials but shows also how various exhaust gas species behave under transient temperature modulation.

Conductometric Soot Sensors: Internally Caused Thermophoresis as an Important Undesired Side Effect

“…At the end after 250 s, a temperature increase of 19 K occurs. According to Figure 7 , applying 60 V between the electrodes leads then to a heat generation of 0.6 W. If we compare that with our experiences concerning the heater structure, materials data and simulations on similar sensor setups [ 51 ], an increase of 19 K seems plausible. As a conclusion, the temperature increase with higher applied voltages has a direct and an indirect origin.…”

Exhaust Gas Analysis of Firewood Combustion Processes: Application of a Robust Thermoelectric Gas Sensor

“…The sensor shows a linear characteristic curve. Its sensitivity dependence is well understood and the whole sensor behavior can even be described by modelling [6]. By operation at different temperatures Tabs, different gas components can be distinguished from each other due to their individual activation energies of combustion [7].…”

A pathway to eliminate the gas flow dependency of a hydrocarbon sensor for automotive exhaust applications

“…It keeps the catalyst active. For more details, see , Ritter et al (2017), Hagen et al (2017) and Ojha et al (2017a). …”

“…Secondly, such sensors give a direct response on temperature gradients on the device originating from outer impacts like the gas flow inhomogeneity. Therefore, the thermoelectric sensors are ideal candidates for this investigation, presuming that the evaluated effects also appear with any other kind of gas sensor (Ritter et al, 2017).…”

“…Recently developed thermoelectric hydrocarbon (HC) sensors Ritter et al, 2017) should especially benefit from an application in a bypass setup. Their major drawback -the flow dependency of the signal -needs to be resolved.…”