Mixed Potential Gas Sensor with Au,Pt-YSZ Electrode for Optimization of Wood Combustion Processes: Validation of a Signal Stabilization Concept By Repeated Electrochemical Treatments
Authors: Zhang Xin , Ojha Binayak , Schwab Christoph , Kohler Heinz 
Published: July 2021

Introduction

Small scaled wood-log fueled furnaces used for heating of domestic households are widely used and well known to contribute considerably to air pollution and climate change by toxic emissions of un-/partly combusted exhaust gas components (CO/HC) as well as particulate matter (PM) loaded with organics [1]. Effective reduction of the emissions of those wood-log fueled firing appliances can be achieved if advanced combustion air stream control concepts are applied. The control of the airstreams based on combustion temperature and residual oxygen partial pressure in the flue gas is the current state of art. However, it has been shown that combustion air stream control based on these two parameters is not sufficient for optimum control of the combustion process. Only when additionally the sensor signals representing CO/HC content in the exhaust gas are taken into account, the quality of combustion can be substantially improved leading to a reduction of toxic gas emissions of about 80% compared to the manual operation of the furnace [2].
Despite those inspiring results, lack of appropriate and long-term stable CO/HC gas sensors hindered the effectuation of those control systems in the market of advanced firing systems up to now. However, a commercially available mixed potential type CO/HC sensor with Au,Pt-YSZ electrode (CarboSen, LAMTEC GmbH, Walldorf, Germany) is a promising candidate. As recently shown, the sensitivity of such type of sensors can be checked by electrochemical impedance spectroscopy (EIS) and the sensitivity can be almost fully regenerated by a cyclic cathodic polarization sequence (CV). Both electrochemical treatments were done at synthetic air conditions [3]
Based on these results, we propose a novel sensitivity regeneration and signal stabilization concept for those mixed potential CO/HC-sensors when applied in wood combustion firing processes. First results of the validation of this concept are reported in this paper.

Method

For a first verification of the concept, a CarboSen-type sensor, which had been already operated in the flue gas of a wood-log fueled fireplace (SF10SK, Brunner GmbH, Eggenfelden) and had lost its sensitivity after some operation in flue gas, was tried to be regenerated by cyclic cathodic polarizations. The details of this procedure are described in [3]. For investigation of the long-term signal stability, the sensitivity was checked in CO model gases before and after the sensor regeneration. In an additional step, another two new unused sensor individuals were characterized by sensitivity and EIS-measurements, separately before operation in any batch firing experiment. One of these two fresh sensors at first was pretreated by a cathodic polarization sequence as stated above to verify whether this pre-treatment could enhance the signal stability from the very beginning of the operation in flue gas. The development of the sensitivity- and the EIS-characteristics of these two sensors under exposure to flue gas is just under investigation.

Results and Conclusions

The development of the sensor signals (sensitivity was checked periodically in CO/O2/N2 model gas) during operation in the flue gas of wood-log combustion processes is visualized in Fig. 1. After 12h of operation in the flue gas, the sensor signal clearly declined, but is raised after cathodic polarization to values even higher than the ones measured in the very beginning. This indicates, that reaction sites of the electrode for mixed potential formation are re-activated by the cathodic polarization treatment [3]. Surprisingly it could now be shown, that this regeneration effect of gas sensitivity is stable for at least 48 hours of further operation in the flue gas. Moreover it was found that even the sensing performance of a fresh sensor after the cathodic polarization treatment was improved in terms of the response amplitude and the response time.
So far, these results are very promising and offer an excellent prospect of the proposed signal stabilization concept. This means at a break between two consecutive batch firing processes, periodic sensitivity checks by EIS measurement and if required, on-site regeneration of sensitivity under ambient air conditions are conducted, as illustrated schematically in Fig. 2. An electronic device in hard- and software has been developed and was recently finalized to automate this concept, namely on-site of operation of the mixed potential gas sensor. The details of this setup for field applications and the corresponding results, i.e. the long-term behavior of differently pre-treated mixed potential gas sensor elements at operation in the flue gas with respect to sensitivity and signal stability, will be discussed and a final proposal for operation of such sensor elements in firing systems with automated combustion air control will be presented.

References

[1] M. Tapanainen, P.I. Jalava, M.-R. Hirvonen et.al., Physicochemical characterization of fine particles from small-scale wood combustion, Atmospheric Environment 45 (2011) 7546-7554. doi: 10.1016/j.atmosenv.2011.02.072
[2] H. Kohler, B. Ojha, M. Dambacher et al., In situ high-temperature gas sensors: continuous monitoring of the combustion quality of different wood combustion systems and optimization of combustion process, Journal of Sensors and Sensor Systems 7 (2018) 161 – 167. doi: doi.org/10.5281/zenodo.1189998
[3] X. Zhang, H. Kohler, U. Guth et al., Stability Improvement of Layered Au,Pt-YSZ Mixed-potential Gas Sensing Electrodes by Cathodic Polarization: Studies by Steady State and Dynamic Electrochemical Methods, Sensors and Actuators B: Chemical, submitted Dec. 2020

Figure 1