This study presents a quasi-online method for monitoring of dissolved volatile fatty acids (VFAs) in biogas fermentation processes with a carrier gas probe by use of thermo-cyclically operated metal oxide gas sensor arrays. Each of the two sensor arrays comprises a pure SnO2 and three different SnO2/additive-composites (additives: alumina, YSZ, NASICON) but differ by SnO2 synthesis routes, namely Flame Spray Pyrolysis (FSP) and Sol-Gel (SG) technique, respectively. This allowed comparative studies of the influence of layer morphology on VFA sensing characteristics. For sensitive determination of the dissolved VFAs besides high concentrations of biogas components like CO or CH4, first a pre-treatment routine of the fermentation sample was introduced to remove those physically dissolved gases without losing VFAs. The Conductance-over-Time-Profiles (CTPs) of eight different sensing layers were measured simultaneously at exposure to the gases extracted from the fermentation sample at different pH conditions. Almost all the investigated SnO2/additive-composites show CTP-features clearly correlating with the undissociated VFA even at concentrations below 120 ppm as referenced by GC-analysis. The lower detection limit is well below inhibitory concentration for fermentation processes. As expected, most pronounced CTPs representing actual VFAs situation were measured at pH 3, well below the pKa of the VFAs. The FSP-layers highlighted clearly better sensitivity and CTP specificity of higher quality compared to SG-layers. Among the SnO2/additives, the CTP-features of the SnO2(FSP)/NASICON and SnO2(SG)/NASICON layers showed the best specificity to acetic and propionic acid. For the first time, quasi-online analysis of VFAs using metal oxide gas sensors for early warning of VFA-development in biogas fermentation processes was demonstrated.

In view of the tremendous emissions of toxic gases and particulate matter (PM) by low-power firewood-fueled fireplaces, there is an urgent need for effective measures to lower emissions to keep this renewable and economical source for private home heating available in the future. For this purpose, an advanced combustion air control system was developed and tested on a commercial fireplace (HKD7, Bunner GmbH, Eggenfelden, Germany), complemented with a commercial oxidation catalyst (EmTechEngineering GmbH, Leipzig, Germany) placed in the post-combustion zone. Combustion air stream control of the wood-log charge combustion was realized by five different control algorithms to describe all situations of combustion properly. These control algorithms are based on the signals of commercial sensors representing catalyst temperature (thermocouple), residual oxygen concentration (LSU 4.9, Bosch GmbH, Gerlingen, Germany) and CO/HC-content in the exhaust (LH-sensor, Lamtec Mess- und Regeltechnik für Feuerungen GmbH & Co. KG, Walldorf (Germany)). The actual flows of the combustion air streams, as calculated for the primary and secondary combustion zone, are adjusted by motor-driven shutters and commercial air mass flow sensors (HFM7, Bosch GmbH, Gerlingen, Germany) in separate feedback control loops. For the first time, the residual CO/HC-content (CO, methane, formaldehyde, etc.) in the flue gas is in-situ monitored with a long-term stable AuPt/YSZ/Pt mixed potential high-temperature gas sensor, which allows continuous estimation of the flue gas quality with an accuracy of about ±10%. This parameter is not only an essential input for advanced combustion air stream control but also provides monitoring of the actual combustion quality and logging of this value over a whole heating period. By many firing experiments in the laboratory and by field tests over four months, it could be demonstrated that with this long-term stable and advanced automated firing system, depression of the gaseous emissions by about 90% related to manually operated fireplaces without catalyst could be achieved. In addition, preliminary investigations at a firing appliance complemented by an electrostatic precipitator yielded PM emission depression between 70% and 90%, depending on the firewood load.

For a comparative study of the gas sensing performance to dissolved volatile organic compounds (VOCs), the SnO2-powders have been prepared using two different fabrication routes; the flame-spray-pyrolysis (FSP) and the sol-gel (SG) route; and were admixed with the same additive-powders (alumina, YSZ, and NASICON). The morphology of the two different SnO2/additive families were investigated by ESEM analysis and Energy-dispersive X-Ray Spectroscopy (EDS). Both SnO2/additive material families were separately deposited as thick-film-layers on two four-fold sensor-chips which were simultaneously thermo-cyclically operated in a measurement cell combined with a carrier gas probe, which enables sensing tests with evaporated VOCs (acetic acid, propionic acid, ethanol, acetone) dissolved in water. The resulting Conductance-over-Time-Profiles (CTPs) highlight better sensitivities of most of the FSP-layers to all the analytes compared to the SG-prepared layers. Furthermore, the CTP shapes of the FSP layers show clearly enhanced specificity representing the individual analyte components. This was interpreted to be the consequence of the extremely fine, scarcely agglomerated grain morphology of FSP-prepared powders and their very narrow grain size distribution which provide better conditions for enhanced gas specific surface reactions. Results promise a better chemical analysis capability of dissolved VOCs by numerical analysis of the CTP of FSP-prepared gas sensitive layers.

A novel method for quasi-continuous tar monitoring in hot syngas from biomass gasification is reported. A very small syngas stream is extracted from the gasifier output, and the oxygen demand for tar combustion is determined by a well-defined dosage of synthetic air. Assuming the total oxidation of all of the combustible components at the Pt-electrode of a lambda-probe, the difference of the residual oxygen concentrations from successive operations with and without tar condensation represents the oxygen demand. From experiments in the laboratory with H2/N2/naphthalene model syngas, the linear sensitivity and a lower detection limit of about 70 ± 5 mg/m3 was estimated, and a very good long-term stability can be expected. This extremely sensitive and robust monitoring concept was evaluated further by the extraction of a small, constant flow of hot syngas as a sample (9 L/h) using a Laval nozzle combined with a metallic filter (a sintered metal plate (pore diameter 10 µm)) and a gas pump (in the cold zone). The first tests in the laboratory of this setup—which is appropriate for field applications—confirmed the excellent analysis results. However, the field tests concerning the monitoring of the tar in syngas from a woodchip-fueled gasifier demonstrated that the determination of the oxygen demand by the successive estimation of the oxygen concentration with/without tar trapping is not possible with enough accuracy due to continuous variation of the syngas composition. A method is proposed for how this constraint can be overcome.

The stability of a high temperature electrochemical CO/HC sensor (mixed potential type) can be clearly improved by sensitivity check with electrochemical impedance spectroscopy and regeneration by cyclic cathodic polarization under synthetic air conditions. Based on these results, a signal stabilization concept was proposed, and an instrument was built up to improve the quality and stability of automatic control of wood combustion processes. First results showed a clearly more stable sensing behavior and considerable emission reduction of a wood-log fueled fireplace with such control.

Motivation: Thermochemical conversion of biomass to syngas by gasification of biological wastes like food or wood residuals wins more and more importance for energy supply because this kind of fuel is sustainable and generally available on demand. The syngas is composed of fuel components like CO, CH4, H2 and light hydrocarbons, but also contains undesired components like particulate matter and particularly tar. The latter constituents represent a complex mixture of aromatic compounds like toluene, phenol or naphthalene, which vary in relative composition and absolute concentration related to the composition of the biomass and the actual quality of the gasification process. Tar content in the raw syngas leads to complications like tar deposition on the walls, clogging of pipes in the equipment of the associated processes e.g. for generation of electric energy in which syngas is used as a fuel. The continuous monitoring of the tar in the syngas even at low concentrations is, therefore, of major importance to enable installation of a feedback operation control of the gasifying process and achieve minimization of the tar content. In the past, analysis of tar in syngas is well established, but expensive analysis methods like FTIR, NMR and GC/MS were published [1]. Also, tar analysis by LED induced fluorescence spectroscopy was reported in [2], but preliminary tests were conducted in the liquid phase at relatively high concentrations of phenol (8-10%). Recently, online monitoring of the tar concentration in producer syngas by use of a photo ionization detector (PID) [3] or a flame ionization detector (FID) [4] was described. In the latter concept the FID difference signal based on the syngas stream without and with condensation of the tar (cooled filter at 20°C-100°C) represents the tar concentration. This concept of tar analysis seems to be quite accurate for tar concentrations higher than 5g/m3. PID promises to enable tar component detection and to be very sensitive, however, the long-term stability of this monitoring concept seems to be problematic. New concept of more sensitive tar-monitoring in syngas: In this paper, for the first time, a novel concept of continuous tar monitoring in syngas is introduced. The setup for evaluation in the laboratory is schematically given in Fig.1a. It allows accurate measurements of toluene (used as a model tar) even below 1000ppm. The sensing principle is based on the estimation of the residual oxygen demand for tar combustion. First, a small flow (100ml/min) of the hot syngas stream is extracted from the gasifier and dosed with synthetic air to adjust stoichiometric combustion conditions (l=1) by use of an electronic mass flow controller (MFC) operated in a feedback loop with the signal of a wide band high-temperature Pt/8YSZ/Pt - oxygen sensor (LSU 4.9, Bosch GmbH). In the second step the synthetic air flow is kept constant, but now the syngas is lead over a condensation unit (T=-32°C) and again conducted to the oxygen sensor. The sensor provides a signal (coulometric current Ip) proportional to the excess oxygen concentration measured after tar condensation (l>1). The difference signal DIp = Ip (l>1) – Ip (l=1) represents the excess oxygen concentration after tar condensation which is directly related to the oxygen demand for tar oxidation. Discussion and outlook: Of course, this method of tar monitoring does not provide an analysis of the tar content because the measured oxygen demand related to the tar concentration depends on the specific mixture of aromatic compounds forming the tar. In addition, even at a temperature of approximately -32°C volatile components like toluene, which is one of the major constituents of tar, can only be condensate to a residual saturation concentration of about 800 ppm according to Clausius-Clapeyron equation, which corresponds to a concentration of about 3g/m3 (25°C). This means, at this temperature other components of tar with higher evaporation enthalpies like naphthalene and phenol will be estimated at a much lower sensitivity limit and, therefore, will represent the tar concentration even at considerably lower values than 1000ppm. In the next step this novel procedure of tar monitoring was experimentally evaluated, and tested with naphthalene as model tar. The results demonstrated the very good sensitivity and long-term stability of this monitoring concept. The automated system recorded a clear signal even at the lowest naphthalene concentration (14ppm/74mgm-3). These experimental results from laboratory will be presented in detail, modifications of the setup necessary for analysis of real syngas of a wood gasifier and some preliminary experiments will be reported and limits of detection in context with some technical advantages/restrains will be discussed. Acknowledgement This work is part of the EBIPREP collaboration project (www.ebiprep.eu) financed by the EU International Programme INTERREG V Oberrhein 2017-2020. References [1] Rudy Michel, Sergio Rapagnà, Philippe Burg, Giuseppe Mazziotti di Celso, Claire Courson, Thierry Zimny, René Gruber; Steam gasification of Miscanthus X Giganteus with olivine as catalyst production of syngas and analysis of tars (IR, NMR and GC/MS) Biomass & Bioenergy 35 (2011) 2650 [2] Sean Capper, Zakir Khan, Prashant Kamble, James Sharp, Ian Watson; Progression towards online tar detection systems, Energy procedia 142 (2017) 892 [3] Mozhgan Ahmadi, Harrie Knoef, Bert Van de Beld, Truls Liliedahl, Klas Engvall; Development of a PID based on-line tar measurement method – Proof of concept, Fuel 113 (2013) 113 [4] A. Gredinger, R. Spörl and G. Scheffknecht; Comparative measurements of tar concentrations in gasification systems between an online method and the tar protocol; Proceedings 24th European Biomass Conference and Exhibition, 6.-9. June 2016, Amsterdam, p. 466 Figure 1

Motivation: Thermochemical conversion of biomass to syngas by gasification of biological wastes like food or wood residuals wins more and more importance for energy supply because this kind of fuel is sustainable and generally available on demand. The syngas is composed of fuel components like CO, CH4, H2 and light hydrocarbons, but also contains undesired components like particulate matter and particularly tar. The latter constituents represent a complex mixture of aromatic compounds like toluene, phenol or naphthalene, which vary in relative composition and absolute concentration related to the composition of the biomass and the actual quality of the gasification process. Tar content in the raw syngas leads to complications like tar deposition on the walls, coking and clogging of pipes in the equipment of the associated processes in which syngas is used as a fuel. The continuous monitoring of the tar in the syngas even at low concentrations is, therefore, of major importance to enable installation of a feedback operation control of the gasifying process and achieve minimizing of the tar content. In the past, for analysis of tar in syngas well established, but expensive analysis methods like FTIR, NMR and GC/MS were published [1]. Also, tar analysis by LED induced fluorescence spectroscopy was reported in [2], but preliminary tests were conducted in the liquid phase at relatively high concentrations of phenol (8-10%). Recently, online monitoring of the tar concentration in producer syngas by use of a flame ionization detector (FID) was described in [3]. The FID difference signal based on the syngas stream without and with condensation of the tar (cooled filter at 20°C-100°C) represents the tar concentration. This concept of tar analysis seems to be quite accurate for tar concentrations higher than 5g/m3. New concept of more sensitive tar-monitoring in syngas: In this paper, for the first time, a novel concept of continuous tar monitoring in syngas is introduced which has been tested in a lab setup (Fig. 1a). It allows accurate measurements of toluene (used as a model tar) even below 1000ppm. The sensing principle is based on the estimation of the residual oxygen demand for tar combustion. First, a small flow (100ml/min) of the hot syngas stream is extracted from the gasifier and dosed with synthetic air to adjust stoichiometric combustion conditions (λ=1) by use of an electronic mass flow controller (MFC) operated in a feedback loop with the signal of a classical high-temperature Pt/8YSZ/Pt - oxygen concentration cell. In the second step the synthetic air flow is kept constant, but now the syngas is lead over a condensation unit (T=-32°C) and again conducted to the oxygen concentration cell (KS1D, Lamtec GmbH) or to a broadband lambda-probe (LSU 4.9, Bosch GmbH). The latter sensor provides a signal (coulometric current Ip) proportional to the excess oxygen concentration measured after tar condensation (λ>1). The difference signal DIp = Ip(λ>1) – Ip(λ=1) represents the excess oxygen concentration after tar condensation which is directly related to the oxygen demand for tar oxidation. Discussion and outlook: Of course, this method of tar monitoring does not provide an analysis of the tar content because the measured oxygen demand related to the tar concentration depends on the specific mixture of aromatic compounds forming the tar. In addition, even at temperature of approximately -32°C volatile components like toluene, which is one of the major constituents of tar, can only be condensate to a residual saturation concentration of about 800 ppm according to Clausius-Clapeyron equation, which corresponds to a concentration of about 3g/m3 (25°C). This means, at this temperature other components of tar with higher evaporation enthalpies like naphthalene, phenol will be estimated at a much lower sensitivity limit and, therefore, will represent the tar concentration even at considerably lower values than 1000ppm. In the next step this novel procedure of tar monitoring will be automated, tested with naphthalene (model tar) and finally, first experiments will be conducted with syngas of a wood gasifier. These results will be reported as well and limits of detection will be discussed in context with some technical advantages/restrains. Acknowledgement This work is part of the EBIPREP collaboration project (www.ebiprep.eu) financed by the EU International Programme INTERREG V Oberrhein 2017-2020. References [1] Rudy Michel, Sergio Rapagnà, Philippe Burg, Giuseppe Mazziotti di Celso, Claire Courson, Thierry Zimny, René Gruber; Steam gasification of Miscanthus X Giganteus with olivine as catalyst production of syngas and analysis of tars (IR, NMR and GC/MS) Biomass & Bioenergy 35 (2011) 2650 [2] Sean Capper, Zakir Khan, Prashant Kamble, James Sharp, Ian Watson; Progression towards online tar detection systems, Energy procedia 142 (2017) 892 [3] A. Gredinger, R. Spörl and G. Scheffknecht; Comparative measurements of tar concentrations in gasification systems between an online method and the tar protocol; Proceedings 24th European Biomass Conference and Exhibition, 6.-9. June 2016, Amsterdam, p. 466 Figure 1

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 proposed 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 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, 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 in Fig. 2. 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: [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, under preparation Figure 1

1. Introduction and Motivation Considering the global climate change, the efficient use of clean and renewable sources of energy is one of the key research areas. In this context, the biological production of methane and other combustible biogas produced by anaerobic fermentation of organic wastes is gaining growing importance. These fermentation processes attract more and more interest mainly due to the possibility to use different types of residues like food waste, dairy wastes and many other organic wastes as feeding substrates. However, efficient biomass conversion to biogas is only possible if the process parameters taking direct influence on the fermentation process are continuously and reliably monitored. This allows efficient process control. One of such key process parameters are the volatile fatty acids (acetic, propionic, butanoic) formed during the biomass conversion process. The reliable monitoring of such organic acids and other volatile organic compounds (VOCs) gives valuable information and their analysis even at low concentrations (<2000 ppm) allows to model the actual microbial state and to adapt the feeding to keep their concentration and their inhibiting influence on the fermentation process low. Conventionally, the analysis of the organic acid composition in anaerobic fermentation processes is done by sophisticated methods like gas chromatography [1], infrared spectroscopy [2], and high pressure liquid chromatography (HPLC) [3]. However, the major disadvantages of these methods are their complicated and time consuming sample pre-treatment routines and high costs. In this paper, an automated measuring system developed by combining a silicon rubber membrane based carrier gas probe (Fig. 1) with a thermo-cyclically operated metal oxide gas sensor array [4] is introduced. This automated system might enable in-situ monitoring of different VOCs developing during the bio-fermentation processes in time periods of about one hour. 1. Method of VOC monitoring Metal oxide gas sensors (MOGs) are well established as gas sensing devices for monitoring of VOCs and oxidizable gases like CO, H2 and CH4. Thus, before measurement of dissolved VOCs, CH4 and other to cross-sensitivity contributing gas components present in the biogas fermentation sample must be driven out. In a first step, a small amount of the fermentation liquid (about one liter) is extracted from the main reactor and its pH is shifted to an alkaline value by dosage of potassium hydroxide. This allows transformation of the dissolved organic acids to the dissociated state and enables purging out CH4, H2 and all other non-acidic, physically dissolved gas components contributing to the sensor signal, by a high flow of N2. Then, pH is shifted to a value near or even lower than the pKa value of the organic acids by dosage of phosphorous acid. Now the dissolved organic acids are in the undissociated molecular state and equilibrate with the gas state (Henry´s law). This enables the uptake of molecular dissolved organic acids from the liquid state into the constant flow of synthetic air (carrier gas: 5ml/min) by permeation through the gas permeable silicon rubber membrane of the gas carrier probe. By the help of the carrier gas the organic acids are transported to the metal oxide gas sensor array (Fig 1a) for analysis. 1. Results and Discussion Several SnO2/additive gas sensing composites have been prepared and the most interesting candidates with respect to their remarkably high sensitivity and most characteristic conductance vs. time profile (CTP) shapes when operated in thermocyclic mode [4,5] will be reported. When the carrier gas probe is immersed in acetic acid/deionized water admixtures some metal oxides show characteristic CTP-shapes (Fig. 1b) representing the individual surface reaction processes with acetic acid. In addition, some first experiments with real fermentation liquids resulted in some unexpected gas developments with pH and time well indicated by characteristic CTP-changes, which will highlight the presentation. 1. Conclusions and outlook By some extended in-situ monitoring experiments of acetic acid in deionized water as a model substance using a gas carrier probe combined with a 4-fold metal oxide sensor array, several metal oxide materials could be identified as good candidates for monitoring of organic acids in biogas fermentation processes. Monitoring experiments in real fermentation samples and analysis of the CTPs yielded gas formation changing with pH and time. These gas formation processes have to be referenced by simultaneous Gas Chromatograph-Mass Spectrometry (GC-MS) analysis and the results will be reported as well in context with the MOG sensitivity data. 1. Acknowledgement This work is part of the EBIPREP collaboration project (www.ebiprep.eu/). It is financed by the EU International Programme INTERREG V Oberrhein 2017-2020. References [1] V. Diamantis, P. Melidis, A. Aivasidis; Continuous determination of volatile products in anaerobic fermenters by on-line capillary gas chromatography, Analytica Chimica Acta 573-574(2006)189-194 [2] H.M. Falk, P. Reichling, C. Andersen, R. Benz; Online monitoring of concentration and dynamics of volatile fatty acids in anaerobic digestion processes with mid-infrared spectroscopy, Bioprocess Biosyst Eng (2015) 38:237-249 [3] P.v. Zumbusch, T. Meyer-Jens, G. Brunner, H. Märkl; On-line monitoring of organic substances with high-pressure liquid chromatography (HPLC) during the anaerobic fermentation of waste-water, Appl Microbiol Biotechnol (1994)42:140-146 [4] K. Frank, V. Magapu, V. Schindler (†), H. Kohler, H.B. Keller, R. Seifert; Chemical analysis with tin oxide gas sensors: choice of additives, method of operation and analysis of numeric signal, 7th East Asian Conference on Chemical Sensors, Dec. 3-5, 2007, Singapore, SENSOR LETTERS 6 (2008) 908-911. [5] Navas Illyaskutty, Jens Knoblauch, Matthias Schwotzer, Heinz Kohler; Thermally modulated multi sensor arrays of SnO2/additive/electrode combinations for enhanced gas identification, Sensors and Actuators B 217 (2015) 2-12 Figure 1

The sensing characteristics and long-term stability of different kinds of CO ∕ HC gas sensors (non-Nernstian mixed potential type) during in situ operation in flue gas from different types of low-power combustion systems (wood-log- and wood-chip-fuelled) were investigated. The sensors showed representative but individual sensing behaviour with respect to characteristically varying flue gas composition over the combustion process. The long-term sensor signal stability evaluated by repeated exposure to CO ∕ H2 ∕ N2 ∕ synthetic air mixtures showed no sensitivity loss after operation in the flue gas. Particularly for one of the sensors (Heraeus GmbH), this high signal stability was observed in a field test experiment even during continuous operation in the flue gas of the wood-chip firing system over 4 months. Furthermore, it was experimentally shown that the signals of these CO ∕ HC sensing elements yield important additional information about the wood combustion process. This was demonstrated by the adaptation of an advanced combustion airstream control algorithm on a wood-log-fed fireplace and by the development of a combustion quality monitoring system for wood-chip-fed central heaters.

The design and fabrication of a miniaturized calorimetric-type gas sensor in a single chip arrangement is presented. Active and passive thin-film Pt meanders are integrated in a single platform (7 × 7 mm2) together with a temperature sensor and a thin-film microheater at the reverse side. Active meanders are covered by a porous Al2O3/2 wt % Pt thick-film layer. The selection of substrate, position of meanders, and active catalysts (especially their concentration) play a crucial role in directing sensor performance. The presented results show that the sensor signal (Wheatstone bridge voltage) is generated by diffusion-limited exothermic reactions which point towards catalytically enhanced combustion reactions mainly inside the active porous layer. By extrapolation of the linear sensitivity curves, the sensitivity limit was estimated to be 4 ppm for propene and to be 18 ppm for CO. In general, the one-chip-sensing concept has high potential to be used as a gas sensor for analysis of combustible gases; however, further optimization of the meander design and the catalyst material as well as investigations of the sensing behavior under varying ambient temperatures are necessary before such applications shall be considered.

Dataset supplementing H. Kohler, B. Ojha, N. Illyaskutty, I. Hartmann, C. Thiel, K. Eisinger, M. Dambacher: 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 (JSSS), 2018

Effective reduction of the emissions of low-power wood-log fueled firing appliances is only possible, if advanced combustion air stream control concepts are applied. Such an advanced method of automated estimation and control of combustion air streams is based on combustion temperature, residual oxygen concentration and content of the un-/partly combusted exhaust gas components and separate control loops for adjustment of the combustion air streams using air mass stream sensors and motor driven air stream limiting shutters. In the past lack of appropriate CO/HC gas sensors providing satisfying long-term signal stability hindered the effectuation of those control systems in the market of advanced firing systems. In this paper an electrochemical method is reported, which may solve the problem. It enables checking of the current status of CO/HC gas sensitivity of a commercial mixed potential type gas sensor and, if required, regeneration of sensitivity at the place of operation under ambient air conditions.

ZusammenfassungDurch Regelung der Verbrennungsluft in mit Scheitholz befeuerten Anlagen

In order to optimize firewood combustion in low-power firewood-fuelled fireplaces, a novel combustion airstream control concept based on the signals of in situ sensors for combustion temperature, residual oxygen concentration and residual un-combusted or partly combusted pyrolysis gas components (CO and HC) has been introduced. A comparison of firing experiments with hand-driven and automated airstream-controlled furnaces of the same type showed that the average CO emissions in the high-temperature phase of the batch combustion can be reduced by about 80 % with the new control concept. Further, the performance of different types of high-temperature CO / HC sensors (mixed-potential and metal oxide types), with reference to simultaneous exhaust gas analysis by a high-temperature FTIR analysis system, was investigated over 20 batch firing experiments (∼ 80 h). The distinctive sensing behaviour with respect to the characteristically varying flue gas composition over a batch firing process is discussed. The calculation of the Pearson correlation coefficients reveals that mixed-potential sensor signals correlate more with CO and CH4; however, different metal oxide sensitive layers correlate with different gas species: 1 % Pt / SnO2 designates the presence of CO and 2 % ZnO / SnO2 designates the presence of hydrocarbons. In the case of a TGS823 sensor element, there was no specific correlation with one of the flue gas components observed. The stability of the sensor signals was evaluated through repeated exposure to mixtures of CO, N2 and synthetic air after certain numbers of firing experiments and exhibited diverse long-term signal instabilities.

In this paper, the combustion and emission situations of batch wise firewood fueled fireplaces is discussed.Difference in CO-emissions of a hand operated and automatically air stream controlled firewood combustion process is illustrated.An air stream control algorithm is introduced which directs all phases of the firing process: ignition, high temperature and burn out phase.The combustion air stream control concept is based on motor driven shutters combined with air mass stream sensors and on flue gas analysis by sensors for combustion temperature, residual oxygen concentration (ROC) and residual un-or partly combusted pyrolysis gas components (CO/HC) Different commercially available high temperature CO/HC sensors along with an indigenously developed metal oxide (MOG) sensor array are evaluated in batch firing experiments with reference to the data sampled by a HT-FTIR analysis system.Finally, the signal stability of the sensors was investigated by repeated exposure to CO/air gas mixtures.