Very Sensitive Tar Monitoring in Syngas Streams By Estimation of Oxygen Demand – a Preliminary Study
Authors: Ojha Binayak , Kohler Heinz , Turad Stefan , Jochum Joachim 
Published: July 2021

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