Introduction

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Figure 1 - Slideshow: Explanation of electrochemical flue gas purification

The electrochemical reduction of NO was at first proven possible in 1975 by Pancharatnam et al. Using a zirconia based oxide ion conducting electrolyte they reduced NO to elemental nitrogen and oxide anions on a Pt-electrode. The main obstacle with this technique is that the presence of oxygen lowers the activity of this type of system towards the reduction of NO and thereby increases the current consumption. Only a few types of cathodes are known that can reduce NO in the presence of oxygen with a sufficiently high current efficiency. The main problems with the known electrodes materials are that they are expensive or even poisonous.

Electrochemical oxidation of soot has been demonstrated by the Danish company Dinex Filter Technology A/S. The idea is to capture the soot in a porous structure consisting of an oxide ionic conducting electrolyte and a pair of electronic conducting ceramic electrodes. Reactive oxide anions are pumped to the anode where they react with the soot particles forming CO₂.

The electrochemical oxidation of hydrocarbons in an all solid state electrochemical cell has been studied by several authors. The main problem is that not all the hydro-carbons are converted to CO₂, but that CO and partially oxidized hydrocarbons also are formed.

In principle it is possible to combine all the processes in one filter as shown in figure 2. The filter is porous and consists of alternating layers of electrodes and electrolyte.



Figure 2 - A sketch of a layered electrochemical filter to be used for removal of soot, hydrocarbons and NO_X. The driving force for the reactions is an external potential difference imposed between the top and the bottom of the filter. The filter converts NO to elemental nitrogen and oxide anions at the electrodes (1). The oxide anions generated, from the reduction of NO, is transported through the electrolyte (2) to the next electrode and is used to oxidise the soot and the hydrocarbons. By using alternating current the electrodes is acting as both anodes and as cathodes.