Catalytic Denitrification Unit with upstream Sodium Hydroxide Scrubber for Cleaning the Exhaust Air of Etching Process in the Production of Solar Cells

Posing the tasks

In the production of solar cells silicon wafers are etched by adding hydrofluoric acid and nitric acid. The exhaust air flow contains HF, SiF₄ and high concentrations of nitrogen oxides NO and NO₂.

To fulfil the emission limit levels an effective exhaust air cleaning system has to be installed. Instead of scrubbers for NOx removal, which only shifts the pollutants from exhaust air to the waste water, the customer preferred a selective catalytic denitrification system (SCR) because of the final destruction of the nitrogen oxides.

Solution / Highlights

In the sodium hydroxide scrubber the acidic pollutant HF is absorbed to fall below the emission limit level. Simultaneously nearly all SiF₄ has to be absorbed as SiF₄ reacts with the SCR catalyst and causes a significant decrease of the catalytic activity.

After separating the droplets in a mist eliminator the exhaust air is heated up to the operating temperature of the catalyst via a heat exchanger and a natural gas burner.
Urea solution as the reducing agent is controlled injected, atomised and vaporised. Due to the required high degree of NOx separation of more than 98% an almost homogeneous distribution of the reducing agent in the exhaust gas air is essential. This is ensured by several static mixing elements and efficient inflow of the reactor, which houses the catalyst in several layers.

Highly active honeycomb elements of TiO₂ with inlays of WO₃ and V₂O₅ are used as catalyst. On the internal surface of the elements the reaction of the NOx with the reducing agent takes place. Nitrogen, water vapour and a small amount of carbon dioxide are the products of the reaction.
The discontinuous etching process produces extreme peaks of the NOx concentration for short time. Even during these peaks especially the NO₂ has to be removed to the greatest extent, as higher concentrations of NO₂ result in yellow-brown colouration of the exhaust air plume and cause complaints from the neighbourhood.
In order to achieve this, elaborate systems with short response times for analysing NO and NO₂ and the control of dosed urea have been installed.