The SNOX™ process is an innovative process which removes sulfur dioxide, nitrogen oxides and particulates from flue gases. The sulfur is recovered as commercial grade concentrated sulfuric acid and the nitrogen oxides are reduced to free nitrogen.
The process is based on catalytic reactions and does not consume water or absorbents. Neither does it produce any waste, except for the separated dust.
Due to recycling of the heat generated by the SNOX™ process to be used by the client as preheated air to his boilers, the overall efficiency and economics is improved considerably.
In addition, the SNOX™ can handle other sulfurous waste streams. This is highly interesting in refineries, where e.g. H2S gas, sour water stripper gas and Claus tail gas can be led to the SNOX™ plant, and thereby investment in other waste gas handling facilities can be saved.
The flue gas desulfurization process
The SNOX™ process includes the following steps:
- dust removal
- catalytic reduction of NOx by adding NH3 to the gas upstream the SCR DeNOx reactor
- catalytic oxidation of SO2 to SO3 in the oxidation reactor
- cooling of the gas to about 100ºC whereby the H2SO4 condenses and can be withdrawn as a concentrated sulfuric acid product
Dust removal is an important feature of the SNOX™ technology. Dust (ashes and soot) must be removed to a very low level. This takes place in an electrostatic precipitator or in bag filters. Residual dust will be trapped in the SO2 conversion catalyst from where it can be removed. In this way, the cleaned gas will be virtually dust free.
Since the flue gas is heated up by means of the reacted flue gas after the SO2 converter, the absence of leaks between the two sides of the heat exchanger is important. Topsoe has experience with vendors supplying efficient, large-scale, gas tight heat exchangers.
The SNOX™ condenser
The design of the air-cooled, multi-tube SNOX™ condenser allows controlled cooling of the process gas and condensation of the sulfuric acid without formation of acid mist. The vertical glass tubes, the brick-lined bottom section and the coated tube-plates of the condenser are designed for long service life and avoidance of corrosion.
Our proprietary catalyst
Another important element of the SNOX™ process is the proprietary Topsoe catalysts - the DNX series and the VK-W series - developed for reduction of NOx and for the conversion of SO2 into SO3 in the presence of water vapor. The catalysts feature high activity and stability.
Topsoe has established an extensive technical service programme available to our SNOX™ technology clients. In addition to frequent contact the service programme includes activity testing of catalyst samples, evaluations of catalyst performance, plant optimization, trouble shooting and management of catalyst replacement.
The performance catalysts we supply for this application are listed below.
- CatalystsVK-WD is optimized for lean SO2+SO3 gases, such as in a SNOX™ plant or in the bed(s) after the intermediate condenser in a WSA-DC plant.
- CatalystsVK-WH is a cesium-promoted catalyst designed for continuous operation down to 370°C/700°F in medium- to high-strength SO2+SO3 gasses. A top-layer of VK-WH can significantly improve operating flexibility by widening the bed temperature control window, especially in plants struggling with fluctuating...
- CatalystsVK-WL is a cesium-promoted catalyst designed for continuous operation down to 370°C/700°F in lean SO2+SO3 gasses (less than approximately 2 mole%). At these conditions, VK-WL offers unmatched high activity throughout the entire operating temperature range. The very high activity provides the...
- CatalystsThe VK-WSA formulation offers excellent activity over a wide range of operating conditions and can be used in any bed of a WSA or WSA-DC converter. VK-WSA is available in the 12 mm Daisy shape and as a dust-protection catalyst in the large 25 mm Daisy shape. 12 mm Daisy: improved energy efficiency...
- CatalystsThe VK-WSX formulation was developed to offer higher activity in SO2 gasses that are more than 90% converted. Typically VK-WSX is used in the last bed of a WSA converter. For this service, VK-WSX is more active than VK-WSA and even more active than cesium-promoted VK-WH catalyst. The high activity...