DNX® - Topsøe SCR DeNOx catalysts 

Oxidation of SO2 into SO3

Sulphur is an omnipresent component of both coal and oil and 

results in a significant environmental impact when combusted 

and the management of sulphur in fossil-fuelled power plants is 

therefore of great importance.

During the combustion of coal in the boiler virtually all sulphur 

is converted into SO2

 but a small fraction is further converted 

or oxidised into SO3

. The SO3 will react with residual ammonia 

to form ammonia bisulphate, ABS, which can foul downstream 

equipment. Also, SO3

 will react with water vapour as the flue 

gas cools down and forms sulphuric acid mist which cannot be 

removed in common FGDs and therefore ends up in the stack as 

a visible opaque plume.

It is a general fact that the active component in SCR DeNOx 

catalysts, vanadium, is also catalysing the formation of SO3

 from 

SO2

. The choice of SCR DeNOx catalyst and the selection of the 

operating conditions are therefore crucial to the power plant owner.

As shown in Figure 1 there are two possible ways of controlling 

the SO2

 oxidation: By varying the percentage of vanadium in the 

catalyst or by limiting the temperature. Most often the temperature 

is not a variable in power plants and the designer of an SCR 

DeNOx unit will therefore have to carefully adjust the catalyst 

properties to reach his goal.

Figure 1 explains the designer’s dilemma: Reducing the SO2

oxidation by lowering the percentage of vanadium in the catalyst 

also lowers the specific activity and the total catalyst volume will 

therefore increase and the plant become more costly.

1.0

Relative activity 0.5

0.0

250

High-vanadium type

Low-vanadium type

300 350

Temperature, °C

400 450

Figure 1: DeNOx activity and SO2 oxidation characteristics.
DNX® - Topsøe SCR DeNOx catalysts 

Oxidation of SO2 into SO3

As illustrated in Figure 2 the DeNOx reaction takes place on 

the internal surface in the pores of the catalyst and, by some 

approximation, the activity (ADeNOx) becomes 

(1) ADeNOx = k1 × As × Ca

 × η

where As is the specific active surface area (m2

/m3), C is the 

percentage of vanadium in the catalysts and η is the effectiveness 

factor accounting for the resistance towards diffusion of NOx and 

ammonia to the catalyst active sites.

In contrast, the oxidation of SO2 is not limited by diffusion and thus 

takes place in the entire catalyst mass and we get

(2) ASO2

 = k2 × W × C

where ASO2

 is the activity towards SO2 oxidation and W is the bulk 

density of the catalyst.

It follows from the above that the best possible SCR DeNOx 

catalyst for a coal-fired power plant minimises

(3) ASO2

/ADeNOx = k × W/As × C1-a × η-1

The special way of manufacturing the Topsøe DNX® catalyst 

allows for careful control of the pore structure within the catalyst 

and this has been optimised with a tri-modal pore structure as 

shown in Figure 2. 

Through large macro-pores in addition to the meso- and micropores the access to the interior of the catalyst is enhanced and 

this maximises the effectiveness factor η. The elaborated pore 

system further increases the porosity (As/W) and gives the lowest 

possible SO2-oxidation as per (3). A direct comparison of the 

bulk density of DNX® with other catalysts will serve to illustrate 

this fact. 

The low SO2

 oxidation of the Topsøe DNX® catalyst has been 

verified by testing in the industry. Figure 3 illustrates such a 

comparison where standard extruded catalysts and plate-type 

catalysts with a much lower porosity show a much inferior ratio 

between SO2 oxidation and DeNOx activity than Topsøe’s DNX®

catalyst.

Topsøe is capable of tailor-making catalysts for any application 

and for any level of sulphur in the fuel and can therefore provide 

the best compromise between NOx removal and SO2 oxidation.

Topsøe has demonstrated superior performance together with 

low SO2 oxidation under industrial conditions and has provided 

guarantees for SO2 oxidation down to 0.1%.

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Meso-pore

Macro-pore

Micro-pore

FLUE GAS FLOW

SO3

NO NO

NO

N2 N2

N2

SO2

CATALYST WALL

Relative SO2 oxidation

Catalyst 1

Plate type

Catalyst 6

Plate type

Catalyst 4

Extruded

Catalyst 5

Extruded

Catalyst 3

Extruded

Catalyst 2

Extruded

Haldor Topsøe

Corrugated type

Relative SCR DeNOx activity

Figure 2.

Figure 3.