# DNX ® 

# - Topsøe SCR DeNOx catalysts Oxidation of SO 2 

# into SO 3

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 SO 2 but a small fraction is further converted or oxidised into SO 3 . The SO 3 will react with residual ammonia to form ammonia bisulphate, ABS, which can foul downstream equipment. Also, SO 3 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 SO 3 from SO 2 . 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 SO 2 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 SO 2 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.



## 1.0Relative activity



## 0.5

0.0 250 High-vanadium type Low-vanadium type

300 350 Temperature, °C 400 450

Figure 1: DeNOx activity and SO 2 oxidation characteristics.

# DNX ® 

# - Topsøe SCR DeNOx catalysts Oxidation of SO 2 

# into SO 3

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 (A DeNOx) becomes (1) 

### A DeNOx= k 1 × A s × C a × η where A s is the specific active surface area (m 2 /m 3 ), 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 SO 2 is not limited by diffusion and thus takes place in the entire catalyst mass and we get (2) 

### A SO 2 = k 2 × W × C where A SO 2 is the activity towards SO 2 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) 

### A SO 2 / 

### A DeNOx= k × W/A s × C 1-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 (A s /W) and gives the lowest possible SO 2 -oxidation as per (3). A direct comparison of the bulk density of DNX ® with other catalysts will serve to illustrate this fact.

The low SO 2 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 SO 2 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 SO 2 oxidation. Topsøe has demonstrated superior performance together with low SO 2 oxidation under industrial conditions and has provided guarantees for SO 2 oxidation down to 0.1%. WWW.TOPSOE.COM

Meso-pore Macro-pore Micro-pore FLUE GAS FLOW

SO 3 NO NO NO

N 2 N 2 N 2
SO 2 CATALYST WALL

Relative SO 2 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.