The most effective process to date for flue gas NOx removal in power plants is known as selective catalytic reduction (SCR). It operates at temperatures of between 300 °C and 400°C on the reaction principle that is shown in Fig. 1 and may be summarized by the following equations:
Before the flue gas enters the reactor, ammonia is added in the form of a NH3 / air mixture, which promotes the reduction of nitrogen oxides when the gas comes into contact with the catalyst.
The DENOX unit can be installed downstream of the boiler between the economizer (feed water pre-heater) and the combustion air preheater and is known as the “high-dust” configuration.
When the unit is located downstream of the electrostatic precipitator this result is the so-called “low-dust” configuration. In this configuration the DENOX unit may be also installed downstream of the desulphurization system. This result is the so-called “tail-end” configuration.
THE DESIGN
Taking into account the Customer specifications and the amount of space available in each case, the size of individual reactors is optimized with the aid of pilot plant tests and with computational fluid dynamic models. The criteria of particular importance include the thorough mixture of NH3 and NOx molecules in the reactor hood and a constant gas flow in the vertical part of the reactor.
The key design parameter in a reactor of this type is the so-called space velocity (SV). This is a measure of the residence time of the flue gas mixture (at STP) within the catalyst volume.
Calculation of the space velocity takes into account the following factors:
- Efficiency of the DENOX reaction
- Temperature
- Allowable ammonia slip
- Flue gas analysis
- Dust analysis
The SV is used in calculating the amount of catalyst required per unit time for a given volume of flue gas. In coal-fired power plants, the SV is normally between 1000 and 3000 per hour whereas for oil – and gas – fired boilers it will be higher, resulting in a smaller quantity of catalyst being required for the NOx reduction.
Extra space is normally provided to allow catalyst to be added when the overall performance begins to decrease, thereby raising the effective service life of the catalyst. In this way, it is possible to extend the time between complete replacements of the catalyst.
The catalyst composition and cells number or pitch can be varied to accommodate individual requirements.
Used catalyst can either be disposed of, or recycled to recover useful materials.
The ammonia storage tanks may be either above or below ground as required, the ammonia being vaporized in heat exchangers of either the water bath or the tube bundle type.
Safety aspects, including sprinkler systems, ground slabs and the disposal of gaseous and liquid residues, are also taken into consideration.
ADVANTAGES OF THE CATALYST
- Approximate stoichiometric operation due to high catalyst activity.
- Low pressure drop and avoidance of dust accumulation.
- Low SO2 to SO3 conversion rate due to the high selectivity of the catalyst.
- Honeycomb ceramic catalyst elements, higher mechanical stability.
- High resistance to temperature change.
- High resistance to erosion.
Easy handling of catalyst modules due to their sturdy construction. Individual catalyst elements are then stacked together to form units or modules that are generally arranged in the reactor in discrete layers. As a result of the high packing density, it is possible to achieve an optimum catalyst-to-space ratio.
