Secondary techniques for NO X reduction

[1, UBA, 2001] Directive 2000/76/EC requires a daily average NOX (as NO2) clean gas value of 200 mg/Nm³. In order to achieve compliance at this level, it is common for secondary measures to be applied. For most processes the application of ammonia or derivatives of ammonia (e.g.

urea) as reduction agent has proved successful. The nitrogen oxides in the flue-gas basically consist of NO and NO2 and are reduced to nitrogen N2 and water vapour by the reduction agent.

Reaction equations:

4 NO + 4 NH3 + O2 4 N2 + 6 H2O 2 NO2 + 4 NH3 + O2 3 N2 + 6 H2O

Two processes are important for the removal of nitrogen from flue-gases - the Selective Non-Catalytic Reduction (SNCR) and the Selective Non-Catalytic Reduction (SCR).

Both NH3 and urea are applied in aqueous solutions. NH3 is normally, for safety reasons, delivered as a 25 % solution.

2.5.5.2.1 Selective Non-Catalytic Reduction (SNCR) process

In the Selective Non-Catalytic Reduction (SNCR) process nitrogen oxides (NO + NO2) are removed by selective non-catalytic reduction. With this type of process the reducing agent (typically ammonia or urea) is injected into the furnace and reacts with the nitrogen oxides. The reactions occur at temperatures between 850 and 1000 °C, with zones of higher and lower reaction rate within this range.

Figure 2.48: SNCR operating principle [1, UBA, 2001]

Reducing NOX by SNCR more than 60 – 80 %, requires a higher addition of the reducing agent.

This can lead to emissions of ammonia, also known as ammonia slip. The relationship between NOX reduction, ammonia slip and reaction temperature is given in Figure 2.49 below:

Figure 2.49: Relationship between NOX reduction, production, ammonia slip and reaction temperature for the SNCR process

[Austria, 2002 #3] [64, TWGComments, 2003]

In Figure 2.49, it is shown that, at a reaction temperature of, for example, 1000 °C, the reduction of NOX would be about 85 %, and there would be an ammonia slip of about 15 %. In addition, at this temperature there would be a production of NOX, from the incineration of the injected NH3, of about 25 %.

Figure 2.49 also shows that, at higher temperatures (with ammonia), the percentage of NOX

reduction is higher, and while the ammonia slip is lower, the NOX produced from the ammonia rises. At high temperatures, (>1200 °C) NH3 itself oxidises and forms NOX. At lower operational temperatures the NOX reduction is less efficient, and ammonia is slip higher

Application of urea instead of ammonia in SNCR leads to relatively higher N2O emissions in comparison with ammonia reduction. [64, TWGComments, 2003]

In order to ensure an optimum utilisation of ammonia at varying degrees of load, which cause varying temperatures in the combustion chamber, NH3 can be injected at several layers.

When used with wet scrubbing systems, the excess ammonia may be removed in the wet scrubber. The ammonia can then be recovered from the scrubber effluent using an ammonia stripper and fed back to the SNCR feed system.

Important for optimisation of the SNCR process is the effective mixing of flue-gases and NOX

reduction reagent, and sufficient gas residence time to allow the NOX reduction reactions to occur.

In the case of pyrolysis and gasification processes, optimisation of SNCR is achieved by injecting the reagent into the syngas combustion zones with a well controlled temperature and effective gas mixing.

2.5.5.2.2 Selective Catalytic Reduction (SCR) process

Selective Catalytic Reduction (SCR) is a catalytic process during which ammonia mixed with air (the reduction agent) is added to the flue-gas and passed over a catalyst, usually a mesh (e.g.

platinum, rhodium, TiO2, zeolites). [74, TWGComments, 2004] When passing through the catalyst, ammonia reacts with NOX to give nitrogen and water vapour.

To be effective, the catalyst usually requires a temperature of between 180 and 450 °C. The majority of systems used in waste incinerators currently operate in the range 230 - 300 °C.

Below 250 °C more Catalyst volume is necessary and there is a greater risk of fouling and catalyst poisoning. In some cases catalyst temperature regulated bypasses are used to avoid damage to the SCR unit. [74, TWGComments, 2004]

The SCR process gives high NOX reduction rates (typically over 90 %) at close to stoichiometric additions of the reduction agent. For waste incineration, SCR is mainly applied in the clean gas area i.e. after de-dusting and acid gas removal. For this reason, the flue-gases generally require reheating to the effective reaction temperature of the SCR system. This adds to the energy requirements of the flue-gas treatment system. However, when SOX levels in the flue-gas have already been reduced to a very low value at the inlet of the SCR section, reheating may be reduced substantially, or even omitted. Heat exchangers are used to reduce additional energy demand.

After a wet FGT system, droplets may be removed to prevent salt deposits inside the catalyst.

Due to risk of ignition, safety measures are of importance, e.g. by passes, CO control, etc. [74, TWGComments, 2004]

Low-temperature SCR requires catalyst regeneration due to salts formation (especially ammonium chloride and ammonium sulphate). The regeneration may be critical because of the salt sublimation may lead to exceedences of the applied ELV for releases to air for some pollutants e.g. HCl, SO2, NOX. [74, TWGComments, 2004]

SCR is sometimes positioned directly after the ESP, to reduce or eliminate the need for reheating in the flue-gas. When this option is used, the additional risk of PCDD/F formation in the ESP (typically when the ESP operated at temperatures above 220 - 250 ºC) must be considered. Such operation can result in increased PCDD/F emissions to ESP residues and higher concentrations in the gas stream leaving the ESP and passing to the SCR unit.. SCR can also be used for PCDD/F destruction. Multi layer SCR systems are used to provide combined NOX and PCDD/F control.

Figure 2.50: SCR operating principle [3, Austria, 2002]

The flue-gases discharged by the reactor may be directed through a gas-gas heat-exchanger to preheat the entering gases in order to maintain the operating temperature of the catalyst and to save a part of the imported energy (see diagrams in Section 4.4.4.1).

2.5.6 Techniques for the reduction of mercury emissions