Selective catalytic reduction

Synonyms, abbreviations and/or process names

• SCR

 

Removed components

• NO_x

 

Diagram

 

Process description

During the SCR process (Selective Catalytic Reduction) NO_x is reduced to N₂ and H₂O by adding NH₃ or ureum in the presence of a catalyst, as in the following reaction diagram: 

The optimum process temperature lies between 320 – 500 °C depending on the catalyst.

Oxides of vanadium, wolfram, molybdenum or other metals are often used as a catalyst, with titanium oxide as the carrying material. An optimum mix in the catalyst bed and a molar ratio of 1:1 for NH₃/NO_x are the most important process parameters for ensuring the most comprehensive reaction and thus avoiding unwanted ammonia emissions (“NH₃ slip”).

The lifespan of the catalyst is primarily determined by the erosion caused by the flue gas, deactivation caused by poisoning (NH₄HSO₄) and the build-up and fouling by the catalyst.

Special attention must be paid to start-up and “shut-down” of the installation. It is important to stop NH₃ injection when the temperature drops below a particular level, in order to prevent ammonium water sulphate building-up on the catalyst.

 

Variants 

The SCR installation can be placed immediately after the boiler (“high-dust” switching).

A second possibility is to later employ a SCR unit after the fabric filters or scrubbers (“low-dust” switching). This requires the flue gas to be heated to reaction level.

 

Efficiency

NO_x: 90-94%

Residual emissions: Possible aerosol-forming from ammonium chloride and ammonium sulphate.

 

Boundary conditions

• Flow rate: 1 000 000 Nm³/h
• Temperature: 200 - 500 °C depending on the catalyst
• NO_x: In the range g/Nm³
• NH₃/NO_x ration: < 1:1

In the presence of SO₃ the ammonium is collected via the forming of ammonium sulphate. The ammonium sulphate can block the catalyst and is emitted as a difficult-to-separate aerosol.

 

Auxiliary materials

Reductants: 25% solution of ammonium or ureum

Steam: To aerate the reductant before it is injected.

Catalyst: V₂O₅ (vanadium pentoxide) and/or TiO₂ (titanium dioxide), etc.

 

Environmental aspects

• Possible NH₃ emissions due to the process not being completed effectively (NH₃ slip).
• Used catalyst
• Laughing gas (N₂O) can be produced as a by-product.
• In "high-dust" switching, the flue gases are laden with NH₃;

 

Energy use

Energy use for heating flue gases in "low-dust configuration”.

 

Cost aspects

Investment

• 7 500 -32 000 EUR for 1 000 Nm³/h (strongly determined by application) [1,2]
• For a household-waste incineration plant with a capacity of ca. 80 000 ton per year and a flue gas flow rate of 60 000 Nm³/h, the investment costs amount to ca.  5 000 000 EUR [6].

Operating costs

• Personnel costs: ca. 20 000 EUR per year
• Auxiliary and residual materials: Up to 370 kg NH₃ solution per ton NO_x removed
• Cost aspects NH_3: 150 EUR/ton

For flue gases from a household-waste incineration plant, the average gas composition for NO_x is 335 mg/Nm³ (VITO measurement 1999). This is equivalent to an average emission of 2 141 g/ton waste. For an installation of 18 ton/h, this corresponds to 38 kg/h NH₃. In order to reduce NO_x emission by 50% the use of NH₃ amounts to an average of 19 kg/h or 465 kg/day. On an annual basis, this means ca. 17 ton per year per 1 000 Nm³/h or 2 550 EUR per year per 1 000 Nm³/h.

 

Advantages and disadvantages

Advantages

• Higher NO_x reduction yield compared to SNCR
• Relatively simple installation

Disadvantages

• For relatively high SO₃ contents, the process must be performed at a relatively high temperature in order to avoid condensation.
• In "high-dust" switching, the flue gases are laden with NH₃.
• Cost aspects of catalyst
• Possibility of blocking and poisoning of the catalyst
• In "low-dust" switching, flue gases need to be heated.
• Catalyst may suffer from erosion caused by flue gases.
• High investment cost compared to SNCR

 

Applications

Selective catalytic reduction is used within combustion installations in the following sectors:

• Waste incineration;
• Energy plants;
• Metal industry;
• Greenhouse horticulture.

 

References

1. Factsheets on Air-emission reduction techniques, www.infomil.nl, Infomil
2. Common waste water and waste gas treatment and management systems in the chemical sector. BREF document, European IPPC Bureau, http://eippcb.jrc.es
3. Elslander H., De Fré R., Geuzens P., Wevers M. (1993). Comparative evaluation of possible gas purification systems for the combustion of household waste. In: Energie & Milieu, 9
4. Vanderreydt I. (2001). Inventory of the waste incineration sector in Flanders. Vito, 2001/MIM/R/030
5. Work-book on environmental measures: “Metal and electro-technical industry” (1998 ). VNG publishers
6. Supplier information
7. VDI 3927, Abgasreinigung, Abscheidung von Schwefeloxiden, Stickstoffoxiden und Halogeniden aus Abgasen (Rauchgasen) von Verbrennungsprozessen

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