Non-selective catalytic reduction

Synonyms, abbreviations and/or process names

—  NSCR

Removed components

—  NOx, CO, hydrocarbons

Diagram

 

Process description

In non-selective catalytic reduction, CO, NOx and hydrocarbons are converted into CO2 and N2 via a catalyst. This technique does not need additional reagents to be injected because the unburnt hydrocarbons are used as a reductant. Though gases must not possess more than 0.5% oxygen.

NOx removal takes place in two sequential phases, with the following reactions:

The reactions in phase 1 remove excess oxygen because this reacts better with CO and hydrocarbons than NOx. For this reason, the oxygen concentration in flue gases must be kept below 0.5%.

This is why NSCR can only be used with rich combustion mixes. For poor mixes, one must change to SCR .

The catalysts used are normally based on platinum.

Non-selective catalytic reduction is mainly used as a three-way catalyst in the automotive industry.

Variants 

Certain suppliers use natural gas as a reductant for NOx. This allows the permitted oxygen concentration to be raised to maximum 2%.

Efficiency

NOx removal efficiencies of 90 – 98 % can be realised [1].

NOx emissions of 25 ppmv can be realised [2].

Boundary conditions

—  Oxygen content maximum 0.5% (combustion almost stoichiometric) to 2% (when natural gas is used as reductant).
      This means it can only be used for choked mix engines and not for self-combustors like diesel engines.
—  There is a lack of experience in treating biogas from digesters or waste gas. The H2S and other substances could
      possibly poison the catalyst.
—  Temperature of flue gases: 375 - 825 °C (425 – 650 °C for yields > 90 %) [2]

Auxiliary materials

The catalyst must be periodically replaced. A life-span of 2 – 3 years is normally guaranteed [2].

Environmental aspects

The catalyst must be periodically replaced and forms a residue.

The use of NSCR could possibly result in higher CO levels due to the engine’s need for a rich mix, to ensure that CO is available to the catalyst for the removal of NOx. If the CO level is too high after the catalyst, it may be necessary to later employ an oxidation catalyst to oxidise the CO into CO2

Energy use

Extra fuel is only used due to the catalyst’s higher pressure drop. The extra use amounts to 0 – 5%, depending on the design of the catalyst. The capacity of the engine also decreases by 1 – 2 % [2].

Cost aspects

Investment costs: [2]

Engine size (HP)

Cost in 1 000 USD

80 - 500

15 - 27

501 – 1 000

27 - 41

1 001 – 2 500

41 - 87

2 501 – 4 000

87 - 132

4 001 – 8 000

132 - 253

Total annual cost: [2]

Calculated at 8000 h/year, incl. maintenance and amortisation.

Engine size (HP)

Cost in 1 000 USD

80 - 500

69 - 79

501 – 1 000

79 - 90

1 001 – 2 500

90 - 124

2 501 – 4 000

124 - 158

4 001 – 8 000

158 - 244

Cost-effectiveness: [2]

Engine size (HP)

Cost in USD per ton NOx

80 - 500

1 260 – 6 900

501 – 1 000

750 - 1 260

1 001 – 2 500

395 - 750

2 501 – 4 000

315 - 395

4 001 – 8 000

240 - 315

The catalyst must be periodically replaced. The used catalyst can sometimes be sold, which helps to reduce the price for replacement.

Advantages and disadvantages

  •  Advantages
    —  No extra reductants needed
  • Disadvantages
    —  Needs engine control based on oxygen level ( lambda probe)
    —  Limited application

 Applications

Primarily used in the automotive industry. Can be used for applications where combustion is almost stoichiometric, as in stationary engines to generate energy or to drive.

This means it can only be used for choked mix engines and not for self-combustors like diesel engines. There is also a lack of experience with biogas from digesters or waste gas. The H2S and other substances could possibly poison the catalyst.

References

  1. EPA factsheets: Nitrogen oxides (NOx); why and how they are controlled
  2. EPA: Alternative Control Techniques (ACT) Document - Internal Combustion NOx Part 1 & 2 (EPA-453/R-93-032)