H+H SCR Technology for lower NOx emissions

SCR Technology

What is SCR and how does SCR work?

SCR = Selective Catalytic Reduction

The SCR method is intended to control the NOx emissions by means of a selective catalytic reduction. This is the most common method
for removal of NOx in combustion engines and non-engine combustion processes (biomass, glass furnace, SNCR-SCR combination, industrial boilers).

This is injected in the exhaust gas where it evaporates and releases NH3 – ammonia. On the SCR catalyst surface the reduction occurs
and converts the NOx to harmless N2 which is inert and a natural component of the air. The equally harmless H2O is a by-product.

What chemical reactions are behind the SCR technology?

Nitrogen oxides are converted to inert, non-toxic gas components with the help of a reducing agent. The catalyst reduces
the amount of energy which is necessary to allow the reaction to proceed under the given temperature conditions.

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With the SCR technology, the nitrogen monoxide is reduced by ammonia and oxygen to nitrogen and water:
4NH3 + 4NO + O2 → 4N2 + 6H2O

The reaction also takes place in the significantly slower reaction without oxygen:
4NH3 + 6NO → 5N2 + 6H2O

However, most of the exhaust gases contain an excess of oxygen, so this reaction is hardly relevant. If the two
nitrogen oxides NO and NO2 are equimolar, the fastest of the reactions will take place:
4NH3 + 2NO + 2NO2 → 4N2 + 6H2O

Depending on the thermodynamic equilibrium and oxygen content, however, there is usually an NO content greater than 90%,
so that this reduction also plays a subordinate role. In particular, vanadium, pentoxide-tungsten, dioxide-titanium, dioxide catalysts
are used, since these can be produced as full extrudates in different cell densities. 

Depending on the space velocity (≈ 10000 h-1), pressure loss and temperature, the geometry and vanadium content are adapted
to the respective application

For which temperature range are the
H+H SCR systems suitable?

Our H+H SCR systems are suitable for operating temperatures between 180 and 540°C. The catalytic converter must be operated at a minimum temperature, but must not exceed the maximum. While the minimum temperature depends on the sulfur content, the maximum temperature is based on the thermal limit of the catalyst. The minimum temperature is a design criteria and an important parameter for the NOx conversion. The lower the temperature, the more catalyst is needed.

The operating temperature for combustion engines (gas / diesel / heavy oil) is at least
260-320°C and at most 540°C.

The H+H SCR systems require at least 180° C for non-engine combustion processes (biomass, glass furnace, SNCR-SCR combination, industrial boilers).

Which catalysts do we use for our H+H SCR plants?

For our H+H SCR systems we use SCR and oxidation catalysts.

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SCR catalysts
For our H+H SCR plants we mostly use homogeneously extruded honeycomb catalysts. These SCR honeycomb catalysts consist entirely of catalytically
active materials - titanium oxide, tungsten oxide and vanadium pentoxide - and are therefore more active than coated SCR catalysts. The necessary mechanical strength was achieved by adding glass fibers. The honeycomb SCR catalytic converter is delivered pre-installed in element frames or modules to ensure durability and easy handling.

In some applications, we still use coated metal catalysts, which are mostly integrated directly into the exhaust duct.

Oxidation catalysts
The oxidation catalysts used for our H+H SCR systems consist of coated metal foils and the catalytic coating, usually precious
metals such as platinum and palladium.

The oxidation catalyst will be preinstalled in a single round casing or together with the SCR honeycomb catalysts described above in the
same dimensions in just one round, or square shape casing.

Which components are used for a H+H SCR system?



The H+H SCR systems consists of the following components:
+ Reducing agent pump or pump station
+ Reducing agent dosing unit
+ Mixing unit with injection lance 
+ Catalyst housing (reactor) including SCR catalysts
+ Control unit

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Optionally the following components can be provided:
+ Compressor
+ Reducing agent tank
+ NOx Analyser / sensor
+ Ethene gas chromatograph
+ Sootblower


Do our H+H SCR systems meet the limit values ​​of the (inter) national emission regulations?


Our H+H SCR systems comply with the following regulations in the stationary sector

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Our H+H SCR systems comply with the following regulations in the maritime sector

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Our H+H SCR systems comply with the following regulations in the inland waterway vessel

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How do I submit a request for quotation for a H+H SCR project?

Our SCR systems are tailor made and will be designed to fit to your application. Therefore we need some
technical data to your project, respectively emission source. Please send us an e-mail to info@huhes.de.

Does the DENOx / SCR system start up automatically?

Yes, in automatic mode the system will recognize the engine or system start and then automatically starts
injection once the catalyst reached its operating temperature.

Does SCR affect fuel efficiency? 

No. If the SCR is sized correctly and backpressure is at the acceptable level, there should be no loss in fuel economy.
The H+H engineered solution ensures proper backpressure and system integration without loss of effectiveness.

Does the SCR catalyst also reduce formaldehyde?

Yes, up to 20 – 30 %, moreover DOC (oxidation catalyst) can almost completely reduce formaldehyde.

Substantial Expericence

Personal and reliable

Our service engineers have been working for more than 30 years in the field
of exhaust gas and air treatment technology. Should you have any question,
we would be pleased to support you.