Catalysts

  

 

 

Introduction

Exomission has extensive experience in the design, substrate selection, coating formulation and the mechanical and chemical service life of the most varied catalytic converter systems. We offer both standard solutions and components that are modified especially for your area of use. We would be happy to advise you and look forward to your query.

The oxidation catalyst (OC) for use in engines in lean-burn operation. The catalyst oxidises carbon monoxide (CO) and non-combusted hydrocarbons (HC) efficiently. Efficiency rates of 90% and more are often achieved. The hydrocarbons include compounds such as the following:

  •     polycyclic aromatic hydrocarbons
  •     volatile organic compounds
  •     non-methane hydrocarbons
  •     aldehydes, for example formaldehyde, also known as methanal (CH2O)

 

Special forms

The diesel oxidation catalyst (DOC) oxidises carbon monoxide (CO) and non-combusted hydrocarbons (HC) in diesel engines. A small part of the particulate mass emitted is also reduced by oxidation of the hydrocarbons (HC) condensed out on the soot particles.

The DOC is also required in conjunction with particulate filters in order to convert the nitrogen oxides (NOx) that are contained in the exhaust gas into nitrogen dioxide (NO2); this makes it possible to use passive regenerating particulate filters – see particulate filter technology.  We can both deliver and optimise coatings that prevent the formation of NO2.

 

Our DOCs are available in the most varied shell versions.

The biogas catalyst (BOC) is an oxidation catalyst that is used in combustion engines (lean-burn or pilot injection procedure) which are operated using gases of biological origin. These include biogas, mine gas, gas from purification plants and landfill gas.

 

The BOC oxidises carbon monoxide and hydrocarbons. Formaldehyde is also one of the hydrocarbons.

Formaldehyde is a by-product of the combustion of methane and is therefore incurred principally in the exhaust gas of biogas engines.

Formaldehyde is classified as carcinogenic, meaning that a limit value of 1 mg/Nm³ based on 5 % O2 in the exhaust gas must be complied with in accordance with Germany’s TA-Luft. However, there is neither an exhaust gas aftertreatment that is proven to comply with this limit value, nor is there measuring technology that can definitely show whether this limit value is complied with or exceeded under field conditions.

The German federal committee for air pollution control (LAI) therefore decided in the framework of the EEG2009 (Renewal Energy Sources Act 2009) to establish 40 mg/Nm³ as the tightened limit value to be kept below and to compensate for the additional expense required for this (biogas treatment and exhaust gas aftertreatment) with an additional 1 ct/kWel.

A problem when using BOC is the catalyst poisons contained in the combustion gas, particularly the hydrogen sulphide H2S. Sulphur compounds in the bio combustion gas combust in the engine, forming SO2, which leaves the engine as an exhaust gas and damages the catalytic coating of the catalyst substrate. SO2 oxidises to become SO3 to some extent in the catalyst converter itself.

The SO2 forms sulphurous acid with the water vapour that is also contained in the exhaust gas; the SO3 formed forms sulphuric acid with the water vapour. The sulphuric acid formed in this way condenses due to the low dew point, especially in the exhaust gas heat exchanger of a CHP unit, and this corrodes/destroys even high-grade steels due to its corrosiveness.

It is therefore absolutely essential that combustion gas desulphurisation takes place.

Exomission BOCs also have a special catalytic coating that has increased resistance to sulphur compounds and generally reduces the oxidation of sulphur compounds.

This increases the resistance to catalyst poisoning on the one hand and reduces the formation of sulphuric acid (and therefore the corrosion of the exhaust gas heat exchanger) on the other hand.

 

Our BOC is available in the most varied shell versions.

The ammonia slip catalyst (ASC) is responsible for converting any excess ammonia back into nitrogen and water. This is required because gaseous NH3 has a very low smell threshold. Odour emission occurs from an ammonia slip of as little of 15 ppm. NH3 is also poisonous and leads to irritation of eyes, airways and skin at higher concentrations.

 – For more about the ammonia slip catalytic convertor, see NOX reduction using SCR.

The three-way catalyst (TWC) for use in λ-controlled petrol engines. The catalyst oxidises carbon monoxide (CO) and non-combusted hydrocarbons (HC) while simultaneously reducing nitrogen oxides. The desired reactions take place optimally at λ = 1. This is referred to as a stoichiometric mixture. For petrol, a mass ratio (air/fuel) of 14.7:1 applies

The SCR catalyst (selective catalytic reduction) for reducing nitrogen oxides. SCR technology inserts ammonia (NH3) in the form of a substrate (AdBlue®) into the exhaust gas system before an SCR catalyst converter, which is used to help reduce nitrogen oxides selectively to elemental nitrogen with conversion rates of up to  and above.

The currently available SCR catalytic convertors distinguish themselves both with respect to their structure and with respect to the chemical composition of the active components. Due to the differing activity and stability, they are each only suitable for particular fields of use and temperature ranges and must therefore be designed system-specifically.

A distinction is also made between substrate and full catalysts in SCR catalyst technology.

Full catalysts are extruded porous ceramics, which contain their active components in the ceramic. They have a smaller specific surface area than substrate catalysts; however, their storage capacity is higher, as they are composed completely of active material. However, they have the disadvantage that the active material dissolves faster than for substrate catalysts with these catalysts in dynamic operation (increased temperature or space velocity). Due to this characteristic, full catalysts are usually used on stationary-operated engines and systems. Substrate catalysts have proven themselves in mobile SCR use thanks to their superior storage characteristics.

Depending on the system conditions, we establish the best possible catalysts for you depending on costs, conversion rates, service life and installation space conditions. If you require the SCR dosing system as well as the SCR catalysts, you can find [further information here].

 

Coatings and substrate versions

A catalyst system is made up of the shell, the substrate (monolith), the intermediate layer (wash-coat) and the active catalytic coating applied over this. With the help of the wash-coat structure, a correspondingly high specific surface, which is critical for the catalyst activity, is made available. Aluminium oxides, silicon oxides and titanium oxides are used as a wash-coat, although the interaction is very low for aluminium oxide coatings and very pronounced for titanium oxide. A distinction can be made between ceramic and metal substrates. The structure of the substrate is characterised by narrow canals or cells, which usually run parallel to each other and through which the exhaust gas flows. Metal substrates tend to offer the advantage of thinner wall thicknesses and higher cell densities. This makes it possible to reduce the installation space required for the catalyst and the pressure lost. Due to the lower heat capacity, metal substrates have a more favourable heat-up characteristic in dynamic operation. Catalytic converters are positioned as close as possible to the engine, so that they reach the temperature required to function efficiently (light-off temperature). Using structured metal sheets makes it possible to reduce the catalytic converter volume by up to 30% using the turbulence generated. This also means that the high-grade metal quantity required for coating is lower.

 

Shell versions

Shell versions are possible in the most varied forms for integrating the catalytic converter.

   
  1. Directly flanged catalytic converter shell

 

 

 


  1. Catalytic converter flanged with tapered shell

 

 

 


  1. Catalytic converter with tapered shell welded into the exhaust pipe

 

 

 


  1. Catalytic converter with slide-in/changeable shell

 



 

 

Recycling

Particularly when biogas is used, the catalysts are subject to toxic processes that make it necessary to replace the substrate after certain operating times. When a substrate is replaced, we will take the used monoliths back and recycle them on the condition that these were originally supplied by us or there is a certificate of high-grade metal for a substrate from a different company. As we work together with a renowned specialist company for catalyst recycling, we can reimburse you for a large part of the recycled high-grade metal.

 

                       

We look forward to hearing from you.

 

 

 

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