A Selective Catalytic Reduction (SRC) device i.e. a catalytic converter is often associated with a muffler in a reciprocating internal combustion engine (heat engine) exhaust line, particularly if it comes to a power plant with a high capacity, therefore with significant fumes flow rates.
Because then, from an environmental protection perspective, it is not only necessary - for such hardware whose operation is based on a combustion phenomenon - to reduce noise, but also to limit pollutant emissions - resulting from the combination of oxygen (O2) with nitrogen (N2) by means of a chemical oxidation reaction - what are nitrogen oxides (NOx):
- nitric oxide NO
- nitrogen dioxide NO2
Unlike devices for oxidizing carbon particles (and unburned carbon monoxide (CO) to obtain carbon dioxide (CO2) which only requires inserting a cartridge, requiring a removable part for the silencer, to allow its possible replacement, the catalyst intended for the oxidation of nitrogen oxides (NOx) is generally dissociated, constituting an independent subassembly (e.g. in the form of a skid).
Selective Catalytic Reduction (SCR) is a géneral technique for treating exhaust gases i.e. with combustion products such as coal, oil, gas.
A catalytic converter limits the release to the atmosphere of toxic elements such as hydrocarbons, carbon monoxide (CO), typically at a concentration of less than 650 mg / Nm3, and nitrogen oxides by means of a specific chemical reaction.
For what concerns industrial heat engines - such as those used for the propulsion of boats or for the production of electricity (i.e. generator sets) -, it mainly involves reducing nitrogen oxide (NOx) emissions, typically at a concentration of less than 250 mg/Nm3 (the reduction rate is generally around 80% if not more), by performing a chemical transformation (oxidation-reduction) towards compounds such as nitrogen (N2) and water (H2O). [1]
The process requires:
- a nitrogen reducing agent: often urea CO(NH2)2 in aqueous solution
- a catalyst (when it is not zeolite, a micoporous natural mineral: precious metals e.g. tungsten W or vanadium V on a support based on titanium oxide TiO2 in the form of modular elements i.e. cassettes)
The best performance requires a chemical reaction at a temperature - that of the hot exhaust gases - which is optimal when between 380°C and 470°C, which can - when required - be as low as 250°C, but must not exceed 520 °C so as not to damage the metal catalytic supports.
A Selective Catalytic Reduction (SCR) device i.e. a catalytic converter for a thermal engine exhaust line mainly comprises:
- a urea tank (sometimes: daily, if a large capacity tank is provided elsewhere, upstream) with at least one associated pump
- a mixer i.e. a pipe in which the mixture of the exhaust gases and the urea is carried out (by means of injectors, with compressed air)
- a reactor in which the catalyst cassettes are installed, through which the aforementioned gas mixture passes
- an electrical cabinet with an automaton for managing the urea dosage according to information received from specific sensors
The assembly is in the form of a metal construction, on a mechanically welded frame, lending itself well to a realization according to the concept "plug and play" ("ready to be connected").
Accessories are possibly:
- upstream:
- expansion joint
- variable piping elements to adapt to the engine interface configuration (elbow, manifold)
- primary silencer (reactive)
- downstream:
- secondary silencer (reactive/dissipative)
- variable piping elements depending on the context (elbow, chimney with or without anti rain device)
Among the many technical aspects related to the performance of a Selective Catalytic Reduction (SCR) device i.e. a catalytic converter, the main selection criteria are:
- the efficiency in terms of reduction rate and or content of nitrogen oxides (NOx) e.g. in mg/Nm3
- the total pressure loss which must remain compatible with optimized motor operation: this involves not altering the efficiency of an electrical production installation (whether with continuous operation or not - peak shedding, emergency -)
- maintenance cost and overall durability
ITS markets Selective Catalytic Reduction (SCR) devices i.e. catalytic converters for industrial engines exhausts which are the best in this field, while the limitation of certain pollutants (e.g. NOx) in the atmosphere is an increasingly pressing need as the number and power of combustion installations grow on a planetary scale, the origin of their emissions and on the other hand the awareness of the related environmental issues..
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Selective Catalytic Reduction (SCR) device, catalytic converter: all equipment that one can source from Isolation Technologie Services (ITS).
[1] main chemical equations involved are:
2 NO + 2 NH3 + 1/2 O2 → 2 N2 + 3 H2O
NO2 + 2 NH3 + 1/2 O2 → 3/2 N2 + 3 H2O
NO + NO2 + 2 NH3 → 2 N2 + 3 H2O