How to carry out the depollution of exhaust gases from internal combustion engines with an SCR process ?

Products of a combustion process, the exhaust gases from internal combustion engines contain atmospheric pollutants such as nitrogen oxides (NOx):

  • nitric oxide NO
  • nitrogen dioxide NO2

It is well known that these molecules are toxic, posing serious health problems (e.g. concerning the respiratory system of humans), and that they have harmful effects for the environment (e.g. contributing to the phenomenon of acid rain).

The depollution of combustion engine exhaust gases is possible in particular through Selective Catalytic Reduction (SCR).

It is a question of causing chemical reactions specific to recombining (with nitrogenous compounds) the molecules constituting the undesirable gases that NOx are, to obtain compounds as harmless as nitrogen (N2) and water (H2O) usually contained in (wet) air. [1]

Urea CO(NH2)2 in aqueous solution is a nitrogenous compound widely used for the Selective Catalytic Reduction (SCR) of nitrogen oxides (NOx) contained in the exhaust gases of industrial engines.

A precious alloy e.g. tungsten W or vanadium V on a support based on titanium oxide TiO2 often serves as a catalyst for chemical reactions which generally occur at temperatures between 250°C and 520°C.

The depollution of exhaust gases from internal combustion engines can then be carried out by means of metal cassettes whose internal, porous parts allow the circulation of their mixture with the nitrogenous compound.

A Selective Catalytic Reduction (SCR) module - requiring a continuous supply of urea, with dosage managed by the means of an automaton and of sensors - can thus complete the noise reduction devices of an exhaust line that reactive (i.e. whose operating principle is based on changes in the geometry of the internal parts with chambers) or dissipative (i.e. whose effectiveness is linked to the presence of porous materials that absorb sound) silencers are.

Although it is generally a very distinct sub-assembly, the Selective Catalytic Reduction (SCR) module can, in somes cases, be combined with a silencer to obtain an assembly offering the features of limitation of emissions of polluting compounds and of noise.

The dimensioning of such components of an industrial combustion engine exhaust line (including for gensets) must be the subject of an overall study, taking into account the combination of :

  • in terms of acoustics: the insertion loss of each component (for silencers, depending on their design: at low and/or medium and/or high frequencies; for the SCR Selective Catalytic Reduction device: only at high frequency)
  • in terms of aerodynamics: the total pressure loss of each component (it is not uncommon for it to be of the same order of magnitude)
  • in terms of general arrangement: the size of each component (it is not uncommon for it to be of the same order of magnitude, and for the part dedicated to the mixture between the nitrogen compound and the exhaust gases - upstream of the reactor – to have to be combined with a primary silencer in view of compactness)

Experience counts to offer in all contexts an optimized solution for high-performance equipment built to last, for the pollution control of industrial combution engines exhaust gases with a Selective Catalytic Reduction (SCR) process of nitrogen oxides (NOx) such as for noise reduction.


[1] the principle of depollution of combustion engine exhaust gases by means of a Selective Catalytic Reduction (SCR) process is based on the following typical chemical equations :

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
Description

Which noise reduction of control valves ?

When it comes to limiting noise emission due to discharge, noise reduction of control valves can be obtained by means of a suitable silencer, installed in the end of the piping line.

Such silencers are generally composed of a diffuser (upstream) and of a dissipative stage (downstream).

The diffuser is a perforated element, at which a change occurs (that is desired to be downwards) of the turbulence noise and shock noise, the presence of small diameter perforations causing a peak in noise generation for  high frequency. In addition, the presence of the diffuser causes a total loss of pressure on which attention must be paid (*).

The dissipative stage, in turn, consists of a lining (preferably: with high sound absorption), often used as filling of splitters, sometimes concentric (else: transverse), allowing for noise attenuation in a frequency range more or less extended namely depending on the acoustic characteristics of the porous medium and of its possible surfacing, on the geometry of the dissipative stage and on the nature and passage speed of the fluid.

In addition, the presence of the dissipative floor causes a total loss of pressure (usually less compared to that of the diffuser) on which attention must be paid (*) and generates self noise for which it is important to make sure that it is compatible with the noise reduction target to be considered as part of a project for which the implementation of a silencer is envisaged.

* Especially vis-à-vis the operating conditions of the valve on the downstream pressure which affects (upward)

Link to learn more about control valves noise calculation

What are the main parameters influencing control valve noise ?

Control valves (a fortiori when operating under conditions of high pressure drop) can contribute significantly to industrial & process plant noise, namely due to aerodynamic noise generation depending on valve data & process data, main parameters being as follows:

  • Valve inlet absolute pressure
  • Valve outlet absolute pressure
  • Liquid pressure recovery factor of a valve with or without attached fittings
  • Pressure differential ratio factor of a control valve with or without attached fittings at choked flow
  • Valve style modifier
  • (Required) Flow coefficient
  • Acoustic power ratio or Valve correction factor for acoustical efficiency
  • Molecular mass of flowing fluid
  • Inlet absolute temperature
  • Density of fluid at inlet
  • Specific heat ratio
  • Mass flow rate
  • Strouhal number
  • Outlet absolute temperature
  • Density of fluid at outlet
  • Speed of sound at downstream conditions
  • Valve outlet diameter
  • Internal downstream pipe diameter
  • Contraction coefficient for valve outlet or expander inlet

When valve style modifier is not an available explicit input data, additional parameters must be accounted

  • Area of a single flow passage 
  • Wetted perimeter of a single flow passage
  • Number of independent and identical flow passages in valve trim

When flow indicators are not available explicit input data, additional parameters must be accounted

  • Liquid pressure recovery factor of a valve without attached fittings
  • Upstream inside pipe diameter

Links to learn more about control valves noise calculation

What are the main fluids of which flow rate can me changed by a control valve ?

A control valve can change the flow rate of many fluids used in various industrial processes:

  • Acetylene
  • Air
  • Ammonia
  • Argon
  • Benzene
  • Isobutane
  • n-Butane
  • Isobutylene
  • Carbon dioxide
  • Carbon monoxide
  • Chlorine
  • Ethane
  • Ethylene
  • Fluorine
  • Freon 11 (trichloromonofluoromethane)
  • Freon 12 (dichlorodifluoromethane)
  • Freon 13 (chlorotrifluoromethane)
  • Freon 22 (chlorodifluoromethane)
  • Helium
  • n-Heptane
  • Hydrogen
  • Hydrogen chloride
  • Hydrogen fluoride
  • Methane
  • Methyl chloride
  • Natural gas
  • Neon
  • Nitric oxide
  • Nitrogen
  • Octane
  • Oxygen
  • Pentane
  • Propane
  • Propylene
  • Saturated steam
  • Sulphur dioxide
  • Superheated steam

What is the definition and what are the main noisy control valve types ?

A control valve is a power operated device which changes the fluid flow rate in a process control system. It consists of a valve (i.e. an assembly forming a pressure retaining envelope containing internal means) connected to an actuator that is capable of changing the position of a closure member in the valve in response to a signal from the controlling system.

Main control valve types are as follows:

  • Globe, single port
    • 3 V-port plug
    • 4 V-port plug
    • 6 V-port plug
    • Contoured plug (linear and equal percentage)
    • 60 equal diameter hole drilled cage
    • 120 equal diameter hole drilled cage
    • Characterized cage, 4-port
  • Globe, double port
    • Ported plug
    • Contoured plug
  • Globe, angle
    • Contoured plug (linear and equal percentage)
    • Characterized cage, 4-port
    • Venturi
  • Globe, small flow trim
    • V-notch
    • Flat seat (short travel)
  • Rotary
    • Eccentric spherical plug
    • Eccentric conical plug
  • Butterfly (centered shaft)
    • Swing-through (70°)
    • Swing-through (60°)
    • Fluted vane (70°)
  • Butterfly (eccentric shaft)
    • Offset seat (70°)
  • Ball
    • Full bore (70°)
    • Segmented ball

More Articles