From the historical point of the development of incineration
systems the Catalytic Oxidation (CatOx) followed the classic
thermal oxidizer (TO). It was invented for exhaust air containing
only low concentrations of solvents (0,0 up to 5,0 g/Nm³)
which being cleaned in the thermal oxidizer (TO) would need
much more energy.
By inserting a catalyst into the burning chamber the oxidation
temperature compared to the recuperative Thermal Oxidizer
(TO, 750 °C) can be lowered to less than half (300-350
°C) which reduces the necessary amount of primary energy
for the burning system.
Function
of the catalyst:
The catalyst attracts the C-atoms of the hydrocarbons and
the O-atoms of the surrounding raw gas and unifies these atoms
in the shortest time. The atoms do not need to find each other
on the long way through the combustion chamber which represents
the dwell time of a thermal oxidizer.
This attraction effect shortens the oxidation process and
the lower oxidation temperature (lower than in the thermal
oxidizer) results in considerable energy saving.
To start the catalytic reaction the complete system has to
be pre-heated to the starting temperature of the catalyst
which is at approx. 320 - 330 °C (reaction starting temperature).
Attention:
Poisons for the catalyst:
Catalysts are uncritical when it comes to the oxidation of
singular hydrocarbons as coming from e.g. a flexo or rotogravure
printing process or coating process. Oppositely, the catalysts
react very sensibly to even small additions of lead, cadmium,
mercury, fluorine, chlorine, silicone, sulfur, micro dust,
carbon black and acids. The reactivity of the catalyst is
reduced quickly, sometimes within days: at this point the
catalyst is contaminated and cannot be used for the given
process. Before installing a catalytic system it has therefore
carefully to be checked if the solvents in the exhaust air
do not carry these unwanted additions or if suitable catalysts
or pre-filtering methods are available.
Lifetime
of the catalyst:
The normal lifetime is approx. 5-7 years. After this time
the complete catalyst or part of it need to be exchanged against
a new one.
It is also possible that due to certain contaminants the reactivity
of the catalyst may be reduced over the lifetime of the catalyst.
In this case the catalyst may be refreshed in its reactivity
by a heating process which is a kind of „cooking out“
of unwanted particles on the reactive surface to achieve a
nearly original reactivity.
Application
of the catalytic oxidation:
Before selecting a catalytic oxidizer it has to be checked carefully
if the advantages of the lower oxidation temperature of the
catalytic system definitely exist in comparison to the today
available regenerative oxidizer (RTO). This comparison should
be made over a lifetime of both systems of approx. 15 years.
Design
and function of the RekuKAT:
If the solvent laden exhaust air (raw gas) would directly
flow to the combustion chamber a very high energy amount would
be necessary to raise the relatively low raw gas (exhaust
air) temperature of approx. 50 – 150 °C onto the
oxidation temperature of approx. 300 - 350 °C. A catalytic
oxidizer therefore uses the energy coming from the oxidation
process to pre-heat the cold raw gas. This preheating is realized
by a recuperative heat exchanger. Recuperative heating means
that the heat flow of the hot gas from the combustion chamber
and the cold flow of the raw gas happen at the same time.
This process results in a preheating effect of approx. 70
% up to the combustion temperature.
The CatOx is made in a vessel-like combustion chamber.
Preheating is realized with an external air pre-heater.
Keeping the combustion temperature at 300 – 350 °C
at all possible variations of solvent concentration, raw
gas quantity and raw gas temperature is realized by the
heating energy of the incinerated solvents and feeding of
a controlled amount of natural gas or propane. Should by
high VOC concentration the oxidation temperatures exceed
the maximum of 450 - 500°C the incoming raw gas flow
will be diluted by sucking-in additional fresh air in front
of the catalyst to avoid overheating.
Secondary
heat recovery:
The recuperative air preheater allows for a max. heat exchange
of 70 %. The residual 30 % of the energy quantity from the combustion
chamber is coming out at a temperature of 150 – 200 °C
and would go to atmosphere or, as it happens in most cases,
is used in a secondary heat exchanger to heat up hot water or
hot air for process heat. Doing this the economy of the CatOx
system is improved.
Applicability
of the catalytic oxidation:
Highest air cleaning function
Low reaction temperature
Oxidation without additional formation of NOx
Optimum energy recovery
High efficient catalysts
Wide experience in selecting the right catalyst for industrial
applications
Air cleaning
function and prescribed clean gas data::
precondition for any thermal incineration is that the solvents
in question can be burned or oxidized. The cleaning of the solvent
laden air is a transversion of a) solvents consisting of carbon-hydoxides
(-CH-) and b) oxygen (O2) being in the surrounding raw gas (exhaust
air) to
c) CO2 and d) water (H2O). In a catalytic oxidizer (CatOx) this
happens at reaction temperatures of approx. 300 - 350 °C
and a very short residence time. This transversion performs
nearly completely but not at 100 %. Small quantities of residual
components of various hydrogen connections (total C) and connections
generated during the oxidation like CO and NOx in accordance
with the principle of best available technique are allowed but
limited by law.
In the European Community the „EU council
directive 1999/13/EC” (in Germany “TA-Luft”)
has fixed certain limits of residual combinations depending
on the kind of oxidation process: When using a catalytic oxidation
process (which is a non-thermal oxidation) the quantities
of components in the clean gas (measured in the stack) should
not exceed following values:
Total C : 50 mg/Nm³
CO : 50 mg/Nm³
NOx : 50 mg/Nm³
Application
and advantages of the CatOx:
At low oxigen content
At known raw gas composition
At absence of catalytic poisons and at low content of
fine dust
At low to medium exhaust air volumes (1.000 – 20.000
Nm³/h)
At fast start up requirements: A cooled down unit will
be operative after only approx. 30 – 60 minutes depending
on the size of the oxidizer. This makes the CatOx very suitable
for short time operations, e.g. for 1-shift operation or
as a standby plant which has to take over in case of emergency
if another air abatement system fails.
OXIDATION OF SOLVENTS WITH NITROGEN GROUPS: A very high
advantage for the catalytic oxidizer is given when solvents
with nitrogen groups have to be oxidized. In a normal thermal
oxidation process (750 – 800 °C) the nitrogen
groups (-N,-NO) would create NOx connections. Oppositely,
the catalytic oxidation with using special catalysts and
their low reaction temperature (300 – 350 °C)
oxidizes the hydrocarbon groups, isolates the N-atoms and
gives them out into the clean gas as being mainly N2. This
kind of oxidation process is only possible via the catalytic
method. – Typical solvents of this type are:
Anilin C5 H7 N Dimethylacetamid
DMAc C4 H9 NO
Di-ethylamin C4 H11 N Methyl-2-pyrrolidon
C5 H9 NO
Methylamin C H5 N n-Ethylpyrrolidon
C6 H11 NO
Pyrridin C5 H5 N
