The Regenerative thermal
oxidation (RTO) is the most modern design of thermal incineration.
The convincing argument for the RTO is its very low energy
consumption.
Design and function
of the RTO:
The difference to the classic TO lies mainly in the much
higher pre-heating of the raw gas to be incinerated. This
is realized by heating up of heat storage stones which in
turn are used to pre-heat the relatively cold raw gas (by
ceramic heat exchangers).
For the TO in the recuperative heat exchanger the raw gas
can be by max. 70 %, at the RTO heating up efficiencies
of 90 – 95 % are possible, in some cases even more
depending on the mass of the heat storage.
In most cases the RTO consists of 3 chambers, filled with
ceramic storage media. The hot gas coming from the burning
chamber (800 °C) flows from top to bottom through one
of the storage beds and heat the bed up („Regeneration
of the beds“). After switching the cycle direction
the stored energy is used to heat up the cold raw gas flowing
from bottom to top. This process works in change. The switching
of the air directing change (cycle) happens every 1,5 -
2 minutes.
Normally the RTO has 3 chambers, for bigger RTO’s
even more. At the same time one bed is used for pre-heating
of the cold raw gas, the second bed is taking energy from
the combustion and the thirds bed serves the cleaning of
the ceramic storage mass from residual amounts of non-combusted
solvents which are still resting in the storage mass. They
are blown separately into the combustion chamber for final
incineration.
Directing of the different air volumes is made by inlet
and outlet duct systems with pneumatically operated opening
and closing valves which are controlled via a PLC-programme.
Secondary
heat recovery:
For low solvent concentrations of 0 – 5 g/Nm³ the
RTO compared to the TO is in any case the less energy consuming
alternative. Already at concentrations of approx. 2 g/Nm³
the RTO runs in autotherm which means that the energy content
of the solvents contained in the exhaust air deliver enough
energy to keep the combustion temperature and the gas burner
is switched out. Depending of the solvent concentration a
higher and higher combustion temperature will be achieved
which is not a problem for the RTO because all fire contacted
areas are made of ceramic materials such as the inside insulation
and the ceramic honeycomb regenerator heat stones. Being in
the condition above the autothermal point the RTO can take
out a certain part of the surplus energy. This is a very hot
gas which leaves the RTO through the so-called “hot
bypass” to avoid overheating. Normally this energy is
not going straight to the stack. It is recovered in a secondary
heat exchanger to heat e.g. thermal oil, steam, hot water
or air.
Operating an RTO with extreme high solvent concentrations
becomes more economic as the RTO for its own process does
not need primary energy and at the same time recovers heat
for the production process so that at existing heat generators
(steam, thermal oil, fresh air heating) immediately energy
is saved.
Working in this way and seen over the lifetime of approx.
15 years the RTO has a positive balance.
Air
cleaning function and clean gas data:
A 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 Regenerative Thermal Oxidizer
(TO) this happens at combustion chamber temperatures of nearly
800 °C and a residence time in the process of approx.
0,8 sec. 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 thermal 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 thermal afterburning
process the quantities of components in the clean gas (measured
in the stack) should not exceed following values:
Total C : 20 mg/Nm³
CO : 100 mg/Nm³
NOx : 100 mg/Nm³
Application and advantages
of the RTO:
Comparing the RTO (regenerative thermal oxidizer) with the
TO (recuperative thermal oxidizer) the biggest advantage
for the RTO is the much lower gas consumption and the recovery
of heat after the RTO has passed its “autothermal
point“. By using much less energy the RTO has also
a lower CO2 output.
