The Thermal Oxidation
(TO) has been the first form of a VOC post combustion system
(at around 1960) and is therefore traditionally known as “Thermal
incinerator” or “Thermal afterburner”.
Design and function of the TO:
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 temperature
of approx. 50 – 150 °C onto the combustion chamber
temperature of approx. 750 °C. A thermal incinerator therefore
uses the energy coming from the combustion 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. 55 – 70 % up to the
combustion temperature.
The TO is made as a vessel-like combustion chamber. Preheating
is realized with an external or integrated air pre-heater.
Keeping the combustion temperature at 750 °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 by a controlled amount of
additional energy like gas or heating oil. Should by high
VOC concentration the combustion temperature exceeds the normal
level of 750°C the raw gas partly will automatically bypass
the pre-heater. This means that in total the raw gas is only
partly preheated resulting in a lower preheating effect which
again avoids overheating of the combustion chamber.
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 would go to atmosphere or, as it happens
in most cases, is used in a secondary heat exchanger to heat
up thermal oil, steam, hot water or hot air for process heat.
Doing this the economy of the TO is improved.
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 Thermal oxidizer (TO) this
happens at combustion chamber temperatures >750 °C
and a residence time in the process of approx. 1,0 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 TO:
Normally the air pre-heater of the TO is designed to achieve
a maximum of preheating of the incoming raw gas (exhaust air);
the rest of the total energy remains for the secondary heat
exchanger.
An advantage for the TO is to alternatively distribute the
quantities of the total heat for the primary and secondary
heat exchanger in such a way that at first instance in the
secondary heat exchanger a required amount of energy is recovered
(for process energy) and the primary heat exchanger is only
supplied by the remaining energy quantity for pre-heating
the exhaust air. In this configuration the TO becomes mainly
an energy generator which nearby oxidizes solvents. This design
due to lower preheating of the incoming exhaust air needs
higher gas consumption but there is a constant high output
of process heat.
