industry and environment - DTIE

Chemicals management
Annual energy export is approximately 160 GWh
as district heating and 50 GWh as electricity.
The off-air plant
Even very low concentrations of volatiles will
often create unacceptable odours, causing inconvenience
with respect to the internal and (if the
odours are strong) external environment. To deal
effectively with local emissions of volatiles (e.g.
hydrocarbons from unloading activities, processes
in one of the drum-emptying plants), an “off-air
plant” was established in 1993 (Figure 5).
Eleven sections of the plant area were defined,
including about 50 spots where emissions would
normally occur during operations. The pipe and
control system from the off-air plant is widely
spread throughout the whole area. Ventilated
volatiles will effectively be sucked by the mediumpressure
fan located near the incinerator plants.
To maintain a constant gas flow, ventilated VOCs
are mixed with ambient air, entering the suction
part as the last input point. The fan distributes the
volatiles further to one of the three SCCs, where
high-temperature incineration takes place.
The closed system is designed for -200 to +400
mBar. It operates with a pressure of -80 to -100
mBar on the suction part and +50 to +150 mBar
Figure 5
Off-air plant
VOC - sources
after the fan. The galvanized pipe system is
designed to withstand a pressure of 10 bars.
Differences in tank volumes (and thereby pressure)
during filling and emptying periods are regulated
in such a way that the off-air plant will
ventilate the compressed tank gases in the ratio
25/20 mBar, and the inert supply system will add
nitrogen in the ratio 10/15 mBar.
To eliminate potential risk of flame propagation,
the ex-proof system is equipped with more than 50
static flame arresters, redundant monitoring
equipment, etc. The off-air plant is designed to a
minimum velocity of 17 metres/second and the
SCC nozzles ensure a minimum velocity of 55
metres/second.
Removal of bromine and iodine from
flue gases
When waste containing bromine or iodine is
incinerated, the flue gas will contain these substances,
which are brown and purple in colour.
There are no official EU flue gas limits for bromine
and iodine. However, due to their colour they cannot
be accepted in the flue gas. Kommunekemi has
therefore installed special bromine/iodine cleaning
columns in all three incineration lines. Bromine/
iodine cleaning works as follows:
-0.7/40 mbar
Br 2 + SO 3
2- + H 2 O → 2Br - + SO 4
2- + 2H + (1)
and
I 2 + SO 3
2- + H 2 O → 2I - + SO 4
2- + 2H + (2)
From the chemical reaction, it can be seen that
the reduction of bromine/iodine to bromide and
iodide causes lower pH and sodium hydroxide
must therefore be added to keep the pH at approximately
7.5.
Unfortunately the sulphite will also be consumed
by the remaining oxygen (approximately
10 %) in the flue gas:
O 2 + 2SO 3
2- → 2SO 4
2- (3)
This reaction does not cause any change in pH.
However, expensive raw material (sodium sulphite)
is consumed without any useful effect on the
process.
Exactly similar chemical reactions will also take
place in the SO 2 scrubber (i.e. when the concentration
of SO 2 in the flue gas is high enough, there
will be sufficient sulphite to reduce bromine/
iodine to bromide and iodide). It can therefore be
concluded that as long as the SO 2 load in the flue
gas is high, there is no need for the bromine/ iodine
stage. However, it is Kommunekemi’s experience
that the sulphur content in hazardous waste has
fallen during the last year, thereby creating a need
for a bromine/iodine stage.
To reduce the amount of “wasted” sulphite, as
described in chemical reaction (3) above, an automatic
analyzing device for bromine and iodine has
been installed in clean flue gas lines. When the bromide
concentration exceeds 40 mg/Nm 3 or iodine
exceeds 80 mg/Nm 3 (both levels clearly below the
visible levels), the bromine stage automatically
starts. Immediately afterward, the concentration
of bromine and iodine in the cleaned flue gas will
begin to decrease.
SCC SCC SCC
25/20 mbar
Carbon
filter
Off-air plant
10/15 mbar
Typical installation for a tank unit
N2
Notes
1. Directive 96/61. See http://europa.eu.int/
comm/environment/ippc/index.htm, and (for
Annex) http://eippcb.jrc.es/pages/Directive.htm
#annex
2. See http://eippcb.jrc.es.
3. Kommunekemi a/s was established in Nyborg,
Denmark, in 1971. It collects and treats hazardous
waste from industries and households. The company
produces and delivers almost 100% of the
district heat used in the city of Nyborg and 15-
20% of that city´s electricity consumption. ◆
UNEP Industry and Environment April – September 2004 ◆ 55