394
O. Petzoldt et aI.
Elemental mercury is retained practically not at all in wet scrubbers and only
to a small degree with flyash, meaning that a separate process step is necessary to
remove Hgo from flue gas.
Adsorption methods employing activated carbon are particularly well suited
for removal of mercury. The absorbent used may be impregnated with iodine,
silver, sulphuric acid or elemental sulphur (Henning et al. 1988)
Sulphur- and sulphuric-acid-impregnated activated carbon has been used in
fixed-bed adsorbers for several years with great success to clean natural gas
(Daza 1993). In contrast with natural gas, however, precleaned flue gases
generally contain a certain amount of S02 that is similarly adsorbed on the
carbon and is catalytically oxidized by the residual oxygen in the flue gas to S03'
which reacts with the moisture present in the gas to form sulphuric acid.
This sulphuric acid absorbs additional moisture until the activated carbon
finally becomes soaked its adsorptive capacity declines and corrosion problems
set in.
As a means of overcoming these problems, activated carbon is practically no
longer employed in fixed-bed adsorbers for cleaning flue gases. Instead, the
adsorbent is continuously exchanged, as in the moving-bed or turbulent-contact
method. These methods actually utilize the formation of sulphuric acid for
additional removal of mercury. The spent adsorbents can either be incinerated in
the same plant, if suitable facilities for reduction of pollutant levels are provided,
or they can be sent for external disposal together with the flyash from the
upstream dust collectors.
Another fundamental problem associated with use of carbonaceous adsorbents
for cleaning gases containing oxygen is the risk of fire. Heat generated in the
adsorber may not be completely removed, resulting in formation of hot spots
within the bed, which may lead to spontaneous ignition. Licensing authorities
therefore usually make the use of carbon-containing adsorbents contingent on
the provision of appropriate safety measures, which in Germany are summarized
in the LIS Report NO.97 issued by the North Rhine-Westphalian State Institute
for Source Emission Control (1991). Commonly, this covers carbon monoxide
monitoring, automatic fire extinguishing and inertization facilities.
Owing to these problems, efforts have long been directed at developing nonflammable adsorbents that preferably do not retain either S02 or moisture and
are catalytically inactive.
3
Process Development, Pilot Plants, Test Results
In the quest for a suitable alternative to flammable carbonaceous adsorbents,
Lurgi first undertook preliminary trials in 1990 to 1992 on a laboratory scale with
a wide variety of inorganic substances. These were subsequently continued in a
focused direction and in close cooperation with Degussa using a modified zeolite
specially developed by the latter.
O. Petzoldt et aI.
Elemental mercury is retained practically not at all in wet scrubbers and only
to a small degree with flyash, meaning that a separate process step is necessary to
remove Hgo from flue gas.
Adsorption methods employing activated carbon are particularly well suited
for removal of mercury. The absorbent used may be impregnated with iodine,
silver, sulphuric acid or elemental sulphur (Henning et al. 1988)
Sulphur- and sulphuric-acid-impregnated activated carbon has been used in
fixed-bed adsorbers for several years with great success to clean natural gas
(Daza 1993). In contrast with natural gas, however, precleaned flue gases
generally contain a certain amount of S02 that is similarly adsorbed on the
carbon and is catalytically oxidized by the residual oxygen in the flue gas to S03'
which reacts with the moisture present in the gas to form sulphuric acid.
This sulphuric acid absorbs additional moisture until the activated carbon
finally becomes soaked its adsorptive capacity declines and corrosion problems
set in.
As a means of overcoming these problems, activated carbon is practically no
longer employed in fixed-bed adsorbers for cleaning flue gases. Instead, the
adsorbent is continuously exchanged, as in the moving-bed or turbulent-contact
method. These methods actually utilize the formation of sulphuric acid for
additional removal of mercury. The spent adsorbents can either be incinerated in
the same plant, if suitable facilities for reduction of pollutant levels are provided,
or they can be sent for external disposal together with the flyash from the
upstream dust collectors.
Another fundamental problem associated with use of carbonaceous adsorbents
for cleaning gases containing oxygen is the risk of fire. Heat generated in the
adsorber may not be completely removed, resulting in formation of hot spots
within the bed, which may lead to spontaneous ignition. Licensing authorities
therefore usually make the use of carbon-containing adsorbents contingent on
the provision of appropriate safety measures, which in Germany are summarized
in the LIS Report NO.97 issued by the North Rhine-Westphalian State Institute
for Source Emission Control (1991). Commonly, this covers carbon monoxide
monitoring, automatic fire extinguishing and inertization facilities.
Owing to these problems, efforts have long been directed at developing nonflammable adsorbents that preferably do not retain either S02 or moisture and
are catalytically inactive.
3
Process Development, Pilot Plants, Test Results
In the quest for a suitable alternative to flammable carbonaceous adsorbents,
Lurgi first undertook preliminary trials in 1990 to 1992 on a laboratory scale with
a wide variety of inorganic substances. These were subsequently continued in a
focused direction and in close cooperation with Degussa using a modified zeolite
specially developed by the latter.