396
O. Petzoldt et al.
metallic mercury is adsorbed and then reacts with the sulphur to form mercuric
sulphide, a non-volatile water-insoluble compound, thereby becoming fixed.
Long-term tests conducted at Lurgi's research laboratories have delivered
important results in removal of metallic mercury. As an example, when treating
raw gas containing 1 to 5 mg/m3 of mercury, the concentration measured in the
clean gas remained lower than 0.01 to 0.03 mg/m3 even after 13 000 h of
operation.
4
Description of Process and Plant
With the Medisorbon process, chemisorptive removal of mercury from the flue
gas of incineration plants takes place at a temperature of about 60-85 °C. Higher
temperature entails the risk of sulphur being lost from the adsorbent, while lower
temperature results in slowing of the conversion reaction and moreover requires
additional energy to reheat the flue gases before they are released through the
stack.
In order to avoid increased retention of water by the adsorbent, relative
humidity of the gases must not exceed 85%. This also rules out condensation of
moisture in the adsorber. Optimum utilization of the theoretical adsorbent run
time requires that the dust burden be kept as small as possible. Although finest
dust particles mostly pass through the adsorbent bed, dust concentration in the
gas to be cleaned should not be more than 10 mg/m3 so as to reliably preclude
excessive pressure drop across the bed and, with it, the necessity to replace the
adsorbent prematurely.
The heart of a Medisorbon plant is the zeolite-packed adsorber. This is a
horizontal-flow vertical adsorber (see Fig. 2). A particular advantage of the
vertical type is the possibility of replacing only the first, possibly thin layer of the
bed in the event of a high dust burden causing premature increased pressure
drop, leaving the remaining layers in place. Furthermore, when using different
adsorbents, it is possible to keep them separate, which simplifies their
regeneration. Another obvious point is easier discharge of the separate layers
from a vertical adsorber, which can be accomplished by gravity through a
rotating-vane gate on a vertical adsorber. This type of adsorber is built as a very
compact unit and does not contain any valves or similar controls needing to be
actuated regularly in normal operation. The adsorbents employed for removal of
mercury from gases by the Medisorbon process are now exclusively modified
zeolites manufactured by Degussa.
The gas heater necessary for conditioning the gas is used together with fresh
air intake during startup for heating the adsorber to operating temperature
before admitting the moist gas as a means of preventing condensation in the
adsorber. Depending on whether the gas blower is installed upstream or
downstream of the Medisorbon system, auxiliary provision of a fresh air blower
may be required for startup, but it may be of much smaller capacity than the gas
O. Petzoldt et al.
metallic mercury is adsorbed and then reacts with the sulphur to form mercuric
sulphide, a non-volatile water-insoluble compound, thereby becoming fixed.
Long-term tests conducted at Lurgi's research laboratories have delivered
important results in removal of metallic mercury. As an example, when treating
raw gas containing 1 to 5 mg/m3 of mercury, the concentration measured in the
clean gas remained lower than 0.01 to 0.03 mg/m3 even after 13 000 h of
operation.
4
Description of Process and Plant
With the Medisorbon process, chemisorptive removal of mercury from the flue
gas of incineration plants takes place at a temperature of about 60-85 °C. Higher
temperature entails the risk of sulphur being lost from the adsorbent, while lower
temperature results in slowing of the conversion reaction and moreover requires
additional energy to reheat the flue gases before they are released through the
stack.
In order to avoid increased retention of water by the adsorbent, relative
humidity of the gases must not exceed 85%. This also rules out condensation of
moisture in the adsorber. Optimum utilization of the theoretical adsorbent run
time requires that the dust burden be kept as small as possible. Although finest
dust particles mostly pass through the adsorbent bed, dust concentration in the
gas to be cleaned should not be more than 10 mg/m3 so as to reliably preclude
excessive pressure drop across the bed and, with it, the necessity to replace the
adsorbent prematurely.
The heart of a Medisorbon plant is the zeolite-packed adsorber. This is a
horizontal-flow vertical adsorber (see Fig. 2). A particular advantage of the
vertical type is the possibility of replacing only the first, possibly thin layer of the
bed in the event of a high dust burden causing premature increased pressure
drop, leaving the remaining layers in place. Furthermore, when using different
adsorbents, it is possible to keep them separate, which simplifies their
regeneration. Another obvious point is easier discharge of the separate layers
from a vertical adsorber, which can be accomplished by gravity through a
rotating-vane gate on a vertical adsorber. This type of adsorber is built as a very
compact unit and does not contain any valves or similar controls needing to be
actuated regularly in normal operation. The adsorbents employed for removal of
mercury from gases by the Medisorbon process are now exclusively modified
zeolites manufactured by Degussa.
The gas heater necessary for conditioning the gas is used together with fresh
air intake during startup for heating the adsorber to operating temperature
before admitting the moist gas as a means of preventing condensation in the
adsorber. Depending on whether the gas blower is installed upstream or
downstream of the Medisorbon system, auxiliary provision of a fresh air blower
may be required for startup, but it may be of much smaller capacity than the gas