Method and arrangement for purifying a carbon-containing adsorption medium

In a method for purifying a carbon-containing adsorption medium used for treating flue gases within a flue gas scrubbing process the adsorption medium is introduced into an adsorption medium reactor where the adsorption medium adsorbs pollutants from the flue gas. The adsorption medium laden with pollutants is then removed from the adsorption medium reactor and conveyed into a thermal regeneration apparatus. At temperatures of 350.degree. to 650.degree. C. the pollutants are desorbed in the thermal regeneration apparatus, resulting in a desorption gas. In the desorbing step, pollutants in the form of heavy metals are vaporized and pollutants other than heavy metals are released from the adsorption medium and optionally decomposed. The desorption gas is then guided into an adsorption apparatus where the heavy metals are removed from the desorption gas by adsorption. The desorption gas substantially free of heavy metals is subjected to further treatment, preferably is reintroduced into the flue gas scrubbing process upstream of a flue gas scrubbing apparatus.

BACKGROUND OF THE INVENTION 
The invention relates to a method for purifying a carbon-containing 
absorption medium that has been spent in a flue gas scrubbing process and 
which has been removed from a flue gas treatment reactor wherein the 
removed adsorption medium is subjected to a desorption process in a 
separate thermal regeneration/desorption apparatus for separating and 
decomposing pollutants and wherein the pollutant-rich desorption gas is 
subjected to a further treatment. Furthermore, the invention relates to an 
arrangement for performing this method. 
With the increasing number of flue gas scrubbing devices, with respect to 
number and capacity the amount of adsorption medium used within such 
scrubbing devices has also steadily increased, these adsorption media 
being partially heavily loaded with pollutants. A combustion of the 
carbon-containing adsorption medium is not feasible or feasible only with 
a considerable expenditure whereby it has not been possible to date to 
dispose of especially heavy metals and dioxin in a suitable manner. 
From U.S. Pat. No. 4,500,501 a method is known in which the separation of 
SO.sub.2 and NO.sub.x by catalytic adsorption is performed in two serially 
connected moving bed reactors. The carbon-containing adsorption medium 
removed from the second moving bed reactor in line is regenerated in a 
regenerator arranged downstream, partially replaced and turned into the 
first flue gas treatment reactor. With this known process it is possible 
to reactivate the adsorption medium; on the other hand, the contents of 
toxic substances, especially dioxin, furane, and heavy metals, especially 
mercury, within the recycled adsorption medium can increase. 
Under the given circumstances the pollutant-rich adsorption medium spent 
during flue gas scrubbing often must be disposed of at special hazardous 
waste depositories at a respective cost expenditure. 
From German published document 3 426 059 a desorption method for 
carbon-containing adsorption media is known in which temperatures of more 
than 1000.degree. C. are deemed necessary. In the described method there 
is further the danger of introducing oxygen into the desorption apparatus. 
It is therefore an object of the invention to dispose of heavily polluted 
carbon-containing adsorption media spend during flue gas scrubbing at 
comparatively low costs and in an environmentally safe manner. 
SUMMARY OF THE INVENTION 
According to the inventive method this object is solved by: subjecting the 
removed adsorption medium in a separate thermal regeneration apparatus to 
a desorption process whereby the treatment temperature is selected such 
that pollutants are separated from the adsorption medium, i.e., especially 
dioxins are decomposed, adsorbed SO.sub.2, HCl, HF, etc. are released, and 
mercury is vaporized; by guiding the pollutant-rich desorption gas into an 
adsorption apparatus and treating it by removing mercury from it; by 
guiding the essentially mercury-free desorption gas of the adsorption 
apparatus to further treatment, especially returning it into the flue gas 
scrubbing process. 
The arrangement for performing this method is inventively characterized by 
providing a thermal regeneration apparatus downstream of the flue gas 
treatment reactor, the thermal regeneration apparatus provided with means 
for heating the adsorption medium, a removal unit at the bottom for 
removing the treated adsorption medium and a gas and steam removing device 
for removing the pollutant-rich adsorption gas, the gas and steam removal 
device connected to an adsorption apparatus which is designed to remove 
mercury from the desorption gas. 
With the invention the adsorption medium loaded to a greater or lesser 
extent with pollutants is first substantially freed from pollutants in a 
desorption process performed by supplying heat. The thus treated 
adsorption medium can then be burned without further substantial 
expenditure or regenerated for reuse. Residues of the previously cleaned 
adsorption medium, after combustion, can be disposed of without problems 
and correspond to only small amounts. 
The treatment temperature and the treatment time depend on the amount of 
pollutants and the desired efficiency of the desorption process. The 
treatment temperature in a preferred embodiment is within a temperature 
range of 350.degree. to 650.degree. C., preferably within a range between 
450.degree. and 550.degree. C. 
For comparatively inexpensive carbon-containing adsorption media, for 
example, for active brown coal, the expenditure of adsorption medium 
regeneration is in general not cost effective. Accordingly, such an 
adsorption medium is burned after desorption. 
When using stone coal pellets as adsorption media it can however be 
expedient to return the spent adsorption medium into the flue gas 
treatment process after treatment in a thermal regeneration apparatus 
(desorption apparatus) and, optionally after replacement of losses, reuse 
it for the flue gas scrubbing process. 
In a preferred embodiment of the present invention, the adsorption medium 
is introduced into an adsorption medium reactor where the adsorption 
medium adsorbs pollutants from the flue gas. The adsorption medium laden 
with pollutants is then removed from the adsorption medium reactor and 
conveyed into a thermal regeneration apparatus. At temperatures of 
350.degree. to 650.degree. C. the pollutants in the thermal regeneration 
apparatus are desorbed, resulting in a desorption gas, whereby pollutants 
in the form of heavy metals are vaporized and pollutants other than heavy 
metals are released from the adsorption medium and optionally decomposed. 
The desorption gas is subsequently guided into an adsorption apparatus 
where the heavy metals are removed from the desorption gas by adsorption. 
The desorption gas substantially free of heavy metals is then subjected to 
further treatment. Advantageously, the desorption gas is recycled into the 
flue gas scrubbing process. 
The inventively selected temperature range is sufficient to decompose 
dioxins and furanes, to vaporize low-boiling metals and to release 
adsorbed SO.sub.2, HCl, HF etc. Furthermore, by indirectly heating the 
thermal regeneration apparatus, the introduction of oxygen into the 
desorption apparatus (thermal regeneration apparatus) is prevented. 
Preferably, the adsorption medium is burned after desorption. 
In the alternative, the adsorption medium is returned via a conveying 
device into the adsorption medium reactor after the desorption step. 
The amount of lost adsorption medium can be replaced within the conveying 
device or directly within the adsorption medium reactor. 
The inventive device for purifying a carbon-containing adsorption medium 
used for treating a flue gas stream within a flue gas scrubbing process 
comprises an adsorption medium reactor receiving an adsorption medium for 
adsorbing pollutants from the flue gas stream. The adsorption medium 
reactor has a removal device for removing the adsorption medium laden with 
pollutants from it. A thermal regeneration apparatus is connected to the 
removal device of the adsorption medium reactor; it serves to desorb at 
temperatures of 350.degree. to 650.degree. C. the pollutants from the 
adsorption medium in the form of a desorption gas, whereby the pollutants 
in the form of heavy metals are vaporized and pollutants other than heavy 
metals are released from the adsorption medium and optionally decomposed. 
The thermal regeneration apparatus comprises a heating means for 
indirectly heating (without directly supplying a heating gas) the thermal 
regeneration apparatus, a removal unit for removing the adsorption medium 
from the thermal regeneration apparatus, and a gas and steam removing 
device for removing the desorption gas form the thermal regeneration 
apparatus. The inventive device further comprises an adsorption apparatus 
having an inlet and an outlet. The inlet is connected to the gas and steam 
removing device for removing by adsorption mercury from the desorption 
gas. Subsequently, the desorption gas substantially free of mercury is 
subjected to further treatment. 
The device advantageously further has a return line connected to the outlet 
of the adsorption apparatus for returning the desorption gas into the flue 
gas stream upstream of the adsorption medium reactor. 
Preferably, the inventive device comprises a combustion device connected to 
said removal unit of said thermal regeneration apparatus. 
The inventive device may further comprise a conveying device connected to 
the removal unit of the thermal regeneration apparatus. The removal unit 
is preferably located at a bottom portion of said thermal regeneration 
apparatus.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The adsorption medium reactor 1 (flue gas treatment reactor), schematically 
represented in the drawing of the described example, serves to separate 
sulfur oxides (SO.sub.2 /SO.sub.3), hydrogen chloride (HCl), hydrogen 
fluoride (HF), chloro-organic components, especially dioxins and furanes, 
and heavy metals from the flue gases. It is positioned at a suitable 
location, in general behind the flue gas scrubbing apparatus 20, within 
the flue gas line 2. The illustrated adsorption medium reactor 1 
represents a number of adsorption and/or reduction stages of any desired 
number which can be serially and/or parallel connected within the flue gas 
stream. In the shown embodiment the adsorption medium reactor is a 
so-called transverse flow reactor in which the flue gas stream flows in 
the direction of arrows 3 transverse through a catalytically active 
carbon-containing adsorption medium bed 4. The pellet-shaped, lumpy, or 
granular adsorption medium is filled and distributed as uniformly as 
possible over the cross-section of the reactor via the feeding device 5 in 
order to form the adsorption medium bed 4. The adsorption medium travels 
vertically from the top to the bottom. Spent adsorption medium is removed 
via a removal device 7 comprised of a plurality of removing pipes 7 at the 
bottom of the reactor 1. The removing pipes are closeable by suitable 
means so that the removal of spent adsorption medium can be performed 
continuously or discontinuously, i.e., in batches. 
Spent adsorption medium is guided from the reactor 1 via a conveying device 
8, represented as a simple conduit, into a thermal regeneration apparatus 
10. In the thermal regeneration apparatus 10 the pollutant-laden 
adsorption medium is heat-treated for a suitable treatment period at such 
a temperature that a desorption process occurs in which the pollutants are 
separated from the adsorption medium, especially such that dioxins are 
decomposed, gaseous components such as SO.sub.2, HCl, HF, etc. are 
released and low boiling heavy metal, especially mercury, is vaporized. 
The heat supplied to the thermal regeneration apparatus, in the following 
also called desorber, is supplied via a heat circuit 11, the heat energy 
of which in general is derived from a combustion unit of the device, i.e., 
the exhaust gas of the combustion unit is not directly used for heating 
the adsorption medium. 
The desorber has a removal unit 12 at the bottom via which the essentially 
pollutant-free adsorption medium is removed for further treatment, 
respectively, combustion, and further has a gas and steam removing device 
13 for removing the pollutant-rich desorption gas. The removing device 13 
is connected to an adsorption apparatus 14 which primarily serves to 
remove mercury vapors from the desorption gas. Only after this treatment, 
the pollutant-rich but mercury-free desorption gas can be recycled via a 
line 15 into the flue gas stream before the flue gas scrubbing apparatus 
20. The mercury removal within the adsorption apparatus 14 has the 
advantage that the desorption gas free of mercury can not additionally 
enrich the adsorption medium within the reactor 1 with mercury. 
In general, the adsorption medium present at the removal unit 12 of the 
desorber 10 and freed of pollutants is burned within a combustion device, 
not represented in the drawing. With high-quality stone coal containing 
adsorption media, for example, in the form of stone coal pellets, at least 
a portion of the adsorption medium amount treated within the desorber 10, 
optionally after screening (16) and after further regeneration and 
replacement of losses, can be returned into the flue gas treatment 
circuit. For returning the thermally regenerated carbon-containing 
adsorption medium a conveying device 17 is provided which returns the 
granular or pellet-type adsorption media particles from the top via the 
feeding device 5 into the reactor 1. 
In principle, the invention can be used with the same advantages with all 
adsorption and reduction stages which operate with carbon-containing 
adsorption media. The exact embodiment of the conveying devices 8 and 17 
as well as of the feeding and removing devices determining the adsorption 
medium throughput are not represented in the drawing and are of no 
decisive consequence for the invention. It is however important that 
temperature and treatment time within the desorber 10 are adjusted such 
that the desorption process is reliably performed and the load of 
pollutants in the adsorption medium is almost entirely removed. 
The present invention is, of course, in no way restricted to the specific 
disclosure of the specification and drawings, but also encompasses any 
modifications within the scope of the appended claims.