Method for purification of waste gases

A method which provides purification of waste gases by adding particles of reagent and/or absorbent, which react with the pollutants in the waste gases, to the gases, and by introducing the gases into a wetting reactor for activating the reagent or absorbent contained in the gases. Gases are conveyed to at least two levels in the reactor through inlets so that a first portion of the gases is introduced into a wetting zone and a second portion of the gases below the wetting zone. A high density of particles is maintained in the wetting zone by recycling to the wetting zone particles separated from the gas above the wetting zone.

The present invention relates to a method of purification of waste gases 
which are produced in, for example, combustion, gasification, or some 
chemical or metallurgical processes. Sulfur dioxides, ammonia, chlorine 
and fluorine compounds and condensing hydrocarbon compounds are typical 
pollutants contained in these gases. The present invention especially 
relates to a method in which reagent or absorbent which reacts with 
pollutants contained in the gases is activated by leading the gases into a 
wetting reactor. The reagent or absorbent is added to the process itself 
or to the gases discharged from the process. The reagent or absorbent 
particles which have reacted either completely or partially are separated 
from the gases. Carbonates, oxides or hydroxides of, e.g., either alkali 
metals or alkaline earth metals are used as reagents or absorbents. 
The present invention also relates to an apparatus for purification of 
waste gases. Especially, it relates to a wetting reactor which is provided 
with an inlet for waste gases and for reagent and/or absorbent, with spray 
means for water or steam, said spray means forming a wetting zone for 
activating the absorbent, with a filter disposed above the wetting zone in 
the upper section of the wetting reactor, for separating solid particles 
from the gases, and a gas outlet connected to the filter, and with an 
outlet or outlet duct for particles separated from the gases, disposed in 
the lower section of the wetting reactor. 
As known, combustion of fossile fuels produces flue gases which contain 
sulfur dioxide and cause environmental acidification. The sulfur content 
of the flue gases varies depending on the sulfur content of the fuel. 
Efforts are made to find means for employing fuels which contain more and 
more sulfur even though the restrictions on sulfur emissions become 
tighter and tighter. Waste incineration plants, the number of which is 
continuously increasing, also produce sulfur-containing flue gases which 
have to be purified so as to be within the set limits. The flue gases 
produced in waste incineration plants when, e.g., plastic compounds are 
burnt contain, besides SO.sub.2 and SO.sub.3 emissions, also hydrochloric 
and hydrofluoric acids and other harmful gaseous and solid compounds. 
Process gases produced in various gasification processes may also contain 
harmful amounts of sulfuric or other compounds which have to be separated 
from the gases prior to further treatment thereof. 
Several methods have been developed for cutting down sulfur emissions of 
combustion plants. The most common method used so far is wet scrubbing in 
which method the gases are scrubbed with a water suspension of a reagent, 
such as lime, reacting with, e.g., sulfur dioxides. The water suspension 
is sprayed into a gas flow in a scrubber arranged after a combustor, 
whereby sulfur is absorbed into the water suspension and sulfur dioxide 
reacts with lime, forming calcium sulphate or calcium sulphite 
EQU CaO+SO.sub.2 +1/20.sub.2 .fwdarw.CaSO.sub.4 
EQU or CaO+SO.sub.2 .fwdarw.CaSO.sub.3. 
Water suspension is sprayed in such an amount that sulfur compounds thus 
formed have not enough time to dry, but they are discharged as a slurry 
from the lower section of the scrubber. The wet scrubbing process is 
complicated as it requires means for preparing water suspension and means 
for after-treatment thereof. Furthermore, the method usually requires 
additional energy for drying the produced slurry in a slurry 
after-treatment plant. Therefore, the water suspension is usually fed into 
the system as dry as possible in order to minimize the energy requirement. 
Due to the considerable amount of water suspension used, the gas may be 
cooled to a relatively low temperature in the scrubber and, consequently, 
the gas discharged from the scrubber may cause corrosion and clogging of 
filters. Further, energy is consumed for reheating the flue gases prior to 
leading them out of the system. In the wet scrubbing system the separation 
degree of, for example, SO.sub.2, is about 95%. 
During the last few years, semi-dry scrubbing methods have been developed, 
in which a fine alkali suspension, e.g., calcium hydroxide suspension is 
sprayed through nozzles into a hot flue gas flow in a contact reactor 
where sulfur oxides dissolve in water and, when the suspension dries, are 
bound to the lime compound. Water is evaporated in the contact reactor so 
as to form a solid waste, whereby reaction products of, for example, 
sulfur and lime are readily separable from the gases by means of a filter. 
It is attempted to maintain the consistency of the calcium hydroxide 
suspension on such a level that the heat content of the flue gases is 
sufficient for evaporating the water therefrom. The thick lime suspension, 
however, easily deposits layers on the reactor walls and especially around 
the spray nozzles, and may finally clog the nozzles entirely. The reactors 
have to be dimensioned relatively large for minimizing the drawbacks 
caused by deposits. Furthermore, as separate equipment is required for the 
production of lime suspension, a considerable amount of equipment will be 
needed in the semi-dry scrubbing method as well, and the gas purification 
will be fairly expensive. A further drawback is the wearing effect of the 
lime suspension on the nozzles. 
The semi-dry scrubbing method is advantageous for the process because the 
pollutants in the gases may be removed as dry waste. The process has 
drawbacks of being difficult to control and providing a sulfur absorption 
below 90%, which is less than in wet scrubbing. A still further drawback 
is that inexpensive limestone cannot be used in the semi-dry method 
because it is very slow to react with sulfur. Either calcium oxide or 
calcium hydroxide, which are much more expensive, have to be used instead. 
In big combustion plants, the cost of absorbent is remarkable. 
Addition of limestone already into the actual combustion or gasification 
stage has also been suggested. As a result of such addition, limestone is 
calcined into calcium oxide in accordance with the following reaction 
EQU CaCO.sub.3 .fwdarw.CaO+CO.sub.2. 
Calcium oxide is then capable of reacting already in the combustor with the 
sulfur oxides formed therein. The reaction takes place as follows: 
EQU CaO+SO.sub.2 +1/2 O.sub.2 .fwdarw.CaSO.sub.4. 
When the reactions proceed, calcium sulphate or calcium sulphite layers, 
however, cover the surface of the calcium oxide particles preventing 
sulfur from penerating the particles, thereby slowing down and finally 
preventing the reactions between sulfur and lime. Thus, lime will not 
react completely and will not, therefore, be optimally utilized. Many 
other parameters, such as Ca/S mole ratio, temperature and retention time 
also affect sulfur absorption. 
The closer to the dew point the reactions take place, the higher the 
reactivity of alkali compounds becomes. Better reactivity is caused by the 
fact that, in a wetted particle, reactions take place in a water phase as 
fast ionic reactions. Close to the dew point, the particles stay wetted 
and the reactivity also remains on a desired level for a longer time. The 
moistness of the particles is preferably maintained on such a high level 
that water surrounds the particles, also penetrating them. As the water 
penetrates the lime particles, the sulphate or sulphite layer deposited on 
them will be broken, thereby revealing new reactive lime area. Sulfur 
dioxide contained in the gases dissolves in the water surrounding the 
particles and reacts with calcium compounds in the liquid phase. 
Finnish patent specification 78401 discloses a method in which sulfur 
dioxide of flue gases is caused to react in a reaction zone and to be 
thereby transformed into solid sulphates and sulphites separable from the 
flue gases. The flue gases are conducted into the lower section of a 
vertical, lengthy contact reactor. Additionally, powdered lime and water 
are separately brought into the reactor from several points for the sulfur 
to be absorbed by lime. Flue gas suspensions are discharged from the upper 
section of the flow-through reactor and are further conducted to a dust 
separation stage. By feeding the powdered lime and water separately into 
the reactor, production, treatment, and spraying of a water suspension are 
avoided. According to the specification, this method, when used in sulfur 
absorption with calcium oxide, results in about 80% SO.sub.2 reduction 
with a mole ratio of Ca/S=1.56 and about 90% SO.sub.2 reduction with a 
mole ratio of Ca/S=2.22. The 98% SO.sub.2 reduction is not achieved until 
the mole ratio is Ca/S=4. In this method either, the temperature of the 
flue gas flow must not be allowed to drop optimally close to the dew point 
as the solids contained in the flue gas suspension then would deposit 
layers on the walls of the tubes and other equipment, thus causing 
troubles in dust separation. 
European patent specification 0 104 335 discloses another two-phase, 
semi-dry flue gas purification system. In this method, dry reagent is fed 
into the flue gases in a contact reactor in a first stage and water or an 
aqueous solution, to which dissolved reagent has been added, in a second 
stage. In the first stage, an inactive surface layer is formed on the 
reagent particles. The layer slows down or prevents reactions between the 
reagent and, e.g., sulfur oxide. By adding water in the second stage, the 
reagent is reactivated. In this manner, the reagent is utilized more 
completely. The gas temperature is allowed to decrease to a level on which 
it always stays above the dew point, for example, to 105.degree. C. The 
gas temperature must not, in this method either, be allowed to decrease 
too close to the dew point because any wetted particles possibly formed 
would cause difficulties in the long run, even if the reactivity of the 
reagent at a lower temperature would be much better. According to the 
method, the required amount of reagent may be reduced by recycling 
reagent-containing solid material which has been separated from the gas at 
a later stage and then regenerated by either grinding or some other way. A 
drawback of this method is, however, separate equipment needed for 
handling and storing of the recycled solids. 
U.S. patent specification 4,509,049 suggests a dry gas purification system 
in which lime is added to flue gases in a boiler and the lime is then 
allowed to react with the flue gases in a reactor. The lime, which has 
partly reacted with the pollutants in the flue gases, is separated from 
the gases in a filter in the upper section of the reactor. The dry lime 
thus separated from the gases is accumulated in the base portion of the 
reactor or into a separate chamber where it is ground and treated with dry 
steam in order to increase the reactivity of the dry lime, whereafter the 
lime is recycled into the gas flow at a location prior to the reactor. The 
dry steam treatment of lime takes 2 to 24 hours, which is a long time 
involving high consumption of energy. 
An object of the present invention is to provide an improved method of 
purification of waste gases, such as sulfur, chlorine and fluorine 
compounds or other condensable compounds. 
Another object of the invention is to provide a method by which, e.g., 
sulfur reduction can be considerably improved, preferably even so that the 
amount of the reagent need not be increased. 
A further object of the invention is to provide a method by which a gas to 
be purified may be wetted very close to the dew point, for example, 
0.degree.-20.degree. C. therefrom, in a wetting reactor, the method still 
allowing the particles separated from the gases to be removed in a dry 
state in the wetting reactor. 
A still further object of the invention is to provide an improved apparatus 
in comparison with the prior art for purification of flue gases. 
Especially, an object of the invention is to provide an apparatus where the 
waste gases to be purified may be wetted very close to the dew point, the 
apparatus still allowing the particles to be separated from the gases to 
be discharged in dry condition. 
For achieving the objects described above, it is characteristic to the 
method according to the invention that 
the gases are introduced into the wetting reactor to at least two levels so 
that a first portion of the gases is fed into a wetting zone where the 
suspension produced of gas and reagent and/or absorbent is wetted with 
water and/or steam, and a second portion of the gases is fed into a second 
zone disposed below the wetting zone, 
in the wetting zone, a particle suspension is maintained the particle 
density of which is higher than the particle density of the gas fed into 
the wetting reactor, by recycling particles separated from the gas to the 
wetting zone and that 
the gases are discharged from the wetting reactor from above the wetting 
zone. 
The second portion of the gases preferably serves as a drying gas and is 
brought into contact with and to dry wetted particles flowing downwardly 
from the wetting zone. At least a part of the downwardly flowing particles 
is then carried away by the upwardly flowing drying gas and conveyed back 
upwards into the wetting zone in order to activate the still unreacted 
reagent or absorbent. In the upper section of the wetting reactor, the 
particles are separated from the gases by means of a filter and are then 
returned to the lower section of the reactor. In this way, an internal 
circulation of reagent or absorbent particles is brought about in the 
wetting reactor and a relatively high density of particles is maintained 
therein. 
The particles are separated from the gas in a fabric filter, electric 
filter or some other equivalent type of separator. Particles are detached 
from the filter either intermittently or continuously, e.g., by pulse 
flushing, backwash or shaking, whereby the particles drop either 
separately or in lumps downwards in the wetting reactor. 
At least a part of the particles stick to each other in the wetting zone or 
at the filter and form bigger agglomerates and pass thereafter downwards 
through the wetting zone all the way to the lower section of the reactor, 
whereas single small particles are easily carried away by the upwardly 
flowing gas and are conveyed from the wetting zone into the upper section 
of the reactor. Bigger lumps of particles and wet, heavy particles are 
dried and ground into finer particulates by the drying gas or by other 
mixing when they reach the lower section of the reactor. 
The drying gas is preferably introduced into the lower section of the 
reactor, first as downwardly directed sprays. The drying gas dries, grinds 
and causes whirling of the particles accumulated in the lower section of 
the reactor. Thorough mixing of the particles in the lower section of the 
reactor gives a positive effect, equalizing heat and moistness in the 
particle suspension. As the particles are ground smaller, their reactive 
area increases. After this, at least a part of the particles are carried 
away with the drying gases, passing again through the wetting zone, 
whereby the particles are activated and will again be capable of absorbing 
sulfur in the reaction zone. 
Mixing and recycling of the particles increases the retention time, dust 
density, Ca/S mole ratio and total surface area of the lime particles in 
the reaction zone, thereby decreasing the need for new reagent. According 
to the invention, an average particle density is maintained by internal 
circulation in the wetting reactor, which density is clearly higher than 
the particle density in the gas introduced into the reactor. The internal 
circulation can be controlled by regulating the amount and velocity of the 
gas introduced into the drying section. The location of the feeding point 
of the drying gas also has an effect on the recycling. The shorter the 
distance from which the gas spray is directed to the particle layer, the 
stronger the whirling effect of the spray. 
Part of the particles is preferably removed from the reactor through an 
outlet disposed in the lower section of the wetting reactor below the 
drying zone. Part of the discharged particles may be returned to the 
wetting reactor if desired. Thus, external circulation of particles may 
also be provided in connection with the wetting reactor. Particles may be 
treated outside the reactor, for example, to regenerate some reagent. The 
particle density may also be controlled in the reactor by regulating the 
amount of particles removed from the lower section of the reactor. 
External particle circulation in the wetting reactor may be provided by 
connecting a filter or an equivalent particle separator, which is either 
totally or partly disposed outside the reactor, to the upper section of 
the wetting reactor. In such a filter or particle separator, reacted and 
still unreacted absorbent particles are separated from the gases, at least 
part of which particles is directly returned to the lower section of the 
wetting reactor, preferably to the drying zone. Particles may be detached 
from the filter either continuously or intermittently and be returned to 
the lower section of the wetting reactor. Part of the material separated 
by means of the particle separator may be totally removed from the system. 
By the method according to the invention, it is possible to decrease the 
average temperature of the gases in the wetting reactor to a level which 
is about 0.degree.-20.degree. C., preferably 0.degree.-10.degree. C., from 
the dew point, and even to the actual dew point, and still to avoid the 
drawbacks caused by too wet particles in the upper or lower sections of 
the reactor. The particles wetted in the wetting zone and falling 
downwardly are dried by the drying gas in the drying zone, thereby not 
causing any trouble in the lower section of the reactor. Due to recycling, 
the differences in temperature and moistness are very small also above the 
wetting zone, at various cross-sectional points of the reactor. In this 
way, local troubles caused by wetted particles or water drops are avoided. 
The relative amounts of gas introduced into different zones of the wetting 
reactor may vary according to the temperature and composition of the 
gases. The ratio of the amount of gas introduced into the wetting zone to 
the amount of drying gas is about 10:1-1:5. Mostly, it is advantageous to 
introduce more gas into the wetting zone than into the drying zone, e.g., 
so that about 60% of the gas is fed into the wetting zone. A small part of 
the gas, preferably &lt;10%, may be fed to a zone above the wetting zone to 
make sure that the upwardly flowing gas suspension is dry enough when 
entering the filter. The absorbent cake or absorbent layer formed on the 
filter contains partly reactive absorbent, which is capable of absorbing a 
significant part of the sulfur contained in this added gas. 
In accordance with a preferred embodiment of the invention, layers formed 
by wetted particles on the walls of the wetting reactor may be avoided in 
such a manner that at least a part of the gas fed into the wetting zone is 
conducted into the wetting reactor as jacket flow so that the gases, 
either indirectly or directly, heat the reactor walls. The gas is 
conducted into the reactor through ducts disposed, e.g., in the walls, 
whereby the hot gas flowing in the ducts prevents the walls from cooling 
and thereby solids from depositing layers on the walls. The gases may also 
be injected directly to the inside of the reactor and caused to flow 
downwardly along the walls, protecting the walls. Thereby, the wetted 
particles are either directed away from the wall or they dry when passing 
through the jacket flow prior to touching the wall. The jacket flow is 
brought about by feeding gas, e.g., into a cylindrical reactor via an 
annular opening in its wall. 
Removing of deposits from the walls may also be intensified by shaking or 
by constructing the walls of flexible material, whereby pressure 
fluctuations normally occuring in the system will shake the walls, causing 
the deposits to fall down. 
Especially in big reactors, gas may also be introduced into the inner part 
of the wetting zone for providing a gas distribution as even as possible 
in the reactor. Gas may be fed, e.g., through a plurality of nozzles or 
slots disposed in the gas duct in the middle part of the reactor. Gas may 
also be fed into the wetting reactor from more than two levels. 
Gas may be introduced also into the drying zone as jacket flow or it may be 
introduced into the inner part of the drying section for ensuring even 
distribution of the gas. 
By sprays of water or steam, a wetting zone is provided in the upper or 
middle section of the wetting reactor. Water is preferably sprayed into 
the flue gases, mainly downwards from above the gas inlets. Sprays of 
water or steam are preferably so arranged that as much as possible of the 
gas flow is evenly covered. 
An apparatus for implementing the method of the invention is preferably a 
wetting reactor, which is characterized in that gas inlets are disposed in 
the wetting reactor at least at two different elevations. At least one 
inlet, e.g., an annular or rectangular opening following the outer wall of 
the reactor, is disposed in the wetting zone. The wetting zone may also be 
provided with one or more inlets into the inner part of the reactor so 
that gas will be distributed evenly over the entire crosssectional area of 
the reactor. At least one second inlet for the reactor is disposed below 
the wetting zone in the drying or mixing zone. This second inlet opening 
may be disposed in the reactor wall or inside the reactor, or in both of 
them. 
The wetting reactor may be either totally or partly doublewalled so that in 
the wall there is formed an inlet duct or inlet ducts for the gas to be 
fed into the reactor. 
The wetting zone of the wetting reactor is preferably provided with 
downwardly directed water or water vapor nozzles, disposed, for example, 
in the support members running horizontally through the wetting reactor. 
The filter disposed in the upper section of the wetting reactor is 
preferably a fabric filter such as a hose or cassette filter, or possibly 
an electric or some other equivalent type of filter, wherefrom particles 
are returned to the lower section of the reactor by shaking or backblowing 
the filter. PG,14 
The lower section of the reactor is preferably provided with a mechanical 
mixer, mixing solid material accumulated in the lower section of the 
reactor. Mixing of solid material intensifies the equalization of the 
moistness and heat of the particles, whereby the particles which are still 
wet will be dried when coming into contact with drier and hotter 
particles. At the same time, the mixer breaks the lumps of particles so as 
to facilitate them to be conveyed upwards in the reactor by the gas flow. 
Thus, the mixer intensifies the effect of the drying gas for bringing 
about internal circulation of particles in the reactor. The speed of the 
mixer is adjustable, and together with the gas flow entering the mixing 
area, a wide range of adjustment of particle circulation is thereby 
provided. 
The lower section of the wetting reactor is provided with means for 
discharging particles from the reactor. Particles are preferably 
discharged by the mixer described above. The blades of the mixer can be 
directed askew so that they gradually move particles to one end of the 
lower section of the reactor, wherefrom the particles can be removed dry 
through a suitable sealing means. They may also be removed by a separate 
discharge screw or a discharge conveyor. Particles are discharged from the 
wetting reactor preferably in such a dry state that they can be further 
conveyed, for example, pneumatically. 
If necessary, the lower section of the wetting reactor may be provided with 
a separate feeding point for reagent or absorbent. Several different 
reagents may be introduced into the wetting reactor for removing harmful 
substances from the gases in one stage. 
The arrangement according to the invention provides e.g. the following 
advantages over the earlier known arrangements: 
Several functions, such as sulfur absorption, wetting of reagent, particle 
separation and drying, may be concentrated in one apparatus. Wetting of 
gas may be arranged in the same space as the existing ash separation, 
whereby neither extra devices nor separate reactors are needed for each 
partial process. 
By the present invention, it is possible to operate very close to the dew 
point, even almost at the dew point, as the filter is directly arranged in 
the reactor, and no gas ducts are needed, whereby the problem of layers 
depositing on the walls of such gas ducts is avoided in conveyance of gas 
which becomes wet when close to the dew point. The possibility of 
operating close to the dew point results in a highly efficient elimination 
of SO.sub.2, SO.sub.3, HCl and HF emissions. 
Internal circulation of particles through the wetting zone cuts down the 
consumption of reagent or absorbent. By this method, the retention time of 
the absorbent in the reactor becomes essentially longer, preferably about 
2 to 10 times longer in comparison with earlier known oncethrough 
reactors. 
Fine ash is also separated from the gases in this apparatus. Ash and 
consumed absorbent may be recovered dry and in a common step. Only one ash 
removal system and ash treatment is needed. Dry ash and absorbent may be 
conveyed pneumatically. 
In the earlier known methods, only if the SO.sub.2 content of the inlet gas 
has been &lt;40 ppm, almost complete sulfur absorption has been provided in 
the wetting stage with SO.sub.2 containing gases. By the method of the 
invention, complete sulfur removal is possible even though the SO.sub.2 
content of the inlet gas is &gt;100 ppm. 
The method and the apparatus are simple. 
In the arrangement according to the invention, three main factors having an 
positive effect on absorbing reactions may be used simultaneously and 
optimally: 
cooling of gas to a temperature level which is close to the dew point in 
order to provide fast reactions; 
high Ca/S mole ratio in the reaction zone; and 
long retention time for optimal utilization of the absorbent.

FIG. 1 discloses a wetting reactor 10 provided with gas inlets 12 and 14, a 
gas outlet duct 16 and a discharge duct 18 for particles separated from 
the gas. The wetting reactor is also provided with nozzles 20 for spraying 
water or steam into the wetting reactor above the gas inlets. The upper 
section of the reactor is provided with a filter 22 for separating 
particles from the upwardly flowing gas. 
The wetting reactor according to the invention may be disposed in the flue 
gas duct after the combustion chamber of a grate furnace, pulverized fuel 
combustor or fluidized bed combustor, such as a circulating fluidized bed 
reactor, whereby the wetting reactor is preferably disposed after the heat 
recovery boiler. Prior to entering the wetting reactor, the flue gases are 
cooled to &lt;300.degree. C., preferably to &lt;150.degree. C. For removing 
sulfur oxides from the flue gases, absorbent, such as limestone, has been 
fed into the combustion chamber or fluidized bed reactor or thereafter. 
The absorbent is at least partly calcined in hot flue gas to calcium 
oxide, which absorbs sulfur as calcium sulphate and calcium sulphite. The 
lime/sulfur ratio of 1.5-2.1 produces about 80 to 95% sulfur reduction in 
a circulating fluidized bed reactor. The flue gases still contain sulfur 
as well as unreacted lime when entering the wetting reactor. An important 
object of the wetting reactor according to the invention is to activate 
lime or other absorbent in the flue gases so that the rest of the sulfur 
will also be removable from the flue gases. 
In the arrangement shown in FIG. 1, flue gases containing sulfur and lime 
are conveyed through pipe 24 into the wetting reactor. Prior to feeding 
the flue gases into the reactor, they are divided into two separate flue 
gas flows in ducts 26 and 28. The flue gas flow in duct 26 is conducted 
into the reactor,- substantially to the same level as the water sprays 20. 
The flue gas flow in duct 28 is conducted to a substantially lower level. 
The main flue gas flow is conducted into the wetting reactor substantially 
to the same level as the water sprays, either above or below or to exactly 
the same level as the water sprays. It is essential that the gas fed into 
the reactor is well mixed with the water spray. Both the gas and the water 
are preferably fed into the reactor as a downwardly flowing spray, which, 
at a small distance from the inlet, turns upwards. In this manner, 
vortices of gas and water spray are provided in the wetting zone and 
thereby also a good mixing effect. 
The water sprays constitute a wetting zone 30 in the wetting reactor. In 
this wetting zone, the flue gases are wetted and cooled as close to the 
dew point thereof as possible, preferably to about 0.degree.-3.degree. C. 
therefrom. In the wetting zone, the lime particles are wetted, whereby 
sulfur is absorbed by the particles and fast ionic reactions between 
sulfur and calcium can take place in the liquid phase. 
Water is preferably sprayed from nozzles, which produce small drops, 
preferably &lt;100 .mu.m in size, and which are large-angled so that the 
reactor cross-section and the gas flow are well covered. Water is sprayed 
downwardly. The wetting zone covers the vertical zone of the reactor which 
preferably equals the hydraulic diameter of the reactor. 
In the embodiment shown in FIG. 1, flue gas is introduced into the reactor 
as jacket flow. From duct 26 the gas is first conveyed into a tubular duct 
32 surrounding the reactor. From the tubular duct, the gases are further 
conveyed into one or more downwardly directed ducts 36 defined by the 
reactor wall 34. The reactor is doublewalled so as to form an inlet duct 
36 for flue gas between the walls 34 and 38. From ducts 36, the flue gases 
are conveyed through inlets 12 into the wetting zone 30 in the reactor. 
Correspondingly, gas is conducted from the lower gas duct 28 to a tubular 
duct 42 surrounding the reactor and therefrom further to a downwardly 
directed duct 46 defined by the reactor walls 44. From that duct 46, the 
flue gases flow into the lower section i.e. the drying or mixing zone 40, 
of the reactor. 
Introduction of gas into the wetting reactor is controllable, e.g., by 
means of dampers 27 and 29 in ducts 26 and 28. Introduction of gas is also 
controllable by means of an adjustable slot 48 in the duct 46. 
Drying gas can be fed into the drying section to such an extent that the 
particles accumulated in the lower section of the reactor stay mainly dry. 
The temperature in the lower section of the reactor is then maintained 
above the dew point for providing efficient drying. The gases flow from 
the drying zone upwards, thereby drying particles flowing downwardly from 
the filter and the wetting zone. The flow of drying gas is automatically 
adjustable by members 47 and 49, in accordance with the temperature of the 
gas in the lower section of the reactor or the temperature of the 
particles to be discharged. 
Further, the lower section of the reactor is equipped with mechanical 
mixers 50. The embodiment shown in FIG. 1 has two such mixers lying on the 
bottom of the reactor and being provided with blades 52. The mixers break 
the lumps of particles falling down to the lower section of the reactor. 
At the same time, they equalize the temperature and moistness between the 
particles. The mixer blades are preferably so disposed that they, when 
rotating, move particles to one end of the lower section of the reactor, 
said end being provided with a discharge duct 18 for particles. The 
particles preferably flow over an over-flow plate, not disclosed, into the 
discharge duct. In this manner, a "buffer" of particles, which equalizes 
the temperature and moistness of the down-flowing particles, is always 
maintained in the reactor. 
FIG. 2 shows a wetting reactor 10 similar to that of FIG. 1, except that 
gas is introduced into the drying section via a gas inlet duct 54 disposed 
inside the reactor. The gas inlet duct is provided with downwardly 
directed nozzles 56, through which the gas first flows towards the 
particles accumulated in the lower section of the reactor and thereafter 
upwards. In this way, mixing is provided among the particles accumulated 
in the lower section of the reactor. 
In the reactor according to FIG. 2, the amount of water fed into the 
wetting zone is regulated by a member 21 according to the temperature of 
the gas in the upper section of the reactor. The wetting reactor may be 
provided with water nozzles on several different levels if required for 
the gas to be evenly wetted. 
In FIGS. 1 and 2, the reactors are made up of hose filter chambers, each of 
which has a standard filter and, in the lower section of the chamber, a 
wetting zone and a drying zone. 
FIG. 3 illustrates a reactor in which a filter 60 is disposed immediately 
outside the reactor chamber. Thus, in addition to internal circulation, 
also external circulation of particles is effected in the reactor. Some of 
the particles wetted in the wetting zone 30 separate from the gases by 
themselves and flow, because of their weight, down to the drying section, 
where they become under the influence of the drying gas. After drying, the 
particles again flow upwards, entrained with the gases, thereby 
constituting internal circulation. Part of the wetted particles follow the 
gases to the upper section of the reactor and to the filter 60 and will be 
returned via duct 62 to the drying section 40. If necessary, particles may 
be removed from the circulation by outlet means 64, which may be closed by 
a valve 66. Particles may also be wetted outside the wetting reactor. 
In FIG. 3, the flue gas inlet ducts 26 and 28 may be connected to different 
points of the combustion processes, for example so that, the gas brought 
into the reactor via duct 26 has been more cooled than the gas brought via 
duct 28, which duct may bring hotter gas for ensuring the drying process. 
Compared with the prior art, the invention provides much better sulfur 
absorption of flue gases with much lower lime consumption, as indicated by 
the accompanying results of tests made on certain coal and limestone 
grades. 
EXAMPLE 
Means in accordance with FIG. 1 was used in the test run. The wetting 
reactor was supplied with flue gases of about 870.degree. C. from a 
circulating fluidized bed reactor, which had been supplied with limestone 
the mole ratio Ca/S being 1.41-2.33. The theoretic SO.sub.2 content of the 
flue gases was 860 to 960 ppm. The sulfur contained in the flue gases 
reacted already in the circulating fluidized bed reactor prior to the 
wetting reactor in such a manner that the SO.sub.2 content of the flue 
gases entering the wetting reactor was about 60 to 201 ppm. The gases were 
conducted into the wetting reactor at a temperature of about 130.degree. 
to 160.degree. C. The theoretic dew point of the gases in the wetting 
reactor was about 54.degree. C. 
The test results are shown by the table below. 
______________________________________ 
Temp. SO.sub.2 
SO.sub.2 
after before after SO.sub.2 
Ca/S reactor reactor reactor 
abs. 
mol/mol .degree.C. 
ppm ppm % 
______________________________________ 
1.88 55 201 27 97 
1.91 55 111 2 100 
1.95 55 107 0 100 
1.94 57 105 0 100 
2.33 57 129 2 100 
1.93 59 60 0 100 
1.41 61 183 83 91 
1.87 63 121 25 97 
2.00 66 136 61 93 
2.08 81 77 53 95 
______________________________________ 
The test results clearly indicate that, by the method according to the 
invention, sulfur absorption is almost complete even with very low Ca/S 
mole ratios when the final reactions take place nearly at the dew point, 
i.e. 1.degree.-5.degree. C. from the dew point. Very good results are 
achieved even with the highest temperatures, i.e. 10.degree.-30.degree. C. 
from the dew point, and with much lower lime consumption than in earlier 
known methods. 
According to information in literature, the wetting reactors of prior art 
have given about 90% SO.sub.2 reduction with a mole ratio of Ca/S=2.22. 
About 98% SO.sub.2 reduction has not been achieved until the mole ratio 
has been Ca/S=4. 
FIG. 4 shows the ratio of SO.sub.2 reduction to Ca/S mole ratio received in 
the above described series of test runs when applying the method according 
to the invention. As a comparison, the figure also shows the ratio of 
SO.sub.2 reduction to the Ca/S mole ratio when the test run is performed 
without the wetting reactor. 
As a conclusion, the present invention enables combining of various stages 
of several different processes into a whole: 
A wetting reactor, made up of the space below the filter cassettes or the 
like. A nozzle system disposed in this space sprays water for wetting the 
ash and absorbent particles and for dropping the flue gas temperature 
close to the dew point, i.e. 0.degree.-20.degree. C. therefrom. 
A fabric filter or the like, which operates either on the ordinary 
counterflow cleaning principle, with pressure pulses, backwash or shaking. 
Combined mixing and transfer means for ash and absorbent, disposed, for 
example, in the receiving hopper at the bottom of the reactor. The mixing 
means preferably rotates at such a high velocity that it breaks the 
deposits which, when wet, fall down from the walls and filter, and which 
are dried by the hot gas flow. 
Circulation of ash and absorbent, which is brought about by blowing part of 
the incoming flue gas into the reactor via the lower section thereof. Gas 
may also be blown into the reactor from below the mixers in such a manner 
that the gas fluidizes the particle mass accumulated in the lower section 
of the reactor. The gas introduced into the reactor from the lower section 
thereof together with the main gas flow coming from the side walls dries 
the wet lumps of particles falling down from the upper section of the 
wetting reactor. The gases catch part of the particles back into the 
wetting zone, thereby resulting in an internal circulation of particles in 
the wetting reactor.