Process of cleaning flue gases from heating plants

By a process of cleaning flue gases, both solid particles and nitrogen oxides and sulphur dioxide are removed from the flue gas prior to the discharging into the atmospheric air. In order to ensure that almost all the content of sulphur dioxide and nitrogen oxides is removed from the flue gas, said flue gas is initially carried through a shower of aqueous liquid and subsequently through a heat exchanger, in which its temperature is lowered to about 20.degree.-30.degree. C., and then through a film of alkaline liquid, and finally through a second shower of aqueous liquid to pass a sulphuric acid neutralizing liquid in a container before the discharging into the atmospheric air. As a result a simultaneous possibility of utilizing the heat of the flue gas is ensured by means of the heat exchanger. A cleaning plant for carrying out the process comprises an apparatus (2), through which the flue gas cleaned of solid particles is carried. When seen in the flow direction, this apparatus comprises a first flue scrubber (8), in which the flue gas is subjected to a shower of liquid, a two-step heat exchanger (9, 10) permitting a reduction of the temperature of the flue gas to 20.degree.-30.degree. C. After the two-step heat exchanger (9, 10), the apparatus (2) comprises a rotating, conical disc (11) providing a film of milk of lime, through which the flue gas is forced to pass during the continuous flow through the apparatus. After this film of milk of lime, the flue gas passes a shower of liquid emitted from a second flue scrubber (14), and upon said scrubber the flue gas is forced to pass a limy liquid in a container (15).

TECHNICAL FIELD 
The invention relates to a process of cleaning flue gases from heating 
plants such as oil burners, whereby it is substantially ensured that solid 
particles such as fly ash are initially removed from the flue gas, 
whereafter nitrogen oxides and sulphur dioxide are removed from the flue 
gas prior to the discharging to the atmospheric air, optionally through a 
neutralizing device. The invention furthermore relates to a plant for 
carrying out the process. 
BACKGROUND ART 
It is known to clean flue gases both of solid particles and of nitrogen 
oxides and sulphur dioxide, and it is known to utilize the heat of the 
flue gas. The previously known plants ensure a reduction of about 90% of 
the amount of sulphur dioxide in the flue gas. The remaining amount of 
sulphur dioxide assures that the flue gas discharged into the atmospheric 
air is particularly aggressive, said flue gas usually being discharged at 
a temperature of about 60.degree.-70.degree. C. 
DISCLOSURE OF INVENTION 
The object of the invention is to provide a process ensuring a reduction of 
the amount of sulphur dioxide of 98-100% at the same time as the flue gas 
is cooled almost to room temperature. 
The process according to the invention is characterised in that initially 
the flue gas cleaned of solid particles is carried through an optionally 
intermittent, first shower of aqueous liquid, subsequently through a heat 
exchanger, in which the temperature of the flue gas is reduced to about 
20.degree.-30.degree. C., optionally through a film of an alkaline liquid 
such as milk of lime or chloride of lime, through a second shower of 
aqueous liquid, and finally through a sulphuric acid neutralizing liquid 
in a container prior to the exhaust to the atmospheric air or an 
additional neutralizing device. 
As a result, the discharged flue is cleaned to such an extent that it is 
comparable with the atmospheric air as to the content of combustion gases. 
The content of sulphur dioxide and sulphuric acid is almost completely 
removed, i.e. only from 0 to 2% thereof remain. Only a small amount of 
nitrogen and carbon dioxide remain, and the result of the cleaning is that 
the expenses involved in building large chimneys are completely avoided. 
At the same time the heat of the flue gas can be completely utilized as 
low as to 20.degree.-30.degree. C. for heating purposes since a 
temperature of at least about 150.degree. C. to 160.degree. C. in the flue 
gas is no longer required during the passage of said flue gas through a 
chimney. This high temperature was necessary in connection with the 
previous plants in order to ensure that the content of sulphuric acid of 
the flue gas did not turn aggressive within the chimney. This effect is 
due to a co-operation of the various liquids and the great reduction of 
the temperature which the flue gas is subjected to. A reduction of the 
temperature of the partly cleaned flue gas after the first treatment with 
a shower of aqueous liquid involves an intensely accelerating effect 
concerning the conversion of SO.sub.2 --SO.sub.3 into H.sub.2 SO.sub.3 and 
H.sub.2 SO.sub.4. The film with alkaline liquid ensures a neutralization 
of the previously formed H.sub.2 SO.sub.4 and an oxidation of present 
nitrogen oxides. The later shower of aqueous liquid ensures a conversion 
of the remaining amounts of SO.sub.2 and SO.sub.3 into H.sub.2 SO.sub.3 
and H.sub.2 SO.sub.4 being removed during the passage through the 
sulphuric acid neutralizing liquid in a container before the discharging 
into the atmospheric air. As an additional security measure, the flue gas 
cleaned may optionally pass through an additional neutralizing device. The 
low temperature of the cooled flue gas ensures an essential increase of 
the capacity of the sulphur dioxide of being absorbed in the amount in 
question of aqueous liquid. By a usual liquefaction of sulphur dioxide at 
0.degree. C. with water, the dissolving or absorbing is carried out with 
the following ratios, viz. 1 part of water to 80 parts of SO.sub.2, and 
each time an increase of temperature of 20.degree. C. takes place, the 
number of parts of SO.sub.2 is halved. The present 20.degree.-30.degree. 
C. of the flue gas thus implies that considerably higher amounts of 
SO.sub.2 can be absorbed by the aqueous liquid than in the previously 
known plants, whereby the consumption of aqueous liquid is relatively low 
compared to usual scrubbers operating at temperatures of about 70.degree. 
C. to 80.degree. C. 
The heat exchanger permits heating of for instance water for district 
heating or of boiler water, as well as a preheating of air for combustion. 
The preheating of the air for combustion improves the combustion, said 
combustion being improved by 1.4% per 10.degree. increase of the 
temperature of the air for the combustion. The loss of heat from the plant 
is thereby very low compared to the loss occurring in connection with 
plants with chimneys. The lower amount of necessary liquid renders it 
possible to make the plant necessary for carrying out the process very 
small and compact. Furthermore, the higher efficiency per m.sup.3 of flue 
gas and the efficient utilization of the heat reduce the working expenses. 
According to the invention the flue gas may together with the first shower 
of liquid be carried in a substantially vertical direction downwards 
through the heat exchanger until it is contacted with the film of alkaline 
liquid and the second shower of liquid. As a result it is possible to 
utilize the gravity efficiently and to obtain a particularly good heat 
transition to the heat exchanger. 
Furthermore according to the invention, the liquid of the first and the 
second shower of aqueous liquid may be a mixture of water and hydrogen 
peroxide, whereby a particularly efficient oxidation of the sulphur 
dioxide into the sulphur trioxide is ensured. 
According to the invention it is particularly preferred that the second 
shower of liquid is emitted in a direction opposite the flow direction of 
the gas. 
The invention furthermore relates to a cleaning plant for carrying out the 
process, and this cleaning plant comprises an apparatus, in which it is 
ensured that solid particles such as fly ash are removed from the flue 
gas, and an apparatus for the removal of nitrogen oxides and sulphur 
dioxide from the flue gas, the latter apparatus optionally being coupled 
to a neutralizing device. This cleaning plant is according to the 
invention characterised in that the apparatus for the removal of nitrogen 
oxides and sulphur dioxide, when seen in the flow direction, comprises a 
first flue scrubber subjecting the passing flue gas to a shower of liquid, 
a two-step heat exchanger lowering the temperature of the flue gas to 
20.degree.-30.degree. C. and comprising both a high temperature heat 
exchanger and a low temperature heat exchanger, whereby said high 
temperature heat exchanger is adapted to transfer heat from the flue gas 
to water such as boiler water or district heating water, and whereby said 
low temperature heat exchanger is adapted to transfer heat from the flue 
gas to air supplied to the combustion, that after the two-step heat 
exchanger means are located for spreading a film of supplied alkaline 
liquid such as milk of lime or chloride of lime across the apparatus on 
the place in question, and that a second flue scrubber is provided after 
these means, said second flue scrubber spraying a shower of liquid on the 
passing flue gas, and that the apparatus for the removal of nitrogen 
oxides and sulphur dioxide at the discharge end comprises a container 
provided with outlets and catching the liquid from the flue scrubbers and 
the means, and in which the flue gas is forced to pass the liquid present 
in the container before the discharging. 
This cleaning plant operates particularly efficiently and when employing 
appropriate sensors and regulating means, a very reliable control of this 
plant is rendered possible. In this connection it is noted that the flue 
gas being treated as mentioned initially is cleaned of solid particles 
such as fly ash. This cleaning may be performed in many various manners, 
e.g. by means of a particle-separating apparatus in the form of a 
multicyclon or filters. In case of oil heating, the particle-separating 
apparatus may be replaced by an emulsion plant located in front of the oil 
burner. In this emulsion plant, the water is in a manner known per se 
emulsified in the oil in such a manner that the surface tension of the oil 
is changed, and a usual drop of oil is changed into about 10 to 20 small 
drops of oil within the burner. In this manner a better division into fine 
particles of the oil during the combustion is obtained, which implies that 
all carbon particles disappear from the flue gas and an optimum combustion 
is obtained. 
According to the invention the high temperature heat exchanger may be 
adapted to cause a reduction of the temperature of the flue gas to about 
70.degree.-80.degree. C., and the low temperature heat exchanger may be 
adapted to cause an additional reduction of the temperature of the flue 
gas to about 20.degree.-30.degree. C., whereby a particularly efficient 
utilization of the heat is obtained. Finally according to the invention, 
the first flue scrubber, the two-step heat exchanger, the means, and the 
second flue scrubber may be located successively and substantially 
vertically on top of each other with the first flue scrubber at the top, 
whereby the plant is particularly compact.

BEST MODE FOR CARRYING OUT THE INVENTION 
The flue gas cleaning plant illustrated in FIG. 1 shows in succession a 
particle-separating apparatus 1, an apparatus 2 for the removal of sulphur 
dioxide and nitrogen oxides as well as for utilization of the heat of the 
flue gas, a neutralizing device 3, and a drop collector 4 such as a 
cyclone drop collector. From this drop collector, the flue gas flows 
directly into the atmosphere. The neutralizing device or optionally only 
the bottom of the apparatus 2 is downwardly connected to a separation 
apparatus 5 in which the milk of lime used within the neutralizing device 
and co-operating in neutralizing the sulphuric acid in the flue gas, is 
separated from CaSO.sub.4 in a generally known manner. The hydrated 
calcium sulphate is discharged whereas the remaining milk of lime is 
returned through a pipe 6 for use in the cleaning plant, optionally during 
addition of fresh milk of lime. In the illustrated embodiment, the bottom 
of the apparatus 2 communicates as mentioned with the neutralizing device 
3 through a pipe 7, whereby liquid collected at the bottom of the 
apparatus 2 can flow into the neutralizing device 3. Since it is only as a 
security measure that the plant according to the invention may be coupled 
to a neutralizing device, the connection 7 may be adapted to carry liquid 
directly downwards to the separation apparatus 5, which is a generally 
known type too such as a separator. The drop collector 4 removes possible 
residues of milk of lime from the cleaned flue gas prior to the 
discharging into the atmosphere. 
As illustrated in FIG. 2, which is a vertical, sectional view through the 
apparatus 2 for the removal of sulphur dioxide and nitrogen oxides and for 
the utilization of the heat of the flue gas, the flue gas flowing from the 
particle-separating apparatus is contacted with a first flue scrubber 8 at 
the inlet of the apparatus 2. This first flue scrubber is followed by a 
two-step heat exchanger comprising a high temperature heat exchanger 9 and 
a low temperature heat exchanger 10. A short distance below the low 
temperature heat exchanger 10, a conical disc 11 of a generally known type 
is located, and which by means of an electromotor 12 is caused to rotate. 
Immediately above the centre of this conical disc 11, a pipe 13 ends for 
the supply of an alkaline liquid such as milk of lime or chloride of lime 
to the rotating disc 11. The apparatus 2 ends in a container-forming bend 
15 receiving liquid from the two flue scrubbers 8 and 14 and the rotating 
disc. Since this part of the apparatus 2 forms a bend, the flue gas is at 
an appropriately regulated liquid level forced to flow through the liquid 
collected in said bend before it flows out through the outlet 16 of the 
apparatus. As illustrated, the container-forming bend 15 is downwardly 
provided with outlets for the liquid collected, which is carried away 
through the previously mentioned pipe 7. The two flue scrubbers 8 and 14 
are of any appropriate type adapted to spray a shower of water containing 
hydrogen peroxide on the passing flue gas, the flow direction of said flue 
gas everywhere being indicated by means of substantially vertical arrows. 
The first flue scrubber 8 furthermore sprays the shower of liquid on the 
low temperature heat exchanger, and together with the flue gas said liquid 
gives off heat to the water passing through a pipe system into a boiler or 
a district heating. Before flowing into the apparatus 2, the flue gas has 
a temperature of 200.degree.-260.degree. C., and upon passage through the 
high temperature heat exchanger said flue gas has a temperature of 
70.degree.-80.degree. C., whereby the temperature of the water within the 
heat exchanger has risen to 95.degree.-105.degree. C. After the high 
temperature heat exchanger, the liquid and the flue gas are contacted with 
the low temperature heat exchanger preferably present in the form of a 
large number of transverse glass tubes preferably made of Pyrex glass.RTM. 
standing up to high temperatures and sulphuric acid. Atmospheric air 
passes these glass tubes at 10.degree.-25.degree. C., and during the 
passage through the heat exchanger the temperature of said air is risen to 
45.degree.-55.degree. C., whereby it is suited for use as air for the 
combustion. Having passed the low temperature heat exchanger, the 
temperature of the flue gas is 20.degree.-30.degree. C., and between the 
rotating disc and the wall 17 of the apparatus here being funnel-shaped, 
the flue gas is contacted with a film of milk of lime or chloride of lime 
emitted from the rotating disc so as immediately thereupon to be contacted 
with the shower of liquid from the second flue scrubber 14. 
During the passage of the flue gas through the upper portion of the 
apparatus 2, i.e. the portion comprising the first flue scrubber 8 and the 
high temperature heat exchanger 9, part of the sulphur dioxide content is 
contacted with the water from the flue scrubber and converted into H.sub.2 
SO.sub.3, whereas other parts of the sulphur dioxide are oxidized by the 
hydrogen peroxide into SO.sub.3 and subsequently converted into H.sub.2 
SO.sub.4 during the contact with the water. The water of the flue scrubber 
simultaneously ensures a continuous rinsing of the two heat exchangers. 
During the passage of the flue gas through the portion of the apparatus 2 
comprising the low temperature heat exchanger 10, the temperature of said 
flue gas is as mentioned lowered to about 20.degree.-30.degree. C. At this 
temperature the capacity of the water of absorbing sulphur dioxide is 
greatly increased compared to the capacity thereof at higher temperatures. 
At 20.degree. C., 40 volumes of SO.sub.2 are dissolved in one volume of 
water. At the present temperatures, a high degree of dissolving is 
therefore ensured though the water consumption is very small. At the 
contact of the flue gas with the film of milk of lime or the like, a 
considerable part of the sulphuric acid now present in the flue gas is 
neutralized, and the present part of nitrogen is caught and oxidized. 
During the further flow of the flue gas past the second flue scrubber 14, 
the remaining amounts of SO.sub.2 and SO.sub.3 are caught and oxidized and 
converted into H.sub.2 SO.sub.4. In the container-forming bend 15 of the 
apparatus, the flue gas is subsequently forced into intense contact with 
the limy liquid present therein, said liquid neutralizing the remaining 
residues of H.sub.2 SO.sub.4. Subsequently, the flue gas flows out of the 
apparatus 2 through the outlet 16, and here the flue gas only comprises 
from 0 to about 2% of sulphur dioxide. This unusually low amount of 
sulphur dioxide can, if desired, be additionally reduced by means of the 
neutralizing device illustrated in FIG. 1. 
The invention has been described with reference to a preferred embodiment. 
Many modifications may be performed without thereby deviating from the 
scope of the invention. The hydrogen peroxide used in the two flue 
scrubbers is a 3% hydrogen peroxide. The operation and capacity of the 
individual members such as the flue scrubbers and the rotating disc are 
adjusted in an appropriate manner by means of sensors and regulating means 
not shown. 
The rotating disc may be replaced by other means for spreading an alkaline 
liquid.