Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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6600-104
A process for removing gaseous sulfur compounds and sulfur
dioxide from the flue gases of a furnace
The present invention relates to a process for removing gaseous
sulfur compounds, and especially sulfur dioxide, from the flue
gases of a furnace which burns sulfur-containing fuels, such as
coal or oil.
It is previously known to decrease the sulfur dioxide content of
the flue gases of a furnace by feeding calcium oxide, calcium
carbonate or some other alkaline compound into the combustion
chamber of the furnace. In a fluidized-bed furnace with a circul-
ating bed it is possible by means of a calcium addition to de-
crease the sulfur dioxide content of the flue gases by as much as
90 ~ when the furnace is operating within the temperature range
which is optimal for the chemical reactions, i.e. 800-1000 C.
The sulfur dioxide thus absorbed leaves the furnace in the form
of gypsum, along with the fly ash.
In other furnaces, in which it is necessary to use temperatures
higher than those mentioned above and in which the retention of
the additive is short due to the nature of the combustion, it is
to be expected that the decrease in the sulfur dioxide content of
the flue gases remain substantially lower, about 50 ~ or less, and
therefore this process has not been applied on an industrial
scale to such furnaces.
On the other hand, it is known that the sulfur dioxide content
of flue gases can be decreased by various absorption processes
outside the furnace. One such process, known _er se, is the
so-called semidry process, in which the flue gas emerging from the
furnace is led into a separate reactor, into which an aqueous
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slurry of calcium hydroxide is sprayed in the form of small droplets through
specific nozzles. The reactor is typically a rather large vessel, in which
the velocity of the flue gases is allowed to decrease and the aqueous slurry
is sprayed downwards from above, from the upper part of the vessel. The
temperature of the reactor is at this time about 50-80C, and the control
of the spraying of the aqueous slurry of calcium hydroxide is very important,
since drops which are too large will remain as liquid on the bottom of the
reactor. The thickness of the aqueous slurry of calcium hydroxide should
be maintained thick enough that the heat content in the flue gases would
evaporate the water entering the reactor, so that the adsorption product
can be recovered in the form of dry powder. By this process it is possible
to remove as much as 90~ of the sulfur dioxide. The disadvantages of the
process include the tendency of the nozzle to become clogged, an extra
preparing and batching apparatus for the aqueous slurry of calcium hydroxide
which raises the investment costs, and problems of controlling the drop
size during the spraying.
The goal of the present invention is to provide a process for
removing gaseous sulfur compounds, such as sulfur dioxide, from the flue
gases of a furnace, a process by which the gaseous sulfur compounds can be
converted to solid sulfur compounds which can easily be separated from the
gases and thereby effectively removed from the flue gases of the furnace in
a simple and economical manner.
According to the present invention there is provided a process
for removing a gaseous sulfur compound from a flue gas of a furnace
comprising:
a) feeding a pulverous alkali metal hydroxide or alkaline earth
metal hydroxide, in addition to the flue gas to be burned and an oxygen-bearing
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gas, into a furnace into the sulfur dioxide containing flue gases which
leave the furnace,
b) separately spraying water or steam into the furnace or into
the flue gas, and finally
c) separating the solicl obtained as a reaction product, which
contains alkali metal or alkaline earth metal sulfate, from the gases.
The gaseous sulfur compound to be removed is preferably sulfur
dioxide. The quantity of pulverous hydroxide used is preferably an excess
in proportion to the amount of sulfur present in the flue gas. Preferably
the water or steam is sprayed while the temperature of the flue gas is
50 to 800C, more preferably 90 to 200C, and the quantity of water
or steam preferably does not exceed the amount which can be evaporated
by the thermal energy of the flue gas and by the reactions in the furnace.
Preferably the hydroxide is calcium hydroxide or a calcium/magnesium
hydroxide mixture.
in the process according to the present invention, a material
which reacts with gaseous sulfur compounds, and particularly with sulfur
dioxide, and water are fed into the process separately, whereby the
problems of preparing, handling and feeding in a slurry are avoided,
in the following manner:
a) a pulverous alkali metal hydroxide and/or alkaline earth
metal hydroxide is fed, in addition to the sulfur-containing material
to be burned and the oxygen-containing gas, into the furnace or into
the sulfur dioxide containing flue gases emerging from the furnace,
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b) water and/or steam is sprayed separately into the furnace
and/or into the flue gases, and finally
c) a solid which contains alkali metal and/or alkaline
earth metal sulfate, and possibly sulfite, is separated from the gases.
The basic idea of the invention is thus that the hydroxide is
fed into the flue gases in the form of powder and is activated only
~n situ in the flue gases by means oE water and/or steam, whereupon it
reacts with sulfur dioxide and forms a sulfate/sulfite mixture which
can thereafter be removed effectively from the flue gases by conventional
physical ash separation methods.
A pulverous hydroxide is fed into the combustion chamber of the
furnace, or into the flue gases emerging from the Eurnace, in accordance
with the sulfur content of the fuel in such a way that the amount of
alkali and/or alkaline earth metal in the molar ratio according to the
reaction formula, is at least the amount equivalent to the sulfur,
preferably, however, greater than the amount necessary for the reaction.
By feeding hydroxide in the form of powder separately into the
combustion chamber, or into the flue gas conduit, it is possible
to use simple feeding devices, e.g. pneumatic ones, whereby the
clogging of the nozzles and the use of extra preparing and batching
devices for an aqueous slurry are avoided. Respectively, the feeding
of water and steam through nozzles is uncomplicated and easy.
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The feeding of water or steam into the flue gases is in practice
carried out at a temperature range 50-800 C, preferably within
the temperature range 90-200 C. If it is desired to recover the
product of absorption substantially in the form of dry powder,
water is used in the spraying only in such an amount that the
thermal energy of the flue gases suffices to evaporate it.
The invention is described below in greater detail with reference
to the accompanying drawing, which depicts diagrammatically an
apparatus suitable for carrying out the process according to the
invention.
In the drawing the furnace in general is indicated by reference
numeral 1. The sulfur-containing material 4 to be burned, an
oxygen-bearing gas 5, and a pulverous calcium and/or magnesium
hydroxide 6, are fed into the combustion chamber of the furnace 1,
preferably in excess in proportion to the sulfur dioxide gas pro-
duced in the combustion chamber. By the expression "in excess" is
meant in this context that the amount of the calcium, magnesium,
or calcium and magnesium compound is greater than would in theory
be needed, according to the reaction formula, to react with all of
the sulfur dioxide fed into the combustion chamber.
The hydroxide fed into the furnace is first dehydrated in the
furnace to oxide. The oxide, for its part, can react with the
sulfur dioxide, first forming sulfite and, when oxidizing there-
after, sulfate. Owing to the short retention time in the furnace
only part of the oxide has time to react with the sulfur dioxide
at a temperature sufficiently high in terms of the reaction, and
for this reason, calcium oxide and/or magnesium oxide bearing flue
gases 8 which contain combustion residue and steam, and also
unabsorbed sulfur dioxide leave, the combustion chamber of the
furnace through the flue gas conduit 7
In practice the temperature of the flue gases 8 is so low that
the reaction between -the calcium and/or magnesium oxide and sulfur
oxide is relatively weak, and under such conditions the oxides
can be regarded as inactive in terms of sulfur removal. When the
temperature of the flue gases decreases, the oxides may react
with the steam present in the flue gases and reconver to hydroxi-
de. Thus it is preferable to feed the pulverous hydroxide directly
into the flue gas conduit 7 or into the reactor 2 subsequent to
it. In addition, the flue gases 8 can be used in the heat ex-
changer 12 to heat the air 5 to be fed into the furnace 1.
The sulfur dioxide containing flue gases emerging from the com-
bustion chamber of the furnace 1, which contain calcium and/or
magnesium oxide and possibly calcium and/or magnesium hydroxide,
are thereafter directed into a reactor, which is generally indi-
cated by reference numeral 2. In order to activate the oxide and/
or hydroxide, water or more steam is sprayed into the flue gases
of the reactor 2, and this water or steam reacts with the calcium
and/or magnesium oxide and forms the respective hydroxide, acti-
vating it and also activating the hydroxide possibly already pre-
sent in the flue gases. The hydroxide for its part reacts with the
sulfur dioxide still present in the flue gases 8, thereby forming
the respective sulfite, which, in the presence of oxygen, at least
in part further oxidiæes to the respective sulfate. The amount of
water 9 fed into the reactor 2 is adjusted to so low a level that
the heat of the flue gases 8 will suffice to evaporate the water
fed into the reactor 2. Thereupon the substantially dry, fly-
ashlike reaction product can be removed, in the same manner as
other dust, in a conventional dust separator 3, from which the
flue gases 11 are further directed into the flue 13 and the sepa-
rated dust 12 is directed to a possible further treatment.
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The order in which the water and the hydroxide are added is in no
way critical. Thus it is possible, for example, to feed the water
or steam into the furnace and the pulverous hydroxide only at a
point subsequent to the furnace, either into the flue gas conduit
or into -the reactor subsequent to it.
One of the further advantages of the present invention is that the
process can be applied to a furnace provided with any type of
burner. The size of the furnace is not a restricting factor, and
it is not necessary to circulate the calcium and/or magnesium
hydroxide in the combustion chamber, whereby the expensive circu-
lating-bed alternative with its complicated circulation devices,
and at the same time the excessive dust due to its operating
principle and also dust separation are avoided. Compared with the
prior known spray method, the spraying of water or steam into the
reactor 2 is, furthermore, considerably less complicated and
easier to implement than when using a slurry which clogs the
nozzles and is difficult to mix.
The invention is described below in greater detail with the aid
of examples.
Example 1
Coal having a sulfur content of 1.4 % is fed at a rate of 70 tn/h
into a 600 MW pulverized-coal furnace, while operating at full
capacity. An excess of combustion air is fed in , so that the
oxygen content in the flue gases is 4 %.
Calcium hydroxide having a calcium hydroxide content of 90 % is
fed into the furnace at a certain varying proportion to the
sulfur amount entering the furnace in the fuel. The theoretical
equivalent amount is about 2.5 tn/h of the said calcium hydroxide.
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Calcium hydroxide and water and/or steam are sprayed into the
flue gases either in the flue gas duct or in a separate reactor
located at a point subsequent to the flue gas duct.
In terms of energy economy it is most advantageous to increase
the moisture content of the flue gases by spraying water into
them in a separate reactor located at a point subsequent to all
heat recovery surfaces.
The increased moisture content of the flue gases makes the
calcium hydroxide highly reactive, whereupon it rapidly reacts
with the oxides present in the flue gases. The higher the moisture
content of the flue gases upon leaving, the more effectively the
the sulfur dioxide becomes removed from the flue gases. In terms
of energy economy it is, however, advantageous to operate in such
a way that the heat released in the chemical reactions will
suffice to evaporate the water amount added. If it is desired to
raise the final temperature of the flue gases, this is done either
by using external heat or by means of a warm flue gas flow.
The results are shown in the table below, which shows in percent
how much sulfur dioxide was removed from the flue gases when
varying amounts of calcium hydroxide were fed into the furnace in
accordance with the present invention; the amounts of the calcium
hydroxide are indicated as molar proportions of the calcium con-
tent of the pulverous calcium hydroxide to the sulfur content of
the fuel fed into the furnace. The temperatures of the flue gases
were measured immediately prior to the feeding point of the water
or steam, except at 800 C, at which the water or steam was fed
directly into the furnace.
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Table 1
B)
Ca/S Flue gas Flue gas SO
temperature ternperature reduction
o A) o
0.48 800 C 108 C 42 %
o o
0.52 500c 650c 56 %
1.52 202 C 74 C 77 %
o o
1.56 90 C 68 C 82 %
o ' o
2.20 200 C 72 C 87 %
o o
2.22 120 C 62 C 96 %
o o
2.3 llQ C 68 C 93 %
o o
2.5 90 C 66 C 97 %
o o
4.1 800 C 110 C 72 %
4.0 120 C 68C 98 %
A) water or steam fed into the furnace
B) Immediately prior to the feeding point of water
Example 2
A calcium-magnesium hydroxide which contains calcium hydroxide
45 ~, magnesium hydroxide 45 % and impurities 10 % is fed into the
furnace according to Example 1, under corresponding conditions.
Calcium-magnesium hydroxide and water and/or steam are fed into
the flue gases either in the furnace or in a separate reactor
located at a point subsequent to the furnace.
The increase in the moisture content makes particularly calcium
hydroxide highly reactive, whereupon it rapidly reacts with the
oxides of sulfur present in the flue gases. If the molar ratio
of the calcium contained in the calcium hydroxide to the sulfur
is at least 1, the reaction occurs primarily between the calcium
hydroxide and the oxides of sulfur, and the magnesium hydroxide,
being slower, passes through the reaction zone substantially
unchanged.
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If the calcium-magnesium hydroxide is fed into hot flue gases,
the consequence may be either that the magnesium hydroxide breaks
down into magnesium hydroxide and water or that the entire
calcium-magnesium hydroxide breaks down into calcium oxide and
magnesium oxide and water. In this case each oxide can as such
react witll the oxides of sulfur and, when the flue gases cool and
the moisture content increases, re-form hydroxide, which further
reacts with the oxides of sulfur. If the molar proportion of the
calcium to the sulfur is at least 1, the reaction results and
conditions are substantially in accordance with the corresponding
values in Table 1, owing to the ability of calcium compound to
react more rapidly.
