METHOD OF PURIFYING A GAS CONTAINING SULPHUR DIOXIDE COMPONENTS THEREIN

METHODE DE PURIFICATION D'UN GAZ CONTENANT DE L'ANHYDRIDE SULFUREUX

English Abstract

ABSTRACT OF THE DISCLOSURE
Purification of gases from sulphur dioxide components according to
the dry method means that the gas is contacted with an
absorbent in a solid phase, which yields a relatively low
reaction rate. In order to increase the reaction rate
according to the invention, the gas is contacted with a
mixture consisting partially of a solid absorbent phase and
partially a liquid phase on the surface of the solid phase.
At the purification of flue gases from sulphur dioxide hot gas
is contacted with an absorption liquid in a drying apparatus,
where evaporation of the water supplied and a reaction
between sulphur dioxide and the absorption substance take
place simultaneously, whereafter the solid product phase is
separated from the gas in a dust separator and a slower,
secondary absorption reaction is operative. In order to
achieve a higher reaction rate in the dust separator, a
substance is added to the flue gas which, together with other
chemical species derived from the gas or the solid absorbent
forms a liquid phase distributed over the surface of the
solid phases. The liquid phase usually contains water and has
a good solubility for reactants and reaction products.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method for purifying a gas containing sulphur
dioxide components therein, which method comprises contacting
said gas with a mixture consisting of (i) a solid absorbent
phase and (ii) a liquid, hydrous phase, which liquid phase
is derived from a hydroscopic salt selected from the group
consisting of FeCl3, Fe(NO3)3, Fe(NO3)2, Al(NO3)3, Ca(NO3)2,
Mg(ClO3)2, MnCl2, K2SO4°MgSO4, NaAl(SO4)2, NaClO, Na3PO4,
Na2SiO3, Na2SO4, and Zn(No3)2 which hygroscopic salt is
thermodynamically stable at the prevailing water partial
pressure and temperature of said gas and which at least partly
covers said solid absorbent phase, and preparing said mixture
by adding to said solid absorbent phase said hygroscopic
salt.
2. A method as defined in claim 1, wherein said
gas consists of flue gas, said purifying process comprises
a drying and absorption step, and a dust separation and
secondary absorption step, and said solid absorbent is in
liquid form, and said absorption liquid and said liquid
phase are added to said flue gas, said hygroscopic salt
under conditions prevailing in the dust separator being adapted
to distribute said liquid phase over the surface of said
solid phase.
3. A method as defined in claim 1 or 2, wherein
said sulphur dioxide components of said gas react with said
absorbent to produce reaction products, and said liquid
phase is aqueous and has good solubility for reactants and
reaction products.
4. A method as defined in claim 2, wherein said
added hydroscopic salt constitutes a maximum of 20 percent
by volume of said solid phase present at said dust separator
step.

5. A method as defined in claim 1 or 2, wherein
said absorbent is selected from the group consisting of
calcium hydroxide, calcium carbonate, magnesium calcium
carbonate, sodium carbonate, sodium hydroxide or mixtures
thereof.
6. A method as defined in claim 2, wherein the
added hygroscopic salt is at least partially extracted from
fly ash separated from the flue gas.
7. A method as defined in claim 2, wherein the
added hygroscopic salt is at least partially extracted
from gaseous substances in the flue gas.
8. A method as defined in claim 2, wherein said
contacting step comprises atomizing said absorption liquid,
and said hygroscopic salt is mixed in said absorption liquid
prior to its atomization.
9. A method as defined in claim 2, wherein the
added hygroscopic salt is added to the flue gas by means
of a special atomization device.
10. The method of claim 2, wherein said added
hygroscopic salt constitutes a maximum of 5 percent by
volume of said solid phase present at said dust separator.

Description

0 ~ ~
Gases can be purified from sulphur dioxide c~nents by contacting
with an absorbent in a liquid phase twet method) or in a
solid phase (dry method). The present invention relates to
a substantially dry method. As the dry method includes
reaction steps implying diffusion into a solid phase, one
of its disadvantages is a relatively low reaction rate.
The object of the present invention is to reduce
the impact of the said disadvantage.
In general terms, the present invention provides
a method for purifying a gas containing sulphur dioxide
components therein, which method comprises contacting said
gas with a mixture consisting of (i~ a solid absorbent
phase and (ii) a liquid, hydrous phase, which liquid phase
is derived from a hygroscopic salt selected from the group
consisting of FeC13, Fe(N03)3, Fe(N03)2, Al(N03)3, Ca(N03)2,
Mg(C103)2, MnC12, K2SO40MgSO4, NaAl(S04)2, NaC10, Na3P04,
Na2SiO3, Na2S04, and ~n(N03)2 which hygroscopic salt is
thermodynamically stable at the prevailing water partial
pressure and temperature of said gas and which at least
partly covers said solid absorbent phase, and preparing said
mixture by adding to said solid absorbent phase said
hygroscopic salt.
By effecting a liquid aquous phase with a high
solubility for the participating reactants on the surface of
the solid absorbent particles, the reaction steps, viz
dissolving of the sulphur dioxide component, diffusion,
separation of absorbent and precipitation of reaction
product, proceed substantially more rapidly than the correspond-
ing reaction steps when only gaseous and solid reactants are
present. The absorbent may be an oxide, a carbonate or a
hydroxide of an alkali metal. The liquid phase, may be
derived from a hygroscopic salt, which at the prevailing gas
temperature and gas water content is a molten hydrate.

~ ~2031
In recent years a process for absorbt~on of sulphur dioxide
from a hot flue sas has come into use which in the literature
~s referred as "dry scrubb~ng". The process substantially
co~prises a drying step and a filtering step, and it operates
as follows: The ho~ gas is contac~ed in a drying apparatus
with an atomized aqueous suspension or an aqueous solution
capable to react with sulphur dioxide. In the
drying apparatus evaporation of the supplied water ~akes
place and simultaneously a react~on between sulphur dioxide
and the absorbent occurs. As the gas leaves the drying
apparatus, the major part of the supplied water has
evaporated, the gas temperature has decreased, the water
vapour conten~ of the gas has increased, and part of the
sulphur dioxide has been absorbed and now is bound to the
solid reaction product. The gas thereafter passes a dust
separator where the solfd product remain~ng absorbent and any
o~her solid ma~erial i5 separa~ed from the gas. Part of the
separated dust may be further recycled ~nto the absorption
liquid in order to increase ~he u~ilization of the absorbent.
This process often results in inco~plete utilization of the
absorben~ during the driving period. After said period the
reaction between the sulphur d~oxide and the absorbent
proceeds in the dust separator. This reac~ion, however,
proceeds relatively slowly, since the reaction steps occur in
solid phases.
The present invent~on has also ~he object of bringing about a
h~gher reaction rate during said latter process step.

1 1~2031
This higher reaction rate is obtained by adding to the gas a limited
amount of substances such as hygrosoopic salts which under the conditions
prevail~ng in the dust separ~tor form on the surface of the
solid particles a l~quid ~queous phase w~h sufficient
solubility for the reactants. As the reactlon steps, viz.
solution of sulphur dioxide, diffusion of reactants and
precip~tation of product, proceed substant~ally more rapid
than the corresponding steps in a sol~d phase, the net e~fect
thereof is an increase ot the ~otal reaction rate. In order
to prevent the operation of the dus~ separator from being
jeopardized, the volume of the 7iqu~d phase must be small in
relatlon to the volume of the solid phases, and in order to
achieve a good effect, the ~quid phase ~ust be well
d1str~bu~ed over ~he ~urface of ~he solid phases.
For example, the added hygroscopic substance ma~ constitute a maximum
of about 20 percent, and preferably is no greater than about 50 percent
by volume of said solid phase present at said dust separator. Examples of
compounds~ the hydrate of which has the property of, under
the conditions preva~ling a~ ~he absorption of sulphur
dioxide from flue gases of fossil-fired boi7ers, being ~n
liquid phase are a.o. FeC13, FetN03~3~ Fe(N03)2,
Al(NO3)3, Ca(N0)3)2, Mg(C103)2, MnCl~9 K2S04 -
MgS04, NaAl(S04)2, NaClO, Na3P04, Na2SiO3, Na2S04,
Zn(N03)2O
E~ample
__
In a pilo~ plan~ with a capacity of 10 000 Nm /h comprising
a dry~ng apparatus, into which atomized absorption ligu~d is
~njected, and a fabric filter as the dust separator, tests
were carried out for determining the capacity with and w1thout
the addition of so~e of the aforesaid substances. Calcium
hydroxide was used as absorbent which was added in
an amount correspond~ng to a mole ratio Ca/S02 = 1,1
(calculated vn ingoing gas). The ~ngoing gas had a temperature
of 140C and an S0~-content of 850 ppm.
--3--
>,~

~ 1~203~
r~he filtering velocit~ of the fabric filter was 92 m/h. The gas
temperature after passing through the fabric filter was 70C. The
following S02 absorption efficiencies were measured.
Test NoO Addition Eff~e~ency
- 70
2 FeCl 3 74
3 FeS04 83
4 Fe(N03)3 81
Al (N03)3 71
6 Ca (N03 ) 2 76
7 NaAl (S04)2 78
8 Na2S04 74
9 Zn~N03)~ 77
:
~:
::
~:
--4--