Note: Descriptions are shown in the official language in which they were submitted.
CA 02261037 1999-01-18
' ? ~ ?~
Method for Preventinq Hiqh Tem~erature Corrosion
The invention relates to a method for preventing high
temperature chlorine corrosion in combustion chambers of
firing means and waste incineration plants and reducing the
flue dust portion contained in the combustion offgases.
Known methods for preventing corrosion were aimed at reducing
corrosion in incineration plants and, in particular, on the
heat exchanger surfaces of boilers by effecting reactions in
the gas phase in order to deactivate corrosive substances. To
this end, magnesium oxide is usually nozzled in, thereby
enabling primarily high temperature sulfate corrosion to be
substantially reduced at temperatures of above 480~C. This is
obtained in that, at an excess of MgO contained in the
deposits forming, for instance, on the superheater tubes,
MgSO4 instead of alkali pyrosulfates is formed with the SO2 of
the smoke gas. At temperatures of above 480~C alkali
pyrosulfates will, in fact, dissolve the layer of scale, thus
leading to catastrophic corrosion damages.
MgO is ineffective against corrosive damages caused by
chlorine forming during the sulfation of chlorides.
That type of corrosion has been increasingly encountered in
the firing space regions of refuse incineration plants during
the past years, since the composition of refuse has changed on
account of refuse separation, on the one hand and a higher
smoke gas temperature must be observed as in accordance with
legal provisions, on the other hand (800~C for at least 2
seconds within the combustion chamber).
From WO 95/11287 it has already become known to nozzle cerium
compounds such as iron cerium, cerium oxides and/or cerium
oxide hydrates in powder form. According to that previous
proposal, it was possible to nozzle in such cerium compounds
together with magnesium oxide, primarily aiming at
CA 02261037 1999-01-18
substantially reducing the cleaning expenditures involved in
the removal of deposits from the internal sides of plant parts
and thereby enabling longer campaign times than in known
processes. In particular, it was intended to preserve the
action of, and no longer deteriorate, passivating layers
already formed on the upper surfaces of plant parts for the
protection of the same. Those cerium compounds such as iron
cerium, cerium oxide or cerium oxide hydrate were to safeguard
reducing zones in the vicinity of the walls in order to
thereby reduce corrosion. The effect of cerium compounds in
the manner of oxidation catalysts induces afterburning in
reducing regions of the smoke gas, thereby reliably preventing
the reduction of an oxide layer of scale or rust already
formed for passivation. The cerium compounds also were to
become active against chloride ion corrosion in order to
thereby ensure oxidation to basically less dangerous chlorine
gas in molecular form.
The use of additives of this type, however, involves
relatively high costs and has proved to be insufficiently
effective, in particular in connection with high temperature
chlorine corrosion, in which chlorine gases dissociate again.
Attempts to admix to the charging material to be burnt
filtering aids and, in particular, inorganic filtering aids
based on active silicic acid containing silanol groups
likewise have proved to be relatively expensive, since
additives will show effects only in percentage ranges due to
their being inhomogenously distributed in refuse.
For recycling into the circuit of a melting chamber firing
means pollutant-loaded residues such as, for instance, filter
ashes, adsorbents or the like while adding used glass and/or
limestone chips, DE-A-4 021 362 has already proposed to
introduce the absorber into the circuit of a melting chamber
firing means with complete ash recycling. The absorber may be
fed into the the smoke gas path and/or into the ash
CA 02261037 1999-01-18
recirculation and/or into the slag discharge and/or along with
coal, wherein broken used glass and/or limestone chips may be
dosed in.
From GB-A-1 307 127, the use of 85 % by weight of calcium
bentonite, 10 ~ by weight of sodium phosphate and 5 % by
weight of sodium borate in oil, gas and coal firings may be
taken for granted. US-A-3 249 047 proposes the intoduction of
antimony compounds and silicates having large specific
surfaces in the overheating zone of coal-fired boilers in
order to reduce SO3 corrosion.
Finally, GB-A-800 445 suggests to use burnt bentonite and
other refractory materials in order to enhance the
distribution of oil droplets in gas turbines.
The invention aims at providing a method of the initially
defined kind, in which even the slightest amounts of an
additive may be employed with alkali and the content of metal
chlorides in the smoke gas being reducible as quickly as
possible. The reaction is to start as rapidly as possible and
to occur even at high temperatures without releasing
elementary chlorine.
To solve this object, the method according to the invention
for reducing high temperature chlorine corrosion in firing
means and incineration plants, wherein additives are nozzled
into the gas space, essentially consists in that acidically
activated bentonite is nozzled into the gas space at gas
temperatures of above 750~C, preferably 800~C. Due to the high
reactivity of acidically activated bentonite it is even
feasible already at relatively high temperatures to effect the
desired reaction for binding alkalis, the desired reactions
proceeding rapidly and quantitatively even at temperatures of
above 900~C. Very quick alkali binding, therefore, is achieved
if the additive is nozzled in along with secondary air closely
above the burner plane or the grate, for instance in the plane
,
CA 02261037 1999-01-18
of the secondary air supply, so as to be able to substantially
reduce the distance over which a corrosive attack may happen
at all. In order to enhance the distribution of the acidically
activated bentonite, the latter optionally may be nozzled in
together with silicon dioxide, SiO2 becoming active primarily
as a diluent. In any event, acidically activated bentonite in
mixtures with SiO2 is to be used in amounts of more than 50 %
by weight, based on the mixture, in order to rapidly ensure
the desired reaction.
By nozzling in glass dust it can be ensured that the glass
dust or glass powder will rapidly melt completely, wherein an
effective protection of the boiler walls to be proteced
against corrosion may be enhanced in that the glass dust or
glass powder is introduced in the direction of the walls of
the gas space via directed nozzles or spraying discs. In this
manner, a rinsing flow of glass melt is safeguarded along the
walls of the gas space.
In accordance with the invention, glass dust or glass powder
advantageously is nozzled into the gas space in amounts of
from 0.3 kg/ton to 30 kg/ton refuse, wherein ground used glass
such as, e.g., window glass or bottle glass may simply be
used. Advantageously, glass ground to grain sizes of about 50
~m is used for the method according to the invention.
Advantageously, it is proceeded in a manner that acidically
activated bentonite optionally mixed with sio2 is nozzled into
refuse incineration plants in amounts of from 0.5 to 3.0
kg/ton refuse.
A particularly economic process control enabling further
reduction of pollutant emissions may be achieved in that
acidically activated bentonite is used for adsorbing harmful
substances such as Hg or dioxin in the cooled offgases of
incineration plants, in particular in a flue flow process, and
subsequently is nozzled into the combustion chamber.
.. _ . . . ..
CA 02261037 1999-01-18
- 5 -
Due to the conditions prevailing within the firing chamber,
alkaline chloride is formed in quite considerable amounts at
the high chlorine contents usually present in refuse these
days. The ratio between chlorine and sulfur with refuse in
smoke gas has shifted in favor of chlorine during the past
years, elevated amounts of metal chlorides getting into the
smoke gas undecomposed and being converted into sulfates only
there or in the deposits. Under the given thermodynamic
conditions such a reaction, which is also called sulfation
reaction, results in sodium sulfate and elementary chlorine
and hence in a strong corrosive attack. The chlorine even
reaches the tube surface and there can destroy steel while
forming iron chloride. Such a sulfation reaction in the firing
chamber mainly is to be observed behind and closely above the
brickwork, and the nozzling in of acidically activated
bentonite according to the invention renders feasible an
extremely rapid reduction of the metal chloride content.
Proposals to nozzle sulfur into the smoke gas for that purpose
bring about an acceleration of chloride sulfation, yet
sulfation will occur only at lower temperatures and the amount
of released chlorine will be the same. Acidically activated
bentonite, due to its chemical and physical properties, is
able to react with alkaline compounds in the smoke gas
extremely rapidly even at elevated temperatures, in particular
at temperatures of above 900~C, wherein alkali can be bound
and HCl is formed. Thus, no elementary chlorine is released
and the risk of high temperature chlorine corrosion is
substantially lowered.
According to the invention, a cost-effective and simple
additive to be additionally used for nozzling into the
combustion chambers of firing means and refuse incineration
plants, which enables boiler campaigns to be substantially
increased and which, at the same time, aims to drastically
reduce the portion of flue dust contained in the offgas, which
is still relatively high in case of known additives, was
.. . .
CA 02261037 1999-01-18
obtained from oxide melts, in particular glass powder or glass
dust having a melting point of below 1000~C. Glass powder or
glass dust, which preferably is nozzled in in counterflow to
the smoke gases, melts at the temperatures prevailing in the
combustion chambers, binding flue dust to water insoluble
glasses. At the same time, surprisingly also alkali is rapidly
bound into such melting glass powder or glass dust particles
with a rapid reduction of the chlorine content in the offgases
likewise having been observed in a surprising manner. All of
these corrosive components of the combustion offgases are,
thus, effectively bound by the melting glass powder or glass
dust particles with the additional advantage being obtained of
the melt forming a dense deposit on the boiler walls, which
will prevent corrosive attacks, flowing down along the walls
in the direction towards the slag as a liquid melt film. Such
a rinse of the boiler walls by the molten glass melt in
addition to a corrosion-reducing effect on the boiler walls to
be protected, thus, also has the advantage of a number of
noxious substances being effectively dischargeable by the
downwardly flowing melt. Such an additional additive may be
nozzled in along with acidically activated bentonite, thereby
enhancing the effects to go even beyond the sum of the
individual effects.
Advantageously, glass powder or glass dust having a melting
point of below 800~C is used, complete melting and the safe
delivery of the noxious matter dissolved in the melt being
ensured in'that the glass powder or glass dust has a mean
grain size of 30 to 60 ~m, preferably 40 ~m.
In the following, the invention will be explained in more
detail by way of two diagrams. In Fig.l of the drawing the
amount of sodium chloride as well as the SO2 equilibrium
partial pressure are plotted over the temperature for the
firing chamber conditions indicated below. PCO2 = 0.2 bar,
pH2O = 0.2 bar, PO2 = 0.05 bar, pHCl = 10-5 bar and pCl = 10-5
bar. From that illustration according to Fig. 1 it is apparent
, , . . ~
CA 02261037 1999-01-18
that the formation of sodium chloride in the firing chamber
occurs primarily in the high temperature range and it is,
therefore, aimed at eliminating sodium chloride from the smoke
gas already at high temperatures.
In Fig. 2 the amount of sodium chloride in mg/m3 is plotted
over the temperature. Curve 1 indicates the reduction to be
attained by nozzling in 1 kg of acidically activated bentonite
per ton of refuse. Curve 2 shows the effect that would be
achievable by nozzling in 2 kg of sulfur, this clearly
illustrating the superiority of nozzling in acidically
activated bentonite for the purpose of lowering the sodium
chloride portion at high temperatures. On the other hand,
Curve 3 clearly shows the effect if only 1 kg of sulfur is
nozzled in and Curve 4 indicates the effect with pure smoke
gas.
The additive is in powder form and may readily be prepared in
a grinding fineness that allows the additive to be nozzled
directly into the firing chamber via secondary air.
When simultaneously nozzling in glass, corrosion-resistant
linings of the boilers were formed and the pollutant emission
was lowered, the service life of the means following upon the
firing chamber, such as superheaters or the like, having been
improved as well.
