allied environmental
technologies, inc.
Ammonia
Conditioning
of
Flue
Gases
A White Paper
allied environmental
technologies, inc.
Ammonia
Conditioning
of
Flue
Gases
A White Paper
Numerous investigators
reported use of the ammonia conditioning with various degrees of success. Ammonia conditioning usually works best with
the acidic fly ash. If injected ahead
of the air heater tendency to plug (unless air heater temperatures were
unusually high - >400EF) the air heater was reported as an
additional, but unwanted result. Ammonia
injection downstream of the air heater was reported to be unpredictable.
No effect of the
ammonia on the fly ash resistivity was reported with any degree of
certainty. Instead, the effect of
ammonia appeared to be an enhancement of the space-charge component of the
electric field. In addition, increase
of the cohesiveness of collected ash and a reduction of the re-entrainment
appears to be another "major player" in successful application of the
ammonia conditioning.
This paper presents
results of the literature review to identify the "what's..." and
"why's..." of the ammonia conditioning. It will attempt to focus on a mechanism of the ammonia
conditioning and specifics of its successful or unsuccessful applications. The presence of trace amounts of the alkali
materials may contribute appreciably to the conductivity of the fly ash
particles.
I. Ammonia Flue Gas Conditioning
II. Chemical Reactions Involved with Ammonia Conditioning
III. Mechanisms of the Ammonia COnditioning
III.1 Effects of Ammonia on the Electrical Resistivity
III.2 IV.2 Precipitator Performance Improvement
III.2.1 Sulfuric Acid Attachment
III.2.3 Fly Ash Cohesivity Control
Electrostatic
precipitation is a process using electric forces to separate suspended
particles form gases. Fortunately, the
precipitation process can be effectively carried out over a wide range of dust
and gas properties and conditions. In
cases where precipitation is difficult, steps are usually taken to improve the
dust or gas properties and thereby increase the precipitability of the
dust. The most known and widely used
technique is to add trace amounts of a "conditioner", which when
mixed with the gas and dust under the right conditions, alters their properties
so as to move them into a range of good precipitation.
Because this is and
electric process, the electrical conductivity of the material being collected
is of paramount importance. Highly
conductive dusts with resistivity of less than 104 ohm-cm can be
charged and collected very easily.
However, because they are so conductive they bled of their charge to the
grounded collection plate rapidly and are prone to escape the collection zone,
either because of electrical repulsion back into the gas stream, or because of
simple re-entrainment be the passing gas.
Highly resistive dusts with resistivity over 1011 ohm-cm
exhibit opposite phenomena. Once
collected on the collecting plates, the collected dust layer will be tightly
held in place by a corona current, however, because the high resistivity dust
the voltage drop can build up to such levels as to cause the well known back
corona phenomenon, which in its most severe form can reduce precipitator
collecting efficiency to nearly zero.
By far the largest body
of work in the art of the fly gas conditioning ash been devoted to attempts to
lower the dust resistivity.
Conditioning is as old as the art of precipitation itself. As early as 1912 it was discovered that
copper converter dust precipitation was considerably enhanced by the presence
of sulfur trioxide or increased moisture of exhaust gases.
Progress in the
application of conditioning agents for precipitator performance enhancement has
been erratic. In the petroleum industry
ammonia conditioning of catalyst cracker gases has been accepted for many years. Conditioning of utility boiler flue gases
has been proven over and over and is a widely accepted technology today. It has been proven now that chemical
conditioning affords an alternative to increased precipitator size as a
solution to fly ash collection form boilers burning low sulfur coals.
The value of ammonia as
a conditioning agent was reportedly discovered in 1942 in efforts to deal with
high resistivity of catalyst dust in the petroleum industry. The use of the ammonia for treating fly ash
in coal-fired power plants, on the other hand, is "comparatively"
new. First reports of the intermittently successful use of the
ammonia as a conditioning agent were reported in Australia by Watson and
Blecher in 1966. The results of this
study showed good results while using the ammonia as a conditioning agent. There the SO3 flue gas conditioning
was proved to be ineffective due to the highly acidic ash (pH of 3.5-4) and,
thus, speculations were that conditioning with a basic rather than acidic agent
might prove more effective. The initial
pilot plant trials with ammonia conditioning conducted at the Tallawarra plant
in 1959 found to be very successful. Similar promising results were obtained at
the No. 5 Tallawarra precipitator.
However, subsequent ammonia conditioning at the No. 6 precipitator at
the Tallawarra did not produce the same encouraging results that were obtained
on No. 5.
The electrical behavior
of the precipitators with ammonia conditioning changed considerable within minutes
after the introduction of ammonia into the flue gas. Increases of 50 to 100% in electrode potential and corresponding
decreases in electrode current were maintained consistently and tent to persist
for several hours after ammonia injection is stopped.
Further resistivity
measurements were carried out, however, these presented rather perplexing
results when ammonia-conditioned dust was evaluated. It is generally accepted that the increase in precipitation efficiency
brought about by flue gas conditioning is caused be a reduction in the particle
surface resistivity and the elimination of back corona, a phenomenon reported
to be quite common with new South Wales fly ash. The resistivity of the ammonia-conditioned ash, however, showed
no appreciable difference from that of non-conditioned fly ash. Both were in the range of 1013 to
1014 ohm-cm, which is certainly well above the critical level of 1010
ohm-cm suggested by Dr. White.
Watson and Blecher then
went on with a hypothesis that, perhaps ammonia gas conditioning changed flue
gas characteristics sufficiently to eliminate back corona and/or the forces of
particle adhesion and cohesion sufficiently to bring about cleaner electrodes
and, hence, better performance.
Shortly thereafter
Baxter reported success with ammonia conditioning several plants in the United
States, where apparent effect was to lower ash resistivity in some instances
and to increase it in the other instances.
Initially, he injected sulfur trioxide with the unsatisfactory
results. Performance improvement was
attained only by the use of excessive conditioning which allowed an
objectionable acid vapor plume to escape the stack. The presence of the acid plume indicates that the necessary
adsorption on the dust particles was not occurring. Where sulfur trioxide conditioning has been effective, stack
measurements show no increase in effluent SO3 concentration, rather
the injected gaseous agent attaches to and is discharged with the dust
collected in the precipitator as reported by Durby and Hendrich. Analyses of the collected ash indicated that
SO3 flue gas conditioning reduced laboratory measured resistivity
form 1012 to 1011 ohm-cm and increased acidity of the
dust from a pH of 5.5 to 3.5.
Subsequent trials with
ammonia injection reportedly produced marked precipitator performance improvement. Dust analyses showed decrease in resistivity
similar to that mentioned with sulfur trioxide conditioning, however, the pH of
the ash increased to a neutral condition.
Although he reported varying success with precipitator performance,
improvement has been accompanied by air heater plugguage at each test
location.
Finally, the tests
reported at the power plant with two identical cyclone fired boilers produced
still unexplained results, i.e. units reacted in very explicitly different
manner. One of the units responded well
to the ammonia injection, while the other reacted rather poorly. Moreover, chemical and physical analyses of
the dust produced no reasons for such a difference. The resistivity was measured and actually increased. Dust ph also increased but remained quite
acidic. Unfortunately authors did not
publish detailed coal and analyses.
Furthermore, it is worth noted that, according to the author, "No
correlation can be established between effectiveness and initial ash
resistivity, nor between ammonia injection and treated ash resistivity".
In 1968, Reese and
Greco were successful in the use of ammonia to treat the fly ash from high
sulfur coal (they concluded that the effect of ammonia was to overcome an
unfavorably low resistivity stemming from an excess of naturally occurring
sulfur trioxide).
Some of the trials with
ammonia flue gas conditioning have been successful, but others have not
produced the desired results. One
source of difficulty in the use of the ammonia conditioning has been the
absence of basic information on mechanisms of conditioning be this compound and
the circumstances in the power plant to favor its use.
Elementary explanations
of ammonia conditioning effects have assumed that the chemical compounds formed
when ammonia is injected, either concurrently with injection of sulfur trioxide
or when the latter is formed by "natural" (combustion) processes in
sufficient quantities, are only ammonium bisulfate NH4HSO4
or, if the stoichiometric ratio is appropriate, the normal sulfate (NH4)2SO4. Besides these, the existence of intermediate
chemical compounds is possible. The
melting points change continuously according to the H2SO4/NH3
stoichiometric ratio, and there is a possibility that compounds with very low
melting points may be formed in the process.
In addition, the ones which are stable in solid form at temperatures
above 150 oC (300 oF) are those with stoichiometric
ratios 1.2 and above.
Ammonia flue gas conditioning is very little understood, especially as to the mechanism involved in improving ash resistivity. The chemical reaction postulated by Rendle and Wildson teaches that ammonia will react with any natural sulfur trioxide present and moisture to produce ammonium bi-sulfate:
In the presence of
excess ammonia this reaction will slowly continue to produce ammonium sulfate:
Lacking excess ammonia
or as cooling occurs the liquid bi-sulfate will freeze at temperatures below
297 ºF into a sticky solid. It is this adherent nature of the resultant that has been a
"suspect" to play a major role in the precipitator performance
improvement.
The use of ammonia (NH3)
for modification of dust resistivity has not been developed to the same degree
as SO3. Some experiments
have shown that ammonia injection improved the collection of fly ash in ESP's,
but it is not clear whether the improvement resulted from resistivity
modification or other effects. Other
trials have shown little or no change in fly ash resistivity or ESP
performance.
There has been much
debate by many authors regarding the use of ammonia alone for modifying the electrical
resistivity of fly ash. One argument
suggests that for high resistivity ashes a reduction in resistivity might be
produced, permitting higher voltages and currents to be maintained without
sparking or back corona. On the other
hand, for lower resistivity ash an increase in resistivity due to
neutralization of excess acid might permit a sufficient electric field to exist
in the ash on the precipitator collection electrodes to overcome losses from
rapping reentrainment.
While successes have
been reported using ammonia as the sole conditioning agent for some low sulfur
Australian coals, U. S. experience has been improved precipitator performance
for some low sulfur coals and not for the others. It has been theorized that ammonia conditioning is more effective
for acidic ashes in terms of the relative concentrations of acidic and basic
oxides present (acid/base ratio) but this has not been true in every case.
Ashes having high
acid/base ratios with small to moderate amounts of sulfur trioxide present in
the flue gas can have high resistivity and it may be that resistivity is
reduced as a result of ammonium bisulfate acting to facilitate sulfur trioxide
attachment. If little or no sulfur
trioxide were present, ammonia alone might have little effect. This hypothesis is somewhat speculative at
present in view of the small number of observations documented.
A study of the effects
of the ammonia on the electrical resistivity of the silica-alumina catalysts
may shed some light on the possible mechanism of the conductivity improvement
on one hand and, perhaps, answer numerous questions why ammonia is very
unpredictable in its conditioning nature.
It has been
demonstrated that sodium and potassium increase the conductivity of
silica-alumina materials (note: fly ash is predominantly silica-alumina
compound) at about 400 ºF or
higher. It was further speculated that
the increased conductivity results from ionic conduction of sodium and
potassium ions. Ammonia and amines are
reported to markedly increase the resistivity of silica-alumina catalysts below
200 ºF. The
increased conductivity is closely associated with large amounts of physical
adsorption, which probably increases the mobilities of the current
carriers. The irreversible adsorption
of ammonia below 200 ºF is independent of
temperature and is measure of the acid sites on the particulate. The reversible adsorption increases with
decreasing temperatures. The increase
in conductivity depends on the properties of both adsorbate and the
particulate.
The chemisorption of
ammonia on silica-alumina compounds like fly ash particles results in the
formation of ammonium ions from reaction of ammonia with protons on the ash
particles surface. This, perhaps, could
be responsible for the power resistivity over the temperature range of 300-500 ºF. In
ammonia it seems reasonable to conclude that most of conductivity occurs on the
surface. The physically adsorbed
ammonia offers an excellent medium for the passage of ions. In addition, the relative size of the
ammonia ion, the internal structure of silica-alumina and the temperature
regions where the precipitators are operating are not favorable for ionic
conduction (by ammonium ions) in the bulk.
The exact mechanism of the increased surface conductivity resulting form
ammonia adsorption is difficult to deduce but one could speculate that presence
of traces of alkali metals would contribute appreciable to the conductivity of
the fly ash particles.
Ammonia in the presence
of sulfur trioxide may work in a variety of ways. Although the exact mechanisms responsible for improved
precipitator performance are not completely defined, they appear to include such
phenomena as an improved acid attachment, space‑charge effects, agglomeration
and increased cohesiveness of the fly ash.
Some evidence suggests
that injection of ammonia promotes improved attachment of available sulfuric
acid to fly ash particles, making it more effective for resistivity
control. It creates a thin conductive
film on the ash surface believed to consist of low melting point ammonium
sulfate products, which contributes to resistivity modification. It has been
suggested that the addition of ammonia improves upon the efficiency of the
sulfur trioxide conditioning by an additional charge carrier, the NH4+
ions.
In some low sulfur
coals the percentage of acidic compounds in the ash (aluminum oxide, iron
oxide, and silicon oxide) is high (above about 90 percent). Here, the injection of sulfur trioxide alone
to reduce resistivity loses some of its effectiveness because the acidic sulfur
trioxide cannot readily attach to the highly acidic ash at higher temperatures.
Substantial amounts of excess sulfur
trioxide must be injected while treating such ashes to produce a conductive
film on the surface.
Since the conditioning
process is concerned with application of an acid film to the surface of the fly
ash particles, it is clear that the coal ash content will affect the amount of
acid to be injected. Acquisition of an
acid film by the fly ash particles is presumed to take place by a
condensation/partial pressure deposition process and would therefore be
expected to be temperature sensitive.
Dr. Frisch has shown
that the surface composition of the ash particles can affect acid attachment
[17]. Dr. Bickelhaupt also showed that
a "threshold" effect was exhibited by certain ashes, evidenced by a
lack of change in resistivity at ambient acid concentrations below the
threshold level [18]. These phenomena
are clearly related to ash chemistry, and call for inclusion of all possible
combinations of ash chemical properties as variables to be tested for
significance.
Those who have studied
factors affecting the resistivity of fly ash in a flue gas atmosphere are very
familiar with the hump-backed characteristic of resistivity as a function of
temperature. Generally, resistivity has
a maximum value at a temperature around 300 oF with decreasing
values above and below the maximum point.
In determining the amount of SO3 to be required to reduce the
ash resistivity to a desired lower value, one would think that the maximum rate
would be required at the maximum unconditioned resistivity point, with
decreasing amounts at higher and lower temperatures. That is, a hump-backed curve of injection rate reflecting the
unconditioned resistivity characteristic would be expected. An interesting discovery made in the
development of SO3 flue gas conditioning is that the amount of SO3
required to attain a desired level of resistivity follows the expected
dome-shaped curve with respect to temperature only up to a point, after which
it breaks off to a rapidly rising characteristic [19].
The inflection point
between the two portions of the curve is a function of the surface chemistry of
the ash, occurring at relatively low temperatures for acidic ashes and at
higher temperatures for basic ashes.
The range of variation of the inflection point temperature appears to be
approximately from 250 oF to 400 oF for coals available
worldwide. If the flue gas temperature
is above the inflection point, the portion of the injected SO3,
which is greater than the level of the dome-shaped portion of the curve, does
not attach to the ash and will be passed through the ESP. In the rare cases where a wet scrubbing
system follows the ESP the excess SO3 will be captured; otherwise an
objectionable blue plume will be formed if the emitted SO3
concentration approaches 10 ppmV or more.
The inflection point
effect explains some of the difficulties various experimenters have
encountered in attempting adjustment of ash resistivity with sulphur trioxide
alone. In some low sulfur coals the
percentage of acidic compounds in the ash (aluminum oxide, iron oxide, and
silicon oxide) is high (above about 90 percent). Here, the injection of sulfur trioxide alone to reduce
resistivity loses some of its effectiveness because the acidic sulfur trioxide
cannot readily attach to the highly acidic ash at higher temperatures.
Substantial amounts of excess sulfur trioxide must be injected while treating
such ashes to produce a conductive film on the surface. Furthermore, if the operating temperature is
much higher than the inflection point, no reasonable amount of injected SO3
will be effective in reducing the resistivity.
The temperature at
which inflection occurs is a function of ash surface conditions, i.e. a
measure of susceptibility to attachment of acid. The increased requirement for sulfur trioxide at temperature
above the inflection point indicates the required partial pressure of sulfur
trioxide needed to attach sufficient sulfuric acid to the particles to obtain
the desired ash resistivity. The
difference between the injection level at low temperatures and that at a high
temperature is a measure of excess sulfur trioxide, which will escape from the
stack at the higher temperature. If
this difference is on the order of 10 ppmV or more, it is likely that a condensation
plume will form which may be visible, depending upon the concentration of fly
ash in the emitted stream.
It has been found that
the simultaneous injection of ammonia (NH3) with the SO3
will condition the ash surface to accept resistivity adjustment without excess
SO3 being required [21]. Ammonia FGC tends to overcome this problem by
allowing greater adhesion of the residual sulfur trioxide (again, one has to
presume that there is a sufficient supply of natural SO3 present) to the acidic ash particle surfaces
to optimize its resistivity. This
allows for proper fly ash resistivity adjustment without using excess sulfur
trioxide. It seems logical that the
temperature at which inflection occurs is a function of ash surface conditions
- a measure of susceptibility to attachment of acid, perhaps, inasmuch as the
lower inflection temperatures occur for ashes having low base-to-acid ratios
and the higher points correspond to higher ratios.
When ammonia is
injected into a gas stream in the presence of the sulfur trioxide a fine fume
consisting of a variety of ammonium sulfate particles, all much less than 1
micron in size, is produced. These
particles alter the electrical characteristics of the flue gas between the
discharge and collecting electrodes and produce a space‑charge
enhancement of the electric field.
This well documented effect arises when fine fume is charged in the
precipitator and the electric field is thereby increased. Moderate, strictly controlled quantities of
fine particles would therefore, increase the charge level of fly ash particles
and the field near the collecting plates.
The higher field increases collection efficiency. The space charge effect is usually more
pronounced in the front fields than in those following. This happens because a large portion of the
particles is collected in the front fields and the remaining small quantity is
not sufficient to produce a significant space charge.
Ammonia can generate
low melting point substances depending on the H2SO4/NH3
stoichiometric ratio; at low stoichiometric ratios low melting point substances
having high moisture absorbency are generated; this promotes cohesion. This cohesion, applied properly and strictly
controlled, improves overall precipitator efficiency.
The cohesive properties
of fly ash influence stack opacity and precipitator efficiency by agglomerating
particles and by reducing re-reentrainment.
To simplify, the ammonia combines with sulfur trioxide to form ammonium
bisulfate. The melting point for this
material, as discussed earlier, is about 147 oC (297 oF),
so it is semi-liquid at typical flue gas temperatures and acts as a binding
agent when mixed with fly ash. Because
the agglomerated ash groupings are heavier, rapping losses are minimized.
For high resistivity
ashes it has been found that variations of cohesion due to additives are of
minor importance to precipitation efficiency.
For low resistivity ashes where the use of an additive has little or no
effect on resistivity it has been found that changes in efficiency of precipitation
are associated with variations of cohesion.
High cohesion has resulted in high efficiency due to reduced rapping
losses and re-reentrainment or more effective initial capture of particles. The electrical forces of attraction between
particles in an ash layer on the collecting electrode in an energized
precipitator are the predominant component of the cohesive strength of a
highly resistive ash layer and the variation of the mechanical component due to
the additive is then relatively unimportant.
With low resistivity ash the electrical forces are reduced and may even
become repulsive. Under these circumstances
mechanical forces causing re-reentrainment become important.
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