United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-87/020 Apr. 1988
Project Summary
Influence of Coal Mineral
Matter on the Effectiveness of
Dry Sorbent Injection for 862
Control
D. M. Slaughter, W. J. Thomson, T. W. Peterson, S. L. Chen, W. R. Seeker,
and D. W. Pershing
This report summarizes the results of
a detailed examination of the interac-
tions between calcium based sorbents
and chemical species which could
either enhance or detract from the
sorbents' ability to capture SO2 in
pulverized coal flames. Previous,
limited experimental studies had indi-
cated that sorbent performance would
be influenced by various inorganic
compounds, particularly those occur-
ring in coal mineral matter.
This Project Summary was devel-
oped by EPA's Air and Energy Engi-
neering Research Laboratory. Research
Triangle Park. NC, to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report
ordering information at back),
Program Objectives
The overall program objectives were to:
(1) determine the influence of mineral
ash constituents on the sulfur capture
performance of calcium based sorbents,
(2) define the chemical compounds
which can enhance the ability of the
calcium based sorbents to capture SO2
and identify the mechanism(s) of promo-
tion, (3) establish the influence of mineral
ash on the effectiveness of sorbent
promoters, and (4) optimize the process
for adding promoter materials to the
sorbents.
Program Scope
A combined experimental and theoret-
ical approach was used in this study.
Programs examined the physical and
chemical changes of a wide variety of
sorbents with carefully selected chem-
ical additives under different composition
environments and over the range of
time/temperature histories. Experimen-
tal testing was conducted in furnace
facilities ranging from laboratory to small
pilot scale with both natural gas and coal
as the primary fuel. Thermal histories
ranging from near isothermal to highly
quenched conditions were considered.
Fundamental mechanistic information
on the interactions between the pro-
moters and calcium based sorbents were
obtained from a dynamic x-ray diffrac-
tometer (DXRD) and from extensive
investigation of solid samples extracted
from the combustion furnaces. Chemical
and elemental analysis techniques
included electron microscopy (scanning
and transmission), energy dispersion
spectroscopy, and x-ray diffractometry.
DXRD was used to trace the formation
of chemcial species and the destruction
of solid, promoted and unpromoted,
calcium compounds as a function of time
and temperature. Microscopy and spec-
troscopy techniques were used to char-
acterize the physical morphology and
elemental spatial distribution of the
promoter, calcium sorbent, and chemical
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constituents of the mineral ash. X-ray
analysis was used to quantify chemical
species present. Equilibrium calculations
based on free energy minimization were
used to characterize the probable gas and
solid phase species. The MAEROS com-
puter program was used to involve the
multicomponent population balance
equations governing the coagulation,
fragmentation, condensation, and
nucleation of the ash and promoter
aerosols system.
Coal/Sorbent Interactions
Under certain limited conditions, coal
mineral matter can influence the effec-
tiveness of calcitic sorbents. If the
mineral matter and sorbent are in direct
intimate contact (particularly by simul-
taneous pulverization) prior to injection
and if they are injected through the
burner into the temperature regions of
a boiler, sorbent performance may be
reduced. The controlling mechanism
appears to involve the formation of low
melting eutectics: the calcium reacts
chemically with various alumina and
silica components of the ash and
becomes unavailable for subsequent
sulfation. Because of this interaction
(and because of the deleterious effects
of the flame zone on the sorbent itself)
burner injection cannot be recom-
mended. If, however, the sorbent is
injected separately from the pulverized
coal (either with the secondary air or
downstream) then essentially no sorbent
deactivation due to mineral matter
occurs. Modelling calculations indicate
that particle/particle collision frequen-
cies are simply not adequate to deacti-
vate a significant number of the sorbent
particles. Figure 1 shows data obtained
in the 293 kW boiler simulator furnace
(BSF) with both limestone (i.e., Vicron)
and hydrated (i.e., Longview) sorbents
injected with the fuel and downstream.
A wide range of normal and cleaned coals
were tested in this sequence, and the
results compare favorably with the data
taken firing natural gas. No evidence of
ash deactivation was observed with any
coal. Therefore, the performance of the
current limestone injection multistage
burner (LIMB) concept (separate down-
stream sorbent injection) should not be
significantly degraded by mineral ash
effects.
Promoter/Sorbent Interactions
A major series of promoter screening
tests indicated that chromium series
transition elements and alkali metals can
Vicron
W/Fuel Downstream
Longview
W/Fuel Downstream
60
§50
§30
o
10
American
Boiler (AB)
Profile
O Natural Gas
• Indiana #3
m Illinois
A Kentucky
* Lower Kittannir
• Lower
Kittanning
Clean Coal
(1-35)
123 123 123 12
Ca/S Ca/S Ca/S Ca/S
Figure 1. effect of fuel type on SOi capture with Vicron and Longview.
effectively increase the ability of calcium
based sorbents to capture SO2. Figure 2
summarizes data obtained in the bench
scale Control Temperature Tower (CTT)
furnace with limestone and indicates
that all of the alkali metals significantly
enhance the measured SO2 capture. The
primary enhancement mechanism is not
chemical catalysis of the SOz to SO3
reaction step; rather, these materials
react chemically with the CaO to produce
local physical changes in the sorbent
which enhance sorbent capture. In
particular, both the chromium series
elements and the alkali metals appear
to induce particle fragmentation, crea-
tion of large cracks, and pore enlarge-
ment, all of which increase the acces-
sibility of the interior calcium and
decrease the rate of site loss by pore
mouth plugging. In addition, the pro-
moters often produce low melting eutec-
tics with the CaO, and this liquid phase
may enhance the local transport of the
SOz to the unreacted CaO. In addition
to affecting the sorbent morphology, the
alkali metal promoters also directly react
with SO2 to produce sulfate products;
they effectively function as sorbents
themselves. The chromium compounds
do not exhibit this behavior.
Coal/Promoter Interactions
Unfortunately, the effectiveness of
promoted sorbents for SOz control may
be significantly less in actual, coal fired
systems than in SOz doped, natural gas
tests due to the presence of coal mineral
matter, in particular the submicron ash
aerosol. Figure 3 summarizes a wide
series of tests conducted to evaluate the
influence of coal mineral matter constit-
uents on the promotion of limestone and
hydrated sorbents by both NaHC03 and
Cr2O3. These data indicate that under all
conditions tested both the sodium and
the chromium produced significant
increases in sorbent utilization in the ash
free experiments (natural gas firing).
However, with all of the sodium promo-
tion cases and with the chromium cases
where the sorbent was added within the
burner zone, the promotion effect was
greatly reduced with coal firing. Only
when the sorbent was added down-
stream and the promoter was Cr203, was
the beneficial effect retained in the
presence of coal firing. With sodium
promotion external contact between the
promoted sorbent and the coal is not
required; separate injection still leads to
deactivation. EDS analysis confirmed the
presence of sodium on the mineral ash
as well as the sorbent, suggesting that
the controlling mechanism is vaporiza-
tion and uniform sodium condensation.
Condensation modeling calculations
indicated that more than 90% of the
added sodium could condense on the
submicron ash aerosol if the sorbent
particles are large and the internal
surface area is not readily avialable.
X-ray diffractograms indicated that the
condensed sodium reacted with the
refractory oxides in the coal ash to form
sodium silicates, thereby effectively
reducing the sodium available for direct
reaction with SOZ. Thus, the effective-
ness of sodium promotion is reduced in
coal fired systems simply because the
promoter itself condenses on the submi-
cron ash aerosol and the amount avail-
able for enhancement of the calcium
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70
60
50
40
I
O 30
20
10
Control
Temperature
Tower
Natural Gas
1500 K Injection
£± Vfcron + LiiCOt
D Vicron + NaHCO3
Q Vicron + K2CO3
Q Vicron
are almost certainly environmentally
unacceptable. Figure 4 illustrates data
obtained with a chromium-promoted
pressure-hydrated calcitic sorbent and
indicates that capture levels in excess of
70% were achieved at a Ca/S ratio of
2.0. Unfortunately, to date it has not been
possible to identify an environmentally
acceptable promoter capable of inducing
the morphological changes produced by
chromium with a sufficiently low vola-
tility to avoid vaporization and subse-
quent scavenging by the ash aerosol.
Ca/S
Figure 2. Summary of SOz capture with alkali metals.
sorbents is greatly reduced. The coal ash
has less influence on chromium promo-
tion because chromium is a far less
volatile promoter. EDS analysis indicated
that the chromium was primarily asso-
ciated with the calcium; little chromium
was found with the ash, particularly in
the downstream injection cases.
Application Considerations
Parametric process studies indicated
that the chemical form of the sodium
promoter is not significant. Particle size
can influence promotion; large particles
may not be completely vaporized and
dispersed in the residence time available.
Because of the vapor phase transport of
the alkali metals, they need not be
injected with the sorbent particles to
provide effective promotion (in the
absence of an ash aerosol).
Various, externally generated pro-
moted sorbents were produced and
evaluated in an attempt to avoid the
deleterious effects of the ash aerosol.
However, even hydration in the presence
of sodium did not overcome this problem;
at normal LIMB injection temperatures
the sodium still tends to vaporize initially
anda large fraction ultimately condenses
on the submicron aerosol. Conversely,
chromium promotion can be optimized to
produce extremely high capture levels,
although chromium promoted sorbents
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Sorbent + NaHCO3
Burner Downstream
Sorbent + Cr2O3
Burner Downstream
60
50
c
?40
!
330
s
-------
80
70
60
SO
$ 40
I
30
20
JO
I
I.Ox 10'Btu/hr (290 kW)
2200~230O°Ff120O-
1260°C) Injection
Q EER-PH* (Illinois Coal)
£ EER-PH + Cr2Os (Indiana Coal)
Ca/Cr = IS
Boiler Simulator Furnace
/'{Energy and Environmental
Research Corporation
Pressure Hydrate
Figure 4.
1 2 3
Ca/S (molar ratio)
Chromium-promoted, catcitic pressure hydrate.
D. Slaughter, W. Thomson, T. Peterson, S. Chen, W. Seeker, and D. Pershing
are with Energy and Environmental Research Corporation, Irvine, CA 92718-
2798.
David A. Kirchgessner is the EPA Project Officer (see below).
The complete report, entitled "Influence of Coal Mineral Matter on the
Effectiveness of Dry Sorbent Injection for SO2 Control," (Order No. PB 88-
178 587/AS; Cost: $25.95. subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
. GOVERNMENT PRINTINGOfHCfe 1983/548-153/67094
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
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IS 3 EJIVIR PROTECTION AGENCY
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