United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
*
Research and Development
EPA/600/S7-87/013 June 1987
&EPA Project Summary
Effective Mixing Processes for
SOX, Sorbent, and Coal
Combustion Products
B. M. Cetegen, T. R. Johnson, R. Payne, D. K. Moyeda, M. S. Sheldon, W. Li,
and G. S. Kindt
This report describes an evaluation of
the mixing of jets of calcium based
sorbents injected into large wall-fired
coal burning utility boiler furnaces for
sulfur dioxide (SO2) emission control.
Isothermal flow experiments evalu-
ated the mixing characteristics of single
and multiple jets into idealized cross-
flow streams and into flow fields
simulating the upper regions of boiler
furnaces, where gas temperatures are
most conducive to sulfur capture. Pilot
scale combustion experiments at 1.41
and 24 MW thermal input tested the
dependence of overall sulfur removal
efficiency on injection parameters and
sorbent type. Several computational
and empirical models of jet behavior
were adapted for use in sorbent injec-
tion system design.
An effective injection system design
methodology is based on a hierarchy
of design and interpretation proce-
dures. The methodology includes: (1)
the use of simple empirical jet relation-
ships to provide preliminary estimates
for injection system parameters
(number, size, and velocity of jets); (2)
isothermal physical flow modeling of
the boiler to evaluate and optimize
injection system mixing parameters;
and (3) coupling of jet mixing results
with experimental sorbent sulfation
data, or sulfation calculations, to
predict the impact of mixing on overall
SO 2 capture.
This Project Summary was deve-
loped 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).
Introduction
Control of S02 emissions from large
utility boilers burning high sulfur fuels
has gained considerable attention
because SOa is believed to be a major
precursor to acid rain. Many efforts are
focused on the use of dry desulfunzation
processes which have shown promise of
economical SOa emission control, espe-
cially as a retrofit technology.
One dry desulfurization process
involves injecting calcium based sor-
bents, such as CaCO3 or Ca(OH)2, directly
into the combustion chamber. When
heated, sorbent particles decompose into
CaO particles which react with SO2 in
the furnace gas stream to form CaSO.*.
This process is limited to a narrow
temperature range (window) where
sulfation reactions proceed rapidly. The
upper temperature limit of this window
is about 1200°C and is dictated by the
thermodynamic stability of the product
CaSO4 and the loss of sorbent reactivity
at high temperatures. The lower temper-
ature is about 800°C, below which the
sulfation reaction rates are too slow for
the available residence times. This
temperature zone in utility boilers lies in
the upper furnace, beginning around the
radiant furnace exit extending into the
convective passes. Because the flue gas
loses heat rapidly in the convective
passes of the boilers, the residence times
in this temperature window are usually
in the order of 1 sec. Studies on sulfation
of calcium based sorbents indicate that
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residence times of the order of 1 sec are
required in the sulfation temperature
zone for the sulfation reactions to
proceed effectively. Therefore, it is
important that sorbents are dispersed
and mixed with the boiler flue gas in the
smallest space and shortest time to
effectively utilize the available residence
time in the sulfation window. The mixing
time cannot be extended by injecting the
sorbent at higher temperatures, because
sorbent reactivity is reduced by deacti-
vation processes (sintering, dead burn-
ing) at high temperatures.
Results and Discussion
Studies were conducted on the mixing
of the sorbent stream with the flue gas
stream of wall-fired utility boilers. The
key objective of the studies was to devise
a sorbent injection system design metho-
dology for wall-fired boilers.
Isothermal Modeling Studies of
Jet Injection
Flow modeling studies of jet injection
into boiler geometries were conducted in
several isothermal flow models. Figure
1 is an example of a single jet injected
into a wall-fired boiler upper furnace at
the nose elevation. In the flow field of
such enclosure, the jet penetrates a
certain depth into the furnace cross-flow
after which it is carried by the furnace
flow toward the convective sections of
the boiler. Key results were:
• The two key jet parameters determin-
ing the jet trajectory in boiler-like flow
fields are the jet-to-cross-flow
momentum flux ratio and the jet
diameter.
• The trajectories of the jets injected
from the front wall were affected by
such enclosure characteristics as
nose confinement factor and the
injection location in relation to the
nose point. The side spacing of a row
of jets appeared to have negligible
impact on jet trajectories for the
typical boiler injection configurations.
• In the flow field of the boiler around
the nose, the initial bending of the jets
injected from the front wall can be
well predicted by the average jet
trajectory equation. However, as the
furnace flow turns to the horizontal
passes, the jet trajectory naturally
deviates from the predictions.
Figure 1. A single 2.54 cm (1.0 in.)
diameter jet injected into a
boiler geometry at a momen-
tum flux ratio of 100.
• Injection from the rear wall atthe nose
location can be an effective way to
introduce the sorbent into the bulk of
the flow near the nose. However, this
region of the boiler Is generally
inaccessible.
• Injection from the roof of the boiler
or steeply down-angled from the front
wall require very high injection veloc-
ities to penetrate the opposing flow.
Although roof injection may be a
viable way to control the injection
temperature, high velocities make it
impractical. There also appears to be
marginal or no gain in residence time
due to the increased path of the jet
because of the high injection
velocities.
• Effects of sorbent particles can be
incorporated in the bulk jet properties;
i.e., jet density and momentum.
Photographic evidence and particle
dispersion measurements support
this view. If limestone particles are
small (less than 20 fjm) and the
particle loadings are not high (less
than 107 particles/cc), the dispersion
characteristics of particle laden jets
and the equivalent air jets are similar.
• Simulation of cold dense jets into a
hot combustion environment in an
isothermal study requires modifying
the jet diameter with the square root
of the free stream to jet density ratio
if near field dilution rates are to be
properly simulated. However, in the
far field (greater than 10-15 jet
diameters from the jet nozzle), differ-
ent scaling approaches give similar
results. For example, geometric scal-
ing of jet diameters can be employed.
• Comparison of measured isothermal
and combustion dispersion patterns,
suggest approximate similarity of the
dispersion patterns with similar dis-
tributions of the tracer concen-
trations.
Computational Modeling of Jet
Injection
A computational model of a gas jet
injected into a cross flow was developed
to evaluate temperature/time and con-
centration/time profiles in sorbent jets.
This model, based on integral mass,
momentum, and energy balances, was
used to describe the temperature and
concentration histories early in the
injection jets. The profiles of tempera-
ture/time and concentration/time were
input into a sulfation model to predict the
sulfur capture. The final value of the
sulfur capture compared favorably with
the range of the experimentally mea-
sured values. The jet modeling was
included as part of the sorbent injection
design methodology to evaluate the
dynamic behavior in the sorbent jets.
Pilot Scale Experiments in a
Tower Furnace
Sulfur capture tests were conductec
in a 1.2 MWth (4.2 x 106 Btu/hr) pilot
scale furnace to evaluate the impact 01
sorbent injection parameters on sulfui
capture. The furnace modeled the uppei
furnace section of a wall-fired boiler witr
sorbent injection through nozzles locatec
near the nose section. The followine
results were obtained from thes<
experiments
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• The sorbent injection velocity (or jet
momentum) had virtually no effect on
sulfur capture. In these tests, a
commercial atmospheric hydrate (Lin-
wood) was injected through six injec-
tors on the front wall at three different
velocities, corresponding to three
momentum flux ratios. It was shown
that the dependence on the jet velocity
is in fact small, if the jet penetration
is not greatly impaired.
• Sorbent dispersion, as influenced by
a number of sorbent jets, had an
impact on sulfur capture levels.
Decreasing the number of jets from
six to one reduced the sulfur capture
by about 55%. However, the disper-
sion effect became important when
the sorbent was highly maldistributed
in the furnace flow.
• An empirical scheme, based on isoth-
ermal dispersion experiments and
small scale sorbent injection tests,
was developed to explain the disper-
sion effects.
Large Pilot Scale Experiments
A series of sorbent injection tests were
conducted in the Large Watertube Sim-
ulator (LWS) furnace, in a geometry
resembling that of a small wall-fired
utility boiler. Sorbents were injected
through 4.4 and 9.8 cm diameter jets in
the upper furnace. The results from these
tests showed:
• Upper furnace injection showed a
dependence of sulfur capture on the
jet velocity and momentum.
• The effect of dispersion altered by the
number of injection jets was found to
be small for these injection
configurations.
Although both results appear to con-
tradict the smaller pilot scale tests, the
results were due to the nature of the flow
field in the LWS model and its interaction
with sorbent jets. Isothermal flow model-
ing studies in a 1/15 scale model of the
furnace and the observations in the LWS
furnace itself revealed that the flow
patterns in the furnace caused the
sorbent jets to be entrained to the lower
furnace where high temperatures pre-
vailed. A simple modeling of the observed
phenomena explained the sulfur capture
test results. These tests and the subse-
quent data analysis provided useful
information in the development of the
sorbent injection system design
methodology.
Injection System Design
Methtidology
The ultimate goal of this program was
to develop a design methodology that can
be applied to utility boilers with relative
ease. The elements, described earlier,
were used to formulate such a metho-
dology. The methodology, a hierarchy of
design and interpretation procedures to
be used in the actual design and per-
formance prediction phases, consists of
three levels of sophistication:
(I) Simple empirical relationships
which were developed and tested
during this study. These refined
formulas can be used to determine
the injection system parameters
(number, size, and velocity of injec-
tion jets) based on the operational
characteristics of the boiler and such
other constraints as the availability
of injector sites.
(II) Isothermal modeling which can be
used, together with Level I results,
to study the effects of varying the
injection system parameters roughly
determined in Level I and optimize
mixing in an isothermal model. This
requires constructing an isothermal
model of the boiler furnace (or at
least the upper furnace) and studying
the dispersion of the sorbent jets in
the furnace flow. This scheme also
provides an empirical capability
which can be utilized to predict the
sulfur capture potential for particular
injection systems.
(Ill) Computational models of jet injec-
tion and calcination/sulfation pro-
cesses which can be used to inves-
tigate and predict the time-
dependent nature of the sorbent
injection process.
B. Cetegen, T. Johnson, R. Payne. D. Moyeda, M. She/don, W. Li, and G. Kindt
are with Energy and Environmental Research Corporation, Irvine. CA 92718-
4190.
Nicholas Kresovich is the EPA Project Officer (see^below).
The complete report, entitled "Effective Mixing Processes for SO,, Sorbent,
and Coal Combustion Products," (Order No. PB87-188 892/AS; Cost: $36.95)
will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA22161
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
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Environmental Protection
Agency
Center for Environmental Research
Information
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Official Business
Penalty for Private Use $300
EPA/600/S7-87/013
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