United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-82-060 Jan. 1983
Project Summary
Use of Sorbents to Reduce
SOg Emissions from Pulverized
Coal Flames Under Low-N0x
Conditions (Progress Report)
P. Case, M. Heap, J. Lee, C. McKinnon, R. Payne, and D. Pershing
This summarizes a special progress
report that describes data obtained to
date under an EPA contract concerned
with the use of dry sorbents to reduce
sulfur oxide (SOx) emissions when
pulverized coal is burned under condi-
tions which limit the formation of
nitrogen oxides (NOx). The full report
both summarizes the data obtained to
date, and assesses their significance.
However, readers should bear in mind
the preliminary nature of the data.
This Project Summary was devel-
oped by EPA's Industrial Environmen-
tal 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).
The Problem
Metallic oxides form sulfates under
combustion conditions. The goal of this
program is to ascertain whether calcium
containing sorbents (or other suitable
materials) can be used in economic
quantities to provide combined NOx/SOx
control for pulverized-coal-fired boilers
without reducing boiler availability.
Previous studies with dry limestone
injection into utility boilers were not
very encouraging. This was attributed to
a combination of deadburning and non-
uniform distribution of the limestone in
the boiler. Early tests were carried out
with pre-NSPS high-turbulent burners;
however, when pulverized coal is
burned under low-NOx conditions, peak
flame temperatures are reduced and
there are enlarged fuel-rich zones.
These difference?, together with inject-
ing the sorbent with either the staged
air or the coal to ensure even distribution,
could provide conditions which are
conducive to sulfur capture by dry
sorbents.
The Approach
Two parallel investigations are under-
way at different scales to determine
whether it is possible to effectively
control NOx and SOx emissions from
pulverized-coal-fired boilers using low-
NOx burners and sorbent injection:
• Bench-scale investigations are being
conducted using a boiler simulator
furnace (BSF) to determine the
phenomena controlling sulfur cap-
ture in pulverized-coal flames. In
addition, these studies will provide
information on the impact of sorbent
injection on slagging and fouling.
• Pilot-scale investigations are involv-
ing full-scale low-NOx coal burners
tested in an.environment designed to
simulate the burner zone heat release
rate of the small utility boiler being
used as the host for the demonstra-
tion of EPA's low-NOx coal burner.
The intention is to conduct a program
in which the results of the bench-
scale studies can be readily trans-
ferred to pilot-scale and any anoma-
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lies encountered at pilot-scale can be
investigated cost-effectively at bench-
scale.
Results
Figure 1 summarizes data obtained to
date in the study at both scales. Data
have been obtained with the EPA
prototype at 70 x 106 Btu/hr burning
two coals: a low-sulfur (0.6%) Utah coal,
and a medium-sulfur (2.5%) Indiana
coal. Sulfur capture, as expected, is
dependent on coal sulfur content: for
the Indiana coal, captures of 50% were
possible at full load with calcium-to-
sulfur molar ratio of 2. Figure 1 also
shows the range of sulfur captures
obtained in the BSF for the Indiana coal
with the same sorbent used in the pilot-
scale studies, a commercial calcium
carbonate. The parameters which
caused this wide range in captures
were: heat extraction in the radiant
zone, load, and tertiary (staged) air
velocity.
Future Work
The bench-scale studies have been in
progress for 3 months, and initial
screening studies will be completed
within 6 months. These studies will de-
termine the influence of: fuel-rich
conditions, detailed temperature history,
coal-type, and sorbent type/size.
During this time, pilot-scale studies
will be mainly concerned with tests at
higher firing densities. After the con-
trolling parameters have been defined
by these screening studies, more de-
tailed measurements will be made to
enable the results to be explained and to
determine whether high captures can
ever be obtained in real boilers.
Although coal is the most abundant
U.S. source of fossil fuel energy, its
use poses several serious problems for
society. Mining coal, either from deep or
surface mines, involves environmental,
safety, and health considerations. Once
coal is mined, problems associated with
transportation, storage, and energy
conversion must be faced. In the
foreseeable future, coal will be used
primarily for the generation of electricity
in thermal power plants which burn coal
in suspension after it has been pulver-
ized. Coal is not a pure hydrocarbon, and
the impurities can give rise to the
generation of atmospheric pollutants
when it is burned. Assuming an efficient
combustor, atmospheric pollutants
are produced from the impurities in
coal: nitrogen, sulfur, and inorganic
material. The inorganic material forms
100
O EPA Low-NO* Burner Utah Coal
O EPA Low-NOx Burner Indiana Coal
80
I
60
Range of Data
Obtained in BSF
Indiana Coal
O
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fuels can be mixed to reduce SOX
emissions; e.g., coal/oil mixtures
and coal blending.
4. Prevention of the formation of gas-
eous pollutants. NOx emissions can
be reduced by limiting flame temper-
atures and controlling oxygen avail-
ability in the initial heat release zone.
SOx emissions can be reduced by
retaining sulfur as part of the solid
effluent. Thus, there is the potential to
prevent the formation of gaseous
pollutants by optimizing conditions in
the heat release zone.
This report describes a series of pilot-
scale experiments which were designed
to provide information on this final
approach: the use of sorbents to reduce
SOx emissions under combustion condi-
tions which also minimize NOX forma-
tion.
NOx emissions from pulverized-coal-
fired power plants are due primarily to
the oxidation of fuel-bound nitrogen
which is partitioned between the
volatile and char fraction when coal is
decomposed. The fractional conversion
of gaseous nitrogen specie (XN) to NO is
controlled by oxygen availability be-
cause of two competing reaction paths:
Oxidizing XN + . . . — NO + . . .
Reducing XN + . . . — N2+ . . .
In addition, an optimum gas-phase
stoichiometry (about 70% theoretical
air) maximizes N2 production by the
second path. In contrast, little is known
about the oxidation of nitrogen in the
char other than under oxidizing condi-
tions conversion to NO is low but finite.
NOx is also formed by the fixation of
molecular nitrogen. This formation path
can be restricted by limiting flame and
bulk gas temperatures since the rate of
fixation is very strongly dependent on
temperature. Thus NOx formation in
pulverized-coal flames can be reduced
by modifying the combustion process to
ensure that the coal reacts initially in an
oxygen deficient region and that peak
flame temperatures are limited. This
can be achieved by dividing the furnace
into a fuel-rich and an oxidizing burnout
zone. An alternative approach, one that
is being supported by the EPA, uses
burner design and outboard staged-air
ports to provide a flame with a fuel-rich
inner core with a complete oxidizing
envelope.
For solid-fuel combustion, sulfur in
the fuel does not necessarily convert
quantitatively to SOa/SOa in the com-
bustion products. Coals with high alkali
metal oxides contents retain significant
amounts of sulfur in the ash as sulfates.
Thus, there is the potential to mix a
sorbent with the fuel either prior to or
during combustion which will capture
gaseous sulfur species and reduce the
emissions of SOa because some of the
sulfur will be removed in the paniculate
collector as a solid. Figure 2 is a
schematic of a combined NOx/SOx
control system applied to a pulverized-
coal-fired power plant. Coal is fired in
low-NOx burners to minimize NOx
emissions, and SOx emissions are
reduced by a combination of in-furnace
capture and downstream cleanup. The
sorbent could be:
1. Mixed with the coal prior to the
pulverizer, or mixed with the coal
after .grinding and fed to the furnace
through the burner.
2. Mixed with one of the combustion air
streams (secondary or tertiary staged
air) and then injected into the
furnace.
The addition of sorbent will increase the
total solids loading in the furnace and
can also cause problems due to slagging
and fouling. In Figure 2, a dust collector
downstream of the air heater will
remove a large fraction of the sorbent
which will then be used in a wet or dry
contactor to further reduce flue gas S02
content.
Figure 3 places the results obtained to
date in perspective with other pilot-
scale studies. Data are compared from
the bench- and pilot-scale tests and
data obtained with Steinmuller in a
refractory tunnel furnace with an
approximate firing rate of 10 x 106
Btu/hr. The data shown in Figure 3 for
the BSF represents data obtained with
heat extraction in the radiant zone. This
figure shows similar captures for
various scales; however, it fails to
answer two basic questions: (1) What
factors will allow data generated at one
scale to be interpreted in terms of
another scale? and (2) Are these
captures possible in real systems?.
To Paniculate
Removal
Solid Disposal
Figure 2. Combined /VO«/SO« emission control for pulverized-coal-fired boilers.
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LWS
O Prototype Burner - Indiana Coal
O Prototype Burner - Utah Coal
O Steinmuller Burner - Indiana Coal
BSF
d Indiana Coal
IFRF (Steinmuller Burner)
A 7.03% 5 Coal
• 2.42% S Coal
80
70
*. 60
I
8. 50
I
ti 40
30
20
10
JL
72345
Molar Ratio Calcium/Sulfur
Figure 3. Summary of pilot- and
bench-scale data.
Since this is a progress report, it is
inappropriate to draw conclusions from
a study that is in its infancy. Data have
been obtained which show that calcium
utilization efficiencies of 25% are
possible if the thermal history of the
sorbent is controlled. However, the data
obtained to date are limited. The bench-
scale studies have not investigated
conditions which would allow reactions
involving HaS to become important. The
studies have been exclusively cause
and effect: no information has been
gathered which will explain the ob-
servables. Information is required on:
Sorbent particle temperature as a
function of time.
Sorbent residence time distribution in
the radiant furnace.
Sorbent reactivity as a function of
time.
Gas-phase sulfur speciation and
concentration (i.e., related to sul-
fur evolution from the coal).
The form of the calcium/sulfur solid
(sulfate, sulfide, or sulfide coated with
sulfate).
Decomposition of the sulfide or
sulfate producing SO2.
This type of information will be obtained
after the bench-scale screening studies
to determine the effect of fuel-rich
conditions, thermal environment, sor-
bent type, and coal type have been
defined.
Further work at pilot-scale must be
limited to screening studies which will
concentrate mainly on the impact of
thermal environment. The EPA burner
will be fired in a test tunnel which has a
much higher exit temperature and gives
total thermal histories which approxi-
mate those used in the IFRF studies and
in real boilers.
The preceding discussion does not
address the impact of sorbent addition
on boiler operability. Even though
sorbent injection could be used to
reduce SOx emissions, it would be an
unacceptable technology if it seriously
impaired boiler availability or increased
the cost of other pollution control
equipment. Thus, further work is needed
to assess the impact of sorbent addition
on: slagging, fouling rates and the
nature of the deposit (i.e., if it can be
removed by soot blowers), furnace exit
temperatures, and precipitator efficien-
cy.
Even though the results obtained to
date are preliminary, they do indicate
that the technology is sufficiently
promising to warrant more detailed
study to establish the precise conditions
under which it can be applied in
practice.
P. Case, M. Heap, J. Lee. C. McKinnon. R. Payne, and D. Pershing are with
Energy and Environmental Research Corp., Santa Ana, CA 92705.
Dennis C. Drehmel is the EPA Project Officer (see below).
The complete report, entitled "Use of Sorbents to Reduce S02 Emissions from
Pulverized-CoalFlames Under Low-NO* Conditions (Progress Report), "(Order
No. PB 83-131 045; Cost: $11.50; subject to change) 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:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
• U S. GOVERNMENT PRINTING OFFICE: 1983 659-O17/O888
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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