SEPA
United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-82-059 Dec. 1982
Project Summary
Application of LIMB to
Pulverized Coal Boilers
A Systems Analysis
Limestone Feed and
Boiler Systems
C. W. Arnold and R. C. Burt
The report gives results of a systems
analysis of the application of Lime-
stone Injection Multistaged Burner
(LIMB) technology to pulverized coal
boilers. This study was conducted as
part of a larger U. S. EPA program to
develop the LIMB technology. The
report evaluates alternative limestone
handling, preparation, and injection
methods and boiler system impacts
associated with LIMB applications.
Downstream emission control sys-
tems are not addressed.
LIMB simultaneously reduces sulfur
oxides (SOx) and nitrogen oxides (NOX)
emissions from pulverized coal boilers.
It is based on using low NO* combus-
tion techniques in combination with
dry limestone injection into the furnace
for simultaneous SOX control. Study
aims were to evaluate alternative
concepts for application of the tech-
nology, assess potential system prob-
lems related to its application, and
identify engineering solutions to those
problems. Further goals were to
identify information needs and recom-
mend studies to acquire this informa-
tion.
Conceivably, all new boilers could
be designed to handle any foreseeable
impacts associated with LIMB; how-
ever, the practicality of LIMB as a
retrofit technology depends on its
compatibility with existing boiler
systems. Sufficient information is not
yet available to accurately gauge its
applicability as a retrofit technology;
however, the study identified potential
system impacts, situations where the
impacts will likely be minimized, and
design or operating procedures for
dealing with them.
This Project Summary was devel-
oped by EPA 's Industrial Environ-
mental 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
EPA's LIMB program is an effort to
develop an effective but inexpensive
emission control technology for pul-
verized coal boilers that will simul-
taneously reduce sulfur oxides (SO,)
and nitrogen oxides (NO,) emissions.
The technology is based on using low
NO* combustion techniques in com-
bination with dry limestone injection
into the furnace for simultaneous SOX
control.
The need for effective low-cost tech-
niques for control of SO, and NOX
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emissions has become apparent in
recent years because of increased
public awareness of the problems
associated with acid rain. The long-
range transport and transformation of
SOx and NO, are believed to be major
factors contributing to acid rain forma-
tion. In addition, these pollutants also
degrade local air quality, present health
hazards, and participate in photo-
chemical smog formation. To address
these problems, effective low-cost
control techniques are needed that are
applicable to both new and existing
sources of these pollutants. The LIMB
program seeks to develop such a tech-
nique for pulverized coal boilers which
represent a major source of S0« and
NOx emissions.
The multiyear LIMB program, initi-
ated in fiscal year 1981, is based on the
results of earlier studies conducted on
low NOx combustion techniquesand dry
sorbent injection. The program is
planned to result in a complete LIMB
technology design package. To accom-
plish this objective EPA has initiated
research and development activities
including fundamental studies and
bench- and pilot-scale experiments.
Along with these research activities
EPA also initiated a systems analysis
task that includes this report. A major
objective of the analysis task is to
evaluate alternative concepts for ap-
plication of the technology to pulverized
coal boilers, to assess potential system
problems related to its application, and
to identify engineering solutions to
those problems. This includes analyses
of alternative limestone preparation and
injection schemes, boiler system im-
pacts associated with LIMB, and im-
pacts on downstream emission control
systems. (The current report does not in-
clude analyses of downstream control
systems.) Additional goals are to identify
information gaps related to system im-
pacts and to recommend studies or
experiments to acquire the needed in-
formation.
Project Analyses
The report gives detailed results of
several investigations/evaluations
carried out as part of the project.
To form a basis for later technology
evaluations, investigations were first
made of the physical and chemical
properties of coals commonly burned
in U. S. boilers and of the types of lime-
stones potentially available for use with
LIMB. These properties are summarized
and discussed in the report for a range
of coal and limestone types.
Investigations were also conducted
into the design and operation of various
types of raw material handling, prepara-
tion, and injection equipment. The
report describes how the equipment
design and operation are influenced by
such factors as raw material grindabil-
ity, hardness, moisture content, and
desired ultimate particle size. Based on
this information, evaluations were
made of several potential limestone
preparation and injection schemes and
their relative advantages and disad-
vantages were compared. The basic
schemes analyzed were:
(1) Crushed limestone and raw coal
mixed, pulverized together, and
injected through the burners.
(2) Ground limestone and raw coal
mixed, passed through the coal
pulverizers together, and injected
through the burners.
(3) Ground limestone pneumatically
mixed with pulverized coal and
primary air and injected through
the burners.
(4) Separate furnace injection of
ground limestone through com-
bustion air ports or other ports in
the furnace walls.
Also, several variations of these four
basic schemes were considered.
Investigations were also conducted
into the design and operation of various
types of pulverized coal boilers in use
today. The influence of such factors as
design coal type and boiler age on
various design variables (e.g., furnace
size, convection pass spacing, soot-
blower design, and ash removal system
design) is described in the report. Also
described are the likely influences of
LIMB on boiler operation, taking into
account such factors as coal type and
boiler design. This includes evaluations
of the impacts of LIMB on the boiler ash
loading and ash removal system opera-
tion, slagging and fouling tendencies,
and energy requirements. Critical
information needs and methods of
dealing with potential operating prob-
lems were also identified.
The analyses described above led to
several major conclusions and recom-
mendations.
Conclusions
Conceivably, all new boilers could be
designed to handle any foreseeable
impacts associated with LIMB; how-
ever, the practicality of LIMB as a retrofit
technology depends on its compatibility
with existing boiler systems. To be
compatible, technology application
should not require major boiler design
modifications or severe boiler operation
impairments. LIMB is still under develop-
ment so sufficient information is not yet
available to accurately gauge its ap-
plicability as a retrofit technology;
however, the study identified potential
system impacts associated with LIMB,
situations where these impacts will
likely be minimized, and design or oper-
ating procedures for dealing with
these impacts.
The major factors influencing the
compatibility of LIMB appear to be the
coal properties and designs of the boiler
furnace, convection section, and ash
removal systems. Depending on these
factors, potential problems arising from
LIMB applications include increased
furnace slagging, plugging of tight
convection section passes, overloading
or plugging of ash removal systems, and
incomplete coal combustion. If not
severe, these problems can be dealt
with through alterations in boiler
operating procedures or minor system
design modifications.
No major system problems are ex-
pected with application of limestone
handling, preparation, and injection
equipment other than potential space
limitations at some existing facilities.
Limestone feed systems will have to
handle limestone feed rates of about 10-
30% of the coal feed rate. Several
potential limestone preparation and
injection schemes are possible which
would allow injection through the
burners, secondary or tertiary air ports,
or other ports installed in the furnace
walls Any one of these schemes could
prove best for a particular application
depending on 562 control considera-
tions and the existing coal feed system
design. Not much data are available on
the dry grinding characteristics of
limestone, but measurements of lime-
stone properties should ensure ade-
quate limestone feed system design.
Applications where LIMB appears
most compatible with existing boiler
systems are those which fire low rank
coals. In these applications, the coal
properties and designs of the furnace,
convection section, and ash removal
system all appear well suited for LIMB.
However, system impacts will be
larger with bituminous coal units.
Bituminous coals generally have higher
sulfur levels than lower rank coals, and
hence require injection of larger quanti-
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ties of limestone. These larger injection
rates require larger limestone feed
system capacities, result in larger feed
system power requirements, increase
boiler thermal losses, and increase ash
loadings handled by the boiler and ash
removal systems. Also, limestone
injection in these units may increase
the tendencies for furnace slagging,
convection pass erosion, and plugging
of convection passes and ash removal
systems.
The potentially most critical problems
of those mentioned above concerning
the application of LIMB technology to
bituminous coal units result from
interactions between limestone and ash
particles. These interactions may result
in decreased ash fusion temperatures
which, together with the increased ash
loadings, may cause increased furnace
slagging and plugging of tight convec-
tion passes. This effect is not likely to
occur with low rank coals due to
differences in ash chemistry. Another
related problem is that many older
bituminous coal units remove econo-
mizer ash with wet sluicing systems.
With LIMB, this ash will contain large
quantities of calcium and sulfates.
When exposed to water it will likely ex-
hibit cementitious properties causing
plugging of sluicing systems. Wet sluic-
ing systems for economizer ash are not
typically used on units firing low rank
coals.
Thus it appears that LIMB will be most
easily retrofit on units firing low rank
coals. However, about 75% of the
existing boiler population on a total
capacity basis were designed to fire
bituminous coal. Further, bituminous
coal units account for an even larger
percentage of existing SOx emissions.
For these reasons, LIMB must be
applied to bituminous coal units in order
to have a large impact on existing SOX
emissions. In this study, some potential
problems were identified concerning
the application of LIMB to bituminous
coal units. Further evaluations are
needed to define the magnitudes of
these problems and to engineer solu-
tions to them.
Recommendations
Recommendations from this study
include: (1) research and development
activities that might be undertaken to
better understand and respond to
system impacts associated with the
application of LIMB technology; and (2)
additional engineering analyses which
would further define system impacts
and problems, engineering solutions to
those problems, and the compatibility
of LIMB with the existing boiler popula-
tion.
Research and Development
Needs
Several research needs related to the
limestone feed and boiler systems were
identified in this study: measurements
of limestone grinding characteristics
and of the impacts of LIMB on boiler
slagging and fouling tendencies, ash
entrainment, ash physical properties,
and convection section erosion.
The most critical research need is to
determine the effect of LIMB on slag-
ging and fouling, especially in bitumi-
nous coal units. Limestone injection tests
in some bituminous coal units have
resulted in severe deposit buildup
and plugging in the convection passes.
This buildup could result from a com-
bination of increased solids flow,
reduced ash fusion temperatures, and
tight tube spacmgs. Some reports from
these tests indicated that the deposits
were sticky at normal gas temperatures
but became friable when cooled, indi-
cating that the ash melting properties
were important.
Correlations of ash fusion tempera-
tures with composition and various
slagging indices are available to predict
the effects of increased calcium content
on ash slagging tendencies. Also, direct
laboratory measurements have been
made of the impacts of increased levels
of limestone on thefusion temperatures
of ash/limestone mixtures. However,
these types of tools or tests presume
100% interaction between the ash and
injected limestone particles. None of
them model the dynamics of limestone/
ash interactions in the furnace. The true
extent of interaction is unknown, but is
probably related to the conditions under
which the limestone and ash particles
come into contact
As a result, bench- or pilot-scale tests
should be considered in which lime-
stone particles are injected into a
furnace operated so that a variety of
peak flame temperatures and time/
temperature relationships occur. These
tests should include injection through
the burners and through ports located at
other furnace temperature zones. Air
cooled probes can be placed down-
stream in various temperature zones
reflective of radiant and convection
sections in order to measure ash
deposition rates. Also, various ash
strength and friability tests can be
conducted on the deposited samples.
Other research objectives should
include measurements of the influence
of LIMB on total ash entrainment and
ash physical properties since these
factors can influence ash removal
system design and operation. Tests
should include collection of ash sam-
ples from various points in the furnace
and convection section. The samples
should be analyzed for calcium content
and mixed with water to determine the
ash settling and cementitious properties,
both of which are especially important
in the operation of wet ash removal
systems (the degree of ash entrainment
determines the necessary ash removal
system design capacity). Entrainment
and physical properties should be
measured as functions of both injected
limestone particle size and combustion
gas flow rate.
Another useful test objective is to
determine the influence of LIMB on
erosion in the convection passes, but
this would require relatively long term
testing. Limestone injection will sig-
nificantly increase the total flue gas
solids loading. Also, limestone inerts
include those compounds thought to
contribute most to erosion (SiOz and
FeaOa). Testing would involve measure-
ment of the change in weight of metal
probe samples in the convection section
of a pilot unit.
Other research needs include studies
of limestone grinding characteristics.
As indicated in the report, simultaneous
grinding of coal and limestone may be a
desirable alternative at facilities with
excess pulverizer capacity. However,
the effects of simultaneous grinding are
relatively unknown. Tests might be
considered of simultaneous grinding for
ranges of coal and limestone types, inlet
limestone rock sizes, and pulverizer
designs. The test objective would be to
observe: the adequacy of mixing of coal
and limestone; the effects of limestone
addition rate on pulverizer capacity,
power requirements, and reliability; and
the effects of simultaneous grinding on
resulting coal and limestone particle
size distributions. A related research
need (which may prove valuable in
design of limestone feed systems) is to
develop a data base of Hardgrove
Grindability Index, hardness, and parti-
cle specific gravity values for a range of
commercially available limestone types.
Engineering Analysis Needs
Several areas have been identified in
which additional engineering analyses
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would be useful. These include ad-
ditional analyses of the boiler system,
analyses of downstream emission
control systems, and overview type
analyses of the system as a whole.
Important information for assessing
the usefulness of LIMB as a retrofit
technology includes the design profile
of the existing boiler population. Limited
material on this subject is presented in
the report; however, more specific data
are available. Specifically, computer
data bases containing substantial
information on utility boilers are availa-
ble. Conceivably, this information could
be cross-referenced with information
from available FGD system data bases
or survey reports to provide a fairly
complete picture of the existing boiler
population and the nationwide potential
for LIMB retrofit applications.
An additional worthwhile effort
would be to research the furnace
dimensions, plan areas, or heat release
rates of the existing boiler population.
This information would provide insight
about potential unit derating resulting
from retrofit of LIMB technology. Such
derating may be brought about by the
large flame volumes and/or burner
dimensions of the low NOX burner
techniques instrumental to the applica-
tion of LIMB technology.
For a complete systems analysis,
engineering evaluations are also needed
of the impact of LIMB on downstream
particulate control systems. Flue gas
solids loadings are greatly increased
and the particle characteristics are
altered by LIMB. Analyses should be
conducted to evaluate which particulate
control systems are best suited for LIMB
applications and to choose the best
methods for upgrading existing control
if U S. GOVERNMENT PRINTING OFFICE. 1982 659-O17/O876
systems in LIMB retrofit applications. A
related need is to evaluate alternatives
for recovering unused sorbent from the
fly ash and recycling it to the boiler.
Finally, overall system material and
energy balances and cost analyses
should be conducted for a number of
cases involving different coal types,
system sizes, limestone injection rates,
injection system designs, and particu-
late control system designs. These
analyses would serve to summarize
total system impacts resulting from
LIMB application and to highlight areas
of technological and cost benefits
associated with LIMB.
C. W. Arnold and R. C. Burt are with Radian Corporation, Durham, NC 27705.
James H. Abbott is the EPA Project Officer (see below).
The complete report, entitled "Application of LIMB to Pulverized Coal Boilers—
A Systems Analysis: Limestone Feed and Boiler Systems," (Order No. PB
83-114 553; Cost: $ 17.50, 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:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use S300
0000329
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