United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S8-81 -017 Dec. 1981
Project Summary
Limestone FGD Scrubbers:
User's Handbook
D. S. Henzel, B. A. Laseke, E. 0. Smith, and D. O. Swenson
Among the many available processes
for desulfurizing flue gas from utility
boilers, the limestone wet scrubbing
process is widely used and is gaining
increasing acceptance as technologi-
cal advances are developed. The Lime-
stone FGD Scrubbers: User's Hand-
book is intended for use by utility
project managers and project engineers.
It provides guidance in selecting,
installing, and operating a limestone
FGO system, covering all phases from
inception of the project through
design, procurement, operation, and
maintenance. The Handbook gives
detailed accounts of utility experience
with operational limestone scrubbing
systems. It also describes in detail
numerous optional process features
and recent innovative modifications
that enhance the efficiency of the
system.
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
Flue gas desulf urization (FGD) systems
are widely used by utilities to comply
with current limitations on emissions of
sulfur dioxide (SOz) from power plant
boilers. Among the available FGD
processes, wet limestone scrubbing has
met widespread acceptance: about 63
percent of the systems now operational,
under construction, or planned use
limestone as the absorption reagent.
Because substantial increases in FGD
usage are projected for the next several
years, there is need for an instructive
handbook that deals on a practical level
with the major aspects of limestone
scrubbing. The User's Handbook there-
fore is designed to provide guidance for
utility project managers, engineers, and
others involved in selecting, procuring,
installing, and operating a limestone
scrubbing system.
The Handbook is based in part on
current, well-proven engineering/
management practices and in part on
the records and experience of utilities
with operational limestone systems.
Since 1972 the operation of prototype
scrubbers at the EPA/TVA Shawnee
test facility has generated a large
amount of data on limestone scrubber
design, performance, operating param-
eters, and reliability. The Handbook
incorporates practical guidance based
on these data and on experience gained
at-the Shawnee facility.
Figure 1 shows an FGD project
coordination scheme, from inception of
the project through design, procurement,
operation, and maintenance of the
system. This scheme also provides the
basis for the structure of the Handbook.
The Basic Process and Optional
Process Features
The basic limestone process handles
boiler flue gas from which fly ash has
been removed by treatment in a particu-
late collection device such as an
electrostatic precipitator or a fabric
filter. The flue gas is brought into
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Assignment of Limestone FGD Project
Overall System Design
• Power Plant Considerations
• Design Basis
• Material and Energy Balances
• System Configuration Options
• Computerized Design Guides
Equipment Design
Scrubber Modules
Limestone Slurry Preparation
Liquid Flow Equipment
Flue Gas Flow Equipment
Sludge Processing Equipment
Process Control and Instrumentation
Procurement
• Prebid Considerations
• Preparation of Specifications
• Evaluation of Proposals
• Engineering Design, Installation,
Startup, and Testing
Operation and Maintenance
Standard Operations
Initial Operations
System Startup and Shutdown
System Upsets
Preventive Maintenance Programs
Unscheduled Maintenance
Figure 1. FGD project coordination
sequence.
contact with the limestone slurry in a
simple scrubber tower. Chemical re-
action of limestone with SOa in the flue
gas produces waste solids, which must
be removed continuously from the
scrubbing loop. These waste solids are
concentrated in a thickener and then
dewatered in a vacuum filter to produce
a filter "cake," which is mixed with fly
.ash. The resulting stabilized mixture is
then transported to a landfill. The
limestone scrubbing system is called a
"throwaway" process because the
product sludge is disposed of rather
than regenerated to recover sulfur.
The limestone FGD process has been
enhanced with the advent of recent
technology improvements, which provide
options to the basic limestone process.
The capabilities of these optional
process features are being verified by
current commercial experience. Among
these features are:
• Useofapresaturatortocoolthehot
flue gas.
• Installation of a venturi scrubber
ahead of the main scrubber module.
• Operation of a combination scrub-
ber (venturi/spray) with a charged
particulate scrubber.
• Placement of an additional tank in
the process flow, a "two-loop"
system.
• Operation of a two-loop system
with a single scrubber module.
• Forced oxidation of the scrubber
sludge.
• Use of chemical additives.
Although the utility management
staff does not perform detailed system
design, their informed decisions con-
cerning the system are prerequisite to
effective dealings with architectural/
engineering consultants and with FGO
system suppliers. The Handbook
presents a systematic approach to
managerial decision-making through
discussion of process and equipment
options.
Among the major design considera-
tions are those related to the power
plant: coal properties and supply, steam
generator design, power demand,
factors related to the power plant site,
and environmental regulations.
The primary factors serving as the
design basis are those that determine
the size and configuration of the FGD
system: flow rate, composition, pressure,
and temperature of the flue gas from the
boiler. These parameters, together with
the pollutant removal requirements,
reagent stoichiometric ratio, and com-
positions of the limestone and makeup
water, provide the basis for FGD system
design.
After determination of the major
design basis and material/energy
factors, several system configuration
options must be considered. Most
critical among these are the options for
(1) particulate removal (separate versus
integral), (2) location of the flue gas fan,
(3) flue gas bypass (versus no bypass),
(4) reheat (versus no reheat), and (5)
sludge disposal. Redundancy and system
flexibility, which are closely interrelated,
are needed to some extent in every
limestone FGD system. They are dis-
cussed as options because decisions
must be made as to the degree of
redundancy and flexibility to be incor-
porated into the system design.
Three major computer programs are
available for use in the planning and
operational stages of a limestone FGD
project. The TVA Lime/Limestone
Scrubbing Computer Model is structured
to generate a complete conceptual
design package for either a lime or
limestone scrubbing system, together
with a breakdown of cost requirements.
The PEDCo Flue Gas Desulfurization
Information System is an ongoing
survey of design, performance, and
process data on operational FGD systems
in the United States. The Bechtel-
Modified Radian Equilibrium program
gives simplified equations for calculation
of gypsum saturation levels as an index
of scaling potential.
The FGD System
A substantial portion of the Handbook
gives detailed descriptions of key
components of the FGD system including
materials of construction and process
control. Major emphasis is on equipment
items that most strongly affect the
operation and performance of a lime-
stone FGD system: the scrubber, mist
eliminator, reheater, and fans, together
with equipment used in slurry prepara-
tion and sludge treatment. Each of these
major equipment items is presented in a
similar format: first, a description of the
unit and its function, followed by
discussion of the basic equipment
types, including major design variations.
Then follows a review of the principal
design considerations, and finally a
brief survey of actual applications in
operational systems in the U.S.
Table 1, showing the basic types of
scrubbers available for use in a limestone
FGD system, exemplifies the Handbook's
treatment in discussions of the scrubber
and of individual components; it lists
generic and specific types and identifies
common trade names. These descrip-
tions are then, used throughout the
technical discussion of each component,
as exemplified in Table 2, which
concerns gas velocities through the
scrubber. Details of the key parameters
in operational and planned systems,
together with accounts of on-line
operational experience, provide a basis
for assessment and decision by the
project utility staff. •'
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Table. 1 Basic Scrubber Types for Commercial Limestone FGD Systems
Generic type
Specific type
Trade or common name
Venturi Variable-throat/bottom-entry
liquid distribution disk
Variable-throat/side-movable blades
Variable-throat/side-movable blocks
Variable-throat/vertically adjustable
rod decks
Variable-throat/adjustable drum
Spray Open/countercurrent spray
Open/crosscurrent spray
Tray Sieve tray
Packed Static bed
Mobile bed
Rod deck
Grid
Combination Spray/packed
Venturi/spray
Flooded-disk scrubber
Rod scrubber
Radial flow venturi
Vertical spray tower
Horizontal spray chamber
Marble-bed scrubber
Turbulent contact
absorber fTCAJ
Ventri-sorber
Table 2. Limestone Scrubber Design Gas Velocities
Gas velocity, ft/s*
Operational Planned
Scrubber type
Venturi
Variable-throat/side-movable
blocks
VariaNe-throat/vertically
adjustable rod decks
Spray
Open/countercurrent
Open/ crosscurrent
Tray
Sieve
Packed
Static bed
Mobile bed
Rod deck
Grid
Combination
Spray/packed
No.'
2
1
2
1
2
1
2
1
1
2
Range
90-130
80
10
22
10-15
31
13-15
13
12
7-10
Avg. /Vo.b Range Avg.
110
80
10 3 8-10 9
22 2 22 22
13
31
14
13
12
81 10 10
required capital investment and annual
operating costs. After selection of the
successful bidder, the utility project
management staff coordinates the
activities of suppliers and architectural/
engineering consultants in detailed
engineering design, fabrication, instal-
lation, startup, and testing.
Operation and Maintenance
Reliable operation of the limestone
scrubbing system is achieved by ad-
herence to a sound program of operation
and maintenance. The Handbook pro-
vides guidance for standard operating
practices, routine start-up and shut
down, and system upsets. It outline
procedures for preventive and erne1
gency maintenance and gives guideliru
on scrubber unit staffing.
'Multiply ft/s by 0.3048 to convert to m/s.
"The number of systems for which design scrubber L/G is known.
Procurement of the System
Procurement planning involves defi-
nition of the scope of supply, allocation
of procurement packages among poten-
tial bidders, and selection of bidders.
Specifications are then prepared in
three categories: (1) bidding require-
ments, which inform the bidders re-
garding the information required,
proposal format, number of copies, and
similar requirements; (2) contract
requirements, consisting of contract
regulations and provisions for bonds,
guarantees, and payments; and (3)
technical requirements, which constitute
a full and detailed statement of the
utility's requirements for chemical and
engineering parameters of the FGD
system.
Proposals are evaluated in terms of
technical, commercial, and economic
factors, the latter including details of
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D. S. Henzel and B. A. Laseke are with PEDCo Environmental. Inc.. Cincinnati.
OH. 45246; E. O. Smith andD. 0. Swenson are with Black and Veatch. Kansas
City. MO 64114.
Robert H. Borgwardt is the EPA Project Officer (see below).
The complete report, entitled "Limestone FGD Scrubbers: User's Handbook."
(Order No. PB 82-106 212; Cost: $37.50, 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
U.S. GOVERNMENT PRINTING OFFICE; 1981 — 599-017/7410
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 $300
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