EPA-450/3-75-027
March 1975
FIELD SURVEILLANCE
AND ENFORCEMENT GUIDE:
WOOD PULPING INDUSTRY
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Air and Waste Management
t
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina 27711
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EPA-450/3-75-027
FIELD SURVEILLANCE
AND ENFORCEMENT GUIDE:
WOOD PULPING INDUSTRY
Environmental Science and Engineering, Inc,
2324 SW 34th Street
Gainsville, Florida 32601
Contract No. 68-02-0618
EPA Project Officer: H.G.Richter
Prepared for
ENVIRONMENTAL PROTECTION AGENCY
Office of Air and Waste Management
Office of Air Quality Planning and Standards
Research Triangle Park, N. C. 27711
March 1975
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This report is issued by the Environmental Protection Agency to report .
technical data of interest to a limited number of readers. Copies are
available free of charge to Federal employees, current contractors
and grantees, and nonprofit organizationss - as supplies permit - from
the Air Pollution Technical Information Center, Environmental Protection
Agency, Research Triangle Park, North Carolina 27711; or, for a
fee, from the National Technical Information Service, 5285 Port Royal
Road, Springfield, Virginia 22161.
This report was furnished to the Environmental Protection Agency by
Environmental Science and Engineering, Inc. , Gainsville, Fla, in fulfill-
ment of Contract No. 68-02-0618. The contents of this report are reproduced.
herein as received from Environmental Science and Engineering, Inc.
The opinions, findings, and conclusions expressed are those of the author
and not necessarily those of the Environmental Protection Agency. Mention
of company or product names is not to be considered as an endorsement
by the Environmental Protection Agency.
Publication No. EPA-450/3-75-027
11
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TABLE OF CONTENTS
LIST OF FIGURES
LIST OF TABLES
CHAPTER 1. - INTRODUCTION
1.1 - OBJECTIVES AND SCOPE
1.2 - ROLE OF FIELD INSPECTION IN AN ENFORCEMENT PROGRAM
CHAPTER 2. - THE CHEMICAL WOOD PULPING INDUSTRY
2.1 - INTRODUCTION
2.2 - ECONOMIC POSITION
2.3 - PRESENT GEOGRAPHIC DISTRIBUTION
2.4 - FORECASTS
2.5 - REFERENCES
CHAPTER 3. - PROCESS DESCRIPTIONS
3.1 - DEFINITIONS
3.2 - THE KRAFT PROCESS
3.3 - DESCRIPTION OF THE NSSC PROCESS
3.4 - DESCRIPTION OF SULFITE PROCESS
3.5 - SULFITE RECOVERY SYSTEMS, PRESENT AND FUTURE
PROSPECTS
3.6 - SUMMARY OF AIR POLLUTION PROBLEMS
CHAPTER 4. - MONITORING PROCESS AND CONTROL EQUIPMENT VARIABLES
4.1 - KRAFT PROCESS INSTRUMENTATION
4.2 - INSTRUMENTATION FOR EMISSION MONITORING
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TABLE OF CONTENTS
^(Continued)
4.3 - CONTINUOUS SULFUR DIOXIDE MONITORS
4.4 - GAS/LIQUID CHROMATOGRAPHIC ANALYSIS OF REDUCED
SULFUR COMPOUNDS
4,5 - ELECTROLYTIC TITRATION OF SULFUR COMPOUNDS
4.6 - PARTICULATE MONITORS
4.7 - LITERATURE CITED
CHAPTER 5. - AIR POLLUTION CONTROL SYSTEMS SUMMARY
5.1 - INTRODUCTION
5.2 - GENERAL DESCRIPTION OF CONTROL EQUIPMENT
5.3 - POWER AND COMBINATION BOILERS
5.4 - SULFITE SOURCES
5.5 - NSSC SOURCES
5.6 - REFERENCES
CHAPTER 6. - FIELD ENFORCEMENT EQUIPMENT
6.1 - BASIC EQUIPMENT REQUIRED
6.2 - RECORDS AND FORMS REQUIRED
6.3 - PROCEDURES AND EQUIPMENT FOR SOURCE SAMPLING
CHAPTER 7. - UTILIZATION OF FIELD DATA AND RECORDS
7.1 - GENERAL INFORMATION
7.2 - FILE OF FIELD DATA
7.3 - ADDITIONAL SOURCE INFORMATION
7.4 - AGENCY RECORD KEEPING REQUIREMENTS
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. TABLE OF CONTENTS
(Continued)
CHAPTER 8. - SITE INSPECTION-ENFORCEMENT PROCEDURE
8.1 - GENERAL.
8.2 - OFF SITE OBSERVATIONS
8.3 - INTERVIEW WITH PLANT PERSONNEL
8.4 - IN-PLANT OBSERVATIONS
8.5 - CRITIQUE AND MEETING WITH PLANT MANAGEMENT
8.6 - SAFETY PRECAUTIONS
8.7 - SPECIAL PROCEDURES FOR KRAFT MILLS
8.8 - SPECIAL PROCEDURES FOR SULFITE MILLS
8.9 - SPECIAL PROCEDURES FOR NSSC MILLS
APPENDIX
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LIST OF FIGURES
FIGURE
2-1. REGIONAL DISTRIBUTION OF KRAFT PULP MILLS IN THE U.S.
2-2. REGIONAL DISTRIBUTION OF SULFITE AND NSSC PULP MILLS IN THE U.S.
2-3. PROJECTION OF PRODUCTION OF CHEMICAL PULPS IN THE U.S.
3-1. KRAFT PULPING AND RECOVERY PROCESS
3-2. TWO STAGE VENTURI EVAPORATION-SCRUBBING SYSTEM
3-3. CASCADE PRECIPITATOR SYSTEM
3-4. VENTURI EVAPORATOR-SCRUBBER SYSTEM
3-5. SINGLE STAGE VENTURI SCRUBBING SYSTEM
3-6. SECONDARY VENTURI SCRUBBING SYSTEM (FOLLOWING RECIPITATOR)
3-7. SECONDARY VENTURI SCRUBBING SYSTEM (FOLLOWING EXISTING VENTURI
SYSTEM)
3-8. COMPARISON OF C.E. AND B&W RECOVERY BOILERS
3-9. LAMINAIR AIR HEATER USED IN C.E. DESIGN
3-10. SCHEMATIC ARRANGEMENT OF THE COMMERCIAL OXIDATION SYSTEM
3-11. OXIDATION SYSTEM FOR STRONG BLACK LIQUOR
3-12. S-F VENTURI SCRUBBER
3-13. LIME KILN VENTURI SCRUBBER SYSTEM
3-13A. TOTAL REDUCED SULFUR VERSUS EXCESS OXYGEN
3-14. SCHEMATIC DIAGRAM OF NSSCa PROCESS WITH NO CHEMICAL RECOVERY
3-15. SCHEMATIC DIAGRAM OF CROSS RECOVERY SYSTEM FOR REGENERATING
NSSC COOKING CHEMICALS
3-16. FLOW SHEET OF TAMPELLA PROCESS
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LIST OF FIGURES
(Continued)
FIGURE
3-17. COPELAND RECOVERY SYSTEM
3-18. MEAD PROCESS SIMPLIFIED FLOW SHEET
3-19. INSTITUTE PROCESS SIMPLIFIED FLOW SHEET
3-20. WESTERN PRECIPITATION PROCESS SIMPLIFIED FLOW SHEET
3-21. SULFUR DIOXIDE REDUCTION WITH INCREASE IN Na/S RATIO
3-22. SULFITE PULPING FLOW SHEETS
3-23. EFFECTS OF AMMONIA-BASE LIQUOR SOLIDS CONTENT OF FLAME
TEMPERATURE
3-24. VISCOSITY OF AMMONIA AND MAGNESIUM LIQUORS
3-25. SECONDARY RECOVERY SYSTEM PERFORMANCE
3-26. SECONDARY RECOVERY SYSTEM PERFORMANCE
3-27. WEIGHT RATIO, NaOH:S02
3-28. JENSSEN EXHAUST SCRUBBER PROCESS FLOW
3-29. SULFITE PULPING PROCESS, MAGNESIUM BASE RECOVERY .
3-30. MAGNESIUM BISULFITE PROCESS FLOW
3-31. DIRECT OXIDATION RECOVERY
3-32. BISULFITE SULFITATION RECOVERY
3-33. FLUID BED PYROLYSIS RECOVERY
3-34. SOLID CARBONATION RECOVERY
3-35. GREEN LIQUOR CARBONATION RECOVERY
4-1. PICTORIAL INSTRUMENTATION SYMBOLS
4-2. BASIS INSTRUMENT DESIGN
4-3. INSTRUMENTATION FOR KRAFT BATCH DIGESTER
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LIST OF FIGURES
(Continued)
FIGURE
4-4. INSTRUMENTATION FOR A CONTINUOUS DIGESTER
4-5. HOT STOCK SCREENING INSTRUMENTATION
4-6. PULP WASHER INSTRUMENTATION
4-7. FOUR STAGE BLEACH PLANT.INSTRUMENTATION
4-8. SODIUM HYPOCHLORITE BLEACH LIQUOR PREPARATION INSTRUMENTATION
4-9. CHLORINE DIOXIDE BLEACH LIQUOR PREPARATION INSTRUMENTATION
4-10. INSTRUMENTATION FOR MULTIPLE-EFFECT EVAPORATOR SYSTEM
4-11. INSTRUMENTATION TO CONTROL COMBUSTION IN THE LIME RECOVERY
PROCESS
4-12. INSTRUMENTATION OF HIGH CAPACITY RECOVERY FURNACE
4-13. INSTRUMENTATION FOR LIQUOR CAUSTICIZING IN ALKALINE COOKING
4-14. GAS CHROMATOGRAPHY SYSTEM
4-15. PROBE FOR SOURCES CONTAINING ENTRAINED WATER
4-16. BATCH SAMPLING SYSTEM
4-17. CONTINUOUS SAMPLING SYSTEM
4-18. BARTON SAMPLING SYSTEM
4-19. DIAL READ OUT BOLOMETER
4-20. CONDUCTIVITY MONITOR
5-1. PRECIPITATOR FOR RECOVERY BOILER
5-2. VENTURI SCRUBBER WITH CYCLONIC SEPARATOR
5-3. CYCLONIC SCRUBBER
. 5-4. IMPINGEMENT SCRUBBER
5-5. PACKED TOWER SCRUBBER AND PACKINGS
5-6. LARGE DIAMETER CYCLONE COLLECTOR
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LIST OF FIGURES
(Continued)
FIGURE
. 5-7. MULTI-TUBE COLLECTOR
6-1. GAS SAMPLING SYSTEM
6-2. BARTON SAMPLING SYSTEM
7-1. ASEXTUPLE EFFECT EVAPORATOR
7-2. TYPICAL CONDITIONS FOR RECOVERY UNIT MATERIAL AND HEAT
BALANCE
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LIST OF TABLES
TABLE
2-1. SUMMARY CHEMICAL PULP MILL CAPACITIES
2-2. PRODUCTION OF TEN LEADING PULP PRODUCING NATIONS
2-3. ANNOUNCED AND ESTIMATED EXPANSION AND PHASING OUT PLANS
THROUGH 1980
3-1. SUMMARY OF MAJOR SOURCES AND TYPES OF CHEMICAL LOSSES
3-2. CHEMICAL EMISSIONS FROM THE RECOVERY FURNACE
3-3. OPERATING AND PERFORMANCE CHARACTERISTICS OF SYSTEMS
FOLLOWING RECOVERY FURNACE
3-3A. OPERATING PERFORMANCE OF SECONDARY SCRUBBING SYSTEMS .
3-4. LIME KILN EMISSIONS RELATED TO OPERATIONAL VARIABLES
3-5. EMISSIONS FROM NSSC PULPING
3-6. AMMONIA-BASE LIQUOR BURNING TESTS
3-7. RECOVERY UNIT PERFORMANCE
3-8. RECOVERY UNIT PERFORMANCE
3-9. JENSSEN TOWER GAS ANALYSIS
3-10. 'SUMMARY OF EMISSION DATA SULFITE PROCESS
4-1. KRAFT MILL INSTRUMENT SYMBOLS
4-2. LETTER INSTRUMENTATION SYMBOLS
5-1. TREATMENT EQUIPMENT CHARACTERISTICS
5-2. PARTICLE SIZE DISTRIBUTION OF FLY ASH FROM VARIOUS BOILERS
7-1. WOOD PULPING DIRECTORY EXAMPLE
7-2. OPERATING CONDITIONS IN A SEXTUPLE-EFFECT EVAPORATOR
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LIST OF TABLES
(continued)
TABLE
. 7-3. COMBUSTION OF BLACK LIQUOR
7-4. DUST COLLECTOR EFFICIENCY TEST
7-4A. CYCLONE EFFICIENCY VERSUS TUBE SIZE, AND SIZE DISTRIBUTION
OF BARK CHAR
7-5. POWER BOILER EMISSION DATA
7-5A. POWER BOILER EMISSION DATA (PARTICLE SIZE)
7-6. SUMMARY OF TESTS ON BARK BOILER COLLECTORS
7-7. VENTURI SCRUBBER SPECIFICATIONS
7-7A. VENTURI SCRUBBER EFFICIENCY TEST
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C H A P T E R 1
INTRODUCTION
OBJECTIVES AND SCOPE
This manual provides guidelines and background information for use
by personnel of state and local air pollution control agencies in
their surveillance and enforcement activities related to the major
types of chemical pulp mills. The three major types of mills dis-
cussed are Kraft, Sulfite, and Neutral.Sulfite Semi-Chemical (NSSC).
For each type of mill, the process is described; the emissions,.
both gaseous and particulate, are characterized; and the types of
applicable control equipment are delineated. Field enforcement
inspection, reporting and enforcement procedures to be followed in
. each type of mill by control agency personnel are suggested.
2 ROLE OF FIELD INSPECTION IN AN ENFORCEMENT PROGRAM
In any size control agency, it is necessary frequently to determine
whether sources are in compliance with applicable regulations.
This is the role of the field enforcement officer (FEO) in an air
pollution control program. It is his responsibility, also, to carry
out the functions of field inspection and enforcement. "
One of the most important aspects of a control agency's enforcement
activity is frequent and direct contact with the owners and opera-
tors of processes which emit air pollutants. Frequently, the FEO
is the person to whom this task falls.
The FEO also may be involved in a variety of assignments depending
on the size and organization of the agency and the regulations which
are involved. In all approved state implementation plans, a permit
or registration system is required. These programs assure that no
.new sources of air pollution are constructed, reconstructed, or
altered without the knowledge of the agency. They also have provi-
sions for the review of plans for construction or modification to
insure that a proposed system is capable of operating in compliance
with the regulations and makes it possible to develop and maintain
emission inventories.
To insure that operating sources continue to comply with air pollu-
tion regulations, implementation plans provide for the audit of all
sources periodically. The interval may range from once every six
months to once every five years, although the usual time period is
once each year. During these periodic checks, the FEO determines
that the source is in compliance with permit requirements, reviews
the emission data and inspects the site to insure good operating
and maintenance practices.
Surveillance of a source may be conducted in a number of ways. The
FEO may detect observable violations of the regulations from out-
side the source premises, by on-site inspections, and by emission
measurements. The FEO has primary responsibility for the success of
a source surveillance program even though he may not conduct the
monitoring himself.
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CHAPTER2
THE CHEMICAL HOOD PULPING INDUSTRY
2.1 INTRODUCTION
The pulp and paper industry is the ninth largest manufacturing indus-
try in the United States. This industry accounts for nearly four
percent of the value of all manufacturing.
Although paper is one of the oldest manufacturing industries, it
is an industry that is expanding faster than the general economy.
Consumption of paper rises with increased affluence. The per capita
consumption of paper in the U.S. is now over 550 pounds per year
compared to about 412 pounds only 12 years ago. There are no signs
that per capita consumption is leveling off.
The pulp and paper industry is comprised of three distinct segments:
(1) pulp, (2) primary paper and paperboard (cardboard, etc.), and
(3) converted, paper and paperboard products.
PulpMost pulp is made by integrated companies and consumed cap-
tively without moving through the marketplace. About ten percent
of the total pulp produced is, however, made by independent pulp
producers without their own paper making facilities or by integrated
companies producing surpluses for market. About three percent of
all pulp produced is consumed outside the industry for such products
as cellophane, rayon, cellulose esters and ethers, and their deri-
vatives. Eighty percent of the pulp used for making paper comes
from wood and 20 percent is made from waste paper or such fibers
as cotton and bagasse.
Primary Paper and Paperboard--This s.egment of the industry produces
paper, paperboard, and building paper and board. A portion of this
production is sold directly to industrial users such .as newspapers,
book publishers, and printers, or to building and other users.
Converted Paper and Paperboard ProductsAbout 70 percent of the
primary paper production is further processed by paper converters
into such products as containers, bags, sanitary tissue products,
and stationery.
Wood pulp is prepared either mechanically or chemically. In the
mechanical processes (groundwood, defibered and exploded) wood is
shredded or separated by physical means. In the chemical processes--
kraft, sulfite, neutral sulfite semi-chemical (NSSC), soda, and
dissolvingwood is treated with chemical reagents which form solu-
ble compounds with the noncellulosic materials, thus leaving resi-
dual cellulose. The NSSC process involves treating the wood first
with a mild chemical and then mechanically separating the fibers.
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Data are presented in Table 2-1 which show the number of mills and
total mill capacities for the five chemical pulping processes. In
Table 2-1, all capacities are based on air dried tons of pulp;
annual capacities are based on operating at rated capacity for 350
days per year, allowing for normal maintenance and scheduled shut-
downs. It is emphasized that these figures represent production
capability and do not portray actual production data.
Three of the chemical wood pulping processes display a potential
to cause air pollution. These are the kraft, sulfite, and NSSC
processes. Collectively, these three processes account for about
80 percent of the total wood pulp produced in the United States.
To define the air pollution problem posed by the chemical wood
pulping industry, it is necessary to establish the geographical
distribution of existing production capacity and to identify trends
which might cause a redistribution of production capacity in the
future.
2.2 ECONOMIC POSITION
The United States pulp and paper industry includes more than 400
pulp mills of all types, mechanical and chemical. Estimates for
1967 indicate 37 companies, each with pulp and paper sales at the
manufacturer's level of at least $100 million, accounted for $10.24
billion in sales, or 49 percent of the industry's total of $20.88
billion.
Table 2-2 shows the wood pulp production in 1971 for the ten lead-
ing pulp producing nations of the world (]_). The other 60 pulp
producing nations individually produced less than one million tons
of pulp and collectively produced 14,441,000 tons of pulp in 1971.
From these data, it can be determined that the U.S. and Canada
produced 52 percent of the world's wood pulp in 1971. This massive
capacity, coupled with the contiguous features of the U.S. and
Canada, place these countries in a leading position in terms of
production.
It is reported (2J that North American industry is planning to
build 65 new pulp mills in the 1970's--39 in the U.S. and 26 in
Canada. It seems reasonable to conclude, therefore, that the U.S.
and Canada will remain the dominant nations in wood pulp produc-
tion at least for the next two or three decades.
2.3 PRESENT GEOGRAPHIC DISTRIBUTION
A compilation of data on current wood pulping practice in the U.S.
was made by searching available published reports, such as Lock-
wood 's Directory of the Paper and Allied Trades, Post's Pulp and
Paper Directory, and Southern Pulp and Paper Manufacturer's Southern
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TABLE 2-1
SUMMARY - U.S.A.
CHEMICAL PULP MILL CAPACITIES (UNBLEACHED)
AS OF DECEMBER 31, 1971
Process
Kraft
Sulfite
NSSC
Dissolving
Soda
TOTALS
Number
of
Mills
128
44
48
9
4
233
*
Capacity
ADT/Day
93,600
6,650
11,300
5,100
920
117,570
Annual*
Capacity
Tons
32,800,000
2,324,000
3,950,000
1,780,000
322,000
41,176,000
1972
Production
Tons
.30,250,000
2,150,000
3,650,000
1,650,000
300,000
38,000,000
These figures represent capacity and not actual production. ADT stands
for air-dried tons of unbleached pulp per day; air-dried pulp contains
10 percent moisture.
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TABLE 2-2
PRODUCTION OF TEN LEADING
PULP PRODUCING NATIONS - 1971
Nation
United States
Canada
Sweden
Japan
Finland
USSR
Norway
Mainland China
France
West Germany
Million Short Tons
45.20
20.70
9.20
9.00
7.72
6.78
2.59
2.30
1.77
1.73
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Mill Directory. Information originally tabulated included plant
location, owner, pulping process employed, rated mill capacity,
type of wood pulped, and age of original mill. These data were
brought up to date based on in-house information, available NAPCA -
NCASI reports, and communications with industry representatives.
Using these data as a base, two maps have been prepared to illus-
trate geographically the distribution of chemical wood pulping mills
throughout the United States. These maps are presented here as
Figures 2-1 and 2-2.
2.4 FORECASTS
2.4.1 6ROHTH AND PROCESS TRENDS
The United States and Canada produce more than 52 percent of the
world's supply of pulp, with the U.S. in 1972 furnishing nearly 46
million short tons. Of this amount, 38 million tons were chemical
pulp. Approximately 75 percent of this was produced by the kraft
process, nine percent by the sulfite, and ten percent by neutral
sulfite semichemical (NSSC). .
A number of forecasts have been made which attempt to portray the
. future demand for wood pulp (all grades) in the United States.
These forecasts range from a low of 61 million tons per year in
1985 as given by Forest Research Report No. 17, 1965 (3), to a
high of 89 million tons per year in 1985 as given by Resources in
America, 1963 (4_). A middle of the road forecast has been made by
the American Paper Institute. Based on these data, plus numerous
other sources, H. W. Meakin of the J. E. Sirrine Company has pro-
jected chemical pulp production through 1985. These projections
are reproduced here as Figure 2-3.
Viewed together, these data show that through 1985, the production
of soda pulp and dissolving pulps will remain reasonably constant;
sulfite pulp production will decrease slightly; NSSC production will
nearly double, and kraft pulp will increase to approximately 2-1/2
times the 1968 amount.
In 1985, kraft and NSSC processes are expected to dominate chemical
pulping in the United States. Kraft production is projected to
account for-about 85 percent of the chemical (about 70 percent of
total wood pulp, all grades, production), and NSSC for about 9 per-
cent of the total chemical pulp production. The total production
of chemical pulp is expected to slightly more than double. Regional
distribution of pulping capacity is expected to remain in the same
relative proportions as it is today.
Table 2-3 has been included to summarize announced and estimated
expansion and phasing-out operations through 1980.
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REGION I
REGION n
REGION m
REGION IV
REGION I
REGIONAL DISTRIBUTION
OF KRAFT PULP MILLS
IN THE U. S.
FIGU";
-------
NSSC MILLS
O- 200 TPO
200 - 300 TPD
>. - 300 TPD
SULFITE MILLS
A 0- 100 TPD
A 100-200 TPD
200-400 TPD
> -400 TPD
REGION I
REGION H
REGION ffl
REGION IS
REGION Z
REGIONAL DISTRIBUTION
OF SULFITE AND NSSC PULP MILLS
IN THE U. S.
2-8
FIGURE 2-2
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FIGURE 2-3
PROJECTION OF PRODUCTION
OF CHEMICAL PULPS-IN THE U. S.
TOTAL
KRAFT
o --"-t-
1960
1965
1970
1980
_-*NSSC
ISULFITE
r> DISSOLVING
1985
YEAR
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Detailed breakdowns of Table 2-3 may be found in Appendix A
(Tables A-4, A-5, A-6, and A-7).
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T A B L E 2-3
1 ANNOUNCED AND ESTIMATED EXPANSION
AND PHASING OUT PLANS THROUGH 1980
I. Current and Planned New Plant Construction as of December 31, 197P '
. CAPACITY APT/DAY
Kraft Sulfite NSSC
New
Expansion
5,866 (12)
2,135 (5)
830 (2)
0
750 (3)
568 (2)
TOTAL 8,001 (17) 830 (2) 1,318 (5)
II. Estimate of Phased Out Operations
CAPACITY ADT/DAY
Time Period
In 1968
In 1969-70
In 1970-80
TOTAL
(b) Figures in ( ) indicate number of mills
Kraft
205 (1)
85 (1)
209 (2)
Sulfite
835 (5)
503 (3)
1,562 (17)
2,900 (25)
NSSC
235 (1)
235 (1)
Soda
60 (1)
140 (2)
200 (3)
In addition to the current and planned new plant construction
shown above, there are at least twelve proposed or tentative
mills in the talking stage of development. These twelve mills
would, if brought to production, supply in excess of an addi-
tional 3,000 tons per day of pulp.
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2.5 REFERENCES
1. Staff, "19th Annual Horld Review," Pulp and Paper. 43^(7),
7- , .(June 25, 1969). .
2. Staff, "Expansion/Modernization/Acquisition Report," Pulp
iand Paper. 42^(51), 43- , (December 16, 1968).
3. "Timber Trends in the United States," Forest Service Report
No. 17, U.S. Department of Agriculture, Washington, 1965.
4. "Resources in America's Future," Landsberg, Fishman and
Fisher, -New York, 1963.
5. Josephson, H.R. (Director, Division of Forest Economics
and Marketing Research, Forest Service, USDA), "Availability
of Wood Supplies for the Pulp and Paper Industry," Paper.
presented at 1968 Annual Meeting of Pulp and Raw Materials
Division of American Paper Institute, New York, February 20,
1968.
6. Slatin, Benjamin (Economist, API), "Future Demands for Pulp
and Paper as They Influence Pulp Wood Requirements," Paper
presented at fall meeting of the Southeastern Technical
Division of the American Pulp Wood Association, Atlanta,
November 21, 1968.
7.. Staff, "U.S. New Capacity Additions Will be Modest Through
1975," Paper. Trade Journal, Vol. 156, No. 46, pp. 50-54,
November 6, 1972.
8. Slatin, B., "The Paper Industry in the South: Running Full,"
Southern Pulp and Paper Manufacturer, Vol. 35, No. .10, pp.
11-16, October 1, 1972.
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CHAPTER 3
PROCESS DESCRIPTIONS
3.1 DEFINITIONS
The following definitions are provided for the benefit of those
not intimately familiar with the chemical wood pulping industry.
In some instances definitions are given because of special
meanings which a term may have in this report.
Particles - Any material which exists as a solid in a gas steam
at duct conditions and is collected in accordance with EPA sampling
procedures.
Trace - A quantitative expression of emissions which is less than
0.01 pounds per ton of air dry unbleached pulp.
Standard Conditions - 29.92 inches of mercury and 70 degrees F.
Sulfidity - An expression of the percentage makeup of chemicals in
kraft cooking liquor obtained by the formula
Na2S + NaOH X 10°
where the sodium compounds are expressed as Na20.
Yield - The percentage of a specific weight of bone dry wood that is
converted to bone dry pulp.
Weak Hash - A liquid stream in the kraft process which results from
washing of the lime mud. It is used mainly for dissolving smelt.
Smelt - The molten chemicals from the kraft recovery furnace con-
sisting mostly of sodium sulfide and sodium carbonate.
Oxidation Efficiency - The percentage of sodium sulfide in the kraft
black liquor which is oxidized by air introduced into the liquor.
The Na2S is usually expressed in grams per liter of black liquor.
Roundwood - Logs as delivered to the mill with bark attached and cut
to specified lengths (up to 10 feet).
Board - A heavy sheet made with single or multiple plies of pulp
formed on a board machine such as a fourdrinier.
Linerboard - A laminated container board usually made of kraft pulp.
It consists of a base sheet which is coarse strong pulp and a top
liner sheet which is fine pulp. The top liner gives the container
board a more finished exposed surface than the base sheet.
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Top Liner - A sheet, usually produced from kraft pulp, which is
added as a laminate to the base sheet to produce linerboard. The
pulp may in some cases be bleached.
Base Stock - A sheet, usually produced from unbleached kraft pulp,
formed into linerboard on a fourdrinier machine.
Corrugating Medium - A sheet, usually made from NSSC pulp, which is
corrugated to form a cushioning layer when attached to a single sheet
or between two boards. The corrugating sheet is usually 0.009 inches
thick and traditionally is referred to as "9 point."
Bark Boiler - A combustion unit used to produce steam for process
or electrical energy which is designed to burn mainly bark and wood
residues.
Combination Boiler - A combustion unit used to produce a steam for
process or electrical energy which is designed to burn bark and at
least one other fuel.
Power Boiler - A combustion unit used to produce steam for process
or electrical energy which is designed to burn oil, coal, or gas.
Recovery Boiler - A combustion unit used to produce steam for pulping
and recovery operations, and to recover the chemicals from the spent
cooking liquor.
3.2 THE KRAFT PROCESS
The kraft process produces a dark colored fiber. Therefore, the
market for the unbleached pulp is usually limited to its use in board,
wrapping, and bag papers. If kraft pulp is to be used in the manu-
facture of fine white papers, its fibers must be treated additionally
in a bleach plant.
The presence of caustic soda in the cooking liquor permits the pulp-
ing of practically all wood species. The other active chemical,
sodium sulfide, creates a chemical reaction during cooking that im-
parts the strength characteristics to kraft fibers, producing fibers
that are stronger than those made from NSSC or sulfite processes.
Small amounts of sodium sulfide react with lignin and carbohydrates
in the wood to form odorous compounds which may cause a reduction
of air quality.
In the kraft pulping process, shown in Figure 3-1, the white liquor
--a solution of sodium sulfide and sodium hydroxide in wateris
utilized to dissolve the lignin from wood and render cellulose fibers
usable for pulp manufacture. The white liquor is mixed with wood
chips in a digester where the chips are cooked for about three hours
with 110 psig steam. During the cooking period, the digester is
-14-
-------
Steam
1
© ©
t
Recovery
Boiler
Smelt 1
60-70%
Evaporator
BL
i
- 1
Oxida-
tion
C.,_4-_
To Stack
*
-i I
t !_
Weak Wash
Dissolving
Tank
1
Scrubber
System
©
Green Liquor
Causti-
cizing
White Liquor
©
I
To Stack
|Lime Kiln
CaO
Ca(OH),
Scrubber
Slaking
System
7
To
ta
Lj
Scrubber 1
J *
m
H
L_, 45-55% BL
Chips Steam
-xl 1 r*
It 1 !
Digester
Pulp f
20 and \
~a rLiquor
Washer
M.E. Evaporator
\G
vl
Conden-
sate
5a)
Weak
Liquor
)
*
Oxi-
dizer
^
*
1
)xidation
System
}
i
k
Ox
We
.i<
Condensate
Mud Washer
Pulp
Air
*Refer to Table 3-1 for
explanations.
_____ _>v Indicates
Gas Streams
FIGURE 3-1. KRAFT PULPING AND RECOVERY PROCESS
-------
relieved periodically to reduce the pressure buildup of various gases
generated within. When cooking is complete, the digester contents are
forced into a blow tank where a major portion of the spent cooking
liquor containing the dissolved lignin is drained. The pulp passes
through the knotter, goes through various stages of washing and
bleaching, and is then pressed and dried into the finished product.
The kraft recovery process essentially consists of the following major
operations:
1. Concentration of weak black liquor from approximately
17 to 50 percent solids, in multiple effect evaporators.
2. Further concentration to 65 to 70 percent solids in an
additional effect known as a concentrator or using the
heat content of the recovery furnace flue gases in a
direct contact evaporator.
3. .The burning of the combustible concentrated black liquor
in the recovery furnace to recover a portion of the heat
by oxidation of the dissolved lignin and to recover the
inorganic chemicals in the form of molten smelt.
4. The preparation of green liquor by dissolving the smelt
(a mixture of sodium sulfide and sodium carbonate) with
water and weak wash from the causticizing plant.
5. The preparation of white liquor in a causticizer where
the sodium carbonate (from green liquor) is converted to
sodium hydroxide by the addition of calcium hydroxide.
6. The conversion of the precipitated calcium carbonate
to calcium hydroxide for reuse in the causticizer by
burning the calcium carbonate in a lime kiln and then
slaking the product calcium oxide.
In the kraft process, chemicals in the form of solids, mists, odorous
and nonodorous gases may be emitted into the atmosphere, as summarized
in Table 3-1. The recovery furnace potentially is the largest source
of emission of solids and sulfur bearing gases. Digester relief gases,
blow gases, and off gases from the evaporators represent a considerably
smaller volume, but have a potentially higher nuisance value. Not only
are these chemical losses costly, but they are air pollutants as well.
The recovery furnace, multiple effect evaporator, lime kiln, and blow
gases are the major source of heat loss.
In the following sections, the sources of emission, the theoretical
explanation for the emissions, and the various processes and equipment
used to reduce the chemicals and heat losses will be discussed in
greater detail.
-16-
-------
TABLE 3-1
SUMMARY OF MAJOR SOURCES AND
TYPES OF CHEMICAL LOSSES
Source
1. Recovery Furnace
2. Direct contact
evaporator
3. Lime kiln
4. Dissolving tank
5. Multiple effect
evaporator
6. Digester
Type of Chemical Loss
Odorous and
Solids
Sodium carbonate
Sodium sulfate
Carbon
Same as recovery
furnace
Lime dust
Soda dust
Sodium carbonate
Sodium sulfate
None
None
7. Blow Tank
None
Nonodorous Gases
Carbon monoxide
Carbon dioxide
Sulfur dioxide
Hydrogen sulfide
Methyl mercaptan
Dimethyl sulfide
Dimethyl disulfide
Same as digester
Sulfur dioxide
Hydrogen sulfide
Hydrogen sulfide
Same as digester
Sulfur dioxide
Hydrogen sulfide
Methyl mercaptan
Dimethyl sulfide
Dimethyl disulfide
Same as digester
-17-
-------
TABLE 3-2
CHEMICAL EMISSIONS FROM THE RECOVERY FURNACE
Chemical
Sodium salts
Na2SO, Na2C03
Hydrogen
sulfide
Methyl
mercaptan
Dimethyl
sulfide
Sulfur
dioxide
Emission
4.5 -
SCF
900 -
200 -
60 - 1
300 -
6.0 grains/
dry
1000 ppm
1000 ppm
50 ppm
500 ppm
Ib/ton
of pulp
150 - 200
25 - 28
8 - 40
3 - 7.5
25 - 40
Ib/day*
75,000 -
100,000
12,500 -
14,000
4,000 -
20,000
1,500 - 3,750
12,500 -
20,000
*
For 500 tons/day pulp mill
-18-
-------
3.2.1 DIGESTER AND BLOW TANK
During the cooking period, the digester is relieved periodically to
reduce the pressure buildup of various gases within. These relief
gases essentially contain steam, hydrogen sulfide, methyl mercaptan,
dimethyl sulfide, and dimethyl disulfide. When cooking is completed,
the digester contents are forced into the blow tank where a major
portion of the spent cooking liquor containing dissolved lignin is
drained. Gases similar in composition to the digester relief
gases are released.
The formation of mercaptan and dimethyl sulfides in the digester re-
sult from reactions between sulfide and hydrosulfide ions and methoxyl
group of lignin in the wood being pulped. During the digester relief
when the pressure is suddenly released, mercaptan and sulfides will
be carried off with the steam. Hydrogen sulfide will also be released
due to a stripping reaction.
The quantity of dimethyl sulfide formed depends on the quantity of
methyl mercaptide ion present. In long cooks at high temperature and
sulfidity, the amount of dimethyl sulfide may exceed the methyl mer-
captan. The quantity of mercaptan and sulfide formed is directly
proportional to the digestion temperature and the sodium sulfide con-
centration in the cooking liquor. The quantity of mercaptan and sul-
fide is higher when pulping hardwoods than softwoods.
Various sulfur compounds of digester relief in Ibs/ton of pulp include:
Hydrogen sulfide, 0.01 to 0.05; Methyl mercaptan, 0.02 to 0.04; Di-
methyl sulfide, 0.5_to_1.0; and Dimethyl disulfide, 0.1 to 0.51 Blow
tank Ibs/ton of pulp figures for the same gases are: Hydrogen sulfide,
0.1 to 0.5; Methyl mercaptan, 0.5 to 1.5; Dimethyl sulfide, 1.0 to 1.5;
and Dimethyl disulfide, 0.5 to 2.0.
Various processes used in the industry to reduce and recover the sulfur
loss and the odor include:
1. Oxidation of the noncondensible gases by burning in the
recovery furnace, lime kiln, or other suitable devices.
2. Absorption by suitable alkaline solution such as white
liquor using a venturi or packed tower type absorber.
3. Destruction of sulfur compounds by chlorination.
As the batch digester is relieved intermittently to avoid surging condi-
tions, some mills collect the.gases in a suitable container and then feed
continuously to either a lime kiln or a recovery furnace. When a con-
tinuous digester is used, the surges are largely eliminated and no con-
tainment vessel is required. When burning these sulfur bearing gases
at or above 1,400°F, complete combustion occurs of the organic sulfides
and hydrogen sulfide to sulfur dioxide, carbon dioxide and water vapor.
Since the firing end of the lime kiln operates at a temperature above
-19-
-------
2,000°F, combustion of these gases in the lime kiln is most suitable.
The sulfur dioxide generated will be efficiently absorbed in the venturi
type lime kiln scrubber.
Chlorination, followed by caustic absorption, is a less effective means
for treating the noncondensible sulfur compounds. Chlorine, being a
powerful oxidizing agent, readily oxidizes organic sulfides and hydrogen
sulfide to free sulfur, sulfonyl or sulfoxy compounds. The gases leaving
the chlorination stage are then absorbed in a caustic solution.
These methods of treatment effectively eliminate the odor caused by
sulfur bearing gases. Where combustion is practiced, a suitable scrubber
will efficiently recover the sulfur compounds formed as a result of
combustion products.
Where wood with a turpentine content is cooked, there is a loss of tur-
pentine whenever a digester is relieved. The resinous materials in
certain pulpwoods, particularly the southern pines, give off turpentine
vapor. The heat and turpentine are recovered simultaneously by con-
densing the. digester relief gases in a condenser.
-20-
-------
3.2.2 BROWN-STOCK WASHING
Two types of washing equipment used for removal of black liquor are
the diffuser and the rotary-drum vacuum filter. In comparing the
advantages of the vacuum washer and diffuser systems, some pulp mill
operators consider that the time involved in the use of diffusers is
a distinct advantage not present with vacuum washers in that the
longer period of contact with wash water permits removal of soda from
the pulp by leaching or diffusion.
The rotary-drum vacuum filter arranged'for multistage countercurrent
washing has become standard equipment in most of the pulp mills in
the United States and Canada. Diffusers were generally used until
about 1935; previously, vacuum washers had been considered unsatis-
factory for washing pulp from resinous woods because of the serious
foaming conditions which resulted. Foaming trouble was overcome by
improvement in filter design, which prevented air from becoming en-
trained with the liquor.
For some time there have been no installations of diffusers, adoption
of the rotary washer being general. Two, three, or four washers are
used in series for multistage countercurrent washing, which reduces
the amount of wash water required for the desired pulp cleanliness.
The rotary vacuum filter with multiple port valves has been designed
for two and even more stages of washing on the same drum. These sys-
tems are more complex and require close and accurate control owing
to the splitting of the filtrate and the time element required for
the liquor to pass through the sheet and reach the valve. These
washers have met with success where the throughput is maintained within
an extremely limited range.
3.2.2.1 The Washing Operation
A typical brown-stock operation is made up of a series of rotary-drum
vacuum filters with intermediate repulpers for redilution and agita-
tion of the pulp sheet. Almost all the systems in the United States
use a single liquor displacement on each washer. Although the wash-
ing system and its operation may be complex, the fundamentals are as
simple as those of the age-old washing of clothes. When a limited
amount of water is available, washing is best accomplished by scrub-
bing the clothes in part of the water, wringing thoroughly, and re-
peating several times. In pulp washing, this is the essential process,
except that it is on a continuous basis, with a small amount of water
added in the final stage and a like amount flowing countercurrent to
the middle stage, or stages, and then to the first stage. The liquid
entering the system with the blow and the amount of water entering at
the last stage minus the water in the sheet then carry the soluble
solids out of the first stage to the evaporating system, and the washed
pulp is discharged from the final washer,
-21-
-------
The vacuum filter is excellent for this soluble solids reduction be-
cause it allows the liquid entering each stage to be displaced through
the thin mat of stock as it travels over the exposed area of the fil-
ter. A uniform application of wash liquor will achieve a higher degree
of displacement of the vat liquor in the sheet and result in a higher
displacement ratio. Although the displacement of wash liquid in the
sheet is important, essential work is done in the intermediate repulp-
ers between the stages where the thickened sheet of pulp is diluted
and scrubbed with the weaker liquor of the subsequent stage. Here
fibers are agitated in as low a consistency of slurry as possible in
order to achieve the minimum concentration of dissolved solids prior
to the extraction on the following vacuum filter. Low vat consistency
promotes diffusion of the strong liquor at the fiber wall.
Emissions from brown stock washers consist of H2S and TRS ranging
from 0.01 to 0.08 and 0.12 to 0.45 Ibs/air dry ton respectively. Con-
trol methods consists of those alternatives employed for other relief
and non-condensable gases.
-22-
-------
3.2.3 BLEACH PLANT
Bleaching of chemical pulps is, in a sense, the continuation of diges-
tion. The placement of the dividing line is sometimes a matter of
relative costs. Thus, a pulp mill with high wood costs but cheap
power and chemicals (some of which may be of mill manufacture) may
decide within limits-- to end the cooks while the pulp retains a
relatively high portion of lignin and to complete the delignification
with the more selective bleaching chemicals as a means of achieving
higher yield. A pulp mill with low wood costs and expensive bleach-
ing chemicals will sacrifice some pulp yield and cook to the lowest
Kappa number which still gives the desired pulp qualities. Whatever
the situation, digestion must be properly and uniformly conducted,
and in most cases the aim is to remove in cooking as much of the
lignin as feasible.
Our commercial cooking processes, most of whicTTproceed in a reducing
system at rather high temperature, cannot be pushed to complete delig-
nification without serious damage to the fibers. All the lignin can,
however, be eliminated by some oxidizers (as for instance chlorine
dioxide), but reaction time is long and costs are high and even in
this case there is always the danger of carbohydrate degradation.
Thus, the production of white wood fibers depends on a combination
of cooking and bleaching.
3.2.3.1 Basic Principles
The discovery of chlorine by Scheele in 1774 and the subsequent pre-
paration of calcium hypochlorite by Tennant initiated the use of
chemicals in the bleaching of cellulose fibers. Initially, equipment
and process were rather crude, and it was only about 150 years later
that a measure of control was established and reproducible results
were obtained. During all this time the mills endeavored to do on
chemical pulps with one bleaching agent (calcium hypochlorite) a job
which, as we know today, cannot be done efficiently ine one operation.
There is at present no single chemical substance which answers all
the requirements of a commercial pulp-bleaching agent, so that it is
necessary to resort to a number of chemicals, which in turn recommends
the division of the process into a series of separate treatments.
Experience gained and studies made since about 1925 furnish a good
foundation for the chemical bleaching of cellulose, and although many
details are still obscure, enough knowledge has been developed to
build bleach plants which yield the desired product.
A bleaching stage normally consists of equipment to bring the stock
to the desired consistency, a mixer that will blend chemical, fibers,
and steam, a retention vessel wherein the reaction may proceed, and
a washer for the separation of the treated fibers from the waste pro-
ducts. A series of such stages is called a bleaching sequence.
-23-
-------
Unfortunately, none of the chemicals at our disposal is absolutely
specific, which means that in every case there is some unwanted action
upon the pulp fibers. It is, therefore, necessary to find out how
much of the respective agent can be efficiently applied, and experi-
ence has confirmed the expectation that smaller amounts are better
than larger ones. On that basis, one would conclude that the more
stages used the better the results. This is quite true, as long as
each stage provides sufficient reaction time for the lenient condi-
tions then existing. However, it will be found that capital, main-
tenance costs, and operating costs put a low ceiling on the number
of stages, and a compromise must be made between academic goals and
practicality.
Elemental chlorine is universally used for lignin fragmentation. It
measures up well to the above requirements. Its specificity for
lignin is excellent, and the reaction proceeds so fast and completely
that any attack upon the carbohydrates can be prevented. Chlorine
dioxide is also suitable in some respectsand is perhaps even su-
periorso that it may be used in substitution if its present cost
differential with chlorine can be reduced.
The products of pulp chlorination and of the oxidative bleaching
stages are more soluble in an alkaline medium than in water, and
they are generally taken out of the system by an alkaline extraction.
For good efficiency, temperature and pH of these extractions must be
high, the latter at least above 10 and frequently above 11. Such
high values are not, however, compatible with those of the bleaching
reactions, so that separate extraction stages are recommended for
all bleaching agents except peroxides. In many cases these extrac-
tion stages also serve the removal of resin and hemicelluloses.
Caustic soda is the preferred agent, but other alkalis have been used.
-24-,
-------
3.2.4 MULTIPLE EFFECT EVAPORATORS
In a multiple effect evaporator, black liquor is concentrated from 16%
solids to about 50 to 55% solids in several successive stages. The
dilute black liquor that enters the evaporator contains some methyl
mercaptan dissolved in the alkaline solution. Douglass and co-workers
(16) reported that about 0.38 pounds of methyl mercaptan per ton of
pulp production and only a negligible quantity of dimethyl sulfide
enter the evaporator. The concentrated black liquor contains less
residual mercaptan than the amount that entered with weak liquor.
This means most of the original mercaptan is stripped off daring evap*
oration. During the final evaporation stage, there is appreciable
formation of methyl mercaptan. This may be a result of the high con-
centration of chemical reactants and the high temperature that exists
in the final evaporation stage. A small quantity of dimethyl sulfide
is also formed. Douglass (16) concluded that most of the mercaptan
found in the vapor and condensate from the multiple effect evaporator
is the same as that present in the black liquor entering the evapora-
tor.
Hydrogen sulfide and methyl mercaptan are the major constituents found
in the noncondensible gases and the condensate leaving the evaporators.
The hydrogen sulfide emission may vary between 1 to 3 Ibs/ton of pulp ,
whereas the mercaptan emission may be as high as 0.5 Ibs/ton of pulp.
The condensate also carries the same odorous compounds.
3.2.4.1 Effect of Oxidation
Weak black liquor oxidation has been found effective in significantly
reducing the release of odorous compounds. Almost 90% reduction can
be attained with essentially 100% oxidation efficiency. In recent
findings, Shah and Stephenson (8) reported that condensate from the
multiple effect evaporators, including both the condensate from the
liquor vapors and the jet condensers, had very little noticeable odor.
The biochemical oxygen demand was reduced by almost 28% and the pH .
was alkaline. As a result, the condensate is suitable for reuse in
the process as hot water.
Chiorination is another means for treating the noncondensible sulfur
compounds. In some cases, the waste hypochlorite bleach liquor is
used to treat these gases. Dimethyl sulfide is oxidized in the pres-
ence of chlorine to a water soluble and essentially odorless compound
dimethyl sulfoxide.
The complex reactions between methyl mercaptan and chlorine result in
high boiling, hence, less volatile compounds. In most cases, the
chlorination stage is followed by caustic absorption. Packed and plate
-25-
-------
towers and low pressure drop venturi type absorbers are widely used"" ""
as chlorine caustic scrubbers. This treatment results in essentially
complete absorption of sulfur compounds, but the sulfur recovered will
be a loss for the pulping process as the resulting solution will be
discharged to the sewer.
In some cases, the malodorous gases are collected and then burned in
either a lime kiln or other suitable furnace. This is a more economical
and more effective means of disposal and/or recovery than bv chlorinatinn,
The chemical reaction are:
2H2S + 302
-> 2S02 + 2H20
2CH3SCH2 +.902
-> 4C02 + 2S02 + 6H20
CH3SH + 302
-> C02 + S02 + 2H20
" » . * ». '
This method of recovery has limited application potential due to certain
explosion dangers. For safe handling, it is recommented these gases be
diluted with large volumes of air. In case these gases are burned in
the lime kiln, the sulfur dioxide produced will be efficiently absorbed
in a venturi type lime kiln scrubber. It these gases are burned in
a separator reactor, a caustic or carbonate scrubber will be most ef-
fective for sulfur dioxide recovery.
3.2.5 BLACK LIQUOR OXIDATION (BLO)
The release of malodorous gases, such as hydrogen sulfide, mercaptan's,
methyl sulfides and disulfides, and sulfur dioxide can be reduced sig-
nificantly if the black liquor is oxidized. In recent years, black
liquor oxidation has become an integral part of the kraft recovery pro-
cess because of its high potential for reducing odor and sulfur losses
from conventional boilers. Black liquor is oxidized either prior to
the recovery boiler or multiple effect evaporation, depending on the
specific application.
The development of various oxidation systems is a direct result of con-
tinuing research to reduce the emission of odorous gases which contri-
bute to the typical characteristic odor associated with kraft pulping.
The black liquor oxidation (BLO) not only reduces odor and chemical
losses from the recovery furnace and the direct contact evaporator, but
also reduces the chemical losses from multiple effect evaporators if
weak black liquor is oxidized.
In the kraft pulping industry, both weak and strong black liquor oxida-
tion are practiced. The oxidation of weak black liquor offers more
benefits such as lower operating costs, reduction in sulfur loss from
multiple effect evaporators, increased soap yield, etc. than strong
black liquor oxidation. The weak black liquor is the liquor at 12 to
-26-
-------
18% solids concentration entering the multiple effect evaporators,
whereas the strong liquor is 45 to 55% solids concentration leaving
the multiple effect evaporators.
The strong black liquor oxidation is practiced greatly in the southern
U.S. mills using southern pine as wood furnish. The weak black liquor
resulting from the pine resinous woods has the tendency to foam ex-
cessively. The soap content of the black liquor essentailly determines
the extent of the foam formation. The foam has been found to be very
difficult to handle. Recently, a successful operation of the commercial
weak black liquor oxidation plant handling 60% pine wood furnish has
been reported.
The oxidation of black liquor and its advantages have been described
in detail. Figures 3-8 and 3-9 show typical weak and strong black
liquor systems showing the various equipments, i.e. oxidation tower,
foam tank, foam breakers and cyclones. The arrangement of the equip-
ment may vary from one supplier to another. The oxidation tower may
be a plate tower, packed tower or other suitable type, and the liquor
and air flows may be cocurrent or countercurrent.
Black liquor oxidation application to southern pine liquors has mainly
been restricted to the compressed air sparger - retention tank system
because of the inherent foam problem. Recently, there have been a few
successful attempts at oxidizing southern kraft weak black liquor in
conventional mass transfer systems.
In the black liquor oxidation, the liquor and oxidizing agent, such as
air, are brought into intimate contact. The oxidized liquor, air, and
foam then enter the foam tank where oxidized liquor is separated from
air and foam. The air and foam then enter the foam breakers where foam
is converted to liquor which is returned to oxidized liquor storage
tanks. The clean air is discharged into the atmosphere after separa-
tion of entrained liquor in a cyclone. The objective of BLO is to pro-
duce a liquor in which the sulfides are less than 0.1 gram per liter.
The chemical reactions involved in black liquor oxidation are complex
since the organic material as well as the sulfur bearing compounds
react with the oxygen content of the air. The overall oxidation reac-
tions are:
2Na2S + 202 + H20
+ Na2S203 + ZNaOH
4CH3S Na + 02 + 2H20
-* 2CH3S SCH3 + 4NaOH
-27-
-------
To Atmosphere
Four Stage
Oxidation
Tower
Stack
Motor
Foam Breaker
Black
Liquor Pump
Foam Tank
FIGURE 3-8 THE SCHEMATIC ARRANGEMENT OF THE
COMMERCIAL OXIDATION SYSTEM
-28-
-------
t
II
Clean
Exit Air
Cyclone
Foam
Breaker
Liquor Sparger
Air Sparger
Air
Oxidation Tank
Black Liquor Oxidized
Feed Pump Black Liquor
o
Air Compressor
FIGURE 3-9 OXIDATION SYSTEM FOR STRONG BLACK LIQUOR
CHAMPION PROCESS
-29-
-------
In the case when black liquor is oxidized at too low a temperature,
elemental sulfur instead of thiosulfate will be formed. This should
be avoided because during evaporation of the liquor, the elemental
sulfur reacts with hydroxide at higher temperatures, reverting par-
tially to sulfide ions.
3.2.5.1 Effect of BLO
The value of black liquor oxidation for a kraft mill operating under
the overload conditions of a recovery furnace really depends upon the
operation of the direct contact evaporator or scrubber installed for
recovering the heat and chemicals from the flue gases. Black liquor
oxidation cannot control hydrogen sulfide emission from the recovery
furnace under the overload conditions, as the hydrogen sulfide emission
is primarily affected by the ratio of air to black liquor solids at the
furnace hearth and the method of air distribution.
To reduce the concentration of hydrogen sulfide in the flue gases
leaving the stack, the direct contact evaporator or scrubber play a
very prominant role. The oxidized black liquor contains sodium thio-
sulfate and essentailly no sodium sulfide. The carbon dioxide and
sulfur dioxide gases do not react with sodium thiosulfate. Thus, hy-
drogen sulfide generation does not take place. Also, the sodium hy-
droxide formed during oxidation reacts with sulfur dioxide and hydro-
gen sulfide in the flue gas, and thus increases the absorption of both
gases. This clearly indicates that oxidation of black liquor reduces
the concentration of sulfur dioxide and hydrogen sulfide in the flue
gases leaving the stack of conventional boilers.
The following explains what happens in the recovery furnace when oxi-
dized liquor is fired.
The sodium thiosulfate will decompose in the furnace to sodium sulfite
and sulfur. In an oxidizing atmosphere, the elemental sulfur is con-
verted to sulfur dioxide which reacts with sodium carbonate to form
sodium sulfite and is further oxidized to sodium sulfate.
The overall reactions are as follows:
Na2S203
+ Na2S03 + S
S + 02
-* so2'
Na2C03 + S02
+ Na2S03 + C02
Na2S03 H
-30-
-------
The amount of sulfur dioxide emission from the recovery furnace is pro-
portional to the concentration of sodium thiosulfate in the black liquor.
Theoretically, for every mole of sodium sulfide oxidized, \ mole of sul-
fur dioxide should be emitted. However, due to high availability of
sodium carbonate in the recovery furnace, the actual emission is less.
It is observed that for a mill practicing oxidation, the sulfur dioxide
concentration in the flue gases leaving the recovery furnace increases.
But in the case of mills using the venturi type evaporator and scrubber,
this sulfur dioxide will be efficiently absorbed. If the reducing at-
mosphere exists in the recovery furnace, then the elemental sulfur will
be converted to hydrogen sulfide instead of sulfur dioxide.
With the oxidation of black liquor, the hydrogen sulfide concentration
leaving the recovery furnace stack can be reduced by 90 to 98%, and a
significant reduction of odor can be attained. With black liquor oxida-
tion efficiency essentially 100%, and by proper operation of the recovery
furnace, it is possible: to reduce concentrations of hydrogen sulfide
to 1 to 2 ppm; methyl mercaptan to 1 to 3 ppm; and dimethyl sulfide to
1 to 2 ppm. Even with these low concentrations, the typical kraft mill
odor can still be detected since these compounds can be sensed down to
the range of a few parts per billion.
3.2.6 THE RECOVERY FURNACE
Potentially, the major source of chemical and heat loss in the kraft
pulping process is the recovery furnace which is the heart of the re-
covery process. Since the inception of the kraft recovery process,
significant progress has been made in reducing the loss of heat and
chemicals from the recovery furnace.
The flue gases leaving the recovery furnace contain not only 90 to 200
pounds of sodium compounds (mainly sodium sulfate and sodium carbonate)
per ton of pulp production, but also 35 to 50 pounds of sulfur bearing
malodorous gases such as hydrogen sulfide, sulfur dioxide, methyl mer-
captan, dimethyl sulfides, and disulfides.
In the recovery furnace, heat is obtained from the combustion of the
organic constituents of the black liquor, and the inorganic constituents
are recovered as a molten smelt. The concentrated black liquor at 65
to 70 percent solids content is sprayed into the furnace where the water
content is flashed off. As a result of pyrolysis, the organic matter
forms volatile products which burn in presence of oxygen. The combus-
tion products, sulfur dioxide and carbon dioxide, react with sodium com-
pounds to form sodium sulfite and sodium carbonate. The hot carbon from
the pyrolized organic matter reduces the sulfur sodium compounds to
sodium sulfide. The smelt that leaves the furnace essentially contains
sodium sulfide and sodium carbonate.
While the black liquor is burned in the furnace, an appreciable quantity
of sodium compounds are sublimed. As the combustion gases cool, the
sublimate condenses and is entrained in the gas stream as a very finely
-31-
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dispersed fume (essentially sodium sulfate" and sodium carbonate). Most
of this is reclaimed by deposition on the heating surfaces of the re-
covery furnace and on the wetted surfaces of the direct contact evap-
orator if one is used. The remainder, in suspension in the flue gas
as very fine solid particles, passes from the unit. The quantity of
sodium (largely as sodium sulfate) in the flue gas, depends upon service
conditions and operating care of the recovery furnace. It will vary
from 90 to 200 pounds with an average of about 150 Ibs/ton of pulp. For
a 1000 ton/day pulp production, this abounts to 75 tons/day of dust
entrained in the flue gases. This chemical loss in not only costlv
but it is also a nuisance as an air pollutant.
The sulfur bearing malodorous gases, such as hydrogen sulfide, sulfur
dioxide, methyl mercaptan, dimethyl sulfides and disulfides, represent
another source of chemical loss and air pollution. Large volumes of
hydrogen sulfide may be formed in the recovery furnace and, unless opti-
mum combustion conditions exist, it will be discharged to the atmosphere.
The hydrogen sulfide emissions from a kraft recovery furnace is primari-
ly affected by the ratio of air to black liquor solids at the furnace
hearth and the method of, air distribution. If a proper balance is main-
tained between the black liquor feed and the air input, a complete com-
bustion of all pyrolysis products can be attained and all reduced sul-
fur gases will be converted to sulfur dioxide. The theoretical air
requirements for complete combustion of a typical kraft black liquor
and production of smelt of the desired composition are of the order of
four to six pounds of air per pound of black liquor solids. As the
kraft recovery furnace is overloaded, the amount of air and its dis-
tribution become limited by the forced draft and induced draft fan capa-
cities. The forced draft fan "pushes" the combustion air into the
furance, and the induced draft fan "pulls" the combustion from the
furnace. Consequently, reducing conditions in the hearth or char bed
become more pronounced, thereby generating increased quantities of
hydrogen sulfide. This increased hydrogen sulfide generation is a
function primarily of gas composition in equilibrium with the liquid
solid phase at the furnace hearth. The air to solids ratio has a very
promounced effect on the concentration of hydrogen sulfide and sulfur
dioxide in the flue gases leaving the recovery furnace.
In present day practice, the recovery furnace functions as both a
chemical reactor and steam generatorthe steam generation function tends
to predominate with older mills. The recovery furnace can be over-
loaded to achieve increased pulp productions and increased steam re-
quirements. Increased pulp productions mean increase in the firing rate
of black liquor. As the amount of black liquor solids fired increases,
the ratio of air to solids decreases, which results in the increased
generation of hydrogen sulfide. To obtain increased quantity of steam,
the flue gas temperature entering the superheater section of the furnace
.has to be maintained at maximum. This can be attained by using the
minimum quantity of air practicable or by using auxiliary fuel. This
results in a decrease in the ratio of air to solids and hence an in-
crease in generation of hydrogen sulfide.
-32-
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Table 3-2 summarizes the type and quantity of chemical losses occurring
at the_recovery furnace. The gaseous emissions_are based on the assump-
tion that the mill does not practice any black liquor oxidation. The
quantities may vary from mill to mill. It can be seen that the chemical
losses from the recovery furnace can vary from 53 to 80 tons per day.
3.2.7 DIRECT CONTACT EVAPORATOR
In the classic kraft process, flue gases leaving the recovery furnace
pass through the direct contact evaporator (DCE), where as a result of
heat and mass transfer, the concentrated liquor from multiple effect
evaporators is further concentrated to 65 to 70 percent. Some mills
built since about 1968 do not utilize direct contact evaporators, but
provide the additional concentration by an additional stage of multiple
effect. In general, four types of direct contact evaporators are used:
the cascade, the cyclone, the P-A venturi, and the two-stage venturi
scrubber. In all four evaporators, the sensible heat content of the
flue gas leaving the recovery furnace is utilized to further concentrate
the black liquor from multiple effect evaporators prior to firing in
the recovery furnace. The carbon dioxide (16 to 17 percent by volume)
and sulfur dioxide (300 to 500 ppm) present in the flue gas react with
the sodium sulfide content of black liquor and release hydrogen sulfide
according to the following reactions:
Na2S + C02 + H20
-* Na2C03 + H2S
Na2S + S02 + H20
+ Na2S03 + H2S
Depending upon the sulfide ion concentration and the pH of the black
liquor and the mixture of gases in the incoming gas stream, the direct
contact evaporator may either emit hydrogen sulfide or absorb sulfur
dioxide and hydrogen sulfide. Some of the sulfur dioxide in the furnace
flue gas is absorbed by reacting with sodium carbonate, thus forming
sodium sulfite:
. Na2C03 + S02
-» Na2S03 + C02
The extent to which sulfur dioxide is absorbed largely depends upon the
type of direct contact evaporator that follows the recovery furnace.
The venturi type evaporators provide the best absorption efficiency as
compared to the cascade and cyclone types.
3.2.8 RECOVERY FURNACE SYSTEM
In recovery furnaces which utilize any type of DCE} it is yery dif-
ficult to distinguish the emissions evolved from the furnace alone
-33-
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as opposed to the combined emissions exiting the DCE and into the
stack. In a case such as this, the equipment combination may be
referred to as the recovery furnace system.
3.?.8.1 Control of Particulate Emissions From the Conventional Recovery
Furnace System
For the recovery of chemicals in paniculate form (sodium sulfate and
other salts) from the flue gas leaving the recovery furnace, there are
four systems offered to the kraft pulping industry:
1. The two-stage evaporator-scrubber system.
2. The cascade-precipitator system.
3. The cyclone-precipitator system.
4. The P-A (Pease-Anthony) venturi evaporator-
scrubber system.
The cascade-precipitator and cyclone-precipitator systems are very
similar in performance and operation.
The two-stage evaporator-scrubber system shown in Figure 3-2 consists
of a venturi evaporator and venturi scrubber. The flue gas from the re-
covery furnace enters the venturi evaporator and contacts the liquor
to be concentrated. As a result of simultaneous heat and mass transfer,
the liquor is concentrated to the desired solids concentration for firing
in the recovery furnace. The make up liquor to evaporator is either
liquor from the multiple effect evaporator or a mixture of such liquor
and bleed from the second stage scrubber, depending on what liquor is
being used in the second stage as a scrubbing medium. The gas then
enters the venturi scrubber where all the energy (pressure drop) is
essentially utilized for scrubbing and optimum dust collection efficiency
is attained.
The cascade-precipitator system shown in Figure 3-3 consists of a cascade
evaporator and an electrostatic precipitator. Further concentration of
black liquor from a multiple effect evaporator is accomplished in a cas-
cade evaporator using the recovery furnace flue gases. The modern cascade
evaporator consists of from one to four wheels in parallel and/or series
configurations. The arrangement and amount of heating surface required
depend upon the weight and temperature of gas and the desired final
concentration of black liquor. The gas temperature leaving the cascade
and cyclone evaporators is normally maintained at 300°F or higher. If
the gas temperature entering the precipator is less than 300°F, local
cold spots develop and result in condensation, fouling and rapid cor-
rosion of the precipitator.
-34-
-------
U1
I
ventun
Evaporator
Recirculating
Pump
To. Cooling
- Tower
Separator
& Cooler
Make-up
Liquor
Recirculating pump
FIGURE 3-2. TWO STAGE VBJTURI EVAPORATION - SCRUBBING SYSTEM
-------
I
oo
cr>
i
To Stack
Cascade Evaporator
Precipi-
ta tor
Black
Liquor
45-55%
Black Liquor 60-70%
Smelt
FIGURE 3-3. CASCADE PRECIPITATOR SYSTEM
-------
The flue gas leaving the contact evaporator then enters the electrostatic
precipitator where the sodium compounds--in dust formare collected. The
precipitator is operated on the principle that a particle suspended in
the flue gas, if subjected to a sufficiently strong electrostatic field,
becomes charged electrically and tends to migrate toward and adhere to
an oppositely charged electrode. The collection electrodes are mechani-
cally rapped periodically to shake the particles loose into a hopper or
a vat of liquor. To energize the precipitator, potentials of 75,000 to
100,000 volts are necessary. The important factors for efficient pre-
cipitator operation are the gas temperature, moisture content, velocity
and the retention time within the area subject to electrical charge.
In the P-A venturi evaporator-scrubber system shown in Figure 3-4, both
the collection of dust from the flue gas and concentration of liquor by
evaporation are accomplished simultaneously in a venturi type device.
The system's operating principle is the collision of dust particles with
liquid droplets formed by atomization and high velocity gas impact with
liquor. The liquid droplets, with their dust particles entrapped, are
then separated from the flue gas in a cyclonic separator.
The operating and performance characteristics of the three systems are
summarized in Table 3-3. With the use of a two-stage evaporator-scrubber
system, the chemical loss in the form of dust can be reduced by more than
90%. The total particulate matter loss can be reduced to 750 to 1,000 Ibs/
day for a 500 tons/day pulp production. Similar performance can be ob-
tained by combining a cascade evaporator or cyclone evaporator with
secondary scrubbing.
The secondary scrubbing systems shown in Figures 3-5 and 3-6, namely
cyclonic and venturi, offer a solution to upgrade the dust collection
and/or thermal efficiencies of the existing cascade-or cyclone-precipi-
tator system. Table 4 summarizes the operating performance of the
secondary scrubbing systems. The cyclonic scrubber system eliminates
precipitator snow out problems land provides efficient heat and chemical
recovery. This system's performance is excellent as a backup scrubber
after an efficient precipitator. When the precipitator is down for
major overhaul or for replacement, and when its performance has deter-
orated, a cyclonic scrubber alone cannot provide .adequate dust col-
lection efficiency. However, if a venturi type scrubber system is
used following the precipitator, and should it become necessary to take
the precipitator off line for some period of time, the venturi scrub-
ber with proper static energy applied may perform adequate emission
control alone. Many mills do not have nor need a secondary scrubber on
this source.
With a precipitator efficiency of only 80 percent, the (secondary) cyclonic
scrubber can provide 90 percent efficiency on emissions leaving the pre-
cipitator, for a total of 88 percent efficiency, at a pressure drop of
4 to 6 inches W.G. Likewise, a (secondary) venturi scrubber system can
-37-
-------
Venturi
Evaporator
Scrubber
oo
c»
Separator
To Stack
Recirculating Pump
FIGURE 3-4. VENTURI EVAPORATOR-SCRUBBER SYSTEM
-------
TABLE 3-3
OPERATING AND PERFORMANCE CHARACTERISTICS
OF SYSTEMS FOLLOWING RECOVERY FURNACE
Gas temperature
°F In
°F Out
Liquor concentration
% Solids In
% Solids Out
Dust loading
GR/SCF dry
In
Out
Na2SOi+ Ib/ton of pulp
In
Out
Efficiency %
Pressure drop "WG"
Snow Out problem
Maintenance
Thermal efficiency
Space requirement
Equipment cost
Operational
(Availability)
Secondary
Scrubber requirement
Two Stage
Evaporator
Scrubber System
600
150
Evap-45 Scrub-! 7
60 - 70 -30
4.0
0.04
150
1.5
99,0+
30 - 35
No
Very little
Very high
Less
Higher
Continuous
Cascade
Precipitator
System
600
300
45 - 50
60 - 70
4.0
0.08 - 0.4
150
3-15
90 - 98
8-10
Yes
High
Low
More
Highest
Non
continuous
P-A Venturi
Scrubber
System
600
190
45 - 50
60 - 65
4.0
0.4 - 0.8
150
15 - 30
80 - 90
35 - 40
No
Low
High
Less
Low
Almost
continuous
No
Yes
Yes
-39-
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Cascade or
Cyclone Evap-
orato/*
Bleed
=i To Cooling
^~ Tower
Separator
& Cooler
Make-up
Liquor
Recirculating Pump
FIGURE 3,5. SINGLE STAGE VBMJRI SCRUBBING SYSTEM
-------
Cascade
Cyclone
orator
or
Evap-
)
Precipitator
Bleed
Venturi
Scrubber
Separator
Make-up
Liquor
Recirculating Pump
FIGURE 3-6. SECONDARY VENTURI SCRUBBING SYSTEM
(FOLLOWING PRECIPITATOR)
-------
Recovery
Boiler
Venturi
Evaporator-
Scrubber
**To Stack
.».»«
Recirculating Pump
Separator
& Cooler
To Process
To Cooling
Tower
Make-up
Liquor
Recirculating Pump
FIGURE 3-7. SECONDARY VENTURI SCRUBBING SYSTEM
(Following Existing Venturi System)
-------
TABLE 3-8 A
OPERATING PERFORMANCE OF
SECONDARY SCRUBBING SYSTEMS
Gas temperature
°F In
°F Out
Dust loading
GR/SCF Dry In
GR/SCF Dry Out
Efficiency %
Pressure Drop "WG"
Liquor Concentration
% Solids In
% Solids Out
Cyclonic
Scrubber
300
160 - 170
0.5 - 1.0
0.05 - 0.1
90
1-3
0 - 10
15
Venturi
Scrubber
300
160 - 170
0.5 - 1.0
0.04
90 - 98
10 - 15
0 - 18
30
-43-
-------
provide 98 percent efficiency at a pressure drop of 12 to 15 inches
W.G., thus an overall efficiency of 98 to 99 percent can be attained.
When the precipitator is taken out, the gases from the evaporator can
be diverted to the venturi system. An overall efficiency of 95 %, plus "
can be attained at a pressure drop of 25 in. w.g. across the scrubber.
When the P-A venturi evaporator-scrubber system is providing only 85
to 90% efficiency at a pressure drop of 35 to 40 in. w.g. and has to
be upgraded to obtain an overall efficiency of 99% plus, it should be
upgraded by conversion into a two-stage system, as shown in Figure 3-7.
The existing separator and fan could be utilized in the converted two-
stage system, by transferring the majority of available static pressure
to the second stage with only a minimum applied to the first (evaporation)
stage. This way, performance equivalent to a two-stage evaporator-scrub-
ber system can be achieved.
3.2.9 RECOVERY FURNACE SYSTEMS WHICH ELIMINATE DIRECT CONTACT BETWEEN
BLACK LIQUOR AND FLUE GASES
In the past five years, there have been several applications of the
"odor free" recovery boiler in the industry, as manufactured by the
two major boiler companies, Babcock and Wilcox (B & W) and Combustion
Engineering (C-E). Essentially, the modification developed by both
companies avoids direct contact of black liquor with boiler flue gases,
as formerly required to achieve liquor concentrations in the 70% solids
range.
By eliminating the D-C evaporator, the B & W approach puts more emphasis
on the multiple effect evaporator train, which heretofore has been only
marginal in raising the solids concentration of weak black liquor. B & W
says its improved design can reach 60 to 65% solids, well above the usual
minimum of 55% that is required for steady, safe operation of the boiler.
B & W's three main design changes are: additional indirect evaporation
stages which uses forced circulation of liquor through the exchangers
(due to the higher viscosity of strong black liquor); a revised tube-
survace., arrangement to offset scaling and poorer rates of heat transfer;
and a steam pressure limit of 35 psig. coming into the multiple effect
train, so that maximum steam pressure in the first effect (where vis-
cosity is highest) to 8 psig. (Usual practice has been to employ 60
psig. steam, but the lower pressure, notes the company, brings the
temperature down to a point where scaling is substantially reduced).
To recover heat from the flue gases, and also to cool gases before ad-
mitting them to the electrostatic precipitator, B & W outfits the
economizer with approximately three times the surface area of a compar-
able unit feeding 50% liquor to a direct contact evaporator. The gas
temperature at the exit of the enlarged economizer is about 50°F higher
than with a direct contact unit. The only advantages claimed for the
higher temperatures are reduction of corrosion in the precipitator and
less steam plume.
-44-
-------
provide 98 percent efficiency at a pressure drop of 12 to 15 inches
W.G., thus an overall efficiency of 98 to 99 percent can be attained.
When the precipitator is taken out, the gases from the evaporator can
be diverted to the venturi system. An overall efficiency of 95 %, plus"
can be attained at a pressure drop of 25 in. w.g. across the scrubber.
When the P-A venturi evaporator-scrubber system is providing only 85
to 90% efficiency at a pressure drop of 35 to 40 in. w.g. and has to
be upgraded to obtain an overall efficiency of 99% plus, it should be
upgraded by conversion into a two-stage system, as shown in Figure 3-7.
The existing separator and fan could be utilized in the converted two-
stage system, by transferring the majority of available static pressure
to the second stage with only a minimum applied to the first (evaporation)
stage. This way, performance equivalent to a two-stage evaporator-scrub-
ber system can be achieved.
3.2.9 RECOVERY FURNACE SYSTEMS WHICH ELIMINATE DIRECT CONTACT BETWEEN
BLACK LIQUOR AND FLUE GASES
In the past five years, there have been several applications of the
"odor free" recovery boiler in the industry, as manufactured by the
two major boiler companies, Babcock and Wilcox (B & W) and Combustion
Engineering (C-E). Essentially, the modification developed by both
companies avoids direct contact of black liquor with boiler flue gases,
as formerly required to achieve liquor concentrations in the 70% solids
range.
By eliminating the D-C evaporator, the B & W approach puts more emphasis
on the multiple effect evaporator train, which heretofore has been only
marginal in raising the solids concentration of weak black liquor. B & W
says its improved design can reach 60 to 65% solids, well above the usual
minimum of 55% that is required for steady, safe operation of the boiler.
B & W's three main design changes are: additional indirect evaporation
stages which uses forced circulation of liquor through the exchangers
(due to the higher viscosity of strong black liquor); a revised tube-
survace arrangement to offset scaling and poorer rates of heat transfer;
and a steam pressure limit of 35 psig. coming into the multiple effect
train, so that maximum steam pressure in the first effect (where vis-
cosity is highest) to 8 psig. (Usual practice has been to employ 60
psig. steam, but the lower pressure, notes the company, brings the
temperature down to a point where scaling is substantially reduced).
To recover heat from the flue gases, and also to cool gases before ad-
mitting them to the electrostatic precipitator, B & W outfits the
economizer with approximately three times the surface area of a compar-
able unit feeding 50% liquor to a direct contact evaporator. The gas
temperature at the exit of the enlarged economizer is about 50°F higher
than with a direct contact unit. The only advantages claimed for the
higher temperatures are reduction of corrosion in the precipitator and
less steam plume.
-44-
-------
C-E ACE System
B&W System ^
*In B&W Design, economizer is enlarged
Gas To Stack
^
Air
Gas
Ljunstrom-Type
Air Heater
Cascade
Evapora
Liquor From Multiple-
Effect Evaporators
Air
i fc
*
ie
~ator
F
urnace
FIGURE 3-10. COMPARISON OF C-E AND B&W RECOVERY BOILERS
SHOWS DESIGN DIFFERENCES WITH OR WITHOUT
THE DIRECT-CONTACT EVAPORATOR
Flue Gas Outlet
Heating
Surface
FIGURE 3-11. DETAIL SHOWS THE LAMINAIRE AIR HEATER
USED IN THE C-E DESIGN
-46-
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3.2.11 LIME KILN
The lime kiln is a major source of chemical loss in the form of particu-
late emissions. The gaseous emissions are not so pronounced. In the
lime recovery process, lime is usually lost at various pointssome with
grits or impurities removed by the slaked lime classifier and some at
storage facilities for reburned lime. The major source of loss is the
exit gases leaving the kiln. The lime dust escaping with the flue gases
is more objectionable than the calcium carbonate and lime lost at the
other end of the kiln because it is reactive lime. The major loss of
particulate matter from this source is alkaline earth oxides.
The other minor chemical loss which creates an odor problem, is the re-
lease of sulfur bearing compounds through exit flue gas. The quantity
of odor from lime kiln gases is very, small as compared with other
sources in the pulp mill. To understand and develop an odor control
system or to minimize the formation and release of odorous compounds,
it is necessary to evaluate the sources contributing to odor emission.
The possible sources of sulfur compounds are:
1. The degree of washing of lime mud and the filter cake
dryness determines the amount of sodium sulfide in the
lime sludge. In cases where liquor sulfidity is high
and washing poor, as much as 5 pounds of sodium sul-
fide per ton of pulp production may be present in lime
sludge fed to the kiln. The high carbon dioxide and
high temperature atmosphere may favor release of hydro-
gen sulfide and sulfur dioxide according to the follow-
ing reactions:
Na2S + S02+ H20
+ Na2C02 + H2S
2H2S + 302
-> 2S02 + 2H20
The hydrogen sulfide emissions will increase with lower-
ing of kiln exit temperature or oxygen content.
2. A high sulfur content fuel oil used in the kiln as a
fuel source results in the release of sulfur dioxide.
3. Where the water supplies are limited and a mill has a
tight water balance, the evaporator condensate is used
as makeup to the kiln scrubbing system. If the mill
does not practice weak black liquor oxidation or strip-
ping of the condensates, this may become a significant
odor source.
4.
Some mills practice burning of the noncondensible gases
released in the digester and evaporation process in the
kiln. These gases with high sulfur content are efficiently
burned in the high temperature zone.
-47-
-------
The use of wet scrubbers, of low sulfur content fuel, and the efficient
control of combustion and the proper washing of lime mud will definitely
reduce the odorous gases and chemical losses in the form of sulfur diox-
ide and hydrogen sulfide.
The flue gas leaving the kiln carries an appreciable quantity of dust.
The chemical composition of the dust is essentially carbonates and
oxides of sodium, calcium and magnesium, sodium sulfate and acid in-
solubles (consisting of aluminium oxide, iron oxide, silica and others).
The amount of chemicals leaving the kiln in the flue gases may range
from 5 to over 20 grains/cubic foot. This represents a loss of 10 to
20 tons/day.
To recover these chemicals, the wet scrubber is very widely used. Of
the various types, the venturi scrubber in Figure 3-12 has proven to
be most efficient. There are about 100 installations throughout the
industry. The lime kiln scrubber should be considered as an essential
and integral part of kraft chemical and heat recovery process, just as
a lime kiln constitutes an integral part of the causticizing and chemi-
cal recovery process.
With the use of the efficient venturi scrubber, these chemical losses
are reduced by 99% plus. The collection of lime dust is not as diffi-
cult as the collection of soda dust. The soda is in the form of very
fine fume and requires higher energy for collection. At a total pres-
sure drop of 15 to 17 in. w.g. across the scrubber system, essentially
all the lime dust can be recovered whereas only 70 to 80% soda fume is
recovered. The sulfur dioxide and hydrogen sulfide will be absorbed
efficiently in the venturi type scrubbers. It is believed that better
mud washing methods and improved control of lime, kiln operation,
coupled with the efficient scrubber operation, can result in a totally
satisfactory solution to this problem.
The lime kiln scrubber system as in Figure 3-12 consists of a venturi
flooded elbow and a cyclonic separator. In the venturi, the scrubbing
liquor enters tangentially into the shelf section and swirls down the
converging walls. It then hits the lip section in the throat which
throws11 the liquor toward the center thus creating a curtain of liquor
across the throat. The dust laden gas enters the venturi through a
thimble, accelerates through the convergent section, impacts with the
liquor curtain thus shattering it into droplets into which the particu-
late matter imbeds and is collected. The cleaned gas and liquor enter
the flooded elbow. The gas then tangentially enters the cyclonic
separator where any entrained liquor is separated. For efficient
separation, both the spin height and the vertical velocity are important
factors for design. If the separator is not properly designed, carry
over of liquor droplets will occur and will result in increased dust
loss to the atmosphere.
-48-
-------
FIGURE 3-12. S-F VENTURI SCRUBBER
-49-
-------
The venturi scrubber has also been found to be an effective device for
the removal of malodorous gases. The scrubbing solution used in the
lime kiln scrubber is alkaline, and the acidic gases such as hydrogen
sulfide, sulfur dioxide, etc., will be efficiently absorbed, provided
solution pH is maintained in proper range.
-50-
-------
3.2.12 THE EFFECT OF PROCESS AND CONTROL EQUIPMENT VARIABLES ON PULP
PRODUCTION AND EMISSIONS
The following sections describe process and emission control equip-
ment variables which affect pulp production rates and attendant
emission levels for the kraft pulping process.
3.2.12.1 Recovery Furnace System
In the kraft process the more conventional chemical recovery complex
consists of the recovery boiler and the direct contact evaporator(s).
In the more recent "low odor" recovery systems, the acidic products
of combustion from the furnace are not directly contacted with the
strong black liquor, thus eliminating the stripping of malodorous
reduced sulfur compounds from this unit process.
NCASI studies (18) have identified black liquor solids firing rates,
available oxygen supply, adequate residence time in the combustion
zone, and turbulence to be important factors in controlling reduced
substances lost from the smelting zone of recovery furnaces. In
other studies, Thoen et.al. (,19) reported that in addition to the above
factors, the ratio of primary to secondary air and liquor spray
firing patterns were significant factors in controlling emissions
from recovery furnaces. In another detailed study, Murray (20)
further related the ratio of available air to liquor flow and sul-
fide content of the fired liquor to reduced sulfur concentrations
in recovery furnace exit gases. Smelt bed depths, although not
considered to be as important as the previously described variables,
can have some effect on both particulate and gaseous emissions from
the furnace. In general, the deeper the smelt bed, the greater the
loss of both particulate and gaseous materials from the smelting
zone.
Black liquor firing rates are directly proportional to pulp
production and also have a profound effect on both particulate
and gaseous sulfur emissions from the furnace itself. High
flue gas velocity, normally attendant to increased liquor firing
rates, may cause the carry-up of small droplets of black liquor
which have been sprayed into the furnace. The increased flue
gas velocity may also entrain additional quantities of the inor-
ganic sodium salts, primarily sodium sulfate and sodium carbon-
ate, which are present in recovery complex emissions.
-51-
-------
The direct contact evaporator(s) receiving the furnace gases
effectively serves as participate reduction devices, thus
minimizing particulate loadings to the subsequent emission
control equipment. Normally, direct contact evaporators will
remove approximately 40 to 60 percent of the particulate
emissions from the recovery furnace. Particulate emissions
from the direct contact evaporator(s) are dealt with most
effectively by employing such control devices as electrostatic
precipitators and Venturi scrubbers. It is normally more ad-
vantageous to control furnace operating conditions to minimize
gaseous emissions rather than particulate emissions. However,
as a general rule, most operating conditions which result in
reduced gaseous emissions also serve to reduce particulate
emissions.
Particulate levels in kraft recovery furnace flue gases, prior
to reaching a reduction device, normally range from 8.0 to
12.0 grains per standard dry cubic foot of flue gas. The
atmospheric emission levels are a function of control device
efficiency which is dependent upon the system design. In con-
ventional recovery furnace systems (those with direct contact
evaporators), particulate emission control consists of: (a)
the contact evaporator; (b) a primary unit which is either a pre-
cipitator or a Venturi recovery unit; and (c) in some cases,
secondary scrubbers. In newer systems where no flue gas direct
contact evaporators are employed, high efficiency (99+%) elec-
trostatic precipitators are used for particulate control almost
exclusively. This does not, however, preclude the addition
of secondary scrubbers to solve special "snowing" problems.
Particulate collection efficiencies for the previously described
recovery furnace complex generally range from 85-99+ percent
with the higher percentages applying to systems installed within
the past few years.
In conventional recovery complex systems, malodorous reduced
sulfur compounds may be present in the furnace exit gases and
also "stripped" from the black liquor in the direct contact
evaporators. Generally, in the absence of black liquor oxi-
dation, the contact evaporators are the major source of malodor-
ous sulfur gas emissions. Sulfide and available alkali content
of the black liquor entering the contact evaporators are the
major variables in controlling the loss of reduced sulfur com-
pounds from this unit process. Numerous studies have shown that
inorganic and organic sulfide stripping is minimized when the
measurable sulfide and mercaptide content of the black liquor
is below 0.1 grams per liter. Hydrogen sulfide accounts for
most of the reduced sulfurs emanating from this operation, al-
-52-
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though there can be methyl mercaptan, dimethyl sulfide, and
dimethyl disulfide present. These organic sulfur compounds
seldom exceed ten percent of the h^S present and normally are
not measurable at all.
In well controlled recovery complex systems, atmospheric
emission levels of reduced sulfur compounds may be as low as
1.0 parts per million (ppm) by volume to several hundred ppm.
The wide range of emission levels is a function of the afore-
described process and control equipment variables.
Historically, there has been little concern for the S02
generated in a kraft recovery furnace. In conventional kraft
recovery furnace system designs concentrations are substan-
tially reduced when furnace exhaust gas passes through the
contact evaporator. After the contact evaporator they are
characteristically below that existing from the combustion
of fossil fuels containing 0.5 percent sulfur, i.e. less than
300 ppm, usually between 50 and 150 ppm. Concentrations of
S02 in recovery furnace exhaust gas range from less than 50
to as high as 700 or 800 ppm. The factors responsible for this
range of concentration are not well identified. Blue and
Llewellyn (21) as well as results of currently unpublished
studies indicate that S02 generation in kraft recovery furnaces
is a function of several variables. One is cooking liquor
sulfidity, and indirect measure of the soda and sulfur ratio
in black liquor fed to the furnace. Others include smelt bed
depth, manner in which liquor is sprayed into the furnace,
ratio of primary to secondary combustion air and possibly
temperature within the furnace itself. As previously stated,
the higher concentrations are of limited practical concern
except in those recovery system designs which eliminate the
contact evaporator.
3.2.12.2 Lime Kilns
The lime kiln is a major source of chemical loss in the form of
particulate emissions. The gaseous emissions are not so pro-
nounced. In the lime recovery process, lime is usually lost at
various pointssome with grits or impurities removed by the
slaked lime classifier and some at storage facilities for re-
burned lime. The major source of loss is the exit gases leaving
the kiln. The lime dust escaping with the flue gases is more
objectionable than the calcium carbonate and lime lost at the
other end of the kiln because it is reactive lime.
-53-
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The other minor chemical loss which creates an odor problem, is
the release of sulfur bearing compounds through exit flue gas.
The quantity of odor from lime kiln gases is very small as com-
pared with other sources in the pulp mill. To understand and
develop an odor control system or to minimize the formation and
release of odorous compounds, it is necessary to evaluate the
sources contributing to odor emission. The possible sources of
sulfur compounds are:
1. The degree of washing of lime mud and the filter
cake dryness determines the amount of sodium sul-
fide in the lime sludge. In cases where liquor
sulfidity is high and washing poor, as much as
5 pounds of sodium sulfide per ton of pulp produc-
tion may be present in lime sludge fed to the kiln.
The high carbon dioxide and high temperature at-
mosphere may favor release of hydrogen sulfide and
sulfur dioxide according to the following reactions:
Na2S + C02 + H20
-> Na2C03 + H2S
2H2S + 302
+ 2S02 + 2H20
The hydrogen sulfide emissions will increase with
lowering of kiln exit temperature or oxygen content.
2. The high sulfur content fuel oil used in the kiln
as a fuel source results in the release of sulfur
dioxide.
3. Where the water supplies are limited and a mill has
a tight water balance, the evaporator condensate is
used as makeup to the kiln scrubbing system. If
the mill does not practice weak black liquor oxida-
tion, this may become a significant odor source.
4. Some mills practice burning of the noncondensable
gases released in the digester and evaporation pro-
cess in the kiln. These gases with high sulfur
content are efficiently burned in the high tempera-
ture zone.
-54-
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The use of wet scrubbers, of low sulfur content fuel and the
efficient control of combustion and the proper washing of lime
mud will definitely reduce the odorous gases and chemical losses
in the form of sulfur dioxide and hydrogen sulfide. A typical
lime kiln scrubbing system is shown in Figure 3-13.
The flue gas leaving the kiln carries an appreciable quantity
of dust. The chemical composition of the dust is essentially
carbonates of sodium, calcium and magnesium, sodium sulfate
and acid insolubles (consisting of aluminum oxide, iron oxide,
silica and others). The amount of chemicals leaving the kiln
in the flue gases may range from 5 to over 20 grains/cubic
foot. This represents a loss of 10 to 20 tons/day (for a 400
TPD mill).
Factors which affect reduced sulfur emissions from the kiln
proper have been identified as stack gas oxygen content, design
dimensions, operating temperature, and effectiveness of the
lime mud washer. Those operating parameters which have been
identified as being responsible for the reduced sulfur contri-
bution from the scrubbing system include make-up water source
and recirculation rate, pH of the scrubbing solution, and the
sulfide content of the particulate matter.
Minimum emission rates which can be consistently obtained
through the optimum control of these variables have not yet
been fully established. Current findings indicate the emission
concentration can be maintained at less than 50 ppm or 0.25
pounds of sulfur per ton of air dried pulp. While kiln operating
variables which affect the emission of hydrogen sulfide have
been identified, minimum emission rates which can be consis-
tently attained through controlled operation have not been fully
established.
Hydrogen sulfide can be emitted from two distinct and separate
points which are:
1. the lime kiln proper
2. the particulate control scrubber
Factors which affect the concentration of reduced sulfur in the
exit gas from the lime kiln include:
1. oxygen content of the exit gas,
2. length to diameter ratio of the kiln,
3. sulfide content of the lime mud,
-55-
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Lime kiln venturi scrubber system.
FIGURE 3-13
FRESH
WATER OR
KILN COOLING
WATER
LIME
SLUDGE
WASHERS
-56-
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4. cold-end operating temperature,
5. burning of green liquor dregs.
Operating variables of the particulate control scrubber which
govern the contribution of hydrogen sulfide include:
1. source of scrubber make-up water,
2. recirculation rate,
3. pH of the scrubbing solution,
4. acid liberated sulfide content of the particulate
collected.
Minimum hydrogen sulfide concentrations in the stack gas of
individual kilns have been measured at oxygen contents of 3.5
to 4.0 percent for units operated under stable conditions.
The hydrogen sulfide values recorded have varied from 10 ppm
to about 100 ppm for properly operated kilns. The higher con-
centrations were associated with "so-called" long kilns operated
on lime mud of high sulfide content. A typical hydrogen sulfide
vs. oxygen content curve is shown in Figure 3-13 A.
Long kilns are characterized as units with lengths of 25 diameters
or more and short kilns as units with lengths of 20 diameters or
less.
The gross effect of lime mud sulfide content on kiln proper
hydrogen sulfide emission rates is presented in Table 3-4 (22).
Operating conditions during intervals II and III on two separate
trials on one kiln were similar, as was the hydrogen sulfide
emitted. During interval IV, the only operating variable
changed was the removal of green liquor dregs from the kiln
lime mud feed. The corresponding reduction experienced in the
hydrogen sulfide emission was about 40 percent.
Particulate emissions from the lime kiln consist of the sodium
salts, calcium carbonate, calcium sulfate, calcium oxide, and
insoluble ash. The presence of the sodium salts may be accounted
for by the sublimation-condensation process, and by dust entrain-
ment within the kiln. However, neither calcium carbonate, nor
calcium oxide will vaporize at temperatures within the kiln and
the presence of calcium carbonate and calcium sulfate must be
explained by the entrainment of the calcium carbonate and calcium
oxide. The particles of calcium oxide may react with either
-57-
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FIGUREW-13 A
TOTAL .REDUCED SULFUR VS. EXCESS OXYGEN
LIME KILN
100
en
CO
CL
r>
cr.
2
ID
CO
Q
L-J
Q
UJ
CC
-J
<
J
o
h-
90 h
80
70
GO
50
40
30
20
10
0
o
0
1.0
2.0 3.0
4.0
5.0
6.0
7.0
8.0
EXCESS OXYGEN
-------
TABLE 3-4
LIME KILN EMISSIONS (Prior to Particulate Control Device)
AS RELATED TO OPERATIONAL VARIABLES (22)
Interval
i
I
II
III
IV
V
Stack Gas
S0? Content
%
0
2.8
2.4
2.8
4.0 - 7.0
Burnings
Dredge
Yes
Yes
Yes
No
No
H?S
ppm
804
506
463
243
60 - 86
Lbs S/Ton
2.9
1.7
1.5
0.9
0.2 - 0.3
-59-
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carbon dioxide or sulfur dioxide within the kiln to yield the
appropriate calcium salt. Calcium carbonate may react with
the sulfur dioxide and then oxygen to yield calcium sulfate.
CaO +
CaO +
CaC03
CaS03
co2 +
+ so2
+ 1/20
CaC03
CaS03
-> CaS03
2 -» CaSO
4
The particulate emissions from the lime kiln (before the control'
equipment) are about one-fourth the emissions from the recovery
furnace but are still significantly large. However, the relative-
ly low gas flow rates through the lime kiln allow installation
of moderately priced high efficiency wet scrubbers which can re-
duce. particulate emissions to low levels. At the present time,
most lime kilns are equipped with wet scrubbers whose efficiencies
range from 80 to 99 percent on the calcium salts. The particu-
late emissions from the lime kilns whose scrubber efficiencies
are in the lower part of the range may be significant.
Other than controlling the rate of material output there appears
to be no well defined method of controlling the particulate
emission for the lime kiln through manipulation of operating
variables.
The previously described particulate control system employed
on most lime kilns can be a source of reduced sulfur losses
dependent upon the source of scrubbing water and the recircula-
tion rate. Although the collected particulates are alkaline,
the kiln exit gases are rich in carbon dioxide and, therefore,
caused a pH drop in the scrubbing medium. In these cases, soluble
sul fides in the scrubbing liquid may be evolved as hydrogen
sulfide. This potential condition then dictates the necessity
of utilizing sulfide free source water for the scrubbing system.
3.2.12.3 Smelt Dissolving Tank
Both particulate material and reduced sulfur compounds are
present in the vented gases from smelt dissolving tanks. Parti-
culates are primarily caused by the entrainment of large parti-
cles in the vent gases. Because of the violent reactions taking
place when the molten smelt from the furnace is discharged into
the tank, the turbulence created will "splash" water droplets
which contain both dissolved and undissolved inorganic salts
-60-
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above the liquid surface of the tank. Therefore, due to the
high temperature of the offgases, the small particles, now in
suspension, are carried out with the gas stream.
Some reduced sulfur compounds are formed by reactions in the
tank and others are distilled from the make-up water, usually
weak filtrate from the lime mud washer, added to the dissolving
tank.
Operational variables most influential on these emissions are
(a) rate of molten smelt discharge from the recovery furnace and
(b) source o»f make-up water utilized. The shatterjet system utilized
on the furnace smelt spouts can also have a significant effect
on particulate emissions from this source.
Wire mesh mist eliminators are currently the most used emission
control devices employed on smelt tank vents. However, more re-
cently, wet scrubbing systems, including packed columns, are now
being installed in many places. These scrubber/absorption sys-
tems can be effective on both particulars and reduced sulfur
emissions when the proper scrubbing medium is used.
3.2.12.4 Multiple Effect Evaporators
The kraft process utilizes multiple effect evaporation to con-
centrate weak black liquor (spent cooking liquor) washed from
the pulp. Removal of large amounts of water from the liquor is
necessary to facilitate combustion of the dissolved organic
material in the recovery furnace. The liquor is concentrated
in the multiple effect evaporators from a solids content of
12-18% to 40-55% (25).
Most kraft mills utilize long-tube vertical shell-and-tube type
evaporators. The weak black liquor is fed to the tube side of
the latter evaporation effects and steam is supplied to the
shell side of the first effect. The liquor proceeds through the
tube side of each effect from last to first, being heated in
each by condensation of the vapor driven off the boiling liquor
in the tube side of each preceding effect.
Evaporated water vapor from the last effect of the set is con-
densed in one of two types of condensers: (a) direct contact
barometric condensers, and (b) surface condensers with steam
ejectors. The condenser must remove vapor fast enough to create
a vacuum in the vapor space of the last effect. Each type of
condenser is equipped with a small steam ejector to remove non-
condensibles.
-61-
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The emissions from the multiple effect evaporators are non-con-
densible reduced sulfur gases which are vaporized or stripped
during the boiling. These non-condensible gases, with vapors
created during boiling, pass to the heating element of the follow-
ing effect. The reduced pressure in the later effects will re-
sult in a higher evolution of the reduced sulfur compounds. This
increased evolution and the steam stripping of the reduced sulfur
compounds are responsible for the emissions from this source.
In order to eliminate an accumulation of non-condensible gases in
the heating element each heating element is provided with a gas
vent. The vents from the heating elements that are under a pres-
sure greater than atmospheric are vented to its vapor head. The
vents from the heating elements under a vacuum are usually valved
to a common header going directly to either a barometric or sur-
face condenser. The condenser type influences the relative
emissions of TRS compounds from this source as indicated in the
following table (limited data):
Emissions (Ib/ADT)
Condenser Type
Surface
Barometric (with loss at
noncondensible jet)
Barometric (losses from
hot well)
H2S Methyl Di-methyl
Mercaptan Sulfide
4.80 1.44
'0.04 0.03
0.13 0.20
0.35
0.04
0.14
Di-methyl
Di-Sulfide
0.62
0
0.02
Sulfidity and pH of the liquor tend to be controlling factors in
the quantity of reduced sulfur compounds emitted. Although black
liquor oxidation can reduce the TRS emissions significantly, the
residual concentrations are such that further processing of non-
condensibles is desirable.
The most common practice, in cases where treatment of these
non-condensibles is employed, is to combine these non-condensible
gases with the digester blow and relief gases and treat in the
same manner as described in section 3.2.12.5.
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3.2.12.5 Digester-Relief and Blow
Digester systems employed in the kraft process consists of the
batch and continuous types previously described.
Although no particulates are generated from these operations,
the relief and blow gases can be the largest potential source
of reduced sulfur emissions within the kraft mill. This is
mainly due to the fact that most of the malodorous organic
sulfur compounds from the kraft process are primarily formed
during the digestion of the wood chips. Further, their first
opportunity to "escape" comes during the cooking cycle via
the relief gases and at the end of the cooking cycle through
the blow gases.
Where turpentine recovery is employed, the relief gases are
normally passed through turpentine recovery systems where the
condensible fractions of the gases are removed. The non-
condensible fractions may then be vented to the atmosphere or
a control system for further treatment. These non-condensible
gases are mainly made up of methyl mercaptan, dimethyl sulfide
and dimethyl disulfide.
Blow gases are released at the end of the cooking cycle and, in
batch systems, normally pass through a blow-heat recovery sys-
tem which is relatively ineffective insofar as removal of organic
sulfur compounds is concerned. In batch digester systems, the
blow-heat recovery tank can be an intermittent, but significant
source of malodorous sulfur emissions.
Several operating variables affect both production and sulfur gas
emissions from these operations. The most important of these
as they relate to emissions are (a) cooking liquor sulfidity
(b) duration of cook (c) wood species and (d) degree (severity)
of the cook which is related to a,b, and c and the desired yield.
Treatment methods utilized to dispose of the malodorous non-
condensible blow and relief gases include thermal oxidation,
chemical oxidation and chemical absorption. The most prevalent
methods currently in use are a combination of chemical absorption
followed by thermal oxidation, most commonly in the lime kiln.
As previously stated in section 3.2.12.2 thermal oxidation in
the lime kiln virtually eliminates malodors produced by these
reduced sulfur compounds. Burning the gases is accomplished
-63-
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in the lime kiln, or a special separate incinerator (23). In
burning, care must be taken to avoid the occurrence of explosive
mixtures. In a batch digestion system, the problem of preventing
large surges of gas to the burning device arises. Gas surge
capacity is provided by using large spherical tanks equipped with
a movable non-porous diaphram (24), or conventional gas holders.
Burning can be a very effect!veTechnique for disposal of these
gases. Data in the section on lime kilns illustrate this point.
Scrubbing the gas stream with a sodium hydroxide solution offers
a partial control method for digester emissions. Effectiveness
is limited to h^S and methyl mercaptan. Some mills use such
scrubbers for preliminary treatment of gases before burning
them. Three objectives are achieved: (a) some sulfur is re-
covered, (b) steam is condensed, and (cj turpentine vapors and
mists are removed, mitigating an explosion hazard.
Scrubbing with chlorine solutions is practiced in some mills.
In the case of bleached kraft mills, chlorine-containing efflu-
ent from the bleach plant may be used to scrub the gases. It
is necessary that residual chlorine be present at all times in
these effluents to maintain the effectiveness of this technique,
which is of limited effectiveness at best.
3.2.12.6 Brown Stock Washer Systems
Brown stock washer systems are commonly characterized as high
volume-low concentration sources of reduced sulfur compounds.
No particulate material is generated from these sources.
Operational variables which affect production and atmospheric
emissions include (a) liquor sulfidity (b) pulp flow rate (c)
wood species (d) wash water source and (e) storage time in blow
tanks (26.).
Emissions from the brown stock washers arise primarily from the
vaporization of the volatile reduced sulfur compounds. No chemi-
cal reactions are believed to take place. However, there is a
shift in the equilibrium for hydrogen sulfide and methyl mercap-
tan. Since the washer water normally has approximately a neutral
pH, the dilution of the liuqor by the water will cause a lowering
of the liquor pH to approximately 10.0.. This pH is below the
equilibrium point for methyl mercaptide ion and results in a
corresponding shift to methyl mercaptan gas. The lower pH will
also cause an increase in the concentration of dissolved hydro-
gen sulfide. However, the equilibrium point of H2S (8.0) will
not be reached.
-64-
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The composition of these sulfur gases is almost exclusively
organic sulfur compounds made up of methyl mercaptan, dimethyl
sulfide and dimethyl disulfide.
Control of these emissions is not commonly practiced at the
present time stemming primarily from the difficulty and expense
of treating the high volume-low concentration sources. However,
newer installations are venting these gases to the recovery
furnaces for utilization as a source of combustion air. There
is a scarcity of available data regarding the effectiveness of
this practice in the overall odor reduction of kraft pulp mill -
operations.
Measured emissions from these sources range between 0.01 and 0.6
with a mean of 0.1 pounds of total reduced sulfur (as H2S) per
ton of air dried pulp (26).
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3.2.13 REFERENCES
1. Douglass, I.B. and L. Price, Abstracts, 52nd Annual Meeting of
TAPPI, p. 40 (1967).
2. Brink, D.L., J.F. Thomas, and D.L. Feuerstein, TAPPI, 50 (6),
p. 276 (1967).
3. Shah, I.S. and L. Mason, TAPPI, 50 (10), p. 27A (1967).
4. Shah, I.S., Chemical Engineering, 74 (7), p. 84 (1967).
5. West, P.H., H.P. Markant, and J. H. Coulter, TAPPI, 44 (10),
p. 710 (1967).
6. Shah, I.S. Paper Trade Journal. 152 (11) p. 65 (1968).
7. Shah, I.S. Paper Trade Journal. 152 (12) p. 58 (1968).
8. Shah, I.S. and W.D. Stephenson, Abstracts, 53rd Annual Meeting of
TAPPI, (1968).
9. Landry, J.E., TAPPI, 46 (12), p. 766, (1963).
10. Hawkins, G., NCSI Technical Bulletin No. 153.
11. Hendrickson, E.R., and C.J. Harding, Journal of APCA, 14 (12),
p. 487 (1964).
12. Trobeck, K.G., W. Lenz, and A. Tirade, TAPPI, 42 (6), p. 425
(1950).
13. Collins, T.T., Jr., TAPPI, 38 (8), p. 172A, (1955).
14. Stuart, H.H., and R.E. Bailey, Southern Pulp of Paper Manufacturing.
p. 46,(September 10, 1965).
15. Navaree, J., NCSI Technical Bulletin No. 26.
16. Douglass, I.B., R.L. Weichman, and L. Price. Unpublished paper,
"Odor Formation in Black Liquor Multiple Effect Evaporator."
17. Tomlinson, G.H., Pulp and Paper Manufacturing, Vol. 1, 3rd Edition,
p. 416, McGraw-Hill, N.Y. (1950).
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18. "Factors Affetting Reduced Sulfur Emissions from the Kraft
Recovery Furnace and Direct Contact Evaporator." Technical
Bulletin No. 44, National Council of the Paper Industry for
Air and Stream Improvement, Inc., New York, December 1969.
19. Thoen, G.N., DeHaas, G., Tallent, R., and Davis, A., "The
Effect of Combustion Variables on the Release of Odorous
Sulfur Compounds from a Kraft Recovery Process," TAPPI jj]_
(8) 329 (1968).
20. Murray, F.E., and Rayner, H.B., "The Emission of Hydrogen
Sulfide from Kraft Recovery Furnaces," Pulp and Paper Maga-
zine of Canada, 69 (5) 71 (1968).
21. Blue, J.D., and Llewellyn, W.F., "Operating Experience of a
Recovery System for Odor Control," TAPPI, 54_, 7, 1143-47.
22. Franklin, M.E., and Caron, A".L., "Factors Effecting Lime
Kiln Reduced Sulfur Emissions," NCASI Southern Regional
Meeting, July 1971.
23. Blosser, R.D., Cooper, B.H., "Current Practices in Thermal
Oxidation of Non-condensible Gases in the Kraft Industry,"
NCASI Atmospheric Pollution Technical Bulletin No. 34
(November 1967).
24. Morrison, J.L., "Collection and Combustion of Non-condensible
Digester and Evaporator Gases." TAPPI, 52-51, December 1969,
pp. 2300-2301.
25. Britt, Kenneth W., Handbook of Pulp and Paper Technology.
Reinhold Publishing Corporation, New York, 1964.
26.. "Factors Affecting Emission of Odorous Reduced Sulfur Compounds
from Miscellaneous Kraft Process Sources," Technical Bulletin
No. 60, NCASI, New York, March 1972.
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3.3 DESCRIPTION OF THE NSSC PROCESS
This process is mainly used for the production of a high yield pulp
having a high crush strength, important for making corrugated board.
In addition, it utilizes hardwood species that are not readily adapt-
able to the other processes. Coniferous woods are considered less
desirable for the NSSC process because of a higher chemical comsump-
tion during cooking, high lignin content for a given yield, and high
energy requirements for refining.
Recovery of chemicals from the spent liquor is not practiced at the
majority of mills and, therefore, may create a water pollution prob-
lem. Incineration of the liquor may, in turn, create an emission
problem because of sulfur dioxide emission from the incinerator and
will depend on the degree of sulfur dioxide recovery.
Neutral sulfite semi chemical pulping is basically a two-stage process.
It involves:
1. A mild chemical treatment of the wood chips in the
presence of a neutral chemical solution within a
digester and followed by
2. A mechanical treatment, called defibering, to
disintegrate the wood chips into pulp.
It derives its name "neutral sulfite" from the fact that the solution
containing the cooking chemicals, consisting of sodium sulfite and
sodium carbonate, is maintained above a pH of 7.0. Aqueous solutions
of sodium sulfite are basic but during digestion wood chips release
organic acids which if not neutralized will acidify the cooking liquor.
Alkali, usually sodium carbonate, is added to the cooking liquor to
neutralize the organic acids, primarily to reduce the corrosiveness of
the liquor. The higher pH retards hydrolysis of carbohydrates, pro-
ducing higher yields of stronger pulp; but increases bleaching costs
because a darker pulp is produced. The name "semichemical" is given
because all of the cementing material is not completely removed by the
chemical reaction and some mechanical disintegration is required to
separate the fibers. Because some of the cementing material remains
with the fibers it follows that the "yield" for this process is higher
than for a conventional full-chemical pulping process. Semichemical
pulping may produce yields of 60 to 80 percent.
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Various types of semi chemical pulps are produced by the acid sulfite,
neutral sulfite, kraft, soda and cold soda pulping processes. The
major process difference from conventional chemical pulping lies in
the use of lower temperatures, more dilute cooking liquor or shorter
cooking time and mechanical disintegration. Semichemical processes
can be categorized as alkaline or acid.
The NSSC process varies somewhat from mill to mill. Some mills recover
heat and chemicals from the spent liquor, others dispose of spent
chemicals through ponding or other means, including blending with kraft
black liquor. This chapter will contain discussions relative to the
most pertinent operations currently utilized.
The cooking process is carried out in either batch or continuous
digesters. Steam maintains the temperature and pressure of the cook
within certain limits depending on the end use of the pulp. During
this cooking stage odorous gases are created within the digester. The
digester may be partially vented continuously throughout the cook.
This serves to remove carbon dioxide from the system and thus helps
maintain a high pH in the cooking liquor. At the completion of the
cooking cycle, residual pressure within the digester is used to dis-
charge the entire contents of the batch digester into a blow tank.
Waste gases, containing the odorous compounds formed in the digester,
are usually vented to the atmosphere.
At the completion of the cooking cycle, the internal pressure within
the reactor is used to discharge the softened chips to a holding
vessel known as the blow tank. The waste gases are usually passed
through a condenser to remove steam and other condensible vapors.
After excess liquor is separated by draining, pressing, or washing,
the softened chips are reduced to pulp through mechanical treatment
in equipment such as rod mills or rotating disc refiners. The pulp
is then diluted and usually pumped to multistage drum filters where
counter-current washing with water is carried out to remove the spent
liquor from the pulp. Other means for washing the pulp are available
and are used by some mills. The resultant filtrate is termed weak
black (spent) liquor. The washed pulp is then upgraded to the degree
desired in subsequent processing steps and ultimately utilized in the
production of paper products.
3.3.1 CHEMICAL RECOVERY OR DISPOSAL METHODS
Over the years and up to the present, five different methods have been
utilized in disposing of or recovery of the spent liquors from NSSC
pulping operations. These five methods consist of 1) dumping the
effluent with no recovery, 2) mixing with kraft black liquors for sub-
sequent processing, 3) cross-recovery integrated with a kraft mill,
-69-
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4) partial or complete recovery utilizing a kraft type recovery boiler
or smelter, and 5) fluidized bed recovery system. The first method
is obviously no longer considered because of pollution abatement laws
and operating economics. For this reason, this method will not be
further discussed. A schematic diagram of this operation is depicted
in Figure 3-14.
3.3.1.1 Disposal By Mixing With Kraft Liquors
If a kraft system is adjoining, the NSSC spent liquor can be mixed with
the spent kraft liquor, up to a limiting percentage, and burned in the
recovery furnace. Formerly, the ideal ratio was about three parts
kraft pulp to one part NSSC pulp. However, increasingly stringent
emissions controls have reduced the salt cake losses from kraft mills
to the point that the chemicals recovered from one ton of NSSC pulp
production will provide sufficient make-up for eight tons of kraft
production. The recovered chemicals are used entirely in the kraft
system. Emissions of both sulfur dioxide and hydrogen sulfide may be
increased from the kraft recovery furnace when NSSC liquor is added.
This is primarily due to the fact that the NSSC liquor is more acid
than kraft liquor. Most of the increased hydrogen sulfiide losses are,
therefore, from the direct contact evaporator.
3.3.1.2 NSSC Cross Recovery With Kraft Recovery Operation
A second class of process involves recovery of NSSC cooking liquor
via a cross recovery system in conjunction with a kraft mill, as indi-
cated in Figure 3-15. There are several processes in this class
(Tampella, Olinkraft, e.g.). These systems generally recover sodium
carbonate from kraft green liquor by crystallization and add sulfur
by absorbing sulfur dioxide produced by a sulfur burner. The economics
of this process depend upon efficient recovery of gases and liquors at
each stage in the operation and usually air emissions from these de-
vices are minimal.
NSSC black liquor from the pulp mill is accumulated in a large storage
tank, from where it is processed through a triple-effect evaporator.
The heavy liquor, at 52 percent solids, is mixed with strong black
liquor from the kraft mill, and the combined stream is run through the
existing oxidation system (if available) followed by burning in the
existing recovery boiler. The semichemical liquor preparation system
functions as follows. Clarified green liquor from the causticizing
plant is sent to aocrystallizer, where it is cooled to promote crys-
tallization of Na2C03. A centrifuge recovers the carbonate crystals,
while the filtrate is sent back to the kraft green liquor system. This
basic scheme is illustrated in Figure 3-16.
-70-
-------
Digester
Vent
Digester
(Batch)
Pulp To Machine
Spent Cooking Liquor
Flash Tank
i
i
Liquor
Preparation
Na2S03
Na2C03
Spent Cooking Liquor
Spent Liquor
Storage
To Sewer
FIGURE 3-14.
SCHEMATIC DIAGRAM OF NSSC PROCESS
WITH NO CHEMICAL RECOVERY
-------
Kraft
Digesters
ro
i
Kraft
Pulp
White
Liquor
Causticizing
Green
Liquor
Cross
Recovery
Process
Kraft
Pulp
Washing
Black
Liquor
Recovery
Boilers
Cooking
Liquor
NSSC
Digesters
(Continuous)
Green
Liquor
Digester
Vent
Blow lank
Kraft
Evaporators
NSSC
Pulp
NSSC
Evaporators
Blow Tank
Off Gases
NSSC
Pulp
Washing
FIGURE 3-15.
SCHEMATIC DIAGRAM OF CROSS RECOVERY SYSTEM
FOR REGENERATING NSSC COOKING CHEMICALS
-------
Sulfur
Burner
High Sulfidity Li
Kraft
Causticizing
FIGURE 3-16.
FLOWSHEET OF THE TAMPELLA PROCESS OF SODIUM SULFITE
CHEMICALS RECOVERY, WHICH CONVERTS GREEN LIQUOR
CHEMICALS INTO SULFITE COOKING LIQUOR
-73-
-------
Subsequently, molten sulfur is burned in an appropriate system to
produce S02. The S02 is combined with the Na2C03 solution in an
absorption tower. Gas emissions from the liquor preparation and
recovery system are run through a fume scrubber before discharge
to the atmosphere.
The above processes are convenient where the NSSC mill is operated
in conjunction with a kraft mill. In cases where the NSSC mill
exists alone, the spent cooking liquor (after evaporation) may be
oxidized in a fluidized bed reactor. The product (a mixture of .
Na2C03 and Na2S04) of this patented process is not suitable for
re-use by the NSSC mill and in fact it usually must be enriched
with sulfur before it is suitable for use in the kraft process.
This is accomplished by adding sulfur to the fluidized bed unit
along with the feed liquor.
3.3.1.3 NSSC Fluidized Bed Recovery Process
Figure 3-17 shows the fluidized bed system for treatment of neutral
sulfite pulping waste liquors. In these mills, a pulping liquor
containing sodium sulfite (Na2S03) and sodium carbonate (Na2C03) is
used as the pulping medium. During pulping, .lignin and other organic
matter is extracted from the wood and the spent pulping liquor con-
tains sodium-sulfur compounds of varied degrees of oxidation in
association with the organic matter extracted from the wood. The
total solids in the spent pulping liquor have a gross heating value
of about 5,500 BTU per pound (dry), contain about 30 percent carbon
and 3 percent hydrogen.
The spent pulping liquor is received from the pulp washing operation
at about 10 percent total solids. In order to support autogenous
combustion of the liquor in a fluidized bed reactor, it is necessary
to concentrate the waste liquor to about 35 percent total solids, at
which point the fuel values in the waste can sustain combustion with-
out addition of external fuel. This concentration is accomplished
in multiple-effect vertical-tube forced-circulation evaporators of
a special design to minimize tube fouling. Nominally, a three-body,
three-effect evaporation unit is employed at a steam economy of about
2.6. The concentrated liquor, at about 35 percent total solids, is
pumped to a storage facility before introduction into the fluidized
bed reactor.
The temperature of the fluidized bed reactor is maintained at about
1,325°F. The concentrated liquor is introduced as a liquid disper-
sion into the freeboard area of the reactor. This method has several
advantages over introduction of the liquid feed below the level of
the fluidized bed. As the liquid feed contacts the heated exhaust
gases rising from the fluidized bed, a portion of the water content
of the feed is evaporated and the remainder of the feed falls into
-74-
-------
MUTIPLE EFFECT FORCED
CIRC. EVAPORATORS
01
I
CONCENTRATED
LIQUOR STORAGE
PELLET STORAGE
TANK
METERING
SCREW -
FIGURE 3-17. COPELAND RECOVERY SYSTEM FOR NSSC PULP MILLS
-------
the fluidized bed. In the bed, the organic portion of the fe~ed is oxi-
dized to carbon dioxide and water vapor and the residual inorganic
chemicals are oxidized to their stable oxidation state; i.e., sodium
sulfate and sodium carbonate. Oxidation of the organic matter provides
sufficient thermal energy to maintain the reaction temperature. The
product of the inorganic chemicals, sodium sulfate and sodium car-
bonate, deposits in the bed and forms the bed material. The chemical
product can be made to pelletize or agglomerate in the fluidized bed,
and the product is discharged as a nearly perfect sphere in the size
range of 20 to 65 mesh.
The exhaust gases, containing entrained particulate material, are
passed through a cyclone separator where entrained material is separated
from the exhaust gases and returned through a screw conveyor or steam
ejector directly to the fluidized bed. The exhaust gases are scrubbed
with weak waste liquor to remove any remaining entrained particles and
to recover a portion of the sensible heat content of the exhaust gases..
Analysis of these exhaust gases, for the presence of offensive gaseous
compounds, indicates that the effluent contains little if any compounds
that might contribute to air pollution. There are no reduced or par-
tially oxidized gaseous sulfur compounds. . ._...,
3.3.1.4 Direct Sulfitation in Recovery Boilers or Smelters
Another cclass of NSSC process is that which recovers sulfite cooking
liquors directly. The I.P.C.,Mead and Western Precipitation processes
are typical of this group. The process essentially is a recovery
boiler (smelter) modified to produce sodium sulfite rather than sodium
sulfide. Some of these are conversions of old kraft recovery boilers,
others may be new equipment designed for this service.
3.3.1.4.1 Spent Chemical Recovery Cycle
The black liquor filtrate separated from the softened chips and mechani-
cal pulp is concentrated in multiple effect and/or direct contact
evaporators to the desired solids concentration. The concentrated black
liquor is sprayed into a furnace, usually with an auxilliary fuel such
as oil. The furnace is maintained at 1,000°F to 1,300°F.
In the furnace, the sprayed liquor droplets flash dry. The carbonaceous
material chars in the upper oxidizing atmosphere of the furnace and the
inorganic carbonate, sulfur salts, and char fall to the bottom of the
furnace where a bed forms. Primary air to the bed is controlled to main-
tain reducing conditions which will convert sulfur-containing salts to
sodium sulfide. Secondary air admitted above the bed burns hydrogen
sulfide to sulfur dioxide and completes the combustion of the carbona-
-------
ceous material. The sodium sulfide and sodium carbonate form a molten
smelt which flows from the furnace and is dissolved in water to form a
solution referred to as "green liquor".
Because of the sulfur compounds in the black liquor the flue gas from
the furnace contains some sulfur dioxide, and may contain reduced sul-
fur compounds. In order to achieve efficient chemical recovery, the
sulfur dioxide must be absorbed from the combustion gases with sodium
carbonate or caustic solution, a process which presents special prob-
lems because of the low sulfur dioxide concentration. Scrubbing can
also help to remove hydrogen sulfide and methyl mercaptan.
The flue gases also contain sodium sulfate dust with some sodium car-
bonate. For efficient chemical recovery, this particulate matter must
be removed by electrostatic precipitators or high efficiency venturi
scrubbers.
3.3.1.4.2 Preparation of Pulping Chemical
The green liquor, rich in sodium carbonate and sodium sulfide, is pumped
to the top of an absorption tower. There the solution is brought in
contact with sulfur dioxide from a sulfur burner. The concurrent scrub-
bing of the S02-laden gas by the green liquor results in the following
reactions:
2Na2C03 + S02 + H20 + Na2S03 + 2NaHC03
Na2S + S02 + H20 -> Na2S03 + H2S
The scrubbing towers are efficient in their absorption of the sulfur
dioxide. There is, however, a large quantity of hydrogen sulfide pro-
duced which must be burned to sulfur dioxide and abosrbed in sodium
carbonate or eliminated in some other way if atmospheric pollution is
to be avoided. The sulfited green liquor emerges from the absorption
tower as "cooking liquor" and is ready for reaction with wood chips in
the digester.
3.3.2 OTHER NSSC RECOVERY PROCESSES
There are several variations of the NSSC recovery processes based on
burning the spent liquor, and three of these will be briefly discussed
under this section, and as shown in Figures 3-18 through 3-20.
-77-
-------
3.3,2.1 Mead Process
The first of the three methods to be discussed is the Mead Process,
developed and patented by The Mead Corporation. Basically, the process
operates by separating the sulfur from the sodium complex by carbonation
of the green liquor with waste flue gas. The sulfur, driven off as H2S,
can then be oxidized by burning in air either in the furnace or in a
special burner, and is then recombined with sodium by washing the SC^ con-
taining gases with the sodium carbonate solution formed in the first
step. Processes similar in general approach have a considerable his-
tory, but this process has several novel features which are what make
it feasible. The scrubbing of the totality of furnace flue gases with
a Na2C03 solution gives a clean, S02 free, C02 rich, gas to use for
carbonation, as well as minimizes sulfur losses. The splitting of the
gas stream between the precarbo'nation and carbonation towers provides
the maximum concentration of H2S to the furnace and still does not re-
sult in sulfur loss.
Following through the process, the concentrated liquor from the multiple
effect evaporators is evaporated further in the venturi scrubber-
evaporator. This liquor is burned in a kraft type furnace, which oper-
ates with a reducing atmosphere in the "hearth" zone. The molten
Na2S - Na2C03 mixture flows into a dissolving tank. The green liquor
is. then clarified.
The flue gases from the furance pass through the superheater, boiler
and economizer for purposes of steam production, and then to the venturi
scrubber where the gas velocity is increased and black liquor is in-
jected into the gas stream. The high gas velocity (approximately 300
fps) atomizes the liquor. This provides a cloud of droplets with a
tremendous surface for heat transfer and on which the dust in the flue
gas is collected. The liquor is spearated from the flue gas in the
cyclone separator, and the clean gas containing N2, C02, 02 and S02
is passed to the sulfiting tower.
The sulfiting tower makes the cooking liquor by absorption of the S02
from the flue gas in a Na2C03 solution. This carbonate solution is
.obtained from the carbonating towers, where it is made by carbonation
of green liquor with a part of the clean, substantially S02 free, flue
gas from the sulfiting tower. It is in these carbonation towers that
the sulfide in the green liquor is removed as H2S.
The gases leaving the carbonating tower contain H2S and the C02 not used
in carbonization. The concentration of this H2S is sufficiently low to
present difficulty in combustion. Therefore, the precarbonation tower
is a device used to bring about an increase in H2S concentration. About
a third of the gas leaving is passed through this tower. The high alka-
linity of the liquor at this point removes the H2S from the gas stream,
forming NaHS. The gases leaving this tower can be discarded with
-78-
-------
FIGURE 3-18. MEAD PROCESS SIMPLIFIED FLOM SHEET
TO
COOKING
LIQUOR
STORAGE
FIGURE 3-19. INSTITUTE PROCESS
SIMPLIFIED FLOW SHEET
BL.LIQ. . S02,C02,N2
TO STACK
f"TO STOCK
OR
HgS H
bU|
SULFIT
TOWER
GREEN LIQL
NG
IOR
SULFUR
BURNER
FIGURE 3^20. WESTERN PRECIPITATION PROCESS SIMPLIFIED FLOW SHEET
FURNACE
PRECIPITATOR
OXIDATION
BLACK
__rrai,.
THICKENER
COCOg
WATER
SMELT I
OQ.
CRYSTAL
J5R7
w
, TO STACK
SULFATING
AlR
R_N02CO
ss.
FILTER
No
PCX,
SOLI
n
'ION
-79-
-------
negligible loss in sulfur, while the H2S thus absorbed will be released
in the carbonation tower. Consequently, there is a very substantial
enrichment in H2S content of that portion of the gases returned to the
furnace, insuring satisfactory combustion.
3.3.2.2 I.P.C. Process
The next process to be discussed is one involving direct sulfitation of
the green liquor (Figure 3-19). This process has been developed by the .
Institute of Paper Chemistry. It is a less complex system in that no .
sulfur recovery is attempted. The drawback to the process lies in the
production of some Na2S203Which, although inert in the NSSC cook, re-
sults in higher overall chemical losses and possible difficulties with
bleaching. The sulfur losses are complete unless the H2S formed is
recovered.
3.3.2.3 Western Precipitation Process
Basically, the process uses the furnace as a carbonator and, by increas-
ing green liquor concentration, Na2CC>3 is salted out. The sulfide rich
mother liquor is recycled to the multiple effect evaporators and added
to the entering black liquor. The sulfide is stabilized by oxidation
to prevent losses through evolution of H2S during evaporation and sub-
sequent venting. A simplified flow sheet based on the published infor-
mation is shown in Figure 3-20 By scrubbing the flue gases.from the
recovery unit with the carbonate solution, this process could achieve
high recovery efficiencies. It is no simpler than the other processes,
with its oxidation and crystallization replacing gas absorption and
stripping, but it has some advantages. One simplification is that no
H2S has to be piped from a tower to a burner.
3.3.3 EMISSIONS FROM NSSC PULPING
Because of the milder pulping conditions employed, the emissions from
semichemical plants are less intense than those from conventional pulping
processes. The emissions will also vary depending on the process em-
ployed; e.g., neutral sulfite, or kraft. In the latter case, the air
pollution problems will parallel those described under kraft pulping
but will be of a lower order of magnitude. Particulate problems from
recovery furnaces are much the same throughout the industry.
Published data on emissions from NSSC pulping are virtually nonexistent.
Certainly information exists in the files of various mill engineering
departments and perhaps in the files of state and local control agencies.
This data is proprietary however and not available. The data in Table
3-5 have been excerpted from various technical and commercial publica-
tions. The data for newer technology represent improvements made in
-80-
-------
TABLE 3-5
EMISSIONS FROM NSSC PULPING (IBM/TON ADP)
Source po1 Tutant . Techno! ogy
Recovery Furnace S0?
or Smelter H?S
RSH
Total
Sulfiting Tower S0?
H2S
Total
Blow Tank S02
H?S
RSH
Other Organic
Total
Dissolving Tank H2S
S0?
Total
Evaporator H2S
Total Organic S
Total
Mashers H?S
S02
Total
FUndized Bed ' S02
Total Organic S
Total
Cope! and Process SO
Total Organic S
Total
8.20
4.20
0.30
12.70
0.80
8.55
9.35
0.30
' 4.20
1.56
3.12
9.18
0.10
0.04
.0.14
0.13
0.09
0.22
0.05
0.01
0.06
Newer
Technology
2.45 - 1.40
1.10 - 0.20
0.10 - 0.05
3.65 - 1.65
0.30
0.30
0.25
2.10
0.78
1.50
4.63
0.10
0.04
0.14
0.13
0.09
0.22
0.05
0.01
0.06
.004 ~ .008
.004 - .007
.008 - .015
0.06 - 0.20
0.18 - 0.32
0.24 - 0.52
-81-
-------
the past five to six years. Combination of NSSC spent liquor with kraft
black liquor prior to evaporation and combustion results in an increased
sulfur dioxide and hydrogen sulfide emission from the kraft recovery
system. No quantitative data on this increased emission rate are avail-
able.
3.3.3.1 LITERATURE CITED
1. O'Donoghue, Roderick, TAPPI 38 No. 6 162A-7A (1955).
2. Haywood, Gerald, TAPPI 37 No. 2 134A-6A (1954).
3. Collins, T.T., Jr., SOUTHERN PULP AND PAPER MANUFACTURER 19 No. 1
94-106 (1956).
4. Bauer, T.W., Dorland, R.M., CAN. J. TECH. 32, 91-101 (1954).
5. Campbell, Jr., Jr., Shiek, P.E., "The Mead Recovery Process"
presented at the Tenth Alkaline Pulping Conference, New
Orleans, Louisiana, (November 14-16, 1956).
6. Mai, K.L., Babb, A.L., IND. ENG. CHEM. 47, 1749-57 (1955).
7. Whitney, R.P., Han, S.T., Davis, J.L., TAPPI 40, 487-94 (1957).
8. Nugent, R.A., Boyer, R.Q., PAPER TRADE J. 140, No. 18, 29-32,
34, 36, 38, 40, 42, (1956).
9. Boyer, R.Q., "Application of Green Liquor Phase Separation to
Neutral Sulfite-Kraft Integration," presented at the Tenth
Annual Pulping Conference, New Orleans, Louisiana, (November
14-16, 1956).
-82-
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3.3.4 THE EFFECT OF PROCESS AND CONTROL EQUIPMENT VARIABLES ON PULP
PRODUCTION AND EMISSIONS
The following sections describe variables in process and emission
control which affect pulp production rates and attendant emission
levels for the NSSC pulping process.
3.3.4.1 Liquor Pyrolysis and Chemical Recovery Operations
Many of the unit operations utilized in both the NSSC and sulfite
processes are similar to those used in kraft operations. There-
fore, the variable relationships and their subsequent effect on
emissions would be similar, although the range magnitude may differ
somewhat.
NSSC spent liquors are treated in a number of ways, and there exists
very little information regarding process variable and control equip-
ment effects on the attendant emissions. The various methods for
recovery have been previously discussed in this chapter, which in-
cluded among other kraft-NSSC cross-recovery, fluidized bed, and a
modified "kraft type" recovery boiler. Kraft-NSSC cross-recovery
has been discussed under kraft recovery boiler operations and
therefore will not be further discussed.
3.3.4.2 Chemical Recovery Furnace
There is a relationship between the Na/S ratio in the liquor and the
emission of sulfur dioxide. When TRS emissions are reduced the Na/s
ratio decreases and sulfidity increases. Consequently, less alkali
is available for recombination with the S02 in the secondary zone
and thus S02 emission increases. Figure 3-21 _gives the experimental
data showing the relationship between Na/S ratio and S02 emissions.
Adequate turbulence and cooling of the gases is essential to promote
efficient recombination and low alkali losses. There is a need of
about a 1000°F drop in temperature for best recombination results,
but it should be rapid to avoid formation of other sulfur compounds.
Cool tertiary air is also considered to be helpful.
3.3.4.3 Sulfitation Towers
Sulfur dioxide from sulfur burners or other make-up sources are
scrubbed with an alkaline solution of sodium base in the amount
required by the Na make-up of the mill. The solution should have
a molar ratio bisulfite/sulfite of 4:1 at a pH of around 7.0.
-83-
-------
2500 t
2000
I 1500
TD
X
o
-o
S-
1000
500
\
2.0
2.1
a By analysis
o Estimated
2.2 2.3
Na/S ratio
2.4
2.5
2.6
FIGURE 3-21
SULFUR DIOXIDE REDUCTION WITH INCREASE IN Na/S RATIO
-84-
-------
The formation of the bisulfite-sulfite solution takes place in the
original sulfitation tower. The bisulfite-sulfite liquor can be
contacted with green liquor instead of directly with the smelt, as
is often done.
3.3.4.4 D-irect Contact Evaporation
In this type recovery operation, where direct contact evaporators
are involved, removal of S02 is an important tool in reducing the
total emission of sulfur from the smelter, since the smelter, per
se, does not favor the best control of sulfide and sulfur oxide
emissions. In order to improve emission control from the smelters,
existing concentrated black liquor used in the venturi evaporators
may be replaced with a weaker black liquor, containing 12-17 percent
solids for instance.
3.3.4.5 Fluidizied Bed Pyrolysis Operations
Emissions associated with the operation of fluidized bed units
were previously discussed. These systems are not nearly so com-
plex as other recovery methods; and, based on the limited data
available, indications are that emissions for these sources are
minor.
The major variables associated with fluidized bed operations,
which could affect emissions, are the feed solids concentration,
combustion bed temperature, fluid bed height, and the feed liquor
sodium/sulfur ratio. The main variables affecting the venturi
evaporator-scrubber performance are pressure drop across the
scrubber and scrubbing liquor solids concentration.
3.4 DESCRIPTION OF SULFITE PROCESS
The sulfite pulping process produces easy bleaching pulps from
non-resinous woods. The characteristics of the pulp make it
suitable for use in many grades of paper, but it is especially
suitable for tissues and fine papers.
Recovery of cooking chemicals and the heat values in the spent
cooking liquors was not widespread until fairly recent years
when the older calcium base has been replaced in many mills by
a soluble base such as sodium, magnesium, or ammonium. Sodium
and magnesium bases require recovery for economic reasons. Of
the two, magnesium is more widespread in its use because on com-
bustion the inorganic constituents break down directly to magne-
sium oxide and sulfur dioxide which can readily be recycled.
-85-
-------
Spent liquors from ammonia base pulping may be incinerated with
recovery of most of the S02- The ammonia burns completely to
nitrogen and water vapor.
3.4.1 GENERAL DESCRIPTION OF SULFITE PROCESS
Sulfite pulping is an acid chemical method of dissolving the lignin that
bonds the cellulose fibers together. Many of the older mills use a
sulfurous acid - calcium bisulfite solution for the cooking acid. Ca.lcium-
base spent liquor, because of problems associated with evaporation and
chemical recovery, is discarded and may result in water pollution problems.
In order to overcome the problem of water pollution, several other acid
bases have been developed, the most important being sodium, magnesium, and
ammonium.
Because sulfite pulp is used in a wide variety of end products, operations
will vary considerably between mills. These products can include pulp for
making high grade book and bond papers or tissues for combining with other
pulps, and for making dissolving pulp for producing cellophane, rayon,
acetate, films, and others.
The pulping operation involves cooking the wood chips in the presence of
an acid within a digester. The heat required for cooking is produced by
the direct addition of steam to the digester or by the steam heating of
the recirculated acid in an external heat exchanger. The cooking liquor,
or acid, is made up of sulfurous acid and a bisulfite of one of the four
above mentioned bases. The sulfurous acid is usually produced by burning
sulfur or. pyrites and absorbing the S02 in liquor. Normally, part of the
sulfurous acid is converted to the base bisulfite to buffer the cooking
action. During the cooking action, it is necessary to vent the digester
occasionally as the pressure rises within the digester. These vent gases
contain large quantities of sulfur dioxide and, therefore, are recovered
for reuse in the cooking acid.
Upon completion of the cooking cycle the contents of the digester, con-
sisting of cooked chips and spent liquor, are discharged into a tank.
During this operation some water vapor and fumes escape to the atmosphere
from the tank vent. The pulp then goes through a washing stage, where
the spent liquor is separated from the fibers. The washed pulp is either
shipped or kept within the plant for further processing. Block diagram
flowsheets of the sulfite pulping processes are contained in Figure 3-22.
The spent liquor that was washed out of the pulp can be discarded or, as
an alternative, can be concentrated by evaporation and run through a re-
covery cycle. The concentrated liquor is sprayed into a furnace where
the organic compounds are burned. The residual inorganic compounds may
be collected and reused in the manufacture of cooking acid.
-86-
-------
A-Tower System
B-Milk-of-Lime System
COMBUSTION
CHAMBER OR
SCRUBBER
1
t
REUEF(b)
SULFUR..
OR
PYRITES
SULFUR.. BURNER!
0 R
PYRITES FURNACj
WATER
F=l
SLAKINS TOWER
| L'ME j 1 | ABSORPTION TANK3_»j EXHAUSTER
ATM.
WASTE LIQUOR
TO SEWER
OR RECOVERY
WASTE LIQUOR
TO SEWER
OR RECOVERY
FIGURE 3-22.
SULFITE PULPING FLOW SHEETS
DIRECT OR INDIRECT COOKING
WITH OR WITHOUT FORCED LIQUOR CIRCULATION
-87-
-------
3.4.2 SULFITE PROCESS VARIATIONS
There are a wide variety of processes in use for recovery and disposal
of spent sulfite pulping liquors. Because for a number of years there
was little attempt to recover chemicals at all, and during the en-
suing years a variety of systems were applied as process modifica-
tions, there is very little available comparing operating variables.
Only recently have several systems been designed for complete recovery
applications in sulfite operations. This section, will be devoted to
briefly discussing some of the operating variables associated with
certain systems.
3.4.2.1 Ammonia-Base Liquor Burning and Sulfur Dioxide Recovery
The use of ammonia-base for sulfite pulping requires consideration
of waste liquor burning and recovery,of sulfur dioxide - the com-
bustion product. Depending on the pulp products desired, an ammonia-
base mill can use any one of three basic pulping techniques: acid
sulfite, bisulfite, and neutral sulfite semichemical.
Coupled with the liquor burning process, one of the combustion
products, sulfur dioxide reacts in an absorption system with anhydrous
or aqueous ammonia makeup chemical to produce ammonium bisulfite
acid. Several mills in North America are burning ammoniabase
liquor without sulfur dioxide recovery.
Limited data from the test on ammonia-base liquor burning indicates
solid loading in the stack gases was 1 Ib per 1000 Ib of flue gas un-i
der stable operating conditions. Limited flue gas analyses showed
1 % sulfur dioxide by volume on a dry basis and 100 to 300 ppm of sul-
fur trioxide. It has been determined small variations in firing
liquor solids content can have a major influence on the heat load and
burning intensity. The higher flame temperature should reduce un-
burned carbon carry over.
Burning intensity is also adversely affected by large particle size,
hence the necessity to obtain good atomization. Good atomization is
affected by liquid viscosity. Table 3-6 and Figures 3-23 and 3-24
relate some of the operating variables associated with ammonia-base
liquor burning.
In absorption system design, interdependent factors affecting absorp-
tion effectiveness must be balanced. Acid concentration, concentra-
tion of sulfur dioxide in the gas, and operating temperature determine
the arrangement and amount of absorption surface.
-88-
-------
Table 3-6. Ammonia-Base Liquor Burning Tests.
Liquor rate, Ibs/hr
Solids concentration, %
Natural gas rate, Ibs/hr
Heat release rate,
million BTU/hr
Total air at stack, %
Air temperature, °F
Liquor temperature, °F
Furnace exit gas tempera-
ture, °F
Stack dust loading,
lbs/1000 Ibs gas
Average S02 at stack
volume, %
Average $03 at stack
volume, ppm
Test 1
773
48.4
180
6.9
105
500
206
2240
0.42
70
Test 2
909
44.6
157
6.7
108
750
165
1800
0.70
0.9
84
Test 3
1100
53.7
0
5.1
110
750
165
1820
1.4
1.13
350
Test 4
815
54.6
0
3.75
114
550
165
1750
0.70
1.01
10
-89-
-------
3600
3?00
o:
LU
D.
LU
2300
2400
100
80 60
Solids, weight °t
40
Figure 3-23. Effect of ammonia-base liquor solids content on flame
temperature.
-90-
-------
. 160
180 200
TEMPERATURE, °F
220
240
Figure 3-24. Viscosity of ammonia and magnesium liquors (54.5%
solids content.)
-91-
-------
Available technology limits absorption system design to a temperature
level of about 100°F, requiring that the combustion gases leaving the
boiler be cooled by dehumidification before entering the absorption
equipment. In one operation, combustion gases were cooled to 104° in
a packed cooling tower, and 99% recovery of sulfur was achieved from
gases containing 0.5-0.7 vol. % sulfur dioxide in the production of
a 6.25 pH solution of ammonium bisulfite and sulfite.
3.4.2.2 Magnesium Oxide Recovery System
The recovery of magnesium in a sample recovery system is made pos-
sible by the chemical and physical properties of the base chemical.
Spent liquor burned at elevated temperature in a controlled oxidizing
atmosphere yields the base in the reusable form of an active magne-
sium oxide. The oxide is readily recombined in a simple secondary
system with sulfur dioxide produced in the combustion to yield cooking
acid for pulping.
The concentration of sulfur dioxide in the stack gas from a recovery
system can be maintained at less than 250 ppm by dry volume. For a
magnefite mill, this is about 3 Ibs sulfur per ton of unbleached
pulp, which is a small amount compared to the total mill sulfur
loss of 70-80 Ibs/ton. The 250 ppm is equivalent to 2-3% of the
sulfur in the liquor feed to the recovery furnace.
The sulfur dioxide recovery efficiency of the three absorption
Venturis has been measured at 97.5% when producing an acid with an
average concentration of 5.7% total sulfur and 3.2% combined sul-
fur dioxide. Concentrations of sulfur dioxide in the gases
entering and leaving each absorption venturi are plotted in Figure
3-25. There is considerable difference in the capacity and per-
formance, of recovery systems designed for the various processes.
This diversity is presented by Tables 3-7 and 3-8, using as a
basis of comparison typical conditions for a kraft recovery unit.
3.4.2.3 Other Magnesium Oxide Sources
Test data from a magnefite pulp recovery system presented by Figure
3-26 illustrates factors to be considered in an evaluation of sec-
ondary system performance. The system consists of a packed tower
for cooling combustion gases to 104°F prior to their entering a
series of three absorption Venturis.
Sulfur dioxide gas from the burning of makeup sulfur enters the re-
covery system at the induced draft fan inlet after passing through
the fortification system and this quantity is included in the gas
analysis at the cooling tower inlet. Digester relief gases enter
the recovery system at the base of the cooking tower.
-92-
-------
oo
ii
00
o
sg
1.2
1.0
0.8
0.6
0.4
d£ 0.2
00 O
INTL
VENTURI
f
2ndiVEN
EN^UIR
IN1,ET } AVG. =
£,
1.05%
. ,
IN^ET f AVG. = 0.765%
T
Figure 3-25.
Secondary recovery system performance for a
product acid composed of 5.75% total SO^
and 3.25% combined S02.
-93-
-------
Table 3-7. Recovery Unit Performance - Relative Comparison for 1-lb
Solids.
Liquor heating value,
BTU/lbs solids
Thermal efficiency,
% total input
Steam generation,
Ib/lb solids
Flue gas leaving
evaporator, Ib/lb solids
Table 3-8. Recovery Unit
Unbleached Pul
Lb solids/a. d. ton
unbleached pulp
Yield, % (approximate)
Solids flow, Ib solids/hr
Total heat input, million
BTU/hr
Steam flow, Ibs/hr
Lb steam, ton pulp
Board
Linerboard
6600
59.7
3.7
7.0
Performance at
P
Linerboard
3000
45
37,500
213
139,000
11,120
Magnesium
and
Magnefite Bisulfite
News Dissolving
Furnish Grade
6800
64.1
4.1
7.0
300 Tons/Day
Magnefite
News
Furnish
1850
60
23,120
169.5
95,000
7,600
8000
68.9
5.2
8.5
Air-dried
Magnesium
and
Bisulfite
Dissolving
Grade
3200
37
40,000
343.0
208,000
16,649
Flue gas leaving cyclone
evaporator, Ibs/hr
262,500
162,000
340,000
-94-
-------
00
ii
c/l
-------
Overall performance data indicates that the concentration of sulfur
dioxide in the gas discharged to the atmosphere gives a more defi-
nite measure of system performance than an efficiency value based
on inlet and outlet concentrations of sulfur dioxide.
3.4.2.4 Jenssen Exhaust Scrubber
Sulfite cooking acid is often produced in a Jenssen-type, two
tower limestone system. Data show that weak tower exhaust
contains an average of 0.5% S02 with trace amounts of sulfur tri-
oxide. These data are summarized in Table 3-9. Typical mass
transfer type scrubbers have resulted in S02 recovery efficiencies
of over 98%, with about 50 ppm S02 in the scrubber exit gases.
Data on the liquid composition of the NaOH-SO?-H20 system as a
function of pH are plotted in Figure 3-27, and a schematic of the
scrubbing system indicating controlled variables is shown in
Figure 3-28.
3.4.2.5 Scrubbing Blowpit Gases
Sulfur dioxide loss from digester blowpit vents is a major emission
source for most sulfite pulp mills. Such losses may be virtually
eliminated by the application of a chemical scrubber on the resid-
ual gas from an existing water absorption S02 recovery system. The
gas collection system for the .scrubber is designed such that a
portion of the high volume, low S02 content gas released during the
initial 45 seconds of the digester blow bypasses the tower and is
vented to the atmosphere. During the remaining 13 to 14 minutes
of the blow, outside air is drawn in, allowing the scrubber to
process a constant volume.
Scrubbing liquor is a mixture of makeup NaOH and a 4.5% solution of
Na2 C03/NaHC03 from the recovery process at a pH of 9-10. Operating
the tower at a rate of 10,500 lbs/ft2 liquor and 2,000 lbs/ft2 gas
(90°F) has resulted in S02 absorption efficiencies of 99 to 99.5%.
Because nearly all of the sulfite mills built in the past 20 years
have used the magnesium-base system, this report will give greater
emphasis to a description of that process. The magnesium-base sys-
tem provides maximum recovery of process chemicals and minimizes
air and water pollution problems connected with the pulping system.
Aside from the liquor recovery system, however, the sulfite process
is independent of the base chemical used. Figure 3-29 illustrates
a typical system.
-96-
-------
Table 3-9. Jenssen Tower Gas Analysis.
Gas
Sulfur Dioxide
Oxygen
Nitrogen
Carbon Dioxide
Sulfur Trioxide
Concentration
Inlet
(%)
20.5
5.1
74.3
0.1
0.03
Concentration
Exhaust
(%)
0.5
5.3
85.3
8.9
0.03
-97-
-------
Q.
90
80
70
60
50
40
06
07 08 09 10
WEIGHT RATIO,1 NaOH SO,
12
Figure 3-27.
Liquid composition of the NaOH S02-HpO system as a
function of pH. By defining two variables, the
third is fixed.
-98-
-------
JEHSSEN
EXHAUST
J
, jsUv ..-^S-L--'1
1 i>NY^A>\J
15% S00
->
ABSORPTION
COLUMN
TO NSCM
PROCESS
Figure 3-28. Jenssen exhaust scrubber process flow. (FIC, flow control; DIG, density
control; pHIC, pH control.)
-------
Mg(HS03)2 + H2S03
Cooking Liquor
Chips
To Atmosphere
_L
MgO To Stack
Blow Gas
Digester
o
o
Blow
Scrubber
Blow
Tank
Pulp
Fi1ter
Pulp
Spent (Red)
Liquor
Slurry
OJ
-Q
-Q
3
i~.
O
CO
Abs.
Tower
(HS03)2
Abs.
Tower
iy (HSOo)-
Abs.
Tower
Cooled Gases
(1% S02)
Scrubber
ID Fan
O
To Atmosphere
Mult.
Effect
Evaporators
Concentrated
Red Liquon
Sulfur
Makp-yp .
MgO Slurry
Magnesia
Make-up _»,
Water -.
V
!
/
SI urry
Tank
Recovery
Furnace
Gases
MgO + S02
Gases
'(1% S02)
Cyclone
FIGURE 3-29. SULFITE PULPING PROCESS
MAGNESIUM BASE RECOVERY
-------
The cyclone collection system, coupled with the wet scrubbers and absorbers
used to remove S02 from the recovery furnace gases, control particulate
losses from this operation. In addition, many mills generate their own
steam and power, and those using fossil fuel and bark are subject to the
normal emissions from this type of equipment.
A summary of emission data relative to various sulfite pulping processes
is contained in Table 3-10.
-101-
-------
TABLE 3-10
SUMMARY OF EMISSION DATA - SULFITE PROCESS
(Emissions in Ibs/ADT)
Digester Blow Pit 40-100
or Dump Tank 10-25
Water Scrubbers Following Blow Pit 0.8-20
Chemical Scrubbers Following Water Scrubbers 0.015-0.4
i
o Oenssen Tower Exhaust 10-15
ro
i
Scrubbers Following Jenssen Tower Exhaust 0.22-0.5
Multiple- Effect Evaporators 5-10
Recovery Furnaces,(Magnesium Sulfite Process) 50-75
Following Scrubbers 3-15
Incinerators, Ammonium Base 250-500
Particulate
-------
3.4.3 PULPING PROCESS
The pulping operation (or cook) is carried out in a pressure vessel called
a digester. Up to the present time, batch digesters have been used ex-
clusively for sulfite pulping. Recently, the first continuous sulfite
digesters have been put into operation. Batch digesters may be as large
as 10,000 cubic feet capacity and hold up to 100 tons of wood chips. The
cooking liquor, containing a mixture of magnesium bisulfite and sulfurous
acid, is pumped into the digester cold and then heated by steam in an ex-
ternal heat exchanger or by direct addition of steam to the digester.
Normal maximum cooking temperatures will range from 250°F. to 300°F. at
pressures up to 110 psi depending on the pulp grade being made. Cooking
times will vary from as low as six hours to as high as 20 hours. Low
temperatures and longer cooking times are associated with a special type
of sulfite pulp known as Mitscherlich pulp for glassine paper. Where
dissolving pulps are being made, careful control at the digesters is ex-
tremely important to final pulp quality.
During the cooking cycle, it is necessary to vent the digester occasionally
as the temperature rises. These "relief" gases are rich in sulfur dioxide
and are collected in accumulators for direct reuse. When the accumulators
are operating properly, little or no air pollution is associated with the
cooking cycle.
Upon completion of the cooking cycle, the digester pressure is reduced to
atmospheric pressure and the pulp is washed into a tank. In some cases,
the dump tank has a perforated, false bottom to allow the pulp to be washed
in the same tank or blow pit. Newer systems dilute the pulp and pump it
to countercurrent drum washers. During the blow, sulfur dioxide and volatile
materials are released. The blow gases are normally passed through an
absorption tower to remove sulfur dioxide. Although the noncondensable
gases from a sulfite mill have a characteristic odor, in general, they have
not been considered offensive and in comparison with the much more serious
problems associated with kraft pulping, little attention has been paid to
the air pollution .aspects of sulfite pulping.
After being discharged from the digester, the pulp goes through a washing
stage. Spent liquor from this stage is sent to evaporators, if chemical
recovery is practiced, and the washed pulp is either sent to the bleach
plant or is used directly in products requiring unbleached pulp.
Up to this point in the system, the process is independent of the base
used.
-103-
-------
3.4.4 SPENT CHEMICAL RECOVERY CYCLE (MAGNESIUM-BASE SYSTEM)
The spent liquor (red liquor) separated from magnesium-base pulp is
concentrated in multiple-effect evaporators to the desired concentra-
tion for firing the recovery furnace. As the strong red liquor is
showered into the furnace, the organics dissolved from the wood burn
away, leaving a magnesium sulfite ash. At the temperature of the
furnace (about 1600°F.), the ash dissociates into magnesium oxide (MgO)
and sulfur dioxide (SC^)- The magnesium oxide is removed from the
flue gas by a cyclonic collection system and is slurried in water.
This slurry is later caused to react with sulfur dioxide to form the
magnesium bisulfite for the cooking acid. The sulfur dioxide is
scrubbed from the flue gases with water to form the sulfurous-acid
solution for cooking acid.
Chemical recovery is efficient for both magnesium oxide and sulfur
dioxide. The wet scrubbers used to recover sulfur dioxide from the
flue gases also remove particulate matter getting past the cyclonic
separators. The major components of a magnesium bisulfite recovery
system are illustrated in Figure 3-30.
3.4.5 PREPARATION.OF PULPING CHEMICALS
The magnesium-oxide slurry from the recovery furnace is fed into an
absorption tower with sulfur dioxide scrubbed from the flue gases or
produced in a sulfur burner. Magnesium bisulfite is formed as follows:
MgO + 2S02 + H20 -* Mg(HS03)2
In the preparation of calcium-base sulfite liquor, S02 from a sulfur
burner is passed through a limestone-packed tower countercurrent with
water to form Ca(HS03)2. An excess of S02 is added to form sulfurous
acid according to the need for the grade of pulp being made. Pulp
characteristics and pulping conditions can be affected drastically by
varying the ratio of "combined" and "free" S02. Some minor SO? losses
may occur around the absorption system, but no data are available with
regard to the extent of such losses.
3.5 SULFITE RECOVERY SYSTEMS, PRESENT AND FUTURE PROSPECTS
The choice of a recovery process for sulfite pulping is not a simple
one. This is in part because sulfite recovery technology is still in
a state of active development, as evidenced by the variety of processes
now in use or proposed for use. This is also in part because of the
widely differing requirements with regard to both cooking liquor com-
position.and load on the recovery over the whole range of sulfite pulp-
.ing processes from high yield semi-chemical pulps to low yield dissolving
pulps. And, this is in part because of the varying interrelationship
-104-
-------
STEAM FOR PROCESS
AND POWER
CHIPS
DIGESTER
RECOVERY
BOILER
S02IN FLUE OAS
STRONG
RED
LIQUOR
STORAGE
FORTIFICATION
TOWER
MECHANICAL
DUST COLLECTOR
MAKEUP
Mg(OH)_
LIQUOR HEATER
MAKEUP
SULFUR
COOKING
ACID
STORAGE
SULFITE
LIQUOR
EVAPORATORS
SULFUR
BURNER
3 STAGE
WASHERS I »
WEAK
RED
LIQUOR
FIGURE 3-30. MAGNESIUM BISULFITE PROCESS FLOM
-105-
-------
- of technical, ecological, and economic factors for each installation, as
a function of size and location as well as of special situations, such
as integration with other pulping processes or the manufacture of by-
products. It is, nevertheless, possible to draw some general conclusions
regarding the present or probable trends in sulfite recovery, based on
the information available today. It is believed that these choices
represent the most direct solutions to the problem of .eliminating air
and stream pollution from spent sulfite liquors.
3.5.1 SULFUR RECOVERY ONLY
For small, independent, low combined acid sulfite mills with either
calcium or ammonia base, the lowest' cost solution would appear to be
based upon recovery of heat values and the possible recovery of sulfur
dioxide, but with no attempt at recovering the base for reuse. _ After
concentration, both liquors may be burned in a conventional boiler
equipped with a Loddby furnace or in a fluidized bed; and the sulfur
dioxide may be recovered from digester blow gases and from the spent
liquor prior to evaporation. When burning in a conventional furnace,
adequate dust collection should be provided and for the ammonia base
liquors, nitrogen oxides may present a problem at higher furnace tern-.
peratures. Both boiler and evaporator investments may be minimized by
use of most of the recovered heat for direct contact liquor evaporation.
For most paper-grade pulps, higher yields may be obtained with soluble
bases, which should justify the use of ammonia base for small installa-
tions.
It might also be noted that Spring Chemicals, Ltd. has recently announced
recoveries for calcium, magnesium or ammonia base acid sulfite or bi-
sulfite liquors, with recovery of both the base and sulfur dioxide. The
first step is precipitation of calcium as calcium sulfite, which reduces
or eliminates scaling and also serves as the base recovery in calcium
base pulping.
3.5.2 MAGNESIUM-BASE ONLY
For larger installations, use of magnesium or sodium with recovery of the
base can be justified. The recovery of the magnesium base can be carried
out by conventional (Babcock & Wilcox and Lenzing) or fluid bed (Copeland)
combustion to give magnesium oxide in a form which may be reused in the
process. These available recoveries are now well established; however,
magnesium base systems are limited to pulping on the acid side and,there-
fore, are not suitable for the newer high strength alkaline sulfite
pulping processes.
Sodium base may be used over the entire pulping pH range. In part, for
this reason, there are a greater variety of sodium base recoveries with
none so well established as those for magnesium base.
-106-
-------
3.5.3 CROSS AND SALTCAKE RECOVERIES
Where location and capacities are suitable, the simplest sodium base
"recovery" is combustion of the spent sulfite liquor as make-up with
kraft liquor in the kraft recovery furnace (cross recovery). For small
isolated mills, sodium base liquors may be burned in a fluid bed to
produce by-product sodium carbonate and sodium sulfate, suitable for
kraft make-up or as alkali in the manufacture of glass (Copeland and
Dorr-Oliver).
3.5.4 CONVENTIONAL SODIUM RECOVERIES
The larger, independent sodium base' pulp mills must have their own re-
coveries. The majority of present sodium base recoveries are based on
the displacement of hydrogen sulfide from kraft-type green liquor with
carbon dioxide, the combustion of the hydrogen sulfide to sulfur dioxide,
and the recombination to form a pulping liquor (Sivola-Lurgi, Stora
Kopparberg, and Tampella). One is based on so-called "shock pyrolysis"
to separate sulfur as hydrogen sulfide from the sodium base, which is
recovered as sodium carbonate (SCA-Billerud). Ion exchange is also
practiced for partial recovery of sodium base (Ontario Paper Co.).
Direct sulfitation of green liquor with recovery only of the base has
also been used (IPC).
3.5.5 NEWER' SODIUM-BASE- RECOVERIES
In their more recent forms, sodium base sulfite recoveries are reported
to be economically competitive with comparable magnesium base recoveries.
They are also of the same order of cost as kraft recovery, in particular,
where the higher yield of the sulfite pulp places a lower organic load
on the recovery system. With further development of sulfite recovery
systems, these costs should be further reduced. Several such systems
are depicted in Figures 3-31 through 3-35.
Among such possible developments are bisulfite sulfitation (IPC), solid
state carbonation (Mitsubishi), a new and simplified green liquor car-
bonation recovery (Owens-Illinois-Vulcan Cincinnati), direct oxidation
of smelt or green liquor (Bratislava-Lurgi and Owens-Illinois), and fluid
bed pyrolysis (Owens-Illinois).
3.6 SUMMARY OF AIR POLLUTION PROBLEMS
The terpene-like odors from the digesters and leaks of SOp in the various
accumulators, storage tanks, washers, dump tanks, absorption towers, etc.,
are the normal sources of air pollution connected with sulfite pulp mills.
Sulfur dioxide can readily be removed from gas streams with water scrubbers,
Most sulfite mills in operation today find it economically sound to re-
cover all the S02 possible.
-107-
-------
CONCENTRATED
SPENT
LIQUOR
\
p RASES*. ,. ^__
SMELT
1 '
GRANULATION
No2CO a Nq2S
STFAM Z U*IUAIIU
^~~~^^ '
NcgCOj i 6L Na2S03
H«0 *> SOriJTION
N^co^a^so,
COOKING LIQUOR Pf
10 ABSORPTION .... n.0
1
;o3a
Na2S03
1 1
EPARATION
FIGURE 3-31. DIRECT OXIDATION RECOVERY
CONCENTRATED CONaCpi!T5ATED
SPENT LIQUOR
LIQUOR I
i
, ^MELT **£mo*
H2o * SOLUTION » H2s BURNER! i No2so3
Na2coja Na23 H s' S02
1 2I . f
BISULFITE 1 1 S02 *
TTPAM < ^. ^A Mullen 1 ^m
STtAM »- R|]| F|TAT|ON -«-NaHSOvl ABSORPTION "* ,TACK
NapSO, NaHSO
i t
COOKING LIQUOR PREPARATION
- - «^ SFOONDARY
r-n»«iF«-^ PYROLY3IS - GASES-^ I
GASES'^ 6ASES-*- COMBUSTION ^
No2C03aC FLUEOA3
sri- ^1
-AIR ». OXIDATION . ». . . »>
- HsO^* ABSORPTION STACK GAS
Na2C03
H20 SOLUTION Na,C03
1 1
N,, C03 MP2S03aNoH303
COOKING LIQUOR PREPARATION
FIGURE 3-32. BISULFITE SULFITATION RECOVERY FIGURE 3-33. FLUID BED PYROLYSIS RECOVERY
-108-
-------
CONCENTRATED
SPENT
LIQUOR
CONCENTRATED
LIQUOR
STACK SA3
FIGURE 3-34. SOLID CARBONATION RECOVERY FIGURE 3-35. GREEN LIQUOR CARBONATION RECOVERY
-109-
-------
CHAPTER 4
MONITORING PROCESS AND CONTROL EQUIPMENT VARIABLES
This chapter presents those major process and control equipment
variables that may be monitored with in-line instrumentation
associated with modern wood pulping operations. The importance
of the measurement of these variables as they relate to pro-
duction and emission levels has been described in Chapter 3.
In addition to standard on-line process instrumentation, applied
to both process and control equipment, emission monitors for
particulate and gaseous components that have proven application
are discussed. Major emphasis has been placed on the kraft
pulping process because of its dominance in the chemical wood
pulping industry.
4.1 KRAFT PROCESS INSTRUMENTATION
Tables 4-1 and 4-2 in concert with Figures 4-1 through 4-13
depict instrumentation symbols and codes and standard kraft
process flow sheets with superimposed in-line instrumentation.
The descriptions and depictions presented herein are for a kraft
pulp mill and includes a typical bleaching operation. Although
the kraft process was utilized for this illustration, the same
approach would apply to both the sulfite and NSSC processes.
-110-
-------
Table 4-1. Kraft Mill Instrument Applications.
DIGESTER ROOM
Liquor drawdown and refill
Total steam flow
Heater condensate level
Heater condensate conductivity
Steaming
Temperature-pressure
Gas-off
Blow tank level
Turpentine condenser temperature
Liquor circulation flow
BROWN STOCK HASHER
Blow tank level
Blow tank motor load dilution
Stock to washer flow
Knotter dilution
Shower
Filtrate tank level
Repulper dilution
Strong liquor baume
Strong liquor filter
Liquor to evaporator - flow
Strong liquor storage level
Washed stock storage level
H. D. chest dilution
SCREEN ROOM
Washed stock storage level
Consistency
Stock flow
Dilution
Screened stock storage level
BLEACH PLANT
Screened stock storage level
Consistency
Chemical flow
Tower levels
Tower temperatures
Dilution
Mixer temperatures
Shower flows
Total steam flow
Total water flow
Fresh water make-up flow
Filtrate overflow to sewer
Bleach stock storage level
H. D. storage dilution
Chemical storage level
Effluent to sewer - flow
Hot water temperature
-111-
-------
Table 4-1. Kraft Mill Instrument Applications.
(Cont'd)
BLOW STEAM HEAT RECOVERY
Condenser temperature
Condenser water make-up temperature
Heat exchanger temperature
Clean hot water tank level
Clean hot water tank temperature
BLACK LIQUOR EVAPORATOR
Strong liquor storage level
Strong liquor baume
Flow ratio - liquor to 5 and 6 effects
Temperatures
Pressure and vacuum
Soap tank level
Steam flow and pressure
Boiling point rise
Condensate level
Thick liquor flash tank level
Condensate diversion
Thick liquor storage level
RECOVERY
Thick liquor storage level
Thick liquor flow
Cascade evaporator level
-112-
Precipitator wet bottom level
Multiple indicating draft gauge
Temperature in & out - cascade
Liquor-salt cake ratio
Primary heater temperature
Secondary heater temperature
Liquor to nozzles-pressure
Feed water control system
Oxygen
Steam temperature and pressure
Air flow
Dissolving tank density
Dissolving tank level
CAUSTICIZING
Liquor storage level
Raw green liquor flow
Green liquor temperature
Hot water temperature
Clarifier rake-torque
Water to lime mud filter
Lime mud density
Lime mud filter shower flow
Filter cake thickness
Lime mud filter vat - level
-------
Table 4-1. Kraft Mill Instrument Applications.
(Cont'd)
CAUSTICIZING (continued)
White liquor flow
KILN
Oil temperature
Oil pressure
Oil storage level
Hot end temperature
Exit gas temperature
Draft control and gauge
Atomizing steam pressure
Kiln speed
Drive motor load
Scrubber level
CHEMICAL MAKE-UP AND HANDLING
Storage levels
Chlorine gas pressure
Chlorine vaporizer temperature
Caustic strength of solution
Strong caustic temperature
Hypo mixing
Chemical flows
Lime feeder
CHLORINE DIOXIDE PLANT
Chemical storage levels
Chemical mixing
Temperature
Cold water make-up
Spent liquor pH
Chlorine dioxide flow
High-low alarms
-113-
-------
Table 4-2 TYPICAL LETTER INSTRUMENTATION SYMBOLS
Process
variable
Analysis3
Burner Flame
Consistency
Density
Electric6
Flow0
Hand
Level
Moisture
Pressure0
Speed
Temperature0
Viscosity
Weight
t letter
CO
t.
Ll_
A-
B-
C-
D-
E-
F-
H-
L-
M-
P-
S-
T-
V-
W-
cn
c
o
TD
t i
-I
AI
BI
CI
DI
El
FI
HI
LI
MI
PI
S-
TI
VI
WI
ricaDL
en
c
-a
i.
o
o
01
CtL
-R
AR
CR
DR
ER
FR
HR
LR
MR
PR
SR
TR
VR
WR
u i ny i
0 HI
en o c
en c -i- o
C T- > -r-
i +-> aj -t->
N n3 T3 rtj
r- t- >
i en to t-
ea oj t/i tii
O C r- _Q
1 -r- C3 O
-Q -G
AQ
EQ
FQ FG
LG
SQ
VG
WQ
pin
c
ca
-T
AT
BT
CT
DT
ET
FT
LT
MT
PT
ST
TT
VT
WT
cUlbllll til
en
c
n3
O
-b
tr
-IT
AIT
BIT
CIT
DIT
EIT
FIT
LIT
MIT
PIT
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HP
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MP
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NOTES
Readily recognized self-defining symbols such as C0?, 0?, pH, and ORP may be used in place of A.
i £ £
Letter subscripts c (current), v (voltage), or w (power) may be used after E to designate type of measurement.
0 Lower case letters r, d, and t may be inserted to distinguish ratio, difference, and time respectively.
Q may also be used with letters I, R, and C where indicating, recording, or control functions also exist.
e Lower case letter subscripts 1 and h may be used to denote low or high alarm functions.
/ If required note (a) could be expanded to cover any first letter, not in conflict with those listed, as long as a description for such a
designation is part of the flow diagram.
-------
LOCALLY
MOJMTEO
o
JNSTIiUM E NT -
SERVICE AND FUNCTION
TRANSMITTER
Hi'
.PRIMARY FLOW
DEVICE
JWCUM ATI C.TRANSMISSION..
JM3I!M'!ML (ELECTRIC
TRANSMISSION THE SAME
EXCEPT FOR TYPE OF
CONNECTION.)
LOCALLY
MOUNTED
BOARD
MOUNTED
COMBINATION INSTRUMENT
OR DEVICE WITH TWO SERVICES
OR FUNCTIONS
DIAPHRAGM
MOTOR VALVE
ELECTRICALLY PISTON
OPERATED
VALVE
(SOLENOID OR
MOTOR)
OPERATED
'VALVE
3-WAY BODY
FOR ANY VALVE
(HYDRAULIC
OR PNEUMATIC)
SAFETY
(RELIEF)
VALVE
PNEUMATIC TRANS-
MlSSjON FROM
_[NSTRU^E_NJ JTO
DIAPHRAGM 'MOTOR
VALVE
SELF ACTUATED
(INTERGHAL)
REGULATING VALVE
INSTRUMENT AIR LINES
INSTRUMENT PROCESS PIPING
INSTRUMENT CAPILLARY TUBING
INSTRUMENT ELECTRICAL LEADS
FIGURE 4-1 TYPICAL PICTORIAL INSTRUMENTATION SYMBOLS
-115-
-------
A-
LOCALLY
MOUNTED FLOW
RECORDER
FLOW TRANSMITTER
WITH PNEUMATIC
TRANSMISSION TO
BOARD-MOUNTED
FLOW RECORDING
CONTROLLER
LEVEL TRANSMITTER WITH
PNEUMATIC TRANSMISSION TO
BOARD-MOUNTED LEVEL
RECORDER CONTROLLER
TEMPERATURE TRANSMITTER WITH
PNEUMATIC TRANSMISSION TO
BOARD-MOUNTED TEMPERATURE
RECORDER CONTROLLER
CASCADE CONTROL LOOP PRESSURE RECORDER
CONTROLLER SETS FLOW RECORDER CONTROLLER
FIGURE 4-2 TYPICAL BASIC INSTRUMENT DIAGRAMS
-11.6-
-------
STEAM
RELIEF
Flftl iPF
("U'^F
\J v> i ^ (-...
i^3TKUiViEMTAT!OM FOR KRAFT BATCH DIGESTER
FT-I
STEAM
F..OV/ TRANSMITTER
FHC-I STKAM FLOW RECORDING
CONTROLLER
LSR-I LOW SELECTOR RELAY
RR-I Ri£VEf?SING RFLAY
FY-I STI-:AM FLOW CONTROL VALVE
PT-2 DIGESTER PHtySUHE TrtAiJGMlT
CPC-2 CAM PRESSURE CCNTROLLER
TPR-2 D!(.-cGT;IR TEf/IP. PRESS. RECORDER
FT-3 RELIEF FLOW TRANSMITTER
PIC-3 RELIEF FLOW INDICATING
380-4 SLOW BACK CONTROLLER
SBV-4 BLOW SACK CONTROL VALVE
-117-
-------
L.P. STEAM
CONDENSER
CXI
I
WHITE fi\
LIQUOR VLi
BLACK
LIQUOR
STEAM
TO
EVAPORATOR
»TO BLOW TANK
COLO BLOW LIQUOR
FIGURE 4-4 INSTRUMENTATION FOR A CONTINUOUS DIGESTER
-------
FIGURE 4-5 HOT STOCK SCREENING INSTRUMENTATION
PRIMARY
BROWN STOCK
WAShER
-------
HOT
WATER
ro
o
y_ ELACK LIQUOR
T TO EVAPORATORS
FIGURE 4-6 PULP WASHER INSTRUMENTATION
-------
i LJ^O^ ,<=^-?^
. \:j ,> -^ f . ' V S
FIGURE 4-7 FOUR-STAGE BLEACH PLANT INSTRUMENTATION
-------
LOW PRESSURE
ALARM
UNLOADING LINE
FIGURE 4-8 SODIUM HYPOCHLORITE BLEACH LIQUOR PREPARATION INSTRUMENTATION.
-------
H.P. AIR
FOR UNLOADING
rv>
CO
SODIUM CHLORATE
TANK CAR
i VAPOR ii; ;'
SPACE :;;;
CHLORINE
DIOXIDE
GENERATOR
LIQUID
ING
R
*
~~
-
0-
J_^
T
£.
TV-
STRIPI
(\-
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r
SPENT
ACID
TANK
COOLING
WATER
LV-I d
H, qvK
LIXATOR
i I
i
I3RINE
1 WEU.
RIME
EU_
}
v 4
BRINE AND
CHLORATE
_L_ TOT
i^nfe*
TO HTPO MAKE-UP
SYSTEM
* EMERGENCY SHUTDOWN SYSTEM
FROM HI-TEUP AT TR-15 NOT
SHOWN
FIGURE .4-9 CHLORINE DIOXIDE BLEACH LIQUOR PREPARATION INSTRUMENTATION
-------
CCNOEH3ATE
COMPENSATE
TO BOILER
FIGURE 4-10 INSTRUMENTATION FOR MULTI RLE-EFFECT EVAPORATOR SYSTEM
-------
ro
tn
L I l.i C k! 'J D
FEfO FKO.U
FluTES
\AAA/ '
/
s
A A
F.ECY:I.E
TO L!a£
U'JB FILTER
REBURNEO LIME TO
SLAKER
FIGURE 4-11 INSTRUMENTATION TO CONTROL COMBUSTION IN THE LIME RECOVERY PROCESS.
-------
"
I I
A B
C
0
E
f
G
H
1 J
,-L I - BOILER
J.TA A / ORUM
TURBINE
(3<-FEED WATES
STEAU
VARIABLE
S^O COU^LIMO
TrllCK
'-1|:-J=R
STOniGE
r
CSEEN LIOUOR TO
RECAUSTICiZiNG PLANT
LV 19
FIGURE 4-12 SOME OF THE MORE IMPORTANT INSTRUMENTATION USED ON A HIGH CAPACITY RECOVERY FURNACE.
-------
i""Vi
NEW '
I
ro
TO i£WES *
FIGURE 4-13 TYPICAL INSTRUMENTATION FOR LIQUOR CAUSTICIZING IN ALKALINE COOKING.
-------
4.2 INSTRUMENTATION FOR EMISSION MONITORING
There are three basic approaches to instrumented source monitoring.
In the first, which may be called extractive, a continuous sample
stream is drawn from the stack and transported to the analyzer, which
can be mounted in any convenient location. This requires a probe
mounted in the stack or duct, and some form of interface system to
provide the analyzer with a sample that is in an appropriate state
of cleanliness, temperature, pressure, and moisture content. This
approach is the oldest and has provided the most experience to date.
The second approach may be called in-situ monitoring. The instrument
is mounted either inside the plenum or just outside the stack. In
the case of optical instruments, the source may be mounted on one
side and the detector on the other, so that the instrument scans the
full width of the stack. This method is most commonly used for
visible particulates or "smoke." A combination of the first and
second methods is to mount the analyzer directly on the stack and
draw the sample through it with little or no preconditioning.
Obviously, this requires an instrument that can accept the sample
in its natural state.
The third approach is to monitor the plume above the stack with a
remote optical instrument. So far, this approach is in the research
stage, whereas the other two methods have been reduced to a more
practical state.
The in-situ across-the-stack approach and the remote method offer
an advantage over the extractive approach in that they provide an
average reading rather than a point reading. However, it is theo-
retically feasible to use multiple extractive probes and obtain an
integrated sample that is representative of the complete cross
section.
The potential pollutants which may be measured continuously in a
kraft'pulp mill include sulfur dioxide, TRS, and particulate matter.
4.3 CONTINUOUS SULFUR DIOXIDE MONITORS
As previously stated, sulfur dioxide (S02) emissions arise from the
oxidation of sulfur bearing compounds normally found in all three
of the chemical wood pulping processes. Generally speaking, S0£
emissions from the kraft process are limited to the recovery furnace
and the ancillary power generating facilities. S02 emissions from
the sulfite and NSSC processes can be significant from a standpoint
of air pollution and reusable chemical losses. The following des-
criptions of continuous S02 monitoring systems can be adapted to
any of the aforedescribed pulping processes.
-128-
-------
Three methods have been used to monitor sulfur dioxide from the
sulfite process. These methods include infrared and ultraviolet
spectroscopy and conductivity.
A non-dispersive infrared analyzer has been used in a magnesium base
acid bisulfite mill with success. The~ reference cell contained all
of the flue gas components less the sulfur dioxide. The flue gas
is passed through the sample cell at 3.5 CFH and the differential
output is related to the S02 in the flue gas. The IR cell windows
must be protected from water. In one instance, a large filter is
followed by a refrigerated dryer reducing the flue gas to -10°F.
Particulates in the flue gas caused frequent plugging of the sample
line and the instrument environment was unduly corrosive for the
electronic circuitry. It is estimated that one installation cost
in excess of $10,500 and has been continually plagued with ap-
proximately 50 percent instrument downtime.
Thoen's ultraviolet system by contrast is quite simple in design,
relatively inexpensive (less than $2,000), and extremely reliable.
The ultraviolet source and detector are mounted externally to the
flue gas duct and protected from the flue gas by quartz windows.
A 2 1/2 inch diameter tube is located at 90° to the flue gas flow
and connects the externally mounted source and detector. One-half
inch holes are placed 2 1/2 inches apart at 90° to the gas flow.
These physical arrangements minimize particulate and water ac-
cumulation in the light path tube. Maintenance is limited pri-
marily to a cleaning of the quartz windows every three weeks.
The instrument has been calibrated externally to the flue gas
duct by passing nitrogen-sulfur dioxide mixtures through the
light path tube. (Possibly a more reliable calibration pro-
cedure would involve the addition of known quantities of sulfur
dioxide to a flue gas matrix rather than the nitrogen gas base
which has been used.)
A variety of conductivity instruments from home-builts to modified
commercial units have been successfully used by several sulfite
mills. Conductivity units require close control of reagent and
sample gas flow to maintain calibration. These instruments are
relatively inexpensive.
Correlation spectrometry and multiple-scan interferometry have
been proposed for use as remote sensing devices to determine the
S02 concentration from industrial and power plant stacks. In
theory the instrument could be used by the emitter at the source
to monitor S02 emissions or by regulatory officials located in
a mobile unit at some distance from the source. Significantly,
this can also be done at night. This instrumentation is con-
siderably more complex than Thoen's ultraviolet system. Thus
initial expense and subsequent maintenance could be significantly
greater.
-129-
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4.4 GAS/LIQUID CHROMATOGRAPHIC ANALYSIS OF REDUCED SULFUR COMPOUNDS
Pulp mill emissions often contain an appreciable amount of water
vapor, water droplets and particulate matter such as fiber, organic
and inorganic chemicals and mists. Proper sample preparation and
handling must precede injection of the sample into a chromatographic
system. Sample conditioning requirements are dictated by the impuri-
ties which must be removed to protect the analytical equipment, or
by necessity to maintain the physical properties of the gas so as
to prevent a material alteration of the components of the gas stream.
Consideration must, therefore, be given to (a) appropriate particu-
late and gas separation schemes, (b) sample collection and convey-
ance systems that do not react with the sample, and (c) sample col-
lection and conveyance systems that maintain the sample above its
dew-point temperature.
4.4.1 OPERATING PRINCIPLES
The function of a gas chromatographic system is twofold: (1) separa-
tion of the sample into its individual components, and (2) determination
of desired isolated components with a suitable detector.
Separation of the sample into its components is effected by a chroma-
tographic column. During analysis, a uniform flow of inert gas
(commonly called carrier gas) is maintained in the system. A small
volatile liquid or gas sample is injected into the inert gas stream
prior to its entry into the column. As the sample components are
carried through the column by the carrier gas stream, absorption and
desorption from the liquid support proceeds in accordance with the
individual component's coefficient of distribution. As the components
are eluted from the column, quantitative analysis is performed by a
detector which monitors a common property of the compounds. Reten-
tion time of the component in the column is used to qualitatively
identify the compound and measured detector response is used to
quantitatively identify the compound.
For proper operation of a gas-liquid chromatographic system, the
sample handling system, injection technique, column and detector
must be carefully selected for the specific application. Temperature
and carrier gas flow control are also important items. Figure 4-14.
a block diagram of a typical gas liquid chromatographic system.
4.4.2 SAMPLING SYSTEMS
The sample handling equipment must be constructed of materials which
will not react with individual components of the gas stream to be
analyzed. Those suitable for use in pulp mill situations dealing
-130-
-------
,...., Op)
FLOW REGULATOR
INERT
GAS
CYLINDER
SAMPLE
INJECTION
VALVE
INJECTOR
HEATING
ZONE
COLUMN
HEATING
ZONE
L C
LUMN
DETECTOR
PETECTOR
HEATING
ZONE
FIGURE 4-14 GASCP.ROMATOGRAPHY SYSTEM
-131-
-------
with sulfur gases, terpenes, and alcohols, in the one ppm volume
basis (v/v) range, are listed below.
Teflon and glass are the least reactive of the materials listed
and are preferred materials of construction for gas handling sys-
tems where batch samples are handled and where any absorbance or
reaction may significantly alter the sample composition. Type 316
stainless steel is satisfactory for continuous flow systems, and
after several exposures, because of saturation characteristics, is
generally suitable for batch sample handling systems.
The other two materials, polyethylene and polypropylene must be used
with care. They are sufficiently reactive to limit their use almost
exclusively to continuous flow systems where any reaction between
the construction material and a gas component alters the gas composi-
tion insignificantly.
CONSTRUCTION MATERIALS FOR GAS
HANDLING SYSTEMS
Least Reactive Teflon
Glass
Satisfactory for Continuous 316 - Stainless Steel
Flow Systems
Must be used with care Polyethylene*
Polypropylene
*Limited to temperatures of 250°F or less.
4.4.3 PREPARATION AND CONDITIONING
Many emission sources at the point of sampling contain liquid mists
(a) which are not properly a portion of the sample since they are
removed prior to discharge, or (b) for which no suitable system can
be devised for their incorporation in the sample.
Even when judgement dictates these droplets should be a portion of
the sample, their entry into a heated sampling system and subsequent
evaporation may cause random, unpredictable and nonrepresentative
gas concentrations in the sample stream. As a matter of course it
is therefore a common practice to remove large liquid droplets from
the gas stream before sampling.
These droplets can be removed by a properly designed entrainment
separator on the end of the sample probe as illustrated in Figure 4-15.
-132-
-------
2-TO SAMPLE LINE
-ENLARGED'PROBE END
.SPLASH PLATE
!£?'" <:'* !*" PCJ
i {.',,. iC»-!'..» i ii
-133-
-------
With this device, water droplets are diverted by the splash plate
and the small amount that circumvents the plate will fall by gravity
from the enlarged low gas flow velocity area incorporated in the
probe design. Nothing precludes the use of a properly designed
cyclone for droplet removal providing it is located in the duct so
that evaporation on heated surfaces of a transfer system can be
avoided.
Coarse particulate removal is a requirement in all continuous sam-
pling systems. Stainless steel tubes of 1 to 2 inches diameter
and 6 to 12 inches long packed with glass wool have been used.
Ceramic or sintered stainless steel- diffuser tubes have also been
used (1 ). Commercial probes are also available (1 ). Either
manual or automatic blowback systems for cleaning are required
if frequent removal from the stack for cleaning is to be avoided.
The blowback cleaning system should isolate the probe from the
remainder of the sampling system to avoid major surges in flow
that may cause damage. Fine particulate filtration following
coarse filtration is a common requirement on continuous flow gas
handling systems. Walther and Amberg report the use of a com-
mercially available system (2 ). One filter which removes
particles down to one micron in size found extensive use for
this purpose. A stainless steel body and sintered stainless
steel filter cartridge make it well suited for use in systems
heated above the gas dew pointf!'
Water vapor is present in most emission sources at concentrations
generally ranging from 20 to 95% (v/v). When cooled to ambient
temperatures either (a) condensation occurs which entraps unknown
and variable amounts of the components of primary interest, or (b)
some of the components may be cooled below their boiling point
resulting in their condensation. To prevent these occurrences the
sample must be maintained above its dewpoint temperature. This can
be done either by (a) using heat traced sampling equipment, or (b)
dilution of the source gas to the point at which its dewpoint is
below ambient temperature.
Even when dilution is used as a means to avoid condensation, seldom
if ever can the use of a heated sampling line be completely avoided.
Care must be taken to assure that the gas sample temperature remains
above its dew point prior to dilution which normally involves some
heat traced sampling line. The sample conditioning equipment for
chromatographic analysis is listed below.
-134-
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SAMPLE CONDITIONING EQUIPMENT
Sample Components Sample Preparation Device
Liquid Droplets Entrainment separators
Participate Coarse filtration and fine
filtration
Water Vapor Heated equipment - Dilution
with an inert gas
4.4.4 COLLECTION AND TRANSPORTING
Batch Systems - One commonly used method of batch sampling involves
drawing a known vacuum on a heated container and subsequently draw-
ing a sample from the source into the heated container until a
pressure equilibrium is established. This technique has major dis-
advantages. Care must be taken to assure that collection lines are
well purged to assure collection of a representative sample. Main-
taining the gas temperature above the dew point when subsequently
transferring to a multiport sampling valve is difficult. If the
gas temperature is permitted to drop, dilution of the sample may
occur as pressure is equalized. Displacement with an inert gas
line leading to the bottom of the container is the common means of
forcing the sample to a multiport sampling valve. This may result
in some dilution. A simple procedure minimizes these problems by
drawing a sample through an adequate probe, a heated sample line, and
heated sample container. This permits purging of the sample con-
tainer, line and probe, as well as allowing the walls of the
sample container to reach equilibrium with the gas components
present. Such a system is diagrammed in Figure 4-16.
The sample collection container is normally a glass burette fitted
with Teflon stopcocks (stopcock grease reacts witH most sulfur
compounds) although any glass container can be used. Either must
be equipped with a heating mantle. It is necessary to heat the
container prior to its use and heating while collecting the sample
to avoid any condensation is also required.
The temperature of the heated sample is maintained above the dew
point prior to injection into the chromatograph. Transfer of the
sample is by drawing or forcing a portion of the sample in the
container into a multiport sample injection valve with care taken
to insure the sample is maintained above its dew point at all times.
Enough sample should be drawn through the valve to replace all gas
in the sample loop and transfer line and yet not allow the signifi-
cant dilution by the incoming inert gas or ambient air in the heated
-135-
-------
FIGURE
BATCH SAMPLING SYSTEM
oo
CTi
I
HEATED LINE
A
VACUUM
P U M P
HEATED CONTAINER
(GLASS)
A. ENTRAPMENT SEPARATOR
SOURCE AND PRIMARY FILTER
-------
container. The multiport sampling valve can be heated in an enlarged
injector oven of the gas chromatograph or in a separate oven. It
is not recommended that this valve be placed in the column oven as
temperature can be varied at this location and can affect the perfor-
mance of most commercially available valves.
Transfer of the sample from the sample container to the chromatograph
injection port can be accomplished by syringe. A septum is placed
on one port of the container, both stopcocks opened and at least one
syringe of gas (more if required to evacuate any dead spaces) is
removed and wasted to condition the syringe. The procedure is sub-
ject to some error due to the condensation that occurs in the syringe
and remains in the needle, as well as the dilution that occurs as
either ambient air or an inert gas replaces the sample taken from
the sample container. This dilution is minimal and probably neg-
ligible if displacement gas is introduced opposite the point of with-
drawal and care is used in rapidly transferring a minimum of sample
volume.
Continuous Sampling Systems - Continuous sampling with the gas
chromatograph may be performed by continuously drawing a conditioned
sample from the desired source through a heated inert line attached
to a multiport sampling valve. Injection of a sample into the col-
umn then consists of operating the sample valve on a routine basis.
Electrically or steam heated Teflon sample lines are available com-
mercially ( 1, 2, 3 ). The system used must include provisions
for removal of particulate matter and entrained liquid as previously
discussed.
Movement of the sample through the continuous flow line and the
sampling valve must be accomplished with an air or water aspirator,
or a vacuum pump since no known suitable inert and leak-proof pump
is known to be available at this time.
Multiport sampling valves are available in six, seven, or eight-
port models constructed of Teflon or 316-stainless steel or other
suitable inert material ( 1, 2, 3 ). The use of Viton and other
rubber or synthetic materials should be avoided since reduced
sulfur gases can become adsorbed on the material, alter the gas
composition, as well as cause mechanical malfunctions of the valve.
Provisions for leak detection in systems operated under a vacuum
must be made. Leaks are checked by isolating the complete system
or sections thereof and placing them under negative head. The
vacuum can be monitored with a manometer or gauge. A leak would
be indicated by loss of vacuum.
The high sensitivity of the flame photometric detector (FPD) which
results in non-linear detector response above a sulfur concentration
-137-
-------
of about 5 ppm (v/v) either required dilution of the sample or
equipment modifications which permit analysis of samples containing
well above 5 ppm concentrations. Stevens et_ a_l_ ( 3) describe a
dilution system used in conjunction with the FPD for continuous
analyses of source gases. While the system is complex it has func-
tioned well in special study application. A drawback of the system
is the use of at least three pumps whose short term performance
casts some doubt on long term performance capability in routine
monitoring situations. NCASI has constructed and used a dilution
apparatus in continuous flow special studies of source emissions.
In this system a previously filtered source gas is maintained at a
temperature above the dew point. -After fine filtration for further
particulate reduction, a known volume of source gas is mixed with
a known volume of inert gas such as nitrogen. Flows of gas are
measured with rota'neters calibrated under the pressure and tempera-
ture conditions of use (e.g. 190 to 200°F, negative and positive
head) and flow rate controlled with micrometering valves. A com-
mercial dilution device to be used in continuous monitoring appli-
cations in conjunction with thair FPD is currently marketed by
Meloy Laboratories, Inc., Springfield, Virginia.
-138-
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4.5 ELECTROLYTIC TITRATION OF SULFUR COMPOUNDS
An electrolytic titrator which is commercially available has found
fairly widespread use in the kraft wood pulping industry, particu-
larly in the more modern installations. This analytical system is
most commonly referred to as a Barton titrator and is commercially
available from the Barton Instrument Company, a division of Inter-
national Telephone and Telegraph Corporation. The system, as
currently marketed, has direct application to the continuous moni-
toring of recovery furnace gases However, with some modifications,
described herein, the titrator is adaptable to most major gaseous
sulfur emissions in the kraft pulp mill.
The concentration of the reactive compounds may be expressed in
terms of the amount of known reagent added. In the Electrolytic
Titrator, the gas sample continuously passes through the titration
cell at a constant flow rate. The reagent for sulfur compounds is
bromine, which is generated by an electrical current passing through
the cell solution. The amount of current supplied to the cell is
automatically adjusted by the solid-state circuit in the control
module. The current provided by the control module produces just
enough bromine to satisfy the demand of the reactive compounds in
the sample and thus continuously maintain the end-point condition.
The amount of current required to maintain the end-point condition
is proportional to the concentration of reactive compounds in the
sample ( 2).
The main functional components of the titration cell consist of a
set of three electrodes arranged to form two functional pairs. One
pair acts as the bromine-generating electrodes and the other pair
serves as the sensing and control electrodes. The anode is common
to both pairs of electrodes. Changes in the bromine level of the
cell electrolyte cause the sensing electrodes to vary the current
supplied to the input of the electronic circuit in the control mod-
ule. As a result of the action of the sensing electrodes and the
electronic circuit, current is allowed to pass through the gene-
rating pair of electrodes only when the bromine level in the cell
falls below the preset operating bromine level ("blank" level).
The current is proportional to the TRS present in the sample.
All compounds oxidizable by bromine will contribute to the bromine
demand as the sample passes through the cell. The various organic
and inorganic sulfur compounds react according to their chemical
nature.
-139-
-------
Sulfur combined in the form of mercaptans or organic sulfide does
not require the same smount of bromine for reaction as sulfur com-
bined as hydrogen sulfide. Furthermore, one compound can react
according to two or more patterns. In a very low concentration
range, the reproducibility of the bromine oxidation reactions is
reliable to about + 10% over reasonable periods of operation on a
given sample source. For a specifically accurate sulfur measure-
ment, it is necessary to titrate compounds each as h^S, RSH, and
RSR separately and apply an appropriate factor to the titration
current required for each. The total then would equal the total
sulfur concentration. This can be done by manual or automatic
filtration using a scrubber solution selective to the compound
in the sample stream. Usually, the total of TRS is expressed as
equivalent
The decrease in titration current that occurs when the compound
is removed in the scrubber solution is a measure of its concentra-
tion. In most applications, however, the selective filter system
is not .required, since it is necessary to know only the variation
in the total bromine generated for all compounds present. The in-
strument may be used as a continuous monitor on wet gas streams
providing the moisture in the gas stream is reduced to the dew
point at ambient conditions and the bulk of the particulate material
is removed before the gas enters the cell. The system is normally
operated to determine the total reduced sulfur concentration (minus
SC>2) with the manual selecte scrubber-filter system to establish
the concentration of individual components.,
Figure 4-17 depicts a system which has functioned well at sources such
as the kraft recovery boiler, kraft recovery stack, and smelt tank
vents. The system consists of (a) a probe which is packed with glass
wool for use where particulate concentration is high, (b) a reservoir
for storing 5 percent potassium biphtal.ate, (c) an orifice which re-
stricts flow of biphthalate solution to 0.5 to 1 ml minute, (d) a
scrubber which removes particulate and S02 and cools the gas stream
to ambient temperatures., (e) the necessary tubing to conduct the
scrubbed gas sample to the cell, and (f) the pump, filter rack and
other equipment used in batch sampling.
Figure 4-18 presents a modified sampling system which may be utilized
as a continuous monitor on the recovery complex as well as other
reduced sulfur sources in pulp mills. The substantial difference in
the two transmittal and sensing systems shown is the thermal oxida-
tion furnace preceding the titration cell. The function of the
furnace is to oxidize the remaining sulfur gases, subsequent to the
S02 scrubber to sulfur dioxide. This accomplishes two major objec-
tives. The reduced sulfur compounds are all oxidized to S02 and
sensed as S02 thus minimizing the calibrations which would be
necessary for the individual compounds and also eliminates the pos-
sibility of "interference" from other non-sulfur bearing organic
conponents .
-140-
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!;; j' i) ;,/'. r,;
i 1.1 ."] T I .-
I I -.' i I L.I...
I
Ov'Ls
-------
4.6 PARTICULATE MONITORS
Continuous particulate sampling and detection systems are currently
receiving more attention and utilization in the wood pulping industry.
To date, although not in widespread use, extractive and in-situ
methods have been employed with varying degrees of success on wood
pulping process units. The following sections describe those sys-
tems which have been used to continuously monitor particulates from
such operations as kraft recovery furnaces, smelt dissolving tank
vents, and lime kiln scrubber exhaust gases. As previously mentioned,
the particulate material from these sources mainly consists of sodium
sulfate, sodium oxide, calcium carbonate, and calcium oxide.
4.6.1 BOLOMETER-IN-SITU MONITOR
A light absorption device, the bolometer, used for many years in
measuring smoke density has been found applicable for measurement of
particulate loadings in kraft mill recovery furnace stack emissions.
Calibration techniques involving wet test methods have shown that the
instrument output signal has a semi-logarithmic relationship to par-
ticulate concentrations (6 ).
The bolometer utilizes a light absorption principle in continuously
measuring the concentration of particulate matter passing a_g.iyen
point in the kraft mill recovery furnace flue gas system. The bolo-
meter instrument consists of a light source and a detector head mounted
on the opposite ends of a gas sample pipe or tube. The gas sample
tube is commonly a length of standard 4-inch pipe with a 5-foot by
3 1/4-inch longitudinal slot cut on both sides of the pipe. This
slot is normally centered in a horizontal or a vertical section of
the ductwork or the stack being monitored. The light source is pro-
jected through the dist laden gases passing through the slot. The
filament of the detector head is connected to a Wheatstone bridge
measuring circuit. Changes in the dust loading or the dust density
result in changes in the electrical resistance of the filament. A
servo motor and measuring slide wire resistor maintain the Wheat-
stone bridge circuit balance and provide the instrument readout on
the recorder. Normal instrument readout supplied by the vendor is
from 0 to 100 percent of full scale (6).
As supplied from the vendor, the bolometer can be used for the
measurement of dust density or smoke density. If the first measure-
ment is desired, the instrument can be calibrated in either of the
two commonly used units of dust loading measurementgrains per
standard cubic foot of dry gas or grains per actual cubic foot of
gas. If smoke density is the desired measurement, as is sometimes
the case on power and/or combustion boilers, the problem of accurate
Ringelmann Number determination is not encountered (5 ).
-142-
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Figure 4-19 depicts the bolometer with a dial readout in a typical
installation. Most of these instruments are installed with a recorded
output on the control panel of the unit process in order that a con-
tinuous and permanent record of the operation may be accomplished.
The continuous measuring aspects of this instrument enable optimiza-
tion of operating variables which normally result in minimizing par-
ticulate .mission levels. Malfunction of control devices (i.e.,
electrostatic precipitators) are rapidly detected which results in
an economic saving of reusable chemicals.
4.6.2 CONDUCTIVITY (EXTRACTIVE) MONITOR
This monitor, Figure 4-20 uses a twin tubed sample probe to provide
a non plugging sample line through which a flue gas sample is drawn.
Water is continuously metered down one line of the sample probe to
a point just back of the sample probe tip where it joins the second
line which is carrying the gas away from the probe inlet. The water
is sheared off by the gas flow and mixed with it until it reaches a
separation-mixing tank. By the time the gas leaves this chamber it
has left most of its heat and almost all of its dust load in the
water. The water runs down an overflow tube to a water cooler and
then to a conductivity cell connected to a continuous recorder. The
conductivity is measured and is related to the sodium content of the
water solution. A gas meter measures the gas drawn off the separa-
tion tank by a vacuum system.
With the water flow, gas flow and conductivity, i.e. sodium concentra-
tion, all known the sodium concentration in the flue gas is found by:
sodium flue gas =
water flow X sodium concentration
sample gas flow
To determine the total losses with the flue gas it is also necessary
to know the volume of flue gas flow. The sodium losses are simply
the flue gas flow times its concentration.
Although just out of the experimental stage, the monitor provides
a continuous record of sodium losses with recovery flue gases. The
principle of using a twin tube wet tip probe may be applicable to
several other tests. By passing a solution of cadmium chloride
through the tip and measuring the resulting turbidity of the final
solution the amount of H2$ in the flue gas may be determined. If
a fuschin solution were used in place of water and the color
monitored, the amount of SO? in the stack gas might be determined.
-143-
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S.V.Ct-D FLAI-iCJiiS FOR
AIR
LIGHT SOURCE
BOLOMETER
SMOKE OR DUST
PASSAGE
SPACED FLANGES FOR
AIR INLET
FIGURE 4-19 -
Bolometer for Measurement of Particulate
Emissions
-144-
-------
\VATl-R SUPPLY
GAS
STREAM
s
1
,
!
[X
p-vru
"V
"i«.i«*0:WV.,,
j
j.
If"**
~.Lj
SEPARATOR
TANK
-j' r 1
J !_ t. (
'~ r~~!
>,- L
TEMP. '"" .J
CONTROL 1
/I
« . -^
--.
r.,rr
' ** I/' ,'*"""'
if"""1
til
J |
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V
iJLr:
tZZZl
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TANK
-V^\-»
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!)
If,-." JJJB < ^£iH«,ni< C
. s>
^
iijj
-^ ^V-wi^ta**
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f_.
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nrr
MVCU,i._->
1
^/DROP LEG
CONDUCTIVITY/ 1 - " /,
CELL -^ lsX\ ! //
| SAMPLE -^ <
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t
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SEAL PIT
SUPPLY
FIGURE 4 -
20 - Titrimetric Instrument for
Measuring Particulate Emissions
-145-
-------
4.6.3 SPECIFIC ION (EXTRACTIVE) MONITORS
At least one kraft mill has experimented with a specific ion
electrode method to continuously monitor particulate emissions from
the recovery furnace complex.
Briefly the system employs a transmittal system which extracts
particulate material from the exhaust gases at or near a previously
determined, by conventional methods, isokinetic rate. The sample
stream then enters a sensing cell equipped with a sodium ion elec-
trode. The sodium ion concentration thus sensed is then translated
and read out as sodium sulfate concentration. As stated, this
method, although appearing to have application to wood pulping
unit processes, has not found wide acceptance and, therefore, must
be considered as still in the applied research and development
stage.
-146-
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4.7 LITERATURE CITED
1. Chapman, Robert L., POLLUTION ENGINEERING, pp. 38-39, September
1972.
2. Section 11A - Gaseous Emissions - Automatic Techniques - Electrolytic
Titration, Technical Bulletin No. 38, NCASI, New York, N.Y., December
1968.
3. A Guide to the Use of Gas Chromatography in Emission Analysis,
Technical Bulletin No. 59, NCASI, New York, N.Y., February 1972.
4. Marks, Lionel S., "Inadequacy of the Ringelmann Chart," MECHANICAL
ENGINEERING, September, 1937.
5. Cooper, S. R., and Haskell, C. F., "Cutting Chemical Ash Losses in
a Kraft Recovery System," PAPER TRADE JOURNAL, March 27, 1967.
6. MacDonald, Wayne G. L., "BCFP Monitor Cuts Recovery Boiler Stack
.- Losses 50%," PULP AND PAPER, April, 1966.
7. "Determination of Collecting Efficiency at Electrostatic Precipita-
tor Plants for Soda Recovery Units," Method of AB Svenska Flakt
fabriken.
-147-
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CHAPTER 5
AIR POLLUTION CONTROL SYSTEMS
5.1 INTRODUCTION
Control methods presently in use in the wood pulping industry consist
of add-on hardware or process modifications. The methods considered
in this section are those which have been in reasonably successful
operation for at least one year at one or more locations. Control
methods are briefly described in general terms and are evaluated under
conditions of specific applications. The evaluations include an
effectiveness study as well as a discussion of engineering factors
which are unique to the application.
A variety of methods is found to be useful in controlling particulate
emissions from pulp mill sources. Efficiencies of 99+% are possible.
The listing which follows identifies those methods most commonly used.
It should be noted that the direct contact evaporator following the
kraft recovery furnace is an important particulate control device.
It should also be noted that where primary or secondary wet scrubbers
are used as particulate collectors on combustion sources, the emis-
sion of gaseous sulfur compounds may be increased or decreased de-
pending on the nature of the scrubber medium.
Kraft Recovery Furnace
Kraft Lime Kiln
Kraft Smelt Dissolving
Tank
Kraft Lime Slaker
Power Boilers
Electrostatic Precipitators
Venturi Evaporator/Scrubbers
Electrostatic Precipitators
plus Secondary Scrubber
Venturi Scrubber
Cyclonic Scrubber
Impingement Baffle Scrubber
Electrostatic Precipitators
(recently)
Mesh Pads
Packed Tower Scrubbers
Orifice Scrubbers
Mesh Pads
Cyclonic Scrubbers
Mechanical Collectors
Electrostatic Precipitators
-148-
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Combination Boilers
- Mechanical Collectors
Cyclonic Scrubbers
Fewer numbers of methods are in use for the control of gaseous emis-
sions, particularly the odorous reduced sulfur compounds. The majority
of odorous emissions can be grouped into three categories: (a) recovery
furnace offgases; (b) low volume high concentration sources such as
the noncondensible gases from the multiple effect evaporators, and di-
gester relief and blow gases; (c) high volume low concentration sources
such as brown stock washers and smelt dissolving tanks. Wet scrubbers
have received limited application on combustion sources in the.kraft
process because of the difficulty-of absorbing effectively all of the
odorous compounds with a single scrubbing medium. Also as indicated
previously, the emission of gaseous sulfur compounds may be increased
or decreased depending on the nature of the scrubbing medium. The
following list identifies those methods most commonly used and which
are described in the chapter:
Kraft Recovery Furnace
- Weak Black Liquor Oxidation
- Strong Black Liquor Oxidation
- Proper Operation
- Venturi Evaporator/Scrubber
- Cyclone Evaporator/Scrubber
- New Recovery System Design
Kraft Smelt Dissolving - Packed Tower Scrubber
Tank - Orifice Scrubber
Kraft Digester Relief
and Blow plus M.E.
Evaporator
Sulfite Acid Tower
Sulfite Blow Pit .
Chlorination
Incineration
Packed Tower Scrubbers
Weak Black Liquor Oxidation
(M.E. Evaporators)
Steam Stripping of Condensates
Air Stripping of Condensates
Additional Absorption Tower
Packed Tower
5.2 GENERAL DESCRIPTION OF CONTROL EQUIPMENT
5.2.1 ELECTROSTATIC PRECIPITATQRS :
An electrostatic precipitator consists in principle of a number of
discharge (emitting) electrodes, collecting electrode plates and a
high-voltage power unit. This power unit comprises high-voltage
-149-
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transformers and rectifiers to convert the available AC power to high-
voltage DC power. Lately, silicon diode rectifiers have almost
exclusively replaced other types of rectifiers.
The dust laden gas enters the electrostatic precipitator and flows in
the passages created by the collecting electrodes. When high-voltage
power is applied to the discharge electrodes located in the passages,
ionization or corona discharge occurs near the surface of the dis-
charge electrodes. The negative ions attach themselves to dust
particles near the electrodes giving the particles a negative electric
charge. The charged dust particles are repelled by the discharge and
attracted to the collecting surfaces connected to ground. Here the
dust particles lose their electrical charge and are deposited on the
collecting electrodes.
The collecting electrodes are provided with a rapping mechanism for
dislodging the dust precipitated on the electrodes. This rapid system
must be designed very carefully to avoid creation of a dust cloud in
the space between the electrodes. Incorrect rapping operation or pro-
gramming may cause the dust to re-entrain which, in turn, would greatly
decrease the dust collecting efficiency of the precipitator. Incor-
rect rapping is the most common cause for so-called "snow-outs." The
rapping mechanism, which is generally operated on an automatic time
cycle basis, is either of vibrator or motor driven fall-hammer type.
Vibrators are preferred for pulp mill applications rather than the ham-
mer rappers which are used more frequently, for example, on fly ash
precipitators.
The high voltage power level is controlled by an automatic control unit.
The discharge electrodes are connected to the negative rather than to
the positive side of the power supply. A higher voltage can thereby
be maintained without excessive arc-overs which, in turn, would cause
waste of electric power and eventually lead to operating difficulties.
The automatic voltage rate of sparking provides a feedback to the con-
trol unit making it possible automatically to operate the electric
system of the precipitator at optimum conditions. Little or no spark-
ing will increase the voltage and too much sparking will reduce the
voltage, thereby preventing arc-overs detrimental to the precipitator.
Gas conditions are never uniform throughout the precipitator. Aside
from unintended irregularities in gas distribution and other conditions,
the dust loading varies from inlet to outlet of the precipitator. To
reach optimum dust collecting efficiencies, the electrical control units
must accomodate these variations. A high dust loading will increase the
sparking and reduce the voltage. If the precipitator would have only
one electric system with one automatic voltage control, the voltage
throughout the precipitator would be limited to the lowest voltage
permissible at any point in the precipitator. The modern precipitator
is, therefore, divided into independent electrical units, each controlled
for maximum voltage depending on the gas conditions in that particular
-150-
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section of the precipitator. The discharge system close to the outlet
of the precipitator will consequently operate at a higher voltage level
than the inlet. This need for sectionalizing has led to increased use
of the horizontal flow, plate-type precipitator. This type has super-
seded the tube-type, vertical flow precipitator, once very common in
the pulp and paper industry. The tube-type precipitator does not
easily lend itself to sectionalizing.
The electric power consumption of a precipitator depends on several
factors. The input of current is necessary to sustain the voltage level.
There is a constant power drain due to ionization in the corona, due
to the controlled sparking and other voltage leakages because of poor
maintenance, dust build-up, et cetera. The collection efficiency de-
pends on the voltage level; but, on the other hand, the higher the
voltage level, the greater the power drain. At 90 percent collection
efficiency, the power consumption is approximately 0.2 kw/1000 CFM of
gas and at 99.9 percent efficiency, the power consumption has risen to
approximately 0.8 kw/1000 CFM.
Most precipitators used in the wood pulping industry on recovery boilers
are designed for collection efficiencies of 90 - 99.9 percent. The pres-
sure drop of the gas passing through the precipitator is usually below
0.5 inch W.G. A typical wet-bottom precipitator with tile shell is
shown in Figure 5-1.
5.2.2 VENTURI SCRUBBERS
The Venturi scrubber consists in principle of a convergent section (throat)
and a divergent section. Dust laden gas enters the convergent section
and is accelerated to high velocity as it approaches the throat. Gas
velocities in the throat section vary from 100 to 500 FPS.
Water or other scrubbing liquid is injected either directly into the
throat section or the top of the Venturi. In the latter case, the scrub-
bing liquid cascades down the walls of the convergent section. The high
velocity gas stream atomized the liquod into a fine mist--the greater the
velocity, the finer the droplets. Collison between the dust particles
and the water droplets takes place and causes the dust to be entrapped
in the water. Further collison between the water droplets occurs. This
will create aggregate droplets of relatively large size. These droplets
are easily separated from the gas stream in a subsequent separator. The
collision or impaction phenomenon is rather complex, but is mainly due
to mass forces created by the great velocity differential between the
dust particles and the water droplets. For submicron particles, the
Brownian molecular movement, diffusion, and electrostatic forces also
play an important role.
In the divergent section, the gas and the dust particles are decelerated
thereby creating a new velocity differential with additional agglomeration.
Finally, in the elbow connecting the Venturi and the separator, as well
-151-
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GAS
OUTLET
yi -- -,-\' ,n
;!'/;': >J;H
BLACK LICHJOUR FTE'
FIGURES.-!
PRECIPITATOR FOR RECOVERY BOILER
1I1LET
-152-
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as in the inlet to the separator (if radial), changes of direction of
the gas flow cause additional impaction and agglomeration.
The gas and liquid enter the separator, usually of the cyclonic type,
where the liquid is thrown to the walls by centrifugal forces and
drains to the bottom by gravity. The clean gas exists through the .up-
per portion of the separator.
The elbow connecting the Venturi with the separator is either of the
regular type or of so-called flooded types. The flooded elbow is
developed to prevent erosion. Here the liquid surface absorbs the
brunt of the water and dust stream. For pulp mill applications, such
as lime kiln scrubbers, the flooded elbow is recommended.
Normally the separator is of the cyclonic type with conical bottom and
either top outlet or side outlet. The type of outlet depends on whether
the fan is located before or after the scrubber. The top outlet lends
itself to a direct mounted stack. In order to facilitate the installation,
the separator is sometimes provided with a flat bottom. This may some-
times create drainage problems. For special purposes a so-called false
bottom is also installed. The false bottom creates an intermediate re-
serve for the scrubbing liquid where de-aeration can take place making
the scrubbing liquid (e.g., black liquor) more suitable to pump. Scrub-
bers for recovery boilers are almost always provided with facilities for
wall wash of the separator. This prevents build-up of black liquor solids
which can be a fire hazard, in addition to being a maintenance problem.
The Venturi scrubber is capable of high efficiency collection of dust
and fumes even in the submicron range. The efficiency of collection is
a function of the pressure drop across the scrubber which in turn is a
function of the gas velocity in the throat and the liquid flow rate
(liquid to gas ratio). The higher the gas velocity or the liquid flow
rate, the greater will be the pressure drop and consequently the
greater the efficiency. There is, however, a cut-off point where in-
creased liquid flow rate will have an adverse effect on the collecting
efficiency. The throat becomes "flooded." This occurs at about 20
gallons per 1000 CFM. In order to attain high collecting efficiency,
the water droplet size distribution has to be in a certain relation to
the dust particle size distribution. Too large droplets would reduce
the probability of collection drastically, and even if collection would
take place the chance for the dust particles to be entrapped in water
would also be reduced.
Increased gas velocity (pressure drop) increases the atomization. A
fine dust will, therefore, require a higher pressure drop than a
coarse dust for the same collecting efficiency.
Scrubbing liquid is injected into the Venturi at low pressure through
relatively large jets or open weirs. In order to be able to recirculate
-153-
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the scrubbing liquid, in other words maintain a high concentration of
solids, the manufacturers try to completely do away with narrow
constrictions and weirs. The liquid rate is normally 3 - 15 gallons
per 1000 CFM of gas. A typical Venturi scrubber is shown in Figure
5-2.
5.2.3 CYCLONIC SCRUBBERS
The cyclonic scrubber is an efficient device for removing dust particles
two microns and larger and is also a relatively good gas absorber. In
this unit, the dust laden gas enters tangentially at the bottom of a
cylindrical tower and spirals upward through the scrubber in a con-
tinuously rotating path. A spray manifold is located axially in the
center of the scrubber with banks of spray nozzles directed radially
toward the walls. The spray sweeps across the path of the gas stream
intercepting and entrapping the dust particles. The centrifugal motion
of the spray impacted by the rotating gas causes the droplets to im-
pinge against the walls of the scrubber and drain to the bottom due to
gravitational forces.
In another type of cyclonic scrubber, the spray nozzles are mounted on
the wall rather than in a central spray manifold. The advantage with
this type is that the nozzles may easily be serviced or replaced while
the scrubber is in operation. .
The mechanism of collecting particles in the cyclonic scrubber is in es-
sence the same as in a Venturi scrubber; namely, impaction and agglomera-
tion of liquid droplets and dust particles with subsequent centrifugal
and gravitational separation.
The liquid pressure ranges from 50 to 400 PSIG. The flow rate is norm-
ally 3-8 gallons per 1000 CFM.
The cyclonic scrubber is most efficient on relatively coarse dust and
the efficiency drops off markedly for particles under two microns. The
pressure drop is considerably less than for a Venturi scrubber and
ranges normally from 0.5 to 3 inches WG. A typical cyclonic scrubber
is shown in Figure 5-3.
5.2.4 IMPINGEMENT BAFFLE SCRUBBERS
The impingement baffle scrubber is a vertical tower equipped with one
or more impingement baffle stages. The impingement baffle consists of
a perforated plate having a multitude of small holes so arranged that
a baffle is located directly above each perforation. A weir on each
plate maintains a level of scrubbing liquid.
The contaminated gas enters radially at .the bottom of the scrubber
and is subjected to a water spray that will precipitate out the coarser
dust particles. The gas then passes through the perforations and impinges
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SEPARATOR OUTLET
GAS
INLET
LIQUID INLETS
THROAT-
SEPARATOR INLET NOZZLE-
FIGURE 5-2
VEMTURI SCRUBP.ER WITH CYCLONIC
SEPARATOR
-155-
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SCRUBBER OUTLET
IET
.-^LIQUID INLET
WALL MOUNTED SPRAYS-
MAY BE CENTER PIPE,
FIGURE 5-3
CYCLONIC SCRUBBER
-156-
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on the baffles. The gas velocity through the perforations, ranging
from 75 to 100 FPS, atomizes the scrubbing liquid and the velocity
differential between gas and liquid results in the impaction and
agglomeration of liquid and particulate matter. The scrubbing liquid
is introduced into the scrubber through low pressure spray nozzles
located below the plates and spraying upward. These sprays not only
deliver liquid to the plates, but also serve to cool and humidify the
gasses; remove coarse particles; and keep the bottom of the plates
clean. For scrubbers with more than one stage, the scrubbing liquid
is drained off the plates downward from stage to stage in the scrubber.
In addition to the multiple impaction, the change of solids concen-
tration in the liquid also contributes to increasing the collecting
efficiency.
The pressure drop for impingement'baffle scrubbers ranges from 2 inches
WG to 8 inches WG depending on the number of stages, size, and numbers
of perforations and baffles. As would be expected, increased number
of stages and smaller perforations result in higher pressure drop and
subsequent higher efficiency. An impingement baffle scrubber is shown
in Figure 5-4.
The impingement baffle scrubbers are normally used in lime kiln appli-
cations. The efficiency is, however, not high enough to comply with
today's air quality requirements. For upgrading of existing impinge-
ment baffle scrubbers, it is possible to install a Venturi and an elbow
ahead of the scrubber and to use the scrubber shell as a cyclonic
separator. The inlet to the scrubber must, in such a case, be changed
to enter the shell tangentially, and all the internal parts have to be
removed.
5.2.5 PACKED TOWER SCRUBBERS
The packed tower scrubber is used primarily as a gas absorber. Its use
for collection of solid particulates is limited because it is rather
inefficient for particles under five microns and is subject to becoming
plugged because of dust build-up. It is, however, an excellent device
for absorption of such gases as HC1, S02, Cl2> H2S, and NH3. Other
advantages of the scrubber are simple design and low manufacturing cost.
The scrubber consists of a vertical cylindrical shell with the gas in-
let at the bottom and the outlet at the top. Above the gas entrance is
a packed section consisting of four or more feet of packing material.
This packing may consist of Raschig rings, Pall rings, saddles, et
cetera, made from stoneware, ceramic, or polypropylene. Water is dis-
tributed uniformly over the packing by means of low pressure spray noz-
zles or weirs located above the packing. Normally, there is a mist
eliminator above the spray nozzles to prevent entrainment of liquid in
the clean gas leaving the tower. Normally a reservoir is located at
the bottom of the scrubber for direct recirculation. A typical packed
tower scrubber and packing are shown in Figure 5-5.
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TARGET PLATE
CLEAN GAS OUTLET
ENTRAPMENT
ELIMINATOR
IMPINGEMENT
BAFFLE PLATE
(SECOND STAGE)
WATER SEAL-
IMPINGEMENT
BAFFLE PLATE
(FIRST STAGE)
WATER LEVEL
ORIFICE PLATED
,DRAIN
FIGURE 5-4
IMPINGEMENT SCRUBBER
IMPINGEMENT SCRUBBER
MECHANISM
LIQUID INLET
WATER SEAL
>s*\
CONTAMINATED
GAS INLET
-------
CLEAN GAS OUTLET
DEfllSTER PAD
CONTACT BED
CONTAMINATED
GAS INLET
LIQUID INLETS
RECYCLE SECTION
RASCHIG
R I NG
SPIRAL
RING
COUNTER-FLOW PACKED TOWER SCRUBBER
LESSING CROSS-PARTITION
RING RING
^F>
BERL
SADDLE
INTALOX
SADDLE
CERAMIC PACKINGS
FIGURE 5-5
^S-
^^)
PALL RING
TELLERETTES*
MASPAC'
If.'TALOX
SADDLE
PLASTIC PACKINGS
-159-
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The contaminated gas enters at the bottom of the tower and moves up-
ward through the packing counter-current to the scrubbing liquid.
The packing forces the gas to follow a tortuous path over the contact
surfaces and interstices creating intimate contact with the descending
liquid.
The pressure drop over the tower depends on the height of packing but
is normally in the order of 1 - 2 inches WG.
As mentioned before, the packed tower is an excellent gas absorber,
but less suitable for dust removal due to dust build-up. When dust
is present in the gas, special attention has to be given to the selection
of packing material. A larger size packing; i.e., 4-inch Pall rings in-
stead of 2-inch should be chosen in order to prevent plugging. The
contact surface will thereby be reduced and the gas absorption efficiency
will drop. This can be compensated for by increasing the height of the
packing but the plugging tendencies will increase simultaneously.
5;2."6 MECHANICAL COLLECTORS
There are a great number of different mechanical collectors in use '
throughout in.dustry. The most common ones within the wood pulping in-
dustry are the large diameter cyclones and the multi-tube collectors.
This description will be limited to these two types.
Cyclone dust collectors are of cylindrical or conical type, utilizing
centrifugal and gravitational forces for separation of dust particles
from a gas stream. The dust laden gas enters the collector tangentially
either directly or via an expanded involute section where the dust
particles are subjected to the separating forces. The centrifugal force
drives the dust particles to the collector wall; the gravitation drives
the concentrated dust dov/nward to the cone outlet; and the dust is dis-
charged into a collection hopper while the cleaned gas flows upward in
an inner vortex to the gas outlet tube.
Two basic types of cyclone collectors are availablethe tangential in-
let type shown in Figure 5-6 and 5-7 and the axial vane type. The former
is sometimes referred to as large diameter cyclone, or cyclone collector,
and the latter one as a tubular collector. In the tangential inlet type,
the gas enters the cyclone through a straight tangential, helical, or
involute inlet section. Axial vane units employ inlet vanes to provide
the spiraling motion to the dust laden gas stream.
Many sizes and designs of cyclone collectors can be provided to meet
specific dust collection problems. The units may be installed in single
or multiple arrangements, in parallel or in series. Cyclone collectors
are generally suitable for separating solid particles in size ranging
from about 3 microns to 200 microns. They can, of course be used for
larger particle sizes also.
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GAS
INLET
GAS
OUTLET
DUST OUTLET
FIGURE 5-6
LARGE DIAMETER CYCLONE COLLECTOR
STANDARD ARRANGEMENT
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GAS
INLET
OUTLET TUBE
SPIN VANES
INLET TUBE
-TUBE COLLECTOR
GAS
OUTLET
FIGURE 5-7
COLLECTOR ELEMENT
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For a given cyclone collector, the overall collecting efficiency can
be determined if certain parameters are known. These parameters in-
clude dust particle size distribution, density, concentration, and
other properties of the dust. Gas temperature, pressure, moisture con-
tent and gas composition, as well as pressure drop limitations and
local conditions, are also factors of consideration. The efficiency
increases with increased dust particle size, density, and concentrations.
Collection efficiency increases with increased pressure drop across the
cyclone. The collector can be designed for pressure drops of less than
2 inches WG for large diameter cyclones and for 2-7 inches WG for
the smaller diameter (high efficiency) units.
Increasing temperature will decrease the efficiency of the cyclone.
However, up to 700°F, the influence of the gas temperature is not great.
For the same pressure drop, increased gas viscosity or density will low-
er the collecting efficiency.
As particle size, dust density, and inlet gas velocity decreases the
efficiency tapers off. Overall efficiency of the collection system can
be increased by arranging the cyclones in series. By reducing the
diameter of the cyclone the centrifugal forces increase, thus increasing
the efficiency. The gas volume capacity will, however, simultaneously
be reduced and the number of cyclones has to be increased. This principle
is utilized in the multi-tube collectors where the tube diameter is from
12 inches or less up to 24 inches. Batteries of tubes are mounted in
the same casing and high efficiencies are attainable.
Since high efficiencies require high radial gas velocities, abrasion
is often a problem for cyclone collectors. Proper selection of material
of construction is imperative. Materials in use include carbon steel,
low alloy steels, aluminum and special materials. Abrasion resistant
linings in castable form or brick linings can also be used.
5.2.7 BLACK LIQUOR OXIDATION
Black liquor oxidation is practiced for the purpose of oxidizing the
sodium sulfide in the liquor. This is accomplished by reacting the
sodium sulfide with oxygen in the air to form sodium thiosulfate.
This compound is relatively stable and will not break down in passing
through the direct contact evaporator. Sodium sulfide reacting with the
carbon dioxide and sulfur dioxide in the flue gases is the cause of
hydrogen sulfide emissions. The chemical reactions of sodium sulfide
(unoxidized liquor) and C02 and S02 in the flue gases are as follows:
Na2S + C02 + H20 -> Na2 .003 + H2S
Na2S.+ S02 + H20 + Na2 S03 + H2S
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Black liquor oxidation can be applied either to weak black liquor or
to concentrated black liquor. The choice is dependent to a considerable
extent on the characteristics of the liquor. Usually weak liquor '
oxidation is used on non-foaming liquor and concentrated liquor oxida-
tion on foaming type liquor. Regardless of the type of oxidation, it
is essential that complete oxidation of the sodium sulfide takes place,
if hydrogen sulfide emissions from the direct contact evaporator are
to be prevented.
Lately, investigations have been conducted into sodium sulfide reversion
taking place after weak liquor oxidation. Instances are known to exist
where, although there is 99 plus percent oxidation during weak liquor
oxidation, the concentrated liquor shows the presence of sodium sulfide.
One solution is to add a second system of concentrated liquor oxidation.
Among those who have installed weak liquor oxidation and who are having
sodium sulfide reversion, there are suggestions that concentrated liquor
oxidation might be the correct installation (14).
There are three types of oxidation systems:
1. Packed Towers
2. Bubble Tray Towers
3. Air Sparged Reactors
Packed Towers
Packed towers have been applied primarily to weak black liquor oxida-
tion. The principal advantage of a packed tower lies in reduced power
costs. The pressure drop through a packed unit is usually about 1 to
2 inches WG, and a tower for a 500 ton-per-day mill will require about
20 hp to operate the required air blowers. In regions of high power
costs, this is an important factor in selecting an oxidation unit. The
disadvantage of this unit is that it has plugging tendencies and has
lower oxidation efficiency.
Bubble Tray Towers
The bubble tray oxidation tower is used for weak black liquor oxidation
only. The liquor is pumped out on a perforated steel plate, under which
air is blown from a fan. The air passes through the perforations and
bubbles through the liquor. The liquor height on the plate is normally
four to six inches and the liquor makes several passes over the plate.
In order to accomodate larger flows of liquor, a number of these aeration
chambers (bubble trays) are connected in parallel and stacked on top
of each other.
The liquor, air, and foam are discharged into a foam tank, where mechan-
ical foam breakers convert the foam to liquor. Certain liquors with
low foam characteristics may not require foam breakers if the tank is
large enough. The air leaving the system through foam breakers carries
some entrained liquor. This liquor is separated from the air in a cy-
clone and returned to the foam tank, while the air is exhausted to the
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atmosphere. This air stream, although relatively small, contains some
hydrogen sulfide, methyl mercaptan, dimethyl sulfide, and dimethyl
disulfide.
The system operates at 25 - 35 inches WG pressure drop. The foam
breakers draw approximately 10 BMP each. Including necessary pump
capacity, the total power consumption runs about 0.25 BHP/GPM.
The air to liquor ratio is 15 - 20 CFM/GPM and a retention of 4 - 6
minutes is required on the perforated plates.
Air Sparger Reactors
The air sparged reacters are of two types--air sparging with agitation
and without agitation.
Air Sparger With Agitation. Air sparging with agitation is used for
both weak and concentrated liquor. This system requires more electric
power, it is more complicated, and the equipment cost is higher than
for the non-agitation system. The liquor is pumped into a tank where
an air header comes in centrally at the top. The header conveys the
air into the sparger located approximately six feet from the bottom of
the tank. The sparger has a number of arms extending radially from the
header and each arm has a number of branches with aeration nozzles. . The
sparger is submerged ten to fifteen feet in the liquor depending on the
desired retention time. The incoming liquor is distributed above the
surface in a system of pipes and nozzles. In addition to being evenly
distributed", the liquor helps to beat down the foam floating on the sur-
face.
Air Sparger Without Agitation. Air sparging without agitation is used
exclusively for concentrated black liquor oxidation. This system
operates at about 10 psig air pressure and draws approximately 1.5 -
2.0 BHP/GPM of liquor. The air to liquor ratio is 20 - 30 CFM/GPM
and the retention time 2-3 hours. Systems with agitation use more
horsepower, but should otherwise be comparable.
Effect of Batch and Continuous Digesters
Mills with continuous digesters usually experience higher sodium sul-
fide loadings than mills with batch digesters. This is attributed
to the fact that batch digester systems expose the liquor to more con-
tact with air (primarily during washing) than continuous digester sys-
tems, thus resulting in more oxidation of the liquor. For batch
digester systems the sodium sulfide content in the weak black liquor
storage tanks may be 50 percent of that in the white liquor storage
tanks. This condition must be considered when selecting oxidation sys-
tems.
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Summary
As stated previously, foaming is a major consideration in the selection
of an oxidation system. Foaming depends on the wood species, which
will contain varying amounts of resin and fatty acid salts. These
compounds will create a foam, that in some cases will preclude the
oxidation of weak liquor. The foaming is less pronounced in strong
liquor. Especially severe is the foaming of liquors from southern
pine. The addition of fuel oil or kerosene to the liquor to reduce the
foam helps in some cases. Foaming characteristics of the liquor in-
fluences the geographical locations of the various oxidation systems.
In general, the air sparger type is located in the South due to the
resinous content of the southern liquors. Weak liquor oxidation has
found extensive application in western United States. The mid-west and
northeastern United States have installed primarily weak liquor oxidation
systems; however, (]_) only one installation is reportedly operating at
a high oxidation efficiency. Currently, the general trend in oxidation
systems appears to be directed towards more acceptance of the air sparger
type for concentrated liquor oxidation.
5.2.8 ORIFICE SCRUBBER
The orifice scrubber is a collection device consisting of a restricted
air passage partially filled with water. The resulting dispersion of
the water causes wetting of the particles and their collection. Pres-
sure drop is comparable to cyclonic scrubbers.
5.2.9 MESH PADS
Mesh pads are collection devices composed of material such as knitted
wire mesh. Dust and liquid droplets are collected on the pads.
A spray washing system is provided for back washing the mesh pads to re-
move accumulated solid particulate. In normal operation, a maximum pres-
sure drop of approximately 0.2 inches WG is maintained by periodic opera-
tion of the spray system.
5.2.10 STRIPPING CONDENSATES
Many condensate streams, especially from the "foul" streams of the MEE,
contain TRS compounds. Countercurrent stripping by either air or
steam has successfully removed more than 95 percent of these compounds.
Thus, the stripped condensate can be used as process water where its use
prior to stripping would have resulted in TRS emissions to the atmosphere.
It can be used as scrubber water in the smelt dissolving tank, and lime
slaking. The stripped compounds are usually incinerated in the recovery
furnace, limekiln, or power boiler.
5.3 POWER AND COMBINATION BOILERS, GENERAL
The emission control equipment used in the steam power plants has been
almost exclusively limited to removal of particulate matter. Lately
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efforts have been made to find ways and means to remove sulfur from
the fuels and alternatively sulfur dioxide from the flue gases. As
yet no commercially economic and feasible method is in operation, but
a number of promising pilot studies have been made. The recognition
of nitrogen oxides as undesirable compounds in the flue gases is re-
latively new and efforts in that direction have only begun.
Power and combination boilers in the pulp and paper industry incorporate
a wide variety of firing equipment. Some of these types are:
Oil and Gas Burners
Pulverized Coal
Spreader Stokers with Traveling Grates
Spreader Stokers with Vibrating Grates
Spreader Stokers with Water Cooled Grates
Spreader Stokers with Stationary Grates
Spreader Stokers with Dump Grates
Dutch Ovens
Aside from coal the main fuels are oil, gas, and bark. Today the burn-
ing of bark for steam generation is confined to locations where bark is
available as a waste or by-product. In producing lumber half of the
wood in the log is often discarded as saw-dust, bark, shavings, slabs
and ends, all of which can be used as fuel. New methods in barking and
pulping, however, are continuously reducing the quantity of waste wood
available.
The devices commonly used for removal of particulate matter in flue gases
of steam boilers are centrifugal collectors and electrostatic pre-
cipitators. The following list indicates the type of dust collectors
normally used for various types of fuel.
Mechanical Electrostatic Precipitators
Coal (pulverized) X X
#6 Oil X Has been used but is not common
Gas
Bark X
Bark + Coal X
Bark + Oil X
Bark + Gas X
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Fly ash may be approximately classified according to type of firing as
shown in Table 5-1. Fly ash from pulverized coal fired boilers is
generally quite fineabout 70 percent (or more) smaller than 325 mesh.
It is usually light gray in color as it contains only a small amount
of unburned carbon. The small particles, having a large surface
area, will impart a definite color to the stack gases. This may,
under certain light conditions, indicate that a sizeable amount of fly
ash is being discharged into the atmosphere. Fly ash from stoker fired
boilers on the other hand is much coarser and has, since it contains
larger quantities of unburnt carbon, a dark gray to black color. This
fly ash does not discolor the flue gas in the same manner as described
above. A clean appea/ing stack discharge from a stoker fired boiler
does not necessarily indicate that there is no emission problem. The
coarse particles will fall out relatively close to the stack and on an
equal weight basis will cause a greater local nuisance than fly ash
from pulverized coal. Thus fine particles make a small plume appear
much denser than coarse particles. It is for this reason that removal
of particles larger than 20 microns has little effect on the appearance
of the plume.
Electrostatic Precipitators
The performance of an electrostatic precipitator is directly related.to
the level of power input maintained. The amount of electric charge that
the dust particles will acquire depends upon the conductivity or re-
sistivity of the dust. When the resistivity exceeds approximately
2 x 10^0 Ohm centimeters excessive sparking is to be expected with sub-
sequent drop in collecting efficiency. Research has shown that the re-
sistivity of the fly ash is closely related to the presence of water,
soluble sulfates, and free sulphuric acid formed on the surfaces of the
dust particles by absorption of 803 from the flue gas. If the sulfur
content is greater than 1 percent in the coal there is likely to be
sufficient 863 in the flue gas to create favorable conditions for an
acceptable resistivity. However, $03 is formed by oxidation of SO?
which occurs between 750° and 1500°F. The amount of SO? formed will de-
pend on the design of the boiler and the rate of combustion. The lower
limit of soluble sulfur compounds in the coal ash, necessary to create
an acceptable resistivity has been found to be between 0.5 and 1.0 per-
cent. The resistivity also varies with the flue gas temperature and
reaches a peak at approximately 300°F for average fly ash. The peak
resistivity of some fly ash is high enough to adversely effect the pre-
cipitator performance^ Yet 300°F is a favorable temperature level for
boiler and evaporator operation. Thus, requirements for the boiler
and for the precipitators are contradictory. The amount of magnesium
and aluminum in the coal ash is also believed to be a factor contributing
to high resistivity. Fly ash from strip mined coal usually has higher
resistivity than fly ash from deep mined coal.
Fly ash containing more than 15 percent carbon will effect precipitator
performance adversely. The reason for this is that carbon is a relatively
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.cr>
UD
TOTAL
TABLE 5-1
TYPICAL PARTICLE SIZE DISTRIBUTION OF FLY ASH FROM BOILER GASES OF VARIOUS BOILERS
(Percent by Weight)
Particle
Size
Microns
0-10
10-20
20-30
30-40
40-74
74-149
+ 149
COAL FIRED BOILER
Pulverized
Coal
25%
24
16
14
13
6
2
Cyclone
Furnace
72%
15
6
2
5
Spreader Traveling
Stoker Grate
11%
12
9 11
10
12 12
17 30
29 47
Underfed
Stoker
7%
8
6
9
8
19
43
100 Percent Bark Fired
Boiler
12%
10
7
6
14
16
35
100
100
100
100
100
100
Data frpm Bituminous Coal Research Association, Pittsburgh, Penn. and the Industrial
Gas Cleaning Institute, "Criteria for the Application of Dust Collectors to Coal
Fired Boilers."
-------
good conductor and will acquire, but also loose, an electrical charge
very easily. When a particle with high carbon content precipitates
on the collector plate it loses its charge immediately. Owing to
a phenomenon referred to as "charge by induction," the particle as-
sumes a weak positive charge. It is repelled from the collector
plate and reentrains into a gas stream. This phenomenon is repeated
over and over again until the particle is carried out by the gas stream
and escapes from the precipitator. In order to overcome this problem,
the collector plates are given a special form and the so-called pocket
or screen grid collecting electrodes have proven to be very effective.
There is also a certain correlation between the carbon content and the
coarseness of the fly ash. A coarse fly ash has normally a high carbon
content.
Recent improvements in the design of electrostatic precipitators have
reduced many of the advantages of combinations of electrostatic pre-
cipitators and mechanical collection systems, so common some years ago.
The plate electrode precipitators did not have sufficient collecting
efficiency on high carbon fly ash with problems of "snow outs" from the
stack during soot blowing and collector plate rapping periods. Pocket
electrodes and continuous rapping have, to a large extent, circumvented
these problems. Another advantage of a combined system installation
was that the mechanical collector wou.ld still remove a major portion
of the fly ash should the precipitator be out of order for some reason.
Sectionalized design and more reliable rectifiers have shortened the
shut-down time considerably on modern precipitators.
Mechanical Collectors
Bark char is a difficult material to collect. While the particle size
is relatively large, the specific gravity is very low and the sliver
shape of the particles makes collection difficult. Bark char has a
specific gravity in the 0.2 range; pulverized coal ash for example, is
10 - 15 times heavier. In addition to the unique particle shape, the
low specific gravity and the very fragile particles, the bark boiler
collector must also be able to handle abrasive sand accompanying the
bark. There are consequently a number of considerations to make when
selecting a collector for a bark boiler.
Electrostatic precipitators are not generally suitable for use on bark
boilers due to (1) the poor electrical characteristics of bark char
and (2) the possibility of fires.
Some studies report that the bark char disintegrates into fine
particles in a multi-tube collector due to the high centrifugal forces
imposed. These fine particles coupled with their low specific gravity
have a tendency to float through the outlet tube of the collector and
out the stack. This phenomenon is also attributed to the short dis-
tance from the wall of the collecting tube to the outlet tube. A sur-
vey shows that there are more large diameter cyclone installations
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than multi-tube units operating on bark boilers in the wood pulping
industry. It should be noted, however, that both collection devices
can give sufficient efficiency providing they are selected properly.
Many bark char collectors in the southern part of the United States
are operating with reinjection systems that are not equipped with
provisions for separation of sand from the char before reinjection.
Sand separation is essential for reducing excessive erosion of the
collector and the boiler tubes. It is not unusual for a system
operating on bark from southern pine to accumulate a number of truck
loads of sand in an operating ,day. Some mills are now selling the
collected bark for manufacturing of charcoal briquettes.
In many multi-tube collectors recovery vanes are installed to reduce
pressure drop and collector size.- The experience shows that collectors
operating on boilers firing oil in combination with bark must be de-
signed without recovery vanes. Some oil fractions having high dew-
points combine with the bark char particles and plug the recovery
vanes.. One single plugged outlet tube can by-pass some 10 times its
normal share of flue gas uncleaned into the stack. It is consequently
imperative to prevent plugging of the tubes.
Studies show that substantial improvement of the collecting efficiency
of bark char collectors can be attained by changing the normally coni-
cal bottom outlet of the cyclone (tube). By using straight cylindrical
tubes with a peripheral discharge rather than conventional conical dis-
charge, the ash is removed from the tube before the gas reverses into
the inner vortex and in sufficient distances from the turning point.
This prevents re-entrainment of already collected particles.
The volume handled by a given diameter collector tube depends on the
shape of the inlet guide vane. Shallow pitched vanes give more spin
and handle less volume. Steep pitched vanes reduce the spin and can
handle more volume. The shape of the inlet guide vane can make a
difference in volume capacity of the tube at a ratio of 1 to 2. The
vanes are of obvious importance for collector size as well as operating
efficiency.
Bark char collectors should be easily accessible for inspection, main-
tenance and repair because of the difficult operating condition under
which they work. One always has to bear in mind the abrasiveness of
the dust and possibility of clogging. Large diameter cyclone collectors
are easily accessible for maintenance. Some modern multi-tube col-
lectors also offer design with accent on maintenance. The place sub-
ject to wear in a well designed collector is limited to the collection
tube or cyclone cone. Hard cast iron is often used in tubes. Hardness
of 420 Brinell has proven to be a maximum, because hard tubes are sub-
ject to thermal shock damage. Cyclone cones, as well as the entire
cyclone, are sometimes furnished with abrasion resistant linings.
This lining is usually installed over a hexagonal steel liner.
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Both mechanical collectors and electrostatic precipitators are
sensitive to uneven gas and dust distribution. The entrance to the
collector is normally provided with devices for correction of uneven
gas distribution. The space for proper distribution ahead of the col-
lector is almost always very limited. Sharp bends and short transitions
tend to produce uneven distribution across the entrance. Various de-
vices such as spreaders, turning vanes and perforated baskets in the
collector entrance are used in efforts to overcome these problems.
Hopper fires occur in bark char collectors. Unlike ash from coal
fire boilers, bark char must be continuously and completely removed.
A small air leakage is enough for accumulated char to catch fire. Gas
tight construction is essential in bark char collectors.
Design must consider the possibility and effect of fires. This removal
from the hopper must be accomplished without a back flow of air. This
can be done most successfully with rotary steel valves.
Bark boiler dust collectors should be carefully sized for correct
pressure drop inlet velocity. While efficiency generally increases with
increased pressure drop (and velocity) there are limits beyond which
fragile material like bark char will be broken into smaller particles
which has a detrimental effect on the collecting efficiency.
The low specific gravity of bark char, when the boiler is operated on
100 percent bark without reinjection, or the finer particle size dis-
tribution when the ash is reinjected, create operating conditions which
limit the application of large diameter cyclones on these installations.
Only when other conditions are superimposed over these conditions can
the large diameter cyclone approach 92 percent efficiency in a single
stage unit. One of the conditions which will increase the efficiency
is the amount of sand in the bark char.
When operating on 100 percent bark, the efficiency will vary greatly de-
pending upon the amount of sand imbedded in the bark. Without rein-
jection, much of the imbedded sand remains in the bark char. This
greatly increases the average specific gravity of the bark char, making
the centrifugal separation easier with higher efficiencies.
Example: 80% Bark Specific Gravity (as char) 0.3
20% Sand Specific Gravity 2.8
S. G. = (0.8) (0.3) + (0.2) (2.8) = 0.24 + 0.56 = 0.80
For a large diameter cyclone where the specific gravity is assumed at
o.3, the most practical unit would give efficiencies in the 70 to 75
percent range. If, as in the example above, the char contained 20
sand, the efficiency would increase to 85 to 89 percent.
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The performance of mechanical collectors, with 40 percent bark and 60
percent oil feed to the boiler, is greatly improved due to the higher
specific gravity of the fly ash. The nature of this dust, however, in
multi-tube collectors creates a plugging problem. The higher specific
gravity and tendency not to plug make the large diameter cyclone
practical for combination fired boilers. It is possible to achieve
the 92 percent design efficiency in both multi-tube and large dia-
meter cyclones with single-stage units. The 96 percent design ef-
ficiency is possible with two-stage large diameter or multi-tube units.
5.4 SULFITE SOURCES
The evaluation of control methods for sulfite sources is limited to
those which are distinct from the kraft sources. The reader is refer-
red to the previous discussions o-f kraft sources for the following
sulfite sources: Washer vents, evaporators, power boilers, and com-
bination boilers. It should also be noted that control methods have
not been considered for the following sulfite sources: Dump tank
vent, Venturi absorbers, absorption towers, and ammonia incineration.
These sources are either of minor importance from an emission stand-
point, or there are no control methods presently in use.
5.4.1 ACID TOHER
5.4.1.1 Application
The pressure accumulator is vented into the acid storage tank because
the two tanks are nearby. The acid storage tank is then vented to the
absorption tower. This system effectively prevents emissions from
these items of equipment, thereby leaving the acid absorption tower
as the only significant source of emission in the acid system.
The construction of most absorption towers incorporates a mesh pad
distribution system for absorption liquid. Therefore, additional
mesh pads are deemed necessary.
The efficiency of absorption of most sulfiting towers exceeds 90 per-
cent. Some mills have placed a second absorption tower in series with
the sulfiting unit for scrubbing exhaust gases. This method has been
tried only where an existing second tower was available and could be
used economically. An example would be the conversion of calcium
base, requiring two towers, to ammonium base which requires one tower.
5.4.2 BLOW PIT
5.4.2.1 Application
Two systems were considered to replace the existing multiple wooden
blow stacks and showers with a high efficiency S02 recovery system as
follows:
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1. Condenser with Cyclone and Absorption Tower
2. Packed Tower
Flash steam, sulfur dioxide, and inert gases are released in the blow
pit during a digester blow. These gases then exit from the blow pit
and after the steam is condensed, the noncondensible gases, mainly
sulfur dioxide, are absorbed in a packed tower. The recovered sulfur
dioxide is reused in the process and the condensate creates a source
of hot water.
5.5 NSSC SOURCES
Evaluations of control.methods applied to NSSC sources are not in-
cluded due to the lack of emission data and application experi-
ence. The reader is referred to the previous kraft discussions for
the following NSSC sources: washer vents, evaporators, combination
boiler, and power boiler.
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5.6 REFERENCES
1. Shah, I. S. and Stephenson, W. D., "Weak Black Liquor Oxidation
System: Its Operation and Performance," TAPPI 51 (a), 87 -,
1968.
2. "Steam, Its Generation and Use," 37th Edition, The Babcock and
Wilcox Company, New York.
3. Fryling, G. R., "Combustion Engineering," Revised Edition First
Impression, Combustion Engineering, Inc., New York.
4. MacDonald, R. G., Editor, "Pulp and Paper Manufacture - Volume
I, The Pulping of Wood," Second Edition, McGraw-Hill, New York
1969.
5. Whitney, R. P., Editor, "Chemical Recovery in Alkaline Pulping
Processes," TAPPI Monograph Series No. 32, TAPPI, New York, 1968.
6. Theon, G. N., DeHaas, G. G., Tallent, R. G., and Davis, A. S.,
"The Effect of Combustion Variables on Release of Orodous Com-
pounds from Kraft Recovery Furnaces," TAPPI 51 (8), 329-,
1968.
7. Harding, C. I. and Galeano, S. F., "Using Weak Black Liquor for
Sulfur Dioxide Removal and Recovery," TAPPI 50 (10), 48A-,
1967.
8. Tomlinson, G. H., Chapter 5, page 419, "Pulp and Paper Manu-
facture - Volume I, Preparation and Treatment of Wood Pulp,"
First Edition, McGraw-Hill, New York, 1950.
9. Thomas, E., Broadus, S., and Ramsdell, E.W., "Air Pollution
Abatement at S. D. Warren's Kraft Mill in Westbrook Maine,"
TAPPI 50 (8), 81a-, 1967.
10. Hough, G. W. and Gross, L. J., "Air Emission Control in a Modern
Pulp and Paper Mill," American Paper Industry, 36-, February
1969.
11. Mullen, J. F., "A Method for Determining Combustible Loss, Dust
Emission, and Recirculated Refuse for a Solid Fuel Burning System,"
Paper presented at ASME Winter Annual Meeting, New York, November
29 - December 4, 1964.
12. Wrist, Peter, Mead Corporation, Verbal Communication, Liaison
Meeting, New York, November 6, 1969.
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13. Carlton-Jones, Dennis and Schneider, H.B., "Tall Chimneys,"
Chemical Engineering, 75. October 14, Iy68.
14. Martin, F., "Secondary Oxidation Overcomes Odor from Kraft
Recovery," Pulp and Paper, 43, June 1969.
15. Industrial Gas Cleaning Institute, "Test Procedure for Gas
Scrubbers," Publication No. 1, IGCI, Box 448, Rye, New York.
1964.
16. Industrial Gas Cleaning Institute, "Criteria for Performance
Guarantee Determinations," Publication E-P3, IGCI, Box 448,
Rye, New York, 1965.
17. NAPCA, "Tall Stacks - Various Atmospheric Phenomena and Related
Aspects," Publication No. APTD 69-12.
18. Wrist, Peter, Mead Corporation, Personal Communication, Sept.
2, 1969.
19. Blosser, R. 0., and Cooper, H.B.H., "Current Practices in
Thermal Oxidation of Noncondensible Gases in the Kraft Industry,"
Atmospheric Pollution Technical Bulletin No. 34, National
Council for Air and Stream Improvement, Inc., New York.
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C H A .P T E R 6
. FIELD ENFORCEMENT EQUIPMENT
When the FEO visits a.pulp mill in order to perform a routine or special
inspection, it is not anticipated that he will perform any substantial
amount of source sampling. He should, however, carry certain basic equip-
ment and records pertaining to the facility.
6.1 BASIC EQUIPMENT REQUIRED
Normally the FEO will have an official vehicle equipped with a two-way
radio to provide adequate transportation and communications equipment.
The car should always contain the basic equipment which will make it
possible for the FEO to perform his inspection properly, and comply with
all applicable safety rules and regulations. The plant manager is
responsible for. insuring that the OSHA regulations are complied with by
his own employees and visitors. Thus the FEO should be prepared to comply
with these regulations also. In certain areas it is required that all
personnel wear safety shoes, hard-hats, and safety glasses.
The following list indicates the minimum basic equipment which the FEO
should carry with him and use: hard-hat, safety glasses, safety shoes,
asbestos gloves, coveralls, Ringlemann chart or opacity guide, Polaroid
camera, compass, wind speed indicator, flashlight, thermometers covering
the range 50-800°F, stop watch, 50-foot tape measure, 6-foot rule, manometer,
or pressure/vacuum gauge (0-30 inches Hg and 0-10 H?0), sample containers,
thermocouple with portable millivolt meter.
6.2 RECORDS AND FORMS REQUIRED
Prior to the time of his visit, the FEO should review carefully any pre-
vious field inspection forms concerning the installation and the permit
application form. He should also carry with him on each inspection a copy
of the previous field inspection form, a copy of the air pollution code, and
a copy of the emergency episode procedures. He also should carry with him.
on a clipboard a form for recording his observations as well as data obtained.
from the mil 1.
6.3 PROCEDURES AND EQUIPMENT FOR SOURCE SAMPLING
In the event that the results of a mill inspection and survey suggest, that
source .sampling is required to evaluate a particular unit, the FEO may have
his own personnel conduct the sampling or may require mill personnel to con-
duct the sampling with an agency employee observing. Regardless of who does
the sampling, it is important for the FEO to assure himself that the equipment
to be used has been properly calibrated. Generally the agency will be inter-
ested in sampling for particulate matter and gaseous sulfur compounds.
6.3.1 PARTICULATE SAMPLING PROCEDURES (AND EQUIPMENT SPECIFICATIONS)
Particulate sampling procedures and the equipment utilized, should essentially
conform to that outlined in the Federal Register (EPA). Vol. 36, No. 274, Part
II (Dec. 23, 1971), "Standards of Performance for New Stationary Sources."
-177-
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6.3.2 GASEOUS (SULFUR) SAMPLING PROCEDURES (AMD EQUIPMENT SPECIFICATIONS)
This section will outline procedures and equipment descriptions which have
been utilized for sampling the major gaseous constituents emanating from
wood pulping processes. Currently two basic methods have been developed,
sanctioned by the EPA and various states for sampling gaseous emissions,
the Barton Titrator and Gas Liquid Chroma-tograph (GLC).
6.3.2.1 Gas/Liquid Chromatograph (GLC)
Figure 6-1 illustrates the system which may be employed in conveying
.the gases from the source to the GLC sensing equipment. The stainless
steel probe and Teflon sampling line are maintained at temperatures ex-
ceeding the dew point of the source gases. The sampling line consists of
an insulated, electrically heated 1/4-inch Teflon tube. The sample gases
are transmitted to the heated dilution box where they are split into two
separate streams. One stream is conveyed to the vacuum source and wasted
to minimize lag time in the sampling line. The remainder of the flow is
diluted with nitrogen by an amount sufficient to lower the dew point of
the gases below ambient temperature. A portion of this diluted sample is
injected into the Chromatograph through the Gas/Liquid Chromatograph (GLC)
sampling valve. The remainder of the diluted gas is wasted through the
vacuum source.
Gaseous sulfur concentrations may be determined with a Gas/Liquid Chromat-
ograph. This unit is equipped with a flame photometric detector which
is specific for sulfur compounds. Two analytical columns are utilized
in the separation and analysis of the gaseous sulfur compounds. One
is a 36-foot by 1/8-inch OD Teflon column packed with polyphenyl ether
(liquid phase) on a solid support of granular Teflon. The second column,
constructed of identical materials, is 8 feet long. Both columns are
operated at 50°C.
The 36-foot column is utilized for analyzing hydrogen sulfide, sulfur
dioxide, and methyl mercaptan while the 8-foot column facilitates the
analysis of dimethyl sulfide and dimethyl disulfide.
Several sampling loops of different volumes may be employed on the GLC
sampling valve to accommodate a wide range of gaseous sulfur concentrations.
The Chromatograph is calibrated for hydrogen sulfide, sulfur dioxide,
methyl mercaptan, dimethyl sulfide and dimethyl disulfide, using the
"spinning syringe" technique or permeation tubes.
6.3.2.2 Barton Titrator
Figues 6-2 illustrates the system which is employed in conveying the gases
from the source to the Barton Titrator. The stainless steel probe and
Teflon sampling line are maintained at temperatures exceeding the dew point
of the stack gases. -The sampling line is identical to the sampling line
used with GLC. The sample gases are transmitted to the Barton Titrator by
a vacuum source.
-178-
-------
Total reduced sulfur (TRS) concentrations are analyzed using a Barton
Titrator, Model 400.. Furnace gases are scrubbed through a 3% solution
of potassium acid phthalate (KHP) which removes sulfur dioxide and a
large fraction of water vapor from the sample gases. The scrubbed gases
are then passed through a combustion tube maintained above 1500°F where
all the remaining reduced sulfur compounds are oxidized to sulfur dioxide.
The sample gas is then introduced to a coulometric titration cell which
utilizes hydrobromic acid (HBr) as an electrolyte. The electrolytic cell
generates bromine from the HBr electrolyte which reacts with the sulfur
compounds entering the titration cell. The quantity of current required
to generate the excess bromine, to consume the sulfur compound, is pro-
portional to the gaseous sulfur concentrations introduced. The current
required to operate the titration cell is sensed and transmitted to a
recorder where a continuous readout is accomplished. The recorded output
is converted to TRS concentrations, as S02» from calibration data generated
with the "spinning syringe" technique.
-179-
-------
Glass Wool
Source
CJ>
to
oo
r
u
Dilution System
O)
n
O)
" _ _ ', I
i i
n
u
Chart
Recorder
In
Out
Gas/Liquid
Chromatograph
Carrier Gas
.J
Vacuum Pump^
Dilution Nitrogen
GAS SAMPLING SYSTEM
Figure 6-1
-------
I
oo
I
Source
Glass Wool
Heated
Probe
QJ
c'
1
n^
^j>
c
Q.
f-~;
(O
CO
-a
QJ
1 i
ro
CU
3C
1
so2
Scrubbers
<
Combustion
Furnace
Data
Recorder
Barton
Titration
Cell
-1-
O)
*->
o>
Drying
Meter
Valve
Vacuum Pump
BARTON SAMPLING SYSTEM
Figure 6-2
-------
CHAPTER?
UTILIZATION OF FIELD DATA AND RECORDS
7.1 GENERAL INFORMATION
Most states have air implementation plans approved by the Administrator of
EPA which contain provisions for a permit system. Normally, a permit is
required for both construction and operation of an air pollution source.
The plans also require a limitation on emissions of certain compounds .to the
atmosphere as well as pre-planned strategy for meeting an air pollution
emergency.
The application for permit usually requires complete information on the
site, a description of the process and process design, and information on
the air pollution abatement strategy which is intended to be used. The
application states the level of emissions from all parts of the process
without any abatement strategy. In an application for a permit to construct,
the applicant is required to make an estimate of the level of emission to
be expected with the proposed control equipment whereas a permit to operate
may not be granted until performance tests are conducted on the source and
it can be shown that the source will meet applicable regulations. Usually
the regulations require that any modification to the process or any modifi-
cation to the control devices be reported to the agency. Usually the
regulations also require immediate notification of the agency in the case
of an upset or emergency condition.
These requirements implicitly call for a continuing program of reports by
the holder of the operating permit plus a continuing program of inspection
and observation of each source. A visit by the FEO may be triggered by
complaints, by reports from the mill, or a routine periodic inspection. Ad-
ditional information such as complete plans and specifications, operating
conditions and similar data may be required by the director of the state
agency. All of the information previously cited will form the basis for a
file on a specific source.
7.2 FILE OF FIELD DATA
A file on each company (source) should be opened as soon as an inquiry from
an official of the company is received at the state agency. The file should
contain the completed application for a permit to construct and/or operate
with any supporting data submitted by the mill (such as flow diagrams).
This file should also contain emissions data collected by the state pollution
control staff or the mill staff, copies of permit, copies of complaints,
and/or observed violations, and reports on inspections by the FEO. In some
instances, the complaint file may be a separate file, depending on the num-
ber received, but copies should be placed in the general file for that source.
7.2.1 INDUSTRY DIRECTORIES " "
Information of a general nature relative to a particular plant may be
obtained from one of several pulp and paper industry directories, such
as those of Post and Lockwood. An example of pertinent information which
-182-
environmental science and engineering, inc.
-------
may be found in these directories is as follows: (1) Company name and or
specific division, location, executive office location, and phone numbers.
(2) Production capacity, types of wood pulping processes and raw material
consumption. (3) Management personnel and their area of responsibility.
(4) Summary of process equipment such as the number of individual units,
capacities, manufacturer, and miscellaneous information.
A review of information contained in these sources should be invaluable
in assessing the extent and type of survey which should be performed as
well as an indication of the technical review required prior to outlining
the site inspection.
7.3 ADDITIONAL SOURCES OF INFORMATION
In building up the technical information file on a specific company, and in
preparation for an on-site inspection, the following additional sources of
information are suggested. A suggested procedure for the on-site inspection
is described in the next chapter.
Most pulp mills maintain excellent operating records. The daily operating
statistical report and the operating log sheets from the major process
areas are somewhat similar from mill to mill. These reports can be utilized
to provide the FEO with a profile of operating levels for various mills.
Many pulp mills routinely conduct source sampling tests to determine the
performance level of air control equipment.- This test may be conducted
periodically or continously as indicated in previous chapters. The reports
are usually kept on file in the technical department. In some states the
results of emission monitoring are required to be submitted to the state
agency on a periodic basis.
7.3.1 PROCESS EQUIPMENT SPECIFICATIONS
Quite often, pulp mill equipment capacities are specified by the number of
pulp production "tons" which they will support, and this does not relate to
the true engineering design criteria. For instance, identical capacity
equipment will support a variable pulp production rate, depending on the pulp
yield range which is being attained.
Almost all pulp mill production equipment is specified and designed based
on pounds of dry solids per unit time processed. These design capacities
may be expressed as Ibs/hr, tons/day, etc., with further qualifications
regarding operating temperatures, inlet and outlet concentrations, etc.
With proper conversion factors and pertinent knowledge, it is possible
to express specified design capacities in more convenient units for com-
parisons or other reasons. The Appendix of this report contains information
which will assist in converting process material units.
Pulp mill technical personnel may quote "nominal" equipment capacities and
specifications when questioned in this regard, which may or may not reflect
-183-
-------
design and/or actual operating rates. Therefore, the only sure way to
determine the actual design specifications is to request a review of the
quoted equipment specifications contained in the engineering file, as well
as any designed modifications which may have altered the original.
Examples of the type information which may be outlined and summarized in
engineering design specifications is illustrated in Tables 7-1 and 7-2, and
Figures 7-1 and 7-2 for an evaporator set and recovery boiler.
7.3.2 EMISSION CONTROL EQUIPMENT SPECIFICATIONS
For the major pulping processes, emission control equipment specifications
will be tied closely to the process equipment specifications, particularly
where the control equipment is an integral part of a "system." In'a recently
constructed mill, these specifications would be included in a file with the
major process equipment. In the case of an older mill, in all probability
the control equipment has been updated by. modification or replacement, and
engineering data and specifications would be contained in a separate file.
Control equipment on the miscellaneous sources, because of the various avail-
able-alternative methods, are usually specified separately as additions to
the process. An exception to this may be the case with mills which were
built very recently. (See Table 7-3 and Figures 7-3 and 7-4.)
Control equipment design specifications normally stipulate a performance
guarantee based on inlet gas volume, grain loading, moisture content, and
temperatures. In the case of sub-micron particulate "fume" and gaseous
constituents,further stipulation may include quantitative limits' on compon-
ents contained in the process input feed or streams. Design criteria also
state static pressure and pressure drop relationships as well as liquid rates
and pH in the case of scrubber applications.
Tables 7-4, 7-5, 7-6, and 7-7, contain results of performance tests on
several different types of control devices. The data and nomenclature contained
in these tables is similar to that which would be itemized in design
specifications for similar control devices. More recently the trend has
been to specify control systems as a "system" as opposed to individual
components. Where a system has been engineered as individual components,
it is often difficult to determine the precise performance guarantee point,
and where the responsibility and liability lies.
7.4 AGENCY RECORD KEEPING REQUIREMENTS
The FEO must be cognizant of agency record keeping requirements and/or
plans. The next chapter of this manual will contain recommended data
format sheets, sequential inspection outline, and other pertinent informa-
tion. The final reporting of data for files, however, may require some
individual judgement.
If the agencies involved have not provided specific guidelines in these
regards, the inspector may want to make recommendations or suggestions to
make files more functional for future reference and site inspection compar-
isons. These factors will become much more obvious after several pulp mill
inspections have been performed. Inasmuch as agency requirements are
-184-
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constantly changing, it would not be appropriate to make concrete reconrr
mendations relative to these in this manual.
-185-
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Table 7-1. Operating Conditions in a Sextuple-Effect Evaporator
Flash
Conditions Tank IA IB II III IV V VI
Steam pressure .
psi 31.5 31.5 9.4 1.2
vacuum, in. Hg . 8.3 16.2 22.0
Steam temperature, °F 276 276 238 216 196 175 152
Working Temp, drop, °F - 24 26 13 13 15 17 24
Liquor Temp., °F 232 252 250 225 203 181 158 128
Boilino pt. rise, °F 14 13 11 7 5 4 3-3
Saturated vapor temp., °F 218 239 239 218 198 177 . 155 125
Vapor pressure
psi 9.8 9.8 1.8
Vacuum, in. Hg 7.4 15.6 21.4 26.0
Latent heat of vapor, Btu/lb 953 953 967 979 992 1005 1023
Pressure drop
psi 0.4 0.4 0.6
vacuum, in. Hg 0,9 0.6 0.6
Vapor pressure '
Next effect, psi 9.4 9.4 1.2
Vacuum, in. Hg
Feed, Ib/hr
Discharge, Ib/hr
Solids in, %
Solids out, %
Heating surface, ft2
Heating transfer coefficient,
Btu/(hr)/(ft)2/(°F) . 174 242 392 386 316 240 190
90,605
89,200
51.1
52.0
109,775
90,605
42.2
51.1
4,400
135,825
109,755
34.2
42.2
4,400
178,815
135,825
26.0
34.6
8,800
8.3
219,585
178,875
21.2
26.0
8,800
16.2
251,995
219,585
18.4
21.2
8,800
22.0
166,520
130,475
13.9
17.8
8,800
166,520
121,520
13.9
19.1
8,800
-------
Water
STEAM
CO
I
THICK
LIQUOR
-£
fcOMBINED
i CONDENSATE
STEAM
CONDENSATE
Figure 7-1. A sextuple-effect evaporator.
-------
TABLE 7-2
COMBUSTION OF BLACK LIQUOR
Typical Operating Performance
Capacity* black liquor at 50%
solids . lb/24hr 3,000,000
Black liquor solids lb/24hr 1,500,000
Steam temperature °F .825
Steam pressure S.H.. outlet psig 850
Feedwater temperature °F 275
Air temperature at F
-------
TABLE 7-2 (cont.)
Distribution
Water vapor loss all sources 1593.2 22.51
Dry gas loss 319.6 4.52
Heat of reaction correction 392.8 5.55
Reduction of salt cake 119.8 1.69
Heat of fusion and sensible
heat in smelt 233.7 3.30
Radiation 29.7 0.42
Unaccounted for 273.6 3.86
Total unavailable . 2962.4 41.85
Heat available for steam 4116.4 58.15
Total distribution 7078.8 100.0
Enthalpy in steam, Btu/lb 1410
Enthalpy in feedwater, Btu/lb 244
Steam, Ib/hr 220,600
Quantity
Steam to liquor heaters, Ib/hr 2,790
Flue gas leaving evaporator, Ib/hr 429,400
Air to air heater, Ib/hr . 318,400
Temperature
Flue gas leaving economizer, °F 648
Smelt temperature, °F . . 1530
Liquor to furnace, °F 240
Liquor solids
To evaporator, % 50
From evaporator, % 68
Reduction, % 95
Sulfidity, % 25
* Before the addition of raw and precipitated salt cake.
-189-
-------
o
I
COMBUSTION GASES
At exit gas temperature
Dry gas, Moisture & Fume
(Na Compounds)
FEEDWATER
RADIATION
AIR FOR
COMBUSTION
STRONG BLACK LIQUOR
TO DIRECT CONTACT
EVAPORATOR
50% Solids
200 °F
Elemental analysis
weight percent dry solids
Carbon
Hydrogen
Sulfur
Sodium
Inert oxides
Oxygen
SALT CAKE MAKEUP TO MIX TANK
125 Ib / ton pulp
Equivalent to
0.0417 Ib / Ib solids
at 3000 Ib solids / ton pulp
reduce to Na$ in furnace
STEAM
INFILTRATION
AIR
STEAM TO DIRECT CONTACT
LIQUOR HEATER
SMELT TO GREEN LIQUOR
DISSOLVING TANK
99% of Total
Na Compounds
and Inerts
Na2S 24.7%
Na2S04 1.3% y as Nc
Na2C03 70.0%_
Figure 7-2,
Typical conditions
and heat balance.
for recovery unit material
-------
TABLE 7-3
BARK BOILER
DUST COLLECTOR EFFICIENCY TEST
Date of Test:
BOILER PRIMARY SECONDARY
OUTLET COLL. OUTLET COLL. OUTLET
Draft Loss 2.0" 2.0"
Gas Temperature (Dry Bulb) °F 640 640 470
Gas Temperature (Wet Bulb) °F 165 170 175
Static Pressure Inches H20 -2.5 -4.5 +0.1
Density @ Conditions #/Cu. Ft. 0.0326 0.0315 0.0363
Density @ 70° F. #/Cu. Ft. ' 0.0681 0.0662 0.0637
Gas Velocity in Duct Ft./Min. 1609 1714 4233
Gas Flow in Duct @ Conditions CFM 279,000 245,960 211,650
Gas Flow in Duct @ 70°F. SCFM 133,613 117,200 120,619
Volume Gas Sampled Cu. Ft. 260.19 213.81 270.48
Sampling Time Min. 35 27 30
Dust Sample Taken Grams . 22.7924 1.5658 1.0956
Grain Loading @ Conditions Grains/Cu. Ft. 1.3517 0.1130 0.0625
Grain Loading @ 70° F. Grains/Cu. Ft, 2.8055 0.2345 0.1097
Collector Efficiency % 91.64 53.22
Total System Efficiency % ^ - 96.09
Stack Dust Emission #/Min. 1.89
Total Stack Emission per 24 Mrs. #/Day 2722
-191-
-------
Figure 7-3.
TOO
90
C
QJ
O
O
O
d)
80
70
60
50
9 In.;Tubes
.0.5
1.0 1.5 2.0 2.5 .3.0
Pressure Drop-In. Water
Cyclone Efficiency Vs. Tube Size
3.5
4.0
Collection Tubes
To capably evaluate dust collectors, the engineer must be aware that
there are a wide range of collecting tube designs available, affecting
volume of gas handled, draft loss, efficiency, operation, and price.
The tube itself is usually of cast iron in a size range between 6 and
24 inches in diameter, nominally 9 inches in diameter for bark boiler
service. The smaller 6-inch diameter tube is more efficient, but this
is offset by the higher tendency to plug and the higher wear rate of
smaller tubes.
-192-
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-------
TABLE 7-4
Power Boiler Emission Data
FUEL: NO. 6 OIL (NO ADDITIVES)
LBS. STEAM: 100,000 #/HR.
Gas Temperature (Wet) (F°)
Gas Temperature (Dry) (F°)
Gas .Volume @ Standard Conditions (S.C.F.M.)
Gas Density @ Standard Conditions (Lbs/Cu. Ft.)
Grain Loading @ Standard Conditions (Grains/S.C.F.)
Gas Volume @ Conditions (A.C.F.M.)
Gas Density @ Conditions (Ibs./Cu.Ft.)
Grain Loading @ Conditions (Grains'/A.C.F.)
Gas Volume Sampled (Cu. Ft.)
Gas Velocity (Ft./Min.)
Time Sampled (Minutes)
Dust Loading (Lbs./lOOO Lbs. Gas)
Dust Emission (Lbs./Min.)
Dust Emission (Lbs./Hr.)
INLET
125
330
29,509
0.0722
0.111
43,695
0.0494
0.07502
10,508
2819 .
840
0.2167
0.4683
28
OUTLET
125
325
30,492
0.0732
0.0137
45,429
0.0491
0.0092
10,855
2912
840
0.0268
0.06
3.6
Collector Efficiency: 87%
0=3%
CO = 13.8%
15.4% Excess Air
-194-
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TABLE 7-4A
Power Boiler Emission Data
PARTICLE SIZE-ANALYSIS
(INLET SAMPLE TO COLLECTOR)
The equivalent micron size is determined from the Bahco analyzer
having been standardized with a dust of 2.7 specific gravity but
corrected to the actual specific gravity of 1.22.
MICRON SIZE % BY HEIGHT
1.64 and Below .66%
1.65 - 2.53 1.79%
2.54 - 5.50 8.78%
5.51 - 10.41 7.80%
10.42 - 15.62 11.26%
15.63 - 24.54 . 10.94%
24.55 - 31,24 2.16%
31.25 - 35.70 .72%
35.71 - .44.00 .53%
45.00 - 74.00 28.56%
75.00 -105.00 13.34%
106.00 -250.00 11.36%
251.00 -420.00 1.82%
421 and Above .28%
100.00%
-195-
-------
TABLE 7-5
SUMMARY OF TESTS ON BARK BOILER COLLECTORS
Type Collector - Mechanical,
Number Tubes
Type Vane
Design CFM
Design Temperature
Design Draft Loss
Type Fuel
Rated Steam Load
Bark
Bark & Aux. Fuel
Aux- Fuel
Steam Load, #/Hr.
Actual Oper. Temp.
Actual Oper. D. L.
MM BTU/Hr. Fired
Max. Rated #/Hr. of Bark
Volume:
ACFM, inlet
SCFM, inlet
Dust Loading:
Inlet, #/Day
Outlet, #/Day
Inlet, #/M # Gas
Outlet, #/M # Gas
Inlet, #/MM BTU's
Outlet #/MM BTU's
Grain Loading, .gr/SCF:
Inlet
Outlet
Efficiency of Collection
Allowable Emissions, #MM BTU:
Georgia and NYC Code
Screen Mesh Size
Notes :
Multiple Tube
204
NR/A
152,530
500
2.73
bark & gas
150,000
150,000
165,000
200,000
150,000
460
228
54,700
150,190
85,323
60,120
4,096
6.797
.497
10.98
0.75
3.426
0.242
93.19
.35
24
285
NR/A
215,000
725
2.5
bark, gas ,oil
300 ,000
340 ,000
270,000
738
408
60,000
267,181
119,911
93.432
7,464
7.566
0.606
9.54
.76
3.788
0.3027
92.12
.45
30
384
NR/A
230,000
450
2.5
bark & oil
300,000
300,000
420
3.0
457
222,713
134,000
13,940
1,056
1.029
.0707
1.29
.0965
.5056
.0343
93.36
.44
14
35% bark
hardwood
65% oil
384
NR/A
230,000
450
2.5
bark & oil
300,000
268,000
410
2.5
407
188,510
114,800'
20,799
1,455
1,804
.1052
2.13
.149
.8805
.05286
94.12
.45
14
79% bark
pine
21% oil
344
NR/XD
293,000
679
3.0
bark & oil
300,000
300,000
455-425
3.5
457
241,618
1 39 ,500.
47,902
3,509
3.376
.2658
4.36
.317
1.6688
.13018
. 92.25
.44
340
NR/A
297,542
725
3.0
bark & gas
300,000
300 ,000
640
2.0
457
279,000
133,613
77,592
5,718
5.92
0.513
7.07
0.52
2.8055
0.2345
91.64
.44
20
-------
TABLE 7-6
SPECIFICATIONS
Venturi Scrubber
DESIGN INLET CONDITIONS
Volume
Temperature
Humidity
Dust Loading
Density
DESIGN EXIT CONDITIONS
Volume
Temperature
Humidity
Dust Loading
Density
PRESSURE DROP ALLOWANCES
Waste. Heat Evaporator
Kiln
Duct Work
Venturi Throat
Separator
TOTAL
HATER REQUIREMENTS
Scrubbing Water
Water Evaporated
Bleed-Off at
% Solids
Make-Up Water
Water Recirculation
MATERIALS OF CONSTRUCTION
Venturi Throat
Flooded Elbow
Separator
Duct Work
Stack
CFM
°F
#W.V./#D.G.
Grs/SCFD
#/Ft.3
.CFM
°F
#W.V./#D.G.
Grs/SCFD
#/Ft.3
11 W.G.
" W.G.
" W.-G.
" W.G.
11 W.G.
11 W.G.
GPM
GPM
GPM
GPM
GPM
-197-
-------
TABLE 7-6A
Venturi Scrubber Efficiency Test
°F
°F
Draft Loss (inches H20)
Gas Temperature (Dry Bulb)
Gas Temperature (Wet Bulb)
Static Pressure (inches H20)
Density @ Conditions (#/cu.ft.) .
Density @ Standard Conditions (70°F.) 3/cu.ft.
Gas Velocity in Duct (ft/min.)
Gas Flow in Duct @ Conditions A.C.F.M.
Gas Flow in Duct @ Standard Conditions (70°F.) S.C.F.M.
Volume Gas Sampled Cu. Ft.
Sampling Time Minutes
Dust Sample Taken Grams .
Grain Loading @ Conditions (grains/A.C.F.)
Grain Loading @ Standard Conditions (70°F.) grains/S.C.F.
Collector Efficiency % .
Stack Emission (#/min.)
Stack Emission (#/hr.)
Stack Emission (#/day)
Dust Loading (#/1000 # Gas)
Georgia Code in #/hr. emissions based on process input.
6000 #/hr.
Inlet
400
152
23
Outlet
22.6
152
152
.0454
.0695
.275
.0583
.0673
2011
23207
15159
102.
. 24
28,
4,
72
4204
2692
6.5319
14.1524
849.144
20379
13.4146
1528
19208
16640
195
60
0,
0,
0,
99,
0,
4,
114.
0,
3633
0287
0331
49
0788
728
0704
8.56
-198-
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Table 7-7
Information Required For Complete Precipitator Survey
COMPANY
MILL
ADDRESS
PERSON TO CONTACT FOR ANY CLARIFICATION
PHONE NUMBER
Please fill-in or check appropriate blanks:
A. Recovery information:
1. Boiler Manufacturer.
2. Is this a conventional unit with a direct contact evapora-
tor? Yes No
B. Precipitator Design Data:
1
2.
3.
4. Inlet loading to precipitator, grains/AWCF
5.~ Outlet loading from precipitator, grainf/AWCF
6. Guaranteed efficiency @ A.2. gas volume
Manufacturer.
Gas volume, actual wet cubic feet per min.
Gas temperature, °F.
(AWCFM)
7. Gas velocity in precipitator, feet per second (FPS)
Precipitator Construction:
Number of chambers.
Number of fields.
1.
2. .. ..
3. Dry bottom or wet bottom? Wet Dry
4. If dry bottom, is it a hopper bottom or drag bottom?
Hopper Drag
5. Does precipitator have a weatherproof enclosure for trans-
former rectifiers? Yes No
6. Is the precipitator a steel or tile shell? Steel
Tile
7. If steel shell, do you have a shell heating system?
Yes No
8. Is the I. D. fan upstream or downstream from precipitator?
Up Down
D. Collection System:
.1. Total collecting surface area, square feet .
2. Height of collecting platers, feet .
3. Number of collecting rappers .
E. High Voltage Discharge System:
1. Number of transformer rectifiers.
2. Total output capacity of all transformer rectifiers
milliamps.
3. Number of discharge rappers.
Mechanical Hammer
F. Rappers:
1. Manufacturer
2. Type: Electric Pneumatic
G. Inlet and Outlet Boxes:
1. Do you have gas distribution devices, at precipitator in-
let? Yes No At precipitator outlet? Yes
No
2. Type of distribution devices:
Perforated Plate (Inlet, outlet or both)
Adjustable Channesl " " "
Other " " "
None " " "
-3. Do you experience pluggage of these"devices at precipitator
Inlet? Yes Ho
4. Do you have rappers on these devices, if any? Yes
No
Shut-Off Gates:
1. Manufacturer.
2. Type of gate (slide, multivane damper or other)
3. Do your gates perform satisfactorily? Yes No
Precipitator Operating Performance: (Please complete the
following if possible.)
1. Date of precipitator start-up.
2. Initial performance:
Efficiency %
Gas Flow
Inlet Grain Loading
Outlet Grain Loading
Gas Temperature
_AWCFM
_Grains/AMCFM
_Grains/AWCFM
3. Performance approximately one (1) year after start-up.
Efficiency %
Gas Flow AWCFM
Inlet Grain Loading Grains/AWCFM
Outlet Grain Loading Grains/AWCFM
Gas Temperature __^ °F.
4. Performance approximately two (2) years after start-up.
Efficiency %
Gas Flow AWCFM
Inlet Grain Loading Grains/AWCFM
Outlet Grain Loading Grains/AWCFM
Gas Temperature °F
-------
Table 7-7
(cont.)
5. Performance approximately three (3) years after start-up.
Efficiency %
Gas Flow AWCFM
Inlet Grain Loading Grains/AWCFM
Outlet Grain Loading Grains/AWCFM
Gas Temperature "F.
6. Performance approximately four (4) years after start-up.
Efficiency
Gas Flow
Inlet Grain Loading
Outlet Grain Loading
Gas Temperature
5. Please describe any other problem areas:
_AWCFM
_Grains/AWCFM
_Grains/AWCFM
J. Precipitator Testing:
1.
Yes
Were guaranteed performance tests run on the unit?
No
What type of particulate test do you perform for guarantee?
(Check one) Wet impinger
Dry thimble
Dry thimble & glass filter_
Other
Do you routinely test the precipitator? Yes No
If yes, is this weekly, monthly, yearly, or what?
What type of particulate test do you perform for routine
precipitator testing, if any? (Check one)
Wet -impinger
Dry thimble
Dry thimble & glass filter
Other
K. Precipitator Operation: (Please give your opinion)
1. uo vou have internal corrosion in your precipitator?
Y^s" iic.
2. .lava yuu experienced any corrosion or deterioration of your
shall that you consider serious? Yes No If
yes, please explain.
3. If a dry bottom unit, do you have any plugging problems in
conveying salt cake to salt cake mix tank? Yes No
4. Do you consider any of the following to be problem areas?
(Yes or NO) Wire breakage
Collecting plate
ment
discharge electrode align-
Salt Cake Buildup
If yes, explain where buildup occurs
Wet bottom agitators
Dry bottom hopper
Rapper controls
Transformer-recti fiers
Transformer-rectifier controls
Shut-off gates
Rappers
Is precipitator showing a problem on your unit? Yes
No
Was your precipitator adequately sized for your require-
ments? Yes ___ No If no, is this because your
boiler is operated above rated capacity? Yes No
Miscellaneous:
Did the precipitator vendor erect your unit? Yes
No
Do you believe the precipitator vendor should erect the
precipitator? Yes No Makes no difference
Did you obtain a satisfactory erection job? Yes No
4. Did the precipitator vendor stand behind his unit? Yes
No
5. Approximately how many hours per year is one or more of your
precipitator chambers out of service because of the pre-
cipitator? hours
6. Approximately what have you averaged spending per year for
precipitator maintenance? $
7. Precipitator Location: (Check one)
On Bui Idi.ng .Roof On Ground. In Between
8. Approximate stack height: Above Precipitator Roof Feet
Above Grade Feet
Comments:
1. What suggestions would you offer to improve precipitator ocr-
formance and reliability?
2. Other Comments:
-------
CHAPTER 8
SITE INSPECTION-ENFORCEMENT PROCEDURE
8.1 GENERAL
The data to be collected and the procedures to be followed are determined
by the purpose of the inspection and whether this is an initial or a
subsequent, inspection.
Specific procedures, especially the citation of violations will depend
on the control program and policies of the air pollution agency involved.
In general, the inspector performs the following functions: (1) Reporting
or verifying progress made by the mill in meeting compl iance plan schedules.
.(2) Verifying any changes from the conditions of the permit. (3) Assuring
that day--to-day operation and maintenance practices serve to minimize pol-
lution wherever possible. (4) Reporting breakdown, shut down, or bypassing
of process equipment and changes in pulp production schedules or cooking
and chemical recovery procedures.! (5) Citing violations that are clearly
flagrant in nature. (6) Correlating public complaints with air quality
measurements, emission rates, and control practices. (7) Noting any new
sources, new or modified control devices, and changes in pollution control
personnel since the last visit. (8) Reviewing the emission data collected
by the plant staff and comparing it with reports submitted to the control
agency.
In all cases certain general procedures should be followed including: (1)
off-site observations, (2) interview with plant personnel, (3) in-plant
observations, (4) critique and meeting with plant management, and (5) ob-
serving safety precautions.
8.2 OFF-SITE OBSERVATIONS ' ;
Before entering the plant, the inspector should observe and note any
visible emissions and odors, dustfall in the surrounding area, possible
vegetation damage, and/or effects on materials and paint. These findings
should be correlated with public reactions in neighboring communities and
with specific mill operations wherever possible.. ,
The inspector should note wherever possible wind directions, wind speed,
humidity, and atmospheric stability in his observations, and he should
develop a systematic procedure for patrolling and noting the location,
quality and intensity of the odor. Odors may be noted on an^intensity
scale or a scentometer may be used. The inspector may organize a com-
munity odor panel consisting of carefully selected citizens who may
help to establish the significance of day-to-day ground level variations
within the normal odor intensity range. An expert odor panel working under
controlled, closed room conditions may also help to isolate the effect of
possible changes in mill operations (1). .
Sulfur compounds (particularly FLS) can discolor and damage lead based paints
and.paints containing mercury-based fungicides and may accelerate tarnishing
of silver and copper. Discoloration usually takes the form of browning or
blackening of materials and is enhanced if the surfaces have been moistened
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or weathered, depending on the lead content. Fungi will also darken un-
protected paints, so the inspector should, in doubtful cases, have the
cause of damage verified by experts. Vegetation damage also may result
from sulfur dioxide emitted from boiler equipment, as well as from hydrogen
.sulfide.
Deposits of particulates may result from the fallout of saltcake particles,
fly ash, soot, burned particles, and lime, which are usually confined to
within 1/4 to 2 miles from the plant. Some of the.fallout is of a compara-
tively caustic nature and can damage vegetation, materials and painted sur-
faces particularly on vehicles located near the plant. Particles may be
sampled and taken to the laboratory for analysis to help identify the process
sources responsible. Particles high in calcium are generally emitted from
the lime kiln; particles high in sodium may originate from the recovery
furnace and smelt tank; charred or insoluble material originates from hog
burning equipment. Sodium salts, which tend to be more volatile than other
particulates, may be due to flocculation of particles in the electrostatic
precipitator. These consist of fluffy aggregates up to 1 millimeter in
diameter (5). The largerraggregates are largely responsible for damage to
paint and vegetation.
All observations should be carefully documented including the date and time,
and weather conditions (especially wind speed and direction). In reading
visible emissions, the recommended procedures regarding location of observer
in relation to plume and sun must be followed.
Points of emissions in a pulp mill include stacks and vents at roof level,
the recesses between plant structures, inside the structures, and ground
level. The obvious emissions are the stream plumes and mists, particularly
from the recovery furnace. These can be voluminous and may travel over
great distances and altitudes.. The steam and vapor emissions make it dif-
ficult to isolate and read the dry portions of the plumes. Long plumes, how-
ever, are always suspect in view of the relatively rapid dissipation of steam.
Reading visible emissions,.while.taking into account the dry and liquid
contaminants is tenable, but the practice will depend on the policy of the
air pollution control agency involved.
The inspector should become familiar with the.quality and intensities of
odors and the visual character of mill emissions as they vary through the
day and with weather conditions. Where abnormal conditions occur, source
testing should be requested. Where a. large number of public complaints
are reported, the possibility of odors being released with intermittent
emissions from starting up and shutting down of equipment, opening of
digesters, condenser or heat recovery system failure, and overloading of
recovery furnace, should be investigated. Vents on bleaching and other mill
operations, also, should not be ignored, .as chlorine gas can be accidentally
released. , .
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8.3 INTERVIEW WITH PLANT PERSONNEL
Direct discussion with both management and operating personnel will
enable, the enforcement officer to gain a complete understanding of the
plant schedule and operation, and allow plant personnel to understand
the officer's purpose for this inspection. If a violation has already
occurred, the responsible persons should be notified and the reason
for the violation determined.
Establishing a good rapport with the plant staff will be most important
in obtaining process operating information, descriptions of start-up
conditions, number and extent of upsets, equipment breakdown problems,
and related information.
.In pulp mills, the interview is usually conducted with staff that has
been permanently assigned to the air pollution .problems of the plant.
The interview should be primarily directed at ascertaining factors which
affect process emissions, control progress and changes in operating
procedures which cannot be noted during the physical inspection. Record-
ing charts, logs and charts relating to process performance and emissions
can be examined in the office with the air pollution staff of the mill.
The inspector should clearly establish normal operating procedures in
order to be able to identify abnormal or changing conditions. A pro-
cedure should be adopted and practiced by which the mill reports upsets
and outages to the enforcement agency. Scheduled maintenance, especially
if it involves taking control equipment out of operation,should be an-
nounced in advance.
The exact nature of an interview will vary with the purpose of the visit..
For an initial visit an extensive review of the process equipment, layout,
and operating schedule will be required. Subsequent visits will require
less time and consist only of a review to make sure nothing has changed,
or if changes have occurred, to check their effect on emissions.
Information obtained by interview should include: .
1. Type of Wood Used: resinous content, softwood, hardwood
extent of use of saw dust, etc. The methyl group con-
tained in certain sulfur.based emissions derive from the
methoxyl content of woods. Hardwoods contain more methoxyls
than softwoods, arid have greater process odor potential (7).
2. Cooking Variables.. Malodorous emissions can be increased
when cooks are shortened and pressure and temperatures are
increased.
3. Pulping Capacity of the mill should be determined in terms
of tons of unbleached air dried pulp. The inspector
should become familiar with variations in pulping capacity.
Substantial increases in pulping output may cause the capaci-
ties of existing process equipment to be exceeded, including
overloading of the recovery furnace, and excessive emissions
from power plant and other supporting equipment.
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4. Plant program for monitoring emissions. The inspector
should be familiar with the mill schedule for conducting
source testing and maintenance of continuous monitors,
such as the titrator, discussed later. It is extremely
important that the equipment be kept in good working
order. The inspector should clearly establish any
modifications made by the plant to the instrumentation,
and. should assure that a formal procedure for maintaining
the accuracy of continuous monitors be adopted.
5. Emission inventories, negotiated compliance plans and
permits. The inspector should have available with him
information concerning the equipment and emission inven-
tories of the plant to determine if significant changes
have been made in any of the major sources that will
affect emissions and odors. It is desirable to compare
operating permit, inventories, process flow diagrams, and
plot plans on file with existing conditions. This can
be accomplished either before or after the tour through
the plant. The liquor flow, the exact points of-emission
and firing rates to the recovery furnace, the lime kiln
and the power boilers and hog burners, in particular,
should be noted.
6. Frequency of conditions under which excessive emissions
might occur, such as start-up, soot blowing, ash removal,
peak loads, etc. should,be discussed and noted.
Interviews also present a good opportunity to discuss future
plans for changes in the operation such as expansions and/or
major modifications. The personnel interview is also an excellent
time to review emergency action plans and discuss the feasibility
and time schedule required to implement the plan.
8.4 IN-PLANT OBSERVATIONS
i
The field enforcement officer upon entering the plant property should
ask for the plant manager and state his name, affiliation, and purpose.
In most cases, his visit will have been prearranged and he will have a
firm appointment. However, on occasion, a random visit or a visit based
. on an off-site observation will be made. During such times, the plant
manager may not be available and a. lower ranking member of management or
technical staff should be requested.
While inside the plant boundary, the inspector should again note the
general "housekeeping" practices within the plant; i.e., is the plant
equipment well maintained, are the roadways dust covered or littered,
is equipment kept clean and painted. These general observations will
give the inspector a feel for the care taken in preventing excessive
atmospheric emissions.
i '
Additional .visible emissions and/or odor observations can be made on-
site, especially if vents were not visible from the plant boundary.
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8.4.1 EQUIPMENT INSPECTION
The field enforcement officer should visibly inspect the process
equipment, any control devices, and all 'appurtenances. Nameplate
data and ratings should be checked and the degree of maintenance
and operability of all equipment noted. Operational data such as
temperatures, pressure drop, steam rates, auxiliary fuel use, etc.
should be noted where available. He should be especially cognizant
of unusual operating conditions such as excessively high or low load.
A visual inspection will also .indicate the operability and care given
to the instruments and control systems used to monitor and operate the
process.
8.4.2 DETERMINATION OF OPERATING CONDITIONS
An assessment of the operating conditions, and their related effect
on emissions, can be performed in a,comprehensive manner provided
adequate preliminary evaluations have been performed, i.e., following
the pre-survey review of the mill, review of state permit applications,
and review of the process and control equipment specifications. If
these reviews are performed comprehensively in the proper sequence,
it is conceivable that in the case of well maintained mills with good
operating records, and with the assistance of proper data, a site
evaluation could be performed, requiring very little time in the process
areas.1 The source of this information and operating data is the daily .
operating statistical report and the operating log sheets from the
major process areas.
Figures 8-1 through 8-7 contain plots of some of the more pertinent .
measured variables and their relation to emission levels associated with the
-kraft recovery furnace operation. Similar relationships for other kraft
processes, and other pulping processes, may be found in Chapters 2 and
3, or may be derived from the discussions contained in these same
chapters. Tables 8-1 and 8-2 have been prepared to assist field enforce-
ment personnel in conducting their surveillance of operating variables
and conditions relating to atmospheric emissions.
8.4.3 .ASSESSING CONTROL EQUIPMENT OPERATING CONDITIONS
Pertinent control equipment operating variables are also monitored
with in-line instrumentation which are alsp^shown on process flow-
sheet instrumentation schematics as previously discussed. The impor-
tant variable values are also recorded on the appropriate process
operating log sheets.
The optimum level of these variables, as required to maintain maxi-
mum performances, are normally stipulated in the formal design
specifications. Principles of operation for all types of control
devices have been discussed ^'n Chapter 4 and effects of operating
variables have been discussed in Chapters 2 and 3.
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8.4.4 ESTIMATING EMISSION RATES
The appendix contains a series of graphs developed by NCASI, which relate
atmospheric emissions from various sources in the kraft pulping process,
to pulp production rates. The basis used to derive these relationships
is clear-ly stated, and correction factors must be applied to these data
if the basic process basis differs appreciably from that stated. This
data can be used effectively to'determine nominal levels, to compare
operating data and estimate emission levels where data is not available.
8.4.5. SOURCE SAMPLING RESULTS CONDUCTED BY MILLS
Many pulp mills routinely conduct source sampling tests periodically
to determine the performance level of their control equipment. The.
results of these tests are usually the subject of an informal report
for inter-company use. These reports are kept on file in the technical
department and should be available for review by enforcement officials.
This is another source of information which may be used to assist
enforcement officials in developing their site inspection plan.
8.5 CRITIQUE AND MEETING WITH PLANT;MANAGEMENT
An effective air pollution control program cannot be conducted at
a pulp mill unless it has the whole-hearted endorsement of management.
It is important therefore that the inspector discuss all of his major
findings both good and bad with; top mill management. At the least, this
should include the mill manager and the technical director. Those
personnel who accompany this inspector on his inspection should also be
present. It is especially important to inform the top management of the
mill of any adverse findings as well as the need for new equipment or
new programs. It is also important to inform them of especially effective
work done by the mill employees..
8.6 SAFETY PRECAUTIONS
The field enforcement officer must obey all safety precautions during
his visits, whether.or not they are required by the plant. As a rule
the enforcement officer should not sign accident waivers when entering
a plant. He should, however, have his own personal safety equipment
such as a hard hat, eye and ear protection, and safety shoes, etc.,
as detailed previously. On a first visit to a plant, he should review
all safety rules with plant personnel. The inspector should never
tour the plant without an escort, nor should he open furnace doors,
manipulate valves or controls, or in any way try to change the operating
characteristics of the unit. When observing the combustion chamber
interior, he should always have the operator open the viewing ports,
and always use proper eye and face shields.
Due to a large number of fans,'belt and chain drives, electrical
motors, etc., the inspector should be especially aware of the hazards
associated with this type of equipment.
. -206-
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The inspector should not climb on ladders, scaffolds, etc. unless
they meet the appropriate safety standards.
As a general rule, the inspector should not take any action which he
feels could in the slighest way>cause a safety hazard to himself or
other plant personnel.
8.7 SPECIAL PROCEDURES FOR KRAFT MILLS . .
The physical inspection should be organized around two closed systems:
(1) The black liquor system including the digesters, relief valves,
accumulators* oxidation towers (if used), multiple effect evaporators,
direct contact evaporator., recovery furance, and smelt tank. (2) The
lime recycling system including the mud filter, lime kiln, and slaker.
The bleaching plant should also be inspected, particularly the brown
stock washer, where water vapor and malodorous compounds may be collected
by a hood and discharged to the atmosphere.
The principal odors from a kraft mill are characteristic in terms of
their quality and pervasiveness. Tables 8-2 and 8-3 illustrate odor.
thresholds, and qualities of sulfur compounds.(1). Other odors such
; as terpenes and methanol differ significantly from hydrogen sulfide
and mercaptans, and are easily recognized. These have the odor of
turpentine or are "medicinal" ,in character, and tend to be localized
around process equipment, particularly stock washers, and oxidation
towers. The sulfides and mercaptans, by contrast,can be detected many .
miles from the plant.
8.7.1 BLACK LIQUOR SYSTEM
Although the recovery furnace is the most significant unit from the
viewpoint of the emission of particulate matter and TRS compounds,
the inspector should visit all the other units previously described
for this system. The inspector should become familiar with the vents,
valves and. condensers, the type of. condensers, and associated odors
and emissions. The design., condition, and operation of all other units
should be noted as these have a bearing on the release of odors.from
the furnace. Spills, indications of poor maintenance, and low level
.odors should be noted at each unit. It is necessary also to deter-
mine what treatment is applied to malodorous gas streams.
1
Not only is the recovery furnace the most important source of emissions
in the black liquor system, but the operator of the furnace maintains .
a daily operating log which contains information from other parts of
the system.
The inspector should become familiar with the specific design and oper-
ation of the recovery furnace in the plant to which he is assigned.
Many factors enter into achieving optimum combustion in the furnace
from an air pollution standpoint, including the firing rate, secondary
air flow, percent excess oxygen in the flue gas, black liquor spray
droplet .size, time, and turbulence. In the recovery furnace control
-207-
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room, the following items (usually entered hourly) should be checked on
the operator's log sheet or by instrument readings: (1) Sulfidity of
white liquor (in some mills the sulfidity of the green liquor is mea-
sured), (2) percent solids of the black liquor, (3) sodium sulfide
content of black liquor in and out of the black liquor oxidation unit
(if one is used), (4) excess oxygen in flue gas, (5) rate of excess
air and secondary air, (6) black liquor firing rate, (7) combustibles,
(8) TRS emissions (Barton), (9). frequency and time of soot blowing, (10)
steam production, and (11) voltage and amperage readings of the
electrostatic precipitator.
All of the above items should be correlated with recovery furnace
emissions, so that overloads can clearly be recognized from the
performance data.
It should be noted that increased sulfidity usually results in higher
TRS emissions. Also it is important to note the relationship of
TRS, combustibles, and excess oxygen. When TRS increases, combustibles
also show an increase, but excess oxygen will show a decrease, and
vice versa. Excess oxygen readings below about 3% may indicate furnace
overloading, and/or may indicate an increase in TRS emissions.
The inspector should check to see whether the air ports in the recovery
furnace are regularly rodded to prevent plugging and that excess oxygen
and proper operating temperature (usually around 1100°F) are main-
tained in the upper oxidation zone of the furnace. A decrease in
secondary air for any reason can cause an increase in emissions. The
inspector should recognize,however, that steps may have to be taken
on occasion to prevent dangerous situations such as furnace reaction
explosion..
8.7.2 LIME RECYCLING SYSTEM
The lime recycling system may be a source of both particulate and
odorous emissions. The makeup rate for salt cake (pounds of salt
cake added per ton of pulp produced) indicates how much of the sodium
and salt cake is being lost to the atmosphere and to wastewater reuse
or treatment. A loss of 100 pounds of salt cake per ton.of pulp may
be reasonable, but the proper figure must be established with each
plant. This information generally will be found in the recovery furnace
log sheet. Hydrogen sulfide may be emitted from, a kiln stack, particu-
larly in long kilns, due to reactions of carbon dioxide with sodium
sulfide in the carbonate sludge, and to the introduction of carbonate
at the cooler end of the kiln. In some plants non-condensible gases are
carried to the lime kiln for incineration.
i
Operational data for this system usually will be found in the operators
log at the lime kiln control room. The following lime kiln operational
data should be obtained from the log sheet: (1) sodium and sulfide con-
tent of washed cake (usually taken once per day or per shift), (2)
scrubber water flow rate and source (usually taken once an hour),(3)
scrubber pressure drop by manometer, and (4) TRS emissions (Barton).
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It should be noted that large variations occur in emissions from different
mills using the same process, making the same products, from the same
wood species.. Even in the same mill large variations may occur from
hour to hour.
8.8 SPECIAL PROCEDURES FOR SULFITE MILLS
The sulfite segment of the pulp'industry, unfortunately, cannot be as
readily categorized as the kraft segment. There are four chemical bases
for cooking liquor, four pH ranges for the liquor, and about twenty
schemes for recovery of the cooking liquor. Thus the number of
,possible combinations far exceeds the 33 sulfite mills reportedly
operating in the United States i,n 1973.
Whillte (JJL) has published a comprehensive review of sulfite and bisulfite.
recovery systems. He concluded that the only proven technology for
recovery of both base and process sulfur is for magnesium base sulfite
liquors.
Generally speaking, the principal emissions are sulfur dioxide and
particulate matter and the major, source of S02 is the blow tank of an
uncontrolled mill. In most sulfite mills the SO,, emissions from the
various sources are recirculated several times tnrough the absorbers to
boost the sulfite content of the cooking liquor.
i
For these reasons, an inspection; itinerary which would be applicable-to
even a majority of the mills is virtually impossible. Since there are
no more than 9 or 10 sulfite mills in any state, it is recommended that the
FEO work closely with the technical 'Staff of each mill to establish an
inspection protocol.
8.9 SPECIAL PROCEDURES FOR NSSC MILLS
' I ' .
As in the case of sulfite mills, there is considerable difference among
NSSC mills- In fact, many of the existing mills were converted to
NSSC from other types of pulping. Thus, categorization of this segment
of the chemical pulp industry is virtually impossible.
Emissions to the atmosphere from NSSC pulping depend largely on the
method of handling the spent cooking liquor. Nearly half of the NSSC
mills is the U. S. sewer the liquor and treat it as a liquid waste.
Nearly as many mills attempt recovery of heat and chemicals by intro-
ducing the spent liquor into a smelter or recovery furnace (either its
own or that of a neighboring kraft mill). Since the spent NSSC liquor has
a lower pH than the spent kraft liquor, a mixture of the two results in
an increase in emissions of S02 and I^S from the kraft recovery furnace.
. According to Galeano (1_3_), the principal emissions are particulate matter,
S0_ and H^S. The major sources appear to be the recovery furnace or
smelter, the sulfiting tower, and the blow tank.
Because of the lack of data on emissions from the NSSC process, especially
on the variation between mills, an inspection itinerary which would be .
" -209-
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applicable to even the majority of the U.S. mills is impossible. Since
there are no more than 4 mills using the NSSC process in any state
(roughly 40 in the U. S. in 1973), it is recommended that the FEO
work closely with the technical staff at each mill to establish an
inspection protocol.
-210-
-------
TABLE 8-1
KRAFT PULP MILL
PERTINENT NOMENCLATURE AND TYPICAL VALUES
Active alkali on ovendry wood, Ib/lb 0.167
Active alkali concentration, lb/ft3 7.0
Effective alkali on ovendry wood, Ib/lb 0.14
Normalizing value of effective alkali, Ib/lb 0.15
Capacity of digesters (a.d. tons), tons/batch 14.5
Specific heat of wood solids, Btu/(lb)(°F) 0.33
Caustic conversion efficiency, nondimen 0.78
Evaporator steam economy, nondimen 4.66
Solids fraction in liquor to furnace, nondimen 0.545
Solids fraction in liquor to evaporator,
,. nondimen 0._!88
Blow flow rate, Ib/ton a.d. pulp 1516.'"
Water in liquor from evaporator, Ib/ton a.d.
pulp : 2519.
Evaporation rate from evaporator, Ib/ton a.d.
pulp . 10,549.
Water in .liquor to evaporator, Ib/ton a.d.
pulp. ' 13,069.
Heat capacity of digester contents, (Btu/°F)/
ton a.d. pulp 11,845
Heat of combustion of lignin, Btu/lb 8100.
Heat of fusion of smelt, Btu/lb . 530.
Heat produced in recovery process, Btu/ton a.d.
pulp 7.96 x 106
Total sulfur loss fraction, nondimen 0.337
Total sodium loss fraction, nondimen 0.0931
Number of digesters, nondimen 5.
Production rate, tons a.d. pul-p/day 500.
Furnace reduction ratio, nondimen 0.976
Liquor-to-wood ratio, Ib/lb 2.8
Sulfidity of cooking liquor ' 0.323
"Effective" sulfide in cooking liquor, Ib/lb wood ,0539
Solids in smelt, Ib/ton a.d. pulp 1208.
Black liquor solids in liquor to evap., Ib/tons
a.d. pulp . 3018.
Usage of kiln fuel, MBtu/ton NaOH 8.84
Usage of makeup lime, Ib/lb NaOH 0.0276
Volume of black liquor to digesters, ft3/ton
a.d. pulp 13.6 .
Volume of total cooking liquor, ft3/ton a.d.
pulp . . . 104.
Volume of white liquor, ft3/ton a.d. pulp 90.4
Weight of wood fiber, Ib/ton a.d. pulp 1800.
Weight of wood lignin, Ib/ton a.d. pulp . 1989
-211-
-------
KRAFT PULP MILL
(Continued)
Moisture content of wood (fraction), Ib/lb
Weight of wood solids, Ib/ton a.d. pulp
Total weight of wet wood, Ib/ton a.d. pulp
Weight of wood water, Ib/ton a.d. pulp
Weight of caustic (as Na20) in cook, liquor
Ib/ton a.d. pulp
Weight of sulfide (as Na20) in cook liquor,
Ib/ton a.d. pulp
Weight of sulfate (as Na20) in cook liquor,
Ib/ton a.d. pulp
Weight of carbonate (as Na20) in cook liquor
Ib/ton a.d. pulp '
Weight of makeup salt cake (as Na20), Ib/ton
a.d. pulp :
Weight of makeup carbonate (as Na20), Ib/ton
a.d. pulp
Pulping yield, nondimen
0.52
3789.
7895.
4105.
428.4
204.4
5.0
120.8
70.58
0.06
0.475
-212-
-------
Table 8-2
Pertinent Operating Parameters Which May Affect
Atmospheric Emissions from Kraft Pulping Operation
(To Be Used By FEO In Developing Site Inspection
Route And Data Collection Format)
Digester Room Log (Pulp Production)
Chip weight charged, tons
White liquor charged, gallons
Actual cooking time, minutes
Number of cooks blown per 24 hour period,
Hard Stock
Soft Stock
White liquor sulfidity, %
Recovery Boiler Log
Steam flow, 1000 Ibs per hour
Gas temperature boiler outlet, °F
I.D. fan setting
I.D. fan speed
Total air flow, CFM or pounds per hour
Primary air flow, CFM or pounds per hour
Secondary air flow, CFM or pounds per hour
Tertiary air flow, CFM or pounds per hour
I.D. fan inlet draft, inches water gage
Precipitator inlet draft, inches water gage
Dissolver tank vent draft, inches water gage
Demister Pads
Black liquor flow to burners, Ibs per hour
Black liquor pressure, Ibs per square inch
Black liquor solids, percent
Fuel oil flow, gallons per minute
Saltcake feed, pounds per hour
Combustibles, percent
Excess air, percent
Precipitator Operation
Inlet, volts, amperes, milliamps and spark
Outlet, volts, amperes, milliamps and spark
-213-
-------
Table 8-2 (Continued)
Lime Kiln Operating Log
Feed mud solids, percent
Feed flow, GPM
Mass flow, tons per day
Oil pressure, psi
Cold end temperature, °F
Hot end temperature, °F
I.D. fan inlet vacuum, inches water gage
Scrubber inlet pressure, inches water gage
Scrubber outlet pressure, inches water gage
Fresh lime rate, tons per'day
Barker Boiler Operation
Steam from oil, Ibs per hour
Steam from bark, Ibs per hour
Oil flow, gallons per minute
Screw feeder speed, RPM
Bin level, feet
C02> percent
Over fire air pressure, inches water gage
I.D. fan draft at inlet, inches water gage
Gas temperature leaving boiler, °F
Power Boiler Operation
Fuel oil supply pressure, psi
Under-grate air flow, ration (or oven-fire)
Oil burner windbox pressure, inches water gage
I.D. fan inlet draft, inches water gage
C02» percent
Gas temperature leaving boiler, °F
-214-
-------
TABLE 8-3
ODOR THRESHOLDS OF KRAFT MILL GASEOUS
SULFUR COMPOUNDS IN AIR
. ppm. (by volume)
Sulfur Dioxide 1.0-5.0
Hydrogen Sulfide 0.009-0.0085
Methyl Mercaptan 0.0021-0.040
Dimethyl Sulfide, 0.0001-0..0036
TABLE 8-4
CHARACTERISTICS OF KRAFT MILL GASEOUS
SULFUR COMPOUNDS '
Compound
Characteristic
Odor
Explosive Limits (air)
Color Lower Upper
Sulfur Dioxide
Hydrogen Sulfide
Methyl Mercaptan
Dimethyl Sulfide
strong, suffocating none
rotten eggs none
rotten cabbage none
vegetable sulfide none
Not explosive
4.3% 46%
3.9% 22%
2.2% 9%
-215-
-------
to
(O
Q.
Q.
C
O
800
600
400
200
s-
o
CVJ
S o
Figure 8-1
EFFECT OF CHLORINE IN BLACK LIQUOR
o
c/o
E
7
-------
Figure 8-2
EFFECT OF BLACK LIQUOR HEATING VALUE
2000
1600
to
to
_a
b
a
a.
a.
c
o
to
to
1200
800
CVJ
o
oo
400
0
5200
5600 6000 6400
Heating Value of Solids to Furnace, Btu/lb
6800
-217-
-------
o
GO
fO
en
OJ
o
o
CL)
12
10
8
6
4
2
0
Figure 8-3
EFFECT OF TOTAL AIR SUPPLIED TO THE UNIT
CO
fO
-d
1500
.1000
CL
Q.
I 500
CM
O
100
80
60
40
20
0
OJ
ro
Tl
?
CD
DJ
4 o
ro
cr
CO
_i.
CO
oo
in
50
45
40
35
30
100 110 120 130 140
Total Air to Unit, percent of theoretical
2000
1900
1800
03
n>
Q.
-3
TJ
ft)
01
fD
150
1700
-218-
-------
Figure 8-4
EFFECT OF PRIMARY AIR
o
1/1
Q
to
c
fO
o>
o
(O
o
QJ
14
12
10
8
6
1500
to
ItJ
o 1000
Q.
Q.
I 500
(/)
to
OsJ
O
100
80 ^
60 ^
=3'
40 ".c1
fD
20 ^
0
45
40
oo
£ 35
oo
30
35
40 45 50 55
Primary Air, percent of total
2000
03
CD
ft>.
CU
1800
60
1600
-219-
-------
Figure 8-5
EFFECT OF PRIMARY AIR .TEMPERATURE
600
l/l
fO
Q.
Q.
c:
o
1/5
l/l
CM
O
00
500
400
300
200
100
I
fa
O
n>
10
3
O
oo
O
100
200
300
400
500
Primary Air Temperature, °F
-220-
-------
03
8000
6000
£ 4000
o
C '
-J
CD
o
o>
CO
-s
a>
0
oo
o
OO
OJ
TOO
80
60
40
20
I
0.2
0.4
0.6
0.3
i.o-
1.2
Molar Ratio S/Na2 in Black Liquor
-221-
1.4
-------
Figure 8-7
Effect of. Black Liquor Solids Concentration
1/5
fO
600
500
400
Q.
Q.
° 300
1/5
-------
8.5 REFERENCES
1. Hendrickson, E.R., J.E. Roberson, and J.B. Koogler. Control of
Atmospheric Emissions in the Wood Pulping Industry, Vol. 1-3,
Final Report. Contract No. CPA 22-69-18. March 15, 1970.
2. A Report to the Washington Air Pollution Control Board Prepared
in Conjunction with Rules and Regulations for Kraft Pulp Mills
in Washington Office of Air Quality Control. Washington State
Department of Health, Seattle, Washington. May, 1969. [Figure
7.2.1 based on Figure 1 of reference 4, p. 42.]
3. Hough, G.W. and L. V. Gross. Air Emission Control in a Modern
Pulp and Paper Mill. Portland, Oregon, Fall Pacific Coast
Division Meeting. Paper Industry Management Association.
December 5-7, 1968.
4. Douglass, I.B. The Chemistry of Pollutant Formation in Kraft
Pulping. In: Proceedings of the International Conference on
Atmospheric Emissions foom Sulfate Pulping, E.R. Hendrickson
(ed.). PHS, National Council for Stream Improvement. University
. of Florida, April 28, 1966.
5. Kenline, P.A., and J.M. Hales. Air Pollution and the Kraft
Pulping Industry, and Annotated Bibliography. Environmental
Health Series. DHEW, PHS, November 1963.
6. Knudson, J.C. Air Pollution Controls to Meet Washington State
Kraft Mill Standards. Department of Ecology, Redmond, State
of Washington. Spokane, Washington, International Section,
Air Pollution Control Association, November 16-18, 1970.
7. Major, W.D. Variations in Pulping Practices which may Affect
Emissions. In: Proceedings of the International Conference
on Atmospheric Emissions from Sulfate Pulping, E.R. Hendrickson
(ed.). PHS, National Council for Stream Inprovement. Univer-
sity of Florida. April 28, 1966.
8. Pulp and Paper Titrator, Model 400,installation and Operation
Manual. Barton ITT, Process Instruments and Controls. Monterey
Park, California.
9. Barton Titrators, Training Manual, Model 400, Installation and
Operation Manual. Barton ITT, Process Instrument and Controls.
Monterey Park, California.
10. Knudson, J.C. Air Pollution Controls to Meet Washington State
Kraft Mill Standards. Department of Ecology, Redmond, State of
Washington, International Section, Air Pollution Control Associa-
tion, November 16-18, 1970.
-223-
-------
11. Ayer, C. A Report on the Kraft Pulping Industry's Progress in
Complying with the Emission Regulations of April, 1969. Oregon
Department of Environmental Quality (unpublished).
12. Hhillte, D.'J., "Sulfite and Bisulfite Pulp Mill Recovery Systems,"
TAPPI, 54., pp. 1074-1088, 1971.
13. Galeano, S.F., and Dillard, B.M., "Process Modifications for Air
Pollution Control in Neutral Sulfite Semi-Chemical Mills,"
Journ. APCA, 22. 195, 1972.
-224-
-------
APPENDIX
This appendix contains a series of graphical relationships,
tables, and standard calculations which should be useful in assisting
the FED to estimate emission quantities. Insofar as process opera-
tions are concerned, the bulk of th.ese data pertain to kraft pulping
operations. Further in this regard, these data may only be used for
estimating emission levels relative to the basis indicated.
-225-
-------
APPENDIX
LIST OF TABLES
TABLE 1. FLUE GAS PARTICULATE CONCENTRATION AND EMISSION RATE
RELATIONSHIPS FOR KRAFT RECOVERY FURNACES
TABLE 2. GAS CONCENTRATION AND SULFUR EMISSION RATE RELATION-
SHIPS FOR KRAFT RECOVERY FURNACES
TABLE 3. FLUE GAS PARTICULATE CONCENTRATION AND EMISSION. RATE
RELATIONSHIPS FOR LIME KILNS
TABLE 4. FLUE GAS SULFUR CONCENTRATION AND SULFUR EMISSION RATE
RELATIONSHIPS FOR LIME KILNS
TABLE 5. VENT GAS PARTICULATE CONCENTRATION AND EMISSION RATE
RELATIONSHIPS FOR SMELT DISSOLVING TANKS
TABLE 6. VENT GAS SULFUR CONCENTRATION AND EMISSION RATE RELA-
TIONSHIPS FOR SMELT DISSOLVING TANKS
TABLE 7. SULFUR DIOXIDE CONCENTRATIONS IN POWER BOILER FLUE GAS
AS A. FUNCTION OF SULFUR CONTENT OF FUEL
TABLE 8. CONVERSION OF DUST LOADING IN BOILER FLUE GAS
LIST OF FIGURES
FIGURE 1. PROCESS WEIGHT AND EQUIVALENT PULP PRODUCTION RELATION-
SHIPS FOR KRAFT RECOVERY FURNACES
FIGURE 2. PROCESS WEIGHT AND EQUIVALENT PULP PRODUCTION RELATION-
SHIPS FOR LIME KILNS
FIGURE 3. PROCESS WEIGHT AND EQUIVALENT PULP PRODUCTION RELATION-
SHIPS FOR SMELT TANKS
FIGURE 4. PARTICULATE EMISSION RATE PER UNIT OF PULP PRODUCTION
AS A FUNCTION OF CONCENTRATION IN KRAFT RECOVERY FURNACE
FLUE GAS
-226-
-------
LIST OF FIGURES
(continued)
FIGURE 5. EMISSION RATE IN UNIT TIME AS A FUNCTION OF CONCENTRA-
TION IN KRAFT RECOVERY FURNACE FLUE GAS
FIGURE 6. PARTICULATE EMISSION RATES AS A FUNCTION OF EQUIVALENT
PULP PRODUCTION AND CONCENTRATION FOR KRAFT RECOVERY
FURNACES
.FIGURE 7. GAS CONCENTRATION AND SULFUR EMISSION RATE RELATIONSHIPS
FOR KRAFT RECOVERY FURNACES
FIGURE 8. PARTICULATE EMISSION RATE PER UNIT OF PULP PRODUCTION AS
A FUNCTION OF CONCENTRATION IN LIME KILN FLUE GAS
FIGURE 9. PARTICULATE EMISSION RATE IN UNIT TIME AS A FUNCTION OF
CONCENTRATION IN LIME KILN FLUE GAS
FIGURE 10. PARTICULATE EMISSION RATE AS A FUNCTION OF EQUIVALENT
PULP PRODUCTION AND CONCENTRATION FOR LIME KILNS
FIGURE 11. FLUE GAS CONCENTRATION AND SULFUR EMISSION RATE RELATION-
SHIPS FOR LIME KILNS
FIGURE 12. PARTICULATE EMISSION RATE PER UNIT OF PULP PRODUCTION AS
A FUNCTION OF CONCENTRATION IN SMELT DISSOLVING TANK
.VENT GAS
FIGURE 13. PARTICULATE EMISSION RATE IN UNIT TIME AS A FUNCTION OF
CONCENTRATION IN SMELT DISSOLVING TANK VENT GAS
FIGURE 14. PARTICULATE EMISSION RATE AS A FUNCTION OF EQUIVALENT
PULP PRODUCTION AND CONCENTRATION FOR SMELT DISSOLVING
TANK VENTS
FIGURE 15. VENT GAS SULFUR CONCENTRATION AND EMISSION RATE RELATION-
SHIPS FOR SMELT DISSOLVING TANKS
FIGURE 16. PARTICULATE COLLECTION EFFICIENCY AND EMISSION RATE
RELATIONSHIPS FOR KRAFT RECOVERY .FURNACES
FIGURE "17. PARTICULATE COLLECTION EFFICIENCY AND EMISSION -RATE
RELATIONSHIPS FOR LIME KILNS
FIGURE 18. PARTICULATE COLLECTION EFFICIENCY AND EMISSION RATE
RELATIONSHIPS FOR SMELT DISSOLVING TANKS
FIGURE 19. SPECIFIC GRAVITY OF SPENT LIQUORS
FIGURE 20. CHEMICAL BALANCE IN KRAFT RECOVERY CYCLE
-227-
-------
TABLE 1. FLUE GAS PARTICULATE CONCENTRATION AND EMISSION RATE
RELATIONSHIPS FOR KRAFT RECOVERY FURNACES
Grain s/SDCF
0.01
0.02
0.03
.0.04
0.05
0.06
0.07
0.08
0.09
. 0.10
0.20
0.40
0.50
0.60
0.80
1.00
2.00
4.00
6.00
8.00
10.00
Ibs/Ton Pulp
0.473
0.946
1.42
1.89
2.37
2.84
3.31
3.78
4.26
4.73
9.46
18.9
23.7
28.4
37.8
47.3
94.6
189
284
378
473
Ibs/Hour (ONE TON/DAY MILL)
0.0197
0.0394
0.0591
0.0788
0.0985
0.118
0.138
0.158
0.177
0.197
0.394
0.788
0.985
1.182
1.58
1.97
3.94
7.88
11.8
15.8
19.7
Note: Interpolate for values not shown, or refer to
Figures 1, 2 and 3.
-228-
-------
TABLE 2. GAS CONCENTRATION AND SULFUR EMISSION RATE RELATION-
SHIPS FOR KRAFT RECOVERY FURNACES
PPfrTby Volume
1
5
10
20
40
60
80
100
200
400
600
800
1000
Pounds/Ton Pulp as Sulfur
0.0.274
0.137
0.274
0.548
1.10
1.64
2.19
2.74
5.48
11.0
16.4 .
21.9
27.4
One pound of Sulfur/Ton pulp = 36.4 PPM
-229-
-------
TABLE 3. FLUE GAS PARTICULATE CONCENTRATION AND EMISSION RATE
REL/iTIONSHIPS FOR .LIME KILNS
Grains/SDCF
0.01
0.02
0.03
0.04
. 0.05
0.06
0.07
0.08
0.09
0.10
0.20
0.40
0.60
0.80
1.00
2.00
4.00
6.00
8.00
10.00
IS. 00
20.00
Ibs/Ton
0.0822
0.164
0.247
0.328
0.411
0.493
0.575
0.658
0.740
0.822
1.64
3.29
4.93
6.58
8.22
16.44
32.9
49.3
65.8
82.2
123
164
Ibs/Hour (ONE TON/DAY MILL)
0.00343
0.00686 .
0.0103
0.0137
0.0172
0.0209
0.0240
0.0274
0.0309
0.0343
0.0686
0.137
0.206
0.274
0.343
0.686
1.37
2.06
2.74
3.43
5.15
6.86
-230-
-------
TABLE 4. FLUE GAS SULFUR CONCENTRATION AND' SULFUR EMISSION RATE
RELATIONSHIPS FOR LIME KILNS
PPM by Volume
1
5
10
20
40
50
60
80
100
200
400
500
600
800
1000
Pounds/Ton Pulp as Sulfur
0.00476
0.0238
0.0476
0.0952
0.190
0.238
0.286
0.381
0.476
0.952
1.90
2.38
2.86
3.81
4.76
One pound of Sulfur/Ton = 210 PPM.
-231-
-------
TABLE 5. VENT GAS PARTICULATE CONCENTRATION AND EMISSION RATE
RELATIONSHIPS FOR SMELT DISSOLVING TANKS
Grains/SDCF
0.01
0.02
0.03
0.04
0 .' 05
0.06
0.07
0.08
0.09
0.10
0.20
0.40
0.60
0.80
1.00
1.50
2.00
Ibs/Ton Pulp
0.0576
0.115
0.173
0.230
0.288
0.346
0.403
0.461
0.518
0.576
1.15
2.30
3.46
4.61
5.76
8.64
11.5
Ibs/Hour (ONE TON/DAY MILL)
0.0024
. 0.0048
0.0072
0.0096
0.0120
0.0144.
0.0168
0.0192
0.0216
0.0240
0.0480
0.0960
0.144
0.192
0.240
0.360
0.480
-232-
-------
Gaseous Sulfur Emissions^
Table 6 presents gaseous sulfur emission rates expressed as
pounds of sulfur corresponding to concentrations in parts per
million parts by volume dry gas. This vent gas has a mean mois-
ture content of about 40% by volume. These relationships 'shown
in Table #6 assume that the gaseous molecule contains only one
atom of sulfur. An average value of 28 SDCFM/TPD was used in
calculating emission rates. Figure 15 presents these data in
graphical form.-
TABLE 6. VENT GAS SULFUR CONCENTRATION AND EMISSION RATE RE-
LATIONSHIPS FOR SMELT DISSOLVING TANKS
PPM by Volume
1
5
10
15
20
25
50 ;
7.5
100
200
500
700
1000
Pounds/Ton as Sulfur
0.0033
0.0167
0.0333
0.0500
0.0667
0.0834
0.167
0.250
0.333
0.667
1.67
2.33
3.33
One pound of sulfur/ton =300 PPM
-233-
-------
TABLE 7. SULFUR DIOXIDE CONCENTRATIONS IN POWER BOILER FLUE
GAS AS A FUNCTION OF SULFUR CONTENT OF FUEL
Fuel Sul
fur Content
Percent
0.
0.
0.
1.
1.
2.
2.
3.
3.
4.
5.
10
25
50
00
50
00
50
00
50
00
00
PPM SO?
20%
Excess
Air
77
190
380
770
1150
1530
1910
2300
2680
3060
,3830
(v/v)
50%
Excess
Air
62
150
300
610
920
1210
1510
1810
2110
2420
3020
- Coal
~12~%
CO?
Content
63
160
310
620
940
1240
1550
1780
2180
2490
3100
PPM SO?
20%
Excess
Air
55
140
280
550
830
1100
1380
1660
1930
2210
2760
(v/v) - Oil
12%
CO 2
Content
51
130
260
510
770
1030
1280
1540
1800
2060
2580
Note: Interpolate for values not shown.
-234-
-------
TABLE 7
(continued)
1. Coal
Heat value - 13,000 BTU/pound
Carbon Content - 72.8%
Theoretical combustion air =
7.6 pounds/10,000 BTU
Theoretical air = 9.9 pounds/pound of coal
205 Excess Air = 11.85 pounds air/
pound of coal
At 70°F, 1 Atm., 11.85 pounds of air =
158 ft3 dry air
1 pound of coal of 1% sulfur content produces
0.121 ft3 of SO2 assuming 100%
combustion and no sulfur discharged with ash
PPM S02 (1% sulfur) = 0.121/158 x 106 = .
765 PPM
Pounds of SC-2 produced =
(2) (% Sulfur) (Pounds of Coal)
. 100
2. Oil (No. 6 or "Bunker C")
Heat value - 18,500 BTU/pound
Carbon.Content = 88.4%
Theoretical combustion air =
7.46 pounds/10,000 BTU
Theoretical air = 13.80 pounds/pound of oil
20% excess air - 16.5 pounds/pound of oil
At 70°F, 1 Atm., 16.55 pounds of air ~
-235-
-------
TABLE 7
(continued)
220 ft3 dry air.
1 pound of oil at 1% sulfur produces
0.121 ft3 of S02 assuming 100%
combustion.
PPM S02 (1% Sulfur) = 0.121/220 x 106 =
552 PPM
Pounds of SC>2 produced =
(2) (% Sulfur) (Pounds of Oil).
100.0
-236-
-------
TABLE 8
CONVERSION OF DUST LOADING IN BOILER FLUE GAS
FROM GR./SCF TO IB./MM BTU OF HEAT INPUT
AND VICE VERSA
Federal guidelines for participate emission rates are expressed in
Ib./MM BTU of heat input while many state regulations are expressed
in gr./SCF. The following formulas and curves can be used to relate
these two values.
1. COAL FIRED BOILER:
Step 1 : If percent 02 in flue gas is not known, determine per-
cent 02 from Figure 1 by assuming percent excess air or percent
C02- Generally one of these three quantities are known.
Step 2: Determine multiplication factor A-| from Figure 2.
Step 3: Determine multiplication factor B-| from Figure 3.
Step 4: Calculate #/MM BTU from formula below:
Lb/MM BTU = (Gr/SCF) (A1 ) (B^ or
2. OIL FIRED BOILER:
Step 1 : If percent Op in flue gas is not known, determine per-
cent Op from Figure 1 by assuming percent excess air or percent
C02. Generally one of these three quantities are known.
Step 2: Determine multiplication factor A2 from Figure 4.
Step 3: Calculate #/MM BTU from formula below:
Lb/MM BTU = (Gr/SCF) x (A2) or
Gr/SCF = Lb/MM BTU x
3. WOOD BARK OR BAGASSE FIRED BOILER:
Step 1 : If percent Op in flue gas is not known, determine percent
Op from Figure 1 by assuming percent excess air or percent C02-
Generally one of these three quantities are known.
-237-
environmental science and engineering, inc.
-------
Step 2: Determine multiplication factor A3 from Figure 5.
Step 3: Determine multiplication factor 63 from Figure 6.
Step 4: Calculate #/MM BTU from formula below:
Lb/MM BTU = (Gr/SCF) (A3) (63) or
Gr/SCF = Lb/MM BTU J- ]
Results from the above calculations should be accurate within five
percent of the true answer. Naturally, some generalization had to
be made in simplifying the above calculation procedures which explain
this accuracy range. Average values were utilized for the approxi-
mate analysis of the fuel in deriving the multiplication factor (A).
Likewise a heat value was assumed for the generated steam of 1050
BTU/Lb. The multiplication factor (A) originates from thermodynamic
combustion calculations and are compiled on a strictly theoretical
level. Boiler efficiency does not enter into the above calculations
and therefore only minimum assumptions must be made. Generally,
either the percent oxygen in the flue gas, the percent carbon dioxide
in the flue gas or the excess air is known, so it was decided that
this would be a good starting point for the above computations. For
oil and coal fired boilers the excess air would generally be in the
neighborhood of thirty percent while for wood waste fired boilers
this value might be as high as fifty percent. Notice that the family
of curves for the coal fired boiler in Figure 1 varies with percent
volatile matter.
-238-
environmental science and engineering, inc.
-------
Table 8
Figure 1
'ositjion For
"acTt'e~anarrfgrffCe
srcent Vol
sture
atile Matter
Fret
Dash Lines wi
-fletejrmi nation
and Air Weigh
Q_Pe.rcent_
-a
cu
(/I
fO
CO
c
o
i.
O)
D-
a
c
o
a.
«=c
o
il
O)
Q.
to
o
0 10 20 30 40 50 60 70 80 90 100
Excess Air-Percent
-239-
-------
Table 8
O
CQ
Q
LU
cc
'O
C_3
O
I
o
2 - 4 . 6
8 10
12
% Oxygen in Flue Gas
MULTIPLICATION FACTOR "A" FOR COAL FIRED BOILER
Figure 2
-240-
-------
Table 8
1.00
20 30
40
% H20 in Fuel
MULTIPLICATION FACTOR "B" FOR COAL FIRED BOILER
Figure 3.
-241-
-------
Table 8
ro
ro
i
cu
o
CQ
CD
-a
o
o
i.
o
q-
ro
-------
TABLE 8
(continued)
Factor F
10 20 30 40 50 60 70 80 90
%H20 in Fuel
Multiplication Factor B3 For Wood Fired Boiler
Figure 6
-243-
-------
FOR KRAFT RECOVERY FURi\7ACES
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PROCESS WEIGHT - 1000 POUNDS/HOUR
ICO
1000
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FIGURE 2. PROCESS WEIGHT.AND EQUIVALENT PULP PRODUCTION RELATIONSHIPS FOR LIME KILNS
IUUU
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10
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1000-
A
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BASIS: 75D IBS/TON
10-
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PROCESS HEIGHT - 1000 POUNDS/HOUR
10
100
-------
ir'lUUKE 4. PARTICULATE EMIS Si GYRATE PER UNIT OF PULP PRODUCTION AS
FUNCTION OF CONCENTRA^TON IN KRAFT RECOVERY FURNACE FLUE GAS
10.00-
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EMISSION RATE - POUNDS/TON PULP
IOOO.O
-------
FIGURE 5. EMISSION RATE IN UNIT TIME AS FUNCTION OF CONCENTRATION
IN KRAFT RECOVERY FURNACE FLUE GAS
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EMISSION RATE - POUi-lDS/HOUR
100.0
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1000-
pTETl CURATE EffT!TsiOrr^ATE^S ATC^CTlTl^bF ^QlTlV^SENT PULP* PRODUCTION
AND CONCENTRATION FO^^RAFT RECOVERY. FURNACES
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BASIS: 230 SDCFri/TPD
SDCF = DRY GAS; 70°F, 1 ATM,
10
EMISSION RATE - POUNDS/HOUR
100
1000
-------
FIGURED. GAS CONCENTRATION AND SULFUR EMISSION RATE RELATIONSHIPS FOR .KRAFT RECOVERY FURNACES
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EMISSION RATE - POUNDS OF SULFUR/TON PULP
100.0
-------
FIGURE 8. PARTICULATE EMISSION RATE PER UNIT OF PULP PRODUCTION AS FUNCTION OF
CONCENTRATION IN LIME KILN FLUE GAS
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.EMISSION RATE - POU.'IDS/TOiI PULP
100.0
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IN LIME KI FLUE GAS
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EMISSION RATE - POUNDS/HOUR
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',00-
FIGURE 10. PARTICULATE EMISSION RATE - AS FUNCTION OF EQUIVALENT PULP
PRODUCTION AND CONCENTRATION FOR LIME KILNS
E
A.
X
A-
/Q.Oi GRAin/SDCF.
P.05 GRAIN/SOCFy
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BASIS: AO SDCFfVTPD
SDCF = DRY GAS; 70°F; 1 ATM,
10-
1.0
10
EMISSION RATE - POUNDS/HOUR
100
1000
-------
FIGURE 11. FLUE GAS CONCENTRATION AND SULFUR EMISSION RATE RELATIONSHIPS FOR LIKE KILNS
or
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EMISSION .RATE - POUNDS OF-SULFUR/TON PULP
-------
FIGURE 12. ' PARTICULAR EMISSION -RATS PER UNIT OF PULP PRODUCTION AS
FUNCTION OF CONCENTRATION IN SMELT DISSOLVING TANK VENT GAS
I
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01
en
i
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BASIS: 28 SDCFM/TPD
SDCF = DRY GAS; /0°F; i ATM,
0.0!
0.01
0.10
1.00 10.00
EMISSION RATE - POUNDS/TON PULP
100.00
-------
"ICURE 13. PARTICULATE EMISSION RATE IN U2W TIME AS FUNCTION OF CONCENTRATION
UjjK
iSWW
IN SMELT DISSlWlNG TANK VENT GAS
I
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:F = DRY GAS; 70°F; 1
LL SIZE - ONE TON/DAY
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EMISSION RATE - POlTiDS/HOUR
-------
-FIGURE 14. PARTICULATE EMISSION RATE AS-FUNCTION.OF EQUIVALENT'PULP
. PRODUCTION AND CONCENTRATION FOR SMELT DISSOLVING TANK VENTS
1000-
_J1
A
A
17
a
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Z
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0.05 GRAIN
0.0! GRAIN/SDCf
0.5 GRAI
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GRA!^/SDCF
0.1-
X
BASIS: 28 SDCFM/TPD
. SDCF = DRY GAS; 70°F; 1 ATM,
0.1
-1.0
EMISSION RATE - POUilDS/HOUR
10
100
-------
FIGURE 15. VENT GAS SULFUR CONCENTRATION AND EMISSION RATE
RELATIONSHIPS FOR SMELT DISSOLVING TANKS
"
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E,r!ISSIOn RATE - POUNDS SULFUR/TOM PULP
-------
FIGURE 16 - r
3G.!
LATE COLLECTION EFFICIENCY AND EMISSIO^RATE RELATIONSHIPS FOR KRAFT RECOVERY FURNA
!
t
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BAS i s :
SDCF =
"A" -WIT
"B" -WIT
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70° F DRY .GAS; 1 ATM,
H CONTACT EVAPORATOR- INLET CONCENTRATION -5 GR/SDCF
HOUT CONTACT EVAPORATOR - INLET CONCENTRATION -10 GR/SDCF
1
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POUilDS/TO;! PULP
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FIGURE 18 - PARTICULATE COLLECTION EFFICIENCY AND'EMISSION RATE RELATIONSHIPS FOR SMELT DISSOLVING TANKS
*t \x'.V>
t;
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LIME KILN
ARTICULATE REMOVAL EFFICIENCY
AS is: 40 SDCFfVTPD
DCF = DRY GAS; 70°F; 1 ATM, -
LET CONCENTRATION - 20 GR/SDCF
. ' . 1
J
(
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1 I
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1.0
10.0
100
.pou:iDs/To;i PULP
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o
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10
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LU
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1.36
NEUTRAL SULFITE 1
' 2
i 3 ,
4 .
KRAFT 5
0
10
20 30 40
CONCENTRATION, % SOLIDS
Figure 19. Specific gravity of spent liquors.
-262-
-------
4.0
3.72 TONS CHIPS 1 TON A. D. PULP
1 @ 48.5% MOISTURE . j 18 Ib Na20 LOSS
DIGESTER
'
656
557
25% .
PO
CO
FROM SCRUBBER
32.5 Ib CaO
fi.5 Ih NaoO .
47% YIELD
> WASHING
Ib Na?0
Ibs A A
BULFIDITY
WHITE 802 ..5 Ib
linnnR ^NaoO
CLARIFIER ^00.5 Ib CaO
1
14.5%SOLIDS
Ib Na20
SLAKE R
2.25 Ib INSERTS L
; 2.25]b CaO LOSS
0.5 Ib Na20 LOSS574>5 ]h , T ^
Ih Na 01 (TV DUST
MULTIPLE
EFFECT
EVAPORATORS
, ^165 11
10 Ib
803 Ib Na20
22.5 IbPUR
"""LIME @ 905
ABILITY -!
. COLLECTOR
48.8%SOLIDS
628 Ib Na20
D SOAP
Na20 LOSS
GREEN
LIQUID
CLARIFIER
CHASED
', AVAIL- *~
?0.25 Ib CaO
- 500.5 Ib CaO I @ 84% AVAILABILITY 13.5 1b Na 0
". . T146.51b Na?0 J482.5 Ib CaO ] T8 1b Ca§ L0ss
MUD
WASHER
533 Ib CaO LIME !
10 Ib Na20* KILN
1
- TO WEAK WASH
. 143 Ib Na20
iO.5 Ib CaO
10 Ib Na20
SCRUBBER
* TO MUD WASH '
32.5 Ib CaO
6.5 Ib Na20
96 Ib SALT CAKE MAKEUP
42.0 Ib Na20
,. ' u
DIRECT
CONTACT
EVAPORATOR!
CROM WEAK WAJ
163 Tb Na2C
826 Ib Na20.
23 Ib Na20
MAKEUF
i '
DREGS
WASHER
"tt l
f RECOVERY
i BOILER
65% SOLIDS
2794 Ib SOLIDS
PLUS DUST v
PICKUP
.u 666 Ib NaoO
>H | <-
)--y v
nTcc;ni \/TWP. .<«.T n ih NT n i n^r
UlooULVliNU ' '"O . U ID INUoU LUo^
TANK
TO WEAK WASH
' 20 Ib Na20
T DREGS TO SEWER
3 Tb Na20 LOSS
Figure 20. Chemical balance in recovery cycle.
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA 450/3-75-027
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
Field Surveillance and Enforcement Guide:
Wood Pulping Industry
5. REPORT DATE
Issue: March 1975
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
S. Oglesby, E. R. Hendrickson
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Environmental Science and Engineering, Inc.
Post Office Box 13454 - University Station
Gainesville, Florida 32604
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
68-02-0618
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Protection Agency
Research Triangle Park, North Carolina 27711
13. TYPE OF REPORT AND PERIOD COVERED
.Final
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT . .
This manual provides guidelines and background information for use by personnel
of state and local air pollution control agencies in their surveillance and
enforcement activities related to the major types of chemical pulp mills. The
three major types of mills discussed are Kraft, Sulfite, and Neutral Sulfite
Semi-Chemical (NSSC). For each type of mill, the process is described; the emissions,
both gaseous and particulate, are characterized; and the types of applicable
control equipment are delineated. Field enforcement, inspection, reporting and
enforcement procedures to be followed in each type of mill by control agency
personnel are suggested.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS c. COSATI Field/Group
Emission Sources, Control Methods,
Kraft Pulping, Odors, Black Liquor Oxi-
dation, Mercaptans, Sulfur Dioxide arid
Paper.Manufacture
Pulp Manufacturing
13. DISTRIBUTION STATEMENT
Release Unlimited
19. SECURITY CLASS (This Report!
Unclassified
21. NO. OF PAGES
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
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INSTRUCTIONS
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significant bibliography or literature survey, mention it.here.
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(a) DESCRIPTORS - Select from the Thesaurus of Engineering and .Scientific Terms the proper authorized terms that identify the major
concept of the research and are sufficiently specific and precise to be used as index entries for cataloging.
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ended terms written in descriptor form for those subjects for which no descriptor exists.
t .
(c) COSATI FIELD GROUP - Field and group assignments are to be taken from the 1965 COSATI Subject Category List. Since the ma-
jority .of documents are multidisciplinary in nature, the Primary Field/Group assignment(s) will be specific discipline, area of human
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EPA Form 2220-1 (9-73) (Reverse)
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