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18 Wood Processing Industry
Chemical Wood Pulping Mechanical Pulping
Paper and Paperboard Manufacture Arun V. Someshwar and John E. Pinkerfon
The processing of wood to yield such varied products as paper, paperboard, lumber, plywood, and reconstituted building board ranks among the ten largest industrial activi- ties in the United States. To produce paper or paperboard, the wood is first pulped. Pulps are made from wood chips, whole tree chips, sawmill residues, or logs. Pulps can be produced through chemical or mechanical means or by a combination of both. The pulp may then be bleached to various degrees of brightness. Finally, bleached or un- bleached pulp is processed into thick sheets of paperboard or paper. The manufacture of building boards, with a few exceptions, involves processing the wood itself into sheets or boards, with the aid of adhesives.
The following material is intended to provide a broad overview of selected segments of the wood processing in- dustry. The production process, atmospheric emissions, and emission control technologies employed are described for (l), several chemical wood pulping processes, ( 2 ) mechani- cal wood pulping processes, and (3) paper and paperboard manufacture. It should be recognized that this manual only provides information on the most common manufacturing processes and control technologies. There are a large num- ber of variations of the basic processes and control systems; thus the descriptions here are necessarily generic in nature. Also, this presentation does not cover solid wood products manufacturing operations.
Chemical Wood Pulping Arm V. Someshwar and John E. Pinkerton
Chemical wood pulping involves cooking wood chips or sawdust in an aqueous solution of pulping chemicals, result-
ing in the exiraction of cellulose from the wood by dissolv- ing h e lignin that binds the cellurose fibers together. The pulping chemicals may be alkaline, acidic, or neutral. Three principal chemical wood pulping processes currently in use are (1) kraft, (2) acid sulfite, and (3) neutral sulfite semi- chemical. Of these three, kraft pulping accounts for nearly 80% of the chemical pulp produced in the United States. In 1988, 126 kraft, 18 sulfite, and 15 other chemical pulp mills were in operation in this country.'
The dominant wood pulping process today is the kraft process. The term kraft or sulfate has been in use since 1879, when sodium sulfate replaced sodium carbonate (soda process) as the makeup chemical. For reasons that include the comparative simplicity and rapidity of the pro- cess, its insensitivity to variations in wood condition, and its applicability to all wood species, as well as the valuable properties of the pulp produced, the kraft process is ex- pected to continue as the dominant chemical wood pulping process.'
The production of pulp by the kraft process can be divided into three areas, namely, (1) the making of pulp, ( 2 ) the recovery of cooking chemicals, and (3) the bleaching of pulp. Figure 1 provides a schematic representation of the kraft pulping and chemical-recovery process.
The Making of Pulp In the kraft process, wood chips are cooked at an elevated temperature (340-360°F) and pressure (100-135 psig) in an aqueous solution of sodium hydroxide and sodium sulfide (also called white liquor). The sodium sulfide in the cook-
.L . ,,
836 Wood Processing Industry
CnPS CONDENSER VENT
L \ COOKING - UQUOP
SCREENS i WASHED PULP
TO BLEACH PLANT OR PAPER MILL
PULP MILL VENT GASES LIQUOR
NDCE RECOVERY FURNACE
'I MIX TANK \
WHTE LlOUOR TO DIGESTER LIME
FIGURE 1. Schematic Diagram of the Kraft Pulping and Recovery Process
ing liquor serves to buffer and sustain the cooking reaction, while the sodium hydroxide is consumed by reaction with the lignin and carbohydrates in the wood. Once the cooking is complete, the wood is broken down into two phases: a soluble phase containing the lignin and alkali-soluble hemicellulose and an insoluble phase containing the alpha cellulose or pulp. The wood chips are cooked in large vertical vessels called digesters, which may be operated in either the "batch" or the "continuous" mode.
In the case of batch digesters, air trapped with the chips and gases formed during digestion are relieved in- termittently during cooking. In softwood pulping, the relief gases are condensed for turpentine recovery before venting. The cooking period ranges from two to six hours. At the end of the cook, the digester contents are transferred to an atmospheric tank, called the blow tank. Gases leaving the blow tanks pass through a condenser to remove moisture, and the uncondensed gases are incinerated in a combustion device.
In continuous digesters, which are extremely tall and large cooking vessels (see Figure 2 ) , an uninterrupted flow of wood chips and cooking liquor enters from the top. Pulp is withdrawn continuously from the bottom into a blow tank, while the spent liquor is drawn off and transferred to a flash tank. Steam from the flash tank is used to presteam the wood chips after which it passes through a condenser. FIGURE 2. Continuous Digester
Chemical Wood Pulping 837
FIGURE 3. Brown-Stock Washers (Vacuum Drum Type)
Noncondensible gases (NCGs) are vented, and in the case of softwoods, turpentine is decanted from the condensate.
Newer digesters may utilize a “cold blow” as compared with the conventional “hot blow.” In the cold blow process, the pulp leaves the digester at a much lower temperature, resulting in a significant reduction in blow gas emissions.
From the blow tank, the pulp and spent cooking liquor are diluted and pumped to a series of brown-stock washers, where the spent liquor is separated from the pulp, usually by countercurrent washing. Brown-stock washers are typically of the rotary-drum vacuum filter type (see Figure 3), although the newer diffusion-type washers are becoming increasingly common, especially with continuous digesters. The washed pulp, still brown in color, may then be sub- jected to a bleaching sequence, before being pressed and dried to yield the finished product.
Recovery of Cooking Chemicals The spent liquor (called black liquor) is extracted from the washers in a dilute phase (12-18% dissolved solids) and the rest of the chemical-recovery process is concerned with recovering and regenerating the cooking chemicals dis- solved in this liquor. The weak black liquor is concentrated in multiple-effect evaporators to about 55% solids. Further concentration to about 65% solids is accomplished in two distinct ways, depending on the configuration of the recov- ery furnace in which the 65% solids liquor is burned. In the older recovery furnaces, the hot combustion gases are uti- lized to concentrate the liquor in a “direct-contact evapora- tor” (DCE). Since the hot, acidic flue gases (primarily CO?) directly contacting the black liquor react with its reduced sulfur content and cause odorous total reduced sulfur (TRS) compounds to be stripped, the liquor is usually subject to oxidation prior to concentration in the DCE. Weak or strong black liquor oxidation by either air or molecular oxygen is carried out in gas-liquid contactors. This results in oxida- tion ot the Na2S species to Na2S203 or other polysulfides with a higher oxidation state for sulfur, which, in turn, leads to significantly reduced TRS emissions from the DCE. Most furnaces built since the early 1970s and many of the
older furnaces that have since been modified are designed without a DCE (also called noncontact furnaces or NDCE). A concentrator is used in such instances to concentrate the black liquor from about 55% to over 65% solids prior to burning in the furnace.
Kraft recovery furnaces are much larger and nearly three times as expensive as fossil-fuel-fired boilers of compara- ble heat input capacity (see Figures 4 and 5 ) . Concentrated black liquor is sprayed into the furnace and the organics in the black liquor, which are derived from pulping the wood, are combusted. This generates sufficient energy to produce steam and, more important, to reduce the sodium sulfate in the liquor to sodium sulfide, a cooking chemical. The bulk of the inorganic molten smelt that forms and collects in the furnace bottom consists of sodium carbonate and sodium sulfide in about a 3: 1 weight ratio. The smelt is con- tinuously withdrawn through smelt spouts into a smelt- dissolving tank. The alkali-fume-laden gas streams from both the DCE and NDCE furnaces pass through a particu- late control device, usually an electrostatic precipitator (ESP). The ESP serves not only to control particulate emis- sions, but also to recover and recycle the predominantly sodium sulfate particulate catch. +
In the smelt-dissolving tank, jets of water are used to quench the molten smelt to form green liquor, which con- sists of an aqueous solution of Na2C03 and Na2S. The
FIGURE 4. Kraft Recovery Fumace
v I ..
838 Wood Processing Industry
N E E RECOVERY FURNACE
Oxidizing - zone
Dtying / zone
-Reducing zone Primary Air
To Smelt Tank
Strong liquor from concentrator
FIGURE 5. Kraft Recovery Furnace Schematic
quenching of molten smelt results in large quantities of steam leaving the tank, carrying with it small amounts of particulate matter, containing mainly Na2C03 and Na2S. The green liquor is then transferred to a causticizing tank where quicklime (calcium oxide) is added to convert the Na2C03 to NaOH. This results in a white liquor solution containing NaOH, Na2S, and lime mud precipitate (mainly CaC03). The white liquor is then recycled to the digesters and the entire process repeated.
The limc mud from the causticizing tank is washed, dried (6C-80% solids), and then calcined in a lime kiln to regenerate quicklime. Large rotary kilns are typically used in the kraft pulp industry, although a few fluidized-bed calciners are also being used. Fossil fuels, mainly natural gas or residual fuel oil, are used to provide the energy required for calcining the limestone. Particulate emissions in the lime kiln exhaust gases are controlled by venturi scrubbers or ESPs. The wet or dry particulate catch is recovered and returned to the system for calcining.
The two principal commercial by-products of the kraft pulping industry are crude sulfate turpentine and crude tall oil. Further refining of these products is normally done elsewhere.
Pulp Bleaching Pulp bleaching imparts whiteness or brightness to the pulp, in addition to yielding certain desirable physical and chemi- cal properties. Nearly 60% of all chemical pulps are bleached. Kraft and other chemical pulps are usually sub- jected to multistage “lignin-removing’’ methods as com- pared with single- or two-stage “lignin-bleaching’’ methods used for mechanical and some semichemical pulps.’ Chlorination in one or two stages brings about the degrada- tion of lignin. An alkali stage usually follows chlorination to dissolve and extract the degradation products. The
bleaching process is typically finished by one or mor- oxidative stages with intermediate alkali extraction. Oxida tion by chlorine dioxide, hypochlorite, or peroxide is es; sential to remove the final discoloration and obtain full higll brightness.’ Significant changes in bleaching sequences, bleaching conditions, and use of bleaching chemicals have occurred in recent years. mostly in response to environmen.. tal concerns resulting from formation of certain bleaching by-products. These by-products include pdychorinated di-. benzodioxins and -furans (in the aqueous phase) and chlor- oform. The substitution of chlorine dioxide for chlorine andl the elimination or reduction in the use of hypochlorite are: among a number of bleach plant modifications already! implemented or planned for the near future. Relative to a i r emissions from bleach plant vents, chlorine, chlorine di-. oxide, and the by-product chloroform are the principal compounds.
Air Emissions Characterization
Historically, odor and visible particulate emissions from kraft pulp mills have received considerable attention. Initial state and federal emission regulations addressed TRS and particulate emissions. The characteristic odor of a kraft pulp mill results from TRS compound emissions, which are the by-products of reactions between the wood lignin and sul- fide ions in kraft liquors. Hydrogen sulfide (H2S) and three organic sulfur compounds-methyl mercaptan (CH3SH), dimethyl sulfide ((CH&S), and dimethyl disulfide ((CH3)2S+constitute the major TRS compounds. Besides TRS emissions, other gaseous by-products from the pulping process, such as methanol and acetone, are present in NCGs formed in the pulping and evaporation area. TRS, acetone, and methanol are present in digester relief gases or vent gases from turpentine condensers that treat digester relief gases, in noncondensible blow gases after the condenser, in gases vented from brown-stock washers, in seal tank and pulp knotter vents, and in NCGs from the evaporator- concentrator system. Mills that collect their foul con- densates and either steam or air-strip them generate an additional gaseous stream containing these chemical by- products.
Table 1 presents estimates of uncontrolled emission fac- tors for TRS, acetone, and methanol in pulp mill and evaporator NCGs. For purposes of uniformity, the emission factors in Table 1 and all succeeding tables are expressed in units of pounds per air-dried ton of pulp. It should be noted. however, that mills differ greatly with respect both to the pulping processes and to type of wood pulped. Thus based on conversion factors alone, some variability in emission factors among mills is to be expected.
Mills operating recovery furnaces with DCEs often prac- tice black liquor oxidation (BLO). This is typically ac- complished by employing air-sparging reactors in order to provide the necessary air-liquid contact. Estimates of un- controlled emissions of TRS compounds and volatile Or-
Chemical Wood Pulping 839
TABLE 1. Uncontrolled Noncondensible Gas Emissions from Kraft Pulping
Acetone Emissions, Iblton Methanol Emissions lbiton TRSb Emissions lbiton as sulfur Type" of
Source Digester Range Average Reference Range Average Reference Range Avenge ~ ~ f ~ ~ ~ ~ , - e
Relief gases or turpentine condenser vent gases
Blow gases Condenser vent gases, hardwoodC Condenser vent gases. softwood Brown-stock washersd
Fresh water Condensate
Evaporator gases Miscellaneous vent gases' Condensate stipptng systems
B C C
0.47-1, I5 0.01-0.04 0.30-0.55
0. I 6 5
0.90 5 0.03 1 0.42 5
0.05 7 NA
0.02 5 0.05 7 NA
0.80-1.40 0.02-0. I O
0 .OS-0.35 NA NA
I .20 0.05 NA
0.30 0.48 NA
5 0.29-0.97 7 0.20-0. 84
7 0.01-0.79 0.13-0.85
5 0.03-5.93 7 NA
0.64 0.47 NA
0.16' 0.43 I .09 0.18 1.10
3 3 3 3
All factors are in Ib/(air-dried ton pulp) and are valid for untreated gases only. For gases that are collected and burned in a combustion device (lime kiln. power boiler. incinerator. etc.). almost all the methanol. acetone. and TRS wil'l be destroyed. NA-not available. aB for batch and C for continuous digesters. TRS (includes H2S. methyl mercaptan. dimethyl sulfide. and dimethyl disullide). 'Estimates are for continuous digesters without diffusion washers only. dlncludes gases to roof vents and from undervents (vacuum pump exhausts and filtrate seal tanks). TRS emissions for vacuum-drum-type washing systems: TRS lor newer diffusion washers are <O.Ool Ib/ADTP.h 'Estimates for fresh water use only: includes washer seal tanks and pulp knotter vents. Wntreated stripped gases.
TABLE 2. Emissions from Black Liquor Oxidation Tower Vents
VOC Emissions. Ibiton as carbon TRS Emissions, Ib/ton as sulfur
Black Liquor Oxidation Range Average Reference Range Average Reference
Weak liquor 0.42-1.94 0.80" 7 0.02-0 21 0.10 15 Strong liquor 0.23-0.44 0.34 8 0.01-0.17 0.08 15
All factors are tn Ib/(atr-dried ton pulp) VOC and TRS emissions wil l be destroyed 11 vent gases are sent to a combustton device 'Includes methanol. acetone. and alpha pinene only
ganic compounds (VOCs) from BLO tower vents are given in Table 2 .
Major emission sources in the chemical-recovery area include recovery furnaces, smelt-dissolving tanks, and lime kilns. Besides TRS compounds, these sources emit particu- lates, sulfur dioxide (SOz), carbon monoxide (CO), and nitrogen oxides (NO,). The particulate matter emitted from kraft recovery furnaces is mainly Na2S04 (about 80%), with smaller amounts of KzS04. Na2C03, and NaCI. Sig- nificant alkali fuming action in the lower furnace causes over 10% of the sodium input to the furnace to vaporize. Sodium vapors are most likely to react rapidly with oxygen and carbon dioxide to form submicron-sized Na2C03 fume particles, which, in turn, scavenge the sulfur dioxide result- ing from black liquor combustion. The net result is that typically less than 5% of the total sulfur entering the furnace via the kraft black liquor escapes as SO2 and over 85% of the particulate catch that is recycled is made up of alkali sulfates.
Finely divided smelt (Na2C03 and NalS) entrained in
water vapor accounts for the particulate emissions from smelt-dissolving tanks. Lime kiln particulate emissions are mainly sodium salts, calcium carbonate, and calcium oxide. with uncontrolled emissions containing mainly calcium compounds and controlled emissions comprising mainly sodium salts. Sodium salts result from the residual sodium sulfide in the lime mud after washing.
Table 3 gives estimates of uncontrolled and controlled particulate emissions from these three kraft mill SOUrCeS in units of pounds per ton of pulp. Particulate emission limits for kraft sources subject to New Source Performance Stan- dards (NSPS) are expressed in units other than pounds per ton of pulp (see Table 8). However, for purposes of corn- parison, the NSPS values, after using typical conversion factors, correspond to about 2.0, 0.30,0.40 I b m pulp for kraft recovery furnaces, smelt-dissolving tanks. and lime kilns firing gaseous and liquid fuels respectively. Table 4 gives the cumulative mass percent less than Or equal to 10 p m (PMlo) and 2.5 p m (PM2.5) in Pafliculate emis- sions from uncontrolled and controlled recoveV t'rnaces.
840 Wood Processing Industry
TABLE 3. Particulate Emissions from Recovery Sources in Kraft Pulping
lblton Type of Units Control Range Average Tested Reference
Recovery fumace with Untreated direct-contact evaporator (DCE) Venturi scrubber
Noncontact recovery furnace Untreated without DCE E S P
E S P Smelt-dissolving tank Untreated
Mesh pad Scrubber
Lime kiln Untreated Venturi scrubber'
140-313 14-1 15
0.25-1 I . 4 0.66-1 .S6 204-743
0.62-4.33 0.50-2.66 0.19-23.7 0.05-2.3 0.07-0.29 41.5-7 1.3 0.23-1. I
1.98 I .67 7.0 I .o 0 . 2
I O I O 20 IO 19 13 19 I O 6
10 2 5
18 19 21 20 21
3 3 4 3 4
All factors are in Ib/(air-dried ton pulp). Paniculate emissions from ESPs on recovery furnaces were convened from units of gr/acf and gdscf to Ib/ADTP using conversion factors of 400 acfm/TPD (range. 370 to 3-10) and 301 sct"/TPD (range. 252 to 344) respectively. "1971 survey of ESPs on DCE furnaces started between 1966 and 1970.'R b1979 survey of ESPs on DCE furnaces started between 1973 and 1977." '1974 survey of ESPs started between 1969 and 1973." '1979 survey of ESPs started between 1974 and 1978." 'Data from five kilns during the NSPS review and development program.'
smelt-dissolving tanks, and lime kilns. It is to be noted that the estimates given in Table 4 are based on extremely limited data.4 It can be seen from Table 4 that nearly 75% of the particulate emissions from ESPs on DCE and NDCE furnaces are 5 10 pm in size, whereas this fraction in- creases to 98% and over 89% for controlled emissions from lime kilns and smelt-dissolving tanks respectively.
The presence of sulfides in kraft liquors invariably leads to some TRS compound emissions from these three sources. TRS emission factors for kraft recovery furnaces, smelt- dissolving tanks, and lime kilns are shown in Table 5 . Over
TABLE 4. Mill Sources
PMlo and PM2.5 Emissions from Kraft
Cumulative Mass % 5 Stated Size, wm
10 pm 2.5 pm Type of Control
DCE fumace Uncontrolled 93.5 83.5 ESP 75.0 53.8
NDCE furnace Uncontrolled NA 78.0 ESP 74.8 67.3
Lime kiln Uncontrolled 16.8 10.5 Venturi scrubber 98.3 96.0
ESP 88.5 83.0
Smelt-dissolving tank Uncontrolled 88.5 73.0 Packed tower 95.3 85.2
Venturi scrubber 89 .5 81.3
Source: Reference 4.
the past 20 years, there have been major reductions in kraft pulp mill TRS emissions as a result of equipment upgrades and replacements, federal and state emission regulations, and a desire to minimize odor complaints in mill communi- ties.
The combustion of sulfur-containing black liquor is ex- pected to result in sulfur dioxide emissions. These emis- sions from kraft recovery furnaces are a complex function of liquor sulfidity, liquor characteristics, fumace design and loading, combustion air flow and distribution, and so on. Sulfur dioxide emissions from most well-designed furnaces of the DCE and NDCE type are below 500 ppm, with several operating consistently at levels below 100 ppm. However, for reasons not totally understood, on a given day the SO2 emissions can vary considerably. The emissions of SO2 from smelt-dissolving tanks and lime kilns are gener- ally insignificant. Moderate to low quantities of NO,, VOCs, and CO are also released from recovery furnaces and lime kilns. Available emission factors for SO2, NO,, VOC and CO from kraft recovery furnaces, smelt dissolv- ing tanks and lime kilns are also presented in Table 5. Other kraft recovery fumace emissions that are occasionally tested for, but for which insufficient data exist at present, include PCDD/Fs (dioxins and furans), certain organic gases, and trace metals. Preliminary test results indicate that the PCDD/F levels range from extremely low to nondetec- table.25
If pulp bleaching is practiced at a kraft mill, uncontrolled bleach plant vent gases may contain chlorine (C12), chlorine dioxide (C102), and chloroform (CHC13). Emission factors for uncontrolled C12 and C102 in bleach plant vent gases are
Chemical Wood Pulping 841
TABLE 5. SO*, NO,, CO, VOC, and TRS Emissions from Kraft Recovery Sources
SO?, NO,, co , v o c , TRS , Ib S02/ton lb Noliton Ib CO/ton Ib CHdton Ib Siton
Recovery furnace DCE with BLO” Range Average Number of furnaces Reference
Recovery furnace NDCE Range Average Number of furnaces Reference
Range Average Number of SDTs Reference
Lime kiln Range Aver age Number of kilns Reference
0.005-0. I l e 0.04
I I 24
0.9-3.3 I .8b I O I I
0.9-3.3 I .8b I O I I
NA NA NA -
0.2-3.7 1.2 6 IO
NA NA NA -
0.7-1. I 0.83
NA NA NA -
0.01-0.15d NA NA 22
0.0 1-0.05‘ NA NA 23
0.01-0. IOd NA
All factors are in Ib/(air-dried ton pulp). NA-not available. ‘Values are for “new design” furnaces: “old design” DCE furnaces (built before 1965) have TRS emission limit guidelines of 0.6 Ib/ADTPzZ and emission factors for NO,, CO. and VOC are unavailable. bAverage from IO units tested, five NDCE and five DCE.” For all IO units. the percent solids in black liquor were below 70%. More recent furnaces firing higher solids liquor could have higher NO, emission levels. ‘Average value for five units. two NDCE and three DCE9 dBased on TRS emission guidelines for existing kraft pulp mills” ‘Based on test data obtained from different mills. ‘Based on NSPS for SDTs of 0.016 g/Kg bls as H2S”
TABLE 6. Chlorine and Chlorine Dioxide Emissions from Pulp Bleaching
Chlorine, Ib/ton Chlorine Dioxide, Ib/ton Type of
Source Control Range Average Reference Range Average Reference
Bleach plant Uncontrolled 0.01-10.1 0.70 5 0.00-2.90 0.50 5
C102 generators’ Absorbers 0.80-14.0 - 5 0.23-6.10 2.30 5 scrubbers a a 5 b b 5
Factors are in Ib/(air-dried ton pulp) for bleach plant sources and in lbiton CIO? generated for C102 generators T I 2 removal efficiencies by various scrubbing fluids range from 75% to 99%.’ bCIOz removal efficiencies by various scrubbing fluids range from 50% to 99%.’ ‘Range applies to various ClO? generating processes with absorbers that are followed by a caustic scrubber if required? the pound CIOl used per ton pulp varies considerably, ranging from 0 to as much as 60 (three separate C102 stages).
presented in Table 6 and those for CHC13 are given in Table 7. Bleach plant emissions of C12, C102, and CHCl3 are extremely site specific and variable. Ranges and median values of available data should be used with caution. The Hypochlorite Use Range Average Reference
TABLE 7. Chloroform Emissions from
Chloroform Emissions, Ib/ton
emission of these gases depends on the application rates of
the bleaching sequences and bleaching conditions em- ployed. Chlorination, oxidation, and alkali extraction may
Cl2, C102, and hypochlorite during bleaching, and also on 0.1-<0.5Q 0. O W . 67 0.27 5 0.5- 2.0% 0.11-1.10 0.53 5
>2.0% 0.22-2.01 0.80 5
each make up a separate bleaching stage. Figure 6 gives a *I1 In Ib’(au-dned ton Pulp). Table 7 only applies to bleaching sequences that use hypochlonte in excess of 0. I % , where percent usage IS expressed as (pounds hypochlonte used per pound oven dry much-simplified diagram Of a stage. Gas-
phase emissions may occur from each stage’s tower, washer brown-stock pulp) x iw.
842 Wood Processing Industry
BLEACHING TOWER VENT
In 1979. the U.S. Environmental Protection Agency (EPA) issued retrofit emission guidelines for the control of TRS emissions at existing facilities not subject to NSPS, and
ping. Power boilers firing either wood residues, coal, oil, or natural gas are utilized for the purpose of generating steam and power. Air emissions from such sources have already been dealt with elsewhere in this text.
A i TANK j VENT
4 WASHER ; A
Air Pollution Control Measures i i
these are also shown in Table 8.*' In addition to particulate
hood, and seal tank vent. The various emission points may or may not be combined into one or more common vents. Scrubbers utilizing various scrubbing fluids, such as bleach- plant extraction-stage filtrate, sodium hydroxide solution, sodium bisulfite solution, alkaline wash water from causticizing operations (weak wash), white liquor, and chilled water, have been installed for the control of Clz and C102 air emissions. Chloroform formed during bleaching can also be released to the environment in the wastewater treatment area. A study3' by the National Council of the Paper Industry For Air and Stream Improvement showed that between 10% and 94% (average 48%) of CHC13 that is formed as a by-product is released to the wastewater treat- ment area, where most of it is stripped or volatilized.
The production of steam in kraft recovery fumaces is usually sufficient to meet the steam demand for the pulping and evaporation functions at a mill. Additional steam is required for several other operations within a kraft mill, including bleaching, pulp drying, paper making, and strip-
Control of TRS Emissions The collection and treatment of NCGs from kraft pulp mill sources have been practiced for over 40 years. While most early systems were designed to control TRS emissions from the digester and multiple-effect evaporator NCGs only, during the past 20 years, efforts to include such sources as the brown-stock washer hood vents, condensate stripper system vents, turpentine decanter vents, and a number of other minor sources have been made." Incineration in ex- isting combustion devices, namely, lime kilns and power boilers, is the treatment most commonly used. Incinerators dedicated to this purpose, while used to a lesser degree, are becoming more prevalent. However, the oxidation of TRS results in SOz formation and a caustic scrubber is often installed following the incinerator for SO1 removal. Other combustion devices, such as recovery fumaces and flares, are also used. Alternative disposal practices employed in- clude venting with the bleach-plant chlorination or chlorine
TABLE 8. NSPS and Emission Guidelines for Kraft Pulp Mills
Total Reduced Sulfur
Guidelines" Source Particulate Matter NSPS
Recovery furnace New design Old design
Cross recovery Smelt-dissolving tank Lime kiln
Digester system BSW system BLO system MEE system Condensate stnpper cystem
0.044 gridscf (Z 8% O2
0 044 gridscf f i 8% O2 0.20 lbiton bls
0.067 gridacf f i I O % O2 0.130 gddacf Ci 10% O2
- - - - -
5 ppmdv 6, 8% O2
25 ppmdv ( i ~ 8% Or 0.032 Ib/ton bls
8 ppmdv k( I O % O2 8 ppmdv (ii 10% O2 5 ppmdv G 10% O2 5 ppmdv (u I O % O2
No control 5 ppmdv (u 10% O2 5 ppmdv 6 IO% O2
5 ppmdv Ca 8% O2 20 ppmdv Ca 8% O2 25 ppmdv Ca 8% O2
20 ppmdv (ir 10% O2 20 ppmdv Ca 10% 0 2
No control No control No control
5 ppmdv 6 10% Or 5 ppmdv 6 10% O2
"EPA guidelines for TRS emission from existing kraft pulp ini l ls nut wbject to NSPS": ppmdv-ppm dry on volume basis
Chemical Wood Pulping 843
dioxide stage washer vent gases for chemical oxidation of the sulfur compounds, and, during process upsets, dis- charge of selected streams to stacks for improved disper- sion, thus minimizing the potential for worker exposure to high concentrations of TRS.”
The non-condensible gases in the kraft pulping process are of two types-low-volume, high-concentration (LVHC) and high-volume, low-concentration (HVLC). The LVHC gases include those from the digester area, condensate strippers, evaporators, and so on, while HVLC gases in- clude those from brown-stock washers, pulp knotters. and washer seal tanks. The LVHC gases are typically burned in lime kilns, boilers, or incinerators, while HVLC gases are burned in boilers that can accept such large gas vol- umes.
Vent gases from BLO systems are not required to be controlled under NSPS due to the prohibitive cost- effectiveness of control and the declining use of BLO. l 3
The TRS emissions from the newer diffusion brown-stock washers are extremely small as compared with the older vacuum-drum-type washing systems.6 The use of fresh water or stripped condensate as a washing medium in brown-stock washers (as opposed to dirty condensate) also leads to reduced TRS emissions. The TRS emissions from smelt-dissolving tanks are similarly most effectively con- trolled by the choice of a water (both for smelt dissolving and particulate control) containing minimal amounts of re- duced sulfur compounds.
The TRS emissions from kraft recovery furnaces are most efficiently controlled by maintaining sufficient ox- ygen, residence time, and turbulence, and avoiding over- loading. Furnaces with DCEs rely additionally on BLO for TRS emission control. By oxidizing the Na2S in the liquor to Na2S203 before it enters the DCE, the reactions between the combustion gases and black liquor in a DCE that gener- ate H2S are inhibited.
Lime kiln TRS control mainly involves achieving a high degree of lime mud washing. Lime mud is the precipitate resulting from the causticizing reaction when Na2C03 in the green liquor is converted to NaOH and lime is converted to CaC03. Sodium sulfide entrained in the lime mud reacts with C 0 2 in the cold end of the kiln, giving rise to H2S emissions. Proper operation of the lime mud filter to pre- vent Na2S from entering the lime kiln and allow for oxida- tion of residual Na2S to NaZS203 is usually sufficient to keep H2S levels below about 8 ppm. The use of sulfide-free streams such as fresh water or clean condensate for makeup in the scrubber also prevents H2S formation by contact with kiln gases.
Control of Particulate Emissions Particulate control on furnaces with DCEs and on NDCE furnaces is achieved predominantly by ESPs. Electrostatic- precipitator particulate-removal efficiencies range from about 90% in older installations to well over 99% in newer units. The range and average particulate emission factors
for furnaces with ESPs are shown in Table 3. Direct-contact evaporators used to concentrate black liquor also serve to scrub the particulates leaving the furnace, removing from 20% to 50% of the particulate load prior to the ESP. A few scrubbers have. been installed following older ESPs to obtain satisfactory levels of particulate removal.
Particulate control of smelt-dissolving tank vent gases is accomplished by installing demister pads, packed towers, or venturi scrubbers. Most lime kilns are controlled by venturi scrubbers, with pressure drops ranging from 17 to 34 inches of water, although ESPs are increasingly being used in new installations. Typical emission factors for smelt-dissolving tank vents and lime kilns with control are also shown in Table 3.
Fugitive emissions from sources or areas of importance in a kraft mill include coal piles, paved and unpaved roads, bulk materials handling (lime, limestone, starch, etc.), and wood handling. Control strategies include wetting; the use of chemical agents, building enclosures, and windscreens; paving or wetting roads; and modifying handling equip- mer~t.‘~
Control of SO,, NO,, CO, andVOC Emissions Besides power boilers in which sulfur-containing fuels are fired, SO2 emissions from a kraft mill occur principally through kraft-recovery-furnace flue gases. Unlike power boilers firing fossil fuel, the combustion of black liquor in a kraft recovery furnace results in SO2 emissions that are extremely variable and depend on a variety of factors, which include (1) liquor properties such as sulfidity (or sulfur-to-sodium ratio), heat value, and solids content; (2) combustion air and liquor firing patterns; (3) furnace design features; and (4) other operational parameters. l 6
Table 5 gives SO2 emission factors for DCE and NDCE fumaces developed from averaging recent, long-term, con- tinuous emission monitoring SOz data. l 6 Strategies to (1) lower liquor sulfidity and (2) optimize liquor and com- bustion air properties and firing patterns so as to yield maximum and uniform temperatures in the lower furnace have been used to minimize kraft-recovery-furnace SO2 emissions. Flue gas desulfurization is capital and energy intensive and its efficacy is uncertain, considering the generally low concentrations and rapidly fluctuating levels of SO2 in the furnace flue gases. Sulfur dioxide may be formed in the lime kiln when fuel oil is combusted. The regenerated quicklime in the kiln acts as an in-situ scrubb- ing agent and the venturi scrubber that normally follows the kiln augments the SO2 removal process. Limited SOz measurements following ESPs on fuel-oil-burning kilns suggest that over 90% of the SO2 is captured by the time the flue gas exits the ESP.
The NO, emissions from recovery furnaces and lime kilns result from black liquor and fossil fuel combustion respectively. The emissions from recovery furnaces are mainly attributed to “fuel NO,,” resulting from partial oxidation of the black liquor nitrogen content. Kraft recov-
’ 844 Wood Processing Industry
ery fumaces operate with a reducing zone in the lower part of the furnace (for reduction of Na2S04 to Na2S) and an oxidizing zone farther up in the region of the liquor spray guns and secondary air. This staged combustion is a natural deterrent to excessive NO, formation, with the result that most existing kraft recovery fumaces emit less than about 100 ppm NO,. However, the current trend for new units or existing units undergoing upgrading is to bum liquors with increasingly higher solids content (>70% solids). Available emission factors for NO, from kraft recovery fumaces (shown in Table 5) are based on tests on older units buming liquors with <70% solids. These emission estimates are being reviewed, considering the somewhat higher NO, formation that could result from more intense burning of the higher-solids liquor. Coincidentally, higher temperatures in the lower furnace have been documented to yield signifi- cant other environmental (such as lower ,502 and TRS) and furnace operational (such as smelt-bed stability) advantages. l6 Higher temperatures in the lower fumace zone can result from several factors, including firing higher- solids liquor, firing liquor with higher heat content, and operating the furnace at higher than the design load.
The VOC and CO emissions from recovery furnaces and lime kilns result from incomplete combustion of the organic matter in the fuel. Lime kiln CO and VOC emissions are generally small, as seen from Table 5. Recovery-fumace CO and VOC emissions are a function of the level of excess air used and the degree of mixing achieved within the furnace. Control strategies to minimize CO and VOC emis- sions involve increasing the residence time, oxygen con- tent, temperature, and level of turbulence in the furnace combustion zone. Unfortunately, an increase in excess air, residence time, and temperature has the opposite effect on NO, formation.
Control of Bleach Plant Emissions As shown in Table 6 , the uncontrolled Cl2 and C102 emis- sion from bleach plants exhibit a very broad range. These emissions are expected from bleach towers, washer hood vents, and seal tank vents of the chlorination and chlorine dioxide stages of bleaching respectively. Emissions of Cl2 and C102 are also expected from the C102 generator ab- sorbers.
Studies have indicated that maintaining low-bleaching chemical residuals in the pulp leaving the bleaching towers results in minimal C12 and C102 emissions from washer hoods and seal tanks.” Application rates for Clz and C102 govern the C12 and CIOz emission rates from their respec- tive tower vents. Packed-tower scrubbers are used to control Cl2 and C102 emissions from washer hoods and seal tanks. Smaller scrubbers are used to control emissions from tower vents, especially those of an upflow-downflow design.30 These smaller scrubbers are designed with chemical recov- ery in mind. Some mills use the larger packed scrubbers to scrub combined vent gases from tower vents, washer hoods, and seal tanks. Estimates for the removal efficiencies of Clz
and C102 emissions from bleach plant vent gases after alkali scrubbing are given in Table 6 .
The formation of by-product chloroform (CHC13) during pulp bleaching is a much more complex phenomenon. Lab- oratory and field studies have shown that CHC13 may be formed in the chlorination (C), extraction (E), and hypo- chlorite (H) stages of bleaching3’ The use of hypochlorite in the bleaching sequence seems to have the most impact on CHC13 formation. Elimination of hypochlorite from the bleaching sequence has been used to reduce CHCl3 generation3’ at a number of bleach plants. A reduction in the chlorine factor (defined as percent chlorine applied/ kappa number, where kappa number is a measure of the residual lignin content) also helps in minimizing chloroform by-product formation. On an average, slightly over one half of the CHC13 formed in the bleach plant is expected to be released through the bleach plant vent^.^' Table 7 gives the CHCl3 air emission factors for bleaching sequences with different levels of hypochlorite usage. Emission estimates for bleaching sequences using <O. 1 % hypochlorite are more complex to determine and are given elsewhere.’ The feasibility of gas-phase scrubbing of CHC13 from bleach plant vents has not yet been demonstrated. A reduction in CHC13 by-product formation would also lead to decreased emissions from the wastewater treatment area.
ACID SULFITE PULPING
In the early 19OOs, the sulfite process was predominant as it yielded the brightest unbleached pulp and the most easily bleached one. However, over the years, owing to its sensitivity to the wood raw material and the difficulty of recovering cooking chemicals and utilizing process waste products, the sulfite pulping process has been steadily on the decline.
The production of pulp by the acid sulfite process is carried out in a manner similar to that of kraft pulp, except that the cooking chemicals NaOH and Na2S are replaced by sulfurous acid. To buffer the cooking solution, a bisulfite solution of one of four bases (ammonium, calcium, magne- sium, or sodium) is used. The liquor (pH 1 to 6 ) is prepared by reacting SO2 with the base solution in one or more absorption devices.
Wood chips are cooked in the acidic cooking solution in batch or continuous digesters at high pressures (90-100 psi) and temperatures (260-320°F). At the end of the cooking period (somewhat longer than kraft cooks), the digester contents are either discharged under pressure into a blow tank or pumped at lower pressures to a dump tank. The spent sulfite liquor (also called red liquor) drains through the bottom of the tank and is either processed to recover certain organic materials, concentrated and incinerated, or sent to a recovery area where the cooking chemicals and
8 bi w
Chemical Wood Pulping 845
TO HEAT a b CHEMICAL
BROWNSTOCK WASHERS \ ,,(T
F RECOVERY SYSTEM 1 EXHAUST GAS GASES -
4 r - 7 \ EVAPORATORS
MAKEUP A MAKEUP RECOVER
I I / WEAK RED
STRONG - 4 RED LIQUOR
COMBUSTION AIR I 1 SCREENS
1 PULP I I GAS
WASHED PULP TO BLEACH P I A M
OR PAPER MILL
FIGURE 7. Pulping and Recovery in a Typical Magnesium-Based Sulfite Process
liquor heat content are recovered. The pulp is washed, screened, and centrifuged (to remove knots). It may sub- sequently be bleached, pressed, and dried. The pulp is used in a variety of fine papers or specialty paper products or is converted to numerous nonpaper end uses.
The magnesium-based process is currently the most widely used form of sulfite pulping. A typical magnesium- based sulfite cooking process is shown schematically in Figure 7. Older sulfite mills typically used calcium-based chemicals for cooking. As a result of maintenance problems associated with scale deposition on equipment and, more important, the impracticality of recovering the cooking chemicals, there has been a gradual trend in the industry away from calcium-based sulfite pulping. Existing calcium- based mills usually either process the spent liquor to recover selected organics or sewer the spent liquor or incinerate it, using the ash for by-products. In NH3-based cooking, the spent liquor is concentrated in evaporators and burned in recovery furnaces. Heat is recovered, but the ammonium base itself is consumed during combustion. However, the sulfur is recombined with aqueous ammonia and recycled to
cooking liquor. In sodium- and magnesium-based pulping, it is feasible to recover heat, sulfur, and base economically.
For acid sulfite mills, a sulfur burner is the usual source of SOz. Sulfur is burned in rotary or spray burners and the gases are cooled by heat exchangers and a water spray and then absorbed in a variety of different scrubbers into solu- tions of CaC03 or one of the other base chemicals. The amount of sulfur burned in a mill depends on whether chemical recovery is practiced and also on the efficiency of the SO2 emission control equipment.
If chemical recovery is practiced, the spent red liquor is concentrated in multiple-effect evaporators and a DCE to between 55% and 60% solids. This strong liquor is then sprayed into a boiler or furnace, where the organic content in the liquor is burned, producing steam to operate the digesters, evaporators, and so on. The inorganic content undergoes different fates, depending on the base chemical.
When magnesium-based liquor is burned, a flue gas containing magnesium oxide (MgO) and SO2 is produced, and the MgO is recovered in a multiple cyclone as a fine white powder. The MgO is then water slaked and used as
846 Wood Processing lndilstry
the circulating liquor in a series of venturi scrubbers, de- signed to absorb the SO2 from the recovery-furnace flue gases leaving the DCE. The bisulfite solution formed is fortified in the acid plant by the SO2 formed by buming makeup sulfur.
When a sodium-based liquor is burned, the inorganic compounds in the liquor are recovered as a molten smelt containing Na2S and Na2C03 (just as in kraft recovery furnaces). This smelt may be further processed to absorb SO2 from the flue gas and sulfur burner. In some sodium- based mills, the smelt may be sold to a nearby kraft mill as raw material for preparing green liquor.
When NH3-based liquor is burned, the small amount of inorganics is removed as furnace slag. The ammonia is decomposed to nitrogen and water. The SO2 is absorbed in a heat-recovery unit with aqueous ammonia. The resulting bisulfite solution is further acidified with SO2 from the buming of elemental sulfur to make the cooking liquor.
There are several recovery processes for magnesium- and sodium-based liquors, including the B&W magnefite process, the Copeland process, the STORA process, the SCA-Billerud process, the Tampella process, and the CE Sevola process.
Air Emissions Characterization and Control Measures
Sulfur dioxide is considered the major pollutant of concern from sulfite pulping. The characteristic odor of kraft pulp- ing is generally absent, as TRS compounds are not formed during the lignin-bisulfite reactions. However, sodium- based sulfite pulping operations practicing recovery may experience some TRS emissions. Particulate, NO,, and CO emissions from all recovery furnaces constitute the other pollutant emissions. Emissions from bleaching operations may be similar to those from kraft pulp bleaching, but are very dependent on the bleach sequence, the wood source, and the amount of residual lignin entering the bleach stage.
The digester and blow tank areas are a major source of SO;?. The relief gases from the digester and NCG gases from the presteaming vessel and flash evaporators are nor- mally all returned to the acid-preparation system for SO2 recovery. Considerable quantities of SO2 (ranging from 10 to 70 pounds SOz per ton of pulp32) could be flashed during a hot blow, with uncontrolled emissions comprising nearly 95% water vapor, 3% SO2, and 2% C02.33 The quantity of SO2 actually evolved and emitted in the blow gases is usually much less, depending on the pH of the cooking liquor and the pressure at which the digester contents are discharged. Raising the pH of the digester contents before blow lowers the free SO2 in solution. Lowenng the digester pressure before blow (to as low as 3 psi) and pumping instead of blowing out the contents are added measures for reducing SO2 emissions. The SO2 released to the atmo- sphere, however, depends on the effectiveness of the heat- recovery and SO*-absorption systems employed for SOz
recovery. The SO2 in blow gases can be scrubbed with an alkaline solution of the base and returned to acid prepara- tion, with recoveries as high as 97%.33 This method is viable with sodium and NH3 bases. However. magnesium and calcium bases require slurry scrubbers, which are less practical.33 The SO2 released during a cold blow from a blow tank is of a lesser magnitude (4-20 pounds SO2 per ton of pulp35), which makes scrubbing impractical.
Many sulfite mills currently utilize displacement or dif- fusion washers. quite similar to those in kraft mills. Un- controlled emissions from acid bisulfite washers and screens can be as high as 16 pounds SO2 per ton of pulp,33 although typical losses range from 1 to 4 Ib/ton.” Scrub- bing of SO2 from the washers is generally accomplished by hooding and directing the collected gases to a direct-contact scrubber.
Spent sulfite liquor (SSL) is evaporated in multiple- effect evaporators similar to kraft liquors. The SO2 gener- ated during evaporation is pH dependent, ranging from 40 to 60 pounds SO2 per ton of pulp for acid bisulfite cooking (pH < 2 ) to less than 2 pounds SOz per ton of pulp for bisulfite (pH 2 to 6 ) and neutral sulfite (pH 6 to 9) cooking.” Sulfur dioxide from acid bisulfite liquors is usually treated by absorption in an alkaline solution of the base or sent to the acid plant for SO2 recovery. Some mills practice weak SSL neutralization (usually magnesium- based process), which practically eliminates SO2 emissions during e~aporation.~’
There exist a number of SSL recovery processes for bisulfite liquors using magnesium, sodium, and NH3 bases. Either boilers or fluidized-bed reactors are used for SSL combustion. Efficient recovery of SO2 from the combustion gases is critical for economical operation, as concentrations exceeding 1% (10,000 ppm) SO2 can result in gases from SSL combustion. Recovery systems at most mills are closed and include the recovery furnace, DCE, multiple-effect evaporator, acid fortification tower, and SOz-absorption scrubbers. Generally, there exists only one emission point for this recovery system. For magnesium-based liquors, multicyclones and venturi scrubbers are used to control recovery-system SOz emissions to levels below about 9 pounds SO2 per ton of pulp.32 Gases from NH3-based liquor combustion are treated in an aqueous ammonia-absorption- heat-recovery tower. Glass-fiber filters for particulate and mist elimination are used before final emission to the atmo- sphere. The SO2 emissions from NH3-based recovery sys- tems amount to less than 7 pounds SO2 per ton of pulp.32 The gases from sodium-based liquor-recovery furnaces are scrubbed in a sodium carbonate scrubber and recovery- system SO2 emissions are < 2 Ib/ton pulp.32
Particulate emissions in the sulfite process result only from SSL combustion. Typical emission factors from recovery systems with additional scrubbing or absorption units for magnesium-, NH,-, and sodium-based liquors are 2 , 0.7, and 4 Ib/ton pulp3* respectively. The lower factor for NH3-based liquors results from the NH3 breaking down
Chemical Wood Pulping 847
to N2 and H 2 0 and becoming nonrecoverable. However, the sulfur content is emitted as SO2 and recycled by absorp- tion in aqueous ammonia to generate ammonium bisulfite solution. This is then used to make cooking liquor. A single set of tests on the uncontrolled exhaust gases from a sulfite- recovery furnace showed that 98% and 78% of the particu- late emissions were less than 10 p m (PM,,) and less than 2.5 p m (PM,.,) in size r e ~ p e c t i v e l y . ~ ~
NEUTRAL SULFITE SEMICHEMICAL PULPING
Semichemical pulping is a process for obtaining high yields of pulps with characteristics suitable for certain end uses. Conventional chemical pulping achieves yields of 50% to 55%, whereas semichemical pulping can achieve from 60% to 80% yields. The yield is defined as the fraction of wood that results in unbleached pulp, both on a dry basis. The lignin is removed only partially during the cook and a second stage involving mechanical disintegration is neces- sary. Various types of semichemical pulps are produced by the acid sulfite, neutral sulfite, kraft, soda, and cold soda pulping processes. The major process difference from con- ventional chemical pulping lies in the use of lower tempera- tures, more dilute cooking liquor or shorter cooking time, and mechanical disintegration. 37
The neutral sulfite semichemical (NSSC) process is the most widely used semichemical pulping process today. In this method, wood chips are cooked in a neutral solution of sodium sulfite and sodium carbonate. Sulfite ions react with the wood lignin while the carbonate buffers the reaction, neutralizing the organic acid formed and maintaining a neutral pH of about 7. After cooking, the contents are discharged into a blow tank and excess liquor is separated by draining, pressing, or washing. Next, the softened chips are reduced to pulp by mechanical treatment in such equip- ment as rod mills or rotating-disk refiners.37 The pulp is then washed in multistage drum filters and the weak liquor separated. Weak black liquor is either disposed of, recov- ered in furnaces, or blended with kraft liquors to “cross- recover” NazS and Na2C03 from the liquor.
Air Emissions Characterization
Because of the milder pulping conditions, NSSC processes are expected to generate lesser amounts of pollutants than kraft or other full chemical pulping processes. Particulate emissions are a potential problem only when recovery fur- naces are involved and are similar to those for kraft recov- ery furnaces. Fluidized-bed reactors are also utilized to burn NSSC spent liquor when a kraft furnace is not available. When recovered in a furnace, the combustion of sulfur containing NSSC liquor can result in TRS and SO2 emis- sions that are somewhat higher than those from kraft black
liquors. Lower NSSC liquor heat content resulting from lesser amounts of organic matter extracted from the wood and higher ratios of sulfur to sodium are believed to be responsible. With proper instrumentation and control of the furnace operation, Galeano et al.38 have shown that NSSC recovery furnaces should be able to operate just as efficient- ly (with respect to air emissions) as comparable kraft recov- ery furnaces.
Preparation of the pulping chemical is carried out in a manner similar to the acid sulfite process. Sulfur dioxide from sulfur burners is absorbed in the green liquor contain- ing Na2C03 and Na2S in scrubbing towers. Significant quantities of H2S are also produced during this process, and they need to be further oxidized to SO2 and scrubbed by Na2C03 or some other base. Potential emission points of SO2 are absorbing towers, digester/blow tank systems, and recovery furnaces. Owing to the great variations in the type of NSSC processes practiced and the paucity of measured emission data, emission factors are not readily available.
References I .
Lockwood Post’s Directory of the Pulp, Paper and Allied Trades, f988, Miller Freeman Publications, San Francisco, 1988. S. A. Rydholm, Pulping Processes, 1st ed., Interscience Publishers, New York, 1965. Atmospheric Emissions from the Pulp and Paper Manufactur- ing Industp-Report of NCASI-EPA Cooperative Study Pro- ject. Technical Bulletin No. 69, National Council of the Paper Industry for Air and Stream Improvement, New York, 1974. H. Modetz and M. Murtiff, Kraft Pulp Industry Particulate Emissions: Source Category Report, EPbJ600/7-87/006, U.S. Environmental Protection Agency, Research Triangle Park. NC, 1987. NCASI Handbook of Chemical Specific Information for SARA Section 313 Form R Reporting, National Council of the Paper Industry for Air and Stream Improvement, New York, April 1990. Emission of Reduced Sulfur Compounds from Kraft Process Brownstock Diffusion Washer Vents. Technical Bulletin No. 406, National Council of the Paper Industry for Air and Stream Improvement, New York, 1983. J . C. Walther and H. R. Amberg, “A positive air quality control program at a new kraft mill,” J Air Pollut. Contr. Assoc. 20( 1):9 (1970). TGNMO Emission Potential from Kraft Process Heavy Black Liquor Oxidizers Operated on Liquors from Western Wood Species, Technical Bulletin No. 37 I , National Council of the Paper Industry for Air and Stream Improvement, New York, 1982. Carbon Monoxide Emissions from Selected Combustion Sources Based on Short-term Monitoring Records, Technical Bulletin No. 416, National Council of the Paper Industry for Air and Stream Improvement, New York, 1984. A Stu& of Nitrogen Oxides Emissions for Lime Kilns. Tech- nical Bulletin No. 107. National Council of the Paper In- dustry for Air and Stream Improvement, New York, 1980.
848 Wood Processing Industry
11. A Study of Nitrogen Oxides Emissions from Large Kraft 29. Fugitive Dust Emission Factors and Control Methods Impor-
Recovery Furnaces, Technical Bulletin No. 1 1 1, National Council of the Paper Industry for Air and Stream Improve- ment, New York, 1981. A Study of Kraj? Recovery Furnace Total Gaseous Non- Methane Organic Emissions, Technical Bulletin No. 1 12, National Council of the Paper Industry for Air and Stream Improvement, New York, 198 I . Review of New Source Performance Standards for Kraji Pulp Mills, EPA-45013-83-017, U.S . Environmental Protection Agency, Research Triangle Park, NC, 1983. K. E. McCance and H. G. Burke, “Contaminated condensate stripping-an industry survey ,” Pulp Paper Canada 81(11):78 (1980). Factors Affecting Emission of Odorous Reduced Sulfur Com- pounds from Miscellaneous Kraft Process Sources, Technical Bulletin No. 60, National Council of the Paper Industry for Air and Stream Improvement, New York, 1972. Summary of Long-term CEMS SO2 Data and Review of Fac- tors Aflecting Sulfur Dioxide Emissions from Kraft Recovery Furnaces, Technical Bulletin No. 604, National Council of the Paper Industry for Air and Stream Improvement, New York, 1991. A Study of Kraji Process Lime Kiln Total Gaseous Non- Methane Organic Emissions, Technical Bulletin No. 358, National Council of the Paper Industry for Air and Stream Improvement, New York, 1981. J. S. Henderson and J. E. Roberson, “1971 precipitator survey,” TAPPI J., 56(4):91 (1973). J. S. Henderson, “Final survey results for direct contact evaporator recovery boiler electrostatic precipitators ,” TAPPI J . , 65(3):91 (1982). J. S. Henderson, “Precipitator survey on non-contact recov- ery boilers,” TAPPI J., 58(5):86 (1975). J . S. Henderson, “Final survey results for non-contact recov- ery boiler electrostatic precipitators,” TAPPI J . , 63( 12):71 (1980). Kraft Pulping-Control of TRS Emissions from Existing Mills, EPA-45012-78-003b. U.S. Environmental Protection Agency, Research Triangle Park, 1979. Standards of Performance for New Stationary Sources: Kraft Pulp Mills, U .S . Environmental Protection Agency [Federal Register 51 185441, May 20, 1986. Data File Information, National Council of the Paper Industry for Air and Stream Improvement, New York, 1990. National Dioxin Study Tier 44ombust ion Sources, Project Summary Report, EPA-45014-84-0 14h, 1987. BACT-LAER Clearinghouse-A Compilation of Control Technology Determinations (4th Supplement to I985 Edi- tion), U.S . Environmental Protection Agency, Office of Air Quality Planning and Standards, June 1989. Collection and Burning of Kraft Non-Condensible Gases- Current Practices, Operating Experience and Important Aspects of Design and Operation, Technical Bulletin No. 469. National Council of the Paper Industry for Air and Stream Improvement, New York, 1985. A Review of Preconstruction Air Quality Permits Issued for Pulp Mill Emission Sources, Special Report No. 90-03, National Council of the Paper Industry for Air and Stream Improvement, New York, May 1990.
tant to Forest Products Industry Manufacturing Operations, Technical Bulletin No. 424, National Council of the Paper Industry for Air and Stream Improvement, New York, March 1984.
30. A. K. Jain and V. J. Dallons, “Control of chlorine and chlorine dioxide emissions from bleach plants,” Proceedings of the 1989 TAPPI Environmental Conference, TAPPI, 1989.
31. Results of Field Measurements of Chloroform Formation and Release from Pulp Bleaching, Technical Bulletin No. 558. National Council of the Paper Industry for Air and Stream Improvement, New York, 1988.
32. Compilation of Air Pollutant Emission Factors, Volume I: Stationary Point and Area Sources (AP-42), U . S . Environ- mental Protection Agency, Research Triangle Park, NC, 1986, Chapter 10.1.
33. H. Edde, Environmental Control for Pulp and Paper Mills. Pollution Technology Review No. 108, Noyes Publication, Park Ridge, NJ, 1984, pp 148-156.
34. A. L. Caron, “Practices used by the sulphite pulping industry in the handling and treatment of sulfur dioxide from mis- cellaneous sources,” Proceedings of the I976 NCASI Cen- tral-Lake States Regional Meeting, Special Report No. 77- 02, National Council of the Paper Industry for Air and Stream Improvement, New York, 1977.
35. Environmental Pollution Control. Pulp and Paper Industry, Part I: Air, EPA-62517-76-001, U . S . Environmental Protec- tion Agency, Washington, DC, October 1976.
36. Air Emissions Species Manual, Vol. II: Particulate Matter Species Profiles, U . S . Environmental Protection Agency, Research Triangle Park, NC, 1988, p 174.
37. M. Benjamin, I. B. Douglas, G. A. Hansen, et al., “A general description of commercial wood pulping and bleach- ing processes,” J. Air Pollut. Conrr. Assoc. 19(3):155-161 ( 1969).
38. S. F. Galeano, D. C. Kahn, and R. A. Mack, “Air pollution: Controlled operation of a NSSC recovery furnace,” TAPPI J., 54(5):741 (1971).
pp 507-5 12.
MECHANICAL PULPING Arun V. Someshwar and John E. Pinkerton
Mechanical pulping, as the name implies, relies mainly on mechanical energy to convert wood to pulp. Mechanical pulping dates back to 1840, and the invention of the pulp- wood grinder‘ and the stone-ground wood (SGW) process. In addition to the SGW process, current mechanical pulp manufacturing processes comprise several high-energy refining systems for the production of pulp from chips. These include the refiner mechanical pulping (RMP) pro- cess, the thermomechanical pulping (TMP) process, the chemimechanical pulping (CMP) process, and the chemithermomechanical pulping (CTMP) process.
In the RMP process, chips are refined directly at atmo- spheric pressure. Chemicals are sometimes added at the various stages of refining. In the TMP process, the chips are usually steamed under a pressure of 20-40 psi for two to four minutes prior to refining. In some modifications of this process, the refiners are operated under pressure. In the mid-l970s, chemically modified versions of RMP and TMP were introduced. Up until the present, the chemical treat- ment used has been almost exclusively sulfonation. ’ This may be carried out in several ways, including the treatment of wood chips prior to refining, the treatment of coarse pulp between refining stages, the treatment of completely refined pulp, and the treatment of long fiber. Chips are usually cooked (softened) in a sodium sulfite solution (pH between 4 and 9) for about 30 minutes at temperatures between 270°F and 320°F. In the CTMP process, the chips are treated with chemicals for softening and refined under pressure. The usual level of addition of sodium sulfite is kept between 1% and 4% Na2S03 on a bone-dry pulp basis.
The CMP and CTMP processes reduce power consump- tion at the refiners. The RMP and TMP processes are known for their high consumption of electric energy. Pulp yields of 95% or higher are common with the TMP and RMP processes, whereas the CMP and CTMP processes exhibit yields of about 90%.
AIR EMISSIONS CHARACTERIZATION ANDCONTROLMEASURES
Large quantities of steam are generated from mechanical and chemimechanical pulping processes. Moisture emission rates are reported to range from 4000 to 7500 lb/ton of pulp for systems not equipped with heat r e ~ o v e r y . ~ As wood contains a fair amount of organic material, volatile organic compounds (VOCs) are likely to be released along with the steam during the cooking and refining process.
Heat recovery from steam emissions is extensively prac- ticed. A study3 by the National Council of the Paper In- dustry for Air and Steam Improvement on estimating VOC emissions from TMP processes showed the following: (1) Emissions of VOCs from the TMP process operated on western white wood species ranged from 1.09 to 1.73 and averaged I .4 pounds of carbon per ton pulp; from west- em pine species, it ranged from 0.83 to 3.84 and averaged 1.9 pounds of carbon per ton; and from southem pine species, it ranged from 2.2 to 7.6 pounds of carbon per ton. (2) Emission rates for VOCs were proportional to moisture emission rates, indicating that those heat-recovery systems that drop the exhaust gas temperature well below the boiling point of water would also be expected to reduce VOC emissions by an unknown amount. No emissions other than VOCs are expected from mechanical pulping processes.
Paper and Paperboard Manufacture 849
References D. Atack, “Mechanical pulping-some highlights of the second 75 years,” Pulp Paper Canada, 9O:lO (1989). D. M. Mackie and J. S. Taylor, “Review of the production and properties of alphabet pulps,” Pulp Paper Canada, 89(2):58 1988). TGNMO Emissions from the Therm&”hanical Pulping Pro- cess, Technical Bulletin No. 410, .National Counci! of the Paper Industry for Air and Stream Improvement, New York, 1983.
PAPER AND PAPERBOARD MANUFACTURE
Arun V. Someshwar and John E. Pinkerton
Pulping operations using chemical or mechanical processes were discussed in earlier sections of this chapter. Following the washing operation (and bleaching, if performed), the pulp or stock is pumped to high-densiry storage tanks. From the pulp slurry, thin sheets (paper) or mats (paperboard) are manufactured in the paper mill area on a paper machine (see Figure 1). Individual pulp mills may or may not have paper machines, and mills with stand-alone operations may pur- chase baled pulp for paper and paperboard making. To obtain paper or board sheet from a pulp suspension, three operations are performed: stock preparation, sheet forma- tion, and drying. Further operations, finishing and convert- ing, yield a final product. Converting is often carried out in stand-alone plants located in the consuming districts.
Stock preparation involves mixing of various types of pulps and additives and beating of the pulp fibers. Pulps that are
FIGURE 1. A Typical Paper Machine
850 Wood Processing Industry
high density blending storage tank chest head
box press wire section , \
I-J 1 ” - h//
1 PULP FROM WASHERS
wQ white water pit
FIGURE 2. Typical Layout of a Paper Mill
derived from virgin wood are termed “virgin” pulp. In recent times, there has been a large increase in the use of “secondary” fiber, derived from recycling paper, for pulp and paper products. Post- or pre-consumer recycled paper is commonly grouped into five categories: deinked, mixed paper, pulp substitutes, newspaper, and corrugated.’ De- pending on the category, one or more of the following processes-asphalt dispersion, cleaning, deinking , pulping, and screening-are used for pulp production. Pulping chemicals used in recycled paper deinking may include caustic as a difibering agent, sodium silicate as a stabilizer, other deinking chemicals as dispersants and collectors, calcium chloride for water conditioning, and hydrogen peroxide for bleaching and preventing yellowing of groundwood. ‘ Process chemicals include defoamers and acid for pH adjustment. Bleaching can range from minimal (involving H202 during pulping and hydrosulfite at end of the deinking process) to extensive (such as a chlorination- hypochlorite or chlorination-extraction-hypochlorite se- quence). ‘
Figure 2 shows a typical layout of a paper and paper- board mill. Stock from the high-density storage tanks is mixed in blending chests to control consistency and the mixed pulp is sent to a system of “beaters” or “refiners,” which further treat the fibers to obtain the desired finished- product characteristics, such as paper strength. After stock preparation, the stock is pumped to the head boxes of the paper machine prior to which it is diluted to about 0.5% consistency. From the head boxes, the dilute stock is uni- formly distributed across the “wire” of the paper machine. About 98% of the water is removed by gravity and vacuum while the stock is on the wire. The newly formed sheet, still very soft and wet, then passes through the press section of the paper machine, where the sheet is smoothed and addi- tional moisture is removed. The sheet then enters the “dryer” section, where steam-heated drying cylinders are
DRYING calendar stack
dryer section \
PAPER OR PAPERBOARD ROLLS d
used to evaporate the rest of the moisture in the sheet. After drying, many printing grades of paper and paperboard are surface coated with an aqueous suspension of pigments (such as clay) in adhesives (such as starch) in order to improve surface properties. After coating or sizing, the solvent, usually water, is removed from the coating by evaporative drying, first by air impingement or radiant heating and then over steam-heated drums. Finally, the coated sheet is “ironed” in the calendar stack and then wound in large rolls, ready for shipment.
AIR EMISSIONS CHARACTERIZATION
Emissions from the paper machine consist mainly of water vapor; little or no particulate matter is emitted from the dryers.* Many papers and paperboards contain noncellu- losic additives that improve the use properties of the final product. Additives may be put into the paper stock either before the sheet is formed or later at the size press, the calendar stack, or in a subsequent converting ~pe ra t ion .~ The beater or wet-end method of incorporating additives is generally preferred. Thus only small fractions of these addi- tives are expected to be retained in the sheet before they enter the dryer section. Additionally, because of their high boiling points, very small quantities are likely to volatilize and result in air emissions. Chemicals that may volatilize during the making of paper or paperboard include formal- dehyde and phenol present in resins; ammonia added as a coating or for pH adjustment; chemicals such as hexane, xylene, petroleum naphtha, and toluene that may be used periodically in cleaning operations; and minute quantities of the free, unreacted monomeric constituents of the various additives. Because of the variety of chemicals used and conditions of usage, paper machine vent emission estimates have not been developed. Ammonia and formaldehyde are two compounds that have been identified as being present in
paper machine vents where ammonia- or formaledhyde- based additives are used in the papermaking process. Es- timates of air emissions during the recovery of secondary fiber are not available. However, these are expected to be minimal.
AIR POLLUTION CONTROL MEASURES
Control techniques for paper machine vents are considered impractical because of the high moisture content and high volume of the vent exhaust gases and the minimal pollutant concentrations. Waste minimization techniques, such as reduced chemical usage and modifications in the mode and
Paper and Paperboard Manufacture 851
location of chemical addition, may be considered on a site-specific basis where reductions are desirable.
References 1. L. A. Broeren; “New technology, economic benefits give
boost to secondary fiber use,” Pulp Paper, 69 (November 1989).
2. Compilation of Air Pollutant Emission Factors, Volume I : Stationary Point and Area Sources (AP42) , U.S. Environ- mental Protection Agency, Research Triangle Park, NC, 1986, Chapter 10.2.
3 . Pulp and Paper Science and Technology, Vol II. Paper; C . E. Libby, Ed.; McGraw-Hill Book Co., New York, 1962, p 113.
Air Pollution Engineering Manual
- - -- - I -
AIR & WASTE MANAGEMENT A S S O C I A T I O N
Edited by Anthony J. Buonicore
Wayne T. Davis
VAN NOSTRAND REINHOLD New York
This project has been funded in part by the United States Environmental Protection Agency under assistance agreement T901763 to the Air and Waste Management Association. The contents of this document do not necessarily reflect the views and policies of the Environmental Protection Agency, nor does mention of trade names or commercial products constitute endorsement or recommendation for use.
Copyright 8 1992 by Van Nostrand Reinhold
Library of Congress Catalog Card Number 91-46007 ISBN 0-442-00843-0
All rights reserved. No part of this work covered by the copyright hereon may be reproduced or used in any form or by any means-graphic, electronic, or mechanical, including photocopying, recording, taping, or information storage and retrieval systems-without written permission of the publisher.
Manufactured in the United States of America
Published by Van Nostrand Reinhold 115 Fifth Avenue New York, New York 10003
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16 IS 14 13 12 11 1 0 9 8 7 6 5 4 3 2 I
Library of Congress Cataloging-in-Publication Data Air pollution engineering manual I Air & Waste Management Association
; edited by Anthony J. Buonicore, Wayne Davis.
Includes bibliographical references and index.
I . Air-Pollution-Equipment and supplies. 2. Gases, Asphyxiating
and poisonous-Environmental aspects. 3. Particles-Environmental aspects. I. Buonicore, Anthony J. 11. Davis, Wayne T. 111. Air & Waste Management Association. TD889.A39 1992
628.5'3-dcZO 9 1-46007 CIP