Biermann Ch. Handbook of Pulping and Papermaking

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284 11. ENVIRONMENTAL IMPACT

with certainty an exact level of a chemical which

will cause one cancer in a million people exposed

to this chemical over a lifetime at low levels is not

being reasonable. Science is simply not that

precise by any means. For example, if

two

people

each smoke 40 cigarettes per day for 40 years,

about one million cigarettes are smoked. We

might expect one of these people to develop

cancer. In other words about one million ciga-

rettes might induce cancer in a person. Does that

mean a person who smokes a single cigarette in

his life has a one in a million chance of getting

cancer from it? We will never know until 25

million sets of identical twins are willing to subject

themselves to a life-long experiment where every

detail of their lives are controlled. However, it is

known that the risk of cancer in people who quit

smoking decreases significantly and after many

years approaches the risk level of people who have

never smoked, compared to their colleagues who

continue to smoke. This indicates that a certain

levels of toxins are required to induce cancer.

In the case of a compound like dioxin which

was found to be highly toxic in laboratory studies,

only later were studies done on humans who have

been exposed to it through industrial exposure

over many years, accidents, etc. Even these

studies are often done on small sample sizes by

people overly eager to advance their careers by

being able to report something "controversial".

By the time responsible, long term studies of

reasonably large populations are finished, the

damage has long been done, such as in the overly

high estimated toxicity of dioxin.

Third, some chemicals behave much differ-

ently in rats than in humans. The chemistry of

cancer is much more complex than the chemistry

of cyanide ion, which binds to hemoglobin and

prevents oxygen exchange by the blood where one

can expect the same result in a rat as in a human

as in any other animal with hemoglobin. But that

is certainly not true with long term carcinogens,

especially at very low rates of cancer mortality.

Fourth, the fact is that people in the medical

community tend to be aggressive; that is a factor

in the selection factor. Many studies get published

with statistical analysis, but statistically insignifi-

cant results. Let me explain. If one correlates 20

different would-be "dependent" variables to one

independent variable and finds there is a 95%

chance that one of

these

dependent variables corre-

lates to the independent variable, does this mean

anything? The answer is probably not since there

is a 5% chance that the correlation is just due to

chance. Since you started with 20 variables the

chances are that one of these 20 (which is 5%) is

bound to show a correlation just by chance at the

95%

confidence level. Stated another way, one

out of 20 researchers can expect to observe a

correlation at the 95% confidence level that is

merely due to chance for each correlation tested.

And these are the studies that use statistical analy-

sis!

Mark Twain is quoted as saying "there are

lies,

damn lies, and statistics!"

In the news media, which should not be

construed as scientifically (or otherwise) meaning-

ful,

one week coffee (caffeine) increases risk of

heart attach and cancer, and one week it does not

increase your risk. Along these lines, an article in

the New England Journal of Medicine (Fingerhut,

1991) indicates that earlier studies indicating the

toxicity of dioxin overstated the problem due to

small sample sizes and other reasons.

The EPA level of permissible dioxin emission

is 13 parts per quintillion in 1991; yet it is unde-

tectable below 10 parts per quadrillion (10,000

parts per quintillion). Do not shrug off these

incredibly small numbers without thinking about

them. One part in a quadrillion is only this much:

0.000000000000001! An olympic-sized pool

might have on the order of 250,000 gallons of

water or 1 billion grams. The level of arsenic in

drinking water is on the order of 0.01 parts per

million corresponding to 10 grams in the pool.

The measurable total amount of dioxin in the pool

corresponds to 0.00001 g; the proposed regulation

would be 0.000000013 g, 0.000013 mg, or 0.013

lig in the entire pool. (It would take more than 10

lifetimes for a person to drink the water in this

pool.) The area of low-level, long-term toxicity is

all very new ground. It is difficult to know what

rules,

regulations, and technologies will come out

of all of this. Certainly, however, it must be

handled in a more rational manner in the future.

11.2 WATER POLLUTION

It is useful to trace some of the important

U.S.

Federal legislation in the area of water. The

WATER POLLUTION 285

first federal law was the Water Pollution Control

Act of

1948,

which dealt with conventional forms

of water pollution (BOD, TSS, and pH), such as

municipal wastes. The Water Pollution Control

Act

Amendments

of 1956 and 1965 funded munic-

ipalities to build sewage treatment plants. The

Federal Water Pollution Control Act of 1972 in-

creased grants to municipalities for sewage treat-

ment plants, established the

Environmental

Protec-

tion Agency (EPA) as the organization to approve

discharge permits, and set deadlines for pollution

control. The Clean Water Act of 1977 clarified

conventional versus toxic water pollutants and

postponed some of the deadlines of the 1972

legislation. The Water

Quality

Act of 1987 (or the

Clean Water Act) continued federal grants to

municipalities, established a program for toxic

pollutants, postponed some deadlines, and set new

requirements for non-point sources such as storm

water runoff for industrial and urban sites.

Undesirable characteristics of pulp and paper

mill effluents include:

The presence of a food source for microor-

ganisms which would deplete oxygen in

rivers if not at least partially removed

(BOD).

Color discharges which detract from the

appearance of a river, lake, etc.

Suspended solids which make water appear

murky.

Other materials such as fillers, acids, bases,

and sludges, which may be sent to the sewer

during mill upsets.

The severity of the problem depends on the type

of mill. Mills producing bleached kraft pulp dis-

charge over 30,000 gallons per ton of pulp. The

digester and evaporator condensates of kraft mills

have BOD producing compounds that cause water

pollution and odorous compounds that cause air

pollution. Mechanical pulp mills discharge about

5,000

to 7,000 gallons of water per ton of pulp.

In TMP mills the principal pollutants are derived

from wood extractives including resin acids.

Resin acids may be as high as 50-200 mg/L in

untreated effluent, but may be decreased to 0.5

mg/L with a high level of secondary treatment. In

the U.S., mill effluents are typically diluted at

least 1:100 into the receiving body of water.

Because of

the

importance of oxygen in water

in the environment, the solubility of oxygen in

water over air (0.2098 atm Oj/atm pressure) with

and without the effect of water vapor displacement

is shown in Fig. 11-1; for example, the vapor

pressure of oxygen at atmospheric pressure in the

atmosphere over water at 100**C (212°F) is 0

since the atmosphere is all steam.

11.3 WATER QUALITY TESTS

Biological oxygen demand, BOD

BOD is the amount of oxygen necessary for

bacteria to consume the organic material in water.

Organic material that is discharged into natural

waters causes a rapid increase in the growth of

microorganisms that deplete the oxygen required

for other aquatic life. BOD is determined in the

laboratory by measuring the oxygen depletion of a

diluted aqueous sample incubated with microorgan-

isms.

Oxygen is measured with a probe much like

when determining a pH. BOD5 is the BOD deter-

mined after 5 days incubation.

Regulatory agencies in North America de-

crease the allowable levels of BOD discharge in

summer months for most mills. There are two

important reasons for this. First, at this time of

year, the water is warmer than other times of the

year and the oxygen solubility is decreased. Sec-

ond, the amount of water in most rivers and lakes

is much lower in summer than other times of the

year.

Chemical

oxygen demand, COD

Chemical oxygen demand is a measure of the

components that can be oxidized in a chemical

reaction. The amount of sodium permanganate or

sodium dichromate as a chemical oxidant indicates

the oxygen demand. This procedure can be done

quickly and gives an estimate of the BOD, but,

because it does not involve a biological system, it

is only loosely correlated to BOD5.

Total suspended

solids,

TSS

Total suspended solids are all of

the

materials

in water that are not soluble. They contribute to

the turbidity (cloudiness or murkiness) of water

according to their amount and size distribution.

286 11. ENVIRONMENTAL IMPACT

16n

1/1-

10-

^ IH-

/1-

'•^

,, 'v>s

\;\^^

•^

Water vapor

—i—

'^"\

displacing au r

20 30 70 80

90 100

40 50 60

Temperature, Celsius

Fig. 11-1. The solubility of

in water. The upper curve is a constant partial pressure of 0.2098

atm.

The lower curve has partial pressure at 0.2098 atm minus oxygen displaced by water vapor.

Total organically bound

chlorine,

TOCl

TOCl is a measure of chlorine in organic

material (chlorinated organic compounds), which

are considered to be toxic pollutants, although the

toxicity varies tremendously among the millions of

possible compounds. TOCl can be decreased by

improving delignification during pulping, proper

washing of

the

pulp before bleaching, using substi-

tutes for chlorine, avoiding over-chlorination (by

efficient mixing of chlorine added to pulp or using

several smaller additions of chlorine to avoid

localized high chlorine concentrations), or effec-

tively removing TOCl from effluents. For exam-

ple,

TOCl decreases linearly with chlorine dioxide

substitution of chlorine in bleaching. Dioxin pro-

duction decreases if chlorine is added in small

amounts during bleaching; this effect is probably

a result of decreasing over-chlorination.

Color units

Color in water is a measure of the absorbed

light, by definition, at 465 nm wavelength. The

color of

waste

water

measured using color units;

500 color units corresponds to 0.14 absorbance

(70%

transmittance. Section 14.6 explains this in

more detail) at 465 nm. Allowable color dis-

charge (of nontoxic materials) should take into

account local geography. In some areas many of

the rivers tend to be muddy to begin with, so color

discharge should not be as much of a concern as

in other areas where water is very clear.

Dioxins

Dioxins deserve special mention only because

of the notoriety they have received in the media.

This is an example of the media creating the news

in terms dioxin and the pulp and paper industry.

WATER QUALITY TESTS 287

Fig. 11-2. The structure of the dioxm 2,3,7,8-

TCDD.

_ CK

Fig. 11-3. Fonnation of 2,3,7,8-TCDD from

two molecules of trichlorophenol.

Dioxins are chlorinated derivatives of

dibenzo-;?-dioxin. Indeed, there are 75 different

dioxins with

one

to eight chlorine molecules differ-

ing in toxicity and other properties. By far, the

most toxic dioxin is 2,3,7,8-tetrachlorodibenzo-/>-

dioxin (2,3,7,8-TCDD); it is this compound that is

called dioxin, although the term is misleading as

dioxins are a family of related compounds. Fig.

11-2 shows the structure of 2,3,7,8-TCDD.

Dioxins occur naturally and as by-products of

man's activities. They are not made deliberately.

The production of trichlorophenol and 2,4,5 T

involves side reactions leading to undesired con-

taminants. For example, trichlorophenol is the

precursor for the production of

the

herbicide 2,4,5

T (2,4,5-triclilorophenoxyacetic acid). (This

herbicide was used in the defoliant Agent Orange

by the U.S. during the Vietnam war and was

implicated as the cause of symptoms described by

veterans.) Trichlorophenol is reacted under

alkaline conditions with monochloroacetic acid to

make this herbicide. Unfortunately this allows the

side reaction (Fig. 11-3) to occur, making signifi-

cant amounts of 2,3,7,8-TCDD as an impurity (on

the order of one to several parts per million).

This has resulted in a decrease in the use of 2,4,5-

T and 2,4,5-trichlorophenol in the U.S. Incinera-

tion of municipal wastes is one of the largest

human-made sources of dioxins, but this tends to

be octachlorodioxin, which is millions of times

less toxic than 2,3,7,8-TCDD. Forest fires are a

large source of natural dioxins.

In the pulp and paper industry most dioxins

are formed by the action of chlorine during

bleaching on lignin. Processes not using elemental

chlorine do not produce measurable levels of

dioxin. Certain impurities associated with addi-

tives such as defoamers have also been implicated

as dioxin precursors. When identified, these

materials are removed from the process. Many

mills have substituted part of the chlorine with

chlorine dioxide to reduce dioxin formation.

Cooking to lower kappa numbers, using oxygen

delignification, and other techniques have further

decreased the amount of dioxins produced at

bleached pulp mills. There is much evidence that

the precursors to formation of dioxin and chlori-

nated dibenzofurans in chlorine bleaching are the

unchlorinated compounds dibenzo-/7-dioxin and

dibenzofiiran, which means these two compounds

may be present in the lignin before chlorine

bleaching or formed during bleaching.

Presently dioxin can be detected in water at

the level of 13 parts per quadrillion (13 parts in

10^^).

This is no small feat and each sample costs

about $1000 to process. The ability to measure

such small amounts of materials is a tribute to the

highly selective and sensitive instrumentation of

modem physics. Yet many U.S. states have im-

posed regulations requiring diluted mill effluents to

be below 13 parts per quintillion (13 parts in 10^*)

of dioxin.

11.4 AQUEOUS EFFLUENT TREATMENTS

Pretreatment

Water is pretreated by screening with coarse

filters, grit removal, and pH adjustment. The

adjustment of pH is made by metering effluents

from various parts of the mill together. The acid

line from the chlorination stage raises the pH of

other lines that tend to be alkaline. Washed grits

fi-om kraft liquor recovery may be ground and

used to raise the pH.

Primary treatment, primary clarifiers

In primary treatment, solids are removed

from water by allowing them to settle from the

288

11.

ENVIRONMENTAL IMPACT

water. Primary clarifiers came into widespread

use in the pulp and paper industry in the late

1950s in the U.S. A relatively coarse screen may

be used to remove large materials that might

otherwise clog pumps and other equipment. The

most commonly used device is the mechanical

clarifier. This is a circular, concrete tank with a

rotating sludge scraper mechanism which rakes the

sludge to the center sump of the tapered bottom.

The clarified waste water is removed at the outer

edge of

the

tank. Floating debris may be collected

by a surface skimmer. The sludge is collected

and dewatered. It is then disposed of in a landfill.

Occasionally the sludge is incinerated or spread on

agricultural fields. Some mills use settling ponds

for primary clarification. The ponds must be

dredged periodically (perhaps once each year).

A second method of solids removal employs

air flotation where fine air bubbles attach them-

selves to suspended solids and cause them to float.

The floating layer is then skinmied. The method

is very effective; however, the method is also

expensive. This method is used to reuse effluents

from the processing of recycled fiber with

deinking operations and was discussed in Chapter

10.

In most cases effluents then undergo a sec-

ondary treatment process.

Secondary treatment, biological treatment

Secondary treatment came into widespread

use by the U.S. pulp and paper industry during the

1960s. Secondary treatment involves reaction of

the effluent (after primary treatment) with oxygen

and microorganisms (bacteria and

fimgi)

to remove

oxygen-consuming materials. The effluent is

prepared by pH balancing and addition of nutri-

ents.

In principle, microorganisms use the food

source before the effluent is discharged; otherwise

the rapid growth of microorganisms in the body of

water in which the effluent is discharged leads to

rapid oxygen depletion and the death of other

aquatic life. The BOD is typically reduced by 80-

90%

during secondary treatment. The "toxicity"

of the effluent is also decreased considerably,

although this is much more subjective and difficult

to define let alone measure. Secondary treatment

is usually accomplished by oxidation basins,

aerated stabilization

basins,

or the activated sludge

process. The words basin,

pond,

and lagoon are

used to describe a body of water which is usually

three to five feet deep. This is a pond constructed

so sunlight, microorganisms, and oxygen interact

to break down the organic materials.

The oxidation basin is merely a treatment

pond that relies on natural aeration from the atmo-

sphere to supply oxygen. This is suited to mills

with low production, processes producing relative-

ly low amounts of BOD, and large areas for

treatment ponds. Long retention times are re-

quired.

Aerated stabilization basins, ASB, are the

most common method used (Plate 40). These are

similar to oxidation basins except that oxygen

transfer into the water is improved by use of

aerators. This greatly improves the efficiency of

the system. Retention times for treatment are

typically 6-14 days. Periodic dredging (on the

order of once in several years) is required to

remove the sludge that tends to build.

For maximum growth of the organisms and

BOD reduction, the nitrogen to phosphorus ratio

should be about 5:1 in the nutrients added prior to

treatment (assuming neither of these are present

already), and the ammonia level should be at least

0.5 ppm in the final effluent discharge. In some

mills energy reductions have led to decreased

effluent temperatures that have decreased microor-

ganism growth, especially in winter. Increasing

the retention time of the effluent may help this

condition. It is always important to make sure the

ASB is not "short circuited" so that some parts of

the basin are not being fiilly used and the retention

time is not what one would calculate based on the

size of the basin. High levels of dissolved oxygen

and nutrients in some parts of the basin are indica-

tors that this may be the case. The use of baffles

or several basins in series are useful to prevent

this condition. Proper operation of the ASB will

mean fewer expensive dredgings will be required.

In the activated sludge process (Fig. 11-4),

decomposing bacteria are purposely added along

with agitation to dissolve oxygen. The microor-

ganisms are then removed in a secondary clarifier.

Some of this supplies the source of additional

bacteria. The remaining sludge represents a con-

siderable disposal problem and is either landfiUed

or sometimes dried and burned. This method is

relatively expensive and generally used where

AQUEOUS EFFLUENT TREATMENT 289

space is limited or where BOD5 removal above

90%

is required. This method works best with a

constant feed source of fairly highly concentrated

BOD.

Retention times of 6-8 hours are typical for

secondary fiber effluent.

Rotating biological filters or rotary disks are

occasionally used for areas with space limitations.

The partially submerged disks provide surfaces for

the microorganisms to prosper with a high surface

area for oxygen exchange. These systems are

very efficient but require high capital investment

and operating costs. They look a bit like large

disk filters that are used to thicken or recover

stock.

Tertiary treatments

Tertiary treatments are advanced treatments

following secondary treatment. They are often

considered for color removal, since primary and

secondary treatments are not particularly effective

in this area. Secondary treatments may remove on

the order of

50%

of the color. Much research has

been done on decoloring mill effluents, but very

little is done conmiercially. In bleached kraft

mills,

most of the color comes from the first alkali

extraction and the chlorination stages. A few

mills decolorize these streams specifically by

ultrafiltration or sedimentation with polymers.

Most mills do not practice decolorization. Oxygen

delignification offers the potential for reducing

color emission by allowing the first bleaching

stream to be used in the brown stock washers so

it ultimately goes to the recovery boiler (if a mill

is not recovery limited).

Ultrafiltration uses a semipermeable mem-

brane and reverse osmosis. The effluent is forced

through the membrane under high pressure while

large molecules remain

behind.

Flocculation with

alum removes much material but creates a large

sludge-disposal problem. Massive lime addition is

similar to flocculation with alum. Polymers can

be used for flocculation but are very expensive to

use.

Carbon adsorption can be used for decolor-

ization, but it is also very expensive.

Fig. 11-4. An activated sludge process with retention time of 48 hours. One of the sludge presses

is shown to the right and the clarifier is in the middle.

290

11.

ENVIRONMENTAL IMPACT

Fate of paper machine waste water

Water from the paper machine must be recy-

cled to recover fiber, to reduce treatment of fresh

water, and to reduce the pollution load. Much of

the fiber is recovered in disk filter save alls such

as that shown in Fig. 10-17. This also cleans the

water for reuse. Thus it may take 30,000 gallons

of water to make a ton of paper, but most of this

water is recycled water. The consumption of

water may be as low as 1000 gallons per ton of

paper.

11.5 AIR POLLUTION

Materials contributing to air pollution include

particulate matter from the boilers, lime kiln, and

smelt dissolving tank; gaseous combustion pollut-

ants such as carbon monoxide, nitrogen oxides,

and volatile organics from power boilers, recovery

furnaces, and the lime kiln; odor consisting of

reduced sulfur compounds in the kraft process

arising from the digesters, evaporators, recovery

boiler, smelt dissolving tank, and lune kiln; sulfur

dioxide firom the recovery furnace and boilers

using fiiel containing sulfur; and volatile organic

chemicals from miscellaneous sources. In the

sulfite process the primary pollutants are organics,

oxides of sulfur, and particulates.

11.6 AIR QUALITY TESTS AND CONTROL

Total reduced

sulfur,

TRS

Reduced sulfur compounds include hydrogen

sulfide (HjS), the mercaptans, including methane-

thiol (CH3SH, alias methyl mercaptan or methyl

sulfhydrate), methyl sulfide (CH3SCH3, alias

dimethylsulfide), dunethyl disulfide (CH3SSCH3),

and other compounds. They are formed during

kraft pulping (Fig. 11-5) by reaction of sulfides

with methoxy groups of lignin via nucleophilic

substitution reactions.

At low concentrations, TRS is more of a

nuisance than a serious health hazard. Hydrogen

sulfide and organic sulfides are detectable at a

concentration of a few parts per billion by the

olfactory sense, so air pollution is always a con-

cern. Reduced sulfides originate from the black

liquor of the kraft process. Sources of odor

pollution from the kraft process in decreasing

0.0-.-

O-CH,

RSCH,

OR ~ OR

Fig. 11-5. The formation of mercaptans and

organic sulfides.

order of importance are 1) black liquor combus-

tion, 2) black liquor evaporation, and 3) the diges-

tion process. The EPA limits emission for NSPS

lime kilns to below 8 ppm in new equipment.

High emission levels in these new systems have

been traced by NCASI to 1) high soluble sulfides

in particulate control scrubber solutions, 2) low

fresh water addition rates at the precoat filter, and

3) changes in combustion conditions due to firing

of noncondensible gases in the lime kiln. About

0.1-0.4 kg of TRS is emitted per dry ton of pulp

(0.2-0.8 lb/ton) at 5 ppm in the recovery boiler

flue

gases.

The total TRS production has gone

from 3 kg/t (6.5 lb/ton) pulp in 1960 to less than

0.5 kg/t (1 lb/ton) in the early 1990s.

Oxidation of most of the sulfides before

liquor concentration, weak liquor oxidation, was

first practiced in 1956 in the Soviet Union. Intro-

duction of oxygen at 7.5 g/L black liquor at 70°C

(158°F) with a retention time of about one minute

converts most of the sulfides to nonvolatile thio-

sulfates. The concentrations of the mercaptans

and methyl sulfides are decreased as well. Partial

oxidation is required with direct contact evapora-

tors but not with concentrators. As the solids

content increases, the vapor pressure of the sul-

fides increases. The pH of the black liquor is

another parameter which determines how much

sulfide will be liberated in the atmosphere. A pH

of 12 or higher decreases hydrogen sulfide and

methyl mercaptan emission by keeping these

compounds in their salt forms, decreases lignin

precipitation as a cause of evaporator plugging,

and decreases scaling. This pH is maintained in

indirect contact evaporation, but not in direct

contact evaporators. In the case of direct contact

evaporation, residual NaOH of the black liquor

reacts with CO2 to produce Na2C03; this lowers

the pH by several units. Black liquor oxidation

does cause heating of

the

black liquor and increas-

QUALITY TESTS

AND

CONTROL

291

Table 11-2. The effect of black liquor oxidation on its chemical composition (EPA, 1976).

Location

1 Reactor inlet |

1 Reactor outlet |

Storage outlet

Concentration, (g/L)

Na^S

4.56

0.45

0.19

NajSjOj

0.80

4.35

4.61

Na2S04

0.53

0.70

0.63

Total

5.89

5.50

5.43

Account, %

100.0

93.7

92.8

es the temperature up to 10°C. A representative

change in Uquor composition is shown in Table

11-2.

The ratio of sulfide converted to thiosulfate

depends on the oxidation method, the purity of the

oxygen, and the amount of oxygen used.

Final odor treatment of gases with alkaline

scrubbing liquids can remove compounds like HjS,

HSCH3,

SO2 (which forms H2SO3), and sulfur

trioxide (which forms H2SO4) that have ionizable

protons. For example HSCH3 + NaOH forms

NaSCHs, which is a nonvolatile saU. Other

organic sulfur compounds such as (CH3S)2 or

(0113)28 cannot be removed by alkali scrubbing as

they do not have ionizable protons and do not

form nonvolatile salts.

Nuisance odors of hydrogen sulfide result

from small amounts of H2S in aqueous effluents

that sometimes can be overcome by the addition of

iron ions to precipitate the HjS as FeS, which has

a very low water solubility.

Sulfur

dioxide,

SO2

Sulfur dioxide and, to a much smaller extent,

sulfur trioxide are formed by oxidation of sulfur

compounds in the recovery boiler and the burning

of sulfiir-containing fuels in power boilers. When

properly operated, modern recovery boilers should

have emissions below 100 parts per million.

Sulfur emissions from power boilers are controlled

by using fuels of low sulfur content. Some sulfur

dioxide is emitted by the lime kiln and smelt dis-

solving tank.

Nitrogen oxides (NOJ

The majority of the oxides of sulfur and

nitrogen originate under the oxidative conditions of

the lime kiln and kraft recovery furnace. Oxides

of nitrogen can form in any combustion process,

but particularly during high temperature combus-

tion. The principal oxides of nitrogen formed

during combustion processes are nitric oxide (NO)

and nitrogen dioxide (NO2), with the latter ac-

counting for 5-10% of the total NO^ by volume.

These form by the reactions

+ O2

^F^

2N0 and

2N0 + 02?^ 2NO2. Table 11-3 shows the

production of nitric oxide NO as a function of

combustion temperature. Typical NO^, emissions

by volume are 32 ppm for the kraft recovery

boiler and 200 ppm for the rotary lime kiln. The

recovery boiler produces about 0.5 kg (1.2 lb) of

NO^

per ton black liquor solids. The lime kiln

emissions are probably relatively low for the high

combustion temperature because the gases are

slowly cooled before exiting the kiln allowing the

equilibrium to shift back to N2. Power boilers

emit from 100-450 ppm

NO^^

by volume depending

on the temperature of combustion and percentage

of excess air.

Table 11-3. Flame temperature vs. nitric oxide

equilibrimn concentration and reaction time.*

Gas

temperature of

combustion 1

1 2430

1930

1760

1430

1090

4400

3500

3200

2600

2000

NO '

Cone,

(ppm,

vol.)

19,000

14,000

4,000

500

Reaction

time, (s)

0.004

0.090

0.700

21.0

162.0

^Reactant gas of 77% N2, 15% O2, and 8% inert

gases.

Ref. 1 with data from Ref. 3.

292 11- ENVIRONMENTAL IMPACT

Noncondensible Gas, NCG

Noncondensible gases consist of TRS and

smaller amounts of other volatile organic chemi-

cals such as methanol, terpenes, phenols, and

hydrocarbons liberated from the wood. Liberated

organic chemicals may directly be pollutants or

may form pollutants as they undergo photochemi-

cal reactions (formation of smog).

Because pulp mills must run at full capacity,

and because the limiting factor in batch digesting

is the condensing or heat recovery unit, often

pollutants emitted during surges of gases formed

during blowing are not ftiUy collected in the

condensers. For example, during blowing of a

200 m^ digester from 78 psig to atmospheric

pressure, 20 tons of pulp and 20 tons of steam

(34,000 m^) are produced in about 20 minutes, a

huge amount. There is also loss of recoverable

heat if the condensing system is not adequate to

meet these needs. Factors which may compromise

the necessary condensing rate include large

digesters, short blow periods, high blow pressures,

dirty or fouled condensers, and high water temper-

atures. Mills with pollution problems caused by

inadequate condensation of blow gases can limit

the problem by the usual methods for increasing

condensation efficiency, including increasing pump

capacity for the condensers, increasing the heat

exchanger surface area, adding a secondary con-

denser, and so forth. Because the time scale is

longer for continuous pulping processes, condensa-

tion is usually not a critical problem. Most of the

TRS and much of the BOD can be removed from

the condensates by steam stripping of the conden-

sates or other treatments.

In some cases noncondensible gases are sent

to the primary air inlet of the lime kiln [with at

least 50:1 dilution and an inlet velocity of over 9

m/s (30 ft/s) to prevent flameout] where they are

oxidized to sulfite and form the calcium salt. This

is not possible in a highly closed recovery system.

Particulate matter

Particulate matter originates primarily in the

kraft recovery ftirnace, the smelt dissolving tank,

and the lime kiln. The first of these is the largest

potential source, on the order of 1 kg/t (2 lb/ton)

pulp.

Sodium sulfate is the major constituent of

particulates, with smaller amounts of sodium car-

bonate, sodium chloride, and other salts.

Particulates vary in size from 0.1 /im to 1 mm for

uncontrolled emissions to 0.1 to 10 ^m for highly

controlled emissions. Two major types of particu-

late control devices are the electrostatic precip-

itators (ESP, Section 4.4) and venturi-type

evaporator-scrubbers (Fig. 4-18).

Operation of

the

kraft recovery boiler

Proper design and operation of the kraft

recovery boiler is necessary to limit air pollution

(VanHeiningen, 1992). The upward flow of gases

should be uniform, while combustion air mixes

completely with combustion

gases.

Computational

fluid dynamics (CFD) helps achieve this goal.

Continuous monitoring of emissions should

be done for SO2 (below 150 ppm), NO^ (below 80

ppm),

CO (50-200 ppm), and TRS (below 5 ppm).

Stack opacity should be below 20%.

Firing liquor above 75% solids leads to high

emissions of particulates and high levels of NO^

caused by high combustion temperatures. Com-

bustion temperatures can be lowered by using less

preheating of the combustion air intake.

Attempting to limit NO^ emissions from the

recovery boiler by limiting the excess oxygen

leads to a large increase in carbon monoxide and

TRS emissions, so CO should be monitored also.

NO^

emissions can be decreased, sometimes, by

increasing tertiary air and decreasing air.

SO2 emission is controlled by generating

sodium ftime by reducing secondary air to develop

a large bed.

Gas analysis for pollution control and correlations

between gas constituents

In order to control pollution, one has to

measure the composition of various components of

stack gases. The basis of measuring certain gases

is given here. The following gases can be mea-

sured continuously, on-line to measure at the

source of pollution {point sampling) as follows:

oxygen is measured by paramagnetic torque,

carbon dioxide and carbon monoxide are measured

by infrared absorption, total hydrocarbons are

measured by flame ionization (although different

hydrocarbons, alcohols, etc. have different detec-

tor responses for a given amount of material),

nitrogen oxides are measured by chemilumines-

AIR QUALITY TESTS AND CONTROL 293

cence, sulfiir dioxide is measured by flame pho-

tometry, and sulfur dioxide and total reduced

sulfur are measured by UV absorbance. Sulfur

dioxide can be measured separately from the TRS

if a citrate scrubber is used to remove TRS.

More sophisticated tests can be used to quan-

tify individual components (i.e., of hydrocarbons

and TRS) without interference or to measure low

levels of pollutants downwind of the mill or from

diffuse sources of pollution (ambient sampling),

such as TRS emissions from aeration

basins.

For

example, hydrocarbons and reduced sulfur com-

pounds can be trapped from a portion of effluent

and measured by gas chromatography.

There is some correlation among the individ-

ual constituents of gaseous effluents. For exam-

ple,

an elevated level of CO indicates a somewhat

reducing environment and is expected to be ac-

companied by high levels of TRS from recovery

boiler gases. Conversely, one might see a high

level of TRS, CO, and hydrocarbons with only

excess oxygen, but a relatively low level of

these materials with 5% excess oxygen, although

this might mean a high level of

SO2

emissions.

11.7 SOLID WASTE DISPOSAL

Sludges that are produced at paper mills must

be disposed. These include the sludge from the

primary clarifiers (usually a high percentage of

wood fiber and calcium deposits from the kraft

recovery

process),

sludge removed from secondary

treatment of effluent, and sludge from clarification

of deinking mill effluent. Sludges can be thick-

ened by use of screw presses (Fig. 11-6). Chemi-

cal additives (alum or polymers) are also used.

Usually these sludges are sent to landfills.

About 20% are spread on agricultural fields, and

some is burned. In 1991, after being sued by

"pro-environmental" organizations, the EPA

proposed a new set of rules that would affect mills

that use chlorine bleaching and use land applica-

tion of sludge. To summarize the rules, the

concentration of dioxin and furan in sludges would

have to be measured to insure that the soil con-

centrations would not exceed 10 ppt. Sludge

application within 10 m (33 ft) of bodies of water

will not be allowed. There are numerous other

rules that apply to land application of sludges.

Predraining Screen

Compression Screen Motor

Discharge Finger f \

Coupling < 1^

Bearing

V-Beits

and Pulleys

GearReducer

Inlet Chamber Screen / Plug Housing \ \ Bearing

Pressure Screw Assembly Discharge Thrust Bearing

Fig. 11-6. Horizontal screw press fit for thickening sludge. Courtesy of Andritz Sprout-Bauer