Herbert Sixta 6 страница

Pulp & Paper International, 2004 (5).

22 Stigsson, L., et al., Recovery of strongly

alkaline chemicals for the NovaCell Process.

International Chemical Recovery

Conference. Charlston, SC, USA:

TAPPI/PAPTEC, 2004.

23 Lindblom, M., An overview over Chemrec

process concepts. Colloquium on

Black Liquor Combustion and Gasification.

Park City, UT, USA, 2003.

24 Mansour, M.N., R.R. Chandran, L.

Rockvam. The evolution of and

advances in steam reforming of black

liquor. TAPPI Fall Technical Conference.

San Diego, CA, USA, 2002.

25 Larson, E.D., et al., A cost-benefit

assessment of BLGCC technology. Tappi

J., 2000; 83(6): 57.

26 Thorp, B., Agenda 2020. Reachable

Goals Can Double Industry Cash Flow.

PIMA’s Leadership Conference. New

Orleans, LA, USA, 2004.

27 Nohlgren, I., S. Sinquefield, X. Zeng. In

situ caustificization for low temperature

black liquor gasification. Colloquium on

Black Liquor Combustion and Gasification.

Park City, UT, USA, 2003.

28 Thorp, B., Industry/Government Partnership

Targets New Energy Strategies.

Solutions! for People, Processes and Paper,

2003(9): 35–36.

Environmental Aspects of Pulp Production

Hans-Ulrich Suss

10.1

Introduction

Since the production of pulp uses the renewable resource wood, the environmental

impact starts with the selection of the tree species and their planting. All forestry

operations have an environmental impact. Forestry certification, for example

by the Forest Stewardship Council (FSC), is used to describe “best” conditions.

Logging processes and the transport of wood similarly have an impact, as can

wood storage and debarking. Bark can be described as the natural protection of

trees against biological activity. Fungi and bacteria use wood as nutrition source,

and therefore trees naturally produce compounds (e.g., resin acids) that have a

certain toxicity and a very high concentration of poorly biodegradable organic matter

to hinder rapid decay. The leaching of bark with water can result in a rather

high level of toxicity in such an effluent, and dry debarking is therefore preferred.

Bark is burned and used as energy source. Because bark is generally richer in

minerals than the corresponding wood [1], the ash content is usually more than

10% – which is ten times higher than that in wood. The resulting ashes contain

high levels of elements such as calcium, magnesium, potassium, sodium, iron,

manganese, zinc, and phosphorus. These trace elements are important to the

nutrition of trees, and should be returned into the forest, though this is not always

permitted.

This chapter will focus briefly on the impact of the pulping process. The dominant

process for chemical pulp production is the kraft or sulfate process. Apart

from some specific differences, other processes such as sulfite pulping or soda

pulping have a rather comparable environmental impact. Although the operation

of a pulping process requires energy. modern mills do not require fossil fuel as

the source for all energy is combustion of the compounds dissolved during the

pulping process. The resultant emissions are the release of volatile compounds

during the high-temperature pulping process and during brown stock washing.

Evaporation and combustion of the recovered liquor causes additional emissions.

In alkaline pulping the operation of the lime kiln represents an emission source.

Handbook of Pulp. Edited by Herbert Sixta

Copyright © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ISBN: 3-527-30999-3

©2006 WILEY-VCHVerlag GmbH&Co.

Handbook of Pulp

Edited by Herbert Sixta

The standard measures to reduce the impact of volatile and odorous compounds

is by the sophisticated management of these sources. This comprises their collection,

combustion and scrubbing of the remaining noncondensable gases. These

methods are applied similarly for the on-site generation of bleaching chemicals

such as chlorine dioxide and ozone. The build-up of nonprocess elements (chloride

ions, potassium, or transition metals) in the process requires the introduction

of cleaning steps that cause additional liquid or solid emissions. Washing of the

fibers in the bleach plant removes organic matter, these dissolved compounds

require a biological treatment of the effluent.

The impact of pulping, and the measures to control and limit the emissions,

have been described in a variety of publications, with the European Commission

publishing standards in 2000 [2]. Integrated Pollution and Prevention Control

(IPPC) is used to describe the “best available techniques in the pulp and paper

industry”. Similarly guidelines for a potential kraft pulp mill in Tasmania [3]

describe the requirements for an environmentally sound operation.

10.2

A Glimpse of the Historical Development

Over the years, pulp mills have become significantly larger with the development

of technology. During the late nineteenth century, a mill with an annual production

above 10 000 tons of fiber was considered to be a huge operation. Recovery of

the pulping chemicals and treatment of the effluent was not usual [4], and for the

sulfite process in some regions this approach was maintained for almost 100

years. The cheap pulping chemicals – limestone and sulfur – did not require

expensive recovery, and consequently all wood compounds dissolved during pulping

were discharged into the rivers. Assuming a pulping yield (on wood) of less

than 50%, more dissolved material such as carbohydrates and sulfonated lignin

was discharged than fiber produced.

This situation was different in kraft pulping, however, as the rather expensive

caustic soda made recovery economical. The process acquired its other name –

sulfate pulping – from the addition of sodium sulfate as sulfur make-up ahead of

the reduction and combustion steps. As the processes changed slowly to provide

better recoveries, huge amounts of organic material were removed from the effluent.

Today, in sulfite pulping magnesium is the cation of choice. Following evaporation

and combustion of the dissolved wood material, it is recovered as MgO

from the dust filters. The combustion gases contain SO2, which is recovered by

absorption by a magnesium oxide slurry. The small number of calcium sulfite

mills remaining in operation also include an evaporation stage, and produce different

types of lignosulfonates as valued byproducts. The sulfur dioxide remaining

in the combustion gases is recycled and used to prepare fresh pulping acid solution.

Therefore, sulfite mills do not contribute as excessively as in the past to the

problem of “acid rain”, and sulfur dioxide emission using the best available techniques

(BAT) is now running at 0.5–1.0 kg t–1 pulp (as sulfur) [2].

998 10 Environmental Aspects of Pulp Production

In alkaline pulping, the intensity of pulping chemical recovery has been

increased significantly. This had a pronounced impact on the color of the effluent.

In general the effectiveness of brownstock washing and measures to avoid spills

were intensified. Special basins are required to handle spills. The use of an oxygen

stage as an “extension” of the pulping process has become common practice, as

this permits to combust additional amounts of lignin dissolved by the oxygen

treatment. This requires a higher evaporation and boiler capacity, but has the

advantage of reducing the amount of dissolved organic material from the bleach

plant.

Initially, the bleaching of chemical pulp was limited to treatment with hypochlorite

in a hollander, and effluent from the bleach plant was discharged without

further treatment. Sulfite pulp responds much better to bleaching than do kraft

pulps, and by the end of the nineteenth century the demand for bright paper was

satisfied by the wide use of sulfite pulp. The intensity of the exposure to hypochlorite

cannot be enforced without damaging the fiber properties, however. A multistage

treatment with intermediate washing reduced the demand for hypochlorite

in a HEH treatment and permitted higher brightness at about 80% ISO (using

current scales for better comparison).

Bleaching in towers with elemental chlorine was first introduced during the

mid-1920s, whereupon the standard bleaching sequence became a CEH configuration.

Because chlorination causes less fiber damage at low temperature, and

the solubility of chlorine gas in water is higher at low temperature, chlorine was

applied using and discharging large amounts of water. Indeed, for the very bleachable

sulfite pulp this sequence remained standard until the early 1970s.

During the 1950s, chlorine dioxide became the standard chemical for the production

of brighter kraft pulp. Initially, chlorine dioxide was applied simply as an

additional final stage (CEHD). This procedure rapidly gained acceptance, due

mainly to its high effectiveness in brightening without causing fiber damage.

Consequently, bleaching sequences such as CEHDED became commonplace.

Increasing pulp production resulted in increasing effluent volumes and loads.

The discharge without any treatment became a significant problem, especially for

mills located on streams with poor water flow. The need to reduce the amount of

organic material was most pronounced in highly populated countries, where filtered

river water was used as source for drinking water. In other countries, a low

availability of water did not allow high pollution levels. Already in the late 1960s,

Sappi in South Africa began to develop oxygen delignification with the target of

cutting the demand for bleaching chemical and decreasing the remaining discharge

of organic compounds [5]. In Germany, the effluent of the extraction stage

was evaporated together with the calcium sulfite pulping liquor at Schwabische

Zellstoff [6]. The aim of another project was to adsorb all higher molecular-weight

compounds in the effluent on aluminum oxide [7], and then to reactivate the

adsorbent by thermal treatment in a rotating kiln. However, serious corrosion

problems as a result of high levels of chloride ions and hydrochloric acid stopped

this project. In a Canadian project, the target was an effluent-free pulp mill, and

water consumption and the bleach plant configuration and operation were modi-

10.2 A Glimpse of the Historical Development 999

fied stepwise over a number of years [8]. During these investigations, many lessons

were learned about corrosion and bleaching efficiency in narrow loops, and

it became obvious that the bleaching process was extremely difficult to operate in

a “highly closed” mode.

The biological treatment of an effluent from chlorine bleaching, for example

with CEH or CEDED, is of limited effectiveness. Some of the halogenated compounds

produced are toxic, and most are poorly biodegradable [9]. Their amounts

could be measured using absorption on to activated carbon. After washing to

remove inorganic chloride, combustion of the loaded carbon provides an indication

of the amount of absorbable organically bound halogen (AOX). The use of

chlorine in the bleaching process led to the generation of very large amounts of

organically bound chlorine. As a rule of thumb, about 10% of the chlorine applied

in a C stage was detected as AOX in the effluent (50 kg Cl2 t–1 pulp generated

about 5 kg AOX) [10]. The different reaction of hypochlorite with lignin led to the

generation of only about half of the AOX for the same amount of active chlorine.

Chlorine dioxide reacts with lignin dominantly as an oxidant. Typically, only 0.5–

1% of the active chlorine is converted into halogenated compounds (50 kg active

chlorine would generate about 250–500 g of AOX). There appeared to be no correlation

between the AOX value and effluent toxicity [11], and whilst some toxic

compounds contain halogen atoms, they are not all necessarily toxic.

During the 1980s, however, the detection of polychlorinated dioxins and furans

in chlorination effluent [12] led to the relatively rapid development of alternative

bleaching processes. Chlorine was eliminated from most bleaching sequences,

the initial intention being the complete replacement of all chlorinated compounds

(termed “totally chlorine-free”, or TCF bleaching). This could be easily achieved

with sulfite pulps, which typically have good bleachability. The conversion of sulfite

bleaching sequences from CEH (or CEHD) to shorter two-stage processes

with PP stages took only a few years, and put an end to discussions about the relevance

of AOX quantities in the description of potential or real hazards. The

absence of chlorinated products in the effluent allowed for effective biodegradation.

A sulfite mill having a lower brightness target applied the option to use magnesium

oxide as an alkalization source for the peroxide stage. This permitted

countercurrent water flow from the bleach plant to the pulping chemical recovery,

which in turn led to very low amounts of organic material remaining and the mill

being described as a (nearly) closed-cycle process [13].

Kraft pulp mills have been converted predominantly to elemental chlorine-free

(ECF) bleaching, with such processes using modified pulping and oxygen delignification

to achieve low residual lignin levels. In bleaching, chlorine dioxide and

oxidative supported extraction stages (Eop and Ep) are applied alternately, which

results in sequences such as DEopDEpD or DEopDP. Some mills use ozone to

further reduce the demand for active chlorine as chlorine dioxide, and this has

resulted in very low AOX discharge levels, whilst maintaining the pulp quality.

Depending on the intensity of the use of other chemicals and the temperature

applied in the chlorine dioxide treatment, ECF bleaching can become ECF-“light”

bleaching. This is reflected in the detectable residuals of halogenated compounds

1000 10 Environmental Aspects of Pulp Production

10.2 A Glimpse of the Historical Development

remaining in the fully bleached pulp. Such residuals are analyzed in similar manner

to AOX, and labeled as OX (“halogenated residual”). An ECF-“light” pulp can

have a residual of “OX” comparable to the “OX” background value of halogenated

organic material present in TCF pulp.

TCF bleaching of kraft pulp is much more complicated because the condensed

lignin is difficult to bleach without effective electrophilic oxidation. The compounds

available – ozone or peracid (peracetic acid or Caro’s acid) – are much less

selective than chlorine dioxide, and cannot be applied in large amounts without

risking fiber damage and pulp yield loss. Thus, high brightness or top strength

targets are typically difficult to meet simultaneously.

In 1990, only about 5% of the world’s bleached pulp was produced using ECF

bleaching sequences, but by 2002 this level had increased to more than 75%, representing

64 million tons of pulp [14]. The level of pulp still bleached with chlorine

has been affected by a slower than anticipated conversion of pulp mills in

Japan, and its relative high use in the multitude of small non-wood pulp mills in

Asia (mainly China and India). The huge kraft pulp mills in Western Europe, in

the Americas, in Australia and New Zealand, as well as those in South Africa, are

operating in the ECF mode at a level above 90%. These mills reach annual pulp

production rates of between 400 000 and 1 000 000 tons. The mills that still use

chlorine are much smaller, and produce pulp in amounts between 1000 and several

10 000 tons. These are typically old-fashioned, non-wood mills pending an

upgrade or closure. Some of these mills do not even operate a chemical recovery

unit, and therefore pollution caused by the bleaching process can unfortunately

be labeled as minor compared to the total discharge of waste.

Currently, the extent of TCF bleaching remains static, though the process is

maintaining its niche market position at around 5% of pulp production. TCF

bleaching remains the method of choice for sulfite mills, though the slow

decrease in sulfite operations and the continuing construction of new pulp mills

using ECF bleaching will in time lead to a fall in the share of TCF bleaching.

In developed countries, kraft pulp mills began to use biodegradation plants for

effluent treatment at an early stage. These included the use of large lagoons with

aerators and anaerobic decomposition zones, while activated sludge systems were

less common. The chemical oxygen demand (COD) or total organic carbon (TOC)

are decreased by bacteria which consume the organic material. Today, rather narrow

limits are set for COD and BOD (biochemical oxygen demand). The AOX are

removed in these plants predominantly by adsorption [11], and not by actual biodegradation.

Compared with the pollution levels of the early pulping operations – and also

compared to the situation just 10 to 20 years ago – pulp mills have more recently

developed into very clean operations. Their typical water demand has fallen from

more than 50 m3 t–1 pulp to below 30 m3 t–1, while some mills operate with volumes

below 10 m3 t–1. However, at such low levels of water usage other problems

such as scaling may become a serious threat to the operation. Although water saving

may lead to additional environmental emissions – perhaps via a higher

demand for bleaching chemicals or a higher energy input – sophisticated water

10 Environmental Aspects of Pulp Production

management and “kidneys” for the separation of nonprocess elements and pollutants

become essential for such an operation.

New mills are typically erected based on the continuing development of technology,

and current terms used to describe the state of the art are “accepted modern

technology” (AMT) or “best available technology” (BAT). As new or “emerging”

technology will develop into “accepted technology” once its applicability is proven,

pulping and bleaching technology cannot be described as established processes.

Indeed, all processes are undergoing continual development and further improvement.

10.3

Emissions to the Atmosphere

Typically, the combustion processes in the recovery or the bark boiler or lime kiln

result in the emission of particulate matter. Such emission may be controlled

using effective scrubbers or electrostatic precipitators.

During the pulping process, volatile organic compounds (VOC = volatile

organic carbon) are generated. The typical odor of the kraft process is caused by

mercaptans. These can be released either during the blow (discharge) of the digester

or during brownstock washing or evaporation of the black liquor. All gaseous

emissions must be collected and sent to a combustion unit. These malodorous

emissions are described as “total reduced sulfur” (TRS) and expressed as hydrogen

sulfide (H2S). These sulfur compounds are removed by oxidation to sulfur

dioxide. During the combustion process, nitrogen is oxidized to a mixture of nitrogen

oxides; they are described as NOx. The amount of sulfur dioxide or nitrogen

oxides within the gaseous effluent must not exceed certain threshold values that

vary slightly different depending upon the type of combustion unit and the fuel

material. The emission sources, the emissions and the interrelation between the

different process steps are shown schematically in Fig. 10.1.

The emissions of all these compounds are controlled using either BAT or AMT.

This could be achieved either by combustion or by washing in scrubbers with suitable

liquids. The combustion itself must be controlled to avoid the emission of

carbon monoxide or incomplete incineration. Likewise, nitrogen oxide and sulfur

dioxide emission must be controlled. Examples of the emissions permitted from

the recovery boiler and the lime kiln, the most important parts of the process, are

listed in Tab. 10.1.

10.3Emissions to the Atmosphere

wood handling

malodorous

compounds

volatile organic

compounds

bark boiler

cooking

pulp

washing

pulp

screening

bleaching pulp drying

chlorine

compounds

particles

particles,

SO2, NOx

lime kiln

particles, TRS

SO2, NOx

evaporation

recovery

boiler

recausticizing

bleaching

chemical

preparation

chlorine

compounds

malodorous

compounds

malodorous

compounds

particles, TRS

SO2, NOx

particles, SO2,

NOx

malodorous

compounds

tanks

auxiliary

boiler(s)

(oil, gas, etc)

Fig. 10.1 Sources and emissions to the atmosphere from kraft pulp mills [2].

Tab. 10.1 Selected emission limits to the atmosphere described

in 2004 for a potential pulp mill in Tasmania using “accepted

modern technology” (AMT) [3].

Emission point Pollutant Units Annual/monthly average

Recovery boiler PM mg NDm–3 50 @3% O2

TRS mg H2S NDm–3 7@ 3% O2

PCDD/PCDF pg I-TEQ NDm–3 100@ 3% O2

Lime kiln PM mg NDm–3 40@ 3% O2

TRS mg H2S NDm–3 16@ 3% O2

PCDD/PCDF pg I-TEQ NDm–3 100@ 3% O2

All sources NOx kg NO2 adt–1 1.3

All sources SO2 kg S adt–1 0.4

PM = Particulate matter (or dust).

TRS = Total reduced sulfur.

NOx = Nitrogen oxides.

SO2 = Sulfur dioxide.

PCDD/PCDF = Polychlorinated dioxins and furans.

NDm3 = normal cubic meter of dry gas.

pg I-TEQ = pg International Toxicity Equivalents.

10 Environmental Aspects of Pulp Production

Detailed data on the typical emissions and the impact of different measures

using BAT, for example, wet scrubbing only or with an electrostatic precipitator,

are available in the European Commission report [2].

10.4

Emissions to the Aquatic Environment

Washing following the pulping and bleaching process requires water, the demand

for which has decreased significantly during the past few decades. This is the

result of more effective washing procedures with improved equipment, and a

higher degree of countercurrent water flow. The process, which is typically

described as “loop closure”, became possible with the elimination of chlorine and

hypochlorite from the bleaching sequences, both of which required a low treatment

temperature and therefore cold dilution water. ECF bleaching allows a

rather constant high temperature range to be maintained from brownstock washing

to the dryer machine. The positive side effect is a lower demand for energy to

heat the process water, while the downside is a higher tendency for scaling, for

example with barium sulfate and calcium oxalate.

Cleaning of the effluent begins with the sedimentation of suspended solids,

which describes mostly the recovery of fiber losses; this step is labeled as “primary

effluent treatment”.

As mentioned previously, wood handling and debarking should be made without

generating a higher volume of effluent. If de-icing or log washing is required,

the effluent must be made nontoxic by a biological treatment. An example of this

is woodyard effluent (rain water), which must be collected and treated biologically

(unlike other rain water).

The pulping liquor should be recovered very effectively. Screening and brownstock

washing should be conducted in a closed loop mode. Spills and leakages

should be avoided by an optimized process control, but if they do occur enough

temporary storage volume and evaporation capacity must be made available to

deal with these problems. Brownstock washing following an oxygen delignification

step must remove dissolved organic compounds and inorganic pulping

chemicals effectively. Typically, the level of washing efficiency asked for is recovery

of 99% of the dissolved organic material. Similarly, the lime mud generated in the

green liquor clarification requires efficient washing.

Evaporation condensates must be stripped and appropriately reused. The condensates

of acidic pulping processes (e.g., sulfite pulping) contains compounds

such as methanol, acetic acid and furfural. These compounds may be separated, and

represent a valuable byproduct; alternatively, they can be used as an energy source in a

boiler. Water can be saved by recycling all clean cooling and sealing water.

The typical secondary effluent treatment unit for kraft pulp mills uses aerated

lagoons in which the biomass is degraded by bacteria. Zones with a low oxygen

content allow anaerobic fermentation and sludge decomposition, which decreases

the generation of excess biomass (sludge).

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