Herbert Sixta 5 страница

generated is 3.5 tons per ton of black liquor solids. Note that some of the generated

steam is consumed by the boiler itself. Sootblowing steam, steam for air/

liquor pre-heating and feedwater preparation need to be deducted from the gross

steam generation to obtain the net steam quantity available for the mill.

The data in Tab. 9.4 show that the humidity of the flue gas accounts for a considerable

energy loss from the boiler. The humidity comes mainly from the water

in the black liquor, from water formed out of hydrogen in organic material, and

from sootblowing steam. Increasing the dry solids concentration of the black

liquor, and thereby reducing the water input to the boiler, leads to a higher steam

generation per mass unit of black liquor solids (Fig. 9.9).

9.2 Chemical Recovery Processes

Tab. 9.4 Simplified recovery boiler heat balance.

System input/output Mass

[kg ton–1 dry solids]

Specific enthalpy

[kJ kg–1]

Enthalpy

[MJ ton–1 dry solids]

Enthalpy of input/output streams

Black liquor 1.333 2.8. 130 485

Pre-heated air (dry) 4.909 1.0. 120 589

Humidity of pre-heated air 70 2.725 190

Sootblowing steam 100 2.820 282

Flue gas (dry) 5.137 0.96. 180 –888

Humidity of flue gas 827 2.840 –2.349

Smelt 448 1.500 –672

Reaction enthalpy

HHVof black liquor solids 1.000 14.000 14.000

Reduction to Na2S 89 13.090 –1.170

Reduction to K2S 25 9.625 –244

Losses –300

Heat to steam 9.923

Feedwater/steam

Feedwater 3.494 510 1.782

Total steam generation 3.494 3.350 11.705

90%

95%

100%

105%

110%

60% 70% 80% 90%

Relative steam generation

Black liquor solids concentration, wt.-%

Fig. 9.9 Steam generation in a recovery boiler as a function of

the black liquor solids concentration; typical curve normalized

to 100% at 75% solids concentration.

9.2.3.2 Causticizing and Lime Reburning

9.2.3.2.1 Overview

The causticizing and lime reburning operations target at the efficient conversion

of sodium carbonate from the smelt to sodium hydroxide needed for cooking. As

a part of the cooking chemical cycle, the preparation of white liquor consists of

several process steps, and is accompanied by a separate chemical loop, the lime

cycle (Fig. 9.10). The generated white liquor ought to contain a minimum of residual

sodium carbonate in order to maintain the dead solids load in the cooking

chemical cycle as low as possible.

Cooking /

washing

COOKING

CHEMICAL

CYCLE

Evaporation

Recovery

boiler

Smelt

dissolving

Green liquor

filtration /

Slaking clarification

Causticising

Whilte liquor

filtration

LIME

CYCLE

Lime reburning

Lime mud

washing

Fig. 9.10 Major unit operations of causticizing and lime

reburning in the context of the kraft chemical recovery cycle.

Process wise, the smelt coming from the smelt spouts of the recovery boiler

drops into the smelt dissolving tank and becomes dissolved in weak wash, thereby

forming green liquor. Since the smelt carries impurities which disturb the subsequent

process steps, those must be removed by clarification or filtration of the

green liquor. Then follow slaking, causticizing and white liquor filtration. After

separation from the white liquor, the lime is washed and reburned for re-use in

causticizing.

9.2.3.2.2 Chemistry

The basic chemical reactions in the causticizing plant and lime kiln start with the

exothermic slaking reaction, where burned lime, CaO, is converted into calcium

hydroxide, Ca(OH)2 (slaked lime):

CaO _ H2O→Ca_OH_2 DH _ _65 kJ kmol_1 _12_

986 9 Recovery

Then the causticizing reaction transforms sodium carbonate from the smelt,

Na2CO3, to sodium hydroxide needed for cooking, thereby giving rise to calcium

carbonate, CaCO3:

Na2CO3 _ Ca_OH_2 _ 2NaOH _ CaCO3 DH≈ 0 kJ kmol_1 _13_

Calcium carbonate is separated from the white liquor and reburned at a temperature

above 820 °C following the endothermic calcination reaction:

CaCO3→CaO _ CO2 DH _ _178 kJ kmol_1 _14_

From a chemical perspective, white liquor is fundamentally characterized by

active or effective alkali concentration, by sulfidity, as well as by causticizing and

reduction efficiencies (see Section 4.2.2). In the causticizing plant, the total titratable

alkali (TTA) is also of interest. Causticizing efficiency, CE, and TTA are

defined as follows:

CE _

NaOH

NaOH _ Na2CO3 _ 100_ _15_

TTA _ NaOH _ Na2S _ Na2CO3 _16_

The concentrations of the sodium salts in Eqs. (15) and (16) are, by convention,

expressed in g L–1 and in terms of NaOH or Na2O equivalents.

Lime (CaO), calcium hydroxide (Ca(OH)2) and calcium carbonate (CaCO3), also

referred to as “lime mud”, all have a very low solubility in water. Reactions related

to these components are basically happening in the solid phase.

The equilibrium of the slaking reaction is far on the product side of Eq. (12),

and slaking is completed within 10–30 min. In contrast, the equilibrium of the

causticizing reaction in a typical kraft pulp mill would be reached at a causticizing

efficiency of about 85–90% (Fig. 9.11). The equilibrium conversion rate depends

mainly on total alkali, sulfidity, lime quality, and temperature. A higher TTA and

sulfidity reduce the equilibrium causticizing efficiency through product inhibition.

As the retention time proceeds, the causticizing reaction is increasingly limited

by the diffusion of reactants and reaction products through the increasingly

thicker layer of CaCO3 around the hydroxide core of the particle. The equilibrium

efficiency can be reached only with an excess of lime and at very long retention

times.

Actual mill operations deal with time restrictions, and must avoid over-liming

in order to maintain good filterability of the white liquor. As a consequence, the

average achievable causticizing efficiency on mill scale is 3–10% lower than the

equilibrium efficiency. Typical total retention times provided in the causticizers

are around 2.5 h.

9.2 Chemical Recovery Processes 987

70% 75% 80% 85% 90% 95% 100%

Total titrable alkali (TTA), g/L as NaOH

Causticising efficiency

Total titrable alkali (TTA), g/L as Na2O

Typical operation window

0% sulphidity

30% sulphidity

Fig. 9.11 Equilibrium causticizing efficiencies at 0% and 30%

sulfidity [14] and typical operating window for kraft mill causticizing

systems. Sulfidity in this diagram is defined as

NaOH/(Na2S + NaOH + Na2CO3).

9.2.3.2.3 White Liquor Preparation Processes and Equipment

The preparation of white liquor begins with smelt dissolving. Weak wash and

smelt form the green liquor, a solution of mainly sodium carbonate and sodium

sulfide. The green liquor carries some unburned carbon and insoluble inert material

from the smelt, which are detrimental to the downstream recovery and pulping

processes if not removed, together with lime mud particles. While the removal

of these so-called “dregs” was traditionally carried out by sedimentation, the

requirements of today’s increasingly closed mills are best met with green liquor

filtration. Since filters retain much smaller particles than clarifiers, the levels of

insoluble metal salts are kept low. Several types of filters with and without lime

mud filter aid are in use, such as candle filters, cassette filters, crossflow filters or

disk filters.

The dregs separated from the green liquor are subjected to washing for recovery

of valuable cooking chemicals. Dregs washers are typically rotary drum filters

with a lime mud precoat. As the filter drum rotates, the dregs are dewatered,

washed, and finally discharged from the drum at 35–50% dry solids by a slowly

advancing scraper together with a thin layer of precoat. The consumption of lime

mud for the precoat amounts to at least the same quantity as the dregs. This consumption

is, however, not considered a loss because some lime mud must be

sluiced from the lime cycle anyway for process reasons. Otherwise, nonprocess

elements would accumulate in the lime cycle to problematical levels.

Clear green liquor coming from clarification or filtration proceeds to slaking.

An example of a slaker is shown in Fig. 9.12. Lime mud and green liquor enter

the equipment from the top of the cylindrical slaker bowl and are intensely mixed

988 9 Recovery

by an impeller. Not only the slaking reaction, but also a major part of causticizing

occurs in the slaker. The slurry flows from the slaker to the classifier section, from

where it overflows to the causticizers. Grits – that is, heavy insoluble particles

such as sand and overburned lime – settle in the classifier. These are transported

by an inclined screw conveyor through a washing zone, and leave the cooking

chemical cycle for landfill, together with dregs.

Fig. 9.12 A slaker [16].

The slurry from the slaker enters the first of typically three causticizers, each of

which is divided into two or three compartments (Fig. 9.13). The slurry flows

from one unit to the next by gravity. Minimum backmixing between compartments

ensures that the causticizing efficiency advances to a maximum. Agitation

in slakers and causticizers needs special attention in order to avoid particle disintegration,

since small lime mud particles reduce the white liquor filterability.

Fig. 9.13 A causticizer train [17].

After causticizing, the lime mud is removed from the slurry by pressure disk

filters or candle (pressure tube) filters. The use of clarifiers for that purpose is fading

out. A pressure disk filter, where the slurry from the last causticizer enters the

filter vessel at the bottom of the horizontal shell, is shown in Fig. 9.14. White

liquor is pushed by gas pressure through the precoat filter medium on the rotat-

9.2 Chemical Recovery Processes 989

ing disks into the center shaft, and flows to the filtrate separator. There, the white

liquor and gas are separated. While the white liquor proceeds to storage, a fan

blows back the gas from the top of the filtrate separator to the shell of the disk

filter to provide the driving force for filtration. Lime mud accumulates on the filter

medium as it rotates submerged in slurry, is then washed and continuously

scraped from the surface of the disc at 60–70% dry solids. The lime mud is then

discharged through a number of chutes into the mud mix tank. The washing step

leads to a minor dilution of the white liquor, but reduces the requirements of

downstream lime mud washing.

Fig. 9.14 A pressure disk filter [18].

The examples of equipment solutions described above are what will most likely be

found in a new mill. Existing causticizing plants are likely to appear quite different, as

they may have seen certain pieces of equipment taken into different service over time

as causticizing capacity increased. Equipment with potential application in changed

positions includes rotary drum filter for the washing of dregs or lime mud; candle

filters for white liquor filtration or lime mud washing; and sedimentation clarifiers

for clarification of green liquor or white liquor, or for lime mud washing.

9.2.3.2.4 Lime Cycle Processes and Equipment

Lime mud from the white liquor filter is pumped to storage and then washed on a

rotary drum filter for the removal of soluble liquor constituents. The wash filtrate

resulting fromlimemud washing, termed “weakwash”, is used for smelt dissolving.

The lime mud coming from the lime mud washer contains 75–85% dry solids.

It is either dried with flue gas in a separate, pneumatic lime mud dryer or is fed

directly to the rotary kiln for drying and subsequent calcination. The diagram in

Fig. 9.15 shows how solids and gas flow countercurrently through a lime kiln

with a drying zone. Lime mud enters the refractory lined kiln at the cold end. The

kiln slopes towards the firing end, and the solids move downwards as the kiln

990 9 Recovery

slowly rotates. At first, water is evaporated from the lime mud in the drying section,

and then the carbonate is brought to calcination temperature in the heating

zone; finally, the calcination reaction takes place in the calcination zone. The high

lime temperature at the firing end causes agglomeration and slight sintering. The

overall retention time in the lime kiln is typically 2–4 h. Before leaving the kiln,

the lime is cooled in tubular satellite coolers and in turn heats up fresh combustion

air. After that, the larger lime particles are crushed and the lime is stored in a

silo for re-use in slaking.

The quality of the burned lime is characterized mainly by the amount of residual

calcium carbonate, typically 2–4%, and by the lime availability – that is, the

percentage of lime which reacts with acid, typically 85–95%. Lime make-up

requirements are usually in the range of 3–5%.

1.000

1.500

2.000

SOLIDS FLOW

Drying Heating Calcination

GAS FLOW

FLUE GAS

LIME MUD

FUEL, AIR

BURNED LIME

Flame

Rotary kiln AIR

Satellite

cooler

Temperature,.C

Solids

Gas

Fig. 9.15 Schematic of a lime kiln with temperature profiles of solids and gas.

The energy supply for the very endothermic calcination reaction usually comes

from firing of fuel oil or natural gas. Approximately 150 kg of fuel oil or 200 Nm3

natural gas are needed per ton of lime product. The oxygen for fuel combustion is

supplied by air. The flame extends into the calcination zone, where the major part

of the energy is transferred by radiation. As the flue gas passes through the kiln,

its temperature falls gradually. Only about one-half of the chemical energy in the

fuel is consumed by the calcination reaction, while about one-quarter is needed

for evaporation of water from the lime mud. The remainder of the energy is lost

with the flue gas and via the kiln shell. The flue gas which exits the kiln carries

dust and, depending on the type of fuel, also sulfur dioxide. It is cleaned in an

electrostatic precipitator for the elimination of particulates and, if needed, in a wet

scrubber for SO2 removal.

9.2 Chemical Recovery Processes 991

9.2.3.3 The Future of Kraft Chemical Recovery

9.2.3.3.1 Meeting the Industry’s Needs

The core technology of the Tomlinson-type chemical recovery boiler was developed

in the 1930s. Various improvements have been made since then, and especially

the energy efficiency has improved dramatically. However, certain inherent disadvantages

of today’s recovery systems are inflexibility regarding the independent

control of sodium and sulfur levels in white liquor, as well as the safety risk connected

to explosions caused by smelt/water contact.

Future recovery technologies are challenged by the technical requirements of

modern kraft cooking processes with regard to liquor compositions, by increasingly

stringent environmental demands, and last – but not least – by the industry’s

everlasting strive for improving the economic efficiency of pulping. In fact,

the ongoing developments address all of these issues, and novel recovery techniques

provide for the appealing long-term perspective that product diversification

will once make pulp mill economics less dependent on pulp prices alone.

With regard to the near future, the two technologies which have conceivable

potential to change the face of kraft chemical recovery are black liquor gasification

(BLG) and in-situ causticization. Major achievements have been made in these

fields since the 1990s, and commercialization is currently in progress [19–22].

9.2.3.3.2 Black Liquor Gasification

Black liquor gasification is founded chemically on the pyrolysis of organic material

under reducing conditions, or on steam reforming with the objective to form

a combustible product gas of low to medium heating value. The main gasification

products are hydrogen, hydrogen sulfide, carbon monoxide, and carbon dioxide.

Gasification processes are divided into low-temperature techniques, where the

inorganics leave the reactor as solids, and into high-temperature techniques,

which produce a slag. Both gasification types allow the separation of sodium and

sulfur, and both bear insignificant risk of smelt/water incidents.

High-temperature gasification occurs at about 1000 °C in an entrained flow reactor.

Air is used as oxidant in low-pressure gasifiers, whereas high-pressure systems

operate with oxygen. The black liquor decomposes in the reactor to form product

gas and smelt droplets, both of which are quenched after exiting the reaction

chamber. The smelt droplets dissolve in weak wash to form green liquor. The

product gas serves as fuel after particulate removal and cooling [23].

Low-temperature gasification is carried out in an indirectly heated fluidized bed

reactor, with sodium carbonate bed material and at a temperature around 600 °C.

Superheated steam provides for bed fluidization, and the required energy is supplied

by burning a portion of the product gas in pulsed tubular heaters immersed

in the bed. Green liquor is produced from surplus bed solids. The product gas

proceeds to cleaning and further on to utilization as a fuel [24].

Due to the separation of sulfur to the product gas, the salts recovered from gasification

have a high carbonate content. Despite the flexibility in producing cooking

992 9 Recovery

liquors of different compositions, the overall mill balance for sodium and sulfur

must be observed. Selective scrubbing of hydrogen sulfide from the product gas

and absorption in alkaline liquor is a must for kraft mills which operate at typical

sulfidity levels. Depending on the set-up of hydrogen sulfide absorption, mills

may run into increased loads on causticizing and lime kiln processes [25].

BLG is particularly energy efficient when applied together with combined-cycle

technology – that is, when the product gas is burned in a gas turbine with subsequent

heat recovery by steam generation (BLGCC; see Fig. 9.16). In such a case,

the yield of electrical power from pulp mill operations can be increased by a factor

of two compared with the conventional power generation by steam turbines alone.

The related benefits range from the income generated from selling excess electrical

power to the environmental edge of replacing fossil fuel elsewhere.

BLACK LIQUOR

GASIFIER

GAS COOLING

AND CLEANING

SULPHUR

RECOVERY

Product gas

Condensed phase

(salts, smelt)

Thick

black

liquor

Clean syngas

GAS TURBINE

Sulphur

COOKING LIQUOR

PREPARATION

EVAPORATION HEAT RECOVERY STEAM TURBINE

Exhaust gas

Electricity

Steam

MILL USERS

AND EXPORT

Electricity

Flue gas to stack Steam to process

Fig. 9.16 The principle of black liquor gasification with combined cycle (BLGCC).

At present, some BLG installations are operating on a mill scale, mostly providing

incremental capacity for handling black liquor solids. The encountered difficulties

are mainly the adequate carbon conversion for the low-temperature process

and the choice of materials for the high-temperature process. With regard to

black liquor solids, the capacities of the currently installed systems are less than

10% of today’s largest recovery boilers. When the process and material issues are

settled, appropriate scale-up will be the next challenge. Nevertheless, it is expected

that gasifier-based recovery systems will operate a number of reactors in parallel

because physical limitations restrict the maximum size of a unit.

With regard to the combined-cycle systems, the BLG processes must be followed

by efficient gas-cleaning steps. Cleaning of the synthesis gas is especially needed

9.2 Chemical Recovery Processes 993

because some volatile tar is formed during gasification, and this must be kept

from entering the gas turbine.

9.2.3.3.3 In-Situ Causticization

The second group of technologies on the brink of commercialization includes

autocausticization. This is based on the formation of sodium hydroxide in the

recovery furnace by means of soluble borates circulating in the cooking chemical

cycle. Under certain conditions of causticizing or lime kiln limitations, partial

autocausticizing can remove a bottleneck. Mill trials have demonstrated the technical

feasibility, improved causticizing efficiency, and energy savings in lime

reburning, and the process has been applied in one mill [26].

Another technique of in-situ causticization is that of direct causticizing. The process

is still in the conceptual phase, and builds on the formation of sodium titanates

or manganates in combination with BLG. The reactions in the gasifier

release carbon dioxide to the product gas. Titanates are more efficient at converting

carbonates to carbon dioxide than are manganates. The metal oxides proceed

from the gasifier to a leaching step, where sodium hydroxide is formed in the

presence of water. The insoluble metal oxides are then separated from the liquor

and returned to the gasifier [27].

9.2.3.3.4 Vision Bio-Refinery

Although a general breakthrough in novel recovery techniques is not expected

before 2010–2015, it is likely that over the next decades a number of technologies

will emerge to match certain applications. Fully commercialized BLGCC applications

will add substantial flexibility to pulp mill operations, and will represent a

most important step towards the pulp mill as a bio-refinery. Developments in the

future may then involve the production of liquid biofuels from product gas, export

of pure hydrogen or fabrication of hydrogen-based products [28].

9.2.4

Sulfite Chemical Recovery

The recovery of cooking chemicals from the sulfite pulping process can be split

into primary and secondary recovery steps. This definition relates to the recovery

of sulfur dioxide (see Fig. 9.17). In sulfite cooking, gas is continuously relieved

from the digester for pressure control during the time at temperature, and the

cook is also terminated by a pressure relief. The resulting relief gas contains considerable

amounts of sulfur dioxide together with a bulk of water vapor and some

NCG such as carbon oxides. In the primary recovery system, this gas is subjected

to countercurrent absorption by fresh cooking acid in a number of vessels operated

under staged pressure levels.

994 9 Recovery

Cooking /

washing COOKING

CHEMICAL

CYCLE

Evaporation

Recovery

boiler

Secondary

recovery -

SO2 absorption

from flue gas

Primary

recovery -

SO2 absorption

from relief gas

RELIEF GAS

RECYCLE

Fig. 9.17 The sulphite cooking chemical cycle.

Following evaporation, thick spent sulfite liquor is usually fired in a recovery

boiler under an oxidative environment. Sulfur leaves the boiler in the form of SO2

with the flue gas, and is subsequently absorbed from the flue gas in the secondary

recovery system. The design of both recovery boilers and secondary recovery systems

is largely different between sulfite cooking bases. While magnesium and

sodium bases can be recovered from the spent cooking liquor and re-used for

cooking acid preparation, the recovery of calcium and ammonium bases is not

practicable.

The sulfite pulping process is of declining relevance. New developments in the

area of sulfite recovery are minor and very site-specific. They target mainly at

reduced emissions to atmosphere and at more flexibility regarding combined and

free SO2 in the cooking acid.

References 995

References

1 Sjostrom, E., Wood Chemistry: Fundamentals

and Applications. Academic

Press, Inc.: San Diego, California, 1981:

200–202.

2 Alen, R., Conversion of cellulose-containing

materials into useful products.

In Cellulose Sources and Exploitation –

Industrial Utilization, Biotechnology and

Physico-Chemical Properties. Ellis Horwood

Ltd: Chichester, England, 1990:

453–464.

3 Alen, R., Analysis of degradation products:

a new approach to characterizing

the combustion properties of kraft black

liquors. J. Pulp Paper Sci., 1997; 23(2):

J62–J66.

4 Lisboa, S.A., et al., Isolation and structural

characterization of xylans from

eucalyptus globulus kraft black liquors.

In Eighth European Workshop on Lignocellulosics

and Pulp. Riga, Latvia,

2004.

5 Zeng, L., A.R.P.v. Heiningen, Pilot fluidized-

bed testing of kraft black liquor

gasification and its direct causticization

with TiO2. J. Pulp Paper Sci., 1997;

23(11): J511–J516.

6 Soderhjelm, L., M. Hupa, T. Noopila,

Combustibility of black liquors with different

rheological and chemical properties.

J. Pulp Paper Sci., 1989; 15(7):

J117–J122.

7 Forssen, H., M. Hupa, P. Hellstrom,

Liquor-to-liquor differences in combustion

and gasification processes: nitrogen

oxide formation tendency. TappiJ.,

1999; 82(3): 221–227.

996 9 Recovery

8 Frederick, J., Black liquor properties. In

Kraft Recovery Boilers, T.N. Adams, Ed.

TAPPI Press: Atlanta, GA, 1997: 61–99.

9 Ryham, R., High solids evaporation of

kraft black liquor using heat treatment.

In Proceedings, Engineering Conference.

Seattle, WA, 1990.

10 Fricke, A.L., Physical properties of kraft

black liquors: Final Report – Phase I

and II. DOE Report no. DOE /CE

40606-T5 (DE88002991), 1987.

11 Zamann, A.A., S.A. Tavares, A.L. Fricke,

Studies on the heat capacity of slash

pine kraft black liquors: effect of temperature

and solids concentrations.

J. Chem. Eng. Data, 1996; 41: 266–271.

12 Chemical Pulping. Papermaking Science

and Technology, Book 6B, J. Gullichsen,

H. Paulapuro, Eds. Helsinki: Fapet Oy,

2000.

13 Kocurek, M.J., Ed. Alkaline pulping. Pulp

and paper manufacture, 3rd edn. Vol. 5.

Atlanta, Montreal: The joint textbook

committee of the paper industry

(TAPPI/CPPA), 1989.

14 Hough, G., Ed. Chemical Recovery in the

Alkaline Pulping Processes. TAPPI:

Atlanta, GA, USA, 1985: 196.

15 Kocurek, M.J., Ed. Sulfite Pulping Science

and Technology. Pulp and Paper Manufacture,

3rd edn., Vol. 4. Atlanta, Montreal:

The joint textbook committee of the

paper industry (TAPPI/CPPA), 1989.

16 Caustec Lime slaker (product leaflet).

Kvaerner Pulping: Karlstad, Sweden,

2003.

17 Caustec Causticizers (product leaflet).

Kvaerner Pulping: Karlstad, Sweden,

2003.

18 Caustec PDW filter (product leaflet).

Kvaerner Pulping: Karlstad, Sweden,

2000.

19 Kordsachia, O., Stand und Perspektiven

der Schwarzlaugenvergasung. Das

Papier, 2002; 10: 50–53.

20 Patrick, K., Gasification edges closer to

commercial reality with three new N.A.

mill startups. PaperAge, 2003; October:

30–33.

21 Jopson, N., What tomorrow may bring.

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