Improvement of Pulp Mill Energy Efficiencyin an Integrated Pulp and Paper Mill

Leena Kilponen, Helsinki University o/TechnologyPekka Ahtila, Helsinki University ofTechnology

Juha Parpala, Stora Enso OyjMatti Pihko, Stora Enso Oyj

ABSTRACT

Finland is among the most advanced countries in the world in the development anduse of energy efficient technologYe Minimizing the specific energy demand of industrialprocesses is one of the main objectives& This paper examines a case focusing on theimprovement of pulp mill energy efficiency by saving steam with process modifications~ Amethod of simple rules based on thermodynamic principles and process integration isapplied~

this particular study, the pulp heat demand would decrease by 7.6 %. Thespecific heat demand of pulp would reduce from the current 9.2 GJ/ADt of bleached pulp tothe value 8.5 GJ/ADt, which, according to literature, exceeds the potential of the existingmills in the late 1990s$ The results imply that similar savipgs can be found in other existing

provided with a corresponding comprehensive analysis0

Introduction

Energy costs a significant operating costs of the pulp and paperdifferent evaporation dewatering processes energy is transferred into

low-grade secondary energy usually in form of warm or hot water, which in turn can be__.&. ,._..Il..A..&. conditions. secondary energy is high

_.L"'''"'''~_J~A.. it can replace use of steama case a designing an improved

the mill is necessary"_""~'ll~_."l!:r different

hllril,r1o'M l1~ir~ra·tC'l to

___.........a.JL,... Jloooo I!JJlbVV''''''U.Il".IWlVIoJ' are construction0 Companiesauditing and follow-up studies are not avail Ie. Due

to of evidence, there is great prejudice against the profitability of energyconservation" Positive examples would encourage companies to proceed with their energy

conservation investments and include energy efficiency auditing as part of their standardoperations.

Methodology

In this study energy conservation calculations are based on the general rules ofthermodynamics and process integration. To avoid complexity and large investment costs,the studies were made according to the following principles:

4» Utilize the whole temperature potential of streams; don't mix streams at differenttemperatures.

• Avoid heating with primary energy at temperatures below the hottest availablesecondary energy.

.. Study the availability of secondary energy, secure the operation ofprocesses at alltimes.

(8 Calculate the profits and investment costs for each design. In the case of differentalternatives, choose the best design according to its net present value~

@ Study the effects of energy conservation on the whole process.the beginning of the studies the intention was to use pinch analysis in the

calculations. During the background studies, however, it was found that the pinch analysiswould have been too time-consuming. The advantage of pinch analysis is that it also revealsutilities where heat is transferred across the pinch, against the rules of the minimum use ofenergy. In an existing process following the pinch rules requires .extensive work in theoptimization of investment costs. addition~ the availability of secondary energy is rarely arestriction mills.

of Saved Steam

In this research the pricing method for saved steam is based on a publication of theForest Industries Federation (1976), and has later been modified by Pihko (2000).

fITst step in the calculations is to defme the price of saved high pressure steam accordingprice ofpurchased fuels and their combustion efficiencies$

Saving steam should be based on the most expensive fuel used within the milt A ratioterm is needed if there are more than one fuel that is cut back. For example, oil usually serves

as cannot without the other. The price of highsteam is the following:

(1)

is the price of high pressure steam, .xi the ratio of the fuel i, PEURlt,i the price ofi, PEURIMWh,i the price of fuel i, 17i the combustion efficiency of the fuel i, and Hi the

heating value of the fuel i. The equation has two forms according to the unitconversion of the fuel price.

The prices of saved middle and low pressure steams depend on the price of savedhigh pressure steam, power generation, and the price of purchased electricity. The steamconsumption of a back pressure turbine depends almost linearly on the relative load of the

364

turbine. The most important characteristic in back pressure power generation is the power toprocess heat ratio:

(2)

where Pg is the power generation at the alternator terminals and Qt the process heat demand.The power to heat ratios of steams in different pressures must be calculated

separatelyG The quantity of energy extracted in the turbine is proportional to the difference inthe energy of the steam passing through the turbine& The power generation of the turbine atthe alternator tennmals is the following:

where Qtl is the turboaltemator mechanical and electrical losses, mb boiler steam flow, hb

the boiler steam enthalpy, and h2 the steam enthalpy after the turbine.The definition ofprocess heat is the following:

is process heat mt the steam to process, ht the process steam enthalpy,

me process condensate return, he the process condensate enthalpy..AJl.AJIl.'-l'o-.JlI._ pressure or steam is following:

=

=

+---

+---

power to heat

(5)

of middle

(6)

to heat ratio low pressureof each steam can be calculated

economic evaluation of a project's desirability requires a stipulation of a decisionaccepting or rejecting investment projects. The next sections present the financial

u.""',.....Il.U.ll.'VJl..II. rules for wealth maximization: the net present value, payback period, and the1n1"i:::J!lrn'~1 rate ofreturn..

365

Net present value. An investment proposal's net present value is derived by discounting thenet cash receipts at a rate which reflects the value of the alternative use of the funds,summing them over the life of the proposal and deducting the initial outlay (Levy & Sarnat1990)0 In this study, the net present value (NPV) can be defined as follows (Uusi-Rauva et aL1993):

Vo =an/i 0 S , and

(7)

(8)

(9)

where NPV is the net present value, /0 the initial investment outlay, Vo the net profit presentvalue, S the net cash receipt at the end of year n, n the operation time, i the discount rate io eothe required minimum rate ofreturn on new investment, and anli the present value coefficient.

the outcome of the net present value is positive, the investment is acceptable" Netpresent value is the most reliable feasibility indicator since it reflects the absolute magnitudeof the project (Levy & Sarnat 1990). The larger the value is the better the profits.

Payback period~ The payback period gives the number of years required to recover thefrom a project's future cash flows. payback can be calculated

Sarnat 1990):

Paybackinvestment=------'----

Annual cash receipts(10)

The method obvious defects. It concentrates solely on the receipts within thepayback period; receipts later years are ignored.

The payback period is often falsely used as an argument for rejecting profitableenergy conservation investments. For example, Diesen (1998) suggests that the payback timenll"l".....1I118T"a1 be less than 3-4 years to accept an investment. The reason for this is that a higher

the implementation cannot achieved due to limitations, complications,bottlenecks the implemented investment reveals elsewhere in the production line. In

other words, Diesen's recommendation includes a certain margin for an unknown risk"

The internal rate of return (IRR) is defined as the rate. of discount,equates the net present value of the cash flow to zero (Levy & Sarnat 1990)~ If the

internal rate of return is greater than the required rate of return of the investment, the'lI_"l,·.t::!l>t"'1"1~o.1~1I" is acceptable. The internal rate of return can be compared with the cost of fundsto a 'margin of profit'. Mutually exclusive decisions should be dictated by differencesbetween the alternative proposals' net present values and not by their internal rates of return.

366

Subject Analysis

In this study the subject of investigation is the Stora Enso Fine Papers Oy Qulu Mills(Figure 1). The mills include a sulfate pulp mill, a paper mill with two paper machines PM6and PM?, and a sheeting plant. The capacity of the pulp mill is 370 000 t/a of elementarychlorine free (ECF) pulp. The paper mill produces coated art paper, using as its raw materialfully bleached softwood and hardwood .pulps from the pulp mill. The capacity of PM6 is420 000 t/a and PM? 382 000 tla. The capacity of the sheeting plant is 300 000 t/a.

were analyzing and looking forstreams. Separate flow sheets were drafted

warm water system (see Appendices 1-3).'t.o"ln..,~o.-rni-llI"il"'O levels, flow rates, and the distribution of the main

streams the process. The flow sheet of the pulp mill wann water system wasimportant because it revealed the availability of secondary energy for anyuse.

According to Gullichsen and Fogelholm (2000), the heat demand of the existing pulpmills is on average 11-12 GJ/ADt of bleached pulp. The average heat demand of the Quiupulp mill is only 9.2 GJ/ADt, which is very close to the 8.5-9 GJ/ADt potential of theexisting mills in the late 1990s. In this sense, the Qulu pulp mill is already energy efficient.

367

The current study focused on five potential energy saving opportunities found byinvestigating the pulp mill steam utilities. The preheating of chemically treated water withscrubber water would require adding a third heat exchanger between the two current heatexchangers in the drying department. The circulation water used in wood deicing could beheated indirectly with scrubber water. Scrubber water could also be use'd for the preheatingof the recovery boiler combustion air. The whole temperature potential of the hot water fromthe turpentine condenser could be utilized by connecting the stream directly to displacementbleaching.. The rest of the hot water could be used for preheating of the circulation water inthe mill district heating system.

To evaluate the feasibility of the found opportunities, it was necessary to analyze theduration .curves of all process variables affecting the designs. The ,investment costs of theproposed modifications were also determined.

Results

The results with three investment calculation methods are shown in Table 1. Thediscount rate of the net present value is 15 % and the operation times are 10 to 15 years. Theprice of purchased electricity is so decisive on the results that the calculations were madewith two prices 25 ..2 and 33.6 €/MWh according to the estimated future minimum andmaximum prices of electricity in the Nordic Power Exchange (Nord Pool). As seen inTable 1, the negative impact of loss of cogeneration power is the greater the higher the priceofpurchased. electricity·

The designs with positive net present values are profitable. Only one design, thepreheating ofthe recovery boiler combustion air, was found to be uneconomic due to its largeinvestment costso The costs would be considerably lower in a new mille The total net presentvalue of the profitable designs is 815 000 - 1 561 000 € depending on the price of electricity 0

preheating of chemically treated water is .the most profitable, hot water tank by-pass thenext etco

Table 2 presents the estimated potential for saving heat with the five proposedmodificationso The annual savings in heat energy would be 70.9 GWh. This is equal to 7.6 %

the pulp mill he demand. The pulp mill heat demand would drop to 8.5 GJ/ADt whichexceeds the estimated potential of the existing mills in the late 1990se

Electricity production decreases by 200 MW due to the loss of cogeneration power..is to 1 % of the Quiu Mills total power production. There will also be a small

increase (166.kW) in the pulp mill power demand in winter due to the increased pumping ofthe scrubber water"

Emissions reduction in the Quiu Mills is based on the use of peat. The use of peatdecreases by 15.3 % which is equal to 26 600 t/a. Accordingly, the carbon dioxide emissionsreduce by 25 100 t/a, the sulfur dioxide emissions by 53 t/a, and the nitrogen oxides by740 t/a.

reduction in the QuIu Mills' CO2 balance is 5.1 %. The total C02 balanceincludes fossil fuels in use at the mills: peat, oil, and liquefied petroleum gas (LPG).. Oil isused in the lime kiln, and as backup fuel in the recovery and fluidized bed boilers. LPG isused in the IR dryers of the two paper machines. A flow sheet of the pulp mill wann watersystem after the changes is presented in Appendix 4.

368

Table 1~ Results of Investment Calculations

Case I:Electricity 33.6 EUR/MWh

Preheatmg of chemtcally treated waterHot water tank by-passWood deicingMill district heatingPreheating of SK7 combustion air*)

Case II:Electricity 25.2 EUR/MWh

Preheating of cheImcal1y treated waterHot water tank by-passWood deicingMill district heatingPreJtlea1tIng of SK7 combustion air*)

*) uneconomic, not included in the max. values

Net presentvalue

[BUR]547 300173 6005630037300

-487900Max. 814600

Net presentvalue[EUR]

939200283 70024070097500

-251 600

Paybackperiod

[a]1.00.54.73.812.1

Paybackperiod

[a]

Internal rateofretum

[%]10121620263

Internal rateofretum

[%]15934035429

Table Estimated Heat Energy Savings per Year

ProcessrnodTI5cation K4vvh7a

Preheating ofchemically treated water

Preheating ofSK7 combustion air*)

W cod deicing

Hot water tank by-pass

Mill district heating

*) uneconorru~c" not included in the rnax$ value

Comments and LonC.IUSIOI1S

39000

24400

15800

11 000

5 100

J!-""'''''''''' __ ~'''h.J' integration has a substantial profit potiential inparticular case, applying a method of simple rules

A.VI.JI'Il,.o3.JI."""""'lI,.lI. a % a pulp performing according to the~11?"r01''\1" standardss

thorough evaluation the whole mill is a prerequisite to any process integrationThe effects of the proposed modifications have to be carefully analyzed to avoid

bottlenecks which WOllld ultimately restrict the operation time of the mvestments8 After

369

determining the best opportunities, alternative proposals should be evaluated against theircosts, risks, and profits by using the net present value as the primary criterion.

When companies are striving for better competitiveness, the improvement of energyefficiency is often mentioned as one of the tools. In practice, few companies are active inutilizing their full potential. In conclusion, energy conservation is an area which will requireincreasing resources in the future - an aspect that has yet to be widely recognized.

Acknowledgements

This research was financed by Stora Enso Oyj and is the second phase of the researchproject "The Effective Use of Bio-Fuels in Combined Heat and Power Production inIntegrated Pulp and Paper Mills" led by Professor Pekka Ahtila at Helsinki University ofTechnology. The authors wish to thank the personnel of Stora Enso Fine Papers Oy QuiuMills for their helpful comments.

References

Diesen, M .. 1998. Economics of the Pulp and Paper Industry. Papermaking Science andTechnology, Part 1. Jyvaskyla, Finland: Finnish Paper Engineers' Association, andTAPPI.

a..."III~lI"'l"lllC"h Forest Industries Federation. 1976. Energian han/dnta ja hinta [Purchase and PriceofEnergy]. Helsinki, Finland: (in Finnish)

p.-A., Asblad, A., and Bemtsson, 1999. "Process Integration Applications in thePulp and Paper Industry." In Proceedings of P/'99 International Conference onProcess Integration, Volume L Papers. Copenhagen, Denmark. March 7-10, 1999.Vanlose, Denmark: Nordic Energy Research Programme and International EnergyAgency.

and Fogelholm, 2000. Chemical pulping. Papermaking Science andTechnology, Part 6B. Jyvaskyla, Finland: Finnish Paper Engineers' Association and

1990. Energiankiiyton tehostamismahdollisuudet Suomen massa- japaperiteollisuudessa [Opportunities to Improve Energy Usage in the Finnish Pulpand Paper Industry}. Helsinki, Finland: Jaakko Poyry Oy. (in Finnish)

and Sarnat, M. 1990. Capital Investment & Financial Decisions. 4th edition.Cambridge, United Kingdom: Prentice Hall.

Haverila, and Kouri. 1993& Teollisuustalous [Industrial Economics}. Tampere,Finland: Infacs Johtamistelmiikka Oy. (in Finnish)

370

APPENDIX 1: Flow Chart of Primary Heat, Summer 1 Jul- 30 Nov 1999

Production 1086 tid 90 % bleached pulpAverage outside temperature 10°C

Pulp mill 25.8 MW

RECOVERYBOILER

SK7

T5

2.8 kgls

okgls

okgls

77.9 kg/s

15.0 kgls

0.2 kg/s

4.1 kg/s

3.8 kgls

WOODHANDLING

COOKING

WASHINGSCREENING

1.lkg/sBLEACHING

22.8 kgls1.2 kg/s

OTHER USE

PULPING,DRYING

u~ 0.5 kg/s--e Ii------i

ai~ 6.0 kglsa ......------1

J:J~("f')

8..7 MW

1.0 kgls

T6

1.9

FLUIDIZED ];ui 1.6 kg/s 19.8 kg/s~

BED a ~EVAPORATION

BOILERV") ..0~

K3~ l': U

~ U~ <

482°C, 89 bar abs.41.3 kgls

SK7 &K3CONSUMPTION3..4MW

MAKE-UP WATER3ft4

5.4 kg/s

1.9

42.7 kg/s

1.5 kg/s

3.8 kgls

2.8 kgls

PAPER MILL7(l.t MW

CHEMICALINDUSTRYS(la5MW

73.5 kg/s

4.4 kgls

FEED WATER TANKS

Power demandPower generationPower procurement

150MW58MW92MW

371

APPENDIX 2: Flow Chart of Primary Heat, Winter 1 Dec 1999 - 31 Mar 2000

Production 1032 tid 90 % bleached pulpAverage outside temperature --6°C

Pulp min 25.3 MW454°C, 82 bar abs.71.8 kg/s

T5

2.4 kgls

okg/s

38.1MW

59.1 kg/s

15.1 kg/s

_2._1_kg/_s-. WOOD

HANDLING

6.3 kg/s

COOKING

4.3 kgls

WASHINGSCREENING

6.6 kg/s

1-----1 BLEACHING

1.1 kg/s

OTHER USE

EVAPORATION

I CAUSTICIZING I

20.7 kgls ~-------.

u~ 0.7 kgls~ .....-----1

rJi~ 7.3 kg/sa1------1 PULPING,..0 DRYING~M

~ 1.5 kgls

]~

l':.,.....,..,.....,.

2607MW

T6

okgls

okg/s

FLUIDIZEDBED

BOILERK3

498°C, 92 bar abs.53.1 kg/s

2.6 kgls

1.6 kg/s

3.9 kg/s

SK7 &K3CONSUMPTION3$8 MW

MAKE-UP WATER40.9 gls

6.6 kgls

1.7 kgls

t--44_.9_k_g/_s-t PAPER MILL68,.4MW

CHEMICAL1------""'*"'"'--_---.1._---1~-----~ INDUSTRY

4~t6MW

FEED WATER TANKS

81.5 kg/s

Power demandPower generationPower procurement

143MW63MW80MW

372

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26·70

24·80

12 .. 80

39-60

103·65

195 .. 46

Condensate A 60 .. 80

20-42

76-80

1-00

t4Sx20

25430 lP steam

Blo-siudgo treattrnlntI ,

207-56

Wastewater

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49· 75

174·56

Secondarycondenser

32591

Wash liquidcoofer26082

Black liquorcooler 23200 I 49 • 83 i

Turpentinecondenser 23177 I 76 _86 ....

Primary condensert----_ ......

• 8. 32590

100-0

S26~OI

~

--145-0

Condensate a,W~W 341".0

Wash liquor

cooter 23155 100 .. 21

Warm water system, 1 Dec 99 ... 31 Mar 00Balance units Va .. °CProduction of bleached pulp 1032 adVd

~Fd~

~~~tie

_.=~S·~ff)tB1

~=~e~=fi'l'It'.f'DIllIIirJ)~tilfill+.(1)

a~

~(1)IiIIi

nt:r'

==CIQ1:

_!.:g--_..._~.Debarking IRaw water ------

I ,. ._~ .. IIsoITothesea

196·48

165 .. 34

Condensate A

Warm water system after changesBalance units lis .. °C

106-71

-..------...-.........-......l?:!L1~=~1 I62·78

130·30

34·56

treatment

201-56

Wastewater

"'----,~_ ..:~ ..-:::..-..,,._~.:.It~..,"\/ \/ \(: : 30 -15

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WashllquofcooI8r23165

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£)wrldng

. . r:~-:'~:-J:~\-~:J....P9_·..1! -"I I V V \J I II I I I! 25«Oindrying~····-·"·····' lI ,--.---------,. I

, · i',' , 80 - 4S J'&!.._-- .....-"", \ 1\ r • .4,." ... -"""' ..........--•••••••••••-.----••-._ I• V V .., : I

: .. "'.,"'.,..... ',.. ...._ .... .J ;

20-42 . IJ VV~ ~ v J~ Jt i 498· 19 JChemical water

treatment

TurpentineI I .1 condenser231nIi....

I Primary condensar.----......341-0 I ~ 32590

100 .. 0

wt.",J~