Improvement ofPulp Mill Energy Efficiency in an Integrated Pulp and Paper Mill

Leena Kilponen, Helsinki University o/Technology Pekka Ahtila, Helsinki University ofTechnology Juha Parpala, Stora Enso Oyj Matti Pihko, Stora Enso Oyj

ABSTRACT

Finland is among the most advanced countries in the world in the development and use of energy efficient technologYe Minimizing the specific energy demand of industrial processes is one of the main objectives& This paper examines a case focusing on the improvement of pulp mill energy efficiency by saving steam with process modifications~ A method of simple rules based on thermodynamic principles and process integration is applied~ this particular study, the pulp heat demand would decrease by 7.6 %. The specific heat demand ofpulp would reduce from the current 9.2 GJ/ADt ofbleached pulp to the value 8.5 GJ/ADt, which, according to literature, exceeds the potential of the existing mills 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 paper different evaporation dewatering processes energy is transferred into low-grade secondary energy usually in form ofwarm or hot water, which in turn can be

__.&. ,._..Il..A..&. conditions. secondary energy is high _.L"'''"'''~_J~A.. it can replace use ofsteam a 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 Companies auditing and follow-up studies are not avail Ie. Due to of evidence, there is great prejudice against the profitability of energy conservation" Positive examples would encourage companies to proceed with their energy conservation investments and include energy efficiency auditing as part of their standard operations. Methodology

In this study energy conservation calculations are based on the general rules of thermodynamics 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 ofstreams; don't mix streams at different temperatures. • Avoid heating with primary energy at temperatures below the hottest available secondary energy. .. Study the availability ofsecondary energy, secure the operation ofprocesses at all times.

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

@ Study the effects ofenergy 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 analysis would have been too time-consuming. The advantage ofpinch analysis is that it also reveals utilities where heat is transferred across the pinch, against the rules of the minimum use of energy. In an existing process following the pinch rules requires .extensive work in the optimization ofinvestment costs. addition~ the availability ofsecondary energy is rarely a restriction mills.

ofSaved Steam

In this research the pricing method for saved steam is based on a publication of the Forest Industries Federation (1976), and has later been modified by Pihko (2000). fITst step in the calculations is to defme the price ofsaved high pressure steam according price ofpurchased fuels and their combustion efficiencies$ Saving steam should be based on the most expensive fuel used within the milt A ratio term is needed ifthere are more than one fuel that is cut back. For example, oil usually serves as cannot without the other. The price of high steam is the following:

(1)

is the price of high pressure steam, .xi the ratio of the fuel i, PEURlt,i the price of i, 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 unit conversion ofthe fuel price. The prices of saved middle and low pressure steams depend on the price of saved high pressure steam, power generation, and the price of purchased electricity. The steam consumption 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 to process 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 ofenergy extracted in the turbine is proportional to the difference in the energy of the steam passing through the turbine& The power generation of the turbine at the 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:

= +--- (5)

power to heat of middle

= +--- (6)

to heat ratio low pressure of each steam can be calculated

economic evaluation ofa project's desirability requires a stipulation ofa decision accepting 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 the 1n1"i:::J!lrn'~1 rate ofreturn..

365 Net present value. An investment proposal's net present value is derived by discounting the net 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 & Sarnat 1990)0 In this study, the net present value (NPV) can be defined as follows (Uusi-Rauva et aL 1993):

(7)

Vo = an/i 0 S, and (8)

(9) where NPV is the net present value, /0 the initial investment outlay, Vo the net profit present value, S the net cash receipt at the end ofyear n, n the operation time, i the discount rate io eo the 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" Net present value is the most reliable feasibility indicator since it reflects the absolute magnitude ofthe 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 the from a project's future cash flows. payback can be calculated Sarnat 1990):

investment Payback =------'---- (10) Annual cash receipts

The method obvious defects. It concentrates solely on the receipts within the payback period; receipts later years are ignored. The payback period is often falsely used as an argument for rejecting profitable energy conservation investments. For example, Diesen (1998) suggests that the payback time

nll"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 ofreturn can be compared with the cost of funds to a 'margin ofprofit'. Mutually exclusive decisions should be dictated by differences between the alternative proposals' net present values and not by their internal rates ofreturn.

366 Subject Analysis

In this study the subject ofinvestigation 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 PM6 and PM?, and a sheeting plant. The capacity of the pulp mill is 370 000 t/a of elementary chlorine free (ECF) pulp. The paper mill produces coated art paper, using as its raw material fully bleached softwood and hardwood .pulps from the pulp mill. The capacity of PM6 is 420 000 t/a and PM? 382 000 tla. The capacity ofthe sheeting plant is 300 000 t/a.

were analyzing and looking for streams. 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 ofthe main streams the process. The flow sheet of the pulp mill wann water system was important because it revealed the availability of secondary energy for any use. According to Gullichsen and Fogelholm (2000), the heat demand of the existing pulp mills is on average 11-12 GJ/ADt of bleached pulp. The average heat demand of the Quiu pulp mill is only 9.2 GJ/ADt, which is very close to the 8.5-9 GJ/ADt potential of the existing 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 by investigating the pulp mill steam utilities. The preheating of chemically treated water with scrubber water would require adding a third heat exchanger between the two current heat exchangers in the drying department. The circulation water used in wood deicing could be heated indirectly with scrubber water. Scrubber water could also be use'd for the preheating ofthe recovery boiler combustion air. The whole temperature potential ofthe hot water from the turpentine condenser could be utilized by connecting the stream directly to displacement bleaching.. The rest of the hot water could be used for preheating ofthe circulation water in the mill district heating system. To evaluate the feasibility ofthe found opportunities, it was necessary to analyze the duration .curves of all process variables affecting the designs. The ,investment costs of the proposed modifications were also determined.

Results

The results with three investment calculation methods are shown in Table 1. The discount rate ofthe net present value is 15 % and the operation times are 10 to 15 years. The price of purchased electricity is so decisive on the results that the calculations were made with two prices 25 ..2 and 33.6 €/MWh according to the estimated future minimum and maximum prices of electricity in the Nordic Power Exchange (Nord Pool). As seen in Table 1, the negative impact ofloss ofcogeneration power is the greater the higher the price ofpurchased. electricity· The designs with positive net present values are profitable. Only one design, the preheating ofthe recovery boiler combustion air, was found to be uneconomic due to its large investment costso The costs would be considerably lower in a new mille The total net present value ofthe profitable designs is 815 000 - 1 561 000 € depending on the price ofelectricity 0 preheating ofchemically treated water is.the most profitable, hot water tank by-pass the next etco Table 2 presents the estimated potential for saving heat with the five proposed modificationso 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 which exceeds the estimated potential ofthe 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 of the scrubber water" Emissions reduction in the Quiu Mills is based on the use of peat. The use of peat decreases by 15.3 % which is equal to 26 600 t/a. Accordingly, the carbon dioxide emissions reduce by 25 100 t/a, the sulfur dioxide emissions by 53 t/a, and the nitrogen oxides by 740 t/a. reduction in the QuIu Mills' CO2 balance is 5.1 %. The total C02 balance includes fossil fuels in use at the mills: peat, oil, and liquefied petroleum gas (LPG).. Oil is used in the lime kiln, and as backup fuel in the recovery and fluidized bed boilers. LPG is used in the IR dryers of the two paper machines. A flow sheet of the pulp mill wann water system after the changes is presented in Appendix 4.

368 Table 1~ Results ofInvestment Calculations

Case I: Net present Payback Internal rate Electricity 33.6 EUR/MWh value period ofretum [BUR] [a] [%] Preheatmg of chemtcally treated water 547 300 1.0 101 Hot water tank by-pass 173 600 0.5 216 Wood deicing 56300 4.7 20 Mill district heating 37300 3.8 26 Preheating ofSK7 combustion air*) -487900 12.1 3 Max. 814600

Case II: Net present Payback Internal rate Electricity 25.2 EUR/MWh value period ofretum [EUR] [a] [%] Preheating of cheImcal1y treated water 939200 159 Hot water tank by-pass 283 700 340 Wood deicing 240700 35 Mill district heating 97500 42 PreJtlea1tIng ofSK7 combustion air*) -251 600 9

*) uneconomic, not included in the max. values

Table Estimated Heat Energy Savings per Year ProcessrnodTI5cation K4vvh7a

Preheating ofchemically treated water 39000 Preheating ofSK7 combustion air*) 24400 W cod deicing 15800 Hot water tank by-pass 11 000 Mill district heating 5 100

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

Comments and LonC.IUSIOI1S

J!-""'''''''''' __ ~'''h.J' integration has a substantial profit potiential in particular 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 integration The 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 their costs, risks, and profits by using the net present value as the primary criterion. When companies are striving for better competitiveness, the improvement of energy efficiency is often mentioned as one of the tools. In practice, few companies are active in utilizing their full potential. In conclusion, energy conservation is an area which will require increasing 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 ofthe research project "The Effective Use of Bio-Fuels in Combined Heat and Power Production in Integrated Pulp and Paper Mills" led by Professor Pekka Ahtila at Helsinki University of Technology. The authors wish to thank the personnel of Stora Enso Fine Papers Oy Quiu Mills for their helpful comments.

References

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

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

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

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

1990. Energiankiiyton tehostamismahdollisuudet Suomen massa- ja paperiteollisuudessa [Opportunities to Improve Energy Usage in the Finnish Pulp and 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 ofPrimary Heat, Summer 1 Jul-30 Nov 1999

Production 1086 tid 90 % bleached pulp Average outside temperature 10°C Pulp mill 25.8 MW

0.2 kg/s WOOD T5 HANDLING

77.9 kg/s 4.1 kg/s COOKING RECOVERY BOILER 2.8 kgls 15.0 kgls 3.8 kgls SK7 okgls WASHING SCREENING okgls 1.lkg/s BLEACHING 1.2 kg/s 22.8 kgls ai ~ 1.0 kgls 6.0 kgls a ...... ------1 PULPING, J:J DRYING ~ 482°C, 89 bar abs. ("f') 41.3 kgls 8.. 7 MW u ~ 0.5 kg/s OTHER USE T6 --e Ii------i

ui 1.6 kg/s 19.8 kg/s FLUIDIZED ~ BED ]; EVAPORATION V") a BOILER ~ ..0 ~ ~ l': U K3 ~ U ~ <

1.9 42.7 kg/s 5.4 kg/s PAPER MILL SK7 &K3 7(l.t MW CONSUMPTION 1.5 kg/s 3.. 4MW 1.9 3.8 kgls CHEMICAL MAKE-UP WATER INDUSTRY 3ft4 2.8 kgls S(la5MW 73.5 kg/s

Power demand 150MW 4.4 kgls Power generation 58MW FEED WATER TANKS Power procurement 92MW

371 APPENDIX 2: Flow Chart ofPrimary Heat, Winter 1 Dec 1999 - 31 Mar 2000

Production 1032 tid 90 % bleached pulp Average outside temperature --6°C Pulp min 25.3 MW 454°C, 82 bar abs. 71.8 kg/s 38.1MW _2._1_kg/_s-. WOOD T5 HANDLING

59.1 kg/s 6.3 kg/s COOKING

2.4 kgls 15.1 kg/s 4.3 kgls

okg/s WASHING SCREENING

1-----1 BLEACHING 1.1 kg/s 6.6 kg/s rJi ~ okg/s 7.3 kg/s a1------1 PULPING, ..0 DRYING ~ 498°C, 92 bar abs. M 53.1 kg/s 2607MW u ~ 0.7 kgls OTHER USE T6 ~ .....-----1

~ 1.5 kgls ~------. FLUIDIZED 20.7 kgls BED ]~ EVAPORATION BOILER .,.....,.l': K3 .,.....,.

I CAUSTICIZING I okgls

t--44_.9_k_g/_s-t PAPER MILL 6.6 kgls SK7 &K3 68,.4MW CONSUMPTION 1.6 kg/s 3$8 MW 1.7 kgls 3.9 kg/s 1------""'*"'"'--_---.1._---1 CHEMICAL MAKE-UP WATER ~-----~ INDUSTRY 4~t6MW 40.9 gls 2.6 kgls 81.5 kg/s

Power demand 143MW Power generation 63MW FEED WATER TANKS Power procurement 80MW

372 ~ fud ~ ~

26·70 ><~ ~ 20-42 103·65 e=Ii11 ~ ~ S26~O I ....n> ~ ~ 195 .. 46 -- 00 145-0 ~ tI'J , Condensate a t4Sx20 ft

W VIa Wastewater Blo-siudgo treattrnlnt ~ I , W 341".0 Primary condenser t----_...... • 8. 32590 t::I (f) Secondary n ~ condenser 207-56 \C 32591 ... '" tas .. 35 't 266 .. 45.1 Recovery line canal \C 174·56 to the sea \C 100-0 I Black liquor t4J

cooler 23200 I 49 • 83 i 1-00 ... 12 .. 80 ~ 25430 lP steam Turpentine ...= condenser 23177 I 76 _86 .... 39-60 ~ Q 76-80 Q= Wash liquid 24·80 coofer26082 49· 75 Condensate A 60 .. 80

Wash liquor Warm water system, 1 Dec 99 ... 31 Mar 00 cooter 23155 100 .. 21 Balance units Va .. °C Production of bleached pulp 1032 adVd ~ Fd ~ ~ ~ £)wrldng ~ tie . . r:~-:'~:-J:~\-~:J ....P9_·..1! -" I I V V \J I I I I I I ~ ! 25«Oindrying~····-·"·····' l I ,--.------,. I =~ ,·&!.._-- ..... -"",i','\ 1\ r • .4,.", ... -"""' ...... --•••••••••••80 - 4S -.----••-._ I • V V .., : J'I :.. "'.,"'.,.....',...... _ .... .J ; e 20-42 . I ~ =fi'l'It'. f'D J VV~ ~ v ~ Jt 498· 19 JChemical water J i IllIIi 196·48 treatment rJ) ~ til fill+. 130·30 I ,. ._~ .. IIsoITothesea (1) w a Wastewater treatment t.",J ~ ~ I Primary condensar.----...... 341-0 I ~ 32590 ~ _!.:g--_..._~. Debarking I (1) 201-56 34·56 106-71 Raw water IiIIi secondary ------n condenser t:r' 32591 165 .. 34 t74·56 CIQ= 100 .. 0 =_. Black liquor "'----,~_ .. :~ ..-:::..-.., 1: ,._~.:.It~..,"\/ \/ \(: : 30-15 cooler 23200 :"iiisiridiiOOti~) = I - ~ Turpentine S· ~ I I .1 condenser 231nIi.... ff) tB1 62·78 WashUquid Condensate A cooler 26082 -..------...-...... -...... l?:!L1~=~1 I Washllquof cooI8r23165 Warm water system after changes Balance units lis .. °C