I CO-GENERATION

Approach and Findings in Energy Generationat Pulp and Paper Mills- A New LookNarsimha Rao G. R.

ABSTRACT

The last decade has witnessed a sea of development in thetechnologies and optimization of energy utilization by the pulp andpaper industry sector. However with all this Indian mills lagfar behindin terms of specific energy and water consumptive figures whencompared to the mills from the developed countries. This itself is anindicator for existence of further scope for optimization of resources.The co-generation by the mills is one of the options, while for existingco-generation facilities it would be advantageous to enhance MWcapacity utilization. With such an arrangement" the mills at developedcountries utilize only 30-40% of energy requirements from externalenergy source, whereas Indian mills depend to the extent of 70%.

The economics of co-generation is largely dictated by a carefulbalancing of heat and power output requirements and cost of purchasedpower from SEB's. The present case covered by the article outlinesa situation where purchased unit cost of power from grid was muchless than that of the mill's co-generation plant. However, in depthenergy audit of the plant helped in deciding the issues such as optionsfor increased generation from co-generation plant, cost and optimalmix of various sources of power. No doubt a well-established energyaudit procedure for pulp and paper industry is already in vogue. TataEnergy Research Institute (TERI) being pioneer in energy conservationstudies, it has been a constant endeavor of the company to help theclients in optimizing their resources especially in energy (1J'Idwater.In the present context of competitive market, it is necessary to lookfor new avenues for energy saving opportunities, including scenarioarising out of yearly power tariff revisions from utilities. This paperalso highlights the necessity of hiring 'experienced consultants toconduct instrumented audit, leading to in depth analysis, which willhelp in deciding the overall benefits to the mills.

INTRODUCTION for their unit paper prodUction in comparison to thepaper mills in developed countries. The systematic

Closer look for energy saving potential in pulpand paper mill is a must as now the energy cost nextto raw material cost of the producing a tonne ofpaper. With the technologies available. Indian millson an average are consuming lS0-20001Ohigher energy

Tata Energy Research Institute (TERI),P.O. Bos No. 1541011, Palace RoadBangalore-560 052 (Karnataka)

IPPTA VoI.-12. No.-2.Jooe 2000 1

energy audit will help to understand more about theenergy and fuel utilization in industry and inidentifying the areas and where scope for improvementexists.

Thermal energy requirement in paper mills isnormally met by coal and black liquor generated inthe process. Since the steam generation optimizationleads to reduction of energy input. The followingpoints need verification for individual plants.

• Feasibility for co-generation option in theexisting set-up.

• Heat to power ratio for co-generation.

• Present unit supply cost from utilities (SEB's).

Today, co-generation could be considered as twofaces of a coin ultimately benefiting both the industryas well as for the electrical supply utilities. For theindustry, it represents a major energy conservationopportunity and a means to reduce their productioncosts. While for the cash starved utilities, it wouldact as a means for deferring or avoiding of newcapacity addition, requiring J.!l1ajorcapital investment.

The recent audit study done by TERI highlightsthe opportunity for optimizing the co-generation atthe existing pulp and paper mill, Which has helpedthem to have reliable power supply.

LITEF' \TURE

It is an energy audit case of an integratednewsprint mill located in South India, with newsprintproduction capacity of one lakh MT per year for 49to 45 GSM. The present output is generally for 49GSM paper. The mill has a unique of combiriationboth chemical pulp and chemi-mechanical pulp units.

Energy Sources: The major direct energy sourcesto the plant are electricity, coal, furnace oil and blackliquor (produced from wood). The plant has averageload of 16 MW, with peak demand of 18 MW. Outof this, generally coal based co-generation plant withits nominal rating of 15 MW meets 10.5 MW loadand the balance of 5.5 MW (average) is met byexternal grid. For emergency requirements 2.5 MVAcapacity DG set is also available, which meets veryessential loads requirements in case of black out.

Steam Generation and Co-generation Plant: Planthas three fluidized bed combustion (FBC) boilers ofrated capacity of 60 TPH each and pressure 60 kg!

2

I CO-GENERATION

em". It has one recovery boiler of 19.5 TPH capacitywith generating pressure same as that of power boilers.Steam turbines (15 MW) used is of double extractioncum condensing type, which is presently run as singleextraction mode. The super heated steam generatedfrom the boilers is supplied to a common header. Thesteam to turbine is tapped from the header. About75 TPH steam is passed through 15 MW turbine tohave 10.5 MW power generation, which is the presentoptimal level of power generation due to variousconstraints exists in the power plant.

Based on the detailed observation for steamgeneration and utilization areas, some of the energyconservation measures identified were:

• Use of on-line instruments for continuousmonitoring of flue gas analysis and control ofexcess air.

• Optimizing the boiler feed water delivery pressurefrom the pump.

• Optimizing the steam generating pressure ofrecovery boiler.

• Integration of plate heat exchanger (in place ofshell and tube exchanger), in the blow downheat recovery system of the power boilers toutilize the heat to raise the feed water temperatureby 5-10oC more than present temperature.

The detailed steam balance was carried out usingultrasonic flow meter to measure feed water andcondensate flow at different sections. Steamconsumption at equipment level was also worked outwith heat and mass balance.

As the mill works for 24 hours round the clockrequires a reliable power supply to meet desiredtargeted output. However, power receivable duringsummer months from the grid is not uniform (shortagefor hydro-electric generation), requiring examining ofpossibility of additional generation from co-generationplant itself. This required in depth analysis for fullcapacity utilization of 15 MW turbine, which ispresently delivering only 67% of its nominal rating.

CO-GENERATION ANDOPERATING PRACTICES

PRESENTI'.

For any co-generation plant, the operation of theturbine and quantum of power generation is dictatedby the following consideration.

IPPTA Vol.-12, No.-2, June 2000

TABLE-l

STEAM GENERATION AND UTILISATION

Steam Generation Steam Utilisation

Boiler TPH Area/User TPH

PB#1 37.0 58.5 kg/em" steam to TG 75.0

PB#2 33.0 TG condensate flow 22.0

RB 16.0 3.5 kg/em" Extracted steam 53.0 (55.0)

Total 86.0

RB internal steam consumption soot 0.5Blower 18.5 kg/em'Air preheating 30 kg/em" 0.5Paper machine thermo-compressor 21 kg/em" 1.5TG internal consumption 16 kg/ern? 1.0

CP Digester's (Avg.) 5.5

3.5 kg/em" extracted steam utilizationPaper mlc 24.0CMP 2.0Recovery old evaporator 11.0Recovery new evaporator 4.0Dearator 12.0

Condensate return from processPaper mlc 15.0Old Evaporator 10.0New Evaporator 3.5CP 3.0TG condensate 22.0

FiCUre-lOperation Mode of Captive Power Generation

,

-.I----l.~Condensate(22-20TPH)

58 kglcm2

75-80 TPH

PRY

••5TPH(Avg.)

10.5 kglcm2

CPl.5 kglcm2

(53-55 )Process ,steam

IPPTA Vol.-12, No.-2, June 2000 3

I CO-GENERATION

detailed observation during audit revealed thefollowing:

• Quantity of process steam requirements.

• Heat to power generation ratio of steam turbine• The heat removal rate at condenser was

inefficient.• Tariff of the grid power for optimal mix.

The cost of power generation during the auditperiod was calculated at Rs 1.76 per unit as againstRs 2.00 if availed from grid, leading to saving of Rs0.24 per unit.

• The cooling water pumps of condenser circuitwas loaded to only 60%.

• Power generation was only at 70% of its ratingi.e. 10.5 MW (this was true even when stand-by pump was also in operation)IDENTIFICATION OF SCOPE FOR

INCREASING CAPTIVE POWERGENERATION All these observations, along with analysis lead

to the conclusion that turbine condenser was unableto take 40TPH condensate required to get additional3.0 MW Power. This will also lead to a situation ofsteam venting if extraction steam quantity was morethan process requirement. A total review for thepossible options for improving the self-generation was

From the system configuration of steam turbine,with 95 TPH of super heated steam input, generationof 13.5 MW (3 MW more) is possible. Taking processsteam requirement of 55 TPH, possible condensatefrom turbine could be estimated at 4OTPH. However

FIGURE-2IMPROVED SELF GENERATION

58 kg/crrr'95TPH

PRY

14.5 MW5TPH(Avg.)

10.5 kg/ern/

CondenserP

3.5 kg/ern'55TPH

For 40 TPHCooling Water flow

1900 m3/h

564 m3/h-l-----, NEW ERA

~OLDERA504 m3/h

Process

C.T

PumpPower consumption

118kWERA= Evaporator

4 IPPTA Vot.-1~l.No-.-2, June 2000

the necessity, which are carried out as given:

Option-Is Avoid inter mixing of process coolingwater retllrn with co-generation condenser retllr"circllit pipe.

The present configuration of return water pipecircuit, process colling water return mixes with thecondensate circuit return water from the co-generation,which was identified as major hindrance forimprovement of water flow. It was essential to getthe design water flow of 1900 m3/h through thecondenser return circuit, which was at 1200m3/h. Itwas suggested to make use of a separate dedicatedcooling tower for process side cooling water (withoutinter mixing with co-generation) and utilize presentcooling tower for co-generation alone. With this itwould be possible to get extra generation of 3.0 MW,though unit rate will go up to Rs. 2.36/unit (Rs. 0.60per unit more).

Option-He Fllrlher improving generation torated capacity of 14.5 MW

The first extraction of 12 kg/em" steam can bepassed through a downstream turbine of 2.5 MW.Chemical pulp (CP) unit require 10.5 kg/em? pressuresteam and presently this steam is availed by passing58 kg/em" pressure steam through pressure reducingvalve (PRY). By running present steam turbine indouble extraction mode, CP steam requirement canbe met through first extraction. The total first steamextraction is around 30 TPH. The maximum steamrequirement at CP is 12 TPH, 10.5 kg/em- pressureat any given time. As the operation of CP digestersare in batches, steam consumption will vary between3 to 12TPH. Hence, power generation potential is 1.5to 2.5 MW for steam flow of 15 to 22 TPH respectivelythrough the proposed downstream turbine. Due tovarying load the downstream turbine should besynchronized with grid (infinite source), so as toavoid crippling effect on the electrical system.

To generate 14.5 MW power from present steamturbine in double extraction mode, steam flow raterequired is around 120 TPH matching to the existingboiler steam generating capacity. Two power boilerscould be loaded to 85-90% to generate 100-105 TPHsteam, as the contribution of recovery boiler to mainheader is 15 TPH after its internal steam consumption.By operating FBC boilers at 85 to 90% load, theefficiency of the boiler improves by at least 2%.

With this extra generation of nearly 4 MW fromthe present level, the unit power cost may go up to

tPPTAVoJ.-12; No;-2,Jtme: 2OO(t

FIGURE-3INSTALLING 1.5 MW PARALLEL TURBINE

58 kg/ern'120 TPH

14.5 MW

Condensate30TPH

CP5TPH(avg.)

"60TPH3.5 kg/cm2

25TPH12 kg/cur'

2.5MW

Condensate-.-1---" 25 TPH

Rs. 2.57 per kWh. As the present grid power costis cheaper by Rs. 0.57 per unit, the present proposaldoes not look very attractive from the financial angle.But with the anticipated revision of electricity tariff(when new Naptha/Nuclear based units arecommissioned) the unit rate will be much higher thanRs. 30.00 per "Wh, in which case any extra self-generation will be beneficial.

Retrofit: Replacement of PRY for PowerGeneration

CP requires steam of 10.5 kg/em? for cookingin batch digesters. Presently 58 kg/em? steam passthrough PRV to meet this requirement. Installingbackpressure steam turbine in parallel with PRV leadsto power generation, without extra fuel input. As theavailability of present steam quantity varies between5 to 12 TPH at 10.5 kg/em? pressure, power generation

5

potential works out to be 159 kW to 526kW dependingupon the process steam available. The generated powerhas to be synchronized at 415 V plant bus level.Installation cost of this retrofit works out to Rs. 25.0lakh with a simple payback period of 18 months.

DISCUSSIONS

The outcome of energy audit has been reviewedafter one year. Plant personnel have implemented lowcost investment proposals. By controlling excess airand continuous monitoring of flue gas analysis resultedin 1% efficiency improvement in boilers and reductionin coal consumption was also reported. Feed waterpump pressure settings were changed during the studyperiod and reduction in power consumption has beenmeasured. Plant has installed variable speed drives(VSDs) for boiler induced draft fans and realizedsubstantial energy savings. Water pumps and processpumps modifications has done and electrical savingsare obtained. Due to tariff revision by supply utilityby Rs 0.80 (present rate Rs. 2.80/unit). the paybackperiods have improved further than anticipated earlier.If the plant completes the measures under option- Iwill definitely leads to additional generation by 3 MW

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and higher cost saving benefits than anticipated atthe time of audit. due to revision in board tariff.

CONCLUSION

With the ever-growing cost of electricity fromElectricity Board and other utilities. improvedperformance of existing co-generation plants even by10% will lead to better resource utilization. Unlessaudit is done in systematic manner. such improvementopportunities at co-generation plants may be lostpermanently there by not able to meet the main aim!objective of the audit itself. Instrumented audit.expertise and experience of the auditors play vital rolein audit part. which ultimately lead to un-roll thehidden of the mill performance in achieving a goalof "rational use of energy".

REFERENCES

1. Strategies for Rational Use of Energy "CaseStudy on Co-generation in Indian Industry" -TERI Project Code No. 941E53.

2. Comprehensive Energy Audit- TERI Project CodeNo. 98B073

IPPTA Vol.-12. No.-2. June 2000