Cogeneration: A Latest Developmentin the Manufacturing ofAnhydrous Ammonia

The latest developments in the manufacture of anhydrous ammoniain the past several years have been projects directed at reducingthe energy required to produce a ton of ammonia. This paperdescribes a unique method to generate electric power at 5,700Btu/kWh with a gas turbine and utilize the heat in the exhaustfrom the gas turbine to preheat various streams in an adjacentammonia plant, thus reducing the amount of fuel energy requiredin production. This unit has operated successfully since December,1986.

Paul J. BadameArcadian Corporation, Geismar, LA

Arcadian Corporation operates fertilizer com-plexes at Geismar, Louisiana and LaPlatte,Nebraska* Each location produces anhydrousammonia, nitric acid, ammonium nitrate (AN)and urea. The AN and urea are mixed to pro-duce a 32% urea-ammonium nitrate (UAN) solu-tion or URAN'**'. The Geismar site also in-cludes a phosphate operation which producessul furic acid, phosphoric acid, superphospho-ric acid (SPA), ammonium Dolyphosphates (11-37-0) solution or POLY-NW and diammoniumphosphate (DAP). While the major products areURAN, SPA and POLY-N for the fertilizer indus-try, some of the intermediates used in thefertilizer manufacture are further processedto be supplied to the industrial chemical mar-ket. These products are aqua ammonia, indus-trial ammonia, 65% nitric acid, 50-70% ureasolution, 93-99% sul furic acid, 93-99% lowiron sulfuric acid, 54% Pg05 phosphoric acidand HQ phosphoric acid ( a low fluoride, lowsolids 54% P2Û5). Arcadian also producesspecialty fertilizer products such as SURANWa 28-0-0-2(5) solution, NZNW a 15-0?0(Zn)solution, a foliar fertilizer TRISERT̂ 'a 13-3-4-0.5(5) solution, and a liquid lawn fertil-izer N-SUREWa 28-0-0 solution

The ammonia plant at the Geismar, Louisi-ana site, which is located on the MississippiRiver about 20 miles south of Baton Rouge,Louisiana, is the location which is the sub-ject of this report.

Arcadian Corporation recently installed a22 MW gas turbine driven generator as part ofa cogeneration project to supply most of itselectrical power requirements. The overallproject schedule from time of conception tofirst electrical power generation was twenty(20) months; however, it took six (6) monthsto obtain information and evaluate which gasturbine best fit the overall cogeneration con-figuration. Once the gas turbine and genera-tor were selected, it took fourteen (14)months from the date the turbine was orderedto first electrical generation. Installedcost for the system was approximately $350 perKW at a payout of less than two years.

The project included a Sulzer-Escher WyssLtd. Type 10 gas turbine, driving a BrownBoveri generator, via a high efficiencyBayeriscke Berg-Hutten and Salzwerke AG (BHS)planetary gearbox, along with heat recoveryequipment, which were successfully integratedinto the operation of the ammonia plant.Electricity was generated at a reduced energyconsumption and cost, and waste heat was re-covered by preheating streams in the existingammonia plant; thus reducing the energy re-quirements and operating cost of the ammoniaplant and nitrogen area.

Several unique and innovative featureswere included in the Arcadian cogenerationproject. Some of these features are chillingand filtering the air inlet to the gas tur-bine, selecting higher efficiency componentsfor the cogeneration system, integrating the

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exhaust waste heat recovery design into theammonia process, erection of the gas turbinepackage in a minimum period, etc. Several ofthe above features have been included on apatent application which is pending.

Arcadian's process of recovering wasteheat from the exhaust of a gas turbine by in-tegration in an ammonia plant can be easilyutilized in other chemical or petrochemicalplants. A basic understanding of the processplant and properly designed controls are allthat is required.

Based on the highly successful installa-tion and operation of the above cogenerationconfiguration, Arcadian Corporation is willingto license its cogeneration technology and"know-how".

The latest developments in the manufac-ture of anhydrous ammonia in the past severalyears have been retrofit projects directed atreducing the energy required to produce a tonof ammonia and thus reduce costs. Projectssuch as lower energy requirement C02 removalprocesses, make-up synthesis gas drying, re-duced pressure drop in the synthesis gas loop,etc. have been implemented successfully. Also,retrofits to lower energy consumption and in-crease capacity and new low energy consumptionprocesses requiring less than 25 MM BTU (LHV)per ton of ammonia have been constructed andstarted-up successfully. As shown in TableNo. 1, the energy or cost savings for this Co-generation scheme is equivalent to construct-ing such a plant; however, at a capital costof less than one-tenth the cost of a modernlow energy ammonia plant.

In the past, gas turbines have been util-ized in ammonia plants as mechanical drivesfor compressors with the gas turbine hot ex-haust gas being used to heat combustion airfed to the primary reformer, drives for elec-trical generators with the hot exhaust gasesused to generate steam, etc. This paper des-cribes a unique method to generate electricpower at less than 5,700 BTU/KWH with a gasturbine and utilize the exhaust gases to pre-heat various streams in an adjacent ammoniaplant; thus, reducing the amount of fuelenergy required in the ammonia plant. Theunit was successfully started up in December1986. The electricity generated was used tooffset purchased electricity and thus reducethe operating cost for fertilizer manufacture.The importance of reducing electrical cost isexplained in the next paragraph.

In early 1984, the projected cost ofelectricity, from the local utility company,indicated that electricity would double incost from 1984 to 1988. As s result, ArcadianCorporation, whose electric bill in 1983 wasover 6.0 MM$ per year, could not afford a 100%increase in its electric bill to 12.0 MM$ peryear and remain competitive in the fertilizerindustry. Thus, studies were initiated todetermine what could be done to reduce elec-trical costs.

Initial studies included many items suchas cogeneration, conversion of electric motordrives to steam turbine drives, operate onsteam turbine drives versus electric driveswhere already available, improve processefficiencies, etc. Besides the many smallprojects implemented, the biggest reductionwas in converting an 8,000 HP electric motor(3,000 HP normal operating) to a steam tur-bine drive in October 1985. Besides reducingelectrical costs, this project helped to im-prove the plant steam balance (most of whichis derived from waste heat recovery).

Several cogeneration projects were stud-ied, including installing cogeneration in thesul furic acid plant and in the nitrogen areaof the complex which is the largest energyand electrical consuming section of the plant.Fossil fuel was used in the auxiliary boilerand convection section of the ammonia plantto provide high pressure steam generation andsuperheat and preheat for various streams inthe ammonia plant. The objective was to de-termine how best to use this fossil fuel in acogeneration system.

After completion of an ammonia plantheat and material balance, certain deficien-cies in preheating feed streams and otherbottlenecks were defined. These deficiencieswere integrated into the exhaust conditionsof several different gas turbine vendors. Themost economical design from an installed cost,electrical generation load, and heat recoverybasis was a Sulzer-Escher Wyss Ltd. Type 10gas turbine unit which was rated for approxi-mately 22.0 MW. This unit is manufactured inWinterthur, Switzerland.

All cogeneration configurations studiedwere based on a topping cycle configurationand no sale of electrical power to the publicutility. A topping cycle is a facility inwhich the energy input to the facil i ty isfirst used to produce useful power output,and the reject heat from power production isthen used to provide useful thermal energy.

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While in a combined cycle, the gas turbine ex-haust is used to generate high pressure steamin a waste heat boiler, the steam is used toproduce more electricity in a steam turbinedriven generator, arid the exhaust steam isused to meet process requirements.

The purpose of cogeneration, which is"the efficient generation of electricity anduseful heat from a single fuel", is to saveenergy and reduce operating costs. Figure No.1 attached shows the basic concept of cogener-ation. As indicated, the electric utilityconsumes 10,300 BTU (LHV) to generate a KWH ofelectricity (for an overall thermal efficiencyof 33%}, while Arcadian's cogeneration schemeconsumes 5,700 BTU (LHV) to generate the sameKWH of electricity (.for an overall thermalefficiency of 60%). Thus, the energy cost forthe cogenerated electricity is about 45% lessthan electricity generated by the utility com-pany. The overall efficiency of the cogenera-tion scheme including heat recovery is 78%.

Arcadian's cogeneration scheme is shownin Figure No. 2. As indicated, air at ambientconditions is chilled and filtered before en-tering the suction of a gas turbine (6T) unit.The purpose of chilling is to maximize powerproduction and eliminate power fluctuationscaused by changes in ambient air temperaturefrom day to night and summer to winter. Thepurpose of filtering the ambient air is to re-duce fouling of the air compressor (AC) bladeswhich would result in decreased power output.The gas turbine unit consists of a 10 stageaxial flow air compressor which compresseschilled-filtered air to approximately 190 psig.The compressed air enters the combustion sys-tem, which consist of 18 fuel-steam nozzlesand an annular combustion chamber, where natu-ral gas is injected into the air stream to pro-duce a temperature of 1930°F. The hot combus-tion gases then flow to the gas generator tur-bine (GGT) which operates at 9700 rpm and pro-vides the horsepower to drive the air compres-sor. The hot gases then flow to the powerturbine (PT) which operates at 7700 rpm andprovides the horsepower to drive the speed re-duction gear (SRG). The GGT and PT both con-sist of a two-stage hot gas expander. The SRGreduces the PT speed to 1800 rpm and drivesthe generator (GEN), which is rated at 24 MW.As indicated above, the gas turbine unit is asplit shaft unit with the air compressor andgas generator turbine on one shaft and thepower turbine on the other shaft. Figure No.3 shows a cross-section view of the gas tur-bine. The exhaust gases from the PT at 943°Fexhaust to the heat recovery unit (HRU), where

various streams from the ammonia plant arepreheated. The gases then exhaust to the at-mosphere at about 390°F.

The output of the generator at 13.8 KVis fed into the plant electrical distributionsystem along with a purchased power tie-infrom the electric utility. Along with thecogeneration project, a new electrical sub-station was installed to receive standby andpurchased power and an electrical loopingsystem at 13.8 KV was installed.

The gas turbine/generator unit is housedin a preformed, prestressed concrete buildingwith an overhead crane over the gas turbineand is protected by a low-pressure C02 fireprotection system. In designing the gas tur-bine/generator foundation and building,Arcadian developed a construction methodwhereby the unit can be installed in minimumtime; thus, reducing construction costs.

The gas turbine generator is controlledby a computerized system, which is designedfor automatic startup of the system from acold start by going through a sequence ofoperations to full power and from full powerto a controlled shutdown and cooling period.All startup, operating, shutdown and coolingfeatures are automatically controlled andmonitored. A control system is provided toadjust generator load so as to purchase mini-mum power to as low as 0.1 MW. If the plantload requirement is less than the maximumoutput of the generator, the above controlsystem will reduce purchased power to a mini-mum and then reduce generator load. If theplant load requirement is greater than themaximum output of the generator, purchasedpower will increase up to the contract amount.A load shedding control logic is included inthe control scheme to prevent exceeding con-tract power and to prevent generator shutdownif purchased power is not available.

A SCADA (Supervisory Control and DataAcquisition) system was installed to monitorand control the overall cogeneration andelectrical system. Displays were constructedshowing the process flow diagram with alloperating conditions including temperatures,pressures, flows, speeds, vibration levels,purchased and generated electrical power,lube oil temperature, and the one-line elec-trical diagram of the 13.8 KV plant electri-cal system showing all electrical loads. Allof the 13.8 KV circuit breakers can be openedand closed from the SCADA. The SCADA canalso be used to automatically synchronize and

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put the generation on-line. Additional dis-plays include daily 24 hour average summaryreports, historical arrays at 30 minute inter-vals of tie-line power, power factor, feedercurrent, maximum power and generator load, andhistorical arrays at 15 minute intervals ofsteam and gas flow, steam-gas ratio and gener-ator load. The latter display (the ratio) isused to determine if the gas turbine is oper-ating in compliance for NOx emissions.

The process, mechanical and civil design,drafting and purchasing were performed byArcadian personnel. The electrical design anddrafting was performed by an "outside" elec-trical consultant. Project engineering, man-agement and coordination with Sulzer were byArcadian personnel. Arcadian personnel alsoacted as the general construction contractorby sub-contracting various installation pack-ages. Sulzer provided mechanical and electri-cal erection advisors and inspectors, pre-commission, training and commission engineers,along with personnel to assist in running theperformance tests. Arcadian operating super-vision provided detailed overall training forthe operating technicians.

After a series of gas turbine and genera-tor tests, the cogeneration system has opera-ted on a continuous fu l l load basis sincemid-December 1986, with the exception of asmall number of scheduled and ammonia plantassociated shutdowns. The first inspectionwas in July 1987, at which time the un i t hadaccumulated over 5,000 operating hours. Atthat time, several design modifications andimprovements were carried out. As of August1988, the unit has accumulated over 13,000operating hours. The next major inspection isscheduled for January 1989 prior to expirationof the two year warranty period.

The unit has operated at an availabil i tyfactor of over 99%. The major gas turbinedowntime is due to planned shutdowns for "wash-outs".

Since the unit has been on stream, duringnormal operation in 1987 the amount of purchas-ed power averaged 12.6% of the total power re-quirement. During this period of time, theplant 's electric bill decreased about $500,000per month, while the unit cost of purchasedpower continues to increase. Arcadian's actualenergy cost continues to be less than 50% ofthe utilities energy cost. The above perform-ance was at a steam-to-fuel ratio (SFR) of 0.4to 0.6. In late 1987, a larger steam line wasinstalled allowing SFR's up to 1.5. Operating

at a SFR of 0.8 in 1988 has reduced purchasedpower to less than 5% of the total plant re-quirements. As a result of the higher SFR's,rates up to 24.0 MW have been experiencedwith a daily average of over 23.0 MW.

Gas Turbine Characteristics - A gas tur-bine 'T~p1ëTf^rliimc¥Tsn'̂ tëTliTTSO conditions(15°C or 59°F and 1,013 m bar or 14.69 psiaand no suction or discharge losses). Any de-viation from these conditions, except lowersuction temperature, results in a loss ofpower. In order to obtain the maximum poweroutput for the unit Arcadian installed, thefollowing items were optimized:

(1) Air Inlet Temperature - The inletair filtration system was designed to bothpretreat and chil l the air before f inal f i l -tration. Thus, the design air suction temper-ature to the gas turbine will average lessthan 59°F on an annual basis.

(2) Air Inlet Pressure Losses - A lowpressure drop inlet air chil ler/f i l ter wasinstalled.

(3) Exhaust Pressure Losses - A lowpressure drop waste heat recovery system wasinstalled.

Figure No. 4 shows a typical daily cyclepower output plot for Arcadian's system andanother gas turbine design. As indicated,due to suction ch i l l ing , Arcadian's power out-put does not vary with time-of-day (or ambienttemperature) whi le the other design varies byabout 10%. This reduction in power eitherhas to be purchased or production rates de-creased during peak hours of the day.

Other characteristics of gas turbinesare the fouling of compressor/turbine bladesfrom atmospheric pollutants and the decay ofpower generation with age. The fouling re-sults in reduced power output and can be over-come by either a crank-soak washing with asolution of detergent or a fi red-wash withcondensate. The crank-soak washing requiresa turbine shutdown and cooling off of severalhours prior to washing, whi le a fi red-washcan be performed at reduced turbine load(about 33%) in about 30 minutes. Arcadian'sexperience to date has been about three fi red-washes and one crank-soak per month. The de-cay of power generation with age results in achangeout of some abradable seals, coincidentwith changing "hot parts" after approximately40,000 hours operation. After 12,000 hoursof operation, we have not detected any loss

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in power.

Another characteristic of a gas turbineto increase power output is mass flow; thehigher the mass flow through the turbine, thehigher the power output. This may be achievedby several different methods such as the fol-1owi nq:

(1) Minimize seal air flow.(2) Replace seal air flow with steam.(3) Increase steam flow.

Arcadian and Sulzer chose to use steam flow.Stack emission testing in August 1986 indicat-ed acceptable NOx and CO emissions at steam-to-fuel ratios (SFR) up to the capacity of thesteam line (2 inches) at a SFR of 1.0. Withthe installation of the larger steam line,SFR's up to 1.5 were .obtained. At this SFRand Arcadian's suction chilling, system, a rateof 24.5 MW can be obtained with this unit.

In addition, Sulzer is currently develop-ing a steam injection system that can be usedsimply for additional power as opposed tobasically emissions reduction. This systemwill permit steam flows up to SFR of 3.5 whichwill increase the power output of the gas tur-bine to 28.6 MW.

Operating experience has shown that, withthe control systems utilized, during startups,upsets, and shutdowns, the operation of thecogeneration system is insensitive to theammonia plant; and the operation of the ammoniaplant is insensitive to the cogeneration sys-tem.

Based on the performance thus far, thecost to produce power with Arcadian's designis less than 2 cents per KWH with $2.00 perMM BTU natural gas cost.

In order to obtain an operating permit,gas emissions exit the gas turbine stack hadto be determined by analysis. Stack analysiswas performed by an outside contractor for NOxand CO at various gas turbine operating ratesand steam injection rates. Steam injectioninto the combustion chamber is used to reduceNOx emissions. The above data was used togenerate curves showing emission levels versusoperating rates and steam injection rates.These curves are used to determine if the gasturbine exhaust is in compliance of the oper-ating permits.

With a single operating generator, it wasimportant that backup electrical service forstandby and scheduled and unscheduled mainten-ance be available at a reasonable cost. To

encourage the more efficient use of fuel orcogeneration, the federal government passedThe Public Utility Regulatory Policies Act(PURPA) of 1978, which allowed a cogeneratorwith certification from the Federal EnergyRegulatory Commission (FERC) as a qualifyingfacility (QF) to fall under the PURPA provi-sions.

General requirements for qualification.Cogeneration facility is a qualifying facil-ity if it: (1) Meets applicable operatingand efficiency standards. (2) Meets owner-ship criteria.

Criteria for qualifying cogenerationfacilities. (a) Operating and efficiencystandards for topping-cycle facilities.(1) Operating Standard - The useful thermalenergy output of the facility must, duringany calendar year period, be no less thanfive percent of the total energy output.(2) Efficiency Standard - The useful poweroutput 'of the facility plus one-half the use-ful thermal energy output, during any calen-dar year period, must be no less than 42.5percent of the total energy input.

Ownership criteria, (a) General Rule -A cogenerati on f aci l i ty may not be owned bya person primarily engaged in the generationor sale of electric power, (b) Ownership Test- A cogeneration facility shall be consideredto be owned by a person primarily engaged inthe generation or sale of electric power ifmore than 50 percent of the equity interestin the facility is held by an electricutility.

Electric utility obligations, (a) Obi i -gàti on tö purchase from qua! i f yi ng f aci l i ti es .Each electric utility shall purchase anyenergy and capacity which is made availablefrom a qualifying facility, (b) Obligationto sell to qua! i fyi ng f aci 1 i ti es . Each elec-tric utility shall sell to any quäl i fyi ngfacility any energy and capacity requested bythe qualifying facility, (c) Obligation tointércönnject. Any electric utility shallmake such interconnections with any qualify-ing facility as may be necessary to accom-plish purchases or sales, (d) Parallel oper-ation. Each electric utility shall offer tooperate in parallel with a qualifying facil-ity.

Arcadian's contract with the local elec-tric utility provides for both firm andstandby power including scheduled and un-scheduled maintenance.

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In order to remain competitive in thefertilizer industry, Arcadian has installedand successfully demonstrated how to integratea cogeneration system in an ammonia plant. Theelectrical energy is used in the nitrogen areaof the complex to reduce the manufacturingcost of the end product URANW and the ther-mal energy is recovered by the ammonia plantto reduce its energy requirements.

As indicated, the cogeneration unit avail-ability during the first eighteen months ofoperation has been over 99%. The cost of co-generated power has been less than 2.0 centsper KWH, compared to 4.7 cents per KWH forpurchased power. The annual savings result ina payout of less than two years.

With additional steam injection and suc-tion chilling, a machine ISO rated for 22.0MW can produce over 24.0 MW at the generatortermi nals.

ELECTRICITY

HEATRECOVERY

ELECTRIC UTILITYBTU/KWH

10.300

COGENERATION TOTALHEAT RECOVERY

ELECTRICITY

10,381-4,702

5.679

Arcadian continues to evaluate other smalland large potential cogeneration applicationsin its fertilizer complex to further reducecosts. The energy cost charged to electricalgeneration in these applications would be min-imal, since waste heat generated steam isavailable for reduction through a steam tur-bine-generator unit. An example is cogenera-tion in sul furic acid and nitric acid plants.

Table 1. Latest development in ammonia plantdesign.

DESIGN

1980

RETROFITS

1965

COGENERATION

ENERGY COSTCONSUMPTION SAVINGS

MM BTU(LHV) /TON MM $/YR.(l)

32

29

25

2.07

4.83

4.83

COSTMM $

11

100

11

Figure 1. Cogeneration.

AFUEL.

HRU

rAIR

C/FAC

LUMB

L

GOT

1FT SRG GEN

SYMBOL DESCRIPTION

C/F CHILLER / FILTERAC AIR COMPRESSOR

COMB COMBUSTORGOT GAS GENERATOR TURBINE

PT POWER TURBINE

SRG SPEED REDUCTION GEARGEN GENERATOR

HRU HEAT RECOVERY UNIT

(1) $2.00 / MM BTU AND 345,000 TPY NH3. Figure 2. Cogeneration process.

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185

DISCUSSIONA. DETfEGE, Kemira, S.A., Tertre, Belgium: Youmentioned that chilling of the conditioner for the turbineis necessary to stabilize the output of this machine.What does it look like when you take into accountthe energy required to produce this chilling activityon your air?

P.J. BÄDAME, Arcadian Corporation, Geismar, LA:I couldn't hear most of the question.

DETIEGE: Are you chilling with chilled water orammonia? To produce this chilled water or torecondense this ammonia evaporated by the chilling,you need energy somewhere, so that there could bea trade between the saving or the increased powerfrom the gas turbine and the consumption of energysomewhere else.

BADÂME: We had some low-level waste steam (50psig) from the ammonia plant and installed a lithiumbromide absorption refrigerator unit, which uses low-pressure waste steam to achieve our refrigeration,which is chilled water.

DETIEGE: You have a heat recovery unit as well asair compressor. You should have an excess of steam,which was steam used previously for the turbine ofthe air compressor. Do you transfer that into fuelsaving?

BADAME: I didn't understand where the air compres-sor comes in. Did you mean the gas turbine aircompressor?

DETIEGE: Yes. From what I understand, you drivethe air compressor of your unit with part of yourmachine.

BADAME: No, it's being driven by the expander. Theair is compressed up to 190 psig, and then it combineswith fuel and generates temperatures up to 1,900°.The gases go through a two-stage hot gas expanderthat drives the air compressor.

DETiEGE: Last point, due to the heat recovery unit,does this system have an impact on your stack

temperature from your furnace? If not, there could bejust a trade between fuel gas burned in the turbineand fuel gas burned in the furnace.

BADAME: No, as a matter of fact, test data show thatbecause it was a different time of the year the primaryreformer stack temperature was lower. We reduce fuelin the auxiliary boiler and superheat burners, so weare putting less Btu's in the reformer section.

DETIEGE: But, the global effect is a net reduction ofthe fuel gas required for the ammonia plant.

BADAME: Yes, the fuel gas for the ammonia plantis drastically reduced.

DETIEGE: Thank you.

R. LeBLANC, The M. W. Kellogg Company, Houston,TX: Could you comment on what change, if any, therehas been in the operability or serviceability of theammonia plant with the integration?

BADAME: We learned that the operation of theammonia plant was insensitive to the cogenerationunit, whose operation in turn was insensitive to theoperation of the ammonia plant. We have seen thisa couple of times. Fortunately, about 90 million Btuare transferred into the boiler feedwater system andsome into the gas streams. And this transfer helpsstart-up. It makes the start-up with the ammonia planteasier, because you are putting in Btu's from anexternal source.

LeBLANC: Has the normal performance been goodfrom a reliability standpoint?

BADAME: Excellent, in fact. During 1987 the ammoniaplant had some shutdowns which caused the gasturbine to shut down. The gas turbine cannot operatewithout the ammonia plant because of heat recovery.On the other hand, when the ammonia plant doesnot operate, the urea plant, the ammonium nitrate plant,or nitric acid plants, which are the major electricalusers, do not operate.

LeBLANC: Thank you.

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