Biogas measurement for the supply of a combined heat and power plant

•Methanegasproductionintheararegionbernagwastewaterplant•UltrasonicflowmeasurementofdigestergaswithahighCO2content•Measurementoftheenergytransportfromdigestiontowertogasometerwithnopressureloss

1. Backgroundararegionbernag(arabern)operatesawastewatertreatmentplantinHerrenschwanden,Switzerland.Theplantisalsousedtogeneraterenewableenergy.ThecompanyobtainsCO2-neutralbiogas(methane)frombiogenicwasteandsludge.Usingacombinedheatandpowerplant(CHP),theoperatorisabletoconvertaportionofthisbiogasintoelectricalenergy,covering24%ofitspowerrequirementsfromitsownproduction.Theheatenergyreleasedintheprocessisusedtogeneratethermalenergy.Anotherportionofthebiogasisconvertedtonaturalgasqualitybiomethaneinatreatmentplantinordertothenbeinjectedintothepublicgasgrid.

2. Measurement requirements

Toproducedigestergas,araberndeliversthesludgefromtheprimarytreatmentstageandthebiologicalpurificationstagetodigestiontowerswhereitiscirculatedfor25daysandheatedupto35°C/95°F.Thebiogasmixturereleasedintheprocesscontainsmethane(65vol.%).Othercomponentsincludecarbondioxide(> 25 vol.%)andwater(5-7vol.%).Immediatelyfollowingproduction,thebiogasrunsthroughvariousfiltersandistransportedthroughapipelinesystemat24°C/75,2°FtoagasometerwhichthensuppliesthegasmotoroftheCHPwithenergy.Fortheproductionmeasurementofthebiogasflowfromthedigestiontowertothegasometer,arabernrequiresatechnicalsolutionthatprovidesreliableandaccuratebiogasvolumemeasurements(0...2000Nm³/h/70621scf/h)despiteveryhighmoistureinthegasflowandverylowpressure(about35mbar/0,5psi).TheapplicationallowsvirtuallynopressurelossandalsorequiresapprovalinaccordancewiththeATEXdirectiveonprotectionagainstexplosion(Zone1).

APPLICATION REPORT Water & Wastewater

arabern digestion towers

Application 357 EN 5.indd 1 04.12.2012 9:09:05 Uhr

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3. KROHNE solution

TomeasurethebiogasKROHNEprovidedtheOPTISONIC7300F.Wheninstallingtheultrasonicflowmeter,thestainlesssteelpipelinewasbroughtfromDN200/8"downtoDN150/6"toincreasetheflowvelocityofthebiogasmixture.Thefreemeasuringtubemeansnolossofpressure.ThemeasuringinstrumentwasprovidedinEx-versioninaccordancewithATEX.

TheOPTISONIC7300determinesthebiogasflowbywayofthetransittimedifferentialmeasuringprinciple.Thevelocityofsoundofthegasismeasuredatthesametime.Onthisbasisandwiththeinputofgastemperature,adiabaticindexandthemolargasconstant,thesignalconverteralsocalculatesthemolarmasswhichisusedtodeterminethemethanecontentwith2%accuracy.

TheOPTISONIC7300complementsaKROHNEGFM700ultrasonicdevicewhichhasbeenusedforyearsandwhichisstillusedforabypassmeasurement.AnotherGFM700isinstalledinfrontoftheflarestackwhereitmeasurestheflowofthemethanegasthatisburnedwhentheCHPisswitchedoff.

4. Customer benefits

TheOPTISONIC7300providesarabernwithpreciseflowmeasurementofthebiogasproducedon-site.Theintegratedcalculationofthemethanecontentinthebiogasallowstheoperatortoaccuratelydeterminetheenergyproductionofhiswastewaterplant.ThisisalsoimportantforaraberntoknowastheengineoftheCHPrunssmoothlyonlywithaminimummethanecontent.

arabernbenefitsinparticularfromtheflexibilityoftheultrasonicdeviceasitprovidesreliablemeasurementsinapressure-sensitiveenvironmentandindependentofthegascomposition.WhilemechanicalmeasuringdevicescannotbeusedinthisapplicationduetothelowpressureandthemeasurementstabilityofotherultrasonicdevicesisnegativelyaffectedbythemoistgasandthehighCO2content,theOPTISONIC7300anditstitaniumsensorsprovidegoodsignalstrengthandthushighmeasuringaccuracy.Themeasuringinstrumentismaintenance-freeandboastslong-termstability.

5. Product used

OPTISONIC 7300 •Ultrasonicflowmeterforgases•Integratedcalculationofstandardvolumeandmethanecontent•Nopressureloss•CertificationforhazardousareasaccordingtoATEX•NominalsizesDN50...DN600/2”…24”•Long-termstabilityandaccuracy•Lowinvestment,lowoperatingcosts

Contact

KROHNEMesstechnikGmbHLudwig-Krohne-Str.547058DuisburgGermanyTel.: +49(0)2033010Fax: +49(0)20330110389info@krohne.de

PleasevisitourwebsiteforacurrentlistofallKROHNEcontactsandaddresses.

Biogas measurement with the OPTISONIC 7300

Bypass measurement with the KROHNE GFM 700

APPLICATION REPORT

Application 357 EN 5.indd 2 04.12.2012 9:09:05 Uhr