Bentzen, Jens Dall

Henriksen, Ulrik Birk

Hindsgaul, Claus

Condensate, produced when gas from downdraft biomass gasifier is cooled, contains organic compounds that inhibit nitrifiers. Treatment with activated carbon removes most of the organics and makes the condensate far less inhibitory. The condensate from an optimised two-stage gasifier is so clean that the organic compounds and the inhibition effect are very low even before treatment with activated carbon. The moderate inhibition effect relates to a high content of ammonia in the condensate. The nitrifiers become tolerant to the condensate after a few weeks of exposure. The level of organic compounds and the level of inhibition are so low that condensate from the optimised two-stage gasifier can be led to the public sewer

Online Research Database In Technology

                       General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.  • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal   If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.     Downloaded from orbit.dtu.dk on: Dec 17, 2017Condensate from a two-stage gasifierBentzen, Jens Dall; Henriksen, Ulrik Birk; Hindsgaul, ClausPublished in:Proceedings of 1. World Conference and Exhibition on Biomass for Energy and IndustryPublication date:2000Document VersionEarly version, also known as pre-printLink back to DTU OrbitCitation (APA):Bentzen, J. D., Henriksen, U. B., & Hindsgaul, C. (2000). Condensate from a two-stage gasifier. In Proceedingsof 1. World Conference and Exhibition on Biomass for Energy and Industry Sevilla, Spain.Published by Kyritsis, S., Beenackers, A.A.C.M., Helm, P., Grassi, A., Chiaramonti, D. (eds) 2001,1st World Conference on Biomass for Energy and Industry. Proceedings of the Conferenceheld in Sevilla, Spain, 5-9 June 2000, London, James & James (Science Publishers) Ltd.1CONDENSATE FROM A TWO-STAGE GASIFIERJens Dall Bentzen* and Ulrik HenriksenTechnical University of Denmark, Department of Energy Engineering, 2800 Lyngby, Denmark, uh@et.dtu.dk*COWI Consulting Engineers and Planners AS, Parallelvej 15, 2800 Lyngby, Denmark, jdb@cowi.dkABSTRACT: Condensate, produced when gas from downdraft biomass gasifier is cooled, contains organic compounds thatinhibit nitrifiers. Treatment with activated carbon removes most of the organics and makes the condensate far less inhibitory.The condensate from an optimised two-stage gasifier is so clean that the organic compounds and the inhibition effect are verylow even before treatment with activated carbon. The moderate inhibition effect relates to a high content of ammonia in thecondensate. The nitrifiers become tolerant to the condensate after a few weeks of exposure.The level of organic compounds and the level of inhibition are so low that condensate from the optimised two-stage gasifiercan be led to the public sewer.1. BACKGROUNDSince the energy production from biomass is CO2-neutral,biomass gasifiers have a future as environmentally friendlyenergy supply. Therefore it is essential that newenvironmental problems are not created when biomassgasification is introduced to the market.Until now, there has been too little focus on the potentialenvironmental problems that condensate from gasifiers maycreate.When the product gas from a gasifier is cooled below thedew point of the gas, a condensate is produced. Thiscondensate may contain toxic organic compounds andinhibit nitrifiers in waste water treatment plants.If the condensate needs to be disposed it is necessary toinvestigate the amount of toxic compounds and the amountof inhibition of nitrifying bacteria.The two-stage gasification process was originallydeveloped to gasify straw. When a dry fuel as straw isgasified, it is possible to evaporate and recycle thecondensate to the process, thus preventing the process fromproducing tar water.If wood chips, which have a high moisture content, are thefuel in a gasifier it is necessary either to- dry the fuel and recycle the condensate- clean and discharge the water into the public seweragesystem.- optimise the gasification process so that the condensatebecomes so clean that it can be led to the public sewerwithout treatment.For technical and financial reasons, the last two solutionsmay be preferable, and condensate and possibilities ofcleaning the condensate from the two-stage gasifier weretherefore investigated.2. PURPOSEThe purpose of this work was to investigate if thecondensate produced from a two-stage gasifier was cleanenough, or with simple means could be cleaned enough, tobe led to the public sewer.3. TESTSCondensate from two test runs with the 100 kW (thermal)two-stage gasifier was analysed for organic compounds andfor toxicity to the nitrifiers (a specific group of bacteriaconverting ammonia into nitrate) in the waste watertreatment plants.Both samples were analysed before and after treatment withactivated carbon from the gasifier. It is well known thatactivated carbon can remove certain organic compoundsincluding PAH from waste water [2], [3].The two samples of condensate were different since thegasifier was reconstructed and optimised between the twotests [1]. The condensates that were treated and analysedwere from tests in March, 1998, and in September, 1998,when the gasifier had been reconstructed and optimised.The four tested samples are numbered as follows:1a: Condensate before optimisation of gasifier, filtered.1b: Condensate before optimisation of gasifier, treated withcrushed activated carbon and filtered.2a: Condensate after optimisation of gasifier, filtered.2b: Condensate after optimisation of gasifier, treated withcrushed activated carbon and filtered.Treatment: For each sample of condensate one part wasfiltered through a 0,45 µm filter and one part was treatedwith activated carbon.Investigation of char from the two-stage gasifier documentsthat the char is an activated carbon with very good2adsorption data. [1] [4]. It was therefore decided toinvestigate the condensate before and after treatment withactivated carbon produced by the two-stage gasifier.Activated carbon from a test of the 100 kW two-stagegasifier in January 1998 was used.For the treatment, 5 grams of crushed activated carbonwere stirred in one litre of condensate for one hour andthen filtered through a 0,45 µm filter.4. ORGANIC SUBSTANCE IN THE CON-DENSATE FROM A TWO-STAGE GASIFIERPhenols and other organic compounds are formed duringpyrolysis and Polyaromatic Hydrocarbons (PAH) areformed during further reactions. Some PAH's can causecancer and are therefore not wanted in waste water and influegasses. The contents of Phenols, PAH and otherorganics have been investigated at RISØ and at VKI inDenmark.5.1 Organic substance in the condensate before andafter cleaning with activated carbonTable 1. Organic compounds of condensate from a two-stage gasifier before and after treatment with activatedcarbonSample pH NVOCmg C/LPhenolmg/LNaphthalenmg/L1a1b2a2b9,079,299,179,1556314,86,3120,01*0,001*0,0005*0,120,0020,002*0,02*maxIn Table 1 it is seen that the amount of organics in thecondensate is dramatically reduced after the reconstructionand optimisation of the gasifier. Results from analysis ofcondensate from later tests (1999) are similar to the resultsafter the optimisation shown in Table 1 (September 1998).Further, it is seen in Table 1 that treatment of condensatefrom downdraft gasifiers that are not optimised will reducethe content of toxic organic compounds effectively.5.2  PAH's in condensateTwo samples of condensate from different days on thesame test after the optimisation of the gasifier wereanalysed for 27 PAH's.Table 2. Content of 27 PAH components in thecondensate from the test in September 1998.9/9-98µg/l10/9-98µg/lNaphthalen 21 1,02-Methylnaphthalen < 0,1 < 0,11-Methylnaphthalen < 0,1 < 0,1Biphenyl < 0,1 < 0,1Acenaphthylen < 0,3 < 0,5Acenaphthen < 0,1 < 0,2Dibenzofuran 0,2 < 0,1Flouren 0,3 0,51-Methylflouren < 0,1 < 0,1Phenanthren 1,9 3,1Anthracen < 0,2 < 0,32-Methylphenanthren 0,2 < 0,12-Methylanthracen < 0,1 < 0,11- Methylphenanthren < 0,1 < 0,1Fluranthen 0,3 0,3Pyren 0,2 0,31-Benzoflouren < 0,1 < 0,1Benzo(a)anthracen < 0,3 < 0,5Chrysen < 0,3 < 0,5Benzo(b+j+k)flouranthen < 0,3 < 0,5Benzo(e)pyren < 0,1 < 0,1Benzo(a)pyren < 0,2 < 0,4Indeno(1,2,3-cd)pyren < 0,2 < 0,4Dibenzo(a,h)anthrecen < 0,1 < 0,2Benzo(g,h,i)perylen < 0,2 < 0,4Sum af 27 PAH’er 24 5It is seen in Table 2 that the content of most of thecomponents is so small that it is below the detection level.The most important difference between the samples of the9th and the 10th September, 1998, is the content ofNaphtalen. Naphtalen is toxic, and therefore the lowestvalue should be reached either by process optimisation orby treatment with activated carbon.5. INHIBITION OF NITRIFIERS FROMCONDENSATEWaste water that is led to the public sewer should notinhibit the biological process in the waste water treatmentplant. The most sensitive of the biological processes isnormally nitrification.To measure the inhibition of nitrifiers, a sample of wastewater and sludge from a waste water treatment plant isused. The waste water and the sludge are mixed, and it ismeasured how much the nitrifying bacteria are inhibited inpercentage.In Denmark there are two limit values for 20% samplemixed with 80% sludge. If the inhibition is more than 50%,the waste water should not be allowed to be led to thepublic sewer.If the inhibition is between 20-50%, further investigationsof the waste water should be made. It should then beinvestigated if the sludge will become habituated to the3waste water, so that the inhibition will be reduced aftersome time.If the inhibition is less than 20%, the waste water should beallowed to be led to the public sewer.The experiments of the inhibition of nitrifying bacteria aremade by Jes La Cour Jansen at Lunds Universitet, Sweden,[5].4.1 Inhibition of condensate from the two-stage gasifier-200204060801000,1 1 10 100Inhibition of nitrifiers % 1a1b2a2bConditional acceptanceAcceptancePercentage of condensateFigure 1. Inhibition of nitrifiers from condensate froma two-stage gasifier.In Figure 1, the results of the inhibition tests are shown.Condensate labelled 1 is from the gasifier before theoptimisation and label 2 is condensate from the gasifierafter the optimisation. Label a is untreated condensate, andlabel b is condensate treated with activated carbon.It is seen in Figure 1 that untreated condensate before theoptimisation inhibits very much, but it is also seen thatsimple treatment with activated carbon reduces theinhibition so much that the cleaned condensate is in thezone where further investigations, “a tolerance test”, arenecessary.The inhibition curves from the treated and the untreatedcondensate after the optimisation are the same as from thetreated condensate before optimisation. Treatment of thecondensate after the optimisation with activated carboncould not reduce the inhibition further.5.1.2 Tolerance test of the condensateInhibition tests are made of the condensate from thegasifier after the optimisation to investigate if the sludgewill become habituated to the waste water, so that theinhibition will be reduced after some time.The tolerance test was made during three weeks. At start,there were small concentrations of condensate, but thesewere increased and kept at a level so that the condensatewas about 40% of the water in the test.In Figure 2 is shown the inhibiting effect of the condensatefrom the normal sludge that is used for the inhibition testsand from the sludge that has been exposed to thecondensate.-200204060801000,1 1 10 100Inhibition of nitrifiers % Not exposed sludgeExposed sludgeConditional acceptanceAcceptancePercentage of condensateFigure 2. Inhibition of nitrifiers from condensatemixed in ordinary sludge and from sludge that hasbeen exposed to condensate for three weeks.In Figure 2 is seen that there is a moderate inhibition in theordinary sludge equivalent to the results in Figure 1. Theinhibiting effect of the exposed sludge is reduced to a levelthat will be accepted so the condensate can be led to thepublic sewer.The content of ammonia in the condensate is high (1340mg/l) so it was investigated if the inhibition was caused bythe content of ammonia.5.1.3 Inhibition test of distilled condensateTo investigate if the inhibition was caused by the highammonia content, 1000 ml of the condensate was distilled.Since the boiling point of ammonia is lower than that ofwater, ammonia will be distilled of first. The inhibition wasthen examined of the following three fractions of thedistillate: The first 10% distillate: the next 70%, and thelast 20%. See Table 3 and Figure 3.Table 3. Simple chemical characterisation of condensatefrom the optimised two-stage gasifier before and afterdistillation.Type of condensate CODmg/lAmmoniamg/lpHCondensateFirst 10 % distillateNext 70 % distillateLast  20 % distillate5632102091395817553109,089,659,529,764-200204060801000,1 1 10 100Inhibition of nitrifiers %CondensateFirst 10 %Next 70 %Last 20 %Conditional acceptanceAcceptancePercentage of condensateFigure 3. Inhibition of nitrifiers from condensate and threefractions of distilled condensate.In Table 3 and Figure 3 is seen that the condensatecontains 1395 mg ammonia pr. litre, and that the inhibitionis similar to earlier tests (Figures 1 and 2). The first 10%distillate, which contains 6 times the concentration ofammonia, is inhibiting very much, while the other fractionsinhibit less than required [5].6. CONCLUSIONSRegarding inhibition of nitrifiers, the condensate from thetwo-stage gasifier can be led to the public sewer afteroptimisation. Almost all the inhibition in the condensate ofthe two-stage gasifier can be related to the ammoniacontent [5].The condensate from the gasifier before the optimisationwas strongly inhibiting, but a simple treatment withactivated carbon produced by the gasifier itself reduced theinhibition to an acceptable level.The content of PAH and other organic compounds is verylow in the condensate of the two-stage gasifier afteroptimisation.To reduce the content even further and to ensure a lowlevel, the condensate can be led through an activatedcarbon filter.From these investigations of the condensate from a two-stage gasifier, the condensate from an optimised two-stagegasifier is considered to be so clean that it can be led to thepublic sewer in full compliance with the environmentallegislation.ACKNOWLEDGEMENTSThis work was financed by the Danish Energy Agency.REFERENCES[1] J. D. Bentzen and U. Henriksen. Optimised two-stage gasifier. 1st World Biomass Conference andTechnology Exhibition, 5-9 June, Sevilla, Spain.[2] NORIT. "Introduction to Activated Carbon".[3] P. Hasler, P. Morf, R. Buehler, T. Nussbaumer.Gas Cleaning and Waste Water Treatment forSmall Scale Biomass Gasifiers. Swiss FederalOffice of Energy, September 1998.[4] J. D. Bentzen and U. Henriksen. Investigations of atwo-stage gasifier. Olle Lindström symposium.KTH Stockholm 1999[5] J. la C. Jansen and K. Jönsson; Nitrificationinhibition of tar-water from wood chips gasifiers;1.st World Biomass Conference and TechnologyExhibition. 5-9 June, Sevilla, Spain.

Condensate from a two-stage gasifier

http://orbit.dtu.dk/ws/files/3594373/sevilla2000_v8-86.pdf

Condensate from a two-stage gasifier

Abstract

Similar works

Full text

Available Versions

Online Research Database In Technology